Patent Publication Number: US-2023145283-A1

Title: Coordinated multi-user transmissions with multiple access points

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 17/490,847 (now U.S. Pat. No. 11,546,021) entitled “Coordinated Multi-User Transmissions with Multiple Access Points,” filed on Sep. 30, 2021 which is a continuation of U.S. patent application Ser. No. 16/827,483 (now U.S. Pat. No. 11,146,311), entitled “Coordinated Multi-User Transmissions with Multiple Access Points,” filed on Mar. 23, 2020, which claims the benefit of U.S. Provisional Patent Application No. 62/821,936, entitled “Access Point (AP) Coordinated Orthogonal Frequency Multiple Access (OFDMA),” filed on Mar. 21, 2019, U.S. Provisional Patent Application No. 62/837,106, entitled “Access Point (AP) Coordinated Orthogonal Frequency Multiple Access (OFDMA),” filed on Apr. 22, 2019, U.S. Provisional Patent Application No. 62/934,452, entitled “Access Point (AP) Coordinated Orthogonal Frequency Multiple Access (OFDMA),” filed on Nov. 12, 2019. All of the applications referenced above are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF TECHNOLOGY 
     The present disclosure relates generally to wireless communication systems, and more particularly to coordination of multiple access points in multiple wireless local area networks. 
     BACKGROUND 
     Wireless local area networks (WLANs) have evolved rapidly over the past two decades, and development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11 Standard family has improved single-user peak data rates. One way in which data rates have been increased is by increasing the frequency bandwidth of communication channels used in WLANs. For example, the IEEE 802.11n Standard permits aggregation of two 20 MHz sub-channels to form a 40 MHz aggregate communication channel, whereas the more recent IEEE 802.11ax Standard permits aggregation of up to eight 20 MHz sub-channels to form up 160 MHz aggregate communication channels. Work has now begun on a new iteration of the IEEE 802.11 Standard, which is referred to as the IEEE 802.11be Standard, or Extremely High Throughput (EHT) WLAN. The IEEE 802.11be Standard may permit aggregation of as many as sixteen 20 MHz sub-channels (or perhaps even more) to form 320 MHz aggregate communication channels (or perhaps even wider aggregate communication channels). 
     As the density of IEEE 802.11 WLANs increases over time, it tends to become more difficult for an access point (AP) to find several 20 MHz sub-channels that are idle and that can be aggregated together to form a larger aggregate channel. One way to increase the likelihood of WLANs being able to take advantage of wider frequency bandwidths is to allow APs of neighboring networks to coordinate the use of sub-channels amongst the WLANs. 
     SUMMARY 
     In an embodiment, a method for wireless communication by a first access point (AP) associated with one or more first client stations includes: generating, at the first AP, an announcement frame that announces a coordinated multi-user (MU) transmission involving multiple APs including the first AP and one or more second APs, each of the second APs associated with a respective one or more second client stations, wherein the announcement frame is generated to indicate one or more respective sets of communication parameters to be used by the one or more second APs for communicating with the respective one or more second client stations during the coordinated MU transmission; transmitting, by the first AP, the announcement frame to the one or more second APs to initiate the coordinated MU transmission; and participating, by the first AP, in the coordinated MU transmission while the one or more second APs also participate in the coordinated MU transmission. 
     In another embodiment, a first access point AP associated with one or more first client stations comprises a wireless network interface device having one or more integrated circuit (IC) devices. The one or more IC devices are configured to: generate an announcement frame that announces a coordinated MU transmission involving multiple APs including the first AP and one or more second APs, each of the second APs associated with a respective one or more second client stations, wherein the announcement frame is generated to indicate one or more respective sets of communication parameters to be used by the one or more second APs for communicating with the respective one or more second client stations during the coordinated MU transmission; control the wireless network interface device to transmit the announcement frame to the one or more second APs to initiate the coordinated MU transmission; and control the wireless network interface device to participate in the coordinated MU transmission while the one or more second APs also participate in the coordinated MU transmission. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1 A  is a block diagram of an example communication system that includes multiple access points (APs) that participate in coordinated multi-user (MU) transmissions, according to an embodiment. 
         FIG.  1 B  is a block diagram of an example AP in the communication system of  FIG.  1 A , according to an embodiment. 
         FIG.  1 C  is a block diagram of an example client station in the communication system of  FIG.  1 A , according to an embodiment. 
         FIG.  2    is a diagram of an example coordinated MU downlink (DL) transmission implemented by the communication system of  FIG.  1 A , according to an embodiment. 
         FIG.  3    is a diagram of another example coordinated MU DL transmission implemented by the communication system of  FIG.  1 A , according to another embodiment. 
         FIG.  4    is a diagram of another example coordinated MU DL transmission implemented by the communication system of  FIG.  1 A , according to another embodiment. 
         FIG.  5    is a diagram of an example acknowledgment procedure used in a coordinated MU DL transmission such as in  FIGS.  2 - 4   , according to an embodiment. 
         FIG.  6    is a diagram of another example acknowledgment procedure used in a coordinated MU DL transmission such as in  FIGS.  2 - 4   , according to another embodiment. 
         FIG.  7    is a diagram of another example acknowledgment procedure used in a coordinated MU DL transmission such as in  FIGS.  2 - 4   , according to another embodiment. 
         FIG.  8    is a diagram of an example coordinated MU uplink (UL) transmission implemented by the communication system of  FIG.  1 A , according to an embodiment. 
         FIG.  9    is a diagram of another example coordinated MU UL transmission implemented by the communication system of  FIG.  1 A , according to another embodiment. 
         FIG.  10    is a diagram of another example coordinated MU UL transmission implemented by the communication system of  FIG.  1 A , according to another embodiment. 
         FIG.  11    is a diagram of an example acknowledgment procedure used in a coordinated MU UL transmission such as in  FIGS.  8 - 10   , according to an embodiment. 
         FIG.  12    is a diagram of an example coordinated MU UL transmission followed by a coordinated MU DL transmission, according to an embodiment. 
         FIG.  13    is a diagram of an example coordinated MU DL transmission followed by a coordinated MU UL transmission, according to an embodiment. 
         FIG.  14    is a flow diagram of an example method for coordinated wireless communications involving multiple APs, according to an embodiment. 
         FIG.  15    is a flow diagram of another example method for coordinated wireless communications involving multiple APs, according to another embodiment. 
         FIG.  16    is a flow diagram of another example method for coordinated wireless communications involving multiple APs, according to another embodiment. 
         FIG.  17    is a flow diagram of another example method for coordinated wireless communications involving multiple APs, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In various embodiments described below, access points (APs) of neighboring wireless local area network (WLANs) coordinate the use of wireless sub-channels. For example, one AP may act as a “master AP” and one or more other APs may act as “slave APs,” and the master AP may coordinate synchronized transmissions in respective WLANs, the synchronized transmissions using respective frequency segments. Such synchronized transmissions are sometimes referred to as coordinated orthogonal frequency division multiple access (C-OFDMA). 
     As part of coordinating a C-OFDMA transmission, the master AP generates and transmits a C-OFDMA announcement (C-OFDMA-A) frame to one or more slave APs, according to some embodiments. The C-OFDMA-A frame advertises a start of a coordinated uplink or downlink OFDMA transmission involving multiple WLANs, according to an embodiment. The C-OFDMA-A frame includes information regarding the coordinated OFDMA transmission such as one of, or any suitable combination of two or more of, i) a respective frequency bandwidth to be used in a respective WLAN, ii) a respective frequency resource unit (RU) to be used in a respective WLAN, iii) a duration (in time) of the coordinated OFDMA transmission, iv) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission in a respective WLAN, etc., according to various embodiments. 
     For a C-OFDMA downlink (DL) transmission, the C-OFDMA-A frame transmitted by the master AP prompts one or more slave APs to transmit respective DL OFDMA transmissions as part of the C-OFDMA transmission, according to some embodiments. For a C-OFDMA uplink (UL) transmission, the C-OFDMA-A frame transmitted by the master AP prompts one or more slave APs to transmit respective trigger frames, which in turn prompt respective sets of client stations to transmit respective UL OFDMA transmissions as part of the C-OFDMA transmission, according to some embodiments. 
       FIG.  1 A  is a diagram of an example communication system  10  that includes multiple WLANs, including a WLAN  20  and a WLAN  30 . Although two WLANs are illustrated in  FIG.  1 A , the communication system  10  includes other suitable numbers of WLANs such as three, four, five, etc., in various embodiments. 
     The WLAN  20  comprises an AP  34  and a plurality of client stations  38 . The AP  34  acts as a master AP that coordinates synchronized transmissions in respective WLANs, as will be described in more detail below. For example, the master AP  34  transmits instructions, information, etc., regarding a C-OFDMA transmission to one or more slave APs, according to some embodiments. 
     The WLAN  30  comprises an AP  44  and a plurality of client stations  48 . The AP  44  acts as a slave AP that participates in a C-OFDMA transmission coordinated by the master AP  34 . For example, the slave AP  44  receives instructions, information, etc., from the master AP  34  regarding a C-OFDMA transmission, and the slave AP  44  participates in the C-OFDMA transmission according to the instructions, information, etc., received from the master AP  34 , in some embodiments. 
     The master AP  34  comprises a C-OFDMA controller  60  that determines parameters for a C-OFDMA transmission, generates data units for setting up a C-OFDMA transmission, controls the timing of transmissions by the master AP  34  during a C-OFDMA transmission, etc. according to various embodiments. The C-OFDMA controller  60  is described in more detail below. 
     The slave AP  44  comprises a C-OFDMA controller  70  that receives parameters for a C-OFDMA transmission from the master AP, generates data units for the C-OFDMA transmission, controls the timing of transmissions by the slave AP  44  during a C-OFDMA transmission, etc., according to various embodiments. The C-OFDMA controller  70  is described in more detail below. 
     In some embodiments, one or more client stations  38 ,  48  include a C-OFDMA controller  80  that receives frames transmitted by the master AP  34  and/or the slave AP  44  as part of setting up a C-OFDMA transmission (or by another AP (not shown) as part of setting up another C-OFDMA transmission in another set of WLANs (not shown)), and uses information in such frames for purposes such as determining whether a communication medium is idle, according to various embodiments. The C-OFDMA controller  80  is described in more detail below. 
       FIG.  1 B  is a block diagram of an example AP  114  that may be used as the master AP  34  and/or the slave AP  44 , in various embodiments. In some embodiments, the AP  114  is configured to operate as a master AP at some times, and a slave AP at other times. A master AP generally allocates frequency resource units (RUs) and/or spatial streams, etc., to slave APs for a C-OFDMA transmission, and initiates the C-OFDMA transmission. On the other hand, a slave AP generally participates in the C-OFDMA transmission in response to a prompt from a master AP and uses an RU and/or one or more spatial streams for the C-OFDMA transmission that were allocated to the slave AP by the master AP. 
     The AP  114  comprises a host processor  118  coupled to a wireless network interface device  122 . The wireless network interface device  122  includes one or more medium access control (MAC) processors  126  (sometimes referred to herein as “the MAC processor  126 ” for brevity) and one or more physical layer (PHY) processors  130  (sometimes referred to herein as “the PHY processor  130 ” for brevity). The PHY processor  130  includes a plurality of transceivers  134 , and the transceivers  134  are coupled to a plurality of antennas  138 . Although three transceivers  134  and three antennas  138  are illustrated in  FIG.  1   , the AP  114  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  134  and antennas  138  in other embodiments. In some embodiments, the AP  114  includes a higher number of antennas  138  than transceivers  134 , and antenna switching techniques are utilized. 
     The wireless network interface device  122  is implemented using one or more integrated circuits (ICs) configured to operate as discussed below. For example, the MAC processor  126  may be implemented, at least partially, on a first IC, and the PHY processor  130  may be implemented, at least partially, on a second IC. As another example, at least a portion of the MAC processor  126  and at least a portion of the PHY processor  130  may be implemented on a single IC. For instance, the wireless network interface device  122  may be implemented using a system on a chip (SoC), where the SoC includes at least a portion of the MAC processor  126  and at least a portion of the PHY processor  130 . 
     In an embodiment, the host processor  118  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a random access memory (RAM), a read-only memory (ROM), a flash memory, etc. In an embodiment, the host processor  118  may be implemented, at least partially, on a first IC, and the wireless network device  122  may be implemented, at least partially, on a second IC. As another example, the host processor  118  and at least a portion of the wireless network interface device  122  may be implemented on a single IC. 
     In various embodiments, the MAC processor  126  and/or the PHY processor  130  of the AP  114  are configured to generate data units, and process received data units, that conform to a WLAN communication protocol. For example, the MAC processor  126  is configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  130  is configured to implement PHY functions, including PHY functions of the WLAN communication protocol. For instance, the MAC processor  126  is configured to generate MAC layer data units such as MAC service data units (MSDUs), MAC protocol data units (MPDUs), etc., and provide the MAC layer data units to the PHY processor  130 , according to some embodiments. The PHY processor  130  is configured to receive MAC layer data units from the MAC processor  126  and encapsulate the MAC layer data units to generate PHY data units such as PHY protocol data units (PPDUs) for transmission via the antennas  138 , according to some embodiments. Similarly, the PHY processor  130  is configured to receive PHY data units that were received via the antennas  138 , and extract MAC layer data units encapsulated within the PHY data units, according to some embodiments. The PHY processor  130  provides the extracted MAC layer data units to the MAC processor  126 , which processes the MAC layer data units, according to some embodiments. 
     PHY data units are sometimes referred to herein as “packets”, and MAC layer data units are sometimes referred to herein as “frames”. 
     In connection with generating one or more RF signals for transmission, the PHY processor  130  is configured to process (which may include modulating, filtering, etc.) data corresponding to a PPDU to generate one or more digital baseband signals, and convert the digital baseband signal(s) to one or more analog baseband signals, according to an embodiment. Additionally, the PHY processor  130  is configured to upconvert the one or more analog baseband signals to one or more RF signals for transmission via the one or more antennas  138 . 
     In connection with receiving one or more RF signals, the PHY processor  130  is configured to downconvert the one or more RF signals to one or more analog baseband signals, and to convert the one or more analog baseband signals to one or more digital baseband signals. The PHY processor  130  is further configured to process (which may include demodulating, filtering, etc.) the one or more digital baseband signals to generate a PPDU. 
     The PHY processor  130  includes amplifiers (e.g., a low noise amplifier (LNA), a power amplifier, etc.), an RF downconverter, an RF upconverter, a plurality of filters, one or more analog-to-digital converters (ADCs), one or more digital-to-analog converters (DACs), one or more discrete Fourier transform (DFT) calculators (e.g., a fast Fourier transform (FFT) calculator), one or more inverse discrete Fourier transform (IDFT) calculators (e.g., an inverse fast Fourier transform (IFFT) calculator), one or more modulators, one or more demodulators, etc., that are not shown in  FIG.  1    for purposes of brevity. 
     The PHY processor  130  is configured to generate one or more RF signals that are provided to the one or more antennas  138 . The PHY processor  130  is also configured to receive one or more RF signals from the one or more antennas  138 . 
     The MAC processor  126  is configured to control the PHY processor  130  to generate one or more RF signals, for example, by providing one or more MAC layer data units (e.g., MPDUs) to the PHY processor  130 , and optionally providing one or more control signals to the PHY processor  130 , according to some embodiments. In an embodiment, the MAC processor  126  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a read ROM, a flash memory, etc. In another embodiment, the MAC processor  126  includes a hardware state machine. 
     The MAC processor  126  includes the C-OFDMA controller  60  and/or the C-OFDMA controller  70  of  FIG.  1 A . In some embodiments, the C-OFDMA controller  60  is configured to generate C-OFDMA-A frames and to prompt the PHY processor  130  to transmit the C-OFDMA-A frames, as will be described in more detail below. In some embodiments, the C-OFDMA controller  70  is configured to receive a C-OFDMA-A frame from another AP and to process the C-OFDMA frame, as will be described in more detail below. 
       FIG.  1 C  is a block diagram of an example client station  154  that may be used as one or more of the client stations  38 / 48  of  FIG.  1 A , in various embodiments. In other embodiments, one or more of the client stations  38 / 48  have a suitable structure different than the client station  154 . For example, one or more of the client stations  38 / 48  are legacy client stations that do not include the C-OFDMA controller  80  of  FIG.  1 A . 
     The client station  154  includes a host processor  158  coupled to a network interface device  162 . The network interface device  162  includes one or more MAC processors  166  (sometimes referred to herein as “the MAC processor  166 ” for brevity) and one or more PHY processors  170  (sometimes referred to herein as “the PHY processor  170 ” for brevity). The PHY processor  170  includes a plurality of transceivers  174 , and the transceivers  174  are coupled to a plurality of antennas  178 . Although three transceivers  174  and three antennas  178  are illustrated in  FIG.  1   , the client station  154  includes other suitable numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  174  and antennas  178  in other embodiments. In some embodiments, the client station  154  includes a higher number of antennas  178  than transceivers  174 , and antenna switching techniques are utilized. 
     The network interface device  162  is implemented using one or more ICs configured to operate as discussed below. For example, the MAC processor  166  may be implemented on at least a first IC, and the PHY processor  170  may be implemented on at least a second IC. As another example, at least a portion of the MAC processor  166  and at least a portion of the PHY processor  170  may be implemented on a single IC. For instance, the network interface device  162  may be implemented using an SoC, where the SoC includes at least a portion of the MAC processor  166  and at least a portion of the PHY processor  170 . 
     In an embodiment, the host processor  158  includes a processor configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a ROM, a flash memory, etc. In an embodiment, the host processor  158  may be implemented, at least partially, on a first IC, and the network device  162  may be implemented, at least partially, on a second IC. As another example, the host processor  158  and at least a portion of the network interface device  162  may be implemented on a single IC. 
     In various embodiments, the MAC processor  166  and the PHY processor  170  of the client station  154  are configured to generate data units, and process received data units, that conform to the WLAN communication protocol or another suitable communication protocol. For example, the MAC processor  166  is configured to implement MAC layer functions, including MAC layer functions of the WLAN communication protocol, and the PHY processor  170  is configured to implement PHY functions, including PHY functions of the WLAN communication protocol. The MAC processor  166  is configured to generate MAC layer data units such as MSDUs, MPDUs, etc., and provide the MAC layer data units to the PHY processor  170 , according to some embodiments. The PHY processor  170  is configured to receive MAC layer data units from the MAC processor  166  and encapsulate the MAC layer data units to generate PHY data units such as PPDUs for transmission via the antennas  178 , according to some embodiments. Similarly, the PHY processor  170  is configured to receive PHY data units that were received via the antennas  178 , and extract MAC layer data units encapsulated within the PHY data units, according to some embodiments. The PHY processor  170  provides the extracted MAC layer data units to the MAC processor  166 , which processes the MAC layer data units, according to some embodiments. 
     The PHY processor  170  is configured to downconvert one or more RF signals received via the one or more antennas  178  to one or more baseband analog signals, and convert the analog baseband signal(s) to one or more digital baseband signals, according to an embodiment. The PHY processor  170  is further configured to process the one or more digital baseband signals to demodulate the one or more digital baseband signals and to generate a PPDU. The PHY processor  170  includes amplifiers (e.g., an LNA, a power amplifier, etc.), an RF downconverter, an RF upconverter, a plurality of filters, one or more ADCs, one or more DACs, one or more DFT calculators (e.g., an FFT calculator), one or more IDFT calculators (e.g., an IFFT calculator), one or more modulators, one or more demodulators, etc., that are not shown in  FIG.  1    for purposes of brevity. 
     The PHY processor  170  is configured to generate one or more RF signals that are provided to the one or more antennas  178 . The PHY processor  170  is also configured to receive one or more RF signals from the one or more antennas  178 . 
     The MAC processor  166  is configured to control the PHY processor  170  to generate one or more RF signals by, for example, providing one or more MAC layer data units (e.g., MPDUs) to the PHY processor  170 , and optionally providing one or more control signals to the PHY processor  170 , according to some embodiments. In an embodiment, the MAC processor  166  includes a processor (not shown) configured to execute machine readable instructions stored in a memory device (not shown) such as a RAM, a ROM, a flash memory, etc. In an embodiment, the MAC processor  166  includes a hardware state machine (not shown). 
     The MAC processor  166  includes the C-OFDMA controller  80  of  FIG.  1 A . In some embodiments, the C-OFDMA controller  80  is configured to receives frames transmitted as part of setting up a C-OFDMA transmission, and to use information in such frames for purposes such as determining whether a communication medium is idle, according to various embodiments. 
       FIG.  2    is a diagram of an example C-OFDMA DL packet exchange  200  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  2    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA DL packet exchange  200  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     A master AP (e.g., the master AP  34 ) generates and transmits a C-OFDMA-A frame  204  to one or more slave APs (e.g., the slave AP  44 ). The C-OFDMA-A frame advertises a start of a DL C-OFDMA transmission involving multiple WLANs, according to an embodiment. The C-OFDMA-A frame  204  includes information regarding the DL C-OFDMA transmission such as one of, or any suitable combination of two or more of, i) indicators of one or more WLANs that are to participate in the DL C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN for the DL C-OFDMA transmission, iii) a respective frequency RU to be used in a respective WLAN for the DL C-OFDMA transmission, iv) a duration (in time) of the DL C-OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission (which is part of the DL C-OFDMA transmission) in a respective WLAN, etc., according to various embodiments. 
     The C-OFDMA-A frame  204  is configured to prompt one or more slave APs  44  to transmit respective DL OFDMA transmissions as part of the DL C-OFDMA transmission, according to some embodiments. 
     In an embodiment, the C-OFDMA-A frame  204  is a MAC layer data unit transmitted within a PHY data unit (e.g., a packet) not shown in  FIG.  2   . In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates, the C-OFDMA controller  60  generates, etc.) the C-OFDMA-A frame  204 . In an embodiment, the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) a packet that includes the C-OFDMA-A frame. In an embodiment, the C-OFDMA controller  60  generates the C-OFDMA-A frame  204 , provides the C-OFDMA-A frame  204  to the PHY processor  130 , and controls the PHY processor  130  to transmit the C-OFDMA-A frame  204  within a packet. 
     A defined time period after an end of transmission of the C-OFDMA-A frame  204  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  204 ), the master AP and one or more slave APs transmit as part of a DL C-OFDMA transmission  208 . In an embodiment, the defined time period is a short interframe space (SIFS) as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     Responsive to receiving the C-OFDMA-A frame  204  and as part of the DL C-OFDMA transmission, one or more slave APs generate and transmit respective downlink orthogonal frequency division multiple access (DL OFDMA) transmissions  212  in respective frequency RUs to respective one or more sets of client stations of the one or more slave APs. Although a DL-OFDMA transmission  212  from one slave AP is illustrated in  FIG.  2    to simplify the diagram, multiple slave APs transmit multiple DL OFDMA transmissions  212  in respective frequency RUs in some scenarios. 
     As an illustrative embodiment, in response to receiving the C-OFDMA-A frame  204 , the slave AP  44  determines (e.g., the network interface  122  determines, the MAC processor  126  determines, the C-OFDMA controller  70  determines, etc.) whether the slave AP  44  is to participate in the DL C-OFDMA transmission  208  by analyzing information in the C-OFDMA-A frame  204 , such as one or more indicators of one or more WLANs (e.g., one or more basic service set (BSS) identifiers) that are to participate in the DL C-OFDMA transmission  208 . In response to determining that the slave AP  44  is to participate in the C-OFDMA transmission  208 , the slave AP  44  determines (e.g., the network interface  122  determines, the MAC processor  126  determines, the C-OFDMA controller  70  determines, etc.) a frequency segment that the slave AP  44  is to use for the DL C-OFDMA transmission  208  by analyzing information in the C-OFDMA-A frame  204 , such as an indicator of the frequency segment to be used by the slave AP  44 , a frequency RU to be used by the slave AP  44 , etc. 
     Also in response to determining that the slave AP  44  is to participate in the C-OFDMA transmission  208 , the slave AP  44  generates the DL-OFDMA transmission  212 . In an embodiment, the slave AP  44  generates the DL-OFDMA transmission  212  according to parameters in the C-OFDMA-A frame  204  such as one of, or two or more of, an indicator of a duration (in time) of the DL C-OFDMA transmission  208 , an indicator of a length (in bits, octets, words, etc.) of the DL OFDMA transmission  212  by the slave AP  44 , etc., according to various embodiments. The AP  44  generates (e.g., the network interface  122  generates, the MAC processor  126  generates, etc.) a plurality of MAC data units for the DL OFDMA transmission  212  and provides the plurality of MAC data units to the PHY processor  130 , the plurality of MAC data units for client stations  48  in a WLAN managed by the slave AP  44 . The AP  44  also generates and transmits (e.g., the network interface  122  generates and transmits, the PHY processor  130  generates and transmits, etc.) the DL OFDMA transmission  212  to include the plurality of MAC data units. Thus, the DL OFDMA transmission  212  includes a plurality of MPDUs for client stations  48  in the WLAN managed by the slave AP  44 . In some embodiments, the DL OFDMA transmission  212  includes a multi-user multiple input, multiple output (MU-MIMO) transmissions to multiple client stations  48  via a plurality of spatial streams. In some embodiments, DL OFDMA transmission  212  is replaced by an MU-MIMO transmission to multiple client stations  48  via a plurality of spatial streams. 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the DL OFDMA transmission  212  so that the DL OFDMA transmission  212  begins substantially simultaneously (i.e., within 5% of) with a beginning of a DL OFDMA transmission  216  by the master AP  34 , according to an embodiment. For example, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the DL OFDMA transmission  212  so that the DL OFDMA transmission  212  begins a defined time period after an end of reception of the C-OFDMA-A frame  204  (or after an end of reception of the packet that includes the C-OFDMA-A frame  204 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     Simultaneous with the DL OFDMA transmission(s)  212  by the slave AP(s)  44 , the master AP  34  transmits the DL OFDMA transmission  216  in a frequency segment different than the frequency segment(s) used by the slave AP(s)  44  for the DL OFDMA transmission(s)  212 . The DL OFDMA transmission  216  is to a plurality of client stations  38  in a WLAN managed by the master AP  34 . The AP  34  generates (e.g., the network interface  122  generates, the MAC processor  126  generates, etc.) a plurality of MAC data units for the DL OFDMA transmission  216  and provides the plurality of MAC data units to the PHY processor  130 , the plurality of MAC data units for the plurality of client stations  38  in the WLAN managed by the master AP  34 . The AP  34  also generates and transmits (e.g., the network interface  122  generates and transmits, the PHY processor  130  generates and transmits, etc.) the DL OFDMA transmission  216  to include the plurality of MAC data units. Thus, the DL OFDMA transmission  216  includes a plurality of MPDUs for client stations  38  in the WLAN managed by the master AP  34 . In some embodiments, the DL OFDMA transmission  216  includes a multi-user multiple input, multiple output (MU-MIMO) transmissions to multiple client stations  38  via a plurality of spatial streams. In some embodiments, DL OFDMA transmission  216  is replaced by an MU-MIMO transmission to multiple client stations  38  via a plurality of spatial streams. 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the DL OFDMA transmission  216  so that the DL OFDMA transmission  216  begins substantially simultaneously (i.e., within 5% of) with a beginning of the DL OFDMA transmission(s)  212  by the slave AP(s)  44 , according to an embodiment. For example, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the DL OFDMA transmission  216  so that the DL OFDMA transmission  216  begins a defined time period after an end of transmission of the C-OFDMA-A frame  204  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  204 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In response to receiving the DL OFDMA transmission  212 , client stations  48  in the WLAN managed by the slave AP  44 , the client stations  48  transmit acknowledgment (ACK) information and/or block acknowledgment (BA) information in an UL transmission  232 . In an embodiment, the UL transmission  232  is transmitted in the same frequency segment in which the DL OFDMA transmission  212  was transmitted. 
     The slave AP  44  receives (e.g., the network interface  122  receives, the MAC processor  126  receives, the PHY processor  130  receives, etc.) the UL transmission  232 . In an embodiment, slave AP  44  receives the UL transmission  232  via the same frequency segment in which the DL OFDMA transmission  212  was transmitted. 
     In response to receiving the DL OFDMA transmission  216 , client stations  38  in the WLAN managed by the master AP  34 , the client stations  38  transmit ACK information and/or BA information in an UL transmission  236 . In an embodiment, the UL transmission  236  is transmitted in the same frequency segment in which the DL OFDMA transmission  216  was transmitted. 
     The master AP  34  receives (e.g., the network interface  122  receives, the MAC processor  126  receives, the PHY processor  130  receives, etc.) the UL transmission  236 . In an embodiment, master AP  34  receives the UL transmission  236  via the same frequency segment in which the DL OFDMA transmission  216  was transmitted. 
     The UL transmission  236  and the UL transmission(s)  232  by the slave AP(s)  44  are transmitted simultaneously, in an embodiment. 
     In an embodiment, a duration of the UL transmission  232  is specified in the C-OFDMA-A frame  204 . For example, the C-OFDMA-A frame  204  includes an indication of the duration of the UL transmission  232 , according to an embodiment. 
     In an embodiment, the slave AP  44  includes (e.g., the network interface  122  includes, the MAC processor  126  includes, the C-OFDMA controller  70  includes, etc.) the indicator of the duration of the UL transmission  232  in the DL OFDMA transmission  212 , and the client station  48  uses (e.g., the network interface  122  uses, the MAC processor  126  uses, the C-OFDMA controller  80  uses, etc.) the indicator of the duration of the UL transmission  232  to generate the UL transmission  232  to have the indicated duration. In another embodiment, the client station  48  receives the C-OFDMA-A frame  204  and uses (e.g., the network interface  122  uses, the MAC processor  126  uses, the C-OFDMA controller  80  uses, etc.) the indicator of the duration of the UL transmission  232  in the C-OFDMA-A frame  204  to generate the UL transmission  232  to have the indicated duration. 
       FIG.  3    is a diagram of another example C-OFDMA DL packet exchange  300  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to another embodiment.  FIG.  3    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA DL packet exchange  300  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     In the packet exchange  300 , the slave AP  44  generates and transmits an ACK  304  that acknowledges the C-OFDMA-A frame  204  in response to receiving the C-OFDMA-A frame  204 . In an embodiment, the slave AP  44  generates and transmits a packet that includes the ACK  304 , the packet spanning a same frequency bandwidth that the C-OFDMA-A frame  204  spans. When the C-OFDMA-A frame  204  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective ACKs  304  via different spatial streams using UL MU-MIMO, the respective transmissions spanning the same frequency bandwidth that the C-OFDMA-A frame  204  spans, according to an embodiment. For example, in an embodiment, the C-OFDMA-A frame  204  indicates respective spatial streams that the multiple slave APs  44  are to use to transmit the ACKs  304 . 
     In another embodiment, when the C-OFDMA-A frame  204  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective ACKs  304  at different times, the respective transmissions spanning the same frequency bandwidth that the C-OFDMA-A frame  204  spans. For example, in an embodiment, the C-OFDMA-A frame  204  indicates an order in which the multiple slave APs  44  are to transmit the ACKs  304 . 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the ACK  304  so that transmission of the ACK  304  (or a packet that includes the ACK  304 ) begins a defined time period after an end of reception of the C-OFDMA-A frame  204  (or after an end of reception of the packet that includes the C-OFDMA-A frame  204 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the DL OFDMA transmission  212  so that transmission begins a defined time period after an end of transmission of the ACK  304  (or after an end of transmission of the packet that includes the ACK  304 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. When multiple slave APs  44  transmit multiple ACKs  304  at different times, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the DL OFDMA transmission  212  so that transmission begins a defined time period after an end of transmission of the last occurring ACK  304  (or after an end of transmission of the packet that includes the last occurring ACK  304 ). 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the DL OFDMA transmission  216  so that transmission begins a defined time period after an end of transmission of the ACK  304  (or after an end of transmission of the packet that includes the ACK  304 ). When multiple slave APs  44  transmit multiple ACKs  304  at different times, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the DL OFDMA transmission  216  so that transmission begins a defined time period after an end of transmission of the last occurring ACK  304  (or after an end of transmission of the packet that includes the last occurring ACK  304 ). 
       FIG.  4    is a diagram of yet another example C-OFDMA DL packet exchange  400  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to yet another embodiment. In some embodiments, the C-OFDMA DL packet exchange  400  is useful in situations involving a channel switch in one or more WLANs participating in the C-OFDMA transmission. 
       FIG.  4    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA DL packet exchange  400  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     In the packet exchange  400 , the slave AP  44  generates and transmits a C-OFDMA-A frame  404  in response to receiving the C-OFDMA-A frame  204 . In an embodiment, the C-OFDMA-A frame  404  is a copy of the C-OFDMA-A frame  204 . The slave AP  44  generates and transmits a packet that includes the OFDMA-A frame  404 , the packet spanning a frequency segment indicated in the C-OFDMA-A frame  204  (e.g., the frequency segment that the slave AP  44  is to use for the C-OFDMA transmission  208 . When the C-OFDMA-A frame  204  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective C-OFDMA-A frames  404  in respective frequency segments, the respective C-OFDMA-A frames  404  being copies of the C-OFDMA-A frame  204 , according to an embodiment. For example, in an embodiment, the C-OFDMA-A frame  204  indicates respective frequency segments that the multiple slave APs  44  are to use for the C-OFDMA transmission  208 . 
     Additionally, the master AP  34  generates a C-OFDMA-A frame  408 , and transmits the C-OFDMA-A frame  408  (e.g., within a packet) simultaneously with transmission of the C-OFDMA-A frame  404 . In an embodiment, the C-OFDMA-A frame  408  is a copy of the C-OFDMA-A frame  204 . 
     In an embodiment, generating a packet that includes the C-OFDMA-A frame  204  includes scrambling (e.g., by a scrambler circuit of the PHY processor  130 ) the C-OFDMA-A frame  204  according to a scrambling algorithm and using a first scrambling seed (e.g., an initial value to seed the scrambling algorithm implemented by the scrambler circuit); and generating a packet that includes the C-OFDMA-A frame  404 / 408  includes scrambling (e.g., by a scrambler circuit of the PHY processor  130 ) the C-OFDMA-A frame  404 / 408  according to the scrambling algorithm and using a second scrambling seed (e.g., an initial value to seed the scrambling algorithm implemented by the scrambler circuit). In an embodiment, the first scrambling seed is the same as the second scrambling seed. In another embodiment, the first scrambling seed is different than the second scrambling seed. In an embodiment, generating a packet that includes the C-OFDMA-A frame  404 / 408  includes using one of, or any suitable combination of two or more of: i) a same modulation and coding scheme (MCS) used for the packet that included the C-OFDMA-A frame  204 , ii) a same data rate used for the packet that included the C-OFDMA-A frame  204 , iii) a same number of spatial streams used for the packet that included the C-OFDMA-A frame  204 , iv) a same PPDU format used for the packet that included the C-OFDMA-A frame  204 , etc. 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the C-OFDMA-A frame  404  so that transmission of the C-OFDMA-A frame  404  (or a packet that includes the C-OFDMA-A frame  404 ) begins a defined time period after an end of reception of the C-OFDMA-A frame  204  (or after an end of reception of the packet that includes the C-OFDMA-A frame  204 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the C-OFDMA-A frame  408  so that transmission of the C-OFDMA-A frame  408  (or a packet that includes the C-OFDMA-A frame  408 ) begins a defined time period after an end of transmission of the C-OFDMA-A frame  204  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  204 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     After transmission of the C-OFDMA-A frame  404  and the C-OFDMA-A frame  408 , the master AP transmits a C-OFDMA trigger frame  420  to prompt the slave AP(s)  44  to transmit as part of the C-OFDMA transmission  208 . In an embodiment, the C-OFDMA trigger frame  420  includes some or all of the same information included in the C-OFDMA-A frame  204 , such as one of, or any suitable combination of two or more of: i) indicators of one or more WLANs that are to participate in the DL C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN for the DL C-OFDMA transmission, iii) a respective frequency RU to be used in a respective WLAN for the DL C-OFDMA transmission, iv) a duration (in time) of the DL C-OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission (which is part of the DL C-OFDMA transmission) in a respective WLAN, etc., according to various embodiments. 
     The C-OFDMA trigger frame  420  is configured to prompt one or more slave APs  44  to transmit respective DL OFDMA transmissions as part of the DL C-OFDMA transmission  208 , according to some embodiments. 
     In an embodiment, the C-OFDMA trigger frame  420  is a MAC layer data unit transmitted within a PHY data unit (e.g., a packet) not shown in  FIG.  2   . In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates, the C-OFDMA controller  60  generates, etc.) the C-OFDMA trigger frame  420 . In an embodiment, the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) a packet that includes the C-OFDMA trigger frame  420 . In an embodiment, the C-OFDMA controller  60  generates the C-OFDMA trigger frame  420 , provides the C-OFDMA trigger frame  420  to the PHY processor  130 , and controls the PHY processor  130  to transmit the C-OFDMA trigger frame  420  within a packet. 
     A defined time period after an end of transmission of the C-OFDMA trigger frame  420  (or after an end of transmission of the packet that includes the C-OFDMA trigger frame  420 ), the master AP and one or more slave APs transmit as part of the DL C-OFDMA transmission  208 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
       FIG.  5    is a diagram of an example acknowledgment packet exchange  500  for a DL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  5    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the acknowledgment packet exchange  500  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The acknowledgment packet exchange  500  is used in connection with any of the DL C-OFDMA transmissions of  FIGS.  2 - 4   , or with other suitable DL C-OFDMA transmissions, according to various embodiments. 
     In the acknowledgment packet exchange  500 , respective sets of client stations corresponding to respective WLANs transmit respective acknowledgement information at different times. In some embodiments, the C-OFDMA-A frame  204  includes an indication of an order in which slave APs  44  are to prompt respective sets of client stations to transmit respective acknowledgement information. In some embodiments that utilize a C-OFDMA trigger frame  420  ( FIG.  4   ), the C-OFDMA trigger frame  420  additionally or alternatively includes the indication of the order in which slave APs  44  are to prompt respective sets of client stations to transmit respective acknowledgement information. 
     After the DL C-OFDMA transmission  208 , the master AP  34  generates and transmits a multi-user block acknowledgment request (MU-BAR) frame  504 . In an embodiment, the MU-BAR frame  504  is included in a packet (not shown). In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates) the MU-BAR frame  504 , and the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) the packet that includes the MU-BAR frame  504 . The MU-BAR frame  504  is configured to prompt client stations  38  of the WLAN managed by the master AP  34  to transmit to the master AP  34  acknowledgment information regarding the DL OFDMA transmission  216  in an UL transmission  508  (e.g., an UL OFDMA transmission, an UL MU-MIMO transmission, etc.). Responsive to the MU-BAR frame  504 , client stations  38  of the WLAN managed by the master AP  34  transmit acknowledgment information regarding the DL OFDMA transmission  216  in the UL transmission  508 . 
     In an embodiment, the packet including the MU-BAR frame  504  and the UL transmission  508  are transmitted in a same frequency segment in which the DL OFDMA transmission  216  was transmitted. 
     After the UL transmission  508 , the slave AP  44  generates and transmits an MU-BAR frame  520 . In an embodiment, the MU-BAR frame  520  is included in a packet (not shown). In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates) the MU-BAR frame  520 , and the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) the packet that includes the MU-BAR frame  520 . The MU-BAR frame  520  is configured to prompt client stations  48  of the WLAN managed by the slave AP  44  to transmit to the slave AP  44  acknowledgment information regarding the DL OFDMA transmission  212  in an UL transmission  524  (e.g., an UL OFDMA transmission, an UL MU-MIMO transmission, etc.). Responsive to the MU-BAR frame  520 , client stations  48  of the WLAN managed by the slave AP  44  transmit acknowledgment information regarding the DL OFDMA transmission  212  in the UL transmission  524 . 
     In an embodiment, the packet including the MU-BAR frame  520  and the UL transmission  524  are transmitted in a same frequency segment in which the DL OFDMA transmission  212  was transmitted. 
       FIG.  6    is a diagram of another example acknowledgment packet exchange  600  for a DL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to another embodiment.  FIG.  6    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the acknowledgment packet exchange  600  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The acknowledgment packet exchange  600  is used in connection any of the DL C-OFDMA transmissions of  FIGS.  2 - 4   , or with other suitable DL C-OFDMA transmissions, according to various embodiments. 
     In the acknowledgment packet exchange  600 , respective sets of client stations corresponding to respective WLANs transmit respective acknowledgement information simultaneously as part of an UL C-OFDMA transmission. 
     After the DL C-OFDMA transmission  208 , the master AP  34  and the slave AP(s)  44  transmit MU-BAR frames as part of a further DL C-OFDMA transmission  604 . In an embodiment, the packet including the MU-BAR frame  504  and the UL transmission  508  are transmitted in a same frequency segment in which the DL OFDMA transmission  216  was transmitted; and the packet including the MU-BAR frame  520  and the UL transmission  524  are transmitted in a same frequency segment in which the DL OFDMA transmission  212  was transmitted. 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the MU-BAR frame  504  so that transmission of the MU-BAR frame  504  (or a packet that includes the MU-BAR frame  504 ) begins a defined time period after an end of transmission of the DL OFDMA transmission  216 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the MU-BAR frame  520  so that transmission of the MU-BAR frame  520  (or a packet that includes the MU-BAR frame  520 ) begins a defined time period after an end of transmission of the DL OFDMA transmission  212 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
       FIG.  7    is a diagram of yet another example acknowledgment packet exchange  700  for a DL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to another embodiment.  FIG.  7    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the acknowledgment packet exchange  700  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The acknowledgment packet exchange  700  is used in connection any of the DL C-OFDMA transmissions of  FIGS.  2 - 4   , or with other suitable DL C-OFDMA transmissions, according to various embodiments. 
     The acknowledgment packet exchange  700  is similar to the acknowledgment packet exchange  600  of  FIG.  6   , except that the master AP  34  generates and transmits a further C-OFDMA-A frame  704  in connection with the DL C-OFDMA transmission  604 . 
       FIG.  8    is a diagram of an example C-OFDMA uplink (UL) packet exchange  800  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  8    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA UL packet exchange  800  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     A master AP (e.g., the master AP  34 ) generates and transmits a C-OFDMA-A frame  804  to one or more slave APs (e.g., the slave AP  44 ). The C-OFDMA-A frame advertises a start of a UL C-OFDMA transmission involving multiple WLANs, according to an embodiment. The C-OFDMA-A frame  804  includes information regarding the UL C-OFDMA transmission such as one of, or any suitable combination of two or more of, i) indicators of one or more WLANs that are to participate in the UL C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN for the UL C-OFDMA transmission, iii) a respective frequency RU to be used in a respective WLAN for the UL C-OFDMA transmission, iv) a duration (in time) of the UL C-OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission (which is part of the UL C-OFDMA transmission) in a respective WLAN, etc., according to various embodiments. 
     The C-OFDMA-A frame  804  is configured to prompt one or more slave APs  44  to transmit respective trigger frames to prompt respective sets of client stations to transmit as part of the UL C-OFDMA transmission, according to some embodiments. 
     In an embodiment, the C-OFDMA-A frame  804  is a MAC layer data unit transmitted within a PHY data unit (e.g., a packet) not shown in  FIG.  8   . In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates, the C-OFDMA controller  60  generates, etc.) the C-OFDMA-A frame  804 . In an embodiment, the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) a packet that includes the C-OFDMA-A frame  804 . In an embodiment, the C-OFDMA controller  60  generates the C-OFDMA-A frame  804 , provides the C-OFDMA-A frame  804  to the PHY processor  130 , and controls the PHY processor  130  to transmit the C-OFDMA-A frame  804  within a packet. 
     A defined time period after an end of transmission of the C-OFDMA-A frame  804  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  204 ), the master AP and one or more slave APs transmit as part of a DL C-OFDMA transmission  808 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     Responsive to receiving the C-OFDMA-A frame  804  and as part of the DL C-OFDMA transmission  808 , one or more slave APs generate and transmit respective trigger frames  820 / 824  in respective frequency RUs to respective one or more sets of client stations of the one or more slave APs. Although one trigger frame  824  from one slave AP is illustrated in  FIG.  8    to simplify the diagram, multiple slave APs transmit multiple trigger frames in respective frequency RUs in some scenarios. 
     As an illustrative embodiment, in response to receiving the C-OFDMA-A frame  804 , the slave AP  44  determines (e.g., the network interface  122  determines, the MAC processor  126  determines, the C-OFDMA controller  70  determines, etc.) whether the slave AP  44  is to participate in the DL C-OFDMA transmission  808  by analyzing information in the C-OFDMA-A frame  804 , such as one or more indicators of one or more WLANs (e.g., one or more BSS identifiers) that are to participate in the UL C-OFDMA transmission announced by the C-OFDMA-A frame  804 . In response to determining that the slave AP  44  is to participate in the DL C-OFDMA transmission  808 , the slave AP  44  determines (e.g., the network interface  122  determines, the MAC processor  126  determines, the C-OFDMA controller  70  determines, etc.) a frequency segment that the slave AP  44  is to use for the DL C-OFDMA transmission  808  by analyzing information in the C-OFDMA-A frame  804 , such as an indicator of the frequency segment to be used by the slave AP  44 , a frequency RU to be used by the slave AP  44 , etc. 
     Also in response to determining that the slave AP  44  is to participate in the DL C-OFDMA transmission  808 , the slave AP  44  generates (e.g., the network interface  122  generates, the MAC processor  126  generates, etc.) the trigger frame  824 . In an embodiment, the slave AP  44  generates the trigger frame  824  according to parameters in the such as one of, or two or more of, an indicator of a frequency RU to be used for the UL C-OFDMA transmission, an indicator of a duration (in time) of the UL C-OFDMA transmission, etc., according to various embodiments. For example, the trigger frame  824  is generated to specify respective frequency RUs, within the frequency RU indicated by the C-OFDMA frame  804 , that client stations of the slave  44  are to use for the UL C-OFDMA transmission, according to an embodiment. As another example, the trigger frame  824  is generated to specify a duration of the UL C-OFDMA transmission indicated by the C-OFDMA-A frame  804 , according to an embodiment. 
     Also in response to determining that the slave AP  44  is to participate in the DL C-OFDMA transmission  808 , the slave AP  44  generates and transmits (e.g., the network interface  122  generates and transmits, the PHY processor  130  generates and transmits, etc.) a packet that includes the trigger frame  824 . 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the trigger frame  824  (or of transmission of the packet that includes the trigger frame  824 ) so that transmission of the trigger frame  824  (or of the packet that includes the trigger frame  824 ) begins substantially simultaneously (i.e., within 5% of) with a beginning of a transmission by the master AP  34  of the trigger frame  820  (or of a packet that includes the trigger frame  820 ), according to an embodiment. For example, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the packet that includes the trigger frame  824  so that the packet begins a defined time period after an end of reception of the C-OFDMA-A frame  804  (or after an end of reception of the packet that includes the C-OFDMA-A frame  804 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     Simultaneous with the transmission of the trigger frame(s)  824  by the slave AP(s)  44 , the master AP  34  transmits the trigger frame  820  (or a packet that includes the trigger frame  820 ) in a frequency segment different than the frequency segment(s) used by the slave AP(s)  44  for the trigger frame(s)  824 . In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the trigger frame  820  (or the packet that includes the trigger frame  820 ) so that transmission of the trigger frame  820  (or the packet that includes the trigger frame  820 ) begins substantially simultaneously (i.e., within 5% of) with a beginning of the transmission of the trigger packet  824  (or the packet that includes the trigger frame  824 ) by the slave AP(s)  44 , according to an embodiment. For example, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the trigger frame  820  so that the trigger frame  820  (or the packet that includes the trigger frame  820 ) begins a defined time period after an end of transmission of the C-OFDMA-A frame  804  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  804 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     The trigger frame  820  from the master AP  34  and the trigger frame(s)  824  from the slave AP(s)  44  prompt an UL C-OFDMA transmission  812  by client stations  34 / 38  in WLANs managed by the master AP  34  and the slave AP(s)  44 . The UL C-OFDMA transmission  812  comprises an UL OFDMA transmission  840  by client stations  38  in the WLAN managed by the master AP  34 , and one or more UL OFDMA transmission(s)  844  by client stations  38  in one or more respective WLANs managed by one or more respective slave APs  44 . 
     For instance, responsive to the trigger frame  820 , client stations  38  in the WLAN managed by the master AP  34  transmit as part of the UL OFDMA transmission  840 . For example, the trigger frame  820  transmitted by the master AP  34  is configured to prompt at least a subset of client station  38  to transmit as part of an UL OFDMA transmission  840 . In various embodiments, the trigger frame  820  is generated by the master AP  34  indicate one of, or any suitable combination of two or more of: i) which client stations  38  are to participate in the UL OFDMA transmission  840 , ii) respective frequency RUs client stations  38  are to use for the UL OFDMA transmission  840 , iii) respective spatial streams client stations  38  are to use for the UL OFDMA transmission  840 , iv) a duration of the UL OFDMA transmission  840 . 
     Similarly, responsive to the trigger frame  824 , client stations  48  in the WLAN managed by the slave AP  44  transmit as part of the UL OFDMA transmission  844 . For example, the trigger frame  824  transmitted by the slave AP  44  is configured to prompt at least a subset of client station  48  to transmit as part of an UL OFDMA transmission  844 . In various embodiments, the trigger frame  824  is generated by the slave AP  44  to indicate one of, or any suitable combination of two or more of: i) which client stations  38  are to participate in the UL OFDMA transmission  844 , ii) respective frequency RUs client stations  38  are to use for the UL OFDMA transmission  844 , iii) respective spatial streams client stations  38  are to use for the UL OFDMA transmission  844 , iv) a duration of the UL OFDMA transmission  844 . 
     Client stations  38  participating in the UL OFDMA transmission  840  are configured to transmit, as part of the UL OFDMA transmission  840 , simultaneously with transmissions by client stations  48  participating in the UL OFDMA transmission  844 , and vice versa, according to an embodiment. For example, client stations  38  participating in the UL OFDMA transmission  840  are configured to begin transmitting, as part of the UL OFDMA transmission  840 , a defined time period (e.g., SIFS or another suitable time period) after an end of reception of the trigger frame  820  (or an end of reception of a packet that includes the trigger frame  820 ), according to an embodiment. Similarly, client stations  48  participating in the UL OFDMA transmission  844  are configured to begin transmitting, as part of the UL OFDMA transmission  844 , a defined time period (e.g., SIFS or another suitable time period) after an end of reception of the trigger frame  824  (or an end of reception of a packet that includes the trigger frame  824 ), according to an embodiment. 
       FIG.  9    is a diagram of another example C-OFDMA UL packet exchange  900  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to another embodiment.  FIG.  9    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA DL packet exchange  900  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     In the packet exchange  900 , the slave AP  44  generates and transmits an ACK  904  that acknowledges the C-OFDMA-A frame  804  in response to receiving the C-OFDMA-A frame  804 . In an embodiment, the slave AP  44  generates and transmits a packet that includes the ACK  904 , the packet spanning a same frequency bandwidth that the C-OFDMA-A frame  804  spans. When the C-OFDMA-A frame  804  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective ACKs  904  via different spatial streams using UL MU-MIMO, the respective transmissions spanning the same frequency bandwidth that the C-OFDMA-A frame  804  spans, according to an embodiment. For example, in an embodiment, the C-OFDMA-A frame  804  indicates respective spatial streams that the multiple slave APs  44  are to use to transmit the ACKs  904 . 
     In another embodiment, when the C-OFDMA-A frame  804  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective ACKs  904  at different times, the respective transmissions spanning the same frequency bandwidth that the C-OFDMA-A frame  804  spans. For example, in an embodiment, the C-OFDMA-A frame  804  indicates an order in which the multiple slave APs  44  are to transmit the ACKs  904 . 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the ACK  904  so that transmission of the ACK  904  (or a packet that includes the ACK  904 ) begins a defined time period after an end of reception of the C-OFDMA-A frame  804  (or after an end of reception of the packet that includes the C-OFDMA-A frame  804 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the trigger frame  824  (or a packet that includes the trigger frame  824 ) so that transmission begins a defined time period after an end of transmission of the ACK  904  (or after an end of transmission of the packet that includes the ACK  904 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. When multiple slave APs  44  transmit multiple ACKs  904  at different times, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the trigger frame  824  (or a packet that includes the trigger frame  824 ) so that transmission begins a defined time period after an end of transmission of the last occurring ACK  904  (or after an end of transmission of the packet that includes the last occurring ACK  904 ). 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the trigger frame  820  (or a packet that includes the trigger frame  820 ) so that transmission begins a defined time period after an end of transmission of the ACK  904  (or after an end of transmission of the packet that includes the ACK  904 ). When multiple slave APs  44  transmit multiple ACKs  904  at different times, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the trigger frame  820  (or a packet that includes the trigger frame  820 ) so that transmission begins a defined time period after an end of transmission of the last occurring ACK  904  (or after an end of transmission of the packet that includes the last occurring ACK  904 ). 
       FIG.  10    is a diagram of yet another example C-OFDMA UL packet exchange  1000  in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to yet another embodiment. In some embodiments, the C-OFDMA DL packet exchange  1000  is useful in situations involving a channel switch in one or more WLANs participating in the C-OFDMA transmission. 
       FIG.  10    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the C-OFDMA DL packet exchange  1000  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     In the packet exchange  1000 , the slave AP  44  generates and transmits a C-OFDMA-A frame  1004  in response to receiving the C-OFDMA-A frame  804 . In an embodiment, the C-OFDMA-A frame  1004  is a copy of the C-OFDMA-A frame  804 . The slave AP  44  generates and transmits a packet that includes the C-OFDMA-A frame  1004 , the packet spanning a frequency segment indicated in the C-OFDMA-A frame  804  (e.g., the frequency segment that the WLAN managed by the slave AP  44  is to use for the UL OFDMA transmission  844 ). When the C-OFDMA-A frame  804  is addressed to multiple slave APs  44 , the multiple slave APs  44  transmit respective C-OFDMA-A frames  1004  in respective frequency segments, the respective C-OFDMA-A frames  1004  being copies of the C-OFDMA-A frame  804 , according to an embodiment. For example, in an embodiment, the C-OFDMA-A frame  804  indicates respective frequency segments that the multiple slave APs  44  are to use for the C-OFDMA transmission  1004 . 
     Additionally, the master AP  34  generates a C-OFDMA-A frame  1008 , and transmits the C-OFDMA-A frame  1008  (e.g., within a packet) simultaneously with transmission of the C-OFDMA-A frame  1004 . In an embodiment, the C-OFDMA-A frame  1008  is a copy of the C-OFDMA-A frame  804 . 
     In an embodiment, generating a packet that includes the C-OFDMA-A frame  804  includes scrambling (e.g., by a scrambler circuit of the PHY processor  130 ) the C-OFDMA-A frame  804  according to a scrambling algorithm and using a first scrambling seed (e.g., an initial value to seed the scrambling algorithm implemented by the scrambler circuit); and generating a packet that includes the C-OFDMA-A frame  1004 / 1008  includes scrambling (e.g., by a scrambler circuit of the PHY processor  130 ) the C-OFDMA-A frame  1004 / 1008  according to the scrambling algorithm and using a second scrambling seed (e.g., an initial value to seed the scrambling algorithm implemented by the scrambler circuit). In an embodiment, the first scrambling seed is the same as the second scrambling seed. In another embodiment, the first scrambling seed is different than the second scrambling seed. In an embodiment, generating a packet that includes the C-OFDMA-A frame  1004 / 1008  includes using one of, or any suitable combination of two or more of: i) a same MCS used for the packet that included the C-OFDMA-A frame  804 , ii) a same data rate used for the packet that included the C-OFDMA-A frame  804 , iii) a same number of spatial streams used for the packet that included the C-OFDMA-A frame  804 , iv) a same PPDU format used for the packet that included the C-OFDMA-A frame  804 , etc. 
     In an embodiment, the slave AP  44  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  70  controls, etc.) timing of the transmission of the C-OFDMA-A frame  1004  so that transmission of the C-OFDMA-A frame  1004  (or a packet that includes the C-OFDMA-A frame  1004 ) begins a defined time period after an end of reception of the C-OFDMA-A frame  804  (or after an end of reception of the packet that includes the C-OFDMA-A frame  804 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In an embodiment, the master AP  34  controls (e.g., the network interface  122  controls, the MAC processor  126  controls, the C-OFDMA controller  60  controls, etc.) timing of the transmission of the C-OFDMA-A frame  1008  so that transmission of the C-OFDMA-A frame  1008  (or a packet that includes the C-OFDMA-A frame  1008 ) begins a defined time period after an end of transmission of the C-OFDMA-A frame  804  (or after an end of transmission of the packet that includes the C-OFDMA-A frame  804 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     After transmission of the C-OFDMA-A frame  1004  and the C-OFDMA-A frame  1008 , the master AP transmits a C-OFDMA trigger frame  1020  to prompt the slave AP(s)  44  to transmit as part of the C-OFDMA transmission  808 . In an embodiment, the C-OFDMA trigger frame  1020  includes some or all of the same information included in the C-OFDMA-A frame  804 , such as one of, or any suitable combination of two or more of: i) indicators of one or more WLANs that are to participate in the DL C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN for the DL C-OFDMA transmission, iii) a respective frequency RU to be used in a respective WLAN for the DL C-OFDMA transmission, iv) a duration (in time) of the DL C-OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission (which is part of the DL C-OFDMA transmission) in a respective WLAN, etc., according to various embodiments. 
     The C-OFDMA trigger frame  1020  is configured to prompt one or more slave APs  44  to transmit respective DL OFDMA transmissions as part of the DL C-OFDMA transmission  808 , according to some embodiments. 
     In an embodiment, the C-OFDMA trigger frame  1020  is a MAC layer data unit transmitted within a PHY data unit (e.g., a packet) not shown in  FIG.  2   . In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates, the C-OFDMA controller  60  generates, etc.) the C-OFDMA trigger frame  1020 . In an embodiment, the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) a packet that includes the C-OFDMA trigger frame  1020 . In an embodiment, the C-OFDMA controller  60  generates the C-OFDMA trigger frame  1020 , provides the C-OFDMA trigger frame  1020  to the PHY processor  130 , and controls the PHY processor  130  to transmit the C-OFDMA trigger frame  1020  within a packet. 
     A defined time period after an end of transmission of the C-OFDMA trigger frame  1020  (or after an end of transmission of the packet that includes the C-OFDMA trigger frame  1020 ), the master AP and one or more slave APs transmit as part of the DL C-OFDMA transmission  808 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
       FIG.  11    is a diagram of an example acknowledgment packet exchange  1100  for an UL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  11    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the acknowledgment packet exchange  1100  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The acknowledgment packet exchange  1100  is used in connection any of the UL C-OFDMA transmissions of  FIGS.  8 - 10   , or with other suitable UL C-OFDMA transmissions, according to various embodiments. 
     A defined time period after an end of transmission of the UL C-OFDMA transmission  812 , the master AP  34  begins transmitting a packet that includes the C-OFDMA-A frame  1104 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     The master AP  34  generates a C-OFDMA-A frame  1104  and, after an end of transmission of the UL C-OFDMA transmission  812 , the master AP  34  transmits the C-OFDMA-A frame  1104  to one or more slave APs (e.g., the slave AP  44 ). The C-OFDMA-A frame  1104  advertises a start of the DL C-OFDMA transmission  856 , according to an embodiment. The C-OFDMA-A frame  1104  includes information regarding the DL C-OFDMA transmission  856  such as one of, or any suitable combination of two or more of, i) indicators of one or more WLANs that are to participate in the DL C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN for the DL C-OFDMA transmission, iii) a respective frequency RU to be used in a respective WLAN for the DL C-OFDMA transmission, iv) a duration (in time) of the DL C-OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission (which is part of the DL C-OFDMA transmission) in a respective WLAN, etc., according to various embodiments. In some embodiments, the RUs used for the DL C-OFDMA transmission  856  are the same as the RUs used for the UL C-OFDMA transmission  812 . 
     The C-OFDMA-A frame  1104  is configured to prompt one or more slave APs  44  to transmit respective ACK or BA frames regarding the UL C-OFDMA transmission  812 , according to some embodiments. 
     In an embodiment, the C-OFDMA-A frame  1104  is a MAC layer data unit transmitted within a PHY data unit (e.g., a packet) not shown in  FIG.  11   . In an embodiment, the network interface device  122  generates (e.g., the MAC processor  126  generates, the C-OFDMA controller  60  generates, etc.) the C-OFDMA-A frame  1104 . In an embodiment, the network interface device  122  generates and transmits (e.g., the PHY processor  130  generates and transmits) a packet that includes the C-OFDMA-A frame  1104 . In an embodiment, the C-OFDMA controller  60  generates the C-OFDMA-A frame  1104 , provides the C-OFDMA-A frame  1104  to the PHY processor  130 , and controls the PHY processor  130  to transmit the C-OFDMA-A frame  1104  within a packet. 
     In an embodiment, transmission of a packet that includes the C-OFDMA-A frame begins a defined time period after an end of the UL C-OFDMA transmission  812 . In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     Responsive to receiving the C-OFDMA-A frame  1104  and as part of the DL C-OFDMA transmission  856 , one or more slave APs generate and transmit respective ACK or BA frames  860 / 864  in respective frequency RUs to respective one or more sets of client stations of the one or more slave APs. In an embodiment, the DL C-OFDMA transmission  856  begins a defined time period after an end of transmission of the C-OFDMA-A frame  1104  (or after an end of transmission of a packet that includes the C-OFDMA-A frame  1104 ). In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
       FIG.  12    is a diagram of an example packet exchange  1200  involving an UL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  12    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the packet exchange  1200  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The packet exchange  1200  is used in conjunction with any of the UL C-OFDMA transmissions of  FIGS.  8 - 10   , or is used in conjunction with other suitable UL C-OFDMA transmissions, according to various embodiments. 
     As a variation from the UL C-OFDMA packet exchanges of  FIGS.  8 - 10   , a DL C-OFDMA transmission  1204  immediately follows the UL C-OFDMA transmission  812 , where the DL C-OFDMA transmission  1204  does not merely contain ACK/BA information for the UL C-OFDMA transmission  812 . A defined time period after an end of transmission of the UL C-OFDMA transmission  812 , the master AP  34  begins a DL-OFDMA transmission  1220 , and the slave AP  44  begins a DL-OFDMA transmission  1224 . The DL-OFDMA transmission  1220  does not merely contain ACK/BA information for the UL C-OFDMA transmission  812 , and the DL-OFDMA transmission  1224  also does not merely contain ACK/BA information for the UL C-OFDMA transmission  812 . For example, the DL-OFDMA transmission  1220  includes user data for client stations  38 , and the DL-OFDMA transmission  1224  includes user data for client stations  48 , according to an embodiment. 
     In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In various embodiments, the DL C-OFDMA transmission  1204  is acknowledged (not shown in  FIG.  12   ) by client stations  38 / 48  using acknowledgment techniques such as described with reference any of  FIGS.  2 - 7   , or using other suitable acknowledgment techniques. 
       FIG.  13    is a diagram of an example packet exchange  1300  involving a DL C-OFDMA transmission in a communication system such as the communication system  10  of  FIG.  1 A , or another suitable communication system, according to an embodiment.  FIG.  13    is described with reference to  FIGS.  1 A-C  for explanatory purposes. In some embodiments, however, the packet exchange  1300  is implemented in other suitable communication systems and/or with suitable communication devices different than the example communication devices of  FIGS.  1 B-C . 
     The packet exchange  1300  is used in conjunction with any of the DL C-OFDMA transmissions of  FIGS.  2 - 7   , or is used in conjunction with other suitable DL C-OFDMA transmissions, according to various embodiments. 
     As a variation from the DL C-OFDMA packet exchanges of  FIGS.  2 - 7   , an UL C-OFDMA transmission  1304  immediately follows the DL C-OFDMA transmission  208 , where the UL C-OFDMA transmission  1304  does not merely contain ACK/BA information for the DL C-OFDMA transmission  208 . A defined time period after an end of transmission of the DL C-OFDMA transmission  208 , client stations  38  of the master AP  34  begin an UL-OFDMA transmission  1320 , and client stations  44  of the slave AP  44  begins a DL-OFDMA transmission  1324 . The UL-OFDMA transmission  1320  does not merely contain ACK/BA information for the DL C-OFDMA transmission  216 , and the UL-OFDMA transmission  1324  also does not merely contain ACK/BA information for the DL C-OFDMA transmission  212 . For example, the UL-OFDMA transmission  1320  includes user data from client stations  38 , and the UL-OFDMA transmission  1324  includes user data from client stations  48 , according to an embodiment. 
     In an embodiment, the defined time period is SIFS as defined by the IEEE 802.11 Standard. In other embodiments, the defined time period is a suitable time period different than SIFS. 
     In some embodiments, the DL C-OFDMA transmission  216  includes a trigger frame that prompts client stations  38  to transmit the UL C-OFDMA transmission  1320 , and the DL C-OFDMA transmission  212  includes a trigger frame that prompts client stations  48  to transmit the UL C-OFDMA transmission  1324 . 
     In various embodiments, the UL C-OFDMA transmission  1304  is acknowledged (not shown in  FIG.  13   ) by the master AP  34  and the slave AP  44  using acknowledgment techniques such as described with reference any of  FIGS.  8 - 11   , or using other suitable acknowledgment techniques. 
     In some embodiments, the C-OFDMA-A frame discussed with reference to  FIGS.  2 - 13    comprises a control frame. For example, the IEEE 802.11 Standard defines a frame header with a type subfield and a subtype field. In an embodiment, the type subfield of the C-OFDMA-A frame is set to a value to indicate a control-type frame, and the subtype subfield of the C-OFDMA-A frame is set to a value to indicate the control frame is a C-OFDMA-A frame. 
     In some embodiments, the C-OFDMA-A frame discussed with reference to  FIGS.  2 - 13    comprises a trigger frame, which is a control frame subtype. For example, the type subfield of the C-OFDMA-A frame is set to a value to indicate a control-type frame, and the subtype subfield of the C-OFDMA-A frame is set to a value to indicate the control frame is a trigger frame. The current draft of the IEEE 802.11ax Standard defines a trigger frame format with a trigger type subfield, and the trigger type subfield can be set to one of a plurality of values to indicate one of a plurality of different types of trigger frames. In an embodiment, a C-OFDMA-A frame includes a trigger type subfield set to a value to indicate the trigger frame is a C-OFDMA-A frame. 
     As discussed above, the C-OFDMA-A frame includes information regarding an announced C-OFDMA transmission such as one of, or any suitable combination of two or more of: i) identifier(s) of WLAN(s) of slave AP(s) to participate in the C-OFDMA transmission, ii) a respective frequency bandwidth to be used in a respective WLAN, iii) a respective frequency RU to be used in a respective WLAN, iv) a duration (in time) of the coordinated OFDMA transmission, v) a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission in a respective WLAN, etc., according to various embodiments. In some embodiments, the C-OFDMA-A frame additionally or alternatively includes one of, or any suitable combination of two or more of: an indication a type of long training field (LTF) to be included in PHY preambles of the C-OFDMA transmission (e.g., where a communication protocol provides for a plurality of different types of LTFs), an indication a number of LTFs to be included in PHY preambles of the C-OFDMA transmission (e.g., where the communication protocol provides for different numbers of LTFs), an indication a length of a signal field (e.g., where the signal field is a variable length signal field) to be included in PHY preambles of the C-OFDMA transmission, etc. 
     For DL C-OFDMA transmissions such as described above with reference to  FIGS.  2 - 7   , the C-OFDMA-A frame additionally or alternatively includes one or both of: i) a UL acknowledgement type (e.g., whether the DL C-OFDMA transmission is to be acknowledged by client stations  38 / 48  by an immediate acknowledgement (e.g., which is solicited by trigger frames in the DL C-OFDMA transmission), such as shown in  FIGS.  2 - 4   ; whether the DL C-OFDMA transmission is to be acknowledged by client stations  38 / 48  by a BA solicited by an MU-BAR separate from the DL C-OFDMA transmission, such as shown in  FIGS.  5 - 7   , etc.); ii) a duration of the UL ACK/BA (e.g., if the DL C-OFDMA transmission is to be acknowledged by client stations  38 / 48  by an immediate acknowledgement); according to various embodiments. In some embodiments, indicator(s) of the duration(s) of the UL ACK/BA transmissions  232  and  236  are included elsewhere, such as in trigger frames included in the DL C-OFDMA transmissions ( 212 / 216 ), in MU-BAR frames  504 / 520 , etc. 
     In some embodiments, frequency bandwidth and/or RUs are indicated in the C-OFDMA-A frame in units of 20 MHz. In some embodiments, frequency bandwidth and/or RUs are indicated in the C-OFDMA-A frame in units of 40 MHz. In some embodiments, when the frequency bandwidth allocated to a particular WLAN for the C-OFDMA transmission is less than or equal to 160 MHz, the frequency bandwidth and/or RU allocated to the WLAN are indicated in the C-OFDMA-A frame in units of 20 MHz, whereas when the frequency bandwidth allocated to a particular WLAN for the C-OFDMA transmission is greater than 160 MHz, the frequency bandwidth and/or RU allocated to the WLAN are indicated in the C-OFDMA-A frame in units of 40 MHz. 
     In an embodiment, the C-OFDMA-A frame includes one or more resource allocation fields corresponding to one or more respective WLANs to participate in the C-OFDMA transmission. As an illustrative example, each resource allocation field of the C-OFDMA-A frame includes i) an identifier of the BSS (e.g., a 48-bit MAC address of the slave AP, a color ID of the BSS, a 5-bit hash of the MAC address of the slave MAC address and a 6-bit color ID of the BSS, or another suitable identifier), ii) a frequency bandwidth of the frequency segment to be used for the C-OFDMA transmission, and iii) a starting 20 MHz subchannel of the frequency segment, according to an illustrative embodiment. In some embodiments in which a DL C-OFDMA transmission immediately follows an UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA-A frame includes i) an indication of a frequency bandwidth and starting subchannel for the UL C-OFDMA transmission, and ii) an indication of a frequency bandwidth and starting subchannel for the DL C-OFDMA transmission. In other embodiments in which a DL C-OFDMA transmission immediately follows an UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA-A frame includes only one indication of a frequency bandwidth and starting subchannel for both the UL C-OFDMA transmission and the DL C-OFDMA transmission, i.e., the UL C-OFDMA transmission and the DL C-OFDMA transmission use the same frequency segment. 
     For a DL C-OFDMA transmission, each resource allocation field of the C-OFDMA-A frame further includes one of, or any suitable combination of two or more of: an indicator of a type of LTF to be included in PHY preambles of the DL C-OFDMA transmission, an indicator of a number of LTFs to be included in PHY preambles of the DL C-OFDMA transmission, an indication a length or duration of a signal field to be included in PHY preambles of the DL C-OFDMA transmission, an indicator of a duration of the DL C-OFDMA transmission, etc., according to various embodiments. In some embodiments (such as illustrated in  FIG.  4   ) in which the master AP  34  further transmits a C-OFDMA trigger frame (e.g., the C-OFDMA trigger frame  420 ) to prompt the DL C-OFDMA transmission, the C-OFDMA-A frame does not include the indicator of the type of LTF, the indicator of the number of LTFs, the length or duration of the signal field, and the length or duration of the DL C-OFDMA transmission. For example, such information is instead included in the C-OFDMA trigger frame. In other embodiments (such as illustrated in  FIG.  4   ) in which the master AP  34  further transmits a C-OFDMA trigger frame (e.g., the C-OFDMA trigger frame  420 ) to prompt the DL C-OFDMA transmission, the C-OFDMA-A frame includes one of, or any suitable combination of two or more of: the indicator of the type of LTF, the indicator of the number of LTFs, the length or duration of the signal field, and the length or duration of the DL C-OFDMA transmission. 
     For an UL C-OFDMA transmission, each resource allocation field of the C-OFDMA-A frame further includes one of, or any suitable combination of two or more of: an indication a type of LTF to be included in PHY preambles of the UL C-OFDMA transmission, an indication a number of LTFs to be included in PHY preambles of the UL C-OFDMA transmission, an indication a duration of the UL C-OFDMA transmission, etc., according to various embodiments. In other embodiments in which a DL C-OFDMA transmission immediately follows the UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA-A frame further includes, for the DL C-OFDMA transmission one of, or any suitable combination of two or more of: an indication a type of LTF to be included in PHY preambles of the DL C-OFDMA transmission, an indication a number of LTFs to be included in PHY preambles of the DL C-OFDMA transmission, an indication a length or duration of a signal field to be included in PHY preambles of the DL C-OFDMA transmission, an indication a duration of the DL C-OFDMA transmission, etc., according to various embodiments. 
     In some embodiments (such as illustrated in  FIG.  10   ) in which the master AP  34  further transmits a C-OFDMA trigger frame (e.g., the C-OFDMA trigger frame  1020 ) to prompt the slave APs to transmit trigger frames to trigger the UL C-OFDMA transmission, the C-OFDMA-A frame does not include the indicator of the type of LTF, the indicator of the number of LTFs, and an indicator of the length or duration of the UL C-OFDMA transmission. For example, such information is instead included in the C-OFDMA trigger frame. In other embodiments (such as illustrated in  FIG.  10   ) in which the master AP  34  further transmits a C-OFDMA trigger frame (e.g., the C-OFDMA trigger frame  1020 ) to prompt the UL C-OFDMA transmission, the C-OFDMA-A frame includes one of, or any suitable combination of two or more of: the indicator of the type of LTF, the indicator of the number of LTFs, the indicator of the length or duration of the UL C-OFDMA transmission. 
     In some embodiments, the C-OFDMA-A frame is transmitted in a 20 MHz-wide legacy PPDU (sometimes referred to in the IEEE 802.11 Standard as a “non-HT PPDU”), and duplicates of the PPDU are transmitted in each 20 MHz subchannel (sometimes referred to in the IEEE 802.11 Standard as a “non-HT duplicate PPDU”) to generate the full bandwidth C-OFDMA-A transmission. As an illustrative example, eight duplicates of the C-OFDMA-A frame are transmitted in eight non-HT duplicate PPDUs in eight 20 MHz subchannels to generate a 160 MHz transmission. In some embodiments in which the C-OFDMA-A frame is transmitted in a non-HT PPDU (i.e., a legacy PPDU), a data rate at which the C-OFDMA-A frame is transmitted is limited to a data rates from a set of mandatory data rates defined by a communication protocol (e.g., the IEEE 802.11 Standard). In some embodiments in which the C-OFDMA-A frame is transmitted in a non-HT PPDU (i.e., a legacy PPDU), a data rate at which the C-OFDMA-A frame is transmitted is limited to data rates from a set of common data rates supported by both the master AP  34  and the one or more slave APs  44  that are to participate in the C-OFDMA transmission. 
     In other embodiments, the C-OFDMA-A frame is transmitted in another suitable PPDU (e.g., a PPDU that conforms to the current draft of the IEEE 802.11ax Standard, a PPDU that conforms to the IEEE 802.11be Standard now under development, etc.) that is 20 MHz wide, and duplicates of the PPDU are transmitted in each 20 MHz subchannel to generate the full bandwidth C-OFDMA-A transmission. In some embodiments in which the C-OFDMA-A frame is transmitted in a PPDU that conforms to the current draft of the IEEE 802.11ax Standard or to the IEEE 802.11be Standard now under development, an MCS and a number of spatial streams used for transmitting the C-OFDMA-A frame is limited to MCS/number of spatial stream combinations that the IEEE 802.11ax Standard/IEEE 802.11be Standard define as mandatory. In other embodiments in which the C-OFDMA-A frame is transmitted in a PPDU that conforms to the current draft of the IEEE 802.11ax Standard or to the IEEE 802.11be Standard now under development, an MCS and a number of spatial streams used for transmitting the C-OFDMA-A frame is limited to MCS/number of spatial stream combinations from a set of common MCS/number of spatial stream combinations supported by both the master AP  34  and the one or more slave APs  44  that are to participate in the C-OFDMA transmission. 
     In other embodiments, the C-OFDMA-A frame is transmitted in a single PPDU that spans the full bandwidth of the C-OFDMA-A transmission. 
     Referring now to  FIGS.  4  and  10   , a C-OFDMA trigger frame, such as C-OFDMA trigger frame  420  and C-OFDMA trigger frame  1020 , comprises a trigger frame, which is a control frame subtype. For example, the type subfield of the C-OFDMA trigger frame is set to a value to indicate a control-type frame, and the subtype subfield of the C-OFDMA trigger frame is set to a value to indicate the control frame is a trigger frame. The current draft of the IEEE 802.11ax Standard defines a trigger frame format with a trigger type subfield, and the trigger type subfield can be set to one of a plurality of values to indicate one of a plurality of different types of trigger frames. In an embodiment, a C-OFDMA trigger frame includes a trigger type subfield set to a value to indicate the trigger frame is a C-OFDMA trigger frame. 
     The C-OFDMA trigger frame includes information regarding a C-OFDMA transmission such as one of, or any suitable combination of two or more of: i) identifier(s) of WLAN(s) of slave AP(s) to participate in the C-OFDMA transmission, ii) an indicator of a respective frequency bandwidth to be used in a respective WLAN, iii) an indicator of a respective frequency RU to be used in a respective WLAN, iv) an indicator of a duration (in time) of the coordinated OFDMA transmission, v) an indicator of a respective length (in bits, octets, words, etc.) of a respective OFDMA transmission in a respective WLAN, etc., according to various embodiments. In some embodiments, the C-OFDMA trigger frame additionally or alternatively includes one of, or any suitable combination of two or more of: an indicator of a type of long training field (LTF) to be included in PHY preambles of the C-OFDMA transmission (e.g., where a communication protocol provides for a plurality of different types of LTFs), an indicator of a number of LTFs to be included in PHY preambles of the C-OFDMA transmission (e.g., where the communication protocol provides for different numbers of LTFs), an indicator of a length of a signal field (e.g., where the signal field is a variable length signal field) to be included in PHY preambles of the C-OFDMA transmission, etc. 
     In some embodiments, frequency bandwidth and/or RUs are indicated in the C-OFDMA trigger frame in units of 20 MHz. In some embodiments, frequency bandwidth and/or RUs are indicated in the C-OFDMA trigger frame in units of 40 MHz. In some embodiments, when the frequency bandwidth allocated to a particular WLAN for the C-OFDMA transmission is less than or equal to 160 MHz, the frequency bandwidth and/or RU allocated to the WLAN are indicated in the C-OFDMA trigger frame in units of 20 MHz, whereas when the frequency bandwidth allocated to a particular WLAN for the C-OFDMA transmission is greater than 160 MHz, the frequency bandwidth and/or RU allocated to the WLAN are indicated in the C-OFDMA trigger frame in units of 40 MHz. 
     In an embodiment, the C-OFDMA trigger frame includes one or more resource allocation fields corresponding to one or more respective WLANs to participate in the C-OFDMA transmission. As an illustrative example, each resource allocation field of the C-OFDMA trigger frame includes i) an identifier of the BSS (e.g., a 48-bit MAC address of the slave AP, a color ID of the BSS, a 5-bit hash of the MAC address of the slave MAC address and a 6-bit color ID of the BSS, or another suitable identifier), ii) a frequency bandwidth of the frequency segment to be used for the C-OFDMA transmission, and iii) a starting 20 MHz subchannel of the frequency segment, according to an illustrative embodiment. In some embodiments in which a DL C-OFDMA transmission immediately follows an UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA trigger frame includes i) an indication of a frequency bandwidth and starting subchannel for the UL C-OFDMA transmission, and ii) an indication of a frequency bandwidth and starting subchannel for the DL C-OFDMA transmission. In other embodiments in which a DL C-OFDMA transmission immediately follows an UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA trigger frame includes only one indication of a frequency bandwidth and starting subchannel for both the UL C-OFDMA transmission and the DL C-OFDMA transmission, i.e., the UL C-OFDMA transmission and the DL C-OFDMA transmission use the same frequency segment. 
     For a DL C-OFDMA transmission, each resource allocation field of the C-OFDMA trigger frame further includes one of, or any suitable combination of two or more of: an indicator of a type of LTF to be included in PHY preambles of the DL C-OFDMA transmission, an indicator of a number of LTFs to be included in PHY preambles of the DL C-OFDMA transmission, a length or duration of a signal field to be included in PHY preambles of the DL C-OFDMA transmission, an indicator of a duration of the DL C-OFDMA transmission, etc., according to various embodiments. 
     For an UL C-OFDMA transmission, each resource allocation field of the C-OFDMA trigger frame further includes one of, or any suitable combination of two or more of: an indication of a type of LTF to be included in PHY preambles of the UL C-OFDMA transmission, an indication a number of LTFs to be included in PHY preambles of the UL C-OFDMA transmission, an indication a duration of the UL C-OFDMA transmission, etc., according to various embodiments. In other embodiments in which a DL C-OFDMA transmission immediately follows the UL C-OFDMA transmission (such as illustrated in  FIG.  12   ), the C-OFDMA trigger frame further includes, for the DL C-OFDMA transmission one of, or any suitable combination of two or more of: an indication a type of LTF to be included in PHY preambles of the DL C-OFDMA transmission, an indication a number of LTFs to be included in PHY preambles of the DL C-OFDMA transmission, an indication a length or duration of a signal field to be included in PHY preambles of the DL C-OFDMA transmission, an indication of a duration of the DL C-OFDMA transmission, etc., according to various embodiments. 
     In various embodiments, the C-OFDMA trigger frame does not include one of, or any two of, i) the identifier of the BSS, ii) the indicator of the frequency segment, iii) the indicator of the frequency bandwidth of the frequency segment to be used for the C-OFDMA transmission, iv) the starting 20 MHz subchannel of the frequency segment, v) the indicator of the type of LTF, vi) the indicator of the number of LTFs, vii) the indicator of the length or duration of the signal field, vii) the indicator of the length or duration of the DL C-OFDMA transmission, etc. For example, such information is instead included in the C-OFDMA-A frame. 
     In some embodiments, the C-OFDMA trigger frame is transmitted in a 20 MHz-wide legacy PPDU (sometimes referred to in the IEEE 802.11 Standard as a “non-HT PPDU”), and duplicates of the PPDU are transmitted in each 20 MHz subchannel (sometimes referred to in the IEEE 802.11 Standard as a “non-HT duplicate PPDU”) to generate the full bandwidth C-OFDMA trigger transmission. As an illustrative example, eight duplicates of the C-OFDMA trigger frame are transmitted in eight non-HT duplicate PPDUs in eight 20 MHz subchannels to generate a 160 MHz transmission. In some embodiments in which the C-OFDMA trigger frame is transmitted in a non-HT PPDU (i.e., a legacy PPDU), a data rate at which the C-OFDMA trigger frame is transmitted is limited to a data rates from a set of mandatory data rates defined by a communication protocol (e.g., the IEEE 802.11 Standard). In some embodiments in which the C-OFDMA trigger frame is transmitted in a non-HT PPDU (i.e., a legacy PPDU), a data rate at which the C-OFDMA trigger frame is transmitted is limited to data rates from a set of common data rates supported by both the master AP  34  and the one or more slave APs  44  that are to participate in the C-OFDMA transmission. 
     In other embodiments, the C-OFDMA trigger frame is transmitted in another suitable PPDU (e.g., a PPDU that conforms to the current draft of the IEEE 802.11ax Standard, a PPDU that conforms to the IEEE 802.11be Standard now under development, etc.) that is 20 MHz wide, and duplicates of the PPDU are transmitted in each 20 MHz subchannel to generate the full bandwidth C-OFDMA trigger transmission. In some embodiments in which the C-OFDMA trigger frame is transmitted in a PPDU that conforms to the current draft of the IEEE 802.11ax Standard or to the IEEE 802.11be Standard now under development, an MCS and a number of spatial streams used for transmitting the C-OFDMA trigger frame is limited to MCS/number of spatial stream combinations that the IEEE 802.11ax Standard/IEEE 802.11be Standard define as mandatory. In other embodiments in which the C-OFDMA trigger frame is transmitted in a PPDU that conforms to the current draft of the IEEE 802.11ax Standard or to the IEEE 802.11be Standard now under development, an MCS and a number of spatial streams used for transmitting the C-OFDMA trigger frame is limited to MCS/number of spatial stream combinations from a set of common MCS/number of spatial stream combinations supported by both the master AP  34  and the one or more slave APs  44  that are to participate in the C-OFDMA transmission. 
     In other embodiments, the C-OFDMA trigger frame is transmitted in a single PPDU that spans the full bandwidth of the C-OFDMA trigger transmission. 
     Referring now to  FIGS.  2 - 4   , in some embodiments, the C-OFDMA-A frame  204  includes an indicator of a duration of the UL ACK/BA transmissions  232  and  236 . Client stations  38 / 48  use the indicator of the duration of the UL ACK/BA transmissions  232  and  236  to generate the UL ACK/BA transmissions  232  and  236  according to the indicated duration, according to some embodiments, so that the UL ACK/BA transmissions  232  and  236  end at substantially the same time (e.g., within 5%). Referring now to  FIG.  2   , in some embodiments, the C-OFDMA trigger frame  420  includes an indicator of a duration of the UL ACK/BA transmissions  232  and  236 . Client stations  38 / 48  use the indicator of the duration of the UL ACK/BA transmissions  232  and  236  to generate the UL ACK/BA transmissions  232  and  236  according to the indicated duration, according to some embodiments. 
     In other embodiments, the C-OFDMA-A frame  204  and the C-OFDMA trigger frame  420  do not includes an indicator of duration(s) of the UL ACK/BA transmissions  860  and/or  864 . For example, the UL ACK/BA transmissions  232  and  236  are permitted to have different durations. In some embodiments, indicator(s) of the duration(s) of the UL ACK/BA transmissions  232  and  236  are included elsewhere, such as in trigger frames included in the DL C-OFDMA transmissions ( 212 / 216 ), in MU-BAR frames  504 / 520 , etc. 
     Referring now to  FIGS.  8 - 10   , in some embodiments, the C-OFDMA-A frame  804  includes an indicator of a duration of the DL ACK/BA transmissions  860  and  864 . The slave AP(s)  44  use the indicator of the duration of the DL ACK/BA transmissions  860  and  864  to generate the DL ACK/BA transmission  864  according to the indicated duration, according to some embodiments, so that the DL ACK/BA transmissions  860  and  864  end at substantially the same time (e.g., within 5%). Referring now to  FIG.  10   , in some embodiments, the C-OFDMA trigger frame  1020  includes an indicator of a duration of the DL ACK/BA transmissions  860  and  864 . The slave AP(s)  44  use the indicator of the duration of the DL ACK/BA transmissions  860  and  864  to generate the DL ACK/BA transmission  864  according to the indicated duration, according to some embodiments, so that the DL ACK/BA transmissions  860  and  864  end at substantially the same time (e.g., within 5%). 
     In other embodiments, the C-OFDMA-A frame  804  and the C-OFDMA trigger frame  1020  do not includes an indicator of duration(s) of the DL ACK/BA transmissions  860  and/or  864 . For example, the master AP  34  and the slave AP(s)  44  select suitable durations for the DL ACK/BA transmissions  860  and  864 , e.g., the DL ACK/BA transmissions  860  and  864  are permitted to have different durations. 
     In some embodiments, an RU allocated to a slave AP  44  for a C-OFDMA transmission is required to include a primary channel of the slave AP  44 . In an embodiment, if the RU allocated to the slave AP  44  for the C-OFDMA transmission spans a frequency bandwidth of 160 MHz or less, the RU is required include the 20 MHz primary channel of the slave AP  44 , whereas if the RU allocated to the slave AP  44  for the C-OFDMA transmission spans a frequency bandwidth greater than 160 MHz, the RU is required include the 40 MHz primary channel of the slave AP  44 . In other embodiments, a RU allocated to a slave AP  44  for a C-OFDMA transmission is not required to include a primary channel of the slave AP  44 . 
     In some embodiments, the master AP  34  and the slave AP  44  have a same primary channel. When the master AP  34  and the slave AP  44  have the same primary channel, a target wake time (TWT) subchannel selective transmission (SST) is used where an AP announce channels for client stations to receive a trigger frame or a downlink multi-user signal field for C-OFDMA operation, according to an embodiment. In another embodiment, the master AP  34  announces schedule information for the master AP  34  and slave AP(s)  44 . 
     In other embodiments, the master AP  34  and the slave AP  44  have different primary channels. When the master AP  34  and the slave AP  44  have different primary channels, client stations  38  of the master AP  34  listen to the primary channel of the master AP  34  for RU allocation information for the C-OFDMA transmission; and client stations  48  of the slave AP  44  listen to the primary channel of the slave AP  44  for RU allocation information for the C-OFDMA transmission, according to an embodiment. 
     In some embodiments, the aggregate communication channel used for the C-OFDMA transmission is required to be included within an operating channel of the master AP  34 . In some embodiments, the aggregate communication channel used for the C-OFDMA transmission is required to be included within either the operating channel of the master AP  34  or the operating channel of the slave AP  44 . In other embodiments, the aggregate communication channel used for the C-OFDMA transmission is required to include the operating channel of the master AP  34 . In other embodiments, the aggregate communication channel used for the C-OFDMA transmission is required to include both the operating channel of the master AP  34  and the operating channel of the slave AP  44 . 
     In some embodiments, the master AP  34  is permitted to transmit the C-OFDMA-A frame in 20 MHz subchannels that the master AP  34  determines are idle. In an embodiment, the master AP  34  determining (e.g., the network interface  122  determining, the MAC processor  126  determining, etc.) that a set of subchannels are idle includes i) determining that a network allocation vector (NAV) timer (e.g., implemented using a timer circuit, a counter circuit, etc., included in the network interface  122 , the MAC processor  126 , etc.) is zero, ii) determining that a PHY clear channel assessment (CCA) (e.g., implemented using an energy measurement circuit included in the network interface  122 , the PHY processor  130 , etc.) indicates a primary subchannel (e.g., a 20 MHz primary channel, a 40 MHz primary channel, etc.) of the master AP  34  is idle, and iii) determining that a PHY CCA is idle within a defined time period (e.g., a point coordination function (PCF) interframe space (PIFS) defined by the IEEE 802.11 Standard, or another suitable time period) for non-primary subchannels (e.g., 20 MHz channels, 40 MHz channels, etc.) before transmission of the C-OFDMA-A frame is to begin. 
     In some embodiments, the slave AP  44  is permitted to transmit (e.g., a trigger frame, a DL C-OFDMA transmission, etc.) in response to a C-OFDMA-A frame in any of the subchannels allocated to the slave AP  44  by the C-OFDMA-A frame determined by the slave AP  44  to be idle. In an embodiment, the slave AP  44  determining (e.g., the network interface  122  determining, the MAC processor  126  determining, etc.) that a set of subchannels are idle includes i) determining that a NAV timer is zero, ii) determining that a primary subchannel of the slave AP  44  is idle, comprising one of: a) determining that a PHY CCA indicates the primary subchannel is idle a predetermined time period (e.g., PIFS or another suitable time period) before transmission of the C-OFDMA-A frame began, or b) determining that the PHY CCA indicates the primary subchannel is idle a predetermined time period (e.g., SIFS or another suitable time period) before the slave AP  44  is to begin transmitting (e.g., a trigger frame, a DL C-OFDMA transmission, etc.), and iii) determining that one or more non-primary subchannels are idle comprising one of: a) determining that a PHY CCA indicates the non-primary subchannel is idle a predetermined time period (e.g., PIFS or another suitable time period) before transmission of the C-OFDMA-A frame began, or b) determining that the PHY CCA indicates the nonprimary subchannel is idle a predetermined time period (e.g., SIFS or another suitable time period) before the slave AP  44  is to begin transmitting (e.g., a trigger frame, a DL C-OFDMA transmission, etc.). 
     In other embodiments, the slave AP  44  transmits (e.g., a trigger frame, a DL C-OFDMA transmission, etc.) in response to a C-OFDMA-A frame in the subchannels allocated to the slave AP  44  by the C-OFDMA-A frame without the slave AP  44  first checking whether any of the subchannels are idle. In other embodiments, the C-OFDMA-A frame included information that indicates whether the slave AP  44  is to determine whether subchannels are idle prior to transmitting in the subchannels in response to a C-OFDMA-A frame. 
     In some embodiments, the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that encompasses the C-OFDMA transmission. In other embodiments in which the C-OFDMA-A frame prompts a DL C-OFDMA transmission followed by an UL C-OFDMA transmission (e.g., such as the examples of  FIGS.  2 - 4  and  13   ), the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that ends prior to an end of the DL C-OFDMA transmission  208 , such as when trigger frames in the DL C-OFDMA transmission  208  include information that indicates the client stations  38 / 48  are to check whether subchannels are idle prior to transmitting as part of the UL C-OFDMA transmission. In other embodiments in which the C-OFDMA-A frame prompts a DL C-OFDMA transmission followed by an UL C-OFDMA transmission (e.g., such as the examples of  FIGS.  2 - 4  and  13   ), the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that ends prior to the UL C-OFDMA transmission, such as when trigger frames in the DL C-OFDMA transmission  208  include information that indicates the client stations  38 / 48  are to check whether subchannels are idle prior to transmitting as part of the UL C-OFDMA transmission. 
     In other embodiments, a client station  38 / 48  is configured to ignore a NAV counter set by a C-OFDMA-A frame when transmitting a UL C-OFDMA transmission in response to DL C-OFDMA transmission, which in turn was transmitted in connection with the C-OFDMA-A frame. In other embodiments, a client station  48  is configured to ignore a NAV counter set by a C-OFDMA-A frame when the C-OFDMA-A addresses a slave AP  44  with which the client station  48  is associated. 
     In other embodiments in which the C-OFDMA-A frame prompts an UL C-OFDMA transmission (e.g., such as the examples of  FIGS.  2 - 4  and  13   ), the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that ends prior to an end of the DL C-OFDMA transmission  208 , such as when trigger frames in the DL C-OFDMA transmission  208  include information that indicates the client stations  38 / 48  are to check whether subchannels are idle prior to transmitting as part of the UL C-OFDMA transmission. In other embodiments in which the C-OFDMA-A frame prompts a DL C-OFDMA transmission followed by an UL C-OFDMA transmission (e.g., such as the examples of  FIGS.  8 - 12   ), the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that ends prior to the UL C-OFDMA transmission. In an embodiment, the master AP  34  sets a duration subfield in the C-OFDMA-A frame to indicate a duration that ends prior to an end of a DL OFDMA transmission that includes trigger frames that prompt the UL C-OFDMA transmission. 
     In other embodiments, a client station  38 / 48  is configured to ignore a NAV counter set by a C-OFDMA-A frame when transmitting a UL C-OFDMA transmission in response to the C-OFDMA-A frame. In other embodiments, a client station  48  is configured to ignore a NAV counter set by a C-OFDMA-A frame when the C-OFDMA-A addresses a slave AP  44  with which the client station  48  is associated. 
     In some embodiments, a client station  38 / 48  maintains a first NAV counter (an intra-BSS NAV counter) for intra-BSS transmissions (e.g., for transmissions within the WLAN or BSS to which the client station  38 / 48  belongs), and a second NAV counter (an inter-BSS NAV counter) for inter-BSS transmissions (e.g., for transmissions from WLANs or BSSs to which the client station  38 / 48  does not belong). In an embodiment, when a client station  48  receives a C-OFDMA-A frame from the master AP  34  (and the client station is not associated with the master  34 ), the client station  48  determines (e.g., the network interface  162  determines, the MAC processor  166  determines, etc.) whether the C-OFDMA-A frame includes, among a set of network identifiers that indicate slave APs  44  that are to participate in a C-OFDMA transmission, a network identifier (e.g., a MAC address, a BSS ID, or another suitable identifier) of the slave AP  44  with which the client station  48  is associated; and when the C-OFDMA-A frame includes the network identifier of the slave AP  44  with which the client station  48  is associated, the client station  48  sets (e.g., the network interface  162  sets, the MAC processor  166  sets, etc.) the intra-BSS NAV counter using duration information in the C-OFDMA-A frame. 
     In some embodiments, prior to a C-OFDMA transmission, slave APs  44  transmit to the master AP  34  resource request information regarding the C-OFDMA transmission. In various embodiments, the resource request information includes one of, or any suitable combination of two or more of: an indication of a requested frequency bandwidth, an indication of a requested duration of a PPDU to be transmitted during the C-OFDMA transmission, an indication of a type of LTF(s) to be included in the PPDU, an indication of a number of LTFs to be included in the PPDU, an indication of a requested duration of a signal field to be included in the PPDU (when the PPDU is to be part of a DL C-OFDMA transmission, etc. In an embodiment, the slave AP  44  is configured to (e.g., the network interface  122  is configured to, the MAC processor  126  is configured to, the C-OFDMA controller is configured to, etc.) generate a frame that includes the resource request information, and the slave AP  44  is configured to (e.g., the network interface  122  is configured to, the PHY processor  130  is configured to, etc.) transmit the frame in a packet to the master AP  34 . 
     In some embodiments, the slave AP  44  is configured to contend for a wireless communication medium and transmit the resource request information to the master AP  34  in response to obtaining the wireless communication medium. 
     In other embodiments, the master AP  34  is configured to poll slave APs  44  for resource request information. For example, the master AP  34  is generate and send a trigger frame (e.g., a resource request trigger) to slave APs  44 , the trigger frame being configured to prompt slave APs  44  to transmit resource request information to the master AP  34 . In an embodiment, the resource request trigger includes, for each of one or more slave APs  44 , a network identifier of the slave AP  44 . When a slave AP  44  receives the resource request trigger, the slave AP  44  determines whether the network ID of the slave AP  44  is included in the resource request trigger, and when the network ID of the slave AP  44  is included in the resource request trigger, the slave AP  44  transmits resource request information to the master AP  34 , according to an embodiment. 
     In an embodiment, the network ID of the slave AP  44  comprises a MAC address of the slave AP  44 . In another embodiment, the network ID of the slave AP  44  comprises a BSS ID corresponding to the slave AP  44 . In another embodiment, the network ID of the slave AP  44  comprises a hash value generated by applying a known hash function of the MAC address of the slave AP  44 . In various embodiments, the hash value has a length of 11 bits, or another suitable number of bits. 
     In another embodiment, the network ID of the slave AP  44  comprises i) a BSS color ID of the slave AP  44  (or a subset of bits of the BSS color ID (such as 6 bits of the BSS color ID)), and ii) bits (such as 5 bits, 6 bits, etc.) taken or generated (e.g., by applying a hash function to the MAC address) from the MAC address of the slave AP  44 . 
     The master AP  34  analyzes the resource request information received from the slave AP(s)  44 , and determines an allocation of frequency segments to the master AP  34  and the slave APs  44  based on the resource request information received from the slave AP(s)  44 . In some embodiments, the master AP  34  analyzes the resource request information received from the slave AP(s)  44 , and determines a duration of the C-OFDMA transmission (e.g. a DL C-OFDMA transmission, an UL C-OFDMA transmission) based on the resource request information received from the slave AP(s)  44 . 
     In some embodiments, the master AP  34  analyzes the resource request information received from the slave AP(s)  44 , and determines whether a cascading C-OFDMA operation (e.g., a DL C-OFDMA transmission followed by an UL C-OFDMA transmission as in  FIG.  13   , an UL C-OFDMA transmission followed by a DL C-OFDMA transmission as in  FIG.  12   , etc.) should be allocated based on the resource request information received from the slave AP(s)  44 . 
     In some embodiments, an AP announces (e.g., in a management frame such as beacon frame or a probe response frame (or another suitable management frame), in frames addressed to neighboring APs, etc.) whether the AP supports C-OFDMA transmissions. In some embodiments, the AP also announces (e.g., in the same frame or a different frame) whether the AP supports a master role, and/or whether the AP supports a slave role. 
     In some embodiments, APs negotiate by exchanging frames (e.g., public Action frames or other suitable frames) which AP will be the master AP and which AP(s) will be the slave APs; and the APs retain the same roles until renegotiated. 
     In other embodiments, an AP that obtains access to a channel medium automatically becomes the master AP, and announces to other APs (e.g., via public Action frame(s), a management frame (e.g., a beacon frame, a probe response frame, etc.) or another suitable frame) that the other APs may participate in a C-OFDMA transmission as slave APs. 
     In embodiments in which the master AP  34  and the slave AP(s)  44  are part of an extended service set (ESS), the master AP  34  is configured to schedule one or more APs  44  (that are capable of C-OFDMA transmissions) for a C-OFDMA transmission. In other embodiments, a first AP notifies a second AP (e.g., via a management frame or another suitable frame) whether the second AP is permitted to schedule the first AP for C-OFDMA transmissions. 
     In an embodiment, APs are configured to form a static group of APs that are configured to participate in C-OFDMA transmissions. For example, APs in an ESS and that are configured to participate in C-OFDMA transmissions implicitly form the group, according to an embodiment. In other embodiments, APs negotiate to form the group by exchanging management frames, for example. Once the group is formed, any AP in the group may act as the master AP  34  by, for example, initiating a C-OFDMA transmission and allocating frequency RUs to other APs (acting as slave APs  44 ) for the C-OFDMA transmission, according to an embodiment. 
     Although examples described above involve an AP transmitting to multiple client stations or multiple client stations transmitting to an AP as part of a C-OFDMA transmission, in some embodiments the AP transmits to a single client station or a single client station transmits to the AP as part of the C-OFDMA transmission. 
     Although examples described above involve coordinated OFDMA transmissions, in other embodiments the packet exchanges, techniques, etc., described above utilize additionally or alternatively utilize coordinated MU-MIMO transmissions. For example, as part of a coordinated DL transmission the master AP  34  may transmit in a frequency segment using one or more first spatial streams while the slave AP  44  transmits in the same frequency segment using one or more second spatial streams. As another example, as part of a coordinated UL transmission the one or more client stations  38  of the master AP  34  may transmit in a frequency segment using one or more first spatial streams while one or more client stations  48  of the slave AP  44  transmit in the same frequency segment using one or more second spatial streams. Thus, the example C-OFDMA packet exchanges, techniques, etc., described above are merely illustrative embodiments of coordinated multi-user (MU) transmissions and associated techniques. In other embodiments, the coordinated MU transmissions comprise coordinated MU-MIMO transmissions. Similarly, the C-OFDMA-A frames described above are merely illustrative examples of announcement frames that announce a coordinated multi-user (MU) transmission, which may be used in conjunction with C-OFDMA transmissions and coordinated MU-MIMO transmissions, for example. Similarly, the C-OFDMA trigger frames described above are merely illustrative examples of trigger frames for use with coordinated MU transmissions, which may be used in conjunction with C-OFDMA transmissions and coordinated MU-MIMO transmissions, for example. Similarly, the C-OFDMA controllers  60 / 70 / 80  described above are merely illustrative examples of controllers for use with coordinated MU transmissions, which may be used in conjunction with C-OFDMA transmissions and coordinated MU-MIMO transmissions, for example. 
     Although examples described above involve synchronized transmissions by or to multiple APs that begin at substantially a same time, in other embodiments, the example packet exchanges, techniques, etc., are modified to permit transmissions by or to different APs to begin at different times. Similarly, although examples described above involve synchronized transmissions by or to multiple APs that end at substantially a same time, in other embodiments, the example packet exchanges, techniques, etc., are modified to permit transmissions by or to different APs to end at different times. For example, in an illustrative embodiment, transmissions by or to different APs overlap in time and occur during a same time window, but do not necessarily begin at a substantially same time and/or do not necessarily end at a substantially same time. 
       FIG.  14    is a flow diagram of an example method  1400  for wireless communications involving multiple APs, according to an embodiment. The method  1400  is implemented by a master AP having a structure such as described with reference to  FIG.  1 B , and  FIG.  14    is described with reference to  FIG.  1 B  for ease of explanation. In other embodiments, however, the method  1400  is implemented by an AP having a suitable structure different than illustrated in  FIG.  1 B . 
     In various embodiments, the method  1400  is utilized in connection with any of the frame exchanges discussed in connection with any of  FIGS.  2 - 13   , and/or in connection with any of the techniques discussed above. 
     The method  1400  is implemented by a master AP associated with one or more first client stations. 
     At block  1404 , the master AP generates (e.g., the network interface  122  generates, the MAC processor  126  generates, the controller  60  generates, etc.) an announcement frame that announces a coordinated MU transmission (e.g., a C-OFDMA transmission, a coordinated MU-MIMO transmission, etc.) involving multiple APs including the master AP and one or slave APs, each of the slave APs associated with a respective one or more second client stations. In an embodiment, the announcement frame is generated at block  1404  to indicate respective one or more frequency RUs allocated to the one or more slave APs for the coordinated MU transmission. 
     At block  1408 , the master AP transmits (e.g., the network interface  122  transmits, the PHY processor  130  transmits, etc.) the announcement frame to the one or more slave APs to initiate the coordinated MU transmission. 
     At block  1412 , the master AP participates in the coordinated MU transmission while the one or more second APs participate in the coordinated MU transmission. 
     In some embodiments, participating in the coordinated MU transmission at block  1412  comprises: the master AP transmitting (e.g., the network interface  122  transmits, the PHY processor  130  transmits, etc.) a first DL transmission to at least one first client station among the one or more first client stations, while one slave AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     In some embodiments, generating the announcement frame comprises generating the announcement frame to indicate the one slave AP is allocated a first frequency RU; and transmitting the first DL transmission as part of participating in the coordinated MU transmission at block  1412  comprises transmitting the first DL transmission in a second frequency RU while the one slave AP transmits the second DL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     In some embodiments, generating the announcement frame at block  1404  comprises generating the announcement frame to indicate the one slave AP is allocated a first frequency RU and one or more first spatial streams; and transmitting the first DL transmission as part of participating in the coordinated MU transmission at block  1412  comprises transmitting the first DL transmission in the first frequency RU using one or more second spatial streams while the one slave AP transmits the second DL transmission in the first frequency RU using the one or more first spatial streams. 
     In some embodiments, generating the announcement frame at block  1404  comprises generating the announcement frame to include an indication of a duration of a signal field to be included in a PHY header of the second DL transmission; and the method  1400  further comprises the master AP generating the first DL transmission to include a signal field in a PHY header of the first DL transmission, the signal field in the PHY header of the first DL transmission having the duration of the signal field in the PHY header of the second DL transmission. 
     In some embodiments, participating in the coordinated MU transmission at block  1412  comprises: the master AP transmitting a first trigger frame to at least one first client station among the one or more first client stations, while one slave AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and receiving a first UL transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the one slave AP in response to the second trigger frame. 
     In some embodiments, generating the announcement frame at block  1404  comprises generating the announcement frame to indicate the one second AP is allocated a first frequency RU; and participating in the coordinated MU transmission at block  1412  comprises receiving the first UL transmission in a second frequency RU while the at least one second client station transmits the second UL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     In some embodiments, generating the announcement frame at block  1404  comprises generating the announcement frame to indicate the one slave AP is allocated a first frequency RU and one or more first spatial streams; and participating in the coordinated MU transmission at block  1412  comprises receiving the first UL transmission in the first frequency RU via one or more second spatial streams while the at least one second client station transmits the second UL transmission in the first RU via the one or more first spatial streams. 
     In some embodiments, generating the announcement frame at block  1404  comprises generating the announcement frame to include an indication of a duration of the second trigger frame; and participating in the coordinated MU transmission at block  1412  comprises the master AP generating the first trigger frame to have the duration of the second trigger frame. 
     In some embodiments, the method  1400  further comprises: the master AP receiving resource request information from the one or more slave APs; and the master AP allocating the one or more frequency RUs to the one or more slave APs for the coordinated MU transmission based on the resource request information from the one or more second APs. 
       FIG.  15    is a flow diagram of another example method  1500  for wireless communications involving multiple APs, according to another embodiment. The method  1500  is implemented by a slave AP having a structure such as described with reference to  FIG.  1 B , and  FIG.  15    is described with reference to  FIG.  1 B  for ease of explanation. In other embodiments, however, the method  1500  is implemented by an AP having a suitable structure different than illustrated in  FIG.  1 B . 
     In various embodiments, the method  1500  is utilized in connection with any of the frame exchanges discussed in connection with any of  FIGS.  2 - 13   , and/or in connection with any of the techniques discussed above. 
     The method  1500  is implemented by a slave AP associated with one or more first client stations. 
     At block  1504 , the slave AP receives (e.g., the network interface  122  receives, the MAC processor  126  receives, the controller  70  receives, etc.) an announcement frame from a master AP associated with one or more second client stations. In an embodiment, the announcement frame announces a coordinated MU transmission (e.g., a C-OFDMA transmission, a coordinated MU-MIMO transmission, etc.) involving at least the slave AP and the master AP. In an embodiment, the announcement frame includes an indicator of a frequency RU allocated to the slave AP for the coordinated MU transmission. 
     At block  1508 , the slave AP participates in the coordinated MU transmission using the frequency RU indicated by the announcement frame while the slave AP participates in the coordinated MU transmission. 
     In an embodiment, participating in the coordinated MU transmission at block  1508  comprises: the slave AP transmitting (e.g., the network interface device  122  transmitting, the PHY processor  130  transmitting, etc.) a first DL transmission to at least one first client station among the one or more first client stations, while the master AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     In some embodiments, the method  1500  further comprises: the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the slave AP; and participating in the coordinated MU transmission at block  1508  comprises transmitting the first DL transmission in the first frequency RU while the second AP transmits the second DL transmission in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency. 
     In some embodiments, the method  1500  further comprises: the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the slave AP; and the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; and participating in the coordinated MU transmission at block  1508  comprises transmitting the first DL transmission in the first frequency RU using one or more first spatial streams while the second AP transmits the second DL transmission in the first frequency RU using the one or more second spatial streams. 
     In some embodiments, the method  1500  further comprises: the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a duration of a signal field based on an indicator, in the announcement frame, of a signal field duration for the coordinated MU transmission, the signal field to be included in a physical layer (PHY) header in the first DL transmission; and participating in the coordinated MU transmission at block  1508  comprises the slave AP generating (e.g., the network interface device  122  generating, the PHY processor  130  generating, etc.) the first DL transmission to include, in the PHY header of the first DL transmission, the signal field having the duration. 
     In some embodiments, participating in the coordinated MU transmission at block  1508  comprises: the slave AP transmitting (e.g., the network interface device  122  transmitting, the PHY processor  130  transmitting, etc.) a first trigger frame to at least one first client station among the one or more first client stations, while the master AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and the slave AP receiving (e.g., the network interface device  122  receiving, the PHY processor  130  receiving, etc.) a first UL transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the master AP in response to the second trigger frame. 
     In some embodiments, the method  1500  further comprises the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; and participating in the coordinated MU transmission at block  1508  comprises: transmitting the first trigger frame in the first frequency RU while the master AP transmits the second trigger frame in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency; and receiving the first UL transmission in the first frequency RU while the at least one second client station transmits the second UL transmission in the second RU. 
     In some embodiments, the method  1500  further comprises: the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; and the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; and the slave AP generating (e.g., the network interface device  122  generating, the MAC processor  126  generating, etc.) the first trigger frame to instruct the one or more first client stations to transmit, during the first UL transmission, in the first frequency RU via the one or more first spatial streams. In some embodiments, participating in the coordinated MU transmission at block  1508  comprises: the slave AP receiving the first UL transmission in the first frequency RU via one or more first spatial streams while the at least one second client station transmits the second UL transmission in the first RU via one or more second spatial streams. 
     In some embodiments, the method  1500  further comprises: the slave AP determining (e.g., the network interface device  122  determining, the MAC processor  126  determining, the controller  70  determining, etc.) a duration of the first trigger frame based on an indicator, in the announcement frame, of the duration of the first trigger frame; and the slave AP generating (e.g., the network interface device  122  generating, the MAC processor  126  generating, etc.) the first trigger frame to have the determined duration. 
     In some embodiments, the method  1500  further comprises, prior to receiving the announcement frame: the slave AP generating (e.g., the network interface device  122  generating, the MAC processor  126  generating, etc.) resource request information to request an RU for the coordinated MU transmission; and the slave AP transmitting (e.g., the network interface device  122  transmitting, the PHY processor  130  transmitting, etc.) the resource request information to the second AP. 
       FIG.  16    is a flow diagram of another example method  1600  for wireless communications involving multiple APs, according to another embodiment. The method  1600  is implemented by a master AP having a structure such as described with reference to  FIG.  1 B , and  FIG.  16    is described with reference to  FIG.  1 B  for ease of explanation. In other embodiments, however, the method  1600  is implemented by an AP having a suitable structure different than illustrated in  FIG.  1 B . 
     In various embodiments, the method  1600  is utilized in connection with any of the frame exchanges discussed in connection with any of  FIGS.  2 - 13   , and/or in connection with any of the techniques discussed above. 
     At block  1604 , a first AP determines (e.g., the network interface  122  determines, the MAC processor  126  determines, the controller  60  determines, etc.) that the first AP is to be a master AP for a coordinated MU transmission (e.g., a C-OFDMA transmission, a coordinated MU-MIMO transmission, etc.) involving multiple APs including the first AP and one or more second APs acting as slave APs. 
     At block  1608 , the first AP receives (e.g., the network interface  122  receives, the MAC processor  126  receives, the controller  60  receives, etc.), from the one or more second APs, resource request information regarding access to a wireless communication medium by the one or more second APs for the coordinated MU transmission. 
     At block  1612 , the first AP allocates (e.g., the network interface  122  allocates, the MAC processor  126  allocates, the controller  60  allocates, etc.) frequency RUs to the first AP and the one or more second APs for the coordinated MU transmission based on the resource request information received at block  1608 . 
     At block  1616 , the first AP generates (e.g., the network interface  122  generates, the MAC processor  126  generates, the controller  60  generates, etc.) an announcement frame regarding the coordinated MU transmission. In an embodiment, the announcement frame includes allocation information regarding the RUs allocated to the one or more second APs for the coordinated MU transmission. 
     At block  1620 , the first AP transmits (e.g., the network interface  122  transmits, the PHY processor  130  transmits, etc.) the announcement frame to provide the one or more second APs with the allocation information regarding the RUs allocated to the one or more second APs. 
     At block  1624 , the first AP participates in the coordinated MU transmission while the one or more second APs participate in the coordinated MU transmission. 
     In some embodiments, receiving resource request information from one of the second APs at block  1608  comprises: the first AP receiving a packet from the one second AP, the packet sent in response to the second AP contending for the wireless communication medium in order to transit the packet and obtaining the wireless communication medium. In an embodiment, the packet includes resource request information from the one second AP. 
     In some embodiments, the method  1600  further includes: the first AP generating (e.g., the network interface  122  generating, the MAC processor  126  generating, the controller  60  generating, etc.) a trigger frame configured to prompt at least one second AP among the one or more second APs to transmit resource request information; and the first AP transmitting (e.g., the network interface  122  transmitting, the PHY processor  130  transmitting, etc.) the trigger frame to prompt the at least one second AP to transmit resource request information. In an embodiment, receiving the resource request information at block  1608  comprises receiving resource request information from the at least one second AP responsive to transmitting the trigger frame. 
     In some embodiments, generating the trigger frame comprises: including, in the trigger frame, an identifier of one second AP; wherein receiving the resource request information at block  1608  comprises receiving resource request information from the one second AP. 
     In some embodiments, the identifier of the one second AP in the trigger frame comprises: a MAC address of the one second AP. In other embodiments, the identifier of the one second AP in the trigger frame comprises: a first set of bits from a BSS color identifier of the one second AP; and a second set of bits generated from a MAC address of the one second AP. In an embodiment, the second set of bits generated from the MAC address of the one second AP comprises: a set of bits generated by applying a hash function to the MAC address of the one second AP. 
     In some embodiments, receiving the resource request information at block  1608  comprises receiving, from one second AP: an indicator of a frequency bandwidth requested by the one second AP for the coordinated MU transmission. 
     In some embodiments, receiving the resource request information at block  1608  comprises receiving, from one second AP: an indicator of a duration of a packet to be transmitted during the coordinated MU transmission. 
     In some embodiments, receiving the resource request information at block  1608  comprises receiving, from one second AP: an indicator of a duration of a signal field in a PHY header of a packet to be transmitted during the coordinated MU transmission. 
       FIG.  17    is a flow diagram of another example method  1700  for wireless communications involving multiple APs, according to another embodiment. The method  1700  is implemented by a client station having a structure such as described with reference to  FIG.  1 C , and  FIG.  17    is described with reference to  FIG.  1 C  for ease of explanation. In other embodiments, however, the method  1700  is implemented by a client station having a suitable structure different than illustrated in  FIG.  1 C . 
     In various embodiments, the method  1700  is utilized in connection with any of the frame exchanges discussed in connection with any of  FIGS.  2 - 13   , and/or in connection with any of the techniques discussed above. 
     The method  1700  is implemented by a client station associated with a first AP. 
     At block  1704 , the client station receives (e.g., the network interface  162  receives, the MAC processor  166  receives, the controller  80  receives, etc.) an announcement frame transmitted by a second AP with which the client station is not associated. The announcement frame announces a coordinated MU transmission involving the second AP and one or more other APs. The announcement frame includes one or more respective network identifiers of the one or more other APs, and the announcement frame further includes a duration field indicating a time duration corresponding to the coordinated MU transmission. 
     At block  1708 , in response to receiving the announcement frame, the client station sets (e.g., the network interface  162  sets, the MAC processor  166  sets, the controller  80  sets, etc.) a NAV counter of the client station based on a value of the duration field in the announcement frame. 
     At block  1712 , the client station determines (e.g., the network interface  162  determines, the MAC processor  166  determines, the controller  80  determines, etc.) that the announcement frame includes a network identifier of the first AP. 
     At block  1716 , the client station determines (e.g., the network interface  162  determines, the MAC processor  166  determines, the controller  80  determines, etc.) that the client station is to transmit to the first AP in a communication channel as part of the coordinated MU transmission. 
     At block  1720 , the client station determines (e.g., the network interface  162  determines, the MAC processor  166  determines, the controller  80  determines, etc.) that the communication channel is idle, including ignoring the NAV counter in response to determining that the NAV counter was set in response to the announcement frame that includes the network identifier of the first AP; and 
     At block  1724 , the client station transmits (e.g., the network interface  162  transmits, the PHY processor  170  transmits, etc.) as part of the coordinated MU transmission in response to determining that the communication channel is idle. 
     In some embodiments, the coordinated MU transmission includes respective downlink transmissions by the first AP and the second AP; and transmitting, at block  1724 , as part of the coordinated MU transmission comprises transmitting to the first AP after the respective downlink transmissions by the first AP and the second AP. 
     In some embodiments, the method  1700  further comprises: the client station maintaining (e.g., the network interface  162  maintaining, the MAC processor  166  maintaining, etc.) a first NAV counter for transmissions in a basic service set (BSS) managed by the first AP; and the client station maintaining (e.g., the network interface  162  maintaining, the MAC processor  166  maintaining, etc.) a second NAV counter for transmissions not within the BSS managed by the first AP; wherein setting the NAV counter of the client station based on the value of the duration field in the announcement frame comprises the client station setting (e.g., the network interface  162  setting, the MAC processor  166  setting, etc.) the first NAV counter in response to determining that the announcement frame transmitted by the second AP includes the network identifier of the first AP. 
     Embodiment 1: A method for wireless communication by a first access point (AP) associated with one or more first client stations, the method comprising: generating, at the first AP, an announcement frame that announces a coordinated multi-user (MU) transmission involving multiple APs including the first AP and one or more second APs, each of the second APs associated with a respective one or more second client stations, wherein the announcement frame is generated to indicate respective one or more frequency resource units (RUs) allocated to the one or more second APs for the coordinated MU transmission; transmitting, by the first AP, the announcement frame to the one or more second APs to initiate the coordinated MU transmission; and participating, by the first AP, in the coordinated MU transmission while the one or more second APs also participate in the coordinated MU transmission. 
     Embodiment 2: The method of embodiment 1, wherein participating in the coordinated MU transmission comprises: transmitting, by the first AP, a first downlink (DL) transmission to at least one first client station among the one or more first client stations, while one second AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     Embodiment 3: The method of embodiment 2, wherein: generating the announcement frame comprises generating the announcement frame to indicate the one second AP is allocated a first frequency RU; and transmitting the first DL transmission comprises transmitting the first DL transmission in a second frequency RU while the one second AP transmits the second DL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     Embodiment 4: The method of embodiment 2, wherein: generating the announcement frame comprises generating the announcement frame to indicate the one second AP is allocated a first frequency RU and one or more first spatial streams; and transmitting the first DL transmission comprises transmitting the first DL transmission in the first frequency RU using one or more second spatial streams while the one second AP transmits the second DL transmission in the first frequency RU using the one or more first spatial streams. 
     Embodiment 5: The method of any of embodiments 2-4, wherein generating the announcement frame comprises: generating the announcement frame to include an indication of a duration of a signal field to be included in a physical layer (PHY) header of the second DL transmission; and generating, at the first AP, the first DL transmission to include a signal field in a PHY header of the first DL transmission, the signal field in the PHY header of the first DL transmission having the duration of the signal field in the PHY header of the second DL transmission. 
     Embodiment 6: The method of embodiment 1, wherein participating in the coordinated MU transmission comprises: transmitting, by the first AP, a first trigger frame to at least one first client station among the one or more first client stations, while one second AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and receiving, at the first AP, a first uplink (UL) transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the one second AP in response to the second trigger frame. 
     Embodiment 7: The method of embodiment 6, wherein: generating the announcement frame comprises generating the announcement frame to indicate the one second AP is allocated a first frequency RU; and receiving the first UL transmission comprises receiving the first UL transmission in a second frequency RU while the at least one second client station transmits the second UL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     Embodiment 8: The method of embodiment 6, wherein: generating the announcement frame comprises generating the announcement frame to indicate the one second AP is allocated a first frequency RU and one or more first spatial streams; and receiving the first UL transmission comprises receiving the first UL transmission in the first frequency RU via one or more second spatial streams while the at least one second client station transmits the second UL transmission in the first RU via the one or more first spatial streams. 
     Embodiment 9: The method of any of embodiments 6-8, wherein: generating the announcement frame comprises generating the announcement frame to include an indication of a duration of the second trigger frame; and participating in the coordinated MU transmission comprises generating, at the first AP, the first trigger frame to have the duration of the second trigger frame. 
     Embodiment 10: The method of any of embodiments 1-9, further comprising: receiving, at the first AP, resource request information from the one or more second APs; allocating, at the first AP, the one or more frequency RUs to the one or more second APs for the coordinated MU transmission based on the resource request information from the one or more second APs. 
     Embodiment 11: The method of any of embodiments 1-10, further comprising: after transmitting the announcement frame, receiving, at the first AP, one or more respective copies of the announcement frame from the one or more second APs; and simultaneously with receiving the one or more respective copies of the announcement frame, transmitting, by the first AP, a further copy of the announcement frame. 
     Embodiment 12: The method of embodiment 11, further comprising: after receiving the one or more respective copies of the announcement frame, transmitting, by the first AP, a trigger frame to the one or more second APs to further initiate the coordinated MU transmission. 
     Embodiment 13: A first access point (AP) associated with one or more first client stations, the first AP comprising: a wireless network interface device comprising one or more integrated circuit (IC) devices. The one or more IC devices are configured to: generate an announcement frame that announces a coordinated multi-user (MU) transmission involving multiple APs including the first AP and one or more second APs, each of the second APs associated with a respective one or more second client stations, wherein the announcement frame is generated to indicate respective one or more frequency resource units (RUs) allocated to the one or more second APs for the coordinated MU transmission; control the wireless network interface device to transmit the announcement frame to the one or more second APs to initiate the coordinated MU transmission; and control the wireless network interface device to participate in the coordinated MU transmission while the one or more second APs also participate in the coordinated MU transmission. 
     Embodiment 14: The first AP of embodiment 13, wherein the one or more IC devices are configured to control the wireless network interface device to participate in the coordinated MU transmission at least by: controlling the wireless network interface device to transmit a first downlink (DL) transmission to at least one first client station among the one or more first client stations, while one second AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     Embodiment 15: The first AP of embodiment 14, wherein the one or more IC devices are configured to: generate the announcement frame to indicate the one second AP is allocated a first frequency RU; and control the wireless network interface device to transmit the first DL transmission in a second frequency RU while the one second AP transmits the second DL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     Embodiment 16: The first AP of embodiment 14, wherein the one or more IC devices are configured to: generate the announcement frame to indicate the one second AP is allocated a first frequency RU and one or more first spatial streams; and control the wireless network interface device to transmit the first DL transmission in the first frequency RU using one or more second spatial streams while the one second AP transmits the second DL transmission in the first frequency RU using the one or more first spatial streams. 
     Embodiment 17: The first AP of any of embodiments 14-16, wherein the one or more IC devices are configured to: generate the announcement frame to include an indication of a duration of a signal field to be included in a physical layer (PHY) header of the second DL transmission; and generate the first DL transmission to include a signal field in a PHY header of the first DL transmission, the signal field in the PHY header of the first DL transmission having the duration of the signal field in the PHY header of the second DL transmission. 
     Embodiment 18: The first AP of embodiment 13, wherein the one or more IC devices are configured to control the wireless network interface device to participate in the coordinated MU transmission at least by: controlling the wireless network interface device to transmit a first trigger frame to at least one first client station among the one or more first client stations, while one second AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and receiving a first uplink (UL) transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the one second AP in response to the second trigger frame. 
     Embodiment 19: The first AP of embodiment 18, wherein the one or more IC devices are configured to: generate the announcement frame to indicate the one second AP is allocated a first frequency RU; and receive the first UL transmission in a second frequency RU while the at least one second client station transmits the second UL transmission in the first RU, wherein the second RU does not overlap the first frequency RU in frequency. 
     Embodiment 20: The first AP of embodiment 18, wherein the one or more IC devices are configured to: generate the announcement frame to indicate the one second AP is allocated a first frequency RU and one or more first spatial streams; and receive the first UL transmission in the first frequency RU via one or more second spatial streams while the at least one second client station transmits the second UL transmission in the first RU via the one or more first spatial streams. 
     Embodiment 21: The first AP of any of embodiments 18-20, wherein the one or more IC devices are configured to: generate the announcement frame to include an indication of a duration of the second trigger frame; and generate the first trigger frame to have the duration of the second trigger frame. 
     Embodiment 22: The first AP of any of embodiments 13-21, wherein the one or more IC devices are further configured to: receive resource request information from the one or more second APs; allocate the one or more frequency RUs to the one or more second APs for the coordinated MU transmission based on the resource request information from the one or more second APs. 
     Embodiment 23: The first AP of any of embodiments 13-22, wherein the one or more IC devices are further configured to: after transmitting the announcement frame, receive one or more respective copies of the announcement frame from the one or more second APs; and control the wireless network interface device to transmit a further copy of the announcement frame simultaneously with receiving one or more respective copies of the announcement frame from the one or more second APs. 
     Embodiment 24: The first AP of embodiment 23, wherein the one or more IC devices are further configured to: control the wireless network interface device to transmit a trigger frame to the one or more second APs to further initiate the coordinated MU transmission after receiving the one or more respective copies of the announcement frame. 
     Embodiment 25: A method for wireless communication by a first access point (AP) associated with one or more first client stations, the method comprising: receiving, at the first AP, an announcement frame from a second AP associated with one or more second client stations, the announcement frame announcing a coordinated multi-user (MU) transmission involving at least the first AP and the second AP, wherein the announcement frame includes an indicator of a frequency resource unit (RU) allocated to the first AP for the coordinated MU transmission; and participating, by the first AP, in the coordinated MU transmission using the frequency RU indicated by the announcement frame while the second AP also participates in the coordinated MU transmission. 
     Embodiment 26: The method of embodiment 25, wherein participating in the coordinated MU transmission comprises: transmitting, by the first AP, a first downlink (DL) transmission to at least one first client station among the one or more first client stations, while the second AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     Embodiment 27: The method of embodiment 26, further comprising: determining, at the first AP, a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; wherein transmitting the first DL transmission comprises transmitting the first DL transmission in the first frequency RU while the second AP transmits the second DL transmission in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency. 
     Embodiment 28: The method of embodiment 26, further comprising: determining, at the first AP, a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; determining, at the first AP, one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; and transmitting the first DL transmission comprises transmitting the first DL transmission in the first frequency RU using one or more first spatial streams while the second AP transmits the second DL transmission in the first frequency RU using the one or more second spatial streams. 
     Embodiment 29: The method of any of embodiments 26-28, further comprising: determining, at the first AP, a duration of a signal field based on an indicator, in the announcement frame, of a signal field duration for the coordinated MU transmission, the signal field to be included in a physical layer (PHY) header in the first DL transmission; and generating, at the first AP, the first DL transmission to include, in the PHY header of the first DL transmission, the signal field having the duration. 
     Embodiment 30: The method of embodiment 25, wherein participating in the coordinated MU transmission comprises: transmitting, by the first AP, a first trigger frame to at least one first client station among the one or more first client stations, while the second AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and receiving, at the first AP, a first uplink (UL) transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the second AP in response to the second trigger frame. 
     Embodiment 31: The method of embodiment 30, further comprising: determining, at the first AP, a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; wherein transmitting the first trigger frame comprises transmitting the first trigger frame in the first frequency RU while the second AP transmits the second trigger frame in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency; and wherein receiving the first UL transmission comprises receiving the first UL transmission in the first frequency RU while the at least one second client station transmits the second UL transmission in the second RU. 
     Embodiment 32: The method of embodiment 30, further comprising: determining, at the first AP, a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; determining, at the first AP, one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; and generating, at the first AP, the first trigger frame to instruct the one or more first client stations to transmit, during the first UL transmission, in the first frequency RU via the one or more first spatial streams; wherein receiving the first UL transmission comprises receiving the first UL transmission in the first frequency RU via one or more first spatial streams while the at least one second client station transmits the second UL transmission in the first RU via one or more second spatial streams. 
     Embodiment 33: The method of any of embodiments 30-32, further comprising: determining, at the first AP, a duration of the first trigger frame based on an indicator, in the announcement frame, of the duration of the first trigger frame; and generating, at the first AP, the first trigger frame to have the determined duration. 
     Embodiment 34: The method of any of embodiments 25-33, further comprising, prior to receiving the announcement frame: generating, at the first AP, resource request information to request an RU for the coordinated MU transmission; and transmitting, by the first AP, the resource request information to the second AP. 
     Embodiment 35: The method of any of embodiments 25-34, further comprising: after receiving the announcement frame, transmitting, by the first AP, a copy of the announcement frame. 
     Embodiment 36: The method of embodiment 35, further comprising: after transmitting the copy of the announcement frame, receiving, at the first AP, a trigger frame from the master AP in connection with the coordinated MU transmission; wherein participating in the coordinated MU transmission is responsive to the trigger frame. 
     Embodiment 37: A first access point (AP) associated with one or more first client stations, the first AP comprising: a wireless network interface device comprising one or more integrated circuit (IC) devices. The one or more IC devices are configured to: receive an announcement frame from a second AP associated with one or more second client stations, the announcement frame announcing a coordinated multi-user (MU) transmission involving at least the first AP and the second AP, wherein the announcement frame includes an indicator of a frequency resource unit (RU) allocated to the first AP for the coordinated MU transmission; and control the wireless network interface device to participate in the coordinated MU transmission using the frequency RU indicated by the announcement frame while the second AP also participates in the coordinated MU transmission. 
     Embodiment 38: The first AP of embodiment 37, wherein the one or more IC devices are further configured to control the wireless network interface device to participate in the coordinated MU transmission at least by: controlling the wireless network interface device to transmit a first downlink (DL) transmission to at least one first client station among the one or more first client stations, while the second AP transmits a second DL transmission to at least one second client station among the one or more second client stations. 
     Embodiment 39: The first AP of embodiment 38, wherein the one or more IC devices are further configured to: determine a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; control the wireless network interface device to transmit the first DL transmission in the first frequency RU while the second AP transmits the second DL transmission in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency. 
     Embodiment 40: The first AP of embodiment 38, wherein the one or more IC devices are further configured to: determine a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; determine one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; and control the wireless network interface device to transmit the first DL transmission in the first frequency RU using one or more first spatial streams while the second AP transmits the second DL transmission in the first frequency RU using the one or more second spatial streams. 
     Embodiment 41: The first AP of any of embodiments 38-40, wherein the one or more IC devices are further configured to: determine a duration of a signal field based on an indicator, in the announcement frame, of a signal field duration for the coordinated MU transmission, the signal field to be included in a physical layer (PHY) header in the first DL transmission; and generate the first DL transmission to include, in the PHY header of the first DL transmission, the signal field having the duration. 
     Embodiment 42: The first AP of embodiment 37, wherein the one or more IC devices are further configured to control the wireless network interface device to participate in the coordinated MU transmission at least by: controlling the wireless network interface device to transmit a first trigger frame to at least one first client station among the one or more first client stations, while the second AP transmits a second trigger frame to at least one second client station among the one or more second client stations; and receiving a first uplink (UL) transmission from the at least one first client station while the at least one second client station transmits a second UL transmission to the second AP in response to the second trigger frame. 
     Embodiment 43: The first AP of embodiment 42, wherein the one or more IC devices are further configured to: determine a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; control the wireless network interface device to transmit the first trigger frame in the first frequency RU while the second AP transmits the second trigger frame in a second frequency RU, wherein the second frequency RU does not overlap the first frequency RU in frequency; and receive the first UL transmission in the first frequency RU while the at least one second client station transmits the second UL transmission in the second RU. 
     Embodiment 44: The first AP of embodiment 42, wherein the one or more IC devices are further configured to: determine a first frequency RU based on the indicator, in the announcement frame, of the RU allocated to the first AP; determine one or more first spatial streams based on an indicator, in the announcement frame, of one or more spatial streams allocated to the first AP for the coordinated MU transmission; generate the first trigger frame to instruct the one or more first client stations to transmit, during the first UL transmission, in the first frequency RU via the one or more first spatial streams; and receive the first UL transmission in the first frequency RU via one or more first spatial streams while the at least one second client station transmits the second UL transmission in the first RU via one or more second spatial streams. 
     Embodiment 45: The first AP of any of embodiments 42-44, wherein the one or more IC devices are further configured to: determine a duration of the first trigger frame based on an indicator, in the announcement frame, of the duration of the first trigger frame; and generate the first trigger frame to have the determined duration. 
     Embodiment 46: The first AP of any of embodiments 37-45, wherein the one or more IC devices are further configured to, prior to receiving the announcement frame: generate resource request information to request an RU for the coordinated MU transmission; and control the wireless network interface device to transmit the resource request information to the second AP. 
     Embodiment 47: The first AP of any of embodiments 37-46, wherein the one or more IC devices are further configured to: control the wireless network interface device to transmit a copy of the announcement frame after receiving the announcement frame. 
     Embodiment 48: The first AP of embodiment 47, wherein the one or more IC devices are further configured to: after transmitting the copy of the announcement frame, receive a trigger frame from the master AP in connection with the coordinated MU transmission; and control the wireless network interface device to participate in the coordinated MU transmission responsive to receiving the trigger frame. 
     Embodiment 49: A method for coordinating transmissions in multiple wireless communication networks, the method comprising: determining, at a first access point (AP), that the first AP is to be a master AP for a coordinated multi-user (MU) transmission involving multiple APs including the first AP and one or more second APs; receiving, from the one or more second APs, resource request information regarding access to a wireless communication medium by the one or more second APs for the coordinated MU transmission; allocating, at the first AP, frequency resource units (RUs) to the first AP and the one or more second APs for the coordinated MU transmission based on the resource request information received from the one or more second APs; generating, at the first AP, an announcement frame regarding the coordinated MU transmission, the announcement frame including allocation information regarding the RUs allocated to the one or more second APs for the coordinated MU transmission; transmitting, by the first AP, the announcement frame to provide the one or more second APs with the allocation information regarding the RUs allocated to the one or more second APs; and participating, by the first AP, in the coordinated MU transmission while the one or more second APs participate in the coordinated MU transmission. 
     Embodiment 50: The method of embodiment 49, wherein receiving resource request information from one of the second APs comprises: receiving, by the first AP, a packet from the one second AP, the packet sent in response to the second AP contending for the wireless communication medium in order to transit the packet and obtaining the wireless communication medium, wherein the packet includes resource request information from the one second AP. 
     Embodiment 51: The method of embodiment 49, further comprising: generating, at the first AP, a trigger frame configured to prompt at least one second AP among the one or more second APs to transmit resource request information; and transmitting, by the first AP, the trigger frame to prompt the at least one second AP to transmit resource request information; and wherein receiving the resource request information comprises receiving resource request information from the at least one second AP responsive to transmitting the trigger frame. 
     Embodiment 52: The method of embodiment 51, wherein generating the trigger frame comprises: including, in the trigger frame, an identifier of one second AP; wherein receiving the resource request information comprises receiving resource request information from the one second AP. 
     Embodiment 53: The method of embodiment 52, wherein the identifier of the one second AP in the trigger frame comprises: a media access control (MAC) address of the one second AP. 
     Embodiment 54: The method of claim  52 , wherein the identifier of the one second AP in the trigger frame comprises: a first set of bits from a basic service set (BSS) color identifier of the one second AP; and a second set of bits generated from a media access control (MAC) address of the one second AP. 
     Embodiment 55: The method of embodiment 54, wherein second set of bits generated from the MAC address of the one second AP comprises: a set of bits generated by applying a hash function to the MAC address of the one second AP. 
     Embodiment 56: The method of any of embodiments 49-55, wherein receiving the resource request information comprises receiving, from one second AP: an indicator of a frequency bandwidth requested by the one second AP for the coordinated MU transmission. 
     Embodiment 57: The method of any of embodiments 49-56, wherein receiving the resource request information comprises receiving, from one second AP: an indicator of a duration of a packet to be transmitted during the coordinated MU transmission. 
     Embodiment 58: The method of any of embodiments 49-57, wherein receiving the resource request information comprises receiving, from one second AP: an indicator of a duration of a signal field in a physical layer (PHY) header of a packet to be transmitted during the coordinated MU transmission. 
     Embodiment 59: A communication device, comprising: a wireless network interface device implemented on one or more ICs, the one or more ICs configured to implement any of the methods of embodiments 49-58. 
     Embodiment 60: A method for wireless communication by a client station associated with a first access point (AP), the method comprising: receiving, at the client station, an announcement frame transmitted by a second AP with which the client station is not associated, the announcement frame announcing a coordinated multi-user (MU) transmission involving the second AP and one or more other APs, wherein the announcement frame includes one or more respective network identifiers of the one or more other APs, and wherein the announcement frame further includes a duration field indicating a time duration corresponding to the coordinated MU transmission; in response to receiving the announcement frame, setting a network allocation vector (NAV) counter of the client station based on a value of the duration field in the announcement frame; determining, at the client station, that the announcement frame includes a network identifier of the first AP; determining, at the client station, the client station is to transmit to the first AP in a communication channel as part of the coordinated MU transmission; determining, at the client station, that the communication channel is idle, including ignoring the NAV counter in response to determining that the NAV counter was set in response to the announcement frame that includes the network identifier of the first AP; and transmitting, by the client station, as part of the coordinated MU transmission in response to determining that the communication channel is idle. 
     Embodiment 61: The method of embodiment 60, wherein: the coordinated MU transmission includes respective downlink transmissions by the first AP and the second AP; and transmitting, by the client station, as part of the coordinated MU transmission comprises transmitting to the first AP after the respective downlink transmissions by the first AP and the second AP. 
     Embodiment 62: The method of either of embodiments 60 or 61, further comprising: maintaining, at the client station, a first NAV counter for transmissions in a basic service set (BSS) managed by the first AP; and maintaining, at the client station, a second NAV counter for transmissions not within the BSS managed by the first AP; wherein setting the NAV counter of the client station based on the value of the duration field in the announcement frame comprises setting the first NAV counter in response to determining that the announcement frame transmitted by the second AP includes the network identifier of the first AP. 
     Embodiment 63: A communication device, comprising: a wireless network interface device implemented on one or more ICs, the one or more ICs configured to implement any of the methods of embodiments 60-62. 
     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 suitable computer readable memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, etc. The software or firmware instructions may include machine readable instructions that, when executed by one or more processors, cause the one or more processors 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), a programmable logic device (PLD), 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.