Patent Publication Number: US-8542776-B2

Title: Apparatus and method for transmitting/receiving data in a multi-antenna system, and system using the same

Description:
PRIORITY 
     This application is a Continuation of U.S. patent application Ser. No. 11/896,479, which claims priority under 35 U.S.C. §119(e) of U.S. Patent Provisional Application No. 60/841,246, filed Aug. 31, 2006, in the United States Patent and Trademark Office, and claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 2007-73155, filed Jul. 20, 2007, in the Korean Intellectual Property Office, the entire disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a data transmission/reception apparatus and method in a multi-antenna system, and a system using the same. More particularly, the present invention relates to an apparatus and method for transmitting/receiving data by antenna grouping, and a system using the same. 
     2. Description of the Related Art 
     Conventionally, the wireless channel environment, unlike the wired channel environment, shows a lower reliability due to multipath interference, shadowing, propagation loss, time-varying noise, interference, etc. This is the typical cause of obstruction to an increase in a data transfer rate, or a data rate, in mobile communication. Therefore, to implement a high-speed wireless environment for providing high-rate services, there is an urgent need for a solution to the foregoing problem. 
     A Multi Input Multi Output (MIMO) multi-antenna system is a conventional technology that has been proposed for addressing the problem of lower reliability in a wireless channel environment. The proposed MIMO multi-antenna system has an advantage of being capable of increasing performance of the system without addition of power and spectrum. 
     Generally, the multi-antenna system supports a Single-User (SU) mode and a Multi-User (MU) mode. A multi-antenna system supporting the SU mode transmits data to the same user via multiple transmission antennas, and a multi-antenna system supporting the MU mode transmits data to multiple users via multiple transmission antennas. The multi-antenna system supporting the MU mode has been proposed to obtain improved performance compared to the multi-antenna system supporting the SU mode, while minimizing an increase in the number of antennas and an increase in complexity of the hardware structure. In addition, the MU mode lays emphasis on improvement of the system&#39;s transmission capacity using Spatial Division Multiple Access (SDMA) scheduling. 
     The multi-antenna system is classified into a closed-loop scheme that depends on feedback information for resource allocation, and a closed-loop that does not depend on feedback information. For the closed-loop scheme, the most important issue is to prepare a scheme for minimizing feedback information for efficient resource allocation. Particularly, in the MU mode, there has long been a need for a reduction in the amount of feedback information. 
     Meanwhile, due to the diversification of the wireless communication services, there is a probability that terminals having different characteristics will coexist in the same service area. Therefore, a base station should be able to support various communication schemes. For example, the base station should be able to selectively support not only the SU mode but also the MU mode. In addition, for signal detection, the base station should be able to support both a terminal using a linear detection technique and a terminal using a nonlinear detection technique. 
     Accordingly, there is a need for an improved apparatus and method for generating and transmitting feedback information in a multi-antenna system using grouped antennas, and a data transmission apparatus and method for transmitting a data stream of a user according to a transmission mode selected depending on the feedback information. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect, a method for receiving/transmitting data in a closed-loop multi-antenna system defines multiple antenna groups by antenna grouping on multiple antennas, and performs data transmission individually via each of the multiple antenna groups. The method includes receiving, by a transmitter, feedback information from at least one receiver, and calculating, by the transmitter, each of a sum rate (R_MU) in a multi-user mode and a sum rate (R_SU) in a single-user mode using the feedback information. The method also includes selecting, by the transmitter, one of the single-user mode and the multi-user mode as a transmission mode using a comparison result between the sum rate (R_MU) in the multi-user mode and the sum rate (R_SU) in the single-user mode, and transmitting, by the transmitter, a data stream according to the selected transmission mode. The method also includes, acquiring, by the at least one receiver, channel quality information (CQI) for each data stream transmitted individually via each antenna group corresponding to the transmission mode. The method includes determining, by the at least one receiver, CQI associated information from the CQI acquired individually for each antenna group, and transmitting, by the at least one receiver, the determined CQI associated information to the transmitter as the feedback information. The CQI associated information includes maximum CQI, an antenna group index associated with the maximum CQI, rank information, and remaining CQI associated the rank information. 
     In accordance with another aspect, a closed-loop multi-antenna system defines multiple antenna groups by antenna grouping on multiple antennas, and performs data transmission individually via each of the multiple antenna groups. The closed-loop multi-antenna system includes a transmitter including a feedback information processor and a first transmit unit, and at least one receiver including a channel estimator, a feedback information generator and a second transmit unit. The feedback information processor for receiving feedback information from the at least one receiver, calculating each of a sum rate (R_MU) in a multi-user mode and a sum rate (R_SU) in a single-user mode using the feedback information, and selecting one of the single-user mode and the multi-user mode as a transmission mode depending on a comparison result between the sum rate (R_MU) in the multi-user mode and the sum rate (R_SU) in the single-user mode. The first transmit unit for transmitting a data stream according to the selected transmission mode. The channel estimator for acquiring, CQI for each data stream transmitted individually via each antenna group. The feedback information generator for determining, CQI associated information from the CQI acquired individually for each antenna group corresponding to the transmission mode, and the second transmit unit for transmitting the feedback information to the transmitter. The CQI associated information includes maximum CQI, an antenna group index associated with the maximum CQI, rank information, and remaining CQI associated the rank information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and advantages of certain embodiments of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a diagram illustrating a closed-loop multi-antenna system according to an embodiment of the present invention; 
         FIG. 2  is a diagram illustrating an example of a detailed structure of a reception apparatus according to an embodiment of the present invention; 
         FIG. 3  is a diagram illustrating an exemplary operation of a reception apparatus for generating feedback information according to an embodiment of the present invention; 
         FIG. 4  is a diagram illustrating an operation performed in a reception apparatus with, for example, two antenna groups according to an embodiment of the present invention; 
         FIG. 5  is a diagram illustrating an example of a detailed structure of a transmission apparatus according to an embodiment of the present invention; and 
         FIG. 6  is a diagram illustrating a control flow performed in a transmission apparatus according to an embodiment of the present invention. 
     
    
    
     Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures. 
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention and are 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. 
     Preferred embodiments of the present invention will now be described in detail with reference to the annexed drawings. 
     A reception apparatus proposed by an exemplary embodiment of the present invention, described below, generates feedback information including the best channel quality information out of channel quality information associated with each data stream group, an index of a data stream group associated with the best channel quality information, the number (or RANK value) of activated transmission stream groups, which is information additionally needed for a flexible SU mode, and channel quality information based on the number (RANK value) of activated transmission stream groups. The index of the data stream group is information necessary during base station scheduling for supporting the MU mode. The flexible SU mode can be defined including both a transmission mode for transmitting a data stream associated with one user via one antenna group and a transmission mode for transmitting the data stream via multiple antenna groups. Therefore, in an exemplary embodiment of the present invention described below, the SU mode should be construed as the above-defined flexible SU mode. 
     It can be assumed that for generation of a channel quality value for the MU mode and generation of a channel quality value for the flexible SU mode, the reception apparatus can use either the same receiver or different receivers according to its own conditions. For example, it can be assumed that the reception apparatus can use a linear receiver for generation of a channel quality value for the MU mode, and a nonlinear receiver, such as Successive Interference Cancellation (SIC) receiver and Modified Maximum Likelihood (ML) receiver, for generation of a channel quality value for the flexible SU mode. 
     A detailed description will be made herein of a multi-antenna system supporting both the SU mode and the MU mode based on antenna grouping. Further, a detailed description will be made of a structure and operation of a reception apparatus for generating and transmitting feedback information according to a data stream transmitted by means of antenna grouping, and a detailed description will also be made of a structure and operation of a transmission apparatus for transmitting a data stream via an antenna group grouped by means of a transmission mode determined based on the feedback information. 
     A. Multi-Antenna System 
       FIG. 1  illustrates a closed-loop multi-antenna system according to an exemplary embodiment of the present invention. The multi-antenna system shown in  FIG. 1  includes one transmission apparatus  110  and multiple reception apparatuses  120 - 1  and  120 -N. The transmission apparatus  110  can be assumed to be a base station, and the multiple reception apparatuses  120 - 1  and  120 -N each can be assumed to be user terminals. The following description will be made with reference to one reception apparatus  120 - 1 , and the same can be applied to the other reception apparatuses. 
     Referring to  FIG. 1 , the transmission apparatus  110  includes N t  antennas, and the reception apparatus  120 - 1  includes N r  antennas. The N t  antennas included in the transmission apparatus  110  are grouped by a predetermined number of antennas. This is called ‘antenna grouping’, and an antenna bundle obtained by the grouping is called an ‘antenna group’. However, a multi-antenna system using a precoding matrix can apply grouping for columns, and grouping for beams. In this case, the present invention can be implemented by means of “column group” or “beam group” rather than the “antenna group”. Therefore, the “antenna group” as used herein should be construed to include the “column group” and the “beam group”. For convenience, the number of antenna groups herein is assumed to be 2. Therefore, each antenna group is composed of N t /2 antennas. 
     The transmission apparatus  110  transmits a data stream for each user via an antenna group based on feedback information provided from the reception apparatus  120 - 1 . That is, the transmission apparatus  110  determines a transmission mode for transmitting the data stream depending on the feedback information. The transmission mode is classified into the SU mode and the MU mode. The SU mode is divided into two transmission modes depending on rank information. In addition, the transmission apparatus  110  determines a Modulation &amp; Coding Selection (MCS) level depending on the feedback information. The data streams transmitted via multiple antennas constituting the antenna group will be referred to herein as a ‘data stream group’. 
     The reception apparatus  120 - 1  receives a signal via at least one antenna, and estimates a channel characteristic for each antenna group (or data stream group) through channel estimation on the received signal. The reception apparatus  120 - 1  acquires Channel Quality Information (CQI) for each antenna group based on the estimated channel characteristic. The CQI is a value based on which channel quality between each antenna group and each reception antenna can be estimated. 
     In addition, the reception apparatus  120 - 1  generates feedback information based on channel quality information associated with each antenna group. The feedback information includes a maximum CQI, an antenna group index associated with the maximum CQI, rank information, and a Remain CQI associated with the rank information. The rank information is a value designating the number of antenna groups via which the reception apparatus  120 - 1  will transmit a data stream to one user in the SU mode. If the rank information is ‘1’, it indicates a request for transmission of a data stream for one user via one antenna group. If the rank information is ‘2’, it indicates a request for transmission of a data stream for one user via two antenna groups. 
     The transmission apparatus  110  determines a transmission mode depending on feedback information provided from each of the reception apparatuses  120 - 1  and  120 -N. That is, the transmission apparatus  110  can selectively use the SU mode and the MU mode. 
     To this end, the reception apparatus  120 - 1  includes at least one reception antenna Ant_rx # 1 , Ant_rx # 2 , . . . , Ant_rx #N r , a channel estimator  122 - 1 , and a feedback information generator  124 - 1 . The transmission apparatus  110  includes multiple transmission antennas (Ant_tx # 1 , Ant_tx # 2 , . . . , Ant_tx #N t ), a feedback information processor  114 , and a data transmitter  112 . 
     Regarding the reception apparatus  120 - 1 , a signal received from at least one reception antenna Ant_rx # 1 , Ant_rx # 2 , . . . , Ant_rx #N r  is input to the channel estimator  122 - 1 . The channel estimator  122 - 1  estimates a channel characteristic for each of all channels (i.e. data stream groups or antenna groups) over which the signal is transmitted. In addition, based on the estimated channel characteristic, the channel estimator  122 - 1  calculates CQI information associated with each of the data stream groups transmitted via each antenna group by means of a predetermined signal detection technique. 
     The signal detection technique is classified into a linear detection technique and a nonlinear detection technique. A Minimum Mean Square Error (MMSE) technique can be a typical example of the linear detection technique, and an SIC technique can be a typical example of the nonlinear detection technique. An exemplary embodiment of the present invention will be described with reference to the MMSE technique, the SIC technique, and a beam-forming technique. 
     The channel estimator  122 - 1  can selectively use the MMSE technique, the SIC technique, and the beam-forming technique. The channel estimator  122 - 1  provides CQI information of each antenna group estimated by the MMSE technique, CQI information of each antenna group measured by the SIC technique, and CQI information of each antenna group measured by the beam-forming technique, to the feedback information generator  124 - 1 . 
     The feedback information generator  124 - 1  generates feedback information depending on the CQI information for each antenna group provided from the channel estimator  122 - 1 . The function of the channel estimator  122 - 1  for acquiring CQI information by means of the various signal detection techniques can also be implemented to be performed in the feedback information generator  124 - 1 . 
     The feedback information, as stated above, includes a maximum CQI, antenna group index associated with the maximum CQI, rank information, and a Remain CQI associated with the rank information. The maximum CQI is the best CQI among the CQIs acquired for each antenna group. The rank information corresponds to the number of antenna groups. For example, if the number of antenna groups is assumed to be 2, the rank information can be ‘1’ or ‘2’. The Remain CQI can be differently defined according to the rank information. For example, if the rank information is ‘1’, the Remain CQI is CQI that can be obtained for the remaining data stream group after turning off an antenna group (or data stream group) having the minimum CQI by means of the beam-forming technique. If the rank information is ‘2’, the Remain CQI is CQI that can be obtained for the remaining antenna group (or data stream group) after removing the maximum CQI by means of the SIC technique. 
     The reception apparatus  120 - 1  transmits the feedback information generated by the feedback information generator  124 - 1  to the transmission apparatus  110 . Preferably, the transmission of the feedback information is periodically performed by the reception apparatus  120 - 1 . However, if the transmission time is previously agreed upon between the transmission apparatus  110  and the reception apparatus  120 - 1 , the feedback information can be aperiodically transmitted. 
     Regarding the transmission apparatus  110 , the feedback information received from all the reception apparatuses  120 - 1  and  120 -N is provided to the feedback information processor  114 . The feedback information processor  114  determines a transmission mode, a coding technique, and an MCS level based on the feedback information received from each reception apparatus. 
     The feedback information processor  114  calculates a sum rate (R_MU) in the MU mode and a sum rate (R_SU) in the SU mode to determine the transmission mode. The calculation of the R_MU and the R_SU is made depending on the feedback information. In addition, the feedback information processor  114  determines a coding technique and an MCS level for supporting the determined transmission mode. 
     The data transmitter  112  transmits, via an antenna group, at least one user data stream selected depending on the transmission mode, the coding technique, and the MCS level provided by the feedback information processor  114 . 
     B. Structure and Operation of Reception Apparatus 
       FIG. 2  illustrates an example of a detailed structure of a reception apparatus according to an exemplary embodiment of the present invention. It is assumed in  FIG. 2  that two data stream groups are transmitted, that is data streams are transmitted from a transmission apparatus via two antenna groups. 
     Referring to  FIG. 2 , a signal received via N R  antennas is provided to a channel estimator &amp; feedback information generator  210  and a group signal detector  220 . 
     The group signal detector  220  detects multiple data streams from the received signal by applying a predetermined signal detection technique. The predetermined signal detection technique can be any one of a nonlinear signal detection technique and a linear signal detection technique. The SIC technique can be applied as an example of the predetermined signal detection technique. The multiple data streams, data streams transmitted via an antenna group of a transmitting entity (or the transmission apparatus), are classified into two data stream groups. However, when the data streams are transmitted by the transmitting entity by means of the beam-forming technique, the multiple data streams can be classified into one data stream group. The beam-forming technique corresponds to a signal transmission technique for transmitting data streams via one antenna group in a concentrated manner. 
     The multiple data streams detected by the group signal detector  220  are provided to a spatial demultiplexing block  230 . The spatial demultiplexing block  230  demultiplexes the multiple data streams separately for each data stream group, and outputs a data stream associated with each data stream group. 
     To this end, the spatial demultiplexing block  230  can be composed of a spatial demultiplexer associated with each antenna group (i.e. each data stream group). In  FIG. 2 , because the number of antenna groups is assumed to be 2, the spatial demultiplexing block  230  is composed of two spatial demultiplexers  232  and  234 . Therefore, the spatial demultiplexers  232  and  234  each demultiplex the multiple data streams provided from the group signal detector  220 , and output one data stream corresponding to a unique data stream group. 
     The data stream of each data stream group, output from the spatial demultiplexing block  230 , is provided to a demodulation block  240 . The demodulation block  240  performs demodulation on the data stream provided separately for each data stream group. 
     The demodulation block  240  is composed of multiple demappers  242  and  244  associated with each data stream group. The multiple demappers  242  and  244  each perform demodulation, that is demapping, on the data streams provided from their associated spatial demultiplexers  232  and  234 , and output demodulated data streams. 
     The demodulated multiple data streams output from the demodulation block  240  are provided to a decoding block  250 . The decoding block  250  performs decoding on the demodulated data streams for each data stream group by means of a predetermined decoding technique. A turbo decoding technique can be used as an example of the predetermined decoding technique. 
     The decoding block  250  is composed of multiple decoders  252  and  254  associated with each data stream group. The multiple decoders  252  and  254  each perform decoding on the demodulated data streams provided from their associated demappers  242  and  244 . 
     The channel estimator &amp; feedback information generator  210  estimates a channel characteristic associated with each data stream group by channel estimation on the received signal provided via the N R  antennas. In addition, the channel estimator &amp; feedback information generator  210  acquires CQI associated with each data stream group based on the channel characteristic estimated separately for each data stream group. The CQI can be represented by an Effective Signal-to-Noise Ratio (ESN). In the following description, the CQI and the ESN will be used together. Nevertheless, application of an exemplary embodiment of the present invention shall not be limited to the CQI or ESN. Meanwhile, various signal detection techniques can be used to acquire an ESN from the channel characteristic. An exemplary embodiment of the present invention uses the MMSE technique, the SIC technique, and the beam-forming technique. 
     In addition, the channel estimator &amp; feedback information generator  210  generates feedback information based on the ESN (where j is a data stream group index or an antenna group index) acquired separately for each data stream group. The feedback information includes a maximum ESN (MAX-ESN), an antenna group index (MAX group index) associated with the MAX-ESN, rank information (RANK), and a Remain ESN (Remain-ESN) associated with the rank information. The MAX-ESN is the best ESN among the ESNs acquired separately for each data stream group, and the MAX group index is an antenna group index (i.e. data stream group index) of the MAX-ESN. In addition, the rank information, indicative of the number of data streams (i.e. the number of data stream groups) transmitted by the transmitting entity via an antenna group, is information used for determining a transmission mode of the transmitting entity. The Remain-ESN can be acquired by means of a signal detection technique designated individually for each rank information. 
     For example, the channel estimator &amp; feedback information generator  210  estimates an ESN associated with each data stream group by applying three different signal detection techniques. That is, the channel estimator &amp; feedback information generator  210  acquires a MAX-ESN and a MAX group index depending on the best MMSE-ESN m     —     best  among the ESNs (MMSE-ESNs) estimated by means of a first signal detection technique among the three signal detection techniques. The MMSE technique can be used as the first signal detection technique, which is a linear detection technique. 
     Further, the channel estimator &amp; feedback information generator  210  acquires an ESN n  (SIC-ESN n , where n is different from m_best) associated with a particular data stream group by means of a second signal detection technique available for rank information=‘2’ among the three signal detection techniques. The SIC technique can be used as the second signal detection technique, which is a nonlinear detection technique. 
     In addition, the channel estimator &amp; feedback information generator  210  acquires an ESN n  (OFF-ESN n , where n is different from m_best) associated with a particular data stream group by means of a third signal detection technique (for example, the beam-forming technique) available for rank information=‘1’ among the three signal detection techniques. The beam-forming technique can be used as the third signal detection technique. The beam-forming technique turns off the remaining antenna groups except for one antenna group among the antenna groups, and performs channel estimation only on the data stream transmitted via the one non-turned-off antenna group. The turned-off data stream group corresponds to the data stream group having the minimum channel quality. 
     Further, the channel estimator &amp; feedback information generator  210  determines one of the SIC-ESN n  and the OFF-ESN n  as a Remain-ESN. For example, the channel estimator &amp; feedback information generator  210  determines, as a Remain-ESN, an ESN capable of obtaining a higher rate among a rate (R SIC ) calculated depending on the SIC-ESN n  and a rate (R OFF ) calculated depending on the OFF-ESN n . The channel estimator &amp; feedback information generator  210  further considers the MAX-ESN for calculation of the R SIC . The channel estimator &amp; feedback information generator  210  determines, as rank information, a rank value associated with the signal detection technique used for acquiring the ESN determined as the Remain-ESN. 
     A detailed description of an operation of the channel estimator &amp; feedback information generator  210  for acquiring the feedback information will be made in the description of an operation of the reception apparatus. Although the structure for channel estimation and the structure for generating feedback information are united in a single structure in  FIG. 2 , they can be implemented with separate structures. 
       FIG. 3  illustrates an exemplary operation of a reception apparatus for generating feedback information according to an embodiment of the present invention. The operation shown in  FIG. 3  can be performed by a channel estimator &amp; feedback information generator in the reception apparatus. 
     Referring to  FIG. 3 , in step  310 , a reception apparatus estimates a channel characteristic H associated with each antenna included in a transmitting entity from a signal received via antennas. That is, if the number of antennas in the transmitting entity is assumed to be N T , the channel characteristic H can be defined as [h 1 , h 2 , . . . , h NT ]. 
     In step  312 , the reception apparatus calculates an MMSE-ESN j  of each antenna group by means of the MMSE technique based on the estimated channel characteristic H. In step  314 , the reception apparatus selects, as a MAX-ESN, the best MMSE-ESN from among the MMSE-ESN calculated for each antenna group, and selects, as a MAX group index, an antenna group index associated with the selected MAX-ESN. 
     Thereafter, in step  316 , the reception apparatus calculates an SIC-ESN and an OFF-ESN. That is, the reception apparatus calculates the SIC-ESN by means of the SIC technique based on the estimated channel characteristic H. In addition, the reception apparatus calculates the OFF-ESN by means of the beam-forming technique based on the estimated channel characteristic H. The SIC-ESN is calculated for one of the antenna groups, and the antenna group index associated with the SIC-ESN is different from the MAX group index. The OFF-ESN is calculated for one of the antenna groups, and the antenna group index associated with the OFF-ESN is different from the antenna group index associated with the SIC-ESN. 
     In step  318 , the reception apparatus calculates a rate R SIC  that it can obtain when applying the SIC technique, and a rate R OFF  that it can obtain when applying the beam-forming technique. For the calculation of the R SIC , the MAX-ESN and the SIC-ESN are used, and for the calculation of the R OFF , the OFF-ESN is used. 
     In step  320 , the reception apparatus determines a RANK and a Remain-ESN, each corresponding to the feedback information. For the Remain-ESN, the reception apparatus compares the R SIC  with the R OFF , and selects a higher one of them. Further, the reception apparatus checks a signal detection technique associated with the selected rate, and determines, as a Remain-ESN, the ESN calculated for the checked signal detection technique. The RANK is determined according to the determined Remain-ESN. The RANK is previously designated for the signal detection technique. 
     For example, if the R SIC  is higher than the R OFF , the reception apparatus determines the SIC-ESN as a Remain-ESN, and determines ‘2’ associated with the SIC technique as a RANK value. However, if R OFF  is higher than the R SIC , the reception apparatus determines the OFF-ESN as a Remain-ESN, and determines ‘1’ associated with the beam-forming technique as a RANK value. 
     In step  322 , the reception apparatus generates feedback information. The feedback information includes the MAX-ESN and the MAX group index determined in step  314 , and the RANK and the Remain-ESN determined in step  320 . Further, the reception apparatus transmits the feedback information to the transmission apparatus. 
       FIG. 4  illustrates an operation performed in a reception apparatus with, for example, two antenna groups according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 4 , in step  410 , a reception apparatus estimates a channel characteristic H associated with each antenna included in a transmitting entity from a signal received via antennas. That is, if the number of antennas in the transmitting entity is assumed to be N T , the channel characteristic H can be defined as [h 1 , h 2 , . . . , h NT ]. 
     In step  412 , the reception apparatus calculates an MMSE-ESN 1  and an MMSE-ESN 2  by means of the MMSE technique based on the estimated channel characteristic H. The MMSE-ESN 1  associated with a first antenna group can be calculated by Equation (1), and the MMSE-ESN 2  associated with a second antenna group can be calculated by Equation (2). 
     
       
         
           
             
               
                 
                   
                     
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     Herein, ρ m  denotes a CQI estimated for an m th  transmission antenna, and a capacity function can be defined as ƒ r (ρ)=log 2 (1+Γρ), where Γ denotes a performance difference between the actual processing performance and the Shannon capacity. 
     As defined in Equation (1) and Equation (2), the CQI associated with each antenna group is calculated by a sum of CQIs of the individual antennas constituting each antenna group. 
     The ρ m  by the MMSE technique is calculated by Equation (3). 
     
       
         
           
             
               
                 
                   
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     In step  414 , the reception apparatus decides a better MMSE-ESN from among the MMSE-ESN 1  and the MMSE-ESN 2 . That is, the reception apparatus determines whether the MMSE-ESN 1  is greater than the MMSE-ESN 2 . If the MMSE-ESN 1  is greater than the MMSE-ESN 2 , the reception apparatus proceeds to step  416 . 
     In step  416 , the reception apparatus sets the MAX group index to ‘1’ indicating the first antenna group, and sets the MAX-ESN to MMSE-ESN 1 . 
     In the above-described manner, the reception apparatus calculates an MMSE-ESN in association with each antenna group and a mapping relationship between the MAX-ESN set by the calculated MMSE-ESN. The MAX group index is shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 antenna group index 
                 calculated ESN 
               
               
                   
                   
               
             
            
               
                   
                 Group 1 
                 MMSE-ESN 1   
               
               
                   
                 Group 2 
                 MMSE-ESN 2   
               
               
                   
                 MAX-ESN 
                 MMSE-ESN 1   
               
               
                   
                 MAX group index 
                 1 
               
               
                   
                   
               
            
           
         
       
     
     In step  418 , the reception apparatus calculates an SIC-ESN 2  and an OFF-ESN 1 , and sets them to SIC-ESN and OFF-ESN, respectively. To calculate SIC-ESN, the reception apparatus should first detect the data stream transmitted by the first antenna group (an antenna group corresponding to the MAX group index). The reason is because MMSE-ESN 1  is set to MAX-ESN. In addition, the reception apparatus acquires an ESN (SIC-ESN 2 ) associated with the data stream transmitted by the second antenna group from the received signal from which the data stream component transmitted by the first antenna group is removed. To acquire the OFF-ESN 1 , the reception apparatus turns off the antennas belonging to the second antenna group, and acquires an ESN (OFF-ESN 1 ) associated with the data stream transmitted by the first antenna group. 
     However, if MMSE-ESN 2  is greater than the MMSE-ESN 1  in step  414 , the reception apparatus proceeds to step  420 . It is provided in  FIG. 4  that if the MMSE-ESN 1  is equal to the MMSE-ESN 2 , the reception apparatus proceeds to step  420 . Alternatively, however, if the MMSE-ESN 1  is equal to the MMSE-ESN 2 , the reception apparatus can be implemented to proceed to step  416 . 
     In step  420 , the reception apparatus sets the MAX group index to ‘2’ indicating the second antenna group, and sets the MAX-ESN to MMSE-ESN 2 . 
     In the above-described manner, the reception apparatus calculates an MMSE-ESN in association with each antenna group and a mapping relationship between the MAX-ESN set by the calculated MMSE-ESN. The MAX group index is shown in Table 2. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 antenna group index 
                 calculated ESN 
               
               
                   
                   
               
             
            
               
                   
                 Group 1 
                 MMSE-ESN 1   
               
               
                   
                 Group 2 
                 MMSE-ESN 2   
               
               
                   
                 MAX-ESN 
                 MMSE-ESN 2   
               
               
                   
                 MAX group index 
                 2 
               
               
                   
                   
               
            
           
         
       
     
     In step  422 , the reception apparatus calculates an SIC-ESN 1  and an OFF-ESN 2 , and sets them to SIC-ESN and OFF-ESN, respectively. To calculate the SIC-ESN, the reception apparatus should first detect the data stream transmitted by the second antenna group (an antenna group corresponding to the MAX group index). The reason is because the MMSE-ESN 2  is set to MAX-ESN. Further, the reception apparatus acquires an ESN (SIC-ESN 1 ) associated with the data stream transmitted by the first antenna group from the received signal from which the data stream component transmitted by the second antenna group is removed. To acquire the OFF-ESN 2 , the reception apparatus turns off the antennas belonging to the first antenna group, and acquires an ESN (OFF-ESN 2 ) associated with the data stream transmitted by the second antenna group. 
     A further detailed description will now be made of an operation of calculating the SIC-ESN and the OFF-ESN. 
     The reception apparatus calculates the SIC-ESN assuming that the rank is 2, and calculates the OFF-ESN assuming that the rank is 1. On this consumption, the SIC-ESN and the OFF-ESN can be calculated by Equation (4). 
     
       
         
           
             
               
                 
                   
                     ρ 
                     
                       g 
                       , 
                       min 
                     
                     Remain 
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               ρ 
                               
                                 g 
                                 , 
                                 
                                   I 
                                   
                                     g 
                                     , 
                                     min 
                                   
                                 
                               
                               BF 
                             
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               r 
                             
                             = 
                             1 
                           
                         
                       
                       
                         
                           
                             
                               ρ 
                               
                                 g 
                                 , 
                                 
                                   I 
                                   
                                     g 
                                     , 
                                     max 
                                   
                                 
                               
                               SIC 
                             
                             , 
                           
                         
                         
                           
                             
                               if 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               r 
                             
                             = 
                             2 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     where I= g,min =arg min{ρ g,1 ,ρ g,2 }, and ρ g,min   Remain  denotes a Remain-ESN. 
     The ρ g,m   BF  calculated by Equation (4) is an SINR received by means of the beam-forming (BF) technique, and can be expressed as Equation (5) for m=1 and 2. 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       r 
                     
                     ⁡ 
                     
                       ( 
                       
                         ρ 
                         
                           g 
                           , 
                           m 
                         
                         BF 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         
                           
                             2 
                             ⁢ 
                             m 
                           
                           - 
                           1 
                         
                       
                       
                         2 
                         ⁢ 
                         m 
                       
                     
                     ⁢ 
                     
                       
                         f 
                         r 
                       
                       ⁡ 
                       
                         ( 
                         
                           ρ 
                           l 
                           BF 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     where ρ m   BF  denotes a CQI of an m th  transmission antenna to which beam-forming is applied. 
     The ρ m   BF  is calculated by Equation (6). 
     
       
         
           
             
               
                 
                   
                     ρ 
                     m 
                     BF 
                   
                   = 
                   
                     
                       
                         h 
                         m 
                         H 
                       
                       ( 
                       
                         
                           
                             ∑ 
                             
                               
                                 l 
                                 ≠ 
                                 m 
                               
                               , 
                               
                                 l 
                                 ∈ 
                                 
                                   A 
                                   l 
                                 
                               
                             
                             
                                 
                             
                           
                           ⁢ 
                           
                             
                               h 
                               l 
                             
                             ⁢ 
                             
                               h 
                               l 
                               H 
                             
                           
                         
                         + 
                         
                           
                             2 
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               N 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                             
                           
                           ⁢ 
                           I 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       h 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     where 
     
       
         
           
             
               A 
               l 
             
             = 
             
               
                 { 
                 
                   
                     
                       2 
                       ⁡ 
                       
                         [ 
                         
                           
                             l 
                             + 
                             1 
                           
                           2 
                         
                         ] 
                       
                     
                     - 
                     1 
                   
                   , 
                   
                     2 
                     ⁡ 
                     
                       [ 
                       
                         
                           l 
                           + 
                           1 
                         
                         2 
                       
                       ] 
                     
                   
                 
                 } 
               
               . 
             
           
         
       
     
     In addition, the ρ g,m   SIC  in Equation (4) is an SINR received by means of the SIC technique, can be expressed as Equation (7) for m=1 and 2. 
     
       
         
           
             
               
                 
                   
                     
                       f 
                       t 
                     
                     ⁡ 
                     
                       ( 
                       
                         ρ 
                         
                           g 
                           , 
                           m 
                         
                         GSIC 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       ∑ 
                       
                         l 
                         = 
                         
                           
                             2 
                             ⁢ 
                             m 
                           
                           - 
                           1 
                         
                       
                       
                         2 
                         ⁢ 
                         m 
                       
                     
                     ⁢ 
                     
                       
                         f 
                         r 
                       
                       ⁡ 
                       
                         ( 
                         
                           ρ 
                           l 
                           GSIC 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where ρ m   GSIC  denotes a CQI calculated for an m th  antenna group after removing the signal transmitted by the remaining antenna group except for the m th  antenna group. 
     The ρ m   GSIC  is calculated by Equation (8). 
     
       
         
           
             
               
                 
                   
                     ρ 
                     m 
                     GSIC 
                   
                   = 
                   
                     
                       
                         h 
                         m 
                         H 
                       
                       ( 
                       
                         
                           
                             ∑ 
                             
                               
                                 l 
                                 ≠ 
                                 m 
                               
                               , 
                               
                                 l 
                                 ∈ 
                                 
                                   A 
                                   i 
                                 
                               
                             
                           
                           ⁢ 
                           
                             
                               h 
                               l 
                             
                             ⁢ 
                             
                               h 
                               l 
                               H 
                             
                           
                         
                         + 
                         
                           
                             4 
                             
                               S 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               N 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               R 
                             
                           
                           ⁢ 
                           I 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       h 
                       m 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     After acquiring the SIC-ESN and the OFF-ESN in step  418  and/or step  422 , the reception apparatus proceeds to step  424  where it calculates a rate R SIC  that it can obtain when applying the SIC technique, and a rate R OFF  that it can obtain when applying the beam-forming technique. The R SIC  and the R OFF  can be calculated by Equation (9).
 
 R   BF =ƒ r (ρ g,I     g,max     BF )
 
 R   GSIC =ƒ r (ρ g,I     g,max   )+ƒ r (ρ g,I     g,min     GSIC )  (9)
 
     In step  426 , the reception apparatus determines whether the R OFF  is higher than the R SIC . If the R OFF  is higher than the R SIC , it means that transmitting the data stream by means of the beam-forming technique is superior in terms of the transmission efficiency. Otherwise, if the R SIC  is higher than the R OFF , transmitting the data stream by means of the SIC technique is superior in terms of the transmission efficiency. 
     If the R OFF  is higher than the R SIC , the reception apparatus proceeds to step  428 . However, if the R SIC  is higher than the R OFF , the reception apparatus proceeds to step  430 . It is provided in  FIG. 4  that if the R OFF  is equal to the R SIC , the reception apparatus proceeds to step  430 . Alternatively, however, if the R OFF  is equal to the R SIC , the reception apparatus can be implemented to proceed to step  428 . 
     In step  429 , the reception apparatus sets the rank information to a value ‘1’ corresponding to the use of the beam-forming technique, and sets the Remain-ESN to the calculated OFF-ESN. Otherwise, in step  430 , the reception apparatus sets the rank information to a value ‘2’ corresponding to the use of the SIC technique, and sets the Remain-ESN to the calculated SIC-ESN. 
     In the above-described manner, the reception apparatus acquires the MAX-ESN, the MAX group index, the RANK, and the Remain-ESN, generates feedback information depending on the acquired MAX-ESN, MAX group index, RANK and Remain-ESN, and transmits the feedback information to the transmission apparatus. 
     C. Structure and Operation of Transmission Apparatus 
       FIG. 5  illustrates an example of a detailed structure of a transmission apparatus according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , a feedback information processor  510  receives feedback information from all reception apparatuses and controls the overall operation of transmitting data streams depending on the feedback information. 
     More specifically, the feedback information processor  510  determines a transmission mode, a coding technique, and an MCS level based on the feedback information received from each reception apparatus. The transmission mode is classified into the SU mode and the MU mode. 
     The feedback information processor  510  calculates an R_MU and an R_SU to determine the transmission mode. The feedback information processor  510  calculates the R_MU depending on a MAX-ESN and a MAX group index in the received feedback information. The feedback information processor  510  calculates the R_SU depending on a RANK, a Remain-ESN, and a MAX-ESN in the received feedback information. 
     For example, to calculate the R_MU, the feedback information processor  510  should collect a MAX-ESN individually for each antenna group depending on the MAX group index. Further, the feedback information processor  510  selects a best CQI (MAX-ESN i,j , where i denotes a user index and j denotes a antenna group index) individually for each antenna group from among the MAX-ESNs collected in association with each antenna group, and calculates the R_MU by the sum of MAX-ESN i,j  selected individually for each antenna group. 
     To calculate the R_SU, the feedback information processor  510  should collect a CQI (MAX-ESN or Remain-ESN) associated with each antenna group taking RANK into account. Further, the feedback information processor  510  calculates a sum rate (R_SU_i, where i denotes a user index) individually for each transmission apparatus based on the CQI collected in association with each antenna group, and calculates the best rate among the R_SU_i as the R_SU. 
     Thereafter, the feedback information processor  510  compares the R_MU with the R_SU, and selects one of the MU mode and the SU mode as a transmission mode according to the comparison result. For example, if the R_SU is equal to or less than the R_RU, the feedback information processor  510  selects the MU mode as a transmission mode. However, if the R_SU is greater than the R_MU, the feedback information processor  510  selects the SU mode as a transmission mode. 
     In addition, the feedback information processor  510  determines an MCS level and user scheduling information for supporting the SU mode and/or the MU mode. The user scheduling information is control information used for selecting a user data stream that the transmission apparatus will transmit according to the transmission mode. 
     A user stream processor  520  receives data associated with each of multiple users User # 1  to User #K, groups the data of each user under the control of the feedback information processor  510 , and outputs the grouped data as at least one data stream. The user stream processor  520  selects the received user data depending on the transmission mode and the user scheduling information provided from the feedback information processor  510  and outputs the selected data as at least one data stream. If the MU mode is designated, the user stream processor  520  outputs as many user data streams as the number of antenna groups. However, if the SU mode is designated, the user stream processor  520  outputs the selected user data as one data stream. 
     An encoding block  530  includes as many encoders as the number of antenna groups. In  FIG. 5 , as the number of antenna groups is assumed to be two, the encoding block  530  is composed to two encoders  532  and  534 . The encoding block  530  performs encoding on the at least one data stream provided from the user stream processor  520 . In this case, the encoding block  530  takes into account the MCS level provided from the feedback information processor  510 . That is, a coding rate in the encoding block  530  is determined depending on the MCS level. 
     A modulation block  540  modulates the at least one data stream encoded by the encoding block  530  taking the MCS level into consideration. The modulation block  540  is composed of as many mappers as the number of antenna groups. In  FIG. 5 , as the number of antenna groups is assumed to be two, the modulation block  540  is composed to two mappers  542  and  544 . 
     A spatial multiplexing block  550  is composed of spatial multiplexers  552  and  554  associated with their corresponding antenna groups. The spatial multiplexers  552  and  554  multiplex the modulated data streams provided from the modulation block  540  in association with their antenna groups. The number of data streams output from each of the spatial multiplexers  552  and  554  corresponds to the number of antennas constituting each antenna group. The data streams output from the spatial multiplexers  552  and  554  are each transmitted via their associated antennas. 
     Although the feedback information processor  510  and the user stream processor  520  are implemented with separate structures in  FIG. 5  by way of example, it is also possible to implement to process the operation of the feedback information processor  510  and the operation of the user stream processor  520  by means of a single structure. 
       FIG. 6  illustrates a control flow performed in a transmission apparatus according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 6 , in step  610 , a transmission apparatus receives feedback information transmitted separately for each user. The feedback information includes MAX-ESN i , MAX group index i , RANK i , and Remain-ESN i . Table 3 shows an example of feedback information that the transmission apparatus has received separately for each user. In Table 3, the number of users is assumed to be three, and the number of antenna groups is assumed to be two. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 MAX group 
                   
                   
               
               
                   
                 MAX-ESN i   
                 index i   
                 RANK i   
                 Remain-ESN i   
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 USER 1 
                 MAX-ESN 1   
                 2 
                 1 
                 Remain-ESN 1   
               
               
                 USER 2 
                 MAX-ESN 2   
                 1 
                 2 
                 Remain-ESN 2   
               
               
                 USER 3 
                 MAX-ESN 3   
                 2 
                 1 
                 Remain-ESN 3   
               
               
                   
               
            
           
         
       
     
     In step  612 , the transmission apparatus collects a MAX-ESN i  using a MAX group index i  based on the feedback information received from each user. That is, the transmission apparatus collects the MAX-ESN i  separately for each user (or reception apparatus) taking the MAX group index i  into account. 
     Table 4 shows an example of MAX-ESN i,j  collected by the transmission apparatus that has received the feedback information shown in Table 3. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 USER 1 
                 USER 2 
                 USER 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 GROUP 1 
                 — 
                 MAX-ESN 2,1   
                 — 
               
               
                   
                 GROUP 2 
                 MAX-ESN 1,2   
                 — 
                 MAX-ESN 3,2   
               
               
                   
                 m_best 
                 2 
                 1 
                 2 
               
               
                   
                   
               
            
           
         
       
     
     As can be appreciated from Table 4, the transmission apparatus checks MAX-ESN i  and MAX group index i  individually for each user, and maps the checked MAX-ESN i  to the antenna group designated by the MAX group index i . The MAX-ESN i  mapped in this rule is expressed as MAX-ESN i,j  taking the antenna group index into consideration, where j denotes an antenna group index. Further, in Table 4, m_best corresponds to the MAX group index checked separately for each user. 
     For example, MAX-ESN 1  received from USER  1  is mapped to MAX-ESN 1,2  because its MAX group index is ‘2’, and MAX-ESN 2  received from USER  2  is mapped to MAX-ESN 2,1  because its MAX group index is ‘1’. 
     In step  614 , the transmission apparatus calculates an R_MU depending on the collected information. To this end, the transmission apparatus selects the best MAX-ESN i,j  from among MAX-ESN i,j  collected separately for each antenna group, and calculates a serviceable rate depending on the MAX-ESN i,j  selected individually for each antenna group. The transmission apparatus sets the calculated rate as R_MU. 
     Table 5 shows an example of R_MU calculated based on the MAX-ESN i,j  collected as shown in Table 4. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 USER 1 
                 USER 2 
                 USER 3 
                 Maximum 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 GROUP 1 
                 — 
                 MAX- 
                 — 
                 MAX-ESN 2,1   
               
               
                   
                   
                 ESN 2,1   
               
               
                 GROUP 2 
                 MAX-ESN 1,2   
                 — 
                 MAX-ESN 3,2   
                 MAX-ESN 1,2   
               
               
                 M_best 
                 2 
                 1 
                 2 
                 R_MU 
               
               
                   
               
            
           
         
       
     
     According to Table 5, MAX-ESN 2,1  is selected for the first antenna group, and MAX-ESN 1,2  is selected for the second antenna group. The reason why the MAX-ESN 1,2  is selected is because the MAX-ESN 1,2  has higher quality than the MAX-ESN 3,2 . In addition, the transmission apparatus sets, as R_MU, the rate supportable by the MAX-ESN 2,1  selected for the first antenna group and the MAX-ESN 1,2  selected for the second antenna group. 
     In step  616 , the transmission apparatus collects MAX-ESN i  and Remain-ESN i  using RANK i  based on the feedback information received from each user. That is, the transmission apparatus collects the MAX-ESN i  and Remain-ESN i  separately for each antenna group taking the RANK i  into account. 
     Table 6 shows an example of MAX-ESN i,j  and Remain-ESN i  collected based on the RANK i  by the transmission apparatus that has received the feedback information shown in Table 3. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                 USER 1 
                 USER 2 
                 USER 3 
               
               
                   
                 (RANK = 1) 
                 (RANK = 2) 
                 (RANK = 1) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 GROUP 1 
                 OFF 
                 MAX-ESN 2,1   
                 Remain-ESN 3,1   
               
               
                   
                 GROUP 2 
                 Remain-ESN 1,2   
                 Remain-ESN 2,2   
                 OFF 
               
               
                   
                   
               
            
           
         
       
     
     As can be appreciated from Table 6, the transmission apparatus collects only the Remain-ESN i  for the user with RANK i =1, and collects MAX-ESN i,j  and Remain-ESN i  for the user with RANK i =2. This is because the SU mode by the beam-forming technique is requested for the RANK i =1, and the MU mode by the SIC technique is requested for the RANK i =2. Therefore, the transmission apparatus collects Remain-ESN i  associated with one of two antenna groups for the USER  1  with RANK i =1 and the USER  3  with RANK i =1. Further, the transmission apparatus turns off the remaining antenna group. That is, for USER  1 , the transmission apparatus collects Remain-ESN 1,2  in association with the second antenna group and turns off the first antenna group. For USER  3 , the transmission apparatus collects Remain-ESN 3,1  in association with the first antenna group and turns off the second antenna group. 
     However, for USER  2  with RANK i =2, the transmission apparatus collects MAX-ESN i,j  for one of the two antenna groups and collects Remain-ESN i,j  for the remaining antenna group. That is, for USER  2 , the transmission apparatus collects MAX-ESN 2,1  for the first antenna group and collects Remain-ESN 2,2  for the second antenna group. 
     Thereafter, in step  618 , the transmission apparatus calculates an R_SU depending on the collected information. To this end, the transmission apparatus calculates a serviceable rate R_SU_i depending on the Remain-ESN i,j  or MAX-ESN i,j  and Remain-ESN i,j  collected separately for each user. 
     Table 7 shows an example of serviceable rates R_SU_ 1 , R_SU_ 2 , and R_SU_ 3  calculated separately for each user based on the collected information shown in Table 6. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 USER 1 
                 USER 2 
                 USER 3 
               
               
                   
                 (RANK = 1) 
                 (RANK = 2) 
                 (RANK = 1) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 GROUP 1 
                 OFF 
                 MAX-ESN 2,1   
                 Remain-ESN 3,1   
               
               
                 GROUP 2 
                 Remain-ESN 1,2   
                 Remain-ESN 2,2   
                 OFF 
               
               
                 SUM RATE 
                 R_SU_1 
                 R_SU_2 
                 R_SU_3 
               
               
                   
               
            
           
         
       
     
     The transmission apparatus compares the rates R_SU_ 1 , R_SU_ 2 , and R_SU_ 3  calculated individually for each user, and selects the highest one of them. Further, the transmission apparatus sets the selected rate as R_SU. 
     Table 8 shows an example of setting R_MU on the assumption that among the rates R_SU_ 1 , R_SU_ 2 , and R_SU_ 3  calculated separately for each user as shown in Table 7, R_SU_ 3  is the highest rate. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 USER 1 
                 USER 2 
                 USER 3 
                   
               
               
                   
                 (RANK = 1) 
                 (RANK = 2) 
                 (RANK = 1) 
                 Maximum 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 GROUP 1 
                 OFF 
                 MAX-ESN 2,1   
                 Remain-ESN 3,1   
                 Remain- 
               
               
                   
                   
                   
                   
                 ESN 3,1   
               
               
                 GROUP 2 
                 Remain-ESN 1,2   
                 Remain-ESN 2,2   
                 OFF 
                 OFF 
               
               
                 SUM 
                 R_SU_1 
                 R_SU_2 
                 R_SU_3 
                 R_SU = 
               
               
                 RATE 
                   
                   
                   
                 R_SU_3 
               
               
                   
               
            
           
         
       
     
     In step  620 , the transmission apparatus compares the R_MU with the R_SU to determine a supportable transmission mode. That is, the transmission apparatus compares the R_MU with the R_SU and determines a higher rate as a supportable transmission mode. 
     If the R_SU is higher than the R_MU, the transmission apparatus transmits a data stream of the corresponding user by means of the SU mode in step  622 . That is, the transmission apparatus transmits the data stream of the corresponding user via an antenna group associated with the rate set as the R_SU among the multiple antenna groups. Here, the number of antenna groups via which the data stream of the user will be transmitted is determined according to RANK designated by the corresponding user. For example, if RANK is designated as ‘1’, the transmission apparatus transmits the data stream of the corresponding user via one antenna group. In this case, the antennas corresponding to the remaining one antenna group are turned off. However, if RANK is designated as ‘2’, the transmission apparatus transmits the data stream of the corresponding user via two antenna groups. According to Table 8, the transmission apparatus transmits data streams of USER  3  via the first antenna group at the rate R_SU_ 3 . 
     Otherwise, if the R_MU is higher than the R_SU, the transmission apparatus transmits data streams of the users selected for each antenna group by means of the MU mode in step  624 . That is, the transmission apparatus transmits the data streams of the corresponding users via an associated antenna group among the multiple antenna groups. The rate used at this time is a rate capable of supporting MAX-ESN i,j  corresponding to each user selected separately for each antenna group. According to Table 5, data streams of USER  2  are transmitted via the first antenna group at the rate capable of supporting MAX-ESN 2,1 , and data streams of USER  1  are transmitted via the second antenna group at the rate capable of supporting MAX-ESN 1,2 . 
     A description of the operation in which the R_SU is equal to the R_MU has not been provided herein. It is assumed in  FIG. 6  that if the R_SU is equal to the R_MU, the transmission apparatus operates in the MU mode. However, the transmission apparatus can also be implemented such that when the R_SU is equal to the R_MU, it operates in the SU mode. 
     Although an exemplary embodiment of the present invention generates feedback information depending on a data stream of a user and transmits the feedback information, by way of example, exemplary embodiments of the present invention can also be implemented using a signal (for example, a pilot signal) predefined for each user instead of using the data stream of the user. In addition, although the number of antenna groups herein is assumed to be 2, the number of antenna groups is subject to change. In this case, it is necessary to newly define RANK information according to the changed number of antenna group. For example, if an exemplary embodiment of the present invention is implemented with three antenna groups, the RANK information should be defined depending on the information based on which one to three antenna groups can be selected. In addition, exemplary embodiments of the present invention can be applied regardless of the number of users. 
     As is apparent from the foregoing description, according to exemplary embodiments of the present invention, multiple receivers each extract channel quality of an antenna group connected to a transmitter, generate feedback information using the extracted channel quality, and then transmit the feedback information to the transmitter, thereby reducing the amount of feedback information compared to the case of feeding back channel quality of each antenna. 
     In addition, the transmitter according to an exemplary embodiment of the present invention is configured to receive channel qualities fed back from multiple receivers, thereby enabling scheduling such that a user data stream is transmitted individually via each antenna group. This contributes to an increase in the transmission capacity of the multi-antenna system. 
     Further, the feedback information proposed by exemplary embodiments of the present invention is similar in size to the feedback information proposed in each of the conventional SU mode and MU mode. Therefore, exemplary embodiments of the present invention can support both the SU mode and the MU mode without increasing the size of the feedback information. Moreover, because the feedback information includes a rank value indicating the beam-forming mode and/or the SIC mode, exemplary embodiments of the present invention can adaptively control the use of the feedback information when operating in the SU mode. 
     While the invention has been shown and described with reference to a 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 and their equivalents.