Patent Publication Number: US-8111679-B2

Title: Wireless communication apparatus and wireless communication method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-226653, filed on Aug. 23, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to wireless communication and transmission directivity. 
     2. Description of the Related Art 
     For the transmission and reception of data using multiple antennas, a beamforming technique has been employed. Beamforming transmits a single datum via multiple antennas imparting directivity by the multiple antennas by staggering phases on transmission. 
       FIG. 9  is a schematic showing a conventional wireless communication apparatus.  FIG. 9  shows the partial configuration of the wireless communication apparatus on the base station side of a mobile communication system. Data from multiple channels (four channels shown in the figure) is input to a precoder  901 , and each channel is given a fixed phase difference to form multiple beams. A transmitting unit  902  includes transmission circuits  902   a  to  902   d  for each channel, and transmits data via an antenna  903 . 
     While the configuration shown in  FIG. 9  is the configuration of a beamformer that attains a given directivity using the multiple antenna  903 , a similar technique that transmits data by imparting phase differences to each channel exists, a multiple input multiple output (MIMO) technique, in which different data is transmitted by each channel (for example, Published Japanese translation of a PCT application No. 2005-522086). 
     In the configuration shown in  FIG. 9 , however, there is phase variation among the transmission circuits  902   a  to  902   d  for each channel of the transmitting unit  902 . The existence of the phase variation causes, at the transmitting unit  902 , distortion of the beam formed at the precoder  901 , and expected directivity cannot be obtained. 
       FIG. 10  illustrates gain reduction of multi-beam transmission. As shown on the left side of the figure, under a condition in which there is no phase variation, expected directivity is obtained and transmission can be done with an improved gain (up to 6 decibels (dB)) by the effect of the multi-beam method. Inversely, in the case that phase variation exists in the transmission circuits  902   a  to  902   d  of the transmitting unit  902 , directivity becomes inadequate, e.g., broaden, and the gains of each beam is reduced (down to 4 dB). 
     Therefore, in order to eliminate phase variation, phase adjustment has formerly been required so that phase variation does not occur by comparing the outputs of the transmission circuits  902   a  to  902   d  of the transmitting unit  902  and a reference signal using an external calibrator. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least solve the above problems in the conventional technologies. 
     According to one aspect of the present invention includes, a wireless communication apparatus, on a base station side, that performs beamforming and transmits data to a mobile station using a plurality of antennas, includes a code book that stores beamforming information; a precoder that reads the code book and performs processing to form a multi-beam pattern for the data; and a control unit that, based on feedback information for a compensation of a phase variation in each transmission circuit of a plurality of transmission circuits that each respectively corresponds to a channel, controls a retrieval of the beamforming information that compensates the phase variation. 
     According to another aspect of the present invention, a wireless communication apparatus, on a mobile station side, that receives a plurality of beams of data from a base station using a plurality of antennas, includes a code book that stores beamforming information identical to that stored in a code book of the base station; and a control unit that, when notified that a phase variation compensation is required by the base station, searches the beamforming information, based on a signal condition of each of the beams, for the beamforming information that enables optimal beamforming, and transmits the selected beamforming information to the base station as feedback information. 
     According to still another aspect of the present invention, a wireless communication method, on a base station side, of performing beamforming and transmitting data to a mobile station using a plurality of antennas, includes reading beamforming information stored in a code book; forming a multi-beam pattern for the data; and retrieving, based on feedback information for a compensation of a phase variation in each transmission circuit of a plurality of transmission circuits that each respectively correspond to a channel, the beamforming information to compensate the phase variation. 
     According to yet another aspect of the present invention, a wireless communication method of a wireless communication apparatus on a mobile station side that, using a plurality of antennas, receives a plurality of beams of data from a base station, includes searching, based on a signal condition of each of the beams when notified that a phase variation compensation is required by the base station, a code book for beamforming information that enables optimal beamforming, the code book storing beamforming information identical to that in a code book provided in the base station; and transmitting the beamforming information that enables optimal beamforming to the base station as feedback information. 
     The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic showing the configuration of wireless communication apparatuses according to a first embodiment of the present invention; 
         FIG. 2  is a schematic showing detailed configuration of the beam measuring unit shown in  FIG. 1 ; 
         FIG. 3  is a flowchart showing processes related to phase variation compensation; 
         FIG. 4  is a schematic showing a state of a code book during the search; 
         FIG. 5  is a schematic showing the state of a code book during a partial search; 
         FIG. 6  is a timing chart showing the transition of searching status; 
         FIG. 7  is a schematic of a wireless communication apparatus according to a second embodiment of the present invention; 
         FIG. 8  is a timing chart showing search status transition; 
         FIG. 9  is a schematic showing a conventional wireless communication apparatus; and 
         FIG. 10  illustrates gain reduction of multi-beam transmission. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the accompanying drawings, exemplary embodiments according to the present invention are explained in detail below. 
       FIG. 1  is a schematic showing the configuration of wireless communication apparatuses according to a first embodiment of the present invention. Wireless communication apparatuses  10  and  50  of a base station (transmitter) and of a mobile station (receiver), respectively, are shown of mobile communication system. 
     The wireless communication apparatus  10  on the base station side is explained. Data (data stream) for multiple channels (four channels shown in the figure) is input to a precoder  11 , and given phase differences are assigned to each channel to form multiple beams. A code book  21  stores 16-way (equal to the number of combination of indices) compensation values to give weights to the data of each channel according to the multi-beam pattern. 
     The pilot signal of an orthogonal component (orthogonal pilot) is individually added to the output data of each channel of the precoder  11  by an adding unit  12 . A transmitting unit  13  includes transmission circuits  13   a  to  13   d  for individual channels, and modulates data at the oscillation frequency (RF) of an oscillator  14  ( 14   a  to  14   d ) and outputs the data via multiple antennas  15 . 
     A control unit  22  compensates phase variation based on the data input by the wireless communication apparatus  50  of a mobile station and selects a compensation value from the code book  21  and inputs the value to the precoder  11  to compensate phase variation. 
     The wireless communication apparatus  50  on the mobile station side is explained. Data received through multiple channels (two channels shown in the figure) via antennas  51  is input to a channel estimation unit  52 , and the channel estimation unit  52  estimates the channels using orthogonal pilot components included in the transmitted data, and a demodulator  53  demodulates stream data and outputs demodulated data. Although the system shown in  FIG. 1  is the example of single input multiple output (SIMO) configuration, it can be applied similarly to an MIMO system. 
     The wireless communication apparatus  50  on the mobile station side is also equipped with a code book  61  that contains the same contents as that on the base station side. A beam measuring unit  62 , based on channel values estimated by the channel estimating unit  52  and the contents of the code book  61 , alters estimated channel values necessary for the compensation of the variation in the received beams. At this time, a beam indicating the maximum signal to interference plus noise ratio (SINR) (signal to interference ratio (SIR)) is measured, and the optimal transmission beam and the number of streams are selected by a selecting unit  63 , and the information is fed back to the wireless communication apparatus  10  of the base station. Feedback information only includes the transmission beam and the number of streams, and thus can easily be transmitted to the base station side without compressing the band of an upstream transmission path. The beam measuring unit  62  and the selecting unit  63  constitute a control unit in the wireless communication apparatus  50  on the mobile station side. 
       FIG. 2  is a schematic showing detailed configuration of the beam measuring unit shown in  FIG. 1 . The beam measuring unit includes a channel value estimating unit  62 A and an SINR estimating unit  62 B. The channel value estimating unit  62 A computes an estimate channel value HW for each beam based on an estimate channel value H output by the channel estimating unit  52  and a weight W set in the code book  61 . The SINR estimating unit  62 B estimates the level, e.g., SINR, of each beam using the estimate channel value HW of each beam. The estimation can be done not only by SINR but also by signal power. 
     The selecting unit  63  compares the level of each beam with a threshold value, and ranks the beams in ascending order of level. Transmission beam identifiers (ID&#39;s), which are assigned to each high ranking beam, and the determined number of streams are transmitted as feedback information to the wireless communication apparatus  10  on the base station side. 
     The code books  21  and  61  include weights to form the beam pattern of multiple beams and to compensate phase variation. The weight W for multi-beam generation is expressed by the following equation 1 (j and −j are imaginary numbers). 
     
       
         
           
             
               
                 
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                   = 
                   
                     
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                               1 
                             
                           
                           
                             
                               w 
                               2 
                             
                           
                           
                             
                               w 
                               3 
                             
                           
                           
                             
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                         ⁡ 
                         
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     As code book information for precoding, the weight W is defined from W(0, 0) to W(3, 3) in the following sixteen ways according to combinations of the index numbers: W(0, 0), W(0, 1), W(0, 2), W(0, 3), W(1, 0), W(1, 1), W(1, 2), W(1, 3), W(2, 0), W(2, 1), W(2, 2), W(2, 3), W(3, 0), W(3, 1), W(3, 2), W(3, 3). 
     A code book to obtain high gains is prepared by compensating phase variations existing in the channels of the transmission circuits  13   a  to  13   d  of the transmitting unit  13 . In the case of four indices as the above, when a 4×4 diagonal matrix is described as D(m 3 , m 4 ) and the phase difference is ninety degrees, the matrix is expressed as equation 2. By assigning the two of four matrix arrangements that are diagonally arranged from the upper left to the lower right, as 1 and 1, virtual straight lines positioned at angles differing by 90° are created. Terms for approaching the virtual straight line are φ(m 3 ) and φ(m 4 ) 
     
       
         
           
             
               
                 
                   
                     
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     Equation 3 is computed by normalizing the diagonal matrix after discrete Fourier transform (DFT) and Hermitian transposition.
 
 W ( m   3   ,m   4 )=DFT[ D ( m   3   ,m   4 )] H /√{square root over (4)}  (3)
         H: Hermitian transposition       

     As a result, a unitary matrix for precoding is obtained and is stored in the code books  21  and  61 . 
     Consequently, the beam measuring unit  62  of the wireless communication apparatus  50  on the reception side, using the code book  61 , computes the following equation 4, where phase terms to compensate phase variation are added to a third and a fourth antenna of the antennas  15  for transmission. 
     
       
         
           
             
               
                 
                   
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                     conjugate 
                   
                 
               
               
                 
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     The beam number wi of the maximum SINR and the phase number thereof (matrix number) m 3  and m 4  are obtained and are transmitted as feedback information to the wireless communication apparatus  10  on the base station side. 
       FIG. 3  is a flowchart showing processes related to phase variation compensation. Processes by wireless communication apparatuses of both a base and a mobile station are shown. 
     The base station notifies or informs the mobile station of incomplete calibration if calibration for phase variation compensation is not completed (step S 1 ). Receiving the notification or information, the mobile station searches the entire code book  61  (step S 11 ).  FIG. 4  is a schematic showing a state of a code book during the search. The entire 16-way code book is searched. 
     The mobile station then informs the base station of a matrix number as the result of the search for phase variation compensation and a beam number based on beam measurement by the beam measuring unit  62  as feedback information.  FIG. 4  shows an example in which matrix number nine and beam number two are reported. 
     The base station completes calibration based on the feedback information (step S 2 ). At this time, a matrix number and a beam number indicated by the feedback information are read from the code book  21  and are input to the precoder  11 , and thus optimum beamforming can be performed. The base station notifies or informs the mobile station of the completion of calibration. 
     After calibration is completed, the mobile station performs a partial search of the code book  61  (step S 12 ).  FIG. 5  is a schematic showing the state of a code book during a partial search, “partial” meaning searching for a beam number within the matrix number nine that is obtained by the search of an entire code book. Once phase variation is compensated, only the search of an optimum beam number that constantly indicates the maximum SINR is required without searching an entire code book.  FIG. 5  shows an example in which beam number three (search result) is reported to the base station. 
     Subsequently, the base station transmits data (step S 3 ). At this time, the data transmission is performed using the matrix number and beam number reported by the mobile station and one cycle of the process is thus completed. 
       FIG. 6  is a timing chart showing the transition of searching status. After the search of the entire code book  61  (step S 11 ), only partial searching (step S 21 ) needs to be performed at a fixed time intervals. It is not necessary to search the entire code book  61  at every calibration process, and thus time for partial searching can be increased. In this way, the search process of the code book  61  in the wireless communication apparatus  50  on the mobile station side can be simplified, and the processing load involved in the search can be reduced. 
     According to the first embodiment, the phase variation of each transmission circuit  13   a  to  13   d  of the transmitting unit  13  in the wireless communication apparatus  10  of the base station is compensated on the base station side using the feedback information that is output by the wireless communication apparatus  50  of the mobile station based on the receiving condition. The compensation of phase variation can be performed simply without any specific calibration circuit. 
       FIG. 7  is a schematic of a wireless communication apparatus according to a second embodiment of the present invention. A single oscillator, an oscillator  14 , is connected to the multiple channels of the transmitting unit  13 , differentiating the apparatus from that of  FIG. 1  according to the first embodiment. The oscillator  14  supplies a local signal that is common to each transmission circuit of the transmitting unit  13 . Therefore, phase variation among the transmission circuits  13   a  to  13   d  is not generated. 
       FIG. 8  is a timing chart showing search status transition. According to the second embodiment, the same process as shown in  FIG. 3  is also performed by the wireless communication apparatus  50  on the mobile station side. Similarly, a partial search (step S 21 ) is performed after the entire code book  61  is searched (step S 11 ) at the mobile station. 
     According to the second embodiment, as the oscillator  14  supplies the transmitting unit  13  with a common local signal and thus phase variations among channels are not induced, a single search of the entire code book  61  at a calibration process is sufficient. After a single search of the entire code book  61  is performed, the duration of the partial search can be longer (the relative proportion of the partial search time can be increased) compared to the first embodiment. Search of the entire code book  61  at fixed time intervals (for example, thirty-minute cycles) based on temperature shift, and compensation, also at a fixed time intervals based on temperature shift, of phase variation induced at the transmitting unit  13  may be performed. The wireless communication apparatus  50  on the mobile station side recognizes, by information transmitted from the base station side, that the transmitting unit  13  of the wireless communication apparatus  10  on the base station side has a configuration employing a common local signal. 
     According to the second embodiment, a single oscillator, the oscillator  14 , supplies a common local signal to each of the transmission circuits  13   a  to  13   d  of the transmitting unit  13  in the wireless communication apparatus  10  of the base station, and phase variation is compensated on the base station side using the feedback information that is output by the wireless communication apparatus  50  of the mobile station based on the receiving condition. As compensation of phase variation can be performed simply without any specific calibration circuit, the search cycle of an entire code book can be increased, and the partial search time can be relatively increased. Furthermore, simplification of the configuration and reduction of the processing load to the mobile station can be realized. 
     Although each embodiment above is explained with the example of an SIMO configuration that receives data input by a single user as a single stream, performs beamforming to transmit the data in multiple beams. The embodiments can also be applied to an MIMO configuration that receives multi-stream data input by multiple users, and transmits the data by multiple beams. 
     The method related to the wireless communication explained in the present embodiments can be realized by executing a prepared program on a computer, such as a personal computer or work station. The program is recorded on computer-readable recording medium such as a hard disk, flexible disk, compact disk read-only memory (CD-ROM), magneto optical disk (MO), or digital versatile disk (DVD), and is read and executed by a computer. The program can be a transmittable medium that can be distributed via networks, such as the internet. 
     The wireless communication apparatus according to the embodiments effects improvement of gain on the mobile station side and expected multi-beam directivity by compensating phase variation with a simple configuration to merely control the reading of optimum code book on the base station side without any specific calibrator. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.