Abstract:
A method for receiving a signal in a wireless communication system based on Multiple-Input Multiple Output (MIMO)—Orthogonal Frequency Division Multiplexing (OFDM) through multiple antennas. In the system, weight for a zero forcing based equalization matrix is determined by inverting a channel transfer matrix for each subcarrier of the signals. The original data transmitted is recovered by combining data subcarriers from different receive antennas using the calculated weights thus reducing the system complexity.

Description:
BACKGROUND 
       [0001]    The claimed embodiment relates to a wireless communication system, and more particularly to detecting and decoding a signal in an Orthogonal Frequency Division Multiplexing (OFDM) Multiple Input Multiple Output (MIMO) communication system. 
         [0002]    A MIMO communication system transmits and receives data using multiple transmit antennas and multiple receive antennas. A MIMO channel, formed by transmit and receive antennas, is divided into a plurality of independent spatial subchannels. Because the MIMO system employs multiple antennas, it outperforms a single antenna system by having greater channel capacity. 
         [0003]    Multiple antenna techniques also allow spatial multiplexing of data streams originating from one or multiple users. MIMO detection schemes used in the latest recommendations from standards organization use equalizers having weight matrices for every subcarrier and multiplication of the OFDM subcarrier received signals from different antennas to get estimates of the transmitted data symbols. The performance and complexity of such systems will depend on the number of antennas used. As more antennas are used, the greater the complexity of the system. Greater complexity may typically require additional computational blocks. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0004]    Additional objects and features as defined by the claims will be more readily apparent from the following detailed description and appended claims when taken in conjunction with the drawings, in which: 
           [0005]      FIG. 1  depicts a simplified block diagram of a receiver of the OFDM system to which a signal detection and decoding system of the claimed embodiment is applied. 
           [0006]      FIG. 2  is a component diagram of the equalizer used in the detection and decoding an OFDM MIMO system. 
           [0007]      FIG. 3  is a flow diagram of the method for detecting and decoding signals in an OFDM MIMO system. 
           [0008]      FIG. 4  is an algorithm used by the equalizer in the detection and decoding system. 
       
    
    
     DESCRIPTION 
       [0009]      FIG. 1  depicts a structure of a receiver  100  of a MIMO OFDM system to which signal detection and decoding is applied. In  FIG. 1 , receiver  100  includes a plurality of antennas  102  (A-D) respectively coupled via Analog Front-End (AFE)  104 (A-D) and Fast Fourier Transform (FFT) blocks  106  (A-D) to an input of MIMO equalizer circuit  108  and demodulator circuit  110 . MIMO equalizer circuit  108  has an output coupled to demodulator circuit  110 . The output of demodulation circuit  110  is coupled to decoder circuit  112  which generates data bits for a computer or any processing device (not shown) for further handling. 
         [0010]    During operation packetized analog signals from antennas are transformed to a baseband signal on AFE (Analog Front-End) blocks  102  (A-D). The resulting baseband signal is transformed into different sub-carriers as a frequency domain representation using FFT blocks  106  (A-D) and is fed to MIMO equalizer  108  and to Demodulator block  110 . Equalizer  108  performs an estimation of channel coefficients (out of scope), forms an equalization matrix and sends matrix information to demodulator  110  which equalizes the transformed signal using an equalization matrix. In one embodiment the equalization matrix may be sent during the reception of a special block in the received packet. Using this block it may be possible to perform an estimation/calculation of channel coefficients (e.g. channel matrix H). In another embodiment the equalization matrix may be a zero forcing based equalization matrix. The equalization matrix may be determined by determining a weight by inverting at least one channel transfer matrix for each subcarrier of the signals. Data may be recovered by combining data subcarriers from different receive antennas using the determined weights. Further details of the equalization matrix are described in  FIGS. 3 and 4 . 
         [0011]    The output of the equalization matrix  108  is feed to the demodulator  110  which restores the original modulated signals and transforms them into a series of soft bits that are transferred to decoder  112 . Decoder  112  parses and decodes the soft bits to obtain the original transmitted data from the symbols. The output of decoder  112  may then be transmitted to a computer device (not shown) for further processing. 
         [0012]      FIG. 2  further depicts an embodiment of an illustrative implementation of certain components of FFT blocks  106 , in which MIMO equalization circuit  200  may be used to equalize the transformed signal with demodulator circuit  110  and decoder circuit  112 . The equalization circuit  200  may have process capabilities and memory suitable to store and execute computer-executable instructions. In this embodiment, the equalization circuit  200  includes one or more processors  202  and memory  204 . The memory  204  may include volatile and nonvolatile memory, removable and non-removable media implemented in any method or technology for storage of information, such as computer-readable instructions, data structures, program modules or other data. Such memory includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, RAID storage systems, or any other medium which can be used to store the desired information and which can be accessed by a computer system. 
         [0013]    Stored in memory  204  are modules  208 - 212 . Modules  208 - 212  are implemented as software or computer-executable instructions that are executed by one or more processors  202 . Receiver module  208  receives the signal from antennas  102  and generates an FFT transformed signal. 
         [0014]    Detection module  210  detects one or more symbols from the received signal using an equalization matrix. The detection module  210  may determine the equalization matrix by inverting at least one channel transfer matrix for each subcarrier of the signals. At least one channel transfer matrix may be an N×N Matrix H. Matrix H may be defined by the following equation (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                     = 
                     
                       ( 
                       
                         
                           
                             A 
                           
                           
                             B 
                           
                         
                         
                           
                             C 
                           
                           
                             D 
                           
                         
                       
                       ) 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     A 
                     ∈ 
                     
                       C 
                       KXK 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     B 
                     ∈ 
                     
                       
                         C 
                         KXM 
                       
                        
                       C 
                     
                     ∈ 
                     
                       C 
                       MXK 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     D 
                     ∈ 
                     
                       
                         C 
                         MXM 
                       
                        
                       m 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     
                       K 
                       + 
                       M 
                     
                     = 
                     n 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0015]    Matrix H may be inverted by multiplying S 1 ×S 2 ×S 3 . S 1 −S 3  may be defined by the following equations (2)-(4) respectively: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       S 
                       1 
                     
                     = 
                     
                       ( 
                       
                         
                           
                             I 
                           
                           
                             0 
                           
                         
                         
                           
                             X 
                           
                           
                             
                               α 
                                
                               
                                   
                               
                                
                               I 
                             
                           
                         
                       
                       ) 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     X 
                     ∈ 
                     
                       C 
                       MXK 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       S 
                       2 
                     
                     = 
                     
                       ( 
                       
                         
                           
                             
                               β 
                                
                               
                                   
                               
                                
                               I 
                             
                           
                           
                             Y 
                           
                         
                         
                           
                             0 
                           
                           
                             I 
                           
                         
                       
                       ) 
                     
                   
                   , 
                   
                     
 
                   
                    
                   
                     Y 
                     ∈ 
                     
                       C 
                       KXM 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
             
               
                 
                   
                     S 
                     3 
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             adj 
                              
                             
                               ( 
                               A 
                               ) 
                             
                           
                         
                         
                           0 
                         
                       
                       
                         
                           0 
                         
                         
                           
                             adj 
                              
                             
                               ( 
                               E 
                               ) 
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0016]    For equations (2)-(4), I is an identity Matrix, 0 is a zero Matrix, and X=−Cadj(A), E=αD+XB, Y=−Badj(E). The term “adj” determines an adjoint matrix, which may be calculated via the algebraic cofactors of matrix H. A Matrix V=S 1 S 2 S 3  inverts Matrix H. Two scaling gains may need to be computed for the output of equalization matrix by using equation (5), where “det” is a determinate function, to avoid a division operation in demodulator block  110 . 
         [0000]      α= det ( A ), β= det ( E )   (5) 
         [0017]    Recovery module  212  may recover the original data transmitted using the symbols and by combining data subcarriers from different receive antennas using weights calculated with the equalization matrix. The weights for each subcarrier may be calculated with the recovery module  212  by inverting the channel transfer matrix for each subcarrier. 
         [0018]      FIG. 3  depicts a process for the detecting and decoding signals in an OFDM MIMO system using an equalization matrix. The process is depicted as a collection of blocks in a logical flow diagram, which represents a sequence of operations that can be implemented in hardware, software, and a combination thereof. In the context of software, the blocks represent computer-executable instructions that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the process. For discussion purposes, the process is described with reference to the system  100  of  FIG. 1 , although it may be implemented in other system architectures. 
         [0019]    In process  300  a signal is received from the antennas  102  using receiver module  508  in block  302 . Receiver module  508  receives the signal from antennas  102  and generates an FFT transformed signal. In block  304  an equalization matrix is determined using detection module  210 . A gain is also determined from the channel matrix of received signal in block  306 . Further details of determining the equalization matrix and determining gain are described in  FIG. 4 . In block  308 , the symbols are detected from the FFT transformed signal and the determined equalization matrix and gain using detection module  210 . The original data is recovered from the symbols with recovery module  212  in block  310 . 
         [0020]      FIG. 4  depicts an illustrative algorithm  400  for determining an output matrix 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       C 
                       2 
                     
                   
                   
                     
                       D 
                       2 
                     
                   
                 
                 
                   
                     
                       K 
                       2 
                     
                   
                   
                     
                       F 
                       2 
                     
                   
                 
               
               ) 
             
               
           
         
       
     
         [0000]    shown in block  430  with gains of α, and β using exemplary channel transfer matrix H H . A Hermitian matrix G may be formed from H by the following: G=H H H or G=H H H+N where N is a noise covariance matrix calculated during the channel estimation time interval (block  402 ) provided to detection module  210 . Gains α, and β may be required for scaling of constellation points fed to the demodulator  110 . These gains may be required for scaling of constellation points on the demodulator side if the processing device does not have dividing unit. In this exemplary embodiment, matrix G is assigned matrix 
         [0000]    
       
         
           
             G 
             = 
             
               ( 
               
                 
                   
                     A 
                   
                   
                     B 
                   
                 
                 
                   
                     
                       B 
                       H 
                     
                   
                   
                     D 
                   
                 
               
               ) 
             
           
         
       
     
         [0000]    in block  404  and channel transfer matrix H H  is assigned matrix 
         [0000]    
       
         
           
             
               H 
               H 
             
             = 
             
               ( 
               
                 
                   
                     C 
                   
                   
                     L 
                   
                 
                 
                   
                     K 
                   
                   
                     F 
                   
                 
               
               ) 
             
           
         
       
     
         [0000]    in block  406 . In block  408 , alpha (A) is assigned using equation (6). 
         [0000]      α= a   11   a   22   −a   12   a   21    (6) 
         [0021]    The congregate of A, X, E, the congregate of E and Y are determined by equations (7)-(11) in blocks  410 - 418  respectively. 
         [0000]    
       
         
           
             
               
                 
                   
                     adj 
                      
                     
                       ( 
                       A 
                       ) 
                     
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             a 
                             22 
                           
                         
                         
                           
                             - 
                             
                               a 
                               12 
                             
                           
                         
                       
                       
                         
                           
                             - 
                             
                               a 
                               21 
                             
                           
                         
                         
                           
                             a 
                             11 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
             
               
                 
                   X 
                   = 
                   
                     
                       - 
                       
                         B 
                         H 
                       
                     
                      
                     
                       adj 
                        
                       
                         ( 
                         A 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
             
               
                 
                   E 
                   = 
                   
                     
                       α 
                        
                       
                           
                       
                        
                       D 
                     
                     + 
                     XB 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     adj 
                      
                     
                       ( 
                       E 
                       ) 
                     
                   
                   = 
                   
                     ( 
                     
                       
                         
                           
                             e 
                             22 
                           
                         
                         
                           
                             - 
                             
                               e 
                               12 
                             
                           
                         
                       
                       
                         
                           
                             - 
                             
                               e 
                               21 
                             
                           
                         
                         
                           
                             e 
                             11 
                           
                         
                       
                     
                     ) 
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   Y 
                   = 
                   
                     
                       - 
                       B 
                     
                      
                     
                         
                     
                      
                     
                       adj 
                        
                       
                         ( 
                         E 
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
         [0022]    From equation (8) and H H , K 1  and F 1  are determined in equation (12) in block  420  as follows: 
         [0000]        K   1   =XC+αK    (12) 
         [0000]    
       
      
       F 
       1 
       =XL+αF  
      
     
         [0023]    Using equation 12 and equation 10, K 2  and F 2  are determined in equations (13) in block  422 . 
         [0000]        K   2   =adj ( E ) K   1    (13) 
         [0000]        F   2   =adj ( E ) F   1    
         [0024]    C 1  and D 1  are derived from equations (11) and (13) using equations (14) in block  424 , and C 2  and D 2  are derived from equations (7) and (14) using equations 15 in block  426 . 
         [0000]        C   1   =βC+YK   1    (14) 
         [0000]    
       
      
       D 
       1 
       =βL+YF 
       1  
      
     
         [0000]        C   2   =adj ( A ) C   1    (15) 
         [0000]        D   2   =adj ( A ) D   1    
         [0025]    β or the gain of the equalization matrix is determined by equation (16) in block  428  to complete the matrices for the output 
         [0000]    
       
         
           
             
               ( 
               
                 
                   
                     
                       C 
                       2 
                     
                   
                   
                     
                       D 
                       2 
                     
                   
                 
                 
                   
                     
                       K 
                       2 
                     
                   
                   
                     
                       F 
                       2 
                     
                   
                 
               
               ) 
             
               
           
         
       
     
         [0000]    depicted in block  430 . 
         [0000]      β= e   11   e   22   −e   12   e   21    (16) 
         [0026]    In closing, although the invention has been described in language specific to structural features and/or methodological acts, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or acts described. Rather, the specific features and acts are disclosed as illustrative forms of implementing the claimed invention.