Abstract:
In a multiple-input/multiple-output wireless communications system, a stream of data symbols is demultiplexed into M sub-streams, where M is greater than one. Each sub-stream is space-time transmit diversity encoded into a pair of transmit signals. Power is dynamically allocated to each transmit signal according to corresponding feedback signal received from a receiver of the transmit signal.

Description:
FIELD OF THE INVENTION  
         [0001]    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  
         [0002]    Transmit diversity is one of the key contributing technologies in 3 rd  generation wireless communications (3G) systems, such as wideband code division multiple access (W-CDMA) and CDMA2000. Transmit diversity reduces the impact of channel fading by transmitting multiple independent copies of a digitally modulated signal to a receiver. The likelihood that all copies of the signal will fade simultaneously is very small. Therefore, transmit diversity can improve the system performance in the presence of fading channels.  
           [0003]    As shown in FIG. 1A, an open loop solution for transmit diversity is used to maximize the diversity gain. This scheme uses two antennas  101 - 102  for transmission and a single antenna  103  for reception. In such a system, every two symbols X 1  and X 2    110  of the transmitted data are encoded by a space-time transmit diversity (STTD) encoder  120  to generate four encoded symbols  140 , two symbols for each antenna  101 - 102 . Each antenna transmits different symbol streams through the channel to gain diversity. The transmitted symbols are given by  
             [           X   1           X   2               -     X   2   *             X   1   *           ]           (   1   )                               
 
           [0004]    where * is a complex conjugate. Each row of the STTD output matrix in equation (1) represents the output to a transmit antenna, as shown in FIG. 1.  
           [0005]    As shown in FIG. 1B, adaptive power allocation according to feedback information  152 , combined with the STTD encoder  120 , is known, see Huawei “ STTD with Adaptive Transmitted Power Allocation,”  3GPP TSG-R WG1 document, TSGR1#26 R1-02-0711, Gyeongju, Korea May 13-16, 2002. There, a weight calculator  150  determines weights w 1  and w 2    151 , which are real, positive functions of propagation channel coefficients h 1  and h 2    153  from each respective transmit antenna  101 - 102  to the receive antenna  103 . The weight functions perform the transmitted power allocation to the transmit antennas, in a way that maximizes the receiver performances. Hence, the condition a W 2   1 +W 2   2 =1 should always be satisfied. The weights are calculated from the feedback channel information  152  from user equipment (UE). The feedback channel information can be carried by feedback indicator (FBI) bits within the uplink dedicated physical control channel (DPCCH), as it is done for the existing TxAA closed loop transmit diversity modes, which is defined in 3GPP standard specifications.  
           [0006]    Theoretical analysis and simulation results prove that such an adaptive STTD (ASTTD) provides, compared with the current STTD, about a 1.55 dB performance gain measured on the raw bit error rate (BER) at all UE velocities, and from 1.0 to 0.7 dB on the decoded BER in the range of velocities between 20 and 120 kmph. The ASTTD also requires simpler feedback information compared with standard closed-looped transmit diversity modes.  
           [0007]    Multiple input multiple output (MIMO) technologies have been proposed in the 3GPP standards for high speed downlink packet access (HSDPA) in W-CDMA systems. MIMO uses multiple antennas for both transmission and reception. Because multiple antennas are deployed in both transmitters and receivers, higher capacity or transmission rates can be achieved. However, the transceiver complexity is higher.  
           [0008]    This is because the simultaneous transmitted signals from multiple antennas can interfere with the desired signal, and therefore, an advanced and more complicated receiver is necessary to detect the received signals. On the other hand, current 3G standards already specify the transmitter configurations for voice and low data rate users. It becomes an important issue to design a MIMO system for high speed data users, which is backward compatible with the current 3G systems. With the backward compatibility, the entire system complexity can be reduced, while the number of users within a cell can also be increased.  
         SUMMARY OF THE INVENTION  
         [0009]    In a multiple-input/multiple-output wireless communications system, a stream of data symbols is demultiplexed into M sub-streams, where M is greater than one. Each sub-stream is space-time transmit diversity encoded into a pair of transmit signals. Power is dynamically allocated to each transmit signal according to corresponding feedback signal received from a receiver of the transmit signal, so that the total allocated power is constant.  
           [0010]    The feedback is determined in a receiver by a channel estimation unit, and a weight calculation unit, which computes one weighting parameter for each transmitted signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1A is a block diagram of a prior art STTD transmitter;  
         [0012]    [0012]FIG. 1B is a block diagram of a prior art STTD transmitter with adaptive power control;  
         [0013]    [0013]FIG. 2 is a block diagram of a MIMO transmitter according to the invention; and  
         [0014]    [0014]FIG. 3 is a block diagram of a MIMO receiver according to the invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    [0015]FIG. 2 shows a transmitter  200  for a multiple-input/multiple-output wireless communications system (MIMO) according to the invention. The transmitter  200  include a demultiplexer (DEMUX)  210  coupled to multiple STTD encoders  230 . Each STTD encoder  230  produces two output signals  231 . The power of each pair of output signals  231  is weighted  250 . The weighted signals are coupled to M pairs of antennas  240 . The output of the STTD encoder at i th  group of antenna can be represented by  
             [           X     i                 1             X     i                 2                 -     X     i                 2     *             X     i                 1     *           ]           (   2   )                               
 
         [0016]    where [X i1 X i2 ] is the input  211  to the STTD encoder at i th  group of antenna, as shown in FIG. 2.  
         [0017]    The power allocated at the i th  group of antennas is determined by a weight selection block  260  as [W i1 , W i2 ], i=1, 2, . . . , M. The values for the weights W are based on a feedback signal  261  from the receivers  300 , with a constraint that the total transmit power is fixed, i.e.,  
                   ∑     i   =   1     M                     w     i                 1     2       +     w     i                 2     2       =     consant   .             (   3   )                               
 
         [0018]    The weight selection block  260  makes the final decision on the weight selections when system resource cannot meet power requirements according to the feedback signal  261 .  
         [0019]    [0019]FIG. 3 shows the receiver  300  in greater detail. The receiver uses R antennas  301  for reception. At each antenna, the received signal r i (n)  302 , i=1, . . . , R, is fed into M STTD decoders  310 , where M is equal to the number of STTD encoders at the transmitter side.  
         [0020]    The outputs for decoder j at antenna i, S i   j (n), are given by 
           S   j   i ( n )= h   *   (2j−1),i r i ( n )+ h   2j,i r i   * ( n− 1)   n= 2,4, 
           S   j   i ( n +1)= h   *   (2j−1),i r i ( n −1)− h   2j,i r *   i ( n )   n= 2,4, 
         [0021]    where h ji  is the channel coefficient from the j th  transmit antenna to the i th receive antenna. Here the channel coefficients can be estimated  320  from the signals received at each antenna. Based on the estimated channel coefficients, the power allocation weights W for each transmit antenna can be calculated  330  and signaled  261  back to the transmitter  200  of FIG. 2.  
         [0022]    The outputs of the decoder j at each antenna are further combined  340  based on a maximum ratio combining (MRC) method to form the inputs to an interference supression block  350 . An interference suppression process, such as iterative minimum mean square error (MMSE) can be implemented to maximize the signal to interference-and-noise ratio (SINR) at the output of the interference supression block  350 . The parallel outputs from the interference supression block are converted  360  into a serial data stream  309  to form the input for demodulation and channel decoding.  
         [0023]    This present invention is an improvement over a prior art MIMO systems described in the “Technical Specification Group Radio Access Network; Physical layer aspects of UTRA High Speed Downlink, Packet Access, Technical Report,” 3GPP TR 25.848 V4.0.0, March 2001 (TR 25.848). The system as described above has a lower complexity. With the use of STTD encoder at the transmitter, the more complicated receiver structure, such as layed receiver structure (VBLAST), is not necessary for receiver design, see TR 25.848 FIG. 7, at page 17.  
         [0024]    The system as described is less sentive to correlated fading channels, whereas the prior art MIMO systems is sensitive to channel correlations, and independent diversity for transmit antennas is generally assumed to achieve higher diversity gains. In the prior art MIMO system, the number of receive antennas has to be geater or equal to the number of transmit antennas. There are no such restrictions with the present invention. In addition, the present MIMO system with adaptive power allocation is backward compatible with 3G W-CDMA systems.  
         [0025]    It is to be understood that various other adaptations and modifications may 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.