Methods and devices for non-primary channel access coordination in multi-AP operation

Abstract

Multi-Access Point, MAP, coordination is used to control secondary channel access of other basic service sets (BSS) when sharing a TXOP. A sharing AP transmits a MAP MU-RTS trigger frame reserving a TXOP and allocating the primary channel to its BSS and secondary channels to shared BSSs. The frame triggers a switching, at the shared AP and stations (STA) of the shared BSSs, from a primary channel access (PCA) scheme to a non-primary channel access scheme where medium access in the shared BSS is performed on the allocated secondary channel. The shared AP, once switched, transmits a clear-to-send (CTS) frame over the secondary channel and another CTS frame, repeating the allocation scheme, over the primary channel. Each shared BSS operates on its allocated secondary channel during the shared TXOP, independently to each other and to the sharing BSS operating on the primary channel. The stations switch back to the PCA scheme upon the TXOP ending. At a STA of a second BSS receive from a first AP managing a first BSS a control frame. In response to the control frame, switch from a PCA scheme to a non-primary channel access scheme.

Description

FIELD OF THE INVENTION The present invention generally relates to wireless communications and more specifically to coordination between multiple access points (APs) in multi-AP operations. BACKGROUND OF THE INVENTION The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section. 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 WLAN (Wireless Local Area Network) technology based on the IEEE (Institute of Electrical and Electronics Engineers - RTM) 802.11 family standards provides a very simple distributed channel access mechanism. Distributed channel access means that a device, in IEEE 802.11 terminology known as a station (STA), either access point (AP) or a non-access point (non-AP), tries to access an operating channel when it has data to send, usually using contention schemes on a so-called primary channel. A centralized channel access is also proposed to improve the channel usage and limit collisions due to contention in the BSS (basic service set) cell. For instance, the AP managing a BSS may control the channel access with its BSS by obtaining itself the access to the channel (using contention) and then managing Multi-User transmissions, either uplink or downlink or direct link, over the obtained operating channel. An operating channel of 40, 80, 160 or 320MHz bandwidth (or even more in the future) is usually made of a primary channel and one or more secondary channels, each channel being 20MHz-wide or a multiple thereof. The primary channel is used for signalling (including channel access procedure such as the contention-based channel access method called Enhanced Distributed Channel Access - EDCA) and backwards compatibility while the secondary channels are only used to extend throughput when sending data at full speed. All the known features for efficient medium usage assume that the primary channel is idle: if the primary channel of a BSS is busy for instance due to interference, stations of that BSS are not allowed to send any frame even if the secondary channels are idle. In other words, mandating the primary channel in the data transmission processes imposes limitations on network efficiency because the secondary channels are prevented from being utilized when the primary channel is occupied. Often, many stations do not support the full bandwidth option supported by their AP (i.e., the AP of the BSS to which they have registered). Where new-generation APs may support an operating channel up to 320 MHz, associated stations may not. As an example, former 802.11 standards may support only 80 MHz, or 160 MHz or even less. Also, stations of the same generation as the APs may have less bandwidth capacities than the APs for cost reasons. As a result, when contention is won on the primary channel of the operating channel by the AP, not all secondary channels of the operating channel are accessible by the associated stations to communicate. That is why submissions, including publications IEEE 802.11 -23 / 0797r0 and 802.11 -23 / 2005r0, have been made to introduce an alternative channel access, known as “non primary channel access” or NPCA. In the first mentioned publication, a station listens to its primary channel an RTS / CTS frame exchange sent by an OBSS station and responsively sets its NAV counter from the PPDU. It then switches to an auxiliary primary channel declared by a management frame sent by its AP, and, after switching to the auxiliary primary channel, invokes a new back-off procedure after sensing the wireless medium on the auxiliary primary channel. After the NAV counter is expired, the station switches back to the primary channel. In the other mentioned publication, the AP does backoff on the auxiliary primary channel (named “second primary channel” in this publication) and initiates a transmission opportunity TXOP with a control frame to target stations. A station of the BSS managed by this AP also moves to the auxiliary primary channel if the first primary channel is busy for NAV time duration. The station waits for a control frame from the AP to possibly be involved in a communication over the auxiliary primary channel. Each BSS or corresponding AP manages the NPCA on its own side, resulting in potential interference between BSSs that elect the same auxiliary primary channel. There is a need to improve this situation. SUMMARY OF INVENTION Coordination of channel usage between BSSs is utilized to improve network performance. The IEEE 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 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. 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 network performance by smartly managing the interference due to OBSSs. It is a broad objective of the present disclosure to overcome some of the foregoing concerns and to improve MAP coordination to take higher benefit of the NPCA mechanism. In this respect, an aim of the disclosure is to improve the use of the medium within the operating channel shared by multiple BSSs I APs. The inventors have contemplated coordinating NPCA operations for APs of a MAP coordinated set even when the primary channel is occupied by a sharing AP of the same set. This AP may initiate a sharing of the operating channel within the MAP coordinated set, triggering the other (shared) APs for NPCA. In this context, a communication method is proposed, comprising at a station of a second Basic Service Set (BSS): receiving, from a first access point (AP) managing a first BSS different from the second BSS, a control frame, and responsive to the control frame, switching from a primary channel access scheme to a non-primary channel access scheme where medium access is performed on a secondary channel. Correspondingly, a communication method is proposed, comprising at a first access point (AP) of a first Basic Service Set (BSS): transmitting a control frame triggering, at one or more stations of a second BSS different from the first BSS, a switching from a primary channel access scheme to a non-primary channel access scheme where medium access in the second BSS is performed on a secondary channel of the second BSS. In that way, the sharing AP having an overall view of the MAP Coordination set and sending the control frame can organize the NPCA operations in other MAP BSSs with a view of reducing interference, hence improving network efficiency. Of course, the sharing AP may change over the time from amongst the AP forming the MAP Coordination set. Furthermore, performing NPCA-based communication over the secondary channel advantageously avoids any relationship with the former primary channel. Hence, there is no need to synchronize the frames between the two channels. The one or more stations may preferably include all the stations of the second BSS, including the second AP managing that BSS. This is to have the entire BSS using NPCA in order to have efficient communications within this BSS. Of course, intermediate approaches may rely on a part only of those stations to switch to the NPCA. Optional features are defined below with reference to methods, while they can be transposed into device features. In embodiments, the control frame allocates the secondary channel to the second BSS or AP. In that way, the sharing (first) AP explicitly indicates the targeted channel for the NPCA operations of the shared (second) AP or APs. This mere indication may be seen as an implicit triggering indication. Of course, in variants, explicit triggering indication may additionally be embedded in the control frame. In other embodiments, the control frame allocates a primary channel to the first BSS or AP. Assuming the operating channel encompasses the primary channels of the first and second BSSs and the secondary channel (or channels if multiple of them are allocated to other BSSs), the control frame by the first AP can reserve a TXOP over the operating channel. Due to the allocation of the secondary channel to the second BSS, the stations of the first BSS (including the first AP) are forced to operate, during the TXOP, on a reduced bandwidth compared to the entire reserved operating channel. Usually, the primary channel of the first BSS but also the primary channel of the second BSS are included in the bandwidth allocated by the control frame to the first BSS. Note that the first and second BSS may have the same primary channel or may have different ones, providing that the first AP obtains a TXOP encompassing the primary channel of the second BSS, to require the switching of NCPA scheme for the second BSS. The first AP may thus check such encompassing before deciding to trigger a switching at the second AP. In this respect, the method at the first AP may further comprise determining, based on channel access information provided by a second AP managing the second BSS (e.g., through a listened management frame or through direct exchanges), whether a channel to be allocated to a third BSS not managing the second BSS (e.g. the first BSS) by the control frame encompasses a primary channel of the second BSS. Also, the method at the first AP may further comprise determining the secondary channel, based on channel access information provided by a second AP managing the second BSS. With an overall view of the OBSSs thanks to the channel access information provided by multiple APs, the first AP can smartly direct the various BSSs to various different secondary channels for NPCA operations. Network efficiency is further increased. In multiple-BSS embodiments, the control frame allocates another secondary channel to a third BSS different from the first and second BSSs and triggers, at stations of the third BSS, a switching from a primary channel access scheme to a non-primary channel access scheme where medium access in the third BSS is performed on the other secondary channel. This illustrates that the same sharing AP may simultaneously control NPCA operations for multiple BSSs. In some embodiments, the control frame includes a switch duration at the end of which the station of the second BSS switch back to the primary channel access (PCA) scheme where medium access is performed on a primary channel of the second BSS. The sharing (first) AP keeps higher control on the NPCA operations within the other BSS. In particular embodiments, the control frame requests a transmission opportunity (TXOP) for a signalled TXOP duration, wherein the switch duration is based on the TXOP duration. In particular, it is sought that the second BSS switches back to the PCA scheme at last with the end of the TXOP (i.e., simultaneously to or prior to a predefined delay). In some embodiments, the control frame includes a multi-user request-to-send trigger frame (MU-RTS TF) reserving a transmission opportunity (TXOP) over an operating channel encompassing primary channels of the first and second BSSs (which may be one and the same primary channel, or different ones) and the secondary channel. Other known TFs (MU BSRP, Basic, NFRP TFs) may be used alternatively, provided that they can signal the secondary channel for the second BSS. For example, the control frame includes at least two User Info fields for respectively the first and second BSS. Each User Info field has an identifier of the respective BSS (BSSID, Identifier or MAC address of the first or second AP, and so on.) and has a bandwidth field specifying a reduced bandwidth compared to the operating channel. The reduced bandwidth for the first BSS includes the primary channel of the first and second BSSs, while the reduced bandwidth for the second BSS includes the secondary channel. A 20MHz-wide control frame transmitted over a primary channel, such as a trigger frame, can be duplicated I replicated over each secondary channel forming, all together -including the primary channel and also the secondary channel for NPCA in the second BSS -, the operating channel the first AP requests for a TXOP. In some embodiments, the method at the station of the second BSS may further comprise sending a secondary control response frame to the first AP over the secondary channel, in response to the control frame. This is to confirm, for all surrounding stations including the first AP, the reservation of the secondary channel. Correspondingly, the method at the first AP may further comprise receiving a secondary control response frame from a station of the second BSS over the secondary channel. In particular embodiments, the secondary control response frame is a clear-to-send (CTS) frame. In other particular embodiments, the secondary control response frame is sent a SIFS after the control frame is received (i.e., immediately, without contending for medium access over the secondary channel used for NPCA operations). In variants, for example when the secondary channel is busy, the secondary control response frame is sent by the station of the second BSS upon gaining access to the secondary channel using a medium access contention scheme. NPCA operations are therefore conducted by this station before being able to respond to the first AP. In other particular embodiments, the method at the station of the second BSS may further comprise sending a frame over the secondary channel immediately after (e.g., a SIPS after) the secondary control response frame. This frame aims at resynchronizing all the stations within the second BSS, after the NPCA switching. As an example, this resynchronizing frame may be a mere (basic) trigger frame to organize multi-user transmissions over the secondary channel, or a mere downlink multi-user PPDU. In some embodiments, the method at the station of the second BSS may further comprise sending a primary control response frame to the first AP over a primary channel of the first AP. The frame is sent in response to the control frame over the primary channel used to transmit such control frame. This allows those (hidden) neighbouring stations not having received the control frame to be aware of the reservation of the primary channel by the first AP. Correspondingly, the method at the first AP may further comprise receiving a primary control response frame over a primary channel of the first AP. In particular embodiments, the primary control response frame includes channel allocation information mirroring channel allocation information contained in the control frame. This allows the (hidden) neighbouring stations to be aware of NPCA switches for other BSSs, in particular to determine whether they have to perform the NPCA switching and to which secondary channel. In other particular embodiments, the primary control response frame is a clear-to-send (CTS) frame. In other particular embodiments, the primary control response frame is sent a SIFS after the control frame is received (i.e., immediately, without contending for medium access over the secondary channel used for NPCA operations). Hence the primary control response frame may be sent independently to the secondary control response frame (e.g., sent after the station gains access to the secondary channel through contention). Preferably, the station of the second BSS that sends the first and secondary control response frames and optionally the resynchronizing frame is a second AP managing the second BSS. In some embodiments, the control frame is transmitted within a Target Wake Time service period (TWT SP) previously signalled by the first AP to the second AP or agreed between the first and second APs. More generally, a TWT schedule may be negotiated in a MAP Coordination set. Thanks to the TWT schedule, the first AP has a scheduled SP where no OBSS interferes and the first AP can therefore easily organise the transmissions in its own BSS and the other OBSSs using the NPCA switching on appropriate secondary channels. In some embodiments, the control frame includes padding to have a length of the control frame at least equal to a channel switch time length. This is to allow the stations of the second BSS to have enough time to perform the channel access scheme switch to be ready to use the secondary channel with NPCA as soon as the control frame ends. In particular embodiments, the channel switch time length is based on channel switch time information provided by a second AP managing the second BSS to the first AP. Hence, the first AP may obtain the channel switch time information by exchanging with the second AP (more generally with the APs of the MAP Coordination set), correspondingly determines the channel switch time length appropriate for the second BSS, and then provide padding in the control frame corresponding to the determined channel switch time length. The padding may be adjusted to align the control frame length to the determined channel switch time length. Alternatively, a prefixed padding may be added or not according to the determined length. Altogether, above features may result in a communication method over a wireless medium having an operating channel comprising a primary channel and one or more secondary channels, that is shared by first and second BSSs, the method comprising at a station of the second BSS, all or subparts of: receiving, from a first AP managing a first BSS, a MU RTS trigger frame reserving a TXOP on the operating channel for a TXOP duration, the MU RTS trigger frame having an allocation scheme allocating the primary channel to the first BSS and a secondary channel to the second BSS, for a TXOP duration, responsive to the MU RTS trigger frame, switching from a primary channel access scheme to a non-primary channel access scheme where medium access is performed on the allocated secondary channel, where the station is a second AP of the second BSS, transmitting a first CTS frame to the first AP over the allocated secondary channel and optionally a second CTS frame over the primary channel, at least the second CTS frame repeating the allocation scheme. The CTS frame over both channels can be transmitted immediately after the MU RTS trigger frame (i.e., a SIFS after), or the CTS frame over the allocated secondary channel only can be transmitted after the second AP gains access to the secondary channel using contention, operating on the allocated secondary channel using the non-primary channel access scheme - meaning performing backoff procedures on that secondary channel to gain access to it, at the end of the TXOP duration, switching back to the primary channel access scheme. The corresponding method at the first AP of the first BSS comprises all or subparts of: transmitting a MU RTS trigger frame reserving a TXOP on the operating channel for a TXOP duration, the MU RTS trigger frame having an allocation scheme allocating the primary channel to the first BSS and a secondary channel to the second BSS to trigger a switching, at stations of the second BSS, from a primary channel access scheme to a non-primary channel access scheme where medium access in the second BSS is performed on the allocated secondary channel, receiving, in response to the MU RTS trigger frame and from a second AP of the second BSS, a first CTS frame over the allocated secondary channel and optionally a second CTS frame over the primary channel, at least the second CTS frame repeating the allocation scheme, operating on a channel, including the allocated primary channel, that has a reduced bandwidth compared to the operating channel reserved by the MU RTS trigger frame, at the end of the TXOP duration, operating back on the entire operating channel. 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, using a timeline, an exemplary use by a station (e.g., 802.11bn / UHR STA) of the NPCA scheme based on an initial NAV set; Figures 3 and 4 illustrate, using flowcharts, exemplary steps of non-primary channel access (NPCA) coordination in a MAP Coordination set, respectively at the sharing AP sharing a TXOP and at a station of a shared BSS; Figures 5 and 6 illustrate, using flowcharts, other exemplary steps of non-primary channel access (NPCA) coordination in a MAP Coordination set, respectively at the sharing AP sharing a TXOP and at a station of a shared BSS; Figure 6a illustrates, using a flowchart, embodiments to handle the transmission of response control frames such as CTS frames; Figure 6b illustrates, using a flowchart, exemplary steps of non-primary channel access (NPCA) coordination in a MAP Coordination set at a non-AP station of a shared BSS in case the steps of Figure 6 are performed by the shared AP managing the shared BSS; Figure 7 illustrates a scenario of transmission sequence implementing a TXOP coordination for reduced OBSS interference in MAP operation, according to embodiments of the disclosure; Figure 8a illustrates the format of an 802.11 Trigger frame; Figure 8b illustrates format of a User Info field in the Trigger frame; Figure 8c illustrates format of a Common Info field in the Trigger frame; Figure 9a shows a schematic representation a communication device in accordance with embodiments of the present invention; and Figure 9b shows a schematic representation of a wireless communication device in accordance with embodiments of the present 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, 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 or “operating” channel thus granted for transmission comprises the primary channel and optionally secondary channels. According to the 802.11 standard family, the primary channel is the common channel of operation for all stations that are members of the BSS. Usually, in a 20 MHz, 40 MHz, 80 MHz, 160 MHz, 80+80 MHz, 320 MHz BSS, the primary channel is a primary 20 MHz channel. Correspondingly, a non-primary channel is any 20 MHz channel other than the primary 20 MHz channel. A secondary channel is a channel associated with a primary channel used to create an operating channel wider than the primary channel. In a 40 MHz, 80 MHz, 160 MHz, 80+80 MHz or 320 MHz BSS, the secondary channel is a secondary 20 MHz channel. However, the notions of primary and secondary channels have been extended to channels having more than 20 MHz width. For example, the primary 40 MHz (resp. 80 MHz, 160 MHz) channel is the 40 MHz channel (resp. 80 MHz, 160 MHz) in a 80 MHz, 160 MHz, 80+80 MHz or 320 MHz BSS, formed by the primary channel (20 MHz) and one or more adjacent and aggregated secondary channels (20 MHz each), that is used to transmit 40 MHz (resp. 80 MHz, 160 MHz) physical layer (PHY) protocol data units (PPDUs). Similarly, a secondary X MHz channel (X = 20, 40, 80 or 160) is defined in a 2X (or more) MHz BSS as the X MHz channel adjacent to the primary X MHz channel that together form a primary 2X MHz channel (or the entire 2X MHz operating channel). The primary channel is used for signalling and backwards compatibility while the secondary channels are only used to extend throughput when sending data at full speed. This channel access based on the primary channel is known as the Primary Channel Access or “PCA”. Efficient medium usage within one operating channel having an operation bandwidth (up to 320MHz in the latest 802.11 be D5.0 standard, but may be wider in future amendments) has evolved along the evolution of the IEEE 802.11 standards. For example, dynamic bandwidth signalling feature was introduced in the IEEE 802.11ac amendment, preamble puncturing feature was introduced in the IEEE 802.11 ax standard and further evolved in the IEEE 802.11 be amendments. Also, 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. 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 resource units forming the communication channel. 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. 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 802.11 be, as illustrated by draft IEEE P802.11 be / D5.0 of November 2023, introduce the Multi-Link Operation (MLO) when it comes to MAC layer operation. The MLO allows multi-link devices (MLD) to establish or setup multiple links and operate them simultaneously. The present disclosure 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. 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 or “shared” communication channel or wireless medium. The common communication channel may correspond to a part (e.g., 20 MHz) or all of their operating channels (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 110, 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 BSS exchange data frames over the communication channel 100, under the management of the AP, using the primary channel of the BSS and optional secondary channels aggregated to the primary channel. Note that for the present disclosure, the multiple BSSs use the same primary channel, either same primary 20 MHz channel or same 40 MHz channel or same 80 MHz channel or same 160 MHz channel, provided that an adjacent secondary 20-40-80-160 MHz channel is also used. The APs 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 -not shown (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 primary channel using EDCA (Enhanced Distributed Channel Access) contention to access the communication channel in order to be granted a transmission opportunity (TXOP) over an operating channel made of the primary channel and a secondary channel to increase bandwidth. The TXOP may then be used to transmit (single-user, SU) data frames or to implement multi-user (MU) transmissions. It turns out that this conventional Primary Channel Access (PCA) scheme does not allow the stations to use any secondary channel while the primary channel is busy. This induces a waste of communication resources in case of primary channel use, even more massive as the operating channel has a large bandwidth (40 / 80 / 160 / 320 MHz). To meet low latency requirements as well as to increase efficiency of bandwidth usage, efforts have been made by the 802.11 bn group to reuse the lost secondary channels through a so-called non-primary channel access (NPCA) scheme, sometimes called Secondary Channel Access (SCA). The basic idea is for stations of a BSS (including the AP) to opportunistically operate (in the meaning of performing contention, e.g. through backoff procedure) on a non-primary channel, i.e., a secondary channel, while the primary channel of that BSS is busy and thus blocked (e.g., due to an OBSS transmission). A typical usage of NPCA is when two stations (e.g., a non-AP station and its AP) have their network allocation vector (NAV) set from the OBSS transmission. They can both decide starting to operate on the non-primary channel, by switching their channel access mechanism (e.g., medium sensing and backoff procedure) on the non-primary channel they elect. It is expected that the stations have capability to monitor the non-primary channel at least to the extent of performing Clear Channel Assessment (CCA) to detect the presence of energy in the medium while it is not transmitting on the primary channel and vice-versa. The duration for which the two stations operate on the non-primary link is expected to be limited to the time for which the NAV in the primary channel was set by the OBSS transmission. Already-mentioned publications IEEE 802.11-23 / 0797r0 and 802.11-23 / 2005r0, disclose NPCA schemes. The “non-primary channel for NPCA” can also be referred to as the “secondary channel for NPCA” or the “temporary primary channel” since NPCA is temporary or the “auxiliary primary channel” since NPCA use the non-primary channel as an auxiliary channel to the conventional primary channel or the “fallback primary channel” since the stations fall back to that channel while the conventional primary channel is busy. Of course, any other terminology can be used. Figure 2 illustrates, using a timeline, an exemplary use by a station (e.g., 802.11bn / UHR STA) of the NPCA scheme based on an initial NAV set. In the example, the operating channel of the BSS is 160 MHz or 80+80 MHz wide, made of 80 MHz channel 1 and 80 MHz channel 2. 80 MHz channel 1 is a primary 80 MHz channel since it includes the primary channel where the backoff procedure is made, while 80 MHz channel 2 is a secondary 80 MHz channel. This organization of the operating channel may be announced by the AP of the communications system 200 (e.g. administrated by AP 110, 120 or 130) through for instance beacon frames. Usually, the AP and the stations of the BSS perform the backoff procedure in the primary channel only. The primary channel (hence the primary 80 MHz channel 1) is busy due to a neighbour BSS's TXOP. Knowing this situation where their NAV is set for the TXOP, the stations of the BSS switch their channel access scheme to another channel (a secondary channel) where they can enable a backoff procedure. The stations switch from a PCA to a NPCA on the secondary channel elected, and by sensing the various secondary channels, they can enjoy using the secondary 80 MHz channel 2. Frame exchanges can be performed in the subchannel(s) other than the subchannel with the busy primary channel, during the duration of the TXOP set in the primary channel. On subchannel 1, an overlapping BSS (OBSS) starts a TXOP, blocking any backoff procedure of the stations of the BSS for the TXOP duration. The stations switch their channel access scheme to subchannel 2 for possible frame exchanges on subchannel 2. On subchannel 2, the stations perform CCA for PPDU detection and do the backoff procedure. Once gaining access to subchannel 2 (backoff counter becomes 0), a first one of the stations transmits an RTS frame 201. The other station responds with a CTS frame 202 responsive to the reception of the RTS frame 201. The first station transmits a data frame 211 after receiving the CTS frame. The other station responds with an acknowledgement (ACK) frame 212 after receiving the data frame. At the end of the TXOP, the stations switch back their channel access scheme to the primary channel. Keeping longer the channel access scheme on the secondary channel might mean the stations lose medium access on the primary channel. It also means that the AP remains unavailable to any legacy associated station which would try to reach the AP after the NAV set by OBSS transmission is over. However, even when switching back under these circumstances, the stations may be desynchronized with the activities on the primary channel because they may not be aware of any change of transmission activities. For instance, there are some situations where the frame exchanges on the primary channel terminate earlier than expected, hence returning earlier the primary channel to conventional contention. There are also some situations where additional frame exchanges are needed and occur, exceeding the anticipated duration, in which case the primary channel remains busy when the station switch back to the PCA scheme. Furthermore, each BSS or corresponding AP manages the NPCA on its own side (e.g. activate it or not for the entire BSS), resulting in potential interference between BSSs that elect the same secondary channel for NPCA. There is a need to improve this situation. In this respect, MAP coordination is contemplated to take higher benefit of the NPCA mechanism and to organise the NPCA operations over multiple BSSs with an overall view of reducing interference, hence improving network efficiency. Back to Figure 1 depicting multiple APs, Multi-AP (MAP) coordination relies on mechanisms 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. The AP sharing the channel it has been granted may be referred to as “sharing AP” (hence sharing BSS for the corresponding BSS), while the APs benefiting of the sharing may be referred to as “shared APs” (hence shared BSSs) or “participating” APs. MAP sharing of the common communication channel can be 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. In publication 802.11-19 / 1582r1 (entitled “Coordinated AP Time / Frequency Sharing in a Transmit Opportunity in 11be”), a time / frequency resource sharing mechanism is proposed made of three phases: first, a TXOP owner AP initially coordinates the participating APs; secondly, it informs the participating APs about their allocated resources and a TX start time; in the third phase, the participating APs transmit on their respective allocated resources in the TXOP, beginning at the TX start time.. 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 beacon frames or dedicated broadcasted frames, to advertise the other APs of their MAP coordination capability. The MAP Coordination group contains multiple APs. To take advantage of the NPCA mechanism in a MAP coordination environment, it is proposed for the sharing AP to send a control frame triggering, at one or more stations of a shared BSS, a switching from the primary channel access scheme to the non-primary channel access scheme where medium access in the shared BSS is performed on a secondary channel. The sharing AP having an overall view of the MAP BSSs can efficiently organise the NPCA operations in the shared BSSs, reducing OBSS interference and increasing network efficiency. As an example, the sharing AP may send an allocation frame (e.g. an augmented MU RTS frame as described below) allocating a secondary channel for a shared AP (one secondary channel per AP in case of a plurality of shared APs) for a period during which the primary channel is determined to be busy (e.g., TXOP duration). The stations of the shared BSS can then switch their channel access scheme to the NPCA scheme on the allocated secondary channel, to operate afterwards on the allocated secondary channel. The present disclosure will be illustrated with general term shared BSSs or APs, and one may note that this includes physical AP devices, but also other declinations such as the following non-exhaustive list: - a virtual AP managing a virtual BSS (referred to as a nontransmitted BSSID AP in the IEEE specification). The nontransmitted BSSID is considered as a shared BSS, for which the transmitted BSSID AP may be the sharing AP; - a software access-point (soft-AP) or Wi-Fi Direct Group Owner, allowing P2P communications to operate independently of an infrastructure BSS. The P2P communication group is thus considered as forming a shared BSS; - a non-AP station acting in a Peer-to-peer session (like a Tunneled Direct Link Setup (TDLS)) and exchanging Channel Usage elements to inform the sharing AP about the resource needs indicative of channels to perform off-channel operations for the sake of the P2P session. The P2P session is thus considered as forming a shared BSS. In this respect, the off-channel may amount to the secondary channel for NPCA in the meaning of the invention. Figures 3 and 4 illustrate, using flowcharts, exemplary steps of NPCA coordination in a MAP Coordination set, respectively at the sharing AP and at a station of a shared BSS. At step 300, the sharing AP obtains access to the operating channel through contention on the primary channel. In other words, the sharing AP operates on the operating channel. At step 310, the sharing AP transmits a control frame triggering, at the stations of a shared BSS, a switching from the PCA scheme to the NPCA scheme where medium access in the shared BSS is performed on a secondary channel. In embodiments, the same control frame can trigger such switch in two or more shared BSSs. The control frame may be a broadcast or multicast or unicast frame. The secondary channel (or channels) to be used for NPCA may be signalled in the control frame or, alternatively, be predefined. The shared subchannel (comprising the secondary channel for NPCA, and not the primary channel, e.g. subchannel 2 in Figure 2) on which the shared BSS will operate using NPCA can be known by the sharing AP. Next, at step 320, the sharing AP continues to operate on the subchannel comprising the primary channel (e.g. subchannel 1 in Figure 2), i.e., on a reduced bandwidth compared to the operating channel. Correspondingly, at step 400, a station (non-AP station or AP) of the shared BSS, preferably all the stations, receive, from the sharing AP, the control frame. At step 410, the station, responsive to the control frame, switches from the PCA scheme to the NPCA scheme where medium access is performed on the secondary channel. At step 420, the station operates on the secondary channel for NPCA, more particularly operates on a subchannel of the operating channel that includes the secondary channel (e.g. subchannel 2 in Figure 2) and performs the backoff procedure on the secondary channel. Figures 5 and 6 illustrate, using flowcharts, exemplary steps of NPCA coordination in a MAP Coordination set, respectively at the sharing AP and at a station of a shared BSS, according to other embodiments. Figure 7 shows, using a timeline, an illustrative scenario of the NPCA coordination when implementing the steps of Figures 5 and 6. The process at the sharing AP (e.g. AP2 in Figure 7) starts at step 500 where it obtains channel access information about the shared AP / BSS or APs / BSSs (e.g. AP1 and AP3 in Figure 7) and more generally about the APs of the MAP Coordination set. The channel access information for an AP / BSS includes its primary channel and its secondary channel or channels (hence the operating channel), its capability to perform NPCA, and optionally the secondary channel or channels on which the concerned BSS can perform NPCA. More generally, the channel access information for an AP may indicate a bandwidth part of the medium on which the AP is operating. The sharing AP may obtain the information from local memory or from a third party. In some embodiments however, the information is obtained from the shared AP itself. As an example, the sharing and shared APs may perform initial setup for channel access coordination, by exchanging management frames. For example, any (shared) AP of the MAP Coordination set sends channel access information to each other AP (including the sharing AP). In embodiments, all or part of the channel access information for an AP is conveyed in a broadcast message transmitted by the AP and thus received by the sharing AP (e.g. Beacon frames 750 in Figure 7). A beacon frame may then be analysed to derive a bandwidth part and / or one or more channels on which the sending AP operates. Alternatively or in addition, all or part of the channel access information for the AP is conveyed in a unicast handshake message exchange between the AP and the sharing AP (e.g. frames 760 / 761 in Figure 7). Alternatively or in addition, all or part of the channel access information for the AP is conveyed in a multicast handshake message exchange performed in a group of APs which includes the AP and the sharing AP (e.g. the MAP Coordination set). In this exchange, the AP sends a multicast message to the other APs of the group, and at least some of the other APs respond to the multicast message. The channel access information may then be determined from these responses. Step 500 thus allows the sharing AP to have an overall view of the operating channels and NPCA capabilities of the various APs of the MAP Coordination set. Although they collaborate to share a communication channel, they may have different primary (20 MHz) channels which however interfere because belonging to the same primary 40-80-160 MHz channel within a wider operating channel. Step 500 may be repeated over time for the sharing AP (more generally any AP) to have an up-to-date view of the MAP Coordination set in terms of NPCA opportunities. At step 510, the sharing AP operates on its maximum bandwidth, i.e., the whole operating channel including its primary channel and the secondary channel or channels. The entire operating channel (maximum bandwidth) is considered, even if no associated station can support this bandwidth. This operation includes carrier sensing to gain access to the primary channel. This may involve performing a CCA procedure to assess whether the medium is occupied. In addition, the sharing AP scans all secondary channels (where it intends to provide sharing to some other AP(s)) to detect activity (e.g. control messages) on the secondary channels. This allows the sharing AP to determine whether it can request access to the secondary channels in addition to the primary channel (to be granted the widest bandwidth). It also allows the sharing AP to have knowledge of which secondary channel or channels are available for sharing to the other APs. At step 520, the sharing AP determines the secondary channel to allocate to each shared AP, within the next TXOP the sharing AP will obtain. This first includes determining which other APs can no longer use their primary channel if the sharing AP gains access to the medium and allocates a first channel that encompasses the primary channel of the other APs. As an example, the sharing AP may decide to use the gained medium over a reduced bandwidth only (including its own primary channel and optionally one or more additional secondary channels), meaning that the sharing AP allocates itself a first channel through the control frame to be sent. Of course, a first channel allocated to any AP (for sharing purposes) may encompass the primary channel of another AP which thus should perform NPCA on a secondary channel. In that respect, the sharing AP determines, based on the channel access information obtained for the other APs, whether a channel to be allocated to a third (e.g. the sharing one) AP / BSS by the control frame to be sent encompasses the primary channel of the other AP / BSS. In the affirmative, the sharing AP will try to trigger a switching at these other AP / BSS to the NPCA scheme. However, appropriate secondary channels must be available. In that respect, the sharing AP may determine or select the secondary channel to be allocated to another AP based on the channel access information obtained for that other AP. As the sharing AP has an overall view of all the other APs, it may smartly select the most appropriate secondary channels for specific other APs. Doing so, the sharing AP allocates secondary channel 791 to AP1 while allocating secondary channel 792 to AP3, in the scenario of Figure 7. Depending on the secondary channels available and the channel access information, the number of shared APs may vary from one to multiple. At step 530, when the primary channel is free and contention succeeded, the AP gains access to the medium over the operating channel and emits a control frame (e.g. frame 700 in Figure 7) towards the sharing and shared BSSs (hence including the sharing AP and the one or more shared APs as determined at step 520 as well as their associated stations). The control frame may be duplicated over each 20 MHz channel forming the solicited operating channel. This control frame aims at triggering, at stations of the shared BSSs, a switching from the PCA scheme to the NPCA scheme where medium access in the shared BSS is performed on a secondary channel allocated, by the control frame, to the shared BSS. An explicit triggering signalling may be provided in the control frame or alternatively it may be implicit, e.g. responsive to the allocation of the secondary channel. This control frame is typically a trigger frame for MAP coordination with a view of triggering the NPCA switching in the shared BSS or BSSs. Therefore, the control frame can be a MAP trigger frame for NPCA. As an example, the MAP trigger frame for NPCA is a multi-user request-to-send MU-RTS trigger frame variant, meaning it also triggers a CTS response from the addresses of the trigger frame (i.e., those stations that are allocated resources through the trigger frame). The trigger frame reserves a transmission opportunity (TXOP) over the operating channel encompassing the primary channels of the sharing and shared APs / BSSs (which may be one and the same primary channel) and secondary channels. The trigger frame contains an identification of secondary channels to be used by the shared AP(s), along with the allocation of those channels to each shared AP. In that way, the control frame allocates a secondary channel to each participating shared AP. This may be done using the User Info field of an 802.11 trigger frame. The control frame signals the secondary channel the shared AP should use for NPCA. In embodiments, the control frame also includes a new operational bandwidth for the shared AP which may be the same as the sole secondary channel for NCPA or be wider than the sole secondary channel for NPCA, meaning additional secondary channel or channels are allocated to the shared AP together with the secondary channel for NCPA. In other words, the control frame may indicate the secondary 20 MHz channel for NPCA as well as a secondary 40-80-160 MHz (encompassing the channel for NPCA) to be used for operation by the shared AP. In some embodiments, the sharing AP keeps the ownership of the primary channel for the TXOP, for its BSS (including the sharing AP) to be able to conduct communications during the TXOP. In this respect, the control frame also allocates the primary channel to the sharing AP / BSS. For instance, a dedicated User Info field is used in the trigger frame. While the MU-RTS trigger frame is used in the present example for illustrative purposes, other MU trigger frames, such as the MU BSRP, Basic or NFRP trigger frames, can be used alternatively. Figures 8a, 8b and 8c illustrate trigger frames that can be used as the above control frame (e.g. frame 700 in Figure 7). Most of the fields keep their use as defined in the standard IEEE P802.11 REVme / D5.0. A trigger frame 800 is a trigger frame in the meaning of the 802.11 standards, i.e., it is a MAC frame having Type value ‘01’ and Subtype value ‘0010’ in the Frame Control field 801 of the MAC header. Such trigger frame allocates resources for one or more PPDUs. The Trigger frame also carries other information required by the responding station to send an HE / EHT / UHR TB PPDU, such as a CTS - clear to send - frame. Trigger frame 800 is made up of the following conventional fields: - Frame Control field 801 to indicate mainly the type of the frame. Type value ‘01 ’ and Subtype value ‘0010’ in this field identify a Trigger frame, - Duration field 802 to set a duration of the transmission, generally in ps (microseconds). This value allows the receivers to set their network allocation vector (NAV) which is an indication of the duration that a station prevents from accessing the medium. As the trigger frame reserves a TXOP on the medium, this duration is also known as the TXOP Duration, - RA (Receiver Address) field 803 to identify the addressee or addressees of the frame. RA field 803 is set to a broadcast address to target all stations and other APs (e.g. for MAP operation, as in the scenario of Figure 7), or to a station address to perform unicast transmission to a target station, - TA (Transmitter Address) field 804 to identify the transmitting station. It is set to the address of the transmitting station (sharing AP AP2 in the scenario of Figure 7), - Common Info field 810 further described below with reference to Figure 8c, - User Info List field 805 that contains zero or more User Info fields 830 to respectively define zero or more RU or channel allocations. User Info field 830 is further described below with reference to Figure 8b, - Optional Padding field 806 to extend the frame length to give the recipient stations enough time to prepare a response for transmission a SIFS after the Trigger frame is received, and - FCS field 807 to contain a 32-bit CRC. Figure 8b illustrates a format of a User Info field 830 (HE variant) in the Trigger frame format. The EHT variant of the User Info field format (not illustrated) is the same except that the EHT variant includes a PS160 subfield (in replacement of reserved bit B39) which is used in complement to RU allocation and UL BW subfields for instance to handle 320MHz bandwidth channel. User Info field 830 includes the following subfields: - AID12 subfield 831 encoded as described in the following table: AID12 subfield Description 0 User Info field allocates one or more contiguous random-access RUs for associated STAs 1-2007 User Info field is addressed to an associated STA whose AID is equal to the value in the AID12 subfield 2008-2044 Reserved 2045 User Info field allocates one or more contiguous random-access RUs for unassociated STAs 2046 Unallocated RU 2047-4094 Reserved 4095 Start of Padding field - RU Allocation subfield 832 along with UL BW subfield 815 in Common Info field 810 to identify the size and the location of the RU allocated through the current User Info field. If the AID12 subfield is in the range 1 to 2007, then the RU Allocation subfield indicates the RU is allocated to the STA identified by the AID12 subfield. If the AID12 subfield is 0 or 2045, then the RU Allocation subfield indicates the starting RU of one or more contiguous RA-RUs (random access) allocated by the User Info field. If the AID12 subfield is 2046, then the RU Allocation subfield indicates an unallocated RU, - UL FEC Coding Type subfield 833 to indicate the code type of the solicited HE TB PPDU, - UL HE-MCS subfield 834 to indicate the HE-MCS of the solicited HE TB PPDU, - UL DCM subfield 835 (absent in the EHT variant) to indicate DCM (dual carrier modulation) of the solicited HE TB PPDU. - subfield 836 corresponds to the RA-RU Information subfield if the AID12 subfield is either 0 or 2045; otherwise this subfield corresponds to the SS Allocation subfield. - UL Target Receive Power subfield 837 to indicate the expected receive signal power, measured at the AP. - Reserved bit B39 (PS160 bit in the EHT variant), and - Optional Trigger Dependent User Info subfield 839 to convey additional information. Its presence depends on the value of the Trigger Type field 811 in Common Info field 810 (i.e., depends on the type of Trigger frame). Figure 8c illustrates formats of HE variant and the EHT variant of Common Info field 810 in the Trigger frame format. Common Info field 810 includes the following subfields (not exhaustive list for conciseness): - Trigger Type subfield 811 to identify the Trigger frame variant. For example, value 0 defines a Basic trigger frame, value 1 a Beamforming Report Poll trigger frame, value 2 a MU-BAR trigger frame, value 3 a MU-RTS trigger frame, value 4 a BSRP trigger frame, value 5 a GCR MU-BAR trigger frame, value 6 a BQRP trigger frame, value 7 a NFRP trigger frame, value 8 a Ranging trigger frame. In some embodiments, value 9 can be used to define a MAP trigger frame for MAP collaboration, e.g. MAP MU-RTS trigger frame if it further solicit a CTS response. In some embodiments with explicit signalling of the NPCA switch triggering, value 9 (or any available 9-15 values) is used to define a MAP trigger frame for NPCA (or a NPCA Switch trigger frame), - UL Length subfield 812 to indicate the value of the L-SIG LENGTH field of the solicited response, - More TF subfield 813 to indicate whether or not a subsequent Trigger frame is scheduled for transmission, - CS Required subfield 814 to define specific rules for channel sensing, - UL BW subfield 815 to indicate bandwidth in the HE-SIG-A of the response, - subfield 816 corresponds to the Triggered TXOP Sharing Mode subfield if the Trigger type 811 indicates an MU-RTS Trigger Frame (value 3); otherwise field 716 is the Gl And HE-LTF Type subfield. The values available for the Triggered TXOP Sharing Mode subfield are shown in the Figure (value 3 is reserved according to 802.11 standards). As mentioned just above, Trigger Type subfield 811 may be used to explicitly signal a MAP trigger frame for NPCA. For example, value 9 in that subfield can be used to that end. Preferably, this trigger frame is also of a MU-RTS type, meaning it also solicits a CTS response (e.g., 710, 711 in Figure 7) from the multiple addressed APs in different (secondary) channels and so to protect more widely (i.e., in a wider area) the subsequent communications in each dedicated channel. In that case, the trigger frame is a MAP MU-RTS trigger frame for NPCA (or MU-RTS NPCA Switch trigger frame). Alternative signalling options to Trigger Type subfield 811 can be used, alone or in combination. For example, Triggered TXOP Sharing Mode subfield 816 as shown in Figure 7c, has been extended to the MAP usage through value 3 where an AP can be allocated an RU or channel (i.e., “scheduled”). Such value may be interpreted as a request for NPCA switching, in which case the trigger frame is a MAP MU-RTS trigger frame or MAP MU-RTS TXS trigger frame, for NPCA. Another signalling option includes augmenting User Info field 830 or Common Info field 831 by including a new MAP ID field (not shown) in either a reserved field or in Trigger Dependent User / Common Info field 839 / 817. This identifier may carry any identifier of the MAP Coordination set to which the APs pertain, hence signalling a MAP trigger type, which is of the MU-RTS type if Trigger Type subfield 811 equals 3 (or 9). Further to the signalling of the MAP MU-RTS trigger frame for NPCA, the secondary channels allocated to the shared APs for NPCA have to be signalled / identified. It is recalled that each allocation may be defined through a User Info field 830, including a self-allocation of one or more channels for the sharing AP. However, Common Info field 810 can be used to define common information to all the allocations. In embodiments, the following use of conventional subfields in User Info field 830 and Common Info field 810 is made to signal the allocations of the secondary channels for NPCA: UL BW field 815 in the Common Info field indicates the bandwidth of the PPDU carrying the MAP MU-RTS Trigger frame for NPCA 700. This corresponds to the operating channel encompassing the primary and secondary channels. UL Length field 812 (normally reserved) is optionally augmented to convey a switch duration for NPCA, i.e., the duration allowed for NPCA at the end of which the stations having switched to the NPCA scheme switch back to the PCA scheme where medium access is performed on their primary channel. This use of UL Length 812 is optional since it may be decided that the NPCA switching can last up to the end of the TXOP. In that case, the TXOP duration as specified in Duration field 802 can be used: the trigger frame requests a transmission opportunity (TXOP) for a signalled TXOP duration, and the switch duration is based on the TXOP duration, in particular to align the switching back to the PCA scheme to the end of the TXOP. RU Allocation field 832 in the User Info field addressed to a station of the sharing BSS (e.g., the sharing AP itself) indicates in which channel the solicited CTS response has to be transmitted, indicating either the primary 20 MHz channel, primary 40 MHz channel, primary 80 MHz channel, primary 160 MHz channel, or 80+80 MHz channel (HE only) or 320 MHz channel (using values from 61 to 68 for a MU-RTS trigger frame). For the allocation of secondary channels to the shared APs, RU Allocation field 832 can be used to indicate the secondary 20Mz channel allocated to the shared AP where to perform NPCA and emit the solicited CTS response. Values different from 61 to 68 may be used to uniquely identify each 20 MHz channel within the operating channel defined by UL BW field 815. AID12 field 831 in the User Info field includes an identifier of the shared AP or BSS concerned by the channel allocation of that User Info field. Such an identifier may be provided to the APs when creating or joining the MAP Coordination Set. In some embodiments, Trigger Dependent User info field 839 may further include a BW subfield to indicate the bandwidth (in multiple of 20 MHz) that is allocated to the shared AP or BSS concerned by the channel allocation. In a variant, the roles of RU Allocation field 832 and the BW subfield in Trigger Dependent User info field 839 may be inverted. Anyway, the shared AP is then able to know the secondary channel for NPCA as well as the total bandwidth (including the secondary channel for NPCA) that is allocated to its shared BSS for communication using NPCA. Alternatively to the augmented use of conventional subfields, a new format can be defined for the User Info field to declare an allocation of a secondary channel for NPCA. This new format is illustrated by reference 830b in Figure 8b. The new User Info field 830b includes the following subfields: AP_ID (renaming former AID12 subfield) subfield 831 to convey the identifier of the shared AP or BSS concerned by the channel allocation, RU Allocation subfield 832 to indicate the secondary 20 MHz channel to be used for NPCA, and new BW subfield 890 to indicate the bandwidth (in multiple of 20 MHz) that is allocated to the concerned shared AP or BSS. Obviously, the allocated channel encompasses the secondary 20 MHz channel for NPCA. BW subfield 890 may thus specify a secondary 40 or 80 or 160 MHz channel that includes the secondary channel for NPCA. In a variant, as the MAP MU-RTS trigger frame is clearly identified as a new format (through fields 811 and / or 816), RU Allocation field 832 may rather identify the secondary channel for NPCA using a Channel Number as defined in 802.11 REVme D4.0 Annex E. Such identification by a Channel Number provides the advantage of relying on a legacy format that is simple and widely used for example in Management frames. The exemplary signalling of Figures 8a, 8b and 8c thus allows the sharing AP to signal and the shared AP to identify a MAP MU-RTS trigger frame that triggers NPCA switching at the shared BSSs, to operate on a signalled secondary 20 MHz channel and communicate on a signalled 20-or-more MHz channel that includes the secondary channel for NPCA. Back to Figure 5, once the sharing AP has sent the control frame, it operates at step 540 on its primary channel (for channel sensing such as CCA, and backoff procedure), while considering a bandwidth reduced compared to the original operating channel, due to the secondary channels allocated to the shared APs. Operation with limited bandwidth continues up to the end of the TXOP (step 550), in which case the sharing AP recovers its classical operating channel (step 560 similar to step 510). The process at stations of the shared BSS (e.g. AP1 or AP3 in Figure 7) is illustrated in Figure 6 and starts at step 600. Preferably, this process is implemented at the shared AP and not its associated stations. However, in alternative embodiments, any (all) station of the shared BSS can implement the process of Figure 6. Step 600 corresponds to step 500 where the shared AP communicates its channel access information to the sharing AP. This step is optional since other stations of the shared BSS do not implement it and also because the sharing AP may alternatively obtain this information from other sources. At step 610, the station operates in a conventional way, operating (channel sensing and backoff procedure) on the primary channel of its BSS and using the operating channel (20 to 320 MHz) to perform communication. At step 620, the station receives the control frame sent by a sharing AP at step 530. The reception can happen while the station contends for access to the medium. In some embodiments however, the control frame is transmitted and thus received by the station within a Target Wake Time service period (TWT SP) previously signalled by the sharing AP or agreed in the MAP Coordination set. Indeed, negotiated TWT in favour of the sharing AP may make the other APs being aware that the sharing AP will access the medium and warning their stations not to access the medium at this time. Hence, the stations associated with the other APs of the MAP Coordination set avoid performing medium access when they know it will become time for the sharing AP to request and be granted a MAP TXOP. TWT negotiation within MAP coordination has been mentioned in publications 802.11-22 / 1530r0 (entitled “Multi AP coordination for next-generation Wi-Fi”) and US 2022 / 408355, as a medium-access avoidance scheme to reduce contention. They propose a TWT coordination for MAP operation, that reduces OBSS interference, using the Restricted Target Wake Time (R-TWT) procedure. The R-TWT feature is used to avoid OBSS interference in between the BSSs of the MAP Coordination set. 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 stations fall in a geographic area that overlaps a neighbour BSS, then the neighbouring APs in the MAP Coordination set will operate such as to quiet their administrated stations in their own BSS. Once the station receives the control frame, e.g. the above-mentioned MAP MU-RTS trigger frame for NPCA 700 (identified thanks to the above signaling, e.g. Trigger Type subfield 811), the station determines whether it can take part of the current shared TXOP. This may be done by determining whether its BSS or managing AP (if not itself) is allocated a secondary channel by the control frame. As an example, the station scrutinizes the User Info fields 830 to find whether or not one of them bears an identification of its BSS or managing AP in field 831. In the affirmative, the station (and more generally its entire BSS) is concerned by a NPCA switching. The station then determines the secondary channel where to switch NPCA, and optionally the new operational channel for NPCA-based communications during the TXOP. They may be predefined. Alternatively, they may be determined from the control frame, and more particularly from Common Info field 810 together with the User Info field 830 assigned to its BSS: the secondary channel where to switch NPCA can be retrieved from field 832, while the new operational channel (e.g. bandwidth) for NPCA-based communications during the shared TXOP may be obtained from Trigger Dependent User Info field 839 or from BW field 890 (Figure 8b). As mentioned above, the new operational channel has a reduced bandwidth compared to the initial operating channel. The station can also determine at this step the duration of the NPCA switching, e.g. from UL Length field 812 or Duration field 802. Once the channels and duration are known, the station switches from the PCA access scheme (performed on the primary channel) to the NPCA access scheme performed on the allocated secondary channel for NPCA. It also configures itself to be ready to use the new operational channel (e.g. BW field 890) for NPCA-based communications during the shared TXOP. This is step 630. The station next operates NPCA on the secondary channel (channel sensing and backoff procedure) to use the new operational channel. This is step 640. This step may include sending one or more control response frames to the sharing AP in response to the control frame. This is step 641, an embodiment of which is shown in Figure 6a. The control response frame confirms, for all surrounding stations, the reservation of the channel over which it is transmitted. In embodiments, CTS frames are used as response to the control frame, in particular of the MU-RTS trigger frame type. CTS frames 710, 711 are exemplary response frames in the scenario of Figure 7. Preferably, a control response frame is sent over the allocated secondary channel, i.e., over the secondary channel to which the station has switched its NPCA and which is now used for channel sensing and backoff procedure (e.g. channel 791 for AP1 and channel 792 for AP3 in the scenario of Figure 7). Such control response frame can thus be referred to as “secondary control response frame” with reference to the secondary channel. The secondary control response frame secures the reservation of the secondary channel for NPCA. A control response frame can also be sent over the primary channel of the sharing AP (e.g. channel 790 in the scenario of Figure 7). Such control response frame can thus be referred to as “primary control response frame” with reference to the primary channel. The primary control response frame allows the (hidden) neighbouring stations not having received the control frame to be aware of the reservation of the primary channel by the sharing AP. Next to step 641 is step 642 (still within step 640) where the station uses the new operational channel. The station can perform channel sensing and backoff procedure on the secondary channel - this is NPCA - and communicate over the new operational channel when NPCA-based access to the medium is gained. Operation with limited bandwidth continues up to the end of the switch duration, typically the end of the shared TXOP (step 650), in which case the station switches back to the PCA scheme where medium access is performed on the primary channel of its shared BSS. This is step 660 mirroring step 630 in the reverse way. The switching back allows the station to recover its initial operating channel, and thus to operate on its primary channel within the maximum bandwidth. This is step 670 similar to step 610. Figure 6a illustrates, using a flowchart, embodiments to handle the transmission of response control frames such as CTS frames. The process starts when the station receives the control frame at step 620. As mentioned above, the involvement in a NPCA switching, the secondary channel for NPCA and the new operational channel for NPCA-based communications during the TXOP shared by the control frame are obtained by the station, which switches (step 630) its channel access to NPCA on the allocated secondary channel. Also, the station sends at step 641 one or more control response frames to the sharing AP in response to the control frame. A secondary control response frame may be sent to the sharing AP over the allocated secondary channel (e.g., CTS frame 711 sent by AP1 over secondary channel 791 and CTS frame 712 sent by AP3 over secondary channel 792 in the scenario of Figure 7). This is step 6400 within step 641. A primary control response frame may also be sent to the sharing AP over the primary channel of the sharing AP (e.g. CTS frame 710 sent by AP1 over primary channel 790 or by AP3 over primary channel 790 in the scenario of Figure 7). This is step 6410 within step 641. As apparent from the Figure, the transmission of the secondary control response frame (during step 6400) is performed depending on the occupancy of the allocated secondary channel for NPCA. Therefore, the station determines, after the NPCA switching, whether the allocated secondary channel for NPCA (i.e., where it now contends for medium access) is busy or not - step 6401. Note that a secondary channel granted through the sending of the secondary control response frame is not considered as been busy in the viewpoint of the station, because it has been emitted by the sharing AP of the MAP Coordination set known by the station. In the negative (secondary channel available), the station directly (e.g., a SIFS after the control frame is received) emit back the response (CTS) frame over the secondary channel, to acknowledge the reservation. This is step 6402 and is illustrated by CTS frame 711 sent by AP1 over secondary channel 791 in the scenario of Figure 7. In a variant, the station may decide to content for access to the secondary channel (i.e., using a backoff procedure in the NPCA mode) before sending the CTS frame. In that case, the secondary control response frame is sent by the station upon gaining access to the secondary channel using a medium access contention scheme. A conventional CTS frame made of a Frame Control field (two bytes), a Duration field (two bytes), a RA field (six bytes) and an FCS field (four bytes), may be sent. In some embodiments, the CTS frame is augmented to include the channel allocation information contained in the control frame. For example, it may binary repeat or copy the whole MAC payload of the control frame (Figure 8a) or only the User Info fields and optionally the Common Info field where the resource allocations are defined. This is for the other stations receiving the CTS frame but not the control frame to be aware of the shared TXOP and how the operating channel is shared to various shared BSSs. In the affirmative of test 6401 (secondary channel not available), the station has to wait for the secondary channel to be released, hence to be idle. It is not possible to immediately emit back the CTS frame to acknowledge the reservation. The station therefore monitors the availability of the secondary channel during the shared TXOP. The station may stop monitoring such availability when approaching to the end of the switch duration (or TXOP duration). It may happen that this secondary channel is never won by the station (or the shared BSS to which it belongs). In that case, the sharing tentative has failed, but at least a station (e.g. the shared AP) has tried. When the secondary channel becomes available, the station may contend for access to it, using a backoff procedure on that channel (hence NPCA) and send the CTS frame over that channel when access is gained. This is step 6403 and is illustrated by CTS frame 712 sent by AP3 over secondary channel 792 after NPCA contention in the scenario of Figure 7. After either step 6402 or 6403, the shared BSS to which the station belongs owns the secondary channel up to the remaining allocation duration. The station then operates on the allocated secondary channel and use the new operational channel, at step 642. In some embodiments, the station sends a frame over the secondary channel immediately after (e.g., a SIFS after) the secondary control response frame. This is illustrated by frame 720 sent by AP1 over secondary channel 791 immediately after CTS frame 711 and by frame 721 sent by AP3 over secondary channel 792 immediately after CTS frame 712 in the scenario of Figure 7. This frame aims at resynchronizing all the stations within the shared BSS on the new secondary channel to which they have NPCA-switched. Preferably, this frame is sent by the shared AP (managing the shared BSS) for centralized and efficiency approach. In some embodiments, this resynchronizing frame is a mere (basic) trigger frame to organize multi-user transmissions over the secondary channel, or a mere downlink multi-user PPDU. In parallel to the transmission 6400 of the secondary control response frame, the station may send the first control response frame, during step 6410. The frame is immediately sent (e.g., a SIFS after) over the primary channel after the control frame is received, to acknowledge the reservation. One may thus note that such first control response frame may not be aligned in time with the secondary control response frame, as illustrated through primary CTS frame 710 sent over primary channel 790 prior to secondary CTS frame 712 over secondary channel 792, in the scenario of Figure 7. A conventional CTS frame may be sent. Preferably, the primary CTS frame is an augmented CTS frame to include the channel allocation information contained in the control frame. For example, it replicates the User Info fields and optionally the Common Info field, for the other stations receiving the CTS frame but not the control frame to be aware of the shared TXOP and how the operating channel is shared to various shared BSSs. As mentioned above, the process of Figure 6 may be implemented by any station in the BSS of the MAP Coordination set. However, in some embodiments, only the (shared) APs of the MAP Coordination set perform those steps. In that case, their associated stations (i.e., the non-AP stations of the shared BSSs) can implement a lighter process as depicted in Figure 6b. The process has quite the same steps as steps 610, 620, 630, 642, 650, 660 and 670 of Figure 6, meaning a non-AP station does not exchange channel access information with the sharing AP at step 600 nor send the control response frames at step 641. Furthermore, as the non-AP station may be out of the transmission range of the sharing AP it may not receive the control frame sent at step 530. To be aware of the NPCA switching and of the channel allocation, the non-AP station can also rely on the control response frames sent by e.g., its managing AP at steps 6400, 6410. That is why the process of Figure 6b includes step 620’ (instead of step 620), where the station receives a control frame or a control response frame, which frame includes a signalling to trigger, at the non-AP station, a switching from the PCA to the NPCA. Such signalling may be the channel allocation information such as the User Info fields and optionally the Common Info field. To illustrate it, in the scenario of Figure 7, the non-AP station may receive either the MAP MU-RTS trigger frame for NPCA 700 or the primary CTS frame 710 or both. This is one advantage of the CTS frame 710 to replicate the allocation, in order that all administrated stations can receive the information (in case that some are too far to receive the initial control frame 710 from the sharing AP). In embodiments, the operation of step 642 may require for the non-AP station to wait for a first frame (e.g. trigger frame) sent by its managing shared AP after the secondary CTS frame, to guarantee resynchronization in the shared BSS. The scenario of Figure 7, already referred to above, regards a transmission sequence implementing a TXOP 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 scenario involves three BSSs in the MAP Coordination set, e.g., BSS1, BSS2 and BSS3 of Figure 1. First, the three APs directly exchange (above steps 500 and 600) their channel access information and optionally TWT scheduling information. This may be a negotiation between each pair of APs. MAP coordination frames 760 / 761 or management frames 750 / 751 (e.g. Beacon frames or Probe Response frames) can be used to that end. Of course, any AP can be involved and in charge of initiating this negotiation phase. Once the negotiation is completed, the three APs continue their own operations on their proper BSS. The TXOP NPCA coordination starts when an AP - here AP2 - decides to act as a sharing AP. The TXOP NPCA coordination starts with an enhanced MU-RTS / CTS sequence procedure initiated by AP2. This procedure allows AP2 to initiate a TXOP for a given TXOP duration and to protect the TXOP frame exchange sequences thanks to the NAV. AP2 can determine when to contend on the wireless medium for MAP operation. In embodiments, this can be performed during a TWT SP negotiated by the MAP group: this advantageously avoid any contention by neighbour APs of the MAP Coordination set when each knows their turn to perform medium access for a given TWT SP. Alternatively, no coordination for TWT SP is envisaged. In the illustrated example, AP2 monitors an 80MHz operating channel, composed of channels 790 to 793, where channel 790 is the primary channel for PCA. Of course, any other operating channel width be considered. In the scenario, among that operating channel, AP2 does not detect any communication activity within its range. As a consequence, it performs a backoff countdown after CCA sensing period 701 to gain access to the medium using conventional PCA, and initiates a transmission when it gains access to the medium (backoff count down to 0). AP2 intends to provide channel 2 (791) to BSS1 managed by AP1, and channel 3 (793) to BSS3 managed by AP3. It can also keep a channel, here channel 1 (790), for its own BSS. Sharing AP AP2 (TXOP owner, on primary channel 790) sends the MAP MU-RTS trigger frame for NPCA 700 addressed (in the User Info fields) to itself (preferably for primary channel), AP1 and AP3 (surrounded APs or APs belonging to an already created MAP group). The MAP MU-RTS Trigger frame for NPCA 700 is identified using any signalling method as described above. This frame 700 as a trigger frame reserves the TXOP on the operating channel and allocates resources (here secondary 20 MHz channels 791,792) to other APs. This frame as a MU-RTS frame solicits simultaneous CTS (Clear-To-Send) frame transmissions from one or more addressed stations, including the non-AP stations of BSS2 as well as the shared APs. Upon reception of this MAP MU-RTS Trigger frame for NPCA 700, non-AP stations of BSS2 as well as addressed AP1 and AP3 have to respond with a CTS frame back to AP2. Non-AP stations of BSS1 immediately respond with CTS frames 710 over the primary channel 790 (which frames superimpose one over the other, to be seen as a single response by AP2). AP1 and AP3 receives the MAP MU-RTS trigger frame for NPCA 700 in the primary channel 790, from where they decode it and detect it is also duplicated among the whole bandwidth. Therefore, regardless of whether some secondary channels are busy or not, they can decode frame 700 and determine where they are allocated a secondary channel for NPCA. In the affirmative, they (and also their associated non-AP stations receiving frame 700) switch to NCPA scheme (SW1 and SW3 in the Figure), and they send back one or more CTS frames to acknowledge the medium access. According to embodiments, a (secondary) CTS frame is sent only on the allocated secondary channel. This is important to signal the acknowledgment of medium access by a shared AP on the channel where it is allocated. As a result, AP1 which also performs CCA on secondary channels knows that the allocated channel (791) is free. Upon reception of this MAP MU-RTS Trigger frame for NPCA 700, AP1 responds with CTS frame 711 to AP2 over secondary channel 791. As the wireless medium (secondary channel) is obtained directly by AP2, there is no need for local contention on that secondary channel 791. Any station receiving MAP MU-RTS Trigger frame for NPCA 700 or CTS response frame 710 or both sets their NAV. Thereby, the MU-RTS / CTS procedure protects each channel of the medium to be accessed by unexpected stations and then protects any subsequent triggerbased transmission operated by the TXOP owner or the shared stations. The CTS response 711 sent by the addressed APs advantageously allows a larger area than the MU-RTS itself to be protected. Next, once the medium is protected on the secondary channel 791, AP1 operates normally: it can send a legacy Basic Trigger frame 720 to stations of its BSS that have also switched on this channel. This is illustrated by STA11 that has also received MAP MU-RTS trigger frame for NPCA 700. Trigger frame 720 can be also named “Trigger Frame To Self” because it is intended to the shared BSS (here BSS1). In embodiments, AP1 also sends a (primary) CTS frame 710 on the primary channel 790, a SIFS after the MAP MU-RTS trigger frame for NPCA 700. This frame, sent in the non-HT duplicate format, superimposes over the (identical) CTS frames 710 (if any) sent by the non-AP stations of sharing BSS2. Secondary CTS frame 711 is preferably a conventional CTS frame while primary CTS frame 710 is preferably an augmented CTS frame, i.e., including the channel allocation information. Conventional format for secondary CTS frame 711 makes this frame to be a kind of usual CTS-to-self, that is sufficient since all the non-AP stations of the shared BSS (BSS1) have already switched to NPCA on secondary channel (791). Similarly to AP1, AP3 performs CCA on secondary channels and determines that the allocated channel (792) is busy due to OBSS interference. Upon reception of MAP MU-RTS Trigger frame for NPCA 700, since the allocated secondary channel is not available, AP3 waits for the channel to become idle where it then performs local contention 702, i.e., a backoff countdown after CCA sensing period, to gain access to the secondary channel (hence using NPCA). Upon gaining access to the secondary channel 792, AP3 responds to frame 700 with (secondary) CTS frame 712 to AP2 over secondary channel 792. Secondary CTS frame 712 may be the same as secondary CTS frame 711, in particular to be a conventional CTS frame (i.e., not augmented with the channel allocation information). Next, once the medium is protected on the secondary channel 792, AP3 operates normally: it can send a legacy Basic Trigger frame 721 to stations of its BSS that have also switched on this channel. Like AP1, AP3 may also send a (primary) CTS frame 710 (in the non-HT duplicate format) on the primary channel 790, a SIFS after MAP MU-RTS trigger frame for NPCA 700. Although the above contemplates AP3 and its associated non-AP stations to switch to the NPCA upon receiving frame 700, embodiments may drive these stations to perform the NPCA switching only when the allocated secondary channel for NPCA 792 becomes idle. The above timing shows that any shared AP must be ready to send a secondary CTS frame 711 on its allocated secondary channel immediately after (a SIFS after) MAP MU-RTS trigger frame for NPCA 700. However, the shared AP (as well as its associated stations) need some time to configure itself for NPCA on the allocated secondary channel. This timing delay or “channel switch time" may be known in advance by the sharing AP or exchanged by each AP within the MAP Coordination set (e.g.. during steps 500 / 600). A maximum channel switch time may be defined for any station to switch to NPCA. Typically, any AP may receive Channel Switch Timing elements from its associated stations that contain information regarding the channel switch time needed by the respective associated station, in units of microseconds (defined in section 9.4.2.62 of IEEE P802.11-REVme / D4.0, August 2023). This timing delay can then be used by sharing AP AP2 to proceed to padding of MAP MU-RTS trigger frame for NPCA 700. This is to extend the frame length to give the recipient STAs and APs enough time to prepare their NPCA switching onto the allocated secondary channel, and especially for the shared AP to be ready to send the CTS response a SIFS after frame 700 is received. Optional Padding field 806 (Figure 8a) can be used to that end. The field may have a variable length, depending on the timing delay to be obtained. In embodiments, a User Info field 830 may be provided in User Info List field 805, which User Info field has an APID / AID field 831 set to a predefined value (e.g. 4095) to advise a padding is provided in Padding field 806. These embodiments allow all relevant User Info fields to be decoded before the padding. In some embodiments, a User Info field 830 having an APID / AID field set to another predefined value (e.g. 4094) can be used to advise that User Info field is specific in that it further contains a 32-bit CRC calculated based on previous Common Info field 810 and User Info fields 830. In the present disclosure, this 32-bit CRC allows receivers to perform an immediate check of the integrity of the fields preceding the padding field 806 without waiting the complete reception of the frame 800. This allows the stations to start the NPCA switching earlier, as soon as the secondary channel allocation is known and the 32-bit CRC is checked. In addition to the padding field 806, a hash field (not represented on the Figure) may be considered to embed a hash value computed based on the fields preceding the padding field 806, along with an identifier of the shared AP (e.g. its SSID) in order to authenticate the trigger frame 800 as been emitted by the sharing AP. Each of the three BSSs (more generally the sharing and shared BSSs) have their own operations over their allocated channel or channels during the shared TXOP, independently to the other BSSs. Communications 730,731 are for example conducted over the primary channel within BSS2 using PCA, while communication 732 is conducted over secondary channel 791 within BSS1 using NPCA. As a result, the proposed mechanism provides that a coordinated-BSS medium reservation is performed on the primary channel, while distinct BSS communications are isolated on their own allocated secondary channels. In a sense, this corresponds to the objective of NPCA operation due to OBSS, but with an optimized way: the sharing AP triggers operation on secondary channels due to its own activity (OBSS in shared APs’ viewpoint). At the end of the switch duration or TXOP, AP1 and AP2 as well as their associated stations switch back to their PCA (SWB1 and SWB3 in the Figure). In the above scenario, sharing AP2 allocates itself primary channel 790 for the gained TXOP, hence triggering the other BSSs to switch to NPCA on secondary channels. In variants, sharing AP2 may allocate the primary channel 790 to one other AP (different from itself) of the MAP Coordination set, which also requires the other BSSs to switch to NPCA on secondary channels. The newly proposed NPCA triggered mechanism through MAP coordination provides several advantages compared to known techniques for NPCA (or secondary channel access) triggered within a single BSS. Firstly, while the known techniques require that all stations have to hear the OBSS interference in order to start and end at the same times, the proposed MAP MU-RTS / CTS scheme enables PCA-based communications for the sharing BSS and NPCA-based communications for the shared BSSs that take place on a reduced bandwidth and are independent one to the other, during the shared TXOP. Secondly, the proposed MAP MU-RTS / CTS scheme ensures a fully-determined time period where the NPCA is performed in the shared BSSs because triggered by the sharing BSS, hence the NPCA scheme remains simple and does not need any specific sensing means to monitor whether the medium is freed back in the primary channel (due to NAV shortened) as mandated by other NPCA schemes. Thirdly, while the known techniques require that each station has the capability to monitor the non-primary channel at least to the extent of performing CCA through ED (Energy Detection) check while it is not transmitting on the primary channel, and vice-versa, the proposed MU-RTS / CTS scheme makes only the APs of the BSSs taking action on the channels, and issuing resynchronization of their associated non-AP stations later. Fourthly, while the known techniques require monitoring ED in case of OBSS, the proposed MU-RTS / CTS scheme immediately (i.e., without ED monitoring) informs that a BSS will own the primary channel (due to corresponding allocation). Therefore, the other BSSs can immediately switch to the NPCA scheme on the allocated secondary channel, hence saving time when this secondary channel is free (no need to wait for detection) Lastly, the proposed MU-RTS / CTS scheme synchronizes dynamically the secondary channel allocation through the augmented MAP MU-RTS trigger frame for NPCA, whereas known techniques are relatively static and need further indications as in Beacon frames. Figure 9a schematically illustrates a communication device 900 configured to implement at least one embodiment of the present disclosure, for instance any of the (AP and non-AP) stations shown in Figure 1. The communication device 900 may preferably be a device such as a microcomputer, a workstation or a light portable device. The communication device 900 comprises a communication bus 913 to which there are preferably connected: a central processing unit 901, such as a processor, denoted CPU; a memory 903 for storing an executable code of methods or steps of the methods according to embodiments as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least one communication interface 902 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 904. Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 900 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 900 directly or by means of another element of the communication device 900. 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 902, in order to be stored in the memory of the communication device 900 before being executed. In an embodiment, the device is a programmable apparatus which uses software to implement embodiments of the disclosure. However, alternatively, embodiments of the present disclosure may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Figure 9b is a block diagram schematically illustrating the architecture of the communication device 900, adapted to carry out, at least partially, the present disclosure. As illustrated, device 900 comprises a physical (PHY) layer block 923, a MAC layer block 922, and an application layer block 921. The PHY layer block 923 (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 922 preferably comprises a MAC 802.11 layer 924 implementing conventional 802.11 be MAC operations, and additional block 925 for carrying out, at least partially, the invention. The MAC layer block 922 may optionally be implemented in software, which software is loaded into RAM 903 and executed by CPU 901. Preferably, the additional block 925, referred to as MAP NPCA managing module which has different operations to implement parts of the present disclosure, depending on the role played by the communication device 900. As the same device can play different roles over time, the additional block 925 is preferably designed to selectively perform the different operations. For instance, and not exhaustively, operations for the communication device 900 acting as an AP include: exchanging frames 760 / 761 for a MAP NPCA negotiation, determining BSSs that interfere with a target BSS and are pertaining a channel allocation for MAP coordination; reserving a wider band for operation compared to possibilities of local non-AP stations, and allocating a secondary channel to shared BSS(s) of the MAP Coordination set. To that end, sending MAP MU-RTS trigger frame for NPCA 700, acknowledging any reservation for its BSS by exclusively issuing a response frame, such as CTS frame 711 or 712, on their allocated secondary channel when the channel is available; performing EDCA contention 702 onto the allocated channel during the allocated TXOP, and switching back to primary channel upon expiry of the allocated TXOP duration. Operations for the communication device 900 acting as a non-AP station in a BSS interfering with a target BSS include receiving the MAP MU-RTS trigger frame for NPCA 700 from a first AP and its acknowledgment CTS frame from a second AP to which the station is associated; determining the medium access rules / policy and switching onto a secondary channel allocated to its associated (second) AP; and switching back to primary channel at the end of the allocation. MAC 802.11 layer 924 and MAP NPCA Managing module 925 interact one with the other in order to process accurately communications over the medium, e.g., over secondary channels addressed to multiple BSSs according to embodiments of the invention. On top of the Figure, application layer block 921 runs an application that generates and receives data packets, for example data packets such as a video stream. Application layer block 921 represents all the stack layers above MAC layer according to ISO standardization. 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 comprising, at a station of a second Basic Service Set (BSS):receiving, from a first access point (AP) managing a first BSS different from the second BSS, a control frame, andresponsive to the control frame, switching from a primary channel access scheme to a non-primary channel access scheme where medium access is performed on a secondary channel.

2. A communication method comprising, at a first access point (AP) of a first Basic Service Set (BSS):transmitting a control frame triggering, at one or more stations of a second BSS different from the first BSS, a switching from a primary channel access scheme to a non-primary channel access scheme where medium access in the second BSS is performed on a secondary channel of the second BSS.

3. The method of Claim 1 or 2, wherein the control frame allocates the secondary channel to the second BSS or AP.

4. The method of Claim 3, wherein the control frame allocates a primary channel to the first BSS or AP.

5. The method of Claim 2, further comprising determining, based on channel access information provided by a second AP managing the second BSS, whether a channel to be allocated to a third AP not managing the second BSS by the control frame encompasses a primary channel of the second BSS.

6. The method of Claim 2, further comprising determining the secondary channel, based on channel access information provided by a second AP managing the second BSS.

7. The method of Claim 1 or 2, wherein the control frame allocates another secondary channel to a third BSS different from the first and second BSSs and triggers, at stations of the third BSS, a switching from a primary channel access scheme to a non-primary channel access scheme where medium access in the third BSS is performed on the other secondary channel.

8. The method of Claim 1 or 2, wherein the control frame includes a switch duration at the end of which the station of the second BSS switch back to the primary channel access (PCA) scheme where medium access is performed on a primary channel of the second BSS.

9. The method of Claim 8, wherein the control frame requests a transmission opportunity (TXOP) for a signalled TXOP duration, wherein the switch duration is based on the TXOP duration.

10. The method of Claim 1 or 2, wherein the control frame includes a multi-user request-to-send trigger frame (MU-RTS TF) reserving a transmission opportunity (TXOP) over an operating channel encompassing primary channels of the first and second BSSs and the secondary channel.

11. The method of Claim 1, further comprising sending a secondary control response frame to the first AP over the secondary channel, in response to the control frame.

12. The method of Claim 2, further comprising receiving a secondary control response frame from a station of the second BSS over the secondary channel.

13. The method of Claim 11 or 12, wherein the secondary control response frame is a clear-to-send (CTS) frame.

14. The method of Claim 11, wherein the secondary control response frame is sent a SIFS after the control frame is received.

15. The method of Claim 11, wherein the secondary control response frame is sent by the station of the second BSS upon gaining access to the secondary channel using a medium access contention scheme.

16. The method of Claim 11, further comprising sending a frame over the secondary channel immediately after the secondary control response frame.

17. The method of Claim 11, further comprising sending a primary control response frame to the first AP over a primary channel of the first AP.

18. The method of Claim 12, further comprising receiving a primary control response frame over a primary channel of the first AP.

19. The method of Claim 17 or 18, wherein the primary control response frame includes channel allocation information mirroring channel allocation information contained in the control frame.

20. The method of Claim 17 or 18, wherein the primary control response frame is a clear-to-send (CTS) frame.

21. The method of Claim 17, wherein the primary control response frame is sent a SIFS after the control frame is received.

22. The method of Claim 1 or 2, wherein the control frame is transmitted within a Target Wake Time service period (TWT SP) previously signalled by the first AP to the second AP or agreed between the first and second APs.

23. The method of Claim 1 or 2, wherein the control frame includes padding to have a length of the control frame at least equal to a channel switch time length.

24. The method of Claim 23, wherein the channel switch time length is based on channel switch time information provided by a second AP managing the second BSS to the first AP.

25. A wireless communication device comprising at least one microprocessor configured for carrying out the method of Claim 1 or 2.

26. 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 Claim 1 or 2.Application No: GB2405379.5 Examiner:Contract Unit ExaminerClaims searched: 1-26Date of search: 18 December 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X Y X: 1-21, 25, 26 Y: 22-24 US2020 / 245352 Al (SEOK Y0NGH0 ET AL) abstract, paragraph [0003] - paragraph [0004]; figures 1-19, paragraph [0051] - paragraph [0063], paragraph [0070] - paragraph [0087] Y 22-24 IEEE DRAFT, vol 802.11, 2024, YUE ZHAO (HUAWEI), "SP-based non-primary channel access", pages 1-10 URL: https: / / mentor.ieee.org / 802.1 l / dcn / 24 / 11-24-0538-00-00bn-sp-based-non-primary-channel-access.pptx page 3 - page 7; figures 1-7 A - IEEE DRAFT, vol 802.11, 2024, SALVATORE TALARICO (SONY), "Considerations on Non-Primary Channel Access", pages 1-11 URL: https: / / mentor.ieee.org / 802.1 l / dcn / 24 / 11-24-0458-00-00bn-considerations-on-non-primary-channel-access.pptx the whole documentCategories:X Document indicating lack of novelty or inventive step A Document indicating technological background and or state of the art. Y Document indicating lack of inventive step if combined with one or more other documents of same category'. P Document published on or after the declared priority date but before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority' date earlier than, the filing date of this application.Field of Search:Search of GB, EP, WO &US patent documents classified in the following areas of the UKCX :Worldwide search of patent documents classified in the following areas of the IPC____________H04W____________________________________________The following online and other databases have been used in the preparation of this search reportInternational Classification:Subclass Subgroup Valid From H04W 0074 / 0816 01 / 01 / 2024 H04W 0074 / 00 01 / 01 / 2009 H04W 0084 / 12 01 / 01 / 2009

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