Abstract:
A calibration apparatus and method for supporting a Relay Station (RS) in a multiple antenna communication system are provided. The calibration method in the multiple antenna communication system which supports the RS includes, when a modem stage sends a downlink calibration signal, receiving, at a calibrator, the downlink calibration signal and estimating a downlink path channel, when the calibrator sends an uplink calibration signal, receiving, at the modem stage, the uplink calibration signal and estimating an uplink path channel, determining, at the modem stage, calibration coefficients per path not to generate a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel, and applying the calibration coefficients per path to corresponding antennas respectively.

Description:
PRIORITY 
       [0001]    This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Sep. 25, 2008 and assigned Serial No. 10-2008-0094031, the entire disclosure of which is hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a multiple antenna system. More particularly, the present invention relates to a method and an apparatus for calibration in a multiple antenna system that supports a Relay Station (RS). 
         [0004]    2. Description of the Related Art 
         [0005]    System performance and capacity of mobile communication systems are limited by interference signals of the same channel between cells or within a cell, and radio channel characteristics such as multipath fading and the Doppler effect. To address the performance degradation factors, one technique for expanding the system capacity is a beamforming technique. 
         [0006]    The beamforming technique directs a radio signal toward each terminal by multiplying downlink traffic transmitted to each terminal by a particular coefficient. To apply the beamforming technique to a Base Station (BS), it is necessary to calibrate a phase and amplitude difference caused by the characteristic difference of Radio Frequency (RF) elements of the downlink and the uplink of the BS. 
         [0007]    A conventional calibration method relatively calibrates the phase and amplitude difference of the uplink per path. Likewise, the conventional calibration method relatively calibrates the phase and amplitude difference of the downlink per path. That is, the phase and the amplitude are calibrated equally per downlink and uplink based on Equation (1). Herein, four transmit antennas and four receive antennas are under consideration. 
         [0000]    
       
      
       H 
       d1 
       *W 
       d1 
       =H 
       d2 
       *W 
       d2 
       =H 
       d3 
       *W 
       d3 
       =H 
       d4 
       *W 
       d4  
      
     
         [0000]        H   u1   *W   u1   =H   u2   *W   u2   =H   u3 *W u3   =H   u4 *W u4    (1) 
         [0008]    In Equation (1), the channel of the downlink path  1  is H d1 , the channel of the uplink path  1  is H u1 , the channel of the downlink path  2  is H d2 , the channel of the uplink path  2  is H u2 , the channel of the downlink path  3  is H d3 , the channel of the uplink path  3  is H u3  the channel of the downlink path  4  is H d4 , and the channel of the uplink path  4  is H u4 . W dx  denotes a beamforming calibration coefficient for the downlink path x and W ux  denotes a calibration coefficient for the uplink path x. 
         [0009]    The conventional method determines the calibration coefficients W d1 , W d2 , W d3 , W d4 , W u1 , W u2 , W u3  and W u4  to establish Equation (1). Moreover, the conventional method establishes the relationship of H d1 /H u1 =H d2 /H u2 =H d3 /H u3 =H d4 /H u4 =α (an unknown complex value including both the phase and the amplitude). 
         [0010]    When the same unknown phase and amplitude difference α between the downlink and the uplink exists in each path, the beamforming coefficient is expressed by Equation (2). 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       w 
                       t 
                     
                     = 
                     
                       
                         α 
                          
                         
                             
                         
                          
                         
                           h 
                           t 
                           * 
                         
                       
                       
                         
                           
                             
                                
                               
                                 h 
                                 1 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 h 
                                 2 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 h 
                                 3 
                               
                                
                             
                             2 
                           
                           + 
                           
                             
                                
                               
                                 h 
                                 4 
                               
                                
                             
                             2 
                           
                         
                       
                     
                   
                   , 
                   
                     t 
                     = 
                     1 
                   
                   , 
                   2 
                   , 
                   3 
                   , 
                   4 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0011]    In Equation (2), w t  denotes the t-th beamforming coefficient and h t  denotes the t-th channel matrix, h t * denotes the conjugate of h t . 
         [0012]    The receive signal beamformed based on Equation (2) is given by Equation (3). 
         [0000]        r =α√{square root over (|h 1 | 2   +|h   2 | 2   +|h   3 | 2   +|h   4 | 2 )}·s+n   (3) 
         [0013]    In Equation (3), α denotes a phase difference, s denotes a transmit vector, h t  denotes the t-th channel matrix, and n denotes a noise vector. 
         [0014]    To maximize a Signal to Noise Ratio (SNR) of the receive signal, a receiver of the terminal calibrates the phase of the receive signal. Accordingly, the value corresponding to the phase difference disappears from the value α, and merely the amplitude difference remains. In an ideal case, the SNR difference corresponds to the square of the amplitude difference and does not affect the beamforming performance. 
         [0015]    The above-mentioned calibration method does not incur any problem in a BS that does not use a Relay Station (RS), but causes a problem in a system supporting an RS. The problem that occurs in a conventional system supporting an RS is now explained by referring to  FIGS. 1A and 1B . 
         [0016]    In  FIG. 1A , when a Mobile Station (MS)  130  is far away from a BS  100 , that is, when an RS  110  is propagationally isolated from the BS  100 , such as being underground, on an “island”, or in a shadow area, the beams  150  and  155  are formed in accordance with the sounding channel although the sounding is incoming via the RS  110 . The beam corresponding to the interval between the RS  110  and the MS  130  is formed to the RS  110  and the BS  100  at the same time. However, since one radio resource is allocated to only one MS  130  in the region covering the BS  100  and the RS  110 , there is no problem in the beamforming. 
         [0017]    In  FIG. 1B , when the MS  130 , which is located between the BS  100  and the RS  110 , simultaneously transmits sounding signals to the BS  100  and the RS  110 , both of the uplink sounding signals  135  and  140  transmitted from the MS  130  are received at the BS  100  and the RS  110  and their combined signal is input to the BS  100 . When a radio channel from the MS  130  to the BS  100  is h and a radio channel from the MS  130  to the RS  110  is h′, the beamforming coefficient may be determined at the BS  100  using the current algorithm based on Equation (4). 
         [0000]    
       
         
           
             
               
                 
                   
                     w 
                     t 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             α 
                              
                             
                                 
                             
                              
                             
                               h 
                               t 
                             
                           
                           + 
                           
                             βh 
                             t 
                             ′ 
                           
                         
                         ) 
                       
                       * 
                     
                     
                       
                         
                           
                              
                             
                               
                                 h 
                                 1 
                               
                               + 
                               
                                 h 
                                 1 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 2 
                               
                               + 
                               
                                 h 
                                 2 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 3 
                               
                               + 
                               
                                 h 
                                 3 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 4 
                               
                               + 
                               
                                 h 
                                 4 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0018]    The signal received at the MS  130  is given by Equation (5). 
         [0000]    
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                               
                           
                            
                           
                             { 
                             
                               
                                 
                                   
                                      
                                     
                                       h 
                                       t 
                                     
                                      
                                   
                                   2 
                                 
                                  
                                 
                                   α 
                                   * 
                                 
                               
                               + 
                               
                                 
                                   
                                      
                                     
                                       h 
                                       t 
                                       ′ 
                                     
                                      
                                   
                                   2 
                                 
                                  
                                 
                                   β 
                                   * 
                                 
                               
                               + 
                               
                                 
                                   h 
                                   t 
                                   * 
                                 
                                  
                                 
                                   h 
                                   t 
                                   ′ 
                                 
                                  
                                 
                                   α 
                                   * 
                                 
                               
                               + 
                               
                                 
                                   h 
                                   t 
                                 
                                  
                                 
                                   h 
                                   t 
                                   
                                     ′ 
                                     * 
                                   
                                 
                                  
                                 
                                   β 
                                   * 
                                 
                               
                             
                             } 
                           
                         
                         
                           
                             
                               
                                  
                                 
                                   
                                     h 
                                     1 
                                   
                                   + 
                                   
                                     h 
                                     1 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     2 
                                   
                                   + 
                                   
                                     h 
                                     2 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     3 
                                   
                                   + 
                                   
                                     h 
                                     3 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     4 
                                   
                                   + 
                                   
                                     h 
                                     4 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                           
                         
                       
                       · 
                       s 
                     
                     + 
                     n 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0019]    In Equation (5), α denotes the phase and amplitude difference of the downlink and the uplink at the BS, β denotes the phase and amplitude difference of the downlink and the uplink at the RS, h t  denotes the t-th channel matrix, s denotes a transmit vector, and n denotes a noise vector. 
         [0020]    In Equation (4) and Equation (5), when the MS between the RS and the BS transmits the sounding signals, the SNR differs depending on α and β. In other words, the beamforming performance is influenced by α and β. 
         [0021]    As discussed above, since the phase and amplitude difference α of the downlink and the uplink of the BS  100  may differ from the phase and amplitude difference β of the downlink and the uplink of the RS  110 , the multiple antenna system including the RS is subject to beamforming performance degradation. 
       SUMMARY OF THE INVENTION 
       [0022]    An aspect of the present invention is to address at least the above mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for calibration to support a Relay Station (RS) in a multiple antenna communication system. 
         [0023]    Another aspect of the present invention is to provide an apparatus and a method for beamforming in a multiple antenna communication system supporting an RS. 
         [0024]    According to one aspect of the present invention, a calibration method in a multiple antenna communication system which supports an RS is provided. The method includes, when a modem stage sends a downlink calibration signal, receiving, at a calibrator, the downlink calibration signal and estimating a downlink path channel, when the calibrator sends an uplink calibration signal, receiving, at the modem stage, the uplink calibration signal and estimating an uplink path channel, determining, at the modem stage, calibration coefficients per path not to generate a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel, and applying the calibration coefficients per path to corresponding antennas respectively. 
         [0025]    According to another aspect of the present invention, a calibration method in a multiple antenna communication system which supports an RS is provided. The method includes estimating, at a modem stage, a downlink path channel by transmitting and receiving a downlink calibration signal, switching, at a calibration controller, so that the modem stage transmits and receives one of an uplink calibration signal and the downlink calibration signal, estimating, at the modem stage, an uplink path channel by transmitting and receiving the uplink calibration signal, determining, at the modem stage, calibration coefficients per path not to generate a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel, and applying the calibration coefficients per path to corresponding antennas respectively. 
         [0026]    According to yet another aspect of the present invention, a calibration method in a multiple antenna communication system which supports an RS is provided. The method includes, when a modem stage sends a downlink calibration signal, receiving, at a calibrator, the downlink calibration signal and estimating a downlink path channel, when the calibrator sends an uplink calibration signal, receiving, at the modem stage, the uplink calibration signal and estimating an uplink path channel, determining, at the modem stage, calibration coefficients per path to make a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel be a first value, and applying the calibration coefficients per path to corresponding antennas respectively. 
         [0027]    According to still another aspect of the present invention, a calibration method in a multiple antenna communication system which supports an RS is provided. The method includes estimating, at a modem stage, a downlink path channel by transmitting and receiving a downlink calibration signal, switching, at a calibration controller, so that the modem stage transmits and receives one of an uplink calibration signal and the downlink calibration signal, estimating, at the modem stage, an uplink path channel by transmitting and receiving the uplink calibration signal, determining, at the modem stage, calibration coefficients per path to make a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel be a first value, and applying the calibration coefficients per path to corresponding antennas respectively. 
         [0028]    According to a further aspect of the present invention, a calibration apparatus in a multiple antenna communication system which supports an RS is provided. The apparatus includes a modem stage for, when a downlink calibration signal is transmitted, receiving the downlink calibration signal from a calibrator and estimating a downlink path channel, and the calibrator for, when an uplink calibration signal is transmitted, receiving the uplink calibration signal from the modem stage and estimating an uplink path channel. In an exemplary implementation, the modem stage may determine calibration coefficients per path not to generate a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel, and apply the calibration coefficients per path to corresponding antennas respectively. 
         [0029]    According to a further aspect of the present invention, a calibration apparatus in a multiple antenna communication system which supports a RS is provided. The apparatus includes a modem stage for estimating a downlink path channel by transmitting and receiving a downlink calibration signal, and a calibration controller for switching so that the modem stage transmits and receives one of an uplink calibration signal and the downlink calibration signal. In an exemplary implementation, the modem stage may estimate an uplink path channel by transmitting and receiving the uplink calibration signal, determine calibration coefficients per path not to generate a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel, and the apply the calibration coefficients per path to corresponding antennas respectively. 
         [0030]    According to a further aspect of the present invention, a calibration apparatus in a multiple antenna communication system which supports an RS is provided. The apparatus includes a modem stage for, when a downlink calibration signal is transmitted, receiving the downlink calibration signal from a calibrator and estimating a downlink path channel, and the calibrator for, when an uplink calibration signal is transmitted, receiving the uplink calibration signal from the modem stage and estimating an uplink path channel. In an exemplary implementation, the modem stage may determine calibration coefficients per path to make a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel be a first value, and apply the calibration coefficients per path to corresponding antennas respectively. 
         [0031]    According to a further aspect of the present invention, a calibration apparatus in a multiple antenna communication system which supports an RS is provided. The apparatus includes a modem stage for estimating a downlink path channel by transmitting and receiving a downlink calibration signal, and a calibration controller for switching so that the modem stage transmits and receives one of an uplink calibration signal and the downlink calibration signal. In an exemplary implementation, the modem stage may estimate an uplink path channel by transmitting and receiving the uplink calibration signal, determine calibration coefficients per path to make a phase and amplitude difference with respect to the estimated uplink path channel and the estimated downlink path channel be a first value, and apply the calibration coefficients per path to corresponding antennas respectively. 
         [0032]    Other aspects, advantages, and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    The above and other aspects, features and advantages of certain exemplary embodiments the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings, in which: 
           [0034]      FIGS. 1A and 1B  illustrate a diagram of a beamforming technique in a conventional multiple antenna system supporting a Relay Station (RS); 
           [0035]      FIG. 2  illustrates a block diagram of a Base Station (BS) in a multiple antenna system supporting an RS according to an exemplary embodiment of the present invention; and 
           [0036]      FIG. 3  illustrates a flowchart of calibration operations of a BS in a multiple antenna system supporting an RS according to an exemplary embodiment of the present invention. 
       
    
    
       [0037]    Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements, features and structures. 
       DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0038]    The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
         [0039]    Exemplary embodiments of the present invention provide a method and an apparatus for calibration in a multiple antenna system based on a Relay Station (RS). 
         [0040]      FIG. 2  illustrates a block diagram of a Base Station (BS) in a multiple antenna system supporting an RS according to an exemplary embodiment of the present invention. 
         [0041]    The BS of  FIG. 2  includes a modem stage  200 , a transceiver  210 , a Radio Frequency (RF) distributer  220 , a calibrator  230 , and a calibration controller  240 . 
         [0042]    The modem stage  200  converts time-domain signals output from the transceiver  210  to frequency-domain signals using Fast Fourier Transform (FFT), selects data of subcarriers to actually receive from the frequency-domain data, and demodulates and decodes the selected data at a preset modulation level (i.e., Modulation and Coding Scheme (MCS) level). The modem stage  200  encodes and modulates a transmit signal at the preset modulation level (the MCS level) and converts the frequency-domain signal to a time-domain sample signal using Inverse FFT (IFFT). In addition, the modem stage  200  receives a calibration reference signal of the uplink from the transceiver  210  and estimates the uplink path channel. The modem stage  200  determines the calibration coefficients for the same value of the phase and amplitude difference of the uplink and the downlink using the downlink path channel estimate value provided from the calibrator  230  and its estimated uplink path channel estimate value (see Equation (6)). That is, the modem stage  200  equally calibrates the phase and amplitude difference per path of the downlink and the uplink. 
         [0000]        H   d1   *W   d1   =H   d2   *W   d2   =H   d3   *W   d3   =H   d4   *W   d4   =H   u1   *W   u1   =H   u2   *W   u2   =H   u3   *W   u3   =H   u4 *W u4    (6) 
         [0043]    In Equation (6), the downlink path channel of the path  1  between the antenna  1  and the RF distributor  220  is H d1 , the channel of the uplink path  1  is H u1 , the channel of the downlink path  2  between the antenna  2  and the RF distributor  220  is H d2 , the channel of the uplink path  2  is H u2 , the channel of the downlink path  3  between the antenna  3  and the RF distributor  220  is H d3 , the channel of the uplink path  3  is H u3 , the channel of the downlink path  4  between the antenna  4  and the RF distributor  220  is H d4 , and the channel of the uplink path  4  is H u4 . 
         [0044]    Thus, the calibration coefficients W d1 , W d2 , W d3 , W d4 , W u1 , W u2 , W u3  and W u4  satisfying Equation (6) are acquired. 
         [0045]    When the phase and amplitude difference in every downlink and uplink is equally calibrated as expressed in Equation (6), the beamforming coefficient is given by Equation (7). 
         [0000]    
       
         
           
             
               
                 
                   
                     w 
                     t 
                   
                   = 
                   
                     
                       
                         ( 
                         
                           
                             h 
                             t 
                           
                           + 
                           
                             h 
                             t 
                             ′ 
                           
                         
                         ) 
                       
                       * 
                     
                     
                       
                         
                           
                              
                             
                               
                                 h 
                                 1 
                               
                               + 
                               
                                 h 
                                 1 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 2 
                               
                               + 
                               
                                 h 
                                 2 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 3 
                               
                               + 
                               
                                 h 
                                 3 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                         + 
                         
                           
                              
                             
                               
                                 h 
                                 4 
                               
                               + 
                               
                                 h 
                                 4 
                                 ′ 
                               
                             
                              
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0046]    The receive signal is given by Equation (8). 
         [0000]    
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                               
                           
                            
                           
                             
                                
                               
                                 
                                   h 
                                   t 
                                 
                                 + 
                                 
                                   h 
                                   t 
                                   ′ 
                                 
                               
                                
                             
                             2 
                           
                         
                         
                           
                             
                               
                                  
                                 
                                   
                                     h 
                                     1 
                                   
                                   + 
                                   
                                     h 
                                     1 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     2 
                                   
                                   + 
                                   
                                     h 
                                     2 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     3 
                                   
                                   + 
                                   
                                     h 
                                     3 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                             + 
                             
                               
                                  
                                 
                                   
                                     h 
                                     4 
                                   
                                   + 
                                   
                                     h 
                                     4 
                                     ′ 
                                   
                                 
                                  
                               
                               2 
                             
                           
                         
                       
                       · 
                       s 
                     
                     + 
                     n 
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0047]    In Equation (8), α denotes the amplitude and phase difference of the downlink and the uplink at the BS, β denotes the amplitude and phase difference of the downlink and the uplink at the RS, h t  denotes the t-th channel matrix between the t-th BS and the MS, h t ′ denotes the t-th channel matrix between the BS and the RS, s denotes a transmit vector, and n denotes a noise vector. 
         [0048]    In reception, the transceiver  210  down-converts the RF signal received over the antennas to a baseband analog signal, converts the analog signal to a digital signal, and outputs the digital signal to the modem stage  200 . In transmission, the transceiver  210  converts the sample signal to an analog signal, up-converts the baseband signal to an RF signal, and transmits the RF signal via the antennas. For transmission or reception by the transceiver  210 , the BS operates in a Time Division Duplex (TDD) manner. For example, in transmission, switches  214 - 1  through  214 - 5  are switched to transmission blocks  211 - 1  through  211 - 4  to propagate data and control signals received from the modem stage  200  via the antennas. In reception, the switches  214 - 1  through  214 - 5  are switched to reception blocks  213 - 1  through  213 - 4  to output the receive signals to the modem stage  200 . 
         [0049]    In various implementations, the transmission blocks  211  and the reception blocks  213  may operate in a Frequency Division Duplex (FDD) manner. 
         [0050]    The RF distributor  220  is interfaced with the multiple antennas. The RF distributor  220  receives the calibration reference signal of the downlink transmitted via the transceiver  210 , distributes the receive signals per antenna, and sends the distributed signals to the calibrator  230 . The RF distributor  220  outputs the calibration reference signal of the uplink to the transceiver  210 . 
         [0051]    The calibrator  230  estimates the downlink path channel by receiving the calibration reference signal of the downlink per antenna from the RF distributor  220  and outputs the estimated channel to the modem stage  200 . Under the control of the calibration controller  240 , the calibrator  230  outputs the calibration reference signal of the uplink to the RF distributor  220 . 
         [0052]    The calibration controller  240  determines the channel estimation time of the uplink and the channel estimation time of the downlink and controls the calibrator  230  and the transceiver  210 . For example, in the uplink path channel estimation, the calibration controller  240  controls the calibrator  230  to output the calibration reference signal of the uplink and the transceiver  210  to receive the calibration reference signal of the uplink. In the downlink path channel estimation, the calibration controller  240  controls the transceiver  210  to output the calibration reference signal of the downlink output from the modem stage  200  over the antennas and controls the calibrator  230  to receive the calibration reference signal of the downlink. 
         [0053]    In various exemplary embodiments, the modem stage  200  may function as the calibrator  230 . For doing so, the calibration controller  240  can control the modem stage  200  to transmit and receive the downlink calibration signal and the uplink calibration signal respectively. 
         [0054]    The present exemplary algorithm is applicable to a case where the additional calibrator  230  is not provided. In that case, the modem stage  200  executes the functions of the calibrator  230  through some switches. 
         [0055]      FIG. 3  is a flowchart of a calibration method in a multiple antenna system supporting an RS according to an exemplary embodiment of the present invention. 
         [0056]    In step  300 , the modem stage  200  sequentially transmits the calibration reference signals per path to calibrate the downlink. 
         [0057]    In step  302 , the calibrator  230  receives the calibration reference signals of the downlink and determines the channel estimate values per downlink path. 
         [0058]    In step  304 , the calibrator  230  sends the determined channel estimate values of the downlink to the modem stage  200 . 
         [0059]    In step  306 , the modem stage  200  stores the channel estimate values of the downlink received from the calibrator  230 . 
         [0060]    In step  308 , the calibrator  230  sends the calibration reference signal to the modem stage  200  to calibrate the uplink. 
         [0061]    In step  310 , the modem stage  200  receives the calibration reference signal of the uplink and extracts the channel estimate values per uplink path. 
         [0062]    In step  312 , the modem stage  200  determines the calibration coefficients to make the phase and amplitude differences per path of the downlink and the uplink the same based on the channel estimate values of the downlink and the channel estimate values of the uplink (see Equation (6)). 
         [0063]    In various exemplary embodiments, the calibration coefficient for the uplink path may be first determined and then the calibration coefficient for the downlink path may be determined by referring to the calibration coefficient for the uplink path. Alternatively, the calibration coefficient for the downlink path may be first determined and then the calibration coefficient for the uplink path may be determined by referring to the calibration coefficient for the downlink path. 
         [0064]    Next, the present process is finished. 
         [0065]    While the links are calibrated by estimating the downlink path channel and then the uplink path channel in  FIG. 3 , the links may be calibrated by estimating the uplink path channel and then the downlink path channel. The calibration order can be determined by the calibration controller  240 . 
         [0066]    As set forth above, in a multiple antenna system supporting an RS, the channels of the downlink and the uplink are estimated in order and the calibration coefficients are computed by comparing the estimated downlink path channel value and the estimated uplink path channel value. Therefore, the beamforming performance can be enhanced. 
         [0067]    While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.