Abstract:
There is provided a wireless communication system, including a first terminal equipped with N antennas, N being an integer of at least two, a second terminal equipped with M antennas, M being an integer of at least one, and a third terminal equipped with L antennas, L being an integer of at least one. The first terminal includes a matrix calculation portion that calculates a transmission weight matrix suitable for transmitting data from the first terminal to the second terminal, and a matrix multiplication portion that multiplies a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted from the first terminal to the second terminal, and the second data stream being transmitted from the first terminal to the third terminal.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention generally relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program. The present invention particularly relates to a wireless communication system, a wireless communication device, a wireless communication method and a computer program that are capable of simultaneously transmitting data to a plurality of terminals. 
         [0003]    2. Description of the Related Art 
         [0004]    Typical examples of wireless local area network (LAN) systems include IEEE802.11a and IEEE802.11b/g that are standardized by the Institute of Electrical and Electronic Engineers (IEEE). In the IEEE802.11a/g wireless LAN system, an orthogonal frequency division multiplexing (OFDM) system is used, which provides high frequency use efficiency and which is robust against frequency selective fading. The maximum bit rate in the IEEE802.11a/g wireless LAN system is 54 Mbps. 
         [0005]    However, in the next generation of wireless LAN systems, a higher bit rate is desired. To address this, in the IEEE systems, IEEE802.11n that uses a multi-input multi-output (MIMO) communication system is being standardized. 
         [0006]    In the MIMO communication system, the transmission capacity can be increased in accordance with the number of antennas, without increasing the frequency band. For example, if it is assumed that the number of antennas on the transmission side is N and the number of antennas on the receiving side is M, data transmission can be performed using streams that are spatially multiplexed between a transmitter and a receiver, and it is possible to form spatial streams corresponding to the smaller number (MIN [N, M]) of the transmission and reception antennas. In summary, the transmission capacity can be increased up to MIN [N, M] times with respect to a known transmission system that is not spatially multiplexed. 
         [0007]    Mathematical expressions and the like that relate to the MIMO communication system are described in Japanese Patent Application Publication No. JP-A-2007-318727, Japanese Patent Application Publication No. JP-A-2007-318728 and Japanese Patent Application Publication No. JP-A-2007-318729. As described in JP-A-2007-318727, JP-A-2007-318728 and JP-A-2007-318729, the MIMO communication system estimates a channel matrix H of transmission paths using a given method, and multiplies signals by an antenna weight matrix on both the transmitter and receiver sides (in some cases, only on the receiver side), thereby achieving space multiplexing transmission. 
         [0008]    Further, Japanese Patent Application Publication No. JP-A-2007-74318 discloses a technology that improves effective speed in a wireless network by suppressing interference within the same cell in downlink, in the next generation of wireless communication system in which a plurality of antennas are used and a plurality of mobile stations perform communication at the same time using the same frequency. 
       SUMMARY OF THE INVENTION 
       [0009]    As one of the methods that perform multiplication by transmission antenna weight on the transmitter side, there is an SVD-MIMO system that utilizes singular value decomposition (SVD) of the channel matrix H. In the SVD-MIMO system, a matrix V that is obtained by performing the singular value decomposition of the channel matrix H is used as a transmission antenna weight matrix, and a data signal x that is desired to be transmitted is multiplied by the matrix V, thereby generating a transmission signal x′. 
         [0010]    However, in the known art, modulation scheme assignment of each element of the data signal x that is desired to be transmitted is determined based on differences in magnitude of singular values λ, which are elements of a diagonal matrix D obtained by performing the singular value decomposition of the channel matrix H. Therefore, even if equivalent transmission power is consumed for transmission, there is an element that can only send signals of a low transmission rate to the receiver side. 
         [0011]    In light of the foregoing, it is desirable to provide a novel and improved wireless communication system, wireless communication device, wireless communication method and computer program capable of using transmission power more efficiently for data transmission. 
         [0012]    According to an embodiment of the present invention, there is provided a wireless communication system, including a first terminal equipped with N antennas, N being an integer of at least two, a second terminal equipped with M antennas, M being an integer of at least one, and a third terminal equipped with L antennas, L being an integer of at least one. The first terminal includes a matrix calculation portion that calculates a transmission weight matrix suitable for transmitting data from the first terminal to the second terminal, and a matrix multiplication portion that multiplies a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted from the first terminal to the second terminal, and the second data stream being transmitted from the first terminal to the third terminal. 
         [0013]    According to such configuration, a first terminal equipped with N antennas, a second terminal equipped with M antennas, a third terminal equipped with L antennas. In the first terminal, a matrix calculation portion calculates a transmission weight matrix suitable for transmitting data from the first terminal to the second terminal, and a matrix multiplication portion multiplies a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted from the first terminal to the second terminal, and the second data stream being transmitted from the first terminal to the third terminal. As a result, there is capable of using transmission power more efficiently for data transmission. 
         [0014]    The first data stream may be mapped according to a modulation scheme assigned based on a singular value decomposition (SVD)-multi-input multi-output (MIMO) system, and the second data stream may be mapped according to a modulation scheme determined in advance between the first terminal and the third terminal. 
         [0015]    The first terminal may transmit, once in a plurality of times, only data addressed to the third terminal. 
         [0016]    The third terminal may send back, to the first terminal, information indicating that data has been received correctly, collectively for a plurality of data. 
         [0017]    The third terminal may send back, to the first terminal, information indicating that data has been received correctly, collectively for a plurality of data, at a time point when the first terminal transmits only data addressed to the third terminal. 
         [0018]    Based on information from the third terminal indicating that data has been received correctly, the first terminal may preferentially retransmit data that has not been received by the third terminal. 
         [0019]    The first data stream received by the second terminal and the second data stream received by the third terminal may be combined and used. 
         [0020]    Information from the second terminal indicating that data has been received correctly, and information from the third terminal indicating that data has been received correctly may be shared by each other, and the information including also the information of a sharing counterpart is transmitted from one of the M antennas and the L antennas. 
         [0021]    According to another embodiment of the present invention, there is provided a wireless communication device, including N antennas, N being an integer of at least two, a matrix calculation portion that calculates a transmission weight matrix suitable for transmitting data to a first terminal equipped with M antennas, M being an integer of at least one, and a matrix multiplication portion that multiplies a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted to the first terminal, and the second data stream being transmitted to a second terminal equipped with L antennas, L being an integer of at least one. 
         [0022]    The first data stream may be mapped according to a modulation scheme assigned based on an SVD-MIMO system, and the second data stream may be mapped according to a modulation scheme determined in advance with the second terminal. 
         [0023]    The N antennas may transmit, once in a plurality of times, only data addressed to the second terminal. 
         [0024]    Based on information from the second terminal indicating that data has been received correctly, the N antennas may preferentially retransmit data that has not been received by the second terminal. 
         [0025]    According to another embodiment of the present invention, there is provided a wireless communication method, including the steps of calculating a transmission weight matrix suitable for transmitting data from N antennas, N being an integer of at least two, to a first terminal equipped with M antennas, M being an integer of at least one, and multiplying a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted to the first terminal, the second data stream being transmitted to a second terminal equipped with L antennas, L being an integer of at least one. 
         [0026]    According to another embodiment of the present invention, there is provided a computer program that comprises instructions that command a computer to execute the steps of calculating a transmission weight matrix suitable for transmitting data from N antennas, N being an integer of at least two, to a first terminal equipped with M antennas, M being an integer of at least one, and multiplying a first data stream and a second data stream by the transmission weight matrix, the first data stream being transmitted to the first terminal, the second data stream being transmitted to a second terminal equipped with L antennas, L being an integer of at least one. 
         [0027]    According to the embodiments of the present invention described above, there are provided the wireless communication system, the wireless communication device, the wireless communication method and the computer program that are novel and improved and that are capable of using transmission power more efficiently for data transmission. More specifically, the first data stream to be transmitted from the first terminal to the second terminal and the second data stream to be transmitted from the first terminal to the third terminal are both multiplied by the transmission weight matrix suitable for transmitting data from the first terminal to the second terminal. Thus, transmission power can be used more efficiently for data transmission. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is an explanatory diagram illustrating a configuration of a wireless communication system according to an embodiment of the present invention; 
           [0029]      FIG. 2  is an explanatory diagram illustrating a configuration of a MIMO transmission operation portion  130 ; 
           [0030]      FIG. 3  is an explanatory diagram illustrating an example of a transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention; 
           [0031]      FIG. 4  is an explanatory diagram illustrating an example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention; 
           [0032]      FIG. 5  is an explanatory diagram illustrating an example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention; 
           [0033]      FIG. 6  is an explanatory diagram illustrating an example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention; 
           [0034]      FIG. 7  is an explanatory diagram illustrating a modified example of the wireless communication system according to the embodiment of the present invention; 
           [0035]      FIG. 8  is an explanatory diagram illustrating a configuration of a known wireless communication system; 
           [0036]      FIG. 9  is an explanatory diagram illustrating a transmission/reception timing relationship in the known wireless communication system; and 
           [0037]      FIG. 10  is an explanatory diagram illustrating a transmission/reception timing relationship in the known wireless communication system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
         [0039]    An exemplary embodiment of the present invention will be described in detail in the following order. 
         [0040]    1. Explanation of a Wireless Communication System That Uses a Known SVD-MIMO System 
         [0041]    2. Explanation of a Wireless Communication System According to an Embodiment of the Present Invention
       2-1. Configuration of the Wireless Communication System According to the Embodiment of the Present Invention   2-2. Operation of the Wireless Communication System According to the Embodiment of the Present Invention       
 
         [0044]    3. Explanation of a Modified Example of the Wireless Communication System According to the Embodiment of the Present Invention 
         [0045]    4. Conclusion 
         [0046]    1. Explanation of a Wireless Communication System That Uses a Known SVD-MIMO System 
         [0047]    First, a wireless communication system that uses a known SVD-MIMO system will be explained. 
         [0048]    As described above, in the SVD-MIMO system, the matrix V that is obtained by performing the singular value decomposition of the channel matrix H is used as a transmission antenna weight matrix, and the data signal x that is desired to be transmitted is multiplied by the matrix V, to thereby generate the transmission signal x′. The transmission signal x′ passes through a transmission path of the channel matrix H, and is received as a received signal y′ on the receiver side. 
         [0049]    On the receiver side, a matrix U that is also obtained by performing the singular value decomposition of the channel matrix H is used as a reception antenna weight matrix, and a data signal y that is received by multiplying the received signal y′ with the matrix U is generated. After noise components have been removed from the received data signal y, the resultant portion of the received data signal y can be expressed as Dx, by utilizing the diagonal matrix D that is also obtained by performing the singular value decomposition of the channel matrix H. Note that the diagonal matrix D is a diagonal matrix that has, as a diagonal element, the square root of each of the singular values λ obtained by the singular value decomposition. Thus, depending on the magnitude of the singular values, attained communication quality of each element of the data signal x that is desired to be transmitted varies in each element of the received data signal y. In the SVD-MIMO system, the modulation scheme assignment of each element of the data signal x that is desired to be transmitted is performed based on this communication quality, thereby improving the communication quality efficiently. 
         [0050]    Theoretically, it is possible to obtain the maximum communication capacity by performing such modulation scheme assignment. 
         [0051]      FIG. 8  is an explanatory diagram illustrating a configuration of a known wireless communication system. As shown in  FIG. 8 , the known wireless communication system includes a transmitter  1  and a receiver  2 . The transmitter  1  includes antennas  11   a  and  11   b,  a data input/output terminal  12 , a MIMO transmission operation portion  13  and a divider  15 . The receiver  2  includes antennas  21   a  and  21   b,  a data input/output terminal  22 , a MIMO reception operation portion  24  and a coupler  26 . 
         [0052]    First, a connection relationship between the transmitter  1  and the receiver  2  will be explained. The data input/output terminal  12  is connected to the divider  15  provided inside the transmitter  1 . The divider  15  is connected to the MIMO transmission operation portion  13 . The MIMO transmission operation portion  13  is connected to the antennas  11   a  and  11   b . The antennas  21   a  and  21   b  are connected to the MIMO reception operation portion  24  provided inside the receiver  2 . The MIMO reception operation portion  24  is connected to the coupler  26 . The coupler  26  is connected to the data input/output terminal  22 . 
         [0053]    Next, operations of the transmitter  1  and the receiver  2  will be explained. Data S for transmitting to the receiver  2  is input to the divider  15  provided inside the transmitter  1 . The divider  15  divides the data S into data S 0  and data S 1  based on the ratio between a modulation scheme for S 0  and a modulation scheme for S 1  that are derived in advance by the channel matrix H between the transmitter  1  and the receiver  2 . Then, the divider  15  outputs the data S 0  and the data S 1  to the MIMO transmission operation portion  13 . 
         [0054]    The MIMO transmission operation portion  13  performs matrix operation processing on the data S 0  and S 1  that are mapped according to the modulation schemes assigned based on the concept of the SVD-MIMO system, using the matrix V that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  1  and the receiver  2 . The MIMO transmission operation portion  13  generates the transmission signals x′ to be transmitted from the antennas  11   a  and  11   b,  by performing the matrix operation processing. The transmission signals x′ generated by the MIMO transmission operation portion  13  are sent to the antennas  11   a  and  11   b,  and output from the antennas  11   a  and  11   b  to wireless transmission paths. 
         [0055]    The receiver  2  sends, to the MIMO reception operation portion  24 , the received signals y′ that are received by the antennas  21   a  and  21   b  through the wireless communication paths. The MIMO reception operation portion  24  performs matrix operation processing on the input received signals y′, using the matrix U that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  1  and the receiver  2  (or using an inverse matrix calculated from the received signals y′). The data S 0  and S 1  are generated by the matrix operation processing performed by the MIMO reception operation portion  24 , and output to the coupler  26 . The coupler  26  combines the data series of the data S 0  and S 1  input from the MIMO reception operation portion  24 , in accordance with the ratio between the modulation schemes used for modulating the data S 0  and the data S 1 , respectively, and outputs the resultant data to the data input/output terminal  22 . 
         [0056]      FIG. 9  is an explanatory diagram illustrating a transmission/reception timing relationship in the known wireless communication system. With reference to  FIG. 9 , a case will be explained in which a terminal  1  corresponds to the transmitter  1  in  FIG. 8 , and a terminal  2  and a terminal  3  correspond to the receiver  2  in  FIG. 8  (i.e., a case in which there are two receivers  2  in the wireless communication system shown in  FIG. 8 ). 
         [0057]    First, a notation system used in  FIG. 9  will be explained. In  FIG. 9 , transmission/reception states of the “terminal  1 ”, the “terminal  2 ” and the “terminal  3 ” are shown, and each horizontal axis represents passage of time. The data “&lt;terminal  2 - 1 &gt;” on the right side of the “terminal  1 ” is surrounded by the solid line, and this indicates a signal transmitted from the terminal  1 . Further, at the same time point, the data “&lt;terminal  2 - 1 &gt;” surrounded by the broken line is present in the “terminal  2 ”, and this indicates that the terminal  2  has received the signal (in other words, the data has been output to the data input/output terminal). The data denoted by “&lt;terminal *-*&gt;” is a signal indicating transmission data, and the data denoted by “&lt;Ack*&gt;” is a response signal for notifying the transmission side that each data has been received successfully. Note that the first subscript of each signal indicates the destination terminal number, and if there is the second subscript, it indicates the packet number of the signal series. 
         [0058]    Next, referring to  FIG. 9 , the transmission/reception timing relationship in the known wireless communication system will be explained as a time series. First, the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  is transmitted from the terminal  1 . The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” transmitted from the terminal  1 . Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” has been received correctly. When the terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , it can confirm that the terminal  2  has correctly received the data “&lt;terminal  2 - 1 &gt;” for the terminal  2 . 
         [0059]    Continuously, the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  is transmitted from the terminal  1 . The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1 . Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 . 
         [0060]    In a similar manner, the data “&lt;terminal  2 - 3 &gt;” for the terminal  2  is transmitted from the terminal  1 . The terminal  2  receives the data “&lt;terminal  2 - 3 &gt;” transmitted from the terminal  1 . Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  2 - 3 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 . Then, the data “&lt;terminal  3 - 1 &gt;” for the terminal  3  is transmitted from the terminal  1 . The terminal  3  receives the data “&lt;terminal  3 - 1 &gt;” transmitted from the terminal  1 . Next, the terminal  3  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  3 - 1 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  3 . 
         [0061]    Specific operations will be explained separately for each terminal. First, at the terminal  1 , a general protocol, namely a “method to transmit the next data when Ack is received” is used for data transmission to the terminal  2 . Further, the general protocol, the “method to transmit the next data when Ack is received” is also used for data transmission to the terminal  3 . 
         [0062]    At the terminal  2 , a general protocol, namely a “method to send back Ack when data is received successfully” is used for data reception from the terminal  1 . Also at the terminal  3 , the general protocol, the “method to send back Ack when data is received successfully” is used for data reception from the terminal  1 . 
         [0063]    In the transmission/reception timing relationship in the known wireless communication system, data transmission to one terminal is performed in one transmission, and the terminal that has received the data sends Ack back to the transmission source sequentially when the data is received successfully. If Ack is not received on the transmission side and time out occurs, retransmission is performed a plurality of times (in some cases, retransmission is not performed). If retransmission time out occurs, the corresponding data packet is discarded, and transmission of the next data packet is performed. 
         [0064]      FIG. 10  is an explanatory diagram illustrating a transmission/reception timing relationship in the known wireless communication system. With reference to  FIG. 10 , in a similar manner to  FIG. 9 , a case will be explained in which the terminal  1  corresponds to the transmitter  1  in  FIG. 8 , and the terminal  2  and the terminal  3  correspond to the receiver  2  in  FIG. 8  (i.e., a case in which there are two receivers  2  in the wireless communication system). 
         [0065]    A notation system used in  FIG. 10  is the same as the notation system used in  FIG. 9 , and a detailed explanation is therefore omitted. 
         [0066]    Next, referring to  FIG. 10 , the transmission/reception timing relationship in the known wireless communication system will be explained as a time series. First, the data “&lt;terminal  1 - 1 &gt;” for the terminal  1  is transmitted from the terminal  3 . The terminal  1  receives the data “&lt;terminal  1 - 1 &gt;” transmitted from the terminal  3 . Next, the terminal  1  transmits the data “&lt;Ack 3 &gt;” in order to notify the terminal  3  that the data “&lt;terminal  1 - 1 &gt;” has been received successfully. The terminal  3  receives the data “&lt;Ack 3 &gt;” transmitted from the terminal  1 . 
         [0067]    Continuously, the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  is transmitted from the terminal  1 . The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” transmitted from the terminal  1 . Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 . 
         [0068]    In a similar manner, the data “&lt;terminal  1 - 2 &gt;” for the terminal  1  is transmitted from the terminal  3 . The terminal  1  receives the data “&lt;terminal  1 - 2 &gt;” transmitted from the terminal  3 . Next, the terminal  1  transmits the data “&lt;Ack 3 &gt;” in order to notify the terminal  3  that the data “&lt;terminal  1 - 2 &gt;” transmitted from the terminal  3  has been received successfully. The terminal  3  receives the data “&lt;Ack 3 &gt;” transmitted from the terminal  1 . Continuously, the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  is transmitted from the terminal  1 . The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1 . Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 . 
         [0069]    Specific operations will be explained separately for each terminal. First, at the terminal  1 , the general protocol, the “method to transmit the next data when Ack is received” is used for data transmission to the terminal  2 . Further, the general protocol, the “method to send back Ack when data is received successfully” is used for data reception from the terminal  3 . 
         [0070]    At the terminal  2 , the general protocol, the “method to send back Ack when data is received successfully” is used for data reception from the terminal  1 . Also at the terminal  3 , the general protocol, the “method to transmit the next data when Ack is received” is used for data transmission to the terminal  1 . 
         [0071]    In the transmission/reception timing relationship in the known wireless communication system, after performing transmission for sending back Ack, if there is new data to be transmitted from the terminal itself, it is necessary to perform transmission processing according to a series of transmission procedures. In  FIG. 10 , the transmission of the data “&lt;terminal  2 -*&gt;” after the data “&lt;Ack 3 &gt;” has been transmitted from the terminal  1  corresponds to the above case. Accordingly, when it is necessary to transmit data to a plurality of terminals, the data cannot be simultaneously transmitted, resulting in reduced communication efficiency. 
         [0072]    The modulation scheme assignment of each element of the data signal x that is desired to be transmitted is determined based on differences in magnitude of the singular values λ that are elements of the diagonal matrix D. Therefore, even if equivalent transmission power is consumed for transmission, there is an element that can only send signals of a low transmission rate to the receiver side. 
         [0073]    To address this, the present invention makes it possible to simultaneously transmit different data to a plurality of terminals on the receiving side. Thus, transmission power can be used for data transmission more efficiently, and also an efficient communication protocol can be formed. 
         [0074]    2. Explanation of a Wireless Communication System According to an Embodiment of the Present Invention 
         [0075]    Hereinafter, the exemplary embodiment of the present invention will be explained using examples. 
         [0076]    2-1. Configuration of the Wireless Communication System According to the Embodiment of the Present Invention 
         [0077]      FIG. 1  is an explanatory diagram illustrating a configuration of the wireless communication system according to the embodiment of the present invention. Hereinafter, the configuration of the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 1 . 
         [0078]    As shown in  FIG. 1 , the wireless communication system according to the embodiment of the present invention includes a transmitter  100 , and receivers  200  and  300 . 
         [0079]    The transmitter  100  includes antennas  110   a  and  110   b,  data input/output terminals  120  and  121 , and a MIMO transmission operation portion  130 . The receiver  200  includes antennas  210   a  and  210   b,  a data input/output terminal  220  and a MIMO reception operation portion  240 . Similarly, the receiver  300  includes antennas  310   a  and  310   b,  a data input/output terminal  320  and a MIMO reception operation portion  340 . 
         [0080]    Here, referring to  FIG. 1 , a connection relationship between the transmitter  100 , the receiver  200  and the receiver  300  will be explained. The data input/output terminals  120  and  121  are connected to the MIMO transmission operation portion  130  provided inside the transmitter  100 . The MIMO transmission operation portion  130  is connected to the antennas  110   a  and  110   b.  The antennas  210   a  and  210   b  are connected to the MIMO reception operation portion  240  provided inside the receiver  200 . The MIMO reception operation portion  240  is connected to the data input/output terminal  220 . The antennas  310   a  and  310   b  are connected to the MIMO reception operation portion  340  provided inside the receiver  300 . The MIMO reception operation portion  340  is connected to the data input/output terminal  320 . 
         [0081]    Next operations of the transmitter  100 , the receiver  200  and the receiver  300  will be explained. 
         [0082]    In the transmitter  100 , data S 0  for transmitting to the receiver  200  and data S 1  for transmitting to the receiver  300  are respectively input, via the input/output terminals  120  and  121 , to the MIMO transmission operation portion  130  provided inside the transmitter  100 . The MIMO transmission operation portion  130  performs matrix operation processing on the data S 0  that is mapped according to a modulation scheme assigned based on the concept of the SVD-MIMO system, using the matrix V that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  100  and the receiver  200 . At the same time, the MIMO transmission operation portion  130  performs the matrix operation processing, using the matrix V, also on the data S 1  that is mapped according to a modulation scheme that is determined separately in advance between the transmitter  100  and the receiver  300 . By performing the matrix operation processing, the MIMO transmission operation portion  130  generates transmission signals x′ to be transmitted from the antennas  110   a  and  110   b . The transmission signals x′ generated by the MIMO transmission operation portion  130  are sent to the antennas  110   a  and  110   b , and output to wireless transmission paths. 
         [0083]    The receiver  200  sends, to the MIMO reception operation portion  240 , received signals y′ that are received by the antennas  210   a  and  210   b  through the wireless communication paths. The MIMO reception operation portion  240  performs matrix operation processing on the input received signals y′, using the matrix U that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  100  and the receiver  200  (or using an inverse matrix calculated from the received signals y′). Then, from among the data obtained as a result of the matrix operation processing performed by the MIMO reception operation portion  240 , the MIMO reception operation portion  240  outputs, to the data input/output terminal  220 , only the data S 0  that is data addressed to the receiver  200 . 
         [0084]    The receiver  300  sends, to the MIMO reception operation portion  340 , received signals y″ that are received by the antennas  310   a  and  310   b  through the wireless communication paths. The MIMO reception operation portion  340  performs matrix operation processing on the input received signals y″, using an inverse matrix calculated from the received signals y″. From among the data obtained as a result of the matrix operation processing performed by the MIMO reception operation portion  340 , the MIMO reception operation portion  340  outputs, to the data input/output terminal  320 , only the data S 1  that is data addressed to the receiver  300 . 
         [0085]    It should be noted herein that, when only the data S 0  that is the data addressed to the receiver  200  is output by, for example, the transmitter  100 , data that describes identification information indicating that the data S 0  is addressed to the receiver  200  may be added to the beginning portion of the data S 0 . In a similar way, when only the data S 1  that is the data addressed to the receiver  300  is output by, for example, the transmitter  100 , data that describes identification information indicating that the data S 1  is addressed to the receiver  300  may be added to the beginning portion of the data S 1 . 
         [0086]    Although not particularly shown in  FIG. 1 , the transmitter  100  is provided with an operation portion that has an equivalent function to the MIMO reception operation portions  240  and  340  that are respectively included in the receiver  200  and the receiver  300 . Because the transmitter  100  is provided with the operation portion, when data is transmitted from the receiver  200  or the receiver  300 , it can extract data by performing the matrix operation processing. In a similar way, the receiver  200  and the receiver  300  are provided with an operation portion that has an equivalent function to the MIMO transmission operation portion  130  included in the transmitter  100 . Because the receiver  200  and the receiver  300  are provided with the operation portion, data can be transmitted from the receiver  200  or the receiver  300 . 
         [0087]    This completes the explanation of the configuration of the wireless communication system according to the embodiment of the present invention. Next, a configuration of the MIMO transmission operation portion  130  included in the transmitter  100  according to the embodiment of the present invention will be described. 
         [0088]      FIG. 2  is an explanatory diagram illustrating the configuration of the MIMO transmission operation portion  130  included in the transmitter  100  according to the embodiment of the present invention. Hereinafter, the configuration of the MIMO transmission operation portion  130  will be explained with reference to  FIG. 2 . 
         [0089]    As shown in  FIG. 2 , the MIMO transmission operation portion  130  included in the transmitter  100  according to the embodiment of the present invention includes a matrix calculation portion  140  and a matrix multiplication portion  142 . 
         [0090]    The matrix calculation portion  140  performs, in advance, singular value decomposition of the channel matrix H between the transmitter  100  and the receiver  200 , and thereby generates the matrix V with two rows and two columns that is used in matrix operation processing in the matrix multiplication portion  142 . The channel matrix H between the transmitter  100  and the receiver  200  can be estimated in advance using a known pattern (for example, a reference signal) transmitted by the receiver  200 . A known method can be used as an estimation method of the channel matrix H, or as a method for generating the matrix V by performing the singular value decomposition of the channel matrix H. A detailed explanation of these methods is therefore omitted here. 
         [0091]    The matrix multiplication portion  142  performs the matrix operation processing, using the matrix V generated by the matrix calculation portion  140 , on the data S 0  that is the data addressed to the receiver  200  and that is mapped according to the modulation scheme assigned based on the concept of the SVD-MIMO system, and on the data S 1  that is the data addressed to the receiver  300  and that is mapped according to the modulation scheme determined separately in advance between the transmitter  100  and the receiver  300 . 
         [0092]    As a result of the matrix operation processing in the matrix multiplication portion  142 , transmission signals x′ to be transmitted from the antennas  110   a  and  110   b  are generated. The transmission signals x′ generated by the matrix multiplication portion  142  include both the data addressed to the receiver  200  and the data addressed to the receiver  300 . By transmitting such data from the antennas  110   a  and  110   b,  it is possible to improve communication efficiency. 
         [0093]    In the present embodiment, the data S 0  is the data that is mapped according to the modulation scheme assigned based on the concept of the SVD-MIMO system. However, it is needless to mention that, in the present invention, the data S 0  is not limited to this example. Further, the data S 1  is the data that is mapped according to the modulation scheme determined separately in advance between the transmitter  100  and the receiver  300 . However, it is needless to mention that, in the present invention, the data S 1  is not limited to this example. 
         [0094]    This completes the explanation of the configuration of the MIMO transmission operation portion  130  included in the transmitter  100  according to the embodiment of the present invention. Next, an operation of the wireless communication system according to the embodiment of the present invention will be explained. 
         [0095]    2-2. Operation of the Wireless Communication System According to the Embodiment of the Present Invention 
         [0096]      FIG. 3  is an explanatory diagram illustrating an example of a transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention. Hereinafter, the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 3 . 
         [0097]    In  FIG. 3 , the “terminal  1 ” corresponds to the transmitter  100  in the wireless communication system shown in  FIG. 1 , the “terminal  2 ” corresponds to the receiver  200 , and the “terminal  3 ” corresponds to the receiver  300 . 
         [0098]    First, a notation system used in  FIG. 3  will be explained. In  FIG. 3 , transmission/reception states of the “terminal  1 ”, the “terminal  2 ” and the “terminal  3 ” are shown, and each horizontal axis represents passage of time. The data “&lt;terminal  2 - 1 &gt;” and the data “&lt;terminal  3 - 1 &gt;” on the right side of the “terminal  1 ” that are surrounded by the solid line indicate signals transmitted from the terminal  1 . The data “&lt;terminal  2 - 1 &gt;” in the upper section corresponds to the data S 0  in  FIG. 1 , and the data “&lt;terminal  3 - 1 &gt;” in the lower section corresponds to the data S 1  in  FIG. 1 . Further, at the same time point, the data “&lt;terminal  2 - 1 &gt;” surrounded by the broken line is present in the “terminal  2 ” and the data “&lt;terminal  3 - 1 &gt;” surrounded by the broken line is present in the “terminal  3 ”, and these data indicate that the terminal  2  and the terminal  3  have received signals addressed thereto (in other words, these data have been output to the data input/output terminals). Further, the data denoted by “&lt;terminal *-*&gt;” is a signal indicating the content of the transmission data, and the data denoted by “&lt;Ack*&gt;” or “&lt;BlockAck*&gt;” is a response signal for notifying the transmission side that each data has been received successfully. Note that the first subscript of each signal indicates the destination terminal number, and if there is the second subscript, it indicates the packet number of the signal series. The signals that are not dividedly shown in the upper and lower sections, such as “&lt;Ack*&gt;”, indicate that they are signals that have been transmitted/received using a known system such as that shown in  FIG. 8 . 
         [0099]    Next, referring to  FIG. 3 , the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained as a time series. 
         [0100]    First, the terminal  1  transmits data including the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 1 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 1 &gt;” via the antennas  310   a  and  310   b.  At this time, the MIMO reception operation portion  240  of the terminal  2  (the receiver  200 ) disregards or discards the data addressed to the terminal  3  (the receiver  300 ), and the MIMO reception operation portion  340  of the terminal  3  disregards or discards the data addressed to the terminal  2 . 
         [0101]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0102]    In response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 2 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 2 &gt;” via the antennas  310   a  and  310   b.    
         [0103]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0104]    In a similar manner, in response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 3 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 3 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 3 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 3 &gt;” via the antennas  310   a  and  310   b.    
         [0105]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 3 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0106]    Next, the terminal  1  transmits only the data “&lt;terminal  3 - 4 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  3  receives the data “&lt;terminal  3 - 4 &gt;” via the antennas  310   a  and  310   b.  Then, the terminal  3  transmits the data “&lt;BlockAck 1 &gt;” from the antennas  310   a  and  310   b,  in order to notify the terminal  1  that the data “&lt;terminal  3 - 1 &gt;” to “&lt;terminal  3 - 4 &gt;” have been received successfully. The terminal  1  receives the data “&lt;BlockAck 1 &gt;” transmitted by the terminal  3 , via the antennas  110   a  and  110   b.  Note that the terminal  2  may receive the data “&lt;terminal  3 - 4 &gt;” via the antennas  210   a  and  210   b.  However, this is not the data addressed to the terminal  2 . Therefore, even if the terminal  2  receives the data “&lt;terminal  3 - 4 &gt;”, the terminal  2  disregards or discards that data. 
         [0107]    Note that, in the example shown in  FIG. 3 , the terminal  2  may receive the data “&lt;BlockAck 1 &gt;” transmitted by the terminal  3  for the terminal  1 . Similarly, the terminal  2  disregards or discards that data. Further, the terminal  3  may receive the data “&lt;Ack 1 &gt;” transmitted by the terminal  2  for the terminal  1 . Similarly, the terminal  3  disregards or discards that data. 
         [0108]    Specific operations will be explained separately for each terminal. 
         [0109]    At the terminal  1 , the general protocol, the “method to transmit the next data when Ack is received” is used for data transmission to the terminal  2 . Further, a protocol called BlockAck, namely a “method to collectively receive and process Ack for a plurality of data transmissions” is used for data transmission to the terminal  3 . Note that there are separately determined various systems for processing, such as retransmission caused by the information of BlockAck collectively sent back from the terminal  3 . They do not have a direct relationship with the present invention, and a detailed explanation is therefore omitted. As the number of the “plurality of data transmissions”, a selected number can be determined in advance between the terminal  1  and the terminal  3 . In the example shown in  FIG. 3 , when the terminal  3  receives data from the terminal  1  four times, the terminal  3  sends BlockAck back to the terminal  1  in response to the fourth reception. 
         [0110]    The terminal  2  performs reception processing on the signals transmitted from the terminal  1 , and while extracting only the data S 0  addressed to the terminal itself, the terminal  2  sequentially sends back the data “Ack” indicating successful reception to the terminal  1 . Further, at a timing when data addressed to the terminal itself is not present (for example, at the timing when the data “&lt;terminal  3 - 4 &gt;” is transmitted from the terminal  1 ), the terminal  2  remains in a standby state in a similar manner to the existing operation. 
         [0111]    The terminal  3  performs reception processing on the signals transmitted from the terminal  1 , and while extracting only the data S 1  addressed to the terminal itself, the terminal  3  waits to send the data “Ack” indicating successful reception. After receiving a determined number of data, which is determined in advance with the terminal  1 , the terminal  3  collectively sends back the data “BlockAck”. Note that, the number of received data for sending back BlockAck is “ 4 ” in  FIG. 3 . However, there are some cases in which the terminal  3  fails to receive the data “&lt;terminal  3 - 1 &gt;” to “&lt;terminal  3 - 3 &gt;” and the data “&lt;terminal  3 - 4 &gt;”. In such cases, the number of the received data may be different from the determined number of data that has been determined with the terminal  1 . When there is a difference, occurrence of conflict with Ack that is sequentially sent back from the terminal  2  is conceivable. Therefore, in order to avoid the conflict, the terminal  3  may send the data “BlockAck” back to the terminal  1  only when the data addressed to the terminal itself is sent (in  FIG. 3 , when the data “&lt;terminal  3 - 4 &gt;” is transmitted from the terminal  1 ). 
         [0112]    With the use of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention shown in  FIG. 3 , it is possible to avoid the conflict between Acks sent back from the terminal  2  and the terminal  3 . Further, with the use of the above-described transmission/reception timing relationship, it is possible to simultaneously transmit data to the terminal  2  and the terminal  3 , while the transmission rate to each terminal is maintained evenly. Particularly, when the modulation scheme assigned to the data S 1  that is addressed to the terminal  3  is higher for transmission to the terminal  3  than for transmission to the terminal  2 , an overall transmission rate at which data can be transmitted from the terminal  1  can be maintained at a high level. Thus, frequency use efficiency is expected to be improved. 
         [0113]    The transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention is explained above with reference to  FIG. 3 . Next, another transmission/reception timing relationship will be explained.  FIG. 4  is an explanatory diagram illustrating another example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention. Hereinafter, the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 4 . 
         [0114]      FIG. 4  illustrates a case in which a reception error occurs in a portion of data in the transmission/reception timing relationship shown in  FIG. 3 . 
         [0115]    First, a notation system used in  FIG. 4  will be explained. Note that an explanation of the same portions as those in  FIG. 3  is omitted. The data “&lt;terminal  2 - 3 &gt;” indicates that it has not been correctly transmitted from the terminal  1  to the terminal  2  due to some reason such as noise, and a reception error has occurred. Note that the characters “[reception error]” are added to the data “&lt;terminal  2 - 3 &gt;” as annotation. 
         [0116]    Next, referring to  FIG. 4 , the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained as a time series. 
         [0117]    First, the terminal  1  transmits data including the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 1 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 1 &gt;” via the antennas  310   a  and  310   b.  At this time, the MIMO reception operation portion  240  of the terminal  2  (the receiver  200 ) disregards or discards the data addressed to the terminal  3  (the receiver  300 ), and the MIMO reception operation portion  340  of the terminal  3  disregards or discards the data addressed to the terminal  2 . 
         [0118]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0119]    In response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 2 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 2 &gt;” via the antennas  310   a  and  310   b.    
         [0120]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0121]    Next, in response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 3 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 3 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  Here, the terminal  2  fails to receive the data “&lt;terminal  2 - 3 &gt;” due to some reason such as noise, and a reception error occurs. On the other hand, the terminal  3  receives the data “&lt;terminal  3 - 3 &gt;” via the antennas  310   a  and  310   b.    
         [0122]    The terminal  2  does not send the data “&lt;Ack 1 &gt;” back to the terminal  1 , because the data “&lt;terminal  2 - 3 &gt;” transmitted from the terminal  1  has not been received correctly. Although the terminal  1  is waiting for the transmission of the data “&lt;Ack 1 &gt;” from the terminal  2 , if the data “&lt;Ack 1 &gt;” is not transmitted for a predetermined time, the terminal  1  performs time out processing and proceeds to the next operation. 
         [0123]    Next, the terminal  1  transmits only the data “&lt;terminal  3 - 4 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  3  receives the data “&lt;terminal  3 - 4 &gt;” via the antennas  310   a  and  310   b.  Next, the terminal  3  transmits the data “&lt;BlockAck 1 &gt;” from the antennas  310   a  and  310   b,  in order to notify the terminal  1  that the data “&lt;terminal  3 - 1 &gt;” to “&lt;terminal  3 - 4 &gt;” have been received successfully. The terminal  1  receives the data “&lt;BlockAck 1 &gt;” transmitted from the terminal  3 , via the antennas  110   a  and  110   b.  Note that, in some cases, the terminal  2  may receive the data “&lt;terminal  3 - 4 &gt;” via the antennas  210   a  and  210   b . However, this is not the data addressed to the terminal  2 . Therefore, even if the terminal  2  receives the data “&lt;terminal  3 - 4 &gt;”, the terminal  2  disregards or discards that data. 
         [0124]    Then, the terminal  1  retransmits, from the antennas  110   a  and  110   b , the data “&lt;terminal  2 - 3 &gt;” for the terminal  2  that has not been received correctly by the terminal  2  due to a transmission error. Further, the terminal  1  transmits the data “&lt;terminal  3 - 5 &gt;” for the terminal  3  simultaneously from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 3 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 5 &gt;” via the antennas  310   a  and  310   b.    
         [0125]    This time, the terminal  2  receives the data “&lt;terminal  2 - 3 &gt;” correctly. Therefore, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 3 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0126]    Note that, in the example shown in  FIG. 4 , the terminal  2  may receive the data “&lt;BlockAck 1 &gt;” transmitted by the terminal  3  for the terminal  1 . However, in this case, the terminal  2  disregards or discards that data. Further, the terminal  3  may receive the data “&lt;Ack 1 &gt;” transmitted by the terminal  2  for the terminal  1 . Similarly, in this case, the terminal  3  disregards or discards that data. 
         [0127]    In this manner, in the wireless communication system according to the embodiment of the present invention, if a reception error occurs at the transmission destination, whether to retransmit data or to transmit data only to another terminal can be determined based on the timing of the occurrence of the reception error. 
         [0128]    The other example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention is explained above with reference to  FIG. 4 . Next, yet another transmission/reception timing relationship will be explained.  FIG. 5  is an explanatory diagram illustrating yet another example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention. Hereinafter, the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 5 . 
         [0129]    A notation system used in  FIG. 5  is the same as the notation system used in  FIG. 3  and  FIG. 4 , and a detailed explanation is therefore omitted.  FIG. 5  illustrates another example of the case in which a reception error occurs in a portion of data in the transmission/reception timing relationship shown in  FIG. 3 . 
         [0130]    Next, referring to  FIG. 5 , the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained as a time series. 
         [0131]    First, the terminal  1  transmits data including the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 1 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 1 &gt;” via the antennas  310   a  and  310   b.  At this time, the MIMO reception operation portion  240  of the terminal  2  (the receiver  200 ) disregards or discards the data addressed to the terminal  3  (the receiver  300 ), and the MIMO reception operation portion  340  of the terminal  3  disregards or discards the data addressed to the terminal  2 . 
         [0132]    Next, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0133]    In response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , continuously, the terminal  1  transmits the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  and the data “&lt;terminal  3 - 2 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  Here, the terminal  2  fails to receive the data “&lt;terminal  2 - 2 &gt;” due to some reason such as noise. On the other hand, the terminal  3  receives the data “&lt;terminal  3 - 2 &gt;” via the antennas  310   a  and  310   b.    
         [0134]    The terminal  2  does not send the data “&lt;Ack 1 &gt;” back to the terminal  1 , because the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1  has not been received correctly. Although the terminal  1  is waiting for the transmission of the data “&lt;Ack 1 &gt;” from the terminal  2 , if the data “&lt;Ack 1 &gt;” is not transmitted for a predetermined time, the terminal  1  performs time out processing and proceeds to the next operation. 
         [0135]    Continuously, the terminal  1  transmits data to the terminal  2  and the terminal  3 . 
         [0000]    However, the data “&lt;terminal  2 - 2 &gt;” to be transmitted to the terminal  2  from the terminal  1  has not been transmitted successfully. Therefore, the terminal  1  retransmits the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  from the antennas  110   a  and  110   b.  At the same time, the terminal  1  transmits the data “&lt;terminal  3 - 3 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” via the antennas  210   a  and  210   b,  and the terminal  3  receives the data “&lt;terminal  3 - 3 &gt;” via the antennas  310   a  and  310   b.    
         [0136]    This time, the terminal  2  has received the data “&lt;terminal  2 - 2 &gt;” correctly. Therefore, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0137]    Next, in response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits only the data “&lt;terminal  3 - 4 &gt;” for the terminal  3  from the antennas  110   a  and  110   b.  The terminal  3  receives the data “&lt;terminal  3 - 4 &gt;” via the antennas  310   a  and  310   b.  Next, the terminal  3  transmits the data “&lt;BlockAck 1 &gt;” from the antennas  310   a  and  310   b,  in order to notify the terminal  1  that the data “&lt;terminal  3 - 1 &gt;” to “&lt;terminal  3 - 4 &gt;” have been received successfully. The terminal  1  receives the data “&lt;BlockAck 1 &gt;” transmitted from the terminal  3 , via the antennas  110   a  and  110   b.  Note that, in some cases, the terminal  2  may receive the data “&lt;terminal  3 - 4 &gt;” via the antennas  210   a  and  210   b . However, this is not the data addressed to the terminal  2 . Therefore, even if the terminal  2  receives the data “&lt;terminal  3 - 4 &gt;”, the terminal  2  disregards or discards that data. 
         [0138]    Specific operations will be explained separately for each terminal. 
         [0139]    When data transmission to the terminal  2  fails, the terminal  1  performs processing for retransmission to the terminal  2 , independently from transmission to the terminal  3  (without being affected by the order of the signal series based on the packet number), and performs data transmission to the terminal  3  following the previous transmission. Note that the terminal  1  may perform the same processing also when retransmission to the terminal  3  occurs by the data “BlockAck” from the terminal  3 . 
         [0140]    In this manner, in the wireless communication system according to the embodiment of the present invention, if a reception error occurs at the transmission destination, whether to retransmit data concurrently with data transmission to another terminal, or to transmit data only to the other terminal can be determined based on the timing of the occurrence of the reception error. 
         [0141]    The yet another example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention is explained above with reference to  FIG. 5 . Next, a further transmission/reception timing relationship will be explained.  FIG. 6  is an explanatory diagram illustrating a further example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention. Hereinafter, the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 6 . 
         [0142]    A notation system used in  FIG. 6  is the same as the notation system used in  FIG. 3  to  FIG. 5 , and a detailed explanation is therefore omitted.  FIG. 6  illustrates a case in which data is transmitted from the terminal  3  to the terminal  1 , and the terminal  1  sends data back to the terminal  3  at the same time as transmitting data to the terminal  2 . 
         [0143]    Next, referring to  FIG. 6 , the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention will be explained as a time series. 
         [0144]    First, the terminal  3  transmits the data “&lt;terminal  1 - 1 &gt;” for the terminal  1  from the antennas  310   a  and  310   b.  The terminal  1  receives the data “&lt;terminal  1 - 1 &gt;” transmitted from the terminal  3  via the antennas  110   a  and  110   b.    
         [0145]    The terminal  1  transmits the data “&lt;Ack 3 &gt;” from the antennas  110   a  and  110   b,  in order to notify the terminal  3  that the data “&lt;terminal  1 - 1 &gt;” transmitted from the terminal  3  has been received correctly. At the same time, the terminal  1  transmits the data “&lt;terminal  2 - 1 &gt;” for the terminal  2  from the antennas  110   a  and  110   b.    
         [0146]    The terminal  2  receives the data “&lt;terminal  2 - 1 &gt;” transmitted from the terminal  1 , via the antennas  210   a  and  210   b.  The terminal  3  receives the data “&lt;Ack 3 &gt;” sent back from the terminal  1 , via the antennas  310   a  and  310   b.  Then, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 1 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0147]    Next, in response to the reception of the data “&lt;Ack 3 &gt;” sent back from the terminal  1 , the terminal  3  transmits the data “&lt;terminal  1 - 2 &gt;” for the terminal  1  from the antennas  310   a  and  310   b.  The terminal  1  receives the data “&lt;terminal  1 - 2 &gt;” transmitted from the terminal  3  via the antennas  110   a  and  110   b.    
         [0148]    The terminal  1  transmits the data “&lt;Ack 3 &gt;” from the antennas  110   a  and  110   b,  in order to notify the terminal  3  that the data “&lt;terminal  1 - 2 &gt;” transmitted from the terminal  3  has been received correctly. At the same time, in response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 2 &gt;” for the terminal  2  from the antennas  110   a  and  110   b.    
         [0149]    The terminal  2  receives the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1 , via the antennas  210   a  and  210   b.  The terminal  3  receives the data “&lt;Ack 3 &gt;” sent back from the terminal  1 , via the antennas  310   a  and  310   b.  Then, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 2 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0150]    Next, in response to the reception of the data “&lt;Ack 3 &gt;” sent back from the terminal  1 , the terminal  3  transmits the data “&lt;terminal  1 - 3 &gt;” for the terminal  1  from the antennas  310   a  and  310   b.  The terminal  1  receives the data “&lt;terminal  1 - 3 &gt;” transmitted from the terminal  3 , via the antennas  110   a  and  110   b.    
         [0151]    The terminal  1  transmits the data “&lt;Ack 3 &gt;” from the antennas  110   a  and  110   b,  in order to notify the terminal  3  that the data “&lt;terminal  1 - 3 &gt;” transmitted from the terminal  3  has been received correctly. At the same time, in response to the reception of the data “&lt;Ack 1 &gt;” sent back from the terminal  2 , the terminal  1  transmits the data “&lt;terminal  2 - 3 &gt;” for the terminal  2  from the antennas  110   a  and  110   b.    
         [0152]    The terminal  2  receives the data “&lt;terminal  2 - 3 &gt;” transmitted from the terminal  1 , via the antennas  210   a  and  210   b.  The terminal  3  receives the data “&lt;Ack 3 &gt;” sent back from the terminal  1 , via the antennas  310   a  and  310   b.  Then, the terminal  2  transmits the data “&lt;Ack 1 &gt;” from the antennas  210   a  and  210   b,  in order to notify the terminal  1  that the data “&lt;terminal  2 - 3 &gt;” transmitted from the terminal  1  has been received successfully. The terminal  1  receives the data “&lt;Ack 1 &gt;” transmitted from the terminal  2 , via the antennas  110   a  and  110   b.    
         [0153]    Specific operations will be explained separately for each terminal. 
         [0154]    At the terminal  1 , the general protocol, the “method to transmit the next data when Ack is received” is used for data transmission to the terminal  2 . Further, the general protocol, the “method to send back Ack when data is received successfully” is used for data reception from the terminal  3 . However, unlike the known wireless communication system, when the terminal itself transmits the data “Ack”, it generates a transmission signal by combining simultaneous data transmission to another terminal. 
         [0155]    The terminal  2  performs reception processing on the signals transmitted from the terminal  1 , and while extracting only the data S 0  addressed to the terminal itself, the terminal  2  sequentially sends back Ack indicating successful reception. Note that, in  FIG. 6 , when data that is not addressed to the terminal itself is transmitted, for example, when the data “&lt;terminal  1 -*&gt; is transmitted, the terminal  2  remains in a standby state in a similar manner to the existing operation. 
         [0156]    At the terminal  3 , the general protocol, the “method to transmit the next data when Ack is received” is used for data transmission to the terminal  1 . 
         [0157]    With the use of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention shown in  FIG. 6 , data transmission to another terminal can be performed at the transmission timing when the data “Ack” is sent back. Thus, it is possible to eliminate waiting time for avoiding interference. Particularly, when the modulation schemes assigned to the data S 0  and S 1  for the terminal  2  are significantly different from each other (for example, when S 0 &gt;S 1 ), even if the data “Ack” for the terminal  3  is assigned to the data S 1 , reduction in the transmission rate per packet for the terminal  2  is suppressed. Further, with the use of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention, it is possible to perform data transmission at shorter intervals. Thus, frequency use efficiency is expected to be improved. 
         [0158]    The further example of the transmission/reception timing relationship in the wireless communication system according to the embodiment of the present invention is explained above with reference to  FIG. 6 . Next, a modified example of the wireless communication system according to the embodiment of the present invention will be explained.  FIG. 7  is an explanatory diagram illustrating a modified example of the wireless communication system according to the embodiment of the present invention. Hereinafter, the modified example of the wireless communication system according to the embodiment of the present invention will be explained with reference to  FIG. 7 . 
         [0159]    3. Explanation of a Modified Example of the Wireless Communication System According to the Embodiment of the Present Invention 
         [0160]    As shown in  FIG. 7 , the modified example of the wireless communication system according to the embodiment of the present invention includes the transmitter  100  and a receiver  200 ′. 
         [0161]    The receiver  200 ′ includes the antennas  210   a  and  210   b,  antennas  211   a  and  211   b,  the data input/output terminal  220 , the MIMO reception operation portion  240 , a MIMO reception operation portion  241 , and a coupler  260 . 
         [0162]    Here, referring to  FIG. 7 , a connection relationship of the receiver  200 ′ will be explained. The antennas  210   a  and  210   b  are connected to the MIMO reception operation portion  240  provided inside the receiver  200 ′. The antennas  211   a  and  211   b  are connected to the MIMO reception operation portion  241  provided inside the receiver  200 ′. The MIMO reception operation portions  240  and  241  are connected to the coupler  260 . The coupler  260  is connected to the data input/output terminal  220 . 
         [0163]    Next operations of the transmitter  100  and the receiver  200 ′ will be explained. 
         [0164]    In the transmitter  100 , data S for transmitting to the receiver  200 ′ is input to a divider  150  provided inside the transmitter  100 . The divider  150  divides the data S into data S 0  and data S 1  based on the ratio between a modulation scheme for S 0  and a modulation scheme for S 1 . The modulation scheme for S 0  is derived in advance by the channel matrix H between the transmitter  100  and the receiver  200 ′ on the MIMO reception processing portion  240  side. The modulation scheme for S 1  is determined separately in advance between the transmitter  100  and the receiver  200 ′ on the MIMO reception processing portion  241  side. The data S 0  and the data S 1  divided by the divider  150  are sent to the MIMO transmission operation portion  130 . 
         [0165]    The MIMO transmission operation portion  130  performs matrix operation processing on the data S 0  that is mapped according to the modulation scheme assigned based on the concept of the SVD-MIMO system, using the matrix V that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  100  and the receiver  200 ′ on the MIMO reception operation portion  240  side. At the same time, the MIMO transmission operation portion  130  performs the matrix operation processing, using the matrix V, also on the data S 1  that is mapped according to the modulation scheme that is determined separately in advance between the transmitter  100  and the receiver  200 ′ on the MIMO reception operation portion  241  side. The MIMO transmission operation portion  130  generates transmission signals x′ to be transmitted from the antennas  110   a  and  110   b,  by performing the matrix operation processing. The transmission signals x′ generated by the MIMO transmission operation portion  130  are sent to the antennas  110   a  and  110   b,  and output to wireless transmission paths. 
         [0166]    The receiver  200 ′ on the MIMO reception operation portion  240  side sends, to the MIMO reception operation portion  240 , received signals y 0 ′ that are received by the antennas  210   a  and  210   b  through the wireless communication paths. The MIMO reception operation portion  240  performs matrix operation processing on the input received signals y 0 ′, using the matrix U that is obtained in advance by performing the singular value decomposition of the channel matrix H between the transmitter  100  and the receiver  200 ′ on the MIMO reception operation portion  240  side (or using an inverse matrix calculated from the received signals y 0 ′). Then, among the data obtained as a result of the matrix operation processing performed by the MIMO reception operation portion  240 , the MIMO reception operation portion  240  outputs, to the coupler  260 , only the data S 0  that is data addressed to the receiver  200 ′ on the MIMO reception operation portion  240  side. 
         [0167]    Further, the receiver  200 ′ on the MIMO reception operation portion  241  side sends, to the MIMO reception operation portion  241 , received signals y 1 ′ that are received by the antennas  211   a  and  211   b  through the wireless communication paths. The MIMO reception operation portion  241  performs matrix operation processing on the input received signals y 1 ′, using an inverse matrix calculated from the received signals y 1 ′. Among the data obtained as a result of the matrix operation processing performed by the MIMO reception operation portion  241 , the MIMO reception operation portion  241  outputs, to the coupler  260 , only the data S 1  that is data addressed to the receiver  200 ′ on the MIMO reception operation portion  241  side. 
         [0168]    The coupler  260  combines the data series of the data S 0  input from the MIMO reception operation portion  240  and the data S 1  input from the MIMO reception operation portion  241 , in accordance with the ratio between the modulation schemes used for modulating the data S 0  and the data S 1 , respectively. The obtained data that has been combined by the coupler  260  is output to the data input/output terminal  220 . 
         [0169]    It should be noted herein that, when only the data S 0  that is the data addressed to the receiver  200 ′ on the MIMO reception operation portion  240  side is output by, for example, the transmitter  100 , data that describes identification information indicating that the data S 0  is addressed to the receiver  200 ′ on the MIMO reception operation portion  240  side may be added to the beginning portion of the data S 0 . In a similar way, when only the data S 1  that is the data addressed to the receiver  200 ′ on the MIMO reception operation portion  241  side is output, data that describes identification information indicating that the data S 1  is addressed to the receiver  200 ′ on the MIMO reception operation portion  241  side may be added to the beginning portion of the data S 1 . 
         [0170]    The configuration of the modified example of the wireless communication system according to the embodiment of the present invention is described above. The above-described operation of the wireless communication system according to the embodiment of the present invention can be applied as it is to the modified example of the wireless communication system according to the embodiment of the present invention shown in  FIG. 7 . More specifically, if the receiver  200 ′ on the MIMO reception operation portion  240  side is taken as the receiver  200  shown in  FIG. 1 , and the receiver  200 ′ on the MIMO reception operation portion  241  side is taken as the receiver  300  shown in  FIG. 1 , the above-described operation of the wireless communication system according to the embodiment of the present invention can be applied as it is. Moreover, when the receiver  200 ′ sends the data “Ack” back to the transmitter  100 , the data “Ack 1 ” indicating correct reception by the MIMO reception operation portion  240  and the data “BlockAck 1 ” indicating correct reception by the MIMO reception operation portion  241  may be shared in the receiver  200 ′ so as to combine each other&#39;s information, and the receiver  200 ′ may transmit the combined data from the antennas  210   a  and  210   b  or from the antennas  211   a  and  211   b.    
         [0171]    Note that, in the present embodiment, one transmitter and two receivers are provided and two data streams are used for explanation. However, it is needless to mention that the present invention is not limited to this example. Further, it is needless to mention that the number of the antennas of the transmitter  100  or the receivers  200  and  300  is not limited to two. Furthermore, in the present embodiment, error correction processing may be performed on the two data streams at an appropriate point (for example, in the divider  150  or the coupler  260 ). 
         [0172]    Moreover, the operations of the transmitter  100  and the receivers  200  and  300  according to the embodiment of the present invention may be performed such that a read only memory (ROM) and another storage medium that store computer programs are provided inside the transmitter  100  and the receivers  200  and  300 , and a central processing unit (CPU) and another control unit provided inside the transmitter  100  and the receivers  200  and  300  sequentially read and execute the computer programs. 
         [0173]    4. Conclusion 
         [0174]    As described above, the wireless communication system according to the embodiment of the present invention makes it possible to simultaneously transmit different data to a plurality of terminals on the receiving side. Thus, transmission power can be used for data transmission more efficiently. In addition, it is possible to form an efficient communication protocol, while achieving efficient data transmission. Therefore, the wireless communication system according to the embodiment of the present invention makes it possible to improve communication quality and communication efficiency, as compared to the known wireless communication system. 
         [0175]    The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2008-264110 filed in the Japan Patent Office on Oct. 10, 2008, the entire contents of which is hereby incorporated by reference. 
         [0176]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.