Patent Publication Number: US-8982803-B1

Title: Systems and methods for link adaption in wireless communication systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/716,300 filed on Mar. 3, 2010, which claims the benefit of U.S. Provisional Application No. 61/157,591, filed on Mar. 5, 2009, U.S. Provisional Application No. 61/170,778, filed on Apr. 20, 2009, and U.S. Provisional Application No. 61/176,083, filed on May 6, 2009. This application also claims the benefit of U.S. Provisional Application No. 61/287,098, filed on Dec. 16, 2009, U.S. Provisional Application No. 61/295,473 filed Jan. 15, 2010, and U.S. Provisional Application No. 61/310,105 filed on Mar. 3, 2010. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD 
     The present disclosure relates to wireless communication systems and more particularly to systems and methods for link adaptation in wireless communication systems. 
     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 the work 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. 
     Wireless communication systems may transmit and/or receive data via radio frequency (RF) signals at various frequencies. For example, conventional Wi-Fi communication systems may include less than a 0.5 gigahertz (GHz) spectrum allocated between 2 GHz and 6 GHz (e.g., 2.4 GHz Wi-Fi). This small spectrum may provide for transmission of, for example, up to 300 megabits per second (Mb/s). 
     60 gigahertz (GHz) wireless communication systems, on the other hand, may include an unlicensed 7 GHz spectrum centered at or near 60 GHz (e.g., 57-64 GHz). The large 7 GHz spectrum may provide for multi-gigabit (Gb) RF links (e.g., &gt;1.0 Gb/s). For example, multi-Gb RF links may allow for transmission of high-definition (HD) media such as HD video. 
     60 GHz wireless communication systems may also provide more secure transmissions. More specifically, oxygen attenuates high frequency signals (e.g., 57-64 GHz signals), thus limiting the distance of transmissions. In other words, these high frequency signals may rarely be intercepted by other antennas located beyond an intended target antenna. 
     Moreover, 60 GHz wireless communication systems may include high gain, narrow beam antennas that accurately transmit these short distances to intended target antennas (i.e., unidirectional, as opposed to wide-beam). 60 GHz wireless communication systems, therefore, may include densely distributed antennas that may transmit at the same frequency with minimal interference and increased security. The large number of unidirectional antennas, however, may result in a lack of “channel reciprocity.” 
     Channel reciprocity is based on the property that electromagnetic waves traveling in both directions undergo the same physical perturbations (e.g., reflection, refraction, diffraction, etc.). Thus, when an RF link operates at the same frequency band in both directions, the impulse response of the channel observed between any two antennas may be the same regardless of the direction. Application of the channel reciprocity principle, therefore, may remove the necessity for a continuous feedback of channel estimates. 
     Two stations in a 60 GHz wireless communication system, however, may include different numbers of antennas. Each station, therefore, may transmit at a different power. Moreover, in addition to oxygen attenuation, transmissions between the stations may be affected by precipitation. For example, the effect of rainfall on 60 GHz transmissions may be greater than the effect of oxygen attenuation. For example only, lower data rates may be required during rainfall to achieve a desired throughput. Therefore, multiple modulation and coding schemes (MCSs) may be required depending on operating conditions. For example, an MCS may include a modulation scheme and/or a data rate. 
       FIG. 1  illustrates a conventional 60 GHz wireless communication system  10 . The 60 GHz wireless communication system may include a first node  12  (Station A, or “STA”) and a second node  14  (Station B, or “STB”) that communicate via RF signals. In other words, Station A  12  may transmit and/or receive RF signals to/from Station B  14 . Similarly, Station B  14  may transmit and/or receive RF signals to/from Station A  12 . 
     As previously described, Station A  12  and Station B  14  may include different numbers of antennas. For example only, Station A  12  includes 36 antennas and Station B includes 2 antennas. Station A, therefore, may transmit RF signals at a much higher power than Station B. For channel reciprocity to apply, channel metrics in the forward link direction (ρ F ) equal channel metrics in the reverse link direction (ρ R ). However, due to different numbers of antennas and thus different MCS and/or data rates that may be used, channel reciprocity may not apply (e.g., ρ F ≠ρ R ) in the example communication system of  FIG. 1 . 
     SUMMARY 
     A first wireless device includes a transmitter and a receiver. The transmitter is configured to transmit a request for parameters to a second wireless device via a communication channel, wherein the parameters include at least one of (i) an estimated quality of the communication channel and (ii) a modulation and coding scheme (MCS) for transmission via the communication channel. The transmitter is further configured to, based on at least one of the parameters, select a MCS for transmission to the second wireless device via the communication channel. The receiver is configured to receive a response from the second wireless device via the communication channel, wherein the response includes the parameters. 
     A method includes transmitting a request for parameters from a first wireless device to a second wireless device via a communication channel, wherein the parameters include at least one of (i) an estimated quality of the communication channel and (ii) a modulation and coding scheme (MCS) for transmission via the communication channel, based on at least one of the parameters, selecting a MCS for transmission from the first wireless device to the second wireless device via the communication channel, and receiving a response from the second wireless device at the first wireless device via the communication channel, wherein the response includes the parameters. 
     In still other features, the systems and methods described above are implemented by a computer program executed by one or more processors. The computer program can reside on a tangible computer readable medium such as but not limited to memory, nonvolatile data storage, and/or other suitable tangible storage mediums. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a schematic of a wireless communication system according to the prior art; 
         FIG. 2  is a schematic of a 60 gigahertz (GHz) wireless communication system according to one implementation of the present disclosure; 
         FIG. 3  is a functional block diagram of a wireless device that includes a receiver according to one implementation of the present disclosure; 
         FIG. 4  is a functional block diagram of a wireless device that includes a transmitter according to one implementation of the present disclosure; 
         FIG. 5  is a timeline of packet transmissions during link adaptation according to one implementation of the present disclosure; 
         FIG. 6  is a flow diagram of a method for link adaptation according to one implementation of the present disclosure; 
         FIG. 7  is a schematic of a beamforming operation according to one implementation of the present disclosure; 
         FIGS. 8A-8C  are diagrams of media access control (MAC) layers for sector sweeping, beam refinement, and beamforming completion operations, respectively, according to one implementation of the present disclosure; 
         FIG. 9  is a flow diagram of a method for link adaptation during beamforming operations according to one implementation of the present disclosure; 
         FIG. 10  is a timeline of packet transmissions during a beam tracking operation according to one implementation of the present disclosure; 
         FIG. 11  is a flow diagram of a method for link adaptation during beam tracking operations according to one implementation of the present disclosure; 
         FIG. 12  is a functional block diagram of a wireless device that includes a transmitter according to one implementation of the present disclosure; 
         FIG. 13  is a functional block diagram of a wireless device that includes a receiver according to one implementation of the present disclosure; 
         FIG. 14  is a timeline of packet transmissions during a first requestable link adaptation operation according to one implementation of the present disclosure; 
         FIG. 15  is a diagram of packet fields used to request link adaptation according to one implementation of the present disclosure; 
         FIG. 16  is a timeline of packet transmissions during a second requestable link adaptation operation according to one implementation of the present disclosure; 
         FIG. 17  is a timeline of packet transmissions during a third requestable link adaptation operation according to one implementation of the present disclosure; and 
         FIG. 18  is a flow diagram of a method for requesting and performing link adaptation according to one implementation of the present disclosure. 
     
    
    
     DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module may refer to, be part of, or include an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and/or memory (shared, dedicated, or group) that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
       FIG. 2  illustrates a 60 gigahertz (GHz) wireless communication system  20 . As previously explained, 60 GHz wireless communication systems may suffer from a lack of channel reciprocity. Accordingly, 60 GHz wireless communication systems may require explicit feedback from a receiver (at a second station) to a transmitter (at a first station). The feedback provides information for adaptation of a radio frequency (RF) link between the first and second stations to improve data throughput. 
     Conventional 60 GHz wireless communication systems slowly adapt an RF link between two stations based on learned information about a channel between the two stations. For example, conventional 60 GHz wireless communication systems may adjust transmissions based on amount of data lost (i.e., expected throughput minus actual throughput) during a predetermined period. The slow adaptation process, however, may result in decreased throughput prior to adaptation (or optimization) of the RF link. 
     Therefore, systems and methods are presented that provide for faster adaptation of an RF link in a 60 GHz wireless communication system. For example, the 60 GHz wireless communication system  20  may implement the systems and/or methods of the present disclosure. The systems and methods may improve data throughput of a 60 GHz wireless communication system by determining a modulation and coding scheme (MCS) for transmission. In other words, for example, the systems and methods may determine a modulation scheme and/or a data rate for transmissions to optimize data throughput. 
     More specifically, the wireless communication system  20  includes a first wireless device  22  that communicates with a second wireless device  24  via a communication channel  26  (“channel 26”). The channel  26  may include a spectrum of RF signals ranging from, for example only, 57-64 GHz. Thus, the channel  26  may be referred to as a “60 GHz channel” due to the spectrum being centered at or near 60 GHz. For example only, the first and second wireless devices  22 ,  24  may include wireless access points (e.g., a wireless router) and wireless adapters (e.g., a wireless network interface card). 
     The first wireless device  22  includes a transmitter  28  and a receiver  32 . The transmitter  28  transmits RF signals via antennas  30 - 1 ,  30 - 2 , . . . , and  30 - n  (collectively antennas  30 ). The receiver  32  receives RF signals via the antennas  30 . The second wireless device  24  also includes a receiver  34  and a transmitter  38 . The receiver  64  receives RF signals via antennas  36 - 1 ,  36 - 2 , . . . , and  36 - m  (collectively antennas  36 ). The first wireless device  22  can include a different number of antennas relative to the second wireless device  24  (e.g., n≠m). Additionally, for example, the antennas  30 ,  36  may be high gain, narrow band (or unidirectional) antennas. 
     The transmitters  28 ,  38  may transmit data according to an MCS. For example, the MCS can include a modulation scheme and/or a data rate. Thus, the transmitters  28 ,  38  can encode data streams according to the modulation scheme for transmission via the antennas  30 ,  36 . The antennas  30 ,  36  can then transmit signals based on the encoded data streams. For example, the modulation scheme may be one of single carrier (SC) modulation and orthogonal frequency-division multiplexing (OFDM) modulation. The transmitters  28 ,  38  may also control a rate of transmission according to the data rate. For example, the data rate may be one of a plurality of data rates ranging from 1.7 MB/s to 7 Gb/s. 
     Referring now to  FIG. 3 , the second wireless device  24  includes receiver  34  and transmitter  38 . The second wireless device  24  may implement the systems and/or methods of the present disclosure. More specifically, receiver  34  includes a channel estimation module  52  that estimates a quality of the channel  26  based on a received training packet (e.g., from transmitter  28  in the first wireless device  22 ). The estimated quality can include one or more of a plurality of signal strength metrics. The plurality of signal strength metrics can include, but are not limited to a signal-to-noise ratio (SNR) and a receiver signal strength index (RSSI). For example, the SNR may indicate a ratio of a power of an RF signal to a power of noise corrupting the RF signal. For example, the RSSI may indicate a measurement of power present in a received RF signal. In the subsequent descriptions, the term “channel quality” is used to represent one or more than one of the metrics such as SNR, RSSI or other signal quality metrics. 
     Receiver  34  also includes a preferred MCS determination module  54  that determines a preferred MCS based on the training packet (or the estimated channel quality). The preferred MCS can include a preferred modulation scheme and/or a preferred data rate. For example, the preferred modulation scheme may include one that indicates SC modulation (e.g., 0) or OFDM modulation (e.g., 1). Additionally, for example, the preferred data rate may include a data rate ranging from 1.7 MB/s to 7 Gb/s. 
     The preferred data rate, however, may also depend on the preferred modulation scheme. Therefore, multiple bits may be used to indicate both the preferred modulation scheme and the preferred data rate. For example, multiple bits may indicate various data rates for SC modulation (e.g., SC1, SC2, . . . , SCX) and various data rates for OFDM modulation (e.g., OFDM1, OFDM2, . . . , OFDMY). 
     The preferred MCS determination module  54  may also determine the preferred MCS based on other parameters. For example, the other parameters may include channel delay (e.g., average root mean square, or RMS delay). Accordingly, the other parameters may affect the determination of the preferred MCS. Thus, additionally or alternatively, the preferred MCS may include a quantized version of a channel delay profile. Additionally or alternatively, the first wireless device  22  may also include the channel estimation module  52  and the preferred MCS determination module  54  (e.g., in receiver  32 ). 
     In one embodiment, receiver  34  selects the MCS to use in subsequent transmissions and then transmits the selection to the first wireless device  22  (via transmitter  38 ). The process of selecting the MCS, however, may be computationally intensive. The receiver  34 , therefore, may require additional processing capabilities (i.e., complexity) in order to properly select the MCS. Moreover, the receiver  34  may not have information regarding power operational amplifiers (PAs) in the transmitter  28 , and, thus, the receiver  34  may not accurately select the MCS without knowing how the power of the transmitter  28  may be adjusted. 
     Therefore, in one implementation, the second wireless device  24  transmits the estimated channel quality and/or the preferred MCS to transmitter  28  (via transmitter  38 ). More specifically, the receiver  34  can output the estimated channel quality and/or the preferred MCS to the transmitter  38 . The transmitter  38  then transmits the estimated channel quality and/or the preferred MCS to the first wireless device  22 . For example, the estimated channel quality and/or the preferred MCS may be transmitted as part of an acknowledgement packet or through a special control or management packet or an aggregation of an acknowledgment packet and a special control or management packet. Additionally or alternatively, for example, the feedback may be included in existing feedback packets during beamforming or beam tracking operations. The packet with the estimated channel quality and/or the preferred MCS information may be called link feedback packet in general. 
     The first wireless device  22  may receive the link feedback packet (via receiver  32 ) and send the link feedback information (i.e. the estimated channel quality and/or the preferred MCS) to transmitter  28 . The transmitter  28  may then perform the MCS selection process. The preferred MCS (in addition to the estimated channel quality) may help the transmitter  28  (in the first wireless device  22 ) in selecting the MCS to be used in subsequent transmissions. Receiver  34 , therefore, may be less complex, thus reducing costs and complexity of the 60 GHz wireless communication system  20 . 
     Referring now to  FIG. 4 , the first wireless device  22  includes receiver  32  and transmitter  28 . The first wireless device  22  may implement the systems and methods of the present disclosure. More specifically, transmitter  28  may include an MCS selection and power control module  72  that selects the MCS and/or transmit power level to be used in subsequent transmissions based on the estimated channel quality and/or the preferred MCS received from the second wireless device  24 . For example, if SNR feedback is very strong, the MCS selection and power control module  72  may determine that a lower transmit power may achieve the same data rate. A transmission control module  74  may then apply the selected MCS and/or transmit power level to subsequent transmissions of data. Additionally or alternatively, the second wireless device  24  may also include the MCS selection and power control module  72  and the transmission control module  74  (e.g., in transmitter  38 ). 
     Receiver  32  receives the link feedback packet from transmitter  38  (see  FIG. 3 ). The link feedback packet may include the estimated channel quality and/or the preferred MCS, as determined by the second wireless device  24  (i.e., the receiver  34 ). The receiver  32  outputs the link feedback information (i.e., the estimated channel quality and/or the preferred MCS) to the MCS selection and power control module  72  of transmitter  28 . 
     The MCS selection and power control module  72  selects the MCS to use in subsequent transmissions based on the estimated channel quality and/or preferred MCS. More specifically, the selected MCS may include a modulation scheme and/or a data rate. For example, the selected MCS may include one of SC modulation and OFDM modulation and/or one of a plurality of data rates (e.g., ranging from 1.7 MB/s to 7 Gb/s). 
     The transmission control module  74  receives the selected MCS. The transmission control module  74  controls transmission of data according to the selected MCS and/or transmit power level. Specifically, the transmission control module  74  may encode data according to the selected modulation scheme and/or may control transmission of the data according to the selected data rate. Furthermore, for example, transmit power level control may include adjusting the power amplifier (PA) operational level, and/or turning on/off transmit antenna elements. The transmission control module  74  may also control generation and transmission of the training packet. For example, the training packet may be a predetermined packet known by the second wireless device  24 . 
       FIG. 5  illustrates a timeline of a link adaptation. Specifically, link adaptation between the first wireless device  22  and the second wireless device  24  (see  FIG. 2 ) is shown. The link adaptation, however, may be implemented between other wireless devices. 
     The first wireless device  22  transmits a training packet (via transmitter  28 ) to the second wireless device  24 . The second wireless device  24  receives the training packet (via receiver  34 ). The second wireless device  24  then estimates a channel quality using the training packet. More specifically, for example, the training packet may be a predetermined packet known by the second wireless device  24 . Thus, the second wireless device  24  may estimate channel quality based on a quality of the received training packet. 
     Additionally or alternatively, the second wireless device  24  may determine a preferred MCS. For example, the preferred MCS may include a preferred modulation scheme (e.g., SC or OFDM) and/or a preferred data rate (e.g., 1.7 MB/s to 7 Gb/s). For example only, the preferred MCS may include one bit indicating the preferred modulation scheme (e.g., 0 for SC modulation and 1 for OFDM modulation). Alternatively, for example only, the preferred MCS may include multiple bits indicating the preferred modulation scheme and the preferred data rate (e.g., SC1, SC2, . . . , SCX, OFDM1, OFDM2, . . . , OFDMY). 
     The second wireless device  24  may then transmit an acknowledgement packet or any other packet (via transmitter  38 ) back to the first wireless device  22 . More specifically, this packet may include the estimated channel quality and/or the preferred MCS. The first wireless device  22  may receive the acknowledgement packet (via receiver  32 ). 
     The first wireless device  22  may then select an MCS and/or transmit power level based on the acknowledgement packet. More specifically, the first wireless device  22  may select the MCS based on the estimated channel quality and/or the preferred MCS, and/or may select the transmit power level based on the estimated channel quality. After selecting the MCS and/or transmit power level, the first wireless device  22  may begin transmitting data packets according to the selected MCS. In other words, the first wireless device  22  may encode data using the selected modulation scheme and/or transmit data according to the selected data rate. 
     Referring now to  FIG. 6 , a method for adapting an RF link in a 60 GHz wireless communication system begins at  100 . At  100 , the first wireless device  22  generates and transmits a training packet. At  104 , the second wireless device  24  receives the training packet and estimates channel quality and/or determines a preferred MCS based on the training packet. While the first and second wireless devices  22 ,  24  are described, the method may be implemented using other wireless devices. 
     At  108 , the second wireless device  24  generates and transmits an acknowledgement packet that includes the estimated channel quality and/or the preferred MCS. At  112 , the first wireless device  22  receives the acknowledgement packet and selects an MCS and/or transmit power level based on the acknowledgement packet. At  116 , the first wireless device  22  may transmit according to the selected MCS and/or transmit power level. In other words, the first wireless device  22  may encode data according to the selected modulation scheme and/or transmit the data according to the selected data rate, and/or transmit with the selected power level. 
     Referring again to  FIG. 2 , noise in the channel  26 , such as electromagnetic interference (EMI), inter-channel crosstalk, etc., may interfere with the signals transmitted through the channel  26 . Thus, transmitters  28 ,  38  may use a technique called beamforming to reduce the effects of noise on signals transmitted through the channel  26 . Beamforming may increase SNR of signals received at receivers  32 ,  34 . Consequently, beamforming may allow the transmitters  28 ,  38  to increase data rates during transmission. Moreover, beamforming may be required in 60 GHz wireless communication systems due to the high gain, narrow beam antennas that are typically implemented. 
     During a beamforming operation, the transmitters  28 ,  38  may map data streams to antennas  30 ,  36 , respectively, to change a spatial gain of transmitted signals. Mapping the data streams to the antennas  30 ,  36  to change the spatial gain of transmitted signals is referred to herein as “steering a signal.” The transmitters  28 ,  38  may map a data stream to the antennas  30 ,  36 , respectively, using complex weighting values. Each of the complex weighting values may include an amplitude portion and a phase shift portion. The complex weighting values used to map the data stream to the antennas  30 ,  36  may be included in a steering matrix. Accordingly, the transmitters  28 ,  38  may map the data stream to the antennas  30 ,  36 , respectively, based on the steering matrix. 
     For example, a beamforming operation may be further divided into three stages: sector sweeping, beam refinement, and beamforming completion. Sector sweeping includes each transmitter (i.e., each wireless device) determining a best sector (i.e., direction) for transmission based on feedback from receivers (i.e., other wireless devices) in different sectors. Beam refinement, on the other hand, includes an iterative loop of transmitting training packets and adjusting the steering matrix based on feedback from a wireless device in the best sector. Accordingly, in one implementation, beam refinement includes continually adjusting (or steering) signals to various antennas to improve an overall quality of transmission. Lastly, beamforming completion includes transmitting and feeding back a final feedback packet, after which normal transmission of data packets may begin. 
       FIG. 7  illustrates a beamforming operation that includes the link adaptation of the present disclosure. For example, a beamforming operation between the first wireless device  22  and the second wireless device  24  (see  FIG. 2 ) is shown. The beamforming operation, however, may be implemented between other wireless devices. 
     The beamforming operation begins with a sector sweep operation. More specifically, the first wireless device  22  may transmit to different sectors and determine a best sector based on feedback from wireless devices in the different sectors. As shown, for example, the first wireless device  22  transmits to a sector that includes the second wireless device  24 . 
     The second wireless device  24  transmits feedback to the first wireless device  22 . The feedback may also include estimated channel quality and/or a preferred MCS according to the link adaptation of the present disclosure. The first wireless device  22  may determine, for example, that the second wireless device  24  is located in a best sector. 
     The beamforming operation may continue with a beam refinement operation between the first wireless device  22  and the second wireless device  24  (i.e., on top of the best sector). The beam refinement operation may include an iterative process of transmitting receive training packets (Rx Train), which includes a receive training sequence (TRN-R) at the end of the packet, and transmitting transmit training packets (Tx Train), which includes a transmit training sequence (TRN-T) at the end of the packet, and refining (i.e., steering) the beam based on feedback with respect to TRN-T. More specifically, for example, the first wireless device  22  may transmit training packets to the second wireless device  24 . 
     The second wireless device  24  may transmit feedback to the first wireless device  22 , such as channel strength information (CSI) or a steering matrix. The feedback may also include estimated channel quality and/or a preferred MCS according to the link adaptation of the present disclosure. For example, the channel quality may include the channel quality (e.g. SNR) corresponding to each subsequence of the transmit training sequence TRN-T. The first wireless device  22  may then adjust (i.e., steer) the transmit beam based on the received feedback, meanwhile the first wireless device  22  may adjust its MCS and/or transmit power level for the subsequent transmitted packets, based on the channel quality and/or MCS feedback. The same process above then may be conducted on the reverse direction. In other words, the second wireless device  24  starts to transmit receive training packets Rx Train and transmit training packets Tx Train to first wireless device  22 , which follows the same feedback and MCS and/or transmit power selection procedure as described above. The process of transmitting training packets and adjusting the beam based on feedback may continue until the beam is refined. For example, the beam may be refined when the feedback indicates transmission quality is greater than a quality threshold. In each iteration the MCS and/or transmit power of the Rx Train or Tx Train packets from first wireless device  22  to second device  24 , or on the reverse direction, may be adjusted based on the channel quality and/or MCS feedback in the previous iteration, as described above. 
     The beamforming operation may conclude with a beamforming completion operation between the first wireless device  22  and the second wireless device  24 . The beamforming completion operation may include transmitting a final feedback packet in both directions. For example, transmitting and feeding back a final packet may verify beam refinement. In other words, the first wireless device  22  may transmit the final packet to the second wireless device  24 . The second wireless device  24  may receive the final packet and transmit a final feedback packet back to the first wireless device  22 . The final feedback packets in both directions may include the estimated channel quality and/or preferred MCS according to the link adaptation of the present disclosure. 
     After the completion of the beamforming operation, the first wireless device  22  and second wireless device  24  may then select an MCS and/or transmit power level for subsequent transmissions in both directions, respectively, based on the received channel quality feedback and/or MCS feedback during the beam refinement iteration(s). In other words, in both directions, respectively, the first wireless device  22  and second wireless device  24  may encode the data according to the selected modulation scheme and/or transmit the data according to the selected data rate and transmit using the selected transmit power. 
     Referring now to  FIGS. 8A-8C , media access control (MAC) layer packet format for sector sweeping, beam refining, and beamforming completion operations are shown. These formats, however, are examples, and thus other subfields of  FIGS. 8A-8C  may be varied according to applications; however, the proposed subfields for similar concepts may be fixed in whatever format to be applied.  FIG. 8A  illustrates an example of the corresponding portion of MAC layer format in a packet for a sector sweeping operation. The estimated channel quality and/or preferred MCS may be included in an SNR feedback field (“SNR-FB”) which has a length of 8 bits.  FIG. 8B  illustrates an example of the corresponding portion of MAC layer format in a packet for a beam refinement operation. The estimated channel quality and/or preferred MCS may be included in a transmission feedback field (“T-FBCK”) which has a variable bit length.  FIG. 8C  illustrates an example of the corresponding portion of MAC layer format in a packet for a beamforming completion operation. The estimated channel quality and/or preferred MCS may be included in a transmission feedback field (“T-FBCK”) which has a variable bit length (similar to  FIG. 8B ). 
     Referring now to  FIG. 9 , a method for link adaptation during beamforming operations begins at  200 . At  200 , the first wireless device  22  performs a sector sweep operation to determine a best sector for transmission. At  204 , the second wireless device  24  responds to the sector sweep by feeding back a sector index indicating the best sector. For exemplary purposes the second wireless device  24  may be located in the best sector. The second wireless device  24  may also include an estimated channel quality and/or a preferred MCS in the best sector index feedback to the first wireless device  22 . While the first and second wireless devices  22 ,  24  are described, the method may be implemented using other wireless devices. 
     At  208 , the first wireless device  22  may begin a beam refinement operation. More specifically, the first wireless device  22  may generate and transmit training packets to the second wireless device  24 . At  212 , the second wireless device  24  may feedback CSI and/or a steering matrix to the first wireless device  22 . The second wireless device  24  may also feedback an estimated channel quality and/or a preferred MCS to the first wireless device  22 . 
     At  216 , the first wireless device  22  may refine the beam using the CSI and/or steering matrix received from the second wireless device  24 . For example, the first wireless device  22  may adjust an existing steering matrix or generate a new steering matrix based on the received CSI and/or steering matrix. Meanwhile the first wireless device  22  may select appropriate MCS and/or transmit power level for the subsequent packets transmitted from itself to the second device  24  (e.g. another iteration of training packets). At  220 , the first wireless device  22  determines whether the beam is refined. For example, the first wireless device  22  may determine that the beam is refined when a quality of transmission (via feedback from the second wireless device  24 ) is greater than a quality threshold. If true, control may proceed to  224 . If false, control may return to  208  and beam refinement may continue. The same process from  208  to  220  may be conducted one more time in the reverse direction (i.e. from second wireless device  24  to first wireless device  22 ) in each of the beam refinement iterations (i.e. in each iteration forward and reverse training process are both conducted). 
     At  224 , the first wireless device  22  and second wireless device  24  may conclude the beamforming operation with a beamforming completion operation in both directions, respectively. More specifically, the first wireless device  22  and second wireless device  24  may transmit a final feedback packet to the second wireless device  24 . At  228 , the second wireless device  24  may receive the final feedback packet and send the final feedback packet on the reverse direction. In both of the final feedback packets, the first wireless device  22  and the second wireless device  24  may include an estimated channel quality and/or preferred MCS in the final feedback packets in both directions respectively. 
     At  232 , the first wireless device  22  and the second wireless device  24  may select an MCS and/or transmit power level for subsequent transmissions based on the received transmit training feedback and/or final feedback packets in both directions during the whole beam refinement iterations. More specifically, the first wireless device  22  and second wireless device  24  may select the MCS and/or transmit power level based on the estimated channel qualities and/or the preferred MCSs received. For example, the first wireless device  22  and second wireless device  24  may select an MCS and/or transmit power level based on averages of the estimated channel qualities and/or a most preferred MCS in some or all of the transmit training feedback packets during the iterations (in both directions). Alternatively, for example, the first wireless device  22  and second wireless device  24  may select an MCS based on a most recent estimated channel quality and/or preferred MCS (e.g., the final feedback packets). The first wireless device  22  and second wireless device  24  may then encode data according to the selected modulation scheme and/or transmit data according to the selected data rate, and transmit using the selected transmit power. 
     Referring now to  FIG. 10 , a timeline of a link adaptation during a beam tracking operation is shown. Beam tracking refers to a beam refinement operation triggered after an initial beamforming operation. In other words, data throughput may decrease when a channel changes and thus a beam tracking operation may be required to refine the beam and increase data throughput. For example, beam tracking may occur during normal operation of the system by aggregating beam refinement information onto existing data and/or acknowledgement packets. 
     A wireless device that initiates a beam tracking operation may be referred to as a “beam tracking initiator.” A wireless device that responds to a beam tracking operation may be referred to as a “beam tracking responder.” For example, the beam tracking initiator and the beam tracking responder may include the first and second wireless devices  22 ,  24 . 
     The beam tracking initiator may initiate a beam tracking operation via an acknowledgement packet that includes a beam tracking request. The acknowledgment packet may then be transmitted to the beam tracking responder. For example, the beam tracking initiator may initiate beam tracking when a quality of transmission decreases below a threshold. The beam tracking responder may receive the acknowledgement packet and begin beam tracking when the acknowledgement packet includes a beam tracking request. 
     The beam tracking responder may aggregate beam refinement frame onto a data packet for transmission to the beam tracking initiator. For example, the aggregated information for transmission may be referred to as an aggregated MAC protocol data unit, or AMPDU. Additionally, a receive training sequence TRN-R is appended at the end of the same AMPDU packet, so on receiving this AMPDU packet, the beamtracking initiator will be able to refine its receive antenna array. For example, TRN-R may include a known training sequence for the purpose of receive antenna array training, and thus no feedback for TRN-R is required. 
     Subsequently, the beam tracking initiator may request another beam tracking request for transmit training, then the beam tracking responder, on receiving the second tracking request, may send another aggregate data and beam refinement frame with a TRN-T sequence appended at the end of the packet. To respond the TRN-T, after receiving the AMPDU packet, the beam tracking initiator may transmit another AMPDU with a beam refinement frame to the beam tracking responder, in which the estimated CSI/steering matrix are contained, and in addition channel quality and/or a preferred MCS may be included in the transmission feedback (“T-FBCK”) field of the beam refinement frame in the same AMPDU. And then the beam tracking responder on receiving the feedback may adjust its transmit beam based on the CSI/steering matrix feedback. In addition, the beam tracking responder may select appropriate MCS and/or transmit power level for the subsequent transmissions. The beam tracking initiator may continue requesting beam tracking until transmission quality increases (e.g., above a threshold). The same beam tracking process may be conducted in the reverse direction, i.e. initiated by the original beam tracking responder, therefore the original beam tracking initiate is also able to receive channel quality and/or MCS feedbacks and adjust its own MCS and/or transmit power accordingly. 
     Referring now to  FIG. 11 , a method for adapting an RF link during a beam tracking operation begins at  300 . At  300 , the beam tracking initiator may determine whether a beam tracking operation is necessary (i.e., whether the channel has changed). If true, control may proceed to  304 . If false, control may return to  300 . 
     At  304 , the beam tracking initiator generates an acknowledgement packet that includes a beam tracking request. The beam tracking initiator may then transmit the acknowledgement packet to the beam tracking responder. At  308 , the beam tracking responder receives the acknowledgement packet. The beam tracking responder may generate and transmit an AMPDU that includes a bean refinement (BR) frame with appended TRN-R fields. More specifically, the beam tracking responder may aggregate the receive training information onto an existing data packet for transmission. 
     At  312 , the beam tracking initiator may receive the aggregated packet with the receive training information. For example, the receive training information may be a known sequence in the appended TRN-R field(s). The beam tracking responder may then refine the receive antenna array based on. 
     At  316 , the beam tracking responder may determine whether the beam tracking initiator is requesting beam tracking. More specifically, the beam tracking initiator may be requesting transmit training. If true, control may proceed to  320 . If false, control may return to  316 . 
     At  320 , the beam tracking responder may generate an AMPDU that includes a BR frame with appended TRN-T fields. More specifically, the beam tracking responder may aggregate the transmit training information onto an existing data packet for transmission. 
     At  324 , the beam tracking initiator may receive the aggregated packet with the transmit training information. For example, the transmit training information may include information in the appended TRN-T field(s). For example, the information may include CSI and/or a steering matrix. The beam tracking initiator may then refine the transmit beam based on the information. 
     At  328 , the beam tracking initiator may determine whether beam tracking should be requested again. For example, the beam tracking initiator may determine that beam tracking is no longer required when transmission quality increases above a threshold. If true, control may return to  308 . If false, control may proceed to  332 . 
     At  332 , the beam tracking initiator may generate and transmit an AMPDU that includes a BR frame with feedback information (T-FBCK). For example, the feedback information may include estimated channel quality (e.g., SNR, RSSI, etc.) and/or a preferred MCS. 
     At  336 , the beam tracking responder may select an MCS and/or a transmit power level based on the feedback information. The beam tracking responder may then encode data according to the selected modulation scheme and/or transmit data according to the selected data rate and/or the selected transmit power level. 
     Referring again to  FIG. 2 , link adaptation may also be explicitly requested. In other words, the first wireless device  22  may solicit link feedback information from the second wireless device  24 . For example, link adaptation may be required more often than merely during beamforming or beamtracking operations. Therefore, systems and methods are presented that provide for requestable adaptation of an RF link in a 60 GHz wireless communication system. For example, the 60 GHz wireless communication system  20  may implement the systems and/or methods of the present disclosure. 
     Referring now to  FIG. 12 , the first wireless device  22  includes a receiver  32  and a transmitter  28 . The first wireless device  22  may implement the systems or methods of the present disclosure. More specifically, transmitter  28  may include a request generation module  76  that generates a request for link feedback information. Specifically, the request generation module  76  may communicate the request to the second wireless device  24  using the transmission control module  74 . For example, the request generation module  76  may generate the request for link feedback information when link adaptation has not been performed for a predetermined period. Additionally or alternatively, for example, the request generation module  76  may generate the request for link feedback information based on an external request. For example, the external request may be input by a user (i.e., user commanded link adaptation). 
     The first wireless device  22  may then perform link adaptation based on the received link feedback information. As shown, the transmitter  28  may also include the MCS selection and power control module  72  and the transmission control module  74 . The MCS selection and power control module  72  may select at least one of a modulation scheme, a data rate for transmission, and a transmission power level based on link feedback information from the second wireless device  24 . The transmission control module  74  may also receive data for transmission of data packets to the second wireless device  24 . 
     Referring now to  FIG. 13 , the second wireless device  24  includes a receiver  34  and a transmitter  38 . The second wireless device  24  may implement the systems or methods of the present disclosure. More specifically, the receiver  34  receives the request for link feedback information from the first wireless device  22 . The second wireless device  24  includes the channel estimation module  52 , the preferred MCS determination module  54 , and the link margin determination module  56 . The channel estimation module  52  estimates the quality of the communication channel  26 . The preferred MCS determination module  54  determines a preferred MCS based on the estimated quality. 
     Additionally or alternatively, the link feedback information may include a link margin. Specifically, the link margin may include a difference between an SNR of a transmission and a minimum required SNR for transmission. The link margin determination module  56 , therefore, may determine the link margin. For example, the SNR of the transmission may be based on the estimated channel quality. The minimum required SNR, on the other hand, may be determined based on previous transmission data. Alternatively, however, the minimum required SNR may be predetermined or input by the user. 
     The transmitter  38  receives the estimated channel quality, the preferred MCS, and/or the link margin. The transmitter  38  transmits the link feedback information to the first wireless device  22 . For example, the link feedback information may be included in a data packet. The first wireless device  22  may then perform link adaptation (as previously described) based on the received link feedback information. 
     Referring now to  FIG. 14 , a timeline of a first requestable link adaptation operation is shown. Specifically, the first wireless device  22  requests link adaptation by sending a packet including a link adaptation request frame  400  to the second wireless device  24 . For example, the packet may include a Physical Layer Convergence Procedure (PLCP) protocol data unit (also known as a PLCP protocol data unit, or PPDU). The link adaptation request frame  400  may include channel sounding information to instruct the second wireless device  24  to determine and/or store the link feedback information before decoding data. 
     For example, the channel sounding information may include a channel sounding bit in a PHY header preamble. A preamble that has the sounding bit set (i.e., sounding bit=1) may be also be referred to as a sounding preamble. Thus, the sounding bit may also be referred to as a link adaptation bit because it may be set when the link adaptation request frame  400  is included in a packet. Additionally, the sounding preamble may be included in either the link adaptation request frame  400  or a regular data frame. 
     Referring now to  FIG. 15 , a packet format is shown for requestable link adaptation according to the present disclosure. The depicted format, however, is an example, and thus other fields of  FIG. 13  may be varied according to applications; however, the proposed fields for similar concepts may be fixed in whatever format to be applied. Specifically, the packet format may include the following fields: a preamble  450 , a header  460 , data  470 , and transmitter or receiver training field  480  (hereinafter referred to as “TRN-T/R”). For example, the TRN-T/R field  480  is typically used during beamforming and beamtracking operations. 
     Link adaptation may be requested by including additional information in one or more of the fields  450 - 480 . For example, the preamble  450  may be a sounding preamble. Additionally or alternatively, the TRN-T/R field  480  may be used to request link adaptation. More specifically, the header  460  may indicate that the TRN-T/R field  480  is to be used to link adaptation and not for beamforming or beamtracking. For example, the header  460  may include a bit indicating whether the TRN-T/R field  480  is being used for link adaptation. Alternatively, for example, a length of the header  460  may be adjusted and the adjusted length may be interpreted to indicate whether the TRN-T/R field  480  is being used for link adaptation. Additionally or alternatively, the header  460  may indicate a length of the TRN-T/R field  480 . 
     When specified, the TRN-T/R field  480  may then be used for link adaptation. Specifically, the link feedback information may be determined using the TRN-T/R field  480 . For example, the antenna gain control (AGC) subfield of the TRN-T/R field  480  may include four repetitions of an AGC sequence. During link adaptation, however, the AGC subfield is not used to steer the antennas such as during beamforming or beamtracking. Additionally, for example, the TRN-T/R subfield of the TRN-T/R field  480  may include five repetitions of a training sequence. 
     Moreover, both the sounding preamble and TRN-T/R field  480  may be used during link adaptation. More specifically, the sounding preamble may initiate determination of a first set of link feedback parameters and the TRN-T/R field  480  may initiate determination of a second set of link feedback parameters. For example, the first and second sets of link feedback parameters may be mutually exclusive. 
     In other words, the sounding bit may initiate determination of a first set of link feedback parameters and the TRN-T/R field  480  may initiate determination of a second set of link feedback parameters. For example, the first set of link feedback parameters may include SNR and preferred MCS and the second set of link feedback parameters may include link margin. The first and second sets of link feedback parameters, however, may not be mutually exclusive. In other words, the same link feedback parameters may be determined more than once (i.e., initiated by both the sounding bit and the TRN-T/R field  480 ) which may improve accuracy of the link feedback information due to added redundancy. 
     Referring now to  FIG. 16 , a timeline of a second requestable link adaptation operation is shown. Specifically, the first wireless device  22  requests link adaptation by sending a packet including a link adaptation request frame  500  to the second wireless device  24 . For example, the packet may be a PPDU. The link adaptation request frame  500  may request acknowledgement (“ACK”) from the second wireless device  24 . The second wireless device  24  sends an ACK packet  510  to the first wireless device  22  upon receiving the link adaptation request frame  500 . The ACK packet  510  from the second wireless device  24 , however, is optional. In other words, the link adaptation request frame  500  may not request ACK from the second wireless device  24  (described more below). 
     After receiving the ACK packet  510  from the second wireless device  24 , the first wireless device  22  may wait for a predetermined period before sending a regular data packet  520 . For example, the predetermined period may include a period between an acknowledgement and a subsequent data frame, also known as a Short Inter-Frame Space (SIFS). The predetermined period, however, may have a different duration depending upon the application. 
     After the predetermined period has elapsed, the first wireless device  22  may send the regular data packet  520 . The second wireless device  24  may determine the link feedback information based on the regular data packet  520 . The second wireless device  24  may then send a packet including a link adaptation response frame  530  to the first wireless device  22 . The link adaptation response frame  530  includes the link feedback information for link adaptation. For example, the link feedback information may include estimated channel quality, preferred MCS, and/or link margin. The first wireless device  22  may then, based on the link feedback information, select a modulation scheme, a data rate for transmission, and/or a transmit power level for subsequent transmissions. 
     Referring now to  FIG. 17 , a timeline of a third requestable link adaptation operation is shown. Specifically, the first wireless device  22  requests link adaptation by sending a packet including a link adaptation request frame  550  to the second wireless device  24 . For example, the packet may be a PPDU. As shown, the link adaptation request frame  550  does not request ACK from the second wireless device  24  (i.e., the ACK packet is optional, as previously described). After sending the packet including the link adaptation request frame  550 , the first wireless device  22  may wait for a predetermined period before sending a regular data packet  560 . For example, the predetermined period may include an SIFS. The predetermined period, however, may have a different length depending upon the application. 
     After the predetermined period has elapsed, the first wireless device  22  may send the regular data packet  560 . The second wireless device  24  may determine the link feedback information based on the regular data packet  560 . The second wireless device  24  may then send a packet including a link adaptation response frame  570  to the first wireless device  22 . The link adaptation response frame  570  includes the link feedback information. For example, the link feedback information may include estimated channel quality, preferred MCS, and/or link margin. The first wireless device  22  may then, based on the link feedback information, select a modulation scheme, a data rate for transmission, and/or a transmit power level for subsequent transmissions. 
     Referring now to both  FIGS. 16 and 17 , the communication protocol of the second and third requestable link adaptation operations may be adjusted or redefined to include a predetermined period before transmitting the regular data packet. For example, the protocol may be defined such that the first wireless device  22  waits for the predetermined period after sending the packet including a link adaptation request frame or after receiving an ACK packet from the second wireless device  24  (i.e., if ACK is requested by the first wireless device  22 ). 
     Referring now to  FIG. 18 , a method for requesting and performing link adaptation according to the present disclosure begins at  600 . At  600 , a first wireless device determines whether link adaptation is required. If true, control may proceed to  604 . If false, control may return to  600 . 
     At  604 , the first wireless device sends a packet including a link adaptation request frame to a second wireless device. For example, the link adaptation may be requested using a sounding bit and/or TRN-T/R field(s). At  608 , the second wireless device may, upon receiving the link adaptation request frame, send an ACK to the first wireless device. Sending an ACK to the first wireless device, as previously described, is optional. In other words, the link adaptation request frame may or may not request ACK from the second wireless device. 
     At  612 , the first wireless device determines whether a predetermined period has elapsed. For example, the predetermined period may begin either after sending the link adaptation request frame or after receiving the ACK from the second wireless device. If true, control may proceed to  616 . If false, control may return to  612 . 
     At  616 , the first wireless device sends a normal data packet to the second wireless device. At  620 , the second wireless device determines link feedback information based on the received normal data packet. For example, the link feedback information may include estimated channel quality (e.g., SNR, RSSI, CSI, etc.), a preferred MCS, and/or link margin. 
     At  624 , the second wireless device may send a packet having a link adaptation response frame to the first wireless device. The link adaptation response frame includes the link feedback information. At  628 , the first wireless device may then, based on the link feedback information, select a modulation scheme, a data rate for transmission, and/or a transmit power level for subsequent transmissions. Control may then return to  600 . 
     The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims.