GB2540184A

GB2540184A - Dynamic ajusting of contention mechanism for access to random resource units in an 802.11 channel

Info

Publication number: GB2540184A
Application number: GB1511968.8A
Authority: GB
Inventors: Nezou Patrice; Baron Stéphane; Guignard Romain; Viger Pascal
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2015-07-08
Filing date: 2015-07-08
Publication date: 2017-01-11
Anticipated expiration: 2035-07-08
Also published as: GB201511968D0; GB2540184B

Abstract

In an 802.11ax network with an access point, a trigger frame offers random resource units to nodes for data uplink communication to the access point. To dynamically adapt the contention mechanism used by the nodes to access the random resource units, the AP updates a correcting parameter at each new TXOP (Transmission Opportunity) and includes the updated adjusting parameter in the trigger frame for the next TXOP. The correcting parameter is a function of the number of unused random resource units and the number of collided random resource units. The nodes use the adjusting parameter to generate a local random parameter from the conventional 802.11 backoff value. Next, the nodes contend for access to the random resource units using the local random parameter. The dynamic adjustment of the contention mechanism allows utilisation of the available sub-channels while minimising collisions on these sub-channels. A second embodiment describes sending padding data on a random resource unit to maintain synchronisation. Use of the above methods are described within a WLAN operating very high throughput (VHT) by combining contiguous channels.

Description

DYNAMIC AJUSTING OF CONTENTION MECHANISM FOR ACCESS TO RANDOM RESOURCE UNITS IN AN 802.11 CHANNEL

FIELD OF THE INVENTION

The present invention relates generally to wireless communication networks and more specifically to the random allocation for Uplink communication of OFDMA sub-channels (or Resource Units) forming for instance a communication composite channel. One application of the method regards wireless data communication over a wireless 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

The IEEE 802.11 MAC standard defines the way Wireless local area networks (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 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 which is deemed to be highly susceptible to interference to the 5GHz band, thereby allowing 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 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 20, 40 or 80MHz, the composite channels being made of one or more 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 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 20MHz 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-802.11 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 20MHz channels, some secondary channels are named tertiary or quaternary channels.

In 802.11ac, 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 802.11η or 802.11ac (or 802.11ax) is that the 802.11η and 802.11ac-compliant nodes (i.e. HT nodes standing for High Throughput nodes) and the other legacy nodes (i.e. non-HT nodes compliant only with for instance 802.11a/b/g) have to co-exist within the same wireless network and thus have to share the 20MHz channels.

To cope with this issue, the 802.11η and 802.11ac 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 802.11a legacy format (called as “non-HT”) to establish a protection of the requested TXOP over the whole composite channel.

This is for any legacy 802.11a node that uses any of the 20MHz channel involved in the composite channel to be aware of on-going communications on the 20MHz 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 802.11n/ac node.

As originally proposed by 802.11η, a duplication of conventional 802.11a or “non-HT” transmission is provided to allow the two identical 20MHz 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 802.11ac to allow duplication over the channels forming an 80MHz or 160MHz 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 20MHz channel(s) of the (40MHz 80MHz or 160MHz) operating band.

In practice, to request a composite channel (equal to or greater than 40MHz) for a new TXOP, an 802.11n/ac node does an EDCA backoff procedure in the primary 20MHz 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 802.11 ax task group, as the successor of 802.11ac. The primary goal of the 802.11 ax task group consists in seeking for an improvement in data speed to wireless communicating devices used in dense deployment scenarios.

Recent developments in the 802.11 ax 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 802.11 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 20MHz 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 20MHz channel band. Typically, a 2MHz or 5MHz sub-channel may be contemplated as a minimal width, therefore defining for instance 9 subchannels or resource units within a single 20MHz channel.

To support multi-user uplink, i.e. uplink transmission to the 802.11ax access point (AP) during the granted TxOP, the 802.11 ax AP has to provide signalling information for the legacy nodes (non-802.11ax nodes) to set their NAV and for the 802.11 ax 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 802.11 ax nodes to trigger uplink communications.

The document IEEE 802.11-15/0365 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 responses to the Trigger frame. All transmitters can send data at the same time, but using disjoint sets of RUs (i.e. of frequencies in the OFDMA scheme), resulting in transmissions with less interference.

The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is added. The TF frame is sent over the primary 20MHz channel and duplicated (replicated) on each other 20MHz 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 802.11ac 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 indicated node does not need to perform contention on accessing a scheduled RU reserved to it.

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 802.11-15/0604 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.

SUMMARY OF INVENTION

As the nodes access the RUs on a random basis, the risk that either nodes collide on the same RU, or some RUs are not used, or both is high.

For instance, there is no guarantee that the Scheduled and 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.

On the other, depending on the contention procedure used by the nodes to randomly access the Random RUs, it may happen that nodes select the same RUs and thus collide.

To reduce the risk, a desired access rule may be deployed over the nodes to drive the random access as desired. For instance, the same mapping may be implemented in each node to map a local random value, such as the conventional local backoff counter, onto the RU having the same index value in the composite channel (for instance based on an ordering index of the RUs within the composite channel), which mapped RU is thus selected for access by the node.

However, the use of an access rule may not be satisfactory to efficiently reduce the risk, in particular because the network evolves: the number of nodes registered in the AP evolves over time, the number of nodes having data to upload to the AP, etc. Due to such network evolution, an access rule relevant at a first time may prove not to be relevant at a later time.

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 particular to provide wireless communication methods having more efficient usage of the network bandwidth (of the RUs) with limited risks of collisions.

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 uplink transmission to the AP of an IEEE 802.11 ax network (and future version).

First main embodiments of the invention provide, from the access point’s perspective, a wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at the access point: sending one or more trigger frames to the nodes, each trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, each trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining statistics (i.e. at least one item of information) on random resource units not used by the nodes during the one or more transmission opportunities and/or random resource units on which nodes collide during the one or more transmission opportunities; determining a correcting parameter based on the determined statistics, sending, to the nodes, a next trigger frame for reserving a next transmission opportunity, the next trigger frame including the determined correcting parameter.

The next trigger frame (TF) is not necessarily adjacent to one previous TF having a correcting parameter. For instance, a conventional TF may be sent there between. Also conventional RTS/CTS exchanges may occur between two trigger frames according to the first main embodiments (i.e. including the correcting parameter).

The same first main embodiments of the invention provide, from the node’s perspective, a wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at one of said nodes: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network and including a correcting parameter, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining, based on the correcting parameter and on one random parameter local to the node, one of the random resource units (this step corresponds to the way the nodes contend for access to the random resource units according to the first embodiments of the invention); transmitting data to the access point using the determined random resource unit.

In these first main embodiments, a correcting parameter is exchanged between the access point and the nodes. On one hand, it is used by the nodes to adjust how the local random parameter impacts the choice of the random RUs to be used. This is why the parameter is named “correcting”. On the other hand, this correcting parameter is calculated by the access point based on statistics related to the use of the Random RUs (unused or collided RUs). This is because the access point has an overall view of the network, as the nodes only communicate with it.

It results that the contention scheme used by the nodes to access the Random RUs can be dynamically adapted to the network environment. As a consequence, more efficient usage of the network bandwidth (of the RUs) with limited risks of collisions can be achieved.

Correlatively, the invention provides a communication device acting as an access point in a wireless network also comprising a plurality of nodes, the communication device acting as an access point comprising at least one microprocessor configured for carrying out the steps of: sending one or more trigger frames to the nodes, each trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, each trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining statistics on random resource units not used by the nodes during the one or more transmission opportunities and/or random resource units on which nodes collide during the one or more transmission opportunities; determining a correcting parameter based on the determined statistics, sending, to the nodes, a next trigger frame for reserving a next transmission opportunity, the next trigger frame including the determined correcting parameter.

From the node’s perspective, the invention also provides a communication device in a wireless network comprising an access point and a plurality of nodes, the communication device being one of the nodes and comprising at least one microprocessor configured for carrying out the steps of: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network and including a correcting parameter, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining, based on the correcting parameter and on one random parameter local to the node, one of the random resource units; transmitting data to the access point using the determined random resource unit.

Optional features of embodiments of the invention are defined in the appended claims. Some of these features are explained here below with reference to a method, while they can be transposed into system features dedicated to any node device according to embodiments of the invention.

In embodiments, the correcting parameter is function of the number of unused random resource units and of the number of collided random resource units. These embodiments make it possible to dynamically adapt to various deficient network environments.

In embodiments, the correcting parameter is function of the number of nodes having data to transmit during the next transmission opportunity. In some situations, such number of nodes having data to transmit may be approximate to the number of nodes transmitting in the one or more (previous) transmission opportunities.

Such number of nodes having data to transmit directly impacts the risk of collisions and/or of unused RUs, in particular if the number of RUs forming the composite channel is known in advance.

In embodiments, the method at the access point further comprises modifying the number of random resource units within the communication channel for the next transmission opportunity, based on the determined statistics (this is equivalent to being based on the transmitted correcting parameter). The AP may thus adjust the number of Random RUs as the network conditions evolve.

In embodiments from the access point’s perspective, the correcting parameter includes a value to apply to a random parameter local to each node, for the node to determine which one of the random resource units to access. For instance, the random parameter may be based on a backoff value used by the node to contend for access to the communication channel. This backoff value is for instance the conventional 802.11 backoff counter used to contend for network access to the 20MHz channels.

These embodiments keep compliance with the 802.11 standard, as the backoff counter is still used. In addition, they provide an efficient random mechanism for contention that can be dynamically adjusted in a very simple way.

In variants, the correcting parameter includes a number of random resource units not used during the transmission opportunity or a ratio of this number to the total number of random resource units in the communication channel.

In other variants, the correcting parameter includes a number of random resource units on which nodes collide during the transmission opportunity or a ratio of this number to the total number of random resource units in the communication channel. The two variants may be combined.

In embodiments from the node’s perspective, the random parameter local to the node is based on a backoff value used by the node to contend for access to the communication channel (i.e. a value corresponding to the number of time-slots the node waits before accessing the communication medium).

In embodiments from the node’s perspective, the random resource units have respective unique indexes (for instance an ordering index), and determining one of the random resource units includes applying the correcting parameter to the local random parameter, the result of which identifying the index of the random resource unit to be used to transmit the data to the access point. Note that, as described above, the local random parameter can be the backoff counter used by the node to contend for access to the communication channel.

These embodiments provide a simple way to perform random contention on the RUs, while keeping compliance with 802.11 standard.

In a specific embodiment, applying the correcting parameter to the local random parameter includes dividing the local random parameter by the correcting parameter and outputting an integer rounding of the division result. This is to provide a simple mechanism to dynamically adjust the contention scheme to the network conditions (through the use of the statistics and correcting parameter at the access point).

In embodiments still from the node’s perspective, the method may further comprise the steps of: determining a first time instant based on the random parameter local to the node; and sending padding data on the determined random resource unit from the determined first time instant up to the end of a predetermined time window after having received the trigger frame, start transmitting the data on the determined random resource unit when the predetermined time window ends.

These embodiments offer an efficient contention mechanism while keeping synchronization between the nodes. Indeed, all the nodes start transmitting their data to the access point from the same time instant (when the time window ends). Such synchronization is particularly important in case of OFDMA RUs.

Various declinations of these embodiments are defined and explained below with reference to the second main embodiments.

Second main embodiments of the invention provide a wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at one of said nodes: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining a first time instant based on one random parameter local to the node; sending padding (or dummy) data on a first one of the random resource units from the determined first time instant up to the end of a predetermined time window after having received the trigger frame (the determining and sending steps thus forming a mechanism for contending for access to the RUs according to embodiments of the invention); starting transmitting data to the access point on the first random resource unit when the predetermined time window ends (it defines a predefined second time instant).

The second embodiments define a new contention mechanism for access to RUs composing a conventional communication channel, for instance a 20MHz 802.11 channel. They are mainly implemented at the nodes.

They particularly apply to OFDMA RUs. This is because, due to synchronization requirements between the OFDMA symbols (or PPDUs), the nodes implementing the second embodiments of the invention only send padding data. The padding data are sent up to a time point (predefined second time instant) at which all the nodes having data to transmit simultaneously start transmitting the data. Synchronization is thus saved, while having an efficient contention scheme to access the Random RUs.

Note that the nodes being allocated with a respective Scheduled RU in the communication channel should also wait for the end of the time window before transmitting their data. “Wait” may also mean sending padding data on the Scheduled RU.

Correlatively, the invention provides a communication device in a wireless network comprising an access point and a plurality of nodes, the communication device being one of the nodes and comprising at least one microprocessor configured for carrying out the steps of: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining a first time instant based on one random parameter local to the node; sending padding (or dummy) data on a first one of the random resource units from the determined first time instant up to the end of a predetermined time window after having received the trigger frame (the determining and sending steps thus forming a mechanism for contending for access to the RUs according to embodiments of the invention); starting transmitting data to the access point on the first random resource unit when the predetermined time window ends (it defines a predefined second time instant).

In embodiments, the local random parameter is based on a backoff value used by the node to contend for access to the communication channel (i.e. a value corresponding to the number of time-slots the node waits before accessing the communication medium, for instance a 20MHz channel). This is a simple way to obtain a local random parameter, while keeping compliancy with the 802.11 standard.

In specific embodiments, the first time instant is determined as a linear function of the backoff value (local random parameter) within the time window. As an example, the method may further comprise decrementing the backoff value (local random parameter) each elementary time unit within the time window, and the first time instant is the time instant at which the backoff value (local random parameter) reaches zero. In other words, the nodes may perform contention on the Random RUs using their conventional 802.11 backoff counter. Note that the elementary time units used to decrement the backoff value during contention to access the RUs may be different in size (in particular shorter) compared to the time units used when contending for access to the (20MHz) communication channel. This is to shorten the required time window and thus to increase the actual transmission duration dedicated to useful data.

In specific embodiments, if the backoff value (local random parameter) does not reach zero at the end of the time window, no random resource unit is selected for sending padding data and transmitting data within the transmission opportunity.

In embodiments, the time window is calculated based on a number of elementary time units corresponding to the number of random resource units in the communication channel. For instance, the same number of elementary time units as the number of random resource units may be used. This is to avoid that too many nodes try to access a limited number of Random RUs.

In a particular embodiment, the time window is further calculated based on an adjusting parameter, which adjusting parameter is function of statistics on random resource units not used by the nodes during one or more previous transmission opportunities and/or random resource units on which nodes collide during one or more previous transmission opportunities. In other words, the time window size is adjusted according to the network conditions (statistics). The statistics may be defined and used as described above with reference to the first embodiments of the invention.

In embodiments, the method may further comprise sensing an use of the random resource units during the time window (in particular until the first time instant). Use of a random RU means that an OFDM symbol is detected by the node on the RU. Note that the implementation of the second embodiments of the invention results in having OFDM symbols made of padding data.

In a particular embodiment, the method further comprises selecting one of the random resource units sensed as unused to send the padding data and transmit the data. This is to efficiently use the network bandwidth with limited collisions.

According to a specific implementation, the random resource units are ordered within the communication channel (they have respective unique indexes), and the selected unused random resource unit is the first one of the sensed unused random resource units according to the order. With this approach, only one random resource unit is newly used each time the local random parameter is evaluated anew. A control may thus be achieved to propose a new unused random resource unit at each new evaluation of the local random parameter within the time window.

In another particular embodiment (which may be combined), the method further comprise, upon sensing a new random resource unit as used, updating the local random parameter. This provision makes it possible to speed up the RU allocation for the remaining time (for instance if the update consists in decreasing the local random parameter).

According to a specific implementation, the local random parameter is updated based on at least one correcting parameter specified in the trigger frame received from the access point. Such correcting parameter may be as defined above with reference to the first main embodiments. This configuration helps optimizing the use of the Random RUs, since such correcting parameter may be set by the access point based on statistics representative of the network environment.

For instance, the correcting parameter is function of statistics on random resource units not used by the nodes during one or more previous transmission opportunities and/or random resource units on which nodes collide during one or more previous transmission opportunities.

In a yet other particular embodiment (which also may be combined), as soon as all the random resource units of the at least one communication channel are sensed as used, stopping the sensing step (also the decrementing step when implemented). This is to avoid useless processing as soon as no further Random RUs is available.

In embodiments, a backoff value used by the node to contend for access to the communication channel is updated based on the value taken by the local random parameter at the determined first time instant. The backoff value may be the conventional 802.11 backoff counter used to contend for access to the 20MHz channels.

This provision optimizes use of the network. This is because, since the local random parameter has evolved while been evaluated over the time window, some nodes have already sent their data. It results that they are less chances that nodes succeed in contending for access to the communication channel in the first next backoff time slots. To avoid wasting such first backoff time slots, the backoff counter of the nodes may thus be updated according to the evolution of their local random parameter.

As noted above, no first time instant may be obtained for some nodes, for instance if the contention mechanism does not give an access to those nodes during the time window. For such nodes, a backoff value used by the node to contend for access to the communication channel is also updated based on the value taken by the local random parameter at the end of the time window in case no first time instant has been determined.

In embodiments, the duration of the time window is specified in the trigger frame received from the access point. It makes it possible for the access point to efficiently drive the contention mechanism at the nodes.

In embodiments, the received trigger frame includes a correcting parameter, and the method further comprises determining the first random resource unit based on the correcting parameter and on the local random parameter. As described above for the first main embodiments, this configuration helps adapting dynamically the contention scheme used by the nodes to access the Random RUs, to the network environment. As a consequence, more efficient usage of the network bandwidth (of the RUs) with limited risks of collisions can be achieved.

Of course, all the embodiments described above with reference to the first main embodiments may apply to this configuration.

In embodiments regarding both first and second main embodiments, the random resource units are accessed using OFDMA within the communication channel. This complies with 802.11 ax multi-user uplink communication.

Another aspect of the invention relates to a wireless communication system having an access point and at least one node as defined above.

Another aspect of the invention relates to a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a device of a wireless network, causes the device to perform any 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.

Another aspect of the invention relates to a wireless communication method in a wireless network comprising an access point and a plurality of nodes, substantially as herein described with reference to, and as shown in, Figure 7a, or Figure 7b, or Figures 7a and 8, or Figures 7b and 9, or Figures 7a and 10, or Figures 7b and 10, or Figures 7a, 7b and 10, or Figures 7a, 7b, 8, 9 and 10 of the accompanying drawings.

At least parts of the methods according to the invention may be computer implemented. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "circuit", "module" or "system". Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium.

Since the present invention can be implemented in software, the present invention can be embodied as computer readable code for provision to a programmable apparatus on any suitable carrier medium. A tangible 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.

BRIEF DESCRIPTION OF THE DRAWINGS

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.

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 3 illustrates 802.11ac channel allocation that support channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz as known in the art;

Figure 4 illustrates an example of 802.11ax uplink OFDMA transmission scheme, wherein the AP issues a Trigger Frame for reserving a transmission opportunity of OFDMA sub-channels (resource units) on an 80 MHz channel as known in the art;

Figure 5 shows a schematic representation a communication device or station in accordance with embodiments of the present invention;

Figure 6 shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention;

Figure 7a illustrates, using a flowchart, general steps of a wireless communication method at one of the nodes (not the AP) according to a first exemplary embodiment of the invention;

Figure 7b illustrates, using a flowchart, general steps of a wireless communication method at one of the nodes (not the AP) according to a second exemplary embodiment of the invention;

Figure 8 illustrates exemplary communication lines according to the first exemplary embodiment of Figure 7a;

Figure 9 illustrates exemplary communication lines according to the second exemplary embodiment of Figure 7b;

Figure 10 illustrates, using a flowchart, general steps of a wireless communication method at the AP adapted to the first and/or second exemplary embodiments; and

Figure 11 illustrates an exemplary format for an information Element dedicated to the transmission of parameter values from the AP to the nodes in both the first and/or second exemplary embodiments.

DETAILED DESCRIPTION

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

Figure 1 illustrates a communication system in which several communication nodes (or stations) 101-107 exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN), under the management of a central station, or access point (AP) 110. The radio transmission channel 100 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 or transmitting 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 exchange consists in 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 a 20 MHz channel of the 802.11 medium: transmitting 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 TO, 23 as shown in the Figure).

Channel sensing is for instance performed using Clear-Channel-Assessment (CCA) signal detection which is a WLAN carrier sense mechanisms defined in the IEEE 802.11-2007 standards.

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 802.11η 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, 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 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 T1, the timer expires, the corresponding node 20 requests 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, atT1, 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 802.11n 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 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 using the backoff procedure anew.

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 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 receiving node 21 does not receive RTS frame 210 correctly due to a message/frame collision or to fading. Even if it does receive it, receiving node 21 may not always respond with a CTS 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 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 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 transmission attempts from various wireless nodes lead to collisions. The 802.11 backoff procedure was first introduced for the DCF mode as the basic solution for collision avoidance. In the emerging IEEE 802.11n/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, 802.11ac is targeting larger bandwidth transmission through multi-channel operations. Figure 3 illustrates 802.11ac channel allocation that support composite channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz IEEE 802.11ac introduces support of a restricted number of predefined subsets of 20MHz channels to form the sole predefined composite channel configurations that are available for reservation by any 802.11ac node on the wireless network to transmit data.

The predefined subsets are shown in the Figure and correspond to 20 MHz, 40 MHz, 80 MHz, and 160 MHz channel bandwidths, compared to only 20 MHz and 40 MHz supported by 802.11η. Indeed, the 20 MHz component channels 300-1 to 300-8 are concatenated to form wider communication composite channels.

In the 802.11ac standard, the channels of each predefined 40MHz, 80MHz or 160MHz 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 160 MHz channel bandwidth is composed of two 80 MHz channels that may or may not be frequency contiguous. The 80 MHz and 40 MHz channels are respectively composed of two frequency adjacent or contiguous 40 MHz and 20 MHz channels, respectively. A node is granted a TxOP through the enhanced distributed channel access (EDCA) mechanism on the “primary channel” (300-3). Indeed, for each composite channel having a bandwidth, 802.11ac designates one channel as “primary” meaning that it is used for contending for access to the composite channel. The primary 20MHz 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 20MHz channel of such composite channel.

As addressed earlier, the IEEE 802.11ac standard enables up to four, or even eight, 20 MHz channels to be bound. Because of the limited number of channels (19 in the 5 GHz band in Europe), channel saturation becomes problematic. Indeed, in densely populated areas, the 5 GHz band will surely tend to saturate even with a 20 or 40 MHz bandwidth usage per Wireless-LAN cell.

Developments in the 802.11ax 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 20MHz channel (300-1 to 300-4) into sub-channels 410 (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 Figure 4.

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 802.11 networks.

Contrary to downlink OFDMA wherein the AP can directly send multiple data to multiple stations (supported by specific indications inside the PLOP 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 802.11 ax AP has to provide signalling information for both legacy stations (non- 802.11 ax nodes) to set their NAV and for 802.11 ax nodes to determine the Resource Units allocation.

In the following description, the term legacy refers to non-802.11ax nodes, meaning 802.11 nodes of previous technologies that do not support OFDMA communications.

As shown in the example of Figure 4, the AP sends a trigger frame (TF) 430 to the targeted 802.11 ax nodes. The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is added. The TF frame is sent over the primary 20MHz channel and duplicated (replicated) on each other 20MHz 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 802.11ac 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.

Based on an AP’s decision, the trigger frame TF may define a plurality of resource units (RUs) 410, or “Random RUs”, which can be randomly accessed by the nodes of the network. 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.

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

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 = 1) or Random (tag = 0). 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). Furthermore, a node identifier, such as the Association ID (AID) assigned to each node upon registration, is added in association with each Scheduled RU in order to explicitly indicate the node that is allowed to use each Scheduled RU.

In the example of Figure 4, each 20MHz channel is sub-divided in frequency domain into four sub-channels or RUs 410, typically of size 5 Mhz. These sub-channels (or resource units) are also referred to as “sub-carriers” or “traffic channels”.

Of course the number of RUs splitting a 20MHz channel may be different from four. For instance, between two to nine RUs may be provided (thus each having a size between 10MHz and about 2MHz).

Once the nodes have used the RUs to transmit data to the AP, the AP responds with an acknowledgment (not show in the Figure) to acknowledge the data on each RU.

The present invention regards more particularly the Random RUs used to provide fair use of the network in dense wireless environments. Efficiency of the random uplink accesses through the Random RUs strongly depends on the allocation schemes used to allocate the OFDMA RUs to the nodes.

Few allocation schemes are known in the prior art. For instance, the publication “Generalized CSMA/CA for OFDMA Systems” (Hojoong Kwon et al. [IEEE GLOBECOM 2008, ISBN 978-1-4244-2324-8]) proposed a CSMA/CA protocol for OFDMA systems providing a random access scheme based on backoff mechanism.

Unfortunately, the proposed scheme is not compliant with the conventional 802.11 random access. In particular, this is because the proposed scheme does not keep considering the 20MHz channel as the main communication entity to allocate to the nodes. Furthermore, the use of the sub-channels is not optimum: as a random access, some collisions may occur on some sub-channels and some other subchannels may remain empty or unused even if some nodes have data to transmit (because their associated backoff is not equal to zero).

The present invention finds a particular application in enhancements of the 802.11ac standard, and more precisely in the context of 802.11ax wherein dense wireless environments are more ascertained to suffer from previous limitations.

The present invention provides improved wireless communications with more efficient use of the OFDMA Random RUs while limiting the risks of collisions on these RUs. All of this is preferably kept compliant with 802.11 standards.

An exemplary wireless network is an IEEE 802.11ac network (and upper versions). However, the invention applies to any wireless network comprising an access point AP 110 and a plurality of nodes 101-107 transmitting data to the AP through a multi-user transmission. The invention is especially suitable for data transmission in an IEEE 802.11ax 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 Figures 1 to 4.

First embodiments of the invention provide a dynamic control by the AP of parameters used by the nodes to contend for access to the Random RUs. Following one or more trigger frames reserving a transmission opportunity on at least one communication channel of the wireless network, each trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme, the wireless communication method according to the first embodiments has specific steps.

At the access point AP, they include: determining statistics on random resource units not used by the nodes during the one or more transmission opportunities and/or random resource units on which nodes collide during the one or more transmission opportunities; determining a correcting parameter based on the determined statistics, sending, to the nodes, a next trigger frame for reserving a next transmission opportunity, the next trigger frame including the determined correcting parameter.

At the nodes, they include: determining, based on the correcting parameter and on one random parameter local to the node, one of the random resource units (this step corresponds to the way the nodes contend for access to the random resource units according to the first embodiments of the invention); transmitting data to the access point using the determined random resource unit.

All of this shows that a correcting parameter is exchanged between the access point and the nodes. On one hand, it is used by the nodes to adjust how the local random parameter impacts the choice of the random RUs to be used. On the other hand, this correcting parameter is calculated by the access point based on statistics related to the use of the Random RUs (unused or collided RUs) in one or more previous transmission opportunities. This is because the access point has an overall view of the network, as the nodes only communicate with it.

Second embodiments of the invention provide a progressive contention scheme in the nodes for access to the Random RUs. Following a trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme, the wireless communication method according to the first embodiments has specific steps.

At the nodes (not the AP), they include: determining a first time instant based on one random parameter local to the node; sending padding (or dummy) data on a first one of the random resource units from the determined first time instant up to the end of a predetermined time window after having received the trigger frame (the determining and sending steps thus forming a mechanism for contending for access to the RUs according to embodiments of the invention); starting transmitting data to the access point on the first random resource unit when the predetermined time window ends (it defines a predefined second time instant).

This new contention mechanism particularly applies to OFDMA RUs. This is because, due to synchronization requirements between the OFDM symbols, the nodes implementing the second embodiments of the invention only send padding data. The padding data are sent up to a time point (predefined second time instant) at which all the nodes having data to transmit simultaneously start transmitting the data. Synchronization is thus saved, while having an efficient contention scheme to access the Random RUs.

The first and second embodiments can be implemented separately, or in combination as further described below to provide a progressive contention mechanism with dynamic adaptation to the network conditions.

Figure 5 schematically illustrates a communication device 500 of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 500 may preferably be a device such as a microcomputer, a workstation or a light portable device. The communication device 500 comprises a communication bus 513 to which there are preferably connected: • a central processing unit 511, such as a microprocessor, denoted CPU; • a read only memory 507, denoted ROM, for storing computer programs for implementing the invention; • a random access memory 512, 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 502 connected to the radio communication network 100 over which digital data packets or frames or control frames are transmitted, for example a wireless communication network according to the 802.11ac protocol. The frames are written from a FIFO sending memory in RAM 512 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 512 under the control of a software application running in the CPU 511.

Optionally, the communication device 500 may also include the following components: • a data storage means 504 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention; • a disk drive 505 for a disk 506, the disk drive being adapted to read data from the disk 506 or to write data onto said disk; • a screen 509 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 510 or any other pointing means.

The communication device 500 may be optionally connected to various peripherals, such as for example a digital camera 508, each being connected to an input/output card (not shown) so as to supply data to the communication device 500.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 500 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 500 directly or by means of another element of the communication device 500.

The disk 506 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 507, on the hard disk 504 or on a removable digital medium such as for example a disk 506 as described previously. According to an optional variant, the executable code of the programs can be received by means of the communication network 503, via the interface 502, in order to be stored in one of the storage means of the communication device 500, such as the hard disk 504, before being executed.

The central processing unit 511 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 non-volatile memory, for example on the hard disk 504 or in the read only memory 507, are transferred into the random access memory 512, 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).

Figure 6 is a block diagram schematically illustrating the architecture of a communication device or node 500, either the AP 110 or one of nodes 100-107, adapted to carry out, at least partially, the invention. As illustrated, node 500 comprises a physical (PHY) layer block 603, a MAC layer block 602, and an application layer block 601.

The PHY layer block 603 (here an 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20MHz channel or the composite channel, and thus sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance medium access trigger frames TF 430 to reserve a transmission slot, MAC data and management frames based on a 20 MHz width to interact with legacy 802.11 stations, as well as of MAC data frames of OFDMA type having smaller width than 20 MHz legacy (typically 2 or 5 MHz) to/from that radio medium.

The MAC layer block or controller 602 preferably comprises a MAC 802.11 layer 604 implementing conventional 802.11ax MAC operations, and an additional block 605 for carrying out, at least partially, the invention. The MAC layer block 602 may optionally be implemented in software, which software is loaded into RAM 512 and executed by CPU 511.

Preferably, the additional block, referred to as the OFDMA RU random allocation module 605, implements the part of the invention that regards node 500, for instance, not exhaustively, gathering statistics on use of the Random RUs, computing a correcting parameter and optionally a time window size, adjusting the number of Random RUs, at the AP; using such information from the AP to contend for access to the RUs, calculating a local multichannel backoff value for such contention, sensing use or not of the Random RUs before accessing one of them, at the nodes. The OFDMA RU random allocation module 605 also performs transmitting and receiving operations on RUs.

On top of the Figure, application layer block 601 runs an application that generates and receives data packets, for example data packets of a video stream. Application layer block 601 represents all the stack layers above MAC layer according to ISO standardization.

The present invention is now illustrated using various exemplary embodiments from the nodes’ perspective (Figures 7) and from the AP’s perspective (Figure 10). In these exemplary embodiments, the trigger frame includes a correcting parameter or Collision Risk Factor (CRF) used to optimize the OFDMA Random RUs allocation for the next OFDMA TXOP.

Figure 7a illustrates, using a flowchart, general steps of a wireless communication method at one of the nodes (not the AP) according to a first exemplary embodiment of the invention. In this first exemplary embodiment, the random resource units (Random RUs) have respective unique indexes (for instance an ordering index), and the correcting parameter CRF is applied to a local random parameter to obtain a result, the result identifying the index of the random resource unit to be used by the node to transmit the data to the access point.

In this example, the random parameter local to the node is based on the conventional backoff value (or counter) of the node used to contend for access to the communication channel (i.e. a value corresponding to the number of time-slots the node waits before accessing the communication medium).

In other words, the correcting parameter CRF is used (with the local backoff counter) to allocate the Random RUs.

Upon receiving of a trigger frame from the AP (700), the node STA extracts the correcting parameter CRF value and the number of RUs subject to random allocation, from the trigger frame.

By default, a transmitting 802.11 node has its own (local) backoff counter different from zero (otherwise it would have accessed the medium).

In this first exemplary embodiment, the transmitting node computes a multichannel backoff value (i.e. a local random parameter) based on the current value of the standard 802.11 backoff counter value and based on the extracted correcting parameter CRF value. This is step 701.

For instance, to speed up the backoff decrement over time as explained below (step 702) and to tend to allocate all the Random RUs, the local multichannel backoff value may be equal to the standard 802.11 backoff value divided by the correcting parameter CRF value. A rounding operation is used to obtain an integer, if appropriate. This approach can be implemented in a simple way, which is particularly adapted to low resource nodes.

Of course, operations other than a division (e.g. multiplication, more complex mathematical functions) may be used, and the correcting parameter sent by the AP can be adapted to the operation used by the nodes.

To increase the number of nodes (i.e. multichannel backoff values) that can access the Random RUs, all the multichannel backoff values below a predefined threshold (for instance NxM where N is an integer and M is the number of Random RUs) can be kept and a modulo M operation can be applied to them in order to map each kept multichannel backoff value on one of the Random RUs. Depending on the network conditions, this approach may increase the risk of collisions on the Random RUs.

Once the local multichannel backoff value has been computed, step 702 consists for the node to determine whether or not it is selected for contenting on a random RU.

One solution for the selection of contenting nodes is to compare the local multichannel backoff value with the number of RUs to be allocated. For instance, when the number of RUs to be allocated is 8 (as an example, a 40MHz band, wherein each 20MHz channel band contains 4 OFDMA RUs), all the transmitting nodes with a local multichannel backoff value less than 8 are considered as eligible for having access onto a Random RU. On the other hand, the other transmitting nodes are not selected for Random RU allocation in the current TXOP and must wait for another transmission opportunity (OFDMA TXOP or standard TXOP) before sending their data.

Next step is step 703 in which the node selects the Random RU to be used. In this exemplary embodiment, the Random RU having an index equal to the local multichannel backoff value computed at step 702 is selected.

Next, at step 704, the node transmits at least one 802.11 PPDU frame in a 802.11ax format in the selected Random RU.

Then, it waits for an acknowledgment of the transmitted PPDU frame from the AP. This is step 705.

This exemplary embodiment is illustrated through Figure 8.

Figure 8 illustrates exemplary communication lines according to such exemplary random allocation procedure that is used by the nodes to access the

Random RUs indicated in the TR. As explained above, this random allocation procedure is based on the reuse of the conventional 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 and including the correcting parameter CRF. In the example of the Figure, eight RUs with the same bandwidth are defined for a 40MHz composite channel, and the TF 430 is duplicated on the two 20 MHz channels forming the composite channel. In other words, the network is configured to handle four OFDMA Resource Units per each 20MHz 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 802.11 backoff value (800), based on which it computes the local multichannel backoff value (801), using the correcting parameter CRF (802). CRF=2 in this example. For instance, node STA2 has an 802.11 backoff value equal to 6, and using CRF=2, it obtains a local multichannel backoff value equal to 3.

The random allocation procedure 810 of Figure 8 comprises, for a node of a plurality of nodes having an active backoff and calculating a local multichannel backoff value using the correcting parameter specified in the TF, a first step of determining from the trigger frame the Random sub-channels or RUs of the communication medium available for contention, a second step of verifying if the value of the multichannel backoff value local to the considered node is not greater than the number of detected-as-available Random RUs, and then a step of sending data is performed on the RU whom number equals the local multichannel 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 local multichannel backoff value of the node.

As shown in the Figure, some Random Resource Units may not be used, for instance RUs indexed 2 (410-2), 5, 7 and 8. 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 2, 5, 7 or 8 when the TF is sent.

To base the Random RU allocation on the conventional 802.11 backoff value allows maintaining the access priority defined in the 802.11 standard. Another advantage is that the Random RU allocation keep relying on classical random generation resources present in conventional 802.11 hardware.

While the above example selects the Random RUs based on its RU index matching the local multichannel backoff value, other approaches, for instance selecting randomly the Random RU, can be implemented. In any case, the overall allocation is randomized since the local multichannel backoff value is intrinsically randomly computed.

Figure 7b illustrates, using a flowchart, general steps of a wireless communication method at one of the nodes (not the AP) according to a second exemplary embodiment of the invention. In this second exemplary embodiment, the AP defines a time window size, denoted ΔΤ (specified in the TF), in which the nodes can perform contention on the Random RUs. Once the time window ends, all the nodes to which a RU has been allocated (thus including the Scheduled RUs) start transmitting their data simultaneously. This is to keep OFDMA synchronization between the nodes.

As an alternative to an explicit indication in the TF, the time window size may be determined locally on each node using the same determination scheme.

Upon receiving a trigger frame from the AP (780), the node STA extracts, from the trigger frame, the correcting parameter CRF value, the ΔΤ period and the number of RUs subject to random allocation.

By default, a transmitting 802.11 node has its own (local) backoff counter different from zero (otherwise it would have access the primary 20MHz channel).

In this second exemplary embodiment, the transmitting node computes a multichannel backoff value (i.e. a local random parameter) based on the current value of the standard 802.11 backoff counter value and based on the extracted correcting parameter CRF value. This is step 721.

Other variants as described above with reference to Figure 7a may also be implemented. In addition, the standard 802.11 backoff value may also be used as the local multichannel backoff value when the contention scheme of Figure 7b is implemented.

Upon reception of the trigger frame, after a SIFS time, the local multichannel backoff is decremented by one at each multichannel backoff time interval (typically the 802.11 ax standard value: 9 ps) during the ΔΤ period. This is the loop 722-740-output ‘no’ at 741.

Through this loop, as long as the medium is sensed as idle on Random RUs, the local multichannel backoff value is counted down until it goes to 0 (test 740). This makes it possible to determine a first time instant based on the random parameter local to the node (i.e. the local multichannel backoff value).

At each multichannel backoff time interval, if the multichannel backoff of the STA is not equal to 0 (test 741), the RU distribution is analysed. It means that the node continuously senses the use of the random resource units during the time window. This is step 723.

If a new Random RU is sensed as busy during the current time interval (test 750), the local multichannel backoff value may be updated at step 751. This is to speed up the RU allocation for the remaining time.

The local multichannel backoff value may be updated based on at least one correcting parameter specified in the trigger frame received from the access point, for instance the CRF factor defined above. For instance, the starting formula to compute the local multichannel backoff value may be applied again on the current local multichannel backoff value: new local multichannel backoff value = current multichannel backoff / CRF value. Of course other embodiments may be used.

Steps 750-751 are optional. If they are not implemented, the loop from output ‘no’ at step 741 directly goes to step 722.

During the countdown of the local multichannel backoff value, it is determined whether or not at least one Random RU is still available. This is step 724. Indeed, as soon as all the random resource units of the at least one communication channel are sensed as used, the node may stop the process, i.e. sensing the use of the Random RUs, the countdown of its local multichannel backoff value. This is to avoid useless processing as soon as no further Random RUs is available.

In the example of the Figure, upon detecting all the Random RUs used, the process goes to the optional step 730.

If the local multichannel backoff value does not reach zero at the end of the time window ΔΤ (the ΔΤ period expires - test 722), no random resource unit is selected for the node within the transmission opportunity. The process thus goes to optional step 730.

As no Random RU has been allocated to the node after expiry of ΔΤ period or if all Random RUs have been allocated, the node comes back to conventional 802.11 contention for access to the network. At step 730, the 802.11 standard backoff value is set to the current local multichannel backoff value, i.e. to the value taken by the local random parameter at said first time instant. This is to speed up access to the network for the node, since a number of backoff values have already has access to the network during the ΔΤ period. Next to step 730, the process ends.

Back to test 741, if the local multichannel backoff value of the node reaches zero, a first time instant has thus been determined. At this time instant, a Random RU is selected and allocated to the node at step 760.

In particular a Random RU is selected from the available Random RUs. In other words, one of the random resource units sensed as unused is selected.

In embodiments, the selection can be controlled by using the first available Random RUs. In these embodiments, the random resource units are ordered within the communication channel (they have respective unique indexes), and the selected unused random resource unit is the first one of the sensed unused random resource units according to the order.

Next to step 760, the node starts sending padding data on the selected random resource unit, at step 761. In particular it sends the padding data from the determined first time instant up to the end of a predetermined time window ΔΤ (loop 762). Sending dummy data (i.e. padding) in the selected Random RU ensures this RU is sensed as busy by other nodes.

Note that the dummy/padding data are sent by the nodes on the allocated Random RUs to ensure the OFDM symbol to be synchronized between the transmitting nodes. This requires that the same padding is also performed for any Scheduled RU in the composite channel forming the TXOP.

At the end of the ΔΤ period (test 762), the node stop sending padding data and starting transmitting data to the access point on the selected random resource unit. At step 763, the node thus sends at least one real data 802.11 PPDU frame during the OFDMA TXOP in an 802.11ax format in the selected RU.

Preferably, when the node ends sending the data intended to the access point, the node may continue emit a signal, for instance by sending new padding data, on the selected RU until the end of the TXOP. This is to ensure a correct energy level to be detected by legacy node on the 20MHz channel including the selected RU.

Next at step 764, the node waits for an acknowledgment response from the AP before the next data transmission TXOP.

The process then ends.

This exemplary embodiment is illustrated through Figure 9.

The 802.11 backoff values 800 are converted into local multichannel backoff values 801 using CRF factor 802 as explained above.

From a SIFS after the TF 430, the countdown of the local multichannel backoff values starts, for the ΔΤ period.

The first node having a local multichannel backoff value reaching zero (at time t1) is allocated the first Random RU (#1), on which it starts sending padding data (900). This is STA2.

Next a second node, STA3, has its local multichannel backoff value reaching zero at t2. It then selects the second Random RU (#2, first available one), on which it starts sending padding data (901).

The countdown is done for the whole ΔΤ period. In the example of the Figure, a third node, STAn, has its local multichannel backoff value reaching zero before the end of the ΔΤ period, at t3. It then selects the third Random RU (#3, first available one), on which it starts sending padding data (902).

At the end of the ΔΤ period, nodes STA1 and STAn-1 have non-zero local multichannel backoff values: they are not allocated with a Random RUs.

Simultaneously, STA2, STA3 and STAn start transmitting their data using OFDMA on their respective selected Random RU. They send data up to the end of the TXOP (possibly using padding data if necessary). An ACK then follows, sent by the AP.

Turning now to the operations performed by the access point, Figure 10 illustrates, using a flowchart, general steps of a wireless communication method at the AP adapted to the first and/or second exemplary embodiments introduced above.

One skilled in the art will unambiguously identify which parts of Figure 10 are required for the first exemplary embodiments of Figure 7a and which parts of Figure 10 are required for the second exemplary embodiments of Figure 7b. In particular, the AP is configured to compute, update and send the correcting parameter CRF used to optimize the OFDMA Random RU allocation for the next OFDMA TXOP in the first exemplary embodiments, and to compute, update and send the ΔΤ value in some embodiments of the second exemplary embodiments. In any case, such information is encapsulated inside a new Trigger Frame (TF) sent by the AP.

Upon receiving an uplink OFDMA frame (1001), the AP is in charge of sending an acknowledgment frame to acknowledge safe reception of transmitted data by all or part of the nodes over the OFDMA RUs (1002).

At step 1003, the AP analyses the number of collided and empty (i.e. unused) OFDMA RUs. It may perform this step by sensing each RU forming the composite channel. These values are used to update OFDMA statistics. In particular, the AP determines statistics on random resource units not used by the nodes during the transmission opportunity and/or random resource units on which nodes collide during the transmission opportunity.

The OFDMA statistics are used by the AP at steps 1004-1006 to determine various parameters to dynamically adapt (from one TXOP to the other) the contention scheme for access to the Random RUs.

It includes determining the correcting parameter CRF for the next OFDMA transmission (1004) at least for the first exemplary embodiments.

It may also include determining and thus modifying the number of random resource units within the communication channel for the next transmission opportunity (1005).

It also includes determining the size of the ΔΤ period (1006).

For the first exemplary embodiments of Figure 7a, steps 1004-1005 thus dynamically adapt (from one TXOP to the other) the contention scheme for access to the Random RUs, by both adjusting the CRF value and the number of Random RUs available for the nodes.

To illustrate such dynamical adaptation, it may be considered the case where all (or more than 80%) OFDMA Random RUs are used in the last OFDMA TXOP (or N previous OFMDA TXOPs, N being integer). It means that many nodes are requesting to transmit data. As a consequence, the number of Random RUs for the next OFDMA transmission can be increased by the AP (for instance by 1 up to a maximum number), while the correcting parameter CRF value can remain the same.

In addition, if collisions occur on several used OFDMA Random RUs (at least for instance more than a third), it means that the correcting parameter CRF value should be decreased to minimize the collisions between the nodes during the RU allocation. For instance, the CRF value may be decreased by about 30%. A drawback of decreasing the CRF value (used as a divisor of the 802.11 backoff value by the nodes) is that the Random RU allocation is less optimized.

On the other hand, if several OFDMA Random RUs remain unused (at least for instance more than a third - or less than 50% of the RUs are used), the correcting parameter CRF value can be increased, for instance by 30%, and/or the number of Random RUs for the next OFDMA transmission can be decreased by the AP (for instance by 1) to optimize the OFDMA Random RU allocation. A drawback of increasing the CRF value is that the collisions during the Random RU allocation may increase.

This illustrates that, upon termination of each uplink OFDMA TXOP, the updating of the correcting parameter CRF value is a trade-off between minimizing collisions during Random RU allocation and optimizing the filling of the OFDMA Random RUs.

To be precise, at step 1004, the AP computes a new correcting parameter CRF value based on the determined OFDMA statistics, optionally further based on the number of nodes transmitting on the random resource units during the previous transmission opportunity. Note that the OFDMA statistics may be statistics on the previous TXOP only or on N (integer) previous TXOPs.

For instance, as introduced in the first embodiments of Figure 7a, the correcting parameter CRF includes a value to apply to a random parameter local to each node, for the node to determine which one of the random resource units to access. For instance, the random parameter can be based on a backoff value used by the node to contend for access to the communication channel.

In embodiments, a good starting correcting parameter CRF value is 2 (used as a divisor of the 802.11 backoff value by the nodes). This value substantially increases the speed of the backoff counter, with limited risk of addition collisions.

However this value can be adjusted as the OFDMA statistics show that too many collisions occur on the Random RUs or too many Random RUs remain unused.

Next, at step 1005, the AP determines the number of Random RUs to consider for the next multi-user TXOP about to be granted (because the AP can preempt the wireless medium over the nodes, since it must wait for the medium to be idle during a shorter duration than the waiting duration applied by the nodes).

The determination of step 1005 can based on the BSS configuration environment, that is to say the basic operational width (namely 20MHz, 40MHz, 80MHz or 160MHz composite channels that include the primary 20MHz channel according to the 802.11ac standard).

For the sake of simplicity, one may consider that a fixed number of OFDMA RUs is allocated per 20 MHz band by the 802.11ax standard: in that case, it is sufficient that the Bandwidth signalling is added to the TF frames (i.e. 20, 40, 80 or 160 MHz values is added). Typically, such information is signaled in the SERVICE field of the DATA section of non-HT frames according the 802.11 standard. As a consequence, compliance with 802.11 is kept for the medium access mechanism.

For the second exemplary embodiments of Figure 7b, step 1006 dynamically adapts the ΔΤ value to the network conditions. This adaptation may thus be based on the OFDMA statistics, i.e. on the number of random resource units not used by the nodes during one or more previous transmission opportunities and/or of random resource units on which nodes collide during one or more previous transmission opportunities. It is also adjusted based on the number of available Random RUs provided in the TXOP.

For instance, the ΔΤ value is computed as a multiple of the multichannel backoff time interval (used by the nodes when decrementing their local multichannel backoff values - equal to 9 ps). As an example, the multiplicity may be equal to the number of available Random RUs (determined at step 1005 for the next TXOP). Generally, one can consider the following equation: ΔΤ = (number of available Random RUs x elementary 9 ps time unit) * k wherein k is an adjusting parameter function of the OFDMA statistics.

Typically ‘k’ value can be set to 2. Its minimum value is 1 to allow the allocation of one Random RU at least at each backoff decrement (9 ps).

The ‘k’ value can be adjusted depending on the number of empty Random RUs on the past OFDMA TXOP: for instance, if a third of the Random RUs remain unused in the last OFDMA TXOP, the ‘k’ value may be increased by 30%.

However, the ΔΤ value is kept below a predefined threshold, in order to avoid having it too high. This is to avoid spending too much time for the Random RU allocation.

Next to step 1006, steps 1007/1008 consist for the AP to build and send the next trigger frame with the above determined information: Random RUs information, CRF value, ΔΤ value.

It is expected that every nearby node (legacy or 802.11ac, i.e. which is neither STA1 nor STA2) 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 reserved by the AP.

Figure 11 illustrates an exemplary format for an information Element dedicated to the transmission of the CRF value and the ΔΤ value within the TF.

The ‘CRF Information Element’ (1110) is used by the AP to embed additional information within the trigger frame TF related to the OFDMA TXOP.

The proposed format follows the ‘Vendor Specific information element’ format as defined in IEEE 802.11-2007 standard.

The ‘CRF Information Element’ (1110) is a container of the correcting parameter CRF attributes (1120), having each a dedicated attribute ID for identification. The header of CRF IE can be standardized (and thus easily identified by stations 500) through the Element ID.

The CRF attributes 1120 are defined to have a common general format consisting of a 1-byte CRF Attribute ID field, a two-byte Length field and a CRF attribute body (1130) including the CRF value computed by the AP.

As for the CRF attributes, the ΔΤ attribute is built on the same way. It is defined to have a common general format consisting of a 1-byte ΔΤ Attribute ID field, a 2-byte Length field and a ΔΤ attribute body (1140) including the ΔΤ value computed by the AP.

The usage of the 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 in that it keeps legacy compliancy for the medium access mechanism. This is because any modification performed inside the PHY header of the 802.11 frame would have inhibited any successful decoding of the MAC header (the Duration field would not have been decoded, so the NAV would not have been set by legacy devices).

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.

In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims

1. A wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at the access point: sending one or more trigger frames to the nodes, each trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, each trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining statistics on random resource units not used by the nodes during the one or more transmission opportunities and/or random resource units on which nodes collide during the one or more transmission opportunities; determining a correcting parameter based on the determined statistics, sending, to the nodes, a next trigger frame for reserving a next transmission opportunity, the next trigger frame including the determined correcting parameter.

2. A wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at one of said nodes: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network and including a correcting parameter, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining, based on the correcting parameter and on one random parameter local to the node, one of the random resource units; transmitting data to the access point using the determined random resource unit.

3. The wireless communication method of Claim 1 or 2, wherein the correcting parameter is function of the number of unused random resource units and of the number of collided random resource units.

4. The wireless communication method of Claim 1 or 2, wherein the correcting parameter is function of the number of nodes having data to transmit during the next transmission opportunity.

5. The wireless communication method of Claim 1, wherein further comprising modifying the number of random resource units within the communication channel for the next transmission opportunity, based on the determined statistics.

6. The wireless communication method of Claim 1, wherein the correcting parameter includes a value to apply to a random parameter local to each node, for the node to determine which one of the random resource units to access.

7. The wireless communication method of Claim 6, wherein the random parameter is based on a backoff value used by the node to contend for access to the communication channel.

8. The wireless communication method of Claim 2, wherein the random parameter local to the node is based on a backoff value used by the node to contend for access to the communication channel.

9. The wireless communication method of Claim 2, wherein the random resource units have respective unique indexes, and determining one of the random resource units includes applying the correcting parameter to the local random parameter, the result of which identifying the index of the random resource unit to be used to transmit the data to the access point.

10. The wireless communication method of Claim 9, wherein applying the correcting parameter to the local random parameter includes dividing the local random parameter by the correcting parameter and outputting an integer rounding of the division result.

11. The wireless communication method of Claim 2, further comprising the steps of: determining a first time instant based on the random parameter local to the node; and sending padding data on the determined random resource unit from the determined first time instant up to the end of a predetermined time window after having received the trigger frame, start transmitting the data on the determined random resource unit when the predetermined time window ends.

12. A wireless communication method in a wireless network comprising an access point and a plurality of nodes, the method comprising the following steps, at one of said nodes: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining a first time instant based on one random parameter local to the node; sending padding data on a first one of the random resource units from the determined first time instant up to the end of a predetermined time window after having received the trigger frame; starting transmitting data to the access point on the first random resource unit when the predetermined time window ends.

13. The wireless communication method of Claim 12, wherein the local random parameter is based on a backoff value used by the node to contend for access to the communication channel.

14. The wireless communication method of Claim 12, wherein the first time instant is determined as a linear function of the backoff value within the time window.

15. The wireless communication method of Claim 12, further comprising decrementing the backoff value each elementary time unit within the time window, and the first time instant is the time instant at which the backoff value reaches zero.

16. The wireless communication method of Claim 15, wherein if the backoff value does not reach zero at the end of the time window, no random resource unit is selected for sending padding data and transmitting data within the transmission opportunity.

17. The wireless communication method of Claim 12, wherein the time window is calculated based on a number of elementary time units corresponding to the number of random resource units in the communication channel.

18. The wireless communication method of Claim 17, wherein the time window is further calculated based on an adjusting parameter, which adjusting parameter is function of statistics on random resource units not used by the nodes during one or more previous transmission opportunities and/or random resource units on which nodes collide during one or more previous transmission opportunities.

19. The wireless communication method of Claim 12, further comprising sensing an use of the random resource units during the time window.

20. The wireless communication method of Claim 19, further comprising selecting one of the random resource units sensed as unused to send the padding data and transmit the data.

21. The wireless communication method of Claim 20, wherein the random resource units are ordered within the communication channel, and the selected unused random resource unit is the first one of the sensed unused random resource units according to the order.

22. The wireless communication method of Claim 19, further comprising, upon sensing a new random resource unit as used, updating the local random parameter.

23. The wireless communication method of Claim 22, wherein the local random parameter is updated based on at least one correcting parameter specified in the trigger frame received from the access point.

24. The wireless communication method of Claim 23, wherein the correcting parameter is function of statistics on random resource units not used by the nodes during one or more previous transmission opportunities and/or random resource units on which nodes collide during one or more previous transmission opportunities.

25. The wireless communication method of Claim 19, wherein as soon as all the random resource units of the at least one communication channel are sensed as used, stopping the sensing step.

26. The wireless communication method of Claim 12, wherein a backoff value used by the node to contend for access to the communication channel is updated based on the value taken by the local random parameter at the determined first time instant.

27. The wireless communication method of Claim 12, wherein a backoff value used by the node to contend for access to the communication channel is updated based on the value taken by the local random parameter at the end of the time window in case no first time instant has been determined.

28. The wireless communication method of Claim 12, wherein the duration of the time window is specified in the trigger frame received from the access point.

29. The wireless communication method of Claim 12, wherein the received trigger frame includes a correcting parameter, and the method further comprises determining the first random resource unit based on the correcting parameter and on the local random parameter.

30. The wireless communication method of Claim 1 or 2 or 12, wherein the random resource units are accessed using OFDMA within the communication channel.

31. A communication device acting as an access point in a wireless network also comprising a plurality of nodes, the communication device acting as an access point comprising at least one microprocessor configured for carrying out the steps of: sending one or more trigger frames to the nodes, each trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, each trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining statistics on random resource units not used by the nodes during the one or more transmission opportunities and/or random resource units on which nodes collide during the one or more transmission opportunities; determining a correcting parameter based on the determined statistics, sending, to the nodes, a next trigger frame for reserving a next transmission opportunity, the next trigger frame including the determined correcting parameter.

32. A communication device in a wireless network comprising an access point and a plurality of nodes, the communication device being one of the nodes and comprising at least one microprocessor configured for carrying out the steps of: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network and including a correcting parameter, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining, based on the correcting parameter and on one random parameter local to the node, one of the random resource units; transmitting data to the access point using the determined random resource unit.

33. A wireless communication system having an access point according to Claim 31 and at least one node according to Claim 32.

34. A communication device in a wireless network comprising an access point and a plurality of nodes, the communication device being one of the nodes and comprising at least one microprocessor configured for carrying out the steps of: receiving a trigger frame from the access point, the trigger frame reserving a transmission opportunity on at least one communication channel of the wireless network, the trigger frame defining resource units forming the communication channel including a plurality of random resource units that the nodes access using a contention scheme; determining a first time instant based on one random parameter local to the node; sending padding data on a first one of the random resource units from the determined first time instant up to the end of a predetermined time window after having received the trigger frame; starting transmitting data to the access point on the first random resource unit when the predetermined time window ends.

35. A wireless communication system having an access point and at least one node according to Claim 34.

36. A non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a device of a wireless network, causes the device to perform the method of Claim 1 or 2 or 12.

37. A wireless communication method in a wireless network comprising an access point and a plurality of nodes, substantially as herein described with reference to, and as shown in, Figure 7a, or Figure 7b, or Figures 7a and 8, or Figures 7b and 9, or Figures 7a and 10, or Figures 7b and 10, or Figures 7a, 7b and 10, or Figures 7a, 7b, 8, 9 and 10 of the accompanying drawings.