GB2567813A

GB2567813A - Improved station power saving during an 802.11ax multi-user TXOP having cascaded trigger frames

Info

Publication number: GB2567813A
Application number: GB1717323.8A
Authority: GB
Inventors: Sevin Julien; Viger Pascal; Baron Stéphane; Nezou Patrice
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2017-10-20
Filing date: 2017-10-20
Publication date: 2019-05-01
Anticipated expiration: 2037-10-20
Also published as: GB201717323D0; GB201802908D0; GB2567906B; GB2567813B; GB2567906A

Abstract

The invention provides improved power saving management at the stations STA1, STA2, STA3 of an 802.11ax network. When transmitting cascaded trigger frames TF#1, TF#2, TF#3, TF#4, the access point signals, for each scheduled resource unit whether the corresponding scheduled station will be offered either additional scheduled resource units or additional transmission opportunity through random resource units in the subsequent trigger frames of the same TXOP 590. The access point may signal in a Common Info field of the trigger frames whether further random resource units are expected in the subsequent trigger frames. The scheduled stations may read the Common Info field and decide to enter a doze mode early. Also, the stations remain awake as long as scheduled resource units are expected for them. This avoids wasting resource units. Also disclosed is assigning scheduled resource units to stations based on load information representing an amount of data to be sent by each station. Stations with low load are granted resource units early in the transmission opportunity TXOP 590.

Description

IMPROVED STATION POWER SAVING DURING AN 802.11 AX MULTI-USER TXOP HAVING

CASCADED TRIGGER FRAMES

FIELD OF THE INVENTION

The present invention relates generally to wireless communication networks comprising an access point (AP) and stations and more specifically to the management of station power, and corresponding devices.

The invention finds application in the management of power saving of 802.11ax stations during a multi-user transmission opportunity granted to the access point that provide cascaded communication periods triggered by respective trigger frames.

BACKGROUND OF THE INVENTION

The IEEE 802.11 MAC family of standards (a/b/g/n/ac/etc.) defines a way wireless local area networks (WLANs) work at the physical and medium access control (MAC) level over a 2.4 or 5 or 60 GHz frequency band. Typically, the 802.11 MAC (Medium Access Control) operating mode implements the well-known Distributed Coordination Function (DCF) which relies on a contention-based mechanism based on the so-called “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA) technique.

More recently, Institute of Electrical and Electronics Engineers (IEEE) officially approved the 802.11 ax task group, as the successor of 802.11ac. The primary goal of the 802.11ax task group consists in seeking for an improvement in data speed to wireless communicating devices (or stations) used in dense deployment scenarios.

In this context, multi-user (MU) transmission has been considered to allow multipie simultaneous transmissions to/from different stations (i.e. users) registered to the AP, in both downlink (DL) and uplink (UL) directions from/to the AP, during a transmission opportunity granted to the AP over a 20MHz (or more) communication channel.

In the uplink, multi-user transmissions are used to mitigate the collision probability. This is because multiple non-AP stations are allowed to transmit simultaneously.

To actually perform such multi-user transmission, it has been proposed to split a granted communication channel (or transmission opportunity granted to the AP) into subchannels, also referred to as resource units (RUs), that are usually shared in the frequency domain between multiple users (non-AP stations/nodes), based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

Both multi-user Downlink OFDMA and Uplink OFDMA mechanisms offer overhead reduction as key benefit.

To perform multi-user (MU) Uplink OFDMA transmission, the AP usually sends a control frame, known as Trigger Frame (TF), to the stations prior they can access the RUs. Some RUs, known as scheduled RUs, are assigned by the access point to respective specific stations. The assignment of scheduled RUs to the stations is signalled in a similar way as above (for Downlink transmission using AIDs), but >n the payload of the TF packet. Other RUs, known as random RUs, are offered by the AP to station contention, i.e. the access to the random RUs is made by the stations using contention.

As a station is usually provided with a single transceiver, assignment of multiple scheduled RUs to one and the same station shall not be allowed in 802.11ax, for both multiuser Downlink and Uplink transmissions. Of course a station with multiple transceivers could be assigned multiple scheduled RUs or use multiple RUs.

Thus, at most one RU is generally used by a station in the 802.11ax context. All the stations are offered the same RU length even if the RU width may vary from one station to the other.

To improve network efficiency, the access point may send consecutively multiple trigger frames during the same granted transmission opportunity, without contending each time for a new access to the communication channel. This offers multiple opportunities for the stations to have a resource unit to perform uplink transmissions.

This operating mode is known as trigger frame cascading.

Each cascaded trigger frame defines a period for uplink data transmission over the channel and announces whether or not other trigger frame or frames are following the uplink transmission period within the same transmission opportunity. As a trigger frame, it also defines a plurality of (scheduled and/or random) resource units forming the communication channel during the associated uplink transmission period.

Power saving is more than an issue in 802.11ax: it is a goal. In that respect, some features have been introduced to optimize power consumption at the stations (often batterypowered mobile devices) and save device battery. A main feature is to drive such devices to enter a doze or standby mode/state when no more data is to be exchanged.

Cascaded trigger frames during a transmission opportunity do not make the power saving management easy as the stations are not able to know in advance 'whether new resource units will be available for them during the remainder of the transmission opportunity data. The station thus has to remain in an active state as long as the transmission opportunity continues. This is very costly from power saving perspective, in particular for these stations that have no data to exchange,

A first solution is proposed in the current version 2.0 of the 802.11 ax standard. It is proposed to signal, in the current trigger frame, whether the following/subsequent cascaded trigger frame or frames within the granted transmission opportunity provide other random resource units or not.

The signalling is thus used by the stations to enter a doze mode for the remainder of the transmission opportunity, for instance if no more random RU is provided.

The signalling is made using a 1-bit signalling flag, known as “Random Access RU information” or “No further RA RU”, introduced at the 31^th bit position in the User Info field defining one random resource unit in the current trigger frame according to 802,11ax standard (version 2.0).

The No-further-RA-RU approach is not satisfactory.

First, as the scheduled stations stop parsing the User Info fields as soon as they find an RU assigned to them in the same current trigger frame, they never read the “No further RA RU” flag in the User Info field defining a random RU. Failing to have such information, the stations that are scheduled in the same trigger frame as the one conveying the “No further RA RU” flag never enter the doze mode during the transmission opportunity. This may be very costly from power saving perspective.

Second, the stations that are not scheduled in the trigger frame can enter the doze mode if no further random RU during the transmission opportunity is announced by the “No further RA RU” flag. However, the access point may schedule an RU for such an in-doze-mode station, in which case the latter is not able to use the scheduled RU. RUs are thus lost.

SUMMARY OF INVENTION

It is a broad objective of the present invention to efficiently improve this situation, i.e. to overcome some or all of the foregoing limitations. In particular, the present invention seeks to provide a more efficient power saving of the stations.

In particular, better signalling of further MU transmission opportunities for specific stations is provided.

In this context, the inventors propose enhanced station power saving methods in a wireless network comprising an access point and stations.

In embodiments, the method comprises, at the access point:

transmitting a plurality of cascaded trigger frames (over time) during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and signalling, using a signalling flag in a current trigger frame assigning a scheduled resource unit to a station during an associated communication period, when no subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station. Thus, it is signalled when such station is not assigned any other scheduled resource unit by any subsequent cascaded trigger frame within the granted transmission opportunity.

From station perspective, the method comprises, at one of the stations:

receiving, from the access point, a current trigger frame during a multi-user transmission opportunity granted to the access point, the current trigger frame indicating one or more cascaded trigger frames are subsequent to the current trigger frame within the transmission opportunity to provide further cascaded periods for data communication with the access point over a communication channel, each trigger frame defining a plurality of resource units forming the communication channel during an associated communication period;

identifying, in the current trigger frame, a scheduled resource unit assigned to the station for data communication;

retrieving, from the current trigger frame, a signalling flag indicating whether at least one subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station or not; and entering a doze mode depending on the retrieved signalling flag.

Thanks to the signalling flag provided by the AP for a respective scheduled station, the latter is able to determine more efficiently when no further MU transmission opportunity will be offered to it. As a consequence, the station can enter a doze mode early during the granted transmission opportunity, while avoiding wasting scheduled RUs.

In other embodiments, the method comprises, at the access point:

transmitting a plurality of cascaded trigger frames (over time) during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and signalling, using a 1-bit signalling flag in the Common Info field frame according to 802.11 ax standard (version 2.0) of a current trigger frame, whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention. Thus, it is signalled, at Common Info field level, when no further random RU is provided within the granted transmission opportunity.

From station perspective, the method comprises, at one of the stations:

receiving, from the access point, a current trigger frame during a multi-user transmission opportunity granted to the access point, the current trigger frame indicating one or more cascaded trigger frames are subsequent to the current trigger frame within the transmission opportunity to provide further cascaded periods for data communication with the access point over a communication channel, each trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and retrieving, from the Common Info field frame according to 802.11 ax standard of the current trigger frame, a 1-bit signalling flag indicating whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention; and entering a doze mode depending on the retrieved 1-bit signalling flag.

For instance, the 1-bit signalling flag may be positioned at the 64^th bit in the Common Info field frame.

By providing the signailing flag in the Common Info field of the 802,11 ax trigger frame, the proposed embodiments guarantee any station, scheduled or not, have access to the signalling flag. Furthermore, such a flag is also provided by the AP even if no random RU is defined in the trigger frame. This makes it possible to repeat this information in all subsequent trigger frames. Thus scheduled stations can also enter the doze mode during the transmission opportunity. Power saving is thus improved.

Also, there is provided a wireless communication device forming access point in a wireless network comprising an access point and stations. The device forming access point comprises at least one microprocessor configured for carrying out the steps defined above for the methods from AP perspective.

Also, there is provided a wireless communication device forming station in a wireless network comprising an access point and stations. The device forming station comprises at least one microprocessor configured for carrying out the steps defined above for the methods from station perspective.

Optional features of these embodiments are defined in the appended claims with reference to methods. Of course, same features can be transposed into system features dedicated to any device according to the embodiments of the invention.

In embodiments, the signalling flag is included in control information defining the scheduled resource unit assigned to the station from among the plurality of resource units defined in the current trigger frame. This approach helps providing, in a simple way, specific signalling to a given scheduled station. Indeed, as this control information is dedicated to that station, the targeted signalling according to the invention does not need to designate the station again.

In specific embodiments, the signalling flag is made of a 1-bit signalling flag positioned at the 40^fh bit in the User Info field defining the scheduled resource unit in the current trigger frame according to 802.11ax standard (e.g. in version 2,0). This approach avoids modifying the standard as the 40¹¹¹ bit is not yet used and thus is available for such signalling use.

In other embodiments, the signalling flag only indicates whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station. Thus, it is only signalled whether or not the station is not assigned any other scheduled resource unit within the remainder of the granted transmission opportunity.

In embodiments, the method further comprises, at the station, enabling a first status flag local to the station when the retrieved signalling flag indicates no subsequent cascaded trigger frame within the granted transmission opportunity assigns a scheduled resource unit to the station. This status flag in a memory local to the station makes it possible for the station to separately manage several criteria to be met to enter the doze mode.

In some embodiments, the method further comprises, at the access point, signalling, in one trigger frame of the granted transmission opportunity (it may be the current trigger frame), when no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention.

Based on such information, the stations, be scheduled by a previous trigger frame or not scheduled, can fully determine whether they will have another MU transmission opportunity during the granted TXOP.

In specific embodiments, signalling no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames use a single 1 -bit signalling flag. This optimizes signalling costs.

In that case, from station perspective, the station may enter the doze mode when (e.g. as soon as) the signalling flag indicates both no subsequent cascaded trigger frame assigns a scheduled resource unit to the station and no subsequent cascaded trigger frame defines one random resource unit the access of which being made by the stations using contention. This approach may use the single 1 -bit signalling flag provided by the AP.

In variants, signalling no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames use two respective 1bit signalling flags. This approach makes it possible to tune more finely the entering into doze mode. This is because the stations may obtain the two signalling flags from different trigger frames.

In that case, from station perspective, the method further comprises, at the station, retrieving, from a trigger frame received during the granted transmission opportunity (it may be the current trigger frame), a second signalling flag indicating whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention, wherein entering a doze mode further depends on the retrieved second signalling flag.

To memorize this information, the method may further comprise, at the station, enabling a second status flag local to the station when the retrieved second signalling flag indicates no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention, i.e. no further random resource unit will be provided by the AP during the remainder of the granted transmission opportunity. This status flag in a memory local to the station makes it possible for the station to separately manage two indications (no more random RU and no more scheduled RU) to enter the doze mode.

The second signalling flag may thus be retrieved from the received trigger frame when the second status flag is disabled.

In specific embodiments, the station enters the doze mode when (e.g. as soon as) the first and second status flags are enabled.

In some embodiments, the second signalling flag to signal no further random resource unit within the granted transmission opportunity is made of one from among:

a 1-bit signalling flag positioned at a bit position from the 27ⁱⁿ bit to the 3T^h bit in the User Info field defining one random resource unit in the current trigger frame according to 802.11 ax standard; and a 1-bit signalling flag positioned at the 64^!il bit in the Common Info field of the current trigger frame according to 802.11 ax standard.

The first proposed position complies with the actual version 2.0 of the 802.11ax standard, while the second proposed position can advantageously be read by any station, be it scheduled by the same trigger frame or not.

To take advantage of this improved station powering management, the method may further comprise, at the access point:

obtaining, for each station of a plurality of stations, load information representative of an amount of data to be transmitted by the station to the access point (preferably during the granted transmission opportunity); and assigning scheduled resource units to stations based on the obtained load information for the stations, for instance assigning scheduled resource units early in the granted transmission opportunity (i.e. through the first one or ones of the cascaded trigger frames) to stations having low load according to the obtained load information. Conventional mechanisms may be used by the AP to poll the stations regarding their buffer loads.

This approach aims at increasing the number of stations that can enter the doze mode early in the granted transmission opportunity. This offers a better power saving management of a fleet of stations.

Such approach may define, independently, other embodiments related to a method in a wireless network comprising an access point and stations, the method comprising, at the access point:

transmitting a plurality of cascaded trigger frames during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a scheduled resource unit assigned to a station during a data communication period, wherein an order of assigning scheduled resource units defined in the plurality of cascaded trigger frames to stations is based on an amount of data to be sent by said stations.

Same embodiments relate to a wireless communication device forming access point in a wireless network comprising an access point and stations, the device forming access point comprising at least one microprocessor configured for carrying out the steps defined above for the method.

Optional features may be contemplated.

For instance, scheduled resource units may be assigned in to stations having relatively low amount of data to transmit. Priority may mean here that other stations with high amount of data to transmit may not be assigned any scheduled resource unit in the TXOP at the end. Priority may also include the idea of providing early scheduled resource units within the TXOP.

In some embodiments, scheduled resource units are assigned in priority to stations that necessitate relatively a low number of cascaded trigger frames to transmit their corresponding amount of data.

In specific embodiments, scheduled resource units are assigned first to stations that can transmit their corresponding amount of data in a resource unit of only one trigger frame.

In other embodiments, the amount of data is based on load information obtained by the access point from the stations.

In yet other embodiments, the multi-user transmission opportunity corresponds to a Target Wake up Time Service Period according to 802.11ax standard.

The invention also relates to a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a device, causes the device to perform any method as defined above.

The non-transitory computer-readable medium may have features and advantages that are analogous to those set out above and below in relation to the methods and devices.

At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, microcode, etc.) or an embodiment combining software and hardware aspects that may ail generally be referred to herein as a circuit, module or system. Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.

Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible carrier medium may comprise a storage medium such as a hard disk drive, a magnetic tape device or a solid state memory device and the like, A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g. a microwave or RF signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will become apparent to those skilled in the art upon examination of the drawings and detailed description. Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings.

Figure 1 illustrates a typical wireless communication system in which embodiments of the invention may be implemented;

Figure 2 illustrates 802.11ac channel allocation that support channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz as known in the art;

Figure 3 illustrates an example of 802.11 ax uplink OFDMA transmission scheme, wherein the AP issues a Trigger Frame for reserving a transmission opportunity of OFDMA subchannels (resource units) on an 80 MHz channel as known in the art;

Figure 4 depicts various fields composing a trigger frame;

Figure 5 illustrates, through an exemplary situation of MU data transmission in a WI_AN, drawbacks of the power saving management at the stations according to a recent resolution taken within the 802.11ax standard;

Figure 6 shows a schematic representation a communication device in accordance with embodiments of the present invention;

Figure 7 shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention;

Figure 8 illustrates, using a flowchart, general steps for an access point to build trigger frames according to embodiments of the invention;

Figure 9 illustrates, using a flowchart, general steps for the access point to generate a cascaded sequence of trigger frames according to embodiments of the invention;

Figure 10a illustrates, using a flowchart, steps for the access point to generate a “no further scheduled RU” indicator according to embodiments of the invention;

Figure 10b illustrates, using a flowchart, steps for the access point to generate a “no further transmission opportunity” indicator according to embodiments of the invention;

Figure 10c illustrates, using a flowchart, steps for the access point to generate a “no further random RU” indicator according to embodiments of the invention;

Figure 11 illustrates, using a flowchart, general steps of a power saving management at a station based on signalling provided by the access point, according to embodiments of the invention;

Figure 12 illustrates, using a flowchart, general steps of a power saving management at a station based on signalling provided by the access point, according to other embodiments of the invention; and

Figure 13 illustrates the benefits of using the newly proposed signalling to improve power saving management at the non-AP stations.

DETAILED DESCRIPTION

The invention will now be described by means of specific non-limiting exemplary embodiments and by reference to the figures.

Figure 1 illustrates a communication system in which several communication nodes (or stations) 101-107 exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN), under the management of a central station, or access point (AP) 110. The radio transmission channel 100 is defined by an operating frequency band constituted by a single channel or a plurality of channels forming a composite channel.

Access to the shared radio medium to send data frames is based on the CSMA/CA technique, for sensing the carrier and avoiding collision by separating concurrent transmissions in space and time.

Carrier sensing in CSMA/CA is performed by both physical and virtual mechanisms. Virtual carrier sensing is achieved by transmitting control frames to reserve the medium prior to transmission of data frames.

Next, a source or transmitting station, including the AP, first attempts through the physical mechanism, to sense a medium that has been idle for at least one DIFS (standing for DCF InterFrame Spacing) time period, before transmitting data frames.

However, if it is sensed that the shared radio medium is busy during the DIFS period, the source station continues to wait until the radio medium becomes idle.

To access the medium, the station starts a countdown backoff counter designed to expire after a number of timeslots, chosen randomly in a contention 'window range [0, CW], CW (integer) being also referred to as the Contention Window size and defining the upper boundary of the backoff selection interval (contention 'window range). This backoff mechanism or procedure is the basis of the collision avoidance mechanism that defers the transmission time for a random interval, thus reducing the probability of collisions on the shared channel. After the backoff time period, the source station may send data or control frames if the medium is idle.

One problem of 'wireless data communications is that it is not possible for the source station to listen while sending, thus preventing the source station from detecting data corruption due to channel fading or interference or collision phenomena. A source station remains unaware of the corruption of the data frames sent and continues to transmit the frames unnecessarily, thus wasting access time.

The Collision Avoidance mechanism of CSMA/CA thus provides positive acknowledgement (ACK) of the sent data frames by the receiving station if the frames are received with success, to notify the source station that no corruption of the sent data frames occurred.

The ACK is transmitted at the end of reception of the data frame, immediately after a period of time called Short InterFrame Space (SIFS).

To meet the ever-increasing demand for faster wireless networks to support bandwidth-intensive applications, 802.11ac and later versions (802.11ax for instance) implement larger bandwidth transmission through multi-channel operations. Figure 2 illustrates an 802.11ac channel allocation that supports composite channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz.

IEEE 802.11ac introduced support of a restricted number of predefined subsets of 20MHz channels to form the sole predefined composite channel configurations that are available for reservation by any 802.11ac (or later) station on the wireless network to transmit data.

The predefined subsets are shown in the Figure and correspond to 20 MHz, 40 MHz, 80 MHz, and 160 MHz channel bandwidths, compared to only 20 MHz and 40 MHz supported by 802.11η. Indeed, the 20MHz component channels 200-1 to 200-8 are concatenated to form wider communication composite channels.

In the 802.11ac standard, the channels of each predefined 40MHz, 80MHz or 160MHz subset are contiguous within the operating frequency band, i.e. no hole (missing channel) in the composite channel as ordered in the operating frequency band is allowed.

The 160 MHz channel bandwidth is composed of two 80 MHz channels that may or may not be frequency contiguous. The 80 MHz and 40 MHz channels are respectively composed of two frequency adjacent or contiguous 40 MHz and 20 MHz channels, respectively. However the present invention may have embodiments with either composition of the channel bandwidth, i.e. including only contiguous channels or formed of non-contiguous channels within the operating band.

A station (including the AP) is granted a transmission opportunity (TxOP) through the enhanced distributed channel access (EDCA) mechanism on the “primary channel” (200-3). Indeed, for each composite channel having a bandwidth, 802.11ac designates one channel as “primary” meaning that it is used for contending for access to the composite channel. The primary 20MHz channel is common to ali stations (STAs) belonging to the same BSS, i.e. managed by or registered with the same local Access Point (AP),

However, to make sure that no other legacy station (i.e. not belonging to the same set) uses the secondary channels, it is provided that the control frames (e.g, RTS frame/CTS frame or trigger frame described below) reserving the composite channel are duplicated over each 20MHz channel of such composite channel.

The IEEE 802.11ac standard enables up to four, or even eight, 20 MHz channels io be bound. Because of the limited number of channels (19 in the 5 GHz band in Europe), channel saturation becomes problematic. Indeed, in densely populated areas, the 5 GHz band will surely tend to saturate even with a 20 or 40 MHz bandwidth usage per Wireless-LAN cell.

Developments in the 802.11ax standard seek to enhance efficiency and usage of the wireless channel for dense environments.

In this perspective, one may consider multi-user (MU) transmission features, allowing multiple simultaneous transmissions to different users in both downlink (DL) and uplink (UL) directions, once a transmission opportunity has been reserved and granted to the AP. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple non-AP stations to simultaneously transmit to the AP.

To actually perform such multi-user transmission, it has been proposed to split a granted 20MHz channel (200-1 to 200-4) into at least one sub-channel, but preferably into a plurality of sub-channels 310 (elementary sub-channels), also referred to as sub-carriers or resource units (RUs) or “traffic channels”, that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

This is illustrated with reference to Figure 3.

The multi-user feature of OFDMA allows the AP to assign different RUs to different stations in order to increase competition within a reserved transmission opportunity TXOP. This may help to reduce contention and collisions inside 802.11 networks.

In this example, each 20 MHz channel (200-1, 200-2, 200-3 or 200-4) is subdivided in the frequency domain into four OFDMA sub-channels or RUs 310 of size 5MHz. Of course the number of RUs splitting a 20 MHz channel may be different from four, and the RUs may have different sizes. For instance, between two to nine RUs may be provided (thus each having a size between 10 MHz and about 2 MHz). It is also possible to have a RU width greater than 20 MHz, when included inside a wider composite channel (e.g. 80 MHz).

Regarding the MU uplink transmissions, the AP must control when and how (in which RU) the stations must emit data.

A trigger mechanism has been adopted for the AP to trigger MU uplink communications from various non-AP stations. This is for the AP to have such control on the stations.

To support a MU uplink transmission (during a TXOP pre-empted by the AP), the 802.11ax AP has to provide signalling information for both legacy stations (i.e. non-802.11ax stations) to set their NAV and for 802,11 ax stations to determine the Resource Units allocation.

In the following description, the term legacy refers to non-802.11ax stations, meaning 802,11 stations of previous technologies that do not support OFDMA communications.

As shown in the example of Figure 3, the AP sends a trigger frame (TF) 330 to the targeted 802.11 ax stations to reserve a transmission opportunity. The bandwidth or width of the targeted composite channel for the transmission opportunity is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is signalled.

The TF frame is a control frame, according to the 802.11 legacy non-HT format, and is sent over the primary 20MHz channel and duplicated (replicated) on each other 20MHz channels forming the targeted composite channel. Due to the duplication of the control frames, it is expected that every nearby legacy station (non-HT or 802.11ac stations) receiving the TF on its primary channel, then sets its NAV to the value specified in the header of the TF frame. This prevents these legacy stations from accessing the channels of the targeted composite channel during the TXOP.

Based on an AP’s decision, the trigger frame TF may define a plurality of resource units (RUs) 310. The multi-user feature of OFDMA allows the AP to assign different RUs to different stations in order to increase competition. This may help to reduce contention and collisions inside 802.11 networks.

The information about the RU distribution in the requested transmission opportunity and about the assignment of stations to the RUs is indicated in the payload of the MAC frame. Indeed, the MAC payload is basically empty for classical control frames (such as RTS or CTS frame), but is enhanced with an information structure for Trigger Frames: an RU allocation field defines the allocated RUs (i.e. RU distribution in the TXOP) while one or more User Info fields indicates the information related to each respective RU (in the same order as provided by the RU allocation info field). In particular, the Address field in each User Info field provides the AID of the station to which the corresponding RU is assigned.

These various fields are shown in Figure 4 described below.

The trigger frame 330 may define “Scheduled” RUs, that may be reserved by the AP for certain stations in which case no contention for accessing such RUs is needed for these stations. Such scheduled RUs and their corresponding scheduled stations are indicated in the trigger frame (the Address field of the User Info field for the scheduled RU is set to the AID of the station). This explicitly indicates the station that is allowed to use each Scheduled RU. Such transmission mode is concurrent to the conventional EDCA mechanism.

If a station finds that there is no User Info field for Scheduled RUs in the Trigger frame 330 carrying its AID in its Address field, then the station should not be allowed to transmit in a Scheduled RU of the TXOP triggered by the TF.

The trigger frame TF 330 may also designate “Random” RUs, in addition to or in replacement of the “Scheduled” RUs. The Random RUs can be randomly accessed by stations. In other words, Random RUs designated or allocated by the AP in the TF may serve as basis for contention between stations willing to access the communication medium for sending data. A collision occurs when two or more stations attempt to transmit at the same time over the same RU.

Such random RUs are signalled in the TF by using specific reserved AID in the Address field of the User Info field corresponding to the RU. For instance, an AID equal to 0 is used to identify random RUs available for contention by stations associated with the AP emitting the trigger frame.

Note that several random RUs with AID=0 may be provided by the same TF.

A random allocation procedure may be considered for 802.11 ax standard based on an additional backoff counter (OFDMA backoff counter, or OBO counter or RU counter) for random RU contention by the 802.11ax non-AP stations, i.e. to allow them for performing contention between them to access and send data over a Random RU. The RU backoff counter is distinct from the classical EDCA backoff counters (as defined in 802.11e version). However data transmitted in an accessed OFDMA RUs 310 is assumed to be served from same EDCA traffic queues.

The RU random allocation procedure comprises, for a station of a plurality of 802.11ax stations having an positive RU backoff value (initially drawn inside an RU contention window range), a first step of determining, from a received trigger frame, the RUs of the communication medium available for contention (the so-called “random RUs” identified by AID=0), a second step of verifying if the value of the RU backoff value local to the considered station is not greater than the number of detected-as-available random RUs, and then, in case of successful verification, a third step of randomly selecting a RU among the detected-asavailable RUs to then send data. In case the second step is not verified, a fourth step (instead of the third) is performed in order to decrement the RU backoff counter by the number of detected-as-available random RUs.

As one can note, a station having no Scheduled RU is not guaranteed to perform OFDMA transmission over a random RU for each TF received. This is because at least the RU backoff counter is decremented upon each reception of a Trigger Frame by the number of proposed Random RUs, thereby deferring data transmission to a subsequent trigger frame (depending of the current value of the RU backoff number and of the number of random RUs offered by each of further received TFs).

Back to Figure 4 showing the various fields forming the Trigger Frame 330 to declare the RUs. Reference 400 indicates the Trigger Frame 330. It includes a single Common Info field 410 and a plurality of User Specific fields 420 each dedicated to a specific RU defined in the Common Info field 410.

The single Common Info field 410 contains a Cascade Indication field 411 (the 17^fhbit in the Common Info field, noted B16) the use of which is described below, and a 1-bit Reserved field (the 64^th bit in the Common Info field, noted B63) 412 used in embodiments of the present invention. This Reserved field 412 is referred below as “Reserved Common Info” field.

The other fields of the Common Info field 410 are of less importance here.

The User Specific fields 420 define information related to each RU of the trigger frame 330.

Each User Specific field 420 includes:

an AID field 423 which indicates the AID of the station to which the corresponding RU is assigned. The value of the AID field 423 is set to 0 for the random RUs, the access of which being made using contention;

an RU allocation field 424 which indicates the RU concerned, in particular the position and the width of the RU within the communication channel;

a 1-bit Reserved field (the 40^th bit in field 420, noted B39) 421 used by the present invention. This Reserved field is referred below as “Reserved User Info” field; and a field called “SS Allocation/Random Access RU Information” 422 (6 bits). The meaning of this field depends on the type of the corresponding RU. If the RU is a scheduled RU, field 422 corresponds to “SS Allocation” field and indicates the spatial streams used (6-bit information). If the RU is a random RU, field 422 corresponds to “Random Access RU Information” and only one bit is used, referred to as “No further RA RU”.

Based on the resource distribution provided in the Common Info field and each corresponding User Specific field, a station can determine which resource unit it can directly use (scheduled RU) or access through contention (random RU).

Back to Figure 3, it results from the various possible accesses to the RUs that some of them are not used (31 Ou) because no station with an RU backoff value less than the number of available random RUs has randomly selected one of these random RUs, whereas some other RUs have collided (as example 310c) because at least two of these stations have randomly selected the same random RU. This shows that due to the random determination of random RUs to access, collision may occur over some RUs, while other RUs may remain free.

Once the stations have used the RUs to transmit data to the AP, the AP responds with an acknowledgment frame (not show in the Figure) to acknowledge the data received. As for the other control frames, the acknowledgment frame is duplicated over each 20MHz channel if necessary. Preferably, the acknowledgment frame performs a block acknowledgment, meaning that it acknowledges simultaneously reception of data transmitted over a plurality (e.g. all) of the RUs.

802.11ax provides a priority to the Access Point to cascade several MU Uplink transmission periods within the pre-empted TXOP.

Figure 5 illustrates this option given by the standard. To do so, the AP has to send successively separate trigger frames 330-1 to 330-4 to trigger successive MU Uplink transmission periods 580-1 to 580-4,

The AP first gains access to the communication medium through conventional EDCA and pre-empt a new transmission opportunity TXOP 590. Then it can send a first TF 3301 to trigger a first MU Uplink transmission period 580-1. During period 580-1, some stations access the RUs and upload their data to the AP (uplink transmission 550-1) and the AP acknowledges reception thereof (570-1).

Next, the AP relaxes the communication medium but immediately, after a PIFS time period, sends another and second TF 330-2 to trigger a second MU Uplink transmission period 580-2. The PIFS time period is less than the DIFS duration necessary for the stations managed by the AP to start a new contention procedure. It ensures that the AP keeps control of the communication channel. That is why, for the stations, the same transmission opportunity TXOP is continuing.

During period 580-2, some stations access the RUs and upload their data to the AP (uplink transmission 550-2) and the AP acknowledges reception thereof (570-2).

Next, third and fourth MU Uplink transmission periods 580-3 and 580-4 may be triggered by the AP within the same TXOP 590, using the same mechanism (TFs 330-3 and 330-4 after a PIFS time period).

In this cascaded scheme, the AP transmits a plurality of cascaded trigger frames 330-1 to 330-4 (over time) during a multi-user transmission opportunity TXOP 590 granted to the access point to provide cascaded periods 580-1 to 580-3 for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period.

The intention of the AP to send successive trigger frames 330-1 to 330-4 within the same TXOP 590 (i.e. using a priority by possibly waiting only for a single first PIFS duration) can be specified in the trigger frames 330-1 to 330-4 using the Cascade Indication field 411 introduced above with reference to Figure 4. The Cascade Indication makes it possible for the stations to keep listening to the medium even if they do not access RUs of a previous trigger frame.

As shown in Figure 5, the Cascade Indication (Cl) takes the value 1 when one or more subsequent trigger frames 330 are envisioned in the same TXOP 590, while it takes the value 0 for the last trigger frame 330-4 in the TXOP.

The usage of the “No further RA RU” bit 422 in case of random RUs is now explained still with reference to Figure 5. This bit is used by the AP to indicate whether the subsequent trigger frames in the current TXOP 590 will contain random RUs or not. If yes (subsequent TFs are expected), its value is 1; otherwise it is 0.

First trigger frame 330-1 contains a User Info field indicating one scheduled RU 551-1 for station STA1, a User Info field indicating one scheduled RU 552-1 tor station STA2 and a User info field indicating a random RU 553-1.

Moreover, the field “No further RA RU” 422 associated with random RU 553-1 is set to zero, meaning that subsequent trigger frames (here 330-2) will indicate one or more random RUs. Only STA3 can read this “No further RA RU” 422 as STA1 and STA2 stop parsing the User Info fields as soon as they have identified their own scheduled RUs.

Depending on the UL OFDMA-based random access scheme used, station STAS may access the random RU (STA1 and STA2 have their own scheduled RUs).

Consequently, as shown in the time lines 510-520-530, station STA1 sends a UL MU PPDU (uplink multi-user PPDU) inside the scheduled RU 551-1, station STA2 sends a UL MU PPDU inside the scheduled RU 552-1 and station STA3 sends a UL MU PPDU inside the random RU 553-1.

In response to the data transmission, the AP sends a Multi-User Block Acknowledgment frame 570-1 to acknowledge the data received on each RU.

At this stage, STAS does not enter a doze mode as it may be interested by the one or more random RUs announced through the “No further RA RU” information 422. STA1 and

STA2 cannot be concerned by the doze mode as they do not read the “No further RA RU” information 422.

Next, second trigger frame 330-2 (announced through the Cascade Indication Cl of TF 330-1) contains a User Info field indicating one scheduled RU 551-2 for station STA2, a User Info field indicating one scheduled RU 552-2 for station ST A3 and a User Info field indicating a random RU 553-2.

Moreover, the field “No further RA RU” 422 associated with random RU 553-2 is now set to 1, meaning that no further random RU will be proposed by the subsequent trigger frames during TXOP 590. Only STA1 can read this “No further RA RU” 422 as STA2 and STA3 stop parsing the User Info fields as soon as they have identified their own scheduled RUs.

Depending on the UL OFDMA-based random access scheme used, station STA1 may access the random RU (STA2 and STA3 have their own scheduled RUs). In the present example, STA1 is not allowed to access the random RU.

Consequently, as shown in the time lines 510-520-530, station STA2 sends a UL MU PPDU (uplink multi-user PPDU) inside the scheduled RU 551-2 and station STAS sends a UL MU PPDU inside the scheduled RU 552-2.

In response to the data transmission, the AP sends a Multi-User Block Acknowledgment frame 570-2 to acknowledge the data received on each RU.

At this stage, according to the 802.11 standard, STA1 can enter a doze mode as it is aware that no more random RU will be provided in the remainder of TXOP 590 (“No further RA RU” information 422 set to 1). For instance STA1 actually enters the doze mode up to the end of TCOP 590, as shown by arrow 591. STA2 and STA3 cannot be concerned by the doze mode as they do not read the “No further RA RU” information 422 embedded in TF 330-2.

Next, third trigger frame 330-3 (announced through the Cascade Indication Cl of TF 330-2) is only received by STA2 and STA3 (STA1 being sleeping).

TF 330-3 contains a User Info field indicating one scheduled RU 551-3 for station STA1 and a User Info field indicating one scheduled RU 552-3 for station STA2.

As station STA1 is sleeping (doze mode), only station STA2 actually sends a UL MU PPDU (uplink multi-user PPDU) inside the scheduled RU 552-3. In response to the data transmission, the AP sends a Multi-User Block Acknowledgment frame 570-3 to acknowledge the data received on the RUs,

Thus scheduled RU 551-3 is wasted.

No further “No further RA RU” information 422 being provided in TF 330-3 (because it defined no random RU), STA3 cannot enter a doze mode if wished.

Next, fourth and final trigger frame 330-4 (announced through the Cascade Indication Cl of TF 330-3) is only received by STA2 and STA3.

TF 330-4 contains a User Info field indicating one scheduled RU 551-4 for station STA1 and a User Info field indicating one scheduled RU 552-4 for station STA2.

As above, station STA1 being sleeping (doze mode), only station STA2 actually sends a UL MU PPDU (uplink multi-user PPDU) inside the scheduled RU 552-4. In response to the data transmission, the AP sends a Multi-User Block Acknowledgment frame 570-4 to acknowledge the data received on the RUs.

Thus scheduled RU 551-4 is again wasted.

To summarize, station STA1 enters the doze mode at a wrong instant, causing waste of scheduled RLJs.

Station STA2 does not enter a doze mode. This is an appropriate behaviour as it receives multiple scheduled RUs during all TXOP 590.

Station STA3 does not enter a doze mode, but this does not appear optimal. Indeed, STA3 should have entered the doze mode prior to both MU Uplink transmission periods 580-3 and 580-4 because it was never offered scheduled RUs during these periods. Thus power has been lost in STA3 due to a weak power saving strategy.

The present invention seeks to overcome such drawbacks of the known power saving managing schemes.

To that end, embodiments of the present invention proposes to signal, using a signalling flag in a current trigger frame 330 assigning a scheduled resource unit 310 to a station during an associated communication period 580, when no subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station. The embodiments thus signal when such station is not assigned any other scheduled resource unit by any subsequent cascaded trigger frame within the granted transmission opportunity.

This makes it possible for the station to decide to enter the doze mode only if no more scheduled RU will be offered in the current TXOP. As a result, no scheduled RU should be wasted.

Also, embodiments provide to signal, in one trigger frame of the granted transmission opportunity (it may be the current trigger frame), when no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention. Combined with the first signalling, the stations can enter the doze mode for power saving reasons only when they are sure no more opportunity to send data will be offered to them. This approach optimizes the saving of power at the stations while efficiently using the medium.

Some embodiments also provide that signalling no further random RU (similarly to the known “No further RA RU” information 422) can be made using a 1-bit signalling flag in the Common Info field frame according to 802.11ax standard (version 2.0) of a current trigger frame. By providing such signalling at Common Info field level, all the stations, be them scheduled or not by the current trigger frame, are become aware of the absence of further random RU within the TXOP. They can thus enter the doze mode earlier, compared to prior art.

Figure 6 schematically illustrates a communication device 600, either a non-AP station 101-107 or the access point 110, of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 600 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 600 comprises a communication bus 613 to which there are preferably connected:

« a central processing unit 611, such as a microprocessor, denoted CPU;

« a read only memory 607, denoted ROM, for storing computer programs for implementing the invention;

» a random access memory 612, denoted RAM, for storing the executable code of methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing methods according to embodiments of the invention; and « at least one communication interface 602 connected to the radio communication network 100 over which digital data packets or frames or control frames are transmitted, for example a wireless communication network according to the 802.11ax protocol. The frames are written from a FIFO sending memory in RAM 612 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 612 under the control of a software application running in the CPU 611.

Optionally, communication device 600 may also include the following components:

« a data storage means 604 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention;

» a disk drive 605 for a disk 606, the disk drive being adapted to read data from the disk 606 or to write data onto said disk;

« a screen 609 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 610 or any other pointing means.

The communication device 600 may be optionally connected to various peripherals, such as for example a digital camera 608, each being connected to an input/output card (not shown) so as to supply data to the communication device 600.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 600 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 600 directly or by means of another element of the communication device 600.

The disk 606 may optionally be replaced by any information medium such as for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, a USB key or a memory card and, in general terms, by an information storage means that can be read by a microcomputer or by a microprocessor, integrated or not into the apparatus, possibly removable and adapted to store one or more programs whose execution enables a method according to the invention to be implemented.

The executable code may optionally be stored either in read only memory 607, on the hard disk 704 or on a removable digital medium such as for example a disk 606 as described previously. According to an optional variant, the executable code of the programs can be received by means of the communication network 603, via the interface 602, in order to be stored in one of the storage means of the communication device 600, such as the hard disk 604, before being executed.

The central processing unit 611 is preferably adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to the invention, which instructions are stored in one of the aforementioned storage means. On powering up, the program or programs that are stored in a non-volatile memory, for example on the hard disk 604 or in the read only memory 607, are transferred into the random access memory 612, which then contains the executable code of the program or programs, as well as registers for storing the variables and parameters necessary for implementing the invention.

In a preferred embodiment, the apparatus is a programmable apparatus which uses software to implement the invention. However, alternatively, the present invention may be implemented in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC).

Figure 7 is a block diagram schematically illustrating the architecture of the communication device 600, either the AP 110 or one of stations 101-107, adapted to carry out, at least partially, the invention. As illustrated, device 700 comprises a physical (PHY) layer block 703, a MAC layer block 702, and an application layer block 701.

The PHY layer block 703 (here an 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20MHz channel or the composite channel, and thus sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance medium access trigger frames TF 330 (Figure 3) to reserve a TXOP, MAC data and management frames based on a 20MHz width to interact with legacy 802,11 stations, as well as of MAC data frames of OFDMA type having smaller width than 20MHz legacy (typically 2 or 5 MHz) to/from that radio medium.

The MAC layer block or controller 702 preferably comprises a MAC 802.11 layer 704 implementing conventional 802.11ax MAC operations, and additional block 705 for carrying out, at least partially, the invention. The MAC layer block 702 may optionally be implemented in software, which software is loaded into RAM 612 and executed by CPU 611.

Preferably, the additional block 705, referred to as power saving module, to provide appropriate signalling from AP perspective or to use such signalling at the stations to enter a doze mode when appropriate as explained below according to embodiments of the invention.

When entering the doze mode, the power saving module may drive a switch off of the MAC 802.11 layer 704 (and other module of the station) in order to save power.

On top of the Figure, application layer block 701 runs an application that generates and receives data packets, for example data packets of a video stream. Application layer block 701 represents all the stack layers above MAC layer according to ISO standardization.

Embodiments of the present invention are now illustrated using various exemplary embodiments in the context of IEEE 802.11 ax by considering cascaded MU Uplink transmission periods, each offering multiple OFDMA RUs.

Figures 8 to 10 illustrate operations at the access point, e.g. AP 110 of Figure 1, to provide an enhanced signalling for the station to perform efficient power saving, in particular to enter a doze mode when appropriate.

In the context of MU Uplink transmissions, the AP sends a cascaded sequence of trigger frames 330 during a granted TXOP 590.

First, the AP draws (810) a table or list of scheduled and random resource units that may be useful or required by the stations. For instance, this may be based on load information obtained, for each station of a plurality of stations, that is representative of an amount of data to be transmitted by the station to the access point. For instance, the AP may poll each station to know their transmission buffer load.

The TXOP may correspond to a TWT SP (Target Wake up Time Service Period) as defined in the 802.11ax standard. In that case, the plurality of stations is formed of the stations that will wake up for this service period (as agreed with the AP previously).

The AP can then organise the required scheduled and random RUs in the cascaded sequence of trigger frames defining the next granted TXOP. It can then build a list of RUs (with associated scheduled station 'when appropriate) for the whole TXOP.

With a view of ordering the RUs in the list, the AP may preferably assign scheduled resource units to stations based on the obtained load information, for instance in priority to stations having low load, for instance early in the transmission opportunity (i.e. at the beginning of the list, corresponding to the first one or ones of the cascaded trigger frames) to stations having low load according to the obtained load information. This is to have these stations early in the doze mode to optimize power saving.

The ordered list may then be split for instance by taking into account RU constraints, such as a maximum of 9 RUs per each 20MHz channel. This forms n sub-lists:, list RU-i . list RU₂,.,., list RU_n - n being the number of cascaded trigger frames to be sent during the TXOP (list RU, corresponds to the list of RUs to be defined by the I^th trigger frame 330-i).

Next, at step 820, the AP generates the cascaded trigger frames 330-1 to 330-n using corresponding sub-lists list RU-, to list_RU_n. This step is described below with reference to Figure 9. Next, at step 830, the cascaded trigger frames are provided to the MAC 802.11 layer of the AP with a view of transmitting them over the communication channel.

Figure 9 thus illustrates, using a flowchart, exemplary steps for the AP to generate (step 820) a cascaded sequence of trigger frames according to embodiments of the invention. The process of this Figure includes a loop over each cascaded trigger frame.

First, at step 910, the generation of a first trigger frame 330-1 is initiated. In particular, its Common Info field 410 is generated.

The Cascade Indication field 411 is set to 0 if it is a single trigger frame to be sent in the next granted TXOP. Otherwise, the Cascade Indication field 411 is set to 1.

The “Reserved Common Info” field 412 defined above is initialized to 0. This 1-bit field is used, according to embodiments of the invention, to convey a signalling flag signalling whether or not further random RUs are to be defined in the subsequent trigger frames cascaded in TXOP 590. Field 412 is thus liable to be modified during a next step, for instance step 930.

Next, at step 920, a first RU of the current RU list iist__RU_k (selected either at step 910 or at step 970 for generating the k^th trigger frame 330-k) is selected.

Next, at step 930, the User Info field 420 for the RU selected at step 920 is generated.

A different processing is made depending on whether the selected RU is a scheduled RU or a random RU.

In case the RU selected at step 920 is a scheduled RU assigned to a scheduled station, a signalling flag is preferably provided in the corresponding User Info field 420 to signal when no subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station., i.e. when no further scheduled RU will be provided for the scheduled station. The signalling flag is thus included in control information defining the scheduled resource unit assigned to the station from among the plurality of resource units defined in the current trigger frame.

Figures 10a and 10b illustrate the provision of such a signalling flag as a 1-bit signalling flag positioned at the 40^fh bit of the User Info field defining the scheduled resource unit in the current trigger frame according to 802.11ax standard (e.g. in version 2,0), i.e. as the “Reserved User Info” field 421 defined above.

Figure 10a illustrates general steps for generating the “Reserved User Info” field 421 of a scheduled RU according to a first embodiment where the signalling flag 421 only indicates whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station.

In detail, the process starts at step 1010 by checking whether the scheduled RU is the last scheduled RU assigned to the scheduled station during the TXOP 590 or not. This check may consist in analyzing all the scheduled RUs throughout the subsequent RU lists, i.e. from list RU_k+1 to list RU_n.

If no further scheduled RU assigned to the scheduled station is found in those RU lists, the check 1010 is negative and next step is step 1011 during which the “Reserved User Info” field 421 is set to 1 in the User Info field 420 of the considered scheduled RU. The AP thus signals “No further scheduled RU” for the scheduled station. The latter, upon receiving such information, will be able to adapt its power saving strategy, and to enter a doze mode if necessary as explained below.

Otherwise, if at least one scheduled RU assigned to the scheduled station is found in those RU lists, the check 1010 is positive and next step is step 1012 during which the “Reserved User Info” field 421 is set to 0 in the User Info field 420 of the considered scheduled RU. The AP thus signals that the scheduled station will have another scheduled RU in the remainder of TXOP 590 (i.e. during MU UL period 580-k+1 to 580-n). Upon receiving such information, the scheduled station will thus not decide to enter the doze mode.

In a variant, Figure 10b illustrates general steps for generating the “Reserved User Info” field 421 of a scheduled RU according to a second embodiment. In this embodiment, the access point also signals when no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit, and signalling no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames use a single 1-bit signalling flag, namely the Reserved User Info” field 421.

In detail, the process starts at step 1030 similar to step 1010 above by checking whether the scheduled RU is the last scheduled RU assigned to the scheduled station during the TXOP 590 or not. This check may consist in analyzing all the scheduled RUs throughout the subsequent RU lists, i.e. from list__RU_k+1 to list__RU_n.

If no further scheduled RU assigned to the scheduled station is found in those RU lists, the check 1030 is negative and next step is step 1031. The latter consists for the AP in checking whether or not a random RU will be provided in the subsequent MU UL periods 580k+1 to 580-n, i.e. a random RU is defined in the subsequent trigger frames within TXOP 590. This check may consists in analyzes all the RUs throughout the subsequent RU lists, i.e. from list RUk+i to list RUn, in order to find at least one random RU.

If no random RU is found, the check 1031 is negative and next step is step 1032 during which the “Reserved User Info” field 421 is set to 1 in the User Info field 420 of the considered scheduled RU. The AP thus signals “No further transmission opportunity” for the scheduled station (including no more scheduled RU but also no more random RU). The scheduled station, upon receiving such information, will be able to adapt its power saving strategy, and to enter a doze mode if necessary as explained below.

Otherwise, if at least one random RU is found, the check 1031 is positive and next step is step 1033 during which the “Reserved User Info” field 421 is set to 0 in the User Info field 420 of the considered scheduled RU. The AP thus signals that the scheduled station will have another opportunity to transmit (here through a random RU, although the scheduled station will not be able to infer it) within the remainder of TXOP 590 (i.e. during MU UL period 580-k+1 to 580-n). Upon receiving such information, the scheduled station will thus not decide to enter the doze mode.

If no further scheduled RU assigned to the scheduled station is found at step 1030 in the subsequent RU lists, the check 1030 is positive and next step is step 1033 already described. The “Reserved User Info” field 421 is set to 0. However in that case it signals that the scheduled station will have another opportunity to transmit but here through a new scheduled RU (although the scheduled station will not be able to infer it).

A specific process may be applied in case the RU selected at step 920 is a random RU, if the AP wishes to signa! separately the situation where no further random RU will be offered in the remainder of TXOP 590. In this situation, signalling no other scheduled resource unit (using for instance Figure 10a) and signalling no further random resource unit in the subsequent cascaded trigger frame or frames thus use two respective 1-bit signalling flags.

Figure 10c illustrates the provision of such second signalling flag to signal no further random resource unit within the granted transmission opportunity.

This second signalling flag may be a 1-bit signalling flag in the User Info field 420 corresponding to the random RU, e.g. the “Reserved User Info” field 421 positioned at the 40^!nbit in the User Info field 420 or the “No Further RA RU” bit positioned at a bit position from the 27^th bit to the 31^th bit in the User Info field.

in a variant, this second signalling flag may be a 1-bit signalling flag in the Common Info field 410, for instance the Reserved Common Info” field 412 positioned at the 64^th bit.

In detail, the process starts at step 1050 by checking whether it is the last random RU for TXOP 59 or not, i.e. whether or not a random RU will be provided in the subsequent MU UL periods 580-k+1 to 580~n (or subsequent trigger frames). This check may consists in analyzes ail the RUs throughout the subsequent RU lists, i.e. from list_RU_k+i to iist_RU_n, in order to find at least one random RU.

If no random RU is found, the check 1050 is negative and next step is step 1051 during which the second signalling bit is set to 1, in first embodiments, this is either the “Reserved User Info” field 421 or the “No further RA RU” field 422 in the User Info field 420 of the considered random RU that is set to 1. In that case, the AP thus signals “No further random RU” for all non-scheduied stations. The non-scheduied station, upon receiving such information, will be able to adapt its power saving strategy, and to enter a doze mode if necessary as explained below.

In second embodiments, this is the “Reserved Common Info” field 412 that is set to 1. In that case, the AP thus signals “No further random RU” for all stations (scheduled or not). The station, upon receiving such information, will be able to adapt its power saving strategy, and to enter a doze mode if necessary as explained below.

Otherwise, if at least one random RU is found, the check 1050 is positive and next step is step 1052 during which the second signalling flag defined above is set to 0. The AP thus signals that the stations will have one or more random RUs to transmit data within the remainder of TXOP 590 (i.e. during MU UL period 580-K+1 to 580-n). Upon receiving such information, the scheduled station will thus not decide to enter the doze mode.

Once the User Info field 420 and appropriate signalling have been generated (using Figure 10a, 10b and/or 10c), the AP goes to step 940 where it checks whether or not the RU considered is the last one in the current RU list list_RU_k. If no, next step is step 950 where the AP selects the next RU in the current RU list list RU_k and loops back to step 930 to build the corresponding User Info field 420.

Otherwise, next step is step 960 where the AP checks whether or not the trigger frame currently generated is the last trigger frame to be generated for TXOP 590. It may consist in verifying whether or not list_RU_k is the last RU list (i.e. whether k n or not).

In the affirmative, next step is 980 where the process ends. Otherwise, next step is step 770 where the AP switches to the next trigger frame (i.e. to the next RU list: k=k+1). Similar to step 910, the generation of the next trigger frame 330-k is initiated; its Common Info field 410 is generated.

It may be noted that its Cascade Indication field 411 is set to 1 if one or more subsequent trigger frames are scheduled. Otherwise (the current trigger frame is the last one in TXOP 590), the Cascade Indication field 411 is set to 0.

Figures 11 and 12 illustrate operations at any station according to two embodiments. Figure 11 regards the case where two separate signalling flags are used to signal no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames (e.g, when Figure 10a and 10c are used together). Figure 12 regards the case where a single signalling flag is used (also as Figure 10b).

Figure 11 illustrates, using a flowchart, exemplary operations done by a station (non-AP) to enter a doze mode according to a first embodiment of the invention. These operations can be made upon receiving each new trigger frame during the TXOP as long as the station does not enter the doze mode.

As explained above with reference to Figure 10a, the AP may have signalled, in each User Info field 420 corresponding to a scheduled RU, that another or no further scheduled RU will be assigned to the same scheduled station in the subsequent TFs of TXOP 590, This is made using for instance the 1 -bit “Reserved User Info” field 421.

As explained above with reference to Figure 10c, the AP may also have signalled whether another or no further random RU will be provided by the subsequent TFs.

In this example, each station locally (in local memory) maintains two status flags (e.g. 1-bit flags) to store over time (during a given TXOP 590) whether another scheduled RU is announced for the station within the same TXOP 590 or not, or whether another random RU is announced within TXOP 590 or not, respectively. The first status flag is denoted “No further scheduled RU” status flag and the second status flag is denoted “No further random RU” status flag. These local status flags help the station in deciding to enter the doze mode.

These two status flags are initially disabled, i.e, set to 0 or false.

The process starts at step 1105 when a given station receives a trigger frame 330.

At step 1110, the station checks whether, in the current trigger frame, a scheduled resource unit is assigned to the station for data communication. This amounts to check whether a User Info field 420 in the trigger frame has an AID 423 equal to the station AID.

In the affirmative, next step is step 1115 where the station retrieves, from the current trigger frame, a signalling flag indicating whether at least one subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station or not. This may be done by retrieving the “No further scheduled RU” field set during the process of Figure 10a, e.g. the “Reserved User info” filed 421 for the identified User Info field 420.

The value of this field 421 is then checked at step 1120.

If this field 421 indicates another scheduled RU is planned in the subsequent trigger frames of TXOP 590 (field 421 is false or 0), the station shall not enter the doze mode and the process ends (step 1150). The reception process of 802.11ax can go on as defined in the IEEE standard 802.11 ax.

If this field 421 indicates no further scheduled RU is planned in the subsequent trigger frames of TXOP 590 (field 421 is true or 1), the station enables the first status flag local to the station, i.e. it sets the “No further scheduled RU” status flag to true or 1, at step 1125.

Next, at step 1130, the station checks 'whether the “No further random RU” status flag (second status flag) is enabled (i.e. set to true or 1) or not. This is to determine whether the AP has already announced (in a previous TF in the same TXOP 590) that no further random RU will be provided in TXOP 590. This status flag is updated at step 1165 described below.

in the affirmative, no further opportunity will be given to the station (no further scheduled RU and no further random RU). Consequently, the station can enter a doze mode, at step 1135, until the end of TXOP 590.

in the negative, the station has to determine whether the current trigger frame announces or not a further random RU in the subsequent trigger frames of TXOP 590, This is checked at step 1140, in this step, the station retrieves, from the trigger frame, a second signalling flag indicating whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one (or more) random resource unit. This may be done by retrieving the “No further random RU” field set during the process of Figure 10c. In an embodiment, this field is stored in the “Reserved Common Info” field 412 positioned at the 64^thbit in the Common Info field 410 of the trigger frame. In another embodiment, it is stored in the “Reserved User Info” field 421 positioned at the 40^th bit in a User Info field 420 dedicated to a random RU (i.e. with AID = 0) in the current trigger or in the “No Further RA RU” bit positioned at a bit position from the 27^th bit to the 31^th bit in this User Info field 420. In that case, if no random RU is found in the current trigger frame, the “No further random RU” field is considered to be false.

Thanks to step 1130, the second signalling flag is retrieved from the received trigger frame at step 1140 only when the second status flag is disabled.

Next step is step 1145 where the retrieved “No further random RU” field is checked.

If this retrieved field indicates another random RU is planned in the subsequent trigger frames of TXOP 590 (field is false or 0), the station shall not enter the doze mode and the process ends (step 1150).

If this retrieved field indicates no further random RU is planned in the subsequent trigger frames of TXOP 590 (field is true or 1), no further opportunity will be given to the station (no further scheduled RU detected at 1120 and no further random RU detected at 1145). Consequently, the station can enter a doze mode, at step 1135, until the end of TXOP 590.

Thus, entering a doze mode depending on the two retrieved 1 -bit signalling flags.

In particular, the station enters the doze mode when (e.g. as soon as) the first and second status flags are enabled.

Back to test 1110, in the negative of this test (i.e. no scheduled RU is assigned to the station performing the process), next step is step 1155 similar to step 1140 in order to retrieve the “No further random RU” field set during the process of Figure 10c.

The retrieved “No further random RU” field is checked at step 1160 similar to step 1145.

if this retrieved field indicates no further random RU is planned in the subsequent trigger frames of TXOP 590 (field is true or 1), the station enables the second status flag local to the station, i.e, it sets the “No further random RU” status flag to true or 1, at step 1165,

Next, at step 1170, the station checks whether the “No further scheduled RU” status flag (first status flag) is enabled (i.e. set to true or 1) or not. This is to determine whether the AP has already announced (in a previous TF in the same TXOP 590) that no further scheduled RU will be provided to the station during TXOP 590. This status flag is updated at step 1125 described above.

In the affirmative, no further opportunity will be given to the station (no further scheduled RU detected at 1120 and no further random RU detected at 1160). Consequently, the station can enter a doze mode, at step 1135, until the end of TXOP 590.

In the negative, the station still considers that a further scheduled RU may be assigned to it in the subsequent trigger frames of TXOP 590. The process then ends (step 1150).

Figure 12 illustrates, using a flowchart, exemplary operations done by a station (non-AP) to enter a doze mode according to a second embodiment of the invention. These operations can be made upon receiving each new trigger frame during the TXOP as long as the station does not enter the doze mode.

As explained above with reference to Figure 10b, the AP may have signalled no other scheduled resource unit for the station and no further random resource unit in the subsequent cascaded trigger frames using a single 1-bit signalling flag, namely the Reserved User Info” field 421 of the User Info field 420 corresponding to a scheduled RU assigned to the station in the current trigger frame. These exemplary operations do not use status flags, contrary to the first embodiment above.

Similar to the process of Figure 11, the process starts at step 1210 when a given station receives a trigger frame 330. At step 1220, the station checks whether, in the current trigger frame, a scheduled resource unit is assigned to the station for data communication. This amounts to check whether a User Info field 420 in the trigger frame has an AID 423 equal to the station AID.

In the negative, no signalling according to the second embodiment is provided by the AP to the station. As a result, the station is not aware whether or not other opportunities will be offered to it before the end of TXOP 590. The process thus ends (step 1260).

In the affirmative, next step is step 1230 where the station retrieves, from the current trigger frame, a signalling flag indicating both no subsequent cascaded trigger frame (during TXOP 590) assigns a scheduled resource unit to the station and no subsequent cascaded trigger frame defines one random resource unit the access of which being made by the stations using contention. This may be done by retrieving the “No further transmission opportunity” field set during the process of Figure 10c, e.g, the “Reserved User Info” filed 421 for the identified User Info field 420.

The value of this field 421 is then checked at step 1240.

If this field 421 indicates another transmission opportunity will be offered by the subsequent trigger frames in TXOP 590 (field 421 is false or 0), the station shall not enter the doze mode and the process ends (step 1260). The reception process of 802.11 ax can go on as defined in the IEEE standard 802,11 ax.

If this field 421 indicates no further transmission opportunity will be offered during TXOP 590 (field 421 is true or 1), the station can enter a doze mode, at step 1250, until the end of TXOP 590.

Figure 13 illustrates the impact of these various embodiments on a power saving management at the stations. Figure 13 uses the same scenario as Figure 5 introduced above.

In the first TF 330-1 triggering the first MU Uplink transmission period 580-1, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 551-1 for station STA1 is set to 0. This is to indicate, to STA1, that a further scheduled RU in the first embodiment or a further transmission opportunity in the second embodiment will be offered to it during TXOP 590, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 552-1 for station STA2 is also set to 0, and the User Info field indicating a random RU 553-1 indicates that a further random RU will be provided (in the first embodiment only). This signalling flag is set to 0 in the “Reserved User Info” field 421 or the “No further RA RU” bit 422. In a variant, such indication is carried by the “Reserved Common Info” field 412 in TF 330-1. In this variant, STA1 and STA2 are able to read this field 412 to know whether further random RUs will be provided in TXOP 590.

The stations then transmit data as shown in timelines 510, 520, 530 (STA3 accesses the random RU 553-1).

Next, in the second TF 330-2 triggering the second MU Uplink transmission period 580-2, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 551-2 for station STA2 is set to 0. This is to indicate, to STA2, that a further scheduled RU in the first embodiment or a further transmission opportunity in the second embodiment will be offered to it during TXOP 590, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 552-2 for station STA3 is set to 1 in the first embodiment. This is because this is the last scheduled RU that will be assigned to STA3 during TXOP 590. In the second embodiment, field 421 is also set to 1 because there will be no more scheduled RU for STA3 nor further random RU, and the User Info field 420 indicating a random RU 553-2 may be set to indicate that no further random RU wiii be provided by the subsequent trigger frames of TXOP 590 (in the first embodiment only). For instance, the “Reserved User Info” field 421 or the “No further RA RU bit 422 in the User Info field 420 is set to 1. In a variant, the “Reserved Common Info” field 412 in TF 330-2 is set to 1.

STA2 and STA3 then transmit data as shown in timelines 520, 530.

STA1 and STA2 have not been indicated that they will not have further scheduled RUs. Thus, they remain awake. Thus, contrary to the prior art (Figure 5), STA1 does not enter a doze mode.

STA3 has been indicated that no more scheduled RU and random RU (first embodiment) will be offered to it or has been indicated that no further transmission opportunity (second embodiment) will be offered to it. As a consequence, STA3 may decide to enter the doze mode (592) until the end of TXOP 590.

Next, in the third TF 330-3 triggering the third MU Uplink transmission period 5803, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 551-3 for station STA1 is set to 0. This is to indicate, to STA1, that a further scheduled RU in the first embodiment or a further transmission opportunity in the second embodiment will be offered to it during TXOP 590 (here during transmission period 580-4), the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 552-3 for station STA2 is also set to 0.

STA1 and ST2 then transmit data as shown in timelines 510, 520.

As the two stations will have another transmission opportunity (or scheduled RU) during TXOP 590, they remain awake.

Contrary to the prior art (Figure 5), the scheduled RU 551-3 is not wasted.

Next, in the fourth and last TF 330-4 triggering the fourth MU Uplink transmission period 580-4, the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 551-3 for station STA1 is set to 0. This signalling is not crucial as TXOP 590 is about to end. The stations still awake will not enter the doze mode.

the “Reserved User Info” field 421 of the User Info field indicating one scheduled RU 552-3 for station STA2 is also set to 0.

STA1 and ST2 then transmit data as shown in timelines 510, 520.

Contrary to the prior art (Figure 5), the scheduled RU 551-4 is not wasted.

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention.

Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate.

In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A method in a wireless network comprising an access point and stations, the method comprising, at the access point:

transmitting a plurality of cascaded trigger frames during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and signalling, using a signalling flag in a current trigger frame assigning a scheduled resource unit to a station during an associated communication period, when no subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station.

2. A power saving method in a wireless network comprising an access point and stations, the method comprising, at one of the stations:

3. The method of Ciaim 1 or 2, wherein the signalling flag is included in control information defining the scheduled resource unit assigned to the station from among the plurality of resource units defined in the current trigger frame.

4. The method of Ciaim 3, wherein the signalling flag is made of a 1-bit signalling flag positioned at the 40^th bit in the User Info field defining the scheduled resource unit in the current trigger frame according to 802.11ax standard.

5. The method of Claim 1 or 2, wherein the signalling flag only indicates whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station.

6. The method of Claim 2, further comprising, at the station, enabling a first status flag local to the station when the retrieved signalling flag indicates no subsequent cascaded trigger frame within the granted transmission opportunity assigns a scheduled resource unit to the station.

7. The method of Claim 1, further comprising, at the access point, signalling, in one trigger frame of the granted transmission opportunity, when no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention.

8. The method of Claim 7, wherein signalling no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames use a single 1 -bit signalling flag.

9. The method of Claim 2, wherein the station enters the doze mode when the signalling flag indicates both no subsequent cascaded trigger frame assigns a scheduled resource unit to the station and no subsequent cascaded trigger frame defines one random resource unit the access of which being made by the stations using contention.

10. The method of Claim 7, wherein signalling no other scheduled resource unit and signalling no further random resource unit in the subsequent cascaded trigger frame or frames use two respective 1 -bit signalling flags.

11. The method of Claim 2, further comprising, at the station, retrieving, from a trigger frame received during the granted transmission opportunity, a second signalling flag indicating 'whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention, wherein entering a doze mode further depends on the retrieved second signalling flag.

12. The method of Claim 11, further comprising, at the station, enabling a second status flag local to the station when the retrieved second signalling flag indicates no subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention.

13. The method of Claims 6 and 11, wherein the station enters the doze mode when the first and second status flags are enabled.

14. The method of Claim 10 or 11, wherein the second signalling flag to signal no further random resource unit within the granted transmission opportunity is made of one from among:

a 1-bit signalling flag positioned at a bit position from the 27^th bit to the 31^tn bit in the User Info field defining one random resource unit in the current trigger frame according to

802.11 ax standard; and a 1-bit signalling flag positioned at the 64^tn bit in the Common info field of the current trigger frame according to 802.11 ax standard.

15. The method of Claim 1, further comprising, at the access point:

obtaining, for each station of a plurality of stations, load information representative of an amount of data to be transmitted by the station to the access point; and assigning scheduled resource units early in the granted transmission opportunity to stations having low load according to the obtained load information.

16. A method in a wireless network comprising an access point and stations, the method comprising, at the access point:

transmitting a plurality of cascaded trigger frames during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and signalling, using a 1-bit signalling flag in the Common Info field frame according to

802.11 ax standard of a current trigger frame, whether or not at least one subsequent cascaded trigger frame within the granted transmission opportunity defines one random resource unit the access of which being made by the stations using contention.

17. A power saving method in a wireless network comprising an access point and stations, the method comprising, at one of the stations:

18. The method of Claim 16 or 17, wherein the 1-bit signalling flag is positioned at the 64^th bit in the Common Info field frame.

19. A method in a wireless network comprising an access point and stations, the method comprising, at the access point:

20. The method of Claim 19, wherein scheduled resource units are assigned in priority to stations having relatively low amount of data to transmit.

21. The method of Claim 19, wherein scheduled resource units are assigned in priority to stations that necessitate relatively a low number of cascaded trigger frames to transmit their corresponding amount of data.

22. The method of Claim 21, wherein scheduled resource units are assigned first to stations that can transmit their corresponding amount of data in a resource unit of only one trigger frame.

23. The method of Claim 19, wherein the amount of data is based on load information obtained by the access point from the stations.

24. The method of Claim 19, wherein the multi-user transmission opportunity corresponds to a Target Wake up Time Service Period according to 802.11ax standard.

25. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a device, causes the device to perform the method of Claim 1,2, 16, 17 or 19.

26. A wireless communication device forming access point in a wireless network comprising an access point and stations, the device forming access point comprising at least one microprocessor configured for carrying out the steps of:

transmitting a plurality of cascaded trigger frames during a multi-user transmission opportunity granted to the access point to provide cascaded periods for data communication with the stations over a communication channel, each cascaded trigger frame defining a plurality of resource units forming the communication channel during an associated communication period; and signalling, using a signalling flag in a current trigger frame assigning a scheduled resource unit to a station during an associated communication period, 'when no subsequent cascaded trigger frame within the granted transmission opportunity assigns another scheduled resource unit to the station.

27. A wireless communication device forming station in a wireless network comprising an access point and stations, the device forming station comprising at least one microprocessor configured for carrying out the steps of:

28. A wireless communication device forming access point in a wireless network comprising an access point and stations, the device forming access point comprising at least one microprocessor configured for carrying out the steps of:

29. A wireless communication device forming station in a wireless network comprising an access point and stations, the device forming station comprising at least one microprocessor configured for carrying out the steps of:

39. A wireless communication device forming access point in a wireless network comprising an access point and stations, the device forming access point comprising at least one microprocessor configured for carrying out the step of: