Abstract:
A receiver comprises a plurality of antenna elements for receiving a data signal. Each antenna element has a plurality of Rake fingers. Each Rake finger processes a received multipath component of the received data signal of its antenna element by applying a complex weight gain to that received multipath component. A complex weight gain generator determines the complex weight gain for each Rake finger for each antenna element using an input from all the Rake fingers. A summer combines an output of each Rake finger to produce an estimate of the data signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. provisional application Ser. No. 60/507,874, filed Sep. 30, 2003, which is incorporated by reference as if fully set forth. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to the field of wireless communications. More specifically, the present invention relates to a code division multiple access (CDMA) receiver.  
       BACKGROUND  
       [0003]     A received CDMA signal, r l (t), at l th  (I≦L) receiver antenna element out of an L element array is denoted as per Equation 1:  
                       r   l     ⁡     (   t   )       =       ⁢         ∑     k   =   1     K     ⁢       ∑     n   =     -   ∞       ∞     ⁢       ∑     m   =   1     M     ⁢       A   k     ⁢     s     k   ,   n       ⁢       p   k     ⁡     (     t   -   nT   -     τ     k   ,   m         )       ⁢       h     k   ,   m   ,   l       ⁡     (   t   )               +                     ⁢     n   ⁡     (   t   )                     Equation   ⁢           ⁢   1             
 
 where A k  is the signal amplitude of k th  user, s k,n  is the n th  symbol of k th  user, p k  (t) is the signature waveform, including the spread code and pulse shaping waveform, of k th  user. h k,m,l  (t) is the channel response of m th  path from l th  antenna of k th  user. n(t) is the combined interference which is typically due to the interference from other cells and additive channel noise. As is typical, this interference has the statistics of white Gaussian noise. The n th  symbol of the k th  user is of interest and the user index k and the symbol index n are dropped. After despreading the received signal for the k th  user and n th  symbol and for all M paths and all L antennas, Equation 2 is derived as follows: 
 
 d   m,l   =Ah   m,l   s+z   m,l    Equation 2 
 
 where z m,l  is the residual signal at the despreader for m th  path and l th  receiver antenna. 
 
         [0006]     It is traditionally and commonly assumed that all z m,l  (1≦m≦M, 1≦l≦L) are Gaussian variables, and they are mutually uncorrelated across different multipath components and across different antennas. This assumption leads to a very simple and traditional receiver called a “Rake receiver” as shown in  FIG. 1 , where each Rake, or each branch in  FIG. 1 , estimates the complex channel weight gain (CWG) independently. As shown in  FIG. 1 , the antenna array has L elements,  110   1  to  110   L . For each element  110 , a group of delays  112   11  to  112   LN , produce a group of delayed versions of the vector received by that element  110 . Each delayed version is despread by a respective despreader  115   11  to  115   LN . Each despread output is input into a respective CWG generation circuit  105   11  to  105   LN . The derived CWGs are respectively applied to each despread output via respective multipliers  120   11  to  120   LN . The weighted outputs are combined by a combiner  125 . The combiner  125  usually uses the maximum-ratio combining (MRC) in order to achieve the maximum signal-to-noise ratio at the combiner output. Mathematically, each Rake receiver estimates the channel gain g m,l , where g m,l  is an estimate of Ah m,l , and noise variance σ m,l   2 , where σ m,l   2 , is an estimate of the power of z m,l ) If MRC is used, the combiner generates  
         ∑       1   ≤   m   ≤   M       1   ≤   l   ≤   L         ⁢           d     m   ,   l       ⁢     g     m   ,   l     *         σ     m   ,   l     2       .         
 
 Since g m,l  is an estimate of Ah m,l  and σ m,l   2 , is an estimate of the power of z m,l , the generation of g m,l  for any one particular Rake receiver is independent of all other Rake receivers. This approach assumes that all z m,l  (1≦m≦M,1≦l≦L) are zero mean Guassian variables, which are mutually uncorrelated across different multipath components and accross different antennas. However, there is correlation across the multipath components and antennas, which result in inter symbol interference (ISI). Also, due to correlation between multiple user also over the multipath components and antennas, multiple access interference (MAI) is also increased. Accordingly, the receiver performance is degraded. 
 
         [0008]     Accordingly, it is desirable to have alternate receiver configurations.  
       SUMMARY  
       [0009]     A receiver comprises a plurality of antenna elements for receiving a data signal. Each antenna element has a plurality of Rake fingers. Each Rake finger processes a received multipath component of the received data signal of its antenna element by applying a complex weight gain to that received multipath component. A complex weight gain generator determines the complex weight gain for each Rake finger for each antenna element using an input from all the Rake fingers. A summer combines an output of each Rake finger to produce an estimate of the data signal. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     A more detailed understanding of the invention may be had from the following description, given by way of example and to be understood in conjunction with the accompanying drawings wherein:  
         [0011]      FIG. 1  is a prior art Rake receiver;  
         [0012]      FIG. 2  is a block diagram of a Rake-based receiver with two receiver antennas operating in accordance with the present invention;  
         [0013]      FIG. 3  is a block diagram of a CWG generation device used in conjunction with the receiver of  FIG. 2 ;  
         [0014]      FIG. 4  is a block diagram of a circuit used to implement R estimation in conjunction with the receiver of  FIG. 2 ;  
         [0015]      FIG. 5  compares the block error rate (BLER) at 50 km/hr between a conventional Rake receiver and the Rake receiver of  FIG. 2 ; and  
         [0016]      FIG. 6  compares the BLER at 120 km/hr between a conventional Rake receiver and the Rake receiver of  FIG. 2 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]     The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. Hereafter, a wireless transmit/receive unit (WTRU) includes, but is not limited, to a user equipment, a mobile station, a fixed or mobile subscriber unit, a pager, or any other type of device capable of operating in a wireless environment. When referred to hereafter, a base station includes, but is not limited to, a base station, a Node-B, a site controller, an access point, or any other interfacing device in a wireless environment. The multiple antenna element Rake receiver can be used in a WTRU, base station or both.  
         [0018]     Using L receiver antenna elements, all Rake finger outputs are organized into groups having the same de-spread symbol into the same vector. Each Rake finger output is denoted as vector d=[d 1,1 ,d 1,2 , . . . ,d 1,L ,d 2,1 ,d 2,2 , . . . ,d M,1 ,d M,2 , . . . ,d M,L ] T . Similarly, the noise vector at each Rake finger output is denoted as z=]z 1,1 ,z 1,2 , . . . ,z 1,L ,z 2,1 ,z 2,2 , . . . ,z M,1 ,z M,2 , . . . ,z M,L ] T , and the channel vector for all Rake fingers are denoted as. h=[h 1,1 ,h 1,2 , . . . ,h 1,L ,h 2,1 ,h 2,2 , . . . ,h M,1 , M,2 , . . . ,h M,L ] T . Thus, Equation 3 is derived as follows: 
 
 d=AhS+z    Equation 3 
 
         [0019]     The noise correlation matrix is derived as per Equation 4: 
 
 R=E ( zz   H )= E ( dd   H )− A   2   E|s|   2   hh   H    Equation 4 
 
 where for binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) modulation, E|s| 2 =1, and Equation 4 is further simplified as per Equation 5: 
 
 R=E ( zz   H )= E ( dd   H )− A   2   hh   H    Equation 5 
 
         [0021]     An optimal receiver in terms of maximizing the log-likelihood function provides the data detection as denoted as per Equation 6: 
 
 v =( R   −1   h ) H   d    Equation 6 
 
         [0022]      FIG. 2  is a block diagram of a Rake-based element  200  using a CWG generation device  205  in conjunction with L receiver antenna elements  210   1  to  210   L . The components of  FIG. 2  can be implemented on a single integrated circuit (IC), multiple ICs, discrete components or combination of integrated circuits and discrete components. For each element  210 , a group of delays  212   11  to  212   LN , produce a group of delayed versions of the vector received by that element  210 . Each delayed version is despread by a respective despreader  215   11  to  215   LN . All despreader outputs from the L antenna elements  210  for all multipaths are fed to a complex weight gain (CWG) generation device  205  (see  FIG. 3 ), within which a channel estimation h is calculated  320 , correlation matrix R is calculated  305  based on the data from all of the despreaders  215  and the channel estimation h, the inverse of R is calculated  310 , and then the weight is calculated as (R −1 h) H    315 . Each element of the calculated (R −1 h) is applied as a CWG at each multiplier  220   11  to  220   LN  of each Rake finger. These weighted components are summed by a summer  225  to produce soft symbols. Accordingly, the CWG generated for any one Rake finger is derived from all of the despreaders  215 .  
         [0023]     Since the correlation matrix R considers each path for each antenna element, the complex weighting corrects for the ISI. Additionally, since this correction is also applied to other user signals, MAI is also suppressed to some extent across the antennas and paths.  
         [0024]     The noise correlation matrix can be estimated, R, as per Equation 7:  
               R   ^     =         1   N     ⁢       ∑     k   =   1     N     ⁢       d   ⁡     (   k   )       ⁢       d   ⁡     (   k   )       H           -       1   N     ⁢       ∑     k   =   1     N     ⁢         h   ^     ⁡     (   k   )       ⁢         h   ^     ⁡     (   k   )       H                     Equation   ⁢           ⁢   7             
 
 where d(k) is the vector d for a k th  symbol, ĥ(k) is the channel estimation (which is also an estimate of vector Ah) for a k th  symbol, N is the estimation length in symbols. 
 
         [0026]     In  FIG. 4 , an embodiment of the R matrix estimation  305  is shown. The channel estimation h is vector multiplied  400  by its complex conjugate transpose (Hermetian), producing h(k)h(k) H . The multiplied results are averaged  405 ,  
         1   N     ⁢       ∑     K   =   1     N     ⁢       h   ⁡     (   k   )       ⁢         h   ⁡     (   k   )       H     .             
 
 The data from each despreader  215  is vector multiplied  410  by its Hermetian, producing d(k)d(k) H . The results are averaged  415 ,  
         1   N     ⁢       ∑     k   =   1     N     ⁢       h   ⁡     (   k   )       ⁢         h   ⁡     (   k   )       H     .             
 
 A matrix subtraction  420  of the averaged channel estimate from the averaged data is performed, producing {circumflex over (R)} as per Equation 7.  FIG. 5  compares simulation results between a conventional Rake receiver and a Rake-based receiver using an International Telecommunications Union (ITU) voice activity factor (VA) channel model operating in accordance with the present invention at a vehicular speed of 50 km/hr.  FIG. 6  compares simulation results between the conventional Rake receiver and the Rake-based receiver using an ITU VA channel model operating in accordance with the present invention at a vehicular speed of 120 km/hr. The simulations compare the performance of a traditional Rake with one antenna element “Rake(1RxAnt)”, two correlated antenna elements “RakeReceiver(2RxAnt−cor)”, two uncorrelated antenna elements and “RakeReceiver(2RxAnt−uncor)” to an uncorrelated embodiment of the present invention “NewReceiver(2RxAnt−uncor)” and a correlate embodiment “NewReceiver(2RxAnt−cor)”. In each case, the receiver operating in accordance with the present invention provides much better performance than the conventional Rake receiver. 
 
         [0029]     While the present invention has been described in terms of the preferred embodiment, other variations which are within the scope of the invention as outlined in the claims below will be apparent to those skilled in the art.