Communication device

Abstract

This communication device serving as an aggregation station comprises: a signal processing unit that generates a main signal and a control signal; and a subcarrier multiplexing unit that arranges the main signal and the control signal in each of a first sideband and a second sideband with a carrier frequency as a reference, and generates an intensity-modulated optical signal including the control signal having opposite phases in the first sideband and the second sideband. This communication device serving as an extension station comprises: a photoelectric converter that converts, into an electrical signal, an intensity-modulated optical signal based on a main signal and a control signal arranged at the same frequency; a first filter that generates the main signal by passing, among the converted electrical signals, an electrical signal of a first frequency; a second filter that passes, among the converted electrical signals, an electrical signal of a second frequency; and a control signal generation unit that generates the control signal on the basis of the electrical signal of the first frequency and the electrical signal of the second frequency.

Description

Communication device

[0001] This invention relates to a communication device.

[0002] In a radio communication system using analog RoF (Radio over Fiber), one or more branch stations are deployed in the radio communication area. A centralized station accommodates the communications of each branch station. In such a radio communication system, it is necessary to multiplex the main signal and control signals.

[0003] Wavelength Division Multiplexing (WDM) is a method for multiplexing multiple signals (see Non-Patent Document 1). Furthermore, to suppress the increase in wavelength resources, a method is to use both subcarrier multiplexing (SCM) and wavelength division multiplexing for the main signal and control signal (see Non-Patent Document 2).

[0004] Hirotaka Ono, Katsuhiro Shimano, Mitsunori Fukutoku, and Shigeru Kuwano, “An EDFA Gain Control and Power Monitoring Scheme for Fault Detection in WDM Networks by Employing a Power-Stabilized Control Channel”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 20, NO. 8, AUGUST 2002.Y. Yamamoto et al., “Experimental results of multiplexing transmission of 256-QAM 5G NR signal with control signal by SCM over 20-km analog RoF link”, [online], 11 July 2024, Optics Communications, Vol.570, [Searched on November 6, 2020] Internet<URL: https: / / doi.org / 10.1016 / j.optcom.2024.130862>

[0005] The main signal and the control signal are generated for each communication carrier or wireless system. Therefore, there is a problem that the main signal and the control signal cannot be multiplexed without increasing the wavelength resources so that the communications of multiple communication carriers or wireless systems can be accommodated in the concentration station.

[0006] In view of the above circumstances, an object of the present invention is to provide a communication device capable of multiplexing a main signal and a control signal without increasing wavelength resources.

[0007] One aspect of the present invention is a communication device including a signal processing unit that generates a main signal and a control signal, and a subcarrier multiplexing unit that arranges the main signal and the control signal in each of a first sideband and a second sideband based on a carrier frequency, and generates an optical intensity modulation signal including the control signal having opposite phases between the first sideband and the second sideband.

[0008] One aspect of the present invention is a communication device including a photoelectric converter that converts an optical intensity modulation signal based on a main signal and a control signal arranged at the same frequency into an electrical signal, a first filter that generates the main signal by transmitting the electrical signal of a first frequency among the converted electrical signals, a second filter that transmits the electrical signal of a second frequency among the converted electrical signals, and a control signal generation unit that generates a control signal based on the electrical signal of the first frequency and the electrical signal of the second frequency.

[0009] According to the present invention, it is possible to multiplex a main signal and a control signal without increasing wavelength resources.

[0010] It is a diagram showing a configuration example of a communication system in an embodiment. It is a diagram showing an example of a main signal and a control signal arranged at the same frequency in an embodiment. It is a diagram showing a configuration example of a concentration station in an embodiment. It is a flowchart showing an operation example of a concentration station in an embodiment. It is a flowchart showing an operation example of an outstation in an embodiment. It is a diagram showing a configuration example of a concentration station in a modification example of an embodiment. It is a diagram showing a hardware configuration example of a communication device in an embodiment.

[0011] Embodiments of the present invention will be described in detail with reference to the drawings. Figure 1 is a diagram showing an example configuration of the communication system 1 in an embodiment. The communication system 1 is a wireless communication system to which analog RoF is applied. The communication system 1 comprises a central station 2, an optical transmission line 3, and one or more branch stations 4. Hereinafter, the direction from the central station to the branch station will be referred to as "downlink". The direction from the branch station to the central station will be referred to as "uplink".

[0012] The aggregation station 2 is a communication device acting as the master unit for analog RoF, and is, for example, a base station, CS (Central Station), CU (Central Unit), DU (Distributed Unit), or uplink RU (Radio Unit).

[0013] The aggregation station 2 comprises N signal processing units 21 (where N is an integer of 1 or more) and a subcarrier multiplexing unit 22. The aggregation station 2 comprises, for example, a signal processing unit 21 for each telecommunications carrier or wireless system.

[0014] Each signal processing unit 21 generates a main signal and a control signal. Each signal processing unit 21 outputs the main signal and the control signal to the subcarrier multiplexing unit 22. The subcarrier multiplexing unit 22 (Non-Orthogonal Multiple Access (NOMA) unit) generates an optical intensity modulated signal (subcarrier multiplexing signal) in which the main signal and the control signal are arranged at the same frequency. Multiplexing techniques such as polarization multiplexing, mode multiplexing, and multicore fiber may be combined in the optical intensity modulated signal. The subcarrier multiplexing unit 22 outputs the optical intensity modulated signal, in which the main signal and the control signal are arranged at the same frequency, to the optical transmission line 3.

[0015] Figure 2 shows an example of a main signal and control signal arranged at the same frequency in an embodiment. The lower and upper sidebands are at the carrier frequency "f Ｃ This is a bandwidth with "[ ]" as the reference (boundary). The main signal 101-1-1 and the control signal 201-1-1 are placed at the same frequency in the lower sideband. Also, the main signal 101-2-1 and the control signal 201-2-1 are placed at the same frequency in the upper sideband.

[0016] The same applies to the main signal 101-1-n (where n is an integer from 1 to N) and the control signal 201-1-n in the lower sideband. Similarly, the same applies to the main signal 101-2-n and the control signal 201-2-n in the upper sideband.

[0017] Returning to Figure 1, let's continue the explanation of the details of the communication system 1. The optical transmission path 3 transmits optical intensity modulated signals to the outgoing station 4 using optical fibers. The optical fibers are not limited to a specific type, but include, for example, single-mode fibers, multimode fibers, multicore fibers, or distributed-shift fibers. The optical signal transmission method in the optical transmission path 3 is, for example, RFoF (Radio Frequency over Fiber) using the radio frequency band, or IFoF (Intermediate Frequency over Fiber) using the intermediate frequency band.

[0018] The extension station 4 is connected to the aggregation station 2 via the optical transmission path 3. The extension station 4 is a communication device acting as a slave unit of analog RoF, and is, for example, a TRP (Transmission and Reception Point), RRU (Remote Radio Unit), RRH (Remote Radio Head), downstream RU (Radio Unit), distributed antenna (DA), antenna, panel, TP (Transmission Point), or RP (Reception Point).

[0019] The extension station 4 comprises a photoelectric converter 41, N demodulation units 42, and N antennas 43. The extension station 4 may have demodulation units 42 and antennas 43 that are equal to or greater than the number of signal processing units 21 "N". The extension station 4 may have demodulation units 42 and antennas 43 that are equal to or less than the number of signal processing units 21 "N". The extension station 4 may also include an amplifier (not shown) and a switch (not shown). The demodulation unit 42 comprises a first filter 421, a second filter 422, a control signal generation unit 423, and a control unit 424.

[0020] The optical intensity modulation signal transmitted downward from the concentration station 2 is input to the photoelectric converter 41 via the optical transmission line 3. The optical intensity modulation signal "OE" input to the photoelectric converter 41 ｉｎ is expressed as in Equation (1).

[0021]

[0022] Here, for simplicity of explanation, the number "N" of the signal processing units 21 is 1. "f" Ｃ represents the frequency of the optical carrier wave (carrier frequency). "f" Ｓ represents the frequency of the main signal converted into an electrical signal. "A" Ｓ is the main signal, and in particular represents the amplitude of the main signal. "A" Ｔ is the control signal, and in particular represents the amplitude of the control signal. "cos(2πf" ｃ t)" represents the optical carrier wave of the control signal.

[0023] "A" Ｓ cos{2π(f" Ｃ - f" Ｓ )t}" represents the main signal arranged in the lower sideband. "A" Ｔ cos{2π(f" Ｃ - f" Ｓ )t}" represents the control signal arranged in the lower sideband. In contrast, "A" Ｓ cos{2π(f" Ｃ + f" Ｓ )t}" represents the main signal arranged in the upper sideband. "A" Ｔ cos{2π(f" Ｃ + f" Ｓ )t - π}" represents the control signal arranged in the upper sideband. As such, the phase of the control signal in the upper sideband is inverted with respect to the phase of the control signal in the lower sideband.

[0024] The electrical signal "OE" output from the photoelectric converter 41 ｏｕｔ is expressed as in Equation (2).

[0025]

[0026] Here, "R" represents the sensitivity of the photoelectric conversion. "M" represents the photoelectric conversion coefficient. Also, in Equation (2), "(A" Ｓ + AＴ ) cos(2πf Ｃ t) cos {2π(f Ｃ -f Ｓ )t} + (A Ｓ -A Ｔ ) cos(2πf Ｃ t) cos {2π(f Ｃ +f Ｓ )t}" can be expressed as shown in equation (3).

[0027]

[0028] Here, "A Ｓ cos(2πf) Ｓ "t)" represents the frequency component of the main signal. The first filter 421 (band-pass filter) filters the electrical signal input to the first filter 421, removing the main signal "A Ｓ The following is output to the control signal generation unit 423 and the control unit 424. Also, in equation (2) "(A Ｓ +A Ｔ ) (A Ｓ -A Ｔ ) cos {2π(f Ｃ -f Ｓ )t}cos{2π(f Ｃ +f Ｓ )t}" can be expressed as shown in equation (4).

[0029]

[0030] Here, "(A Ｔ ２ -A Ｓ ２ ) cos(2π(2f Ｓ )t) / 2" is the frequency of the main signal "f Ｓ This represents the second harmonic component of the main signal. The second filter 422 (band-pass filter) filters the electrical signal input to the second filter 422 for the second harmonic component of the main signal frequency "(A Ｔ ２ -A Ｓ ２ ) cos(2π(2f Ｓ The signal t) / 2 is output to the control signal generation unit 423.

[0031] The control signal generation unit 423 generates the frequency component "A" of the main signal. Ｓ cos(2πf) ＳThe first filter 421 acquires the second harmonic component of the main signal frequency "(A Ｔ ２ -A Ｓ ２ ) cos(2π(2f Ｓ The second filter 422 obtains the second harmonic component of the main signal frequency "(A)t) / 2". The control signal generation unit 423 obtains the second harmonic component of the main signal frequency "(A)t) / 2". Ｔ ２ -A Ｓ ２ ) cos(2π(2f Ｓ )t) / 2" and the frequency component of the main signal "A Ｓ cos(2πf) Ｓ Based on t), the control signal "A Ｔ The control signal generation unit 423 generates the control signal "A Ｔ The command " is output to the control unit 424.

[0032] The control unit 424 receives the main signal "A Ｓ The control unit 424 receives the control signal "A" from the first filter 421. Ｔ The control signal is obtained from the control signal generation unit 423. The control unit 424 may control the transmission of the main signal from the antenna 43 based on the control signal. For example, the antenna 43 may transmit the main signal (radio signal) by beamforming based on the control signal.

[0033] Next, the details of the aggregation station 2 will be described. Figure 3 is a diagram showing an example configuration of the aggregation station 2 in the embodiment. The subcarrier multiplexing unit 22 includes a branching unit 23, a first intensity modulation unit 24, a second intensity modulation unit 25, a reverse phase processing unit 26, a third intensity modulation unit 27, and an optical coupler 28. The first intensity modulation unit 24 includes a frequency division multiplexing unit 241 and an electrophotoconverter 242. The second intensity modulation unit 25 includes a frequency division multiplexing unit 251, an electrophotoconverter 252, and an optical filter 253. The third intensity modulation unit 27 includes a frequency division multiplexing unit 271, an electrophotoconverter 272, and an optical filter 273.

[0034] The branching unit 23 receives control signals from each signal processing unit 21. The branching unit 23 generates a first branch signal and a second branch signal for each signal processing unit 21 by branching the control signals. The branching unit 23 outputs the first branch signal of each signal processing unit 21 to the frequency division multiplexing unit 251. The branching unit 23 outputs the second branch signal of each signal processing unit 21 to the inverse phase processing unit 26.

[0035] The frequency division multiplexer 241 receives the main signals from each signal processing unit 21. The frequency division multiplexer 241 performs frequency division multiplexing on the main signals from each signal processing unit 21. The photoelectric converter 242 converts the main signals, which have undergone frequency division multiplexing by the frequency division multiplexer 241, into light. The photoelectric converter 242 outputs the converted light as a first optical signal to the optical coupler 28.

[0036] The frequency division multiplexer 251 receives the first branch signals from each signal processing unit 21 via the branching unit 23. The frequency division multiplexer 251 performs frequency division multiplexing on the first branch signals from each signal processing unit 21. The photoelectric converter 252 converts the first branch signals, which have undergone frequency division multiplexing by the frequency division multiplexer 251, into light. The photoelectric converter 252 outputs the converted light to the optical filter 253. The optical filter 253 outputs the light in the first sideband as a second optical signal to the optical coupler 28.

[0037] The inverse phase processing unit 26 receives the second branch signals from each signal processing unit 21 via the branching unit 23. The inverse phase processing unit 26 generates an inverse-phase second branch signal for each signal processing unit 21 by inverting the phase of the second branch signal relative to the phase of the first branch signal. The frequency division multiplexing unit 271 receives the inverse-phase processed second branch signals from the inverse phase processing unit 26 for each signal processing unit 21. The photoconverter 272 converts the inverse-phase second branch signals, which have undergone frequency division multiplexing by the frequency division multiplexing unit 271, into light. The photoconverter 272 outputs the converted light to the optical filter 273. The optical filter 273 outputs the light in the second sideband as a third optical signal to the optical coupler 28. The photoconverters 242, 252, and 272 may use a common light source (not shown) to convert electrical signals into optical signals.

[0038] The optical coupler 28 receives the first optical signal from the photoelectric converter 242. The optical coupler 28 receives the second optical signal from the optical filter 253. The optical coupler 28 receives the third optical signal from the optical filter 273. The optical coupler 28 combines the first optical signal, the second optical signal, and the third optical signal. The optical coupler 28 outputs the combined signal as an optical intensity modulated signal to the optical transmission line 3.

[0039] Next, an example of the operation of the communication system 1 will be described. Figure 4 is a flowchart showing an example of the operation of the aggregation station 2 in the embodiment. The signal processing unit 21 generates the main signal and the control signal (step S101). The first intensity modulation unit 24 sets the carrier frequency "f Ｃ A first optical signal is generated based on the main signals placed in the first and second sidebands, respectively, with the reference to " (step S102).

[0040] The branching unit 23 generates a first branch signal and a second branch signal by branching the control signal (step S103). The second intensity modulation unit 25 places the first branch signal at the frequency of the main signal (step S104). The second intensity modulation unit 25 generates a second optical signal based on the first branch signal placed at the frequency of the main signal in the first sideband (step S105).

[0041] The phase inversion processing unit 26 generates a second branch signal with inverted phase (step S106). The third intensity modulation unit 27 positions the second branch signal with inverted phase at the frequency of the main signal (step S107). The third intensity modulation unit 27 generates a third optical signal based on the second branch signal with inverted phase positioned at the frequency of the main signal in the second sideband (step S108).

[0042] The optical coupler 28 combines the first optical signal, the second optical signal, and the third optical signal (step S109). The optical coupler 28 outputs the combined signal as an optical intensity modulated signal to the optical transmission line 3 (step S110).

[0043] Figure 5 is a flowchart showing an example of the operation of the extension station 4 in the embodiment. The optical intensity modulated signal transmitted downstream from the aggregation station 2 is input to the photoelectric converter 41 via the optical transmission line 3. The photoelectric converter 41 converts the optical intensity modulated signal, based on the main signal and control signal arranged at the same frequency, into an electrical signal (step S201). The first filter 421 filters the main signal to the first frequency "f Ｓ It is generated from the electrical signal of " (step S202). The second filter 422 filters the converted electrical signal, for example, the second frequency "2f Ｓ The electrical signal of " is transmitted (step S203).

[0044] The control signal generation unit 423 generates a control signal "A" based on the first frequency electrical signal and the second frequency electrical signal. Ｔ The control unit 424 generates the signal (step S204). Based on the control signal, the control unit 424 controls the transmission of the main signal (step S205).

[0045] As described above, in the aggregation station 2 (communication device as the master unit for analog RoF), the signal processing unit 21 generates the main signal and control signal. The subcarrier multiplexing unit 22 sets the carrier frequency "f Ｃ One or more main signals and control signals are placed in each of the first and second sidebands, which are based on "[ ]". The subcarrier multiplexer 22 generates an optical intensity modulated signal. The optical intensity modulated signal includes control signals whose phases are reversed in the first and second sidebands. For example, the first intensity modulator 24 places main signals in each of the first and second sidebands. Here, the first sideband may be the lower sideband and the second sideband may be the upper sideband, or the first sideband may be the upper sideband and the second sideband may be the lower sideband. The first intensity modulator 24 generates a first optical signal based on the main signals placed in each of the first and second sidebands.

[0046] The branching unit 23 generates a first branch signal and a second branch signal by branching the control signal. The second intensity modulation unit 25 positions the first branch signal at the frequency of the main signal. The second intensity modulation unit 25 generates a second optical signal based on the first branch signal positioned at the frequency of the main signal in the first sideband.

[0047] The phase inversion processing unit 26 generates a second branch signal with inverted phase by inverting the phase of the second branch signal relative to the phase of the first branch signal. The third intensity modulation unit 27 positions the second branch signal with inverted phase at the frequency of the main signal. The third intensity modulation unit 27 generates a third optical signal based on the second branch signal with inverted phase positioned at the frequency of the main signal in the second sideband. The optical coupler 28 generates an optical intensity modulated signal based on the main signal and control signal positioned at the same frequency by combining the first optical signal, the second optical signal, and the third optical signal.

[0048] This makes it possible to generate optical signals from which control signals can be extracted through simple processing. It also allows for the suppression of increased wavelength resources and simplifies the configuration of the telescope station.

[0049] In the extension station 4 (a communication device acting as a slave unit for analog RoF), the photoelectric converter 41 converts the optical intensity modulated signal, based on the main signal and control signal arranged at the same frequency, into an electrical signal. The first filter 421 filters the converted electrical signal to a first frequency "f Ｓ The main signal is generated by allowing the electrical signal of "" to pass through. The second filter 422 allows the electrical signal of the second frequency from the converted electrical signal to pass through. The second frequency is the frequency output by the photoelectric conversion, and is, for example, twice the frequency of the first frequency "2f Ｓ For example, the second frequency is set to a frequency "2f" so that a control signal can be generated based on the component of the first term of equation (4). Ｃ " is also acceptable. The control signal generation unit 423 generates a control signal "A" based on the first frequency electrical signal and the second frequency electrical signal. Ｔ The control unit 424 may control the transmission of the main signal based on the control signal.

[0050] This makes it possible to multiplex the main signal and control signal without increasing wavelength resources. Furthermore, since the control signal can be extracted from the optical signal through simple processing, the configuration of the extension station can be simplified.

[0051] (Modification) In the above embodiment, phase inversion processing was performed at the electrical signal processing stage. In the modification, the main difference from the above embodiment is that phase inversion processing is performed at the optical signal processing stage. The modification will be explained focusing on the differences from the above embodiment.

[0052] Figure 6 shows an example of the configuration of the aggregation station 2 in a modified embodiment. The subcarrier multiplexing unit 22 comprises a first intensity modulation unit 24, a second intensity modulation unit 25, and an optical coupler 28. The second intensity modulation unit 25 comprises a frequency division multiplexing unit 251, an electrophotoconverter 252, an optical filter 253, a branching unit 254, an optical filter 255, and an inverse phase processing unit 256.

[0053] The first intensity modulation unit 24 generates a first optical signal based on the main signals arranged in the first and second sidebands, respectively. Here, the first sideband may be the lower sideband and the second sideband may be the upper sideband, or the first sideband may be the upper sideband and the second sideband may be the lower sideband.

[0054] The frequency division multiplexer 251 performs frequency division multiplexing on the control signals of each signal processing unit 21. The photoelectric converter 252 converts the control signals, which have undergone frequency division multiplexing by the frequency division multiplexer 251, into light. The branching unit 254 generates a third branch signal and a fourth branch signal by branching the converted light. The branching unit 254 outputs the third branch signal to the optical filter 253. The branching unit 254 outputs the fourth branch signal to the optical filter 255.

[0055] The optical filter 253 outputs the third branch signal in the first sideband from the third branch signal output from the branching unit 254 to the optical coupler 28 as the second optical signal. The optical filter 255 outputs the fourth branch signal in the second sideband from the fourth branch signal output from the branching unit 254 to the inverse phase processing unit 256. The inverse phase processing unit 256 generates an inverse phase control signal by inverting the phase of the fourth branch signal in the second sideband with respect to the phase of the third branch signal (second optical signal) in the first sideband. The inverse phase processing unit 26 outputs the inverse phase control signal as the third optical signal to the optical coupler 28.

[0056] As described above, the first intensity modulation unit 24 places a main signal in the first sideband and the second sideband, respectively. The first intensity modulation unit 24 generates a first optical signal based on the placed main signals. The second intensity modulation unit 25 places a control signal at the frequency of the main signal. The second intensity modulation unit 25 generates a second optical signal based on the control signal placed at the frequency of the main signal in the first sideband. The second intensity modulation unit 25 generates a third optical signal by inverting the phase of the control signal in the second sideband with respect to the phase of the second optical signal. The optical coupler 28 generates an optical intensity modulated signal based on the main signal and control signal placed at the same frequency by combining the first optical signal, the second optical signal, and the third optical signal.

[0057] This makes it possible to multiplex the main signal and control signal without increasing wavelength resources. Furthermore, since the control signal can be extracted from the optical signal through simple processing, the configuration of the extension station can be simplified.

[0058] (Hardware Configuration) Figure 7 shows examples of the hardware configuration of the communication device 10 in each embodiment. The examples of the hardware configuration of the communication device 10 correspond to the hardware configuration example of the aggregation station 2 and the hardware configuration example of the extension station 4 in each embodiment.

[0059] The communication device 10 is implemented as software by a processor 11, such as a CPU (Central Processing Unit), executing a program stored in a storage device 12 having a non-volatile recording medium (non-temporary recording medium) and a memory 13. The program may be recorded on a computer-readable recording medium. A computer-readable recording medium is a non-temporary recording medium such as a portable medium such as a flexible disk, magneto-optical disk, ROM (Read Only Memory), CD-ROM (Compact Disc Read Only Memory), or a storage device such as a hard disk or solid-state drive (SSD) built into a computer system. The communication unit 14 performs predetermined communication processing.

[0060] The communication device 10 may be implemented via hardware including electronic circuits (or circuits) using, for example, LSI (Large Scale Integrated Circuit), ASIC (Application Specific Integrated Circuit), PLD (Programmable Logic Device), or FPGA (Field Programmable Gate Array).

[0061] While 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.

[0062] The present invention is applicable to wireless communication systems that utilize analog RoF, etc.

[0063] 1...Communication system, 2...Aggregation station, 3...Optical transmission line, 4...Output station, 21...Signal processing unit, 22...Subcarrier multiplexing unit, 23...Branching unit, 24...First intensity modulation unit, 25...Second intensity modulation unit, 26...Phase inversion processing unit, 27...Third intensity modulation unit, 28...Optical coupler, 41...Photoelectric converter, 42...Demodulation unit, 43...Antenna, 101...Main signal, 201...Control signal, 241...Frequency division multiplexing unit, 242...Electron photoconverter, 251...Frequency division multiplexing unit, 252...Electron photoconverter, 253...Optical filter, 254...Branching unit, 255...Optical filter, 256...Phase inversion processing unit, 271...Frequency division multiplexing unit, 272...Electron photoconverter, 273...Optical filter, 421...First filter, 422...Second filter, 423...Control signal generation unit, 424...Control unit

Claims

1. A communication device comprising: a signal processing unit that generates a main signal and a control signal; and a subcarrier multiplexing unit that arranges the main signal and the control signal in a first sideband and a second sideband, respectively, based on the carrier frequency, and generates an optical intensity modulated signal that includes the control signal with inverse phase between the first sideband and the second sideband.

2. The communication device according to claim 1, wherein the subcarrier multiplexing unit comprises: a first intensity modulation unit that places the main signal in the first sideband and the second sideband respectively and generates a first optical signal based on the placed main signal; a branching unit that generates a first branch signal and a second branch signal by branching the control signal; a second intensity modulation unit that places the first branch signal at the frequency of the main signal and generates a second optical signal based on the first branch signal placed at the frequency of the main signal in the first sideband; an inverse phase processing unit that generates an inverse phase second branch signal by inverting the phase of the second branch signal with respect to the phase of the first branch signal; a third intensity modulation unit that places the inverse phase second branch signal at the frequency of the main signal and generates a third optical signal based on the inverse phase second branch signal placed at the frequency of the main signal in the second sideband; and an optical coupler that generates an optical intensity modulated signal based on the main signal and the control signal placed at the same frequency by combining the first optical signal, the second optical signal and the third optical signal.

3. The communication device according to claim 2, wherein the first intensity modulation unit includes a frequency division multiplexing unit that performs frequency division multiplexing on the main signal and an electro-photoconverter that converts the main signal on which frequency division multiplexing has been performed into light and outputs the converted light as the first optical signal to the optical coupler; the second intensity modulation unit includes a frequency division multiplexing unit that performs frequency division multiplexing on the first branch signal and an electro-photoconverter that converts the first branch signal on which frequency division multiplexing has been performed into light and an optical filter that outputs the light in the first sideband as the second optical signal to the optical coupler; and the third intensity modulation unit includes a frequency division multiplexing unit that performs frequency division multiplexing on the second branch signal in the opposite phase and an electro-photoconverter that converts the second branch signal on which frequency division multiplexing has been performed into light and an optical filter that outputs the light in the second sideband as the third optical signal to the optical coupler.

4. The communication device according to claim 1, wherein the subcarrier multiplexing unit comprises: a first intensity modulation unit that places the main signal in the first sideband and the second sideband and generates a first optical signal based on the placed main signal; a second intensity modulation unit that places the control signal at the frequency of the main signal, generates a second optical signal based on the control signal placed at the frequency of the main signal in the first sideband, and generates a third optical signal by inverting the phase of the control signal in the second sideband with respect to the phase of the second optical signal; and an optical coupler that generates an optical intensity modulated signal based on the main signal and the control signal placed at the same frequency by combining the first optical signal, the second optical signal and the third optical signal.

5. A communication device comprising: a photoelectric converter that converts an optical intensity modulated signal based on a main signal and a control signal arranged at the same frequency into an electrical signal; a first filter that generates the main signal by allowing the electrical signal of a first frequency to pass through the converted electrical signal; a second filter that allows the electrical signal of a second frequency to pass through the converted electrical signal; and a control signal generation unit that generates a control signal based on the electrical signal of the first frequency and the electrical signal of the second frequency.

6. The communication device according to claim 5, further comprising a control unit that controls the transmission of the main signal based on the control signal.

7. The communication device according to claim 5, wherein the electrical signal of the first frequency includes the components of the main signal, the electrical signal of the second frequency includes the components of the main signal and the control signal, and the control signal generation unit obtains the components of the main signal from the electrical signal of the first frequency, obtains the components of the main signal and the control signal from the electrical signal of the second frequency, and generates the control signal based on the components of the main signal and the components of the main signal and the control signal.

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