Patent Publication Number: US-9408090-B1

Title: Signaling guard interval capability in a communication system

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/102,727, now U.S. Pat. No. 8,665,908, entitled “Signaling Guard Interval Capability in a Communication System,” filed May 6, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/333,690, filed on May 11, 2010. Both of the applications referenced above are hereby incorporated by reference herein in their entireties. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to communication networks and, more particularly, to communicating device capabilities between devices in a wireless network. 
     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. 
     Development of wireless local area network (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. Work has begun on a new standard, IEEE 802.11ac, that promises to provide even greater throughput. 
     SUMMARY 
     In one embodiment, a method for generating a data unit for transmission in a wireless network is disclosed. Communication devices in the wireless network are configured to use a first guard interval between symbols or a second guard interval between symbols, wherein the first guard interval has a length shorter than a length of the second guard interval. The method includes generating a field to indicate a set of one or more modulation and coding schemes (MCSs) supported by a first device in the wireless network and to indicate whether each of the one or more MCSs is supported when using the first guard interval. The method also includes generating a data unit to include the field and causing the data unit to be transmitted to a second device in the wireless network. 
     In another embodiment, an apparatus for use in a wireless network is disclosed. The wireless network is configured to use a first guard interval between symbols or a second guard interval between symbols, wherein the first guard interval has a length shorter than a length of the second guard interval. The apparatus comprises a wireless network interface configured to generate a field to indicate a set of one or more modulation and coding schemes (MCSs) supported by the wireless network interface and to indicate whether each of the one or more MCSs is supported when using the first guard interval. The wireless network interface is further configured to generate a data unit to include the field and cause the data unit to be transmitted to another device in the wireless network. 
     In yet another embodiment, a method for determining capabilities of a communication device in a wireless network is disclosed. The wireless network is configured to use a first guard interval between symbols or a second guard interval between symbols, wherein the first guard interval has a length shorter than a length of the second guard interval. The method includes analyzing a field in a data unit received from a communication device to determine a set of one or more modulation and coding schemes (MCSs) supported by the communication device and to determine whether one or more MCSs in the set of one or more MCSs is supported by the communication device when using the first guard interval. Additionally, the method includes utilizing i) one MCS in the set of one or more MCSs and ii) the first guard interval a) when communicating with the communication device and b) when it is determined that the one MCS is supported by the communication device when using the first guard interval. 
     In still another embodiment, an apparatus for use in a wireless network is disclosed. The wireless network is configured to use a first guard interval between symbols or a second guard interval between symbols, wherein the first guard interval has a length shorter than a length of the second guard interval. The apparatus comprises a wireless network interface configured to analyze a field in a data unit received from a communication device to determine a set of one or more modulation and coding schemes (MCSs) supported by the communication device and to determine whether one or more MCSs in the set of one or more MCSs is supported by the communication device when using the first guard interval. The wireless network interface is further configured to utilize i) one MCS in the set of one or more MCSs and ii) the first guard interval when communicating with the communication device and when it is determined that the one MCS is supported by the communication device when using the first guard interval. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example wireless local area network (WLAN) that utilizes techniques for communicating capabilities between devices, according to an embodiment. 
         FIG. 2  is a block diagram of an example physical layer (PHY) processing unit, according to an embodiment. 
         FIG. 3  is a diagram of an example orthogonal frequency division multiplexing (OFDM) symbol, according to an embodiment. 
         FIG. 4  is a diagram of a field included in an example data unit, according to an embodiment. 
         FIG. 5  is a flow diagram of an example method for generating a data unit to communicate capabilities with other communication devices in a wireless network, according to an embodiment. 
         FIG. 6  is a flow diagram of an example method  600  for determining capabilities of a communication device, 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 data streams to one or more client stations. According to an embodiment, symbols transmitted by the AP include guard intervals to prevent or minimize intersymbol interference at the receiver caused by multipath propagation in the communication channel. The length of the guard interval needed to mitigate interference generally depends on the delay spread of the particular channel being utilized. Consequently, in some embodiments and/or scenarios, the guard interval utilized by the AP is a long guard interval (LGI), while in other embodiments and/or scenarios, the guard intervals utilized is a short guard interval (SGI). The short guard interval has an advantage of reducing idle time between symbols and thus increasing transmission data rate. However, in some situations, the increased data rate associated with the shorter guard interval is not supported by a particular client station, and in these situations the longer guard interval needs to be utilized even if the delay spread of the channel allows for a shorter guard interval to be used. Therefore, a client station, in establishing communication with an AP, communicates to the AP data rate capabilities of the client station in various scenarios and/or embodiments. For example, in an embodiment, the client station communicates to the AP information that allows the AP to determine if a short guard interval is supported by the client station for a particular channel bandwidth and/or a particular modulation and coding scheme (MCS). In an embodiment, the AP utilizes this information to determine the proper guard interval based on the bandwidth and MCS being utilized for communicating with the client station. Similarly, the AP communicates to the client station data rate capabilities of the AP in other various scenarios and/or embodiments. Additionally, a first client station communicates to a second client station data rate capabilities of the first client station in other various scenarios and/or embodiments. 
       FIG. 1  is a block diagram of an example embodiment of a wireless local area network (WLAN)  10  that utilizes techniques described herein for communicating capabilities among devices, according to an embodiment. An AP  14  includes a host processor  15  coupled to a network interface  16 . The network interface  16  includes a medium access control (MAC) processing unit  18  and a physical layer (PHY) processing unit  20 . The PHY processing unit  20  includes a plurality of transceivers  21 , and the transceivers are coupled to a plurality of antennas  24 . Although three transceivers  21  and three antennas  24  are illustrated in  FIG. 1 , the AP  14  can include different numbers (e.g., 1, 2, 4, 5, etc.) of transceivers  21  and antennas  24  in other embodiments. In one embodiment, the MAC processing unit  18  and the PHY processing unit  20  are configured to operate according to a first communication protocol (e.g., the IEEE 802.11ac Standard, now in the process of being standardized). The first communication protocol is also referred to herein as a very high throughput (VHT) protocol. In another embodiment, the MAC processing unit  18  and the PHY processing unit  20  are also configured to operate according to at least a second communication protocol (e.g., the IEEE 802.11n Standard, the IEEE 802.11a Standard, etc.). 
     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. At least one of the client stations  25  (e.g., client station  25 - 1 ) is configured to operate at least according to the first communication protocol. 
     The client station  25 - 1  includes a host processor  26  coupled to a network interface  27 . The network interface  27  includes a MAC processing unit  28  and a PHY processing unit  29 . The PHY processing unit  29  includes a plurality of transceivers  30 , and the transceivers  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  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 all of the client stations  25 - 2 ,  25 - 3  and  25 - 4 , have a structure the same as or similar to the client station  25 - 1 . In these embodiments, the client stations  25  structured the same as or similar to 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. 
     In various embodiments, the PHY processing unit  20  of the AP  14  is configured to generate data units conforming to the first communication protocol. The transceiver(s)  21  is/are configured to transmit the generated data units via the antenna(s)  24 . Similarly, the transceiver(s)  24  is/are configured to receive the data units via the antenna(s)  24 . The PHY processing unit  20  of the AP  14  is configured to process received data units conforming to the first communication protocol, according to an embodiment. 
     In various embodiments, the PHY processing unit  29  of the client device  25 - 1  is configured to generate data units conforming to the first communication protocol. The transceiver(s)  30  is/are configured to transmit the generated data units via the antenna(s)  34 . Similarly, the transceiver(s)  30  is/are configured to receive data units via the antenna(s)  34 . The PHY processing unit  29  of the client device  25 - 1  is configured to process received data units conforming to the first communication protocol, according to an embodiment. 
       FIG. 2  is a block diagram of an example PHY processing unit  200  configured to operate according to the VHT protocol, according to an embodiment. Referring to  FIG. 1 , the AP  14  and the client station  25 - 1 , in one embodiment, each include a PHY processing unit such as the PHY processing unit  200 . 
     The PHY unit  200  includes a scrambler  204  that generally scrambles an information bit stream to reduce the occurrence of long sequences of ones or zeros, according to an embodiment. In another embodiment, the scrambler  204  is replaced with a plurality of parallel scramblers located after an encoder parser  208 . In this embodiment, each of the parallel scramblers has a respective output coupled to a respective one of a plurality of FEC encoders  212 . The plurality of parallel scramblers operate simultaneously on a demultiplexed stream. In yet another embodiment, the scrambler  204  comprises a plurality of parallel scramblers and a demultiplexer that demultiplexes the information bit stream to the plurality of parallel scramblers, which operate simultaneously on demultiplexed streams. These embodiments may be useful, in some scenarios, to accommodate wider bandwidths and thus higher operating clock frequencies. 
     The encoder parser  208  is coupled to the scrambler  204 . The encoder parser  208  demultiplexes the information bit stream into one or more encoder input streams corresponding to one or more FEC encoders  212 . In another embodiment with a plurality of parallel scramblers, the encoder parser  208  demultiplexes the information bit stream into a plurality of streams corresponding to the plurality of parallel scramblers. 
     Each encoder  212  encodes the corresponding input stream to generate a corresponding encoded stream. In one embodiment, each FEC encoder  212  includes a binary convolutional encoder. In another embodiment, each FEC  212  encoder includes a binary convolutional encoder followed by a puncturing block. In another embodiment, each FEC encoder  212  includes a low density parity check (LDPC) encoder. In yet another embodiment, each FEC encoder  212  additionally includes a binary convolutional encoder followed by a puncturing block. In this embodiment, each FEC encoder  212  is configured to implement any of: 1) binary convolutional encoding without puncturing; 2) binary convolutional encoding with puncturing; or 3) LDPC encoding. 
     A stream parser  216  parses the one or more encoded streams into one or more spatial streams for separate interleaving and mapping into constellation points. Corresponding to each spatial stream, an interleaver  220  interleaves bits of the spatial stream (i.e., changes the order of the bits) to prevent long sequences of adjacent noisy bits from entering a decoder at the receiver. Also corresponding to each spatial stream, a constellation mapper  224  maps an interleaved sequence of bits to constellation points corresponding to different subcarriers of an OFDM symbol. More specifically, for each spatial stream, the constellation mapper  224  translates every bit sequence of length log 2(M) into one of M constellation points. The constellation mapper  224  handles different numbers of constellation points depending on the MCS being utilized. In an embodiment, the constellation mapper  224  is a quadrature amplitude modulation (QAM) mapper that handles M=2, 4, 16, 64, 256, and 1024. In other embodiments, the constellation mapper  224  handles different modulation schemes corresponding to M equaling different subsets of at least two values from the set {2, 4, 16, 64, 256, 1024}. 
     In an embodiment, a space-time block coding unit  228  receives the constellation points corresponding to the one or more spatial streams and spreads the spatial streams to a greater number of space-time streams. In some embodiments, the space-time block coding unit  228  is omitted. A plurality of CSD units  232  are coupled to the space-time block unit  228 . The CSD units  232  insert cyclic shifts into all but one of the space-time streams (if more than one space-time stream) to prevent unintentional beamforming. For ease of explanation, the inputs to the CSD units  232  are referred to as space-time streams even in embodiments in which the space-time block coding unit  228  is omitted. 
     A spatial mapping unit  236  maps the space-time streams to transmit chains. In various embodiments, spatial mapping includes one or more of: 1) direct mapping, in which constellation points from each space-time stream are mapped directly onto transmit chains (i.e., one-to-one mapping); 2) spatial expansion, in which vectors of constellation point from all space-time streams are expanded via matrix multiplication to produce inputs to the transmit chains; and 3) beamforming, in which each vector of constellation points from all of the space-time streams is multiplied by a matrix of steering vectors to produce inputs to the transmit chains. 
     Each output of the spatial mapping unit  236  corresponds to a transmit chain, and each output of the spatial mapping unit  236  is operated on by an IDFT unit  240  that converts a block of constellation points to a time-domain signal. Outputs of the IDFT units  240  are provided to GI insertion and windowing units  244  that prepend, to each OFDM symbol, a guard interval (GI) portion, which is a circular extension of the OFDM symbol in an embodiment, and smooth the edges of each symbol to increase spectral decay. Outputs of the GI insertion and windowing units  244  are provided to analog and RF units  248  that convert the signals to analog signals and upconvert the signals to RF frequencies for transmission. The signals are transmitted in a 20 MHz, a 40 MHz, an 80 MHz, a 120 MHz, or a 160 MHz bandwidth channel, in various embodiments and/or scenarios. 
       FIG. 3  is a diagram of an example OFDM symbol  300  generated by the PHY processing unit  200 , according to an embodiment. The OFDM symbol  300  includes a guard interval portion  302  and a data portion  304 . For example, the guard interval comprises a cyclic prefix repeating an end portion of the symbol, according to an embodiment. Further, according to one embodiment, the guard interval portion  302  is either a short guard interval or a long guard interval, depending on mode of transmission to be utilized. In an embodiment, the short guard interval (SGI) has a length of 0.4 μs, and the long guard interval (LGI) has a length of 0.8 μs guard interval. In an embodiment, the data portion  304  has a length of 3.2 μs. In other embodiments, other suitable lengths for the SGI, the LGI, and the data portion  304  are utilized. In some embodiments, the SGI has a length that is 50% of the length of the LGI. In other embodiments, the SGI has a length that is 75% or less of the length of the LGI. In other embodiments, the SGI has a length that is 50% or less of the length of the LGI. 
     In an embodiment, the data rate of data units processed by the PHY processing unit  200  depends on the channel bandwidth, the particular MCS being utilized, and the guard interval length. For example, in an embodiment, the channel bandwidth determines the number of data tones, and the MCS defines the constellation size, the coding rate, and the number of spatial streams utilized. In an embodiment, the guard interval length determines the total time over which a symbol is transmitted. For example, when the SGI has a length of 0.4 μs, the LGI has a length of 0.8 μs guard interval, and the data portion  304  has a length of 3.2 μs, the OFDM symbol  300  has a length of 3.6 μs when the SGI is utilized and a length of 4.0 μs when the LGI is utilized. 
     The number of sub-carriers (or tones) in an OFDM symbol generally depends on the bandwidth (BW) of the channel being utilized, in some embodiments. For example, an OFDM symbol for a 20 MHz channel corresponds to a size 64 IDFT and includes 64 tones, whereas an OFDM symbol for a 40 MHz channel corresponds to a size 128 IDFT and includes 128 tones, according to an embodiment. In an embodiment, the tones in an OFDM symbol include guard tones for filter ramp up and ramp down, DC tones for mitigating radio frequency interference, and pilot tones for frequency offset estimation. The remaining tones can be used to transmit data (“data tones”), according to an embodiment. More specifically, continuing with the same example, if a size 64 IDFT is used to generate an OFDM symbol, and seven tones are used as guard tones, one tone is used as a DC tone, four tones are used as pilot tones, the remaining 52 tones are then used as data tones. As another example, an OFDM symbol for an 80 MHz channel corresponds to a size 256 IDFT and may include 230 data tones according to an embodiment. In general, more tones are available for data transmission in higher bandwidth channels resulting in higher data rates generally associated with the wider bandwidths. Various example transmission channels and tone mappings that are utilized in some embodiments of the present disclosure are described in U.S. patent application Ser. No. 12/846,681, entitled “Methods and Apparatus for WLAN Transmission”, filed on Jul. 29, 2010, which is hereby incorporated by reference herein in its entirety. 
     With reference to  FIG. 2 , a particular MCS defines the coding rate for the FEC encoders  212 , the number of spatial streams created by the stream parser  216 , and the number of constellation points used by the constellation mapper  224 , in an embodiment. Generally, higher coding rates, more spatial streams, and larger constellations result in higher data rates. Conversely, a guard interval (e.g., inserted at unit  244  of  FIG. 2 ) extends the symbol transmission time, thereby decreasing the data rate. A longer guard interval decreases throughput more than a shorter guard interval. As a specific example, in an embodiment, a data stream generated using 64-QAM modulation and 5/6 FEC coding rate, transmitted using 8 spatial streams in an 80 MHz channel with a long GI (0.8 μs) is transmitted at approximately 2.3 Gbps. The same data unit but with a short GI (0.4 μs) is transmitted at approximately 2.5 Gbps. 
     Referring again to  FIG. 2 , depending on the particular data rate, different numbers of encoders  212  operate in parallel in various embodiments and/or scenarios. For example, according to one embodiment, the PHY processing unit  200  includes four encoders  212 , and one, two, three, or four encoders operate simultaneously depending on the particular MCS, bandwidth, and guard interval being utilized. In another embodiment, the PHY processing unit  200  includes five encoders  212 , and one, two, three, four, or five encoders operate simultaneously depending on the particular MCS, bandwidth, and guard interval being utilized. In another embodiment, the PHY unit  200  includes up to ten encoders  212 , and one, two, three, four, five, six, seven, eight, nine or ten encoders operate simultaneously depending on the particular MCS, bandwidth, and guard interval being utilized. In an embodiment, the number of encoders operating simultaneously increments at multiples of the data rate, e.g., every 600 Mbps. In other words, one encoder is utilized for data rates up to 600 Mbps, two encoders are utilized for data rates between 600 Mbps and 1200 Mbps, as an example. In an illustrative example, a data stream encoded with the coding rate of 3/4, modulated using 256-QAM modulation (with 234 data tones), and transmitted on 4 spatial streams in an 80 MHz channel requires three encoders  212  to operate in parallel, in an embodiment. 
     As discussed above, the PHY processing unit  200  ( FIG. 2 ) is utilized to encode and transmit data units, according to an embodiment. In some embodiments, the PHY processing unit  200  is also configured for receiving and decoding data units. The number of decoders utilized to decode a data stream generally corresponds to the number of encoders used to encode the data stream. Therefore, an AP (such as the AP  14 ) and/or a client station (such as the client station  25 - 1 ) generally includes an equal number of encoders and decoders. In some embodiments, however, the number of encoders is different than the number of decoders. In an embodiment, the number of decoders operating simultaneously increments at multiples of the data rate, e.g., every 600 Mbps. In other words, one decoder is utilized for data rates up to 600 Mbps, two decoders are utilized for data rates between 600 Mbps and 1200 Mbps, as an example. In an illustrative example, a data stream encoded with the coding rate of 3/4, modulated using 256-QAM modulation (with 234 data tones), and transmitted on 4 spatial streams in an 80 MHz channel requires three decoders to operate in parallel, in an embodiment. 
     According to an embodiment, for a particular MCS and bandwidth, the number of encoders  212  (or decoders) that operate in parallel to encode (decode) a data stream is the same regardless of whether a short guard interval or a long guard interval is used to generate the symbols. According to another embodiment, for a particular MCS and bandwidth, more encoders (or decoders) are needed to encode (decode) a data stream with short guard intervals than a data stream with long guard intervals. For example, an MCS that defines a 64-QAM with 6 spatial streams encoded at the rate of 5/6 and transmitted in an 80 MHz channel corresponds to 1.725 Gbps data rate when a long guard interval of 0.8 μs is used, according to one embodiment. If the number of encoders (or decoders) increments at 600 Mbps, this data rate then requires three encoders (decoders) to be used in parallel. Continuing with the same example, in this embodiment, the data rate is approximately 1.9 Gbps when a short guard interval of 0.4 μs is used. In this case, four encoders (or decoders) need to operate in parallel to process the data unit. As just another example, a 80 MHz 64-QAM data stream with coding rate of 5/6 and 2 spatial streams requires one encoder (or decoder) when a long guard interval of 0.8 μs is used, but two encoders (decoders) are needed for the same MCS and BW when a short guard interval of 0.4 μs is used. Consequently, certain MCSs are supported at the AP  14  and/or the client station  25 - 1  when a long guard interval is utilized, but are not supported at the AP  14  and/or the client station  25 - 1  when a short guard interval is utilized in various embodiments and/or scenarios. 
     In an embodiment, a client station such as client station  25 - 1 , in establishing communication with the AP  14 , signals to the AP  14  a highest data rate capability of the client station  25 - 1  based, at least in part, on the number of encoders available at the client station to process data streams. For example, in establishing communication with the AP  14 , the client station  25 - 1  transmits an association frame to the AP  14 , where the association frame includes an indicator of the highest data rate of the client station  25 - 1 , according to one embodiment. The AP  14  then utilizes the indicator of the highest data rate of the client station  25 - 1  to determine whether an SGI or an LGI can be used with a particular MCS and/or a particular bandwidth when communicating with the client station  25 - 1 , as will be described in more detail below. For example, when a particular MCS at a particular BW and with an SGI results in a data rate that exceeds the highest data rate of the client station  25 - 1 , the AP  14  determines that SGI cannot be used with the particular MCS and the particular BW, in an embodiment. 
     In some embodiments, the client station  25 - 1  also receives an indication of a highest data rate capability of the AP  14  from the AP  14 . The client station  25 - 2  then utilizes the indicator of the highest data rate of the AP  14  to determine whether an SGI or an LGI can be used with a particular MCS and/or a particular bandwidth when communicating with the AP  14 . For example, when a particular MCS at a particular BW and with an SGI results in a data rate that exceeds the highest data rate of the AP  14 , the client station  25 - 1  determines that SGI cannot be used with the particular MCS and the particular BW, in an embodiment. 
       FIG. 4  is a diagram of a field  400  that is utilized by a first communication device to transmit to a second communication device an indication of a set of one or more MCSs supported by a first device and whether each of the one or more MCSs is supported when using the SGI, according to an embodiment. The field  400  is included in an association request frame that the client station  25 - 1  is configured to transmit to the AP  14 , according to an embodiment. In other embodiments, the field  400  is included in one or more of a beacon, association request, association response, reassociation request, reassociation response, probe request, and probe response frames or any other initial capability inquiry and/or response frame that the station  25 - 1  is configured to transmit to the AP  14  or vice versa. In some embodiments, the field  400  is included in an initial capability inquiry and/or response frame that the AP  14  is configured to transmit to the client station  25 - 1  or vice versa. The field  400  includes an Rx MCS map subfield  402 , and an Rx highest supported data rate subfield  404 . According to an embodiment, the Rx MCS map subfield  402  indicates one or more supported MCS sets for one or more of a plurality of spatial streams to be received. For example, in an embodiment, one or more supported MCS sets is indicated for one or more of up to eight spatial streams to be received. In one such embodiment, if a particular device is only able to receive a subset of the eight spatial streams (e.g., if a device is only able to receive 1, 2, 3, or 4 of the 8 spatial streams), the device indicates, in the Rx MCS map subfield  402 , a supported MCS set for each of the number of supported spatial streams, and also indicates that there is no support for the remaining numbers of spatial streams. The Rx highest supported data rate subfield  404  indicates a highest data rate that the device is capable of receiving. 
     Further, the field  400  includes a Tx MCS set defined subfield  406 , a Tx MCS map subfield  408  and Tx highest supported data rate subfield  410 . The Tx MCS set defined subfield  406  indicates whether the subfields  408  and  410  are valid. For example, when the Tx MCS set defined subfield  406  indicates the subfields  408  and  410  are not valid, a device receiving the field  400  determines that the device that transmitted the field  400  has transmission capabilities according to the subfields  402  and  404 . When the Tx MCS set defined subfield  406  is appropriately set, the subfields  408  and  410  signal the MCS and data rate capabilities of the station for transmission. The field  400  also includes a reserved subfield  412 . 
     In another embodiment, the station  25 - 1  communicates to the AP  14  in one or more of the association request, association response, reassociation request, reassociation response, probe request, probe response, or any other initial capability inquiry and/or response frame the maximum data rate that the client station  25 - 1  supports for reception and/or a number decoders of the client station  25 - 1 , along with supported MCSs, enabling the AP  14  to determine the station&#39;s SGI capabilities for a particular MCS and/or a particular bandwidth. In yet another embodiment, the station  25 - 1  communicates to the AP  14  in one or more of the beacon, association request, association response, reassociation request, reassociation response, probe request, probe response, an indicator of supported MCSs for different bandwidths and guard interval lengths (SGI or LGI). In other words, for each supported MCS, the indicator may indicate whether the client station  25 - 1  supports that MCS with an SGI. 
       FIG. 5  is a flow diagram of an example method  500  for generating a data unit that includes information to indicate transmission and/or reception capabilities for a communication device in a wireless network, according to an embodiment. With reference to  FIG. 1 , the method  500  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  500 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  500 . With continued reference to  FIG. 1 , in yet another embodiment, the method  500  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  500  is implemented by other suitable network interfaces. 
     At block  504 , a field is generated to indicate a set of one or more MCSs supported by the communication device (by which the method  500  is being implemented). The field also indicates whether each of the one or more MCSs is supported when using a first guard interval. For example, Rx MCS Map field  402  ( FIG. 4 ) is generated to indicate supported MCSs and the RX Highest Supported Data Rate is generated to indicate whether each of the supported MCSs is supported by the communication device when a first guard interval (e.g., the SGI discussed above with reference to  FIG. 3 ) is used. In an embodiment, the field is the field  400  of  FIG. 4 . In other embodiments, another suitable field is utilized. 
     At block  508 , a data unit which includes the field is generated. At block  512  the data unit is transmitted to another device in the wireless network. For example, according to an embodiment, the data unit is transmitted to the AP  14  ( FIG. 1 ) in an association request frame or another suitable data unit. In an embodiment, a processing unit causes the data unit to be transmitted, such as a PHY processing unit, a MAC processing unit, or another suitable processing unit. 
       FIG. 6  is a flow diagram of an example method  600  for determining capabilities of a communication device, according to an embodiment. With reference to  FIG. 1 , the method  600  is implemented by the network interface  16 , in an embodiment. For example, in one such embodiment, the PHY processing unit  20  is configured to implement the method  600 . According to another embodiment, the MAC processing  18  is also configured to implement at least a part of the method  600 . With continued reference to  FIG. 1 , in yet another embodiment, the method  600  is implemented by the network interface  27  (e.g., the PHY processing unit  29  and/or the MAC processing unit  28 ). In other embodiments, the method  600  is implemented by other suitable network interfaces. 
     At block  604 , a first communication device receives a data unit which includes capabilities information from a second communication device in a wireless network regarding capabilities of the second communication device. The capabilities information is a field such as the field  400  of  FIG. 4  or another suitable field, according to an embodiment. 
     At block  608 , the first communication device determines one or more MCS sets supported at the second communication device based on the information received at block  604 . In an embodiment, a supported set is determined for each of a plurality of spatial streams. 
     At block  612 , it is determined whether the one or more MCSs in the one or more of the supported MCS sets are supported at the second communication device when a first guard interval is utilized. In an embodiment, the guard interval support determination is made based on an indication of a highest supported data rate included in the data unit received at block  604  (e.g., in the Rx highest data rate subfield  404 ,  FIG. 4 ). In an embodiment, the first guard interval is a short guard interval (e.g., the short guard interval discussed above with reference to  FIG. 3 ). 
     At block  616 , one of the supported MCS s is selected to be utilized when communicating with the second communication device. 
     At block  624 , it is determined whether the first guard interval is supported for the selected MCS. If it is determined at block  624  that the first guard interval is supported for the selected MCS, then the first guard interval is utilized when communicating with the second communication device at block  628 . On the other hand, if it is determined at block  624  that the first guard interval is not supported for the selected MCS, then a second guard interval (e.g., the long guard interval discussed above with reference to  FIG. 3 ) is utilized when communicating with the second communication device at block  628  at block  632 . 
     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), a programmable logic device (PLD), etc. 
     While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, changes, additions and/or deletions may be made to the disclosed embodiments without departing from the scope of the invention.