Abstract:
Methods and apparatus are disclosed for receiving a diversity transmitted signal using a Rake or G-Rake receiver in a radio system. Signals are diversity transmitted in a first and second symbol period by two or more antennas. One or more receive antennas receive the diverse transmit signals. Weights are generated to account for the correlation between the channels over which the diverse transmit signals are transmitted in the first and second symbol period and are used to weight the received signals. The weighted signals are then fed to a mutual receiver.

Description:
TECHNICAL FIELD 
     The present invention relates to a method and a device for receiving radio signals. In particular the present invention relates to a RAKE or GRAKE receiver and a method for receiving signals in a RAKE or a GRAKE receiver. 
     BACKGROUND 
     In WCDMA, the open loop downlink transmission diversity employs a Space Time block coding based Transmit Diversity (STTD), see 3GPP TS 25.211, “Physical channels and mapping of transport channels onto physical channels (FDD)”, which is based on the Alamouti Space-Time Block Code (STBC), see S. M. Alamouti, “A simple transmit diversity technique for wireless communications”, IEEE Journal on Selected Areas in Communications, Volume 16, Issue 8, October The STTD encoding is optional in UTRAN (UMTS Terrestrial Radio Access Network) UMTS is the Universal Mobile Telecommunications Systems and STTD support is mandatory in the User Equipment (UE). 
     Space-time block coding is a technique used in wireless communications to transmit multiple copies of a data stream across a number of antennas and to exploit the various received versions of the data to improve the reliability of data-transfer. The first STBCs were designed for a two transmitting antenna, which is a rate-1 code. It takes two time-slots to transmit two symbols, and there are two copies of each symbol transmitted. 
     In  FIG. 1  such a diversity scheme is depicted as a baseband representation. The scheme is designed for two transmitting antennas and one receiving antenna. At a given symbol period, two signals are simultaneously transmitted from the two antennas. The signal transmitted from antenna zero is denoted by s 0  and from antenna one by −s 1 *. During the next symbol period signal s 1  is transmitted from antenna zero, and signal s 0  is transmitted from antenna one where * is the complex conjugate operation. This sequence is shown in Table 1 below. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 The encoding and transmission sequence for the two-branch 
               
               
                 transmit diversity scheme 
               
             
          
           
               
                   
                 Antenna 0 
                 Antenna 1 
               
               
                   
                   
               
             
          
           
               
                   
                 1 st  symbol period 
                 s 0   
                 −s 1 * 
               
               
                   
                 2 nd  symbol period 
                 s 1   
                 s 0 * 
               
               
                   
                   
               
             
          
         
       
     
     In Table 1 the encoding is done in space and time (space-time coding). The encoding, however, may also be done in space and frequency. Instead of two adjacent symbol periods, two adjacent carriers may for example be used for the two symbol periods (space-frequency coding). 
     Moreover in the U.S. Pat. No. 6,754,253, a RAKE receiver structure is proposed for the transmit diversity in CDMA system where the mobile terminal includes a first rake receiver matched to a first transmitting antenna and a second rake receiver matched to a second transmitting antenna. 
     Also, the generalized RAKE (GRAKE) receiver, see G. E. Bottomley, T. Ottosson, and Y.-P. E. Wang, “A generalized RAKE receiver for interference suppression,” IEEE J. Select. Areas Commun., vol. 18, pp. 1536-1545, August 2000 is a good means for suppressing the colored interference from the own base station in a CDMA system. The GRAKE receiver is also extended to MIMO system. see S. J. Grant, K. J. Molnar, and G. E. Bottomley “Generalized RAKE receivers for MIMO systems”, VTC 2003-Fall. 2003 IEEE 58 th , Volume I and Wang Hai, Miao Qingyu, “GRAKE2 modeling”, EAB-06:039491, pB. Also GRAKE for STTD in WCDMA system is discussed in Y.-P. Eric Wang, G. E. Bottomley, and A. S. Khayrallah, “Transmit Diversity and Receiver Performance in a WCDMA System”, submitted to globecom 2007. 
     In  FIG. 2  a typical RAKE receiver structure  200  is depicted. In the existing structure  2  receivers are used, the first receiver detects the signal in the first symbol period and the second receiver detects the signal in the second symbol period. In the end, the signal is combined. Hence in the existing receiver structure, the receiver is configured to first obtain the channel coefficient h with channel estimation. The weighting factors w 1 * for the first symbol period is then obtained using the channel coefficient h, the power, the interference and noise in the first symbol period. The signal received by a first antenna  201  in the first symbol period is fed to the taps  205  of the first receiver of the RAKE/GRAKE receiver  200 . The signal is despread in despreading modules  209  for each tap  205 . Each despreaded signal is then multiplied with the corresponding weighting factors w 1 * for the first symbol period. A received signal for the first symbol period is then formed in a receiver  215 . Next, the weighting factors w 2 * for the second symbol period are calculated with the channel coefficient h, the power, the interference and noise in the second symbol period. The detected signal in the second symbol period with the weighting factor w 2 *for the second symbol period is then obtained in the same manner as the signal in the first symbol period. Hence, the signal associated with the second symbol period received by a second antenna  203  is despread in despreading modules  209  for each tap  207 . Each despreaded signal is then multiplied with the corresponding weighting factors w 2 * for the second symbol period. A received signal for the second symbol period is then formed in a receiver  217 . Finally, the received signal in the first symbol period and the received signal in the second symbol period are combined to for a combined received signal in a combiner  219 . 
     There is a constant demand to improve the efficiency in existing telecommunications systems and to utilize resources in a more cost efficient way. Hence, there exist a need for a method and a system that is able to improve over existing telecommunications systems. 
     SUMMARY 
     In one aspect, the present invention provides a receiver structure for a RAKE and GRAKE receiver that is more efficient and which requires less resources compared to existing structures. 
     According to one or more embodiments, in a RAKE or GRAKE receiver, a combined set of weight factors are used to combine the signal received in a first symbol period with the signal received in a second symbol period. The output from the first and second symbol period is then fed to a single, mutual receiver. 
     In accordance with one embodiment the method of receiving a signal is extended to multiple receiving antennas. 
     The invention also extends to a RAKE and GRAKE receiver structure for receiving signals in accordance with the method. In the receiver structure, only one receiver is needed for two symbol periods. In the existing receiver structures, two receivers are needed for the two symbol periods separated. Hence, one receiver can be saved. Also, with the proposed receiver structure. Multiple Input Multiple Output (MIMO) receivers can be reused for the STTD. 
     Another advantage provided by the method and the receiver in accordance with the present invention is that if there is correlation between the signals in the two symbol periods any such existing correlation can be utilized, which is not the case for existing RAKE and GRAKE receivers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will now be described in more detail by way of non-limiting examples and with reference to the accompanying drawings, in which: 
         FIG. 1  is a view illustrating a STBC transmission system, 
         FIG. 2  is view of an existing RAKE/GRAKE receiver for receiving STBC transmitted data, 
         FIG. 3  is view of an exemplary RAKE/GRAKE receiver for receiving STBC transmitted data in accordance with the invention, and 
         FIG. 4  is a flowchart illustrating steps performed when receiving transmitted signals. 
     
    
    
     DETAILED DESCRIPTION 
     From  FIG. 1  and table 1 it can be seen that the received signals at the two receiving symbol periods are,
 
 r   10 ( n )=√{square root over (i P 0 )} h   10 ( n )* s   0 ( n )−√{square root over ( P   1 )} h   11 ( n )* s   1 *( n )+√{square root over ( N   0 )} z   0 ( n )  (1)
 
 r   11 ( n )=√{square root over ( P   0 )} h   10 ( n )* s   1 ( n )+√{square root over ( P   1 )} h   11 ( n )* s   0 *( n )+√{square root over ( N   0 )} z   1 ( n )  (2)
 
     Where P j  is the transmission power from antenna j for the STTD user. Assume that,
 
 y   0 ( n )= r   10 ( n )=√{square root over ( P   0 )} h   10 ( n )* s   0 ( n )−√{square root over ( P   1 )} h   11 ( n )* s   1 *( n )+√{square root over ( N   0 )} z   0 ( n )  (3)
 
 y   1 ( n )= r   11 *( n )=√{square root over ( P   1 )} h   11 *( n )* s   0 ( n )+√{square root over ( P   0 )} h   10 *( n )* s   1 *( n )+√{square root over ( N   0 )} z   1 *( n )  (4)
 
if {tilde over (s)} 0 (n)=s 0 (n) and {tilde over (s)} 1 (n)=s 1 *(n) are regarded as two streams, and the two symbol periods are regarded as two receiving antennas, then the alamouti diversity with 2 transmitting antennas and 1 receiving antenna is equivalent to a 2-by-2 MIMO system with common scrambling code (but the bandwidth is halved). The composite multi-path channels can be
 
     
       
         
           
             
               
                 
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     Assume that there are S streams in the system. Stream  1  and stream  2  are the STTD streams. Pu k  is the transmission power of the desired code channel on stream k. The co-variance matrix for the received signals can be constructed as, 
     
       
         
           
             
               
                 
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     The detailed calculation can be found in Wang Hai, Miao Qingyu, “GRAKE2 modeling”, EAB-06:039491, pB1. 
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     Where, h N0  and h N1  are the multi-path channels between the 2 transmitting antennas and Nth receiving antenna. 
     And the covariance matrix can be extended to, 
     
       
         
           
             
               
                 
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                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   j 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           … 
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               1 
                             
                           
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               2 
                             
                           
                         
                         
                           … 
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     j 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   j 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           … 
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                             
                           
                         
                         
                           
                             R 
                             
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                               ⁢ 
                               
                                 ( 
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     ] 
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     Where, R (2i-1)(2j-1)  is the cross-correlation between the impairment components on fingers assigned to antenna i and antenna j in the first symbol period, R (2i)(2j)  is the cross-correlation between the impairment components on fingers assigned to antenna i and antenna j in the second symbol period. 
     R (2i-1)(2j)  is the cross-correlation between the impairment components on fingers assigned to antenna i in the first symbol period and antenna j in the second symbol period, R (2i)(2j-1)  is the cross-correlation between the impairment components on fingers assigned to antenna i in the second symbol period and antenna j in the second symbol period. 
     This 2-by-2N receiver structure for STTD can be combined with RAKE or GRAKE receiver. 
     For RAKE receiver, the weights can be 
                     [           w     1   ,   k                 w     2   ,   k               …           …             w       (       2   ⁢   N     -   1     )     ,   k                 w       2   ⁢   N     ,   k             ]     =     [             h   ~       1   ,   k                   h   ~       2   ,   k               …           …               h   ~         (       2   ⁢   N     -   1     )     ,   k                   h   ~         2   ⁢   N     ,   k             ]             (   10   )               
the GRAKE weights can be
 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             w 
                             
                               1 
                               , 
                               k 
                             
                           
                         
                       
                       
                         
                           
                             w 
                             
                               2 
                               , 
                               k 
                             
                           
                         
                       
                       
                         
                           … 
                         
                       
                       
                         
                           … 
                         
                       
                       
                         
                           
                             w 
                             
                               
                                 ( 
                                 
                                   
                                     2 
                                     ⁢ 
                                     N 
                                   
                                   - 
                                   1 
                                 
                                 ) 
                               
                               , 
                               k 
                             
                           
                         
                       
                       
                         
                           
                             w 
                             
                               
                                 2 
                                 ⁢ 
                                 N 
                               
                               , 
                               k 
                             
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       Ru 
                       k 
                       
                         - 
                         1 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         
                           
                             
                               
                                 h 
                                 ~ 
                               
                               
                                 1 
                                 , 
                                 k 
                               
                             
                           
                         
                         
                           
                             
                               
                                 h 
                                 ~ 
                               
                               
                                 2 
                                 , 
                                 k 
                               
                             
                           
                         
                         
                           
                             … 
                           
                         
                         
                           
                             … 
                           
                         
                         
                           
                             
                               
                                 h 
                                 ~ 
                               
                               
                                 
                                   ( 
                                   
                                     
                                       2 
                                       ⁢ 
                                       N 
                                     
                                     - 
                                     1 
                                   
                                   ) 
                                 
                                 , 
                                 k 
                               
                             
                           
                         
                         
                           
                             
                               
                                 h 
                                 ~ 
                               
                               
                                 
                                   2 
                                   ⁢ 
                                   N 
                                 
                                 , 
                                 k 
                               
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     By using this receiver structure for STTD, i.e. mapping the transmit diversity to a MIMO system, only one receiver is needed for the two symbol periods, which is flexible and also saves resources. 
     In  FIG. 3  a RAKE/GRAKE receiver structure  300  designed to utilize weight is accordance with the present invention is depicted. Hence the receiver can comprise two receiving antennas  301  and  303  for receiving a first signal via a first antenna  301  transmitted in a first symbol period and a second antenna  303  for receiving a second signal transmitted in second symbol period. The receiver  300  further comprises a set of taps  305  and  307  associated with a receiving antenna. Each tap is associated with a particular time delay τ. The taps  305  and  307  are coupled to despreaders  309  configured to despread the signals received by the antennas  301  and  303  fed through the respective set of taps  305  and  307 , respectively. Each despreaded sub-signal is then weighted with the corresponding weights calculated in accordance with the above thereby forming a weighted subsignal for each tap of the receiver structure  300  in sets of weighing units  311  and  313  respectively. The subsignals from both sets of weighing units  313  and  315  are then received by a receiver  315  as the received signal. 
     The receiver structure depicted in  FIG. 3  can be extended to any number N of antennas. Only one receiving antenna may also be used. The receiver structure  300  may be either a RAKE receiver or a GRAKE receiver. In case the receiver structure is a RAKE receiver the weights used in the units  311  and  313  can advantageously be the weights in accordance with equation 10 above. In case the receiver structure is a GRAKE receiver the weights used in the units  311  and  313  can advantageously be the weights in accordance with equation 11 above. 
     In  FIG. 4  the steps performed when receiving a signal transmitted in two symbol periods as received by the receiver structure  300  are shown. First in a step  401   a  first signal transmitted in a first symbol period and a second signal transmitted in second symbol period are received by two antennas. Next in a step  403  each received signal is fed trough a set of taps of the RAKE/GRAKE receiver. Each tap is associated with a particular time delay T. The output from all taps is then despread in a step  405 . Thereupon each despreaded tap output is weighted with a particular weight in a step  407 . Finally, all weighted signals are received by a receiver for forming the received signal in a step  409 . 
     The method described in conjunction with  FIG. 4  can be extended to any number N, one or many receiving antennas. The receiver structure used may be either a RAKE receiver or a GRAKE receiver. In case the receiver structure is a RAKE receiver the weights used can advantageously be the weights in accordance with equation 10 above. In case the receiver structure is a GRAKE receiver the weights can advantageously be the weights in accordance with equation 11 above. 
     By using the RAKE and GRAKE receiver structure as described herein only one receiver is needed for two symbol periods. In the existing receiver structures, two receivers are needed for the two symbol periods separated. Also, with the proposed receiver structure, Multiple Input Multiple Output (MIMO) receivers can be reused for the STTD. Another advantage obtained is that if there is correlation between the signals in the two symbol periods any such existing correlation can be utilized.