Abstract:
A method for transmitting a stream of data symbols in a multiple-input/multiple-output (MIMO) wireless communications system including N r  transmitting antennas. The stream of data symbols is first demultiplexed into M sub-streams, where M=N r /2. Then, space-time transmit diversity encoding is applied to each sub-stream to generate a pair of transmit signals. Power is allocated dynamically to each transmit signal of each pair of transmit signals according to a corresponding feedback signal received from a receiver of the transmit signal. The feedback signal including a ratio of magnitude sums of channel coefficients for channels used for the transmit signals.

Description:
RELATED APPLICATION  
       [0001]    This is a continuation-in-part application of U.S. patent application Ser. No. 10/217,919 “MIMO Systems with STTD Encoding and Dynamic Power Allocation,” filed by Horng et al., on Aug. 13, 2002. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to wireless communications, and more particularly to multiple input/multiple output wireless communications systems with dynamic power allocation.  
         BACKGROUND OF THE INVENTION  
         [0003]    Transmit diversity is one of the key technologies used in third generation (3G) wireless communications systems. By transmitting the signal through multiple transmit antennas to multiple receive antennas spatial diversity gains can be achieved to enhance the system capacity. Space-time transmit diversity (STTD) is an open loop technique in which the symbols are modulated using space-time block code, see Alamouti, “ A simple transmit diversity technique for wireless communications ,” IEEE J. Select. Areas Commun., 16:1451-1468, 1998.  
           [0004]    The STTD scheme is adopted by 3GPP due to its simple implementation and maximal diversity gains. Transmit adaptive array (TXAA) is a closed loop transmit diversity technique adopted by 3GPP. The mobile receiver feedbacks the estimated optimal transmit weights to the basestation. The base station uses this feedback information to adjust the power level of the transmitted signal so that the received power at the desired mobile receiver is maximized.  
           [0005]    Simulation results show that the STTD is robust at higher velocities, while TXAA provides the biggest benefits at lower velocities, see Derryberry et al., “ Transmit Diversity in  3 G CDMA Systems ,” IEEE Comm. Magazine, vol.40, no.4, pp. 68-75, April 2002.  
           [0006]    A mixture of open and closed loop diversity technique could be used to combat both fast and slow fading. Recently, an adaptive STTD (ASTTD) scheme has been described, which combines STTD with adaptive transmit power allocation in order to improve the performances of the STTD systems, see Huawei “ STTD with Adaptive Transmitted Power Allocation ,” 3GPP TSG-R WG1 document, TSG-R1#26 R1-02-0711, Gyeongju, Korea May 13-16, 2002.  
         SUMMARY OF THE INVENTION  
         [0007]    A method for transmitting a stream of data symbols in a multiple-input/multiple-output (MIMO) wireless communications system including N r  transmitting antennas.  
           [0008]    The stream of data symbols is first demultiplexed into M sub-streams, where M=N r /2. Then, space-time transmit diversity encoding is applied to each sub-stream to generate a pair of transmit signals.  
           [0009]    Power is allocated dynamically to each one of the pairs of transmitted signal according to a corresponding feedback signal received from a receiver of the transmit signal.  
           [0010]    The feedback signal including a ratio of magnitude sums of channel coefficients for channels used for the transmitted signals. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a block diagram of a multiple input/multiple output wireless communications systems that uses the invention; and  
         [0012]    [0012]FIG. 2 is a block diagram of a receiver of the system of FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]    [0013]FIG. 1 shows a MIMO system  100  that can use the invention. From a single input signal  101 , an STTD encoder  110  produces multiple output signals  102  using demultiplexing  103 . The power of each of the output signals is weighted  120  by a power amplifier before sent to multiple transmitter antennas  120  through channels having responses h to receiver antennas  201  of a receiver  200 , see FIG. 2.  
         [0014]    The weights, w, and w 2 , are real positive numbers which are selected to maximize the SNR at the receiver  200  with the constraint that w 1   2 +w 2   2 =1. The STTD encoding  110  is a space-time block code which encodes the input signal [X 1  X 2 ] T    101  into output signals 102 
               [           X   1           -     X   2   *                 X   2           X   1   *           ]     ,           (   1   )                               
 
         [0015]    where * denotes complex conjugate and each row of the matrix is assigned to the same transmit antenna  120 .  
         [0016]    [0016]FIG. 2 show the receiver  200  in greater detail. The receiver includes a combiner  220 , an STTD decoder  230 , and an interference cancellation stage  240 .  
         [0017]    Assume there are Nr receiver antennas  201  at the receiver  200 . A received signal r i (n)  210  at the ith receiver antenna can be expressed as  
                   [             r   1          (   n   )                   r   1   *          (     n   +   1     )                   r   2          (   n   )                   r   2   *          (     n   +   1     )               ⋮               r     N   r            (   n   )                   r     N   r     *          (     n   +   1     )             ]       r        (   n   )         =           [             w   1          h   11               w   2          h   21                   w   2          h   21   *               -     w   1            h   11   *                   w   1          h   12               w   2          h   22                   w   2          h   22   *               -     w   1            h   12   *               ⋮       ⋮               w   1          h     1        N   r                   w   2          h     2        N   r                       w   2          h     2        N   r       *               -     w   1            h     1        N   r       *             ]       H   ~              [           X   1               X   2           ]     X       +       [             v   1          (   n   )                   v   1   *          (     n   +   1     )                   v   2          (   n   )                   v   2   *          (     n   +   1     )               ⋮               v     N   r            (   n   )                   v     N   r     *          (     n   +   1     )             ]       v        (   n   )             ,           (   2   )                               
 
         [0018]    where h ij  are the channel coefficients of the channels between the i th  transmitter antennas and the j th  receiver antennas, and v(n) is the additive white Gaussian noise sample at time instant n, which is assumed to be independent at all receiver antenna elements.  
         [0019]    The received signals  210  from all antennas  201  are first combined  220  before passed to the STTD decoder  230 . Therefore, the output {tilde over (r)} of the STTD decoder  230  corresponding to the two successive transmitted symbol in one space-time coding block is given by  
                       r   ~          (   n   )       =       [             r   ~     1                 r   ~     2           ]     =       H   *          r        (   n   )                       =       [           h   11   *           h   21           h   12   *           h   22         ⋯         h     1        N   r       *           h     2        N   r                   h   21   *           -     h   11             h   22   *           -     h   12           ⋯         h     2        N   r       *           -     h     1        N   r                 ]          r        (   n   )                     =           H   *          H   ~                   X     +       H   *          v        (   n   )           =         [         A       B             -     B   *           A         ]          [           X   1               X   2           ]       +       v   ~          (   n   )                         (   3   )                         where                 A     =         w   1            ∑     i   =   1       N   r                              h     1                   N   r              2         +       w   2            ∑     i   =   1       N   r                              h     2                   N   r              2             ,                            B   =         w   2            ∑     i   =   1       N   r              h     1        N   r       *          h     2        N   r               -       w   1            ∑     i   =   1       N   r              h     1        N   r       *          h     2        N   r                                 (   4   )               and        
              v   ~          (   n   )       =         H   *          v        (   n   )         =     [               ∑     i   =   1       N   r              h     1                 i     *            v   i          (   n   )           +       ∑     i   =   1       N   r              h     2                 i              v   i   *          (     n   +   1     )                           ∑     i   =   1       N   r              h     2                 i     *            v   i          (   n   )           +       ∑     i   =   1       N   r              h     1                 i              v   i   *          (     n   +   1     )                   ]                 (   5   )                               
 
         [0020]    To cancel the cross-interference term B in Equation (4), the cross-interference cancellation stage  240  maximizes the SNR of the STTD decoded symbols. The output {circumflex over (X)}  250  is given by  
                     [             X   ^     1                 X   ^     2           ]     =                [         A       B             -     B   *           A         ]     *                       r   ~          (   n   )                     =                [                  A        2     +          B        2           0           0                A        2     +          B        2             ]          [           X   1               X   2           ]       =              [           A   *           -   B               B   *         A         ]                       v   ~          (   n   )                         (   6   )                               
 
         [0021]    Thus, the conditional SNR of the output signal can be obtained by  
                 SNR                   l     h     i                 j           =         (            A        2     +          B        2       )          E   s           σ   v   2            ∑   j            ∑   i                 h     i                 j            2               ,           (   7   )                               
 
         [0022]    where Es is the transmitted symbol energy and σ v   2  he additive white noise power.  
         [0023]    Because the term A in Equation (4) contributes dominantly to the desired signal energy, one can maximize A instead of maximizing the SNR in Equation (7).  
         [0024]    Thus, the optimum weight function is found by letting dA/dw 1 =0 with the fixed power constraint, w 1   2 +w 2   2 =1.  
               w   1     =         1       1   +       (         ∑     i   =   1       N   r                   h     2                   N   r              2           ∑     i   =   1       N   r                   h     1                   N   r              2         )     2                           w   2       =       1       1   +       (         ∑     i   =   1       N   r                   h     1                   N   r              2           ∑     i   =   1       N   r                   h     2                   N   r              2         )     2           .               (   8   )                               
 
         [0025]    Equation 8 shows that just the ratio of the magnitude sum of the channel coefficients are sufficient for the transmitter to calculate the optimum transmit weight. This is much simpler than the prior art closed loop techniques (TXAA), which uses the amplitude and phase information of the propagation channels to calculate the transmit weights. It also leads to a reduced number of bits or more reliable transmission of feedback information in our invention.  
         [0026]    In there are no limits on the feedback size, then an eigen-mode can be used as an alternative to calculate transmit weights. The transmit weight vector is chosen as the principal eigenvector corresponding to a maximum eigenvalue of the channel correlation matrix R, where  
               R   =       H   ^                       H   ^     H         ,       and                   H   ^       =       [           h   11           h   12         ⋯         h     1        N   r                   h   21           h   22         ⋯         h     2        N   r               ]     .               (   9   )                               
 
         [0027]    The performance of the adaptive STTD can provide about a 0.8 dB SNR gain for a BER=10 −3 , and a 1.0 dB gain for FER=10 −1 . It should be noted that the invention can also be used for multiple transmit antennas and a single receive antenna.  
         [0028]    Although the invention has been described by way of examples of preferred embodiments, it is to be understood that various other adaptations and modifications can be made within the spirit and scope of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.