Patent Description:
The IEEE <NUM> MAC standard defines the way Wireless local area networks (WLANs) must work at the physical and medium access control (MAC) level. Typically, the <NUM> 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 <NUM> medium access protocol standard or operating mode is mainly directed to the management of communication nodes 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 <NUM>. 11ac standard provides very high throughput (VHT) by, among other means, moving from the <NUM> band which is deemed to be highly susceptible to interference to the <NUM> band, thereby allowing for wider frequency contiguous channels of <NUM> to be used, two of which may optionally be combined to get a <NUM> channel as operating band of the wireless network.

11ac standard also tweaks control frames such as the Request-To-Send (RTS) and Clear-To-Send (CTS) frames to allow for composite channels of varying and predefined bandwidths of <NUM>, <NUM> or <NUM>, the composite channels being made of one or more channels that are contiguous within the operating band. The <NUM> composite channel is possible by the combination of two <NUM> composite channels within the <NUM> operating band. The control frames specify the channel width (bandwidth) for the targeted composite channel.

A composite channel therefore consists of a primary channel on which a given node performs EDCA backoff procedure to access the medium, and of at least one secondary channel, of for example <NUM> each. The primary channel is used by the communication nodes to sense whether or not the channel is idle, and the primary channel can be extended using the secondary channel or channels to form a composite channel.

Sensing of channel idleness is made using CCA (clear channel assessment), and more particularly CCA-ED, standing for CCA-Energy Detect. CCA-ED is the ability of any node to detect non-<NUM> energy in a channel and back off data transmission. An ED threshold based in which the energy detected on the channel is compared is for instance defined to be 20dB above the minimum sensitivity of the PHY layer of the node. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy becomes below the threshold anew.

Given a tree breakdown of the operating band into elementary <NUM> channels, some secondary channels are named tertiary or quaternary channels.

11ac, all the transmissions, and thus the possible composite channels, include the primary 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 channel only. The other channels are assigned as secondary channels, on which the nodes have only capability of CCA (clear channel assessment), i.e. detection of an idle or busy state/status of said secondary channel.

An issue with the use of composite channels as defined in the <NUM>. 11n or <NUM>. 11ac (or <NUM>. 11ax) is that the <NUM>. 11n and <NUM>. 11ac-compliantnodes (i.e. HT nodes standing for High Throughput nodes) and the other legacy nodes (i.e. non-HT nodes compliant only with for instance <NUM>. 11a/b/g) have to co-exist within the same wireless network and thus have to share the <NUM> channels.

To cope with this issue, the <NUM>. 11n and <NUM>. 11ac standards provide the possibility to duplicate control frames (e.g. RTS/CTS or CTS-to-Self or ACK frames to acknowledge correct or erroneous reception of the sent data) in an <NUM>. 11a legacy format (called as "non-HT") to establish a protection of the requested TXOP over the whole composite channel.

This is for any legacy <NUM>. 11a node that uses any of the <NUM> channel involved in the composite channel to be aware of on-going communications on the <NUM> channel. As a result, the legacy node is prevented from initiating a new transmission until the end of the current composite channel TXOP granted to an <NUM>. 11n/ac node.

As originally proposed by <NUM>. 11n, a duplication of conventional <NUM>. 11a or "non-HT" transmission is provided to allow the two identical <NUM> non-HT control frames to be sent simultaneously on both the primary and secondary channels forming the used composite channel.

This approach has been widened for <NUM>. 11ac to allow duplication over the channels forming an <NUM> or <NUM> composite channel. In the remainder of the present document, the "duplicated non-HT frame" or "duplicated non-HT control frame" or "duplicated control frame" means that the node device duplicates the conventional or "non-HT" transmission of a given control frame over secondary <NUM> channel(s) of the (<NUM> <NUM> or <NUM>) operating band.

In practice, to request a composite channel (equal to or greater than <NUM>) for a new TXOP, an <NUM>. 11n/ac node does an EDCA backoff procedure in the primary <NUM> channel. In parallel, it performs a channel sensing mechanism, such as a Clear-Channel-Assessment (CCA) signal detection, on the secondary channels to detect the secondary channel or channels that are idle (channel state/status is "idle") during a PIFS interval before the start of the new TXOP (i.e. before the backoff counter expires).

More recently, Institute of Electrical and Electronics Engineers (IEEE) officially approved the <NUM>. 11ax task group, as the successor of <NUM>. The primary goal of the <NUM>. 11ax task group consists in seeking for an improvement in data speed to wireless communicating devices used in dense deployment scenarios.

Recent developments in the <NUM>. 11ax standard sought to optimize usage of the composite channel by multiple nodes in a wireless network having an access point (AP). Indeed, typical contents have important amount of data, for instance related to high-definition audio-visual real-time and interactive content. Furthermore, it is well-known that the performance of the CSMA/CA protocol used in the IEEE <NUM> standard deteriorates rapidly as the number of nodes and the amount of traffic increase, i.e. in dense WLAN scenarios.

In this context, multi-user transmission has been considered to allow multiple simultaneous transmissions to/from different users in both downlink and uplink directions. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple nodes to simultaneously transmit.

To actually perform such multi-user transmission, it has been proposed to split a granted <NUM> channel into sub-channels (elementary sub-channels), also referred to as resource units (RUs), that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

OFDMA is a multi-user variation of OFDM which has emerged as a new key technology to improve efficiency in advanced infrastructure-based wireless networks. It combines OFDM on the physical layer with Frequency Division Multiple Access (FDMA) on the MAC layer, allowing different subcarriers to be assigned to different nodes in order to increase concurrency. Adjacent sub-carriers often experience similar channel conditions and are thus grouped to sub-channels: an OFDMA sub-channel or RU is thus a set of sub-carriers.

As currently envisaged, the granularity of such OFDMA sub-channels is finer than the original <NUM> channel band. Typically, a <NUM> or <NUM> sub-channel may be contemplated as a minimal width, therefore defining for instance <NUM> sub-channels or resource units within a single <NUM> channel.

To support multi-user uplink, i.e. uplink transmission to the <NUM>. 11ax access point (AP) during the granted TxOP, the <NUM>. 11ax AP has to provide signalling information for the legacy nodes (non-<NUM>. 11ax nodes) to set their NAV and for the <NUM>. 11ax nodes to determine the allocation of the resource units RUs.

It has been proposed for the AP to send a trigger frame (TF) to the <NUM>. 11ax nodes to trigger uplink communications.

The document IEEE <NUM>-<NUM>/<NUM> proposes that a 'Trigger' frame (TF) is sent by the AP to solicit the transmission of uplink (UL) Multi-User (OFDMA) PPDU from multiple nodes. In response, the nodes transmit UL MU (OFDMA) PPDU as immediate response to the Trigger frame. All transmitters can send data at the same time, but using disjoint sets of RUs, resulting in transmissions with less interference.

The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the <NUM>, <NUM>, <NUM> or <NUM> value is added. The TF frame is sent over the primary <NUM> channel and duplicated (replicated) on each other <NUM> channels forming the targeted composite channel. As described above for the duplication of control frames, it is expected that every nearby legacy node (non-HT or <NUM>. 11ac nodes) receiving the TF on its primary channel, then sets its NAV to the value specified in the TF frame in order. This prevents these legacy nodes from accessing the channels of the targeted composite channel during the TXOP.

A resource unit RU can be reserved for a specific node, in which case the AP indicates, in the TF, the node to which the RU is reserved. Such RU is called Scheduled RU. The nodes do not need to perform contention on accessing scheduled RUs.

In order to better improve the efficiency of the system in regards to unmanaged traffic to the AP (for example, uplink management frames from associated nodes, unassociated nodes intending to reach an AP, or simply unmanaged data traffic), the document IEEE <NUM>-<NUM>/<NUM> proposes a new trigger frame (TF-R) above the previous UL MU procedure, allowing random access onto the OFDMA TXOP. In other words, the resource unit RU can be randomly accessed by more than one node. Such RU is called Random RU and is indicated as such in the TF. Random RUs may serve as a basis for contention between nodes willing to access the communication medium for sending data.

The random resource selection procedure is not yet defined. All that is known is that the trigger frame may define only Scheduled RUs, or only Random RUs within the targeted composite channel.

There is no guarantee that the Scheduled or Random RUs will be used by the nodes.

It is particularly the case for the Random RUs because any rule used by the nodes to select a Random RU may result in having RUs not allocated at all to any node. Also, the AP does not know whether or not some nodes need bandwidth. In addition, some RUs provided by the AP may not be accessible for some nodes because of hidden legacy nodes.

It is also the case for the Scheduled RUs (which are reserved by the AP because some nodes have explicitly requested bandwidth) if the specified nodes do not send data.

It results that the channel bandwidth is not optimally used.

Furthermore, the more unused RUs within a <NUM> channel, the lower the average energy over this <NUM> channel.

However, since the legacy nodes not registered to the AP use this average energy over their primary <NUM> to sense whether it is idle or busy, the presence of unused RUs increases the risk that legacy nodes sense the corresponding <NUM> channel as idle. The legacy nodes may then transmit data on this <NUM> channel, thus colliding with the data traffic conveyed over the used RUs.

The document "Multi channel availability for UL-OFDMA; <NUM>-<NUM>-<NUM>-<NUM>-00ax-multi-channel-availability-for-ul-ofdma" by Woojin Ahn discloses prior art relating to the invention.

It is a broad objective of the present invention to provide wireless communication methods and devices in a wireless network. The wireless network includes an access point and a plurality of nodes, all of them sharing the physical medium of the wireless network.

The present invention has been devised to overcome one or more foregoing limitations.

In this context, the present invention seeks to provide wireless communication methods having improved mechanisms against collisions in communication channels.

The invention can be applied to any wireless network in which an access point provides the registered nodes with a plurality of sub-channels (or resource units) forming a communication channel. The communication channel is the elementary channel on which the nodes perform sensing to determine whether it is idle or busy.

The invention is especially suitable for data transmission to the AP of an IEEE <NUM>. 11ax network (and future version).

The present invention proposes a communication apparatus as defined in Claim <NUM>.

The communication apparatus performs wireless communication according to Institute of Electrical and Electronics Engineers, IEEE <NUM> standard, and comprises:.

The invention also provides a communication method as defined in Claim <NUM>, comprising steps corresponding to the above-mentioned means.

The present invention also proposes a communication apparatus as defined in Claim <NUM>.

Another aspect of the invention relates to a non-transitory computer-readable medium storing a program which, when executed by a communication apparatus, causes the communication apparatus to perform the method as defined above.

The non-transitory computer-readable medium may have features and advantages that are analogous to those set out above and below in relation to the methods and node devices.

Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible carrier medium may comprise a storage medium such as a hard disk drive, a magnetic tape device or a solid state memory device and the like. A transient carrier medium may include a signal such as an electrical signal, an electronic signal, an optical signal, an acoustic signal, a magnetic signal or an electromagnetic signal, e.g. a microwave or RF signal.

Further advantages of the present invention will become apparent to those skilled in the art upon examination of the drawings and detailed description. Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings.

The invention will now be described by means of specific non-limiting exemplary embodiments and by reference to the figures.

<FIG> illustrates a communication system in which several communication nodes (or stations) <NUM>-<NUM> exchange data frames over a radio transmission channel <NUM> of a wireless local area network (WLAN), under the management of a central station, or access point (AP) <NUM>. The radio transmission channel <NUM> is defined by an operating frequency band constituted by a single channel or a plurality of channels forming a composite channel.

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 [<NUM>, 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 <NUM> standard.

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

<FIG> illustrates the behaviour of three groups of nodes during a conventional communication over a <NUM> channel of the <NUM> medium: transmitting or source node <NUM>, receiving or addressee or destination node <NUM> and other nodes <NUM> not involved in the current communication.

Upon starting the backoff process <NUM> prior to transmitting data, a station e.g. source node <NUM>, initializes its backoff time counter to a random value as explained above. The backoff time counter is decremented once every time slot interval <NUM> for as long as the radio medium is sensed idle (countdown starts from T0, <NUM> as shown in the Figure).

Channel sensing is for instance performed using Clear-Channel-Assessment (CCA) signal detection.

CCA is a WLAN carrier sense mechanisms defined in the IEEE <NUM>-<NUM> standards as part of the Physical Medium Dependant (PMD) and Physical Layer Convergence Protocol (PLCP) layer. It involves two functions:
Carrier Sense (CCA-CS) which is the ability of the receiving node to detect and decode an <NUM> frame preamble. From the PLCP header field, the time duration for which the medium will be occupied can be inferred and when such <NUM> frame preamble is detected, a CCA flag is held busy until the end of data transmission.

Energy Detect (CCA-ED) which is the ability of the receiving node to detect non-<NUM> energy in a specific <NUM> channel and back off data transmission. In practice, a level of energy over the <NUM> channel is sensed and compared to an ED threshold discriminating between a channel state with or without <NUM> energy channel. The ED threshold is for instance defined to be 20dB above the minimum sensitivity of a PHY layer of the node. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy becomes below the threshold anew.

The time unit in the <NUM> standard is the slot time called 'aSlotTime' parameter. This parameter is specified by the PHY (physical) layer (for example, aSlotTime is equal to <NUM> for the <NUM>. 11n standard). All dedicated space durations (e.g. backoff) add multiples of this time unit to the SIFS value.

The backoff time counter is 'frozen' or suspended when a transmission is detected on the radio medium channel (countdown is stopped at T1, <NUM> for other nodes <NUM> 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 T2, <NUM> as soon as the transmission opportunity TXOP granted to source node <NUM> ends and the DIFS period <NUM> elapses. DIFS <NUM> (DCF inter-frame space) thus defines the minimum waiting time for a source node before trying to transmit some data. In practice, DIFS = SIFS + <NUM> * aSlotTime.

When the backoff time counter reaches zero (<NUM>) at T1, the timer expires, the corresponding node <NUM> requests access onto the medium in order to be granted a TXOP, and the backoff time counter is reinitialized <NUM> using a new random backoff value.

In the example of the Figure implementing the RTS/CTS scheme, at T1, the source node <NUM> that wants to transmit data frames <NUM> 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 <NUM>") and the duration for which the radio medium is to be reserved for transmitting the control frames (RTS/CTS) and the data frames <NUM>.

Upon receiving the RTS frame and if the radio medium is sensed as being idle, the receiving node <NUM> responds, after a SIFS time period <NUM> (for example, SIFS is equal to <NUM> for the <NUM>. 11n 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 <NUM> as an acknowledgment of its request to reserve the shared radio medium for a given time duration.

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

The source node <NUM> is thus allowed to send the data frames <NUM> upon correctly receiving the CTS frame <NUM> and after a new SIFS time period <NUM>.

An ACK frame <NUM> is sent by the receiving node <NUM> after having correctly received the data frames sent, after a new SIFS time period <NUM>.

If the source node <NUM> does not receive the ACK <NUM> 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 using the backoff procedure anew.

Since the RTS/CTS four-way handshaking mechanism <NUM>/<NUM> is optional in the <NUM> standard, it is possible for the source node <NUM> to send data frames <NUM> immediately upon its backoff time counter reaching zero (i.e. at 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 <NUM>" in <FIG>) 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 <NUM> or CTS <NUM>) not addressed to itself, a listening node <NUM> updates its NAVs (NAV <NUM> associated with RTS and NAV <NUM> associated with CTS) with the requested transmission time duration specified in the control frame. The listening nodes <NUM> thus keep in memory the time duration for which the radio medium will remain busy.

Access to the radio medium for the other nodes <NUM> is consequently deferred <NUM> by suspending <NUM> their associated timer and then by later resuming <NUM> the timer when the NAV has expired.

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

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

The RTS/CTS four-way handshaking mechanism is very efficient in terms of system performance, in particular with regard to 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 <NUM> (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 <NUM> MAC always sends an acknowledgement (ACK) frame <NUM> after each data frame <NUM> received.

However, such collisions limit the optimal functioning of the radio network. As described above, simultaneous transmission attempts from various wireless nodes lead to collisions. The <NUM> backoff procedure was first introduced for the DCF mode as the basic solution for collision avoidance. In the emerging IEEE <NUM>. 11n/ac/ax standards, the backoff procedure is still used as the fundamental approach for supporting distributed access among mobile stations or nodes.

To meet the ever-increasing demand for faster wireless networks to support bandwidth-intensive applications, <NUM>. 11ac is targeting larger bandwidth transmission through multi-channel operations. <FIG> illustrates <NUM>. 11ac channel allocation that support composite channel bandwidth of <NUM>, <NUM>, <NUM> or <NUM>.

IEEE <NUM>. 11ac introduces support of a restricted number of predefined subsets of <NUM> channels to form the sole predefined composite channel configurations that are available for reservation by any <NUM>. 11ac node on the wireless network to transmit data.

The predefined subsets are shown in the Figure and correspond to <NUM>, <NUM>, <NUM>, and <NUM> channel bandwidths, compared to only <NUM> and <NUM> supported by <NUM>. Indeed, the <NUM> component channels <NUM>-<NUM> to <NUM>-<NUM> are concatenated to form wider communication composite channels.

In the <NUM>. 11ac standard, the channels of each predefined <NUM>, <NUM> or <NUM> subset are contiguous within the operating frequency band, i.e. no hole (missing channel) in the composite channel as ordered in the operating frequency band is allowed.

The <NUM> channel bandwidth is composed of two <NUM> channels that may or may not be frequency contiguous. The <NUM> and <NUM> channels are respectively composed of two frequency adjacent or contiguous <NUM> and <NUM> channels, respectively.

A node is granted a TxOP through the enhanced distributed channel access (EDCA) mechanism on the "primary channel" (<NUM>-<NUM>). Indeed, for each composite channel having a bandwidth, <NUM>. 11ac designates one channel as "primary" meaning that it is used for contending for access to the composite channel. The primary <NUM> channel is common to all nodes (STAs) belonging to the same basic set, i.e. managed by or registered to the same local Access Point (AP).

However, to make sure that no other legacy node (i.e. not belonging to the same set) uses the secondary channels, it is provided that the control frames (e.g. RTS frame/CTS frame) reserving the composite channel are duplicated over each <NUM> channel of such composite channel.

As addressed earlier, the IEEE <NUM>. 11ac standard enables up to four, or even eight, <NUM> channels to be bound. Because of the limited number of channels (<NUM> in the <NUM> band in Europe), channel saturation becomes problematic. Indeed, in densely populated areas, the <NUM> band will surely tend to saturate even with a <NUM> or <NUM> bandwidth usage per Wireless-LAN cell.

Developments in the <NUM>. 11ax standard seek to enhance efficiency and usage of the wireless channel for dense environments.

In this perspective, one may consider multi-user transmission features, allowing multiple simultaneous transmissions to different users in both downlink and uplink directions. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple nodes to simultaneously transmit.

To actually perform such multi-user transmission, it has been proposed to split a granted <NUM> channel (<NUM>-<NUM> to <NUM>-<NUM>) into sub-channels <NUM> (elementary sub-channels), also referred to as sub-carriers or resource units (RUs), that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

This is illustrated with reference to <FIG>.

The multi-user feature of OFDMA allows the AP to assign different RUs to different nodes in order to increase competition. This may help to reduce contention and collisions inside <NUM> networks.

Contrary to downlink OFDMA wherein the AP can directly send multiple data to multiple stations (supported by specific indications inside the PLCP header), a trigger mechanism has been adopted for the AP to trigger uplink communications from various nodes.

To support an uplink multi-user transmission (during a pre-empted TxOP), the <NUM>. 11ax AP has to provide signalling information for both legacy stations (non-<NUM>. 11ax nodes) to set their NAV and for <NUM>. 11ax nodes to determine the Resource Units allocation.

In the following description, the term legacy refers to non-<NUM>. 11ax nodes, meaning <NUM> nodes of previous technologies that do not support OFDMA communications.

As shown in the example of <FIG>, the AP sends a trigger frame (TF) <NUM> to the targeted <NUM>. 11ax nodes. The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the <NUM>, <NUM>, <NUM> or <NUM> value is added. The TF frame is sent over the primary <NUM> channel and duplicated (replicated) on each other <NUM> channels forming the targeted composite channel. As described above for the duplication of control frames, it is expected that every nearby legacy node (non-HT or <NUM>. 11ac nodes) receiving the TF on its primary channel, then sets its NAV to the value specified in the TF frame in order. This prevents these legacy nodes from accessing the channels of the targeted composite channel during the TXOP.

The trigger frame TF may designate at least one resource unit (RU) <NUM>, or "Random RU", which can be randomly accessed by more than one node. In other words, Random RUs designated or allocated by the AP in the TF may serve as basis for contention between nodes willing to access the communication medium for sending data. An exemplary embodiment of such random allocation is illustrated by <FIG>.

The trigger frame TF may also designate Scheduled resource units, in addition or in replacement of the Random RUs. Scheduled RUs may be reserved for certain nodes in which case no contention for accessing such RUs is needed.

In this context, the TF includes information specifying the type (Scheduled or Random) of the RUs. For instance, a tag may be used to indicate that all the RUs defined in the TF are Scheduled (tag = <NUM>) or Random (tag = <NUM>). In case, Random RUs and Scheduled RUs are mixed within the TF, a bitmap (or any other equivalent information) may be used to define the type of each RU (the bitmap may follow a known order of the RUs throughout the communication channels).

In the example of <FIG>, each <NUM> channel is sub-divided in frequency domain in four sub-channels or RUs <NUM>, typically of size <NUM>. These sub-channels (or resource units) are also referred to as "sub-carriers" or "traffic channels".

Of course the number of RUs splitting a <NUM> may be different from four. For instance, between two to nine RUs may be provided (thus each having a size between <NUM> and about <NUM>).

<FIG> illustrates exemplary communication lines according to an exemplary random allocation procedure that may be used by the nodes to access the Random RUs indicated in the TR. This random allocation procedure is based on the reuse of the backoff counter values of the nodes for assigning an RU to a node of the network to send data.

An AP sends a trigger frame TR defining RUs with random access. In the example of the Figure, eight RUs with the same bandwidth are defined for a <NUM> composite channel, and the TF <NUM> is duplicated on the two <NUM> channels forming the composite channel. In other words, the network is configured to handle four OFDMA Resource Units per each <NUM> channel.

Each node STA1 to STAn is a transmitting node with regards to receiving AP, and as a consequence, each node has at least one active backoff value.

The random allocation procedure comprises, for a node of a plurality of nodes having an active backoff, a first step of determining from the trigger frame the sub-channels or RUs of the communication medium available for contention, a second step of verifying if the value of the active backoff local to the considered node is not greater than the number of detected-as-available RUs, and then a step of sending data is performed on the RU whom number equals the backoff value.

In other words, the Random RUs may be indexed in the TF, and each node uses the RUs having an index equal to the backoff value of the node.

As shown in the Figure, some Resource Units may not be used, for instance RUs indexed <NUM> (<NUM>-<NUM>), <NUM>, <NUM> and <NUM>. This is due to the randomization process, and in the present example, due to the fact that none of the nodes has a backoff value equal to <NUM>, <NUM>, <NUM> or <NUM> when the TF is sent.

The legacy <NUM>. 11a/n/ac nodes that operate at a <NUM> channel-width granularity can detect the Trigger Frame in various ways.

In case a legacy node (<NUM>. 11a/n/ac) has its primary channel operating on one of the <NUM> channels (<NUM>) on which the TF is duplicated, the node can defer its activity using Clear Channel Assessment (CCA). To be precise, the node uses a full CCA on the primary channel, including preamble packet detection (called Signal Detection SD), and performs both physical carrier sensing and virtual carrier sensing. In other words, the node decodes the detected PLCP (Physical Layer Convergence Protocol) preamble from the TF received on its primary channel and use that information to set its NAV (Network Allocation Vector) counter.

In case a legacy node (<NUM>. 11n/ac) does not have its primary channel within the composite channel used by the AP, but has secondary channel(s) in the composite channel, the node uses a reduced CCA (called energy detection (ED) as the signal is not decodable) on the secondary channel and thus does not set the NAV counter.

CCA on the primary channel is set only if the legacy node has successfully received the Trigger Frame. Note that further OFDMA RU transmissions are not decodable by legacy nodes.

An issue arises with new comers in the network, or more classically with nodes experiencing hidden nodes. Such nodes may perform CCA sensing anew during OFDMA TXOP (i.e. after the TF has been transmitted).

However, a legacy node may not be able to detect a significant signal onto its primary channel if the measurement of total received RF (radiofrequency) power or energy within the defined <NUM> channel bandwidth suffers from free RUs during TXOP <NUM>. The problem mostly comes from the fact that <NUM> legacy nodes assess availability of the medium based on <NUM> portions, while UL OFDMA assignments could be narrower and varying across the BSS coverage.

The aforementioned issue of under-usage of Resource Units should be handled with care, as the resulting signal energy on a specific <NUM> channel could drop under the energy detection (ED) threshold used by the legacy nodes (for example, the energy detection threshold is -62dBm for a <NUM> channel width).

Indeed, collisions may occur on under-used <NUM> channel (i.e. channel in which some RUs are unused) as soon as the legacy nodes do not detect enough signal energy. In other words, an OFDMA TXOP <NUM> having unused RUs is a factor of increasing of collisions (conducting to a new kind of collision), which is opposite to the intended usage of the Random RUs.

Unuse of Scheduled RUs may lead to the same issue of having legacy nodes colliding the OFDMA traffic on some RUs.

The present invention finds a particular application in enhancements of the <NUM>. 11ac standard, and more precisely in the context of <NUM>. 11ax wherein dense wireless environments are more ascertained to suffer from previous limitations.

The present invention provides improved wireless communications with more efficient use of bandwidth while limiting the risks of collision.

An exemplary wireless network is an IEEE <NUM>. 11ac network (and upper versions). However, the invention applies to any wireless network comprising an access point AP <NUM> and a plurality of nodes <NUM>-<NUM> transmitting data to the AP through a multi-user transmission. The invention is especially suitable for data transmission in an IEEE <NUM>. 11ax network (and future versions) requiring better use of bandwidth.

An exemplary management of multi-user transmission in such a network has been described above with reference to <FIG>.

First embodiments of the present invention provide that, following a trigger frame reserving at least one communication channel of the wireless network and defining a plurality of resource units forming the communication channel, one or more devices in the network perform the following steps:.

Preferably, the device involved is the AP. Alternatively, one of nodes <NUM>-<NUM> may be involved.

The overall energy level over the <NUM> communication channel can thus be raised above the ED threshold. It results that no legacy node is about to sense this channel as idle. Collisions are avoided.

Second embodiments of the present invention are directed to the cases where duplicated trigger frames are sent to reserve a plurality of communication channels each made of an ordered plurality of resource units, the duplicated trigger frames defining one or more scheduled resource units on which respective specified nodes are allowed to transmit data and one or more random resource units that the nodes access on a random basis (i.e. using a contention scheme). In the second embodiments, it is provided that the scheduled resource units and the random resource units are substantially uniformly distributed over the plurality of communication channels. "substantially uniformly distributed" means that it is sought to have substantially the same number of scheduled resource units over the channels (i.e. the difference in number of scheduled RUs between two channels is at most <NUM>).

As the scheduled RUs are more liable to be used by the associated nodes, the second embodiments reduce the risk that a communication channel has a very low overall energy level. The legacy nodes will statistically sense more often the channels as busy, thus avoiding collisions to occur.

The first and second embodiments can be implemented separately, or in combination as further described below.

<FIG> schematically illustrates a communication device <NUM> of the radio network <NUM>, configured to implement at least one embodiment of the present invention. The communication device <NUM> may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device <NUM> comprises a communication bus <NUM> to which there are preferably connected:.

Optionally, the communication device <NUM> may also include the following components:.

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

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device <NUM> 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 <NUM> directly or by means of another element of the communication device <NUM>.

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

The central processing unit <NUM> is preferably 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 <NUM> or in the read only memory <NUM>, are transferred into the random access memory <NUM>, 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 a preferred embodiment, the apparatus is a programmable apparatus which uses software to implement the invention. However, alternatively, the present invention may be implemented in hardware (for example, in the form of an Application Specific Integrated Circuit or ASIC).

<FIG> is a block diagram schematically illustrating the architecture of a communication device or node <NUM>, either the AP <NUM> or one of nodes <NUM>-<NUM>, adapted to carry out, at least partially, the invention. As illustrated, node <NUM> comprises a physical (PHY) layer block <NUM>, a MAC layer block <NUM>, and an application layer block <NUM>.

The PHY layer block <NUM> (here an <NUM> standardized PHY layer) has the task of formatting, modulating on or demodulating from any <NUM> channel or the composite channel, and thus sending or receiving frames over the radio medium used <NUM>, such as <NUM> frames, for instance medium access trigger frames TF <NUM> to reserve a transmission slot, MAC data and management frames based on a <NUM> width to interact with legacy <NUM> stations, as well as of MAC data frames of OFDMA type having smaller width than <NUM> legacy (typically <NUM> or <NUM>) to/from that radio medium.

The PHY layer block <NUM> includes CCA capability to sense the idle or busy state of <NUM> channels and to report the result to the MAC <NUM> according to <NUM> standard. Upon detecting a signal with significant received signal strength, an indication of channel use is generated.

The MAC layer block or controller <NUM> preferably comprises a MAC <NUM> layer <NUM> implementing conventional <NUM>. 11ax MAC operations, and an additional block <NUM> for carrying out, at least partially, the invention. The MAC layer block <NUM> may optionally be implemented in software, which software is loaded into RAM <NUM> and executed by CPU <NUM>.

Preferably, the additional block, referred to as the OFDMA RU energy detection module <NUM>, implements the part of the invention that regards node <NUM>, i.e. detecting use of OFDMA RUs and energy over a <NUM> channel based on PHY layer <NUM>. The OFDMA RU energy detection module <NUM> also performs transmitting and receiving operations on RUs.

On top of the Figure, application layer block <NUM> runs an application that generates and receives data packets, for example data packets of a video stream. Application layer block <NUM> represents all the stack layers above MAC layer according to ISO standardization.

<FIG> illustrates, using two flowcharts, general steps of embodiments of the present invention. These embodiments provide efficient management of (uplink) multi-user OFDMA transmissions (RUs) in an <NUM>. 11ax wireless medium, to reduce risk of collisions with legacy nodes.

The method of <FIG> is implemented by at least one node <NUM>. one node is the access point AP <NUM> to perform flowchart 8a, and one or more other nodes or the same AP can be involved to perform flowchart 8b.

Flowchart 8a illustrates the algorithm performed to prepare a trigger frame TF and transmit it on the wireless channel. This algorithm is performed by the access point AP.

According to embodiments of the present invention, the TF presents a efficient distribution of the OFDMA RUs, in particular a Trigger Frame profile having the scheduled resource units and the random resource units substantially uniformly distributed over the plurality of communication channels composing the targeted composite channel. In particular, both Random and Scheduled RUs are interleaved one with each other.

A goal of this approach is to average signal energy over a <NUM> channel, if possible above the minimum energy detection threshold of the legacy node, rather than having all the energy concentrated on few <NUM> channels.

Flowchart 8b illustrates the behaviour of at least one node <NUM> (either AP <NUM> or any node <NUM>-<NUM>) for monitoring the energy of active RUs forming an uplink OFDMA transmission slot, and performing a signal strength analysis per each <NUM> communication channel. It means that the node evaluate overall signal strength over the communication channel during the sensing period (from a predefined start time at which the node or nodes start transmitting data on the resource unit or units).

In case of insufficient signal (i.e. energy below the ED threshold), the monitoring node <NUM> is allowed to communicate over at least one empty RU, i.e. unused RU. In other words, the monitoring node emits a signal on the sensed unused resource unit or units depending on the evaluated overall signal strength.

Preferably, the monitoring node <NUM> is the access point AP itself, because it is the destination of the uplink traffics and is well positioned to detect the received signals. In a variant, it may be one of the transmitting nodes <NUM>-<NUM>.

Note that a transmitting and monitoring node <NUM> has preferably two or more transmissions chains in order to simultaneously send data in one RUs and monitor usage of other RUs according to the invention. A node having only one transmission chain may still be designated by the AP to act as the monitoring node, but will not be capable to simultaneously send data in one RU: it is only allowed to monitor the energy of one or more communication channels.

While the example of <FIG> combines the interleaving of Scheduled and Random RUs with the emission of a signal on unused RUs, these two features may be used alone since both contribute to raise the overall energy per channel over the composite channel, and thus to reduce risks of having legacy nodes transmitting on a channel having RUs used.

Although not limited in this respect, one way to implement the algorithm 8a is as follows.

At step <NUM>, the AP determines the number of Resource Units to consider for the multi-user TXOP upon being granted. This determination is based on the BSS configuration environment, that is to say the basic operational width (namely <NUM>, <NUM>, <NUM> or <NUM> channels that include the primary <NUM> channel according the <NUM>. 11ac standard).

For the sake of simplicity, one may consider that a fixed number of OFDMA RUs is allocated per <NUM> band by the <NUM>. 11ax standard (for instance nine): in that case, Bandwidth signalling in the TF frames (i.e. <NUM>, <NUM>, <NUM> or <NUM> values is added) is sufficient for the nodes to know the number of RUs. Typically, such information is signaled in the SERVICE field of the DATA section of non-HT frames according the <NUM> standard, thus keeping compliance with the medium access mechanism for the legacy nodes.

The TF may include an information element indication that the multi-user TXOP includes either or both random type and scheduled Resource Units <NUM>; that is to say multiple nodes can access a RU inside the OFDMA TXOP either randomly by a random allocation procedure or at a fixed RU location attributed by the AP. In other words, the trigger frame defines which resource unit or units of the communication channel(s) are reserved for specified nodes and which resource unit or units of the communication channel the nodes access on a random basis (using a contention scheme).

In embodiments of the invention, if the allocation includes some fixed allocation (i.e. Scheduled RUs), those fixed allocations are uniformly distributed among the entire composite channels in order to guarantee an average repartition of the channel energy. For instance, if eight Scheduled RUs have to be allocated among <NUM> RUs forming four <NUM> channels, two Scheduled RUs may be positioned in each one of the four <NUM> channels.

Next to step <NUM>, step <NUM> consists for the AP to determine one node to be declared as the monitoring station for a given <NUM> channel.

In a preferred embodiment, the AP considers itself as the monitoring station for all <NUM> channels forming the composite channel in which case the flowchart of <FIG> is performed by the AP only.

In a variant, the AP can select a registered node per <NUM> channel for it to monitor the considered <NUM> channel in which case the flowchart of <FIG> is performed by this node for the considered <NUM> channel. Thanks to the registration process of the nodes to the AP, the AP may identify, for the appropriate <NUM> channel, the corresponding monitoring node by its association identifier (AID), in the TF. In that case, the trigger frame indicates a specific node which is allowed to emit a signal on each unused resource unit of the considered communication channel.

In other variants, rather than associating a monitoring node per <NUM> channel, a monitoring node may be defined for each RU. In that case, the trigger frame indicates a specific node per resource unit, which is allowed to emit a signal on this resource unit if unused.

<FIG> proposes a format for an Information Element inside a TF frame to indicate the allocation of a monitoring node with a given Resource Unit.

Note that other variants may consider a dynamic procedure to assign which node is a monitoring node, without having the AP specifying it in the TF. For instance, the dynamic procedure may assign a node transmitting on one RU to monitor the adjacent or next RU in the same <NUM> channel. In other words, the node may transmit data (uploading data to the AP) on one resource unit during the sensing period, and determine on which unused resource unit or units to emit the signal, based on which resource unit the node transmits data. If the resource units are ordered within the communication channel, the unused resource unit or units on which to emit the signal may be further determined relative to the order of the resource units, for instance the next resource unit or units (note that the AP may be in charge of the first RU or RUs of the <NUM> channel if unused).

Next to step <NUM>, step <NUM> consists for the AP to send a TF frame (and possibly duplicates thereof if the composite channel includes more than one <NUM> channel) with an indication of the bandwidth of the targeted TXOP.

The TF also defines the RUs and their types (Scheduled or Random).

When appropriate, the TF also include an indication of which node is a monitoring node for specific <NUM> channel(s) and/or RUs.

It is expected that every nearby node (legacy or <NUM> ac) can receive the TF on its primary channel. Each of these nodes then sets its NAV to the value specified in the TF frame: the medium is thus theoretically reserved by the AP.

To avoid legacy nodes not receiving the Trigger Frame to erroneously sense a <NUM> channel as available, the algorithm continue with step <NUM> during which the AP waits for the start of OFDMA transmissions by the nodes in the RUs. This predefined start time at which the node or nodes start transmitting data on the RUs occurs for instance a SIFS interval after the emission (or reception) of the TF frame.

The predefined start time starts the sensing or monitoring period as defined above.

Next, at step <NUM>, it is checked whether or not the AP is a monitoring node.

In the affirmative, the AP performs the monitoring and signal emission steps (<NUM> to <NUM>) according the embodiments of the invention. These steps are described below with reference to Flowchart 8b.

Next to the steps <NUM>-<NUM> and in the negative of test <NUM> (the AP does not monitor any <NUM> channel), the AP waits (step <NUM>) for the end of the OFDMA TXOP <NUM> as defined in the TF.

Next, at step <NUM>, the AP sends an acknowledgment frame (ACK frame) related to the received MPDUs from the multiple nodes within the OFDMA TXOP <NUM>.

Preferably, the ACK frame is transmitted in a non-HT duplicate format in each <NUM> channel of the composite channel. This acknowledgment is necessary for the multiple transmitting nodes to determine if the destination (AP) has well received the OFDMA MPDUs, as the transmitting nodes are not be able to detect collisions inside their selected RUs (for instance collision in RU#<NUM> of <FIG> since two nodes have the same backoff value equal to <NUM>).

Turning now to Flowchart 8b, it illustrates the behaviour of at least one monitoring node <NUM> (either AP <NUM> or transmitting node <NUM>-<NUM>) for monitoring the energy due by the active RUs over a <NUM> channel, when uplink OFDMA transmission is implemented.

In embodiments, if the signal strength analyzed on a <NUM> channel is less than a legacy threshold (as example -62dBm), then the monitoring node sends an OFDMA transmission in relation to the missing level of energy. For instance, it emits a signal on the unused resource unit or units.

In variant, the signal strength analysis may be avoided, and a signal is automatically emitted on each unused resource unit or units.

As described above with reference to <FIG>, the AP acting as a monitoring node, only performs steps <NUM>-<NUM> (if test <NUM> is positive). All the transmitting nodes registered to the AP perform the whole Flowchart 8b (test <NUM> described below discriminating between the monitoring nodes and the other nodes).

The process starts at step <NUM> during which a transmitting node <NUM> verifies whether or not it has received an <NUM>. 11a frame in a non-HT format. Preferably, the type/sub-type indicates a trigger frame TF type, and the Receiver Address (RA) of the TF is a broadcast or group address (this is not a unicast address corresponding to the node <NUM> MAC address).

Upon receiving the trigger frame TF, the composite channel width occupied by the TF control frame is signaled in the SERVICE field of the <NUM> data frame (The DATA field is composed of SERVICE, PSDU, tail, and pad parts).

At step <NUM>, the transmitting node <NUM> verifies whether or not it has to act as a monitoring node for at least one <NUM> channel of the composite channel width.

For instance it may search for any Information Element inside the trigger frame that specifies the transmitting node <NUM> is designated (the format of such indication is provided as example in regards to <FIG>) as a monitoring node for a specific <NUM> channel.

In a variant based on a dynamic procedure, the transmitting node is automatically a monitoring node as soon as it transmits on one RU. In the example above, it is in charge of the next RU or RUs in case they are not used by other nodes.

A positive verification conducts to apply steps <NUM> to <NUM>.

In case of the transmitting node has not to act as a monitoring node, the algorithm stops at step <NUM>. That is to say, the node continues any usual action independent of the current invention: the node STA responds to the received TF with at least one <NUM> PPDU frame (PPDU means PLCP Protocol Data Unit, with PLCP for Physical Layer Convergence Procedure; basically a PPDU refers to an <NUM> physical frame) in an <NUM>. 11ax format after a SIFS period in a Scheduled Resource Unit of the OFDMA TXOP <NUM> if it is dedicated to it, or in a Random RU if an Random RU allocation scheme allocates it with such a Random RU.

Focus is now on steps <NUM>-<NUM> performed by the monitoring node (either AP or transmitting node).

At step <NUM>, the monitoring node monitors the energy level of the considered <NUM> channel using conventional sensing mechanism. Thanks to the loop <NUM>, the sensing or monitoring period lasts for a predefined duration, e.g. two aSlotTime time units, from a predefined start time (e.g. SIFS after the TF) at which the node or nodes start transmitting data on the resource unit or units. This duration corresponds to 'DIFS - SIFS', wherein the DIFS (standing for DCF InterFrame Spacing) corresponds to the time period that an <NUM> node should sense a medium as idle before transmitting new data frames.

The sensing or monitoring period has thus a known duration.

During the monitoring period, module <NUM> computes the signal energy over the <NUM> channel assigned to the monitoring node for monitoring.

Once the sensing period ends, the monitoring node has evaluated signal energy for the <NUM> channel, and can compare it to an ED threshold at step <NUM>.

If there is enough signal energy, the process continues at step <NUM>.

Otherwise, the monitoring node determines (step <NUM>) which (Scheduled and/or Random) RUs within the monitored <NUM> channel are unused. This may be done by analysing the OFDMA signal received to detect which OFDMA slots are unused (i.e. without an UL OFDMA PPDU).

Next, at step <NUM>, the monitoring emits a signal on sensed unused resource unit or units. And the signal is emitted until an end time at which all the nodes stop transmitting on all the resource units forming the communication channel. This is the end time of TXOP <NUM> as specified in the TF.

The signal is preferably made of padding data (i.e. without content intelligible for the AP). However, in a variant, it may be provided that the signal includes data to the access point (plus possible padding data to reach the transmission duration of TXOP <NUM>).

In a first embodiment, each RU is maintained busy for the time indicated by AP (TXOP <NUM>). It means that a signal is emitted on any RU detected as empty (case 910a of <FIG>). The monitoring node emits the signal with non-AP station power requirements (which are often less than allowed power value for access points).

A padding transmission may be equivalent to the A-MPDU padding as defined in <NUM>. 11ac specifications, which is used if a node does not have enough data to fill the available PSDU bytes.

To save energy, the signal emitted on the unused RU or RUs is with own signal strength so that overall signal strength over the <NUM> communication channel is above the ED threshold. In other words, the additional signal emitted by the monitoring node is slightly above the lack of energy detected at step <NUM> (i.e. equivalent for reaching the ED threshold).

In a second embodiment, if at least two RUs are detected as unused within the <NUM> channel, the monitoring node may emit a signal on a subpart only of the unused RUs of the <NUM> channel, provided that the own signal strength of the emitted signal makes that resulting overall signal strength over the <NUM> communication channel is above the ED threshold.

In a variant, the monitoring node may aggregate several contiguous RUs for sending a single padding data (case 910b of <FIG>).

Note that in one embodiment in which no energy detection is made on the <NUM> channel, steps <NUM>-<NUM> may be avoided. The monitoring node automatically emits a signal on the unused RUs.

Next to step <NUM>, step <NUM> consists for the monitoring node to stop emitting the signal (padding transmission) upon reading the end of TXOP duration.

Algorithm thus ends for the transmitting nodes (test <NUM>). As far as the AP is concerned, it now returns to usual mode, that is to say sending ACK for received data in used RUs (step <NUM>).

<FIG> illustrates exemplary communication lines according to the invention. Although these examples show a WLAN system using a multi-channel including a <NUM> bandwidth composite channel having a set of <NUM> OFDMA resource units, the number of <NUM> bands forming the overall composite channel and/or the number of OFDMA resource units per <NUM> channel bandwidth thereof may vary.

Also the application of the invention is raised through examples that use the trigger frame mechanism sent by an AP for multi-user uplink transmissions according to <NUM>. Of course, equivalent mechanisms may be used in an ad-hoc environment (no AP), meaning that a TF is sent by a node. Examples of monitoring nodes include the nodes to which scheduled RUs are allocated.

An AP sends a trigger frame for multi-user uplink transmissions on an overall exemplary <NUM> composite channel (meaning the TF <NUM> is duplicated on four <NUM> channels). This example suggests that the network is configured to handle four OFDMA Resource Units per each <NUM> channel (all nodes are aware of this configuration, or elsewhere the configuration is specified by the Trigger Frame).

Some Resource Units are not used (as example the indexes <NUM>, <NUM>, <NUM>, <NUM> to <NUM>, <NUM> and <NUM>) during the period <NUM> due to the RU allocation scheme implemented by the nodes.

Reference <NUM> indicates the sensing or monitoring period corresponding to the loop between steps <NUM> to <NUM> of Flowchart 8b.

Next, after sensing period <NUM> is over, step <NUM> conducts to obtain various padding signals <NUM>. 910a is a single padding over one RU. 910b is an exemplary padding performed over aggregated contiguous RUs.

Reference <NUM> illustrates an optional embodiment corresponding to the case of total inactivity over a secondary <NUM> channel <NUM>. As no OFDMA communication at all occurs, it is possible for the monitoring node executing Flowchart 8b to decide to free the whole <NUM> channel in order to ensure environment fairness. In other words, if the monitoring node senses that all the resource units forming a communication channel are unused, the node does not emit a signal on those resource units forming the communication channel.

<FIG> presents the format of a 'RU Information Element' (<NUM>), which may be used according to embodiments of the present invention.

The 'RU Information Element' (<NUM>) is used by the AP to embed additional information inside the trigger frame related to the OFDMA TXOP. Its format follows the 'Vendor Specific information element' format as defined in IEEE <NUM>-<NUM> standard.

The 'RU Information Element' (RU IE, <NUM>) is a container of one or several RU attributes (<NUM>), having each a dedicated attribute ID for identification. The header of RU IE can be standardized (and thus easily identified by the nodes) through the Element ID, OUI, OUI Type values.

The RU attributes <NUM> are defined to have a common general format consisting of a one-byte RU Attribute ID field, a two-byte Length field and variable length attribute specific information fields.

The usage of Information Element inside the MAC frame payload is given for illustration only, any other format may be supportable.

The choice of embedding additional information in the MAC payload is advantageous for keeping legacy compliancy with the medium access mechanism, because any modification performed inside the PHY header of the <NUM> frame would have inhibited any successful decoding of the MAC header by legacy devices.

In regards to step <NUM>, the access point may want to designate a transmitting node as a monitoring node for an entire <NUM> channel. The trigger frame contains a list of RU attributes <NUM>, each one being used to specify the transmitting node responsible for a given <NUM> channel.

To do so, the TF contains a specific information element <NUM> in the frame body of the <NUM> MAC frame, which contains the RU attribute <NUM> according to <FIG>.

As shown in the Figure, a dedicated RU attribute follows the following format:.

As one can note, the various alternative embodiments presented in <FIG> and <FIG> are compatible one with each other, and may be combined to take advantage of their respective advantages.

Although the present invention has been described hereinabove with reference to specific embodiments, the present invention is not limited to the specific embodiments, and modifications will be apparent to a skilled person in the art which lie within the scope of the present invention.

Many further modifications and variations will suggest themselves to those versed in the art upon making reference to the foregoing illustrative embodiments, which are given by way of example only and which are not intended to limit the scope of the invention, that being determined solely by the appended claims. In particular the different features from different embodiments may be interchanged, where appropriate.

Claim 1:
A communication apparatus (<NUM>) configured to perform wireless communication according to Institute of Electrical and Electronics Engineers, IEEE <NUM> standard, the communication apparatus comprising:
a receiving means (<NUM>) for receiving, from an access point, a trigger frame including first information indicating a plurality of resource units splitting a communication channel in a frequency domain and second information for determining whether a sensing of energy of the communication channel is required;
a sensing means (<NUM>) for sensing the energy of the communication channel in a case where the communication apparatus is identified by an Association ID, AID, included in the trigger frame and the second information indicates that the sensing of the energy of the communication channel is required; and
a transmission control means (<NUM>) for performing control such that the communication apparatus does not transmit a signal in a case where the sensed energy exceeds a specific threshold, and the communication apparatus transmits a signal on one or more resource units included in the plurality of resource units otherwise.