# Patent

## Title
Method and apparatus for generating beamforming feedback

## Abstract

First gain and phase information corresponding to an estimate of a communication channel from a second communication device to a first communication device are determined at the first communication device. First transmit beamforming information are calculated using the first gain and phase information at the first communication device, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via multiple antennas. The first transmit beamforming information are converted, at the first communication device, to second gain and phase information corresponding to the estimate of the communication channel. Feedback is generated at the first communication device using the second gain and phase information, the feedback to be transmitted to the second communication device.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
The present application is a continuation of U.S. patent application Ser. No. 13/947,985, now U.S. Pat. No. 8,630,376, entitled “Method and Apparatus for Generating Beamforming Feedback,” filed Jul. 22, 2013, which is a continuation of U.S. patent application Ser. No. 13/674,752, now U.S. Pat. No. 8,494,093, entitled “Method and Apparatus for Generating Beamforming Feedback,” filed Nov. 12, 2012, which is a continuation of U.S. patent application Ser. No. 12/396,323, now U.S. Pat. No. 8,311,160, entitled “Explicit Beamformee Method and Apparatus for MIMO and MISO Systems with Single Data Stream,” filed on Mar. 2, 2009, which claims the benefit of U.S. Provisional Application No. 61/037,543, entitled “Fast Explicit Beamformee Algorithm and Architecture for MIMO and MISO Systems with Single Data Stream,” filed on Mar. 18, 2008. All of the applications referenced above are hereby incorporated by reference herein in their entireties.


FIELD OF DISCLOSURE
The present disclosure relates generally to wireless communication systems and, more particularly, to an apparatus and method for performing beamforming calculations for a multiple-input, multiple-output wireless communication system.
BACKGROUND
An ever-increasing number of relatively inexpensive, low power wireless data communication services, networks and devices have been made available over the past number of years, promising near wire speed transmission and reliability. Various wireless technology is described in detail in the 802.11 IEEE Standard, including for example, the IEEE Standard 802.11a (1999) and its updates and amendments, the IEEE Standard 802.11g (2003), as well as the IEEE Standard 802.11n now in the process of being adopted, all of which are collectively incorporated herein fully by reference. These standards have been or are in the process of being commercialized with the promise of 54 Mbps or more effective bandwidth, making them a strong competitor to traditional wired Ethernet and the more ubiquitous “802.11b” or “WiFi” 11 Mbps mobile wireless transmission standard.
Generally speaking, transmission systems compliant with the IEEE 802.11a and 802.11g or “802.11a/g” as well as the 802.11n standards achieve their high data transmission rates using Orthogonal Frequency Division Modulation or OFDM encoded symbols mapped up to a 64 quadrature amplitude modulation (QAM) multi-carrier constellation. The use of OFDM divides the overall system bandwidth into a number of frequency sub-bands or channels, with each frequency sub-band being associated with a respective sub-carrier upon which data may be modulated. Thus, each frequency sub-band of the OFDM system may be viewed as an independent transmission channel within which to send data, thereby increasing the overall throughput or transmission rate of the communication system.
Generally, transmitters used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n standards as well as other standards such as the 802.16e IEEE Standard, perform multi-carrier OFDM symbol encoding (which may include error correction encoding and interleaving), convert the encoded symbols into the time domain using Inverse Fast Fourier Transform (IFFT) techniques, and perform digital to analog conversion and conventional radio frequency (RF) upconversion on the signals. These transmitters then transmit the modulated and upconverted signals after appropriate power amplification to one or more receivers, resulting in a relatively high-speed time domain signal with a large peak-to-average ratio (PAR).
Likewise, the receivers used in the wireless communication systems that are compliant with the aforementioned 802.11a/802.11g/802.11n and 802.16e IEEE standards generally include an RF receiving unit that performs RF downconversion and filtering of the received signals (which may be performed in one or more stages), and a baseband processor unit that processes the OFDM encoded symbols bearing the data of interest. Generally, the digital form of each OFDM symbol presented in the frequency domain is recovered after baseband downconverting, conventional analog to digital conversion and Fast Fourier Transformation of the received time domain analog signal. Thereafter, the baseband processor performs demodulation (phase rotation) and frequency domain equalization (FEQ) to recover the transmitted symbols, and these symbols are then processed in a viterbi decoder to estimate or determine the most likely identity of the transmitted symbol. The recovered and recognized stream of symbols is then decoded, which may include deinterleaving and error correction using any of a number of known error correction techniques, to produce a set of recovered signals corresponding to the original signals transmitted by the transmitter.
In wireless communication systems, the RF modulated signals generated by the transmitter may reach a particular receiver via a number of different propagation paths, the characteristics of which typically change over time due to the phenomena of multi-path and fading. Moreover, the characteristics of a propagation channel differ or vary based on the frequency of propagation. To compensate for the time varying, frequency selective nature of the propagation effects, and generally to enhance effective encoding and modulation in a wireless communication system, each receiver of the wireless communication system may periodically develop or collect channel state information (CSI) for each of the frequency channels, such as the channels associated with each of the OFDM sub-bands discussed above. Generally speaking, CSI is information defining or describing one or more characteristics about each of the OFDM channels (for example, the gain, the phase and the SNR of each channel). Upon determining the CSI for one or more channels, the receiver may send this CSI back to the transmitter, which may use the CSI for each channel to precondition the signals transmitted using that channel so as to compensate for the varying propagation effects of each of the channels.
An important part of a wireless communication system is therefore the selection of the appropriate data rates, and the coding and modulation schemes to be used for a data transmission based on channel conditions. Generally speaking, it is desirable to use the selection process to maximize throughput while meeting certain quality objectives, such as those defined by a desired frame error rate (FER), latency criteria, etc.
To further increase the number of signals which may be propagated in the communication system and/or to compensate for deleterious effects associated with the various propagation paths, and to thereby improve transmission performance, it is known to use multiple transmit and receive antennas within a wireless transmission system. Such a system is commonly referred to as a multiple-input, multiple-output (MIMO) wireless transmission system and is specifically provided for within the 802.11n IEEE Standard now being adopted. Generally speaking, the use of MIMO technology produces significant increases in spectral efficiency and link reliability, and these benefits generally increase as the number of transmission and receive antennas within the MIMO system increases.
In addition to the frequency channels created by the use of OFDM, a MIMO channel formed by the various transmit and receive antennas between a particular transmitter and a particular receiver includes a number of independent spatial channels. As is known, a wireless MIMO communication system can provide improved performance (e.g., increased transmission capacity) by utilizing the additional dimensionalities created by these spatial channels for the transmission of additional data. Of course, the spatial channels of a wideband MIMO system may experience different channel conditions (e.g., different fading and multi-path effects) across the overall system bandwidth and may therefore achieve different SNRs at different frequencies (i.e., at the different OFDM frequency sub-bands) of the overall system bandwidth. Consequently, the number of information bits per modulation symbol (i.e., the data rate) that may be transmitted using the different frequency sub-bands of each spatial channel for a particular level of performance may differ from frequency sub-band to frequency sub-band.
However, instead of using the various different transmit and receive antennas to form separate spatial channels on which additional information is sent, better transmission and reception properties can be obtained in a MIMO system by using each of the various transmission antennas of the MIMO system to transmit the same signal while phasing (and amplifying) this signal as it is provided to the various transmission antennas to achieve beamforming or beamsteering. Generally speaking, beamforming (also referred to as beamsteering) creates a spatial gain pattern having one or more high gain lobes or beams (as compared to the gain obtained by an omni-directional antenna) in one or more particular directions, while reducing the gain over that obtained by an omni-directional antenna in other directions. If the gain pattern is configured to produce a high gain lobe in the direction of each of the receiver antennas, the MIMO system can obtain better transmission reliability between a particular transmitter and a particular receiver, over that obtained by single transmitter-antenna/receiver-antenna systems.
There are many known techniques for determining a steering matrix specifying the beamsteering coefficients that need to be used to properly condition the signals being applied to the various transmission antennas so as to produce the desired transmit gain pattern at the transmitter. As is known, these coefficients may specify the gain and phasing of the signals to be provided to the transmitter antennas to produce high gain lobes in particular or predetermined directions. These techniques include, for example, transmit-MRC (maximum ratio combining) and singular value decomposition (SVD). While it is known to perform beamforming in a MIMO system in a manner that uses all of the available receiver and transmitter antennas in the MIMO system, which thereby maximizes transmission reliability, the calculations and processing overhead associated with computing the beamsteering coefficients may be significant, depending on the number of transmit and/or receive antennas increases. As a result, beamforming in a MIMO system may significantly increase the system complexity and processing overhead when a relatively large number of transmit and receive antennas are present.
SUMMARY
In one embodiment, a method includes determining, at a first communication device, first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas. The method also includes calculating, at the first communication device, first transmit beamforming information using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas. Additionally, the method includes converting, at the first communication device, the first transmit beamforming information to second gain and phase information corresponding to the estimate of the communication channel from the second communication device to the first communication device. The method further includes generating, at the first communication device, feedback using the second gain and phase information, the feedback to be transmitted to the second communication device.
In another embodiment, a first communication device comprises one or more integrated circuits configured to: determine first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas, calculate first transmit beamforming information using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas, convert the first transmit beamforming information to second gain and phase information corresponding to the estimate of the communication channel from the second communication device to the first communication device, and generate feedback using the second gain and phase information, the feedback to be transmitted to the second communication device.
In yet another embodiment, a non-transitory, tangible computer-readable medium stores a plurality of instructions that, when executed by one or more processors of a first communication device, cause the one or more processors to: convert first transmit beamforming information to first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, the first transmit beamforming information having been calculated by the first communication device using second gain and phase information corresponding to the estimate of the communication channel, wherein the second gain and phase information was determined by the first communication device, and the first transmit beamforming information corresponds to transmit beamforming by the second communication device via multiple antennas of the second communication device. The plurality of instructions, when executed by one or more processors of a first communication device, further cause the one or more processors to generate feedback using the first gain and phase information, the feedback to be transmitted to the second communication device.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram illustrating explicit beamforming implemented in a prior art beamformee transceiver;
FIG. 2 is a block diagram illustrating explicit beamforming implemented in a beamformee transceiver according to one embodiment;
FIG. 3 is a block diagram of an example steering vector calculator that may be utilized in the example transceiver of FIG. 2;
FIG. 4 is a graph illustrating example angle quantization to 8 possible angles; and
FIG. 5 is a block diagram of a wireless MIMO communication or transmission system that implements a transmitter beamforming technique using matrix equalizer information of a receiver;



DETAILED DESCRIPTION
While the beamforming techniques described herein for processing and effecting a wireless data transmission are described as being used in communication systems that use one of the IEEE Standard 802.11x communication standards, these techniques may be used in various other types of wireless communication systems and are not limited to those conforming to one or more of the IEEE Standard 802.11x standards. As just some examples, the beamforming techniques may also be used in conjunction with systems that conform to the IEEE 802.16e/m standards, the Long Term Evolution (LTE) standard (now in the process of being adopted) of the Third Generation Partnership Project (3GPP), etc.
MIMO may achieve spatial multiplexing gain when multiple streams are transmitted over MIMO channel to increase the data rate. MIMO may also achieve diversity gain when a single stream is transmitted separately on multiple independent wireless links to increase reliability. In a MIMO system where reliability dominates the overall system performance (e.g., when the signal-to-noise ratio (SNR) is low), it may be appropriate to transmit only a single steam and to utilize diversity gain. On the other hand, in a multiple input, single output (MISO) system (i.e., multiple transmit antennas and a single receive antenna), the physical configuration limits the number of streams that can be transmitted between the two stations to one. The transmit beamforming techniques described herein may be utilized in MIMO and MISO systems in which only a single stream is transmitted.
With transmit beamforming in the context of a single stream, the transmitted signal can be modeled as:

y=Hvsteers+n  (Equation 1)

where y is a NR×1 received signal vector, H is an NR×NT matrix corresponding to the MIMO/MISO channel response, vsteer is a single column steering vector, s is the data symbol transmitted on the single stream, n is an NR×1 additive noise vector, NR is the number of receive antennas, and NT is the number of transmit antennas. vsteer is applied at the transmitter, and is designed according to the current channel state information (CSI), i.e., H. In other words, knowledge of the channel is leveraged to increase signal directivity (translates to receive SNR gain) and exploit spatial diversity gain. In an OFDM system, the model according to Equation 1 may be applied for each subcarrier, or for each of a plurality of groups of adjacent subcarriers. With a properly designed steering vector vsteer, the received strength (or SNR) on the single stream can be greatly improved, so that the reliability of the signal or the operable range of the wireless system can be extended. Both increased reliability and extended range are often valuable in wireless communication systems such as WLAN and WiMAX systems.

But the channel, H, will typically vary over time. Thus, the steering vector vsteer should be updated as H changes. There are two options for a transmitter to maintain a steering vector that tracks a changing channel: implicit beamforming and explicit beamforming. In implicit beamforming, the beamformer (i.e., the station that transmits a beamformed signal) will estimate the forward channel (beamformer to beamformee (i.e., the station that receives the beamformed signal) from reverse channel sounding (i.e., the beamformee sends sounding (or training) signals to the beamformer, and the beamformer estimates the reverse channel estimation). The beamformer may then estimate the forward channel by transposing the reverse channel estimate (assuming reciprocity between the forward channel and the reverse channel). By periodically implicitly estimating the forward channel, the beamformer can seek to maintain an up-to-date estimate of the forward channel, and thus an up-to-date beamsteering vector. However, the reverse and forward channels may not be fully reciprocal due to the mismatch between transmit and receive radio frequency (RF) circuitry, for example.
In explicit beamforming, the beamformer sends sounding signals to the beamformee. The beamformee then estimates the forward channel and feeds back the CSI of the forward channel to the beamformer. Optionally, the beamformee may compute the steering vector vsteer and feed vsteer back to the beamformer. By periodically receiving CSI for the forward channel, or a steering vector, from the beamformee, the beamformer can seek to maintain an up-to-date estimate of the forward channel and/or an up-to-date beamsteering vector. The beamforming techniques described further below utilize explicit beamforming.
A more detailed explanation of explicit beamforming in the context of a system operating according to the IEEE 802.11n Standard is now provided. It will be understood, however, that explicit beamforming in other types of systems will operate similarly. First, the beamformer sends a sounding packet to the beamformee. Upon receiving the sounding packet, the beamformee may estimate H (i.e., CSI) for the forward channel based on the received sounding packet. Then, the beamformee may transmit the CSI back to the beamformer. Alternatively, the beamformee may compute the steering vector vsteer based on the CSI, and may then transmit vsteer back to the beamformer. If the beamformer receives the CSI, the beamformer may compute the steering vector vsteer based on the CSI, and then apply vsteer for transmitting subsequent packets (Equation 1). If the beamformer receives vsteer from the beamformee, the beamformer may merely apply vsteer for transmitting subsequent packets.
The IEEE 802.11n Standard specifies that, if H is fed back in explicit beamforming, encoding on H should be utilized. Similarly, the IEEE 802.11n Standard specifies that, if vsteer is fed back, vsteer should be either encoded or compressed. Thus, the beamformee may be required to not only compute H and/or vsteer, but also to performing encoding or compression on H or vsteer. Typically, vsteer calculation and encoding/compression are implemented by a processor executing software instructions. FIG. 1 is a block diagram illustrating explicit beamforming implemented in a prior art beamformee transceiver 10. The prior art device includes hardware (HW) for implementing physical layer operations such as receiving signals, demodulating the received signals, and decoding the received signals to retrieve data transmitted by the beamformer. The prior art device also includes a general purpose processor that executes software instructions (SW) to perform media access control (MAC) layer operations. In FIG. 1, a dotted line indicates the demarcation between hardware and software in the beamformee 10.
The beamformee 10 includes a CSI buffer 14 for storing CSI for the forward channel. A hardware/software interface 18 generally permits communication of data between the hardware and software portions of the beamformee 10. The software portion includes a module 22 for calculating the steering matrix and encoding or compressing the steering matrix when the steering matrix is to be fed back to the beamformer. The software portion also includes a module 26 for encoding the CSI when the CSI is to be fed back to the beamformer. The software portion also may include a module 30 for selecting steering vector feedback or CSI feedback. The software portion also may include a module 34 for forming a feedback packet that includes the steering vector or the CSI.
In operation, the hardware stores the estimated CSI for the forward channel in the CSI buffer 14. Next, the CSI stored in the CSI buffer 14 is provided to the software portion via the hardware/software interface 18. Next, the software portion processes the CSI by either encoding the CSI (block 26), or calculating the steering matrix and encoding/compressing the steering matrix (block 22). Then, either the encoded CSI or the encoded/compressed steering matrix is transmitted back to the beamformer.
The performance of the system 10 is generally limited by the software processing. For example, the CSI usually includes a large quantity of data, and the hardware/software interface 18 is a relatively slow and limited, shared resource. For example, the hardware/software interface 18 may need to exchange a variety of data between the hardware and software due to various MAC layer processing. Thus, the CSI data will typically be transferred intermittently, block-by-block by the hardware/software interface 18. Additionally, steering matrix calculation, encoding, and compression are time intensive operations. Delay between receiving the sounding packet and transmitting the CSI or steering matrix feedback will adversely affect performance of beamforming. In particular, the forward channel is time varying, and thus the CSI or steering matrix feedback may be stale by the time the beamformer receives it. This is may be more pronounced when the beamformee is a handheld device (e.g. a mobile phone).
FIG. 2 is a block diagram illustrating explicit beamforming implemented in a beamformee transceiver 100. The explicit beamforming implemented in the beamformee transceiver 100 is for a scenario in which one stream of data is to be transmitted. The transceiver 100 includes hardware (HW) for implementing physical layer operations such as receiving signals, demodulating the received signals, and decoding the received signals to retrieve data transmitted by the beamformer. The transceiver 100 also includes a processor that executes software instructions (SW) to perform, for example, MAC layer operations. The processor may include or be coupled to a memory in which machine readable instructions are stored. When executed by the processor, the machine readable instructions may cause the processor to perform various operations to be described below with respect to the software portion of FIG. 2. In FIG. 2, a dotted line indicates the demarcation between hardware and software in the beamformee 100.
The beamformee 100 includes a CSI buffer 104 for storing CSI for the forward channel. The CSI may correspond to a sounding packet transmitted by the beamformer, for example, and may be generated by the hardware. A steering vector calculator 108 is coupled to the CSI buffer 104. The steering vector calculator 108 generates a steering vector based on the CSI in the CSI buffer 104 using, for example, a cophasing algorithm. The result of the cophasing algorithm is a set of cophasing angles for each OFDM subcarrier (or for each group of adjacent subcarriers). The number of cophasing angles per subcarrier (or group of adjacent subcarriers) may correspond to the number of transmit antennas. The steering vector calculator 108 may also quantize each cophasing angle, such as quantizing each cophasing angle to a desired number of bits. For example, the steering vector calculator 108 may quantize each cophasing angle to three bits. Of course, cophasing angles may be quantized by some other degree, such as by two bits, four bits, five bits, six bits, etc.
A hardware/software interface 112 permits the cophasing angles generated by the steering vector calculator 108 to be communicated to the software portion (i.e., the portion that is implemented by a processor executing software instructions) of the beamformee 100. The software portion includes a steering vector feedback mapping module 116 for mapping the quantized cophasing angles generated by the hardware into either encoded or compressed steering vector elements. As will be discussed subsequently, mapping the quantized cophasing angles generated by the hardware into either encoded or compressed steering vector elements may be implemented using a look-up table, for example.
The software portion includes a CSI feedback mapping module 120 for mapping the quantized cophasing angles generated by the hardware into CSI feedback elements. As will be discussed subsequently, mapping the quantized cophasing angles generated by the hardware into CSI feedback elements may be implemented using a look-up table, for example.
The software portion also includes logic 124 for selecting either steering vector feedback or CSI feedback. Although illustrated in FIG. 2 as a multiplexer that selects between the output of the module 116 and the module 120, the logic 124 may alternatively select which of the module 116 and the module 120 is executed. After the steering vector feedback or the CSI feedback has been generated, a module 128 may form a feedback packet or frame for transmission to the beamformer.
An example cophasing algorithm that can be used for implementing the block 108 of FIG. 2 will now be described. Cophasing (which is also sometimes referred to as equal gain combining (EGC) is a single-stream steering calculation approach. Cophasing attempts to make the phase of the channel coefficients from different transmit antennas to one receive antenna identical. In the case of one receive antenna, the forward channel can be represented as:

H=[h1h2 . . . hNT]  (Equation 2)

so the steering vector can be designed as:





  
    
      
        
          v
          steer
        
        =
        
          [
          
            
              
                
                  e
                  
                    jΔ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        
                          N
                          T
                        
                        ⁢
                        1
                      
                    
                  
                
              
            
            
              
                
                  e
                  
                    jΔ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        
                          N
                          T
                        
                        ⁢
                        2
                      
                    
                  
                
              
            
            
              
                ⋮
              
            
            
              
                
                  e
                  
                    jΔ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        
                          N
                          T
                        
                        ⁡
                        
                          (
                          
                            
                              N
                              T
                            
                            -
                            1
                          
                          )
                        
                      
                    
                  
                
              
            
            
              
                1
              
            
          
          ]
        
      
    
    
      
        (
        
          Equation
          ⁢
          
            
          
          ⁢
          3
        
        )
      
    
  




where ΔθNTi=θNT−θi, for i=1, . . . NT−1, and θi is the angle of the complex channel coefficient hi. So the “steered” transmitted signal model can be modeled as:





  
    
      
        y
        =
        
          
            
              
                
                  [
                  
                    
                      h
                      1
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      h
                      2
                    
                    ⁢
                    
                      
                    
                    ⁢
                    ...
                    ⁢
                    
                      
                    
                    ⁢
                    
                      h
                      
                        N
                        T
                      
                    
                  
                  ]
                
                ⁡
                
                  [
                  
                    
                      
                        
                          e
                          
                            jΔ
                            ⁢
                            
                              
                            
                            ⁢
                            
                              θ
                              
                                
                                  N
                                  T
                                
                                ⁢
                                1
                              
                            
                          
                        
                      
                    
                    
                      
                        
                          e
                          
                            jΔ
                            ⁢
                            
                              
                            
                            ⁢
                            
                              θ
                              
                                
                                  N
                                  T
                                
                                ⁢
                                2
                              
                            
                          
                        
                      
                    
                    
                      
                        ⋮
                      
                    
                    
                      
                        1
                      
                    
                  
                  ]
                
              
              ⁢
              s
            
            +
            n
          
          =
          
            
              
                (
                
                  
                    e
                    
                      jθ
                      
                        N
                        T
                      
                    
                  
                  ⁢
                  
                    
                      ∑
                      
                        i
                        =
                        1
                      
                      
                        N
                        T
                      
                    
                    ⁢
                    
                      
                      
                        h
                        i
                      
                      
                    
                  
                
                )
              
              ⁢
              s
            
            +
            n
          
        
      
    
    
      
        
          (
          
            Equation
            ⁢
            
              
            
            ⁢
            4
          
          )
        
        ⁢
        
          
        
      
    
  




The received SNR is enhanced due to the combining. The steering vector of Equation 3 cophases to the last transmit antenna (i.e., the NT-th antenna), but it will be understood by those of ordinary skill in the art that, in other implementations, cophasing to any of the other transmit antenna may also be implemented.

In another embodiment, one or more of the blocks 116, 120, 124 and 128 may also be implemented in hardware. For example, all of the blocks 116, 120, 124 and 128 may be implemented in hardware configured to implement MAC layer operations. An “all hardware” implementation of steering vector feedback/CSI feedback generation may be particularly useful for scenarios in which very fast feedback is desired.
FIG. 3 is a block diagram of an example steering vector calculator 200 that may be utilized for the steering vector calculator 108 of FIG. 2. It will be understood, however, that a steering vector calculator other than the steering vector calculator 200 may also be utilized for the steering vector calculator 108 of FIG. 2.
If the beamformee includes multiple receive antennas, the steering vector calculator 200 may include a selection block 204 for selecting one of the receive antennas to which to cophasing is performed. For example, the receive antenna with the strongest forward channel response may be selected. Determining which receive antenna is the strongest may include calculating the row norm of the forward channel H matrix (based on the CSI) associated with each receiver antenna. The row norm for the l-th antenna, gl, may be calculated as:




  
    
      
        
          
            g
            l
          
          =
          
            
              ∑
              
                i
                =
                1
              
              
                N
                T
              
            
            ⁢
            
              
              
                h
                li
              
              
            
          
        
        ,
        
          l
          =
          
            1
            ⁢
            
              
            
            ⁢
            ...
            ⁢
            
              
            
            ⁢
            
              N
              R
            
          
        
      
    
    
      
        (
        
          Equation
          ⁢
          
            
          
          ⁢
          5
        
        )
      
    
  




Then, the receive antenna corresponding to the largest row norm may be selected as the antenna to which cophasing is to be performed. If there is only one receive antenna, the block 204 may be omitted.

The row coefficients h1, h2, . . . , hNT corresponding to the one receive antenna, or if there are multiple receive antennas, the selected receive antenna, are provided to a plurality of coordinate rotation digital computer (CORDIC) blocks 208. Each CORDIC block 208 generally computes the phase of the corresponding row coefficient hi. Although in FIG. 3 a separate CORDIC block 208 is illustrated for each row coefficient, it will be understood that one or some other number less than NT of CORDIC blocks 208 may be utilized by, for example, time sharing the CORDIC block(s) 208. In other implementations, hardware that implements an algorithm other than the CORDIC algorithm may be utilized to calculate the phases of the row coefficients.
Outputs of the CORDIC blocks 208 are provided to a plurality of subtractors 212. The subtractors 212 calculate subtractions ΔθNTi=θi+1−θi, for i=1, 2, . . . , NT−1. Although in FIG. 3, NT−1 subtractors 212 are illustrated, it will be understood that one or some other number less than NT−1 of subtractors 212 may be utilized by, for example, time sharing the subtractor(s) 212.
Outputs of the subtractors 212 are provided to a plurality of quantizers 216 that quantize the outputs of the subtractors to some desired number of bits. Quantization with a relatively small number Na of bits (e.g., Na=3 bits) for each of the cophasing angles results in a steering vector that achieves acceptable beamforming performance gain. In an OFDM system, the total amount of data per subcarrier delivered to the software portion through the hardware/software interface is now Na*(NT−1) bits. This is far less data than if the whole channel matrix H per subcarrier is sent through the hardware/software interface. FIG. 4 is a graph illustrating example angle quantization for Na=3 bits, resulting in 8 possible angles (indicated in FIG. 4 by the black circles on the circumference of the larger circle). The outputs of the quantizers 216 are output by the example steering vector calculator 200 as NT−1 cophasing angles.
Although in FIG. 3, NT−1 quantizers 216 are illustrated, it will be understood that one or some other number less than NT−1 of quantizers 216 may be utilized by, for example, time sharing the quantizer(s) 216.
Referring again to FIG. 2, the software portion of the beamformee 100 receives, via the hardware/software interface 112, the cophasing angles generated by the steering vector calculator 108. The software portion of the beamformee 100 generally uses the cophasing angles to generate the steering vector feedback or the CSI feedback. In one implementation, the software portion of the beamformee 100 may include pre-stored feedback content that corresponds to all of the possible cophasing angles. The steering vector or CSI feedback may then be generated by selecting from the pre-stored feedback content using the cophasing angles. For example, the feedback may be generated using one or more look-up tables (LUTs). In such implementations, feedback can be generated without performing encoding or compression each time feedback needs to be provided.
For example, if encoded steering vector feedback according to the IEEE 802.11n standard is to be generated (i.e., non-compressed steering vector feedback), the cophasing angles generated by the steering vector calculator 108 translate into the steering vector according to Equation 3. The real part and the imaginary part of each steering vector element can each be represented by Nb bits, for example. Table 1 is an example LUT for Na=3 and Nb=2 and for a non-compressed steering vector feedback compliant with the IEEE 802.11n standard.










TABLE 1







3 bit Angle ΔθNTi
i-th complex element in vsteer









0 (b000)
1



1 (b001)
1 + j



2 (b010)
j



3 (b011)
−1 + j



4 (b100)
−1



5 (b101)
−1 − j



6 (b110)
−j



7 (b111)
1 − j










The quantized cophasing angles can be used as indices into the LUT of Table 1.

As another example, if compressed steering vector feedback according to the IEEE 802.11n standard is to be generated, actual compressing need not be performed each time the compressed steering vector feedback is to be generated. Rather, a LUT similar to Table 1 may be utilized to lookup compressed steering vector feedback elements based on the cophasing angles. For example, the quantized cophasing angles can be used as indices into the LUT.
As yet another example, if CSI feedback according to the IEEE 802.11n standard is to be generated, the cophasing angles may be converted into CSI information. In particular, the CSI feedback, HFB, may be estimated by:

HFB=vsteerH  (Equation 6)





  
    
      
        
          H
          FB
        
        =
        
          [
          
            
              
                
                  e
                  
                    
                      -
                      j
                    
                    ⁢
                    
                      
                    
                    ⁢
                    Δ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        N
                        
                          T
                          1
                        
                      
                    
                  
                
              
              
                
                  e
                  
                    
                      -
                      j
                    
                    ⁢
                    
                      
                    
                    ⁢
                    Δ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        N
                        
                          T
                          2
                        
                      
                    
                  
                
              
              
                ...
              
              
                
                  e
                  
                    
                      -
                      j
                    
                    ⁢
                    
                      
                    
                    ⁢
                    Δ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      θ
                      
                        
                          N
                          T
                        
                        ⁡
                        
                          (
                          
                            
                              N
                              T
                            
                            -
                            1
                          
                          )
                        
                      
                    
                  
                
              
            
          
          ]
        
      
    
    
      
        (
        
          Equation
          ⁢
          
            
          
          ⁢
          7
        
        )
      
    
  




If the beamformer then uses a classical approach to calculating the steering vector using HFB, such as transmit maximum ratio combining (TxMRC) or cophasing, the beamformer will then generate its steering vector as the same vsteer used to generate HFB in Equation 6.

The real part and the imaginary part of each CSI feedback element in HFB can each be represented by Nb bits, for example. Table 2 is an example LUT for Na=3 and Nb=4 and for CSI feedback compliant with the IEEE 802.11n standard.








TABLE 2





3 bit Angle ΔθNTi
i-th complex element in HFB
















0 (b000)
4


1 (b001)
4 − 4j


2 (b010)
−4j


3 (b011)
−4 − 4j


4 (b100)
−4


5 (b101)
−4 + 4j


6 (b110)
4j


7 (b111)
4 + 4j









The quantized cophasing angles can be used as indices into the LUT of Table 2.

Referring now to FIG. 5, a MIMO communication system 300 is illustrated in block diagram form as generally including a single transmitter 312 having multiple transmit antennas 314A-314N and a single receiver 316 having one or more receive antennas 318A-318M. The number of transmit antennas 314A-314N can be the same as or more than the number of receive antennas 318A-318M. As shown in FIG. 5, the transmitter 312 may include a controller 320 coupled to a memory 321, a symbol encoder and modulator unit 322 and a beamforming block 324, also referred to herein as a transmit beamforming network. The transmitter 312 may also include a matrix equalizer 325 and a symbol demodulator and decoder unit 326 to perform demodulation and decoding of signals received via the antennas 314A-314N in a receive mode. Additionally, the transmitter 312 may include a steering matrix calculation unit 328. The steering matrix calculation unit 328 may generate a steering matrix based on explicit forward channel CSI feedback from the receiver 316.
The controller 312 may be any desired type of controller and both the controller 312 and the steering matrix calculation unit 328 may be implemented as one or more standard multi-purpose, programmable processors, such as micro-processors, as application specific integrated circuits (ASICs), etc., or may be implemented using any other desired types of hardware, and/or programmable processors (i.e., processors that execute software and/or firmware instructions). Likewise, the beamforming network 324, and the matrix equalizer 325 may be implemented using known or standard hardware and/or programmable processor elements. If desired, various of the transmitter components, such as the controller 320, the modulator unit 322, the demodulator unit 326, the steering matrix calculation unit 328, the beamforming network 324 and the matrix equalizer 325 may be implemented in the same or in different hardware devices, such as in the same or different processors. Additionally, each of these components of the transmitter 312 may be disposed in a housing 329 (shown in dotted relief in FIG. 5). Still further, the routines or instructions for implementing the functionality of any of these components may be stored in the memory 321 or within other memory devices associated with the individual hardware used to implement these components.
During operation, information signals Tx1-Txn which are to be transmitted from the transmitter 312 to the receiver 316 are provided to the symbol encoder and modulator unit 322 for encoding and modulation. Of course, any desired number of signals Tx1-Txn may be provided to the modulator unit 322, with this number generally being limited by the modulation scheme used by and the bandwidth associated with the MIMO communication system 300. Additionally, the signals Tx1-Txn may be any type of signals, including analog or digital signals, and may represent any desired type of data or information. Additionally, a known test or control signal Cx1 (which may be stored in the memory 321) may be provided to the symbol encoder and modulator unit 322 so that it can be transmitted to the receiver 316. The receiver 316 can then utilize the control signal Cx1 to determine forward channel CSI describing the characteristics of the forward channel from the transmitter 312 to the receiver 316. The same control signal or a different control signal may be used to determine the CSI for each frequency used in the MIMO communication system 300. The control signal Cx1 may be referred to as a sounding packet.
The symbol encoder and modulator unit 322 may interleave digital representations of the various signals Tx1-Txn and Cx1 and may perform any other known type(s) of error-correction encoding on the signals Tx1-Txn and Cx1 to produce a stream of symbols to be modulated and sent from the transmitter 312 to the receiver 316. While the symbols may be modulated using any desired or suitable QAM technique, such as using 64 QAM, these symbols may be modulated in any other known or desired manner including, for example, using any other desired phase and/or frequency modulation techniques. In any event, the modulated symbol stream is provided by the symbol encoder and modulator unit 322 to the beamforming network 324 for processing before being transmitted via the antennas 314A-314N. While not specifically shown in FIG. 5, the modulated symbol streams may be up-converted to the RF carrier frequencies associated with an OFDM technique (in one or more stages) before being processed by the beamforming network 324. Upon receiving the modulated signals, the beamforming network 324 processes the modulated signals by injecting delays and/or gains into the modulated signals based on a steering vector provided by the controller 20 or the steering matrix calculation unit 328, to thereby perform beamforming via the transmit antennas 14A-14N.
The signals transmitted by the transmitter 312 are detected by the receive antennas 318A-318M and may be processed by a matrix equalizer 335 within the receiver 316 to enhance the reception capabilities of the antennas 318A-318M. As will be understood, the processing applied at the receiver 316 (as well as at the transmitter 312) may be based on, for example, the CSI developed by the receiver 316 in response to the transmission of the test or control signal Cx1 (i.e., sounding packet). In any event, a symbol demodulator and decoder unit 336, under control of a controller 340, may decode and demodulate the received symbol strings as processed by the matrix equalizer 335. In this process, these signals may be downconverted to baseband. Generally, the matrix equalizer 335 and the demodulator and decoder unit 336 may operate to remove effects of the channel based on the CSI as well as to perform demodulation on the received symbols to produce a digital bit stream. In some cases, if desired, the symbol demodulator and decoder unit 336 may perform error correction decoding and deinterleaving on the bit stream to produce the received signals Rx1-Rxn corresponding to the originally transmitted signals Tx1-Txn.
A steering matrix calculation unit 348 may generate explicit forward channel beamforming feedback based on the control signal Cx1 as described above. In one implementation, the steering matrix calculation unit 348 may include both the hardware and software portions in FIG. 2. In another implementation, the steering matrix calculation unit 348 may include the hardware portion of FIG. 2, and the controller 340 may include the software portion of FIG. 2. Additionally, the steering matrix calculation unit 348 may include the hardware/software interface 112 (or a portion thereof). In another implementation, the controller 340 may include the hardware/software interface 112 (or a portion thereof). The controller 340 may cause the explicit forward channel beamforming feedback to be transmitted to the transmitter 312.
As shown in FIG. 5, the receiver 316 may also include a memory 341 and a symbol encoder and modulator unit 346 which may receive one or more signals TR1-TRm which may be encoded and modulated using any desired encoding and modulation techniques. If the receiver 316 includes multiple antennas 318A-318N, the encoded and modulated symbol stream may then be upconverted and processed by a beamforming network 334 to perform beamsteering based on a steering matrix. If the receiver 316 only includes a single antenna, the beamforming network 334 may be omitted. Then, the upconverted signals are transmitted via the antenna(s) 318A-318N to, for example, the transmitter 312, thereby implementing the reverse link. As shown in FIG. 5, each of the receiver components may be disposed in a housing 349.
The matrix equalizer 325 and the demodulator/decoder unit 326 within the transmitter 312 operate similarly to the matrix equalizer 335 and the demodulator/decoder unit 336 of the receiver 316 to demodulate and decode the signals transmitted by the receiver 316 to produce the recovered signals RR1-RRm. Here again, the matrix equalizer 325 may process the received signals in any known manner to enhance the separation and therefore the reception of the various signals transmitted by the antenna(s) 318A-318M. As noted above, the CSI, steering vectors/matrices, beamforming and other programs and data used by the units 328 and 348 and by the controllers 320 and 340 may be stored in the memories 321 and 341.
As is generally known, beamforming or beamsteering typically includes applying appropriate phases and gains to the various signals as sent through the multiple transmit antennas 314A-314N, in a manner with causes the signals sent from the different transmit antennas 314A-314N to constructively interact (add in phase) in certain predetermined directions and to deconstructively interact (cancel) in other directions. Thus, beamforming typically produces a beam pattern having high gain regions (referred to as high gain lobes) in various predetermined directions and low gain regions (typically referred to as nulls) in other directions. The use of beamforming techniques in a MIMO system enables a signal to be sent with high gain (as compared to an omni-directional antenna) in certain directions, and to be sent with low gain (as compared to an omni-directional antenna) in other directions. Thus, in the MIMO system 300 of FIG. 5, beamforming may be used to enhance signal directivity towards the receive antenna(s) 318A-318M, which improves the SNR of the transmissions and results in more reliable transmissions. In this case, the beamforming technique will generally form high gain lobes in the direction of propagation at which the highest gain is desired, and in particular in the directions of propagation from the transmitter 312 to each of the one or more receive antennas 318A-318M of the receiver 316.
At least some of the various blocks, operations, and techniques described above may be implemented in hardware, in a processor executing firmware instructions, in a processor executing software instructions, or any combination of thereof. When implemented using a processor that executes software/firmware instructions, the software/firmware 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 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 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 may include machine readable instructions that are capable of causing one or more processors to perform various acts.
When implemented in hardware, the hardware may comprise one or more of discrete components, an integrated circuit, an application-specific integrated circuit (ASIC), etc.
While the present invention has been described with reference to specific examples, which are intended to be illustrative only and not to be limiting of the invention, it will be apparent to those of ordinary skill in the art that changes, additions or deletions in addition to those explicitly described above may be made to the disclosed embodiments without departing from the spirit and scope of the invention.

## Claims

1. A method, comprising: determining, at a first communication device, first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas; calculating, at the first communication device, first transmit beamforming information, including cophasing angles, using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas; converting, at the first communication device, the first transmit beamforming information, including the cophasing angles, to second gain and phase information corresponding to the estimate of the communication channel from the second communication device to the first communication device; and generating, at the first communication device, feedback using the second gain and phase information, the feedback to be transmitted to the second communication device.

2. The method of claim 1, wherein the cophasing angles are calculated with respect to a second antenna of, or coupled to, the first communication device.

3. The method of claim 2, wherein:
the first communication device includes, or is coupled to, multiple second antennas; and
the cophasing angles are calculated with respect to a selected one of the multiple second antennas.

4. The method of claim 1, wherein converting the cophasing angles to second gain and phase information comprises looking up, at the first communication device, the second gain and phase information in a lookup table using the cophasing angles.

5. The method of claim 1, further comprising quantizing, at the first communication device, the transmit beamforming information,
wherein converting the transmit beamforming information to second gain and phase information comprises using transmit beamforming information that has been quantized.

6. The method of claim 1, further comprising transmitting the feedback from the first communication device to the second communication device.

7. A method comprising: determining, at a first communication device, first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas; calculating, at the first communication device, first transmit beamforming information using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas; determining, at the first communication device, whether i) beamforming information or ii) gain and phase information is to be fed back to the second communication device; converting, at the first communication device, the first transmit beamforming information to second gain and phase information corresponding to the estimate of the communication channel from the second communication device to the first communication device when gain and phase information is to be fed back to the second communication device; in response to determining that gain and phase information is to be fed back to the second communication device, generating, at the first communication device, feedback using the second gain and phase information, the feedback to be transmitted to the second communication device; and generating, at the first communication device, feedback i) using the first transmit beamforming information and ii) without using the second gain and phase information in response to determining that beamforming information is to be fed back to the second communication device.

8. The method of claim 7, further comprising converting, at the first communication device, the first transmit beamforming information to second transmit beamforming information when it is determined that beamforming information is to be fed back to the second communication device,
wherein generating the feedback i) using the first transmit beamforming information and ii) without using the second gain and phase information comprises generating the feedback using the second transmit beamforming information.

9. The method of claim 8, wherein generating the feedback using the second transmit beamforming information comprises looking up, at the first communication device, the second beamforming information in a lookup table using the first transmit beamforming information.

10. The method of claim 7, wherein converting the first transmit beamforming information to second gain and phase information is in response to determining that beamforming information is to be fed back to the second communication device.

11. A first communication device, comprising:
one or more integrated circuits configured to: determine first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas, calculate first transmit beamforming information, including cophasing angles, using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas, convert the first transmit beamforming information, including the cophasing angles, to second gain and phase information corresponding to the estimate of the communication channel from the second communication device to the first communication device, and generate feedback using the second gain and phase information, the feedback to be transmitted to the second communication device.

12. The first communication device of claim 11, wherein the one or more integrated circuits are configured to calculate the cophasing angles with respect to a second antenna of, or coupled to, the first communication device.

13. The first communication device of claim 12, wherein:
the first communication device includes, or is coupled to, multiple second antennas; and
the one or more integrated circuits are configured to calculate the cophasing angles with respect to a selected one of the multiple second antennas.

14. The first communication device of claim 11, wherein the one or more integrated circuits are configured to look up the second gain and phase information in a lookup table using the cophasing angles.

15. The first communication device of claim 11, wherein the one or more integrated circuits are configured to:
quantize the transmit beamforming information, and
convert the transmit beamforming information to second gain and phase information using transmit beamforming information that has been quantized.

16. The first communication device of claim 11, wherein the one or more integrated circuits are configured to transmit the feedback from the first communication device to the second communication device.

17. The first communication device of claim 16, further comprising one or more second antennas coupled to the one or more integrated circuits.

18. A first communication device comprising:
one or more integrated circuits configured to: determine first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, wherein the second communication device includes multiple first antennas, determine whether i) beamforming information or ii) gain and phase information is to be fed back to the second communication device, calculate first transmit beamforming information using the first gain and phase information corresponding to the estimate of the communication channel, wherein the first transmit beamforming information corresponds to transmit beamforming by the second communication device via the multiple first antennas, generate feedback using the second gain and phase information, the feedback to be transmitted to the second communication device, in response to determining that gain and phase information is to be fed back to the second communication device, and generate feedback i) using the first transmit beamforming information and ii) without using the second gain and phase information in response to determining that beamforming information is to be fed back to the second communication device.

19. The first communication device of claim 18, wherein the one or more integrated circuits are configured to:
convert the first transmit beamforming information to second transmit beamforming information when it is determined that beamforming information is to be fed back to the second communication device; and
generate the feedback using the first transmit beamforming information by using the second transmit beamforming information.

20. The first communication device of claim 19, wherein the one or more integrated circuits are configured to look up the second beamforming information in a lookup table using the first transmit beamforming information.

21. The first communication device of claim 18, wherein the one or more integrated circuits are configured to convert the first transmit beamforming information to second gain and phase information in response to determining that beamforming information is to be fed back to the second communication device.

22. A non-transitory, tangible computer-readable medium storing a plurality of instructions that, when executed by one or more processors of a first communication device, cause the one or more processors to: convert first transmit beamforming information, including cophasing angles, to first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, the first transmit beamforming information, including the cophasing angles, having been calculated by the first communication device using second gain and phase information corresponding to the estimate of the communication channel, wherein the second gain and phase information was determined by the first communication device, and the first transmit beamforming information corresponds to transmit beamforming by the second communication device via multiple antennas of the second communication device; and generate feedback using the first gain and phase information, the feedback to be transmitted to the second communication device.

23. A non-transitory, tangible computer-readable medium storing a plurality of instructions that, when executed by one or more processors of a first communication device, cause the one or more processors to convert first transmit beamforming information to first gain and phase information corresponding to an estimate of a communication channel from a second communication device to the first communication device, the first transmit beamforming information having been calculated by the first communication device using second gain and phase information corresponding to the estimate of the communication channel, wherein the second gain and phase information was determined by the first communication device, and the first transmit beamforming information corresponds to transmit beamforming by the second communication device via multiple antennas of the second communication device; determine whether i) beamforming information or ii) gain and phase information is to be fed back to the second communication device; and generate feedback i) using the first transmit beamforming information and ii) without using the second gain and phase information in response to determining that beamforming information is to be fed back to the second communication device, wherein generating feedback using the first gain and phase information is in response to determining that gain and phase information is to be fed back to the second communication device.

24. The non-transitory, tangible computer-readable medium of claim 23, further storing instructions that, when executed by the one or more processors of the first communication device, cause the one or more processors to:
convert the first transmit beamforming information to second transmit beamforming information when it is determined that beamforming information is to be fed back to the second communication device,
wherein generating the feedback i) using the first transmit beamforming information and ii) without using the second gain and phase information comprises generating the feedback using the second transmit beamforming information.

25. The non-transitory, tangible computer-readable medium of claim 24, further storing instructions that, when executed by the one or more processors of the first communication device, cause the one or more processors to look up the second beamforming information in a lookup table using the first transmit beamforming information.

26. The non-transitory, tangible computer-readable medium of claim 23, further storing instructions that, when executed by the one or more processors of the first communication device, cause the one or more processors to convert the first transmit beamforming information to second gain and phase information in response to determining that beamforming information is to be fed back to the second communication device.