Method and apparatus for transmission in random access in TXS timeslots in a wireless network

Abstract

The present invention concerns a method of communication in a wireless network, comprising the following steps at a station comprised in a group of stations for transmitting a Physical layer Protocol Data Unit, PPDU: receiving from an access point, AP, a Multi User – Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to the group of stations for transmission; determining that a predefined field of the MU-RTS TXS TF indicates a random access for the group of stations to the TXS timeslots; and contending for access to one of the TXS timeslots for transmitting the PPDU. The MU-RTS TXS TF may define a plurality of TXS timeslots, and the method may further comprise selecting a TXS timeslot of the plurality of TXS timeslots for transmitting the PPDU. The contending for access to one of the TXS timeslots may be performed according to an enhanced distributed channel access, EDCA, process. The EDCA may comprise determining a random backoff accessing the TXS timeslot based on EDCA parameters.

Description

FIELD OF THE INVENTION The present invention relates generally to communication networks and more specifically to wireless communication methods in a wireless network comprising an access point (AP) and a plurality of non-AP stations, and corresponding devices. The invention applies to contention-based channel access methods using backoff values. BACKGROUND OF 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 multipleaccess 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 802.11 family of standards adopted by the Institute of Electrical and Electronics Engineers (IEEE) provides a great number of mechanisms for wireless communications between stations. 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 are being developed to allow a single access point (AP) to schedule MU transmissions, i.e., multiple simultaneous transmissions to or from non-AP stations, in the wireless network. For example, one of such MU schemes has been adopted by the Institute of Electrical and Electronics Engineers (IEEE) in the 802.11 ax standard. Thanks to the MU feature, a 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 where the latter can perform multiple simultaneous elementary transmissions, over so-called resource units (RUs), to various non-AP stations. As an example, the resource units split a communication channel of the wireless network in the frequency domain (e.g., a 20MHz channel is composed of maximum 9 users with 26-tone RUs), based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique. The assignment of the RUs to the 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, where various non-AP stations can simultaneously transmit to the AP over the resource units forming the MU UL transmission. To control the MU UL transmission by the non-AP stations, the AP sends a control frame, known as a Trigger Frame (TF), by which it allocates the resource units to the non-AP stations using 16-bit Association I Dentifiers (AlDs) assigned to them upon registration to the AP and / or using reserved AIDs designating a group of non-AP stations. The trigger-based allocation of resources prevents the medium from collisions between stations involved in the service periods since the AP is managing the resource allocation through a trigger frame. In order to protect large MU UL and DL transmissions, the AP initiates a MU transmission sending a MU-RTS frame (Multi-User Request-To-Send, one variant of the trigger frame), which includes information about user stations involved in the upcoming MU transmission and informs about the width of the primary channels of the expected CTS frames. User stations reply with simultaneous CTS frames on their primary channels (i.e., one or more 20 MHz channels). In case not all CTS frames are received, the AP may decide to distribute the channel resources only between the user stations that have replied. In order to meet specific requirements of users, the IEEE802.11be standard provides enhancement of the MU-RTS scheme, such as a triggering the sending of single user physical layer protocol data unit (SU-PPDU), both for Uplink scenario or uplink and Peer-to-Peer (P2P) scenario. The procedure is called “Triggered TXOP Sharing (TXS)” procedure and allows an AP to allocate a portion of an obtained TXOP to one associated non-AP EHT STA for transmitting one or more non-TB PPDUs (the triggered PPDU is non-Triggered-based, because it does not follow OFDMA format but the legacy OFDM having a bandwidth multiple of 20MHz). Due to the reduction of collisions (medium access centralized by the AP), the protection by CTS frames, and the legacy data frame format, the TXS scheme is candidate to enhanced scenarios for 802.11 bn such as multiple AP communication. However, with the current implementation of Triggered TXOP Sharing procedure, an AP needs to get the identity of the user stations and the information about the availability of their packets waiting for transmission (in real time, or expected by traffic specification means), which may represent a significant overhead. The present embodiments aim to provide a random allocation scheme for TXS. SUMMARY OF THE INVENTION The present invention has been devised to address one or more of the foregoing concerns. The inventors have noticed that the existing triggering procedures can be extended to support random allocation. New ways to transmit an SU PPDU in a TXOP period allocated by a trigger frame (MU-RTS frame), especially that triggers and allocates random time resources to other stations or APs, are proposed in the present disclosure. According to a first aspect of the invention there is provided a method of communication in a wireless network, comprising the following steps at a station comprised in a group of stations for transmitting a Physical layer Protocol Data Unit, PPDU: - receiving from an access point, AP, a Multi User - Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to the group of stations for transmission; - determining that a predefined field of the MU-RTS TXS TF indicates a random access for the group of stations to the TXS timeslots; and - contending for access to one of the TXS timeslots for transmitting the PPDU. According to embodiments, the MU-RTS TXS TF defining a plurality of TXS timeslots, the method further comprises: - selecting a TXS timeslot of the plurality of TXS timeslots for transmitting the PPDU. According to embodiments, the selection of a TXS timeslots of the plurality of TXS timeslots is performed randomly. According to embodiments, the selection of a TXS timeslots of the plurality of TXS timeslots is performed using timeslot contention parameters. According to embodiments, the timeslot contention parameters are updated depending on the success of the transmission of the PPDU. According to embodiments, contending for access to one of TXS timeslots is performed according to an enhanced distributed channel access, EDCA, process. According to embodiments, the EDCA comprises determining a random backoff accessing the TXS timeslot based on EDCA parameters. According to embodiments, the EDCA parameters are updated depending on the success of the transmission of the PPDU. According to embodiments, a single User Info field of the MU-RTS TXS TF is used to define several contiguous TXS timeslots. According to embodiments, the method comprises: - sending a CTS response to the MU-RTS TXS TF for indicating that the station intends to transmit the PPDU in one of the TXS timeslots. According to another aspect of the invention there is provided a method of communication in a wireless network, comprising the following steps at an access point, AP: - generating a Multi User - Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to a group of stations of the wireless network for transmission; - setting a predefined field of the MU-RTS TXS TF to indicate a random access for the group of stations to the TXS timeslots; and - transmitting the MU-RTS TXS TF. According to embodiments, the method further comprises: - determining that one of the allocated TXS timeslots is not used by any station in the group of stations; and - transmitting data in the unused TXS timeslot. According to another aspect of the invention there is provided a computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to the invention, when loaded into and executed by the programmable apparatus. According to another aspect of the invention there is provided a computer-readable storage medium storing instructions of a computer program for implementing a method according to the invention. According to another aspect of the invention there is provided a computer program which upon execution causes the method of the invention to be performed. According to another aspect of the invention there is provided a station device for communicating in a wireless network, the station being comprised in a group of stations, the station device comprising a processor configured for: - receiving from an access point, AP, a Multi User - Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to the group of stations for transmission; - determining that a predefined field of the MU-RTS TXS TF indicates a random access for the group of stations to the TXS timeslots; and - contending for access to one of the TXS timeslots for transmitting a Physical layer Protocol Data Unit, PPDU. According to another aspect of the invention there is provided an access point, AP, device for communicating in a wireless network, the AP device comprising a processor configured for: - generating a Multi User- Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to a group of stations of the wireless network for transmission; - setting a predefined field of the MU-RTS TXS TF to indicate a random access for the group of stations to the TXS timeslots; and - transmitting the MU-RTS TXS TF. 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, non-transitory carrier medium may comprise a storage medium such as a floppy disk, a CD-ROM, 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 a typical wireless communication system in which embodiments of the invention may be implemented; Figure 2a describes a conventional MU UL communication triggered by an AP; Figure 2b describes another exemplary trigger-based communication scenario with TXOP sharing soliciting UL PPDU; Figure 2c describes yet another exemplary trigger-based communication scenario with TXOP sharing soliciting UL PPDU and / or Direct-link transmission; Figure 3a illustrates the format of an 802.11 Trigger frame, as defined in the standards IEEE P802.11REVme / D3.0; Figure 3b illustrates a format of a HE variant User Info field in the Trigger frame of Figure 3a; Figures 3c and 3d illustrate formats of respectively the HE variant and the EHT variant of a Common Info field in the Trigger frame of Figure 3a; Figure 3e illustrates the various values available for the Trigger Type subfield in a conventional Common Info field; Figure 3f illustrates the various values available for the Triggered TXOP Sharing Mode subfield in a conventional Common Info field; Figure 4a shows a schematic representation of a communication device in embodiments of the invention; Figure 4b illustrates schematically the architecture of the communication device of Figure 4a; Figure 5 illustrates, using a flowchart, general steps of a transmitting method according to embodiments of the invention; Figure 6 illustrates a first example of frame exchange based on the RA MU-RTS TXS procedure, according to first embodiments; Figure 7 illustrates a second example of frame exchange based on the RA MU-RTS TXS procedure, according to embodiments; Figure 8 illustrates, using a flowchart, general steps of a transmitting method according to second embodiments of the invention; Figure 9 illustrates a third example of frame exchange based on the RA MU-RTS TXS procedure, according to second embodiments; Figures 10a and 10b illustrate a augmented format of the Common Info field, and the HE / EHT variants of the User Info field, according to embodiments; Figure 11 illustrates an augmented format of the Trigger dependent User Info field, according to embodiments; and Figure 12 illustrates possible formats of elements for advertising the RA TXS parameters. DETAILED DESCRIPTION OF THE INVENTION 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 SingleCarrier 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 BSS (and thus corresponding WLANs): each BSS is thus uniquely identified by a specific basic service set identification, BSSID and managed by a separate virtual AP implemented in the physical AP. Each STA is identified within a BSS thanks to an identifier, AID, assigned to it by the AP upon registration. The 802.11 family of standards define various media access control (MAC) mechanisms to drive access to the wireless medium. For example, in order to address the issue of increasing bandwidth and decreasing latency requirements that are demanded for wireless communications systems in high-density environments, multi-user (MU) schemes have been developed to allow a single access point (AP) managing a Basic Service Set (BSS) to schedule MU transmissions, i.e., multiple simultaneous transmissions to or from non-AP stations of the BSS, in the wireless network. A MU scheme has been adopted in the 802.11ax-2021 standard, published on May 2019. Thanks to the MU feature, a non-AP station has the opportunity to gain access to the wireless medium via two access schemes: the MU scheme and the conventional Enhanced Distributed Channel Access - EDCA (Single User) scheme. Each BSS defines a main elementary channel of the wireless medium (known as a primary channel, usually a 20 MHz channel or a multiple of 20 MHz channel) on which the stations (including the AP) perform EDCA contention using generally legacy EDCA parameters (defined in an EDCA Parameter Set provided by the AP). To increase bandwidth for the forthcoming transmission, the stations can simultaneously contend for additional 20 MHz channels, known as secondary channels. The communication channel thus granted for transmission comprises the primary channel and optionally secondary channels. The 802.11 ax standard allows a MU downlink (DL) transmission to be performed by the AP when gaining access to the wireless medium for a transmission opportunity (TXOP). During the MU DL transmission on the granted communication channel, the AP performs multiple simultaneous elementary transmissions, over so-called resource units (RUs), to various non-AP stations. As an example, the resource units split the communication channel of the wireless network in the frequency domain, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique. The assignment of the RUs to the non-AP stations is signalled 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 802.11ax standard also allows a MU uplink (UL) transmission to be triggered by the AP when gaining access to the wireless medium. During the MU UL transmission, various non-AP stations can simultaneously transmit data to the AP over the resource units forming the communication channel. To control the MU UL transmission by the non-AP stations, the AP previously sends a control frame, known as a Trigger Frame (TF). The Trigger Frame allocates the resource units to the non-AP stations of the same BSS, using 16-bit Association IDentifiers (AlDs) assigned to them upon registration to the AP and / or using reserved Al Ds 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 P802.11be / D4.1 of September 2023, introduce the Multi-Link Operation (MLO) when it comes to MAC layer operation. The MLO allows multi-link devices to establish or setup multiple links and operate them simultaneously. A Multi-Link Device (MLD) is a logical entity and has more than one affiliated STA (STA) and has a single medium access control (MAC) service access point (SAP) to logical link control (LLC), which includes one MAC data service. An Access Point MultiLink Device (or AP MLD) then corresponds to a MLD where each STA affiliated with the MLD is an AP, hence referred to as “affiliated AP”. A non-Access Point Multi-Link Device (or non-AP MLD) corresponds to a MLD where each STA affiliated with the MLD is a non-AP STA, referred to as “affiliated non-AP STA”. Depending on the literature, “multilink device”, “ML Device” (MLD), “multilink logical entity”, “ML logical entity” (MLE), “multilink set” and “ML set” are synonyms to designate the same type of ML Device. Multiple affiliated non-AP STAs of a non-AP MLD can then setup communication links with multiple affiliated APs of an AP MLD, hence forming a multi-link channel. This is for instance done through the conventional association procedure: ML Discovery, followed by ML Authentication and finally by ML Setup where the non-AP MLD associates with the AP MLD (hence obtained an Association IDentifier, AID) and sets up the ML links for its affiliated non-AP STAs with the APs affiliated with the AP MLD. The links established (or “enabled links”) for MLDs are theoretically independent, meaning that the channel access procedure (to the communication medium) and the communication are performed independently on each link. Hence, different links may have different data rates (e.g., due to different bandwidths, number of antennas, etc.) and may be used to communicate different types of information (each over a specific link). A communication link or “link” thus corresponds to a given channel (e.g., 20 MHz, 40 MHz, and so on) in a given frequency band (e.g., 2.4 GHz, 5 GHz, 6 GHz) between an AP affiliated with the AP MLD and a non-AP STA affiliated with the non-AP MLD. The affiliated APs and non-AP STAs operate on their respective channels in accordance with one or more of the IEEE 802.11 standards (a / b / g / n / ac / ad / af / ah / aj / ay / ax / be / bn) or other wireless communication standards. Thanks to the multi-link aggregation, traffic associated with a single MLD can theoretically be transmitted across multiple parallel communication links, thereby increasing network capacity and maximizing utilization of available resources. The description below mostly concentrates on a single link for ease of explanation. However, similar considerations can be made with respect to each link forming a multiple link set for MLD devices. Therefore, the term STA or “station” may refer to one affiliated STA of a non-AP MLD (non-AP STAs of a non-AP MLD), and AP may refer to one affiliated AP of an AP MLD. The invention will now be described by means of specific non-limiting exemplary embodiments and by reference to the figures. Figure 1 illustrates an exemplary network environment in which embodiments of the present disclosure can be implemented. The illustrated wireless network environment comprises a multi-AP system 100 formed by a group of neighbouring wireless networks that operate over a common communication channel or wireless medium. The common communication channel may correspond to a part (e.g., 20 MHz) or all of an operating channel (e.g., 20 MHz, 40 MHz, 80 MHz, 160 MHz or 320 MHz). A first wireless network (or Basic Service Set) BSS1 comprises an access point (AP) 110 and three non-AP stations (STAs) 111, 112 and 113 associated to 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 BSS3 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 wireless network exchange data frames over the communication channel, under the management of the AP. A primary channel, usually 20 MHz channel, is defined per wireless network on which the management frames are exchanged. The other 20 MHz channels of the communication channel, if any, are known as secondary channels. In the context of the invention, the APs can also communicate one with each other, either using a communication channel of their BSS that is common to the other BSSs or using separate communication links (such as a separate wireless network or channel, an Ethernet backhaul connecting all the APs, direct links, and so on). Each non-AP STA 1x1-1x3 registers to the AP 1x0 of one wireless network BSSx during an association procedure. During the association procedure over the primary channel, the AP assigns a specific Association IDentifier (AID) to the requesting station. For example, the AID is a 16-bit value uniquely identifying the station. The stations (including the AP) compete one against another over the communication channel (including the primary channel and optionally secondary channels to increase bandwidth) using EDCA (Enhanced Distributed Channel Access) contention to access the communication channel in order to be granted a transmission opportunity (TXOP). The TXOP may then be used to transmit (single-user, SU) data frames or to implement multi-user (MU) transmissions. In the MU scheme, a single station, usually the AP of the wireless network BSSx, is allowed to schedule a MU transmission, i.e., multiple simultaneous transmissions to or from other stations of the wireless network. One implementation of such a MU scheme has been for example adopted in IEEE 802.11ax amendment standard, known as the Multi-User Uplink and Downlink OFDMA (MU UL and DL OFDMA) procedures. In the MU scheme, resources are defined over the 20 MHz channel or channels used, known as resource units. More generally, the resources may include space, frequency and time resources and may be obtained according to different multiplexing schemes. Examples of those schemes include Spatial Division Multiple Access (SDMA) system, Time Division Multiple Access (TDMA) system, Orthogonal Frequency Division Multiple Access (OFDMA) system, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) system. In the IEEE 802.11 wireless local area networking standards, the multi-AP system 100 may correspond to an extended service set (ESS) and each of the wireless networks to a basic service set (BSS). Although the description of embodiments of the invention is given in the context of IEEE 802.11, the embodiments are not limited thereto and they may apply to other types of wireless networks and protocols. To meet low latency requirements in 802.11be as well as to increase efficiency of the MU operation, existing mechanisms have been reused and improved, including the Triggered TXOP Sharing procedure. The Triggered TXOP Sharing procedure or Triggered TXS procedure allows an AP to allocate a portion of an obtained TXOP to one of its associated non-AP STA for transmitting its own data. This time sharing is triggered based on a MU-RTS trigger frame formerly defined by 802.11 ax but with a new TXOP Sharing mode subfield. This subfield informs whether the MU-RTS trigger frame is used according to its previous meaning defined by the 802.11 ax standard (Triggered TXOP Sharing Mode is set to 0) or for the new 802.11 be TXOP Sharing procedure (Triggered TXOP Sharing Mode is set to 1 or 2). In the latter, the TXOP Sharing is either limited to Uplink Traffic when the Triggered TXOP Sharing Mode is set to 1 or dedicated to Uplink and / or direct link traffic when the Triggered TXOP Sharing Mode is set to 2. Therefore, with this new mechanism, the AP is now able to allocate its associated non-AP stations with frequency resource units (e.g., Basic Trigger Frame) or temporal resource units. Since its initial versions, the IEEE 802.11 has developed or is developing wireless communication technology to improve the quality of service (QoS), compatibility of an access point (AP) protocol, security enhancement, radio measurement or radio resource measurement, wireless access in vehicular environment, fast roaming, and the like. With the constant increase of the number of wireless devices, the optimization of the medium access is an important factor to increase the useful bandwidth. Thereby, the 802.11 standard has moved from a random-access mechanism, called EDCA, in which each station individually contends to get the medium and then transmit, to a trigger-based medium access mechanism largely controlled by the AP for its associated non-AP stations. The triggering procedure was introduced by the 802.11 ax amendment and refined in some parts in the 802.11 be amendment It allows various non-AP stations to simultaneously transmit data over resource units of an operating channel that are allocated to these stations by the AP through a control frame, known as a Trigger Frame (TF). The allocation is made using the Association IDentifiers (AlDs) assigned to the stations upon registration to the AP (for scheduled RUs) and / or using reserved Al Ds designating a random access to an RU (for random OFDMA RUs, according to the so-called Uplink OFDMA-based Random Access (UORA)). The Trigger frame also defines the start of the multi-user (MU) transmission by the non-AP stations as well as the length thereof. The non-AP stations can transmit data to the AP (so-called MU UL transmission) or between them (so-called Direct Link or DiL or Peer-to-peer transmission). Although random access schemes such as distributed coordination function (DCF) or enhanced distributed channel access (EDCA) are used to occupy or share radio resources in the previous WLAN standard, they are not applicable to the OFDMA system. Therefore, uplink OFDMA-based random access (UORA), which is a new feature for random channel access in OFDMA-based WLANs, has been introduced in IEEE 802.11 ax. In the UORA mechanism, as the channel is divided into several sub-carrier groups referred to as resource units (RUs), multiple STAs can transmit data frames at the same time with different randomized RUs. For the operation of multi-user transmission, UORA introduces the OFDMA contention window (OCW) and OFDMA backoff (OBO) counter. To transmit a frame, each STA selects a random OBO counter within the OCW value and decreases it by the number of RUs available for UORA. If the decreased OBO counter becomes less than or equal to zero, the STA is allowed to transmit the frame with an arbitrarily available RU. Similar to DCF or EDCA, the performance of UORA is very sensitive to the number of contending STAs and the OCW range. In contrast to DCF and EDCA operating in time domain, UORA suffers from interoperability issue in between participants of the MU Uplink session that requires fine frame synchronisation. Figures 2 to 3 illustrate trigger-based medium access procedures. Figures 2a, 2b and 2c illustrate frame exchanges occurring in typical triggered procedure as defined in the 802.11 ax and 802.11 be standards. Figure 2a describes a Multi-User Uplink (MU UL) communication triggered by the AP. The frame exchange starts with an optional MU-RTS / CTS sequence procedure. This procedure allows the AP to initiate a TXOP (transmission opportunity) and to protect the TXOP frame exchange sequences thanks to the NAV. The AP may transmit an MU-RTS (Ready-To-Send) Trigger frame 210 to solicit simultaneous CTS (Clear-To-Send) frame 211, 212 transmissions from one or more non-AP STAs as illustrated by MU-RTS frame 210 sent by one of the APs 110, 120 or 130 in the example of Figure 1. The MU-RTS Trigger frame 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 of the MAC header. The MU-RTS trigger frame is further described in relation with Figures 3a-3f. When the stations received Mll-RTS Trigger Frame 210 (Trigger Frame type field 311 set to 3, on Figure 3c) from their associated AP with a User Info Field 330 which is addressed to them (i.e., with the AID12 subfield 331 equals to the 12-LSB of their AID), then the stations send back a CTS (Clear-to-send) frame to the transmitting AP. In the scenario shown, STA1 and STA2 send respectively simultaneous CTS frames 211 and 212 to the AP. The CTS frame is sent on the 20 MHz channel(s) indicated by the RU Allocation subfield 332 of the appropriate User Info field, that means in each 20Mhz channel occupied by the MU-RTS frame. This procedure allows the subsequent transmission to be protected. Indeed, all the stations receiving either the MU-RTS frame from the AP or one of the CTS frames from the stations set their NAV to the duration indicated in the frame, which prevent the stations from accessing the medium during the duration. Those durations computed by the AP corresponds to time to transmit the complete sequence of frames, i.e., Trigger frame, data and acknowledgement. Next, once access to the medium has been gained (confirmed by the CTS frames received), the AP sends a Trigger frame to solicit simultaneous immediate response frames from the stations addressed by the Trigger frame. In the scenario shown, the AP sends Basic trigger frame 213 (Trigger frame type set to 0) including User Info fields with the AID12 subfields 331 corresponding to STA1 and STA2 respectively. In response to Trigger frame 213, STA1 and STA2 send uplink data (214 for STA1 and 215 for STA2) to the AP on the RU allocated by the respective RU Allocation subfield 332. Next, the AP may acknowledge the reception of the uplink data by sending a Multi-STA Block Ack frame 216 over the operating channel that includes an acknowledgement for STA1 and STA2. Figure 2b describes another exemplary trigger-based communication scenario with TXOP sharing soliciting UL PPDU according to 802.11 be. In this example, an MU-RTS TXS Trigger frame with Triggered TXOP Sharing Mode subfield value equal to 1 (as defined in Table of Figure 3f) is used. This procedure allows an AP to initiate a TXOP and then to share its TXOP with a single one associated non-AP station before getting the medium back to send data to a non-AP station. This procedure may optionally be preceded by the sending of a CTS-to-self frame (not shown) by the AP to protect the TXOP frame exchange sequences. In the scenario, the AP sends MU-RTS Trigger Frame 220 (Trigger Frame Type field 311 set to value 3) having Triggered TXOP Sharing Mode field 316 set to value 1. It means the Trigger frame is a MU-RTS TXS Trigger frame that solicits allocation of a portion of an obtained TXOP (by the AP) to a single one associated non-AP EHT STA for transmitting one or more PPDUs (non-TB PPDU). Upon receiving Trigger frame 220, the scheduled station STA1 (i.e., identified in AID12 subfield 331 included in User Info field 330 (when RA field 303 is set to a broadcast address)) transmits to the AP a CTS response frame 221 and then starts to transmit to the AP uplink data in a non-trigger based PPDU 222. Next, the AP may acknowledge the reception of the uplink data by sending a Block Ack frame 223 to STA1. STA1 may start again the transmission of a new non-TB PPDU toward AP 224 which acknowledges the data with a subsequent Block Ack frame 225. After the sequence of UL transmission from STA1 (the AP finds out the sequence ends because the medium becomes idle), the AP may use the end of its TXOP for its own operation, e.g., to send data 230 to another station at or even before the end of the TXOP Sharing duration allocated to the scheduled station STA1. Figure 2c describes another exemplary trigger-based communication scenario with TXOP sharing soliciting UL PPDU and / or DiL transmission (i.e., to another station for direct or P2P transmission). In this example, an MU-RTS TXS Trigger frame with Triggered TXOP Sharing Mode subfield value equal to 2 (as defined in Table of Figure 3f) is used. This procedure allows an AP to initiate a TXOP and then share its TXOP with an associated non-AP station which uses it to indifferently perform UL transmissions toward the AP or DiL transmissions toward another STA. The procedure may optionally be preceded by the sending of a CTS-to-self (not shown) to protect the TXOP frame exchange sequences. The scenario starts as in Figure 2b with the sending of MU-RTS TXS Trigger Frame 250, this time with Triggered TXOP Sharing Mode field 316 set to value 2, followed by CTS response frame 221, uplink data in non-TB PPDU 222 and Block Ack frame 223. Then, scheduled station STA1 may decide to send DiL (or P2P) data in non-TB PPDU 251 to another station (STA2 in the scenario). Upon receiving these DiL data from STA1, STA2 may acknowledge the reception of the data by sending a Block Ack frame 252 to STA1. At the end of the duration allocated to the scheduled station STA1, the AP may use the end of its TXOP for its own operation (not shown), e.g., to transmit a new MU-RTS TXS to share again its TXOP with other stations or to merely send data. Figure 3a illustrates the format of an 802.11 Trigger frame, as defined in the standards IEEE P802.11REVme / D3.0, April 2023 and its amendment IEEE P802.11be / D4.1, September 2023. A Trigger frame (except MU-RTS trigger frame) allocates frequency resources (tone sets) for data and thus solicits one or more TB PPDU transmissions. The Trigger frame also carries other information required by the responding STAto send an HE TB PPDU. An MU-RTS trigger frame allocates resources for transmitting a CTS frame (this is a non-TB PPDU, as CTS follows legacy non-HT format or non-HT Duplicate format when bandwidth is greater than 20MHz). According to 802.11ax, only CTS frames 211 / 212 can be triggered by an MU-RTS TF. Trigger frame 300 is made up of the following fields: - Frame Control field 301 to indicate mainly the type of the frame. Type value ‘01’ and Subtype value ‘0010’ in this field identify a Trigger frame, - Duration field 302 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, - RA (Receiver Address) field 303 to identify the addressee or addressees of the frame. It is set to the non-AP address of the station identified by the AID12 subfield 331 of the User Info field 330 when there is only one User Info field 330 in the User Info list 305. Otherwise, if there are more than one User Info field 330 in the User Info list 305 or one User Info field 330 with the AID12 331 that allocates an RA-RU, RA field 303 is set to a broadcast address to target all stations, - TA (Transmitter Address) field 304 to identify the transmitting station. It is set to the address of the transmitting station (AP) if the frame is addressed to stations that belongs to the same BSS or is set to the transmitted BSSID AP if the frame is addressed to stations that belongs to several BSSs of a multiple BSSID set, - Common Info field 310 further described below with reference to Figures 3c and 3d for the HE and EHT variants respectively, - User Info List field 305 that contains zero or more User Info fields 330 to respectively define zero or more RU allocations. User Info field 330 is further described below with reference to Figure 3b, - Optional Padding field 306 to extend the frame length to give the recipient STAs enough time to prepare a response for transmission a SIFS after the Trigger frame is received, and - FCS field 307 to contain a 32-bit CRC. Figure 3b illustrates a format of the User Info field 330 (HE variant) in the Trigger frame format according to the 802.11 standard. The EHT variant of the User Info field format (not illustrated) is the same except that the EHT variant includes a PS160 subfield (in place of a reserved bit) which is used in complement to RU allocation and UL BW subfields for instance to handle 320MHz bandwidth channel. User Info field 330 includes the following subfields: - AID12 subfield 331 encoded as described in the following table: AID12 subfield Description 0 User Info field allocates one or more contiguous OFDMA RA-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 RA-RUs for unassociated STAs (OFDMA RA-RU) 2046 Unallocated RU 2047-4094 Reserved 4095 Start of Padding field - RU Allocation subfield 332 along with UL BW subfield 315 in Common Info field 310 identifies 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 OFDMA RU) 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 333 to indicate the code type of the solicited HETB PPDU, - UL HE-MCS subfield 334 to indicate the HE-MCS of the solicited HE TB PPDU, - UL DCM subfield 335 (absent in the EHT variant) to indicate DCM (dual carrier modulation) of the solicited HE TB PPDU. - subfield 336 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. The RA-RU information is made up of two subfields: o The Number Of RA-RU subfield (not shown bits 26 to 30) indicates the number of contiguous RUs allocated for UORA (Uplink OFDM Random-Access). The value of the Number Of RA-RU subfield is equal to the number of contiguous RA-RUs minus 1. o The More RA-RU subfield (not shown bit 31) is set to 1 to indicate that RA-RUs of the type indicated by the AID12 subfield in this User Info field are allocated in subsequent Trigger frames that are sent until the end of a TWT SP in which the Trigger frame carrying this field is sent, - UL Target Receive Power subfield 337 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 339. Its presence depends on the value of the Trigger Type field 311 in Common Info field 310 (i.e., depends on the type of Trigger frame). Figures 3c and 3d illustrate formats of respectively the HE variant and the EHT variant of Common Info field 310 in the Trigger frame format according to the 802.11 standard. Common Info field 310 includes the following subfields (not exhaustive list for conciseness): - Trigger Type subfield 311 to identify the Trigger frame variant. The values available for this field, corresponding to the various variants or types, are shown in Figure 3e, - UL Length subfield 312 to indicate the value of the L-SIG LENGTH field of the solicited TB PPDU, - More TF subfield 313 to indicate whether or not a subsequent Trigger frame is scheduled for transmission, - CS Required subfield 314 to define specific rules for channel sensing, - UL BW subfield 315 to indicate bandwidth in the HE-SIG-A of the HE TB PPDU, - subfield 316 corresponds to the Triggered TXOP Sharing Mode subfield if the Trigger type 311 indicates an MU-RTS Trigger Frame (value 3); otherwise, field 316 is the Gl And HE-LTF Type subfield. The values available for the Triggered TXOP Sharing Mode subfield are shown in Figure 3f, - AP Tx Power subfield 321 to indicate the AP’s combined transmit power at the transmit antenna connector of all the antennas used to transmit the triggering PPDU in units of dBm / 20 MHz, - UL Spatial Reuse subfield 324 to carry the values to be included in the Spatial Reuse fields in the HE-SIG-A field of the solicited HE TB PPDUs, - Optional Trigger Dependent Common Info subfield 328. Its presence depends on the value of the Trigger Type field, hence of the type of Trigger frame. Common Info field 310 in the HE variant also includes Number Of HE / EHT-LTF Symbols subfield 318, LDPC Extra Symbol Segment subfield 320, Pre-FEC Padding Factor subfield 322, PE Disambiguity subfield 323 and Reserved bit B63 327. The HE variant also specifically carries MU-MIMO HE-LTF Mode subfield 317a, UL STBC subfield 319a, Doppler subfield 325a and UL HE-SIG-A2 Reserved subfield 326a, while the EHT variant carries Reserved bit B22 317b, Reserved bit B26 319b, Reserved bit B53 325b, HE / EHT P160 subfield 326b, Special User Info Field Flag subfield 350 and EHT Reserved bits B56-B62 351. These subfields are of less importance for the present disclosure. However, Special User Info Field Flag subfield 350is always set to 0 in an EHT-variant Common Info field, indicating that a Special User Info field is included in the Trigger frame that contains the EHT-variant Common Info field. Back to Figure 1 depicting multiple APs, a Multi-AP (MAP) technology has emerged where the APs 110, 120, 130 collaborate to share the common communication channel once one of them is granted access to it. To do so, the APs exchange messages one with each other to create a MAP group (MAP Coordination Set) and coordinate the MAP communications, and thus to avoid interference. In addition, each BSS managed by APs 110, 120, 130 are composed of several STAs that share also the common communication channel. Sharing of the common communication channel is resource-based. An amount of a shared resource can be measured in time units, frequency bandwidth, 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 STAs or any other AP through the MAP technology. Random access is a usual mechanism in 802.11, especially when the TXOP granter (e.g., an AP) does not know what are the expectations of the granted devices (e.g., the associated STAs). The present Mll-RTS TXS provides some enhancements in medium access reservation but is too limited: only one time slot is provided to a single designated associated non-AP STA. A need thus exists to extend the TXS trigger-based mechanisms to support randomization for inter and intra BSS operations. Invention relates to extending signaling within the MU-RTS TXS to allow intimeslot-based random access (CSMA / CA). In case of multiple timeslots defined in the MU-RTS TXS, a further random selection of a given random timeslot may be considered to reduce risk of collisions. As will become apparent in the following description, embodiments are suitable for implementation within a standard environment, and especially in the transmission state machine of an 802.11 device. Figure 4a schematically illustrates a communication device 400 configured to implement at least one embodiment of the present invention, for instance any of AP or AP MLD as well as non-AP STA or non-AP MLD STA shown in Figure 1. The communication device 400 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 400 comprises a communication bus 413 to which there are preferably connected: a central processing unit 401, such as a processor, denoted CPU; a memory 403 for storing an executable code of methods or steps of the methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least one communication interface 402 connected to a wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards and / or Wireless-Fidelity (Wi-Fi) specifications, via transmitting and receiving antennas 404. Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 400 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 400 directly or by means of another element of the communication device 400. The 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 402, in order to be stored in the memory of the communication device 400 before being executed. In an embodiment, the device is a programmable apparatus which uses software to implement embodiments of the invention. However, alternatively, embodiments of the present invention may be implemented, totally or in partially, in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC). Figure 4b is a block diagram schematically illustrating the architecture of the communication device 400, adapted to carry out, at least partially, some embodiments of the invention. As illustrated, device 400 comprises a physical (PHY) layer block 423, a MAC layer block 422, and an application layer block 421. The PHY layer block 423, here a plurality of 802.11 standardized PHY layer modules, has the task of formatting, modulating on or demodulating from any 20MHz channel or composite channel. The PHY layer thus sends or receives frames over the radio medium NETW, such as 802.11 frames. These frames may be for instance medium access trigger frames to reserve a transmission slot, MAC data and management frames based on a 20MHz width to interact with legacy 802.11 stations and with legacy Wi-Fi Direct specification, 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 422 preferably comprises a Multi-Link MAC 802.11 layer 424 implementing conventional 802.11 MAC operations. It may comprise additional block 425 for carrying out, at least partially, embodiments of the invention. The MAC layer block 422 may optionally be implemented in software, which software is loaded into RAM 403 and executed by CPU 401. The ML MAC 802.11 layer 424 may implement an Upper-MAC stack along with a series of Lower-MAC modules. Of course, single-link architecture is supported (whereas not illustrated here) Preferably, the additional block 425, referred to as TXS random access module, which has different operations to implement parts of the invention, depending on the role played by the communication device 400. As the same device can play different roles over time, the additional block 425 is preferably designed to selectively perform the different operations relative to random TXS. MAC 802.11 layer 424 and TXS random medium access module 425 interact one with the other in order to establish and process accurately communications over a Link of the wireless medium, in between a non-AP MLD station and an AP MLD forming an infrastructure BSS according to embodiments of the invention or AP MLDs forming a MAP network. In other words, the communications are preferably over single-user timeslots addressed to multiple stations according to embodiments of the invention, the multiple stations could be one affiliated non-AP STA of a non-AP MLD or one affiliated AP of an AP MLD that operate on a Link. The MAC 802.11 layer 424, may comprise a single upper MAC layer 424a handling a plurality of lower MAC layer modules 424b. On top of the Figure 4b, application layer block 421 runs an application that generates and receives data packets, for example data packets such as a video stream. Application layer block 421 represents all the stack layers above MAC layer according to ISO standardization. Embodiments of the present invention are now illustrated using various exemplary embodiments. Figure 5 illustrates, using a flowchart, the main steps of a transmitting method according to embodiments of the invention. The description focuses on MU-RTS TXS Trigger frame, that provides the sharing of the operating channel made on a time basis (over a channel composed of multiple of 20 MHz bands). In embodiments, a dedicated signalling that the Trigger frame is a Random Access TXS Trigger frame is provided. Exemplary signalling is illustrated below with reference to Figures 10a and 10b. A TF is a control frame, according to the IEEE 802.11 legacy non-HT format, that is sent over the primary 20 MHz channel and duplicated (replicated) on each other 20 MHz channel forming the targeted composite channel. Due to the duplication of the control frame, it is expected that every nearby station (any 802.11 station or AP) receiving the TF on its primary channel, sets its NAV to the value specified in the header of the TF. This prevents these stations from accessing the channels of the targeted composite channel during a transmission opportunity (TXOP). According to embodiments, the RA Mll-RTS TXS Trigger frame is emitted by a triggering or sharing device, typically an AP. The RA TXS TF frame can be received from an AP of a basic service set to trigger a single-user (non-TB PPDU in a SU mode) transmission. The RA TXS TF frame allocates at least one TXS slot for SU transmission that the triggered STA(s) of the BSS can access for sending a data frame. The RA TXS Trigger frame (for BSS operation) is received by the triggered non-AP STAs at step 500. The RA TXS TF frame can be used for MAP operation (refer to Figure 10). In a MAP group, the sharing AP sends a RA TXS Trigger frame including the resource allocation for the coordinated APs that may be identified through a specific group identifier MAP ID. Such an MAP ID may be provided to the APs when creating or joining a MAP group (MAP Coordination Set). The RA TXS Trigger frame (for MAP operation) is received by the coordinated APs at step 500. For the following, the term receiving STA will be used in order to designate the device operating the algorithm, and could be either a non-AP STA in a BSS or a coordinated AP in a MAP group. If the receiving STA is not concerned by anyAID12 subfield 1021 (not pertain to a group identified by a predefined value of AID12, or the TXS slot is a scheduled one meaning no random scheme), then the algorithm stops. At step 510, the receiving STA analyses the contents of the received RA TXS TF frame, to determine the number of TXS slots that are allocated by the TF. As illustrated, several TXS slots can be envisaged: per the number of User Info fields 1020, but also if a number is provided in the Number Of RA-TXS subfield 1024 (Figures 10a, 10b). The receiving STA only considers TXS slot(s) allocated to a group to which the STA pertains (as example BSSID, or MAP ID). In case of a scheduled AID12 that identifies uniquely the receiving STA, the algorithm ends. When several TXS slots are found, then one or more, possibly all, TXS slots can be considered for transmission. According to alternative embodiments, illustrated by Figure 8 refines the determination step by the use of second contention parameter sets. In steps 520-530-540-560, it is envisaged that a contention is performed inside a TXS slot. Therefore, in step 520, the receiving STA has to retrieve the first contention parameters set to set up the contention. In one option, the parameters are those legacy EDCA ones already used outside the TXS operation. Advantage is the simplicity, against even worse performance (as example, for MAP, contention in between participants (APs) of a same TXS slot has nothing to compare with EDCA medium access in a BSS, and EDCA parameters can be completely different in between BSSs managed by participant APs, which results in unfairness). In a second and preferred option, the parameters are specific to the TXS operation, and may be named TXS-Time parameters. Advantage is to offer accurate contention against the number of participants, which is managed by AP. According to the TXS-Time parameters, the AP can configure and advertise a TCW range (TCWmin and TCWmax) by broadcasting management frames such as beacon or fast initial link setup discovery frames. The TCW range can also be contained in several unicast management frames (e.g., probe response, association response, and re-association response frames). These various management frames contain two 3-bit OCW range fields, ETCWmin and ETCWmax. On receiving these frames, the STA sets TCWmin=2ETCWmhl-1 and TCWmax=2ETCWmax-1. If the STA does not receive the TCW range field of TXS-Time parameters from the AP, it uses the default TCW range (i.e., TCWmin = 7 and TCWmax = 31). In step 530, the contention starts. In embodiment, the contention directly starts at the beginning of the TXS timeslot, that is to say at a SIFS after the preceding frame. Using a SIFS delay (or even more reduced delay like RIFS, 2psec) is to drive communication faster, and not go to the legacy scheme that consists to wait the DIFS deferring before invoking a backoff. Principle is to respect a SIFS interframe between last frame of previous TXS slot and new frame of the starting TXS slot. According to first option, up to the 4 EDCA backoff counters corresponding to each AC queue can count down. Instead, only one backoff counter may decrement according to a TID or AC class specified in Trigger Dependent User Info field 1026. According to second option, a single new TBO backoff is armed based on received TXS parameters and countdowns. With such dedicated backoff to be used for several successive TXS slots, one may either consider redrawing the TBO for each TXS slot. Alternatively, the STA ends the algorithm and goes in power saving until the end of the TXOP. In any option, when the backoff counter reaches zero, a communication is performed in step 550: the allocation field 1022 indicates the allowed duration for the timeslot, so that the winner STA has to consider this time limit for communication exchanges with a peer STA. If the communication goes well, the CW is updated accordingly (step 560) such as set to its minimum value TCWmin. As one can note, a successful transmission concerns both the medium access but also the acknowledgment by receiving STA: If a STA transmits a non-TB PPDU that contains a frame that solicits an immediate response in a TXS slot and the expected response is not received, the transmission is considered unsuccessful. Otherwise, the transmission is considered successful. An AP may set the More RA-RLI subfield 313 in the User Info field to 1 to indicate it intends to transmit additional RA TXS Trigger frames, so the STA may wait the next TF and restarts the algorithm. Figure 6 illustrates a first example of frame exchange based on the RA MU-RTS TXS procedure, according to first embodiments of Figure 5; In this example, an RA MU-RTS TXS Trigger frame with Triggered TXOP Sharing Mode subfield value equal to 2 to consider both UL and Direct-link communications. This procedure allows an AP to initiate a TXOP and then share its TXOP with associated non-AP stations which uses it to indifferently perform UL transmissions or DiL transmissions. The procedure may optionally be preceded by the sending of a CTS-to-self (not shown) to protect the TXOP frame exchange sequences. The scenario starts with the sending of RA TXS Trigger Frame 600, with Triggered TXOP Sharing Mode field set to value 2, followed by CTS response frame 611. In some embodiments, only the receiving stations intending to participate to the random access transmission in a TXS timeslot defined in the RA TXS TF 600 are transmitting a CTS response frame 611. The CTS response frames 611 sent by different stations are identical and synchronized. Therefore, the AP is able to receive this CTS response, but the AP cannot determine which station has responded. Nevertheless, the CTS response is useful as it may be received by hidden stations that have not received the RA TXS TF 600, thus preventing these hidden stations to transmit during the TXOP. The transmission of the CTS response by the stations intending to participate to the random access transmission improves the protection of the TXOP and indicates this intention to stations in the neighbourhood of the transmitting station. The RA TXS TF 600 is addressed to a group of stations (composed of STA1, STA2 and STA3 as example pertaining to a BSSID), and indicates two TXS slots for random access (e.g., AID12 subfield equals this BSSID value). The scenario shows that STA1 has won contention of first TXS slot, and STA3 has won the second TXS slot. Then, winner station STA1 may decide to send UL data 612 to AP. Upon receiving these data from STA1, UL may acknowledge the reception of the data by sending a Block Ack frame 613 to STA1. Then, for the next timeslot, STA2 and STA3 perform contention, and finally STA3 wins. Thus, winner station STA3 may decide to send Direct-Link (or P2P) data 614 to another station (STA2 in the scenario). Upon receiving these DiL data from STA1, STA2 may acknowledge the reception of the data by sending a Block Ack frame 615 to STA1. At the end of the duration allocated to the scheduled station STA1, the AP may use the end of its TXOP for its own operation (not shown), e.g., to transmit a new MU-RTS TXS to share again its TXOP with other stations or to merely send data. Figure 7 illustrates a second example of frame exchange based on the RA MU-RTS TXS procedure, according to embodiments of Figures 5 and 6. As the TXS slots are allocated for groups of stations performing random access, a contention period 701 is performed at the beginning of each slot. Therefore, it is possible that backoff countdown by more than one station can be down to zero at the same time (that means within same backoff slot duration). Figure 7 illustrates two data communications 702 and 703 by two stations, that collide in the TXS slot. This is very damageable, as the complete TXS slot is lost. This appears depending of the population of STAs pertaining to a given group (e.g., BSSID). Of course, an AP may manage the CW used by STAs within that group to try to reduce collisions. One complementary way to reduce contention is to randomly distribute TXS slots for a given group. This is the second embodiment illustrated in relation to Figure 8. Figure 8 illustrates, using a flowchart, general steps of a transmitting method according to second embodiments of the invention; The algorithm of Figure 5 is now augmented with randomization procedure for selecting a TXS slot. Especially, steps 510 to determine a TXS slot and step 560 after the transmission has occurred are modified. According to a first aspect, a simple random selection of a TXS slot can be performed. As example, the receiving can count the number of TXS slots allocated in the RA TXS TF (including considering Number Of RA-TXS subfield 1024). It picks randomly one of the slots among this obtained number of slots. In a second aspect, specific contention parameters (called TXS-Slot contention parameters) are used for determining a TXS slot. The AP can configure and advertise a SCW (txs-Slot Contention Window) range (SCWmin and SCWmax) by broadcasting management frames such as beacon or fast initial link setup discovery frames. The SCW range can also be contained in several unicast management frames (e.g., probe response, association response, and re-association response frames). These various management frames contain two 3-bit OCW range fields, ESCWmin and ESCWmax. On receiving these frames, the STA sets SCWmin=2ESCWmin-1 and SCWmax=2ESCWmax-1. If the STA does not receive the SCW range field of TXS-Slot parameters from the AP, it uses a default SCW range (i.e., SCWmin = 7 and SCWmax = 31). The STA maintains an internal SCW and an internal SBO counter for driving the determination of TXS slot. Each time the STA associates with a different AP (or MAP), and prior to the initial attempt of RA-TXS transmission towards it, the STA shall set the value of SCW to the SCWmin value and shall initialize its SBO counter in the range 0 to SCW. Preferably, if the RA TF is for a several group of STAs that the STA is associated with, then individual instances of SCW / SBO are used by the client STA. With a SBO armed with a random value (SBO counter set to an integer value randomly selected from a uniform distribution in the range 0 to SCW), several methods are proposed to determine a TXS slot. Two non-limitative examples of such methods are proposed for the sake of illustration. A first method referenced 800 consists in: if the SBO counter of the STA is not greater than the number of provided TXS slots in a RA TXS Trigger frame from the AP, then the STA shall select one TXS slots corresponding to SBO (e.g., TXS_slot number minus SBO value), and set its SBO counter to zero. Otherwise, the STA decrements its SBO counter by the number of TXS slots provided in the Trigger frame. A second method referenced 810 consists in: if the SBO counter of the STA is not greater than the number of provided TXS slots in a RA TXS Trigger frame from the AP, then the STA shall set its SBO counter to zero and randomly select one of the TXS slots to be considered for transmission. Otherwise, the STA decrements its SBO counter by the number of TXS slots provided in the Trigger frame. If a TXS slot was selected, then rest of steps 520 to 560 can be proceeded (otherwise algorithm ends). Upon ending its communication inside the selected TXS slot, in addition to the operations disclosed by step 560 (the CW is updated accordingly such as set to its minimum value TCWmin when transmission was success), there is also a need to update the contention window related to TXS Slots, the SCW, accordingly to the transmission success. After each successful non-TB PPDU transmission in a RA TXS slot, a STA shall set the value of SCW to the SCWmin obtained from the most recent SCWmin indicated in the TXS-Slot Parameter Set element from the AP or default values (if TXS-Slot Parameter Set element was not received) and shall initialize its SBO counter to an integer value randomly selected from a uniform distribution in the range 0 to SCW (step 562). If the non-TB PPDU is not successfully transmitted in the selected TXS slot, then the STA shall update its SCW to 2 x SCW + 1 when the SCW is less than the value of SCWmax, and shall randomly select its SBO counter in the range 0 and SCW. Once the SCW reaches SCWmax for successive retransmission attempts, the SCW shall remain at the value of SCWmax until the SCW is reset. Figure 9 illustrates another example of frame exchanges based on the RA MU-RTS TXS procedure, according to embodiments of Figures 8. By using several TXS slots (e.g., User Info fields) for random access for a given group of STAs to be allowed to contend in given TXS slots, it may appear that a given TXS slot (such as TXS slot 2 in the figure) are not occupied by a receiving STA. (no STA selects this TXS slot during step 510). The AP can detect this inactivity, and decides to operate inside the slot. To do so, a TXS Start Guard Time 900 indicates the time duration during which the receiving STA(s) is allowed for initiating transmission during this TXS slot. After that delay, the AP has to take the medium to remain as TXOP holder. We recall invention provides accessing the TXS slot directly (or after a RIFS, respecting a SIFS in between 2 frames of distinct TXS slots). TXS Start Guard Time 900 can be a period multiple of DCF slot time (determined by aSlotTime, usually 9psec). Typically, the DIFS can be considered: A 802.11 station must sense the status of the wireless medium before transmitting. If it finds that the medium is continuously idle for DCF Interframe Space (DIFS) duration, it is then permitted to transmit a frame. If the channel is found busy during the DIFS interval, the station should defer its transmission. No legacy station can access the medium during DIFS (34psec, equivalent to around 4 backoff countdown). In embodiments, as the medium is already granted to the AP for the TXOP, the TXS Start Guard Time could be larger (could be required when lot of STAs have to contend in 701). Preferably, the AP uses the TXS slot for its DL data towards receiving STAs. In option, the AP can send a frame to truncate the TXS slot (that means goes to next slot). This could be a QoS_Null Data frame, or a Cf-End frame, both with a duration field embedding the remaining duration of the TXOP (typically reduced from the non-consumed time duration remaining in the current TXS timeslot). In another option, the AP may send a CF-Poll (with or without data, and possibly broadcast or addressed to group corresponding to AID12 value: e.g., when AID12=BSSID index, then destination MAC address is set to BSSID) to allow starting the next TXS slot in the list. Turning now to the signalling of the RA TXS Trigger frame, Figure 10a presents format variants of Common Info field format 310 and Figure 10b illustrates format variants of User Info field formats 1020-A and 1020-B (respectively based on HE and EHT formats). As already discussed, the RA TXS TF can be used for intra or inter BSS operations. According to a first aspect, a first augmented format of the Common Info field 1010 includes a multi-AP bit (or field) taking a first value when the RA TXS Trigger frame allocates timing resources to stations associated with the transmitting AP only, and taking a second value when the Trigger frame is a multi-AP TXS Trigger frame allocating timing resources to another BSS or AP. As shown in the example of the Figure, the exemplary frame format is augmented with one non-AP / AP field 1019 (any other name can be used) in place of reserved bit B63. This field allows to discriminate between a MAP TXS Trigger frame dedicated to Multi-AP triggering (inter-BSS) or a conventional Trigger frame inside the BSS to trigger the stations associated to the AP transmitting this Trigger frame. If the field is set to 0, the RA TXS Trigger frame solicits the stations associated to the transmitting AP; otherwise, the Trigger frame solicits AP(s) in its vicinity or stations associated to APs in its vicinity (i.e., it is a MAP RA TXS Trigger frame). This additional field advantageously allows all the existing Trigger frame types to be reused. As alternative embodiment, the Common Info field 1010 includes a Trigger Type field whose value distinguishes between conventional 802.11ax types of Trigger frames allocating resources to stations associated to the BSS of the transmitting AP only and extended types (MAP types) of Trigger frames allocating resources to another BSS or AP than the transmitting BSS or AP. As a result, a new Trigger Type values from 9 to 15 (formerly reserved values) is used to advertise a RA MU-RTS TXS Trigger frame (not illustrated). According to a second aspect, a second augmented format of the Common Info field 1010 includes using the UL Length field 312 (that can be renamed as TXS Length), as a collector for obtaining whole period of time of the shared TXS allocation (this avoids any receiver to perform determination of each TXS slot duration and compute the resulting whole duration, especially for receivers not addressed by the TF). This whole duration may be different from the duration filed value of MAC header, as the latter one represents a TXOP duration which may include several RA TXS TFs). The RA TXS TF 1000 is augmented by one or more User Info fields 1020. Advantage of several User Info fields is to provide a different Allocation Duration 1023. A User Info field has the purpose of identifying group of clients for resource allocation (instead of a single client addressed per User Info according to legacy scheme, the AID12 subfield 1021 is equal to the 12 LSBs of the AID of the STA), with the use of predefined values for Random Access. The AID12 subfield can be made of: - Value 0: User Info field allocates one or more contiguous TXS slot(s) for associated STAs of a BSS; - Value 2045: User Info field allocates one or more contiguous TXS slot(s) for unassociated STAs of a BSS. - Value set to a BSSID index: User Info field allocates one or more contiguous TXS slot(s) for associated STAs of the BSS specified by BSSID Index; - MAP group ID: User Info field allocates one or more contiguous TXS slot(s) for a group of APs that pertain to a MAP group identified by the specified MAP group ID. Any other value indicative of a group of devices may be envisaged. As example, a specific value could be envisaged for P2P groups, wherein it indicates that all devices having an established P2P session can try to access a TXS slot. There shall not have any Unallocated TXS slot (e.g., value 2046 is forbidden). The Number Of RA-TXS subfield 1024 indicates the number of contiguous TXS slots allocated for RA TXS procedure according to embodiments of the invention. Advantageously, the value of the Number Of RA-TXS subfield is equal to the number of contiguous RA-TXS slots minus 1. If there is more than one RA-TXS slots (i.e., the Number Of RA-TXS subfield of this User Info field has a value greater than 0), then the allocated slots are contiguous. Preferably, all TXS slots in the RA TXS TF have the same size (duration and BW size). A condition for a recipient addressed by a RA TXSTF (receiving non-AP STA or receiving AP of a MAP set) of not responding nor transmitting a non-TB PPDU in an allocated TXS slot is when the TF contains one or more subfields in the Common Info field or in the User Info field(s) addressed to or selected by the recipient with values that are not recognized, are not supported, or cannot be satisfied by the recipient device. As example of error, the RU allocation field 1022 indicates a TXS slot that is not multiple of 20MHz. While the present description is focused on RA User Info field, one may envisage also providing allocation (mixed allocation of scheduled and random timeslots) of TXS slot to a specific device (not for random access according to embodiments): that is to say a non-AP STA or AP is providing one User Info with AID12 set to a legacy value for the non-AP STA or to an AP ID, in addition to random access User Info fields. Thus, a non-AP STA addressed by a User Info field in a RA TXS Trigger frame (i.e., the AID12 subfield is equal to the 12 LSBs of the AID of the non-AP STA) may ignore the remainder of User Info fields in the Trigger frame. With that possibility of mixed User Info fields, one may envisage adding an explicit bit (like bit B38 1025b) for indication of the User Info field has to be considered for Random Access. An efficient implementation may allocate any remaining time within a TxOP following scheduled timeslot(s) to any station for random access timeslot(s). This is more beneficial to use the remaining time instead of keeping it unused or releasing the TxOP. One may also consider the inclusion of a Trigger Dependent User Info field 1026, as illustrated by Figure 11. According to some embodiments of the invention, the Trigger Dependent User Info takes the format illustrated by the field 1026 of Figure 11. The Trigger Dependent User Info 1026 is one octet length and is present in RA TXS TF. It is used to indicate the MPDUs allowed in an A-MPDU carried in the non-TB PPDU conveyed in the TXS slot when the recipient wins the contention. The Trigger Dependent Type subfield 1026-1 intends to indicate the type of traffic that will be identified by subfield 1026-2. It could be either: a stream classification service identifier (SCSID) information, a TID information, a User priority, aTCLAS (Traffic Classification) information, etc. One may envisage new services, like Buffer Status report for MAP operation. A zero value indicates no restriction of any type of traffic. The presence of Trigger Dependent User Info field 1026 may be conditioned to the assertion of one presence bit in its corresponding User Info field 1020 (not illustrated). The format herein proposed is for illustration only, any other combination of Presence Bit and / or variant for coding the Trigger Dependent Type subfield could be envisaged inside a RA TXS Trigger frame format Figure 12 illustrates possible formats of elements for advertising the RA TXS parameters, that is to say a TXS Parameters Set Element supporting TXS-Time parameters and TXS-Slot parameters. Within a BSS context, the TXS parameter Set element 1200 can be transmitted in any management frame issued from an AP managing its BSS. Within a MAP group context, the TXS parameter Set element 1200 can be transmitted in any management frame issued in between APs of the group, like beacons or dedicated broadcasted frames, for advertisement of the multi-AP coordination capability (group formation is out of scope of the invention). The TXSS Parameter Record 1220 indicates the minimum and maximum values of the SCW (TXS Slot contention window): two 3-bit OCW range subfields, ESCWmin 1221 and ESCWmax 1222. The ESCWmin subfield indicates the minimum value of SCW for the initial non-TB PPDU transmission using RA TXS. The SCWmin parameter is used by a STA either for an initial transmission or following a successful non-TB PPDU transmission and is derived as follows: SCWmin = 2ESCWmin - 1 Where ESCWmin is the value in the ESCWmin subfield 1221. The ESCWmax subfield indicates the maximum value of SCW for UORA. The SCWmax parameter used by a STA for its retransmission attempts of RA TXS and is derived as follows: SCWmax = 2ESCWmax - 1 Where ESCWmax is the value in the ESCWmax subfield 1222. In preferred embodiment, there is only one TXSS Parameter Record 1220. The formats of TXS_AC_BE, TXS_AC_BK, TXS_AC_VI, and TXS_AC_VO Parameter Record fields have the same structure 1250. They relate to TXS-Time parameters for applying backoff contention 701 (Figure 7) inside any RA TXS slot. In the illustration, the Parameter Records are assigned to each AC queue, but any other assignment may be envisaged (to do so, the Traffic Identifier field 1260 may take the same format as Trigger Dependent User Info 1026, in order to identify several types of traffic). The number of TXS-Time parameters sets is not limited to the four illustrated TXS-Time parameters sets. As illustrated, The ETCWmin and ETCWmax subfields take same format as ECWmin / ECWmax of EDCA Parameter Set. This is because they are addressing also a backoff contention. The ETCWmin and ETCWmax subfields 1271, 1272 encode the values of TCWmin and TCWmax, respectively, in an exponent form. The ETCWmin and ETCWmax values are defined so that TCWmin = 2ETCWmin - 1 TCWmax = 2ETCWmax - 1 Hence the minimum encoded value of TCWmin and TCWmax is 0, and the maximum value is 32 767. By combining use of TXS timeslot random access and backoff contention inside a TXS timeslot, an AP may manage easily the contention inside a group of stations it administrates. Advantage of the scheme aims to provide simple parameter sets for the contentions, compared to handle large traffic characteristics with scheduling of stations. The RA TXS scheme is useful for low-end AP devices that do not embed smart schedulers. They can request various information, such as BSR, SCS Requests, P2P event reports, channel usages, OBSS TWT Request frame, etc. in a TDMA-like manner where the TXS / TDMA slots are randomly accessed. Any step of the algorithms of the invention may be implemented in software by execution of a set of instructions or program by a programmable computing machine, such as a PC (“Personal Computer”), a DSP (“Digital Signal Processor”) or a microcontroller; or else implemented in hardware by a machine or a dedicated component, such as an FPGA (“Field-Programmable Gate Array”) or an ASIC (“Application-Specific Integrated Circuit”). Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention. Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate. Each of the embodiments of the invention described above can be implemented solely or as a combination of a plurality of the embodiments. Also, features from different embodiments can be combined where necessary or where the combination of elements or features from individual embodiments in a single embodiment is beneficial. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A method of communication in a wireless network, comprising the following steps at a station comprised in a group of stations for transmitting a Physical layer Protocol Data Unit, PPDU:- receiving from an access point, AP, a Multi User - Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to the group of stations for transmission;- determining that a predefined field of the MU-RTS TXS TF indicates a random access for the group of stations to the TXS timeslots; and- contending for access to one of the TXS timeslots for transmitting the PPDU.

2. The method of claim 1, wherein the MU-RTS TXS TF defining a plurality of TXS timeslots, the method further comprises:- selecting a TXS timeslot of the plurality of TXS timeslots for transmitting the PPDU.

3. The method of claim 2, wherein the selection of a TXS timeslots of the plurality of TXS timeslots is performed randomly.

4. The method of claim 2, wherein the selection of a TXS timeslots of the plurality of TXS timeslots is performed using timeslot contention parameters.

5. The method of claim 4, wherein the timeslot contention parameters are updated depending on the success of the transmission of the PPDU.

6. The method of any one claims 1 to 5, wherein contending for access to one of TXS timeslots is performed according to an enhanced distributed channel access, EDCA, process.

7. The method of claim 6, wherein the EDCA comprises determining a random backoff accessing the TXS timeslot based on EDCA parameters.

8. The method of claim 7, wherein the EDCA parameters are updated depending on the success of the transmission of the PPDU.

9. The method of claim 2, wherein a single User Info field of the MU-RTS TXS TF is used to define several contiguous TXS timeslots.

10. The method of any one claims 1 to 9, wherein the method comprises:- sending a CTS response to the MU-RTS TXS TF for indicating that the station intends to transmit the PPDU in one of the TXS timeslots.

11. A method of communication in a wireless network, comprising the following steps at an access point, AP:- generating a Multi User-Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to a group of stations of the wireless network for transmission;- setting a predefined field of the MU-RTS TXS TF to indicate a random access for the group of stations to the TXS timeslots; and- transmitting the MU-RTS TXS TF.

12. The method of claim 11, wherein the method further comprises:- determining that one of the allocated TXS timeslots is not used by any station in the group of stations; and- transmitting data in the unused TXS timeslot.

13. A computer program product for a programmable apparatus, the computer program product comprising a sequence of instructions for implementing a method according to any one of claims 1 to 12, when loaded into and executed by the programmable apparatus.

14. A computer-readable storage medium storing instructions of a computer program for implementing a method according to any one of claims 1 to 12.

15. A computer program which upon execution causes the method of any one of claims 1 to 12 to be performed.

16. A station device for communicating in a wireless network, the station being comprised in a group of stations, the station device comprising a processor configured for:- receiving from an access point, AP, a Multi User - Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to the group of stations for transmission;- determining that a predefined field of the MU-RTS TXS TF indicates a random access for the group of stations to the TXS timeslots; and- contending for access to one of the TXS timeslots for transmitting a Physical layer Protocol Data Unit, PPDU.

17. An access point, AP, device for communicating in a wireless network, the AP device comprising a processor configured for:- generating a Multi User- Ready To Send Triggered TXOP Sharing trigger frame, MU-RTS TXS TF, for reserving a transmission opportunity, TXOP, on a communication channel, the MU-RTS TXS TF defining at least one TXS timeslot within the TXOP allocated to a group of stations of the wireless network for transmission;- setting a predefined field of the MU-RTS TXS TF to indicate a random access for the group of stations to the TXS timeslots; and- transmitting the MU-RTS TXS TF.

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