GB2567896A - Energy correction in an uplink OFDMA 802.11 channel - Google Patents

Abstract

A station in a wireless network identifies a resource unit in a multi-user uplink transmission opportunity 340 which has been dedicated to padding data, then transmits on this resource unit to provide energy correction. The station may detect the energy transmitted during the preamble period 350 of the transmission opportunity, and transmit the energy corrective data if this energy falls below a certain threshold. The energy correction resource units may be signalled by the access point in a trigger frame 330, and may be distributed homogenously over the channel with one at the centre of the frequency band. The station may be the access point or a non-access-point station. Where the station is the access point it may comprise two antennae, the first for receiving the uplink transmission and the second for transmitting the corrective data. The scheme is intended for interference reduction in an IEEE 802.11ax WLAN, with the energy correction used to prevent legacy stations making erroneous determinations of an idle channel.

Description

ENERGY CORRECTION IN AN UPLINK OFDMA 802.11 CHANNEL

FIELD OF THE INVENTION

The present invention relates generally to wireless communication networks comprising a plurality of stations, including an access point station, and more specifically to a method and device for correcting the energy of a multi-user uplink communication.

The invention finds application in the interference reduction management within an 802.11 ax network.

BACKGROUND OF THE INVENTION

The IEEE 802.11 MAC family of standards (a/b/g/n/ac/etc.) defines a way wireless local area networks (WLANs) work at the physical and medium access control (MAC) level over a 2.4 or 5 or 60 GHz frequency band. 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.

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

In 802.11ax, a feature, known as multi-user transmission, has been considered to allow multiple simultaneous transmissions to/from different users in both downlink and uplink directions over a communication channel. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple stations to simultaneously transmit.

To actually perform such multi-user transmission, it has been proposed to split a granted channel, which may be of a single 20 MHz channel or multiple 20 MHz channels (known together as a composite communication channel), into sub-channels (elementary subchannels), also referred to as resource units (RUs), that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

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

As currently envisaged, the granularity of such OFDMA RUs is finer than the original 20MHz channel band composing the 802.11ac channels. Typically, a 2MHz or 5MHz

RU may be contemplated as a minimal width, therefore defining for instance 9 RUs within a single 20MHz channel composing the composite communication channel.

The composite channel comprises a primary 20 MHz channel on which a station senses the medium as idle or busy and thus on which it performs EDCA backoff procedure to access the medium (frequency bands other than 20MHz may be contemplated). The primary channel may be extended into a composite communication channel by appending one or more secondary 20 MHz channels.

To support multi-user uplink, i.e. uplink transmission to the 802.11 ax access point (AP) during a granted transmission opportunity (TxOP), the 802.11 ax AP has to provide signalling information for the legacy stations (non-802.11 ax stations) to set their NAV (Network Allocation Vector) and for the 802.11 ax stations to determine the allocation of the resource units.

As signalling information, it has been proposed for the AP to send a trigger frame (TF) to the 802.11 ax stations to trigger uplink communications.

The bandwidth or width of the targeted composite channel is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is added. The TF frame is sent over the primary 20MHz channel and duplicated (replicated) on each other 20MHz channels forming the targeted composite channel. It is expected that every nearby legacy station receiving the TF on its primary channel, then sets its NAV to the value specified in the TF frame. This should prevent these legacy stations from accessing the channels of the targeted composite channel during the TXOP.

A resource unit, RU, can be reserved for a specific station or be randomly accessed by stations.

The reserved RU, also known as Scheduled RU, is indicated by the AP in the TF by indicating the station to which the RU is reserved. The stations do not need to perform contention to access scheduled RUs.

The random RU is also indicated by the AP in the TF. Random RUs may serve as a basis for contention between nodes willing to access the communication medium for sending data.

A trigger frame thus defines a set of RUs selected from Scheduled RUs and Random RUs, to form the targeted composite channel.

Upon receiving the trigger frame, the stations access their RUs (through contention for random RUs) and then transmit their own data to the access point (uplink transmission) over their respective RU, after a preamble period. During this preamble period, all the transmitting stations simultaneously transmit the same data, known as “preamble”, each over the entire 20 MHz channel, and not over their respective RU only. The transmitted preambles thus superpose each other over the 20 MHz channel.

In practice, there is no guarantee that the Scheduled or Random RUs will actually be used by the stations.

A Random RU may simply not be selected by the RU contention mechanisms at the stations. Some RUs may also not be accessible for some stations because of hidden legacy stations.

A Scheduled RU may not be used if the specified station (i.e. to which the RU has been assigned by the AP) has not correctly received by the latter (e.g. because of interference or because the station entered a doze mode) or has no data to send.

The channel bandwidth may thus not be optimally used. Usually, the AP still receive data from stations using the RUs.

However, in some situations, unused RUs may dramatically impact the reception of data by the AP. This is mainly because the more unused RUs within a 20 MHz channel, the lower the average energy over this 20 MHz channel.

In particular, as the legacy stations not registered to the AP use this average energy over their primary 20 MHz to sense whether it is idle or busy, the presence of unused RUs increases the risk that such legacy stations sense the corresponding 20 MHz channel as idle. Consequently, these legacy stations try to access and transmit data over this 20 MHz channel, resulting in collisions with the uplink transmission within the multi-user uplink TXOP (i.e. data traffic conveyed over the used RUs).

This is clearly not a satisfactory situation.

Publication GB 2,539,693 discloses a solution to that issue by allowing a station to detect an unused resource unit, i.e. an RU on which no data transmission is in progress during a sensing period after the preamble period, and then to emit padding data on the detected unused RU.

As a result, the overall energy level over the 20MHz communication channel is raised, reducing risks forthe legacy stations to sense this channel as idle.

However, this solution has some drawbacks.

In particular, it does not prevent the legacy stations from accessing the medium during the sensing period (if the preamble period plus the sensing period is longer than a socalled DIFS period). Thus, there remain risks that the legacy stations create interference. Consequently, a better solution is still needed.

SUMMARY OF INVENTION

It is a broad objective of the present invention to improve this situation, i.e. to overcome some or all of the foregoing limitations. In particular, the present invention seeks to provide a more efficient use of the communication channel, in particular by reducing or avoiding risks of interference.

To achieve this objective, the present invention seeks to provide an improved energy correction or compensation over the RUs during the multi-user uplink TXOP.

In this context, the inventors propose an enhanced method for energy correction in a wireless network comprising a plurality of stations, including an access point station. The method comprises, at one of the stations:

identifying an energy correction resource unit dedicated to padding only from a plurality of resource units forming a communication channel during a multi-user uplink transmission opportunity granted to the access point station; and sending energy corrective data over the identified energy correction resource unit right after an uplink preamble period, the energy corrective data being padding data.

The padding data are preferably sent during the whole multi-user uplink transmission opportunity (like the other stations make with their uplink data over the other RUs).

Thanks to the provision (usually in a trigger frame) of a specific resource unit, namely the energy correction resource unit, for padding data that can be identified, the station can send padding data earlier, thereby reducing risks of having a legacy station accessing the medium.

In other embodiments, the inventors propose an enhanced method for energy correction in a wireless network comprising a plurality of stations, including an access point station. The method comprises, at one of the stations:

determining energy emitted by stations during an uplink preamble period preceding, within a multi-user uplink transmission opportunity granted to the access point station, an uplink data transmission used by the emitting stations to transmit data over respective resource units forming part of a communication channel; and responsive to a level of the determined energy, sending energy corrective data over a free resource unit of the communication channel.

Using the uplink preamble period to detect satisfactory or unsatisfactory level of transmitted energy, makes it possible to early provide additional energy by sending energy corrective data, e.g. padding data. As a result, risks of having a legacy station accessing the medium are reduced.

Also, there is provided a wireless communication device forming station in a wireless network comprising a plurality of stations, including an access point station. The device forming station comprises at least one microprocessor configured for carrying out the steps defined above for any of the methods.

Optional features of these embodiments are defined in the appended claims with reference to methods. Of course, same features can be transposed into system features dedicated to any device according to the embodiments of the invention.

In some embodiments, the sending station is the access point station. This approach advantageously secures the reception of uplink data by the AP. Indeed, as the interferences within the uplink data transmission are liable to impact only the AP (because it is the AP that receives the uplink data), energy correction signal sent by the AP allows the nodes or stations that may fall within the transmission range of the AP to be kept aware of medium use.

In a variant, the sending station is a non-access-point station. This advantageously allows a more simple AP to be used (e.g. a single antenna for the AP is needed, or no postprocessing on the energy correction RU, for instance masking, is required by the AP).

These embodiments also advantageously offer better protection of the network area (covered by the AP) from interference since one or more of these sending stations may be located on or close to the edge of the network area. Indeed, in that case such a sending station eliminates risks of hidden nodes in its vicinity within the network area.

In some embodiments, the plurality of resource units is defined in a trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity, the trigger frame identifying the energy correction resource unit from among the plurality of resource units. This explicit indication of the energy correction RU advantageously offers flexibility in how the additional energy may be spread over the channels, and thus in which profile of RUs the trigger frame may be based on.

In embodiments, the trigger frame defines a single energy correction resource unit. This optimizes usage of the communication channel as a maximum of resource units are available for uplink data transmission by the other stations.

In variants, the trigger frame defines a plurality of energy correction resource units. This provides a better management of energy at the stations, as the provision of energy corrective data on the channel may be shared between two or more stations.

According to a specific feature, the plurality of energy correction resource units is homogenously distributed over the communication channel. The distribution is made in the frequency domain. It means that energy correction resource units are not concentrated around the same frequencies, but spread over the whole frequency band of the channel. This offers a more regular signal strength over the whole frequency band and thus a reduced dynamicity of the resulting received signal for the access point.

In some embodiments, one energy correction resource unit (it may be the single one) is positioned at the centre of the frequency band of the communication channel. This has a double advantage. On one hand, the central RU in the majority of RU profiles available at the AP for OFDMA signalling is of the smallest possible size (26 tones). Thus, this approach allows the AP to still have the choice between a large number of possible RU profiles, while keeping the maximum bandwidth for uplink transmission.

On the other hand, by knowing the central position of the energy correction RU, the AP may use predetermined filters (masks) at its side when receiving the OFDMA uplink transmissions.

In other embodiments, the energy correction resource unit or units are defined in the trigger frame using an AID equal to 2046. No change is thus needed in the trigger frame signalling compared to the current version of 802.11 ax as this AID is already known to identify the RUs the non-AP stations cannot use.

In some embodiments, the method further comprises receiving, using a first antenna, data from stations over resource units during the multi-user uplink transmission opportunity, wherein the sending of energy corrective data is made using a second and distinct antenna. Use of two antennas by the access point is not an issue as latest versions of the

802.11 standard imposes the usage of several antennas (ΜΙΜΟ configurations).

In some embodiments, the method further comprises, at the station, determining energy emitted by other stations during the uplink preamble period, wherein the sending of energy corrective data depends on the determined energy.

This approach optimizes the energy management at the stations, since their use to provide energy correction or compensation is adjusted based on the determined energy.

For instance, deciding to send energy corrective data or not may depend on whether the determined energy is below a threshold or not, i.e. be responsive to a level of the determined energy. In other words, if enough energy is determined over the channel during the uplink preamble period, the station may avoid sending energy corrective data, thereby saving own energy. On the opposite, if there is not enough energy to ensure legacy stations see the medium as busy, the station may decide to send the energy corrective data.

In another approach which may be combined with the previous one, an energy level used by the station to send the energy corrective data depends on the determined energy, i.e. is responsive to a level of the determined energy. For instance, the lowest the determined energy, the highest the energy used by the station to send energy corrective data. This optimizes energy consumption at the station in order to reach a target energy level (to ensure legacy stations see the medium as busy).

In yet another approach which may also be combined, an energy level used by the station to send the energy corrective data depends on an energy level specified in the trigger frame. For instance, the station may determine a sum of elementary target energy levels defined for resource units reserved for energy correction in the trigger frame. An example of such elementary target energy level is the RSSI indication provided at RU level in the trigger frame. This sum corresponds to the energy the AP wishes to receive.

Next, the station determines transmission attenuation as a difference between a signal strength of the received trigger frame and a level on energy used by the AP to transmit this trigger frame (this level is indicated in the trigger frame itself).

Then, the station sets its transmission power or energy level based on both the expected reception energy level for the AP and the determined transmission attenuation, for instance as the sum of both values.

In yet another approach which may also be combined, an energy level used by the station to send the energy corrective data is a prefixed energy level, i.e. not requiring calculation based on what is indicated in the trigger frame and/or on an energy level of the trigger frame.

In some embodiments, the method further comprises comparing the determined energy with a target energy level defined in a trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity. This approach contributes to optimized energy management. Indeed, the AP usually defines the RUs in the trigger frame with associated energy levels which together ensure an overall energy level avoiding the legacy stations to sense the medium as idle. Thus sending the energy corrective data so as to comply with this overall energy level avoids risks of interference while not sending with too much energy.

For instance, the target energy level defined in a trigger frame may be a sum of elementary target energy levels defined for respective resource units in the trigger frame. An example of such elementary target energy level is the RSSI indication provided at RU level in the trigger frame.

In some embodiments, the method further comprises filtering a signal received through the first antenna to attenuate the signal (preferably only) on its frequency sub-band corresponding to the energy correction resource unit. The frequency sub-band is to be considered relatively to the other frequency sub-bands (RUs) of the communication channel.

This filtering approach maintains the dynamicity of the received signal in a range compatible with the tolerance of the components in charge of the decoding of the received signal (typically the Analog to Digital Converters called ADCs has a limited dynamic range).

In some embodiments, the method further comprises sending the same preamble data as other stations in superimposition to these other stations over the whole communication channel during the uplink preamble period. Thus the transmissions of the stations are made simultaneously.

The invention also relates to a non-transitory computer-readable medium storing a program which, when executed by a microprocessor or computer system in a device, causes the device to perform any method as defined above.

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

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, microcode, 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 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 3 illustrates an example of 802.11 ax uplink OFDMA transmission scheme, wherein the AP issues a Trigger Frame for reserving a transmission opportunity of OFDMA subchannels (resource units) on an 80 MHz channel as known in the art;

Figure 4 depicts various fields composing a trigger frame;

Figures 5a and 5b illustrate, using flow charts, general steps at the AP to manage a Multi user uplink session and general steps at a non-AP station to participate to a Multi user uplink session, respectively;

Figure 6 shows a schematic representation a communication device 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 8a illustrates, using a flowchart, general steps for energy correction at the access point, according to embodiments of the invention;

Figure 8b illustrates, using a flowchart, general steps for energy correction at the access point, according to other embodiments of the invention;

Figure 8c illustrates, using a flowchart, general steps for energy correction at the access point, according to yet other embodiments of the invention;

Figure 9a illustrates, using a flowchart, general steps at the access point when energy correction is performed at a non-AP station, according to embodiments of the invention;

Figure 9b illustrates, using a flowchart, general steps for energy correction at the non-AP station, according to embodiments of the invention;

Figure 9c illustrates, using a flowchart, general steps for energy correction at the non-AP station, according to other embodiments of the invention;

Figure 9d illustrates, using a flowchart, general steps for energy correction at the non-AP station, according to yet other embodiments of the invention; and

Figure 10 illustrates an example of 802.11 ax uplink OFDMA transmission scheme, implementing the invention to provide energy correction in 20MHz channels.

DETAILED DESCRIPTION

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

Figure 1 illustrates a communication system in which several communication nodes (or stations) 101-107 exchange data frames over a radio transmission channel 100 of a wireless local area network (WLAN), under the management of a central station, or access point (AP) 110. The radio transmission channel 100 is defined by an operating frequency band constituted by a single channel or a plurality of channels forming a composite channel.

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

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

Next, a source or transmitting station, including the AP, 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 station continues to wait until the radio medium becomes idle.

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

CCA is a WLAN carrier sense mechanisms defined in the IEEE 802.11-2007 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 station to detect and decode an

802.11 frame preamble sent by another station on the network. 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, a CCA flag is held busy until the end of data transmission.

Energy Detect (CCA-ED) is the ability of the station to detect non-802.11 energy in a specific 20MHz channel and back off data transmission. In practice, a level of energy over the 20MHz channel is sensed and compared to an ED threshold discriminating between a channel state with or without 802.11 energy channel. The ED threshold is for instance defined to be 20dB above the minimum sensitivity of a PHY layer of the node. If the in-band signal energy crosses this threshold, CCA is held busy until the medium energy becomes below the threshold anew.

To access the medium, the station starts a countdown backoff counter (as long as the medium is sensed as idle) to expire after a number of timeslots, chosen randomly in a contention window range [0, CW], CW (integer) being also referred to as the Contention Window size and defining the upper boundary of the backoff selection interval (contention window range). 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 station 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 station to listen while sending, thus preventing the source station from detecting data corruption due to channel fading or interference or collision phenomena. A source station 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 station if the frames are received with success, to notify the source station 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).

To meet the ever-increasing demand for faster wireless networks to support bandwidth-intensive applications, 802.11ac and later versions (802.11 ax for instance) implement larger bandwidth transmission through multi-channel operations. Figure 2 illustrates an 802.11 ac channel allocation that supports composite channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz.

IEEE 802.11ac introduced support of a restricted number of predefined subsets of 20MHz communication channels to form the sole predefined composite channel configurations that are available for reservation by any 802.11ac (or later) station on the wireless network to transmit data.

The predefined subsets are shown in the Figure and correspond to 20 MHz, 40 MHz, 80 MHz, and 160 MHz channel bandwidths, compared to only 20 MHz and 40 MHz supported by 802.11η. Indeed, the 20MHz component channels 200-1 to 200-8 are concatenated to form wider communication composite channels.

In the 802.11ac standard, the channels of each predefined 40MHz, 80MHz or 160MHz subset are contiguous within the operating frequency band, i.e. no hole (missing channel) in the composite channel as ordered in the operating frequency band is allowed.

The 160 MHz channel bandwidth is composed of two 80 MHz channels that may or may not be frequency contiguous. The 80 MHz and 40 MHz channels are respectively composed of two frequency adjacent or contiguous 40 MHz and 20 MHz channels, respectively. However the present invention may have embodiments with either composition of the channel bandwidth, i.e. including only contiguous channels or formed of non-contiguous channels within the operating band.

A station (including the AP) is granted a transmission opportunity (TxOP) through the enhanced distributed channel access (EDCA) mechanism on the “primary channel” (200-3). Indeed, for each composite channel having a bandwidth, 802.11ac designates one channel as “primary” meaning that it is used for contending for access to the composite channel. The primary 20MHz channel is common to all stations (STAs) belonging to the same BSS, i.e. managed by or registered with the same local Access Point (AP).

However, to make sure that no other legacy station (i.e. not belonging to the same set and thus possibly having an own primary channel corresponding to a secondary channel of the BSS) uses the secondary channels, it is provided that control frames (e.g. RTS frame/CTS frame or trigger frame described below) reserving the composite channel are duplicated over each 20MHz channel of such composite channel.

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

Developments in the 802.11ax standard seek to enhance efficiency and usage of the wireless channel for dense environments.

In this perspective, one may consider multi-user (MU) transmission features, allowing multiple simultaneous transmissions to or from different stations in both downlink (DL) and uplink (UL) directions, once a transmission opportunity has been reserved and granted to the AP. In the uplink, multi-user transmissions can be used to mitigate the collision probability by allowing multiple non-AP stations to simultaneously transmit to the AP.

To actually perform such multi-user transmission, it has been proposed to split a granted 20MHz channel (200-1 to 200-4) into at least one sub-channel, but preferably into a plurality of sub-channels 310 (elementary sub-channels), also referred to as sub-carriers or resource units (RUs) or “traffic channels”, that are shared in the frequency domain by multiple users, based for instance on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

Each RU thus corresponds to a distinct frequency sub-band within the 20MHz band of the channel. This is illustrated with reference to Figure 3.

The multi-user feature of OFDMA allows the AP to assign different RUs to different stations in order to increase competition within a reserved transmission opportunity TXOP. This may help to reduce contention and collisions inside 802.11 networks.

In this example, each 20 MHz channel (200-1, 200-2, 200-3 or 200-4) is subdivided in the frequency domain into four OFDMA sub-channels or RUs 310 of size 5MHz. Of course the number of RUs splitting a 20 MHz channel may be different from four, and the RUs may have different sizes. For instance, between two to nine RUs may be provided (thus each having a size between 10 MHz and about 2 MHz). It is also possible to have a RU width greater than 20 MHz, when included inside a wider composite channel (e.g. 80 MHz).

Regarding the MU uplink transmissions, the AP must control when and how (in which RU) the stations must emit data.

A trigger mechanism has been adopted for the AP to trigger MU uplink communications from various non-AP stations. This is for the AP to have such control on the stations.

To support a MU uplink transmission (during a TXOP pre-empted by the AP), the

802.11 ax AP has to provide signalling information for both legacy stations (i.e. non-802.11ax stations) to set their NAV and for 802.11 ax stations to determine the Resource Units allocation.

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

As shown in the example of Figure 3, the AP sends a trigger frame (TF) 330 to the targeted 802.11ax stations to reserve a transmission opportunity TXOP 340. The bandwidth or width of the targeted composite channel for the transmission opportunity is signalled in the TF frame, meaning that the 20, 40, 80 or 160 MHz value is signalled.

The TF frame is a control frame, according to the 802.11 legacy non-HT format, and is sent over the primary 20MHz channel and duplicated (replicated) on each other 20MHz channels forming the targeted composite channel. Due to the duplication of the control frames, it is expected that every nearby legacy station (non-HT or 802.11ac stations) receiving the TF on its primary channel, then sets its NAV to the value specified in the header of the TF frame. This prevents these legacy stations from accessing the channels of the targeted composite channel during TXOP 340.

Based on an AP’s decision, the trigger frame TF may define a plurality of resource units (RUs) 310. The multi-user feature of OFDMA allows the AP to assign different RUs to different stations in order to increase competition. This may help to reduce contention and collisions inside 802.11 networks.

The information about the RU distribution in the requested transmission opportunity and about the assignment of stations to the RUs is indicated in the payload of the MAC frame. Indeed, the MAC payload is basically empty for classical control frames (such as RTS or CTS frame), but is enhanced with an information structure for Trigger Frames: an RU allocation field defines the allocated RUs (i.e. RU distribution in the TXOP) while one or more User Info fields indicates the information related to each respective RU (in the same order as provided by the RU allocation info field). In particular, the Address field in each User Info field provides the AID of the station to which the corresponding RU is assigned.

These various fields are shown in Figure 4 described below.

The trigger frame 330 may define “Scheduled” RUs, that may be reserved by the AP for certain stations in which case no contention for accessing such RUs is needed for these stations. Such scheduled RUs and their corresponding scheduled stations are indicated in the trigger frame (the Address field of a so-called User Info field for the scheduled RU is set to the AID of the station). This explicitly indicates the station that is allowed to use a given Scheduled RU. Such transmission mode is concurrent to the conventional EDCA mechanism.

If a station finds that there is no User Info field for Scheduled RUs in the Trigger frame 330 carrying its AID in its Address field, then the station should not be allowed to transmit in a Scheduled RU of the TXOP triggered by the TF.

The trigger frame TF 330 may also designate “Random” RUs, in addition to or in replacement of the “Scheduled” RUs. The Random RUs can be randomly accessed by stations. In other words, Random RUs designated or allocated by the AP in the TF may serve as basis for contention between stations willing to access the communication medium for sending data (uplink OFDMA communication). A collision occurs when two or more stations attempt to transmit at the same time over the same RU.

Such random RUs are signalled in the TF by using specific reserved AID in the Address field of the User Info field corresponding to the RU. For instance, an AID equal to 0 is used to identify random RUs available for contention by stations associated with the AP emitting the trigger frame.

Note that several random RUs with AID=0 may be provided within the same TF.

A random allocation procedure may be considered for 802.11 ax standard based on an additional backoff counter (OFDMA backoff counter, or OBO counter or RU counter) for random RU contention by the 802.11 ax non-AP stations, i.e. to allow them for performing contention between them to access and send data over a Random RU. The RU backoff counter is distinct from the classical EDCA backoff counters (as defined in 802.11e version). However data transmitted in an accessed OFDMA RUs 310 is assumed to be served from same EDCA traffic queues.

The RU random allocation procedure comprises, for a station of a plurality of

802.11 ax stations having an positive RU backoff value (initially drawn inside an RU contention window range), a first step of determining, from a received trigger frame, the RUs of the communication medium available for contention (the so-called “random RUs” identified by AID=0), a second step of verifying if the value of the RU backoff value local to the considered station is not greater than the number of detected-as-available random RUs, and then, in case of successful verification, a third step of randomly selecting a RU among the detected-asavailable RUs to then send data. In case the second step is not verified, a fourth step (instead of the third) is performed in order to decrement the RU backoff counter by the number of detected-as-available random RUs.

As one can note, a station having no Scheduled RU is not guaranteed to perform OFDMA transmission over a random RU for each TF received. This is because at least the RU backoff counter is decremented upon each reception of a Trigger Frame by the number of proposed Random RUs, thereby deferring data transmission to a subsequent trigger frame (depending of the current value of the RU backoff number and of the number of random RUs offered by each of further received TFs).

When, responsive to the reception of a trigger frame 330, a station determines it can access and use one RU 310 (a Scheduled RU or after contention for a Random RU), the station initiates the uplink data transmission 360 by sending, during an uplink preamble period 350, preamble data 351 over the whole 20Mhz channel that includes the RU 310 on which the station will transmit data. It means the preamble data 351 is a 20MHz wide signal. Then the station starts sending its data over its respective RU during the uplink data transmission period 360.

802.11 ax provides that all the stations about to use an RU in the 20MHz channel transmit simultaneously the very same 20MHz-wide preamble data 351. In other words, the preamble data 351 transmitted by the stations superimpose one each other in a correct way so as to allow the AP to efficiently receive it.

Thus, the energy received by the access point on the 20Mz during the uplink preamble period 350 (i.e. the superimposed or added energies from the various stations) mirrors the energy that will be received during the uplink data transmission period 360. The energy received by the AP on each 20MHz channel (200-1 to 200-4) are potentially different depending on the RUs left empty due to the unpredictable non transmission of some stations.

Back to Figure 4 showing the various fields forming the Trigger Frame 330 to declare the RUs. Reference 400 indicates the Trigger Frame 330. It includes, inter alia, a Duration field (indicating the estimated duration of TXOP 340), a RA field (receiver address usually a broadcast address for trigger frames), a TA (transmitter address - typically the MAC address of the AP), a single Common Info field 410 and a plurality of User Specific fields 420 each dedicated to a specific RU defined in the Common Info field 410.

The single Common Info field 410 includes various fields used by all the stations receiving the trigger frame 330. For instance it indicates the width (20, 40, 80 or 160MHz) of the composite channel asked for reservation, and also specifies (in an AP tx power or Txpwr_AP subfield) the combined average power per 20 MHz band referenced to the antenna connector.

The User Specific fields 420 define information related to respective RUs of the trigger frame 330. Each User Specific field 420 corresponding to a specific RU includes:

an AID12 field 421 which indicates the AID of the station to which the corresponding RU is assigned. The value of the AID12 field 421 is set to 0 for the random RUs, the access of which being made using contention;

an RU allocation field 422 which indicates the RU concerned, in particular the position and the width of the RU within the composite communication channel; and a target RSSI field 423 which indicates a target received signal power desired by the AP for the uplink data transmission (i.e. PPDU) over the specific RU.

Based on the resource distribution provided in the Common Info field and each corresponding User Specific field, a station can determine which resource unit it can directly use (scheduled RU) or access through contention (random RU).

Figure 5a illustrates, using a flow chart, general steps at the AP to manage a Multi user uplink session, while Figure 5b illustrates, using a flow chart, general steps at a non-AP station to participate to a Multi user uplink session.

The process at the AP starts at step 501 where the AP creates a trigger frame 330 based on previously received information from the stations (for instance buffer load obtained through Buffer Report Status packets). The trigger frame 330 signals one or more RUs assigned to dedicated stations and potentially one or more random RUs (not assigned to a specific station), using the above mentioned signalling fields. As defined in 802.11 ax, some RUs may not be defines as Scheduled or Random RUs, and are left unused. This can be done using a specific AID value = 2046.

Next, at step 502, the AP sends the trigger frame 330 so created on the wireless medium, after conventional EDCA contention.

It is up to the station to act. The process as any non-AP station starts at step 520 where it receives the trigger frame (if no trigger frame is received or it is received with error, no process is performed).

At next step 521, the station considered (i.e. each station performs the same process) determines, based on the trigger frame content, whether or not it can access one RU defined by the trigger frame 330. It may be one RU assigned to it or one random RU (if of course the station has some data queued for uplink transmission) in case the RU random allocation procedure used allocates this RU to it.

If the station will not use an RU in the TXOP 340 reserved by the received trigger frame 330, the station enters an inactive mode as least during TXOP 340 (due to the NAV value) and then waits for a new trigger frame.

Otherwise, if the station can use one of the RUs, the station determines, at step 522, the RU to be used (thanks to the scheduling or to the RU random allocation procedure).

Next, at step 523, the station determines an energy level to be used for the UL data transmission 360.

To do so, the station uses the target RSSI field 423 of the User Info field 440 corresponding to the determined RU.

To determine the transmission power (Txpwr_STA) to apply, the station uses the following formula: Txpwr_STA = PLDL + TargetRSSI, where

PLDL is the Path Loss Down Link. PLDL = Txpwr_AP - DLRSSI with Txpwr AP being the power used by the AP to transmit the trigger frame (as specified in the Common Info field 410 of the trigger frame 330) and DLRSSI is the signal power of the trigger frame 330 as received by the station. Thus, PLDL is the loss of power between the AP and the station; and

TargetRSSI is the target received signal power desired by the AP as specified in target RSSI field 423 of the determined RU to be used.

If Txpwr_STA is upper than the station capacity, then the station transmits at its maximum power.

Once the transmission power is known, the station creates, at step 524, the Aggregated MPDU (A-MPDU) to be sent using the data queued for transmission.

Next, at step 525, the station emits the preamble data 351 over the whole 20MHz band containing the selected RU (within the composite communication channel) using the transmission power Txpwr_STA determined. It is reminded that the preamble data 351 are the same for all the stations emitting data on a RU 310 of the same 20MHz channel.

Simultaneously, the AP detects, at step 503, the initiation of the uplink data transmission 360. In particular, the AP starts receiving the preamble data 351.

At the end of the uplink preamble period 350 (detected through test 504), the AP calibrates its reception chain (including its reception antenna) at step 505 based on the preamble data received. This calibration configures the AP to be ready to receive and decode the uplink data sent during the uplink data transmission period 360.

Immediately after the uplink preamble period 350, the station, sends, at step 526, the body of the PPDU on the determined RU, still using the same transmission power Txpwr_STA normalized on 20MHz as determined above. As defined in 802.11 ax, if the length of the PPDU is lower than the length required to occupy the channel during the required uplink data transmission period 360, optional step 527 may be executed (to reduce risks of interference) where the station continues sending over the RU, but this time padding data until the end of period 360.

Simultaneously, right from the end of the uplink preamble period 350, the AP actually receives and decodes the uplink data (from the various stations). This is step 506.

Once the uplink data transmission period 360 (detected through test 507), the AP builds and transmits, to the stations, its Acknowledgement packet corresponding to the correctly received packet. This is step 508.

Back to Figure 3, it results from the various possible accesses to the RUs that some of them are not used (31 Ou) because no station with an RU backoff value less than the number of available random RUs has randomly selected one of these random RUs, whereas some other RUs have collided (as example 310c) because at least two of these stations have randomly selected the same random RU. This shows that due to the random determination of random RUs to access, collision may occur over some RUs, while other RUs may remain free.

Also some Scheduled RUs may not have been used because, for instance, either the scheduled station has not correctly received the trigger frame due to collision for instance, or is not ready to transmit (station in sleep mode for instance, or off).

Once the stations have used the RUs to transmit data to the AP, the AP responds with an acknowledgment frame (not show in the Figure) to acknowledge the data received. As for the other control frames, the acknowledgment frame is duplicated over each 20MHz channel if necessary. Preferably, the acknowledgment frame performs a block acknowledgment, meaning that it acknowledges simultaneously reception of data transmitted over a plurality (e.g. all) of the RUs.

Besides the waste of bandwidth, the non-use of the RUs may have dramatic impacts, up to interferences with legacy stations and thereby loss of the whole TXOP.

This is not usually the case with legacy stations (802.11a/n/ac) that have their primary channel operating on one of the 20MHz channels (300) on which the TF is duplicated. This is because these stations can defer their activity using Clear Channel Assessment (CCA). To be precise, the stations use a full CCA on the primary channel, including preamble packet detection (called Signal Detection SD), and perform both physical carrier sensing and virtual carrier sensing. In other words, the stations decode the detected PLCP (Physical Layer Convergence Protocol) preamble from the TF received on their primary channel and use that information to set their NAV (Network Allocation Vector) counter.

The story is different for legacy stations (802.11 n/ac) that do not have their primary channel within the composite channel used by the AP, but has secondary channels) in the composite channel. In that case, the stations use a reduced CCA (called energy detection (ED) as the signal is not decodable) on the secondary channel and thus do not set their NAV counter.

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

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

However, a legacy station may not be able to detect a significant signal onto its primary channel if the measurement of total received RF (radiofrequency) power or energy within the defined 20MHz channel bandwidth suffers from free RUs during TXOP 340. The problem mostly comes from the fact that 802.11 legacy stations assess availability of the medium based on 20MHz portions, while UL OFDMA assignments could be narrower and varying across the BSS coverage.

The aforementioned issue of under-usage of Resource Units should be handled with care, as the resulting signal energy on a specific 20MHz channel could drop under the energy detection (ED) threshold used by the legacy stations (for example, the energy detection threshold is -62dBm for a 20 MHz channel width).

Indeed, collisions may occur on under-used 20MHz channel (i.e. channel in which some RUs are unused) as soon as the legacy stations do not detect enough signal energy. In other words, an OFDMA TXOP 340 having unused RUs has high risks of collisions (conducting to a new kind of collision), which is opposite to the intended usage of the Random RUs.

Non-use of Scheduled RUs may lead to the same issue of having legacy stations colliding the OFDMA traffic on some RUs.

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

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

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

The present invention deals with these various issues by providing new methods for energy correction or compensation in the wireless network. This is to increase the energy level in the 20MHz channel, systematically or upon needs.

Embodiments are based on the ingenuous provision of one or more energy correction (EC) resource units dedicated to energy correction only in the TXOP. Thus any station, be it the AP or a non-AP station, may:

identify an energy correction resource unit dedicated to padding only from the plurality of resource units forming the 20MHz communication channel during a multi-user uplink transmission opportunity granted to the access point station; and send energy corrective data over the identified energy correction resource unit right after an uplink preamble period, the energy corrective data being padding data.

The provision of such EC RU thus lets no room (or substantially decreases risks) for interference during the uplink data transmission period 360. Of course and as described below, the power of energy used to send padding data may be tune to appropriately ensure all legacy nodes will detect activity on the corresponding 20MHz channel.

Other embodiments provide an ingenuous approach to detect whether or not error correction is required. In these embodiments, a station, be it the AP or a non-AP station, may:

determine the energy emitted (from the station perspective of course, i.e. energy received) by stations during an uplink preamble period 350 preceding, within a multi-user uplink transmission opportunity TXOP 340 granted to the access point, an uplink data transmission 360 used by the emitting stations to transmit data over respective resource units 330 forming part of a communication channel 200-1 to 200-4; and responsive to a level of the determined energy, send energy corrective data over a free resource unit of the communication channel. This free RU is thus an energy correction (EC) resource unit.

Thanks to the superimposition, during the preamble period, of energies from the various emitting stations, the listening station (AP or not) has a good estimate of the signal energy/power that will exist during the forthcoming uplink data transmission period 360. Thus efficient measures can be taken to ensure sufficient power/energy on the 20MHz channel as soon as the uplink data transmission period 360 starts.

Figure 6 schematically illustrates a communication device 600, either a non-AP station 101-107 or the access point 110, 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 802.11 ax 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, 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 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 the communication device 600, either the AP 110 or one of stations 101-107, adapted to carry out, at least partially, the invention. As illustrated, device 700 comprises a physical (PHY) layer block 703, a MAC layer block 702, and an application layer block 701.

The PHY layer block 703 (here an 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20MHz channel or the composite channel, and thus sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance medium access trigger frames TF 330 (Figure 3) to reserve a TXOP, MAC data and management frames based on a 20MHz width to interact with legacy 802.11 stations, as well as of MAC data frames of OFDMA type having smaller width than 20MHz legacy (typically 2 or 5 MHz) to/from that radio medium.

The PHY layer block 703 includes CCA capability to sense the idle or busy state of 20Mhz channels and to report the result to the MAC 702 according to 802.11 standard. Upon detecting a signal with significant received signal strength, an indication of channel use is generated.

In the case of an AP performing the error correction mechanism according to the invention (i.e. sending padding data over an EC RU during the uplink data transmission period 360), the PHY layer block 703 is connected to at least two antennas. This is for the AP to be able to receiving, using a first antenna, uplink data from the stations over resource units during the multi-user uplink transmission opportunity, and to send the energy corrective data over the EC RU using a second and distinct antenna.

The MAC layer block or controller 702 preferably comprises a MAC 802.11 layer 704 implementing conventional 802.11 ax MAC operations, and 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 705, referred to as OFDMA Energy correction module, implements the part of the invention concerning device 600, e.g. signalling the EC RUs in the trigger frames at the AP, sending energy corrective data in an announced EC RU, measuring an overall energy level during the uplink preamble in order to adapt the energy corrective data sending. Other details of operations are provided below.

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 to ISO standardization.

The present invention is now illustrated using various exemplary embodiments in the context of IEEE 802.11 ax.

Figures 8a to 8c illustrates different embodiments where the energy correction is performed by the AP itself. The non-AP stations thus operate as in the prior art (Figure 5b).

Figure 8a illustrates a general and simple embodiment where the operations performed by the AP are limited.

The process at the AP starts at step 800 by determining whether an energy correction is required. This step may appear interesting to avoid wasting an energy correction RU if such correction is not required. Thus, a variant may consist in systematically providing energy correction through a dedicated EC RU. This may be the case if the usage of the energy correction is a system parameter set by an administrator for instance.

To perform step 800, the AP can take into account history of past trigger frame results, in particular, the energy received in response to those past trigger frames compared to their respective target RSSIs requested by the AP at that time.

Once the AP has decided performing energy correction (otherwise the process ends), the AP reserves a dedicated EC RU in TXOP 340 at step 801, meaning that the AP builds a trigger frame 330 with the indication of a reserved EC RU (i.e. left unused). Typically, the AID12 field 421 in the User Info field of this RU can be set to a specific value (for instance AID12=2046).

Such EC RU will be used for the energy correction/compensation mechanism and will receive the additional energy sent here by the AP. The position of the EC RUs can be dynamically determined by the AP, for instance based on history (to determine close to which RUs - random RUs or Scheduled RUs assigned to specific stations that often do not use their RUs). In embodiments, the energy correction resource unit is positioned at the centre/middle of the frequency band of the 20MHz communication channel.

The trigger frame is then sent.

Next, after the uplink preamble period 350, the AP emits an energy corrective signal (typically made of padding data) over the EC RU. An energy level used by the AP to send the energy corrective data may be a prefixed energy level, e.g. corresponding to 15dBm.

One easily understands that the 15dBm energy corrective data raises the energy of the signal sensed by the legacy stations. It results that risks to have such legacy stations to sense the medium as idle is reduced, thereby risks of interference are reduced.

Figure 8b shows a slightly more complicated embodiment where the AP further determines or measures energy emitted by other stations during the uplink preamble period, in order to adapt the sending of energy corrective data depending on the determined energy.

As for the process of Figure 8a, steps 800 and 801 determine the opportunity to provide energy correction and accordingly the provision of an EC RU.

Once the trigger frame has been sent, the AP receives the preamble data 351 from the various stations that will transmit over one RU within the same 20MHz channel. The AP may thus determine the energy received during this uplink preamble period 350, which energy is representative of the overall energy expected from the stations during the uplink data transmission period 360. This is step 852.

This measured energy may be used in a lot of different ways as described below with reference to Figure 8c, in order for the AP to adapt its action for energy correction.

For instance, deciding to send energy corrective data or not may depend on whether the determined energy is below a threshold or not. Indeed, if the AP considers there is enough energy to minimize risks that legacy stations interfere, the AP may decide not sending EC signal. Otherwise, the AP may emit an energy corrective signal during step 854.

Of course, the AP may decide to emit an energy corrective signal during step 854 regardless of the measured energy.

In both cases, a prefixed energy level may be used as described above with reference to Figure 8a.

However, as the energy expected from the other stations is known, more specific schemes, including finer tuning of the energy to be used by the AP, may be contemplated. This is to avoid wasting energy while not saturating station reception chains.

For instance, an energy level used by the AP to send the energy corrective data depends on the measured energy.

The difference between the measured energy and a target energy level may be used to set the energy level for the energy corrective signal. The target energy level may be defined in the trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity, for instance as the sum of elementary target energy levels (such as RSSI indications 423) defined for respective resource units in the trigger frame.

Variants may contemplate setting the energy level for the energy corrective signal is such a way the energy during the uplink data transmission period 360 is substantially the energy of the trigger frame (as indicated in field Txpwr_AP of the Common Info field 410). In that case, the target energy level is Txpwr_AP.

More detailed embodiments are those of Figure 8c, where the same steps as in Figure 5a have the same references.

As for the process of Figure 8a or 8b, steps 800 and 801 determine the opportunity to provide energy correction and accordingly the provision of an EC RU in a trigger frame.

The trigger frame 330 so built is sent at step 502.

The preamble data are received through tests 503-504.

Then, the AP performs parallel processing:

on one hand, steps 805, 506 to 508 performed by a receiver chain (including a first antenna), and on the other hand, steps 851-854 performed by an transmission chain (including a second and different antenna).

Steps 851-854 regard the energy correction mechanism according to the invention. These steps include at any moment (not shown) before the EC data transmission 854, configuring the transmission chain of the AP to be ready to transmit the EC data (thus some corrective energy) on the EC RU.

At step 851, the AP determines if the energy level measured during the uplink preamble period 350 is conform to the expectation of the AP. This determination can be done by evaluating the resulting signal strength corresponding to the sum of the target RSSI signal levels 423 of the different RUs defined in the trigger frame 330. If the difference A_sig between the expected signal strength (target energy level) and the effective signal strength of the preamble data (as measured) is significantly high (e.g. above a threshold THR_sig (predefined by the system administrator, for instance THR_sig = 6 dBm), it means that not enough energy will be conveyed during the uplink data transmission period 360. Thus, additional corrective energy is required. Otherwise, no energy is required, ending the process for the second antenna.

If step 851 determines that additional corrective energy is required, step 852 is executed where the AP determines the level of energy to apply on the EC RU.

This level of energy can be for instance the energy level of the trigger frame or be determined based on a desired distance over which the station wishes the signal energy to be detectable (formulas, such as Friis formula, linking signal attenuation - and thus a signal transmission power required - to a transmission distance).

In a variant, a predefined constant value (typically 15dBm) can be used for the energy level to be used.

Next, at step 854, the AP emits the energy corrective signal (typically padding data) over the EC RU with the energy determined at step 852, during the whole uplink data transmission period 360.

One may note that in the embodiment of Figure 8a, steps 851 and 852 are not implemented. Thus there are optional in the context of the present invention.

Steps 805, 506 to 508 performed by the receiving chain regards the reception of uplink data during the uplink data transmission period 360.

At step 805, the AP calibrates its receiver chain to prepare the reception of uplink data. This may be similarly to step 505 of Figure 5a.

However, in embodiments, the receiver chain may be configured to filter the signal received from the transmitting stations during period 360 to attenuate the signal (preferably only) on its frequency sub-band corresponding to the energy correction resource unit used. This may be implemented using internal reception filters to mask the frequency occupied by the EC RU used. This is thus to mask, in reception, the error corrective signal the AP is about to emit.

This filtering/masking maintains the dynamicity of the received signal in a range compatible with the tolerance of the components in charge of the decoding of the received signal (typically the Analog to Digital Converters called ADCs has a limited dynamic range).

An efficient way to easily mask the error corrective signal emitted by the AP itself, is to position the EC RU at a fixed position in each 20MHz channel, and to use predefined signal filters having a profile with a strong attenuation on the frequency corresponding to the EC RU position. A typical predefined position for an EC RU can be a 26 tones RU centred in the centre of the RU.

Once calibrated, the receiver chain is used to receive the uplink data (step 506 as described above) up to the end of the uplink data transmission period 360 (test 507). An acknowledgment of the received data is then sent by the AP (step 508).

Figures 9a to 9d illustrates other embodiments where the energy correction is performed by a non-AP station. The AP has thus to provide one or more EC RUs in the trigger frames and possibly to designate which station or stations will perform the energy correction. This is Figure 9a. A non-AP station performing the energy correction (EC station below) may proceed according to various schemes, substantially based on the same approaches as Figures 8a and 8b above from AP perspective. This is Figures 9b and 9c. More detailed embodiments for an EC station are shown in Figure 9d.

The signalling of an EC RU by the AP may simply performed through steps 800801-502 to 508 (step 805 being step 505 in that case) of Figure 8c.

Figure 9a is another example in which after having determined the need for energy correction (step 800), the AP elects one or more stations to perform energy correction. This is step 899. Each selected station is offered a respective EC RU when building the trigger frame at step 801. In embodiments, the AP specifies in the RSSI field 423 of the User Info field 420 associated with the EC RU, which energy level the EC station will have to use to emit the energy corrective signal.

Optionally an identification of the selected stations may be specified in the trigger frame.

In a variant, the selecting step 899 can be performed by sending specific control frame like OMI frames to the selected stations (in this case, upon reception of the OMI control frame, the selected station enables a local flag to indicate EC correction).

In another variant, those stations (possible a single one) involved in a MU RTS/CTS mechanism with the AP and that provide CTS frames to the AP may be designated as responsible for the energy correction over the EC RUs included in the 20MHz channels on which they were in charge of the CTS emission.

In other variants, the selections of the stations may also take into account station geographical position (typically on the edge of the cell), available station power headroom, quantity of data in station’s emission queues (e.g. stations having low or no data to transmit are good candidates to operate as an energy correcting station).

Figure 9b shows a simple energy correction scheme for an EC station. Any station can perform this process.

It starts at step 830 where the station determines whether or not it is an EC station. In other words, whether or not it has been selected by the AP for energy correction. Of course, this determining step depends on the election mechanism used by the AP and the possible signalling used.

If the AP identifies, in the trigger frame, the selected stations, then the station only analyses the received trigger frame.

If the AP used an OMI control frame, then the station checks whether its local flag to indicate EC correction is enabled.

If the AP used MU RTS/CTS option, then the station determines if it was responsible of the emission of the CTS frame during the last MU RTS/CTS sequence.

Note that the station may be devoted to energy correction (for example it may be decided with the AP during the station registering process), in which case step 830 is not required (it knows that it will emit systematically EC data).

After step 830, the EC station emits preamble data 351 at step 825 during the uplink preamble period 350 (the same preamble data as the other stations using the RUs in a conventional way). Next, the EC station emits the energy corrective signal (typically padding data) on the EC RU the station is responsible for. An energy level used by the EC station to send the energy corrective data may be a prefixed energy level, e.g. corresponding to 15dBm, or may be the one specified in RSSI field 423 of the corresponding User Info field 420 (using the formula mentioned above for step 523.

Figure 9c shows a slightly more complicated embodiment where the EC station further determines or measures energy emitted by other stations during the uplink preamble period, in order to adapt the sending of energy corrective data depending on the determined energy.

The station may determine whether or not it is an EC station at step 830 (see above description).

Then instead of emitting preamble data 351 during period 350, it listens to the preamble data emitted by other stations to measure the energy level expected during uplink data transmission period 360. This is step 852 already described above from AP perspective.

Next, the EC station sends the energy corrective signal at step 854, already described above from AP perspective.

More detailed embodiments are those of Figure 9d, where the same steps as in Figure 5b have the same references.

The process of the Figure may be done by each station.

The process starts at step 520 where the station receives a trigger frame.

The station may directly enter the energy correction process of steps 830-825-854 (dashed arrow from 820 to 830).

Otherwise, the station determines, based on the trigger frame content, whether or not it can access one RU defined by the trigger frame 330 to send its uplink data. This is step 521 described above. In the affirmative, steps 522-527 described above are performed.

Otherwise, the station is available for energy correction, and enters the corresponding process of steps 830-825-584.

At step 830, the station determines whether or not it is an EC station (see above description). The station may also determine based on the trigger frame content (User Info field of the different RUs) if the trigger frame defines one or more RUs for energy correction (i.e.one or more EC RUs).

If the station is not an EC station or if there is no EC RU in the trigger frame, the algorithm stops.

Otherwise (EC station and EC RU provided), the station emits an energy corrective signal (typically padding data) on the EC RU or RUs identified. This is step 854 already described. Any of the schemes described above to determine an energy level for EC data emission can be implemented (prefixed energy level, level specified in RSSI field, energy level to reach a target energy level for the whole 20MHz channel, and so on).

Figure 10 illustrates exemplary communication lines resulting from an implementation of the invention. These exemplary communication lines are drawn in a similar way to Figure 3. A TXOP 340 over an 80MHz composite channel is reserved is reserved by duplicating trigger frame 330 on the four 20MHz channels.

In this example, each 20MHz channel is split into four RUs. Within the 80MHz composite channel, the sixteen RUs are numbered from #1 to #16.

Resources units #2 for the first 20MHz channel 200-3, #5 and #6 for the second 20MHz channel 200-4, #11 for the third 20MHz channel 200-2 and #14 and #15 for the fourth 20MHz channel 200-1 are indicated as EC RUs in trigger frame 330 (using AID=2046 for instance).

In use, resource units #1, #4, #7, #9, #12 and #16 are accessed by some stations to transmit uplink data to the AP during the uplink data transmission period 360 (black-greyed

RUs). On the other hand, resource units #3, #8, #10 and #13 are not used during the uplink data transmission period 360 (white RUs), for instance due to the RU allocation scheme implemented by the stations or to erroneous trigger frame reception.

Thanks to the invention, as soon as the uplink preamble period 350 ends, the AP and/or EC station(s) in charge of energy correction performs step 854 to emit energy corrective signals over the EC RUs (RUs hashed vertically), in particular padding data during the whole uplink data transmission period 360.

Energy correction may be performed over a single RU at a time, such as over RU #2, or may be aggregated over two (or more) contiguous RUs, such as RU#14 and #15.

In this example, the 20 MHz channels would have one or two RUs used during the uplink data transmission period 360, in case the invention is not implemented. This may amount to a low level of energy over the channel, letting room for the legacy stations to sense the 20MHz channel as idle and thus to transmit over it during period 360.

On the contrary, thanks to the invention, additional energy (corrective energy) is provided through the EC RUs, resulting here in having three RUs used in each 20MHz channel. The legacy stations have thus substantially less chances to sense the 20MHz channel as being idle.

Most of the explanation above is directed to energy correction when a single energy correction resource unit is provided per trigger frame.

However, as proposed in the example of Figure 10, the trigger frame may define a plurality of energy correction resource units, that may be used (for energy correction) by a single station (or AP) or by multiple stations (possibly including the AP) using the mechanisms described above. When a plurality of energy correction resource units is provided, they are preferably homogenously distributed over the communication channel.

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

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

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

Claims (24)

1. A method for energy correction in a wireless network comprising a plurality of stations, including an access point station, the method comprising, at one of the stations:

identifying an energy correction resource unit dedicated to padding only from a plurality of resource units forming a communication channel during a multi-user uplink transmission opportunity granted to the access point station; and sending energy corrective data over the identified energy correction resource unit right after an uplink preamble period, the energy corrective data being padding data.

2. A method for energy correction in a wireless network comprising a plurality of stations, including an access point station, the method comprising, at one of the stations:

determining energy emitted by stations during an uplink preamble period preceding, within a multi-user uplink transmission opportunity granted to the access point station, an uplink data transmission used by the emitting stations to transmit data over respective resource units forming part of a communication channel; and responsive to a level of the determined energy, sending energy corrective data over a free resource unit of the communication channel.

3. The method of Claim 1 or 2, wherein the sending station is the access point station.

4. The method of Claim 1 or 2, wherein the sending station is a non-accesspoint station.

5. The method of Claim 1 or 2, wherein the energy corrective data is sent over an energy correction resource unit and the plurality of resource units is defined in a trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity, wherein the trigger frame identifies the energy correction resource unit from among the plurality of resource units.

6. The method of Claim 5, wherein the trigger frame defines a single energy correction resource unit.

7. The method of Claim 5, wherein the trigger frame defines a plurality of energy correction resource units.

8. The method of Claim 7, wherein the plurality of energy correction resource units is homogenously distributed over the communication channel.

9. The method of Claim 5, wherein one energy correction resource unit is positioned at the centre of the frequency band of the communication channel.

10. The method of Claim 5, wherein the energy correction resource unit or units are defined in the trigger frame using an AID equal to 2046.

11. The method of Claim 3, further comprising receiving, using a first antenna, data from stations over resource units during the multi-user uplink transmission opportunity, wherein the sending of energy corrective data is made using a second and distinct antenna.

12. The method of Claim 11, further comprising filtering a signal received through the first antenna to attenuate the signal on its frequency sub-band corresponding to the energy correction resource unit.

13. The method of Claim 1, further comprising, at the station, determining energy emitted by other stations during the uplink preamble period, wherein the sending of energy corrective data depends on the determined energy.

14. The method of Claim 13, wherein deciding to send energy corrective data or not depends on whether the determined energy is below a threshold or not.

15. The method of Claim 13, wherein an energy level used by the station to send the energy corrective data depends on the determined energy.

16. The method of Claim 13, wherein an energy level used by the station to send the energy corrective data is a prefixed energy level.

17. The method of Claim 13, further comprising comparing the determined energy with a target energy level defined in a trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity.

18. The method of Claim 17, wherein the target energy level defined in a trigger frame is a sum of elementary target energy levels defined for respective resource units in the trigger frame.

19. The method of Claim 18, wherein the elementary target energy level of a resource unit is an RSSI, Received Signal Strength Indication, indication provided in the trigger frame in association with the resource unit.

20. The method of Claim 1 or 2, wherein an energy level used by the station to send the energy corrective data depends on an energy level specified in a trigger frame sent by the access point station to reserve the multi-user uplink transmission opportunity.

21. The method of Claim 20, further comprising, at the station, determining a sum of elementary target energy levels defined for energy correction resource units in the trigger frame;

determining transmission attenuation as a difference between a signal strength of the trigger frame as received by the station and a level on energy used by the access point station to transmit the trigger frame; and obtaining the energy level to be used by the station to send the energy corrective data as the determined sum of elementary target energy levels augmented by the determined transmission attenuation.

22. The method of Claim 1 or 2, further comprising sending the same preamble data as other stations in superimposition to these other stations over the whole communication channel during the uplink preamble period.

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

24. A wireless communication device forming station in a wireless network comprising a plurality of stations, including an access point station, the device forming station comprising at least one microprocessor configured for carrying out the steps of the method of Claim 1 or 2.