EP4696086A1

EP4696086A1 - Methods and devices for twt coordination in multi-ap operation

Info

Publication number: EP4696086A1
Application number: EP24718448.4A
Authority: EP
Inventors: Pascal Viger; Stéphane Baron; Patrice Nezou
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2023-04-14
Filing date: 2024-04-09
Publication date: 2026-02-18
Also published as: KR20250164217A; WO2024213545A1; CN121080099A; JP2026513743A

Abstract

To reduce OBSS interference, a neighbouring AP in a MAP coordination set of APs receives, from a target AP managing a target BSS, a MAP coordination frame specifying a TWT schedule provided in the target BSS. The neighbouring AP defines an OBSS TWT schedule having TWT SPs at least partially overlapping with TWT SPs of the TWT schedule in the target BSS. The OBSS TWT schedule forbids the stations of the neighbouring BSS to access the wireless medium during the OBSS TWT SPs. Alternatively, it defines reduced communication activities for those stations during the OBSS TWT SPs, by setting an OBSS EDCA Parameter Set to be used. In a further alternative, it indicates a temporary operating channel for stations of the neighbouring BSS to switch to the temporary operating channel for their communications during the OBSS TWT SPs.

Description

METHODS AND DEVICES FOR TWT COORDINATION IN MULTI-AP OPERATION

FIELD OF THE INVENTION

The present invention generally relates to wireless communications and more specifically to coordination in multi-AP operations.

BACKGROUND OF THE INVENTION

Wireless communication networks are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc. These wireless networks may be multiple-access networks capable of supporting multiple users by sharing the available network resources. Examples of such multiple-access networks include Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA) networks.

The IEEE (Institute of Electrical and Electronics Engineers - RTM) 802.11 be draft standard Task Group addresses a so-called Multi-Access Point (Multi-AP or MAP) technology which aims at providing some collaboration between neighbouring access points (APs) managing separate BSSs in order to have a more efficient utilization of time, frequency and spatial resources available. This is particularly important when the neighbouring APs operate over the same selected communication channel (or channel sufficient close to communicate which each other) in which interference may occur. In that case, the BSSs are referred to as overlapping BSSs or OBSSs

The proposed MAP mechanisms allow two or more neighbouring APs to share resources in terms of frequency and/or time and, in this way, they intend to prevent interferences.

The MAP topic is now addressed as a main feature of 802.11 bn Task Group, the successor of the 802.11 be Task Group. The scope of the MAP topic is extended to optimized coordination, not only regarding shared transmissions but also regarding alternative mechanisms for OBSS Interference reduction. In other words, MAP coordination becomes one of emerging features for interference management in WLAN networks: multiple APs can cooperate together to enhance the performance of the network by smartly managing the interference due to OBSSs.

A contribution, namely contribution 11-22-1530 (entitled “Multi AP coordination for next-generation Wi-Fi”), has been made to the 802.1 1 UHR Study Group to propose a TWT coordination that reduces OBSS interference. Publication US 2022/408355 discloses the same mechanism.

Target Wake Time (TWT) mechanism, originally defined in the IEEE 802.1 1 ah, is one of the important features of the IEEE 802.11 ax amendment, referenced 802.11 ax-2021 . TWT enables wake time negotiation between an AP and an associated station (STA) for improving power efficiency. With TWT operation, a STA has only to wake up at a pre-scheduled time negotiated with another STA or AP in the network. TWT has been adapted to be included in the IEEE 802.11 be standard (last version D3.0). An adaptation is known as the Restricted Target Wake Time (R-TWT) which schedules dedicated (and protected) service periods (SPs) for stations (possibly affiliated with a non-AP multi-link device, MLD, as introduced in IEEE 802.11 be) to convey their latency sensitive traffic(s) over their BSS. An R-TWT agreement is nothing more than a Broadcast TWT agreement negotiated between an AP and an associated non-AP station of the BSS on a given link. The non- AP station establishes with the AP membership in the Broadcast TWT (or R-TWT) schedule. The R-TWT Service Periods SPs of the R-TWT schedule are advertised in broadcast management frames (e.g., beacons), using R-TWT information about the negotiated R-TWT SPs, typically a Broadcast TWT identifier (bTWT ID).

Above contribution and publication US 2022/408355 intend to coordinate MAP with the R-TWT procedure. The R-TWT feature is used to avoid OBSS interference in between the BSSs of the MAP coordinated group, i.e., MAP coordination set of APs.

Indeed, in a dense deployment scenario where neighbouring BSSs corresponding to two or more APs overlap each other, if one AP has an R-TWT schedule for which the corresponding scheduled STAs fall in a geographic area that has overlap with a neighbour BSS, then the R-TWT scheduled STAs would possibly face interference from the neighbouring BSS's operation.

To provide better protection for latency-sensitive traffic for its member STAs, the solution proposed in the contribution and publication handles interference between neighbouring BSS activities during a restricted TWT service period by managing a Quiet interval in the neighbouring BSS.

To do so, when a target BSS has one or more R-TWT schedules, it sends Quieting requests to the neighbouring APs in its coordination set of APs and also shares its R-TWT schedules with them. If a neighbouring AP accepts the quieting request from the target AP, it sends a Quiet element to its associated stations, the Quiet element corresponding to the R-TWT schedule for which the Quieting request was sent by the target AP.

Quiet element is a key component of the IEEE 802.1 1 h standard, defining an interval during which no transmission occurs in the channel. This is originally used to perform channel measurements, specially to avoid interfering with certain military radar technologies.

The MAP R-TWT coordination based on Quiet element is however not satisfactory.

A first drawback of this coordination scheme is the difficulty for the neighbouring APs to locally (in their own BSSs) schedule a Quiet period (interval) corresponding to a target BSS’s R-TWT SPs. Indeed, the TBTTs (Target Beacon Transmission Time) in between two BSSs are not aligned (in time) and may not last the same duration. Furthermore, the Quiet element and the TWT element do not use the same timing reference: the Quiet element is based on the TBTT (hence the beacon frames) while the TWT element is based on the TSF (Time Synchronization Function) timer synchronized between all the stations of the BSS. A second drawback is closely linked to the signalling of the Quiet element. As it is signalled in the beacon frame with a Quiet Count strictly positive, the Quiet Period can be effective only after the next beacon frame. It turns out that the Quiet-based coordination cannot have immediate application in the neighbouring BSS.

A third drawback comes from the fact that the Quiet scheme is very aggressive in that all the stations of the neighbouring BSS are forbidden to transmit. As a result, the medium is poorly used by the stations of the neighbouring BSS.

A fourth drawback is the unmanaged situation of legacy devices (before 802.11 ax) when several Quiet elements are specified during a TBTT. Generally, 802.11 ah measurements were at most envisaged once per TBTT. Thus, an appropriate use of Quiet element should be limited to a single occurrence per TBTT, which may not meet the R-TWT schedule of the target BSS.

A fifth drawback is that the Quiet-based approach does not appear compatible with the multi-link devices (MLDs). Indeed, IEEE802.11 be provides some restrictions on how the MLDs operate, in particular that an EHT AP affiliated with an AP MLD shall not include in its transmitted Beacon or Probe Response frames any Quiet elements that overlap quiet intervals that are scheduled and advertised by other APs affiliated with the same AP MLD. Therefore, it seems extremely difficult to apply a Quiet interval of time that encompasses R-TWT SPs in at least one neighbouring BSS (i.e., on one link of the MLD) but at the same time does not encompass a Quiet period on another neighbouring BSS of the same MLD (i.e., on another link of the same MLD).

Therefore, there is a need to provide more efficient TWT coordination in MAP operation, and more generally to provide more efficient MAP operation and MAP coordination.

SUMMARY OF INVENTION

It is a broad objective of the present invention to overcome some of the foregoing concerns. The inventors have noticed that replicating the R-TWT or TWT SPs of the target BSS into corresponding TWT SPs in the neighbouring BSS allows the neighbouring AP to adjust a medium access policy in its BSS that reduces or even avoids OBSS interferences during the R- TWT SPs of the target BSS.

In this context, a communication method over a medium (or channel) is proposed in a wireless network, comprising, at a second access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from a first AP managing a first Basic Service Set, BSS, a MAP coordination frame specifying a Target Wake Time, TWT schedule provided in the first BSS, and responsive to the receiving, sending, to stations of a second BSS managed by the second AP, a frame including an overlapping BSS (OBSS) TWT element defining second TWT Service Periods, SPs, timely aligned with first TWT SPs of the TWT schedule in the first BSS. “Timely aligned” or “time aligned” does not mean that the first and second TWT SPs perfectly overlap. By “timely aligned” or “time aligned”, it is understood that the first and second TWT SPs overlap in such a way absence of communication in the second BSS due to the second TWT SP benefits to the first TWT SP. In that respect, the overlap is preferable organized over at least the beginning of the first TWT SP. For example, the first and second TWT SPs may start simultaneously, meaning their starting times are fully aligned in time. In this context, the second TWT SP may be shorter than the corresponding first TWT SP, while both SPs starts at the same time.

By using the second TWT SP, the second (neighbouring) AP has the same timing reference as the first (target) AP. Furthermore, the signaling of the TWT schedule can be made by the second AP not only in beacon frames but also in other management frames, thus allowing the second TWT SPs to be declared and to start immediately. Multiple second TWT SPs can also be scheduled during each TBTT, while the quiet-period-based restriction for MLDs no longer applies.

Furthermore, the second AP may also adjust the medium access policies for its associated stations during the second TWT SPs, hence providing a less aggressive scheme that known techniques, to reduce OBSS interferences.

Correspondingly, a second access point (AP) device is also provided in a multi-AP (MAP) coordination set of APs that manages a second Basic Service Set, BSS, in a wireless network having a medium, the second AP comprising: a receiver configured to receive, from a first AP managing a first BSS a MAP coordination frame specifying a Target Wake Time, TWT schedule provided in the first BSS, and a transmitter configured to send, responsive to the reception of the MAP coordination frame, to stations of the second BSS, a frame including an overlapping BSS (OBSS) TWT element defining second TWT Service Periods, SPs, timely aligned with first TWT SPs of the TWT schedule in the first BSS.

This second (neighbouring) AP has the same advantages as the method described above.

Optional features are defined below with reference to a method, while they can be transposed into device features.

In some embodiments, the OBSS TWT element limits communication activity of the stations of the second BSS during the first TWT SPs of the first BSS. Exemplary limitation of communication activity includes driving a restricted access to the medium as explained below. In that way, the second AP keeps control on the risks of OBSS interference.

In some embodiments, the OBSS TWT element forbids the stations of the second BSS to access the medium during the second TWT SPs. As a consequence, risks of OBSS interference are substantially reduced during the TWT SPs of the first BSS. In some embodiments, the OBSS TWT element indicates a temporary operating channel for stations of the second BSS to switch to the temporary operating channel for their communication during the second TWT SP.

In specific embodiments, a Minimum TWT Wake Duration field within the OBSS TWT element is set to 0.

In some embodiments, the OBSS TWT element defines a waiting time for the stations of the second BSS to access the medium that is larger than a waiting time for stations of the first BSS to access the medium during the first TWT SPs. This configuration provides a more permissive scheme than a medium access prohibition. In the same time, priority is kept for the stations of the first BSS to gain access to the medium during time-aligned TWT schedule within the first BSS. Hence, communication within the second BSS is improved during the OBSS TWT schedule, while risks of interference with the first BSS are substantially reduced thanks to the priority given to the stations of that first BSS.

In some embodiments, the waiting time for the stations of the second BSS is computed based on a Minimum TWT Wake duration specified in the MAP coordination frame. The first AP then keeps control on how the neighbouring BSS can offer access to the medium during the TWT schedule. The first AP can therefore dynamically adjust the waiting time for those neighbouring BSSs over time.

In some embodiments, the OBSS TWT element includes a field setting a minimum waiting time before the stations of the second BSS can access the medium after a second TWT SP starts.

In other embodiments, the OBSS TWT element includes an OBSS EDCA Parameter Set, different from a legacy EDCA Parameter Set, to be applied by the stations of the second BSS to contend for access to the medium during the second TWT SPs. This approach advantageously extends known mechanisms to OBSS parameters, for the second AP to efficiently drive the waiting time of its stations when accessing the medium during the OBSS TWT SPs.

In specific embodiments, the OBSS EDCA Parameter Set includes a degraded ECWmin value. The second AP thus forces its stations to statistically wait for a longer time than in a conventional operating mode. This results in less medium access attempts by its stations, hence in less OBSS interference with the first BSS.

In other embodiments, the OBSS EDCA Parameter Set includes a degraded AIFSN value. The second AP thus drives its stations to wait for a longer time before starting decrementing their backoff counter to gain access to the medium. This results in giving priority for medium access to stations having a lower AIFSN, basically those of the first BSS, hence results in less OBSS interference with the first BSS.

In some embodiments, the degraded AIFSN value is set by the second AP based on a minimum waiting time specified by the first AP in the MAP coordination frame. In particular, the degraded AIFSN value may be set by the second AP to a Minimum TWT Wake Duration negotiated with the first AP and specified in the MAP coordination frame. In other embodiments, the degraded AIFSN value is to be added to a legacy AIFSN value by the stations of the second BSS to obtain an AIFSN value to be applied when contending for access to the medium during the second TWT SPs.

In some embodiments, the OBSS EDCA Parameter Set includes an OBSS EDCA Timer indicating a duration of time during which the stations of the second BSS use OBSS EDCA parameters before switching back to legacy EDCA parameters, wherein the OBSS EDCA Timer is based on a Minimum TWT Wake Duration field specified in the MAP coordination frame. A fine control of the stations of the second BSS to reduce risks of OBSS interference (through the use of the OBSS EDCA Parameters) is therefore provided.

In some embodiments, the waiting time is specified in the MAP coordination frame. In particular, the waiting time may be set to a duration of the first TWT SPs.

In some embodiments, the second AP obtains information about interfering stations of its second BSS that are interfering with the first BSS and free stations of its second BSS that are not interfering with the first BSS, wherein the OBSS TWT element limits communication activity over the medium for the interfering stations only. This configuration optimizes the use of the second BSS as stations not interfering with the first BSS are not subject to OBSS limitation according to the invention. Hence, they can keep going with a normal use of the medium, without risks of OBSS interference with the first BSS.

In specific embodiments, the frame including the OBSS TWT element is sent to the interfering stations only.

In embodiments, the second AP checks whether the first TWT SPs do not overlap TWT SPs initially scheduled by the second AP within the second BSS.

In specific embodiments, the method further comprises sending, to the first AP, a positive response in case of no overlapping; otherwise sending a negative response.

In some embodiments, the method further comprises converting timing information of the received TWT schedule in the first BSS that is based on a first clock applicable to the first BSS, into timing information based on a second clock applicable to the second BSS, wherein timing information within the OBSS TWT element is second-clock-based timing information.

In some embodiments, the frame including the OBSS TWT element is a beacon frame broadcast by the second AP in the second BSS. This allows the second AP to periodically inform all the stations about the limitation of their communication activity during the OBSS TWT SPs.

In some embodiments, the beacon frame includes an OBSS TWT Update counter that is incremented each time an OBSS TWT parameter is updated in the OBSS TWT element.

In other embodiments, the frame including the OBSS TWT element is a probe response frame sent by the second AP in the second BSS. This allows the second AP to early specify the OBSS TWT schedule and the OBSS TWT Parameter Set to the stations, in particular well before the next beacon frame. In specific embodiments, the probe response frame is broadcast to the stations of the second BSS.

In some embodiments, the OBSS TWT element is a restricted TWT element having a Restricted TWT Traffic Info section comprising an OBSS TWT field that is enabled. The Restricted TWT Traffic Info section is defined in the series of IEEE802.11 be standards, e.g., D3.0. The proposed signaling hence relies on an existing section, without requiring new bits to be added to existing formats.

In some embodiments, the MAP coordination frame is a beacon frame broadcast by the first AP within the first BSS.

In particular embodiments, the TWT schedule received by the second AP is defined by a TWT element addressed to stations of the first BSS within the beacon frame. In that case, a conventional TWT element, e.g., a R-TWT element, is reused, without incurring additional signaling costs.

In some embodiments, the MAP coordination frame is a dedicated unicast frame addressed by the first AP to the second AP over the medium or using an alternate communication medium. Using an alternate communication medium advantageously avoids any signaling cost in the BSSs. In that case, the link or network impacted by the communication interference and for which the TWT coordination is expected may be identified in the MAP coordination frame.

In other embodiments, the MAP coordination frame is included in a negotiation process between the first and second APs to establish a MAP TWT agreement. In that way, only required TWT information may be sent, and not the entire conventional TWT element.

In yet other embodiments, the MAP coordination frame is exchanged between a third AP co-affiliated with the first AP to the same first AP multi-link device, MLD, and a fourth AP coaffiliated with the second AP to the same second AP MLD, wherein the third and fourth APs operate on a link separate from the link on which the first and second APs operate. This takes advantage of the MLD operation. The third AP therefore sends the MAP coordination MAP on behalf of the first AP experiencing the OBSS interference. The fourth AP receiving the MAP coordination frame may then forward it to the second AP, within the second AP MLD, for the second AP to conduct the TWT coordination according to the invention.

The invention also provides a communication method over a medium in a wireless network, comprising, at a station of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from the AP, a frame including an overlapping BSS (OBSS) TWT element defining TWT Service Periods, SPs, switching from a legacy EDCA Parameter Set to an OBSS EDCA Parameter Set to contend for access to the medium during one of the TWT SPs.

Correspondingly, a station device of a Basic Service Set, BSS, managed by an access point (AP) is also provided in a multi-AP (MAP) coordination set of APs, in a wireless network having a medium, the station comprising: a receiver configured to receive, from the AP, a frame including an overlapping BSS (OBSS) TWT element defining TWT Service Periods, SPs, a communication module configured to switch from a legacy EDCA Parameter Set to an OBSS EDCA Parameter Set to contend for access to the medium during one of the TWT SPs.

The station and associated method advantageously reduce risks of OBSS interference with another BSS having scheduled TWT SPs, by using a dedicated OBSS EDCA Parameter Set. As mentioned above, the TWT SPs of the OBSS TWT schedule are indeed defined time aligned with TWT SPs (to be protected) scheduled in the other BSS.

In some embodiments, the switching takes place so that the OBSS EDCA Parameter Set starts to be used when the TWT SP starts.

In some embodiments, the OBSS EDCA Parameter Set is included in a beacon frame broadcast by the AP.

In variants, the OBSS EDCA Parameter Set is included in the OBSS TWT element.

In some embodiments, the OBSS EDCA Parameter Set includes a degraded AIFS value and/or a degraded ECWmin value.

In specific embodiments, the degraded AIFS value is added to a legacy AIFS value when contending for access to the medium during the TWT SP.

In some embodiments, the method further comprises switching back to the legacy EDCA Parameter Set after an OBSS duration defined by a Minimum TWT Wake Duration field specified in the OBSS TWT element or when an OBSS EDCA Timer specified in the OBSS TWT element elapses.

In some embodiments, the method further comprises, at the station, ending any ongoing transmission over the medium, before the start of one (each) of the TWT SPs.

The invention also provides a communication method over a medium in a wireless network, comprising, at a station of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from the AP, a frame defining a TWT Service Period (SP), and switching from an initial operating channel to a second operating channel to contend for access to the medium over the second operating channel during the TWT SP.

Correspondingly, a station device of a Basic Service Set, BSS, managed by an access point (AP) is also provided in a multi-AP (MAP) coordination set of APs, in a wireless network having a medium, the station comprising: a receiver configured to receive, from the AP, a frame defining a TWT Service Period

(SP), and a communication module configured to switch from an initial operating channel to a second operating channel to contend for access to the medium over the second operating channel during the TWT SP.

The station and associated method advantageously avoid OBSS interference with another BSS having scheduled TWT SPs by switching to second (e.g., temporary) operating channel, a dedicated OBSS EDCA Parameter Set. The TWT SP may be at least partially overlapping a TWT SP (to be protected) in another BSS managed by another AP of the MAP coordination set.

In some embodiments, the received frame includes an overlapping BSS (OBSS) TWT element defining the TWT SP.

The invention also provides a communication method over a medium in a wireless network, comprising, at an access point (AP) in a multi-AP (MAP) coordination set of APs and managing a Basic Service Set, BSS, the following step: sending, to stations of the BSS, a legacy EDCA Parameter Set defining EDCA parameters to be used by the stations when contending for access to the medium, a Multi-User (MU) EDCA Parameter Set defining MU EDCA parameters to be temporarily used by the stations when contending for access to the medium after they made UpLink MU transmission, and an overlapping BSS (OBSS) EDCA Parameter Set defining OBSS EDCA parameters to be temporarily used by the stations when contending for access to the medium during an OBSS Service Period, defined by the AP for the BSS, that is time aligned with neighbouring TWT SPs of a TWT schedule in a neighbouring BSS.

In this respect, the AP now broadcasts at least three distinct sets of EDCA parameters to be used by its associated stations in respective situations: after UL MU HE TB PPDU transmission and during the MU EDCA Timer duration for the MU EDCA Parameter Set; after an OBSS TWT SP starts and optionally during an OBSS EDCA Timer duration for the OBSS EDCA Parameter Set; in all other cases for the legacy EDCA Parameter Set.

Correlatively, the invention also provides a wireless communication device comprising at least one microprocessor configured for carrying out any method as described above.

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 wireless device, causes the wireless device to perform any method as described above.

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

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which:

Figure 1 illustrates an exemplary network environment in which embodiments of the present disclosure can be implemented;

Figure 2 illustrates a format of a Quiet element according to the 802.11 REVme D2.1 standard;

Figures 3a and 3b illustrate formats of a Target Wake Time, TWT, element adapted to be used for r-TWT according to the 802.1 1 be D3.0 standard;

Figure 4a illustrates, using a flowchart, general steps at an AP of a communication method providing TWT coordination in MAP operation according to embodiments of the invention;

Figure 4b illustrates, using a flowchart, general steps at a non-AP station of a communication method according to embodiments of the invention;

Figure 5 illustrates a transmission sequence implementing a TWT coordination for reduced OBSS interference in MAP operation, according to embodiments of the invention;

Figures 6 and 7 illustrate formats of a Target Wake Time, TWT, element adapted to be used for OBSS TWT according to embodiments;

Figure 8a shows a schematic representation a communication device in accordance with embodiments of the present invention;

Figure 8b shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention;

Figure 9a illustrates, using a flowchart, general steps at an AP of a communication method providing announcement of channel switching corresponding to a TWT coordination in MAP operation according to embodiments of the invention;

Figure 9b illustrates, using a flowchart, general steps at a non-AP station of a communication method according to alternative embodiments of the invention;

Figure 10 illustrates a second transmission sequence implementing a TWT coordination for reduced OBSS interference in MAP operation, according to alternative embodiments of the invention;

Figure 11a illustrates format of a Channel Switch Announcement, CSA, frame adapted to be used for OBSS TWT according to embodiments of the invention; Figure 11 b illustrates formats of a Channel Switch Announcement, CSA, element adapted to be used for OBSS TWT according to embodiments of the invention; and

Figure 12 illustrates format of a Target Wake Time, TWT, element including a Channel Switch Announcement, CSA, and adapted to be used for OBSS TWT according to embodiments of the invention.

DETAILLED DESCRIPTION OF EMBODIMENTS

The techniques described herein may be used for various broadband wireless communication systems, including communication systems that are based on an orthogonal multiplexing scheme. Examples of such communication systems include Spatial Division Multiple Access (SDMA) system, Time Division Multiple Access (TDMA) system, Orthogonal Frequency Division Multiple Access (OFDMA) system, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) system. An SDMA system may utilize sufficiently different directions to simultaneously transmit data belonging to multiple user terminals, i.e., wireless devices or stations. A TDMA system may allow multiple user terminals to share the same frequency channel by dividing the transmission signal into different time slots or resource units, each time slot being assigned to different user terminal. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which is a modulation technique that partitions the overall system bandwidth into multiple orthogonal sub-carriers or resource units. These sub-carriers may also be called tones, bins, etc. With OFDM, each sub-carrier may be independently modulated with data. An SC-FDMA system may utilize interleaved FDMA (IFDMA) to transmit on sub-carriers that are distributed across the system bandwidth, localized FDMA (LFDMA) to transmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA) to transmit on multiple blocks of adjacent sub-carriers.

The teachings herein may be incorporated into (e.g., implemented within or performed by) a variety of apparatuses (e.g., stations). In some aspects, a wireless device or station implemented in accordance with the teachings herein may comprise an access point (so- called AP) or not (so-called non-AP station or STA).

An AP may comprise, be implemented as, or known as a Node B, Radio Network Controller (“RNC”), evolved Node B (eNB), 5G Next generation base station (gNB), Base Station Controller (“BSC”), Base Transceiver Station (“BTS”), Base Station (“BS”), Transceiver Function (“TF”), Radio Router, Radio Transceiver, Basic Service Set (“BSS”), Extended Service Set (“ESS”), Radio Base Station (“RBS”), or some other terminology.

A non-AP station may comprise, be implemented as, or known as a subscriber station, a subscriber unit, a mobile station (MS), a remote station, a remote terminal, a user terminal (UT), a user agent, a user device, user equipment (UE), a user station, or some other terminology. In some implementations, a STA may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol (“SIP”) phone, a wireless local loop (“WLL”) station, a personal digital assistant (“PDA”), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or smart phone), a computer (e.g., a laptop), a tablet, a portable communication device, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a global positioning system (GPS) device, or any other suitable device that is configured to communicate via a wireless or wired medium. In some aspects, the non-AP station may be a wireless node. Such wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as the Internet or a cellular network) via a wired or wireless communication link.

An AP manages a set of STAs (registered to it or associated with it) that together organize their accesses to the wireless medium for communication purposes. The STAs (including the AP to which they register) form a service set, here below referred to as basic service set, BSS (although other terminology can be used). A same physical STA acting as an access point may manage two or more BSSs (and thus corresponding WLANs): each BSS is thus uniquely identified by a specific basic service set identification, BSSID and managed by a separate virtual AP implemented in the physical AP. Each STA is identified within a BSS thanks to an identifier, AID, assigned to it by the AP upon registration.

The 802.11 family of standards define various media access control (MAC) mechanisms to drive access to the wireless medium.

For example, in order to address the issue of increasing bandwidth and decreasing latency requirements that are demanded for wireless communications systems in high-density environments, multi-user (MU) schemes have been developed to allow a single access point (AP) managing a Basic Service Set (BSS) to schedule MU transmissions, i.e., multiple simultaneous transmissions to or from non-AP stations of the BSS, in the wireless network. A MU scheme has been adopted in the 802.11 ax-2021 standard, published on May 2019.

Thanks to the MU feature, a non-AP station has the opportunity to gain access to the wireless medium via two access schemes: the MU scheme and the conventional Enhanced Distributed Channel Access - EDCA (Single User) scheme.

Each BSS defines a main elementary channel of the wireless medium (known as a primary channel, usually a 20 MHz channel or a multiple of 20 MHz channel) on which the stations (including the AP) perform EDCA contention using generally legacy EDCA parameters (defined in an EDCA Parameter Set provided by the AP). To increase bandwidth for the forthcoming transmission, the stations can simultaneously contend for additional 20 MHz channels, known as secondary channels. The communication channel thus granted for transmission comprises the primary channel and optionally secondary channels.

The 802.11 ax standard allows a MU downlink (DL) transmission to be performed by the AP when gaining access to the wireless medium for a transmission opportunity (TXOP). During the MU DL transmission on the granted communication channel, 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 the communication channel of the wireless network in the frequency domain, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique. The assignment of the RUs to the non-AP stations is signaled at the beginning of the MU Downlink frame, by providing an association identifier (AID) of a non-AP station (individually obtained by each station during its association procedure with the AP) for each RU defined in the transmission opportunity.

The 802.11 ax standard also allows a MU uplink (UL) transmission to be triggered by the AP when gaining access to the wireless medium. During the MU UL transmission, various non-AP stations can simultaneously transmit data to the AP over the resource units forming the communication channel. To control the MU UL transmission by the non-AP stations, the AP previously sends a control frame, known as a Trigger Frame (TF). The Trigger Frame allocates the resource units to the non-AP stations of the same BSS, using 16-bit Association IDentifiers (AIDs) assigned to them upon registration to the AP and/or using reserved AIDs designating a group of non-AP stations. The TF also defines the start of the MU UL transmission by the non- AP stations as well as the length thereof. After a non-AP station makes an MU UL transmission, it performs EDCA contention on the medium using temporarily a different (from the legacy ones) set of EDCA parameters, known as MU EDCA parameters (defined in a Multi-User (MU) EDCA Parameter Set provided by the AP).

The current discussions in the task group 802.1 1 be, as illustrated by draft IEEE P802.11 be/ D3.0 of March 2023, introduce the Multi-Link Operation (MLO) when it comes to MAC layer operation. The MLO allows multi-link devices to establish or setup multiple links and operate them simultaneously.

A Multi-Link Device (MLD) is a logical entity and has more than one affiliated STA (STA) and has a single medium access control (MAC) service access point (SAP) to logical link control (LLC), which includes one MAC data service. An Access Point Multi-Link Device (or AP MLD) then corresponds to a MLD where each STA affiliated with the MLD is an AP, hence referred to as “affiliated AP”. A non-Access Point Multi-Link Device (or non-AP MLD) corresponds to a MLD where each STA affiliated with the MLD is a non-AP STA, referred to as “affiliated non-AP STA”. Depending on the literature, “multilink device”, “ML Device” (MLD), “multilink logical entity”, “ML logical entity” (MLE), “multilink set” and “ML set” are synonyms to designate the same type of ML Device.

Multiple affiliated non-AP STAs of a non-AP MLD can then setup communication links with multiple affiliated APs of an AP MLD, hence forming a multi-link channel.

The links established (or “enabled links”) for MLDs are theoretically independent, meaning that the channel access procedure (to the communication medium) and the communication are performed independently on each link. Hence, different links may have different data rates (e.g., due to different bandwidths, number of antennas, etc.) and may be used to communicate different types of information (each over a specific link). A communication link or “link” thus corresponds to a given channel (e.g., 20 MHz, 40 MHz, and so on) in a given frequency band (e.g., 2.4 GHz, 5 GHz, 6 GHz) between an AP affiliated with the AP MLD and a non-AP STA affiliated with the non-AP MLD.

The affiliated APs and non-AP STAs operate on their respective channels in accordance with one or more of the IEEE 802.11 standards (a/b/g/n/ac/ad/af/ah/aj/ay/ax/be/bn) or other wireless communication standards.

Thanks to the multi-link aggregation, traffic associated with a single MLD can theoretically be transmitted across multiple parallel communication links, thereby increasing network capacity and maximizing utilization of available resources.

The description below mostly concentrates on a single link for ease of explanation. However, similar considerations can be made with respect to each link forming a multiple link set for MLD devices. Therefore, the term STA or “station” may refer to one affiliated STA of a non- AP MLD (non-AP STAs of a non-AP MLD), and AP may refer to one affiliated AP of an AP MLD.

Figure 1 illustrates an exemplary network environment in which embodiments of the present disclosure can be implemented.

The illustrated wireless network environment comprises a multi-AP system 100 formed by a group of neighbouring wireless networks that operate over a common communication channel or wireless medium. The common communication channel may correspond to a part (e.g., 20 MHz) or all of an operating channel (e.g., 20 MHz, 40 MHz, 80 MHz, 160 MHz or 320 MHz).

A first wireless network (or Basic Service Set) BSS1 comprises an access point (AP) 110 and three non-AP stations (STAs) 111 , 112 and 113 associated with the AP 110 (i.e., registered with it). A second wireless network BSS2 comprises an AP 120 and three associated non-AP STAs 121 , 122 and 123. A third wireless network BSS2 comprises an AP 130 and three associated non-AP STAs 131 , 132 and 133. In the following, BSSx represents any of the wireless networks, while 1x1 , 1x2 and 1x3 any of the non-AP stations. Of course, another number of wireless networks and any number of non-AP stations per wireless network can be contemplated. In the present disclosure, APs 1 10, 120 and 130 are also referred to, respectively, as AP1 , AP2 and AP3. A device may act as an AP of one wireless network and at the same time may belong to another wireless network as an associated STA.

All or part of the APs may be affiliated APs to the same AP MLD. They also can be separate devices. Any AP broadcasts management frames, such as beacon frames, to share parameters to be used for the functioning of its BSS.

The stations (AP and non-AP) of each wireless network exchange data frames over the communication channel 100, under the management of the AP. A primary channel, usually 20 MHz channel, is defined per wireless network on which the management frames are exchanged. The other 20 MHz channels of the communication channel, if any, are known as secondary channels. In the context of the invention, the APs can also communicate one with each other, either using a communication channel of their BSS that is common to the other BSSs or using separate communication links (such as a separate wireless network or channel, an Ethernet backhaul connecting all the APs, direct links, and so on).

Each non-AP STA 1x1 -1x3 registers to the AP 1x0 of one wireless network BSSx during an association procedure. During the association procedure over the primary channel, the AP assigns a specific Association IDentifier (AID) to the requesting station. For example, the AID is a 16-bit value uniquely identifying the station.

The stations (including the AP) compete one against another over the communication channel (including the primary channel and optionally secondary channels to increase bandwidth) using EDCA (Enhanced Distributed Channel Access) contention to access the communication channel in order to be granted a transmission opportunity (TXOP). The TXOP may then be used to transmit (single-user, SU) data frames or to implement multi-user (MU) transmissions. In the MU scheme, a single station, usually the AP of the wireless network BSSx, is allowed to schedule a MU transmission, i.e., multiple simultaneous transmissions to or from other stations of the wireless network. One implementation of such a MU scheme has been for example adopted in IEEE 802.11 ax amendment standard, known as the Multi-User Uplink and Downlink OFDMA (MU UL and DL OFDMA) procedures. In the MU scheme, resources are defined over the 20 MHz channel or channels used, known as resource units.

More generally, the resources may include space, frequency and time resources and may be obtained according to different multiplexing schemes. Examples of those schemes include Spatial Division Multiple Access (SDMA) system, Time Division Multiple Access (TDMA) system, Orthogonal Frequency Division Multiple Access (OFDMA) system, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) system.

In the IEEE 802.11 wireless local area networking standards, the multi-AP system 100 may correspond to an extended service set (ESS) and each of the wireless networks to a basic service set (BSS).

Although the description of embodiments of the invention is given in the context of IEEE 802.11 , the embodiments are not limited thereto and they may apply to other types of wireless networks and protocols.

To meet low latency requirements in 802.11 be as well as to increase efficiency of the UL MU operation, existing mechanisms have been reused and improved, which are summarized now.

Quiet element

The Quiet element is a key component of the IEEE 802.11 h standard. It defines an interval during which no transmission occurs in the current channel. This interval might be used to assist in making channel measurements without interference from other STAs in the BSS. The format of the Quiet element 200 is shown in Figure 2 taken from Figure 9-345 (entitled “Quiet element format”) of 802.11 REVme D3.1 . Quiet Count field 210 is set to the number of TBTTs until (i.e., preceding) the beacon interval during which the next quiet interval starts. The value 0 is reserved, therefore the Quiet element can only apply after the next beacon frame issuance.

In case the Quiet interval is to be repeated, Quiet Period field 220 is set to the number of beacon intervals between the start of regularly scheduled quiet intervals defined by this Quiet element. A Quiet Period field set to 0 indicates that no periodic quiet interval is defined.

Quiet Duration field 230 is set to the duration of the quiet interval, expressed in Time Units (one TU = 1024ps), so that stations can reserve their NAV accordingly.

Quiet Offset field 240 can be used to specify an additional offset after the start time, which has to be shorter than one beacon interval. The Quiet Offset field is set to the offset of the start of the quiet interval from the TBTT specified by the Quiet Count field, expressed in TUs. The value of the Quiet Offset field is less than one beacon interval.

As mentioned above, US 2022/408355 proposes to send a Quiet element in a neighbouring BSS that overlaps an R-TWT schedule in a target BSS. This aims at protecting service periods for R-TWT member ST As against other STAs in the BSS which are not members of the R-TWT schedule.

TWT

The Target Wake Time (TWT) mechanism, originally defined in the IEEE 802.11 ah and 802.1 1 ax standards, has been adapted to be included in the 802.11 be D3.0 standard. An adaptation is known as the Restricted Target Wake Time (R-TWT) which schedules dedicated (and protected) service periods (SPs) for stations (affiliated with a non-AP MLD) to convey their latency sensitive traffic(s) over their BSS. An R-TWT agreement is nothing more than a Broadcast TWT agreement negotiated between an AP and an associated non-AP station of the BSS of a given link. The non-AP station establishes with the AP membership in a Broadcast TWT (or R- TWT) schedule. The schedule may be defined for some TIDs. The R-TWT Service Periods SPs of the R-TWT schedule are advertised in broadcast management frames (e.g., beacons), using an R-TWT information about the negotiated R-TWT SPs, typically a Broadcast TWT ID (bTWT ID).

Mechanisms like TWT or R-TWT are negotiated per link in case of ML operation, that is to say between an initiator affiliated STA of the non-AP MLD and the corresponding affiliated AP of the AP MLD.

For a given link, a non-AP station establishes membership in broadcast TWT schedules of the AP, while the AP delivers broadcast TWT parameter sets to the non-AP stations. The non-AP station is said to be the TWT scheduled station, while the AP is said to be the TWT scheduling station.

Negotiations to become a member of or to terminate membership in an R-TWT schedule (more generally a broadcast TWT) are performed with an exchange of frames that carry TWT elements as discussed below, having the Negotiation Type subfield set to 3 (Broadcast TWT). In particular, a non-AP STA MLD may request to become a member of a TWT schedule by transmitting a TWT Setup frame to its associated AP MLD that contains a TWT element for a given R-TWT schedule.

The AP then advertises the scheduled broadcast TWT (or R-TWT) using broadcast TWT elements in its management frames, typically in the beacon frames, FILS Discovery frames and broadcast Probe Response frames.

Figures 3a and 3b illustrate formats of a TWT element 300 adapted to be used for TWT according to the D3.0 standard.

TWT element 300 is identified by Element ID 301 and comprises “Control” field 310 and field 320 for conveying TWT parameter information.

“Control” field 310 informs, through Negotiation Type field 311 , whether the TWT is a broadcast TWT or an individual TWT agreement. The MSB of the Negotiation Type subfield 31 1 is the Broadcast field, therefore the TWT element 300 is referred to as Broadcast TWT element when MSB of subfield 311 is 1 (otherwise it is a single Individual TWT element). Other fields are of less importance for present description.

“TWT Parameter Information” field 320 contains a single ‘Individual TWT Parameter Set’ field if individual TWT (320b, Figure 3b), and one or more ‘Broadcast TWT Parameter Set’ fields having format 320a shown in the Figure 3a if Broadcast TWT (when Broadcast field of the “Negotiation Type” subfield is 1).

First field in “Broadcast TWT Parameter Set” field 320a is Request Type field 330 comprising:

TWT Request subfield 331 set to 1 when issued by the TWT scheduled STA. Otherwise, set to 0 when issued by the TWT scheduling STA (AP);

TWT Setup Command subfield 332 to indicate the type of TWT command: Request, Suggest, Demand, Reject when issued by a non-AP STA; or Accept, Alternate, Dictate, Reject when issued by a TWT scheduling AP;

Trigger field 333 to indicate whether or not the TWT SP indicated by the TWT element 300 includes triggering frames (the Trigger subfield equals to 1 for trigger-enabled R- TWT). Such a TWT SP is named triggered-enabled TWT SP, and a non-AP station cannot start transmitting data within it without previous triggering by the AP;

Broadcast TWT Recommendation field 336 set to 4 to indicate the TWT described in Broadcast TWT element 300 is a restricted TWT (r-TWT). In that case, Broadcast TWT element 300 is also referred to as a restricted TWT element (r-TWT IE), while Broadcast TWT Parameter Set 320a is referred to as Restricted TWT Parameter Set.

Other subfields of Request Type field 330 are of less importance:

Last Broadcast Parameter Set subfield 334 is set to 0 to indicate that another Broadcast TWT Parameter set follows this set. The Last Broadcast Parameter Set subfield is set to 1 to indicate that this is the last broadcast TWT Parameter set in the broadcast TWT element;

Flow Type subfield 335 indicates whether the TWT is announced (the TWT scheduling AP will wait to receive a frame from TWT scheduled STA to signal its awake state) or not (Flow Type subfield equals to 0 for r-TWT, for "announced" mode, because r-TWT is a trigger- enabled TWT);

A TWT Wake Interval Exponent subfield 337; and

A reserved subfield 338.

Other fields in Restricted TWT Parameter Set field 320a are used to define time Parameters for the R-TWT schedule, as follows:

Target Wake Time (TWT) field 340 indicates the next time (in microseconds) at which the station participating in the R-TWT schedule should wake up for the next R-TWT SP;

Nominal Minimum TWT Wake Duration field 350 indicates the minimum amount of time that the TWT scheduled STA is expected to be awake since the starting time of the TWT SP in order to complete the frame exchanges for the period of TWT Wake Interval. The TWT Wake Interval of the R-TWT SP is the value calculated from the TWT Wake Interval Mantissa 360 and TWT Wake Interval Exponent 337. It is expressed in number of units as defined in Wake Duration Unit subfield 312 of Control field 310, e.g., typically 256 ps.

Other fields in Restricted TWT Parameter Set field 320a are used to define parameters specific to the Broadcast and Restricted nature of the R-TWT SP.

As to Broadcast TWT Info field 370, it conveys the identifier of the R-TWT schedule, namely the Broadcast TWT ID 373 (bTWT ID), that is used to identify the R-TWT SPs belonging to the same R-TWT schedule. This identifier, which is not 0, hence allows an AP to schedule multiple sets of Broadcast TWT SPs with different sets of TWT parameters; it specifies, through Broadcast TWT Persistence subfield 374, the number of Target Beacon Transmission Times (TBTT) during which the Broadcast TWT SPs corresponding to this Restricted (more generally Broadcast) TWT Parameter set are present; it also signals, through Restricted TWT Schedule Full subfield 372, when set to 1 , that the r-TWT scheduling AP is unlikely to accept a request from a STA in the BSS to establish a new membership in the corresponding schedule (identified by bTWT ID 373); finally, it also signals, through Restricted TWT T raffic Info Present field 371 , whether Restricted TWT Traffic Info field 380 is present (field 371 to 1) or not.

As to Restricted TWT Traffic Info field 380 specific to the restriction of the Broadcast TWT to specific traffics, it is mandatory (hence field 371 is mandatorily set to 1) when the broadcast TWT is related to a LL (low latency) stream (the Traffic Info is related to TIDs). It comprises the following fields:

Traffic Info Control field 381 that indicates whether the following fields 382 and 383 are provided (i.e., “valid”). DL TID Bitmap Valid subfield 3811 (respectively UL TID Bitmap Valid subfield 3812) indicates whether the Restricted TWT DL TID Bitmap field 382 (respectively Restricted TWT UL TID Bitmap field 383) has valid information. Subfield 3813 is reserved. Restricted TWT DL TID Bitmap field 382 (respectively Restricted TWT UL TID Bitmap field 383) that identifies TIDs as latency sensitive traffic for DL (respectively UL) directions, i.e., TIDs that are allowed in the R-TWT defined by Restricted TWT element 300. The TIDs may be those defining LL streams. A value of 1 at bit position k in the bitmap indicates that TID k is classified as latency sensitive traffic stream for the concerned transmission direction.

The TWT SR of an R-TWT schedule is uniquely identified by the <bTWT ID, MAC address of TWT scheduling AP> tuple, where the TWT scheduling AP is the affiliated AP of concerned link of AP MLD.

Thanks to this TWT element included into a TWT Setup frame, an initiator STA can request its AP to become an R-TWT scheduled STA, by negotiating R-TWT SPs for its low latency traffics. For example, initiator STA (e.g., affiliated STA of a non-AP MLD) may negotiate the wake TWT, wake interval and the TIDs to be allowed in the R-TWT. The AP (e.g., affiliated AP of the AP MLD on that link) provides a TWT Response frame accepting or refusing the request. In other words, the STA requests membership in an R-TWT schedule.

A TWT Setup frame carries TWT elements with the Negotiation Type subfield 311 set to 3 and the TWT Setup Command field 332 set to Request TWT, Suggest TWT, or Demand TWT. The Restricted TWT Parameter set 320a indicates the Broadcast TWT ID 373 of the R- TWT schedule that the STA is requesting to join. The AP has possibility to answer (TWT Response frame) with no new R-TWT schedule for the bTWT ID (keep existing one), or offering an alternative set of parameters to those indicated in TWT Setup frame, or creating a new R-TWT schedule with a new bTWT ID.

Once the negotiation and membership are completed, a conventional TWT/R-TWT scheduled STA that is in awake state may enter, thanks to the advertising of the R-TWT SPs (e.g., through beacon frames), the doze state after receiving a Beacon frame with a Restricted TWT element indicating the existence of an R-TWT schedule and switch back to the awake state at the R-TWT start times. The Beacon frame indicates an R-TWT SP during which the TWT scheduling AP intends to send Trigger frames on that link, or DL BUs (Bufferable Units) to the TWT scheduled STAs.

At the beginning of each TWT/R-TWT service period, and expecting the TWT scheduled stations are in the awake state, TWT scheduling AP usually uses OFDMA Multi-User techniques (e.g., MU UL trigger-based transmission, MU DL transmission) to manage the R-TWT SP and possibly offer resource units to all or part of the TWT scheduled stations in the awake state.

Figure 3b illustrates the case of an individual TWT element for an individual TWT session (i.e., initiated by the non-AP station). First, there is a negotiation phase in which the AP and the target station agree on a common set of parameters, which are basically the same as Broadcast TWT (the most relevant ones are: Target Wake Time 340, TWT Wake Interval Exponent 337 and Minimum TWT wake duration 350). TWT parameter Information field 320b for Individual TWT adds the following information:

TWT Group Assignment field 351 which provides information to a requesting STA about the TWT group to which the STA is assigned. This field contains the TWT Group ID, Zero Offset of Group (optional), TWT Unit, and TWT Offset subfields. The TWT Group ID subfield is an unsigned integer and indicates the identifier of the TWT group to which the requesting STA is assigned. The value zero in the TWT Group ID subfield is used to indicate the unique TWT group, which contains all ST As in the BSS.

TWT Channel field 354 which defines the channel a station can use temporarily as the primary one, according to the individual agreement provided by 320b.

NPD Paging field 355 which is an optional field (used for power saving purpose).

Link ID Bitmap field 356 (indicates the links to which the TWT element sent by a STA affiliated with a Multi-Link Device (MLD) applies).

Request Type field 3330 is different from the Request Type field 330 of the broadcast TWT. In particular,

Implicit subfield 3301 is set to 1 to indicate an implicit TWT and is set to 0 to indicate an explicit TWT;

Flow type subfield 3302 specifies whether it is Announced (the scheduled STA signals its awake state at the beginning of the period) or Unannounced;

TWT Flow Identifier subfield 3303 contains a 3-bit value that identifies the specific information for this TWT request uniquely from other requests made between the same TWT requesting STA and TWT responding STA pair.

TWT Protection subfield 3304 specified the mechanism used to protect a TWT session from transmissions of external (to the TWT schedule) stations, such as the RTS/CTS.

Back to Figure 1 depicting multiple APs, a Multi-AP (MAP) technology has emerged where the APs 110, 120, 130 collaborate to share the common communication channel once one of them is granted access to it. To do so, the APs exchange messages one with each other to coordinate the MAP communications, and thus to avoid interference.

MAP sharing of the common communication channel is resource-based. An amount of a shared resource can be measured in time units, frequency band width, number of streams, amount of data or traffic (e.g., number of bytes) and/or any other suitable unit, depending on the type of resources as defined above. In this perspective, “shared resources”, “shared frequency band”, “shared channels” and “shared resource units” are synonyms and designate those resources offered by one of the APs to any other AP through the MAP technology.

To coordinate the MAP communications, the APs may be part of an inter-AP coordination group or MAP coordination set of APs, the formation of which is out of scope of the present invention. As an example, the APs willing to collaborate may previously issue management frames, like beacons or dedicated broadcasted frames, to advertise the other APs of their MAP coordination capability. The coordination group is also referred to as the AP Candidate Set for the MAP sharing. The coordination set may contain one or more APs.

Embodiments described below provide a more efficient TWT coordination in MAP operation than contribution 11-22-1530 and corresponding publication US 2022/408355, to mitigate the interference between neighbouring APs.

In these embodiments, (restricted) TWT service periods of a target BSS pertaining to the MAP collaborative set are replicated in time through a new TWT period, called OBSS TWT, in a second (neighbouring) BSS of the set, wherein activities of the OBSS TWT are limited for STAs of the second (neighbouring) BSS.

To do so, a second AP managing the second (neighbouring) BSS first receives, from a first AP managing the first target BSS, a MAP coordination frame specifying a TWT schedule provided in the first BSS. Next, the second AP, responsive to the receiving, sends, to stations of the second BSS, a frame including an overlapping BSS (OBSS) TWT element defining second TWT Service Periods, SPs, that are preferably aligned in time, or at least partially overlap, with first TWT SP of the TWT schedule in the first target BSS. Note that the overlap may be partial or full over the first TWT SP. A full overlap may correspond to a second TWT SP aligned with, or encompassing, the first TWT SP. It should be noted that the use of the OBSS TWT element for defining second TWT SPs relates to a preferred implementation; of course, other types of elements or other fields in the sent frame may be used for defining the second TWT SPs.

By handling the OBSS TWT schedule in a synchronous manner with the TWT schedule of the first target BSS, the second (neighbouring) AP can adjust the communication policy of its own stations during the TWT schedule of the first target BSS, hence can adjust risks of interference. A TWT coordination is therefore achieved.

Restrictive communication policy favors very low risks of interference to the detriment of communication efficiency in the second (neighbouring) BSS. On the other hand, too permissive communication policy will help having more efficient communication within the BSS, but with the downside of higher risks of interference with the overlapping BSSs.

As disclosed below, the proposed OBSS TWT mechanism according to embodiments of the invention is preferably based on Broadcast TWT, however distinct from the actual R-TWT.

Figure 4a illustrates, using a flowchart, general steps of a communication method providing TWT coordination in MAP operation according to embodiments of the invention. The communication method takes place at a neighbouring AP of the MAP coordination set.

It is assumed that a first AP of the MAP coordination, e.g., AP1 of BSS1 in Figure 1 , has planned a TWT or R-TWT schedule in its own “first” BSS, which requires TWT coordination with the other BSSs of the MAP coordination set. Traditionally, the Broadcast TWT operation allows the first AP to set up a shared TWT session for a group of stations within its BSS, and to periodically specify the TWT parameters set within the beacon frames it broadcasts. The stations of a TWT Broadcast agreement are required to wake up to receive only the beacon frames containing instructions for the TWT Broadcast sessions they belong to. The AP may advertise existing TWT Broadcast agreements so that the stations may ask membership to existing TWT sessions, or send requests to create new ones, as explained above.

The first AP, AP1 , may decide to share this information about its TWT schedule or schedules to the other (neighbouring) APs of the MAP coordination set. In the description below, the first AP and its corresponding first BSS may also be referred to as target AP and target BSS respectively. In the same way, the other AP and corresponding BSS may be referred to second or neighbouring AP and second or neighbouring BSS.

Hence, at step 400, a neighbouring AP of the MAP coordination set receives a MAP coordination frame specifying a TWT schedule from the target AP of the same MAP coordination set. The neighbouring AP that receives TWT scheduling information thus handles a BSS interfering with the target BSS of the target AP. The neighbouring AP may be for example AP2 of BSS2 in Figure 1.

The scheduling information may belong to a TWT element, regardless of whether it refers to an individual or Broadcast or Restricted TWT in the target BSS. Various ways of receiving such TWT scheduling information (TWT element) of the target BSS may be envisaged.

In first embodiments, the TWT element shared by the target AP with the neighbouring AP is the TWT element 300 used by the target AP within its own target BSS to announce the TWT schedule. A description thereof is provided above (Figures 3a and 3b).

It is known that the target AP, AP1 in the example, regularly informs the stations of its target BSS, BSS1 , about the TWT schedule by including such a TWT element (having the format 320a e.g.) in the beacon frames it broadcasts in its BSS. In this situation, the MAP coordination frame is a beacon frame broadcast by the first (target) AP within the first (target) BSS. In particular, the TWT schedule information received by the second (neighbouring) AP is defined by the TWT element addressed to stations of the first (target) BSS within the beacon frame.

Of course, any type of IEEE802.11 frame may be used to convey the TWT element 300 from target AP AP1 to neighbouring AP AP2. These could be actions frame specific to a MAP scheme which are exchanged in single-user mode between both APs, as shown, for illustrative purposes, by references 560 and 561 in Figure 5.

The 802.11 frames may be exchanged over the wireless medium shared between BSS1 and BSS2, or exchanged over another (wireless or wired) medium. For instance, an Ethernet backhaul may be built as an off-link or separate network between the two APs (more generally between the APs forming the MAP coordination set) fortransporting such 802.11 frames conveying the TWT element. This off-link or separate network means that frames 560 and 561 (Figure 5) are exchanged over a link distinct to the wireless links of Figure 1 (forming the various BSSs), and contain a link identification identifying the link or separate network over which the interferences impact the communications. In these various options, the MAP coordination frame is a dedicated unicast frame addressed by the first (target) AP to the second (neighbouring) AP over the medium or using an alternate communication medium.

In second embodiments, the MAP coordination frame is included in a negotiation process between the first and second APs to establish a MAP TWT agreement. That means the TWT Element is provided through a TWT agreement in between the two APs that may be used to initiate a (MAP) TWT session in between the two APs.

A negotiation phase is conducted between the two APs in which the target AP (AP1) and the neighbouring AP (AP2) agree on a minimum set of parameters related to a TWT schedule, including:

- a Target Wake Time (TWT - as field 340) defining the next time in microseconds at which the stations participating in the TWT-based communication should wake up for that TWT session in its BSS. This information is used, in the present embodiments, as an indication for the neighbouring (receiving) AP about the start of the next TWT service period;

- a TWT Wake interval (defined by fields 337 and 360 in relation with field 312) defining the time interval between subsequent TWT sessions. Its value is higher than 0 when the TWT is periodic; and

- a Minimum TWT Wake duration (as field 350) defining the minimum time duration a station shall stay awake since the starting time of the TWT session so as to be able to receive frames from the other station(s). This information is used, in the present embodiments, to drive a waiting period (idle) for the stations of the neighbouring BSS before trying to issue medium access request. As described later in the description, it aims at reducing the activity of the neighbouring stations of that neighbouring BSS on the medium, hence reducing the risks of OBSS interference.

Other conventional subfields of a TWT element are of less importance, as the neighbouring (receiving) AP does not need to know all the transmission rules for the TWT SPs that are dedicated to the target BSS only. For example, knowledge of a trigger-enabled operation during the TWT SPs is not useful for the neighbouring AP to implement the MAP coordination according to embodiments of the invention. Hence it is not required in the MAP coordination frame.

If several TWT SPs have to be protected, hence to be notified in MAP coordination frames, the MAC addresses of the target and neighbouring APs and the TWT Flow Identifier 3303 of a successful TWT MAP setup between those two APs may be used to uniquely identify a TWT MAP agreement.

To create a new MAP TWT session, a Request and Accept frames is a minimum set of messages exchanged by the two APs. However, multiple pairs of Suggest/Demand and Alternate/Dictate frames may be exchanged before reaching a final MAP TWT agreement. The negotiation advantageously allows avoiding scheduling concurrent period of activities (TWT SPs) in the two BSSs, i.e., respective TWT schedule that could overlap.

In some embodiments, an individual TWT element as depicted in Figure 3b may be used in frames 560 and 561 . In that case, TWT Group Assignment field 351 may be used to indicate this is a MAP TWT exchange in between two APs. As an example, TWT Group ID subfield within TWT Group Assignment field 351 indicates an index of a MAP coordinated session or group (if it exists).

The frames exchanged by the two APs during the negotiation can be identified by each of them, as coming from a device outside its own BSS by merely analyzing the MAC frame header. For example, AP2 may identify the MAC address source of a received frame is AP1 MAC address, which does not belong to BSS2. Optionally or alternatively, a SSID can be randomly generated for the MAP coordination set at the time the set is formed, and such exchanged frame can be identified when the SSID used for address 3 of MAC header is not set to BSS2 nor BSS1 but to the randomly generated SSID for the MAP coordination set.

Third embodiments are directed to the specific case of MLDs where multiple links exist. In these embodiments, the MAP coordination frame is exchanged between a third AP coaffiliated, with the target AP, to the (same) first AP MLD and a fourth AP co-affiliated, with the neighbouring AP, to the (same) neighbouring AP MLD. Hence the frame is conveyed on a separate link than the link on which the target and neighbouring APs operate and that creates OBSS interference, i.e., it is forwarded from another affiliated AP of the AP1 MLD. The third AP sends the MAP coordination MAP on behalf of the target AP experiencing the OBSS interference, while the fourth AP receiving the MAP coordination frame may then forward it to the neighbouring AP, within the neighbouring AP MLD, for the neighbouring AP to conduct the TWT coordination.

This could be the case for 802.11 be/bn devices (multi-link capable) according to coverage in wireless bands. For instance, 2.4GHz offers greater coverage than 6GHz. There could then have OBSS interferences in between a subset of stations of two BSSs of a 6GHz band, while the two corresponding APs of two AP MLDs are out of range of each other. In that case, AP1 operating on the 2.4GHz band for the first AP MLD may inform AP2 operating on the same band for the second AP MLD, about such OBSS interference, i.e., the MAP coordination frame informing of the TWT coordination is conveyed on another link than the 6GHz link experiencing the OBSS interference. In this context, frames 560 and 561 (Figure 5) are exchanged on a distinct link, and contain the link identification where the interferences impact the communications.

Once the MAP coordination frame including TWT information about the TWT schedule of BSS1 is received by the neighbouring AP at step 400, the neighbouring AP performs (optionally) clock mapping of the timing information included in the received TWT information, at step 410. Indeed, in practice, the TWT time fields shared by the target AP are expressed using a time or clock reference that is specific to that target AP (AP1) and known by the target BSS only. The TWT time fields are therefore not linked at all with any time or clock reference used by the neighbouring AP (AP2). For instance, the target AP sending a beacon frame (conveying a TWT element for example) sets the value of the beacon frame’s timestamp to a value of its local Time Synchronization Function (TSF) timer corresponding to the time when the data symbol containing the first bit of the timestamp is transmitted to the PHY plus the target AP’s delays required for the path through its local PHY from the MAC-PHY interface to its interface with the wireless medium (WM).

Therefore, at step 410, the neighbouring AP converts timing information of the received TWT schedule in the target BSS that is based on a first clock (or time reference) applicable to the target BSS, into timing information based on a second clock (or time reference) applicable to the neighbouring BSS. Next, as explained below, timing information within the OBSS TWT element sent within the neighbouring BSS will be second-clock-based timing information.

Main field concerned by such time conversion includes the Target Wake Time (TWT - as field 340).

For the neighbouring AP to have knowledge of the time shift (or TSF offset) between the two clocks (and then to determine that time shift for time correction/conversion), the target AP may advertise a TSF timestamp in the TWT element it sends to the neighbouring AP within the MAP coordination frame. Exemplary illustrations are provided below with reference to Figures 6 and 7 (elements 6813, 711 and 684).

Alternatively (not represented), the target AP may advertise a time standard by transmitting a Timing Advertisement element in TWT-conveying frames (Time Advertisement element is described in section 9.4.2.60 of 802.11 REVme D2.1), such as the MAP coordination frame, including the frames exchanges during the MAP TWT negotiation. This element may describe the source of time corresponding to the time standard, and an estimate of the offset between that time standard and the indicated TSF timer (timestamp) provided in the MAP coordination frame. This Time Advertisement element may be included in a TWT element (e.g., in subfields 6813 or 684) used for TWT coordination.

In other embodiments where the two APs are directly connected through a backhaul (e.g., Ethernet backhaul), step 410 can be avoided as there may exist a Cross-AP synchronization by synchronizing the APs via the LAN (Local Area Network) formed by the backhaul.

Once the time shift (TSF offset) in between the two BSSs is determined, the time characteristics of an OBSS TWT schedule within the neighbouring BSS (BSS2) to protect the TWT schedule of the target BSS (BSS1) are known, in particular the TWT timing information expressed using the clock reference of the target BSS. As mentioned above, the OBSS TWT schedule aims at defining OBSS TWT SPs that are preferably aligned with, or overlap (either fully or partially over the beginning of), the TWT SPs of the target BSS.

With this knowledge, the neighbouring AP verifies, at step 420, whether the OBSS information complies with existing reserved periods on the neighbouring BSS. Indeed, the neighbouring AP may not impose limited communication policies within its BSS during service periods dedicated to intensive communication for its BSS, in particular with respect to low latency data traffic (for R-TWT scheduled in that neighbouring BSS). Hence, the second AP checks whether the first TWT SPs (of the TWT scheduled in the target BSS) do not overlap TWT SPs initially scheduled by the neighbouring AP within the neighbouring BSS. To do so, the neighbouring AP determines whether the TWT parameters (starting time point and end time point) obtained from the MAP coordination frame (after conversion if needed) from the target AP do not encompass any period scheduled by the neighbouring AP in its own BSS. Such period may typically be any TWT service period already scheduled in the neighbouring BSS or similar period that favors communication with the neighbouring BSS. In other embodiments, the neighbouring AP may use the timing of the TWT SPs of the TWT scheduled in the target BSS to set up or modify the setting of its own TWT SPs within its neighbouring BSS.

The MAP coordination may be accepted by the neighbouring AP in case no interference between scheduled periods is detected. Hence, the neighbouring AP may send, to the target AP, a positive response in case of no overlapping (positive check); otherwise send a negative response to the target AP. The response may be a TWT Response frame with an Accept (or Refusal) code. In case of refusal, the process stops.

In case of acceptance, the neighbouring AP next computes or builds, at step 430, an OBSS TWT schedule for its BSS that is adapted to the TWT schedule of the target BSS. To do so, it builds an OBSS TWT element.

Two issues are dealt with at this step.

First issue regards the timing of the OBSS TWT SPs of the OBSS TWT schedule. As mentioned above, they are preferably time aligned with the TWT SPs scheduled in the target BSS.

In embodiments, they may have the same starting time, meaning the TWT (field 340) is the same, given the determined time shift (TSF offset). Variants may contemplate having OBSS TWT SPs starting a little before the TWT SPs scheduled in the target BSS.

The OBSS TWT SPs may have the same length as the TWT SPs scheduled in the target BSS, meaning for example that the Minimum TWT Wake Duration fields are the same. Variants may define a larger or shorter Minimum TWT Wake duration for the OBSS TWT schedule compared to the TWT schedule of the target BSS. In the case of encompassing, the Minimum TWT Wake duration is larger. In the case of overlapping, the Minimum TWT Wake duration may be larger or shorter.

Second issue regards the communication activity the neighbouring AP can allow to the stations of its BSS to ensure TWT coordination, i.e., reduced OBSS interference with the target BSS. It is indeed sought to limit communication activity of the stations of the neighbouring BSS during the TWT SPs scheduled in the target BSS. In addition, it is expected that the stations of the neighbouring BSS end their transmissions before the starting of the TWT SPs scheduled in the target BSS.

In first embodiments regarding the second issue, the OBSS TWT element forbids the stations of the second BSS to access the medium during the second TWT SPs. It thus announces a TWT service period where no station of the second BSS can access the medium. It aims at offering a TWT schedule that stations of the BSS cannot try to join, or if they intend, the AP will always refuse. In other words, the TWT schedule may correspond to an idle TWT schedule in which the stations of the BSS cannot participate. An OBSS TWT element 600 signaling such first embodiments is disclosed in Figure 6 that has similar fields to Figures 3a and 3b.

The OBSS TWT element 600 remains a Broadcast TWT. Preferably, it is not an R- TWT element anymore, because R-TWT operation aims to provide protected access to a set of stations. Optionally, the OBSS TWT element lies to a specific declination of an R-TWT element (as described later with regards to fields 670 and 680).

Control field 610 is similar to Control field 310 above with Negotiation Type field 611 having its MSB set to 1 (broadcast TWT).

TWT Parameter Information field 620 has the same format as TWT Parameter Information field 320. In particular, Request Type field 630, similar to Request Type field 330, includes Trigger field 633 set to 0 to indicate the OBSS SPs of the OBSS schedule are not trigger- enabled. Other fields within field 630 (631 , 632, 634, 635, 636, 637) are of less importance and are similar to corresponding fields 331 , 332, 334, 335, 336 and 337.

Some other fields in TWT Parameter Set field 620 are used to define time parameters for the OBSS TWT schedule, as follows:

- Target Wake Time (TWT) field 640 indicates the next time (in microseconds) at which the next OBSS TWT SP will start. As mentioned above, it may correspond to the TWT specified in the MAP coordination frame that is converted in the time reference of the neighbouring BSS;

- Nominal Minimum TWT Wake Duration field 650 indicates the minimum amount of time that a TWT scheduled station in the neighbouring BSS would be expected to be awake since the starting time of the OBSS TWT SP in order to complete the frame exchanges for the period of TWT Wake Interval. As the embodiments required no transmission from the stations, the duration may be set to 0: a Minimum TWT Wake Duration field within the OBSS TWT element 600 is set to 0.

While some other fields in TWT Parameter Set field 620 are used to define parameters specific to the Broadcast and Restricted nature of the TWT SP, including Broadcast TWT Info field 670 similar to above-described Broadcast TWT Info field 370 and including optional Restricted TWT Traffic Info field 680 specific to the restriction of the Broadcast TWT to specific traffics.

Regarding field 670, Broadcast TWT ID 673 (bTWT ID) used as a non-zero identifier of the OBSS TWT schedule, allows the neighbour AP to schedule multiple sets of Broadcast TWT SPs with different sets of OBSS TWT parameters. Broadcast TWT Persistence subfield 674 specifies the number of Target Beacon Transmission Times (TBTT) during which the Broadcast TWT SPs corresponding to this Restricted (more generally Broadcast) TWT Parameter set are present; it is based on the calculation made by the neighbouring AP at step 410. Restricted TWT Schedule Info subfield 672 indicates whether the neighbouring AP is likely to accept a request from a station of its (neighboring) BSS to establish a new membership in the corresponding OBSS TWT schedule. Preferably, value 2 is selected to indicate that it is unlikely that such a request from a station of the BSS be accepted to establish a new membership.

Regarding optional Restricted TWT Traffic Info field 680, its use in an OBSS TWT schedule may be as follows: Restricted TWT DL TID Bitmap field 682 and respectively Restricted TWT UL TID Bitmap field 683 identify no traffic. An additional subfield in Traffic Info Control field 681 , namely OBSS TWT subfield 6813, may explicitly indicate that the present TWT element is an OBSS TWT element for an OBSS TWT schedule. In that way, the OBSS TWT element 600 is a restricted TWT element having a Restricted TWT Traffic Info section 680 comprising an OBSS TWT field 6813 that is enabled.

In some embodiments where the target AP wants to advertise a TSF timestamp in the TWT element it sends to the neighbouring AP of the MAP coordination set (discussed above with respect to step 410), a dedicated Timestamp field 684 may be provided within the Restricted TWT Traffic Info field 680 of the OBSS TWT element 600. OBSS TWT subfield 6813 may be used, in addition to indicating the OBSS nature of the TWT element, to signal the presence of a TSF timestamp in Timestamp field 684.

Timestamp field 684 represents the timing synchronization function (TSF) of a TWT frame’s source. The length of the Timestamp field is 8 octets, and represents the number of microseconds the AP has been active. When the timestamp value reaches its maximum value (2^A64 microsecond or ~580,000 years), it is reset to 0.

Turning now to second embodiments regarding the second issue (communication activity for the stations during the OBSS TWT SPs), the OBSS TWT element 600 announces a TWT service period where the stations of the neighbouring BSS (BSS2) can access the medium with a less aggressive medium access scheme than the first embodiments (i.e., lower priority). It is proposed that the OBSS TWT element 600 defines a waiting time for the stations of the neighbouring BSS to access the medium that is larger than a waiting time for stations of the target BSS to access the medium during the first TWT SPs. The stations of the neighbouring BSS thus take more time to issue a medium access. So, they perform a CCA (channel sensing) during a larger time (corresponding to a larger AIFS as explained below) that lets stations of the target BSS (BSS1) to have priority to access the medium.

In some embodiments, the waiting time for the stations of the second BSS is computed based on a Minimum TWT Wake duration (as field 350) specified in the MAP coordination frame (e.g., in the TWT element broadcast by the target AP in its beacon frames or the duration agreed on during the MAP TWT agreement established through frame exchange 560/561).

An OBSS TWT element 600 signaling such second embodiments is disclosed in Figure 7 that is a variant to Figure 6. This variant includes an additional section, namely OBSS TWT Info field 790, to TWT Parameter Information field 720 (620 in Figure 6 and 320a/b in Figures 3a and 3b). The aim of this additional section is to provide the OBSS TWT parameters related to the waiting time, and optionally to the TSF timestamp mentioned above. As depicted, Timestamp field 684 (as described above) can be provided in new section OBSS TWT Info 790. The presence of this field is signaled through a dedicated 1 -bit field (Timestamp Valid field 711) of Traffic Info Control field 791 starting OBSS TWT Info 790.

In some embodiments, OBSS TWT Info 790 of OBSS TWT element 600 may directly include a field (not shown) setting a minimum waiting time before the stations of the neighbouring BSS can access the medium after the OBSS TWT SP (described by the OBSS TWT element) starts.

In preferred embodiments, the waiting time is indirectly encoded through a new set of EDCA parameters, referred to as OBSS EDCA Parameter Set. In this context, OBSS TWT Info 790 of OBSS TWT element 600 includes an OBSS EDCA Parameter Set 793, different from a legacy EDCA Parameter Set, to be applied by the stations of the neighbouring BSS to contend for access to the medium during the OBSS TWT SPs. The neighbouring AP can then drive the waiting time of its stations by adjusting the EDCA parameters of the OBSS EDCA Parameter Set. It also means that those stations have to use this specific set of parameters upon each new OBSS TWT SP starting and to switch back to the legacy EDCA Parameter Set upon the OBSS TWT SP ending or upon a dedicated timer elapsing. The presence of the OBSS EDCA Parameter Set field is signaled through a dedicated 1 -bit field (EDCA Param Valid 712) of Traffic Info Control field.

OBSS EDCA Parameter Set 793 thus provides information needed by the stations for proper operation of medium access during the OBSS TWT SP embedding this element 793. In other words, for an infrastructure BSS such as BSS2, the OBSS EDCA Parameter Set element is used by AP2 to establish policy (by changing default MIB attribute values), to change policies when member stations (STA21 , STA22, STA23) perform medium access.

Various embodiments of OBSS EDCA Parameter Set 793 are depicted in Figure 7.

First embodiments corresponding to format 793a reuse the legacy format for the EDCA Parameter Set element, as follows.

The QoS Info and Update EDCA Info fields are reserved (not used). The formats of AC_BE, AC_BK, AC_VI, and AC_VO Parameter Record fields 730 are the conventional ones.

ACI/AIFSN subfield 731 typically contains an ACI subfield and AIFSN subfield. The value of the AC index (ACI) references the AC to which all parameters in this record correspond. The AIFSN subfield indicates the number of slots after a SIFS a STA defers before either invoking a backoff or starting a transmission. AIFSN stands for “Arbitration InterFrame Space Number”.

ECWmin/ECWmax subfields 732 encode the values of CWmin and CWmax, respectively, in an exponent form. ECW stands for “EDCA Contention Window”.

The TXOP Limit field 733 shall not be specified as a TWT SP is considered. Thus, a TXOP Limit field set to 0 (that has a special meaning) can be used.

In embodiments seeking to provide less aggressive medium access than pure prohibition, one may envisage using a greater ECWmin than the default one in the BSS. Hence, the OBSS EDCA Parameter Set includes a degraded ECWmin value. In other embodiments still seeking to provide less aggressive medium access, the AIFSN value may be adjusted (basically increased) to delay the backoff decrement by the stations of the neighbouring BSS, hence to delay any medium access. In that case, the OBSS EDCA Parameter Set includes a degraded AIFSN value.

Second embodiments corresponding to format 793b reuse the known format of the MU EDCA Parameter Set element. While the conventional MU EDCA Parameter Set element is used by an AP to control the use of EDCA by non-AP 802.11 be/HE stations following particular UL MU HE TB PPDU transmissions, it is proposed here for the neighouring AP of a MAP coordination set to use it in order to control the use of EDCA by its non-AP stations inside a particular OBSS TWT SP. The element is used as follows.

The QoS Info field is reserved (not used). The formats of MU AC_BE, MU AC_BK, MU AC_VI, and MU AC_VO Parameter Record fields 740 are the conventional ones.

Each field element 740 contains the same subfields ACI/AIFSN 731 and ECWmin/ECWmax 732, plus a MU EDCA Timer subfield 743. The MU EDCA Timer field shall be set to value 0 as reserved. Subfields ACI/AIFSN 731 and ECWmin/ECWmax 732 are described above and may carry a degraded ECWmin value and/or a degraded AIFSN value.

However, in some embodiments, a timer defined by this field 743 may be used to drive the stations of the neighbouring BSS to switch back from the OBSS EDCA Parameter Set to the legacy EDCA Parameter Set. This is to restore the legacy parameters once the OBSS TWT SP protecting a TWT SP scheduled in the target BSS has ended. In these embodiments, the OBSS EDCA Parameter Set includes an OBSS EDCA Timer indicating a duration of time during which the stations of the neighbouring BSS use OBSS EDCA parameters before switching back to legacy EDCA parameters.

For example, the MU EDCA Timer field 743 (carrying the OBSS EDCA Timer value) can be based on a Minimum TWT Wake Duration field (as field 350) specified in the MAP coordination frame, in particular set to the value of that field. The MU EDCA Timer field 743 indicates the duration of time, in units of 8 TUs, during which the stations use the OBSS EDCA parameters for the corresponding AC. Each TU may be defined by Wake Duration Unit field 312.

In these embodiments, the stations of the neighbouring BSS may implement an independent backoff procedure during the OBSS TWT SP to control medium access based on the parameters of the OBSS EDCA Parameter Set.

Third embodiments corresponding to format 793c reuse ACI/AIFSN field 731 alone as forming the OBSS EDCA Parameter Set 793. In that field, only the AIFSN subfield may be significant (ACI subfield is not used) to indicate the number of slots after a SIFS a station defers before either invoking a backoff or starting a transmission. The indicated AIFSN value applies to all ACs during the OBSS TWT SP.

In any of these embodiments (793a, 793b or 793c), the AIFSN value may be used alone to defer invocation of the backoff decrement. In a variant, the AIFSN value may define additional slots that the station adds to the legacy EDCA AIFSN value of each AC category, to defer such invocation. In other words, the degraded AIFSN value (specified in field 731) is to be added to a legacy AIFSN value (e.g., defined in the legacy EDCA Parameter Set) by the stations of the neighbouring BSS to obtain an AIFSN value to be applied when contending for access to the medium during the OBSS TWT SPs.

The neighbouring AP has to determine the minimum value of the AIFSN subfield(s) whatever the embodiment 793a, 793b or 793c.

In some embodiments, a minimum value of the AIFSN subfield 731 can be set to 2, in particular when added to the legacy AIFSN value. This value provides a shift in medium access slot countdown in favor of the target BSS (BSS1): each AC queue of the stations in the neighbouring BSS (BSS2) is shifted two slots later compared to the target BSS.

In other embodiments, a minimum value of the AIFSN subfield 731 can be set to the maximum AIFSN value used by the stations of the target BSS (BSS1). As an example, a default value for AC_BK may be set to 7. This allows to prioritize all queues from the target BSS (BSS1) compared to the highest priority queue in the neighbouring BSS (BSS2). Of course, larger values can also be envisaged. They can ensure medium access is always granted to the stations of the target BSS. As an example, the Minimum TWT Wake Duration (as in field 350) provided in the MAP coordination frame can be considered for aligning an AIFSN close to this delay. In that case, the waiting time is specified in the MAP coordination frame, and the waiting time is set to a duration of the TWT SPs scheduled in the target BSS or set to the negotiated Minimum TWT Wake Duration.

Although the embodiments above provide the OBSS EDCA Parameter Set (793) within OBSS TWT element 600, variants may contemplate providing such Parameter Set in the beacon frames periodically broadcast by the neighbouring AP. Should different OBSS EDCA Parameter Sets be used for different OBSS TWT schedules of the neighbouring BSS, the OBSS EDCA Parameter Sets may be provided in the beacon frames in association with the bTWT ID of the respective OBSS TWT schedule. In these variants, it turns out that the AP broadcasts various EDCA Parameter sets.

Therefore, a communication method over a medium is also provided in a wireless network, wherein the method comprises, at an AP in a MAP coordination set of APs and managing a BSS the following step: sending, to stations of the BSS, a legacy EDCA Parameter Set defining EDCA parameters to be used by the stations when contending for access to the medium, a Multi-User (MU) EDCA Parameter Set defining MU EDCA parameters to be temporarily used by the stations when contending for access to the medium after they made UpLink MU transmission, and an overlapping BSS (OBSS) EDCA Parameter Set defining OBSS EDCA parameters to be temporarily used by the stations when contending for access to the medium during an OBSS Service Period, defined by the AP for the BSS, that is aligned or overlapping with neighbouring TWT SPs of a TWT schedule in a neighbouring BSS. Preferably, all these three sets are included in the same beacon frame broadcasted by the AP within its BSS.

Reverting back to Figure 4a, once the OBSS TWT element 600 has been prepared at step 430, the neighbouring AP (AP2) sends, to associated stations of its BSS, a frame including the OBSS TWT element 600 which defines OBSS TWT SPs aligned or overlapping with the TWT SPs scheduled in the target BSS (BSS1). This is step 440 corresponding to the announcement of the OBSS TWT schedule in the neighbouring BSS.

In some embodiments, the frame including the OBSS TWT element 600 is a beacon frame broadcast by the neighbouring AP in the neighbouring BSS. Hence all the stations of that BSS can receive the OBSS TWT schedule and then can be limited in their communication activity during the OBSS TWT SPs. These beacon-based embodiments allow the neighbouring AP to periodically specify the TWT parameters set.

In alternative embodiments, the frame including the OBSS TWT element 600 is a probe response frame sent by the neighbouring AP in the neighbouring BSS. Although that probe response frame can be broadcast to all stations of the BSS, it can also be addressed to a specific subset thereof. This probe-based approach allows the neighbouring AP to specify more rapidly the OBSS TWT schedule and the OBSS TWT Parameter Set (in advance to the next TBTT) compared to the beacon frame.

Hence, decision for the neighbouring AP to use the conventional beacon frame or a probe response frame can be based on the starting time of the next OBSS TWT SP. In particular, a Probe Response frame including the OBSS TWT parameters is preferably emitted if the starting time of the next OBSS TWT SP is scheduled prior to the next TBTT (beacon frame).

Although the neighbouring AP can send the OBSS TWT element 600 to all its associated stations, hence limiting the communication activity of all these stations during the OBSS TWT SPs, there are situations where such limitation is excessive. This is the case for instance forthose stations that are far enough from the target BSS so they cannot generate OBSS interference with the target BSS. It would be worth having these stations keeping their entire communication activity during the OBSS TWT SPs, although other stations of the same neighbouring BSS are limited in their communication activity. The neighbouring AP may then try to make those interfering stations joining the OBSS TWT schedule.

In this respect, the neighbouring AP (AP2) may first obtain information about interfering stations of its BSS (BSS2) that are interfering with the target BSS (BSS1) and free (or not interfering) stations of its BSS that are not interfering with the target BSS. Next, the neighbouring AP may decide to send the OBSS TWT element 600 to the interfering stations only. This may be done using individually-addressed probe response frames or individually-addressed TWT Setup frames with a command value of TWT Grouping (not shown in the Figures) in the TWT Setup Command field 332 and with the TWT Request field 331 equal to 0 (transmitting AP is a TWT scheduling station). As a result, the OBSS TWT element limits communication activity over the medium for the interfering stations only.

Turning now to the stations of the neighbouring BSS (BSS2), Figure 4b illustrates, using a flowchart, general steps, at such a station, of a communication method according to embodiments of the invention.

As explained above, the OBSS TWT element 600 does not define explicit restriction on the class of traffic (i.e., EDCA Access Category) to be transmitted within an OBSS TWT SP, but rather defines a restriction of the medium access itself (i.e., EDCA Access).

At step 450, a station of BSS2 receives, from its AP (AP2), an OBSS TWT element 600 (either conforming to Figure 6 or 7) in a beacon frame or a Probe Response frame. The OBSS TWT element defines an OBSS TWT schedule with OBSS TWT SPs that are aligned or overlapping with TWT SPs (to be protected) scheduled in the target BSS (BSS1).

Depending on the embodiments, the OBSS TWT element may include an OBSS EDCA Parameter Set providing the parameters for the station to limit its communication activity during the OBSS TWT SPs.

Step 460 consists for the station in waiting the start of the next OBSS TWT SP.

In the first embodiments corresponding to Figure 6 above, the OBSS TWT element 600 indicates that the OBSS TWT schedule is not available for accepting new membership. In that case, the station does not request to establish membership in such OBSS TWT schedule and stays in a doze state (i.e., with no communication activity) during the OBSS TWT SP.

In second embodiments corresponding to Figure 7 above, the station is authorized to have some transmission activity in the BSS according to restriction or limitation defined in the OBSS TWT Element 600 (typically according to the OBSS EDCA Parameter Set).

Note that for both cases, the station is expected to stop any transmission activity before an OBSS TWT scheduled SP.

In that second embodiments, at step 470, the station applies the parameters of OBSS EDCA Parameter Set 793 advertised by its AP (AP2). As mentioned above, the station may then apply a degraded AIFSN value 731 and/or a degraded ECWmin value 732 and optionally an OBSS EDCA Timer 743, to control the use of EDCA during the OBSS TWT SP. Reference is made here to the above description in relation to Figure 7. Step 470 thus consists for the station to switch from a legacy EDCA Parameter Set to an OBSS EDCA Parameter Set to contend for access to the medium during one of the OBSS TWT SPs. The switch takes place so that the OBSS EDCA Parameter Set starts to be used when the OBSS TWT SP starts.

Step 480 consists for the station in waiting the end of the OBSS TWT SP.

At the end of the SP, the legacy EDCA parameters are restored (step 490). To do so, the station switches back (from the OBSS EDCA Parameter Set) to the legacy EDCA Parameter Set. It may be at the end of the OBSS TWT SP, i.e., after an OBSS duration defined by a Minimum TWT Wake Duration field 650 specified in the OBSS TWT element 600. In a variant where the OBSS EDCA Timer 743 is used, it is made when the OBSS EDCA Timer 743 specified in the OBSS TWT element elapses.

Next, the station invokes a new (conventional) EDCA backoff procedure.

Figure 5 illustrates a transmission sequence implementing a TWT coordination for reduced OBSS interference in MAP operation, according to embodiments of the invention. Although it is described using 802.11 single user frames, it may alternatively use equivalent frames in multi-user operation.

The proposed example involves two BSSs in a MAP coordination set, e.g., BSS1 and BSS2 of Figure 1 .

First, the two APs directly exchange (above step 400) their TWT scheduling information through frames 560 and 561 . This may be a negotiation between the two APs.

According to some embodiments, AP1 provides a MAP TWT scheduling which follows the format of an individual TWT (320b) or Broadcast TWT (320a) in its own BSS1. The MAP TWT scheduling is included in MAP coordination frame 560 for example.

Once the MAP scheduling negotiation is completed, AP1 and AP2 continue their own operations on their proper BSS.

Typically, AP1 sends regular beacon frames 500, at each TBTT1 duration interval. In that example, TBTT1 is shorter than TBTT2 (used by AP2 in its own BSS2). As an alternative or complement to frame exchange 560-561 , the MAP TWT scheduling can also be obtained by AP2 directly from beacon frame 500. This is because TWT element 320 is present in such beacon frame for the stations of BSS1 . Note that this TWT element may also result from the negotiation 560-561 (if any). Such a TWT element 320 in the beacon frame is usually a broadcast TWT element or a restricted -TWT element (which is a particular broadcast TWT element).

In the example of the Figure, beacon frame 500 is sent by AP1 in BSS1 at TO and T4, and indicates the TWT SPs that follow for BSS1 : TWT SP 540 and TWT SP 541 , each corresponding to timing T1 -T2 and T5-T6. TWT SPs 540 and 541 are the period suffering from OBSS interferences with BSS2 and that require protection using a TWT coordination between AP1 and AP2.

In BSS2, beacon frame 550 is sent by AP2 at T3.

Embodiments of the TWT coordination according to the invention operate as follows. Once the MAP TWT negotiation 560/561 is completed, AP2 can determine that first TWT SP 540 (T1-T2) in BSS1 will take place before the next TBTT T3 (i.e., before the beacon frame to be sent).

A MAP TWT protection scheme can be provided before T3 by sending Probe Response frame 551 that includes OBSS TWT element 520 informing about the OBSS TWT schedule (and thus SPs) within BSS2. Probe Response frame 551 may be broadcast to reach all stations of BSS2 or may be sent to a specific subset of stations within BSS2, e.g., stations determined as interfering with BSS1 . In the example of the Figure, OBSS TWT element 520 announces an OBSS TWT SP 530 (starting time aligned with TWT SP 540) where the stations of BSS2 can access the medium using a degraded medium access scheme, e.g., relying on above-discussed degraded AIFSN value and/or degraded ECWmin value (optionally included in OBSS EDCA Parameter Set 793). OBSS TWT element 520 may be as explained above with respect to Figure 7. As a result, a subset of stations may conduct communication in BSS2 during OBSS TWT SP 530, stations of which the CCA sensing 599 (Minimum TWT Wake Duration 650 as defined in the OBSS TWT element 600) takes more time than for stations of BSS1 in TWT SP 540. This ensures that BSS1 has priority over BSS2 in accessing the medium during scheduled TWT SP 540.

In the example, STA22 does not detect any communication activity within its range. As a consequence, it may perform a backoff countdown after CCA sensing period 599 and initiate a transmission if it gains access to the medium.

Of course, OBSS TWT element 520 may alternatively announce an OBSS TWT SP where medium access is forbidden to the stations of BSS2.

This is for example the case with beacon frame 550 sent by AP2 that includes OBSS TWT element 520 announcing next OBSS TWT SP 531 and specifying medium access is forbidden to the stations of BSS2. OBSS TWT element 520 may be as explained above with respect to Figure 6. In that case, there is no communication at all in BSS2 during OBSS TWT SP 531 , hence avoiding any OBSS interference with BSS1 during TWT SP 541 .

CCA sensing period 599 corresponds to T1 -T2’, and could be set so that T2’ can reach T2. For example, Minimum TWT Wake Duration 650 as defined in the OBSS TWT element 600 may be set to Minimum TWT Wake Duration 350 of the TWT SPs within target BSS1 , i.e., as defined in the TWT element 300 of the beacon frames 500 transmitted by AP1 .

This scenario shows that two different communication activity policies are used for OBSS TWT SPs 530 and 531 . Of course, one and the same policy (access prohibition or controlled access with waiting time) can be used over multiple OBSS TWT SPs of the same OBSS TWT schedule.

Situations may require that AP1 and AP2 update their negotiation 560/561 (not shown in the figure). To handle such updates over time, beacon frames 550 may advise an OBSS TWT Update Counter field to follow the updates, that is associated with an OBSS TWT schedule (e.g., using identifier bTWT ID). OBSS TWT Update counter field may be defined as an unsigned integer initialized to 0 that is incremented each time an update of the OBSS TWT parameters is made. In that case, the stations of BSS2 become aware of any important change in the OBSS TWT schedule and may retrieve the updated parameters from beacon frame 550.

Figure 8a schematically illustrates a communication device 800 configured to implement at least one embodiment of the present invention, for instance any of the (AP and non- AP) stations shown in Figure 1 . The communication device 800 is either a coordinator device, a coordinated device or a mere station managed by the coordinator or coordinated device of a Multi- AP set. The communication device 800 may preferably be a device such as a microcomputer, a workstation or a light portable device. The communication device 800 comprises a communication bus 813 to which there are preferably connected: a central processing unit 801 , such as a processor, denoted CPU; a memory 803 for storing an executable code of methods or steps of the methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least one communication interface 802 connected to a wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards, via transmitting and receiving antennas 804.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 800 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 800 directly or by means of another element of the communication device 800.

The executable code may be stored in a memory that may either be read only, a hard disk or on a removable digital medium such as for example a disk. According to an optional variant, the executable code of the programs can be received by means of the communication network, via the interface 802, in order to be stored in the memory of the communication device 800 before being executed.

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

Figure 8b is a block diagram schematically illustrating the architecture of the communication device 800, adapted to carry out, at least partially, the invention. As illustrated, device 800 comprises a physical (PHY) layer block 823, a MAC layer block 822, and an application layer block 821 .

The PHY layer block 823 (here an 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20MHz channel or the common communication channel, and thus sending or receiving frames over the wireless radio medium used, such as 802.11 frames, for instance medium access trigger frames TF to reserve a transmission slot, 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 822 preferably comprises a MAC 802.11 layer 824 implementing conventional 802.11 be MAC operations, and additional block 825 for carrying out, at least partially, the invention. The MAC layer block 822 may optionally be implemented in software, which software is loaded into RAM 803 and executed by CPU 801 . Preferably, the additional block 825, referred to as multi-AP interference managing module which has different operations to implement parts of the invention, depending on the role played by the communication device 800. As the same device can play different roles over time, the additional block 825 is preferably designed to selectively perform the different operations.

For instance, and not exhaustively, operations for the communication device 800 acting as an AP include: exchanging frames for a MAP TWT negotiation 560/561 , converting timing of received TWT schedule information into timing information of its local clock (TSF); setting up an OBSS TWT element defining OBSS TWT SPs time aligned or overlapping with TWT SPs of a target BSS; declaring a communication activity policy for OBSS TWT SPs; determining stations of its own BSS that interfere with a target BSS; providing OBSS TWT elements in beacon frames or Probe Response frames.

Operations for the communication device 800 acting as a non-AP station in a BSS interfering with a target BSS include receive an OBSS TWT element from its AP; determining the medium access rules/policy; retrieving degraded AIFSN/ECWmin or an OBSS EDCA Parameter Set; switching to the degraded values for an OBSS TWT SP and switching back to legacy EDCA parameters after; performing EDCA contention with the degraded values during the OBSS TWT SP.

MAC 802.11 layer 824 and multi-AP interference managing module 825 interact one with the other in order to process accurately communications over the medium, e.g., over OFDMA RUs addressed to multiple stations according to embodiments of the invention.

On top of the Figure, application layer block 821 runs an application that generates and receives data packets, for example data packets such as a video stream. Application layer block 821 represents all the stack layers above MAC layer according to ISO standardization.

Figures 9a, 9b and 10 illustrate third embodiments (regarding the second issue), wherein communication activity for the stations during the OBSS TWT SPs is off-loaded to a distinct channel. The OBSS TWT element may include a channel (or list of) indication along with an OBSS EDCA Parameter Set providing the parameters for the station to limit its communication activity during the OBSS TWT SPs.

In other words, the OBSS TWT element 600 announces a TWT service period where the stations of the neighbouring BSS (BSS2) can access the medium but over another channel, different from the operating channel initially in use by stations of the neighbouring BSS. Thus, instead of stopping or reducing access to the medium by the stations of the neighbouring BSS according to first and second embodiments, communications can be maintained while preventing interference with the target BSS during the first TWT SP. In the following, the channel initially in use by stations is referred to as initial operating channel and the other channel as second channel or temporary operating channel. The initial operating channel is usually a primary channel (channel over which beacon frames are exchanged), but may also be a secondary channel.

Therefore, a channel switch is attempted to move the BSS2 to the temporary operating channel.

The decision to switch to a new operating channel in an infrastructure BSS is made only by the AP of the BSS. An AP may make use of the information in Supported Channels elements and the results of measurements undertaken by the AP and other STAs in the BSS to assist the selection of the new channel. The Supported Channels information element, according to IEEE802.11 series of standards, describes sub-bands that are supported by a station (it consists of a first channel number, which is the lowest channel in a supported sub-band, followed by the number of channels in the sub-band). Generally, a channel switch is used very rarely by an AP. The third embodiments of the invention advantageously consider using this mechanism when OBSS TWT is going to occur.

In complement, an AP may make use of the information in Supported Channels elements provided by other APs of the MAP coordination group. The negotiation of an elected channel among the supported channels may be provided by frame exchanges 560/561 , wherein each AP (here AP1 or AP2) exchanges information about at least one second channel used for channel switching during an OBSS TWT. According to some embodiments where APs provide a MAP TWT scheduling (frames 560/1060/561/1061) that follows the format of an individual TWT (320b), the TWT Channel field 354 may be used to indicate the negotiated or proposed second channel. Of course, the neighbouring BSS (BSS2) may still decide of the second channel as temporary operating channel by its own, without any advertisement or negotiation with other AP(s) in the MAP group.

Figure 9a illustrates, using a flowchart, general steps at an AP of a communication method providing announcement of channel switching corresponding to a TWT coordination in MAP operation according to third embodiments of the invention.

In the illustration of Figure 10, the neighbouring (second) AP remains the AP2 and executes the method of Figure 9a.

Similarly to step 400 of Figure 4a, wherein the neighbouring (second) AP of the MAP coordination set receives a MAP coordination frame specifying a TWT schedule from the target (first) AP of the same MAP coordination set, here the neighbouring AP of the MAP coordination set receives channel information from the target AP (step 900). Various types of channel information may be envisaged. The channel information may concern the information in Supported Channels elements advertised by the target AP for its BSS (through its emitted management frames like beacon frames). In a variant or in addition, the channel information may concern the information of interfered channels in results of measurements undertaken by the AP and other STAs in the target AP’s BSS. In a variant or in addition, the channel information may concern the information of negotiated channel (or list of) as a result of channel allocation for OBSS TWT within the MAP set. In step 910, the AP attempts to select a second channel that is supported by all associated STAs and that’s compliant with the received channel information from the target AP. In other words, the AP determines if a channel can be accommodated locally compared to a received list of channels. If a channel can be accommodated, it’s selected as second (or temporary) channel and is announced to the BSS.

In step 920, the neighbouring AP of the MAP coordination informs its associated STAs that it is switching from the initial operating channel to the second operating channel channel. It is to be noted that the association with the STAs is maintained when advertising the switch.

Three exemplary mechanisms to announce a channel switch information (CSA, Channel Switch Announcement) can be envisaged: using Channel Switch Announcement frames (or equivalent) until the intended channel switch time; format of such frame is illustrated by 1100 in Figure 11a. As it will be described later, this would mandate having two frames, the first one sent before the OBSS TWT to indicate when to switch on the temporary operating channel, and the second one (during the OBSS TWT) to alert when to switch back on the initial operating channel; using Channel Switch Announcement elements (or equivalent) in Beacon frames or Probe Response frames; format of such element is illustrated by 1150 in Figure 11 b. As will be described later, this would mandate having two elements, the first one used to switch onto the temporary operating channel and the second one to switch back on the initial operating channel; using a combination of Channel switch information and OBSS TWT schedule, as further illustrated by the element 1280 of Figure 12. In this exemplary mechanism, steps 920 and 930 are combined into one single step as this element combines the two information to be advertised.

Advantage of the first CSA mechanism (frame 1100, Figure 11a) is that the advertisement frames are compliant with legacy devices. By configuring existing elements in IEEE802.11 specifications, so that their parameter values correspond to the start and to end of the OBSS TWT timing, legacy stations would adopt the appropriate channel switch.

Advantage of the third CSA mechanism is the optimization in framing; less messaging overhead (basically timing information only conveyed in Beacon frames) and reduced overhead in the Beacon or Management frame (needs only indication of the channel and TWT timing). However, this mechanism can only be used by new generation of stations (with updated software) as legacy stations would not decode the new format.

One may envisage any combination of the three CSA mechanisms, so that the AP intends to make CSA Information understood and applicable to legacy devices or not.

Next, the step 930 is equivalent to step 440 corresponding to the announcement of the OBSS TWT schedule in the neighbouring BSS. Various frame and element formats are now disclosed for supporting a CSA operation.

Figure 11a illustrates a format of a Channel Switch Announcement, CSA, frame 1100 adapted to be used for OBSS TWT according to embodiments.

Illustration is provided with the legacy format of Extended Channel Switch Announcement frame (as per 9.6.7.7 of IEEE P802.11-REVme/D4.0, August 2023), so that legacy stations can follow the advertisement and operate switching. Any other format may be envisaged (e.g., Channel Switch Announcement frame, as per 9.6.2.6 of IEEE P802.11 - REVme/D4.0, August 2023, or any proprietary frame).

The Channel Switch Mode field 1101 indicates any restrictions on transmission until a channel switch. As a result, by using a Channel switch advertisement frame, the AP may force stations in the BSS to stop transmissions until the channel switch takes place by setting the Channel Switch Mode field in the Channel Switch Announcement element to 1. The channel switch frame should be scheduled so that part or all STAs in the BSS, including STAs in power save mode, have the opportunity to receive the Channel Switch Announcement element before the switch.

The New Channel Number field 1102 identifies the second (temporary) 20 MHz channel where to operate after the switch.

The Channel Switch Count field 1103 field indicates the number of target beacon transmission times (TBTTs) until the AP sending the frame switches to the new (temporary) channel. Preferably, value 0 is used that indicates that the switch occurs at any time after the frame containing the element is transmitted. So, the frame 1100 has to be sent prior to the switching, with respect to a guard interval to be considered as minimum duration to let time for stations to perform their switching (see more details with regards to periods 1099 illustrated in Figure 10).

If present, the Wide Bandwidth Channel Switch field 1105 indicates the BSS bandwidth after channel switching.

Other fields are of less importance and can be omitted or ignored.

Optionally, other frames may be considered for advertising a CSA operation. As an example, a channel usage procedure may be used by the AP to assist the stations to operate a channel switch during the OBSS TWT by exchanging Channel Usage Request and Response frames, or by the AP emitting unsolicited Channel Usage Response frames.

For example, the Channel Usage frame exchange is only performed by the AP with some of its administrated stations that interfere with the first AP.

The Channel Usage Request/Response frames may contain zero or more TWT elements along with one or more Channel Usage Elements. Therefore, an OBSS TWT agreement can be established between a STA and its associated AP, by exchanging Channel Usage Request and Response frames, that includes OBSS TWT Elements and Timeout Interval Element fields (600).

The Channel Usage element (as per 9.4.2.84 of IEEE P802.11-REVme/D4.0, August 2023) defines the channel usage information for BSSs. The format of the Channel Usage element is composed of a Usage Mode field and a list of Channel Entry fields. One may envisage that the Usage Mode field number (values listed in table 9-268 in the above reference), that identifies the usage of the recommended channels listed in the Operating Class/Channel Number pair fields of the Channel Usage element, is provided a new value defining an OBSS TWT usage mode. For example, Usage Mode field may be set to a new value 5.

Figure 11 b illustrates formats of a Channel Switch Announcement, CSA, element 1150 adapted to be used for OBSS TWT according to embodiments.

The Channel Switch Announcement element 1150 is used by an AP to advertise when it is changing to a new channel and the channel number of the new channel.

The Channel Switch Mode field 1101 indicates any restrictions on transmission until a channel switch. The AP may force STAs in the BSS to stop transmissions until the channel switch takes place by setting the Channel Switch Mode field in the Channel Switch Announcement element to 1. Value 0 is preferred here because, as emitted in a beacon frame, it is not intended to disallow any transmission on the original channel before the switching.

The New Channel Number field 1102 is set to the number of the channel to which the STA is moving.

The Switch Time field 1104 indicates the maximum time delta between the TBTT of the last Beacon frame transmitted by the AP in the current channel and the switching date.

Optionally, other formats (or combination thereof) may be envisaged: the legacy Channel Switch Announcement element (described in Figure 9-219 of IEEE P802.11 -REVme/D4.0, August 2023), that contains fields 1101 , 1102 and a Channel Switch Count field, set to the number of TBTTs until the STA sending the Channel Switch Announcement element switches to the second channel (Value 0 indicates that the switch occurs at any time after the frame containing the element is transmitted).

A Wide Bandwidth Channel Switch subelement (defined in 9.4.2.159 of IEEE P802.11 -REVme/D4.0, August 2023) may be present if the channel switching changes the BSS operating channel frequency and bandwidth compared to the original bandwidth on the initial channel.

A Secondary Channel Offset element (defined in 9.4.2.18 of IEEE P802.11- REVme/D4.0, August 2023). Figure 12 illustrates a format of a Target Wake Time, TWT, element including a Channel Switch Announcement, CSA, and adapted to be used for OBSS TWT according to third embodiments.

This enhanced TWT element 1280, as a variant of the elements of Figure 6 and Figure 7, includes additional fields 9814 and 985, related to the information of a temporary channel.

As depicted in Figure 12, Target Channel field 985 may be provided in the added section OBSS TWT Info 790 (Figure 7) or in the Restricted TWT Traffic Info field 680 (Figure 6) . The presence of this field is signaled through a dedicated 1 -bit field (Channel Switch Valid field 1214) of Traffic Info Control field.

The Target Channel field indicates a channel number for second channel operation during OBSS TWT SP, which is interpreted in the context of the BSS operating class. Channel numbers are defined in 802.1 I REVme D4.0 Annex E. Optionally (not shown), an Operating Bandwidth field allows indicating an operating bandwidth greater than 20 MHz.

Turning now to the stations of the neighbouring BSS (BSS2), Figure 9b illustrates, using a flowchart, general steps, at one of those stations, of a communication method according to third embodiments of the invention.

At step 950, a station of BSS2 receives, from its AP (AP2), an OBSS TWT element 600 in a beacon frame or a Probe Response frame, and a CSA information (1100, 1150 or 1280).

A legacy station may apply the algorithm when the received CSA information (1100, 1150) is advertised according to the first and/or second mechanism(s). A non-legacy station may apply the algorithm when the received CSA information (1100, 1150 or 1280) is advertised according to the first, second and/or third mechanism(s).

According to optional embodiments where the OBSS TWT is conveyed through Channel usage element, upon receipt of a Channel Usage element in the Probe Response or in unsolicited Channel Usage Response frame, the receiving station uses the channel usage information as part of channel selection process to start an off-channel OBSS TWT communication.

Result of step 950 is an indication of a selected target (second) channel to switch to, and the appropriate timing for the switching.

Step 960 consists for the station to wait for the start of a next OBSS TWT SP.

At step 970, the station (including the AP) selects the second channel, and operates the switching. In embodiments applying a restriction or a limitation defined in the OBSS TWT Element 600 (typically according to the OBSS EDCA Parameter Set), the station may conduct backoff procedure for access on the selected channel with provided EDCA parameters (793), or may wait for receiving a frame on this channel (e.g., a Trigger Frame emitted by AP2).

Step 980 consists for the station to wait for the end of the OBSS TWT SP. At the end of the SP, the station switches back onto the initial operating channel (step 990). In embodiments, the legacy EDCA parameters are restored.

Some variants may be considered for the switching steps corresponding to starting/ending dates (steps 960 and 980). The waiting step 960 may preferably be an effective target date, therefore meaning the channel switching has to be terminated at this target date. The waiting step 980 may be a starting date when the switching can start to be operated.

At step 990, the station terminating second channel operation returns to normal operation on the initial operating channel. The station may have further data, packets or frame to transmit.

Figure 10 illustrates a second transmission sequence implementing a TWT coordination for reduced OBSS interference in MAP operation, according to third embodiments of the invention.

The sequence is similar to the Figure 5, except that a channel switch is performed for the OBSS TWT SP.

First, the two APs directly exchange (above step 900) their TWT scheduling information along with CSA information through frames 1060 and 1061 . This may be a negotiation between the two APs. The neighbouring APs of the MAP coordination set receive channel information from each other.

Similarly to the example of the Figure 5, beacon frame 1000 is sent by AP1 in its BSS1 to indicate the TWT SPs that follow for BSS1 (TWT SP 1040).

In BSS2, beacon frame 1050 and Probe Response frame 1051 are sent by AP2.

Those management frames contain OBSS TWT element 520 supplemented by at least one CSA indication (1150). Preferably, a first CSA indication 1150 provides the date of starting TWT (T1) whereas a second CSA indication indicates when to switch back on original channel (T3).

When the CSA indication takes the format 1280 (instead of 1150), only one element 1280 is provided in management frames.

In the sequence, the Channel Switch Announcement frames are also illustrated by 1100a and 1100b, that can be sent respectively before and after each OBSS TWT SP. The first one (1100a) is sent before the OBSS TWT to indicate when switching on the temporary second channel, and the second one (1100b, emitted on second channel during the OBSS TWT) to alert when to switch back onto the original channel where the BSS operates.

As already noticed, if a non-AP STA in a BSS receives in a frame 1 100 a Channel Switch Mode field (1101) that has the value 1 , it is not allowed to transmit any more frames on the channel until the scheduled channel switch occurs. Therefore, the frames 1100a and 1 100b have to be emitted close to their own timing deadlines, with respect to a delay 1099 detailed hereafter. This solution is safer compared to any protection of the OBSS TWT, including the Quiet Element that was considered in the prior-art.

One may note the timing delay 1099 in the figure, that represents the time it takes for a station to switch its operating channel. Typically, an AP may receive Channel Switch Timing elements from its administrated stations that contain information regarding the needed channel switch time, in units of microseconds (defined in section 9.4.2.62 of IEEE P802.11 -REVme/D4.0, August 2023). In a variant where the OBSS EDCA Timer 743 is not used for EDCA parameters, the AP may decide to use the OBSS EDCA Timer field 743 specified in the OBSS TWT element as an indication of the maximum switch time to consider when entering/exiting an OBSS TWT SP. That means a receiving station is not expected to communicate nor receive frames during that maximum switch time.

In case where the switching is allowed to be performed after the start of the OBSS TWT (date T1), and if second and third embodiments are both considered (that is to say some stations will execute according to second embodiment whereas some others, like legacy stations, execute according to third embodiment), then the duration of delay 1099 (Figure 10) may be set in order to correspond to CCA sensing period 599 (Figure 5).

In embodiments, a station that receives a switching announcement from its associated AP may not transmit a frame to the AP on the second channel until it receives a frame on the second channel from the AP. Therefore, AP considers sending first an initial frame when the switching as occurred (illustrated by frame 1110). Preferably, a trigger frame may be considered to be emitted for triggering stations that have also switched their operating channel.

The third embodiment has been illustrated by using Channel Switch Announcement. In practice, several mechanisms may be envisaged to apply the channel switch operation:

BSS channel switch as illustrated, but also, more recent technologies or protocols such as:

Secondary Channel Access (SCA) operation, where the primary channel is not operated until sensed as free;

Subchannel Selective Transmission (SST) operation (including SST element in Beacon frames); and/or

Dynamic Sub-band Operation (DSO).

Combination of first, second and third embodiments can be envisaged. A channel switch is an attempt to move a BSS to a new operating channel. It is an objective that disruption to the BSS be minimized in this process, although it should be recognized that a channel switch might not successfully move all STAs. Therefore, the AP may decide which of its administrated stations have to perform channel switch. In an implementation, the AP may request only legacy stations that cannot understand OBSS TWT and/or cannot apply degraded EDCA parameters to switch their operating channel. In this implementation, the AP selects a second channel that is supported those legacy stations. 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 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

1. A communication method over a medium in a wireless network, comprising, at a second access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from a first AP managing a first Basic Service Set (BSS), a MAP coordination frame specifying a first Target Wake Time (TWT) Service Period (SP) provided in the first BSS, and responsive to the receiving, sending, to stations of a second BSS managed by the second AP, a frame defining a second TWT SP at least partially overlapping with the first TWT SP of the first BSS.

2. The method of Claim 1 , wherein the frame sent to the stations includes an overlapping BSS (OBSS) TWT element for defining the second TWT SP.

3. The method of Claim 1 , wherein the second TWT SP is time aligned with the first TWT SP.

4. The method of Claim 1 , wherein the starting time of the second TWT SP is aligned with the starting time of the first TWT SP.

5. The method of Claim 2, wherein the OBSS TWT element limits communication activity of stations of the second BSS during the first TWT SP of the first BSS.

6. The method of Claim 2, wherein the OBSS TWT element forbids stations of the second BSS to access the medium during the second TWT SP.

7. The method of Claim 2, wherein the OBSS TWT element indicates a temporary operating channel for stations of the second BSS to switch to the temporary operating channel for their communication during the second TWT SP.

8. The method of Claim 6, wherein a Minimum TWT Wake Duration field within the OBSS TWT element is set to 0.

9. The method of Claim 5, wherein the OBSS TWT element defines a waiting time for stations of the second BSS to access the medium that is larger than a waiting time for stations of the first BSS to access the medium during the first TWT SP.

10. The method of Claim 9, wherein the waiting time for the stations of the second BSS is computed based on a Minimum TWT Wake duration specified in the MAP coordination frame.

11. The method of Claim 9, wherein the OBSS TWT element includes a field setting a minimum waiting time before the stations of the second BSS can access the medium after a second TWT SP starts.

12. The method of Claim 9, wherein the OBSS TWT element includes an OBSS EDCA Parameter Set, different from a legacy EDCA Parameter Set, to be applied by the stations of the second BSS to contend for access to the medium during the second TWT SP.

13. The method of Claim 12, wherein the OBSS EDCA Parameter Set includes a degraded ECWmin value and/or a degraded AIFSN value.

14. The method of Claim 13, wherein the degraded AIFSN value is set by the second AP based on a minimum waiting time specified by the first AP in the MAP coordination frame.

15. The method of Claim 14, wherein the degraded AIFSN value is set by the second AP to a Minimum TWT Wake Duration negotiated with the first AP and specified in the MAP coordination frame.

16. The method of Claim 13, wherein the degraded AIFSN value is to be added to a legacy AIFSN value by the stations of the second BSS to obtain an AIFSN value to be applied when contending for access to the medium during the second TWT SP.

17. The method of Claim 12, wherein the OBSS EDCA Parameter Set includes an OBSS EDCA Timer indicating a duration of time during which the stations of the second BSS use OBSS EDCA parameters before switching back to legacy EDCA parameters, wherein the OBSS EDCA Timer is based on a Minimum TWT Wake Duration field specified in the MAP coordination frame.

18. The method of Claim 9, wherein the waiting time is specified in the MAP coordination frame.

19. The method of Claim 2, wherein the second AP obtains information about interfering stations of its second BSS that are interfering with the first BSS and free stations of its second BSS that are not interfering with the first BSS, wherein the OBSS TWT element limits communication activity over the medium for the interfering stations only.

20. The method of Claim 19, wherein the frame including the OBSS TWT element is sent to the interfering stations only.

21. The method of Claim 1 , wherein the second AP checks whether the first TWT SP does not overlap TWT SP initially scheduled by the second AP within the second BSS.

22. The method of Claim 21 , further comprising sending, to the first AP, a positive response in case of no overlapping; otherwise sending a negative response.

23. The method of Claim 2, further comprising converting timing information of the received first TWT SP in the first BSS that is based on a first clock applicable to the first BSS, into timing information based on a second clock applicable to the second BSS, wherein timing information within the OBSS TWT element is second-clock-based timing information.

24. The method of Claim 2, wherein the frame including the OBSS TWT element is a beacon frame broadcast by the second AP in the second BSS.

25. The method of Claim 24, wherein the beacon frame includes an OBSS TWT Update counter that is incremented each time an OBSS TWT parameter is updated in the OBSS TWT element.

26. The method of Claim 2, wherein the frame including the OBSS TWT element is a probe response frame sent by the second AP in the second BSS.

27. The method of Claim 2, wherein the OBSS TWT element is a restricted TWT element having a Restricted TWT Traffic Info section comprising an OBSS TWT field that is enabled.

28. The method of Claim 1 , wherein the MAP coordination frame is a beacon frame broadcast by the first AP within the first BSS.

29. The method of Claim 28, wherein the first TWT SP received by the second AP is defined by a TWT element addressed to stations of the first BSS within the beacon frame.

30. The method of Claim 1 , wherein the MAP coordination frame is a dedicated unicast frame addressed by the first AP to the second AP over the medium or using an alternate communication medium.

31. The method of Claim 1 , wherein the MAP coordination frame is included in a negotiation process between the first and second APs to establish a MAP TWT agreement.

32. The method of Claim 1 , wherein the MAP coordination frame is exchanged between a third AP co-affiliated with the first AP to the same first AP multi-link device, MLD, and a fourth AP co-affiliated with the second AP to the same second AP MLD, wherein the third and fourth APs operate on a link separate from the link on which the first and second APs operate.

33. A second access point (AP) device in a multi-AP (MAP) coordination set of APs that manages a second Basic Service Set, BSS, in a wireless network having a medium, the second AP comprising: a receiver configured to receive, from a first AP managing a first BSS, a MAP coordination frame specifying a first Target Wake Time (TWT) Service Period (SP) provided in the first BSS, and a transmitter configured to send, responsive to the reception of the MAP coordination frame, to stations of the second BSS, a frame defining second TWT SP at least partially overlapping with the first TWT SP of the first BSS.

34. A communication method over a medium in a wireless network, comprising, at a station of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from the AP, a frame including an overlapping BSS (OBSS) TWT element defining a TWT Service Period (SP), and switching from a legacy EDCA Parameter Set to an OBSS EDCA Parameter Set to contend for access to the medium during the TWT SP.

35. The method of Claim 34, wherein the switching takes place so that the OBSS EDCA Parameter Set starts to be used when the TWT SP starts.

36. The method of Claim 34, wherein the OBSS EDCA Parameter Set is included in a beacon frame broadcast by the AP.

37. The method of Claim 34, wherein the OBSS EDCA Parameter Set is included in the OBSS TWT element.

38. The method of Claim 34, wherein the OBSS EDCA Parameter Set includes a degraded AIFS value and/or a degraded ECWmin value.

39. The method of Claim 38, wherein the degraded AIFS value is added to a legacy AIFS value when contending for access to the medium during the TWT SP.

40. The method of Claim 34, further comprising switching back to the legacy EDCA Parameter Set after an OBSS duration defined by a Minimum TWT Wake Duration field specified in the OBSS TWT element or when an OBSS EDCA Timer specified in the OBSS TWT element elapses.

41. The method of Claim 34, further comprising, at the station, ending any ongoing transmission over the medium, before the start of the TWT SP.

42. A station device of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, in a wireless network having a medium, the station comprising: a receiver configured to receive, from the AP, a frame including an overlapping BSS (OBSS) TWT element defining a TWT Service Period (SP), and a communication module configured to switch from a legacy EDCA Parameter Set to an OBSS EDCA Parameter Set to contend for access to the medium during the TWT SP.

43. A communication method over a medium in a wireless network, comprising, at a station of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, the following steps: receiving, from the AP, a frame defining a TWT Service Period (SP), and switching from an initial operating channel to a second operating channel to contend for access to the medium over the second operating channel during the TWT SP.

44. The method of Claim 43, wherein the TWT SP is at least partially overlapping a TWT SP in another BSS managed by another AP of the MAP coordination set.

45. The method of Claim 43, wherein the received frame includes an overlapping BSS (OBSS) TWT element defining the TWT SP.

46. A station device of a Basic Service Set, BSS, managed by an access point (AP) in a multi-AP (MAP) coordination set of APs, in a wireless network having a medium, the station comprising: a receiver configured to receive, from the AP, a frame defining a TWT Service Period (SP), and a communication module configured to switch from an initial operating channel to a second operating channel to contend for access to the medium over the second operating channel during the TWT SP.

47. A communication method over a medium in a wireless network, comprising, at an access point (AP) in a multi-AP (MAP) coordination set of APs and managing a Basic Service Set, BSS, the following step: sending, to stations of the BSS, a legacy EDCA Parameter Set defining EDCA parameters to be used by the stations when contending for access to the medium, a Multi-User (MU) EDCA Parameter Set defining MU EDCA parameters to be temporarily used by the stations when contending for access to the medium after they made UpLink MU transmission, and an overlapping BSS (OBSS) EDCA Parameter Set defining OBSS EDCA parameters to be temporarily used by the stations when contending for access to the medium during an OBSS Service Period, defined by the AP for the BSS, that is at least overlapping with neighbouring TWT SPs of a TWT schedule in a neighbouring BSS.

48. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a wireless device, causes the wireless device to perform the method of any one of Claims 1 to 32, 34 to 41 , 43 and 47.