Optical communication system, aggregation station, and communication method

Abstract

An optical communication system in which an aggregation station and a plurality of extension stations are connected in series with one another with the aggregation station at the front, wherein: said aggregation station outputs a downlink optical signal generated by subjecting a wireless signal to an E / O conversion to the plurality of extension stations, and obtains a wireless signal by subjecting an uplink optical signal inputted from the plurality of extension stations to an O / E conversion; each of the extension stations transmits a wireless signal obtained by subjecting the downlink optical signal received from the aggregation station to an O / E conversion, transmits an uplink optical signal generated by subjecting the received wireless signal to an E / O conversion to the aggregation station; and the wavelengths of the uplink optical signals and downlink optical signals assigned to the plurality of extension stations are identical.

Description

[Technical Field]

[0001] The present invention relates to an optical communication system, a central station, and a communication method. [Background technology]

[0002] Millimeter-wave radio waves have attracted attention due to their potential for high-speed transmission. However, because millimeter-wave radio waves have large propagation losses, they require the deployment of wireless base stations at high density.

[0003] As a method for deploying wireless base stations, Non-Patent Document 1 discloses a method of applying analog RoF (Radio-over-Fiber). Analog RoF is a technology in which an optical signal is intensity-modulated with a wireless signal and the intensity-modulated optical signal is transmitted over an optical fiber, and the original wireless signal can be extracted by O / E converting the transmitted optical signal.

[0004] By applying analog RoF, wireless base stations can be separated into a central station that processes signals and a base station that has antenna functions, and the signal processing function can be consolidated into the central station. This simplifies the base station and makes it easier to install.

[0005] Non-Patent Document 2 discloses that the central station and the base stations are connected in a cascade configuration in order to more easily deploy the base stations. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Kodai Ito, Mizuki Suga, Hirofumi Shirato, Naoki Kita, Takeshi Onisawa, "Efficient Accommodation of Diverse High-Frequency Band Wireless Systems Using Analog Radio Fiber," NTT Technical Journal, 32(3), 15-17, 2020. [Non-patent document 2] E. -S. Kim, M. Sung, JH Lee, JK Lee, S. -H. Cho and J. Kim, "Coverage Extension of Indoor 5G Network Using RoF-Based Distributed Antenna System," IEEE Access, vol. 8, pp. 194992-194999, 2020. Summary of the Invention [Problem to be solved by the invention]

[0007] According to Non-Patent Document 2, two wavelengths are assigned to each base station for the upstream and downstream optical signals. When two wavelengths are assigned to each base station, the design differs for each base station, which can result in high design costs. The present invention provides an optical communication system, an aggregation station for the optical communication system, and a communication method that can be designed at lower costs. [Means for solving the problem]

[0008] One aspect of the present invention is an optical communication system in which a central station and multiple base stations are connected in series with the central station at the head, wherein the central station outputs downstream optical signals generated by E / O converting radio signals to the multiple base stations, and obtains radio signals by O / E converting upstream optical signals input from the multiple base stations, each of the base stations transmits a radio signal obtained by O / E converting the downstream optical signal received from the central station, and transmits an upstream optical signal generated by E / O converting the received radio signal to the central station, and the wavelengths of the upstream optical signals and the downstream optical signals assigned to the multiple base stations are the same.

[0009] One aspect of the present invention is an aggregation station in an optical communication system in which a plurality of base stations are connected in series with a aggregation station at the head, the aggregation station transmitting only optical signals of a first wavelength to the base station and receiving only optical signals of a second wavelength, which is different from the first wavelength, from the base station.

[0010] One aspect of the present invention is an optical communication system in which a central station is at the head and a plurality of base stations are connected in series, This communication method includes a downstream optical signal generating step in which the central station performs E / O conversion on a radio signal to generate a downstream optical signal; an output step in which the downstream optical signal is output to the plurality of base stations; a radio signal acquiring step in which the central station performs O / E conversion on an upstream optical signal input from the plurality of base stations to acquire a radio signal; a radio signal transmitting step in which the base station transmits a radio signal obtained by O / E conversion of the downstream optical signal received from the central station; and an optical signal transmitting step in which the central station transmits an upstream optical signal generated by E / O conversion of the received radio signal to the central station, wherein the wavelengths of the upstream optical signal and the downstream optical signal assigned to the plurality of base stations are the same. [Effects of the Invention]

[0011] The optical communication system of the present invention can be designed at a lower cost. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an optical communication system in which a central station and a base station are connected in a cascade configuration. [Figure 2] FIG. 2 is a diagram illustrating the configuration of a central station. [Figure 3] FIG. 1 is a diagram illustrating the configuration of an ith base station (i is an integer satisfying the condition 1≦i≦N). [Figure 4] 10 is a flowchart showing a transmission method of the aggregation station. [Figure 5] 10 is a flowchart showing a receiving method of the aggregation station. [Figure 6] 10 is a flowchart showing a method of communication between a base station and an upstream device. [Figure 7] 10 is a flowchart showing a method of communication between a base station and a downstream device. DETAILED DESCRIPTION OF THE INVENTION

[0013] Optical communication system FIG. 1 is a diagram showing the configuration of an optical communication system 1 in which an aggregation station 10 and a base station 11 are connected in a cascade configuration. The optical communication system 1 has one aggregation station 10 and N base stations 11 (N is an integer equal to or greater than 2). In the optical communication system 1, the aggregation station 10 is at the head, and the N base stations 11 are connected in series. Hereinafter, when distinguishing between the N base stations 11, each base station 11 will be referred to as the i-th base station 11-i (i is an integer satisfying 1≦i≦N). However, when describing features common to all the i-th base stations 11-i, the base station 11 will simply be referred to as the base station 11. Note that the ordinal number i is equal to the number of hops from the aggregation station 10. In the optical communication system 1, the direction from the aggregation station 10 to the base station 11 is referred to as downstream, and the direction from the base station 11 to the aggregation station 10 is referred to as upstream.

[0014] The central station 10 performs E / O conversion on the generated wireless signal and transmits the generated downstream optical signal to the first base station 11-1. The first base station 11-1 branches the received downstream optical signal, transmits part of the optical signal to the second base station 11-2, processes the remaining optical signal within itself, and transmits a wireless signal according to the optical signal. The first base station 11-1, for example, O / E converts the optical signal and radiates the resulting wireless signal into space via an antenna. The second base station 11-2 to the N-1th base station 11-(N-1) also perform the same operation as the first base station 11-1. In other words, the ith base station 11-i (i is an integer satisfying 2≦i≦N−1) branches the optical signal received from the i-1th base station 11-(i-1), which is an upstream device, and transmits part of the optical signal to the i+1th base station 11-(i+1), which is a downstream device.

[0015] The ith base station 11-i (i is an integer satisfying 2≦i≦N−1) generates an upstream optical signal by E / O converting a radio signal received via an antenna, for example. The ith base station 11-i combines the optical signal received from a downstream device with the upstream optical signal and transmits the combined signal to an upstream device. In other words, the ith base station 11-i combines the optical signal received from the (i+1) base station 11-(i+1) with an optical signal obtained by multiplexing the radio signal received by the ith base station, and transmits the combined optical signal to the (i−1) base station 11-(i−1).

[0016] That is, the optical signal transmitted from the central station 10 travels downstream through the first base station 11-1 and the second base station 11-2, and finally reaches the Nth base station 11-N. The optical signal transmitted from the Nth base station 11-N travels upstream to the N-1th base station 11-(N-1), and finally reaches the central station 10. The central station 10 performs O / E conversion on the received optical signal to obtain a radio signal. The radio signal is subjected to, for example, signal processing.

[0017] As described above, in the optical communication system 1, N base stations 11 are connected in series with the central station 10 at the head. Radio signals received at the base stations 11 are converted into optical signals without undergoing signal processing at the base stations 11 and are then aggregated at the central station 10. The optical signals aggregated by the central station 10 are converted into radio signals, and the radio signals received at the base stations 11 are extracted. Because the base stations 11 do not need to include a configuration for processing radio signals, the base stations 11 can be easily implemented.

[0018] 2 is a diagram showing the configuration of the central station 10. The central station 10 includes a transmitting unit 100, an E / O converting unit 101, a multiplexing unit 102, an O / E converting unit 103, and a receiving unit 104.

[0019] The transmitter 100 transmits an electrical signal to the E / O converter 101. The E / O converter 101 converts the received electrical signal into an optical signal and transmits the optical signal to the multiplexer 102. The wavelength of the optical signal transmitted from the E / O converter 101 to the multiplexer 102 is set to λ DL The multiplexing unit 102 multiplexes the optical signals received from the E / O conversion unit 101 and transmits the multiplexed optical signals to the first base station 11-1. The multiplexing unit 102 also demultiplexes the optical signals received from the first base station 11-1 and transmits the demultiplexed optical signals to the O / E conversion unit 103.

[0020] The O / E converter 103 converts the received optical signal into an electrical signal and transmits the electrical signal to the receiver 104. The O / E converter 103 converts the wavelength of the received optical signal into λ UL The receiving unit 104 receives the electrical signal from the O / E conversion unit 103. The electrical signal received by the receiving unit 104 is subjected to, for example, signal processing.

[0021] 〈Exhibition Bureau〉 3 is a diagram showing the configuration of the ith base station 11-i (i is an integer satisfying 1≦i≦N). The ith base station 11-i includes an ith optical splitter 110-i, an ith multiplexing unit 111-i, an ith O / E conversion unit 112-i, an ith transmitting amplifier 113-i, an ith circulator 114-i, an ith antenna 115-i, an ith receiving amplifier 116-i, and an ith E / O conversion unit 117-i. Like the i-th base station 11-i, the i-th optical splitter 110-i, the i-th multiplexing unit 111-i, the i-th O / E conversion unit 112-i, the i-th transmitting amplifier 113-i, the i-th circulator 114-i, the i-th antenna 115-i, the i-th receiving amplifier 116-i, and the i-th E / O conversion unit 117-i will also be referred to as the optical splitter 110, the multiplexing unit 111, the O / E conversion unit 112, the transmitting amplifier 113, the circulator 114, the antenna 115, the i-th receiving amplifier 116, and the E / O conversion unit 117, respectively, when explaining their characteristics that are independent of the position of the base station 11.

[0022] The ith optical splitter 110-i branches an optical signal received from an upstream device and transmits the branched optical signal to the ith multiplexer 111-i and the (i+1)th base station 11-(i+1). The first optical splitter 110-1 branches an optical signal received from the aggregation station 10, and the ith optical splitter 110-i (i is an integer satisfying 2≦i≦N−1) branches an optical signal received from the (i−1)th base station 11-(i−1). The i-th optical splitter 110-i combines the optical signals transmitted from the i-th multiplexer 111-i and the i+1-th base station 11-(i+1), which is a downstream device, and transmits the combined optical signal to an upstream device. The first optical splitter 110-1 transmits the combined optical signal to the aggregation station 10, and the i-th optical splitter 110-i (i is an integer satisfying 2≦i≦N−1) transmits the combined optical signal to the i-1-th base station 11-(i−1).

[0023] The i-th multiplexing unit 111-i is configured by, for example, an optical multiplexer / demultiplexer. The i-th multiplexing unit 111-i demultiplexes the optical signal received from the i-th optical splitter 110-i, and separates the optical signal having a wavelength λ DLThe i-th multiplexer 111-i extracts the signal of wavelength λ from the optical signal received from the i-th E / O converter 117-i and outputs it to the i-th O / E converter 112-i. UL The optical signal transmitted from the i-th E / O conversion unit 117-i is finally received by the receiving unit 104.

[0024] The i-th O / E converter 112-i converts the optical signal received from the i-th multiplexer 111-i into an electrical signal and transmits it to the i-th transmitting amplifier 113-i. The i-th transmitting amplifier 113-i amplifies the electrical signal received from the i-th O / E converter 112-i and transmits the amplified electrical signal to the i-th circulator 114-i.

[0025] The i-th circulator 114-i transmits the electrical signal received from the i-th transmitting amplifier 113-i to the i-th antenna 115-i, and also transmits the electrical signal received from the i-th antenna 115-i to the i-th receiving amplifier 116-i.

[0026] The i-th antenna 115-i radiates the electrical signal received from the i-th circulator 114-i into space, and also transmits the received electrical signal to the i-th circulator 114-i.

[0027] The i-th receiving amplifier 116-i amplifies the electrical signal received from the i-th circulator 114-i and transmits the amplified electrical signal to the i-th E / O converter 117-i. The i-th E / O converter 117-i converts the electrical signal received from the i-th receiving amplifier 116-i into λ UL The optical signal is converted into an optical signal of the wavelength of the signal and transmitted to the ith multiplexing unit 111-i.

[0028] The N-th base station 11-N has almost the same configuration as the i-th base station 11-i (i is an integer satisfying 1≦i≦N−1), but does not need to include an optical splitter.

[0029] In the i-th optical splitter 110-i, the wavelength λ transmitted from the i-th multiplexing unit 111-i is ULand the optical signal of wavelength λ transmitted from the i+1th base station 11-(i+1). UL Since the optical signals are multiplexed, the optical signals may interfere with each other. To reduce the influence of interference on the optical signals, the i-th optical splitter 110-i may switch between multiplexing and not multiplexing the optical signals. The optical signal transmitted from the (i+1)-th base station 11-(i+1) may be output to an upstream device without being multiplexed with the optical signal transmitted from the i-th multiplexer 111-i.

[0030] Furthermore, if the central station 10 has a delayed wave removal function for removing waves (delayed waves) contained in the input optical signal that have a delay time of a predetermined time or more, the effect of interference components of the optical signal can be reduced by this function.

[0031] 4 is a flowchart showing a transmission method of the central station 10. The transmitter 100 transmits an electrical signal to the E / O converter 101 (step S1). The E / O converter 101 converts the electrical signal into an optical signal (step S2). The multiplexer 102 multiplexes the optical signal (step S3). The multiplexed optical signal is transmitted to the first base station 11-1.

[0032] 5 is a flowchart showing a receiving method of the central station 10. The multiplexing unit 102 demultiplexes the optical signal received from the first base station 11-1 (step S11). The O / E conversion unit 103 converts the optical signal into an electrical signal (step S12). The receiving unit 104 receives the electrical signal (step S13).

[0033] 6 is a flowchart showing a method of communication between the base station 11 and an upstream device. The optical splitter 110 branches an optical signal received from the upstream device (step S20). The branched optical signal is transmitted to the multiplexer 111 and the downstream device. The multiplexer 111 demultiplexes the optical signal received from the optical splitter 110 (step S21). The O / E converter 112 converts the optical signal into an electrical signal (step S22). The transmission amplifier 113 amplifies the electrical signal (step S23). The electrical signal is radiated from the antenna 115 via the circulator 114 (step S24).

[0034] 7 is a flowchart showing a method of communication between the base station 11 and downstream devices. The antenna 115 receives an electrical signal (step S31). The electrical signal is passed through the circulator 114 and amplified by the receiving amplifier 116 (step S32). The E / O converter 117 converts the electrical signal into an optical signal (step S33). The multiplexer 111 multiplexes the optical signal (step S34). The optical splitter 110 multiplexes the optical signal received from the multiplexer 111 with the optical signal received from the downstream device, and transmits the combined signal to the upstream device (step S35).

[0035] The wavelength of the optical signal transmitted from the central station 10 to the base station 11 is λ DL The wavelength of the optical signal received from the base station 11 and the aggregation station 10 is λ UL Therefore, the base station 11 transmits only the wavelength λ DL and processes the optical signal of wavelength λ UL In other words, the wavelengths of the upstream and downstream optical signals assigned to all base stations 11 are the same. As a result, all the multiplexing units 111 provided in the base stations 11 may be the same. Furthermore, when the branching ratios of all the optical splitters 110 are made the same, all the base stations 11 may have the same configuration. The optical communication system 1 can be particularly used in high-frequency band distributed antenna technology, which installs a large number of antennas and ensures line-of-sight for any one of the antennas, or enables gain to be achieved by using multiple antennas. The optical communication system 1 may perform phase adjustment in the base station 11 to in-phase combine signals transmitted and received by multiple antennas. When the central station and each base station transmit and receive the same signal, there is no need to allocate two wavelengths to each base station. As a result, it is possible to reduce the difference in design between base stations, and to provide an optical communication system at lower cost.

[0036] The ith optical splitter 110-i may branch the received optical signal so that the magnitude ratio is 1:(Ni). The ith optical splitter 110-i may transmit the branched optical signal to the ith multiplexer 111-i and the (i+1) base station 11-(i+1), respectively, so that the magnitude ratio between the optical signal to be transmitted to the ith multiplexer 111-i and the optical signal to be transmitted to the (i+1) base station 11-(i+1) is 1:(Ni). This makes it possible to equalize the magnitudes of the optical signals transmitted to the multiplexers 111 of all base stations 11.

[0037] Other Embodiments One embodiment of the present invention has been described above in detail with reference to the drawings, but the specific configuration is not limited to this embodiment, and includes designs within the scope of the present invention that do not deviate from the gist of the present invention.

[0038] In the optical communication system 1 according to the above-described embodiment, the string of multiple cascaded base stations 11 does not branch midway. However, in other embodiments, the string of base stations 11 may branch midway. That is, two or more base stations 11 may be connected to the downstream side of a certain base station 11. In this case, the branching ratio of the base station 11 is the ratio of the number of base stations 11 constituting the string of base stations connected to the downstream side. Furthermore, regarding the optical splitter of the base station in the downstream direction after branching, the ratio of the magnitude of the optical signal input to the jth base station (j is an integer satisfying 1≦j≦M) to the magnitude of the optical signal output is (M−j+1):(Mj), where M is the number of base stations in the downstream direction after branching and the first base station, second base station, ... are in order from the side closest to the branching point.

[0039] There are no particular limitations on the method of multiplexing the optical signals by the multiplexing units 102 and 111. For example, the multiplexing units 102 and 111 may multiplex the optical signals by frequency division multiplexing or time division multiplexing.

[0040] The multiplexing unit 111 may be an optical circulator. The circulator outputs the optical signal input from the optical splitter 110 to the O / E conversion unit 112, and outputs the optical signal input from the E / O conversion unit 117 to the optical splitter 110.

[0041] The base station 11 may include a control unit 118. When the multiplexing unit 111 multiplexes optical signals by time division multiplexing, the circulator 114 may be a TDD switch, and the control unit 118 may control the TDD switch to switch between the connection between the transmitting amplifier 113 and the circulator 114 and the connection between the circulator 114 and the receiving amplifier 116. When the multiplexing unit 111 multiplexes optical signals by time division multiplexing, the wavelength λ of the optical signal transmitted by the E / O conversion unit 101 may be 1 / 2. DL and the wavelength λ of the optical signal received by the O / E conversion unit 103. UL may be the same wavelength.

[0042] The antenna 115 is an array antenna with a controllable beam direction, and the control unit 118 may control the antenna 115 to change the beam direction in which the antenna 115 transmits and receives.

[0043] The control unit 118 may be controlled based on a control signal received from the central station 10. The control signal may be subcarrier-multiplexed, polarization-multiplexed, or wavelength-multiplexed onto a downstream optical signal transmitted by the central station 10, and then transmitted to the base station 11. The wavelength of the control signal is λ DL and λ UL In this case, a circulator or the like may be provided between the multiplexing unit 111 and the E / O conversion unit 117 to prevent a control signal from being input from the multiplexing unit 111 to the E / O conversion unit 117.

[0044] The control unit 118 may control whether or not to combine optical signals at the base station 11. In other words, the control unit 118 may control the optical splitter 110 so that the optical splitter 110 outputs an optical signal input from a downstream device to an upstream device without combining the optical signal with the optical signal input from the multiplexer 111.

[0045] The base station 11 may transmit to the aggregate station 10 a signal including information regarding the quality of the signal to be output to the aggregate station 10. The base station 11 may subcarrier-multiplex, polarization-multiplex, or wavelength-multiplex the signal including the information onto an upstream optical signal and transmit the signal to the aggregate station 10. The aggregate station 10 may transmit a control signal based on the signal including the information. For example, the aggregate station 10 may select a base station 11 that outputs a signal with good quality based on the signal including the information, and transmit a control signal to receive signals only from that base station 11.

[0046] The optical communication system 1 employs, but is not limited to, an analog RoF. The optical communication system 1 may employ a digital RoF (for example, CPRI or eCPRI) or digital coherent transmission, in which the central station 10 includes an A / D converter, each base station 11 includes a D / A converter, and the central station 10 transmits a digital signal.

[0047] The present invention can be applied even when the central station 10 and the base station are connected by a mobile fronthaul in the optical communication system 1. [Explanation of symbols]

[0048] 10 aggregation station, 11 base station, 100 transmitting unit, 101 E / O conversion unit, 102 multiplexing unit, 103 O / E conversion unit, 104 receiving unit, 110 optical splitter, 111 multiplexing unit, 112 O / E conversion unit, 113 transmitting amplifier, 114 circulator, 115 antenna, 116 receiving amplifier, 117 E / O conversion unit

Claims

1. An optical communication system including a central station and a plurality of base stations, the plurality of base stations being connected in series with the central station at the head, the central station outputs downstream optical signals and control signals generated by E / O converting wireless signals to the plurality of base stations, and O / E converts upstream optical signals input from the plurality of base stations to obtain wireless signals; each of the base stations transmits a radio signal obtained by O / E converting a downstream optical signal received from the central station, and transmits to the central station an upstream optical signal generated by E / O converting the received radio signal and a signal including information on the signal quality of the upstream optical signal; the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the control signal is based on information about the quality of upstream optical signals input from the plurality of base stations, Optical communication system.

2. A central station in an optical communication system in which a plurality of base stations are connected in series with a central station at the head, outputting downstream optical signals and control signals generated by E / O converting wireless signals to the plurality of base stations, and obtaining wireless signals by O / E converting upstream optical signals input from the plurality of base stations; the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the control signal is based on information about the quality of upstream optical signals input from the plurality of base stations, Aggregation station.

3. In an optical communication system in which a central station is at the head and multiple base stations are connected in series, a downstream optical signal generating step of E / O converting a radio signal by the central station to generate a downstream optical signal and a control signal; an output step of outputting the downstream optical signal to the plurality of base stations; a radio signal acquisition step of O / E converting upstream optical signals input from the plurality of base stations to acquire radio signals; a radio signal transmitting step of transmitting, by the base station, a radio signal obtained by O / E converting the downstream optical signal received from the central station; an optical signal transmitting step of transmitting to the central station an upstream optical signal generated by E / O converting the received radio signal and a signal including information on the quality of the upstream optical signal; Including, the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the control signal is based on information about the quality of upstream optical signals input from the plurality of base stations, Communication method.

4. An optical communication system comprising a central station and a plurality of base stations, the plurality of base stations being connected in series with the central station at the head, the central station outputs downstream optical signals generated by E / O converting wireless signals to the plurality of base stations, and acquires wireless signals by O / E converting upstream optical signals input from the plurality of base stations; each of the base stations transmits a radio signal obtained by O / E converting a downstream optical signal received from the central station, and transmits an upstream optical signal generated by E / O converting the received radio signal to the central station; the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the base station includes an optical splitter; the optical splitter switches between merging an upstream optical signal generated by E / O converting the received radio signal and an upstream optical signal received from a downstream base station, and Optical communication system.

5. An aggregation station in an optical communication system in which a plurality of base stations are connected in series with an aggregation station at the head, comprising: outputting downstream optical signals generated by E / O converting wireless signals to the plurality of base stations, and obtaining wireless signals by O / E converting upstream optical signals input from the plurality of base stations; the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the base station includes an optical splitter; the optical splitter switches between merging an upstream optical signal generated by E / O converting the received radio signal and an upstream optical signal received from a downstream base station; Aggregation station.

6. In an optical communication system in which a plurality of base stations are connected in series with a central station at the head, a downstream optical signal generating step of E / O converting a radio signal by the central station to generate a downstream optical signal; an output step of outputting the downstream optical signal to the plurality of base stations; a radio signal acquisition step of O / E converting upstream optical signals input from the plurality of base stations to acquire radio signals; a radio signal transmitting step of transmitting, by the base station, a radio signal obtained by O / E converting the downstream optical signal received from the central station; an optical signal transmitting step of E / O converting the received radio signal to generate an upstream optical signal and transmitting the upstream optical signal to the aggregation station; Including, the wavelengths of the upstream optical signals assigned to the plurality of base stations are the same, the wavelengths of the downstream optical signals assigned to the plurality of base stations are the same, the base station includes an optical splitter; the optical splitter switches between merging an upstream optical signal generated by E / O converting the received radio signal and an upstream optical signal received from a downstream base station, and Communication method.

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