Optical wireless communication system, signal processing device, wireless communication device, optical wireless communication method, signal processing method, and wireless communication method

Abstract

To simplify the internal configuration of a device that wirelessly transmits a signal received by light.SOLUTION: A signal processing device includes a signal generation unit that generates a polarization multiplexed multilevel phase modulated signal of light of a first frequency, and a multiplexing unit that outputs signal light obtained by multiplexing the generated polarization multiplexed multilevel phase modulated signal and light of a second frequency to a wireless communication device. The wireless communication device includes a separation unit that separates the signal light received from the multiplexing unit into a first polarized signal light and a second polarized signal light, and a photoelectric conversion unit that converts the first polarized signal light and the second polarized signal light into electrical signals having a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted electrical signal of the first polarization from a first antenna, and transmits the converted electrical signal of the second polarization from the second antenna.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical wireless communication system, a signal processing device, a wireless communication device, an optical wireless communication method, a signal processing method, and a wireless communication method. [Background technology]

[0002] High-speed wireless access systems such as 5G (fifth generation mobile communication system), which are currently becoming widely used, use a network configuration called a Centralized Radio Access network (C-RAN) configuration to accommodate a large number of user terminals. Figure 6 is a diagram showing an example of a wireless communication system with a C-RAN configuration. In a C-RAN configuration, a large number of base stations equipped with radio signal transceivers that cover a small area are deployed, and these base stations are accommodated by a large-scale base station, which performs scheduling processing and the like.

[0003] The C-RAN configuration is characterized by a simple configuration of the many base stations deployed, and a division of functions that concentrates them in the central station. Currently, a configuration is being considered in which the base station is equipped with not only an antenna but also signal processing and decoding functions, and some processing is performed by the base station, while the remaining processing is performed by the central station. An analog RoF method is a method that further expands this C-RAN configuration (see, for example, Non-Patent Document 1).

[0004] Figure 7 is a diagram of an optical wireless communication system using the RoF method. In downstream communications, the radio signal processing unit on the central station side generates a radio signal, and the optical transmitter directly converts the generated radio signal from electrical to optical (E / O) and transmits the optical signal over fiber. In the base station, the optical receiver converts the propagated optical signal from optical to electrical (O / E) and transmits the radio signal directly from the antenna. Similarly, in upstream communications, the optical transmitter in the base station converts the radio signal received by the antenna into an optical signal and transmits the optical signal over fiber to the central station. By using this method, the signal processing function can be removed from the base station.

[0005] The terahertz band generally refers to the frequency band from approximately 100 GHz (gigahertz) to 10 THz (terahertz). Currently, no frequencies have been allocated to the terahertz band under the Radio Wave Act. Due to its high carrier frequency and wide frequency range, the terahertz band is a candidate technology for future high-speed wireless communication systems such as 6G (sixth generation mobile communication system), as it is expected to enable high-speed wireless communication. Until now, the terahertz region has been an unexplored area in terms of technology and applications compared to the radio wave region (<100 GHz) and the light wave region (>10 THz). One of the reasons for this is that the technology to efficiently generate terahertz waves has not yet been established.

[0006] However, with recent technological advances, many techniques for efficiently generating terahertz waves have been reported. Methods for generating terahertz waves can be broadly divided into two approaches. The first involves using electronic circuits such as CMOS (Complementary Metal Oxide Semiconductor) integrated circuits to input a local oscillator signal and an intermediate frequency band modulation signal into a mixer and perform frequency conversion to generate terahertz waves in the electrical domain (see, for example, Non-Patent Document 2). The second involves inputting two wavelengths into a nonlinear optical crystal such as GaSe (gallium selenide) and generating terahertz waves through difference frequency generation, a nonlinear optical effect (see, for example, Non-Patent Document 3). Another approach involves inputting two light beams with different wavelengths (frequencies) at a frequency of 3 dB (decibels) and then inputting the resulting optical beat into a uni-traveling carrier photodiode (UTC-PD) to generate terahertz waves in the optical domain through photoelectric conversion (see, for example, Non-Patent Document 4). Thus, multiple methods for generating terahertz signals have been gradually proposed, and studies on systematization are also beginning to progress.

[0007] FIG. 8 is a diagram showing communication between an aggregation station and a base station that is currently being considered (see, for example, Non-Patent Document 5). In FIG. 8, a transceiver that uses a digital coherent receiving method (see, for example, Non-Patent Document 6) that is in practical use in core networks and the like is used for communication between the aggregation station and the base station. That is, a two-core digital coherent transceiver equipped with a DSP (Digital Signal Processor) is placed in the aggregation station. The two-core digital coherent transceiver is a general digital coherent transceiver.

[0008] In downstream communications, a dual-core digital coherent transceiver in the central station transmits a polarization-multiplexed optical signal. The dual polarization coherent receiver (DP-Coh.Rx) in the base station converts the polarization-multiplexed optical signal received from the central station into an electrical baseband (BB) signal. The base station converts the BB polarization-multiplexed optical signal into the terahertz band using a local oscillator (LO) signal generated using the electronic circuit described above, and transmits it via antennas corresponding to the X and Y polarizations. In upstream communications, the base station downconverts the terahertz band signals received from user terminals via each antenna to baseband using the LO signal, then converts them into an optical signal using a dual polarization Mach-Zehnder modulator (DP-MZN) and transmits them to the central station. This enables high-speed signal transmission and reception using existing digital coherent transceivers while eliminating signal processing at the base station. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] Nippon Telegraph and Telephone Corporation, "Efficient accommodation of diverse high-frequency band wireless systems using analog RoF," NTT Technical Journal, March 2020, pp. 15-17, [online], [Retrieved June 6, 2022], Internet,<URL: https: / / journal.ntt.co.jp / wp-content / uploads / 2020 / 05 / JN20200315.pdf> [Non-patent document 2] M. Fujishima, "Study on sub-terahertz-band wireless system with fiber-optic speed", Impact, vol. 2020, no. 1, pp. 41-43, Feb. 2020 [Non-patent document 3] Zhiming Huang, Jinxing Lu, Jingguo Huang, Bingbing Wang, Yun Hou, Xuemin Shen, Junhao Chu, "Terahertz generation from DFG and TPG configurations", 2011 International Conference on Infrared, Millimeter, and Terahertz Waves, 2011. [Non-patent document 4] H. Ito, T. Furuta, F. Nakajima, K. Yoshino, and T. Ishibashi, "Photonic Generation of Continuous THz Wave Using Uni-Traveling-Carrier Photodiode", Journal of Lightwave Technology, Vol.23, No.12, pp.4016-4021, 2005. [Non-patent document 5] C. Castro, R. Elschner, T. Merkle, C. Schubert and R. Freund, "100 Gb / s Real-Time Transmission over a THz Wireless Fiber Extender Using a Digital-Coherent Optical Modem", 2020 Optical Fiber Communications Conference and Exhibition (OFC), 2020 [Non-patent document 6] OIF, "OIF-IC-TROSA-01.0", August 2019, [Retrieved June 6, 2022], Internet,<URL: https: / / www.oiforum.com / wp-content / uploads / OIF-IC-TROSA-01.0.pdf> Summary of the Invention [Problem to be solved by the invention]

[0010] The above technology requires a complex digital coherent receiver at the base station. In addition, the base station must convert the received signal to a baseband signal once, so it must handle signals separated into I and Q for each polarization between the antenna and the coherent receiver, and between the antenna and the modulator. This increases the number of electrical elements, such as amplifiers, required at the base station.

[0011] In view of the above circumstances, the present invention aims to provide an optical wireless communication system, a signal processing device, a wireless communication device, an optical wireless communication method, a signal processing method, and a wireless communication method that can simplify the internal configuration of a device that wirelessly transmits a signal received by light. [Means for solving the problem]

[0012] One aspect of the present invention is an optical wireless communication system having a signal processing device and a wireless communication device, wherein the signal processing device comprises a signal generation unit that generates a polarization-multiplexed multi-level phase-keyed signal of light at a first frequency, and a multiplexing unit that outputs signal light obtained by multiplexing the polarization-multiplexed multi-level phase-keyed signal generated by the signal generation unit with light of a second frequency to the wireless communication device, and the wireless communication device comprises a separation unit that separates the signal light received from the multiplexing unit into signal light of a first polarization and signal light of a second polarization, and an opto-electrical conversion unit that converts the signal light of the first polarization and the signal light of the second polarization into electrical signals of a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted electrical signal of the first polarization from a first antenna, and transmits the converted electrical signal of the second polarization from a second antenna.

[0013] One aspect of the present invention is a signal processing device comprising: a signal generating unit that generates a polarization multiplexed multi-level phase-keyed signal of light at a first frequency; a multiplexing unit that outputs to the wireless communication device signal light that is generated by multiplexing the polarization multiplexed multi-level phase-keyed signal generated by the signal generating unit with light at a second frequency whose difference frequency from the first frequency is the frequency of a radio signal transmitted from a wireless communication device; and a demodulating unit that converts radio signals received by two different antennas into first and second received signals of intermediate frequencies, respectively, modulates light at a third frequency that is separated from the first frequency by the intermediate frequency using the first and second received signals, and receives from the wireless communication device a polarization multiplexed multi-level phase-keyed signal of an intermediate frequency that is generated by modulating the light at a third frequency that is separated from the first frequency by the intermediate frequency, using the first and second received signals, and demodulates the received polarization multiplexed multi-level phase-keyed signal using the light at the first frequency.

[0014] One aspect of the present invention is a wireless communication device comprising: a separation unit that receives, from a signal processing device, a signal light obtained by multiplexing a polarization-multiplexed multi-level phase-keyed signal of light of a first frequency and light of a second frequency, and separates the received signal light into a signal light of a first polarization and a signal light of a second polarization; an optoelectric conversion unit that converts the first polarization and the second polarization signal light into electrical signals of a difference frequency between the first frequency and the second frequency, respectively, and transmits the converted first polarization electrical signal by radio from a first antenna and transmits the converted second polarization electrical signal from a second antenna; a frequency conversion unit that converts a radio signal received by a third antenna into a first received signal of an intermediate frequency, and converts a radio signal received by a fourth antenna into a second received signal of the intermediate frequency; and a modulation unit that modulates light of a third frequency that is separated from the first frequency by the intermediate frequency using the first received signal and the second received signal converted by the frequency conversion unit, and transmits the polarization-multiplexed multi-level phase-keyed signal of the intermediate frequency generated by the modulation to the signal processing device.

[0015] One aspect of the present invention is an optical wireless communication method in an optical wireless communication system having a signal processing device and a wireless communication device, the optical wireless communication method comprising: a signal generation step in which the signal processing device generates a polarization multiplexed multi-level phase-keyed signal of light of a first frequency; a combining step in which the signal processing device outputs signal light obtained by combining the polarization multiplexed multi-level phase-keyed signal with light of a second frequency to the wireless communication device; a separation step in which the wireless communication device receives the signal light output in the combining step and separates the received signal light into signal light of a first polarization and signal light of a second polarization; and an opto-electrical conversion step in which the wireless communication device converts the signal light of the first polarization and the signal light of the second polarization into electrical signals of a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted electrical signal of the first polarization from a first antenna, and transmits the converted electrical signal of the second polarization from a second antenna.

[0016] One aspect of the present invention is a signal processing method including: a signal generation step of generating a polarization multiplexed multi-level phase-keyed signal of light at a first frequency; a combining step of combining the polarization multiplexed multi-level phase-keyed signal with light at a second frequency whose difference frequency from the first frequency is the frequency of a radio signal transmitted from a radio communication device, and outputting the resulting signal light to the radio communication device; and a demodulation step of converting radio signals received by two different antennas into first and second received signals of intermediate frequencies, respectively, modulating light at a third frequency that is separated from the first frequency by the intermediate frequency using the first and second received signals, generating polarization multiplexed multi-level phase-keyed signals of an intermediate frequency, and demodulating the received polarization multiplexed multi-level phase-keyed signal using the light at the first frequency.

[0017] One aspect of the present invention is a wireless communication method including: a separation step of receiving, from a signal processing device, signal light obtained by multiplexing a polarization-multiplexed multi-level phase-keyed signal of light of a first frequency and light of a second frequency, and separating the received signal light into signal light of a first polarization and signal light of a second polarization; an optoelectric conversion step of converting the first polarization and the second polarization signal light into electrical signals of a difference frequency between the first frequency and the second frequency, respectively, wirelessly transmitting the converted first polarization electrical signal from a first antenna, and transmitting the converted second polarization electrical signal from a second antenna; a frequency conversion step of converting a wireless signal received by a third antenna into a first received signal of an intermediate frequency, and converting a wireless signal received by a fourth antenna into a second received signal of the intermediate frequency; and a modulation step of modulating light of a third frequency that is separated from the first frequency by the intermediate frequency, using the first received signal and the second received signal, and transmitting the polarization-multiplexed multi-level phase-keyed signal of the intermediate frequency generated by the modulation to the signal processing device. [Effects of the Invention]

[0018] According to the present invention, it is possible to simplify the internal configuration of a device that wirelessly transmits a signal received by light. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram illustrating a configuration of an optical wireless communication system according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram illustrating the configuration of a wireless terminal according to the embodiment. [Figure 3] FIG. 10 is a diagram illustrating a configuration of an optical wireless communication system according to a second embodiment. [Figure 4] FIG. 10 is a diagram illustrating a configuration of an optical wireless communication system according to a modified example of the second embodiment. [Figure 5] FIG. 10 is a diagram illustrating a configuration of an optical wireless communication system according to a modified example of the second embodiment. [Figure 6] FIG. 1 is a diagram illustrating an example of a wireless communication system having a C-RAN configuration. [Figure 7] FIG. 1 is a diagram illustrating an example of an optical wireless communication system using the RoF method. [Figure 8] FIG. 2 is a diagram illustrating communication between a central station and a base station. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0021] An optical wireless communication system according to an embodiment of the present invention is configured to communicate between a central station and a base station equipped with a wireless antenna. For downstream communication from the central station to a wireless terminal, the central station combines an optical signal with frequency f1 generated by an optical coherent transceiver (TRx) and light with frequency f2 output from a local oscillator light source, and transmits the combined signal to the base station. The base station is equipped with an opto-electrical converter that transmits a terahertz signal corresponding to the difference frequency between these two wavelengths. For upstream communication from the wireless terminal to the central station, the base station converts the terahertz signal received from the wireless terminal to an IF (Intermediate Frequency) band. The base station transmits the converted IF band signal to the central station using a signal optical wavelength that is separated by the IF band frequency from the oscillation wavelength of the local oscillator light source used by the optical coherent transceiver in the central station. This allows for the use of an existing optical coherent transceiver in the central station, while simplifying the configuration of the base station and achieving miniaturization and cost reduction.

[0022] [First embodiment] 1 is a configuration diagram of an optical wireless communication system 100 according to a first embodiment of the present invention. The optical wireless communication system includes a central station device 200 and a base station device 300. Although only one base station device 300 is shown in the figure, multiple base station devices 300 can be connected to one central station device 200. The optical wireless communication system 100 is a system in which the central station device 200 and the base station device 300 communicate transmitted and received signals of the same wavelength via a dual-core fiber.

[0023] The central station device 200 and the base station device 300 are connected by a transmission path 401 and a transmission path 402. The transmission path 401 and the transmission path 402 are each different cores of a two-core fiber. The transmission path 401 and the transmission path 402 may each be different fibers. Here, it is assumed that the base station device 300 performs terahertz communication in a wireless section, but the wireless section may be in any frequency band.

[0024] The aggregation station device 200 includes a digital coherent transceiver (TRx) 210, a local oscillation light source 220, and an optical multiplexing / demultiplexing unit 230. An existing general two-core digital coherent TRx can be used as the digital coherent TRx 210. The digital coherent TRx 210 transmits and receives polarization-multiplexed multilevel phase-shift keyed signals such as DP (Dual Polarization)-QPSK (Quadrature Phase Shift Keying) and DP-16QAM (Quadrature Amplitude Modulation). The digital coherent TRx 210 includes a light source 211, an optical splitter 212, a DSP 213, a plurality of digital-to-analog converters (DACs) 214, a polarization multiplexing IQ modulator 215, a polarization diversity receiver 216, and a plurality of analog-to-digital converters (ADCs) 217. The plurality of DACs 214 are referred to as DACs214, and the plurality of ADCs 217 are referred to as ADCs217.

[0025] The light source 211 outputs light having a light source frequency f1. The optical splitter 212 splits the light output by the light source 211 into two. The optical splitter 212 outputs one of the split light beams to the polarization multiplexing IQ modulator 215 and the other to the polarization diversity receiver 216. The DSP 213 generates downstream signals of XI, XQ, YI, and YQ components of a digital signal and outputs them to the DACs 214. The XI component and the XQ component are the in-phase (I) component and the quadrature (Q) component of the X polarization, respectively, and the YI component and the YQ component are the in-phase (I) component and the quadrature (Q) component of the Y polarization, respectively. The X polarization and the Y polarization are orthogonal to each other. The DACs 214 converts the downstream signals of the XI, XQ, YI, and YQ components generated by the DSP 213 from digital signals to analog signals and outputs them to the polarization multiplexing IQ modulator 215. The polarization multiplexed IQ modulator 215 modulates the light input from the optical splitter 212 using the analog signals of the XI, XQ, YI, and YQ components output by the DACs 214, and generates a polarization multiplexed multi-level phase-keyed signal of the optical signal. The polarization multiplexed IQ modulator 215 outputs the generated downstream polarization multiplexed multi-level phase-keyed signal to the optical multiplexing / demultiplexing unit 230.

[0026] The polarization diversity receiver 216 receives the polarization multiplexed multi-level phase-shift keyed IF signal output from the base station device 300 via the transmission path 402. The polarization diversity receiver 216 demodulates the polarization multiplexed multi-level phase-shift keyed signal using the optical signal input from the optical splitter 212 to obtain the XI, XQ, YI, and YQ components of the upstream signal. The polarization diversity receiver 216 outputs the demodulated electrical signals of the XI, XQ, YI, and YQ components to the ADCs 217. The ADCs 217 converts the input electrical signals of the XI, XQ, YI, and YQ components from analog to digital and outputs them to the DSP 213. The DSP 213 performs reception processing such as decoding on the upstream signals of each component received from the ADCs 217 to obtain data.

[0027] The local oscillation light source 220 generates local oscillation light with an oscillation frequency f2. The oscillation frequency f2 is a frequency that is separated from the light source frequency f1 of the light source 211 by the transmission radio frequency fc in the base station device 300. The optical multiplexing / demultiplexing unit 230 multiplexes a downstream signal, which is a polarization multiplexed multilevel phase-shift keyed signal with the signal light frequency f1 output by the digital coherent TRx 210, with the local oscillation light with the frequency f2 output by the local oscillation light source 220, and outputs the resulting signal light to the transmission path 401.

[0028] When the central station device 200 is connected to a plurality of base station devices 300, the central station device 200 includes a set of a digital coherent TRx 210, a local oscillation light source 220, and an optical multiplexing / demultiplexing unit 230 corresponding to each of the base station devices 300. Note that one local oscillation light source 220 may be shared by the optical multiplexing / demultiplexing units 230 corresponding to each of the plurality of base station devices 300.

[0029] The base station device 300 includes an optical receiving unit 310, an antenna unit 320, an antenna unit 330, and an optical transmitting unit 340. The optical receiving unit 310 includes a polarization separation element 311, an opto-electric conversion element 312-1, and an opto-electric conversion element 312-2. The antenna unit 320 includes an antenna 321-1 and an antenna 321-2. The antenna unit 330 includes an antenna 331-1 and an antenna 331-2. The optical transmitting unit 340 includes a mixer 341-1, a mixer 341-2, an oscillator 342, a multiplier 343, a light source 344, and a polarization multiplexing IQ modulator 345.

[0030] The polarization separation element 311 receives the signal light transmitted through the transmission line 401 and separates the input signal light into orthogonal X-polarized and Y-polarized signal light. The polarization separation element 311 outputs the X-polarized signal light to the photoelectric conversion element 312-1 and the Y-polarized signal light to the photoelectric conversion element 312-2. The photoelectric conversion elements 312-1 and 312-2 are the above-mentioned UTC-PDs or resonant tunneling diodes. The photoelectric conversion elements 312-i (i = 1, 2) convert the optical signal of frequency λ1 multiplexed into the input signal light into an electrical signal of a frequency band including a frequency fc corresponding to the difference frequency |λ2 - λ1| between the optical signal of frequency λ1 and the optical signal of frequency λ2 multiplexed into the input signal light. The converted frequency band is, for example, the terahertz band. The photoelectric conversion element 312-i outputs the frequency-converted electrical signal to the antenna 321-i. The antenna 321-i wirelessly transmits the electrical signal input from the photoelectric conversion element 312-i.

[0031] The antennas 321-i (i = 1, 2) receive radio signals and output the received radio signals to the mixers 341-i. The radio signal received by the antenna 321-1 is a multilevel phase-modulated signal of X-polarized terahertz waves, and the radio signal received by the antenna 321-2 is a multilevel phase-modulated signal of Y-polarized terahertz waves. The oscillator 342 outputs a local oscillation signal (LO) of frequency fd. The multiplier 343 converts the local oscillation signal of frequency fd output by the oscillator 342 into a local oscillation signal of frequency Afd, which is multiplied by A (A is an integer equal to or greater than 1), and outputs the local oscillation signal to the mixers 341-1 and 341-2. The frequency Afd of the light source 211 included in the digital coherent TRx 210 of the aggregation station device 200 is a frequency that is P [GHx] away from the light source frequency f1.

[0032] The mixers 341-i (i = 1, 2) down-convert the signal input from the antenna 331-i using the local oscillation signal of frequency Afd input from the multiplier 343, converting it into a signal in the IF band including frequency P. The mixers 341-i output the down-converted signal to the polarization multiplexing IQ modulator 345. The light source 344 generates light of frequency f3. The difference frequency between frequency f3 and the light source frequency f1 of the light source 211 included in the digital coherent TRx 210 of the aggregation station device 200 is P. The polarization multiplexing IQ modulator 345 modulates the light input from the light source 344 using the signal input from the mixers 341-1 and 341-2 to generate a polarization multiplexed multilevel phase-keyed signal in the IF band. The polarization multiplexing IQ modulator 345 outputs the generated multilevel phase modulated signal to the transmission path 402 .

[0033] The base station device 300 may not include the multiplier 343, and the oscillator 342 may directly emit light of the frequency used for down-conversion in the mixers 341-1 and 341-2. To ensure sufficient transmission intensity, the base station device 300 may include an optical amplifier before the polarization separation element 311, and an electric amplifier before the antennas 321-1 and 321-2. An electric amplifier may be arranged after the mixers 341-1 and 341-2 in accordance with the modulation amplitude of the polarization multiplexing IQ modulator 345.

[0034] The optical wireless communication system 100 performs downstream communication as follows. The optical splitter 212 in the aggregation station device 200 splits the light of frequency f1 output by the light source 211. The DSP 213 generates downstream signals of XI, XQ, YI, and YQ components. The DACs 214 converts the generated downstream signals of XI, XQ, YI, and YQ components from digital to analog signals. The polarization multiplexed IQ modulator 215 modulates the light input from the optical splitter 212 using the analog signals of XI, XQ, YI, and YQ components output by the DACs 214 to generate a polarization multiplexed multi-level phase-shift keyed signal. The optical multiplexing / demultiplexing unit 230 multiplexes the polarization multiplexed multi-level phase-shift keyed signal of frequency f1 generated by the polarization multiplexed IQ modulator 215 with the local light of frequency f2 output by the local oscillation light source 220, and outputs the resulting signal light to the transmission path 401.

[0035] The polarization separation element 311 of the base station device 300 receives the signal light transmitted through the transmission path 401. The polarization separation element 311 separates the received signal light into X-polarized and Y-polarized signal light and outputs them to the photoelectric conversion elements 312-1 and 312-2, respectively. The photoelectric conversion element 312-1 converts the X-polarized signal light into an electrical signal in a frequency band including frequency fc. Similarly, the photoelectric conversion element 312-2 converts the Y-polarized signal light into an electrical signal in a frequency band including frequency fc. The frequency fc corresponds to the difference frequency |λ2-λ1| between the optical signal of frequency λ1 and the optical signal of frequency λ2 multiplexed into the signal light. The antennas 321-1 and 321-2 wirelessly transmit the terahertz-band electrical signals received from the photoelectric conversion elements 312-1 and 312-2, respectively.

[0036] The optical wireless communication system 100 performs uplink communication as follows. The antenna 321-1 of the base station device 300 receives a radio signal that is a multi-level phase-modulated signal of X-polarized terahertz waves, and the antenna 321-2 receives a radio signal that is a multi-level phase-modulated signal of Y-polarized terahertz waves. The multiplier 343 multiplies the local oscillation signal of frequency fd output from the oscillator 342 by A (A is an integer equal to or greater than 1) and outputs the multiplied signal to the mixers 341-1 and 341-2. The mixers 341-1 and 341-2 down-convert the signals input from the antennas 331-1 and 331-2, respectively, using the local oscillation signal input from the multiplier 343, converting them into signals in the IF band including the frequency P. Polarization multiplexing IQ modulator 345 modulates the light of frequency f3 input from light source 344 using the signal input from mixer 341-1 and the signal input from mixer 341-2 to generate a polarization multiplexed multi-level phase-modulated signal in the IF band. Polarization multiplexing IQ modulator 345 outputs the generated multi-level phase-modulated signal to transmission path 402.

[0037] The polarization diversity receiver 216 of the aggregation device 200 receives the polarization multiplexed multi-level phase-shift keyed signal output by the base station device 300 via the transmission path 402. The polarization diversity receiver 216 demodulates the polarization multiplexed multi-level phase-shift keyed signal using the light of frequency f1 received from the optical splitter 212 to obtain electrical signals of XI, XQ, YI, and YQ components. The ADCs 217 convert each of the electrical signals of the XI, XQ, YI, and YQ components from analog to digital. The DSP 213 performs reception processing on the upstream signals of each component converted into digital signals to obtain data.

[0038] As described above, during downstream communication, the local oscillation light source 220 of the central station device 200 emits light at an oscillation frequency f2, which is different from the frequency f1 of the optical signal output by the digital coherent TRx 210 by the frequency fc of the radio signal transmitted by the base station device 300. The sign of the frequency difference fc relative to the frequency f1 may be either positive or negative. That is, f2 = f1 - fc or f2 = f1 + fc may be used. The photoelectric conversion elements 312-1 and 312-2 of the base station device 300 output electrical signals in a frequency band corresponding to the frequency difference fc and transmit them from the antennas 321-1 and 321-2. According to the configuration of this embodiment, optical signals can be converted into terahertz band signals without installing complex equipment such as a coherent receiver in the base station device 300.

[0039] In upstream communication, radio signals of any frequency received by the antennas 331-1 and 331-2 of the base station device 300 are downconverted to the IF band by the mixers 341-1 and 341-2, modulated by the polarization multiplexing IQ modulator 345, and transmitted to the central station device 200 in the IF band. In a conventional method in which the base station device downconverts signals received wirelessly to the BB band, IQ signals corresponding to each polarization are required as signals to be input to the polarization multiplexing IQ modulator. In contrast, in this embodiment, since IF band transmission is performed from the base station device 300 to the central station device 200, the base station device 300 only needs to input signals corresponding to the X and Y polarizations to the polarization multiplexing IQ modulator. This reduces the number of required electrical elements by half compared to the conventional technology. However, an electrical bandwidth equivalent to the IF band is required.

[0040] Setting the signal wavelength is crucial for implementing this embodiment. In downstream communications, as described above, a frequency difference fc corresponding to the terahertz frequency is established between the light output by the light source 211 and the local light output by the local oscillator light source 220, and the light is converted to the terahertz band using the characteristics of the photoelectric conversion elements 312-1 and 312-2. In upstream communications, as described above, signals are assumed to be transmitted in the IF band from the base station device 300 to the central station device 200. In this case, a typical digital coherent receiving TRx is assumed to receive baseband signals. However, as shown in FIG. 1 , when an IF band signal is transmitted, a modulation spectrum occurs in the optical section at a position separated by the IF frequency P[GHx] from the optical center wavelength f3. Therefore, reception is not possible when the light source frequency f3 of the base station device 300 and the light source frequency f1 of the central station are the same, as in a configuration in which the digital coherent TRx 210 are arranged opposite each other.

[0041] Therefore, in this embodiment, the light source 344 of the remote station device 300 is operated at an oscillation wavelength that is ±P [GHz] away from the light source frequency f1 of the central station device 200. As a result, the polarization diversity receiver 216 converts the modulation spectrum present in the IF band into a BB signal by utilizing the wavelength difference of the light source. This makes it possible to receive IF band signals without changing the configuration of the central station device 200. In this case, wavelength control accuracy is required so that the frequency offset is within the range that can be compensated for by the DSP 213 after reception. In the configuration shown in FIG. 1, only the upper or lower spectrum of the modulation spectrum is used, resulting in a decrease in SN (signal-to-noise ratio). To avoid this, the polarization multiplexing IQ modulator 345 may apply SSB (single sideband) modulation.

[0042] 2 is a diagram showing an example configuration of a wireless terminal 500 according to this embodiment. The wireless terminal 500 includes antennas 511-1 and 511-2, mixers 512-1 and 512-2, an oscillator 513, a multiplier 514, multiple analog-to-digital converters (ADCs) 515, a DSP 531, multiple digital-to-analog converters (DACs) 551, mixers 552-1 and 552-2, an oscillator 553, a multiplier 554, antennas 555-1 and 555-2. The multiple ADCs 515 are referred to as ADCs515, and the multiple DACs 551 are referred to as DACs551.

[0043] The antenna 511-i (i = 1, 2) receives a terahertz wave radio signal transmitted by the antenna 321-i of the base station device 300 and outputs the received radio signal to the mixer 512-i. The oscillator 513 outputs a local oscillation signal. The multiplier 514 converts the local oscillation signal output by the oscillator 513 into a local oscillation signal of a multiplied frequency and outputs the local oscillation signal to the mixers 512-1 and 512-2. The mixer 512-i (i = 1, 2) down-converts the received signal input from the antenna 511-i using the local oscillation signal input from the multiplier 514 and outputs the down-converted signal to the ADCs 515. The ADCs 515 converts the received signal input from the mixer 512-1 and the received signal input from the mixer 512-2 from digital signals to analog signals and outputs the down-converted signals to the DSP 531. The DSP 531 performs reception processing on the received signal input from the ADCs 515 to obtain the data transmitted by the central station device 200 .

[0044] The DSP 531 also generates upstream transmission signals to be transmitted from each of the antennas 555-1 and 555-2 and outputs them to the DACs 551. The DACs 551 converts the transmission signals input from the DSP 531 from digital signals to analog signals. The DACs 551 outputs transmission signals to be transmitted from the antennas 555-i (i = 1, 2) to the mixers 552-i. The oscillator 553 outputs a local oscillation signal. The multiplier 554 converts the local oscillation signal output from the oscillator 553 into a local oscillation signal of a multiplied frequency and outputs the local oscillation signal to the mixers 552-1 and 552-2. The mixers 552-1 and 552-2 up-convert the signals input from the DACs 551 using the local oscillation signal input from the multiplier 554, and convert them into transmission signals in the frequency band of the radio signal. Mixer 552-i (i=1, 2) outputs the up-converted transmission signal to antenna 555-i, which wirelessly transmits the transmission signal input from mixer 552-i.

[0045] As described above, the wireless terminal 500 converts the wireless signal received from the base station device 300 to an arbitrary frequency and then performs AD (analog-to-digital) conversion. Similarly, during transmission, the wireless terminal 500 converts the transmission signal of an arbitrary frequency to DA (digital-to-analog), and then converts it to the frequency band of the wireless signal using mixers 552-1 and 552-2.

[0046] Since the format of the signals transmitted and received by the central station device 200 conforms to the signal format of the optical transceiver, the wireless terminal 500 needs to be equipped with a DSP having the same function.

[0047] [Second embodiment] A second embodiment of the present invention will be described. As described in the first embodiment, the optical wireless communication system of this embodiment generates and receives electrical signals in any frequency band by controlling the optical oscillation frequency. Therefore, setting the oscillation frequency is important. Generally, digital coherent transceivers have a wavelength changing function. Therefore, in the second embodiment, the frequencies of various light sources can be set based on the frequency of the wireless section given from an external information transmission path. The second embodiment will be described, focusing on the differences from the first embodiment described above.

[0048] Fig. 3 is a configuration diagram of an optical wireless communication system 101 according to the second embodiment. In Fig. 3, the same components as those in the optical wireless communication system 100 according to the first embodiment shown in Fig. 1 are denoted by the same reference numerals, and their description will be omitted. The optical wireless communication system 101 includes a central station device 201 and a base station device 301. The base station device 301 and a wireless terminal 501 communicate wirelessly in the same manner as the wireless terminal 500 and base station device 300 according to the first embodiment. Furthermore, the central station device 201 and the base station device 301 are connected to the wireless terminal 501 by a control signal transmission path 600.

[0049] The central station side device 201 differs from the central station side device 200 of the first embodiment shown in FIG. 1 in that it further includes a frequency control unit 250. The frequency control unit 250 acquires wireless section frequency information. The wireless section frequency information indicates a frequency used in the wireless section between the base station side device 301 and the wireless terminal 501. For example, the frequency control unit 250 receives the wireless section frequency information from the wireless terminal 501 via a control signal transmission path 600. Alternatively, the frequency control unit 250 may receive the wireless section frequency information from an external device different from the wireless terminal 501 via an external information transmission path, or may store the wireless section frequency information in advance. In this case, the central station side device 201 does not need to be connected to the wireless terminal 501. The frequency control unit 250 determines the frequency f1 of the signal output by the digital coherent TRx 210 of its own device and the oscillation frequency f2 of the local oscillation light source 220 using the downlink wireless section frequency fc indicated in the received wireless section frequency information. That is, the frequency control unit 250 determines the frequency f1 and the oscillation frequency f2 so that fc = |f1 - f2|. Note that the frequency f1 or the oscillation frequency f2 may be fixed. The frequency control unit 250 sets the determined frequency f1 in the digital coherent TRx 210, and sets the determined oscillation frequency f2 in the local oscillation light source 220.

[0050] The base station device 301 differs from the base station device 300 of the first embodiment shown in FIG. 1 in that it further includes a frequency control unit 350. The frequency control unit 350 acquires either or both of wireless section frequency information and aggregation station device frequency information. The aggregation station device frequency information indicates the frequency f1 of the light source 211 of the digital coherent TRx 210 of the aggregation station device 201 and the oscillation frequency f2 of the local oscillation light source 220. Note that the aggregation station device frequency information does not need to include information on the oscillation frequency f2 of the local oscillation light source 220. For example, the frequency control unit 350 receives the wireless section frequency information from the wireless terminal 501 via the control signal transmission path 600. Alternatively, the frequency control unit 350 may receive the wireless section frequency information from an external device different from the wireless terminal 501 via an external information transmission path, or may store the wireless section frequency information in advance. In this case, the base station device 301 does not need to be connected to the wireless terminal 501. The frequency control unit 350 may also receive the central station side device frequency information from an external device via an external information transmission path. Alternatively, instead of receiving the central station side device frequency information, the frequency control unit 350 may calculate the frequency f1 of the light source 211 and the oscillation frequency f2 of the local oscillation light source 220 based on the wireless section frequency information using an algorithm similar to that used by the frequency control unit 250 of the central station side device 201.

[0051] The frequency control unit 350 determines the frequency fd of the oscillator 342 and the frequency f3 of the light source 344 based on the frequency f1 of the light source 211 and the frequency of the uplink wireless section. That is, the frequency control unit 350 determines the frequency fd and the frequency f3 so that the difference frequency between the frequency f1 and the frequency f3 becomes the frequency P after downconversion of the radio signal obtained based on the frequency Afd and the frequency of the uplink wireless section. The frequency control unit 350 sets the determined frequency fd in the oscillator 342 and sets the frequency f3 in the light source 344. Note that the frequency control unit 350 may externally acquire the transmission frequency from the optical transmitting unit 340 and set the frequency fd and the frequency f3 in the optical transmitting unit 340 based on the transmission frequency.

[0052] 2 in that the wireless terminal 501 further includes a control signal transmitting unit 560. The control signal transmitting unit 560 transmits wireless section frequency information to the central station device 201 and the base station device 301 via a control signal transmission path 600.

[0053] 4 is a configuration diagram of an optical wireless communication system 102 according to a modified example of the second embodiment. In FIG. 4, the same components as those in the optical wireless communication system 100 according to the first embodiment shown in FIG. 1 are denoted by the same reference numerals, and their description will be omitted. The optical wireless communication system 102 includes a central station device 202 and a base station device 302. The central station device 202 differs from the central station device 200 of the first embodiment shown in FIG. 1 in that it further includes a frequency control unit 260. The base station device 302 also differs from the base station device 300 of the first embodiment shown in FIG. 1 in that it further includes a frequency control unit 360. The frequency control unit 260 of the base station device 302 and the frequency control unit 360 of the base station device 302 transmit and receive control information via a control information transmission path 601.

[0054] The frequency control unit 260 of the central station device 202 acquires wireless section frequency information in the same manner as the frequency control unit 250 of the central station device 201 shown in Fig. 3, and determines the frequency f1 of the signal output by the digital coherent TRx 210 of its own device and the oscillation frequency f2 of the local oscillation light source 220, using the wireless section frequency fc indicated by the wireless section frequency information. The frequency control unit 260 notifies the frequency control unit 360 of the base station device 302 of control information via a control information transmission path 601. The control information is central station device frequency information, or wireless section frequency information and central station device frequency information. The frequency control unit 360 of the base station device 302 performs processing similar to that of the frequency control unit 350 of the base station device 301 shown in Fig. 3, using the received control information.

[0055] As a transmission path for control information, for example, as described in Reference 1, an AMCC (Auxiliary Management and Control Channel) signal may be used, in which the control information is superimposed on the main signal using an external intensity modulator, and the AMCC signal is selectively received by the receiver of the base station device.

[0056] (Reference 1) Ryo Igarashi, Ryo Koma, Kazutaka Hara, Kazuaki Honda, Jun-ichi Kani, and Tomoaki Yoshida, "Simultaneous Reception of ASK-based AMCC Signals and QPSK Signals with Single Coherent Receiver", 2021 Optical Fiber Communications Conference and Exhibition (OFC), 2021

[0057] Fig. 5 is a configuration diagram of an optical wireless communication system 103 according to a modified example of the second embodiment. In Fig. 5, the same components as those in the optical wireless communication system 100 according to the first embodiment shown in Fig. 1 are denoted by the same reference numerals, and their description will be omitted. The optical wireless communication system 103 includes a central station device 203 and a base station device 303. The central station device 203 and the base station device 303 transmit and receive control signals using AMCC signals. The AMCC signal is a low-frequency signal that is superimposed on a main signal and transmitted.

[0058] The central station side device 203 differs from the central station side device 200 of the first embodiment shown in Fig. 1 in that it further includes a frequency control unit 270 and an intensity modulator 271. The frequency control unit 270 performs the same processing as the frequency control unit 240 of the central station side device 201 shown in Fig. 3. However, the frequency control unit 270 outputs control information to be transmitted to the remote station side device 302 to the intensity modulator 271. The intensity modulator 271 superimposes an AMCC signal, in which the control information input from the frequency control unit 270 is set, on the signal light output by the optical multiplexing / demultiplexing unit 230. The intensity modulator 271 outputs the signal light on which the AMCC signal is superimposed to the transmission path 401. The control information is wireless section frequency information, or wireless section frequency information and central station side device frequency information.

[0059] The base station device 303 differs from the base station device 300 of the first embodiment shown in Fig. 1 in that it further includes an optical multiplexing / demultiplexing unit 371, an AMCC signal receiving unit 372, and a frequency control unit 373. The optical multiplexing / demultiplexing unit 371 receives signal light transmitted through the transmission path 401 and demultiplexes it based on wavelength. The optical multiplexing / demultiplexing unit 371 outputs signal light having a wavelength set for the AMCC signal to the AMCC signal receiving unit 372 and outputs signal light having a wavelength set for the main signal to the optical receiving unit 310. The AMCC signal receiving unit 372 detects the AMCC signal from the signal light received from the optical multiplexing / demultiplexing unit 371 and outputs control information set in the AMCC signal to the frequency control unit 373. The frequency control unit 373 uses the control information received from the AMCC signal receiving unit 372 to perform processing similar to that of the frequency control unit 350 of the base station device 301 shown in Fig. 3.

[0060] The above-described central station device 201, central station device 202, central station device 203, base station device 301, base station device 302, and base station device 303 each include a processor, memory, auxiliary storage device, and the like, which are connected by a bus. The processor may execute a program to realize the functions of the frequency control unit 250, frequency control unit 260, frequency control unit 270, frequency control unit 350, frequency control unit 360, and frequency control unit 373. The processor may be, for example, a central processing unit (CPU) or a graphics processing unit (GPU). Note that all or part of the functions of the frequency control unit 250, frequency control unit 260, frequency control unit 270, frequency control unit 350, frequency control unit 360, and frequency control unit 373 may be realized using hardware such as an application specific integrated circuit (ASIC), a programmable logic device (PLD), or a field programmable gate array (FPGA). The programs for frequency control units 250, 260, 270, 350, 360, and 373 may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, and storage devices such as hard disks built into computer systems. The programs for frequency control units 250, 260, 270, 350, 360, and 373 may be transmitted via telecommunications lines.

[0061] According to the above-described embodiment, the optical wireless communication system includes a signal processing device and a wireless communication device. For example, the signal processing device corresponds to the central station device 200, 201, 202, and 203 of the embodiment, and the wireless communication device corresponds to the base station device 300, 301, 302, and 303 of the embodiment. The signal processing device includes a signal generation unit and a multiplexing unit. For example, the signal generation unit corresponds to the polarization multiplexed IQ modulator 215 of the embodiment, and the multiplexing unit corresponds to the optical multiplexing / demultiplexing unit 230 of the embodiment. The signal generation unit generates a polarization multiplexed multi-level phase-keyed signal of light with a first frequency f1. The multiplexing unit multiplexes the polarization multiplexed multi-level phase-keyed signal generated by the signal generation unit with light with a second frequency f2, and outputs the resulting signal light to the wireless communication device. The wireless communication device includes a demultiplexing unit and an opto-electrical conversion unit. For example, the separator corresponds to the polarization separator element 311 in the embodiment, and the photoelectric converter corresponds to the photoelectric converters 312-1 and 312-2 in the embodiment. The separator separates the signal light received from the multiplexer into a first polarized signal light and a second polarized signal light. The photoelectric converter converts the first polarized signal light and the second polarized signal light into electrical signals with a difference frequency fc between the first frequency f1 and the second frequency f2, and wirelessly transmits the converted first polarized electrical signal from the first antenna and transmits the converted second polarized electrical signal from the second antenna. For example, the first antenna and the second antenna correspond to the antennas 331-1 and 331-2 in the embodiment.

[0062] The wireless communication device may further include a frequency converter and a modulator. For example, the frequency converter corresponds to the mixers 341-1 and 341-2 of the embodiment, and the modulator corresponds to the polarization multiplexing IQ modulator 345 of the embodiment. The frequency converter converts the radio signal received by the third antenna into a first received signal of intermediate frequency P, and converts the radio signal received by the fourth antenna into a second received signal of intermediate frequency P. The modulator modulates light of a third frequency f3, which is separated from the first frequency f1 by the intermediate frequency P, using the first and second received signals converted by the frequency converter, and transmits the polarization multiplexing multi-level phase-shift keyed signal of the intermediate frequency generated by the modulation to the signal processing device. The signal processing device includes a demodulator. For example, the demodulator corresponds to the polarization diversity receiver 216 of the embodiment. The demodulator demodulates the polarization multiplexing multi-level phase-shift keyed signal received from the modulator using light of the first frequency f1.

[0063] The first antenna and the second antenna may each wirelessly transmit an electrical signal in the terahertz band, and the third antenna and the fourth antenna may each receive a wireless signal in the terahertz band. [Explanation of symbols]

[0064] 100, 101, 102, 103 Optical wireless communication system 200, 201, 202, 203 Aggregation station side equipment 210 Digital Coherent Transmitter / Receiver 211 Light source 212 Optical Splitter 214 Digital-to-analog converter 215 Polarization Multiplexing IQ Modulator 216 Polarization diversity receiver 217 Analog-to-Digital Converter 220 Local Oscillator Light Source 230 Optical multiplexing / demultiplexing section 240, 250, 260, 270 Frequency control section 271 Intensity Modulator 300, 301, 302, 303 Outgoing station side equipment 310 Optical receiver 311 Polarization Separation Element 312-1, 312-2 Photoelectric conversion element 320 Antenna section 321-1, 321-2 Antennas 330 Antenna section 331-1, 331-2 Antenna 340 Optical transmitter 341-1, 341-2 Mixer 342 Oscillator 343 Multiplier 344 Light source 345 Polarization Multiplexing IQ Modulator 350, 360, 373 Frequency control section 371 Optical multiplexing / demultiplexing section 372 AMCC signal receiver 401, 402 transmission lines 500, 501 Wireless terminal 511-1, 511-2, 555-1, 555-2 antennas 512-1, 512-2, 552-1, 552-2 Mixers 513, 553 oscillators 514, 554 Multiplier 515 Analog-to-Digital Converter 551 Digital-to-Analog Converter 560 Control signal transmitter 600 Control signal transmission path 601 Regulatory Signaling Pathways

Claims

1. An optical wireless communication system having a signal processing device and a wireless communication device, The signal processing device includes: a signal generator that generates a polarization multiplexed multilevel phase-modulated signal of a first frequency; a multiplexing unit that multiplexes the polarization multiplexed multilevel phase-shift keyed signal generated by the signal generating unit with light of a second frequency and outputs the resulting signal light to the wireless communication device, The wireless communication device a demultiplexer that demultiplexes the signal light received from the multiplexer into a first polarized signal light and a second polarized signal light; an opto-electrical conversion unit that converts the first polarized signal light and the second polarized signal light into electrical signals having a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted first polarized electrical signals from a first antenna, and wirelessly transmits the converted second polarized electrical signals from a second antenna; The wireless communication device a frequency conversion unit that converts a radio signal received by the third antenna into a first received signal of an intermediate frequency, and converts a radio signal received by the fourth antenna into a second received signal of the intermediate frequency; a modulation unit that modulates light of a third frequency that is separated from the first frequency by the intermediate frequency using the first received signal and the second received signal converted by the frequency conversion unit, and transmits a polarization multiplexed multilevel phase keyed signal of the intermediate frequency generated by the modulation to the signal processing device, The signal processing device includes: a demodulation unit that demodulates the polarization multiplexed multi-level phase-shift keyed signal received from the modulation unit using light of the first frequency, Optical wireless communication system.

2. the first antenna and the second antenna each wirelessly transmit the electrical signal in the terahertz band; the third antenna and the fourth antenna each receive the wireless signal in the terahertz band. The optical wireless communication system according to claim 1 .

3. a signal generation unit that generates a polarization multiplexed multilevel phase-modulated signal of light at a first frequency; a multiplexing unit that multiplexes the polarization multiplexed multi-level phase-shift keyed signal generated by the signal generating unit with light of a second frequency, the difference frequency of which from the first frequency is the frequency of a radio signal transmitted from a radio communication device, and outputs the resulting signal light to the radio communication device; a demodulation unit that converts radio signals received by two different antennas into first and second received signals of intermediate frequencies, respectively, receives from the wireless communication device a polarization multiplexed multi-level phase-shift keyed signal of intermediate frequency generated by modulating light of a third frequency that is separated from the first frequency by the intermediate frequency using the first and second received signals, and demodulates the received polarization multiplexed multi-level phase-shift keyed signal using the light of the first frequency; A signal processing device comprising:

4. a separation unit that receives signal light obtained by multiplexing a polarization-multiplexed multilevel phase-keyed signal of light of a first frequency and light of a second frequency from the signal processing device, and separates the received signal light into signal light of a first polarization and signal light of a second polarization; an opto-electrical conversion unit that converts the first polarized signal light and the second polarized signal light into electrical signals having a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted first polarized electrical signals from a first antenna, and wirelessly transmits the converted second polarized electrical signals from a second antenna; a frequency conversion unit that converts a radio signal received by the third antenna into a first received signal of an intermediate frequency, and converts a radio signal received by the fourth antenna into a second received signal of the intermediate frequency; a modulation unit that modulates light of a third frequency that is separated from the first frequency by the intermediate frequency using the first received signal and the second received signal converted by the frequency conversion unit, and transmits a polarization multiplexed multilevel phase keyed signal of the intermediate frequency generated by the modulation to the signal processing device; A wireless communication device comprising:

5. An optical wireless communication method in an optical wireless communication system having a signal processing device and a wireless communication device, a signal generation step in which the signal processing device generates a polarization multiplexed multilevel phase modulated signal of light of a first frequency; a combining step in which the signal processing device combines the polarization multiplexed multilevel phase-modulated signal and light of a second frequency and outputs the combined signal light to the wireless communication device; a separating step in which the wireless communication device receives the signal light output in the combining step and separates the received signal light into a signal light of a first polarization and a signal light of a second polarization; an opto-electrical conversion step in which the wireless communication device converts the first polarized signal light and the second polarized signal light into electrical signals having a difference frequency between the first frequency and the second frequency, wirelessly transmits the converted first polarized electrical signal from a first antenna, and wirelessly transmits the converted second polarized electrical signal from a second antenna; a frequency conversion step in which the wireless communication device converts the radio signal received by the third antenna into a first received signal of an intermediate frequency, and converts the radio signal received by the fourth antenna into a second received signal of the intermediate frequency; a modulation step in which the wireless communication device modulates light of a third frequency that is separated from the first frequency by the intermediate frequency using the first received signal and the second received signal converted in the frequency conversion step, and transmits a polarization multiplexed multilevel phase keyed signal of the intermediate frequency generated by the modulation to the signal processing device; a demodulation step in which the signal processing device demodulates the polarization multiplexed multi-level phase modulated signal received from the wireless communication device using light of the first frequency; An optical wireless communication method comprising:

6. a signal generation step of generating a polarization multiplexed multilevel phase modulated signal of light of a first frequency; a combining step of combining the polarization multiplexed multilevel phase-shift keyed signal with light of a second frequency, the difference frequency of which from the first frequency is the frequency of a radio signal transmitted from a radio communication device, and outputting the resulting signal light to the radio communication device; a demodulation step of converting radio signals received by two different antennas into first and second received signals of intermediate frequencies, respectively, receiving from the wireless communication device a polarization-multiplexed multi-level phase-shift keyed signal of intermediate frequency generated by modulating light of a third frequency that is separated from the first frequency by the intermediate frequency using the first and second received signals, and demodulating the received polarization-multiplexed multi-level phase-shift keyed signal using the light of the first frequency; A signal processing method comprising:

7. a separating step of receiving, from the signal processing device, signal light obtained by multiplexing a polarization multiplexed multilevel phase-modulated signal of light of a first frequency and light of a second frequency, and separating the received signal light into signal light of a first polarization and signal light of a second polarization; an opto-electrical conversion step of converting the first polarized signal light and the second polarized signal light into electrical signals having a difference frequency between the first frequency and the second frequency, wirelessly transmitting the converted first polarized electrical signals from a first antenna, and wirelessly transmitting the converted second polarized electrical signals from a second antenna; a frequency conversion step of converting the radio signal received by the third antenna into a first received signal of an intermediate frequency, and converting the radio signal received by the fourth antenna into a second received signal of the intermediate frequency; a modulating step of modulating light of a third frequency that is separated from the first frequency by the intermediate frequency using the first received signal and the second received signal, and transmitting a polarization multiplexed multilevel phase-shift keyed signal of the intermediate frequency generated by the modulation to the signal processing device; A wireless communication method comprising:

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