Optical communication system, master station device, slave station device, and optical communication method

Abstract

Provided is an optical communication system for performing transmission / reception of an optical signal between a master station device and a slave station device and performing transmission / reception of a wireless signal between the slave station device and one or a plurality of communication terminals. The master station device determines a first local frequency and transmits the first local frequency to the slave station device. The slave station device receives an uplink wireless signal including a plurality of first wireless signals. The slave station device generates a first electric signal by performing uplink conversion processing on the uplink wireless signal. The slave station device transmits an uplink optical signal including data of the generated first electric signal to the master station device. The master station device acquires the first electric signal from the uplink optical signal. The first local frequency is the frequency of a first local signal used in the uplink conversion process, in which process the frequencies of the plurality of first wireless signals are down-converted, with each of the plurality of first wireless signals corresponding to each of a plurality of frequency bands.

Description

Optical communication system, master station device, slave station device, and optical communication method

[0001] This application claims priority based on Japanese Patent Application No. 2024-097925, filed on June 18, 2024, the disclosure of which is incorporated herein in its entirety by reference.

[0002] Conventionally, optical communication systems that can be shared and used by multiple telecommunications carriers have been developed. For example, Patent Document 1 (JP 2022-185171 A) discloses the following communication relay system. The communication relay system transmits signals related to wireless communication between base stations of multiple telecommunications carriers and mobile stations that use different frequency bands. The communication relay system includes a master unit capable of transmitting and receiving signals with base stations of the multiple telecommunications carriers, and a slave unit that transmits signals between the master unit and wirelessly communicates with the mobile station. The communication relay system also includes a timing detection unit that detects the timing of transmission and reception between the base station of each telecommunications carrier and the mobile station, and a signal suppression unit that suppresses interference waves using a signal that is transmitted to the mobile station at a timing tailored to a desired telecommunications carrier based on the timing detected by the timing detection unit for the signal received from the mobile station.

[0003] JP 2022-185171 A JP 2019-213014 A JP 2018-186353 A

[0004] The optical communication system disclosed herein is an optical communication system in which optical signals are transmitted and received between a master station device and a slave station device, and radio signals are transmitted and received between the slave station device and one or more communication terminals, wherein the master station device determines a first local frequency and transmits the first local frequency to the slave station device, the slave station device receives an upstream radio signal including a plurality of first radio signals, the slave station device generates a first electrical signal by performing upstream conversion processing on the upstream radio signal, the slave station device transmits an upstream optical signal including data of the generated first electrical signal to the master station device, the master station device obtains the first electrical signal from the upstream optical signal, the first local frequency is the frequency of the first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

[0005] One aspect of the present disclosure can be achieved not only as an optical communication system including such a characteristic processing unit, but also as a program for causing a computer to execute the steps of such a characteristic process.

[0006] One aspect of the present disclosure can be realized not only as a master station device having such a characteristic processing unit, but also as an optical communication method including such characteristic processing steps, or as a program for causing a computer to execute such steps. Furthermore, one aspect of the present disclosure can be realized as a semiconductor integrated circuit that enables part or all of the master station device.

[0007] One aspect of the present disclosure can be realized not only as a slave station device having such a characteristic processing unit, but also as an optical communication method including such characteristic processing steps, or as a program for causing a computer to execute such steps. Furthermore, one aspect of the present disclosure can be realized as a semiconductor integrated circuit that enables part or all of the slave station device.

[0008] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating an example of a frequency spectrum of an RF signal transmitted and received by a slave station device in the optical communication system according to an embodiment of the present disclosure. FIG. 3 is a diagram illustrating the configurations of a master station device and a slave station device in the optical communication system according to an embodiment of the present disclosure. FIG. 4 is a diagram illustrating a frequency characteristic Fres of distortion of an upstream multiplexed analog signal in the optical communication system according to an embodiment of the present disclosure. FIG. 5 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a master station device in the optical communication system according to an embodiment of the present disclosure. FIG. 6 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in the optical communication system according to an embodiment of the present disclosure. FIG. 7 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. FIG. 8 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in the optical communication system according to an embodiment of the present disclosure. FIG. 9 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in the optical communication system according to an embodiment of the present disclosure. FIG. 10 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. FIG. 11 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure. FIG. 12 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure. FIG. 13 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. FIG. 14 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a master station device in an optical communication system according to an embodiment of the present disclosure. FIG. 15 is a flowchart defining an example of an operation procedure when a master station device according to an embodiment of the present disclosure determines an upstream frequency fu of an upstream local signal Lu. FIG. 16 is a diagram illustrating an example of a sequence of adjustment of each of the upstream local signal Lu and the downstream local signal Ld in the optical communication system according to an embodiment of the present disclosure.

[0009] <Problem to be Solved by the Present Disclosure> There is a need for a technology that goes beyond the technologies described in Patent Documents 1 to 3 and that is capable of improving communication quality with a simple configuration without requiring a wideband analog circuit.

[0010] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide an optical communication system, a master station device, a slave station device, and an optical communication method that can improve communication quality with a simple configuration without requiring a wideband analog circuit.

[0011] <Effects of the Present Disclosure> According to the present disclosure, communication quality can be improved with a simple configuration without requiring a wideband analog circuit.

[0012] The contents of the embodiments of the present disclosure will be listed and explained. (1) An optical communication system according to an embodiment of the present disclosure is an optical communication system in which optical signals are transmitted and received between a master station device and a slave station device, and radio signals are transmitted and received between the slave station device and one or more communication terminals, wherein the master station device determines a first local frequency and transmits the first local frequency to the slave station device, the slave station device receives an upstream radio signal including a plurality of first radio signals, the slave station device generates a first electrical signal by performing upstream conversion processing on the upstream radio signal, the slave station device transmits an upstream optical signal including data of the generated first electrical signal to the master station device, the master station device obtains the first electrical signal from the upstream optical signal, the first local frequency is a frequency of a first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, the first local signal is common to the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

[0013] In this configuration, the master station determines the first local frequency, and the slave station performs upstream conversion processing using the local signal of the first local frequency. This allows the frequency of the first electrical signal transmitted using the upstream optical signal to be flexibly changed in consideration of, for example, the quality of upstream communication. Therefore, communication quality can be improved with a simple configuration without requiring a broadband analog circuit.

[0014] (2) In the above (1), the master station device may determine the first local frequency based on a signal quality of the first electrical signal in a predetermined frequency range.

[0015] With this configuration, the frequency of the first electrical signal can be adjusted to, for example, a band that improves signal quality, thereby improving the throughput in the optical communication system.

[0016] (3) In the above (1) or (2), the master station device may determine the first local frequency based on a traffic volume of communications using the upstream wireless signal.

[0017] With this configuration, for example, when a slave station device receives an upstream radio signal including multiple first radio signals, the frequency of the first electrical signal can be changed depending on, for example, the magnitude relationship of the communication traffic in each frequency band, thereby improving the throughput in the optical communication system.

[0018] (4) In any one of (1) to (3) above, the first local signal may be common to the plurality of first radio signals.

[0019] By performing uplink conversion processing for the first radio signal using a common local signal, the number of analog circuits required for frequency conversion can be reduced compared to a configuration in which each first radio signal is frequency converted using an individual local signal.

[0020] (5) In any of (1) to (3) above, the parent station device may determine a plurality of the first local frequencies, and the child station device may perform the uplink conversion process using a plurality of first local signals in a time-division manner, and each of the plurality of first local signals may correspond to each of the plurality of first local frequencies determined by the parent station device.

[0021] With this configuration, for example, when a slave station device receives an upstream radio signal including a plurality of first radio signals, it is possible to average the throughput of each of the first radio signals in the optical communication system.

[0022] (6) In any of (1) to (5) above, the parent station device may determine a second local frequency and transmit the second local frequency to the child station device; the parent station device may be capable of transmitting a downstream optical signal including data of a second electrical signal in which a plurality of analog signals are multiplexed to the child station device; the child station device may acquire the second electrical signal from the downstream optical signal; the child station device may generate a downstream wireless signal by performing downstream conversion processing on a signal including data received from the parent station device and transmit the generated downstream wireless signal; the second local frequency may be a frequency of a second local signal used in the downstream conversion processing; the downstream conversion processing may upconvert the frequency of the acquired second electrical signal; the second local signal may be common to the plurality of analog signals; and each of the plurality of analog signals may correspond to a respective one of a plurality of frequency bands.

[0023] By performing downstream conversion processing for analog signals using a common local signal, the number of analog circuits required for frequency conversion can be reduced compared to a configuration in which each analog signal is frequency converted using an individual local signal. Also, by having the master station determine the second local frequency and the slave station perform downstream conversion processing using the local signal of the second local frequency, the slave station can transmit downstream wireless signals in a predetermined frequency band while flexibly changing the frequency of the second electrical signal transmitted using the downstream optical signal, for example, in the master station, taking into account the quality of downstream communication.

[0024] (7) In any of (1) to (6) above, the parent station device may determine the first local frequency using a correspondence table showing a correspondence relationship between the first local frequency and a set of frequency ranges of the first radio signal and an analog signal generated by down-converting the frequency of the first radio signal.

[0025] The correspondence table is stored in, for example, a storage unit in the master station. By using the correspondence table, the local frequency can be determined efficiently, thereby improving the throughput in the optical communication system.

[0026] (8) A master station device according to an embodiment of the present disclosure is a master station device that transmits and receives optical signals to and from slave station devices, and includes: a determination unit that determines a first local frequency; a transmission unit that transmits frequency information indicating the first local frequency determined by the determination unit to the slave station device; and an optical receiving unit that receives an upstream optical signal including data of a first electrical signal from the slave station device and acquires the first electrical signal from the received upstream optical signal.

[0027] The configuration in which the first local frequency is determined and frequency information indicating the determined first local frequency is transmitted to the slave station device allows the slave station device to perform upstream conversion processing using a local signal of the first local frequency, so that the frequency of the first electrical signal transmitted using the upstream optical signal can be flexibly changed in consideration of, for example, the quality of upstream communication. Therefore, communication quality can be improved with a simple configuration without requiring a wideband analog circuit.

[0028] (9) A slave station device according to an embodiment of the present disclosure is a slave station device that transmits and receives optical signals to and from a master station device and transmits and receives radio signals to and from one or more communication terminals, and includes: a radio receiving unit capable of receiving an upstream radio signal including a plurality of first radio signals; a frequency information receiving unit that receives frequency information indicating a first local frequency from the master station device; a converting unit that generates a first electrical signal by performing an upstream conversion process on the upstream radio signal; and an optical transmitting unit that transmits an upstream optical signal including data of the first electrical signal generated by the converting unit to the master station device, wherein the first local frequency is the frequency of a first local signal used in the upstream conversion process, and the upstream conversion process down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

[0029] By performing upstream conversion processing using a local signal having a first local frequency indicated by frequency information received from the master station, the frequency of the first electrical signal transmitted using the upstream optical signal can be flexibly changed in consideration of, for example, the quality of upstream communication. Therefore, communication quality can be improved with a simple configuration without requiring a wideband analog circuit.

[0030] (10) An optical communication method according to an embodiment of the present disclosure is an optical communication method in an optical communication system in which a master station device and a slave station device transmit and receive optical signals and the slave station device transmit and receive wireless signals to and from one or more communication terminals, the method including the steps of: the master station device determining a first local frequency and transmitting the first local frequency to the slave station device; the slave station device receiving an upstream wireless signal including a plurality of first wireless signals; the slave station device generating a first electrical signal by performing upstream conversion processing on the upstream wireless signal; the slave station device transmitting an upstream optical signal including data of the generated first electrical signal to the master station device; and the master station device acquiring the first electrical signal from the upstream optical signal, wherein the first local frequency is a frequency of a first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first wireless signals, and each of the plurality of first wireless signals corresponds to a respective one of a plurality of frequency bands.

[0031] By using a method in which the master station determines a first local frequency and the slave station performs upstream conversion processing using a local signal of the first local frequency, the frequency of the first electrical signal transmitted using the upstream optical signal can be flexibly changed in consideration of, for example, the quality of upstream communication, thereby improving communication quality with a simple configuration without requiring a broadband analog circuit.

[0032] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals, and the description thereof will not be repeated. At least some of the embodiments described below may be combined in any manner.

[0033] [Configuration and Basic Operation] Fig. 1 is a diagram showing the configuration of an optical communication system according to an embodiment of the present disclosure. Referring to Fig. 1, an optical communication system 401 includes a master station device 101 and a slave station device 201. The master station device 101 and the slave station device 201 are connected via an optical fiber 191.

[0034] The master station device 101 and the slave station device 201 transmit and receive optical signals, including analog signals, via the optical fiber 191. Hereinafter, an optical signal transmitted from the master station device 101 to the slave station device 201 will also be referred to as a downstream optical signal, and an optical signal transmitted from the slave station device 201 to the master station device 101 will also be referred to as an upstream optical signal.

[0035] The master station device 101 is connected to the base station devices 301A, 301B, 301C, and 301D via a transmission line 181. The transmission line 181 is, for example, a coaxial cable. The base station devices 301A, 301B, 301C, and 301D are provided in base stations of different telecommunications carriers. Hereinafter, each of the base station devices 301A, 301B, 301C, and 301D will also be referred to as a base station device 301. The master station device 101 is not limited to a configuration in which it is connected to four base station devices 301, and may be connected to two, three, five, or more base station devices 301.

[0036] The slave station device 201 includes an antenna 211. The slave station device 201 transmits and receives wireless signals, i.e., RF (Radio Frequency) signals, to and from communication terminals 311A, 311B, 311C, and 311D via the antenna 211. The communication terminals 311A, 311B, 311C, and 311D are communication terminals of users who have concluded communication contracts with the telecommunications carriers corresponding to the base station devices 301A, 301B, 301C, and 301D, respectively. Hereinafter, each of the communication terminals 311A, 311B, 311C, and 311D will also be referred to as a communication terminal 311. The communication terminals 311 may be mobile terminals or fixed terminals. In the optical communication system 401, the direction from the base station device 301 to the parent station device 101, the child station device 201, and the communication terminal 311 is also referred to as the "downstream" direction, and the direction from the communication terminal 311 to the child station device 201, the parent station device 101, and the base station device 301 is also referred to as the "upstream" direction.

[0037] 2 is a diagram illustrating an example of a frequency spectrum of an RF signal transmitted and received by a slave station device in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0038] 2 , the slave station device 201 is capable of transmitting RF signals RdA, RdB, RdC, and RdD via the antenna 211. The slave station device 201 is also capable of transmitting RF signals RuA, RuB, RuC, and RuD, respectively transmitted by the communication terminals 311A, 311B, 311C, and 311D, via the antenna 211. Hereinafter, the downstream RF signals RdA, RdB, RdC, and RdD are also referred to as downstream RF signals, and the upstream RF signals RuA, RuB, RuC, and RuD are also referred to as upstream RF signals. The upstream RF signals are an example of a first radio signal.

[0039] The frequency band of each of the downlink RF signal RdA and the uplink RF signal RuA is frequency band FrA, the frequency band of each of the downlink RF signal RdB and the uplink RF signal RuB is frequency band FrB, the frequency band of each of the downlink RF signal RdC and the uplink RF signal RuC is frequency band FrC, and the frequency band of each of the downlink RF signal RdD and the uplink RF signal RuD is frequency band FrD. The frequency bands FrA, FrB, FrC, and FrD are frequency bands assigned to the base station devices 301A, 301B, 301C, and 301D, respectively. The center frequencies of the frequency bands FrA, FrB, FrC, and FrD are center frequencies CrA, CrB, CrC, and CrD, respectively, with the center frequencies CrA, CrB, CrC, and CrD decreasing in this order. Hereinafter, the frequency bands FrA, FrB, FrC, and FrD are also referred to as uplink and downlink frequency bands. The bandwidth of the upstream and downstream frequency bands is, for example, 400 MHz. Part of the upstream and downstream frequency bands may overlap with part of another upstream and downstream frequency band.

[0040] 1 , for example, the optical communication system 401 is used as a fronthaul in a mobile communication system conforming to the TDD (Time Division Duplex) method. In this case, in the optical communication system 401, a downlink period for downlink communication in which the slave station device 201 transmits a downlink RF signal to the communication terminal 311 and an uplink period for uplink communication in which the slave station device 201 receives an uplink RF signal from the communication terminal 311 are switched and alternately repeated.

[0041] More specifically, during the upstream period, the slave station device 201 can receive an upstream multiplexed RF signal including upstream RF signals RuA, RuB, RuC, and RuD via the antenna 211. The upstream multiplexed RF signal is an example of an upstream wireless signal.

[0042] The slave station device 201 performs an upstream conversion process to down-convert the frequency of the received upstream multiplexed RF signal, thereby generating an upstream multiplexed analog signal, for example, in the IF (Intermediate Frequency) band. In the upstream conversion process, a common local signal (upstream local signal) is used for each upstream RF signal included in the upstream multiplexed RF signal. The upstream multiplexed analog signal is an example of a first electrical signal. The slave station device 201 transmits an upstream optical signal including data of the generated upstream multiplexed analog signal to the master station device 101 via the optical fiber 191.

[0043] The master station device 101 receives an upstream optical signal from the slave station device 201 via the optical fiber 191 and acquires an upstream multiplexed analog signal from the received upstream optical signal. The master station device 101 acquires analog signals corresponding to the base station devices 301A, 301B, 301C, and 301D from the acquired upstream multiplexed analog signal. The master station device 101 generates digital signals by digitally converting the acquired analog signals. The master station device 101 acquires communication data from the generated digital signals and transmits the acquired communication data to the base station devices 301A, 301B, 301C, and 301D via the corresponding transmission lines 181. Hereinafter, each upstream analog signal will also be referred to as an upstream analog signal, and each upstream digital signal will also be referred to as an upstream digital signal.

[0044] The master station 101 receives communication data from each of the base stations 301A, 301B, 301C, and 301D via the corresponding transmission line 181. The master station 101 converts digital signals based on the received communication data into analog signals, thereby generating analog signals, for example, in the IF band. Hereinafter, each of the downstream analog signals will also be referred to as a downstream analog signal. The master station 101 transmits a downstream optical signal containing data of a downstream multiplexed analog signal, in which the generated downstream analog signals IdA, IdB, IdC, and IdD are multiplexed, to the slave station 201 via the optical fiber 191. The downstream multiplexed analog signal is an example of a second electrical signal.

[0045] The slave station device 201 receives a downstream optical signal from the master station device 101 via the optical fiber 191 and acquires a downstream multiplexed analog signal from the received downstream optical signal. The slave station device 201 performs downstream conversion processing to upconvert the frequency of the acquired downstream multiplexed analog signal, thereby generating a downstream multiplexed RF signal including downstream RF signals RdA, RdB, RdC, and RdD. In the downstream conversion processing, a common local signal (downstream local signal) is used for each downstream analog signal included in the downstream multiplexed analog signal. During the downstream period, the slave station device 201 transmits the generated downstream multiplexed RF signal to the communication terminal 311 via the antenna 211. The downstream multiplexed RF signal is an example of a downstream radio signal.

[0046] [Problem] In an optical communication system 401, a technique is desired that can improve communication quality with a simple configuration without requiring a wideband analog circuit.

[0047] For example, in configuration 1, in upstream communication of the slave station device 201, an upstream multiplexed RF signal including multiple upstream RF signals is frequency converted using an upstream local signal common to each upstream RF signal and a common mixer 32. In configuration 2, each upstream RF signal is frequency converted using an individual local signal. In configurations 1 and 2, analog circuits such as a mixer and a VCO (Voltage Controlled Oscillator) are required for frequency conversion. The number of analog circuits in configuration 1 is smaller than that in configuration 2, but configuration 1 requires wideband analog circuits compatible with multiple telecommunications carriers.

[0048] For example, if the slave station device 201 is installed on the ceiling of a room such as a conference room, and the coverage area of ​​the antenna 211 is within the space of the room, it is assumed that there is a low possibility that communication terminals 311A, 311B, 311C, and 311D of each telecommunications carrier will communicate in parallel within the coverage area of ​​the antenna 211. In this case, the throughput of the slave station device 201 only needs to satisfy a target value that is set in advance based on, for example, the size of the room in which the slave station device 201 is installed and the capacity of the room, and there is less need for the slave station device 201 to simultaneously provide high throughput to each communication terminal 311 corresponding to all telecommunications carriers.

[0049] The optical communication system 401 according to the embodiment of the present disclosure solves the above problems by the following configuration.

[0050] (Configuration of Master Station Device and Slave Station Device) FIG. 3 is a diagram illustrating the configuration of a master station device and a slave station device in an optical communication system according to an embodiment of the present disclosure. Referring to FIG. 3 , the master station device 101 includes a storage unit 10, a digital processing unit 11, a DA conversion unit 12, a multiplexing unit 13, a control unit 14, a multiplexing unit 15, an optical modulation unit 16, an optical coupler 17, an optical demodulation unit 18, a signal acquisition unit 19, and an AD conversion unit 20. The digital processing unit 11 is an example of a determination unit. The optical modulation unit 16 is an example of a transmission unit. The optical demodulation unit 18 is an example of an optical reception unit. Some or all of the functions of the digital processing unit 11 and the control unit 14 are achieved by, for example, a processing circuit including one or more processors. The storage unit 10 is, for example, a non-volatile memory included in the processing circuit.

[0051] The slave station device 201 includes an optical coupler 21, an optical demodulator 22, a separator 23, an amplifier 24, a mixer 25, a voltage-controlled oscillator 26, an amplifier 27, a band-pass filter (BPF) 28, an RF transmitter / receiver 29, a band-pass filter (BPF) 30, an amplifier 31, a mixer 32, a voltage-controlled oscillator 33, an amplifier 34, an optical modulator 35, and a controller 36. The RF transmitter / receiver 29 is an example of a radio receiver. The mixer 32 is an example of a converter. The optical modulator 35 is an example of an optical transmitter. The controller 36 is an example of a frequency information receiver. Some or all of the functions of the controller 36 are achieved by, for example, a processing circuit including one or more processors.

[0052] 4 is a diagram illustrating the frequency characteristic Fres of the distortion of an upstream multiplexed analog signal in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency and the vertical axis represents the amount of distortion.

[0053] 4, the storage unit 10 stores quality information that is created based on the frequency characteristic Fres and indicates the signal quality of an upstream multiplexed analog signal in a predetermined frequency range Fif. More specifically, the storage unit 10 stores quality information that indicates the quality rank of an upstream multiplexed analog signal in each of frequency ranges R1, R2, R3, R4, R5, R6, and R7 that are obtained by dividing the frequency range Fif into 400 MHz bandwidths.

[0054] In the quality information, the quality rank of the upstream multiplexed analog signal in frequency ranges R4 and R5 is "A," the quality rank of the upstream multiplexed analog signal in frequency ranges R3 and R6 is "B," the quality rank of the upstream multiplexed analog signal in frequency ranges R1 and R2 is "C," and the quality rank of the upstream multiplexed analog signal in frequency range R7 is "D." Thus, in frequency range Fif, the quality of the upstream multiplexed analog signal in the frequency ranges at both ends is inferior to the quality of the upstream multiplexed analog signal in the central frequency range. For example, the quality rank of the upstream multiplexed analog signal outside frequency range Fif is inferior to "D." The quality rank is determined based on the magnitude of distortion of the upstream multiplexed analog signal indicated by the frequency characteristic Fres, with "A," "B," "C," and "D" indicating better signal quality of the upstream multiplexed analog signal in that order.

[0055] For example, the frequency characteristic Fres is determined according to the characteristics of analog circuits such as the amplifiers 31 and 34 and the mixer 32 in the slave station device 201, as well as the characteristics of the optical modulator 35 in the slave station device 201 and the optical demodulator 18 in the master station device 101. The quality information is created in advance by an administrator of the optical communication system 401 and stored in the storage unit 10.

[0056] 3 , in the master station 101, the digital processing unit 11 receives communication data from each of the base station devices 301A, 301B, 301C, and 301D via the corresponding transmission line 181. The digital processing unit 11 outputs a digital signal based on the received communication data to the DA conversion unit 12.

[0057] The DA conversion unit 12 converts the digital signal received from the digital processing unit 11 into analog to generate downstream analog signals IdA, IdB, IdC, and IdD each containing communication data, and outputs the generated downstream analog signals IdA, IdB, IdC, and IdD to the multiplexing unit 13.

[0058] The multiplexing unit 13 generates a downstream multiplexed analog signal by multiplexing the downstream analog signals IdA, IdB, IdC, and IdD received from the DA conversion unit 12 , and outputs the generated downstream multiplexed analog signal to the multiplexing unit 15 .

[0059] 5 is a diagram illustrating an example of the frequency spectrum of an analog signal generated in a master station in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0060] 5, the frequency bands of the downstream analog signals IdA, IdB, IdC, and IdD are frequency bands FidA, FidB, FidC, and FidD, respectively. The center frequencies of the frequency bands FidA, FidB, FidC, and FidD are downstream center frequencies CidA, CidB, CidC, and CidD, respectively, and the downstream center frequencies CidA, CidB, CidC, and CidD decrease in this order. Hereinafter, each of the frequency bands FidA, FidB, FidC, and FidD will also be referred to as a downstream frequency band.

[0061] 3 , the control unit 14 generates TDD information and a digital signal. The TDD information indicates an uplink period and a downlink period. The digital signal includes synchronization information indicating a reference clock signal. The control unit 14 outputs the generated digital signal to the multiplexing unit 15.

[0062] The multiplexing unit 15 generates an electrical signal in which the downstream multiplexed analog signal received from the DA conversion unit 12 and the digital signal received from the control unit 14 are frequency-multiplexed, and outputs the electrical signal to the optical modulation unit 16 .

[0063] The optical modulation unit 16 receives the electrical signal from the multiplexing unit 15 and optically modulates the received electrical signal to generate a downstream optical signal. The optical modulation unit 16 transmits the downstream optical signal to the slave station equipment 201 via the optical coupler 17 and the optical fiber 191.

[0064] In the slave station device 201, the optical demodulator 22 receives the downstream optical signal from the master station device 101 via the optical fiber 191 and the optical coupler 21. The optical demodulator 22 generates an electrical signal at a level corresponding to the intensity of the received downstream optical signal and outputs the electrical signal to the separator 23.

[0065] The separator 23 separates the downstream multiplexed analog signal and the digital signal contained in the electrical signal received from the optical demodulator 22. The separator 23 outputs the separated downstream multiplexed analog signal to the amplifier 24, and also outputs the separated digital signal to the controller 36.

[0066] The amplifier 24 amplifies the downstream multiplexed analog signal received from the separator 23 and outputs the amplified downstream multiplexed analog signal to the mixer 25 .

[0067] The control unit 36 ​​acquires TDD information and synchronization information from the digital signal received from the demultiplexer 23, and based on the acquired TDD information and synchronization information, switches between RF signal transmission and reception operations by the RF transceiver unit 29. More specifically, based on the TDD information and synchronization information, the control unit 36 ​​generates a TDD switching signal for switching between RF signal transmission and reception operations by the RF transceiver unit 29, and outputs the generated TDD switching signal to the RF transceiver unit 29.

[0068] The control unit 36 ​​outputs an analog downstream control voltage to the voltage-controlled oscillator 26 to adjust the downstream frequency fd of the downstream local signal to be supplied to the mixer 25. For example, in the initial state, the control unit 36 ​​outputs a downstream control voltage of a predetermined level to the voltage-controlled oscillator 26. The downstream frequency fd is an example of a local frequency in the downstream direction.

[0069] The voltage controlled oscillator 26 receives a downstream control voltage from the control unit 36 ​​and outputs to the mixer 25 a downstream local signal having a frequency fd according to the level of the received downstream control voltage.

[0070] The mixer 25 generates a downstream multiplexed RF signal by performing downstream conversion processing to up-convert the frequency of the downstream multiplexed analog signal separated by the separation unit 23 using a downstream local signal common to the downstream analog signals IdA, IdB, IdC, and IdD. More specifically, the mixer 25 multiplies the downstream multiplexed analog signal received from the amplification unit 24 by the downstream local signal received from the voltage controlled oscillator 26 to generate a downstream multiplexed RF signal and outputs it to the amplification unit 27.

[0071] The amplifier 27 amplifies the downstream multiplexed RF signal received from the mixer 25 and outputs the amplified downstream multiplexed RF signal to the BPF 28 .

[0072] The BPF 28 receives the downstream multiplexed RF signal from the amplifier 27 and attenuates components outside a predetermined passband in the received downstream multiplexed RF signal.

[0073] In accordance with the TDD switching signal received from the control unit 36, the RF transceiver unit 29 transmits the downstream multiplexed RF signal that has passed through the BPF 28 to the communication terminal 311 via the antenna 211 during the downstream period.

[0074] (Uplink Communication) The RF transceiver 29 can receive an uplink multiplexed RF signal including uplink RF signals RuA, RuB, RuC, and RuD via the antenna 211. The RF transceiver 29 receives an uplink multiplexed RF signal including uplink RF signals corresponding to one or more communication carriers via the antenna 211 during an uplink period in accordance with a TDD switching signal received from the control unit 36. The RF transceiver 29 outputs the received uplink multiplexed RF signal to the BPF 30.

[0075] The BPF 30 receives the upstream multiplexed RF signal from the RF transceiver 29 and attenuates components outside a predetermined passband in the received upstream multiplexed RF signal.

[0076] The amplifier 31 amplifies the upstream multiplexed RF signal that has passed through the BPF 30 , and outputs the amplified upstream multiplexed RF signal to the mixer 32 .

[0077] The control unit 36 ​​outputs an upstream control voltage to the voltage-controlled oscillator 33 to adjust the upstream frequency fu of the upstream local signal to be supplied to the mixer 32. For example, in the initial state, the control unit 36 ​​outputs an upstream control voltage of a predetermined level to the voltage-controlled oscillator 33. The upstream frequency fu is an example of a local frequency in the upstream direction.

[0078] The voltage controlled oscillator 33 receives an upstream control voltage from the control unit 36 ​​and outputs to the mixer 32 an upstream local signal with an upstream frequency fu according to the level of the received upstream control voltage.

[0079] The mixer 32 generates an upstream multiplexed analog signal by performing an upstream conversion process that down-converts the frequency of the upstream multiplexed RF signal received by the RF transceiver 29 using an upstream local signal that is common to the upstream RF signals RuA, RuB, RuC, and RuD. More specifically, the mixer 32 multiplies the upstream multiplexed RF signal received from the amplifier 31 by the upstream local signal received from the voltage-controlled oscillator 33 to generate an upstream multiplexed analog signal and outputs it to the amplifier 34.

[0080] 6 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0081] 6 , the upstream multiplexed analog signal includes one or more upstream analog signals IuA, IuB, IuC, and IuD obtained by down-converting the frequencies of the upstream RF signals RuA, RuB, RuC, and RuD, respectively. The frequency bands of the upstream analog signals IuA, IuB, IuC, and IuD are frequency bands FiuA, FiuB, FiuC, and FiuD, respectively. The upstream center frequencies of the frequency bands FiuA, FiuB, FiuC, and FiuD are upstream center frequencies CiuA, CiuB, CiuC, and CiuD, respectively, with the upstream center frequencies CiuA, CiuB, CiuC, and CiuD decreasing in this order. Hereinafter, each of the frequency bands FiuA, FiuB, FiuC, and FiuD will also be referred to as an upstream frequency band.

[0082] Referring back to FIG. 3, the amplifier 34 amplifies the upstream multiplexed analog signal received from the mixer 32 and outputs the amplified upstream multiplexed analog signal to the optical modulator 35 .

[0083] The optical modulator 35 generates an upstream optical signal including the upstream multiplexed analog signal generated by the mixer 32, and transmits the generated upstream optical signal to the parent station 101. More specifically, the optical modulator 35 receives the upstream multiplexed analog signal from the amplifier 34, and generates an upstream optical signal by optically modulating the received upstream multiplexed analog signal. The optical modulator 35 transmits the upstream optical signal to the parent station 101 via the optical coupler 21 and the optical fiber 191.

[0084] In the master station 101, the optical demodulator 18 receives an upstream optical signal including data of the upstream multiplexed analog signal from the slave station 201 and acquires the upstream multiplexed analog signal from the received upstream optical signal. More specifically, the optical demodulator 18 receives the upstream optical signal from the slave station 201 via the optical fiber 191 and the optical coupler 21. The optical demodulator 18 generates an upstream multiplexed analog signal, which is an electrical signal at a level corresponding to the intensity of the received upstream optical signal, and outputs the upstream multiplexed analog signal to the signal acquirer 19.

[0085] The signal acquirer 19 acquires upstream analog signals IuA, IuB, IuC, and IuD, which are signals in the upstream frequency bands FiuA, FiuB, FiuC, and FiuD, from the downstream multiplexed analog signal received from the optical demodulator 18. The signal acquirer 19 outputs the acquired upstream analog signals IuA, IuB, IuC, and IuD to the AD converter 20.

[0086] The AD conversion unit 20 digitally converts the upstream analog signals IuA, IuB, IuC, and IuD received from the signal acquisition unit 19 to generate digital signals, and outputs the generated digital signals to the digital processing unit 11 .

[0087] The digital processing unit 11 receives the digital signals from the AD conversion unit 20 and acquires communication data from the received digital signals. The digital processing unit 11 transmits the acquired communication data to the base station devices 301A, 301B, 301C, and 301D via the corresponding transmission lines 181.

[0088] (Adjustment of Uplink Frequency fu of Uplink Local Signal Lu) In the master station device 101, the digital processing unit 11 determines the uplink frequency fu of the uplink local signal Lu used in the uplink conversion process.

[0089] For example, the digital processing unit 11 determines the upstream frequency fu of the upstream local signal Lu based on the traffic volume of the communication using the upstream multiplexed RF signal and the frequency characteristic Fres.

[0090] More specifically, communication terminal 311 located in the coverage area of ​​antenna 211 periodically or irregularly transmits buffer information indicating the amount of transmission data buffered in communication terminal 311 in an uplink RF signal.

[0091] The digital processing unit 11 acquires buffer information of the corresponding communication terminal 311 from the upstream digital signal received from the AD conversion unit 20 .

[0092] The digital processing unit 11 calculates the upstream buffer amounts of the communication terminals 311A, 311B, 311C, and 311D at calculation timings according to a predetermined calculation cycle, based on buffer information acquired from the upstream digital signals. Hereinafter, each upstream buffer amount is also referred to as an upstream buffer amount. Each upstream buffer amount may be the total value of the buffer amount of transmission data in each of the multiple communication terminals 311.

[0093] The digital processing unit 11 calculates the traffic volume of each communication using the uplink RF signals RuA, RuB, RuC, and RuD based on the calculated uplink buffer volume. The digital processing unit 11 also calculates the total value of the uplink traffic volume. Hereinafter, each of the uplink traffic volumes will also be referred to as the uplink traffic volume.

[0094] The digital processing unit 11 assigns a priority to the corresponding RF signals in descending order of the calculated upstream traffic volume. The digital processing unit 11 also counts the number of upstream RF signals corresponding to upstream traffic volumes greater than zero as the number of upstream communication carriers.

[0095] 7 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. Referring to FIG. 7, the storage unit 10 stores a correspondence table TBu1 indicating a correspondence relationship between a set of frequency ranges of an upstream RF signal and an upstream analog signal generated by down-converting the frequency of the upstream RF signal, and an upstream frequency fu of an upstream local signal Lu.

[0096] The digital processing unit 11 determines the upstream frequency fu of the upstream local signal Lu based on the priority and the number of upstream communication carriers of each upstream RF signal and the correspondence table TBu1 in the storage unit 10.

[0097] (Specific Example 1 of Adjusting the Uplink Frequency fu of the Uplink Local Signal Lu) Consider a case where the number of upstream communication carriers is "1." In order to assign the upstream frequency band Fiu of the upstream analog signal Iu corresponding to the upstream RF signal for upstream communication to the frequency range with the highest quality rank, the digital processing unit 11 determines the uplink frequency fu.

[0098] More specifically, consider a case where the number of upstream communication carriers is "1" and, for example, the priority of the upstream RF signal RuD is the highest. In order to assign the upstream frequency band FiuD of the upstream analog signal IuD to frequency range R4, which has a quality rank of "A," the digital processing unit 11 determines the upstream frequency fu of the upstream local signal Lu to be 26.2 GHz. In a case where the number of upstream communication carriers is "1" and, for example, the priority of the upstream RF signal RuD is the highest, the digital processing unit 11 may determine the upstream frequency fu of the upstream local signal Lu to be 25.8 GHz in order to assign the upstream frequency band FiuD of the upstream analog signal IuD to frequency range R5.

[0099] Referring again to Figure 3, when the digital processing unit 11 determines the uplink frequency fu to be 26.2 GHz, it generates uplink frequency information indicating the determined uplink frequency fu and outputs the generated uplink frequency information to the control unit 14 and the signal acquisition unit 19.

[0100] The control unit 14 receives the upstream frequency information from the digital processing unit 11 , generates a digital signal further including the received upstream frequency information, and outputs the generated digital signal to the multiplexing unit 15 .

[0101] The multiplexing unit 15 generates an electrical signal in which the downstream multiplexed analog signal received from the DA conversion unit 12 and the digital signal received from the control unit 14 are frequency-multiplexed, and outputs the electrical signal to the optical modulation unit 16 .

[0102] The optical modulator 16 transmits upstream frequency information indicating the upstream frequency fu determined by the digital processor 11 to the slave station equipment 201. More specifically, the optical modulator 16 generates a downstream optical signal by optically modulating the electrical signal received from the multiplexer 15, and transmits the generated downstream optical signal to the slave station equipment 201 via the optical coupler 17 and the optical fiber 191.

[0103] In the slave station device 201, the control unit 36 ​​receives the upstream frequency information from the master station device 101. More specifically, the control unit 36 ​​acquires the upstream frequency information from the digital signal received from the demultiplexer 23. The control unit 36 ​​generates an upstream control voltage for setting the upstream frequency fu of the upstream local signal Lu to be provided to the mixer 32 to 26.2 GHz indicated by the acquired upstream frequency information, and outputs the generated upstream control voltage to the voltage-controlled oscillator 33.

[0104] The voltage controlled oscillator 33 receives an upstream control voltage from the control unit 36 ​​and outputs an upstream local signal Lu of 26.2 GHz to the mixer 32 .

[0105] The mixer 32 performs an upstream conversion process using an upstream local signal Lu having an upstream frequency fu indicated by the upstream frequency information received by the control unit 36. More specifically, the mixer 32 multiplies the upstream multiplexed RF signal received from the amplifier unit 31 by the 26.2 GHz upstream local signal Lu received from the voltage controlled oscillator 33 to generate an upstream multiplexed analog signal and outputs it to the amplifier unit 34.

[0106] In the parent station device 101, the signal acquisition unit 19 acquires an upstream analog signal IuD, which is a signal in the frequency range R4, from the downstream multiplexed analog signal received from the optical demodulation unit 18 based on the upstream frequency information received from the digital processing unit 11, and outputs the acquired upstream analog signal IuD to the AD conversion unit 20.

[0107] 8 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0108] 8, the upstream analog signal Iu includes an upstream analog signal IuD obtained by down-converting the frequency of the upstream RF signal RuD. The upstream center frequency CiuD of the upstream frequency band FiuD of the upstream analog signal IuD is included in a frequency range R4 having a quality rank of "A." This makes it possible to prevent degradation of the signal quality of the upstream analog signal IuD due to the characteristics of analog circuits, etc.

[0109] For example, in the optical communication system 401, if the signal quality of the upstream analog signal Iu deteriorates, the throughput may decrease due to retransmission of the communication data or adaptive change of the transmission method of the communication data. In response to this, the throughput can be improved by improving the signal quality of the upstream analog signal Iu corresponding to the upstream RF signal actually used for communication within the limited bandwidth of the analog circuit.

[0110] (Specific Example 2 of Adjusting the Uplink Frequency fu of the Uplink Local Signal Lu) Referring again to Figure 7, consider a case where the number of upstream communication carriers is two or more and the proportion of the traffic volume of the upstream RF signal with the highest priority to the total traffic volume in the upstream direction is equal to or greater than a predetermined value. Under the condition that the upstream frequency band Fiu of the upstream analog signal Iu corresponding to each upstream RF signal used for upstream communication falls within the frequency range Fif, the digital processing unit 11 determines the upstream frequency fu in order to assign the upstream frequency band Fiu of the upstream analog signal Iu corresponding to the upstream RF signal with the highest priority to the frequency range with the highest quality rank.

[0111] More specifically, consider a case where the number of upstream communication carriers is "2," the priorities of the upstream RF signals RuD and RuB are greatest in this order, and the proportion of the upstream traffic volume of the upstream RF signal RuD to the total upstream traffic volume is equal to or greater than a predetermined value. To allocate the upstream frequency band FiuB, FiuD of the upstream analog signals IuB, IuD to the frequency range R5 with a high quality rank while keeping the upstream frequency bands FiuB, FiuD of the upstream analog signals IuB, IuD within the frequency range Fif, the digital processing unit 11 determines the upstream frequency fu of the upstream local signal Lu to be 25.8 GHz.

[0112] Referring again to Figure 3, when the digital processing unit 11 determines the uplink frequency fu to be 25.8 GHz, it generates uplink frequency information indicating the determined uplink frequency fu and outputs the generated uplink frequency information FRQ1 to the control unit 14 and the signal acquisition unit 19.

[0113] The control unit 14 receives the upstream frequency information from the digital processing unit 11 , generates a digital signal further including the received upstream frequency information, and outputs the generated digital signal to the multiplexing unit 15 .

[0114] The multiplexing unit 15 generates an electrical signal in which the downstream multiplexed analog signal received from the DA conversion unit 12 and the digital signal received from the control unit 14 are frequency-multiplexed, and outputs the electrical signal to the optical modulation unit 16 .

[0115] The optical modulation unit 16 generates a downstream optical signal by optically modulating the electrical signal received from the multiplexing unit 15 , and transmits the generated downstream optical signal to the slave station equipment 201 via the optical coupler 17 and the optical fiber 191 .

[0116] In the slave station device 201, the control unit 36 ​​acquires upstream frequency information from the digital signal received from the demultiplexer 23. The control unit 36 ​​generates an upstream control voltage for setting the upstream frequency fu of the upstream local signal Lu to be provided to the mixer 32 to 25.8 GHz indicated by the acquired upstream frequency information, and outputs the generated upstream control voltage to the voltage-controlled oscillator 33.

[0117] The voltage controlled oscillator 33 receives an upstream control voltage from the control unit 36 ​​and outputs an upstream local signal Lu of 25.8 GHz to the mixer 32 .

[0118] Mixer 32 multiplies the upstream multiplexed RF signal received from amplifier 31 by the 25.8 GHz upstream local signal Lu received from voltage controlled oscillator 33 to generate an upstream multiplexed analog signal, which is output to amplifier 34 .

[0119] In the parent station device 101, the signal acquisition unit 19 acquires an upstream analog signal IuB, which is a signal in the frequency range R1, and an upstream analog signal IuD, which is a signal in the frequency range R5, from the downstream multiplexed analog signal received from the optical demodulation unit 18, based on the upstream frequency information received from the digital processing unit 11, and outputs the acquired upstream analog signals IuB and IuD to the AD conversion unit 20.

[0120] 9 is a diagram illustrating an example of a frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0121] 9, the upstream multiplexed analog signal includes upstream analog signals IuB and IuD obtained by down-converting the frequencies of the upstream RF signals RuB and RuD. The upstream center frequency CiuD of the upstream frequency band FiuD of the upstream analog signal IuD is included in a frequency range R5 having a quality rank of "A." The upstream center frequency CiuB of the upstream frequency band FiuB of the upstream analog signal IuB is included in a frequency range R1 having a quality rank of "C." This makes it possible to prevent degradation of the signal quality of the upstream analog signal IuD due to the characteristics of analog circuits, etc. Furthermore, it is possible to keep degradation of the signal quality of the upstream analog signal IuB within an acceptable range.

[0122] (Specific example 3 of adjusting the uplink frequency fu of the uplink local signal Lu) Referring again to Figure 7, the digital processing unit 11 determines multiple frequencies fu when the number of uplink communication carriers is "2" or more and the proportion of the traffic volume of the uplink RF signal with the highest priority in the total value of the traffic volume in the uplink direction is less than a predetermined value.

[0123] 10 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. Referring to FIG. 10, the storage unit 10 stores a correspondence table TBu2 indicating a correspondence relationship between a combination of upstream RF signals Ru for upstream communication and an upstream frequency fu of an upstream local signal Lu.

[0124] Referring to Figure 10, when the number of upstream communication carriers is "2" or more and the proportion of the traffic volume of the upstream RF signal Ru with the highest priority in the total value of the traffic volume in the upstream direction is less than a predetermined value, the digital processing unit 11 obtains the upstream frequency fu corresponding to the combination of upstream RF signals Ru for which upstream communication is performed in the correspondence table TBu2.

[0125] More specifically, when the number of upstream communication carriers is "4" and the proportion of the upstream traffic volume of the upstream RF signal Ru, which has the highest priority, to the total value of the upstream traffic volume is less than a predetermined value, the digital processing unit 11 determines the frequency fu of the upstream local signal Lu to be 25.3 GHz and 24.9 GHz.

[0126] Referring again to Figure 3, when the digital processing unit 11 determines the uplink frequency fu to be 25.3 GHz and 24.9 GHz, it generates uplink frequency information indicating the determined uplink frequency fu and outputs the generated uplink frequency information to the control unit 14 and the signal acquisition unit 19.

[0127] The control unit 14 receives the upstream frequency information from the digital processing unit 11 , generates a digital signal further including the received upstream frequency information, and outputs the generated digital signal to the multiplexing unit 15 .

[0128] The multiplexing unit 15 generates an electrical signal in which the downstream multiplexed analog signal received from the DA conversion unit 12 and the digital signal received from the control unit 14 are frequency-multiplexed, and outputs the electrical signal to the optical modulation unit 16 .

[0129] The optical modulation unit 16 generates a downstream optical signal by optically modulating the electrical signal received from the multiplexing unit 15 , and transmits the generated downstream optical signal to the slave station equipment 201 via the optical coupler 17 and the optical fiber 191 .

[0130] The slave station equipment 201 performs an upstream conversion process using a plurality of upstream local signals Lu corresponding to the plurality of upstream frequencies fu determined by the master station equipment 101 in a time-division manner.

[0131] More specifically, in the slave station device 201, the control unit 36 ​​acquires upstream frequency information from the digital signal received from the demultiplexer 23. The control unit 36 ​​generates an upstream control voltage Vu1 for setting the upstream frequency fu of the upstream local signal Lu to be provided to the mixer 32 to 25.3 GHz indicated by the upstream frequency information, and an upstream control voltage Vu2 for setting the upstream frequency fu to 24.9 GHz indicated by the upstream frequency information. The control unit 36 ​​outputs the upstream control voltages Vu1 and Vu2 alternately to the voltage-controlled oscillator 33 at a one-to-one time ratio.

[0132] For example, the base station devices 301A, 301B, 301C, and 301D periodically transmit communication data including broadcast information to the master station device 101. The control unit 36 ​​switches the uplink control voltage output to the voltage-controlled oscillator 33 around the timing when the slave station device 201 transmits a downlink multiplexed RF signal including the broadcast information. This reduces the impact of switching the uplink frequency fu of the uplink local signal Lu on the uplink communication.

[0133] The voltage-controlled oscillator 33 receives an upstream control voltage Vu1 from the control unit 36 ​​and switches the upstream frequency fu of the upstream local signal Lu to be output to the mixer 32 to 25.3 GHz. The voltage-controlled oscillator 33 also receives an upstream control voltage Vu2 from the control unit 36 ​​and switches the upstream frequency fu of the upstream local signal Lu to be output to the mixer 32 to 24.9 GHz.

[0134] Mixer 32 multiplies the upstream multiplexed RF signal received from amplifier 31 by the upstream local signal Lu received from voltage controlled oscillator 33 to generate an upstream multiplexed analog signal, and outputs the signal to amplifier 34 .

[0135] In the parent station device 101, the signal acquisition unit 19 acquires upstream analog signals IuA, IuB, IuC, and IuD from the downstream multiplexed analog signal received from the optical demodulation unit 18 based on the upstream frequency information received from the digital processing unit 11, and outputs the acquired signals to the AD conversion unit 20.

[0136] 11 and 12 are diagrams illustrating an example of the frequency spectrum of an analog signal generated in a slave station device in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0137] 11 and 12 , the upstream multiplexed analog signal includes upstream analog signals IuA, IuB, IuC, and IuD obtained by down-converting the frequencies of the upstream RF signals. The upstream frequency bands FiuA, FiuB, FiuC, and FiuD are alternately assigned to frequency ranges R2, R3, R4, and R7, respectively, and frequency ranges R1, R2, R3, and R6, respectively. This makes it possible to equalize the signal qualities of the upstream analog signals IuA, IuB, IuC, and IuD, thereby averaging the throughput of each upstream RF signal.

[0138] (Adjustment of Downstream Frequency fd of Downstream Local Signal Ld) In the master station device 101, the digital processing unit 11 determines the downstream frequency fd of the downstream local signal Ld used in the downstream conversion process.

[0139] For example, the digital processing unit 11 determines the downlink frequency fd of the downlink local signal Ld based on the traffic volume of the communication using the downlink multiplexed RF signal and the signal quality of the downlink multiplexed analog signal in the frequency range Fif. The signal quality is, for example, frequency characteristics.

[0140] More specifically, each base station device 301 periodically or irregularly transmits buffer information indicating the amount of data buffered in the base station device 301 for transmission to the communication terminal 311 to the master station device 101 .

[0141] The digital processing unit 11 receives buffer information from the base station device 301 via the corresponding transmission line 181 .

[0142] The digital processing unit 11 calculates the downlink buffer amounts of the base station devices 301A, 301B, 301C, and 301D at calculation timings according to a predetermined calculation cycle, based on the buffer information received from the base station device 301. Hereinafter, each of the downlink buffer amounts will also be referred to as a downlink buffer amount.

[0143] The digital processing unit 11 calculates the traffic volume of each of the downlink RF signals RdA, RdB, RdC, and RdD based on the calculated downlink buffer volume. The digital processing unit 11 also calculates the total value of the downlink traffic volume. Hereinafter, each of the downlink traffic volumes will also be referred to as the downlink traffic volume.

[0144] The digital processing unit 11 assigns a priority to the corresponding RF signals in descending order of the calculated downstream traffic volume. The digital processing unit 11 also counts the number of downstream RF signals corresponding to downstream traffic volumes greater than a predetermined value as the number of downstream communication carriers.

[0145] 13 is a diagram illustrating an example of a correspondence table stored in a storage unit in a master station device according to an embodiment of the present disclosure. Referring to FIG. 13, the storage unit 10 stores a correspondence table TBd1 indicating a correspondence relationship between a downstream RF signal generated by up-converting the frequency of a downstream analog signal, a frequency range of the downstream analog signal, and a downstream frequency fd of a downstream local signal Ld.

[0146] The digital processing unit 11 determines the downstream frequency fd of the downstream local signal Ld based on the priority and the number of downstream communication carriers of each downstream RF signal and the correspondence table TBd1 in the storage unit 10.

[0147] (Specific Example 1 of Adjusting the Downlink Frequency fd of the Downlink Local Signal Ld) When the number of downstream communication carriers is "1", the digital processing unit 11 assigns the downstream frequency band of the downstream analog signal corresponding to the downstream RF signal for downstream communication to the frequency range with the highest quality rank. The digital processing unit 11 determines the downlink frequency fd for up-converting the frequency of the downstream analog signal to the frequency of the downstream RF signal.

[0148] More specifically, when the number of downstream communication carriers is "1" and, for example, the priority of the downstream RF signal RdD is the highest, the digital processing unit 11 assigns the downstream frequency band FidD of the downstream analog signal IdD to frequency range R4, which has a quality rank of "A." The digital processing unit 11 determines the downstream frequency fd for up-converting the frequency of the downstream analog signal IdD to the frequency of the downstream RF signal RdD to be 26.2 GHz. When the number of downstream communication carriers is "1" and, for example, the priority of the downstream RF signal RdD is the highest, the digital processing unit 11 may assign the downstream frequency band FidD of the downstream analog signal IdD to frequency range R4 and determine the downstream frequency fd of the downstream local signal Ld to be 25.8 GHz.

[0149] Referring again to Figure 3, when the digital processing unit 11 assigns the downstream frequency band FidD of the downstream analog signal IdD to the frequency range R4, it performs predetermined signal processing to generate a digital signal for generating the downstream analog signal IdD, which includes communication data and has the downstream frequency band FidD in the frequency range R4, and outputs the digital signal to the DA conversion unit 12.

[0150] The DA conversion unit 12 converts the digital signal received from the digital processing unit 11 into analog to generate a downstream analog signal IdD in which the downstream frequency band FidD has a frequency range R4, and outputs the generated downstream analog signal IdD to the multiplexing unit 13.

[0151] When the digital processing unit 11 determines the downstream frequency fd to be 26.2 GHz, it generates downstream frequency information indicating the determined downstream frequency fd and outputs the generated downstream frequency information to the control unit 14 .

[0152] The control unit 14 receives the downstream frequency information from the digital processing unit 11 , generates a digital signal further including the received downstream frequency information, and outputs the generated digital signal to the multiplexing unit 15 .

[0153] The multiplexing unit 15 generates an electrical signal in which the downstream multiplexed analog signal received from the DA conversion unit 12 and the digital signal received from the control unit 14 are frequency-multiplexed, and outputs the electrical signal to the optical modulation unit 16 .

[0154] The optical modulation unit 16 transmits downstream frequency information indicating the downstream frequency determined by the digital processing unit 11 to the slave station equipment 201. More specifically, the optical modulation unit 16 generates a downstream optical signal by optically modulating the electrical signal received from the multiplexing unit 15, and transmits the generated downstream optical signal to the slave station equipment 201 via the optical coupler 17 and the optical fiber 191.

[0155] In the slave station device 201, the control unit 36 ​​receives downlink frequency information from the master station device 101. More specifically, the control unit 36 ​​acquires the downlink frequency information from the digital signal received from the demultiplexer 23. The control unit 36 ​​generates a downlink control voltage for setting the downlink frequency fd of the downlink local signal Ld to be provided to the mixer 25 to 26.2 GHz indicated by the acquired downlink frequency information, and outputs the generated downlink control voltage to the voltage-controlled oscillator 26.

[0156] The voltage controlled oscillator 26 receives a downstream control voltage from the control unit 36 ​​and outputs a downstream local signal Ld of 26.2 GHz to the mixer 25 .

[0157] The mixer 25 performs downstream conversion processing using the downstream local signal Ld of the downstream frequency fd indicated by the downstream frequency information received by the control unit 36. More specifically, the mixer 25 multiplies the downstream multiplexed analog signal received from the amplifier unit 24 by the 26.2 GHz downstream local signal Ld received from the voltage controlled oscillator 26 to generate a downstream multiplexed RF signal and outputs it to the amplifier unit 27.

[0158] 14 is a diagram illustrating an example of the frequency spectrum of an analog signal generated in a master station in an optical communication system according to an embodiment of the present disclosure, where the horizontal axis represents frequency [GHz] and the vertical axis represents signal level.

[0159] 14, a part or all of the downstream frequency band FidD of the downstream analog signal IdD is included in a frequency range R4 having a quality rank of "A." This makes it possible to prevent degradation of the signal quality of the downstream analog signal IdD due to the characteristics of the analog circuit, etc.

[0160] For example, in the optical communication system 401, if the signal quality of the downstream analog signal deteriorates, the throughput may decrease due to retransmission of the communication data or adaptive change of the transmission method of the communication data. In response to this, the throughput can be improved by improving the signal quality of the downstream analog signal corresponding to the downstream RF signal actually used for communication within the limited bandwidth of the analog circuit.

[0161] (Another Example of Adjustment of Downlink Frequency fd of Downlink Local Signal Ld) Referring again to FIG. 13 , consider a case where the number of downstream communication carriers is two or more, and the ratio of the traffic volume of the downstream RF signal with the highest priority to the total traffic volume in the downstream direction is equal to or greater than a predetermined value. Under the condition that the downstream frequency band of the downstream analog signal corresponding to the downstream RF signal for which downstream communication is performed is contained within the frequency range Fif, the digital processing unit 11 determines the downlink frequency fd so as to assign the downlink frequency band of the downstream analog signal corresponding to the downlink RF signal with the highest priority to the frequency range with the highest quality rank. The digital processing unit 11 generates a downstream digital signal and downlink frequency information, as in "Specific Example 1 of Adjustment of Downlink Frequency fd of Downlink Local Signal Ld."

[0162] The digital processing unit 11 determines multiple downlink frequencies fd when the number of downlink communication carriers is "2" or more and the proportion of the traffic volume of the downlink RF signal with the highest priority in the total value of downlink traffic volume is less than a predetermined value.

[0163] More specifically, the storage unit 10 stores a correspondence table TBd2 that indicates the correspondence between combinations of downstream RF signals used in downstream communication and the downstream frequency fd of the downstream local signal Ld.

[0164] When the number of downstream communication carriers is two or more and the ratio of the traffic volume of the downstream RF signal with the highest priority to the total traffic volume in the downstream direction is less than a predetermined value, the digital processing unit 11 acquires the downstream frequency fd corresponding to the combination of downstream RF signals for which downstream communication is performed from the correspondence table TBd2. The digital processing unit 11 generates the downstream digital signal and the downstream frequency information, as in "Specific Example 1 of Adjusting the Downstream Frequency fd of the Downstream Local Signal Ld."

[0165] [Operation Flow] FIG. 15 is a flowchart defining an example of an operation procedure when the master station device according to the embodiment of the present disclosure determines the upstream frequency fu of the upstream local signal Lu.

[0166] Referring to FIG. 15, first, the parent station device 101 waits for the calculation timing according to the calculation period (NO in step S11), and when the calculation timing arrives (YES in step S11), it calculates the total value tuSUM of each upstream buffer volume bu and the upstream traffic volume based on the buffer information (step S12).

[0167] Next, the master station device 101 calculates each upstream traffic volume based on each upstream buffer volume bu, and assigns priorities to the corresponding RF signals in descending order of the calculated upstream traffic volumes (step S13).

[0168] Next, the master station 101 counts the number of upstream communication carriers (step S14).

[0169] Next, the master station device 101 determines the upstream frequency fu of the upstream local signal Lu based on the priority of each RF signal, the number of upstream communication carriers, the total value of the upstream traffic volume, and the correspondence table TBu1 (step S15).

[0170] Next, the master station device 101 generates upstream frequency information FRQ1 indicating the determined upstream frequency fu, and transmits the generated upstream frequency information FRQ1 to the slave station device 201 (step S16).

[0171] Next, the master station device 101 waits for a new calculation timing to arrive (NO in step S11).

[0172] FIG. 16 is a diagram illustrating an example of a sequence for adjusting each of an upstream local signal Lu and a downstream local signal Ld in an optical communication system according to an embodiment of the present disclosure.

[0173] Referring to FIG. 16, first, the master station device 101 determines the upstream frequency fu of the upstream local signal Lu used in the upstream conversion process and the downstream frequency fd of the downstream local signal Ld used in the downstream conversion process (step S21).

[0174] Next, the parent station device 101 generates a downstream optical signal including a digital signal Sdd containing upstream frequency information FRQ1 and downstream frequency information FRQ2 indicating the determined upstream frequency fu and downstream frequency fd, respectively, and data of the downstream multiplexed analog signal Ad (step S22).

[0175] Next, the master station 101 transmits the generated downstream optical signal to the slave station 201 via the optical coupler 17 and the optical fiber 191 (step S23).

[0176] Next, the slave station equipment 201 receives the downstream optical signal from the master station equipment 101 via the optical fiber 191, and acquires the digital signal Sdd and the downstream multiplexed analog signal Ad from the received downstream optical signal (step S24).

[0177] Next, the slave station device 201 acquires the upstream frequency information FRQ1 and the downstream frequency information FRQ2 from the digital signal Sdd (step S25).

[0178] Next, the slave station device 201 generates a downlink multiplexed RF signal Md by performing a downlink conversion process to upconvert the frequency of the downlink multiplexed analog signal Ad using a downlink local signal Ld having the downlink frequency fd indicated by the downlink frequency information FRQ2 (step S26).

[0179] Next, in the downlink period, the slave station device 201 transmits the generated downlink multiplexed RF signal Md to the communication terminal 311 via the antenna 211 (step S27).

[0180] Next, the slave station device 201 receives the upstream multiplexed RF signal Mu via the antenna 211 during the upstream period (step S28).

[0181] Next, the slave station device 201 performs an upstream conversion process to down-convert the frequency of the upstream multiplexed RF signal Mu using the upstream local signal Lu having the upstream frequency fu indicated by the upstream frequency information FRQ1, thereby generating an upstream multiplexed analog signal Au (step S29).

[0182] Next, the slave station device 201 generates an upstream optical signal including the data of the upstream multiplexed analog signal Au (step S30).

[0183] Next, the slave station device 201 transmits the generated upstream optical signal to the master station device 101 via the optical coupler 17 and the optical fiber 191 (step S31).

[0184] Next, the master station 101 receives the upstream optical signal from the slave station 201 via the optical fiber 191, and acquires the upstream multiplexed analog signal Au from the received upstream optical signal (step S32).

[0185] Next, the master station device 101 acquires the upstream analog signal Iu from the upstream multiplexed analog signal Au (step S33).

[0186] Although the optical communication system 401 according to the embodiment of the present disclosure has been described as including one slave station device 201, the present invention is not limited to this. The optical communication system 401 may be a distributed antenna system including multiple slave station devices 201. In this case, the master station device 101 and the multiple slave station devices 201 are connected in a one-to-multiple configuration via an optical fiber 191 and an optical splitter (not shown). The digital processing unit 11 in the master station device 101 determines, for each slave station device 201, the upstream frequency fu of the upstream local signal Lu and the downstream frequency fd of the downstream local signal Ld.

[0187] In the master station 101, the digital processing unit 11 is configured to determine the upstream frequency fu of the upstream local signal Lu and the downstream frequency fd of the downstream local signal Ld, but this is not limiting. The digital processing unit 11 may be configured not to determine either the upstream frequency fu or the downstream frequency fd.

[0188] In the master station device 101, the digital processing unit 11 is configured to determine the upstream frequency fu of the upstream local signal Lu based on the traffic volume of the communication using the upstream multiplexed RF signal and the frequency characteristic Fres, but this is not limited to this. The digital processing unit 11 may be configured to determine the upstream frequency fu without using the traffic volume of the communication using the upstream multiplexed RF signal, or may be configured to determine the upstream frequency fu without using the frequency characteristic Fres.

[0189] In the master station device 101, the digital processing unit 11 is configured to determine multiple upstream frequencies fu when the number of upstream communication carriers is two or more and the proportion of the upstream traffic volume of the upstream RF signal with the highest priority in the total upstream traffic volume is less than a predetermined value, but this is not limited to this.The digital processing unit 11 may also be configured to determine a single upstream frequency regardless of the number of upstream communication carriers and the proportion of the upstream traffic volume of the upstream RF signal with the highest priority in the total upstream traffic volume.

[0190] In the slave station device 201, when the control unit 36 ​​generates the uplink control voltages Vu1, Vu2 based on the uplink frequency information FRQ1, the control unit 36 ​​outputs the uplink control voltages Vu1, Vu2 alternately to the voltage-controlled oscillator 33 at a time ratio of 1:1. However, this is not limiting. For example, when the digital processing unit 11 in the master station device 101 determines multiple uplink frequencies fu, the digital processing unit 11 may determine a time ratio for using each uplink frequency fu in the uplink conversion process in accordance with the magnitude relationship of the uplink traffic volume of each uplink RF signal used in communication, in order to make the throughput of each uplink RF signal more uniform, and transmit ratio information indicating the determined ratio to the slave station device 201. In this case, the control unit 36 ​​in the slave station device 201 outputs the uplink control voltages Vu1, Vu2 alternately to the voltage-controlled oscillator 33 at a time ratio according to the ratio information. Similarly, when a plurality of downlink frequencies fd are determined, the digital processing unit 11 may be configured to determine the ratio of the time for using each downlink frequency fd in the downlink conversion process and transmit ratio information indicating the determined ratio to the slave station device 201.

[0191] In the master station device 101, the storage unit 10 is configured to store quality information created based on the frequency characteristic Fres, but this is not limiting. Instead of the frequency characteristic Fres, the storage unit 10 may be configured to store quality information created based on the frequency characteristic of the gain deviation for each predetermined bandwidth in the analog signal, or may be configured to store quality information created based on the frequency characteristic of high-order distortion components in the analog signal.

[0192] The above-described embodiments should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0193] Each process (each function) in the above-described embodiments is achieved by a processing circuit (circuitry) including one or more processors. The processing circuit may be configured as an integrated circuit or the like that combines one or more memories, various analog circuits, and various digital circuits in addition to the one or more processors. The one or more memories store programs (instructions) that cause the one or more processors to execute each of the processes. The one or more processors may execute each of the processes according to the program read from the one or more memories, or may execute each of the processes according to a logic circuit designed in advance to execute each of the processes. The processor may be any of various processors suitable for computer control, such as a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a DSP (Digital Signal Processor), an FPGA (Field Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit). Note that the physically separated processors may cooperate with each other to execute the processes. For example, the processors installed in the physically separated computers may cooperate with each other via a network such as a LAN (Local Area Network), a WAN (Wide Area Network), or the Internet to execute the processes. The program may be installed into the memory from an external server device or the like via the network, or may be distributed in a state stored on a recording medium such as a CD-ROM (Compact Disc Read Only Memory), a DVD-ROM (Digital Versatile Disc Read Only Memory), or a semiconductor memory, and then installed into the memory from the recording medium.

[0194] The above description includes the following additional features: [Supplementary Note 1] An optical communication system in which optical signals are transmitted and received between a master station device and a slave station device, and radio signals are transmitted and received between the slave station device and one or more communication terminals, wherein the master station device determines a first local frequency and transmits the first local frequency to the slave station device, the slave station device receives upstream radio signals including a plurality of first radio signals, the slave station device generates a first electrical signal by performing upstream conversion processing on the upstream radio signals, the slave station device transmits an upstream optical signal including data of the generated first electrical signal to the master station device, the master station device obtains the first electrical signal from the upstream optical signal, the first local frequency is a frequency of a first local signal used in the upstream conversion processing, the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands, an optical communication system in which the master station device determines the first local frequency using a correspondence table that indicates a correspondence relationship between the first local frequency and a set of frequency ranges of the first radio signal and an analog signal generated by down-converting the frequency of the first radio signal.

[0195] 10 Memory unit 11 Digital processing unit 12 DA conversion unit 13 Multiplexing unit 14 Control unit 15 Multiplexing unit 16 Optical modulation unit 17 Optical coupler 18 Optical demodulation unit 19 Signal acquisition unit 20 AD conversion unit 21 Optical coupler 22 Optical demodulation unit 23 Separation unit 24 Amplification unit 25 Mixer 26 Voltage controlled oscillator 27 Amplification unit 28 BPF 29 RF transmission / reception unit 30 BPF 31 Amplification unit 32 Mixer 33 Voltage controlled oscillator 34 Amplification unit 35 Optical modulation unit 36 ​​Control unit 101 Master station device 201 Slave station device 211 Antenna 181 Transmission line 191 Optical fiber 301, 301A, 301B, 301C, 301D Base station device 311, 311A, 311B, 311C, 311D Communication terminal 401 Optical communication system RdA, RdB, RdC, RdD RF signal (downstream RF signal) Md Downstream multiplexed RF signal Ru, RuA, RuB, RuC, RuD RF signal (upstream RF signal) Mu Upstream multiplexed RF signal IdA, IdB, IdC, IdD Analog signal (downstream analog signal) Ad Downstream multiplexed analog signal Iu, IuA, IuB, IuC, IuD Analog signal (upstream analog signal) Au Upstream multiplexed analog signal Ld Downstream local signal Lu Upstream local signal FrA, FrB, FrC, FrD Frequency band (upstream and downstream frequency band) FidA, FidB, FidC, FidD Downstream frequency band fd Downstream frequency Fiu, FiuA, FiuB, FiuC, FiuD Upstream frequency band fu Upstream frequency CrA, CrB, CrC, CrD Center frequency CidA, CidB, CidC, CidD Downstream center frequency CiuA, CiuB, CiuC, CiuD Upstream center frequency Fres Frequency characteristics FRQ1 Upstream frequency information FRQ2 Downstream frequency information R1, R2, R3, R4, R5, R6, R7 Frequency range Fif Frequency range bu Upstream buffer amount tuSUM Sum of the amount of traffic in the upstream direction TBu1, TBu2, TBd1, TBd2 Correspondence table Sdd Digital signal

Claims

1. An optical communication system in which optical signals are transmitted and received between a master station device and a slave station device, and radio signals are transmitted and received between the slave station device and one or more communication terminals, wherein the master station device determines a first local frequency and transmits the first local frequency to the slave station device, the slave station device receives an upstream radio signal including a plurality of first radio signals, the slave station device generates a first electrical signal by performing upstream conversion processing on the upstream radio signal, the slave station device transmits an upstream optical signal including data of the generated first electrical signal to the master station device, the master station device obtains the first electrical signal from the upstream optical signal, the first local frequency is the frequency of a first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

2. The optical communication system according to claim 1, wherein the master station determines the first local frequency based on the signal quality of the first electrical signal in a predetermined frequency range.

3. An optical communication system according to claim 1 or 2, wherein the master station device determines the first local frequency based on the amount of traffic in communications using the upstream wireless signal.

4. An optical communication system according to any one of claims 1 to 3, wherein the first local signal is common to the plurality of first radio signals.

5. An optical communication system according to any one of claims 1 to 3, wherein the master station device determines a plurality of the first local frequencies, and the slave station device performs the upstream conversion processing using a plurality of first local signals in a time-division manner, each of the plurality of first local signals corresponding to a respective one of the plurality of first local frequencies determined by the master station device.

6. The optical communication system according to any one of claims 1 to 5, wherein the parent station device determines a second local frequency and transmits the second local frequency to the child station device; the parent station device is capable of transmitting to the child station device a downstream optical signal containing data of a second electrical signal in which a plurality of analog signals are multiplexed; the child station device acquires the second electrical signal from the downstream optical signal; the child station device generates a downstream wireless signal by performing downstream conversion processing on a signal containing data received from the parent station device and transmits the generated downstream wireless signal; the second local frequency is the frequency of a second local signal used in the downstream conversion processing; the downstream conversion processing upconverts the frequency of the acquired second electrical signal; the second local signal is common to the plurality of analog signals; and each of the plurality of analog signals corresponds to a respective one of a plurality of frequency bands.

7. An optical communication system according to any one of claims 1 to 6, wherein the master station device determines the first local frequency using a correspondence table indicating the correspondence between the first local frequency and a set of frequency ranges of the first radio signal and an analog signal generated by down-converting the frequency of the first radio signal.

8. A master station device that transmits and receives optical signals to and from a slave station device, comprising: a determination unit that determines a first local frequency; a transmission unit that transmits frequency information indicating the first local frequency to the slave station device; and an optical receiving unit that receives an upstream optical signal including data of a first electrical signal from the slave station device and acquires the first electrical signal from the received upstream optical signal.

9. A slave station device that transmits and receives optical signals to and from a master station device and transmits and receives radio signals to and from one or more communication terminals, comprising: a radio receiving unit capable of receiving upstream radio signals including a plurality of first radio signals; a frequency information receiving unit that receives frequency information indicating a first local frequency from the master station device; a converting unit that performs upstream conversion processing on the upstream radio signals to generate a first electrical signal; and an optical transmitting unit that transmits an upstream optical signal including data of the first electrical signal generated by the converting unit to the master station device, wherein the first local frequency is the frequency of the first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

10. An optical communication method in an optical communication system in which optical signals are transmitted and received between a master station device and a slave station device, and radio signals are transmitted and received between the slave station device and one or more communication terminals, comprising the steps of: the master station device determining a first local frequency and transmitting the first local frequency to the slave station device; the slave station device receiving an upstream radio signal including a plurality of first radio signals; the slave station device generating a first electrical signal by performing upstream conversion processing on the upstream radio signal; the slave station device transmitting an upstream optical signal including data of the generated first electrical signal to the master station device; and the master station device obtaining the first electrical signal from the upstream optical signal, wherein the first local frequency is the frequency of a first local signal used in the upstream conversion processing, and the upstream conversion processing down-converts the frequencies of the plurality of first radio signals, and each of the plurality of first radio signals corresponds to a respective one of a plurality of frequency bands.

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