Abstract:
An apparatus and a method for improving the performance of an error correction code in response to the influence of error propagation are disclosed. A receiver of a mobile communication system, which transmits/receives data at a high speed by means of the plurality of transmission antennas and the plurality of reception antennas, estimates a transmission signal of a specific path from a first received signal according to a preset criterion, measures an error component for each symbol of the estimated transmission signal, performs an error correction for symbols having a corresponding error component exceeding a preset value, detects transmission data from all symbols through a predetermined signal reverse-processing procedure, reconstructs a transmission signal from the transmission data, subtracts the reconstructed transmission signal from the received signal to generate a second received signal, and repeats the above operations until transmission data of all paths are detected from the second received signal.

Description:
PRIORITY 
     This application claims the benefit under 35 U.S.C. 119(a) of an application entitled “Apparatus And Method For Canceling Interference Signal In Mobile Communication System Using Multiple Antenna” filed in the Korean Intellectual Property Office on Dec. 2, 2003 and assigned Ser. No. 2003-86931, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a multi-input multi-output (multiple antennas) mobile communication system. More particularly, the present invention relates to an apparatus and a method for improving the performance of an error correction code in response to the influence of error propagation. 
     2. Description of the Related Art 
     A conventional mobile communication system provides a voice-service and mainly uses channel coding to overcome unfavorable channel conditions. However, a multimedia service of high quality increases the necessity for a next generation wireless transmission technology that can transmit a large quantity of data with few errors. In particular, high speed transmission is more important in a forward link having a high transmission quantity of data. However, in a mobile communication system, the reliability of a signal is largely reduced due to fading, shadow, wave attenuation, noise, interference, etc. In particular, fading due to a multi-path causes severe signal distortion due to the sum of signals which are received through different paths and have different phases and sizes. Since the fading must be overcome to support high speed data communication, research into the fading has been actively pursued. Accordingly, a multi-input multi-output (‘MIMO’) technology using a plurality of transmission/reception antennas has been proposed. The MIMO simultaneously transmits data to a transmitter and a receiver by means of multiple antennas, thereby transmitting a large quantity of data without increasing transmission bandwidth. 
       FIG. 1  is a block diagram illustrating a conventional MIMO system. As shown in  FIG. 1 , a transmitter includes a demultiplexer  100 , a signal processor  102  and transmission antennas  104 ,  106  and  108  and a receiver includes reception antennas  110 ,  112  and  114  and a signal processor  116 .  FIG. 1  shows only elements necessary for describing the principle of the MIMO system. Further, a plurality of inter-antenna channels are formed between the transmission antennas  104 ,  106  and  108  and the reception antennas  110 ,  112  and  114 . 
     Referring to  FIG. 1 , the demultiplexer  100  demultiplexes a transmitted data stream into the same number of data streams as the number of the transmission antennas  104 ,  106  and  108 , and outputs the multiplexed data streams. That is, the demultiplexer  100  duplicates each of the transmitted user data streams into the same number of data streams as the number of transmission antennas. Each of the user data streams is overlappingly transmitted through a multiple antenna in this manner, so that the error probability for the user data stream is reduced. Therefore, the reliability of the received user data stream can be improved. In other cases, the demultiplexer  100  receives the same number of data as the number of antennas and can output the received user data streams to transmission antennas. 
     The user data streams sent from the demultiplexer  100  experience a predetermined processing by the signal processor  102  and are then output to the transmission antennas  104 ,  106  and  108 . The transmission antennas  104 ,  106  and  108  transmit the received user data streams to the reception antennas  110 ,  112  and  114 . The reception antennas  110 ,  112  and  114  receive the user data streams transmitted from the transmission antennas  104 ,  106  and  108 . That is, the reception antenna  110  receives the user data stream transmitted from the transmission antennas  104 ,  106  and  108 , and the reception antenna  112  receives the user data stream transmitted from the transmission antennas  104 ,  106  and  108 . Similarly, the reception antenna  114  receives the user data stream transmitted from the transmission antennas  104 ,  106  and  108 . The reception antennas  110 ,  112  and  114  sends the received user data streams to the signal processor  116 . The signal processor  116  performs a predetermined processing such as coding and modulation for the received user data streams. 
     The MIMO system may employ a bell labs layered space-time (‘BLAST’) scheme and a per-antenna rate control (PARC) scheme. Hereinafter, the BLAST scheme will be first described. 
     A transmitter of the BLAST scheme demultiplexes a user data stream into the same number of data streams as the number of transmission antennas and the transmission antennas use the same data rate. The BLAST scheme may be classified into a diagonal BLAST (‘DBLAST’) scheme, a vertical BLAST (‘VBLAST’) scheme and a horizontal BLAST (HBLAST) scheme. The DBLAST scheme performs a specific block coding for a user data stream transmitted from each transmission antenna, thereby improving efficiency. However, it is difficult to realize the DBLAST scheme. The VBLAST scheme performs an independent coding for a user data stream transmitted from each transmission antenna. In such a VBLAST scheme, the number of reception antennas is equal to or larger than that of transmission antennas and a receiver uses a maximum likelihood detection (‘ML’) scheme. In the ML scheme, symbols having a minimum error are selected through substitution of all symbols transmittable in all transmission antennas, so as to greatly improve performance of the antennas. 
     However, since the calculation amount increases due to the increase of the number of the transmission antennas, it is difficult to realize the ML scheme. 
     Meanwhile, the PARC scheme assigns data rates differently according to channel states experienced by each transmission antenna. The channel state may be expressed by a signal-to-interference and noise ratio (SINR). 
       FIG. 2  is a block diagram showing a structure of a transmitter of a MIMO system using a PARC scheme.  FIG. 2  shows a system capable of simultaneously transmitting J×M user data streams by means of J spreading codes and M transmission antennas. 
     The user data stream is transmitted to a demultiplexer  200 . The demultiplexer  200  divides the user data stream by the J number of data according to the number of the transmission antennas and sends the divided user data streams to signal processors  210 ,  212  and  214 . The signal processors  210 ,  212  and  214  perform a predetermined signal processing for the received user data streams. 
     Further, the signal processors  210 ,  212  and  214  perform a coding, an interleaving, a modulation, etc., for the received user data streams by means of preset data rates, respectively. The signal processors  210 ,  212  and  214  send the processed user data streams to spreaders  220 ,  222  and  224 . Herein, the J output data streams processed by the signal processor  210  are respectively output to the spreaders  220 ,  222  and  224 . Similarly, the signal processor  212  outputs the J output data streams to the spreaders  220 ,  222  and  224  and the signal processor  214  outputs the J output data streams to the spreaders  220 ,  222  and  224 . 
     The spreaders  220 ,  222  and  224  use different spreading codes. The spreader  220  performs spreading for the user data streams sent from the signal processors  210 ,  212  and  214  by means of the same spreading code  1 , the spreader  222  performs spreading for the user data streams sent from the signal processors  210 ,  212  and  214  by means of the same spreading code  2 , and the spreader  224  performs spreading for the user data streams sent from the signal processors  210 ,  212  and  214  by means of the same spreading code J. 
     The user data streams that experienced the spreading by the spreaders  220 ,  222  and  224  are output to adders  230 ,  232  and  234 . Herein, the user data streams (having experienced the coding/interleaving/modulation) processed by the same signal processor are output to the same adder. Specifically, the user data stream processed by the same signal processor  210  is output to the adder  230 , the user data stream processed by the same signal processor  212  is output to the adder  232 , and the user data stream processed by the same signal processor  214  is output to the adder  234 . 
     The data stream added by the adder  230  according to each antenna is subjected to an additional signal processing (i.e., frequency up-conversion) of the transmitter and is then transmitted through a radio channel by a first transmission antenna  240  as a signal S 1 (t). Herein, since the additional signal processing is not directly associated with the main scope of the present invention, a detailed description will be omitted. Next, the data stream added by the adder  232  according to each antenna is subjected to the additional signal processing of the transmitter and is then transmitted through a radio channel by a second transmission antenna  242  as a signal S 2 (t). Last, the data stream added by the adder  234  according to each antenna is subjected to the additional signal processing of the transmitter and is then transmitted through a radio channel by an M th  transmission antenna  244  as a signal S M (t). 
       FIG. 3  is a block diagram showing a structure of a receiver of an MIMO system using a PARC scheme. The structure of the receiver shown in  FIG. 3  corresponds to the structure of the transmitter shown in  FIG. 2 . 
     Referring to  FIG. 3 , a reception antenna  300  receives the user data streams sent from the transmission antennas  240 ,  242  and  244 . Referring to  FIG. 2 , the reception antenna  300  receives the signal sent from the transmission antennas  240 ,  242  and  244 . Further, the reception antenna  302  receives the signal sent from the transmission antennas  240 ,  242  and  244  and the reception antenna  304  receives the signal sent from the transmission antennas  240 ,  242  and  244 . 
     The reception antenna  300  sends the received signal to despreader  320  to  322 , the reception antenna  302  sends the received signal to despreaders  323  to  325 , and the reception antenna  304  sends the received signal to despreaders  326  to  328 . Spreading codes used in the despreaders  320  to  328  are the same as those used in the spreaders  220 ,  222  and  224  of the transmitter. That is, the despreaders  320 , the despreaders  323 , the despreaders  326  and the spreader  220  of the transmitter use the same spreading codes. Further, the despreaders  321 , the despreaders  324 , the despreaders  327  and the spreader  222  of the transmitter use the same spreading codes. Similarly, the despreaders  322 , the despreaders  325 , the despreaders  328  and the spreader  224  of the transmitter use the same spreading codes. 
     The signal despreaded by the despreader  320  is output to a mean minimum square error (‘MMSE’) receiver  330 , the signal despreaded by the despreader  321  is output to an MMSE receiver  332 , the signal despreaded by the despreader  322  is output to an MMSE receiver  334 , the signal despreaded by the despreader  323  is output to an MMSE receiver  330 , the signal despreaded by the despreader  324  is output to an MMSE receiver  332 , the signal despreaded by the despreader  325  is output to an MMSE receiver  334 , the signal despreaded by the despreader  326  is output to an MMSE receiver  330 , the signal despreaded by the despreader  327  is output to an MMSE receiver  332 , and the signal despreaded by the despreader  328  is output to an MMSE receiver  334 . 
     The MMSE receivers  330 ,  332  and  334  detect user data streams by a preset rule according to a spreading code of a specific transmission antenna. The detected user data streams of the specific transmission antenna are output to a multiplexer  340 . The multiplexer  340  multiplexes the received user data streams of the specific transmission antenna and outputs the multiplexed data streams to a signal reverse-processor  350 . The signal reverse-processor  350  detects the received data streams according to a preset antenna index sequence and performs a predetermined signal reverse-processing such as a demodulation, a deinterleaving, a decoding, etc. Herein, it is assumed that data streams are detected in a sequence of the first transmission antenna  240 , the second transmission antenna  242  and the J th  transmission antenna  244 . Accordingly, in the first step, the transmission signal of the first transmission antenna  240  is detected. 
     The data stream of the first transmission antenna  240  reverse-processed by the signal reverse-processor  350  is output to the next terminal  370 . In addition, the reverse-processed data stream of the first transmission antenna  240  is output to a signal processor  360 . The signal processor  360  performs the signal processing equal to that of the transmitter for the data stream of the first transmission antenna  240  sent from the signal reverse-processor  350 . The signal processing includes a coding, an interleaving and a modulation. In this manner, the signal processing is performed, so that the transmission signal estimated as a signal transmitted from the first transmission antenna  240  is reconstructed. 
     The reconstructed transmission signal of the first transmission antenna  240  is output to subtracters  310 ,  312  and  314 . The subtracters  310 ,  312  and  314  subtract the reconstructed transmission signal of the first transmission antenna  240  from the signal received in the reception antennas  300 ,  302  and  304 , and provides the subtraction result to the despreader  320  to  328 . The aforementioned process is repeatedly performed up to the transmission signal of the J th  transmission antenna. Therefore, the receiver can exactly receive the transmission signals sent from the transmitter while sequentially reducing the influence by the multiple transmission antenna. 
     In the conventional MIMO communication system as described above, a transmission signal of an M th  transmission antenna is estimated by means of an estimated transmission signal of an (M-1) th  transmission antenna. A scheme of estimating a transmission signal of a transmission antenna in this way is called a successive interference cancellation (‘SIC’) scheme. However, when an error occurs in estimating the transmission signal of the (M-1) th  transmission antenna, an error also occurs in all following transmission signals estimated by means of the transmission signal of the M th  transmission antenna. Accordingly, it is necessary to propose a scheme for solving the aforementioned problem caused by the characteristics of the SIC reception scheme. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an object of the present invention to provide an apparatus and a method for reducing the influence of an error that occurred in a previous step in finding information of the next step. 
     It is another object of the present invention to provide an apparatus and a method for minimizing an error occurring on a radio channel by efficiently using information found in a previous step. 
     It is further another object of the present invention to provide a sequential interference cancellation apparatus and method having an error verification and correction function in a mobile communication system using a multi-input multi-output (MIMO) technology. 
     In order to accomplish the aforementioned objects, according to one aspect of the present, there is provided a method for receiving a plurality of signals that passed through a plurality of paths from a plurality of transmission antennas to a plurality of reception antennas in a mobile communication system that transmits/receives data at a high speed by means of the plurality of transmission antennas and the plurality of reception antennas, the method comprising the steps of a) estimating a transmission signal of a specific path from a first received signal received through each of the plurality of reception antennas according to a preset criterion; b) measuring an error component for each symbol of the estimated transmission signal; c) performing an error correction for symbols having a corresponding error component exceeding a preset value; d) detecting transmission data from all symbols including the error-corrected symbols through a predetermined signal reverse-processing procedure; e) reconstructing a transmission signal from the transmission data through a predetermined signal processing procedure; f) subtracting the reconstructed transmission signal from the received signal and generating a second received signal; and g) repeating steps a) through f) until transmission data of all paths are detected from the second received signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram illustrating a conventional multi-input multi-output (MIMO) mobile communication system; 
         FIG. 2  is a block diagram illustrating a structure of a transmitter of an MIMO mobile communication system; 
         FIG. 3  is a block diagram illustrating a structure of a receiver of an MIMO mobile communication system; 
         FIG. 4  is a block diagram illustrating a structure of a receiver of an MIMO mobile communication system according to an embodiment of the present invention; 
         FIG. 5  is flow diagram illustrating an operation in a receiver of an MIMO mobile communication system according to an embodiment of the present invention; 
         FIG. 6  is a graph comparing a bit error rate (‘BER’) for a signal-to-noise ratio (SNR) for an embodiment of the present and to the prior art; and 
         FIG. 7  is another graph comparing a bit error rate for a signal-to-noise ratio for an embodiment of the present and to the prior art. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configuration incorporated herein will be omitted for conciseness. 
     In a multi-input multi-output (MIMO) system in accordance with an embodiment of the present specification, a transmitter transmits data by means of M number of transmission antennas and J number of spreading codes and a receiver receives the data by means of N number of reception antennas. Hereinafter, the structure of a minimum mean square error successive interference cancellation (MMSE-SIC) receiver according to an embodiment of the present invention will be described with reference to  FIG. 4 . For convenience of description, a detailed description on a received signal that passed through each block and an operation of a conventional MMSE reception unit will be omitted. 
     Referring to  FIG. 4 , in a state in which a transmission signal of any transmission antenna is cancelled, a signal received through a first reception antenna  400  will be called r (0) (1), a signal received through a second reception antenna  402  will be called r (0) (2), and a signal received through an N th  reception antenna  404  will be called r (0) (N). Here, it is apparent that the r (0) (N) may be expressed by a combination of signals s 1  to s m  having experienced channels between M number of transmission antennas and a specific reception antenna. Here, the s m  denotes a signal sent from an M th  transmission antenna. Similarly, a signal received through an N th  reception antenna after a (i-1) th  interference cancellation step passes will be called r (i) (N). 
     The first reception antenna  400  sends the received signal to despreader  420  to  422 , the reception antenna  402  sends the received signal to despreaders  423  to  425 , and the Nth reception antenna  404  sends the received signal to despreaders  426  to  428 . Spreading codes used in the despreaders  420  to  428  are the same as those used in the spreaders  220 ,  222  and  224  of the transmitter of  FIG. 2 . That is, the despreaders  420 , the despreaders  423 , the despreaders  426  and the spreader  220  of the transmitter use the same spreading codes. Further, the despreaders  421 , the despreaders  424 , the despreaders  427  and the spreader  222  of the transmitter use the same spreading codes. Similarly, the despreaders  422 , the despreaders  425 , the despreaders  428  and the spreader  224  of the transmitter use the same spreading codes. 
     The signal despreaded by the despreader  420  is output to a first MMSE receiver  430 , the signal despreaded by the despreader  421  is output to a second MMSE receiver  432 , the signal despreaded by the despreader  422  is output to an J th  MMSE receiver  434 , the signal despreaded by the despreader  423  is output to the first MMSE receiver  430 , the signal despreaded by the despreader  424  is output to the second MMSE receiver  432 , the signal despreaded by the despreader  425  is output to the J th  MMSE receiver  434 , the signal despreaded by the despreader  426  is output to the first MMSE receiver  430 , the signal despreaded by the despreader  427  is output to the second MMSE receiver  432 , and the signal despreaded by the despreader  428  is output to the J th  MMSE receiver  434 . 
     The MMSE receivers  430 ,  432  and  434  detect user data streams of each transmission antenna using a predetermined rule. Hereinafter, the functions of the MMSE receivers  430 ,  432  and  434  will be briefly described. 
     The following equation 1 denotes an k th  signal received in an entire reception antenna: 
                   r   =               α   2     M       ⁢   H   ⁢       ∑     j   =   1     J     ⁢       c   ⁡     (   j   )       ⁢     b   ⁡     (   j   )             +   n     =             α   2     M       ⁢     H   ·   s       +   n               Equation   ⁢           ⁢   1               
α 2  is a normalized value of the power of a received signal, the c(j) denotes an j th  spreading code, the b(j) denotes a signal input to an j th  spreader, and the n denotes noise on a radio channel. Further, the s denotes [s(1), s(2), . . . , s(m)] and the s(m) denotes the signal sent from the M th  transmission antenna. The channel matrix H denotes a channel characteristic between all transmission/reception antennas and a channel characteristic between the M th  transmission antenna and the N th  reception antenna is H mn .
 
     A signal obtained by despreading the received signal r may be expressed by the following equation 2: 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           z 
                           ⁡ 
                           
                             ( 
                             j 
                             ) 
                           
                         
                         = 
                         
                           
                             
                               c 
                               * 
                             
                             ⁡ 
                             
                               ( 
                               j 
                               ) 
                             
                           
                           ⁢ 
                           r 
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               c 
                               * 
                             
                             ⁡ 
                             
                               ( 
                               j 
                               ) 
                             
                           
                           ⁢ 
                           
                             ( 
                             
                               
                                 
                                   
                                     
                                       α 
                                       2 
                                     
                                     M 
                                   
                                 
                                 ⁢ 
                                 H 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   c 
                                   ⁡ 
                                   
                                     ( 
                                     j 
                                     ) 
                                   
                                 
                                 ⁢ 
                                 
                                   b 
                                   ⁡ 
                                   
                                     ( 
                                     j 
                                     ) 
                                   
                                 
                               
                               + 
                               n 
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             
                               
                                 
                                   α 
                                   2 
                                 
                                 M 
                               
                             
                             ⁢ 
                             
                               H 
                               · 
                               
                                 b 
                                 ⁡ 
                                 
                                   ( 
                                   j 
                                   ) 
                                 
                               
                             
                           
                           + 
                           
                             n 
                             ′ 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     In equation 2, the z(j) denotes a signal obtained by despreading the reception signal of the entire reception antenna by an j th  despreader and the c*(j) denotes a conjugate of the j th  spreading code. Here, the despreaded signal z is a signal obtained by canceling a spreading code component contained in a transmission signal. Therefore, in order to obtain exact data transmitted from a transmission side, the channel component H must be cancelled. Accordingly, an MMSE reception unit including the multiple MMSE receivers  430 ,  432  and  434  calculates an MMSE linear transformation matrix as the following equation 3 in order to cancel the H component and minimize an error with the transmission signal: 
     
       
         
           
             
               
                 
                   w 
                   = 
                   
                     
                       
                         M 
                         
                           α 
                           2 
                         
                       
                     
                     ⁢ 
                     
                       
                         H 
                         ⁡ 
                         
                           ( 
                           
                             
                               H 
                               * 
                               H 
                             
                             + 
                             
                               
                                 M 
                                 
                                   α 
                                   2 
                                 
                               
                               ⁢ 
                               I 
                             
                           
                           ) 
                         
                       
                       
                         - 
                         1 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     The calculated W is an N×M matrix. Accordingly, an estimated value {tilde over (s)}=W*·z of an entire transmission signal s is calculated by means of the W and is then output to a multiplexer. 
     Here, the z denotes [z(1), z(2), . . . ,z(j)] and is an N×J matrix. Further, the same number of the MMSE receivers as the number J of spreading codes are provided. Also, an j th  MMSE receiver performs an operation for a z(j) vector of N×1 and M rows of W* denoting a channel component between the transmission antenna and the N th  reception antenna. 
     When an MMSE result value for which a soft decision has been performed in an i th  sequential interference cancellation step is a {tilde over ( )}b(i), a multiplexer  400  in a first interference cancellation step multiplexes J number of received MMSE result values and generates an estimated value {tilde over ( )}b(1). The estimated value {tilde over ( )}b(1) is output to a signal reverse-processor  450 . 
     Hereinafter, the construction and the operation of the signal reverse-processor  450  will be described. 
     The signal reverse-processor  450  performs a modulation, a deinterleaving, a decoding, etc., for the received estimated value, generates a hard decision result value, and outputs the hard decision result value to an error detector  460 . The following equation 4 denotes a process by which a hard decision is performed:
 
   b   ( j )= sgn ( b ′( j ))   Equation 4
 
     The error detector  460  detects an error component from the received hard decision result value. The following equation 5 denotes the error component detected by the error detector  460 : 
     
       
         
           
             
               
                 
                   
                     e 
                     ⁡ 
                     
                       ( 
                       j 
                       ) 
                     
                   
                   = 
                   
                     
                       z 
                       ⁡ 
                       
                         ( 
                         j 
                         ) 
                       
                     
                     - 
                     
                       
                         
                           
                             α 
                             2 
                           
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                       ⁢ 
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                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         
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                           _ 
                         
                         ⁡ 
                         
                           ( 
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                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   5 
                 
               
             
           
         
       
     
     The e(j) denotes the size of the error component that occurred in a signal spread by a spreading code j on a radio channel. Accordingly, the size of an error component for a transmission signal of a transmission antenna m may be expressed by the following equation 6: 
     
       
         
           
             
               
                 
                   E 
                   = 
                   
                     
                       [ 
                       
                         
                           e 
                           ⁡ 
                           
                             ( 
                             1 
                             ) 
                           
                         
                         ⁢ 
                         
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                           ⁡ 
                           
                             ( 
                             2 
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                         ⁢ 
                         
                             
                         
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                           ⁡ 
                           
                             ( 
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                       ] 
                     
                     = 
                     
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                               2 
                             
                             M 
                           
                         
                         ⁢ 
                         H 
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                           B 
                           _ 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     The Z denotes [z(1)z(2) . . . z(j)] and the  B  denotes [  b (1)  b (2) . . .  b (J)]. When an absolute value for an error for the j th  spreading signal is obtained by means of the E, the absolute value may be expressed by the following equation 7: 
     
       
         
           
             
               
                 
                   
                     
                       ɛ 
                       j 
                     
                     = 
                     
                       
                          
                         
                           e 
                           j 
                         
                          
                       
                       = 
                       
                         
                           
                              
                             
                               
                                 z 
                                 j 
                               
                               ⁢ 
                               
                                 
                                   
                                     α 
                                     2 
                                   
                                   M 
                                 
                               
                               ⁢ 
                               H 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 
                                   b 
                                   _ 
                                 
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                           ⁢ 
                           
                               
                           
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                           J 
                         
                         = 
                         i 
                       
                     
                   
                   , 
                   2 
                   , 
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                   ⁢ 
                   
                       
                   
                   , 
                   J 
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
           
         
       
     
     When the error for the j th  spreading code is larger than a reference value k, it can be recognized that an error for the MMSE result value has occurred in the j th  spreading code. 
     Hereinafter, a construction and an operation of an embodiment of the present invention for interference cancellation will be described. 
     First, a signal-to-interference and noise ratio (SINR) of each transmission antenna is calculated for the interference cancellation. Next, data are detected and interference is cancelled in a sequence of a transmission antenna having a high SINR and a transmission antenna having a low SINR. If it is assumed that transmission signal power of each transmission antenna is equal to each other, an SINR may be calculated by the following equation 8: 
     
       
         
           
             
               
                 
                   
                     SINR 
                     ⁡ 
                     
                       ( 
                       m 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         
                           α 
                           2 
                         
                         M 
                       
                       ⁢ 
                       
                         
                            
                           
                             
                               w 
                               m 
                               * 
                             
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                               h 
                               m 
                             
                           
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                         2 
                       
                     
                     
                       
                         
                           
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                             2 
                           
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                         ⁢ 
                         
                           
                             ∑ 
                             
                               
                                 k 
                                 = 
                                 1 
                               
                               , 
                               
                                 k 
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                   8 
                 
               
             
           
         
       
     
     Further, a transmission antenna symbol sequence having the highest SINR is detected from transmission symbols of each transmission antenna and a maximum likelihood detection (ML) scheme is applied to the detected symbol sequence. Herein, the ML scheme is not performed for all detected transmission symbols of each transmission antenna, but performed for only a symbol in which the size of the error component for the received symbol shown in equation 6 exceeds a preset value. 
     For instance, for the size of the error component for the received symbol shown exceeds the preset value and others do not exceed the predetermined value, in equation 6, when only a size e(1) of an error component for a first code symbol the ML scheme is performed for only the first code symbol. 
     If a Quaternary Phase Shift Keying (QPSK) modulation scheme has been used, a possible symbol combination F={v 1 , v 2 , . . . , v q } of the first code symbol will be {00, 01, 10, 11}. That is, the receiver puts the all possible symbols into the first code symbol and determines an optimal symbol. Then, the receiver replaces the first code symbol with the optimal value, detects the transmission antenna symbol sequence having the highest SINR, and outputs the transmission antenna symbol sequence. Hereinafter, a symbol of data which has been spread by the j th  spreading code and transmitted through a first transmission antenna will be called b 1j  and an estimated transmission symbol will be called v q . 
     Accordingly, a value of a signal transmitted from the first transmission antenna and estimated by means of the estimated transmission symbol becomes  B . When the b 1j  is estimated as the v q  by the ML scheme, a size of an error component for the estimated symbol may be expressed by the following equation 9: 
     
       
         
           
             
               
                 
                   
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                   9 
                 
               
             
           
         
       
     
     The following equation 10 denotes a case in which there exist three reception symbols in which a size of an error component exceeds the preset value when a SINR size of a transmission signal is aligned in a sequence of a transmission antenna index in a system having a transmission antenna (M=4) and a spreading code (J=8). That is, e(2), e(3), e(6) are larger than the preset value: 
     
       
                 
         
             
             
         
      
     
     As shown in equation 10, since the first transmission antenna has the highest SINR, an ML process is performed for the transmission symbol of the first transmission antenna. Accordingly, the receiver performs the ML process for b 12 , b 13  and b 16 . The transmission symbol sequence of the first transmission antenna obtained by replacing the b 12 , b 13  and b 16  with the optimal v q  is output to a signal reconstruction unit  470 . 
     The signal reconstruction unit  470  performs a predetermined signal processing for the transmission symbol sent from the error detector  460  and reconstructs a transmission signal estimated as a signal transmitted from a specific transmission antenna. The signal processing is equal to the processing that was performed for the transmission symbol sequence of the first transmission antenna in the transmitter and includes a coding, an interleaving, a modulation, etc. 
     The symbols having passed through the signal processing are output to subtracters  410 ,  412  and  414 . The subtracters  410 ,  412  and  414  perform a function of canceling the symbols that passed through the signal processing from user data streams received through reception antennas. Signals output from the subtracters  410 ,  412  and  414  are sent to the despreaders  420  to  428 . 
     The above embodiment has described an example in which an ML process is applied to only a symbol sequence of a transmission antenna having the highest SINR (i.e., of a first path), but a path (i.e., the number of transmission antennas) to which the ML process is applied may be changed according to a selection. 
       FIG. 5  is flow diagram illustrating an operation in a receiver according to a preferred embodiment of the present invention. 
     Referring to  FIG. 5 , in step  500 , the receiver initializes the number i of times of searching for a transmission signal of a transmission antenna to be 0 and sets the number M of transmission antennas and a preset value t of an error component to which an ML process is to be applied. In step  502 , signals sent from each transmission antenna are received in the receiver. The receiver includes two or more reception antennas and each reception antenna receives the transmission signal from each transmission antenna. In step  504 , the receiver determines whether or not transmission data have been extracted from the transmission signals from all transmission antennas. As a result of the determination, when the transmission data have not been extracted from the transmission signals from all transmission antennas, step  506  is performed. In contrast, when the transmission data have been extracted from the transmission signals from all transmission antennas, the receiver ends all procedures. 
     In step  506 , the receiver performs a despreading process for a received signal of each reception antenna. Spreading codes used in the despreading process are equal to those used in a transmitter. That is, the spreading codes includes spreading codes  1  to J and the received signal of each reception antenna is despread by the spreading codes  1  to J. 
     In step  508 , the receiver performs an MMSE process for the signals despread by the same spreading codes by the same number as the number of the spreading codes. That is, the MMSE process is performed for the received signals despread by the spreading code  1  and the MMSE process is performed for the received signals despread by the spreading code  2 . In step  510 , the receiver measures an SINR of each transmission antenna. Then, step  512  is performed. That is, in step  512 , among the measured SINRs of each transmission antenna, the receiver searches for transmission signals corresponding to the preset number of transmission antennas from the highest SINR. 
     In step  514 , the receiver counts the number i of times of searching for the transmission signal of the transmission antenna. Then, step  516  is performed. That is, in step  516 , when the i is larger than the number M of transmission antennas to which the ML process is to be applied, step  524  is performed. In contrast, when the i is not larger than the number M of transmission antennas to which the ML process is to be applied, step  518  is performed. In step  518 , the receiver measures an error component of the transmission signal. Then, step  520  is performed. That is, in step  520 , the receiver selects symbols in which the error component measurement result exceeds a preset value t from the searched symbols of the transmission symbol sequence of the transmission antenna. When there are symbols in which the error component measurement result exceeds the preset value t, step  522  is performed. In contrast, when there are no symbols in which the error component measurement result exceeds the preset value t, step  524  is performed. 
     In step  522 , the receiver performs an error correction for the symbols in which the error component measurement result exceeds the preset value t according to the ML scheme. Then, step  524  is performed. That is, in step  524 , the receiver extracts transmission data sent from the searched transmission antenna from the error-corrected symbol sequence. Then, step  526  is performed. Herein, a symbol sequence already stored in a previous time point may be used. In step  526 , the receiver reconstructs a transmission signal from the extracted transmission data of the transmission antenna. Then, step  528  is performed. Herein, a method of reconstructing the transmission signal can be obtained by applying the signal processing method used in the transmitter to the extracted transmission data. In step  528 , the receiver cancels the reconstructed transmission signal of the transmission antenna from an antenna reception signal in a previous time. Then, returns to step  504 . In step  504 , the receiver determines whether or not transmission data have been extracted from the transmission signals from all transmission antennas. Then, the aforementioned processes are repeated. 
     A process of canceling an exact signal in which an error has been corrected according to the method as proposed above is sequentially performed, so a received signal estimation process is performed for all transmission antennas. 
       FIGS. 6 and 7  are graphs comparing the embodiment of the present invention with the prior art.  FIGS. 6 and 7  show a bit error rate (‘BER’) for a signal-to-noise ratio (SNR). Specifically,  FIG. 6  shows a case in which a Binary Phase Shift Key (BPSK) modulation scheme is used and  FIG. 7  shows a case in which a QPSK modulation scheme is used.  FIGS. 6 and 7  show cases in which a Zero Forcing (ZF) scheme, an MMSE scheme, an MMSE-SIC scheme, an enhanced MMSE-SIC scheme (‘EMMSE-SIC scheme’) according to an embodiment of the present invention, and an ML scheme are used. Referring to  FIGS. 6 and 7 , the EMMSE-SIC scheme according to an embodiment of the present invention has the lowest BER except for the ML scheme. Further, when it is considered that the ML scheme has a very high complexity, the EMMSE-SIC scheme according to an embodiment of the present invention can obtain the highest efficiency. 
     As described above, in an embodiment of the present invention, when the size of an error component for a received signal is larger than a preset value, the error for the received signal is partially cancelled, so that the reliability for the received signal can be improved. Further, in an embodiment of the present invention, another signal is estimated by means of the signal having a partially improved reliability, so that error improvement can be realized. 
     Although a certain embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, including the full scope of equivalents thereof.