WO2025127741A1 - Method for transmitting and receiving data in wireless communication system, and wireless communication terminal using same - Google Patents

Abstract

The present invention provides a method and a device for operating a wireless communication terminal. Specifically, a wireless communication terminal according to the present invention may receive a physical layer protocol data unit (PPDU) from an overlapping basic service set (OBSS) access point (AP), which is not associated with the wireless communication terminal, in a primary channel of a bandwidth in which the wireless communication terminal operates. The preamble of the PPDU includes a transmission opportunity (TXOP) field and a duration field related to a TXOP duration configured by the OBSS AP. Thereafter, when a specific condition is satisfied, the wireless communication terminal may perform channel access by switching a channel from the primary channel of the bandwidth to a non-primary channel.

Description

Method for transmitting and receiving data in a wireless communication system and a wireless communication terminal using the same

The present invention relates to a channel access procedure in overlapping operating channels.

Recently, as the spread of mobile devices has expanded, wireless LAN technology that can provide fast wireless Internet services to them has been receiving a lot of attention. Wireless LAN technology is a technology that allows mobile devices such as smart phones, smart pads, laptop computers, portable multimedia players, and embedded devices to wirelessly connect to the Internet at home, in businesses, or in specific service areas based on short-range wireless communication technology.

Since supporting the initial wireless LAN technology using the 2.4 GHz frequency, IEEE (Institute of Electrical and Electronics Engineers) 802.11 has commercialized or is developing various technology standards. First, IEEE 802.11b supports a communication speed of up to 11 Mbps while using the 2.4 GHz band frequency. IEEE 802.11a, which was commercialized after IEEE 802.11b, reduces the impact of interference compared to the considerably crowded 2.4 GHz band frequency by using the 5 GHz band frequency instead of the 2.4 GHz band, and improves the communication speed up to 54 Mbps by using OFDM (orthogonal frequency division multiplexing) technology. However, IEEE 802.11a has a shorter communication distance than IEEE 802.11b. And IEEE 802.11g, like IEEE 802.11b, uses the 2.4GHz band to achieve a communication speed of up to 54Mbps and satisfies backward compatibility, which has attracted considerable attention. It is also superior to IEEE 802.11a in terms of communication distance.

And in order to overcome the limitations of communication speed, which has been pointed out as a vulnerability in wireless LAN, there is IEEE 802.11n, a technical standard. IEEE 802.11n aims to increase the speed and reliability of networks and to extend the operating range of wireless networks. More specifically, IEEE 802.11n supports high throughput (HT) with a data processing speed of up to 540 Mbps, and is based on MIMO (Multiple Inputs and Multiple Outputs) technology that uses multiple antennas at both the transmitter and receiver to minimize transmission errors and optimize data rates. In addition, this standard can use a coding method that transmits multiple redundant copies to increase data reliability.

As wireless LAN becomes more widespread and its applications diversify, there is a growing need for a new wireless LAN system that supports a higher throughput (Very High Throughput, VHT) than that supported by IEEE 802.11n. Among these, IEEE 802.11ac supports a wide bandwidth (80MHz to 160MHz) at the 5GHz frequency. Although the IEEE 802.11ac standard is defined only for the 5GHz band, early 11ac chipsets will also support operation in the 2.4GHz band to maintain backward compatibility with existing 2.4GHz band products. In theory, according to this standard, the wireless LAN speed of multiple stations will be at least 1Gbps, and the maximum single link speed will be at least 500Mbps. This is achieved by extending the radio interface concepts accepted in 802.11n, such as wider radio frequency bandwidth (up to 160 MHz), more MIMO spatial streams (up to 8), multi-user MIMO, and high-density modulation (up to 256 QAM). In addition, there is IEEE 802.11ad, which transmits data using the 60 GHz band instead of the existing 2.4 GHz/5 GHz. IEEE 802.11ad is a transmission standard that provides a speed of up to 7 Gbps using beamforming technology, and is suitable for high bitrate video streaming such as large-capacity data or uncompressed HD video. However, the 60 GHz frequency band has a disadvantage in that it has difficulty passing through obstacles, so it can only be used between devices in short distances.

Meanwhile, the IEEE 802.11ax (High Efficiency WLAN, HEW) standard, which is a wireless LAN standard following 802.11ac and 802.11ad, is nearing the development stage to provide high-efficiency and high-performance wireless LAN communication technology in high-density environments where APs and terminals are densely packed. In an 802.11ax-based wireless LAN environment, high-frequency-efficient communication must be provided indoors and outdoors in the presence of high-density stations and APs (Access Points), and various technologies have been developed to implement this.

In addition, new wireless LAN standards have been developed to increase the maximum transmission speed to support new multimedia applications such as high-definition video and real-time games. The 7th generation wireless LAN standard, IEEE 802.11be (Extremely High Throughput, EHT), is currently under development with the goal of supporting transmission rates of up to 30 Gbps in the 2.4/5/6 GHz bands through wider bandwidth, increased spatial streams, and multi-AP cooperation.

Recently, discussions have begun on ultra-high reliability (UHR) wireless LAN communication technology to overcome reliability issues that have been pointed out as limitations of wireless LAN as a wireless LAN standard after 802.11be. The ultra-high reliability wireless LAN standard is currently being developed with the goal of supporting low latency and low jitter in wireless LAN traffic with a high probability (e.g., 99.9999% or more).

The present invention aims to provide a method for improving channel accessibility for overlapping channels.

In addition, the present invention has a purpose of providing a method for performing a channel access procedure through a non-primary channel rather than a primary channel when the state of the primary channel (or primary channel) is busy.

The technical problems to be achieved in this specification are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

In a non-AP (Access Point) multi-link device (MLD) including a plurality of stations according to the present invention, a processor includes a transceiver; and a processor, wherein the processor receives a preamble of an inter-BSS PPDU (Physical Layer Protocol Data Unit) from an OBSS (Overlapping Basic Service Set) AP (Access Point) that is not associated with the wireless communication terminal on a primary channel of a bandwidth in which the wireless communication terminal operates, the preamble of the inter-BSS PPDU including a TXOP field related to a transmission opportunity (TXOP) duration set by the OBSS AP, and when a specific condition is satisfied, switches the channel from the primary channel of the bandwidth to a non-primary channel, wherein the specific condition is whether a length of a remaining OBSS TXOP calculated based on the TXOP field or the duration field is greater than a minimum duration threshold value.

Additionally, in the present invention, the state of the primary channel is busy by the OBSS AP.

Additionally, in the present invention, the minimum duration threshold is a minimum value for the wireless communication terminal to switch a channel to the non-primary channel.

Additionally, in the present invention, the processor receives a management frame from an AP associated with the wireless communication terminal, wherein the management frame includes list information of adjacent APs.

Additionally, in the present invention, the list information includes BSS color information and/or MAC address of each of the APs.

Additionally, in the present invention, the channel switch to the non-primary channel is performed when the primary channel is occupied by one of the APs.

Additionally, in the present invention, the processor compares the BSS color information or MAC address of the inter-BSS PPDU with the BSS color information or MAC address included in the list information.

Additionally, in the present invention, it is determined whether the specific condition is satisfied by additionally considering the channel switch delay of the wireless communication terminal in addition to the length of the remaining OBSS TXOP.

Additionally, in the present invention, the processor transmits a primitive from the MAC layer to the PHY layer to perform the channel switch to the non-primary channel.

In addition, the present invention provides a method including the steps of receiving a PPDU (Physical Layer Protocol Data Unit) from an OBSS (Overlapping Basic Service Set) AP (Access Point) that is not associated with the wireless communication terminal on a primary channel of a bandwidth in which the wireless communication terminal operates, wherein a preamble of the PPDU includes a TXOP field and a Duration field related to a transmission opportunity (TXOP) duration set by the OBSS AP; and performing channel access by switching a channel from the primary channel of the bandwidth to a non-primary channel when a specific condition is satisfied, wherein the specific condition is whether a specific value obtained based on a value indicated by the TXOP field or the Duration field is greater than a minimum duration threshold value.

One embodiment of the present invention has the effect of efficiently performing channel access in an environment where operating channels overlap.

In addition, according to the present invention, since MLD performs channel access through a plurality of BSSs operating in overlapping operation channels, even if the primary channel of a specific BSS among the plurality of BSSs is occupied by another device, channel access rights can be obtained through the primary channel of another BSS.

Additionally, according to one embodiment of the present invention, when the state of the primary channel is busy, a channel access procedure can be performed through a non-primary channel.

The effects obtainable from the present invention are not limited to the effects mentioned above, and other effects not mentioned will be clearly understood by those skilled in the art to which the present invention belongs from the description below.

Figure 1 illustrates a wireless LAN system according to one embodiment of the present invention.

Figure 2 illustrates a wireless LAN system according to another embodiment of the present invention.

Figure 3 shows the configuration of a station according to one embodiment of the present invention.

Figure 4 shows the configuration of an access point according to one embodiment of the present invention.

Figure 5 schematically illustrates the process by which a station establishes a link with an access point.

Figure 6 shows an example of the CSMA (Carrier Sense Multiple Access)/CA (Collision Avoidance) method used in wireless LAN communications.

FIG. 7 shows various standard generation-specific physical layer protocol data unit (PPDU) formats according to an embodiment of the present invention.

Figure 8 shows an EHT/UHR PPDU format according to an embodiment of the present invention.

FIG. 9 shows a transmission/TXOP protection method using an RTS frame and a CTS frame according to an embodiment of the present invention.

FIG. 10 shows a transmission/TXOP protection method using an MU-RTS frame and a CTS frame according to an embodiment of the present invention.

Figure 11 shows a mapping table of user priority and access category.

Figure 12 shows an example of a channel access procedure through a secondary channel when the state of the primary channel is busy.

Figure 13 illustrates an example of a transmission length limitation of a PPDU transmitted after performing channel access through a subchannel.

FIG. 14 illustrates an embodiment of a method for limiting the length of a TXOP obtained through channel access via a sub-channel.

FIG. 15 illustrates a method for an AP to manage a primary operating channel by obtaining TXOPs through primary and secondary channels according to one embodiment of the present invention.

FIG. 16 illustrates an example of a method for configuring an AP MLD including a primary AP and an auxiliary AP having overlapping operating channels and setting an operating channel according to one embodiment of the present invention.

FIG. 17 illustrates an example of a procedure for an AP MLD to obtain a TXOP using a primary BSS and a secondary BSS according to one embodiment of the present invention.

FIG. 18 illustrates an example of a format of an RNR element transmitted by an AP MLD to indicate an auxiliary AP according to one embodiment of the present invention.

FIG. 19 illustrates a method in which a channel switch of an auxiliary BSS is instructed/performed together when a channel switch for a primary BSS is performed according to one embodiment of the present invention.

FIG. 20 illustrates a configuration of an AP MLD including a primary AP and a secondary AP having continuous operation channels and a method for setting operation channels according to one embodiment of the present invention.

FIG. 21 illustrates a channel access method of an MLD that operates an STA on two links having continuous operation channels according to one embodiment of the present invention.

FIG. 22 illustrates an embodiment of a method for performing resource unit allocation to each STA using a resource unit allocation subfield and a resource unit allocation subfield indicated through a preamble of a PPDU and a method for indicating a content channel.

Figure 23 illustrates the ambiguity problem of allocation RU interpretation of an STA that receives a preamble on a subchannel.

FIG. 24 illustrates a method in which an AP performing channel access through a subchannel indicates BW and RU allocation information of a PPDU according to one embodiment of the present invention.

Figure 25 illustrates an embodiment of a transmission/TXOP protection method using MU-RTS frame and CTS frame.

Figure 26 illustrates the format of a trigger frame.

Figure 27 illustrates an example of the format of the common information field of a trigger frame.

Figure 28 illustrates an example of the format of the user information field of a trigger frame.

FIG. 29 illustrates an example of channel access in a non-primary channel when the primary channel is occupied according to one embodiment of the present invention.

FIG. 30 illustrates an example of operation of STAs in a non-primary channel based on information related to OBSS transmitted from an AP according to an embodiment of the present invention.

FIG. 31 illustrates an example of a method for determining whether to perform an operation on a non-primary channel considering the TBTT of a BSS according to an embodiment of the present invention.

FIG. 32 illustrates an example of a channel access method according to reception of an RTS frame of an AP according to an embodiment of the present invention.

FIG. 33 illustrates another example of a channel access method according to reception of an RTS frame of an AP according to an embodiment of the present invention.

FIG. 34 illustrates an example of a method for transmitting a CTS-to-self frame based on reception of an RTS frame of an AP according to an embodiment of the present invention.

FIG. 35 illustrates an example of information acquired by a STA and a method for setting NAV based on reception of an OBSS PPDU according to an embodiment of the present invention.

FIG. 36 illustrates an example of a method for setting up NAV and a method for using a subchannel when an STA receives an OBSS PPDU according to an embodiment of the present invention.

FIG. 37 illustrates an example of a primitive exchange procedure for channel access in a non-primary channel of an STA according to an embodiment of the present invention.

FIG. 38 is a flowchart showing an example of a channel access procedure performed by a terminal according to an embodiment of the present invention.

The terms used in this specification are selected from the most widely used and general terms possible while considering the functions of the present invention, but they may vary depending on the intentions of engineers in the field, customs, or the emergence of new technologies. In addition, in certain cases, there are terms arbitrarily selected by the applicant, and in such cases, their meanings will be described in the description of the relevant invention. Therefore, it should be noted that the terms used in this specification should be interpreted based on the actual meaning of the terms and the overall contents of this specification, not simply the names of the terms.

Throughout the specification, when a component is said to be "connected" to another component, this includes not only the case where it is "directly connected" but also the case where it is "electrically connected" with another component in between. Also, when a component is said to "include" a particular component, this does not mean that it excludes other components, but rather that it may include other components, unless specifically stated otherwise. In addition, the limitation of "more than" or "less than" with respect to a particular threshold value may be appropriately replaced with "more than" or "less than", respectively, depending on the embodiment.

Hereinafter, in the present invention, fields and subfields may be used interchangeably.

Figure 1 illustrates a wireless LAN system according to one embodiment of the present invention.

A wireless LAN system includes one or more Basic Service Sets (BSS), which represent a set of devices that are successfully synchronized and can communicate with each other. In general, BSS can be divided into infrastructure BSS and independent BSS (IBSS), and Fig. 1 shows an infrastructure BSS among them.

As illustrated in FIG. 1, an infrastructure BSS (BSS1, BSS2) includes one or more stations (STA1, STA2, STA3, STA4, STA5), an access point (AP-1, AP-2) which is a station providing a distribution service, and a distribution system (DS) that connects multiple access points (AP-1, AP-2).

A station (STA) is any device that includes a medium access control (MAC) and a physical layer interface for a wireless medium that complies with the provisions of the IEEE 802.11 standard, and broadly includes both non-AP stations and access points (APs). In addition, the term 'terminal' in this specification may be used as a term referring to a non-AP STA or an AP, or both. A station for wireless communication includes a processor and a communication unit, and may further include a user interface unit and a display unit, etc., depending on the embodiment. The processor may generate a frame to be transmitted through a wireless network or process a frame received through the wireless network, and may perform various processes for controlling the station. In addition, the communication unit is functionally connected to the processor and transmits and receives frames through the wireless network for the station. In the present invention, a terminal may be used as a term including a user equipment (UE).

An Access Point (AP) is an entity that provides access to a distribution system (DS) via a wireless medium for stations associated with it. In an infrastructure BSS, communication between non-AP stations is in principle performed via an AP, but direct communication is also possible between non-AP stations when a direct link is established. Meanwhile, in the present invention, an AP is used as a concept including a PCP (Personal BSS Coordination Point), and in a broad sense, it can include concepts such as a centralized controller, a base station (BS), a node-B, a base transceiver system (BTS), or a site controller. In the present invention, an AP may also be referred to as a base wireless communication terminal, and in a broad sense, a base wireless communication terminal can be used as a term including an AP, a base station, an eNodeB (eNB), and a transmission point (TP). In addition, the base wireless communication terminal may include various types of wireless communication terminals that allocate communication medium resources and perform scheduling in communications with multiple wireless communication terminals.

Multiple infrastructure BSSs can be interconnected via a distribution system (DS). In this case, multiple BSSs connected via the distribution system are called an extended service set (ESS).

Fig. 2 illustrates an independent BSS, which is a wireless LAN system according to another embodiment of the present invention. In the embodiment of Fig. 2, parts that are the same or corresponding to those in the embodiment of Fig. 1 will be omitted for redundant description.

Since BSS3 illustrated in Fig. 2 is an independent BSS and does not include an AP, all stations (STA6, STA7) are not connected to an AP. An independent BSS is not allowed to connect to a distribution system and forms a self-contained network. In an independent BSS, each station (STA6, STA7) can be directly connected to each other.

FIG. 3 is a block diagram showing the configuration of a station (100) according to one embodiment of the present invention. As illustrated, the station (100) according to the embodiment of the present invention may include a processor (110), a communication unit (120), a user interface unit (140), a display unit (150), and a memory (160).

First, the communication unit (120) transmits and receives wireless signals such as wireless LAN packets, and may be built into or externally installed in the station (100). According to an embodiment, the communication unit (120) may include at least one communication module using different frequency bands. For example, the communication unit (120) may include communication modules of different frequency bands such as 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. According to one embodiment, the station (100) may include a communication module using a frequency band of 7.125 GHz or higher and a communication module using a frequency band of 7.125 GHz or lower. Each communication module may perform wireless communication with an AP or an external station according to the wireless LAN standard of the frequency band supported by the corresponding communication module. Depending on the performance and requirements of the station (100), the communication unit (120) may operate only one communication module at a time or may operate multiple communication modules simultaneously. When the station (100) includes multiple communication modules, each communication module may be provided in an independent form, or multiple modules may be integrated into one chip. In the embodiment of the present invention, the communication unit (120) may represent an RF communication module that processes RF (Radio Frequency) signals.

Next, the user interface unit (140) includes various types of input/output means provided in the station (100). That is, the user interface unit (140) can receive user input using various input means, and the processor (110) can control the station (100) based on the received user input. In addition, the user interface unit (140) can perform output based on a command of the processor (110) using various output means.

Next, the display unit (150) outputs an image on the display screen. The display unit (150) can output various display objects such as content executed by the processor (110) or a user interface based on a control command of the processor (110). In addition, the memory (160) stores a control program used in the station (100) and various data according to the control program. The control program may include a connection program required for the station (100) to perform a connection with an AP or an external station.

The processor (110) of the present invention can execute various commands or programs and process data within the station (100). In addition, the processor (110) can control each unit of the above-described station (100) and control data transmission and reception between the units. According to an embodiment of the present invention, the processor (110) can execute a program for connection with an AP stored in the memory (160) and receive a communication setup message transmitted by the AP. In addition, the processor (110) can read information on the priority conditions of the station (100) included in the communication setup message and request connection to the AP based on the information on the priority conditions of the station (100). The processor (110) of the present invention may refer to the main control unit of the station (100), and according to an embodiment, may refer to a control unit for individually controlling some components of the station (100), such as the communication unit (120). That is, the processor (110) may be a modem or a modulator and/or demodulator that modulates and/or demodulates a wireless signal transmitted and received from the communication unit (120). The processor (110) controls various operations of transmitting and receiving wireless signals of the station (100) according to an embodiment of the present invention. A specific embodiment thereof will be described later.

The station (100) illustrated in FIG. 3 is a block diagram according to one embodiment of the present invention, and the blocks shown separately are logically distinguished to illustrate elements of the device. Accordingly, the elements of the device described above may be mounted as one chip or as multiple chips depending on the design of the device. For example, the processor (110) and the communication unit (120) may be implemented as one chip or as separate chips. In addition, in the embodiment of the present invention, some components of the station (100), such as the user interface unit (140) and the display unit (150), may be selectively provided in the station (100).

FIG. 4 is a block diagram showing the configuration of an AP (200) according to one embodiment of the present invention. As shown, the AP (200) according to the embodiment of the present invention may include a processor (210), a communication unit (220), and a memory (260). In FIG. 4, a redundant description of parts of the configuration of the AP (200) that are identical or corresponding to the configuration of the station (100) of FIG. 3 will be omitted.

Referring to FIG. 4, the AP (200) according to the present invention has a communication unit (220) for operating a BSS in at least one frequency band. As described above in the embodiment of FIG. 3, the communication unit (220) of the AP (200) may also include a plurality of communication modules using different frequency bands. That is, the AP (200) according to the embodiment of the present invention may have two or more communication modules of different frequency bands, for example, 2.4 GHz, 5 GHz, 6 GHz, and 60 GHz. Preferably, the AP (200) may have a communication module using a frequency band of 7.125 GHz or higher and a communication module using a frequency band of 7.125 GHz or lower. Each communication module may perform wireless communication with a station according to the wireless LAN standard of the frequency band supported by the corresponding communication module. The communication unit (220) may operate only one communication module at a time or may operate a plurality of communication modules simultaneously, depending on the performance and requirements of the AP (200). In an embodiment of the present invention, the communication unit (220) may represent an RF communication module that processes RF (Radio Frequency) signals.

Next, the memory (260) stores a control program used in the AP (200) and various data according to the control program. The control program may include a connection program that manages the connection of the station. In addition, the processor (210) controls each unit of the AP (200) and may control data transmission and reception between the units. According to an embodiment of the present invention, the processor (210) may execute a program for connection with a station stored in the memory (260) and transmit a communication setup message to one or more stations. At this time, the communication setup message may include information on the connection priority conditions of each station. In addition, the processor (210) performs connection setup according to a connection request of the station. According to one embodiment, the processor (210) may be a modem or a modulator and/or demodulator that modulates a wireless signal transmitted and received from the communication unit (220). The processor (210) controls various operations of wireless signal transmission and reception of the AP (200) according to an embodiment of the present invention. Specific examples of this will be described later.

Figure 5 schematically illustrates the process by which a station establishes a link with an access point.

Referring to FIG. 5, the link between STA (100) and AP (200) is established largely through three stages: scanning, authentication, and association. First, the scanning stage is a stage in which STA (100) acquires access information of BSS operated by AP (200). As a method for performing scanning, there is a passive scanning method in which information is acquired only by utilizing a beacon message (S101) periodically transmitted by AP (200), and an active scanning method in which STA (100) acquires access information by transmitting a probe request to AP (S103) and receiving a probe response from AP (S105).

In the scanning step, the STA (100) that successfully receives wireless access information transmits an authentication request (S107a) and receives an authentication response from the AP (200) (S107b) to perform the authentication step. After the authentication step is performed, the STA (100) transmits an association request (S109a) and receives an association response from the AP (200) (S109b) to perform the association step. In this specification, association basically means wireless association, but the present invention is not limited thereto, and association in a broad sense may include both wireless association and wired association.

Meanwhile, an additional 802.1X-based authentication step (S111) and an IP address acquisition step (S113) via DHCP may be performed. In Fig. 5, the authentication server (300) is a server that processes STA (100) and 802.1X-based authentication, and may be physically connected to the AP (200) or may exist as a separate server.

Figure 6 shows an example of the CSMA (Carrier Sense Multiple Access)/CA (Collision Avoidance) method used in wireless LAN communications.

A terminal performing wireless LAN communication performs carrier sensing before transmitting data to check whether the channel is busy. If a wireless signal of a certain strength or higher is detected, the channel is determined to be busy, and the terminal delays access to the channel. This process is called clear channel assessment (CCA), and the level that determines whether or not the signal is detected is called the CCA threshold. If a wireless signal of a CCA threshold or higher received by the terminal is directed to the terminal as a receiver, the terminal processes the received wireless signal. On the other hand, if no wireless signal is detected in the channel or a wireless signal of a strength lower than the CCA threshold is detected, the channel is determined to be idle.

When the channel is determined to be idle, each terminal having data to transmit performs a backoff procedure after an IFS (Inter Frame Space), such as AIFS (Arbitration IFS), PIFS (PCF IFS), etc., according to the situation of each terminal. Depending on the embodiment, the AIFS may be used as a configuration replacing the existing DIFS (DCF IFS). Each terminal waits while decreasing the slot time by a random number determined for the terminal during the interval of the idle state of the channel, and a terminal that has exhausted all of the slot times attempts to access the channel. The period during which each terminal performs the backoff procedure is called a contention window period. At this time, the random number may be referred to as a backoff counter. That is, the initial value of the backoff counter is set by an integer, which is a random number obtained by the terminal. When the terminal detects that the channel is idle during the slot time, the terminal may decrease the backoff counter by 1. Additionally, if the backoff counter reaches 0, the terminal may be allowed to perform channel access on the channel. Accordingly, the terminal's transmission may be allowed if the channel is idle during the AIFS time and the slot time of the backoff counter.

If a specific terminal successfully accesses the channel, the terminal can transmit data through the channel. However, if the terminal attempting access collides with another terminal, the collided terminals are each assigned a new random number and perform a backoff procedure again. According to one embodiment, the random number newly assigned to each terminal can be determined within a range twice (2*CW) of the random number range (contention window, CW) previously assigned to the terminal. Meanwhile, each terminal performs the backoff procedure again in the next contention window period to attempt access, and at this time, each terminal performs the backoff procedure from the slot time remaining in the previous contention window period. In this way, each terminal performing wireless LAN communication can avoid collisions with each other for a specific channel.

<Various PPDU format examples>

FIG. 7 shows various standard generation-specific physical layer protocol data unit (PPDU) formats according to an embodiment of the present invention.

More specifically, FIG. 7(a) illustrates an embodiment of a legacy PPDU format based on 802.11a/g, FIG. 7(b) illustrates an embodiment of a HE PPDU format based on 802.11ax, and FIG. 7(c) illustrates an embodiment of a non-legacy PPDU (i.e., EHT PPDU) format based on 802.11be. In addition, FIG. 7(d) illustrates a detailed field configuration of L-SIG and RL-SIG commonly used in the above PPDU formats.

Referring to FIG. 7(a), the preamble of the legacy PPDU includes a Legacy Short Training field (L-STF), a Legacy Long Training field (L-LTF), and a Legacy Signal field (L-SIG). In an embodiment of the present invention, the L-STF, L-LTF, and L-SIG may be referred to as a legacy preamble.

Referring to FIG. 7(b), the preamble of the HE PPDU additionally includes an RL-SIG (Repeated Legacy Short Training field), a HE-SIG-A (High Efficiency Signal A field), a HE-SIG-B (High Efficiency Signal B field), a HE-STF (High Efficiency Short Training field), and a HE-LTF (High Efficiency Long Training field) in addition to the legacy preamble. In an embodiment of the present invention, the RL-SIG, HE-SIG-A, HE-SIG-B, HE-STF, and HE-LTF may be referred to as a HE preamble. The specific configuration of the HE preamble may be modified according to the HE PPDU format. For example, HE-SIG-B may be used only in the HE MU PPDU format.

Referring to FIG. 7(c), the preamble of the EHT PPDU additionally includes an RL-SIG (Repeated Legacy Short Training field), a U-SIG (Universal Signal field), an EHT/UHR-SIG-A (Extremely High Throughput/Ultra High Reliability Signal A field), an EHT/UHR-SIG-A (Extremely High Throughput/Ultra High Reliability Signal B field), an EHT-STF (Extremely High Throughput Short Training field), and an EHT-LTF (Extremely High Throughput Long Training field) in addition to the legacy preamble. In an embodiment of the present invention, the RL-SIG, EHT-SIG-A, EHT-SIG-B, EHT-STF, and EHT-LTF may be referred to as an EHT preamble. The specific configuration of the non-legacy preamble may be modified according to the EHT PPDU format. For example, EHT-SIG-A and EHT-SIG-B can only be used in some of the EHT PPDU formats.

In this way, the PPDU used in the UHR standard may have a format similar to the PPDU format used in the EHT standard. This is because the EHT PPDU format defined in 802.11be includes a U-SIG field that multiple wireless LAN generations have agreed to use in common. At this time, the value of the PHY Version Identifier field of the U-SIG field included in the EHT PPDU may be 0, and the value of the PHY Version identifier field of the U-SIG field included in the UHR PPDU may have a non-zero value, such as 1. The EHT PPDU includes an EHT-STF (Extremely High Throughput Short Training field) field in the STF field, and an EHT-LTF (Extremely High Throughput Long Training field) field in the LTF field. The UHR PPDU includes a UHR-STF field (Ultra High Reliability Short Training field) in the STF field, and a UHR-LTF (Ultra High Reliability Long Training field) field in the LTF field.

The L-SIG field included in the preamble of the PPDU applies 64FFT OFDM and consists of a total of 64 subcarriers. Of these, 48 subcarriers, excluding the guard subcarrier, DC subcarrier, and pilot subcarrier, are used for data transmission of L-SIG. Since BPSK, Rate=1/2 MCS (Modulation and Coding Scheme) is applied to L-SIG, it can include a total of 24 bits of information. Fig. 7(d) shows the 24-bit information configuration of L-SIG.

Referring to Fig. 7(d), L-SIG includes an L_RATE field and an L_LENGTH field. The L_RATE field consists of 4 bits and represents an MCS used for data transmission. Specifically, the L_RATE field represents one of the transmission speeds of 6/9/12/18/24/36/48/54 Mbps, which combine modulation methods such as BPSK/QPSK/16-QAM/64-QAM and inefficiencies such as 1/2, 2/3, and 3/4. Combining the information in the L_RATE field and the L_LENGTH field can represent the total length of the corresponding PPDU. In the non-legacy PPDU format, the L_RATE field is set to the minimum speed of 6 Mbps.

The unit of the L_LENGTH field is byte, and a total of 12 bits are allocated, allowing signaling up to 4095. In combination with the L_RATE field, it can indicate the length of the corresponding PPDU. At this time, legacy terminals and non-legacy terminals can interpret the L_LENGTH field in different ways.

First, the method for a legacy terminal or a non-legacy terminal to interpret the length of the PPDU using the L_LENGTH field is as follows. If the value of the L_RATE field is set to indicate 6Mbps, 3 bytes (i.e., 24 bits) can be transmitted during 4us, which is the duration of one symbol of 64FFT. Therefore, if 3 bytes corresponding to the SVC field and the Tail field are added to the L_LENGTH field value and divided by 3 bytes, which is the transmission amount of one symbol, the number of symbols based on 64FFT after L-SIG is obtained. If the obtained number of symbols is multiplied by 4us, which is the duration of one symbol, and then 20us required for transmission of L-STF, L-LTF, and L-SIG is added, the length of the PPDU, i.e., the reception time (RXTIME) is obtained. If this is expressed as a formula, it is as shown in Mathematical Expression 1 below.

는 x보다 크거나 같은 최소의 자연수를 나타낸다. L_LENGTH 필드의 최대값은 4095이므로 PPDU의 길이는 최대 5.484ms까지로 설정될 수 있다. 해당 PPDU를 전송하는 논-레거시 단말은 L_LENGTH 필드를 아래 수학식 2와 같이 설정해야 한다.At this time,

represents the smallest natural number greater than or equal to x. Since the maximum value of the L_LENGTH field is 4095, the length of the PPDU can be set to a maximum of 5.484 ms. A non-legacy terminal transmitting the PPDU must set the L_LENGTH field as in the mathematical expression 2 below.

Here, TXTIME is the total transmission time that constitutes the corresponding PPDU, as shown in the mathematical expression 3 below. In this case, TX represents the transmission time of X.

Referring to the above formulas, the length of the PPDU is calculated based on the rounded up value of L_LENGTH/3. Therefore, for any value of k, three different values of L_LENGTH={3k+1, 3k+2, 3(k+1)} indicate the same PPDU length.

Referring to Fig. 7(e), the U-SIG (Universal SIG) field continues to exist in EHT/UHR PPDU and subsequent generation wireless LAN PPDUs, and plays a role in distinguishing which generation of PPDU it is, including EHT/UHR. In addition, the U-SIG field can play a role in facilitating spatial reuse of EHT/UHR and subsequent generation wireless LANs. U-SIG is an OFDM 2 symbol based on 64FFT and can convey a total of 52 bits of information. Of these, 43 bits, excluding 9 bits of CRC/Tail, are largely divided into a VI (Version Independent) field and a VD (Version Dependent) field.

The VI bit maintains the current bit configuration in the future so that even when a subsequent generation PPDU is defined, the current EHT/UHR terminals can obtain information about the corresponding PPDU through the VI fields of the corresponding PPDU. For this purpose, the VI field consists of PHY version, UL/DL, BSS Color, TXOP, and Reserved fields. The PHY version ID field is 3 bits and sequentially distinguishes the EHT/UHR and subsequent generation wireless LAN standards by version. The PHY version ID field of the EHT (11be) PPDU has a value of 000b, and the PHY version ID field of the UHR PPDU has a value other than 000b. The UL/DL field distinguishes whether the corresponding PPDU is an uplink/downlink PPDU. BSS Color means an identifier for each BSS defined in 11ax and has a value of 6 bits or more. TXOP stands for Transmit Opportunity Duration transmitted in the MAC header. By adding it to the PHY header, the length of the TXOP containing the corresponding PPDU can be inferred without having to decode the MPDU, and has a value of 7 bits or more.

The VD field of EHT is signaling information useful only for PPDU of 11be version, and can be composed of fields commonly used in any PPDU format, such as PPDU format and BW, and fields defined differently for each PPDU format. PPDU format is a delimiter that distinguishes EHT SU (Single User), EHT MU (Multiple User), EHT TB (Trigger-based), and EHT ER (Extended Range) PPDU.

The BW field largely signals five basic PPDU BW options of 20, 40, 80, 160 (80+80), and 320 (160+160) MHz (a BW that can be expressed in the form of an exponential of 20*2 can be called a basic BW) and various remaining PPDU BWs configured through Preamble Puncturing. In addition, some 80 MHz can be signaled in a punctured form after being signaled at 320 MHz. In addition, the punctured and modified channel form can be signaled directly in the BW field, or can be signaled using the BW field and a field appearing after the BW field (for example, a field in the EHT-SIG field). If the BW field has 3 bits, a total of 8 BW signaling is possible, and therefore only a maximum of 3 puncturing modes can be signaled. If the BW field is 4 bits, a total of 16 BW signalings are possible, so the puncturing mode can signal up to 11.

The VD field of UHR is a field that indicates signaling information that is useful only for UHR PPDU. However, the information indicated by each field included in the VD field of UHR PPDU may be the same as or more extended than the information indicated by the field that plays the same role as the VD field of EHT (11be). For example, the field indicating the puncturing pattern included in the VD field of UHR PPDU may indicate various types of patterns than the field indicating the puncturing pattern included in the VD field of EHT PPDU. Alternatively, the field indicating the puncturing pattern included in the VD field of UHR PPDU may be interpreted in combination with the BW field. Through this, more various types of puncturing patterns may be indicated.

Figure 8 shows an EHT/UHR PPDU format according to an embodiment of the present invention.

The EHT/UHR PPDU format can be indicated by the PPDU Format field of the U-SIG field of the PPDU. Fig. 8 (a) shows an EHT/UHR SU PPDU according to an embodiment of the present invention. The EHT/UHR SU PPDU is a PPDU used for single-user transmission between an AP and a single station, and may include an EHT-SIG-A field for additional signaling after the U-SIG.

Fig. 8(b) shows an EHT/UHR Trigger-based PPDU according to an embodiment of the present invention. An EHT/UHR Trigger-based PPDU is an uplink PPDU used for transmission in response to a trigger frame, and may not have a separate EHT/UHR-SIG-A field after U-SIG.

Fig. 8(c) shows an EHT/UHR MU PPDU according to an embodiment of the present invention. An EHT/UHR MU PPDU is a PPDU used for transmission to one or more terminals. The EHT/UHR MU PPDU format may include a HE-SIG-B after the U-SIG field.

Fig. 8(d) shows an EHT/UHR ER SU PPDU according to an embodiment of the present invention. The EHT/UHR ER SU PPDU is used for single-user transmission to stations in an extended range. The EHT/UHR ER SU PPDU format allows U-SIG to be repeated on the time axis.

The EHT/UHR MU PPDU described through (c) of FIG. 8 can be used by an AP to perform downlink transmission to multiple stations. At this time, the EHT/UHR MU PPDU can include scheduling information for multiple stations to simultaneously receive the PPDU. At this time, the EHT/UHR MU PPDU can convey AID information of a receiver or transmitter of the corresponding PPDU through a user specific field of EHT/UHR-SIG-B. A station receiving the EHT/UHR MU PPDU can perform a spatial reuse operation based on the AID information acquired from the preamble of the PPDU. More specifically, the resource unit allocation (RA) field of EHT/UHR-SIG-B can include information on a resource unit (RU) partitioning form in a specific bandwidth (e.g., 20 MHz) in the frequency domain. Additionally, information about the station assigned to each partitioned resource unit may be conveyed via a user-specific field of EHT/UHR-SIG-B. The user-specific field may include one or more user fields corresponding to each partitioned resource unit.

A receiver's or sender's AID may be inserted into a user field corresponding to a resource unit in which data transmission is performed among the multiple divided resource units. A pre-specified null STA ID may be inserted into a user field corresponding to the remaining resource units in which data transmission is not performed.

Two or more PPDUs described through Fig. 8 may be indicated by the same PPDU format. For example, the value of the U-SIG PPDU format subfield indicating EHT/UHR SU PPDU and the value of the U-SIG PPDU format subfield indicating EHT/UHR MU PPDU may be the same.

Some of the fields or some information of the fields included in the format of the PPDU described above may be omitted. This may be referred to as compression mode or compressed mode.

<Wi-Fi terminal channel access method>

Since Wi-Fi terminals (APs, non-AP STAs, etc.) perform communications using unlicensed bands, they check whether the channel on which they want to perform transmission is being used by another device before transmitting a frame. CSMA (Carrier Sense Multiple Access) is a channel access method in which a terminal that wants to transmit a packet performs carrier sense to check whether the channel is being used by another device, and performs transmission only if the channel is determined to be not being used by another device (Idle). Since a terminal using CSMA can perform an action of not attempting transmission at least when it is confirmed that another device is using the medium (channel) (when determined to be busy), transmission that has been started first can be protected from other devices.

However, multiple terminals that recognize that the medium is occupied by another device experience a transmission collision by simultaneously attempting to transmit packets when it is confirmed that the medium occupation from the other device has ended (the medium has changed to Idle). That is, as multiple other terminals attempt to transmit packets at the same time that a specific terminal attempts to transmit packets, a terminal that should receive the packet transmitted by the specific terminal cannot properly receive and decode the packet that it should receive due to interference caused by the transmissions performed by the multiple other terminals.

As described above, CSMA/CA (CSMA with collision avoidance) is a channel access mechanism that prevents multiple terminals that detect that the medium has changed to Idle from simultaneously attempting packet transmission. Terminals accessing the medium (channel) using CSMA/CA attempt to transmit after waiting for a random amount of time when the state of the medium observed by the terminals changes to Idle. The random amount of time may be an aslottime (typically 9 us) as long as a random number (random backoff counter) generated by each terminal attempting to transmit. In other words, terminals accessing the medium using CSMA/CA attempt to transmit after waiting for different random amounts of time, so they attempt to transmit at different times, unlike when only CSMA is used. At this time, when a specific terminal that waited for the shortest random amount of time after the medium changed to Idle attempts to transmit first, other terminals can recognize that the medium is occupied (changed to busy) by the specific terminal and stop the channel access procedure. At this time, the specific terminal may perform an operation of decreasing the backoff counter maintained by it by 1 every aslottime while the medium is maintained as Idle, and may attempt transmission when the backoff counter becomes 0, or when aslottime has elapsed after the backoff counter becomes 0. At this time, the specific terminal that performed the transmission may generate a new random number (new backoff counter) after the transmission is terminated, and may attempt transmission when the new random number becomes 0 again, or after it becomes 0.

The CSMA/CA and random backoff procedures briefly explained above are applied to DCF (Distributed coordination function) and EDCAF (Enhanced distributed channel access), which are basic functions used by Wi-Fi terminals when attempting to access a channel. Since these are well-known and widely used unlicensed band channel access methods, a detailed explanation will be omitted.

The DCF and EDCAF utilized by the MAC of the Wi-Fi terminal evaluate the channel status by considering not only the channel status (idle/busy) confirmed by each terminal directly performing a physical CS (Carrier Sense) but also the result of a virtual CS. In more detail, even if the result of the physical CS performed on the channel is idle, the Wi-Fi terminal considers the channel status to be busy if the result of the virtual CS is busy. At this time, the Virtual CS is a channel evaluation method that determines the channel to be busy if the NAV (Network allocation vector) is not 0. The NAV may be a value maintained for future traffic that is predicted to occupy the medium. To explain in more detail, when the MAC of Wi-Fi receives an RTS/CTS frame, it can set the NAV (NAV count) based on the duration information of the received frame, for example, the value of the duration field, and maintain the NAV as a non-zero value for the expected time that the medium will be occupied after the RTS/CTS frame exchange. In other words, the value maintained as the NAV decreases over time. If the NAV value of a specific MAC is 0, it can be interpreted that the future traffic recognized by the specific MAC no longer occupies the medium. If the NAV is 0, the MAC can determine the virtual CS result as Idle. In this case, the MAC of Wi-Fi can also set the NAV based on the duration value obtained from not only the RTS/CTS frame but also other received MAC frames.

The channel estimation method (determine the state of the medium) that considers the results of both the physical CS and virtual CS briefly described above is also one of the well-known Wi-Fi MAC functions, so a detailed description is omitted.

<EDCA and TXOP>

EDCA provides a mechanism to manage traffic by differentiating it into four types of ACs (access categories) according to the characteristics of the traffic. At this time, the four types of ACs are AC_VO (AC Voice), AC_VI (AC Video), AC_BE (AC Best effort), and AC_BK (AC Background), and each AC can have different CW (contention window), TXOP (transmit opportunity), and AIFSN parameters. Simply put, EDCA is a mechanism that differentiates the CW, TXOP, and AIFSN parameters for the four types of ACs and controls the transmission priority of the traffic transmitted using each AC. To this end, EDCA can map the traffic (MSDU) that the MAC must service to one of the four ACs according to the TC (traffic category) or TS (traffic stream). At this time, the traffic mapped to one of the four ACs by EDCA is divided and managed into four queues for each AC. At this time, the four queues may be logically separated rather than physically separated.

AC_VO is an AC that can be used for traffic that is vulnerable to transmission delay, although the absolute amount of traffic, such as voice traffic, is not large, and has relatively small CW and AIFSN parameter values to increase the probability of being serviced preferentially over traffic from other ACs. The TXOP parameter of AC_VO is limited to a relatively smaller value than the TXOP parameters of other ACs, so that only a shorter transmission time is guaranteed than that of other ACs.

AC_VI is an AC that is more tolerant to transmission delay than voice traffic, but can still be used for traffic such as video that requires low-latency transmission and a large amount of traffic. AC_VI has larger CW and AIFSN parameter values than AC_VO but smaller than other ACs, and instead, TXOP is about twice as long as AC_VI.

AC_BE is an AC that can be utilized for traffic that is robust to transmission delay, and most general traffic except voice data and streaming video data can be classified as AC_BE. AC_BE uses CW and AIFSN parameters with values greater than AC_VO and AC_VI. In addition, AC_BE does not have a separate TXOP. Therefore, traffic corresponding to AC_BE cannot be utilized in the TXOP transmission sequence that transmits a PPDU, receives an ACK in response, and then transmits a PPDU again after SIFS.

AC_BK is a traffic that is robust to transmission delay similar to AC_BE, but can be utilized for traffic with a lower priority than BE traffic. AC_BK utilizes the same CW parameter values as AC_BE, and the AIFSN parameter values are larger than AC_BE. In addition, traffic corresponding to AC_BK does not have a separate TXOP like AC_BE, so it cannot be utilized in the TXOP transmission sequence.

The four types of EDCA AC described above are mapped to the UP (user-priority) of 802.1D, and the EDCA AC is determined according to the UP value of the traffic received through the wire or the TID of the MSDU indicated from the upper layer. At this time, if the TID of the MSDU indicates a value of 0 to 7, the value indicated by the TID can correspond 1:1 with the UP.

In addition, the four types of EDCA AC described above have default CW (CWmin, CWmax), AIFSN, and TXOP parameters defined in the standard, and the parameter values of each AC can be changed by the AP and different values can be used for each BSS.

With the EDCA mechanism, Wi-Fi traffic is stored in one of four queues corresponding to four ACs, and can be transmitted to a destination device only when the AC containing the traffic wins the channel access contention with other ACs. At this time, in the channel access contention between ACs, each AC performs the contention using the access parameters (CW[AC], AIFSN[AC]) assigned to it, and the channel access contention operation performed by each AC is the same as DCF. At this time, if a specific AC does not have any traffic to transmit in the queue, the specific AC may not participate in the contention.

However, as described above, since the CW and AIFSN parameter values utilized by each AC are different, the AC_VO with the smallest CW and AIFSN parameters is more likely to win the channel access competition with other ACs, and thus the traffic of AC_VO is more likely to be serviced with priority over the traffic of other ACs.

In addition, the EDCA mechanism stipulates internal competition rules such as when an (internal) collision occurs between ACs, the AC with the highest priority wins and increases the CW of the other AC that caused the collision, and rules for composing a PPDU including traffic from an AC other than the AC that won the competition (primary AC), but a detailed description is omitted because it is not closely related to the proposal of the present invention.

As described above, EDCA provides the EDCA TXOP (EDCA Transmission Opportunity) function along with the function of operating differentiated AC according to the type of traffic (frame, packet, etc.) to enhance QoS. EDCA TXOP refers to the time during which the EDCAF (EDCA Function) of a specific AC can control the medium without being disturbed by other devices during the TXOP period (duration) when it obtains a channel access opportunity, that is, becomes a TXOP holder. At this time, the EDCA TXOP can be restricted by the TXOP limit advertised by the AP. The TXOP holder must ensure that its transmission and the transmission of the response frame responded to by its transmission can be terminated within the TXOP limit.

A TXOP holder can transmit multiple frames (multiple PPDUs) during an EDCA TXOP interval. If transmission of each frame is performed within the acquired TXOP interval, the TXOP holder can transmit multiple frames consecutively without performing a separate channel access procedure, for example, a backoff procedure, between transmissions of each frame. In this case, if the multiple frames are MPDUs or A-MPDUs (Aggregated MAC protocol data units) that do not request immediate ack, transmission of the multiple frames can be performed at a short interframe space (SIFS) or reduced interframe space (RIFS) interval. In this case, if there is an MPDU or A-MPDU requesting immediate ack among the multiple frames, the TXOP holder can transmit a frame requesting immediate ack, receive the ack, and transmit the next frame after SIFS.

At this time, traffic (packets, frames, etc.) of other ACs other than the specific AC that is the TXOP holder may also be transmitted together within the TXOP acquired by the TXOP holder (specific AC) when specific conditions are satisfied. The transmission of traffic of other ACs other than the TXOP holder within the TXOP may be an operation due to TXOP sharing between ACs, and detailed information regarding the specific conditions is omitted because it is not related to the present invention.

As described above, the TXOP holder can perform continuous frame transmission without performing a separate channel access procedure within the TXOP. This may be an operation that can be achieved when other terminals understand and protect the TXOP section acquired by the TXOP holder. That is, in order for the TXOP holder to acquire medium control authority for the EDCA TXOP section, a procedure may be required to notify other terminals so that they can recognize the acquired TXOP section.

To this end, a terminal (AC) that becomes a TXOP holder or initiates transmission after completing a channel access procedure may attempt to allow other terminals to recognize the TXOP section by transmitting an RTS frame. At this time, the RTS frame means a frame in which the Type subfield (the fourth bit (B3), the third bit (B2) of the Frame Control field) of the Frame Control field of the MAC frame header is set to 01b (Type = Control frame) and the Subtype subfield (the eighth bit (B7), the seventh bit (B6), the sixth bit (B5), the fifth bit (B4) of the Frame Control field) of the Frame Control field is set to 1011b. Another terminal that receives an RTS frame from a TXOP holder may set an NAV based on information related to a duration included in the RTS frame, for example, the value of the Duration field. The set NAV may be maintained as a non-zero value for a time corresponding to the TXOP of the TXOP holder. However, the terminal indicated as the destination device of the RTS frame must respond with a CTS frame instead of setting the NAV based on the information of the RTS frame. At this time, the destination device of the RTS frame transmitted to start TXOP is a TXOP responder and must transmit a CTS frame in response to the RTS (after SIFS when the RTS frame is received). At this time, the Duration field of the responding CTS frame is set to a value calculated as the value indicated in the Duration field of the received RTS frame - the CTS frame transmission time - SIFS. The terminals receiving the CTS frame can set the NAV based on the information related to the duration included in the CTS frame (for example, the value of the Duration field).

Therefore, the NAV of the terminal that received the RTS frame from the TXOP holder and the terminal that received the CTS frame from the TXOP responder are set to 0 after the TXOP acquired by the TXOP holder is terminated. Through this, the Wi-Fi MAC mechanism can protect the TXOP holder and the TXOP responder so that they can exchange multiple frames without interruption during the TXOP.

However, if the TXOP holder transmits an RTS frame as a non-HT duplicate PPDU over the primary 80 MHz band, but a CTS frame (non-HT duplicate PPDU) responded to from the TXOP responder is responded to only in the primary 40 MHz band, the TXOP holder may use only the bandwidth of the primary 40 MHz or less than the primary 40 MHz, for example, the primary 20 MHz, for frame exchange during the acquired TXOP. The CH_BANDWIDTH (a type of TXVECTOR parameter) of the PPDU transmitted by the TXOP holder shall be set to a value equal to or smaller than the CH_BANDWIDTH_IN-NON_HT (a type of RXVECTOR parameter) of the received CTS frame. In this case, the RTS frame may be an RTS frame that allows the CTS frame to be responded to with a BW smaller than the BW in which the RTS frame was transmitted. An RTS frame may be an RTS frame transmitted with DYN_BANDWIDTH_IN_NON_HT (a type of TXVECTOR parameter) set to Dynamic. If DYN_BANDWIDTH_IN_NON_HT is set to Static and the RTS frame is transmitted from the TXOP holder, the TXOP responder may have to respond with a CTS frame with the same BW as the BW in which the RTS frame was received.

FIG. 9 shows a transmission/TXOP protection method using an RTS frame and a CTS frame according to an embodiment of the present invention.

Before transmitting a PPDU, the first station (STA1) transmits an RTS frame to the second station (STA2), which is the destination of the PPDU, and the second station (STA2) recognizes that the received RTS frame is an RTS frame destined for itself and responds with a CTS frame after SIFS.

STA1_Neighbor, a neighbor station of the first station (STA1), sets its NAV based on the value indicated by the Duration field of the RTS frame after receiving the RTS frame transmitted by the first station (STA1). STA2_Neighbor, a neighbor station of the second station (STA2), sets its NAV based on the information indicated by the Duration field of the CTS frame after receiving the CTS frame transmitted by the second station (STA2). STA1_Neighbor and STA2_Neighbor determine that the virtual CS is busy while the set NAV (counter) is maintained as a non-zero value after receiving the RTS/CTS frame, and perform operations such as not decreasing the backoff counter. As a result, the neighboring terminals that have received the RTS/CTS frame do not attempt transmission during the period in which the NAV is maintained as a non-zero value. Therefore, the first station (STA1) and the second station (STA2) can be exchanged without being disturbed by surrounding terminals while exchanging PPDU and Ack frames.

Even if the first station (STA1) and STA2_Neighbor are in a relationship where signals due to each other's transmissions are not detected (hidden), STA2_Neighbor can perform an operation that takes into account that the channel (channel, WM, Wireless medium) is in use while the first station (STA1) transmits a PPDU.

Meanwhile, a Wi-Fi terminal (non-AP STA) can transmit a UL PPDU to the AP without directly acquiring a TXOP or performing channel access through DCF and EDCAF. More specifically, a non-AP STA can transmit a UL PPDU using an RU allocated to it after receiving a trigger frame transmitted by the AP. At this time, the UL PPDU is a TB (trigger-based) PPDU.

An STA that responds with a UL PPDU after receiving a trigger frame can obtain more transmission opportunities than an STA that does not transmit a UL PPDU based on the trigger frame because it can perform transmission without obtaining direct channel access opportunities through DCF and EDCAF. Therefore, an STA that transmits a UL PPDU through the trigger frame may cause a fairness issue in terms of channel access. To address this fairness issue, 11ax defines a constraint to perform EDCAF using the MU (Multi-user)-EDCA parameter when a HE non-AP STA successfully transmits at least one MPDU through the UL PPDU transmitted after receiving the trigger frame. Accordingly, an STA that transmits a UL PPDU through the trigger frame must perform channel access using the MU-EDCA parameter, not the EDCA parameter. MU-EDCA parameters include parameters related to the size of contention windows for each of AC_VO, AC_VI, AC_BE, and AC_BK and the MU EDCA timer, and the contention window included in MU-EDCA can be set to be larger than the parameters of EDCA. An STA that transmits a TB PPDU through a trigger frame and succeeds in transmitting at least one MPDU performs channel access within the time interval corresponding to the MU EDCA timer by performing channel access using the MU EDCA parameters instead of the EDCA parameters, thereby succeeding in channel access with a lower probability than an STA that uses the EDCA parameters. In this way, by lowering the channel accessibility of an STA that transmits a UL PPDU (TB PPDU) based on the trigger frame, the fairness problem in channel accessibility between an STA that transmits a UL PPDU without performing direct channel access and an STA that does not transmit a UL PPDU based on the trigger frame can be resolved/mitigated.

<TXOP Protection Using MU-RTS Trigger Frame>

In 11ax (6th generation Wi-Fi, Wi-Fi6, HEW, High Efficiency WLAN), the MU-RTS Trigger/CTS frame exchange procedure is defined, and a function to enable the AP to start TXOP and protect the TXOP frame exchange procedure using the MU-RTS trigger frame (hereinafter referred to as MU-RTS, MU-RTS frame) is added. The MU-RTS frame is a type of trigger frame. When the MU-RTS frame is received, a station whose AID12 (LSB 12 bits of Association ID) is indicated from the User field included in the MU-RTS frame simultaneously responds with a CTS frame. When the AP protects TXOP using the MU-RTS frame, since multiple stations respond with CTS frames, the TXOP can be protected from peripheral devices of each of the multiple stations, which are the destination devices of the DL MU PPDU (Down link multi user PPDU). In addition, the MU-RTS frame can be used to protect the UL MU PPDU. In more detail, before requesting TB (Trigger based) PPDU to multiple stations through a trigger frame, the AP can transmit an MU-RTS frame to cause multiple stations to respond with CTS frames to the TB PPDU. At this time, the CTS frames responded to by the multiple stations cause the surrounding stations of each station to set NAVs to protect the TB PPDU and the Ack frame (Ack, Block Ack, etc.) to be transmitted after the TB PPDU, and through this, legacy stations STAs that cannot recognize (interpret, decode) the trigger frame and TB PPDU may not perform channel access during the packet exchange sequence section (or TXOP) initiated through the trigger frame.

FIG. 10 shows a transmission/TXOP protection method using an MU-RTS frame and a CTS frame according to an embodiment of the present invention.

In the embodiment of FIG. 10, before transmitting an MU PPDU, the AP transmits an MU-RTS frame to the first station (STA1) and the second station (STA2), which are the destination devices of the MU PPDU, and the first station (STA1) and the second station (STA2) receive the MU-RTS frame and, after SIFS, each respond to the MU-RTS frame with a CTS frame.

STA1_Neighbor, a neighboring station of the first station (STA1), sets its NAV based on the information indicated by the Duration field of the CTS frame after receiving the CTS frame transmitted by the first station (STA1). STA2_Neighbor, a neighboring station of the second station (STA2), sets its NAV based on the information indicated by the Duration field of the CTS frame after receiving the CTS frame transmitted by the second station (STA2). STA1_Neighbor and STA2_Neighbor perform operations such as not decreasing the back-off counter, determining that the Virtual CS (Virtual Carrier Sense) is busy while the NAV (counter) set after receiving the CTS frame is maintained as a non-zero value. Therefore, neighboring terminals that have received the CTS frame do not attempt to transmit during the period in which the NAV is maintained as a non-zero value. This allows the AP to transmit MU PPDUs and avoid interference from surrounding terminals while the first station (STA1) and the second station (STA2) transmit Ack frames.

The trigger frame described above is a frame type defined in 11ax, in which the Type (fourth bit (B3) and third bit (B2)) and Subtype (eighth bit (B7), seventh bit (B6), sixth bit (B5), and fifth bit (B4)) subfields of the Frame Control field are set to 01b and 0010b, respectively. The trigger frame is a frame of the Control Type in which the Type subfield of the Frame Control field is 01b, and the Subtype value 0010 indicates that it is a Trigger frame type. In 11ax, the trigger frame is defined so that an AP can request a response frame from multiple stations at once, and the MU-RTS frame is used so that an AP can request a CTS frame from multiple stations (non-AP STAs). Trigger Types other than the MU-RTS frame include the Basic Tigger frame requesting UL MU PPDU, the BRP trigger frame requesting Beamforming Report (Beamforming Report Poll Tigger frame), the MU-BAR Tigger frame (BlockAck request), the BSRP trigger frame requesting Buffer Status Report (Buffer Status Report Poll Tigger frame), the GCR MU-BAR trigger frame, the BQRP (Bandwidth Query Report Poll) trigger frame, and the NDP Feedback Report Poll trigger frame. Trigger Types other than the MU-RTS frame are not related to the content of the present invention, so a detailed description is omitted.

<Multi-link Device (MLD)>

MLD is defined in EHT (Extremely High Throughput) of Wi-Fi 7. MLD means a Logical entity that includes one or more STAs. One or more APs (AP STAs) can be affiliated to an AP MLD, and one or more non-AP STAs can be affiliated to a non-AP (STA) MLD.

Each AP belonging to an AP MLD can operate an independent BSS (Basic Service set), and the operating bandwidth (Operating BW) and operating channel (Operating channel) of the BSSs operated by the APs can be different from each other. When an AP MLD and a non-AP MLD are associated, setup can be performed between multiple APs belonging to a single AP MLD and multiple non-AP STAs belonging to a single non-AP MLD. At this time, since each AP belonging to the AP MLD operates a BSS in its own Link (Operating channel), the non-AP MLD associated with each of the multiple APs belonging to the single AP MLD is considered to have performed a Multi-Link setup. In other words, the AP MLD and the non-AP MLD defined in Wi-Fi 7 can perform a Multi-Link setup connected in multiple Links.

Each MLD can have up to 15 STAs (AP STAs, non-AP STAs). That is, 15 APs can belong to the AP MLD, and the 15 APs each operate an independent BSS. At this time, each AP belonging to the AP MLD provides a service equivalent to a conventional Wi-Fi AP. That is, each AP belonging to the AP MLD can function as an independent AP and can also perform a service for non-AP STAs (e.g., legacy non-AP STAs) that do not belong to the MLD. At this time, each AP belonging to the AP MLD is operated in an independent Link, and the meaning of the Link only means the operating channel on which each AP is operated, and does not mean a Link that distinguishes between 2.4/5/6 GHz. That is, the first AP belonging to the AP MLD can be operated in the first Link, and the second AP can be operated in the second Link. At this time, it is possible for both the first link, where the first AP operates, and the second link, where the second AP operates, to be located in the 6 GHz band.

In addition, AP MLD and non-AP MLD can complete setup in multiple Links through the Multi-Link setup procedure performed in a specific Link. At this time, the Multi-Link setup procedure means Multi-Link Probe Request/Response, Multi-Link Association Request/Response frame exchange, etc. performed to establish a connection for one or more Links. In the present invention, the procedure for performing Multi-Link setup between AP MLD and non-AP MLD is not important, so a detailed description thereof will be omitted.

When two MLDs are connected through multiple Links, the two MLDs can operate the Traffic to be transmitted/received through each Link separately. This can be achieved by TID-to-Link mapping negotiation performed between the two MLDs or by applying the TID-to-Link mapping status indicated by the AP MLD. At this time, the TID-to-Link mapping status that the AP MLD indicates to the non-AP MLDs is indicated by the Management frame (e.g. Beacon, Probe Response frame) transmitted by the AP MLD, and the non-AP MLDs associated with the AP MLD through at least one Link must operate each Link according to the TID-to-Link mapping indicated by the AP MLD. However, when a new TID-to-Link mapping negotiation is performed between the AP MLD and the non-AP MLD, the Traffic (MPDU) of each TID can be transmitted/received through different Links according to the method determined by the new TID-to-Link mapping negotiation. For example, if an AP MLD and a non-AP MLD are connected through two Links, and TIDs 0 to 3 are mapped to Link 1 and TIDs 4 to 7 are mapped to Link 2, the AP MLD and the non-AP MLD should transmit/receive only MPDUs with TIDs 0 to 3 through Link 1, and transmit/receive MPDUs with TIDs 4 to 7 through Link 2.

If the AP MLD does not indicate a separate TID-to-Link mapping state and there is no TID-to-Link mapping performed between the AP MLD and the non-AP MLD, the AP MLD and the non-AP MLD have the Default TID-to-Link mapping state. The Default TID-to-Link mapping state means that all TIDs are mapped to each Link, and in this case, the AP MLD and the non-AP MLD transmit/receive MPDUs of all TIDs (TID = 0 to 7) on each Link.

Since Wi-Fi 8 (UHR, Ultra High Reliability) is expected to be developed based on Wi-Fi 7, the MLD concept, connection procedures between MLDs, and link operation methods through TID-to-Link mapping will still be inherited in Wi-Fi 8. In other words, it is possible for an AP belonging to an AP MLD to be a UHR STA, and it is also possible for a non-AP STA belonging to a non-AP MLD to be a UHR STA.

<MLD Channel Access>

Figure 11 shows a mapping table of user priority and access category.

Each STA belonging to MLD performs channel access in the same way as a conventional Wi-Fi terminal. More specifically, each STA performs channel access using EDCA (Enhanced Distributed Channel Access).

Channel access mechanism using EDCA is a commonly used method for channel access in unlicensed bands.

EDCA provides a mechanism to manage traffic by differentiating it into four types of ACs (access categories) according to its characteristics. The four types of ACs are AC_VO (AC Voice), AC_VI (AC Video), AC_BE (AC Best effort), and AC_BK (AC Background), and each AC can have different CW (contention window), TXOP (transmit opportunity), and AIFSN parameters. Simply put, EDCA is a mechanism to control the transmission priority of traffic transmitted using each AC by differentiating the CW, TXOP, and AIFSN parameters for the four types of ACs. To this end, EDCA can map traffic (MSDU) that a MAC must service to one of the four ACs according to the TC (traffic category) or TS (traffic stream). At this time, traffic mapped to one of the four ACs by EDCA is divided into four queues for each AC and managed. At this time, the above four queues may not be physically separated but may be logically separated queues. At this time, packets mapped to each AC and stored in the Transmission queue are transmitted when each AC completes the backoff procedure and acquires channel access rights. At this time, the method by which the AC performs the backoff procedure and acquires channel access rights is described through Fig. 6, so a detailed description is omitted.

AC_VO is an AC that can be used for traffic that is vulnerable to transmission delay, although the absolute amount of traffic, such as voice traffic, is not large, and has relatively small CW and AIFSN parameter values to increase the probability of being serviced preferentially over traffic from other ACs. However, the TXOP parameter of AC_VO is limited to a relatively smaller value than the TXOP parameters of other ACs, so only a shorter transmission time is guaranteed than that of other ACs.

AC_VI is an AC that is more tolerant to transmission delay than voice traffic, but can still be used for traffic such as video that requires low-latency transmission and a large amount of traffic. AC_VI has larger CW and AIFSN parameter values than AC_VO but smaller than other ACs, and instead, TXOP is about twice as long as AC_VI.

AC_BE is an AC that can be utilized for traffic that is robust to transmission delay, and most general traffic except voice data and streaming video data can be classified as AC_BE. AC_BE uses CW and AIFSN parameters with values larger than AC_VO and AC_VI. In addition, AC_BE does not have a separate TXOP, and therefore cannot utilize the TXOP transmission sequence that transmits a PPDU, receives an ACK in response, and then transmits a PPDU again after SIFS.

AC_BK is a type of AC that is robust to transmission delays, similar to AC_BE, but can be used for lower priority traffic than BE traffic. AC_BK uses the same CW parameter values as AC_BE, and uses larger AIFSN parameter values than AC_BE. In addition, AC_BK does not have a separate TXOP like AC_BE, so it cannot use the TXOP transmission sequence.

The four types of EDCA AC described above are mapped to the UP (user-priority) of 802.1D, and the EDCA AC is determined according to the UP value of the traffic received through the wire or the TID of the MSDU indicated from the upper layer. At this time, if the TID of the MSDU indicates a value of 0 to 7, the value indicated by the TID can correspond 1:1 with the UP.

The rules for mapping 802.1D UP and EDCA AC are described in the UP-to-AC mappings table shown in Figure 43.

In addition, the four types of EDCA AC described above have default CW (CWmin, CWmax), AIFSN, and TXOP parameters defined in the standard, and the parameter values of each AC can be changed by the AP and different values can be used for each BSS.

By utilizing the EDCA mechanism, Wi-Fi traffic is stored in one of four queues corresponding to four ACs, and can be transmitted to a destination device only when the AC it is included in wins the channel access competition with other ACs. At this time, in the channel access competition between the ACs, each AC performs the competition using the access parameters (CW[AC], AIFSN[AC]) assigned to it, and the channel access competition operation performed by each AC is the same as DCF. At this time, if a specific AC does not have any traffic to transmit in the queue, the specific AC may not participate in the competition.

However, as described above, since the CW and AIFSN parameter values utilized by each AC are different, the AC_VO with the smallest CW and AIFSN parameters is more likely to win the channel access competition with other ACs, and thus the traffic of AC_VO is more likely to be serviced with priority over the traffic of other ACs.

In addition, the EDCA mechanism stipulates internal competition rules such as when an (internal) collision occurs between ACs, the AC with a higher priority (see Fig. 11) wins and increases the CW of the other AC that caused the collision, and rules for composing a PPDU including traffic from an AC other than the AC that won the competition (primary AC), but a detailed description is omitted because it is not closely related to the proposal of the present invention.

As described above, each STA belonging to the MLD performs channel access in the same manner as a conventional Wi-Fi terminal. That is, when each STA belonging to the MLD is observed in each Link, each STA belonging to the MLD performs channel access in the same manner as a non-MLD STA (QoS STA) that does not belong to the MLD performs channel access. This can be said to be a rule defined in consideration of fairness with the existing non-MLD STAs that were operating in each Link when developing Wi-Fi 7.

However, there is an exception defined for MLDs operating on Nonsimultaneous transmit and receive (NSTR) Link pairs. More specifically, an STA of an MLD operating on a Nonsimultaneous transmit and receive (NSTR) Link pair is allowed to postpone the initiation of a transmission until the backoff procedure is completed in order to synchronize the transmission initiation timing with the transmission performed on the other Link.

An NSTR Link pair refers to a Link pair that causes strong interference to the remaining Links when the MLD performs transmission on a specific Link among the Link pairs on which the STAs of the MLD operate. For example, if Link1 and Link2 are an NSTR link pair of a non-AP MLD, when non-AP STA1 of the non-AP MLD operating on Link1 performs transmission, non-AP STA2 of the non-AP MLD operating on Link2 experiences strong interference. Accordingly, the non-AP STA2 cannot determine whether Link2 is IDLE/BUSY or normally receive the received PPDU while the non-AP STA1 is performing transmission. In this case, the non-AP MLD has a problem that normal operation of the other Link is impossible when transmission is performed on one Link even though it operates STAs on two Links. To alleviate this problem, Wi-Fi 7 introduces a mechanism by which the non-AP MLD can initiate simultaneous transmission on the NSTR link pair. Briefly, the mechanism that can initiate simultaneous transmission is a mechanism that enables simultaneous transmission to be initiated on the first and second links by postponing transmission until the backoff procedure performed on the second link is completed, even if the non-AP MLD has completed the backoff procedure on the first link.

Additionally, an exception rule is defined that allows a response frame (e.g., CTS frame) not to be responded to even if a frame requesting a response (e.g., RTS frame) is received on the other link of the NSTR link pair while a PPDU is being received on one link of the NSTR link pair.

As described above, since an NSTR link pair is characterized in that interference caused by transmission performed by an STA operating on a specific Link makes it impossible for an STA operating on another Link to operate normally (unable to receive CCA and/or PPDU), the same Link pair may be an NSTR link pair for a specific MLD and an STR link pair (Simultaneous transmit and receive) for another MLD. In this case, an STR link pair means a Link pair in which transmission performed by each STA operating on each Link of the STR link pair does not affect STAs operating on other Links, and therefore, PPDU reception is possible on another Link while PPDU transmission is performed on a specific Link.

In this way, each link pair can be an STR link pair to a specific MLD or an NSTR link pair to another MLD depending on the interference shielding capability of each MLD. However, if the Operating channels of a specific Link pair overlap, the specific Link pair cannot help but become an NSTR link pair regardless of the characteristics/performance of the MLD. Accordingly, in Wi-Fi 7, when an AP MLD and a non-AP MLD perform a Multi-Link setup connected through multiple Links, the Operating Channels of the BSSs operated in the Links where the setup is performed are regulated so as not to overlap with each other. In other words, the Operating Channels of the Links where the AP MLD and the non-AP MLD perform the Multi-Link setup do not overlap with each other.

<Primary channel (or main channel) dependency problem in conventional Wi-Fi channel access procedures>

The reason why MLD was introduced is 1) to increase throughput by using multiple links, and 2) to obtain channel access rights faster by performing channel access procedures through multiple links than performing channel access through a single link (quickly obtain rights by performing channel access through one of multiple links). However, the method for improving channel access opportunities by performing channel access procedures simultaneously on multiple links may be a method that causes large power consumption as the number of links performing channel access procedures increases. In other words, MLD can perform channel access on multiple links to increase the channel access probability (frequency), but it has a problem due to the power consumption required when performing channel access on multiple links. Therefore, as a solution for increasing the channel access probability (frequency), the method of using multiple links can be considered a limited solution, and a method for increasing the success probability (frequency) of the channel access procedure performed on each link is needed. For example, in order to obtain channel access rights faster than performing the channel access procedure through a single link, the channel access procedure can be performed through one or more links, but a certain number of links can be used in consideration of power consumption. For example, if a channel access procedure cannot be performed on a primary channel, the channel access procedure can be performed by selecting one of the idle non-primary channels (such as a non-primary channel or a secondary channel) without waiting until the primary channel changes to an idle state.

Therefore, in order to support the development goal of Wi-Fi 8, which is 'Ultra High Reliability (UHR),' it is necessary to optimize the utilization of multiple links, which is a characteristic of MLD, as well as a method to support STAs operating on each link to obtain channel access opportunities in the best possible way.

In this context, it is necessary to analyze the channel accessibility issue of Wi-Fi terminals performing channel access on each link. The Wi-Fi standard has achieved significant throughput performance improvements over generations, and the Wi-Fi 7 standard, which is recently nearing standardization, supports throughput exceeding 30 Gbps. One of the reasons why the Wi-Fi 7 standard can support extremely high throughput compared to legacy Wi-Fi standards is the wide operating BW of Wi-Fi 7. While conventional Wi-Fi terminals use a 20 MHz band as their operating BW, Wi-Fi 7 operates with an operating BW of up to 320 MHz. This means that the maximum throughput increase effect achieved by simply expanding the maximum operating BW supported by the Wi-Fi standard is up to 16 times. However, the maximum throughput of the Wi-Fi standard increased as the operating BW is expanded is only a nominal value, and it is difficult for it to actually lead to improved performance of Wi-Fi terminals.

In other words, although the maximum supportable Operating BW is continuously expanding through the development of Wi-Fi terminals/standards, the impact on the actual performance of Wi-Fi terminals is relatively small. This is because, when a Wi-Fi terminal performs channel access, the probability that the entire wide band included in the maximum Operating BW will be identified as Idle is low, and the method by which a Wi-Fi terminal performs channel access has an excessively high Primary 20 MHz channel dependency. Among these, the problem that the probability that the entire wide band included in the Operating BW will be identified as Idle may be an inherent problem because the frequency band in which the Wi-Fi terminal operates is an unlicensed band. In other words, it is natural for the medium to be occupied by other devices operating in the unlicensed band, and it is impossible to increase the channel access probability of the Wi-Fi terminal by improving this problem. However, the problem of excessively high Primary 20 MHz channel dependency is not a feature of Wi-Fi that has been maintained for harmonious operation with heterogeneous devices, but a feature that has been inherited in the process of maintaining the channel access technique traditionally used from existing Wi-Fi. To explain in more detail, the Wi-Fi standard was designed to perform channel access to a 40 MHz channel by extending the channel access technique used when the operating BW is 20 MHz. More specifically, the method of accessing the 40 MHz channel according to the method defined in the Wi-Fi standard performs access to the 40 MHz band (the 40 MHz band including the Primary 20 MHz and Secondary 20 MHz bands) if the Secondary 20 MHz channel is confirmed to be IDLE for the last PIFS (Priority Inter Frame Space, aSIFSTime (16 us) + aSlotTime (9 us)) when the backoff procedure is completed in the Primary 20 MHz channel. Similarly, a method for accessing an 80 MHz channel according to a method defined in the Wi-Fi standard is to perform access to the 80 MHz band (80 MHz band including the Primary 20 MHz, Secondary 20 MHz, and Secondary 40 MHz bands) if the Secondary 20 MHz channel and the Secondary 40 MHz channel have been identified as IDLE for the last PIFS (Priority Inter Frame Space, aSIFSTime (16 us) + aSlotTime (9 us)) when the backoff procedure is completed in the Primary 20 MHz channel. In this way, the Wi-Fi Wide Band Operation method for accessing subchannels identified as IDLE during the PIFS when the backoff procedure is completed in the Primary 20 MHz channel is applied in the same manner when accessing 320 MHz BW defined in Wi-Fi 7. The reason why this method has been repeatedly used is because it enables wide bandwidth access in a more energy-efficient and less hardware-implementation manner by performing backoff on only one Channel (primary channel) and determining whether other subchannels are accessible within a minimum time interval.

However, this method of channel access using the primary channel has a major drawback in that when the Primary 20 MHz channel on which the Wi-Fi terminal performs the backoff procedure is determined to be busy, even if all sub-channels except the Primary 20 MHz channel are available (not occupied by other devices), the backoff procedure of the Wi-Fi terminal cannot be completed, and thus channel access to wide Idle sub-channels is also impossible.

As described above, the problem of channel access of a Wi-Fi terminal supporting wideband operation being limited depending on the CCA result of the Primary 20 MHz subchannel is not a new problem in UHR. However, in the case of UHR, which succeeds Wi-Fi 7 supporting an ultra-wideband operation up to 320 MHz, the loss to be suffered may be greater than that of the existing WiFi standards due to the dependency on the Primary 20 MHz subchannel described above. Furthermore, the next-generation standards after UHR may also experience performance degradation issues due to the dependency on the Primary 20 MHz subchannel described above, and therefore it is clear that there is a need to solve the channel access problem related to the Primary 20 MHz subchannel described above.

For this reason, the present invention provides a method and procedure for a terminal supporting wideband operation to perform communication using a subchannel other than the primary 20 MHz subchannel determined to be BUSY when the CCA result for the primary 20 MHz subchannel is BUSY.

For example, if a preamble of a PPDU is received through a primary 20 MHz subchannel and the primary 20 MHz subchannel is determined to be busy as a result of performing CCA based on the received preamble, a channel access procedure may be performed by selecting one of the non-primary 20 MHz subchannels other than the primary 20 MHz subchannel. In addition to the primary 20 MHz subchannel, the other subchannel on which the channel access procedure is performed may be included in an operating channel that is the same as or different from the primary 20 MHz.

<Access to channels that do not utilize the primary channel>

As the simplest method for resolving the dependency problem on the aforementioned Primary 20 MHz sub-channel (hereinafter referred to as the P20 channel), a method of performing channel access using (through) a sub-channel (a non-primary channel (or sub-channel)) other than the P20 channel may be considered. At this time, performing channel access using (through) a non-primary channel means performing a backoff procedure based on whether the non-primary channel is idle/busy. At this time, there may be one or more non-primary channels on which the terminal can perform the backoff procedure. That is, the terminal can perform the backoff procedure through the P20 channel or multiple non-primary channels (e.g., the first non-primary channel or the second non-primary channel, etc.). At this time, the channels on which the terminal can perform the backoff procedure (i.e., the P20, the first non-primary channel, the second non-primary channel, the third non-primary channel, etc.) may be 20 MHz sub-channels included in different 80 MHz subblocks, respectively. That is, the first non-primary channel may be located in an 80 MHz subblock other than the 80 MHz subblock including the P20 channel. That is, when the terminal selects a backoff channel (non-primary channel) other than P20, the terminal must select another backoff channel (non-primary channel) from among the 20 MHz subchannels of the 80 MHz subblock (i.e., the subblock excluding the primary 80 MHz subblock) that does not include the P20 subchannel. At this time, each of the different backoff channels selected by the terminal may be located in a different 80 MHz subblock. That is, the first non-primary channel may be a subchannel located in a different 80 MHz subblock from the second non-primary channel. At this time, the selection restriction of the non-primary channel with respect to the aforementioned 80 MHz subblock may be applied only when the operating channel of the BSS is included in the 5 GHz or 6 GHz band.

At this time, channel access using a non-primary channel may be performed only during a time period in which the P20 channel is determined to be BUSY. That is, channel access using a non-primary channel (hereinafter, referred to as non-primary channel access) may be performed only when the P20 channel is determined to be Busy as a result of Physical CCA (ED, Energy detection) and PD, Virtual CCA. In addition, non-primary channel access may be permitted only to STAs (AP STAs, non-AP STAs) in which the P20 channel is BUSY and the frame identified on the P20 channel is not a frame destined for itself. Accordingly, the non-primary channel access procedure may be permitted only when the preamble of a PPDU received on the P20 channel is successfully detected or the MPDU is successfully decoded. Accordingly, the non-primary channel access procedure may be permitted only when the STA successfully receives the frame received on the P20 channel.

That is, when an MLD (AP MLD or non-AP MLD) operates on one P20 and one or more non-primary channels, one STA among the STAs (AP or non-AP) constituting the MLD can perform a channel access procedure on the P20 channel. In this case, the STA can receive a preamble of a PPDU on the P20 channel and perform a CCA based on the preamble. If the P20 channel is determined to be busy as a result of the CCA and the PPDU received on the P20 channel is transmitted from an overlapping BSS (OBSS), the STA can select one non-primary channel among one or more non-primary channels and perform a channel access procedure through the selected non-primary channel. The state of the non-primary channel on which the channel access procedure is performed may be an idle state.

That is, backoff performance and channel access procedures using subchannels other than the P20 channel may be restricted to cases where the PPDU identified in the P20 channel is a PPDU of the OBSS. Accordingly, backoff performance and channel access procedures using other subchannels may be restricted to cases where the PPDU identified in the P20 channel is a PPDU whose destination device is not the device that received the PPDU.

To this end, the backoff performing and channel access proceeding procedure using a subchannel other than the P20 channel can be initiated after confirming whether the PPDU identified on the P20 channel is the destination device or OBSS by decoding the preamble of the PPDU to confirm the BSS Color of the HE-SIG and/or U-SIG, confirming the STA-ID of the EHT-SIG and/or UHR-SIG, or decoding the first MAC frame of the PPDU to confirm the destination device. That is, the STA can confirm the BSS Color included in the SIG field (e.g., HE-SIG (HE-SIG-A or HE-SIG-B), or U-SIG) included in the preamble of the PPDU, or confirm the station identifier (STA-ID) included in the SIG field (e.g., HE-SIG-B, EHT-SIG, or UHR-SIG)) to determine whether the received PPDU was transmitted from the OBSS. Alternatively, the STA can identify the destination device by decoding the first MAC frame of the PPDU.

At this time, if the MAC addresses of the sender/receiver of a specific PPDU and the sender/receiver of the frame included in the PPDU are the APs with which it is associated, the specific PPDU can be distinguished as a PPDU (Intra-BSS PPDU) rather than an OBSS PPDU. At this time, a backoff procedure using a subchannel other than the P20 channel may need to be initiated after confirming whether the other subchannel is IDLE during DIFS. At this time, if the confirmed BSS Color is not its own BSS Color, STA-ID and MAC frame decoding performed to specify the target device may be omitted. At this time, a method of confirming whether the other subchannel is IDLE may be to confirm by performing PHY CCA (Energy detection and/or Packet detection) performed for a preset time period. At this time, the preset time may be PIFS (Priority Inter Frame Space) or DIFS (Distributed Inter Frame Space) or MediumSync time. At this time, MediumSync time can be a time interval with a different name, and means the time that a device that intends to perform a backoff procedure on a non-primary channel (or a sub-channel) must perform CCA to determine whether the medium is IDLE/BUSY. MediumSync time can be several ms long and is shorter than MaxPPDU length (5.484 ms). A terminal that performs CCA by applying MediumSync time can set NAV using information included in PPDU (frame) received while performing CCA. At this time, the NAV set by the terminal based on the PPDU (frame) received on the S20 channel (a sub-channel other than the primary 20 MHz channel) can be a NAV other than the two NAVs (Basic NAV, Intra-BSS NAV) used in conventional Wi-Fi. At this time, the other NAV is a timer set by a frame (PPDU) received through S20, and is a NAV used for Virtual CCA of the S20 channel when performing channel access through the S20 channel. That is, the other NAV is set by a frame (PPDU) received on the S20 channel, and even if the value of the other NAV is not 0, the terminal can determine the result of the CCA performed on the P20 channel as IDLE. That is, the other NAV is a NAV for the S20 channel, not the P20 channel. At this time, the other NAV can be called a secondary NAV.

Accordingly, if the terminal performing channel access via the S20 channel is an AP STA, the AP may need to manage both the basic NAV, which is set based on the received frame (PPDU) while occupying the primary 20 MHz subchannel, and the secondary NAV, which is set based on the received frame (PPDU) (i.e., received via the S20 channel) without occupying the primary 20 MHz subchannel. That is, the AP must perform a backoff procedure considering the basic NAV when performing channel access via the P20 channel, and must perform a backoff procedure considering the secondary NAV when performing channel access via the S20 channel. At this time, the secondary NAV may be a timer that is initialized to the Mediumsync time value when the terminal performing channel access via the P20 channel decides to perform channel access via the S20 channel. That is, when the AP decides to perform channel access via the S20 channel, it may need to initialize the secondary NAV to the Mediumsync time value at the same time as starting CCA for the S20 channel.

In addition, an STA that performs frame exchange after performing channel access through a subchannel other than the Primary 20 MHz subchannel may need to perform a procedure to check whether the Primary 20 MHz subchannel is occupied by another BSS and another device when initiating a channel access procedure on the Primary 20 MHz subchannel after the frame exchange performed through the other subchannel is terminated. At this time, a method for the STA to check whether the P20 channel is occupied by another BSS and another device may be to perform CCA for the P20 channel during the MediumSync time. At this time, if the STA receives a valid PPDU (frame) while performing the CCA during the MediumSync timer, the STA may set the NAV for the P20 channel based on the information acquired through the received PPDU (frame). In this case, the STA may resume the channel access procedure performed on the Primary 20 MHz subchannel when the set NAV is released (when the NAV timer becomes 0).

At this time, the backoff procedure using another subchannel can be compensated for the delayed time for decoding the preamble of the PPDU confirmed on the P20 channel or for decoding the MAC frame. In one embodiment, if 3-slot time (e.g., 27 us) is consumed to confirm the destination device (or BSS Color) of the PPDU confirmed on the P20 channel, an operation of decreasing the backoff counter used for channel access using a subchannel other than the P20 channel by 3 at once can be permitted. Alternatively, since channel access using a subchannel other than the P20 channel is an additional function that is not utilized by conventional devices, the operation of decreasing the backoff counter all at once can be prohibited, and the backoff counter can be sequentially decremented by 1 after confirming the destination device of the PPDU confirmed on the P20 channel. In other words, compensation for the delayed backoff procedure may not be performed separately in the process of confirming the destination device of the PPDU confirmed on the P20.

In the embodiments of the present invention described below, the process of identifying the destination device of a PPDU received on the above-described P20 channel may be omitted for convenience of explanation. Accordingly, even if not described separately, it should be understood that the channel access procedure performed using a subchannel other than the P20 channel includes the process of identifying the destination device of a PPDU received on the above-described P20 channel.

Figure 12 shows an example of a channel access procedure through a non-primary channel when the state of the primary channel is busy.

Referring to FIG. 12, if the CCA result of the primary 20 MHz subchannel is determined to be busy, the terminal can perform channel access using a non-primary 20 MHz subchannel other than the primary 20 MHz subchannel.

Specifically, an STA (AP STA, non-AP STA) configuring an MLD may perform channel access using a channel other than the P20 channel when the P20 channel is determined to be BUSY (for example, when the P20 channel is determined to be busy based on a CCA result based on a preamble of a received PPDU). At this time, the operation of performing the channel access may be performing a backoff according to the CCA result of the other channel. At this time, the backoff operation may be an operation of decreasing a backoff counter by 1 when the result of CCA performed in each slot on the other channel is IDLE. In addition, the backoff operation may be an operation of maintaining the backoff counter without decreasing it when the result of CCA performed in each slot on the other channel is BUSY.

The S20 channel that can be utilized for channel access using a 20 MHz sub-channel other than the above-described P20 channel is not limited to a specific S20 channel, but multiple S20 channels can be utilized. For example, assuming that an STA performs a 320 MHz operation as in one embodiment of FIG. 12, channel access may be possible not only through the S20_1 channel included in the Secondary 80 MHz subblock as illustrated in FIG. 12, but also through S20_2 and S20_3 included in the Secondary 160 MHz subblock. In this case, the number of S20 channels utilized for channel access by each STA may be determined according to the capability of each STA, or may be limited to one or two specific S20 channels. However, the STA may perform backoff on only one non-primary sub-channel per each 80 MHz subblock. At this time, each non-primary subchannel on which the STA performs backoff is a subchannel determined by the AP, and is therefore indicated through a management frame transmitted by the AP. That is, the AP can indicate information on another subchannel (S20) on which backoff can be performed when the primary channel (P20) is busy through a management frame transmitted by the AP (e.g., Beacon, Probe Response, Association Response frame, etc.), and the other subchannel is one of the subchannels included in an 80 MHz subblock other than the Primary 80 MHz subblock.

Referring to Fig. 12 (a), a backoff counter utilized when performing channel access on a P20 channel and a backoff counter utilized when performing channel access on an S20 channel may exist and be managed separately. In this case, the backoff counter utilized by each channel may be changed to a new value of the backoff counter only when transmission is performed as a result of the channel access performed on each channel (when backoff is completed). At this time, changing to the new value means changing to a new backoff counter extracted using CW_min if transmission is successful, or changing to a new backoff counter extracted using CW x 2 if transmission fails. In other words, it does not mean an operation of reducing the backoff counter as a CCA result. At this time, as described above, if there are multiple S20 channels performing channel access, each of the multiple S20 channels may have its own backoff counter. At this time, the backoff counter for each S20 channel may exist by Access Category. That is, the terminal may need to separately manage a backoff counter for each AC for each S20 channel on which it can perform backoff. That is, a terminal performing a backoff procedure through an S20 channel can perform a channel access procedure internally using four ACs.

Referring to Fig. 12 (b), all S20 channels performing channel access, including P20, can utilize a common backoff counter. As illustrated in Fig. 12 (b), as a result of the channel access operation performed on the P20 channel, the backoff counter is decreased from 5 to 3, and then the P20 channel is changed to BUSY. According to an embodiment proposed in the present invention, if the P20 channel is BUSY, the S20 channel can perform the channel access procedure, and as shown in Fig. 13 (b), the backoff counter, which P20 reduced to 3, can continue to be decreased according to the CCA result performed on S20_1. If S20_1 is also determined to be BUSY while decreasing the backoff counter for channel access, the backoff counter can be maintained as is until the channel access using S20_2 is started, or until P20 to S20_1 are determined to be IDLE. At this time, if the channel connection using the above S20_2 is continued, it can be understood as an embodiment in which there are two or more S20 channels used for the channel connection, and if the backoff counter is maintained until the above P20 to S20_1 are determined to be IDLE, it can be understood as an embodiment in which there is only one S20 channel used for the channel connection.

<Restrictions on transmission through primary channels>

As described above, an STA (non-AP STA or AP) that constitutes an MLD (non-AP MLD or AP MLD) operating on one primary channel and one or more non-primary channels can attempt a channel access procedure on the primary channel. At this time, if the STA performs CCA to perform the channel access procedure on the primary channel and the primary channel is in an idle state, the STA can perform the channel access procedure on the primary channel. However, if the primary channel is determined to be busy as a result of the CCA performed based on the preamble of the PPDU received on the primary channel, the STA cannot perform the channel access procedure until the primary channel changes to idle. Therefore, in this case, the STA can select one non-primary channel in an idle state among one or more non-primary channels and perform the channel access procedure without waiting until the primary channel changes to idle.

In this way, an STA that performs a channel access procedure using a non-primary channel (S20 channel) rather than a primary channel may be restricted in the length of a PPDU it transmits. The above-mentioned PPDU transmission length restriction may be necessary for two reasons. In this case, the length of a PPDU transmitted after performing a backoff procedure on the S20 channel may be restricted so that it ends earlier than the end time of a PPDU recognized by a PPDU (and/or frame) confirmed on the P20 channel. That is, when an STA that performs channel access on a non-primary channel transmits a PPDU (second PPDU) on the non-primary channel, the length of the PPDU transmitted on the non-primary channel may be restricted to the length of a PPDU (first PPDU) received for CCA on the primary channel. For example, the length of the second PPDU may be equal to or shorter than the length of the first PPDU. At this time, the length of the first PPDU to limit the length of the second PPDU can be confirmed by the length field included in the preamble of the first PPDU.

One aspect that requires the above restriction may be to prevent a problem that may occur when the transmission of an OBSS device that occupied the P20 channel is terminated while the transmission started using the S20 channel continues. More specifically, the transmission that is performed after performing channel access through the S20 channel will be performed through sub-channel(s) excluding the P20 channel, and there is a problem that the AP cannot provide any service such as transmission/reception, scanning, etc. for the P20 channel while performing the transmission. Therefore, if the transmission of S20 is terminated later than the OBSS PPDU confirmed on the P20 channel, the AP cannot confirm and receive other STA UL PPDU or OBSS PPDU that can be confirmed on the P20 channel. In this case, in addition to the problem that the UL PPDU of the STA cannot be received, the NAV setting by the OBSS PPDU cannot be performed either, which may cause a problem in the overall operation of the BBS. Another aspect that requires the above PPDU transmission length restriction may be to alleviate the fairness problem. If a length limit is not applied to transmissions made after performing channel access through the S20 channel, it may cause fairness problems with conventional WiFi STAs that perform channel access only through the P20 channel. Therefore, in addition to limiting the number of S20 channels that can perform channel access through the aforementioned S20 channel, it is necessary to alleviate the fairness problems with conventional WiFi STAs by limiting the length of PPDUs transmitted through channel access through the S20 channel.

In addition, a device that performs channel access using a non-primary channel (S20 channel) rather than a primary channel (P20 channel) may be restricted in the length of a TXOP acquired after performing a backoff procedure on a sub-channel (S20) other than the primary 20 MHz channel. In this case, the length of the TXOP acquired after performing the backoff procedure on the S20 channel may be restricted so as to end earlier than the TXOP end time of the OBSS recognized by the PPDU (and/or frame) received on the P20 channel. In other words, an STA that performs channel access on a non-primary channel (S20 channel) rather than the primary channel (P20 channel) may be restricted in the length of a TXOP acquired through the channel access procedure. For example, the length of a TXOP (second TXOP) acquired by an STA through a channel access procedure of a non-primary channel may be equal to or shorter than the length of a TXOP (first TXOP) based on a PPDU (first PPDU) of a primary channel. In this case, the length of the first TXOP may be acquired based on a TXOP field included in a preamble of the first PPDU.

The above TXOP length limitation may be necessary for two aspects. One aspect of the limitation may be to prevent a problem that may occur when the TXOP of an OBSS device occupying the P20 channel is terminated while the TXOP acquired using the S20 channel (a sub-channel other than the primary 20 MHz channel) continues. More specifically, the TXOP acquired through the S20 channel is applied to a frequency section occupying the sub-channel(s) excluding the P20 channel, and while frame exchange is performed using the TXOP, the terminals may not be able to perform transmission/reception, CCA, etc. for the P20 channel. Accordingly, if the transmission of the S20 channel is terminated later than the TXOP of the OBSS confirmed on the P20 channel, the terminals cannot confirm and receive other OBSS PPDUs that can be confirmed on the P20 channel. In this case, since the UL PPDU of the STA cannot be received, and NAV setting by OBSS PPDU cannot be performed, a problem may occur in the overall operation of the BBS. Another aspect that requires the above PPDU transmission length limitation may be to alleviate the fairness problem. If the length limitation of the TXOP obtained after performing channel access through the S20 channel is not applied, it may cause a fairness problem with the conventional Wi-Fi terminal that performs channel access through only the P20 channel. Therefore, in addition to limiting the number of S20 channels that can perform channel access through the aforementioned S20 channel, it may be necessary to alleviate the fairness problem with the conventional WiFi STA by limiting the length of the TXOP obtained by channel access through the S20 channel.

Figure 13 illustrates an example of a transmission length limitation of a PPDU transmitted after performing channel access through a non-primary channel.

Referring to FIG. 13, the length of a PPDU transmitted by an STA through a non-primary channel (S20 channel) rather than the primary channel (P20 channel) described above may be limited to be the same as or shorter than the length of a PPDU transmitted on the primary channel (e.g., a PPDU received by the STA through the primary channel for CCA).

Specifically, a situation is illustrated in which a terminal, which has determined that a P20 channel is BUSY, completes a backoff procedure through an S20_1 channel and transmits a PPDU. At this time, the backoff counter of the backoff procedure performed in the S20_1 channel may be a backoff counter commonly used with the backoff counter used in the P20 channel, or may be a separate backoff counter utilized for channel access through the S20_1 channel. In Fig. 13, the S20 channels are illustrated as being in the same 80 MHz subblock as the P20 channel, but the S20 channels that perform a backoff when the P20 channel is BUSY may be located in a different 80 MHz subblock from the 80 MHz subblock that includes the P20 channel.

Before starting channel access through S20_1 channel, STA can detect preamble of OBSS PPDU during channel access attempt through P20 channel and confirm that P20 channel is occupied by OBSS (BUSY). In this situation, STA can perform decoding on preamble of OBSS PPDU and confirm how long OBSS PPDU will last. At this time, operation of STA performed after confirming how long OBSS PPDU will last may be setting NAV. At this time, STA can confirm L-SIG and Length fields of detected preamble or confirm duration of OBSS PPDU based on value indicated through TXOP field included in U-SIG field and/or HE-SIG field of OBSS PPDU. Alternatively, STA can confirm duration of OBSS PPDU based on Duration/ID field of MAC frame included in PPDU.

Thereafter, the STA can determine the length of the PPDU to be transmitted after performing channel access through the S20 channel based on the end time of the OBSS PPDU confirmed on the P20 channel. At this time, the length of the PPDU to be transmitted after performing channel access through the S20 channel can be limited/adjusted to end the same as or earlier than the predicted end time of the OBSS PPDU confirmed on the P20 channel. At this time, a method of limiting the length of the PPDU may be such that the end time of a Response frame expected to be responded to the PPDU is adjusted to end the same as or earlier than the end time of the OBSS PPDU. At this time, the Response frame means a PPDU including an ACK frame or a Block ACK frame, a TB (trigger-Based) PPDU, etc. In other words, the end time of the PPDU responded to by the PPDU can be limited to be earlier than or equal to the predicted end time of the OBSS PPDU confirmed on the P20 channel.

FIG. 14 illustrates an embodiment of a method for limiting the length of a TXOP obtained through channel access via a non-primary channel.

Referring to FIG. 14, the length of a TXOP based on a PPDU transmitted through a non-primary channel (S20 channel) by an STA that performs a channel access procedure rather than the primary channel (P20 channel) described above may be limited to be equal to or shorter than the length of a TXOP based on a PPDU transmitted on the primary channel (e.g., a PPDU received by the STA through the primary channel for CCA).

Specifically, a situation is illustrated in which a terminal, which has determined that the P20 channel is BUSY, completes a backoff procedure through a subchannel (S20 channel) included in a secondary 80 MHz subblock and acquires a TXOP. At this time, a backoff counter of the backoff procedure performed through the S20 channel may be a backoff counter commonly used with the backoff counter used in the P20 channel, or may be a separate backoff counter used when performing a backoff operation in a subchannel included in the secondary 80 MHz subblock.

When a terminal wants to perform a backoff procedure through S20 because the P20 channel is confirmed to be BUSY, the terminal performs CCA on the S20 channel during a time period corresponding to MediumSync time in order to protect the communication of the OBSS that may be transmitting/receiving using the S20 channel. In Fig. 14, the state of S20 observed by the terminal during the time period corresponding to MediumSync time is IDLE, and therefore the terminal starts a backoff procedure on the S20 channel.

When the terminal completes the backoff procedure performed through the S20 channel, the TXOP acquired by the terminal is set to end at the same time as the TXOP (OBSS TXOP) including the frame (RTS/CTS frame of FIG. 14) transmitted by occupying the P20 channel. At this time, although the two TXOPs are illustrated as ending at the same time in the embodiment of FIG. 14, the TXOP acquired after performing the backoff through the subchannel included in the Secondary 80 MHz subblock may be set to end earlier than the OBSS TXOP.

After performing backoff through S20 channel, a terminal acquiring TXOP can utilize Duration/ID field of (MU-)RTS and/or CTS and/or BSRP (Buffer Status Report Poll)/BSR (Buffer Status Report) frame received on Primary 20 MHz subchannel when checking the TXOP length of OBSS occupying P20 channel. That is, when a frame transmitted by STA of OBSS is received through Primary 20 MHz subchannel, the terminal checks the TXOP length of OBSS based on information indicated through Duration/ID field of the received frame. In this case, the terminal sets a NAV corresponding to the Primary 20 MHz subchannel based on the confirmed length, completes the channel access procedure (backoff procedure) through the subchannel included in the Secondary 80 MHz subblock, and then, when acquiring a TXOP, adjusts its TXOP length so that the TXOP it has acquired ends at the same time as or before the TXOP of the confirmed OBSS.

After the TXOP acquired through the S20 channel is terminated, the terminal performs CCA for the P20 channel during the MediumSync time to resume channel access through the P20. If a valid frame (PPDU) is received before the time corresponding to the MediumSync time elapses, the terminal sets NAV based on the information acquired through the field related to the length of the received frame (PPDU), and can resume the channel access procedure through the P20 channel when the NAV is released. In the example of Fig. 14, P20 was observed in the IDLE state until the time corresponding to the MediumSync time elapsed, and therefore, the terminal resumed the channel access procedure after the time corresponding to the MediumSync time expired.

<Restrictions on channel access procedures through S20 channel (non-primary channel)>

A series of channel access procedures performed through the above-described S20 channel may be channel access procedures allowed only to AP STAs. In other words, a channel access procedure through the S20 channel, not the P20 channel, may be a channel access procedure that cannot be performed by non-AP STAs.

The reason why the channel access procedure via the S20 channel is allowed only to AP STAs is because, even among STAs belonging to a single BSS, the status of the P20 channel confirmed by each STA may be different from each other. Assuming that a first non-AP STA and a second non-AP STA are associated with a BSS operated by an AP, the first non-AP STA may recognize that the P20 channel is BUSY after receiving a PPDU transmitted from an OBSS, but the AP and the second non-AP STA may recognize that the P20 channel is IDLE because the PPDU was not received. In addition, there may also be a case where the first non-AP STA does not receive the PPDU received by the second non-AP STA. In this way, the status of the P20 channel confirmed by each non-AP STA belonging to the same BSS may be different from each other, and the status of the P20 channel confirmed by each STA and the status of the P20 channel confirmed by the AP may also be different. In this case, even if a non-AP STA, which determines that the P20 channel is busy, performs channel access through the S20 channel and transmits a UL PPDU, the AP may determine that the P20 is idle and still proceed with the channel access procedure on the P20 channel. In other words, the PPDU transmitted by the non-AP STA after performing channel access on S20 cannot be normally received by the AP if the P20 channel confirmed by the AP is idle. Furthermore, while a non-AP STA, which determines that the P20 channel is BUSY, performs channel access on the S20 channel and transmits a UL PPDU, the AP may transmit a PPDU to the non-AP STA after completing the channel access on the P20 channel. In this case, the non-AP STA encounters a problem that it cannot normally perform a PPDU received on the P20 channel due to the PPDU transmission performed on the S20 channel. In this way, since the status of the P20 channel confirmed by each STA (AP STA and non-AP STA) belonging to the BSS may be different from each other, each non-AP STA may be restricted from performing a channel access procedure on the S20 channel even if it determines that the P20 channel it has confirmed is in a BUSY state. However, the non-AP STA may have to wait for reception of a PPDU that may be transmitted on the S20 channel when it determines that the P20 channel is in a BUSY state.

In the case of an AP, after performing channel access on the S20 channel, in order to check whether a target device to receive frames to be transmitted by the AP is capable of receiving on the S20 channel, a frame of a preset format may need to be transmitted as the first frame transmitted after performing channel access on the S20 channel. At this time, the preset format may be RTS (Request To Send), MU-RTS (Multi-User RTS), BSRP (Buffer Status Report Poll), or another type of trigger frame. In other words, the AP may need to transmit a frame of the preset format included in the first PPDU transmitted after performing channel access through the S20 channel.

That is, an AP that performs a channel access procedure on an S20 channel, not a P20 channel, may transmit a frame in a format that is a preset format first after the channel access procedure. For example, an AP that performs a channel access procedure on an S20 channel may transmit a frame that does not include data first after the channel access procedure. That is, after the channel access procedure, the AP may transmit a frame in a specific format that does not include data first after the channel access procedure, since the channel of an STA associated with the AP may not have been changed yet.

At this time, the frame of the preset format transmitted by the AP is a frame that requests a response of an immediate response frame from one or more STAs, and therefore, when the AP transmits a frame of the preset format, a response frame is sent in response from the STAs that receive it.

When response frames are received in response to a frame of a preset format that the AP transmitted, the AP can recognize that the STAs that transmitted the response frames are STAs that can support reception of the PPDU transmitted via S20. Accordingly, after transmitting the frame of the preset format, the AP can transmit frames targeting the STAs that responded with a response frame to the frame via the next PPDU.

As a more specific example, after performing channel access through S20, the AP can request a response frame from the first STA and the second STA through a frame in a preset format that is transmitted. At this time, if the response frame is responded to from the first STA and the response frame is not responded to from the second STA, the AP can include an MPDU whose destination device is the first STA in the PPDU that is transmitted next, and may not include an MPDU whose destination device is the second STA. That is, if the AP does not receive a response frame for the frame in the preset format transmitted through S20 from a specific STA, the AP must not include an MPDU whose destination device is (individually addressed) the specific STA in the PPDUs that are transmitted through the channel access.

<Normative behavioral restrictions for channel access via S20>

All terminals performing communications using unlicensed bands must perform normative channel access operations to ensure that each terminal can access the medium in a harmonized and fair manner. The ETSI (European Telecommunications Standards Institute) BRAN (Broadband Radio Access Networks) committee has defined a harmonized standard related to the channel access method of terminals utilizing unlicensed bands, and Wi-Fi terminals must also follow the standards defined by ETSI BRAN because they perform operations using unlicensed bands. The EDCA mechanism, which is a channel access method used in conventional Wi-Fi, is defined as one of the normative channel access methods by ETSI BRAN, and therefore conventional Wi-Fi terminals perform channel access through the normative operation defined by ETSI BRAN.

Among the standards defined in relation to the EDCA mechanism, there is a regulation regarding the change of the channel (primary operating channel) performing the backoff operation via the EDCA mechanism. The specific regulation is that a terminal performing channel access via EDCA must not change the primary operating channel more than once per second. In other words, a terminal performing channel access via EDCA must perform channel access using the channel for at least 1 second when the channel performing the backoff procedure (primary operating channel) is changed. In this case, the time for performing channel access using a specific subchannel may be calculated based on the start time of transmission of the first PPDU transmitted among the PPDUs after performing channel access using the specific subchannel. In other words, if the start time of the first PPDU transmitted after the terminal changes the primary operating channel to S20 is T1, the terminal may change the primary operating channel to a subchannel other than S20 (primary 20 MHz subchannel or another subchannel) from a time point 1 second after T1.

Accordingly, the channel access operation via the S20 channel provided in the present invention may be limitedly usable when the channel access operation via the P20 channel is maintained for 1 second or more than 1 second. In addition, a terminal that has performed the channel access operation via the S20 channel must attempt the channel access operation via the S20 channel for 1 second or more than 1 second, and it is possible to switch to performing the channel access via the P20 channel while maintaining the channel access operation via the S20 channel for 1 second or more than 1 second. In this case, the 1 second is for example, and it is also possible for the same operation/restriction to be performed by applying another previously agreed time length.

That is, in order to perform a channel access procedure on a non-primary channel, the terminal must attempt the channel access procedure on the primary channel for a certain period of time (e.g., 1 s), and thereafter, when performing the channel access procedure on a non-primary channel, the terminal must attempt the channel access procedure on the non-primary channel for a certain period of time (e.g., 1 s). For example, when performing a channel access procedure on a non-primary channel, the terminal must perform an operation (e.g., the channel access procedure) on the non-primary channel for a certain period of time (e.g., 1 s).

That is, non-AP STAs that recognize that the AP has performed channel access through the S20 channel may have to perform operations considering the S20 channel as the default channel until the AP can perform channel access through the P20 channel. That is, non-AP STAs that recognize that the AP has performed channel access through the S20 channel must perform channel access on the S20 channel or wait for reception of PPDUs transmitted from the AP until the AP switches to a state in which it can perform channel access on the P20 channel. At this time, the AP can instruct the non-AP STAs with information related to the point in time when it performed channel access through the S20 channel or the point in time when it can switch to channel access operation through the P20 channel using the Management frame (e.g., Beacon frame) that it transmits. That is, the AP can instruct the non-AP STAs with information related to the point in time when it switches to channel access operation through the P20 channel during the time period in which it must perform channel access through the S20 channel, through the Management frame that it transmits. When Non-AP STAs receive a Management frame transmitted by an AP, if the Management frame includes information related to the point in time when the AP switches to channel access operation via a P20 channel, the Non-AP STAs can recognize whether the channel access execution channel of the AP is a P20 channel or an S20 channel based on the indicated information.

FIG. 15 illustrates a method for an AP to manage a primary operating channel by obtaining a TXOP through a primary channel and a non-primary channel according to one embodiment of the present invention.

Referring to Figure 15, in a situation where OBSS1 and OBSS2 exist in the Primary 80 MHz subblock and the Secondary 80 MHz subblock, respectively, the AP performs channel access through a channel not occupied by the OBSS by changing the primary operating channel. The description below explains the channel access operation of the AP in chronological order (from left to right based on the drawing).

When the AP acquires TXOP1, it acquires channel access rights by completing the channel access procedure performed through P20. Since the TXOP of OBSS2 is in progress in the Secondary 80 MHz subblock at the time when the AP acquires TXOP, the AP acquires channel access rights only for the Primary 80 MHz, and therefore TXOP1 is applied only for the Primary 80 MHz subblock. After that, the AP completes the channel access procedure again in P20 and acquires TXOP2, which is a TXOP for the 160 MHz band. After TXOP2 is terminated, while the AP is performing the channel access procedure to acquire TXOP3, the TXOP of OBSS1 is acquired in the form of occupying P20, which is the AP's primary operating channel, and therefore the AP's backoff procedure is stopped. At this time, the AP recognizes that S20 is also occupied by the TXOP of OBSS2 and maintains the primary operating channel without changing it. However, before the channel association procedure of the AP that resumed after the first TXOP of OBSS1 ended is completed, the TXOP of OBSS1 started again, and the AP decides to change the primary operating channel to S20. At this time, the AP is able to change the primary operating channel to S20 because the time it maintained P20 as the primary operating channel was 1 second or longer than 1 second.

Accordingly, the AP performs channel access through S20 during the second TXOP of OBSS1 and acquires TXOP3. After the TXOP of OBSS2 is terminated, the AP completes the channel access procedure performed in S20 and acquires TXOP4, which is a TXOP for the 160 MHz band. After this, the channel access procedure performed by the AP in S20 is interrupted as the TXOP of OBSS2 starts. At this time, the AP confirms that P20 is IDLE and decides to change the primary operating channel to P20. At this time, the AP can change the primary operating channel to P20 because the time it maintained S20 as the primary operating channel was 1 second or longer than 1 second.

<Multi-subchannel access using multi-link>

As mentioned above, if the primary operating channel is allowed to be changed only once per second according to the normative operation defined by the ETSI BRAN committee, it is difficult to say that the dependency problem on the primary channel, which is the basic idea of the present invention, is completely resolved. This is because, after a specific terminal changes the primary operating channel, whether or not the channel access is possible is determined based on whether the changed primary operating channel is IDLE/BUSY for at least one second. In other words, the dependency problem on the primary channel that the Wi-Fi terminal has can be alleviated only once per second at most, and the same dependency problem occurs on the changed primary operating channel for the remaining time interval. In other words, the problem to be solved by the present invention is not the dependency problem on the Primary 20 MHz subchannel of the Wi-Fi terminal, but the dependency problem on the primary operating channel of the Wi-Fi terminal (the channel used for channel access, the reference channel for performing the backoff procedure). Therefore, the method of changing the primary operating channel from the P20 channel to the S20 channel, and then again from the S20 channel to the P20 channel or another S20 channel, only provides the effect of changing the channel designated as the primary operating channel, but does not resolve the limitation that channel access for the entire Operating BW is restricted when a specific 20 MHz sub-channel (primary operating channel) is determined to be BUSY.

As a method for solving the dependency problem on the designated primary operating channel (i.e., the channel that performs the channel access, i.e., the EDCA backoff procedure) as described above, a method of lowering the importance of the designated primary operating channel can be considered. As the simplest method for lowering the importance of the designated primary operating channel, designating multiple primary operating channels for a single operating channel can be considered. When multiple primary operating channels are designated for a single operating channel, the channel access dependency problem of a Wi-Fi terminal for each primary operating channel can be alleviated in proportion to the number of designated primary operating channels.

<How to specify multiple primary action channels for a single action channel>

The Operating BW supported by the Wi-Fi standard has been continuously increasing, and the Wi-Fi7 standard supports an Operating BW of up to 320 MHz BW. Even when accessing an Operating channel with such a wide BW (e.g., 40, 80, 160, 320 MHz channels), a Wi-Fi terminal can perform channel access only when it has completed the backoff procedure on the Primary 20 MHz subchannel (main operating channel). At this time, since the backoff procedure can be completed only when the Primary 20 MHz subchannel is determined to be IDLE, the Wi-Fi terminal cannot obtain access to a wide channel up to the 320 MHz band when the Primary 20 MHz subchannel is determined to be BUSY. This is due to the dependency problem of the Wi-Fi terminal on the primary operating channel mentioned above, and the reason why the existing Wi-Fi standard has maintained a channel access procedure with such a dependency problem is to minimize the increase in complexity of the Wi-Fi terminal.

More specifically, the ETSI regulation defines two different normative channel access methods for wide-bandwidths (BW exceeding 20 MHz). The first normative channel access method is to obtain channel access rights by using EDCA (or a channel access mechanism operating under rules similar to EDCA) for each 20 MHz sub-channel included in the wide-bandwidth to be accessed, and to perform access to one or more 20 MHz channels for which channel access rights have been obtained simultaneously. That is, a terminal performing channel access for a band exceeding 20 MHz can perform a channel access procedure for each 20 MHz sub-channel separately, and access the 20 MHz channels for which channel access rights have been obtained by completing the channel access procedure. The second normative channel access method for wide-bandwidth defined in the ETSI regulation is to simultaneously access other 20 MHz channels that have been determined to be idle in measurements performed for more than 25 us when the channel access procedure on one primary operating channel (one 20 MHz channel) has been completed. The method by which conventional Wi-Fi terminals access the 40 MHz, 80 MHz, 160 MHz, and 320 MHz bands utilizes the second normative channel access method defined in the ETSI regulation.

If a Wi-Fi terminal uses the first normative channel access method defined in the above-mentioned ETSI regulation, the Wi-Fi terminal can more flexibly access each subchannel determined as IDLE within the Operating BW without a dependency issue on a specific subchannel. However, performing a separate channel access procedure for each 20 MHz channel included in the Operating BW increases the hardware/operational complexity of the Wi-Fi terminal and causes excessive energy consumption for the channel access procedure, which incurs significant costs in other aspects.

For this reason, even in Wi-Fi7, the latest Wi-Fi standard, a method of performing channel access on each 20 MHz subchannel to access each 20 MHz subchannel was not introduced, but rather the channel access method of the conventional Wi-Fi was inherited, in which channel access was performed on the Primary 20 MHz channel, which is the main operating channel, and then channel access was performed for other subchannels that were determined to be IDLE when the channel access procedure on the Primary 20 MHz channel was completed.

The overlapping operating channel configuration method of MLD, which is explained through the embodiments of the present invention described below, is a method of lowering the dependency on the main operating channel while maintaining the channel access method of conventional Wi-Fi in which each terminal attempts channel access to a wide bandwidth using the main operating channel.

<Auxiliary AP/Link/BSS>

As explained above, MLD (Multi-link Device) is defined in Wi-Fi7, and AP MLD and non-AP MLD can be in an ML-setup (Multi-link setup) state where setup is performed through multiple links. At this time, the Operating channels of each link where the AP MLD and non-AP MLD perform setup must be setup so as not to overlap each other. At this time, when the AP MLD and non-AP MLD are setup through multiple links, the BSS of the AP (AP belonging to the AP MLD) operated in each of the multiple links must be a BSS operated through a non-overlapping frequency range.

However, the Operating BWs of each AP belonging to the AP MLD are allowed to overlap with each other. The rules in terms of the operating channel of the multi-link setup performed by the AP MLD and the non-AP MLD can be explained with a simpler example as follows. The AP MLD can operate the first AP, the second AP, and the third AP in link 1, link 2, and link 3, respectively. At this time, the Operating channels of the two BSSs operated by the first and second APs overlap (partially or fully overlapped), and the Operating channel of the BSS operated by the third AP does not overlap with the Operating channels of the BSSs operated by the first and second APs. The AP MLD performs ML setup with the non-AP MLD, but allows ML setup states such as [link 1, link 3] and [link 2, link 3], and does not allow ML setup states such as [link 1, link 2] and [link 1, link 2, link 3]. This is because, among the links on which the non-AP MLD performed setup, link 1 and link 2 have overlapping Operating channels. In summary, the BSS Operating channels of each AP operated by the AP MLD can overlap with each other, but the BSS Operating channels of multiple links on which the non-AP MLD performed ML setup are not permitted to overlap with each other. This is because, if the BSS Operating channel of a specific link on which the non-AP MLD performed ML setup overlaps with the BSS Operating channel of another link on which the ML setup was performed, the BSSs of the specific link and the other link act as OBSS (Overlapping BSS) and are BSSs that cannot provide service at the same time.

According to one embodiment of the present invention, AP MLD may set operating channels (channels through which APs operate BSS) of two or more APs to overlap for the purpose of strengthening channel access for non-primary channels. At this time, one of the APs whose operating channels overlap may be set as a primary AP, and the remaining APs may be set as secondary APs. A method of performing channel access using a primary AP and a secondary AP is described through <Channel Access Using Overlapping AP> described below.

In the description of the present invention described below, the links on which the Primary AP and the Auxiliary AP operate are respectively named Primary link and Auxiliary link, the BSS on which the Primary AP operates is named Primary BSS, and the BSS on which the Auxiliary AP operates is named Auxiliary BSS. Since the operating channels of the Primary BSS and the Auxiliary BSS overlap each other, in the description of the present invention described below, the two APs are expressed as having an overlapping AP relationship.

A secondary AP operated by an AP MLD has a paired Primary AP, and the operating channels of the paired secondary AP and the Primary AP overlap. The Primary AP and the secondary AP support different levels of operations. For example, the Primary AP may periodically transmit a Beacon frame (including a Beacon frame and/or other types of Management frames, and a Group addressed frame), but the secondary AP may not transmit a Beacon frame (including a Beacon frame and/or other types of Management frames, and a Group addressed frame). For another example, the Primary AP may support services for legacy STAs (i.e., STAs that follow standards previous to Wi-Fi 8, such as Wi-Fi 7 and Wi-Fi 6), but the secondary AP may be restricted from performing services for legacy STAs. In addition, the BSS operated by the secondary AP (secondary BSS) may be a BSS in which only STAs of a non-AP MLD to which STAs that are members of the BSS (Primary BSS) operated by the Primary AP that is paired with the secondary AP belong may become members.

That is, in order for a non-AP MLD to be setup on a secondary link, it must be setup together with the primary link that is paired with the secondary link. That is, when a non-AP MLD requests setup of a secondary link to the AP MLD, it must also request setup for the primary link that is paired with the secondary link. In other words, a non-AP MLD must not transmit a frame (e.g., Association Request frame) that requests setup only for the secondary link among a pair of primary and secondary links. However, when a non-AP MLD and an AP MLD have already performed setup through the primary link, the non-AP MLD can perform an additional setup request for the secondary link to the AP MLD. In this case, the frame that the non-AP MLD transmits to request addition of the secondary link may be a Link Reconfiguration Request frame.

That is, only a non-AP MLD that is set up with the Primary link can be set up with the secondary link that is paired with the Primary link.

That is, when the AP MLD receives a request for a multi-link setup from a non-AP MLD, if the non-AP MLD requests only the setup for the secondary link and does not request the setup for the Primary link that is paired with the secondary link, the AP MLD should not accept the setup for the secondary link. However, the non-AP MLD can request the AP MLD to perform setup for only the Primary link among the pair of Primary and Secondary links. In this case, if the non-AP MLD requests the setup for both the Primary link and the third link (requests ML setup), the ML setup connected through the AP MLD, the Primary link, and the third link can be performed.

<Overlapping AP>

The reason why AP MLD operates two (primary AP and auxiliary AP) or more than two APs (primary AP and first auxiliary AP, second auxiliary AP, etc.) with overlapping operating channels is to perform channel access through the primary 20 MHz channels of the BSS operated by each AP and to support services for non-AP MLDs through the AP that has completed channel access. At this time, the primary AP and the auxiliary AP may be APs that use independent radios or APs that are operated using a single radio. However, AP MLD performs channel access through overlapping APs, and performs channel access through only one AP at a specific time. At this time, performing channel access means performing a series of procedures for channel access, such as reducing the backoff counter according to the EDCA rules.

There may be an implementation where the Primary AP and the Secondary AP are configured using independent Radios. In this case, the Primary AP can perform PPDU transmission through the Primary 20 MHz subchannel, while the Secondary AP can receive other PPDUs received through the Secondary channel (the Primary channel of the Secondary BSS), or perform CCA, PD (Packet detection), etc. In this way, the Secondary AP with independent Radios can have the Capability to function as a general AP when it is not designated as a Secondary AP by the AP.

Also, an implementation in which the Primary AP and the Secondary AP share a single Radio may be possible. In this case, when the Primary AP transmits a PPDU through the Primary 20 MHz subchannel, the Secondary AP cannot perform transmission/reception, CCA, PD, etc. for its primary channel (secondary channel). This is because if the Radio commonly used by the two APs is used for the operation of the Primary AP, there is no Radio that can be used by the Secondary AP. In other words, the Primary AP and the Secondary AP are APs that exist logically separately, and can be APs that are physically operated using a single device (Radio, RF chain, antenna, etc.). The reason why the AP MLD can operate multiple APs (Primary AP and Secondary APs) using a single Radio is because the Radio is utilized for only one AP among the multiple APs at a specific time. That is, AP MLD may not support channel access/PPDU transmission/reception for auxiliary APs when the channel access procedure is performed by the Primary AP or when PPDU transmission/reception is performed. In addition, when the channel access procedure is performed or PPDU transmission/reception is performed through a specific auxiliary AP, multiple APs share one radio and operate in a way that support for the Primary AP and other auxiliary APs is not required. That is, AP MLD can perform Single-Radio Multi-Link operation for the Primary link and the auxiliary link.

At this time, an AP MLD operating on multiple links using one Radio may lose MediumSync for the remaining links while performing channel access or PPDU transmission/reception on a specific link among the multiple links. At this time, the loss of MediumSync means that the management of the NAV timer, which should have been managed by receiving other PPDUs (frames) transmitted/received on each link, was not performed. Therefore, the AP MLD that lost MediumSync for the remaining links due to an operation performed on a specific link may need to recover MediumSync before performing channel access on the other links. At this time, the AP MLD may need to perform CCA on the link that lost MediumSync for a time corresponding to the MediumSync time in order to recover MediumSync for the link that lost MediumSync. Alternatively, the AP MLD may instruct a specific non-AP MLD to transmit a specific frame (e.g., MediumSync Recovery frame) on the link that lost MediumSync in order to recover MediumSync for the link that lost MediumSync. In this case, the non-AP MLD, which has been instructed to transmit the specific frame from the AP MLD, must transmit the specific frame to the AP MLD through the link when the link to which the AP MLD instructed to transmit the specific frame is determined to be idle (idle as a result of PHY CCA and Virtual CCA). If the instructed link is maintained in a busy state until the instructed time or a preset time has elapsed from the time when the transmission of the specific frame is instructed, the non-AP MLD may not transmit the specific frame to the AP MLD.

FIG. 16 illustrates an example of a method for configuring an AP MLD including a primary AP and an auxiliary AP having overlapping operating channels and setting an operating channel according to one embodiment of the present invention.

Referring to Fig. 16(a), three AP MLDs belong to the AP MLD. At this time, AP1 functions as the Primary AP, AP2 functions as the Auxiliary AP paired with the Primary AP, and AP3 is a general AP.

Referring to Fig. 16(b-1), the Primary AP and the Auxiliary AP operate on exactly the same Operating channel (specifically 320 MHz). However, the Primary 20 MHz subchannel of the Primary AP is located at the lowest 20 MHz subchannel within the Operating channel, and the Primary 20 MHz subchannel (Auxiliary channel) of the Auxiliary AP is located at the highest 20 MHz subchannel within the Operating channel.

Referring to Fig. 16(b-2), the operating channel of the auxiliary AP is included in the operating channel of the primary AP. However, even in this case, the primary 20 MHz subchannel of the primary AP and the primary subchannel (auxiliary channel) of the auxiliary AP are set to different 20 MHz.

Referring to Fig. 16(b-3), the Operating channel of the secondary AP and the Operating channel of the primary AP partially overlap with each other. However, the Primary 20 MHz subchannel of the primary AP and the Primary subchannel (auxiliary channel) of the secondary AP are located in the band where the Operating channels of the two APs overlap.

The embodiments of the present invention described below were written assuming a situation where the operating channels of the Primary AP (BSS) and the Secondary AP (BSS) completely overlap each other, as in Fig. 16 (b-1). However, it should be understood that operating channel configurations such as Fig. 16 (b-2) and (b-3) are also Primary/Secondary BSS operating channel configuration methods that can be used to improve accessibility to specific operating channel(s) using the method provided by the present invention.

<Channel Access Using Overlapping AP>

The AP MLD channel access procedure performed using the Primary BSS and the Auxiliary BSS with an operating channel that overlaps with the Primary BSS is as follows.

The primary BSS and the secondary BSS operate using the same operating channel (for example, both BSSs operate on the same 320 MHz channel). At this time, the primary BSS and the secondary BSS set different 20 MHz subchannels as the primary 20 MHz subchannel. At this time, the operating channel of the secondary BSS can be a subset of the operating channel of the primary BSS.

The primary channel (secondary channel) of the secondary BSS may be located in an 80 MHz subblock other than the 80 MHz subblock in which the primary channel of the Primary BSS is located. That is, a 20 MHz subchannel included in the Primary 80 MHz subblock of the Primary BSS cannot be set as the primary channel (secondary channel) of the secondary BSS.

If multiple auxiliary APs form an AP pair with the Primary AP, the multiple auxiliary APs must each have their primary channel (auxiliary channel) set to a different 80 MHz subblock. For example, if there are three auxiliary BSSs (auxiliary AP1, auxiliary AP2, auxiliary AP3) paired with a Primary BSS of 320 MHz BW, the three auxiliary BSSs must each have their primary channel (auxiliary channel1, auxiliary channel2, auxiliary channel3) set to the 80 MHz subblock corresponding to the lower frequency among the Secondary 80 MHz subblock and the Secondary 160 MHz subblock of the Primary BSS, and the 80 MHz subblock corresponding to the higher frequency among the Secondary 160 MHz.

If the Primary 20 MHz subchannel of the Primary BSS is determined to be IDLE, the AP MLD performs channel access through the Primary 20 MHz subchannel of the Primary BSS and then transmits the PPDU through the Primary AP. That is, the frames included in the PPDU transmitted through the Primary AP have the MAC address (TA) of the transmitting device set to the MAC address (or BSSID) of the Primary AP. In addition, the Address 3 field of a frame transmitted from the Primary BSS (transmitted by the Primary AP or transmitted by an STA that is a member of the Primary BSS) may be set to the BSSID of the Primary AP depending on the values of the To DS and From DS subfields. In addition, the Address 1 field (RA field) of a frame transmitted by a non-AP STA that is a member of the Primary BSS is set to the BSSID of the Primary BSS when the To DS and From DS subfields are 1 and 0, respectively. In addition, frames included in the PPDU transmitted through the Primary AP have the MAC address (RA) of the destination device set to the MAC address of the non-AP STA operating on the Primary link among the non-AP STAs of the non-AP MLD. At this time, if the PPDU is transmitted as a HE/EHT/UHR PPDU, the BSS Color indicated through the preamble of the PPDU is set based on the ID of the BSS operated by the Primary AP. At this time, the secondary BSS is maintained in an inactive state during the section in which channel access is performed through the Primary 20 MHz subchannel of the Primary BSS.

The secondary BSS remaining inactive means that no PPDU transmission/reception is performed through the secondary AP.

During the period in which the secondary BSS remains inactive, the secondary AP must not perform channel access through its primary channel (secondary channel). In this case, not performing channel access means that the backoff counter managed for performing channel access cannot be decreased during the inactive time period.

A secondary AP with an independent radio may be able to perform PHY CCA or Packet Detection even during periods when it is inactive. In this case, the secondary AP can set the NAV for its primary channel (secondary channel) based on the information obtained through the received PPDU (frame).

When the secondary AP performs the backoff procedure using the secondary channel, the backoff counter can be reduced according to the EDCA rules, and the backoff counter can be reduced only when the secondary AP is active. In other words, even if the backoff counter can be reduced according to the EDCA rules, the backoff counter of the secondary AP (more precisely, the backoff counter for each of the four Access Categories of the secondary AP) is not reduced when the secondary AP is inactive.

The secondary AP may not be able to perform PHY CCA and PD, etc. when it is inactive. This may be because there is no RF (radio frequency front end) that the secondary AP can use when it is inactive. This situation may occur when the secondary AP is a logically different AP from the primary AP, but is configured to physically share one radio. In this case, the secondary AP may need to set NAV for the time period corresponding to the MediumSync time when starting a channel connection. That is, even if no PPDU (frame) is received after starting a channel connection, the channel connection should be performed by considering the medium as BUSY (busy as a result of Virtual CCA) during the time period corresponding to the MediumSync time.

When a PPDU (frame) of another BSS is received through the Primary 20 MHz sub-channel of the Primary BSS, the AP MLD switches the Primary AP (BSS) to the inactive state and the secondary AP to the active state during the TXOP period of the other BSS confirmed through the received PPDU. However, when the PPDU of the other BSS is received, if the state of the primary channel (secondary channel) of a specific secondary AP confirmed (determined) through PHY CCA is BUSY, the specific secondary AP may be maintained in the inactive state. If the IDLE secondary channel (primary channel of the secondary APs paired with the Primary AP) confirmed through PHY CCA does not exist, the Primary AP may not be switched to the inactive state and may be maintained in the active state. In this case, the operation of the Primary AP switched to the inactive state may be the same as/similar to the operation performed when the above-mentioned secondary AP is in the inactive state.

The time period during which the Primary BSS remains inactive may be until the TXOP of the other BSS ends. The time period during which the Primary BSS remains inactive may be until the end time of the PPDU of the other BSS.

The time period during which the Primary BSS remains inactive may be until a PPDU from another BSS is received through the Secondary BSS.

The time period during which the above Primary BSS remains inactive may be until after the PPDU transmitted through the channel access performed through the primary channel (secondary channel) of the secondary BSS occupies the primary channel of the Primary AP.

The time period during which the Primary BSS remains inactive may be limited to before the next TBTT (Target Beacon Transmission Time) of the Primary BSS. That is, when the next TBTT of the Primary BSS arrives, the Primary BSS is in an active state and the secondary BSSs are in an inactive state.

The time period during which the Primary BSS is maintained in an inactive state may be limited to before the start time of the R-TWT (Restricted Target Wake Time) SP (Service period) operated by the Primary BSS. That is, when the start time of the R-TWT SP of the Primary BSS arrives, the Primary BSS is in an active state and the secondary BSSs are in an inactive state.

While the secondary AP (BSS) remains active, the AP MLD performs channel access through the primary channel (secondary channel) of the secondary BSS. When the channel access procedure performed through the secondary channel is completed, the AP MLD transmits a PPDU through the secondary AP. That is, the frames included in the PPDU transmitted through the secondary AP have the MAC address (TA) of the transmitting device set to the MAC address of the secondary AP. In addition, the Address 3 field of a frame transmitted from the secondary BSS (transmitted by the secondary AP or transmitted by an STA that is a member of the secondary BSS) may be set to the BSSID of the secondary AP depending on the values of the To DS and From DS subfields. In addition, the Address 1 field (RA field) of a frame transmitted by a non-AP STA that is a member of the secondary BSS is set to the BSSID of the secondary BSS when the To DS and From DS subfields are 1 and 0, respectively. In addition, frames included in the PPDU transmitted through the secondary AP have the MAC address (RA) of the destination device set to the MAC address of the non-AP STA operating in the secondary link among the non-AP STAs of the non-AP MLD. At this time, if the PPDU is transmitted as a HE/EHT/UHR PPDU, the BSS Color indicated through the preamble of the PPDU is set based on the color of the BSS operated by the secondary AP. At this time, the color of the BSS operated by the secondary AP may be the same as the BSS color of the Primary AP. At this time, the Primary AP (BSS) is maintained in an inactive state during the section in which channel access is performed through the primary channel (secondary channel) of the secondary AP (BSS).

The Primary BSS remains inactive, meaning that no PPDU transmission/reception is performed through the Primary AP.

During the period in which the Primary BSS remains inactive, the Primary AP must not perform channel access through its primary channel. In this case, not performing channel access means that the backoff counter managed for performing channel access cannot be decreased during the inactive time period.

A Primary AP in a configuration with an independent Radio may be able to perform PHY CCA or Packet Detection even during periods when it remains inactive. In this case, the Primary AP can set the NAV for its primary channel based on the information obtained through the received PPDU (frame).

When the Primary AP performs the backoff procedure using the primary channel, the backoff counter can be reduced according to the EDCA rules, and the backoff counter can be reduced only when the Primary AP is active. In other words, even if the backoff counter can be reduced according to the EDCA rules, the backoff counter of the Primary AP (more precisely, the backoff counter for each of the four Access Categories of the Primary AP) is not reduced when the Primary AP is inactive.

The Primary AP may not be able to perform PHY CCA and PD, etc. when it is inactive. This may be because there is no RF (radio frequency front end) that the Primary AP can use when the Primary AP is inactive. This situation may occur when the Primary AP is a logically different AP from the Secondary AP, but physically shares a single radio. In this case, the Primary AP may need to set NAV for the time period corresponding to the MediumSync time when starting a channel connection. That is, even if no PPDU (frame) is received after starting a channel connection, the channel connection should be performed by considering the medium as BUSY (busy as a result of Virtual CCA) during the time period corresponding to the MediumSync time.

The TXOP acquired by the secondary AP may be terminated earlier than the TBTT of the primary BSS. This is a restriction to ensure that the Primary AP can transmit the Beacon frame according to the TBTT of the primary BSS. Therefore, the secondary AP must manage its TXOP so that its TXOP is terminated earlier than the next TBTT of the primary BSS. At this time, the TXOP of the secondary AP may be terminated at least T earlier than the TBTT of the primary BSS. At this time, T is a time interval including the time (delay) required when the primary AP (BSS) in the inactive state transitions to the active state. At this time, T may be a time interval including the time required to recover MediumSync after the primary AP transitions to the active state (for example, MediumSync time).

The TXOP acquired by the secondary AP may be terminated earlier than the start time of the R-TWT SP of the primary BSS. This is a restriction to ensure that the primary AP can perform channel access according to the R-TWT SP of the primary BSS. Therefore, the secondary AP must manage its TXOP so that its TXOP is terminated earlier than the start time of the R-TWT SP operated in the primary BSS. At this time, the TXOP of the secondary AP may be terminated at least T earlier than the start time of the R-TWT SP of the primary BSS. At this time, T is a time interval including the time (delay) required when the primary AP (BSS) in the inactive state transitions to the active state. At this time, T may be a time interval including the time required to recover MediumSync after the primary AP transitions to the active state (for example, MediumSync time).

The operation of performing channel access by the Primary AP and the Secondary AP according to the methods 1. to 4. described above is a normative channel access operation in which each AP performs channel access through its own primary channel and does not change the channel through which the channel access is performed (primary operating channel).

However, according to one embodiment of the present invention, AP MLD can secure multiple access paths for a single Operating channel by setting the Operating channels of multiple APs in an overlapped form (overlapping and identically) and differentiating the primary operating channels of each AP. Accordingly, AP MLD can have a lower primary operating channel dependency than conventional Wi-Fi when accessing a specific Operating channel that operates multiple APs.

At this time, APs belonging to a link pair (AP pair) consisting of a primary AP and secondary AP(s) have the characteristic that at a certain point in time, only one AP is maintained in an active state, and the remaining APs are maintained in an inactive state.

At this time, TBTT means the time that AP is promised to transmit Beacon frame. AP periodically transmits Beacon frame through the primary channel of BSS that it operates, and the Beacon frame transmission cycle is Beacon Interval. Therefore, each TBTT has Beacon Interval interval.

At this time, R-TWT SP is a type of Broadcast TWT, and is a service section for low latency traffic introduced in the Wi-Fi7 standard. During R-TWT SP, low latency traffic is processed with priority. At this time, for each R-TWT SP, the ID of the traffic considered as low latency traffic is indicated by the AP. That is, when R-TWT SP is operated in a specific BSS, the traffic corresponding to the TID (Traffic ID) indicated by the AP during the R-TWT SP section is serviced with priority within the BSS.

FIG. 17 illustrates an example of a procedure for an AP MLD to obtain a TXOP using a primary BSS and a secondary BSS according to one embodiment of the present invention.

Referring to Figure 17, the AP MLD performs channel access through the Primary 20 MHz subchannel (P20) of the Primary BSS. That is, the Primary BSS is active and the Secondary BSS is inactive.

AP MLD, which acquired TXOP1 through Primary BSS, performs transmission/reception with non-AP MLD through Primary link. After TXOP1 is terminated, AP MLD confirms that P20 channel of Primary AP is occupied by OBSS, switches Primary AP to inactive state and switches secondary AP to active state.

AP MLD performs channel access through the primary channel (A20) of the secondary AP and acquires TXOP2. During TXOP2, AP MLD performs transmission/reception with non-AP MLD through the secondary link. At this time, TXOP2 is terminated earlier than the TXOP of the OBSS confirmed by the primary AP.

When the TXOP2 acquired through the secondary AP is terminated, AP MLD switches the Primary AP to the active state and switches the secondary AP to the inactive state to perform channel access through the Primary AP.

After that, AP MLD performs channel access through P20 of Primary AP and acquires TXOP3 when channel access is completed. During TXOP3, AP MLD performs transmission/reception with non-AP MLD through Primary link, which is an active link.

<Non-AP MLD operation associated with overlapping APs>

According to one embodiment of the present invention described above, the AP MLD can set the operating channels of a plurality of APs as overlapping operating channels, and operate one AP among the plurality of APs as a Primary AP and the remaining APs as Secondary APs. In this way, among the plurality of APs operated in the overlapping operating channels, when a specific AP is active, other APs are maintained in an inactive state, making PPDU transmission/reception impossible. In addition, an AP in an inactive state may be an AP that is in a state in which not only PPDU transmission/reception but also CCA (Virtual CCA and/or Physical CCA) is impossible.

Among the links on which the non-AP MLD performed ML setup, if there is an inactive link (a link of an inactive AP), the non-AP MLD must not transmit a UL PPDU through the inactive link. This is because, even if the non-AP MLD transmits a UL PPDU through the inactive link, the AP that should receive the UL PPDU does not support reception of the PPDU. In other words, the UL PPDU transmission of the non-AP MLD performed on the inactive link is a transmission that is sure to fail, and can be understood as an unnecessary operation.

Therefore, among the links that performed AP MLD and ML setup, if there is a primary link and a secondary link, the non-AP MLD may need to determine whether to perform transmission and/or channel access in a different manner from the link on which a regular AP is operated when performing channel access through the primary link or the secondary link.

For example, a non-AP MLD should not perform UL PPDU transmission via EDCA on the secondary link of an AP MLD. A frame that the non-AP MLD can transmit on the secondary link may be limited to a response frame to a frame received from the secondary AP. In this case, the type of the response frame transmitted by the non-AP MLD on the secondary link includes at least one of a CTS frame transmitted after receiving a RTS/MU-RTS frame, a BSR frame transmitted after receiving a BSRP trigger frame, and frames included in a TB PPDU transmitted after receiving a trigger frame.

The reason why the non-AP MLD is restricted from performing transmission via EDCA on the secondary link may be that it is difficult for the non-AP MLD to accurately determine whether the secondary AP is active or inactive. Therefore, the non-AP MLD can be restricted to perform UL PPDU transmission only when a frame requesting a response frame from the secondary AP is received, instead of attempting UL PPDU transmission, which is likely to fail.

However, there may be an exception that allows a non-AP MLD to transmit a UL PPDU after performing EDCA channel access through a secondary link. As an example of the exception, a non-AP MLD that obtains information from an AP MLD to determine whether a specific secondary link is active may perform channel access (channel access using an EDCA mechanism, i.e., channel access performed independently, not trigger-based channel access) through a non-AP STA operating on the specific secondary link and then attempt to transmit a UL PPDU. The information that allows the determination whether the specific secondary link is active may be information indicated through a frame transmitted through another AP belonging to the AP MLD. In this case, the information indicated through the other AP belonging to the AP MLD may be information indicating which AP (which link) among the Primary AP and the secondary APs is active. A non-AP MLD that performs channel access via a secondary link (channel access using the EDCA mechanism, channel access performed independently rather than trigger-based channel access) may need to transmit an RTS frame as the first frame transmitted on the secondary link. In other words, a non-AP MLD may need to transmit an RTS frame as the first frame transmitted on the secondary link. If a CTS frame in response to the RTS frame transmitted as the first frame is not received from the AP, the non-AP MLD may not perform additional channel access on the secondary link.

Similar to AP MLD, non-AP MLD can use independent radios for the non-AP STA (Primary non-AP STA) operating in the Primary and the non-AP STA (Secondary non-AP STA) operating in the Secondary, or can operate the non-AP STAs of both links using a single radio.

That is, there can be an implementation in which the primary non-AP STA and the secondary non-AT STA are configured using independent radios. In this case, the primary non-AP STA can transmit/receive PPDUs through the primary 20 MHz subchannel while the secondary non-AP STA can receive other PPDUs received through the secondary channel (primary channel of the secondary BSS) or perform CCA, PD (Packet detection), etc. In this way, the secondary non-AP STA having the independent radio can have the capability to function as a general non-AP STA when associated with an AP that is not the secondary AP (for example, a Regular AP that is not the primary/secondary AP).

In addition, an implementation may be possible in which the primary non-AP STA and the secondary non-AP STA share a single radio. In this case, when the primary non-AP STA transmits/receives a PPDU through the primary 20 MHz subchannel, the secondary non-AP STA cannot perform transmission/reception, CCA, PD, etc. for its primary channel (secondary channel). This is because if the radio commonly used by two non-AP STAs is used for the operation of the primary non-AP STA, there is no radio that can be used by the secondary non-AP STA. In other words, the primary non-AP STA and the secondary non-AP STA are non-AP STAs that exist only logically separately, and may be non-AP STAs that are physically operated using a single device (radio, RF chain, antenna, etc.). The reason why the non-AP MLD can operate multiple non-AP STAs (primary non-AP STA and auxiliary non-AP STAs) using one radio is because the radio is utilized for only one non-AP STA among the multiple non-AP STAs at a specific point in time. That is, the non-AP MLD may not support channel access/PPDU transmission/reception, etc. for auxiliary non-AP STAs when the channel access procedure is performed by the primary non-AP STA or PPDU transmission/reception is performed. In addition, when PPDU transmission/reception is performed through a specific auxiliary non-AP STA, multiple non-AP STAs share one radio and operate in a way that support for the primary non-AP STA and other auxiliary non-AP STAs is not required. That is, the non-AP MLD can perform Single-Radio Multi-link operation for the primary link and the auxiliary link.

Meanwhile, a non-AP MLD may be subject to a restriction that it can only be setup for one auxiliary link among a pair of auxiliary links for each Primary link. For example, when a first Primary link and a first auxiliary link and a second auxiliary link, which are pairs of the first Primary link, exist, a specific non-AP MLD may have an ML setup state including the first Primary link and the first auxiliary link, or may have an ML setup state including the first Primary link and the second auxiliary link, and may not have an ML setup state including both the first auxiliary link and the second auxiliary link. That is, a non-AP MLD may have to be setup only through one auxiliary link among the multiple auxiliary links corresponding to a specific Primary link.

A non-AP STA operating on a secondary link may terminate its TXOP before the next TBTT of the primary link (the TBTT of the primary BSS) when it has acquired a TXOP via EDCA. This is the same/similar behavior as the secondary AP terminating its TXOP before the TBTT of the primary BSS, so a detailed description is omitted.

<Auxiliary AP Discovery>

In this way, since the auxiliary link (auxiliary AP, auxiliary BSS) has different characteristics from the general AP and primary link, the AP MLD must indicate which of its affiliated APs is the AP operating in the auxiliary link through the management frames it transmits. In addition, the non-AP MLD that wants to perform ML setup with the AP MLD must, after receiving the management frame transmitted by the AP MLD, determine whether each AP operated by the AP MLD is a general AP, a primary AP, or a secondary AP based on the information included in the management frame. For example, a non-AP MLD that recognizes that a specific AP of the specific AP MLD is a secondary AP by receiving a Beacon frame transmitted by a specific AP MLD must not perform Multi-link setup through the specific AP (auxiliary AP). In other words, the non-AP MLD must not transmit a (ML) Probe Request frame and/or a (ML) Association Request frame to the secondary AP.

One way for the AP MLD to indicate what type of AP each AP is through the management frames it transmits could be by using the RNR element (Reduced Neighbor Report).

For example, the AP MLD can indicate that an AP corresponding to a Neighbor AP Information field configured/set in a different manner from the Neighbor AP Information fields corresponding to other APs (third AP (normal AP) and Primary AP) among the Neighbor AP Information fields included in the RNR element that it transmits is a secondary AP. A specific method by which the AP MLD indicates a secondary AP using the RNR element is described in more detail with reference to an embodiment of FIG. xx.

FIG. 18 illustrates an example of a format of an RNR element transmitted by an AP MLD to indicate an auxiliary AP according to one embodiment of the present invention.

Figure 18 (a) illustrates the Reduced Neighbor Report element format. The Reduced Neighbor Report element may include multiple Neighbor AP Information Fields, and the length of the element is indicated through the Length field.

Neighbor AP Information Fields include a Neighbor AP Information field corresponding to each AP belonging to the AP MLD transmitting the corresponding element. At this time, the Neighbor AP Information field for the AP transmitting the RNR element is not included.

Fig. 18(b) illustrates a method for setting a Neighbor AP Information field corresponding to an auxiliary AP belonging to an AP MLD. The Neighbor AP Information Field is the same regardless of the characteristics of the corresponding AP, but the setting of the TBTT Information Header and the size of the TBTT Information Set field may be different between a general AP and an auxiliary AP. More specifically, as illustrated in Fig. 18(b), in the Neighbor AP Information field corresponding to the auxiliary AP, the TBTT Information field Type of the TBTT Information Header field may be indicated as a non-zero value (e.g., 1 or 2). This is a setting method that is differentiated from the TBTT Information field Type of the general AP being indicated as 0. In addition, in the Neighbor AP Information field corresponding to the auxiliary AP, the TBTT Information Length indicated in the TBTT Information Header field is set to 3. That is, the TBTT information Set field is indicated as having a size of 3-octet. At this time, the TBTT Information Set field includes an MLD Parameters subfield.

Fig. 18 (c) illustrates the MLD Parameters subfield format corresponding to the auxiliary AP. The MLD Parameters subfield corresponding to the auxiliary AP may be set to a value whose link ID is larger than that of the normal AP. That is, if there are multiple APs (primary AP and normal APs) and one auxiliary AP in the AP MLD, it is recommended that the link ID of the auxiliary AP be set to a value larger than that of the other APs.

The MLD Parameters subfield corresponding to the secondary AP includes a Link ID of Primary link subfield. The Link ID of Primary link subfield is a subfield indicating a link (primary link) ID of the Primary AP paired with the secondary AP. The Secondary Link Indication subfield is a subfield indicating whether the AP corresponding to the corresponding MLD Parameters subfield is a secondary AP, and is indicated as 1 when included in the MLD Parameters subfield corresponding to the secondary AP. The Separated Radio subfield is a subfield indicating whether the secondary AP corresponding to the corresponding MLD Parameters subfield has a configuration having a radio independent from the Primary AP. That is, a secondary AP with the Separated Radio subfield set to a specific value (for example, 1) can avoid losing medium sync for the primary channel (secondary channel) of the secondary BSS even when transmission/reception is performed through the Primary AP.

At this time, non-AP MLDs can recognize that the Neighbor AP Information field corresponds to a secondary AP by checking the Neighbor AP Information field in which the TBTT Information field Type included in the RNR element is not 0 when receiving the RNR element transmitted by the AP MLD. Alternatively, non-AP MLDs can recognize whether the AP corresponding to the MLD Parameters subfield is a secondary AP based on a specific bit included in the MLD Parameters subfield (the secondary link Indication of FIG. 18 (c)).

As another example, the AP MLD can indicate that the AP corresponding to the Per-STA Profile subelement configured/configured in a different way from the Per-STA Profile subelements corresponding to other APs (third AP (normal AP) and primary AP) among the Per-STA Profile subelements included in the Multi-link element that it transmits is a secondary AP by configuring/configuring them differently. The Per-STA Profile subelement configured/configured in a different way may have a subfield (bit) indicated as a specific value (for example, 0 or 1) in the Per-STA Profile subelement corresponding to the normal AP set to a different value (for example, 1 or 0) for the secondary AP, thereby distinguishing the secondary AP from the normal AP.

A non-AP MLD that recognizes that a secondary AP is in operation must not transmit a Probe Request frame to the AP MLD through the secondary link. In this case, the method by which the non-AP MLD recognizes that the secondary link is in operation may be based on information indicated through the Management frame transmitted by the AP MLD.

Meanwhile, AP MLD can change the AP that functions as the Primary AP among the Overlapping APs (Primary AP and Auxiliary APs). For a simple example, the AP that initially functions as the Primary AP can be changed to the Auxiliary AP, and the AP that functions as the Auxiliary AP can be changed to the Primary AP. There can be various reasons for changing the AP that functions as the Primary AP among the Overlapping APs, and one of the reasons can be that AP MLD wants to change the channel that transmits the Beacon frame.

When the AP MLD wants to change an AP (link, BSS) that will function as a Primary AP (link, BSS) among the Overlapping APs (links, BSS), it must instruct the non-AP MLDs to do so. At this time, the AP MLD can instruct, through the Management frame (e.g., Beacon frame) that it transmits, information about a new Primary AP (an AP that is a secondary AP at the time of transmitting the Management frame) and/or information related to the time point at which the Primary AP is changed. At this time, the information that the AP MLD instructs through the Management frame can include the ID of the link on which the secondary AP to be changed to the Primary AP is operated (i.e., the link ID of the secondary link). At this time, the AP MLD can instruct the number of TBTTs remaining until the time point at which the Primary AP is changed in order to instruct the time point at which the Primary AP is changed. That is, if there are 5 TBTTs of the current Primary AP until the point in time when the AP MLD wants to change the Primary AP, the AP MLD can set the value of the field related to the point in time when the Primary AP is changed to a setting (for example, 5 or 4(5-1)) based on the number of remaining TBTTs. That is, if there are no TBTTs remaining until the point in time when the AP MLD wants to change the Primary AP, the AP MLD sets the value of the field related to the point in time when the Primary AP is changed to 1 or 0 and transmits it, and the next Beacon frame will be transmitted through the new Primary AP.

An AP MLD that wishes to change its Primary AP can transmit a BSS Transition Management Request frame via the existing Primary AP (the AP that will become the secondary AP after the Primary AP is changed). At this time, the BSS Transition Management Request frame may be a frame that instructs non-AP STAs and non-AP MLDs that are connected (set up) only via the existing Primary AP that the service of the BSS operated by the Primary AP will be terminated. At this time, the BSS Transition Management Request frame transmitted by the AP MLD may be a BSS Transition Management Request frame in which the link Removal Imminent subfield is set to 0 and the Primary link Change field is set to 1. The Primary link Change field is a field that is set to 1 when a link designated as the Primary link among a pair of Primary links (APs) and secondary links (APs) having overlapping operating channels is changed (and/or is scheduled to be changed). A non-AP MLD that receives a BSS Transition Management frame Request with the Primary link Channel field set to 1 may interpret the BSS Transition Management Request frame not as an end of service for the Primary BSS, but as an indication that the Primary link (AP) and the Secondary link (AP) will be changed. In this case, the AP MLD must update the Neighbor AP Information field for the Secondary link and the Neighbor AP Information field for the Primary link of the RNR element transmitted after the Primary link and the Secondary link are changed, taking into consideration the changed Primary link.

Alternatively, the AP MLD may perform a Channel Switch to change the primary channel of a BSS operated by an AP functioning as a Primary AP (link) among multiple APs (links) having overlapping operating channels. At this time, the AP MLD may transmit an (Extended) Channel Switch Announcement element to change the primary channel of the BSS operated by the Primary AP, and a new primary channel is indicated through a New Channel Number field included in the element. At this time, the new primary channel is one of the subchannels included in the existing operating channel (BW). In this way, when the Channel Switch Announcement element is used to change the subchannel used by the Primary BSS as the primary channel without changing the operating channel, it is possible for the new primary channel to be designated as a subchannel used by the secondary BSS, which is a pair of the Primary BSS, as the primary channel (secondary channel). If the primary channel of a new Primary BSS (indicated by the New Channel Number field) is a subchannel that the secondary BSS previously used as a primary channel (secondary channel), the AP MLD must transmit together the (Extended) Channel Switch Announcement element for changing the primary channel (secondary channel) of the secondary BSS. That is, the AP MLD that wants to change the primary channel of the Primary BSS to the primary channel (secondary channel) of the secondary BSS may need to change the primary channel (secondary channel) of the secondary BSS together to manage that the primary BSS and the secondary BSS do not use the same subchannel as a primary channel. A non-AP MLD that receives together the (Extended) Channel Switch Announcement element for instructing the change of the primary channel of the Primary BSS and the (Extended) Channel Switch Announcement element for instructing the change of the primary channel (secondary channel) of the secondary BSS from the AP MLD can recognize that the primary channels of the two BSSs will be switched. At this time, the two Channel Switch Announcement elements received together indicate the same value through the Channel Switch Count field. At this time, the Channel Switch Mode subfield of the Channel Switch Announcement element transmitted by the AP MLD to change only the primary channel without changing the Operating channel of the Primary BSS may be set to a value (for example, 2) different from the value indicated when a general Channel Switch is performed (when the Operating channel is changed).

<Channel change of AP (link, BSS) pair with overlapping operation channels>

When it is desired to change the operating channel of multiple APs (primary AP and paired secondary APs) having overlapping operating channels, the AP MLD can transmit only the (Extended) Channel Switch Announcement element for the Primary AP. That is, the Channel Switch Announcement element for the Primary AP (link, BSS) may be a Channel Switch Announcement element that is commonly applied to multiple APs (primary AP and paired secondary APs) having overlapping operating channels. For example, if the Operating channel of the Primary BSS is changed from the first 320 MHz channel to the second 320 MHz channel through the Channel Switch Announcement element, the Operating channel of the secondary BSS that is paired with the Primary BSS also changes from the first 320 MHz channel to the second 320 MHz channel. That is, when an operating channel change is instructed for a specific AP (e.g., primary AP) in overlapping operating channels (overlapping AP(link, BSS) pair), an operating channel change can be implicitly instructed for another AP (e.g., secondary AP). Accordingly, when the non-AP MLD is instructed to change the operating channel of the Primary AP from the AP MLD, the non-AP MLD can recognize that the operating channels of the secondary AP(s) paired with the Primary AP are also changed to the same as that of the Primary AP. At this time, the new primary channel (secondary channel) of the secondary AP(s) that has moved to the changed operating channel can be instructed through the RNR element included in the management frame (frame transmitted by the AP MLD) transmitted after the completion of the Channel Switch of the Primary BSS.

FIG. 19 illustrates a method in which a channel switch of an auxiliary BSS is instructed/performed together when a channel switch for a primary BSS is performed according to one embodiment of the present invention.

Referring to Figure 19, the Primary BSS and the Auxiliary BSS operate on an Operating channel whose initial Center Frequency is 'X'.

AP MLD is transmitted by including the (Extended) Channel Switch Announcement element in the Beacon frame transmitted by the Primary AP to change the overlapping operating channel.

(Extended) Channel Switch Announcement element contains information about when the Primary BSS's Channel Switch starts and the new Operating channel.

Since the Channel Switch Count value of the (Extended) Channel Switch Announcement element included in Beacon#1 is 2 and the Channel Switch Count value included in Beacon#2 is 1, the Primary AP starts Channel Switch after transmitting Beacon#2. At this time, although the Channel Switch Announcement element for the secondary AP (BSS) is not directly included in Beacon#1 and Beacon#2, the same information as the Channel Switch information indicated for the Primary AP (BSS) is implicitly included in the secondary AP (BSS). This is because the secondary AP is an AP that is paired with the Primary AP that will perform the Channel Switch.

Therefore, when the Channel Switch of the Primary BSS starts, the Channel Switch of the Secondary BSS also starts, and the changed center frequency of the Primary BSS is the same as the changed center frequency of the Secondary BSS. In other words, the Operating Channels of the two BSSs remain overlapping even after the Channel Switch is completed.

The Primary AP transmits a Beacon#3 frame on a new operating channel, and Beacon#3 includes an RNR element indicating information about a new primary channel (auxiliary channel, A20 in FIG. 19) of the auxiliary AP. Therefore, the non-AP MLD can check information about the new primary channel (auxiliary channel) of the auxiliary BSS by receiving the Beacon frame transmitted by the AP MLD after completing the Channel Switch of the Primary BSS.

<Features and limitations of auxiliary AP/link/BSS>

As described above, the AP MLD can operate multiple APs with overlapping operating channels in order to improve accessibility to a specific operating channel. One AP among the multiple APs functions as a primary AP, and the remaining APs except for the primary AP function as secondary APs. The primary AP transmits a Beacon frame every Beacon interval, and supports (ML) setup procedures with non-AP STAs and non-AP MLDs, thereby supporting functions more similar to those of a general AP. On the other hand, the secondary AP is an AP that has many restrictions, such as not transmitting a Beacon frame and not being able to be setup with a non-AP MLD that is not associated with the paired primary AP. This is because the reason why the AP MLD operates multiple APs with overlapping operating channels is to improve accessibility to the overlapping operating channels, and not to operate the secondary APs as general APs.

In this way, since the secondary AP (link, BSS) is an AP that performs only limited operations and has a dependency on the Primary AP (link, BSS), the AP MLD must utilize the information indicated through the Primary AP and the parameters used by the Primary AP in operating the secondary AP.

<TSF timer of auxiliary link>

As mentioned above, the auxiliary AP does not transmit the Beacon frame. The Beacon frame of Wi-F is a frame transmitted for various purposes, such as indicating various information that non-AP STAs associated with the AP transmitting the Beacon frame should recognize, allowing non-AP STAs that are not associated to recognize (discover) the existence of the AP, and even allowing them to recognize some information about other APs (Neighbor APs) adjacent to the AP. In addition, in 11be, by transmitting a multi-link element through the Beacon frame, it is expanded to include additional various information that non-AP MLDs should obtain, such as indicating information that the AP that transmitted the Beacon frame belongs to the AP MLD and information about other APs (links) belonging to the AP MLD.

The Timestamp field, one of the fields included in the Beacon frame, is a field that indicates a value related to the timing synchronization function (TSF) timer of the AP. It is a field that supports the non-AP STA to adjust its own TSF timer according to the TSF timer value of the AP to synchronize timing between the AP and the non-AP STA. All timing-based operations of the BSS are synchronized based on the TSF timer of the AP, that is, the AP functions as a timing master. Therefore, each non-AP STA must synchronize timing with the AP using the Timestamp field included in the Beacon frame transmitted by the AP. However, in the case of the secondary AP, it does not transmit a Beacon frame for its own BSS, and therefore, there is a problem that the non-AP STAs included in the BSS of the secondary AP (secondary BSS) cannot synchronize time with the secondary AP. As a solution to this, it is possible to consider having the Primary BSS (AP, link) and the secondary BSS (AP, link) use a common TSF timer.

According to one embodiment of the present invention, the secondary APs that are paired with the primary AP may have a common TSF timer. Alternatively, the TSF timers of the primary AP and the secondary APs may be the same. In this case, the value of the commonly used TSF timer is indicated to non-AP STAs (non-AP STAs and non-AP MLDs) through the Beacon frame transmitted by the primary AP. Therefore, the non-AP MLD can maintain time synchronization with the secondary APs by receiving the Beacon frame transmitted by the primary AP.

<Mapping TID (Traffic ID) of auxiliary link>

As described above, MLD is defined in Wi-Fi 7, and TID-to-link mapping agreement can be performed between MLDs where ML setup is performed. As an example of simple TID-to-link mapping, a non-AP MLD setup with an AP MLD over two links can perform an agreement with the AP MLD to map TID0 to TID3 to the first link and TID4 to TID7 to the second link. In this case, the non-AP MLD and the AP MLD transmit only MPDU/MSDUs with TIDs 0 to 3 over the first link, and transmit MPDU/MSDUs with TIDs 4 to 7 over the second link. In this case, it is possible to apply different TID-to-link mappings to the direction in which the AP MLD transmits (DL, Down link) and the direction in which the non-AP MLD transmits (UL, Up link). AP MLD and non-AP MLD where TID-to-link mapping is not performed have a Default TID-to-link mapping agreement state. The Default TID-to-link mapping state means that all TIDs are mapped to all setup links, that is, all types of TIDs can be transmitted without restriction through all setup links. At this time, the Default TID-to-link mapping mode is applied to both DL and UL directions.

In general, the TID-to-link mapping agreement between an AP MLD and a non-AP MLD can be freely set through the agreement between the two MLDs, except for the constraint that all TIDs must be mapped to at least one link. In other words, there is no constraint that only certain TIDs must be mapped to certain links, or that certain TIDs must not be mapped to certain links. However, this is reasonable when it is assumed that all setup links are links on which general APs operate, and if there are auxiliary links among the setup links, additional constraints may be applied.

According to one embodiment of the present invention, only TIDs mapped to a pair of Primary links can be mapped to a secondary link. Furthermore, TIDs mapped to a pair of Primary links can be identically mapped to a secondary link. That is, a TID that is not mapped to a pair of Primary links cannot be mapped to a secondary link. That is, a TID-to-link mapping state of a secondary link is identical to a TID-to-link mapping state of a pair of Primary links. The reason why a separate TID-to-link mapping constraint exists for a secondary link may be because a secondary link is a link that cannot be kept active at the same time as a primary link.

To be more specific, the secondary link can be active only when the primary link is inactive, so the primary link remains inactive while transmission is performed on the secondary link. If a specific TID is mapped only to the primary link and not to the secondary link, the TID that is not mapped to the secondary link becomes a TID that cannot be transmitted during the time period when the primary link is inactive. This means that the TIDs that each MLD can transmit are restricted depending on whether the active link is the primary link or the secondary link, which is a very inappropriate restriction for maintaining traffic flow. Therefore, by restricting the secondary link to always have the same TID mapped to the primary link, it is possible to maintain a state where all TIDs can be transmitted through at least one setup link regardless of whether the primary link or the secondary link is active.

Therefore, when AP MLD and non-AP MLD perform TID-to-link mapping, they must perform a TID-to-link mapping request in the form of always mapping the TID mapped to the primary link to the secondary link. At this time, the constraints on the mapping are applied to both directions (DL/UL).

Alternatively, when AP MLD and non-AP MLD perform TID-to-link mapping, it may be considered that the TID mapping indicated for the primary link is applied equally to the secondary link without separately indicating TID mapping information for the secondary link. That is, AP MLD and non-AP MLD may not perform separate TID-to-link mapping negotiation for the secondary link. In this case, the TID mapping state for the non-negotiated secondary link may be applied equally to the TID mapping state negotiated for the primary link that is paired with the secondary link. That is, the TID mapping for the secondary link is not directly indicated/negotiated, and the TID mapping for the primary link may be applied equally. This may be interpreted that the TID-to-link mapping negotiation for the secondary link is implicitly performed/completed by the TID-to-link mapping negotiation performed for the paired primary link.

<Traffic Indication of Auxiliary AP>

As described above, TID-to-link mapping negotiation can be performed between two MLDs associated through multiple links. If TID-to-link mapping negotiation is performed between an AP MLD and a non-AP MLD, the AP MLD may need to indicate Traffic indication considering the TID-to-link mapping negotiated with the non-AP MLD. Similarly, the non-AP MLD may also need to perform an operation for MSDU reception considering the TID-to-link mapping when receiving a Traffic indication from the AP MLD. The reason why the AP MLD and the non-AP MLD need to perform Traffic indication indication and MSDU reception considering the TID-to-link mapping is because of the traffic transmission restriction related to the TID-to-link mapping. As described above for the TID-to-link mapping, an MLD that has performed TID-to-link mapping negotiation with a counterpart MLD must perform transmission only through the link to which the TID of the traffic to be transmitted is mapped when performing transmission to the counterpart MLD. Therefore, if the AP MLD and the non-AP MLD perform TID-to-link mapping of the non-default mode for the DL direction, the AP MLD should transmit the MSDU to the non-AP MLD only through the link on which the TID of the MSDU being queued is mapped for the DL direction in order to transmit the MSDU. In other words, the non-AP MLD should transmit the PS-Poll frame only through the link on which the MSDU it is to receive can be transmitted, and if the non-AP MLD transmits the PS-Poll frame on the link on which the TID of the MSDU queued on the AP MLD side is not mapped for the DL direction, the MSDU cannot be received from the AP MLD. In this case, the non-AP MLD switches the link on which MSDU reception is impossible to the Awake, which reduces the PS efficiency and also causes a delay in MSDU transmission, making it difficult to effectively perform the operation/support of the PS mode.

Therefore, in order to prevent the problem of Queuing MSDU (BU) transmission impossibility due to TID-to-link mapping, the AP MLD must indicate to each non-AP MLD through the TIM element whether there is a Queuing MSDU to be transmitted, and additionally indicate information regarding whether the MSDU should be transmitted on a link. At this time, for a non-AP MLD that has performed TID-to-link mapping negotiation in which all TIDs are mapped in the DL (Down link, the direction in which the AP transmits to the non-AP) direction for at least one link, no additional indication other than the presence or absence of a Queuing MSDU may be performed. This may be because a non-AP MLD with a default TID-to-link mapping state or a link in which all TIDs are mapped in the DL direction can receive the Queuing MSDU without the problem of transmission impossibility due to the TID by transmitting a PS-Poll frame through the link to which all TIDs are mapped in order to receive the Queuing MSDU.

The method in which an AP MLD indicates to a non-AP MLD information related to whether a corresponding MSDU (BU, Queuing frame, etc.) should be transmitted in a link may be to transmit an element indicating a TID or a link ID together with a TIM element. In this case, the element transmitted to indicate a TID or a link ID to the non-AP MLD may be named a Multi-link TIM element (or Multi-link Traffic Indication element). The Multi-link TIM element indicates information related to whether each non-AP MLD, which is indicated by the TIM element to have a Queuing MSDU, can receive the MSDU in a link. If a separate link-related information indication is not required to the non-AP MLD (if it is in the default TID-to-link mapping state or if TID-to-link mapping negotiation is performed in which there is a link in which all TIDs are mapped in the DL direction), separate link-related information may not be indicated in the Multi-link TIM element. A non-AP MLD that is not instructed with separate link-related information through a multi-link TIM element must transmit a PS-Poll frame on a link to which all TIDs are mapped for the DL direction to receive BUs (queuing MSDUs). In the case of a non-AP MLD that uses the default TID-to-link mapping, it can be understood that there is no separate PS-Poll frame transmission link selection restriction because all links have all TIDs mapped for the DL direction.

As described above, in the case of the secondary link, the same TID as that mapped to the primary link is mapped. Therefore, the traffic (MSDU/MPDU) of the TID that can be transmitted on the primary link according to the TID-to-link mapping status means that it is traffic that can also be transmitted on the secondary link. In addition, the non-AP MLD that is set up through the secondary link is always set up through the primary link as well, and there is no non-AP STA that is associated only through the secondary link.

If the AP MLD indicates through the Multi-link Traffic Indication element that an MSDU queued on the AP MLD side can be received through the primary link, it is self-evident that the MSDU can also be received through the secondary link. In addition, it is self-evident that an MSDU that the AP MLD can transmit through the secondary link can also be transmitted through the primary link.

Therefore, when AP MLD indicates that BUs (bufferable units, MSDUs) to be transmitted on each link are queued through the Multi-link Traffic Indication element, information on the secondary link may not be indicated. Instead, the Traffic indication for the secondary link can be implicitly indicated through the Traffic indication for the primary link. In this case, the link ID bitmap (Per-link Traffic Indication Bitmap subfield) included in the Multi-link Traffic Indication element transmitted by AP MLD may not include a bit corresponding to the secondary link.

In other words, each bit of the Per-link Traffic Indication Bitmap included in the Multi-link Traffic Indication element transmitted by the AP MLD corresponds to the link ID of the general AP and/or the Primary AP, and may not correspond to the link ID of the secondary AP. That is, the Traffic Indication for the secondary AP is not indicated by the AP MLD.

A non-AP MLD, which has been instructed by the AP MLD through the TIM (traffic indication map) element, Multi-link Traffic Indication element, that there is an MSDU to be received on a specific primary link, can attempt to receive a buffered BU from the AP MLD by transmitting a PS-Poll frame through the specific primary link or a secondary link paired with the specific primary link.

<BSS Color setting of auxiliary BSS>

The secondary BSS may need to use the same BSS color as that of the primary BSS. That is, the AP MLD sets the BSS Color field (included in HE-SIG-A of HE PPDU, in U-SIG of EHT/UHR PPDU) of the PPDU transmitted through the primary AP and the HE/EHT/UHR PPDU transmitted through the secondary AP to the same value. In addition, the non-AP MLD shall also set the BSS Color field of the PPDU transmitted through the primary BSS and the PPDU transmitted through the secondary BSS to the same value.

Therefore, when the Primary AP sets the BSS color of the Primary BSS to a new value by transmitting the BSS Color Channel Announcement element, the BSS color of the Secondary BSS is changed to the new value.

However, the Primary BSS and the Auxiliary BSS must be set to have different BSSIDs. In this case, setting the BSSIDs of the two BSSs to different values means that the BSSID of the Primary BSS and the BSSID of the Auxiliary BSS are randomly determined.

<Parameter update of auxiliary BSS>

As mentioned above, the secondary AP cannot transmit Beacon frames (and Probe response frames), and therefore cannot announce changes in the parameters of the secondary BSS to the member STAs of the secondary BSS.

Instead, parameter update of the secondary BSS can be performed via management frames transmitted by the Primary AP paired with the secondary AP, or by other APs belonging to the AP MLD. More specifically, when the parameters of the secondary BSS are updated, the AP MLD can indicate that the parameter update of the secondary BSS is in progress by using the update indication field corresponding to the secondary BSS (e.g., BSS Parameter Change Count subfield (included in the MLD Parameters subfield corresponding to the secondary AP)) included in the RNR element of the management frame transmitted by the AP MLD. In this case, the non-AP MLD can recognize that the parameters of the secondary BSS have been changed via the received RNR element. Thereafter, the non-AP MLD can receive and update the parameters of the secondary BSS via the Per-STA Profile subelement of the secondary AP included in the Multi-link element transmitted by the AP MLD.

In addition, some present bits included in the Per-STA Profile subelement corresponding to the secondary AP must always be set to 0. The Beacon Interval Present subfield of the Per-STA Profile subelement corresponding to the secondary AP is set to 0. The TSF Offset Present subfield of the Per-STA Profile subelement corresponding to the secondary AP is set to 0. The DTIM Info Present subfield of the Per-STA Profile subelement corresponding to the secondary AP is set to 0. At this time, the reason why the above three subfields are set to 0 is because the secondary AP does not transmit a Beacon frame and uses the TSF with the same value as the Primary AP. That is, since the information indicated when the Present subfields are indicated as 1 is information that is not defined or does not need to be indicated for the secondary AP (BSS, link), the above three Present subfields are always set to 0 for the secondary AP.

Certain operating parameters of the secondary BSS may be dependent on the operating parameters of the primary BSS. For example, the operating BW of the secondary BSS may be changed to be the same as the operating BW of the primary BSS when the operating BW of the primary BSS is changed.

<How to operate the modified auxiliary link>

As described above, AP MLD and non-AP MLD may have an ML setup state that includes multiple links operating on overlapping operating channels, for the purpose of improving accessibility to specific Operating channels.

Alternatively, instead of setting up multiple links with overlapping operating channels, a method can be utilized to set up multiple links with operating channels (bandwidths) located sequentially.

More specifically, the AP MLD and the non-AP MLD may attempt to enhance channel accessibility for a consecutive operating channel by establishing an ML setup connected through multiple links having consecutive operating channels within a specific channel bandwidth. As an example, the AP MLD and the non-AP MLD may attempt to enhance accessibility for a specific 160 MHz channel by performing the ML setup through two links that use each of two 80 MHz channels included in the specific 160 MHz channel as an Operating channel. As another example, the AP MLD and the non-AP MLD may attempt to enhance accessibility for a specific 320 MHz channel by performing the ML setup through two links that use each of two 160 MHz channels included in the specific 320 MHz channel as an Operating channel. As another example, an AP MLD and a non-AP MLD may wish to enhance accessibility to a particular 320 MHz channel by performing ML setup over four links, each using one of the four 80 MHz channels included in that particular 320 MHz channel as its Operating channel.

In this case, the AP MLD and/or the non-AP MLD may perform an operation synchronized with the transmission of the second STA operating in the continuous Operating channel when performing transmission through the first STA (AP STA and/or non-AP STA) operating in the continuous Operating channel. For example, when the AP MLD transmits a PPDU through the first AP operating in the first 80 MHz included in the specific 160 MHz, the AP MLD may perform a transmission synchronized with the second AP operating in the second 80 MHz included in the specific 160 MHz. In this case, the 80 MHz PPDUs transmitted by the first AP and the second AP, respectively, may be understood as the first segment and the second segment of the 160 MHz PPDU. At this time, the first segment means a PPDU transmitted in the first 80 MHz segment included in the specific 160 MHz, and the second segment means a PPDU transmitted in the second 80 MHz segment included in the specific 160 MHz. At this time, the meaning that the AP MLD synchronizes the transmission of the first AP and the transmission of the second AP means that the transmission start time and the PPDU length of the PPDU transmitted through the first AP and the PPDU transmitted through the second AP are managed to be the same. In addition, when the first AP and the second AP transmit synchronized PPDUs, it is possible for the TXOPs acquired by the first AP and the second AP to also be synchronized. That is, when the AP MLD synchronizes and transmits 80 MHz PPDUs through the first AP and the second AP, respectively, it is possible for the two transmitted 80 MHz PPDUs to have the same form as a single 160 MHz PPDU. That is, when the AP MLD transmits synchronized PPDUs through the first AP and the second AP, the start time and length of the TXOP acquired by the first AP and the TXOP acquired by the second AP are set to be the same.

In addition, as described above, when a non-AP MLD performs transmission through a first STA (AP STA and/or non-AP STA) operating on a continuous Operating channel, it can perform an operation synchronized with the transmission of a second STA operating on a continuous Operating channel. In this case, a specific description of the synchronized transmission performed by the non-AP MLD is omitted because it is the same as in the case of the AP MLD described above.

Among multiple links having continuous Operating channels, one link is set as a Primary link, and the remaining links(s) are set as Secondary links. An AP operating on the Primary link periodically transmits Beacon frames, while an AP operating on the Secondary link does not transmit Beacon frames. At this time, an operating method identical/similar to that of the Primary link and Secondary link operating on the aforementioned overlapping operating channels may be applied to the Primary link and Secondary link operating on the continuous Operating channel. The same TID is mapped to the Primary link and Secondary link operating on the continuous Operating channel, and the APs and STAs operating on the Secondary link commonly use the TSF timers of the APs and STAs operating on the Primary link.

FIG. 20 illustrates a configuration of an AP MLD including a primary AP and a secondary AP having continuous operation channels and a method for setting operation channels according to one embodiment of the present invention.

Referring to Figure 20(a), three AP MLDs belong to the AP MLD. At this time, AP1 functions as the Primary AP, AP2 functions as the Auxiliary AP paired with the Primary AP, and AP3 is a general AP.

Referring to Figure 20(b), the Primary AP and the Secondary AP operate on consecutive Operating channels (two 160 MHz channels located on a specific 320 MHz channel).

AP MLD and non-AP MLD connected through multiple links with continuous operating channels can communicate in the following manner.

1. STAs (AP STA and non-AP STA) operating on each link (primary link and secondary link(s)) perform a backoff procedure through the primary channel of each link. STAs operating on the secondary link maintain the backoff counter at 0 when the backoff procedure is completed.

2-1. When the backoff procedure of the STA (AP STA and non-AP STA) operating on the primary link is completed, the MLD including the STA initiates transmission through the primary link and the secondary link for which the backoff procedure has already been completed (including links completed simultaneously with the primary link). At this time, the MLD performs the transmissions performed on the primary link and the secondary link as synchronized transmissions.

2-2. When the backoff procedure of an STA (AP STA and non-AP STA) operating on the primary link is interrupted, the MLD including the STA performs transmission through the secondary link when the backoff procedure of the STA operating on the secondary link is completed. If, at the time when the backoff procedure of the STA operating on the primary link is interrupted, the backoff procedure of the STA operating on the secondary link has already been completed and the backoff counter was maintained at 0, the MLD must newly generate a backoff counter of the STA operating on the secondary link and then perform the backoff procedure. At this time, the Retry counter and CW (Contention Window) of the STA operating on the secondary link are not changed. At this time, the interruption of the backoff procedure means a case where the decrease of the backoff counter is interrupted, such as a situation where the primary channel is occupied by an OBSS.

That is, when the backoff procedure of the primary link is completed, as a result of 2-1 described above, the MLD can occupy consecutive operating channels simultaneously by initiating synchronized transmission through the primary link and the secondary links for which the backoff procedure has already been completed.

If the backoff procedure of the primary link is interrupted, the MLD can initiate transmission through the secondary link as described in 2-2 above. The MLD that initiates transmission through the secondary link terminates the TXOP acquired through the secondary link before the time at which the backoff procedure on the primary link is expected to be able to continue (for example, the time at which the NAV of the STA operating on the primary link is expected to become 0).

FIG. 21 illustrates a channel access method of an MLD that operates an STA on two links having continuous operation channels according to one embodiment of the present invention.

Referring to Figure 21, the primary link and the secondary link, each having an 80 MHz Operating BW, are operated within a specific 160 MHz channel.

The procedure for obtaining the first TXOP by MLD is as follows.

MLD first completes the channel access procedure (backoff procedure) on A20 (Primary 20 MHz channel of BSS operated on the secondary link) of the secondary link, but does not initiate transmission on the secondary link, and performs an operation of maintaining the backoff counter of the secondary link to 0 in order to perform transmission synchronized with the primary link. When MLD completes the channel access procedure on P20 (Primary 20 MHz channel of BSS operated on the primary link) of the primary link, it initiates transmission not only through the primary link but also through the secondary link whose backoff counter is already 0. At this time, MLD performs synchronized transmission through STAs of the primary link and the secondary link. At this time, the two 80 MHz PPDUs transmitted by MLD through the two links are indicated as having a BW field of 160 MHz, so that external terminals can recognize them as one 160 MHz PPDU.

The procedure for obtaining the second TXOP by MLD is as follows.

When MLD completes the channel access procedure (backoff procedure) on A20 of the secondary link (the primary 20 MHz channel of the BSS operating on the secondary link), it initiates transmission on the secondary link, considering that the primary link is occupied by the OBSS. At this time, the TXOP (or PPDU) initiated on the secondary link is terminated before the time when the TXOP of the OBSS occupying the primary link is expected to end.

The procedure for obtaining the third TXOP by MLD is as follows.

When MLD completes the channel access procedure (backoff procedure) on P20 of the primary link, it initiates TXOP on the primary link. After MLD initiates transmission on the primary link, it loses the CCA capability for the secondary link, and therefore the backoff counter of the secondary link may not decrease below 3. However, MLD capable of performing CCA on the secondary link may also continuously decrease the backoff counter of the secondary link.

<MU PPDU transmission/reception method of terminals performing nonprimary (secondary) channel access using a single link>

As described above, a channel access method using a non-primary channel can be considered as a method for improving the channel accessibility of a Wi-Fi STA, and in the case of MLD, a method for enhancing channel accessibility using the characteristics of MLD (such as the above-described Overlapping BSS) can be utilized. The common feature of the various channel access methods provided in the present invention is that a transmitting device does not perform channel access only through one primary channel, but performs channel access using another channel (such as a non-primary channel or a primary channel of an auxiliary link) when the primary channel is occupied by another device.

In this way, since a transmitting device can initiate transmission on a channel other than the primary channel, a receiving device must also wait for PPDU reception on a subchannel other than the primary channel (such as a non-primary channel or the primary channel of an auxiliary link) when the state of the primary channel it observes is BUSY.

A PPDU transmitted by an STA (AP STA and non-AP STA) that has performed channel access through a non-primary channel is not transmitted by occupying the primary channel, but a PPDU transmitted by an STA that has performed channel access through the primary channel can be transmitted by occupying the non-primary channel. This is because the state of the primary channel determined by an STA that has performed channel access through a non-primary channel is always BUSY, and an STA that has performed channel access through the primary channel performs channel access through the primary channel regardless of the state of the non-primary channel.

Accordingly, if the state of the primary channel determined by the transmitting device is IDLE, the transmitting device can transmit a PPDU occupying both the primary channel and the non-primary channel, and the receiving device, which determines that the primary channel is BUSY, can receive the PPDU via the non-primary channel. That is, the device receiving the PPDU via the non-primary channel can receive both the PPDU transmitted by the transmitting device after performing channel access via the primary channel and the PPDU transmitted by the transmitting device after performing channel access via the non-primary channel, and can have difficulty in interpreting which index of its RU (or MRU (Multiple-RU)) is indicated by the RU Allocation subfield of the received PPDU. That is, the transmitting device and the receiving device can have different judgments regarding the state (IDLE or BUSY) of the primary channel, and accordingly, the channel through which the transmitting device performed channel access and the channel through which the receiving device starts receiving the PPDU can be different from each other. The procedure by which a conventional Wi-Fi STA instructs and acquires RU Allocation information is briefly described through an embodiment of FIG. 22. Hereinafter, RU may be interpreted to collectively mean not only conventional RUs such as 26, 52, 106, 242, 484, 996, 996 x N (N is a natural number greater than 2)-tone size RUs, but also MRUs (e.g., 52+26, 106+26, 484+242, 996+484-tone size RUs, etc.).

That is, the transmitting device can determine the primary channel and the non-primary channel as idle, but the receiving device can determine the primary channel as busy and the non-primary channel as idle. In this case, the transmitting device can transmit a PPDU that occupies both the primary channel and the non-primary channel. However, the receiving device cannot receive the PPDU on the primary channel because the primary channel is busy, and can receive the PPDU through the non-primary channel. Since the receiving device received the PPDU through the non-primary channel, it can interpret the fields included in the PPDU (e.g., RU allocation subfield, etc.) based on the non-primary channel. However, the transmitting device can generate the fields based on the primary channel and include them in the PPDU. Therefore, in this case, if the receiving device interprets the fields included in the received PPDU based on the non-primary channel, it can interpret the fields included in the PPDU differently from the transmitting device. Therefore, in this case, it is necessary to enable the receiving device to interpret the fields included in the PPDU based on the primary channel.

To this end, if the transmitting device transmits the PPDU using both the primary channel and the non-primary channel, the receiving device can interpret the fields included in the PPDU based on the primary channel even if the receiving device receives the PPDU on the non-primary channel.

In another embodiment of the present invention, instruction information indicating that a non-primary channel should interpret fields included in a PPDU based on a primary channel may be included and transmitted in the PPDU.

FIG. 22 illustrates an embodiment of a method for performing resource unit allocation to each STA using a resource unit allocation subfield and a resource unit allocation subfield indicated through a preamble of a PPDU and a method for indicating a content channel.

Fig. 22 (a) represents a situation in which two 484-tone RUs and one 996-tone RU in the 160 MHz band are allocated to STAs A, B, C, and D. STA A and STA B are allocated RU#1 (484-tone size RU) located in the lowest frequency region, STA C is allocated RU#2 (484-tone size RU), and STA D is allocated RU#3 (996-tone size RU).

As shown in Fig. 22 (a), a transmitting device transmitting a PPDU that allocates RUs indicates the RU configuration and information of an STA to which each RU is allocated through a Signaling field (e.g., HE/EHT/UHR SIG field) included in a DL PPDU. At this time, the Signaling fields transmitted through each 20 MHz subchannel may include the same or different Content channels.

Fig. 22 (b) illustrates the configuration of Content channels. Each Content channel includes an RU Allocation subfield and a User field. At this time, the RU Allocation subfields included in Content channel#1 correspond to the odd-numbered 20 MHz subchannels (see Fig. 22 (a)), and the RU Allocation subfields included in Content channel#2 correspond to the even-numbered 20 MHz subchannels (see Fig. 22 (a)).

Each RU Allocation subfield included in a content channel indicates RU information for each corresponding 20 MHz subchannel. More specifically, the first and second RU Allocation subfields included in content channel#1 indicate RU information corresponding to 20 MHz #1 and 20 MHz #3 subchannels, respectively, and the first and second RU Allocation subfields included in content channel#2 indicate RU information corresponding to 20 MHz #2 and 20 MHz #4, respectively.

Referring to Fig. 22 (a), the 20 MHz #1 subchannel corresponds to RU#1, and therefore, the first RU Allocation subfield included in Content channel#1 indicates information about RU#1. At this time, the RU Allocation subfield also indicates the number information of the User to be allocated the corresponding RU, and among the User fields indicated after the RU Allocation subfields, a User field of a designated number is interpreted as having been allocated the RU indicated through the corresponding RU Allocation subfield.

In the example of Fig. 22 (b), the first RU Allocation subfield included in Content channel#1 indicates 1 User, and therefore RU#1 is allocated to STA A corresponding to the first User field included in Content channel#1. Since the second RU Allocation subfield included in Content channel#1 indicates RU#2 corresponding to 20 MHz#3 and indicates 1 User, RU#2 is allocated to STA C corresponding to the second User field included in Content channel#1.

Content channels#1 and #2 appear alternately every 20 MHz band, and the specific indicated positions can be confirmed through (c) of Fig. 22. Therefore, an STA that wishes to confirm the position of the RU allocated to it receives two types of Content channels received through a specific 40 MHz band (e.g., a specific 40 MHz band included in the Primary 40 MHz, Secondary 40 MHz, or Secondary 80/160 MHz band), confirms whether its corresponding User field is included in the Content channel, and then confirms the information of the RU indicated by the RU Allocation subfield corresponding to its User field, thereby obtaining the RU information allocated to it. However, Content channels located in different 80 MHz Segments may include different contents even if they have the same index. For example, Content channel#1 indicated at 20 MHz#1 and Content channel#1 indicated at 20 MHz#5 may have different contents.

Figure 23 illustrates the ambiguity problem of allocation RU interpretation of an STA that receives a preamble on a non-primary channel.

Case 1 and Case 2 of Figure 23 illustrate the cases of a DL MU PPDU transmitted by an AP performing channel access on a primary 20 MHz channel and a DL MU PPDU transmitted by an AP performing channel access on a secondary 20 MHz channel, respectively.

Case 1 occurs when the AP determines that the primary channel (primary 20 MHz subchannel) is IDLE and the STA determines that the primary channel is Busy. Case 2 occurs when both the AP and STA determine that the primary channel is Busy.

In both Case 1 and Case 2, the STA waits for the reception of PPDU through S20, receives the preamble, and by receiving the Content channel included in the preamble, confirms that the RU allocated to it is a 242-tone size RU located in the lowest frequency band.

However, in the case of Case 1 and Case 2, the location of the 242-tone size RU located in the lowest frequency band is different, and therefore, the STA has a problem in that it cannot determine whether the RU allocated to it is the RU corresponding to Case 1 or the RU corresponding to Case 2.

The following two methods can be used to resolve the ambiguity in the interpretation of allocation RUs of such receiving devices.

1) First, when a receiving device checks the RU indicated by the RU Allocation subfield of a received PPDU, the receiving device interprets the RU Allocation subfield in different ways by considering whether the device that transmitted the received PPDU transmitted it after performing channel access through the primary channel or after performing channel access through the non-primary channel. At this time, the different ways of interpreting the RU Allocation subfield mean that the frequencies used as a reference when interpreting the positions of the RUs indicated by the RU Allocation subfield are different. That is, the subchannel to which a specific order of RU Allocation subfields indicated by the UHR-SIG (or EHT-SIG, HE-SIG) field corresponds may be different depending on whether the device transmitting the PPDU including the corresponding SIG field performed channel access on the primary channel or on the non-primary channel.

2) Second, the transmitting device can apply a setting to the signaling fields (e.g., Bandwidth, RU Allocation subfield, Puncturing information, etc.) of the PPDU transmitted after performing channel access on the non-primary channel, indicating that the band including the primary channel is punctured in the PPDU transmitted after performing channel access through the primary channel. That is, the transmitting device can set the signaling fields of the non-primary channel PPDU in the same manner as the PPDU transmitted after performing channel access through the primary channel, but can indicate that the band including the primary channel (e.g., Primary 80 MHz segment) is punctured. In this case, the receiving device can obtain the RU information allocated to it by interpreting the received PPDU as if it were transmitted by the device performing channel access on the primary channel, regardless of which channel the device transmitting the received PPDU used to perform channel access.

As described in the first method above, the receiving device must determine whether the received PPDU is transmitted by the transmitting device that completed channel access through the primary channel or by the transmitting device that completed channel access through the non-primary channel, and interpret the RU Allocation subfield in different ways. In this case, the RU Allocation subfield means the RU Allocation subfield included in the Signaling field (e.g., included in the HE-SIG, EHT-SIG, UHR-SIG fields, etc.) located in the Preamble of the MU PPDU and/or the RU Allocation subfield located in the User field included in the trigger frame.

The method by which a receiving device determines on which channel a transmitting device has transmitted a PPDU after performing a channel connection may utilize direct information or indirect information indicated by the transmitting device.

According to one embodiment of the present invention, an STA (AP STA and non-AP STA) that transmits a first PPDU after performing channel access through a non-primary channel may set a specific field of the first PPDU in a different manner from a specific field of a second PPDU transmitted after performing channel access through the primary channel. At this time, the specific subfield may be a field included in a U-SIG (Universal SIG field). For example, an STA that transmits a PPDU after performing channel access through the primary channel may set a specific field of the U-SIG to 0, and an STA that transmits a PPDU after performing channel access through a non-primary channel may set the specific field of the PPDU to a non-zero value. In this case, a device that has received a PPDU may determine, based on the specific field, whether the device that transmitted the PPDU performed channel access through the primary channel and transmitted the PPDU or performed channel access through the non-primary channel and transmitted the PPDU.

According to another embodiment of the present invention, even if the transmitting device does not indicate whether the channel access was performed through a primary channel or a non-primary channel, it may be possible for the receiving device to determine a channel access method of the transmitting device on its own. For example, a receiving device that receives an 80 MHz PPDU occupying a secondary 80 MHz segment from an STA (AP STA and non-AP STA) of a BSS having an operating BW of 160 MHz can recognize that the STA transmitted the 80 MHz PPDU after performing channel access through a non-primary channel located in the secondary 80 MHz segment. That is, the receiving device can obtain information on the channel on which the transmitting device performed channel access based on BW information of the received PPDU.

A receiving device of a PPDU interprets the RU Allocation subfield included in the received PPDU in the following manner based on whether the received PPDU was received from a device performing Channel Access over a primary channel or from a device performing Channel Access over a non-primary channel.

First, when it is determined that a received PPDU was transmitted by a device that performed channel access on the primary channel, the receiving device interprets the RU Allocation subfield received on a non-primary channel in the same way as the RU Allocation subfield received on the primary channel.

Second, when it is determined that the received PPDU was transmitted by a device performing channel access on a non-primary channel, the receiving device interprets the RU Allocation subfield received on the non-primary channel as if it was transmitted by a device using the non-primary channel as a primary channel. That is, the receiving device interpreting the RU Allocation subfield received on the non-primary channel must interpret the received RU Allocation subfield considering that the receiving device is a primary channel and that an 80 MHz segment including the non-primary channel is a primary 80 MHz segment. In this process, when the receiving device interprets the position of the RU indicated through the RU Allocation subfield received on the non-primary channel, the receiving device can confirm the position of the RU allocated to it by interpreting that the position of the RU indicated through the RU Allocation subfield has been changed by a frequency offset between its primary channel and the non-primary channel.

When the second method described above is used, when transmitting a PPDU after performing channel access over a non-primary channel, the transmitting device sets the signaling fields of the PPDU in the same manner as when transmitting a PPDU after performing channel access over the primary channel. However, since the transmitting device performs secondary channel access during a time period in which the primary channel is determined to be busy, the PPDU transmitted after performing channel access over the non-primary channel is always transmitted without occupying the primary channel. That is, when transmitting a PPDU after performing channel access on a non-primary channel, the transmitting device must transmit the PPDU in a form of puncturing the primary channel (in a form that does not occupy the primary channel).

For example, a transmitting device that performs channel access through a non-primary channel located in a secondary 80 MHz segment band because the primary channel is determined to be busy must set the BW field of the PPDU to 160 MHz when transmitting a PPDU through the secondary 80 MHz segment band and indicate that Preamble Puncturing is applied to the subchannel where the primary 20 MHz channel is located. At this time, a method of applying Preamble Puncturing to the subchannel located in the primary 20 MHz channel may be to puncture the primary 20 MHz subchannel or to puncture the entire band including the primary 20 MHz subchannel (e.g., the primary 40 MHz band or the primary 80 MHz segment, etc.).

That is, the limitation of conventional Wi-Fi that Preamble Puncturing should not be applied to the Primary 20 MHz subchannel may not be applied to PPDUs transmitted after performing channel access on a non-primary channel.

In this case, the receiving device can determine whether the transmitting device performed channel access through the primary channel or the non-primary channel based on the RU Allocation subfield included in the PPDU. More specifically, the receiving device can recognize that the device transmitting the PPDU performed channel access through the non-primary channel when the RU Allocation subfield of the received PPDU indicates that the RU located on the primary channel has been punctured. At this time, the RU Allocation subfield indicating that the RU has been punctured means the RU Allocation subfield interpreted as 'Punctured 242-tone RU'. At this time, the 20 MHz subchannel indicated as the Punctured 242-tone RU is a subchannel that is not occupied because puncturing is applied to the PPDU.

However, when a receiving device obtains information about RUs allocated to it through the RU Allocation subfield included in the PPDU, the receiving device can interpret the RU Allocation subfield in the same way regardless of whether the channel used by the device transmitting the PPDU for channel access is a primary channel or a non-primary channel.

FIG. 24 illustrates a method for an AP performing channel access through a non-primary channel to indicate BW and RU allocation information of a PPDU according to one embodiment of the present invention.

Referring to FIG. 24, the AP performs channel access through a non-primary channel and then transmits an 80 MHz PPDU occupying a secondary 80 MHz segment. At this time, the AP indicates through the U-SIG of the PPDU that the BW of the PPDU is 160 MHz. At this time, the indicated PPDU BW can be determined as a BW that includes the frequency band actually occupied by the PPDU and the primary channel.

In addition, since the PPDU that the AP actually transmits does not occupy the Primary 80 MHz segment band, the AP does not allocate RUs through the RU Allocation subfields corresponding to the Primary 80 MHz segment band among the RU Allocation subfields included in the PPDU. At this time, the AP indicates that the corresponding subchannel is punctured through the RU Allocation subfield corresponding to the Primary 80 MHz segment.

An STA receiving a PPDU from an AP recognizes that the received PPDU is a PPDU of 160 MHz BW, and checks the RU Allocation subfields to confirm the RU allocated to it. The STA confirms that the subchannel corresponding to the RU Allocation subfield corresponding to its User field is the 20MHz #5 subchannel included in the Secondary 80 MHz Segment. Therefore, the STA can recognize that the RU allocated to it is the RU located at 20MHz #5. At this time, the STA can confirm that the RU allocated to it does not exist in the Primary 80 MHz segment by receiving the RU Allocation subfields corresponding to the Primary 80 MHz segment among the received RU Allocation subfields. That is, since the content channel includes an RU Allocation subfield indicating that the subchannel corresponding to the Primary 80 MHz segment has been punctured, the STA can clearly recognize that the band corresponding to the RU Allocation subfield corresponding to its User field is 20 MHz #5.

<Method for transmitting trigger frames and responding to trigger frames by terminals performing non-primary channel access procedures using a single link>

A trigger frame is a Wi-Fi Control frame that causes a response from a receiving device in the manner instructed/requested by the transmitting device. For example, an AP can instruct a STA to respond with a TB PPDU by transmitting a Basic trigger frame to the STA.

The basic trigger frame includes a Common Info field and a User Info field, and a brief description of the Common Info field and the User Info field is as follows. The Common Info field is a field that includes information that is commonly indicated to multiple STAs that will respond to the TB PPDU, such as the length of the TB PPDU, whether an additional trigger frame is transmitted, whether the Carrier sensing result should be considered when responding to the TB PPDU, and UL BW information that should be recorded in the BW field of the TB PPDU. The User Info field indicates information about the RU to which each STA must respond to the TB PPDU, the FEC Coding method, MCS information to be applied to the TB PPDU, DCM (dual carrier modulation), target Rx power information, etc., and each STA responds to the TB PPDU based on the information obtained through the User Info field corresponding to itself.

In addition, the MU-RTS trigger frame that can be utilized for protection is also a type of trigger frame, and an AP that wants to transmit a DL MU PPDU can perform protection with multiple STAs at once by transmitting an MU-RTS frame requesting a CTS frame response from one or more STAs. A detailed description of the MU-RTS trigger frame is as follows.

MU-RTS is a type of trigger frame. When receiving an MU-RTS trigger frame, STAs whose AID12 (LSB 12 bits of Association ID) is indicated by the User field included in the MU-RTS frame must simultaneously respond with a CTS frame. When the AP performs TXOP protection using the MU-RTS frame, since multiple STAs respond with the CTS frame, there is an advantage in that the TXOP can be protected from the adjacent devices of each of the multiple STAs, which are the destination devices of the DL MU PPDU (Down link multi user PPDU). In addition, the MU-RTS frame can also be used for the purpose of protecting the UL MU PPDU. In more detail, before requesting a TB (trigger based) PPDU from multiple STAs through the trigger frame, the AP can transmit an MU-RTS frame to cause the multiple STAs, which are to respond with the TB PPDU, to respond with the CTS frame. At this time, the CTS frame to which the above multiple STAs responded serves to induce the neighbor STAs of each STA to set NAVs that protect the TB PPDU and the Ack frame (Ack, Block Ack, etc.) to be transmitted after the TB PPDU, and through this, even the legacy STAs that cannot recognize (interpret, decode) the trigger frame and TB PPDU may not perform channel access during the packet exchange sequence section (or TXOP) that starts through the trigger frame.

Figure 25 illustrates an embodiment of a transmission/TXOP protection method using MU-RTS frame and CTS frame.

Referring to FIG. 25, before transmitting an MU PPDU, the AP transmits an MU-RTS frame to STA1 and STA2, which are the destination devices of the MU PPDU, and STA1 and STA2 receive the MU-RTS frame and respond with a CTS frame each after SIFS.

STA1_Neighbor, a neighbor STA of STA1, sets its NAV based on the information indicated in the Duration field of the CTS frame after receiving the CTS frame transmitted by STA1. STA2_Neighbor, a neighbor STA of STA2, sets its NAV based on the information indicated in the Duration field of the CTS frame after receiving the CTS frame transmitted by STA2. STA1_Neighbor and STA2_Neighbor perform operations such as not decreasing the backoff counter, considering that the Virtual CS (Virtual Carrier Sense) is busy while the NAV (counter) set after receiving the CTS frame is maintained as a non-zero value. As a result, since the neighbor terminals that received the CTS frame do not attempt transmission during the period while the NAV is maintained as a non-zero value, the AP may not be interrupted by the neighbor terminals while transmitting the MU PPDU and STA1 and STA2 respond with an Ack frame.

The trigger frame described above is a frame type defined in 11ax, and is a frame type in which the Type (B3 B2) and Subtype (B7 B6 B5 B4) subfields of the Frame Control field are set to 01 and 0010, respectively. That is, the trigger frame is a frame of the Control Type in which the Type subfield of the Frame Control field is 01, and the Subtype value 0010 is used to indicate that it is a trigger frame type. In 11ax, a trigger frame is defined so that an AP can request response frames from multiple STAs at a time, and the MU-RTS frame described above (a type of trigger frame) is used so that an AP can request CTS frames from multiple STAs (non-AP STAs). Trigger Types other than MU-RTS include Basic Tigger frame (UL MU PPDU request), Beamforming Report Poll Tigger frame (Beamforming Report request), MU-BAR Tigger frame (BlockAck request), Buffer Status Report Poll Tigger frame (Buffer Status Report request), GCR MU-BAR Tigger frame, Bandwidth Query Report Poll Tigger frame, and NDP Feedback Report Poll Tigger frame.

Figure 26 illustrates the format of a trigger frame.

A trigger frame consists of a MAC Header including a Frame Control field, a Common Info field, a User Info List field, a Padding field, and an FCS field. The Frame Control field includes Type and Subtype subfields, and in the trigger frame, the two subfields are set to 01 and 0010, respectively. The Common Info field includes a Trigger Type subfield for indicating the Type of the trigger frame, a UL Length subfield for indicating the length of a UL transmission to be responded to, and the details are described in detail through an embodiment of FIG. 27.

The User Info List field includes 0 or 1 or more User Info fields including information for indicating a target device of a trigger frame. At this time, the User Info field includes, in addition to the information for indicating the target device, parameter information (UL DCM, UL MCS, etc.) that the target device must utilize when transmitting a response frame after receiving the trigger frame, depending on the Type of the trigger frame. The details of the User Info field are described in detail through an embodiment of FIG. 28.

The padding field is added to ensure that destination devices of the trigger frame have time to prepare a response frame (e.g., UL TB PPDU, CTS frame, etc.) after receiving the trigger frame, and the AP transmitting the trigger frame can adjust the length of the padding field considering the performance of the destination devices. In addition, in 11be (Wi-Fi 7, EHT), the end time of a PPDU including a trigger frame may be added/adjusted to align it with another PPDU, but since it is not related to the content that the present invention intends to provide, a detailed description is omitted.

The FCS (Frame Check Sequence) field contains a 32-bit CRC (Cyclic redundancy code), and is a calculated value including the MAC Header and Frame Body fields. The function and setting method of the FCS field of the trigger frame are the same as the function and setting method of the FCS field included in the conventional MAC frame, so a separate explanation is omitted.

Figure 27 illustrates an example of the format of the common information field of a trigger frame.

The trigger Type subfield (4 bits) is used to indicate the type (type, variant) of the trigger frame. If the trigger Type subfield is set to 0, it indicates Basic, 1 indicates BFRP (Beamforming Report Poll), 2 indicates MU-BAR, 3 indicates MU-RTS, 4 indicates BSRP (Buffer Status Report Poll), 5 indicates GCR MU-BAR, 6 indicates BQRP (Bandwidth Query Report Poll), and 7 indicates NFRP (NDP Feedback Report Poll).

The UL Length subfield indicates the value that should be set in the L-SIG LENGTH field of the TB PPDU responded to via the trigger frame.

The More TF subfield is used to indicate whether there are more trigger frames to be transmitted after the corresponding trigger frame.

The CS Required subfield indicates whether the destination device of the trigger frame must perform CS (Physical & Virtual CS, ED & NAV) when transmitting a response frame. An STA that transmits a response frame after receiving a trigger frame with the CS Required subfield set to 1 must perform CS.

The UL BW subfield indicates the BW value that STAs responding with a TB PPDU after receiving a trigger frame must indicate in the Preamble (e.g. HE-SIG-A or U-SIG).

The GI And HE/EHT-LTF Type/Triggered TXOP Sharing Mode subfield indicates the GI (Guard interval) and HE (EHT)-LTF values of the TB PPDU to be responded to, or the Sharing mode when the MU-RTS trigger frame is utilized for TXOP sharing using the MU-RTS TXS (TXOP Sharing) trigger frame.

The MU-MIMO HE(EHT)-LTF Mode subfield indicates information regarding the HE(EHT)-LTF mode that should be applied to the TB PPDU to be responded to.

The Number Of HE/EHT-LTF Symbols subfield indicates the number of HE(EHT)-LTF symbols that should be applied to the TB PPDU when the Doppler subfield is indicated as 0, and indicates the number of HE(EHT)-LTF symbols and information related to the periodicity of midamble when the Doppler subfield is indicated as 1.

The LDPC Extra Symbol Segment subfield indicates whether an LDPC extra symbol segment should appear in the responding TB PPDU. If the LDPC Extra Symbol Segment subfield is set to 1, an LDPC extra symbol segment must appear in the TB PPDU.

The AP Tx Power subfield indicates a value related to the transmit power of the AP used when transmitting the trigger frame. The STA can perform Power Control when responding with a response frame based on the value indicated in the AP Tx Power subfield.

The Pre-FEC Padding Factor and PE Disambiguity subfields indicate whether the Pre-FEC Padding Factor is 1, 2, 3, or 4, and information to clarify the length of the PE (Packet Extension).

The UL Spatial Reuse subfield consists of four Spatial Reuse subfields and indicates the values to be set in the Spatial Reuse fields (HE-SIG-A) of the HE TB PPDU to be responded to.

The Doppler subfield indicates whether the TB PPDU to be responded to includes a midamble. However, the trigger frame that responds with an EHT TB PPDU may have the Doppler subfield reserved. In this case, the subfield being reserved may mean that an STA that responds with an EHT TB PPDU after receiving a trigger frame operates without considering the existence and setting value of the subfield.

The HE/EHT P160 subfield indicates whether the responded TB PPDU is responded as a HE TB PPDU or an EHT TB PPDU on the channel corresponding to P160 MHz.

The Special User Info Field Present subfield indicates whether the User Info field with the AID12 subfield set to 2007 appears among the User Info fields.

The Trigger Dependent Common Info subfield is a field that appears only when the type of trigger frame indicated by the Trigger Type field is a Basic trigger frame or an NFRP trigger frame.

Figure 28 illustrates an example of the format of the user information field of a trigger frame.

Referring to FIG. 28, the AID12 subfield indicates information related to which STA the User Info field is for. That is, an STA for which a value identical to its own AID is indicated in the AID12 subfield of a specific User Info field can recognize that the trigger frame includes itself as a target device. At this time, the AID12 subfield can be set to 1 to 2006 (1 to 2007 in case of an HE trigger) when indicating one associated STA.

At this time, the AID12 subfield can be set to 0 when one or more RA-RU (Random Access RU) is to be allocated to associated STAs. That is, STAs associated with an AP can attempt TB PPDU transmission using RA-RU if there is no User Info field of AID12 in which its AID is indicated in the received trigger frame and there is a User Info field in which AID12 is indicated as 0.

At this time, the AID12 subfield can be set to 2045 or 2044 when one or more RA-RUs are to be allocated to unassociated STAs. That is, STAs that are not associated with an AP can attempt to transmit a TB PPDU using RA-RU if a User Info field in which AID12 is indicated as 2045 and 2044 exists in the received trigger frame. At this time, an STA responding to a TB PPDU through RA-RU must respond with a HE TB PPDU if AID12 is indicated as 2045, and must respond with an EHT TB PPDU if AID12 is indicated as 2044.

At this time, the AID12 subfield can be set to a preset value such as 4095 or 4094, and if the AID12 subfield is indicated as a preset value, it means that the padding field starts from the corresponding AID12 subfield. That is, if the AID12 subfield of the trigger frame is indicated as a value (preset) indicating the start of the padding field, the STA can recognize that the padding field has started and may not attempt to parse the remaining part of the MAC frame.

At this time, the User Info field in which the AID12 subfield is indicated as 2046 may include information about an unallocated RU. More specifically, if the AID12 subfield of a specific User Info field is indicated as 2046, the RU indicated by the RU Allocation subfield included in the specific User Info field may be an unallocated RU.

Except for the MU-RTS trigger frame, the RU Allocation subfield of other trigger frames indicates the size and location information of the RU (Resource Unit)/MRU (Multiple Resource Unit) allocated to the destination device (STA indicated via the AID12 subfield) of the corresponding User Info field. However, the RU Allocation subfield of the MU-RTS trigger frame is utilized to indicate the channel on which the destination device of the corresponding User Info field should respond to the CTS frame. More specifically, the RU Allocation subfield of the MU-RTS frame (in the User Info field) indicates whether the destination STA should respond to the CTS frame only on the Primary 20 MHz channel or on the Primary 40 MHz / Primary 80 MHz / Primary 160 MHz / 80 + 80 MHz / (Primary) 320 MHz (in case of transmission by EHT/UHR AP) channel. More specifically, the AP may indicate one of values 61 to 64 through B7-B1 of the RU Allocation subfield in the User Info field of a specific STA to indicate that the specific STA will respond to the CTS frame through Primary 20 MHz, may indicate 65 or 66 through B7-B1 of the RU Allocation subfield to indicate that the specific STA will respond to the CTS frame through Primary 40 MHz, may indicate 67 through B7-B1 of the RU Allocation subfield to indicate that the CTS frame will be responded to through Primary 80 MHz, and may indicate 68 through B7-B1 of the RU Allocation subfield to indicate that the CTS frame will be responded to through Primary 160 MHz. The indication to respond to the CTS frame through Primary 320 MHz channel is performed by indicating 69 through B7-B1. At this time, the PS160 subfield of the User Info field, which instructs to respond to the CTS frame through Primary 20/40/80/160 MHz, is set to 0, and the PS160 subfield of the User Info field, which instructs to respond to the CTS frame through (Primary) 320 MHz, is set to 1.

The UL FEC Coding Type subfield indicates the code type of the TB PPDU to be responded to. If the UL FEC Coding Type subfield is 0, it indicates BCC (binary convolution coding), and if it is 1, it indicates LDPC (low density parity check).

The UL EHT-MCS subfield indicates the EHT-MCS to which the responding TB PPDU should be applied.

The SS Allocation/RA-RU Information subfield can be used as the RA-RU Information subfield when the AID12 subfield is not a value indicating that RA-RU is allocated, that is, when it is indicated as 0, 2044, or 2045, and can be used as the SS Allocation subfield when the AID12 subfield is indicated as a value other than 0, 2044, or 2045. When used as the SS Allocation subfield, the 6 bits corresponding to the SS Allocation subfield can be used as the Starting Spatial Stream subfield 4 bits and the Number Of Spatial Streams subfield 2 bits.

The UL Target Receive Power subfield indicates the predicted signal power value at which the TB PPDU to be responded to will be received at the antenna side of the AP. Therefore, when the STA responds with a TB PPDU, it may need to adjust the transmission power of the TB PPDU according to the value of the UL Target Receive Power subfield so that its TB PPDU can be received with the power predicted by the AP.

The PS160 subfield is used together with the RU Allocation subfield, and indicates information related to the location and size of the RU/MRU (Multiple-RU) allocated through the User Info field.

However, UL EHT-MCS, UL FEC Coding Type, UL DCM, SS Allocation/RA-RU Information, and UL Target Receive Power fields are not used in the MU-RTS trigger frame. That is, they are reserved subfields.

As described above, the AP transmits an MU-RTS frame, and indicates the STAs that should respond to the CTS frame through the User Info field, while also indicating the bands on which each STA should respond to the CTS frame, including the Primary 20 MHz channel.

However, if the non-primary channel access method provided in the present invention is utilized, the AP may perform channel access through the non-primary channel and then transmit an MU-RTS frame through channels other than the Primary 20 MHz channel. In this case, an STA that responds with a CTS frame after receiving the MU-RTS frame must also respond with the CTS frame in a form that does not occupy the Primary 20 MHz channel.

Therefore, an AP that transmits an MU-RTS frame after performing channel access on a non-primary channel must respond with a CTS frame through the User Info field.

This is a different operation from the conventional Wi-Fi STA, which performed transmission only while occupying the Primary 20 MHz channel, and therefore, an STA that received an MU-RTS frame from an AP that performed channel access through a non-primary channel must respond with a CTS frame that does not occupy the primary channel.

However, as explained above, an AP transmitting an MU-RTS frame and instructing an STA on a channel to respond to the CTS frame has a limitation in that it can only instruct a channel type occupying the Primary 20 MHz channel, such as Primary 20/40/80/160/320 MHz. Therefore, in order to allow an AP transmitting an MU-RTS frame after performing channel access through a non-primary channel to instruct a response to a CTS frame occupying only the non-primary channel, more information than the existing MU-RTS trigger frame may need to be utilized between the AP/STA performing the non-primary channel operation. In this case, the more information may mean information on the channel used by the AP for channel access and/or more diverse instruction methods for instructing a channel on which the STA will respond to the CTS frame.

According to one embodiment of the present invention, an AP transmitting a trigger frame may indicate whether it has performed channel access through a primary channel or a non-primary channel. More specifically, the trigger frame may include a subfield that is indicated with a different value when transmitted by an AP that has performed channel access through a primary 20 MHz channel and when transmitted by an AP that has performed channel access through a non-primary channel (a 20 MHz subchannel other than the primary 20 MHz channel).

By setting a specific subfield included in the Common Info field of the trigger frame to a specific value, the AP can indicate to STAs receiving the trigger frame that it has transmitted the trigger frame after performing channel access through the primary channel. When the AP has performed channel access through the non-primary channel, the AP can indicate that it has not performed channel access through the primary channel by setting the specific subfield to a different value. In this case, if the specific subfield has a size of 1 bit, the specific subfield can be indicated as 0 to indicate that the AP has performed channel access on the primary channel, and can be set to 1 to indicate that the AP has performed channel access on a channel other than the primary channel. At this time, if the specific subfield has a size of 2 bits, the specific subfield may be indicated as 0 to indicate that the AP performed channel access on the primary channel, set to 1 to indicate that the AP performed channel access through the first non-primary channel, set to 2 to indicate that the AP performed channel access through the second non-primary channel, and set to 3 to indicate that the AP performed channel access through the third non-primary channel. At this time, the first, second, and third non-primary channels may be subchannels located in different 80 MHz segments.

When the AP indicates information related to the channel on which it performed Channel Access through the Common Info field of the trigger frame, the STA must interpret the RU Allocation subfield included in its User Info field based on the information on the channel on which the AP performed Channel Access when interpreting the RU Allocation subfield included in its User Info field. In other words, the STA that has received the MU-RTS frame must utilize both the channel information used for channel access indicated by the AP through the Common Info field and the information indicated through the RU Allocation subfield of its User Info field to confirm the frequency band to which it will respond with the CTS frame. Briefly explaining how the STA utilizes the information indicated in the Common Info field and the information indicated through the RU Allocation subfield, it may obtain a bandwidth on which it should respond with the CTS frame through the RU Allocation subfield, and confirm the location of the subchannel on which the CTS frame should be responded based on the information indicated in the Common Info field.

For example, when the RU Allocation subfield included in the User Info field of an STA is indicated as a value meaning Primary 80 MHz, if the channel on which the AP performed Channel Access is indicated as a Primary 20 MHz channel, the STA responds with a CTS frame through the Primary 80 MHz channel, and if the channel on which the AP performed Channel Access is indicated as a non-primary channel, the STA responds with a CTS frame on an 80 MHz channel located in the 80 MHz segment that includes the non-primary channel.

Alternatively, the AP can indicate both the frequency band and location information to which each STA will respond to the CTS frame, through the User Info field of each STA. That is, the AP can indicate an RU that does not include the Primary 20 MHz channel by setting the RU Allocation subfield included in the User Info field to a value other than 61 to 69. For example, the AP can indicate that the CTS frame will be responded to on one of the four 20 MHz subchannels located in the Secondary 80 MHz segment by setting the RU Allocation subfield to 71 to 74, can indicate one of the two 40 MHz channels located in the Secondary 80 MHz segment by setting the RU Allocation subfield to 75 and 76, or can indicate the Secondary 80 MHz channel by setting it to 77. That is, the RU Allocation subfield included in the User Info field of the MU-RTS trigger frame can have the function of indicating a frequency range excluding the Primary 20 MHz channel.

Alternatively, the RU Allocation subfield may be configured using the conventional setting method, but may be interpreted in combination with other subfields indicated in the User Info field, so that the AP can instruct the STA to respond to the CTS frame in a band other than the Primary 20 MHz channel. For example, the AP may indicate a value corresponding to 20 MHz (e.g., 61 to 64) through the RU Allocation subfield and indicate a first 80 MHz segment through the specific subfield, so as to instruct the STA to respond to the CTS frame through a specific 20 MHz subchannel located in the first 80 MHz segment. That is, the STA may determine the subchannels on which it should respond to the CTS frame by combining the indicated segment information and the indicated CTS frame response BW information.

<Efficient Operation Method of Non-Primary Channel Access (NPCA)>

The two channel access methods described in the present invention, 1) a channel access method using an auxiliary link and 2) a channel access method using a sub-channel other than a primary channel, are different channel access methods that can be used to achieve the same purpose. The difference between these two channel access methods exists in whether an STA performing a channel access changes the channel on which the channel access procedure is performed (e.g., EDCA) or whether the channel on which the channel access procedure is performed is changed by changing an active link.

That is, both channel access methods are essentially the same in that they perform channel access by performing channel switching to a sub-channel (non-primary channel) other than the primary channel when the primary channel is occupied, thereby having the same effect. Therefore, the methods for efficiently utilizing the two channel access methods are also the same. Hereinafter, the efficient operation method of non-primary channel access proposed in the present invention can be applied to both the non-primary channel access method in which a single STA changes the channel on which it performs channel access from the primary channel to another sub-channel (e.g., non-primary channel or sub-channel) instead of changing the channel from the primary channel to the primary channel, and the channel access method using overlapping BSS (the method using the primary link and auxiliary link described above). However, for the convenience of explanation, the efficient operation method of non-primary channel access described below will be mainly explained in terms of the method in which a single STA (AP and/or non-AP STA) performs channel access by using another sub-channel by channel switching to a sub-channel other than the primary channel. That is, the conditions for performing the non-primary channel access method described below can be applied identically to the conditions for switching the primary link to the inactive state and switching the secondary link to the active state. That is, the operation/judgment method of the STA performing the non-primary channel access described below can be applied identically to the operation/judgment method of the MLD using the overlapping BSS.

An STA performing non-primary channel access can improve its channel access capability by performing a channel access procedure through a subchannel other than the primary channel even when the primary channel is occupied by an OBSS. However, when performing a channel access procedure on a subchannel other than the primary channel, a procedure for protecting transmissions of other STAs that may be in progress on the subchannel first must be performed (for example, applying Mediumsync time), and after completing the channel access procedure, the acquired TXOP must be terminated at a time that is the same as or earlier than the TXOP of the OBSS that occupied its primary channel.

That is, when the primary channel is occupied by an AP or a non-AP STA of another OBSS and is in a busy state, the STA can improve the channel access capability by switching the channel to another sub-channel (sub-channel or non-primary channel) within the same bandwidth and performing a channel access procedure on the channel. In addition, since the problem that other sub-channels of the bandwidth are not used due to the occupied state of the primary channel can be resolved, the channel can be used efficiently. In this case, in order to protect the operations of other STAs in the switched sub-channel, the STA must perform the channel access procedure in the switched channel within the TXOP set in the primary channel. Accordingly, if the channel access procedure cannot be performed within the TXOP, the STA cannot perform the channel access procedure in the switched sub-channel. Therefore, the STA can determine the remaining length of the set TXOP based on a frame transmitted from an AP or a non-AP STA of another OBSS in the primary channel before channel switching in order to determine whether the channel access procedure can be performed in the switched sub-channel. In addition, before determining the remaining duration of TXOP after receiving a frame, the STA may determine whether the received frame is a frame transmitted from an AP associated with it or an AP not associated with it. At this time, the STA may determine the remaining duration of TXOP set for the primary channel based on the value of the TXOP field or duration field included in the received frame.

Therefore, an STA performing non-primary channel access needs to perform more additional operations (e.g., applying Mediumsync time, changing the operating frequency from the primary channel to another subchannel, etc.) than when performing channel access on the primary channel, whereas the TXOP length that can be obtained through the non-primary channel access may be further limited. That is, considering the cost consumed to perform non-primary channel access, a situation may occur where the length of the TXOP that can be obtained through the non-primary channel access is too short, and in this case, it may be more advantageous for the STA not to perform the non-primary channel access. Accordingly, even an STA with the Capability to perform non-primary channel access may not always perform the non-primary channel access when the primary channel is occupied by the OBSS.

As described above, if the STA determines that the primary channel of the bandwidth is occupied by the OBSS AP, the STA can perform channel switching to another subchannel (non-primary channel) of the bandwidth. At this time, the STA can determine whether the frame (or PPDU) it received is a frame transmitted by the OBSS AP (i.e., whether it is an inter-BSS frame or an intra-BSS frame). For example, it can determine whether the received frame is a frame transmitted from the OBSS AP through BSS color information of the received frame, or the MAC address, etc.

If the received frame is a frame transmitted from an OBSS AP, the primary channel is occupied by the OBSS AP and is in a busy state, so the STA cannot perform a channel access procedure on the primary channel. Therefore, the STA can perform a channel access procedure by switching the channel to a non-primary channel within the bandwidth described above.

At this time, the STA can determine whether to perform a non-primary channel access based on the TXOP length of the OBSS that occupies the primary channel. More specifically, the STA can perform a non-primary channel access only when the TXOP length of the OBSS that occupies its primary channel is longer than (or equal to or longer than) a specific value (threshold). At this time, the length of the OBSS TXOP that the STA considers may mean the length from the time when it recognizes the OBSS TXOP to the time when the OBSS TXOP ends. That is, the STA's determination is made based on the remaining time length of the OBSS TXOP remaining at the time of determining whether to perform a non-primary channel access, not the total length of the OBSS TXOP. At this time, the method by which the STA obtains the remaining time length of the OBSS TXOP may be based on the NAV (network allocation vector) set based on the frame transmitted by the OBSS STAs, or on the time indicated by the TXOP field included in the preamble of the OBSS PPDU.

At this time, the STA can determine whether to perform a non-primary channel access based on the PPDU length of the OBSS that occupies the primary channel. More specifically, the STA can perform a non-primary channel access only when the PPDU length of the OBSS that occupies its primary channel is longer than (or equal to or longer than) a specific value (Threshold). At this time, the length of the OBSS PPDU that the STA considers may mean the total length of the OBSS PPDU or the length of time remaining from the time when the STA recognizes the OBSS PPDU to the time when the OBSS PPDU ends. That is, the STA can decide to perform a non-primary channel access when the total length of the OBSS PPDU that the STA has confirmed is longer than a specific value (Threshold), or to perform a non-primary channel access when the length of the remaining OBSS PPDU from the time when the STA recognizes the OBSS PPDU is longer than a specific value (Threshold). At this time, the STA can obtain information on the total length of the OBSS PPDU or the remaining time length until the end point of the OBSS PPDU from the RXVECTOR parameter generated by the OBSS PPDU. The specific process of obtaining information related to the length of the OBSS PPDU from the RXVECTOR parameter is described through the embodiments of the present invention described later, so a detailed description is omitted.

In other words, when a PPDU is received from another STA (AP or non-AP STA), the STA can recognize whether the received PPDU is a PPDU transmitted from a BSS to which the STA belongs (intra-BSS PPDU) or a PPDU transmitted from a BSS to which the STA does not belong (inter-BSS PPDU) based on information included in the received PPDU (e.g., BSS color, BSSID, or MAC address). If the received PPDU is an inter-BSS PPDU, the STA can confirm that the primary channel of the bandwidth in which it operates is occupied through a CCA operation, and in this case, the STA cannot perform frame exchange (or channel access procedure) on the primary channel. Therefore, the STA can perform frame exchange or channel access by switching the channel to a channel other than the primary channel of the bandwidth in which it operates (e.g., a non-primary channel, etc.). In this case, in order for the STA to perform channel switching to another channel, the TXOP set by the received PPDU (TXOP set by another OBSS) must remain sufficiently for channel switching and frame exchange (or channel access). To determine this, the STA can compare a specific value related to the received PPDU with a minimum duration threshold. If the specific value is greater than (or greater than or equal to) the minimum duration threshold, the STA can perform a frame exchange or channel access procedure by switching the channel to another channel.

At this time, the specific value related to the received PPDU may be either the length of the received PPDU or the duration of the remaining TXOP. The length of the received PPDU may be the total length of the PPDU transmitted from the OBSS or the length of the remaining PPDU from the time the STA recognizes the PPDU, and the length of the remaining PPDU may be obtained based on the length field and the rate field included in the PPDU. The duration of the remaining TXOP may mean the length of the remaining TXOP from the time the STA recognizes the PPDU transmitted from the OBSS, and may be obtained by adding the length of the remaining PPDU to the value of the TXOP field included in the PPDU.

Alternatively, the STA can recognize the total length of the PPDU or the length of the remaining PPDU by the RXVECTOR parameter generated at the PHY layer by the received PPDU and transmitted to the MAC layer. The following embodiments describe a specific value based on the remaining TXOP duration, but may be performed based on not only the remaining TXOP duration but also the total length of the PPDU or the length of the remaining PPDU.

The specific value may be a value indicated by the AP via a Management frame transmitted by the AP (e.g., a Beacon frame and/or a Probe Response frame and/or an Association Response frame and/or a Management frame (a type of Operating Mode Notification frame) indicating whether to utilize a non-primary channel connection). Alternatively, the specific value may be a preset value (e.g., 1 ms or 2 ms, etc.).

At this time, the method for the STA to check the TXOP length of the OBSS that occupies the primary channel may be to use the value indicated through the TXOP field included in the Preamble (e.g. HE-SIG-A or U-SIG) of the received PPDU or the Duration/ID field of the MAC Header.

In this case, the STA can recognize the duration of the remaining TXOP after reception of the frame based on the duration field or TXOP field included in the received frame. That is, the STA can recognize the remaining duration from the end time of the received frame to the end time of the TXOP set by the OBSS based on the value indicated by the duration field or TXOP field included in the frame. Accordingly, the STA can determine whether it can perform a channel access procedure by channel switching to a non-primary channel within the set TXOP based on the remaining duration, and if it is determined that channel switching and the channel access procedure (or frame exchange) can be performed within the remaining duration, the STA can perform the channel access procedure (or frame exchange) by channel switching to the non-primary channel. At this time, the STA compares the remaining duration (or, the total length of the PPDU, the length of the remaining PPDU, etc.) with a threshold value to determine whether the channel switching and channel access procedures can be performed within the remaining duration. If the remaining duration is greater than (or equal to or greater than) the threshold value, it can be determined that the channel switching and channel access procedures can be performed. For example, if the value obtained by adding the value indicated by the TXOP field or the duration field to the duration of the frame (e.g., the received PPDU) is greater than the threshold value, the STA can determine that the channel access procedure on the non-primary channel is possible. At this time, the length of the frame (or the duration of the PPDU) can be obtained based on the length field and the rate field included in the frame. At this time, the threshold value can mean the minimum duration for the STA to perform channel switching and channel access.

Alternatively, the method for determining whether an STA performs channel access on a non-primary channel may be performed based on the length of a TXOP that it determines it can obtain through channel access on the non-primary channel, rather than based on the TXOP length of the OBSS occupying the primary channel. That is, the STA may determine to perform channel access on the non-primary channel only when the length of a TXOP that it can obtain through channel access on the non-primary channel is longer than (or equal to or longer than) a specific value (threshold). In this case, while performing a channel access procedure on a subchannel other than the primary channel, if the length of a TXOP that it can obtain through non-primary channel access becomes shorter than the specific value (calculated based on the end time of the TXOP of the OBSS occupying the primary channel), the STA may switch back to the primary channel and perform the channel access procedure. That is, if an STA fails to complete the channel access procedure of a non-primary channel within a certain time period from the end time of the TXOP of the OBSS that occupies its primary channel, the STA may give up access to the non-primary channel and switch back to the channel access procedure using the primary channel.

The method based on the length of the OBSS TXOP described above and the method based on the length of the TXOP that can be obtained are different methods for determining whether the TXOP that can be obtained through the non-primary channel access is sufficient (whether it is sufficiently efficient). An STA that determines whether to attempt the non-primary channel access can use one of the two methods or both methods together to make a final decision on whether to perform the non-primary channel access.

Additionally, when determining whether to perform a channel access procedure on a non-primary channel, the STA may consider a switching delay or transition delay for switching to the non-primary channel operation to determine whether to switch to the non-primary channel operation. In more detail, even if the STA is instructed that the TXOP of the OBSS occupying its primary channel is maintained for a time longer than a threshold value, if the STA considers the time required to switch to the non-primary channel operation mode and the time that the STA can operate on the non-primary channel is shorter than the threshold value, the STA may not switch to the non-primary channel operation mode. That is, the STA may determine whether to switch to the non-primary channel operation mode based on whether the remaining 'time length of OBSS TXOP - Transition delay' is longer than (or equal to) the threshold value.

For example, in order to determine whether a channel access procedure is possible in the non-primary channel described above, the STA may additionally consider a switching delay or a transition delay. That is, the STA compares a value obtained based on a value indicated by a duration field or a TXOP field included in a received frame (or PPDU) (e.g., a value obtained by adding the duration of the PPDU to the value indicated by the TXOP field or the duration field value) with a threshold value, and the threshold value may be set to a value obtained by adding the switching delay or a transition delay to a minimum value for performing channel switching and channel access procedures.

FIG. 29 illustrates an example of channel access in a non-primary channel when the primary channel is occupied according to one embodiment of the present invention.

Referring to FIG. 29, an STA can perform a channel access procedure by switching the channel to a non-primary channel (secondary channel) when the primary channel is in a busy state due to a TXOP set by an OBSS. FIG. 29 illustrates a channel state and a channel access procedure confirmed from the perspective of an STA capable of performing non-primary channel access.

Before the STA completes the backoff procedure on the primary channel (P20), the STA receives an RTS/CTS frame that the OBSS STA exchanges to initiate OBSS TXOP1. Based on the information included in the received frame, the STA can determine that the remaining time length of the OBSS TXOP1 is longer than a threshold value. Since the length of the OBSS TXOP1 is sufficiently long, the STA performs a channel access procedure through a channel other than the primary channel (S20_1 of FIG. 29), acquires the TXOP, and then performs a frame exchange.

An STA that terminates its TXOP before OBSS TXOP1 terminates performs a channel access procedure through the primary channel (P20 of FIG. 29) again, and OBSS TXOP2 starts before the backoff procedure is completed. The STA confirms that the length of the OBSS TXOP2 it has confirmed is shorter than the threshold value, and instead of performing a channel access procedure on a channel other than the primary channel, it waits for OBSS TXOP2 to terminate on the primary channel. Thereafter, when OBSS TXOP2 terminates, the channel access procedure continues on the primary channel.

As described above, it is possible to determine whether performing a non-primary channel connection is efficient or inefficient based on the length of a TXOP that can be obtained through the non-primary channel connection, and the STA can decide whether to perform the non-primary channel connection based on the determination result.

Another type of inefficiency that can occur due to non-primary channel access occurs when the primary channel statuses recognized by the AP and the non-AP STA are different from each other. As explained above, the neighboring STAs of the AP and the non-AP STA may be different from each other, and there may be cases where the signal of the OBSS occupying the primary channel of the AP is not received by a specific non-AP STA. In addition, there may also be cases where the signal of the OBSS occupying the primary channel of a specific non-AP STA is not received by the AP.

In this way, when the status of the primary channel confirmed by the AP and the status of the primary channel confirmed by the non-AP STA are different from each other, the channels through which the AP and the non-AP STA perform channel access may be different from each other. For example, the AP determines that the primary channel is IDLE and transmits a PPDU after completing channel access on the primary channel, but the non-AP STA determines that the primary channel is BUSY due to the OBSS and may be performing a channel access procedure on a subchannel other than the primary channel or waiting to receive a PPDU from the AP. Similarly, the non-AP STA determines that the primary channel is IDLE and transmits a PPDU after completing channel access on the primary channel, but the AP determines that the primary channel is BUSY due to the OBSS and may be performing a channel access procedure on a subchannel other than the primary channel or waiting to receive a UL PPDU. In this way, when the AP and non-AP STA have different views of the primary channel, PPDU transmission fails due to channel mismatch issues between the transmitting and receiving devices.

This is because the introduction of the non-primary channel access procedure has caused a problem that was not previously experienced by Wi-Fi STAs that operated only through the primary channel, and therefore, the situations in which such problems occur must be reduced in order to maximize the benefits and reduce losses that can be obtained from non-primary channel access. However, the fact that the channel environments of STAs operating at different locations are different is a natural phenomenon that occurs due to the limited reach of wireless transmission signals, and therefore, the phenomenon in which two STAs may have different judgments about a specific channel is an unresolvable phenomenon.

However, based on information previously exchanged between two STAs, it is possible to help a specific STA determine a point in time when another STA has the same view as itself. For a simple example, if a specific STA has instructed another STA in advance that a signal transmitted by STA1 has been received by itself, the other STA can anticipate that the signal of STA1 will also be received by the specific STA while the signal of STA1 is being received by itself. That is, based on information provided by the specific STA, the other STA can determine that it and the specific STA have the same view at a specific point in time.

According to one embodiment of the present invention, an AP may instruct a non-AP STA about information related to an adjacent BSS (Overlapping), and the non-AP STA may determine whether to perform a non-primary channel access (move to a subchannel other than the primary channel for performing a channel access procedure and/or waiting for reception (e.g., performing CCA and PD (Preamble/Packet detection)) based on the information instructed to the AP. That is, the AP may transmit information on an adjacent OBSS to associated non-AP STAs by including it in a management frame. For example, the AP may transmit a management frame including list information on OBSS APs, and when the non-AP STA performs a non-primary channel access operation, if an AP that has transmitted the frame is included in the list information of the management frame based on information related to an AP included in a received frame (or PPDU), the non-AP STA may perform the non-primary channel access procedure described above. That is, when a PPDU is transmitted from an OBSS AP, a non-AP STA can compare the AP-related information (BSS color information or MAC address) included in the PPDU with the list information included in the management frame. When the list information includes information related to the AP, a non-AP STA can perform a channel access procedure on a non-primary channel as described above.

Specifically, the AP can notify non-AP STAs of information (a type of list) related to OBSSs (i.e., OBSSs that change the primary channel status of the AP to busy) that can occupy its primary channel. At this time, the information related to the OBSS that the AP notifies to the non-AP STAs may be BSS color information of the OBSSs and/or MAC addresses of Neighbor APs operating the OBSS. At this time, the number of Neighbor APs that the AP indicates in relation to non-primary channel access may be smaller than the number of APs that the AP indicates through the RNR element (Reduced Neighbor Report) of the beacon frame. This is because the Neighbor APs that the AP indicates in relation to non-primary channel access are limited to APs that operate a BSS that can occupy the primary channel of the AP.

A non-AP STA that has received information related to OBSS from an AP can determine whether to switch to non-primary channel operation (performing a channel access procedure using a subchannel other than the primary channel and/or waiting to receive on a subchannel other than the primary channel) by checking whether the OBSS occupying its primary channel is included in the OBSS list indicated by the AP when its primary channel is switched to BUSY.

Specifically, a non-AP STA may decide to perform non-primary channel operation when its primary channel is occupied by an OBSS and the color of the OBSS is included in the OBSS color list indicated by the AP. That is, the non-AP STA can know that the OBSS that changed the state of its primary channel to BUSY is the OBSS that also changed the primary channel state of the AP to BUSY, and it is expected that the AP will also switch to non-primary channel operation when the non-AP STA switches to non-primary channel operation.

That is, the prior information about the OBSS transmitted by the AP through the management frame can be used by the non-AP STA to determine whether the OBSS occupying its primary channel causes the AP to start the non-primary channel operation. If the OBSS occupying its primary channel is an OBSS not included in the list information transmitted by the AP, the non-AP STA must not switch to the non-primary channel operation. In this case, the AP also must not switch to the non-primary channel operation if the primary channel is occupied by an OBSS not included in the list information of the OBSS transmitted by the AP.

The process by which an AP acquires/instructs information about an OBSS and determines whether or not to perform non-primary channel operation of each non-AP STA based on the instructed information can be summarized as follows.

1) When a PPDU transmitted while occupying its primary channel is received, the AP records information of the OBSS that transmitted the PPDU. At this time, the OBSS information recorded by the AP may be BSS color information of the OBSS or the MAC address of the AP operating the OBSS. i) For BSS color information, the AP can obtain it through the BSS color field (included in the SIG field (e.g., HE-SIG-A or U-SIG, etc.)) of the received PPDU (e.g., HE PPDU, EHT PPDU, UHR PPDU). ii) MAC information can be obtained through the TA or RA field (included in the MAC header) of the MAC frame included in the received PPDU (e.g., HE PPDU, EHT PPDU, UHR PPDU).

2) The AP can announce the list information, which is information of the OBSS that switches the state of its primary channel to busy, through the frame that it transmits. At this time, i) the OBSS information may be BSS color information of each OBSS and/or the MAC address of the AP operating the OBSS. ii) The AP can indicate BSS information through the beacon frame and/or notification frame (e.g., Operating Mode Notification frame) that it transmits. At this time, the Notification frame may be a frame that indicates whether the AP is going to perform non-primary channel access.

3) A non-AP STA may perform a non-primary channel access operation based on list information (e.g., color information or MAC address) of OBSSs transmitted from an AP through a management frame. For example, i) a non-AP STA may perform a non-primary channel access operation if the BSS color of an OBSS that occupies its primary channel or the MAC address of an OBSS AP matches the BSS color or MAC address included in the list information acquired from the AP. ii) However, a non-AP STA may not perform the non-primary channel access operation if the BSS color of an OBSS that occupies its primary channel or the MAC address of an OBSS AP does not match the BSS color or MAC address included in the list information acquired from the AP.

4) The AP manages the information of the OBSS that switches the primary channel to the busy state (i.e., the OBSS that occupies the primary channel) by applying a timeout. That is, if the PPDU of a specific OBSS is not received within a certain time (timeout), the AP deletes the information related to the specific OBSS from the list information of the OBSS. That is, the list information of the OBSS newly transmitted by the AP does not include the information of the specific OBSS. A non-AP STA may determine whether to perform a non-primary channel access operation based on the list information of the OBSS most recently transmitted from the AP.

As described above, if information about OBSSs that cause an AP to perform non-primary channel operation is notified to non-AP STAs, non-AP STAs whose primary channel is occupied by an OBSS other than the notified OBSS can be prevented from malfunctioning (non-AP STAs performing non-primary channel operation alone in a situation where the AP performs primary channel operation).

FIG. 30 illustrates an example of operation of STAs in a non-primary channel based on information related to OBSS transmitted from an AP according to an embodiment of the present invention.

Referring to FIG. 30, a non-AP STA can perform a non-primary channel access operation based on a frame transmitted from an OBSS included in the list information of the OBSS included in a management frame transmitted from the AP.

Specifically, as illustrated in Fig. 30, the AP announces information (Neighbor BSS list of Fig. 30) of the OBSS that it occupies the primary channel. At this time, the AP can transmit the Neighbor BSS list, which is information of the OBSS, by including it in a management frame. The BSS colors of the announced BSS list are x and y.

When the TXOP of the OBSS whose BSS color is x starts, the primary channels confirmed by the AP, STA1, and STA2 are all switched to BUSY. STA1 and STA2, which confirmed that the BSS color of the OBSS that occupied their primary channel is x, confirm that the BSS color announced by the AP includes x and perform non-primary channel operation during the OBSS TBOP1.

Afterwards, although the primary channel of STA2 is occupied by an OBSS whose BSS color is a, STA2 does not perform non-primary channel operation because the BSS color of the OBSS that occupied its primary channel is not included in the BSS colors announced by the AP. In other words, STA2 does not switch to non-primary channel operation because the BSS that occupied its primary channel is not included in the OBSS list announced in advance by the AP, even though its primary channel is occupied by OBSS TXOP2.

As OBSS TXOP3 starts, the primary channels of AP and STA2 are switched to the BUSY state, and STA2 can recognize that the AP has also switched its primary channel to BUSY based on the information previously acquired from the AP. Therefore, STA2 switches to the non-primary channel operation to perform transmission/reception with the AP. At this time, since STA1 is operating in a location where the signal of the OBSS with BSS Color y does not reach, it determines that the primary channel is IDLE and continues the primary channel operation.

As described above, based on the OBSS information previously announced by the AP, the non-AP STA can determine whether to switch to the non-primary channel operation mode. As mentioned in the introduction, the above-described conditions for switching between the primary channel operation mode and the non-primary channel operation mode can be equally applied as conditions for switching the primary link and the secondary link to the Active state. That is, based on the OBSS information announced by the AP of the primary link, the non-AP STA MLD can determine whether to maintain the primary link in the Active state or switch the secondary link to the Active state (switch the primary link to Inactive). Since it is easy to understand that the same rule can be applied to two different non-primary channel access methods, a repeated explanation of the non-primary channel access method using the Overlapping BSS is omitted.

According to another embodiment of the present invention, a non-AP STA can indicate information about its adjacent OBSS to the AP. The AP can determine whether to include each non-AP STA in the destination device of a PPDU to be transmitted on a non-primary channel based on the OBSS information indicated from the non-AP STAs. That is, the non-AP STA can transmit BSS color information of its adjacent OBSS and/or the MAC address of the OBSS AP to the AP, and the AP can determine whether to include each non-AP STA in the destination device of a PPDU to be transmitted on a non-primary channel based on the OBSS information transmitted from the non-AP STAs. For example, since non-AP STA 1 has no adjacent OBSS, it can transmit none information to the AP, and since non-AP STA 2 has an adjacent OBSS, it can transmit BSS color information of the OBSS and/or the MAC address of the OBSS AP to the AP. In this case, non-AP STA 1 can perform a channel access procedure on the primary channel because there is no adjacent OBSS. However, non-AP STA 2 cannot perform a channel access procedure on the primary channel if the primary channel is occupied by an OBSS because there is an adjacent OBSS. Therefore, the AP can add non-AP STA 2 to the destination device of the PPDU transmitted on the non-primary channel so that non-AP STA 2 can perform a channel access procedure on the non-primary channel.

For example, if the information of the OBSS indicated from the non-AP STA1 includes BSS color x and the information of the OBSS indicated from the non-AP STA2 does not include BSS color x, when the AP performs the non-primary channel operation due to the TXOP of the OBSS whose BSS color is x, the AP may transmit a PPDU destined for the non-AP STA1 and not transmit a PPDU destined for the non-AP STA2. This is an efficient scheduling method of the AP because, when the primary channel of the AP is occupied by the OBSS whose BSS color is x, the AP may determine that the primary channel of the non-AP STA1 is also likely to be occupied by the same OBSS (i.e., the non-AP STA1 may have switched to the non-primary channel operation mode) and that the primary channel of the non-AP STA2 may not be occupied (i.e., the non-AP STA2 may maintain the primary channel operation mode). At this time, what is meant by the PPDU targeting a specific non-AP STA is that the MAC address of the RA field included in the PPDU indicates the MAC address of the specific non-AP STA, or that the specific non-AP STA is indicated by the user field of the trigger frame included in the PPDU.

In this way, by utilizing the OBSS information indicated from non-AP STAs, the AP can distinguish non-AP STAs that have also transitioned to non-primary channel operation mode when it transitions to non-primary channel operation mode.

However, a non-AP STA can indicate OBSS information by specifying an OBSS that occupies its primary channel among the OBSS list announced by the AP, rather than indicating to the AP information about all OBSSs that have switched its primary channel to busy. To explain with a simple example for easier understanding, if the OBSS information announced from the AP (the OBSS list that occupies the AP's primary channel) includes BSS 1, BSS 2, and BSS 3, and the OBSSs that occupy the non-AP STA's primary channel are BSS 2, BSS 4, and BSS 5, the non-AP STA can indicate only information about BSS 2 to the AP when indicating its OBSS information. This is because BSS 4 and BSS 5 are BSSs that are not OBSSs of AP, and even if information about BSS 4 and BSS 5 (e.g. BSS color) is instructed to AP, AP cannot utilize the information in scheduling process. Therefore, unnecessary information about BSS 4 and BSS 5 can be omitted to reduce overhead.

Meanwhile, the TXOP of an OBSS that has occupied the primary channel may be terminated earlier than the expected end time identified by the PPDU and frames exchanged when the OBSS TXOP starts. For example, a specific STA may determine that the OBSS TXOP will continue for another 5 ms based on information indicated in the TXOP field of the received OBSS PPDU and initiate a non-primary channel operation. The specific STA may complete the non-primary channel operation before the expected end time of the OBSS TXOP and switch back to the primary channel operation, but a situation may occur in which the OBSS TXOP is truncated after lasting for a shorter time than the initially confirmed 5 ms. In this case, the specific STA that has switched to the primary channel operation encounters a situation in which the TXOP of the OBSS that initially occupied the primary channel has already been terminated and the TXOP of another OBSS is in progress. In this case, the specific STA loses the channel access opportunity on the primary channel while performing an operation using a subchannel other than the primary channel during a busy time period of its primary channel. Considering that the non-primary channel operation is more complex and the available BW is narrower than the primary channel operation, the specific STA may suffer a loss while performing the non-primary channel operation.

Such losses can be understood at first glance as an opportunity cost that STAs must pay to perform non-primary channel operations in order to overcome their dependency on the primary channel.

However, if the specific STA is an AP STA and the time point at which the transmission opportunity is lost is a time interval adjacent to the Target Beacon Transmission Time (TBTT), the transmission opportunity on the primary channel that the AP lost has the same meaning as the loss of the opportunity to transmit a Beacon frame at an appropriate time, and therefore, may be an incident that makes it difficult to manage the BSS operated by the AP. For example, among the member STAs of the BSS, STAs performing the Power Save operation transition to Awake at the time point when a Beacon frame is expected to be received from the AP, but because the time point at which the AP transmits the Beacon frame is far from the TBTT, they experience an energy waste problem of maintaining the Awake state for a longer period of time and waiting for TIM frame reception.

Accordingly, an STA (AP and/or non-AP STA) may need to decide not to perform a non-primary channel operation during the TXOP period of an OBSS that occupies its primary channel, if the next TBTT of the BSS to which it belongs (is operating or is a member STA) is scheduled within the TXOP period of the OBSS that occupies its primary channel. This may be a limitation on the utilization of the non-primary channel operation mode that is introduced to transmit/receive a Beacon frame as quickly as possible after the TXOP of the OBSS is terminated, i.e., after the primary channel is switched to IDLE. At this time, the TBTT considered when the STA determines whether to utilize the non-primary channel operation mode may be a TBTT corresponding to the DTIM Beacon frame. At this time, the TBTT considered when the STA determines whether to utilize the non-primary channel operation mode may be all TBTTs.

Similar restrictions can be similarly applied when the start time of the TWT (Target Wake Time) SP (Service Period) of the BSS to which the STA belongs is located within the TXOP interval of the OBSS, or when a Restricted TWT (R-TWT) SP exists. That is, if the TXOP interval of the OBSS that STA occupies its primary channel includes the start time of the TWT and/or R-TWT SP, the STA must not perform non-primary channel operations (performing channel access on a subchannel other than the primary channel (e.g., backoff procedure) or waiting for reception of a PPDU transmitted by a peer STA (e.g., Preamble/Packet detection)) during the TXOP interval of the OBSS. In this case, R-TWT means a TWT that only allows MSDU transmission of a specified TID.

FIG. 31 illustrates an example of a method for determining whether to perform an operation on a non-primary channel considering the TBTT of a BSS according to an embodiment of the present invention.

Figure 31 illustrates the process by which an AP performing a channel access procedure determines whether to switch to a non-primary channel operation mode after confirming an OBSS TXOP occupying the primary channel.

Referring to FIG. 31, the AP, while operating in the primary channel operation mode performing a backoff procedure through the P20 channel, recognizes the start of an OBSS TXOP occupying the P20 channel. Through the received RTS and/or CTS frame of the OBSS, the STA can recognize the time that the TXOP of the OBSS is expected to last, and can confirm that the TXOP of the OBSS includes the next TBTT time point of the BSS to which it belongs (is operating). Since the OBSS TXOP occupying its primary channel includes the TBTT, the STA does not switch to the non-primary channel operation mode during the time period when the primary channel is occupied by the OBSS TXOP, but maintains the primary channel operation mode. When the OBSS TXOP ends, the AP performs a channel access procedure through the primary channel and then transmits a beacon frame. At this time, all STAs of the BSS maintain the primary channel operation mode even during the time that the OBSS TXOP is in progress because the OBSS TXOP includes the TBTT, just like the AP, and receive the beacon frame transmitted by the AP after the OBSS TXOP ends.

A series of procedures for performing channel access and frame exchange using a subchannel other than the primary 20 MHz subchannel (non-primary channel) during the time period when the primary 20 MHz subchannel is occupied by the OBSS may be named Non-Primary Channel Access (NPCA) or Secondary Channel Access (SCA).

As described above, the subchannel for NPCA is indicated by the AP, and may be indicated as one of the subchannels located in a segment other than the P80 segment containing the primary 20 MHz subchannel. In this case, the AP does not indicate a subchannel that is included in the Operating BW of the BSS but is not used by the BSS (i.e., a Disabled subchannel) as a subchannel for NPCA.

<Method for managing frame exchange sequence related to NPCA>

Even if STAs of the same BSS exist in different locations within the coverage of the BSS, each STA may have different views (e.g., different CCA results) of the primary 20 MHz subchannel. As described through the embodiment of FIG. 30 described above, the AP may partially prevent the AP and non-AP STAs from performing different operations by notifying the non-AP STAs of information about the adjacent BSS affecting it and/or the AP operating the adjacent BSS. However, even in a BSS operated by an AP supporting NPCA, there may be member STAs that do not support NPCA, and there may still be non-AP STAs that identify the primary 20 MHz subchannel as IDLE and perform the channel access procedure on the primary 20 MHz subchannel while the AP performs a channel access procedure on a subchannel other than the primary 20 MHz subchannel for the NPCA operation.

That is, while the AP is performing channel access through a subchannel other than the primary 20 MHz subchannel, a situation may occur in which the AP confirms a PPDU transmitted by a non-AP STA that has completed the channel access procedure through the primary 20 MHz subchannel. As a simple example, while the AP is performing a channel access procedure through a subchannel located in the secondary 80 MHz band, a situation may occur in which a non-AP STA that has completed the channel access procedure through the primary 20 MHz subchannel transmits a PPDU to a band including the secondary 80 MHz band. In this case, the AP receives the PPDU transmitted by the non-AP STA on the subchannel located in the secondary 80 MHz band, and if the received PPDU requests a response to the response frame, the AP must determine whether or not to transmit the response frame.

Additionally, a non-AP STA that has transmitted a PPDU to an AP after completing a channel access procedure on the primary 20 MHz channel must adjust its subsequent frame exchange sequence depending on whether a response frame is received from the AP, and must take into account the possibility that the AP may have received the PPDU over a subchannel other than the primary 20 MHz channel.

According to one embodiment of the present invention, an AP that receives a frame requesting a response frame through a subchannel other than the primary 20 MHz subchannel can transmit the response frame through a frequency band excluding the primary 20 MHz subchannel. The method by which the AP transmits the response frame is described in more detail through the embodiments of the present invention described below. In this case, the frame can be appropriately replaced with a PPDU and interpreted.

According to one embodiment of the present invention, a non-AP STA that performs channel access through a primary 20 MHz subchannel and then transmits a frame to an AP can determine a transmission frequency band of the frame it transmits based on a frequency band occupied by a response frame transmitted by the AP. At this time, the determination of the transmission frequency band includes a determination of a width (BW) and/or a location (a band including or not including the primary 20 MHz subchannel) of the frequency band. A method by which a non-AP STA operates based on a frequency band occupied by a response frame received from an AP is described in more detail through embodiments of the present invention described below. At this time, the frame can be appropriately replaced with a PPDU and interpreted.

<AP response frame transmission rules>

When an AP attempting channel access through a subchannel other than the primary 20 MHz channel (hereinafter P20) for NPCA (Auxiliary channel (same as S20 of the present invention described above), hereinafter denoted as A20) receives a frame requesting a response frame, the AP can decide whether to transmit a response frame for the received frame through the following processes (1) and (2). At this time, the frame received on the AP side means a frame in which the destination device of the frame is set to the AP. At this time, the fact that the frame was received through A20 means that the PPDU including the corresponding frame was detected through A20.

(1) If the received frame is a frame transmitted via NPCA, the AP transmits a response frame to the received frame.

A frame transmitted via NPCA refers to a frame transmitted by a non-AP STA that performed channel access via A20. A frame transmitted via NPCA can be distinguished based on information about a frequency range occupied by a PPDU including the frame. For example, if a frame is transmitted via a PPDU that does not occupy P20, the AP can determine that the frame is transmitted via NPCA. As another example, if a frame is transmitted via a PPDU that occupies only a frequency range accessible via A20, the AP can determine that the frame is transmitted via NPCA.

(2) If the received frame is not a frame transmitted via NPCA, the AP determines whether to transmit a response frame to the received frame.

i) If the received frame is transmitted by a non-AP STA that does not support NPCA, i.e., if the non-AP STA identified through the TA field of the received frame is a non-AP STA that does not support NPCA, the AP does not transmit a response frame for the received frame.

ii) If the received frame is transmitted by a non-AP STA supporting NPCA, that is, if the non-AP STA identified through the TA field of the received frame is a non-AP STA supporting NPCA, the AP can transmit a response frame for the received frame. At this time, the AP responds with a CTS frame if ① the received frame is an RTS frame, ② it is received as included in a non-HT or non-HT duplicated PPDU, and ③ it has bandwidth signaling TA and the RXVECTOR parameter DYN_BANDWIDTH_IN_NON_HT of the PPDU is Dynamic. At this time, the duration field of the CTS frame can be set based on the duration/ID field of the received RTS frame or based on the remaining time of the OBSS TXOP. If the time indicated by the Duration field of the received RTS frame is before the end time of the OBSS TXOP, the CTS frame is set based on the Duration field of the RTS frame (e.g., the value of the Duration field of the RTS frame - CTS time (time required to transmit the CTS frame) - SIFS). If the time indicated by the Duration field of the received RTS frame is after the end time of the OBSS TXOP, the CTS frame is set to a time before the end of the OBSS TXOP or a value indicating the end time of the OBSS TXOP. In this case, the non-AP STA that transmitted the RTS frame must end the frame exchange sequence before the time indicated by the Duration field of the CTS frame responded from the AP. C. Alternatively, the AP may not transmit a response frame if the time indicated by the Duration field of the received RTS frame is later than the end time of the OBSS TXOP. In this case, the non-AP STA that transmitted the RTS frame cannot continue the frame exchange sequence.

At this time, the Duration field of the CTS frame can be set based on the Duration/ID field of the received RTS frame or based on the remaining time of the OBSS TXOP. A. If the time indicated by the Duration field of the received RTS frame is before the end time of the OBSS TXOP, the CTS frame is set based on the Duration field of the RTS frame (e.g., the Duration field value of the RTS frame - CTS time (time required to transmit the CTS frame) - SIFS). If the time indicated by the Duration field of the received RTS frame is after the end time of the OBSS TXOP, the CTS frame is set to a time before the end of the OBSS TXOP or a value indicating the end time of the OBSS TXOP. In this case, the non-AP STA that transmitted the RTS frame must terminate the frame exchange sequence before the time indicated by the Duration field of the CTS frame responded from the AP. C. Alternatively, the AP may not transmit a response frame if the time indicated by the Duration field of the received RTS frame is later than the end time of the OBSS TXOP. In this case, the non-AP STA that transmitted the RTS frame cannot continue the frame exchange sequence.

At this time, if the received frame is an RTS frame, is received included in a non-HT or non-HT duplicated PPDU, and has bandwidth signaling TA, the AP responds with a CTS-to-self frame instead of a CTS frame when the RXVECTOR parameter DYN_BANDWIDTH_IN_NON_HT of the PPDU is Static. The CTS-to-self frame may not be considered as a response to the received RTS frame. However, the CTS-to-self frame has the characteristic that it is transmitted after SIFS (Short Inter Frame space) after the end of reception of the RTS frame (i.e., transmitted at the same timing as when the CTS frame is responded to). The TXOP of the AP is acquired for the bandwidth occupied by the transmitted CTS-to-self frame. The TXOP acquired by the AP can be interpreted as the concept that the TXOP of the non-AP STA that transmitted the RTS frame has been granted to the AP. The duration field of the CTS-to-self frame can be set based on the duration/ID field of the received RTS frame or the remaining time of the OBSS TXOP. If the time indicated by the duration field of the received RTS frame is before the end time of the OBSS TXOP, the CTS-to-self frame is set based on the duration field of the RTS frame (e.g., the value of the duration field of the RTS frame - CTStime (time required to transmit the CTS-to-self frame) - SIFS). If the time indicated by the duration field of the received RTS frame is after the end time of the OBSS TXOP, the CTS-to-self frame is set to a time before the end of the OBSS TXOP or a value indicating the end time of the OBSS TXOP.

Alternatively, the AP can also determine whether to respond to the CTS frame based on whether the non-AP STA that transmitted the RTS frame supports reception of the CTS frame responded to on A20. To this end, the non-AP STA instructs the AP with Capability information related to whether it can receive the CTS frame responded to on A20 after transmitting the RTS frame on P20. The AP considers whether the non-AP STA supports NPCA and whether it supports processing of the CTS frame responded to on A20 to determine whether to perform a CTS frame response to the RTS frame received from the non-AP STA (transmitted on P20). That is, if the non-AP STA that transmitted the RTS frame on P20 supports response to the CTS frame responded to on A20, the AP responds with a CTS frame on A20. On the other hand, if a non-AP STA that transmitted an RTS frame through P20 does not support responding to a CTS frame responded to in A20, the AP does not perform a CTS frame response.

An AP that decides to transmit a response frame for a frame received via A20 determines a frequency range in which to transmit the response frame. At this time, determining a frequency range in which to transmit the response frame means determining the bandwidth and transmission frequency (channel) of the PPDU including the response frame.

The AP can transmit a response frame (PPDU) over a frequency band that is not occupied by the OBSS TXOP among the frequency bands occupied by the PPDU containing the frame received via A20. That is, the AP does not transmit a response frame over the band occupied by the TXOP of the identified OBSS.

In addition, when the AP transmits a response frame for a frame received via A20, the AP can transmit the response frame (PPDU) via a frequency band that is allowed to be accessed via A20. For a simple example, if the frequency range accessible via A20 is Secondary 80 MHz (if A20 is a subchannel included in Secondary 80 MHz), the AP responds with a response frame for the received frame within the Secondary 80 MHz band. For another example, if the frequency range accessible via A20 is Secondary 160 MHz (if A20 is a subchannel included in Secondary 160 MHz), the AP responds with a response frame for the received frame within the Secondary 160 MHz band. Of course, even if it is a frequency range that satisfies the above-described condition, the AP does not transmit a response frame in a subchannel for which the CCA result is BUSY.

At this time, the frequency bands that are allowed to be accessed through A20 are limited to some of the frequency bands (Operating BW) that are accessible through P20. The characteristic of the frequency bands that are accessible through A20 is that the remaining frequency bands, excluding the preset bandwidth including P20, are frequency bands that are accessible through A20.

Meanwhile, for the purpose of reducing the complexity of AP operation, it is possible to allow the AP not to transmit a response frame for a frame received via A20. That is, when a frame is received while performing channel access via A20, the AP can decide not to transmit a response frame for the received frame. At this time, the condition under which the AP can decide not to transmit a response frame for the received frame can be limited to when the received frame is not a frame transmitted via NPCA, i.e., a frame transmitted by a non-AP STA that performed channel access via P20.

<Non-AP STA Initial Frame and Subsequent Frame Transmission Method>

A non-AP STA supporting NPCA must perform operations considering that the AP may be operating in A20 when initiating a TXOP for transmitting a UL PPDU. To this end, the non-AP STA may need to check whether a response frame (e.g., a CTS frame) from the AP is responded to through A20 when transmitting an initial Control frame (e.g., an RTS frame) after completing channel access through P20 and the initial Control frame is transmitted by occupying a band including A20. However, a non-AP STA for which all APs included in the OBSS AP list indicated by the AP are confirmed can be certain that it does not have an operation state different from that of the AP (i.e., a P20 operation state or an A20 operation state), and therefore, the rules for non-AP STA operations provided through embodiments of the present invention described below may not be applied.

In addition, when a non-AP STA transmits an initial Control frame through a non-HT duplicated PPDU after completing channel access through P20, if the initial Control frame is transmitted by occupying A20, the TA field of the initial Control frame must be set to bandwidth signaling TA and DYN_BANDWIDTH_IN_NON_HT (TXVECTOR parameter) must be set to Dynamic. However, a non-AP STA that does not intend to receive a response frame of an AP responding through A20 or does not want to acquire a TXOP in a form that does not occupy P20 can set DYN_BANDWIDTH_IN_NON_HT to Static.

If the response frame transmitted from the AP is responded in a form that occupies A20 but not P20, the non-AP STA can recognize that the AP is operating through A20 (Non-P channel operation state of FIG. 30). In this case, even if the response frame of the AP is not received through P20, the non-AP STA can continue the frame exchange sequence by utilizing the frequency band through which the AP transmitted the response frame.

After performing channel access via P20, a Non-AP STA that transmits an initial Control frame can use at least one of the following methods to determine a frequency band that it can utilize when performing a frame exchange sequence based on the response frame of the AP that responded via A20: (A) and (B). For the sake of example, the initial Control frame is described as an RTS frame, and the response frame is described as a CTS frame. However, a similar/identical method may be utilized for other frames/response frames.

A) Method based on the RXVECTOR parameter generated by the CTS frame transmitted by the AP and the location information of A20

A non-AP STA can determine the frequency band of a PPDU to be transmitted based on the RXVECTOR parameter CH_BANDWIDTH_IN_NON_HT of the CTS frame transmitted by the AP and the location information of A20. A specific method is to determine a channel with a BW equal to or smaller than a channel with a BW indicated by the RXVECTOR parameter CH_BANDWIDTH_IN_NON_HT among the channels including A20 as the transmission band of the PPDU. For example, if the RXVECTOR parameter CH_BANDWIDTH_IN_NON_HT of the received CTS frame is 80 MHz, the non-AP STA can transmit the next PPDU through A20 or a 40 MHz Channel including A20 or an 80 MHz Channel including A20. The RXVECTOR parameter generated by a frame received through A20 may be generated separately from the RXVECTOR parameter generated by a frame received through P20. That is, for a frame received in a form that does not occupy P20 but occupies A20, RXVECTOR parameters different from the RXVECTOR parameters of conventional Wi-Fi can be defined/utilized, and it is also possible for a new RXVECTOR parameter generated in relation to the BW of the corresponding frame to be utilized for the purpose of the present invention described above.

B) Method based on the CCA result confirmed when the response frame is received

A non-AP STA can expect that a CTS frame of the AP will be received after SIFS after transmitting its RTS frame, and thus can estimate the frequency band occupied by the CTS frame using the changed CCA result at the time when the CTS frame transmitted by the AP is expected to be received. In this case, the non-AP STA can transmit its next PPDU using all or part of the band occupied by the CTS frame to which the AP responded. As a simple example, a non-AP STA that has transmitted an RTS frame to the AP can estimate that the AP transmitted the CTS frame through the 80 MHz channel including A20 based on the fact that the CCA result of the 80 MHz band including A20 changed to BUSY at the time when the CTS frame is expected to be received from the AP. In this case, the non-AP STA can transmit its next PPDU through A20, the 40 MHz channel including A20, or the 80 MHz channel including A20.

Additionally, a non-AP STA that recognizes that the AP is operating in A20 (non-primary channel access operation in FIG. 30) can adjust its frame exchange sequence length so that the AP can switch to P20 at a scheduled time. In other words, the non-AP STA must adjust its TXOP length so that its TXOP ends earlier than the TXOP of the OBSS that occupies the P20 of the AP, thereby allowing the AP to switch to the P20 operation state (primary channel access operation in FIG. 30) before the TXOP of the OBSS ends.

However, a non-AP STA that has completed the channel access procedure through P20 cannot confirm the transmission of the OBSS that has occupied the AP's P20, and therefore cannot confirm when the OBSS's TXOP will end. Considering this, the AP can use the duration field of the response frame transmitted to the non-AP STA to instruct information related to the time during which the non-AP STA can operate the TXOP, and the non-AP STA can adjust its TXOP length (frame exchange sequence) based on the instructed information.

Specifically, an AP that receives an RTS frame from a non-AP STA sets the duration field of the CTS frame to indicate the maximum time point at which the non-AP STA can operate TXOP. The non-AP STA adjusts its TXOP length based on the time point indicated by the duration field of the CTS frame responded from the AP. At this time, the TXOP length adjustment method may be such that the TXOP is terminated at the same time point indicated by the duration field of the CTS frame, or may be adjusted such that the TXOP is terminated earlier than the time point indicated by the duration field of the CTS frame. That is, a non-AP STA that transmits an RTS frame over P20 and then receives a CTS frame responded to over A20 must adjust its TXOP length based on the time point indicated by the duration field of the CTS frame responded to by the AP, not the time point indicated by the duration field of the RTS frame that it transmitted. At this time, the duration field of the CTS frame responded from the AP may not be set based on the duration field of the RTS frame, but may be set based on the OBSS TXOP end time confirmed by the AP.

After performing channel access through P20, a non-AP STA that transmits an RTS frame may need to check whether the response frame of the AP received through A20 is a CTS frame or a CTS-to-self frame. The non-AP STA may check the RA field of the received frame to check whether the frame transmitted by the AP is a CTS-to-self frame or a CTS frame. A non-AP STA that confirms that the RA field of the response frame received from the AP is set to be identical to the TA field of the RTS frame transmitted by the non-AP STA can confirm that the received frame is a CTS frame, and a non-AP STA that confirms that the RA field of the received response frame is set based on the MAC address of the AP can confirm that the received frame is a CTS-to-self frame.

If the response frame of the AP confirmed by the non-AP STA is a CTS-to-self frame, the non-AP STA should not transmit its own UL PPDU after receiving the CTS-to-self frame, but should wait for reception of the next PPDU of the AP to be received in A20. That is, if it is confirmed that the response frame transmitted by the AP is a CTS-to-self frame, the non-AP STA that transmitted the RTS frame should switch to the operation using A20 (Non-P channel operation of FIG. 30). At this time, the non-AP STA maintains the operation state using A20 until the time indicated by the Duration field of the CTS-to-self frame and then switches to the P20 operation state (P-channel operation of FIG. 30), or switches to the P20 operation state when an explicit instruction is received from the AP.

That is, the AP can perform an explicit instruction to induce non-AP STAs maintaining the A20 operation state to switch to the P20 operation state. At this time, the way for the AP to perform the explicit instruction may be to transmit a CF-END frame. At this time, the way for the AP to perform the explicit instruction may be to transmit a CTS-to-self frame with the Duration field set to 0. At this time, the way for the AP to perform the explicit instruction may be to transmit a frame with the More Data field set to 0. At this time, the way for the AP to perform the explicit instruction may be to transmit a UHR PPDU with a specific subfield of the Preamble set to a specific value. In this case, the non-AP STAs that have confirmed the explicit instruction during the A20 operation must start switching to the P20 operation mode.

FIG. 32 illustrates an example of a channel access method according to reception of an RTS frame of an AP according to an embodiment of the present invention.

Figure 32 illustrates the medium state observed on the AP side. The AP checks the RTS/CTS frame transmitted by the STAs of the OBSS while performing the channel access procedure through P20. After checking the TXOP initiation of the OBSS, the AP moves to A20 located in the secondary 80 MHz band and continues the channel access procedure. At this time, the channel access procedure performed in A20 can be performed using a different contention window size and backoff counter from the channel access procedure performed in P20.

The AP receives the RTS frame transmitted to it while performing the channel access procedure in A20. The AP confirms that the received RTS frame was transmitted by an STA that does not support the NPCA operation and does not perform a CTS frame response. At this time, the AP can use the Capability information (which can be indicated through the UHR Capabilities element) transmitted by the STA to confirm whether the STA that transmitted the RTS frame supports the NPCA operation. If the RTS frame

In the embodiment of FIG. 32, the AP makes a decision to set (i.e., reset) the MediumSyncDelay timer to 0 by the received RTS frame. However, in another embodiment not shown, the RTS frame transmitted by the STA that performed the channel access at P20 may be restricted from being utilized to reset the MediumSyncDelay timer. That is, the STAs that applied MediumSyncDelay at A20 may be restricted from resetting their MediumSyncDelay when they receive the RTS frame transmitted through P20. This restriction is not limited to the embodiment described through FIG. 32 and may be a generally applicable restriction. In this case, the STAs that applied MediumSyncDelay at A20 may not utilize the RTS frame to reset the MediumSyncDelay timer, regardless of whether the received RTS frame was transmitted through P20.

Returning to the embodiment of Fig. 32, the AP does not perform a CTS frame response to the received RTS frame and continues the channel access procedure at A20. After completing the channel access procedure at A20, the AP obtains a TXOP for the band including A20.

Fig. 33 illustrates another example of a channel access method according to reception of an RTS frame of an AP according to an embodiment of the present invention. The description of the embodiment of Fig. 33 may omit the contents described through the previous embodiment.

After receiving the RTS frame from A20, the AP verifies that the STA that transmitted the RTS frame is an STA that supports NPCA and that the RXVECTOR parameter DYN_BANDWIDTH_IN_NON_HT generated by the RTS frame is Dynamic. The AP decides to perform a CTS frame response to the RTS frame and responds with a CTS frame through A20 and the subchannels accessible through A20 (subchannels of the Secondary 80 MHz band in FIG. 33). At this time, the Secondary 40 MHz band is not occupied by the OBSS TXOP and is a subchannel confirmed as IDLE as a result of the CCA, but access through A20 is not permitted, so the CTS frame response is not performed.

A non-AP STA that transmitted an RTS frame recognizes that the AP responded with a CTS frame through A20, and performs a frame exchange sequence with the AP using the subchannel to which the CTS frame was responded. However, the non-AP STA adjusts its TXOP length based on the time indicated by the Duration field of the CTS frame responded from the AP, not the time indicated by the Duration field of the RTS frame.

Through this, the frame exchange sequence between the non-AP STA that performed the channel access procedure through P20 and the AP operating in A20 is performed (TXOP is acquired).

Fig. 34 illustrates an example of a method for transmitting a CTS-to-self frame based on reception of an RTS frame of an AP according to an embodiment of the present invention. The description of the embodiment of Fig. 34 may omit the contents described through the previous embodiment.

P verifies that the RXVECTOR parameter DYN_BANDWIDTH_IN_NON_HT of the RTS frame received from A20 is set to Static, and transmits a CTS-to-self frame SIFS after the end of the RTS frame. That is, it initiates transmission of a CTS-to-self frame, not a CTS frame, at the same time as transmitting a CTS frame in response to an RTS frame.

When the AP determines the frequency band to transmit the CTS-to-self frame, it decides to transmit the CTS-to-self frame only for the subchannels identified as IDLE among the frequency bands accessible through A20. At this time, the Duration field of the CTS-to-self frame is set to a value indicating the end time or before the end time of the OBSS TXOP that occupies the AP's P20.

A non-AP STA that has transmitted an RTS frame to the AP checks whether the AP transmits a CTS frame or a CTS-to-self frame via A20. In the example of Fig. 34, since the AP transmitted a CTS-to-self frame via A20, the non-AP STA can recognize that the AP will control the subsequent frame exchange sequence, and therefore waits for reception of the frame that the AP transmits after the CTS-to-self frame instead of transmitting a UL PPDU.

<A20 channel connection procedure to reduce AP operation complexity>

As described in the embodiments of the present invention described above, an AP that receives an RTS frame in A20 can determine whether to respond with a CTS frame by checking whether the non-AP STA that transmitted the RTS frame is a non-AP STA that supports NPCA, or can make a decision such as transmitting a CTS-to-self frame based on the RXVECTOR of the RTS frame.

However, the AP must complete the above-described decision procedures within the inter-frame space (i.e., SIFS) between the received RTS frame and the next frame it transmits (CTS/CTS-to-self frame), which may result in somewhat increased operational difficulty for the AP.

As a way to reduce the operational complexity of the AP, there may be a way to allow the AP to adopt a consistent response behavior for RTS frames received over A20. That is, the AP may be allowed to always transmit the same response frame (CTS-to-self frame) for a received RTS frame (or other Control frames, e.g., TXOP Sharing (Request) frame), regardless of whether the non-AP STA that transmitted the RTS frame supports NPCA or the RXVECTOR parameter of the RTS frame.

According to one embodiment of the present invention, an AP performing a channel access procedure in A20 can transmit a CTS-to-self frame in response to an initial control frame (e.g., an RTS frame or another initial control frame) transmitted to itself. Accordingly, an initial Control frame transmitted from a non-AP STA that has completed a channel access procedure through A20 has a function of inducing transmission of a CTS-to-self frame by the AP, and acquisition of a TXOP by the non-AP STA may be considered as not occurring or as being transferred to the AP at the same time as acquisition. That is, a non-AP STA that has completed a channel access procedure in A20 transmits an initial Control frame to grant a TXOP to the AP.

As a result, TXOPs acquired from A20 are always managed by the AP, and each TXOP can be acquired by the AP itself (after completing backoff and accessing the channel) or acquired (shared) by the initial Control frame transmitted by a non-AP STA.

Through this process, the AP can initiate a frame exchange sequence (operate TXOP) after transmitting a CTS-to-self frame through A20 and a frequency band to which access via A20 is permitted among the frequency bands occupied by the received initial Control frame, without being restricted by conditions such as the type of non-AP STA that transmitted the initial Control frame and the frequency band occupied by the initial Control frame.

<Operational restrictions of AP operating in A20>

APs operating in A20 (performing channel access and CCA (PD, ED) over 20 MHz subchannels other than the primary 20 MHz channel) may be subject to restrictions that are not applicable when operating in P20. These restrictions are related to the behavior of unassociated STAs that are identified when operating in A20.

An AP operating in A20 can receive a Probe Request frame transmitted from A20. In this case, the AP must not respond with a Probe Response frame even if the Probe Request frame includes a wildcard SSID. This is because the AP that the non-AP STA that transmitted the Probe Request frame intends to scan is an AP that uses the subchannel corresponding to A20 as its primary 20 MHz subchannel, and the AP that received the Probe Request frame through A20 is not a scanning target. In addition, even if the AP responds to the Probe Request frame, when the non-AP STA that received the Probe Request frame transmits an Association Request frame, the AP may be in a state of switching to P20 operation, and the Association attempt of the non-AP STA is likely to fail.

For similar reasons, an AP operating in A20 may be restricted from allocating RA-RUs for Unassociated STAs via Trigger frames transmitted over A20. In this case, RA-RUs for Unassociated STAs mean RA-RUs allocated via the User Info field with AID set to 2045.

This is because the reason for allocating Unassociated RA-RU through Trigger frame is to assist the operation of Unassociated STA that wants to associate with AP that transmits Trigger frame, and it is considered inappropriate for AP that transmits Trigger frame through A20 to perform Association procedure with Unassociated STA. In other words, AP that transmits Trigger frame through A20 is likely to switch to P20 operation before Association with unassociated STA is completed, and therefore there is no reason to allocate RA-RU for unassociated STA. Therefore, a restriction is applied to AP that transmits Trigger frame through A20 that RA-RU for Unassociated STA should not be allocated.

<NPCA operation considering OBSS PPDU length and remaining TXOP>

As described above, an STA performing an NPCA operation can determine a method of performing the NPCA operation based on the remaining TXOP length of an OBSS occupying its primary 20 MHz subchannel. That is, the STA can decide to switch to an operation using the A20 subchannel when the remaining TXOP length of the OBSS exceeds (is equal to or greater than, or is greater than) a threshold value indicated by the AP, and to maintain an operation state using the primary 20 MHz subchannel when the remaining TXOP length of the OBSS is shorter than the threshold value indicated by the AP. In this case, the remaining TXOP length of the OBSS can be the sum of PPDU length information (a value calculated based on the length and rate fields) confirmed by the STA from the L-SIG field of the OBSS PPDU and the TXOP duration indicated by the TXOP field of the OBSS PPDU. At this time, the STA must compare the remaining time of the OBSS TXOP and the threshold value for NPCA at the time when it decides/judges whether to switch to A20 operation, taking into account the time already spent to check the length information of the PPDU (e.g., RL-SIG (4 us), U-SIG (or HE-SIG-A) (8 us), EHT/UHR-SIG (or HE-SIG-B), etc.). That is, the STA can decide to switch to A20 operation when "[the length of the OBSS PPDU indicated by the L-SIG field (the length of the OBSS PPDU calculated using the L-SIG field is the remaining OBSS PPDU length after the L-SIG field, and the actual OBSS PPDU length is 20 us longer than the value calculated using the length field and the rate field of the L-SIG field.)] + [TXOP duration indicated by the TXOP field of the OBSS PPDU] - [the time that has already elapsed until the point at which it performs comparison using the threshold for NPCA operation (since the end of the L-SIG field)]" is greater than the threshold for NPCA operation. At this time, the above [time that has already elapsed until the point in time when it performs comparison using the threshold value for NPCA operation (from the end of the L-SIG field)] may be the same as the time interval from the point in time when the L-SIG field of the OBSS PPDU ends to the point in time when the STA's MAC receives the RXVECTOR parameter (i.e., the point in time when the PHY issues the PHY-RXEND.indication).

That is, as described above, a non-AP STA can compare a value obtained by adding the duration of the PPDU to a value indicated by the TXOP field (or duration field) of the PPDU transmitted from the OBSS AP and the minimum duration threshold value in order to perform a channel access procedure on a non-primary channel. At this time, the duration of the PPDU can be obtained by the length field and the rate field of the PPDU, and the minimum duration threshold value can mean a minimum duration for a non-AP STA to perform a channel access procedure by switching channels from a primary channel to a non-primary channel.

Specifically, a non-AP STA can receive list information of OBSSs capable of performing a channel access procedure on a non-primary channel (e.g., BSS color information of an OBSS and/or MAC address information of an OBSS AP, etc.) through a management frame from an AP associated with the non-AP STA (this operation can be performed selectively). When the non-AP STA receives a PPDU from an OBSS AP, the non-AP STA determines whether the received PPDU is a PPDU transmitted from an AP associated with the non-AP STA (intra-BSS PPDU) or a PPDU transmitted from an OBSS AP (inter-BSS PPDU). If the PPDU is transmitted from an OBSS AP, the primary channel may be occupied by the OBSS AP and may be in a busy state. Therefore, since the non-AP STA cannot perform a channel access procedure on the primary channel, the non-AP STA can determine whether to perform a channel access procedure by channel switching to a non-primary channel. First, if a non-AP STA receives list information of OBSSs from an associated AP, the non-AP STA can compare the list information with the BSS color information or MAC address included in the received PPDU. If the list information includes the BSS color information or the MAC address of the AP that transmitted the PPDU, the non-AP STA can perform the NPCA procedure. However, if the list information does not include the BSS color information or the MAC address of the AP that transmitted the PPDU, the non-AP STA cannot perform the NPCA procedure. This process can be optionally performed when the non-AP STA receives list information of OBSSs from the AP, and is not an essential operation. Therefore, the non-AP STA may not perform the operation of checking the OBSS information. The non-AP STA can compare the value of the TXOP field (or duration field) included in the PPDU plus the duration of the PPDU with the minimum duration threshold value to check whether sufficient TXOPs remain for performing the NPCA operation. The duration of the PPDU can be obtained by the length field and the rate field of the PPDU. If the value of the TXOP field (or Duration field) plus the duration of the PPDU is greater than (or equal to or greater than) the minimum duration threshold, the non-AP STA may switch channels from the primary channel to the non-primary channel and perform a channel access procedure on the switched non-primary channel. However, if the value of the TXOP field (or Duration field) plus the duration of the PPDU is less than or equal to (or less than) the minimum duration threshold, the non-AP STA may not perform the NPCA operation.

The remaining remaining PPDU length from the time when the length of the PPDU was recognized may be utilized for Early NAV setting. That is, the Early NAV setting/update value may be the length of the remaining OBSS PPDU from the time when the length of the OBSS PPDU was recognized and the remaining OBSS TXOP length after the OBSS PPDU indicated by the TXOP field included in the OBSS PPDU.

In this way, the method of performing NPCA operation based on whether the remaining TXOP length of the OBSS is sufficient is intended to prevent (mitigate) inefficiency problems that may occur during the process of switching to A20 operation and then returning to P20 operation when the remaining TXOP length of the OBSS is short.

In the above-described embodiments, it has been explained that the method for the STA to check the remaining TXOP length of the OBSS is to utilize information of the TXOP field included in the Preamble of the OBSS PPDU (if the PPDU is a HE/EHT/UHR PPDU) or to utilize information of the Duration field of the MAC frame included in the OBSS PPDU (included in the MAC Header). However, the information indicated through the above-described fields indicates the length of the remaining TXOP from the time point at which the corresponding OBSS PPDU ends, and therefore, the time point at which the remaining time of the OBSS TXOP can be checked based on the information indicated through the fields is delayed to the time point at which the OBSS PPDU including the corresponding field ends. For example, if an OBSS TXOP having a length of 5 ms is initiated and the transmitted OBSS PPDU has a length of 3 ms, the TXOP field included in the OBSS PPDU indicates a time corresponding to 2 ms. This is because the remaining OBSS TXOP after the OBSS PPDU is terminated has a time length corresponding to 2 ms. Therefore, an STA that receives an OBSS PPDU and checks the TXOP field included in the OBSS PPDU must wait until the end of the OBSS PPDU to check the time point at which the information confirmed in the TXOP field, 2 ms, is applied, and when the end of the OBSS PPDU is confirmed, the STA can decide to switch to the A20 operation state after setting the Basic NAV value it maintains to a value corresponding to 2 ms. In this case, the STA can perform the A20 operation for only a part (2 ms in the example described above) of the total time (5 ms in the example described above) during which its primary 20 MHz subchannel is occupied by the OBSS TXOP, resulting in a problem that the expected benefit obtained through the NPCA operation is limited. In this case, the problem becomes more serious as the length of the OBSS PPDU itself including the TXOP field is longer. As an extreme example, if an OBSS TXOP with a length of 5 ms starts with an OBSS PPDU with a length of 5 ms, the TXOP field included in the OBSS PPDU will indicate a value equal to 0, and the STA will not be able to transition to an operating state via A20 and will remain in the P20 operating state until the OBSS TXOP terminates.

As mentioned above, the problem of NPCA operation inefficiency that occurs when the length of the OBSS PPDU itself is long is caused by the NAV setting method used by the conventional Wi-Fi MAC. The conventional Wi-Fi standard defined that after including a duration field in the header of the MAC frame, a device transmitting the frame is to indicate the length of the remaining TXOP remaining after the PPDU including the corresponding frame ends. (For reference, the duration field setting method defined in Wi-Fi includes single protection (a setting method for protecting a response frame) and multiple protection (a setting method for protecting TXOP), but since what the present invention is intended to deal with is the multiple protection method, the explanation will assume a duration field to which the multiple protection method is applied.)

An STA that is a TXOP holder indicates its remaining TXOP length after the PPDU including the corresponding frame is terminated through the duration field of the frame it transmits, and STAs that have received the frame, other than the destination device of the frame, set their NAVs using the values indicated through the duration field of the corresponding frame. The NAV of each STA set in this way is maintained as a non-zero value until the TXOP of the STA that is the TXOP holder is terminated, and this has the effect of blocking channel access of other STAs (i.e., the Virtual CS results of other STAs are maintained as BUSY) while the TXOP of the STA that is the TXOP holder is in progress. This is the TXOP introduced in Wi-Fi MAC and the MAC protection mechanism to protect it.

If we look a little more closely at how the MAC protection mechanism works, we can find the reason why the value indicated by the Duration field of the MAC frame is set to indicate the remaining TXOP length after the PPDU including the frame is terminated. The details are as follows. The frames transmitted by the TXOP holder are transmitted included in the PPDU, and STAs receiving the PPDU cannot decode the frames included in the PPDU until the reception of the PPDU is complete. In other words, STAs receiving the PPDU can confirm whether they are the destination of the frame included in the PPDU only after the reception of the PPDU is complete and the frame included in the PPDU is decoded. An STA that confirms that it is not the destination of the decoded frame sets the NAV based on the information indicated by the Duration field of the frame, but the time at which the information in the Duration field is acquired is when the reception of the PPDU is already complete (the PPDU transmission of the TXOP holder is terminated). The Wi-Fi standard considers that the point in time when STAs can obtain information (the point in time when they obtain the duration field information of the frame) that can set the NAV is after the transmission of the PPDU is completed (after the reception of the PPDU including the corresponding frame is completed). Therefore, it defines that the TXOP holder is to set the value indicated through the duration field of the frame to the remaining TXOP length after the end of the PPDU including the corresponding frame. Through this, STAs that have received the frame can use the value indicated through the duration field to set (update) the NAV without any additional calculation (without considering the length of the PPDU that they have received).

Meanwhile, the 11ax standard introduced PHY Preamble enhancement to solve the problem that STAs can check the destination device information of the frame included in the PPDU only after completing the reception of the PPDU. If STAs can check the destination device information of the frame after completing the reception of the PPDU, the STAs would have to continuously decode frames for which they are not the destination device, which would be inefficient. Therefore, if STAs can confirm that they are not the destination device of the frame before decoding the frame, the energy required for PPDU reception and frame decoding can be saved. Accordingly, the 11ax standard added the BSS Color and TXOP fields to the PHY Preamble of the HE PPDU, and configured the User Info field (included in the HE SIG field) that can check the destination device information of the DL PPDU to enable an STA that has checked the PHY Preamble of the HE PPDU to determine whether the corresponding PPDU includes a frame transmitted to it. The TXOP field included in the PHY Preamble of the HE PPDU is included to support STAs that have not decoded the frame included in the HE PPDU to set the NAV. When the STA MAC receives the PHY-RXEND.indication (a primitive that the STA's PHY indicates to the MAC at the end of the PPDU) related to the end of the HE PPDU, the STA sets the NAV based on the value indicated by the TXOP field of the corresponding PPDU. At this time, the information of the TXOP field included in the PHY preamble of the PPDU is indicated by the RXVECTOR parameter TXOP_DURATION that the STA's PHY indicates to the MAC. That is, the STA's MAC updates the NAV based on the RXVECTOR parameter TXOP_DURATION information indicated by the PHY when the PHY-RXEND.indication primitive is received from the PHY. At this time, the reason why the point in time when STA (STA's MAC) updates NAV is defined as when the PHY-RXEND.indication primitive is received from STA's PHY is to support performing Spatial Reuse operation before the completion of the HE PPDU transmitted by OBSS when it is confirmed, and to allow operation in a similar manner to the existing method of updating NAV through the duration field. In this case, the STA, which is the TXOP holder, can set the duration field of the frame it transmits and the TXOP field of the HE PPDU including the frame (more precisely, included in the PHY preamble (HE-SIG-A field)) to have the same/similar time values, which has the effect of reducing the operational complexity of the STA, which is the TXOP holder. In addition, among the STAs that have received the HE PPDU, the operations of the STA that sets the NAV based on the information indicated through the duration field of the frame included in the PPDU and the STA that sets the NAV based on the information indicated through the TXOP field included in the PHY preamble are managed to be consistent in terms of the timing of updating the NAV (after the reception of the PPDU is completed) and the NAV value to be updated. The TXOP field, which began to be included in the PHY preamble of the HE PPDU, has been greatly recognized for its advantage of supporting the STA to set the NAV without decoding the frame included in the PPDU, and is included in the PPDU of the generation after EHT. It is defined to be fixedly included in U-SIG (Universal-SIG). Therefore, it is highly likely that the TXOP field will be continuously included in PPDUs of not only EHT PPDU (PPDU of EHT, the 7th generation standard) and UHR PPDU (PPDU of UHR, the 8th generation standard), but also in PPDUs of subsequent generation standards.

With the aforementioned Wi-Fi MAC protection mechanism in mind, the NPCA operation inefficiency problem that occurs when the length of the aforementioned OBSS PPDU is long can be briefly summarized as follows. The values indicated through the TXOP field (PHY Preamble) and Duration field (of the MAC frame included in the PPDU) included in the OBSS PPDU are information on the length of the remaining TXOP remaining from the point in time when the OBSS PPDU ends, and when the length of the OBSS PPDU itself is long, the time indicated through the TXOP field and Duration field of the corresponding PPDU may differ from the actual duration of the OBSS TXOP. Since STAs performing the NPCA operation perform the operation in A20 only until the time when the OBSS TXOP ends (i.e., the time when the NAV set by the OBSS expires) and then return to the P20 operation, they can switch to the A20 operation after setting the NAV, that is, after completing the reception of the OBSS PPDU (after decoding the frame included in the PPDU or receiving the PHY-RXEND.indication primitive from the PHY) and setting the NAV. At this time, if the OBSS PPDU has a long length, the time when the STA performing the NPCA operation sets the NAV is delayed, and as a result, the time available for performing the A20 operation becomes shorter compared to the length of the OBSS TXOP.

Therefore, in order to increase the expected benefit of the NPCA operation, it is important to support the STA performing the NPCA operation to obtain information about the remaining length of the OBSS TXOP as quickly as possible, regardless of the length of the OBSS PPDU it has confirmed. In this case, the STA obtaining information about the remaining length of the OBSS TXOP means setting the NAV corresponding to the TXOP of the OBSS.

FIG. 35 illustrates an example of information acquired by a STA and a method for setting NAV based on reception of an OBSS PPDU according to an embodiment of the present invention.

FIG. 35 illustrates information that the MAC of a STA acquires and how to set the NAV when an OBSS TXOP initiated with a long OBSS PPDU is received at the PHY of the STA.

Referring to FIG. 35, the PHY of the STA detects a Long OBSS PPDU and then issues a PHY-RXSTART.indication primitive based on information acquired from the Preamble of the PPDU. The primitive has a configuration including an RXVECTOR, and among the RXVECTOR parameters, TXOP_DURATION indicates a value set based on the value of the TXOP field included in HE-SIG-A or U-SIG. After the MAC of the STA receives the RXVECTOR parameter TXOP_DURATION from the PHY, it waits until the expected end time of the PPDU and then sets the basic NAV. In the example of FIG. 35, the total length of the OBSS TXOP is longer than the NPCA Threshold, but the remaining time (y us) after the Long OBSS PPDU ends is a value smaller than the NPCA Threshold. The STA MAC decides not to switch to the A20 operation because the remaining time (remaining TXOP) checked through the RXVECTOR parameter TXOP_DURATION is less than the NPCA Threshold (short in time). As a result, the expected gain obtained by the STA by performing the NPCA operation is lost even though the OBSS TXOP is longer than the NPCA Threshold.

<How to set Early NAV for NPCA operation>

According to one embodiment of the present invention, an STA receiving an OBSS PPDU can set a NAV before the OBSS PPDU ends. In this case, the meaning of the STA receiving the OBSS PPDU does not mean decoding a frame included in the OBSS PPDU, but may mean acquiring information from the PHY preamble of the OBSS PPDU. In the case of STAs of the generation after 11ax, which is the 6th generation standard, when a PPDU classified as an OBSS PPDU is received based on BSS color information included in the received PPDU (RXVECTOR Parameter BSS_COLOR), PHY-RXEND.indication (Filtered) is immediately issued following PHY-RXSTART.indication and processing for the corresponding PPDU is stopped. That is, the meaning of the STA receiving the OBSS PPDU means performing the series of procedures described above.

Here is a brief description of how HE/EHT STA updates Basic NAV when OBSS PPDU is received. When a PPDU classified as OBSS PPDU (i.e., when RXVECTOR Parameter BSS_COLOR of the PPDU is different from the COLOR of the BSS that the STA is included in) is received at the STA side, the PHY of the STA issues PHY-RXSTART.indication including RXVECTOR and then sequentially issues PHY-RXEND.indication(Filtered). In this case, the MAC of the STA confirms that PHY-RXEND.indication(Filtered) has been received from the PHY, and sets (updates) Basic NAV at the expected end of the corresponding PPDU. At this time, the STA updates Basic NAV using the time length indicated by the RXVECTOR Parameter TXOP_DURATION of the corresponding PPDU.

In this way, when an OBSS PPDU is received, the STA does not process the OBSS PPDU to the end (such as decoding the frame), but waits until the expected time when the OBSS PPDU ends and then performs MAC protection by updating the Basic NAV only at the expected end time. However, as described above, since the STA performing the NPCA operation needs to evaluate the length of the remaining OBSS TXOP as soon as possible and then decide whether to switch to the A20 operation, it is inappropriate to use the conventional Basic NAV update method by OBSS PPDU as it is.

According to one embodiment of the present invention, the MAC of an STA that has received an OBSS PPDU (i.e., the STA) can update Basic NAV when PHY-RXEND.indication(Filtered) for the OBSS PPDU is received from the PHY (at the time of reception). At this time, the value of the Basic NAV that the STA updates can be (remaining time duration until the expected end of PPDU + duration indicated by TXOP_DURATION (RXVECTOR parameter of PPDU)). At this time, the 'remaining time until the expected end of PPDU' means a time interval from the time when PHY-RXEND.indication(Filtered) is received from the PHY to the expected end of the PPDU.

According to one embodiment of the present invention, it is possible for an STA to update/set a basic NAV before the expected end time of an OBSS PPDU, which means that the basic NAV is updated/set at an earlier time compared to a conventional Wi-Fi STA that updates/sets the NAV at the time when an OBSS PPDU ends (or is expected to end). For simplicity, in this specification, a Basic NAV set at an earlier time according to one embodiment of the present invention will be referred to as an Early NAV.

The operation of an STA that updates its Basic NAV (Early NAV) after receiving an OBSS PPDU and before the end of the OBSS PPDU is briefly summarized as follows. The STA updates its Early NAV to "remaining time duration until the expected end of PPDU + duration indicated by TXOP_DURATION (RXVECTOR parameter of PPDU)" when all of the conditions listed below are satisfied.

- RXVECTOR parameter TXOP_DURATION of OBSS PPDU is not UNSPECIFIED

- Not receiving frames included in OBSS PPDU (frames containing duration field)

- "The remaining time duration until the expected end of PPDU + the duration indicated by TXOP_DURATION (RXVECTOR parameter of PPDU)" is greater than the current basic NAV of the STA.

Meanwhile, as an additional condition, the operation of STA updating/setting basic NAV (Early NAV) before OBSS PPDU ends may be limited to when STA performs NPCA operation. In this case, the meaning of STA performing NPCA operation may mean that the NPCA operation mode of BSS including STA is activated and the NPCA operation mode of STA is also activated. In other words, it means that both AP and STA have performed indication meaning NPCA enable.

Another additional condition to be satisfied for Early NAV update/setting may be that the value indicated by the RXVECTOR parameter BSS_COLOR of the OBSS PPDU must be one of the values included in the OBSS color List indicated in advance by the AP. That is, the STA may be able to perform Early NAV setting only when a PPDU of an OBSS identified by the OBSS color List indicated in advance by the AP is received. This may be because a PPDU of an OBSS not identified by the OBSS color List indicated in advance by the AP is not an OBSS PPDU that causes a transition to A20 operation. That is, the STA can perform Early NAV update/setting and then perform NPCA operation (transition to A20 operation) only when it is confirmed that the OBSS PPDU received by the STA is a PPDU of an OBSS that is a target of NPCA operation indicated in advance by the AP.

According to the above-described method, an STA that has updated/set an Early NAV before the OBSS PPDU ends can determine whether to transition to the A20 operation based on whether the value of the Early NAV (Basic NAV) is greater than (or greater than or equal to) the NPCA Threshold (the minimum Basic NAV time length that allows transition to the A20 operation) indicated by the AP. At this time, the value of the Early NAV being greater than the Threshold means that the time taken until the expiration of the Early NAV is longer than the time length indicated by the Threshold. At this time, a specific method of determining whether to transition to the A20 operation using the Threshold is omitted because it has been described in detail through the above-described embodiments (embodiments that compare the length of the remaining OBSS TXOP with the Threshold to determine whether to transition to the A20 operation).

STAs performing the A20 operation can update/set the Basic NAV (Early NAV) they utilize when performing the P20 operation based on the information instructed from the AP. Specifically, the STAs can update/set the Basic NAV (Basic (Early) NAV set after receiving the OBSS PPDU in the above-described example) they utilize when performing the P20 operation after receiving the frame transmitted by their AP in A20. This is an operation related to the AP instructing the remaining time of the OBSS TXOP it has confirmed and the non-AP STAs performing the A20 operation based on this in the above-described example of the present invention. At this time, the non-AP STAs can adjust their A20 operation state maintenance interval to match the time interval during which the AP maintains the A20 operation state by setting/updating their Basic NAV based on the information instructed by the AP. However, non-AP STAs may not update/set/modify their own Basic NAV values if the value of their own Basic NAV is greater than the value of Basic NAV to be updated based on the information indicated from the AP. In other words, non-AP STAs may need to update their own Basic NAV only when the information indicated from the AP increases the value of their own Basic NAV.

The PHY of the UHR STA can include information in the RXVECTOR parameter to help the MAC calculate the expected end time of the OBSS PPDU, or can indicate the calculated expected end time of the OBSS PPDU with the RXVECTOR parameter. At this time, the indicated information can be information obtained through the L-SIG field of the PPDU (e.g., information of the Length field or the Rate field) or information on the length of the PPDU calculated based on the information included in the L-SIG field (e.g., the RXVECTOR parameter L_LENGTH of the conventional Wi-Fi standard or a new PPDU length-related RXVECTOR parameter). In addition, the indicated information can be indicated together with the RXVECTOR parameter of PHY-RXSTART.indication, or can be the RXVECTOR parameter indicated together when PHY-RXEND.indication(Filtered) is indicated. At this time, the method for PHY to calculate the expected end time of the PPDU based on the information included in the L-SIG field may be 'the length indicated by the Length field included in the L-SIG of the PPDU - (RL-SIG (4 us)) - (U-SIG (8 us) or HE-SIG-A (8 us or 16 us)) - (EHT/UHR-SIG length (variable us) or HE-SIG-B length)). At this time, some of the factors included in the above formula may be omitted during the calculation process. The MAC of the UHR STA can recognize the expected end time of the OBSS PPDU based on the information indicated from the PHY, and can set Early NAV by using the information together with the indicated TXOP_DURATION value. That is, the STA may need to utilize the remaining PPDU length from the time at which it recognized the length of the PPDU for Early NAV setting, considering the PPDU length information (a value calculated based on the Length and Rate fields) determined from the L-SIG field of the OBSS PPDU and the time already taken by the STA to determine the length information of the PPDU (e.g., RL-SIG (4 us), U-SIG (or HE-SIG-A) (8 us), EHT/UHR-SIG (or HE-SIG-B) etc.). That is, the Early NAV setting/update value may be the remaining OBSS PPDU length from the time at which it recognized the length of the OBSS PPDU and the OBSS TXOP length remaining after the OBSS PPDU indicated by the TXOP field included in the OBSS PPDU.

In addition, a UHR STA transmitting a HE/EHT/UHR PPDU may set the TXOP field included in the Preamble of the PPDU it transmits to a value indicating the end time of its TXOP, for the purpose of helping other STAs performing NPCA operation to set accurate Early NAV. That is, the UHR STA may set the TXVECTOR parameter TXOP_DURATION to a value indicating the end time of its TXOP. In addition, the UHR STA may be restricted from setting the TXVECTOR parameter TXOP_DURATION to UNSPECIFIED.

FIG. 36 illustrates an example of a method for setting up NAV and a method for using a subchannel when an STA receives an OBSS PPDU according to an embodiment of the present invention.

FIG. 36 illustrates an operation in which an STA sets an Early NAV before an OBSS PPDU ends and then switches to an operation using an A20 subchannel according to one embodiment of the present invention.

Referring to FIG. 36, the PHY of the STA detects a Long OBSS PPDU and then issues a PHY-RXSTART.indication primitive based on information acquired from the Preamble of the PPDU. At this time, the PHY of the STA verifies that the BSS Color included in the Preamble of the OBSS PPDU does not match the Color of the BSS that the STA is included in, and issues a PHY-RXEND.indication(Filtered) primitive following the PHY-RXSTART.indication primitive. The MAC of the STA can confirm that the PHY has detected/received the OBSS PPDU by verifying that the RXVECTOR parameter BSS_COLOR included in the PHY-RXSTART.indication primitive does not match the BSS color of the BSS to which it belongs. When the MAC of the STA receives PHY-RXEND.indication(Filtered) from the PHY, it updates/sets the Early NAV (Basic NAV). The STA sets Early NAV to the sum of the time indicated by the RXVECTOR parameter TXOP_DURATION and the time indicated by the RXVECTOR parameter REMAINING_PPDU_DURATION. The RXVECTOR parameter REMAINING_PPDU_DURATION is the expected time (or basic information required to calculate the expected time) until the expected end time of the PPDU that the PHY is detecting/receiving, and can be included in one of the two primitives above and indicated/transmitted from the PHY to the MAC. In the example of Fig. 36, the MAC of the STA compares the value of the set Early NAV with the NPCA Threshold and then decides to switch to the operation using the A20 subchannel.

<primitive for NPCA operation>

An STA performing an NPCA operation switches from an operation state using a primary 20 MHz subchannel (P20) to an operation state using an A20 (NPCA primary channel) subchannel, terminates the operation state using the A20 subchannel, and returns to the operation state using the P20. The decision to switch from the P20 operation to the A20 operation and the decision to switch from the A20 operation to the P20 operation are both performed according to the judgment of the STA's MAC, and when the STA's MAC decides to switch, the change of the operation channel must be performed by instructing the STA's PHY to do so.

The MAC of the STA that has decided to switch to the operating state using the A20 channel can issue a PHY-NPCASWITCH.request(PHYCONFIG_VECTOR) primitive to the PHY. At this time, the PHY-NPCASWITCH.request issued from the MAC to the PHY can include parameters (e.g., NPCA_PRIMARY_CHANNEL, NPCA_CHANNEL_WIDTH, NPCA_CENTER_FREQUENCY) that indicate information on the channel to be used for the A20 operation (operating channel and A20 operation primary channel information). That is, the PHY-NPCASWITCH.request(PHYCONFIG_VECTOR) issued by the MAC of the STA can include information that can identify the primary channel (A20 subchannel, NPCA primary channel) used when performing the A20 operation and information that can identify the operating channel (Operating channel) used when performing the A20 operation. At this time, the PHY-NPCASWITCH.request primitive may be the same primitive as the PHY-CONFIG.request primitive used in conventional Wi-Fi. That is, the MAC of the STA that changes the operating channel (from P20 to A20 or from A20 to P20) in relation to the NPCA operation can initiate the change of the operating channel of the PHY by issuing the PHY-CONFIG.request primitive.

Upon receiving the PHY-NPCASWITCH.request primitive, the PHY of the STA initiates a series of procedures to switch the basic operating channel from P20 (primary 20 MHz subchannel) to A20 subchannel. The method by which the PHY performs the above procedures may vary depending on the implementation of the device, and therefore a detailed description thereof is omitted. However, the switching procedure performed by the PHY must be completed within the NPCA switching delay that the STA instructed to the AP.

Upon receiving the PHY-NPCASWITCH.request primitive from the STA MAC, the STA PHY performs a switch to the A20 subchannel based on the information in the PHYCONFIG_VECTOR parameter of the primitive, and issues a PHY-NPCASWITCH.confirm primitive when the switch operation is completed. Upon receiving the PHY-NPCASWITCH.confirm primitive from the PHY, the STA MAC can recognize that the operating channel of the STA PHY has been switched to an operation based on the A20 subchannel.

The MAC of the STA that has decided to transition to the operation state using the A20 channel can issue a PHY-CCARESET.request primitive to the PHY. In this case, the PHY of the STA can evaluate the channel state of A20 as IDLE until PHY-CCA.indication(BUSY) occurs after transitioning to the A20 operation state. At this time, the time point at which the STA MAC issues the PHY-CCARESET.request primitive can be (immediately before or immediately after) when it issues the PHY-NPCASWITCH.request primitive or after the PHY-NPCASWITCH.confirm primitive is received from the PHY. At this time, the STA MAC issues the PHY-CCARESET.request primitive only when the AP indicates that channel access using EDCA on A20 is allowed (when untriggered UL transmission is allowed).

In addition, the MAC of the STA performs a decision to terminate the A20 operation and switch to the P20 operation, and the decision is performed before the Basic (Early) NAV set in the P20 expires. A specific timing for performing the decision may be when the time corresponding to the Basic (Early) NAV value set in the P20 is longer than or equal to its NPCA Switching back delay (delay required to switch from the A20 operation to the P20 operation). In other words, the STA must perform the P20 transition so that the transition to the P20 is completed before the Basic (Early) NAV set in the P20 expires.

Alternatively, the MAC of the STA may have to make a decision to terminate A20 operation and transition to P20 operation when an explicit instruction is received from the AP. That is, a non-AP STA that has been instructed by the AP to transition to P20 operation may have to decide to transition to P20 operation regardless of its Basic NAV value.

The MAC of the STA that decides to end the operation state using the A20 channel and switch back to the operation state using the P20 channel can issue the PHY-NPCASWITCHBACK.request(PHYCONFIG_VECTOR) primitive to the PHY. At this time, the PHY-NPCASWITCHBACK.request issued from the MAC to the PHY can include parameters indicating the operating channel and the primary channel. The NPCASWITCHBACK.request(PHYCONFIG_VECTOR) primitive can have the same configuration as the NPCASWITCH.request(PHYCONFIG_VECTOR) primitive described above. That is, the STA can issue the same primitive when it decides to switch from P20 to A20 and when it decides to switch from A20 to P20.

Alternatively, it is possible for the STA's MAC to have a configuration that does not include the primary PHYCONFIG_VECTOR parameter to instruct the PHY to switch to P20 operation. This is because the P20 operation is the basic operation of the STA, and therefore, various parameters for the P20 operation are already recognized (stored) in the STA's PHY. Accordingly, when the STA's MAC decides to switch to P20 operation, it can issue a PHY-NPCASWITCHBACK.request primitive that does not include a separate parameter, and the PHY that receives it can switch to P20 operation by utilizing various parameters that it already recognizes.

The PHY of the STA that receives the PHY-NPCASWITCHBACK.request primitive initiates a series of procedures to switch the default operating channel from A20 to P20. The method by which the PHY performs the above procedures may vary depending on the implementation of the device, so a detailed description is omitted. However, the switching procedure performed by the PHY must be completed within the NPCA switching back delay that the STA instructed to the AP.

The PHY of an STA that has completed switching to the P20 subchannel issues a PHY-NPCASWITCHBACK.confirm primitive when the switching operation is complete. When the PHY-NPCASWITCHBACK.confirm primitive is received from the PHY, the STA MAC can recognize that the operating channel of the STA PHY has been switched (restored) to operation based on the P20 subchannel.

The STA's MAC, which confirms that the transition to an operational state using the P20 channel is complete by receiving a PHY-NPCASWITCHBACK.confirm primitive from the PHY, issues a PHY-CCARESET.request primitive when its Basic NAV expires (reaches 0).

FIG. 37 illustrates an example of a primitive exchange procedure for channel access in a non-primary channel of an STA according to an embodiment of the present invention.

Referring to FIG. 37, the PHY of the STA detects the Preamble of the OBSS and then issues the PHY-RXSTART.indication primitive. The example of FIG. 37 assumes that the PPDU detected by the PHY is an OBSS PPDU in HE/EHT/UHR PPDU format, and thus the PHY-RXEND.indication primitive is generated together. The MAC that receives the PHY-RXSTART.indication and PHY-RXEND.indication primitives from the PHY sets the Basic NAV (Early NAV) based on the indicated RXVECTOR parameter and decides to switch to the A20 operation. The MAC that decides to switch to the A20 operation issues the PHY-NPCASWITCH.request primitive. At this time, the PHY-NPCASWITCH.request(PHYCONFIG_VECTOR) primitive has a configuration including channel information for A20 operation (such as the location of the A20 subchannel and operating channel information to be used when performing the A20 operation). The PHY that receives the PHY-NPCASWITCH.request primitive from the MAC performs a transition from the P20 operation to the A20 operation, and issues a PHY-NPCASWITCH.confirm primitive when the transition to the A20 operation is completed. The MAC that receives the PHY-NPCASWITCH.confirm primitive can recognize that the PHY has completed the transition for the A20 operation. At this time, the MAC can attempt to perform EDCA on the A20 subchannel by issuing a PHY-CCARESET.request primitive. The MAC of the STA issues the PHY-NPCASWITCHBACK.request primitive before the Basic NAV expires, and the PHY receiving it initiates the transition (return) from the A20 operation to the P20 operation. The transition (return) to the P20 operation performed by the PHY is completed before the expiration of the Basic NAV managed by the MAC. For this, the timing when the MAC issues the PHY-NPCASWITCHBACK.request primitive is managed. The PHY that has completed the transition to the P20 operation issues the PHY-NPCASWITCHBACK.confirm primitive, and the MAC receiving it recognizes that the PHY has completed the transition (return) to the P20 operation. The MAC initiates the CCA procedure (for the EDCA operation) on the primary 20 MHz subchannel by issuing the PHY-CCARESET.request primitive when the Basic NAV it was managing expires.

FIG. 38 is a flowchart showing an example of a channel access procedure performed by a terminal according to an embodiment of the present invention.

FIG. 38 shows that when an STA receives a PPDU transmitted from an OBSS AP on a primary channel, it can switch the channel from the primary channel to a non-primary channel based on whether a specific condition is satisfied, and perform a channel access procedure on the switched non-primary channel.

Specifically, a Physical Layer Protocol Data Unit (PPDU) can be received from an Overlapping Basic Service Set (OBSS) Access Point (AP) that is not associated with a wireless communication terminal (non-AP STA) on a primary channel of a bandwidth in which the wireless communication terminal operates (S38010). A preamble of the received PPDU can include a transmission opportunity (TXOP) field and a duration field related to a duration period set by the OBSS AP.

Thereafter, the non-AP STA may perform channel access by switching the channel from the primary channel of the bandwidth to the non-primary channel if a specific condition is satisfied (S38020). The specific condition is whether a specific value obtained based on the value indicated by the TXOP field or the duration field is greater than the minimum duration threshold value.

At this time, the state of the primary channel is busy because it is occupied by the OBSS AP by the PPDU transmitted by the OBSS AP, and the minimum duration threshold is the minimum value for the wireless communication terminal to perform the channel access by switching the channel to the non-primary channel.

A wireless communication terminal can receive a management frame from an associated AP, and the management frame includes list information of APs that can occupy the primary channel. At this time, the list information can include BSS color information and/or MAC address of each of the APs.

The above channel access on the non-primary channel can be performed when the primary channel is occupied by one of the APs.

A wireless communication terminal can compare the BSS color information or MAC address of the OBSS AP with the BSS color information or MAC address included in the above list information.

A specific value can be obtained based on a value indicated by the TXOP field or the duration field plus a switching delay for switching a channel from a primary channel to the non-primary channel.

The above description of the present invention is for illustrative purposes, and those skilled in the art will understand that the present invention can be easily modified into other specific forms without changing the technical idea or essential characteristics of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive. For example, each component described as a single component may be implemented in a distributed manner, and likewise, components described as distributed may be implemented in a combined form.

The scope of the present invention is indicated by the claims described below rather than the detailed description above, and all changes or modifications derived from the meaning and scope of the claims and their equivalent concepts should be interpreted as being included in the scope of the present invention.

Claims (18)

As a wireless communication terminal, Transmitter and receiver; and Contains a processor, The above processor, Receive a preamble of an inter-BSS PPDU (Physical Layer Protocol Data Unit) on a primary channel of a bandwidth in which the wireless communication terminal operates from an OBSS (Overlapping Basic Service Set) AP (Access Point) that is not associated with the wireless communication terminal, The preamble of the above inter-BSS PPDU includes a TXOP field related to the transmission opportunity (TXOP) duration set by the OBSS AP, When a specific condition is satisfied, the channel is switched from the primary channel of the above bandwidth to a non-primary channel. The above specific condition is a wireless communication terminal wherein the length of the remaining OBSS TXOP calculated based on the TXOP field or the duration field is greater than a minimum duration threshold value. In the first paragraph, A wireless communication terminal whose primary channel status is Busy by the OBSS AP. In the first paragraph, The wireless communication terminal is a wireless communication terminal whose minimum duration threshold is the minimum value for switching a channel to the non-primary channel. In the first paragraph, the processor, Receive a management frame from an AP associated with the above wireless communication terminal, The above management frame is a wireless communication terminal including list information of neighboring APs. In the fourth paragraph, The above list information is a wireless communication terminal including BSS color information and/or MAC address of each of the APs. In the fourth paragraph, A wireless communication terminal in which a channel switch to the above non-primary channel is performed when the primary channel is occupied by one of the above APs. In the fourth paragraph, the processor, A wireless communication terminal that compares the BSS color information or MAC address of the inter-BSS PPDU with the BSS color information or MAC address included in the list information. In the first paragraph, A wireless communication terminal, wherein the above specific condition is determined by additionally considering the channel switch delay of the wireless communication terminal in addition to the length of the remaining OBSS TXOP. In the first paragraph, the processor, A wireless communication terminal that transmits a primitive from the MAC layer to the PHY layer to perform the channel switch to the above non-primary channel. In a method performed by a wireless communication terminal, the method comprises: A step of receiving a preamble of an inter-BSS PPDU (Physical Layer Protocol Data Unit) on a primary channel of a bandwidth in which the wireless communication terminal operates from an OBSS (Overlapping Basic Service Set) AP (Access Point) that is not associated with the wireless communication terminal; The preamble of the inter-BSS PPDU includes a transmission opportunity (TXOP) field related to the duration set by the OBSS AP; and Including a step of switching a channel from the primary channel of the bandwidth to a non-primary channel when a specific condition is satisfied, The above specific condition is whether the length of the remaining OBSS TXOP calculated based on the TXOP field or the duration field is greater than a minimum duration threshold value. In Article 10, The state of the above primary channel is Busy by the above OBSS AP. In Article 10, The above minimum duration threshold is a method in which the wireless communication terminal switches a channel to the non-primary channel. In the 10th paragraph, the method, Further comprising a step of receiving a management frame from an AP associated with the above wireless communication terminal, A method in which the above management frame includes list information of neighboring APs. In Article 13, A method wherein the above list information includes BSS color information and/or MAC address of each of the APs. In Article 13, A method in which a channel switch to the above non-primary channel is performed when the primary channel is occupied by one of the above APs. In the 13th paragraph, the method, A method further comprising the step of comparing the BSS color information or MAC address of the inter-BSS PPDU with the BSS color information or MAC address included in the list information. In Article 10, A method for determining whether the above specific condition is satisfied by additionally considering the channel switch delay of the wireless communication terminal in addition to the length of the remaining OBSS TXOP. In the 10th paragraph, the method, A method further comprising the step of transmitting a primitive from the MAC layer to the PHY layer to perform the channel switch to the non-primary channel.

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