Patent Publication Number: US-9887744-B2

Title: MIMO transmission method and MIMO transmission device using plurality of cells in multi-cell wireless communication system

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims priority under 35 U.S.C. § 365 to International Patent Application No. PCT/KR2013/011800 filed Dec. 18, 2013, entitled “MIMO TRANSMISSION METHOD AND MIMO TRANSMISSION DEVICE USING PLURALITY OF CELLS IN MULTI-CELL WIRELESS COMMUNICATION SYSTEM”, and, through International Patent Application No. PCT/KR2013/011800, to Korean Application No. 10-2012-0148937 filed Dec. 18, 2012, each of which are incorporated herein by reference into the present disclosure as if fully set forth herein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Field 
     The present invention relates to a method and apparatus for downlink Multiple Input Multiple Output (MIMO) transmission using antennas of a plurality of cells in a multi-cell wireless communication system. 
     Background Art 
     In a multi-cell wireless communication system including a plurality of cells, each cell transmits independent data signals to different terminals using its own antenna. In order to apply a Multiple Input Multiple Output (MIMO) transmission mode for simultaneously transmitting a plurality of pieces of data to a terminal in a cell, at least two antennas should be installed in the cell. Accordingly, only cells with antennas corresponding to a minimal number of antennas required for MIMO transmission can perform MIMO transmission. Also, when a cell with the corresponding number of antennas performs MIMO transmission, a total number of data streams that can be simultaneously transmitted to a terminal is limited by the number of antennas included in the cell. 
     DISCLOSURE 
     Technical Problem 
     The present disclosure relates to a method and apparatus for supporting downlink Multiple Input Multiple Output (MIMO) transmission to a specific terminal using antennas of a plurality of cells in a multi-cell wireless communication system. 
     Technical Solution 
     In accordance with an aspect of exemplary embodiments of the present invention, there is provided a method by which a base station performs multiple input and multiple output (MIMO) transmission using antennas of a plurality of cells in a multi-cell wireless communication system, the method including: selecting a rank value and cells for downlink MIMO transmission to a terminal on the basis of measurement information of the base station and the terminal, and determining a precoding matrix according to the rank value; and generating data which is to be transmitted to the terminal as data streams corresponding to the selected rank value, and mapping the data streams to antennas of the selected cells through the precoding matrix so as to transmit the data streams to the terminal. 
     In accordance with another aspect of exemplary embodiments of the present invention, there is provided a method by which a terminal performs multiple input and multiple output (MIMO) reception using antennas of a plurality of cells in a multi-cell wireless communication system, the method including: generating measurement information based on reference signals received from cells constituting a service coverage of a base station; transmitting the measurement information and a sounding signal to the base station; and simultaneously receiving data streams transmitted through cells selected from among the cells constituting the service coverage of the base station based on the measurement information and information acquired by the cells receiving the sounding signal. 
     In accordance with another aspect of exemplary embodiments of the present invention, there is provided a base station which performs multiple input and multiple output (MIMO) transmission using antennas of a plurality of cells in a multi-cell wireless communication system, the base station including: a controller configured to select a rank value and cells for downlink MIMO transmission to a terminal on the basis of measurement information of the base station and the terminal, to determine a precoding matrix according to the rank value, to generate data which is to be transmitted to the terminal as data streams corresponding to the selected rank value, and then to map the data streams to antennas of the selected cells through the precoding matrix so as to control a transmitter to transmit the data streams to the terminal. 
     In accordance with another aspect of exemplary embodiments of the present invention, there is provided a terminal which performs multiple input and multiple output (MIMO) reception using antennas of a plurality of cells in a multi-cell wireless communication system, the terminal including: a controller configured to generate measurement information based on reference signals received from cells constituting a service coverage of a base station, and a transceiver configured to transmit the measurement information and a sounding signal to the base station, and to simultaneously receive data streams transmitted through cells selected from among the cells constituting the service coverage of the base station based on the measurement information and information acquired by the cells receiving the sounding signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view for describing an example of Multiple Input Multiple Output (MIMO) transmission that is performed on a terminal located at a cell boundary using two cells, according to an embodiment of the present disclosure; 
         FIG. 2A  shows an example of a configuration diagram of a base station for MIMO transmission according to an embodiment of the present disclosure; 
         FIG. 2B  shows another example of a configuration diagram of a base station for MIMO transmission according to an embodiment of the present disclosure; 
         FIG. 3  shows still another example of a configuration diagram of a base station that supports MIMO transmission according to an embodiment of the present disclosure; 
         FIG. 4  is a view for describing another example of MIMO transmission according to an embodiment of the present disclosure; 
         FIG. 5  shows an example of a signal flow diagram of MIMO transmission according to an embodiment of the present disclosure; 
         FIGS. 6A and 6B  show examples of transmission systems to which MIMO transmission according to an embodiment of the present disclosure is applied; 
         FIG. 7  is a flowchart showing operations of a base station, according to an embodiment of the present disclosure; and 
         FIG. 8  is a flowchart showing operations of a terminal, according to an embodiment of the present disclosure. 
     
    
    
     BEST MODE 
     Now, the above and other aspects of the present invention will be described in detail through preferred embodiments with reference to the accompanying drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention. 
     Hereinafter, a method and apparatus that support downlink Multiple Input Multiple Output (MIMO) transmission to a specific terminal using antennas of a plurality of cells, according to embodiments of the present disclosure, will be described. 
     Generally, cells configuring a network may be assigned unique cell identifiers, and some of the cells may be assigned the same cell identifier. And, a plurality of cells participating in performing downlink MIMO transmission to a terminal may have the same cell identifier or different cell identifiers. In the following description, for convenience of description, a method of supporting downlink MIMO transmission to a terminal using cells having the same cell identifier will be described. For example, it is assumed that a service coverage of a base station is divided into three sectors, and each sector operates as a cell. In this case, downlink MIMO transmission to a terminal is performed using antennas of the sectors. Each sector may include at least one antenna, and the sectors may include the same number of antennas or different numbers of antennas. In the following description, for convenience of description, the sectors will be referred to as cells. Also, in the following description, for convenience of description, it is assumed that a plurality of cells having the same cell identifier include the same number of antennas. Also, a method of supporting downlink MIMO transmission to a terminal using antennas of a plurality of cells, according to the present disclosure, can be supported through cells having different cell identifiers. 
       FIG. 1  is a view for describing an example of MIMO transmission that is performed on a terminal located at a cell boundary using two cells, according to an embodiment of the present disclosure. 
     Referring to  FIG. 1 , it is assumed that a base station  100  manages three cells, that is, a first cell, a second cell, and a third cell, wherein the first cell includes n 1  antennas, and the second cell includes n 2  antennas. In the example of  FIG. 1 , n 1  and n 2  are assumed to be two. 
     Also, a terminal  110  may be located at a boundary area between the first cell and the second cell. In this case, the base station  100  may decide to transmit data to the terminal  110  using all the antennas included in the first cell and the second cell. Then, a data generator  102  may generate data that is to be transmitted to the terminal  110 , as data streams corresponding to n 1 +n 2  (that is, 4) that is the total number of the antennas included in the first cell and the second cell. Thereafter, the data generator  102  may map the data streams to a first radio unit (RU1)  104  connected to the first cell and a second radio unit (RU2)  106  connected to the second cell, respectively. Then, the RU1  104  and the RU2  106  may transmit the mapped data streams to the terminal  110  through the antennas included in the first and second cells. As a result, the base station  100  can simultaneously transmit a maximum of n 1 +n 2  data streams to the terminal  110 . 
       FIG. 2A  shows an example of a configuration diagram of a base station for MIMO transmission according to an embodiment of the present disclosure. 
     Referring to  FIG. 2A , it is assumed that a base station  200  manages a plurality of cells including a first cell and a second cell, wherein the first cell includes a first antenna  208   a  and the second cell includes a second antenna  208   b . Also, it is assumed that when there is generated data that is to be transmitted to a terminal located at a boundary area between the first cell and the second cell, the base station  200  performs MIMO transmission of the data to the terminal using the first and second antennas  208   a  and  208   b  of the first and second cells. In this case, a data generator  201  may generate the data as first and second data streams corresponding to a total number (that is, 2) of the first and second antennas  208   a  and  208   b  included in the first and second cells. Then, the data generator  201  may transfer the first data stream and the second data stream to a first modulator  202   a  and a second modulator  202   b , respectively. Then, the first modulator  202   a  and the second modulator  202   b  may modulate the first data stream and the second data stream, respectively, and transfer the modulated first and second data streams to a precoder  204 . The precoder  204  may perform precoding on the modulated first and second data streams, map the precoded first and second data streams to a first antenna port connected to the first antenna  208   a  of the first cell and a second antenna port connected to the second antenna  208   b  of the second cell, respectively, and then output the precoded first and second data streams to a RU1  206   a  and a RU2  206   b , respectively. The RU1  206   a  and the RU2  206   b  may transmit signals output mapped to the first and second data streams to the first antenna port and the second antenna port, to the first antenna  208   a  of the first cell and the second antenna  208   b  of the second cell so as to transmit the data steams to the terminal. 
       FIG. 2B  shows another example of a configuration diagram of a base station for MIMO transmission according to an embodiment of the present disclosure. 
     Referring to  FIG. 2B , it is assumed that a base station  210  manages a plurality of cells including a first cell and a second cell, and each of the first and second cells includes two antennas. More specifically, it is assumed that the first cell includes a first antenna  218   a  and a second antenna  218   b , and the second cell includes a third antenna  218   c  and a fourth antenna  218   d . Also, it is assumed that when there is generated data that is to be transmitted to a terminal located at a boundary area between the first cell and the second cell, the base station  210  performs MIMO transmission of the data to the terminal using the antennas  218   a  to  218   d  of the first and second cells. In this case, a data generator  211  may generate the data as first to fourth data streams corresponding to a total number (that is, 4) of the antennas  218   a  to  218   d  included in the first and second cells. Thereafter, the data generator  211  may transfer the first to fourth data streams to first to fourth modulators  212   a  to  212   d , respectively. The first to fourth modulators  212   a  to  212   d  may modulate the first to fourth data streams, respectively, and then transfer the modulated first to fourth data streams to the precoder  214 . The precoder  214  may perform precoding on the modulated first to fourth data streams, map the precoded first to fourth data streams to first to fourth antenna ports respectively connected to the first to fourth antennas  218   a  to  218   d  included in the first and second cells, and then output the first to fourth data streams to a RU1  216   a  and a RU2  216   b . Then, the RU1  216   a  may transmit signals (the first, and second data streams mapped to the first and second antenna ports) output to the first antenna port to the first and second antennas  218   a  and  218   b  of the first cell, respectively, so as to transmit the first and second data streams to the terminal. Likewise, the RU2  216   b  may transmit signals (the third and fourth data streams mapped to the third and fourth antenna ports) output to the second antenna port to the third and fourth antennas  218   c  and  218   d  of the second cell, respectively, so as to transmit the third and fourth data streams to the terminal. 
       FIG. 3  shows still another example of a configuration diagram of a base station that supports MIMO transmission according to an embodiment of the present disclosure. 
     Referring to  FIG. 3 , a base station  300  may include a transmission antenna deciding unit  302 , a precoder and Modulation and Coding Scheme (MCS) deciding unit  304 , and a signal mapping unit  306 . Although not shown in  FIGS. 1, 2A, and 2B , the components constituting the base station  300  may be included as a controller in the base stations  100 ,  200 , and  210  of  FIGS. 1, 2A , and  2 B. Also, the base stations  100 ,  200 , and  210  of  FIGS. 1, 2A, and 2B  may operate according to control information decided by such a controller. 
     Meanwhile, the components included in the base station  300  are divided according to their functions for convenience of description, however, the transmission antenna deciding unit  302 , the precoder and MCS deciding unit  304 , and the signal mapping unit  306  constituting the base station  300  may be configured as one unit, or each divided into at least two sub units. 
     It is assumed that the base station  300  decides to support downlink MIMO transmission to a specific terminal, for example, a terminal  310 . Then, the transmission antenna deciding unit  302  may receive measurement information of the terminal  310  from the terminal  310 . Herein, the measurement information of the terminal  310  may include at least one of a maximum number of streams that can be received by the terminal  310 , channel state information created by the terminal  310  based on a reference signal received from the base station  300 , an amount of data that is to be transmitted to the terminal  310 , and Quality of Service (QoS) required by the terminal  310 . Also, the transmission antenna deciding unit  302  may use measurement information measured by the base station  300 . Herein, the measurement information may include information acquired by receiving a sounding signal transmitted from the terminal  310 . Also, if the base station  300  manages its own cell coverage in units of cells corresponding to a plurality of sectors, as shown in  FIGS. 1, 2A, and 2B , the measurement information may include information acquired by receiving a sounding signal of the terminal  310  for each cell. Also, the measurement information of the terminal  310  and the base station  300  may include information of all terminals located within a service coverage of the base station  300  and information of all terminals located in the adjacent cells, as well as information of the terminal  310 . 
     Also, the transmission antenna deciding unit  302  may decide a group of antennas which support MIMO transmission to the terminal  310 , that is, which is used to transmit downlink data to the terminal  310 , based on the measurement information of the terminal  310  and the base station  300 . At this time, the transmission antenna deciding unit  302  may decide, as the group of antennas, an antenna group including antennas of a cell or antennas of a plurality of cells. Also, a total number of antennas included in the antenna group means a maximum number of data streams that can be simultaneously transmitted to the terminal  310 . 
     The precoder and MCS deciding unit  304  may decide a total number of antennas included in the antenna group decided by the transmission antenna deciding unit  302 , as a maximum value. Also, the precoder and MCS deciding unit  304  may decide a number (that is, a rank value) of downlink data streams that are to be actually transmitted to the terminal  310 , such that the rank value is smaller than or equal to the maximum value. Also, the precoder and MCS deciding unit  304  may select a precoding matrix that is used for transmission from a precoder group configured with precoding matrices corresponding to the decided rank value. Then, the precoder and MCS deciding unit  304  may decide a MCS for each of the data streams that are actually transmitted to the terminal  310 . More specifically, the precoder and MCS deciding unit  304  may use channel state information received from the terminal  310 , information acquired from a sounding signal of the terminal  310 , previous transmission success/failure information (ACK/NACK) related to downlink data transmission with respect to the terminal  310 , etc., to select the precoder and the MCS. Operation of selecting the precoding matrix will be described in more detail, later. 
     If there is generated data that is to be transmitted to the terminal  310 , the signal mapping unit  306  may generate the data as data streams corresponding to the rank value decided by the precoder and MCS deciding unit  304 . Then, the signal mapping unit  306  may map the data streams to the transmission antennas decided by the antenna deciding unit  302 , and enable the data streams to be simultaneously transmitted to the terminal  310  through the transmission antennas. The base station  100  of  FIG. 1  and the base stations  200  and  210  of  FIGS. 2A and 2B , as described above, may also substantially perform operations for MIMO transmission according to instructions from the signal mapping unit  306 . 
       FIG. 4  is a view for describing another example of MIMO transmission according to an embodiment of the present disclosure. Referring to  FIG. 4 , it is assumed that a base station  400  manages its own service coverage in units of three sectors, that is, first, second, and third cells, wherein the first, second, and third cells have the same cell identifier, and each of the first, second, and third cells includes an antenna. Accordingly, the first, second, and third cells may transmit four Common Reference Signals (CRSs). 
     Referring to  FIG. 4 , a first terminal  410  located at a boundary area between the first cell and the second cell may perform MIMO transmission using both the antennas of the first and second cells. That is, it is assumed that the base station  400  decides to perform MIMO transmission of rank 2 to simultaneously transmit two pieces of downlink data to the first terminal  410  through the antennas of the first and second cells using measurement information of the base station  400  and the first terminal  410 . Also, it is assumed that the first terminal  410  includes two reception antennas or more. Also, it is assumed that the base station  400  decides to perform Single Input Multiple Output (SIMO) transmission with respect to a second terminal  420  located in the center area of the third cell using an antenna installed in the third cell. 
     The base station  400  can support MIMO transmission with respect to downlink data of the first terminal  410  although each of cells managed by the base station  400  includes an antenna. 
       FIG. 5  shows an example of a signal flow diagram of MIMO transmission according to an embodiment of the present disclosure. 
     Although not shown in  FIG. 5 , it is assumed that the transmission antenna deciding unit  302  (see  FIG. 3 ) of the base station  400  of  FIG. 4  has decided to perform MIMO transmission of rank 2 with respect to the first terminal  410  using the antennas installed in the first and second cells among the first to third cells managed by the base station  400 . 
     Then, a signal mapping unit  500  of the base station  400  may assign unique virtual mapping vectors  502   a  and  502   b  each having a 4×1 size to the first cell and the second cell, respectively. Then, both the first cell and the second cell may transmit four CRSs of the base station  400  to the first terminal  410 . Although not shown in the drawings, a data generator of the base station  400  may generate the four CRSs, in addition to data streams that are to be transmitted to the first terminal  410 . The four CRSs may pass through the virtual mapping vectors assigned to the respective cells, that is, the virtual mapping vector  502   a  of the first cell and the virtual mapping vector  502   b  of the second cell, and then be transferred to the antennas  506   a  and  506   b  of the first and second cells via RUs of the first and second cells, that is, via a RU  504   a  of the first cell and a RU  504   b  of the second cell. Data of the first terminal  410  may be generated as two data streams by the data generator, pass through the precoder  504 , and then be transferred to the virtual mapping vectors  502   a  and  502   b  of the first and second cells, although not shown in the drawings. The precoder  504  may select a matrix satisfying a condition according to an embodiment of the present disclosure with respect to the virtual mapping vectors  502   a  and  502   b . The virtual mapping vectors  502   a  and  502   b  and the precoder  504  according to an embodiment of the present disclosure will be described in more detail, later. 
     The CRSs passed through the virtual mapping vectors  502   a  and  502   b  may be transferred to the antennas  506   a  and  506   b  of the first and second cells through the RUs  504   a  and  504   b  of the first and second cells. 
     A combination of the virtual mapping vectors  502   a  and  502   b  according to an embodiment of the present disclosure may be expressed by Equation (1), below. In Equation (1), virtual mapping vectors V k  corresponding to three cells managed by the base station  400  are expressed, wherein k is an indicator of each cell. That is, the first, second, and third cells of the base station  400  are mapped to virtual mapping vectors V 1 , V 2 , and V 3 .
 
 V   1 =½[ j  1  j  1]
 
 V   2 =½[ j  −1 1 − j] 
 
 V   3 =½[ j −j  1 −1]  (1)
 
     Meanwhile, the precoder  504  may select a precoding matrix from among matrices satisfying the following condition with respect to the virtual mapping vector  502   a  of the first cell and the virtual mapping vector  502   b  of the second cell. That is, the precoding matrix satisfying the condition may be a matrix that acquires a rank value of 2 when the matrix is multiplied by a 2×4 matrix whose columns are the virtual mapping vectors V 1  and V 2  of cells (that is, the first and second cells) that are used for MIMO transmission of the terminal  410 . Alternatively, the precoding matrix may be a matrix that acquires a value of zero when the matrix is multiplied by a virtual mapping matrix of the remaining cells that are not used for MIMO transmission. 
     For example, if virtual mapping matrices of the first to third cells of the base station  400  are selected to correspond to Equation (1), a matrix satisfying the following Equation (2) may be selected as a precoding matrix of the precoder  504 . 
     
       
         
           
             
               
                 
                   
                     
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     Also, a precoding matrix of the precoder  504  satisfying Equation (2) with respect to the virtual mapping vectors according to Equation (1) may be selected from Table (1), below. Precoding matrices and precoding matrix indexes shown in Table (1) are defined in the Long Term Evolution (LTE) standard of 3rd Generation Partnership Project (3GPP). 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Group of Transmission 
                   
                   
               
               
                 Sectors 
                 Precoding Matrix 
                 Precoding Matrix Index 
               
               
                   
               
             
            
               
                 First and Second Sectors 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                  8 
               
               
                   
               
               
                 Second and Third Sectors 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 10 
               
               
                   
               
               
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     Meanwhile, the base station  400  may inform the terminal  410  of a precoding index corresponding to the precoding matrix used for MIMO transmission. Then, the terminal  410  may decode the data streams received through MIMO transmission according to an embodiment of the present disclosure, using the precoding matrix corresponding to the precoding index. 
     Equation (3) expresses another example of a combination of virtual mapping vectors according to an embodiment of the present disclosure.
 
 V   1 =½[1 −1 − j −j] 
 
 V   2 =½[1+ j  0 0 −1− j] 
 
 V   3 =½[1  j − 1 − j]   (3)
 
     As another example, it is assumed that virtual mapping vectors corresponding to the first to third cells of the base station  400  are mapped to V 1 , V 2 , and V 3  of Equation (3). In this case, a precoding matrix of the precoder  504  satisfying Equation (2) may be selected from Table (2) below. Likewise, precoding matrices and precoding matrix indexes shown in Table (2) are also defined in the LTE standard of 3GPP. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Group of Transmission 
                   
                   
               
               
                 Sectors 
                 Precoding Matrix 
                 Precoding Matrix Index 
               
               
                   
               
             
            
               
                 First and Second Sectors 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 11 
               
               
                   
               
               
                 Second and Third Sectors 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 12 
               
               
                   
               
               
                 Third and First Sectors 
                 
                   
                     
                     
                         
                         
                     
                   
                 
                  0 
               
               
                   
               
            
           
         
       
     
       FIGS. 6A and 6B  show examples of transmission systems to which MIMO transmission according to an embodiment of the present disclosure is applied. Herein, the communication systems are assumed to use 3GPP LTE Release 10 or 11. However, the MIMO transmission according to the present disclosure is not necessarily applied to LTE-based communication systems. 
     Referring to  FIG. 6A , it is assumed that a base station  600  manages three cells, that is, first, second, and third cells, wherein each of the first, second, and third cells includes an antenna. 
     In this case, the base station  600  may decide to support downlink MIMO transmission of rank 2 to the terminal  610  through antennas of the first and second cells, based on measurement information of the base station  600  and measurement information received from a terminal  610 . Also, it is assumed that the terminal  610  includes two antennas or more. In this case, the first cell and the second cell may transmit the same demodulation reference signal (DM-RS) to the terminal  610 . At this time, the DM-RS may be multiplied by a precoding matrix selected for data transmission. 
     Then, the terminal  610  may receive data streams transmitted through the first cell and the second cell based on the received DM-RS, and decode the received data streams. 
     Referring to  FIG. 6B , it is assumed that a base station  620  manages first, second, and third cells, wherein each of the first, second, and third cells includes two antennas. 
     In this case, the base station  620  may decide to support downlink MIMO transmission of rank 4 to a terminal  630  through two antennas of each of the first and second cells, based on measurement information of the base station  620  and measurement information received from the terminal  630 . Also, it is assumed that the terminal  630  includes four antennas or more. In this case, likewise, the first cell and the second cell may transmit the same DM-RS to the terminal  630 . At this time, the DM-RS may be multiplied by a precoding matrix selected for data transmission. 
     Then, the terminal  630  may receive data streams transmitted through the first cell and the second cell based on the received DM-RS, and decode the received data streams. 
     As described above, by supporting downlink MIMO transmission to a terminal using antennas of a plurality of cells according to an embodiment of the present disclosure to increase the number of data streams that are simultaneously transmitted to a terminal regardless of the number of antennas for each cell, it is possible to increase data transmission speed at cell boundary areas. 
       FIG. 7  is a flowchart showing operations of a base station, according to an embodiment of the present disclosure. 
     Referring to  FIG. 7 , a base station may decide cells for downlink MIMO transmission to a terminal and a rank value that is smaller than or equal to a total number of antennas included in the cells, based on measurement information of the base station and the terminal, in operation  700 . At this time, the base station may select sectors constructing its own service coverage as the cells for MIMO transmission. Alternatively, the base station may select cells having a cell identifier that is different from its own cell identifier. Also, the measurement information of the terminal may include at least one of a maximum number of streams that can be received by the terminal, channel state information created by the terminal based on a reference signal received from the base station, an amount of data that is to be transmitted to the terminal, and QoS required by the terminal. Also, the measurement information of the base station may include information acquired by the cells receiving a sounding signal transmitted from the terminal. 
     Also, the base station may determine a precoding matrix according to the rank value, in operation  705 . In order to determine the precoding matrix, the base station may assign unique virtual mapping vectors according to Equation (1) or Equation (3) as described above to the selected cells, respectively. Then, the base station may select a precoding matrix (that is, a precoding matrix according to Equation (2)) corresponding to a rank value calculated by multiplying the precoding matrix by a matrix whose columns are the virtual mapping vectors. Then, the base station may transmit an index of the selected precoding matrix to the terminal. In a system in which the cells transmit DM-RS, the precoding matrix can be freely selected from a group of precoding matrices without using virtual mapping vectors. 
     Thereafter, the base station may generate data that is to be transmitted to the terminal, as data streams corresponding to the selected rank value, map the data streams to the antennas of the selected cells through a precoder, and then transfer the data streams to the terminal, in operation  710 . 
       FIG. 8  is a flowchart showing operations of a terminal, according to an embodiment of the present disclosure. 
     Referring to  FIG. 8 , a terminal may generate measurement information based on reference signals received from cells constructing a service coverage of a base station, in operation  800 . Then, the terminal may transmit the measurement information to the base station, in operation  805 . At this time, the terminal may transmit its own sound signal together with the measurement information to the base station. 
     The terminal may simultaneously receive data streams transmitted through cells selected by the base station based on the measurement information and the sounding signal, in operation  810 . Then, the terminal may decode the simultaneously received data streams using a precoding matrix corresponding to a precoding index received from the base station, from a pre-stored group of precoding matrices, in operation  815 . The precoding matrix may be a matrix that can acquire a rank value (that is, a rank value satisfying Equation (2)) selected by the base station when the precoding matrix is multiplied by a matrix whose columns are unique virtual mapping vectors assigned by the base station to the selected cells according to Equation (1) or Equation (3). In a system in which the cells transmit DM-RS, the precoding matrix may be an arbitrary matrix in a group of precoding matrices. 
     According to the embodiments of the present disclosure as described above, it is possible to provide MIMO transmission to a terminal regardless of the number of antennas included in cells, and to increase speed of data transmission to a terminal, 
     Accordingly, by supporting a downlink MIMO transmission mode of a specific terminal using antennas of a plurality of cells in a multi-cell wireless communication system, it is possible to apply a downlink MIMO transmission mode regardless of the number of antennas installed in a cell, to increase the number of data streams that are simultaneously transmitted to the specific terminal, and to increase data transmission speed at cell boundary areas. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.