Abstract:
In general, in one aspect, the disclosure describes an apparatus that includes a sample averager to construct a preliminary estimate of a spatial covariance matrix from a received communications signal. A time-domain filter with Cholesky decomposition is used to decompose the preliminary estimate of the spatial covariance matrix into product of an upper triangular matrix and complex conjugate of the upper triangular matrix. The time-domain filter with Cholesky decomposition is also used to filter the upper triangular matrix and construct an updated estimate of the spatial covariance matrix using the filtered upper triangular matrix.

Description:
BACKGROUND 
       [0001]    The widespread deployment of local and wide-area wireless networks (wireless local area networks (WLAN) and wireless metropolitan area networks (WMAN)) allows users of mobile equipment to enjoy the benefits of wideband access to the Internet and other digital services. Ubiquitous provision of wireless networking requires the operation of a vast array of base stations and access points. The growth of these services, however, depends on the aggressive reuse of a limited number of frequency channels. For example, certain WMANs may support “frequency reuse one” where all cells operate on the same frequency channel. Co-channel interference (CCI), the inevitable result of such aggressive reuse, is becoming the dominant limit to the growth of these systems and services. 
         [0002]    Many modern wireless standards use Orthogonal Frequency Division Multiplexing (OFDM) as the radio communications method to improve multipath performance. These systems may also use multiple antennas on the transmitters and receivers, referred to as Multiple-Input Multiple-Output (MIMO), to cancel interference from adjacent cells. In such systems, if interfering signals are synchronized with the intended signal, interference can be treated independently on each sub-carrier, and therefore cancelled in a straight-forward manner. However, synchronizing the signals in multiple cells is not always possible. If interfering signals and intended signals are not synchronized (asynchronous CCI), a receiver may estimate the spatial covariance matrix of the interference on each sub-carrier for the purpose of cancellation. The accuracy of this estimation determines the performance of the cancellation process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The features and advantages of the various embodiments will become apparent from the following detailed description in which: 
           [0004]      FIG. 1  illustrates a functional block diagram of an example MIMO OFDM communication system, according to one embodiment; and 
           [0005]      FIG. 2  illustrates a functional block diagram of an example CCI estimator, according to one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]      FIG. 1  illustrates a functional block diagram of an example MIMO OFDM communication system  100 . The system includes a transmitter  110  and a receiver  150 . The transmitter  100  may be included in a first wireless communications device and the receiver  105  may be included in a second wireless communications device. Certain wireless communications devices may include the transmitter and one receiver to allow the device to both send and receive data. 
         [0007]    The transmitter  110  includes an encoder  115 , an interleaver  120 , a serial/parallel (S/P) converter  125 , a space time modulator  130 , one or more Inverse-Fast-Fourier-Transforms (IFFT) with cyclic prefix (CP) adder (IFFT+CP)  135 , and one or more antennas  140 . Data enters the transmitter  110  at the encoder  115 . The encoder  115  performs forward error correction (FEC) encoding on the data to help protect the signal from transmission errors. The encoded data enters the interleaver  120  where it is interleaved (to improve correction of burst errors). The data is then partitioned into blocks in the S/P converter  125 . The data blocks may be modulated in the space-time modulator  130  and then divided into M groups. An IFFT+CP  135  receives a group of modulated data blocks and converts them to a time-domain signal and may add a CP to the time-domain signal to mitigate the effects of multipath-related intersymbol interference. An antenna  140  receives the output of the IFFT+CP  135  and transmits the data therefrom. 
         [0008]    The receiver  150  includes one of more antennas  155 , one or more Fast-Fourier-Transforms (FFT) with CP remover (FFT-CP)  160 , a channel estimator  165 , a CCI estimator  170 , a MIMO detector  175 , a parallel/serial (P/S) converter  180 , a deinterleaver  185 , and a decoder  190 . Data (time domain signals) is received by the one or more antennas  155 . A FFT-CP  160  may remove the cyclic prefix from the received data and transform the time-domain signals into the frequency domain. The channel estimator  165  receives the frequency domain signals and estimates characteristics of transmission channel (channel transfer function) to equalize the received signals. The CCI estimator  170  also receives the frequency domain signals and estimates CCI. The MIMO detector  175  receives the equalized signals and the CCI estimate and cancels the CCI from the estimated signals and demodulates the resulting signals (received data blocks). The P/S converter  180  reconstructs the data blocks into an encoded serial data stream. The deinterleaver  185  deinterleaves the encoded serial stream and the decoder  190  provides error correction. 
         [0009]    In the general case, the narrowband and synchronous signal for each tone (sub-carrier) i received at the receiver  150  can be modeled as Y(i)=H(i)·s(i)+G(i)·x(i)+N(i), where i=1 . . . N FFT , Y(i) is the received signal vector at the i th  tone, s(i) is the transmitted signal, H(i) is the channel matrix (transfer function) of the transmitted signal, x(i) is the interfering signal, G(i) is the channel matrix of the interfering signal, and N(i) is an additive white Gaussian noise (AWGN) vector with variance σ 2  for each element. The goal of the receiver  150  is to estimate the transmitted signal from the received signal. 
         [0010]    For the case of asynchronous interference, the cyclic structure of each interfering OFDM signal is destroyed. In this case, we cannot distinguish the interfering signal from the noise, so we may lump the interference and noise into a single term I(i) to yield Y(i)=H(i)·s(i)+I(i). The goal in this case is to efficiently estimate the spatial covariance of I(i) for each tone (using the CCI estimator  170 ) and use this information to recover the transmitted signal. 
         [0011]    If a space-time block coding (STBC) transmission scheme is used, we can express the equivalent spatio-temporal signal model for a 2×2 case by stacking two receive vector samples (y 1  and y 2 ): 
         [0000]    
       
         
           
             
               [ 
               
                 
                   
                     
                       
                         y 
                         1 
                       
                        
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       
                         y 
                         2 
                       
                        
                       
                         ( 
                         1 
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       
                         y 
                         1 
                         * 
                       
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                 
                 
                   
                     
                       
                         y 
                         2 
                         * 
                       
                        
                       
                         ( 
                         2 
                         ) 
                       
                     
                   
                 
               
               ] 
             
             = 
             
               
                 
                   [ 
                   
                     
                       
                         
                           
                             h 
                             11 
                           
                            
                           
                             ( 
                             1 
                             ) 
                           
                         
                       
                       
                         
                           
                             h 
                             12 
                           
                            
                           
                             ( 
                             1 
                             ) 
                           
                         
                       
                     
                     
                       
                         
                           
                             h 
                             21 
                           
                            
                           
                             ( 
                             1 
                             ) 
                           
                         
                       
                       
                         
                           
                             h 
                             22 
                           
                            
                           
                             ( 
                             1 
                             ) 
                           
                         
                       
                     
                     
                       
                         
                           
                             h 
                             12 
                             * 
                           
                            
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                       
                         
                           - 
                           
                             
                               h 
                               11 
                               * 
                             
                              
                             
                               ( 
                               2 
                               ) 
                             
                           
                         
                       
                     
                     
                       
                         
                           
                             h 
                             22 
                             * 
                           
                            
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                       
                         
                           - 
                           
                             
                               h 
                               21 
                               * 
                             
                              
                             
                               ( 
                               2 
                               ) 
                             
                           
                         
                       
                     
                   
                   ] 
                 
                  
                 
                   [ 
                   
                     
                       
                         
                           s 
                           1 
                         
                       
                     
                     
                       
                         
                           s 
                           2 
                         
                       
                     
                   
                   ] 
                 
               
               + 
               
                 
                   [ 
                   
                     
                       
                         
                           I 
                           1 
                         
                       
                     
                     
                       
                         
                           I 
                           2 
                         
                       
                     
                     
                       
                         
                           I 
                           3 
                         
                       
                     
                     
                       
                         
                           I 
                           4 
                         
                       
                     
                   
                   ] 
                 
                 . 
               
             
           
         
       
     
         [0012]      FIG. 2  illustrates a block diagram of an example CCI estimator  200  (e.g.,  170  of  FIG. 1 ) utilized in an OFDM MIMO receiver (e.g.,  170 ). The CCI estimator  200  includes a sample averager  210  and a time-domain filter with Cholesky decomposition  230 . The CCI estimator  200  may also include a block diagonalizer  220  if STBC is used. 
         [0013]    The sample averager  210  receives frequency domain versions of the signals received by the receiver. The frequency domain signals may be received from an FFT-CP (e.g.,  160 ). The sample averager  210  measures I(i) for each tone. This may be implemented by adding “zero” samples (called zero-padding or training symbols) to various positions in a transmitted packet. The receiver, having knowledge of the position of these zero samples, may make measurements of the interfering signal during these periods. These measurements may be made over several OFDM symbols. Short duration measurement may be required in most applications. An initial spatial covariance matrix R ll  is computed for each tone i as: 
         [0000]    
       
         
           
             
               
                 
                   R 
                   II 
                 
                  
                 
                   ( 
                   i 
                   ) 
                 
               
               = 
               
                 
                   1 
                   K 
                 
                  
                 
                   
                     ∑ 
                     
                       k 
                       = 
                       0 
                     
                     
                       K 
                       - 
                       1 
                     
                   
                    
                   
                       
                   
                    
                   
                     
                       I 
                        
                       
                         ( 
                         i 
                         ) 
                       
                     
                     · 
                     
                       
                         I 
                          
                         
                           ( 
                           i 
                           ) 
                         
                       
                       H 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0014]    where K is the zero-padding or training OFDM symbol number. The structure of the spatial covariance matrix is Hermitian and positive definite. 
         [0015]    The block diagonalizer  220 , employed when STBC is used, may treat the interferences at two successive slots as independent to reduce the number of elements in the covariance matrix that need measuring so that R ll  will have the following block diagonal form: 
         [0000]    
       
         
           
             
               R 
               II 
             
             = 
             
               
                 [ 
                 
                   
                     
                       
                         R 
                         11 
                       
                     
                     
                       
                         R 
                         12 
                       
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       
                         R 
                         21 
                       
                     
                     
                       
                         R 
                         22 
                       
                     
                     
                       0 
                     
                     
                       0 
                     
                   
                   
                     
                       0 
                     
                     
                       0 
                     
                     
                       
                         R 
                         33 
                       
                     
                     
                       
                         R 
                         34 
                       
                     
                   
                   
                     
                       0 
                     
                     
                       0 
                     
                     
                       
                         R 
                         43 
                       
                     
                     
                       
                         R 
                         44 
                       
                     
                   
                 
                 ] 
               
               = 
               
                 
                   [ 
                   
                     
                       
                         
                           R 
                           
                             II 
                              
                             
                                 
                             
                              
                             2 
                             × 
                             2 
                           
                           
                             ( 
                             1 
                             ) 
                           
                         
                       
                       
                         0 
                       
                     
                     
                       
                         0 
                       
                       
                         
                           R 
                           
                             II 
                              
                             
                                 
                             
                              
                             2 
                             × 
                             2 
                           
                           
                             ( 
                             2 
                             ) 
                           
                         
                       
                     
                   
                   ] 
                 
                 . 
               
             
           
         
       
     
         [0016]    If STBC is not used, the interferences may skip the block diagonalizer  220  and be provided directly to the Time-Domain Filter with Cholesky Decomposition  230 . 
         [0017]    The Time-Domain Filter with Cholesky Decomposition  230  makes use of the fact that the spatial covariance matrix is Hermitian and positive definite. The well-known Cholesky Decomposition is used to decompose the matrix into an upper triangular matrix (sometimes referred to as the “square root” matrix) and the conjugate transpose of the same upper triangular matrix for tone i as R ll (i)=U(i) H U(i), where U(i) is an upper triangular matrix. 
         [0018]    The frequency domain power or mutual power spectral density R ll (i)[m,n] corresponds to the time domain auto-correlation or cross-correlation, where m and n are the row and column indices of matrix R ll (i), and used to identify the receiving antennas. Due to the limited channel delay taps and independently transmitted data, the interferences received at antenna m and antenna n are correlated only within a limited time interval. Accordingly, R ll (i)[m,n] needs to be filtered (low pass) in the time domain. 
         [0019]    A time-domain representation of power spectral density r ll [m,n] can be found by computing the Inverse Fast Fourier Transform (IFFT) of the frequency domain power spectral density R ll (i)[m,n], r ll [m,n]=IFFT(R ll [m,n]). The time-domain power spectral density r ll [m,n] may be low pass filtered by setting it equal to zero for time range outside of the maximum delay tap, r ll [m,n](t)=0, for |t|&gt;L, where L is the maximum delay tap of the multi-path channel. 
         [0020]    Since the filtering operation described above would destroy the positive definite structure of R ll (i), U(i) (the square-root of R ll (i), the upper triangular matrix) may be filtered and an updated estimate for R ll (i) may be constructed as {tilde over (R)} ll (i)=Ũ(i) H Ũ(i), where Ũ(i) is the filtered output. 
         [0021]    The filtering operation may also be realized by a weighting operation instead of the IFFT operation. For example, 
         [0000]    
       
         
           
             
               P 
               = 
               
                 FDF 
                 H 
               
             
             , 
             
               
 
             
              
             
               D 
               = 
               
                 diag 
                  
                 
                   ( 
                   
                     d 
                     t 
                   
                   ) 
                 
               
             
             , 
             
               
 
             
              
             
               
                 d 
                 t 
               
               = 
               
                 { 
                 
                   
                     
                       
                         
                           
                             1 
                             , 
                           
                         
                         
                           
                             
                               t 
                               ≤ 
                               L 
                             
                             , 
                             
                               t 
                               ≥ 
                               
                                 
                                   N 
                                   FFT 
                                 
                                 - 
                                 L 
                                 + 
                                 1 
                               
                             
                           
                         
                       
                       
                         
                           
                             0 
                             , 
                           
                         
                         
                           
                             L 
                             &lt; 
                             t 
                             &lt; 
                             
                               
                                 N 
                                 FFT 
                               
                               - 
                               L 
                               + 
                               1 
                             
                           
                         
                       
                     
                      
                     t 
                   
                   = 
                   
                     1 
                      
                     … 
                      
                     
                         
                     
                      
                     
                       N 
                       FFT 
                     
                   
                 
               
             
           
         
       
     
         [0022]    where F is an FFT matrix, and Ũ(i)=P·U(i). The weight matrix F may be pre-computed to reduce computations during real-time operation. 
         [0023]    Once an estimate of the spatial covariance matrix is found, this estimate may be provided to a MIMO detector (e.g.,  175 ) to cancel co-channel interference. 
         [0024]    Although the various embodiments have been illustrated by reference to specific embodiments, it will be apparent that various changes and modifications may be made. Reference to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” or “in an embodiment”appearing in various places throughout the specification are not necessarily all referring to the same embodiment. 
         [0025]    Different implementations may feature different combinations of hardware, firmware, and/or software. It may be possible to implement, for example, some or all components of various embodiments in software and/or firmware as well as hardware, as known in the art. Embodiments may be implemented in numerous types of hardware, software and firmware known in the art, for example, integrated circuits, including ASICs and other types known in the art, printed circuit broads, components, etc. 
         [0026]    The various embodiments are intended to be protected broadly within the spirit and scope of the appended claims.