Abstract:
A communication apparatus includes a radio portion ( 100 ) configured to transmit a modulated transmission signal and to receive a modulated reception signal. The radio portion ( 100 ) includes a transmission and reception antenna ( 103 ) configured to transmit the modulated transmission signal and to receive the modulated reception signal; a reception antenna ( 105 ) configured to receive the modulated reception signal; a transmission filter ( 107 ) interposed between a modulation and demodulation circuit ( 101 ) and the transmission and reception antenna ( 103 ), and configured to pass the modulated transmission signal; a reception filter ( 109 ) interposed between the transmission and reception antenna ( 103 ) and the modulation and demodulation circuit ( 101 ) and between the reception antenna ( 105 ) and the modulation and demodulation circuit ( 101 ), and configured to pass the modulated reception signal; a first circulator ( 111 ) configured to direct the modulated reception signal from the transmission and reception antenna ( 103 ) to the reception filter ( 109 ); and a second circulator ( 113 ) configured to direct the modulated reception signal from the reception antenna ( 105 ) to the reception filter ( 109 ).

Description:
TECHNICAL FIELD 
     The present invention relates to radio communication apparatuses that communicate using frequency division duplexing (FDD) in a radio communication system and, in particular, to radio communication apparatuses that communicate using MIMO (Multiple Input and Multiple Output). 
     BACKGROUND ART 
     In FDD communication, different frequencies are used in transmission and reception. For this reason, a radio communication apparatus needs a transmission antenna and a transmission filter which passes a transmission frequency band, and a reception antenna and a reception filter which passes a reception frequency band. 
     If MIMO, in which plural radio transmission channels and plural radio reception channels are used simultaneously and in parallel, is applied in FDD, the communication apparatus needs the transmission antennas and the transmission filters which pass the transmission frequency band equal to the number of the transmission channels, and the reception antennas and the reception filters which pass the reception frequency band equal to the number of the reception channels. 
     Therefore, increase of the transmission channels and the reception channels in order to accelerate communication speed requires the antennas and the filters equal to the increased number of the channels. As a result, there is a problem that circuit area for installation of the antennas and the filters increases. 
     To cope with the problem, for instance, a technique disclosed in Patent Document 1 may be applied. Patent Document 1 discloses a multiband radio communication apparatus that includes plural antennas, plural filters, plural duplexers, a modulation circuit, and a demodulation circuit. Because a duplexer is connected to a transmission circuit and a reception circuit is connected to one antenna, the apparatus is able to execute both transmission and reception using the one antenna. In the above-mentioned manner, the number of the antennas can be reduced. 
     CITATION LIST 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2007-19939 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, according to the technique in Patent Document 1, the transmission filters equal to the number of the transmission channels and the reception filters equal to the number of the reception channels are required. As a result, even if the technique stated in this Document is applied to MIMO, there remains a problem that the installation area of the filters cannot be reduced. 
     Accordingly, the present invention provides a radio communication apparatus which includes a radio communication unit configured to reduce the installation area of the filters. 
     Solution to Problem 
     In accordance with an aspect of the present invention, a communication apparatus includes: a modulator configured to modulate a transmission signal with a transmission frequency to generate a modulated transmission signal; a demodulator configured to demodulate a modulated reception signal with a reception frequency different from the transmission frequency to generate a reception signal; and at least one transmission and reception set connected to the modulator and the demodulator, and configured to transmit the modulated transmission signal and to receive the modulated reception signal. The transmission and reception set includes: a transmission and reception antenna configured to transmit the modulated transmission signal and to receive the modulated reception signal; a reception antenna configured to receive the modulated reception signal; a transmission filter interposed between the modulator and the transmission and reception antenna, and configured to pass the modulated transmission signal generated by the modulator; a reception filter interposed between the transmission and reception antenna and the demodulator and between the reception antenna and the demodulator, and configured to pass the modulated reception signal received at the transmission and reception antenna and the reception antenna; a first circulator configured to direct the modulated reception signal received at the transmission and reception antenna to the reception filter; and a second circulator configured to direct the modulated reception signal received at the reception antenna to the reception filter. 
     In the configuration, since the one reception filter can be shared with the signal received at the transmission and reception antenna and the signal received at the reception antenna, the reception filter may not be required with respect to each antenna to receive. As a result, the number of the reception filters can be reduced. 
     Preferably, the first circulator is configured to direct the modulated reception signal to the demodulator, the modulated reception signal having passed through the reception filter from the reception antenna via the second circulator, and the second circulator is configured to direct the modulated reception signal to the demodulator, the modulated reception signal having passed through the reception filter from the transmission and reception antenna via the first circulator. 
     In the configuration, the first circulator directs not only the modulated reception signal received at the transmission and reception antenna to the reception filter, but the modulated reception signal which has passed through the reception filter from the reception antenna via the second circulator to the demodulator. The second circulator directs not only the modulated reception signal received at the reception antenna to the reception filter, the modulated reception signal which has passed through the reception filter from the transmission and reception antenna via the first circulator to the demodulator. That is, each circulator can be used for two purposes. As a result, in comparison to cases in which respective wires are set up, the number of wires can be reduced. 
     Preferably, the radio communication apparatus according to the present invention includes plural transmission and reception sets. 
     In the configuration, if the number of the transmission and reception sets of the radio communication apparatus increases and thus the number of the antennas to receive increases, the number of the reception filters can be reduced up to half the number of the antennas to receive. 
     Preferably, the modulator is configured to generate a first modulated transmission signal by modulating a transmission signal with a first transmission frequency and to generate a second modulated transmission signal by modulating a transmission signal with a second transmission frequency, the demodulator is configured to generate a reception signal by demodulating a modulated reception signal with a first reception frequency different from the first transmission frequency and to generate a reception signal by demodulating a modulated reception signal with a second reception frequency different from the second transmission frequency, the transmission and reception antenna is configured to transmit the first modulated transmission signal and the second modulated transmission signal generated by the modulator, the transmission and reception antenna is configured to receive the modulated reception signal modulated with the first reception frequency and the modulated reception signal modulated with the second reception frequency, the reception antenna is configured to receive the modulated reception signal modulated with the first reception frequency and the modulated reception signal modulated with the second reception frequency, the transmission filter comprises a first transmission filter configured to pass the first modulated transmission signal and a second transmission filter configured to pass the second modulated transmission signal, and the reception filter comprises a first reception filter configured to pass the modulated reception signal modulated by the first reception frequency and a second reception filter configured to pass the modulated reception signal modulated by the second reception frequency. Preferably, the radio communication apparatus includes: a transmission switching unit configured to activate any of the first transmission filter and the second transmission filter depending on whether the first transmission frequency or the second transmission frequency is used in transmission; and a reception switching unit configured to activate any of the first reception filter and the second reception filter depending on whether the first reception frequency or the second reception frequency is used in reception. 
     In the configuration, addition of the filters and the switching units can enable communication using plural transmission frequencies and plural reception frequencies, without extra transmission and reception sets for each of the plural transmission frequencies and the plural reception frequencies. As a result, increase of circuit area can be reduced and a circuit structure can be simplified. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows an example of a radio communication unit and a modulator-demodulator unit of a mobile station according to a first embodiment of the present invention. 
         FIG. 2  shows an example of a radio communication unit and a modulator-demodulator unit of a mobile station according to a second embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Reference is made to figures to illustrate the embodiments of the present invention. The following embodiments are not limited to, but are applicable to a radio communication system using SC-FDMA (Single Carrier Frequency Division Multiple Access) for uplink communication and OFDMA (Orthogonal Frequency Division Multiple Access) for downlink communication. 
     First Embodiment 
     A mobile station able to execute MIMO according to a first embodiment of the present invention will be described. 
       FIG. 1  shows a mobile station according to the first embodiment of the present invention. The mobile station includes radio communication units (transmission and reception sets)  100 ,  120  and a modulator-demodulator circuit  101 . The radio communication unit  100  includes a transmission and reception antenna  103 , a reception antenna  105 , a transmission filter  107 , a reception filter  109 , a first circulator  111 , a second circulator  113 , an amplifier  115 , an amplifier  117 , and an amplifier  119 . The radio communication unit  100  is connected to the modulator-demodulator circuit  101 . 
     A configuration of the radio communication unit  120  is equivalent to the configuration of the radio communication unit  100 . Specifically, the radio communication unit  120  includes a transmission and reception antenna  123 , a reception antenna  125 , a transmission filter  127 , a reception filter  129 , a first circulator  131 , a second circulator  133 , an amplifier  135 , an amplifier  137 , and amplifier  139 . The radio communication unit  120  is connected to modulator-demodulator circuit  101 . 
     The modulator-demodulator circuit  101  includes a modulator and a demodulator, which are not shown in  FIG. 1 . 
     The transmission and reception antenna  103  of the radio communication unit  100  transmits a modulated transmission signal and receives a downlink modulated reception signal. The reception antenna  105  receives a downlink modulated reception signal. The transmission filter  107  is a band-limiting filter that passes an uplink frequency. The reception filter  109  is a band-limiting filter that passes a downlink frequency. Each of the first circulator  111  and the second circulator  113  is a three-port circulator which includes three input-output ports. 
     The transmission and reception antenna  123  of the radio communication unit  120  transmits an uplink modulated transmission signal and receives a downlink modulated reception signal. The reception antenna  125  receives a downlink modulated reception signal. The transmission filter  127  is a band-limiting filter which passes the uplink frequency. The reception filter  129  is a band-limiting filter which passes the downlink frequency. Each of the first circulator  131  and the second circulator  133  is a three-port circulator which includes three input-output ports. 
     Next, a transmission process of the uplink modulated transmission signal will be described. 
     The modulator of the modulator-demodulator circuit  101  modulates two different series of the uplink transmission signals with the same uplink frequency to generate two series of the uplink modulated transmission signals. One of the generated two series of the uplink modulated transmission signals is directed to the amplifier  115  of the radio communication unit  100  to be amplified. Subsequently, the one of the series is directed to the transmission filter  107 , passes through the transmission filter  107 , and reaches the transmission and reception antenna  103 . 
     The other one of the series of the uplink modulated transmission signals generated by the modulator of the modulator-demodulator circuit  101  is directed to the amplifier  135  of the radio communication unit  120  to be amplified. Subsequently, the one of the series is directed to the transmission filter  127 , passes through the transmission filter  127 , and reaches the transmission and reception antenna  123 . 
     These two series of the uplink modulated transmission signals are transmitted simultaneously and in parallel from the transmission and reception antenna  103  and the transmission and reception antenna  123  to the base station. The two transmitted series of the uplink modulated transmission signals are spatially multiplexed and received at four reception antennas belonging to the base station, which is not illustrated in the figures. 
     Next, a reception process of the downlink modulated reception signal will be described. 
     Four different series of the downlink modulated reception signals are transmitted simultaneously from four transmission antennas belonging to the base station, which is not illustrated in the figures, at the same downlink frequency different from the uplink frequency, and spatially multiplexed. 
     The spatially multiplexed downlink modulated reception signals are received at the transmission and reception antenna  103  of the radio communication unit  100 . The downlink modulated reception signals received at the transmission and reception antenna  103  are directed to the reception filter  109  by the first circulator  111 , pass through the reception filter  109 , and are directed to the second circulator  113 . Subsequently, the downlink modulated reception signals are directed to the amplifier  119  by the second circulator  113  to be amplified, and directed to the demodulator of the modulator-demodulator circuit  101 . 
     The spatially multiplexed downlink modulated reception signals are received at the reception antenna  105  of the radio communication unit  100 . The downlink modulated reception signals received at the reception antenna  105  are directed to the reception filter  109  by the second circulator  113 , pass through the reception filter  109 , and are directed to the first circulator  111 . Subsequently, the downlink modulated reception signals are directed to the amplifier  117  by the first circulator  111  to be amplified, and directed to the demodulator of the modulator-demodulator circuit  101 . 
     Similarly, in the radio communication  120 , the downlink modulated reception signals received at the transmission and reception antenna  123  and the downlink modulated reception signals received at the reception antenna  125  are directed to the demodulator of the modulator-demodulator circuit  101 . 
     The downlink modulated reception signal transmitted from any one of the four transmission antennas belonging to the base station is received in a spatially multiplexed condition at all of the four reception antennas (i.e., the antennas  103 ,  105  of the radio communication unit  100  and the antennas  123 ,  125  of the radio communication unit  120 ) of the mobile station. In other words, each reception antenna of the mobile station receives the downlink modulated reception signals transmitted from the four transmission antennas of the base station and spatially multiplexed. 
     The spatially multiplexed downlink modulated reception signals received at each of the four reception antennas of the mobile station are respectively directed to the demodulator of the modulator-demodulator circuit  101  via the radio communication unit  100  or the radio communication unit  120  corresponding to the antenna having received the downlink modulated reception signals. 
     Each of the spatially multiplexed downlink modulated reception signals received at the four reception antennas (i.e., the antennas  103 ,  105  of the radio communication unit  100  and the antennas  123 ,  125  of the radio communication unit  120 ) has gone through one of the propagation paths, each of which has different propagation characteristics. As a result, each of the signals experiences different fading. Thus, the demodulator of the modulator-demodulator circuit  101  is able to obtain a channel matrix that contains characteristic values of plural propagation paths from the base station to the mobile station based on each of the spatially multiplexed downlink modulated reception signals. The demodulator of the modulator-demodulator circuit  101  multiplies the spatially multiplexed downlink modulated reception signals by an inversion matrix of the channel matrix to obtain the four series of the downlink modulated reception signals before spatial multiplexing. Subsequently, the demodulator of the modulator-demodulator circuit  101  demodulates the four series of the downlink modulated reception signals with the downlink frequency to generate four series of downlink reception signals. 
     As described above, according to the first embodiment, the first circulator  111  directs the downlink modulated reception signal received at the transmission and reception antenna  103  to the reception filter  109 , and the second circulator  113  directs the downlink modulated reception signal received at the reception antenna  105  to the same reception filter  109 . The same applies to the first circulator  131 , the transmission and reception antenna  123 , the reception filter  129 , the second circulator  133 , and the reception antenna  125 . That is, downlink modulated reception signals received at separate antennas pass through one reception filter, so that a reception filter is not necessary for every antenna to receive. As a result, circuit area corresponding to one reception filter can be reduced per one radio communication unit including two antennas to receive. 
     In the first embodiment described above, the first circulator  111  directs the downlink modulated reception signal to the demodulator of the modulator-demodulator circuit  101 , the downlink modulated reception signal having passed the reception filter  109  from the reception antenna  105  via the second circulator  113 . The same applies to the first circulator  131 , the reception filter  129 , the reception antenna  125 , and the second circulator  133 . The second circulator  113  directs the downlink demodulated reception signal to the demodulator of the modulator-demodulator circuit  101 , the downlink modulated reception signal having passed through the reception filter  109  from the transmission and reception antenna  103  via the first circulator  111 . The same applies to the second circulator  133 , the reception filter  129 , the transmission and reception antenna  123 , and the first circulator  131 . As described above, each circulator is used for two purposes. As a result, in comparison to cases in which respective wires are set up, the number of wires in the circuit can be preferably reduced. 
     Modification of the First Embodiment 
     However, the present invention is not limited the above-described embodiment. The downlink modulated reception signal may be directed to the demodulator of the modulator-demodulator circuit without passing through the first circulator, the downlink modulated reception signal having passed through the reception filter from the reception antenna via the second circulator. The downlink demodulated reception signal may be directed to the demodulator of the modulator-demodulator circuit without passing through the second circulator, the downlink modulated reception signal having passed through the reception filter from the transmission and reception antenna via the first circulator. 
     In the above-described first embodiment, the radio communication unit  100  includes the amplifier  117  which is interposed between the first circulator and the modulator-demodulator circuit  101  and which amplifies the downlink modulated reception signal, and the amplifier  119  which is interposed between the second circulator and the modulator-demodulator circuit  101  and which amplifies the downlink modulated reception signal. The present invention is not limited to the above-described embodiment. The modulator-demodulator  101  may include such amplifiers. Similarly, the radio communication unit  120  includes the amplifier  137  which is interposed between the first circulator and the modulator-demodulator circuit  101  and which amplifies the downlink modulated reception signal, and the amplifier  139  which is interposed between the second circulator and the modulator-demodulator circuit  101  and which amplifies the downlink modulated reception signal. The present invention is not limited to the above-described embodiment. The modulator-demodulator  101  may include such amplifiers. 
     The above-described first embodiment is applied to MIMO according to Space Division Multiplexing (SDM) in which every antenna transmits a different signal. The present invention is not limited to the above-described embodiment. For example, the embodiment may be applied to MIMO according to Eigenbeam-Space Division Multiplexing (E-SDM) in which a different signal is sent through every transmission beam, or other MIMO. 
     The above-described first embodiment is applied to 2×4 mobile station transmission MIMO in which the mobile station uses the two transmission antennas and the base station uses the four reception antennas, and to 4×4 mobile station reception MIMO in which the base station uses the four transmission antennas and the mobile station uses the four reception antennas. The present invention is not limited to the above-described embodiment. The embodiment may be applied to MIMO according to a different antenna configuration, for example, 2×2 mobile station transmission MIMO in which the mobile station uses the two transmission antennas and the base station uses the two transmission antennas. 
     In the above-described first embodiment, the one modulator-demodulator circuit includes the modulator and the demodulator. The present invention is not limited to the above-described embodiment. The modulator and the demodulator may be separately installed in the mobile station. 
     In the above-described first embodiment, the two radio communication units are installed. The present invention is not limited to the above-described embodiment. Only one radio communication unit or more than two radio communication units may be installed. 
     Second Embodiment 
     Next, a mobile station able to execute MIMO according to a second embodiment of the present invention will be described. 
       FIG. 2  shows a mobile station according to the second embodiment of the present invention. The mobile station includes radio communication units (transmission and reception sets)  200 ,  230  and a modulator-demodulator circuit  201 . The radio communication unit  200  includes a transmission and reception antenna  202 , a reception antenna  203 , a first transmission filter  205 , a second transmission filter  207 , a first reception filter  209 , a second reception filter  211 , transmission switches  213 ,  215 , reception switches  217 ,  219 , a first circulator  221 , a second circulator  223 , and amplifiers  225 ,  227 . The radio communication unit  200  is connected to the modulator-demodulator circuit  201 . 
     A configuration of the radio communication unit  230  is equivalent to the configuration of the radio communication unit  200 . The radio communication unit  230  includes a transmission and reception antenna  232 , a reception antenna  233 , a first transmission filter  235 , a second transmission filter  237 , a first reception filter  239 , a second reception filter  241 , transmission switches  243 ,  245 , reception switches  247 ,  249 , a first circulator  251 , a second circulator  253 , and amplifiers  255 ,  257 . The radio communication unit  200  is connected to the modulator-demodulator circuit  201 . 
     The modulator-demodulator circuit  201  includes a modulator and a demodulator, which are not shown in  FIG. 2 . 
     The transmission and reception antenna  202  of the radio communication unit  200  transmits first and second uplink modulated transmission signals and receives first and second downlink modulated reception signals. The reception antenna  203  receives first and second downlink modulated reception signals. The first transmission filter  205  is a band-limiting filter which passes first uplink frequency. The second transmission filter  207  is a band-limiting filter which passes second uplink frequency. The first reception filter  209  is a band-limiting filter that passes the first downlink frequency. The second reception filter  211  is a band-limiting filter that passes the second downlink frequency. Each of the first circulator  221  and the second circulator  223  is a three-port circulator which includes three input-output ports. 
     The transmission and reception antenna  232  of the radio communication unit  230  transmits first and second uplink modulated transmission signals and receives first and second downlink modulated reception signals. The reception antenna  233  receives first and second downlink modulated reception signals. The first transmission filter  235  is a band-limiting filter which passes the first uplink frequency. The second transmission filter  237  is a band-limiting filter which passes the second uplink frequency. The first reception filter  239  is a band-limiting filter which passes the first downlink frequency. The second reception filter  241  is a band-limiting filter which passes the second downlink frequency. Each of the first circulator  251  and the second circulator  253  is a three-port circulator which includes three input-output ports. 
     The first uplink frequency, the second uplink frequency, the first downlink frequency, and the second downlink frequency are different from each other. 
     Next, a transmission process of the uplink modulated transmission signal will be described. First, a case in which the first uplink frequency is used will be described. 
     The modulator of the modulator-demodulator circuit  201  modulates two different series of the uplink transmission signals with the first uplink frequency to generate two series of the first uplink modulated transmission signals. In accordance with application of the first uplink frequency in uplink transmission, the transmission switch  215  directs one of the generated series of the first uplink modulated transmission signals to the amplifier  225  to be amplified. Subsequently, the one of the series is directed to the first transmission filter  205  passing the first uplink frequency, passes through the first transmission filter  205 , and reaches the transmission and reception antenna  202  directed by the transmission switch  213 . 
     The other one of the series of the first uplink modulated transmission signals generated by the modulator of the modulator-demodulator circuit  201  is directed to the transmission switch  245 . In accordance with application of the first uplink frequency in uplink transmission, the transmission switch  245  directs the other one of the generated series of the first uplink modulated transmission signals to the amplifier  255  to be amplified. Subsequently, the other one of the series is directed to the first transmission filter  235  passing the first uplink frequency, passes the first transmission filter  235 , and reaches the transmission and reception antenna  232  directed by the transmission switch  243 . 
     These two series of the first uplink modulated transmission signals are transmitted simultaneously and in parallel from the transmission and reception antenna  202  and the transmission and reception antenna  232  to the base station. The two transmitted series of the first uplink modulated transmission signals are spatially multiplexed and received at four reception antennas belonging to the base station, which is not illustrated in the figures. 
     Next, a case in which the second uplink frequency is used will be described. The case is basically equivalent to the transmission process in which the first uplink frequency is used. A different point is that each transmission switch directs the second uplink modulated transmission signals to be transmitted to each second transmission filter in accordance with application of the second uplink frequency. 
     A reception process of the downlink modulated reception signal will be described. First, a case in which the first downlink frequency is used will be described. 
     Four different series of the downlink modulated reception signals are transmitted simultaneously from four transmission antennas belonging to the base station, which is not illustrated in the figures, at the first downlink frequency, and spatially multiplexed. 
     The first downlink modulated reception signals modulated with the first downlink frequency are received at the transmission and reception antenna  202  of the radio communication unit  200 . The first downlink modulated reception signals received at the transmission and reception antenna  202  are directed to the reception switch  217  by the first circulator  221 . In accordance with application of the first downlink frequency in downlink reception, the reception switch  217  directs the received first downlink modulated reception signals to the first reception filter  209  which passes the first downlink frequency. After passing the first reception filter  209 , the first downlink modulated reception signals are directed reception switch  219 . The reception switch  219  directs the first downlink modulated reception signals to the second circulator  223 . The first downlink modulated reception signals are directed to the demodulator of the modulator-demodulator circuit  201  by the second circulator  223 . The modulator-demodulator circuit  201  includes an amplifier which amplifies the first downlink modulated reception signals and which is not shown in the  FIG. 2 . Alternatively, such an amplifier may be installed in the radio communication unit  200 . 
     The first downlink modulated reception signals modulated with the first downlink frequency are received at the reception antenna  203  of the radio communication unit  200 . The first downlink modulated reception signals received at the reception antenna  203  are directed to the reception switch  219  by the second circulator  223 . In accordance with application of the first downlink frequency in downlink reception, reception switch  219  directs the received first downlink modulated reception signals to the first reception filter  209  which passes the first downlink frequency. After passing the first reception filter  209 , the first downlink modulated reception signals are directed to the reception switch  217 . The reception switch  217  directs the first downlink modulated reception signals to the first circulator  221 . The first downlink modulated reception signals are directed to the demodulator of the modulator-demodulator circuit  201  by the first circulator  221 . The modulator-demodulator circuit  201  includes an amplifier which amplifies the first downlink modulated reception signals and which is not shown in the  FIG. 2 . Alternatively, such an amplifier may be installed in the radio communication unit  200 . 
     Similarly, in the radio communication unit  230 , the first downlink modulated reception signals received at the transmission and reception antenna  232  and the first downlink modulated reception signals received at the reception antenna  233  are directed to the modulator of the modulator-demodulator circuit  201 . The modulator-demodulator circuit  201  includes amplifiers which amplify the first downlink modulated reception signals and which are not shown in the  FIG. 2 . Alternatively, such an amplifier may be installed in the radio communication unit  230 . 
     The first downlink modulated reception signal transmitted at the first downlink frequency from any one of the four transmission antennas belonging to the base station is received in a spatially multiplexed condition at all of the four reception antennas (i.e., the antennas  202 ,  203  of the radio communication unit  200  and the antennas  232 ,  233  of the radio communication unit  230 ) of the mobile station. In other words, each reception antenna of the mobile station receives the first downlink modulated reception signals transmitted at the first downlink frequency from the four transmission antennas of the base station and spatially multiplexed. 
     The spatially multiplexed first downlink modulated reception signals received at each of the four reception antennas of the mobile station are respectively directed to the demodulator of the modulator-demodulator circuit  201  via the radio communication unit  200  or the radio communication unit  230  corresponding to the antenna having received the downlink modulated reception signals. 
     Each of the spatially multiplexed first downlink modulated reception signals received at the four reception antennas (i.e., the antennas  202 ,  203  of the radio communication unit  200  and the antennas  232 ,  233  of the radio communication unit  230 ) has gone through one of propagation paths, each of which has different propagation characteristics. As a result, each of the signals experiences different fading. Thus, the demodulator of the modulator-demodulator circuit  201  is able to obtain a channel matrix that contains characteristic values of plural propagation paths from the base station to the mobile station based on each of the spatially multiplexed first downlink modulated reception signals. The demodulator of the modulator-demodulator circuit  201  multiplies the spatially multiplexed first downlink modulated reception signals by an inversion matrix of the channel matrix to obtain the four series of the first downlink modulated reception signals before spatial multiplexing. Subsequently, the demodulator of the modulator-demodulator circuit  201  demodulates the four series of the first downlink modulated reception signals with the first downlink frequency to generate four series of downlink reception signals. 
     Next, a reception process in which the second downlink frequency is used will be described. The process is basically equivalent to the case in which the first downlink frequency is used. A different point is that each reception switch directs the received second downlink modulated reception signals to each second reception filter in accordance with application of the second downlink frequency. 
     As described above, according to the second embodiment, one radio communication unit corresponds to the first and second uplink frequencies and the transmission switch directs the transmission signal to the transmission filter having a frequency characteristic in which the signal should pass. One radio communication unit corresponds to the first and second downlink frequencies and the reception switch directs the reception signal to the filter having a frequency characteristic in which the signal should pass. As a result, one radio communication unit can execute signal transmission with the first and second uplink frequencies and signal reception with the first and second downlink frequencies. Therefore, in comparison to the configuration that radio communication units are installed for each frequency, increase of radio communication unit area can be reduced and a circuit structure can be simplified. 
     Modification of the Second Embodiment 
     All of the modifications in the first embodiment are applicable to the above-described second embodiment. 
     In the above-described second embodiment, each of the two frequencies (the first uplink frequency and the second uplink frequency) is used as the uplink frequency. The present invention is not limited to the above-described embodiment. More than two uplink frequencies may be used. The same applies to the downlink frequencies. 
     In the above-described second embodiment, each radio communication unit includes the two pairs of the transmission switches, and the two transmission filters each of which has a different frequency characteristic are arranged in parallel between each pair of the transmission switches. The present invention is not limited to the above-described embodiment. Instead of each pair of the transmission switches, only one of the pair of the transmission switches can be included. For example, while the radio communication unit  200  in  FIG. 2  includes the transmission switches  213 ,  215 , the communication unit  200  may include only the transmission switch  213  and the transmission switch  215  may be replaced by wires. Otherwise, the communication unit  200  may include only the transmission switch  215  and the transmission switch  213  may be replaced by wires. The same applies to the reception switches and the reception filters. 
     Third Embodiment (Application to the Base Station) 
     The above-described first and second embodiments are applied to the MIMO-adapted mobile station which transmits the modulated transmission signals to the base station at at least one uplink frequency and receives the modulated reception signals from the base station at at least one downlink frequency. The present invention is not limited to the above-described embodiments. For example, the embodiments may be applied to a MIMO-adapted base station which transmits modulated transmission signals to plural mobile stations at at least one downlink frequency and receives modulated reception signals from the plural mobile stations at at least one uplink frequency. Such application can be realized by modification to the first and second embodiments that the term “mobile station” and the term “base station” are exchanged with each other and the word “uplink” and the word “downlink” are exchanged with each other. 
     Similar to the first and second embodiments, the third embodiment is not limited to, but is applicable to a radio communication system using SC-FDMA for uplink communication and OFDMA for uplink communication. 
     A configuration of the base station, a transmission process, and a reception process according to the third embodiment are equivalent to the MIMO-adapted mobile station and its processes according to the first embodiment. Only a generation process of the modulated transmission signals executed by the modulator-demodulator circuit  101  in the transmission process and a generation process of the modulated reception signals executed by the modulator-demodulator circuit  101  in the reception process are different from the first embodiment. Accordingly, the different points will be described below. 
     The transmission process of the downlink modulated signals from the base station will be described. 
     The modulator of the modulator-demodulator circuit  101  modulates two different series of the downlink transmission signals which represent transmission data for plural users with the same downlink frequency to generate two series of the downlink OFDMA modulated transmission signals. 
     One of the generated two series of the downlink modulated transmission signals is directed to the amplifier  115  of the radio communication unit  100  and the other one of the generated two series of the downlink modulated transmission signals is directed to the amplifier  135  of the radio communication unit  120 . A process after that the generated two series of the downlink modulated signals are respectively directed to the amplifier  115  and the amplifier  135  is equivalent to the transmission process of the first embodiment. 
     Next, the reception process of the uplink modulated reception signals at the base station will be described. 
     Uplink SC-FDMA modulated reception signals are transmitted from each of four transmission antennas belonging to every mobile station, which is not illustrated, and spatially multiplexed. The transmission and reception antennas  103 ,  123  and the reception antennas  105 ,  125  receive the transmitted and spatially-multiplexed signals. A subsequent process that the uplink modulated reception signals are directed to a demodulator via reception filters is equivalent to the reception process of the first embodiment. 
     The demodulator of the modulator-demodulator circuit  201  obtains the uplink SC-FDMA modulated reception signals before spatial multiplexing based on the spatially multiplexed uplink SC-FDMA modulated reception signals. Subsequently, the demodulator of the modulator-demodulator circuit  201  demodulates the uplink modulated reception signals corresponding to respective mobile stations to generate uplink reception signals. 
     The configuration of the base station, the transmission process, and the reception process according to the third embodiment may be equivalent to the MIMO-adapted mobile station and its processes according to the second embodiment. Only a generation process of the modulated transmission signals executed by the modulator-demodulator circuit  201  in the transmission process and a generation process of the modulated reception signals executed by the modulator-demodulator circuit  201  in the reception process are different from the second embodiment in analogy with the above-described base station based on the first embodiment. 
     As described above, according to the third embodiment, the same effect achieved based on the first and second embodiments can be achieved in the base station. 
     In the above-described third embodiment, all of the modifications of the first and second embodiments are applicable when the term “mobile station” and the term “base station” are exchanged with each other and the word “uplink” and the word “downlink” are exchanged with each other. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           100 ,  120  . . . radio communication unit (transmission and reception set) 
           101  . . . modulator-demodulator circuit (modulator, demodulator) 
           103 ,  123  . . . transmission and reception antenna 
           105 ,  125  . . . reception antenna 
           107 ,  127  . . . transmission filter 
           109 ,  129  . . . reception filter 
           111 ,  131  . . . first circulator 
           113 ,  133  . . . second circulator 
           200 ,  230  . . . radio communication unit (transmission and reception set) 
           201  . . . modulator-demodulator circuit (modulator, demodulator) 
           202 ,  232  . . . transmission and reception antenna 
           203 ,  233  . . . reception antenna 
           205 ,  235  . . . first transmission filter 
           207 ,  237  . . . second transmission filter 
           209 ,  239  . . . first reception filter 
           211 ,  241  . . . second reception filter 
           221 ,  251  . . . first circulator 
           223 ,  253  . . . second circulator