Abstract:
A time division duplex (TDD) user equipment (UE) is configured to synchronize to a TDD base station. The UE includes an antenna, a primary synchronization code matched filter, a first plurality of secondary synchronization code matched filters, a second plurality of secondary synchronization code matched filters, and a processor in communication with the first and second plurality of secondary synchronization code matched filters. The first plurality of secondary synchronization code matched filters determines secondary synchronization codes sent on a first carrier and the second plurality of secondary synchronization code matched filters determines secondary synchronization codes sent on a second carrier. The processor is configured to determine a code group assignment and selected timeslot based upon an analysis of the secondary synchronization codes sent on the first and second carriers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/505,575 filed on Aug. 17, 2006, which is a continuation of U.S. patent application Ser. No. 10/695,276 filed on Oct. 28, 2003, which issued on Sep. 5, 2006 as U.S. Pat. No. 7,102,994, which is a continuation of U.S. patent application Ser. No. 09/576,363, filed May 22, 2000, which issued on Apr. 6, 2004 as U.S. Pat. No. 6,717,930. 
     
    
     BACKGROUND 
       [0002]    This invention generally relates to spread spectrum Time Division Duplex (TDD) communication systems using Code Division Multiple Access (CDMA). More particularly, the present invention relates to cell search procedure of User Equipment (UE) within TDD/CDMA communication systems. 
         [0003]      FIG. 1  depicts a wireless spread spectrum TDD/CDMA communication system. The system has a plurality of base stations  30   1  to  30   7 . Each base station  30   1  has an associated cell  34   1  to  34   7  and communicates with user equipments (UEs)  32   1  to  32   3  in its cell  34   1 . 
         [0004]    In addition to communicating over different frequency spectrums, TDD/CDMA systems carry multiple communications over the same spectrum. The multiple signals are distinguished by their respective code sequences (codes). Also, to more efficiently use the spectrum, TDD/CDMA systems as illustrated in  FIG. 2  use repeating frames  38  divided into a number of time slots  36   1  to  36   n , such as sixteen time slots  0  to  15 . In such systems, a communication is sent in selected time slots  36   1  to  36   n  using selected codes. Accordingly, one frame  38  is capable of carrying multiple communications distinguished by both time slot  36   1  to  36   n  and code. 
         [0005]    For a UE  32   1  to communicate with a base station  30   1 , time and code synchronization is required.  FIG. 3  is a flow chart of the cell search and synchronization process. Initially, the UE  32   1  must determine which base station  30   1  to  30   7  and cell  34   1  to  34   7  to communicate. In a TDD/CDMA system, all the base stations  30   1  to  30   7  are time synchronized within a base station cluster. For synchronization with UEs  32   1  to  32   7 , each base station  30   1  to  30   7  sends a Primary Synchronization Code (PSC) and several Secondary Synchronization Code (SSC) signals in the time slot dedicated for synchronization. The PSC signal has an associated chip code, such as an unmodulated 256 hierarchical code, and is transmitted in the dedicated time slot, step  46 . To illustrate, a base station  30   1  may transmit in one or two time slots, such as for a system using time slots  0  to  15  in time slot K or slot K+8, where K is either 0, . . . , 7. 
         [0006]    One technique used to generate a PSC signal is to use two 16 hierarchical sequences, such as X 1  and X 2  in Equations 1 and 2. 
         [0000]      X1=[1, 1, −1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, 1, −1]  Equation 1 
         [0000]      X2=[1, 1, −1, −1, −1, −1, 1, −1, 1, 1, −1, 1, 1, 1, −1, 1]  Equation 2 
         [0000]    Equation 3 illustrates one approach to generate a 256 hierarchal code, y(i), using X 1  and X 2 . 
         [0000]        y ( i )= X 1( i  mod 16)× X 2( i  div 16), where i=0, . . . , 255  Equation 3 
         [0000]    Using y(i), the PSC is generated such as by combining y(i) with the first row of length 256 Hadamarad matrix, h 0 , to produce C p (i) as in Equation 4. 
         [0000]        C   p ( i )= y ( i )× h   0 ( i ), where i=0, . . . , 255  Equation 4 
         [0000]    Since the first row of the Hadamarad matrix is an all one sequence, Equation 4 reduces to Equation 5. 
         [0000]        C   p ( i )= y ( i ), where i=0, . . . , 255  Equation 5 
         [0007]    The C p (i) is used to produce a spread spectrum PSC signal suitable for transmission. 
         [0008]    To prevent the base stations&#39; communications from interfering with each other, each base station  30   1  to  30   7  sends its PSC signal with a unique time offset, t offset , from the time slot boundary  40 . Differing time offsets are shown for time slot  42  in  FIG. 4 . To illustrate, a first base station  30   1  has a first time offset  44   1 , t offset,1  for the PSC signal, and a second base station  30   2 , has a second time offset  44   2 , t offset,2 . 
         [0009]    To differentiate the different base stations  30   1  to  30   7  and cells  34   1  to  34   7 , each base station  30   1  to  30   7  within the cluster is assigned a different group of codes (code group). One approach for assigning a t offset  for a base station using an n th  code group  44   n , t offset,n  is Equation 6. 
         [0000]      t offset,n =n≅71T c   Equation 6 
         [0010]    T c  is the chip duration and each slot has a duration of 2560 chips. As a result, the offset  42   n  for each sequential code group is spaced 71 chips. 
         [0011]    Since initially the UE  32   1  and the base stations  30   1  to  30   7  are not time synchronized, the UE  32   1  searches through every chip in the frame  38  for PSC signals. To accomplish this search, received signals are inputted to a matched filter which is matched to the PSC signal&#39;s chip code. The PSC matched filter is used to search through all the chips of a frame to identify the PSC signal of the base station  30   1  having the strongest signal. This process is referred to as step- 1  of cell search procedure. 
         [0012]    After the UE  32   1  identifies the PSC signal of the strongest base station  30   1 , the UE  32   1  needs to determine the time slot  36   1  to  36   n  in which that PSC and SSC signals are transmitted (referred to as the Physical Synchronization Channel (PSCH) time slot) and the code group used by the identified base station  30   1 . This process is referred to as step- 2  of cell search procedure. To indicate the code group assigned to the base station  30   1  and the PSCH time slot index, the base station  30   1  transmits signals having selected secondary synchronization codes (SSCs), step  48 . The UE  32   1  receives these SSC signals, step  50 , and identifies the base station&#39;s code group and PSCH time slot index based on which SSCs were received, step  52 . 
         [0013]    For a TDD system using 32 code groups and two possible PSCH time slots per frame, such as time slots K and K+8, one approach to identify the code group and PSCH time slot index is to send a signal having one of 64 SSCs. Each of the synchronization codes corresponds to one of the 32 code groups and two possible PSCH time slots. This approach adds complexity at the UE  32   1  requiring at least 64 matched filters and extensive processing. To identify the code group and PSCH time slot index, 17,344 real additions and 128 real multiplications are required in each PSCH time slot and 64 real additions are required for the decision. 
         [0014]    An alternative approach for step- 2  of cell search procedure uses 17 SSCs. These 17 SSCs are used to index the 32 code groups and two possible PSCH time slots per frame. To implement this approach, at least 17 matched filters are required. To identify the code group and time slot, 1,361 real additions and 34 real multiplications are required for each PSCH time slot. Additionally, 512 real additions are required for the decision. 
         [0015]    It would be desirable to reduce the complexity required by a UE  32   1  to perform cell search procedure. 
       SUMMARY 
       [0016]    A time division duplex (TDD) user equipment (UE) is configured to synchronize to a TDD base station. The UE includes an antenna, a primary synchronization code matched filter, a first plurality of secondary synchronization code matched filters, a second plurality of secondary synchronization code matched filters, and a processor in communication with the first and second plurality of secondary synchronization code matched filters. The first plurality of secondary synchronization code matched filters determines secondary synchronization codes sent on a first carrier and the second plurality of secondary synchronization code matched filters determines secondary synchronization codes sent on a second carrier. The processor is configured to determine a code group assignment and selected timeslot based upon an analysis of the secondary synchronization codes sent on the first and second carriers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  illustrates a prior art TDD/CDMA system. 
           [0018]      FIG. 2  illustrates time slots in repeating frames of a TDD/CDMA system. 
           [0019]      FIG. 3  is a flow chart of cell search. 
           [0020]      FIG. 4  illustrates time offsets used by differing base stations sending primary synchronization code signals. 
           [0021]      FIG. 5  is a diagram of the simplified components of a user equipment and a base station using binary phase shift keying modulation for cell search. 
           [0022]      FIG. 6  is a flow chart of secondary synchronization code assignment. 
           [0023]      FIG. 7  illustrates the simplified components of a user equipment and a base station using quadrature phase shift keying modulation for cell search. 
           [0024]      FIG. 8  illustrates the simplified components of a user equipment and a base station reducing the maximum number of transmitted secondary synchronization codes using quadrature phase shift keying modulation. 
           [0025]      FIGS. 9 to 17  are graphs depicting the performance of various synchronization systems under varying simulated channel conditions. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]    The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.  FIG. 5  shows the simplified circuitry of a base station  30   1  and a UE  32   1  for use in cell search. During step- 1  of the cell search, the base station  30   1  generates a PSC signal using a PSC spread spectrum signal generator  66  having the time offset in the time slot  42  associated with the base station  30   1 . The PSC signal is combined by a combiner  63  with M SSC signals. The combined signal is modulated by a modulator  62  to carrier frequency. The modulated signal passes through an isolator  60  and is radiated by an antenna  58  or, alternately, an antenna array. The UE  32   1  receives signals using an antenna  70  or, alternately, an antenna array. The received signals are passed through an isolator  72  where they are demodulated by a demodulator  74  to baseband frequency. During step- 1  of the cell search, the PSC matched filter  76  is used by the processor  80  to search through all the chips of a frame  38  to identify the PSC signal of the base station  30   1  having the strongest signal. 
         [0027]    One approach for detection of a PSC signal location in a frame is as follows. A selected number of positions in the received signal frame, such as forty, having the highest number of accumulated chip matches (i.e. maximum signal strength), are repeatedly correlated at the same positions in subsequent frames  38 . Out of the selected locations, the one having the highest number of cumulative matches (i.e. the maximum signal strength) is identified as the location of the PSC signal. 
         [0028]    For step- 2  of the cell search procedure, the base station  30   1  generates SSC signals, SSC 1  to SSC M , using SSC spread spectrum signal generators  68   1  to  68   M . To reduce the complexity at the UE  32   1 , a reduced number of SSCs are used. By reducing the SSCs, the number of matched filters required at the UE  32   1  is reduced. Additionally, the reduced SSCs decreases the processing resources required to distinguish the different codes. The reduced SSCs also reduces the probability of incorrect detection of a code group number and PSCH time slot index (see  FIGS. 9-15 ). 
         [0029]    One approach to reduce the SSCs is shown in the flow chart of  FIG. 6 . The number of SSCs used, M, is based on the number of code groups and PSCH time slots used per frame, step  54 . The number of SSCs, M, is the log base two of the maximum combination number rounded up to the next higher integer, step  56 , as in Equation 7. 
         [0000]        M =log 2 (# of Code Groups×# of PSCH Time Slots per frame)  Equation 7 
         [0030]    The base station  30   1  generates, using SSC signal generators  68   1  to  68   M , the SSC signals associated with the base station&#39;s code group and the number of PSCH time slots per frame. The SSC signals are combined with each other as well as the PSC signal by combiner  63 . Subsequently, the combined signal is modulated by the modulator  62 , passed through the isolator  60  and radiated by the antenna  58 . The UE  32   1  receives the transmitted signal, passes it through the isolator  72  and demodulates the received signal using the demodulator  74 . Using corresponding SSC 1  to SSC M  matched filters  78   1  to  78   M , the processor  80  determines the binary code that SSCs are modulated. Based on the determined binary code, the base station&#39;s code group and PSCH time slot index in the frame is determined. To illustrate for a system using 32 code groups and two possible time slots per frame, such as slots K and K+8, the number of binary bits needed to modulate SSCs, M, is six (log 2  64). In such a system, the six SSCs are modulated with six bits using binary phase shift keying (BPSK) modulation. The six SSCs are chosen among the 256 rows of Hadamarak matrix, H 8 . The Hadamarak matrix is generated sequentially, such as by Equations 8 and 9. 
         [0000]        H   0 =(1)  Equation 8 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       t 
                     
                      
                     
                       = 
                       
                         [ 
                         
                           
                             
                               
                                 H 
                                 
                                   t 
                                   - 
                                   1 
                                 
                               
                             
                             
                               
                                 H 
                                 
                                   t 
                                   - 
                                   1 
                                 
                               
                             
                           
                           
                             
                               
                                 H 
                                 
                                   t 
                                   - 
                                   1 
                                 
                               
                             
                             
                               
                                 H 
                                 
                                   t 
                                   - 
                                   1 
                                 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                   , 
                   
                       
                   
                    
                   
                     t 
                     = 
                     1 
                   
                   , 
                   … 
                    
                   
                       
                   
                   , 
                   8 
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   9 
                 
               
             
           
         
       
     
         [0031]    A particular code, C k,n (i), where n is the code group number associated with a SSC is produced using Equation 10. The six rows of Hadamarak matrix, H 8 , are r(k)=[24, 40, 56, 104, 120, 136]. 
         [0000]        C   k,n ( i )= b   k   ,n×h   r ( k )( i )× y ( i ), where i=0, 1, . . . , 255 and k=1, . . . , 6  Equation 10 
         [0000]    The value of b 2  to b 6  are depicted in Table 1. 
         [0000]                                                                              TABLE 1                       Code Group (n)   b 6, n     b 5, n     b 4, n     b 3, n     b 2, n                                          1   +1   +1   +1   +1   +1           2   +1   +1   +1   +1   −1           3   +1   +1   +1   −1   +1           . . .   . . .   . . .   . . .   . . .   . . .           32   −1   −1   −1   −1   −1                        
The value of b 1,n  is depicted in Table 2.
 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 PSCH time slot order in the 
                   
               
               
                   
                 frame 
                 b 1, n   
               
               
                   
                   
               
             
             
               
                   
                 K, where K = 0, . . . , 7 
                 +1 
               
               
                   
                 K + 8 
                 −1 
               
               
                   
                   
               
             
          
         
       
     
         [0032]    Each code corresponds to one SSC, SSC 1  to SSC 6 . To distinguish the differing base stations&#39; SSC signals from one another, each of the base stations&#39; SSC signals has the same offset as its PSC signal. At the UE  32   1 , the step- 2  of the cell search procedure (i.e. code group number and PSCH slot order detection) is performed as follows. The received baseband signal is first correlated with C p  as per Equation 4 to obtain phase reference. This correlation is performed by PSC matched filter  76  in  FIG. 5 . The phase reference is obtained by normalizing the correlation value obtained at the output of the PSC matched filter  76 . The received baseband signal is also correlated with C 1 , . . . , C 6  as per Equation 10 to obtain binary data that represent the code group of the base station  30   1  and PSCH slot order in the frame. This correlation is performed by SSC matched filters  78   1 - 78   M  in  FIG. 5 . These matched filter outputs are derotated before BPSK demodulation. The derotation is performed by complex multiplication of the complex conjugate of the phase reference. The derotated SSC matched filter outputs are BPSK demodulated. The BPSK demodulation is performed by a hard limiter on the real part of the derotated SSC matched filter outputs. As a result, if the real part of the derotated SSC matched filter output is greater than zero, it is demodulated as +1. Otherwise, it is demodulated as −1. The demodulated binary data represents the code group of the base station  30   1  and the PSCH time slot order in the frame as depicted in Table 1 and Table 2, respectively. To ease detection of the six SSCs, the UE  32   1  accumulates the derotated outputs of the SSC matched filters  78   1 - 78   M  over a number of the PSCH time slots, such as four or eight. 
         [0033]    Using six SSCs, for 32 code groups and two possible PSCH time slots, requires 653 real additions and 28 real multiplications at the UE  32   1  to identify the code group/PSCH time slot index. For the decision, no additions or multiplications are required. Accordingly, reducing the number of transmitted SSCs in the PSCH time slot reduces the processing at the UE  32   1 . 
         [0034]    Alternately, to reduce the number of SSCs even further quadrature phase shift keying (QPSK) modulation is used. To reduce the SSC number, each SSC signal is sent on either an In-phase (I) or Quadrature (Q) component of the PSCH. One extra bit of data associated with either using the I or Q carrier is used to distinguish the code group/PSCH time slots. As a result, the number of SSCs, M, required by Equation 6 is reduced by one. 
         [0035]    For instance, to distinguish  32  code groups and two possible PSCH time slots, five SSCs (M=5) are required. The code groups are divided in half (code groups  1 - 16  and code groups  17 - 32 ). When the SSCs are transmitted on the I carrier, it restricts the code groups to the lower half (code groups  1 - 16 ) and when the SSCs are transmitted on the Q carrier, it restricts the code groups to the upper half (code groups  17 - 32 ). The five SSCs distinguish between the remaining sixteen possible code groups and two possible PSCH time slots. 
         [0036]    A simplified base station  30   1  and UE  32   1  using QPSK modulation are shown in  FIG. 7 . The base station  30   1  generates the appropriate SSC signals for its code group and PSCH time slot using the SSC spread spectrum signal generators  68   1  to  68   M . Also based on the base station&#39;s code group/PSCH time slot index, switches  90   1  to  90   M  either switch the outputs of the generators  68   1  to  68   M  to an I combiner  86  or a Q combiner  88 . The combined I signal which includes the PSC signal is modulated by an I modulator  82  prior to transmission. The combined Q signal is modulated by a Q modulator  84  prior to transmission. One approach to produce the Q carrier for modulating the signal is to delay the I carrier by ninety degrees by a delay device  98 . The UE  32   1  demodulates the received signals with both an I demodulator  92  and a Q demodulator  94 . Similar to the base station  30   1 , the UE  32   1  may produce a Q carrier for demodulation using a delay device  96 . Obtaining binary data representing the lower or higher half of the 16 code groups and PSCH time slot index is the same as applying BPSK demodulation on the I and Q components of the received signal respectively. The I matched filters  100   1  to  100   M  are used by the processor  80  to determine whether any SSC signals were sent on the I component of the PSCH. A decision variable, I dvar , is obtained such as by using Equation 11. 
         [0000]        I   dvar   =|rx   1   |+|rx   2   |+ . . . +|rx   m |  Equation 11 
         [0037]    |rx i | is the magnitude of the real component (I component) of the i th  SSC matched filter output. Likewise, the Q matched filters  102   1  to  102   M  are used by the processor  80  to determine whether any SSC signals were sent on the Q component of the PSCH. A decision variable, Q dvar , is obtained such as by using Equation 12. 
         [0000]        Q   dvar   =|ix   1   |+|ix   2   |+ . . . +|ix   M |  Equation 12 
         [0000]    |ix i | is the magnitude of the imaginary (Q component) of the i th  SSC matched filter outputs. 
         [0038]    If I dvar  is greater than Q dvar , the SSC signals were transmitted on the I component. Otherwise, the SSC signals were transmitted on the Q component. 
         [0039]    Another approach using QPSK modulation to reduce the number of SSC signals transmitted is depicted in  FIG. 8 . Instead of transmitting the number of SSCs of  FIG. 7 , the number of SSCs, M, representing the code group number and PSCH time slot index is reduced by one. To regain the one lost bit of information by reducing the SSCs, two sets of M SSCs are used. For instance using 32 code groups and two possible PSCH time slots, one set, SSC 11  to SSC 14 , is assigned to the lower code groups, such as code groups  1  to  16 , and the second set, SSC 21  to SSC 24 , is assigned to the upper code groups, such as code groups  17  to  32 . For the lower code group, sending SSC 11  to SSC 14  on the I carrier restricts the code groups to 1 to 8. The Q carrier restricts the code groups to 9 to 16. Likewise, for the upper code group, in phase SSC 21  to SSC 24  restricts the code groups to 17 to 24 and Q SSC 21  to SSC 24  restricts the code groups to 25 to 32. As a result, the maximum number of SSCs transmitted at one time is reduced by one. By reducing the number of SSCs, the interference between SSC signals is reduced. Reduced interference between SSCs allows higher transmission power levels for each SSC signal easing detection at the UE  32   1 . 
         [0040]    A simplified base station  30   1  and UE  32   1  implementing the reduced SSC approach is shown in  FIG. 8 . At the base station  30   1 , two sets of M SSC spread spectrum signal generators  104   11  to  104   2M  generate the SSC signals corresponding to the base station&#39;s code group and PSC time slot. The corresponding SSC signals are switched using switches  106   11  to  106   2M  to either an I  82  or Q modulator  84  as appropriate for that base station&#39;s code group and PSCH time slot. At the UE  32   1 , an I set of matched filters  108   11  to  108   2Q  is used to determine if any of the SSCs were sent on the I carrier. A Q set of matched filters  110   11  to  110   2M  is used to determine if any of the SSCs were sent on the Q carrier. By detecting the transmitted I and Q SSCs, the processor  80  determines the base station&#39;s code group and PSCH time slot. 
         [0041]    One approach to determining which of 32 code groups and two possible PSCH time slots is used by the base station  32   1  follows. After the processor  80  accumulates data from matched filters  110   11  to  110   24 , the code group set, either SSC 11  to SSC 14  or SSC 21  to SSC 24 , is determined using Equations 13 and 14. 
         [0000]      var_set 1=| rx   11   |+|ix   12   |+ . . . +|rx   14   |+|ix   14 |  Equation 13 
         [0000]      var_set 2=| rx   21   |+|ix   22   |+ . . . +|rx   24   |+|ix   24 |  Equation 14 
         [0042]    The values, rx 11  to rx 24 , are the number of accumulated matches for a respective SSC, SSC 11  to SSC 24 , received in the I channel. Similarly, ix 11  to ix 24  are the number of accumulated matches for the Q channel for SSC 11  to SSC 24 . Equations 13 and 14 require a total of 16 real additions. var_set  1  represents the total accumulations of the first SSC set, SSC 11  to SSC 14 . var_set  2  represents the total accumulations of the second SSC set, SSC 21  to SSC 24 . The processor  80  compares var_set  1  to var_set  2  and the larger of the two variables is presumed to be the SSC set transmitted by the base station  32   1 . 
         [0043]    To determine whether the SSCs were transmitted on the I or Q channel, Equations 15 and 16 are used. 
         [0000]      var —   I=|rx   p1   |+ . . . +|rx   p4 |  Equation 15 
         [0000]      var —   Q=|ix   p1   |+ . . . +|ix   p4 |  Equation 16 
         [0044]    If var_set  1  is selected as being larger than var_set  2 , the value of p is one. Conversely, if var_set  2  is larger, the value of p is two. var_I is the accumulated values for the selected set on the I carrier and var_Q is the accumulated values on the Q carrier. The larger of the two variables, var_I and var_Q, is presumed to be the channel that the selected set was transmitted over. By ordering the additions in Equations 13 and 14, the values of var_I and var_Q can be determined simultaneously with var_set  1  and var_set  2 . Accordingly, determining whether the I or Q carrier was used requires no additional additions. As a result, using QPSK modulation and two SSC sets requires 803 real additions and 36 real multiplications in each time slot and 16 real additions for the decision. 
         [0045]      FIGS. 9 to 15  are graphs illustrating the performance for distinguishing 32 code groups/two PSCH time slots of systems using 32 SSCs  128 , 17 SSCs  124  and 6 SSCs  126 . The graphs show the performance for various simulated channel conditions. The simulations accumulated the SSC matches at the UE  32   1  over four or eight PSCH time slots and compared the probability of an incorrect synchronization to the channel&#39;s signal to noise ratio (SNR) in decibels. 
         [0046]    The  FIG. 9  simulation uses an additive white gaussian noise (AWGN) channel and accumulation over eight PSCH time slots. The  FIG. 10  simulation uses a single path Rayleigh fading channel with a six kilohertz (kHz) frequency offset and accumulation over four PSCH time slots. The  FIG. 11  simulation is the same as the  FIG. 10  simulation except the accumulation was over eight PSCH time slots. The  FIG. 12  simulation uses an ITU channel with three multipaths with a UE  32   1  moving at 100 kilometers per hour (km/h) and accumulation over eight PSCH time slots. The  FIG. 13  simulation uses an ITU channel with three multipaths having six kilohertz (kHz) frequency offset and a UE  32   1  moving at 500 km/h with accumulation over eight PSCH time slots. The  FIG. 14  simulation uses a single path Rayleigh channel having 10 kHz frequency offset with accumulation over eight PSCH time slots. The  FIG. 15  simulation uses an ITU channel with three multipaths having 10 kHz frequency offset and the UE  32   1  moving at 500 km/h with accumulation over eight PSCH time slots. 
         [0047]    Under the simulated conditions of  FIGS. 14 and 15 , 6 SSCs  128  outperforms the other techniques  124 ,  126 . As shown in  FIGS. 9 to 13 , 6 SSCs  128  performs favorably in comparison to the other techniques  124 ,  126 . 
         [0048]      FIG. 16  is a graph of the simulated performance of 6 SSCs  114  using BPSK and the two sets of 4 SSCs  112  using QPSK modulation. The simulation used an eight PSCH time slot accumulation of the matches for each SSC and transmission over an AWGN channel. As shown, two set QPSK modulation  112  outperformed 6 SSC BPSK modulation  114 . 
         [0049]      FIG. 17  illustrates the performance of BPSK and two set QPSK modulation accumulating matches over four and eight PSCH time slots. The SSCs were simulated as being transmitted over a single path Rayleigh channel. Performance for both modulation schemes improves with additional time slot correlations. Two set QPSK modulation for four PSCH time slots  116  and eight PSCH time slots  120  outperforms BPSK modulation for four PSCH time slots  118  and eight PSCH time slots  122 , respectively.