GB2581538A - Multi-user RTS/CTS procedure of wireless multi-user transmissions - Google Patents

Abstract

A conditional multi-user (MU) request to send, RTS, and clear to send, CTS, procedure for MU wireless transmissions. A method comprising the steps of defining resources for a MU transmission to one or more stations; and scheduling the MU transmission using the defined resources; wherein an MU-RTS/CTS procedure is used by an access point, AP, prior to the scheduling if one or more conditions are fulfilled, and wherein at least one condition is function of a defined resource or a status of a previously effected MU transmission. The condition may be based on the width of the operating frequency band, and may be fulfilled if the operating frequency band is above a threshold. The condition may be based on the transmission opportunity, TXOP, for example the TXOP being above a threshold. The condition may be a function of the number of data units transmitted or received successfully in a previous communication.

Description

MULTI-USER RTS/CTS PROCEDURE OF WIRELESS MULTI-USER TRANSMISSIONS

FIELD OF THE INVENTION

The present invention generally relates to wireless communications.

BACKGROUND OF THE INVENTION

Wireless communication networks are widely deployed to provide various communication services such as voice, video, packet data, messaging, broadcast, etc. These wireless networks may be multiple-access networks capable of supporting multiple users by sharing the available network resources. Examples of such multiple-access networks include Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, and Single-Carrier FDMA (SC-FDMA) networks.

In order to address the issue of increasing bandwidth and decreasing latency requirements that are demanded for wireless communications systems in high-density environments, multi-user (MU) schemes are being developed to allow a single access point (AP) to schedule MU transmissions, i.e. multiple simultaneous transmissions to (downlink, DL) or from (uplink, UL) non-AP stations (STAs), in the wireless network. For example, such MU schemes have been adopted by the Institute of Electrical and Electronics Engineers (IEEE) in the 802.11ax standard, draft version 4.0 (D4.0) of February 2019.

The 802.11ax standard defines a MU request-to-send (RTS) Trigger/clear-to-send (CTS) frame exchange procedure (referred to as MU-RTS/CTS procedure) that allows an AP to initiate a transmission opportunity (TXOP) and protect the TXOP frame exchanges. The procedure allows to reduce, for example, frame collisions introduced by the hidden node problem.

An AP may transmit a MU-RTS Trigger frame to solicit simultaneous CTS frame responses from one or more non-AP STAs.

However, the MU-RTS/CTS procedure adds overhead due to the sending of the MURTS Trigger and CTS frames. The resources are thus not efficiently used.

SUMMARY OF INVENTION

It is a broad objective of the present invention to improve this situation.

In order to take advantage of the high benefits of the transmission scheduling made by the AP in high density environments, the inventors have contemplated making conditional the use of the MU RTS/CTS procedure. In particular, the conditions are based on a resource defined by the AP for scheduling a MU transmission or a status of previously effected MU transmissions. Certain aspects of the present disclosure provide a method for wireless communication comprising, at an access point, AP: defining resources for a multi-user, MU, transmission to one or more stations; and scheduling the MU transmission using the defined resources; wherein a MU request to send, RTS, and clear to send, CTS, procedure is used by the AP prior the scheduling if one or more conditions are fulfilled, and wherein at least one condition is function of a defined resource or a status of a previously effected MU transmission.

The MU transmission is a downlink, DL, transmission, an uplink, UL, transmission or a combination of both.

In a preferred implementation, a defined resource is an operating frequency band over which the MU transmission is scheduled. Advantageously, a condition is fulfilled if the operating frequency band is above a threshold.

In a preferred implementation, a defined resource is a transmission opportunity duration, TXOP, during which the MU transmission is scheduled. Advantageously, a condition is fulfilled if the transmission opportunity duration TXOP is above a threshold.

In a preferred implementation, the status of a previously effected MU transmission is determined as function of the number of data units transmitted or received successfully by the AP.

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

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

Embodiments of the invention will now be described, by way of example only, and with reference to the following drawings in which: Figure 1 illustrates a typical wireless communication system in which embodiments of the invention may be implemented; Figures 2a illustrates a conventional trigger-based (TB) MU UL OFDMA transmission with acknowledgment according to 802.11ax; Figures 2b illustrates a conventional MU DL OFDMA transmission with acknowledgment according to 802.11ax; Figures 2c illustrates a conventional MU UL/DL OFDMA transmissions with acknowledgment according to 802.11ax; Figure 3 illustrates a channel allocation that supports composite channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz; Figure 4 illustrates the MU-RTS/CTS procedure as defined by the IEEE 802.11ax standard, Figure 5 is a flowchart conceptually illustrating an example of a method of wireless communication according to embodiments of the invention; Figures 6a to 6c illustrate different embodiments for determining one or more conditions for using the MU RTS/CTS procedure; Figures 7a, 7b, 8a and 8b provide detailed embodiments for determining and using conditions based on resources defined by the AP; Figure 9 shows a schematic representation a communication device in accordance with embodiments of the present invention; and Figure 10 shows a schematic representation of a wireless communication device in accordance with embodiments of the present invention.

DETAILLED DESCRIPTION OF EMBODIMENTS

The techniques described herein may be used for various broadband wireless communication systems, including communication systems that are based on an orthogonal multiplexing scheme. Examples of such communication systems include Spatial Division Multiple Access (SDMA) system, Time Division Multiple Access (TDMA) system, Orthogonal Frequency Division Multiple Access (OFDMA) system, and Single-Carrier Frequency Division Multiple Access (SC-FDMA) system. An SDMA system may utilize sufficiently different directions to simultaneously transmit data belonging to multiple user terminals. A TDMA system may allow multiple user terminals to share the same frequency channel by dividing the transmission signal into different time slots or resource units, each time slot being assigned to different user terminal. An OFDMA system utilizes orthogonal frequency division multiplexing (OFDM), which is a modulation technique that partitions the overall system bandwidth into multiple orthogonal sub-carriers or resource units. These sub-carriers may also be called tones, bins, etc. With OFDM, each sub-carrier may be independently modulated with data. An SC-FDMA system may utilize interleaved FDMA (IFDMA) to transmit on sub-carriers that are distributed across the system bandwidth, localized FDMA (LFDMA) to transmit on a block of adjacent sub-carriers, or enhanced FDMA (EFDMA) to transmit on multiple blocks of adjacent sub-carriers.

The teachings herein may be incorporated into (e.g., implemented within or performed by) a variety of apparatuses (e.g., stations). In some aspects, a wireless station implemented in accordance with the teachings herein may comprise an access point (so-called AP) or not (so-called non-AP station).

An AP may comprise, be implemented as, or known as a Node B, Radio Network Controller ("RNC"), evolved Node B (eNB), Base Station Controller ("BSC"), Base Transceiver Station ("BTS"), Base Station CBS"), Transceiver Function ("TF"), Radio Router, Radio Transceiver, Basic Service Set ('BSS"), Extended Service Set ("ESS"), Radio Base Station ("RBS"), or some other terminology.

A non-AP station may comprise, be implemented as, or known as a subscriber station, a subscriber unit, a mobile station (MS), a remote station, a remote terminal, a user terminal (UT), a user agent, a user device, user equipment (UE), a user station, or some other terminology. In some implementations, a non-AP station may comprise a cellular telephone, a cordless telephone, a Session Initiation Protocol ("SIP') phone, a wireless local loop ("WLU) station, a personal digital assistant ("PDA"), a handheld device having wireless connection capability, or some other suitable processing device connected to a wireless modem. Accordingly, one or more aspects taught herein may be incorporated into a phone (e.g., a cellular phone or smart phone), a computer (e.g., a laptop), a tablet, a portable communication device, a portable computing device (e.g., a personal data assistant), an entertainment device (e.g., a music or video device, or a satellite radio), a global positioning system (GPS) device, or any other suitable device that is configured to communicate via a wireless or wired medium. In some aspects, the non-AP station may be a wireless node. Such wireless node may provide, for example, connectivity for or to a network (e.g., a wide area network such as the Internet or a cellular network) via a wired or wireless communication link.

Figure 1 illustrates a wireless communication system in which several communication stations 101-107, 110 exchange data frames over a radio transmission channel 100 of a wireless local area network (VVLAN), under the management of a central station, namely access point (AP) 110 with which the stations have registered. In a variant, direct communications between stations can be implemented without the use of an access point (known as an Ad-hoc mode). 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.

An exemplary wireless network is the 802.11 network according to 802.11ax D4.0 standard (published in February 2019).

Each non-AP station 101-107 registers to the AP 110 during an association procedure. During the well-known association procedure, the AP 110 assigns a specific Association IDentifier (AID) to the requesting non-AP station. An AID is a 16-bit value uniquely identifying the non-AP station. According to IEEE standard, the value of an AID is assigned in the range 1 to 2007 for Directional multi-gigabit non-AP station; the 5 MSBs of the AID are reserved.

All the stations 101-107, 110 compete one against each other using EDCA (Enhanced Distributed Channel Access) contention, to access the wireless medium in order to be granted a transmission opportunity (TXOP) and then transmit data frames.

To increase wireless network efficiency, multi-user (MU) schemes are available to allow a single station, usually the AP 110, to schedule MU transmissions, i.e. multiple simultaneous transmissions to or from other stations, in the wireless network. Such a MU scheme has been adopted in 802.1 lax, as the Multi-User Uplink and Downlink OFDMA (MU UL and DL OFDMA) procedures.

With reference to Figure 2a, to actually perform such MU UL transmission, the 802.11ax standard splits a granted communication channel into resource units 201-204 (RUs) that are shared in the frequency domain by the multiple stations, based on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

To finely control the MU UL transmissions by the non-AP stations 101-107, the AP 110 sends a trigger frame (TF) 210 which defines how the channel is split into RUs and which non-AP station is allowed to transmit over each RU. In this example, trigger frame 210 assigns RU 201 to STA1, RU 202 to STA2, RU 203 to STA3 and RU 204 to STA4. The assignment is made using the AlDs of the non-AP stations. A RU that is reserved for a given non-AP station is referred as "scheduled RU".

Upon reception of trigger frame 210, each non-AP station determines its assigned (scheduled) RU thanks to its own AID and can start transmit MU frames 220 (known as HE TB PPDU) over its assigned RU to the AP after a SIFS period after trigger frame 210.

Note that some resource units may be "Random RUs" which means that they can be randomly accessed by different stations of the network (not illustrated). 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. A collision occurs when two or more stations attempt to transmit at the same time over the same RU. An AID equal to 0 may be used to identify random RUs. Random and Scheduled RUs may be combined in the same TF.

Due to the triggering mechanism, the terms "trigger-based MU UL transmission" are used.

After the parallel transmission of the four HE TB PPDUs by stations STA1 to STA4, AP 110 sends individual block ack (BA) 230 over each RU or a multi-station (M-STA) block ack (BA) 230 over the whole band as described in 802.11ax, D4.0.

Figure 2b illustrates a MU DL transmission in the frequency domain where AP 110 sends data frames (HE PPDUs). As for MU UL transmission, the 802.11ax standard splits a granted communication channel into resource units 201-204 (RUs) that are shared in the frequency domain by the multiple stations, based on Orthogonal Frequency Division Multiple Access (OFDMA) technique.

In the example of the Figure, AP 110 defines four RUs to communicate with the four non-AP stations. Preamble 250 contains a description of the RUs and an ordered list of stations that will be the destination non-AP stations for the DL transmissions over each of the RUs 201204.

Next, AP 110 transmits its data within data frames (HE PPDUs) to the stations: to STA1 over RU1 201, to STA2 over RU2 202, to STA3 over RU3 203 and to STA4 over RU4 204. In addition to the data themselves, AP 110 can include a TRS control subfield in each HE PPDU sent to the non-AP stations. The TRS subfield contains all indications needed by a destination non-AP station (STALSTA4) to acknowledge the received data in the next MU UL transmission (without having to EDCA access the medium), which is for instance triggered after a SIFS (Short Inter Frame Space) after the MU DL transmission 260.

The non-AP stations can decode the PPDU 260 received from the AP, including decoding the content of the TRS subfield of the received PPDU.

The non-AP stations then prepare their block ack (BA) packets 270.

After SIFS (Short Inter Frame Space) after the end of the reception of the PPDU, each non-AP station sends its prepared BA packet 270 over the RU specified in the TRS subfield. An AP may define the TXOP period to cover one MU transmission, such as a MU UL transmission (Figure 2a) or a MU DL transmission (Figure 2b). Of course, a TXOP may cover a succession of MU transmissions, such as MU UL-DL-UL transmissions separated by SIFS periods, as illustrated in Figure 2c.

Figures 2a, 2b and 2c illustrate MU transmissions over an operating band formed by a single 20MHz communication channel. However, in general, the operating frequency band may be a composite channel formed by a plurality of communication channels and may thus have different widths.

Also, four resource units (also referred to as "sub-channels", "sub-carriers" or a set of tones) have been illustrated in Figures 2a and 2b, but the number of RUs splitting a 20 MHz channel may be different from four. For instance, between two to nine RUs may be provided per 20 MHz channel (thus each having a size between 10 MHz and about 2 MHz).

Figure 3 illustrates a channel allocation that supports composite channel bandwidth of 20 MHz, 40 MHz, 80 MHz or 160 MHz (compliant with 802.11ac and 802.11ax standards). IEEE 802.1 lac introduces support of a restricted number of predefined subsets of 20MHz channels to form the sole predefined composite channel configurations that are available for reservation by any 802.11ac station (and hence 802.11ax due to backward compatibility) 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 of the operating band, compared to only 20 MHz and 40 MHz supported by 802.11n.

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.

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.

Figure 4 illustrates the MU-RTS/CTS procedure as defined by the IEEE 802.11ax standard. The illustration is given over a composite channel (and operating band) of 80 MHz, formed by four 20 MHz communication channels; a primary channel 300-3, a secondary channel 300-4, a tertiary channel 300-2 and a quaternary channel 300-1.

The MU-RTS/CTS procedure aims, for an AP, at reserving a TXOP for one or more multi-user transmissions through resource units, by sending a multi-user request to send (MURTS) trigger frame (TF). The MU-RTS frame is duplicated on each requested 20 MHz channel and solicits, on each requested 20 MHz channel, the sending of a response frame or reservation acknowledging frame (CTS response frame) by at least one non-AP STA.

In the example illustrated by Figure 4, the AP duplicates a MU-RTS trigger frame 420 on each 20 MHz channel (sensed as free by the AP) of the intended 80 MHz bandwidth composite channel.

At least one receiving node (STA) replies with a CTS frame 421 on a 20 MHz channel sensed as free by this receiving node. Different nodes may reply with CTS frames on different 20 MHz channels.

If the receiving node(s) has (have) successfully received MU-RTS frames 420 from the entire 80 MHz bandwidth, a total of (at least) four CTS frames 421 is transmitted to cover the requested 80 MHz channel bandwidth, thereby actually reserving the whole 80 MHz for the TXOP 440.

While this example shows a complete reserved band of 80 MHz, there may be that one or up to 3 secondary 20 MHz channels 300-4, 300-2, 300-1 (except the primary 20 MHz channel 300-3 which is mandatorily free) are seen as busy by the AP. In that case, no duplicate of the MU-RTS is sent by the AP over these busy 20 MHz channels. Thus, there may be 20 MHz holes within the requested composite channel.

As mentioned above, thanks to the duplication of the MU-RTS frame, every nearby legacy node (non-HE/non-802.1 lax) receiving it on its primary channel sets its NAV to the value specified in the MU-RTS frame, thereby protecting the channels from these legacy nodes, during the requested TXOP.

Block acks (BA) 432 are sent either by the AP or the non-AP stations depending on whether the MU transmission is UL or DL.

The MU-RTS/CTS procedure shown in Figure 4 may protect two modes of OFDMA transmission (one or the other, or a succession of the two modes).

The first mode is the MU UL transmission. As mentioned above, the 802.11ax standard defines a trigger frame 430 to be sent by the AP on the reserved and granted (through conventional RTS/CTS handshake) channel. Considering Figure 4 to illustrate this mode, the trigger frame 430 defines a plurality of UL resource units 410 forming the one or more communication channels 300-x for UL OFDMA transmission during the reserved transmission opportunity. The trigger frame 430 is now protected through the MU-RTS frame 420 and CTS frames 421.

For example, if the sixteen RUs are numbered from #1 to #16, the following RUs allocation can be considered: resources units #2 for the primary 20 MHz channel 300-3, #5 and #6 for the secondary 20 MHz channel 300-4, #11 for the tertiary 20 MHz channel 300-2 and #14 and #15 for the quaternary 20 MHz channel 300-1 are scheduled RUs reserved for specific non-AP stations for UL transmissions; resource units #1, #4, #7, #9, #12 and #16 are random RUs that can be accessed by stations to transmit uplink data to the AP and for which no scheduled RUs have been reserved (indicated using e.g. AID=0); and resource units #3, #8, #10 and #13 are not used during the uplink data transmission period (white RUs), for instance due to the RU allocation scheme implemented by the stations or to erroneous trigger frame reception.

The second mode is the MU DL transmission. Considering Figure 4 to illustrate this mode (in this case the TF is not transmitted), the AP emits data inside OFDMA DL RUs 410 directly after the duplicated CTS frames 421 are received from the node(s) in response to the MU-RTS frame(s) 420. The use of the MU-RTS frame makes it possible to solicit specific nodes to send the CTS frames, thereby increasing protection of the following OFDMA DL RUs. In this second mode, the RU allocations are defined within the PHY header 451 of the RUs (typically spread over a 20 MHz width). The nodes thus do not require any RU allocation inside the MU-RTS frame, but only expect obtaining channel assignments to nodes for the process of sending CTS frames.

In order to further address the issue of increasing bandwidth and decreasing latency requirements that are demanded for wireless communications systems in high-density environments, aspects of the invention seek to efficiently decide the conditions under which the MU-RTS/CTS procedure is to be performed when the AP has to schedule a MU transmission. To that end, aspects of the invention provide features allowing to determine one or more conditions to initiate a MU-RTS/CTS procedure based on resources defined by the AP, status of previously effected MU transmission(s), or a combination of both.

Figure 5 is a flowchart conceptually illustrating an example of a method of wireless communication according to embodiments of the invention. For clarity, the method is described below with reference to AP 110 of Figure 1.

At 510, the method includes defining by the AP resources necessary to schedule a MU transmission. The MU transmission may be a MU UL transmission, a MU DL transmission or a plurality of MU UL and/or DL transmissions that all need to be protected by a same TXOP reservation.

The resources may include: TXOP duration (cf. for example Figures 2a to 2c, or 4); duration of a MU transmission, i.e. duration of allocated DL or UL RUs in a MU transmission; - operating frequency band (e.g. 20, 40, 80 or 160 MHz); RUs allocation, which may include the number of RUs allocated per communication channel, their position, their width (in MHz or number of tones), their duration and/or their type (scheduled RUs, random RUs, not used RUs), etc. Prior scheduling a MU transmission the AP has to define part or all of these resources, which means that the AP has to choose or set appropriate values for them depending on the traffic to be exchanged between stations of the BSS, i.e. stations managed by or registered with the AP.

At 520, the AP checks one or more conditions for sending a MU-RTS trigger frame.

These one or more conditions are aimed to reflect the tradeoff between the added overhead of the MU-RTS/CTS procedure and the gain in avoiding that the MU transmission(s) be interfered by hidden nodes for example. Different embodiments for determining these conditions are given with reference to Figures 6a to 6c.

If the one or more conditions are fulfilled (test 530 positive, 'Yes'), the AP sends the MU RTS trigger frame at 540 to initiate the MU-RTS/CTS procedure. At 550, CTS response (frames) are received from non-AP stations which allow to determine the requested 20 MHz channels that are reserved and over which the MU transmission can be performed. At 560, the AP schedule a MU transmission, either in UL or DL, or successive MU transmissions over the reserved communication channels. The scheduling is performing using the resources defined by the AP at 510.

If no condition is fulfilled (test 530 negative, 'No'), the AP starts scheduling the one or more MU transmissions at 560 without launching the MU-RTS/CTS procedure.

Figures 6a to 6c illustrate different embodiments for determining one or more conditions used by the checking 520 in Figure 5.

According to embodiments illustrated by Figure 6a, the checking 520 comprises a step 621 of determining at least one condition as a function of a defined resource for the scheduling of the MU transmission(s). A plurality of resources may be considered for determining a plurality of conditions or a single condition. The resources described above may be taken individually or in combination to decide on whether or not to launch a MU-RTS/CTS procedure, i.e. sending a MU RTS trigger frame.

According to embodiments illustrated by Figure 6b, the checking 520 comprises a step 622 of obtaining status of a previously effected MU transmission(s), and a step 623 of determining a condition as a function of the status.

According to one implementation, the status corresponds to whether or not previously transmitted HE PPDUs in DL by the AP have been positively acknowledged by stations, i.e. successfully received. The condition to perform the MU-RTS/CTS procedure may be determined as follows: EK bandwitithDL(k) RUof fic cy_l OPR < RUeff iciency-T HD where: OPB is the operating frequency band; RUband""othoL(k) is the bandwidth occupied by the RU carrying the kth HE PPDU that have been successfully received by a station (the total number of successfully transmitted HE PPDUs being equal to K); RH,/ ftetency_THD is an efficiency threshold below which the condition is considered as fulfilled.

According to a variant, the condition to perform the MU-RTS/CTS procedure may be determined as follows: EK RUbancilvtdthDL(k) ef f iciency_2 < RUe f ficiency_THD

RU

EN RUAllatutedDLO where EN RUAtio,,apf(71) is the sum of bandwidth occupied by all (N) RUs allocated by the AP for DL, i.e. excluding not used RUs.

According to one implementation, the status corresponds to whether or not previously transmitted HE TB PPDUs in UL by stations have been successfully received by the AP. The condition to perform the MU-RTS/CTS procedure may be determined as follows: EK R UbanalviathuL (k) RUef fit cy_3 = PR < RUeff iciency_T HD where:

- PI

--bandwielthUL(k) is the bandwidth occupied by the RU carrying kill HE TB PPDU that have been successfully received by the AP (the total number of successfully received HE TB PPDUs being equal to K); According to a variant, the condition to perform the MU-RTS/CTS procedure may be determined as follows: EK RUbancoviitu atl (k) R f ficiency vç < RUerriciency_THD LIN 1t L'AllucateclUL tn.) where ENRUAllocatedut(n) is the sum of bandwidth occupied by all (N) RUs allocated by the AP, i.e. scheduled and random RUs (thus excluding not used RUs).

According to a variant, the condition to perform the MU-RTS/CTS procedure may be determined as follows: EK bundwidthUL(k) RUef ficrency_5 v Ditu < RU,1 ficiency-THD Seheditled + EU R randOnt where E, RU"hatiea (V) is the sum of bandwidth occupied by scheduled RUs allocated by the AP and EuRlfrandom(u) is the sum of bandwidth occupied by random RUs and 0 < fl < 1.

The parameter 13 reflects the uncertainty about the usage of the random RUs by the stations.

Preferably, fl may take values between 0.3 and 0.5. In one variant, ,6 is chosen substantially equal to 0.38 which represents a typical efficiency of the random access.

According to one implementation, the status includes a first status corresponding to one of the definitions discussed above (successful HE PPDUs or HE TB PPDUs) and a second status corresponding to whether or not the AP has used the MU-RTS/CTS procedure during the last MU transmission(s) or TXOP. Two Rtiefficiency_THD thresholds are defined depending on the second status; a first threshold RUeniciency_THD1 if the AP has used the MU-RTS/CTS procedure during the last MU transmission(s) or TXOP and a second threshold RUeniciTHD2 if the AP has not used the MU-RTS/CTS procedure during the last MU transmission(s) or TXOP, with RUefficienty_THD1> RUefficiency_THD2, as less PPDUs losses are expected when the MURTS/CTS procedure is used.

According to one implementation, the status includes a first status corresponding to one of the definitions discussed above (successful HE PPDUs or HE TB PPDUs) and a second status corresponding to the number of successive previous MU transmission(s) or TX0Ps during which the AP has used the MU-RTS/CTS procedure. A further condition may be added linked to the second status such as for example if the number is below a certain threshold, the previous conditions are considered as fulfilled, i.e. the MU-RTS/CTS procedure is used for the current MU transmission.

According to embodiments illustrated by Figure 6c, the checking 520 comprises a step 622 similar to Figure 6b. At 624, at least one condition is determined considering both a defined resource for scheduling the MU transmission(s) and the status as described above. A plurality of conditions may be determined considering individually the defined resource and the status, or a single condition considering a combination of both.

More detailed embodiments for determining and using conditions based on resources defined by the AP are shown in Figure 7a, 7b, 8a and 8b. All these embodiments may be combined with the embodiments of figures 6a to 6c, and in particular with other conditions that are based on status of one or more previously effected MU transmissions.

Figure 7a is a flowchart conceptually illustrating an example of a method of wireless communication in which the condition is based on the duration of the transmission opportunity (TXOP).

At 710a, the resource corresponding to the TXOP (among possibly other resources) is defined by the AP to prepare the scheduling of a MU transmission (DL and/or UL, single or successive MU transmissions).

At 720a, it is checked whether the TXOP is greater than a threshold TX0P_THD. The greater is the TXOP, the riskier it for the MU transmission to be interfered by a hidden station. Consequently, if TXOP > TX0P_THD (test 730a positive, 'Yes), a MU-RTS trigger frame is sent (740) to initiate a MU-RTS/CTS procedure.

At 750, CTS response (frames) are received from non-AP stations which allow to determine the requested 20 MHz channels that are reserved and over which the MU transmission can be performed. At 760a, the AP schedule a MU transmission, either in UL or DL, or successive MU transmissions over the reserved communication channels. The scheduling is performed during the TXOP period defined by the AP at 710a.

If no condition is fulfilled (test 730a negative, 'No), the AP starts scheduling the one or more MU transmissions at 760a without launching the MU-RTS/CTS procedure.

Figure 7b is a flowchart conceptually illustrating an example of a method of wireless communication in which the condition is based on the duration of the RUs (RUD).

At 710b, the resource corresponding to the duration of the RUs (among possibly other resources) is defined by the AP to prepare the scheduling of a MU transmission (DL and/or UL, single or successive MU transmissions). If successive MU transmissions are to be scheduled, the duration RUD may be the sum of all the durations of the individual MU transmissions.

At 720b, it is checked whether the RUD is greater than a threshold RUD_THD. The greater is the RUD, the riskier it for the MU transmission to be interfered by a hidden station. Consequently, if RUD > RUD_THD (test 730b positive, 'Yes'), a MU-RTS trigger frame is sent (740) to initiate a MU-RTS/CTS procedure.

At 750, CTS response (frames) are received from non-AP stations which allow to determine the requested 20 MHz channels that are reserved and over which the MU transmission can be performed. At 760b, the AP schedule a MU transmission, either in UL or DL, or successive MU transmissions over the reserved communication channels. The scheduling is performed with RUs having the duration defined AP at 710a.

If no condition is fulfilled (test 730b negative, No'), the AP starts scheduling the one or more MU transmissions at 760b without launching the MU-RTS/CTS procedure.

Figure 8a is a flowchart conceptually illustrating an example of a method of wireless communication in which the condition is based on the width of the operating frequency band (OPB).

At 810a, the resource corresponding to the width of the operating frequency band (among possibly other resources) is defined by the AP to prepare the scheduling of a MU transmission (DL and/or UL, single or successive MU transmissions).

At 820a, it is checked whether the OPB is greater than a threshold OPB_THD is performed. The greater is the OPB, the riskier it for the MU transmission to be interfered by a hidden station. In particular, legacy stations may operate in different 20 MHz communication channels. Consequently, if OPB > OPB_THD (test 830a positive, 'Yes), a MU-RTS trigger frame is sent (840) to initiate a MU-RTS/CTS procedure.

At 850, CTS response (frames) are received from non-AP stations which allow to determine the requested 20 MHz channels that are reserved and over which the MU transmission can be performed. At 860a, the AP schedules a MU transmission, either in UL or DL, or successive MU transmissions over the reserved communication channels. The scheduling is performed over the OPB defined by the AP at 810a.

If no condition is fulfilled (test 830a negative, No), the AP starts scheduling the one or more MU transmissions at 860a without launching the MU-RTS/CTS procedure.

Figure 8b is a flowchart conceptually illustrating an example of a method of wireless communication in which the condition is based on the RUs allocation. As discussed above with reference to Figure 5, the RUs allocation may include different parameters, any of them may be considered for determining the condition. In the following illustration, the frequency band (FB) occupied by the allocated RUs is considered as an example as the width of the FB may correlate with the risk of interference. Another example is the number of allocated RUs.

At 810b, the resource corresponding to the width of the frequency band occupied by the allocated RUs (among possibly other resources) is defined by the AP to prepare the scheduling of a MU transmission (DL and/or UL, single or successive MU transmissions). Allocated RUs may represent used RUs in DL, or scheduled and random RUs in UL. In UL, a coefficient fl may also be applied to random RUs as discussed above, for example: = accheduled (V) + /3 I RUrand,m(1) Similar values for coefficient 18 may be considered.

At 820b, checking whether the FB is greater than a threshold FB_THD is performed. The greater is the FB, the riskier it for the MU transmission to be interfered by a hidden station.

Consequently, if FB > FB_THD (test 830b positive, Yes), a MU-RTS trigger frame is sent (840) to initiate a MU-RTS/CTS procedure.

At 850, CTS response (frames) are received from non-AP stations which allow to determine the requested 20 MHz channels that are reserved and over which the MU transmission can be performed. At 860b, the AP schedules a MU transmission, either in UL or DL, or successive MU transmissions over the reserved communication channels. The scheduling is performed over the allocated RUs defined by the AP at 810b.

If no condition is fulfilled (test 830b negative, 'No), the AP starts scheduling the one or more MU transmissions at 860b without launching the MU-RTS/CTS procedure.

Figure 9 schematically illustrates a communication device 1000, representing the access point 110, of the radio network 100, configured to implement at least one embodiment of the present invention. The communication device 1000 may preferably be a device such as a micro-computer, a workstation or a light portable device. The communication device 1000 comprises a communication bus 1013 to which there are preferably connected: a central processing unit 1001, such as a processor, denoted CPU; a memory 1003 for storing an executable code of methods or steps of the methods according to embodiments of the invention as well as the registers adapted to record variables and parameters necessary for implementing the methods; and at least one communication interface 1002 connected to a wireless communication network, for example a communication network according to one of the IEEE 802.11 family of standards, via transmitting and receiving antennas 1004.

Preferably the communication bus provides communication and interoperability between the various elements included in the communication device 1000 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 1000 directly or by means of another element of the communication device 1000.

The executable code may be stored in a memory that may either be read only, a hard disk or on a removable digital medium such as for example a disk. According to an optional variant, the executable code of the programs can be received by means of the communication network, via the interface 1002, in order to be stored in the memory of the communication device 1000 before being executed.

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

Figure 10 is a block diagram schematically illustrating the architecture of the communication device 1000, adapted to carry out embodiments of the invention. As illustrated, device 1000 comprises a physical (PHY) layer block 1023, a MAC layer block 1022, and an application layer block 1021.

The PHY layer block 1023 (here an 802.11 standardized PHY layer) has the task of formatting, modulating on or demodulating from any 20MHz channel or the composite channel, and thus sending or receiving frames over the radio medium used 100, such as 802.11 frames, for instance medium access trigger frames TF 430 (Figure 4) to reserve a transmission slot, 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 MAC layer block or controller 1022 preferably comprises a MAC 802.11 layer 1024 implementing conventional 802.11ax MAC operations, and additional block 1025 for carrying out, at least partially, embodiments of the invention. The MAC layer block 1022 may optionally be implemented in software, which software is loaded into RAM 1012 and executed by CPU 1011.

Preferably, the additional block 1025, referred to as MU-RTS/CTS procedure management module implements the part of the embodiments of the invention for managing the MU-RTS/CTS procedure.

MAC 802.11 layer 1024, management module 1025 interact one with the other in order to process accurately communications over OFDMA RU addressed to multiple stations according to embodiments of the invention.

On top of the Figure, application layer block 1021 runs an application that generates and receives data packets, for example data packets such as a video stream. Application layer block 1021 represents all the stack layers above MAC layer according to ISO standardization. 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 referring 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 fad that different features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be advantageously used.

Claims (10)

1. CLAIMS1. A method for wireless communication comprising, at an access point, AP: defining resources for a multi-user, MU, transmission to one or more stations; and scheduling the MU transmission using the defined resources; wherein a MU request to send, RTS, and clear to send, CTS, procedure is used by the AP prior the scheduling if one or more conditions are fulfilled, and wherein at least one condition is function of a defined resource or a status of a previously effected MU transmission.
2. 2. The method of Claim 1, wherein the MU transmission is a downlink, DL, transmission.
3. 3. The method of Claim 1, wherein the MU transmission is an uplink, UL, transmission.
4. 4. The method of Claim 1, wherein a defined resource is an operating frequency band over which the MU transmission is scheduled.
5. 5. The method of Claim 4, wherein a condition is fulfilled if the operating frequency band is above a threshold.
6. 6. The method of any of Claims 1 to 5, wherein a defined resource is a transmission opportunity duration, TXOP, during which the MU transmission is scheduled.
7. 7. The method of Claim 6, wherein a condition is fulfilled if the transmission opportunity duration TXOP is above a threshold.
8. 8. The method of any of Claims 1 to 7, wherein the status of a previously effected MU transmission is determined as function of the number of data units transmitted or received successfully by the AP.
9. 9. An access point, AP, in a wireless network comprising a microprocessor configured for carrying out the steps of: defining resources for a multi-user, MU, transmission to one or more stations; and scheduling the MU transmission using the defined resources; wherein a MU request to send, RTS, and clear to send, CTS, procedure is used by the AP prior the scheduling if one or more conditions are fulfilled, and wherein at least one condition is function of a defined resource or a status of a previously effected MU transmission.
10. 10. 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 methods of Claim 1 to 8.