Relay station and communication system

Abstract

A relay station comprising: a demultiplexer that demultiplexes a downlink optical signal in which different wavelengths are multiplexed for each extension station in a plurality of extension stations into a plurality of downlink optical signals of different wavelengths for each extension station; a first converter that converts the plurality of demultiplexed downlink optical signals into a plurality of downlink optical signals of a wavelength common to the plurality of extension stations; a first repeater that transmits the plurality of converted downlink optical signals to the plurality of extension stations; a second repeater that acquires a plurality of uplink optical signals of a wavelength common to the plurality of extension stations from the plurality of extension stations; a second converter that converts the plurality of acquired uplink optical signals into a plurality of uplink optical signals of different wavelengths for each extension station; and a multiplexer that multiplexes the plurality of uplink optical signals of different wavelengths for each extension station.

Description

Relay station and communication system

[0001] The present invention relates to a relay station and a communication system.

[0002] With the increasing demand for wireless communication, high-frequency band communication has attracted attention. However, since the propagation loss of wireless signals in free space increases in proportion to the square of the frequency, the transmission distance of wireless signals in high-frequency band communication is short. This limits the communication area of ​​high-frequency band communication.

[0003] One solution to this problem is a communication system using Radio over Fiber (RoF) (hereinafter referred to as "RoF system"). In an RoF system, a central station performs electrical-optical (E / O) conversion on a radio frequency (RF) signal, thereby modulating the optical signal. The central station transmits the modulated optical signal to a base station via optical fiber. The base station generates a high-frequency signal by performing optical-electrical (O / E) conversion on the modulated optical signal. An antenna at the base station transmits a radio signal (radio wave signal) based on the generated high-frequency signal to a wireless communication terminal. In this way, a high-frequency signal is transmitted over long distances in an RoF system.

[0004] A communication system in which the functions of a base station are separated into an aggregation station (signal processing function) and a base station (communication function) for the purpose of reducing the size, power consumption, cost, and ease of installation of the base station is disclosed in Non-Patent Document 1. In addition, Non-Patent Document 2 discloses a passive optical network (PON) as an example of a double star network configuration in which multiple base stations are connected to an aggregation station.

[0005] 3 is a diagram showing a configuration example of a RoF system 10 to which a passive optical network configuration is applied. The RoF system 10 includes a central station 11a, an optical splitter 12, and N (N is an integer equal to or greater than 2) base stations 13. The central station 11a includes a signal processing unit 14 and a converter 15. The base station 13 is a remote radio unit (RRU).

[0006] Hereinafter, the direction from the base station to the aggregation station will be referred to as "upstream," and the direction from the aggregation station to the base station will be referred to as "downstream."

[0007] In downstream transmission, the signal processing unit 14 generates a high-frequency signal (electrical signal). The signal processing unit 14 downstream transmits the generated high-frequency signal to the converter 15. The converter 15 converts the downstream transmitted high-frequency signal into a signal having a predetermined wavelength "λ １ The converter 15 converts the wavelength "λ １ The optical signal of wavelength "λ" is transmitted downstream to the optical splitter 12. １ The optical signal of wavelength "λ １ The base station 13 then transmits a downstream optical signal of "ID 0x0 ...

[0008] In the upstream transmission, each base station 13 transmits a signal with a wavelength of “λ １ The optical signal of wavelength "λ" is transmitted upstream to the optical splitter 12. １ The base station 13 combines the optical signals of " and " and transmits the combined optical signal upstream to the converter 15. The converter 15 converts the combined optical signal into a high-frequency signal. The converter 15 transmits the converted high-frequency signal upstream to the signal processing unit 14. Therefore, for upstream transmission, each base station 13 needs to have an additional function of embedding the identifier of its own base station in the optical signal.

[0009] In addition, all the base stations 13 connected to the central station 11a are connected to the wavelength "λ １Since time division multiplexing (TMD) transmission is performed using ", the transmission capacity of the optical transmission path between the central station 11a and the optical splitter 12 may be limited. Therefore, wavelength division multiplexing (WDM) PONs are being considered to increase the transmission capacity of the optical transmission path (see Non-Patent Document 3). In a WDM-PON, all base stations 13 connected to the central station 11a communicate using optical signals with different wavelengths.

[0010] 4 is a diagram showing an example of the configuration of a RoF system 20 to which a WDM-PON configuration is applied. The RoF system 20 includes a central station 11b, an optical multiplexer / demultiplexer 17, and N base stations 13. The base station 13 is a remote radio unit. The central station 11b includes a signal processing unit 14, N converters 15, and an optical multiplexer / demultiplexer 16.

[0011] In downstream transmission, the signal processing unit 14 generates a high-frequency signal for each base station 13. The converter 15 converts the high-frequency signal addressed to the base station 13-n (n is any integer from 1 to N) into a signal with a wavelength of "λ ｎ The optical multiplexer / demultiplexer 16 multiplexes (wavelength multiplexes) the downstream transmitted optical signals. The optical multiplexer / demultiplexer 17 converts the multiplexed optical signals into optical signals with wavelengths "λ ｎ The base station 13-n splits the wavelength into an optical signal of wavelength "λ ｎ The receiving process is performed on the optical signal ".

[0012] In the upstream transmission, the base station 13-n transmits a signal with a wavelength of "λ ｎ The optical multiplexer / demultiplexer 16 transmits the optical signal of wavelength "λ" to the optical multiplexer / demultiplexer 17. The optical multiplexer / demultiplexer 17 multiplexes (wavelength multiplexes) the optical signals transmitted upstream. The optical multiplexer / demultiplexer 16 multiplexes the multiplexed optical signal of wavelength "λ" to the optical multiplexer / demultiplexer 17. ｎ The converter 15 splits the wavelengths of the optical signals into those of wavelength "λ ｎ The signal processing unit 14 performs reception processing on the electrical signal.

[0013] In the RoF system 20, all base stations 13 connected to the central station 11b communicate using optical signals with different wavelengths. Therefore, it is not necessary to add a function to analyze and embed an identifier to each base station 13, but each base station 13 needs to be able to communicate using each wavelength of the optical signal. １ " to wavelength "λ Ｎ Each base station 13 needs to be provided with a light source (multiple wavelength light source) for generating optical signals of each wavelength up to .

[0014] Furthermore, it is desirable that the light source connected to a converter (not shown) that converts electrical signals into optical signals in the base station 13 be a single-wavelength light source from the viewpoint of reducing the cost of the base station 13. In this case, each base station 13 and the optical multiplexer / demultiplexer 17 must be connected so that the wavelength of each base station 13 matches the wavelength of each port of the optical multiplexer / demultiplexer 17. For this reason, when installing and operating each base station 13, it is necessary to prevent port connection errors so as to prevent communication failures.

[0015] Hirofumi Shirato, Kodai Ito, Mizuki Suga, Kazuto Goto, Hideki Toshinaga, and Naoki Kita, "Proposal of a Millimeter-Wave FWA System Using RoF," IEICE General Conference, B-5-112, 2019. "IEEE Std 802.3ah-2004," IEEE Computer Society, 7 September 2004. Alberto Artuso, et al., "Mobile Optical Pluggables Alliance (MOPA) Technical paper ver.2.1," Ericsson, Nokia, Coherent, Lumentum, Sumitomo Electric and Semtech, March 9, 2023.

[0016] In an RoF system, a double star network is desirable to enable a larger number of base stations to be connected to a central station. However, from the viewpoint of preventing the configuration and installation and operation of base stations from becoming complicated, it is difficult to simply apply existing passive optical network configurations and WDM-PON configurations to an RoF system. Thus, there is a problem in that an RoF system for high-frequency band communication cannot be constructed without complicating the configuration and installation and operation of base stations.

[0017] In view of the above circumstances, an object of the present invention is to provide a relay station and a communication system that can build an RoF system for high frequency band communication without complicating the configuration and installation and operation of the base station.

[0018] One aspect of the present invention is a relay station comprising: a demultiplexer that demultiplexes a downstream optical signal in which different wavelengths are multiplexed for each base station in a plurality of base stations into a plurality of downstream optical signals of different wavelengths for each base station; a first converter that converts the demultiplexed downstream optical signals into a plurality of downstream optical signals of a wavelength common to the plurality of base stations; a first repeater that transmits the converted downstream optical signals to the plurality of base stations; a second repeater that obtains from the plurality of base stations a plurality of upstream optical signals of a wavelength common to the plurality of base stations; a second converter that converts the plurality of upstream optical signals of a wavelength common to the plurality of base stations into a plurality of upstream optical signals of different wavelengths for each base station; and a multiplexer that combines the plurality of upstream optical signals of different wavelengths for each base station.

[0019] One aspect of the present invention is a communication system comprising a central station, a relay station, and a plurality of base stations, wherein the central station uses optical signals in which different wavelengths are multiplexed for each base station in the plurality of base stations to transmit and receive optical signals in which different wavelengths are multiplexed for each base station between the central station and the relay station, the relay station converts optical signals in which different wavelengths are multiplexed for each base station between the optical signals and optical signals of a wavelength common to the plurality of base stations, and the plurality of base stations transmit and receive optical signals of a wavelength common to the plurality of base stations between the central station and the relay station.

[0020] According to the present invention, it is possible to construct an RoF system for high frequency band communication without complicating the configuration and installation and operation of base stations.

[0021] It is a diagram showing a configuration example of an RoF system in an embodiment. It is a diagram showing a configuration example of a relay station in an embodiment. It is a diagram showing a configuration example of an RoF system to which a passive optical network configuration is applied. It is a diagram showing a configuration example of an RoF system to which a WDM-PON configuration is applied.

[0022] An embodiment of the present invention will be described in detail with reference to the drawings. Fig. 1 is a diagram showing an example of the configuration of a RoF system 1 according to the embodiment. The RoF system 1 (communication system) includes a central station 2, a relay station 3, and N base stations 4. As an example, an active double star network configuration is applied to the RoF system 1.

[0023] In the optical transmission path between the central station 2 and the relay station 3, different wavelengths are assigned to each base station 4. In downstream transmission, the central station 2 generates a wavelength-multiplexed optical signal using optical signals with wavelengths for each base station 4. In upstream transmission, the central station 2 performs wavelength division to divide the wavelength-multiplexed optical signal into optical signals for each wavelength.

[0024] On the other hand, in the optical transmission path between the relay station 3 and the base station 4, a wavelength common to all the base stations 4 is assigned to each base station 4. The relay station 3 performs wavelength conversion (optical signal conversion) on the optical signal input from the central station 2, thereby converting the wavelength "λ ｓｉｇ " optical signal is transmitted downstream.

[0025] The central station 2 includes a signal processing unit 21 , N converters 22 , an optical circulator 23 , an optical multiplexer / demultiplexer 24 , and N converters 25 .

[0026] The signal processing unit 21 is realized as software by a processor such as a CPU (Central Processing Unit) executing a program stored in a storage device having a non-volatile recording medium (non-transitory recording medium) and memory. The program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid-state drive (SSD) built into a computer system.

[0027] The signal processing unit 21 may be realized using hardware (accelerator) including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0028] The base station 4 includes an optical circulator 41 , a converter 42 , an optical circulator 43 , an antenna 44 , and a converter 45 .

[0029] First, downstream transmission will be described. The signal processing unit 21 generates a high-frequency signal for each base station 4. The converter 22-n "E / O" converts the high-frequency signal addressed to the base station 4-n into a signal with a wavelength "λ ｎ The optical circulator 23-n converts the wavelength of the optical signal into an optical signal of wavelength "λ ｎ ” to the optical multiplexer / demultiplexer 24 .

[0030] The optical multiplexer / demultiplexer 24 receives the wavelength "λ ｎ" optical signal is input from the optical circulator 23-n. The optical multiplexer / demultiplexer 24 multiplexes the input optical signals of multiple wavelengths. In other words, the optical multiplexer / demultiplexer 24 performs wavelength multiplexing on the optical signals of multiple wavelengths. The optical multiplexer / demultiplexer 24 transmits the wavelength-multiplexed optical signal to the repeater station 3 via the optical transmission path.

[0031] The relay station 3 demultiplexes the wavelength-multiplexed optical signal into optical signals of each wavelength. ｓｉｇ " light source, wavelength "λ ｓｉｇ The relay station 3 generates an optical signal of wavelength "λ ｓｉｇ " to each base station 4. In this way, when the relay station 3 demultiplexes the optical signal transmitted downstream from the central station 2 to each base station 4, it demultiplexes the optical signal having the wavelength "λ ｓｉｇ The wavelength of the optical signal (downstream optical signal) is converted so that

[0032] The optical circulator 41-n outputs a light having a wavelength of "λ ｓｉｇ The converter 42-n "O / E" outputs the optical signal of wavelength "λ ｓｉｇ The optical circulator 43-n converts the optical signal "" into an electrical signal. The optical circulator 43-n outputs the electrical signal to the antenna 44-n. The antenna 44 transmits a wireless signal based on the electrical signal to a wireless communication terminal (not shown).

[0033] Next, upstream transmission will be described. The antenna 44-n receives a radio signal from a radio communication terminal (not shown). The antenna 44 outputs an electrical signal based on the received radio signal to the optical circulator 43-n. The optical circulator 43-n outputs an electrical signal based on the received radio signal to the converter 45-n. The converter 45-n "E / O" converts the wavelength "λ" common to all base stations 4 into an electrical signal. ｓｉｇ ", an electrical signal based on the received radio signal is generated using a light source of wavelength "λ ｓｉｇ The optical circulator 41-n converts the wavelength of the optical signal into an optical signal of wavelength "λ ｓｉｇ The optical signal is transmitted to the repeater station 3 via the optical transmission line.

[0034] The relay station 3 has a wavelength of "λ ｓｉｇThe repeater station 3 converts the wavelength of the optical signal input from the optical circulator 41-n into a wavelength of "λ ｎ The repeater station 3 generates an optical signal of a plurality of wavelengths "λ ｎ That is, the relay station 3 performs wavelength multiplexing on the optical signals of the wavelengths of the respective base stations 4.

[0035] The relay station 3 transmits the wavelength-multiplexed optical signal to the optical multiplexer / demultiplexer 24 via the optical transmission line. In this way, when the relay station 3 multiplexes the optical signals transmitted upstream from each base station 4, the wavelength "λ" of each base station 4 is multiplexed. ｎ The wavelength of the optical signal (upstream optical signal) is converted so that

[0036] The optical multiplexer / demultiplexer 24 demultiplexes the wavelength-multiplexed signal into optical signals of each wavelength. ｎ The converter 25-n "O / E" outputs the optical signal of wavelength "λ ｎ The signal processing unit 21 converts the optical signal "" into an electrical signal. The signal processing unit 21 performs reception processing on the electrical signal.

[0037] Next, a configuration example of the repeater station 3 will be described. Fig. 2 is a diagram showing a configuration example of the repeater station 3 in an embodiment. The repeater station 3 includes an optical multiplexer / demultiplexer 31, N converters 32, N converters 33, N optical circulators 34, N converters 35, N converters 36, and an optical multiplexer / demultiplexer 37.

[0038] In the repeater station 3, downstream optical signals are transmitted by an optical multiplexer / demultiplexer 31, a converter 32, a converter 33, and an optical circulator 34. Furthermore, upstream optical signals are transmitted by the optical circulator 34, a converter 35, a converter 36, and an optical multiplexer / demultiplexer 37.

[0039] In downstream transmission, the wavelength-multiplexed optical signal is input to the optical multiplexer / demultiplexer 31 from the optical multiplexer / demultiplexer 24. The demultiplexed optical signal is input to the converter 32-n "O / E" from the optical multiplexer / demultiplexer 31. The optical multiplexer / demultiplexer 31 divides the wavelength-multiplexed optical signal into wavelengths "λ ｎThe converter 32-n splits the wavelength "λ ｎ The converter 33-n converts the optical signal of the wavelength "λ" common to all base stations 4 into an electrical signal. ｓｉｇ ” light source, and an electrical signal with a wavelength of “λ ｓｉｇ The converter 33-n converts the wavelength "λ ｓｉｇ The optical circulator 34-n outputs an optical signal of wavelength "λ" from the port connected to the base station 4-n. ｓｉｇ " optical signal to the base station 4-n.

[0040] For upstream transmission, the wavelength is ｓｉｇ The optical signal of wavelength "λ" is input to the optical circulator 34-n from the base station 4-n. ｓｉｇ The converter 35-n "O / E" outputs the optical signal of wavelength "λ ｓｉｇ The converter 36-n "E / O" converts the optical signal of the wavelength "λ ｎ ” light source, and an electrical signal with a wavelength of “λ ｎ The optical multiplexer / demultiplexer 37 converts the wavelength "λ ｎ " optical signal is input from converter 36-n. Optical multiplexer / demultiplexer 37 multiplexes the input optical signals of multiple wavelengths. In other words, optical multiplexer / demultiplexer 37 performs wavelength multiplexing on the optical signals of multiple wavelengths. Optical multiplexer / demultiplexer 37 transmits the wavelength-multiplexed optical signal to optical multiplexer / demultiplexer 24 via an optical transmission path.

[0041] As described above, the central station 2 transmits the wavelength “λ 1 ” which is different for each of the multiple base stations 4. ｎ Each base station 4 receives a different wavelength "λ ｎ The relay station 3 transmits and receives an optical signal in which different wavelengths "λ" are multiplexed for each base station 4. ｎ ” and a wavelength “λ ” common to multiple base stations 4. ｓｉｇ The plurality of base stations 4 converts the optical signal of the wavelength "λ ｓｉｇ The optical signal " is transmitted and received between the repeater station 3 and the repeater station 3 .

[0042] In the relay station 3, the optical multiplexer / demultiplexer 31 (demultiplexer) separates the wavelengths “λ 1 ” that are different for each of the multiple base stations 4. ｎ The downstream optical signal multiplexed with the wavelengths "λ" and "λ" is transmitted to each base station 4. ｎ Each converter 32 and each converter 33 (first converter) demultiplexes the demultiplexed downstream optical signals into a plurality of downstream optical signals of wavelengths "λ ｓｉｇ Each optical circulator 34 (first repeater) transmits the converted downstream optical signals to the multiple base stations 4. The optical circulator 34 (second repeater) converts the converted downstream optical signals into a plurality of downstream optical signals of wavelength "λ ｓｉｇ Each converter 35 and each converter 36 (second converter) receives a plurality of upstream optical signals of wavelength "λ" common to the plurality of base stations 4. ｓｉｇ ” are transmitted to each base station at different wavelengths “λ ｎ The optical multiplexer / demultiplexer 37 (multiplexer) converts the wavelengths of the upstream optical signals into a plurality of upstream optical signals of wavelengths "λ" which are different for each base station 4. ｎ " multiple upstream optical signals.

[0043] In this way, the relay station 3 transmits a different wavelength “λ” to each base station 4. ｎ ” and a wavelength “λ ” common to multiple base stations 4. ｓｉｇ This makes it possible to build a RoF system 1 for high-frequency band communication without complicating the configuration and installation and operation of the base station 4.

[0044] That is, because the relay station 3 performs wavelength conversion, all base stations 4 communicate using optical signals of the same wavelength. This eliminates the need to associate the base stations 4 with wavelengths, facilitating the installation of the base stations 4. Furthermore, because the central station 2 identifies the base stations 4 based on the wavelength of the optical signal, the base stations 4 do not need to perform processing based on the identifier.

[0045] (First Modification) In the repeater station 3, the converter 32-n converts the wavelength “λ ｎ The converter 33-n converts the optical signal of the wavelength "λ" common to all base stations 4 into an electrical signal. ｓｉｇ ” light source, and an electrical signal with a wavelength of “λｓｉｇ In this way, the optical signal is converted into an electrical signal. The same applies to upstream transmission.

[0046] Here, a light source used for the electrical-to-optical conversion may be provided for each converter 33. Also, a light source used for the electrical-to-optical conversion may be provided for each converter 36.

[0047] A common light source used for the electrical-to-optical conversion may be provided for each of the multiple converters 33. The light source common to the multiple converters 33 may emit light of wavelength "λ ｓｉｇ The optical signal "1" is generated by each converter 33 as a light source (single wavelength light source).

[0048] A common light source used for electrical to optical conversion may be provided for each of the multiple converters 36. The common light source for the multiple converters 36 may emit light having a wavelength "λ １ " to wavelength "λ Ｎ The converters 36 are light sources (multiple wavelength light sources) for generating optical signals of each wavelength up to .

[0049] (Second Modification) In the above embodiment and the first modification, the relay station 3 temporarily converts the optical signal into an electrical signal. In contrast, in the second modification, if the wavelength used between the central station 2 and the relay station 3 and the wavelength used between the relay station 3 and the base station 4 are mutually convertible, the relay station 3 does not need to convert the optical signal into an electrical signal.

[0050] For example, an optical signal may be input to an optical fiber (nonlinear element) (not shown) having nonlinear wavelength characteristics. An optical carrier for wavelength conversion may be input from a light source to the nonlinear element. Only an optical signal of a desired wavelength may be extracted from the output signal of the nonlinear element using a wavelength filter, thereby converting the wavelength of the input optical signal to a desired wavelength (Reference 1: T. Kato, S. Watanabe, T. Tanimura, T. Richter, R. Elschner, C.S. Langhorst, C.S. Schubert, and T. Hoshida, “THz-Range Optical Frequency Shifter for Dual Polarization WDM Signals Using Frequency Conversion in Fiber,” IEEE ECOC, 2016.). Here, the relay station 3 may include a light source for wavelength conversion for each base station 4, or may include a single light source shared for wavelength conversion among multiple base stations 4. The nonlinear element, wavelength filter, and light source may be included in an optical multiplexer / demultiplexer or an optical circulator.

[0051] As described above, the converters 32, 33, 35, and 36 only need to have a wavelength conversion function, and are not required to have a function of converting to an electrical signal. The converters 32 and 33 (first converters) input the first optical carrier for wavelength conversion and the multiple demultiplexed downstream optical signals to the first nonlinear element. The converters 32 and 33 (first converters) convert the wavelength "λ" common to the multiple base stations 4 into an electrical signal. ｓｉｇ A first wavelength filter is used to extract a plurality of downstream optical signals of different wavelengths "λ" for each base station 4. ｎ (wavelengths of multiple demultiplexed downstream optical signals "λ ｎ ") to a wavelength "λ ｓｉｇ The converters 35 and 36 (second repeaters) convert the second optical carrier for wavelength conversion into a wavelength "λ ｓｉｇ The converters 35 and 36 (second repeaters) convert the wavelengths "λ" different for each base station 4 into a plurality of upstream optical signals.ｎ ” is used to extract a plurality of upstream optical signals of wavelength “λ ” that are common to the plurality of base stations 4. ｓｉｇ ” at each base station 4 using a different wavelength “λ ｎ "

[0052] (Third Modification) In the above embodiment, the wavelength “λ 1 ” used between the relay station 3 and the base station 4 is ｓｉｇ " represents the wavelength "λ" used between the central station 2 and the relay station 3. ｎ " is different from either of these.

[0053] In the third modification, the wavelength “λ 1 ” used between the relay station 3 and the base station 4 is ｓｉｇ " represents the wavelength "λ" used between the central station 2 and the relay station 3. ｎ " may be the same as either "

[0054] (Fourth Modification) In the above embodiment, the wavelength used for upstream transmission and the wavelength used for downstream transmission are the same between the central station 2 and the relay station 3. Furthermore, the wavelength used for upstream transmission and the wavelength used for downstream transmission between the relay station 3 and the base station 4 are the same "λ ｓｉｇ "

[0055] In the fourth modification, the wavelength used for upstream transmission and the wavelength used for downstream transmission may be different between the central station 2 and the relay station 3. Furthermore, the wavelength used for upstream transmission and the wavelength used for downstream transmission may be different between the relay station 3 and the base station 4.

[0056] (Fifth Modification) In a fifth modification, the repeater station 3 may include not only a functional unit that performs wavelength conversion, but also at least one of an optical amplifier, an electric amplifier, an optical switch, and an electric switch.

[0057] (Sixth Modification) When the relay station 3 is provided with a functional unit such as a switch as in the above-described fifth modification, the relay station 3 may also be provided with a control unit that controls the functional unit such as the switch. In this case, the central station 2 transmits a control signal to the relay station 3 for the functional unit such as the switch. The control signal may be, for example, a signal having a wavelength "λ" assigned to the base station 4-n. ｎThe control signal may be multiplexed onto the optical signal of each wavelength "λ" using a subcarrier multiplexing (SCM) technique. ｎ may be transmitted by an optical signal having a different wavelength from that of the signal.

[0058] (Seventh Modification) In the above embodiment, the central station 2 identifies the base station 4 based on the wavelength of the optical signal. Furthermore, WDM transmission is used for transmitting the optical signal between the central station 2 and the relay station 3.

[0059] In the seventh modification, multiplexed transmission other than wavelength and an identification method based on other than wavelength may be used. For example, optical signals may be transmitted between the central station 2 and the relay station 3 using mode multiplexing, and the base station may be identified based on the mode. Alternatively, for example, the central station 2 and the relay station 3 may be connected using a multicore fiber, and the base station 4 may be identified based on which core of the multicore fiber is connected. For example, optical signals may be transmitted between the central station 2 and the relay station 3 using subcarrier multiplexing (SCM), and the base station 4 may be identified based on the subcarrier.

[0060] (Eighth Modification) In the above embodiment, the central station 2 and the relay station 3 are connected in a one-to-one relationship. In the eighth modification, the central station 2 and the relay station 3 may be connected in a one-to-many relationship. When multiple relay stations 3 are connected to the central station 2, the same wavelength may be assigned to the optical signals of each base station 4 connected to different relay stations 3. When the same wavelength is assigned to each base station 4 connected to different relay stations 3, the central station 2 needs to identify the relay station 3. Here, the relay station 3 may be identified based on the identifier assigned to the relay station 3.

[0061] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0062] The present invention is applicable to communication systems.

[0063] DESCRIPTION OF SYMBOLS 1...RoF system, 2...aggregation station, 3...relay station, 4...base station, 10...RoF system, 11a, 11b...aggregation stations, 12...optical splitter, 13...base station, 14...signal processing unit, 15...converter, 16...optical multiplexer / demultiplexer, 17...optical multiplexer / demultiplexer, 20...RoF system, 21...signal processing unit, 22...converter, 23...optical circulator, 24...optical multiplexer / demultiplexer, 25...converter, 31...optical multiplexer / demultiplexer, 32...converter, 33...converter, 34...optical circulator, 35...converter, 36...converter, 37...optical multiplexer / demultiplexer, 41...optical circulator, 42...converter, 43...optical circulator, 44...antenna, 45...converter

Claims

1. A relay station comprising: a splitter that splits downstream optical signals in which different wavelengths are multiplexed for each base station in a plurality of base stations into a plurality of downstream optical signals of different wavelengths for the base station; a first converter that converts the split downstream optical signals into a plurality of downstream optical signals of a wavelength common to the plurality of base stations; a first repeater that transmits the converted downstream optical signals to the plurality of base stations; a second repeater that receives from the plurality of base stations a plurality of upstream optical signals of a wavelength common to the plurality of base stations; a second converter that converts the plurality of upstream optical signals of a wavelength common to the plurality of base stations into a plurality of upstream optical signals of a different wavelength for each base station; and a multiplexer that combines the plurality of upstream optical signals of a different wavelength for each base station.

2. The relay station described in claim 1, wherein the first converter inputs a first optical carrier for wavelength conversion and the multiple demultiplexed downstream optical signals to a first nonlinear element, and uses a first wavelength filter to extract multiple downstream optical signals of a wavelength common to the multiple base stations, to convert the wavelengths that differ for each base station to a wavelength common to the multiple base stations; and the second repeater inputs a second optical carrier for wavelength conversion and multiple upstream optical signals of a wavelength common to the multiple base stations to a second nonlinear element, and uses a second wavelength filter to extract multiple upstream optical signals of a wavelength different for each base station, to convert the wavelengths that are common to the multiple base stations to a wavelength that differs for each base station.

3. A communications system comprising a central station, a relay station, and a plurality of base stations, wherein the central station transmits and receives optical signals in which different wavelengths are multiplexed for each base station in the plurality of base stations between the central station and the relay station, the relay station converts between the optical signals in which different wavelengths are multiplexed for each base station and optical signals of a wavelength common to the plurality of base stations, and the plurality of base stations transmit and receive optical signals of a wavelength common to the plurality of base stations between the central station and the relay station.

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