Abstract:
A method and apparatus for supporting transmit diversity are disclosed. A wireless communication system includes a transmitter having a plurality of transmit antennas and a receiver having at least one receive antenna. The transmitter transmits different pilot code sequences via each of the transmit antennas. The receiver comprises at least one receive antenna for receiving signals transmitted from the transmitter and a plurality of equalizers. Each equalizer is locked onto one of the transmit antennas and processes received samples using a corresponding pilot code sequence. The equalizer may treat user data and pilot code sequence transmitted via all other transmit antennas except the corresponding transmit antenna as interference or alternatively may cancel pilot or pilot and data in parallel or successively.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/760,022 filed Jan. 18, 2006, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention is related to wireless communication systems. More particularly, the present invention is related to a method and apparatus for supporting transmit diversity in a wireless communication system.  
       BACKGROUND  
       [0003]     An adaptive equalizer based receiver, such as a normalized least mean square (NLMS)-based receiver, provides superior performance for high data rate services such as frequency division duplex (FDD) high speed downlink packet access (HSDPA) or code division multiple access (CDMA) 2000 evolution data and voice (EV-DV) over a Rake receiver.  
         [0004]     An NLMS algorithm is used for equalizer filter tap coefficient adaptation to generate and update appropriate filter tap coefficients used by the equalizer filter. Typically, error signal computation, vector norm calculation and leaky integration are performed to generate and update the filter tap coefficients.  
         [0005]     A channel estimation (CE)-NLMS receiver is another example of an advanced receiver that may provide high data rate services. In the CE-NLMS receiver, a channel estimate is used for updating the filter tap coefficients.  
         [0006]     Conventional systems either do not use transmit diversity for adaptive equalization, or use transmit diversity but perform data equalization in a straight forward method for equalization without any joint design and interference cancellation. In a straight forward method for transmit diversity based adaptive filtering, two equalizers using an equalization filter, such as an NLMS equalization filter, are used independently and separately for two transmit antennas. Each equalization filter equalizes the signal distortion of its assigned transmit antenna without considering the interference from the other transmit antenna. However, such a system does not offer optimum performance.  
       SUMMARY  
       [0007]     The present invention is related to a method and apparatus for supporting transmit diversity. A wireless communication system includes a transmitter having a plurality of transmit antennas and a receiver having at least one receive antenna. The transmitter transmits different pilot code sequences via each of the transmit antennas. The receiver comprises at least one receive antenna for receiving signals transmitted from the transmitter and a plurality of equalizers. Each equalizer is locked onto one of the transmit antennas and processes received samples using a corresponding pilot code sequence. The equalizer may treat user data and pilot code sequence transmitted via all other transmit antennas except the corresponding transmit antenna as interference or alternatively may cancel pilot or pilot and data in parallel or successively. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a block diagram of a receiver supporting transmit diversity in accordance with one embodiment of the present invention.  
         [0009]      FIG. 2  is a block diagram of a receiver supporting transmit diversity in accordance with another embodiment of the present invention.  
         [0010]      FIG. 3  is a block diagram of a receiver supporting transmit diversity while implementing parallel interference cancellation (PIC) in accordance with the present invention.  
         [0011]      FIG. 4  is a block diagram of a receiver implementing PIC or successive interference cancellation (SIC) selectively in accordance with the present invention.  
         [0012]      FIG. 5  is a block diagram of a receiver using a joint chip-level equalizer for open loop transmit diversity in accordance with the present invention.  
         [0013]      FIG. 6  is a block diagram of a receiver using a chip-level equalizer for closed loop transmit diversity in accordance with the present invention.  
         [0014]      FIG. 7  is a block diagram of an HSDPA receiver using a chip-level equalizer for open and closed loop transmit diversity in accordance with the present invention.  
         [0015]      FIG. 8  is a block diagram of a receiver including a combined chip-level equalizer for open and closed loop transmit diversity in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     The features of the present invention may be incorporated into an integrated circuit (IC) or be configured in a circuit comprising a multitude of interconnecting components.  
         [0017]     The present invention provides a method and apparatus for transmit diversity processing for a receiver. The present invention is applicable to any wireless communication system including, but not limited to, universal mobile telecommunication system (UMTS) frequency division duplex (FDD) high-speed downlink packet access (HSDPA). The present invention may be implemented using either CE-NLMS or NLMS for either open-loop or close-loop transmit diversity. The transmit diversity may be implemented using any number of transmit antennas, and the receiver may include a single receive antenna or multiple receive antennas for receive diversity and joint processing.  
         [0018]     In accordance with a first embodiment of the present invention, a receiver algorithm for transmit diversity is provided. In accordance with a second embodiment of the present invention, an advanced joint algorithm using interference cancellation is provided. The present invention will be explained with reference to a transmitter having two transmit antennas and a receiver having one or two receive antennas as an example. However, it should be noted that the present invention may be applied to any number of transmit and receive antennas.  
         [0019]     For open-loop transmit diversity, the received signal can be expressed as follows:  
                 r   →     =         1     2       ⁢     H   1     ⁢     x   →       +       1     2       ⁢     H   2     ⁢       x   →     2       +     n   →         ;           Equation   ⁢           ⁢     (   1   )               
 
 where H 1  and H 2  are the channel response matrix for transmit antenna  1  and transmit antenna  2 , respectively. {right arrow over (x)} 1  and {right arrow over (x)} 2  are the transmitted data plus pilot signal of transmit antenna  1  and transmit antenna  2 , respectively. {right arrow over (n)} is a noise vector. 
 
         [0020]     The {right arrow over (x)} 1  and {right arrow over (x)} 2  can be expressed by data and pilot signal as follows:  
                     x   →     1     =       p   1     +       ∑     k   =   1     K     ⁢       y   →     1     (   k   )             ;     ⁢     
     ⁢   and           Equation   ⁢           ⁢     (   2   )                       x   →     2     =       p   2     +       ∑     k   =   1     K     ⁢       y   →     2     (   k   )             ;           Equation   ⁢           ⁢     (   3   )               
 
 where p 1  and P 2  are pilot signals, (or common pilot channel (CPICH) signals), for transmit antenna  1  and transmit antenna  2 , respectively. Let p 1  and P 2  represent CPICH  1  and CPICH  2 , respectively. {right arrow over (y)} 1   (k)  and {right arrow over (y)} 2   (k)  are spread data for user k, (or code k), that are transmitted via transmit antenna  1  and transmit antenna  2 , respectively. Substituting Equations (2) and (3) into Equation (1), Equation (1) can be expressed as follows:  
               r   →     =         1     2       ⁢       H   1     ⁡     (       p   1     +       ∑     k   =   1     K     ⁢       y   →     1     (   k   )           )         +       1     2       ⁢       H   2     ⁡     (       p   2     +       ∑     k   =   1     K     ⁢       y   →     2     (   k   )           )         +       n   →     .               Equation   ⁢           ⁢     (   4   )               
 
         [0021]     For open-loop space time transmit diversity (STTD), {right arrow over (y)} 1   (k)  and {right arrow over (y)} 2   (k)  are the spread data of data symbols {right arrow over (d)} (k)  that are STTD encoded in both space and time domain. For quadrature phase shift keying (QPSK), the STTD encoded data sequences for transmit antenna  1  and transmit antenna  2  are as follows:
 
{right arrow over (d)} 1 =[b 0  b 1  b 2  b 3 ] T ,
 
 and
 
{right arrow over (d)} 2 =[  b 2    b 3  b 0    b 1   ] T .
 
         [0022]     For 16 quadrature amplitude modulation (QAM), the STTD encoded data sequences for transmit antenna  1  and transmit antenna  2  are as follows:
 
{right arrow over (d)} 1 =[b 0  b 1  b 2  b 3  b 4  b 5  b 6  b 7 ] T ,
 
 and
 
{right arrow over (d)} 2 =[  b 4    b 5  b 6  b 7  b 0    b 1    b 2  b 3 ] T .
 
         [0023]     For closed-loop transmit diversity, the received signal can be expressed as follows:  
                 r   →     =         H   1     ⁡     (         1     2       ⁢     p   1       +       s   →     1       )       +       H   2     ⁡     (         1     2       ⁢     p   2       +       s   →     2       )       +     n   →         ;           Equation   ⁢           ⁢     (   5   )               
 
 where  
                     s   →     1     =       ∑     k   =   1     K     ⁢       w   1     (   k   )       ⁢       y   →       (   k   )             ;     ⁢     
     ⁢   and           Equation   ⁢           ⁢     (   6   )                     s   →     2     =       ∑     k   =   1     K     ⁢       w   2     (   k   )       ⁢         y   →       (   k   )       .                 Equation   ⁢           ⁢     (   7   )               
 
         [0024]     For closed-loop transmit diversity, the same data {right arrow over (y)} (k)  are transmitted via the transmit antennas with user-specific weights applied. w 1   (k)  and w 2   (k)  are the weights applied for user k, (or code k), for transmit antennas  1  and  2 , respectively.  
         [0025]      FIG. 1  is a block diagram of a wireless communication system  10  including a transmitter  140  and a receiver  100  for supporting transmit diversity in accordance with one embodiment of the present invention. The transmitter  140  includes a transmit diversity encoder  142  and at least two transmit antennas  150   a ,  150   b . The receiver  100  comprises receive antennas  102   a ,  102   b , a data merger  104 , (if two or more receive antennas are used), a plurality of equalizers  106   a ,  106   b , a plurality of despreaders  110   a ,  110   b  and a closed-loop transmit diversity decoder  120  and/or an STTD decoder  130 . It should be noted that while  FIG. 1  depicts two transmit antennas  150   a ,  150   b  and two receive antennas  102   a ,  102   b  as an example, more than two transmit antennas and any number of receive antennas may be utilized. If multiple receive antennas are used as shown in  FIG. 1 , the data merger  104  is used to combine the received data  103   a ,  103   b  via the receive antennas into one data stream  105 . If only one receive antenna is used, the data merger  104  is not necessary. The transmit diversity encoder  142  may implement open-loop transmit diversity, (i.e., STTD), or closed-loop transmit diversity. The receiver  100  may include only one of the closed-loop transmit diversity decoder  120  and the STTD decoder  130 , or may include both of them and selectively implement the transmit diversity processing.  
         [0026]     The receive antennas  102   a ,  102   b  receive signals transmitted via at least two transmit antennas  150   a ,  150   b . The received signals  103   a ,  103   b  via each of the receive antennas  102   a ,  102   b  are merged into one stream of received data  105  by the data merger  104 . The merged received data  105  is fed into the equalizers  106   a ,  106   b . In the first embodiment, the equalizers  106   a ,  106   b  are NLMS equalizers. Alternatively, any type of adaptive equalizers may be used. Each equalizer  106   a ,  106   b  is locked onto one of the transmit antennas  150   a ,  150   b  of the transmitter  101 . Each equalizer  106   a ,  106   b  performs equalization as if there is only one transmit antenna, (e.g., transmit antenna  150   a ), present and considers transmission by the other transmit antenna, (e.g., transmit antenna  150   b ), as interference.  
         [0027]     For STTD open-loop transmit diversity, a first equalizer  106   a  uses pilot signal p 1 , (e.g., CPICH  1 ), transmitted via the first transmit antenna  150   a , and a second equalizer  106   b  uses pilot signal p 2 , (e.g., CPICH  2 ), transmitted via the second transmit antenna  150   b  as a reference signal, respectively. The first equalizer  106   a  uses pilot signal p 1  for equalizing H 1  to obtain  
           y   →     1     ⁡     (         y   →     1     =       ∑     k   =   1     K     ⁢       y   →     1     (   k   )           )         
 
 and treats signals from the second transmit antenna  150   b  as interference such that:  
                 r   →     =         1     2       ⁢       H   1     ⁡     (       p   1     +       ∑     k   =   1     K     ⁢       y   →     1     (   k   )           )         +     I   2     +     n   →         ;           Equation   ⁢           ⁢     (   8   )               
 
 where I 2  is the interference arising from the second transmit antenna  150   b  including data and pilot transmitted via the second transmit antenna  150   b.  
 
         [0028]     Similarly to obtain  
             y   →     2     ⁡     (         y   →     2     =       ∑     k   =   1     K     ⁢       y   →     2     (   k   )           )       ,       
 
 the second equalizer  106   b  equalizes H 2  using pilot signal p 2  and treats signals from the first transmit antenna  150   a  as interference such that:  
                 r   →     =         1     2       ⁢       H   2     ⁡     (       p   2     +       ∑     k   =   1     K     ⁢       y   →     2     (   k   )           )         +     I   1     +     n   →         ;           Equation   ⁢           ⁢     (   9   )               
 
 where I 1  is the interference arising from the first transmit antenna  150   a  including data and pilot signal transmitted via the first transmit antenna  150   a.  
 
         [0029]     The equalized data outputs  109   a ,  109   b  from the equalizers  106   a ,  106   b  are fed into the despreaders  110   a ,  110   b , respectively. The despreaders  110   a ,  110   b  despread the equalized data  109   a ,  109   b , (i.e., the estimates of {right arrow over (y)} 1  and {right arrow over (y)} 2 ), to obtain the estimates of the transmitted data symbols, {right arrow over (d)} (k) ,  111   a ,  111   b  for user k, (or code k) as follows:  
               d   →     ^     i     (   k   )       =       C       (   k   )     H       ⁢         y   i     →     ^         ,     i   =   1     ,     2   ;         
 
 where C (k)  is the channelization code matrix of user k, (or code k). The estimates of the transmitted data symbols  111   a ,  111   b  are then fed into either the closed-loop diversity decoder  120  or the STTD decoder  130 . 
 
         [0030]     The closed-loop diversity decoder  120  includes a plurality of multipliers  122   a ,  122   b  and a summer  124 . Conjugate  121   a ,  121   b  of the corresponding weights, that are multiplied at the transmit diversity encoder  142  of the transmitter  140 , are multiplied to the data symbols  111   a ,  111   b  by the multipliers  122   a ,  122   b , and the multiplication results  123   a ,  123   b  are combined by the summer  124  to generate data  125  such that:  
                 d     _   ^         (   k   )       =         C       (   k   )     H       ⁡     (         w   1       (   k   )     *       ·       s     _   ^       1       +       w   2       (   k   )     *       ·       s     _   ^       2         )       .             Equation   ⁢           ⁢     (   11   )               
 
         [0031]     The STTC decoder  130  processes the estimates of the transmitted data symbols  111   a ,  111   b  to obtain the data b n    131  for user k, (or code k).  
         [0032]      FIG. 2  is a block diagram of a system  20  including a transmitter  240  and a receiver  200  for supporting transmit diversity processing in accordance with another embodiment of the present invention. The transmitter includes a transmit diversity encoder  242  and at least two transmit antennas  250   a ,  250   b . The receiver  200  includes receive antennas  202   a ,  202   b , a data merger  204 , (if two or more receive antennas are used), a plurality of equalizers  206   a ,  206   b , a plurality of channel estimators  208   a ,  208   b , a plurality of despreaders  210   a ,  210   b  and a closed-loop transmit diversity decoder  220  and/or an STTD decoder  230 . The structure of the receiver  200  is similar to that of the receiver  100  except that the equalizers  206   a ,  206   b  are CE-NLMS equalizers instead of NLMS equalizers. As indicated hereinbefore, more than two transmit antennas and any number of receive antennas may be utilized. If multiple receive antennas are used as shown in  FIG. 2 , the data merger  204  is used to combine the received data  203   a ,  203   b  via the receive antennas into one data stream  205 . If only one receive antenna is used, the data merger  204  is not necessary. The transmit diversity encoder  242  may implement open-loop transmit diversity, (i.e., STTD), or closed-loop transmit diversity. The receiver  200  may include only one of the closed-loop transmit diversity decoder  220  and the STTD decoder  230 , or may include both of them and selectively implement the transmit diversity processing.  
         [0033]     The receive antennas  202   a ,  202   b  receive transmitted signals transmitted via the transmit antennas  250   a ,  250   b . The received signals  203   a ,  203   b  from each of the receive antennas  202   a ,  202   b  are merged into one stream of received data  205  by the data merger  204 . The merged received data  205  is fed into the equalizers  206   a ,  206   b  and the channel estimators  208   a ,  208   b . In this embodiment, the equalizers  206   a ,  206   b  are CE-NLMS equalizers. The CE-NLMS equalizers  206   a ,  206   b  use a channel estimate  215   a ,  215   b  generated by the channel estimators  208   a ,  208   b , respectively, for filter tap coefficients adaptation such that:
 
 {right arrow over (w)}   k   =α{right arrow over (w)}   k-1 +β( E{p·{right arrow over (z)}   k   H   }−{right arrow over (u)}   k   ·{right arrow over (z)}   k   H );  Equation (12)
 
 where  
         β   =     μ              z   _     k          2         ,       
 
 u k  denotes the descrambled equalizer output such that
 
u k ={right arrow over (z)} k {right arrow over (w)} k ;  Equation (13)
 
 where {right arrow over (w)} k  are the filter coefficients of iteration k. The expectation E{p·{right arrow over (z)} k   H } can be obtained from channel estimation or channel state information (CSI). 
 
         [0034]     Each equalizer  206   a ,  206   b  is locked onto one of the transmit antennas  250   a ,  250   b  of the transmitter and performs equalization as if there is only one transmit antenna (e.g., transmit antenna  250   a ) present and considers transmission by the other transmit antenna (e.g., transmit antenna  250   b ) as interference. The equalized data outputs  209   a ,  209   b  from the equalizers  206   a ,  206   b  are fed into the despreaders  210   a ,  210   b , respectively. The despreaders  210   a ,  210   b  despread the equalized data  209   a ,  209   b , (i.e., the estimates of {right arrow over (y)} 1  and {right arrow over (y)} 2 ), to obtain the estimates of the transmitted data symbols, {right arrow over (d)} (k),  211   a ,  211   b  for user k, (or code k). The estimates of the transmitted data symbols  211   a ,  211   b  are then fed into either the closed-loop diversity decoder  220  or the STTD decoder  230  to recover the data as explained hereinbefore. The closed-loop diversity decoder  220  includes a plurality of multipliers  222   a ,  222   b  and a summer  224 . Conjugate  221   a ,  221   b  of the corresponding weights, that are multiplied at the transmit diversity encoder  242 , are multiplied to the data symbols  211   a ,  211   b  at the multipliers  222   a ,  222   b , and the multiplication results  223   a ,  223   b  are combined by the summer  224  to generate data  225 . The STTC decoder  230  processes the estimates of the transmitted data symbols  211   a ,  211   b  to obtain the data b n    231  for user k, (or code k).  
         [0035]      FIG. 3  is a block diagram of a system  30  including a transmitter  351  and a receiver  300  supporting transmit diversity while implementing PIC in accordance with the present invention. The receiver  300  implements interference cancellation based equalization. The transmitter  351  includes a transmit diversity encoder  352  and a plurality of transmit antennas  350   a ,  350   b . The receiver  300  includes a receive antenna  302 , a plurality of equalizers  306   a ,  306   b , a plurality of channel estimators  308   a ,  308   b , a plurality of adders  304   a ,  304   b , a plurality of interference construction units  340   a ,  340   b , a plurality of despreaders  310   a ,  310   b  and a closed-loop transmit diversity decoder  320  and/or an STTD decoder  330 . It should be noted that  FIG. 3  depicts two transmit antennas and one receive antenna as an example, and more than two transmit antennas and/or receive antennas may be utilized. If multiple receive antennas are used, the received signals via each of the receive antennas may be merged into one stream of received data by a data merger (not shown) as shown in  FIGS. 1 and 2 .  
         [0036]     The receive antenna  302  receives signals transmitted via at least two transmit antennas  350   a ,  350   b . The received data  303  is fed into the channel estimators  308   a ,  308   b  and the equalizers  306   a ,  306   b  via the adders  304   a ,  304   b . Each channel estimator  308   a ,  308   b  and each equalizer  306   a ,  306   b  are locked onto one of the transmit antennas  350   a ,  350   b . The channel estimators  308   a ,  308   b  generate channel estimates  315   a ,  315   b  using corresponding pilot signals p 1  and p 2 . The equalizers  306   a ,  306   b  may be NLMS equalizers, CE-NLMS equalizers or any type of adaptive equalizers. If CE-NLMS equalizers are used, the channel estimates  315   a ,  315   b  are fed into the equalizers  306   a ,  306   b  to be used in equalization.  
         [0037]     There are two options for interference cancellation: pilot cancellation only and pilot plus data cancellation. For pilot cancellation only, the interference construction units  340   a ,  340   b  receive pilot signals  307   a ,  307   b  and channel estimates  315   a ,  315   b  generated by the channel estimators  308   a ,  308   b , respectively, and construct a pilot with channel responses, (Ĥ 1 p 1  and Ĥ 2 p 2 )  342   a ,  342   b , respectively. The pilot with channel responses  342   a ,  342   b  are then subtracted by the adders  304   a ,  304   b  from the received data  303 . The subtraction of the constructed pilot with channel response  342   a ,  342   b  is expressed as follows:  
                   r   →     1     =       r   →     -       1     2       ⁢       H   ^     2     ⁢     p   2           ;           Equation   ⁢           ⁢     (   14   )                     r   →     2     =       r   →     -       1     2       ⁢       H   ^     1     ⁢       p   1     .                 Equation   ⁢           ⁢     (   15   )               
 
         [0038]     The resulting pilot-cancelled received signal, ({right arrow over (r)} 1  and {right arrow over (r)} 2 ),  305   a ,  305   b  are then fed into the equalizers  306   a ,  306   b . The pilot cancellation does not require feedback from output of equalizers for interference cancellation. The equalized data  309   a ,  309   b  are then fed to the despreaders  310   a ,  310   b  for despreading. Despread data  311   a ,  311   b  are then fed to the closed loop transmit diversity decoder  320  or the STTD decoder  330  and decoded as explained hereinbefore.  
         [0039]     For pilot and data cancellation, the equalizers  306   a ,  306   b  first equalize H 1  and H 2  separately using pilot signals p 1  and p 2 , respectively. After equalization, data parts, {right arrow over (y)} 1 , {right arrow over (y)} 2  or {right arrow over (s)} 1 , {right arrow over (s)} 2  are estimated and fed into the interference construction units  340   a ,  340   b , respectively. The interference construction units  340   a ,  340   b  construct pilot and data with channel responses  342   a ′,  342   b ′ for transmit antennas  350   a ,  350   b , respectively. The pilot and data with channel responses  342   a ′,  342   b ′ are then subtracted from the received data  303  such that:
 
 {right arrow over (r)}   1   ={right arrow over (r)}−Î   2 ;  Equation (16)
 
 {right arrow over (r)}   2   ={right arrow over (r)}−Î   1 ;  Equation (17)
 
 where Î 1  and Î 2  are estimated interferences arising from the transmit antennas  350   a ,  350   b , respectively. 
 
         [0040]     For open-loop STTD Î 1  and Î 2  are as follows:  
                   I   ^     1     =       1     2       ⁢         H   ^     1     ⁡     (       p   1     +       ∑     k   =   1     K     ⁢       y     →   ^       1     (   k   )           )           ;           Equation   ⁢           ⁢     (   18   )                     I   ^     1     =       1     2       ⁢           H   ^     2     ⁡     (       p   2     +       ∑     k   =   1     K     ⁢       y     →   ^       2     (   k   )           )       .               Equation   ⁢           ⁢     (   19   )               
 
         [0041]     For close-loop transmit diversity Î 1  and Î 2  are as follows:  
                   I   ^     1     =         H   ^     1     ⁡     (         1     2       ⁢     p   1       +       s     →   ^       1       )         ;           Equation   ⁢           ⁢     (   20   )                     I   ^     2     =           H   ^     2     ⁡     (         1     2       ⁢     p   2       +       s     →   ^       2       )       .             Equation   ⁢           ⁢     (   21   )               
 
         [0042]     The resulting pilot/data-cancelled received signal ({right arrow over (r)} 1  and {right arrow over (r)} 2 )  305   a ′,  305   b ′ are then equalized by the equalizers  306   a ,  306   b , respectively. The equalized data  309   a ,  309   b  are then fed to the despreaders  310   a ,  310   b  for despreading. Despread data  311   a ,  311   b  are then fed to the closed loop transmit diversity decoder  320  or the STTD decoder  330  and decoded as explained hereinbefore.  
         [0043]     This embodiment requires channel estimation information. Either NLMS or CE-NLMS may be used as equalization, but CE-NLMS is preferred when channel estimation is available.  
         [0044]     The interference cancellation can be performed either in soft or hard forms depending on the implementation and performance consideration. When the interference cancellation is performed in a soft form for the data, the input to the interference construction units  340   a ,  340   b  is soft samples, which can be obtained from the outputs  309   a ,  309   b  of the equalizers  306   a ,  306   b . If the interference cancellation is performed in a hard form, the inputs to the interference construction units  340   a ,  340   b  are fed to hard decision devices (not shown) first and then the output of the hard decision device is fed to the interference construction units  340   a ,  340   b  sequentially. The hard decision devices restore the samples to the signal constellation according to the transmitted signal constellation, such as quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM). When pilot cancellation is used, the interference construction for pilot should implement the hard form in the interference construction units  340   a ,  340   b  because the pilot sequences are known sequences to the receivers and no estimation is needed for the pilot sequences.  
         [0045]      FIG. 4  is a block diagram of a system  30   a  including a transmitter  351  and a receiver  300   a  implementing PIC or SIC selectively in accordance with the present invention. The structure of the receiver  300   a  is similar to that of the receiver  300  except that the receiver  300   a  further includes an SIC/PIC controller  360  to implement PIC or SIC selectively. The receiver  300   a  implements a PIC between transmit antennas  350   a ,  350   b . When the received power from two transmit antennas  350   a ,  350   b  are not equal, SIC may be advantageous.  
         [0046]     The channel estimators  308   a ,  308   b  measure power of the corresponding pilot signals and the SIC/PIC controller  360  receives the measured power related to the two transmit antennas  350   a ,  350   b  as input and sorts the transmit antennas  350   a ,  350   b  in descending order according to the measured power. The SIC/PIC controller  360  then determines whether the power difference between two transmit antennas  350   a ,  350   b  exceeds a predetermined threshold. If the power difference exceeds the threshold, the SIC/PIC controller  360  selects SIC. Otherwise, the SIC/PIC controller  360  selects PIC. The SIC/PIC selection may be static or dynamic.  
         [0047]     If SIC is selected, the received signal from a transmit antenna with stronger received signal power is equalized first, and the interference of the stronger power transmit antenna signals is constructed and subtracted from the received signal. The resulting signal is then equalized for the transmit antenna having a weaker received signal power.  
         [0048]      FIG. 5  is a schematic block diagram of a receiver  500  using a joint chip-level equalizer for open loop transmit diversity in accordance with the present invention. The receiver  500  includes a joint chip-level equalizer  502 , a plurality of channel estimators  504   a ,  504   b  and a despreader  506 . Received samples  501 , which are generated from received signals from two or more transmit antennas (not shown), are fed to the joint chip-level equalizer  502  and the channel estimators  504   a ,  504   b . Each channel estimator  504   a ,  504   b  is locked onto one of the transmit antennas and generates channel estimates  503   a ,  503   b  using corresponding pilot signals. The joint chip-level equalizer  502  utilizes the channel estimates  503   a ,  503   b  for equalizing the received samples  501 . The equalized received samples  505  are then fed to the despreader  506  for despreading.  
         [0049]      FIG. 6  is a schematic block diagram of a receiver  600  using a chip-level equalizer  602  for closed loop transmit diversity in accordance with the present invention. The structure of the receiver  600  is similar to that of the receiver  500  except the chip level equalizer  602  implements closed-loop transmit diversity. The chip-level equalizer  602  receives the weights  607   a ,  607   b  along with the channel estimates  603   a ,  603   b  generated by the channel estimators  604   a ,  604   b  and outputs equalized received samples  605  multiplied by the weights at chip rate. The equalized received samples  605  are then fed to the despreader  606  for despreading.  
         [0050]      FIG. 7  is a block diagram of an HSDPA receiver  700  using a chip-level equalizer for open and closed loop transmit diversity in accordance with the present invention. The receiver  700  includes a plurality of chip level equalizers  702   a ,  702   b , a plurality of channel estimators  704   a ,  704   b , a plurality of high speed shared control channel (HS-SCCH) despreaders  706   a ,  706   b , a plurality of high speed physical downlink shared channel (HS-PDSCH) despreaders  708   a ,  708   b  and a plurality of decoders  710   a ,  710   b.    
         [0051]     Received samples  701  are fed to the chip-level equalizers  702   a ,  702   b  and the channel estimators  704   a ,  704   b . Each of the chip-level equalizers  702   a ,  702   b  and each of the channel estimators  704   a ,  704   b  are locked onto one of the transmit antennas (not shown). Each of the channel estimators  704   a ,  704   b  generates channel estimates  703   a ,  703   b  using an associated pilot signal  711   a ,  711   b , respectively. Each of the chip-level equalizers  702   a ,  702   b  equalizes the received samples  701  using either the channel estimates  703   a ,  703   b  or pilot signals  711   a ,  711   b  depending on the type of equalizer. If the chip-level equalizers  702   a ,  702   b  are NLMS equalizers, the pilot signals  711   a ,  711   b  are used, and if the chip-level equalizers  702   a ,  702   b  are CE-NLMS equalizers, the channel estimates  703   a ,  703   b  are used.  
         [0052]     The transmit diversity may be either open loop or closed loop. In closed loop transmit diversity, the chip level equalizers  702   a ,  702   b  receive weights  713   a ,  713   b , respectively, and multiples them to the equalized samples at chip rate.  
         [0053]     Each of the equalized received samples  705   a ,  705   b  is fed to the corresponding HS-SCCH despreaders  706   a ,  706   b  and the HS-PDSCH despreaders  708   a ,  708   b , respectively. The HS-SCCH despreaders  706   a ,  706   b  and the HS-PDSCH despreaders  708   a ,  708   b  despread for an HS-SCCH and a high speed downlink shared channel (HS-DSCH). The HS-SCCH despread data  707   a ,  707   b  are fed to the first transmit diversity decoder  710   a  and the HS-DSCH despread data  709   a ,  709   b  are fed to the second transmit diversity decoder  710   b . The transmit diversity decoders may be STTD decoders or closed-loop transmit diversity decoders.  
         [0054]      FIG. 8  is a block diagram of a receiver  800  including a combined chip-level equalizer for open and closed loop transmit diversity in accordance with the present invention. The receiver  800  includes chip-level equalizers  802 , channel estimators  804  and a selector  806 . Multiple channel estimators  804  are provided such that each of the channel estimators is locked on to one of the transmit antennas (not shown) to generate channel estimate  803  using a corresponding pilot signals  811   a ,  811   b . Preferably, the chip-level equalizers  802  include a chip-level equalizer without transmit diversity  802   a , a chip-level equalizer for STTD mode  802   b , a chip-level equalizer for closed-loop mode  802   c . The chip-level equalizers  802   a ,  802   b ,  802   c  receive received samples  801  and channel estimates  803 , and outputs equalized received samples  805 , respectively. The selector  806  selects one of the outputs of the chip-level equalizers  802 . When transmit diversity is not used, the selector  806  selects the output from the chip-level equalizer  802   a , when STTD mode transmit diversity is used, the selector  806  selects the output from the chip-level equalizer  802   b , and when closed loop mode transmit diversity is used, the selector  806  selects the output from the chip-level equalizer  802   c.    
         [0055]     Although the features and elements of the present invention are described in the preferred embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the preferred embodiments or in various combinations with or without other features and elements of the present invention.