GB2585230A - Priority mechanism for short feedback procedures in wireless networks - Google Patents

Abstract

An access point transmits a null data packet (NDP) feedback report poll (NFRP) trigger frame. A station selects one of the resource units (tones) defined by this trigger frame, performs carrier sensing over a defined period, then transmits a NDP feedback poll response if the resource is idle. This sensing may be performed only by lower-priority stations, with higher-priority stations transmitting from the start of the period. The priority level of a station may be determined by the access category of the data it has to send, or by its association identifier (AID). The NFRP trigger frame may include a flag indicating whether this prioritisation mechanism is in use. The sensing period may include a preamble period and a subsequent energy-sensing period: if the lower-priority station detects a PHY preamble it then performs energy sensing, and if the resource remains idle, its NDP response comprises only a transmission of energy (without a preamble). This short feedback procedure may be used in an 802.11ax WLAN.

Description

PRIORITY MECHANISM FOR SHORT FEEDBACK PROCEDURES IN WIRELESS

NETWORKS

FIELD OF THE INVENTION

The present invention relates generally to communication networks and more specifically to wireless communication methods in a wireless network and corresponding communication devices, such as an access point (AP) and non-AP stations.

BACKGROUND OF THE INVENTION

The IEEE 802.11 (RTM) family of standards provides multi-user (MU) schemes to allow a single access point (AP) to schedule MU transmissions, i.e. multiple simultaneous transmissions to or from non-AP stations or "nodes", in the wireless network. This approach increases bandwidth and decreases latency requirements compared to original 802.11 networks.

MU downlink (DL) transmission is allowed where the AP performs multiple simultaneous elementary transmissions, over so-called resource units (RUs), to various non-AP stations. As an example, the resource units split a communication channel of the wireless network in the frequency domain, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

MU uplink (UL) transmissions are also allowed that are triggered by the AP. Various non-AP stations can simultaneously transmit to the AP over the resource units forming the MU UL transmission. To control the MU UL transmission by the non-AP stations, the AP sends a control frame, known as a Trigger Frame (TF), which defines a plurality of resource units for the non-AP stations.

Various variants of trigger frames exist depending on the nature of information the non-AP stations can provide in response. The main variant is the basic trigger frame for the non-AP stations to send any data they wish.

Some RUs may be allocated in a basic trigger frame to specific non-AP stations using 16-bit Association IDentifiers (AIDs) assigned to them upon registration to the AP (so-called scheduled RUs).

Others RUs (known as random RUs) are available to the non-AP stations using a contention-based access scheme. Only three types of trigger frames are known that provide such random access to RUs, namely the Basic Trigger frame, the BQRP Trigger frame and the BSRP Trigger frame. This scheme is known as UL OFDMA-based random access (UORA) scheme.

UORA is useful for wireless networks because it provides opportunities for the non-AP stations to transmit, without the AP having polled them to know their needs for transmission. However, it suffers from various drawbacks.

It suffers from a low maximum efficiency of 37% (successfully used random RUs) to be compared to 37% of unused random RUs and 26% of random RUs with collisions.

The lost random RUs (either unused or collided) occur on large transmission durations (because transmitting non-AP stations have usually substantial amounts of data to transmit during UORA). This substantially decreases network efficiency.

A variant trigger frame to the basic trigger frame is the Null-Data-Packet (NDP) Feedback Report Poll (NFRP) trigger frame implementing the so-called Null-Data-Packet (NDP) Feedback Report procedure. This procedure allows the AP to collect feedback that is not channel sounding from multiple non-AP stations in a more efficient manner than with a basic trigger frame. The AP sends a NFRP Trigger frame to solicit NDP feedback report responses from many non-AP stations that are identified by a range of scheduled AIDS in the NFRP Trigger frame. The NDP feedback report response from a non-AP station is a HE trigger-based (TB) feedback NDP.

The procedure is short compared to the duration of an UL transmission triggered by a basic Trigger frame, mainly because the NDP in response is short. It also has a low and stable latency compared to conventional "Carrier Sense Multiple Access with Collision Avoidance" CSMA-CA mechanisms when used in dense environments.

However, the NDP Feedback Report procedure also suffers from some limitations.

For instance, it can address a limited set of (usually 18 or 36 for a 20Mhz wide operating band) continuous AIDS which may be punctured (some AIDS may not be assigned to non-AP stations or have been released when non-AP stations leave the AP during the lifetime of the network). The limited continuous set of AIDS, one per each RU tone set, is not adapted to the gathering of feedback responses from a high number of non-AP stations, i.e. per BSS basis.

SUMMARY OF INVENTION

In this context, the inventors have contemplated increasing the number of non-AP stations allowed to use each RU tone set, for instance through random access to these RU tone sets.

The competition between non-AP stations to access the same RU tone set introduces risks of collisions, and thus reduced efficiency of the wireless network.

The present invention seeks to overcome some of the foregoing concerns.

In this context, the invention provides a communication method in a wireless network, comprising the following steps at a (non-AP) station: receiving, from an access point, AP, a null data packet, NDP, feedback report poll, NFRP, trigger frame, the NFRP trigger frame reserving a plurality of resource unit, RU, tone sets for NDP feedback report responses by stations, selecting a responding RU tone set from the plurality of RU tone sets, sensing the selected responding RU tone set during a sensing period in which other stations are supposed (i.e. allowed) to send NDP feedback report responses to the NFRP trigger frame, sending a NDP feedback report response on the selected responding RU tone when the latter is sensed idle during the sensing period.

A priority mechanism is therefore introduced where the non-AP station, which is not given priority, determines whether another non-AP station, which is given priority, already use the selected TU tone set. By deferring the use of the network by one non-AP station for the sensing period, the mechanism avoids the two non-AP station to simultaneously start using the same RU tone set, hence reduces risks of collision. Network efficiency is therefore improved.

Correspondingly, the invention also provides a communication method in a wireless network, comprising the following steps at an access point: sending, to (non-AP) stations, a null data packet, NDP, feedback report poll, NFRP, trigger frame, the NFRP trigger frame reserving a plurality of resource unit, RU, tone sets for NDP feedback report responses by stations, and receiving NDP feedback report responses on respective responding RU tone sets, wherein the AP starts receiving a first NDP feedback report response on a first responding RU tone set after an idle period (a sensing period for the corresponding responding station) during which the first responding RU tone set is idle and at least one other NDP feedback report response is received on another responding RU tone set.

Correlatively, the invention also provides a communication device, either the AP or a non-AP station, comprising at least one microprocessor configured for carrying out the steps of any of the above methods.

Optional features of embodiments of the invention are defined in the appended claims. Some of these features are explained here below with reference to a method, while they can be transposed into device features.

In some embodiments, the method further comprises, at the station, determining a level of priority of the station to respond to the NFRP trigger frame, and performing the sensing only when it is determined a low level of priority. The non-AP stations can then take into account a current priority level if the latter is liable to evolve over time. Of course, in a variant where the priority of the non-AP station does not change, it may always sense the selected RU tone set without determining any level of station priority.

According to a feature, when it is determined a high level of priority of the station, the method further comprises, at the station, sending the NDP feedback report response on the selected responding RU tone as from the beginning of the sensing period. This means without sensing the selected responding RU tone during the sensing period. Other low-priority non-AP stations are then able to detect this feedback response on the RU tone set to avoid sending in turn their own feedback responses on the same RU tone set.

In some embodiments, the priority level of the station is based on a (EDCA) traffic access category having a non-empty buffer. This approach makes it possible to define the station priorities based on the type of data the non-AP stations have to send, and thus prioritize low latency traffics. For instance, buffered AC_VO and AC_VI data may define a high-priority non-AP station while buffered AC_BE and AC_BK data may define a low-priority non-AP station that will access an RU tone set only if no high-priority non-AP station has previously access it in response to the NFRP trigger frame. As the EDCA traffic ACs are already ordered by priority in 802.11 DCF (Distributed Coordination Function), the traffic AC to be taken into account may be the non-empty one having the highest 802.11ax priority.

According to a specific feature, the traffic access category is the non-empty traffic access category having the lowest (EDCA) backoff counter from amongst a plurality of traffic access categories having each one a respective backoff counter decremented over time. This approach fulfils the EDCA prioritization scheme.

In other embodiments, the priority level of the station is based on an AID of the station. Some AIDS (e.g. a reserved range of AIDs) may be known as high-priority AID, while others are considered as low-priority AIDs. The priority may be defined by the value of one or more bits of the 12-bit AIDs. As an example, the x LSBs (least significant bits) should match a priority pattern; for instance, the LSB = 1 to have half of high-priority AIDs, or the two LSBs = 11 to have a quarter of high-priority AIDS, or the three LSBs = 111 to have an eighth of high-priority AIDS, and so on.

The NFRP trigger frame may also target a continuous set of AIDS and schedule two or more non-AP stations per RU tone set. In this variant, the one with the lowest (or highest) AID may be considered as the high-priority one, and the others as low-priority ones.

In some embodiments, the method further comprises, at the station, retrieving a priority enablement flag from the NFRP trigger frame, and performing the sensing only if the priority enablement flag is enabled. This allows the AP to decide (through signaling) at NFRP trigger frame level when the priority mechanism according to the invention is to be applied by the non-AP stations.

From AP perspective, a communication method in a wireless network comprises the following steps: generating a NFRP trigger frame reserving a plurality of resource unit, RU, tone sets for NDP feedback report responses by stations, determining whether low-priority stations have to sense a selected responding RU tone set during a sensing period in which high-priority stations are supposed to send NDP feedback report responses to the NFRP trigger frame, setting a priority enablement flag in the NFRP trigger frame depending on the outcome of the determining step, and sending, to stations, the NFRP trigger frame.

In some embodiments, if the priority enablement flag is disabled, sending the NDP feedback report response on the selected responding RU tone as from the beginning of the sensing period. This means without sensing the selected responding RU tone during the sensing period.

In one embodiment, the priority enablement flag is included in a Reserved field of a User Info field of the NFRP trigger frame according to Draft 4.1 of IEEE 802.11ax. This approach keeps retro-compatibility because it keeps unchanged the other fields currently used.

In a variant, the priority enablement flag is included in a Trigger Dependent Common Info field of a Common Info field of the NFRP trigger frame according to Draft 4.1 of IEEE 802.11ax.

In yet another variant, the priority enablement flag is defined by a feedback type field in the NFRP trigger frame. This approach keeps compliance with the current format of the NFRP trigger frame as various values for the 802.11ax Feedback Type field are available for new usages. For instance, feedback type field=1 may upgrade the conventional feedback type field=0 (polling of resource request function of buffered bytes) by differentiating the station behavior between high-priority ones and low-priority ones (which have to sense the selected RU tone set).

In some embodiments, the sensing period comprises at least a preamble period (following the NFRP trigger frame) during which the other stations are supposed to simultaneously transmit a PHY preamble of their NDP feedback report responses. Preamble of a NDP feedback report responses is made of various symbols such a well-known L-STF, L-LTF, L-SIG, RL-SIG, HE-SIG-A and HE-STF fields according to 802.11ax. It is usually transmitted over a (20Mhz) channel formed of the plurality of RU tone sets. Therefore, the duration of the preamble period is defined in 802.11ax and known by all the stations.

In some embodiments, if no PHY preamble is sensed on the selected responding RU tone during the preamble period, the sent NDP feedback report response includes a PHY preamble. This makes it possible for the AP to know the response is a NDP feedback report response, as no PHY preamble has been sent by any other station during the conventional preamble period. All the stations that sense during the preamble period start transmitting the PHY preamble of their responses simultaneously as from the end of the preamble period. This ensures alignment of the PHY preamble all over the (20MHz) channel.

In some embodiments, if a PHY preamble is sensed on the selected responding RU tone during the preamble period, the sensing period further comprises an energy sensing period immediately following the preamble period, and if the selected responding RU tone set is sensed as idle during the energy sensing period, sending the NDP feedback report response includes transmitting only energy, without PHY preamble, on the selected responding RU tone set. This saves process at the station while changing nothing for the AP since the latter has already received a PHY preamble valid forthe plurality of RU tone sets. One understands that the selected responding RU tone set is sensed during the energy sensing period only if a PHY preamble is sensed on the selected responding RU tone during the preamble period.

In some embodiments, the energy sensing period lasts a backoff time, i.e. 9ms in 802.11ax D4.1. This ensures the NFRP procedure to remain as short as in 802.11ax, although a priority mechanism has been added.

In some embodiments, the method further comprises determining, from the NFRP trigger frame, that the RU tone sets are accessed on a random basis, wherein selecting the responding RU tone set comprises randomly selecting the responding RU tone set from the plurality of RU tone sets. The priority mechanism according to the invention is well suitable to random RU tone sets where risks of collision exist.

For instance, the determining step comprises determining whether an association identifier, AID, field (such as the Starting AID field as defined in Draft 4.1 of IEEE 802.11ax) in the received NFRP trigger frame includes a predefined AID value, e.g. 0, defining a random access for the stations to the plurality of RU tone sets.

Another aspect of the invention relates to a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a communication device, causes the communication 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 communication 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, micro-code, etc.) or an embodiment combining software and hardware aspects that may all 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 communication system in which embodiments of the invention may be implemented; Figure 2 illustrates two usages of trigger frames; Figures 2a and 2b illustrate, using flowcharts, corresponding general steps at the access point and at a non-AP station, respectively; Figure 3a illustrates the format of a trigger frame, in particular of NFRP type; Figure 3b illustrates the format of TB NDP PPDU; Figure 4 shows a schematic representation a communication device in accordance with embodiments of the present invention; Figure 5 schematically illustrates functional blocks of a communication device in accordance with embodiments of the present invention; Figure 6 illustrates, using the same scenario as Figure 2, embodiments of the invention providing a priority mechanism when high-priority non-AP stations respond to a NFRP trigger frame of a NDP short feedback report procedure; Figures 6a and 6b illustrate, using flowcharts, corresponding general steps at the access point and at a non-AP station, respectively; and Figure 7 illustrates a variant of Figure 6 in which no high-priority non-AP station responds to the NFRP trigger frame.

DETAILED DESCRIPTION

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

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

Figure 1 illustrates a communication system in which several communication stations (or "nodes") 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, also seen as a station of the network. 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.

In the following, the word "station" refers to any kind of station. The wording "access point station", or in short "access point" (AP), refers to the station playing the role of access point 110. The wording "non-access point station", or in short "non-AP station", or client station (STA) refers to the other stations 101-107. In the following, the terms HE STA and HE AP refer respectively to an 802.11ax non-AP STA and an 802.11ax AP.

Access to the shared radio medium to send data frames is primarily 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.

The wireless communication system of Figure 1 comprises physical access point 110 configured to manage the WLAN BSS (Basic Service Set), i.e. a group of non-AP stations which have previously registered to the AP. A physical access point 110 may be configured to manage two or more WLANs (or BSSs), i.e. two or more groups of station. Each BSS is uniquely identified by a specific basic service set identifier, BSSID, and managed by a virtual AP implemented in the physical AP.

To access the medium, any station, including the AP, starts counting down a backoff counter designed to expire after a number of timeslots when the medium is sensed as idle. The backoff counter is chosen randomly in a so-called contention window [0, CW], where CW is an integer. This backoff mechanism or procedure, also referred to as Distributed Coordination Function (DCF) contention-based channel access scheme, 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 expires (i.e. the backoff counter reaches zero), the source station may send data or control frames if the medium is still idle.

Conventional single-user transmission can occur on at least a primary 20MHz channel (used for contention) and some secondary 20Mhz channels: The resulting bandwidth of an operating channel may be e.g. 20 MHz, 40 MHz, 80 MHz, 80+80 MHz, or 160+160 MHz, or 320 MHz. The channels may include one or more subcarriers or tones, for instance a 20 MHz channel is made of 242 tones.

Management of quality of service (QoS) has been introduced at station level in the wireless networks, through well-known EDCA mechanism defined in the IEEE 802.11e standard. EDCA (Enhanced Distributed Channel Access) mechanism defines four traffic access categories (ACs) or « priorities v to manage access to the medium: a voice access category (AC_VO), a video access category (AC_VI), a best effort access category (AC_BE) for standard applications and a background access category (AC_BK) when traffic is low.

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

In this perspective, multi-user (MU) transmission features have been considered that allow multiple simultaneous transmissions to/from different non-AP stations in both downlink (DL) and uplink (UL) directions from/to the access point. 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 legacy 20MHz channel into at least one subchannel, but preferably a plurality of sub-channels (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. In some embodiments, the bandwidth of the RUs may be based on a number of active data subcarriers. In some embodiments, the bandwidth of the RUs is based on 26, 52, 106, 242 (a whole 20MHz channel), 484 (40MHz channel), 996 (80MHz channel), or 2x996 (80+80Mhz or 160Mhz channel) active data subcarriers or tones.

While the MU DL transmission is fully managed by the AP, the MU UL transmission requires the AP sends a control frame to the non-AP station to trigger the simultaneous MU UL transmissions from the non-AP stations. Such control frame is known as a Trigger Frame (TF), various variants of which exist depending on the usage of the MU UL sub-carriers desired by the AP.

Figure 2 illustrates two usages of trigger frames. In the exemplary embodiment shown a shod feedback report procedure according to 802.11ax (as described in section "26.5.7 NDP feedback report procedure" of Draft D4.1 of IEEE 802.11ax) is shown followed by an UL MU operation (as described in section "26.5.2 UL MU operation" of Draft D4.1 of IEEE802.11ax) based on the results of the short feedback report procedure.

The NDP feedback report procedure allows the AP 110 to collect feedback that is not channel sounding from multiple non-AP STAs 101-107. The AP sends an NFRP Trigger frame to solicit NDP feedback report response from many non-AP STAs that are identified by a range of scheduled AIDs in the NFRP Trigger frame. A non-AP STA uses the information carried in the NFRP Trigger frame to know if it is scheduled, and in this case, may send a NDP feedback report response, usually a HE TB feedback NDP.

Next, based on the received NDP feedback report responses, the AP may, using UL MU operation, solicit simultaneous immediate response frames from one or more of the responding non-AP STAs.

The example shown considers a single 20 MHz channel. Of course, the bandwidth of the channel and the number of RUs splitting a 20 MHz channel may be different from what is depicted. Figures 2a and 2b illustrate, using flowcharts, corresponding general steps at the AP and a non-AP STA, respectively.

The AP is willing to poll non-AP stations using a feedback short procedure. At preliminary step S259, the AP determines NFRP parameters values for NFRP trigger frame 200 to be sent. NFRP trigger frame 200 is a specific trigger frame. It identifies the polled non-AP STAs by a range of scheduled AIDs.

With reference to Figure 3a, like each and every 802.11ax trigger frame, NFRP trigger frame 200 comprises: a frame header with a standardized "Frame Control" field, a standardized "Duration" field, an "RA" field set to a broadcast MAC address, and a "TA" field set to a MAC address of the AP transmitting the trigger frame,

a "Common Info" field 310,

one or more "User Info" fields 350, and

padding and FCS fields.

The "Common Info" field 310 comprises a "Trigger Type" subfield 320 which specifies the type of the trigger frame. For instance, NFRP trigger frame 200 is signaled by a value 7 in the "Trigger Type" subfield 320. It also comprises a 2-bit "UL BW field 330 specifying the bandwidth of the channel considered, e.g. BW=0 to define a 20MHz bandwidth, BW=1 for a 40MHz bandwidth, BW=2 for an 80MHz bandwidth, BW=3 for an 80+80MHz or 160MHz bandwidth (see Table 9-31c of the D4.1 version of 802.11ax). It ends by a Trigger Dependent Common Info subfield 340 of variable length whose content depends on the "Trigger Type" subfield 320. The other fields shown are of less importance for the present invention.

Specific to the trigger frame of NFRP type, a single "User Info" field 350 is provided that comprises a 12-bit Starting AID field 351, a first reserved 9-bit portion 352, a 4-bit feedback type field 353, a second reserved 7-bit portion 354, a 7-bit UL Target RSSI field 355 and a 1-bit

multiplexing flag field 356.

The Starting AID comprises the starting AID of the range of AIDs targeted by the NFRP trigger frame 200, i.e. scheduled to respond to the poll. The range size or width NSTA is defined by the "UL BW" field 330 together the 1-bit multiplexing flag field 356, using the following formula NSTA = 18 x 2Bw x (MultiplexingFlag + 1) For instance, when the MultiplexingFlag is set to 0 (no MIMO), 18 non-AP STAs are requested to answer with a feedback response, per 20MHz operating channel. When the MultiplexingFlag is set to 1, 36 non-AP STAs are scheduled per 20MHz operating channel. It may be noted that some AIDs in the 18 or 36-wide range may not be currently assigned to a non-AP STA.

The multiplexing flag field 356 defines whether spatiality (MIMO) is provided: the flag indicates the number (minus 1) of non-AP STAs that are multiplexed on the same set of tones in the same RU.

The "feedback type" field 353 indicates a type of feedback that is being polled by the AP. For the time being, 802.11ax D4.1 only defines a feedback type equal to 0 that is a resource request. The corresponding polling thus seeks to know whether the responding non-AP STAs 101-107 are requesting UL resources to transmit PPDUs to the AP 110.

At step S259, the AP thus determines the values for StartingAlD field 351, Feedback Type field 353, Multiplexing Flag field 356 and UL BW field 330 and builds the NFRP trigger frame 30 200.

At phase 199, the AP 110 accesses the wireless medium. For example, the AP performs a contention-based method (which may include a clear channel assessment and an EDCA backoff) to acquire access to the wireless medium.

Upon accessing the medium, the AP 110 polls non-AP STAs to know their needs for transmission. To do so, it sends NFRP trigger frame 200 at step S260.

In the example of Figure 2, the NFRP trigger frame 200 is sent (step S260) in a 20MHz primary channel. However, as already discussed, the NFRP trigger frame 200 may also be sent through an extended channel such as 40MHz, 80MHz or larger bands to extend the number of polled stations. By sending trigger frame 200, the AP reserves a transmission opportunity 260 (TXOP) corresponding to the duration specified inside the NFRP trigger frame. If the NFRP trigger frame is sent over an overall width larger than the primary 20MHz channel, the 802.11ax standard envisages that the NFRP trigger frame is duplicated (replicated) on each other 20MHz channels forming the targeted composite channel. Thanks to the duplication of control-type frames in non-HT format, it is expected that every nearby legacy node (non-HT or 802.11ac nodes) receiving the NFRP trigger frame (or a duplicate thereof) on its primary channel, then sets its NAV to the value specified in the NFRP trigger frame. This prevents these legacy nodes from accessing the channels of the targeted composite channel during the TXOP.

Each non-AP STA receiving frame 200 is able to first analyze the received frame to determine whether the non-AP STA is concerned with it, in particular to determine whether the non-AP STA is associated with the BSSID indicated in the TA field of the frame (or if the indicated BSSID pertains to a multiple BSSID set for which the non-AP STA is member of).

In case of positive determination, it then determines whether received frame 200 is a NFRP trigger frame, thanks to the type specified in Trigger Type field 320. These determinations form step S270 (Figure 2b).

Next, the non-AP STA determines whether it is scheduled or "polled" by the received NFRP trigger frame (step S272). It is made by checking whether its AID value (assigned to the non-AP STA by the AP upon registration to the AP) falls within the range [ "Starting AID"; "Starting AID"+ NSTA] as obtained from the fields UL BW 330, Starting AID 351 and Multiplexing flag 356 of the received NFRP trigger frame 200.

When the non-AP STA is not scheduled, nothing more happens at the non-AP station.

If it is polled by the NFRP trigger frame, the scheduled non-AP STA determines a RU tone set index, i.e. a RU tone set 210 on which the non-AP STA will transmit energy in response to the NFRP trigger frame. This is step S274. The non-AP STA usually selects a responding RU tone set based on the position of its AID within the above range, meaning the first RU tone set for the non-AP station having the Starting AID as own AID, and so on.

Table 27-30 of 802.11ax D4.1 describes an example of how the tones forming 80MHz, 40MHz, 20MHz channels are grouped into sets of tones.

For instance, 216 tones (indexed from -113 to -6 and 6 to 113) forming a 20MHz channel are split into six bundles 250 of 36 continuous tones. Next each RU tone set is formed by two tones from each bundle (usually consecutive tones that are collocated from one bundle to the other), thereby resulting in 18 RU tone sets, each having a unique index RU_TONE_SET_INDEX. The two tones obtained from each bundle are assigned to two respective groups forming the RU tone set. It means that each RU tone set is formed of two groups of tones 210a and 210b.

For illustrative purposes, the tone set with RU_TONE_SET_INDEX=6 in a 20MHz channel without spatiality is made of the two following groups of tones (subcarrier indices): Group 210a: -103, -67, -31, 16, 52, 88 Group 210b: -102, -66, -30, 17, 53, 89 In this example, 6 tones are replicated in each group over the 20MHz channel, each tone from one of the six bundles of tones 250.

A RU tone set is thus made of two adjacent groups of tones (-103 is adjacent to - 102, -67 to -66 and so on.), each group being made of non-adjacent tones (-103 not adjacent to -67 and so on.).

Basically, the tone set index for the scheduled non-AP STA is computed from the difference between STA's AID value and "Starting AID" value (usually the difference plus 1). For instance, if this difference plus 1 equals 6, the above-detailed tone set having RU_TONE_SET_INDEX=6 is scheduled for the non-AP STA considered.

Next at step S276, the non-AP STA generates the NDP feedback report response to be sent to the AP.

In particular, the non-AP STA has to transmit energy on the first group 210a of subcarriers or tones to indicate a first response to the feedback type (field 353) polled by the NFRP trigger frame 200, and on the other hand, the non-AP STA must transmit energy on the second group 210b of subcarriers or tones to indicate a second response to the feedback type.

The response is named FEEDBACK_STATUS in the current D4.1 version of 802.11ax. For instance, for the Feedback Type field 353 set to 0 (Resource request), FEEDBACK_STATUS is set to 0 when the non-AP STA requests resource with buffered bytes for transmission between 1 and a resource request buffer threshold. The non-AP station will thus use the first group 210a of tones; FEEDBACK_STATUS is set to 1 when the non-AP STA requests resource with buffered bytes for transmission above the resource request buffer threshold. The non-AP station will thus use the second group 210b of tones.

The non-AP station determines the NDP feedback report response to be sent depending on the feedback type field in the NFRP trigger frame.

Table 27-30 of 802.11ax D4.1 specifies which group of tones within a tone set has to be used depending on the FEEDBACK_STATUS value.

At step S276, the non-AP STA thus determines the FEEDBACK_STATUS value and therefore the group of tones to be used, either 210a or 210b, depending on the feedback it wishes to report to the AP.

Next at step S278, the non-AP STA transmits energy on the group corresponding to the FEEDBACK_STATUS value in the RU tone set of the determined RU_TONE_SET_INDEX.

Technically speaking, the MAC block 502 of the non-AP STA generates a TXVECTOR to cause the non-AP STA to transmit on the RU tone set of the selected RU tone set index and, responsive to the TXVECTOR, the PHY block 503 of the non-AP STA sends an HE TB PPDU 211.

For illustration, Station 1 (corresponding to RU_TONE_SET_INDEX=1) transmits energy on its first group of tones 210a (as consequence, group 210b is represented with a dash line). On the contrary, Station 2 (corresponding to RU_TONE_SET_INDEX=1) transmits energy on its second group of tones 210b.

Technically, the HE TB NDP Feedback PPDU 211 used as a feedback response is a single packet with no real data payload as shown in Figure 3b. The PHY preamble 212 is emitted on 20 MHz width (thus several non-AP STAs may emit the same preamble) and the payload' is composed of a series of HE-LTF symbols 213, emitted on each tone forming the selected group 210a or 210b, to be used for the transmitted feedback (energy).

The non-AP STA commences the transmission of the NDP feedback report response at the SIFS time boundary after the end of the received NFRP trigger frame 200. The transmission duration for the PHY preamble is defined by the fixed amount of symbols to be sent. The duration is referred to as "preamble period" Tpreamble as shown in the Figure. Following the preamble period Tpreamble, the non-AP STA transmits the HE-LTF symbols 213.

Then, the physical layer of the AP receives and decodes (S262) the RU tone sets where energy is present, to provide its MAC layer with a list of used RU_TONE_SET_INDEX and the corresponding Feedback responses (FEEDBACK_STATUS values).

Thanks to the fields UL BW 330, Starting AID 351 and Multiplexing flag 356 of the NFRP trigger frame 200 sent at step S260, the AP is able to retrieve the AID of each responding RU tone set with energy, and thereby retrieve the AID of each non-AP STA responding to the trigger frame 200. The MAC layer entity of the AP is consequently able to determine those NDP-scheduled non-AP STAs who have responded.

At step S264, the AP can send a subsequent trigger frame 220 (Figure 2) to offer new opportunities (RUs) to the responding non-AP STAs, for example a 'Basic' type trigger frame or any convenient type. The 'Basic' type trigger frame is signaled by a "Trigger Type" subfield 320 having value 0.

Based on an AP's decision and the collected feedback responses 211, the trigger frame 220 may define a plurality of data resource units (RUs) 230 (here of 26 tones -of course other numbers of tones may be used). The multi-user feature of OFDMA allows the AP to assign different RUs to different non-AP STAs in order to increase competition. This helps to reduce contention and collisions inside 802.11 networks.

These RUs may be scheduled RUs assigned to the feedback-responding non-AP STAs, using the AIDS retrieved at step S264.

The trigger frame 220 may for instance include a plurality of User Info fields (Figure 3a) for a respective plurality of scheduled RUs, each User Info field setting an AID (so-called AID12 field) of the scheduled non-AP STA for a given RU in the channel.

The non-AP STAs thus receive the subsequent trigger frame 220 and determine whether they are scheduled (step S280).

In the affirmative, the non-AP STA can use the RU scheduled to it (i.e. the one with the AID corresponding to the non-AP STA) and transmit data (HE TB PPDU) to the AP.

According to the exemplary illustration, Station 1 and Station 2 can thus be granted a RU 230. As an example, Station 1 emits a HE TB PPDU 231 in a first RU 230-1, and Station 2 emits a Q0S_Null with Buffer Status Report (the HE TB PPDU is a MAC-PDU with no data payload but with a MAC header containing a BSR) in a second RU 230-2. As the Qos_Null is smaller, the second RU 230-2 is filled in with padding to match the transmission length specified in the trigger frame 220.

Upon receiving the HE TB PPDU 231, the AP acknowledges (or not) the data on each RU by sending a multi-STA block acknowledgment (BA) response (240 -Figure 2), making it possible for each sending non-AP STA to know when its data transmission is successful (reception of the ACK) or not (no ACK after expiry of a time-out). This is step S266.

These explanations show the intent of the NFRP trigger frame mechanism according to the current version of the 802.11ax standard: to receive feedbacks in a short time from a great number of associated non-AP stations.

The overall MU Uplink (UL) medium access sequence, including both NDP Feedback RUs and UL MU scheduled RUs, seems more efficient than conventional EDCA access scheme, especially in dense environments as envisaged by the 802.11ax standard. This is because the number of collisions generated by simultaneous medium access attempts and the overhead due to the medium access are both reduced. The NFRP trigger frame 200 allows information to be requested from 18 non-AP stations per 20 MHz channel (more with spatial multiplexing), and the Basic trigger frame 220 allows RUs to be proposed to up to 9 stations which have shown their interest to be triggered (by responding to the NFRP trigger frame).

However, the Null-Data-Packet (NDP) Feedback Report procedure suffers from limitations, notably because of the number of triggered (scheduled) non-AP stations. As the mechanism is based on fixed range of AIDs (from Starting AID 351, one AID per RU tone set) where some AIDS in the range may not be used (e.g. released during the lifetime of the network cell), several successive NFRP polling phases are required to have better knowledge of the resource needs of more stations with a view of offering an efficient delivery of the data traffic compared to the required polling overhead.

To overcome this situation, the inventors have contemplated allowing a plurality of non-AP stations to concurrently access the same RU tone set.

This may be done by defining (and thus signaling) a random access for the non-AP stations to the plurality of RU tone sets. In that case, a single feedback request may apply for all non-AP stations, possibly of a group (e.g. a given BSS), and the number of non-AP stations that can be targeted by the "random-access" (RA) NFRP trigger frame is no longer limited by the number of RU tone sets.

This may also be done by assigning two or more (let say N) AIDS per RU tone set, using the same mechanism as above: for instance, the polling range is [ "Starting AID"; "Starting AID"+ N x NSTA and AIDS [ "Starting AID" + x.N; "Starting AID"+ x.N -1] are assigned to RU tone set with RU_TONE_SET_INDEX=x. This approach helps reducing the impact of the punctures in the conventional polling range.

However, the competition between various non-AP stations to access the RU tone set may be detrimental to network efficiency because collisions may occur. It would therefore be beneficial to organize the competition. In this respect, the inventors propose a priority mechanism for short feedback procedures.

An underlying idea of the proposal is for some low-priority non-AP stations to sense the RU tone set when the high-priority are supposed to already transmit their feedback responses. A clear RU tone set during the sensing allows the low-priority non-AP stations to transmit their feedback responses on the idle RU tone set. To time align the feedback responses between the high-priority and low-priority non-AP stations, the latter can send a shorter feedback response, for instance deprived of the PHY preamble.

In this respect, a non-AP station that receives a NFRP trigger frame, selects a responding RU tone set, senses the selected responding RU tone set during a sensing period in which the high-priority non-AP stations are supposed or allowed to send their NDP feedback report responses to the NFRP trigger frame, and if the sending shows that the selected responding RU tone is idle, therefore the non-AP station sends its NDP feedback report response on the idle selected responding RU tone. At least two behaviors are defined for the non-AP stations: those responding directly to the NFRP trigger frame and those sensing whether other non-AP stations have started responding to the NFRP trigger frame, before starting sending their own feedback responses.

The AP thus receives various NDP feedback report responses in response to the NFRP trigger frame. Due to the priority mechanism, the AP starts receiving the NDP feedback report response from the above non-AP station after an idle period (corresponding to the sensing period) during which the selected responding RU tone set is idle and at least one other NDP feedback report response is received on another RU tone set from another non-AP station. Figure 4 schematically illustrates a communication device 400 of the radio network 100, either the AP 110 or any non-AP STA 101-107, configured to implement at least one embodiment of the present invention. The communication device 400 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 400 comprises a communication bus 413 to which there are preferably connected: -a central processing unit 411, such as a microprocessor, denoted CPU; -a read only memory 407, denoted ROM, for storing computer programs for implementing the invention; -a random-access memory 412, 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 402 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/be protocols. The frames are written from a FIFO sending memory in RAM 412 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 412 under the control of a software application running in the CPU 411.

Optionally, the communication device 400 may also include the following components: -a data storage means 404 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention; -a disk drive 405 for a disk 406, the disk drive being adapted to read data from the disk 406 or to write data onto said disk; a screen 409 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 410 or any other pointing means.

The communication device 400 may be optionally connected to various peripherals, such as for example a digital camera 408, each being connected to an input/output card (not shown) so as to supply data to the communication device 400.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 400 or connected to it. The representation of the bus is not!imitative and in particular the central processing unit is operable to communicate instructions to any element of the communication device 400 directly or by means of another element of the communication device 400.

The disk 406 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 embodiments of the invention to be implemented.

The executable code may optionally be stored either in read only memory 407, on the hard disk 404 or on a removable digital medium such as for example a disk 406 as described previously. According to an optional variant, the executable code of the programs can be received by means of the communication network 403, via the interface 402, in order to be stored in one of the storage means of the communication device 400, such as the hard disk 404, before being executed.

The central processing unit 411 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 404 or in the read only memory 407, are transferred into the random access memory 412, 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 5 is a block diagram schematically illustrating the architecture of the communication device 400 adapted to carry out, at least partially, the invention. As illustrated, communication device 400 comprises a physical (PHY) layer block 503, a MAC layer block 502, and an application layer block 501.

The PHY layer block 503 (e.g. a 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20 MHz channel or the composite channel, and thus sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance single-user frames, such as control frames (e.g. ACK, Trigger Frame), MAC data and management frames, based on a 20 MHz width to interact with legacy 802.11 stations or with 802.11ax/be in legacy mode (such as for Trigger Frames), as well as MAC data frames of OFDMA type having preferably smaller width than 20 MHz legacy (typically 2 or 5 MHz), as well as NDP frames having preferably a PHY header transmitted on 20MHz width and a short payload consisting on energy located on non-contiguous subcarriers or tones, to/from that radio medium.

The MAC layer block or controller 502 preferably comprises a MAC 802.11 layer 504 implementing conventional 802.11ax/be MAC operations, and an additional block 505 for carrying out, at least partially, embodiments of the invention. The MAC layer block 502 may optionally be implemented in software, which software is loaded into RAM 412 and executed by CPU 411.

Preferably, the additional block 505 referred to as NDP Feedback Management module 505 is configured to implement steps according to embodiments that are performed by the communication device 400, notably transmitting operations for a transmitting/responding station and receiving operations for a receiving station.

Interfaces 506 and 507 are used by the MAC and PHY layer blocks to interact and to exchange information through TXVECTOR (from the MAC to the PHY layer -506) and the 30 RXVECTOR (from the PHY to the MAC block -507). The TXVECTOR and RXVECTOR are defined in the clause 27.2.2 of the draft 4.1 of the 802.11ax standard.

On top of the Figure, application layer block 501 runs an application that generates and receives data packets, for example data packets of a video stream. Application layer block 501 represents all the stack layers above MAC layer according to ISO standardization.

Embodiments of the present invention are now illustrated using various exemplary embodiments.

Although some of the proposed examples use the trigger frames 200 and 220 (see Figure 2) sent by an AP for a multi-user (MU) uplink (UL) transmissions, equivalent mechanisms can be used in a centralized or in an ad hoc environment (i.e. without an AP). It means that the operations described below with reference to the AP may be performed by any station in an ad hoc environment. In particular, subsequent scheduling to provide scheduled transmission opportunities to the NFRP responding non-AP stations may be provided that is different from the 802.11ax UL MU operation.

Figure 6 uses the same timeline as Figure 2 to illustrate some embodiments of the proposed priority mechanism when high-priority non-AP stations respond to the NFRP trigger frame, while Figure 7 illustrates the same embodiments when no high-priority non-AP station responds to the NFRP trigger frame.

Figures 6a and 6b illustrate, using flowcharts, corresponding general steps at the AP and a non-AP STA, respectively. The reference numbers are unchanged when referring to the same elements, frames and steps as in Figures 2.

The AP 110 is willing to poll non-AP stations using a feedback short procedure in order to provide UL MU operation.

At step S659, the AP 110 determines NFRP parameters values for NFRP trigger frame 600 to be sent and builds it.

As the priority mechanism of the invention is used to mitigate risks of collision when accessing the RU tone sets, the NFRP trigger frame 600 has to offer the RU tone sets to a plurality of non-AP stations when the priority mechanism is active.

In one embodiment, the priority mechanism is systematically used, continuously active. In that case no signaling is required. The priority mechanism can be enabled when the stations exchange their capabilities during the setup of the network.

Otherwise, the AP has to signal to the non-AP stations whether the priority mechanism is currently used or not. This may be signaled at NFRP trigger frame level, meaning that the AP can decide to activate or not the priority mechanism at each new NFRP trigger frame 25 600.

First, the AP determines whether the priority mechanism has to be used or not, i.e. it determines whether low-priority stations have to sense a selected responding RU tone set during a sensing period in which high-priority stations are supposed to send NDP feedback report responses to the NFRP trigger frame. The reasons why to activate or not the priority mechanism are out of the scope of the present invention. For illustrative purposes, the AP may consider the number of registered non-AP stations, history statistics on collisions on previous RU tone sets, etc. The AP then sets a priority enablement flag in the NFRP trigger frame depending on the outcome of the determining step. In particular, in the affirmative (priority mechanism to be used), the priority enablement flag is enabled.

Various way to signal the priority enablement flag may be envisioned.

Preferably, it is a 1-bit flag to offer two levels of priorities. Of course, other embodiments may provide a x-bit field to allow up to 2" levels of priorities (and then 2" station behaviors to sense during gradually longer sensing periods).

In one embodiment, the priority enablement flag is included in Reserved field 352 or 354. In a variant, it is included in Trigger Dependent Common Info field 340.

In other embodiments, the Feedback Type (field 353) may indicate simultaneously both the question to be answered by the non-AP stations during the short feedback procedure and the enabling or not if the priority mechanism. The Feedback Type thus acts as a priority enablement flag.

802.11ax D4.1 currently defines Feedback Type=0 for resource request without the priority mechanism of the invention. Another value of the Feedback Type may be used still for resource request but with usage of the priority mechanism. This advantageously keeps retro-compatibility with the current version of 802.11ax. For instance Feedback Type 353 Question to the polled Usage of the priority (priority enablement flag) stations mechanism 0 resource request NO 1 resource request YES While this proposal defines a single level of priority associated with a sensing period as described below, other values of the Feedback Type 353 may help to define a higher number of station priority levels associated with sensing periods having different durations or that are consecutive.

Of course, other questions may be defined, with or without the priority mechanism, using other Feedback Type values.

In yet other embodiments, a range of station AIDS may be reserved for non-AP stations concerned by the priority mechanism. The AP thus assigned these AIDs to the non-AP stations for which it wishes to offer the priority mechanism. Such assignment may be based on the station capabilities exchanged during the registration. Therefore, by targeting some of these reserved AIDS through the StartingAlD field, the AP implicitly indicates when the priority mechanism is enabled. The StartingAlD field thus acts as a priority enablement flag.

In the example of the Figures, the AP decides to use the priority mechanism by setting the priority enablement flag accordingly.

Various options are available for the AP to designate the plurality of non-AP stations that are competing on each RU tone set when the priority mechanism is used.

In some embodiments, the NFRP trigger frame 600 may offer random access to the RU tones sets to a large group of non-AP stations. The NFRP trigger frame 600 is thus referred to as RA-NFRP trigger frame. To that end, the Starting AID field 351 may be set to a predefined AID value defining a random access for the stations to the plurality of RU tone sets. This sharply contrasts with the "Starting AID" field conventionally used which defines the first AID of a restricted range of AIDs scheduled to respond to the RA-NFRP Trigger frame. Thanks to this specific predefined value, the non-AP STAs can know that a random scheme is requested for sending their feedback response to the AP. A decision for the AP to offer random access and to decide which group of non-AP STAs has to be polled may be based on network statistics and/or on history.

The predefined AID value may be set to 0 to target all the non-AP stations yet associated with the AP, or be set to a Basic Service Set Identifier, BSSID, index of a BSS to poll all the non-AP stations belonging to this BSS, or any other value that targets a specific group of non-AP stations.

In a variant to random access, the AP may decide to assign two or more AIDs per RU tone set. For instance, the Starting AID field 351 may keep its meaning while the polling range is extended. The AP may decide on the number N of AlDs assigned per RU tone set. This number may be signaled (by the priority enablement flag or field described above) is various numbers are available. The number of polled stations is thus increased to N x NSTA and the non-AP stations with AIDS belonging to [ "Starting AID" + x.N; "Starting AID"+ x.N -1] are assigned to RU tone set with RU_TONE_SET_INDEX=x.

The other NFRP parameters may be determined in a conventional way.

Next, at step S260, the AP 110 sends the NFRP trigger frame 600 to poll non-AP STAs to know their needs for transmission, by sending the NFRP trigger frame 600.

At step S270, any non-AP station 101-107 receives the NFRP Trigger frame 600 and decodes it. If the receiving non-AP station belongs to a BSS (or virtual BSS) of the transmitting AP, the Trigger Frame is not filtered by the PHY layer as defined in the standard. The filtering is made on so-called "colors" defined in the 802.11ax standard, which mandates that the BSS colors of all the multiple BSSs managed by a single AP are the same.

At step 5671, the non-AP station retrieves the priority enablement flag from the NFRP trigger frame 600. This step is optional depending on whether the AP signals the priority mechanism in the trigger frame.

At step S672, the non-AP STA determines whether it is polled (i.e. targeted) by the NFRP trigger frame 600 or authorized to access the RU tone sets on a random basis.

In case of a RA-NFRP trigger frame, the non-AP station checks StartingAlD 351 to verify whether the non-AP station belongs to the group of stations allowed to contend for access to the RU tone sets.

In case the NFRP trigger frame 600 defines a polling range [StartingAlD, StartingAlD + N x NsTA], the non-AP station only checks whether its AID belongs to the polling range. As mentioned above, the value N may depend on the priority enablement flag or field retrieved at step S671.

At the end of step S672, the non-AP STA knows whether it is allowed to respond to the NFRP trigger frame. In the affirmative, the non-AP STA determines whether it has interest in responding (test S673), for instance depending on whether it has buffered data to transmit. In the affirmative, the process goes on at step S674.

At step S674, the non-AP STA determines the index RU_TONE_SET_INDEX of the RU tone set 210 to be used to transmit its shod NDP feedback report response 211 in response to the NFRP trigger frame 600.

In case of a RA-NFRP trigger frame, the selection of the RU tone set is made on a random basis, by randomly selecting an index from among the available indexes. All the RU tone sets are available for contention. Optionally, only the RU tone sets that fit into station capabilities are eligible for contention (e.g. a station operating on a limited band BW such as a 20 MHz-only station).

The non-AP STA therefore randomly selects a RU tone set Index to send its short feedback: RU_TONE_SET_INDEX = random [ 0, NSTA -1]. Here, it is chosen to start the indexes at 0. In variant, the first index may have another value, e.g. 1 or above, and the provided formulae are modified accordingly.

In case the NFRP trigger frame 600 defines a polling range [StartingAlD, StartingAlD + N x NsTA], the scheduled non-AP STA usually selects a responding RU tone set based on the position of its AID within the above polling range: RU_TONE_SET_INDEX=x when its own AID belongs to [ "Starting AID" + x.N; "Starting AID"+ x.N -1].

Once the random RU tone set (RU_TONE_SET_INDEX) has been randomly selected at step S674, the non-AP STA determines the FEEDBACK_STATUS value, if any, at step S276.

At step S677, the non-AP station determines whether it has to use the priority mechanism. This may be based on the priority enablement flag (various signaling options) retrieved at step S671. In a variant, the priority mechanism may be systematically used.

If the priority mechanism is disabled, the non-AP station can send (step 5278 already described) its NDP feedback report response on the selected responding RU tone as from the beginning of the preamble period Tpreamble.

If the priority mechanism is enabled, the non-AP station determines (step S6770) its level of priority to respond to the NFRP trigger frame 600. Indeed, the level of priority may evolve over time and/or from one NFRP trigger frame to the other.

The priority level can be defined depending on the global status of the non-AP STA.

For instance, the non-AP STA is stated as high-priority station for any data traffics to be sent because a very low latency is required for any data traffics.

In a variant, the priority level is synchronized with the EDCA traffic access category associated with the buffered data to be sent. For instance, the access category AC_VO and AC_VI are stated as high-level access categories. Therefore, the non-AP station have some buffered data in the buffer of one of these ACs, it is considered as a high-priority station.

In a slight variant, the access category to be considered to evaluate the priority level may be the non-empty traffic access category having the lowest (EDCA) backoff counter. For instance, if the EDCA backoff counter associated with AC_BE is the lowest one, the non-AP station may be considered as low-priority, although it has AC_VO buffered data.

In other embodiments, the priority level of the non-AP station is based on an AID of the station.

For instance, the AID of the non-AP station may belong to a range of high-priority AIDs, hence defining the non-AP station as a high-priority station, while the non-AP stations not belonging to this range are considered as low-priority.

In a variant, the x LSB (x being an integer) of its 12-bit AID may match a priority pattern, hence defining the non-AP station as a high-priority station, while the non-AP stations not matching the pattern are considered as low-priority.

Also, the AID of the non-AP station may be the first one (or any other position) in the set of AIDs [ "Starting AID" + x.N; "Starting AID"+ x.N -1] assigned to the selected RU tone set, hence defining the non-AP station as a high-priority station, while the other non-AP stations of the set are considered as low-priority.

If it is determined a high level of priority of the non-AP station (test S6771), the non-AP station can send (step S278 -already described) the NDP feedback report response on the selected responding RU tone as from the beginning of the preamble period Tpreamble. This is illustrated in Figure 6, for instance by Station 1.

If the non-AP station is a low-priority station, the MAC block 502 of the non-AP STA generates and transmits (S6772) a TXVECTOR to cause the PHY 503 of the non-AP STA to transmit over the tones set of the selected RU tone set after a predetermined sensing period Tsensing, should the RU tone set be sensed as idle.

During Tsensing, the PHY block 503 has to sense the medium, in particular those tones of the selected RU tone set.

As shown in the Figures, this predetermined sensing time period Tsensing is divided into two parts: the preamble period Tpreamble to detect PHY preamble 212 sent by others polled non-AP STAs and an energy sensing period Tpriority to detect potential collision on the selected RU tone set. Tpnonty immediately follows Tprearnble. Tprionty may for instance last a backoff time, i.e. 9ms in 802.11ax D4.1.

Conventional sensing techniques may be used.

At step S6773, the non-AP STA determines whether a PHY preamble corresponding to the header 212 of the HE TB Feedback PPDU is detected or not, i.e. whether other polled non-AP stations are simultaneously responding to the NFRP trigger frame 600. For this, the non-AP STA senses the medium during Tpreamble. The PHY preamble is transmitted over the 20MHz channel.

If no PHY preamble is sensed, meaning that no other polled non-AP STA with a high-priority level is currently transmitting a HE TB PPDU over the 20MHz channel, the non-AP STA can send a HE TB Feedback PPDU as described above with reference to Figure 3b, i.e. including the PHY preamble. Indeed, no other non-AP station is currently using the RU tone set selected at step S674. This is illustrated in Figure 7. This avoids the 20MHz channel to be preempted by other legacy non-AP STAs. Preferably, the non-AP station starts sending the HE TB Feedback PPDU as from the end of Tpreamble (i.e. beginning of Tprienly). In a variant, the non-AP station starts sending the HE TB Feedback PPDU as from the end of the predetermined sensing period tensing. If a PHY preamble is sensed at step S6773, meaning that at least one polled high-priority non-AP STA is currently transmitting its NDP feedback report response 211, the non-AP STA continues sensing the tones of its selected RU tone set in order to determine whether it is idle or not.

The sensing is performed during the energy sensing period Tpriorily.

At step S6774, the non-AP STA determines whether the medium is idle on the selected RU tone set.

In the negative, meaning that a polled high-priority non-AP STA has selected the same RU tone set and is currently using it, the non-AP STA cannot use the RU tone set otherwise collision would occur. Therefore, the process ends.

On the other hand, if the medium remains free on the selected RU tone set, meaning that no polled high-priority non-AP STA has selected the same RU tone set, the non-AP STA (which is low-priority) can send (S678) its feedback response. It does it by transmitting only energy (Null Data Packets), i.e. padding without PHY preamble, on the selected responding RU tone set for the remaining time dedicated to the NDP Feedback Report procedure. The transmission preferably starts as from the end of Tpnordy. This is illustrated by the behavior of Station 2 in Figure 6.

As apparent from these explanations, the proposed mechanism reduces risks of collision on the RU tone sets by avoiding any collision between a high-priority station and a low-priority station.

However, some RU tone sets are not selected by any polled non-AP station (e.g. RU tone set 610e) while others may experiment collisions (610c).

The AP receives and decodes (S262) the RU tone sets where energy is present, to provide to its MAC layer a list of used RU_TONE_SET_INDEX and the corresponding Feedback responses (FEEDBACK_STATUS values). At this stage, the AP knows the transmission needs of the polled non-AP stations.

As various non-AP stations can use the same RU tone set, it is not possible for the AP to know exactly which non-AP station has responded on which RU tone set.

At step S664, the AP can send a subsequent trigger frame 620 to offer new opportunities (RUs) to the responding non-AP STAs, for example a 'Basic' type trigger frame or any convenient type. Preferably, the scheduled RUs are of narrow width (26 tones) to offer a maximum of nine RUs per 20MHz channel. The AP may select a subset of the responding non-AP STAs.

By failing to know which non-AP STA has emit energy on a given RU tone set 210, it is impossible for the AP to schedule the responding non-AP STAs through their AIDS in the trigger frame 620.

The AP may thus assign a scheduled resource unit to a responding non-AP station using the index RU_TONE_SET_INDEX of the corresponding responding RU tone set to define the AID (so-called AID12 field) associated with the scheduled RU.

The AP may directly use the index RU_TONE_SET_INDEX as value for the AID12

field.

However, in order to avoid these scheduled index-based AIDs to fall on conventionally-used AIDs (for BSS or for individual non-AP STAs, typically values from 1 to 2007 and some values below 2048 such as 2045 and 2046, and value 4095 is reserved to indicate start of a Padding field), the AID associated with the scheduled resource unit in the subsequent trigger frame may be built from the index RU_TONE_SET_INDEX of the responding RU tone set and from an offset value Offset_AID.

For instance, the AID12 field of a User Info field defining the scheduled RU may be set to the following RA_NDP_AID value: RA_NDP_ AID = Offset_AID + RU_TONE_SET_INDEX + STARTING_STS_NUM x Nfeedback x 26w where STARTING_STS_NUM is parameter handling the spatial multiplexing. It is a station parameter that corresponds to a starting spatial stream number minus 1. It is set to 0 if the MultiplexingFlag 356 of the NFRP trigger frame 600 is set to 0 (no spatial multiplexing), otherwise it is set as follows: STARTING_STS_NUM = entire_value ( RU_TONE_SET_INDEX / Nfeedback / 26W) The Offset_AID parameter is a predetermined offset value known by the non-AP STAs and the AP. In some embodiments, the Offset_AID parameter is transmitted by the AP to the stations in a management frame, e.g. periodically in beacon frames.

Preferably, the Offset_AID parameter is selected such that any subsequent RA_NDP_AID falls outside the legacy range of Association Identifiers (AIDs) provided by AP to associated non-AP STAs. For instance, the offset value is 2048 or above. It is then added to the index RU_TONE_SET_INDEX of the responding RU tone set to form the AID (AID12 field) associated with the scheduled resource unit.

Using an offset value of 2048 to form the 12-bit AID field makes it possible to work on the MSB (set to 1) to easily distinguish between conventional AIDs and those used for the present invention. Furthermore, it allows scheduled RUs for non-AP STAs responding to the NFRP trigger frame 600 to be mixed with scheduled RUs for other non-AP stations directly per their own AID value, with no risk of misunderstanding.

In case of mixing, the subsequent trigger frame 620 first declares all the resource units (it may be a single one) assigned to individual non-AP stations using their own assigned AID, and then declares all resource units (may be a single one) assigned to responding non-AP stations using indexes RU_TONE_SET_INDEX of the responding RU tone sets (preferably using RA_NDP_ AID).

Of course, the subsequent trigger frame 620 may only comprise resource units for non-AP stations responding to the NFRP trigger frame 600 (i.e. RUs with only AID12 set based on RA_NDP_ AID).

In all case, the subsequent trigger frame may only comprise scheduled resource units (assigned to respective individual non-AP stations) without random RUs.

At step 5664, the AP 110 thus sends the subsequent basic trigger frame 620 so built.

Any non-AP STA receiving the subsequent trigger frame 620 thus determines (step S680) whether it is scheduled, i.e. whether a resource unit is assigned to the non-AP station based on the index RU_TONE_SET_INDEX of the responding RU tone set determined and used by the non-AP station at steps S674 and S278 or S678.

The non-AP STA having responded to the NFRP trigger frame 600 uses the formula above to determine its own RA_NDP_ AID and compares it to the AID12 fields specified in the User Info fields of the subsequent trigger frame 620. The non-AP STA thus determines whether an AID associated with a scheduled resource unit in the subsequent trigger frame corresponds to the index RU_TONE_SET_INDEX used given the predefined offset value Offset_AID.

In a preferred embodiment where Offset_AID is set to value 2048, all RUs with MSB set to 1 are analyzed in order than the remaining value (not considering the MSB bit) equals to the RU tone set index RU_TONE_SET_INDEX the non-AP station has previously used.

Of course, in case of mixing RUs with conventional AIDS and index-based AIDS, the non-AP station may be scheduled twice, in which case it should give priority to a scheduled RU with its own AID in order to offer the RU with its index-based AID (if any) to any other colliding non-AP STA having responded on the same RU tone set. In other words, if the non-AP station also determines in the subsequent trigger frame a resource unit that has an associated AID corresponding to an AID assigned by the AP to the non-AP station, the non-AP station discards or disregards the resource unit with the AID corresponding to the index of the selected responding RU tone set to use the resource unit with the assigned AID to send the trigger-based PPDU response. This approach reduces risks of collision in the RUs and may be easily achieved through the order of RU declaration performed by the AP in the subsequent trigger frame 620. Indeed, the non-AP STA may disregard any further User Info fields as soon as it finds one with its own AID.

Therefore, placing the RUs with AIDS assigned upon registration before the RUs with index-based AIDs allows the above priority scheme to be naturally performed.

The non-AP station may thus first determine whether one Resource Unit is allocated to it by positively finding its station AID in the AID12 field of one RU. If not found, a further determination is performed in case that the non-AP station has previously sent a NDP feedback response 211 in response to the NFRP trigger frame 600. The further determination relies on the formula for determining RA_NDP_AID value, considering the RU tone index RU_TONE_SET_INDEX used for the NDP Feedback report response and the predetermined offset value (Offset_AID).

In case of positive determination at step S680, the non-AP STA can use the RU scheduled to it and transmit data 231 (HE TB PPDU) to the AP. This is step 5282. The HE TB PPDU 231 contains the MAC address of the sending non-AP station, making it possible for the AP to identify each sending non-AP station.

As illustrated in Figures 6 and 7: - Station 2 has selected a RU tone set index having value 0 (NDP feedback emitted on the first tone set), then its allocated a scheduled RU 230-1 with RA_NDP_ AID=2048; - Station 20 has selected a RU tone set index having value 3 (NDP feedback emitted on 4th tone set), then its allocated a scheduled RU 230-2 with RA_NDP_ AID=2051; -at least two non-AP stations have selected the same RU tone set index having value 2 (NDP feedback emitted on the third tone set corresponding to 610c), then they are allocated the same RU 230-3 because they use the same RA_NDP_ AID= 2050. As a result, the at least two non-AP stations collide in the RU 230-3.

The AP 110 thus receives the HE TB PPDU 231 over the multiple scheduled RUs. It can then acknowledge (or not) the data on each RU by sending a multi-STA block acknowledgment (BA) response 240, making it possible for each sending non-AP STA to know when its data transmission is successful (reception of the ACK) or not (no ACK after expiry of a time-out). This is step S266.

For instance, it may not acknowledge data over RU 230-3 (Figure 6 or 7) as it detects a collision.

As the acknowledgment (no collision) generally uses the AIDS of the sending non-AP stations, the AP 110 may obtain an AID of the responding non-AP stations using the MAC addresses specified in the HE TB PPDU 231 and thus retrieved therefrom. The non-AP stations are thus only discriminated at this final stage.

The examples of Figures 6 and 7 show a single TXOP 660 during which the NDP Feedback Report procedure and subsequent UL MU operation are both conducted. This ensures that the feedback responses 211 are still relevant when they are exploited by the AP to provide the subsequent UL MU operation based on these responses. In addition, it advantageously avoids a random tone set index to be kept by a non-AP station outside the TXOP; otherwise, this would require keeping in memory this random index for further usage by Data trigger frame TF 620 or until a next NFRP trigger frame 600.

However, TXOP 660 may be split into two separate TXOPs, and/or alternatively several subsequent trigger frames 620 (possibly cascaded) may be issued in order to address more non-AP stations responding to the NFRP trigger frame 600 (as only 9 maximum stations per 20Mhz can be triggered for data RU transmission per Basic trigger frame).

The random-based access scheme provides good efficiency: the collision is largely performed on the NDP feedback responses 211 which are shorter in duration, and the RUs used for HE TB PPDU 231 are never empty.

The best theoretical probabilities for classical random access distributions (such as slotted ALOHA -type) are the following: probability of no collision (success) nearly 37%, compared to 37% for empty, and 26% collisions. This offers a theoretical efficiency ratio of 37% for UORA for instance.

The implementation of the random-based access scheme substantially improves this situation as the random access is moved to the short time NDP Feedback report procedure. It turns that no random RUs (and thus no empty RUs) are met in the subsequent UL MU operation (triggered by trigger frame 620).

Applying a random selection among 18 RU tone set index during the NDP Feedback report procedure provides approximately 6.66 (6) success indexes in addition to 4.68 (4) collisions (for a total of 10 full occupied indexes). As a result, it is expected that most of these occupied slots could be later scheduled in the HE TB PPDU (there is no scheduling for empty RU indexes). Finally, on average, the maximum efficiency for the transmission above 9 RUs would be: 37 / (37+26) = 58.7% as there are no longer empty RUs. This is a high improvement compared to the conventional 37% of UORA scheme.

Although the present invention has been described herein above 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 referring 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 (22)

1. CLAIMS1. A communication method in a wireless network, comprising the following steps at a station: receiving, from an access point, AP, a null data packet, NDP, feedback report poll, NFRP, trigger frame, the NFRP trigger frame reserving a plurality of resource unit, RU, tone sets for NDP feedback report responses by stations, selecting a responding RU tone set from the plurality of RU tone sets, sensing the selected responding RU tone set during a sensing period in which other stations are supposed to send NDP feedback report responses to the NFRP trigger frame, sending a NDP feedback report response on the selected responding RU tone when the latter is sensed idle during the sensing period.
2. 2. The method of Claim 1, further comprising, at the station, determining a level of priority of the station to respond to the NFRP trigger frame, and performing the sensing only when it is determined a low level of priority.
3. 3. The method of Claim 2, wherein when it is determined a high level of priority of the station, the method further comprises, at the station, sending the NDP feedback report response on the selected responding RU tone as from the beginning of the sensing period.
4. 4. The method of Claim 2, wherein the priority level of the station is based on a traffic access category having a non-empty buffer.
5. 5. The method of Claim 4, wherein the traffic access category is the non-empty traffic access category having the lowest backoff counter from amongst a plurality of traffic access categories having each one a respective backoff counter decremented over time.
6. 6. The method of Claim 2, wherein the priority level of the station is based on an AID of the station.
7. 7. The method of Claim 1, further comprising, at the station, retrieving a priority enablement flag from the NFRP trigger frame, and performing the sensing only if the priority enablement flag is enabled.
8. 8. The method of Claim 7, wherein if the priority enablement flag is disabled, sending the NDP feedback report response on the selected responding RU tone as from the beginning of the sensing period.
9. 9. The method of Claim 7, wherein the priority enablement flag is included in a Reserved field of a User Info field of the NFRP trigger frame according to Draft 4.1 of IEEE 802.11ax.
10. 10. The method of Claim 7, wherein the priority enablement flag is included in a Trigger Dependent Common Info field of a Common Info field of the NFRP trigger frame according to Draft 4.1 of IEEE 802.11ax.
11. 11. The method of Claim 7, wherein the priority enablement flag is defined by afeedback type field in the NFRP trigger frame.
12. 12. The method of Claim 1, wherein the sensing period comprises at least a preamble period during which the other stations are supposed to simultaneously transmit a PHY preamble of their NDP feedback report responses.
13. 13. The method of Claim 12, wherein if no PHY preamble is sensed on the selected responding RU tone during the preamble period, the sent NDP feedback report response includes a PHY preamble.
14. 14. The method of Claim 12, wherein if a PHY preamble is sensed on the selected responding RU tone during the preamble period, the sensing period further comprises an energy sensing period immediately following the preamble period, and if the selected responding RU tone set is sensed as idle during the energy sensing period, sending the NDP feedback report response includes transmitting only energy, without PHY preamble, on the selected responding RU tone set.
15. 15. The method of Claims 13 and 14, wherein the selected responding RU tone set is sensed during the energy sensing period only if a PHY preamble is sensed on the selected responding RU tone during the preamble period.
16. 16. The method of Claim 12, wherein the energy sensing period lasts a backoff time.
17. 17. The method of Claim 1, further comprising determining, from the NFRP trigger frame, that the RU tone sets are accessed on a random basis, wherein selecting the responding RU tone set comprises randomly selecting the responding RU tone set from the plurality of RU tone sets.
18. 18. The method of Claim 17, wherein the determining step comprises determining whether an association identifier, AID, field in the received NFRP trigger frame includes a predefined AID value defining a random access for the stations to the plurality of RU tone sets.
19. 19. A communication method in a wireless network, comprising the following steps at an access point: sending, to stations, a null data packet, NDP, feedback report poll, NFRP, trigger frame, the NFRP trigger frame reserving a plurality of resource unit, RU, tone sets for NDP feedback report responses by stations, and receiving NDP feedback report responses on respective responding RU tone sets, wherein the AP starts receiving a first NDP feedback report response on a first responding RU tone set after an idle period during which the first responding RU tone set is idle and at least one other NDP feedback report response is received on another responding RU tone set.
20. 20. The method of Claim 19, wherein the NFRP trigger frame includes a priority enablement flag set an enabled value when low-priority stations have to sense a selected responding RU tone set during a sensing period in which high-priority stations are supposed to send NDP feedback report responses to the NFRP trigger frame.
21. 21. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a communication device, causes the communication device to perform the communication method of Claim 1 or 19.
22. 22. A communication device comprising at least one microprocessor configured for carrying out the steps of the communication method of Claim 1 or 19.