GB2538099A - Method and device for detecting duplicate on sub-channels, wireless communication method and device using the detection - Google Patents

Abstract

An 802.11ac/ax wireless network provides an operating band made of a plurality of sub-channels. A control frame sent on a primary channel by an 802.11ac/ax node is duplicated on each secondary channel. The duplicated frames on the secondary channels are not decoded by the 802.11ac/ax nodes. A need exists to improve detection of collision with the duplicated frames on the secondary channels. A node senses, using Clear Channel Assessment CCA for each sub-channel of the plurality, a change of an idle/busy status of the channel, and stores, in memory, a CCA time profile for each channel; receives a control frame over the primary channel; determine secondary channels whose CCA profiles (regarding the received control frame) match the primary CCA profile. The node may then forms a composite channel made of the primary channel and possibly one or more matching secondary channels, for wireless communication. An embodiment relates to use within a CSMA/CA system to detect multiple Request To Send RTS messages across multiple sub-channels to determine available sub-channels of a composite channel.

Description

METHOD AND DEVICE FOR DETECTING DUPLICATE ON SUB-CHANNELS, WIRELESS COMMUNICATION METHOD AND DEVICE USING THE DETECTION

FIELD OF THE INVENTION

The present invention relates generally to communication networks and more specifically to the detection of duplicate frames on sub-channels forming for instance a communication composite channel. Application of the detection 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

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

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

The 802.11 medium access protocol standard or operating mode is mainly directed to the management of communication nodes 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 RequestTo-Send (RTS) and Clear-To-Send (CTS) frames to allow for composite channels of varying and predefined bandwidths of 20, 40 or 80M Hz, the composite channels being made of one or more sub-channels that are contiguous within the operating band. The 160MHz composite channel is possible by the combination of two 80MHz composite channels within the 160MHz operating band. The control frames 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.

Given a tree breakdown of the operating band into elementary 20MHz channels, some secondary sub-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.

A node or station is allowed to use as much channel capacity (or bandwidth, i.e. of (sub-)channels in the composite channel) as is available. The constraint is that the combined channels need to be contiguous within the operating band for a node having a single antenna (or single spatial stream).

However, if there is noise or interference on one of the 20MHz channel within the wider composite channel, the available bandwidth is reduced. The 802.11ac standard only allows a restricted number of composite channel configurations, i.e. of predefined subsets of 20MHz channel that can be reserved by the 802.11ac nodes to wireless communicate and thus transmit data. These configurations correspond to contiguous composite channels of 20, 40, 80MHz bandwidth (also known as "secondary", "secondary40" and "secondary80" composite channels) in case of single antenna node devices.

Therefore, noise or interference even on a small portion of the composite channel may substantially reduce the available bandwidth of the composite channel to only 40 or 20 MHz, since the resulting reserved bandwidth must meet the 20MHz, 40M Hz, 80MHz or 160MHz channel configurations allowed by the standard.

More recently, Institute of Electrical and Electronics Engineers (IEEE) officially approved the 802.1 lax task group, as the successor of 802.11ac. The primary goal of the 802.11ax task group consists in seeking for an improvement in data speed to wireless communicating devices used in dense deployment scenarios.

The huge gigabit throughputs that are often attributed to 802.11ac are mainly theoretical. In fact, they represent the overall capacity a VVi-Fi network can support, for instance 1.3 Gbps in today's most advanced routers. However, they can occur only in the rarest circumstances where any individual device would actually be able to connect at such high rates.

An issue with the use of composite channels as defined in the 802.11n or 802.11ac (or 802.11ax) is that the 802.11n 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 802.11n and 802.11ac standards provide duplicating control frames (e.g. RTS/CTS or CTS-to-Self) 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 because any legacy 802.11a node that uses any of the 20MHz channel involved in the composite channel can set or update its network allocation vector (NAV) using the Duration/ID field specified in the control frame duplicated 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 802.11n, 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 (sub)channels forming the targeted or requested composite channel.

This approach has been widened for 802.1 lac to allow duplication over the sub-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 each secondary 20MHz (sub)channel of the (40MHz 80MHz or 160MHz) operating band.

In practice, to request a composite channel (equal to or greater than 35 40MHz) for a new TXOP, an 802.11n/ac node does an EDCA backoff procedure in the primary 20MHz (sub-)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).

In other words, only frames on the primary 20MHz (sub-)channel are demodulated and decoded by a receiving node, in particular to determine if they are addressed to it. On the secondary (sub-)channels, no demodulation or decoding of frames is performed.

This processing which is different between the primary (sub-)channel and secondary (sub-)channels is prejudicial to the 802.11ac dynamic bandwidth operation feature according to which two 802.11ac nodes negotiate the bandwidth for their composite channel for wireless communication, using the non-HT RTS/CTS frame handshake.

This is because, as only a PIES period is sensed as free in the secondary (sub-)channels before the RTS frame is emitted, to determine that the secondary (sub-)channels are idle for both 802.11ac nodes, the receiving 802.11ac node is unable to determine whether the activity on the secondary (sub-)channels results from the 802.11ac duplication of the control frame sent on the primary (sub-)channel or it results from other 802.11a legacy traffic occurring after the PIES period.

As being unable to perform such discrimination, the receiving node is not able to detect collisions that may occur with the duplication of the control frame in the secondary (sub-)channel(s). As a result, both 802.1 lac nodes will consider these secondary (sub-)channels as free and thus will use them to form a composite channel for data transmission. Obviously, the data decoded by the receiving node based on such composite channel will be erroneous. This results in having waste of network bandwidth (not only for the mislead 802.1 lac nodes, but also for legacy communication which has been collided).

Thus, in the context of the 802.11ax developments, the need becomes apparent to improve the sensing of duplicate frames on the secondary sub-channels.

This would help to improve the bandwidth usage of the wireless network and thus the efficiency of wireless communication over such a network.

In addition, as mentioned above, 802.11ac provides that if one or more sub-channels of the targeted composite channel are not available, the nodes fallback 35 to a 20MHz or 40MHz or 80MHz composite channel made of sub-channels that are contiguous within the operating band. In other words, the non-contiguous sub-channel or sub-channels within the operating band are not reachable for wireless communication, although they are idle and thus free.

To cope with such limitation, papers have been published in the 802.11ax task group (IEEE 802.11-13/1058r0 "Efficient Wider Bandwidth Operation") to raise the benefit of using all available sub-channels, even if they are non-contiguous within the operation band. However, the papers do not provide any solution to do so.

It is observed that the discrimination issue as mentioned above is even more critical in the case of a TXOP over non-contiguous composite channel, because a higher number of sub-channels are wasted for the duration of the TXOP.

In this context, there is a need for an improved detection of the sub-channels conveying non-HT frames over each sub-channels of a composite channel. An improved wireless communication method over an operating band made of a plurality of sub-channels may in turn be obtained.

It is known publication US 2011/0038441 which discloses a method of detecting whether or not a transmitted signal has been transmitted in a particular transmission mode (typically non-HT duplicate mode). The publication discloses receiving a signal in primary and secondary frequency bands and comparing a first part of a header of the signal in the primary frequency band with a corresponding first part of a header of the signal in the secondary frequency band. The receiver must thus be able to receive and decode 802.11 headers ("signal containing signal components") on several channels.

This approach has too expensive hardware cost for physical chipsets in the scope of the 802.11ac standard having up to eight sub-channels to be considered. This is because, from a hardware point of view, this approach requires duplicating eight times the reception chains per node device, for independent receiver frequency filtering and signal forming.

Furthermore, this approach only focuses on two 20MHz channels for the 802.11n standard in which each node device should support up to two spatial streams (SSs). It is thus not compliant with the requirement of the more recent versions of 802.11. Indeed, in sharp contrast, the 802.11ac standard and thus the 802.11ax task group mandate only a single spatial stream.

In this context, there is also a need that the improved detecting or sensing method complies with single antenna apparatuses, which are obviously not able to demodulate the (duplicated) control frames over each 20MHz channel forming the composite channel or the operating band.

SUMMARY OF INVENTION

The present invention has been devised to overcome the foregoing limitations.

In this context, the invention first provides a method for detecting duplicate frames on sub-channels, the duplicate frames being duplicates of a first frame received over a primary one of the sub-channels, the method comprising, at a first node: sensing, for each of the sub-channels, a change of an idle/busy status of the sub-channel, and storing, in memory, the status changes in association with time information; determining, using the stored time information, one or more other sub-channels that have respective status changes temporally similar to the status changes in the primary sub-channel due to the received first frame, to trigger detection of a duplicate frame on each so-determined sub-channel.

This process can be performed at any one or both of a source node and a receiving node that are about to exchange data over the wireless network.

The time information is for instance representative of the time instant at 20 which the status change occurred.

Correlatively, the invention provides a node device for detecting duplicate frames on sub-channels, the duplicate frames being duplicates of a first frame received over a primary one of the sub-channels, the node device comprising: a channel sensor for sensing, for each of the sub-channels, a change of an idle/busy status of the sub-channel; a memory; and at least one microprocessor configured for carrying out the steps of: storing, in the memory, the status changes sensed by the channel sensor, in association with time information; determining, using the stored time information, one or more other sub-channels that have respective status changes temporally similar to the status changes in the primary sub-channel due to the received first frame, to trigger detection of a duplicate frame on each so-determined sub-channel.

Thanks to the invention, the nodes can efficiently detect the sub-channels conveying frames, such as the non-HT control frames in 802.11ac, duplicated in secondary (i.e. not primary) sub-channels. This is achieved by comparing the status change profiles between the primary sub-channel (used as a reference) and each other sub-channel. This is because each duplicate of the received first frame, for instance a control frame such as RTS, CTS, ACK, etc., has the same time signature as the original received first frame.

As a result, each node improves its ability to detect collision in the secondary sub-channels.

An application of the detecting method regards communication on an appropriate composite channel. For instance, the invention also provides a wireless communication method using sub-channels of an operating band, the method comprising, at a first node: receiving a control frame addressed to the first node by a second node over a primary one of the sub-channels; detecting duplicate frames of the received control frame, on other sub-channels, using the detecting method as defined above; and forming a composite channel for wireless communication, wherein the composite channel is made of the primary sub-channel and possibly one or more determined sub-channels on which a duplicate frame is detected.

The node can determine the composite channel, based on the other sub-channels that are ultimately considered as not being perturbed thanks to the improved detection of duplicate frames according to the invention. It results that non-perturbed composite channels are used more often, thus avoiding waste of TXOP. The use of network bandwidth is consequently substantially improved.

Note that, given some constraints that may exist (for instance the fallback bandwidths defined in 802.11ac), the composite channel may be reduced to the single primary sub-channel.

Depending on the approach adopted, the composite channel may be used by the first and second nodes only for data communication between them, or by them and other nodes if all these nodes have collaboratively decided to share (time and/or sub-channel multiplexing for instance) the TXOP granted on this composite channel.

Other applications of the detecting method may include testing the sub-channels of an operating band made of a plurality of sub-channels. In this application, the first frame may include test data.

Correlatively, the invention provides a node device to wirelessly communication over sub-channels of an operating band, the node device comprising at least one microprocessor configured for carrying out the steps of: receiving a control frame addressed to the first node by a second node over a primary one of the sub-channels; detecting duplicate frames of the received control frame, on other sub-channels, using the detecting method as defined above; and forming a composite channel for wireless communication, wherein the composite channel is made of the primary sub-channel and possibly one or more determined sub-channels on which a duplicate frame is detected.

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 method further comprises determining whether or not the received first frame indicates an operating mode that involves duplicating the first frame over several of the sub-channels; and the step of determining using the stored time information is performed in case of positive operating mode determination. This provision makes it possible to lighten the process at the nodes in case the node that sends the first frame is a 802.11a legacy node, i.e. which cannot use a combine channel for transmission.

In other embodiments, the time information includes a time instant at which the status change for the sub-channel occurs or a time difference between the status change thus sensed and the preceding sensed status change in the same sub-channel. Timestamps generated by a clock internal to the node may be used. Such time information takes very little space in memory of the nodes.

In embodiments, determining one of the sub-channels using the stored time information includes determining whether or not the time difference between an idle-to-busy status change (i.e. a change from idle status to busy status due to the sensing) in the sub-channel considered and an idle-to-busy status change corresponding to the start of the received first frame in the primary sub-channel is less than a first threshold; and the time difference between a busy-to-idle status change in the sub-channel considered and a busy-to-idle status change corresponding to the end of the received first frame in the primary sub-channel is less than a second threshold.

The busy-to-idle status change is the next status change in the sub-channel after the idle-to-busy status change first considered. This is because, the channel status usually has only two states: idle and busy.

Given the time information stored in memory of the node, the above approach for determining the available sub-channels needs low processing resources.

The first and second thresholds may be identical, but this is not mandatory.

As an example, the thresholds may equal few, tenths or hundredths of nanoseconds (ns) because the sensing (e.g. CCA) sensitivity of the 802.11 nodes has the same error tolerance for each sub-channel and thus there should be little or no drift in status change detection among the various sub-channels. For instance, the thresholds are substantially less than the aCCATime defined in 802.11ac as being the "maximum time On microseconds) that the CCA mechanism has available to detect the start of a valid IEEE 802.11 transmission within the primary channel and to assess the energy on the medium within the primary, secondary, secondary40... channels", e.g. substantially less than 1 microsecond In embodiments, the step of sensing a change of the idle/busy status of each sub-channel includes comparing a signal energy level in the sub-channel considered to a predefined threshold.

For instance, the step of sensing a change of the idle/busy status of each sub-channel includes performing a Clear Channel Assessment (CCA).

CCA is a WLAN carrier sense mechanisms defined in the IEEE 802.112007 standards as part of the Physical Medium Dependant (PMD) and Physical Layer Convergence Protocol (PLCP) layer. It involves two functions: Carrier Sense (CCA-CS) which is the ability of the receiving node to detect and decode an 802.11 frame preamble. From the PLCP header field, the time duration for which the medium will be occupied can be inferred and when such 802.11 frame preamble is detected, the CCA flag is held busy until the end of data transmission. Energy Detect (CCA-ED) which is the ability of the receiving node to detect non-802.11 energy in the channel and back off data transmission. The ED threshold 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.

In embodiment, the received first frame includes a channel width defining a target composite channel including the primary sub-channel; and the step of determining is restricted to analysing the stored time information for the sub-channels of the plurality that are included in the target composite channel. This provision allows reducing the processing at the first node.

Embodiments regarding the wireless communication method are now briefly introduced.

In embodiments, the step of receiving the control frame includes demodulating a signal on the primary sub-channel upon sensing the primary sub-channel as busy and decoding the demodulated signal to obtain the control frame. In embodiments, the composite channel formed for wireless communication is made of the primary sub-channel and all the determined sub-channels on which a duplicate frame is detected. This approach means that non-contiguous sub-channels may be reserved for wireless communication, as all the nodes may be aware of which sub-channels are available.

As a result, the invention supports situation in which the control frames are duplicated over a set of sub-channels that is different from the predefined 802.11ac subsets of sub-channels (i.e. the sub-channels are different and the width of the resulting composite channel may also be different).

In embodiments, the operating band is made of an ordered succession of sub-channels, and the composite channel formed for wireless communication is made of one or more sub-channels that are contiguous within the operating band. These sub-channels obviously include the primary sub-channel. This provision is in line with the fallback mechanism of 802.11ac.

Indeed, this standard allows a limited number of contiguous composite channels, i.e. a restricted number of predefined sub-channel subsets that are available for reservation by any wireless node of the wireless network to transmit data, the sub-channel subsets being made of contiguous sub-channels within the operating band. In details, the restricted number of predefined sub-channel subsets for 802.11ac is made of 20MHz, 40MHz, 80MHz and optionally 160MHz bandwidths within the operating band.

The invention also supports composite channels that are different from the predefined 802.11ac subsets of sub-channels (i.e. the sub-channels are different and the width of the resulting composite channel may also be different).

In embodiments, the received control frame is a medium access requesting frame to request reservation of a target composite channel made of sub-channels of the operating band (i.e. for instance a RTS according to 802.11); and the method further comprises using the formed composite channel to send, over each sub-channel of the formed composite channel, the same medium access acknowledging frame acknowledging reservation of the respective sub-channel (i.e. for instance a CTS according to 802.11), to the second node.

It is clearly apparent that the formed composite channel can be different (narrower) from the target composite channel. This is because the first node may have determined some collision in one of the sub-channels forming the target composite channel.

The above provision may take part in the RTS/CTS handshake scheme according to 802.11ac standard, from a receiving node's perspective. The invention thus improves the accuracy of the RTS/CTS handshake scheme since the receiving node is now able to detect collisions with duplicated RTS which were not previously detected.

According to a specific feature, the method further comprises, at the first node, monitoring the determined sub-channel or sub-channels between the reception of the medium access requesting frame and the sending of the medium access acknowledging frame, to sense any change of an idle/busy status of the determined sub-channel or sub-channels; wherein the determined sub-channel or sub-channels for which a change of an idle/busy status is sensed during the monitoring step are discarded from selection to form the composite channel. This additional monitoring increases the reliability of the composite channel so-formed.

In other embodiments, the received control frame is a medium access acknowledging frame acknowledging reservation of the primary sub-channel (i.e. for instance a CTS according to 802.11); and the method further comprises using the formed composite channel to send data in the composite channel.

This provision may take place in the RTS/CTS handshake scheme according to 802.11ac standard, from a source node's perspective. The invention thus improves the accuracy of the RTS/CTS handshake scheme since the source node is now able to detect collisions with duplicated CTS which were not previously detected.

In embodiments, the received control frame is one from a request-to-send (RTS), clear-to-send (CTS), acknowledgment (ACK), Power-save poll (Ps-Poll), block ACK request (BAR), block ACK (BA), contention free-end (CF-end), NULL data packet (NDP) announcement frames as defined in 802.11 standard. This provision shows that the invention may take place at a large number of instants during an 802.11 communication.

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 node device, causes the node device to perform the methods 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 method and node device.

Another aspect of the invention relates to a wireless communication method over sub-channels of an operating band, substantially as herein described with reference to, and as shown in, Figure 8a, or Figures 8a and 8b 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 3a 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 3b illustrates an example of 802.11ac multichannel transmission opportunity on an 80 MHz channel as known in the art; Figure 4 illustrates the fallback mechanism in a 80MHz operating band as known in the art; Figure 5 illustrates two situations of collisions due to co-channel interference or noise that are not detected in the art; Figure 6 shows a schematic representation a communication device or station in accordance with embodiments of the present invention; Figure 7 shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention; Figure 8 illustrates, using flowcharts, general steps of an enhanced method according to embodiments of the present invention, to improve detection of collision in secondary sub-channels of a 802.11ac/ax wireless medium; and Figure 9 and 10 illustrates the benefits of the invention for the two critical situations of Figure 5.

DETAILED DESCRIPTION

The invention is now 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 exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN).The radio transmission channel 100 is defined by an operating band, for instance with a bandwidth available for the communication nodes.

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

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

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

However, if it is sensed that the shared radio medium is busy during the DIFS period, the source node continues to wait until the radio medium becomes idle.

To do so, it starts a countdown backoff counter designed to expire after a number of timeslots, chosen randomly between [0, CVV], 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).

The time unit in the 802.11 standard is the slot time called 'aSlotTime' parameter. This parameter is specified by the PHY (physical) layer (for example, aSlotTime is equal to 9ps for the 802.11n standard). All dedicated space durations (e.g. backoff) 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 Ti, 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 (DOE 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 Ti, 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, at Ti, 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 20 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 Ti).

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 Ti. 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, and further employed by the IEEE 802.11e standard to solve the problem of internal collisions between enhanced distributed channel access functions (EDCAFs). 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.

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

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

The scope of 802.11ac includes single link throughput supporting at least 500 Mbps, multiple-station throughput of at least 1 Gbps and backward compatibility and coexistence with legacy 802.11 devices in the 5 GHz band (i.e. according to 802. 11a/b/g).

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

Contrary to 802.11n where each communication node should support up to two spatial streams (SSs) and an operating band of 40 MHz bandwidth, only one spatial stream (and thus only one antenna par device) is required in 802.11ac or 802.11ax which is the next version, while operating bands of 80 MHz or 160MHz bandwidth are allowed.

One reason for such a change in the new versions of 802.11 is that increasing the number of antennas often results in higher cost. This is because supporting multiple Spatial Streams (SS) has been considered as requiring at least the same number of antennas (and as much reception chains behind these antennas), thus important costs. Consequently, a majority of devices available on the market could only support a single SS.

In 802.11ac, support for only one SS is required so that devices, and especially smartphones, could be labelled as 802.1 lac compliant'. The operating mode with an 80 MHz operating band is made mandatory as a lower cost alternative to the two SS and 40 MHz operating band of 802.11n. Hence, the operating modes that utilize more than one spatial stream are now become optional in 802.11ac.

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

In order to support wider channel bandwidths within the operating band, the operating band in 802.11ac is made of an ordered succession (or series) of sub-channels 300-1 to 300-8 as shown for instance in Figure 3a. IEEE 802.11ac introduces support of a restricted number of predefined subsets of these sub-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 Figure 3a and correspond to 20 MHz, 40 MHz, 80 MHz, and 160 MHz channel bandwidths including the so-called "primary" sub-channel (here 300-3), compared to only 20 MHz and 40 MHz supported by 802.11n. Indeed, the 20 MHz component channels (or sub-channels) 300-1 to 300-8 are concatenated to form wider communication composite channels.

In the 802.11ac standard, the sub-channels of each predefined 40MHz, 80MHz or 160MHz subset are contiguous within the operating band, i.e. no hole (i.e. missing sub-channel) within the sub-channels as ordered in the operating band is allowed, and are defined with respect to the primary sub-channel.

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. The support of 40 MHz and 80 MHz channel bandwidths is mandatory while the support of 160 MHz and 80 + 80 MHz is made optional in the standard (80+80 MHz means that the operating band is made of two non-frequencycontiguous wide channels having each a bandwidth of 80 MHz).

A multi-channel communication node (accessing an 80 MHz operating band in the illustrated example of Figure 3b) is granted a Tx0P through the enhanced distributed channel access (EDCA) mechanism on the "primary sub-channel" (300-3). Indeed, for each channel bandwidth, 802.1 lac designates one sub-channel as "primary," meaning that it is used for control of transmission on that bandwidth.

It shall, however, transmit an 80 MHz PPDU (PPDU means PLCP Protocol Data Unit, with PLOP for Physical Layer Convergence Procedure; basically a PPDU refers to an 802.11 physical frame) only if all other secondary channels have been idle for at least a point coordination function (POE) inter-frame spacing (PIES). If at least one of the secondary sub-channels has not been sensed as idle for a PIES, then the node must either restart its backoff count, or use the obtained Tx0P for 40 MHz or 20 MHz PPDUs.

The vertical aggregation scheme reflects the extension of the payload 230 to all sub-channels. If there is only one collision in one of the sub-channels at a given time, the risks of having a corrupted segment of these sequences are very high despite the error-correcting decoding process. All MPDU (MAC Protocol Data Unit) frames inside the PPDU could thus automatically be considered as incorrect.

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

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

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

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

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

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

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

The source node thus generates the RTS frame with the two-bit field set to the appropriate value.

The RTS frame is sent over the primary 20MHz sub-channel and replicated (duplicated) on each other (i.e. secondary) 20MHz sub-channels forming the target composite channel.

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

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

Before the receiving node replies with a CTS frame, it checks if any of the sub-channels in the 80 MHz operating band is sensed as busy using for instance CCA. The receiving node only replies with a CTS frame on those sub-channels that are sensed as idle (for a PIFS duration preceding the reception of RTS control frame), and signal the total available bandwidth (i.e. the resulting composite channel) in the CTS frame using the same two-bit field as described above for the RTS frame.

If receiving node STA2 has successfully received the RTS frame on the primary sub-channel and if the three secondary sub-channels forming the remainder of the 80MHz operating band are sensed as idle for a PIFS duration prior to RTS reception, a total of four CTS frames are transmitted on the four sub-channels respectively to cover the entire 80MHz operating band.

As with the RTS frame, the CTS frame is sent in an 802.11a (non-HT) PPDU format on the primary sub-channel and is replicated over the different 20 MHz secondary sub-channels that have been sensed as idle by the receiving node.

Thanks to the duplication of the frames on the secondary sub-channels, every other and nearby node (either legacy or 802.11ac nodes, but neither STA1 nor STA2) can receive an RTS/CTS on its primary channel. Each of these nodes then sets its NAV to the duration value specified in the RTS/CTS frame thus received.

If STA1 receives all four CTS frames related to the target 80MHz composite channel, a DATA frame 430 can be transmitted using the whole 80MHz channel bandwidth. However, the assumption of correctly receiving the four CTS frames is performed by STA1 by correctly receiving the replied CTS frame on the primary channel only, and determining that the bandwidth signalling information of the CTS frame indicates an 80MHz channel bandwidth.

This means that STA1 will not detect any collision that may occur with one of the duplicated CTS frames on a secondary sub-channel, and then will use such secondary sub-channel whereas it is busy.

Still referring to Figure 4, a nearby node is already transmitting on the third 20MHz channel (for instance STA2 may detect such transmission) while STA1 starts sending four RTS (410) to reserve an 80MHz Tx0P. Next, STA2 has to inform STA1 by replying with CTSs only in the contiguous idle sub-channels that include the primary sub-channel (two CTS 420 are transmitted in the present example). Next, a DATA frame 430 can be transmitted using only 40MHz channel bandwidth. The ability of IEEE 802.11ac standard to fall back to lower bandwidth modes in case not all the targeted bandwidth is available is known as a fallback mechanism of the "dynamic bandwidth operation" according 802.1 lac.

As addressed earlier, the IEEE 802.11ac standard enables up to four, or even eight, 20 MHz sub-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.

As a result, channel interferences will occur more often: contrary to the example of Figure 4, the fallback mechanism currently provided in the 802.11ac standard will suffer from misleading detection of channel occupation, as further shown in regards to Figure 5.

Still referring to Figure 4, the 802.11ax task group has raised the benefit of using all available sub-channels of the operating band, even if some are noncontiguous.

This approach implies that the CTS frame is also duplicated (420-2) in the fourth sub-channel, which is then followed by a data transmission (430-2) on such reserved sub-channel (the whole data transmission being split between 430 and 430-2 channel allocation for instance).

Both context of 802.11ac and 802.11ax require that the detection of used (or busy) sub-channels be improved to avoid situation in which the source node and the receiving node does not use the same composite channel for data communication. To illustrate this need, Figure 5 shows two examples of channel interferences that are not detected using the conventional mechanisms. In these example, a legacy 802.11a or 802.11n communication in at least one of the secondary MHz sub-channels forming the target composite channel is not detected by the source or receiving node.

In both examples, the medium is sensed as free during a PIES period preceding the (non-HT) RTS transmissions over the 80MHz operating band.

Even if it is known that the RTS/CTS exchange does provide a more robust non-HT signalling mechanism, it is not even proof. There still occur frame collisions in the 20MHz sub-channel context, so it is ascertained this also occurs in secondary sub-channels in the context of a multi-channel transmission (802.11ac or ax).

When the back-off counter of 802.11ac source node STA1 expires in the primary sub-channel, the 802.1 lac source node checks the secondary sub-channels to determine whether or not the sub-channels have been idle for at least PIES time before the back-off timer expiration.

Next, STA1 starts to transmit 20 MHz non-HT duplicate RTS PPDU on each idle sub-channel of the operating band.

In the case of Figure 5a, a legacy communication (520-L) collides with the corresponding non-HT duplicated RTS frame, here in the third sub-channel. This collision event is not detected by receiving node STA2 (i.e. the receiving node specified in RA of the RTS frame in the primary sub-channel) as it does not demodulate the duplicate RTS frame on the secondary sub-channels.

As STA2's NAV indicates idle (due to CSMA/CA performed only on the primary channel), it shall respond with a CTS frame in a non-HT duplicate format after a SIFS period according to 802.11ac.

STA2 specifies the channel width available for communication, in the CTS frame, namely 80MHz in the present example. The channel width considered as available by STA2 includes "any channel width for which CCA on all secondary channels has been idle for a PI FS prior to the start of (receiving) the RTS frame and that is equal to or less than the channel width indicated in the RTS frame".

Next, STA2 duplicates the non-HT CTS PPDU on each sub-channel forming the channel width available, i.e. the whole 80MHz operating band in the example, even if one secondary channel is busy by a legacy communication (but not detected).

Next, STA1 receiving the non-HT CTS frames on the sub-channels demodulates the CTS frame from its primary sub-channel to determine the result of the channel width negotiation for subsequent transmissions within the current transmission opportunity (TXOP). As the channel width has not been reduced compared to the target channel width indicated in the RTS frame, STA1 starts data transmission over the full 80MHz operating band, resulting in collision with legacy data transmission (530L).

It results that the TXOP is wasted.

In addition, in the context of non-contiguous channel usage in the scope of 802.1 lax, even if STA1 detects a legacy communication on the third sub-channel and avoids sending a duplicate of the non-HT RTS frame on this sub-channel (not shown in the Figure), receiving node STA2 would not be able to determine that no RTS frame is sent by STA1 on this sub-channel because it only monitors the primary sub-channel.

As a result, STA1 starts its data transmission over three sub-channels whereas STA2 is analysing the full 80M Hz operational band for erroneously recovering transmitted data.

It would thus be useful if the nodes could be able to discriminate the duplicated RTS frame in order to determine which (non-contiguous) sub-channels form 15 the composite channel, without specific indication of the composite channel composition in the RTS frame.

In the case of Figure 5b, a legacy communication (520-L) collides with the corresponding non-HT duplicated CTS frame, here in the third sub-channel. This collision event is not detected by source node STA1 as it does not demodulate the duplicate CTS frame on the secondary sub-channels.

This CTS-collision situation may correspond to the case where STA1 cannot correctly receive the CTS frame in a secondary sub-channel due to interference with a more powered legacy transmission.

As STA1's NAV on the primary sub-channel is not set, and as the RTS frame was sent previously, and as CCA on all the secondary sub-channels has sensed idle sub-channels for a PIFS prior to the start of the RTS frame, source node STA1 considers the whole operating band as free and available for data transmission, whereas it is not the case.

In addition, in the context of non-contiguous channel usage in the scope of 802.1 lax, even if STA2 detects a legacy communication on the third sub-channel and avoids sending a duplicate of the non-HT CTS frame on this sub-channel (not shown in the Figure), source station STA1 would not be able to determine that no CTS frame is sent by STA2 on this sub-channel because it only monitors the primary sub-channel.

As a result, STA1 starts its data transmission over the full 80MHz operational band whereas STA2 is expecting receiving data only on three sub-channels.

These examples show that the known mechanisms lead to waste of TX0Ps and thus in bandwidth losses over the whole operating bandwidth. This is because these mechanisms do not include means for detecting collision of duplicates of non-HT control frames in the secondary sub-channels.

This collision-detection deficiency is especially problematic with personal devices on which a central organization has little or even no control to select the wireless channels of the 5GHz or the like band. This is ascertained in distributed environments, which are by essence not managed at all.

To alleviate the deficiencies of the prior art, embodiments of the invention provide an improved detecting method for detecting the sub-channels conveying non-HT frames over a composite channel.

Embodiments of the present invention described below fall within an enhancement of the 802.11ac standard, and more precisely into the context of 802.11ax wherein dense wireless environments are more ascertained to suffer from previous limitations.

An exemplary wireless network is an IEEE 802.11ac network (and upper versions) in which an operating band is made of series of sub-channels. However, the invention applies to any wireless network where a source node 101-107 sends data of a data stream to a receiving node 101-107 using multiple channels (see Figure 1). The invention is especially suitable for wireless stations having only one Spatial Stream and labelled as '802.11ac compliant'.

The behaviour of communication nodes during a conventional communication over an 802.11 medium has been recalled above with reference to Figures 1 to 5.

As a first frame, for instance a control frame such as a CTS or RTS frame, is received over a primary one of sub-channels, one aspect of embodiments of the invention provides sensing, for each of the sub-channels, a change of an idle/busy status of the sub-channel, and storing, in memory, the status changes in association with time information; and determining, using the stored time information, one or more other sub-channels that have respective status changes temporally similar to the status changes in the primary sub-channel due to the received first frame, to trigger detection of a duplicate frame on each so-determined sub-channel.

In other words, the detection of the idle or busy sub-channels is made by comparing the status change profiles (i.e. temporal signatures) between the primary sub-channel (used as a reference) and each other sub-channel.

Where the profiles are not temporally similar with respect to the reception of the received frame, the node can infer that the compared secondary sub-channel conveys another communication, and thus is not idle. On the contrary, as long as the profiles are similar, the node can infer the considered secondary sub-channel is idle.

As it will be apparent from the detailed description below of embodiments of the invention, the latter still complies with legacy nodes, i.e. with 802.11 nodes that do not implement the invention. Therefore, the term "legacy station" covers two kinds of situations referring to the 802.11 standard: it may be either a "legacy non-HT" (also called "802.11a legacy") acting as an 802.11a 20MHz station, or either a legacy 802.11n / 802.11ac station not implementing the invention.

A legacy environment typically describes a situation in which the legacy nodes and the nodes implementing the invention coexist and are competing to access the shared wireless channels, possibly using a composite channel.

The invention ensures the legacy nodes still operate in a conventional way, despite some other nodes implement the invention. It means that the detection means according to embodiments of the invention are advantageously transparent for those legacy nodes (i.e. not taken into account).

Figure 6 schematically illustrates a communication device 600 of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 600 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 600 comprises a communication bus 613 to which there are preferably connected: * a central processing unit 611, such as a microprocessor, denoted CPU; * a read only memory 607, denoted ROM, for storing computer programs for implementing the invention; * a random access memory 612, 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 602 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 612 to the network interface for transmission or are read from the network interface for reception and writing into a FIFO receiving memory in RAM 612 under the control of a software application running in the CPU 611.

Optionally, the communication device 600 may also include the following components: * a data storage means 604 such as a hard disk, for storing computer programs for implementing methods according to one or more embodiments of the invention; * a disk drive 605 for a disk 606, the disk drive being adapted to read data from the disk 606 or to write data onto said disk; * a screen 609 for displaying decoded data and/or serving as a graphical interface with the user, by means of a keyboard 610 or any other pointing means.

The communication device 600 may be optionally connected to various peripherals, such as for example a digital camera 608, each being connected to an input/output card (not shown) so as to supply data to the communication device 600.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 600 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 600 directly or by means of another element of the communication device 600.

The disk 606 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 607, on the hard disk 604 or on a removable digital medium such as for example a disk 606 as described previously. According to an optional variant, the executable code of 35 the programs can be received by means of the communication network 603, via the interface 602, in order to be stored in one of the storage means of the communication device 600, such as the hard disk 604, before being executed.

The central processing unit 611 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 604 or in the read only memory 607, are transferred into the random access memory 612, 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 7 is a block diagram schematically illustrating the architecture of a communication device also called node (or station) 600 adapted to carry out, at least partially, the invention. As illustrated, node 600 comprises a physical (PHY) layer block 703, a MAC layer block 702, and an application layer block 701.

The PHY layer block 703 (here a 802.11 standardized PHY layer) has the task of formatting and sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance non-HT control frames such as medium access requesting frames of the RTS type to reserve a transmission slot or medium access acknowledging frames of the CTS type to acknowledge reservation of a transmission slot, as well as of MAC data frames to/from that radio medium.

The PHY layer block 703 includes CCA capability to sense the idle or busy state of sub-channels and to report the result to the MAC 702 according to 802.11 standard. Upon detecting a PLCP preamble of a received frame, the PHY layer block 703 generates and transmits a PHY_CCA.indicafion primitive to the MAC layer block 702. This primitive shall be issued for reception of a signal with significant received signal strength (for the primary channel, it is typically issued prior to correct reception of the PLCP preamble of an 802.11 frame).

Next after having demodulating and Viterbi-decoded a frame header, it generates and transmits another primitive, namely the PHY_RXSTART.indication primitive, to the MAC layer block 702.

In more details, every frame transmission begins with a PLCP training sequence broadcast on the shared channel(s). The PHY layer block 703 generates a PHY-RXSTART.indicafion when it has successfully validated the PLOP header error check CRC at the start of a new PLOP frame.

The RXVECTOR associated with this primitive represents a list of parameters that the PHY layer provides the local MAC layer upon receipt of a valid FLOP header or upon receipt of the last PSDU data bit in the received frame. This vector contains both MAC and MAC management parameters. It includes the SIGNAL field, the SERVICE field, the MAC Protocol Data Units (MPDU) length in bytes (calculated from the LENGTH field in microseconds), the antenna used for receive (RX_ANTENNA), the received signal strength indicator (RSSI) and signal quality. The MAC layer block or controller 702 preferably comprises a MAC 802.11 layer 704 implementing conventional 802.11 MAC operations, and an additional block 705 for carrying out, at least partially, the invention. The MAC layer block 702 may optionally be implemented in software, which software is loaded into RAM 612 and executed by CPU 611.

Preferably, the additional block, referred to as channel usage indication module 705, implements the part of the invention that regards node 600, i.e. transmitting operations for a source node and receiving operations for a receiving 20 node.

Channel usage indication module 705 is in charge of timestamping each CCA indication received from the PHY layer 703 for the sub-channels forming the operating band.

Using the CCA indications and the associated fimestamps, channel usage indication module 705 is able to compare the temporal status profiles of the secondary sub-channels with the primary sub-channel in order to detect possible collisions and thus to detect the sub-channels that are available.

For both source node's perspective and receiving node's perspective, channel usage indication module 705 thus senses which sub-channels are available and contributes to determine if these detected-as-available sub-channels correctly convey non-HT duplicate control frames (e.g. the RTS/CTS control frames supporting the dynamic bandwidth allocation according 802.11ac standard).

In this context, the source and receiving node are designed to determine if a non-HT control frame is correctly duplicated and received in a secondary sub-channel in regards to the original non-HT control frame conveyed on the primary sub-channel.

On top of the Figure, application layer block 701 runs an application that generates and receives data packets, for example data packets of a video stream.

Application layer block 701 represents all the stack layers above MAC layer according ISO standardization.

Figure 8 illustrates, using two flowcharts, general steps of an embodiment of the present invention to enhance channel usage detection for multi-channel transmission in an 802.11ac wireless medium.

Figures 9 and 10 illustrates exemplary communication lines corresponding to the two examples of Figure 5 when the invention is implemented to detect collision of duplicated control frames in the secondary sub-channels.

Although these examples show a WLAN system using a multi-channel including four contiguous sub-channels each having a channel bandwidth of 20 MHz, the number of sub-channels or the channel bandwidth thereof may vary.

Although the interferences shown in the figures are due to one 802.11ac legacy node in one secondary sub-channel, the number of 802.11ac legacy nodes and the number of secondary sub-channels subject to interference is not limited, being noted that a single interference may impact several sub-channels (e.g. 40MHz width for legacy 802.11n communication).

Furthermore, the invention may apply to other mechanisms than the RTS/CTS handshaking mechanism over the multiple channels according to 802.11ac. In particular, it may apply to any 802.11 frame which is transmitted in the non-HT duplicate format, for instance control frames such as ACK, Ps-Poll, CF-End, BAR, NDP announcement, etc. The method of Figure 8 is implemented by a single node 600 which may be either STA1 (source node having data to transmit to STA2) or STA2.

Figure 8b illustrates, using a flowchart, continuous monitoring of the sub-channels that compose the operating band.

By default, the PHY layer 703 is configured to sense wireless signal, to demodulate it in order to receive incoming packet headers (CS/CCA state). Whenever the PHY layer 703 senses the status of a wireless medium changing from channel idle to channel busy or from channel busy to channel idle, it sends a PHY-CCA.indication to the MAC indicating the current state of the medium.

As recalled, CCA is physical carrier sense which listens to received energy on the radio interface with the same energy threshold set over all sub-channels (including the primary one).

At step 810, channel usage indication module 705 receives a PHY-CCA.indication from PHY layer 703.

For instance, upon receiving the transmitted 802.11 PLCP preamble (i.e. beginning of a control frame) in the primary 20 MHz sub-channel, the PHY layer measures a receive signal strength. The CCA mechanism detects a "medium busy" condition for the primary sub-channel and the medium status activity is indicated by the PHY to the MAC via the PHY-CCA.indication primitive with STATUS=busy, prior to correctly receiving the whole PLCP frame.

Similarly, when the PHY stops detecting a signal on the sub-channel, the PHY notifies the MAC by issuing the PHY-CCA.indication primitive with STATUS=idle. For multi-channel PHY as in 802.11n or ac, the PHY15 CCA.indication(STATE, channel-list) primitive is issued as an indication of reception/end-of-reception of a signal in the sub-channels (anyway if the packet will be further decoded or not). The STATE parameter is either "busy" or "idle" depending on the state detected by CCA.

The channel-list is a parameter indicating a list of one or more sub-channels corresponding to the "STATE" parameter. For example, if it is determined that the secondary sub-channel is IDLE, the indication is generated as PHYCCA.indication(IDLE, Secondary). As another example, if it is determined that a composite channel including the primary sub-channel, the sub-secondary channel and the tertiary sub-channel is BUSY, the indication is generated as PHY-CCA.indication(BUSY, Primary, Secondary, Tertiary). As another example, if it is determined that, over a composite channel composed of four sub-channels, only the tertiary sub-channel is BUSY, the indication is generated as PHYCCA.indication(BUSY, Tertiary).

One can note that the CCA sensitivity (in terms of wireless signal level detection and detection time) of the PHY layer has the same error tolerance for each 802.11 channel, thus there should be little or no drift in PHY-CCA.indication assertions among various channel (taking into account the non-HT duplicate frame to analyse is simultaneously duplicated over several channels by a unique PHY transmitter). Most of the time, a single PHY-CCA.indication is captured for several channels through the use of the channel-list parameter.

Next, at step 811, using the received PHY-CCA.indication, module 705 identifies the sub-channels concerned by the received PHY-CCA.indication primitive using the channel-list parameter and their state or status using the STATE parameter. This information is stored in memory 604.

Next, at step 812, module 705 generates a timestamp for every item of information stored at step 811, and stores the timestamps in memory 604 in association with their respective sub-channel and status information.

In a slight variant that reduces use of memory, module 705 generates timestamps only for status change in a sub-channel, and saves such information in memory. For instance, module 705 detects for a sub-channel whether or not the status identified in the last processed PHY-CCA.indication is different from the previous one stored in memory. In other words, the status changes correspond to rising and falling edges in the busy signal indication (meaning both idle and busy notification by the PHY layer).

The time information stored in memory may include the (absolute) time instant (i.e. timestamp) at which the status change for the sub-channel occurs or may include a (relative) time difference between the status change thus sensed and the preceding sensed status change in the same sub-channel.

A method for triggering the required timestamps is to link the PHY-CCA.indication with a simple local clock counter. Alternatively, the counter could be driven by the PHY's existing high-precision reference oscillator and equipped with a means for generating precise timestamps at the moment that a PHY-CCA.indication passes through the PHY-MAC interface. The module 705 could later access the stored timestamp values simply by reading the content of a register without degrading its value because of some uncontrolled notification and/or access delays between PHY-MAC.

In embodiments, the timestamps of at least the current TXOP are stored in memory 604, meaning that the stored values are erased when a new backoff procedure starts.

Knowing the subchannel status changes and corresponding timestamps, module 705 is able to generate CCA profiles, examples of which are shown in Figures 9 and 10.

Figure 8a illustrates, using a flowchart, the algorithm performed to determine which secondary (sub-)channel(s) of the operating band also convey a duplicate of the non-HT control frame received in the primary sub-channel. As apparent from the explanation below, this determination is made relative to the status changes (for instance CCA status profile) of the primary sub-channel. The idea of this process is to determine, using the stored time information or timestamps, one or more other sub-channels of the operating band that have status changes temporally similar to the status changes in the primary sub-channel due to the received non-HT control frame.

Note that the determination process of Fig.8a is asynchronous to the one of Fig.8b, as the determination process is raised upon demodulation of an 802.11 frame on the primary sub-channel. This determination process helps determining the width of the composite channel used for the current TXOP and helps answering to a non-HT control frame request.

As example, receiving station STA2 shall respond to an RTS in the primary sub-channel with a CTS frame in a non-HT duplicate PPDU after a SIFS period but only in each of the secondary channels where a duplicate RTS is detected. As another example, a STA1 shall respond to a CTS in the primary sub-channel with a data PPDU which covers a composite channel made of the sub-channels for which a duplicate CTS is detected.

Referring to the example of Fig.8a, the determination process starts at step 800 in which node 600 (either the source node or the receiving node) receives a 802.11 control frame (e.g. RTS or CTS) on its primary channel. The PHY-CCA.indication notifications related to the reception of this control frame have already been captured by the process of Fig.8b.

At step 800, node 600 verifies that the received control frame: -is intended to it (i.e. the receiver address RA matches node 600); and -is a 802.11a frame in a non-HT format that has a "bandwidth signaling Transmitter Address" (TA) -the network allocation vector (NAV) of the channel indicates idle.

These conditions make it possible to determine whether or not the received control frame indicates an operating mode that involves duplicating the control frame over several of the sub-channels by the TA node.

If the conditions are positively verified, the process continues at step 801.

Otherwise, the control frame is not processed according to the invention, and the process ends.

At step 801, it is determined whether or not the target composite channel specified in the received control frame is strictly more than a single sub-channel (i.e. 20M Hz), in which case the next step is step 802. Otherwise, the process ends.

The support for dynamic or static bandwidth operation (for RTS/CTS frames) and more generally the channel width occupied by the control frame are signaled in the SERVICE field of the 802.11 data frame (The DATA field is composed of SERVICE, PSDU, tail, and pad parts). The composite channel width is thus retrieved directly from the header of the control frame received.

Next, at step 802, the stored timestamp values of each sub-channel composing the target composite channel are retrieved from memory 604. These stored values for a given sub-channel form the CCA timing profile (examples of which are shown in Figures 9 and 10) which displays that channel's usage.

Next, at step 803, the CCA timing profile of the primary sub-channel is selected as a reference for the received control frame.

Next, at steps 804 and 805, the node 600 determines the real sub-channels that have successfully conveyed a duplicate of the non-HT control frame received through the primary sub-channel.

This determination includes checking a CCA profile matching (for instance by comparison) between the CCA profile of the primary sub-channel (used as reference) and the CCA profile of each secondary sub-channel of the target composite channel. The check is made on the portion of the profile that regards the reception of the control frame on the primary sub-channels (for instance a temporal portion with an additional PIFS or 2xPIFS margin). This is because it is considered that the control frame on the primary sub-channel and a duplicate of the same control frame on any secondary sub-channel have the same temporal signature.

The check of a profile matching may include determining whether or not the rising and falling edge timestamps are close enough between the two profiles, so that the drift is negligible (for instance less than a few nano-seconds).

The check for the rising edge may include determining whether or not the time difference between an idle-to-busy status change (i.e. a change from idle status to busy status due to the sensing) in the sub-channel considered and an idle-to-busy status change corresponding to the start of the received control frame in the primary sub-channel is less than a first threshold.

Similarly, the check for the falling edge may include determining whether or not the time difference between a busy-to-idle status change in the sub-channel considered and a busy-to-idle status change corresponding to the end of the received control frame in the primary sub-channel is less than a second threshold.

Of course, to have the same temporal signature through the considered sub-channels, the analysis of step 804 is based on two consecutive edges: a rising edge quite simultaneous to the rising edge due to the received control frame in the primary sub-channel, and the next falling edge.

Based on the time drift between the rising and falling edges for the sub-channels with respect to the primary sub-channel, module 705 consider the sub-channels having a matching profile as pertaining to the same communication.

On the contrary, when the profile matching is not obtained, the corresponding secondary sub-channel is considered as belonging to a distinct communication or as being a collided channel. Thus, this non-matching secondary sub-channel is rejected at step 805.

Figure 9 and 10 illustrates the benefits of the invention in the critical situations of collision due to co-channel interference or noise as explained above with reference to Figure 5.

Figure 9 refers to the case where receiving node STA2 does not detect a collision of the RTS frame duplicated on the third sub-channel.

Receiving node STA2 implementing the invention retrieves the CCA time profiles for all the sub-channels as explained above. Four exemplary CCA time profiles are shown in the Figure.

Based on these CCA time profiles where the CCA time profile for the primary sub-channel is used as a reference, STA2 detects (using the process of Fig.8a) that the falling edge (900) for the third sub-channel does not temporally match the falling edge of the primary (reference) profile. Even if the rising edge timing is considered as correct, the difference of timing 900 informs 3T2 of an unexpected situation (maybe due to a collision). As a consequence, the third sub-channel is considered as a faulty secondary sub-channel, and is rejected.

Thus the MAC module 704 of STA2 now knows through the enhanced CCA detection means of the invention, that it should not answer a CTS in this third sub-channel.

In the context of 802.11ac standard, STA2 then fallbacks to a 40MHz channel width and sends a non-HT duplicate CTS frame on the first two sub-channels (with an indication of the new limited bandwidth inside).

Next, transmitting node STA1 starts its data transmission 430 over the 40M Hz band, and collision with legacy data transmission (930-L) is thus avoided.

In the context of non-contiguous channel usage in the scope of 802.1 lax, the composite channel that can be used for wireless communication can be made of the primary sub-channel and all the profile-matching sub-channels.

For instance, STA2 may consider a 60MHz composite channel made of the first, second and fourth sub-channels that are not contiguous within the 80MHz operating band. In such a case, STA2 sends three non-HT duplicate CTS frames on these three sub-channels, the CTS frame keeping the same 80MHz indication for the bandwidth.

The 60MHz bandwidth may thus be used by the same transmitting node or shared between various nodes that needs to transmit (for instance based on a time division multiplexing or a frequency [here sub-channel] division multiplexing). Communication by a single node over sub-channels that are not frequency contiguous is possible. For instance, document IEEE 802.11-15/0035r1, entitled "Scalable Channel Utilization Scheme", proposes a scalable channel utilization scheme to utilize as many channels as possible, even non-contiguous within the operating band, by turning on/off part of the OFDM tones. The OFDM Tones corresponding to a detected busy channel can be turned off so as to cause no interference with the ongoing transmission. For instance, the D(k) component of the following expression (expressing the OFDM signal) can be set to zero for each sub-channel "k" sensed as busy: N-1 ( N j. 27r. k. n) d(n) = D (k)exp n = 0,1, ... , N -1 k-0 where N is the FFT size.

Figure 10 refers to the case where transmitting node STA1 does not detect a collision of the CTS frame duplicated on the third sub-channel.

Receiving node STA2 implementing the invention retrieves the CCA time profiles for all the sub-channels as explained above. Four exemplary CCA time profiles are shown in the Figure.

Based on these CCA time profiles where the CCA time profile for the primary sub-channel is used as a reference, STA2 detects (using the process of Fig.8a) that all the CCA timing profiles of the secondary sub-channels match the profile of the primary sub-channel on which the RTS has been received. So, STA2 is ready to answer to the RTS with non-HT CTS frame duplicated on the sub-channels available.

Similarly, transmitting node STA1 implementing the invention retrieves the CCA time profiles for all the sub-channels as explained above. Four exemplary CCA time profiles are shown in the Figure.

In the example of the Figure, STA2 has also performed detection of an occupation 1003 on the third sub-channel before sending any CTS. As a consequence, it sends a non-HT duplicated CTS on the three available sub-channels, but not on the third sub-channel (CTS 1020-2 not sent).

Upon receiving the CTS on the primary sub-channel and based on the CCA time profiles where the CCA time profile for the primary sub-channel is used as a reference, STA1 detects (using the process of Fig.8a) that both timing events 1000 and 1001 of the third sub-channel mismatch the primary sub-channel CCA profile. It thus infers that a different communication (from the expected duplicated CTS like in the primary sub-channel) is going on this third sub-channel. As a consequence, the third sub-channel is considered as a faulty secondary sub-channel, and is rejected.

In the context of 802.11ac standard, STA1 thus fallbacks to a 40MHz composite channel and starts transmitting data 1030 on the first two sub-channels (with an indication of the new 40MHz-limited bandwidth inside the 802.1 lac DATA frame).

In the context of non-contiguous channel usage in the scope of 802.1 lax, all the matching sub-channels can be kept to form a 60MHz composite channel to be used for wireless communication.

In another example illustrated by the Figure, STA2 has not detected occupation 1003 on the third sub-channel before sending the CTSs. This is for instance because the 802.11ac standard only mandates analysing the NAV for the primary sub-channel (which is ok) and the CCA before a PIFS period preceding the RTS emission for the secondary sub-channels (which test is also ok).

As a consequence, STA2 sends a non-HT duplicated CTS on the four sub-channels, including CTS 1020-2 on the third sub-channel.

As a result, CTS 1020-2 on the third sub-channel collides the legacy communication.

Upon receiving the CTS on the primary sub-channel and based on the CCA time profiles where the CCA time profile for the primary sub-channel is used as a reference, STA1 detects (using the process of Fig.8a) that the rising edge (1000) of the third sub-channel (900) is different from the primary (reference) profile (1001 may be aligned with the duplicated CTS, not shown on the Figure). The CCA profile of the third sub-channel thus mismatches the primary sub-channel CCA profile.

STA1 thus infers that a different communication (from the expected duplicated CTS like in the primary sub-channel) is going on this third sub-channel. As a consequence, the third sub-channel is considered as a faulty secondary sub-channel, and is rejected.

In the context of 802.11ac standard, STA1 thus fallbacks to a 40MHz composite channel and starts transmitting data 1030 on the first two sub-channels (with an indication of the new 40MHz-limited bandwidth inside the 802.11ac DATA frame).

This fallback information may be retrieved by STA2 using a width newly specified in the PHY VHT-SIG-A header of the next DATA frame sent by STA1.

In the context of non-contiguous channel usage in the scope of 802.1 lax, all the matching sub-channels can be kept to form a 60MHz composite channel to be used for wireless communication. Appropriate signalling may be provided in one header of the next DATA frame sent by STA1 in order for STA2 to be informed of which sub-channels forming the composite channel are being used by STA1.

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.