Method and device for secondary channel access operation

Abstract

An Access Point (AP) device in a wireless communication network operating an operating channel comprising a primary channel and one or more Secondary Channels (SCHs) to communicate with at least one non-AP station, whereby the AP schedules a series of Secondary Channel Access (SCA) Service Periods (SPs), each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time, and transmits to the at least one non-AP station information describing the series of SCA SPs. A non-Access Point device receiving from an AP information describing a series of SCA SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time, and during a period when the primary channel is busy, switching to the at least one secondary channel for communicating with the AP.

Description

FIELD OF THE INVENTION The present invention relates to wireless communications and more specifically to secondary channel access operation. 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. 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 (RTM) 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. 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 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 when sending data at full speed. All of the known features for efficient medium usage assumes that the primary channel is idle. If the primary channel is busy for instance due to interference, stations are not allowed to send any frame even if the secondary channels are idle. Some techniques have been proposed to change the operation bandwidth to a subpart thereof (e.g., from 80MHz to 40MHz) when the primary channel is idle, with the hope that the new operating channel does not suffer from interference. This is however a complex process, while it excludes the future use of the entire initial wider bandwidth in case the interference reduces or stops. The wireless medium thus remains inefficiently used. SUMMARY OF THE INVENTION The present invention has been devised to address one or more of the foregoing concerns. An aim of the invention is to improve the use of the medium within the operating channel. It is achieved by enabling secondary channel access (SCA) operation, i.e., operations on secondary channels (SCHs) even if the primary channel is busy, where one secondary channel is considered among several ones at a given time. According to a first aspect of the invention there is provided a method of communication in a wireless communication network comprising an Access Point, AP, operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the method comprising by the AP, the steps of: - scheduling a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; - transmitting to the group of non-AP stations an information describing the series of at least one SCA SPs. In an embodiment, the scheduling of the series of at least one SCA SPs is based on received SCA capabilities from the non-AP stations of the group. In an embodiment, the scheduling of the series of at least one SCA SPs is relative to Target Beacon Transmission Time, TBTT. In an embodiment, the scheduling of the series of at least one SCA SPs is relative to a timing synchronisation function. In an embodiment, the information is transmitted in an information element in a management frame. In an embodiment, the information element is a SCH Info information element comprising for each SCA SP a starting time, a duration and a number identifying the SCH channel to be used for SCA mechanism. In an embodiment, the information further comprises an operating bandwidth for indicating several SCH channels to be used for SCA mechanism. In an embodiment, the information element is a Target Wake Time, TWT, information element comprising an indication indicating that the corresponding TWT SP is a SCA SP. In an embodiment, the TWT information element comprises for each SCA SP a starting time, a duration and a SCH to be used for SCA mechanism. In an embodiment, the information comprises a guard time indication for indicating a switching interval between two successive SCA period during which no SCA communication may be initiated. According to another aspect of the invention there is provided a method of communication in a wireless communication network comprising an Access Point, AP, operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the method comprising by a non-AP station, the steps of: - receiving from the AP an information describing a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; and - during the period of time, when the primary channel is busy, switching to the at least one secondary channel for communicating with the AP. 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 an Access Point, AP, device in a wireless communication network, the AP operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the AP comprising a processor configured for: - scheduling a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; - transmitting to the group of non-AP stations an information describing the series of at least one SCA SPs. According to another aspect of the invention there is provided a non-Access Point, AP, device in a wireless communication network comprising an AP operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station comprising the non-AP device, the non-AP device comprising a processor configured for: - receiving from the AP an information describing a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; and - during the period of time, when the primary channel is busy, switching to the at least one secondary channel for communicating with the AP. 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 1a illustrates an example of a network system in which some embodiments of the invention may be implemented; Figure 1b illustrates 802.11ac channel allocation that supports operating channels of 20 MHz, 40 MHz, 80 MHz or 160 MHz; Figure 2 illustrates, using a flowchart, exemplary steps performed at stations for SCA operation, according to some embodiments of the invention; Figure 3 illustrates, using a flowchart, an exemplary management frames exchange sequence allowing the two stations to share and negotiate the candidate SCHs for SCA operation in an embodiment of the invention; Figure 4 illustrates, with timeline, an exemplary repartition of SCA Service Periods (SPs) for SCA mechanism in an embodiment of the invention; Figure 5 illustrates, using a flowchart, exemplary steps to operate SCH slot allocation for SCA operation by an AP in an embodiment of the invention; Figure 6 illustrates possible format of information elements for advertising the series of SCA service periods in an embodiment of the invention; Figure 7 illustrates another possible format of information elements for advertising the SCA service periods in another embodiment of the invention; Figure 8 illustrates a timeline example, showing SCA operation over two SCH channels in an embodiment of the invention; Figure 9 illustrates, using flowcharts, exemplary steps of SCA management at both the AP and the non-AP STA, according to some embodiments of the invention; Figure 10 illustrates an example of hardware configuration of both AP and non-AP STA, according to some embodiments of the invention. DETAILED DESCRIPTION OF THE INVENTION Figure 1a illustrates an example of a network system in which some embodiments of the invention may be implemented. For the sake of illustration, Figure 1a represents two 802.11 network (i.e., a WiFi network - RTM) systems 100 and 101. 802.11 network system 100 consists of two wireless devices: an access point station (AP) 100a and a non-AP station (non-AP STA) 110a. Of course, the number of non-AP STAs may be different from one. The AP manages the set of non-AP STAs that together organize their accesses to the wireless medium, known as “operating channel”, for communication purposes. The stations (including the AP) form a service set, here below referred to as basic service set, BSS (although other terminology can be used). A same physical station 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. AP 100a provides wireless connections between stations of the BSS 100 (including non-AP STA 110a and AP 100a) and provide an access to a wider network, such as the Internet. The connection of a non-AP STA 110a to AP 100a may be performed by a standardized process called association. AP 100b provides wireless connections between stations of the BSS 101. It is to be noted that non-AP station 110a, being in the area covered by both BSS 100 and 101 can associate with AP 100a or AP 100b. Once a non-AP STA is associated with the AP, the non-AP STA can access the operating channel, send data over it to other stations of the BSS or to the wider network via the AP and receive data from other stations or from the wider network through the AP. The 802.11 family of standards define various media access control (MAC) mechanisms to drive access to the wireless medium. Figure 1b illustrates 802.11ac channel allocation that supports operating channels of 20 MHz, 40 MHz, 80 MHz or 160 MHz. 802.11be D5.0 has further introduced an operating channel of 320 MHz. Future standards may even propose wider operating channels. An elementary channel 100-1 to 100-8 is 20MHz wide. The IEEE 802.11 family introduces support of a restricted number of predefined subsets of 20MHz channels to form predefined operating channel configurations that are available for the BSSs. The predefined subsets in 802.11ac are shown in Figure 1b and correspond to 20 MHz, 40 MHz, 80 MHz, and 160 MHz channel bandwidths, each grouping 2n elementary 20MHz channels. Indeed, the 20 MHz component channels 100-1 to 100-8 are concatenated to form wider operating channels. As shown the elementary 20MHz channels of the predefined subsets are contiguous. Optionally a predefined subset may be formed by elementary channels that are not all contiguous, e.g., a 160 MHz operating channel may be made of two separate 80 MHz channels. The elementary 20MHz channels for the operating channel are selected from the 20 MHz channels available in the wireless band considered. The number of 20MHz channels per band varies between the 2.4 GHz, 5 GHz and 6 GHz bands. The AP elects one of the elementary 20MHz channel of the operating channel as being the “primary channel” common to all the stations of the BSS. The primary channel is used for signalling (including channel access procedure) and backwards compatibility. Hence, a station (including the AP) is granted a transmission opportunity (TXOP) through the enhanced distributed channel access (EDCA) procedure performed on the primary channel 100-3. The other elementary 20MHz channels of the operating channel are referred to as “secondary channels”. 802.11 network system 101 is another, but similar, 802.11 network system comprising one wireless device: AP 100b. Of course, it may contain one or more non-AP STAs. System 101 operates on the same operating channel as BSS 100. Non-AP STA 110a is within the range of both APs 100a and 100b. AP 100a is within the range of AP 100b. As AP 100a and AP 100b use the same operating channel, they are considered to be “overlapping”, i.e., they have no connection to each other but they interfere with each other. It is said they are in the relationship of OBSS (Overlapping Basic Service Set). APs 100a and 100b 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. It can be a standalone product or it may be integrated in a device, for instance in a broadband remote access server (BRAS). Non-AP STA 110a 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, a user equipment (UE), a user station (STA), or some other terminology. In some implementations, a non-AP STA may be or 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 a smartphone), 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, non-AP station 110a may be a wireless node. Such a 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. Figure 2 illustrates, using a flowchart, exemplary steps performed at a station (including both AP and non-AP STA of the BSSs) for Secondary Channel Access (SCA) operation. The core idea of the proposed SCA operation is to switch the medium access procedure to one or more SCHs (Secondary Channels) that may be pre-agreed between the AP and the non-AP STAs, when the primary channel is detected as busy due to an interference (e.g., OBSS communication, non-Wi-Fi interference). Medium access procedure may be the same procedure as the legacy 802.11 devices conduct on its primary channel (e.g., EDCA) but not limited to it. Thus, AP and non-AP STA may agree on which SCH channel(s) to use beforehand since they cannot communicate if they use different SCHs. Figure 2 illustrates, using a flowchart, the main steps of a communication method in which stations agree on the SCH to be used for SCA and then perform SCA operation. At step 200, the station exchanges with another station one or more management frames to agree on the secondary channel to be used for SCA. In particular, such a frame exchanged between the stations may include SCA support information signalling support of the SCA by the station transmitting the frame and SCA capabilities information advertising the secondary channel or channels eligible for SCA in case of supported SCA. The Management frames may include Beacon frames, Probe Response frames, (Re)Association Response frames for APs and (Re)Association Request frames for non-AP STAs. Hence, the exchange of management frames may take place during an association procedure between the two stations. They may also include other Management frames such as Action Frames, Probe Request frames, Authentication frames but not limited to these frames. Based on the exchanged SCA capabilities, the stations agree on candidate SCH(s) that includes at least one SCH. Step 200 is therefore a “SCA setup procedure”. Various scenarios of SCA setup procedure may be contemplated as described below (e.g., with respect to Figure 3), from an SCH imposed by the AP to a true negotiation of SCHs between the two stations (including multiple back and forth of frames containing proposals and counterproposals). The candidate SCH or SCHs are thus previously agreed between the two stations. As will become apparent later (Figure 3), such negotiation procedure may be only convenient to a very little set of stations; that is to say, this would not be envisaged for BSS with numerous stations. Next at step 201, the station determines when the primary channel of the operating channel becomes busy. Activity on the primary channel may be detected using CCA (Clear Channel Assessment) that includes both PD (Packet detection) and ED (Energy Detection). The primary channel thus becomes not accessible to the station. This triggers the station to operate SCA with the other station (peer station). At step 202 in response to the positive determining of step 201, the station performs SCA on at least one of the agreed candidate SCH(s) determined at step 200. Of course, the peer station may operate in the same way at its side. SCA allows the station to gain access to the wireless medium and then to perform data communication over the SCH or SCHs with the peer station. This forms SCA operation. Of course, any other use of the gained SCH may be contemplated within the SCA operation. At step 203, after having performed the SCA operation, the station returns to conventional operation that uses the primary channel for the medium access procedure. SCA support information and SCA capabilities information of both stations are advertised or signalled in the management frames using Information Elements, lEs. Information Elements are data structures, also called fields, defined for the transmission of information that may be inserted in frames exchanged by stations in the network. These data structures provide a type and a length followed by type specific data. The format and the semantic of the data in an information element are defined by the type of the information element. In the present case, two types of Information Elements are defined for the transmission of respectively SCA support information and SCA capabilities information. In some embodiments, candidate SCH or SCHs are preferably advertised using a bitmap: in that case, the SCA capabilities information includes a SCH bitmap that advertises which secondary channel or channels forming the operating channel are eligible for SCA. In variants where it is known that a single candidate SCH is provided, a SCH field having less bits can be used: In this example, the SCA capabilities information includes a SCH field that comprises an offset value defining a single secondary channel eligible for SCA within the operating channel. As mentioned above, with reference to Figure 2, the SCA setup procedure includes an exchange of management frames. Figure 3 illustrates, using a flowchart, an exemplary management frames exchange sequence allowing the two stations to share and negotiate the candidate SCHs for SCA operation. The sequence takes place during the association procedure of a non-AP station with an AP, that is: AP 100a transmits Beacon 300 periodically (in general at every 100msec interval); non-AP STA 110a transmits Probe Request frame 301 to scan APs in its vicinity; if AP 100a succeeds to receive Probe Request frame 301, it replies with Probe Response frame 302; if non-AP STA 110a decides to associate with AP 100a, it performs Authentication frame exchange (not illustrated) and then transmits Association Request frame 303 to AP 100a; if AP 100a agrees with the association requested by received Association Request frame 303, it replies with Association Response frame 304. The association of non-AP STA 110a with AP 100a is established when Association Response including a status code SUCCESS (0) is correctly received by non-AP STA 110a. After the association ends, AP 100a and non-AP STA 110a may exchange Action frames 305 and / or 306, access the operating channel and exchange data. Preferably, the SCA Support and SCA capabilities lEs are exchanged between AP 100a and non-AP station 110a within any of these Management frames. In some scenarios, AP 100a indicates its SCA support and capabilities (according to its own capabilities) into its Beacon frame 300 or Probe Response frame 302 or both. Non-AP STA 110a receives these Management frames and identifies AP 100a’s SCA support and / or SCA capabilities by parsing the SCA support field and / or SCA capabilities field. Since AP 100a and non-AP STA 110a have to agree on their SCH(s) for SCA, non-AP STA 110a either can directly agree with those proposed by AP 100a if they match its own SCA capabilities, or can negotiate by selecting SCH(s) from the ones proposed from AP 100a that match its own SCA capabilities. Selected SCH(s) is indicated at least within the SCA capabilities included in Association Request frame 303 and optionally in Probe Request frame 301 to inform AP 100a about such non-AP STA’s SCA capabilities. AP 100a may include the same SCA capabilities as those transmitted by non-AP STA in Association Response frame 304 with Status Code SUCCESS (0) and optionally in Probe Response frame 302 to indicate non-AP STA 110a that the proposed SCA capabilities (hence SCHs) are acceptable. On the other hand, if AP 100a does not accept non-AP STA 110a’s proposal, AP 100a may reject the association request by sending Association Response frame 304 with for example a new Status Code such as REJECTED_SCA_NOT_ACCEPTABLE (65534). This status code means the SCA capabilities proposed by non-AP STA 110a are not acceptable to AP 100a. Of course, any other value than 65534 (except those already defined in the latest 802.11 specifications) can be used as Status code rejecting the association based on the SCA capabilities. Non-AP STA 110a can therefore be aware of the detailed reason of the rejection, thanks to this status code. In a BSS, an AP 100a has multiple non-AP STAs associated with it. To provide homogeneous behaviour within the BSS, AP 100a may decide to apply the same SCA capabilities to the non-AP STAs of its BSS. To that end, AP 100a may have a local register to store own SCA capabilities to share with the non-AP STAs. AP 100a may use the negotiation with any non-AP STA to update its SCA capabilities in the local register. In particular, upon agreeing with a non-AP station on one or more SCHs that form a subset of those indicated in the local register, the AP station may update the latter to the subset in order to propose the subset of secondary channels to any other non-AP stations willing to join the BSS. In practise, an AP 100a updates its SCA capabilities field based on the agreed SCA capabilities among the associated non-AP STAs, that results in one or more selected SCHs. Concerning a non-AP STA, one SCH is selected and matches one of the A P’s selected SCHs. Drawback of this individual agreement if that it is done between AP and each non-AP STA, therefore an agreement would not conduct to use a same SCH for another non-AP STA: the AP 100a has to select a subset of SCHs that would fit to the majority. It is evident that non-AP STAs within a BSS do not suffer from same interferences (e.g., due to their location), the resulting SCH or SCHs are thus never satisfactory for some STAs. Action frames 305 and 306 are illustrated in the figure to support an update of the SCH selection (outside association procedure) by the exchange of SCA information elements inserted in these frames used to negotiate an update of the SCA capabilities between the non-AP station and the AP. This renegotiation occurs when the conditions, typically the interferences, affecting the different channels evolve overtime. This scheme provides large overhead, and never ends. As one may understand from the previous description, SCA operation may lead to interoperability issues if STAs and AP do not listen on same SCA channel at a given time. Therefore, it is preferable to advise several STAs to operate a selected SCH. There is need for an enhanced scheme considering several channels for SCA in order to mitigate interferences experienced by various non-AP STAs. It comes that the known SCA mechanisms are not sufficient to provide efficient network communication in wireless networks, in particular to discover and select secondary channels in order to mitigate interferences experienced by various STAs. In this context, the present invention intends to provide new SCA mechanisms that favor time-driven SCH channel indication and operation. Embodiments allow the AP of an infrastructure BSS to gather the SCA capabilities of its communication group constituted by the connected stations in the BSS. Based on these capabilities, the AP schedules a series of at least one SCA service period and allocates one or more SCH to be used for SCA mechanism to each scheduled SCA SP. The AP advertises the scheduled series of SCA SPs to the non-AP STAs. It is to be noted that in some embodiments, the series of SCA SPs may only comprise a single SCA SP. We keep the term series of SCA SPs, even in the case where the series comprises a single SCA SP for generality, while the expression series of SCA SPs means in that case that the AP schedules and advertise one SCA SP. In that way, the STA device solicits SCH assistance to the existing AP. The stations operating on the same channel as the existing AP then become aware of the SCH channel to be used for SCA mechanism without having to scan multiple channels. They may then determine when to switch to the corresponding secondary operating channel when a primary channel is busy. Thanks to this assistance for advertising about the SCH channels along with the corresponding time-slot, the average time needed by stations to switch channels is shorter compared to the prior art. Hence, the setup to establish a SCH communication channel according to SCA has no delay impact nor framing overhead. Figure 4 illustrates, with timeline, an exemplary repartition of SCA Service Periods (SPs) for SCA mechanism. Considering a BSS operating channel 420 constituted in this example of 16 20MHz sub-channels organized into four 80 MHz sub-channels to form an operating band of 320 MHz. One of the 20 MHz sub-channel is the primary channel 410 while the other 20 MHz sub-channels are SCHs. The time is divided into a series of SCA service periods. This division may be based in reference to Target Beacon Transmission Time, TBTT. Beacons are transmitted periodically by the AP and TBTT is the time interval between two consecutive beacon transmission times. STAs should not extend their transmissions over TBTT. Therefore TBTT seems an interesting timing for aligning a series of SCA service periods or SCH time-slots. A SCA service period or SCH time-slot is defined as a period of time during which at least one specific SCH channel has to be considered for SCA mechanism. The series of SCA service periods along with, for each SCA service period the SCH channel selected by the AP for SCA mechanism, is advertised by the AP in an action frame or a beacon frame. Based on this advertisement, any station in the group of stations concerned with the BSS operating channel knows at any time the secondary channel to be considered for SCA mechanism when the primary channel becomes unavailable. As alternative to the TBTT interval, a value of the timing synchronization function,TSF, can be used to define a series of service periods 400 which may not be aligned with the TBTT period in that case. For the sake of illustration, this timeline is illustrated for only one series of SPs at a given time. That means the SP or SCH slot 1 (400) can be allocated to a given set of stations, the stations concerned with the BSS operating channel 420, while a parallel (not illustrated) SCH slot may be considered for another set of STAs operating on another operating channel. This parallel series of SCA service periods is allocated to a distinct SCH channel and may have a distinct duration. Traditionally, according to IEEE802.11ac, the bandwidth is increased according to dynamic bandwidth operation. An RTS frame negotiates a potentially reduced channel width, compared to the channel width indicated by the RTS, for subsequent transmissions within the current transmission opportunity, TXOP, the remaining band being unused. With present embodiments, when the bandwidth is reduced and prevents using the primary channel access, then a secondary channel (411, 412) may be considered, if available, for secondary channel access. During the first time-slot 400, SP1, the sub-channel 411 in the third 80Mhz channel has to be considered for SCA access. During the second timeslot 400, SP2, the sub-channel 412 in the fourth 80Mhz channel has to be considered for SCA access. And so on. As a result, at least one STA is allocated an SCH in at least SCA service period. As further described, a group of STAs is allocated to one or a series of SCA service periods, therefore those STAs will all know which SCH channel they have to consider at a given time. Providing a series of different SCH channels allows to mitigate interferences experienced by various STAs, and this spreading seems fairer for STAs of the set. The illustration is provided for a given link (operating channel and bandwidth). For devices operating multiple links (as IEEE 802.11 be standard), the non-AP STA and AP are considered to be affiliated STAs of respectively a non-AP MLD and AP MLD. Therefore the series of SCA service periods further shall be in reference to channels, bands, and TSF time of the operated link. Figure 5 illustrates, using a flowchart, exemplary steps to operate SCH slot allocation for SCA operation by an AP. The steps are performed when the AP desires to send a Management frame: it may be one of Beacon frame, Probe Response frame, (Re)Association Response frame and Action frame. Part of the steps (e.g., 500 and / or 501) may be performed when a non-AP STA emits one of Probe Request frame, (Re)Association Request frame and Action frame, towards the AP, that indicates a (new) SCA capability. At step 500, the AP retrieves its SCA support information and / or SCA capabilities information from a non-AP STA: the AP may consider non-AP STA information for the Probe Response and (Re)Association Response. If the non-AP STA indicates that it supports SCA, the AP parses the SCA Capability that is included in the received frame or the like. In practice, the AP retrieves the non-AP’s preference of SCH channel or channels (e.g., from an SCH Bitmap field in those received frames). Those capabilities may further be changed depending on the wireless environment. For example, the non-AP station may know which SCH is more suitable for SCA operation knowing the interference or OBSS existence in its surroundings by any means. Using this information, the station may configure its SCA capabilities (e.g., SCH Bitmap). Next in step 501, the AP determines SCA capabilities based on the non-AP station’s SCA capabilities and own SCA capabilities. For example, candidate SCHs can be selected from those advertised in the non-AP station’s SCA capabilities, those already advertised from other non-AP station(s) of the BSS, and that are compatible with own SCA capabilities. Own SCA capabilities may be retrieved from local register. All the stored information that corresponds to the relationship between the non-AP STA and the peer station may be stored in a non-volatile memory, such as a local register, so that non-AP STA can refer to them in a later process. The stored information may be cached and the process may be done once for each non-AP station (e.g., AP) as long as any of the non-AP station does not change its configuration that is relevant to the SCA operation. As the AP is in charge of administrating its BSS, it will only consider SCH channels that still satisfy its hardware capabilities. In one embodiment, AP will select lists of channels that fit within its operating bandwidth. In embodiments, if the AP is able to perform SCA on several SCH channels in parallel, it may allocate several concurrent SCH slots, one per non-AP stations, in addition to successive slots. If the AP knows a non-AP station supports several concurrent SCH channels, it may consider providing allocation for concurrent SCH slots to it. Each non-AP station is provided by the AP one or several SCH channels to be used for the duration of the SCA service period for SCA mechanism, while the SCH channel to be used may be different for two non-AP stations for a given SCA service period. The number of SCA service periods is mainly dependent of the number of non-AP STAs that provide an SCA support indication. With regards to Figure 4, the number ‘n’ relative to SPn 400 may correspond to the number of stations with SCA support in the BSS. Alternatively, this number n can be multiple of this number of non-AP STAs (with SCA support); that means the same SCH to be used can be indicated several times during a TBTT for different SCA service period. Preferably, the determination of which SCH channel to be used by a given non-AP station considers the maximum operating bandwidth of the station. That is to say several SCH channels may be selected according to non-AP STA capabilities: SCH channels close to primary channel for those non-AP STAs with narrow band as a first group of STAs. At step 502, based on the SCH selection computed at step 501, the AP builds the Management / Action frame using any format described in Figures 6 and 7. At step 503, the station transmits the frame so built. An Action frame so built would preferably only be sent in unicast to recipients non-AP STA concerned by the indicated SCH slots. As alternative, the frame so built is a Management frame broadcast to several non-AP STAs. All of the name of the elements and / or fields shown in Figures 6 and 7 are merely examples, meaning alternative names can also be used in variants. In the same way, the order of the fields as shown is for illustrative purposes only, meaning any other order can be used. Figure 6 illustrates possible format of information elements for advertising the series of SCA service periods, namely a list an SCH channel and timing indication. Typically, those parameters are controlled and advertised by the AP through a dedicated information element in its Action frame or Management frames (as example unicast Probe Response and (Re)Association Response frames, or broadcast Beacon frame) in order to update them depending on the load, the collision rates and the SCA Capabilities of its administrated non-AP STAs. In this example, it is not necessary to indicate the non-AP STAs’ identification (e.g., AID of STA) as any recipient of the unicast frame as to be considered as pertaining to the allocation. That is to say, if the frame is unicast, then the single recipient will consider the SCA information. If the frame is a broadcast management frame as a beacon frame, then all stations of the BSS may consider the SCA information. An example of an SCH Info information element 600 is illustrated. The field SCH Info Control 610 is a header indicating the number of channels 611. In the example, the header 610 also provides an SCA Guard time 612 used to take into account the SCH switch time of the stations and prevent any SCA trial during this time between two consecutive SCA service periods. The header 610 is followed by one or several SCH field 620. Each SCH field indicates the parameter of one SCA service period, it is composed in this example of a TSF Time field 630 indicating the starting time of the SCA service period. In this example, the time is indicated using a value representative of the TSF timer, in other words it is the BSS local time. The field may represent the timing synchronization function (TSF) timer (8-octects length, as per section 9.4.1.10 Timestamp field of 802.11REVme D5.0 specification), or a partial timestamp (as exemplary shown in the figure: 2-octets length, representing the lower order 2 octets of the TSF timer value). The field 630 contains a positive unsigned integer corresponding to a TSF time at which the SCH timeslot will start. Alternatively, this field may be based on TBTT and indicate the starting time of the SCA service period as an elapsed time from the TBTT. A duration field 640 contains the duration of the described SCA service period, it may be expressed in milliseconds, or units of 32 or 50 microseconds by example. A SCH Information field 650 indicates the SCH channel(s) to be used by SCA mechanism for the considered SCA service period. The SCH Information field 650 comprises a Target Channel field 662 for indicating the index number of the 20 MHz SCH channel to be used. Optionally, an Operating Bandwidth field 663 allows indicating an operating bandwidth greater than 20 MHz. This permits to indicate several 20 MHz SCH channels, starting from the 20 MHz SCH channel indicated in the Target Channel field 662 for SCA operation. These fields 662 and 663 are common to the two examples illustrated in the figure of the SCH Information field 650, 650-A and 650-B. SCH Information field 650-A further comprises an Operating Class field 661. The Operating Class field 661 indicates an operating class value as defined in 802.1 IREVme D4.0 Annex E, specifying the operating class in which the Target Channel field is valid. The operating class is interpreted in the context of the country specified in the Beacon frame. The Target Channel field indicates a channel number for SCH channel, which is interpreted in the context of the indicated operating class. Channel numbers are defined in 802.11 REVme D4.0 Annex E. As the operating class remains in the context of Beacon frame sent by the AP, it may be not mandatory. The formats of the fields are purely indicative: one may consider different lengths. As example, the Target Channel field may contain an SCH bitmap, each bit therein corresponds to one elementary 20MHz channel forming the operating channel of the BSS. Figure 7 illustrates another possible format of information elements for advertising the SCA service periods. This format relies on the TWT information element format, as it is adapted to be used for SCA service period indication and is suitable to be addressed to the whole BSS. A TWT information element 700 comprises in that order, an Element ID field 701 for identifying the information element, a length field, a Control field 710 and a TWT Parameter Information field 720. The Control field 710 comprises in that order, an NDP Paging Indicator field, a Responder PM Mode field, a Negotiation Type field 711, a TWT Information Frame Disabled field, a Wake Duration Unit field 712, a Link ID Bitmap Present field, the last field being reserved. The Control field 710 informs, through Negotiation Type field 711, whether the TWT is a broadcast TWT or an individual TWT agreement. The MSB of the Negotiation Type subfield 711 is the Broadcast field, therefore the TWT element 700 is referred to as Broadcast TWT element when MSB of subfield 711 is 1 (otherwise it is a single Individual TWT element). The Wake Duration Unit subfield 712 indicates the unit of the Nominal Minimum TWT Wake Duration field. The Wake Duration Unit subfield is set to 0 if the unit is 256 ps and is set to 1 if the unit is a Time Unit (TU, 1024ps). Other fields are of less importance for present description. The TWT element for indication of an SCA service period is preferably a Broadcast TWT. As a result, the following description will be based on that assumption. Then, the “TWT Parameter Information” field 720 contains one or more ‘Broadcast TWT Parameter Set’ fields 725. First field of the “Broadcast TWT Parameter Set” field 725 is a Request Type field 730 first comprising a TWT Request subfield 731 set to 1 when issued by the TWT scheduled STA. Otherwise, set to 0 when issued by the TWT scheduling STA, meaning the AP. In present embodiments, the AP emits an SCH TWT element so the subfield is set to 0. The Request Type field 730 then comprises a TWT Setup Command subfield 732 to indicate the type of TWT command: as example Request, Suggest, Demand, Reject when issued by a non-AP STA; or Accept, Alternate, Dictate, Reject when issued by a TWT scheduling AP. In present embodiments, the AP emits an SCH TWT information element, so ‘Dictate’ or ‘Alternate’ are considered as commands suggested by the AP for SCA mechanism (of course, any other command such as Accept, Suggest or even Grouping, this later not used for TWT, may be considered). The Request Type field 730 then comprises a Trigger field 733 to indicate whether or not the TWT SP indicated by the TWT element 300 includes triggering frames (the Trigger subfield equals to 1 for trigger-enabled R-TWT). Such a TWT SP is named triggered-enabled TWT SP, and a non-AP station cannot start transmitting data within it without previous triggering by the AP; In present embodiments, the SCH TWT element does not preclude any medium access. It is up to the non-AP STA to perform SCA with the allowed data or control frames. So value is set to 0. The Request Type field 730 then comprises a Last Broadcast Parameter Set field 734 is set to 0 to indicate that another Broadcast TWT Parameter set follows this set. The Last Broadcast Parameter Set subfield is set to 1 to indicate that this is the last broadcast TWT Parameter set in the broadcast TWT element. The Request Type field 730 then comprises a Flow Type subfield 735 that indicates whether the TWT is announced, meaning the TWT scheduling AP will wait to receive a frame from TWT scheduled STA to signal its awake state, or not. In present embodiments, Flow Type subfield equals to 0. The Request Type field 730 then comprises a Broadcast TWT Recommendation field 736 that contains a value that indicates recommendations on the types of frames that are transmitted by TWT scheduled STAs and scheduling AP during the broadcast TWT SP, encoded according to the ‘Broadcast TWT Recommendation field for a broadcast TWT element’. In present embodiments, the Broadcast TWT Recommendation field 736 is set to a new value, for example the value 5, to indicate the TWT described in Broadcast TWT element 700 is a SCH timeslot TWT (s-TWT), meaning an SCA service period. In that case, the Broadcast TWT information element 700 is also referred to as a SCH TWT information element (s-TWT or S-TWT IE). In other words, a Broadcast TWT Parameter Set field that has the Broadcast TWT Recommendation field value equal to 5 is referred to as a SCH TWT Parameter Set field. This signals whether SCH TWT Parameter Set field 790 is present or not. The Request Type field 730 then comprises a TWT Wake Interval Exponent field 737 and a reserved field 738. Other fields in the “Broadcast TWT Parameter Set” field 725 are used to define time Parameters for a TWT schedule. As the TWT information element describes an S-TWT schedule, those time parameters act as follows. The “Broadcast TWT Parameter Set” field 725 comprises a Target Wake Time (TWT) field 740 that indicates the next time, in microseconds, at which the station participating in the S-TWT schedule should be ready to apply SCA operation during the next S-TWT SP (SCA SP). The “Broadcast TWT Parameter Set” field 725 comprises a Nominal Minimum TWT Wake Duration field 750 that indicates the minimum amount of time that the TWT scheduled STA is expected to be awake since the starting time of the TWT SP in order to complete the frame exchanges for the period of TWT Wake Interval. The TWT Wake Interval of the TWT SP is the value calculated from the TWT Wake Interval Mantissa field 760 and the TWT Wake Interval Exponent field 737. It is expressed in number of units as defined in Wake Duration Unit subfield 712 of Control field 710. In present embodiments, it refers to the duration of a S-TWT Service period (this is the SCH timeslot, SCA SP duration). Other fields in the “Broadcast TWT Parameter Set” field 725 are used to define parameters specific to the Broadcast and nature of the S-TWT SP. As to Broadcast TWT Info field 770 that conveys the identifier of the S-TWT schedule, namely the Broadcast TWT ID field 773 (bTWT ID), that is used to identify the S-TWT SPs belonging to the same S-TWT schedule. This identifier, which is not 0, hence allows an AP to schedule multiple sets of Broadcast TWT SPs with different sets of TWT parameters. The Broadcast TWT Info field 770 comprises a Broadcast TWT Persistence subfield 774 that specify the number of Target Beacon Transmission Times (TBTT) during which the Broadcast TWT SPs corresponding to this SCH (more generally Broadcast) TWT Parameter set are present. Other fields 771 / 772 in the Broadcast TWT Info field 770 are reserved for an S-TWT element. That means that the Restricted TWT Traffic Info field 780 is absent in an S-TWT). As an option (not shown), the TWT element supports both R-TWT and S-TWT indications, meaning an R-TWT can operate SCA (with indicated SCH channel) during the R-TWT SP. As the described TWT information element provides timing information, the purpose of the TWT Parameter Set field 790 in the “Broadcast TWT Parameter Set” field 725 is to announce the location of an SCH channel. In preferred embodiments, it contains an SCH information field 650, as described in Figure 6. The S-TWT scheduling AP can include one or more TWT parameter set fields in the TWT information element, and each S-TWT parameter set may indicate a periodic occurrence of S-TWTs. Note that AP includes a first broadcast S-TWT element in a Beacon frame that is scheduled at a TBTT. Optionally, several channels are provided in one S-TWT information element: the header 610 is present, along with a set of SCH information 650. In that case, the Target Wake Time (TWT) field 740 indicates the next time of the first SCH time-slot, and the SCH time-slot duration is same for all slots. As a result, SCH Target wake time (SCH-TWT) allows an AP to manage activity in the BSS in order to organize contention between STAs on a secondary channel when the primary channel is busy. By using a broadcast nature of TWT, targeted group of STAs (possibly all) in the BSS are aware of the values of TWT parameters of the S-TWT proposal. Frames transmitted during a S-TWT SP are carried in any PPDU format supported by the pair of STAs. A S-TWT scheduling AP includes a broadcast S-TWT element in the Beacon frame. An S-TWT scheduling AP may include an S-TWT information element with the Negotiation Type subfield equal to 3 in a (Re)Association Response frame or in a TWT setup frame to assign the recipient STA to a broadcast TWT schedule without having received a request from the STA to become a member of the broadcast TWT schedule if that STA has set the Broadcast TWT Support field of HE Capabilities information element it transmits to 1 and also support SCA operation. A non-AP STA shall obtain TWT parameter values from the most recently received TWT information element carried in a Beacon, Probe Response, or (Re)Association Response frame from its associated AP. Individual STAs may have different membership in broadcast S-TWTs as a result of step 501. AP may also notify any update of its S-TWT to the already-associated non-AP STAs, as a critical update by following the BSS parameter critical update procedure defined in the IEEE 802.11be D5.0. That means a non-AP STA, which has detected the critical bit but did not receive any S-TWT prior to it, will refrain to use the SCA mechanism until the next beacon frame indicative of the new SCH allocation. Figure 8 illustrates a timeline example, showing SCA operation over two SCH channels in an embodiment of the invention. An exemplary 80MHz operation band is shown, composed of four 20MHz channels. Channel A is the primary channel. According to embodiments, two SCH timeslots, or SCA SPs, are represented SCA SP T operates between times TO and T1 and SCA SP ‘i+1 ’ operates between times T1 and T3, each SCA SP specifies respectively a SCH channel D and C to be used for SCA mechanism. Therefore, when the primary channel is already busy, as illustrated by communication 810 that may be inside the BSS or in another BSS in the same area, called an Overlapping BSS, OBSS, a non-AP STA that intends to communicate will try to perform SCA operation on an appropriate secondary channel. This SCH to be used for SCA mechanism is prescribed by the SCA SPs. During SCA SP T, the 20MHz channel D is mandated by AP for example. After a successful SCA access on channel D, communication 820 can occur. Note that Acknowledgment frames are not illustrated for the sake of illustration. The communication on SCH channel lasts at most until the end of the communication on the primary channel. That means the communication of a given SCH channel will not stop at T1 even if the SCA SP, and therefore the SCH channel to be used for SCA, has changed at T1. This demonstrates the difference between legacy TWT SPs used for communication and S-TWT SPs used for SCA operation. As illustrated by SCA_GI 850, a SCA switching delay indicating a time or duration needed by the transmitting station to switch from one SCH to another SCH to perform SCA may be optionally considered. To that end, the SCA capabilities information of each non-AP STA may include an SCH switching delay field conveying such delay (This can be seen as a Guard Time or Interval before being able to operate next SCA slot). Since the duration may differ from one station to the other, they have to be notified back to whole BSS. The AP may use the maximum of the switching delays of the non-AP STAs as the prescribed switching delay. For this, the AP computes SCA_GI = MAX {channel switch time of non-AP STAs}. SCA switching delay field may consist of m bits, wherein the possible field values correspond to respective predefined switching delay times. For example, SCA switching delay may have 3 bits and each field value from 0 to 7 may have its pre-defined time such as 0: Ops, 1: 16ps, 2: 32ps, 3: 64ps, ... 7: 1,024ps. Bit size and each matching between a field value and a switching delay time are not limited to this example and may vary. During this guard time, no SCA medium access trial is started to be processed by the non-AP STAs. Then, when the T2 date has expired, STAs have to consider the SCH channel corresponding to SCA SP ‘i+1’ (namely SCH channel C with a 40MHz bandwidth) for any further communication. This is illustrated by transmission 821 that occurs in secondary channel C when communication 811 has own the primary channel during the period T2-T3. Figure 9 illustrates, using a flowchart, the main steps to operate SCA operation at both the AP and the non-AP STA in an embodiment of the invention. These steps are performed when SCA operation is triggered. SCA operation is triggered when the primary channel is not accessible due to busy CCA (including both Packet Detection and Energy Detection) as already mentioned above with reference to step 201. The processes of Figure 9 manage the access to the medium on a SCH for SCA operation, as well as the switching from one SCH to another one for SCA operation when the SCA SP is over, and the switching back to the primary channel. At step 900, the station (any of AP and non-AP STA) selects one candidate SCH among the prescribed SCH according to the previously received SCH timeslot information (600 or 700 in figures 6 and 7 for example). At step 901, the station conducts backoff procedure for access on the selected SCH. At step 902, the station checks whether an interference (i.e., a frame) is detected during one backoff counter period. In the affirmative, next step is step 904 where the station waits for the end of interference. If no other interference is detected, next step is step 905. At step 905, the station checks whether its backoff counter reached 0. In the negative, the station decrements the backoff counter at step 906 and goes back to step 902. In the affirmative, the station that is granted access to the wireless medium over the selected SCH performs SCA operation over the SCH channel, such as transmission of data, packets or frame, at step 907 and then goes to step 910. Step 903 illustrates the checking that the current SCH is still available, meaning allowed to be accessed with regards to the SCA guard interval. In negative case, next step is step 908 where the station checks for a next prescribed SCH available in the agreed SCH timeslots (the period of time is covered by an existing SCH timeslot). In the negative, the process ends. In the affirmative, the station selects the next SCH and proceeds, at step 909, with a switching of channel to the selected next SCH. Switching SCH may consider the values indicated in the SCH switching delay field for both AP and non-AP STA. After the SCH has been switched at step 909, the process loops back to step 901. At step 910, the station terminating SCA operation returns to normal operation on the primary channel, meaning it switches back to the primary channel. The station may have further data, packets or frame to transmit. To do so, the station may loop back to step 900 to continue further SCA on the current SCH. 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 communication network comprising an Access Point, AP, operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the method comprising by the AP, the steps of:- scheduling a series of at least one Secondary Channel Access, SCA, Service Period, SP, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time;- transmitting to the group of non-AP stations an information describing the series of at least one SCA SPs.

2. The method of claim 1, wherein the scheduling of the series of at least one SCA SPs is based on received SCA capabilities from the non-AP stations of the group.

3. The method of claim 1 or 2, wherein the scheduling of the series of at least one SCA SPs is relative to Target Beacon Transmission Time, TBTT.

4. The method of claim 1 or 2, wherein the scheduling of the series of at least one SCA SPs is relative to a timing synchronisation function.

5. The method of any claim 1 to 4, wherein the information is transmitted in an information element in a management frame.

6. The method of claim 5, wherein the information element is a SCH Info information element (600) comprising for each SCA SP a starting time (630), a duration (640) and a number (662) identifying the SCH channel to be used for SCA mechanism.

7. The method of claim 6, wherein the information further comprises an operating bandwidth (663) for indicating several SCH channels to be used for SCA mechanism.

8. The method of claim 5, wherein the information element is a Target Wake Time, TWT, information element (700) comprising an indication (736) indicating that the corresponding TWT SP is a SCA SP.

9. The method of claim 9, wherein the TWT information element (700) comprises for each SCA SP a starting time (725), a duration (750) and a SCH (790) to be used for SCA mechanism.

10. The method of claim 5, wherein the information comprises a guard time (612, 850) indication for indicating a switching interval between two successive SCA periods during which no SCA communication may be initiated.

11. A method of communication in a wireless communication network comprising an Access Point, AP, operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the method comprising by a non-AP station, the steps of:- receiving from the AP an information describing a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; and- during the period of time, when the primary channel is busy, switching to the at least one secondary channel for communicating with the AP.

12. 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 11, when loaded into and executed by the programmable apparatus.

13. A computer-readable storage medium storing instructions of a computer program for implementing a method according to any one of claims 1 to 11.

14. A computer program which upon execution causes the method of any one of claims 1 to 11 to be performed.

15. An Access Point, AP, device in a wireless communication network, the AP operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station, the AP comprising a processor configured for:- scheduling a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time;- transmitting to the group of non-AP stations an information describing the series of at least one SCA SPs.

16. A non-Access Point, AP, device in a wireless communication network comprising an AP operating an operating channel comprising a primary channel and one or more Secondary Channels, SCHs, to communicate with a group of at least one non-AP station comprising the non-AP device, the non-AP device comprising a processor configured for:- receiving from the AP an information describing a series of at least one Secondary Channel Access, SCA, Service Periods, SPs, each SCA SP being associated with at least one secondary channel to be used for SCA and a period of time; and- during the period of time, when the primary channel is busy, switching to the at least one secondary channel for communicating with the AP.29

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