Abstract:
The present invention relates to performance improvement in a cell edge, particularly, to a method for selecting a precoder for a terminal in a multiple antenna system, wherein the method comprises: performing channel estimation for reference signals of a serving base station and another base station; determining a minimum singular value by using the channel estimation result; and if a complete collaborative feedback structure is used, feeding back a table of the determined, minimum singular value to the base stations.

Description:
PRIORITY 
     This application is a National Stage application under 35 U.S.C. §371 of an International application filed on Apr. 2, 2010 and assigned application No. PCT/KR2010/002051, and claims the benefit under 35 U.S.C. §365(b) of a Korean patent application filed Apr. 2, 2009 in the Korean Intellectual Property Office and assigned application No. 10-2009-0028648, the entire disclosure of which is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is for overcoming inter-cell interference in a multi-cell environment and improving performance for a receiver in a cell edge. Particularly, the present invention, which relates to a transmitter suitable to a case of nulling Other Cell Interference (OCI) in a receiver, relates to an apparatus and method for minimizing the entire system error. 
     2. Description of the Related Art 
     A terminal of a cell edge simultaneously receives not only a signal transmitted from its own cell but also signals of neighboring other cells. The signals received from the other cells act as interference, acting as a main cause of limiting the performance of the terminal. That is, there occurs a problem that Inter-Cell Interference (ICI) results in system capacity or throughput performance degradation and the like. 
     And, this problem works more seriously in a Multiple Input Multiple Output (MIMO) system. Accordingly, research on technologies for overcoming this ICI and improving performance is being carried out. This is exemplified as a codebook-based error minimization technique (quantization preprocessor) for a single user, and a codebook-based ICI reduction technique. 
     The quantization preprocessor, which is of a MIMO preprocessing technique of minimizing errors in a single cell, single user environment, does not take interference from neighboring cells into consideration. And, in a case where there are many receive antennas compared to the total number of transmission streams, a receiver can null OCI through a nulling filter. However, a transmission precoder (code word) is selected without considering the OCI-Nulling Filter (OCI-NF), so a problem occurs that the OCI-NF results in performance degradation. 
     The codebook-based ICI reduction technique, which is a technique of minimizing OCI using a precoder, brings about the improvement of a Signal to Noise and Interference Ratio (SINR). But, this technique cannot completely null interference, so there is a problem that, in a case where the size of interference is very large, it is unsuitable to real reception. 
     SUMMARY OF THE INVENTION 
     An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide an apparatus and a method for minimizing errors by a cell edge user in a multi-cell communication system. 
     Another object of the present invention is to provide an apparatus and method for minimizing errors of a cell edge through codebook-based MIMO precoding and a receiver OCI-NF. 
     According to a 1st aspect for achieving the objects of the present invention, a precoder selection method of a terminal in a multiple antenna system is characterized by including performing channel estimation for reference signals of a serving base station and another base station, determining a minimum singular value using the channel estimation result, and, in a case where a full feedback cooperative scheme is used, feeding back a table of the determined minimum singular value to the base station. 
     According to a 2nd aspect for achieving the objects of the present invention, a precoder selection method of a base station in a multiple antenna system is characterized by including receiving setting information from a terminal, in a case where a cooperative scheme is used, exchanging the setting information with setting information of another base station, in a case where the cooperative scheme is used, selecting a code word minimizing errors for the entire system based on the setting information and the setting information of the another base station, and, in a case where the cooperative scheme is not used, selecting a code word minimizing errors for the entire system based on the setting information. 
     According to a 3rd aspect for achieving the objects of the present invention, an apparatus of a terminal of selecting a code word in a multiple antenna system is characterized by including a channel estimator for performing channel estimation for reference signals of a serving base station and another base station, a minimum singular value calculator for determining a minimum singular value using the channel estimation result, and a transmitter for, in a case where a full feedback cooperative scheme is used, feeding back a table of the determined minimum singular value to the base station. 
     According to a 4th aspect for achieving the objects of the present invention, an apparatus of a base station of selecting a precoder in a multiple antenna system is characterized by including a minimum singular value comparator for receiving setting information from a terminal, and exchanging and comparing with setting information of another base station, and a code word selector for, in a case where a cooperative scheme is used, exchanging the setting information with setting information of another base station, selecting a code word minimizing errors for the entire system based on the setting information and the setting information of the another base station and, in a case where the cooperative scheme is not used, selecting a code word minimizing errors for the entire system based on the setting information. 
     Other aspects, advantages and salient features of the invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a diagram illustrating a communication system according to an exemplary embodiment of the present invention; 
         FIG. 2  is a flowchart illustrating an operation process of a base station and a terminal according to an exemplary embodiment of the present invention; 
         FIG. 3  is a block diagram illustrating schematic constructions of a terminal and a base station in a cooperative scheme according to an exemplary embodiment of the present invention; 
         FIG. 4  is a block diagram illustrating schematic constructions of a terminal and a base station in a non-cooperative scheme according to an exemplary embodiment of the present invention; 
         FIG. 5  is a diagram illustrating an example of a code word selected in accordance with a feedback technique according to an exemplary embodiment of the present invention; 
         FIG. 6  is a flowchart illustrating an operation process of a terminal according to an exemplary embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating an operation process of a base station according to an exemplary embodiment of the present invention; 
         FIG. 8  is a block diagram illustrating constructions of a base station and a terminal according to an exemplary embodiment of the present invention; 
         FIG. 9  is a 1st diagram illustrating BER performance according to an exemplary embodiment of the present invention; and 
         FIG. 10  is a 2nd diagram illustrating BER performance according to an exemplary embodiment of the present invention. 
     
    
    
     Throughout the drawings, like reference numerals will be understood to refer to like parts, components and structures. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness. 
     The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
     It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a component surface” includes reference to one or more of such surfaces. 
     By the term “substantially” it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide. 
     Below, the present invention describes an apparatus and method for minimizing errors by a cell edge user in a multi-cell communication system. 
     The present invention selects a precoder for minimizing errors of a cell edge considering a receiver OCI-NF. That is, the present invention nulls OCI through the receiver OCI-NF and selects a codebook-based precoder such that the entire errors of the cell edge are minimized considering the OCI-NF. 
     The present invention is composed of a cooperative technique and a non-cooperative technique depending on the existence or non-existence of information exchange between base stations in code word selection. 
     The cooperative technique is composed of a full feedback or reduced feedback method according to a cooperative code word selection method. The non-cooperative technique is composed of a non-cooperative code word selection method and a simple index feedback method. 
       FIG. 1  is a diagram illustrating a communication system according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 1 , it is a typical environment where ICI exists, and illustrates a case where there are two terminals  115  and  165  allocated the same time and frequency resources in a cell edge. 
     Here, a base station 1   110  transmits data to the terminal 1   115 , and a base station 2   150  to the terminal 2   165 . If defining the number of transmit antennas of the base stations  110  and  150  as n T  and the number of receive antennas of the terminals  115  and  165  as n R , and defining signals received by the terminal 1   115  and the terminal 2   165  as n R ×1 vectors y 1  and y 2 , respectively, it is given as in Equation below. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         
                           
                             y 
                             1 
                           
                         
                       
                       
                         
                           
                             y 
                             2 
                           
                         
                       
                     
                     ] 
                   
                   = 
                   
                     
                       
                         [ 
                         
                           
                             
                               
                                 H 
                                 11 
                               
                             
                             
                               
                                 H 
                                 12 
                               
                             
                           
                           
                             
                               
                                 H 
                                 21 
                               
                             
                             
                               
                                 H 
                                 22 
                               
                             
                           
                         
                         ] 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               
                                 x 
                                 1 
                               
                             
                           
                           
                             
                               
                                 x 
                                 2 
                               
                             
                           
                         
                         ] 
                       
                     
                     + 
                     
                       [ 
                       
                         
                           
                             
                               n 
                               1 
                             
                           
                         
                         
                           
                             
                               n 
                               2 
                             
                           
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Here, ‘x b ’ is a (n T ×1) signal vector transmitted by a base station (b), ‘H mb ’ is a (n R ×n T ) channel matrix between the base station (b) and a terminal (u), and ‘n m ’, which are (n R ×1) noise vectors, are independent from each other and have a normal distribution whose distribution is equal to ‘N 0 ’. 
     The transmit signal (x b ), that a (n S ×1) transmission symbol (s b ) passes through a (n T ×n S ) codebook-based precoder (T b ), can be expressed as in x b =T b s b . 
     If using filters (R 1 ) and (R 2 ) for nulling interference channels (H 12 T 2 ), (H 21 T 1 ) from other cells in the receiver terminal 1   115  and terminal 2   165 , respectively, a reception signal is given as in Equation below. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           [ 
                           
                             
                               
                                 
                                   z 
                                   1 
                                 
                               
                             
                             
                               
                                 
                                   z 
                                   2 
                                 
                               
                             
                           
                           ] 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             
                               
                                 
                                   [ 
                                   
                                     
                                       
                                         
                                           R 
                                           1 
                                         
                                       
                                       
                                         
                                             
                                         
                                       
                                     
                                     
                                       
                                         
                                             
                                         
                                       
                                       
                                         
                                           R 
                                           2 
                                         
                                       
                                     
                                   
                                   ] 
                                 
                                 ⁡ 
                                 
                                   [ 
                                   
                                     
                                       
                                         
                                           H 
                                           11 
                                         
                                       
                                       
                                         
                                           H 
                                           12 
                                         
                                       
                                     
                                     
                                       
                                         
                                           H 
                                           21 
                                         
                                       
                                       
                                         
                                           H 
                                           22 
                                         
                                       
                                     
                                   
                                   ] 
                                 
                               
                               ⁡ 
                               
                                 [ 
                                 
                                   
                                     
                                       
                                         T 
                                         1 
                                       
                                     
                                     
                                       
                                           
                                       
                                     
                                   
                                   
                                     
                                       
                                           
                                       
                                     
                                     
                                       
                                         T 
                                         2 
                                       
                                     
                                   
                                 
                                 ] 
                               
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       s 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     
                                       s 
                                       2 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             
                               [ 
                               
                                 
                                   
                                     
                                       R 
                                       1 
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                       R 
                                       2 
                                     
                                   
                                 
                               
                               ] 
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       n 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     
                                       n 
                                       2 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               [ 
                               
                                 
                                   
                                     
                                       
                                         R 
                                         1 
                                       
                                       ⁢ 
                                       
                                         H 
                                         11 
                                       
                                       ⁢ 
                                       
                                         T 
                                         1 
                                       
                                     
                                   
                                   
                                     
                                         
                                     
                                   
                                 
                                 
                                   
                                     
                                         
                                     
                                   
                                   
                                     
                                       
                                         R 
                                         2 
                                       
                                       ⁢ 
                                       
                                         H 
                                         22 
                                       
                                       ⁢ 
                                       
                                         T 
                                         2 
                                       
                                     
                                   
                                 
                               
                               ] 
                             
                             ⁡ 
                             
                               [ 
                               
                                 
                                   
                                     
                                       s 
                                       1 
                                     
                                   
                                 
                                 
                                   
                                     
                                       s 
                                       2 
                                     
                                   
                                 
                               
                               ] 
                             
                           
                           + 
                           
                             [ 
                             
                               
                                 
                                   
                                     n 
                                     1 
                                     ′ 
                                   
                                 
                               
                               
                                 
                                   
                                     n 
                                     2 
                                     ′ 
                                   
                                 
                               
                             
                             ] 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     A condition for nulling interference channels from other cells is that the number of receive antennas should be N r &gt;=2n S . 
     Dividing into a cooperative technique and a non-cooperative technique, the present invention describes a method of selecting a precoder (T b ) such that errors of a cell edge are minimized in circumstances where interference is completely nulled as in Equation 2 above using an OCI-NF. 
     First, the cooperative scheme is described below. 
     A codebook-based precoder (T b ) to be used in a transmitter selects within a codebook (W={W 1 , W 2 , . . . , W L }) to minimize an error rate in a receiver and feeds back a corresponding index to the transmitter. 
     If a receiver is a linear receiver such as a Zero Forcing (ZF) decoder, a reception SNR of a k th  sub stream of a terminal (u) is given as in Equation below. 
     
       
         
           
             
               
                 
                   
                     SNR 
                     
                       u 
                       , 
                       k 
                     
                     
                       ( 
                       ZF 
                       ) 
                     
                   
                   = 
                   
                     
                       E 
                       S 
                     
                     
                       
                         n 
                         S 
                       
                       ⁢ 
                       
                         
                           
                             N 
                             0 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   ( 
                                   
                                     
                                       R 
                                       u 
                                     
                                     ⁢ 
                                     
                                       T 
                                       uu 
                                     
                                     ⁢ 
                                     
                                       H 
                                       u 
                                     
                                   
                                   ) 
                                 
                                 H 
                               
                               ⁢ 
                               
                                 R 
                                 u 
                               
                               ⁢ 
                               
                                 H 
                                 uu 
                               
                               ⁢ 
                               
                                 T 
                                 u 
                               
                             
                             ) 
                           
                         
                         
                           k 
                           , 
                           k 
                         
                         
                           - 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     Here, an average symbol error probability is bound by a minimum sub stream SNR, so reducing the average symbol error probability needs maximizing the minimum sub stream SNR. The minimum sub stream SNR is bound as in Equation below. 
     
       
         
           
             
               
                 
                   
                     SNR 
                     
                       u 
                       , 
                       min 
                     
                     
                       ( 
                       ZF 
                       ) 
                     
                   
                   ≥ 
                   
                     
                       
                         λ 
                         
                           u 
                           , 
                           min 
                         
                         2 
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             R 
                             u 
                           
                           ⁢ 
                           
                             H 
                             uu 
                           
                           ⁢ 
                           
                             T 
                             u 
                           
                         
                         ) 
                       
                     
                     ⁢ 
                     
                       
                         E 
                         S 
                       
                       
                         
                           n 
                           S 
                         
                         ⁢ 
                         
                           N 
                           0 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Here, the ‘λ u, min  (R u H uu T u )’ is a Minimum Singular Value (MSV) of the R u H uu T u . A code word selection technique based on a minimum sub stream SNR needs to obtain an SNR for each sub stream, so the code word selection technique selects a code word using the MSV as in Equation below. 
     
       
         
           
             
               
                 
                   
                     T 
                     m 
                   
                   = 
                   
                     arg 
                     ⁢ 
                     
                       
                         
                           max 
                         
                       
                       
                         
                           
                             
                               w 
                               l 
                             
                             ∈ 
                             w 
                           
                         
                       
                     
                     ⁢ 
                     
                       
                         λ 
                         
                           u 
                           , 
                           min 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             R 
                             u 
                           
                           ⁢ 
                           
                             H 
                             uu 
                           
                           ⁢ 
                           
                             W 
                             l 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     A transmission precoder (T 1 ) of the base station 1   110  is determined by an effective channel R 1 H 11  (or R 1 H 11 T 1 ). Here, the ‘R 1 ’ is a precoder nulling H 12 T 2 , so it varies depending on a transmission precoder (T 2 ) of the base station 2   150 . 
     For the same reasons, the transmission precoder (T 2 ) of the base station 2   150  also varies depending on the transmission precoder (T 1 ) of the base station 1   110 . That is, each transmission precoder is determined in relation to a different transmission precoder, and the performance of the entire system is determined by a combination [T 1 , T 2 ] of the transmission precoders. 
     Accordingly, the present invention proposes a method of selecting a combination of transmission precoders minimizing an error rate when considering the entire system. By using being bound by a minimum value among an MSV 1  and an MSV 2  of two effective channels R 1 H 11 T 1  and R 2 H 22 T 2 , an average symbol error probability of the entire system selects a code word as in Equation below. 
     
       
         
           
             
               
                 
                   
                     [ 
                     
                       
                         T 
                         1 
                       
                       , 
                       
                         T 
                         2 
                       
                     
                     ] 
                   
                   = 
                   
                     arg 
                     ⁢ 
                     
                       
                         
                           max 
                         
                       
                       
                         
                           
                             
                               [ 
                               
                                 
                                   W 
                                   l 
                                 
                                 , 
                                 
                                   W 
                                   k 
                                 
                               
                               ] 
                             
                             ∈ 
                             
                               w 
                               2 
                             
                           
                         
                       
                     
                     ⁢ 
                     
                       min 
                       ⁡ 
                       
                         ( 
                         
                           
                             
                               λ 
                               
                                 1 
                                 , 
                                 min 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     R 
                                     1 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         H 
                                         12 
                                       
                                       ⁢ 
                                       
                                         W 
                                         k 
                                       
                                     
                                     ) 
                                   
                                 
                                 ⁢ 
                                 
                                   H 
                                   11 
                                 
                                 ⁢ 
                                 
                                   W 
                                   l 
                                 
                               
                               ) 
                             
                           
                           , 
                           
                             
                               λ 
                               
                                 2 
                                 , 
                                 min 
                               
                             
                             ⁡ 
                             
                               ( 
                               
                                 
                                   
                                     R 
                                     2 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         H 
                                         21 
                                       
                                       ⁢ 
                                       
                                         W 
                                         l 
                                       
                                     
                                     ) 
                                   
                                 
                                 ⁢ 
                                 
                                   H 
                                   22 
                                 
                                 ⁢ 
                                 
                                   W 
                                   k 
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     As in Equation 6 above, the present invention selects a code word and determines a combination of transmission precoders. Here, the ‘λ 1,min  (MSV 1 )’ is a value capable of being obtained in the terminal 1 , and the ‘λ 2,min  (MSV 2 )’ in the terminal 2  only, so there is a need to implement information exchange between terminals or between base stations for the sake of code word selection. The ‘MSV 1 ’ and ‘MSV 2 ’ have an L 2  number of values according to a combination of W l  and W k , respectively. 
     To select a code word combination as in Equation 6 above, each user should feed back an ‘L×L’ table composed of MSVs and share through a backhaul. 
     In the cooperative scheme, the terminal 1   115 , the terminal 2   165 , the base station 1   110 , and the base station 2   150  have a detailed structure of  FIG. 3 , and perform a process of  FIG. 2  depending on time. 
     The terminal 1   115  and terminal 2   165  perform channel estimation and, on the basis of estimated channels, determine an MSV 1  table and an MSV 2  table and feed back to the base station 1   110  and base station 2   150 , respectively. 
     The base station 1   110  and base station 2   150  exchange the fedback MSV 1  table and MSV 2  table with each other through a backhaul, and determine precoders of the base station 1   110  and base station  2   150  according to a cooperative code word selection technique of Equation 6 above. Feeding back all of the MSV 1  table and the MSV 2  table as above is called full feedback. 
     Hence, the non-cooperative scheme is described below. 
     In code word selection, unlike the cooperative scheme, the non-cooperative scheme does not exchange information between base stations. The terminal 1   115  and terminal 2   165  have to select code words to be used by the base station 1   110  and base station 2   150  with information of their own MSV 1  and MSV 2  only. However, as mentioned earlier, the MSV 1  and MSV 2  are determined by a combination of code words W l  and W k  of the base station 1   110  and base station 2   150 . 
     That is, the terminal 1   115  selects a code word (W l ) to be used by the base station 1   110  without information of a code word (W k ) used by the base station 2   150 , and the terminal 2   165  selects the code word (W k ) to be used by the base station 2   150  without information of the code word (W l ) used by the base station 1   110 . 
     When considering the entire system, errors are determined by a minimum MSV, so it should be such that a code word combination ([T 1 , T 2 ]=[W l , W k ]) selected by the terminal 1   115  and terminal 2   165  does not have a minimum MSV value to the maximum. 
     
       
         
           
             
               
                 
                   
                     T 
                     1 
                   
                   = 
                   
                     arg 
                     ⁢ 
                     
                       
                         
                           max 
                         
                       
                       
                         
                           
                             
                               W 
                               l 
                             
                             ∈ 
                             w 
                           
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             
                               min 
                             
                           
                           
                             
                               
                                 
                                   W 
                                   k 
                                 
                                 ∈ 
                                 w 
                               
                             
                           
                         
                         ⁢ 
                         
                           
                             λ 
                             
                               1 
                               , 
                               min 
                             
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   R 
                                   1 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       H 
                                       12 
                                     
                                     ⁢ 
                                     
                                       W 
                                       k 
                                     
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 H 
                                 11 
                               
                               ⁢ 
                               
                                 W 
                                 l 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     For this, as in Equation above, the terminal 1   115  obtains an MSV 1  when the code word (W k ) used by the base station 2   150  is worst selected according to all code words (W l ) available by the base station 1   110 , and selects a code word (W l ) maximizing this MSV 1 . 
     
       
         
           
             
               
                 
                   
                     T 
                     2 
                   
                   = 
                   
                     arg 
                     ⁢ 
                     
                       
                         
                           max 
                         
                       
                       
                         
                           
                             
                               W 
                               k 
                             
                             ∈ 
                             w 
                           
                         
                       
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           
                             
                               min 
                             
                           
                           
                             
                               
                                 
                                   W 
                                   l 
                                 
                                 ∈ 
                                 w 
                               
                             
                           
                         
                         ⁢ 
                         
                           
                             λ 
                             
                               2 
                               , 
                               min 
                             
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   R 
                                   2 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   
                                     
                                       W 
                                       l 
                                     
                                     ⁢ 
                                     
                                       H 
                                       21 
                                     
                                   
                                   ) 
                                 
                               
                               ⁢ 
                               
                                 H 
                                 22 
                               
                               ⁢ 
                               
                                 W 
                                 k 
                               
                             
                             ) 
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     Likewise, as in Equation 8 above, the terminal 2   165  obtains an MSV 2  when a code word (W l ) used by the base station 1   110  is worst selected according to all code words (W k ) available by the base station 2   150 , and selects a code word (W k ) maximizing this MSV 2 . 
     This code word selection is a method of selecting its own base station precoder such that performance degradation is not large even in a case where a precoder of an interference base station is set worst, and has an effect of avoiding a minimum MSV value. 
     In the non-cooperative scheme, a terminal has a function block of  FIG. 4 , and has a structure of adding an OCI-NF to a receiver (terminal) in a conventional quantization preprocessor and modifying a code word selector as in  FIG. 4 . 
     The terminal 1   115  and terminal 2   165  perform channel estimation and, on the basis of estimated channels, determine an MSV 1  and an MSV 2  suffering the worst interference (i.e., the worst-interfered MSV 1  and the worst-interfered MSV 2 ) and, on the basis of this, determine precoders of the base station 1   110  and base station 2   150  according to a non-cooperative code word selection technique of Equation 7 and Equation 8 above. 
     In a receiver terminal, a precoder to be used by a transmitter base station is determined, so it feeds back a corresponding code word index. That is, a technique of feeding back each of only a Precoding Matrix Index (PMI) 1  and a PMI 2  is called simple index feedback. This feedback is enough to feed back only an ‘L’ bit, in a case where the size of a codebook is equal to ‘L’. 
     In the cooperative scheme, a cooperative scheme with reduced feedback is described below. 
     In order to prevent a feedback amount from increasing geometrically like L 2  as the size (L) of a codebook increases in cooperative-scheme full feedback, the present invention grafts a non-cooperative code word selection method used in a non-cooperative scheme. 
     Based on code words {circumflex over (T)} 1  and {circumflex over (T)} 2  selected by Equation 7 and Equation 8 above, the present invention selects a code word as in Equation 9 below to determine a transmission precoder combination. 
     
       
         
           
             
               
                 
                   
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     This code word selection has the same effect of avoiding a minimum MSV value as in the non-cooperative scheme, and can improve performance through additional MSV feedback and cooperation. 
     Feeding back a PMI 1 , a PMI 2 , and an MSV corresponding to each PMI as mentioned earlier is called reduced feedback. This feedback feeds back an ‘L’ bit for a PMI and a (2L−1) number of MSVs, in a case where the size of a codebook is ‘L’. 
       FIG. 2  is a flowchart illustrating an operation process of a base station and a terminal according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 2 , a terminal 1   210  and a terminal 2   215  perform channel estimation based on reference signals of a base station 1   250  and a base station 2   255  (steps a, b). 
     After that, the terminal 1   210  and terminal 2   215  each obtain a minimum singular value (MSV 1 , MSV 2 ) table, and feed back the minimum singular value table or a PMI value for each of the base stations  250  and  255  (steps c, d). 
     Next, the base station 1   250  and base station 2   255  select code words for the respective terminals  210  and  215  based on the received MSV (MSV 1 , MSV 2 ) table or PMI, respectively (steps g, h). 
     In a case where this selection step uses full feedback or reduced feedback, as mentioned earlier, an MSV (MSV 1 , MSV 2 ) table can be exchanged with each other between base stations (step f). 
     After that, the base station 1   250  and base station 2   255  transmit data to the respective terminals  210  and  215  using the selected code words (steps i, j). 
       FIG. 3  is a block diagram illustrating schematic constructions of a terminal and a base station in a cooperative scheme according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 3 , a terminal 1  and a terminal 2  do channel estimation and, on the basis of estimated channels, determine an MSV 1  table and an MSV 2  table and feed back to a base station 1  and a base station 2 . 
     The base station 1  and base station 2  exchange the fedback MSV 1  table and MSV 2  table with each other through a backhaul, and determine precoders of the base station 1  and base station 2  according to a cooperative code word selection technique of Equation 5 above. 
       FIG. 4  is a diagram illustrating schematic constructions of a terminal and a base station in a non-cooperative scheme according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , a terminal 1  does channel estimation and, on the basis of an estimated channel, determines an MSV 1  and an MSV 2  suffering the worst interference (the worst-interfered MSV 1  and the worst-interfered MSV 2 ) and, on the basis of this, determines a precoder of a base station 1  according to a non-cooperative code word selection technique of Equation 7 and Equation 8 above. 
     After that, the receiver terminal determines a precoder to be used in the transmitter base station, so it feeds back only a corresponding code word index identically with a conventional quantization preprocessor. It is usable adding an OCI-NF to the receiver (terminal) in the quantization preprocessor and modifying a code word selector as in  FIG. 4 . 
       FIG. 5  is a diagram illustrating an example of a code word selected in accordance with a feedback technique according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , it represents an exemplary implementation of an MSV 1  and an MSV 2  for a case where ‘L’ is equal to ‘4’. In a case (1) of a cooperative scheme with full feedback, a selected precoder combination is equal to [W 2 , W 1 ], and a minimum MSV is equal to ‘min (7, 7)=7’. 
     In a case of a non-cooperative scheme with simple index feedback, a selected precoder combination is equal to [W 3 , W 2 ], and a minimum MSV is equal to ‘min (5, 8)=5’. In a case of a terminal 1   115 , if W l =W 1 , an MSV 1  has a minimum value ‘1’ when W k =W 4  and, if W l =W 2 , the MSV 1  has a minimum value ‘2’ when W k =W 3 . If obtaining the minimum value of the MSV 1  for each precoder W l  as above, it is equal to ‘1’, ‘2’, ‘4’, and ‘2’ in order. This is an MSV 1  when an interference base station precoder W k  of the worst combination with each precoder W l  is selected, and W l =W 3  in which this value is the largest ‘4’ is selected and its index is fed back. Through the same process, even a terminal 2   165  selects W k =W 2  having a minimum value ‘4’ and feeds back its index. Accordingly, a selected precoder combination is equal to [W 3 , W 2 ]. 
     In a case of a cooperative scheme with reduced feedback, a selected precoder combination is equal to [W 3 , W 3 ], and a minimum MSV is equal to ‘min(6, 6)=6’. 
       FIG. 6  is a flowchart illustrating an operation process of a terminal according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , a terminal (assuming a terminal 1   115 ) performs channel estimation for reference signals of a base station 1  and a base station 2  (step  610 ). Here, the base station 1  is a serving base station, and the base station 2  represents a base station other than the serving base station. After that, the terminal determines an MSV 1  for the base station 1  and the base station 2  (step  620 ). 
     If a cooperative scheme is not used (step  630 ), the terminal selects a precoder (code word) index for the MSV 1  and feeds back this to the base station (step  640 ). 
     If the cooperative scheme is used (step  630 ) and full feedback is used (step  650 ), the terminal feeds back the entire table of the determined MSV 1  to the base station (step  660 ). 
     If the cooperative scheme is used (step  630 ) and reduced feedback is used, the terminal selects a code word combination and feeds back the selected code word combination (step  670 ). The code word combination can include a selected MSV table and an index for this. 
       FIG. 7  is a flowchart illustrating an operation process of a base station according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , in a case where a cooperative scheme is used (step  710 ), the base station receives an MSV from a terminal (step  720 ). After that, the base station selects a precoder (code word) to be used, based on the received MSV value (step  730 ). The precoder (code word) selection process is, as aforementioned, composed of a full feedback technique and a reduced feedback technique according to the received MSV. 
     If the cooperative scheme is not used (step  710 ), the base station receives a precoder (code word) index from the terminal (step  740 ). After that, the base station selects a precoder (code word) for the received precoder (code word) index (step  750 ). 
       FIG. 8  is a block diagram illustrating constructions of a base station and a terminal according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , the base station includes a modulator  810 , a precoder  820 , a precoder (code word) selector  830 , a codebook  835 , and an MSV comparator  840 , and the terminal includes a channel estimator  850 , a demodulator  860 , a decoder  870  an MSV calculator  880 , a codebook  885 , and a codebook selector  890 . 
     First, the terminal is described. The channel estimator  850  performs channel estimation for a signal transmitted by the base station, and the demodulator  860  performs a demodulation process, and the decoder  870  a decoding process. 
     The MSV calculator  880  determines, as mentioned earlier, an MSV based on a channel estimation value that the channel estimator  850  performs for a reference signal transmitted by the base station. 
     And, the determined MSV  895  and an index for an MSV selected by the codebook selector  890  of a codebook  885 , i.e., a precoder (code word) index  897  are fed back to the base station. The aforementioned feedback process is implemented by, though not illustrated, a transmitting unit including a transmission modem. 
     Hence, the base station is described. A signal transmitted by the base station is modulated through the modulator  810  and is precoded through the precoder  820  and then, is transmitted through an antenna to airs. 
     The base station receives the MSV  895  or the precoder (code word) index  897 . After that, the MSV comparator  840  compares, as mentioned earlier, its own MSV value with another base station using a full feedback technique or a reduced feedback technique, and provides the comparison result to the precoder (code word) selector  830 . The MSV comparator  840  can exchange the MSV with another base station through a backhaul. 
     The precoder (code word) selector  830  determines a codebook  835  to be used, i.e., a precoder (code word) based on the comparison result, and provides this to the precoder  820 . The precoder  820  performs precoding using the determined precoder (code word). 
     In a case where the non-cooperative scheme is used, the precoder (code word) index  897  is provided to the precoder (code word) selector  830 . The precoder (code word) selector  830  selects a precoder (code word) to be used based on the precoder (code word) index  897 , and provides this to the precoder  820 . The precoder  820  performs precoding using the determined precoder (code word). 
     Hence, the simulation result for the invention is described below. 
     In the present invention, it was assumed that there were two user terminal 1   115  and terminal 2   165  allocated the same time and frequency resources at a boundary of two cells. 
     The environment was that interference acts very greatly because a base station 1   110  transmits data to the terminal 1   115  and simultaneously, a base station 2   150  to the terminal 2   165 . The experiment was conducted for cases where the number (n T ) of transmit antennas of base stations is equal to ‘4’ and the number (n R ) of receive antennas of terminals is equal to ‘2’ or ‘4’, and it was assumed that there is no correlation between antennas. For the sake of the equal comparison, sum-rates of all techniques were standardized as 8 bps/Hz and Bit Error Rate (BER) performance were compared. 
       FIG. 9  is a 1st diagram illustrating BER performance according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 9 , it represents BER performance (two receive antennas, a rank1 codebook, and a 16QAM) of the conventional art and the present invention and, if using a rank1 precoder, it can use an OCI-NF. It can be appreciated that the present invention is superior in diversity order and SNR gain aspects, compared to a conventional quantization preprocessor. 
       FIG. 10  is a 2nd diagram illustrating BER performance according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 10 , it represents BER performance (four receive antennas, a rank2 codebook, and a QPSK) of the conventional art and the present invention and, if using a rank2 precoder, it can use an OCI-NF. The present invention can confirm that diversity order and SNR gain aspects are all improved compared to a conventional quantization preprocessing technique. 
     A non-cooperative scheme (simple index feedback) of the present invention has more superior performance than not only a conventional quantization technique using a 3-bit codebook having the same feedback but also an existing quantization technique using a 6-bit codebook having twice feedback for a similar operation amount. 
     A cooperative scheme of the present invention is more excellent than the non-cooperative scheme by a performance gain through information exchange between base stations. 
     The present invention minimizes errors of a cell edge through a codebook-based MIMO precoding and a receiver OCI-NF. The present invention does not have a loss resulting from the OCI-NF through transmission precoder (code word) selection taking the OCI-NF into consideration, and has an advantage of being capable of additionally reducing an error rate in a cell edge through information exchange between base stations. 
     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.