Patent Publication Number: US-10771126-B2

Title: Uplink multi-user multiple input multiple output for wireless local area network

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/553,982, now U.S. Pat. No. 9,825,678, entitled “Uplink Multi-User Multiple Input Multiple Output for Wireless Local Area Network,” filed on Nov. 25, 2014, which claims the benefit of U.S. Provisional Patent Application No. 61/909,024, entitled “UL MU MIMO MAC Consideration,” filed on Nov. 26, 2013. This application is also related to U.S. patent application Ser. No. 14/955,004, entitled “Uplink Multi-User Multiple Input Multiple Output for Wireless Local Area Network,” filed on Nov. 30, 2015. All of the applications referenced above are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks and, more particularly, to wireless local area networks that utilize multiple input multiple output techniques. 
     BACKGROUND 
     When operating in an infrastructure mode, wireless local area networks (WLANs) typically include an access point (AP) and one or more client stations. WLANs have evolved rapidly over the past decade. Development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps, and the IEEE 802.11ac Standard specifies a single-user peak throughput in the gigabits per second (Gbps) range. Future standards promise to provide even greater throughputs, such as throughputs in the tens of Gbps range. 
     SUMMARY 
     In an embodiment, a method includes: transmitting, by a first communication device, a first communication frame that includes scheduling information corresponding to a time period in which an uplink multi-user transmission is to occur, wherein the scheduling information indicates a start time of the time period, and wherein the first communication frame includes an indication of a group of two or more second communication devices that are to transmit simultaneously during the time period as part of the uplink multi-user transmission; transmitting, by the first communication device, a second communication frame during the time period, wherein the second communication frame is configured to trigger at least some second communication device in the group of two or more second communication devices to transmit simultaneously as part of the uplink multi-user transmission; and receiving, at the first communication device, the uplink multi-user transmission during the time period, wherein the uplink multi-user transmission is responsive to the second communication frame and includes respective simultaneous individual transmissions from respective second communication devices among the group of two or more second communication devices. 
     In another embodiment, an apparatus comprises a network interface device that includes: one or more integrated circuit (IC) devices, and a plurality of transceivers implemented at least partially on the one or more IC devices. The one or more IC devices are configured to: transmit a first communication frame that includes scheduling information corresponding to a time period in which an uplink multi-user transmission is to occur, wherein the scheduling information indicates a start time of the time period, and wherein the first communication frame includes an indication of a group of two or more second communication devices that are to transmit simultaneously during the time period as part of the uplink multi-user transmission; transmit a second communication frame during the time period, wherein the second communication frame is configured to trigger at least some second communication device in the group of two or more second communication devices to transmit simultaneously as part of the uplink multi-user transmission; and receive the uplink multi-user transmission during the time period, wherein the uplink multi-user transmission is responsive to the second communication frame and includes respective simultaneous individual transmissions from respective second communication devices among the group of two or more second communication devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example wireless local area network (WLAN), according to an embodiment. 
         FIG. 2  is a diagram of an example transmission sequence in a WLAN, according to an embodiment. 
         FIG. 3  is a diagram of another example transmission sequence in a WLAN, according to another embodiment. 
         FIG. 4  is a diagram of an example transmission sequence in a WLAN that is initiated by an access point, according to an embodiment. 
         FIG. 5  is a diagram of an example transmission sequence in a WLAN that is initiated by a communication device, according to another embodiment. 
         FIG. 6  is a diagram of an example transmission sequence in a WLAN that is initiated by an access point, according to an embodiment. 
         FIG. 7  is a diagram of another example transmission sequence in a WLAN that is initiated by an access point, according to an embodiment. 
         FIG. 8  is a flow diagram of an example method for communication with a client uplink group initiated by an access point, according to an embodiment. 
         FIG. 9  is a flow diagram of an example method for communication with a client uplink group initiated by a communication device of the client uplink group, according to an embodiment. 
         FIG. 10  is a flow diagram of another example method for communication with a client uplink group initiated by a communication device of the client uplink group, according to an embodiment. 
         FIG. 11  is a flow diagram of an example method for communication with a client uplink group initiated by an access point, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In embodiments described below, a first communication device, such as an access point (AP) of a wireless local area network (WLAN), simultaneously receives multiple independent data streams from multiple second communication devices, such as client stations. The first communication device determines that the second communication devices have data to transmit to the first communication device. Then, the first communication device prompts the second communication devices to simultaneously transmit the data streams during a transmit opportunity period (TXOP) of the first communication device. In an embodiment, a TXOP is a bounded time interval reserved for a communication device in a network during which the communication device can send as many frames as possible (as long as the duration of the transmissions does not extend beyond the PPDU length defined by the first communication device and beyond the TXOP). In an embodiment, other communication devices are generally not permitted to transmit in the TXOP unless the communication device to which the TXOP is assigned specifically permits the other communication device to transmit or unless the other communication device is acknowledging a transmission of the communication device to which the TXOP is assigned. 
       FIG. 1  is a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment. An AP  14  includes a host processor  15  coupled to a network interface  16 . The network interface  16  includes a medium access control (MAC) processing unit  18  and a physical layer (PHY) processing unit  20 . The PHY processing unit  20  includes a plurality of transceivers  21 , and the transceivers are coupled to a plurality of antennas  24 . Although three transceivers  21  and three antennas  24  are illustrated in  FIG. 1 , the AP  14  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  21  and antennas  24  in other embodiments. 
     The WLAN  10  includes a plurality of client stations  25 . Although four client stations  25  are illustrated in  FIG. 1 , the WLAN  10  can include different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations  25  in various scenarios and embodiments. In some embodiments, the AP  14  is configured to transmit independent data to two or more of the client stations  25  simultaneously. In one such embodiment, the two or more client stations  25  are members of a client uplink group created by the AP and identified by a group identifier. In other embodiments, the AP  14  is configured, additionally or alternatively, to receive respective data streams that are transmitted simultaneously by the two or more client stations  25 . In an embodiment, the two or more client stations  25  that transmit simultaneously are members of the client uplink group. In one embodiment, for example, the network interface  16  is configured to transmit independent data simultaneously to multiple client stations  25  via multiple spatial streams using techniques described in U.S. patent application Ser. No. 12/175,526, entitled “Access Point with Simultaneous Downlink Transmission of Independent Data for Multiple Client Stations,” filed on Jul. 18, 2008, which is hereby incorporated by reference. As another example, in another embodiment, the network interface  16 , additionally or alternatively, is configured to receive independent data streams transmitted simultaneously by multiple client stations  25  via different spatial streams using techniques described in U.S. patent application Ser. No. 12/175,501, entitled “Wireless Network with Simultaneous Uplink Transmission of Independent Data from Multiple Client Stations,” filed on Jul. 18, 2008, which is hereby incorporated by reference. 
     A client station  25 - 1  includes a host processor  26  coupled to a network interface  27 . The network interface  27  includes a MAC processing unit  28  and a PHY processing unit  29 . The PHY processing unit  29  includes a plurality of transceivers  30 , and the transceivers are coupled to a plurality of antennas  34 . Although three transceivers  30  and three antennas  34  are illustrated in  FIG. 1 , the client station  25 - 1  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  30  and antennas  34  in other embodiments. 
     In an embodiment, one or more of the client stations  25 - 2 ,  25 - 3 , and  25 - 4  has a structure the same as or similar to the client station  25 - 1 . In these embodiments, the client stations  25  structured like the client station  25 - 1  have the same or a different number of transceivers and antennas. For example, the client station  25 - 2  has only two transceivers and two antennas (not shown), according to an embodiment. 
     In some embodiments, two or more of the client stations  25  are configured to receive respective data streams that are transmitted simultaneously by the AP  14 . In an embodiment, the client stations  25  are members of the client uplink group. In other embodiments, two or more of the client stations  25  additionally or alternatively are configured to transmit corresponding data streams to the AP  14  such that the AP  14  receives the data streams simultaneously. For example, in one embodiment, the network interface  27  is configured to receive a data stream among a plurality of independent data streams transmitted simultaneously by the AP  14  to multiple client stations  25  via multiple spatial streams using techniques described in U.S. patent application Ser. No. 12/175,526. 
     As another example, in another embodiment, the network interface  27 , additionally or alternatively, is configured to transmit a data stream to the AP  14  among a plurality of independent data streams transmitted simultaneously by multiple client stations  25  via different spatial streams using techniques described in U.S. patent application Ser. No. 12/175,501. 
     Before triggering the uplink simultaneously transmission from a group stations, the AP defines the uplink group and notifies the stations in the uplink group about the uplink group allocation, in some embodiments. In an embodiment, the STA sends a communication frame or a management frame that indicates one or more traffic characteristics (e.g., a proposed service interval to be used for an uplink MU-MIMO transmission schedule, a proposed access category data rate to be used for the uplink MU-MIMO transmission schedule, or traffic burst information to be used for the uplink MU-MIMO transmission schedule) to the AP through an enhanced distributed channel access (EDCA) procedure. The AP performs selection of the members of the client uplink group based on the uplink traffic characteristic of the stations or other information, for example, the interference among stations. After finishing the member selection of the uplink group, the AP transmits an uplink group definition frame, through EDCA procedures or other medium access mechanisms, to the members of the uplink group. In an embodiment, the uplink group definition frame includes at least one of i) the member stations in the uplink group, ii) a start time of the uplink MU-MIMO transmission schedule, iii) a duration of the uplink MU-MIMO transmission schedule, or iv) an interval between transmissions of the uplink MU-MIMO transmission schedule. In an embodiment, the start time is indicated as a time offset from a transmission time of the uplink group definition frame until a transmission time of a communication frame to trigger uplink simultaneous transmission, described below, or another suitable time indication. In an embodiment, the duration is indicated as a time duration (e.g., a TXOP for uplink simultaneous transmission), a number of data frames to be transmitted during the schedule. In an embodiment, the interval indicates a time duration (e.g., a number of microseconds) between two adjacent uplink simultaneous transmission TXOPs, an integer number of predetermined intervals (e.g., an integer multiple of a short interframe spacing), or other suitable interval indication. In an embodiment, the AP sets the interval to the proposed service interval received from a client station. In an embodiment, the uplink group definition frame indicates a value for an access category, traffic category, traffic stream, TID, data rate, MCS value, or other suitable parameters to be used for transmissions during the uplink MU-MIMO schedule. In an embodiment, the uplink group definition frame is an action frame having one or more information elements that contain the member station identifiers of the uplink group, MU-MIMO transmission schedule and/or other suitable data. 
       FIG. 2  is a diagram of an example transmission sequence  200  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which an AP prompts a first client station (STA 1 ), a second client station (STA 2 ), and a third client station (STA 3 ) to transmit independent data simultaneously to the AP during a transmission opportunity (TXOP)  202  of the AP. The AP generates and transmits a communication frame  204  that prompts STA 1 , STA 2 , and STA 3  to transmit independent data simultaneously to the AP during the TXOP  202  of the AP. In one embodiment, the AP generates and transmits a communication frame  204  that prompts STA 1 , STA 2 , and STA 3  to transmit independent data simultaneously to the AP via different spatial streams, for example, spatial streams SS- 1 , SS- 2 , and SS- 3 , respectively. In one embodiment, the AP generates and transmits a DL Sync frame  204  to prompt STA 1 , STA 2 , and STA 3  to transmit independent data simultaneously to the AP via different spatial streams. In another embodiment, a DL Sync frame triggers a group of stations to transmit independent data simultaneously to the AP via different OFDM subchannels. 
     In one embodiment, the communication frame  204  includes a duration field (e.g., an UL physical layer protocol data unit (PPDU) duration field) that indicates a maximum duration of UL communication frames (e.g., PPDUs) responsive to the communication frame  204 . In one embodiment, the communication frame  204  comprises a PHY preamble and omits a MAC portion. In this embodiment, the PHY preamble includes a group ID corresponding to a client uplink group and a duration field that indicates a maximum duration of UL communication frames (e.g., PPDUs) responsive to the communication frame  204 . In an embodiment, the communication frame  204  includes station identifiers that indicate which client stations should transmit during the TXOP  202 . In an embodiment, the communication frame  204  includes a number of spatial streams (N ss ) and an index for the corresponding spatial streams to be used by the corresponding client stations. In an embodiment, the communication frame  204  includes a channel bandwidth to be used by the corresponding client stations. In an embodiment, the communication frame includes a modulation and coding scheme (MCS) value that corresponds to a modulation and coding scheme to be used by the corresponding client station. In an embodiment, the communication frame includes a transmission power value to be used by the corresponding client station. 
     Responsive to the communication frame  204 , STA 1 , STA 2 , and STA 3  transmit independent data simultaneously to the AP during the TXOP  202  of the AP. For example, in an embodiment, STA 1  transmits a communication frame  206 , STA 2  simultaneously transmits a communication frame  208 , and STA 3  simultaneously transmits a communication frame  210 . In one embodiment, the communication frame  206 , the communication frame  208 , and the communication frame  210  are transmitted using different spatial streams, for example, spatial streams SS- 1 , SS- 2 , and SS- 3 , respectively. In an embodiment, a duration of the communication frame  206 , a duration of the communication frame  208 , and/or a duration of the communication frame  210  are less than or equal to the maximum duration indicated in the communication frame  204 . Thus, in an embodiment, STA 1 , STA 2 , and STA 3  generate the communication frame  206 , the communication frame  208 , and the communication frame  210  to have a duration less than or equal to the maximum duration indicated in the communication frame  204 . In one embodiment, if the communication frame  208  is less than the maximum duration indicated in the communication frame  204 , STA 2  includes padding  212  to increase the total duration to the maximum duration. In another embodiment, the padding  212  is omitted. In one embodiment, each of communication frames  206 ,  208 , and  210  are padded to the maximum duration if they are shorter than the maximum duration. 
     The AP generates and transmits acknowledgments (ACKs or BlockAcks)  216 ,  218 , and  220  to STA 1 , STA 2 , and STA 3 , respectively, to acknowledge the communication frames  206 ,  208 , and  210 . In the embodiment shown in  FIG. 2 , the AP transmits each of ACK  216 , ACK  218 , and ACK  220  at different times (i.e., staggered times) and in different spatial streams. In an embodiment, the spatial stream in which the ACK is transmitted is the same spatial stream in which the corresponding communication frame is transmitted (i.e., communication frame  206  and ACK  216  are transmitted using spatial stream SS- 1 ). In another embodiment, the AP transmits each of ACK  216 , ACK  218 , and ACK  220  at different times in a same spatial stream. The AP, in some embodiments, continues generating and transmitting communication frames that prompt STA 1 , STA 2 , and/or STA 3  to transmit additional independent data simultaneously to the AP during the remainder of the TXOP  202  of the AP. In one embodiment, STA 1 , STA 2 , and STA 3  include updated pending data queue information in the communication frames  206 ,  208 , and  210 . In this embodiment, the AP utilizes the updated pending data queue information to determine a maximum duration for the additional independent data, and the AP includes information indicating the maximum duration in an additional DL Sync frame (not shown). 
       FIG. 3  is a diagram of another example transmission sequence  300  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which an AP prompts a first client station (STA 1 ), a second client station (STA 2 ), and a third client station (STA 3 ) to transmit independent data simultaneously to the AP during a transmission opportunity (TXOP)  302  of the AP. The transmission sequence  300  is generally the same as the transmission sequence  200 , however the AP utilizes a broadcast acknowledgment (B-ACK)  316  instead of the separate acknowledgments  216 ,  218 , and  220 . In an embodiment, the B-ACK  316  is a control frame that includes the group ID of the client uplink group (e.g., as a receiver address), station identifiers (e.g., AIDs) from which the communication frames  206 ,  208 , and  210  were transmitted, and/or other suitable acknowledgment information which acknowledges the transmissions from STA 1 , STA 2 , and STA 3 . In an embodiment, the B-ACK includes both ACK and BlockAck to acknowledge the transmissions from multiple STAs. In an embodiment, the AP transmits the B-ACK  316  in a single spatial stream. In another embodiment, the AP transmits the B-ACK  316  as multiple beamforming spatial streams. In one such embodiment, the B-ACK  316  is transmitted as separate downlink MU-MIMO data units for each client station using the spatial streams in which each of the communication frames  206 ,  208 , and  210  were transmitted (i.e., spatial streams, SS- 1 , SS- 2 , and SS- 3 , respectively). In another such embodiment, the B-ACK  316  is combined with other data, such as an aggregate media access control protocol data unit (A-MPDU), transmitted to the client stations. In another embodiment, the B-ACK is used to acknowledge the simultaneous transmission in different OFDM subchannels. 
       FIG. 4  is a diagram of an example transmission sequence  400  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which the transmission sequence  400  is initiated by an access point. In a TXOP  402  of the AP, the AP generates and transmits a trigger signal, such as a polling communication frame  404 , to a plurality of client stations, such as client stations STA 1 , STA 2 , and STA 3 . In an embodiment, the polling communication frame  404  is a downlink sync frame (DL Sync). In another embodiment, the polling communication frame  404  is an action frame designated as a polling frame. In an embodiment, the AP sets a value of a duration field of the communication frame  404  to a value corresponding to a remaining duration of the TXOP  402  of the AP, for example, to protect subsequent transmissions in the TXOP  402 . 
     In an embodiment, the AP transmits the polling communication frame  404  in a single spatial stream. In one such embodiment, the polling communication frame  404  has a receiving address that corresponds to a group ID of a client uplink group. In this embodiment, client stations that are members of the client uplink group are configured to respond to the polling communication frame  404  based on the group identifier. In another embodiment, the AP transmits the polling communication frame  404  to each client station communicatively coupled and/or registered with the AP. In an embodiment, the AP previously communicates to the client stations which client stations are in the client uplink group corresponding to the group ID. For example, in one embodiment, the AP previously transmits an uplink group definition frame  408 , described below, that indicates a group ID and also includes a plurality of association IDs (AIDs) that identify client stations that belong to the group corresponding to the group ID. Subsequently, when the AP transmits the polling communication frame  404  with the group ID, the client stations belonging to the group ID recognize that they are being requested to transmit information indicating whether they have data that is to be transmitted to the AP (e.g., traffic information). In this embodiment, the order of AIDs in the group definition frame indicates a spatial stream with which the client stations belonging to the group ID are to transmit feedback frames, as described below. 
     In another embodiment, the polling communication frame  404  includes the group ID and the plurality of AIDs that identify client stations that belong to the client uplink group corresponding to the group ID. In an embodiment, the order of AIDs in the group definition frame indicates a spatial stream with which the client stations belonging to the group ID are to transmit feedback frames. In one embodiment, the polling communication frame  404  is a group definition frame with an indicator (e.g., a field, a flag, etc., in a PHY header or a MAC header) that indicates that client stations in the group corresponding to the group ID should transmit to the AP information regarding whether the client stations have data that is to be transmitted to the AP. 
     In one embodiment in which the polling communication frame  404  includes the group ID, the polling communication frame  404  also includes information (e.g., a bitmap, with each bit corresponding to a client station in the client uplink group) that indicates which stations in the group corresponding to the group ID are to transmit information indicating whether the client station has data that is to be transmitted to the AP. For example, in an embodiment, a polling communication frame  404  includes a group ID and information indicating a subset of client stations in the group that are to transmit information indicating whether there is data that is to be transmitted to the AP. 
     In an embodiment, the polling communication frame  404  includes information that prompts a client station to transmit an uplink traffic characteristic information signal. In an embodiment, the polling communication frame  404  prompts the client station to transmit a proposed service interval to be used for an uplink MU-MIMO transmission schedule, a proposed access category (AC) data rate to be used for the uplink MU-MIMO transmission schedule, or traffic burst information to be used for the uplink MU-MIMO transmission schedule. In an embodiment, the polling communication frame  404  includes a request for the client station to transmit the information indicating whether the client station has data that is to be transmitted to the AP. In an embodiment, the information indicating whether the client station has data that is to be transmitted to the AP is an indication of an amount of data in a queue corresponding to data that is to be transmitted to the AP. In an embodiment, the queue corresponds to a particular traffic category. In an embodiment, the queue corresponds to a particular traffic stream. In an embodiment, the queue corresponds to a particular traffic identifier (TID) such as the TID described in the IEEE 802.11e Standard. In an embodiment, the polling communication frame  404  prompts the client station to transmit information indicating an amount of data in a particular traffic category, traffic stream, or traffic identifier (TID) that is to be transmitted to the AP. In an embodiment, the polling communication frame  404  prompts the client station to transmit information indicating an amount of data in one or more or all traffic categories, traffic streams, or TIDs that are to be transmitted to the AP. 
     In response to the polling communication frame  404 , each client station simultaneously transmits a respective communication frame  406  (i.e., communication frames  406 - 1 ,  406 - 2 , and  406 - 6 ) to the AP during the TXOP  402  of the AP, where the communication frame  406  (referred to herein as an uplink feedback frame or FB frame) includes information indicating whether the client station has data to be transmitted to the AP. In an embodiment, the information includes the uplink traffic characteristic information signal, as described above. In an embodiment, the indication of the amount of data in the queue is field in a MAC header of the FB frame  406 . In an embodiment, the indication of the amount of data in the queue is subfield in a QoS field of the MAC header. In an embodiment, each FB frame  406  indicates an amount of data in the particular traffic category (or categories), traffic stream(s), or TID(s) indicated by the polling frame  404 . In some embodiments, the FB frame is an action frame having an information element that contains the uplink traffic characteristic information signal. In an embodiment, the client station combines the feedback frame  406  with an aggregate media access control protocol data unit in an uplink MU-MIMO data unit. 
     In an embodiment, the AP receives the FB frames  406  that include the information about the station&#39;s uplink characteristic information that is to be transmitted to the AP, and uses this information to determine the uplink group allocation. In the scenario illustrated in  FIG. 4 , the AP determines that multiple client stations have the same or similar traffic characteristic. As a result, the AP selects multiple communication devices as members of a client uplink group based at least on the information indicated by the respective uplink traffic characteristic information signals. In an embodiment, the AP performs selection of the members of the client uplink group and transmits an uplink group definition frame  408  before transmitting the communication frame  404 . In some embodiments, the AP maintains the client uplink group for a period of time, such as until the end of the uplink MU-MIMO schedule, for a predetermined time (e.g., 5 seconds, 2 minutes), or another suitable duration. In an embodiment, the AP maintains the client uplink group until one or more members, or all members, of the client uplink group is dissociated with the AP. 
     In an embodiment, the AP generates and transmits the uplink group definition frame  408  to each member of the client uplink group. In an embodiment, the uplink group definition frame  408  also indicates an uplink multi-user multiple input multiple output (MU-MIMO) transmission schedule for the client uplink group for members of the client uplink group to simultaneously transmit to an access point. In an embodiment, the uplink group definition frame  408  includes at least one of i) the member stations in the uplink group, ii) a start time of the uplink MU-MIMO transmission schedule, iii) a duration of the uplink MU-MIMO transmission schedule, or iv) an interval between transmissions of the uplink MU-MIMO transmission schedule. In an embodiment, the start time is indicated as a time offset from a transmission time of the uplink group definition frame  408  until a transmission time of a communication frame  412 , described below, or another suitable time indication. In an embodiment, the duration is indicated as a time duration (e.g., a remainder of the TXOP  402 ), a number of data frames to be transmitted during the schedule, or other suitable indicators. In an embodiment, the interval between transmissions indicates a time duration (e.g., a number of microseconds), an integer number of predetermined intervals (e.g., an integer multiple of a short interframe spacing), or other suitable interval indication. In an embodiment, the AP sets the interval to the proposed service interval received from a client station (e.g., from communication frame  406 ). In an embodiment, the uplink group definition frame  408  indicates a value for an access category, traffic category, traffic stream, TID, data rate, MCS value, or other suitable parameters to be used for transmissions during the uplink MU-MIMO schedule. In an embodiment, the uplink group definition frame is an action frame having one or more information elements that contain the MU-MIMO transmission schedule and/or other suitable data. 
     In one embodiment, the AP receives the FB frames  406  that include the information indicating whether the client stations in a client uplink group have data that is to be transmitted to the AP, and uses this information to determine whether the client stations have data that is to be transmitted to the AP. In the scenario illustrated in  FIG. 4 , the AP determines that multiple client stations from a client uplink group have data that is to be transmitted to the AP. As a result, the AP selects multiple communication devices from a client uplink group for uplink data frame transmission. The AP generates and transmits a communication frame  412  that prompts the selected multiple client stations to transmit independent data simultaneously to the AP during the TXOP  402  of the AP. In some embodiments, the communication frame  412  is generally the same as the communication frame  204 . In one embodiment, the AP generates and transmits the communication frame  412  to prompt the multiple client stations to transmit independent data simultaneously to the AP via different spatial streams, for example, independent uplink MU-MIMO data units. In one embodiment, the AP generates and transmits a DL Sync frame  412  to prompt the multiple client stations to transmit independent data simultaneously to the AP via different spatial streams. In an embodiment, the DL Sync frame  412  includes an indication of a data unit size  415  to be used for the independent uplink MU-MIMO data units, for example, an indication of a time duration or bit count. In an embodiment, the AP transmits the communication frame  412  according to the UL MU-MIMO transmission schedule. In some embodiments, the communication frame  412  is provided as a legacy PPDU, such as an IEEE 802.11a/b/g (duplicate) PPDU. In an embodiment, the AP sets a value of a duration field of the communication frame  412  to a value corresponding to a remaining duration of the TXOP  402  of the AP, for example, to protect subsequent transmissions in the TXOP  402 . 
     In response to the communication frame  412 , the multiple client stations transmit independent data units  414  simultaneously to the AP during the TXOP  402  of the AP in one or more transmissions. For example, in an embodiment, the multiple client stations generate and transmit one or more UL MU-MIMO data units  414  via different spatial streams. In the embodiment shown in  FIG. 4 , the STA 1  transmits an UL MU-MIMO data unit  414 - 1  to the AP simultaneously with a transmission by the STA 2  of an UL MU-MIMO data unit  414 - 2  to the AP. In an embodiment, the UL MU-MIMO data units  414  correspond to a particular traffic category, traffic stream, or TID indicated by the communication frame  412 . In an embodiment, the UL MU-MIMO data units  414  at least include the traffic category, traffic stream, or TID associated with the TXOP or indicated by the communication frame  412 . In an embodiment, one or more of the UL MU-MIMO data units  414  includes suitable padding to have a suitable data unit size (e.g., the data unit size  415 ). 
     In response to receipt of the UL MU-MIMO data units  414 , the AP transmits a broadcast acknowledgment (B-ACK)  416  to acknowledge the uplink MU-MIMO data units  414 , as described above with respect to B-ACK  316 . In some embodiments, the client stations transmit one or more additional UL MU-MIMO data units  418  during a remainder of the TXOP  402  in response to the B-ACK  416 , for example, according to the previous DL SYNC frame which defines multiple UL MU-MIMO transmissions. In an embodiment, the AP transmits an additional communication frame, such as an additional DL Sync frame  412  to prompt the multiple client stations to transmit the additional UL MU-MIMO data units  418 . In an embodiment, the AP transmits a combined frame that includes the B-ACK  416  and the additional DL Sync frame  412 . In response to receipt of the UL MU-MIMO data units  418 , the AP transmits a broadcast acknowledgment (B-ACK)  420  to acknowledge the uplink MU-MIMO data units  418 , as described above. In an embodiment, the B-ACK  416  is omitted and the B-ACK  420  acknowledges the UL MU-MIMO data units  414  and the UL MU-MIMO data units  418 . 
     In an embodiment, when a transmission time of downlink PPDU(s) between adjacent UL MU PPDUs (i.e., between UL MU-MIMO data units  414  and  418 ) is smaller than an extended interframe space, at least some neighbor devices of the AP (e.g., other access points or client stations) use a value of a duration subfield in the communication frame  404  or the communication frame  412  to set a network allocation vector (NAV). In one such embodiment, neighbor devices of the client stations use the extended interframe space to protect the TXOP  402 . In another embodiment, when the transmission time of downlink PPDU(s) between adjacent UL MU PPDUs is longer than the extended interframe space, additional request to send and/or clear to send messages are used for protection of the TXOP  402 , as described below with respect to  FIG. 5  and  FIG. 7 . 
     In one embodiment, when the multiple client stations transmit independent data simultaneously to the AP during the TXOP  402  of the AP in one or more data units  414 , the client stations disregard a network allocation vector (NAV) in the communication frame  404  or communication frame  412  because the multiple client stations are scheduled by the communication frame  402  or communication frame  412 . When the client stations disregard the NAV, the client stations can transmit communication frames  414  during the TXOP of the AP. In another embodiment, the communication frame  412  includes a reverse direction grant (RDG) indicator that indicates that the client stations are permitted to transmit communication frames  414  during the TXOP  402  of the AP. 
       FIG. 5  is a diagram of an example transmission sequence  500  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which the transmission sequence  500  is initiated by a communication device. The transmission sequence  500  is generally the same as the transmission sequence  400 , however, a client station, such as the STA 2 , transmits a trigger frame, e.g. enhanced request to send (E-RTS) message  510  to the AP. In some embodiments, the enhanced RTS message  510  indicates that the STA 2  has a TXOP  502  (instead of the TXOP  402  of the AP of  FIG. 4 ) and data to be transmitted to the AP (e.g., in an UL MU-MIMO data unit). In an embodiment, the E-RTS message  510  includes i) a length of the TXOP  502  of the STA 2  and ii) an indication of a data unit size  515  for an uplink MU-MIMO data unit to be transmitted by the STA 2 . In an embodiment, the STA 2  is a member of a client uplink group and the indication of the data size unit is a data unit size for an uplink MU-MIMO data unit to be transmitted by the STA 2  simultaneously with transmissions of other members of the client uplink group. In an embodiment, the E-RTS message  510  includes an indication of whether the data unit size  515  should be used until the end of the TXOP  502 . In an embodiment, the client station sets a value of a duration field of the E-RTS message  510  to a value corresponding to a duration of the TXOP  502 , for example, to protect subsequent transmissions in the TXOP  502 . In an embodiment, the STA 2  sends the E-RTS  510  after waiting for a duration of an arbitration interframe spacing (AIFS) and a backoff value. In some embodiments, the E-RTS  510  is provided as a legacy PPDU, such as an IEEE 802.11a/b/g (duplicate) PPDU. 
     In response to the E-RTS message  510 , the AP transmits the communication frame  412 , as described above with respect to  FIG. 4 . In some embodiments, the AP transmits the communication frame  412  after a short interframe space after the E-RTS message  510 . In an embodiment, the communication frame  412  includes the indication of the data unit size  515  from the E-RTS message  510 . In one such embodiment, the UL MU-MIMO data units  414  have a size corresponding to the data unit size  515 . In response to receipt of the UL MU-MIMO data units  414 , the AP transmits the broadcast acknowledgment (B-ACK)  416  to acknowledge the uplink MU-MIMO data units  414 , as described above. In some embodiments, the client stations transmit one or more additional UL MU-MIMO data units  418  during a remainder of the TXOP  502  in response to the B-ACK  416 , for example, according to the uplink MU-MIMO transmission schedule indicated by the previous communication frame  412 . In an embodiment, the client station STA 2  transmits an additional E-RTS message  511  to indicate that the STA 2  has remaining time in the TXOP  502  and additional data to be transmitted to the AP. In an embodiment, the E-RTS message  511  includes an additional data unit size  525  for the additional uplink MU-MIMO data unit to be transmitted by the STA 2 . In some embodiments, the client station uses the frame exchange E-RTS and SYNC for dynamic bandwidth negotiations with the AP. 
     In response to the E-RTS message  511 , the AP transmits an additional communication frame  512 , such as an additional DL sync frame  512 , to prompt the multiple client stations to transmit the additional UL MU-MIMO data units  418 , in an embodiment. In another embodiment, the AP transmits a combined frame that includes the B-ACK  416  and the additional DL sync frame  512 . In an embodiment, the STA transmits a combined frame that includes the UL MU-MIMO data unit  414 - 2  and the E-RTS message  511 . In another embodiment, the E-RTS message  511  and communication frame  512  are omitted and the client stations transmit the UL MU-MIMO data units  418  in response to the B-ACK  416 . 
       FIG. 6  is a diagram of an example transmission sequence  600  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which the transmission sequence  600  is initiated by an access point. In the embodiment of  FIG. 6 , the AP transmits a communication frame  610  that indicates a length of a TXOP  602  of the access point, for example, to protect subsequent transmissions in the TXOP  602  (e.g., to reserve time for a transmission to client station STA 1  and client station STA 2  of the client uplink group). In an embodiment, the communication frame  610  is a clear to send to self (CTS-to-Self) frame  610 . The AP simultaneously transmits respective downlink MU-MIMO data units  612  to the STA 1  and the STA 2 . In the embodiment shown in  FIG. 6 , each downlink MU-MIMO data unit  612  includes i) a respective aggregate media access control protocol data unit (A-MPDU)  613 , and ii) a communication signal  614  to prompt transmission of an independent uplink MU-MIMO data unit by the corresponding client station. For example, in an embodiment, the downlink MU-MIMO data unit  612 - 1  for the STA 1  includes the A-MPDU  613 - 1  and a sync frame  614 - 1 , where the sync frame  614 - 1  prompts the client station STA 1  to transmit an independent uplink MU-MIMO data unit to the AP during the TXOP  602  of the AP (e.g., similarly to the DL sync frame  412 ). In an embodiment, the sync frame  614  includes an indication of a data unit size  615  for an uplink MU-MIMO data unit to be transmitted by the corresponding client station. 
     In response to the respective downlink MU-MIMO data units  612 , the STA 1  and STA 2  simultaneously transmit, and the AP simultaneously receives, respective uplink MU-MIMO data units  616 . Each uplink MU-MIMO data unit  616  includes i) a respective second A-MPDU  617 , and ii) an acknowledgment  618  of the corresponding first A-MPDU  613 . In response to receipt of the UL MU-MIMO data units  616 , the AP transmits a broadcast acknowledgment (B-ACK)  620  to acknowledge the uplink MU-MIMO data units  616 , as described above with respect to B-ACK  420 . 
       FIG. 7  is a diagram of another example transmission sequence  700  in a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment, in which the transmission sequence  700  is initiated by an access point. The transmission sequence  700  is generally the same as the transmission sequence  600 , however the AP utilizes a communication frame  710  to indicate a length of a TXOP  702  of the AP, for example, to reserve time for a transmission to client station STA 1  and client station STA 2  of the client uplink group. In an embodiment, the communication frame  710  is a request to send (RTS) frame  710  that is transmitted to the STA 1 . In response to the communication frame  710 , the STA transmits a communication frame  711  to the AP to confirm receipt of the RTS frame  710 . In an embodiment, the communication frame  711  is a clear to send (CTS) frame  711 . The AP simultaneously transmits the DL MU-MIMO data units  612  in response to the CTS frame  711 , in an embodiment. 
       FIG. 8  is a flow diagram of an example method  800  for communication with a client uplink group initiated by an access point of a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment. The method  800  is implemented by a network interface such as the network interface  16  of the AP  14  of  FIG. 1 , in an embodiment. For example, the network interface  16  is configured to implement the method  800 . In other embodiments, the method  800  is implemented by another suitable communication device. 
     At block  802 , the AP receives an uplink traffic characteristic information signal from each of a plurality of communication devices, in an embodiment. In an embodiment, the uplink traffic characteristic information signal is a management frame and a STA transmits the uplink traffic characteristic information signal using an EDCA procedure. In an embodiment, the uplink traffic characteristic information signal is a feedback frame  406 , as described above with respect to  FIG. 4 . In an embodiment, the uplink traffic characteristic information signal includes at least one of i) a proposed service interval to be used for an uplink MU-MIMO transmission schedule, ii) a proposed access category data rate to be used for the uplink MU-MIMO transmission schedule, or iii) traffic burst information to be used for the uplink MU-MIMO transmission schedule. 
     At block  804 , the AP selects multiple communication devices of the plurality of communication devices as members of a client uplink group based at least on traffic information indicated by the respective uplink traffic characteristic information signal s, in an embodiment, as described above with respect to  FIG. 4 . 
     At block  806 , the AP transmits an uplink group definition frame to each member of the client uplink group, in an embodiment. In one embodiment, the uplink group definition frame is a management frame that is transmitted by an AP through an EDCA procedure. In some embodiments, the uplink group definition frame is the uplink group definition frame  408 , as described with respect to  FIG. 4 . In an embodiment, the uplink group definition frame indicates the uplink MU-MIMO transmission schedule for the client uplink group for members of the client uplink group to simultaneously transmit to the access point. In an embodiment, the uplink group definition frame includes at least one of i) a start time of the uplink MU-MIMO transmission schedule, ii) a duration of the uplink MU-MIMO transmission schedule, or iii) an interval between transmissions of the uplink MU-MIMO transmission schedule. 
     In some embodiments, the AP transmits a communication frame to the plurality of communication devices to prompt transmission of the respective uplink traffic characteristic information signals. For example, in an embodiment, the AP transmits a communication frame  404  to the communication devices. In an embodiment, the communication frame includes a poll indication for members of the client uplink group. In an embodiment, the respective uplink traffic characteristic information signals are uplink MU-MIMO feedback frames that include an indication of available uplink data. In an embodiment, the uplink MU-MIMO feedback frames are quality of service null frames. In another embodiment, the uplink MU-MIMO feedback frames are newly defined control frames (i.e., not currently defined in an IEEE 802.11 standard). 
     In some embodiments, the AP transmits a communication frame to the client uplink group to prompt simultaneous transmission of independent uplink MU-MIMO data units by members of the client uplink group according to the uplink MU-MIMO transmission schedule. In an embodiment, the communication frame is the DL sync frame  412 , as described above with respect to  FIG. 4 . In an embodiment, the communication frame includes an indication of a data unit size to be used for the independent uplink MU-MIMO data units, for example, the data unit size  415 . In an embodiment, the communication frame includes an indication of a maximum duration for a transmission period in which both the independent uplink MU-MIMO data units and corresponding downlink acknowledgments are to be transmitted if a maximum duration for a transmission period in which the downlink acknowledgments are to be transmitted is less than an extended interframe space. 
     In an embodiment, the AP simultaneously receives the independent uplink MU-MIMO data units from the client uplink group. In a further embodiment, the AP transmits a single group acknowledgment frame to the client uplink group. In an embodiment, the single group acknowledgment frame has i) a receiver address corresponding to a group identifier of the client uplink group and ii) a broadcast acknowledgment indicator for each member of the client uplink group, e.g., AID of the client and the indication of the acknowledged frame(s). 
     In an embodiment, the AP receives an independent uplink MU-MIMO data unit from a member of the client uplink group while simultaneously receiving one or more other independent uplink MU-MIMO data units from one or more other members of the client uplink group. In a further embodiment, the AP transmits a downlink MU-MIMO data unit to the member of the client uplink group. In one such embodiment, the downlink MU-MIMO data unit includes i) an acknowledgment for the uplink MU-MIMO data unit and ii) an aggregate media access control protocol data unit. 
       FIG. 9  is a flow diagram of an example method  900  for communication with a client uplink group initiated by a communication device of a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment. The method  900  is implemented by a network interface such as the network interface  27  of the client  25 - 1  of  FIG. 1 , in an embodiment. For example, the network interface  27  is configured to implement the method  900 . In other embodiments, the method  900  is implemented by another suitable communication device. 
     At block  902 , the AP receives an UL trigger message from a first communication device of a client uplink group, in an embodiment. In an embodiment, UL trigger message is the enhanced RTS message  510 . In another embodiment, the UL trigger message is a newly defined control frame. In an embodiment, the enhanced RTS message or the newly defined control frame includes i) a length of a transmission opportunity (TXOP) of the first communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the first communication device. 
     At block  904 , the AP transmits a communication frame to the first communication device and a second communication device of the client uplink group to cause a simultaneous transmission of respective uplink MU-MIMO data units from the first communication device and the second communication device, in an embodiment. In an embodiment, the communication frame is the communication frame  412 . In some embodiments, the communication frame includes the indication of the data unit size which was received in the RTS message. 
     At block  906 , the AP simultaneously receives the respective uplink MU-MIMO data units, in an embodiment. The uplink MU-MIMO data units have the indicated data unit size, in an embodiment. In an embodiment, the uplink MU-MIMO data units are the uplink MU-MIMO data units  414  and/or  418 . 
     In some embodiments, the AP transmits a single downlink MU-MIMO data unit to the first communication device and the second communication device. In an embodiment, the single downlink MU-MIMO data unit is the B-ACK  316 ,  416 , or  420 . In an embodiment, the downlink MU-MIMO data unit include i) an acknowledgment for the uplink MU-MIMO data unit from first communication device and ii) an acknowledgment for the uplink MU-MIMO data unit from the second communication device. 
     In an embodiment, the AP selects a data rate for the communication frame  412  and the downlink MU-MIMO data unit  416  based on members of the client uplink group. In one such scenario, the AP selects the data rate to guarantee that other members of the client uplink group are able to decode the communication frame  412  and the downlink MU-MIMO data unit  416 . In another embodiment, the AP selects a data rate for the communication frame  412  and the downlink MU-MIMO data unit  416  based on a data rate indication included in the RTS message from the first communication device. In one such scenario, the first communication device can more efficiently use its TXOP. 
       FIG. 10  is a flow diagram of an example method  1000  for communication with a client uplink group initiated by a communication device of a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment. The method  1000  is implemented by a network interface such as the network interface  27  of the client  25 - 1  of  FIG. 1 , in an embodiment. For example, the network interface  27  is configured to implement the method  1000 . In other embodiments, the method  1000  is implemented by another suitable communication device. 
     At block  1002 , a communication device of a client uplink group transmits an UL trigger message to an access point of a wireless local area network, in an embodiment. In an embodiment, the UL trigger message is the enhanced RTS message  510  or a newly defined control frame. The enhanced RTS message or the newly defined control frame includes i) a length of a transmission opportunity (TXOP) of the communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the communication device simultaneously with transmissions of other members of the client uplink group, in an embodiment. 
     At block  1004 , the communication device receives a communication frame from the access point, in an embodiment. In an embodiment, the communication frame is the communication frame  412 . The communication frame includes a prompt to transmit an uplink MU-MIMO data unit having the indicated data unit size, in an embodiment. 
     At block  1006 , the communication device generates the uplink MU-MIMO data unit having the indicated data unit size. In an embodiment, the uplink MU-MIMO data unit is the uplink MU-MIMO data unit  414 . At block  1008 , in response to the communication frame, the communication device transmits the uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
     In an embodiment, the communication devices generates the uplink MU-MIMO data unit to include an additional enhanced RTS message that includes i) a remainder of the TXOP length and ii) an indication of a data unit size for another uplink MU-MIMO data unit to be transmitted by the communication device. 
     In some embodiments, the communication device receives a group acknowledgment frame from the access point during the TXOP. In an embodiment, the group acknowledgment frame is the B-ACK  316 ,  416 , or  420 . The group acknowledgment frame includes respective acknowledgments for i) the uplink MU-MIMO data unit transmitted by the communication device and ii) another uplink MU-MIMO data unit transmitted by another communication device of the client uplink group, in an embodiment. The communication device generates an additional uplink MU-MIMO data unit, for example, the uplink MU-MIMO data unit  418 , having the indicated data unit size after receipt of the group acknowledgment frame, in an embodiment. The communication device transmits, in response to the communication frame, the additional uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
       FIG. 11  is a flow diagram of an example method  1100  for communication with a client uplink group initiated by an access point of a WLAN, such as the WLAN  10  of  FIG. 1 , according to an embodiment. The method  1100  is implemented by a network interface such as the network interface  16  of the AP  14  of  FIG. 1 , in an embodiment. For example, the network interface  16  is configured to implement the method  1100 . In other embodiments, the method  1100  is implemented by another suitable communication device. 
     At block  1102 , the access point transmits respective downlink MU-MIMO data units to a first communication device of a client uplink group and a second communication device of the client uplink group, in an embodiment. In an embodiment, the downlink MU-MIMO data unit is the downlink MU-MIMO data unit  612 . Each downlink MU-MIMO data unit includes i) a respective first aggregate media access control protocol data unit, and ii) a communication signal to prompt transmission of an independent uplink MU-MIMO data unit by the corresponding communication device of the client uplink group, in an embodiment. The access point transmits the downlink MU-MIMO data units simultaneously, in various embodiments. 
     At block  1104 , the access point receives, in response to the respective downlink MU-MIMO data units, respective uplink MU-MIMO data units from the first communication device and the second communication device, in various embodiments. In an embodiment, the uplink MU-MIMO data unit is the uplink MU-MIMO data unit  616 . In an embodiment, each uplink MU-MIMO data unit includes i) a respective second aggregate media access control protocol data unit, and ii) an acknowledgment of the corresponding first aggregate media access control protocol data unit. In some embodiments, the access point receives the uplink MU-MIMO data units simultaneously. 
     In an embodiment, the access point transmits a clear to send to self (CTS-to-Self) frame that indicates a length of a transmission opportunity (TXOP) of the access point. In an embodiment, the CTS-to-Self frame is the CTS-to-Self frame  610 . The respective downlink MU-MIMO data units are transmitted during the TXOP and the respective uplink MU-MIMO data units are received during the TXOP, in an embodiment. 
     In another embodiment, the access point transmits a request to send (RTS) message to the first communication device, where the RTS message indicates a TXOP of the access point. In an embodiment, the RTS message is the RTS message  710 . In an embodiment, the access point receives, from the first communication device and in response to the RTS message, a clear to send (CTS) frame that indicates a remaining length of the TXOP. The respective downlink MU-MIMO data units are transmitted during the TXOP after receipt of the CTS frame and the respective uplink MU-MIMO data units are received during the TXOP, in an embodiment. 
     Further aspects of the present invention relate to one or more of the following clauses. 
     In an embodiment, a method includes: receiving an uplink traffic characteristic information signal from each of a plurality of communication devices; selecting multiple communication devices of the plurality of communication devices as members of a client uplink group based at least on traffic information indicated by the respective uplink traffic characteristic information signals; transmitting an uplink group definition frame to each member of the client uplink group, the uplink group definition frame indicating an uplink multi-user multiple input multiple output (MU-MIMO) transmission schedule for the client uplink group for members of the client uplink group to simultaneously transmit to an access point; and triggering the members of the client uplink group to transmit uplink data frames simultaneously. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     The uplink group definition frame includes at least one of i) station identifiers (AIDs) of the members of the client uplink group, ii) a start time of the uplink MU-MIMO transmission schedule, iii) a duration of the uplink MU-MIMO transmission schedule, or iv) an interval between transmissions of the uplink MU-MIMO transmission schedule. 
     The uplink traffic characteristic information signal includes at least one of i) a proposed service interval to be used for the uplink MU-MIMO transmission schedule, ii) a proposed access category data rate to be used for the uplink MU-MIMO transmission schedule, or iii) traffic burst information to be used for the uplink MU-MIMO transmission schedule. 
     The uplink traffic characteristic information signal includes at least one of i) a proposed service interval to be used for the uplink MU-MIMO transmission schedule, ii) a proposed access category data rate to be used for the uplink MU-MIMO transmission schedule, or iii) traffic burst information to be used for the uplink MU-MIMO transmission schedule. 
     The method further includes transmitting a communication frame to the plurality of communication devices to prompt transmission of the respective uplink traffic characteristic information signals, where the communication frame includes a poll indication for members of the client uplink group. 
     The method further includes receiving the respective uplink traffic characteristic information signals through EDCA procedures. 
     The uplink MU-MIMO feedback frames are management frames. 
     The method further includes transmitting a communication frame to the client uplink group to prompt simultaneous transmission of independent uplink MU-MIMO data units by members of the client uplink group. 
     The communication frame includes an indication of a data unit size to be used for the independent uplink MU-MIMO data units. 
     The communication frame includes an indication of a maximum duration for a transmission period in which both the independent uplink MU-MIMO data units and corresponding downlink acknowledgments are to be transmitted if a maximum duration for a transmission period in which the downlink acknowledgments are to be transmitted is less than an extended interframe space. 
     The method further includes: simultaneously receiving the independent uplink MU-MIMO data units from the client uplink group; and transmitting a single group acknowledgment frame to the client uplink group. The single group acknowledgment frame has i) a broadcast address and ii) a station identifier (AID) and acknowledgment indicator of the received data and management frames for each member of the client uplink group. 
     The method further includes receiving an independent uplink MU-MIMO data unit from a member of the client uplink group while simultaneously receiving one or more other independent uplink MU-MIMO data units from one or more other members of the client uplink group; and transmitting a downlink MU-MIMO data unit to the member of the client uplink group. The downlink MU-MIMO data unit includes i) an acknowledgment for the uplink MU-MIMO data unit and ii) an aggregate media access control protocol data unit. 
     The method further includes: transmitting a trigger frame to a plurality of communication devices of the client uplink group to request the uplink traffic characteristic information signals; receiving the uplink traffic characteristic information signals from each of a plurality of communication devices; and selecting some or all of the members of the client uplink group for simultaneous uplink transmissions. 
     The method further includes transmitting an uplink trigger frame to the selected members for the simultaneous uplink transmissions. 
     The respective uplink traffic characteristic information signals are uplink MU-MIMO feedback frames that include an indication of available uplink data. 
     The uplink traffic characteristic information signal is a control frame. 
     In another embodiment, an apparatus includes a network interface device having one or more integrated circuits configured to: receive an uplink traffic characteristic information signal from each of a plurality of communication devices; select multiple communication devices of the plurality of communication devices as members of a client uplink group based at least on traffic information indicated by the respective uplink traffic characteristic information signals; and transmit an uplink group definition frame to each member of the client uplink group, the uplink group definition frame indicating an uplink multi-user multiple input multiple output (MU-MIMO) transmission schedule for the client uplink group for members of the client uplink group to simultaneously transmit to an access point. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The uplink group definition frame includes at least one of i) a start time of the uplink MU-MIMO transmission schedule, ii) a duration of the uplink MU-MIMO transmission schedule, or iii) an interval between transmissions of the uplink MU-MIMO transmission schedule. 
     The uplink traffic characteristic information signal includes at least one of i) a proposed service interval to be used for the uplink MU-MIMO transmission schedule, ii) a proposed access category data rate to be used for the uplink MU-MIMO transmission schedule, or iii) traffic burst information to be used for the uplink MU-MIMO transmission schedule. 
     The one or more integrated circuits are configured to transmit a communication frame to the client uplink group to prompt simultaneous transmission of independent uplink MU-MIMO data units by members of the client uplink group according to the uplink MU-MIMO transmission schedule. 
     The communication frame includes an indication of a maximum duration for a transmission period in which both the independent uplink MU-MIMO data units and corresponding downlink acknowledgments are to be transmitted if a maximum duration for a transmission period in which the downlink acknowledgments are to be transmitted is less than an extended interframe space. 
     The one or more integrated circuits are configured to: simultaneously receive the independent uplink MU-MIMO data units from the client uplink group; and transmit a single group acknowledgment frame to the client uplink group. The single group acknowledgment frame having i) a receiver address corresponding to a group identifier of the client uplink group and ii) a broadcast acknowledgment indicator for each member of the client uplink group. 
     The one or more integrated circuits are configured to: receive an independent uplink MU-MIMO data unit from a member of the client uplink group while simultaneously receiving one or more other independent uplink MU-MIMO data units from one or more other members of the client uplink group; and transmit a downlink MU-MIMO data unit to the member of the client uplink group. The downlink MU-MIMO data unit includes i) an acknowledgment for the uplink MU-MIMO data unit and ii) an aggregate media access control protocol data unit. 
     In an embodiment, a method includes receiving an enhanced request to send (E-RTS) message from a first communication device of a client uplink group. The E-RTS message includes i) a length of a transmission opportunity (TXOP) of the first communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the first communication device. The method further includes transmitting a communication frame to the first communication device and a second communication device of the client uplink group to cause a simultaneous transmission of respective uplink MU-MIMO data units from the first communication device and the second communication device. The communication frame includes the indication of the data unit size. The method also includes simultaneously receiving the respective uplink MU-MIMO data units. The uplink MU-MIMO data units having the indicated data unit size. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     The method further includes transmitting a single downlink MU-MIMO data unit to the first communication device and the second communication device. The downlink MU-MIMO data unit including i) an acknowledgment for the uplink MU-MIMO data unit from first communication device and ii) an acknowledgment for the uplink MU-MIMO data unit from the second communication device. 
     The method further includes selecting a data rate for the communication frame and the downlink MU-MIMO data unit based on members of the client uplink group. 
     The method further includes selecting a data rate for the communication frame and the downlink MU-MIMO data unit based on a data rate indication included in the RTS message from the first communication device. 
     In another embodiment, an apparatus includes a network interface device having one or more integrated circuits configured to receive an enhanced request to send (E-RTS) message from a first communication device of a client uplink group. The E-RTS message includes i) a length of a transmission opportunity (TXOP) of the first communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the first communication device. The one or more integrated circuits are configured to transmit a communication frame to the first communication device and a second communication device of the client uplink group to cause a simultaneous transmission of respective uplink MU-MIMO data units from the first communication device and the second communication device. The communication frame includes the indication of the data unit size. The one or more integrated circuits are also configured to simultaneously receive the respective uplink MU-MIMO data units, the uplink MU-MIMO data units having the indicated data unit size. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The one or more integrated circuits are configured to transmit a single downlink MU-MIMO data unit to the first communication device and the second communication device. The downlink MU-MIMO data unit includes i) an acknowledgment for the uplink MU-MIMO data unit from first communication device and ii) an acknowledgment for the uplink MU-MIMO data unit from the second communication device. 
     The one or more integrated circuits are configured to select a data rate for the communication frame and the downlink MU-MIMO data unit based on members of the client uplink group. 
     The one or more integrated circuits are configured to select a data rate for the communication frame and the downlink MU-MIMO data unit based on a data rate indication included in the RTS message from the first communication device. 
     In an embodiment, a method includes transmitting, by a communication device of a client uplink group, an enhanced request to send (E-RTS) message to an access point of a wireless local area network. The E-RTS message includes i) a length of a transmission opportunity (TXOP) of the communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the communication device simultaneously with transmissions of other members of the client uplink group. The method includes receiving a communication frame from the access point. The communication frame includes a prompt to transmit an uplink MU-MIMO data unit having the indicated data unit size. The method further includes: generating the uplink MU-MIMO data unit having the indicated data unit size; and transmitting, by the communication device and in response to the communication frame, the uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     Generating the uplink MU-MIMO data unit includes generating the uplink MU-MIMO data unit to include an additional E-RTS message that includes i) a remainder of the TXOP length and ii) an indication of a data unit size for another uplink MU-MIMO data unit to be transmitted by the communication device. 
     The method further includes receiving a group acknowledgment frame from the access point during the TXOP. The group acknowledgment frame includes respective acknowledgments for i) the uplink MU-MIMO data unit transmitted by the communication device and ii) another uplink MU-MIMO data unit transmitted by another communication device of the client uplink group. The method also includes: generating an additional uplink MU-MIMO data unit having the indicated data unit size after receipt of the group acknowledgment frame; and transmitting, by the communication device and in response to the communication frame, the additional uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
     In another embodiment, a communication device of a client uplink group includes a network interface device having one or more integrated circuits configured to transmit an enhanced request to send (E-RTS) message to an access point of a wireless local area network. The E-RTS message includes i) a length of a transmission opportunity (TXOP) of the communication device and ii) an indication of a data unit size for an uplink MU-MIMO data unit to be transmitted by the communication device simultaneously with transmissions of other members of the client uplink group. The one or more integrated circuits are configured to receive a communication frame from the access point. The communication frame includes a prompt to transmit an uplink MU-MIMO data unit having the indicated data unit size. The one or more integrated circuits are configured to: generate the uplink MU-MIMO data unit having the indicated data unit size; and transmit, in response to the communication frame, the uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The one or more integrated circuits are configured to generate the uplink MU-MIMO data unit to include an additional E-RTS message that includes i) a remainder of the TXOP length and ii) an indication of a data unit size for another uplink MU-MIMO data unit to be transmitted by the communication device. 
     The one or more integrated circuits are configured to receive a group acknowledgment frame from the access point during the TXOP. The group acknowledgment frame includes respective acknowledgments for i) the uplink MU-MIMO data unit transmitted by the communication device and ii) another uplink MU-MIMO data unit transmitted by another communication device of the client uplink group. The one or more integrated circuits are configured to: generate an additional uplink MU-MIMO data unit having the indicated data unit size after receipt of the group acknowledgment frame; and transmit, in response to the communication frame, the additional uplink MU-MIMO data unit to the access point during the TXOP simultaneously with transmissions of other members of the client uplink group. 
     In an embodiment, a method includes transmitting, by an access point, respective downlink MU-MIMO data units to a first communication device of a client uplink group and a second communication device of the client uplink group. Each downlink MU-MIMO data unit includes i) a respective first aggregate media access control protocol data unit, and ii) a communication signal to prompt transmission of an independent uplink MU-MIMO data unit by the corresponding communication device of the client uplink group. The downlink MU-MIMO data units are transmitted simultaneously. The method includes receiving, in response to the respective downlink MU-MIMO data units, respective uplink MU-MIMO data units from the first communication device and the second communication device. Each uplink MU-MIMO data unit includes i) a respective second aggregate media access control protocol data unit, and ii) an acknowledgment of the corresponding first aggregate media access control protocol data unit. The uplink MU-MIMO data units are received simultaneously. 
     In other embodiments, the method includes any suitable combination of one or more of the following features. 
     The method further includes transmitting a clear to send to self (CTS-to-Self) frame that indicates a length of a transmission opportunity (TXOP) of the access point. The respective downlink MU-MIMO data units are transmitted during the TXOP and the respective uplink MU-MIMO data units are received during the TXOP. 
     The method further includes transmitting a request to send (RTS) message to the first communication device. The RTS message indicates a TXOP of the access point. The method includes receiving, from the first communication device and in response to the RTS message, a clear to send (CTS) frame that indicates a remaining length of the TXOP. The respective downlink MU-MIMO data units are transmitted during the TXOP after receipt of the CTS frame. The respective uplink MU-MIMO data units are received during the TXOP. 
     In another embodiment, an apparatus includes a network interface device having one or more integrated circuits configured to transmit respective downlink MU-MIMO data units to a first communication device of a client uplink group and a second communication device of the client uplink group. Each downlink MU-MIMO data unit includes i) a respective first aggregate media access control protocol data unit, and ii) a communication signal to prompt transmission of an independent uplink MU-MIMO data unit by the corresponding communication device of the client uplink group. The downlink MU-MIMO data units are transmitted simultaneously. The one or more integrated circuits configured to receive, in response to the respective downlink MU-MIMO data units, respective uplink MU-MIMO data units from the first communication device and the second communication device. Each uplink MU-MIMO data unit includes i) a respective second aggregate media access control protocol data unit, and ii) an acknowledgment of the corresponding first aggregate media access control protocol data unit. The uplink MU-MIMO data units are received simultaneously. 
     In other embodiments, the apparatus includes any suitable combination of one or more of the following features. 
     The one or more integrated circuits are configured to transmit a clear to send to self (CTS-to-Self) frame that indicates a length of a transmission opportunity (TXOP) of the access point. The respective downlink MU-MIMO data units are transmitted during the TXOP and the respective uplink MU-MIMO data units are received during the TXOP. 
     The one or more integrated circuits are configured to: transmit a request to send (RTS) message to the first communication device. The RTS message indicating a TXOP of the access point. The one or more integrated circuits configured to receive, from the first communication device and in response to the RTS message, a clear to send (CTS) frame that indicates a remaining length of the TXOP. The respective downlink MU-MIMO data units are transmitted during the TXOP after receipt of the CTS frame and the respective uplink MU-MIMO data units are received during the TXOP. 
     At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. Likewise, the software or firmware instructions may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Thus, the software or firmware instructions may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention.