Patent Publication Number: US-11382132-B1

Title: Methods and devices for communicating in a wireless network with multiple virtual access points

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/963,045, filed Dec. 8, 2015, entitled “METHODS AND DEVICES FOR DETERMINING CHANNEL STATE,” which claims the benefit of U.S. Provisional Patent Application No. 62/089,026, filed Dec. 8, 2014, entitled “BSS COLOR AND MULTIPLE BSSID.” Both of the applications referenced above are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks and, more particularly, to wireless local area networks that utilize orthogonal frequency division multiplexing (OFDM). 
     BACKGROUND 
     The Institute for Electrical and Electronics Engineers (IEEE) 802.11 family of Standards (generally “802.11”) has gone through several iterations over the last decade. In some of the 802.11 standards, such as 802.11ah and beyond, the identity of the Basic Service Set (BSS) (e.g., as managed by an access point (AP) of the BSS) is indicated in a Physical Layer Convergence Procedure (PLCP) Protocol Data Unit (PPDU) by a set of bits that described the “color” of the BSS. The color of a BSS corresponds to an identifier (ID) of the BSS that is shorter than a BSS identifier (BSSID) defined by 802.11. The BSS color may be contained in the Physical Layer (PHY) Signal (SIG) field in a PHY header of a PPDU, whereas the BSSID is typically included in a media access control (MAC) portion of PPDUs. A device (e.g., an AP or client) in a BSS can determine whether a PPDU is from the BSS to which the device belongs (the “same-BSS”) or some other BSS (e.g., an overlapping BSS (OBSS)), a device (e.g., an AP or client) by decoding the SIG field and interpreting BSS color bits included therein. 
     One of the newer implementations of 802.11 being discussed is 802.11ax (sometimes referred to as 802.11 HE or 802.11 HEW). 802.11ax contemplates dynamically adjusting the energy level at which a channel is deemed to be clear depending on whether the energy corresponds to same-BSS signals or to signals from another BSS. Such a scheme helps to promote spatial reuse between neighboring networks. 
     SUMMARY 
     In an embodiment, a method for communicating in a wireless local area network (WLAN) includes: receiving, at a communication device associated with a physical access point (AP) via a wireless communication medium, a physical layer (PHY) data unit having a PHY preamble; determining, at the communication device, a value of a basic service set (BSS) color identifier in the PHY preamble; and performing, at the communication device, a clear channel assessment (CCA) procedure to determine whether the communication device can perform a spatial reuse transmission via the wireless communication medium during reception of the PHY data unit. Performing the CCA procedure to determine whether the communication device can perform a spatial reuse transmission includes: determining whether the BSS color identifier is a color value corresponding to all of multiple virtual APs implemented by the physical AP; and selectively determining whether the communication device can perform the spatial reuse transmission during reception of the PHY data unit as a function of the determination of whether the BSS color identifier is the color value corresponding to all of the multiple virtual APs implemented by the physical AP. The method also includes: when performing the CCA procedure determines that the communication device can perform the spatial reuse transmission during reception of the PHY data unit, transmitting, by the communication device, via the wireless communication medium during reception of the PHY data unit. 
     In another embodiment, a communication device comprises: a wireless network interface device having one or more integrated circuit (IC) devices and one or more transceivers implemented on the one or more IC devices. The one or more IC devices configured to: receive, from a physical access point (AP) via a wireless communication medium, a physical layer (PHY) data unit having a PHY preamble; determine a value of a basic service set (BSS) color identifier in the PHY preamble; and perform a clear channel assessment (CCA) procedure to determine whether the communication device can perform a spatial reuse transmission via the wireless communication medium during reception of the PHY data unit. Performing the CCA procedure to determine whether the communication device can perform a spatial reuse transmission includes: determining whether the BSS color identifier is a color value corresponding to all of multiple virtual APs implemented by the physical AP; and selectively determining whether the communication device can perform the spatial reuse transmission during reception of the PHY data unit as a function of the determination of whether the BSS color identifier is the color value corresponding to all of the multiple virtual APs implemented by the physical AP. The one or more IC devices are further configured to, when performing the CCA procedure determines that the communication device can perform the spatial reuse transmission during reception of the PHY data unit, control the one or more transceivers to transmit via the wireless communication medium during reception of the PHY data unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example wireless local area network (WLAN), according to an embodiment. 
         FIG. 2  is diagram of an example multiple basic service set identifier (BSSID) element that is communicated in a wireless network, according to an embodiment. 
         FIG. 3  is a diagram of two High Efficiency (HE) PPDUs transmitted by different virtual APs, according to an embodiment. 
         FIG. 4  is a message sequence diagram illustrating transmissions by multiple client device associated with different virtual APs, according to an embodiment. 
         FIGS. 5A-5C  are diagrams of example channel allocation schemes in an 80 MHz communication channel, according to various embodiments. 
         FIGS. 6A, 6B, 6C, and 6D  are diagrams illustrating example OFDM sub-channels of an 80 MHz communication channel, according to various embodiments. 
         FIG. 7  is a diagram of an example PHY data unit, according to an embodiment. 
         FIG. 8  is a diagram of an example multi-user PHY data unit, according to an embodiment. 
         FIG. 9  is a message sequence diagram illustrating transmissions among multiple devices, according to an embodiment. 
         FIG. 10A  is a message sequence diagram illustrating multi-user communications, according to an embodiment. 
         FIG. 10B  is a diagram of a downlink orthogonal frequency division multiple access (DL OFDMA) transmission of  FIG. 10A , according to an embodiment. 
         FIG. 11A  is a message sequence diagram illustrating transmissions between two virtual access points and multiple clients, according to an embodiment. 
         FIG. 11B  is a set of diagrams illustrating transmissions of multiple clients as part of a multi-user transmission illustrated in  FIG. 11A , according to an embodiment. 
         FIG. 12  is a message sequence diagram of transmissions among multiple devices, according to an embodiment. 
         FIG. 13A  is a message sequence diagram illustrating transmissions between two virtual access points and multiple clients, according to an embodiment. 
         FIG. 13B  is a set of diagrams illustrating transmissions of multiple clients as part of a multi-user transmission illustrated in  FIG. 13A , according to an embodiment. 
         FIG. 14  is a flow diagram of an example method for informing devices in a wireless network of network identifiers utilized by multiple basic service sets (BSSs) implemented in the wireless network, according to an embodiment. 
         FIG. 15  is a message sequence diagram of transmissions among multiple devices, according to an embodiment. 
         FIG. 16  is a flow diagram of an example method for determining whether a wireless communication medium is busy, according to an embodiment. 
         FIG. 17  is a flow diagram of an example method for performing a multi-user wireless transmission, according to an embodiment. 
         FIG. 18  is a flow diagram of an example method for performing a transmission as part of a multi-user wireless transmission, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In an environment in which a single physical access point (AP) implements a plurality of virtual APs managing a plurality of basic service sets (BSSs), various techniques described below assist a client device associated with one of the virtual APs to quickly determine whether a transmission corresponds to one of the BSSs managed by the AP or a BSS managed by another physical AP, according to various embodiments. For example, physical layer (PHY) headers of PHY data units may include respective identifiers of the respective BSSs to which the PHY data units correspond, according to some embodiments. When receiving a PHY data unit, a client device may examine the identifier of the BSS in the PHY header to determine whether the PHY data unit corresponds to a BSS managed by the physical AP with which the client device is associated, according to some embodiments. Quickly determining whether a transmission corresponds to one of the BSSs managed by the AP or a BSS managed by another physical AP may be useful, for example, in some clear channel assessment (CCA) procedures, according to some embodiments. For instance, as merely an illustrative scenario, for a received signal at a given power level, a client device may determine that a communication channel is busy if the client device determines that the signal corresponds to a transmission in a BSS served by the physical AP to which the client device is associated; whereas if the client device determines that the signal corresponds to a transmission in a BSS served by a different physical AP, the client device may determine that a communication channel is clear, according to an illustrative embodiment. 
     In some embodiments, to facilitate multi-user transmissions having data from multiple BSSs served by a single physical AP, a PHY preamble may include only one BSS identifier. 
       FIG. 1  is a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment. An AP  14  includes a host processor  15  coupled to a network interface device  16 . The network interface device  16  includes a medium access control (MAC) processing unit  18  and PHY processing unit  20 . The PHY processing unit  20  includes a plurality of transceivers  21 , and the transceivers  21  are coupled to a plurality of antennas  24 . Although three transceivers  21  and three antennas  24  are illustrated in  FIG. 1 , the AP  14  includes different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  21  and antennas  24  in other embodiments. 
     In various embodiments, the network interface device  16  is implemented on one or more integrated circuit (IC) devices. For example, in an embodiment, at least a portion of the MAC processing unit  18  is implemented on a first IC device and at least a portion of the PHY processing unit  20  is implemented on a second IC device. As another example, at least a portion of the MAC processing unit  18  and at least a portion of the PHY processing unit  20  are implemented on a single IC device. 
     The WLAN  10  includes a plurality of client stations  25 . Although four client stations  25  are illustrated in  FIG. 1 , the WLAN  10  includes different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations  25  in various scenarios and embodiments. 
     A client station  25 - 1  includes a host processor  26  coupled to a network interface device  27 . The network interface device  27  includes a MAC processing unit  28  and a PHY processing unit  29 . The PHY processing unit  29  includes a plurality of transceivers  30 , and the transceivers  30  are coupled to a plurality of antennas  34 . Although three transceivers  30  and three antennas  34  are illustrated in  FIG. 1 , the client station  25 - 1  includes different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  30  and antennas  34  in other embodiments. 
     In various embodiments, the network interface device  27  is implemented on one or more IC devices. For example, in an embodiment, at least a portion of the MAC processing unit  28  is implemented on a first IC device and at least a portion of the PHY processing unit  29  is implemented on a second IC device. As another example, at least a portion of the MAC processing unit  28  and at least a portion of the PHY processing unit  29  are implemented on a single IC device. 
     In an embodiment, one or more of the client stations  25 - 2 ,  25 - 3 , and  25 - 4  has a structure the same as or similar to the client station  25 - 1 . In these embodiments, the client stations  25  structured like the client station  25 - 1  have the same or a different number of transceivers and antennas. For example, the client station  25 - 2  has only two transceivers and two antennas (not shown), according to an embodiment. 
     In an embodiment, the AP is configured to operate according to a wireless communication protocol that utilizes Orthogonal Frequency Multiple Division Access (OFDMA) technology and/or multi-user multiple input, multiple output (MU-MIMO) technology. The wireless communication protocol is sometimes referred to herein as the IEEE 802.11ax Standard, the high efficiency WiFi protocol, the HEW protocol, the HE protocol, or 802.11 HE. 
     In an embodiment, the AP  14  (e.g., the network interface device  16  of the AP  14 ) is configured to transmit independent data simultaneously to multiple client stations  25  via different spatial streams (e.g., downlink (DL) MU-MIMO) and/or via different OFDM sub-channels (e.g., DL OFDMA). In an embodiment, the AP  14  (e.g., the network interface device  16  of the AP  14 ) is configured to receive independent data simultaneously from multiple client stations  25  via different spatial streams (e.g., uplink (UL) MU-MIMO) and/or via different OFDM sub-channels (e.g., UL OFDMA). In some embodiments, two or more of the client stations  25  are configured to receive respective data streams that are transmitted simultaneously by the AP  14  (e.g., DL OFDMA and/or DL MU-MIMO). For example, in one embodiment, the network interface device  27  is configured to receive a data stream among a plurality of independent data streams transmitted simultaneously by the AP  14  to multiple client stations  25  via different spatial streams and/or via different OFDM sub-channels. In other embodiments, two or more of the client stations  25  additionally or alternatively are configured to transmit corresponding data streams to the AP  14  such that the AP  14  receives the data streams simultaneously (e.g., UL OFDMA and/or UL MU-MIMO). For example, in one embodiment, the network interface device  27  is configured to transmit a data stream while one or more other client devices  25  transmit one or more other independent data streams simultaneously to the AP  14  via different spatial streams and/or via different OFDM sub-channels. 
     In an embodiment, the AP  14  and the client stations  25  contend for communication medium using carrier sense multiple access with collision avoidance (CSMA/CA) protocol or another suitable medium access protocol. In an embodiment, the AP  14  and the client stations employ a clear channel assessment (CCA) procedure, in which the AP/client station determines the energy level of the medium in order to determine whether the medium is busy or idle. If the medium is idle, the device can transmit. If the medium is busy, the device waits a backoff period and then checks the medium again after the backoff period. A threshold energy level for determining whether the medium is idle or busy may depend upon the bandwidth of the channel being used by the device and on whether the energy corresponds to a transmission that conforms to the wireless communication protocol. For example, in 802.11, if the channel bandwidth is 20 Megahertz (MHz), the threshold level is −82 decibel-milliwatts (dBm) for energy from valid 802.11 transmissions. For channel bandwidths of 40 MHz, 80 MHz, and 160 MHz, the threshold levels are −79 dBm, −76 dBm, and −73 dBm, respectively. For energy not identified by the device as a valid 802.11 signal, the threshold level is −62 dBm. 
     In an embodiment, the AP  14  and the client stations  25  employ a dynamic CCA procedure. In the dynamic CCA procedure, the AP/client device may use a higher threshold level for valid 802.11 signals from a different BSS as compared to the threshold level for valid 802.11 signals from the same BSS. For example, an AP/client device might deem a 20 MHz channel to be idle if the energy level of an 802.11 signal from another BSS is less than −62 dBm (i.e., the same threshold level as for energy corresponding to signals that are not valid 802.11 signals), but deem the channel to be busy if the energy level of an 802.11 signal from the same BSS is greater than −82 dBm. Thus, an energy level of −70 dBm of a valid 802.11 signal from a different BSS would result in the device determining that the channel is idle, while an energy level of −70 dBm resulting from same-BSS signals would result in the device determining that the channel is busy. Allowing a higher CCA level for transmissions corresponding to another BSS helps to promote spatial reuse between different BSSs, at least in some embodiments and/or scenarios. In some embodiments, the AP  14  and/or the client stations  25  are able to effectively override dynamic CCA when they determine that an energy level is based on transmissions to or from the same physical AP (although different virtual APs). 
     Further, in an embodiment, the AP  14  or a client station  25  dynamically selects a bandwidth for a transmission based on channels available for the transmission. In some embodiments, communication between the AP  14  and the client stations  25  can occur in a primary channel of the WLAN  10 , in both a primary and a secondary channel of the WLAN  10 , exclusively on a secondary channel of the WLAN  10 , etc. 
     In an embodiment, the AP  14  is configured to transmit different independent data to different client stations  25  simultaneously by generating an OFDMA data unit that includes different independent data modulated in respective sub-channels of a communication channel. In an embodiment, each sub-channel incudes one or more sub-channel blocks, each sub-channel block corresponding to a set of sub-carriers within the OFDMA data unit. In an embodiment, the AP  14  allocates different sub-channels to different client stations and generates the OFDMA data unit that respective data is modulated in sub-channel blocks corresponding to the sub-channels allocated to the client stations. The AP may assign the primary and the non-primary communication channels in any suitable manner to the one or more client stations, in various embodiments. 
     In an embodiment, the AP  14  (e.g., the host processor  15  and/or the network interface device  16  (e.g., the MAC processor device  18 )) sets up a plurality of virtual APs, represented in  FIG. 1  by a first virtual AP  35 - 1  and a second virtual AP  35 - 2 . Each virtual AP uses the resources of the AP  14  to provide a BSS to the WLAN  10 , effectively appearing to the clients  25  as multiple WLANs. To distinguish between the AP  14  and the virtual APs  35 - 1  and  35 - 2 , the AP  14  will sometimes be referred to herein as a “physical AP.” Each virtual AP  35  corresponds to a respective Basic Service Set Identification (BSSID). In an embodiment, the physical AP  14  (e.g., one of the virtual APs) broadcasts a beacon that advertises the BSSIDs of the virtual APs  35 . In an embodiment, the BSSIDs are contained in a multiple BSSID information element of the beacon.  FIG. 2  is a diagram of an example multiple BSSID information element  200 , according to an embodiment. The BSSIDs are included in the field  220 , in an embodiment. 
     Referring again to  FIG. 1 , the virtual APs  35  may utilize a single beacon and may broadcast, via the single beacon, one or more properties, parameter values, etc., shared by the virtual APs, according to various embodiments. For example, in an embodiment, the following information elements in a beacon are common for all of the virtual APs  35  (e.g., each information element provides information that is same for the virtual APs): a Timestamp, a Beacon Interval, a Direct Sequence Spread Spectrum (DSSS) Parameter Set, a Frequency Hopping (FH) Parameter Set, an Independent BSS (IBSS) Parameter Set, a Country, FH Parameters, a FH Pattern Table, a Channel Switch Assignment, an Extended Channel Switch Announcement, Supported Operating Classes, an IBSS Dynamic Frequency Selection (DFS), Extended Rate Physical (ERP) Information, High Throughput (HT) Capabilities, HT Operation elements, etc. Because these elements are common amongst the virtual APs, in an embodiment, the physical AP  14  does not include these elements in the Nontransmitted BSSID Profile field  220  ( FIG. 2 ). In an embodiment, the values of these elements for each nontransmitted BSSID are always the same as the corresponding transmitted BSSID element values. In an embodiment, the first 2 n  bits of the Traffic Indication Map (TIM) are reserved for broadcast/multicast for each BSS, where n is a suitable positive integer. In an embodiment, the value of n is reported in the multiple BSSID element. In an embodiment, the remainder of the Association ID (AID) space is shared by all BSSs. Furthermore, in an embodiment, each BSS (e.g., each virtual AP) has a respective Delivery Traffic Indication Message (DTIM) interval that need not be the same as the other BSSs. 
     In some embodiments, multiple A-MPDUs to/from multiple STAs are included in a single MU transmission. In some embodiments, an A-MPDU in a single MU transmission includes MPDUs to/from multiple STAs. In some embodiments, at most one A-MPDU can be included in a single SU transmission. 
       FIG. 3  is a diagram of an example orthogonal frequency division multiplexing (OFDM) multi-user (MU) transmission corresponding to two PPDUs (e.g., 802.11 HE PPDUs) simultaneously transmitted by a physical AP, where each PPDU corresponds to a different virtual AP having a same BSS color value, according to an embodiment. In the scenario illustrated in  FIG. 3 , a first PPDU  304  is transmitted to a first client device (STA 1 ) via a 20 MHz primary channel of a first BSS served by a first virtual AP; and a second PPDU  308  is transmitted to a second client device (STA 2 ) via a 20 MHz secondary channel of a second BSS served by a second virtual AP. In an embodiment, the first BSS and the second BSS have the same 20 MHz primary channel. 
     The first PPDU  304  includes a first PHY preamble  320  having a first HE signal (SIG) field (HE SIG), and the second PPDU  308  includes a second PHY preamble  324  having a second SIG field (HE SIG). In the scenario illustrated in  FIG. 3 , the first virtual AP has been assigned a first identifier (referred to herein as a “color”), and the second virtual AP has been assigned the same first identifier (or color). In some embodiments, the color is not globally unique. In some embodiments, a range of possible colors is smaller than a range of possible BSSIDs, and thus each color can be represented by less bits than a BSSID. In an embodiment, each HE SIG field includes a color field to indicate the color of the BSS to which the PPDU corresponds. Thus, in an embodiment, the color field in the first HE SIG field in the preamble  320  has a value corresponding to the first identifier (color), and the color field in the second HE SIG field in the preamble  324  has a value corresponding to the same first identifier (color). 
     When different virtual APs have different BSS colors, A-MPDUs transmitted to STAs which are associated with different virtual APs cannot be in one DL MU transmission, and A-MPDUs transmitted from STAs which are associated with different virtual APs cannot be in one UL MU transmission. For instance, in some embodiments, HE SIG fields of MU transmissions use a whole MU transmission bandwidth, e.g. in a 40 MHz MU transmission, HE SIGs from different virtual APs occupy the same 40 MHz bandwidth. 
     On the other hand, an HE SIG of a SU PPDU includes the BSS color of the virtual AP that is the transmitter or the receiver of the SU PPDU. When different virtual APs have different BSS colors, the HE SIGs of SU PPDUs transmitted by different virtual APs have different BSS colors, and the BSS color of SU PPDUs transmitted to different virtual APs have different BSS colors. When the second virtual AP transmits a SU HE PPDU to an associated STA, the HE SIG of the SU HE PPDU includes the BSS color of the second virtual AP. Consequently, devices associated with the first virtual AP may utilize a CCA energy threshold corresponding to an “other BSS” with transmissions from the second virtual AP, and vice versa, even though the transmissions are from the same physical AP. 
       FIG. 4  is a message sequence diagram of multiple transmissions among STA 1 , STA 2 , a first virtual AP (AP 1 ) and a second virtual AP (AP 2 ). In this example, STA 1  is associated with AP 1 , whose BSS color will be referred to as Color  1 , and STA 2  is associated with AP 2 , whose BSS color will be referred to as Color  2 , which is different than Color  1 . At time t 1 , STA 1  begins transmitting an HE PPDU  404  to AP 1 . STA 2  has its own HE PPDU that STA 2  needs to transmit to AP 2 . STA 2  therefore carries out a CCA procedure to determine whether the channel is clear. In carrying out the CCA procedure, STA 2  detects and decodes the HE SIG of HE PPDU  404 , identifies the BSS color associated with HE PPDU  404  as Color  1 , and, because Color  1  is different than the color of AP 2  with which STA 2  is associated, STA 2  uses the dynamic CCA level (i.e., allows for a higher level of 802.11-based energy than a standard CCA level associated with 802.11 transmissions in a same BSS). Using the dynamic CCA level, STA 2  concludes that the channel is clear and, at time t 2 , begins transmitting an HE PPDU  408  to AP 2 . However, because AP 1  and AP 2  exist are implemented by a same physical AP, a collision occurs. 
       FIGS. 5A-5C  are diagrams of example channel allocation schemes in an 80 MHz communication channel, according to various embodiments. In each of  FIGS. 5A-5C , respective 20 MHz sub-channels are allocated to each of four client stations  25  (STA 1 , STA 2 , STA 3  and STA 4 ). In  FIG. 5A , each of the sub-channels, allocated to a particular one of STA 1 , STA 2 , STA 3  and STA 4 , includes of a single sub-channel block of adjacent sub-carriers allocated to the particular station. In  FIG. 5B , each of the sub-channels, allocated to a particular one of STA 1 , STA 2 , STA 3  and STA 4 , includes four respective sub-channel blocks uniformly spaced over the entire 80 MHz channel. In  FIG. 5C , each of the sub-channels includes of four respective non-uniformly (e.g., randomly) spaced over the entire 80 MHz channel. In each of  FIGS. 5B and 5C , each of the sub-channel blocks allocated to a particular client station includes a block of adjacent sub-carriers, wherein the block of adjacent sub-carriers includes a subset of sub-carriers, of the 80 MHz channel, allocated to the particular client station, according to an embodiment. 
     In some embodiments, a sub-channel having a suitable bandwidth less than the smallest bandwidth of the WLAN can be allocated to a client station. For example, in some embodiments in which the smallest channel bandwidth of the WLAN  10  is 20 MHz, sub-channel having bandwidth less than 20 MHz, such as sub-channels having bandwidths of 10 MHz and/or 5 MHz can be allocated to client stations, in at least some scenarios. 
       FIGS. 6A, 6B, 6C, and 6D  are diagrams illustrating example OFDM sub-channels of an 80 MHz communication channel, according to various embodiments. In  FIG. 6A , the communication channel is partitioned into four contiguous sub-channels, each having a bandwidth of 20 MHz. The OFDM sub-channels include independent data streams for four client stations. In  FIG. 6B , the communication channel is partitioned into two contiguous sub-channel channels, each having a bandwidth of 40 MHz. The OFDM sub-channels include independent data streams for two client stations. In  FIG. 6C , the communication channel is partitioned into three contiguous OFDM sub-channels. Two OFDM sub-channels each have a bandwidth of 20 MHz. The remaining OFDM sub-channel has a bandwidth of 40 MHz. The OFDM sub-channels include independent data streams for three client stations. In  FIG. 6D , the communication channel is partitioned into four contiguous OFDM sub-channels. Two OFDM sub-channels each have a bandwidth of 10 MHz, one OFDM sub-channel has a bandwidth of 20 MHz, and one OFDM sub-channel has a bandwidth of 40 MHz. The OFDM sub-channels include independent data streams for three client stations. 
     Although in  FIGS. 6A, 6B, 6C, and 6D  the OFDM sub-channels are contiguous across the communication channel, in other embodiments the OFDM sub-channels are not contiguous across the communication channel (i.e., there are one or more gaps between the OFDM sub-channels). In an embodiment, each gap is at least as wide as one of the OFDM sub-channel blocks. In another embodiment, at least one gap is less than the bandwidth of an OFDM sub-channel block. In another embodiment, at least one gap is at least as wide as 1 MHz. In one embodiment, the AP includes a plurality of radios and different OFDM sub-channel blocks are transmitted using different radios. 
     In  FIGS. 6A, 6B, 6C, and 6D , each sub-channel corresponds to a single sub-channel block of adjacent sub-carriers allocated to a particular client station. In other embodiments, each of at least some sub-channels of an 80 MHz channel corresponds to several sub-channel blocks, each having adjacent sub-carriers, where the several sub-channel blocks collectively comprise the sub-carriers allocated to a particular client station. The several sub-channel blocks corresponding to a particular client station are uniformly or non-uniformly distributed over the 80 MHz channel, for example as described above with respect to  FIGS. 6B and 6C , in some embodiments. In such embodiments, an independent data stream for the particular client station is accordingly distributed over the 80 MHz channel. 
       FIG. 7  is a diagram of an OFDM data unit  700 , according to an embodiment. In some embodiments, an AP (e.g., the AP  14 ) is configured to generate and transmit OFDM data units having a format such as illustrated in  FIG. 7  to client devices (e.g., client devices  25 ), and/or a client station (e.g., the client station  25 - 1 ) is configured to transmit the data unit  700  to the AP (e.g., the AP  14 ). The data unit  700  conforms to the HEW protocol and occupies an 80 MHz band. In other embodiments, data units similar to the data unit  700  occupy different suitable bandwidths such as 20 MHz, 40 MHz, 120 MHz, 160 MHz, or any suitable bandwidth. The data unit  700  is suitable for “mixed mode” situations, such as when a WLAN  10  includes a client station that conforms to a legacy protocol, but not the HEW protocol. The data unit  700  can be utilized in other situations as well. 
     The data unit  700  includes a PHY preamble having four legacy short training fields (L-STFs)  705 ; four legacy long training fields (L-LTFs)  710 ; four legacy signal fields (L-SIGs)  715 ; four first high efficiency WLAN signal fields (HEW-SIGAs)  720 ; four second high efficiency WLAN signal fields (HEW-SIGBs)  722 ; a high efficiency WLAN short training field (HEW-STF)  725 ; and N high efficiency WLAN long training fields (HEW-LTFs)  730 , where N is an integer. The data unit  700  also includes a high efficiency WLAN data portion (HEW-DATA)  740 . The L-STFs  705 , the L-LTFs  710 , and the L-SIGs  715  form a legacy portion of the PHY preamble. The HEW-SIGA  720 , the HEW HEW-SIGBs  722 ; the HEW-STF  725 , and the HEW-LTFs  730  form a high efficiency WLAN (HEW) portion of the PHY preamble. In an embodiment, a color field is included in the HEW-SIGAs  720 . In another embodiment, the color field is included in the HEW-SIGB s  722 . In an embodiment, each STA transmits its PHY SIG only in 20 MHz channels that overlap with the subchannel in which the STA is transmitting. In another embodiment, each STA transmit its PHY SIG in all 20 MHz channels of the MU transmission, and, in an embodiment, the content of the PHY SIGs by multiple STAs in each 20 MHz channel is the same. In an embodiment, the PHY SIGs by multiple STAs are made to have the same content so that other stations can properly decode the content of the PHY SIG. For instance, if the content of different PHY SIGs was different (e.g., different color values), this may garble the content of the PHY SIGs when received by other communication devices, at least in some embodiments and/or scenarios. 
     Each of the L-STFs  705 , each of the L-LTFs  710 , each of the L-SIGs  715 , each of the HEW-SIGAs  720 , and each of the HEW-SIGBs  722  occupy a 20 MHz band, in one embodiment. The data unit  700  is described as having an 80 MHz contiguous bandwidth for the purposes of illustrating an example frame format, but such frame format is applicable to other suitable bandwidths (including noncontiguous bandwidths). For instance, although the preamble of the data unit  700  includes four of each of the L-STFs  705 , the L-LTFs  710 , the L-SIGs  715 , the HEW-SIGAs  720 , and the HEW-SIGBs  722  in other embodiments in which an OFDM data unit occupies a cumulative bandwidth other than 80 MHz, such as 20 MHz, 40 MHz, 120 MHz, 160 MHz, etc., a different suitable number of the L-STFs  705 , the L-LTFs  710 , the L-SIGs  715 , the HEW-SIGAs  720 , and the HEW-SIGBs  722  are utilized accordingly. For example, for an OFDM data unit occupying a 20 MHz cumulative bandwidth, the data unit includes one of each of the L-STFs  705 , the L-LTFs  710 , the L-SIGs  715 , the HEW-SIGAs  720 , and the HEW-SIGBs  722 ; a 40 MHz bandwidth OFDM data unit includes two of each of the fields  705 ,  710 ,  715 ,  720 , and  722 ; a 120 MHz bandwidth OFDM data unit includes six of each of the fields  705 ,  710 ,  715 ,  720 , and  722 ; a 160 MHz bandwidth OFDM data unit includes eight of each of the fields  705 ,  710 ,  715 ,  720 , and  722 , and so on, according to some embodiments. 
     In the example data unit  700 , each of the HEW-STF  725 , the HEW-LTFs  730 , and the HEW-DATA  740  occupy the entire 80 MHz cumulative bandwidth of the data unit  700 . Similarly, in the case of an OFDM data unit conforming to the HEW protocol and occupying a cumulative bandwidth such as 20 MHz, 40 MHz, 120 MHz, or 160 MHz, each of the HEW-STF  725 , the HEW-LTFs  730 , and the HEW-DATA  740  occupy the corresponding entire cumulative bandwidth of the data unit, in some embodiments. 
     In some embodiments, the 80 MHz band of the data unit  700  is not contiguous, but includes two or more smaller bands, such as two 40 MHz bands, separated in frequency. Similarly, for other OFDM data units having different cumulative bandwidths, such as a 160 MHz cumulative bandwidth, in some embodiments the band is not contiguous in frequency. Thus, for example, the L-STFs  705 , the L-LTFs  710 , the L-SIGs  715 , the HEW-SIGAs  720 , and the HEW-SIGBs  722  occupy two or more bands that are separated from each other in frequency, and adjacent bands are separated in frequency by at least one MHz, at least five MHz, at least 10 MHz, at least 20 MHz, for example, in some embodiments. 
     According to an embodiment, each of the L-STFs  705  and each of the L-LTFs  710  have a format as specified in a legacy protocol such as the IEEE 802.11a Standard, the IEEE 802.11n Standard, and/or the IEEE 802.11ac Standard. In an embodiment, each of the L-SIGs  715  has a format at least substantially as specified in legacy protocol (e.g., the IEEE 802.11a Standard, the IEEE 802.11n Standard, and/or the IEEE 802.11ac Standard). In such embodiments, the length and rate subfields in the L-SIGs  715  is set to indicate a duration T corresponding to the remainder of the data unit  700  after the legacy portion. This permits client stations that are not configured according to the HEW protocol to determine an end of the data unit  700  for carrier sense multiple access/collision avoidance (CSMA/CA) purposes, for example. For example, legacy client stations determine the duration of the remainder of the data unit  700  and refrain from accessing the medium (or at least transmitting in the medium) for the duration of the remainder of the data unit  700 , in an embodiment. In other embodiments, each of the L-SIGs  715  has a format at least substantially as specified in legacy protocol (e.g., the IEEE 802.11a Standard, the IEEE 802.11n Standard, and/or the IEEE 802.11ac Standard) but with length field in the L-SIGs  715  set to indicate a duration of the time remaining in a transmission opportunity during which the data unit  700  is transmitted. In such embodiments, client stations that are not configured according to the HEW protocol determine an end of the TXOP and refrain from accessing the medium (or at least transmitting in the medium) for the duration of the TXOP, in an embodiment. 
     In the data unit  700 , frequency domain symbols of the legacy portion are repeated over four 20 MHz subbands of the 80 MHz band. Legacy client stations that are configured to operate with 20 MHz bandwidth will recognize a legacy preamble in any of the 20 MHz subbands. In some embodiments, the modulations of the different 20 MHz subband signals are rotated by different suitable angles. In one example, a first subband is rotated 0 degrees, a second subband is rotated 90 degrees, a third subband is rotated 180 degrees, and a fourth subband is rotated 270 degrees, in an embodiment. In other examples, different suitable rotations are utilized. As just one example, a first subband is rotated 45 degrees, a second subband is rotated 90 degrees, a third subband is rotated −45 degrees, and a fourth subband is rotated −90 degrees, in an embodiment. 
     In some embodiments, the modulations of the HEW-SIGAs  720  in the different 20 MHz subbands is rotated by different angles. In one example, a first subband is rotated 0 degrees, a second subband is rotated 90 degrees, a third subband is rotated 180 degrees, and a fourth subband is rotated 270 degrees, in an embodiment. In other examples, different suitable rotations are utilized. As just one example, a first subband is rotated 45 degrees, a second subband is rotated 90 degrees, a third subband is rotated −45 degrees, and a fourth subband is rotated −90 degrees, in an embodiment. In an embodiment, the same rotations utilized in the legacy portion are utilized for the HEW-SIGAs  720 . In some embodiments, the modulations of the HEW-SIGB s  722  in the different 20 MHz subbands are similarly rotated by different angles. In at least some examples, the HEW-SIGAs  720  are collectively referred to as a single high efficiency WLAN signal field (HEW-SIGA)  720 . In at least some examples, the HEW-SIGBs  722  are collectively referred to as a single high efficiency WLAN signal field (HEW-SIGB)  722 . 
     In an embodiment, the data unit  700  is a single user data unit that includes data for only a single AP or only a single client device  25 . In another embodiment, the data unit  700  is a multi-user data unit that includes independent data streams for multiple client stations  25  over respective spatial streams. In an embodiment in which the data unit  700  is a multi-user data unit, a portion of the data unit  700  (e.g., the L-STFs  705 , the L-LTFs  710 , the L-SIGs  715 , the HEW-SIGAs  720 , and the HEW-SIGBs  722 ) is unsteered or omnidirectional (or “omnidirectional” or “pseudo-omnidirectional”; the terms “unsteered” and “omnidirectional” as used herein are intended to also encompass the term “pseudo-omnidirectional”) and includes data that is common to all intended recipients of the data unit  700 . The data unit  700  further includes a second portion (e.g., the HEW-STF  725 , the HEW-LTFs  730 , and the HEW-DATA portion  740 ) in which beamforming is applied to different spatial streams to shape, or beamform, transmission over the corresponding spatial streams to particular client stations  25 . In some such embodiments, the steered portion of the data unit  700  includes different (e.g., “user-specific”) content transmitted over different spatial streams to different ones of the client stations  25 . 
     In some embodiments, the AP  14  is configured to transmit respective OFDM data units, such as the OFDM data unit  700 , simultaneously to multiple client stations  25  as parts of a downlink OFDMA transmission from the AP  14  to the multiple client stations  25 . In an embodiment, the AP  14  transmits the respective OFDM data units in respective sub-channels allocated to the client stations. Similarly, in an embodiment, multiple client stations  25  transmit respective OFDM data units, such as the OFDM data unit  700 , simultaneously to the AP  14  as parts of an uplink OFDMA transmission from the multiple client stations  25  to the AP  14 . In an embodiment, the client stations  25  transmit the respective OFDM data units in respective sub-channels allocated to the client stations  25 . In an embodiment, a sub-channel allocated to a particular client station corresponds to a single sub-channel block of adjacent sub-carriers of the communication channel (e.g., as illustrated in  FIG. 5A ). In an embodiment, a sub-channel allocated to a particular client station includes several sub-channel blocks of adjacent sub-carriers, each sub-channel block having a set of sub-carriers allocated to the particular client station. In an embodiment, the several sub-channel blocks corresponding to a particular client station are uniformly distributed over the communication channel (e.g., as illustrated in  FIG. 5B ). In another embodiment, the several sub-channel blocks are not necessarily uniformly distributed over the communication channel. For example, the several sub-channel blocks are randomly distributed over the communication channel (e.g., as illustrated in  FIG. 5C ), or are distributed according to another suitable distribution scheme over the communication channel, in some embodiments. 
     In some embodiments, the uplink data units (e.g., transmitted from a client device to an AP) omit HEW-SIGBs. For instance, in some embodiments, the AP instructs client devices regarding which parameters (e.g., MCS, number of spatial streams, etc.) to use when transmitting to the AP, and thus such parameters need not be included in the PHY preamble of uplink data units. Thus, this allows omission of the HEW-SIGBs from uplink data units. 
     In some embodiments, single user (SU) data units omit HEW-SIGBs. For instance, in some embodiments, some parameters in the HEW-SIGB relate to multi-user transmissions, and other parameters (e.g., MCS, number of spatial streams, etc.) in the HEW-SIGB can be included in the HEW-SIGA. Thus, this allows omission of the HEW-SIGBs from SU data units. 
     In some embodiments, the HEW-SIGB(s) are positioned after the HEW-LTFs  730 . In such embodiments, the HEW-SIGB(s) occupy the entire cumulative bandwidth of the data unit  700 . For example, in the case of an OFDM data unit conforming to the HEW protocol and occupying a cumulative bandwidth such as 20 MHz, 40 MHz, 120 MHz, or 160 MHz, the HEW-SIGB(s) the corresponding entire cumulative bandwidth of the data unit, in some embodiments. In such embodiments, beamforming is applied to different spatial streams to shape, or beamform, HEW-SIGBs over the corresponding spatial streams to particular client stations  25 . 
     In some embodiments, further signal (SIG) fields are included in the PHY preamble and positioned after the HEW-LTFs  730 . In such embodiments, beamforming is applied to different spatial streams to shape, or beamform, the further SIG fields over the corresponding spatial streams to particular client stations  25 . 
       FIG. 8  is a diagram of an example OFDMA data unit  800 , according to an embodiment. The OFDMA data unit  800  includes a plurality of OFDM data units  802 . Respective ones of the data units  802  include independent data streams transmitted to, or received from, respective ones of two client devices  25 . In an embodiment, each OFDM data unit  802  is the same as or similar to the OFDM data unit  700  of  FIG. 7 . In an embodiment, the AP  14  transmits the OFDM data units  802  to different client stations  25  via respective OFDM sub-channels within a composite channel spanned by the OFDMA data unit  800 . In another embodiment, different client stations  25  transmit respective OFDM data units  802  to the AP  14  in respective OFDM sub-channels within the composite channel spanned by the OFDMA data unit  800 . In such an embodiment, the AP  14  receives the OFDM data units  802  from the client stations  25  via respective OFDM sub-channels of within the composite channel spanned by the OFDMA data unit  800 . Although the data unit  800  is illustrated in  FIG. 8  as including only two data units  802  transmitted to, or received from, only two client stations  25 , the data unit  800  includes more than two (e.g., 3, 4, 5, 6, etc.) data units  802  transmitted to, or received from, more than two (e.g., 3, 4, 5, 6, etc.) client stations  25 , in other embodiments. 
     Each of the OFDM data units  802  conforms to a communication protocol that defines OFDMA communication, such as the HEW communication protocol, in an embodiment. In an embodiment in which the OFDMA data unit  800  corresponds to a downlink (DL) OFDMA data unit, the OFDMA data unit  800  is generated by the AP  14  such that each OFDM data unit  802  is transmitted to a respective client station  25  via a respective sub-channel allocated for downlink transmission of the OFDMA data unit  800  to the client station. Similarly, an embodiment in which the OFDMA data unit  800  corresponds to an uplink (UL) OFDMA data unit, the AP  14  receives the OFDM data units  802  via respective sub-channels allocated for uplink transmission of the OFDM data units  802  from the client stations, in an embodiment. For example, the OFDM data unit  802 - 1  is transmitted via a first 40 MHz sub-channel, and the OFDM data unit  802 - 2  is transmitted via a second 40 MHz sub-channel, in an embodiment. 
     In an embodiment, each of the OFDM data units  802  includes a preamble including one or more L-STFs  804 , one or more L-LTFs  806 , one or more L-SIGs  808 , one or more HEW-SIG-As  810 , N HEW-LTFs, and a HEW-SIGB  814 . Additionally, each OFDM data unit  802  includes a HEW-DATA portion  818 . In an embodiment, each L-STF field  804 , each L-LTF field  806 , each L-SIG field  808 , each HEW-SIGA field  810 , and each HEW-SIGB field  811  occupies a smallest channel bandwidth supported by the WLAN  10  (e.g., 20 MHz). In an embodiment, if an OFDM data unit  802  occupies a bandwidth that is greater than the smallest channel bandwidth of the WLAN  10 , then each L-STF field  804 , each L-LTF field  806 , each L-SIG field  808 , each HEW-SIGA field  810 , and each HEW-SIGB field  811  is duplicated in each smallest channel bandwidth portion of the OFDM data unit  802  (e.g., in each 20 MHz portion of the data unit  802 ). On the other hand, each HEW-STF field  812 , each HEW-LTF field  814 , and each HEW data portion  818  occupies an entire bandwidth of the corresponding OFDM data unit  802 , in an embodiment. 
     In an embodiment, padding is used in one or more of the OFDM data units  802  to equalize lengths of the OFDM data units  802 . Accordingly, the length of each of the OFDM data units  802  correspond to the length of the OFDMA data unit  802 , in this embodiment. Ensuring that the OFDM data units  802  are of equal lengths facilitates synchronizing transmission of acknowledgment frames by client stations  25  that receive the data units  802 , in an embodiment. In an embodiment, each of one or more of the OFDM data units  802  includes an aggregate MAC protocol data unit (A-MPDU) that includes multiple aggregated MAC protocol data units (MPDUs), which is in turn included in a PHY protocol data unit (PPDU). In another embodiment, each of one or more of the OFDM data units  802  includes a single MPDU, which is in turn included in a PPDU. In an embodiment, padding (e.g., zero-padding) within one or more of the A-MPDUs  802  or single MPDUs  802  is used to equalize the lengths of the data units  802 . 
     In an embodiment, the AP  14  forms groups of client stations  25  for simultaneous downlink transmissions to client stations  25  and/or simultaneous uplink transmissions by client stations  25 . To this end, the AP  14  allocates respective sub-channels to client stations  25  within a group of client stations  25  and/or allocates respective spatial streams to client stations  25 , in embodiments. In an embodiment and/or scenario, the AP  14  then transmits one or more OFDMA data units to the client stations  25  in a group using the respective sub-channels allocated to the client stations  25  within the group and/or transmits one or more MU MIMO data units to client stations  25  in a group using respective spatial streams allocated to the client stations  25  within the group. Each group of client stations  25  includes two or more client stations  25 , in an embodiment. In an embodiment, the AP  14  dynamically groups client stations to one MU transmission without group management. In an embodiment, the AP  14  notifies the groups that a station belongs to a group through a management frame exchange. A particular client station  25  belongs to one or more groups of the client stations  25 , in an embodiment. Thus, for example, a first group of client stations  25  includes the client station  25 - 1  and the client station  25 - 2 , and a second group of client stations  25  includes the client station  25 - 1  and the client stations  25 - 3 , in an example embodiment and/or scenario. Accordingly, the client station  25 - 1  belongs to the first group of client stations  25  and to the second group of client stations  25 , in this example embodiment and/or scenario. 
     In an embodiment, a color field is included in the HEW-SIGAs  810 . In another embodiment, the color field is included in the HEW-SIGBs  811 . 
     In some embodiments, the uplink (UL) OFDMA data units (e.g., transmitted from a client device to an AP) omit HEW-SIGBs. For instance, in some embodiments, the AP instructs client devices regarding which parameters (e.g., MCS, number of spatial streams, etc.) to use when transmitting to the AP, and thus such parameters need not be included in the PHY preamble of uplink data units. Thus, this allows omission of the HEW-SIGBs from UL OFDMA data units. 
     In some embodiments, the HEW-SIGBs are positioned after the HEW-LTFs  814 . In such embodiments, each HEW-SIGB  811  occupies an entire bandwidth of the corresponding OFDM data unit  802 . In such embodiments, beamforming is applied to different spatial streams to shape, or beamform, HEW-SIGBs  811  over the corresponding spatial streams to particular client stations  25 . 
     In some embodiments, further signal (SIG) fields are included in the PHY preamble and positioned after the HEW-LTFs  730 . In such embodiments, beamforming is applied to different spatial streams to shape, or beamform, the further SIG fields over the corresponding spatial streams to particular client stations  25 . 
     In some embodiments and/or scenarios, the same color is assigned to all virtual APs (corresponding to a single physical AP) to address problems with different virtual APs having different colors discussed above. For example, in some embodiments, the physical AP (e.g., the host processor  15 ) is configured to assign the same color to all of the corresponding virtual APs.  FIG. 9  is a message sequence diagram illustrating multiple transmissions among STA 1 , STA 2 , a first virtual AP (AP 1 ) and a second virtual AP (AP 2 ), according to an embodiment. In this embodiment, all virtual APs associated with the same physical AP use the same BSS color. In an embodiment, AP 1  and AP 2  operate like the virtual APs  35 - 1  and  35 - 2  of  FIG. 1 , and STA 1  and STA 2  are configured like the client devices  25 . STA 1  is associated with AP 1 , whose BSS color will be referred to as Color  1 , and STA 2  is associated with AP 2 , which has the same BSS color—Color  1 . At time t 1 , STA 1  begins transmitting an HE PPDU  904  to AP 1 . At time t 2 , AP 1  transmits a Block Acknowledgement (BA)  908  in response to the PPDU  904 . At time t 3 , carries out a CCA procedure to determine whether the channel is clear. In carrying out the CCA procedure, STA 2  detects PPDU  904 , and measures the energy of PPDU  904  as being above the standard CCA threshold level (e.g., −82 dBm), but below the dynamic CCA level (e.g., −62 dBm). STA 2  decodes the color field in the PHY preamble of PPDU  904  and identifies the BSS color associated with PPDU  904  as Color  1 . Because Color  1  is also the BSS color associated with AP 2 , STA 2  uses the standard CCA level (e.g., −82 dBm). Because the measured energy of PPDU  904  is greater than the standard CCA level, STA 2  concludes that the communication medium is busy and initiates a back-off procedure, during which STA 2  will not decrement a backoff counter of STA 2 . In an embodiment, STA 2  also uses virtual carrier sensing to decide whether the backoff counter of STA 2  should be decremented, e.g., when a network allocation vector (NAV) timer is not zero, STA 2  is not permitted to decrement the backoff counter. Just after the end of BA  908 , STA 2  repeats the CCA procedure described above and determines that the communication medium is clear (e.g., overall energy level is below −62 dBm and the 802.11-based energy level is below −82 dBm). After the end of BA  908 , the NAV timer of STA 2  also becomes zero and thus STA 2  can resume decrementing the backoff counter. At time t 4 , based on the determination that the communication medium is clear and that the backoff counter is zero, STA 2  transmits an HE PPDU  912  to AP 2 . At time t 5 , AP 2  transmits a BA  916  in response to the PPDU  912 . 
     In embodiments in which each virtual AP corresponding to a single physical AP has a same color, OFDMA and/or MU-MIMO techniques may be utilized for simultaneous transmissions corresponding to different virtual APs.  FIG. 10A  is a diagram illustrating OFDMA transmissions between three client devices, STA 1 , STA 2 , and STA 3 , and two virtual APs, AP 1  and AP 2 , which are implemented by the same physical AP, according to an embodiment. In the scenario illustrated in  FIG. 10A , the physical AP sends DL traffic from virtual AP 1  and virtual AP 2  in a single DL OFDMA transmission  930 . In  FIG. 10A , STA 1  and STA 3  are associated with AP 1  while STA 2  is associated with AP 2 . The BSS color for AP 1  is a value Color  1  and the BSS color for AP 2  is also the value Color  1 . At time t 1 , the physical AP transmits an A-MPDU  934  (corresponding to virtual AP 1  and with the BSS color field set to Color  1 ) via DL OFDMA  930  to STA 3  via a 20 MHz channel of AP 1 , and an A-MPDU  938  (corresponding to virtual AP 1  and with the BSS color field set to Color  1 ) to STA 1  using a primary 20 MHz channel (which is also the primary channel for AP 2 ). The A-MPDU  934  has a 20 MHz bandwidth, whereas the A-MPDU  938  has a 10 MHz bandwidth. 
     Also at time t 1 , AP 2  transmits an A-MPDU  942  via the DL OFDMA data unit  930  (with the BSS color bits set to Color  1 ) to STA 2  using the primary 20 MHz channel of AP 2 . At time t 2 , STA 1 , STA 2 , and STA 3  transmit a block acknowledgement (BA)  946  to virtual AP 1  and virtual AP 2 . 
       FIG. 10B  is a diagram showing a portion of the DL OFDMA transmission  930  of  FIG. 10A . In particular,  FIG. 10B  illustrates the portion of the DL OFDMA transmission  930  to STA 1  and STA 2 . In particular, because virtual AP 1  and virtual AP 2  utilize the same color value (Color  1 ), a single HE-SIGA  954  (which includes a BSS color field set to Color  1 ) can be utilized for the A-MPDU  938  from virtual AP 1  to STA 1  and the A-MPDU  942  from virtual AP 2  to STA 2 . 
       FIG. 11A  is a diagram illustrating OFDMA transmissions between three client devices, STA 1 , STA 2 , and STA 3 , and two virtual APs, AP 1  and AP 2 , which are implemented by the same physical AP, according to an embodiment. In the scenario illustrated in  FIG. 11A , the AP associations and BSS colors are the same as those described in conjunction with  FIG. 10A . At time t 0 , either AP 1  or AP 2  (or both) transmits SYNC frames  960  to prompt UL OFDMA transmissions from STA 1 , STA 2 , and STA 3  starting at a time t 1 . In response to the SYNC frames  960 , at time t 1 , STA 3  transmits an A-MPDU  964  to AP 1 , STA 2  transmits an A-MPDU  968  to AP 2 , and STA 1  transmits an A-MPDU  972  to AP 1 . A-MPDU  964 , A-MPDU  968 , and A-MPDU  972  form an UL OFDMA data unit  976 . At time t 2 , in response to the UL OFDMA data unit  976 , the physical AP transmits BAs  980  to STA 1 , STA 2 , and STA 3 , e.g., AP 1  transmits BAs to STA 1  and STA 3 , while AP 2  transmits a BA to STA 2 . 
       FIG. 11B  is a set of diagrams illustrating the UL transmissions of STA 1  and STA 2  of  FIG. 11A , according to an embodiment. For example, the transmission  968  is from STA 2  to virtual AP 2 , whereas the transmission  972  is from STA 1  to virtual AP 1 . Transmission  968  includes an HE-SIGA  990  with a BSS color field set to Color  1 , and transmission  972  includes an HE-SIGA  992  with a BSS color field set to Color  1 . Thus, HE-SIGA  990  is the same as HE-SIGA  992 , in some embodiments. 
     Transmission  968  also includes an A-MPDU  996  with data corresponding to AP 2 , and transmission  972  includes an A-MPDU  998  with data corresponding to AP 1 . 
     In some embodiments, a physical AP determines a plurality of respective colors of a plurality of respective BSSs corresponding to a plurality of respective virtual APs implemented by the AP device. For example, in some embodiments, the AP assigns respective colors to respective virtual APs. Then, the physical AP device notifies client devices of the respective colors corresponding to the respective BSSs of the plurality of virtual APs of the physical AP, according to some embodiments. Then, when a client device is performing a CCA procedure, the client device can determine whether a transmission associated with a different color than the BSS with which the client device is associated corresponds to the same physical AP. 
       FIG. 12  is a message sequence diagram illustrating multiple transmissions among STA 1 , STA 2 , a first virtual AP (AP 1 ) and a second virtual AP (AP 2 ), according to an embodiment. In this embodiment, a physical AP that operates multiple virtual APs assigns each virtual AP a BSS color and broadcasts a message  1002  that includes the BSS color of each of the virtual APs. In an embodiment, the colors of the two virtual APs are different—AP 1  having Color  1  and AP 2  having Color  2 , for example. In an embodiment, AP 1  and AP 2  operate like the virtual APs  35 - 1  and  35 - 2  of  FIG. 1 , and that STA 1  and STA 2  are configured like the client devices  25 . STA 1  is associated with AP 1  and STA 2  is associated with AP 2 . 
     At time t 0 , either, or both of, AP 1  and AP 2  broadcasts a message  1002  (e.g., a beacon or another suitable management frame) that lists the BSS colors of each of the virtual APs that are implemented by the physical AP and corresponding BSSIDs, e.g. Color  1  and a BSSID 1  of AP 1 , Color  2  and a BSSID 2  of AP 2 . In the example illustrated in  FIG. 12 , the list of colors and matched BSSIDs in the message includes Color  1  BSSID 1  of AP 1 , as well as Color  2  BSSID 2  of AP 2 . In an embodiment, a first entry in the list indicates that AP 1  has a BSS color of Color  1  and a second entry in the list indicates that AP 2  has a BSS color of Color  2 . In an embodiment, the management frame also indicates a single color to be used in UL MU transmissions by stations, and a single color that will be used by AP 1  and AP 2  in DL MU transmissions. For example, in one embodiment, STAs are configured to recognize that UL MU transmissions should utilize the color of the virtual AP that transmitted the management frame  1002  (e.g., the beacon), and that AP 1  and AP 2  will both use the color of the virtual AP that transmitted the management frame  1002  in DL MU transmissions. In another embodiment, the STAs are configured to utilize a predetermined color value (e.g., a maximum color value, a minimum color value, or some other suitable predetermined color value) for UL MU transmissions; and the STAs are configured to recognize that AP 1  and AP 2  will utilize the same predetermined color value (or another suitable predetermined color value) for DL MU transmissions. 
     At time t 1 , STA 1  begins transmitting an HE PPDU  1004  to AP 1 . At time t 2 , AP 1  transmits a Block Acknowledgement (BA)  1008  in response to the PPDU  1004 . At time t 1 , STA 2  detects the PPDU  1004 , and measures the energy of the PPDU  1004  as being above the standard CCA threshold level (e.g., −82 dBm), but below the dynamic CCA level (e.g., −62 dBm). STA 2  decodes a color field in a PHY preamble of the PPDU  1004  and identifies the BSS color of the PPDU  1004  as being Color  1 . STA 2  then selects which CCA level to use—standard or dynamic—depending on the list of colors provided in the message  1002  at time t 0 . For example, if Color  1  was included in the list of colors in message  1002 , then STA 2  knows that PPDU  1004  corresponds to a BSS implemented by the same physical AP to which STA 2  is associated, and therefore uses the standard CCA level (e.g., −82 dBm). In the example illustrated in  FIG. 12 , at t 1 , the measured energy of the PPDU  1004  is greater than the standard CCA level. STA 2  therefore concludes that the medium is busy. STA 2  also sets NAV timer of STA 2  per a Duration field in a frame in the detected PPDU  1004  if the energy of the PPDU  1004  is higher than the CCA level. At t 3 , STA 2 , which has frame for transmission, initiates a back-off procedure and sets a backoff counter. Just after the end of BA  1008 , STA 2  repeats the CCA procedure described above and determines that the medium is clear (e.g., overall energy level is below −62 dBm and the 802.11-based energy level is below −82 dBm). Therefore, STA 2  begins decrementing the backoff counter. At time t 4 , based on a determination that the backoff counter is zero, STA 2  transmits an HE PPDU  1012  to AP 2 . At time t 5 , AP 2  transmits a BA  1016  in response to the PPDU  1020 . 
     With the help of management frame that notifies the color used in UL MU transmission and DL MU transmission, A-MPDUs from/to multiple virtual APs with different BSS colors can be transmitted in one MU transmission.  FIG. 13A  is a diagram illustrating OFDMA transmissions between three client devices, STA 1 , STA 2 , and STA 3 , and two virtual APs, AP 1  with Color  1  and AP 2  with Color  2 , which are implemented by the same physical AP, according to an embodiment. In the scenario illustrated in  FIG. 13A , Color  1  and Color  2  are different. At time t 0 , AP 1  or AP 2  (or both) broadcasts (e.g., in a beacon  1050  or another suitable management frame) a color value that is to be used for MU transmission, e.g., Color  1 . At time t 1 , either AP 1  or AP 2  (or both) transmits SYNC frames  1060  to prompt UL OFDMA transmissions from STA 1 , STA 2 , and STA 3  starting at a time t 2 . In response to the SYNC frames  1060 , at time t 2 , STA 3  transmits an A-MPDU  1064  to AP 1  and Color  1  is included in a color field of PHY SIG of the PPDU that carry A-MPDU  1064 , STA 2  transmits an A-MPDU  1068  to AP 2  and Color  1  is included in a color field in PHY SIG of the PPDU that carry A-MPDU  1068 , and STA 1  transmits an A-MPDU  1072  to AP 1  and Color  1  is included in a color field of PHY SIG of the PPDU that carry A-MPDU  1072 . A-MPDU  1064 , A-MPDU  1068 , and A-MPDU  1072  form an UL OFDMA data unit  1076 . So the HE SIGs transmitted by STA 1 , STA 2  and STA 3  are same, in an embodiment. At time t 3 , in response to the UL OFDMA data unit  1076 , the physical AP transmits BAs  1080  to STA 1 , STA 2 , and STA 3  and Color  1  is included in a color field of PHY SIG of the PPDU that carries BAs  1080 , e.g., AP 1  transmits BAs to STA 1  and STA 3 , while AP 2  transmits a BA to STA 2 . So the HE SIGs transmitted by AP 1  and AP 2  in connection with the BAs  1080  are the same, in an embodiment. 
       FIG. 13B  is a set of diagrams illustrating the UL transmissions of STA 1  and STA 2  of  FIG. 13A , according to an embodiment. For example, the transmission  1068  is from STA 2  to virtual AP 2 , whereas the transmission  1072  is from STA 1  to virtual AP 1 . Transmission  1068  includes an HE-SIGA  1090  with a BSS color field set to Color  1 , and transmission  1072  includes an HE-SIGA  1092  with a BSS color field set to Color  1 . Thus, HE-SIGA  1090  is the same as HE-SIGA  1092 , in some embodiments. 
     Transmission  1068  also includes an A-MPDU  1096  with data corresponding to AP 2 , and transmission  1072  includes an A-MPDU  1098  with data corresponding to AP 1 . 
       FIG. 14  is a flow diagram of an example method  1300  for informing client devices of a physical AP of BSS identifiers (e.g., colors) of virtual APs implemented by the physical AP, according to an embodiment. In some embodiments, the method  1300  is implemented by the AP  14  ( FIG. 1 ). For example, in some embodiments, the network interface device  16  is configured to implement the method  1300 . As another example, in some embodiments, the host  15  is configured to implement at least a portion of the method  1300 . As another example, in some embodiments, the host  15  and the network interface device  16  are configured to implement the method  1300 . In other embodiments, another suitable communication device is configured to implement the method  1300 . 
     At block  1304 , a plurality of respective identifiers (e.g., colors) of a plurality of respective BSSs corresponding to a plurality of respective virtual APs implemented by a physical AP device are determined. 
     At block  1308 , the plurality of respective identifiers (e.g., colors) determined at block  1304  are communicated to client devices of the physical AP so that client devices of the plurality of virtual APs are made aware of all of the identifiers of the plurality of respective BSSs. In some embodiments, block  1308  also includes indicating to STAs a single color value that is to be used by STAs for MU UL transmissions. In some embodiments, block  1308  also includes indicating to STAs a single color value that will be used by virtual APs for MU DL transmissions. In some embodiments, the single color value that is to be used by STAs for MU UL transmissions is the same as the single color value that will be used by virtual APs for MU DL transmissions. In some embodiments, block  1308  comprises communicating the BSS identifiers of all of the BSSs in a single message. For example, in an embodiment, a list of the BSS identifiers of all of the BSSs (and corresponding BSSIDs) is included in a single physical layer data unit that is broadcast by the physical AP to the client devices of the physical AP. In some embodiments, the single message (e.g., the single PPDU) also indicates the single color value that is to be used by STAs for MU UL transmissions and/or indicates the single color value that virtual APs will use for MU DL transmissions. In some embodiments, the single color value that is to be used by STAs for MU UL transmissions is the same as the single color value that will be used by virtual APs for MU DL transmissions. In some embodiments, the list of the BSS identifiers of all of the BSSs (and optionally one or more of i) the corresponding BSSIDs, ii) the indication of the single color value that is to be used by STAs for MU UL transmissions, and/or iii) the single color value that will be used by virtual APs for MU DL transmissions) is included in a beacon frame or another suitable management frame. In some embodiments, respective portions of the list of the BSS identifiers of all of the BSSs (and optionally one or more of i) the corresponding BSSIDs, ii) the indication of the single color value that is to be used by STAs for MU UL transmissions, and/or iii) the single color value that will be used by virtual APs for MU DL transmissions) are included in respective different physical layer data units transmitted by the physical AP at different times. 
     In some embodiments, the method  1300  further includes assigning the plurality of respective identifiers (e.g., colors) to the plurality of respective BSSs corresponding to the plurality of respective virtual APs. In some embodiments, the method  1300  further includes assigning a plurality of respective BSSIDs to the plurality of virtual APs. In some embodiments, the method  1300  further includes setting up the plurality of virtual APs at the physical AP. 
     According to an embodiment, each virtual AP collocated in the same physical AP uses identical values in a first set of bits (e.g., one of the most significant bits or the least significant bits) of a BSS color value, whereas each virtual AP collocated in the same physical AP uses different values in a second set of bits (e.g., the other of the most significant bits or the least significant bits) of the BSS color value. For instance, in an illustrative embodiment, a BSS color field comprises two parts: m least significant bits (also referred to as a “virtual AP ID part”), which are used to identify virtual APs, and n-m most significant bits (also referred to as the “segment ID part”), which are used to identify a common physical AP for the virtual APs. Each virtual AP, in this embodiment, has identical values for the segment ID part, but a unique (at least for that physical AP) value for the virtual ID part. 
       FIG. 15  is a message sequence diagram illustrating multiple transmissions among STA 1 , STA 2 , a first virtual AP (AP 1 ) and a second virtual AP (AP 2 ), according to an embodiment. At time t 1 , STA 1  begins transmitting an HE PPDU  1304  to AP 1 . At time t 2 , AP 1  transmits a Block Acknowledgement (BA)  1308  in response to the PPDU  1304 . A color field in a PHY preamble of the PPDU  1304  is set to a value Color  1 . In an embodiment, a first set of bits of the value Color  1  corresponds to a segment ID set to a value x, and a second set of bits of the value Color  1  corresponds to a virtual ID set to a value y. 
     At time t 1 , STA 2  detects the PPDU  1304 , and measures the energy of the PPDU  1304  as being above the standard CCA threshold level (e.g., −82 dBm), but below the dynamic CCA level (e.g., −62 dBm). STA 2  decodes the segment ID part of the color field of the PHY preamble of the PPDU  1304  and determines that the segment ID is the same as a segment ID of a color of a BSS to which STA 2  is associated (e.g., Color  2 , where a first set of bits of the value Color  2  corresponds to a segment ID set to the value x, and a second set of bits of the value Color  2  corresponds to a virtual ID set to a value z). STA 2  therefore uses the standard CCA level. Because the measured energy of the PPDU  1304  is greater than the standard CCA level, STA 2  concludes that the channel is busy and sets a NAV timer per duration field(s) in the detected MPDUs carried in PPDU  1304 . At time t 4 , a back-off counter of STA 2  becomes zero. So at time t 5 , STA 2  transmits an HE PPDU  1312  to AP 2 . At time t 6 , AP 2  transmits a BA  1316  in response to the PPDU  1312 . 
       FIG. 16  is a flow diagram of an example method  1500  for performing a CCA procedure, according to an embodiment. In some embodiments, the method  1500  is implemented by the client device  25 - 1  ( FIG. 1 ) and/or the AP  14 . For example, in some embodiments, the network interface device  16  is configured to implement the method  1500 . As another example, in some embodiments, the host  15  and the network interface device  16  are configured to implement the method  1500 . For example, in some embodiments, the network interface device  27  is configured to implement the method  1500 . As another example, in some embodiments, the host  26  and the network interface device  27  are configured to implement the method  1500 . In other embodiments, another suitable communication device is configured to implement the method  1500 . 
     At block  1504 , a signal is received. At block  1508 , it is determined whether the signal is a signal that conforms to a particular one or more communication protocols recognized by the client device  25 - 1 . For example, in an embodiment, it is determined whether the signal conforms to a WiFi communication protocol. As another example, in an embodiment, it is determined whether the signal conforms to a communication protocol defined by the IEEE 802.11 Standard. In other embodiments, it is determined whether the signal conforms to another suitable communication protocol. Block  1508  comprises analyzing the signal to determine whether the signal includes a PHY preamble that conforms to a particular communication protocol, or to one of several particular communication protocols, in various embodiments. 
     If it is determined at block  1508  that the signal is not a signal that conforms to the particular communication protocol (or one of several communication protocols, in some embodiments) recognized by the client device  25 - 1 , the flow proceeds to block  1512 . At block  1512 , an energy level of the signal is compared a first threshold as part of the CCA procedure. The first threshold is lower than a second threshold also used as part of the CCA procedure, as will be described below. 
     On the other hand, if it is determined at block  1508  that the signal is a signal that conforms to the particular communication protocol (or one of several communication protocols, in some embodiments) recognized by the client device  25 - 1 , the flow proceeds to block  1516 . At block  1516 , a color field in a PHY preamble of the signal is decoded to determine a color value corresponding to the signal received at block  1504 . 
     At block  1520 , it is determined whether the color value determined at block  1516  corresponds to a color value corresponding to a physical AP with which the client  25 - 1  is associated. For example, in one scenario, the client  25 - 1  is associated with a first BSS managed by a first virtual AP 1  implemented by the physical AP, where the first BSS has a color value Color  1 . In embodiments in which all BSSs managed by virtual APs implemented by the physical AP have the same color, the client  25 - 1  compares the color value determined at block  1516  with the value Color  1  and if the color value determined at block  1516  is not Color  1 , the client  25 - 1  determines that the signal received at block  1516  does not correspond to the same physical AP. On the other hand, in such embodiments, if the color value determined at block  1516  is Color  1 , the client  25 - 1  determines that the signal received at block  1516  corresponds to the same physical AP. 
     In embodiments in which BSSs managed by virtual APs implemented by the physical AP have different colors, and the physical AP previously provided the client  25 - 1  with the color values of the BSSs of the virtual APs implemented by the physical AP, the client  25 - 1  compares the color value determined at block  1516  with the color values provided by the physical AP. For example, after receiving the color values of the BSSs of the virtual APs implemented by the physical AP, the client device  25 - 1  stores the color values in a memory of or associated with the network interface device  27 . Then, the client device  25 - 1  accesses the color values in the memory as part of implementing block  1520 . If the color value determined at block  1516  does not match any of the color values provided by the physical AP, the client  25 - 1  determines that the signal received at block  1504  does not correspond to the same physical AP. On the other hand, in such embodiments, if the color value determined at block  1516  matches one of the color values provided by the physical AP, the client  25 - 1  determines that the signal received at block  1504  corresponds to the same physical AP. 
     In embodiments in which BSSs managed by virtual APs implemented by the physical AP have colors values in which a respective first part of the each color value has a same value (e.g., a segment ID part) and a respective second part of each color value has a different value (e.g., a virtual ID part), the client  25 - 1  compares a first part of the color value determined at block  1516  with a first part of Color  1 . If the first part of the color value determined at block  1516  is different than the first part of Color  1 , the client  25 - 1  determines that the signal received at block  1504  does not correspond to the same physical AP. On the other hand, in such embodiments, if the first part of the color value determined at block  1516  is the same as the first part of Color  1 , the client  25 - 1  determines that the signal received at block  1504  corresponds to the same physical AP. 
     If it is determined at block  1520  that signal received at block  1504  does not correspond to the same physical AP, the flow proceeds to block  1512 . On the other hand, if it is determined at block  1520  that signal received at block  1504  does correspond to the same physical AP, the flow proceeds to block  1524 . At block  1524 , the energy level of the signal is compared the second threshold as part of the CCA procedure, where the second threshold is higher than the first threshold. 
     In some embodiments, OFDMA transmissions with multiple BSS colors are not permitted. Thus, in some embodiments, a physical AP is not permitted to transmit data corresponding to different BSSs with different colors in a single DL OFDMA PHY data unit. Similarly, in some embodiments, client devices corresponding to different BSSs with different colors are not permitted to transmit as part of a single UL OFDMA transmission. 
     In some embodiments, to avoid the limitation discussed above with respect to OFDMA transmissions and different BSS colors, a physical AP may select a single color value for UL/DL MU transmissions and notifies the STAs of the selected color value. The physical AP sets color fields in PHY preamble of a DL OFDMA data unit to the single value even though the DL OFDMA data unit includes transmissions corresponding to multiple BSSs with different color values. The STAs associated with different virtual APs set color fields in PHY preamble of a UL OFDMA data unit to the selected single value even though the UL OFDMA data unit includes transmissions corresponding to multiple BSSs with different color values. 
       FIG. 17  is a flow diagram of an example method  1600  for performing an OFDMA transmission, according to an embodiment. In some embodiments, the method  1600  is implemented by the AP  14  ( FIG. 1 ). For example, in some embodiments, the network interface device  16  is configured to implement the method  1600 . As another example, in some embodiments, the host  15  is configured to implement at least a portion of the method  1600 . As another example, in some embodiments, the host  15  and the network interface device  16  are configured to implement the method  1600 . In other embodiments, another suitable communication device is configured to implement the method  1600 . 
     At block  1604 , a plurality of identifiers (e.g., color values) of a plurality of respective BSSs are determined, the respective BSSs corresponding to a plurality of respective virtual APs implemented by a physical AP device. In an embodiment, one of the plurality of identifiers is selected for MU transmissions and the AP device notifies STAs of the selected identifier. 
     At block  1608 , an OFDMA PHY data unit is generated, wherein the OFDMA PHY data unit is generated to include data from multiple virtual APs among the plurality of virtual APs. Block  1608  includes generating a PHY preamble that includes one or more fields for specifying an identifier of a BSS, wherein the one or more fields are set to a same value (e.g., the selected identifier). 
     For example, in an embodiment, one identifier from the plurality of respective identifiers of the plurality of respective BSSs is selected, and the one or more fields in the PHY preamble are set to the selected one identifier. 
     As another example, in some embodiments, each identifier includes a respective first set of bits and a respective second set of bits, and block  1604  includes determining the respective first sets of bits to be a same first value and determining the respective second sets of bits to be respective second values. In some embodiments, block  1608 , each of the one or more fields in the PHY preamble has a first set of bits set to the same first value and a second set of bits set to a third value different than all of the respective second values. As an illustrative example, the third value is zero, whereas all of the respective second values are non-zero values, according to an embodiment. In other embodiments, however, the third value is a suitable non-zero value (e.g., all ones). 
     At block  1612 , the OFDMA PHY data unit is transmitted to a group of client devices among the plurality of client devices. 
     To respond to the OFDMA PHY data unit transmitted at block  1612 , in an embodiment, the respective client devices transmit an acknowledgment to the physical AP using the same BSS identifier as the BSS identifier used in the OFDMA PHY data unit transmitted at block  1612 . For example, referring to  FIG. 10A , at time t 2 , all of the stations respond to the DL OFDMA transmission  930  in the single UL OFDMA transmission  946  using the BSS color in the PHY preamble of the DL OFDMA  930 . 
     Referring again to  FIG. 17 , in some embodiments, the method  1600  includes other acts such as one or more of: setting up the plurality of virtual APs, assigning the plurality of identifiers to the BSSs, with the plurality of client devices, etc., according to various embodiments. 
     As another example, in an embodiment, the method  1600  further includes generating a single user PHY data unit corresponding to only one virtual AP, wherein generating the single user PHY data unit comprises generating a PHY preamble of the single user PHY data unit that includes one or more fields for specifying an identifier of a BSS, wherein the one or more fields are set to the identifier corresponding to the one virtual AP, and wherein the identifier corresponding to the one virtual AP is different than the same value in the PHY preamble of the OFDMA PHY data unit; and transmitting the single user PHY data unit to one of the client devices among the plurality of client devices. 
       FIG. 18  is a flow diagram of an example method  1700  for performing an OFDMA transmission, according to an embodiment. In some embodiments, the method  1700  is implemented by the client device  25 - 1  ( FIG. 1 ). For example, in some embodiments, the network interface device  27  is configured to implement the method  1700 . As another example, in some embodiments, the host  26  and the network interface device  16  are configured to implement the method  1700 . In other embodiments, another suitable communication device is configured to implement the method  1700 . 
     At block  1704 , the communication device  25 - 1  associates with a virtual AP implemented by a physical AP, the virtual AP corresponding to a first BSS, wherein the first BSS corresponds to a first BSS identifier (e.g., a first color value (Color  1 )). 
     At block  1708 , as part of an OFDMA transmission (e.g., an UL OFDMA transmission to the physical AP), a signal is generated, the signal having i) a PHY preamble with a field including a second BSS identifier (e.g., a second color value (Color  2 ), and ii) a PHY payload corresponding to the first BSS. 
     For example, in an embodiment, the second BSS identifier corresponds to a second BSS of another virtual AP implemented by the physical AP. For instance, in an embodiment, a set of BSS identifiers is previously received from the physical AP, wherein the set of BSS identifiers include identifiers of BSSs corresponding to BSSs of a set of virtual APs implemented by the physical AP, the second BSS identifier is selected from the set of identifiers received from the physical AP. As an illustrative embodiment, the physical AP indicates that the second BSS identifier, from among the list of BSS identifiers, should be used for UL OFDMA transmissions. For instance, in an embodiment, the second BSS identifier is at a position in (e.g., a top of) the list of BSS identifiers, where the position in the list indicates that the second BSS identifier should be used for UL OFDMA transmissions. In another embodiment, the second BSS identifier is flagged in the list of BSS identifiers, where the flagging of the second BSS identifier indicates that the second BSS identifier should be used for UL OFDMA transmissions. 
     In some embodiments, the physical AP previously informs the client device  25 - 1  to use the second BSS identifier (in PHY preambles) when participating in UL OFDMA transmissions via one or more of a broadcast frame, a management frame, a control frame, a beacon frame, etc. In an embodiment, the physical AP informs the client device  25 - 1  to use the second BSS identifier (in PHY preambles) when participating in the UL OFDMA transmission via a SYNC frame that also prompts the client device  25 - 1  to transmit the signal at block  1708  as part of the UL OFDMA transmission. 
     As another example, in some embodiments, a first set of bits of the second BSS identifier is equal to a corresponding first set of bits of the first BSS identifier; and a second set of bits of the second BSS identifier is different than a corresponding second set of bits of the first BSS identifier. For instance, in an embodiment, the client device  25 - 1  sets the first set of bits of the second BSS identifier equal to the corresponding first set of bits of the first BSS identifier, and sets the second set of bits of the second BSS identifier to a predetermined value that is different than the corresponding second set of bits of the first BSS identifier. In an embodiment, the predetermined value is zero. In other embodiments, the predetermined value is another suitable value other than zero (e.g., all ones). 
     At block  1712 , the signal is transmitted to the physical AP as part of the UL OFDMA transmission. For example, in an embodiment, the signal is transmitted simultaneously with other transmissions from other client devices, the other transmissions also being part of the UL OFDMA transmission. 
     To respond to the OFDMA PHY data unit transmitted at block  1712 , in an embodiment, the physical AP transmits an OFDMA acknowledgment to the client devices using the same BSS identifier as the BSS identifier used in the OFDMA PHY data unit transmitted at block  1712 . For example, referring to  FIG. 11A , at time t 2 , the physical AP responds to the UL OFDMA transmission  976  in the single DL OFDMA transmission  980  using the BSS color that was included in the PHY preamble of the UL OFDMA  976 . 
     Referring again to  FIG. 18 , in some embodiments, the method  1700  also includes one or more other acts, such as receiving a SYNC frame that prompts the transmission at block  1712  as part of the UL OFDMA transmission, exchanging single user PHY data units with the physical AP, wherein the single user PHY data units include PHY preambles, each PHY preamble having a field including the first BSS identifier, etc. In some embodiments in which the method  1700  includes receiving a SYNC frame that prompts the transmission at block  1712 , the SYNC frame indicates that the client device is to use the second BSS identifier in the PHY preamble of the signal generated at block  1708 . 
     In an embodiment, when the BSS colors of all of the virtual APs of a given physical AP are advertised by the physical AP and a selected color to be used for MU transmissions through the use of a Multiple BSS Color element (e.g., a set of BSS color bits for each separate virtual AP transmitted via a beacon, or a set of BSS colors and corresponding BSSIDs for each separate virtual AP transmitted via beacon, and an indication of a selected color for MU transmissions), the client devices send the UL traffic to the virtual APs on a single OFDMA UL transmission using the following rules: (1) The physical AP transmits a management frame that indicates a color for UL OFDMA (e.g. in a Multiple BSS Color element) or a SYNC frame that indicates a BSS color for UL OFDMA. (2) All STAs that associate with a virtual BSS need to receive the SYNC, which may originate from any of the virtual APs on that physical AP (i.e., any virtual AP defined in the Multiple BSSID element transmission—e.g., the beacon). In another embodiment, the STAs do not respond in a single UL OFDMA transmission, but rather in separate single user transmissions. 
     According to an embodiment, each virtual AP collocated in the same physical AP use identical values in a sub-portion (e.g., the most significant bits or the least significant bits) of a BSS color field. In an embodiment, the BSS color field is separated to two parts: m least significant bits (also referred to as a “virtual AP ID part”), which are used to identify virtual APs, and n-m most significant bits (also referred to as the “segment ID part”), which are used to identify a common physical AP for the virtual APs. Each virtual AP in this embodiment has identical values for the segment ID part, but a unique (at least for that physical AP) value for the virtual ID part. In an embodiment, the virtual AP ID part is the same for all virtual APs of a single physical AP. In another embodiment, the virtual AP ID part is different for each virtual AP on the single physical AP. 
     In some embodiments, the physical AP uses one value of the virtual AP ID part for the color value included in a PHY preamble of a DL OFDMA communication, e.g., all zeros in virtual AP ID part, all ones in the virtual AP ID part, etc. 
     In one embodiment, a method for communicating on a wireless network includes, on a physical access AP: setting up a plurality of virtual APs; assigning a basic service set (BSS) color to each of the plurality of virtual APs; and broadcasting a message that identifies each of the plurality of APs and the BSS color assigned to each of the plurality of APs. 
     In other embodiments, the method includes one of, or any suitable combination of two or more of, the following features. 
     The method also includes transmitting, to a first client device on behalf of a first virtual AP of the plurality of virtual APs, a first packet data unit comprising a header, wherein the header comprises data representing the color of the first virtual AP; and transmitting, to a second client device on behalf of a second virtual AP of the plurality of virtual APs, a second packet data unit comprising a header, wherein the header comprises data representing the BSS color of the second virtual AP. 
     The physical AP transmits the first and second packet data units in a single downlink orthogonal frequency division multiple access (OFDMA) transmission. 
     The BSS color assigned to the first virtual AP is identical to the BSS color assigned to the second virtual AP; and the method further includes: transmitting, to a first client device on behalf of a first virtual AP of the plurality of virtual APs, a first packet data unit comprising a header, wherein the header comprises data representing the BSS color; and transmitting, to a second client device on behalf of a second virtual AP of the plurality of virtual APs, a second packet data unit comprising a header, wherein the header comprises data representing the BSS color. 
     The method further includes receiving, in response to the first and second packet data units, a single uplink OFDMA transmission from the first and second client devices, wherein the uplink OFDMA transmission comprises packet data unit from the first client device and a packet data unit from the second client device, and wherein the packet data unit from the first client device includes a header comprising the BSS color and the packet data unit from the second client device includes a header comprising the BSS color. 
     The method further includes selecting a BSS color of the first virtual AP or the BSS color of the second virtual AP; transmitting, on behalf of at least one of the plurality of virtual APs, a SYNC message, wherein the SYNC message identifies the selected BSS color to be the BSS color of the SYNC message; and receiving, in response to the SYNC message, an uplink OFDMA transmission from a plurality of client devices, wherein the uplink OFDMA transmission comprises a header including the selected BSS color. 
     The method further includes transmitting, in response to the uplink OFDMA transmission, an ACK comprising data representing the BSS color. 
     The BSS color assigned to the first virtual AP is identical to the BSS color assigned to the second virtual AP. 
     In another embodiment, a method for communicating on a wireless network, the method includes, on a physical AP: setting up a plurality of virtual APs; assigning a BSS color to each of the plurality of virtual APs, such that: the BSS color of each virtual AP is divided into at least a virtual AP identifier part and a segment identifier part, and the virtual AP identifier is the same for each virtual AP; transmitting, on behalf of the first virtual AP, a first packet data unit to a first client device, wherein a header of the first packet data unit includes the BSS color assigned to the first virtual AP; and transmitting, on behalf of the second virtual AP, a second packet data unit to a second client device, wherein a header of the second packet data unit includes the BSS color assigned to the second virtual AP. 
     In other embodiments, the method includes one of, or any suitable combination of two or more of, the following features. 
     The method further comprises transmitting, on behalf of a first virtual AP of the plurality of virtual APs, a first packet data unit comprising a PHY header, wherein the PHY header comprises data representing the color of the first virtual AP; and transmitting, on behalf of a second virtual AP of the plurality of virtual APs, a second packet data unit comprising a header, wherein the header comprises data representing the BSS color of the second virtual AP. 
     The physical AP transmits the first and second packet data units in a single DL OFDMA transmission. 
     The method further comprises: selecting a BSS color of the first virtual AP or the BSS color of the second virtual AP; transmitting, on behalf of at least one of the plurality of virtual APs, a SYNC message, wherein the SYNC message identifies the selected BSS color to be the BSS color of the SYNC message; and receiving, in response to the SYNC message, an uplink OFDMA transmission from a plurality of client devices, wherein the uplink OFDMA transmission comprises a header including the selected BSS color. 
     The method further comprises transmitting, in response to the uplink OFDMA transmission, an ACK comprising data representing the BSS color. 
     In another embodiment, a method of determining whether a wireless communication medium is clear includes, on client device: associating with a first virtual access point (AP) of a plurality of virtual APs implemented by a physical AP; receiving, from a physical AP, a message identifying the basic service set (BSS) color of each of the plurality of virtual APs implemented by the physical AP; detecting a packet data unit; measuring the energy of the packet data unit; decoding a BSS color from the packet data unit; if the decoded BSS color is the same as a BSS color in the message from the physical AP, then setting an energy threshold to a first level; if the decoded BSS color is not the same as any BSS color in the message from the physical AP, then setting an energy threshold to a second level, wherein the second level is higher than the first level; and transmitting or refraining from transmitting a packet data unit based on a comparison of the measured energy and the energy threshold. 
     In other embodiments, the method includes one of, or any suitable combination of two or more of, the following features. 
     The received message is a beacon; and the method further includes: detecting the packet data unit comprises detecting a packet data unit transmitted from a second virtual AP of the plurality of APs; decoding the BSS color comprises decoding a header of the detected packet data unit; identifying the BSS color as being one of the BSS colors contained in the beacon; and in response to identifying the BSS color as being one of the BSS colors contained in the beacon, setting the energy threshold to first level. 
     The first threshold level is a static clear channel assessment level. 
     The second threshold level is a dynamic clear channel assessment level. 
     In another embodiment, a method of determining whether a wireless communication medium is clear includes, on a client device: associating with a first virtual access point (AP) of a plurality of virtual APs implemented by a physical AP, wherein the first virtual AP has a virtual AP identifier; detecting a packet data unit that includes a header comprising a basic service set (BSS) color field, wherein the BSS color field comprises a virtual AP identifier part and a segment identifier part; measuring the energy of the packet data unit; decoding the virtual AP part; if the decoded virtual AP part is the same as the virtual AP identifier of the first virtual AP, then setting an energy threshold to a first level; if the decoded virtual AP part is not the same as the virtual AP identifier of the first virtual AP, then setting an energy threshold to a second level, wherein the second level is higher than the first level; and transmitting or refraining from transmitting a packet data unit based on a comparison of the measured energy and the energy threshold. 
     In other embodiments, the method includes one of, or any suitable combination of two or more of, the following features. 
     Each of the plurality of virtual APs is assigned the same virtual AP identifier and the same segment identifier. 
     Each of the plurality of virtual APs is assigned a different virtual AP identifier and the same segment identifier. 
     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. Examples of suitable hardware include a microprocessor, microcontroller, application specific integrated circuit (ASIC), field programmable gate array, programmable logic device, etc. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored on a computer readable medium, or media, such as a magnetic disk, an optical disk, a random access memory (RAM), a read only memory (ROM), a flash memory, a magnetic tape, 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 i) discrete components, ii) one or more integrated circuits, iii) one or more application-specific integrated circuits (ASICs), iv) one or more programmable logic devices, etc. 
     While the present disclosure 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 claims.