Communication devices and methods

Abstract

A first communication device that is configured to communicate with one or more second communication devices comprises circuitry configured to detect one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detect a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identify at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.

Description

COMMUNICATION DEVICES AND METHODSBACKGROUNDFIELD OF THE DISCLOSURE

[0001] The present disclosure relates to first and second communication devices, in particular for collision detection and resolution.DESCRIPTION OF RELATED ART

[0002] Enhanced distributed channel access (EDCA) is a main channel access protocol used in WLAN. This protocol is efficient when the number of stations (STA) contending for channel access is low, but it can create very long channel access delays when the number of STAs grows. On the other hand, trigger based (TB) channel access is more efficient whenthe number of STAs is larger, which, however, can also create long delays when the access point (AP) is not aware of data traffic arrival at the STAs. Despite the use of this protocol and the need for performing a backoff procedure, collisions and delays of channel access can occur when multiple STAs access the same channel.

[0003] The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor(s), to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.SUMMARY

[0004] It is an object to reduce the probability of collisions and / or channel access delays, in particular in WLAN systems. It is a further object to provide a corresponding method as well as a corresponding computer program and a non-transitory computer-readable recording medium that stores therein a computer program product for implementing said method.

[0005] According to an aspect there is provided a first communication device configured to communicate with one or more second communication devices, the first communication device comprising circuitry configured to detect one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detect a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identify at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.

[0006] According to a further aspect there is provided a second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device comprising circuitry configured to generate one or more data units having a preamble; provide a collision detection indication within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and transmit the one or more data units including the collision detection indication

[0007] According to still further aspects corresponding methods, a computer program comprising program means for causing a computer to carry out the steps of the method disclosed herein, when said computer program is carried out on a computer, as well as a non-transi- tory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method disclosed herein to be performed are provided.

[0008] Embodiments are defined in the dependent claims. It shall be understood that the disclosed methods, the disclosed computer program, and the disclosed computer-readable recording medium have similar and / or identical further embodiments as the claimed devices and as defined in the dependent claims and / or disclosed herein.

[0009] One of the aspects of the disclosure is to make use a collision detection indication that is transmitted from a second communication device (which may be a STA in an embodiment) to a first communication device (which may be an AP in an embodiment) . The collision detection indication is transmitted in a preamble of a data unit, for instance, in the form of a collision detection field, such as a physical layer (PHY) field, in a data unit (e.g. a physical layer protocol data unit (PPDll)). If a collision or reception failure occurs, the first communication device can identify the second communication device(s) that caused the collision or reception failure from the collision detection indication and may transmit a collision resolution indication that indicates to the second communication device(s) that acollision or reception failure has occurred. This enables a second communication device to detect a collision and, optionally, to take measures for collision resolution, which may be instructed from the first communication device.

[0010] Herein, the term “collision” shall be understood as an event where two or more transmissions by different second communication devices or between a second communication device and another communication device (e.g., a legacy or interfering device that does not use the collision detection indication) are fully or partially overlapped in time and / or frequency domain. For instance, an event where two or more STAs transmit PPDlls that are fully or partially overlapped in time represents a collision. This can occur, for example, when the devices access the wireless medium simultaneously after performing contention-based channel access or receiving an indication from the AP allowing simultaneous transmissions. A reception failure may not only be caused by the device transmitting but rather external factors like interference, channel conditions, etc. that affect the reception of a data unit.

[0011] A collision detection indication shall be broadly understood as an indication that is utilized within a wireless medium where collisions or packet reception failure may occur. Some exemplary applications of a collision detection indication are one or more of detecting collisions and identify colliding STAs during distributed channel access and preemption scenarios and identifying the transmitter of a data unit with a failed packet reception. The collision detection indication may be a signal inserted as a PHY field and composed of one or more OFDM symbols. These OFDM symbols may be truncated to form other periodic signals.

[0012] Contention-based channel access refers to a distributed channel access mechanism like EDCA where the STA that needs to access the channel (or wireless medium) performs carrier sense (CS) and / or clear channel assessment (CCA) to determine if the channel is busy before transmitting. If the channel is busy, a backoff procedure is invoked and the channel access is performed when the backoff counter reaches zero.

[0013] The foregoing paragraphs have been provided by way of general introduction and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWING

[0014] A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:Fig. 1 shows a diagram of a communication scheme using collision resolution with EDCA legacy behavior.Fig. 2 shows a diagram of a communication scheme illustrating an example of a collision in frame-based preemption in a downlink scenario.Fig. 3 shows a diagram of a first embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via a collision detection indication by means of trigger based channel access.Fig. 4 shows a flowchart illustrating an embodiment of the general process of collision detection and resolution via a collision detection field (CoF) at the AP.Fig. 5 shows a diagram of a second embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via CoF by means of a shared TXOP operation.Fig. 6 shows a diagram of a third embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via CoF by means of deferred channel access.Fig. 7 shows a diagram of a fourth embodiment of a communication scheme according to the present disclosure illustrating a response indication to CRF to confirm CoF detection.Fig. 8 shows a flowchart illustrating an embodiment of the general operation of a colliding STA.Fig. 9 shows a diagram of a fifth embodiment of a communication scheme according to the present disclosure illustrating collision resolution with CoF for framebased preemption.Fig. 10 shows a diagram of a sixth embodiment of a communication scheme according to the present disclosure illustrating AP contention before transmitting a collision resolution field (CRF).Fig. 11 show a diagram illustrating an embodiment of the process to set up CoF operation.Fig. 12A shows an embodiment of a PPDll format according to the standard.Fig. 12B shows a first embodiment of a PPDll format according to the present disclosure.Fig. 12C shows a second embodiment of a PPDll format according to the present disclosure.Fig. 13 shows a diagram illustrating another embodiment of the process to generate and distribute CoF signals.Fig. 14 shows a schematic diagram of an embodiment of the layout of a transmitting STA according to the present disclosure.Fig. 15 shows a diagram illustrating a tone plan and a first example of the generation of tone sequences.Fig. 16 shows a diagram illustrating the same tone plan and a second example of the generation of tone sequences.Fig. 17 shows a diagram illustrating a first part of the generation of CoF with tone sequences shown in Fig. 15.Fig. 18 shows a diagram illustrating a second part of the generation of CoF with tone sequences shown in Fig. 15.Fig. 19 shows a diagram illustrating a first part of the generation of CoF with tone sequences shown in Fig. 16.Fig. 20 shows a diagram illustrating a second part of the generation of CoF with tone sequences shown in Fig. 16.Fig. 21 shows a diagram illustrating the generation of CoF with an OFDM symbol sequence mask for the synchronized case.Fig. 22 shows an example of an extended tone plan.Fig. 23 shows a diagram illustrating the generation of CoF with an OFDM symbol sequence mask for the synchronized case with tone shifts using the extended tone plan.Fig. 24 shows a flow chart of an embodiment of a receiver operation to detect a CoF according to the present disclosure.Fig. 25 shows a diagram illustrating an embodiment of a receiver operation in the time domain.Fig. 26 shows a flow chart of an embodiment of a first communication method according to the present disclosure.Fig. 27 shows a flow chart of an embodiment of a second communication method according to the present disclosure.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0015] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, Fig. 1 shows a diagram of a communication scheme using collision resolution with EDCA legacy behavior. A first communication device (in this embodiment an AP) communicates with two second communication devices (in this embodiment stations STA1 and STA2). For Low latency applications the use of EDCA is advantageous because each STA can start contending to access the channel as soon as they have data to transmit, avoiding potential delays of waiting to be triggered by the AP. To avoid collisions, a STA using EDCA must invoke a backoff procedure when the channel is busy. Thus, before transmitting, a STA must sense the channel to be idle for an arbitration interframe space (AIFS) duration and draw a random backoff counter from an interval [0, nCW] where nCW is an integer number that defines the size of the contention window (CW). The STA decrements its backoff counter for each specific time slot durations. When the backoff counter reaches zero, the STA is allowed to transmit. If the STA transmission fails, for example due to a collision or a temporary loss of coverage, the STA must invoke the backoff procedure again, but this time it must double its CW, meaning to draw a new backoff counter from an interval [0, 2 x nCW - 1], This is done to reduce the collision probability, but it may lead to even longer delays as shown in Fig. 1.

[0016] Collisions can also occur in the special scenario of frame-based preemption where protocol changes have been introduced to provide channel access for low latency applications. Fig. 2 shows a diagram of a communication scheme illustrating an example of a collisionin frame-based preemption in a downlink scenario. The AP starts a transmit opportunity (TXOP), and in between specific frames, an IFS gap (denoted as preemption IFS (pIFS) in Fig. 2) is added which has a longer duration than the commonly used short interframe space (SIFS). In these pIFS gaps, other STAs may send a preemptive PPDll (pPPDll) carrying a preemption indication or data. Typically, there is an indication in previous PPDlls announcing an upcoming pIFS gap to STAs with preemptive traffic. Collisions can occur when more than one STA send a pPPDll in the same pIFS gap as illustrated in Fig.2 for STA2 and STA3.

[0017] Since there are specific pIFS gaps where STAs are allowed to send pPPDlls, collisions can happen more frequently than in regular contention scenarios. This can introduce unwanted delays and reduce the efficiency of the TXOP. The frame-based preemption mechanism already has an inherent decrease in efficiency because PPDlls need to have a limited duration to offer regular preemption gaps and support low latency traffic constraints.

[0018] In many cases, when there is a collision, the frames are lost because they cannot be decoded, and it is not known who collided until later. In the case of EDCA, the STAs involved in the collision need to wait for a predetermined timeout, as shown in Fig. 1 , without obtaining a response before they can realize that their transmission failed. Afterwards, the colliding STAs contend again with a new backoff counter having double CW (in Fig. 1 the AIFS is included in the timeout). The collision can be detected at the AP only after one of the STAs is able to successfully transmit a frame containing a retransmission indication.

[0019] In the case of frame-based preemption shown in Fig. 2, the AP needs to start a buffer status report (BSR) procedure to find out which STAs collided. In addition, the collisions can occur in several pIFS gaps during the TXOP. Thus, using BSR procedures to resolve collisions in frame-based preemption cases introduces more delays and reduces the efficiency of the data exchange in the TXOP.

[0020] Fig. 3 shows a diagram of a first embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via a collision detectionindication by means of trigger based (TB) channel access. According to this embodiment a collision detection field (CoF) 10, 20, representing an embodiment of the collision detection indication, is added as a PHY field in a PPDll 11 , 21 which identifies the STA transmitting the PPDll. This enables a receiver STA (i.e. , in this embodiment the AP) to detect a collision. The CoF 10, 20 may, e.g., be located in the preamble of the PPDll 11 , 21 after a legacy portion and at least one signaling field. In an example, the CoF may be added to the first PPDU used to obtain a TXOP, which avoids further delays due to the exponential backoff procedure. The CoF further enables a receiver STA (the AP) to identify which STAs collided even if no other information (including SIG and data fields) can be decoded from the PPDUs 11 , 21 that collided.

[0021] It shall be noted that the AP is assumed to be the intended receiver of a CoF. If the AP identifies at least one of the STAs (STA1 , STA2) that collided, it can modify the channel access to resolve the collision (at least partly) and reduce the channel access delay. For this purpose, it can transmit a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of the STAs which transmitted a PPDU that caused the collision or had a reception failure. In this embodiment, a collision resolution field (CRF) 30 represents the collision resolution indication. A receiver may generally determine that a failed detection has occurred when the likelihood values for the detection test do not reach a certain threshold within the selected detection window. This does not mean that a CoF is present or not, but it simply means that the detection test did not detect a CoF. When this happens a second lower threshold may be used to trigger the start of a detection window adjustment or employ successive interference cancellation methods.

[0022] To support collision resolution mechanisms via CoF the following considerations may be taken. This can be done as part of an initialization or (re-)configuration phase or within the association process for each STA:• Generation and distribution of CoF to all STAs: The AP should know all CoFs and their assignment to each STA. A CoF can be assigned to a single STA or to a group of STAs. A CoF can also be used to indicate a simplified traffic priority of buffer status. For instance, each STAs may have two CoFs, each indicating if the traffic buffer is below or above certain level.• Configuration of CoF to be used by each STA: Duration of CoF, modulation parameters of CoF, sequence type of CoF.• Definition of TB-PPDU or TXOP duration that the AP can set following a collision detection.

[0023] In an embodiment a mode of operation is provided where STAs including CoF in PPDlls to request for channel access, also configure such PPDlls to have equal duration. This makes the legacy signaling field (L-SIG) have equal content and increases their decoding reliability. In case of collisions, if the AP can extract this duration information from the L- SIG, it can adjust the duration of scheduled TB-PPDUs or TXOPs.

[0024] Fig. 4 shows a flowchart illustrating an embodiment of the general process 100 of collision detection and resolution via CoF at the AP. In the following, the proposed mechanisms to detect and resolve collisions with CoF are explained.

[0025] In an embodiment, there are two main assumptions being made about the format of PPDlls carrying a CoF, that is, the PPDU can contain a data field or not. In the first case, a PPDU with data field carries MPDUs containing medium access control (MAC) information in the header like transmitting address (TA) and network allocation vector (NAV) setting indication. Thus, in cases explained below where MAC information items are mentioned, it is assumed that the PPDU contains a data field. In the second case, the PPDU carrying a CoF only contains physical layer (PHY) information and resulting behaviors do not rely on MAC information.

[0026] First, with reference to Fig. 4, a potential AP behavior shall be explained. When energy is detected (L-STF detection), which can be part of carrier sense (CS) or clear channel assessment (CCA) procedures (step 101), the AP process the PPDU as usual (decode preamble and data field; steps 102 and 105, respectively). In addition, it may run (step 103) a detection process for CoF (based on its configuration) in one or more of the following: for first PPDU of a non-established TXOP, for PPDUs transmitted in a service period where specific STAs are allowed to transmit, and for PPDUs within a TXOP where preemptionoperation is allowed (in particular, to PPDlls carrying a preemptive indication or preemptive data). Further, it transmits (step 104) in a subsequent frame an indication if a CoF was detected or not. This indication can be part of the contention resolution / recovery frame (CRF) (30 in Fig. 3), which will be explained below in more detail. The absence of such a transmission may also be an indication. If a CoF is detected, STA ID or ID group is preferably identified by the CoF as well. This frame may also be included in a response PPDll in case the PPDll with CoF also has a data field carrying an MPDll that was successfully decoded.

[0027] The evaluation if a collision has occurred may be done as follows. A collision or packet decoding failure is detected via CoF in the following cases (AP actions follow below). Generally, the CoF can be detected sequentially after the preamble decoding and before the data field decoding or in parallel based on the result from CS or CCA procedures. If one CoF is detected, one of the following cases may occur.

[0028] In a first case, a PPDll including a data field is correctly decoded (step 105) containing at least one MPDU with transmitter address (TA) identifying a transmitter STA that does not match the STA identified or contained in the group of STAs identified by the CoF, which is checked in step 106. In this case a collision is detected (step 107), but the TXOP of the STA sending the correctly decoded MPDU can be established. The AP can send a response frame (step 104) to TXOP initiating STA and allow TXOP to be established. In the response frame the AP can include channel access information to a STA identified by the CoF. Further mechanisms are explained below.

[0029] In a second case, the PHY preamble decoding fails (step 108) or, if the PPDU contains a data field, all MPDUs in the PPDU failed to be successfully decoded (step 105). This case does not necessarily mean that a collision occurred since decoding failure could also be due to an outage in coverage. However, since the CoF is detected, the transmitting STA can be identified (step 109), and the AP can decide to provide channel access or not (step 104) (collision resolution item for mechanism the AP can be used to give channel access).

[0030] If at least two CoFs are detected in step 103, this is interpreted as a collision and the following mechanism explained below for collision resolution may be applied (steps 109, 104, 110).

[0031] In case no collision occurred, it is assumed that the PPDll can be successfully decoded The AP can know (which is detected in step 111) if a CoF was added by an indication in the PPDll preamble or by a previous negotiation phase where it is fixed which STAs will use CoF. If no CoF is detected in step 111 and if a CoF was added in the PPDll, the AP can request (step 110) a CoF configuration change to the STA sending the PPDU or a group of STAs sharing the same CoF configuration or to all STAs in the BSS (step 1 10). Further, the AP can send a CoF report (step 110) to the STA identified by the TA in the decoded PPDU, in a separate frame containing information regarding detection values of CoF (e.g., likelihood ratios of detection test, probability of detection and / or probability of false alarm). It shall be noted that CoF is designed to be as robust or more as the PHY preamble to ensure that a correctly decoded PPDU with no CoF detection should happen with very low probability. Assuming the PPDU contains a data field, if a CoF is detected and if the CoF identifies the same STA identified by the TA of a MPDU carried in the PPDU, no further action with respect to CoF processing is needed, except (optionally) to send a CoF report to the transmitting STA for collecting statistics of CoF detection performance. The PPDU processing shall continue as usual.

[0032] If, in step 105, at least one MPDU has been successfully decoded, an acknowledgement is transmitted if the MPDU is addressed to this STA and the acknowledgement policy requires it (step 112).

[0033] In the following, an embodiment of collision resolution according to the present disclosure will be explained. If a collision or a packet failure is detected via CoF, the AP sends a contention resolution / recovery frame (CRF) a predetermined inter frame space (IFS), denoted as IFS1 in Fig. 3, after the medium is idle (e.g., after the collision occurred) to resolve the collision, or attempts to recover the channel access failure.

[0034] The CRF can enable an uplink TB channel access as illustrated in Fig. 3. The AP triggers (using the CRF itself 30 and / or a trigger frame TF 31) the STAs that were identified in the CoFs 10, 20 to send data in a TB-PPDU 12, 12a, 22, 22a where the TB-PPDU duration can be determined by one of the following: the time duration of the collision (this duration can be estimated as the difference between the end of the collision taken as the point in time the wireless medium is sensed as idle, and the start of the colliding PPDll detection (from e.g., CS / CCA)), or a predetermined fixed value set by the AP or previously negotiated, or duration information in successfully decoded L-SIG in case the colliding PPDlls have equal length. Further, the AP triggers the STAs to collect BSR and continue with uplink TB channel access.

[0035] As shown in Fig. 3, the CRF 30 may act as a trigger frame that schedules the first two PPDlls 12, 22. Since the AP does not fully know how much traffic STA1 and STA2 need to transmit, the length of the first PPDlls 12, 22 may not be sufficient. Thus, a second trigger 31 may be used to allow STA1 and STA2 to finish or continue their data transmission. It can also be that the CRF 30 acts as a trigger but it first collects buffer status from the STAs, so first PPDUs 12, 22, and then triggers the uplink data, i.e. , second PPDUs 12a, 22a.

[0036] In another embodiment the CRF can start a shared TXOP operation with one or more colliding STAs, or other STAs needing to transmit. Fig. 5 shows a diagram of a second embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via CoF by means of a shared TXOP operation. Further, a TXOP can be shared between downlink and / or peer-to-peer (P2P) traffic. The AP can give priority to one or more specific STAs, for example, based on the access category (AC) or Traffic identifier (TID).

[0037] In another embodiment the CRF can defer channel access of a specific group of STAs, e.g., with low priority traffic. Fig. 6 shows a diagram of a third embodiment of a communication scheme according to the present disclosure illustrating collision resolution in uplink via CoF by means of deferred channel access. The AP can ask STA2 to not duplicate CWs 24, 25 compared to the original CW23 to avoid additional delays resulting from the exponential backoff increase.

[0038] In the example illustrated in Fig. 6, STA2 was deferred in favor of STA1 , which gives priority to STA1 . The next TXOP 32 is started by the AP since it typically has short backoff counter. At this point, STA2 starts contending for channel access with a new backoff counter 24 that is not doubled. This modification allows STA2 to avoid having a long waiting time before transmitting. For example, in this case STA1 had a data arrival during the AP’s TXOP 32. Without this modification of the length of the CW24, it would have been likely for STA1 to win contention before STA2.

[0039] In still another embodiment, the AP can move frame exchange to another link.

[0040] In addition, CRF can also indicate on or more of the following:• NAV resetting for all other STAs in case other STAs could extract NAV information from colliding PPDlls, assuming the colliding PPDlls contained a data field carrying MPDlls;• The STAs that participated in the collision (or failed transmission) extracted from detected CoFs and in addition an indication if these STAs are allowed to draw a new backoff counter without doubling CW, an indication if one or more of these STA can use other EDCA parameters (e.g., reduced AIFS), and / or an indication if one or more of these STAs will be triggered in an upcoming TXOP;• to colliding STAs whose CoF were not detected (AP does not know who they are) that they can draw a new backoff counter without double CW, which can only be allowed if a collision is resolved (at least partly), meaning that the CRF gives channel access to at least one colliding STA;• to deferred and / or unscheduled STAs to use only a short frame to start their TXOPs with a mandatory CoF and, in addition, a more robust CoF configuration.

[0041] The expected response to a CRF frame may be configured as follows. Depending on the collision resolution mechanism triggered by the CRF, STAs can reply with a PPDll containing data, or no response as shown in Fig. 6, or with a short response as shown in Fig. 7 showing a diagram of a fourth embodiment of a communication scheme according to the present disclosure illustrating a response indication to CRF to confirm CoF detection.

[0042] In very seldom cases, it can occur that the CoF detected by the AP was incorrect and a wrong STA is identified as a colliding STA. For example, in the example shown in Fig. 7, STA3 is identified as a colliding STA but only STA1 and STA2 collided. To account for this case, the AP can request STAs that are identified as colliding STAs in the CRF 30 to always respond with an indication of whether they participated in the collision or not. This request may be in the form of an additional reply field 36 included in or added to the CRF 30. This response indication can be in a separate short PPDll as shown in Fig. 7 illustrated by the response frame R 16, 46.

[0043] The colliding STAs not identified in the CRF 30 may be allowed to redraw a backoff counter without doubling the CW or optionally with a short AIFS. In the example of Fig. 7, this mechanism allows STA2 to have a shorter contention (compared to double CW) avoiding further delays (e.g., otherwise STA3 would gain channel access before STA2).

[0044] If there is no response to the CRF, the AP should fall back to standard EDCA rules, e.g., to start a new backoff procedure to schedule colliding STAs. No response may, e.g., occur if the STAs addressed in the CRF 30 suffer a momentarily outage (e.g., shadowing effects or external interference prevented them from decoding the CRF).

[0045] The behavior of a colliding STA shall now be explained with reference to Fig. 8 showing a flowchart illustrating an embodiment of the general operation 200 of a colliding STA. Initially, the STA agrees with the AP on CoF usage and configuration. To start, the STA contends for channel access via EDCA (step 201) or obtains channel access in a preemption pIFS gap (step 202). In step 203, the STA adds CoF to a PPDll in one or more of the following cases: for the first PPDll of a non-established TXOP, for PPDUs transmitted in a service period where specific STAs are allowed to transmit, or for PPDUs within TXOP where preemption operation is allowed (as shown in Fig. 9 showing a diagram of a fifth embodiment of a communication scheme according to the present disclosure illustrating collision resolution with CoF for frame-based preemption), in particular to PPDUs carrying a preemptive indication or preemptive data.

[0046] If the colliding STA receives a response frame as usual, which is checked in step 204, and the STA receives an indication that CoF was successfully detected (step 205), this means that there was no collision and the CoF detection works as intended so that no further action is needed, i.e. , the frame exchange can be continued within the TXOP (step 206). If the colliding STA receives an indication that the CoF failed (or absence of CoF information), this means that a PPDll was correctly decoded at the receiver, but the CoF was not detected. Then the colliding STA can modify the CoF configuration at the request of the AP (step 207).

[0047] If the CRF is received (step 208) and conveys actions addressed to the colliding STA, the colliding STA follows instructions of the AP (step 209), for example: respond to TF, respond to frame that schedules a shared TXOP operation (e.g., Mll-RTS), defer channel access by drawing a new backoff counter and (if allowed) do not double the CW and optionally modify AIFs before contending again (this can be in the same channel after the current TXOP ends or can be in a separate link if indicated in the CRF), or modify EDCA parameters, e.g., no doubling of CW, change AIFS duration.

[0048] If the CRF is received but it does not contain actions or information for the colliding STA (step 210) it means the AP does not know that this STA was also part of the collision. If allowed (indicated in the CRF), the STA may not increase CW, draw a new backoff counter, and optionally change EDCA parameters as indicated in CRF. If indicated in the CRF, a short frame with mandatory CoF may be used in the next channel access attempt and a more robust CoF configuration.

[0049] If no response frame is received within a predetermined timeout, the STA shall fall back to the standard backoff (2 x CW - 1) operation (step 201 or 202). the timeout value is set based on the time duration required for an expected response to be received. This can include AIFS, receiver PHY delays and length of response frame.

[0050] For the frame-based preemption case, a collision resolution example is shown in Fig. 9, where STA2 and STA3 are colliding STAs. In this case, the STAs with preemption traffic (STA2, STA3) should include a CoF 20, 40 in the PPDll 27, 47 carrying a preemptionindication or data. This allows the AP to identify which STAs participated in the collision and start a collision resolution mechanism from the ones defined above (as explained for the collision resolution). In the example shown in Fig. 9, the CRF 30 triggers the colliding STAs to reduce the delay that occurs from additional BSR procedures.

[0051] Generally, if there is a collision on the CRF, all STAs should fall back to legacy behavior, doubling CW and waiting AIFS before counting down the backoff counter.

[0052] In the IEEE 802.11 standards, typically, a TXOP is established by a successful exchange of two PPDlls where the STA initiating the exchange obtains channel access via EDCA. To comply with standard operation one of the following rules may be implemented. In the event of a collision that is detected via CoF detection, the AP may perform a backoff procedure before sending the CRF as illustrated in Fig. 10 showing a diagram of a sixth embodiment of a communication scheme according to the present disclosure illustrating AP contention 33 before transmitting CRF 30. In the event of a collision that is detected at the AP via CoF detection, the AP may start a TXOP without performing a backoff procedure as shown, for example, in Fig. 5. In addition, at least one frame in the TXOP should be scheduled to resolve the collision and allow at least one colliding STA to transmit data (e.g., a TF and / or CRF should schedule a colliding STA). The duration of the TXOP that contains transmissions of colliding STAs can be set up to a predetermined duration.

[0053] As explained above, the AP should determine the ID of STAs based on the CoF detection. To achieve this, the AP may correlate the received signal with all possible CoF signals and select the ones whose likelihood values are above a predetermined threshold. This operation can have high complexity if the number of possible CoFs is too large (e.g., more than hundreds). To reduce complexity, it is possible to group several STAs into one CoF signal, and after detection, the AP can trigger the STAs corresponding to this group in other to identify the transmitting STAs.

[0054] In an embodiment of the operation, each CoF signal may identify a different transmitting STA so that several different CoF signals need to be generated. The cross-correlation between CoF signals determines the performance of the detection at the receiver; lowercross-correlation means higher detection reliability. In addition, colliding transmissions are in general not synchronous so that the cross-correlation between CoF signals should also be low in the presence of time and frequency shifts.

[0055] There are two main design parameters for the generation of CoF signals: the time duration of the CoF signal and the number of distinct CoF signals, denoted as Ncof, that shall be generated.

[0056] The time duration of the CoF signal determines the number of samples that the CoF signals has (based, e.g., on the PPDll bandwidth). The more CoF samples, the higher the detection reliability that can be achieved. Furthermore, the greater the number of CoF samples, the easier is it to separate the Ncof signals, in particular, it is desirable that the number of CoF samples is larger or equal to Ncof.

[0057] Fig. 11 show a diagram illustrating an embodiment of the process 300 to set up CoF operation. This process involves the definition of main parameters (e.g., number of STAs), CoF configuration, signal generation and distribution.

[0058] The general procedure to generate and distribute CoF signal may be as follows. In block 302, the time duration of CoF signals within PPDll is defined. Several time durations can be defined to be mapped to several CoF configurations. Thus, it is beneficial to define the time duration in predetermined steps. For example, for a bandwidth of 20MHz, 4ps is a practical step size which contains 80 samples that would correspond to one OFDM symbol for a FFT size of NFFT = 64 plus a guard interval (Gl) of 16 samples.

[0059] In block 301 , the number of distinct CoF signals, denoted as Ncof, is defined that need to be generated. They can be computed as:Ncof = ( ceil (N_contending_STAs I N_STA_groups ) x N_ind_per_STA ) where ceil() denotes a rounding operation to the nearest largest integer value. N_contend- ing_STAs is the number of STAs contending for channel access. N_ind_per_STA is thenumber of indications per STA in case more information than STA identification is to be provided with the CoF, for example: buffer status indications per each STA (each STA can have, for example, two CoFs, one indicating short packets below certain threshold, and the other indicating larger packets above said threshold) and priority level of data needed to be transmitted which can be defined based on TID, AC or stream classification service (SCS) characteristics. N_STA_groups is the number of groups into which several STAs are pulled together for a common CoF assignment.

[0060] In block 303, CoF signals are generated. In particular, complex-value sequences are generated by one of the following:• Create binary sequences and modulate them with PSK modulation (e.g., BPSK or QPSK) to obtain complex-value sequences with low cross-correlation properties.The binary sequences can be common sequences with low cross-correlation properties, like, Gold sequences, or randomly generated sequences where the PSK modulated complex-value sequences have a cross-correlation below a predetermined threshold.• Create complex-value sequences based on DFT or Hadamard matrices.• Encode the bits corresponding to each STA ID using a robust channel coding operation (e.g., BCC with code rate 1 / 2 or 1 / 3) and modulate the encoded bits with a robust modulation (e.g., BPSK) to create complex-value sequences.

[0061] In block 304, the complex-value sequences are modulated into waveforms to be transmitted. This may be done using time domain waveforms, wherein the complex-value sequences can be phase shifted and transmitted with pulse amplitude modulation (PAM). Alternatively, this may be done using OFDM waveforms, wherein the complex-value sequences can be phase shifted and mapped into tones in an OFDM grid that are modulated into OFDM symbols and transmitted.

[0062] In block 305, CoF signals are assigned to STAs. In case each STA has several distinct CoF signals assigned to it (e.g., each having additional indications), signals are chosen that have the larger cross-correlation in comparison to their cross-correlation with CoF signals assigned to other STAs. In case more than one STA is identified by a distinct CoFsignal, STAs that are less likely to contend for channel access at the same time (e.g., if they have regular activity intervals that do not overlap) are grouped.

[0063] In block 306, information is exchanged between AP and all STAs in the BSS so that each STA knows which CoF signal to use (and which additional indications can be conveyed), and the AP knows all CoFs corresponding to each STA. For the AP, all CoF signals and their STA assignment plus additional indications should be known. The generation of CoF signals can be done in a centralized manner at the AP, which in turns sends indications to each STA with information about how to generate its own CoF signal, or the CoF signals themselves.

[0064] Based on the general procedure to create CoF signals explained above, it may be desirable to have several CoF configurations available to dynamically balance reliability, signaling overhead and complexity. Different CoF configurations are obtained by changing one or more of the following parameters: The duration of the CoF signal and / or length of the CoF signal, modulation parameters (the modulation and coding scheme in case the encoded STA ID is used for generating CoF signals; the FFT size, guard interval and tone map to be used in case of OFDM modulation), and sequence type as one of low-cross correlation pseudo random sequence type (e.g., Gold, Kasami, JPL) and orthogonal sequence DTF or Hadamard.

[0065] In many cases, when there is a collision, the frames are lost because they cannot be decoded, and no information can be obtained about which STAs collided. Fig. 12A shows a basic PPDll format in WLAN composed of a PHY preamble and a data field. The more robust part of a PPDU is the legacy short training field (L-STF) which is used for packet detection and initial synchronization. However, this field does not carry any information. The signaling fields, like L-SIG, RL-SIG and SIG, carry basic information about PPDU duration, format, etc. and are modulated with a modulation and coding scheme (MCS) which is robust against channel variations and fading. However, in the event of a collision the decoding of these fields often fails. Furthermore, the identity of the transmitting STA is often included in the MAC header of a MAC protocol data unit (MPDU) carried in the data field with an MCS that is typically less robust than the one used in the PHY preamble.

[0066] In the event of a collision there is no information that the receiver can extract with high reliability to identify this event or the transmitting STAs, making it impossible for the receiver to take actions that could mitigate further delays. According to the present disclosure a PPDll format is defined with the purpose to indicate a reliable channel access request. This PPDll format is shown in Figs. 12B and 12C. It includes a collision detection field (CoF) which can be detected with high reliability even in the event of a collision. The CoF contains a signal, denoted as CoF signal, that identifies the transmitting STA enabling the receiver to implement collision resolution mechanisms and reduce channel access delays.

[0067] The proposed PPDll format contains the same legacy preamble as standard PPDUs for backward compatibility. Legacy STAs use these fields to understand that the channel is busy. The SIG field can be modified to indicate the new PPDll format and the presence of the CoF field which can simplify the receiver operation in case no collision occurs.

[0068] There are two main variants of the proposed PPDU format: one without data field (as shown in Fig. 12B) and one with data field (as shown in Fig. 12C). The first one shown in Fig. 12B is intended to be a PHY only indication with purpose of making a channel access request. In this case, an optional STF can be added to support hardware functionalities like setting of automatic gain control. The packet extension field (PE) is also optionally added in case the receiver needs additional processing time to detect the CoF signals. The second one shown in Fig. 12C contains a data field with MPDUs and the purpose of the CoF is to add reliability to a PPDU transmission in case of a collision. The CoF location can also be placed after the LTF in case a different FFT size from that of the legacy fields is desired.

[0069] An embodiment of the general framework to create and distribute CoF signals shall first be explained. In its most basic form, each CoF signal identifies a different transmitting STA. However, it is also possible to group several STAs to use a common CoF to reduce the receiver complexity. In addition, each STA or group of STAs can use several CoFs to indicate further information like buffer status or priority level of queued traffic. For example, a STA can have two CoFs to indicate if the buffer length of the queued traffic is above or below a predetermined threshold. Similarly, it could be used to indicate if the queued traffic is above or below a predetermined priority level. The priority level can be definedbased on the access category (AC), Traffic identifier (TID) or stream classification service (SCS) characteristics. Thus, several different CoF signals may be generated. The crosscorrelation between CoF signals determines the performance of the detection at the receiver, lower cross-correlation means higher detection reliability.

[0070] Fig. 13 show a diagram illustrating another embodiment of the process 400 to generate and distribute CoF signals. This process is similar to the process 300 shown in Fig. 11 , but there are some differences that will mainly be explained. The general procedure to generate and distribute CoF signal may be as follows.

[0071] Block 401 corresponds to block 301. Block 402 is similar to block 302. It defines CoF configurations based on different time durations of CoF signals, modulation parameters, tone plan and sequence types. More details on CoF configuration are explained below.

[0072] In block 403, CoF signals are generated, involving three main steps (details will be explained below): determining number of periods or OFDM symbols, generation of tone map and tone sequences, and generation of time or symbol sequences. The generated CoF signals can be identified by two main parameters: A tone set index which indicates which tones are needed to form the CoF and a time / symbol sequence index which indicates which time / symbol sequence is used.

[0073] In block 404, which is similar to block 305, CoF signals are assigned to STAs. The CoF signals can be assigned by allocating a tone set index and a time / symbol sequence index to each STA. This can be one based on a STA identifier (ID) like the association identifier (AID) in a static or dynamic fashion. The AP can individually assign each index to each STA in an association or configuration phase, or it can transmit an AID range with a starting AID value from which each STA can identify each index based on its AID number. Matrices that would be known to all STAs may be created to use as a basis for sequence generation, which can be a low complexity approach to generate and assign CoF signals since the AP would only need to indicate row (or column) indices for STAs to use. The row (or column) index can be also selected based on the STA ID to minimize the signaling overhead. In case each STA has several distinct CoF signal assigned to it (e.g., eachhaving additional indications), signals may be chosen that have the larger cross-correlation in comparison to their cross-correlation with CoF signals assigned to other STAs. In case more than one STA is identified by a distinct CoF signal, STAs that are less likely to contend for channel access at the same time (e.g., if they have regular activity intervals that do not overlap) may be grouped. Block 405 corresponds to block 306.

[0074] In the following, the generation of CoF signals based on OFDM for synchronized and unsynchronized collision detection will be explained. At first, synchronization aspects will be discussed.

[0075] Colliding transmissions in EDCA are in general not synchronous, thus the cross-correlation between CoF signals should be also low in the presence of time and frequency shifts. However, scenarios where STAs are expected to contend for channel access or transmit PPDlls at specific times, like in the frame-based preemption application, synchronous operation can be assumed. The design of CoF signals is different depending on whether the collisions can be assumed to be synchronized or not. For synchronized collisions, it is possible to detect different CoF signals in the frequency domain after OFDM demodulation, whereas for unsynchronized collisions, time detection without OFDM demodulation can be more reliable. In the following the CoF design is separated in two cases aimed at time detection (unsynchronized) and frequency detection (synchronized).

[0076] The CoF design for unsynchronized collision cases may be as follows. This case considers periodic OFDM based signals without guard interval where the period of the signal is smaller than the FFT size. This approach does not require to generate full OFDM symbols but rather full periods. Thus, the duration of the CoF signal can be adjusted by increasing or decreasing the number of periods in the signal. Periodic signal can be easier to generate since a basic signal is repeated several times and can be also easier to detect since the receiver can find repetitions equal to the signal period. In addition, OFDM signals with period smaller than an OFDM symbol duration can have low PAPR with makes them less susceptible to hardware distortions. The proposed CoF design separates the CoF signal generation in frequency and time domain to simplify their generation, distribution, and assignment to STAs.

[0077] The proposed CoF design may be as follows. Initially, the number of CoF samples is determined, denoted as Lcof, based on the CoF duration and bandwidth. For example, for an 8 ps CoF duration and 20 MHz bandwidth Lcof = 160 samples. Subsequently, a suitable period is determined. The period, denoted as Pcof, should divide the CoF number of samples in integer parts (e.g., for Lcof = 160, Pcof e {2,4, 5, 8, 10, 16, 20, 32,40}) and the FFT size, denoted as NFFT, is divided in integer parts (e.g., for Lcof = 160 and NFFT = 64, Pcof e {2, 4, 8, 16, 32}). For this example, the possible Pcof values are {2, 4, 8, 16, 32}. It is also possible to have fixed predetermined periods that divide the NFFT size in integer parts, and then determines Lcof and / or CoF duration by a parameter indicating the number of periods in the CoF.

[0078] Subsequently, a tone plan is generated as follows. Assuming the set of NFFT tones are indexed by ke {0, 1, ... , NFFT - 1}, tones are selected within the set kp = • n for n e{0, 1, ..., Pcof - 1} (e.g., for Pcof = 16 and NFFT = 64 tones can be selected from the set kp e {0, 4, 8, 12, ... , 60}). Tones corresponding to guard, DC, or null subcarriers from the tone map are set to zero. The resulting number of selected tones in the tone map is denoted as N_settones. The N_settones tones can be divided into groups of tones, where each group is assigned to one or more different CoF signals.

[0079] A number of complex-value tone sequences of length N_settones, denoted as N_toneseq, can then be determined to be mapped to the generated tone plan as one of the following:• N_toneseq equal to 1; in this case, all CoFs share the same complex-value tone sequence and they are differentiated by the tone assignment (when tones are divided into groups) and / or by the time mask applied (as explained below).• N_toneseq equal to Ncof; in this case, each CoF would have a different complexvalue tone sequence.• N_toneseq equal to the number of indications per STAs or group of STAs.• N_toneseq equal to the number of STAs within each STA group.

[0080] N_toneseq complex-value tone sequences are then generated to be mapped to the selected tones of the generated tone map by means of one of the following:• Complex-value sequence generation.• PSK modulation (e.g., BPSK or QPSK) of pseudo-random binary sequences with low cross-correlation (e.g., well-known sequences like Gold sequences, Kasami sequences, or JPL can be used).• Orthogonal sequence generation by taking rows (or columns) of Hadamard or DTF matrices.

[0081] A complex-value tone sequence and tone allocation are assigned to each CoF signal and the assigned complex-value tone sequences are modulated into ceil(Lcof / NFFT) OFDM symbols to generate time-domain CoF signals. The time-domain CoF signals are truncated to Lcof samples if needed.

[0082] A number of time-mask sequences, denoted as N_timeseq, is determined to apply to the CoF time-domain signals as one of the following:• N_timeseq equal to Ncof.• N_timeseq equal to the number of STAs; in this case, if there are several CoFs assigned to each STA to convey other indications, they will share the same time mask and would be differentiated by the tone-map and / or complex-value tone sequences.• N_timeseq equal to the number of STA groups; in this case, each group will have the same time mask.

[0083] N_timeseq complex-value time sequences are generated from pseudo-random or orthogonal sequences (as mentioned above for the complex-value sequence generation) of length Lcof / Pcof to create a time mask that is applied to the time domain CoF signal. The time mask is applied by multiplying all samples of each period by an element of the time mask. The purpose of the time mask is to further reduce the cross-correlation between CoF signals. In particular, using DTF matrices as a basis to generate the time masks is advantageous because the resulting sequences are orthogonal.

[0084] The CoF design for synchronized collision cases may be as follows. This case considers OFDM based signals where each CoF signal is composed of an integer number of OFDM symbols, each containing a cyclic prefix as a guard interval. This approach assumes thatthe receiver performs OFDM demodulation and detects the CoF signals modulated into specific tones. There are steps shared between this case and the CoF signals for unsynchronized cases as explained above, thus the following steps focus on the differences.

[0085] The proposed CoF design may be as follows. Initially, the number of OFDM symbols, denoted as N_ofdmsym, is determined based on the CoF duration. N_ofdmsym can also be a design parameter and the CoF duration can be extracted from N_ofdmsym and the OFDM parameters (e.g., FFT size, guard interval, bandwidth, subcarrier spacing).

[0086] Subsequently, a tone plan is determined. Tones are selected in integer steps, denoted as N_step, which are power of two and less than half the NFFT value. That is, where N_step = 2nfor n e {0, 1, log2( / VFFT) - 1}. The number of samples for each OFDM symbol (without Gl) is given by NFFT / N_step. This design gives flexibility for obtaining different CoF durations. For example, for n = 0, all tones are used, and the symbols length has NFFT samples. If n = 1, every second tone is used and the symbol length is half, n = 2 uses every fourth tones with length of NFFT / 4 and so on. Tones corresponding to guard, DC, or null subcarriers from the tone map are set to zero. The resulting number of selected tones in the tone map is denoted as N_settones. The N_settones tones can be divided into groups of tones, where each group is assigned to one or more different CoF signals.

[0087] The following three steps are done as in the unsynchronized case: determine N_toneseq complex-value tone sequences, generate complex-value tone sequences, and assign a complex-value tone sequence and tone allocation to each CoF signal. Subsequently, a number of symbol sequences, denoted as N_symseq, is determined to apply to the OFDM symbols of each CoF signal. The number N_symseq is determined is the same way as the time-mask number “N_timeseq” in the unsynchronized case. N_symseq complexvalue symbol sequences are generated from pseudo-random or orthogonal sequences (as explained above for the complex-value sequence generation) of length N_ofdmsym. These sequences are applied to the tones assigned to a CoF signal by multiplying all assigned tones with an element of the complex-value symbol sequence for each OFDM symbol. The resulting assigned tones are modulated with applied complex-value symbol sequence into N_ofdmsym OFDM symbols in time domain. Each OFDM symbol may betruncated by a portion of size 1 / N_step (i.e. , NFFT / N_step samples) and the Gl may be inserted.

[0088] CoF configurations may be designed as follows. It is desirable for each STA to have several CoF configurations available to balance reliability, time overhead, and complexity. The AP can share several CoF configurations with each STA once, or these configurations can be fixed and specified in a standard. The AP can indicate which configuration should be used by each STA, for example based on its STA ID value. The CoF configurations differ in one or more of the following:• Tone set index: different tone maps can have different tone locations and number of non-zero tones; the tone set index can indicate the set of tones allocated to each CoF.• Tone sequence index: indicates which tone sequence is allocated to each CoF; in the case, the CoF share the same sequence (N_toneseq = 1), it can be indicated as part of the tone set index.• Time / symbol sequence index: it indicates the time / symbol sequences allocated to each CoF; it can be indicated as a row of a matrix that contains generated sequences (as shown in Figs. 15 and 16); the sequences can be modified to fit changes in CoF duration and tone plan by either truncation or replication of fixed size sequences with additional phase shifts.• The duration of the CoF signal which can be selected in terms of number of periods or OFDM symbols. It is beneficial to define the time duration in predetermined steps. For example, 4ps is a practical step size, for a bandwidth of 20MHz it contains 80 samples that would correspond to one OFDM symbol for a FFT size of NFFT = 64 plus a guard interval of 16 samples, or alternatively to 5 periods of 16 samples each.• Modulation parameters: FFT size, Gl length, bandwidth.

[0089] In the previous sections, a general design of CoF signals has been presented. In the following, it will be explained how the signals are generated at each STA once a CoF configuration has been selected, and several examples are given. Fig. 14 shows a schematic diagram of an embodiment of the layout of a transmitting STA 500 according to thepresent disclosure for generating CoF signals. Elements that have been modified or added compared to a WLAN transmitter according to IEEE 802.11 have been marked. As shown in Fig. 14, based on the selected tone plan, the tone sequences are modulated into tones (block 501), and for the synchronized design also multiplied with the symbol sequences (block 502). Then cyclic shift diversity (CSD; block 503), spatial mapping (block 504) and IDFT (block 505) is applied resulting in time-domain signals.

[0090] The following three steps are implemented according to the present disclosure as follows. For the unsynchronized case it is possible to only truncate the last OFDM symbol such that the number of samples in the total CoF field is Lcof (block 506). Then the samples in each period are multiplied with the time-mask sequence (block 507). There is no Gl inserted or, if there is, it must be the same length as a period, and the time truncation should correspond to Lcof - Pcof samples. Further, for the unsynchronized case, each OFDM symbol is truncated by 1 / N_step, no time sequence is applied, and Gl is inserted as in regular operation (block 508). Finally, windowing and analog RF operations are performed (block 509), and the signals are transmitted.

[0091] Figs. 15 and 16 show an example of a tone plan generated based on L-STF, having N_settones = 12 selected tones with minimum tone spacing of 4 tones. The tone sequences can be determined by DTF matrices with multiplications in some columns with -1 to add diversity to the first row having otherwise all ones. Two examples on how to generate the tone sequences are given in Figs. 15 and 16, namely tone sequences A (Fig. 15) and B (Fig. 16). Case A is implemented by simply taking rows of the DFT matrix of size N_settones (as shown in Figs. 17 and 18 depicting CoF generation with time sequence mask for unsynchronized case (tone sequence A). Case B assumes a DFT matrix with half the size and maps them to interlaced selected tones (as shown in Figs. 19 and 20 depicting CoF generation with time sequence mask for unsynchronized case (tone sequence B).

[0092] Assuming OFDM modulation with NFFT = 64, the resulting time signals for tone sequences A and B are illustrated in Figs. 17, 18 and Figs. 19, 20, respectively, both having a period of Pcof=16 samples. The duration of the CoF is set to be 4ps, thus in a 20MHz bandwidth Lcof = 160 samples and each CoF signal contains Lcof / Pcof = 10 periods. TheNcof time masks are then generated by using a DFT matrix similar to the ones illustrated in Figs. 15, 16, but of size Lcof / Pcof and multiplying even columns with -1.

[0093] In these examples, the total number of different CoF signals that can be generated is N_settones x (Lcof / Pcof) = 120 accounting for all possible combinations of DTF rows in tone and time sequences.

[0094] Figs. 21 to 23 illustrate the generation of CoF signals based on OFDM symbols with Gl for the synchronized collision case using the tone sequences shown in Fig. 16. In this case the symbol sequence multiplies the modulated tones in the frequency domain before I DFT operation. The symbol sequence is of length Nsym = 4 which means 4 OFDM symbols are modulated. Assuming NFFT = 64 with Gl = 16 and a 20MHz bandwidth the total number of samples would be 320, which means a CoF duration of 16ps.

[0095] Fig. 21 shows a diagram illustrating CoF generation with an OFDM symbol sequence mask for the synchronized case. Each OFDM symbol contains a periodic signal with period equal to 16 samples due to the selected tone plan. Thus, it is possible to truncate each symbol by an integer number of periods to reduce the CoF duration. For example, truncating to 2 periods (32 samples) plus Gl of 16 sample, the total number of samples would 48x4 = 192 which corresponds to a 9.6 ps CoF duration for a 20MHz bandwidth. Furthermore, by selecting the Gl equal to be a period length, the resulting time signal would be a concatenation of 4 periodic signals with 4 periods each.

[0096] Fig. 22 shows an example of an extended tone plan. Fig. 23 shows a diagram illustrating CoF generation with an OFDM symbol sequence mask for the synchronized case with tone shifts using the extended tone plan. According to Fig. 22, the tone map is extended to cover 24 selected tones with minimum spacing of 2 tones, resulting in periodic signals with period of 32 samples. The same tone sequences (as shown in Fig. 16) are used but introducing a different tone mapping operation where 4 instead 2 allocations are available as shown in Fig. 23. This provides two more degrees of freedom to separate CoF in the frequency domain which simplifies detection. If it is desired to obtain a periodic signal, the Gl may then be selected to 32 samples.

[0097] Fig. 24 shows a flow chart of an embodiment of a receiver operation 600 to detect CoF according to the present disclosure. The receiver performs (step 601) carrier sense (CS) or clear channel assessment (CCA) procedures to detect the presence of an arriving PPDll at the PHY layer. These procedures identify a starting point of the PPDll from which a detection window can be identified based on the duration of the Legacy preamble and SIG (optionally STF) fields (step 606). The detection window identifies an interval during which all CoF signals are expected to appear, and it should have a margin at the start and end to account for possible time shift differences between colliding PPDlls.

[0098] Next, the decoding of legacy and SIG fields is performed (step 602), if the decoding is successful, it can be assumed that either no collision occurred or one of the colliding PPDlls has been received with significant more power than the other. A successful preamble decoding indicates that at least one CoF should be correctly detected (step 603) with high reliability. In this case frequency domain detection may be simpler to implement and the detection window may be reduced to fit the CoF size. If the preamble decoding fails, either a collision or outage can be assumed, thus the detection window should maintain its margins. The decoding of the data field (step 605), if present, is done as usual provided the SIG fields where correctly decoded. Before detecting the CoF (step 603 or 607), a detection set should be defined which includes the CoF signals that the receiver will attempt to detect. This can be the entire number of CoFs defined or a subset that excludes STAs that are known to not be contending at that particular time.

[0099] For the case where collisions can be expected to be synchronized within some tolerance margin (e.g., within the Gl), it is advantageous to detect the CoFs in the frequency domain. Thus, OFDM demodulation is performed and the CoF are detected (step 603 or 607) by correlating the received values in the tones and OFDM symbols with each CoF signal in the detection set. The resulting likelihood values that are above the detection threshold result in an indication of successful CoF detection.

[0100] In the time domain CoF detection (step 603 or 607), the CoF signals in the detection set are correlated with the received samples in the detection window. A sliding window approach can be used to correlate the CoFs with different delays until likelihood values above a threshold are identified.

[0101] Once the CoF detection is performed an indication is sent to the MAC layer (step 604 or 609) indicating if a CoF detection was successful, which CoFs were identified and optionally the likelihood values of the detector to share with other devices. The MAC layer would then map the identified CoFs with the STA IDs of the corresponding STAs and additional indications if present.

[0102] Assuming the receiver has a memory to process received samples in several iterations, the detection window can be adjusted (step 614) after a failed detection and / or successive interference cancellation (SIC) (steps 610, 611 and 612) can be employed to improve the detection of more than one CoF in the event of a collision. These functionalities can be triggered if, e.g., the preamble decoding failed. An iterative receiver procedure can start by performing a CoF detection (step 607), if it fails (e.g., likelihood values are below a set detection threshold), the number of CoF detection trials are compared to a set limit (step 13) decremented and the detection window is adjusted by increasing the number of samples and / or its time location. This process can be done iteratively until a successful CoF detection, or the trials limit is reached, in which case an indication of failed detection is sent to the MAC layer (step 609). In case a CoF is successfully detected, based on the likelihood values, it can be determined if SIC should be performed (step 608). As an example, if the CoF signal with maximum likelihood is well above the detection threshold and there is another CoF signal with a significant likelihood value but not above the detection threshold (e.g., above a second SIC threshold), it can be determined (step 608) that SIC would increase the detection probability of a potential second CoF signal.

[0103] The SIC procedure includes first determining a successfully detected CoF signal with the maximum likelihood (step 610) in the CoF set, modifying the CoF set by removing said CoF signal (step 611) and subtracting (step 612) said CoF signal (and STF if present) from the received samples weighted by a channel estimate. Since the CoF signals are designed to be reliable, there are additional degrees of freedom to detect at least a strongest channel path component which is enough for improving the detection performance with SIC.

[0104] When there is no collision, the CoF detection can be done as part of the regular receiver processing. Basically, the CoF detection can be done in two ways: a) as part of asequential PPDll reception (after the preamble detection before the data decoding) and b) as a parallel CoF reception (based on a detection window that runs in parallel to the PPDll receiver operation).

[0105] In Fig. 24, the receiver operation of a PPDll with CoF is illustrated and follows two modes of operation. The first mode corresponds to the case where the CoF detection (step 603) is done as part of the sequential PPDU reception after the preamble decoding (step 602) and before the data field decoding (step 605). This operation mode makes the CoF detection simple in case the preamble decoding is successful (e.g., no collision) since the receiver can already know that a CoF is present based on an indication included in a SIG field. The second mode of operation consists of a parallel CoF reception to the regular PPDU processing to account for cases where either the preamble decoding fails or a colliding PPDU with low power or shifted in time is present. In this case a CoF detection window is determined (step 607) after CS / CCA procedure (step 601) indicating the receiver samples where the CoF detection is to be performed. The two modes of operation can work in parallel and independent of each other. Moreover, the CoF detection indication from PHY to MAC can be done separated or jointly after both CoF detection processes have ended.

[0106] Fig. 25 shows a diagram illustrating an embodiment of a receiver operation 700 in the time domain. The sequential PPDU reception starts with CS or CCA procedures (step 701) where the packet sent by STA 1 is detected and the start of a PPDU is identified. Then, the receiver synchronizes with L-STF and L-LTF and decodes L-SIG, RL-SIG and SIG (steps 702, 703), followed by CoF detection (step 704) regardless of whether SIG fields were successfully decoded or not. In case the SIG field decoding was successful, fine synchronization (step 705) and automatic gain control setting can be done based on STF followed by channel estimation (step 706) with LTF. Finally, the data field is decoded (step 707).

[0107] If the receiver is able to synchronize with the PPDU sent by STA 1 , the above procedure is able to detect the CoF from STA 1 even if the preamble decoding fails. However, if the synchronization fails because of the collision with STA 2, the CoF detection may also fail.Furthermore, the detection of the CoF sent by STA 2 would not be possible if the corresponding PPDll is shifted in time as shown in Fig. 25.

[0108] Thus, the parallel CoF reception is done to reliably detect collisions and identify colliding STAs regardless of the result from synchronization or preamble decoding. In this case, the CoF detection is triggered by the result from CS / CCA where a start of a PPDll is identified. Then a detection window is determined (step 710) based on the duration of signaling fields plus / minus some margin to account for different signaling field durations or time shifts between colliding PPDlls. The legacy fields (L-STF, L-LTF, L-SIG and RL-SIG) have a fixed standardized duration that is known, and the SIG field can have different durations, but they have standardized limits. Finally, the CoF detection operation (step 711) is done within the selected window by correlating the received samples with the CoF signals in the CoF set following a sliding window operation.

[0109] Fig. 26 shows a flow chart of an embodiment of a first communication method 800 of the first communication device (the AP) according to the present disclosure, which may be performed by circuitry or separate units or a programmed computer of processor of the first communication device. In a first step 801 , one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices are detected. In a second step 802, a collision detection indication is detected within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication. In a third step 803, at least one second communication device is identified from which said one or more data units has been transmitted based on the detected collision detection indication.

[0110] Fig. 27 shows a flow chart of an embodiment of a second communication method 900 of the second communication device (any one of the STAs) according to the present disclosure, which may be performed by circuitry or separate units or a programmed computer of processor of the second communication device. In a first step 901 , one or more data units having a preamble are generated. In a second step 902, a collision detection indication isprovided within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication. In a third step 903, the one or more data units including the collision detection indication are transmitted.

[0111] In summary, according to aspects of the present disclosure, a communication device, such as an AP, may detect PHY indications in received PPDlls and extract information identifying the transmitting device for each transmitted PPDll, determine whether a collision between PPDlls or a packet reception failure has occurred, and, if a collision or packet failure occurred, transmit indication to transmitting STAs and other STAs, indicating that a collision or a packet failure has occurred and / or which were the STAs identified in the detected PHY indication.

[0112] A transmitting / colliding communication device, such as a STA, may include a PHY indication at least in a first PPDll transmitted to establish a TXOP or in a service period where specific STAs are allowed to transmit or in a predetermined IFS gap within an established TXOP where preemptive data transmission is allowed, the PHY indication including information identifying the STA as the transmitter, and receive an indication from the AP and determine if it has participated in a collision or a packet failure occurred when transmitting said PPDll.

[0113] According to further aspects of the present disclosure, a transmitting STA may generate a PPDU having a preamble that includes legacy fields and signaling fields, in addition to a CoF (training field that identifies the transmitting STA). Further, it may derive a CoF configuration from an indication transmitted by the AP (does not need to be immediately before) including a tone set index and a time / symbol sequence index. Still further, it may modulate tones indicated by the tone set index by mapping a tone sequence into OFDM symbols and mapping a time / symbol sequence, indicated by the time / symbol sequence index, by multiplying the tones corresponding to OFDM symbols with a time / symbol sequence before modulating them into OFDM symbols to generate a time signal, ormultiplying the OFDM symbols with the time / symbol sequence to generate a time signal. Furthermore, it may insert the generated time signal into the CoF and transmit the PPDll.

[0114] A receiving STA or AP may perform CS or CCA to detect a PPDll arrival, determine a detection window based on the starting point of the PPDll detection, duration of preamble and CoF (starting at the point of the PPDU detection plus the length of the preamble (legacy and SIG) minus a margin and having a duration equal to the CoF plus a margin), and detect the CoF signals contained in CoF and identify the transmitting STA.

[0115] Generation of the CoF signals can be done offline and may include one or of determining number Ncof of CoF signals to generate based on the number of contending STAs and number of indications per STA, selecting a CoF duration that contains a number of samples within a set bandwidth that is greater or equal than the number of CoF signals determining a number of periods or OFDM symbols that fit within the selected CoF duration, generating time sequences with the same length as periods or OFDM symbols, generating a tone plan with a predetermined number of modifiable tones, generating tone sequences with the same length as modifiable tones, generating CoF signals by mapping tone sequences into the generated tone plan and apply time sequences into OFDM symbols before OFDM modulation or time periods after OFDM modulation, and assigning generated CoF signals to a set of STAs.

[0116] The present disclosure proposes a protocol data unit (PPDU) format containing a collision detection PHY field (CoF) to enable collision detection in the PHY layer of a WLAN receiver. The proposed PPDU format is designed to be used as a fast channel access request that can reliably identify the transmitting station (STA) even in the event of a collision.

[0117] The device may be implemented by respective units or circuitry, e.g. a processor, processing circuitry, a computer, dedicated hardware, etc., that carries out the functions of the device. Alternatively, a common unit or circuitry, e.g. a common processor or computer, may implement the various functions of the device, or separate units or elements may be used that together represent the circuitry.

[0118] Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. As will be understood by those skilled in the art, the present disclosure may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present disclosure is intended to be illustrative, but not limiting of the scope of the disclosure, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.

[0119] 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. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0120] In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Further, such a software may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems.

[0121] The elements of the disclosed devices, apparatus and systems may be implemented by corresponding hardware and / or software elements, for instance appropriate circuits or circuitry. A circuit is a structural assemblage of electronic components including conventional circuit elements, integrated circuits including application specific integrated circuits, standard integrated circuits, application specific standard products, and field programmable gate arrays. Further, a circuit includes central processing units, graphics processing units, and microprocessors which are programmed or configured according to software code. A circuit does not include pure software, although a circuit includes the above-described hardware executing software. A circuit or circuitry may be implemented by a single device or unit or multiple devices or units, or chipset(s), or processor(s).

[0122] It follows a list of further embodiments of the disclosed subject matter:1 . First communication device configured to communicate with one or more second communication devices, the first communication device comprising circuitry configured to detect one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detect a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identify at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.2. First communication device according to embodiment 1 , wherein the circuitry is configured to determine a detection window for the detection of the collision detection indication based on a starting point of the detection of the one or more data units and the duration of the preamble including or being arranged before the collision detection indication.3. First communication device according to any one of embodiments 1 to 2, wherein the circuitry is configured to generate and / or indicate to one or more second communication devices one or more configurations of collision detection indications by one or more of: defining the time duration of a collision detection indication within a data unit; defining the number of distinct collision detection indications; determining a number of periods or OFDM symbols that fit within the duration of a collision detection indication; generating time / symbol sequences with the same length as periods or OFDM symbols; defining a time / symbol sequence index that identifies each time / symbol sequence; defining a bandwidth index that identifies the bandwidth occupied by the collision detection indication;generating a tone plan with a predetermined number of modifiable tones; generating a tone plan with a predetermined number of tones that are divided into contiguous or non-contiguous tone sets wherein each tone set is assigned to a distinct collision detection indication; generating tone sequences with equal or smaller length as modifiable tones and / or same length as number of tones in each tone set; defining a tone set index that identifies the tones within a tone set that correspond to a tone sequence and / or assigned to one or more second communication devices; generating collision detection indications by mapping tone sequences into a generated tone plan; creating complex-valued sequences representing or being included in collision detection indications; truncating one or more OFDM symbols included in collision detection indications; applying time / symbol sequences into OFDM symbols before OFDM modulation or time periods after OFDM modulation; modulating complex-valued sequences into waveforms to be transmitted as collision detection indications; assigning one or more collision detection indications to one or more second communication devices; and informing the second communication devices associated with the first communication device of the assignment of one or more collision detection indications to one or more second communication devices.4. First communication device according to any one of embodiments 1 to 3, wherein the circuitry is configured to detect the collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication; determine, based on one or more received collision detection indications, whether a collision or reception failure has occurred; and transmit, if it is determined that a collision or reception failure has occurred, a collision resolution indication indicating that a collision or reception failure has occurred and / orindicating at least one of the one or more second communication devices which transmitted a data unit that caused the collision or reception failure.5. First communication device according to any one of embodiments 1 to 4, wherein the circuitry is configured to detect a collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication after performing contention-based channel access, during a predetermined service period or during a predetermined time gap within an established transmit opportunity.6. First communication device according to any one of embodiments 1 to 5, wherein the circuitry is configured to obtain and / or indicate and / or exchange with the one or more second communication devices information indicating one or more of: the assignment of collision detection indications to the one or more second communication devices; if a collision detection indication shall be included in a subsequent data unit transmitted by one or more second communication devices; if said subsequent data unit can start a transmit opportunity; if a collision detection indication indicates traffic priority and / or buffer status of the respective second communication device; the configuration of collision detection indications; and the maximum length of a data unit including a collision detection indication.7. First communication device according to any one of embodiments 1 to 6, wherein the circuitry is configured to generate and / or indicate to one or more second communication devices one or more configurations of collision detection indications differing in one or more of: tone set index; tone sequence index; time / symbol sequence index; bandwidth index; duration of the collision detection indication; and one or more modulation parameters.8. First communication device according to any one of embodiments 1 to 7, wherein the circuitry is configured to transmit, if a collision detection indication has been detected, information to a MAC layer circuitry indicating one or more of: that detection has been successful; which collision detection indications have been detected; and / or likelihood values of the detection; and wherein the MAC layer circuitry is configured to map detected one or more collision detection indications with identifiers of second communication devices.9. First communication device according to embodiment 2, wherein the circuitry is configured, after a failed detection of a collision detection indication, to adjust the detection window and / or to employ successive interference cancellation.10. Second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device comprising circuitry configured to generate one or more data units having a preamble; provide a collision detection indication within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and transmit the one or more data units including the collision detection indication.11. Second communication device according to embodiment 10, wherein the circuitry is configured to obtain the collision detection indication configuration including a tone set index and / or a time / symbol sequence index and / or a tone sequence index and / or bandwidth index; generate the collision detection indication to be indicated by the tone set index by mapping a tone sequence into OFDM symbols and by mapping a time / symbol sequence indicated by the time / symbol sequence index to generate a time signal; and include the generated time signal into the collision detection indication.12. Second communication device according to embodiment 11 , wherein the circuitry is configured to generate the collision detection indication by multiplying the tones corresponding to OFDM symbols with a time / symbol sequence before modulating them into OFDM symbols to generate the time signal, or multiplying the OFDM symbols with the time / symbol sequence to generate the time signal.13. Second communication device according to any one of embodiments 10 to 12, wherein the circuitry is configured to transmit the data unit including the collision detection indication and / or receive from the first communication device a response to the transmitted data unit or a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of one or more second communication devices that caused the collision or had a reception failure.14. Second communication device according to any one of embodiments 10 to 13, wherein the circuitry is configured to include the collision detection indication into one or more of a first data unit transmitted after performing contention-based channel access ; a data unit transmitted in a service period in which one or more second communication devices are allowed to transmit; and a data unit carrying a preemptive indication and / or preemptive data.15. Second communication device according to any one of embodiments 10 to 14, wherein the circuitry is configured to include into a signaling field of the preamble an indication of the format of the data unit and / or an indication of a presence of a collision detection indication in the data unit and / to include into or replace a training field located within or after the preamble with a collision detection indication.16. Second communication device according to any one of embodiments 10 to 15, wherein the circuitry is configured to use a design of a collision detection indication depending on synchronized or unsynchronized detection of the collision detection indication.17. Second communication device according to any one of embodiments 10 to 16, wherein the circuitry is configured to generate the collision detection indication by truncating one or more OFDM symbols and multiplying the truncated one or more OFDM symbols with a time sequence.18. Second communication device according to any one of embodiments 10 to 17, wherein the circuitry is configured to generate, based on a received indication from the first communication device, one or more configurations of collision detection indications by one or more of: defining the time duration of a collision detection indication within a data unit; defining the number of distinct collision detection indications; determining a number of periods or OFDM symbols that fit within the duration of a collision detection indication; generating time / symbol sequences with the same length as periods or OFDM symbols; defining a time / symbol sequence index that identifies each time / symbol sequence; defining a bandwidth index that identifies the bandwidth occupied by the collision detection indication; generating a tone plan with a predetermined number of modifiable tones; generating a tone plan with a predetermined number of tones that are divided into contiguous or non-contiguous tone sets wherein each tone set is assigned to a distinct collision detection indication; generating tone sequences with equal or smaller length as modifiable tones and / or same length as number of tones in each tone set; defining a tone set index that identifies the tones within a tone set that correspond to a tone sequence and / or assigned to one or more second communication devices; generating collision detection indications by mapping tone sequences into a generated tone plan; creating complex-valued sequences representing or being included in collision detection indications; truncating one or more OFDM symbols included in collision detection indications; applying time / symbol sequences into OFDM symbols before OFDM modulation or time periods after OFDM modulation;modulating complex-valued sequences into waveforms to be transmitted as collision detection indications; assigning one or more collision detection indications to one or more second communication devices; and informing the second communication devices associated with the first communication device of the assignment of one or more collision detection indications to one or more second communication devices.19. First communication method of a first communication device configured to communicate with one or more second communication devices, the first communication method comprising detecting one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detecting a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identifying at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.20. Second communication method of a second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device method comprising generating one or more data units having a preamble; providing a collision detection indication within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and transmitting the one or more data units including the collision detection indication21. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to embodiment 19 or 20 to be performed.22. A computer program comprising program code means for causing a computer to perform the steps of said method according to embodiment 19 or 20 when said computer pro-gram is carried out on a computer.A1. First communication device configured to communicate with one or more second communication devices, the first communication device comprising circuitry configured to detect a collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication; determine, based on one or more received collision detection indications, whether a collision or reception failure has occurred; and transmit, if it is determined that a collision or reception failure has occurred, a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of the one or more second communication devices which transmitted a data unit that caused the collision or had a reception failure.A2. First communication device according to embodiment A1 , wherein the circuitry is configured to detect a collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication after performing contention-based channel access, during a predetermined service period or during a predetermined time gap within an established transmit opportunity.A3. First communication device according to any one of embodiments A1 to A2, wherein the circuitry is configured to obtain and / or indicate and / or exchange with the one or more second communication devices information indicating one or more of: the assignment of collision detection indications to the one or more second communication devices;if a collision detection indication shall be included in a subsequent data unit transmitted by one or more second communication devices; if said subsequent data unit can start a transmit opportunity; if a collision detection indication indicates traffic priority and / or buffer status of the respective second communication device; the configuration of collision detection indications; and the maximum length of a data unit including a collision detection indication.A4. First communication device according to embodiment A3, wherein the circuitry is configured to obtain and / or indicate and / or exchange with the one or more second communication devices information indicating the configuration of collision detection indications including one or more of duration, modulation parameters, sequence type, tone index allocation, time / symbol sequence index allocation, definition of a trigger based data unit and / or transmit opportunity duration that the first communication device can set following the detection of a collision or reception failure.A5. First communication device according to any one of embodiments A1 to A4, wherein the circuitry is configured to determine, based on the one or more received collision detection indications, that a collision or reception failure has occurred by detecting one or more of: transmit information included in the collision detection indication indicating the second communication device as transmitter of the collision detection indication does not match transmit information included in a data unit received and correctly decoded by the first communication device; at least one collision detection indication has been detected and decoding of a preamble and / or data field of one or more data units failed; at least two collision detection indications have been received from different second communication devices.A6. First communication device according to any one of embodiments A1 to A5, wherein the circuitry is configured, if no collision detection indication has been detected from a second communication device, but the first communication device is aware that acollision detection indication should have been transmitted by the second communication device, to perform one or more of: transmit a request to change the configuration of the collision detection indication to one or more second communication devices; and transmit a collision detection report to one or more second communication devices including information regarding detection values of collision detection indications.A7. First communication device according to any one of embodiments A1 to A6, wherein the circuitry is configured, if a collision or reception failure has occurred, to transmit a collision resolution indication that is configured to inform at least the one or more second communication devices indicated by the received collision detection indications of one or more of the following: enable trigger-based channel access; schedule transmissions by one or more second communication devices; start a shared TXOP operation with the first communication device; defer channel access; move data exchange to another link; modify network allocation vector settings; transmit a response if the second communication device participated in the collision or not; indicate if a second communication device is allowed to draw a new backoff counter and / or if it is allowed to draw a new backoff counter without increase of contention window; indicate if a second communication device can use other channel access parameters; indicate if a second communication device will be triggered in an upcoming transmit opportunity; and indicate if a second communication device may use only a short frame to start a transmit opportunity with a mandatory collision detection indication and / or a more robust configuration of the collision detection indication.A8. First communication device according to any one of embodiments A1 to A7,wherein the circuitry is configured, if a collision or reception failure has occurred, to transmit a collision resolution indication in a broadcast data unit that is configured to inform second communication devices that transmitted a data unit before of one or more of the following: modify network allocation vector settings; defer channel access; if a second communication device is allowed to draw a new backoff counter and / or if it is allowed to draw a new backoff counter without increase of contention window; indicate if a second communication device can use other channel access parameters; and indicate if a second communication device may use only a short frame to start a transmit opportunity with a mandatory collision detection indication and / or a more robust configuration of the collision detection indication.A9. First communication device according to any one of embodiments A1 to A8, wherein the circuitry is configured to perform a backoff procedure before the transmission of the collision resolution indication; or start a transmit opportunity, after the wireless medium is identified as idle after a collision or packet error has been determined, without performing a backoff procedure by transmitting a data unit carrying a collision resolution indication that schedules the transmission of a data unit by a second communication device; or preempt an existing transmit opportunity to schedule the transmission of a data unit by a second communication device after transmission of the collision resolution indication by the first communication device.A10. First communication device according to any one of embodiments A1 to A9, wherein the circuitry is configured to generate and / or indicate to one or more second communication devices one or more configurations of collision detection indications by one or more of: defining the time duration of a collision detection indication within a data unit; defining the number of distinct collision detection indications;creating complex-valued sequences representing or being included in collision detection indications; modulating complex-valued sequences into waveforms to be transmitted as collision detection indications; assigning one or more collision detection indications to one or more second communication devices; and informing the second communication devices associated with the first communication device of the assignment of one or more collision detection indications to one or more second communication devices.A11. Second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device comprising circuitry configured to transmit a data unit including a collision detection indication; and receive from the first communication device a response to the transmitted data unit or a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of one or more second communication devices which transmitted a data unit that caused the collision or had a reception failure.A12. Second communication device according to embodiments A11 , wherein the circuitry is configured to receive from the first communication device a collision resolution indication identifying the second communication device as a participant in a collision determined by the first communication device by detecting a collision detection indication.A13. Second communication device according to any one of embodiments A11 to A12, wherein the circuitry is configured to include the collision detection indication into a preamble of a data unit, in particular into a physical layer preamble of a physical layer protocol data unit.A14. Second communication device according to any one of embodiments A11 to A13, wherein the circuitry is configured to transmit the data unit including the collision detection indication after performing contention-based channel access or during a predeterminedservice period or during a predetermined time gap within an established transmit opportunity and / or to include the collision detection indication into one or more of a first data unit transmitted after performing contention-based channel access; a data unit transmitted in a service period in which one or more second communication devices are allowed to transmit; and a data unit carrying a preemptive indication and / or preemptive data.A15. Second communication device according to any one of embodiments A11 to A14, wherein the circuitry is configured to include, into the collision detection indication, transmit information indicating the second communication device as transmitter of the collision detection indication.A16. Second communication device according to any one of embodiments A11 to A15, wherein the circuitry is configured to perform, in response to the received collision resolution indication, one or more of the following: transmit a response and / or a data unit containing data; perform trigger-based channel access by transmitting a trigger-based data unit; transmit traffic according to a received schedule; start a shared TXOP operation with the first communication device; defer channel access; move data exchange to another link; modify network allocation vector settings; transmit a response if the second communication device participated in the collision or not; draw a new backoff counter; draw a new backoff counter without increase of contention window if the second communication device is not indicated or its channel access is deferred in the collision detection indication; use other channel access parameters; use only a short frame to start a transmit opportunity with a mandatory collision detection indication; and use a more robust configuration of the collision detection indication.A17. Second communication device according to any one of embodiments A11 to A16, wherein the circuitry is configured to modulate one or more complex-valued sequences into waveforms to be transmitted as collision detection indication.A18. First communication method of a first communication device configured to communicate with one or more second communication devices, the first communication method comprising: detecting a collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication; determining, based on one or more received collision detection indications, whether a collision or reception failure has occurred; and transmitting, if it is determined that a collision or reception failure has occurred, a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of the one or more second communication devices which transmitted a data unit that caused the collision or had a reception failure.A19. Second communication method of a second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication method comprising: transmitting a data unit including a collision detection indication; and receiving from the first communication device a response to the transmitted data unit or a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of one or more second communication devices which transmitted a data unit that caused the collision or had a reception failure.A20. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to embodiment A18 or A19 to be performed.A21. A computer program comprising program code means for causing a computer to perform the steps of said method according to embodiment A18 or A19 when said computer program is carried out on a computer.

Claims

CLAIMS1 . First communication device configured to communicate with one or more second communication devices, the first communication device comprising circuitry configured to detect one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detect a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identify at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.

2. First communication device according to claim 1 , wherein the circuitry is configured to determine a detection window for the detection of the collision detection indication based on a starting point of the detection of the one or more data units and the duration of the preamble including or being arranged before the collision detection indication.

3. First communication device according to claim 1 , wherein the circuitry is configured to generate and / or indicate to one or more second communication devices one or more configurations of collision detection indications by one or more of: defining the time duration of a collision detection indication within a data unit; defining the number of distinct collision detection indications; determining a number of periods or OFDM symbols that fit within the duration of a collision detection indication; generating time / symbol sequences with the same length as periods or OFDM symbols; defining a time / symbol sequence index that identifies each time / symbol sequence; defining a bandwidth index that identifies the bandwidth occupied by the collision detection indication;generating a tone plan with a predetermined number of modifiable tones; generating a tone plan with a predetermined number of tones that are divided into contiguous or non-contiguous tone sets wherein each tone set is assigned to a distinct collision detection indication; generating tone sequences with equal or smaller length as modifiable tones and / or same length as number of tones in each tone set; defining a tone set index that identifies the tones within a tone set that correspond to a tone sequence and / or assigned to one or more second communication devices; generating collision detection indications by mapping tone sequences into a generated tone plan; creating complex-valued sequences representing or being included in collision detection indications; truncating one or more OFDM symbols included in collision detection indications; applying time / symbol sequences into OFDM symbols before OFDM modulation or time periods after OFDM modulation; modulating complex-valued sequences into waveforms to be transmitted as collision detection indications; assigning one or more collision detection indications to one or more second communication devices; and informing the second communication devices associated with the first communication device of the assignment of one or more collision detection indications to one or more second communication devices.

4. First communication device according to claim 1 , wherein the circuitry is configured to detect the collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication; determine, based on one or more received collision detection indications, whether a collision or reception failure has occurred; and transmit, if it is determined that a collision or reception failure has occurred, a collision resolution indication indicating that a collision or reception failure has occurred and / orindicating at least one of the one or more second communication devices which transmitted a data unit that caused the collision or reception failure.

5. First communication device according to claim 1 , wherein the circuitry is configured to detect a collision detection indication in a data unit received from one or more of the second communication devices configured to transmit a data unit including a collision detection indication after performing contention-based channel access, during a predetermined service period or during a predetermined time gap within an established transmit opportunity.

6. First communication device according to claim 1 , wherein the circuitry is configured to obtain and / or indicate and / or exchange with the one or more second communication devices information indicating one or more of: the assignment of collision detection indications to the one or more second communication devices; if a collision detection indication shall be included in a subsequent data unit transmitted by one or more second communication devices; if said subsequent data unit can start a transmit opportunity; if a collision detection indication indicates traffic priority and / or buffer status of the respective second communication device; the configuration of collision detection indications; and the maximum length of a data unit including a collision detection indication.

7. First communication device according to claim 1 , wherein the circuitry is configured to generate and / or indicate to one or more second communication devices one or more configurations of collision detection indications differing in one or more of: tone set index; tone sequence index; time / symbol sequence index; bandwidth index; duration of the collision detection indication; and one or more modulation parameters.

8. First communication device according to claim 1 , wherein the circuitry is configured to transmit, if a collision detection indication has been detected, information to a MAC layer circuitry indicating one or more of: that detection has been successful; which collision detection indications have been detected; and / or likelihood values of the detection; and wherein the MAC layer circuitry is configured to map detected one or more collision detection indications with identifiers of second communication devices.

9. First communication device according to claim 2, wherein the circuitry is configured, after a failed detection of a collision detection indication, to adjust the detection window and / or to employ successive interference cancellation.

10. Second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device comprising circuitry configured to generate one or more data units having a preamble; provide a collision detection indication within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and transmit the one or more data units including the collision detection indication.

11. Second communication device according to claim 10, wherein the circuitry is configured to obtain the collision detection indication configuration including a tone set index and / or a time / symbol sequence index and / or a tone sequence index and / or bandwidth index; generate the collision detection indication to be indicated by the tone set index by mapping a tone sequence into OFDM symbols and by mapping a time / symbol sequence indicated by the time / symbol sequence index to generate a time signal; and include the generated time signal into the collision detection indication.

12. Second communication device according to claim 11 , wherein the circuitry is configured to generate the collision detection indication by multiplying the tones corresponding to OFDM symbols with a time / symbol sequence before modulating them into OFDM symbols to generate the time signal, or multiplying the OFDM symbols with the time / symbol sequence to generate the time signal.

13. Second communication device according to claim 10, wherein the circuitry is configured to transmit the data unit including the collision detection indication and / or receive from the first communication device a response to the transmitted data unit or a collision resolution indication indicating that a collision or reception failure has occurred and / or indicating at least one of one or more second communication devices that caused the collision or had a reception failure.

14. Second communication device according to claim 10, wherein the circuitry is configured to include the collision detection indication into one or more of a first data unit transmitted after performing contention-based channel access ; a data unit transmitted in a service period in which one or more second communication devices are allowed to transmit; and a data unit carrying a preemptive indication and / or preemptive data.

15. Second communication device according to claim 10, wherein the circuitry is configured to include into a signaling field of the preamble an indication of the format of the data unit and / or an indication of a presence of a collision detection indication in the data unit and / to include into or replace a training field located within or after the preamble with a collision detection indication.

16. Second communication device according to claim 10, wherein the circuitry is configured to generate the collision detection indication by truncating one or more OFDM symbols and multiplying the truncated one or more OFDM symbols with a time sequence.

17. Second communication device according to claim 10, wherein the circuitry is configured to generate, based on a received indication from the first communication device, one or more configurations of collision detection indications by one or more of: defining the time duration of a collision detection indication within a data unit; defining the number of distinct collision detection indications; determining a number of periods or OFDM symbols that fit within the duration of a collision detection indication; generating time / symbol sequences with the same length as periods or OFDM symbols; defining a time / symbol sequence index that identifies each time / symbol sequence; defining a bandwidth index that identifies the bandwidth occupied by the collision detection indication; generating a tone plan with a predetermined number of modifiable tones; generating a tone plan with a predetermined number of tones that are divided into contiguous or non-contiguous tone sets wherein each tone set is assigned to a distinct collision detection indication; generating tone sequences with equal or smaller length as modifiable tones and / or same length as number of tones in each tone set; defining a tone set index that identifies the tones within a tone set that correspond to a tone sequence and / or assigned to one or more second communication devices; generating collision detection indications by mapping tone sequences into a generated tone plan; creating complex-valued sequences representing or being included in collision detection indications; truncating one or more OFDM symbols included in collision detection indications; applying time / symbol sequences into OFDM symbols before OFDM modulation or time periods after OFDM modulation; modulating complex-valued sequences into waveforms to be transmitted as collision detection indications; assigning one or more collision detection indications to one or more second communication devices; andinforming the second communication devices associated with the first communication device of the assignment of one or more collision detection indications to one or more second communication devices.

18. First communication method of a first communication device configured to communicate with one or more second communication devices, the first communication method comprising detecting one or more data units having a preamble transmitted by one or more second communication devices and / or other communication devices; detecting a collision detection indication within or after the preamble of the one or more detected data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and identifying at least one second communication device from which said one or more data units has been transmitted based on the detected collision detection indication.

19. Second communication method of a second communication device configured to communicate with a first communication device that is configured to communicate with one or more second communication devices, the second communication device method comprising generating one or more data units having a preamble; providing a collision detection indication within or after the preamble of the one or more data units according to a predetermined configuration of the collision detection indication, wherein said configuration includes tone sequences mapped into tones of an OFDM symbol of the collision detection indication and / or time / symbol sequences applied to an OFDM symbol of the collision detection indication; and transmitting the one or more data units including the collision detection indication.

20. A non-transitory computer-readable recording medium that stores therein a computer program product, which, when executed by a processor, causes the method according to claim 18 or 19 to be performed.

Images