Link adaptation a communication network

<u>Cross-References to Related Applications</u>

0001This disclosure claims the priority of the following U.S. Provisional Patent Applications: <ul id="ul0001" list-style="none"><li>U.S. Provisional Patent Application No. <patcit id="pcit0001" dnum="US61379322B"><text>61/379,322</text></patcit>, entitled "Link Adaptation in 802.11ac," filed on September 1, 2010;</li><li><patcit id="pcit0002" dnum="US61411857A" dnum-type="L"><text>U.S. Provisional Patent Application No. 61/411,857</text></patcit>, entitled "VHT Fast Link Adaptation," filed November 9, 2010;</li><li><patcit id="pcit0003" dnum="US61416154A" dnum-type="L"><text>U.S. Provisional Patent Application No. 61/416,154</text></patcit>, entitled "VHT Fast Link Adaptation," filed November 22, 2010;</li><li><patcit id="pcit0004" dnum="US61430379A" dnum-type="L"><text>U.S. Provisional Patent Application No. 61/430,379</text></patcit>, entitled "VHT Fast Link Adaptation," filed January 6, 2011; and</li><li><patcit id="pcit0005" dnum="US61451431A" dnum-type="L"><text>U.S. Provisional Patent Application No. 61/451,431</text></patcit>, entitled "VHT Fast Link Adaptation," filed March 10, 2011.</li></ul>

<u>Field of the Disclosure</u>

0002The present disclosure relates generally to communication networks and, more particularly, to communication networks in which devices exchange information to adapt communication links due to changes in channel conditions, for example.

<u>Background</u>

0003The 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.

0004When 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 802.1 1a 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.

0005Throughput Analysis and Improvement of Hybrid Multiple Access in IEEE 802.15.3c mm-Wave WPAN, Pyo, Chang Woo and Harada, Hiroshi, October 2009 relates to throughput analysis and improvement of hybrid multiple access in IEEE 802.15.3c mm-wave WPAN. This document studies the throughput of the IEEE 802.15.3c millimeter-wave (mm-Wave) WPAN system that is based on the hybrid multiple access of CSMA/CA and TDMA. In order to study the throughput of this system, a novel analytical model is constructed by considering hybrid multiple access. The throughput analysis shows that the throughput is significantly affected by the length of access time of CSMA/CA. A large length of access time of CSMA/CA will cause a low data transmission time in TDMA, while a short length of access time of CSMA/CA will cause a large number of data transmission collisions in CSMA/CA. The failure of data transmission channel release due to the collisions will lead to an increase in the unused data transmission time in TDMA. In order to improve the throughput, a private channel-release time is introduced to avoid the failure of data transmission channel release and provide the optimum access time of CSMA/CA, thereby maximizing the throughput. The proposed method can significantly improve the throughput of the IEEE 802.15.3c mm-wave WPAN system.

0006<patcit id="pcit0006" dnum="WO2009088364A1"><text>WO 2009/088364 A1</text></patcit> relates to a method, a device and a computer program product for determining a data transmission characteristic. This document discloses a method for determining a data transmission characteristic, wherein data are transmitted in at least one superframe, wherein each superframe is configured to transmit a plurality of frames, the method comprising: sending, in the superframe, a plurality of data transmission characteristic request messages to a receiver for requesting data transmission characteristic information from the receiver, wherein the data transmission characteristic request messages are command/control messages; receiving, in the superframe, a plurality of data transmission characteristic response messages including data transmission characteristic information from the receiver in response to the data transmission characteristic request messages wherein the data transmission characteristic response messages are command/control messages; determining, from the data transmission characteristic information, at least one data transmission characteristic.

0007Both <patcit id="pcit0007" dnum="WO2008056916A1"><text>WO 2008/056916 A1</text></patcit> and <patcit id="pcit0008" dnum="US20080250294A1"><text>US 2008/0250294 A1</text></patcit> relate to a system and method for wireless communication of uncompressed video having a composite frame format. Each of these documents discloses a system and method for efficiently communicating uncompressed video and for efficiently communicating corresponding acknowledgements in a system for wireless communication of uncompressed video. In one embodiment disclosed in these documents, the method includes aggregating multiple subpackets of different types of data into a composite packet. The different types of data may include video, audio, control data, extraneous data files, and others. A robust composite packet configuration can provide for more flexible and more efficient transmission of data on the high rate channel as well as more efficient transmission of acknowledgements on the low rate channel.

0008<patcit id="pcit0009" dnum="WO2007142482A1"><text>WO 2007/142482 A1</text></patcit> relates to the structure of transmission packet for link adaptation mechanism, and transmitting/receiving device and method using the same. The invention disclosed in this document relates to a structure of a transmission packet for a link adaptation mechanism that can efficiently transmits/receives uncompressed data using a link adaptation mechanism in a high-frequency wireless communication, and an apparatus and method of transmitting/receiving a packet using the same. According to an aspect of the invention disclosed in this document, a structure of a transmission packet for a link adaptation mechanism includes a payload having a plurality of data units obtained through error correction encoding of uncompressed data at a predetermined coding rate, an MAC header being added to the payload and having a link adaptation field and a link adaptation extension field expressing presence/absence of the link adaptation field for transmission/reception of the uncompressed data, and a PHY header being added to the MAC header and including information about the coding rate and information about usability of an unequal error protection mode.

0009It is the technical objective of the present invention to provide an improved technique for link adaptation in a communication system.

0010This objective is solved by the subject matter of the independent claims.

0011Preferred embodiments are defined in the dependent claims.

<u>Brief Description of the Drawings</u>

0012<ul id="ul0002" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a block diagram of an example wireless local area network (WLAN) 10, according to an embodiment.</li><li><figref idref="f0002">Figs. 2A-2C</figref> are diagrams of a prior art control frame format.</li><li><figref idref="f0003">Figs. 3A-3B</figref> are diagrams of an example control frame format, according to an embodiment.</li><li><figref idref="f0004">Figs. 4A-4B</figref> are diagrams of another example control frame format, according to another embodiment.</li><li><figref idref="f0005">Fig. 5</figref> is a diagram of another example control frame format, according to another embodiment.</li><li><figref idref="f0005">Fig. 6</figref> is a diagram of an example communication frame that includes a control frame, according to an embodiment.</li><li><figref idref="f0006">Fig. 7</figref> is a flow diagram of an example method for requesting information to perform link adaptation, according to an embodiment.</li><li><figref idref="f0007">Fig. 8</figref> is a flow diagram of an example method for generating and providing information for performing link adaptation, according to an embodiment.</li><li><figref idref="f0008">Fig. 9</figref> is a flow diagram of an example method for generating and providing unsolicited information for performing link adaptation, in an embodiment.</li><li><figref idref="f0009">Fig. 10</figref> is a flow diagram of an example method for determining a beamforming vector using link quality information, in an embodiment.</li><li><figref idref="f0010">Fig. 11</figref> is a flow diagram of an example method for determining a calibration matrix using link quality information, in an embodiment.</li><li><figref idref="f0011">Fig. 12</figref> is a timing diagram of an example transmission sequence in a downlink multi-user multiple input, multiple output (MIMO) context in which modulation and coding scheme (MCS) and/or link quality feedback is requested by an access point, in an embodiment.</li><li><figref idref="f0012">Fig. 13</figref> is a timing diagram of another example transmission sequence in a downlink multi-user MIMO context in which MCS and/or link quality feedback is requested by an access point, in another embodiment.</li></ul>

<u>Detailed Description</u>

0013In embodiments described below, a first network device, such as a client device in a wireless local area network (WLAN), transmits link quality information to a second network device, such as an access point (AP) of the WLAN. The link quality information is transmitted by the first network device in response to a request for the link quality information from the second network device, in some embodiments and/or scenarios. The link quality information transmitted by the first network device is unsolicited, i.e., transmitted at the initiative of the first network device and not in response to a request from the second network device, in some embodiments and/or scenarios. The link quality information is in the form of one or more of an indication of a modulation and coding set, signal-to-noise ratio (SNR) information, a received signal strength indicator (RSSI), a link margin, etc., in various embodiments.

0014<figref idref="f0001">Fig. 1</figref> is a block diagram of an example wireless local area network (WLAN) 10, according to an embodiment. An AP 14 includes a host processor 15 coupled to a network interface 16. The network interface 16 includes a medium access control (MAC) processing unit 18 and a physical layer (PHY) processing unit 20. The PHY processing unit 20 includes a plurality of transceivers 21, and the transceivers 21 are coupled to a plurality of antennas 24. Although three transceivers 21 and three antennas 24 are illustrated in <figref idref="f0001">Fig. 1</figref>, 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). In another embodiment, the MAC processing unit 18 and the PHY processing unit 20 are also configured to operate according to a second communication protocol (e.g., the IEEE 802.1 In Standard).

0015The WLAN 10 includes a plurality of client stations 25. Although four client stations 25 are illustrated in <figref idref="f0001">Fig. 1</figref>, 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. In some embodiments, at least one of the client stations 25 is not configured to operate according to the first communication protocol but is configured to operate according to the second communication protocol (referred to herein as a "legacy client station").

0016The 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 <figref idref="f0001">Fig. 1</figref>, 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.

0017In an embodiment, one or both of the client stations 25-2 and 25-3, has a structure the same as or similar to the client station 25-1. In an embodiment, the client station 25-4, has a structure similar to the client station 25-1, but is not configured to operate according to the first communication protocol. Rather, in an embodiment, the client station 25-4 is configured to operate according to the second communication protocol. In various 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.

0018According to an embodiment, the client station 25-4 is a legacy client station, i.e., the client station 25-4 is not enabled to receive and fully decode a communication frame that is transmitted by the AP 14 or another client station 25 according to the first communication protocol. Similarly, according to an embodiment, the legacy client station 25-4 is not enabled to transmit communication frames according to the first communication protocol. On the other hand, the legacy client station 25-4 is enabled to receive and fully decode and transmit communication frames according to the second communication protocol.

0019In various embodiments, the MAC processing unit 18 and the PHY processing unit 20 of the AP 14 are configured to generate communication frames conforming to the first communication protocol. The transceiver(s) 21 is/are configured to transmit the generated communication frames via the antenna(s) 24. Similarly, the transceiver(s) 24 is/are configured to receive the communication frames via the antenna(s) 24. The MAC processing unit 18 and the PHY processing unit 20 of the AP 14 are configured to process received communication frames conforming to the first communication protocol, according to various embodiments.

0020In various embodiments, the MAC processing unit 28 and the PHY processing unit 29 of the client device 25-1 are configured to generate communication frames conforming to the first communication protocol. The transceiver(s) 30 is/are configured to transmit the generated communication frames via the antenna(s) 34. Similarly, the transceiver(s) 30 is/are configured to receive communication frames via the antenna(s) 34. The MAC processing unit 28 and the PHY processing unit 29 of the client device 25-1 are configured to process received communication frames conforming to the first communication protocol, according to various embodiments.

0021<figref idref="f0002">Fig. 2A</figref> is a diagram of a prior art control field 60 that the AP 14 and/or the client stations 25-1, 25-2, 25-3 are configured to transmit to the legacy client station 25-4, and vice versa, for link adaptation purposes. The control field 60 is defined in the IEEE 802.1 In Standard. The control field 60 is included in a MAC header of a communication frame. The control field 60 includes a link adaptation control subfield 64 that is utilized to request a modulation coding scheme (MCS) and to transmit an MCS. A calibration position subfield 66 is used to indicate a position in a calibration sounding exchange sequence, and a calibration sequence subfield 68 is used to indicate an instance of a calibration procedure. A subfield 70 is reserved. A channel state information (CSI)/steering subfield 72 is used to indicate a request for CSI or beamforming feedback. A null data packet (NDP) announcement subfield 74 is used to announce that an NDP sounding packet will follow the frame that includes the control field 60. A sounding packet is a communication frame having training fields that enable determining a channel estimate of a multiple input, multiple output (MIMO) communication channel, i.e., that has training fields that sound the available MIMO channel dimensions.

0022The control frame 60 also includes a reserved subfield 76. The control frame 60 also includes an access category (AC) constraint subfield 78 and a reverse direction grant (RDG)/more PPDU subfield 80 related to a transmit opportunity (TXOP) holder permitting another station to transmit data during the TXOP.

0023<figref idref="f0002">Fig. 2B</figref> is a diagram of the link adaptation control subfield 64 of <figref idref="f0002">Fig. 2A</figref>. The link adaptation control subfield 64 includes a reserved subfield 84, and a training request (TRQ) subfield 86. The TRQ subfield 86 indicates whether a responder to the control field 60 is requested to transmit a sounding frame. An MCS request or antenna selection (ASEL) indication (MAI) subfield 88 indicates whether the control subfield 64 should be interpreted as relating to MCS feedback or relating to ASEL. When the MAI subfield 88 is set to the value 14, this indicates that the control subfield 64 is to be interpreted as related to ASEL. When the MAI subfield 88 is set to a value other than 14, this indicates that the control subfield 64 is to be interpreted as illustrated in <figref idref="f0002">Fig. 2C</figref>. Referring to <figref idref="f0002">Fig. 2C</figref>, when the MAI subfield 88 is set to a value other than 14, the MAI subfield 88 is interpreted to include an MCS request (MRQ) subfield 90 and an MRQ sequence identifier (MSI) subfield 92. The MRQ subfield 90 is set to one to request the responder to transmit MCS feedback (MFB) in response to the receiving the control field 60. When the MRQ subfield 90 is set to zero, MFB is not requested. When the MRQ subfield 90 is set to one, the MSI subfield 92 is set to a sequence number in the range 0 to 6 that identifies a specific MRQ. When the MRQ subfield 90 is set to zero, the MSI subfield 92 is reserved.

0024Referring again to <figref idref="f0002">Fig. 2B</figref>, the link adaptation control subfield 64 includes an MCS feedback sequence identifier (MFSI) subfield 94 to indicate an MSI to which MFB in the link adaptation subfield 64 corresponds. For example, a requester may send a request for MCS to a responder using the control frame 60 (<figref idref="f0002">Figs. 2A-2C</figref>), and setting an MSI value in the MSI subfield 92. A responder to the request sends MCS feedback (MFB) using the control frame 60, and sets the MFSI subfield 94 to the MSI value set by the requester.

0025The link adaptation control subfield 64 also includes an MCS feedback and antenna selection command/data (MFB/ASELC) subfield 96. When the MAI subfield 88 is set to the value 14, the MFB/ASELC subfield 96 is interpreted as including ASEL data. On the other hand, when the MAI subfield 88 is set to a value other than 14, the MFB/ASELC subfield 96 is interpreted as indicating a recommended MCS. A value of 127 indicates that no MCS feedback (MFB) is provided.

0026According to the IEEE 802.11n Standard, a network device transmits a frame with the MRQ subfield 90 set to one only when i) the frame is a sounding PHY protocol data unit (PPDU), or ii) in a frame that announces an NDP will follow (i.e., sets the NDP announcement subfield 74 to one) and that is then followed by an NDP sounding packet.

0027In an embodiment, the AP 14 and/or the client stations 25-1, 25-2, 25-3 utilize the control field 60 when performing link adaptation with the legacy client station 25-4, and the legacy client station 25-4 utilizes the control field 60 when performing link adaptation with the AP 14 and/or one of the client stations 25-1, 25-2, 25-3.

0028<figref idref="f0003">Fig. 3A</figref> is a diagram of an example control field 120 that the AP 14 is configured to transmit to one of the client stations 25-1, 25-2, 25-3, and vice versa, for link adaptation purposes, according to an embodiment. The control field 120 includes a subfield 121 to indicate whether the control field 120 should be interpreted according to the format illustrated in <figref idref="f0003">Fig. 3A</figref> or the format illustrated in <figref idref="f0002">Fig. 2A</figref>. For example, if the subfield 121 is set to a first value (e.g., zero), the control field is interpreted according to the format illustrated in <figref idref="f0002">Fig. 2A</figref>. On the other hand, if the subfield 121 is set to a second value (e.g., one), the control field is interpreted according to the format illustrated in <figref idref="f0003">Fig. 3A</figref>.

0029A subfield 122 is utilized to indicate whether the control field 120 includes unsolicited link adaptation feedback, i.e., not sent in response to an MRQ. An MRQ subfield 124 is set to one to request the responder to transmit MCS feedback (MFB) in response to the receiving the control field 120. When the MRQ subfield 120 is set to zero, MFB is not requested. When the MRQ subfield 124 is set to one, an MSI subfield 126 is set to a sequence number from a range 0-6 that identifies a specific MRQ. When the MRQ subfield 124 is set to zero, the MSI subfield 126 is reserved.

0030An MCS feedback sequence identifier (MFSI)/GID-L subfield 128 indicates an MSI to which the MFB in the control field 120 corresponds. For example, a requester may send a request for MCS to a responder using the control frame 120, and setting an MSI value in the MSI subfield 126. A responder to the request, sends MCS feedback (MFB) using the control frame 120, and sets the MFSI/GID-L field 128 to the MSI value set by the requester. When the subfield 122 is set to indicate the control field 120 includes unsolicited link adaptation feedback, the field MFSI/GID-L represents the 3 lowest bits of the Group ID in the referred frame, based on which the unsolicited MFB is computed, in an embodiment.

0031An MFB subfield 130 is utilized for transmitting MFB information and/or other information related to link quality. <figref idref="f0003">Fig. 3B</figref> is a diagram of the MFB subfield 130, according to an embodiment. The MFB subfield 130 includes a subfield 132 to indicate a number of space-time streams to which the MFB information, and/or other information related to link quality, corresponds. An MCS subfield 134 is utilized to provide an indication of a suggested modulation and coding scheme (MCS) for use in transmitting via the link. In an embodiment, a predefined value (e.g., 15 or another suitable value), indicates that MCS feedback is not available. A subfield 136 is utilized to provide information related to link quality other than a suggested MCS and a suggested number of space-time streams. In an embodiment, the subfield 136 is utilized to provide signal to noise ratio (SNR) information related to the link. In an embodiment, the SNR information includes a value indicating an average SNR (e.g., in decibels) averaged over a plurality of orthogonal frequency division multiplexing (OFDM) tones or subchannels (e.g., all tones, all data tones, etc.) and across all spatial streams, as measured by the responder. In one embodiment, the SNR information is determined according to: <maths id="math0001" num="(Equation 1)"><math display="block"><mi mathvariant="italic">SNR</mi><mo>_</mo><mi mathvariant="italic">avg</mi><mo>=</mo><mfrac><mn>1</mn><mrow><msub><mi>N</mi><mi mathvariant="italic">STS</mi></msub><msubsup><mi>N</mi><mi mathvariant="italic">Data</mi><mi mathvariant="italic">Tone</mi></msubsup></mrow></mfrac><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><mi>k</mi><mo>=</mo><mo>−</mo><msub><mi>N</mi><mi mathvariant="italic">SR</mi></msub></mrow><msub><mi>N</mi><mi mathvariant="italic">SR</mi></msub></munderover><mstyle displaystyle="true"><munderover><mo>∑</mo><mrow><msub><mi>i</mi><mi mathvariant="italic">STS</mi></msub><mo>=</mo><mn>1</mn></mrow><msub><mi>N</mi><mi mathvariant="italic">STS</mi></msub></munderover><mrow><mi mathvariant="italic">SN</mi><msub><mi>R</mi><mrow><msub><mi>i</mi><mi mathvariant="italic">STS</mi></msub><mo>,</mo><mi>k</mi></mrow></msub></mrow></mstyle></mstyle><mo>,</mo><mspace width="1ex" /><mi>and</mi></math><img file="EP2612461B1_D0001.tif" /></maths><maths id="math0002" num="(Equation 2)"><math display="block"><mi mathvariant="italic">SNR</mi><mo>_</mo><mi mathvariant="italic">FB</mi><mo>=</mo><mi mathvariant="italic">Quantized</mi><mfenced open="[" close="]" separators=""><mn>4</mn><mfenced separators=""><mi mathvariant="italic">SNR</mi><mo>_</mo><mi mathvariant="italic">avg</mi><mo>−</mo><mn>22</mn></mfenced></mfenced></math><img file="EP2612461B1_D0002.tif" /></maths> where <i>N_STS</i> is the number of spatial streams, <maths id="math0003" num=""><math display="inline"><msubsup><mi>N</mi><mi mathvariant="italic">Data</mi><mi mathvariant="italic">Tone</mi></msubsup></math><img file="EP2612461B1_D0003.tif" /></maths> is the number of data tones, k is a tone index, <i>N_SR</i> is a maximum tone index value, <i>N_SR</i> is a minimum tone index value, <i>i_STS</i> is a spatial stream index, and <i>SNR_{i<sub2>STS</sub2>}</i>,<i>k</i> is an SNR value for the k-th tone of an <i>i_STS</i> -th spatial stream. In Equation 2, the value inside the brackets is quantized to an 8-bit (or to another suitable number of bits), twos-complement number (or another suitable format).

0032In another embodiment, the SNR information includes a single SNR value which is the smallest tone-averaged SNR value among all spatial streams. In another embodiment, the SNR information includes i) a value indicating an average SNR averaged over a plurality of OFDM tones (e.g., all tones, all data tones, etc.) and across all spatial streams, as measured by the responder, and ii) a single SNR value which is the smallest tone-averaged SNR value among all spatial streams. In another embodiment, the SNR information includes a link margin value, i.e., a difference between measured SNR and a sensitivity point of the MCS indicated by the MCS subfield 134.

0033In another embodiment, the subfield 136 includes a received signal strength indicator (RSSI).

0034A GID-H subfield 138 is valid when the subfield 122 is set to indicate the control field 120 includes unsolicited link adaptation feedback, and the field 138 represents the 3 highest bits of the Group ID in the referred frame, based on which the unsolicited MFB is computed, in an embodiment.. A coding type subfield 140 is valid when the subfield 122 is set to indicate the control field 120 includes unsolicited link adaptation feedback, and it represents the error control coding type (BCC or LDPC) in the referred frame, based on which the unsolicited MFB is computed. A feedback Tx type subfield 142 is valid when the subfield 122 is set to indicate the control field 120 includes unsolicited link adaptation feedback, and it represents whether transmit beamforming is applied in the referred frame, based on which the unsolicited MFB is computed.

0035The control field 120 includes a reserved subfield 144. The control frame 120 also includes an access category (AC) constraint subfield 146 and a reverse direction grant (RDG)/more PPDU subfield 148 related to a transmit opportunity (TXOP) holder permitting another station to transmit data during the TXOP.

0036Suitable orderings and numbers of bits of the subfields other than the examples in <figref idref="f0003">Fig 3A and 3B</figref> are used in other embodiments. For example, <figref idref="f0004">Figs. 4A, 4B</figref> and <figref idref="f0005">Fig 5</figref> illustrate other example orderings and numbers of bits.

0037<figref idref="f0004">Fig. 4A</figref> is a diagram of another example control field 200 that the AP 14 is configured to transmit to one of the client stations 25-1, 25-2, 25-3, and vice versa, for link adaptation purposes, according to an embodiment. Similar to the example control field 120 of <figref idref="f0003">Fig. 3A</figref>, the control field 200 includes the subfield 121 to indicate whether the control field 200 should be interpreted according to the format illustrated in <figref idref="f0004">Fig. 4A</figref> or the format illustrated in <figref idref="f0002">Fig. 2A</figref>. For example, if the subfield 121 is set to a first value (e.g., zero), the control field is interpreted according to the format illustrated in <figref idref="f0002">Fig. 2A</figref>. On the other hand, if the subfield 121 is set to a second value (e.g., one), the control field is interpreted according to the format illustrated in <figref idref="f0004">Fig. 4A</figref>.

0038The control field 200 is similar to the control field 120 of <figref idref="f0003">Fig. 3A</figref>, and like-numbered elements are not discussed further. Compared to the control field 120 of <figref idref="f0003">Fig. 3A</figref>, a reserved subfield 204 and an unsolicited FB subfield 208 are in different locations in the control field. Additionally, an MFB subfield 212 has a different format as compared to the MFB subfield 130 of <figref idref="f0003">Fig. 3B</figref>.

0039A subfield 216 is similar to the subfield 136 of <figref idref="f0003">Fig. 3B</figref>, but has two less bits. The subfield 212 also includes a bandwidth (BW) subfield 220 to indicate a bandwidth of the received communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the fields 134 and/or 216. The BW subfield 220 is utilized for unsolicited feedback (i.e., when the unsolicited feedback subfield 208 is set to indicate unsolicited feedback) when it may not be clear to the recipient of the control field 200 the bandwidth of the communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the fields 134 and/or 216. In an embodiment, the BW subfield 220 is reserved when the unsolicited feedback subfield 208 is not set to indicate unsolicited feedback.

0040<figref idref="f0005">Fig. 5</figref> is a diagram of another example control field 250 that the AP 14 is configured to transmit to one of the client stations 25-1, 25-2, 25-3, and vice versa, for link adaptation purposes, according to an embodiment. Similar to the example control fields 120 of <figref idref="f0003">Fig. 3A</figref> and 200 of <figref idref="f0004">Fig. 4A</figref>, the control field 200 includes the subfield 121 to indicate whether the control field 200 should be interpreted according to the format illustrated in <figref idref="f0005">Fig. 5</figref> or the format illustrated in <figref idref="f0002">Fig. 2A</figref>. For example, if the subfield 121 is set to a first value (e.g., zero), the control field is interpreted according to the format illustrated in <figref idref="f0002">Fig. 2A</figref>. On the other hand, if the subfield 121 is set to a second value (e.g., one), the control field is interpreted according to the format illustrated in <figref idref="f0005">Fig. 5</figref>.

0041The control field 250 is similar to the control field 200 of <figref idref="f0004">Fig. 4A</figref> (and to the control field 120 of <figref idref="f0003">Fig. 3A</figref>), and like-numbered elements are not discussed further. As in the control field 200 of <figref idref="f0004">Fig. 4A</figref>, the reserved subfield 204 and the unsolicited FB subfield 208 are in different locations in the control field as compared to the control field 120 of <figref idref="f0003">Fig. 3A</figref>. The MFB subfield 130 has the same format as the MFB subfield 130 of <figref idref="f0003">Fig. 3B</figref>, in an embodiment.

0042When the unsolicited FB subfield 208 is set to a value that does not indicate unsolicited feedback and when the MRQ subfield 124 is set to one, an MSI/multi-user (MU) indicator subfield 254 is set to a sequence number from the range 0-6, as an example, that identifies a specific MRQ. When the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback, a bit of the subfield 254 is reinterpreted to indicate whether the feedback in the subfield 130 is associated with a single user transmission or a multi-user transmission, in an embodiment. In another embodiment, when the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback, the MRQ subfield 124 is reinterpreted to include a bit set to indicate whether the feedback is associated with a single user transmission or a multi-user transmission.

0043When the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback and the subfield 254 indicates that the feedback in the subfield 130 is associated with a single user transmission, two bits of the subfield 258 are reinterpreted to indicate a bandwidth of the communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the subfield 130, and a third bit of the subfield 258 is reinterpreted to be reserved, in an embodiment. When the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback but the subfield 254 indicates that the feedback in the subfield 130 is associated with a multi user transmission, a recipient of the control field 250 can determine the bandwidth of the communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the subfield 130 based on a group identifier in the subfield 258 that indicates a group of stations to which the communication frame was transmitted. In another embodiment, when the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback but the subfield 254 indicates that the feedback in the subfield 130 is associated with a multi user transmission, two bits of the subfield 258 are reinterpreted to indicate a bandwidth of the communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the subfield 130, and a third bit of the subfield 258 is reinterpreted to be reserved. In another embodiment, when the unsolicited FB subfield 208 is set to a value indicating unsolicited feedback and the subfield 124 indicates that the feedback in the subfield 130 is associated with a single user transmission, two bits of the subfield 258 are reinterpreted to indicate a bandwidth of the communication frame used for the MCS selection and/or the link quality determination and corresponding to the values in the subfield 130.

0044<figref idref="f0005">Fig. 6</figref> is a diagram of an example communication frame 280 that is utilized to convey a control frame such as discussed above with respect to <figref idref="f0003">Figs. 3A-C</figref>, <figref idref="f0004">4A</figref>, <figref idref="f0004">4B</figref> and <figref idref="f0005">5</figref>, in some embodiments. The frame 280 includes a MAC protocol data unit (MPDU) 284 and a PHY header 286. The MPDU 284 includes a MAC header 288, a MAC service data unit (MSDU) 290, and a cyclic redundancy check (CRC) field 292. A control field 294 is included in the MAC header 288. In another embodiment, the control field 294 is included in the PHY header 286. In an embodiment, the control frame 294 has a format as in <figref idref="f0003">Fig. 3A</figref>. In another embodiment, the control frame 294 has a format as in <figref idref="f0003">Fig. 3B</figref>. In another embodiment, the control frame 294 has a format as in <figref idref="f0005">Fig. 5</figref>. In other embodiments, the control frame 294 has another suitable format. The PHY header 286 and the MAC header 288 are included in a header portion 296 of the communication frame. Thus, the control field 294 is included in the header portion 296.

0045<figref idref="f0006">Fig. 7</figref> is a flow diagram of an example method 300 for requesting information to perform link adaptation, in an embodiment. The method 300 is implemented by the network interface 16 (e.g., the MAC processing unit 18 and/or the PHY processing unit 20) (<figref idref="f0001">Fig. 1</figref>), in an embodiment. The method 300 is implemented by the network interface 27 (e.g., the MAC processing unit 28 and/or the PHY processing unit 29) (<figref idref="f0001">Fig. 1</figref>), in another embodiment. In other embodiments, the method 300 is implemented by other suitable network interfaces.

0046At block 304, a control field for inclusion in a communication frame is generated. In an embodiment, the control field is a field of a MAC header and is to be included in the MAC header of the communication frame. In an embodiment, the control frame has a format as in <figref idref="f0003">Fig. 3A</figref>. In another embodiment, the control frame has a format as in <figref idref="f0004">Fig. 4A</figref>. In another embodiment, the control frame has a format as in <figref idref="f0005">Fig. 5</figref>. In other embodiments, the control frame has another suitable format.

0047In an embodiment, the block 304 includes a block 308 at which a subfield of the control field is set to indicate a request for link quality information. In an embodiment, the subfield indicates a request for one or more of an MCS, SNR information, an RSSI, link margin information, etc. In some embodiments, block 308 includes setting a value in the MRQ subfield 124 (<figref idref="f0003">Figs. 3A</figref>, <figref idref="f0004">4A</figref>, or <figref idref="f0005">5</figref>) to indicate the request. In some embodiments, block 304 includes setting the MSI subfield 126 (<figref idref="f0003">Fig. 3A</figref> or <figref idref="f0004">4A</figref>), or setting the MSI/MU subfield 254 (<figref idref="f0005">Fig. 5</figref>) to a sequence number (e.g., from a range 0-6 or another suitable range) that identifies a specific request.

0048In an embodiment, the block 304 includes a block 312, at which a subfield is set to a value that indicates that the control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol. In some embodiments, block 312 includes setting the subfield 124 to indicate that the control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol.

0049In an embodiment, block 304 includes both block 308 and block 312. In an embodiment, block 304 includes block 308, but block 304 and the method 300 omit block 312.

0050At block 316, a communication frame is generated that includes the control field generated at block 304. In an embodiment, block 316 includes generating a MAC header of the communication frame that includes the control field generated at block 304.

0051At block 320, the communication frame generated at block 316 is transmitted, or caused to be transmitted, to another device in a communication system.

0052In an embodiment and in some scenarios, the communication frame generated at block 316 and transmitted at block 320 is not a sounding packet and is not a communication frame that includes an indication that a null data packet will be transmitted in a subsequent frame (e.g., a null data packet (NDP) announcement frame). In other embodiments and/or scenarios, the communication frame generated at block 316 and transmitted at block 320 is a sounding packet or an NDP announcement frame.

0053<figref idref="f0007">Fig. 8</figref> is a flow diagram of an example method 400 for generating and providing information for performing link adaptation, in an embodiment. The method 400 is implemented by the network interface 16 (e.g., the MAC processing unit 18 and/or the PHY processing unit 20) (<figref idref="f0001">Fig. 1</figref>), in an embodiment. The method 400 is implemented by the network interface 27 (e.g., the MAC processing unit 28 and/or the PHY processing unit 29) (<figref idref="f0001">Fig. 1</figref>), in another embodiment. In other embodiments, the method 400 is implemented by other suitable network interfaces.

0054At block 404, a first communication frame having a first control field is received. In an embodiment, the first control field is included in a field of a MAC header of the first communication frame. The first control field includes a subfield that includes an indication of a request for link quality information. In an embodiment, the first control frame has a format as in <figref idref="f0003">Fig. 3A</figref>. In another embodiment, the first control frame has a format as in <figref idref="f0004">Fig. 4A</figref>. In another embodiment, the first control frame has a format as in <figref idref="f0005">Fig. 5</figref>. In other embodiments, the first control frame has another suitable format. In some embodiments, the subfield is the MRQ subfield 124 (<figref idref="f0003">Figs. 3A</figref>, <figref idref="f0004">4A</figref>, or <figref idref="f0005">5</figref>) set to indicate a request for link quality information. In some embodiments, the MSI subfield 126 (<figref idref="f0003">Fig. 3A</figref> or <figref idref="f0004">4A</figref>) or the MSI/MU subfield 254 (<figref idref="f0005">Fig. 5</figref>) is set to a sequence number (e.g., from a range 0-6 or another suitable range) that identifies a specific MRQ request.

0055At block 408, a suggested MCS is determined based on the first communication frame received at block 404. A suitable technique for determining a suggested MCS, such as techniques known to those of ordinary skill in the art, is utilized.

0056At block 412, a link quality metric is determined based on the first communication frame received at block 404. A suitable technique for determining a link quality metric, such as techniques known to those of ordinary skill in the art, is utilized. In an embodiment, the link quality metric is a signal to noise ratio (SNR) metric related to the first communication frame received at block 404. In an embodiment, the SNR metric corresponds to a value indicating an average SNR (e.g., in decibels) averaged over a plurality of OFDM tones (e.g., all tones, all data tones, etc.) and across all spatial streams, related to the first communication frame received at block 404 and as measured by the network interface implementing the method 400. In one embodiment, the SNR metric is determined according to Equation 1. In another embodiment, the metric is a single SNR value which is the smallest tone-averaged SNR value among all spatial streams related to the first communication frame received at block 404. In another embodiment, the SNR metric includes i) a value indicating an average SNR averaged over a plurality of OFDM tones (e.g., all tones, all data tones, etc.) and across all spatial streams, related to the first communication frame received at block 404, and ii) a single SNR value which is the smallest tone-averaged SNR value among all spatial streams related to the first communication frame received at block 404. In another embodiment, the link quality metric includes a link margin value related to the first communication frame received at block 404, i.e., a difference between measured SNR and a sensitivity point of the MCS indicated by the MCS subfield 134. In another embodiment, the link quality indicator includes a received signal strength indicator (RSSI) related to the first communication frame received at block 404.

0057At block 416, a second control field for inclusion in a second communication frame is generated to include an indication of the MCS determined at block 408 and an indication of the link quality metric determined at block 412. In an embodiment, the second control field is a field of a MAC header and is to be included in the MAC header of the second communication frame. In an embodiment, the second control frame has a format as in <figref idref="f0003">Fig. 3A</figref>. In another embodiment, the second control frame has a format as in <figref idref="f0003">Fig. 3B</figref>. In another embodiment, the second control frame has a format as in <figref idref="f0005">Fig. 5</figref>. In other embodiments, the second control frame has another suitable format. In an embodiment, the indication of the MCS is included in the subfield 134. In an embodiment, the indication of the link quality metric is included in the subfield 136 (<figref idref="f0003">Fig. 3A</figref>). In another embodiment, the indication of the link quality metric is included in the subfield 216 (<figref idref="f0004">Fig. 4A</figref>). In an embodiment, the indication of the link quality metric is determined according to Equation 2.

0058In an embodiment, block 416 includes setting the MFSI subfield 128 (<figref idref="f0003">Fig. 3A</figref>, <figref idref="f0004">4A</figref>, or <figref idref="f0005">5</figref>) to a sequence number included in the MSI or MSI/MU subfield of the first control field (block 404). In an embodiment, the block 416 includes setting the subfield 122 (<figref idref="f0003">Fig. 3A</figref>) or the subfield 208 (<figref idref="f0004">Fig. 4A</figref>, or <figref idref="f0005">5</figref>) to a value indicating that the link quality information included in the second control field is in response to a request for link quality information (e.g., in response to the first control field in the first communication frame (block 404)).

0059In an embodiment, block 416 includes a block 420, at which a subfield of the second control field is set to a value that indicates that the second control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol. In some embodiments, block 420 includes setting the subfield 124 to indicate that the second control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol. In an embodiment, block 416 and the method 400 omit block 420.

0060At block 424, the second communication frame is generated to include the second control field generated at block 416. In an embodiment, block 424 includes generating a MAC header of the second communication frame that includes the second control field generated at block 416.

0061At block 428, the second communication frame generated at block 424 is transmitted, or caused to be transmitted, to the device that generated the first communication frame (block 404).

0062In an embodiment and in some scenarios, the communication frame received at block 404 is not a sounding packet and is not a communication frame that includes an indication that a null data packet will be transmitted in a subsequent frame. In other embodiments and/or scenarios, the communication frame received at block 404 is a sounding packet.

0063In some embodiments, block 408 is omitted. In some embodiments when block 408 is omitted, block 416 includes setting a subfield in the second control field to a value to indicate that MCS feedback is not available. In an embodiment, the subfield 134 is set to a value to indicate that MCS feedback is not available. In some embodiments when block 408 is omitted, block 416 omits setting a subfield in the second control field to indicate a suggested MCS.

0064<figref idref="f0008">Fig. 9</figref> is a flow diagram of an example method 450 for generating and providing unsolicited information for performing link adaptation, in an embodiment. The method 450 is implemented by the network interface 16 (e.g., the MAC processing unit 18 and/or the PHY processing unit 20) (<figref idref="f0001">Fig. 1</figref>), in an embodiment. The method 450 is implemented by the network interface 27 (e.g., the MAC processing unit 28 and/or the PHY processing unit 29) (<figref idref="f0001">Fig. 1</figref>), in another embodiment. In other embodiments, the method 450 is implemented by other suitable network interfaces.

0065At block 454, it is determined to send unsolicited link quality information to another communication device. Determining whether to send unsolicited link quality information to another communication device is performed in a suitable manner, including using techniques known to those of ordinary skill in the art. For example, in an embodiment, determining to send unsolicited link quality information to another communication device is based on detecting a change in link quality (e.g., increased or decreased SNR, RSSI, PER, etc.).

0066At block 458, a suggested MCS is determined based on a recently received communication frame from the other communication device. A suitable technique for determining a suggested MCS, such as techniques known to those of ordinary skill in the art, is utilized.

0067At block 462, a link quality metric is determined based on based on a recently received communication frame from the other communication device. The recently received communication frame utilized at block 462 is the same as the communication frame utilized at block 458, in an embodiment. In another embodiment, the recently received communication frame utilized at block 462 is different than the communication frame utilized at block 458.

0068A suitable technique for determining a link quality metric, such as techniques known to those of ordinary skill in the art, is utilized. In an embodiment, the link quality metric is a signal to noise ratio (SNR) metric related to the recently received communication frame. In an embodiment, the SNR metric corresponds to a value indicating an average SNR (e.g., in decibels) averaged over a plurality of OFDM tones (e.g., all tones, all data tones, etc.) and across all spatial streams, related to the recently received communication frame and as measured by the network interface implementing the method 450. In one embodiment, the SNR metric is determined according to Equation 1. In another embodiment, the metric is a single SNR value which is the smallest tone-averaged SNR value among all spatial streams related to the recently received communication frame. In another embodiment, the SNR metric includes i) a value indicating an average SNR averaged over a plurality of OFDM tones (e.g., all tones, all data tones, etc.) and across all spatial streams, related to the recently received communication frame, and ii) a single SNR value which is the smallest tone-averaged SNR value among all spatial streams related to the recently received communication frame. In another embodiment, the link quality metric includes a link margin value related to the recently received communication frame, i.e., a difference between measured SNR and a sensitivity point of the MCS indicated by the MCS subfield 134. In another embodiment, the link quality indicator includes a received signal strength indicator (RSSI) related to the recently received communication frame.

0069At block 466, a control field for inclusion in a communication frame is generated to include an indication of the MCS determined at block 458 and an indication of the link quality metric determined at block 462. In an embodiment, the control field is a field of a MAC header and is to be included in the MAC header of the communication frame. In an embodiment, the control frame has a format as in <figref idref="f0003">Fig. 3A</figref>. In another embodiment, the control frame has a format as in <figref idref="f0003">Fig. 3B</figref>. In another embodiment, the control frame has a format as in <figref idref="f0005">Fig. 5</figref>. In other embodiments, the control frame has another suitable format. In an embodiment, the indication of the MCS is included in the subfield 134. In an embodiment, the indication of the link quality metric is included in the subfield 136 (<figref idref="f0003">Fig. 3A</figref>). In another embodiment, the indication of the link quality metric is included in the subfield 216 (<figref idref="f0004">Fig. 4A</figref>). In an embodiment, the indication of the link quality metric is determined according to Equation 2.

0070In an embodiment, block 466 includes setting the subfield 122 (<figref idref="f0003">Fig. 3A</figref>) or the subfield 208 (<figref idref="f0004">Fig. 4A</figref>, or <figref idref="f0005">5</figref>) to a value indicating that the link quality information included in the control field is unsolicited. In an embodiment, when the MCS determined at block 458 and/or the link quality metric determined at block 462 is based on a received single user frame, block 466 includes setting a subfield (e.g., subfield 220 of <figref idref="f0004">Fig. 4B</figref> or subfield 258 of <figref idref="f0005">Fig. 5</figref>) in the control field to indicate the bandwidth of the received single user frame on which the MCS determined at block 458 and/or the link quality metric determined at block 462 are based. In an embodiment, when the MCS determined at block 458 and/or the link quality metric determined at block 462 is based on a received multi user frame, block 466 includes setting a subfield (e.g., subfields 128, 138, 258) in the control field to indicate a group ID of the received multi user frame on which the MCS determined at block 458 and/or the link quality metric determined at block 462 are based.

0071In an embodiment, block 466 includes a block 470, at which a subfield of the control field is set to a value that indicates that the control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol. In some embodiments, block 470 includes setting the subfield 124 to indicate that the control field is to be interpreted according to a first communication protocol and is not to be interpreted according to a second communication protocol. In an embodiment, block 466 and the method 450 omit block 470.

0072At block 474, the communication frame is generated to include the control field generated at block 466. In an embodiment, block 474 includes generating a MAC header of the communication frame that includes the control field generated at block 466.

0073At block 478, the communication frame generated at block 474 is transmitted, or caused to be transmitted, to the device that generated the communication frame utilize to generate the MCS and/or the link quality information.

0074In some embodiments, block 462 is omitted. In some embodiments when block 462 is omitted, block 466 omits setting a subfield in the control field to indicate a link quality metric.

0075Referring to <figref idref="f0007">Figs. 8</figref> and <figref idref="f0008">9</figref>, in some embodiments and/or scenarios, a network interface of a communication device that receives the communication frame transmitted at block 428 or block 479 utilizes the MCS indicated in the control field (if included). In some embodiments and/or scenarios, a network interface of a communication device that receives the communication frame transmitted at block 428 or block 479 utilizes the link quality metric indicated in the control field (if included) to determine an MCS to be used when transmitting to the communication device implementing the method 400/450.

0076<figref idref="f0009">Fig. 10</figref> is a flow diagram of an example method 500 for determining a beamforming vector using link quality information, in an embodiment. The method 500 is implemented by the network interface 16 (e.g., the MAC processing unit 18 and/or the PHY processing unit 20) (<figref idref="f0001">Fig. 1</figref>), in an embodiment. The method 500 is implemented by the network interface 27 (e.g., the MAC processing unit 28 and/or the PHY processing unit 29) (<figref idref="f0001">Fig. 1</figref>), in another embodiment. In other embodiments, the method 500 is implemented by other suitable network interfaces.

0077At block 504, a plurality of communication frames that each request link quality information are generated and transmitted to another communication device. Each of the communication frames are transmitted using a different beamforming vector, in an embodiment. Each of the communication frames are generated using a method such as described with respect to <figref idref="f0006">Fig. 7</figref>, in an embodiment. In an embodiment, the plurality of communication frames transmitted at block 504 are transmitted using a same MCS.

0078At block 508, a plurality of communication frames that each include link quality information are received from the other communication device in response to the plurality of communication frames transmitted at block 504. Each of the other communication frames received at block 508 are generated by the other communication device using a method such as described with respect to <figref idref="f0007">Fig. 8</figref>, in an embodiment.

0079At block 512, a beamforming vector for use in subsequent transmissions to the other communication device is determined based on the link quality information received at block 508. For example, a beamforming vector that provides a highest SNR value is selected, in an embodiment. As another example, a beamforming vector that provides a highest link margin is selected, in an embodiment. As another example, a beamforming vector that provides a highest RSSI is selected, in an embodiment. As another example, a set of beamforming vectors that provide with the highest SNR values, link margins, RSSIs, etc., are utilized to generate a beamforming vector, in an embodiment. The beamforming vector determined at block 512 is utilized to transmit subsequent communication frames to the other communication device.

0080<figref idref="f0010">Fig. 11</figref> is a flow diagram of an example method 600 for determining a calibration matrix for use in implicit beamforming using link quality information, in an embodiment. The method 600 is implemented by the network interface 16 (e.g., the MAC processing unit 18 and/or the PHY processing unit 20) (<figref idref="f0001">Fig. 1</figref>), in an embodiment. The method 600 is implemented by the network interface 27 (e.g., the MAC processing unit 28 and/or the PHY processing unit 29) (<figref idref="f0001">Fig. 1</figref>), in another embodiment. In other embodiments, the method 600 is implemented by other suitable network interfaces.

0081At block 604, a communication frame is received at first communication device from a second communication device. The communication frame received at block 604 is a communication frame having training signals to sound a reverse MIMO channel. At block 608, an estimate of a channel from the second communication device to the first communication device, i.e., a reverse channel estimate, is determined based on the communication frame received at block 604.

0082At block 612, a forward channel estimate, i.e., an estimate of the channel from the first communication device to the second communication device, is determined based on the reverse channel estimate determined at block 604.

0083At block 616, a transmit beamforming vector, i.e., a beamforming vector for transmitting via the forward channel, is determined based on the forward channel estimate determined at block 612.

0084At block 620, a plurality of communication frames that each request link quality information are generated and transmitted to the second communication device. Each of the communication frames are transmitted using the beamforming vector determined at block 616, in an embodiment. Each of the communication frames are transmitted using a different calibration matrix (e.g., a calibration matrix to correct for transmit chain/receive chain mismatches between the forward channel and the reverse channel), in an embodiment. Each of the communication frames are generated using a method such as described with respect to <figref idref="f0006">Fig. 7</figref>, in an embodiment. In an embodiment, the plurality of communication frames transmitted at block 620 are transmitted using a same MCS.

0085At block 624, a plurality of communication frames that each include link quality information are received from the second communication device in response to the plurality of communication frames transmitted at block 620. Each of the communication frames received at block 620 are generated by the second communication device using a method such as described with respect to <figref idref="f0007">Fig. 8</figref>, in an embodiment.

0086At block 628, a calibration matrix for use in subsequent transmissions to the other communication device is determined based on the link quality information received at block 624. For example, a calibration matrix that provides a highest SNR value is selected, in an embodiment. As another example, a calibration matrix that provides a highest link margin is selected, in an embodiment. As another example, a calibration matrix that provides a highest RSSI is selected, in an embodiment. As another example, a set of calibration matrix that provide the highest SNR values, link margins, RSSIs, etc., are utilized to generate a calibration matrix, in an embodiment. The calibration matrix determined at block 512 is utilized to transmit subsequent communication frames to the second communication device.

0087Referring again to <figref idref="f0006">Figs. 7</figref> and <figref idref="f0007">8</figref>, a communication frame that includes a request for link quality information is included in a multi user communication frame, in an embodiment. For example, in a multi user downlink transmission, corresponding control fields that include requests for link quality information are included in respective subframes corresponding to a plurality of client devices. Each client device responds with link quality information in a corresponding uplink transmission. For example, in an embodiment, the link quality information from the plurality of client devices are included in respective acknowledgment frames. As another example, in a multi user downlink transmission, a control field that includes a request for link quality information is included in an NDP announcement frame transmitted to the plurality of client devices. Each client device responds with link quality information in a corresponding uplink transmission. For example, in an embodiment, the link quality information from the plurality of client devices are included in respective acknowledgment frames.

0088<figref idref="f0011">Fig. 12</figref> is a timing diagram 700 of an example transmission sequence in a downlink multi-user MIMO context. After a multi-user sounding/feedback process, the AP transmits a multi-user packet 704 that includes independent data for a plurality of client stations including at least a first client (Client 1) and a second client (Client 2). Independent data for the plurality of client stations is transmitted simultaneously in a suitable manner, such as on different spatial streams, in different frequency portions of the channel, etc. The packet 704 includes an indication of a request for MCS and/or link adaptation feedback, in an embodiment. The packet 704 also includes an indication of which client stations should provide the feedback. In the illustrated embodiment of <figref idref="f0011">Fig. 12</figref>, the packet 704 includes an indication that Client 1 should provide MCS and/or link adaptation feedback (e.g., SNR). Client 1 transmits the MFB/link adaptation feedback (e.g., SNR) in a packet 708. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a control field of the packet 708. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a MAC portion of the packet 708 (e.g., in a high throughput control (HTC) field). In another embodiment, the MFB is included in a control field in a PHY header portion of the packet 708.

0089The packet 708 is an acknowledgment packet in an embodiment. In other embodiments, the packet 708 is another suitable packet for transmitting MFB/link adaptation feedback (e.g., SNR) to the AP. For example, in an embodiment, the packet 708 is a control wrapper frame. In another embodiment, the packet 708 is a management frame.

0090<figref idref="f0012">Fig. 13</figref> is a timing diagram 750 of another example transmission sequence in a downlink multi-user MIMO context. During a multi-user sounding/feedback process, the AP transmits a null data packet announcement (NDPA) packet 754. The NDPA packet 754 includes an indication of a request for MCS and/or link adaptation feedback (e.g., SNR), in an embodiment.

0091In an embodiment, the NDPA packet 754 indicates that all stations in the basic service set (BSS) should provide feedback. In another embodiment, the packet 754 also includes an indication of which client stations should provide feedback. In the illustrated embodiment of <figref idref="f0012">Fig. 13</figref>, the packet 754 includes an indication that Client 1 and Client 2 should provide MFB. For example, the indication that Client 1 and Client 2 should provide MFB is a group ID corresponding to a group of which Client 1 and Client 2 are members.

0092The packet 754 indicates that client stations should provide feedback in response to a subsequent multi-user downlink packet 758 transmitted after the sounding/feedback process. The packet 758 includes independent data for a plurality of client stations including at least a first client (Client 1) and a second client (Client 2). Independent data for the plurality of client stations is transmitted simultaneously in a suitable manner, such as on different spatial streams, in different frequency portions of the channel, etc.

0093Client 1 transmits the MFB/link adaptation feedback (e.g., SNR) in a packet 762. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a control field of the packet 762. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a MAC portion of the packet 762 (e.g., in a high throughput control (HTC) field). In another embodiment, the MFB is included in a control field in a PHY header portion of the packet 762.

0094Client 2 transmits the MFB/link adaptation feedback (e.g., SNR) in a packet 766. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a control field of the packet 766. In an embodiment, the MFB/link adaptation feedback (e.g., SNR) is included in a MAC portion of the packet 766 (e.g., in a high throughput control (HTC) field). In another embodiment, the MFB is included in a control field in a PHY header portion of the packet 766.

0095The packets 762, 766 are acknowledgment packets in an embodiment. In other embodiments, the packets 762, 766 are other suitable packets for transmitting MFB/link adaptation feedback (e.g., SNR) to the AP. For example, in an embodiment, the packets 762, 766 are control wrapper frames. In another embodiment, the packets 762, 766 are management frames.

0096At 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.

0097When 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.

0098While 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.