Patent Publication Number: US-11039440-B1

Title: OFDMA with block tone assignment for WLAN

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 15/924,573, now U.S. Pat. No. 10,462,790, entitled “OFDMA with Block Tone Assignment for WLAN,” filed on Mar. 19, 2018, which is a continuation of U.S. patent application Ser. No. 14/730,651, now U.S. Pat. No. 9,924,512, entitled “OFDMA with Block Tone Assignment for WLAN,” filed on Mar. 24, 2010, which claims the benefit of U.S. Provisional Patent Application No. 61/162,780, entitled “Simple OFDMA with Block Tone Assignment for WLAN,” filed on Mar. 24, 2009. The disclosures of the applications referenced above are incorporated by reference herein 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 background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     When operating in an infrastructure mode, wireless local area networks (WLANs) typically include an access point (AP) and one or more client stations. WLANs have evolved rapidly over the past decade. Development of WLAN standards such as the Institute for Electrical and Electronics Engineers (IEEE) 802.11a, 802.11b, 802.11g, and 802.11n Standards has improved single-user peak data throughput. For example, the IEEE 802.11b Standard specifies a single-user peak throughput of 11 megabits per second (Mbps), the IEEE 802.11a and 802.11g Standards specify a single-user peak throughput of 54 Mbps, and the IEEE 802.11n Standard specifies a single-user peak throughput of 600 Mbps. 
     These WLANs operate in either a unicast mode or a multicast mode. In the unicast mode, the AP transmits information to one client station at a time. In the multicast mode, the same information is transmitted to a group of client stations concurrently. 
     SUMMARY 
     In an embodiment, a method for transmitting to a plurality of client stations in a wireless local area network (WLAN) includes: assigning, at an access point (AP) device, respective blocks of orthogonal frequency division multiplexing (OFDM) tones to the plurality of client stations for an orthogonal frequency division multiple access (OFDMA) data unit; receiving, at the AP device, respective independent data for the plurality of client stations; and generating, at the AP device, a preamble of the OFDMA data unit to include: respective legacy preamble portions in respective sub-channels, wherein each legacy preamble portion includes a legacy signal field that indicates a duration of the OFDMA data unit, and a non-legacy preamble portion. The method also includes: generating, at the AP device, a data portion of the OFDMA data unit to include respective independent data for respective client stations, the respective independent data included within the respective blocks of OFDM tones; and transmitting, by the AP device, the OFDMA data unit. 
     In another embodiment, a wireless communication device comprises: a WLAN network interface device associated with an AP device of a WLAN. The WLAN network interface device comprises one or more integrated circuit (IC) devices, and one or more transceivers. The one or more IC devices are configured to: assign respective blocks of OFDM tones to the plurality of client stations for an OFDMA data unit, and receive respective independent data for the plurality of client stations. The one or more IC devices are configured to generate a preamble of the OFDMA data unit to include: a respective legacy preamble portion in each sub-channel of a plurality of subchannels, wherein each legacy preamble portion includes a legacy signal field that indicates a duration of the OFDMA data unit, and a non-legacy preamble portion. The one or more IC devices are further configured to: generate a data portion of the OFDMA data unit to include respective independent data for respective client stations, the respective independent data included within the respective blocks of OFDM tones; and control the one or more transceivers to transmit the OFDMA data unit. 
     In yet another embodiment, a method for communicating with a plurality of client stations in a WLAN includes: transmitting, by an AP device of a WLAN, a synchronization signal to prompt a plurality of client stations of the WLAN to transmit as part of an uplink OFDMA transmission at a time that begins a defined time period after an end of the synchronization signal; and subsequent to transmitting the synchronization signal, receiving, at the AP device, the uplink OFDMA transmission from the plurality of client stations, wherein the OFDMA transmission is responsive to the synchronization signal. The uplink OFDMA transmission includes: respective legacy preamble portions in respective sub-channels, wherein each legacy preamble portion includes a legacy signal field that indicates a duration of the uplink OFDMA transmission, a non-legacy preamble portion, and respective independent data from respective client stations among the plurality of client stations, the respective independent data included within respective blocks of orthogonal frequency division multiplex (OFDM) tones. 
     In still another embodiment, a wireless communication device comprises: a WLAN network interface device associated with an AP device of a WLAN. The WLAN network interface device comprises: one or more integrated circuit (IC) devices, and one or more transceivers. The one or more IC devices are configured to: control the one or more transceivers to transmit a synchronization signal to prompt a plurality of client stations of a WLAN to transmit as part of an uplink OFDMA transmission at a time that begins a defined time period after an end of the synchronization signal. The WLAN network interface device is further configured to: subsequent to transmitting the synchronization signal, receive the uplink OFDMA transmission from the plurality of client stations, wherein the OFDMA transmission is responsive to the synchronization signal. The uplink OFDMA transmission includes: respective legacy preamble portions in respective sub-channels, wherein each legacy preamble portion includes a legacy signal field that indicates a duration of the uplink OFDMA transmission, a non-legacy preamble portion, and respective independent data from respective client stations among the plurality of client stations, the respective independent data included within respective blocks of orthogonal frequency division multiplex (OFDM) tones. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment; 
         FIGS. 2A, 2B, and 2C  are diagrams illustrating example orthogonal frequency division multiplexing (OFDM) sub-channel blocks for an 80 MHz communication channel, according to an embodiment; 
         FIG. 3  is a diagram of an OFDM symbol that is partitioned into three OFDM sub-channel blocks for an 80 MHz communication channel; 
         FIG. 4  is a block diagram of an example downlink orthogonal frequency division multiple access (OFDMA) signal, according to an embodiment; 
         FIG. 5  is a block diagram of an example downlink OFDMA signal, according to another embodiment; 
         FIG. 6  is a diagram illustrating the transmission of a downlink OFDMA data unit by an access point (AP), and the transmission of acknowledgment signals (ACKs) by client stations in response to the downlink OFDMA data unit, according to an embodiment; 
         FIG. 7  is a diagram illustrating the transmission of a downlink OFDMA data unit by an AP, and the transmission of ACKs by client stations in response to the downlink OFDMA data unit, according to another embodiment; 
         FIG. 8  is a flow diagram of an example method that is implemented by an AP in a WLAN, according to an embodiment; 
         FIG. 9  is a flow diagram of another example method that is implemented by an AP in a WLAN, according to an embodiment; 
         FIG. 10  is a block diagram of an example physical layer (PHY) unit of an AP, according to an embodiment; 
         FIG. 11  is a block diagram of an example physical layer (PHY) unit of an AP, according to another embodiment; 
         FIG. 12  is a diagram illustrating communications in a WLAN during carrier sense multiple access (CSMA) time periods and an OFDMA time period, according to an embodiment; 
         FIG. 13  is a diagram illustrating the transmission of an uplink OFDMA data unit by a plurality of client stations, and the transmission of ACKs by the AP in response to the uplink OFDMA data unit, according to an embodiment; 
         FIG. 14  is a diagram illustrating the transmission of an uplink OFDMA data unit being preceded by the AP transmitting downlink synchronization signals  520 , according to an embodiment; 
         FIG. 15  is a diagram illustrating communications in a WLAN during CSMA time periods and an OFDMA time period, according to an embodiment; 
         FIG. 16  is a flow diagram of an example method that is implemented by an AP in a WLAN, according to an embodiment; and 
         FIG. 17  is a flow diagram of another example method that is implemented by an AP in a WLAN, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In embodiments described below, a wireless network device such as an access point (AP) of a wireless local area network (WLAN) transmits independent data streams to multiple client stations simultaneously. In particular, the wireless device utilizes orthogonal frequency division multiplexing (OFDM) and transmits data for the multiple clients in different blocks of OFDM subchannels. Similarly, in embodiments described below, multiple client stations transmit data to an AP simultaneously, in particular, each client station utilizes OFDM and transmits data to the AP in a different block of OFDM subchannels. 
       FIG. 1  is a block diagram of an example wireless local area network (WLAN)  10 , according to an embodiment. An AP  14  includes a host processor  15  coupled to a network interface  16 . The network interface  16  includes a medium access control (MAC) unit  18  and a physical layer (PHY) unit  20 . The PHY unit  20  includes a plurality of transceivers  21 , and the transceivers are coupled to a plurality of antennas  24 . Although three transceivers  21  and three antennas  24  are illustrated in  FIG. 1 , the AP  14  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  21  and antennas  24  in other embodiments. 
     The WLAN  10  includes a plurality of client stations  25 . Although four client stations  25  are illustrated in  FIG. 1 , the WLAN  10  can include different numbers (e.g., 1, 2, 3, 5, 6, etc.) of client stations  25  in various scenarios and embodiments. Two or more of the client stations  25  are configured to receive corresponding data streams that are transmitted simultaneously by the AP  14 . Additionally, two or more of the client stations  25  are configured to transmit corresponding data streams to the AP  14  such that the AP  14  receives the data streams simultaneously. 
     A client station  25 - 1  includes a host processor  26  coupled to a network interface  27 . The network interface  27  includes a MAC unit  28  and a PHY unit  29 . The PHY unit  29  includes a plurality of transceivers  30 , and the transceivers are coupled to a plurality of antennas  34 . Although three transceivers  30  and three antennas  34  are illustrated in  FIG. 1 , the client station  25 - 1  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  30  and antennas  34  in other embodiments. 
     In an embodiment, one or more of the client stations  25 - 2 ,  25 - 3 , and  25 - 4  has a structure the same as or similar to the client station  25 - 1 . In these embodiments, the client stations  25  structured like the client station  25 - 1  have the same or a different number of transceivers and antennas. For example, the client station  25 - 2  has only two transceivers and two antennas, according to an embodiment. 
     According to an embodiment, the client station  25 - 4  is a legacy client station that is not enabled to receive a data stream that is transmitted by the AP  14  simultaneously with other independent data streams that are intended for other client stations  25 . Similarly, according to an embodiment, the legacy client station  25 - 4  is not enabled to transmit a data stream that to the AP  24  at the same time that other client stations  25  transmit data to the AP  14 . According to an embodiment, the legacy client station  25 - 4  includes a PHY unit that is generally capable of receiving a data stream that is transmitted by the AP  14  simultaneously with other independent data streams that are intended for other client stations  25 . But the legacy client station  25 - 4  includes a MAC unit that is not enabled with MAC layer functions that support receiving the data stream that is transmitted by the AP  14  simultaneously with other independent data streams that are intended for other client stations  25 . According to an embodiment, the legacy client station  25 - 4  includes a PHY unit that is generally capable of transmitting a data stream to the AP  14  at the same time that other client stations  25  transmit data to the AP  14 . But the legacy client station  25 - 4  includes a MAC unit that is not enabled with MAC layer functions that support transmitting a data stream to the AP  14  at the same time that other client stations  25  transmit data to the AP  14 . 
     In an embodiment, the legacy client station  25 - 4  operates according to the IEEE 802.11a and/or the IEEE 802.11n Standards. The legacy client station  25 - 4 , when it communicates with the AP  14 , occupies an entire communication channel. For example, the IEEE 802.11a Standard defines communication channels each having a width of 20 MHz. When the AP  14  and the legacy client station  25 - 4  communicate according to the IEEE 802.11a Standard, the AP  14  transmits data to the legacy client station  25 - 4  in 64 OFDM subchannels that occupy the entire channel, and the legacy client station  25 - 4  transmits data to the AP  14  in the 64 OFDM subchannels. The IEEE 802.11n Standard defines 20 MHz and 40 MHz communications channels. When the AP  14  and the legacy client station  25 - 4  communicate according to the IEEE 802.11n Standard using a 20 MHz channel, the AP  14  transmits data to the legacy client station  25 - 4  in 64 OFDM subchannels that occupy the entire channel, and the legacy client station  25 - 4  transmits data to the AP  14  in the 64 OFDM subchannels. When the AP  14  and the legacy client station  25 - 4  communicate according to the IEEE 802.11n Standard using a 40 MHz channel, the AP  14  transmits data to the legacy client station  25 - 4  in 128 OFDM subchannels that occupy the entire channel, and the legacy client station  25 - 4  transmits data to the AP  14  in the 128 OFDM subchannels. 
     According to the IEEE 802.11a and the IEEE 802.11n Standards, different devices share the communication channel by utilizing a carrier sense, multiple access (CSMA) protocol. Generally speaking, CSMA, according to the IEEE 802.11a and the IEEE 802.11n Standards, specifies that a device that wishes to transmit should first check whether another device in the WLAN is already transmitting. If another device is transmitting, the device should wait for a time period and then again check again to see whether the communication channel is being used. If a device detects that the communication channel is not being used, the device then transmits using the channel. With CSMA, in other words, data that is for a particular device (i.e., not multicast data) can only be transmitted on the channel when no other data is being transmitted on the channel. 
     According to an embodiment, the AP  14  is enabled to transmit different data streams to different client stations  25  at the same time. In particular, the PHY unit  20  is configured to transmit in a communication channel that is wider than specified by the IEEE 802.11a and the IEEE 802.11n Standards. For example, the PHY unit  20  is configured to transmit in one or more of an 80 MHz communication channel, a 120 MHz communication channel, and/or a 160 MHz communication channel, according to an embodiment. As another example, the PHY unit  20  is additionally configured to transmit in one or more of a 200 MHz communication channel, a 240 MHz communication channel, a 280 MHz communication channel, etc., according to an embodiment. 
     According to an embodiment, the AP  14  is configured to partition the wider communication channel into narrower sub-bands or OFDM sub-channel blocks, and different data streams for different client devices  25  are transmitted in respective OFDM sub-channel blocks. According to an embodiment, each OFDM sub-channel block substantially conforms to the PHY specification of the IEEE 802.11a Standard. According to another embodiment, each OFDM sub-channel block substantially conforms to the PHY specification of the IEEE 802.11n Standard. According to another embodiment, each OFDM sub-channel block substantially conforms to a PHY specification of a communication protocol other than the IEEE 802.11a and the IEEE 802.11n Standards. 
     As used herein, the phrase “OFDM sub-channel block substantially conforms to the PHY specification” of a communication protocol or standard generally means that a device (configured according to the communication protocol or standard) that receives the transmitted OFDM sub-channel block is able, generally, to detect and decode the data in the OFDM sub-channel block (signal strength, signal-to-noise (SNR), interference, etc., permitting). For example, an OFDM sub-channel block that substantially conforms to the PHY specification of a communication protocol or standard utilizes the modulation, tone mapping, pilot locations, etc., set forth in the communication protocol or standard, although other aspects of the OFDM sub-channel block do not conform to the PHY specification, according to an embodiment. For example, there may be more zero tones at the edges of an OFDM sub-channel block, reduced power (by frequency domain power allocation) at edge tones, etc., than called for by the communication protocol or standard. Similarly, a used herein, the phrase “a device configured to substantially conform to the PHY specification” of a communication protocol or standard generally means that the device is able to detect and decode a signal that conforms or an OFDM sub-channel block that substantially conforms to the communication protocol or standard (signal strength, signal-to-noise (SNR), interference, etc., permitting). The phrase “a device configured to substantially conform to the PHY specification” of a communication protocol or standard also generally means that the device is able to generate a signal that conforms or an OFDM sub-channel block that substantially conforms to the communication protocol or standard. 
     When an OFDM sub-channel block substantially conforms to the PHY specification of the IEEE 802.11a Standard, for example, a client device  25  corresponding to the OFDM sub-channel block utilizes a PHY unit  29  configured (or substantially configured) according to the IEEE 802.11a Standard to receive the data stream transmitted in the OFDM sub-channel block. When an OFDM sub-channel block substantially conforms to the PHY specification of the IEEE 802.11n Standard, for example, a client device  25  corresponding to the OFDM sub-channel block utilizes a PHY unit  29  configured (or substantially configured) according to the IEEE 802.11n Standard to receive the data stream transmitted in the OFDM sub-channel block. 
     According to an embodiment, each OFDM sub-channel block includes a contiguous block of OFDM sub-channels or tones that can be demodulated at the client station using a fast Fourier transform (FFT) with a width the size of the OFDM sub-channel block. In other words, according to this embodiment, the OFDM sub-channels assigned to client stations are not interleaved such as in the Wi-Max standard. 
       FIGS. 2A, 2B, and 2C  are diagrams illustrating example OFDM sub-channel blocks for an 80 MHz communication channel, according to an embodiment. In  FIG. 2A , the communication channel is partitioned into four contiguous OFDM sub-channel blocks, each having a bandwidth of 20 MHz. The OFDM sub-channel blocks include independent data streams for four client stations. In  FIG. 2B , the communication channel is partitioned into two contiguous OFDM sub-channel blocks, each having a bandwidth of 40 MHz. The OFDM sub-channel blocks include independent data streams for two client stations. In  FIG. 2C , the communication channel is partitioned into three contiguous OFDM sub-channel blocks. Two OFDM sub-channel blocks each have a bandwidth of 20 MHz. The remaining OFDM sub-channel block has a bandwidth of 40 MHz. The OFDM sub-channel blocks include independent data streams for three client stations. 
     Although in  FIGS. 2A, 2B, and 2C , the OFDM sub-channel blocks are contiguous across the communication channel, in other embodiments the OFDM sub-channel blocks are not contiguous across the communication channel (i.e., there are one or more gaps between the OFDM sub-channel blocks). 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 an embodiment, different OFDM sub-channel blocks are transmitted in different channels defined by the IEEE 802.11a and/or 802.11n Standards. In one embodiment, the AP includes a plurality of radios and different OFDM sub-channel blocks are transmitted using different radios. 
     In an embodiment, for a plurality of data streams transmitted by an AP in different OFDM sub-channel blocks, different data streams are transmitted at different data rates when, for example, signal strength, SNR, interference power, etc., varies between client devices. Additionally, for a plurality of data streams transmitted by an AP in different OFDM sub-channel blocks, the amount of data in different data streams is often different. Thus, one transmitted data stream can end before another. In such situations, the data in an OFDM sub-channel block corresponding to data stream that is ended is set to zero or some other suitable predetermined value, according to an embodiment.  FIG. 3  is a diagram of an (n+1)-th OFDM symbol that is partitioned into three contiguous OFDM sub-channel blocks for an 80 MHz communication channel. Two OFDM sub-channel blocks, corresponding to a Station 1 and a Station 3 each have a bandwidth of 20 MHz. The remaining OFDM sub-channel block, corresponding to a Station 2, has a bandwidth of 40 MHz. The OFDM sub-channel blocks include independent data streams for the three stations. The data stream corresponding to Station 2 ended in the n-th OFDM symbol (i.e., the OFDM symbol previous to the (n+1)-th OFDM symbol), whereas the data streams corresponding to Station 1 and Station 2 have not yet ended. Thus, for the (n+1)-th OFDM symbol, data in the OFDM sub-channel block corresponding to a Station 2 is set to zero. 
     An OFDM signal comprising a plurality of OFDM sub-channel blocks to transmit independent data streams as described above is also referred to herein as an orthogonal frequency division multiple access (OFDMA) signals. According to an embodiment, a WLAN utilizes downlink OFDMA signals and uplink OFDMA signals. Downlink OFDMA signals are transmitted synchronously from a single AP to multiple client stations (i.e., point-to-multipoint). An uplink OFDMA signal is transmitted by multiple clients stations jointly to a single AP (i.e., multipoint-to-point). Frame formats and/or signaling schemes for downlink OFDMA and uplink OFDMA are different, according to some embodiments. 
     Embodiments of a PHY frame format for downlink OFDMA signals will now be described. In the following embodiments, OFDM sub-channel blocks have a format substantially similar to the PHY format specified in the IEEE 802.11n Standard. In other embodiments, OFDM sub-channel blocks have a format substantially similar to another communication protocol such as the PHY format specified in the IEEE 802.11a Standard or a communication protocol not yet standardized. 
       FIG. 4  is a block diagram of an example downlink OFDMA signal  100 , according to an embodiment, that is partitioned into four equal-width OFDM sub-channel blocks  102  corresponding to four client stations. In the embodiment of  FIG. 4 , each OFDM sub-channel block  102  has a format substantially similar to the “mixed mode” data unit PHY format specified in the IEEE 802.11n Standard. For example, each OFDM sub-channel block includes a preamble  104  including a legacy short training field (L-STF), a legacy long training field (L-LTF), a legacy signal (L-SIG) field, a high throughput signal (HT-SIG) field, a high throughput short training field (HT-STF), and one or more high throughput long training fields (HT-LTF). Additionally, each OFDM sub-channel block includes a high throughput data field  108  (HT-DATA). The duration of the high throughput portion of the downlink OFDMA signal  100  is T, which corresponds to the longest of the four OFDM sub-channel blocks  102  (i.e.,  102 - 4 ). In other words, the durations of the high throughput portions of the OFDM sub-channel blocks  102 - 1 ,  102 - 2 , and  102 - 3  are shorter than the duration of the high throughput portion of the downlink OFDMA signal  100 . 
     The legacy portion of the preamble  104  (i.e., L-STF, L-LTF, and L-SIG) is identical in all of the OFDM sub-channel blocks  102 , according to an embodiment. For the high throughput portion of the preamble  104  (i.e., starting with HT-SIG), the content of the OFDM sub-channel blocks  102  can be variant for different client stations depending on factors such as rate, data quantity, configuration (e.g., number of antennas, number of supported multiple input, multiple output (MIMO) data streams, etc.) of different clients. 
     According to an embodiment, the AP sets the “reserved bit” in each of the L-SIG fields to “1” (the IEEE 802.11a and 802.11n Standards specify that the “reserved bit” in L-SIG to “0”) to signal the receiver that the current data unit is a downlink OFDMA signal. Additionally, the AP sets the Length and Rate sub-fields in each off the L-SIG fields to correspond to T, the duration of the high throughput portion of the longest OFDM sub-channel block  102  (i.e.,  102 - 4 ). According to another embodiment, the AP sets the “reserved bit” in each of the HT-SIG fields to “0” (the IEEE 802.11n Standard specifies that the “reserved bit” in HT-SIG to “1”) to signal the receiver that the current data unit is a downlink OFDMA signal. 
     In other embodiments, the AP signals that a data unit is a downlink OFDMA signal using techniques other than those described above. For example, according to one embodiment, the AP uses MAC layer signaling to reserve a time period for transmitting a downlink OFDMA signal. In this embodiment, MAC layer signaling is utilized to specify the duration T of the downlink OFDMA signal  100 . In another embodiment, MAC layer signaling does not specify the duration T of the downlink OFDMA signal  100 , but rather specifies different respective times at which respective client stations should send respective acknowledgments of the downlink OFDMA signal  100 . In another embodiment, the AP utilizes MAC layer signaling to specify a single time at which all client stations corresponding to the downlink OFDMA signal  100  should simultaneously transmit respective acknowledgments. 
     In one embodiment, each OFDM sub-channel block  102  in  FIG. 4  has a width of 20 MHz. In another embodiment, each OFDM sub-channel block  102  in  FIG. 4  has a width of 40 MHz. According to an embodiment, if an OFDM sub-channel block has a width of 40 MHz, the legacy portion of the preamble  104  (i.e., L-STF, L-LTF, and L-SIG) is duplicated at upper and lower 20 MHz halves, with the sub-channels in the upper 20 MHz phase shifted by 90 degrees with respect to the sub-channels in the lower 20 MHz. 
       FIG. 5  is a block diagram of an example downlink OFDMA signal  150 , according to an embodiment, that is partitioned into four equal-width OFDM sub-channel blocks  152  corresponding to four client stations. In the embodiment of  FIG. 5 , each OFDM sub-channel block  152  has a format substantially similar to the “Green field mode” data unit PHY format specified in the IEEE 802.11n Standard. For example, each OFDM sub-channel block includes a preamble  154  including an HT-SIG field, and HT-STF field, and one or more HT-LTF fields. Additionally, each OFDM sub-channel block  152  includes a high throughput data field  158  (HT-DATA). The duration of the downlink OFDMA signal  100  is T. The duration of each OFDM sub-channel block  152  is also T. In other words, the AP controls the duration of each OFDM sub-channel block  152  to be T, according to an embodiment. In one embodiment, the AP utilizes zero padding to ensure that each OFDM sub-channel block  152  has a duration of T. In one embodiment, a MAC unit of the AP zero pads one or more MAC service data units (MSDUs) that are included in a MAC protocol data unit (MPDU), which is in turn included in a PHY protocol data unit (PPDU). By zero padding an MSDU, for example, the lengths of the MPDU and the PPDU are increased. 
     In an embodiment, the AP uses MAC layer signaling to reserve a time period for transmitting the downlink OFDMA signal  150 . In one embodiment, MAC layer signaling is utilized to specify the duration T of the downlink OFDMA signal  150 . In another embodiment, MAC layer signaling does not specify the duration T of the downlink OFDMA signal  150 , but rather specifies different respective times at which respective client stations should send respective acknowledgments of the downlink OFDMA signal  150 . In another embodiment, the AP utilizes MAC layer signaling to specify a single time at which all client stations corresponding to the downlink OFDMA signal  150  should simultaneously transmit respective acknowledgments. 
     In another embodiment, a downlink OFDMA signal includes one or more OFDM sub-channel blocks that have a format substantially similar to the “mixed mode” data unit PHY format specified in the IEEE 802.11n Standard and one or more OFDM sub-channel blocks that have a format substantially similar to the “Green field mode” data unit PHY format specified in the IEEE 802.11n Standard. In one implementation, the AP forms the downlink OFDMA signal so that the duration of each of the OFDM sub-channel blocks is the same. In another implementation, the duration of each of the OFDM sub-channel blocks need not be the same. 
     In another embodiment, a downlink OFDMA signal includes one or more OFDM sub-channel blocks that conform to a defined communication protocol specification, such as the IEEE 802.11ac Standard now in the process of being adopted, so that each OFDM sub-channel block in the OFDMA data unit forms an OFDM data unit. In some embodiments, information in preambles of OFDM sub-channel blocks of an OFDMA data unit indicate or signals that each OFDM sub-channel block is part of an OFDMA data unit. In an embodiment, such signaling information is included in a suitable preamble field such as a field that is the same as or similar to the L-SIG field and/or the HT-SIG field specified in the IEEE 802.11a and IEEE 802.11n Standards. 
     In some embodiments, client stations respond with an acknowledgment signal (ACK) or a negative ACK signal (NAK) after the AP transmits each downlink OFDMA data unit or after the AP transmits a plurality of downlink OFDMA data units (referred to as “Block ACK”).  FIG. 6  is a diagram illustrating the transmission of a downlink OFDMA data unit  200  by an AP, and the transmission of ACKs  204  by client stations in response to the downlink OFDMA data unit  200 , according to an embodiment. In the scenario illustrated in  FIG. 6 , four client stations successfully received data transmitted in the downlink OFDMA data unit  200 . In response, each of the four client stations transmits an ACK  204  simultaneous with the transmission of the other ACKs  204 . The ACKs are transmitted after a short inter-frame space (SIFS) interval. In the IEEE 802.11n Standard, SIFS is specified as 16 microseconds, but any suitable SIFS period can be utilized, depending on the particular implementation. In an embodiment, the downlink OFDMA data unit  200  and the ACKs  204  are transmitted in a time period reserved for OFDMA transmissions in the WLAN. According to an embodiment, client stations transmit ACKs/NAKs by an uplink OFDMA data unit, which will be discussed in more detail below. Each client station transmits the ACK  204  in a different OFDM sub-channel block. 
     In one embodiment, each client station determines when to transmit an ACK/NAK based on a determined duration of the OFDMA data unit  200 . As discussed above with respect to  FIG. 4 , the AP can provide information in the OFDMA data unit  200  that indicates the duration of the OFDMA data unit  200 , and a client station can use this information to determine when to transmit the ACK  204 . In another embodiment, the AP assigns a time slot to the client stations in which each client station can transmit an ACK/NAK. For example, a MAC unit in the AP can signal, in an OFDMA data unit previous to the OFDMA data unit  200 , the time period in which the client stations are to transmit ACKs/NAKs. 
       FIG. 7  is a diagram illustrating the transmission of a downlink OFDMA data unit  250  by an AP, and the transmission of ACKs  254  by client stations in response to the downlink OFDMA data unit  250 , according to an embodiment. In the scenario illustrated in  FIG. 7 , four client stations successfully received data transmitted in the downlink OFDMA data unit  250 . In response, each of the four client stations transmits an ACK  254  at different specified times. The downlink OFDMA data unit  250  and the ACKs are transmitted in a time period reserved for OFDMA. A MAC unit of the AP has signaled each of the client stations providing each client station with an indication of the time at which the client station can transmit an ACK/NAK. For example, according to an embodiment, the MAC unit of the AP provides ACK/NAK time slot information to the client stations when providing information regarding the reserved time period for OFDMA. 
     The ACKs are spaced by SIFS intervals. In an embodiment, the downlink OFDMA data unit  250  and the ACKs  254  are transmitted in a time period reserved for OFDMA transmissions in the WLAN. According to an embodiment, client stations transmit ACKs/NAKs by an uplink OFDMA data unit, which is discussed in more detail below. Each client station transmits the ACK  254  in a different OFDM sub-channel block. 
     In an embodiment, the AP assigns the time slots to the client stations in which each client station can transmit the ACKs/NAKs. For example, a MAC unit in the AP can signal, in an OFDMA data unit previous to the OFDMA data unit  250 , the time period in which a client stations is to transmit an ACK/NAK. 
     In another embodiment, ACKs/NAKs are transmitted by the client stations after receiving a plurality of downlink OFDMA data units (referred to as “Block ACK”). In this embodiment, a client station determines when to transmit a Block ACK based on determining a duration of a downlink OFDMA data unit or transmits in a time slot assigned by the AP, for example. 
     In an embodiment, the downlink OFDMA signal is configured to be received and decoded by a legacy client station (e.g., a client station configured to communicate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). In an embodiment, the AP does not signal to at least the legacy client stations that an OFDMA data unit is an OFDMA signal (as opposed to an OFDM data unit according to the legacy protocol (e.g., the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). In an embodiment, at least OFDM sub-channel blocks corresponding to legacy client stations have the same duration as the downlink OFDMA signal so that ACKs/NAKs transmitted by the legacy client station occur at appropriate times with respect to the OFDMA data unit. 
       FIG. 8  is a flow diagram of an example method  300  that is implemented by an AP in a WLAN, according to an embodiment. At block  304 , a plurality of different OFDM sub-channel blocks are assigned to a plurality of different client stations. At block  308 , a plurality of independent data streams (i.e., the streams include different data) are received, wherein each data stream corresponds to a respective client station, and the data streams are to be transmitted to the client stations. At block  312 , downlink OFDM data units are generated such that the plurality of independent data streams are modulated in respective OFDM sub-channel blocks. In an embodiment, generating downlink OFDM data units comprises including an indication in a downlink OFDM data unit that the data unit is an OFDMA data unit (i.e., the data unit includes multiple OFDM sub-channel blocks corresponding to different client stations. In an embodiment, generating downlink OFDM data units comprises including an indication in an OFDM sub-channel block of a duration of a downlink OFDM data unit (i.e., an indication of a duration of the longest duration OFDM sub-channel block in the OFDMA data unit) separate from an indication of the duration of the OFDM sub-channel block. According to an embodiment, the indication of the duration of the downlink OFDM data unit includes an indication of a rate and an indication of a length corresponding to the longest duration OFDM sub-channel block in the OFDMA data unit. 
     At block  316 , the AP transmits the OFDM data units generated at block  312 . 
       FIG. 9  is a flow diagram of an example method  350  that is implemented by an AP in a WLAN, according to an embodiment. In an embodiment, the method  350  is implemented in conjunction with the method  300  of  FIG. 8 . 
     At block  354 , the AP determines a time period that is reserved for downlink OFDMA signals. In one embodiment, the AP also determines a time or times at which client stations can transmit ACKs/NAKs or Block ACKs in response to downlink OFDMA data units. 
     At block  358 , the AP transmits to the client stations data indicative of the time period determined at block  354 . In one embodiment, the AP also transmits data indicative of the time or times at which client stations can transmit ACKs/NAKs or Block ACKs in response to downlink OFDMA data units. 
       FIG. 10  is a block diagram of an example PHY unit  400  of an AP, according to an embodiment. Referring again to  FIG. 1 , the PHY unit  20  of the AP  14  includes the PHY unit  400  of  FIG. 10 , in an embodiment. 
     The PHY unit  400  includes a plurality of processing blocks  404 . In an embodiment, each processing block  404  performs forward error correction (FEC) encoding, modulation, and spatial mapping functions in a manner the same as or similar to such functions described in the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In  FIG. 10 , four processing blocks  404  are illustrated. In other embodiments, a different number of processing blocks  404  are included. For example, in one embodiment, a single processing block  404  is time-shared to process multiple data streams received in parallel. 
     The processing blocks  404  receive a plurality of independent data streams to be transmitted to a plurality of client devices. In the embodiment of  FIG. 10 , each processing block  404  processes a different one of the independent data streams and generates a plurality of constellation points corresponding to a plurality of OFDM sub-channels. 
     Outputs of the processing blocks  404  are provided to a mapping unit  408 . The mapping unit  408  concatenates constellation points from the processing blocks  404  into a larger width OFDM symbol. For example, if the output of each processing block  404  corresponds to a 20 MHz wide (64-point inverse fast Fourier transform (IFFT)) OFDM symbol, then the mapping unit  408  concatenates the outputs of the processing blocks  404  into an 80 MHz wide (256-point IFFT). As another example, if the output of each processing block  404  corresponds to a 40 MHz wide (128-point IFFT) OFDM symbol, then the mapping unit  408  concatenates the outputs of the processing blocks  404  into a 160 MHz wide (512-point IFFT). 
     An IFFT unit  412  generates a time-domain signal from the output of the mapping unit  408 . In an embodiment, the IFFT unit  412  has a width larger than in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, the IFFT unit  412  implements a 256-point IFFT. In another embodiment, the IFFT unit  412  implements a 512-point IFFT. In another embodiment, the IFFT unit  412  implements a suitable width IFFT other than a 256-point IFFT or a 512-point IFFT. 
     A digital processing and digital-to-analog converter (DAC) block  416  processes the output of the IFFT unit  412  and generates an analog signal. In an embodiment, the digital processing and DAC block  416  includes a guard interval insertion unit. In another embodiment, the digital processing and DAC block  416  includes a windowing unit to smooth edges of each OFDM symbol. In an embodiment, the digital processing and DAC block  416  is configured to process signals with a larger bandwidth as compared to a similar processing block in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, the digital processing and DAC block  416  is configured to process signals with a bandwidth of 80 MHz. In another embodiment, the digital processing and DAC block  416  is configured to process signals with a bandwidth of 160 MHz. In another embodiment, the digital processing and DAC block  416  is configured to process signals with a bandwidth different than 80 MHz or 160 MHz. 
     A radio frequency (RF) modulation block  420  generally upconverts the output of the digital processing and DAC block  416  to generate an RF signal, which is transmitted by an antenna  424 . In an embodiment, the RF modulation block  420  is configured to process signals with a larger bandwidth as compared to a similar RF block in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, the RF modulation block  420  is configured to process signals with a bandwidth of 80 MHz. In another embodiment, the RF modulation block  420  is configured to process signals with a bandwidth of 160 MHz. In another embodiment, the RF modulation block  420  is configured to process signals with a bandwidth different than 80 MHz or 160 MHz. 
     In an embodiment, the PHY unit  400  is a sub-unit in a MIMO PHY unit. In this embodiment, the MIMO PHY unit includes a plurality of digital processing and DAC blocks  416  and a plurality of RF modulation blocks  420  corresponding to a plurality of antennas  424 . In another embodiment, the MIMO PHY unit includes a plurality of mapping units  408  and a plurality of IFFT units  412  corresponding to a plurality of transmit chains. In this embodiment, each processing block  404  generates a plurality of outputs corresponding to a plurality of spatially mapped transmit chain signals. In another embodiment, the MIMO PHY unit includes a beamforming unit. 
       FIG. 11  is a block diagram of an example PHY unit  450  of an AP, according to another embodiment. Referring again to  FIG. 1 , the PHY unit  20  of the AP  14  includes the PHY unit  450  of  FIG. 11 , in an embodiment. 
     The PHY unit  450  includes the plurality of processing blocks  404  of  FIG. 10 . A plurality of IFFT units  454  generate time-domain signals from the outputs of the processing blocks  404 . In an embodiment, each IFFT unit  454  has a width such as in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, each IFFT unit  454  implements a 64-point IFFT. In another embodiment, each IFFT unit  454  implements a 128-point IFFT. In another embodiment, each IFFT unit  454  implements a suitable width IFFT other than a 64-point IFFT or a 128-point IFFT. 
     A plurality of digital processing and DAC blocks  458  process the outputs of the IFFT units  454  and generate corresponding analog signals. In an embodiment, each digital processing and DAC block  458  includes a guard interval insertion unit. In another embodiment, each digital processing and DAC block  458  includes a windowing unit to smooth edges of each OFDM symbol. In an embodiment, each digital processing and DAC block  458  is configured to process signals having a bandwidth such as in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, each digital processing and DAC block  458  is configured to process signals with a bandwidth of 20 MHz. In another embodiment, each digital processing and DAC block  458  is configured to process signals with a bandwidth of 40 MHz. In another embodiment, each digital processing and DAC block  458  is configured to process signals with a bandwidth different than 20 MHz or 40 MHz. 
     A plurality of RF modulation blocks  462  generally upconvert the outputs of the digital processing and DAC block  458  to generate RF signals, which are transmitted by respective antennas  466 . In an embodiment, each RF modulation block  462  is configured to process signals having a bandwidth such as in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, each RF modulation block  462  is configured to process signals with a bandwidth of 20 MHz. In another embodiment, each RF modulation block  462  is configured to process signals with a bandwidth of 40 MHz. In another embodiment, each RF modulation block  462  is configured to process signals with a bandwidth different than 20 MHz or 40 MHz. 
     In an embodiment, the PHY unit  450  is a sub-unit in a MIMO PHY unit. In one embodiment, the MIMO PHY unit includes an additional set of one or more of the IFFT units  454 , the digital processing and DAC blocks  458 , and the RF modulation blocks  462  for each of a plurality of transmit chains. In one embodiment, the MIMO PHY unit includes a beamforming unit. 
     In one embodiment, the AP utilizes a single MAC address for the different OFDM sub-channel blocks. In an embodiment, the AP includes a MAC unit that includes a plurality of transmit/receive processing blocks corresponding to the plurality of client devices with which the AP is communicating using OFDMA signals such as described above. The plurality of transmit/receive processing blocks in the MAC unit process the independent data streams simultaneously and/or in parallel. 
     In an embodiment, a client station that supports OFDMA signaling as described above includes a modified PHY unit (as compared to a PHY unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified PHY unit is configured to determine when a received data unit is within an OFDMA data unit, according to an embodiment. For instance, the PHY unit examines the L-SIG and/or HT-SIG “reserved” bits to detect an OFDMA data unit, in an embodiment. As another example, the modified PHY unit is configured to determine when an OFDMA data unit will end, as opposed to an OFDM sub-channel block within the OFDMA data unit that corresponds to the client station, according to an embodiment. For instance, the PHY unit examines the Length and Rate subfields in the L-SIG field to determine a duration of the OFDMA data unit for purposes of determining when to transmit an ACK/NAK, in an embodiment. 
     In an embodiment, a client station that supports OFDMA signaling as described above includes a modified MAC unit (as compared to a MAC unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified MAC unit is configured to interpret MAC signals from the AP regarding when the client can transmit ACKs/NAKs or Block ACKs, according to an embodiment. As another example, the modified MAC unit is configured to interpret MAC signals from the AP regarding periods reserved for OFDMA signals. 
     In an embodiment, a combination of PHY and MAC signaling is utilized for indicating OFDMA data units, the duration of OFMDA data units, and/or reserved time slots for OFDMA signals. In this embodiment, a client station that supports OFDMA signaling as described above includes a modified PHY unit and a modified MAC unit, such as described above. 
       FIG. 12  is a diagram illustrating communications in a WLAN  470  during three time periods: a first CSMA period  474 , an OFDMA time period  478 , and a second CSMA period  482 . In  FIG. 12 , time progresses from left to right so that the first CSMA period  474  occurs first, the OFDMA time period  478  occurs second, and the second CSMA period  482  occurs third. The WLAN includes an AP, a plurality of legacy client stations (LCs), and a plurality of OFDMA client stations (OC). 
     In the first CSMA period  474 , the AP transmits a legacy downlink single to one of the LCs. The OFDMA period  478  is reserved for OFDMA signal transmissions. Thus, in the OFDMA period  478 , the AP transmits a downlink OFDMA signal to a plurality of OCs. The downlink OFDMA signal includes a plurality of OFDM sub-channel blocks corresponding to the plurality of OCs. In the OFDMA period  478 , the plurality of OCs also transmit ACKs/NAKs (not shown) in response to the downlink OFDMA signal, according to an embodiment. In the second CSMA period  482 , an LC transmits a legacy uplink transmission to the AP. 
     Embodiments of a PHY frame format for uplink OFDMA signals will now be described. In the following embodiments, OFDM sub-channel blocks have a format substantially similar to the PHY format specified in the IEEE 802.11n Standard. In other embodiments, OFDM sub-channel blocks have a format substantially similar to another communication protocol such as the PHY format specified in the IEEE 802.11a Standard or a communication protocol not yet standardized. 
     An uplink OFDMA signal comprises a plurality of OFDM sub-channel blocks, a plurality of which are transmitted by different client stations. In one embodiment, each OFDM sub-channel block substantially conforms to the “mixed mode” format as specified in the IEEE 802.11n Standard. In another embodiment each OFDM sub-channel block substantially conforms to the “Green field” format as specified in the IEEE 802.11n Standard. In another embodiment, the OFDM sub-channel blocks in an uplink OFDMA data unit are mixture of “mixed mode” and “Green field” substantially formatted data units. 
     In one embodiment, an uplink OFDMA data unit with mixed mode OFDM sub-channel blocks transmitted by the plurality of clients has a format the same as or similar to the format illustrated in  FIG. 4 . According to one embodiment, the L-SIG “reserved” bit is not set to indicate an OFDMA data unit. According to another embodiment, each client station sets the L-SIG “reserved” bit if the client station is aware that the OFDM sub-channel block that the client station is transmitting is part of an uplink OFDMA data unit. According to another embodiment, each client station does not set the Length and Rate subfields in the L-SIG field differently than when transmitting a CSMA signal. According to another embodiment, each client station sets Length and Rate subfields in the L-SIG field to correspond to the duration of the uplink OFDMA data unit if the client station is aware of the duration of the uplink OFDMA data unit. 
     In one embodiment, the AP reserves a time period for transmission of uplink OFDMA signals. In this embodiment, the AP transmits information regarding the starting time, ending time, and/or duration of the reserved time period to the client stations. 
     According to an embodiment, an AP capable of receiving an uplink OFDMA signal includes an RF demodulation block, an analog-to-digital converter (ADC) and processing block, and an FFT unit that are configured to process signals with a larger bandwidth as compared to similar blocks in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, these blocks are configured to process signals with a bandwidth of 80 MHz. In another embodiment, these blocks are configured to process signals with a bandwidth of 160 MHz. In another embodiment, these blocks are configured to process signals with a bandwidth different than 80 MHz or 160 MHz. 
     In another embodiment, an AP capable of receiving an uplink OFDMA signal includes a plurality of RF demodulation blocks, a plurality of ADC and processing blocks, and a plurality of FFT units that are configured to process signals process signals having a bandwidth such as in a typical AP configured to implement the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In one embodiment, each such block is configured to process signals with a bandwidth of 20 MHz. In another embodiment, each such block is configured to process signals with a bandwidth of 40 MHz. In another embodiment, each such block is configured to process signals with a bandwidth different than 20 MHz or 40 MHz. In these embodiments, the plurality of blocks operate in parallel to process each OFDM sub-channel block, which has a smaller bandwidth than the uplink OFDMA signal, in parallel. 
     In one embodiment, the AP utilizes a single MAC address for receiving different OFDM sub-channel blocks. In an embodiment, the AP includes a MAC unit that includes a plurality of receive processing blocks corresponding to the plurality of client devices with which the AP is communicating using OFDMA signals such as described above. The plurality of receive processing blocks in the MAC unit process the independent data streams received from the client stations simultaneously and/or in parallel. 
     In an embodiment, an AP that supports uplink OFDMA signaling as described above includes a modified PHY unit (as compared to a PHY unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified PHY unit is configured to operate at a wider bandwidth as described above, according to an embodiment. 
     In an embodiment, an AP that supports uplink OFDMA signaling as described above includes a modified MAC unit (as compared to a MAC unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified MAC unit is configured to determine when the AP can transmit ACKs/NAKs or Block ACKs, according to an embodiment. As another example, the modified MAC unit is configured to reserve time periods for uplink OFDMA signals. 
     In an embodiment, a legacy AP is capable of receiving and decoding data transmitted in an OFDM sub-channel block by a client station as part of an uplink OFDMA signal. 
     In an embodiment, a client that supports uplink OFDMA signaling as described above merely implements a PHY data unit that conforms to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard. In another embodiment, a client that supports uplink OFDMA signaling as described above includes a modified PHY unit (as compared to a PHY unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified PHY unit is configured to perform PHY signaling regarding uplink OFDMA. 
     In an embodiment, a client that supports uplink OFDMA signaling as described above includes a modified MAC unit (as compared to a MAC unit configured to operate according to the IEEE 802.11a Standard and/or the IEEE 802.11n Standard). For example, the modified MAC unit is configured to determine when the AP can transmit uplink signals with respect to a reserved time period for uplink OFDMA, according to an embodiment. 
       FIG. 13  is a diagram illustrating the transmission of an uplink OFDMA data unit  500  by a plurality of client stations, and the transmission of ACKs  504  by the AP in response to the uplink OFDMA data unit  500 , according to an embodiment. In the scenario illustrated in  FIG. 13 , four client stations simultaneously transmit respective OFDM sub-channel blocks  508  to the AP. The OFDM sub-channel blocks  508  form the uplink OFDMA data unit  500 . 
     In the scenario illustrated in  FIG. 13 , the AP successfully received each of the OFDM sub-channel blocks  508 . In response, the AP transmits an OFDMA data unit that comprises ACKs  504  corresponding to different OFDM sub-channel blocks. In an embodiment, the OFDMA data unit that comprises the ACKs  504  has a format the same as or similar to a downlink OFDMA data unit as described with respect to  FIG. 5 , the same as or similar to a downlink OFDMA data unit as described with respect to  FIG. 6 , or has another suitable format. 
     The uplink OFDMA data unit  500  and the ACKs  504  are transmitted in a time period reserved for uplink OFDMA. A MAC unit of the AP has signaled each of the client stations providing each client station with an indication of the time at which the client station can transmit the corresponding OFDM sub-channel block  508 . For example, according to an embodiment, the MAC unit of the AP provides uplink OFDMA time slot information to the client stations. 
     The ACKs are spaced from the OFDMA data unit  500  by a SIFS intervals. In an embodiment, a MAC unit of a client station is configured to wait, after the corresponding OFDM sub-channel block, longer than the SIFS interval for an ACK/NAK from the AP. In an embodiment, the MAC unit of the client station is configured to wait for a time out period for retransmission if an ACK/NAK from the AP is not received, wherein the time out period is longer than the SIFS interval. 
     In an embodiment, the AP transmits Block ACKs after receiving several uplink OFDMA data units. 
     According to an embodiment, the AP transmits a synchronization signal to the client stations to help the client stations synchronize for transmitting an uplink OFDMA signal. In an embodiment, the synchronization signal is transmitted to the client stations as a downlink OFDMA signal.  FIG. 14  is a diagram illustrating the transmission of the uplink OFDMA data unit  500  being preceded by the AP transmitting downlink synchronization signals  520 , according to an embodiment. In an embodiment, the synchronization signals  520  are transmitted to the client stations as a downlink OFDMA signal. The synchronization signals  520  have the same duration, according to an embodiment. 
     In the embodiment according to  FIG. 14 , each client station transmits the corresponding OFDM sub-channel block  508  at a determined time duration after receiving the corresponding synchronization signal  520 . 
       FIG. 15  is a diagram illustrating communications in a WLAN  550  during three time periods: a first CSMA period  554 , an OFDMA time period  558 , and a second CSMA period  562 . In  FIG. 15 , time progresses from left to right so that the first CSMA period  554  occurs first, the OFDMA time period  558  occurs second, and the second CSMA period  562  occurs third. The WLAN includes an AP, a plurality of legacy client stations (LCs), and a plurality of OFDMA client stations (OC). 
     In the first CSMA period  554 , the AP transmits a legacy downlink single to one of the LCs. The OFDMA period  558  is reserved for uplink OFDMA signal transmissions. Thus, in the OFDMA period  558 , a plurality of OCs transmit an uplink OFDMA signal to the AP. The uplink OFDMA signal includes a plurality of OFDM sub-channel blocks corresponding to the plurality of OCs. In the OFDMA period  558 , the AP also transmits ACKs/NAKs (not shown) in response to the uplink OFDMA signal, according to an embodiment. According to an embodiment, in the OFDMA period  558 , the AP also transmits synchronization signals (not shown) prior to the uplink OFDMA signal. In the second CSMA period  562 , an LC transmits a legacy uplink transmission to the AP. 
     According to some embodiments, the above discussed OFDMA techniques are utilized in combination with simultaneous downlink transmission (SDT) techniques and simultaneous uplink transmission (SUT) techniques described in U.S. patent application Ser. No. 14/175,526, entitled “Access Point with Simultaneous Downlink Transmission of Independent Data for Multiple Client Stations,” filed on Jul. 18, 2008, and U.S. patent application Ser. No. 14/175,501, entitled “Wireless Network with Simultaneous Uplink Transmission of Independent Data from Multiple Client Stations,” filed on Jul. 18, 2008. Both of U.S. patent application Ser. No. 14/175,526 and U.S. patent application Ser. No. 14/175,501 are hereby expressly incorporated by reference herein in their entireties. 
       FIG. 16  is a flow diagram of an example method  600  that is implemented by an AP in a WLAN, according to an embodiment. At block  604 , a plurality of different OFDM sub-channel blocks are assigned to a plurality of different client stations. At block  308 , OFDM data units are received, wherein each OFDM data unit comprises a plurality of different OFDM sub-channel blocks transmitted by the plurality of client stations simultaneously. In an embodiment, a plurality of independent data streams are modulated in respective OFDM sub-channel blocks. 
     At block  612 , the plurality of independent data streams are demodulated. 
     In another embodiment, the method includes transmitting a synchronization signal from the AP prior to receiving each OFDM signal at block  608 . 
       FIG. 17  is a flow diagram of an example method  650  that is implemented by an AP in a WLAN, according to an embodiment. In an embodiment, the method  650  is implemented in conjunction with the method  600  of  FIG. 16 . 
     At block  654 , the AP determines a time period that is reserved for uplink OFDMA signals. At block  658 , the AP transmits to the client stations data indicative of the time period determined at block  654 . 
     At least some of the various blocks, operations, and techniques described above may be implemented utilizing hardware, a processor executing firmware instructions, a processor executing software instructions, or any combination thereof. When implemented utilizing a processor executing software or firmware instructions, the software or firmware instructions may be stored in any computer readable memory such as on a magnetic disk, an optical disk, or other storage medium, in a RAM or ROM or flash memory, processor, hard disk drive, optical disk drive, tape drive, etc. Likewise, the software or firmware instructions may be delivered to a user or a system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or via communication media. Communication media typically embodies computer readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave or other transport mechanism. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wired media such as a wired network or direct-wired connection, and wireless media such as acoustic, radio frequency, infrared and other wireless media. Thus, the software or firmware instructions may be delivered to a user or a system via a communication channel such as a telephone line, a DSL line, a cable television line, a fiber optics line, a wireless communication channel, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). The software or firmware instructions may include machine readable instructions that, when executed by the processor, cause the processor to perform various acts. 
     When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention.