Patent Publication Number: US-7714783-B2

Title: Method and system for analog beamforming in wireless communications

Description:
FIELD OF THE INVENTION 
   The present invention relates to wireless communications, and in particular, to beamforming transmissions in wireless channels. 
   BACKGROUND OF THE INVENTION 
   With the proliferation of high quality video, an increasing number of electronic devices (e.g., consumer electronics (CE) devices) utilize high-definition (HD) video. Conventionally, most systems compress HD content, which can be around 1 gigabits per second (Gbps) in bandwidth, to a fraction of its size to allow for transmission between devices. However, with each compression and subsequent decompression of the signal, some data can be lost and the picture quality can be degraded. 
   The existing High-Definition Multimedia Interface (HDMI) specification allows for transfer of uncompressed HD signals between devices via a cable. While consumer electronics makers are beginning to offer HDMI-compatible equipment, there is not yet a suitable wireless (e.g., radio frequency (RF)) technology that is capable of transmitting uncompressed HD signals. For example, conventional wireless local area networks (LAN) and similar technologies can suffer interference issues when wireless stations do not have sufficient bandwidth to carry uncompressed HD signals. 
   Antenna array beamforming has been used to increase bandwidth and signal quality (high directional antenna gain), and to extend communication range by steering the transmitted signal in a narrow direction. However, conventional digital antenna array beamforming is an expensive process, requiring multiple expensive radio frequency chains connected to multiple antennas. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a method and system for analog beamforming for wireless communication. In one embodiment, such analog beamforming involves performing channel sounding to obtain channel sounding information, determining statistical channel information based on the channel sounding information, and determining analog beamforming coefficients based on the statistical channel information, for analog beamforming communication over multiple antennas. 
   In one implementation, direction-of-arrival and direction-of-departure information is determined from the statistical channel information. Determining analog beamforming coefficients includes determining transmitter power level coefficients and phase coefficients from the direction-of-departure information. In addition, determining analog beamforming coefficients involves determining receiver power level coefficients and phase coefficients from direction-of-arrival information. A transmitter station performs analog beamforming based on the transmit power level and phase coefficients, and a receiver station performs analog beamforming based on the receiver power level and phase coefficients. 
   These and other features, aspects and advantages of the present invention will become understood with reference to the following description, appended claims and accompanying figures. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of an orthogonal frequency division multiplexing (OFDM) wireless transmitter that implements an analog beamforming method, according to an embodiment of the present invention. 
       FIG. 2  shows a functional diagram of the analog transmit beamforming method of transmitter of  FIG. 1 , according to an embodiment of the present invention. 
       FIG. 3  shows a flowchart of the steps of an analog transmit beamforming process, according to an embodiment of the present invention. 
       FIG. 4  shows a functional diagram of an OFDM wireless station that implements receive analog beamforming, corresponding to the transmit analog beamforming in the wireless station of  FIG. 2 , according to an embodiment of the present invention. 
       FIG. 5  shows a flowchart of the steps of an analog receive beamforming process, according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention provides a method and system for analog beamforming in wireless communications. In one embodiment, the present invention provides a method and system for analog beamforming using statistical channel knowledge for wireless communications between a transmit station and a receive station. An analog domain antenna array beamforming process allows the transmit station and the receive station to perform analog beamforming based on statistical channel information providing direction-of-arrival and direction-of-departure information. The transmit station performs analog beamforming based on direction-of-departure information, and the receive station performs analog beamforming based on direction-of-arrival information. 
   In one example implementation described below, such analog beamforming is utilized for transmission of uncompressed video signals (e.g., uncompressed HD video content), in a 60 GHz frequency band such as in WirelessHD (WiHD) applications. WiHD is an industry-led effort to define a wireless digital network interface specification for wireless HD digital signal transmission on the 60 GHz frequency band, (e.g., for CE devices). 
   For wireless transmission of uncompressed HD video signals due to large bandwidth and low spectrum efficiency, reliable transmission of a single uncompressed video stream is sufficient. Therefore, analog beamforming using an RF chain for multiple antennas in an array (as opposed to an RF chain per antenna in digital beamforming), reduces the RF chain cost while maintaining an antenna array gain. Since the transmission frequency is high, the transmitter antenna spacing is very small. Therefore, in transmitter fabrication, multiple antennas can be mounted in one chip. Using such analog beamforming, a large array gain can be achieved to improve the video transmission quality. 
     FIG. 1  shows a block diagram of a wireless station  100  implementing analog beamforming using statistical (e.g., estimated) channel information, according to an embodiment of the present invention. Such a wireless station is useful in wireless transmission of uncompressed video signals such as in WiHD applications. The wireless station  100  utilizes OFDM, and includes a digital processing section  101 D and an analog processing section  101 A. 
   The digital processing section  101 D has one RF chain including a forward error correction (FEC) encoder  102 , an interleaver  104 , a Quadrature Amplitude Modulation (QAM) mapper  106 , an OFDM modulator  108 , a digital-to-analog converter (DAC)  110  and a controller  111 . The analog section  101 A includes a mixer  112 , a phase (phase shift) array  114 , and an array of multiple power amplifiers (PAs)  116  corresponding to multiple antennas  118 . The controller  111  provides transmit phase and amplitude coefficients to the phase and amplifier arrays  114  and  116 , respectively, for transmit analog beamforming. 
   The FEC encoder  102  encodes an input bit stream, and the interleaver  104  interleaves the encoded bit using block interleaving. Then, the QAM mapper  106  maps the interleaved bits to symbols using a Gray mapping rule. The OFDM modulator  108  performs OFDM modulation on the symbols, and the DAC  110  generates a baseband signal from OFDM modulated symbols. 
   In the analog processing section  101 A, the analog signal from the DAC  110  is provided to the mixer  112  which modulates the analog signal from baseband up to the transmission frequency (e.g., 60 GHz). The modulated signal is then input to the phase array  114 , which in conjunction with the controller  111 , applies a coefficient vector W T  (i.e., weighting coefficients) thereto for transmission beamforming. The weighted signals are then amplified via the PA 116  for transmission through an array of N transmit antennas  118 . 
     FIG. 2  shows an example functional diagram of the analog transmit beamforming method of the wireless station of  FIG. 1 . The FEC encoder  102 , the interleaver  104 , the QAM mapper  106 , and the OFDM modulator  108  in  FIG. 1 , collectively perform transmission baseband digital signal processing, shown as a processing module  150  in  FIG. 2 . 
   The digital output of the processing module  150  is then converted to an analog signal by the DAC  110 , and provided to the mixer  112  which modulates the analog signal to a 60 GHz transmission frequency. The phase array  114 , in conjunction with the controller  111 , applies the coefficient vector W T  to the modulated signal for transmit beamforming. As such, the analog data signals from the DAC  110  are transmitted over a channel via transmit antennas  118  by steering and amplifying the analog data signals using the transmit beamforming vector W T . 
   The transmit beamforming coefficient vector W T  comprises elements e jφ   1 , . . . , e jφ   N , wherein φ 1 , . . . , φ N  are beamforming phase coefficients that are calculated by the controller  111  and controlled digitally at the baseband. Preferably, the coefficient vector W T  is an optimal coefficient. A direction of departure (DoD) function  152  estimates the direction of departure information θ T  based on the statistical channel information obtained during a channel sounding period. 
   A channel sounding period includes a training period, in which a sounding packet exchange can be implemented by generating a training request (TRQ) specifying a number of training fields, and transmitting a TRQ from a transmit station (initiator) having multiple antennas to a receive station (responder) over a wireless channel, wherein the TRQ specifies the number of training fields based on the number of transmit antennas. The receive station then transmits a sounding packet to the transmit station, wherein the sounding packet includes multiple training fields corresponding to the number of training fields specified in the TRQ. Based on the sounding packet, the wireless station transmits a beamforming transmission to the receive station to enable wireless data communication therebetween. This provides a sounding packet format and an exchange protocol for wireless beamforming using statistical channel information. 
   Specifically, the controller  111  determines a transmit channel correlation matrix R T  based on the DoD information θ T  from the channel sounding information. Then, the transmit phase coefficients φ 1 , . . . , φ N  and amplitude (power lever) coefficients [α 1 , . . . , α N ] are determined based on the transmit channel correlation matrix R T  (detailed further below), wherein the transmit beamforming coefficient vector W T =[α 1 e jφ     1   , . . . , α N e jφ     N   ], is related only to the transmit correlation matrix R T . 
   The coefficient vector W T includes  complex numbers as phase (weighting) coefficients, wherein the phase coefficient φ 1 , . . . , φ N  are applied to the frequency band signals by N phase array elements  114 - 1 , . . . ,  114 -N, respectively. Then, the amplitude coefficients [α 1 , . . . , α N ] are applied to the phase shifted signal (i.e., the analog beamformed signal) from the phase array elements  114 - 1 , . . . ,  114 -N, by N power amplifiers  116 - 1 , . . . ,  116 -N, respectively. The signals amplified by the amplifiers  116 - 1 , . . . ,  116 -N are wirelessly transmitted to a receive station via the N antennas  118 - 1 , . . . ,  118 -N. 
     FIG. 3  shows a flowchart of the steps of the example transmit analog beamforming process  160  implemented in  FIG. 2 , including the steps of:
         Step  161 : Perform baseband digital signal processing and convert the resulting data stream to analog data signals.   Step  162 : Perform channel sounding to obtain a channel estimate including direction of departure (DoD) information θ T  based on the sounding period information.   Step  164 : Determine the transmit channel correlation matrix R T  based on the DoD information θ T .   Step  166 : Determine the transmitter beamforming vector W T =[α 1 e jφ     1   , . . . , α N e jφ     N   ] based on the correlation matrix R T .   Step  168 : Determine the transmit beamforming phase coefficients φ 1 , . . . , φ N  and amplitude coefficients [α 1 , . . . , α N ] from the beamforming vector W T =[α 1 e jφ     1   , . . . , α N e jφ     N   ].   Step  170 : Transmit the analog signals to a receive station from a transmit station over transmitter antennas, by steering and amplifying the analog data signals using the phase and amplitude coefficients, respectively. The signals are transmitted via a wireless communication medium (e.g., over RF communication channels).       
     FIG. 4  shows a functional diagram of an OFDM wireless station  200  that implements receive analog beamforming, corresponding to the transmit analog beamforming in wireless station  100 , according to an embodiment of the present invention. The station  200  includes an antenna array  201  (including M receive antennas  201 - 1 , . . . ,  201 -M), a power amplifier array  202  (including M amplifiers  202 - 1 , . . . ,  202 -M), a phase shift array  204  (including M phase elements  204 - 1 , . . . ,  204 -M), a combiner function  205  which coherently combines the outputs of the phase shift array  204 , an analog-to-digital converter (ADC)  206 , a mixer function  208  which down-converts the RF signal from the ADC  206  to baseband for digital signal processing, a direction of arrival (DoA) estimation function  210 , a baseband processing function  214  and a controller  212  that provides receive phase and amplitude coefficients to the amplifier and phase shift arrays  202  and  204 , respectively, for receive analog beamforming. 
   In operation, the transmitted signals are received by the antenna array  201 , and amplified by the amplifier array  202  using receive amplitude (power level) coefficients β 1 , . . . , β M . The amplified signals are processed in the phase shift array  204  using the receive phase coefficients Φ 1 , . . . , Φ M . The receive amplitude and phase coefficients are determined by the controller  212 , and together form a receive beamforming coefficient vector W R =[β 1 e jφ     1   , . . . , β N e jφ     M   ] which comprises elements e jΦ   1 , . . . , e jΦ   M . The output of the phase elements  204 - 1 , . . . ,  204 -M of the phase shift array  204 , representing an analog beamformed signal, is provided to the combiner function  205  which combines them together for high signal power. 
   The output of the combiner function module  205  (i.e., a combined output of the receive analog beamformed signal) is converted to a digital signal by the ADC  206 , and provided to the mixer function  208  for conversion to baseband. The baseband output of the mixer function  208  is provided to the baseband digital signal processor  214  for conventional receiver processing. 
   The output of the mixer function  208  is also provided to the DoA estimator  210  to estimate the DoA information θ R  (i.e., the channel statistical information) from the sounding information (similar to that described above in relation to the station  100 ). The controller  212  uses the DoA information θ R  to determine a receive channel correlation matrix R R . Then, the receive phase coefficients Φ 1 , . . . , Φ M  are determined based on the receive channel correlation matrix R R  (detailed further below). As such, the receive beamforming coefficient vector W R  is related only to the receive correlation matrix R R . 
     FIG. 5  shows a flowchart of the steps of the example receive analog beamforming process  250  implemented in the station  200  of  FIG. 2 , including the steps of:
         Step  251 : Obtain the DoA information θ R  based on the sounding period channel estimation information.   Step  252 : Determine the receive channel correlation matrix R R  based on the DoA information θ R .   Step  254 : Determine the receive beamforming vector W R =[β 1 e jφ     1   , . . . , β N e jφ     M   ] based on the receive correlation matrix R R .   Step  256 : Determine the transmit beamforming amplitude coefficients β 1 , . . . , β M  and phase coefficients φ 1 , . . . , φ N  from the receive beamforming vector.   Step  258 : Receive the analog signals using the receive amplitude and phase coefficients.   Step  260 : The received analog signal is down-converted to a baseband signal for digital signal processing.       
   As noted, the transmitter beamforming coefficient vector W T  is related only to the channel correlation matrix R T , and the receiver beamforming coefficient vector W R  is related only to the channel correlation matrix R R . A channel matrix H can be modeled as:
 
 H=R   R   1/2   H   W   R   T   1/2 ,
 
   wherein elements of matrix H W  are independent and identically distributed (i.i.d.) complex Gaussian distributed, with a zero mean and unit covariance, and wherein: 
   
     
       
         
           
             
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   where θ T , θ R  are the angle of departure from the transmitter and the angle of arrival to the receiver, σ T ,σ R  are angle spreads at the transmitter and the receiver, Δ T ,Δ R  are the distance between the adjacent antenna elements in terms of carrier wavelength: 
   wherein m and n are the element index in each matrix. 
   The transmit beamforming vector W T =e jφ   1 , . . . , e jφ   N  is determined based on the transmit channel correlation matrix R T  as follows. The correlation matrix R T  is used to calculate U T  which is a unitary vector that comprises right singular vectors of R T , such that:
         R T =U T Λ T U T *, wherein * means conjugate transpose.       

   The transmit beamforming vector W T  is determined as W T =U T . 
   Similarly, the receive beamforming vector W R =[β 1 e jφ     1   , . . . , β N e jφ     M   ] is determined based on the receive channel correlation matrix R R  as follows. The receive channel correlation matrix R R  is used to calculate U R  which is a unitary vector that comprises right singular vectors of R R , such that:
 
 R   R   =U   R Λ R   U   R *.
 
   Then, the receiver beamforming vector W R  is determined as W R =U R . 
   An analog domain antenna array beamforming process based on the channel statistical information direction-of-arrival and direction-of-departure information provides simplified and efficient wireless communication, compared to digital beamforming such as eigen-based beamforming techniques which typically require multiple RF chains corresponding to multiple antennas. 
   As is known to those skilled in the art, the aforementioned example architectures described above, according to the present invention, can be implemented in many ways, such as program instructions for execution by a processor, as logic circuits, as an application specific integrated circuit, as firmware, etc. The present invention has been described in considerable detail with reference to certain preferred versions thereof; however, other versions are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.