Abstract:
The present disclosure is used to determine a number of sub-carriers or symbols to be used to estimate N channels for N user equipment (UE)s on a basis of resource allocation information on N reference signals associated with the N UEs, estimate channel values for the N channels by receiving the N reference signals through the sub-carriers or the symbols according to the determined number, and correlate the estimated channel values to use the correlated channel values to estimate each of the N channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) AND CLAIM OF PRIORITY 
       [0001]    The present application is related to and claims the priority under 35 U.S.C. §119(a) to Korean Application Serial No. 10-2014-0053305, which was filed in the Korean Intellectual Property Office on May 2, 2014, the entire content of which is hereby incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    The present disclosure relates to a method and apparatus for channel estimation in a wireless communication system. 
       BACKGROUND 
       [0003]    Wireless communication systems, such as orthogonal frequency division multiplexing (OFDM) communication systems, are based on a physical layer structure in which frequency axis and time axis resources are allocated and used for data transmission. In such wireless communication systems, a transmitter two-dimensionally places reference signal (RS) symbols, such as pilot symbols, in resource elements (REs) within a resource block (RB) to increase channel estimation performance and then transmits the RS symbols. A receiver performs channel estimation on the basis of the RS symbols placed in the REs to enhance the accuracy for the channel estimation. 
         [0004]    Meanwhile, a time-domain/frequency-domain filtering method, an interpolation method, a least square (LS) method, a minimum mean square error (MMSE) method, a 2-dimensional Wiener filtering method, and the like are widely used as a method for channel estimation. The above channel estimation methods may be selected and used on the basis of a wireless channel environment. That is because channel estimation performance varies with the channel estimation method that is used in the corresponding wireless channel environment. Accordingly, an appropriate channel estimation method may be selected and used according to whether the wireless channel environment is a frequency selective fading environment, a flat fading environment, or an environment in which user equipment (UE) moves at a high or low speed. 
         [0005]    In general, when a two-dimensional statistical characteristic (Wide-sense stationary uncorrelated scattering: WSSUS) of a channel is known, the 2-dimensional Wiener filtering method is most effective for channel estimation and is widely used in the actual implementation. However, in cases where the 2-dimensional Wiener filtering method is used in multiple input multiple output (MIMO) communication systems, channel estimation performance is degraded due to interference. That is, in the MIMO communication systems, interference is not completely removed even though an interference removal operation is performed by distinguishing between signals of respective UEs, and therefore the channel estimation performance is affected by the unremoved interference. That is because the existing 2-dimensional Wiener filtering method of performing channel estimation by applying a Wiener filter to a single layer signal does not consider interference existing after signal separation for multiple layer signals. 
       SUMMARY 
       [0006]    To address the above-discussed deficiencies, it is a primary object to provide, for use in a method and apparatus for channel estimation in a wireless communication system. 
         [0007]    Various embodiments of the present disclosure provide a method and apparatus that prevents channel estimation performance from being degraded in a wireless communication system in which a MIMO communication scheme is used. 
         [0008]    Various embodiments of the present disclosure provide a method and apparatus that enables accurate channel estimation to be performed in a receiver, thereby increasing a transmission data rate and enhancing cell throughput. 
         [0009]    In accordance with various embodiments of the present disclosure, a method for channel estimation in a wireless communication system is provided. The method includes: determining a number of sub-carriers or symbols to be used to estimate N channels for N UEs on a basis of resource allocation information on N reference signals associated with the N UEs; estimating channel values for the N channels by receiving the N reference signals through the sub-carriers or the symbols according to the determined number; and correlating the estimated channel values and using the correlated channel values to estimate each of the N channels. 
         [0010]    In accordance with various embodiments of the present disclosure, an apparatus for channel estimation in a wireless communication system is provided. The apparatus includes: a reception unit and a controller configured to determine a number of sub-carriers or symbols to be used to estimate N channels for N UEs on a basis of resource allocation information on N reference signals associated with the N UEs, estimate channel values for the N channels when the N reference signals are received through the sub-carriers or the symbols according to the determined number, and correlate the estimated channel values to use the correlated channel value to estimate each of the N channels. 
         [0011]    The present disclosure prevents performance degradation for channel estimation that can occur when a MIMO communication scheme is used. In addition, the present disclosure allows channel estimation to be performed in consideration of interference that exists after a multiple user interference (MUI) removal operation is performed. Therefore, the present disclosure has the advantages of increasing a transmission data rate and enhancing cell throughput with increasing accuracy for channel estimation. 
         [0012]    Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words and phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or,” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, such a device may be implemented in hardware, firmware or software, or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, those of ordinary skill in the art should understand that in many, if not most instances, such definitions apply to prior, as well as future uses of such defined words and phrases. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts: 
           [0014]      FIG. 1  illustrates an apparatus for channel estimation according to various embodiments of the present disclosure; 
           [0015]      FIG. 2  illustrates a detailed configuration of a multi-signal splitter for performing an MUI removal operation according to various embodiments of the present disclosure; 
           [0016]      FIG. 3  illustrates an apparatus for channel estimation according to various embodiments of the present disclosure; and 
           [0017]      FIG. 4  illustrates a process of performing channel estimation according to various embodiments of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIGS. 1 through 4 , discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged wireless communication device. 
         [0019]    Among the generally used channel estimation methods is a 2-dimensional Wiener filtering method as the best channel estimation method. A Wiener filter for frequency and time axis reference signal (RS) symbols used in the 2-dimensional Wiener filtering method is implemented, for example, as follows. 
         [0020]    In a wireless communication system, such as an orthogonal frequency division multiplexing (OFDM) communication system, the output value of the k-th tone is calculated using the following Equation 1. 
         [0000]        Ĥ   k   =H   k   +n   k   [Equation 1]
 
         [0021]    In Equation 1 above, Ĥ k  denotes the output value of the k-th tone, H k  denotes the channel value of the k-th tone, and n k  denotes the noise component of the k-th tone. 
         [0022]    In a long term evolution (LTE) uplink system, an RS is allocated to twelve consecutive tones per resource block (RB) for a physical uplink shared channel (PUSCH). Accordingly, the channel estimation value of the k-th tone is represented as shown in the following Equation 2. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       ~ 
                     
                     k 
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           0 
                         
                         11 
                       
                        
                       
                         
                           c 
                           i 
                         
                          
                         
                           
                             H 
                             ^ 
                           
                           i 
                         
                       
                     
                     = 
                     
                       
                         
                           ∑ 
                           
                             i 
                             = 
                             0 
                           
                           11 
                         
                          
                         
                           
                             c 
                             i 
                           
                            
                           
                             ( 
                             
                               
                                 H 
                                 i 
                               
                               + 
                               
                                 n 
                                 i 
                               
                             
                             ) 
                           
                         
                       
                       = 
                       
                         
                           c 
                           H 
                         
                          
                         
                           H 
                           ^ 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     2 
                   
                   ] 
                 
               
             
           
         
       
     
         [0023]    In Equation 2 above, Ĥ k  denotes the channel estimation value of the k-th tone, i denotes a tone index, c i  denotes the Wiener filter coefficient of the i-th tone, Ĥ i  denotes the time offset compensation value of the i-th tone, denotes the channel value of the i-th tone, and n i  denotes the noise component for the i-th frequency tone. However, the time offset compensation can be performed according to the presence or absence of a multipath delay component, or a time delay can occur in a receiver. In exemplary embodiments of the present disclosure, considering that a general mobile communication system is involved in an environment in which a time offset exists, a channel estimation scheme, including a structure for compensating for the time offset, will be described. 
         [0024]    The Wiener filter coefficient c, as a value that minimizes the minimum mean square error (MMSE) (i.e., E{|H k −{tilde over (H)} k | 2 }) between a desired channel value (H k ) and a Wiener filter output value (channel estimation value {tilde over (H)} k ), is represented by the following Equation 3. 
         [0000]        c =( E{ĤĤ   H }) −1   E{ĤH*   k }=( R   h +σ 2   I ) −1   E{ĤH*   k }=( R   h +σ 2   I ) −1   r   [Equation 3]
 
         [0025]    In Equation 3 above, R h  denotes the auto-correlation matrix of a channel, r denotes the cross-correlation matrix of the channel, σ denotes noise power, and I denotes an identity matrix. 
         [0026]    When the Wiener filter coefficient c is determined as described above, the channel estimation value of the k-th tone is represented as shown in the following Equation 4. 
         [0000]        {tilde over (H)}   k =(( R   h +σ 2   I ) −1   r ) H   ·Ĥ   [Equation 4]
 
         [0027]    The Wiener filter coefficient is determined from the channel correlation value in this way, and a two-dimensional correlation matrix for a single input multi output (SIMO) is determined as follows. 
         [0028]    When a fading channel response to user equipment (UE) is represented by the following Equation 5, the fast Fourier transform (FFT) of the fading channel response into a channel value in a frequency domain results in the following Equation 6 at (t 1 , f 1 ) (the f 1 -th sub-carrier of the t 1 -th symbol). 
         [0000]    
       
         
           
             
               
                 
                   
                     h 
                      
                     
                       ( 
                       
                         
                           t 
                           1 
                         
                         , 
                         τ 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         1 
                       
                       L 
                     
                      
                     
                         
                     
                      
                     
                       
                         
                           h 
                           l 
                         
                          
                         
                           ( 
                           
                             t 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         δ 
                          
                         
                           ( 
                           
                             τ 
                             - 
                             
                               τ 
                               l 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     5 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     H 
                      
                     
                       ( 
                       
                         
                           
                             t 
                             1 
                           
                           , 
                         
                          
                         
                           f 
                           1 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         1 
                       
                       L 
                     
                      
                     
                         
                     
                      
                     
                       
                         
                           h 
                           l 
                         
                          
                         
                           ( 
                           
                             t 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         W 
                         
                           
                             τ 
                             1 
                           
                            
                           
                             f 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     6 
                   
                   ] 
                 
               
             
           
         
       
     
         [0029]    In Equation 5 above, τ denotes the time variable for which the fading channel exists, τ 1  denotes the delay profile of the l-th reception path, l denotes the index of multiple paths received from a particular UE, and L denotes the total number of paths. In Equation 6 above, 
         [0000]    
       
         
           
             
               W 
               = 
               
                  
                 
                   - 
                   
                     j2π 
                     
                       N 
                       FFT 
                     
                   
                 
               
             
             , 
             
               
                 and 
                  
                 
                     
                 
                  
                 
                   E 
                    
                   
                     [ 
                     
                       
                          
                         
                           
                             h 
                             l 
                           
                            
                           
                             ( 
                             t 
                             ) 
                           
                         
                          
                       
                       2 
                     
                     ] 
                   
                 
               
               = 
               
                 H 
                 l 
               
             
             , 
             
               
                 
                   Σ 
                   1 
                   L 
                 
                  
                 
                   H 
                   l 
                 
               
               = 
               1. 
             
           
         
       
     
         [0030]    Similarly to the above, the channel value at (t 2 , f 2 ) is represented as shown in the following Equation 7. 
         [0000]        H ( t   2   ,f   2 )=Σ l=1   L   h   l ( t   2 ) W   τ     l     f     2     [Equation 7]
 
         [0031]    Based on the channel values of (t 1 , f 1 ) and (t 2 , f 2 ) (preset resources to which an RS sequence is allocated), a two-dimensional correlation matrix is determined using Equation 8 below. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     hh 
                   
                   = 
                   
                     
                       E 
                        
                       
                         [ 
                         
                           
                             H 
                              
                             
                               ( 
                               
                                 
                                   t 
                                   1 
                                 
                                 , 
                                 
                                   f 
                                   1 
                                 
                               
                               ) 
                             
                           
                            
                           
                             
                               H 
                                
                               
                                 ( 
                                 
                                   
                                     t 
                                     2 
                                   
                                   , 
                                   
                                     f 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             * 
                           
                         
                         ] 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           l 
                           ′ 
                         
                         L 
                       
                        
                       
                           
                       
                        
                       
                         
                           ∑ 
                           
                             l 
                             ″ 
                           
                           L 
                         
                          
                         
                           
                             E 
                              
                             
                               [ 
                               
                                 
                                   
                                     h 
                                     l 
                                     ′ 
                                   
                                    
                                   
                                     ( 
                                     
                                       t 
                                       1 
                                     
                                     ) 
                                   
                                 
                                  
                                 
                                   
                                     
                                       h 
                                       l 
                                       ″ 
                                     
                                      
                                     
                                       ( 
                                       
                                         t 
                                         2 
                                       
                                       ) 
                                     
                                   
                                   * 
                                 
                               
                               ] 
                             
                           
                            
                           
                             W 
                             
                               
                                 
                                   τ 
                                   
                                     l 
                                     ′ 
                                   
                                 
                                  
                                 
                                   f 
                                   1 
                                 
                               
                               - 
                               
                                 
                                   τ 
                                   
                                     l 
                                     ″ 
                                   
                                 
                                  
                                 
                                   f 
                                   2 
                                 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     8 
                   
                   ] 
                 
               
             
           
         
       
     
         [0032]    Since channels are independent between multiple paths (i.e., signals are received via different paths for respective UEs), a mean value exists only when the corresponding channels have the same path (l′=l″). In other words, 
         [0000]    
       
         
           
             
               E 
                
               
                 [ 
                 
                   
                     
                       h 
                       
                         l 
                         ′ 
                       
                     
                      
                     
                       ( 
                       
                         t 
                         1 
                       
                       ) 
                     
                   
                    
                   
                     
                       
                         h 
                         
                           l 
                           ″ 
                         
                       
                        
                       
                         ( 
                         
                           t 
                           2 
                         
                         ) 
                       
                     
                     * 
                   
                 
                 ] 
               
             
             = 
             
               
                 δ 
                  
                 
                   ( 
                   
                     
                       l 
                       ′ 
                     
                     - 
                     
                       l 
                       ″ 
                     
                   
                   ) 
                 
               
               · 
               
                 
                   J 
                   0 
                 
                  
                 
                   ( 
                   
                     2 
                      
                     π 
                      
                     
                         
                     
                      
                     
                       f 
                       c 
                     
                      
                     
                       v 
                       c 
                     
                      
                     
                       ( 
                       
                         
                           t 
                           2 
                         
                         - 
                         
                           t 
                           1 
                         
                       
                       ) 
                     
                      
                     T 
                   
                   ) 
                 
               
             
           
         
       
     
         [0000]    is obtained, and the two-dimensional correlation matrix determined from this equation is represented as shown in the following Equation 9. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     hh 
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           l 
                           L 
                         
                          
                         
                           
                             H 
                             l 
                           
                            
                           
                             W 
                             
                               
                                 
                                   τ 
                                   l 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       f 
                                       1 
                                     
                                     - 
                                     
                                       f 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                   
                               
                             
                           
                         
                       
                       
                          
                         
                           freq 
                            
                           
                               
                           
                            
                           domain 
                         
                       
                     
                     · 
                     
                       
                         
                           J 
                           0 
                         
                          
                         
                           ( 
                           
                             2 
                              
                             π 
                              
                             
                                 
                             
                              
                             
                               f 
                               c 
                             
                              
                             
                               v 
                               c 
                             
                              
                             
                               ( 
                               
                                 
                                   t 
                                   2 
                                 
                                 - 
                                 
                                   t 
                                   1 
                                 
                               
                               ) 
                             
                              
                             T 
                           
                           ) 
                         
                       
                       
                          
                         
                           time 
                            
                           
                               
                           
                            
                           domain 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     9 
                   
                   ] 
                 
               
             
           
         
       
     
         [0033]    In this way, the channel correlation matrix is expressed in two dimensions of the frequency axis and the time axis, and the optimum channel estimation value is acquired by estimating the channel profile, namely, the delay profile difference (TO and the moving speed (v) of the multiple paths. 
         [0034]    Hereinafter, a wireless communication system for performing channel estimation in such a manner as described above will be described with reference to  FIG. 1 . 
         [0035]      FIG. 1  illustrates an apparatus for channel estimation according to various embodiments of the present disclosure. 
         [0036]    Referring to  FIG. 1 , the apparatus for channel estimation includes at least one of an RS generator  100 , an RS de-correlator  102 , an FFT device  104 , a multi-signal splitter  106 , a time offset estimator  108 , a time offset compensator  110 , a Wiener filter  112 , and a time offset re-compensator  114 . 
         [0037]    The RS generator  100  generates an RS sequence, and the FFT device  104  performs fast Fourier transform (FFT) on a reception signal to output the transformed signal to the RS de-correlator  102 . The RS de-correlator  102  maps the RS sequence onto an RS symbol. 
         [0038]    The RS de-correlator  102  determines an output direction according to whether a MIMO or SIMO communication scheme is used in a wireless communication system. Specifically, when the MIMO communication scheme is used, the RS de-correlator  102  determines the output direction such that the output of the RS de-correlator  102  is input to the multi-signal splitter  106  for splitting signals for respective UEs. 
         [0039]    The multi-signal splitter  106 , when RBs allocated to respective layers are the same, removes interference between a plurality of UEs that are distinguished by a cyclic shift (CS). The multi-signal splitter  106  distinguishably outputs signals for the multiple layers such that the signals for the multiple layers are subjected to processing (single layer processing). 
         [0040]    The multi-signal splitter  106 , when the RBs allocated to the respective layers are different from each other, removes interference between the plurality of UEs that are distinguished by orthogonal cover codes. The multi-signal splitter  106  distinguishably outputs the signals for the multiple layers such that the signals for the multiple layers are processed. 
         [0041]    Meanwhile, the RS de-correlator  102 , when the SIMO communication scheme is used, determines the output direction such that the output of the RS de-correlator  102  is directly output without being input to the multi-signal splitter  106  (i.e., while bypassing the multi-signal splitter  106 ). 
         [0042]    The time offset estimator  108  estimates a time offset for the reception signal that has been fast Fourier transformed in the time or frequency domain. For example, the time offset estimator  108  estimates the time offset using a method of estimating the location of a sample having a peak value by performing inverse discrete Fourier transform on the fast Fourier transformed reception signal, or using a method of calculating an accumulated average value of phase differences between tones in the frequency domain. 
         [0043]    The time offset compensator  110  compensates for phases according to sub-carriers for the fast Fourier transformed reception signal to output the phase-compensated reception signal to the Wiener filter  112 . The Wiener filter  112  applies a Wiener filter coefficient obtained from a channel correlation value to the received channel value in order to detect the channel estimation value for the phase-compensated reception signal. The time offset re-compensator  114  recovers the phase of the phase-compensated signal as it is on the basis of the channel estimation value. 
         [0044]    When the MIMO communication scheme is used in the apparatus for channel estimation as described above, the multi-signal splitter  106  is used to split signals of multiple layers. The signals split by the multi-signal splitter  106  are used for channel estimation of the corresponding layer through the same procedure as when the SIMO communication scheme is used. 
         [0045]    The channel estimation method using the Wiener filer  112  provides the most excellent channel estimation performance in regard to each layer. However, such a channel estimation method may not be the optimum channel estimation method when the MIMO communication scheme is used. That is because the Wiener filter  112  is applied without consideration of the fact that interference still exists even through a multi user interference (MUI) removal operation is performed to split signals of UEs according to respective layers. Accordingly, in certain embodiments, the channel estimation performance for a MIMO user is degraded. 
         [0046]    The MUI removal operation will be specifically described. In the embodiment of the present disclosure, a case of removing interference between a plurality of UEs, which are distinguished by a CS, when RBs allocated to respective layers are the same will be representatively described. 
         [0047]    An RS used for channel estimation is represented as shown in the following Equation 10. 
         [0000]        r   u,v   (α) ( n )= e   jαn     r     u,v ( n ),0≦ n&lt;M   sc   RS   [Equation 10]
 
         [0048]    In Equation 10 above, u denotes a group number used for group hopping, and uε{0, 1, . . . , 29}. v denotes a base sequence number used for sequence hopping, and vε{0,1} and v=0 for 5 RBs or fewer. a denotes a phase shift value for generating an RS sequence on the basis of a base sequence and is set to 
         [0000]    
       
         
           
             α 
             = 
             
               
                 
                   2 
                    
                   π 
                    
                   
                       
                   
                    
                   
                     n 
                     cs 
                   
                 
                 12 
               
               . 
             
           
         
       
     
         [0000]    M sc   RS  denotes the number of sub-carriers depending on an allocated RB size and is set to M sc   RS =12N I     RB   (u). In certain embodiments, N I     RB   (u) denotes the RB size corresponding to an RB index (IRB). 
         [0049]    Meanwhile, the base sequence is generated using the following Equation 11. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       r 
                       
                         u 
                         , 
                         v 
                       
                     
                      
                     
                       ( 
                       n 
                       ) 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                                
                               
                                 
                                   - 
                                   j 
                                 
                                  
                                 
                                   
                                     π 
                                      
                                     
                                         
                                     
                                      
                                     
                                       qm 
                                        
                                       
                                         ( 
                                         
                                           m 
                                           + 
                                           1 
                                         
                                         ) 
                                       
                                     
                                   
                                   
                                     N 
                                     ZC 
                                     RS 
                                   
                                 
                               
                             
                             , 
                           
                         
                         
                           
                             
                               if 
                                
                               
                                   
                               
                                
                               
                                 M 
                                 sc 
                                 RS 
                               
                             
                             ≥ 
                             
                               3 
                                
                               
                                 N 
                                 sc 
                                 RB 
                               
                             
                           
                         
                       
                       
                         
                           
                             
                                
                               
                                 
                                   jφ 
                                    
                                   
                                     ( 
                                     n 
                                     ) 
                                   
                                 
                                  
                                 
                                   π 
                                   / 
                                   4 
                                 
                               
                             
                             , 
                           
                         
                         
                           Otherwise 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     11 
                   
                   ] 
                 
               
             
           
         
       
     
         [0050]    In Equation 11 above,  r   u,v (n) denotes the base sequence, and m=mod(n,N ZC   RS ), q=└  q +½┘+v·(−1) └2  q ┘ , and  q =N ZC   RS ·(u+1)/31. N ZC   RS  denotes the length of a Zadoff-Chu (ZC) sequence and has a value related to the RB index (I RB ) as listed in the following Table 1. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 RB Index 
                   
               
               
                   
                 I RB   
                 N ZC   RS   
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 11 
               
               
                   
                 1 
                 23 
               
               
                   
                 2 
                 31 
               
               
                   
                 3 
                 47 
               
               
                   
                 4 
                 59 
               
               
                   
                 5 
                 71 
               
               
                   
                 6 
                 89 
               
               
                   
                 7 
                 107 
               
               
                   
                 8 
                 113 
               
               
                   
                 9 
                 139 
               
               
                   
                 10 
                 179 
               
               
                   
                 11 
                 191 
               
               
                   
                 12 
                 211 
               
               
                   
                 13 
                 239 
               
               
                   
                 14 
                 283 
               
               
                   
                 15 
                 293 
               
               
                   
                 16 
                 317 
               
               
                   
                 17 
                 359 
               
               
                   
                 18 
                 383 
               
               
                   
                 19 
                 431 
               
               
                   
                 20 
                 479 
               
               
                   
                 21 
                 523 
               
               
                   
                 22 
                 571 
               
               
                   
                 23 
                 599  
               
               
                   
                 24 
                 647 
               
               
                   
                 25 
                 719 
               
               
                   
                 26 
                 761 
               
               
                   
                 27 
                 863 
               
               
                   
                 28 
                 887 
               
               
                   
                 29 
                 953 
               
               
                   
                 30 
                 971 
               
               
                   
                 31 
                 1069 
               
               
                   
                 32 
                 1151 
               
               
                   
                 33 
                 1193 
               
               
                   
                 34 
                 1291 
               
               
                   
                 35 
                 1536 
               
               
                   
                   
               
             
          
         
       
     
         [0051]    When u, v, and n cs  (the number of cyclic shifts (CSs)) allocated to each UE are input from a higher layer, when the number of UEs is two, the RS generator  100  generates an RS sequence for the first UE that is one of the two UEs. The RS sequence is mapped with an RS symbol in the RS de-correlator  102  and has a channel impulse response (CIR) from sample 0 after IDFT when it is assumed that there is no uplink time offset of all the UEs. In addition, the second UE, which is the other of the two UEs, has a CIR from sample mod(12+n cs   (1) −n cs   (0) ,12)N I     RB     (u) . In the above equation, n cs   (1)  denotes the number of CSs of the second UE, and n cs   (0)  denotes the number of CSs of the first UE. For reference, twelve CSs from 0 to 11 are assigned to distinguish the UEs in the same ZC sequence, and in this case, one of the twelve CSs is designated to each UE. 
         [0052]    The RS sequence generated for the first UE is multiplied by a sub-carrier corresponding to an RS symbol among symbols of a frequency signal output after FFT and is generated as a de-correlated RS sub-carrier as follows. 
         [0000]        {tilde over (r)}   k   r,RS   =r   k   r,RS   r   u,v   (α) ( k ),0≦ k&lt;M   sc   RS   [Equation 12]
 
         [0053]    In Equation 12, {tilde over (r)} k   r,RS  denotes the de-correlated RS sub-carrier, r k   r,RS  denotes the sub-carrier including the RS symbol among the symbols of the frequency signal output after FFT, and r u,v   (α) (k) denotes the RS sequence generated for the first UE. 
         [0054]    Meanwhile, a method of distinguishing between the signal of the first UE and the signal of the second UE will be described as follows. When the RS of the first UE is r u,v   (α) (n)=e jα     1     n   r   u,v (n) 0≦n&lt;M sc   RS  and the RS of the second UE is r u,v   (α) (n)=e jα     2     n   r   u,v (n) 0≦n&lt;M sc   RS , a reception signal is represented as shown in the following Equation 13. 
         [0000]        r   k   r,RS   =H   1   e   jα     1     n     r     u,v ( n )+ H   2   e   jα     2     n     r     u,v ( n )=( H   1   e   jα     1     n   +H   2   e   jα     2     n )   r     u,v ( n )  [Equation 13]
 
         [0055]    The de-correlation result for the RS of the first UE for distinguishing the signal of the first UE from the reception signal is represented as shown in the following Equation 14. 
         [0000]          r     k   r,RS   =r   k   r,RS   e   −jα     1     n     r     u,v ( n )=( H   1   +H   2   e   j(α     2     −α     1     )n )  [Equation 14]
 
         [0056]    The following Equation 15 has to be satisfied to remove an interference component from H 1  and H 2 , which are channels of the first and second UEs, in view of an interval that does not vary with a frequency at every channel estimation interval. 
         [0000]        e   j(α     2     −α     1     )n   =e   j((2πn     cs,2     −2πn     cs,1     )/12)n   =e   j2π(n     cs,2     −n     cs,1     /12)n   =e   j2π(Δn     cs     /12)n =&gt;0  [Equation 15]
 
         [0057]    When as many e j2π(Δn     cs     /12)n  of Equation 15 as NT (an arbitrary integer * a period) tones are added, the interference channel component is removed as shown in the following Equation 16, thereby acquiring the channel of the first UE. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       1 
                       NT 
                     
                      
                     
                       
                         ∑ 
                         
                           n 
                           = 
                           0 
                         
                         
                           NT 
                           - 
                           1 
                         
                       
                        
                       
                         
                           r 
                           ~ 
                         
                         k 
                         
                           r 
                           , 
                           RS 
                         
                       
                     
                   
                   = 
                   
                     
                       
                         H 
                         1 
                       
                       + 
                       
                         
                           
                             H 
                             2 
                           
                           NT 
                         
                          
                         
                           
                             ∑ 
                             
                               n 
                               = 
                               0 
                             
                             
                               NT 
                               - 
                               1 
                             
                           
                            
                           
                              
                             
                               
                                 j2π 
                                  
                                 
                                   ( 
                                   
                                     Δ 
                                      
                                     
                                         
                                     
                                      
                                     
                                       
                                         n 
                                         cs 
                                       
                                       / 
                                       12 
                                     
                                   
                                   ) 
                                 
                               
                                
                               n 
                             
                           
                         
                       
                     
                     ≈ 
                     
                       H 
                       1 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     16 
                   
                   ] 
                 
               
             
           
         
       
     
         [0058]    In Equation 16 above, H 1  denotes the channel of the first UE, H 2  denotes the channel of the second UE, and T denotes the period. When Δ cs   (t) , the CS difference between the first and second UEs, is not 1, 5, or 7, a setting is made to 12/Δ cs   (t)  (when Δ cs   (t)  is smaller than or equal to 6) or 12/(12−Δ cs   (t) ) (when Δ cs   (t)  is larger than 6), and when Δ cs   (t)  is 1, 5, or 7, a setting is made to 12. 
         [0059]    The channel of the first UE is completely acquired in an ideal channel, such as additive white Gaussian noise (AWGN), without time offset and noise. However, in the distinction of the channels of the UEs, more residual interference components are left with increasing selective characteristics and noise, and with increasing tones added (when Δ cs   (t)  is 1, 5, or 7). 
         [0060]    Accordingly, only the channel of the first UE having no interference with the other multiplexed UE cannot be purely deduced, and therefore the best channel estimation performance cannot be ensured even though the optimum Wiener filtering is performed. 
         [0061]      FIG. 2  illustrates a detailed configuration of a multi-signal splitter for performing an MUI removal operation according to various embodiments of the present disclosure. 
         [0062]    Referring to  FIG. 2 , a multi-signal splitter  106  determines a period T when Δ cs   (t) , the CS difference between UEs, is input from a higher layer (such as DSP software, etc.). When a de-correlated signal {tilde over (r)} k   RS     u1    for the RS of the first UE is input, the multi-signal splitter  106  adds T frequency signals from the k-th frequency signal to (k+T−1)-th frequency signal to calculate the mean thereof and outputs the calculated mean value as the k-th MUI-removed signal {tilde over (r)} k   RS     u1   . In addition, when a de-correlated signal r k   RS     u2    for the RS of the second UE is input, the multi-signal splitter  106  adds T frequency signals from the k-th frequency signal to (k+T−1)-th frequency signal to calculate the mean thereof and outputs the calculated mean value as the k-th MUI-removed signal {tilde over (r)} k   RS     u2   . 
         [0063]    As described above, when the method of outputting the MUI-removed signal in the form of calculating the mean value is used, a residual interference component on the channel of each UE may not be completely removed, and therefore the optimum channel estimation performance cannot be expected. 
         [0064]    Accordingly, various embodiments of the present disclosure provide a method and apparatus for preventing performance degradation for channel estimation when a MIMO communication scheme is used. Specifically, the embodiment of the present disclosure provides a method and apparatus for acquiring the optimum Wiener filter coefficient using a residual interference component left after an MUI removal operation. 
         [0065]    Hereinafter, an apparatus for channel estimation, according to various embodiments of the present disclosure, will be described with reference to  FIG. 3 . 
         [0066]      FIG. 3  illustrates the apparatus for channel estimation according to various embodiments of the present disclosure. The apparatus for channel estimation illustrated in  FIG. 3  can be included, for example, in an uplink system of an LTE communication system. 
         [0067]    Referring to  FIG. 3 , the apparatus for channel estimation includes at least one of an RS generator  300 , an RS de-correlator  302 , an FFT device  304 , a time offset estimator  308 , a time offset compensator  310 , a Wiener filter  312 , and a time offset re-compensator  314 . 
         [0068]    In addition, in the apparatus for channel estimation, one controller implements the functions of the RS generator  300 , the RS de-correlator  302 , the FFT device  304 , the time offset estimator  308 , the time offset compensator  310 , the Wiener filter  312 , and the time offset re-compensator  314 . 
         [0069]    Furthermore, the apparatus for channel estimation includes a reception unit for receiving a signal. 
         [0070]    Specifically, the RS generator  300  generates an RS sequence, and the FFT device  304  performs fast Fourier transform (FFT) on a signal to be transmitted to output the transformed signal to the RS de-correlator  302 . The RS de-correlator  302  outputs the RS sequence mapped with an RS symbol. 
         [0071]    The time offset estimator  308  estimates a time offset for the received signal that has been fast Fourier transformed in the time or frequency domain. For example, the time offset estimator  308  estimates the time offset using a method of estimating the location of a sample having a peak value after IDFT, or using a method of calculating an accumulated average value of phase differences between tones in the frequency domain. 
         [0072]    The time offset compensator  310  compensates for phases according to sub-carriers for the fast Fourier transformed reception signal to output the phase-compensated reception signal to the Wiener filter  312 . The Wiener filter  312  applies a Wiener filter coefficient obtained from a channel correlation value to the received channel value in order to detect the channel estimation value for the phase-compensated reception signal. The time offset re-compensator  314  recovers the phase of the phase-compensated signal as it is on the basis of the channel estimation value. 
         [0073]    The Wiener filter  312  is used when a SIMO communication scheme or a MIMO communication scheme is used. In addition, although the time offset estimator  308  and the time offset compensator  310  are illustrated to be physically separated elements in  FIG. 3 , the operations of the time offset estimator  308  and the time offset compensator  310  are integrally performed by the Wiener filter  312 , and in certain embodiments, the time offset estimator  308  and the time offset compensator  310  may not be included in the apparatus for channel estimation. 
         [0074]    Meanwhile, in a general wireless communication system, two-dimensional Wiener filtering is used to determine a Wiener filter coefficient on the basis of a correlation matrix for the SIMO communication scheme. Therefore, when the MIMO communication scheme is used in the general wireless communication system, channel estimation is performed without consideration for MUI, and thus the performance of the channel estimation is degraded. 
         [0075]    Accordingly, various embodiments of the present disclosure proposes a method for Wiener filtering that is effectively used in the MIMO communication scheme as well as the SIMO communication scheme. The method for Wiener filtering proposed by the embodiment of the present disclosure considers MUI to prevent the channel estimation performance from being degraded. 
         [0076]    When two UEs, a first UE and a second UE, are allocated through a collaborative spatial multiplexing method, that is, when a multiple user (MU)-MIMO communication scheme is used, a two-dimensional correlation matrix is calculated. 
         [0077]    The fading channel of the first UE is represented, for example, as shown in the following Equation 17. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       h 
                       1 
                     
                      
                     
                       ( 
                       
                         
                           t 
                           1 
                         
                         , 
                         τ 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         1 
                       
                       
                         L 
                         1 
                       
                     
                      
                     
                       
                         
                           h 
                           
                             1 
                              
                             l 
                           
                         
                          
                         
                           ( 
                           
                             t 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         δ 
                          
                         
                           ( 
                           
                             τ 
                             - 
                             
                               τ 
                               
                                 1 
                                  
                                 l 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     17 
                   
                   ] 
                 
               
             
           
         
       
     
         [0078]    In Equation 17 above, h 1 (t 1 ,τ) denotes the fading channel of the first UE, and δ denotes an impulse function. 
         [0079]    The fading channel of the second UE is represented, for example, as shown in the following Equation 18. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       h 
                       2 
                     
                      
                     
                       ( 
                       
                         
                           t 
                           1 
                         
                         , 
                         τ 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         1 
                       
                       
                         L 
                         2 
                       
                     
                      
                     
                       
                         
                           h 
                           
                             2 
                              
                             l 
                           
                         
                          
                         
                           ( 
                           
                             t 
                             1 
                           
                           ) 
                         
                       
                        
                       
                         δ 
                          
                         
                           ( 
                           
                             τ 
                             - 
                             
                               τ 
                               
                                 2 
                                  
                                 l 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     18 
                   
                   ] 
                 
               
             
           
         
       
     
         [0080]    In Equation 18 above, h 2 (t 1 , τ) denotes the fading channel of the second UE. 
         [0081]    When the MU-MIMO communication scheme is used, the channels of the first and second UEs are received at one time, and therefore the actual reception channel is represented by H 1 (t 1 ,τ)=h 1 (t 1 ,τ)+h 2 (t 1 ,τ). The actual reception channel is represented as shown in the following Equation 19 at (t 1 , f 1 ) in the frequency domain. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       1 
                     
                      
                     
                       ( 
                       
                         
                           t 
                           1 
                         
                         , 
                         
                           f 
                           1 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           1 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               1 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           W 
                           
                             
                               τ 
                               
                                 1 
                                  
                                 l 
                               
                             
                              
                             
                               f 
                               l 
                             
                           
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           2 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               2 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           W 
                           
                             
                               τ 
                               
                                 2 
                                  
                                 l 
                               
                             
                              
                             
                               f 
                               1 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     19 
                   
                   ] 
                 
               
             
           
         
       
     
         [0082]    In Equation 19 above, 
         [0000]    
       
         
           
             
               W 
               = 
               
                  
                 
                   - 
                   
                     j2π 
                     
                       N 
                       FFT 
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    and E[|h l (t)| 2 ]=H l , Σ 1   L H l =1. 
         [0083]    Meanwhile, the signals of the first and second UEs are distinguished from each other through a method of performing the summation of sub-carriers in the frequency domain. This is represented as shown in the foregoing Equation 16, and the signals of the first and second UEs are distinguished from each other by de-correlating the signals as RS sequences of the corresponding UEs, and summing the sub-carriers for a multiple of a period T in order to obtain channel values by distinguishing the signals of the first and second UEs as in the above-described MUI removal method. 
         [0084]    When ZC sequences that are the same but have different CSs are used for the UEs subjected to multiplexing, the RS signals of the two UEs appear in such a form that a difference between sub-carriers is phase-rotated by the interval between the CSs. Equation 14 represents this, and when the sub-carriers are summed for a multiple of a period T, the signal component having a difference of the CS interval from the signal of the corresponding UE disappears and only the signal (H 1 ) of the corresponding UE is left as represented by Equation 16. Considering this, the number of sub-carriers to be summed is determined depending on the allocated CS interval. 
         [0085]    When the number of sub-carriers is determined as described above, the reception channel response values of the sub-carriers are added up. For example, in cases where the CS interval is 6 and two sub-carriers are summed, the total sum of the channel values for the two sub-carriers is as follows. 
         [0086]    The channel (H 1 (t 1 , f 1 +1)) in a sub-carrier subsequent to f 1  is represented as shown in the following Equation 20, and the total sum of the channel values for the two sub-carriers is represented as shown in Equation 21. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       1 
                     
                      
                     
                       ( 
                       
                         
                           t 
                           1 
                         
                         , 
                         
                           
                             f 
                             1 
                           
                           + 
                           1 
                         
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           1 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               1 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           W 
                           
                             
                               τ 
                               
                                 1 
                                  
                                 l 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   f 
                                   1 
                                 
                                 + 
                                 1 
                               
                               ) 
                             
                           
                         
                       
                     
                     - 
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           2 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               2 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           W 
                           
                             
                               τ 
                               
                                 2 
                                  
                                 l 
                               
                             
                              
                             
                               ( 
                               
                                 
                                   f 
                                   1 
                                 
                                 + 
                                 1 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     20 
                   
                   ] 
                 
               
             
             
               
                 
                   
                     
                       H 
                        
                       
                         ( 
                         
                           
                             t 
                             1 
                           
                           , 
                           
                             f 
                             1 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       H 
                        
                       
                         ( 
                         
                           
                             t 
                             1 
                           
                           , 
                           
                             
                               f 
                               1 
                             
                             + 
                             1 
                           
                         
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           1 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               1 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           
                             W 
                             
                               
                                 τ 
                                 
                                   1 
                                    
                                   l 
                                 
                               
                                
                               
                                 f 
                                 1 
                               
                             
                           
                            
                           
                             ( 
                             
                               1 
                               + 
                               
                                 W 
                                 
                                   τ 
                                   
                                     1 
                                      
                                     l 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           
                             2 
                              
                             
                                 
                             
                           
                         
                       
                        
                       
                         
                           
                             h 
                             
                               2 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           
                             W 
                             
                               
                                 τ 
                                 
                                   2 
                                    
                                   l 
                                 
                               
                                
                               
                                 f 
                                 1 
                               
                             
                           
                            
                           
                             ( 
                             
                               1 
                               - 
                               
                                 W 
                                 
                                   τ 
                                   
                                     2 
                                      
                                     l 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     21 
                   
                   ] 
                 
               
             
           
         
       
     
         [0087]    Similarly, the channel value at (t 2 , f 2 ) is represented as shown in the following Equation 22. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                        
                       
                         ( 
                         
                           
                             t 
                             2 
                           
                           , 
                           
                             f 
                             2 
                           
                         
                         ) 
                       
                     
                     + 
                     
                       H 
                        
                       
                         ( 
                         
                           
                             t 
                             2 
                           
                           , 
                           
                             
                               f 
                               2 
                             
                             + 
                             1 
                           
                         
                         ) 
                       
                     
                   
                   = 
                   
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           1 
                         
                       
                        
                       
                         
                           
                             h 
                             
                               1 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               2 
                             
                             ) 
                           
                         
                          
                         
                           
                             W 
                             
                               
                                 τ 
                                 
                                   1 
                                    
                                   l 
                                 
                               
                                
                               
                                 f 
                                 2 
                               
                             
                           
                            
                           
                             ( 
                             
                               1 
                               + 
                               
                                 W 
                                 
                                   τ 
                                   
                                     1 
                                      
                                     l 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                     + 
                     
                       
                         ∑ 
                         
                           l 
                           = 
                           1 
                         
                         
                           L 
                           
                             2 
                              
                             
                                 
                             
                           
                         
                       
                        
                       
                         
                           
                             h 
                             
                               2 
                                
                               l 
                             
                           
                            
                           
                             ( 
                             
                               t 
                               2 
                             
                             ) 
                           
                         
                          
                         
                           
                             W 
                             
                               
                                 τ 
                                 
                                   2 
                                    
                                   l 
                                 
                               
                                
                               
                                 f 
                                 2 
                               
                             
                           
                            
                           
                             ( 
                             
                               1 
                               - 
                               
                                 W 
                                 
                                   τ 
                                   
                                     2 
                                      
                                     l 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     22 
                   
                   ] 
                 
               
             
           
         
       
     
         [0088]    A two-dimensional correlation matrix is determined using the following Equation 23 on the basis of the channel values of (t 1 , f 1 ) and (t 2 , f 2 ) that have been calculated as described above. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     hh 
                   
                   = 
                   
                     
                       
                         1 
                         4 
                       
                        
                       
                         E 
                          
                         
                           [ 
                           
                             
                               ( 
                               
                                 
                                   H 
                                    
                                   
                                     ( 
                                     
                                       
                                         t 
                                         1 
                                       
                                       , 
                                       
                                         f 
                                         1 
                                       
                                     
                                     ) 
                                   
                                 
                                 + 
                                 
                                   H 
                                    
                                   
                                     ( 
                                     
                                       
                                         t 
                                         1 
                                       
                                       , 
                                       
                                         
                                           f 
                                           1 
                                         
                                         + 
                                         1 
                                       
                                     
                                     ) 
                                   
                                 
                               
                               ) 
                             
                              
                             
                               
                                 ( 
                                 
                                   
                                     H 
                                      
                                     
                                       ( 
                                       
                                         
                                           t 
                                           2 
                                         
                                         , 
                                         
                                           f 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                   + 
                                   
                                     H 
                                      
                                     
                                       ( 
                                       
                                         
                                           t 
                                           2 
                                         
                                         , 
                                         
                                           
                                             f 
                                             2 
                                           
                                           + 
                                           1 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                               * 
                             
                           
                           ] 
                         
                       
                     
                     = 
                     
                       
                         
                           1 
                           4 
                         
                          
                         
                           
                             ∑ 
                             
                               l 
                               ′ 
                             
                             
                               L 
                               1 
                             
                           
                            
                           
                             
                               ∑ 
                               
                                 l 
                                 ″ 
                               
                               
                                 L 
                                 1 
                               
                             
                              
                             
                               
                                 E 
                                  
                                 
                                   [ 
                                   
                                     
                                       
                                         h 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                        
                                       
                                         ( 
                                         
                                           t 
                                           1 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       
                                         
                                           h 
                                           
                                             1 
                                              
                                             
                                               l 
                                               ″ 
                                             
                                           
                                         
                                          
                                         
                                           ( 
                                           
                                             t 
                                             2 
                                           
                                           ) 
                                         
                                       
                                       * 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 
                                   W 
                                   
                                     τ 
                                     
                                       1 
                                        
                                       
                                         l 
                                         
                                           
                                             ′ 
                                              
                                             
                                                 
                                             
                                              
                                             
                                               f 
                                               1 
                                             
                                           
                                           - 
                                           
                                             
                                               τ 
                                               
                                                 1 
                                                  
                                                 
                                                   l 
                                                   ″ 
                                                 
                                               
                                             
                                              
                                             
                                               f 
                                               2 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       W 
                                       
                                         τ 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     W 
                                     
                                       - 
                                       
                                         τ 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ″ 
                                           
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           1 
                           4 
                         
                          
                         
                           
                             ∑ 
                             
                               l 
                               ′ 
                             
                             
                               L 
                               2 
                             
                           
                            
                           
                             
                               ∑ 
                               
                                 l 
                                 ″ 
                               
                               
                                 L 
                                 2 
                               
                             
                              
                             
                               
                                 E 
                                  
                                 
                                   [ 
                                   
                                     
                                       
                                         h 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                        
                                       
                                         ( 
                                         
                                           t 
                                           1 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       
                                         
                                           h 
                                           
                                             2 
                                              
                                             
                                               l 
                                               ″ 
                                             
                                           
                                         
                                          
                                         
                                           ( 
                                           
                                             t 
                                             2 
                                           
                                           ) 
                                         
                                       
                                       * 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 
                                   W 
                                   
                                     τ 
                                     
                                       2 
                                        
                                       
                                         l 
                                         
                                           
                                             ′ 
                                              
                                             
                                                 
                                             
                                              
                                             
                                               f 
                                               1 
                                             
                                           
                                           - 
                                           
                                             
                                               τ 
                                               
                                                 2 
                                                  
                                                 
                                                   l 
                                                   ″ 
                                                 
                                               
                                             
                                              
                                             
                                               f 
                                               2 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       W 
                                       
                                         τ 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     W 
                                     
                                       - 
                                       
                                         τ 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ″ 
                                           
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           1 
                           4 
                         
                          
                         
                           
                             ∑ 
                             
                               l 
                               ′ 
                             
                             
                               L 
                               1 
                             
                           
                            
                           
                             
                               ∑ 
                               
                                 l 
                                 ″ 
                               
                               
                                 L 
                                 2 
                               
                             
                              
                             
                               
                                 E 
                                  
                                 
                                   [ 
                                   
                                     
                                       
                                         h 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                        
                                       
                                         ( 
                                         
                                           t 
                                           1 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       
                                         
                                           h 
                                           
                                             2 
                                              
                                             
                                               l 
                                               ″ 
                                             
                                           
                                         
                                          
                                         
                                           ( 
                                           
                                             t 
                                             2 
                                           
                                           ) 
                                         
                                       
                                       * 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 
                                   W 
                                   
                                     τ 
                                     
                                       1 
                                        
                                       
                                         l 
                                         
                                           
                                             ′ 
                                              
                                             
                                                 
                                             
                                              
                                             
                                               f 
                                               1 
                                             
                                           
                                           - 
                                           
                                             
                                               τ 
                                               
                                                 2 
                                                  
                                                 
                                                   l 
                                                   ″ 
                                                 
                                               
                                             
                                              
                                             
                                               f 
                                               2 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       W 
                                       
                                         τ 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     W 
                                     
                                       - 
                                       
                                         τ 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ″ 
                                           
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           1 
                           4 
                         
                          
                         
                           
                             ∑ 
                             
                               l 
                               ′ 
                             
                             
                               L 
                               2 
                             
                           
                            
                           
                             
                               ∑ 
                               
                                 l 
                                 ″ 
                               
                               
                                 L 
                                 1 
                               
                             
                              
                             
                               
                                 E 
                                  
                                 
                                   [ 
                                   
                                     
                                       
                                         h 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                        
                                       
                                         ( 
                                         
                                           t 
                                           1 
                                         
                                         ) 
                                       
                                     
                                      
                                     
                                       
                                         
                                           h 
                                           
                                             1 
                                              
                                             
                                               l 
                                               ″ 
                                             
                                           
                                         
                                          
                                         
                                           ( 
                                           
                                             t 
                                             2 
                                           
                                           ) 
                                         
                                       
                                       * 
                                     
                                   
                                   ] 
                                 
                               
                                
                               
                                 
                                   W 
                                   
                                     τ 
                                     
                                       2 
                                        
                                       
                                         l 
                                         
                                           
                                             ′ 
                                              
                                             
                                                 
                                             
                                              
                                             
                                               f 
                                               1 
                                             
                                           
                                           - 
                                           
                                             
                                               τ 
                                               
                                                 1 
                                                  
                                                 
                                                   l 
                                                   ″ 
                                                 
                                               
                                             
                                              
                                             
                                               f 
                                               2 
                                             
                                           
                                         
                                       
                                     
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     1 
                                     + 
                                     
                                       W 
                                       
                                         τ 
                                         
                                           2 
                                            
                                           
                                             l 
                                             ′ 
                                           
                                         
                                       
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     W 
                                     
                                       - 
                                       
                                         τ 
                                         
                                           1 
                                            
                                           
                                             l 
                                             ″ 
                                           
                                         
                                       
                                     
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     23 
                   
                   ] 
                 
               
             
           
         
       
     
         [0089]    The channels allocated to multiple paths are different from each other, and the channels allocated to the respective UEs are also different from each other. Therefore, a mean value is calculated when l′=l″ in the channel of a particular UE. Arrangement of the first added term 
         [0000]    
       
         
           
             
               1 
               4 
             
              
             
               
                 ∑ 
                 
                   l 
                   ′ 
                 
                 
                   L 
                   1 
                 
               
                
               
                 
                   ∑ 
                   
                     l 
                     ″ 
                   
                   
                     L 
                     1 
                   
                 
                  
                 
                   
                     E 
                      
                     
                       [ 
                       
                         
                           
                             h 
                             
                               1 
                                
                               
                                 l 
                                 ′ 
                               
                             
                           
                            
                           
                             ( 
                             
                               t 
                               1 
                             
                             ) 
                           
                         
                          
                         
                           
                             
                               h 
                               
                                 1 
                                  
                                 
                                   l 
                                   ″ 
                                 
                               
                             
                              
                             
                               ( 
                               
                                 t 
                                 2 
                               
                               ) 
                             
                           
                           * 
                         
                       
                       ] 
                     
                   
                    
                   
                     
                       W 
                       
                         τ 
                         
                           
                             1 
                             ! 
                           
                           
                             
                               ′ 
                                
                               
                                   
                               
                                
                               
                                 f 
                                 1 
                               
                             
                             - 
                             
                               
                                 τ 
                                 
                                   1 
                                    
                                   
                                     l 
                                     ″ 
                                   
                                 
                               
                                
                               
                                 f 
                                 2 
                               
                             
                           
                         
                       
                     
                      
                     
                       ( 
                       
                         1 
                         + 
                         
                           W 
                           
                             τ 
                             
                               1 
                                
                               
                                 l 
                                 ′ 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                    
                   
                     ( 
                     
                       1 
                       + 
                       
                         W 
                         
                           - 
                           
                             τ 
                             
                               1 
                                
                               
                                 l 
                                 ″ 
                               
                             
                           
                         
                       
                     
                     ) 
                   
                 
               
             
           
         
       
     
         [0000]    in Equation 23 yields the following Equation 24. 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       1 
                       4 
                     
                      
                     
                       
                         ∑ 
                         
                           l 
                           ′ 
                         
                         
                           L 
                           1 
                         
                       
                        
                       
                         
                           H 
                           
                             1 
                              
                             
                               l 
                               ′ 
                             
                           
                         
                          
                         
                           
                             
                               W 
                               
                                 τ 
                                 
                                   1 
                                    
                                   
                                     l 
                                     
                                       ′ 
                                        
                                       
                                         ( 
                                         
                                           
                                             f 
                                             1 
                                           
                                           - 
                                           
                                             f 
                                             2 
                                           
                                         
                                         ) 
                                       
                                     
                                   
                                 
                               
                             
                              
                             
                               ( 
                               
                                 1 
                                 + 
                                 1 
                                 + 
                                 
                                   W 
                                   
                                     τ 
                                     
                                       1 
                                        
                                       
                                         l 
                                         ′ 
                                       
                                     
                                   
                                 
                                 + 
                                 
                                   W 
                                   
                                     - 
                                     
                                       τ 
                                       
                                         1 
                                          
                                         
                                           l 
                                           ′ 
                                         
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                           
                           · 
                           
                             
                               J 
                               0 
                             
                              
                             
                               ( 
                               
                                 2 
                                  
                                 π 
                                  
                                 
                                     
                                 
                                  
                                 
                                   f 
                                   c 
                                 
                                  
                                 
                                   v 
                                   c 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       t 
                                       2 
                                     
                                     - 
                                     
                                       t 
                                       1 
                                     
                                   
                                   ) 
                                 
                                  
                                 T 
                               
                               ) 
                             
                           
                         
                       
                     
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           
                             l 
                             ′ 
                           
                           
                             L 
                             1 
                           
                         
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         
                                           
                                             2 
                                              
                                             π 
                                           
                                           
                                             N 
                                             FFT 
                                           
                                         
                                          
                                         
                                           τ 
                                           
                                             1 
                                              
                                             
                                               l 
                                               ′ 
                                             
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                               / 
                               2 
                             
                             · 
                             
                               H 
                               
                                 1 
                                  
                                 
                                   l 
                                   ′ 
                                 
                               
                             
                           
                            
                           
                             W 
                             
                               τ 
                               
                                 1 
                                  
                                 
                                   l 
                                   
                                     ′ 
                                      
                                     
                                       ( 
                                       
                                         
                                           f 
                                           1 
                                         
                                         - 
                                         
                                           f 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                           
                         
                       
                       
                          
                         
                           freq 
                            
                           
                               
                           
                            
                           domain 
                         
                       
                     
                     · 
                     
                       
                         
                           J 
                           0 
                         
                          
                         
                           ( 
                           
                             2 
                              
                             π 
                              
                             
                                 
                             
                              
                             
                               f 
                               c 
                             
                              
                             
                               v 
                               c 
                             
                              
                             
                               ( 
                               
                                 
                                   t 
                                   2 
                                 
                                 - 
                                 
                                   t 
                                   1 
                                 
                               
                               ) 
                             
                              
                             T 
                           
                           ) 
                         
                       
                       
                          
                         
                           time 
                            
                           
                               
                           
                            
                           domain 
                         
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     24 
                   
                   ] 
                 
               
             
           
         
       
     
         [0090]    The second added term in Equation 23 also is arranged similar to Equation 24. For the third and fourth added terms in Equation 23, the channels are different from each other (i.e., h 1 ≠h 2 ) so that the mean value becomes 0. Therefore, a correlation matrix is determined as shown in the following Equation 25. 
         [0000]    
       
         
           
             
               
                 
                   
                     R 
                     hh 
                   
                   = 
                   
                     
                       
                         
                           ∑ 
                           l 
                           
                             L 
                             1 
                           
                         
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   + 
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         
                                           
                                             2 
                                              
                                             π 
                                           
                                           
                                             N 
                                             FFT 
                                           
                                         
                                          
                                         
                                           τ 
                                           
                                             1 
                                              
                                             l 
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                               / 
                               2 
                             
                             · 
                             
                               H 
                               
                                 1 
                                  
                                 l 
                               
                             
                           
                            
                           
                             
                               W 
                               
                                 
                                   τ 
                                   
                                     1 
                                      
                                     l 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       f 
                                       1 
                                     
                                     - 
                                     
                                       f 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             · 
                             
                               
                                 J 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   2 
                                    
                                   π 
                                    
                                   
                                       
                                   
                                    
                                   
                                     f 
                                     c 
                                   
                                    
                                   
                                     v 
                                     c 
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         t 
                                         2 
                                       
                                       - 
                                       
                                         t 
                                         1 
                                       
                                     
                                     ) 
                                   
                                    
                                   T 
                                 
                                 ) 
                               
                             
                           
                         
                       
                       + 
                       
                         
                           ∑ 
                           l 
                           
                             L 
                             2 
                           
                         
                          
                         
                           
                             
                               
                                 ( 
                                 
                                   1 
                                   - 
                                   
                                     cos 
                                      
                                     
                                       ( 
                                       
                                         
                                           
                                             2 
                                              
                                             π 
                                           
                                           
                                             N 
                                             FFT 
                                           
                                         
                                          
                                         
                                           τ 
                                           
                                             2 
                                              
                                             l 
                                           
                                         
                                       
                                       ) 
                                     
                                   
                                 
                                 ) 
                               
                               / 
                               2 
                             
                             · 
                             
                               H 
                               
                                 2 
                                  
                                 l 
                               
                             
                           
                            
                           
                             
                               W 
                               
                                 
                                   τ 
                                   
                                     2 
                                      
                                     l 
                                   
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       f 
                                       1 
                                     
                                     - 
                                     
                                       f 
                                       2 
                                     
                                   
                                   ) 
                                 
                               
                             
                             · 
                             
                               
                                 J 
                                 0 
                               
                                
                               
                                 ( 
                                 
                                   2 
                                    
                                   π 
                                    
                                   
                                       
                                   
                                    
                                   
                                     f 
                                     c 
                                   
                                    
                                   
                                     v 
                                     c 
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         t 
                                         2 
                                       
                                       - 
                                       
                                         t 
                                         1 
                                       
                                     
                                     ) 
                                   
                                    
                                   T 
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                     = 
                     
                         
                       
                         [ 
                         
                           
                             
                               
                                 ∑ 
                                 l 
                                 
                                   L 
                                   1 
                                 
                               
                                
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         1 
                                         + 
                                         
                                           cos 
                                            
                                           
                                             ( 
                                             
                                               
                                                 
                                                   2 
                                                    
                                                   π 
                                                 
                                                 
                                                   N 
                                                   FFT 
                                                 
                                               
                                                
                                               
                                                 τ 
                                                 
                                                   1 
                                                    
                                                   l 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                       ) 
                                     
                                     / 
                                     2 
                                   
                                   · 
                                   
                                     H 
                                     
                                       1 
                                        
                                       l 
                                     
                                   
                                 
                                  
                                 
                                   W 
                                   
                                     
                                       τ 
                                       
                                         1 
                                          
                                         l 
                                       
                                     
                                      
                                     
                                       ( 
                                       
                                         
                                           f 
                                           1 
                                         
                                         - 
                                         
                                           f 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                                
                               
                                 desired 
                                  
                                 
                                     
                                 
                                  
                                 
                                   UE 
                                   &#39; 
                                 
                                  
                                 s 
                                  
                                 
                                     
                                 
                                  
                                 channel 
                               
                             
                           
                           + 
                           
                             
                               
                                 ∑ 
                                 l 
                                 
                                   L 
                                   2 
                                 
                               
                                
                               
                                 
                                   
                                     
                                       ( 
                                       
                                         1 
                                         - 
                                         
                                           cos 
                                            
                                           
                                             ( 
                                             
                                               
                                                 
                                                   2 
                                                    
                                                   π 
                                                 
                                                 
                                                   N 
                                                   FFT 
                                                 
                                               
                                                
                                               
                                                 τ 
                                                 
                                                   2 
                                                    
                                                   l 
                                                 
                                               
                                             
                                             ) 
                                           
                                         
                                       
                                       ) 
                                     
                                     / 
                                     2 
                                   
                                   · 
                                   
                                     H 
                                     
                                       2 
                                        
                                       l 
                                     
                                   
                                 
                                  
                                 
                                   W 
                                   
                                     
                                       τ 
                                       
                                         2 
                                          
                                         l 
                                       
                                     
                                      
                                     
                                       ( 
                                       
                                         
                                           f 
                                           1 
                                         
                                         - 
                                         
                                           f 
                                           2 
                                         
                                       
                                       ) 
                                     
                                   
                                 
                               
                             
                             
                                
                               
                                 MUI 
                                  
                                 
                                     
                                 
                                  
                                 chemist 
                               
                             
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   [ 
                   
                     Equation 
                      
                     
                         
                     
                      
                     25 
                   
                   ] 
                 
               
             
           
         
       
     
         [0091]    Referring to Equation 25, the correlation matrix is determined in consideration of residual interference occurring after the MUI removal operation. When the determined correlation matrix is applied to the above-described Equation 4 as a Wiener filter coefficient used in the Wiener filter  312 , the optimum channel estimation for a MIMO signal is performed. 
         [0092]    Meanwhile, although the case in which the CS interval is 6 has been described above, the method according to the embodiment of the present disclosure can also be applied to other cases in which the CS interval is not 6. In addition, the Wiener filtering can also be used in UEs of a MIMO communication system, such as OCC, having mutually orthogonal RS symbols in a similar manner. 
         [0093]      FIG. 4  illustrates a process of performing channel estimation according to various embodiments of the present disclosure. 
         [0094]    Referring to  FIG. 4 , in step  400 , an apparatus for channel estimation determines the number N of sub-carriers or symbols to add up channel values on the basis of at least one of a CS interval and OCC. For example, when the CS interval is 6, N is 2, or when the OCC is applied, N is 2. 
         [0095]    In step  402 , the apparatus for channel estimation adds up the channel values for respective UEs according to the N sub-carriers or symbols. For example, in the case of 2 UE MIMO, the apparatus adds up the channel value of a first UE (hereinafter, referred to as a ‘first channel value’) and the channel value of a second UE (hereinafter, referred to as a ‘second channel value’) for the N sub-carriers or symbols. 
         [0096]    The apparatus for channel estimation determines a Wiener filter coefficient by correlating the channel values added up for the respective UEs (by correlating the first and second channel values in the above example) in step  404 , and performs channel estimation using the Wiener filter coefficient in step  406 . 
         [0097]    Although the present disclosure has been described with an exemplary embodiment, various changes and modifications may be suggested to one skilled in the art. It is intended that the present disclosure encompass such changes and modifications as fall within the scope of the appended claims.