Patent Publication Number: US-2023155712-A1

Title: Wavelength multiplexing communication system and wavelength multiplexing communication method

Description:
TECHNICAL FIELD 
     The present invention relates to a wavelength multiplexing communication system and a wavelength multiplexing communication method. 
     BACKGROUND ART 
     Functions of a base station in a mobile system are divided into a central unit (CU), a distributed unit (DU), and a remote unit (RU). The RU is responsible for lower layer portions of base station functions. This function of the RU is a radio communication (RF) function with a portion of a physical layer (PHY). The RU performs radio communication with a user equipment (UE). 
       FIG.  9    is a diagram illustrating a configuration in which a wavelength division multiplex-passive optical network (WDM-PON) system is applied to a mobile front hole (MFH). A mobile system includes a CU, N pieces of DUs, and N pieces of RUs. N pieces of DUs are described as DU# 1  to DU #N, and N pieces of RUs are described as RU # 1  to RU #N. The DU #m (m is an integer of 1 or greater and N or less) and RU #m are logically connected on point-to-point basis. The WDM-PON relays a main signal received by the RU#m from the UE by radio communication to the DU#m. The WDM-PON relays a main signal addressed to the UE performing radio communication with the RU #m from the DU #m to the RU #m. As illustrated in  FIG.  9   , the WDM-PON includes an optical line terminal (OLT) with N pieces of optical line terminal-channel terminal (OLT-CT), and N pieces of optical network units (ONUs), and is composed of N pairs of OLT-CT and ONUs. By applying such a WDM-PON configuration, the number of optical fibers of the MFH can be reduced. An OLT-CT #m that is the m-th OLT-CT and an ONU #m that is the m-th ONU transmit and receive optical signals using the wavelength λ U-m  on the uplink and using the wavelength λ D-m  on the downlink. 
       FIG.  10    is an example of a train C that travels at high speed over a track R. It is contemplated that multiple RUs are installed along the track R to provide high-speed radio communication to UEs provided on the train C or present in the train C. However, because the train C travels along the track, the main signal communication is performed only in some RUs at a certain time. For example, in  FIG.  10   , among the RU # 1  to RU #N installed along the track, communication is performed only in the RU # 2  and RU # 3  that can communicate with the UE from the current position of the train C, and communication is not performed in the other RUs. 
     Citation List 
     Non Patent Literature 
     NPL 1: “5G wireless fronthaul requirements in a passive optical network context”, International Telecommunication Union, ITU-T G. Supplement 66,  FIG.  9 - 4   , p. 21, Oct. 2018 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     In order to achieve high-speed radio communication, it is effective to use a high frequency band capable of securing a wide frequency bandwidth. On the other hand, in the case of using the high frequency band, since the radio coverage per RU becomes small, the number of RUs required for converting a certain area into a radio area increases. When the number of RUs increases, the number of required wavelengths, the number of required OLT-CTs, and the number of required ONUs also increase, which increases equipment investment costs. As illustrated in  FIG.  10   , in a use case of a moving body where main signal communication is performed only in some RUs, the actual traffic amount is smaller than the system band, and thus the utilization efficiency of the system band is low. 
     In view of the above circumstances, an object of the present invention is to provide a wavelength multiplexing communication system and a wavelength multiplexing communication method capable of reducing costs of optical communication and improving utilization efficiency of a band. 
     Means for Solving the Problem 
     According to one aspect of the present invention, a wavelength multiplexing communication system comprises: a master station apparatus; and a plurality of slave station apparatuses. The master station apparatus includes a wavelength multiplexing communication unit configured to perform wavelength multiplexing communication with the plurality of slave station apparatuses by using an optical signal of a wavelength in a first wavelength group and an optical signal of a wavelength in a second wavelength group, with the number of wavelengths equal to or less than the number of the plurality of slave station apparatuses, and a slave station apparatus of the plurality of slave station apparatuses includes an optical communication unit configured to, when main signal communication is performed in the slave station apparatus, perform communication of a main signal with the master station apparatus by an optical signal of a wavelength in the first wavelength group, which is different from a wavelength in the first wavelength group used by another slave station apparatus of the plurality of slave station apparatuses, and when the main signal communication is not performed in the slave station apparatus, perform communication of a signal other than the main signal with the master station apparatus by an optical signal of a wavelength in the second wavelength group, which is a wavelength same as a wavelength used by another slave station apparatus of the plurality of slave station apparatuses. 
     According to one aspect of the present invention, a wavelength multiplexing communication method in a wavelength multiplexing communication system including a master station apparatus and a plurality of slave station apparatuses comprises: performing, by the master station apparatus, wavelength multiplexing communication with the plurality of slave station apparatuses by using an optical signal of a wavelength in a first wavelength group and an optical signal of a wavelength in a second wavelength group, with the number of wavelengths equal to or less than the number of the plurality of slave station apparatuses; and performing, by a slave station apparatus of the plurality of slave station apparatuses, when main signal communication is performed in the slave station apparatus, communication of a main signal with the master station apparatus by an optical signal of a wavelength in the first wavelength group, which is different from a wavelength in the first wavelength group used by another slave station apparatus of the plurality of slave station apparatuses, and, when the main signal communication is not performed in the slave station apparatus, communication of a signal other than the main signal with the master station apparatus by an optical signal of a wavelength in the second wavelength group, which is a wavelength same as a wavelength used by another slave station apparatus of the plurality of slave station apparatuses. 
     Effects of the Invention 
     According to the present invention, it is possible to reduce costs of optical communication and improve utilization efficiency of a band. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system according to a first embodiment of the present invention. 
         FIG.  2    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system according to the first embodiment. 
         FIG.  3    is a diagram illustrating another exemplary configuration of the wavelength multiplexing communication system according to the first embodiment. 
         FIG.  4    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system according to a second embodiment. 
         FIG.  5    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system according to the second embodiment. 
         FIG.  6    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system according to a third embodiment. 
         FIG.  7    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system according to the third embodiment. 
         FIG.  8    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system according to a fourth embodiment. 
         FIG.  9    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system according to the related art. 
         FIG.  10    is a diagram illustrating a use case of a moving body according to the related art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. 
     First Embodiment 
       FIG.  1    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system  1  according to a first embodiment. The wavelength multiplexing communication system  1  relays a main signal generated in a mobile system by an optical access system. In the present embodiment, a wavelength division multiplex-passive optical network (WDM-PON) is used as the optical access system. The wavelength multiplexing communication system  1  includes an N (N is an integer of 1 or greater) remote units (RUs)  11 , N optical network units (ONUs)  12 , an optical coupling/splitting unit  13 , a wavelength multiplexing/demultiplexing unit  14 , an optical line terminal (OLT)  15 , a transfer apparatus  16 , n (n is an integer of 1 or greater and N or less) distributed units (DU)  17 , and a central unit (CU)  18 . The N RUs  11 , n DUs  17 , and the CU  18  constitute a mobile system. The RU  11  performs radio communication with the user equipment (UE)  80 . The N ONUs  12 , the optical coupling/splitting unit  13 , the wavelength multiplexing/demultiplexing unit  14 , and the OLT  15  constitute the optical access system. Each of the ONUs  12  and the optical coupling/splitting unit  13  are connected by a transmission line  31 . The optical coupling/splitting unit  13  and the wavelength multiplexing/demultiplexing unit  14  are connected by a transmission line  32 . The transmission line  31  and the transmission line  32  are each, for example, an optical fiber. 
     In the RU  11 , main signal communication is performed by radio communication with the UE  80 . Then, n is the maximum number of RUs  11  in which the main signal occurs at the same time. Since the RU  11  and the ONU  12  are connected on a one-to-one basis, n is also the maximum number of ONUs  12  that can simultaneously perform main signal communication. In the present embodiment, the N RUs  11  are referred to as RU  11 - 1  to RU  11 -N, the N ONUs  12  are referred to as ONU  12 - 1  to ONU  12 -N, and then DUs  17  are referred to as DU  17 - 1  to DU  17 n.  FIG.  1    illustrates an example in a case where N=2n. That is, the wavelength multiplexing communication system  1  includes RU  11 - 1  to RU  11 - 2   n  and ONU  12 - 1  to ONU  12 - 2   n . The RU  11 - j  (j is an integer of 1 or greater and N or less) is connected to the ONU  12 - j . The transmission line  31  between the ONU  12 - j  and the optical coupling/splitting unit  13  is described as a transmission line  31 - j . A direction from the RU  11  to the CU  18  is uplink, and a direction from the CU  18  to the RU  11  is downlink. 
     When the main signal is generated in then RUs  11 , the ONUs  12 - 1  to  12 - 2   n  and the OLT  15  of the present embodiment use the wavelengths λ U-0  to λ U-n  for the uplink communication and the wavelengths λ D-0  to λ D-n  for the downlink communication. Hereinafter, the set of the wavelengths λ U-i  and λ D-i  is referred to as a wavelength λ i  (i is an integer of 0 or greater and n or less). In the present embodiment, one of the wavelengths λ 0  to λ n  is used for the control signal for the mobile system. Here, the wavelength λ 0  is used for control signal communication. The wavelength λ 0  for control signal communication is assigned to all ONUs  12 . The wavelengths λ 1  to λ n  for main signal communication are exclusively assigned to the maximum number n of ONUs  12  in which main signal communication is performed in the same period. In the present embodiment, a wavelength assigned to a certain ONU  12  for main signal communication is always the same. For example, it is assumed that the RUs  11 - 1  to  11 -N are sequentially installed along a route along which a moving body in which n UEs  80  are provided travels. In this case, depending on the time, the combination of RU  11  in which the main signal occurs changes. In any of the combinations that change with this time, wavelengths λ 1  to λ n  are periodically assigned in order from the ONU  12 - 1  such that the wavelengths used for the RU  11  in which the main signal is generated are different from each other. As a result, when j is not a multiple of n, the wavelength λ (j mod(n))  is fixedly assigned in advance to the ONU  12 - j . Then, mod is a modulo operation. When j is a multiple of n, the wavelength λ n  is fixedly assigned in advance to the ONU  12 - j . In  FIG.  1   , all of the ONUs  12 - 1  to  12 - 2   n  use a wavelength λ 0  for the control signal, and the ONUs  12 - 2  to  12 -(n+ 1 ) connected to each of the RUs  11 - 2  to  11 -(n+ 1 ) in which radio communication with the UE  80  occurs exclusively use the wavelengths λ 1  to λ n  for main signal communication. Even when the number of ONUs  12  in which the main signal communication is performed in the same period is less than n, the ONU  12  in which the main signal communication is generated uses a fixed and pre-assigned wavelength for the main signal communication. 
     The RU  11  performs radio communication with the UE  80  existing in a subordinate cell. There are cases where a single radio communication cell is configured by all RUs  11  and when each RU  11  configures one radio communication cell. The RU  11 - j  outputs a control signal of the radio system and an uplink main signal received by radio communication from the UE  80  to the ONU  12 - j . The RU  11 - j  receives the control signal of the radio system and the main signal from the ONU  12 - j , and transmits the received main signal to the UE  80  by radio communication. 
     The ONU  12  includes a lower communication unit  121  and an optical communication unit  122 . The lower communication unit  121  of the ONU  12 - j  receives the control signal for the upstream radio system and the upstream main signal from the RU  11 - j , and outputs them to the optical communication unit  122 . The lower communication unit  121  of the ONU  12 - j  outputs the control signal of the downstream radio system and the downstream main signal received from the optical communication unit  122  to the RU  11 - j.    
     The optical communication unit  122  includes optical transmission/reception units  123  and  124 . The optical transmission/reception unit  123  of the ONU  12 - j  converts the uplink control signal from the electrical signal into an optical signal of a wavelength λ U-0  and outputs the optical signal to the transmission line  31 - j . The uplink control signal includes the uplink control signal of the radio system received by the lower communication unit  121 . The optical transmission/reception unit  123  of the ONU  12 - j  receives a downlink control signal having a wavelength λ D-0  among downlink wavelength-multiplexed signals transmitted through the transmission line  31 - j  and converts the downlink control signal into an electrical signal. The downlink control signal includes a downlink control signal of the radio system. The optical transmission/reception unit  123  outputs a downstream control signal of the radio system to the lower communication unit  121 . The optical transmission/reception unit  124  of the ONU  12 - j  converts the uplink main signal received by the lower communication unit  121  from the RU  11 - j  into an optical signal of a wavelength λ U-(j mod(n))  and outputs the optical signal to the transmission line  31 - j . The optical transmission/reception unit  124  of the ONU  12 - j  receives a downlink main signal having a wavelength λ D-(j mod(n))  among downlink wavelength-multiplexed signals transmitted through the transmission line  31 - j . The optical transmission/reception unit  124  converts the received downlink main signal into an electrical signal and outputs the electrical signal to the lower communication unit  121 . 
     The optical coupling/splitting unit  13  is, for example, a power splitter. The optical coupling/splitting unit  13  receives the uplink optical signals of the wavelengths λ U-0  to λ U-n  from the transmission line  31 - 1  to  31 - 2   n , and outputs a wavelength-multiplexed signal obtained by multiplexing the received uplink optical signals to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal in which the downlink optical signals having the wavelengths λ D-0  to λ D-n  are multiplexed from the transmission line  32 , and splits the received wavelength-multiplexed signal to output the result to the transmission lines  31 - 1  to  31 - 2   n.    
     The wavelength multiplexing/demultiplexing unit  14  is, for example, arrayed waveguide gratings (AWG). The wavelength multiplexing/demultiplexing unit  14  demultiplexes the uplink wavelength-multiplexed signal transmitted through the transmission line  32  into the uplink optical signals of the wavelengths λ U-0  to λ U-n  and outputs the uplink optical signals to the OLT  15 . The wavelength multiplexing/demultiplexing unit  14  multiplexes the downlink optical signals having the wavelengths λ D-0  to λ D-n  output from the OLT  15 , and outputs the multiplexed optical signals to the transmission line  32  as a wavelength-multiplexed signal. 
     The OLT  15  includes (n+ 1 ) OLT-CTs  151 . The OLT-CT  151  terminates the channel of the optical signal. The OLT-CT  151  converts the received upstream optical signal into a signal frame of an electrical signal and outputs the signal frame to the transfer apparatus  16 , and converts the signal frame of the electrical signal received from the transfer apparatus  16  into an optical signal and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The (n+ 1 ) OLT-CTs  151  are referred to as OLT-CTs  151 - 0  to  151 - n . The OLT-CT  151 - i  (i is an integer of 0 or greater and n or less) transmits a downlink optical signal of a wavelength and receives an uplink optical signal of a wavelength λ U-i . The wavelength λ D-0  and the wavelength λ U-0  are for control signal communication and the wavelengths λ D-1  to λ D-n  and the wavelengths λ U-0  to λ U-n  are for main signal communication. That is, the OLT-CT  151 - 0  transmits and receives a control signal, and the OLT-CTs  151 - 1  to  151 - n  transmit and receive a main signal. 
     The transfer apparatus  16  receives the control signal frame in which the uplink control signal is set from the OLT-CT  151 - 0 , and transfers the received control signal frame to the DU  17  that is the destination of the received control signal frame. The transfer apparatus  16  receives the main signal frame in which the uplink main signal is set from the OLT-CTs  151 - 1  to  151 - n , and outputs the main signal frame to the DU  17  that is the destination of the received main signal frame. The transfer apparatus  16  receives a downlink control signal frame and a main signal frame from each DU  17 . When receiving the control signal frame and the main signal frame from the same DU  17 , the transfer apparatus  16  demultiplexes the control signal frame and the main signal frame. The transfer apparatus  16  outputs the control signal frame to the OLT-CT  151 - 0 , and outputs the main signal frame to the OLT-CT  151  corresponding to the wavelength for main signal communication used by the ONU  12  that is the destination. As described above, the transfer apparatus  16  multiplexes and transfers the control signal and the main signal to the same DU  17  in the case of the uplink, and demultiplexes the control signal and the main signal received from the same DU  17  and outputs the signals to different OLT-CTs  151  in the case of the downlink. 
     The DU  17  outputs the uplink main signal received from the transfer apparatus  16  to the CU  18 , and outputs the downlink main signal input from the CU  18  to the transfer apparatus  16 . Each DU  17  is logically connected to the RU  11  on point-to point basis for the main signal. The CU  18  outputs uplink main signals input from the DUs  17 - 1  to  17 - n  to a higher-level apparatus (not illustrated), and outputs downlink main signals received from the higher-level apparatus (not illustrated) to the DUs  17 - 1  to  17 - n.    
       FIG.  2    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system  1 . The right column of  FIG.  2    illustrates the RU  11  communicating with the UE  80  at each time and the wavelength used by the ONU  12 . The left column of  FIG.  2    illustrates signals transmitted and received in the optical access section between each ONU  12  and the OLT  15  at each time. Here, t indicates time. 
     The UE  80  is provided on the train C that moves at high speed on the track R, or is present in the train C. Along the track R, RUs  11 - 1  to  11 -N are installed.  FIG.  2    illustrates RUs up to the RU  11 - 4 . In  FIG.  2   , there are two UEs  80 , and two RUs  11  (n= 2 ) at the maximum perform radio communication in the same period. Wavelengths λ 0  and λ 2  are assigned to the ONUs  12 - 1  and  12 - 3 , and wavelengths λ 0  and λ 1  are assigned to the ONUs  12 - 2  and  12 - 4 . 
     At time T 1 , each of the RUs  11 - 1  and  11 - 2  performs radio communication with the corresponding UE  80 . The uplink communication at time T 1  will be described. The RU  11 - 1  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 1 , and the RU  11 - 2  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 2 . Each RU  11 - j  outputs a control signal of the radio system to the ONU  12 - j . The optical transmission/reception unit  123  of each ONU  12 - j  outputs, to the transmission line  31 - j , an optical signal of a wavelength λ U-0  including an uplink control signal of the radio system received by the lower communication unit  121  from the RU  11 - j . The optical transmission/reception unit  124  of the ONU  12 - 1  converts the uplink main signal received by the lower communication unit  121  from the RU  11 - 1  into an optical signal of a wavelength λ U-1  and outputs the optical signal to the transmission line  31 - 1 , and the optical transmission/reception unit  124  of the ONU  12 - 2  converts the uplink main signal received by the lower communication unit  121  from the RU  11 - 2  into an optical signal of a wavelength  4 - 2  and outputs the optical signal to the transmission line  31 - 2 . 
     The optical coupling/splitting unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing the uplink optical signals of the wavelengths λ U-0  to λ U-2  to the transmission line  32 . The wavelength multiplexing/demultiplexing unit  14  demultiplexes the uplink wavelength- multiplexed signal transmitted through the transmission line  32  into the uplink optical signals of the wavelengths λ U-0  to λ U-2 . The wavelength multiplexing/demultiplexing unit  14  outputs a light signal of a wavelength λ U-0  to the OLT-CT  151 - 0 , an optical signal of a wavelength λ U-1  to the OLT-CT  151 - 1 , and an optical signal of a wavelength λ U-2  to the OLT-CT  151 - 2 . Each of the OLT-CTs  151 - 0 ,  151 - 1 , and  151 - 2  converts the input optical signal into a signal frame of an electrical signal and outputs the signal frame to the transfer apparatus  16 . 
     The transfer apparatus  16  transfers the control signal frame received from the OLT-CT  151 - 0  to the DU  17  that is the destination. The transfer apparatus  16  receives the main signal frames from the OLT-CT  151 - 1  and the OLT-CT  151 - 2  and outputs each of the main signal frames to the DU  17  that is the destination. The DU  17  outputs the uplink main signal received from the transfer apparatus  16  to the CU  18 , and the CU  18  outputs the uplink main signal input from the DU  17  to a higher apparatus (not illustrated). 
     Next, downlink communication at the time T 1  will be described. The CU  18  receives a downlink main signal from a higher apparatus (not illustrated) and outputs the downlink main signal to the DU  17  in accordance with the destination. The DU  17  outputs a signal frame of a down link control signal and a signal frame of a downlink main signal from the CU  18 . The transfer apparatus  16  outputs the downlink control signal frame to the OLT-CT  151 - 0 , and outputs the downlink main signal frame to the OLT-CT  151 - 1  and the OLT-CT  151 - 2  according to the destination. The OLT-CT  151 - 0  converts the downlink control signal frame into an optical signal of a wavelength λ D-0 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The OLT-CT  151 - 1  converts the downlink main signal frame into an optical signal of a wavelength λ D-1 , and outputs the optical signal into the wavelength multiplexing/demultiplexing unit  14 , and the OLT-CT  151 - 2  converts the downlink main signal frame into an optical signal of a wavelength λ D-2 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . 
     The wavelength multiplexing/demultiplexing unit  14  outputs a wavelength-multiplexed signal obtained by multiplexing the downlink optical signals of the wavelengths λ D-0 , λ D-1 , and λ D-2  output by the OLT  15 , to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal from the transmission line  32 , splits the received wavelength-multiplexed signal, and outputs the result to the corresponding transmission line among the transmission lines  31 - 1  to  31 -N. 
     The optical transmission/reception unit  123  of each of the ONUs  12 - 1  to  12 -N receives a control signal of a wavelength λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. The lower communication unit  121  of each ONU  12 - j  outputs the control signal of the radio system to the RU  11 - j . The optical transmission/reception unit  124  of the ONU  12 - 1  selects and receives a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal, and converts the received main signal into an electrical signal. The lower communication unit  121  of the ONU  12 - 1  outputs the main signal converted into the electrical signal to the RU  11 - 1 . The optical transmission/reception unit  124  of the ONU  12 - 2  selects and receives a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal, and converts the received main signal into an electrical signal. The lower communication unit  121  of the ONU  12 - 2  outputs the main signal converted into the electrical signal to the RU  11 - 2 . The RU  11 - 1  transmits by radio communication the main signal received from the ONU  12 - 1  to the UE  80 , and the RU  11 - 2  transmits by radio communication the main signal received from the ONU  12 - 2  to the UE  80 . 
     The train C travels and, at time T 2 , each of the RUs  11 - 2  and  11 - 3  performs radio communication with the corresponding UE  80 . The uplink communication at the time T 2  will be described. The RU  11 - 2  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 2 , and the RU  11 - 3  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 3 . Each RU  11 - j  outputs a control signal of the radio system to the ONU  12 - j . Each ONU  12 - j  outputs, to the transmission line  31 - j , an optical signal of a wavelength λ U-0  including an uplink control signal of the radio system. The ONU  12 - 2  converts the uplink main signal received from the RU  11 - 2  into an optical signal of a wavelength λ U-2  and outputs the optical signal to the transmission line  31 - 2 , and the ONU  12 - 3  converts the uplink main signal received from the RU  11 - 3  into an optical signal of a wavelength λ U-1  and outputs the optical signal to the transmission line  31 - 3 . The subsequent processing is similar to that in the case of the time T 1 . 
     The operation of the downlink communication at the time T 2  is similar to that at the time T 1  up to the part where the optical coupling/splitting unit  13  outputs the wavelength-multiplexed signal received from the transmission line  32  to the transmission lines  31 - 1  to  31 -N. Each of the ONUs  12 - 1  to  12 -N receives a control signal of a wavelength λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. Each ONU  12 - j  outputs a control signal of the radio system to the RU  11 - j . The ONU  12 - 2  selects and receives a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal, converts the received main signal into an electrical signal, and outputs the main signal converted into the electrical signal to the RU  11 - 2 . The ONU  12 - 3  selects and receives a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal, converts the received main signal into an electrical signal, and outputs the main signal converted into the electrical signal to the RU  11 - 3 . The RU  11 - 2  transmits by radio communication the main signal received from the ONU  12 - 2  to the UE  80 , and the RU  11 - 3  transmits by radio communication the main signal received from the ONU  12 - 3  to the UE  80 . 
     The train C travels and, at time T 3 , each of the RUs  11 - 3  and  11 - 4  performs radio communication with the corresponding UE  80 . The uplink communication at the time T 3  will be described. The RU  11 - 3  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 3 , and the RU  11 - 4  outputs an uplink main signal received by radio communication from the UE  80  to the ONU  12 - 4 . Each RU  11   j  outputs a control signal of the radio system to the ONU  12   j . Each ONU  12   j  outputs, to the transmission line  31 - j , an optical signal of a wavelength λ U-0  including an uplink control signal of the radio system. The ONU  12 - 3  converts the uplink main signal received from the RU  11 - 3  into an optical signal of a wavelength λ U-1  and outputs the optical signal to the transmission line  31 - 3 , and the ONU  12 - 4  converts the uplink main signal received from the RU  11 - 4  into an optical signal of a wavelength  4 - 2  and outputs the optical signal to the transmission line  31 - 4 . The subsequent processing is similar to that in the case of the time T 1 . 
     The operation of the downlink communication at the time T 3  is similar to that at the time T 1  up to the part where the optical coupling/splitting unit  13  outputs the wavelength-multiplexed signal received from the transmission line  32  to the transmission lines  31 - 1  to  31 -N. Each of the ONUs  12 - 1  to  12 -N receives a control signal of a wavelength λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. Each ONU  12   j  outputs a control signal of the radio system to the RU  11   j . The ONU  12 - 3  selects and receives a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal, converts the received main signal into an electrical signal, and outputs the main signal converted into the electrical signal to the RU  11 - 3 . The ONU  12 - 4  receives a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal, converts the received main signal into an electrical signal, and outputs the main signal converted into the electrical signal to the RU  11 - 4 . The RU  11 - 3  transmits by radio communication the main signal received from the ONU  12 - 3  to the UE  80 , and the RU  11 - 4  transmits by radio communication the main signal received from the ONU  12 - 4  to the UE  80 . 
     In the wavelength multiplexing communication system  1  illustrated in  FIG.  1   , a network configuration of an MFH section is star type, but may be a bus type as illustrated in  FIG.  3   .  FIG.  3    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system la according to the first embodiment. In  FIG.  3   , parts that are the same as those of the wavelength multiplexing communication system  1  illustrated in  FIG.  1    are denoted by the same reference signs, and description thereof will be omitted. In the wavelength multiplexing communication system la illustrated in  FIG.  3   , the optical coupling/splitting unit  13  is connected to the transmission lines  32  and  34 . The transmission line  34  is provided with a plurality of optical coupling/splitting unit  35 . Each of the ONUs  12  and the optical coupling/splitting unit  35  are connected by a transmission line  36 . The optical coupling/splitting unit  35  outputs the uplink signal received from the ONU  12  to the transmission line  34 . The optical coupling/splitting unit  35  splits the downlink wavelength-multiplexed signal transmitted through the transmission line  34  and outputs the result to the transmission line  36 . The optical coupling/splitting unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing the uplink optical signals of wavelengths transmitted through the transmission line  34  to the transmission line  32 . The optical coupling/splitting unit  13  outputs the downlink wavelength-multiplexed signal transmitted through the transmission line  32  to the transmission line  34 . 
     As described above, in the present embodiment, the OLT and each ONU transmit and receive control signals of the mobile system with a wavelength  2 o, with or without a main signal of the mobile system. For each ONU  12 , main signal communication is performed using the wavelengths λ 1  to λ n  exclusively assigned for main signal communication only for a period during which main signal communication is performed in the RU. The maximum number of RUs in which the main signal communication is performed in the same period is n. Therefore, the number of OLT-CTs and the number of wavelengths used can be reduced from N to (n+ 1 ) in the related art. As a result, system utilization efficiency is improved for band demand, and capital investment costs can be reduced. Since the control signal does not flow in the wavelength for main signal communication, high-speed radio communication with the UE can be achieved. 
     In the embodiment described above, the case where the signal other than the main signal is the control signal has been described as an example, but the signal other than the control signal may be used. In the embodiment described above, one wavelength is assigned to each of the uplink communications and the downlink communications other than the main signal, but a plurality of wavelengths may be assigned. Also in this case, in each of the uplink and the downlink, the total of the number of wavelengths used for communication of signals other than the main signal and the number of wavelengths for main signal communication is set to equal to or less than N. The number of OLT-CTs included in the OLT is the sum of the number of wavelengths used for communication of signals other than the main signal and the number of wavelengths for main signal communication. For example, in the case where the wavelengths λ 0 , λ 0 ′ are assigned to signals other than the main signal, the OLT includes an OLT-CT that transmits and receives wavelengths λ 0 , λ 0 ′, λ 1 , . . . , λ n . Each ONU transmits and receives signals other than the main signal using one or both of wavelengths λ 0  and λ 0 ′. The wavelengths λ 1 , . . . , λ n  are exclusively assigned to an ONU that performs main signal communication. 
     Second Embodiment 
     In the first embodiment, the wavelength used for main signal communication is fixed for each ONU. In a second embodiment, the wavelength for main signal communication used by each ONU is dynamically changed. Hereinafter, differences from the first embodiment will be mainly described. 
       FIG.  4    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system  2  according to the second embodiment. In  FIG.  4   , parts that are the same as those of the wavelength multiplexing communication system  1  illustrated in  FIG.  1    are denoted by the same reference signs, and description thereof will be omitted. The wavelength multiplexing communication system  2  illustrated in  FIG.  4    is different from the wavelength multiplexing communication system  1  illustrated in  FIG.  1    in that the wavelength multiplexing communication system  2  includes ONUs  220  instead of the ONUs  12  and an OLT  250  instead of the OLT  15 . A j-th ONU  220  (j is an integer equal to or greater than 1 and equal to or less than N) is referred to as an ONU  220 - j . As similar to the wavelength multiplexing communication system la illustrated in  FIG.  3   , the network configuration of the MFH section may be a bus type. 
     The ONU  220  is different from the ONU  20  of the first embodiment in that the ONU  220  further includes a wavelength control unit  221 . The wavelength control unit  221  controls the optical transmission/reception unit  124  to dynamically change the wavelength for the main signal communication. The optical transmission/reception unit  124  is, for example, a wavelength variable transceiver. 
     The OLT  250  is different from the OLT  15  of the first embodiment in that the OLT  250  further includes a wavelength assignment unit  251 . The wavelength assignment unit  251  may be provided in an apparatus external to the OLT  250 . The wavelength assignment unit  251  dynamically and exclusively assigns wavelengths λ 1  to λ n  for up to n ONUs  220  in which main signal communication is performed during the same period. For example, the wavelength assignment unit  251  assigns wavelengths such that the ONU  220 - 2  performs main signal communication with the wavelength λ 1 , the ONU  220 - 3  performs main signal communication with the wavelengths λ 2 , . . . , and the ONU  220 -(n+ 1 ) performs main signal communication with the wavelength λ n  in a certain period, and the ONU  220 - 3  performs main signal communication with the wavelengths λ 1 , the ONU  220 - 4  performs main signal communication with the wavelengths λ 2 , . . . , and the ONU  220 -(n+ 2 ) performs main signal communication with the wavelength λ n  in a period different from the certain period. For example, the wavelength assignment unit  251  changes the assignment when the set of ONUs  220  in which the main signal is generated is changed, but may change the assignment at another timing. 
     For example, for the ONU  220  in which the main signal communication is performed, determination is made as follows. The wavelength assignment unit  251  of the OLT  250  stores train operation information in advance. The operation information is information capable of obtaining a correspondence between the time and the position of the train at the time. The wavelength assignment unit  251  periodically refers to the operation information to obtain the current position of the train. Alternatively, the wavelength assignment unit  251  receives position information indicating the current position of the train from an external device if there is a change in position. For example, a camera, a sensor, or the like detects the proximity of the train and notifies the OLT  250  of the position information of the train. The wavelength assignment unit  251  stores correspondence information indicating a correspondence between the position of the train and the ONU  220  or the RU  11  in which the main signal communication is performed. The wavelength assignment unit  251  obtains information on the ONU  220  or the RU  11  corresponding to the current position of the train from the correspondence information. As a result, the wavelength assignment unit  251  obtains information on the ONU  220  in which the main signal is generated and the ONU  220  in which the generation of the main signal is terminated. The wavelength assignment unit  251  exclusively assigns a wavelength for main signal communication to the ONU  220  in which the main signal is generated. 
     Alternatively, each ONU  220  may notify the OLT  250  of generation and termination of the main signal by a control signal. For example, when the wavelength control unit  221  of the ONU  220  detects a signal in which an identifier indicating a main signal is set in a buffer that temporarily stores a signal transmitted from the optical communication unit  122 , the wavelength control unit  221  of the ONU  220  detects generation of the main signal. Furthermore, when the wavelength control unit  221  detects that there is only a signal in which an identifier indicating a control signal is set in a buffer that temporarily stores a signal transmitted from the optical communication unit  122 , the wavelength control unit  221  detects that the generation of the main signal is terminated. For example, the identifier indicating the main signal and the setting contents of the message type “ecpriMessage”can be used as the identifier indicating the main signal. 
     Alternatively, the wavelength control unit  221  of the ONU  220  may monitor the lower communication unit  121 , and detects the occurrence of the main signal communication in a case where the reception rate or the transmission rate exceeds a certain value, and detect the termination of main signal communication in a case where the reception rate or the transmission rate continuously falls below the certain value for a certain period. 
     As described above, when the wavelength assignment unit  251  of the OLT  250  obtains information of the ONUs  220  in which the main signal communication is performed in the same period, the wavelength assignment unit  251  dynamically assigns different wavelengths of the wavelengths λ 1  tp λ n  to the ONUs  220 . The assigned wavelength is notified to the ONU  220  by a control signal, for example. 
       FIG.  5    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system  2 .  FIG.  5    illustrates the RU  11  communicating with the UE  80  at each time and the wavelength used by the ONU  220 . As similar to  FIG.  2   , the UEs  80  are provided in the train C moving at a high speed on the track R, and the RUs  11 - 1  to  11 -N are installed along the track R. A case where there are two UEs  80  provided in the train C or present in the train C, and up to two RUs  11  (n= 2 ) perform radio communication in the same period will be described as an example. 
     At time T 1 , each of the RUs  11 - 1  and  11 - 2  performs radio communication with the corresponding UE  80 . In this case, the wavelength assignment unit  251  of the OLT  250  transmits a control signal for assigning the wavelength λ 1  for main signal communication to the ONU  220 - 1 , and transmits a control signal for assigning the wavelength λ 2  for main signal communication to the ONU  220 - 2 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 1  controls the optical communication unit  122  to transmit and receive the main signal at the wavelength λ 1 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 2  controls the optical communication unit  122  to transmit and receive the main signal at the wavelength λ 2 . The wavelength assignment unit  251  of the OLT  250  may further transmit a control signal for assigning the wavelength λ 0  for control signal communication to all the ONUs  220 . The uplink communication and the downlink communication of the wavelength multiplexing communication system  2  at the time T 1  after the wavelength assignment are similar to the uplink communication and the downlink communication of the wavelength multiplexing communication system  1  at the time T 1  illustrated in  FIG.  2   . 
     At the time T 2 , the RU  11 - 1  terminates the radio communication and the RUs  11 - 2  and  11 - 3  perform radio communication with the UE  80 . The wavelength assignment unit  251  of the OLT  250  detects the termination of the main signal communication in the ONU  220 - 1  and the start of the main signal communication in the ONU  220 - 3 . The wavelength assignment unit  251  transmits a control signal that cancels assignment of the wavelength λ 1  for the main signal communication to the ONU  220 - 1 . The wavelength assignment unit  251  transmits a control signal instructing the ONU  220 - 2  to change the wavelength for main signal communication from the wavelength λ 2  to the wavelength λ 1 . Furthermore, the wavelength assignment unit  251  transmits a control signal to assign a wavelength λ 2  to the ONU  220 - 3 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 1  controls the optical communication unit  122  to stop using the wavelength λ 1 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 2  controls the optical communication unit  122  to transmit and receive the main signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 3  controls the optical communication unit  122  to transmit and receive the main signal with the wavelength λ 2 . The uplink communication and the downlink communication of the wavelength multiplexing communication system  2  at the time T 2  after wavelength assignment is similar to the uplink communication and the downlink communication at the time T 2  illustrated in  FIG.  2    except that a wavelength λ U-1  is used for the uplink main signal from the ONU  220 - 2 , a wavelength λ D-1  is used for the downlink main signal to the ONU  220 - 2 , a wavelength λ U-2  is used for the downlink main signal to the ONU  220 - 3 , a wavelength λ D-2  is used for the downlink main signal to the ONU  220 - 3 . 
     At the time T 3 , the RU  11 - 2  terminates the radio communication and each of the RUs  11 - 3  and the RU  11 - 4  performs radio communication with the corresponding UE  80 . The wavelength assignment unit  251  of the OLT  250  detects the termination of the main signal communication in the ONU  220 - 2  and the start of the main signal communication in the ONU  220 - 4 . The wavelength assignment unit  251  transmits a control signal that cancels assignment of the wavelength λ 1  for the main signal communication to the ONU  220 - 2 . The wavelength assignment unit  251  transmits a control signal instructing the ONU  220 - 3  to change the wavelength for main signal communication from the wavelength λ 2  to the wavelength λ 1 . Furthermore, the wavelength assignment unit  251  transmits a control signal to assign a wavelength λ 2  to the ONU  220 - 4 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 2  controls the optical communication unit  122  to stop using the wavelength λ 1 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 3  controls the optical communication unit  122  to transmit and receive the main signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  221  of the ONU  220 - 4  controls the optical communication unit  122  to transmit and receive the main signal with the wavelength  22 . The uplink communication and the downlink communication of the wavelength multiplexing communication system  2  at the time T 3  after the wavelength assignment are similar to the uplink communication and the downlink communication of the wavelength multiplexing communication system  1  at the time T 3  illustrated in  FIG.  2   . 
     The signals transmitted and received in the optical access section between each ONU of the ONUs  220 - 1  to  220 - 4  and the OLT  250  at each of the times T 1 , T 2 , and T 3  are similar to the signals between each ONU of the ONUs  12 - 1  to  12 - 4  and the OLT  15  illustrated in the left column of  FIG.  2   . 
     In the first embodiment, the wavelength for main signal communication is fixed and assigned to each ONU in advance. However, as similar to the present embodiment, a wavelength assignment unit may be provided in the OLT or the outside of the OLT, and the wavelength assignment unit may assign a wavelength for main signal communication that is different from the wavelength for main signal communication of other ONUs and is fixed to each ONU, to up to n ONUs in which a main signal is generated. 
     Third Embodiment 
     In the first embodiment, both an ONU that performs main signal communication of a mobile system and an ONU that does not perform main signal communication use a wavelength different from that of main signal communication for communication of a control signal of the mobile system. In the present embodiment, the ONU that performs main signal communication of the mobile system also performs communication of the control signal with the wavelength for the main signal communication. Hereinafter, differences from the first embodiment will be mainly described. 
       FIG.  6    is a diagram illustrating an exemplary configuration of a wavelength multiplexing communication system  3  according to a third embodiment. In  FIG.  6   , parts that are the same as those of the wavelength multiplexing communication system  1  illustrated in  FIG.  1    are denoted by the same reference signs, and description thereof will be omitted. The wavelength multiplexing communication system  3  illustrated in  FIG.  6    is different from the wavelength multiplexing communication system  1  illustrated in  FIG.  1    in that the wavelength multiplexing communication system  3  includes ONUs  320  instead of the ONUs  12 , an OLT  350  instead of the OLT  15 , and a transfer apparatus  360  instead of the transfer apparatus  16 . A j-th ONU  320  (j is an integer equal to or greater than 1 and equal to or less than N) is referred to as an ONU  320 - j . As similar to the wavelength multiplexing communication system  1   a  illustrated in  FIG.  3   , the network configuration of the MFH section may be a bus type. 
     In the present embodiment, wavelengths λ 0  to λ n  are used for communication between the ONUs  320  and the OLT  350 , and wavelengths λ 1  to λ n  are used for main signal communication. As similar to the first embodiment, the wavelength assigned to a certain ONU  320  for main signal communication for the ONU  320  is always the same. As similar to the first embodiment, when j is not a multiple of n, a wavelength λ (j mod(n))  is assigned to the ONU  320 - j , and when j is a multiple of n, a wavelength λ n  is assigned to the ONU  320 - j . The present embodiment is different from the first embodiment in that the up to n ONUs  320  that perform the main signal communication perform the main signal communication and the control signal communication by using the wavelength λ (j mod(n))  in a case where j is not a multiple of n, and by using the wavelength λ n  in a case where j is a multiple of n. The ONU  320  that does not perform the main signal communication performs control signal communication by using the wavelength λ 0 . Logically, the RU  11 - 1  and the DU  17 - 1  are connected on a one-to-one basis, and the RU  11 - 2  and the DU  17 - 2  are connected on a one-to-one basis. As similar to this, logically, the RU  11   j  and the DU  17 -(j mod(n)) in which j is not a multiple of n are connected on a one-to-one basis, and the RU  11   j  and the DU  17 - n  in which j is a multiple of n are connected on a one-to-one basis. 
     The ONU  320  is different from the ONU  20  of the first embodiment in that the ONU  320  further includes an optical communication unit  321  instead of the optical communication unit  122 , and a wavelength control unit  323 . The optical communication unit  321  includes an optical transmission/reception unit  322 . The optical transmission/reception unit  322  is a variable optical transceiver. As similar to the optical communication unit  122  of the first embodiment, the optical communication unit  321  may include the optical transmission/reception units  123  and  124 . The optical communication unit  321  uses the optical transmission/reception unit  123  for communication of a control signal when there is no main signal communication, and uses the optical transmission/reception unit  124  for communication of a main signal and a control signal when there is main signal communication. When the main signal communication is not performed in the host ONU, the wavelength control unit  323  controls the optical communication unit  321  to transmit the uplink control signal with the wavelength λ U-0 , and transmit the downlink control signal with the wavelength λ D-0 . When the main signal communication is performed in the host ONU, the wavelength control unit  323  of the ONU  320 - j  in which j is not a multiple of n controls the optical communication unit  321  to transmit an uplink control signal with a wavelength λ U-(j mod(n)) , and transmit the downlink control signal with a wavelength λ D-(j mod(n)) , and the wavelength control unit  323  of the ONU  320 - j  in which j is a multiple of n controls the optical communication unit  321  to transmit the uplink control signal by a wavelength λ U-n , and transmit the downlink control signal by a wavelength λ D-n . 
     The OLT  350  is different from the OLT  15  of the first embodiment in that the OLT  350  further includes a wavelength assignment unit  351 . The wavelength assignment unit  351  may be provided in an apparatus external to the OLT  350 . The wavelength assignment unit  351  instructs up to n ONUs  320 - j  in which main signal communication is performed during the same period to use a wavelength λ (j mod(n))  when j is not a multiple of n, and to use a wavelength λ n  when j is a multiple of n. The wavelength assignment unit  351  instructs the ONU  320  in which the main signal communication is not performed, to use a wavelength λ 0 . The wavelength assignment unit  351  outputs information on the DU  11  in which the main signal is generated and information on the ONU  320  to which any of wavelengths λ 1  to λ n  has been exclusively assigned, to the transfer apparatus  360 . 
     The transfer apparatus  360  does not identify the control signal frame and the main signal frame, and performs distribution on the basis of information on a destination or a transmission source set in the frame and whether main signal communication is performed in the RU  11 . The transfer apparatus  360  may determine that main signal communication is performed in the RU  11   j  that is connected to the ONU  320 - j  to which any of wavelengths λ 1  to λ n  has been exclusively assigned. The transfer apparatus  360  transfers the uplink signal received from the OLT-CT  151  to the DU  17  of the destination set to the uplink signal. The transfer apparatus  360  receives a downlink signal destined for the RU  11   j  in which j is not a multiple of n from the DU  17 -(j mod(n)), and receives a downlink signal destined for the RU  11   j  in which j is a multiple of n from the DU  17 - n . When a main signal is generated in the RU  11   j  of the destination of the downlink signal and any of wavelengths λ 1  to λ n  is exclusively assigned to the ONU  320 - j  connected to the RU  11   j , the transfer apparatus  360  transfers the downlink signal to the OLT-CT  151  corresponding to the wavelength assigned to the ONU  320 - j . When a main signal is not generated in the RU  11   j  of the destination of the downlink signal and the wavelength λ 0  is assigned to the ONU  320 - j  connected to the RU  11   j , the transfer apparatus  360  transfers the downlink signal to the OLT-CT  151 - 0 . 
     The OLT  350  and the ONU  320  can detect the RU  11  or the ONU  220  in which main signal communication is performed in a similar manner to that in the second embodiment. When the wavelength control unit  323  of the ONU  320  detects the occurrence of main signal communication without assignment of a wavelength by the wavelength assignment unit  351  of the OLT  350 , the wavelength control unit  323  of the ONU  320  may control the optical transmission/reception unit  322  to use a wavelength for main signal communication assigned in advance. On the other hand, in the transfer apparatus  360 , it is necessary to refer to the information on the DU  11  in which the main signal is generated and information on the ONU  320  to which any of the wavelengths λ 1  to λ n  has been exclusively assigned. Thus, the wavelength control unit  323  of the ONU  320  notifies the OLT  350  or the transfer apparatus  360  of the occurrence and termination of the main signal communication. 
     The ONU  320  may determine the occurrence and termination of the main signal communication as follows, in addition to the method described in the second embodiment. The OLT  350  notifies the ONU  320  of the operation information of the train. The wavelength control unit  323  of the ONU  320  stores the received operation information, and periodically refers to the operation information to obtain the current position of the train. Alternatively, the wavelength control unit  323  of the ONU  320  may receive position information indicating the current position of the train from the OLT  350 , the camera, or the sensor. The wavelength control unit  323  stores correspondence information indicating correspondence with the ONU  320  or the RU  11  in which the main signal communication is performed, and obtains information on the ONU  320  or the RU  11  according to the current position of the train with reference to the correspondence information. As a result, the wavelength control unit  323  determines the generation of the main signal and the termination of the generation in the host slave station apparatus. 
       FIG.  7    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system  3 .  FIG.  7    illustrates the RU  11  communicating with the UE  80  at each time and the wavelength used by the ONU  320 . As similar to  FIG.  2   , the UEs  80  are provided in the train C moving at a high speed on the track R, and the RUs  11 - 1  to  11 -N are installed along the track R. A case where there are two UEs  80  provided in the train C, and up to two RUs  11  (n= 2 ) perform radio communication in the same period will be described as an example. 
     At time T 1 , each of the RUs  11 - 1  and  11 - 2  performs radio communication with the corresponding UE  80 . In this case, the wavelength assignment unit  351  of the OLT  350  transmits a control signal for assigning a wavelength λ 1  to the ONU  320 - 1 , transmits a control signal for assigning a wavelength  22  to the ONU  320 - 2 , and transmits a control signal for assigning a wavelength λ 0  to the ONUs  320 - 3  to  320 -N. The wavelength assignment unit  351  outputs, to the transfer apparatus  360 , information indicating that the main signal is generated in the RU  11 - 1  and the RU  11 - 2 , and information indicating that the wavelength λ 1  is exclusively assigned to the ONU  320 - 1 , and the wavelength λ 2  is exclusively assigned to the ONU  320 - 2 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 1  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 2  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 2 . When receiving the control signal, the wavelength control unit  323  of each of the ONUs  320 - 3  to  320 -N controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 0 . 
     The uplink communication at time T 1  will be described. The RU  11 - 1  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 1  to the ONU  320 - 1 , and the RU  11 - 2  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 2  to the ONU  320 - 2 . Each RU  11   j  outputs a control signal of the radio system to the ONU  320 - j . The destination of the control signal is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. 
     The optical transmission/reception unit  322  of the ONU  320 - 1  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 1  into an optical signal of a wavelength λ U-1 , and outputs the optical signal to the transmission line  31 - 1 . The optical transmission/reception unit  322  of the ONU  320 - 2  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 2  into an optical signal of a wavelength λ U-2 , and outputs the optical signal to the transmission line  31 - 2 . The optical transmission/reception unit  322  of the ONU  320 - j  excluding j= 1 ,  2  converts the uplink control signal of the optical access system and the uplink control signal of the radio system received by the lower communication unit  121  from the RU  11   j  into an optical signal of a wavelength λ U-0 , and outputs the optical signal to the transmission line  31   j.    
     The optical coupling/splitting unit  13  outputs a wavelength-multiplexed signal obtained by multiplexing the uplink optical signals of the wavelengths λ U-0  to λ U-2  to the transmission line  32 . The wavelength multiplexing/demultiplexing unit  14  demultiplexes the uplink wavelength-multiplexed signal transmitted through the transmission line  32  into the uplink optical signals of the wavelengths λ U-0  to λ U-2 . The wavelength multiplexing/demultiplexing unit  14  outputs a light signal of a wavelength λ U-0  to the OLT-CT  151 - 0 , an optical signal of a wavelength λ U-1  to the OLT-CT  151 - 1 , and an optical signal of a wavelength λ U-2  to the OLT-CT  151 - 2 . Each of the OLT-CTs  151 - 0 ,  151 - 1 , and  151 - 2  converts the input optical signal into an electrical signal and output the electrical signal to the transfer apparatus  360 . 
     The transfer apparatus  360  receives the control signal from the OLT-CT  151 - 0 . The transmission source of the control signal is the ONU  320 - j  excluding the ONUs  320 - 1  and  320 - 2 , and the destination is the DU  17  logically connected to the ONU  320 - j . The destination is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. The transfer apparatus  360  transfers the control signal received from the OLT-CT  151 - 0  to the DU  17  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 1  and transfers these signals to the DU  17 - 1  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 2  and transfers these signals to the DU  17 - 2  that is the destination. The DUs  17 - 1  and  17 - 2  output the main signals to the CU  18 . 
     Next, downlink communication at the time T 1  will be described. The CU  18  outputs a downlink main signal destined for the RU  11 - 1  to the DU  17 - 1 , and outputs a downlink main signal addressed to the RU  11 - 2  to the DU  17 - 2 . The DU  17 - 1  outputs the main signal and the control signal addressed to the RU  11 - 1  to the transfer apparatus  360 , and the DU  17 - 2  outputs the main signal and the control signal addressed to the RU  11 - 2  to the transfer apparatus  360 . The DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ) excluding j= 1 ,  2  outputs the control signal addressed to the RU  11   j  to the transfer apparatus  360 . 
     The transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 1  from the DU  17 - 1 . Since a main signal is generated in the RU  11 - 1  and a wavelength λ 1  is exclusively assigned to the ONU  320 - 1 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 1  to the OLT-CT  151 - 1  corresponding to the wavelength λ 1 . As similar to this, the transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 2  from the DU  17 - 2 . Since a main signal is generated in the RU  11 - 2  and a wavelength λ 2  is exclusively assigned to the ONU  320 - 2 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 2  to the OLT-CT  151 - 2  corresponding to the wavelength λ 2 . The transfer apparatus  360  receives the control signal destined for the RU  11   j  excluding j= 1 ,  2  from the DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ). Since a main signal is not generated in the RU  11   j  and a wavelength λ 0  that is also assigned to another ONU  320  is assigned to the ONU  320 - j , the transfer apparatus  360  transfers the control signal destined for the RU  11   j  to the OLT-CT  151 - 0 . 
     The OLT-CT  151 - 0  converts the downlink control signal destined for each of the ONUs  320 - 3  to  320 -N into an optical signal of a wavelength λ D-0 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The OLT-CT  151 - 1  converts the downlink main signal and the downlink control signal destined for the ONU  320 - 1  into an optical signal of a wavelength λ D-1  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 , and the OLT-CT  151 - 2  converts a downlink main signal and a downlink control signal destined for the ONU  320 - 2  into an optical signal of a wavelength λ D-2  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . 
     The wavelength multiplexing/demultiplexing unit  14  outputs a wavelength-multiplexed signal obtained by multiplexing the downlink optical signals of the wavelengths λ D-0 , λ D-1 , and λ D-2  output by the OLT  350 , to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal from the transmission line  32 , splits the received wavelength-multiplexed signal, and outputs the result to the corresponding transmission line among the transmission lines  31 - 1  to  31 -N. 
     The optical transmission/reception unit  322  of the ONU  320 - 1  receives a control signal and a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  322  of the ONU  320 - 2  receives a control signal and a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  123  of each of the ONUs  320 - 3  to  320 -N receives a control signal of a wavelength of λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. The lower communication unit  121  of the ONU  320 - 1  outputs the main signal and the control signal of the radio system to the RU  11 - 1 , and the lower communication unit  121  of the ONU  320 - 2  outputs the main signal and the control signal of the radio system to the RU  11 - 2 . The lower communication unit  121  of each ONUs  320 - 3  to  320 -N outputs the control signal of the radio system to the corresponding RU among the RUs  11 - 3  to  11 -N. 
     At time T 2 , each of the RUs  11 - 2  and  11 - 3  performs radio communication with the corresponding UE  80 . The wavelength assignment unit  351  of the OLT  350  transmits a control signal for assigning a wavelength λ 2  to the ONU  320 - 2 , transmits a control signal for assigning a wavelength λ 1  to the ONU  320 - 3 , and transmits a control signal for assigning a wavelength λ 0  to the ONU  320 - 1  and the ONUs  320 - 4  to  320 -N. The wavelength assignment unit  351  outputs, to the transfer apparatus  360 , information indicating that the main signal is generated in the RU  11 - 2  and the RU  11 - 3 , and information indicating that the wavelength λ 2  is exclusively assigned to the ONU  320 - 2 , and the wavelength λ 1  is exclusively assigned to the ONU  320 - 3 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 2  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 2 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 3  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  323  of each of the ONUs  320 - 1  and  320 - 4  to  320 -N controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 0 . The wavelength control unit  323  may not transmit a control signal for assigning a wavelength to any of the ONU  320 - 2  and the ONUs  320 - 4  to  320 -N whose wavelengths to be used do not change from the time T 1 . 
     The uplink communication at the time T 2  will be described. The RU  11 - 2  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 2  to the ONU  320 - 2 , and the RU  11 - 3  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 1  to the ONU  320 - 3 . Each RU  11   j  outputs a control signal of the radio system to the ONU  320 - j . The destination of the control signal is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. 
     The optical transmission/reception unit  322  of the ONU  320 - 2  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 2  into an optical signal of a wavelength of λ U-2 , and outputs the optical signal to the transmission line  31 - 2 . The optical transmission/reception unit  322  of the ONU  320 - 3  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 3  into an optical signal of a wavelength of λ U-1 , and outputs the optical signal to the transmission line  31 - 3 . The optical transmission/reception unit  322  of the ONU  320 - j  excluding j= 2 ,  3  converts the uplink control signal of the optical access system and the uplink control signal of the radio system received by the lower communication unit  121  from the RU  11   j  into an optical signal of a wavelength of λ U-0 , and outputs the optical signal to the transmission line  31   j.    
     The operation from when the optical coupling/splitting unit  13  outputs the wavelength-multiplexed signal obtained by multiplexing the upstream optical signals of the wavelengths λ U-0  to λ U-2  to the transmission line  32  to when the OLT-CTs  151 - 0 ,  151 - 1 , and  151 - 2  convert the input optical signals into electrical signals and output the electrical signals to the transfer apparatus  360  is similar to that at the time T 1 . 
     The transfer apparatus  360  receives the control signal from the OLT-CT  151 - 0 . The transmission source of the control signal is the ONU  320 - j  excluding the ONUs  320 - 2  and  320 - 3 , and the destination is the DU  17  logically connected to the ONU  320 - j . The destination is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. The transfer apparatus  360  transfers the control signal received from the OLT-CT  151 - 0  to the DU  17  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 2  and transfers these signals to the DU  17 - 2  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 1  and transfers these signals to the DU  17 - 1  that is the destination. The DUs  17 - 1  and  17 - 2  output the main signals to the CU  18 . 
     Next, downlink communication at the time T 2  will be described. The CU  18  outputs a downlink main signal destined for the RU  11 - 2  to the DU  17 - 2 , and outputs a downlink main signal destined for the RU  11 - 3  to the DU  17 - 1 . The DU  17 - 2  outputs the main signal and the control signal destined for the RU  11 - 2  to the transfer apparatus  360 , and the DU  17 - 1  outputs the main signal and the control signal destined for the RU  11 - 3  to the transfer apparatus  360 . The DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ) excluding j= 2 ,  3  outputs the control signal addressed to the RU  11   j  to the transfer apparatus  360 . 
     The transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 2  from the DU  17 - 2 . Since a main signal is generated in the RU  11 - 2  and a wavelength λ 2  is exclusively assigned to the ONU  320 - 2 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 2  to the OLT-CT  151 - 2  corresponding to the wavelength λ 2 . As similar to this, the transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 3  from the DU  17 - 1 . Since a main signal is generated in the RU  11 - 3  and a wavelength λ 1  is exclusively assigned to the ONU  320 - 3 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 3  to the OLT-CT  151 - 1  corresponding to the wavelength λ 1 . The transfer apparatus  360  receives the control signal destined for the RU  11   j  excluding j= 2 ,  3  from the DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ). Since a main signal is not generated in the RU  11   j  and a wavelength λ 0  that is also assigned to another ONU  320  is assigned to the ONU  320 - j , the transfer apparatus  360  transfers the control signal destined for the RU  11   j  to the OLT-CT  151 - 0 . 
     The OLT-CT  151 - 0  converts the downlink control signal destined for each of the ONUs  320 - 1  and  320 - 4  to  320 -N into an optical signal of a wavelength of λ D-0 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The OLT-CT  151 - 1  converts the downlink main signal and the downlink control signal destined for the ONU  320 - 3  into an optical signal of a wavelength λ D-1  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 , and the OLT-CT  151 - 2  converts a downlink main signal and a downlink control signal destined for the ONU  320 - 2  into an optical signal of a wavelength λ D-2  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . 
     The wavelength multiplexing/demultiplexing unit  14  outputs a wavelength-multiplexed signal obtained by multiplexing the downlink optical signals of the wavelengths λ D-0 , λ D-1 , and  4 - 2  output by the OLT  350 , to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal from the transmission line  32 , splits the received wavelength-multiplexed signal, and outputs the result to the corresponding transmission line among the transmission lines  31 - 1  to  31 -N. 
     The optical transmission/reception unit  322  of the ONU  320 - 2  receives a control signal and a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  322  of the ONU  320 - 3  receives a control signal and a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  123  of each of the ONUs  320 - 1  and  320 - 4  to  320 -N receives a control signal of a wavelength of λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. The lower communication unit  121  of the ONU  320 - 2  outputs the main signal and the control signal of the radio system to the RU  11 - 2 , and the lower communication unit  121  of the ONU  320 - 3  outputs the main signal and the control signal of the radio system to the RU  11 - 3 . The lower communication unit  121  of the ONU  320 - j  excluding j= 2 ,  3  outputs the control signal for the radio system to the RU  11   j.    
     At time T 3 , each of the RUs  11 - 3   11 - 4  performs radio communication with the corresponding UE  80 . The wavelength assignment unit  351  of the OLT  350  transmits a control signal for assigning a wavelength λ 1  to the ONU  320 - 3 , transmits a control signal for assigning a wavelength λ 2  to the ONU  320 - 4 , and transmits a control signal for assigning a wavelength λ 0  to the ONU  320 - j  excluding j= 3 ,  4 . The wavelength assignment unit  351  outputs, to the transfer apparatus  360 , information indicating that the main signal is generated in the RU  11 - 3  and the RU  11 - 4 , and information indicating that the wavelength λ 1  is exclusively assigned to the ONU  320 - 3 , and the wavelength λ 2  is exclusively assigned to the ONU  320 - 4 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 3  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 4  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 2 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - j  excluding j = 3 ,  4  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 0 . The wavelength control unit  323  may not transmit a control signal for assigning a wavelength to the ONU  320 - 1  and the ONUs  320 - 5  to  320 -N whose wavelengths to be used do not change from the time T 2 . 
     The uplink communication at the time T 3  will be described. The RU  11 - 3  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 1  to the ONU  320 - 3 , and the RU  11 - 4  outputs an uplink main signal received by radio communication from the UE  80  and destined for the DU  17 - 2  to the ONU  320 - 4 . Each RU  11   j  outputs a control signal of the radio system to the ONU  320 - j . The destination of the control signal is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. 
     The optical transmission/reception unit  322  of the ONU  320 - 3  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 3  into an optical signal of a wavelength of  44 , and outputs the optical signal to the transmission line  31 - 3 . The optical transmission/reception unit  322  of the ONU  320 - 4  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 4  into an optical signal of a wavelength λ U-2 , and outputs the optical signal to the transmission line  31 - 4 . The optical transmission/reception unit  322  of the ONU  320 - j  excluding j= 3 ,  4  converts the uplink control signal of the optical access system and the uplink control signal of the radio system received by the lower communication unit  121  from the RU  11   j  into an optical signal of a wavelength λ U-0 , and outputs the optical signal to the transmission line  31   j.    
     The operation from when the optical coupling/splitting unit  13  outputs the wavelength-multiplexed signal obtained by multiplexing the upstream optical signals of the wavelengths λ U-0  to λ U-2  to the transmission line  32  to when the OLT-CTs  151 - 0 ,  151 - 1 , and  151 - 2  convert the input optical signals into electrical signals and output the electrical signals to the transfer apparatus  360  is similar to that at the time T 1 . 
     The transfer apparatus  360  receives the control signal from the OLT-CT  151 - 0 . The transmission source of the control signal is the ONU  320 - j  excluding the ONUs  320 - 3  and  320 - 4 , and the destination is the DU  17  logically connected to the ONU  320 - j . The destination is DU  17 -(j mod(n)) when j is not a multiple of n, and DU  17 - n  when j is a multiple of n. The transfer apparatus  360  transfers the control signal received from the OLT-CT  151 - 0  to the DU  17  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 1  and transfers these signals to the DU  17 - 1  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 2  and transfers these signals to the DU  17 - 2  that is the destination. The DUs  17 - 1  and  17 - 2  output the main signals to the CU  18 . 
     Next, downlink communication at the time T 3  will be described. The CU  18  outputs a downlink main signal destined for the RU  11 - 3  to the DU  17 - 1 , and outputs a downlink main signal destined for the RU  11 - 4  to the DU  17 - 2 . The DU  17 - 1  outputs the main signal and the control signal destined for the RU  11 - 3  to the transfer apparatus  360 , and the DU  17 - 2  outputs the main signal and the control signal destined for the RU  11 - 4  to the transfer apparatus  360 . The DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ) excluding j= 3 ,  4  outputs the control signal addressed to the RU  11   j  to the transfer apparatus  360 . 
     The transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 3  from the DU  17 - 1 . Since a main signal is generated in the RU  11 - 3  and a wavelength λ 1  is exclusively assigned to the ONU  320 - 3 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 3  to the OLT-CT  151 - 1  corresponding to the wavelength λ 1 . As similar to this, the transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 4  from the DU  17 - 2 . Since a main signal is generated in the RU  11 - 4  and a wavelength λ 2  is exclusively assigned to the ONU  320 - 4 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 4  to the OLT-CT  151 - 2  corresponding to the wavelength λ 2 . The transfer apparatus  360  receives the control signal destined for the RU  11   j  excluding j= 3 ,  4  from the DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ). Since a main signal is not generated in the RU  11   j  and a wavelength λ 0  that is also assigned to another ONU  320  is assigned to the ONU  320 - j , the transfer apparatus  360  transfers the control signal destined for the RU  11   j  to the OLT-CT  151 - 0 . 
     The OLT-CT  151 - 0  converts the downlink control signal destined for each of the ONU  320 - 1 , ONU  320 - 2 , and the ONUs  320 - 5  to  320 -N into an optical signal of a wavelength of λ D-0 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The OLT-CT  151 - 1  converts the downlink main signal and the downlink control signal destined for the ONU  320 - 3  into an optical signal of a wavelength λ D-1  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 , and the OLT-CT  151 - 2  converts a downlink main signal and a downlink control signal destined for the ONU  320 - 4  into an optical signal of a wavelength λ D-2  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . 
     The wavelength multiplexing/demultiplexing unit  14  outputs a wavelength-multiplexed signal obtained by multiplexing the downlink optical signals of the wavelengths λ D-0 , λ D-1 , and λ D-2  output by the OLT  350 , to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal from the transmission line  32 , splits the received wavelength-multiplexed signal, and outputs the result to the corresponding transmission line among the transmission lines  31 - 1  to  31 -N. 
     The optical transmission/reception unit  322  of the ONU  320 - 3  receives a control signal and a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  322  of the ONU  320 - 4  receives a control signal and a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. Each of the optical transmission/reception unit  123  of the ONUs  320 - 1 ,  320 - 2 , and  320 - 5  to  320 -N receives a control signal of a wavelength of λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. The lower communication unit  121  of the ONU  320 - 3  outputs the main signal and the control signal of the radio system to the RU  11 - 3 , and the lower communication unit  121  of the ONU  320 - 4  outputs the main signal and the control signal of the radio system to the RU  11 - 4 . The lower communication unit  121  of the ONU  320 - j  excluding j= 3 ,  4  outputs the control signal for the radio system to the RU  11   j.    
     The signals transmitted and received in the optical access section between the ONUs  320 - 1  to  320 - 4  and the OLT  350  at each of the times T 1 , T 2 , and T 3  are similar to the signals between the ONUs  12 - 1  to  12 - 4  and the OLT  15  illustrated in the left column of  FIG.  2   . 
     In the present embodiment, only during a period in which main signal communication is performed in the RU, a wavelength for main signal communication is exclusively assigned to the ONU connected to the RU. All ONUs excluding the ONU connected to the RU in which the main signal communication is performed transmit and receive the control signal using the wavelength λ 0 . Thus, the number of OLT-CTs and the number of wavelengths used can be reduced to (n+ 1 ). 
     In the present embodiment, a wavelength-variable transceiver can be used for the ONU, and the ONU does not necessarily need to include a plurality of optical transceivers for transmission and reception of control signals and for transmission and reception of main signals. Therefore, capital investment costs can be reduced. 
     Since the exclusively assigned wavelength can be occupied and utilized by the assigned ONU, a high throughput can be achieved. Since a wait delay with another ONU does not occur, a main signal generated in the RU can be transmitted with a low delay. 
     Fourth Embodiment 
     In the fourth embodiment, the wavelength used by each ONU is fixed. In the fourth embodiment, a wavelength used for an ONU that performs main signal communication is changed. Hereinafter, differences from the third embodiment will be mainly described. 
     The configuration of the wavelength multiplexing communication system of the present embodiment is similar to that of the wavelength multiplexing communication system  3  of the third embodiment illustrated in  FIG.  6   . However, the wavelength assignment unit  351  of the OLT  350  exclusively and dynamically assigns a wavelength for main signal communication to the ONU  320  in which the main signal is generated. The wavelength to be assigned is changed, for example, when the set of ONUs  320  in which the main signal is generated changes, but may be changed at other timings. 
       FIG.  8    is a diagram illustrating a use case of a moving body of the wavelength multiplexing communication system  3  according to the present embodiment.  FIG.  8    illustrates the RU  11  communicating with the UE  80  at each time and the wavelength used by the ONU  320 . As similar to  FIG.  2   , the UEs  80  are provided in the train C moving at a high speed on the track R or present in the train C, and the RUs  11 - 1  to  11 -N are installed along the track R. A case where there are two UEs  80  provided in the train C or present in the train C, and up to two RUs  11  (n= 2 ) perform radio communication in the same period will be described as an example. 
     At time T 1 , each of the RUs  11 - 1  and  11 - 2  performs radio communication with the corresponding UE  80 . The operation of the uplink communication and the downlink communication of the wavelength multiplexing communication system  3  at the time T 1  is similar to that of the third embodiment. 
     At time T 2 , each of the RUs  11 - 2  and  11 - 3  performs radio communication with the corresponding UE  80 . The wavelength assignment unit  351  of the OLT  350  transmits a control signal for assigning a wavelength  80   1  to the ONU  320 - 2 , transmits a control signal for assigning a wavelength λ 2  to the ONU  320 - 3 , and transmits a control signal for assigning a wavelength λ 0  to the ONUs  320 - 1  and  320 - 4  to  320 -N. The wavelength assignment unit  351  outputs, to the transfer apparatus  360 , information indicating that the main signal is generated in the RU  11 - 2  and the RU  11 - 3 , and information indicating that the wavelength λ 1  is exclusively assigned to the ONU  320 - 2 , and the wavelength λ 2  is exclusively assigned to the ONU  320 - 3 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 2  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 1 . When receiving the control signal, the wavelength control unit  323  of the ONU  320 - 3  controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 2 . When receiving the control signal, the wavelength control unit  323  of each of the ONUs  320 - 1  and  320 - 4  to  320 -N controls the optical communication unit  321  to transmit and receive the optical signal with the wavelength λ 0 . 
     The uplink communication at the time T 2  will be described. The operation of the RUs  11 - 1  to  11 -N is similar to that of the wavelength multiplexing communication system  3  of the third embodiment. The optical transmission/reception unit  322  of the ONU  320 - 2  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 2  into an optical signal of a wavelength λ U-1 , and outputs the optical signal to the transmission line  31 - 2 . The optical transmission/reception unit  322  of the ONU  320 - 3  converts the uplink control signal of the optical access system and the uplink control signal and the uplink main signal of the radio system received by the lower communication unit  121  from the RU  11 - 3  into an optical signal of a wavelength λ U-2 , and outputs the optical signal to the transmission line  31 - 3 . The optical transmission/reception unit  322  of the ONU  320 - j  excluding j= 2 ,  3  converts the uplink control signal of the optical access system and the uplink control signal of the radio system received by the lower communication unit  121  from the RU  11   j  into an optical signal of a wavelength λ U-0 , and outputs the optical signal to the transmission line  31   j.    
     The operation from when the optical coupling/splitting unit  13  outputs the wavelength-multiplexed signal obtained by multiplexing the upstream optical signals of the wavelengths λ U-0  to λ U-2  to the transmission line  32  to when the OLT-CTs  151 - 0 ,  151 - 1 , and  151 - 2  convert the input optical signals into electrical signals and output the electrical signals to the transfer apparatus  360  is similar to that at the time T 1 . 
     The transfer apparatus  360  transfers the control signal received from the OLT-CT  151 - 0  similarly to the third embodiment. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 1  and transfers these signals to the DU  17 - 2  that is the destination. The transfer apparatus  360  receives the main signal and the control signal from the OLT-CT  151 - 2  and transfers these signals to the DU  17 - 1  that is the destination. The DUs  17 - 1  and  17 - 2  output the main signals to the CU  18 . 
     Next, downlink communication at the time T 2  will be described. The CU  18  outputs a downlink main signal destined for the RU  11 - 2  to the DU  17 - 2 , and outputs a downlink main signal destined for the RU  11 - 3  to the DU  17 - 1 . The DU  17 - 2  outputs the main signal and the control signal destined for the RU  11 - 2  to the transfer apparatus  360 , and the DU  17 - 1  outputs the main signal and the control signal destined for the RU  11 - 3  to the transfer apparatus  360 . The DU  17 -(j mod(n)) (however, when j is a multiple of n, DU  17 - n ) excluding j= 2 ,  3  outputs the control signal addressed to the RU  11   j  to the transfer apparatus  360 . 
     The transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 2  from the DU  17 - 2 . Since a main signal is generated in the RU  11 - 2  and a wavelength λ 1  is exclusively assigned to the ONU  320 - 2 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 2  to the OLT-CT  151 - 1  corresponding to the wavelength λ 1 . As similar to this, the transfer apparatus  360  receives a main signal and a control signal destined for the RU  11 - 3  from the DU  17 - 1 . Since a main signal is generated in the RU  11 - 3  and a wavelength λ 2  is exclusively assigned to the ONU  320 - 3 , the transfer apparatus  360  transfers the main signal and the control signal destined for the RU  11 - 3  to the OLT-CT  151 - 2  corresponding to the wavelength λ 2 . The transfer apparatus  360  receives the control signal destined for the RU  11   j  excluding j= 2 ,  3  from the DU  17 -(j mod(n)) (however, j is a multiple of n, DU  17 - n ) and transfers the control signal to the OLT-CT  151 - 0 , similarly to the third embodiment. 
     The OLT-CT  151 - 0  converts the downlink control signal destined for each of the ONUs  320 - 1  and  320 - 4  to  320 -N into an optical signal of a wavelength of λ D-0 , and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . The OLT-CT  151 - 1  converts the downlink main signal and the downlink control signal destined for the ONU  320 - 2  into an optical signal of a wavelength λ D-1  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 , and the OLT-CT  151 - 2  converts a downlink main signal and a downlink control signal destined for the ONU  320 - 3  into an optical signal of a wavelength λ D-2  and outputs the optical signal to the wavelength multiplexing/demultiplexing unit  14 . 
     The wavelength multiplexing/demultiplexing unit  14  outputs a wavelength-multiplexed signal obtained by multiplexing the downlink optical signals of the wavelengths λ D-0 , λ D-1 , and λ D-2  output by the OLT  350 , to the transmission line  32 . The optical coupling/splitting unit  13  receives the wavelength-multiplexed signal from the transmission line  32 , splits the received wavelength-multiplexed signal, and outputs the result to the corresponding transmission line among the transmission lines  31 - 1  to  31 -N. 
     The optical transmission/reception unit  322  of the ONU  320 - 2  receives a control signal and a main signal of a wavelength λ D-1  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  322  of the ONU  320 - 3  receives a control signal and a main signal of a wavelength λ D-2  from the wavelength-multiplexed signal and converts the control signal and the main signal into an electrical signal. The optical transmission/reception unit  123  of each of the ONUs  320 - 1  and  320 - 4  to  320 -N receives a control signal of a wavelength of λ D-0  from the wavelength-multiplexed signal and converts the control signal into an electrical signal. Subsequent operation is similar to that of the third embodiment. 
     At time T 3 , each of the RUs  11 - 3  and  11 - 4  performs radio communication with the corresponding UE  80 . The operation of the wavelength multiplexing communication system  3  at the time T 3  is similar to that of the third embodiment. 
     The signals transmitted and received in the optical access section between the ONUs  320 - 1  to  320 - 4  and the OLT  350  at each of the times T 1 , T 2 , and T 3  are similar to the signals between the ONUs  12 - 1  to  12 - 4  and the OLT  15  illustrated in the left column of  FIG.  2   . 
     According to the embodiments described above, the wavelength multiplexing communication system includes a master station apparatus and a plurality of slave station apparatuses. The master station apparatus is, for example, the OLT  15 ,  250 , or  350 . The slave station apparatus is, for example, the ONU  12 ,  220 , or  330 . The master station apparatus includes a wavelength multiplexing communication unit. The wavelength multiplexing communication unit is, for example, the OLT-CT  151 - 0  to  151 -N. The wavelength multiplexing communication unit performs wavelength multiplexing communication with the plurality of slave station apparatuses by wavelengths the number of which is equal to or less than the number of the plurality of slave station apparatuses, using an optical signal of one or more wavelengths included in a first wavelength group and an optical signal having one or more wavelengths included in a second wavelength group. For example, the wavelength in the first wavelength group is the wavelength for the main signal communication, and the wavelength in the second wavelength group is the wavelength for the control signal communication. The slave station apparatus includes an optical communication unit. When the main signal communication is performed in the host slave station apparatus, the optical communication unit performs communication of the main signal with the master station apparatus by an optical signal of a wavelength in the first wavelength group, which is different from a wavelength in the first wavelength group used by another slave station. When the main signal communication is not performed in the host slave station apparatus, the optical communication unit performs communication of a signal other than the main signal with the master station apparatus by an optical signal of a wavelength in the second wavelength group, which is a wavelength same as a wavelength used by another slave station apparatus. Therefore, the number of wavelengths used between the master station apparatus and the slave station apparatuses is equal to or less than the total of the number of slave station apparatuses that perform communication of the main signal at the same time and the number of wavelengths in the second wavelength group. 
     When the main signal communication is performed in the host slave station apparatus, the optical communication unit performs communication of the main signal with the master station apparatus by an optical signal of a wavelength in the first wavelength group, which is different from a wavelength in the first wavelength group used by another slave station, and performs communication of a signal other than the main signal with the master station apparatus by an optical signal of a wavelength in the second wavelength group, which is a wavelength same as a wavelength used by another slave station apparatus. The wavelength within the second wavelength group may be fixed and assigned to each slave station apparatus. 
     When the main signal communication is performed in the host slave station apparatus, the optical communication unit may perform communication of the main signal and communication of a signal other than the main signal with the master station apparatus by an optical signal of a wavelength in the first wavelength group, which is different from a wavelength in the first wavelength group used by another slave station. 
     The wavelength multiplexing communication system may further include a wavelength assignment unit that assigns different wavelengths included in the first wavelength group to each of the plurality of slave station apparatuses in which the main signal is generated. The wavelength assignment unit may dynamically assign a different wavelength included in the first wavelength group to each of the plurality of slave station apparatuses in which the main signal is generated. 
     The wavelength multiplexing communication unit may include a plurality of optical signal terminations that terminate optical signals of different wavelengths. An example of the optical signal terminations includes OLT-CT  151 . The number of optical signal terminations included in the wavelength multiplexing communication unit is the total of the maximum value of the number of the slave station apparatuses that perform communication of the main signal at the same time and the number of wavelengths in the second wavelength group. 
     Although embodiments of the present invention have been described above in detail with reference to the drawings, the specific configurations thereof are not limited to those of the embodiments and also include designs or the like without departing from the spirit of the present invention. 
     Reference Signs List
           1 ,  1   a ,  2 ,  3  Wavelength multiplexing communication system     11 - 1  to  11 - 2   n  RU     12 - 1  to  12 - 2   n ,  220 - 1  to  220 - 2   n ,  320 - 1  to  320 - 2   n  ONU     13  Optical coupling/splitting unit     14  Wavelength multiplexing/demultiplexing unit     15 ,  250 ,  350  OLT     16 ,  360  Transfer apparatus     17 - 1  to  17 - n  DU     18  CU     31 - 1  to  31 - 2   n ,  32 ,  34 ,  36  Transmission line     35  Optical coupling/splitting unit     80  UE     121  Lower communication unit     122 ,  321  Optical communication unit     123 ,  124 ,  322  Optical transmission/reception unit     151 - 0  to  151 - n  OLT-CT     221 ,  323  Wavelength control unit     251 ,  351  Wavelength assignment unit