TECHNICAL FIELD The present invention relates to a device for transmitting a trigger message, e.g. an Access Point (AP) transmitting a Null Data Packet (NDP) trigger frame. Further, the present invention relates to a device, e.g. a station (STA), for receiving such a trigger message, and for responding based on the received trigger message. Accordingly, the two devices can implement a NDP feedback mechanism. The present invention relates also to corresponding methods for transmitting and receiving the trigger message.

BACKGROUND

The NDP feedback (report) mechanism is defined in the P802.11 draft 2.0, and allows a device, e.g. an AP, to trigger and collect short feedbacks from multiple other devices, e.g. from high- efficiency (HE) STAs. The NDP feedback mechanism allows a relatively large number of STAs to respond to a trigger frame sent by the AP, using only a single bit to indicate that they require an UL resource. The feedbacks (e.g. resource requests) are typically sent within an NDP feedback frame in response to the trigger frame. The NDP feedback mechanism is more efficient than e.g. the HE Trigger-Based PPDU (according to which an AP triggers, individually, each of a plurality of STAs to transmit data in the uplink).

Currently, the NDP feedback mechanism is used only by associated STAs (i.e. associated to the AP) for requesting an uplink resource. The number of available NDP feedback resources in a frame depends on the bandwidth and based on a‘Multiplexing Flag’ field. For example, there are either 18 or 36 resources for a 20MHz bandwidth, or 36 or 72 resources for a 40MHz bandwidth, etc.

The trigger frame may specifically be a NDP feedback report poll, which should be transmitted (in the downlink) in the same channel as the requested NDP feedback (uplink). Normally, the trigger frame is transmitted in the‘Legacy duplicate mode’, which means that for bandwidths wider than 20MHz, the NDP Feedback Report Poll is duplicated on each 20MHz.

A user info field within said NDP feedback report poll - as it exists in the current version of the 1 lax standard - is shown in FIG. 7(A). As shown, there are currently 9+7 reserved bits in the NDP feedback report poll. The values supported in the‘Feedback Type’ sub-field (as used today) are shown in FIG. 7(B). That is, currently only resource requests for associated STAs are possible.

Furthermore, the 802.11 standard defines the so-called‘active scanning’ procedure, according to which a non-associated STA may send a probe response request frame that may be responded with a probe response frame by an AP operating on the same, typically 20MHz, channel. The probe response frame is typically transmitted by the AP in the Legacy (20MHz) mode, and is hence only accessible to STAs operating in the same channel as the AP. Moreover, the receiver (destination) address of the probe response frame is typically the address of the soliciting STA.

In High Density (HD) environments (e.g. in large halls, stadiums, etc.) the network overhead from such a network discovery process performed by non-associated STAs may be quite significant. STAs using the above-described active scanning procedure transmit the probe response request frame on each channel at least two times, while every AP operating in the channel is expected to respond with the probe response frame for every received probe response request. It is hence desirable to reduce the large amount of time currently occupied by probe response requests and response messages.

The conventional NDP feedback mechanism does not indicate within the NDP trigger frame where the primary 20MHz channel is, or where secondary e.g. 20/40/80MHz channels are.

The conventional NDP feedback mechanism thus has the following problem regarding non- associated STAs: The NDP trigger frame may be transmitted (duplicated) on multiple 20MHz channels, so that in principle non-associated STAs can decode the NDP trigger frame on any 20MHz channel on which they are listening, even if it is not the primary channel. However, the STAs do not know if the specific 20MHz channel is a primary channel or a secondary channel. Accordingly, non-associated STAs that would respond on the channel they are listening on, would also expect a response on the same channel. Unfortunately, the AP typically wants to transmit the response to unassociated STAs on the primary 20MHz channel only, in order to limit the number of responses on other channels, and to reduce the collision probability. As a consequence, non-associated STAs that are not by chance communicating on the primary channel of the AP cannot receive the response of the AP.

A conventional mechanism also considered for unassociated STAs is to allocate for them a temporary Association ID (AID), until the association procedure is complete and they get a ‘fixed’ AID. According to this mechanism, the NDP feedback resources (and as a consequence their tie to the temporary AID numbering) are numbered from the top to the bottom of all resources (or from bottom to top). However, the primary channel may be located anywhere within the group of 20MHz channels (not necessarily at the top). Problematically, however, non-associated STAs, do not know where the primary/secondary channels are, so they cannot compute which resource/temporary AID they are allocated. For example, the AP may use a 40MHz bandwidth where the primary 20MHz channel is at the bottom and the secondary 20MHz channel is at the top, for which the temporary AID numbering for the primary channel will be the opposite of the case where the primary channel is on top of the secondary 20MHz channel.

SUMMARY

In view of the above-mentioned problems and disadvantages, the present invention aims to improve the conventional mechanism. The present invention has thereby the object to provide devices that enable an improved NDP feedback mechanism and an improved probe response request and probe response procedure. In particular, a probe response request and any feedback from non-associated devices should be supported, regardless of the total bandwidth, and regardless of whether the non-associated device is currently in a primary channel or in a non primary channel within the total bandwidth.

The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.

In particular, in order to improve the NDP feedback mechanism, the present invention proposes that a device, e.g. an AP, indicate via NDP trigger to other devices, e.g. STAs, where a primary channel is, and preferably also where secondary channels are. Further, in order to improve the probe response request and response procedure, the present invention allows the other devices, e.g. the STAs, to send a probe response request or any other unassociated request for resource via the NDP feedback mechanism. That is, the NDP feedback is advantageously enhanced to support probe response requests.

A first aspect of the present invention provides a device configured to transmit a trigger message on a plurality of channels within a determined bandwidth, wherein the trigger message indicates a primary channel of the plurality of channels.

In other words, the trigger message includes information, e.g. a number of bits, identifying the primary channel. The device of the first aspect can accordingly implement the information into the data structure of the trigger frame. The device of the first aspect is preferably an AP. Notably, the determined bandwidth may include multiple channels of pre-defined bandwidth, with one primary channel and at least one secondary channel. Any device, preferably a STA, receiving the trigger frame can determine the primary channel according to the information. Advantageously, the receiving devices can thereupon decide to switch to the primary channel, in order to receive messages from the device of the first aspect, which are only communicated on the primary channel. For instance, even if the receiving device is not associated to the device of the first aspect, it can perform a probe response request and probe response procedure, since it knows the primary channel.

In particular, the trigger message may be an NDP trigger frame sent by an AP, and a non- associated STA determining the primary channel from the trigger frame can send a NDP feedback to the AP that may include a probe response request. The non-associated STA could then receive the following probe response on the primary channel. This is regardless of the channel the non-associated STA currently communicates on. Since in this case the probe response request and probe response procedure is included in the NDP feedback mechanism, probe response requests and probe responses transmitted during conventional network discovery are significantly reduced (hence reducing channel usage), and collision probability is reduced as well. Furthermore, the NDP feedback resources are more efficiently used.

In an implementation form of the first aspect, the trigger message further indicates if at least one resource is allocated within the plurality of channels for a probe response request.

In a further implementation form of the first aspect, the trigger message further indicates a number of resources within the plurality of channels that are allocated for a probe response request. In a further implementation form of the first aspect, the number of resources allocated for the probe response request are on the primary channel only, or are at an end of the determined bandwidth, or are on each of the plurality of channels.

In other words, the trigger message may include information, e.g. a number of bits, identifying the resources and/or number of resources. The device of the first aspect can accordingly implement this information into the data structure of the trigger frame. The information optimizes the implementation of the probe request response and probe response procedure into the NDP feedback mechanism. Devices receiving the trigger message know when to include probe response requests into their feedback. Further, they know how many resources are available. Accordingly, the probe response request and probe response procedure can be implemented efficiently.

In a further implementation form of the first aspect, the trigger message further indicates a location of at least one secondary channel of the plurality of channels with respect to the primary channel. In other words, the trigger message may include information, e.g. a number of bits, identifying the secondary channel or secondary channels. The device of the first aspect can accordingly implement this information into the data structure of the trigger frame. The information can be used by a receiving device, e.g. a STA, to compute which resource/temporary AID it is allocated. In a further implementation form of the first aspect, the trigger message is a NDP feedback report poll including a user info field indicating at least the primary channel.

Thus, a solution tailored for the NDP feedback mechanism is provided, which allows non- associated receiving devices to use probe response requests in the NDP feedback, and to listen to probe response on the primary channel. In a further implementation form of the first aspect, 8 bits in the user info field indicate the primary channel, 1 or 2 bits in the user info field indicate the number of resources allocated for a probe response request, 2-6 bits in the user info field indicate the location of the at least one secondary channel, and 2 bits in the user info field optionally indicate the determined bandwidth. This user info field provides a specific and advantageous implementation for the NDP trigger frame.

A second aspect of the present invention provides a device configured to receive a trigger message on one of a plurality of channels within a determined bandwidth, and determine, from the received trigger message, a primary channel of the plurality of channels.

In other words, the device of the second aspect can extract the information in the trigger message about the primary channel, and can thus identify the primary channel. The device may then decide to switch to the primary channel for further communication. For instance, if the device is a non-associated STA that receives the trigger frame from an AP, it can feedback a probe response request and can then wait for the probe response on the primary channel. Accordingly, even if the device of the second aspect is not currently on the primary channel of the AP, it can participate in the probe response request and probe response procedure implemented e.g. into NDP feedback mechanism.

In an implementation form of the second aspect, the device is further configured to determine, from the received trigger message, a number and/or location of resources within the plurality of channels that are allocated for a probe response request.

In other words, the device of the second aspect can extract the information in the trigger message about the resources. Accordingly, the device knows whether it can transmit a probe response requests or another type of unassociated feedback, and on which resource it can transmit a probe response request or other type of unassociated feedback.

In a further implementation form of the second aspect, the device is further configured to determine, from the received trigger message, a location of at least one secondary channel of the plurality of channels with respect to the primary channel.

In other words, the device of the second aspect can extract the information in the trigger message about the secondary channel(s). Accordingly, the device can compute which resource or temporary AID it is allocated.

In a further implementation form of the second aspect, the device is further configured to receive the trigger message on a secondary channel of the plurality of channels, switch to the primary channel determined from the trigger message, and send a feedback message, preferably a NDP feedback on the primary channel. In this manner, the device of the second aspect can participate in probe response request and probe response procedure via NDP feedback, or other types of unassociated feedback and response, regardless on which channel it is currently on.

In a further implementation form of the second aspect, the device is further configured to send the feedback message on the secondary channel, on which it received the trigger message.

That means, the device can send and NDP feedback either on the secondary channel it is on, or on a primary channel it switches to. It preferably switches to the primary channel at least if it intends to receive a probe response.

A third aspect of the present invention provides a system including a device according to the first aspect or any of its implementation forms and at least one device according to the second aspect or any of its implementation forms.

The system accordingly enjoys the advantages and effects described above for the devices of the first and the second aspect.

A fourth aspect of the present invention provides a method comprising, transmitting a trigger message on a plurality of channels within a determined bandwidth, wherein the trigger message indicates a primary channel of the plurality of channels.

In an implementation form of the fourth aspect, the trigger message further indicates if at least one resource is allocated within the plurality of channels for a probe response request.

In a further implementation form of the fourth aspect, the trigger message further indicates a number of resources within the plurality of channels that are allocated for a probe response request.

In a further implementation form of the fourth aspect, the number of resources allocated for the probe response request are on the primary channel only, or are at an end of the determined bandwidth, or are on each of the plurality of channels.

In a further implementation form of the fourth aspect, the trigger message further indicates a location of at least one secondary channel of the plurality of channels with respect to the primary channel. In a further implementation form of the fourth aspect, the trigger message is a NDP feedback report poll including a user info field indicating at least the primary channel.

In a further implementation form of the fourth aspect, 8 bits in the user info field indicate the primary channel, 1 or 2 bits in the user info field indicate the number of resources allocated for a probe response request, 2 bits in the user info field indicate the location of the at least one secondary channel, and 2 bits in the user info field optionally indicate the determined bandwidth.

The method of the fourth aspect provides the same advantages and effects as the device of the first aspect. A fifth aspect of the present invention provides a method comprising, receiving a trigger message on one of a plurality of channels within a determined bandwidth, and determining, from the received trigger message, a primary channel of the plurality of channels.

In an implementation form of the fifth aspect, the method comprises determining, from the received trigger message, a number and/or location of resources within the plurality of channels that are allocated for a probe response request.

In a further implementation form of the fifth aspect, the method further comprises determining, from the received trigger message, a location of at least one secondary channel of the plurality of channels with respect to the primary channel.

In a further implementation form of the fifth aspect, the method further comprises receiving the trigger message on a secondary channel of the plurality of channels, switching to the primary channel determined from the trigger message, and sending a feedback message, preferably a NDP feedback on the primary channel.

In a further implementation form of the fifth aspect, the method further comprises sending the feedback message on the secondary channel, on which the trigger message is received. The method of the fifth aspect provides the same advantages and effects as the device of the second aspect. A sixth aspect of the present invention provides a computer-implemented data structure embodied on a medium, wherein the data structure is for a trigger message for a NDP and includes information indicating a primary channel of a plurality of channels within a bandwidth.

In an implementation form of the sixth aspect, the data structure for the trigger message further includes information indicating if at least one resource is allocated within the plurality of channels for a probe response request.

In a further implementation form of the sixth aspect, the data structure for the trigger message further includes information indicating a number of resources within the plurality of channels that are allocated for a probe response request. In a further implementation form of the sixth aspect, the number of resources allocated for the probe response request are on the primary channel only, or are at an end of the determined bandwidth, or are on each of the plurality of channels.

In a further implementation form of the sixth aspect, the data structure for the trigger message further includes information indicating a location of at least one secondary channel of the plurality of channels with respect to the primary channel.

In a further implementation form of the sixth aspect, the data structure for the trigger message is a data structure for a NDP feedback report poll including a user info field indicating at least the primary channel.

In a further implementation form of the sixth aspect, 8 bits in the user info field indicate the primary channel, 1 or 2 bits in the user info field indicate the number of resources allocated for a probe response request, 2 bits in the user info field indicate the location of the at least one secondary channel, and 2 bits in the user info field optionally indicate the determined bandwidth.

The data structure of the sixth aspect allows to achieve the advantages and effects with the devices of the first aspect and second aspect, respectively.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which

Fig. 1 shows a device according to an embodiment of the present invention. Fig. 2 shows a device according to an embodiment of the present invention.

Fig. 3 shows in (A), (B), and (C) three examples of data structures for a trigger message, particularly examples of user info fields, according to an embodiment of the present invention.

Fig. 4 shows in (A)-(D) four possible message sequences in a system according to an embodiment of the present invention.

Fig. 5 shows a method according to an embodiment of the present invention. Fig. 6 shows a method according to an embodiment of the present invention.

Fig. 7 shows a conventional data structure for a trigger message, particularly in (A) a user info field for the NDP feedback report poll and in (B) values for the‘Feedback type’ subfield of the user info field of (A). DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a device 100 according to an embodiment of the invention. Preferably, the device 100 is an AP. The device 100 is configured to transmit a trigger message 101 on a plurality of channels 102 within a determined bandwidth. Preferably the trigger message is a NDP trigger frame. The trigger message 101 indicates, e.g. by included information 103, a primary channel 104 of the plurality of channels 102. Here, the channels 102 are labelled with PCh (primary channel 104) and SCh (secondary channel), respectively. Preferably, all channels have a pre- defined bandwidth smaller than the determined bandwidth.

FIG. 2 shows another device 200 according to an embodiment of the invention. Preferably, the device 200 is a STA. The device 200 is configured to receive a trigger message 101 on one of a plurality of channels 102 within a determined bandwidth. Preferably, the trigger message is a NDP trigger frame. Further, the device 200 is configured to determine, from the received trigger message 101, e.g. from information 103 included in this trigger message 101, a primary channel 104 of the plurality of channels 102. Again, these channels 102 are respectively labelled with PCh (primary channel 104) and SCH (secondary channel).

The device 100 of FIG. 1 and the device 200 of FIG. 2 may preferably form a system. That is, the trigger message 101 sent by the device 100 of FIG. 1 is the trigger message 101 received by the device 200 of FIG. 2. The device 200 can extract the information 103 concerning the primary channel 104 from the trigger message 101, wherein this information 103 may have been included before by the device 100. The device 200 can then determine from this information 103, which of the channels 102 is the primary channel 104. Accordingly, it can decide to switch to the primary channel 104.

As mentioned before, the trigger message 101 is preferably a NDP trigger frame like a NDP Feedback Report Poll, which preferably includes a data structure like a user info field 300 (details of which are explained below with respect to FIG. 3) indicating 103 at least the primary channel 104. Accordingly, an NDP feedback mechanism can advantageously be implemented in the system, i.e. between the device 100 and the device 200. The device 200 can feedback to the NDP trigger frame 101. Since it knows the primary channel 104, it can feedback with a probe response request, and can receive the probe response from the device 100 after switching to the primary channel 104. In a similar manner, the device 101 can include (details are also explained below with respect to FIG. 3) information 301 in the trigger message 101 concerning an allocation of resources for a probe response request in the plurality of channels 102, and/or information 302 concerning a location of one or more secondary channels 401 (details are explained below with respect to FIG. 4) of the plurality of channels 102 with respect to the primary channel 104, and/or information 303 concerning the determined bandwidth, i.e. the total bandwidth used. The device 200 is accordingly configured to extract the information 301, 302 and/or 303 from the trigger message 101, and to respectively determine the allocation of resources for a probe response request in the plurality of channels 102, and/or the location of one or more secondary channels 401 of the plurality of channels 102 with respect to the primary channel 104, and/or the determined bandwidth.

In other words, the device 100 may further be configured to indicate to one or more devices 200, where the primary channel 104 and optionally where one or more secondary channels 401 are. In addition, the device 100 may be configured to indicate the number and location of resources allocated for a probe response request, thus allowing for instance a group of (non- associated) devices 200 to send a probe response request or any other non-associated request for resource via NDP feedback (NDP response). The device 100 may in turn respond to this group of devices 200 with, for instance, a single probe response message or frame, e.g. on the primary channel 104, using the broadcast Receiver Address (RA). The devices 200 knowing the primary channel 104 may switch to the primary channel 104 to receive the probe response and become associated with the device 100. If the request of any non-associated device 200 was not a probe response request, but a request for a resource, the device 100 may respond by allocating a resource.

FIG. 3 shows examples (A), (B) and (C) of data structures 300 for a trigger message 101 according to an embodiment of the present invention, particularly as preferred example a user info field 300 of the trigger message 101, specifically of a NDP trigger like a feedback report poll. The three examples of (A), (B) and (C) will be explained in detail below.

In each example (A), (B) and (C), the user info field 300 includes - contrary to the conventional user info field that is shown in FIG. 7(B) - information (bits) 103 that indicate the primary channel 104. Preferably, 8 bits are used. Accordingly, any device 200 - including a non-associated device 200 (i.e. one not yet associated with the device 100, e.g. a STA 200 not associated with an AP 100) operating in a non-primary channel - can be informed by the device 100 about the primary channel 104. The device 200 may then decide upon receiving the trigger message 101, whether to switch channel and to continue a network discovery process on the primary channel 104, or to remain in its current non-primary channel.

In particular, any non-associated device 200 may decode the trigger message 101 in a non primary channel (as part of their scanning). Typically (but not necessarily), a probe response is transmitted by the device 100 in the primary channel 104 only. In this case, only a device 200 located in the primary channel 104 would conventionally have the opportunity to associate with the device 100. However, with the indication 103 of the primary channel 104 according to the present invention, any device 200 can in this case handover (where applicable) to the primary channel 104, and wait for the probe response. Alternatively, the device 200 can also decide not to request the probe response and remain on its current channel.

FIG. 4 shows in this respect possible options envisaged by the invention for operation of the device 100 and the device 200 based on the above. FIG. 4 particularly shows messages (e.g. frames) sent on a primary channel 104 (P20) and a secondary channel 401 (S20), respectively. Thereby, messages shown shadowed are transmitted by the device 200, other messages are transmitted by the device 100.

The first option is shown in FIG. 4(A). The device 100 sends a trigger message (NDP trigger) on all channels 102, i.e. the primary channel 104 and the secondary channel 401. The device 200 accordingly sends a NDP response 402 on the secondary channel 401, which may include a probe response request. Then, the device 200 switches to the primary channel 104, as determined from the trigger message 101. The device 200 then waits until the device 100 sends a probe response on the primary channel 104.

The second option is shown in FIG. 4(B). The device 100 sends a trigger message (NDP trigger) on all channels 102, i.e. the primary channel 104 and the secondary channel 401. The device 200 again sends a NDP response 402 on the secondary channel 401, which may include a probe response request. The device 100 sends a probe response 403 on the primary channel 104. However, only afterwards the device 200 switches to the primary channel 104. Accordingly, the device 200 did not receive yet the probe response, and sends a probe response request 400 or another NDP response 402 on the primary channel 104. This is then followed by another probe response 403 of the device 100, which the device 200 can receive on the primary channel 104.

The third option is shown in FIG. 4(C). The first device 100 sends the trigger message 101 again on all primary and secondary channels 102. The device 200 sends again an NDP response 402 on the secondary channel 401, which may include a probe response request. The device 100 then sends a probe response 403 on all channels, which the device 200 may receive on the secondary channel 401. Afterwards, the device 200 may switch to the primary channel 104.

The fourth option is shown in FIG. 4(D). The first device 100 sends the trigger message 101 again on all primary and secondary channels 102. The device 200 immediately switches to the primary channel 104, and sends a probe response request 400 or a NDP response 402, which may include a probe response request on the primary channel 104. Afterwards, the device 100 sends a probe response 403 on the primary channel 104, which can be received on the primary channel 104 by device 200. Returning to FIG. 3, advantageously, in each example (A), (B), (C) the user info fields 300 include also information (bits) 301 that indicate a number of resources allocated for a probe response request. Preferably 1 or 2 bits are used.

Accordingly, the device 200 can thus be informed by the device 100 whether some of the resources within each channel 102 are allocated for a probe response request. For this indication, 1 or 2 bits may be used: For example, 1 bit may indicate, if the last 1 (or 2) resources, within each of the channels 102 are allocated for a probe response request. Alternatively, 2 bits may be used to indicate how many resources (1/2/3/4) within each of the channels 102, are allocated for a probe response request.

There are different alternatives for implementing by the device 100 - and accordingly for interpreting by the device 200 - the indication 301 of the number of resources for a probe response request in the trigger message 101.

In a first alternative, the number of resources for the probe response request are left only on the primary channel 104 (for example, if there are 4x20MHz channels 102 in an 80MHz bandwidth, the resources available for probe response request are allocated only on the primary 20MHz channel). This means that devices 200 on non-primary channels (secondary channels) cannot send a request for probe response.

In a second alternative, the number of resources for the probe response request are left on the last allocations (for the entire bandwidth; for example, if there are 4x20MHz in an 80Mhz bandwidth, the resources available for the probe response request are allocated at the‘end’ of the available bandwidth). This means that devices 200 that are on a channel 102 that is not the last in the determined bandwidth (counting from either top to bottom or bottom-up) cannot transmit the request for probe response.

In a third alternative, the number of resources are left on each of the channels 102. This means that for every used channel, the last resources of it may be allocated for probe response requests.

Returning to FIG. 3, advantageously, in each example (A), (B), (C) the user info fields 300 include also information (bits) 302 that indicates a location of one or more secondary channels 401 with respect to the primary channel 104. The specific implementations of this information 302 can differ for the different examples of the user info field 300. Optionally, the user info field 300 may also - as shown in the example (A) - include information (bits) 303 about the determined bandwidth.

In particular, if the determined bandwidth is wider than the bandwidth of a channel 102, it is advantageous to include at least information 302 indicating where the secondary channels 402 are, and optionally information 303 indicating what the total (determined) bandwidth is. If the total bandwidth equals the bandwidth of a single channel 102, then such information 302, 303 may not be necessary.

The information 302 may be a secondary channel offset (bits 302 in FIG. 3(A) and (B), bits 302a in FIG. 3(C)). That means, information 302 where a secondary channel 401 is relative to the primary channel 104. If the channel bandwidth is e.g. 20MHZ and the determined bandwidth is e.g. 40MHZ, then information about a secondary channel relative (above/below) the primary channel 104 is relevant. If the determined bandwidth is e.g. 80MHz, information about secondary 20/40MHz channel is relevant. If the bandwidth is e.g. IbOMHz, then information about the secondary 20/40/80MHz channel is relevant, preferably including a center frequency of the 80MHz channel. All relevant information may be contained in other subfields. For the information 303 indicating the total bandwidth (bits 303 in FIG. 3(A)), 2 bits are typically sufficient.

In case of a determined bandwidth of e.g. 80/l60MHz (80MHz+80MHz aggregation included), more information may be required regarding the channel center frequency segments of both primary and secondary 80MHz channels. Such information may be carried in a trigger- dependent common info field. Such a trigger-dependent common info field may for example contain the‘Channel Center Frequency Segment O’ and‘Channel Center Frequency Segment 1’ sub- fields.

In FIG. 3, the first user info field 300 example (A) is suitable directly for bandwidths of 20MHz and 40MHz. For bandwidths of 80MHz and l60MHz, additional information can be carried in a trigger-dependent common info field (carrying channel center frequency segments mentioned above).

The second user info field 300 example (B) is the same as in example (A), but without the bandwidth subfield (not necessarily required, as it may be carried in other signals). The third user info field 300 example (C) is suitable directly for bandwidth of 20/40/80MHz. For a bandwidth of l60MHz, additional information can be carried in a trigger-dependent common info field (carrying channel center frequency segments mentioned above).

FIG. 5 shows a method 500 according to an embodiment of the invention. The method 500 is preferably implemented in a device 100 as shown in FIG. 1, more preferably in an AP. The method 500 includes a step 501 of transmitting a trigger message 101 on a plurality of channels 102 within a determined bandwidth. The method 500 further includes a step 502 of indicating in the trigger message 101 a primary channel 104 of the plurality of channels 102, e.g. by including information bits about the primary channel 104 in the trigger message 101. That is, the transmitted trigger message 101 indicates the primary channel 104. The trigger message 101 is preferably a NDP trigger frame.

FIG. 6 shows a method 600 according to an embodiment of the invention. The method 600 is preferably implemented in a device 200 as shown in FIG. 2, more preferably in a STA. The method 600 includes a step 601 of receiving a trigger message 101 on one of a plurality of channels 102 within a determined bandwidth. Further, the method 600 includes a step 602 of determining, from the received trigger message 101, a primary channel 104 of the plurality of channels 102. The trigger message 101 is preferably a NDP trigger frame.

In summary, the present invention achieves various advantages over conventionally used mechanisms. In particular, by enabling devices 200 to request a probe response via the NDP feedback mechanism (by including a probe response request into a NDP response), several benefits are achieved. A significant reduction of probe response request and probe response messages transmitted during network discovery is achieved, which improves channel usage and reduces collision probability. Further, efficient reuse of NDP feedback resources that are not allocated to associated devices 200 is enabled. Further, a transmission of the NDP trigger frame variant of the trigger message 101 in legacy duplicate mode allows non-associated devices 200 even when located in secondary channels 401 to discover the device 100 not yet collocated with them, particularly without scanning all the channels 102. Hence, the channel discovery time is significantly reduced. Further, the possibility of the“Probe Storm” effect in HD environments is greatly reduced or even eliminated. Furthermore, even unassociated devices 200 (e.g. STAs) can now know the location of the primary channel 104 and the secondary channels 401 , so that they can either transmit their probe request response or request resource on an existing (non-primary) channel or switch to the primary channel 104 and continue there. The devices 200 are also allowed to align to the temporary AID informed by the device 100, by means of providing them preferably with the locations of secondary channels 401 with respect to the primary channel 104.

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word “comprising” does not exclude other elements or steps and the indefinite article“a” or“an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.