GB2542818A - Methods and systems for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group - Google Patents

Abstract

There are provided methods and systems for reserving a transmission opportunity (TXOP) for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network comprising a sending node STA1, an addressee node STA2 and other nodes STA3, STA4. A node STA3 detects at least one request control frame (RTS 1210) for initiating a TXOP reservation for the collaborative group, each RTS being sent on a sub-channel 300-1 to 300-4, by the sending node STA1 to the addressee node STA2, the RTS calling for a first response control frame (CTS 1220) from the addressee node STA2. Node STA3 determines, based on said at least one detected RTS and any detected first CTS, a sub-channel belonging to the composite channel on which no first CTS has been sent and sends a second CTS 1230 on said determined second sub-channel to a collaborative node STA4, for exchanging data on it during the TXOP.

Description

METHODS AND SYSTEMS FOR RESERVING A TRANSMISSION OPPORTUNITY FOR A PLURALITY OF WIRELESS COMMUNICATION DEVICES BELONGING TO A COLLABORATIVE GROUP

FIELD OF THE INVENTION

The present invention relates generally to communication networks and more specifically to methods and devices for data communication over a communication network using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), the network being accessible by a plurality of node devices.

BACKGROUND OF THE INVENTION

Wireless local area networks (WLANs), such as a wireless medium in a communication network using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA), are founded on the principles of collision avoidance. Such networks may also conform to a communication standard such as a communication protocol of 802.11 type e.g. Medium Access Control (MAC).

The IEEE 802.11 MAC standard defines the way WLANs must work at the physical and medium access control (MAC) level. Typically, the 802.11 MAC (Medium Access Control) operating mode implements the well-known Distributed Coordination Function (DCF) which relies on a contention-based mechanism based on the so-called “Carrier Sense Multiple Access with Collision Avoidance” (CSMA/CA) technique.

The 802.11 medium access protocol standard or operating mode is mainly directed to the management of communication nodes (or stations) waiting for the wireless medium to become idle so as to try to access to the wireless medium.

The network operating mode defined by the IEEE 802.11ac standard provides very high throughput (VHT) by, among other means, moving from the 2.4GHz band to the 5GHz band. This is because the 2.4GHz band is deemed to be highly susceptible to interference. This allows for wider frequency contiguous channels of 80MHz to be used, two of which may optionally be combined to get a 160MHz channel as operating band of the wireless network.

The 802.11ac standard also provides control frames such as the Request-To-Send (RTS) and Clear-To-Send (CTS) frames, involved in a well-known RTS/CTS handshake, to allow reservation of composite channels of varying and predefined bandwidths of 20, 40 or 80MHz, the composite channels being made of one or more sub-channels that are contiguous within the operating band.

The 160MHz composite channel is possible by the combination of two 80MHz composite channels within the 160MHz operating band. The control frames (RTS and CTS) specify the channel width (bandwidth) for the targeted composite channel. A composite channel therefore comprises a primary sub-channel on which a given node performs EDCA backoff procedure to access the medium, and at least one secondary sub-channel. Both primary and secondary sub-channels may be of 20MHz. The primary sub-channel is used by the communication nodes to sense whether or not the sub-channel is idle, and the primary sub-channel can be extended using the secondary sub-channel or sub-channels to form a composite channel. Given a tree breakdown of the operating band into elementary 20MHz sub-channels, some secondary sub-channels are named tertiary or quaternary sub-channels.

According to the 802.11ac standard, all the transmissions, and thus the possible composite channels, include the primary sub-channel. This is because the nodes perform full Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) and Network Allocation Vector (NAV) tracking on the primary sub-channel only. The other sub-channels are assigned as secondary sub-channels, on which the nodes have only capability of Clear Channel Assessment (CCA), i.e. detection of an availability state/status (idle or busy) of said secondary sub-channel. A node (also referred to as a station) is allowed to use as much channel capacity (or bandwidth, i.e. of (sub-)channels in the composite channel) as is available. The constraint is that the combined sub-channels need to be contiguous within the operating band for a node having a single antenna (or single spatial stream).

However, noise or interference even on a small portion (e.g. on one of the 20MHz sub-channels) of the composite channel may substantially reduce the available bandwidth of the composite channel to only 40 or 20MHz, since the resulting reserved bandwidth must meet the 20MHz, 40MHz, 80MHz or 160MHz channel configurations allowed by the standard.

Indeed, the 802.11ac standard only allows a restricted number of composite channel configurations, i.e. of predefined subsets of 20MHz sub-channel that can be reserved by the 802.11ac nodes to wireless communicate and thus transmit data. These configurations correspond to contiguous composite channels of 20, 40, 80MHz bandwidth in the case of single antenna node devices.

Consequently, the non-contiguous sub-channel or sub-channels within the operating band are not reachable for wireless communication, although they are idle and thus free.

Thus, a major deficiency of the 802.11ac standard spectral usage lies in that channel utilization is limited to contiguous channels for a single-antenna device. However, the more channels used the greater probability of co-channel operation, especially upon a large number of competing nodes (saturated traffic scenarios, also called dense network scenarios).

This deficiency is even more problematic in a hidden node configuration, wherein a sub-part of a group of nodes may be impacted by the presence of a hidden node that only some nodes can see. It is recalled that the hidden node problem occurs when a node is visible from a first node (for instance a wireless access point), but not from other nodes communicating with that first node (wireless access point).

Due to the hidden node problem, the RTS/CTS handshake may provide partial information of the channel occupancy through the different spatial positions of the nodes since there is a mismatch between the different nodes of the same cell concerning the availability of the medium.

When a sub-channel is seen as busy by a given node and free by the other nodes, the 802.11 standard may for instance allow a communication on the 20MHz primary sub-channel only although the 80MHz are free for the other nodes.

As a result, this leads to an even less optimized usage of the wireless channel capacity, since the disturbance of one node may waste a huge part of spectrum resources.

There is therefore a need to optimize the usage of free sub-channels in the case of hidden node(s), in a context of single antenna devices.

SUMMARY OF THE INVENTION

The present invention has been devised to address one or more of the foregoing concerns.

In this context, according to a first aspect of the invention, there is provided methods for reserving a transmission opportunity (TXOP) for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes.

Such a method comprises, at one of said other nodes: detecting at least one request control frame for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel, by the sending node to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity (TXOP) for the collaborative group on this first subchannel; determining, based on said at least one detected request control frame and any detected first response control frame, at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames; and sending a second response control frame on said at least one determined second sub-channel to a node of the collaborative group, for exchanging data on said at least one determined second sub-channel during said transmission opportunity (TXOP).

Such a method comprises, at said sending node: transmitting at least one request control frame for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity (TXOP) for the collaborative group on this first subchannel; and detecting a second response control frame on at least one second sub-channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, the at least one second sub-channel belonging to said composite channel and said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity (TXOP).

Such a method comprises, at said addressee node: receiving at least one request control frame for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity (TXOP) for the collaborative group on this first sub-channel; and - detecting a second response control frame on at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity (TXOP).

Thanks to the invention, the usage of free or partially free channels composed of at least one sub-channel in a context of single antenna devices is improved, in particular in the case of a hidden node disturbing one of the devices of the collaborative group that are involved in the handshake mechanism for channel reservation.

This is first achieved by requesting a reservation of a transmission opportunity for more than one first node within the collaborative group. As a result, it avoids nodes other than the first node to repeat the RTS/CTS sequence as they can transmit data within the reserved transmission opportunity.

This is also achieved by initiating a transmission of data by a node of the collaborative group on at least one sub-channel (second sub-channel) detected as being busy or not useful by at least one node involved in the handshake mechanism.

This may for example happen because it is a 20 MHz sub-channel and the contiguous sub-channel is seen as busy while a 40 MHz channel is needed for the communication. In this case, the isolated 20 MHz sub-channel is not useful and thus the addressee node may not send a CTS frame on it because there is no need to reserve this sub-channel. Thanks to the present invention, even if this sub-channel is not reserved during operation of the handshake mechanism between the sending node and the addressee node, other devices of the collaborative group can reserve it during the TXOP and use it for exchanging (smaller) data between them.

Also, when the sending node (respectively the addressee node) sees a sub-channel as busy, it does not send an RTS frame (respectively no CTS frame) on it. Consequently, the sub-channel is not reserved during operation of the handshake mechanism between the sending node and the addressee node, but thanks to the present invention, it can be reserved by other devices seeing this sub-channel as free so that they can use the newly reserved sub-channel for their own purpose, without attempting a new medium access procedure. Thus, the bandwidth occupancy is improved.

Optional features of the invention are further defined in the dependent appended claims.

According to embodiments, the at least one first response control frame is sent on a plurality of contiguous first sub-channels that form an aggregated channel.

The sending node can transmit data to the addressee node within the reserved transmission opportunity, even if one or more sub-channels are seen as busy by the sending node or the addressee node.

According to embodiments, the sending node transmits data to the addressee node on at least one of the first sub-channels on which a first response control frame has been received.

According to embodiments, transmitting data to the addressee node is done only on at least one first sub-channel having a quality higher than a given threshold.

For example, the quality of a sub-channel may be analyzed based on the Bit Error Rate (BER) or a Signal on Noise ratio (SNR) measured on the sub-channel.

According to embodiments, transmitting data from the sending node to the addressee node starts as soon as the at least one said first response control frame is received by the sending node.

The primary communication between sending node and addressee node may thus start without waiting for response control frames from the other nodes. The occupancy of the medium is thus optimized without this delaying the primary communication.

According to embodiments, the second response control frame is sent during the transmission of data from the sending node to the addressee node.According to embodiments, no second response control frame is sent on a first sub-channel seen as idle by both sending node and addressee node.

This allows avoiding to send an unnecessary control frame (second control frames) on a first channel that is already reserved for the collaborative group.

According to embodiments, the second response control frame is sent only on the at least one second sub-channel.

Therefore, an additional response control frame is sent only for a subchannel that has not been reserved yet.

According to embodiments, the at least one second sub-channel comprises a plurality of second contiguous sub-channels.

Advantageously, several nodes may occupy the granted composite channel, in such a way that each node occupies a contiguous bandwidth as required for single antenna devices.

According to embodiments, the first and second response control frames are sent within a timeout period defining a duration during which a transmission opportunity reservation may be initiated in response to the at least one request control frame from the sending node.

The band reservation is thus performed by the issue of the request control frames.

According to embodiments, the size of the composite channel is indicated in each of the at least one request control frame.

It is thus possible for each listening node to know the targeted composite channel and also to know whether the actually reserved sub-channel is smaller than the targeted composite channel or not. In this case, there may be one or more subchannels to be reserved by means of second response control frames.

According to embodiments, the method further comprises a step of determining, for each sub-channel of the composite channel and for each node of the collaborative group, an availability status representing the availability of the subchannel for said node.

Thanks to the determination of availability status, each representing the availability of a given sub-channel of the composite channel for a given node of the collaborative group, during operation of the RTS/CTS handshake, it is possible to know, for a given sub-channel of the composite channel, for which nodes it seems free and for which it seems busy.

According to embodiments, the method further comprises, at the other node, detecting another second response control frame sent by a second other node different from the sending and addressee nodes, on the at least one second subchannel, and starting a data exchange with the second other node using the at least one second sub-channel during said transmission opportunity.

Advantageously, there may be a transmission of data between two nodes different from the sending and addressee nodes, in parallel to the primary communication between the later.

According to embodiments, the second response control frame is CTS-to- self.

According to embodiments, the second response control frame comprises a target address corresponding to the collaborative group, thereby allowing broadcasting the second response control frame to all the nodes of the collaborative group.

According to a second aspect of the invention, there is provided a system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes.

According to embodiments, said at least one other node is configured to: detect at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on first sub-channel belonging to said composite channel, by the sending node to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; - determine, based on said at least one detected request control frame and any detected first response control frame, at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames; and - send a second response control frame on said at least one determined second subchannel to a node of the collaborative group, for exchanging data on said at least one determined second sub-channel during said transmission opportunity.

According to embodiments, said sending node is configured to: - transmit at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and - detect a second response control frame on at least one second sub-channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, the at least one second sub-channel belonging to said composite channel and said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

According to embodiments, said addressee node is configured to: receive at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and detect a second response control frame on at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

The second aspect of the present invention provides advantages similar to the first aspect above-mentioned.

The invention also concerns a method for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network as described substantially herein with reference to Figures 10, 11, 12, 13, 14, 15, 16, 17 and 18 of the accompanying drawings, and a system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network as described substantially herein with reference to Figures 10,11,12, 13,14, 15,16, 17 and 18 of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 illustrates a typical wireless communication system in which embodiments of the invention may be implemented;

Figure 2 is a timeline schematically illustrating a conventional communication mechanism according to the IEEE 802.11 standard; - Figure 3a illustrates 802.11ac channel allocation that support channel bandwidths of 20MHz, 40MHz, 80MHz or 160MHz as known in the state of the art; - Figure 3b illustrates an example of 802.11ac multichannel transmission opportunity on an 80MHz channel as known in the state of the art;

Figure 4 illustrates the fallback mechanism in an 80MHz operating band as known in the state of the art;

Figure 5 illustrates three examples of dynamic fallback to narrower channel widths in the presence of co-channel interference or noise that only affects a portion of the larger channel;

Figure 6 illustrates a hidden node configuration and the corresponding channel allocation;

Figure 7 shows a schematic representation a communication device or node in accordance with embodiments of the present invention;

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

Figure 9 illustrates the principle of multiple node simultaneous transmission;

Figure 10 shows an exemplary communication timeline illustrating embodiments of the present invention;

Figure 11 shows, using a flowchart, steps of an algorithm for optimizing channel usage in multi-channel wireless network in accordance with an embodiment of the present invention;

Figures 12 to 18 show exemplary communication timelines illustrating various embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention is now described by means of specific non-limiting exemplary embodiments and by reference to Figures.

Figure 1 illustrates a communication system in which several communication nodes exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN).The radio transmission channel 100 is defined by an operating band, for instance with a bandwidth available for the communication nodes.

Access to the shared radio medium to send data frames is based on the CSMA/CA technique, for sensing the carrier and avoiding collision by separating concurrent transmissions in space and time.

Carrier sensing in CSMA/CA is performed by both physical and virtual mechanisms. Virtual carrier sensing is achieved by transmitting control frames to reserve the medium prior to transmission of data frames.

Next, a source node first attempts through the physical mechanism, to sense a medium that has been idle for at least one DIFS (standing for DCF InterFrame Spacing) time period, before transmitting data frames.

However, if it is sensed that the shared radio medium is busy during the DIFS period, the source node continues to wait until the radio medium becomes idle. To do so, it starts a countdown backoff counter designed to expire after a number of timeslots, chosen randomly between [0, CW], CW (integer) being referred to as the Contention Window. This backoff mechanism or procedure is the basis of the collision avoidance mechanism that defers the transmission time for a random interval, thus reducing the probability of collisions on the shared channel. After the backoff time period, the source node may send data or control frames if the medium is idle.

One problem of wireless data communications is that it is not possible for the source node to listen while sending, thus preventing the source node from detecting data corruption due to channel fading or interference or collision phenomena. A source node remains unaware of the corruption of the data frames sent and continues to transmit the frames unnecessarily, thus wasting access time.

The Collision Avoidance mechanism of CSMA/CA thus provides positive acknowledgement (ACK) of the sent data frames by the receiving node if the frames are received with success, to notify the source node that no corruption of the sent data frames occurred.

The ACK is transmitted at the end of reception of the data frame, immediately after a period of time called Short InterFrame Space (SIFS).

If the source node does not receive the ACK within a specified ACK timeout or detects the transmission of a different frame on the channel, it may infer data frame loss. In that case, it generally reschedules the frame transmission according to the above-mentioned backoff procedure. However, this can be seen as a bandwidth waste if only the ACK has been corrupted but the data frames were correctly received by the receiving node.

To improve the Collision Avoidance efficiency of CSMA/CA, a four-way handshaking mechanism is optionally implemented. One implementation is known as the RTS/CTS exchange, defined in the 802.11 standard.

The RTS/CTS handshake comprises exchanging control frames to reserve the radio medium prior to transmitting data frames during a transmission opportunity called TXOP in the 802.11 standard as described below, thus protecting data transmissions from any further collisions.

Figure 2 illustrates the behaviour of three groups of nodes during a conventional communication over the 802.11 medium: transmitting or sending or source node 20, receiving or addressee or destination node 21 and other nodes 22 not involved in the current communication.

Upon starting the backoff process 270 prior to transmitting data, a station e.g. source node 20, initializes its backoff time counter to a random value as explained above. The backoff time counter is decremented once every time slot interval 260 for as long as the radio medium is sensed idle (countdown starts from time TO (23) as shown in Figure 2).

The time unit in the 802.11 standard is the slot time called ‘aSlotTime’ parameter. This parameter is specified by the PHY (physical) layer (for example, aSlotTime is equal to 9ps for the 802.11n standard). All dedicated space durations (e.g. backoff) are multiples of this time unit.

The backoff time counter is ‘frozen’ or suspended when a transmission is detected on the radio medium channel (countdown is stopped at time T1 (24) for other nodes 22 having their backoff time counter decremented).

The countdown of the backoff time counter is resumed or reactivated when the radio medium is sensed idle anew, after a DIFS time period. This is the case for the other nodes at time T2 (25) as soon as the transmission opportunity TXOP granted to source node 20 ends and the DIFS period 28 elapses. DIFS 28 (DCF inter-frame space) thus defines the minimum waiting time for a source node before trying to transmit some data. In practice, DIFS = SIFS + 2 * aSlotTime.

When the backoff time counter reaches zero (26) at time T1, the timer expires, the corresponding node 20 will request access onto the medium in order to be granted a TXOP, and the backoff time counter is reinitialized 29 using a new random backoff value.

In the example of the Figure implementing the RTS/CTS scheme, at time T1, the source node 20 that wants to transmit data frames 230 sends a special short frame or message acting as a medium access request to reserve the radio medium, instead of the data frames themselves, just after the channel has been sensed idle for a DIFS or after the backoff period as explained above.

The medium access request is known as a Request-To-Send (RTS) message or frame. The RTS frame generally includes the addresses of the source and receiving nodes ("destination 21") and the duration for which the radio medium is to be reserved for transmitting the control frames (RTS/CTS) and the data frames 230.

Upon receiving the RTS frame and if the radio medium is sensed as being idle, the receiving node 21 responds, after a SIFS time period 27 (for example, SIFS is equal to 16 ps for the 802.11η standard), with a medium access response, known as a Clear-To-Send (CTS) frame. The CTS frame also includes the addresses of the source and receiving nodes, and indicates the remaining time required for transmitting the data frames, computed from the time point at which the CTS frame starts to be sent.

The CTS frame is considered by the source node 20 as an acknowledgment of its request to reserve the shared radio medium for a given time duration.

Thus, the source node 20 expects to receive a CTS frame 220 from the receiving node 21 before sending data 230 using unique and unicast (one source address and one addressee or destination address) frames.

The source node 20 is thus allowed to send the data frames 230 upon correctly receiving the CTS frame 220 and after a new SIFS time period 27.

An ACK frame 240 is sent by the receiving node 21 after having correctly received the data frames sent, after a new SIFS time period 27.

If the source node 20 does not receive the ACK 240 within a specified ACK Timeout (generally within the TXOP), or if it detects the transmission of a different frame on the radio medium, it reschedules the frame transmission according to the backoff procedure.

Since the RTS/CTS four-way handshaking mechanism 210/220 is optional in the 802.11 standard, it is possible for the source node 20 to send data frames 230 immediately upon its backoff time counter reaching zero (i.e. at time T1).

The requested time duration for transmission defined in the RTS and CTS frames defines the length of the granted transmission opportunity TXOP, and can be read by any listening node ("other nodes 22" in Figure 2) in the radio network.

To do so, each node has in memory a data structure known as the network allocation vector or NAV to store the time duration for which it is known that the medium will remain busy. When listening to a control frame (RTS 210 or CTS 220) not addressed to itself, a listening node 22 updates its NAVs (NAV 255 associated with RTS and NAV 250 associated with CTS) with the requested transmission time duration specified in the control frame. The listening nodes 22 thus keep in memory the time duration for which the radio medium will remain busy.

Access to the radio medium for the other nodes 22 is consequently deferred 30 by suspending 31 their associated timer and then by later resuming 32 the timer when the NAV has expired.

This prevents the listening nodes 22 from transmitting any data or control frames during that period.

It is possible that the receiving node 21 does not receive the RTS frame 210 correctly due to a message/frame collision or to fading. Even if it does receive it, the receiving node 21 may not always respond with a CTS frame 220 because, for example, its NAV is set (i.e. another node has already reserved the medium). In any case, the source node 20 enters into a new backoff procedure.

The RTS/CTS four-way handshaking mechanism is very efficient in terms of system performance, in particular for large frames since it reduces the length of the messages involved in the contention process.

In detail, assuming perfect channel sensing by each communication node, collision may only occur when two (or more) frames are transmitted within the same time slot after a DIFS 28 (DCF inter-frame space) or when their own back-off counter has reached zero (nearly at the same time T1). If both source nodes use the RTS/CTS mechanism, this collision can only occur for the RTS frames. Fortunately, such collision is early detected by the source nodes since it is quickly determined that no CTS response has been received.

As described above, the original IEEE 802.11 MAC always sends an acknowledgement (ACK) frame 240 after each data frame 230 received.

However, such collisions limit the optimal functioning of the radio network. As described above, simultaneous transmit attempts from a number of wireless nodes lead to collisions. The backoff procedure for the family of IEEE 802.11 standards was first introduced for the DCF mode as the basic solution for collision avoidance, and further employed by the IEEE 802.11e to solve the problem of internal collisions between enhanced distributed channel access functions (EDCAFs). In the emerging IEEE 802.11n/ac standards, the backoff procedure is still used as the fundamental approach for supporting distributed access among mobile nodes.

The rapid growth of smart mobile devices is driving mobile data usage and 802.11 WLAN proliferation, creating an ever-increasing demand for faster wireless networks to support bandwidth-intensive applications, such as web browsing and video streaming. The new IEEE 802.11ac standard is designed to meet this demand, by providing major performance improvements compared to previous 802.11 generations.

The IEEE 802.11ac standard is an emerging very high throughput (VHT) wireless local access network (WLAN) standard that can achieve physical layer (PHY) data rates of up to 7 Gbps for the 5 GHz band.

The scope of 802.11ac includes single link throughput supporting at least 500 Mbps, multiple-station throughput of at least 1 Gbps, and backward compatibility and coexistence with legacy 802.11 devices in the 5 GHz band (i.e. 802.11 a/n).

Consequently, this standard is targeted at higher data rate services such as high-definition television, wireless display (high-definition multimedia interface - HDMI - replacement), wireless docking (wireless connection with peripherals), and rapid sync-and-go (quick upload/download).

In general, 802.11ac could be schematized as an extension of IEEE 802.11η in which the two basic notions of multiple-input, multiple-output (ΜΙΜΟ) and wider channel bandwidth are enhanced for greater efficiency.

Using optional ΜΙΜΟ feature, an access point AP (having several antennae) may communicate with several nodes simultaneously using Spatial Division Multiple Access (SDMA). SDMA multiple access scheme enables multiple stream transmitted to different receivers at the same time to share the same frequency channel. This invention is not devised for such sender device having several antennae.

Contrary to the 802.11η standard where each node should support up to 2 spatial streams (SSs) and 40MHz transmissions, only one spatial stream is required in 802.11ac but support for 80MHz channel bandwidth is added. One reason for such a change is that increasing the number of antennae often results in higher cost. Supporting multiple Spatial Stream (SS) implies having at least the same number of antennae (and as much reception chains behind these antennae), thus important costs.

Consequently a lot of devices available on the market could only support one SS. In 802.11ac standard, support for only one SS is required so that devices, and especially smartphones, could be labelled as ‘802.11ac compliant’. The 80MHz mode is made mandatory as a lower cost alternative to the two SS and 40MHz configuration. Hence, the modes that utilize more than one spatial stream are now optional in 802.11ac standard.

As a result, 802.11ac is targeting larger bandwidth transmission through multi-channel operations. The wider channel aspect will be further described with reference to Figure 3. A MAC mechanism for dynamically protecting and allocating multiple channel will be presented with reference to Figure 4.

In order to support wider channel bandwidths (Figure 3a), IEEE 802.11ac standard introduces support of 20MHz, 40MHz, 80MHz, and 160MHz channel bandwidth, including the so-called “primary” sub-channel (here 300-3), compared to only 20MHz and 40MHz supported by the 802.11η standard. The 20MHz sub-channels 300-1 to 300-8 may be concatenated to form a larger communication channel.

The 160MHz channel bandwidth is composed of two 80MHz channels that may or may not be contiguous. The 80MHz and 40MHz channels are respectively composed of two adjacent (contiguous) 40MHz and 20MHz channels, respectively. The support for 40MHz and 80MHz channel bandwidths is mandatory while the support for 160MHz and 80 + 80MHz is made optional in the standard (80+80MHz means that the operating band is made of two non-frequency-contiguous wide channels having each a bandwidth of 80MHz). A multi-channel node (supporting 80MHz transmissions in the illustrated example of Figure 3b) is granted a TXOP through the enhanced distributed channel access (EDCA) mechanism on the “primary sub-channel” (300-3). Indeed, for each sub-channel bandwidth, the 802.11ac standard designates one sub-channel as “primary sub-channel” meaning that it is used for control of transmission on that bandwidth.

It will, however, transmit a 80MHz PPDU (PPDU means PLCP Protocol Data Unit, with PLCP for Physical Layer Convergence Procedure; basically a PPDU refers to an 802.11 physical frame) only if all other secondary sub-channels have been idle for at least a point coordination function (PCF) inter-frame spacing (PIFS). If at least one of the secondary sub-channels has not been sensed as idle for a PIFS, then the node must either restart its backoff count, or use the obtained TXOP for 40MHz or 20MHz PPDUs.

The vertical aggregation scheme reflects the extension of the payload 230 to all sub-channels. If there is only one collision in one of the sub-channels at a given time, the risk of having a corrupted segment of these sequences is very high despite the error-correcting decoding process. All MPDU (MAC Protocol Data Unit) frames could thus automatically be considered as incorrect. Thus in high load BSSs, a DATA packet split over multiple sub-channel may encounter more collisions, thus wasting the channel resource.

In the description below, the words “channel·’, “20MHz channel·’ or “subchannel·’ mainly refer to the same technical feature, i.e. any channel that complies with 802.11η or older standards. “Composite channel·’ thus refers to the additional feature according to which a composite channel is made of one or more sub-channels that are contiguous within the operating band of the wireless network. In 802.11ac, the composite channels are 20MHz wide (if made of only one sub-channel) or 40MHz wide or 80MHz wide or, optionally, 160MHz wide.

Relying to its multi-channel capability, the 802.11ac standard supports enhanced protection in which the RTS/CTS handshake mechanism is modified to support static or dynamic bandwidth reservation and carry the channel bandwidth information. This is now explained with reference to Figure 4.

Bandwidth signalling is added to the RTS and CTS frames (only 20, 40, 80MHz values). For instance, a two-bit field is provided in the frame header to specify one of the 20, 40, 80 or 160MHz bandwidth values.

As mentioned above, the frame MAC header of control frames (e.g. RTS or CTS) includes the addresses of the source node (TA for transmitter address) and of the receiving node (RA for receiver address).

According to the 802.11ac standard, the TA indicates the presence of additional signalling related to the bandwidth to be used in subsequent transmissions, by using an IEEE medium access control (MAC) individual address of the source node with an Individual/Group bit set to 1 in the frame header also.

The additional information, e.g. support for dynamic or static bandwidth operation and the channel width occupied by the frame, are signalled in the SERVICE field of RTS frames (same case for CTS frame). As mentioned, the conventional 802.11a frame format comprises a PHY preamble (including the legacy short training field (L-STF), legacy long training field (L-LTF) and legacy signal field (L-SIG)) followed by a data payload (including the said SERVICE field, user data (PSDU), pad bits and tail bits).

The additional signalling in the frame header includes a two-bit field set to indicate the bandwidth of the target composite channel for the intended transmission.

The source node thus generates the RTS frame with the two-bit field set to the appropriate value. The RTS frame is sent over the primary 20MHz sub-channel and replicated (duplicated) on each other (i.e. secondary) 20MHz sub-channel forming the target composite channel.

The receiving node replies with a CTS frame on each (sub-)channel sensed as free using for instance CCA. Both nodes follow the non-HT duplicate RTS/CTS frame handshake procedure.

As an example shown in the Figure, prior to transmission of an 80MHz data frame, the source node STA1 may transmit an RTS frame 410 configured to use the 20MHz channel bandwidth to each of the 20MHz sub-channels forming the 80MHz operating band. That is, in association with the 80MHz channel bandwidth, a total of four RTS frames are transmitted in the form of a duplicated PPDU over the four 20MHz sub-channels.

The receiving node STA2 may answer each 20MHz sub-channel in which RTS frames transmitted from the source node STA1 have been successfully received using a CTS frame configured to use the 20MHz sub-channel bandwidth. If the receiving node STA2 has successfully received RTS frames from the entire 80MHz bandwidth, a total of four CTS frames should be transmitted to cover the 80MHz channel bandwidth.

If the source node STA1 receives all the CTS frames related to the 80MHz channel (i.e. if the source node STA1 receives a total of four CTS frames), a DATA 430-1 frame is transmitted using the 80MHz channel bandwidth.

It is expected that every nearby device (legacy or 802.11ac compliant) can receive an RTS frame on its primary sub-channel. Each of these devices then sets its NAV. Before a client replies with a CTS frame, it checks if any of the sub-channels in the 80MHz band is busy. The receiving node STA2 only replies with a CTS frame on those sub-channels that are idle, and reports the total bandwidth of the replicated CTS frames. As with the RTS frame, the CTS frame is sent in an 802.11a (non-HT) PPDU format on the primary sub-channel and is replicated over the different 20MHz secondary sub-channels that have been sensed as idle by the receiving node.

Still referring to Figure 4, a nearby node of receiving node STA2 is already transmitting before source node STA1 starts sending the RTS frames (only interfering with receiving node STA2). Therefore receiving node STA2 has to inform source node STA1 by replying with CTS frames only on the idle sub-channels (three CTS frames are transmitted).

In the well-known hidden node problem, a sub-part of a group of nodes (here the receiving node STA2) may be impacted by the presence of a hidden node (not seen by the other nodes of the group). So, the RTS/CTS handshake may provide partial information of the channel occupancy through the different spatial positions of the nodes. As a result, the usage of the wireless channel capacity could be improved compared to this situation.

Figure 5 represents a scenario of dynamic fallback to narrower channel widths in the presence of co-channel interference or noise that only affects a portion of the larger channel. A channel interference is typically performed by a legacy 802.11a or 802.11η compliant node (transmitting on a 20MHz sub-channel), so that the 802.11ac compliant node may transmit over a fraction of the original requested bandwidth: depending of which 20MHz sub-channel (300) is busy, the channel width of resulting composite channel is reduced from 80MHz to 40MHz (cases 510 and 511), or 20MHz (case 520) whereas a 60MHz bandwidth is available.

It may be noted that the 802.11ac standard does not provide for using such lost bandwidth, as the RTS/CTS control frames embed a bandwidth indication only supporting 20, 40, 80 or 160MHz.

This bandwidth allocation deficiency is especially problematic in a hidden node configuration in which the disturbance of one node can waste a huge part of spectrum resources. For instance in the left part of Figure 6, four nodes: STA1, STA2, STA3 and STA4 (respectively 611, 612, 613 and 614) belonging to the same wireless cell 610 exchange data, but node STA2 is disturbed by a neighbouring cell 600 and especially by a legacy node 601. This situation leads to a mismatch between the different nodes of the same cell 610 concerning the availability of the medium. Thereby as illustrated in the right part of Figure 6, the sub-channel 300-4 can be seen busy by node STA2 612 (630) and free by the other nodes 640. So, according to the standard, if node STA2 is involved in the RTS/CTS handshake mechanism for TXOP reservation, the reservation and thus the communication occurs only on the 20MHz primary subchannel 300-3 although the 80MHz are free for the other nodes.

One embodiment of the present invention provides for forming a group of peer nodes exchanging highly interactive data like live video streams. Those peer nodes are also called collaborative nodes, that is to say they will collaborate in order to provide an efficient channel allocation according to the method of the invention for their group communication. It is assumed that there are also nodes outside of the collaborative group, which may be referred to as legacy 802.11 nodes. A legacy environment typically describes a situation where nodes are independent and do not interact or cooperate with each other, as opposed to the collaborative group of nodes. A peer node may request access to the shared 802.11 type medium according to the 802.11 legacy protocol, and upon grant of access, several peer nodes may communicate with one or more peer nodes according to a collaborative protocol during the reserved talk time. Thus, if the back-off count reaches zero on the primary sub-channel for one peer node among the group, said node reserves a medium access (through the conventional multiple RTS/CTS scheme of 802.11ac standard) for the whole collaborative group onto a composite channel, and lets the group communicates over the composite channel during this granted 802.11 timeslot. It will become apparent that the composite channel according to embodiments of the present invention may be made of more free sub-channels 300 than the 802.11ac prior art.

Figure 7 schematically illustrates a communication device 700 of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 700 may be a device such as a micro-computer, a workstation or a light portable device. The communication device 700 comprises a communication bus 713 to which there are preferably connected: - a central processing unit 711, such as a microprocessor, denoted CPU; - a read only memory 707, denoted ROM, for storing computer programs for implementing the invention; - a random access memory 712, denoted RAM, for storing the executable code of methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing methods according to embodiments of the invention; and - at least one communication interface 702 connected to the radio communication network 100 over which digital data packets or frames are transmitted, for example a wireless communication network according to the 802.11η protocol. The data frames and aggregated frames are written from a FIFO sending memory in RAM 712 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 712 under the control of a software application running in the CPU 711.

Optionally, the communication device 700 may also include the following components: a data storage means 704 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention; a disk drive 705 for a disk 706, the disk drive being adapted to read data from the disk 706 or to write data onto said disk; a screen 709 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 710 or any other pointing means.

The communication device 700 can be connected to various peripherals, such as for example a digital camera 708, each being connected to an input/output card (not shown) so as to supply data to the communication device 700.

The communication bus provides communication and interoperability between the various elements included in the communication device 700 or connected to it. The representation of the bus is not limiting and in particular the central processing unit is operable to communicate instructions to any element of the communication device 700 directly or by means of another element of the communication device 700.

The disk 706 can be replaced by any information medium such as for example a compact disk (CD-ROM), rewritable or not, a ZIP disk, a USB key or a memory card and, in general terms, by an information storage means that can be read by a microcomputer or by a microprocessor, integrated or not into the apparatus, possibly removable and adapted to store one or more programs whose execution enables a method according to the invention to be implemented.

The executable code may be stored either in read only memory 707, on the hard disk 704 or on a removable digital medium such as for example a disk 706 as described previously. According to a variant, the executable code of the programs can be received by means of the communication network 703, via the interface 702, in order to be stored in one of the storage means of the communication device 700, such as the hard disk 704, before being executed.

The central processing unit 711 is adapted to control and direct the execution of the instructions or portions of software code of the program or programs according to the invention, which instructions are stored in one of the aforementioned storage means. On powering up, the program or programs that are stored in a nonvolatile memory, for example on the hard disk 704 or in the read only memory 707, are transferred into the random access memory 712, which then contains the executable code of the program or programs, as well as registers for storing the variables and parameters necessary for implementing the invention.

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

Figure 8 is a block diagram schematically illustrating the architecture of a node 700 adapted to carry out, at least partially, the invention. As illustrated, node 700 comprises a physical (PHY) layer block 803, a MAC layer block 802, and an application layer block 801.

The PHY layer block 803 (here a 802.11 standardized PHY layer) has the task of formatting and sending or receiving frames over the radio medium used 100, such as a medium access request of the RTS type to reserve a transmission slot, a medium access response of the CTS type to acknowledge reservation of a transmission slot, as well as of MAC data frames and aggregated frames to/from that radio medium.

The MAC layer block or controller 802 comprises a standard MAC 802.11 layer 804 and two additional blocks 805 to 806 that may carry out, at least partially, embodiments of the invention. The MAC layer block 802 may be implemented in software, which software is loaded into RAM 712 and executed by CPU 711.

The Group management module 805 implements the management of devices and associated streams related to the collaborative community: typically, an identifier is maintained for a configuration set of registered devices and streams.

The Channel allocation module 806 mainly implements steps of an algorithm according to embodiments of the invention, relating to reaching all free subchannels during the enhanced dynamic channel allocation procedure. In combination with the Group management module 805, it supports a dynamic determination of a data stream allocation for each sub-channel of the granted composite channel for the community of nodes of the invention. In other words, the Channel allocation module 806 and the Group management module 805 perform together a stream allocation that depends on a stream priority. This is advantageous over the prior art in which the communications between the sending node and the addressee node must be performed on the primary channel only, while this cannot be the most efficient in view of the availability of the other sub-channels.

Finally, the application layer block 801 implements an application that generates and receives data packets, for example data packets of a video stream. This application layer block 801 represents all the stack layers above MAC layer according to ISO standardization.

Figure 9 presents an exemplary communication timeline illustrating the principle of multiple node simultaneous transmission.

More specifically, there is provided a transmission method for multiple node sharing the granted frequency channel between them, wherein a first set of nodes communicates over a first set of contiguous sub-channels, and at least one second set of nodes communicates over a second set of contiguous sub-channels (the first and second sets forming a composite channel).

Basically, an 802.11ac compliant node (having a single antenna) cannot use non-contiguous channels. Embodiments of the present invention rely on the idea that at least one pair of second nodes may talk independently in the “not reachable” band, i.e. sub-channels that are seen as busy for at least one of the nodes involved in the handshake mechanism used for TXOP reservation.

In the given examples, a WLAN system is considered using a multi-channel including four contiguous sub-channels having a channel bandwidth of 20MHz. However, it is only an example and the invention is not limited thereto. The number of sub-channels or the channel bandwidth thereof is not limited.

Nodes of the invention use the standard RTS/CTS handshake over the multiple channel according to the 802.11ac standard. A sending node of the invention sends an RTS frame 910 with the bandwidth (20, 40, 80 or 160 MHz) of the intended transmission (replicated on each 20MHz sub-channels forming the targeted bandwidth), a receiving node replies with a CTS frame 920 on each secondary subchannel sensed as free. If the receiving node has successfully received RTS frames from the entire 80MHz bandwidth, a total of four CTS frames can be transmitted to cover the 80MHz channel bandwidth.

Therefore, according to the 802.11ac standard, the sending node is allowed to use the whole 80MHz bandwidth channel for sending data to the receiving node.

In contrast to the 802.11ac standard, in the given example, a source node has reserved a TXOP period, which is further sub-leased to several streams of at least one other source node. As a result, several source nodes may occupy the granted cumulative multi-channel band, in such a way that each source node occupies a contiguous bandwidth. According to this example, source nodes 1 and 2 talk within a 40MHz band (respectively 930-1 and 930-2).

The source device may estimate the TXOP duration prior to it sending the RTS frames and tries to reserve the multi-channel for that duration: thus, the source node preferably communicates over a set of first sub-channels comprising the primary 20MHz sub-channel. That is, for Figure 9, the source node 1 is the source node that sent the RTS frames.

According to the 802.11ac standard, the duration of the TXOP is computed by the sending node, as a function of the total bandwidth of the set of sub-channels seen as free from the sending node standpoint (before sending the RTS frames), and also as a function of the data to be sent. For instance, if the sending node sees a set of four sub-channels as free, thus corresponding to an available bandwidth of 80 MHz, the computed TXOP will be based on this 80 MHz available bandwidth even if a shorter bandwidth is available at the addressee node in the end.

According to embodiments, the TXOP duration is calculated for the set of available sub-channels dedicated to the source node. Thus, in regards to the example shown in Figure 9, the TXOP duration indicated in the RTS frames may be computed based on an amount of data 930-1 to be conveyed on the two sub-channels 300-3 and 300-4.

According to the 802.11ac/n standard, the legacy node, when receiving the RTS frames, retrieves the TXOP duration included in a duration field in the MAC header of the RTS frame and waits until the end of this period before performing clear channel assessment (CCA). Also, when receiving one RTS frame on a given subchannel, an 802.11a legacy node computes a TXOP duration (e.g. using rate and length indicated in the PHY header of RTS frame), and also waits until the end of this period before performing clear channel assessment (CCA).

Consequently, the wireless medium is protected against access by legacy nodes for at least the duration of data 930-1.

The duration of data packet transmission on secondary sub-channels should comply with the occupation duration on the primary 20MHz sub-channel 300-3. This means, in the given example, that the transmission duration of data 930-2 should be shorter or equal to the transmission duration of data 930-1.

Figure 10 represents an exemplary communication timeline illustrating embodiments of the invention.

In the given example, there are four nodes STA1, STA2, STA3 and STA4 forming a collaborative group. The invention is not limited thereto. STA1 is assumed to be a sending node, node STA2 is assumed to be an addressee node and nodes STA3 and STA4 are called “other nodes”. All the nodes belonging to the collaborative group are able to listen (i.e. to detect or sense) control frames that are exchanged between nodes of the collaborative group. A plurality (here four) of request control frames (RTS frames) 1010 are sent by the sending node STA1 to the addressee node STA2, for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a composite channel, here made of four sub-channels 300-1 to 300-4.

As already mentioned, an RTS frame is replicated (duplicated) on the primary 300-3, secondary 300-4, tertiary 300-2 and quaternary 300-1 sub-channels, and each RTS frames indicates the bandwidth for which the TXOP is wanted. Therefore, in embodiments where no RTS frame is sent on some of the sub-channels because the sending node STA1 is disturbed on these sub-channels, the addressee node STA2 and other listening nodes STA3 and STA4 can know the desired bandwidth. Examples of such embodiments will be described with reference to Figures 14 to 17.

Back to the given example, said request control frames 1010 are sent on a plurality or first set of sub-channels belonging to said composite channel. Those subchannels are referred below as “first sub-channels”. In the illustrated example, the plurality or first set of first sub-channels fits the desired bandwidth, i.e. they form the composite channel mentioned above. In other embodiments, as shown in Figures 14 to 17, the plurality of first sub-channels forms a sub-set of the targeted composite channel.

In practice, the RTS frames are addressed to the addressee node STA2, but the other nodes STA3 and STA4 are able to detect (listen or sense) the RTS frames on the medium.

As already mentioned, an RTS frame sent on a first sub-channel calls for a response control frame CTS (first response control frame) from the addressee node STA2 on the same sub-channel, in order to finalise the handshaking process for reserving a TXOP for this first sub-channel. Indeed, this CTS frame indicates the availability of the first sub-channel for the addressee node.

In the given example, the addressee node STA2 does not respond on one of the sub-channels, that is sub-channel 300-4, because it is seen as busy, for instance because of a disturbance by a legacy node. Therefore, addressee node STA2 only responds on first sub-channels 300-1 to 300-3 that are idle (or free) from its standpoint.

According to embodiments, a given node of the collaborative group (typically the other nodes STA3 and STA4) may determine at least one second subchannel belonging to the composite channel seen as busy by said sending node (here STA1) or said addressee node (here STA2). This is done based on the RTS frames sent by the sending node STA1 and the first CTS frames from the addressee node, that are all detected by the given node.

In the given example, no CTS frame is sent by addressee node STA2 on the sub-channel 300-4 whereas an RTS frame has been sent on it by sending node STA1. According to embodiments, the other nodes STA3 and STA4 respond to this RTS by sending a CTS frame (second response control frame) on this second subchannel if it is seen as free from their standpoint. In practice, this may be done periodically or if a hole is detected in the usual RTS/CTS process (i.e. as soon as at least one of the sub-channels 300-1, 300-2, 300-3 or 300-4 is detected as busy for the sending node STA1 and/or the addressee node STA2). In embodiments, the other nodes STA3 and STA4 respond CTS frames on all the first sub-channels seen as free on which an RTS frame has been sent by the sending node STA1.

Therefore, the second sub-channel 300-4, even if it is seen as busy by the sending node STA1 or the addressee node STA2, can be used for transmitting data between the other nodes STA3 and STA4 during said transmission opportunity (TXOP), without requiring another RTS frame to be sent on this second sub-channel. The usage of the operating band is thus improved.

Those control frames (duplicated RTS frames and first and second duplicated CTS frames) allow the determination of the status of the sub-channels (busy or idle/free) for each node of the community.

Thus, a map of links availability 1000 may be created, for example in each collaborative node. This map allows the allocation of the sub-channels for the different applications based on rules known by all the collaborative nodes. For instance, in the example of Figures 6 and 10, node STA2 is disturbed by a legacy node on the secondary sub-channel 300-4 although the other nodes of the community (STA1, STA3 and STA4) consider this sub-channel as free. Thus, contrary to the situation illustrated in Figure 9, while a main application (e.g. video streaming) 1050-2 between sending node STA1 and addressee node STA2 is performed on sub-channels 300-2 and 300-1, a transmission may be initiated for a secondary application 1050-1 (e.g. picture printing) between nodes STA3 and STA4 on sub-channels 300-4 and 300-3 even if sub-channel 300-4 is unavailable for addressee node STA2.

Consequently, embodiments of the invention allow the channel usage to be improved by reusing a part of the operating band already used, for example, by a legacy node located in a different cell.

Figure 11 shows steps of an algorithm for optimizing channel usage in a multi-channel wireless network in accordance with embodiments of the present invention. This exemplary algorithm corresponds to the situation of Figure 12. The method may be implemented by each peer node of the collaborative group.

At step 1100, a control frame (RTS frame) indicating a request to transmit is transmitted on a plurality of first sub-channels by one node of the group called sending node STA1, to another node of the group called addressee node STA2. In this example, the control frame is replicated (1210) over each 20MHz sub-channel (300-1 to 300-4) forming a composite channel. However, in other embodiments, as shown in Figures 14 to 17, the RTS frame may be replicated only on some of the sub-channels (called first sub-channels) forming the composite channel. For instance, it may be because other sub-channels of the composite channel are detected as busy by the sending node STA1. As already mentioned, each RTS frame includes bandwidth information (field called ‘BW of VHT-SIG-A portion of 802.11ac PLCP preamble according to 802.11ac protocol or in the TA field in the non-HT duplicated frame) indicating the bandwidth or size of the composite channel for which the TXOP should be reserved.

Based on techniques such as described in the 802.11ac standard (e.g. energy detection, preamble detection and/or plural channel decoders), the sending node determines which of the communication sub-channels are idle for sending the RTS frames, and applies a fallback mechanism if one of the component sub-channel is detected as busy.

According to embodiments of present invention, the sending node does not systematically apply the fallback scheme but emits a duplicated RTS frame in each idle sub-channel (first sub-channel) of a target composite channel. The bandwidth indication corresponds to the value of the whole targeted bandwidth, i.e. for the targeted composite channel.

In practice, any node of the collaborative group may determine that the TXOP will be shared according to the invention by the analysis of the source address and/or the receiver address of the RTS frames. To do so, receiving nodes STA2, STA3, STA4 belonging to the collaborative group have to check the MAC source address and/or MAC receiver address of RTS frame in order to verify that the source address and/or the receiver address belongs to the group, thus inferring that the current medium access is a collaborative access.

Then, the addressee node STA2 may respond with CTS frames (first response control frames) replicated on each of the first sub-channels (conveying RTS frames) seen as free from its standpoint. The CTS frames thus acknowledge the reservation of the corresponding sub-channels.

Note that in some situations where none of the first sub-channels is sensed as free for the addressee node, the latter does not respond to the RTS frames (see Figure 17).

The original bandwidth indication of the RTS frame or frames is kept inside the CTS header, thus ensuring maximum bandwidth usage. Again, this bandwidth indication may thus encompass sub-channels that are not reserved by the RTS/CTS exchange of step 1100.

At step 1101, a data communication starts between sending node STA1 and addressee node STA2, preferably on the most efficient set of sub-channels of the reserved sub-channels. For instance, the sub-channel allocation to determine such set of sub-channels is performed according to at least one of the following criteria: - the flow prioritization: e.g. a video streaming application has a higher priority than a file transfer application and thus is allocated the most efficient set of subchannels; - the available bandwidth for the aggregated sub-channels: e.g. two contiguous sub-channel (40MHz) should be preferred for priority streams compared to one sub-channel (20MHz); - the quality of the link (BER, SNR...): for instance adjacent sub-channels having a quality higher than a given threshold may be selected.

As previously explained with reference to Figure 10, the video streaming 1050-2 is transmitted on sub-channels 300-1 and 300-2 instead of sub-channels 300-3 (primary sub-channel) and 300-4 in order to have more bandwidth capacity for this huge data transmission.

But contrary to Figure 10, wherein both application data transmissions start simultaneously after “expanded” RTS/CTS procedure, in the example of Figure 12, the primary communication between sending node STA1 and addressee node STA2 starts as soon as the CTS frames have been received from the addressee node STA2 (1220) while the other communication starts after a second and complementary “reservation process” 1103.

At step 1102, any other node STA3 or STA4 determines the availability (busy or free/idle) of the sub-channels of the targeted composite channel for the communication between the sending node STA1 and the addressee node STA2.

This may be done for any of the sub-channels forming the composite channel (as it is the case in Figure 10), for any of the sub-channels of the composite channel that are not yet used by nodes STA1 and STA2 (the case of Figure 12) or for only the sub-channels of the composite channel that are not yet reserved by the RTS/CTS exchange between nodes STA1 and STA2.

In the example of Figure 12, the other nodes STA3 and STA4 are able to detect (listen or sense) the first CTS frames sent by the addressee node STA2 to the sending node STA1. Based on which sub-channels the first CTS frames are conveyed, STA3 and STA4 can determine the so-called first sub-channels on which sending node STA1 and addressee node STA2 will communicate. Thus, also based on the number of replicated RTS frames detected, and/or on the original bandwidth indication located in the RTS frames header, nodes STA3 and STA4 are able to determine on which other sub-channels the determination of channel availability must be performed. In the example of Figure 12, the determination of channel availability must be performed on sub-channels 300-3 and 300-4 since the other sub-channels are about to be used by nodes STA1 and STA2.

Then, at step 1103, the other nodes (STA3 and STA4) of the community respond to the RTS frame by a CTS control frame on the second sub-channel(s) that are unavailable for the primary communication and also on the potentially dropped subchannels. In the example of Figure 12, the sub-channel unavailable for the primary communication is the secondary channel 300-4 and the dropped sub-channel is the primary channel 300-3. Indeed, to take benefits of the maximum of bandwidth, the primary communication is preferably transmitted on a maximum number of contiguous sub-channels, here the tertiary 300-2 and quaternary sub-channels 300-1 instead of the primary 20MHz sub-channel 300-3.

Thanks to the additional CTS frames (second CTS frames) sent by the other nodes (different from STA1 and STA2), additional sub-channels are actually reserved for the collaborative group within the targeted composite channel, compared to the conventional reservation mechanism. The collaborative nodes may thus use such reserved sub-channels. Preferably, a collaborative node may be involved in a data communication on one or more sub-channels on which it has sent CTS frames.

For instance, at step 1104, the other nodes of the community (STA3 and STA4) start their own communication 1050-1 on the recovered sub-channels (300-3 and 300-4) on which CTS frames 1230, 1240.

Figures 12 to 18 represent exemplary communication timelines illustrating various embodiments of the invention.

Figure 12 shows an example wherein the particular goal of the sending node STA1 is to start transmitting 1050-2 (primary communication) as soon as possible, in particular as soon as the first CTS frames 1220 (responsive to the RTS frames 1210 from the sending node STA1) are received from the addressee node STA2 by the sending node STA1, without waiting for the second CTS frames 1230 and 1240 sent by the other nodes STA3 and STA4 on the sub-channels seen as idle (300-3, 300-4) and not busy yet.

Therefore, in the given example, the primary communication 1050-2 starts at the end of the conventional RTS/CTS mechanism between the sending node STA1 and the addressee node STA2 without waiting for the responses of the other collaborative nodes to the RTS frames 1210. The other nodes respond to the RTS frames by second CTS frames 1230 and 1240 on the sub-channels seen as idle, and that are not busy with the primary communication 1050-2.

Thus, while a main application (e.g. video streaming) 1050-2 between sending node STA1 and addressee node STA2 is performed on sub-channels 300-2 and 300-1 as early as possible, a transmission may be initiated for a secondary application 1050-1 (e.g. picture printing) between nodes STA3 and STA4 on subchannels 300-4 and 300-3 even if sub-channel 300-4 is unavailable for addressee node STA2.

Figure 13 shows an example wherein both primary and secondary communications start before the end of a CTS timeout period A.As known in the state of the art, an RTS frame sets a timeout period Δ defining a duration during which a transmission opportunity (TXOP) reservation may be acknowledged by the addressee node STA2 by sending CTS frames in response to the RTS frames from the sending node STA1. For instance, the CTS timeout Δ may be set as follows: Δ = (aSIFSTime) + aPHY-RX-START-Delay + (aSlotTime)

Wherein: aSIFSTime is the time length of the SIFS period (see reference 27 in Figure 2) aPHY-RX-START-Delay is the delay, in microsecond, from a point in time specified by the PHY to the issuance of the PHY-RXSTART.

aSlotTime is a time unit in the 802.11 standard depending on PHY characteristics.

In this example, the addressee node STA2 responds with first CTS frames 1320 to the RTS frames 1310 from the sending node STA1 only on the sub-channel(s) the most adapted to the primary communication (e.g. video streaming) with the sending node STA1, here sub-channels 300-1 and 300-2.

The other nodes of the community STA3 and STA4 respond with second CTS frames on the remaining sub-channels (i.e. on which no CTS frame has been sent yet), before the CTS timeout (1360) in order to keep the band reservation.

Figures 14 to 17 illustrate particular embodiments where no RTS frame is sent on some of the sub-channels of the targeted composite channel that are seen as busy by the sending node STA1, for instance because of a disturbance by a legacy node. The sub-channels on which RTS frames are sent are called first sub-channels. As already mentioned, the addressee node STA2 and other listening nodes STA3 and STA4 can know the desired (targeted) bandwidth anyway since it is indicated in an header of the RTS frames. More specifically, the desired bandwidth may be indicated in a field called ‘BW of VHT-SIG-A portion of 802.11ac PLCP preamble according to the 802.11ac protocol or in the TA field in the non-HT duplicated frame. Therefore, in the embodiments illustrated in Figures 14 to 17, the plurality of first sub-channels on which RTS frames are sent thus forms a sub-set of the composite channel of the desired bandwidth indicated in each RTS frame.

Figure 14 shows an example wherein a plurality (here three) of request control frames (RTS frames) 1410 are sent by the sending node STA1 to the addressee node STA2, for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a targeted composite channel, here made of four subchannels 300-1 to 300-4. As already mentioned, the sending node STA1 sees at least one sub-channel of the composite channel as busy, and thus no RTS frame is sent on it.

In the given example, an RTS frame is duplicated on the primary 300-3, tertiary 300-2 and quaternary 300-1 channels called first sub-channels but not on the secondary channel 300-4 that is seen as busy by the sending node STA1.

Since each RTS frame indicates the bandwidth for which the TXOP is wanted (which is here larger than the set of first sub-channels on which RTS frames are sent), the addressee node STA2 and in particular embodiments, other listening nodes STA3 and STA4, can know the desired bandwidth. In particular, it may deduce that the desired bandwidth differs from the bandwidth of the set of first sub-channels on which RTS frames are sent. In other words, the addressee node STA2 and in particular embodiments, other listening nodes STA3 and STA4 can detect that the sending node sees a sub-channel (called second sub-channel) as busy while a TXOP is wanted for this sub-channel.

Since the addressee node STA2 can know that desired bandwidth for the TXOP, it may send a CTS frame on all the sub-channels of the composite channel seen as idle from its standpoint, including the sub-channel seen as busy by the sending node.

In the given example, it is assumed that the addressee node STA2 sees all the sub-channels of the composite channel as free (idle). Thus, it sends CTS frames on the first sub-channels on which RTS frames have been sent by the sending node STA1 (in this example, first sub-channels 300-1 to 300-3). These CTS frames are called first CTS frames since they are sent by the addressee node STA2 on first sub-channels on which RTS frames have been sent by the sending node STA1. In this example, a CTS frame is also sent on the sub-channel 300-4 (called second sub-channel) on which no RTS frame has been received but for which a TXOP is also wanted according to the desired bandwidth. It should be noted that this CTS frame sent by the addressee node STA2 on the second sub-channel is not a first CTS frame.

Therefore, in this example, even if the sending node STA1 sees the subchannel 300-4 as busy, the addressee node STA2 can send a CTS frame on this subchannel of the composite channel (if it is seen as idle from its standpoint) in order to reserve a TXOP on this second sub-channel for the collaborative group.

In embodiments, another node of the collaborative group, typically the other nodes STA3 and STA4, may also determine such a second sub-channel, i.e. that seems free from its standpoint, but which is seen as busy by the sending node. In practice, the second sub-channel may be determined based on the RTS frames sent by the sending node STA1 to the addressee node STA2 and the first CTS frames from the addressee node, thanks to the listening capacities of the other nodes STA3 and STA4.

In embodiments, they may know the desired bandwidth from the RTS frames listened to and detect a difference with the bandwidth of the set of first subchannels similarly to the addressee node STA2.

In practice, each control frame (RTS frame or CTS frame) comprises the MAC address of the device that sends the control frame and the MAC address of the device targeted by the control frame. Therefore, when listening to the control frames exchanged in the collaborative group, each collaborative node may know the subchannels) available between two collaborative nodes.

In contrast, the quality of the available sub-channel(s) is not known only by listening to the medium. This kind of information must thus be exchanged between the collaborative nodes.

In the given example, no RTS frame is sent on the sub-channel 300-4 (second sub-channel) whereas the desired (targeted) bandwidth includes it. Thus the other nodes STA3 and STA4 send a CTS frame (second response control frame) on this second sub-channel, as well as on all other sub-channels seen as idle to declare those other sub-channels as being idle from their standpoint. This makes it possible for nodes STA3 and STA4 to be involved in communications on any of these first subchannels during the TXOP, if the rules for accessing the medium in the collaborative group allow it.

In embodiments, as illustrated in Figures 15 and 16, second CTS frames are not sent on all the sub-channels seen as idle by the other nodes, but only on the sub-channels not used by the sending node STA1 and the addressee node STA2 for the primary communication.

In practice, this may be done periodically (e.g. every 1ms) or if a hole is detected in the usual RTS/CTS process (i.e. as soon as at least one of the subchannels 300-1, 300-2, 300-3 or 300-4 is detected as busy). Therefore, the second sub-channel 300-4, even if it is seen as busy by the sending node STA1, may be used for transmitting data 1050-1 between the other nodes STA3 and STA4 during said transmission opportunity (TXOP), without need to wait for another RTS frame on the second sub-channel. The usage of the operating band is thus improved.

Those control frames (duplicated RTS frames and first and second duplicated CTS frames) allow the determination of the status of the sub-channels (busy or idle/free) for each node of the community. Thus, a map of links availability may be created as map 1000 of Figure 10, for example in each node. This map allows the allocation of the sub-channels for the different applications (like picture printing 1050-1 and video streaming 1050-2). For instance, collaborative access rules known by all the nodes may determine which nodes may initiate a communication on sub-channels seen as available for these nodes (either they have sent CTS frames or have sent RTS frames for the sending node).

Consequently, embodiments of the invention allow improving the channel usage by reusing a part of the operating band already used, for example, by a legacy node located in a different cell.

Figure 15 shows an example similar to the example shown in Figure 12, but here, both the sending node STA1 and the addressee node STA2 are disturbed on sub-channel 300-4, for instance because of legacy nodes.

As in the example of Figure 12, a particular goal here is to begin a transmission 1050-2 (primary communication) with the sending node STA1 as soon as possible, in particular as soon as the first CTS frames 1520 (responsive to the RTS frames 1510 from the sending node STA1) from the addressee node STA2 are received by the sending node STA1, without waiting for the second CTS frames 1530 and 1540 sent by the other nodes STA3 and STA4 on the sub-channels seen as idle by them (300-3, 300-4) and not busy yet.

Therefore, in the given example, the primary communication 1050-2 starts at the end of the usual RTS/CTS mechanism between the sending node STA1 and the addressee node STA2 without waiting for a response of the other collaborative nodes to the RTS frames 1510. The other nodes react to the RTS frames with second CTS frames 1530 and 1540 on the sub-channels seen as idle, and that are not busy with the primary communication 1050-2.

Thus, while a main application (e.g. video streaming) 1050-2 between sending node STA1 and addressee node STA2 is performed on sub-channels 300-2 and 300-1 as soon as possible, a transmission may be initiated for a secondary application 1050-1 (e.g. picture printing) between nodes STA3 and STA4 on subchannels 300-4 and 300-3 even if sub-channel 300-4 is unavailable for both sending node STA1 and addressee node STA2.

Figure 16 shows an example wherein both primary and secondary communications start before the end of a CTS timeout period Δ. This example is similar to the example shown in Figure 13, but here, the sending node STA1 is disturbed on sub-channel 300-4.

In this example, the addressee node STA2 react to the three RTS frames 1610 sent from the sending node STA1 with first CTS frames 1620 only on the sub channel(s) available for the communication (e.g. video streaming) with the sending node STA1, here sub-channels 300-1 and 300-2.

The other nodes of the community STA3 and STA4 react with second CTS frames on the remaining sub-channels (i.e. on which no CTS frame has been sent yet), before the CTS timeout (1660) in order to keep the band reservation.

Figure 17 illustrates a particular embodiment wherein the sending node STA1 is disturbed on a sub-channel (300-4), and the addressee node STA2 does not react to any RTS frame sent by the sending node STA1 for instance because they are all seen as busy from the addressee STA2 standpoint.

As already mentioned, the other nodes STA3 and STA4 are able to listen (i.e. to detect) control frames (e.g. RTS and CTS) that are exchanged between nodes of the collaborative group.

In the given example, a plurality (here three) of request control frames (RTS) 1710 are sent by the sending node STA1 to the addressee node STA2 on a plurality of first sub-channels, for initiating a transmission opportunity (TXOP) reservation for the collaborative group on a composite channel, here made of four subchannels 300-1 to 300-4. As already mentioned, the sending node STA1 sees at least one sub-channel (here sub-channel 300-4) of the composite channel as busy, and thus no RTS frame is sent on it.

Since each RTS frame indicates the bandwidth for which the TXOP is wanted (which is here larger than the set of first sub-channels on which RTS frames are sent), the listening nodes STA3 and STA4, can know the desired bandwidth. Thus, the listening nodes STA3 and STA4 can detect that the sending node STA1 sees a sub-channel (called second sub-channel) as busy while a TXOP is wanted for this subchannel.

Each listening node STA3 or STA4 may send a CTS frame on all the subchannels of the composite channel seen as idle from its standpoint, including the subchannel seen as busy by the sending node STA1, even if the addressee node STA2 does not react to the RTS frames. In other words, any node of the collaborative group, for instance a listening node, may reserve the transmission opportunity for the whole collaborative group so that the sending node can transmit data (video streaming 1050-2) to one of the listening node (STA3 or STA4) on sub-channels 300-1 and 300-2 and listening nodes (STA3 and STA4) may use the TXOP to exchange data (picture printing 1050-1) in the same time on the other sub-channels 300-3 and 300-4 seen as idle by both of them.

In a general way, nodes STA1 and STA2 exchange one or more RTS frames and CTS frames to actually reserve some first sub-channels of a targeted composite channel indicated in these frames, to reserve a TXOP for the collaborative group. A primary data communication may thus be established between nodes STA1 and STA2 on one or more of these first sub-channels that are preferably contiguous. The primary communication may or may not include the primary sub-channel.

The other nodes of the collaborative group must declare their availability to communicate on any of the other sub-channels (i.e. not necessarily on the first subchannels used by nodes STA1-STA2) of the composite channel. This is made by sending additional CTS frames on the non-used sub-channels (including the subchannels not yet reserved by nodes STA1-STA2 and any reserved first sub-channel not used by nodes STA1-STA2), which in turn may actually reserve additional subchannels for the collaborative TXOP.

Based on all the RTS/CTS exchanged, all the nodes are able to determine which sub-channels are available for which nodes (see for instance the table shown in Figure 10). By applying the same collaborative access rules, one or more communications between collaborative nodes may be initiated on the sub-channels not used by nodes STA1-STA2 for the primary communication.

It may be noted that, in addition to providing frequency division multiplexing access (i.e. various sub-channels of the composite channel are used for various communications), the collaborative access rules may also implement time division multiplexing access, meaning that any communication between a pair of nodes on a set of sub-channels (either the first sub-channels originally used by nodes STA1-STA2 or any sub-channels used by the other nodes) may be followed by another communication between another pair of nodes on the same set of sub-channels, within the granted TXOP.

Figure 18 illustrates a particular embodiment wherein only the sending node STA1 is not disturbed by the legacy node, whereas the addressee node STA2 and the other nodes STA3 and STA4 are all disturbed on the sub-channel 300-4.

In the given example, the sending node STA1 sends four RTS frames 1810 on the set of first sub-channels 300-1 to 300-4 forming the composite channel, to the addressee node STA2.

Since the addressee node STA1 sees the sub-channel 300-4 as busy and a bandwidth of 40 MHz is necessary to perform the primary communication with the sending node STA1, no CTS frames are sent on the sub-channel 300-4 and on the isolated 20 MHz sub-channel 300-3. Therefore, in the given example, first CTS frames 1820 are sent on sub-channels 300-1 and 300-2 only. The primary communication 1015-2 (video streaming) may thus occur between the sending node STA1 and the addressee node STA2.

The other nodes STA3 and STA4 are also disturbed on the sub-channel 300-4. However, they both see the sub-channel 300-3 as idle from their standpoints.

As already mentioned, the other nodes STA3 and STA4 are able to listen (i.e. to detect) control frames (e.g. RTS and CTS) that are exchanged between nodes of the collaborative group. Therefore, they know that no first CTS frame has been sent by the addressee node STA2 on the sub-channel 300-3, and so this sub-channel is not reserved yet.

The other nodes STA3 and STA4 can declare their availability to communicate on this sub-channel not yet reserved and to do so, they send additional CTS frames 1830 and 1840 (second CTS frames) on this sub-channel. The second sub-channel 300-3 is thus reserved for the collaborative TXOP, and a secondary exchange of data 1050-1 (Picture printing) may thus occur, for example between the nodes STA3 and STA4, on this second sub-channel now reserved.

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 which lie within the scope of the present invention will be apparent to a person skilled in the art. 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 as determined by the appended claims. In particular different features from different embodiments may be interchanged, where appropriate.

Claims (22)

1. A method for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, the method comprising, at one of said other nodes: - detecting at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel, by the sending node to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; - determining, based on said at least one detected request control frame and any detected first response control frame, at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames; and sending a second response control frame on said at least one determined second sub-channel to a node of the collaborative group, for exchanging data on said at least one determined second sub-channel during said transmission opportunity.

2. A method for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, the method comprising, at said sending node: transmitting at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and detecting a second response control frame on at least one second sub-channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, the at least one second sub-channel belonging to said composite channel and said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

3. A method for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, the method comprising, at said addressee node: receiving at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and detecting a second response control frame on at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

4. The method according to any one of claims 1 to 3, wherein said at least one first response control frame is sent on a plurality of contiguous first sub-channels that form an aggregated channel.

5. The method according to any one of claims 1 to 4, wherein the sending node transmits data to said addressee node on at least one of said first sub-channels on which a first response control frame has been received.

6. The method according to claim 5, wherein transmitting data to said addressee node is done only on at least one first sub-channel having a quality higher than a given threshold.

7. The method according to any one of claims 5 to 6, wherein transmitting data from said sending node to said addressee node starts as soon as said at least one said first response control frame is received by the sending node.

8. The method according to any one of claims 5 to 7, wherein said second response control frame is sent during said transmission of data from said sending node to said addressee node.

9. The method according to any one of claims 1 to 8, wherein no second response control frame is sent on a first sub-channel seen as idle by both sending node and addressee node.

10. The method according to any one of claims 1 to 9, wherein said second response control frame is sent only on said at least one second sub-channel.

11. The method according to any one of claims 1 to 10, wherein said at least one second sub-channel comprises a plurality of second contiguous sub-channels.

12. The method according to any one of claims 1 to 11, wherein said first and second response control frames are sent within a timeout period defining a duration during which a transmission opportunity reservation may be initiated in response to said at least one request control frame from the sending node.

13. The method according to any one of claims 1 to 12, wherein the size of the composite channel is indicated in each of the at least one request control frame.

14. The method according to any one of claims 1 to 13, further comprising a step of determining, for each sub-channel of the composite channel and for each node of the collaborative group, an availability status representing the availability of said subchannel for said node.

15. The method according to claim 1, further comprising at said other node, detecting another second response control frame sent by a second other node different from the sending and addressee nodes, on the at least one second sub-channel, and starting a data exchange with the second other node using the at least one second sub-channel during said transmission opportunity.

16. The method according to any one of claims 1 to 15, wherein the second response control frame is CTS-to-self.

17. The method according to any one of claims 1 to 16, wherein the second response control frame comprises a target address corresponding to the collaborative group, thereby allowing broadcasting the second response control frame to all the nodes of the collaborative group.

18. A system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, said at least one other node being configured to: - detect at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on first sub-channel belonging to said composite channel, by the sending node to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; determine, based on said at least one detected request control frame and any detected first response control frame, at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames; and send a second response control frame on said at least one determined second subchannel to a node of the collaborative group, for exchanging data on said at least one determined second sub-channel during said transmission opportunity.

19. A system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, said sending node being configured to: - transmit at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and detect a second response control frame on at least one second sub-channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, the at least one second sub-channel belonging to said composite channel and said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

20. A system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network, said collaborative group comprising a sending node, an addressee node and other nodes, said addressee node being configured to: receive at least one request control frame for initiating a transmission opportunity reservation for the collaborative group on a composite channel, each of the at least one request control frame being sent on a first sub-channel belonging to said composite channel to the addressee node, each of the at least one request control frame calling for a first response control frame from said addressee node on the first sub-channel on which it has been sent, to reserve a transmission opportunity for the collaborative group on this first sub-channel; and detect a second response control frame on at least one second sub-channel belonging to the composite channel on which no first response control frame has been sent by said addressee node in response to one of the request control frame or frames, said second response control frame being for exchanging data on said at least one second sub-channel during said transmission opportunity.

21. A method for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network as described substantially herein with reference to Figures 10, 11, 12,13, 14,15,16,17 and 18 of the accompanying drawings.

22. A system for reserving a transmission opportunity for a plurality of wireless communication devices belonging to a collaborative group in a wireless communication network as described substantially herein with reference to Figures 10, 11, 12, 13, 14, 15, 16, 17 and 18 of the accompanying drawings.