Communication device, MIMO system, and communication method

Abstract

The present invention provides a communication device that is a base station included in a MIMO system using hybrid BF and RoF, the MIMO system including an optical amplifier having a constant output, the optical amplifier being provided between a multiplexer that multiplexes RoF signals and a demultiplexer that demultiplexes an output of the multiplexer by wavelengths. The communication device includes: the demultiplexer; a plurality of photoelectric converters that convert incident RoF signals into radio signals; optical branches that branch the incident RoF signals; an optical switch that switches one or both of an output destination of signals demultiplexed by the demultiplexer and an output source of signals to be input to the photoelectric converters; and a control unit that controls an operation of the optical switch on the basis of an optical level of each of RoF signals obtained by the demultiplexer by wavelengths.

Description

Communication equipment, MIMO system, and communication method

[0001] The present invention relates to a communication device, a MIMO system, and a communication method.

[0002] In MIMO (Multi Input Multi Output) systems, there is a technique called hybrid BF (beamforming). Hybrid BF is a technique for using MIMO with low power and low cost, and it can reduce the number of RF chains required compared to digital BF (see Non-Patent Literature 1). Some such systems use optical fibers such as analog RoF (Radio over Fiber) or digital RoF to deploy antennas over a wide area (see Non-Patent Literature 2).

[0003] S. Suyama, T. Okuyama, Y. Inoue, Y. Kishiyama, “5G Multi-antenna Technology,” NTT DOCOMO Technical Journal, Vol.17, No.4, pp.29-39, April 2016.Kota Ito, Mizuki Suga, Yushi Shirato, Naoki Kita, and Takeshi Onizawa, “Efficiently Accommodating High-frequency-band Wireless Systems by Using Analog “Radio-over-fiber”, NTT Technical Review, Vol. 18, No. 5, pp. 19-23, May 2020.

[0004] By the way, there is a time period with low traffic, such as the nighttime period, which is different from daytime. If the traffic is low, the required number of MIMO layers is correspondingly small. Therefore, in a time period with low traffic, from the perspective of power saving, it is expected to reduce the number of operating MIMO layers. However, in a MIMO system, even if the number of MIMO layers is reduced, the power supply to the antenna elements cannot be stopped. As a result, power is supplied to the unused antenna elements, and there is a waste that power is consumed by those antenna elements even though communication using those antenna elements is not performed.

[0005] In view of the above circumstances, an object of the present invention is to provide a technique for reducing the number of antenna elements that consume power even though communication through them is not performed in a MIMO system.

[0006] One aspect of the present invention is a MIMO system using hybrid BF and RoF, and is a communication device comprising a base station of a MIMO system which includes a multiplexer that multiplexes RoF signals, a demultiplexer that separates the output of the multiplexer for each wavelength, and a constant output optical amplifier between them, wherein the demultiplexer comprises a plurality of photoelectric converters that convert the incident RoF signal into a wireless signal, an optical splitter that branches the incident RoF signal, an optical switch that switches either or both of the output destination of the signal after demultiplexing by the demultiplexer and the output source of the signal input to the photoelectric converter, and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the control unit is the first wavelength RoF signal obtained by demultiplexing by the demultiplexer. The communication device controls the operation of the optical switch to direct the output destination of the first signal to the first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to the second photoelectric converter, which is the other of the photoelectric converters, when the optical level of the first signal is above a predetermined optical level and the optical level of the second signal is above a predetermined optical level, when the optical level of the first signal is above a predetermined optical level and the optical level of the second signal is below a predetermined optical level, when the operation of the optical switch controls the output destination of the first signal to the optical splitter, and the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the decoupler.

[0007] One aspect of the present invention is a MIMO system using hybrid BF and RoF, comprising a multiplexer that multiplexes RoF signals, a demultiplexer that separates the output of the multiplexer for each wavelength, and a constant output optical amplifier between them, wherein the base station in the MIMO system comprises the demultiplexer, a plurality of photoelectric converters that convert the incident RoF signal into a radio signal, an optical splitter that branches the incident RoF signal, an optical switch that switches either or both of the output destination of the signal after demultiplexing by the demultiplexer and the output source of the signal input to the photoelectric converter, and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the control unit is the first wavelength RoF signal obtained by demultiplexing by the demultiplexer. This MIMO system controls the operation of the optical switch to direct the output destination of the first signal to the first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to the second photoelectric converter, which is the other of the photoelectric converters, when the optical level of the first signal is above a predetermined optical level and the optical level of the second signal, which is an RoF signal of a second wavelength different from the first wavelength obtained by demultiplexing by the demultiplexer, is above a predetermined optical level. If the optical level of the first signal is above a predetermined optical level and the optical level of the second signal is below a predetermined optical level, the operation of the optical switch controls the output destination of the first signal to the optical splitter, and the output source of the first and second photoelectric converters is controlled to the optical splitter instead of the demultiplexer.

[0008] One aspect of the present invention is a MIMO system using hybrid BF and RoF, and the MIMO system is a base station equipped with an optical amplifier with constant output between a multiplexer that multiplexes RoF signals and a demultiplexer that separates the output of the multiplexer for each wavelength, and the communication device comprises: a demultiplexer; a plurality of photoelectric converters that convert the incident RoF signal into a wireless signal; an optical splitter that branches the incident RoF signal; an optical switch that switches either or both of the output destination of the signal after demultiplexing by the demultiplexer and the output source of the signal input to the photoelectric converter; and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the optical level of the first signal, which is a first wavelength RoF signal obtained by demultiplexing by the demultiplexer, is predetermined The communication method comprises: a first control step in which, if the optical level of the first signal is above a predetermined optical level and the optical level of the second signal, which is an RoF signal of a second wavelength different from the first wavelength obtained by demultiplexing by the demultiplexer, is above a predetermined optical level, the control unit controls the operation of the optical switch to control the output destination of the first signal to the first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to the second photoelectric converter, which is the other of the photoelectric converters; and a second control step in which, if the optical level of the first signal is above a predetermined optical level and the optical level of the second signal is below a predetermined optical level, the control unit controls the operation of the optical switch to control the output destination of the first signal to the optical splitter, and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the demultiplexer.

[0009] One aspect of the present invention is a MIMO system using hybrid BF and RoF, comprising a multiplexer that multiplexes RoF signals, a demultiplexer that separates the output of the multiplexer for each wavelength, and a constant output optical amplifier between them, wherein the base station in the MIMO system comprises the demultiplexer, a plurality of photoelectric converters that convert the incident RoF signals into wireless signals, an optical splitter that branches the incident RoF signals, an optical switch that switches either the output destination of the signals after demultiplexing by the demultiplexer or the output source of the signals input to the photoelectric converter, or both, and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, and a communication method executed by the MIMO system, wherein the optical level of the first signal is the RoF signal of the first wavelength obtained by demultiplexing by the demultiplexer A communication method comprising: a first control step in which, if the light level of the first signal is above a predetermined light level, and the light level of the second signal, which is an RoF signal of a second wavelength different from the first wavelength obtained by demultiplexing by the demultiplexer, is above a predetermined light level, the control unit controls the operation of the optical switch to control the output destination of the first signal to a first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to a second photoelectric converter, which is the other of the photoelectric converters; and a second control step in which, if the light level of the first signal is above a predetermined light level, and the light level of the second signal is below a predetermined light level, the control unit controls the operation of the optical switch to control the output destination of the first signal to the optical splitter, and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the demultiplexer.

[0010] The present invention makes it possible to reduce the number of antenna elements in a MIMO system that consume power even though no communication takes place through them.

[0011] A first explanatory diagram illustrating the MIMO system of the embodiment. A second explanatory diagram illustrating the MIMO system of the embodiment. A third explanatory diagram illustrating the MIMO system of the embodiment. A fourth explanatory diagram illustrating the MIMO system of the embodiment. A diagram showing an example of the hardware configuration of the aggregation station in the embodiment. A diagram showing an example of the configuration of the aggregation station control unit in the embodiment. A flowchart showing an example of the processing flow executed by the aggregation station control unit in the embodiment. A diagram showing an example of the hardware configuration of the base station in the embodiment. A diagram showing an example of the configuration of the base station control unit in the embodiment. A flowchart showing an example of the processing flow executed by the base station control unit in the embodiment. A flowchart showing another example of the processing flow executed by the base station control unit in the embodiment. A first explanatory diagram illustrating the MIMO system in a modified example. A second explanatory diagram illustrating the MIMO system in a modified example.

[0012] (Embodiment) Figure 1 is a first explanatory diagram illustrating the MIMO system 100 of the embodiment. Figure 2 is a second explanatory diagram illustrating the MIMO system 100 of the embodiment. More specifically, the base station 2 shown in Figure 2 is a specific example of the base station 2 shown in Figure 1. The MIMO system 100 in Figures 1 and 2 is a MIMO (Multi Input Multi Output) system that uses BF (beamforming) and RoF (Radio over Fiber). More specifically, the RoF in the examples in Figures 1 and 2 is an analog RoF. That is, the RoF signal in the examples in Figures 1 and 2 is an analog signal.

[0013] The MIMO system 100 comprises a central office 1, a base station 2, and an optical fiber 3 connecting the central office 1 and the base station 2, which is equipped with a constant-output optical amplifier 301. The optical fiber 3 may be a single-core fiber or a multi-core fiber.

[0014] The optical amplifier 301 can be of any type as long as it is an optical amplifier with a constant output. For example, it may be an optical amplifier that maintains a constant output regardless of the input light intensity by adjusting the amplification factor according to the input light intensity.

[0015] In the example in Figure 1, the optical amplifier 301 is located between the aggregation station 1 and the base station 2. However, the optical amplifier 301 does not necessarily need to be located between the aggregation station 1 and the base station 2. The optical amplifier 301 can be located anywhere between the multiplexer (multiplexer 104 in the example in Figure 1) of the aggregation station 1 and the demultiplexer (demultiplexer 201 in the example in Figure 2) of the base station 2. Therefore, the optical amplifier 301 may be installed in, for example, the aggregation station 1 or in the base station 2.

[0016] <Aggregation Station 1> Aggregation Station 1 is an aggregation station in the MIMO system. Aggregation Station 1 comprises an aggregation station control unit 11, a plurality of light sources 101 (light sources 101-1 to 101-4), an RF signal generator 102, a plurality of optical modulators 103 (optical modulators 103-1 to 103-4), a multiplexer 104, a CTL light source 105, and a CTL optical modulator 106.

[0017] The aggregation station control unit 11 performs light source control processing, first-class optical modulation processing, and CTL optical modulator control processing. The light source control processing is the process of controlling the operation of the light source 101. For example, the light source control processing is the process of acquiring data stream count information and controlling the ON / OFF of the light source 101 based on the acquired data stream count information. In the light source control processing, for example, if the average value of the number of data streams falls below a threshold, the operation of a predetermined light source 101 is changed from ON to OFF. In the light source control processing, for example, the ON / OFF of the light source 101 corresponding to the antenna element 207 may be controlled in response to a beam weight change request from the UE (User Equipment).

[0018] The first type of optical modulation processing is a process that controls the RF signal generator 102 and, via the control of the RF signal generator 102, controls the operation of the optical modulator 103. The RF signal generator 102 is an RF (Radio Frequency) signal generator.

[0019] The CTL optical modulator control process is a process that controls the operation of the CTL optical modulator 106 by transmitting control signals such as beamforming signals to the CTL optical modulator 106.

[0020] Light source 101 is a light source that outputs an optical signal. The wavelengths of the optical signals output by light sources 101-1 to 101-4 are different from each other. Specifically, light source 101-1 outputs an optical signal with wavelength λ1, light source 101-2 outputs an optical signal with wavelength λ2, light source 101-3 outputs an optical signal with wavelength λ3, and light source 101-4 outputs an optical signal with wavelength λ4. That is, light source 101-n outputs an optical signal with wavelength λn (where n is an integer from 1 to 4). The light source is, for example, a semiconductor laser.

[0021] The optical modulator 103 is an optical modulator that modulates the incident optical signal. More specifically, the optical modulator 103 is an optical modulator that modulates the intensity of the optical signal output from the light source 101 with a wireless signal. The intensity modulation by the optical modulator 103 generates an RoF signal. Therefore, the optical modulator 103 outputs an RoF signal. More specifically, the optical modulator 103 outputs the RoF signal to the multiplexer 104. In the example in Figure 1, the optical signal output from the light source 101-n is incident on the optical modulator 103-n (where n is an integer from 1 to N).

[0022] In the examples in Figures 1 and 2, N is 4, but N is not necessarily limited to 4; it can be any integer greater than or equal to 2.

[0023] The multiplexer 104 is a multiplexer that multiplexes the incoming RoF signal. The multiplexer 104 may be, for example, a WDM (Wavelength Division Multiplexing) that multiplexes the RoF signal by wavelength, or an SCM (Sub-Carrier Multiplexing) that multiplexes the RoF signal by wavelength. A single-core fiber or a multi-core fiber may be used.

[0024] The CTL light source 105 is a light source that outputs control signals such as beamforming signals, rather than analog RF signals as optical signals. The CTL light source outputs an optical signal with a wavelength of λ5. Wavelength λ5 is different from any of the wavelengths λ1 to λ4. The optical signal output by the CTL light source 105 is incident on the CTL optical modulator 106.

[0025] The CTL optical modulator 106 is an optical modulator that modulates the intensity of the optical signal output from the CTL light source 105 with a control signal such as beamforming. The CTL optical modulator 106 generates an RoF signal from the optical signal output from the CTL light source 105 and outputs the generated RoF signal to the multiplexer 104.

[0026] In Figure 1, “M1,” “M2,” “M3,” and “M4” each represent different data streams. For example, “M1” is a data stream that transmits the text data “Good morning,” and “M2” is a data stream that transmits the text data “Hello.” In the examples in Figures 1 and 2, four light sources 101 are operating, and four types of data streams flow from the aggregation station 1 to the base station 2. When R light sources 101 (where R is an integer less than or equal to N) are operating, R types of data streams flow from the aggregation station 1 to the base station 2.

[0027] <Base Station 2> Base Station 2 comprises a demultiplexer 201, a photodetector 202, a plurality of optical switches 203 (optical switches 203-1 to 203-6), a plurality of optical splitters 204 (optical splitters 204-1 to 204-2), a plurality of photoelectric converters 205 (photoelectric converters 205-1 to 205-4), a plurality of phase shifters 206 (phase shifters 206-1 to 206-8), a plurality of antenna elements 207 (antenna elements 207-1 to 207-8), a CTL photoelectric converter 208, and a base station control unit 21.

[0028] The demultiplexer 201 separates the incoming RoF signal into wavelengths. The signal output from the multiplexer 104 is incident on the demultiplexer 201 via the optical fiber 3. Therefore, it can be said that the demultiplexer 201 separates the output of the multiplexer 104 into wavelengths. Each signal output by the demultiplexer 201 is an RoF signal. The demultiplexer 201 may be, for example, a WDM (Wavelength Division Multiplexing) or an SCM (Sub-Carrier Multiplexing). A single-core fiber or a multi-core fiber may be used.

[0029] The photodetector 202 detects the output of the demultiplexer 201 (for example, the output light from the WDM filter) and acquires the light level for each wavelength. The photodetector 202 is, for example, a photodiode. The information indicating the light level obtained by the photodetector 202 (hereinafter referred to as "light level information") is output to the base station control unit 21.

[0030] The optical switch 203 is an optical switch. The optical switch 203 is, for example, a MEMS (Micro Electro Mechanical System). The optical switch 203 switches either the output destination of the decoupled signal separated by the decoupler 201, or the output source of the signal input to the photoelectric converter 205, or both. In other words, the optical switch 203 is an optical switch that switches either the output destination of the decoupled signal separated by the decoupler 201, or the output source of the signal input to the photoelectric converter 205, or both.

[0031] In the example shown in Figure 1, the demultiplexer 201 and the photoelectric converters 205-1 and 205-2 are connected using optical switches 203-1, 203-2, and 203-3, without passing through the optical splitter 204-1. As a result, the RoF signal with wavelength λ1 propagates to the photoelectric converter 205-1 without passing through the optical splitter 204-1, and the RoF signal with wavelength λ2 propagates to the photoelectric converter 205-2 without passing through the optical splitter 204-1.

[0032] In the example shown in Figure 1, the demultiplexer 201 and the photoelectric converters 203-1 and 205-4 are connected using optical switches 203-4, 203-5, and 203-6, without going through the optical splitter 204-2. As a result, the RoF signal with wavelength λ3 propagates to the photoelectric converter 205-3 without going through the optical splitter 204-2, and the RoF signal with wavelength λ4 propagates to the photoelectric converter 205-4 without going through the optical splitter 204-2.

[0033] As will be described later, switching the optical switch 203 changes the connection relationship between the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205. In the example in Figure 2, the switching of the connection relationship between the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205 is achieved by multiple optical switches 203, but this is solely for the purpose of simplifying the explanation of the signal propagation path. Therefore, it is not necessary to switch the connection relationship between the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205 by multiple optical switches 203, and it may be done by a single optical switch.

[0034] The optical splitter 204 splits the incoming RoF signal. The optical splitter 204 is, for example, an optical coupler. Each optical splitter 204 can be connected to multiple photoelectric converters 205 by switching the connection relationship using optical switches 203-1 to 203-3. In the example in Figure 2, there are two optical splitters 204 because in the example in Figure 1, there are four light sources 101, that is, N=4. As mentioned above, N=4 is just one example. Therefore, for example, N / 2 optical splitters 204 may be provided. Of course, the number of optical splitters 204 may be between 1 and N / 2.

[0035] The photoelectric converter 205 converts the incident RoF signal into a wireless signal. The signal output by each photoelectric converter 205 is incident on one or more phase shifters 206, each different from the others. In the example in Figure 2, an RoF signal with wavelength λn is incident on photoelectric converter 205-n. Also in the example in Figure 2, the signal output by photoelectric converter 205-n is incident on phase shifter 206-m and phase shifter 206-(m+1) (m=2n-1).

[0036] The phase shifter 206 is a phase shifter that changes the phase of the signal output by the connected photoelectric converter 205 by a predetermined amount. In the example in Figure 2, there are four photoelectric converters 205 because N = 4. The number of photoelectric converters 205 should be the same as the number of light sources 101, so N units are sufficient. When BF control is performed, the phase shifter may be controlled by the base station control unit 21 based on a control signal transmitted from the aggregation station control unit 11 to the base station 2 via Ethernet or other external line. Therefore, the signal to control the phase shifter when BF control is performed may be sent via a separate line, or via a dedicated control line through the CTL optical modulator 106.

[0037] Antenna element 207 is an antenna element. Antenna elements 207-k (where k is an integer from 1 to 8) are connected to phase shifters 206-k. Therefore, the number of antenna elements 207 is equal to the number of phase shifters 206. Of the signals output by the photoelectric converter 205, the signals that pass through phase shifters 206-k are propagated to antenna elements 207-k. The set of antenna elements 207-1 to 207-8 may be used as a Massive MIMO antenna or as a Distributed Antenna System (DAS).

[0038] Furthermore, each antenna element 207 may be provided with a power amplifier. In this case, the power amplifier may be provided between the phase shifter 206-k and the antenna element 207-k. That is, the signal that has been amplified after passing through the phase shifter 206-k may propagate to the antenna element 207-k.

[0039] The CTL photoelectric converter 208 is a photoelectric converter that converts the optical signal output from the CTL optical modulator 106 into an electrical signal. The RoF signal with wavelength λ5 obtained by the demultiplexer 201 is incident on the CTL photoelectric converter 208.

[0040] Here, for the sake of simplicity of the following description, some sets of functional units are named. Hereinafter, the set of optical switches 203-1 to 203-6 and optical splitters 204-1 to 204-2 is referred to as a switch unit 291. From this, it can be said that the optical switches 203-1 to 203-6 and the optical splitters 204-1 to 204-2 constitute the switch unit 291.

[0041] Hereinafter, the set of phase shifters 206-1 to 206-8 and antenna elements 207-1 to 207-8 is referred to as an antenna unit 292. From this, it can be said that the phase shifters 206-1 to 206-8 and the antenna elements 207-1 to 207-8 constitute the antenna unit 292.

[0042] The base station control unit 21 executes an optical switch control process. The optical switch control process is a process of controlling the operation of the optical switch 203 based on the optical level information. A further explanation of the optical switch control process will be given using FIGS. 3 and 4 as well. FIGS. 3 and 4 show a MIMO system 100 in which the communication situation such as the traffic volume is different from the scenes in FIGS. 1 and 2. Since the scenes are different, in the MIMO system 100 of FIGS. 3 and 4, some of the components that were operating in the examples of FIGS. 1 and 2 are not operating. Also, compared with the MIMO system 100 of FIGS. 1 and 2 and the MIMO system 100 of FIGS. 3 and 4, a change has occurred in the connection relationship by the optical switch 203.

[0043] FIG. 3 is a third explanatory diagram for explaining the MIMO system 100 of the embodiment. FIG. 4 is a fourth explanatory diagram for explaining the MIMO system 100 of the embodiment.

[0044] The aggregation station 1 in FIG. 3 is different from the aggregation station 1 in FIG. 1 in that the light source 101-2 and the optical modulator 103-2, and the light source 101-4 and the optical modulator 103-4 are not operating. Since the light source 101-2 and the optical modulator 103-2, and the light source 101-4 and the optical modulator 103-4 are not operating, signals of wavelength λ2 and wavelength λ4 do not enter the multiplexer 104.

[0045] As described above, when the R light sources 101 are operating, R types of data streams flow from the aggregation station 1 to the base station 2. Therefore, in the examples of FIGS. 3 and 4, two types of data streams, "M1" and "M2", flow from the aggregation station 1 to the base station 2.

[0046] In the base station 2 of FIG. 4, the optical switch 203-1 connects the demultiplexer 201 and the optical splitter 204-1, and the optical switch 203-2 connects the optical splitter 204-1 and the optoelectronic converter device 205-1. Also, the optical switch 203-3 connects the optical splitter 204-1 and the optoelectronic converter device 205-2. Therefore, unlike the base station 2 in FIG. 2, the signal of wavelength λ1 output from the demultiplexer 201 is incident on the optoelectronic converter device 205-1 and the optoelectronic converter device 205-2 via the optical splitter 204-1.

[0047] Also in the base station 2 of FIG. 4, the optical switch 203-4 connects the demultiplexer 201 and the optical splitter 204-2, and the optical switch 203-5 connects the optical splitter 204-2 and the optoelectronic converter device 205-3. Also, the optical switch 203-6 connects the optical splitter 204-2 and the optoelectronic converter device 205-4. Therefore, unlike the base station 2 in FIG. 2, the signal of wavelength λ3 output from the demultiplexer 201 is incident on the optoelectronic converter device 205-3 and the optoelectronic converter device 205-4 via the optical splitter 204-2.

[0048] In the aggregation station 1 of FIG. 3, since the light source 101-2 is not operating, the signal of wavelength λ2 does not enter the demultiplexer 201. Therefore, the signal of wavelength λ2 also does not exist in the signal output from the demultiplexer 201. Therefore, even if the signal of wavelength λ1 is incident on the optoelectronic converter device 205-1 and the optoelectronic converter device 205-2, no signal collision occurs. Note that signal collision means that signals of a plurality of different wavelengths are simultaneously incident on one optoelectronic converter device.

[0049] In the aggregation station 1 of FIG. 3, since the light source 101-4 is not operating, the signal of wavelength λ4 does not enter the demultiplexer 201. Therefore, the signal of wavelength λ4 also does not exist in the signal output from the demultiplexer 201. Therefore, even if the signal of wavelength λ3 is incident on the optoelectronic converter device 205-3 and the optoelectronic converter device 205-4, no signal collision occurs.

[0050] Thus, the base station 2 in Figure 4 and the base station 2 in Figure 2 differ in the wavelength of the signal incident on the demultiplexer 201, and accordingly, the connection relationships of the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205 are also different.

[0051] These changes in connection relationships are caused by the control of the optical switch 203. The control of the optical switch 203 is performed by the base station control unit 21 through optical switch control processing. Here, a specific example of optical switch control processing will be explained again.

[0052] <Optical Switch Control Processing> In optical switch control processing, for example, the following actions are performed when the first optical level condition is met and when the second optical level condition is met.

[0053] The first optical level condition is that the optical level of the first signal obtained by demultiplexing with the demultiplexer 201 is equal to or greater than a predetermined optical level, and the optical level of the second signal obtained by demultiplexing with the demultiplexer 201 is equal to or greater than a predetermined optical level. On the other hand, the second optical level condition is that the optical level of the first signal is equal to or greater than a predetermined optical level, and the optical level of the second signal is less than a predetermined optical level.

[0054] The first signal is the RoF signal of the first wavelength obtained by demultiplexing with the demultiplexer 201. Wavelengths λ1 and λ3 are examples of the first wavelength. Therefore, the RoF signal of wavelength λ1 output by the demultiplexer 201 and the RoF signal of wavelength λ3 output by the demultiplexer 201 are examples of the RoF signal of the first wavelength obtained by demultiplexing with the demultiplexer 201.

[0055] The second signal is an RoF signal with a second wavelength different from the first wavelength obtained by demultiplexing with the demultiplexer 201. Wavelengths λ2 and λ4 are examples of the second wavelength. Therefore, the RoF signal with wavelength λ2 output by the demultiplexer 201 and the RoF signal with wavelength λ4 output by the demultiplexer 201 are examples of RoF signals with a second wavelength different from the first wavelength obtained by demultiplexing with the demultiplexer 201.

[0056] When the first optical level condition is met, the operation of the optical switch 203 is controlled to direct the output of the first signal to the first photoelectric converter, which is one of the photoelectric converters 205, and to direct the output of the second signal to the second photoelectric converter, which is the other of the photoelectric converters 205. Photoelectric converter 205-1 is an example of the first photoelectric converter, and in this case, photoelectric converter 205-2 is an example of the second photoelectric converter. Also, photoelectric converter 205-3 is an example of the first photoelectric converter, and in this case, photoelectric converter 205-4 is an example of the second photoelectric converter.

[0057] When the second optical level condition is met, the operation of the optical switch 203 is controlled to direct the output destination of the first signal to the optical splitter 204, and the output source of the first and second photoelectric converters is controlled to the optical splitter 204 instead of the demultiplexer 201. This is performed in the optical switch control process.

[0058] The changes in the connection relationships of the demultiplexer 201, optical splitter 204, and photoelectric converter 205 shown in Figures 1 to 4 are concrete examples of the results of such optical switch control processing. More specifically, the connection relationship shown in Figure 2 is an example of the connection relationship when the first optical level condition is met, and the connection relationship shown in Figure 4 is an example of the connection relationship when the second optical level condition is met.

[0059] <Effects of Optical Switch Control Processing> The effects of optical switch control processing will be explained. In a MIMO system, even if the light source 101-2 is not operating, the power supply to the phase shifter 206-3, phase shifter 206-4, antenna element 207-3, and antenna element 207-4 cannot be stopped. Therefore, if the optical splitter 204-1 and optical switches 203-1 to 203-3 are not present and optical switch control processing is not executed, when the operation of the light source 101-2 stops, the phase shifter 206-3, phase shifter 206-4, antenna element 207-3, and antenna element 207-4 will consume power even though they do not contribute to communication. In other words, waste occurs.

[0060] Meanwhile, the MIMO system 100 is equipped with an optical splitter 204-1 and optical switches 203-1 to 203-3, and optical switch control processing is performed. As a result, as shown in Figure 4, an optical signal with wavelength λ1 is incident on the phase shifter 206-3, phase shifter 206-4, antenna element 207-3, and antenna element 207-4. Therefore, even when the light source 101-2 is not operating, the phase shifter 206-3, phase shifter 206-4, antenna element 207-3, and antenna element 207-4 contribute to communication. As the number of antenna elements increases, the quality of communication improves, so in the situation shown in Figure 4, the quality of communication at wavelength λ1 is improved compared to the case in Figure 3.

[0061] The same applies to the light source λ4.

[0062] In this way, by executing optical switch control processing, the number of antenna elements that consume power in the MIMO system, even though no communication takes place through them, is reduced.

[0063] <Other Effects> Incidentally, when the number of data streams changes from four to two, the MIMO system 100 reduces power consumption by stopping the operation of light sources 101-2 and 101-4, as shown in the change from Figures 1 and 2 to Figures 3 and 4. However, because the operation of light sources 101-2 and 101-4 is stopped, there is an effect that phase difference offset is unnecessary.

[0064] The effectiveness will be explained by comparing it with a technology that does not stop the operation of light sources 101-2 and optical modulator 103-2, and light sources 101-4 and optical modulator 103-4, even if the data stream changes to only two types, "M1" and "M2". More specifically, in this comparative technology, "M1" is transmitted using light sources 101-1 and optical modulator 103-1 and light sources 101-2 and optical modulator 103-2, and "M2" is transmitted using light sources 101-3 and optical modulator 103-3 and light sources 101-4 and optical modulator 103-4.

[0065] In other words, "M1" is transmitted using the optical signals of wavelength λ1 and wavelength λ2, and "M2" is transmitted using the optical signals of wavelength λ3 and wavelength λ4. In this case as well, the phase shifter 206-3, phase shifter 206-4, antenna elements 207-3 and 207-4, and the phase shifter 206-7, phase shifter 206-8, antenna elements 207-7 and 207-8 contribute to communication.

[0066] However, in the case of this comparison technology, the same data is transmitted using multiple optical signals with different wavelengths, so a phase difference occurs between the multiple signals transmitting the same data due to chromatic dispersion. Therefore, a technique to offset the phase difference beforehand becomes necessary.

[0067] On the other hand, with the MIMO system 100, one data stream is transmitted at one wavelength, so no phase difference occurs. Therefore, the technique of offsetting the phase difference in advance is unnecessary. The MIMO system 100, which performs optical switch control processing, also achieves this effect.

[0068] <Example of Hardware Configuration of Aggregation Station 1> Figure 5 shows an example of the hardware configuration of aggregation station 1 in the embodiment. Aggregation station 1 includes an aggregation station control unit 11 which is a control unit that has a processor 91 such as a CPU (Central Processing Unit), GPU (Graphics Processing Unit), or NPU (Neural Network Processing Unit) and a memory 92 connected by a bus, and executes a program. By executing the program, aggregation station 1 functions as a device comprising the aggregation station control unit 11, an interface unit 12, a storage unit 13, a light source 101, an RF signal generator 102, an optical modulator 103, a multiplexer 104, a CTL light source 105, and a CTL optical modulator 106.

[0069] More specifically, the processor 91 reads the program stored in the storage unit 13 and stores the read program in the memory 92. By executing the program stored in the memory 92, the processor 91 functions as a device comprising an aggregation station control unit 11, an interface unit 12, a storage unit 13, a light source 101, an RF signal generator 102, an optical modulator 103, a multiplexer 104, a CTL light source 105, and a CTL optical modulator 106.

[0070] The aggregation station control unit 11 performs, for example, the light source control processing, the first type of optical modulation processing, and the CTL optical modulator control processing as described above. The aggregation station control unit 11 also performs, for example, the processing of acquiring information stored in the memory unit 13. Specifically, the processing of acquiring information stored in the memory unit 13 is reading.

[0071] The interface unit 12 is configured to include a communication interface for connecting the aggregation station 1 to an external device. The interface unit 12 communicates with the external device via wired or wireless connection. The external device is, for example, a device that transmits data stream count information. In such a case, the interface unit 12 obtains data stream count information by communicating with the device that transmits the data stream count information. The information obtained by the interface unit 12 is output to, for example, the aggregation station control unit 11.

[0072] The interface unit 12 may include input devices such as a mouse, keyboard, or touch panel. The interface unit 12 may also be configured as an interface connecting these input devices to the aggregation station 1. In this way, the input devices of the interface unit 12 receive various information or signals to the aggregation station 1 via wired or wireless means. Note that the information or signals do not necessarily have to be input to the communication interface of the interface unit 12, but may also be input to the input devices of the interface unit 12.

[0073] The interface unit 12 outputs various types of information, for example. The interface unit 12 includes, for example, a display device such as a CRT (Cathode Ray Tube) display, a liquid crystal display, or an organic EL (Electro-Luminescence) display, as well as a speaker. The interface unit 12 may be configured as an interface connecting these display devices or speakers to the aggregation station 1. Therefore, the interface unit 12 may output information indicated by information or signals input to the input device of the interface unit 12 as an image or sound.

[0074] The storage unit 13 is configured using a computer-readable storage medium (non-transitory computer-readable recording medium) such as a magnetic hard disk drive or a semiconductor storage device. The storage unit 13 stores various information related to the aggregation station 1. For example, the storage unit 13 stores various information generated by the operation of the aggregation station control unit 11.

[0075] Figure 6 shows an example of the configuration of the aggregation station control unit 11 in an embodiment. The aggregation station control unit 11 includes, for example, a light source control unit 111 and a wireless signal processing unit 112. The light source control unit 111 performs light source control processing. The wireless signal processing unit 112 is responsible for downlink signal processing functions. That is, the wireless signal processing unit 112 performs various signal processing that is performed on the downlink. For example, the wireless signal processing unit 112 performs first optical modulation processing and CTL optical modulator control processing.

[0076] The wireless signal processing unit 112 may, for example, acquire stream count information and output the acquired stream count information to the light source control unit 111. The light source control unit 111 may, for example, perform traffic status detection processing and light source on / off control processing in the light source control processing. The traffic status detection processing is a process that estimates the average number of streams per predetermined time period based on the stream count information. The estimated average number may be, for example, a moving average number of streams per predetermined time period. The moving average number of streams is the moving average number of streams.

[0077] The light source on / off control process controls the on / off state of the light source 101 according to a predetermined rule, based on the moving average number of streams obtained by the traffic status detection process.

[0078] Figure 7 is a flowchart showing an example of the processing flow performed by the central control unit 11 in the embodiment. More specifically, Figure 7 is a flowchart showing an example of the processing flow performed by the light source control unit 111. In other words, Figure 7 is a flowchart showing an example of the processing flow performed in the light source control process.

[0079] The aggregation station control unit 11 acquires stream count information (step S101). Next, the aggregation station control unit 11 estimates the average number of streams per predetermined time based on the acquired stream count information (step S102). Next, the aggregation station control unit 11 determines whether the average value obtained in step S102 is less than a predetermined threshold (step S103).

[0080] If the average value obtained in step S102 is less than a predetermined threshold (step S103: YES), the aggregation station control unit 11 controls the light source 101 to turn it off (step S104). That is, the aggregation station control unit 11 stops the operation of the light source 101. More specifically, it stops the operation of a light source 101 that satisfies the conditions of being a single-wavelength light source or a light source whose operation can be controlled for each wavelength. The process ends after step S104.

[0081] On the other hand, if the average value obtained in step S102 is greater than or equal to a predetermined threshold (step S103: NO), the aggregation station control unit 11 controls the light source 101 to turn on (step S105). That is, the aggregation station control unit 11 starts the operation of the light source 101. More specifically, it starts the operation of a light source 101 that satisfies the conditions of being a single-wavelength light source or a light source whose operation can be controlled for each wavelength.

[0082] After the processing in step S105, the process returns to step S101.

[0083] <Example of Hardware Configuration of Base Station 2> Figure 8 shows an example of the hardware configuration of base station 2 in the embodiment. Base station 2 includes a base station control unit 21 which is a control unit connected by a bus and includes a processor 93 such as a CPU (Central Processing Unit), GPU (Graphics Processing Unit), or NPU (Neural Network Processing Unit), and memory 94, and executes a program. Upon execution of the program, base station 2 functions as a device comprising the base station control unit 21, interface 22, storage unit 23, demultiplexer 201, photodetector 202, optical switch 203, optical splitter 204, photoelectric converter 205, phase shifter 206, antenna element 207, and CTL photoelectric converter 208.

[0084] More specifically, the processor 93 reads the program stored in the storage unit 23 and stores the read program in the memory 94. By executing the program stored in the memory 94, the processor 93 functions as a device comprising a base station control unit 21, an interface 22, a storage unit 23, a demultiplexer 201, a photodetector 202, an optical switch 203, an optical splitter 204, a photoelectric converter 205, a phase shifter 206, an antenna element 207, and a CTL photoelectric converter 208.

[0085] The base station control unit 21 performs, for example, the optical switch control processing described above. The base station control unit 21 also performs, for example, the process of acquiring information stored in the memory unit 23. Specifically, the process of acquiring information stored in the memory unit 23 is a read operation.

[0086] The interface unit 22 includes a communication interface for connecting the base station 2 to an external device. The interface unit 22 communicates with the external device via wired or wireless connection. The information acquired by the interface unit 22 is output to, for example, the base station control unit 21.

[0087] The interface unit 22 may include input devices such as a mouse, keyboard, or touch panel. The interface unit 22 may also be configured as an interface connecting these input devices to the base station 2. In this way, the input devices of the interface unit 22 receive various information or signals to the base station 2 via wired or wireless connections. Note that the information or signals do not necessarily have to be input to the communication interface of the interface unit 22, but may also be input to the input devices of the interface unit 22.

[0088] The interface unit 22 outputs various types of information, for example. The interface unit 22 is comprised of a display device such as a CRT (Cathode Ray Tube) display, a liquid crystal display, or an organic EL (Electro-Luminescence) display, as well as a speaker. The interface unit 22 may be configured as an interface for connecting these display devices or speakers to the base station 2. Therefore, the interface unit 22 may output information indicated by information or signals input to the input device of the interface unit 22 as an image or sound.

[0089] The storage unit 23 is configured using a computer-readable storage medium (non-transitory computer-readable recording medium) such as a magnetic hard disk drive or a semiconductor storage device. The storage unit 23 stores various information related to the base station 2. For example, the storage unit 23 stores various information generated by the operation of the base station control unit 21.

[0090] Figure 9 shows an example of the configuration of the base station control unit 21 in an embodiment. The base station control unit 21 includes, for example, an optical switch control unit 211. The optical switch control unit 211 performs optical switch control processing.

[0091] In the optical switch control process, for example, an optical level threshold determination process and an optical switch control command transmission process are executed. The optical level threshold determination process determines whether the optical level of the RoF signal for each wavelength is below a predetermined threshold based on the optical level information. The optical switch control command transmission process transmits a control command to each optical switch 203 to control each optical switch 203 based on the result of the determination in the optical level threshold determination process.

[0092] The content of the control command is determined by the base station control unit 21 based on the configured port information. The configured port information indicates the connection relationship of the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205 before the transmission of the control command. Each optical switch 203 operates according to the received control command. This switches the connection relationship of the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205. The configured port information is stored, for example, in the memory unit 23.

[0093] Figure 10 is a flowchart showing an example of the processing flow executed by the base station control unit 21 in the embodiment. The base station control unit 21 controls the connection relationship of the demultiplexer 201, the optical splitter 204, and the photoelectric converter 205 to a predetermined connection relationship as the initial state (step S101). The predetermined initial state is, for example, a connection relationship in which the output destination of the first signal is the first photoelectric converter and the output destination of the second signal is the second photoelectric converter, as shown in Figure 2.

[0094] Next, the base station control unit 21 acquires optical level information (step S202). Next, the base station control unit 21 determines whether the first optical level condition is met (step S203). If the first optical level condition is met (step S203: YES), the base station control unit 21 controls the output destination of the first signal to the first photoelectric converter and the output destination of the second signal to the second photoelectric converter (step S204). The execution of the process in step S204 is an example of the first control step. The process ends after step S204.

[0095] On the other hand, if the first optical level condition is not met (step S203: NO), the base station control unit 21 determines whether the second optical level condition is met (step S205). If the second optical level condition is met (step S205: YES), the base station control unit 21 controls the output destination of the first signal to the optical splitter 204 and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter 204 (step S206). The execution of the process in step S206 is an example of the second control step. The process ends after step S206.

[0096] On the other hand, if the second light level condition is not met (step S205: NO), the process returns to step S202.

[0097] Figure 11 is a flowchart illustrating another example of the processing flow performed by the base station control unit 21 in the embodiment. For simplicity, Figure 11 will be explained using the control of optical switches 203-1 to 203-3 as an example. Also for simplicity, the explanation will be given in the case where the optical level of the optical signal with wavelength λ1 is above a predetermined optical level.

[0098] The base station control unit 21 acquires the configured port information (step S301). Next, the base station control unit 21 determines, based on the configured port information, whether the demultiplexer 201 and the photoelectric converters 205-1 and 205-2 are connected via the optical splitter 204-1 (step S302). That is, the base station control unit 21 determines, based on the configured port information, whether the output destination of wavelength λ1 is the optical splitter 204-1 and whether the output source of the photoelectric converters 205-1 and 205-2 is the optical splitter 204-1.

[0099] If the demultiplexer 201 and the photoelectric converters 205-1 and 205-2 are connected via the optical splitter 204-1 (step S302: YES), the base station control unit 21 acquires optical level information (step S303). Next, the base station control unit 21 determines whether the optical level of the optical signal at wavelength λ2 is above a predetermined threshold (step S304). As described above, the optical level of the optical signal at wavelength λ1 is above a predetermined optical level, so the process in step S304 is an example of the process of determining whether the first optical level condition is met.

[0100] If the optical level of the optical signal with wavelength λ2 is above a predetermined threshold (step S304: YES), the base station control unit 21 controls optical switches 203-1 to 203-3 to change the connection relationship to one that does not go through optical splitter 204-1 (step S305).

[0101] The connection relationship that does not involve the optical splitter 204-1 is, specifically, a connection relationship in which the output destination of the optical signal with wavelength λ1 is the photoelectric converter 205-1, and the output destination of the optical signal with wavelength λ2 is the photoelectric converter 205-2. The execution of the process in step S305 is an example of the first control step. The process ends after step S305.

[0102] On the other hand, if the optical level of the optical signal at wavelength λ2 is not above a predetermined threshold (step S304: NO), the process returns to step S303.

[0103] If the demultiplexer 201 is connected to the photoelectric converters 205-1 and 205-2 without going through the optical splitter 204-1 (step S302: NO), the base station control unit 21 acquires optical level information (step S306). Next, the base station control unit 21 determines whether the optical level of the optical signal at wavelength λ2 is below a predetermined threshold (step S307). As described above, the optical level of the optical signal at wavelength λ1 is above a predetermined optical level, so the process in step S307 is an example of the process of determining whether the second optical level condition is met.

[0104] If the optical level of the optical signal with wavelength λ2 is below a predetermined threshold (step S307: YES), the base station control unit 21 controls optical switches 203-1 to 203-3 to change the connection relationship to one via optical splitter 204-1 (step S308). Specifically, the connection relationship via optical splitter 204-1 is one in which the output destination of wavelength λ1 is optical splitter 204-1, and the output source of photoelectric converters 205-1 and 205-2 is optical splitter 204-1. The execution of the process in step S308 is an example of the second control step. The process ends after step S308.

[0105] On the other hand, if the optical level of the optical signal at wavelength λ2 is not below a predetermined threshold (step S307: NO), the process returns to step S306.

[0106] As mentioned above, for simplicity, Figure 11 uses the control of optical switches 203-1 to 203-3 as an example. For the control of optical switches 203-4 to 203-6, simply replace λ1 with λ3 and λ2 with λ4 in the explanation in Figure 11. The same applies to other wavelengths.

[0107] The MIMO system 100 configured in this way performs optical switch control processing. As a result, as described in <Effects of Optical Switch Control Processing>, the number of antenna elements that consume power even though no communication is performed through them in the MIMO system can be reduced.

[0108] (Modified Version) Figure 12 is a first explanatory diagram illustrating the MIMO system 100a in a modified version. Figure 13 is a second explanatory diagram illustrating the MIMO system 100a in a modified version. The MIMO system 100a differs from the MIMO system 100 in that the RoF is digital RoF instead of analog RoF. That is, the MIMO system 100a differs from the MIMO system 100 in that the RoF signal is a digital signal instead of an analog signal.

[0109] Due to this difference, MIMO system 100a differs from MIMO system 100 in that it has an aggregation station 1a instead of aggregation station 1, and a base station 2a instead of base station 2.

[0110] The aggregation station 1a differs from the aggregation station 1 in that it has an aggregation station control unit 11a instead of the aggregation station control unit 11. The aggregation station control unit 11a differs from the aggregation station control unit 11 in that it performs a second type of optical modulation process instead of a first type of optical modulation process. The second type of optical modulation process is a process that controls the operation of the optical modulator 103 without going through the RF signal generator 102. Therefore, the aggregation station 1a does not necessarily need to have an RF signal generator 102.

[0111] Base station 2a differs from base station 2 in that it is equipped with multiple D / A converters 209, namely D / A converters 209-1 to 209-4, and multiple frequency converters 210, namely frequency converters 210-1 to 210-4.

[0112] The D / A converter 209-n converts the signal output by the photoelectric converter 205-n into an analog signal. The frequency converter 210-n performs a predetermined frequency conversion on the signal output by the D / A converter 209-n. The output of each frequency converter 210 is incident on one or more phase shifters 206, each different for each frequency converter 210. In the example in Figure 13, the signal output by the frequency converter 210-n is incident on phase shifter 206-m and phase shifter 206-(m+1).

[0113] Furthermore, the aggregation station 1 and aggregation station 1a may be implemented using multiple devices that are connected to each other via a network. Also, for example, each process performed by the aggregation station control unit 11 and the aggregation station control unit 11a may be performed by multiple information processing devices in a distributed manner.

[0114] Furthermore, base stations 2 and 2a may be implemented using multiple devices connected to each other via a network. Also, for example, each process performed by the base station control unit 21 may be performed by multiple information processing devices in a distributed manner.

[0115] Furthermore, all or part of the functions of aggregation station 1, aggregation station 1a, base station 2, and base station 2a may be implemented using hardware such as ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array). The program may be recorded on a computer-readable recording medium. Computer-readable recording media include, for example, portable media such as flexible disks, magneto-optical disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into computer systems. The program may also be transmitted via a telecommunications line.

[0116] Furthermore, the device referred to as a "centralization station" may also be called a "CS (Central Station)," "Base Station," "CU (Central Unit)," "DU (Distributed Unit)," "Radio Unit (RU)," or "master unit," depending on the technical field and context. Additionally, the device referred to as a "base station" may also be called a "wireless base station," "NodeB," "eNodeB," "gNodeB," "Access Point," "cell," "macrocell," "small cell," "femtocell," "picocell," "antenna," or a "slave unit" corresponding to a "master unit," depending on the technical field and context.

[0117] Base station 2 and base station 2a are examples of communication devices that are base stations in a MIMO system, each equipped with a multiplexer that multiplexes RoF signals and a demultiplexer that separates the output of the multiplexer into wavelengths, and a constant-output optical amplifier between them.

[0118] Although embodiments of this invention have been described in detail above with reference to the drawings, the specific configuration is not limited to these embodiments and includes designs and the like that do not depart from the spirit of this invention.

[0119] 100, 100a...MIMO system, 1, 1a...aggregation station, 2, 2a...base station, 3...optical fiber, 11, 11a...aggregation station control unit, 12...interface unit, 13...storage unit, 101...light source, 102...RF signal generator, 103...optical modulator, 104...multiplexer, 105...CTL light source, 106...CTL optical modulator, 111...light source control unit, 112...wireless signal processing unit, 301...optical amplifier, 21...base station control unit, 22...interface unit, 23...storage unit, 201...demultiplexer, 202...photodetector, 203...optical switch, 204...optical splitter, 205...photoelectric converter, 206...phase shifter, 207...antenna element, 208...CTL photoelectric converter, 209...D / A converter, 210...Frequency converter, 211...Optical switch control unit, 212...Attenuator, 291...Switch unit, 292...Antenna unit, 91...Processor, 92...Memory, 93...Processor, 94...Memory

Claims

1. A MIMO system using hybrid BF and RoF, comprising a base station equipped with a constant output optical amplifier between a multiplexer that multiplexes RoF signals and a demultiplexer that separates the output of the multiplexer for each wavelength, wherein the demultiplexer comprises: a plurality of photoelectric converters that convert the incident RoF signal into a wireless signal; an optical splitter that branches the incident RoF signal; an optical switch that switches either or both of the output destination of the signal after demultiplexing by the demultiplexer and the output source of the signal input to the photoelectric converter; and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the control unit A communication device that, when the optical level of the first signal, which is a first wavelength RoF signal obtained by demultiplexing with the demultiplexer, is above a predetermined optical level, and the optical level of the second signal, which is a second wavelength RoF signal different from the first wavelength obtained by demultiplexing with the demultiplexer, is above a predetermined optical level, controls the operation of the optical switch to control the output destination of the first signal to the first photoelectric converter, which is one of the photoelectric converters, and controls the output destination of the second signal to the second photoelectric converter, which is the other of the photoelectric converters; when the optical level of the first signal is above a predetermined optical level, and the optical level of the second signal is below a predetermined optical level, controls the operation of the optical switch to control the output destination of the first signal to the optical splitter, and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the demultiplexer.

2. The communication device according to claim 1, wherein the RoF signal is an analog signal.

3. The communication device according to claim 1, wherein the RoF signal is a digital signal.

4. A MIMO system using hybrid BF and RoF, wherein a MIMO system is provided with a constant output optical amplifier between a multiplexer that multiplexes RoF signals and a demultiplexer that separates the output of the multiplexer for each wavelength, and the base station in the MIMO system comprises: the demultiplexer; a plurality of photoelectric converters that convert the incident RoF signal into a radio signal; an optical splitter that branches the incident RoF signal; an optical switch that switches either or both of the output destination of the signal after demultiplexing by the demultiplexer and the output source of the signal input to the photoelectric converter; and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the control unit MIMO system: If the optical level of the first signal, which is a first wavelength RoF signal obtained by demultiplexing with the demultiplexer, is above a predetermined optical level, and the optical level of the second signal, which is a second wavelength RoF signal different from the first wavelength obtained by demultiplexing with the demultiplexer, is above a predetermined optical level, the operation of the optical switch is controlled to control the output destination of the first signal to the first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to the second photoelectric converter, which is the other of the photoelectric converters. If the optical level of the first signal is above a predetermined optical level, and the optical level of the second signal is below a predetermined optical level, the operation of the optical switch is controlled to control the output destination of the first signal to the optical splitter, and the output source of the first photoelectric converter and the second photoelectric converter is controlled to the optical splitter instead of the demultiplexer.

5. A communication device comprising a base station of a MIMO system using hybrid BF and RoF, the MIMO system comprising a multiplexer that multiplexes RoF signals, a demultiplexer that separates the output of the multiplexer for each wavelength, and a constant output optical amplifier between them, wherein the communication device comprises a demultiplexer, a plurality of photoelectric converters that convert the incident RoF signal into a wireless signal, an optical splitter that branches the incident RoF signal, an optical switch that switches either the output destination of the signal after demultiplexing by the demultiplexer or the output source of the signal input to the photoelectric converter, or both, and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, wherein the optical level of the first signal, which is a first wavelength RoF signal obtained by demultiplexing by the demultiplexer, is above a predetermined optical level, and the optical level of the second signal, which is a second wavelength RoF signal different from the first wavelength obtained by demultiplexing by the demultiplexer, is above a predetermined optical level, A communication method comprising: a first control step in which the control unit controls the operation of the optical switch to control the output destination of the first signal to a first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to a second photoelectric converter, which is the other of the photoelectric converters; and a second control step in which, if the optical level of the first signal is above a predetermined optical level and the optical level of the second signal is below a predetermined optical level, the control unit controls the operation of the optical switch to control the output destination of the first signal to the optical splitter, and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the demultiplexer.

6. A MIMO system using hybrid BF and RoF, comprising a multiplexer that multiplexes RoF signals and a demultiplexer that separates the output of the multiplexer for each wavelength, and a constant output optical amplifier between them, wherein the base station in the MIMO system comprises the demultiplexer, a plurality of photoelectric converters that convert the incident RoF signals into radio signals, an optical splitter that branches the incident RoF signals, an optical switch that switches either the output destination of the signals after demultiplexing by the demultiplexer or the output source of the signals input to the photoelectric converters, or both, and a control unit that controls the operation of the optical switch based on the optical level of each RoF signal obtained by the demultiplexer for each wavelength, and a communication method executed by the MIMO system, A communication method comprising: a first control step in which, if the optical level of a first signal, which is a first wavelength RoF signal obtained by demultiplexing by the demultiplexer, is above a predetermined optical level, and the optical level of a second signal, which is a second wavelength RoF signal different from the first wavelength obtained by demultiplexing by the demultiplexer, is above a predetermined optical level, the control unit controls the operation of the optical switch to control the output destination of the first signal to a first photoelectric converter, which is one of the photoelectric converters, and the output destination of the second signal to a second photoelectric converter, which is the other of the photoelectric converters; and a second control step in which, if the optical level of the first signal is above a predetermined optical level, and the optical level of the second signal is below a predetermined optical level, the control unit controls the operation of the optical switch to control the output destination of the first signal to the optical splitter, and controls the output source of the first photoelectric converter and the second photoelectric converter to the optical splitter instead of the demultiplexer.

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