Patent Publication Number: US-7916774-B2

Title: Method and system for estimating channel of a mobile station in a communication system

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to channel estimation of a Mobile Station (MS) in a communication system. More specifically, the present invention relates to a method and system for channel estimation of a MS in presence of co-channel interference, to improve the performance of sector combining. 
     In a cellular like communication system, sectorization is used as a technique to increase the coverage and capacity of the system or network. A Base Station Transceiver (BTS) comprises multiple sectors, typically 3 sectors are used in a single BTS. A MS is served by a sector of a BTS in which the MS is located in. Each sector of a BTS has information to decode signals received from MSs served by it. The signals may be demodulated at the sector by multiplying the received signal by a pseudo random binary sequence (PRBS) (de-PRBS) which corresponds to the sector. Each sector may use a different PRBS for demodulating its signals. The signals from a MS may be received at one or more collocated or adjacent sectors of a BTS. The signals received at the sector and one or more collocated sectors of the same BTS may be received with different power levels due to different pathloss, shadowing and the angle that the MS makes with the boresight of sector antenna. Therefore, sector combining may be performed to combine the signals received from the MS at the BTS from multiple sectors, which improves the decoding reliability of the signal received from the MS. To achieve sector combining, the channel of the MS has to be estimated at the sectors of the same BTS. The signal received from the MS at one or more collocated sectors may be interfered by a signal received from one or more interfering MSs, which reduces the quality of channel estimation for the MS at one or more collocated sectors. As a result, the link performance of sector combining degrades. 
     SUMMARY 
     Various embodiments provide a method and system for estimating a channel between a desired Mobile Station (MS) and a Base Transceiver Station (BTS) antenna of a one of a plurality of sectors of a BTS in presence of an interfering MS. 
     An embodiment provides a method and system for estimating a channel between a desired MS and a BTS antenna of a one of a plurality of sectors of a BTS. The method includes receiving a composite signal at the BTS antenna. The composite signal comprises a desired MS signal and an interfering MS signal. The composite signal comprises a plurality of pilot symbols and data symbols from each of the desired and interfering MS signals. Thereafter, a known desired random sequence and a known interfering random sequence is applied to pilot symbols of the composite signal, resulting in a first de-randomized signal and a second de-randomized signal. Finally, the channel estimate is obtained by weighting and summing the first de-randomized signal and second de-randomized signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the present invention is provided by reference to the following detailed description when considered in conjunction with the accompanying drawings in which reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a block diagram showing an environment (that is exemplary) in which various embodiments of the invention may function. 
         FIG. 2  is a flowchart of a method for estimating channel between a desired Mobile Station (MS) and a Base Transceiver Station (BTS) antenna in a communication system, in accordance with an embodiment. 
         FIG. 3  is a flowchart of a method for estimating channel between a desired MS and a BTS antenna in a communication system, in accordance with another embodiment. 
         FIG. 4  is a block diagram illustrating channel estimation between a desired MS and a BTS antenna in a communication system, in accordance with an exemplary embodiment. 
         FIG. 5  is a bock diagram showing various components of a BTS for estimating channel between a desired MS and a BTS antenna, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Various embodiments provide methods and systems for estimating channel between a desired Mobile Station (MS) and a Base Transceiver Station (BTS) antenna of a BTS in a communication system. Examples of the communication system include wireless communication systems, for example, GSM, CDMA, OFDMA, and OFDM. The BTS antenna receives a composite signal from the desired MS and an interfering MS. The desired MS signal and the interfering MS signal include overlapping time and frequency data symbols. Additionally, the desired MS signal and the interfering MS include overlapping time and frequency pilot symbols. The BTS antenna demodulates the composite signal by applying a known desired random sequence and a known interfering random sequence to pilot symbols of the composite signal. Demodulation of the composite signal results into a first de-randomized signal and a second de-randomized signal. Finally, channel estimate is obtained by weighting and summing the first de-randomized signal and the second de-randomized signal. 
       FIG. 1  is a block diagram showing an environment  100  (that is exemplary) in which various embodiments of the invention may function. Environment  100  includes a BTS  102 . Base Transceiver Station (BTS)  102  includes a BTS antenna  104 , a BTS antenna  106 , and a BTS antenna  108 . It will be apparent to a person skilled in the art that BTS  102  may include more than three BTS antennas. BTS  102  serves Mobile Stations (MSs) in a plurality of sectors (for example, a sector  110 , a sector  112 , and a sector  114 ) in a geographic region. One or more BTS antennas of BTS  102  serve one or more MSs in the corresponding sector in a geographic region. For example, BTS antenna  104  serves one or more MSs in sector  110 , BTS antenna  106  serves one or more MSs in sector  112 , and BTS antenna  108  serve one or more MSs in sector  114 . 
     A desired MS  116  is served by BTS antenna  104 . Desired MS  116  transmits signal to BTS antenna  104 . The signal transmitted by desired MS  116  may be received at each of BTS antenna  106  and BTS antenna  108 . However, the signal received from desired MS  116  at BTS antenna  106  may be interfered by a signal transmitted by an interfering MS  118 , which is served by BTS antenna  106 . In this case, the channel between desired MS  116  and BTS antenna  106  is estimated in presence of interfering MS  118 . 
       FIG. 2  is a flowchart of a method for estimating channel between desired MS  116  and BTS antenna  106  of sector  112  in a communication system, in accordance with an embodiment. At  202 , BTS antenna  106  receives a composite signal that includes a desired MS signal transmitted by desired MS  116  and an interfering MS signal transmitted by interfering MS  118 . In an embodiment, a second channel may be estimated between desired MS  116  and a second BTS antenna (not shown in  FIG. 1 ) of sector  112 . In this case, the second BTS antenna of sector  112  receives a composite signal that includes a desired MS signal transmitted by desired MS  116  and an interfering MS signal transmitted by interfering MS  118 . The desired MS signal and the interfering MS signal include overlapping time and frequency data symbols, and overlapping time and frequency pilot symbols. Desired MS  116  modulates the desired MS signal using a first Pseudo Random Binary Sequence (PRBS) before transmitting the desired MS signal. The known desired random sequence includes the first PRBS. The first PRBS includes a first plurality of pseudo random binary values. A pseudo random binary value may be one of +1 and −1. Similarly, interfering MS  118  modulates the interfering MS signal using a second PRBS before transmitting the interfering MS signal. The known interfering random sequence includes the second PRBS. The second PRBS includes a second plurality of pseudo random binary values. 
     The composite signal received by BTS antenna  106  from desired MS  116  and interfering MS  118  at a pilot sub-carrier location may be represented by equation (1) given below:
 
 Y   i,j   =C   (1)   i,j   h+C   (2)   i,j   g+n   i,j   (1)
 
where,
         Y i,j , is the composite signal received by the BTS antenna  106  at i th  sub-carrier and j th  OFDM time symbol,   h is the channel response between desired MS  116  and BTS antenna  106 ,   g is the channel response between interfering MS  118  and BTS antenna  106 ,   n i,j  is the noise in the channel at i th  sub-carrier and j th  OFDM time symbol,   C (1)   i,j  is a pseudo random binary value corresponding to a i th  sub-carrier and j th  OFDM time symbol in the first PRBS, and   C (2)   i,j  is a pseudo random binary value corresponding to a i th  sub-carrier and j th  OFDM time symbol in the second PRBS, each of C (1)   i,j  and C (2)   i,j  can have a value of +1 or −1.       

     At  204 , the known desired random sequence and the known interfering random sequence is applied to pilot symbols of the composite signal. This results in a first de-randomized signal and a second de-randomized signal. For estimating the second channel, the known desired random sequence and the known interfering random sequence is applied to pilot symbols of the composite signal received at the second BTS antenna of sector  112 . This results in a first de-randomized signal and a second de-randomized signal corresponding to the composite signal received at the second BTS antenna of sector  112 . The known desired random sequence and the known interfering random sequence may be applied to pilot symbols of the composite signal by multiplying each pseudo random value corresponding to a pilot sub-carrier in the first PRBS and each pseudo random value corresponding to a pilot sub-carrier in the second PRBS with the corresponding pilot symbols of the composite signal. In an embodiment, the first de-randomized signal corresponding to a pilot symbol may be represented by equation (2) given below:
 
 P   i,j   =h+C   (1)   i,j   C   (2)   i,j   g   (2)
 
where,
 
     P i,j , is the first signal at i th  sub-carrier and j th  OFDM time symbol. 
     Similarly, in an embodiment, the second de-randomized signal corresponding to a pilot symbol may be represented by equation (3) given below:
 
 Q   i,j   =C   (1)   i,j   C   (2)   i,j   h+g   (3)
 
where,
 
     Q i,j , is the second signal at i th  sub-carrier and j th  OFDM time symbol. 
     At  206 , weighing and summing of the first de-randomized signal and the second de-randomized signal is performed to obtain an estimate of the channel between desired MS  116  and the BTS antenna  106 . This has been explained in detail in conjunction with  FIG. 3 . For estimating the second channel, weighing and summing of the first de-randomized signal and the second de-randomized signal is performed corresponding to the composite signal received at the second BTS antenna of sector  112 . 
     In an embodiment, a different sector channel may be estimated between desired MS  116  and a first BTS antenna of a different sector of the plurality of sectors of BTS  102 . The different sector may be one of sector  110  and sector  114 . In this case, the first BTS antenna of the different sector receives a composite signal that includes a desired MS signal transmitted by desired MS  116  and an interfering MS signal transmitted by interfering MS  118 . Thereafter, the known desired random sequence and the known interfering random sequence is applied to pilot symbols of the composite signal received at the first BTS antenna of the different sector. This results in a first de-randomized signal and a second de-randomized signal corresponding to the composite signal received at the first BTS antenna of the different sector. Further, to estimate the different sector channel, weighing and summing of the first de-randomized signal and the second de-randomized signal corresponding to the composite signal received at the first BTS antenna of the different sector is performed. Therefore, the different sector channel may be used in conjunction with the channel estimated between desired MS  116  and BTS antenna  106  to achieve sector combining. 
       FIG. 3  is a flowchart of a method for estimating channel between desired MS  116  and BTS antenna  106  in a communication system, in accordance with another embodiment. At  302 , the BTS antenna receives a composite signal from desired MS  116  and interfering MS  118 . Therefore, the composite signal includes the desired MS signal and the interfering MS signal. The desired MS signal and the interfering MS signal have overlapping time and frequency data symbols, and overlapping time and frequency pilot symbols. At  304 , a known desired random sequence and a known interfering random sequence is applied to pilot symbols of the composite signal. This results in the first de-randomized signal and the second de-randomized signal. The known desired random sequence includes the first PRBS and the known interfering random sequence includes the second PRBS. This has been explained in detail in conjunction with  FIG. 2  given above. 
     Further, a PRBS characteristic value corresponding to the first PRBS and the second PRBS is determined at  304 . The PRBS characteristic value is determined by adding the product of each pseudo random binary value corresponding to a pilot sub-carrier in first PRBS and the corresponding pseudo random binary value corresponding to the same pilot sub-carrier in the second PRBS. The PRBS characteristic value may be equal to one of 4, 2, 0, −2, and −4. In an embodiment, the PRBS characteristic value may be represented by equation (4) given below: 
     
       
         
           
             
               
                 
                   
                     PRBS 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     characteristic 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     value 
                   
                   = 
                   
                     
                       ∑ 
                       
                         i 
                         , 
                         j 
                       
                     
                     ⁢ 
                     
                       
                         C 
                         ij 
                         
                           ( 
                           1 
                           ) 
                         
                       
                       ⁢ 
                       
                         C 
                         ij 
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where the summation is over all pairs (i,j) such that the i th  sub-carrier and the j th  OFDM time symbol corresponds to a pilot sub-carrier location. 
     At  306 , sums and differences of the first de-randomized signal and the second de-randomized signal are determined. The sums of the first de-randomized signal and the second de-randomized signal include a sum of each pilot symbol in the first de-randomized signal and the corresponding pilot symbol in the second de-randomized signal. In an embodiment, the sum of the first de-randomized signal and the second de-randomized signal corresponding to a pilot symbol may be represented by equation (5) given below:
 
 P   i,j   +Q   i,j =( h+C   (1)   i,j   C   (2)   i,j   g ) + ( C   (1)   i,j   C   (2)   i,j   h+g )  (5)
 
     The differences of the first de-randomized signal and the second de-randomized signal include a difference of each pilot symbol in the first de-randomized signal and the corresponding pilot symbol in the second de-randomized signal. Further, in an embodiment, the difference of the first de-randomized signal and the second de-randomized signal corresponding to a pilot symbol may be represented by equation (6) given below:
 
 P   i,j   −Q   i,j =( h+C   (1)   i,j   C   (2)   i,j   g )−( C   (1)   i,j   C   (2)   i,j   h+g )  (6)
 
     After determining sums and differences, the sums and differences are weighted with predetermined weights, at  306 . In this case, a power level of interfering MS signal is sensed. If the power level of the interfering MS signal is below a threshold, then the second de-randomized signal is set to zero. Additionally, each of the predetermined weighting is set to ½. 
     In another embodiment, a first weight and a second weight are selected based on the PRBS characteristic value. Thereafter, the sums are weighed with the first weight and the difference is weighed with the second weight. Each of the first weight and the second weight have a value equal to one of ½, ⅓, 1 and 0. A value of ½ is selected for the first weight and a value of 0 is selected for the second weight if the PRBS characteristic value is equal to 4. A value of ⅓ is selected for the first weight and a value of 1 is selected for the second weight if the PRBS characteristic value is equal to 2. Further, a value of ½ is selected for the first weight and a value of ½ is selected for the second weight if the PRBS characteristic value is 0. A value of 1 is selected for the first weight and a value of ⅓ is selected for the second weight if the PRBS characteristic value is equal to −2. A value of 0 is selected for the first predetermined weight and a value of ½ is selected for the second predetermined weight if the PRBS characteristic value is equal to −4. The above listed values of the first weight and the second predetermined weight determined corresponding to the PRBS characteristic value may be represented by Table 1 given below: 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 First and Second weights 
               
            
           
           
               
               
               
            
               
                 
                   
                     
                       
                         
                           
                             
                               
                                 PRBS 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 characteristic 
                               
                             
                           
                           
                             
                               value 
                             
                           
                           
                             
                               
                                 
                                   ∑ 
                                   
                                     i 
                                     , 
                                     j 
                                   
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     C 
                                     ij 
                                     
                                       ( 
                                       1 
                                       ) 
                                     
                                   
                                   ⁢ 
                                   
                                     C 
                                     ij 
                                     
                                       ( 
                                       2 
                                       ) 
                                     
                                   
                                 
                               
                             
                           
                         
                           
                       
                     
                   
                 
                 First predetermined weight (K1) 
                 Second predetermined weight (K2) 
               
               
                   
               
            
           
           
               
               
               
            
               
                 4 
                 ½ 
                 0 
               
               
                 2 
                 ⅓ 
                 1 
               
               
                 0 
                 ½ 
                 ½ 
               
               
                 −2 
                 1 
                 ⅓ 
               
               
                 −4 
                 0 
                 ½ 
               
               
                   
               
            
           
         
       
     
     After selecting the first weight and the second weight based on the PRBS characteristic value, to obtain the weighted sums, the first weight is multiplied with each of the pilot symbol of the sums of the first de-randomized signal and the second de-randomized signal. In an embodiment, the multiplication corresponding to a pilot symbol may be represented by expression (10), as given below:
 
 K 1×( P   i,j   +Q   i,j )  (10)
 
where,
 
     K1 is the weight as shown in Table 1. 
     Similarly, to obtain weighted differences, the second weight is multiplied with each of the pilot symbol of the differences of the first de-randomized signal and second de-randomized signal. In an embodiment, the multiplication corresponding to a pilot symbol may be represented by expression (11), as given below:
 
 K 2×( P   i,j   −Q   i,j )  (11)
 
where,
 
     K2 is the second predetermined weight as shown in Table 1. 
     Thereafter, at step  310 , the weighted sums and the weighted differences are averaged over pilot symbols. The average is obtained by summing the plurality of pilot symbols in the weighted sums and the plurality of pilot symbols in the weighted differences and dividing the sum obtained by the number of pilot symbols. The average gives the estimate of the channel between desired MS  116  and BTS antenna  106 . In an embodiment, the average of the weighted sums and the weighted differences may be represented by expression (12) given below: 
                     (         ∑     i   ,   j       ⁢     K   ⁢           ⁢   1   ⁢     (       P     1   ,   j       +     Q     i   ,   j         )         +       ∑     i   ,   j       ⁢     K   ⁢           ⁢   2   ⁢     (       P     i   ,   j       -     Q     i   ,   j         )           )     /   m           (   12   )               
where,
 
     ‘m’ is the number of pilot symbols used in the signals transmitted by desired MS  116  and interfering MS  118 . 
       FIG. 4  is a block diagram illustrating channel estimation between desired MS  116  and BTS antenna  106  in a communication system, in accordance with an exemplary embodiment. The BTS antenna receives a composite signal from desired MS  116  and interfering MS  118 . The composite signal includes the desired MS signal and the interfering MS signal. The desired MS signal is modulated by desired MS  116  using a first PRBS and the interfering MS signal is modulated by interfering MS  118  using a second PRBS. 
     Referring back to  FIG. 2 , the signal received at BTS antenna  106  at each pilot sub-carrier location may be represented by equations given below:
 
 Y   1,1   =C   (1)   1,1   h+C   (2)   1,1   g=h+g  
 
 Y   1,3   =C   (1)   1,3   h+C   (2)   1,3   g=h+g  
 
 Y   4,1   =C   (1)   4,1   h+C   (2)   4,1   g=h+g  
 
 Y   4,3   =C   (1)   4,3   h+C   (2)   4,3   g=h−g  
 
where,
 
     Y 1,1  is the signal received at first sub-carrier and first OFDM time symbol 
     Y 1,3  is the signal received at first sub-carrier and third OFDM time symbol 
     Y 4,1  is the signal received at fourth sub-carrier and first OFDM time symbol 
     Y 4,3  is the signal received at fourth sub-carrier and third OFDM time symbol 
     In this exemplary embodiment, the values of pseudo random binary values in the first PRBS corresponding to each pilot symbol are given as C (1)   11 =+1, C (1)   13 =+1, C (1)   41 =+1 and C (1)   43 =+1. The values of pseudo random binary values in the second PRBS corresponding to each pilot symbol are given as C (2)   11 =+1, C (2)   13 =+1, C (2)   41 =+1 and C (2)   43 =−1. 
     The signal received at BTS antenna  106  from desired MS  116  and interfering MS  118  is represented by a tile  402 . Tile  402  includes four symbols along a frequency dimension  404  and three consecutive time symbols along a time dimension  406 . 
     Thereafter, the first PRBS is applied on the composite signal to generate a first de-randomized signal (P i,j ). For this, each of the pseudo random binary value (C (1)   11 , C (1)   13 , C (1)   41  and C (1)   43 ) of the first PRBS is multiplied with the corresponding pilot symbol of the composite signal to generate the first de-randomized signal for each pilot symbol which is represented by equations below:
 
 P   1,1   =h+C   (1)   1,1   C   (2)   1,1   g=h+g  
 
 P   1,3   =h+C   (1)   1,3   C   (2)   1,3   g=h+g  
 
 P   4,1   =h+C   (1)   4,1   C   (2)   4,1   g=h+g  
 
 P   4,3   =h+C   (1)   4,3   C   (2)   4,3   g=h−g  
 
     Similarly, BTS  102  applies the second PRBS on the composite signal to generate a second de-randomized signal (Q i,j ). For this, each of the pseudo random binary value (C (2)   11 , C (2)   13 , C (2)   41  and C (2)   43 ) of the second PRBS is multiplied with the corresponding pilot symbol of the composite signal to generate the second de-randomized signal for each pilot symbol which is represented by equations given below:
 
 Q   1,1   =C   (2)   1,1   C   (1)   1,1   h+g=h+g  
 
 Q   1,3   =C   (2)   1,3   C   (1)   1,3   h+g=h+g  
 
 Q   4,1   =C   (2)   4,1   C   (1)   4,1   h+g=h+g  
 
 Q   4,3   =C   (2)   4,3   C   (1)   4,3   h+g=−h+g  
 
     Further, a PRBS characteristic value is determined by adding the product of each pseudo random binary value corresponding to a pilot sub-carrier in the first PRBS and the corresponding pseudo random binary value corresponding to the same pilot sub-carrier in the second PRBS. The PRBS characteristic value is represented by equations given below: 
     
       
         
           
             
               
                 
                   
                     
                       ∑ 
                       
                         
                           i 
                           = 
                           1 
                         
                         , 
                         4 
                         , 
                         
                           j 
                           = 
                           1 
                         
                         , 
                         3 
                       
                     
                     ⁢ 
                     
                       
                         C 
                         ij 
                         
                           ( 
                           1 
                           ) 
                         
                       
                       ⁢ 
                       
                         C 
                         
                           ij 
                           , 
                         
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                   
                   = 
                     
                   ⁢ 
                   
                     
                       
                         C 
                         
                           1 
                           , 
                           1 
                         
                         
                           ( 
                           1 
                           ) 
                         
                       
                       ⁢ 
                       
                         C 
                         
                           1 
                           , 
                           1 
                         
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                     + 
                     
                       C 
                       
                         1 
                         , 
                         3 
                       
                       
                         ( 
                         1 
                         ) 
                       
                     
                     + 
                     
                       C 
                       
                         1 
                         , 
                         3 
                       
                       
                         ( 
                         2 
                         ) 
                       
                     
                     + 
                     
                       
                         C 
                         
                           4 
                           , 
                           1 
                         
                         
                           ( 
                           1 
                           ) 
                         
                       
                       ⁢ 
                       
                         C 
                         
                           4 
                           , 
                           1 
                         
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                     + 
                     
                       
                         C 
                         
                           4 
                           , 
                           3 
                         
                         
                           ( 
                           1 
                           ) 
                         
                       
                       ⁢ 
                       
                         C 
                         
                           4 
                           , 
                           3 
                         
                         
                           ( 
                           2 
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     1.1 
                     + 
                     1.1 
                     + 
                     1.1 
                     + 
                     
                       1. 
                       ⁢ 
                       
                         ( 
                         
                           - 
                           1 
                         
                         ) 
                       
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     1 
                     + 
                     1 
                     + 
                     1 
                     - 
                     1 
                   
                 
               
             
             
               
                 
                   = 
                     
                   ⁢ 
                   
                     + 
                     2 
                   
                 
               
             
           
         
       
     
     Thereafter, sums and differences of the first de-randomized signal and second de-randomized signal are determined. The sums of the first de-randomized signal and the second de-randomized signal may be represented by equations given below:
 
 P   1,1   +Q   1,1 =( h+g ) + ( h+g )=2( h+g )
 
 P   1,3   +Q   1,3 =( h+g ) + ( h+g )=2( h+g )
 
 P   4,1   +Q   4,1 =( h+g ) + ( h+g )=2( h+g )
 
 P   4,3   +Q   4,3 =( h+g ) + (− h−g )=0
 
     The differences of the first de-randomized signal and the second de-randomized signal may be represented by equations given below:
 
 P   1,1   −Q   1,1 =( h+g )−( h+g )=0
 
 P   1,3   −Q   1,3 =( h+g )−( h+g )=0
 
 P   4,1   −Q   4,1 =( h+g )−( h+g )=0
 
 P   4,3   −Q   4,3 =( h+g )−( h+g )=2( h−g )
 
     Thereafter, a first weight (K1) and a second weight (K2) are selected based on the PRBS characteristic value using Table 1. Therefore, the first weight is determined as ⅓ (K1=⅓) and the second weight is determined as 1 (K2=1) corresponding to the PRBS characteristic value of +2. 
     Next, the first weight is applied to the sums of the first de-randomized signal and the second de-randomized signal. The first weight is multiplied with the signal at each of the pilot sub-carrier locations of the sums of the first de-randomized signal and the second de-randomized signal. The multiplication is represented by equations given below:
 
 K 1( P   1,1   +Q   1,1 )=(⅓)·2( h+g )=⅔( h+g )
 
 K 1( P   1,3   +Q   1,3 )=(⅓)·2( h+g )=⅔( h+g )
 
 K 1( P   4,1   +Q   4,1 )=(⅓)·2( h+g )=⅔( h+g )
 
 K 1( P   4,3   +Q   4,3 )=(⅓)·0=0
 
     The above listed equations may be represented as a tile  408 . 
     Similarly, the second weight is applied to the differences of the first de-randomized signal and the second de-randomized signal. The second weight is multiplied with the signal at each of the pilot sub-carrier locations of the differences of the first de-randomized signal and the second de-randomized signal. The multiplication is represented by equations given below:
 
 Q   1,1   =K 2( P   1,1   −Q   1,1 )=(1)·0=0
 
 Q   1,3   =K 2( P   1,3   −Q   1,3 )=(1)·0=0
 
 Q   4,1   =K 2( P   4,1   −Q   4,1 )=(1)·0=0
 
 Q   4,3   =K 2( P   4,4   −Q   4,3 )=(1)·2( h−g )=2( h−g )
 
     The above listed equations may be represented as a tile  410 . 
     Thereafter, linear averaging on the pilot symbols of tile  408  and tile  410  is performed as represented by equation given below: 
     
       
         
           
             
               
                 
                   
                     Average 
                   
                   = 
                     
                   ⁢ 
                   
                     
                       [ 
                       
                         
                           
                             
                               
                                 ( 
                                 
                                   
                                     
                                       2 
                                       / 
                                       3 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         h 
                                         + 
                                         g 
                                       
                                       ) 
                                     
                                   
                                   + 
                                   
                                     
                                       2 
                                       / 
                                       3 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         h 
                                         + 
                                         g 
                                       
                                       ) 
                                     
                                   
                                   + 
                                   
                                     
                                       2 
                                       / 
                                       3 
                                     
                                     ⁢ 
                                     
                                       ( 
                                       
                                         h 
                                         + 
                                         g 
                                       
                                       ) 
                                     
                                   
                                   + 
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     The average is the estimate of the channel between desired MS  116  and BTS antenna  106  at sector  112 . 
       FIG. 5  is a block diagram showing various components of a BTS  500  for estimating channel between desired MS  116  and BTS antenna  106 , in accordance with an embodiment. BTS  500  serves MSs in a plurality of sectors (for example, sector  110 , sector  112 , and sector  114 ) in a geographic region. BTS  500  includes a plurality of BTS antennas (for example, BTS antenna  104 , BTS antenna  106 , and BTS antenna  108 ), a de-randomizing processor  502  operatively coupled to each BTS antenna, and a channel estimating processor  504  operatively coupled to de-randomizing processor  502 . One or more BTS antennas of BTS  500  serve a sector in the plurality of sectors. For example, BTS antenna  106  serves one or more MSs in sector  112  in the plurality of sectors of the BTS  500 . 
     BTS antenna  106  receives a composite signal from desired MS  116  and interfering MS  118 . Desired MS  116  is served by BTS antenna  104  in sector  110  and the interfering MS  118  is served by BTS antenna  106  in sector  112 . The composite signal includes a desired MS signal and an interfering MS signal. The desired MS signal and the interfering MS signal have overlapping time and frequency data symbols. Additionally, the desired MS signal and the interfering MS signal have overlapping time and frequency pilot symbols. Desired MS  116  modulates the desired MS signal using a first PRBS before transmitting the desired MS signal. Similarly, interfering MS  118  modulates the interfering MS signal using a second PRBS before transmitting the interfering MS signal. The first PRBS includes a first plurality of pseudo random binary values and the second PRBS includes a second plurality of pseudo random binary values. A pseudo random binary value in a PRBS is one of +1 or −1 and corresponds to a pilot/data symbol in the tile. 
     After BTS antenna  106  receives the composite signal, de-randomizing processor  502  applies a known desired random sequence and a known interfering random sequence to pilot symbols of the composite signal. This results in a first de-randomized signal and a second de-randomized signal. The known desired random sequence includes the first PRBS and the known interfering random sequence includes the second PRBS. 
     Thereafter, channel estimating processor  504  estimates the channel by weighing and summing the first de-randomized signal and the second de-randomized signal. For this, channel estimating processor  504  determines sums and differences of the first de-randomized signal and second de-randomized signal. This has been explained in detail in conjunction with  FIG. 3  and  FIG. 4 . In an embodiment, channel estimating processor  504  weighs the sums and weighs the differences with predetermined weights to obtain weighted sums and weighted differences. In this case, a power level of interfering MS signal is sensed. If the power level of the interfering MS signal is below a threshold, then the second de-randomized signal is set to zero. Additionally, each of the predetermined weighting is set to ½. 
     In another embodiment, channel estimating processor  504  determines a PRBS characteristic value corresponding to the first PRBS and the second PRBS. The PRBS characteristic value is determined by adding the product of each pseudo random binary value corresponding to a pilot sub-carrier in first PRBS and the corresponding pseudo random binary value corresponding to the same pilot sub-carrier in the second PRBS. Based on the PRBS characteristic value, channel estimating processor  504  selects a first weight and a second weight. This has been explained in conjunction with Table 1 given above. In this case, channel estimating processor  504  weighs the sums with the first weight to obtain the weighted sums and weighs the differences with the second weight to obtain the weighted differences. Finally, channel estimating processor  504  adds the weighted sums and the weighted differences. Channel estimating processor  504  may average the weighted sums and the weighted differences over pilot symbols. The average is obtained by summing the plurality of pilot symbols in the weighted sums and the plurality of pilot symbols in the weighted differences and dividing the sum obtained by the number of pilot symbols. The average gives the estimate of the channel between desired MS  116  and BTS antenna  106 . 
     Various embodiments provide methods and systems estimating a channel between a desired MS and a BTS antenna of a one of a plurality of sectors of a BTS. The system receives and processes signals from the desired MS and an interfering MS to cancel out the interference due to the interfering MS. This may be used to improve the output of sector combining procedure. Further, sector combining may not be performed in case the interference is not cancelled out. Additionally, the system uses the same channel estimator as used in conventional systems in order to obtain the estimate of the channel. This reduces the modifications to be made to the conventional system to achieve the above listed methods and systems.