Abstract:
A method and apparatus for space-time coding signals for transmission on multiple antennas. A received input symbol stream is transformed using a predefined transform and transmitted on a first set of N antennas. The same input symbol stream is then offset by M symbol periods to generate an offset input symbol stream. The offset input symbol stream is then transformed using the predefined transform and transmitted on a second set of N antennas. A third through X th  set of N antennas may be utilized for transmission by successively offsetting the offset input symbol stream by an additional M symbol periods for each additional set of N antennas used, before performing the transform and transmitting on the additional set of N antennas.

Description:
FIELD OF THE INVENTION 
     This invention relates to a method and apparatus for achieving transmit diversity in telecommunication systems and, more particularly, to a method and apparatus for space-time coding signals for transmission on multiple antennas. 
     BACKGROUND OF THE INVENTION 
     As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third and fourth generation systems compared to the first generation analog and second generation digital systems currently in use, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with the system service performance requirements will be equipment design constraints, which will strongly impact the design of mobile terminals. The third and fourth generation wireless mobile terminals will be required to be smaller, lighter, more power-efficient units that are also capable of providing the sophisticated voice and data services required of these future wireless systems. 
     Time-varying multi-path fading is an effect in wireless systems whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks in regard to use for third and fourth generation wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable for sophisticated receiver-to-transmitter feedback techniques that increase mobile terminal complexity. All of these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile terminals. 
     Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In diversity reception, two or more physically separated antennas are used to receive a signal, which is then processed through combining and switching to generate a received signal. A drawback of diversity reception is that the physical separation required between antennas may make diversity reception impractical for use on the forward link in the new wireless systems where small mobile terminal size is desired. A second technique for implementing antenna diversity is transmit diversity. In transmit diversity a signal is transmitted from two or more antennas and then processed at the receiver by using maximum likelihood sequence estimator (MLSE) or minimum mean square error (MMSE) techniques. Transmit diversity has more practical application to the forward link in wireless systems in that it is easier to implement multiple antennas in the base station than in the mobile terminal. 
     Transmit diversity for the case of two antennas is well studied. Alamouti has proposed a method of transmit diversity for two antennas that offers second order diversity for complex valued signals. S. Alamouti, “ A Simple Transmit Diversity Technique for Wireless Communications,” IEEE Journal on Selected Areas of Communications , pp. 1451-1458, October 1998. The Alamouti method involves simultaneously transmitting two signals from two antennas during a symbol period. During one symbol period, the signal transmitted from a first antenna is denoted by s 0  and the signal transmitted from the second antenna is denoted by S 1 . During the next symbol period, the signal −s 1 * is transmitted from the first antenna and the signal s 0 * is transmitted from the second antenna, where * is the complex conjugate operator. The Alamouti method may also be done in space and frequency coding. Instead of two adjacent symbol periods, two orthogonal Walsh codes may be used to realize space-frequency coding. 
     Extension of the Alamouti method to more than two antennas is not straightforward. Tarokh et al. have proposed a method using rate=½, and ¾ SpaceTime Block codes for transmitting on three and four antennas using complex signal constellations. V. Tarokh, H. Jafarkhani, and A. Calderbank, “ Space-Time Block Codes from Orthogonal Designs,” IEEE Transactions on Information Theory , pp. 1456-1467, July 1999. This method has a disadvantage in a loss in transmission rate and the fact that the multi-level nature of the ST coded symbols increases the peak-to-average ratio requirement of the transmitted signal and imposes stringent requirements on the linear power amplifier design. Other methods proposed include a rate=1, orthogonal transmit diversity (OTD)+space-time transmit diversity scheme (STTD) four antenna method. L. Jalloul, K. Rohani, K. Kuchi, and J. Chen, “ Performance Analysis of CDMA Transmit Diversity Methods,” Proceedings of IEEE Vehicular Technology Conference , Fall 1999, and M. Harrison, K. Kuchi, “ Open and Closed Loop Transmit Diversity at High Data Rates on  2  and  4  Elements,” Motorola Contribution to  3 GPP - C 30-19990817-017. This method requires an outer code and offers second order diversity due to the STTD block (Alamouti block) and a second order interleaving gain from use of the OTD block. The performance of this method depends on the strength of the outer code. Since this method requires an outer code, it is not applicable to uncoded systems. For the case of rate=⅓ convolutional code, the performance of the OTD+STTD method and the Tarokh rate=¾ method ST block code methods are about the same. 
     SUMMARY OF THE INVENTION 
     The present invention presents a method and apparatus for space-time coding signals for transmission on multiple antennas. In the method and apparatus, a received input symbol stream is transformed using a predefined transform and transmitted on a first set of N antennas. The same input symbol stream is then offset in time by M symbol periods to generate an offset input symbol stream. The offset input symbol stream may be offset so as to lead or lag the input symbol stream. The offset input symbol stream is then transformed using the predefined transform and transmitted on a second set of N antennas. A third through X th  set of N antennas may be utilized for transmission by successively offsetting the offset input symbol stream by an additional M symbol periods for each additional set of N antennas used, before performing the transform and transmitting on the additional set of N antennas. The transform may be applied in either the time domain or Walsh code domain. 
     At the receiver, the transmitted symbols may be recovered using a maximum likelihood sequence estimator (MLSE) decoder implemented with the Viterbi algorithm with a decoding trellis according to the transmitter. 
     In an embodiment, 4 antennas are used for transmission. Every 2 input symbols in a received input symbol stream are transformed in the time domain by an Alamouti transform and the result is transmitted on antennas  1  and  2  during the time of two symbol periods. The received input symbol stream is also delayed for two symbol periods, and this delayed input symbol stream is input to an Alamouti transform where every two symbols are transformed and the delayed result is transmitted on antennas  3  and  4  during the time of two symbol periods. The transmitted signal may be received and decoded using an MLSE receiver. The method and apparatus provides diversity of order four and outperforms other proposed extensions of the Alamouti method to more than two antennas by approximately ½ to 1 dB for uncoded transmissions. 
     In an alternative embodiment using 4 antennas, every 2 input symbols in a received input symbol stream are transformed in the Walsh code domain. The Alamouti coded symbols are transmitted on two orthogonal Walsh codes, W 1  and W 2  simultaneously on antennas  1  and  2 . Both W 1  and W 2  span two symbol periods, which maintains the transmission rate at two symbol periods. The received input symbol stream is also delayed for two symbol periods and the Alamouti transform is also applied in the Walsh code domain to the delayed input symbol stream. This delayed result is transmitted on antennas  3  and  4  during the time of two symbol periods. 
     In a further alternative embodiment using 8 antennas for transmission, a rate=¾ ST block code is combined with a 4 symbol delay. Every three symbols in an input symbol stream are transformed by the ST block code and transmitted on antennas  1 - 4 . The received input symbol stream is also delayed for four symbol periods, and this delayed input symbol stream is input to the ST block code transform where every three symbols are transformed and the delayed result is transmitted on antennas  4 - 8  during the time of four symbol periods. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a block diagram of portions of a transmitter according to an embodiment of the invention; 
     FIG. 2 shows a block diagram of portions of a receiver according to an embodiment of the invention; 
     FIG. 3 shows a trellis structure used to process signals in the receiver of FIG. 2; 
     FIG. 4 shows a block diagram of portions of a transmitter according to an alternative embodiment of the invention; and 
     FIG. 5 shows a block diagram of portions of a transmitter according to a further alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, therein is illustrated a block diagram of portions of a transmitter  100  according to an embodiment of the invention. Transmitter  100  includes input  102 , offset block  104 , transform block  106 , transform block  108 , spread, filter and modulate (SFM) block  110 , spread, filter and modulate (SFM) block  112 , antenna  114 , antenna  116 , antenna  118  and antenna  120 . Transmitter  100  may be implemented into any type of transmission system that transmits coded or uncoded digital transmissions over a radio interface. 
     In the embodiment of FIG. 1, transmitter  100  receives an input symbol stream X(t) at input  102 . X(t) is split into two identical symbol streams, with one symbol stream X(t) being input to transform block  106  and a second identical symbol stream X(t) being input to offset block  104 . Offset block  104  causes a  2  symbol period delay in the second symbol stream and then the delayed second symbol stream is input to transform block  108 . Every two symbols S 1  and S 2  are processed in transform block  106  using the Alamouti method and the output of the transform is transmitted on antenna  114  and antenna  116 . The input signal may be complex valued and of arbitrary constellation size. The Alamouti transformation performed in transform block  106  can be written in a matrix form as shown below:              [           S   1           S   2               -     S   2   *             S   1   *           ]           Equation  1                                
     The rows in the matrix indicate the antenna the symbol is transmitted on, and the columns indicate the instant they are transmitted. Symbols S 1  and S 2  are transmitted on antenna  114  and antenna  116  at instants t 1  and t 2 , respectively. 
     The second identical symbol stream X(t) input to offset block  104  is offset by two symbol periods and transformed in transform block  108  using the Alamouti transformation as shown below:              [           Sd   1           Sd   2               -     Sd   2   *             Sd   1   *           ]           Equation  2                                
     The output of the transform from transform block  108  is then transmitted on antenna  118  and antenna  120 . The transmitted signal as it will be received during the time period ( 0 ,t 1 ) can be written as follows:                r        (   t1   )       =             E   c     4            [         S   1          α      1       -       S   2   *          α      2       +       S   d1          α      3       -       S   d2   *        α                 4       ]       +     n        (   t1   )                 Equation  3                                
     and, for the time duration (t 1 ,t 2 ) as,                r        (   t2   )       =             E   c     4            [         S   2          α      1       +       S   1   *          α      2       +       S   d2          α      3       +       S   d1   *          α      4         ]       +     n        (   t2   )                 Equation  4                                
     where S d1  and S d2  are the transmitted symbols on the delayed branch and n(t) is the additive white Gaussian noise. 
     The transmitted signal power E c  may be evenly distributed across the four antennas and the channel coefficients α may be modelled as complex Gaussian. 
     This received signal can be decoded using an MLSE receiver. Referring now to FIG. 2, therein is shown a receiver  200  according to an embodiment of the invention. Receiver  200  includes antenna  202 , filter, despread and demodulate block  204 , processor block  206 , and output  208 . 
     In the embodiment, receiver  200  receives the transmitted signal r(t) at antenna  202 , and filters, despreads and demodulates the signal in filter, despread and demodulate block  204 . Processor block  206  then decodes the sequence that minimizes the Eucledian distance D between the transmitted and received signals and outputs the sequence at output  208  according to the following:                    D   =                       r        (   t   )       -     (       x        (   t   )       +     x        (     t   -     2      T       )         )                        =                         r        (   t1   )       -     (         S   1          α      1       -       S   2   *          α      2       +       S   d1          α      3       -       S   d2   *          α      4         )            +                                     r        (   t2   )       -     (         S   2          α      1       +       S   1   *          α      2       +       S   d2          α      3       +       S   d1   *          α      4         )                          Equation  5                                
     Further optimization of the branch metrics can be obtained with the following simplification. Using the equations, 
     
       
           {tilde over (r)} ( t 1)= r ( t 1)−( S   1 α1 −S   2 *α2)  Equation 6 
       
     
     
       
           {tilde over (r)} ( t 2)= r ( t 2)−( S   2 α1 +S   1 *α2)  Equation 7 
       
     
     the following metric can be obtained:                D   2     =                  r   ∼          (   t1   )       -     (         S   d1          α      3       -       S   d2   *          α      4         )            2     +                r   ∼          (   t2   )       -     (         S   d2          α      3       +       S   d1   *          α      4         )            2               Equation  8                                
     This may be further simplified as:                D   2     =                    r   ∼          (   t1   )              (     α      3     )     *       +           r   ∼          (   t2   )       *          α      4       -     S   d1            2     +                  r   ∼          (   t1   )              (     α      4     )     *       -           r   ∼          (   t2   )       *          α      3       +     S   d2   *            2               Equation  9                                
     Symbols S d1 , S d2  may be found separately. In the simplification given by equation 9, only the values S d1  and S d2  need to be modified at each computation stage. This reduces the number of multiplications in the calculation. 
     The input to the Viterbi decoder is the sampled received signal observed over “n” time epochs or n symbol periods, where n=2 for 4 antenna ST codes. The state transitions in the Viterbi decoder occur every “n” time epochs. 
     Referring now to FIG. 3, therein is shown a trellis structure  300  used to process the ST code of the received signal in receiver  200 , according to an embodiment of the invention. Trellis structure  300  is the binary phase shift keying (BPSK) trellis diagram for a 4 antenna space-time (ST) code. Trellis  300  can be described using the following state labelling: 
     
       
         Next state=input symbols ( S   1   ,S   2 )  Equation 10 
       
     
     
       
         Output={previous state, input symbols}={( S   d1   ,S   d2 ), ( S   1   ,S   2 )}  Equation 11 
       
     
     The number of states in the trellis  300  is given by M 2  where M is the signal constellation size. The total number of states shown in trellis  300  is 4. Trellis  300  may be decoded using the Viterbi algorithm. FIG. 3 shows the bpsk case. Other modulation may be used in alternative embodiments. Generally, for the case of a 4-antenna ST code, the decoder has to remember all possible 2 previous symbols (i.e., 4 states for bpsk, and 16 states for qpsk, 64 states for 8-psk and so on) at each state. 
     Referring now to FIG. 4, therein are shown portions of a transmitter according to an alternative embodiment of the invention. FIG.  4 . shows transmitter  400 , which includes input  402 , offset block  404 , space-time spreading (STS) transform block  406 , STS transform block  408 , filter and modulate block  410 , filter and modulate block  412  and antennas  414 ,  416 ,  418  and  420 . In transmitter  400 , the Alamouti transformation is applied in Walsh code domain instead of time domain. The Alamouti coded symbols are transmitted on two orthogonal Walsh codes W 1 , W 2  simultaneously. Both W 1  and W 2  span two symbol periods in this case maintaining the total transmission rate. This method is known as space-time spreading (STS). A delayed copy of the input signal is STS transformed again and transmitted via the other two antennas. 
     In the embodiment of FIG. 4, transmitter  400  receives an input symbol stream X(t) at input  402 . X(t) is split into two identical symbol streams, with one symbol stream X(t) being input to transform block  406  and a second identical symbol stream X(t) being input to offset block  404 . Offset block  404  causes a 2 symbol period delay in the second symbol stream and then the delayed second symbol stream is input to transform block  408 . Every two symbols S 1  and S 2  are processed in transform block  406  using the Alamouti method and the output of the transform is transmitted on antenna  414  and antenna  416 . The input signal may be complex valued and of arbitrary constellation size. The Alamouti transformation performed in STS transform block  406  can be written in a matrix form as shown below:              [         S1W1           S   2        W2                 -     S   2   *          W1             S   1   *        W2           ]           Equation  12                                
     The rows in the matrix indicate the antenna on which the symbol is transmitted. The symbols S 1  and S 2  are transmitted simultaneously on antenna  414  during the same two symbol periods in which the symbols—S 2 * and S 1 * are transmitted simultaneously on antenna  416 . 
     The second identical symbol stream X(t) input to offset block  404  is delayed by two symbol periods and transformed in transform block  408  using the Alamouti transformation as shown below:              [             Sd   1        W1             Sd   2        W2                 -     Sd   2   *          W1             Sd   1   *        W2           ]           Equation  13                                
     The rows in the matrix indicate the antenna on which the symbol is transmitted. The symbols Sd 1  and Sd 2  are transmitted simultaneously on antenna  418  during the same two symbol periods in which the symbols—Sd 2 * and Sd 1 * are transmitted simultaneously on antenna  420 . 
     A receiver for the embodiment of the transmitter of FIG. 4 may be implemented in the same manner as the receiver of FIG. 2, with the filter, despread and demodulate block  204  modified to receive the Alamouti coded symbols that are transmitted simultaneously on the Walsh codes W 1  and W 2 . 
     Various alternative embodiments of the invention are possible. For example, in the case of three transmit antennas, the output of any two of the Alamouti/STS branches can be mapped to the same antenna to obtain a diversity gain of order three. Also, for 6 and 8 antennas the given method can be generalized by using Alamouti transform block combined with 3 and 4 delay diversity branches, respectively. 
     A further alternative embodiment may also be used for  8  transmit antennas. Referring now to FIG. 5, therein is illustrated a block diagram of portions of a transmitter  500  according to a further alternative embodiment of the invention. Transmitter  500  includes input  502 , offset block  504 , transform block  506 , transform block  508 , spread, filter and modulate (SFM) block  510 , spread, filter and modulate (SFM) block  512 , antenna  514 , antenna  516 , antenna  518 , antenna  520 , antenna  522 , antenna  524 , antenna  526  and antenna  528 . Transmitter  500  may be implemented into any type of transmission system that transmits coded or uncoded digital transmissions over a radio interface. 
     In the embodiment of FIG. 5, transmitter  500  receives an input symbol stream X(t) at input  502 . X(t) is split into two identical symbol streams, with one symbol stream X(t) being input to transform block  506 , and a second identical symbol stream X(t) being input to offset block  504 . Offset block  504  causes a 4 symbol period delay in the second symbol stream and then the delayed second symbol stream is input to transform block  508 . Every three symbols S 1 , S 2  and S 3  are processed in transform block  506  using a ¾ rate block code transform and the output of transform block  506  is transmitted on antennas  514 ,  516 ,  518  and  520 . The ¾ rate block code may be as described in the paper by V. Tarokh, H. Jafarkhani, and A. Calderbank, “ Space - Time Block Orthogonal Codes from Orthogonal Designs,” IEEE Transactions on Information Theory , pp. 1456-1467, July 1999. The delayed second input symbol stream is processed in block  508  using the same ¾ rate block code transform and the output of transform block  508  is transmitted on antennas  522 ,  524 ,  526  and  528 . The input signal may be complex valued and of arbitrary constellation size. 
     The ¾ rate ST block code is given by the following transformation.              [           S   1           S   2           S   3         0             -     S   2   *             S   1   *         0         -     S   3                 -     S   3   *           0         S   1   *           S   2             0         S   3   *           -     S   2   *             S   1           ]           Equation  14                                
     The trellis structure for the 8-antenna ST code can be described using the following state labelling. 
     
       
         Next state=input symbols ( S   1   ,S   2   ,S   3 )  Equation 15 
       
     
     
       
         Output label={previous state, input symbols}={( S   d1   ,S   d2   ,S   d3 ), ( S   1   ,S   2   ,S   3 )}  Equation 16 
       
     
     A receiver for the embodiment of the transmitter of FIG. 5 may be implemented in the same manner as the receiver of FIG. 2, with the filter, despread and demodulate block  204  modified to receive the ¾ rate block code symbols. It is assumed that the Viterbi decoder has knowledge of the estimated channel coefficients. For the 8-antenna case of FIG. 5, the decoder has to remember all possible 3 previous symbols at each state (i.e., M 3  states for M-psk). The branch metrics given for the 4-antenna ST code for FIG.1 may be generalized to the 8-antenna case. 
     The described and other embodiments could be implemented in systems using any type of multiple access technique, such as time division multiple access (TDMA), code division multiple access (CDMA), frequency division multiple access (FDMA), orthogonal frequency division multiple access (OFDM), or any combination of these, or any other type of access technique. This could also include systems using any type of modulation to encode the digital data. 
     Thus, although the method and apparatus of the present invention has been illustrated and described with regard to presently preferred embodiments thereof, it will be understood that numerous modifications and substitutions may be made to the embodiments described, and that numerous other embodiments of the invention may be implemented without departing from the spirit and scope of the invention as defined in the following claims.