Abstract:
A plurality of radio access units ( 108 - 1  to  108 - n ) form the single cell (# 120 ) of a first communication system, while forming respective individual multi-cells (# 130 - 1  to # 130 - n ) of a second communication system within the single cell (# 120 ). The plurality of radio access units ( 108 - 1  to  108 - n ) transmit a downstream signal of the first communication system input from a main unit ( 107 ) to a terminal using the first communication system of the single cell (# 120 ), while each transmitting a downstream signal of the second communication system input from the main unit ( 107 ) to a terminal using the second communication system of the corresponding respective one of the multi-cells (# 130 - 1  to # 130 - n ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a communication system, a main unit, a radio access unit and a communication method. More particularly, the present invention relates to a communication system, a main unit, a radio access unit and a communication method for making space where any radio wave signal from a radio base station installed outdoors can not be received, for example, in buildings or underground malls, usable as a coverage area. 
       BACKGROUND ART 
       [0002]    A communication system shown in  FIG. 1  is known as a conventional communication system.  FIG. 1  is a diagram showing conventional communication system  1 . Communication system  1  in  FIG. 1  is mainly formed of radio base station apparatus  2 , main unit  3 , and a plurality of handsets  4 . A plurality of handset  4  define single cell # 10 . 
         [0003]    In communication system  1  in  FIG. 1 , radio base station apparatus  2  transmits a downlink signal to main unit  3  and main unit  3  splits the downlink signal and distributes the downlink signals to a plurality of handsets  4 . Each handset  4  then transmits the distributed downlink signal to a terminal (not shown) in single cell # 10  by radio. A plurality of handsets  4  each receive an uplink signal which is a radio signal transmitted from a terminal (not shown) in single cell # 10  and transmit the received uplink signal to main unit  3 . Main unit  3  then transmits the received uplink signals to radio base station apparatus  2 . Accordingly, conventional communication system  1  can secure a wide coverage in proportion to the number of handsets since a plurality of handsets  4  define single cell # 10 . 
         [0004]    The need for improving a user throughput in packet data communication has been increasing in recent years in addition to the need for securing a wide coverage. A communication system defining only a single cell shown in,  FIG. 1  finds it difficult to meet this need. For example, a problem arises in a situation where a user performing communication involving a high occupancy rate of a radio band shared by a plurality of users exists in a single cell. In this situation, the throughputs of users communicating in the same single cell as the single cell used by the user are lowered. 
         [0005]    The system configuration shown in Patent Literature 1, which uses different communication systems in combination, may be employed as a communication system to solve the problem. According to Patent Literature 1, the system configuration allows the users in the different communication systems to coexist, thereby solving the above problem. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1 
         Japanese Patent Application Laid-Open No. 2002-252867 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    In Patent Literature 1, however, equipment for modulating and demodulating all the different communication systems is installed in a base unit which is the main unit. Accordingly, the system disclosed in Patent Literature 1 has a problem of requiring a significant change in an existing system configuration and not being able to flexibly respond to a change in a system such as a change in the number of cells after the start of operation. 
         [0009]    It is therefore an object of the present invention to provide a communication system, a main unit, a radio access unit and a communication method that can flexibly respond to a change in a system without necessity of significant change in an existing system configuration, thereby making it possible to reduce costs entailed in introduction of the system, secure a wide coverage, and improve a user throughput. 
       Solution to Problem 
       [0010]    A communication system of the present invention is a communication system including a radio base station apparatus, a main unit connected to the radio base station apparatus and a network, and a plurality of radio access units connected to the main unit, and, employs a configuration in which the radio base station apparatus is configured to output a downlink signal of a first communication system to the main unit; the main unit is configured to output the downlink signal of the first communication system received from the radio base station apparatus and a downlink signal of a second communication system received from the network to each of the plurality of radio access units; and the plurality of radio access units are configured to define a single cell of the first communication system; each of the plurality of radio access units is configured to define a multiple cell of the second communication system in the single cell, to transmit the downlink signal of the first communication system received from the main unit to a terminal that uses the first communication system in the single cell, to performs wired protocol signal processing and then wireless protocol signal processing on the downlink signal of the second communication system received from the main unit to generate a resultant signal, and to transmit the resultant signal to a terminal that uses the second communication system in the multiple cell. 
         [0011]    A communication system of the present invention is a communication system including a radio base station apparatus, a main unit connected to the radio base station apparatus and a network, and a plurality of radio access units connected to the main unit, and employs a configuration in which the plurality of radio access units are further configured to define a single cell of a first communication system; each of the plurality of radio access units is further configured to define a multiple cell of a second communication system in the single cell, to output an uplink signal of the first communication system received from a terminal that uses the first communication system in the single cell to the main unit, to perform wireless protocol signal processing and then wired protocol signal processing on an uplink signal of the second communication system received from a terminal that uses the second communication system in the multiple cell to generate a signal, and to output the signal to the main unit; the main unit is further configured to synthesize the uplink signals of the first communication system received from the plurality of radio access units, to output the synthesized signal to the radio base station apparatus, and to output the uplink signals of the second communication system received from the plurality of radio access units to the network; and the radio base station apparatus configured to acquire the synthesized uplink signal of the first communication system from the main unit. 
         [0012]    A main unit according to the present invention employs a configuration to include a splitting section configured to split a received downlink signal of a first communication system into a plurality of downlink signals of the first communication system; and a multiplexing section configured to multiplex each of the downlink signals of the first communication system split in the splitting section with a received downlink signal of a second communication system into a first multiplexed signal, and to output the first multiplexed signal. 
         [0013]    A main unit according to the present invention employs a configuration to include a demultiplexing section configured to acquire a plurality of first multiplexed signals in each of which an uplink signal of a first communication system and an uplink signal of a second communication system are multiplexed, to demultiplex each of the acquired first multiplexed signals into the uplink signal of the first communication system and the uplink signal of the second communication system, and to output the demultiplexed uplink signal of the second communication system; and a synthesis section configured to synthesize the uplink signals of the first communication system demultiplexed in the demultiplexing section, and to output the synthesized signal. 
         [0014]    A radio access unit according to the present invention employs a configuration to include an acquiring section configured to acquire a downlink signal of a first communication system and a downlink signal of a second communication system; a protocol processing section configured to perform wired protocol signal processing and then wireless protocol signal processing on the downlink signal of the second communication system acquired in the acquiring section; and a transmitting section configured to transmit the downlink signal of the first communication system acquired in the acquiring section to a terminal that uses the first communication system in a single cell of the first communication system, the single cell being defined by the radio access unit and the other radio access units, and to transmit the downlink signal of the second communication system which is subjected to the wireless protocol signal processing in the protocol processing section, to a terminal that uses the second communication system in a multiple cell of the second communication system, the multiple cell being defined in the single cell. 
         [0015]    A radio access unit according to the present invention employs a configuration to include a receiving section configured to receive an uplink signal of a first communication system from a terminal that uses the first communication system in a single cell of the first communication system, the single cell being defined by the radio access unit and the other radio access units, and to receive an uplink signal of a second communication system from a terminal that uses the second communication system in a multiple cell of the second communication system, the multiple cell being defined in the single cell; a protocol processing section configured to perform wireless protocol signal processing and then wired protocol signal processing on the uplink signal of the second communication system received in the receiving section; and an outputting section configured to output the uplink signal of the first communication system received in the receiving section and the uplink signal of the second communication system which is subjected to the wired protocol signal processing in the protocol processing section. 
         [0016]    A communication method according to the present invention is a communication method in a communication system including a radio base station apparatus, a main unit connected to the radio base station apparatus and a network, and a plurality of radio access units connected to the main unit, and employs a configuration to include the steps of: outputting, from the radio base station apparatus, a downlink signal of a first communication system to the main unit; outputting, from the main unit, the downlink signal of the first communication system received from the radio base station apparatus and a downlink signal of a second communication system received from the network to each of the plurality of radio access units; defining a single cell of the first communication system by the plurality of radio access units, each of the plurality of radio access units defining a multiple cell of the second communication system in the single cell; transmitting, from each of the radio access units, the downlink signal of the first communication system received from the main unit to a terminal that uses the first communication system in the single cell; performing, by each of the radio access units, wired protocol signal processing and then wireless protocol signal processing on the downlink signal of the second communication system received from the main unit, to generate a signal; and transmitting, from each of the radio access units, the signal, to a terminal that uses the second communication system in the multiple cell. 
         [0017]    A communication method according to the present invention is a communication method in a communication system including a radio base station apparatus, a main unit connected to the radio base station apparatus and a network, and a plurality of radio access units connected to the main unit, and employs a configuration to include the steps of: defining a single cell of a first communication system by the plurality of radio access units, each of the plurality of radio access units defining a multiple cell of a second communication system in the single cell: outputting, from each of the radio access units, an uplink signal of the first communication system received from a terminal that uses the first communication system in the single cell to the main unit; performing, by each of the radio access units, wired protocol signal processing and then wireless protocol signal processing on an uplink signal of the second communication system received from a terminal that uses the second communication system in the multiple cell, to generate a signal; outputting, from each of the radio access units, the signal to the main unit; synsethizing, by the main unit, the uplink signals of the first communication system received from the plurality of radio access units, outputting, from the main unit, the synthesized signal to the radio base station apparatus, outputting, from the main unit, the uplink signals of the second communication system received from the plurality of radio access units to the network; and acquiring the synthesized uplink signal of the first communication system from the main unit by the radio base station apparatus. 
       Advantageous Effects of Invention 
       [0018]    According to the present invention, it is possible to flexibly respond to a change in a system without necessity of significant change in an existing system configuration, thereby making it possible to reduce costs entailed in introduction of the system, secure a wide coverage, and improve a user throughput. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a diagram showing a configuration of a conventional communication system; 
           [0020]      FIG. 2  is a diagram showing a configuration of a communication system according to Embodiment 1 of the present invention; 
           [0021]      FIG. 3  is a block diagram showing a configuration of a main unit according to Embodiment 1 of the present invention; 
           [0022]      FIG. 4  is a block diagram showing a configuration of a radio access unit according to Embodiment 1 of the present invention; 
           [0023]      FIG. 5  is a diagram showing a configuration of a communication system according to Embodiment 2 of the present invention; 
           [0024]      FIG. 6  is a block diagram showing a configuration of a main unit according to Embodiment 2 of the present invention; 
           [0025]      FIG. 7  is a block diagram showing a configuration of a radio access unit according to Embodiment 2 of the present invention; and 
           [0026]      FIG. 8  is a diagram showing a multiplexing processing method of CPRI frames according to Embodiment 2 of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
       Embodiment 1 
       [0028]      FIG. 2  is a diagram showing a configuration of communication system  100  according to Embodiment 1 of the present invention. 
         [0029]    Communication system  100  is mainly formed of radio base station apparatus  102 , core network (CN)  103 , termination unit (OLT)  104 , termination unit (ONU)  105 , router  106 , main unit  107 , and a plurality of radio access units  108 - 1  to  108 - n  (n is any natural number equal to or greater than 2) 
         [0030]    Radio base station apparatus  102  and main unit  107  are connected via one electrical signal transmission cable such as a coaxial cable. Main unit  107  and each of radio access units  108 - 1  to  108 - n  are star-connected via one optical signal transmission cable such as an optical fiber. Each component will be described below. 
         [0031]    In the present embodiment, an example case of employing WCDMA (Wideband Code Division Multiple Access) for the first communication system and LTE (Long Term Evolution) for the second communication system will be described. The WCDMA-based first communication system and the LTE-based second communication system transmit and receive different contents of data. The present embodiment will be described assuming that signals transmitted from radio base station apparatus  102  and core network  103  to radio access units  108 - 1  to  108 - n  are downlink signals, and signals transmitted from radio access units  108 - 1  to  108 - n  to radio base station apparatus  102  and core network  103  are uplink signals. 
         [0032]    Radio base station apparatus  102  is a radio base station for use in WCDMA, and outputs WCDMA downlink signals, which are RF signals, to main unit  107 . Radio base station apparatus  102  also receives WCDMA uplink signals, which are RF signals, from main unit  107 . 
         [0033]    Core network  103  generates LTE IP signals (hereinafter, referred to as “IP signals”) in accordance with an IP protocol using user data and control signals for the second communication system of the LTE scheme and outputs the generated LTE IP signals to termination unit  104 . Core network  103  also receives IP signals from termination unit  104 . 
         [0034]    Termination unit  104  electrooptically converts the IP signals received from core network  103 , and outputs the converted signals to termination unit  105 . Termination unit  104  optoelectrically converts the IP signals received from termination unit  105  and outputs the converted signals to core network  103 . 
         [0035]    Termination unit  105  optoelectrically converts the IP signals received from termination unit  104  and outputs the converted signals to router  106 . Termination unit  105  electrooptically converts IP signals received from router  106  and outputs the converted signals to termination unit  104 . 
         [0036]    Router  106  relays IP signal transmission from termination unit  105  to main unit  107 , or from main unit  107  to termination unit  105 . Main unit  107  splits the WCDMA downlink signal received from radio base station apparatus  102  into a plurality of WCDMA downlink signals and electrooptically converts the split WCDMA downlink signals. Main unit  107  performs wavelength division multiplexing (WDM) on WCDMA downlink signals converted into optical signals and IP signals received from router  106  and outputs the signals to radio access units  108 - 1  to  108 - n . Main unit  107  also demultiplexes wavelength-division-multiplexed signals received from radio access units  108 - 1  to  108 - n , into WCDMA uplink signals and IP signals, and optoelectrically converts the demultiplexed WCDMA uplink signals and IP signals. Main unit  107  synthesizes the WCDMA uplink signals which are converted into electrical signals and outputs the signals to radio base station apparatus  102 . In addition, main unit  107  outputs the IP signals converted into electrical signals to router  106 . Note that the detailed configuration of main unit  107  will be described later. 
         [0037]    A plurality of radio access units  108 - 1  to  108 - n  define single cell # 120 . The plurality of radio access units  108 - 1  to  108 - n  respectively define multiple cells # 130 - 1  to # 1 . 30 - n  in single cell # 120 . That is to say, the plurality of radio access units  108 - 1  to  108 - n  define the number of multiple cells # 130 - 1  to # 130 - n  which is the same as the number of radio access units  108 - 1  to  108 - n . The plurality of radio access units  108 - 1  to  108 - n  demultiplex multiplexed signals received from main unit  107  into WCDMA downlink signals and IP signals and optoelectrically convert the demultiplexed WCDMA downlink signals and IP signals. The plurality of radio access units  108 - 1  to  108 - n  transmit the WCDMA downlink signals converted into electrical signals to a terminal that uses the first communication system in single cell # 120 . The plurality of radio access units  108 - 1  to  108 - n  transmit IP signals converted into electrical signals to a terminal that uses the second communication system in corresponding multiple cells # 130 - 1  to # 130 - n . The plurality of radio access units  1084  to  108 - n  receive WCDMA uplink signals transmitted from the terminal that uses the first communication system in single cell # 120  or IP signals transmitted from the terminal that uses the second communication system in multiple cells # 1304  to # 130 - n . The plurality of radio access units  108 - 1  to  108 - n  electrooptically convert the received WCDMA uplink signals and the IP signals, perform wavelength-division-multiplexing on the WCDMA uplink signals and the IP signals which are converted into optical signals, to generate multiplexed signals, and output the generated multiplexed signals to main unit  107 . Note that the detailed configuration of radio access units  108 - 1  to  108 - n  will be described later. 
         [0038]    Next, the configuration of main unit  107  will be explained using  FIG. 3 .  FIG. 3  is a block diagram showing a configuration of main unit  107 . 
         [0039]    Main unit  107  is mainly formed of splitter  301 , E/O converters  302 - 1  to  302 - n , O/E converters  303 - 1  to  303 - n , media converters  304 - 1  to  304 - n , WDM couplers  305 - 1  to  305 - n , and synthesizer  306 . Optical interface sections  350 - 1  to  350 - n  include E/O converters  302 - 1  to  302 - n , O/E converters  303 - 1  to  303 - n , media converters  304 - 1  to  304 - n , and WDM couplers  305 - 1  to  305 - n.    
         [0040]    Radio base station apparatus  102  and splitter  301  are connected via one electrical signal transmission cable such as a coaxial cable. Radio base station apparatus  102  and synthesizer  306  are connected via one electrical signal transmission cable such as a coaxial cable. WDM couplers  305 - 1  to  305 - n  and radio access units  108 - 1  to  108 - n  are star-connected via optical signal transmission cables such as optical fiber cables, respectively. Each component will be described below. 
         [0041]    Splitter  301  splits a WCDMA downlink signal received from radio base station apparatus  102  into n WCDMA downlink signals and outputs the n WCDMA downlink signals obtained by splitting to E/O converters  302 - 1  to  302 - n , respectively. 
         [0042]    E/O converters  302 - 1  to  302 - n  convert the WCDMA downlink signals received from splitter  301 , which are electrical signals, into optical signals with wavelength λd_RF and output the converted signals to WDM couplers  305 - 1  to  305 - n.    
         [0043]    O/E converters  303 - 1  to  303 - n  convert the WCDMA uplink signals received from WDM couplers  305 - 1  to  305 - n , which are optical signals with wavelength d_RF, into electrical signals and output the converted signals to synthesizer  306 . 
         [0044]    Media converters  304 - 1  to  304 - n  electrooptically convert IP signals received from router  106  into optical signals with wavelength d_IP and output the converted signals to WDM couplers  305 - 1  to  305 - n . Media converters  304 - 1  to  304 - n  convert the received IP signals into optical signals with different wavelengths from the optical signals converted in E/O converters  302 - 1  to  302 - n.    
         [0045]    WDM couplers  305 - 1  to  305 - n  perform wavelength-division-multiplexing on the WCDMA downlink signals with wavelength d_RF received from E/O converters  302 - 1  to  302 - n  and the IP signals with wavelength d_IP received from media converters  304 - 1  to  304 - n , to generate multiplexed signals. WDM couplers  305 - 1  to  305 - n  output the generated multiplexed signals to radio access units  108 - 1  to  108 - n , respectively. WDM couplers  305 - 1  to  305 - n  demultiplex multiplexed signals received from radio access units  1081  to  108 - n  into WCDMA uplink signals with wavelength d_RF and IP signals with wavelength d_IP. WDM couplers  305 - 1  to  305 - n  output the demultiplexed WCDMA uplink signals with wavelength d_RF to O/E converters  303 - 1  to  303 - n  and output the demultiplexed IP signals with wavelength d_IP to media converters  304 - 1  to  304 - n.    
         [0046]    Synthesizer  306  synthesizes WCDMA uplink signals received from O/E converters  303 - 1  to  303 - n  and outputs the synthesized signal to radio base station apparatus  102 . 
         [0047]    The configuration of main unit  107  has been described above. 
         [0048]    Next, the configuration of radio access units  108 - 1  to  108 - n  will be described using  FIG. 4 .  FIG. 4  is a block diagram showing the configuration of radio access unit  108 - 1 . The configuration of radio access units  108 - 2  to  108 - n  is the same as that of radio access unit  108 - 1  in  FIG. 4  and an explanation thereof will be omitted. 
         [0049]    Radio access unit  108 - 1  is mainly formed of WDM coupler  401 , O/E convertor  402 , E/O converter  403 , media converter  404 , radio base station function section  405 , AMP section  406 , and antennas  407 - 1  and  407 - 2 . Each component will be described below. 
         [0050]    WDM coupler  401  demultiplexes a signal received from main unit  107  into a WCDMA downlink signal with wavelength d_RF and an IP signal with wavelength d_IP. WDM coupler  401  outputs the demultiplexed WCDMA downlink signal with wavelength d_RF to O/E converter  402  and outputs the demultiplexed IP signal with wavelength d_IP to media converter  404 . WDM coupler  401  performs wavelength-division-multiplexing on a WCDMA uplink signal with wavelength d_RF received from E/O converter  403  and an IP signal with wavelength d_IP received from media converter  404 , to generate a multiplexed signal. WDM coupler  401  outputs the generated multiplexed signal to main unit  107 . 
         [0051]    O/E converter  402  optoelectrically converts the WCDMA downlink signal with wavelength d_RF received from WDM coupler  401  and outputs the converted signal to AMP section  406 . 
         [0052]    E/O converter  403  converts a WCDMA uplink signal received from AMP section  406  from an electrical signal to an optical signal with wavelength d_RF and outputs the converted signal to WDM coupler  401 . Media converter  404  optoelectrically converts an IP signal with wavelength d_IP received from WDM coupler  401  and outputs the converted signal to radio base station function section  405 . Media converter  404  converts an IP signal received from radio base station function section  405  from an electrical signal to an optical signal with wavelength d_IP and outputs the converted signal to WDM coupler  401 . Media converter  404  converts the received IP signal into an optical signal with a different wavelength from the optical signal converted in E/O converter  403 . 
         [0053]    Radio base station function section  405  performs wired protocol signal processing and then wireless protocol signal processing corresponding to LTE on the IP signal received from media converter  404 , and outputs the resultant signal to AMP section  406  as an RF downlink signal of LTE. Radio base station function section  405  performs wireless protocol signal processing and then wired protocol signal processing corresponding to LTE on the RF uplink signal of LTE received from AMP section  406 , and outputs the resultant signal to media converter  404  as an IP signal of LTE. Radio base station function section  405 , for example, outputs the IP signal of LTE to media converter  404  as an Si interface signal. Radio base station function section  405  outputs a WCDMA uplink signal as an Iuh interface signal. Radio base station function section  405  has the same function as, for example, a femto cell base station. Here, a femto cell is a base station and defines a coverage area for mobile phones, which has a very small range with a radius of approximately several tens of meters. 
         [0054]    AMP section  406  amplifies the WCDMA downlink signal received from O/E converter  402  and transmits, by radio, the amplified signal from antenna  407 - 1  to a terminal that uses the first communication system of single cell # 120 . AMP section  406  amplifies the RF downlink signal of LTE received from radio base station function section  405  and transmits the amplified signal from antenna  407 - 2  to a terminal that uses the second communication system in corresponding multiple cell # 130 - 1 . AMP section  406  amplifies a received signal from a terminal that uses the first communication system of single cell # 120  or a terminal that uses the second communication system of multiple cell # 130 - 1  via antenna  407 - 1  or  407 - 2 , and performs filter processing on the signals, as necessary. That is to say, AMP section  406  extracts a signal in a frequency band used for WCDMA uplink signals or a signal in a frequency band used for RF uplink signals of LTE. AMP section  406  outputs the extracted WCDMA uplink signal to E/O converter  403 . AMP section  406  outputs the extracted RE uplink signal of LTE to radio base station function section  405 . Antennas  407 - 1  and  407 - 2  may be separately used for WCDMA and LTE, or may be shared between WCDMA and LTE. 
         [0055]    The configuration of radio access unit  108 - 1  and the configuration of communication system  100  have been described above. 
         [0056]    Next, a communication method in communication system  100  will be described, A communication method for downlink signals will be first described. 
         [0057]    Radio base station apparatus  102  outputs a WCDMA downlink signal to main unit  107  as an RF signal. 
         [0058]    Next, main unit  107  splits the WCDMA downlink signal received from radio base station apparatus  102  into the same number of WCDMA downlink signals as the number of radio access units  108 - 1  to  108 - n.    
         [0059]    Next, main unit  107  electrooptically converts the WCDMA downlink signals obtained by splitting, and IP signals received via core network  103 , termination unit  104 , termination unit  105 , and router  106 . 
         [0060]    Main unit  107  performs wavelength-division-multiplexing on the WCDMA downlink signals and the IP signals which are converted into optical signals, to generate multiplexed signals, and outputs the generated multiplexed signals to radio access units  108 - 1  to  108 - n.    
         [0061]    Radio access units  108 - 1  to  108 - n  demultiplex the multiplexed signals received from main unit  107  into WCDMA downlink signals and IP signals, and optoelectrically converts the demultiplexed WCDMA downlink signals and IP signals. 
         [0062]    Radio access units  108 - 1  to  108 - n  perform wired protocol signal processing and then wireless protocol signal processing corresponding to LTE on the IP signals converted into electrical signals, and transmit, by radio, the resultant signal to a terminal in multiple cells # 130 - 1  to # 130 - n  as RF downlink signals of LTE. 
         [0063]    Radio access units  108 - 1  to  108 - n  transmit WCDMA downlink signals converted into electrical signals to a terminal that uses the first communication system in single cell # 120 . 
         [0064]    The communication method for downlink signals has been described above. 
         [0065]    A communication method for uplink signals will be described next. 
         [0066]    Radio access units  108 - 1  to  108 - n  receive WCDMA uplink signals from a terminal that uses the first communication system in single cell # 120  and receives RF uplink signals of LTE from a terminal that uses the second communication system in multiple cells # 130 - 1  to # 130 - n.    
         [0067]    Next, radio access units  108 - 1  to  108 - n  perform wireless protocol signal processing and wired protocol signal processing corresponding to LTE on the received RF uplink signal of LTE to generate FP signals of LTE. 
         [0068]    Radio access units  108 - 1  to  108 - n  electrooptically convert the generated IP signals and the received WCDMA uplink signal. Radio access units  108 - 1  to  108 - n  then perform wavelength-division-multiplexing on the WCDMA uplink signals and the IP signals which are converted into optical signals, to generate multiplexed signals, and outputs the generated multiplexed signals to main unit  107 . 
         [0069]    Main unit  107  demultiplexes the multiplexed signals received from radio access units  108 - 1  to  108 - n  into WCDMA uplink signals and IP signals. 
         [0070]    Main unit  107  optoelectrically converts the demultiplexed WCDMA uplink signals and IP signals. 
         [0071]    Main unit  107  then synthesizes the WCDMA uplink signals converted into electrical signals and outputs the synthesized signal to radio base station apparatus  102 . 
         [0072]    In addition, main unit  107  outputs the IP signals converted into electrical signals to core network  103  via router  106 , termination unit  105 , and termination unit  104 . 
         [0073]    The present embodiment provides each radio access unit with a radio base station function section that transmits and receives LTE IP signals by radio, and thereby eliminates the necessity to significantly change an existing system configuration, and also makes it possible to flexibly correspond to a change in a system. This can reduce cost entailed in introduction of the system. According to the present embodiment, it is possible to secure a wide coverage by defining a single cell using a plurality of radio access units. According to the present embodiment, it is possible to improve the user, throughput by defining multiple cells by a plurality of radio access units, respectively. According to the present embodiment, it is possible to efficiency transmit WCDMA signals and LTE signals by performing wavelength-division-multiplexing on the WCDMA signals and LTE signals and transmitting the signals between a main unit and each radio access unit. 
       Embodiment 2 
       [0074]      FIG. 5  is a diagram showing a configuration of communication system  500  according to Embodiment 2 of the present invention. 
         [0075]    Communication system  500  shown in  FIG. 5  includes radio base station apparatus  501  instead of radio base station apparatus  102 , main unit  502  instead of main unit  107 , and radio access units  503 - 1  to  503 - n  instead of radio access units  108 - 1  to  108 - n , as compared to communication system  100  according to Embodiment 1 shown in  FIG. 2 . Parts in  FIG. 5  that are identical in configuration to parts in  FIG. 2  are assigned the same reference signs as in  FIG. 5 , and explanation thereof will be omitted here. 
         [0076]    Communication system  500  is mainly formed of core network (CN)  103 , termination unit (OLT)  104 , termination unit (ONU)  105 , router  106 , radio base station apparatus  501 , main unit  502 , and a plurality of radio access units  503 - 1  to  503 - n.    
         [0077]    Core network  103  and radio base station apparatus  501  are connected via one electrical signal transmission cable such as a coaxial cable. Radio base station apparatus  501  and main unit  502  are connected via one optical signal transmission cable such as an optical fiber cable, and using a CPRI (Common Public Radio Interface) interface. In the present embodiment, a case will be described where the signals of the first communication system and second communication system are multiplexed using a CPRI format for transmitting IQ signals for two antennas, using a single carrier frequency. In this case, a diversity scheme is not employed in transmitting CPRI signals in uplink and downlink, which are outputted between radio base station apparatus  501  and main unit  502 . That is to say, IQ signals for one antenna are outputted. Main unit  502  and each of radio access units  503 - 1  to  503 - n  are star-connected via one optical signal transmission cable such as an optical fiber. Each component will be described below. 
         [0078]    In the present embodiment, a case of employing LTE for the first communication system and employing LTE for the second communication system will be described as an example. The LTE-based first communication system and the LTE-based the second communication system transmit and receive different contents of data. The present embodiment will be described, assuming that signals transmitted from core network  103  to radio access units  503 - 1  to  503 - n  are downlink signals, and signals transmitted from radio access units  503 - 1  to  503 - n  to core network  103  are uplink signals. 
         [0079]    Core network  103  outputs IP signals of LTE in the first communication system to radio base station apparatus  501  and outputs IP signals of the LTE in the second communication system to termination unit  104 . Core network  103  receives IP signals of the LTE in the first communication system from radio base station apparatus  501  and receives IP signals of the LTE in the second communication system from termination unit  104 . 
         [0080]    Radio base station apparatus  501  is a radio base station for the LTE, converts IP signals received from core network  103  into CPR′ downlink signals which are optical signals, performs wavelength-division-multiplexing on the converted CPRI downlink signals and CPRI uplink signals, and outputs the resultant signals to main unit  502 . Radio base station apparatus  501  converts the CPRI uplink signals received from main unit  502 , which are optical signals, and subjected to wavelength-division-multiplexing with CPRI downlink signals, into IP signals and outputs the converted signals to core network  103 . 
         [0081]    Router  106  relays IP signal transmission from termination unit  105  to main unit  502 , or IP signal transmission from main unit  502  to termination unit  105 . 
         [0082]    Main unit  502  demultiplexes CPRI downlink signals received from radio base station apparatus  501 , from CPRI uplink signals. Main unit  502  optoelectrically converts the demultiplexed CPRI downlink signals and branches the CPRI downlink signals converted into electrical signals per frame. Main unit  502  multiplexes the branched CPRI downlink signals with IP signals received from router  106  per frame to generate downlink multiplexed signals, and electrooptically converts the generated downlink multiplexed signals. Main unit  502  performs wavelength-division-multiplexing on the downlink multiplexed signals converted into optical signals with uplink multiplexed signals and outputs the resultant signals to radio access units  503 - 1  to  503 - n . Main unit  502  demultiplexes uplink multiplexed signals received from radio access units  503 - 1  to  503 - n  from downlink multiplexed signals and demultiplexes the demultiplexed uplink multiplexed signals per frame. Main unit  502  adds the uplink multiplexed signals which are demultiplexed per frame to generate CPRI uplink signals, and electrooptically converts the generated CPRI uplink signals. Main unit  502  performs wavelength-division-multiplexing on the CPRI uplink signals converted into optical signals and CPRI downlink signals, and outputs the resultant signals to radio base station apparatus  501 . Note that the detailed configuration of main unit  502  will be described later. 
         [0083]    A plurality of radio access units  503 - 1  to  503 - n  define single cell # 520 . The plurality of radio access units  503 - 1  to  503 - n  respectively define multiple cells # 530 - 1  to # 530 - n  in single cell # 520 . That is to say, the plurality of radio access units  503 - 1  to  503 - n  define the same number of multiple cells # 530 - 1  to # 530 - n  as the number of radio access units  503 - 1  to  503 - n . The plurality of radio access units  503 - 1  to  503 - n  demultiplex downlink multiplexed signals received from main unit  502 , from uplink multiplexed signals, and optoelectrically convert the demultiplexed downlink multiplexed signals. The plurality of radio access units  503 - 1  to  503 - n  demultiplex the downlink multiplexed signals converted into electrical signals, per frame and upconvert the demultiplexed downlink multiplexed signals to generate RF signals having a radio frequency in which a terminal that uses the first communication system in single cell # 520  can receive. The plurality of radio access units  503 - 1  to  503 - n  transmit, by radio, the RF signals to the terminal that uses the first communication system in single cell # 520 . The plurality of radio access units  503 - 1  to  503 - n  convert the demultiplexed downlink multiplexed signals into IP signals and transmit the converted IP signals to a terminal that uses the second communication system in corresponding multiple cells # 530 - 1  to # 530 - n . The plurality of radio access units  503 - 1  to  503 - n  receive RF signals transmitted from the terminal that uses the first communication system in single cell # 520  or RF signals received from the terminal that uses the second communication system in multiple cells # 530 - 1  to # 530 - n . The plurality of radio access units  503 - 1  to  503 - n  downconvert RF signals received from the terminal that uses the first communication system in single cell # 520 . The plurality of radio access units  503 - 1  to  503 - n  perform wireless protocol signal processing and then wired protocol signal processing on the RF signals received from the terminal that uses the second communication system in multiple cells # 530 - 1  to # 530 - n , to generate IP signals. The plurality of radio access units  503 - 1  to  503 - n  multiplex the generated IP signals and the downconverted signals received from the terminal that uses the first communication system in single cell # 520 , per frame, to generate uplink multiplexed signals, and electrooptically convert the generated uplink multiplexed signals. The plurality of radio access units  503 - 1  to  503 - n  electrooptically convert the uplink multiplexed signals converted into optical signals, perform wavelength-division-multiplexing on the uplink multiplexed signals converted into optical signals with downlink multiplexed signals, and output the resultant signals to main unit  502 . Note that the detailed configuration of radio access units  503 - 1  to  503 - u  will be described later. 
         [0084]    Next, the configuration of main unit  502  will be described using  FIG. 6 .  FIG. 6  is a block diagram showing the configuration of main unit  502 . 
         [0085]    Main unit  502  is mainly formed of WDM coupler  601 , O/E converter  602 , E/O converter  603 , signal branch section  604 , signal conversion sections  605 - 1  to  605 - n , frame multiplexing sections  606 - 1  to  606 - n , frame demultiplexing sections  607 - 1  to  607 - n , E/O converters  608 - 1  to  608 - n , WDM couplers  609 - 1  to  609 - n , O/E converters  610 - 1  to  610 - n , and signal adder  611 . REC interface section  650  includes WDM coupler  601 , O/E converter  602 , and E/O converter  603 . Multiplexing/demultiplexing sections  660 - 1  to  660 - n  include signal conversion sections  605 - 1  to  605 - n , frame multiplexing sections  606 - 1  to  606 - n , and frame demultiplexing sections  607 - 1  to  607 - n , respectively. Radio access unit interface sections  670 - 1  to  670 - n  include E/O converters  608 - 1  to  608 - n , WDM couplers  609 - 1  to  609 - n , and O/E converters  610 - 1  to  610 - n , respectively REC refers to an apparatus having a function to perform modulation and demodulation for a radio base station apparatus defined by the specification of CPRI. Each component will be described below. 
         [0086]    WDM coupler  601  demultiplexes CPRI downlink signals, from multiplexed signals obtained by wavelength-division-multiplexing of CPRI downlink signals received from radio base station apparatus  501  and CPRI uplink signals. WDM coupler  601  outputs the demultiplexed CPRI downlink signals to O/E converter  602 . WDM coupler  601  performs wavelength-division-multiplexing on the CPRI uplink signals received from E/O converter  603  and CPRI downlink signals, to generate multiplexed signals, and outputs the generated multiplexed signals to radio base station apparatus  501 . 
         [0087]    O/E converter  602  optoelectrically converts the CPRI downlink signals received from WDM coupler  601  and outputs the converted signals to signal branch section  604 . 
         [0088]    E/O converter  603  electrooptically converts the CPRI uplink signals received from signal adder  611  and outputs the converted signals to WDM coupler  601 . 
         [0089]    Signal branch section  604  branches the CPRI downlink signals received from O/E converter  602  into n CPRI downlink signals and respectively outputs the branched CPRI downlink signals to frame multiplexing sections  606 - 1  to  606 - n.    
         [0090]    Signal conversion sections  605 - 1  to  605 - n  convert the IP signals received from router  106  into pseudo IQ signals and outputs the converted signals to frame multiplexing sections  606 - 1  to  606 - n , respectively. Here, the pseudo IQ signals are continuous signals obtained by inserting a dummy bit into the IP signals received from router  106  such that the signal speed of the IP signals is equivalent to that of IQ signals transmitted and received in an CPRI interface between radio base station apparatus  501  and REC interface section  650 . Signal conversion sections  605 - 1  to  605 - n  remove a dummy bit from pseudo IQ signals received from frame demultiplexing sections  607 - 1  to  607 - n  to convert the received pseudo IQ signals into IP signals and output the converted signals to router  106 . 
         [0091]    Frame multiplexing sections  606 - 1  to  606 - n  multiplex the CPRI downlink signals received from signal branch section  604  with the pseudo IQ signals received from signal conversion sections  605 - 1  to  605 - n  per frame to generate CPRI frame downlink signals. CPRI frame downlink signals are multiplexed in accordance with a CPRI format. Frame multiplexing sections  606 - 1  to  606 - n  output the generated CPRI frame downlink signals to E/O converters  608 - 1  to  608 - n . Frame multiplexing sections  606 - 1  to  606 - n  treat the received CPRI downlink signals and the pseudo IQ signals as signals of different antennas in the CPRI interface. A process in signal conversion sections  605 - 1  to  605 - n  will be described later. 
         [0092]    Frame demultiplexing sections  607 - 1  to  607 - n  demultiplex CPRI frame uplink signals received from O/E converters  610 - 1  to  610 - n  into IQ signals and pseudo IQ signals which are signals of different antennas in the CPRI interface. Frame demultiplexing sections  607 - 1  to  607 - n  output the demultiplexed IQ signals to signal adder  611  and output the demultiplexed pseudo IQ signals to signal conversion sections  605 - 1  to  605 - n.    
         [0093]    E/O converters  608 - 1  to  608 - n  electrooptically convert the CPRI frame downlink signals received from frame multiplexing sections  606 - 1  to  606 - n  and output the converted signals to WDM couplers  609 - 1  to  609 - n , respectively. 
         [0094]    WDM couplers  609 - 1  to  609 - n  perform wavelength-division-multiplexing on the CPRI frame downlink signals received from E/O converters  608 - 1  to  608 - n  with CPRI frame uplink signals and output the resultant signals to radio access units  503 - 1  to  503 - n . WDM couplers  609 - 1  to  609 - n  demultiplex CPRI frame uplink signals from multiplexed signals obtained by wavelength-division-multiplexing CPRI frame downlink signals and CPRI frame uplink signals, and output the demultiplexed CPRI frame uplink signals to O/E converters  610 - 1  to  610 - n.    
         [0095]    O/E converters  610 - 1  to  610 - n  optoelectrically convert the CPRI frame uplink signals received from WDM couplers  609 - 1  to  609 - n  and output the converted signals to frame demultiplexing sections  607 - 1  to  607 - n , respectively. 
         [0096]    Signal adder  611  adds n IQ signals received from frame demultiplexing sections  607 - 1  to  607 - u  to generate a CPRI uplink signal. Signal adder  611  outputs the generated CPRI uplink signal to E/O converter  603 . 
         [0097]    The configuration of main unit  502  has been described above. 
         [0098]    Next, the configuration of radio access units  503 - 1  to  503 - n  will be described using  FIG. 7 .  FIG. 7  is a block diagram showing the configuration of radio access unit  503 - 1 . The configuration of radio access units  503 - 2  to  503 - n  is the same as that of radio access unit  503 - 1  in  FIG. 7  and an explanation thereof will be omitted. 
         [0099]    Radio access unit  503 - 1  is mainly formed of WDM coupler  701 , O/E converter  702 , frame demultiplexing section  703 , TRX section  704 , signal conversion section  705 , radio base station function section  706 , AMP section  707 , frame multiplexing section  708 , E/O converter  709  and antennas  710 - 1  and  710 - 2 . Each component will be described below. 
         [0100]    WDM coupler  701  demultiplexes CPRI frame downlink signals, from multiplexed signals obtained by wavelength-division-multiplexing of CPRI frame downlink signals received from main unit  502  and CPRI frame uplink signals. WDM coupler  701  outputs the demultiplexed CPRI frame downlink signals to O/E converter  702 . WDM coupler  701  performs wavelength-division-multiplexing on. CPRI frame uplink signals received from E/O converter  709  and CPRI frame downlink signals and outputs the resultant signals to main unit  502 . 
         [0101]    O/E converter  702  optoelectrically converts the CPRI frame downlink signals received from WDM coupler  701  and outputs the converted signals to frame demultiplexing section  703 . 
         [0102]    Frame demultiplexing section  703  demultiplexes the CPRI frame downlink signals received from O/E converter  702  into IQ signals and pseudo IQ signals which are signals of different antennas in the CPRI interface. Frame demultiplexing section  703  outputs the demultiplexed IQ signals to TRX section  704  and outputs the demultiplexed pseudo IQ signals to signal conversion sections  705 . 
         [0103]    TRX section  704  upconverts the IQ signals received from frame demultiplexing section  703 , to generate RF signals having a predetermined radio frequency and outputs the upconverted signals to AMP section  707 . TRX section  704  downconverts RF signals received from AMP section  707 , to generate IQ signals and outputs the generated IQ signals to frame multiplexing section  708 . 
         [0104]    Signal conversion section  705  converts the pseudo IQ signals received from frame demultiplexing section  703  into IP signals by removing a dummy bit from the pseudo IQ signals, and outputs the converted IP signals to radio base station function section  706 . Signal conversion section  705  inserts a dummy bit into the IP signals received from radio base station function section  706 , to generate pseudo IQ signals and outputs the generated pseudo IQ signals to frame multiplexing section  708 . 
         [0105]    Radio base station function section  706  performs wired protocol signal processing and then performs wireless protocol signal processing corresponding to LTE, on the IP signals received from signal conversion section  705  and outputs the resultant signals to AMP section  707  as RF downlink signals of LTE. Radio base station function section  706  performs wireless protocol signal processing and then wired protocol signal processing corresponding to LTE, on RF uplink signals of the LTE received from AMP section  707  and outputs the resultant signals to signal conversion section  705  as IP signals of the LTE. Radio base station function section  706 , for example, outputs the IP signals of LTE to signal conversion section  705  as an SI interface signal. Radio base station function section  706  outputs a WCDMA uplink signal as an Iuh interface signal. Radio base station function section  706  has the same function as for example, a femto cell base station. 
         [0106]    AMP section  707  amplifies the RF signals received from TRX, section  704  and transmits the amplified signals from antenna  710 - 1  to a terminal that uses the first communication system by radio AMP section  707  amplifies the RF signals received from radio base station function section  706  and transmits the amplified signals from antenna  710 - 2  to a terminal that uses the second communication system corresponding multi cell # 530 - 1 . AMP section  707  amplifies received signals from the terminal that uses the first communication system in single cell # 520  or the terminal that uses the second communication system in multi cell # 530 - 1  via antennas  710 - 1  and  710 - 2 , and performs filter processing on the signals, as necessary. That is to say, AMP section  707  extracts signals in a frequency band used for the LTE in single cell # 520  and signals in a frequency band used for the LTE in multi cell # 530 - 1 . AMP section  707  outputs the extracted RF signals of the LTE in single cell # 520  to TRX section  704 . AMP section  707  outputs the extracted RF signals of the LTE in multi cell # 530 - 1  to radio base station function section  706 . Antennas  710 - 1  and  710 - 2  may be separately used for each LTE or may be shared with each LTE. 
         [0107]    Frame multiplexing section  708  multiplexes the IQ signals received from TRX section  704  with the pseudo IQ signals received from signal conversion section  705  per frame and generates CPRI frame uplink signals. CPRI frame uplink signals are multiplexed in accordance with a CPRI format. Frame multiplexing section  708  outputs the generated CPRI uplink signals to E/O converter  709 . 
         [0108]    E/O converter  709  electrooptically converts the CPRI frame uplink signals received from frame multiplexing section  708  and outputs the converted signals to WDM coupler  701 . 
         [0109]    The configuration of radio access unit  503 - 1  and the configuration of communication system  100  have been described above. 
         [0110]    Next, a communication method in communication system  500  will be described. A communication method for downlink signals will be first described. 
         [0111]    Radio base station apparatus  501  electrooptically converts CPRI downlink signals acquired from core network  103 , performs wavelength-division-multiplexing on the CPRI downlink signals converted into optical signals and CPRI uplink signals, to generate multiplexed signals. 
         [0112]    Radio base station apparatus  501  outputs the generated multiplexed signals to main unit  502 . 
         [0113]    Main unit  502  demultiplexes the CPRI downlink signals from the multiplexed signals received from radio base station apparatus  501  and optoelectrically converts the demultiplexed CPRI downlink signals. 
         [0114]    Next, main unit  502  branches the CPRI downlink signal converted into an electrical signal into n CPRI downlink signals. 
         [0115]    Main unit  502  converts IP signals received from router  106  into pseudo IQ signals. 
         [0116]    Main unit  502  multiplexes the n CPRI downlink signals and the pseudo IQ signals per frame to generate n CPRI frame downlink signals and electrooptically converts the generated CPRI frame downlink signals. 
         [0117]    Main unit  502  performs wavelength-division-multiplexing on the CPRI downlink signals converted into optical signals and CPRI uplink signals to generate n multiplexed signals, and output the n generated multiplexed signals to radio access units  503 - 1  to  503 - n  respectively. 
         [0118]    Next, radio access units  503 - 1  to  503 - n  demultiplex CPRI downlink signals from the multiplexed signals received from main unit  502 . 
         [0119]    Next, radio access units  503 - 1  to  503 - n  optoelectrically convert the demultiplexed CPRI downlink signals and demultiplex the CPRI downlink signals converted into electrical signals, into IQ signals and pseudo IQ signals. 
         [0120]    Radio access units  503 - 1  to  503 - n  upconvert the IQ signal to generate RF signals and transmits, by radio, the generated RF signals to a terminal that uses the first communication system in single cell # 520 . 
         [0121]    Radio access units  503 - 1  to  503 - n  convert the pseudo IQ signals into IP signals and convert the IF signals into RF signals, and transmit, by radio, the converted signals to a terminal that uses the second communication system in corresponding multiple cells # 530 - 1  to # 530 - n.    
         [0122]    The communication method for downlink signal has been described above. 
         [0123]    A communication method for uplink signals will be described next. 
         [0124]    Radio access units  503 - 1  to  503 - n  receive RF signals from a terminal that uses the first communication system in single cell # 520  and receive RF signals from a terminal that uses the second communication system in multiple cells # 530 - 1  to # 530 - n.    
         [0125]    Radio access units  503 - 1  to  503 - n  downconvert the RF signals received from the terminal that uses the first communication system in single cell # 520 , to generate IQ signals. 
         [0126]    Radio access units  503 - 1  to  503 - n  convert the RF signals received from the terminal that uses the second communication system in multiple cells # 530 - 1  to # 530 - n  into IP signals and convert the IP signals into pseudo IQ signals. 
         [0127]    Radio access units  503 - 1  to  503 - n  multiplex the IQ signals and the pseudo IQ signals per frame to generate CPRI frame uplink signals and electrooptically convert the generated. CPRI frame uplink signals. 
         [0128]    Radio access units  503 - 1  to  503 - n  perform wavelength-division-multiplexing on the CPRI frame uplink signals converted into optical signals and CPRI frame downlink signals, to generate multiplexed signals and output the generated multiplexed signals to main unit  502 . 
         [0129]    Main unit  502  demultiplexes CPRI frame uplink signals from the multiplexed signals received from radio access units  503 - 1  to  503 - n  and optoelectrically converts the demultiplexed CPRI frame uplink signals. 
         [0130]    Main unit  502  demultiplexes the CPRI frame uplink signals converted into electrical signals, into IQ signals and pseudo IQ signals. 
         [0131]    Main unit  502  converts the demultiplexed pseudo IQ signals into IP signals and outputs the IP signals to router  106 . 
         [0132]    Main unit  502  adds n demultiplexed IQ signals to generate uplink CPRI signals and electrooptically converts the generated uplink CPRI signals. 
         [0133]    Main unit  502  performs wavelength-division-multiplexing on the uplink CPRI signals converted into optical signals and downlink CPRI signals to generate multiplexed signals and outputs the generated multiplexed signals to radio base station apparatus  501 . 
         [0134]    The communication method in communication system  500  has been described above. 
         [0135]    A process in frame multiplexing sections  606 - 1  to  606 - n  and frame multiplexing section  708  will be described using  FIG. 8 .  FIG. 8  is a diagram showing a process of multiplexing in a CPRI frame. 
         [0136]    Frame multiplexing sections  606 - 1  to  606 - n  and frame multiplexing section  708  treat IQ signals as signals of antenna # 0  (AC 0 ) and treat pseudo IQ signals as signals of antenna # 1  (AC 1 ) as shown in  FIG. 8 . Frame multiplexing sections  606 - 1  to  606 - n  and frame multiplexing section  708  generate CPRI frame downlink signals and CPRI frame uplink signals, using A×C containers # 0  to # 7  as one group. In this embodiment, a CPRI format for transmitting IQ signals for two antennas, using a single carrier frequency, as an example. The present embodiment uses signals assigned to one of two antennas for radio signals to be transmitted to a terminal that uses the first communication system in a single cell. The present embodiment uses signals assigned to the other antenna for IP signals transmitted to a terminal that uses the second communication system in multiple cells. 
         [0137]    In view of the above, the present embodiment provides each radio access unit with a radio base station function section that transmits and receives LTE IP signals by radio, and thereby eliminates the necessity to significantly change an existing system configuration and also makes it possible to flexibly correspond to a change in a system. This can reduce cost entailed in introduction of the system. According to the present embodiment, it is possible to secure a wide coverage by defining a single cell using a plurality of radio access units. According to the present embodiment, it is possible to improve a user throughput by defining multiple cells by a plurality of radio access units, respectively. According to the present embodiment, a main unit and a radio access unit perform wavelength-division-multiplexing on LTE signals in each communication system and transmit the signals, so that the LTE signals in each communication system can be efficiently transmitted. 
         [0138]    The disclosure of Japanese Patent Application No. 2010-95184, filed on Apr. 16, 2010, including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0139]    A communication system, a main unit, a radio access unit, and a communication method according to the present invention is suitable for making space, for example, in buildings or underground malls usable as a coverage area the space being an area which cannot receive any radio wave signal from a radio base station installed outdoors. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           100  Communication system 
           102  Radio base station apparatus 
           103  Core network 
           104 ,  105  Termination unit 
           106  Router 
           107  Main unit 
           108 - 1  to  108 - n  Radio access unit 
         # 120  Single cell 
         # 130 - 1  to # 130 - n  Multiple cell