Patent Publication Number: US-8125886-B2

Title: System, apparatus, and method for spatial multiplexing with symbol spreading

Description:
The present invention relates to a system, apparatus, and method for spatial multiplexing with symbol spreading rotation that achieves higher order diversity in a MIMO system while maintaining spectral efficiency. 
     In next generation wireless communication systems, the spatially multiplexing (SMX) (or MIMO) is of particular interest since it can exploit the richly scattered channel environment by using multiple transmit and receive antennas. Meanwhile, it can provide significant improvements in spectrum efficiency. 
     Diversity is commonly used in wireless communication systems to improve system performance. Although spatially multiplexing is able to achieve high spectrum efficiency, the number of receiver antennas must be increased to retain the high diversity order. 
     Since it is not practical to implement this type of system having, a different type of diversity to improve the performance of SMX systems is needed. 
     The system, apparatus and method of the present invention provide a technique to achieve more diversity with SMX using a symbol-spreading rotation can be considered as a potential solution based on signal space diversity. The optimal rotations for a single antenna system of a preferred embodiment provide full modulation diversity while maximizing the minimum product distance, see J. Boutros and E. Viterbo, “Signal space diversity: a power- and bandwidth-efficient diversity technique for the Rayleigh fading channel,” IEEE Trans. Information Theory, Vol. 44, pp. 1453-1467, July 1998. For a QAM constellation, real rotation matrices are combined with SMX systems to increase its diversity order while maintaining the relatively low computational complexity. 
    
    
     
         FIG. 1  illustrates the performance improved of the present invention; 
         FIG. 2  illustrates the method according to the present invention; 
         FIG. 3  illustrates an MMSE receiver as T individual MMSE demappers for each channel followed by a joint linear combination; and 
         FIG. 4  illustrates a system architecture including the receiver of  FIG. 3 . 
     
    
    
     It is to be understood by persons of ordinary skill in the art that the following descriptions are provided for purposes of illustration and not for limitation. An artisan understands that there are many variations that lie within the spirit of the invention and the scope of the appended claims. Unnecessary detail of known functions and structure may be omitted from the current descriptions so as not to obscure the present invention. 
     A preferred embodiment applies to multiple-input multiple-output (MIMO) single-carrier systems over fast Rayleigh flat fading channels. In an alternative preferred embodiment, the same system is viewed as a MIMO multi-carrier system whose coherent bandwidth is much less than the channel bandwidth, such as an OFDM-UWB system. The system has M transmitter antennas and N receiver antennas. The N×M channel matrix is denoted by H t , where t is the time instant for a fast fading case or the sub-carrier index for an OFDM case. 
     Define the transmitted symbol vector x t , the received symbol vector y t , the noise vector n t  and the N×M channel matrix H t , respectively, as follows: 
     
       
         
           
             
               
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     Then the general SMX system model can be described as
 
 y   t   =H   t   ·x   t   +n   t , t=1, 2, . . . T.
 
where T is the number of blocks or the number of sub-carriers. The goal is providing certain diversity by combining these T consecutive symbol vectors. The larger T means the higher diversity order. A T×T spreading rotation matrix R is applied to these T consecutive original symbol vectors such that
 
                 s   t     =     [           s     t   ⁢           ⁢   1                 s     t   ⁢           ⁢   2               ⋮             s   iM           ]       ,         
where each component
 
                 [           s     1   ⁢   i                 s     2   ⁢   i               ⋮             s   Ti           ]     =     R   ·     [           x     1   ⁢   i                 x     2   ⁢   i               ⋮             x   Ti           ]         ⁢                           for   ⁢           ⁢   t     =     1   ⁢     ,     ⁢   2       ,   …   ⁢           ,   T   ,     
     ⁢       and   ⁢           ⁢   i     =     1   ⁢     ,     ⁢   2       ,   …   ⁢           ,     M   .           
Then the new transmitting system can be easily described as
 
 y   t   =H   t   ·s   t   +n   t , t=1, 2, . . . , T
 
But the original symbols have been linear-threaded into the new transmitted symbols to achieve higher order diversity. For example, the new 2×2 SMX system with QPSK modulation and T=2 is
 
     
       
         
           
             
               
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     where the spreading rotation matrix 
             R   =       1     5       ⁡     [         2       1           1         -   2           ]             
is used and I 2  is the 2×2 identity matrix.
 
     For the above example, the 4×4 rotation matrix in the formula can be changed to be a unified matrix 
             U   =     [           1   2           1   2         0           2     2               -     1   2             1   2           -       2     2           0           0           2     2           1   2           -     1   2                 -       2     2           0         1   2           1   2           ]           
which can actually maximizes the minimum product MIMO-symbol distance generalized from the aspect of signal space diversity for single antenna system, as presented in J. Boutros and E. Viterbo, “Signal Space Diversity: A Power- and Bandwidth-Efficient Diversity Technique For The Rayleigh Fading Channel,” IEEE Trans. Information Theory, Vol. 44, pp. 453-1467, July 1998. However, both matrices have a simpler computational rotated DFT-matrix, see Wei Zhang; Xiang-Gen Xia; Ching, P. C.; Haiquan Wang; “Rate two full-diversity space-frequency code design for MIMO-OFDM”, Signal Processing Advances in Wireless Communications, 2005 IEEE 6th Workshop on Jun. 2-8, 2005 Page(s):303-307 and cosine matrix, see Soo Ki Choi; Seung Young Park; Chung Gu Kang, “Rotated Multidimensional Modulation For Spatial Multiplexing Systems”, Vehicular Technology Conference, 2003. VTC 2003-Fall. 2003 IEEE 58 th , 6-9 Oct. 2003 Page(s):246-250 Vol. 1.
 
     The spreading rotation matrices can be used here to provide additional diversity for QPSK constellation with T=3 and T=4 and 16QAM with T=2. For example, the new 3×1 SMX system with T=2 and 16QAM will be 
                 [           y   1               y   2           ]     =         [           h   11   1           h   12   1           h   13   1         0       0       0           0       0       0         h   11   2           h   12   2           h   13   2           ]     ·       1     17       ⁡     [           4   ⁢     I   3             I   3               I   3             -   4     ⁢     I   3             ]       ·     [           x   11               x   12               x   13               x   21               x   22               x   23           ]       +     [           n   11               n   12               n   13               n   21               n   22               n   23           ]         ,         
where a 3×3 spreading rotation matrix is used
 
             R   =       1     17       ⁡     [         4       1           1         -   4           ]             
and a I 3  is the 3×3 identity matrix.
 
     The diversity order for this new SMX scheme is T×N.  FIG. 1  compares the bit error rate performance of an uncoded SMX system with an SMX system according to a preferred embodiment with symbol spreading (T=2) for two transmit and two receive antennas. For QPSK mapping, the performance of the present invention is greatly enhanced compared to the conventional SMX system at high SNR. At the BER of 1-4, there is almost 6 dB gain using the signal spreading of the present invention over the conventional SMX scheme. At the same spectral efficiency of 4 bits/sec/Hz, a preferred embodiment of the present invention is approximately 2 dB better than a 16-QAM Alamouti scheme at the BER of 1e-5, also shown in  FIG. 1 . 
     Maximum-likelihood (ML) detection complexity increases exponentially with the diversity order T. In a preferred embodiment, sphere decoding is used in order to reduce the computational load of the ML detection, since sphere decoding can achieve near maximum likelihood (ML) performance at the polynomial complexity with T. Sphere decoding is possible because the present invention is essentially a lattice-based code. Sphere decoding performs a close-point search only over lattice points lying in a certain hyper-sphere centered on a received vector. For a discussion of sphere decoding see, e.g., H. Vikalo and B. Hassibi, “Maximum-Likelihood Sequence Detection of Multiple Antenna Systems over dispersive Channels via Sphere Decoding,” EUROSIP Jour. Appl. Sig. Proc 2002:5, pp. 525-531. 
     Because of the orthogonal structure for spreading rotation matrix R, the MMSE receiver can be easily derived as T individual MMSE demappers or ZF demappers for each channel followed finally by the joint linear combination with R. For example, the MMSE demapper for above 2×2 SMX system with symbol spreading can be derived as 
                       [             x   ^     1                 x   ^     2           ]     MMSE     =       ⁢         1     5       ⁡     [           2   ⁢     I   2             I   2               I   2             -   2     ⁢     I   2             ]       ·       (       [             H   1   h     ⁢     H   1           0           0           H   2   h     ⁢     H   2             ]     +       σ   2     ⁢     I   4         )       -   1                         ⁢       [           H   1   h         0           0         H   2   h           ]     ⁡     [           y   1               y   2           ]                   =       ⁢         1     5       ⁡     [           2   ⁢     I   2             I   2               I   2             -   2     ⁢     I   2             ]       ⁡     [               (         H   1   h     ⁢     H   1       +       σ   2     ⁢     I   2         )       -   1       ⁢       H   1   h     ·     y   1                       (         H   2   h     ⁢     H   2       +       σ   2     ⁢     I   2         )       -   1       ⁢       H   2   h     ·     y   2               ]                   
Similarly, the ZF demapper for the same system can be derived as
 
     
       
         
           
             
               
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     It will have some performance loss due to the property of sub-optimality of MMSE and ZF receivers compared with ML receivers. It can be seen this scheme doesn&#39;t increase the decoding complexity of MMSE or ZF receivers.  FIG. 3  illustrates a receiver having T individual MMSE spatial demappers  301  that receive T blocks over T different MIMO channels  205  and combines the output of the T individual MMSE spatial demappers  301  with R  302  in a joint linear combination which is decoded by a channel decoder  304  and output as estimated bits  305 .  FIG. 4  illustrates combining the transmitter  200  of  FIG. 2  and the receiver  300  of  FIG. 3  in a transceiver system  400  for increasing the diversity of an SMX communication system. 
     To use the preferred embodiment of the SMX scheme with symbol spreading, one performs the steps of buffering  203  of the mapped QAM symbol vectors  202  of the coded input information bits  201  until T blocks have been received and then multiplying each of them with spreading matrix  204  and finally transmitting the output symbol vectors therefrom over different T independent MIMO channels  205 . A flow chart of this method is illustrated in  FIG. 2 . 
     A preferred embodiment of the present invention enables prior art SMX systems to achieve high diversity while maintaining the same transmission rate. One of the immediate applications of the present invention is the next generation (Gigabit) Multi-Band (MB) OFDM UWB system. A preferred embodiment of the present invention enables a 1 Gbps mode that has longer range compared to prior art systems. A preferred embodiment of the present invention can also be used for fast flat fading channels as a coding scheme over multiple blocks. 
     While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that the system, apparatus and methods as described herein are illustrative and various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the present invention. In addition, many modifications may be made to adapt the teachings of the present invention to a particular situation without departing from its central scope. Therefore, it is intended that the present invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out the present invention, but that the present invention include all embodiments falling within the scope of the claim appended hereto.