Transmitter, receiver, communication system and communication method

Abstract

To provide a transmitter, a receiver, a communication system, and a communication method capable of reducing possibility of demodulating a wireless signal by a receiver other than a receiver designated as communication destination of the wireless signal, while preventing communication speed decrease.SOLUTION: In a communication system 1a, a transmitter 2a includes a radio wave signal transmission part 22 for transmitting a main signal using a radio wave as a first carrier, and an optical wave signal transmission part 23a for transmitting a local oscillation signal used for frequency conversion of the main signal in a receiver 3a using an optical wave as a second carrier. The receiver 3a includes a radio wave signal receiving part 30 for acquiring the main signal transmitted by using the radio wave as the first carrier, an optical wave signal receiving part 31a for acquiring the local oscillation signal transmitted by using the optical wave as the second carrier and for performing frequency conversion for the main signal using the local oscillation signal, and a demodulator part 33a for performing demodulation process for the frequency converted main signal.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a transmitter, a receiver, a communication system and a communication method. [Background technology]

[0002] Known wireless communication methods include communication that uses radio waves as a carrier wave for wireless signals (hereinafter referred to as "radio wave communication") (see Non-Patent Document 1), and free-space optics (FSO) that uses light waves such as infrared light as a carrier wave for wireless signals. In high-speed radio wave communication, broadband radio waves such as millimeter waves and terahertz waves are used as the carrier wave for wireless signals. Also, unlike a communication method in which optical signals are transmitted through a closed waveguide (optical fiber communication), in wireless communication, wireless signals are transmitted through free space. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Tadao Nagatsuma, “Ultra-high-speed wireless communication opened up by terahertz waves”, Journal of the Japan Society for Precision Engineering Vol.82, No.3, 2016 Summary of the Invention [Problem to be solved by the invention]

[0004] The higher the frequency of the carrier wave used to transmit the wireless signal, the more linear the carrier wave is. In other words, the higher the frequency of the carrier wave, the narrower the beam width of the carrier wave. For this reason, in free-space optical communication, which uses a light wave with a narrow beam width as the carrier wave of the wireless signal, only a receiver (a receiver designated as the communication destination) existing within the light wave with a narrow beam width can receive the wireless signal. This makes it possible to reduce the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal demodulates the wireless signal. However, free-space optical communication is not suitable for high-speed wireless communication (transmission of a wideband wireless signal) because light waves are easily dispersed due to the effects of the atmosphere in free space and scintillation.

[0005] In contrast, radio wave communication is suitable for high-speed wireless communication because radio waves are not easily dispersed by the influence of the atmosphere in free space. However, in radio wave communication, radio waves with a wide beam width are used as carrier waves of radio signals, so it is not possible to reduce the possibility that a receiver other than the receiver designated as the communication destination of the radio signal will receive the radio signal. Thus, there is a problem in that it is not possible to reduce the possibility that a receiver other than the receiver designated as the communication destination of the radio signal will demodulate the radio signal while suppressing a decrease in communication speed.

[0006] In view of the above circumstances, the present invention aims to provide a transmitter, receiver, communication system, and communication method that are capable of suppressing a decrease in communication speed while reducing the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal will demodulate the wireless signal. [Means for solving the problem]

[0007] One aspect of the present invention is a transmitter comprising an radio wave signal transmitting unit that transmits a main signal using radio waves as a first carrier wave, and a light wave signal transmitting unit that transmits a local oscillator signal used for frequency conversion of the main signal in a receiver using light waves as a second carrier wave.

[0008] One aspect of the present invention is a receiver comprising a radio wave signal receiving unit that acquires a main signal transmitted using radio waves as a first carrier wave, a light wave signal receiving unit that acquires a local oscillator signal transmitted using light waves as a second carrier wave and performs frequency conversion of the main signal using the local oscillator signal, and a demodulation unit that performs demodulation processing on the frequency-converted main signal.

[0009] One aspect of the present invention is a communication system comprising a transmitter and a receiver, wherein the transmitter has a radio wave signal transmitting unit that transmits a main signal using radio waves as a first carrier wave, and a light wave signal transmitting unit that transmits a local oscillator signal used for frequency conversion of the main signal in the receiver using light waves as a second carrier wave, and the receiver has a radio wave signal receiving unit that acquires the main signal transmitted using radio waves as the first carrier wave, a light wave signal receiving unit that acquires the local oscillator signal transmitted using light waves as the second carrier wave and performs frequency conversion of the main signal using the local oscillator signal, and a demodulation unit that performs demodulation processing on the frequency-converted main signal.

[0010] One aspect of the present invention is a communication method performed by a transmitter, the communication method including a radio wave signal transmitting step of transmitting a main signal using radio waves as a first carrier wave, and a light wave signal transmitting step of transmitting a local oscillator signal used for frequency conversion of the main signal in a receiver using light waves as a second carrier wave.

[0011] One aspect of the present invention is a communication method performed by a receiver, the communication method including a radio wave signal receiving step of acquiring a main signal transmitted using radio waves as a first carrier wave, a local oscillator signal acquiring step of acquiring a local oscillator signal transmitted using light waves as a second carrier wave, a light wave signal receiving step of performing frequency conversion of the main signal using the local oscillator signal, and a demodulation step of performing demodulation processing on the frequency-converted main signal.

[0012] One aspect of the present invention is a communication method performed by a communication system having a transmitter and a receiver, in which the transmitter transmits a main signal using radio waves as a first carrier wave, and transmits a local oscillator signal used for frequency conversion of the main signal at the receiver using light waves as a second carrier wave, and the receiver acquires the main signal transmitted using the radio waves as the first carrier wave, acquires the local oscillator signal transmitted using the light waves as the second carrier wave, performs frequency conversion of the main signal using the local oscillator signal, and performs demodulation processing on the frequency-converted main signal. Effect of the Invention

[0013] According to the present invention, it is possible to suppress a decrease in communication speed, and also to reduce the possibility that a receiver other than the receiver designated as the communication destination of a wireless signal demodulates the wireless signal. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram illustrating an example of the configuration of a communication system in a first embodiment. [Diagram 2] 5 is a flowchart showing an example of the operation of the transmitter in the first embodiment. [Diagram 3] 5 is a flowchart showing an example of the operation of the receiver in the first embodiment. [Figure 4] FIG. 11 is a diagram illustrating an example of the configuration of a communication system in a first modified example of the first embodiment. [Diagram 5] FIG. 11 is a diagram illustrating an example of the configuration of a communication system in a second modified example of the first embodiment. [Figure 6] FIG. 11 is a diagram illustrating an example of the configuration of a communication system in a second embodiment. [Figure 7] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a first modified example of the second embodiment. [Figure 8] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a second modified example of the second embodiment. [Figure 9] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a third embodiment. [Figure 10] FIG. 13 is a diagram illustrating a first example of a frequency change in the third embodiment. [Figure 11] FIG. 13 is a diagram illustrating a second example of a frequency change in the third embodiment. [Figure 12] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a fourth embodiment. [Figure 13] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a modified example of the fourth embodiment. [Figure 14] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a fifth embodiment. [Figure 15] FIG. 13 is a diagram illustrating an example of the configuration of a communication system in a sixth embodiment. [Figure 16] FIG. 2 is a diagram illustrating an example of a hardware configuration of a communication device in each embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings. (First embodiment) 1 is a diagram showing an example of the configuration of a communication system 1a in the first embodiment. The communication system 1a is a system that performs wireless communication. The communication system 1a includes a transmitter 2a and a receiver 3a.

[0016] The transmitter 2a includes a generator 20, a first signal generator 21a, a radio signal transmitter 22, and a lightwave signal transmitter 23a. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23a includes a collimator 231.

[0017] The receiver 3a includes a radio signal receiving unit 30, a lightwave signal receiving unit 31a, a mixer 32, and a demodulator 33a. The mixer 32 may be included in the lightwave signal receiving unit 31a. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31a includes a collimator 311, a photoelectric conversion element 312a, an amplifier 313, and a multiplier 314.

[0018] The transmitter 2a transmits, for example, a main signal to the receiver 3a by radio wave communication. The radio wave used as a carrier wave in this radio wave communication is, for example, at least one of a millimeter wave and a terahertz wave. The millimeter wave in radio wave communication is, for example, a radio wave in a frequency band from about 30 GHz to about 100 GHz. The terahertz wave in radio wave communication is, for example, a radio wave in a frequency band from about 100 GHz to 10 THz.

[0019] Radio wave communication using at least one of millimeter waves and terahertz waves as a carrier wave is excellent in terms of high speed and stability of wideband communication. The transmitter 2a transmits a local oscillation signal to the receiver 3a by free space optical communication. In free space optical communication, only the receiver 3a that is in a position (within the beam) capable of receiving the light wave can demodulate the wireless signal, so secure communication is possible. The light wave in free space optical communication is a light wave with a frequency of about 200 THz to about 400 THz, for example, infrared light.

[0020] The receiver 3a receives radio waves corresponding to the main signal from the transmitter 2a through radio communication. The receiver 3a converts the received radio waves into an RF (Radio Frequency: RF) signal. The receiver 3a receives light waves corresponding to the local oscillation signal from the transmitter 2a through free space optical communication. In the receiver 3a, the photoelectric conversion element 312a performs photoelectric conversion on the light waves corresponding to the local oscillation signal. The receiver 3a generates a local oscillation signal (electrical signal) as a result of the photoelectric conversion. Here, the receiver 3a synchronizes the phase of the local oscillation signal transmitted from the transmitter 2a with the phase of the received local oscillation signal, for example, through digital signal processing by a processor (Reference: JP 2020-88598 A).

[0021] The receiver 3a down-converts the frequency of an RF signal (received signal) using a local oscillation signal. For example, the receiver 3a generates an intermediate frequency band signal (IF band signal) by down-converting the frequency of the RF signal to an intermediate frequency (IF). In this way, the receiver 3a performs heterodyne reception. Also, for example, the receiver 3a may generate a baseband signal by down-converting the frequency of the RF signal to baseband. In this way, the receiver 3a may perform intradyne reception.

[0022] Next, the transmitter 2a will be described in detail. The optical frequency comb generator 201 (optical frequency comb generator) has a wavelength interval of Δλ=λ0 2 The optical frequency comb generator 201 generates a multi-wavelength optical signal (optical frequency comb) with wavelength spacing "Δλ" between adjacent wavelengths. Here, "λ0" represents an arbitrary reference wavelength. "f0" represents a frequency determined by the clock frequency of the optical frequency comb generator 201. "C" represents the speed of light. The optical frequency comb generator 201 outputs a multi-wavelength optical signal with wavelength spacing "Δλ" to the optical filter 202.

[0023] The optical filter 202 selects an optical signal of a predetermined wavelength from among multi-wavelength optical signals with wavelength intervals of "Δλ". The optical filter 202 outputs the optical signal of the selected wavelength "λ1" to the first optical modulator 221. The optical filter 202 outputs the optical signal of the selected wavelength "λ2" to the photoelectric conversion element 222.

[0024] Hereinafter, the symbol "n" is an integer equal to or greater than 1. The difference "λ2-λ1" between the wavelengths "λ1" and "λ2" corresponds to the frequency "nf0". The optical filter 202 outputs an optical signal of the selected wavelength "λ3" and an optical signal of the selected wavelength "λ4" to the collimator 231. Hereinafter, the symbol "m" is an integer equal to or greater than 1. The difference "λ4-λ3" between the wavelengths "λ3" and "λ4" corresponds to the frequency "mf0".

[0025] The first signal generating unit 21a generates a main signal (data signal). The first signal generating unit 21a may obtain the main signal from an external information processing device (not shown). The first signal generating unit 21a outputs the main signal to the first optical modulator 221.

[0026] An optical signal of wavelength "λ1" is input to the first optical modulator 221 from the optical filter 202. The first optical modulator 221 modulates the amplitude of the optical signal of wavelength "λ1" using a main signal (data signal). That is, the first optical modulator 221 generates an optical signal corresponding to the main signal by superimposing the main signal on the optical signal of wavelength "λ1". The first optical modulator 221 outputs the optical signal corresponding to the main signal to the photoelectric conversion element 222.

[0027] The photoelectric conversion element 222 receives an optical signal with a wavelength of "λ2" from the optical filter 202. The photoelectric conversion element 222 receives an optical signal corresponding to the main signal from the first optical modulator 221. The photoelectric conversion element 222 performs photoelectric conversion on the optical signal corresponding to the main signal. The photoelectric conversion element 222 generates an electrical signal corresponding to the main signal as a result of the photoelectric conversion. The photoelectric conversion element 222 down-converts the frequency of the electrical signal corresponding to the main signal to a frequency band including a frequency "nf0" that corresponds to the difference "λ2-λ1" between the wavelengths "λ1" and "λ2". The photoelectric conversion element 222 outputs an electrical signal in a frequency band including the frequency "nf0" to the antenna 223.

[0028] The antenna 223 generates a radio signal (terahertz signal) using at least one of millimeter waves and terahertz waves as a carrier wave of the main signal in response to an electrical signal in a frequency band including the frequency "nf0". The antenna 223 transmits the radio signal using radio waves such as terahertz waves as a carrier wave of the main signal as a wideband radio signal. That is, the antenna 223 transmits radio waves corresponding to the main signal as a radio signal in a frequency band including the frequency "nf0".

[0029] An optical signal with a wavelength of "λ3" and an optical signal with a wavelength of "λ4" are input to the collimator 231 from the optical filter 202. The collimator 231 outputs the optical signal with a wavelength of "λ3" and the optical signal with a wavelength of "λ4" as light waves corresponding to a local oscillation signal into free space. Here, the collimator 231 outputs light waves with a beam width equal to or smaller than a predetermined threshold into free space.

[0030] Next, the receiver 3a will be described in detail. Antenna 301 receives a wireless signal in a frequency band including frequency "nf0" from antenna 223 via radio wave communication. Antenna 301 outputs an RF signal (received signal) in a frequency band including frequency "nf0" to amplifier 302. Amplifier 302 amplifies the amplitude (intensity) of the RF signal. Amplifier 302 outputs the RF signal to mixer 32.

[0031] The collimator 311 receives a light wave corresponding to the local oscillation signal from the collimator 231 by free space optical communication. The collimator 311 focuses the received light wave. The amplitude (light intensity) of the focused light wave may be amplified using an optical amplifier (not shown). The collimator 311 outputs the light wave corresponding to the local oscillation signal to the photoelectric conversion element 312a.

[0032] The photoelectric conversion element 312a performs photoelectric conversion on the lightwave corresponding to the local oscillation signal. As a result of the photoelectric conversion, the photoelectric conversion element 312a generates an electrical signal corresponding to the local oscillation signal. In FIG. 1, the photoelectric conversion element 312a converts an optical signal of frequency "mf0" corresponding to the difference "λ4-λ3" between the wavelengths "λ3" and "λ4" into a local oscillation signal (electrical signal) of frequency "mf0". The amplifier 313 amplifies the amplitude (intensity) of the local oscillation signal of frequency "mf0".

[0033] The multiplier 314 up-converts the frequency "mf0" of the local oscillation signal to a predetermined frequency. Here, the multiplier 314 multiplies the frequency "mf0" by "A". The multiplier 314 outputs the local oscillation signal of the frequency "Amf0" to the mixer 32.

[0034] The mixer 32 generates an intermediate frequency band signal in a frequency band including the frequency "(n-Am)f0" by multiplying (mixing) an RF signal in a frequency band including the frequency "nf0" with a local oscillation signal of the frequency "mf0". The mixer 32 outputs the intermediate frequency band signal in a frequency band including the frequency "(n-Am)f0" to the demodulation unit 33a.

[0035] In addition, when the frequency "(n-Am)f0" of the intermediate frequency band signal output from the mixer 32 is equal to the allowable frequency "(nm)f0" of the demodulator 33a (when 1 multiplication "A=1"), the lightwave signal receiving unit 31a does not need to be equipped with the multiplier 314.

[0036] The demodulator 33a (demodulation circuit) performs a predetermined demodulation process on the intermediate frequency band signal in a frequency band including the frequency "(n-Am)f0". The demodulator 33a outputs the main signal obtained as a result of the demodulation process to a predetermined information processing device (not shown).

[0037] Next, an example of the operation of the communication system 1a will be described. 2 is a flowchart showing an example of the operation of the transmitter 2a in the first embodiment. The generator 20 generates a multi-wavelength optical signal (optical frequency comb) (step S101). The generator 20 selects an optical signal of a predetermined wavelength from among the multi-wavelength optical signals (step S102). The first signal generator 21a generates a main signal (step S103). The radio signal transmitter 22 transmits the main signal from the antenna 223 using radio waves as a first carrier wave (step S104). The lightwave signal transmitter 23a transmits a local oscillation signal from the collimator 231 using a lightwave of a predetermined wavelength as a second carrier wave (step S105).

[0038] 3 is a flowchart showing an example of the operation of the receiver 3a in the first embodiment. The radio signal receiving unit 30 acquires a main signal transmitted using radio waves as a first carrier wave (step S201). The light wave signal receiving unit 31a acquires a local oscillation signal transmitted using light waves as a second carrier wave (step S202). The mixer 32 uses the local oscillation signal to perform frequency conversion (down-conversion) of the main signal (step S203). The demodulation unit 33a executes demodulation processing on the frequency-converted main signal (e.g., an intermediate frequency band signal) (step S204).

[0039] As described above, the radio signal transmitter 22 transmits the main signal using radio waves as the first carrier wave. The lightwave signal transmitter 23a transmits the local oscillation signal used for frequency conversion of the main signal in the receiver 3a using light waves as the second carrier wave. For example, the lightwave signal transmitter 23a outputs an optical signal with a wavelength of "λ3" and an optical signal with a wavelength of "λ4" into free space as light waves corresponding to the local oscillation signal.

[0040] The radio signal receiving unit 30 receives a main signal transmitted using radio waves as a first carrier wave. The light wave signal receiving unit 31a receives a local oscillation signal transmitted using light waves as a second carrier wave. The light wave signal receiving unit 31a performs frequency conversion of the main signal using the local oscillation signal. The demodulation unit 33a performs demodulation processing on the frequency-converted main signal.

[0041] In this way, the main signal (wideband wireless signal) is transmitted by radio wave communication using millimeter waves, terahertz waves, or the like, which are excellent in speed and stability. A local oscillator signal (LO signal) required for demodulation is transmitted by free space optical communication, which enables secure communication. Here, only the receiver 3a located within the light wave beam acquires the local oscillator signal and demodulates the wideband wireless signal (main signal) such as terahertz waves.

[0042] This makes it possible to reduce the possibility that a receiver (not shown) other than the receiver 3a designated as the communication destination of the wireless signal demodulates the wireless signal, and also makes it possible to ensure a secure communication path.

[0043] (First Modification of the First Embodiment) The first modification of the first embodiment differs from the first embodiment in that an optical signal of a single wavelength "λ3" is modulated in the transmitter. The first modification of the first embodiment will be described focusing on the differences from the first embodiment.

[0044] 4 is a diagram showing an example of the configuration of a communication system 1b in a first modified example of the first embodiment. The communication system 1b is a system that performs wireless communication. The communication system 1b includes a transmitter 2b and a receiver 3b.

[0045] The transmitter 2b includes a generator 20, a first signal generator 21b, a radio signal transmitter 22, a lightwave signal transmitter 23b, and a second signal generator 24b. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23b includes a collimator 231 and a second optical modulator 232.

[0046] The second signal generating unit 24b generates a modulation signal. The frequency of this modulation signal is frequency "f1". The frequency "f1" is lower than the frequency "nf0" of the radio wave transmitted from the antenna 223. The second signal generating unit 24b may obtain the modulation signal from an external information processing device (not shown). The second signal generating unit 24b outputs the modulation signal to the second optical modulator 232.

[0047] The optical filter 202 outputs an optical signal with wavelength "λ3" to the second optical modulator 232. The second optical modulator 232 modulates the amplitude of the optical signal with wavelength "λ3" using a modulation signal with frequency "f1". The second optical modulator 232 outputs the optical signal modulated by the modulation signal with frequency "f1" to the collimator 231. The collimator 231 outputs the optical signal modulated by the modulation signal with frequency "f1" to free space as a light wave corresponding to the local oscillation signal.

[0048] The collimator 311 outputs a light wave corresponding to the local oscillation signal of frequency "f1" to the photoelectric conversion element 312b. The photoelectric conversion element 312b performs photoelectric conversion on the light wave corresponding to the local oscillation signal. The photoelectric conversion element 312b generates an electrical signal corresponding to the local oscillation signal of frequency "f1" as a result of the photoelectric conversion.

[0049] The mixer 32 generates an intermediate frequency band signal in a frequency band including the frequency "nf0-Af1" by multiplying (mixing) an RF signal in a frequency band including the frequency "nf0" with a local oscillation signal of a frequency "Af1". The mixer 32 outputs the intermediate frequency band signal in a frequency band including the frequency "nf0-Af1" to the demodulation unit 33b.

[0050] As described above, the optical filter 202 outputs an optical signal of a single wavelength to the second optical modulator 232. The second optical modulator 232 modulates the amplitude of the optical signal of a single wavelength using a modulation signal of a predetermined frequency. The second optical modulator 232 outputs the optical signal modulated by the modulation signal of the predetermined frequency to the collimator 231. The collimator 231 outputs the optical signal modulated by the modulation signal of the predetermined frequency to free space as a light wave corresponding to the local oscillation signal.

[0051] The collimator 311 outputs a light wave corresponding to a local oscillation signal of a predetermined frequency to the photoelectric conversion element 312b. The photoelectric conversion element 312b performs photoelectric conversion using the light wave corresponding to the local oscillation signal. The photoelectric conversion element 312b generates an electrical signal corresponding to the local oscillation signal of a predetermined frequency as a result of the photoelectric conversion. The mixer 32 generates an intermediate frequency band signal by multiplying (mixing) the RF signal and the local oscillation signal.

[0052] This makes it possible to reduce the possibility that a receiver other than the intended receiver of the wireless signal demodulates the wireless signal, and also ensure a secure communication path.

[0053] (Second Modification of the First Embodiment) In the first embodiment and the first modified example of the first embodiment, a multi-wavelength optical signal is used in the transmitter. In the second modified example of the first embodiment, a multi-wavelength optical signal (optical frequency comb) is not used, and a terahertz wave electrical signal is generated by the first signal generating unit, which is a difference from the first embodiment. The second modified example of the first embodiment will be described focusing on the difference from the first embodiment.

[0054] 5 is a diagram showing a configuration example of a communication system 1c in a second modified example of the first embodiment. The communication system 1c is a system that performs wireless communication. The communication system 1c includes a transmitter 2c and a receiver 3c.

[0055] The transmitter 2c includes a first signal generating section 21c, a radio wave signal transmitting section 22, a light wave signal transmitting section 23c, and a second signal generating section 24c. The radio wave signal transmitting section 22 includes an antenna 223. The light wave signal transmitting section 23c includes a collimator 231.

[0056] The first signal generating unit 21c generates a main signal (data signal) and a signal for controlling the wavelength of a light wave (hereinafter referred to as a "wavelength control signal"). The first signal generating unit 21c may acquire the main signal and the wavelength control signal from an external information processing device (not shown). The first signal generating unit 21c outputs a main signal (electrical signal) in a frequency band including the frequency "f1" to the antenna 223. The antenna 223 transmits a radio wave corresponding to the main signal as a wireless signal in a frequency band including the frequency "f1".

[0057] The first signal generating unit 21c outputs the wavelength control signal to the second signal generating unit 24c. The second signal generating unit 24c generates an optical signal with a wavelength "λ3" and an optical signal with a wavelength "λ4" based on the wavelength control signal. The second signal generating unit 24c outputs the optical signal with a wavelength "λ3" and an optical signal with a wavelength "λ4" to the collimator 231. The collimator 231 outputs the optical signal with a wavelength "λ3" and an optical signal with a wavelength "λ4" into free space as light waves corresponding to the local oscillation signal.

[0058] The mixer 32 generates an intermediate frequency band signal in a frequency band including the frequency "f1-Amf0" by multiplying (mixing) an RF signal in a frequency band including the frequency "f1" with a local oscillation signal of the frequency "Amf0". The mixer 32 outputs the intermediate frequency band signal in a frequency band including the frequency "f1-Amf0" to the demodulation unit 33b.

[0059] As described above, the first signal generating unit 21c outputs a main signal (electrical signal) in a frequency band including a predetermined frequency to the antenna 223. The antenna 223 transmits radio waves corresponding to the main signal as a wireless signal in a frequency band including the predetermined frequency. The second signal generating unit 24c generates optical signals of each predetermined wavelength based on the wavelength control signal. The second signal generating unit 24c outputs the optical signals of each predetermined wavelength to the collimator 231.

[0060] The mixer 32 generates an intermediate frequency band signal by multiplying (mixing) an RF signal in a frequency band including a predetermined frequency by a local oscillation signal. The mixer 32 outputs the intermediate frequency band signal in a frequency band including the frequency "f1-Amf0" to the demodulation unit 33b.

[0061] This makes it possible to reduce the possibility that a receiver other than the intended receiver of the wireless signal demodulates the wireless signal, and also ensure a secure communication path.

[0062] Second embodiment The second embodiment differs from the first modified example of the first embodiment in that not only a local oscillator signal (LO signal) is transmitted by free space optical communication, but also a sub-signal such as an encryption key used for demodulating or decrypting a main signal (data signal) is transmitted by free space optical communication. The second embodiment will be described focusing on the differences from the first modified example of the first embodiment.

[0063] 6 is a diagram showing an example of the configuration of a communication system 1d in the second embodiment. The communication system 1d is a system that performs wireless communication. The communication system 1d includes a transmitter 2d and a receiver 3d.

[0064] The transmitter 2d includes a generator 20, a first signal generator 21d, a radio signal transmitter 22, a lightwave signal transmitter 23d, and a second signal generator 24d. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23d includes a collimator 231 and a second optical modulator 232.

[0065] The receiver 3d includes a radio signal receiving unit 30, a lightwave signal receiving unit 31d, a mixer 32, and a demodulation unit 33d. The mixer 32 may be included in the lightwave signal receiving unit 31d. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31d includes a collimator 311, a photoelectric conversion element 312d, an amplifier 313, and a multiplier 314. The demodulation unit 33d includes a separation unit 331, a sub-signal demodulation unit 332, and a main signal demodulation unit 333.

[0066] The first signal generating unit 21d generates a main signal (data signal). The first signal generating unit 21d may acquire the main signal from an external information processing device (not shown). The first signal generating unit 21d outputs the main signal to the first optical modulator 221. The first signal generating unit 21d generates a sub-signal. The first signal generating unit 21d outputs the sub-signal to the second optical modulator 232. The sub-signal is, for example, an encryption key (key information) used for demodulation or decryption. The encryption key may be periodically updated. The encryption key may be, for example, a code used for scrambling the main signal, or a code used for decryption processing in the physical layer. The encryption key may be, for example, a hash key. The hash key is output to the second optical modulator 232, for example, at the start of communication.

[0067] The second signal generating unit 24d outputs a modulation signal of frequency "f1" to the second optical modulator 232. The second optical modulator 232 modulates the amplitude of an optical signal of wavelength "λ3" using the modulation signal of frequency "f1" and a sub-signal (e.g., an encryption key). That is, the second optical modulator 232 generates an optical signal corresponding to the sub-signal by superimposing the sub-signal on the optical signal of wavelength "λ3". The second optical modulator 232 outputs an optical signal corresponding to the sub-signal to the collimator 231.

[0068] The separator 331 obtains an intermediate frequency band signal in a frequency band including the frequency "nf0-Af1" from the mixer 32. The separator 331 separates an undemodulated sub-signal (e.g., an encryption key) from the intermediate frequency band signal. The sub-signal demodulator 332 performs a demodulation process on the undemodulated sub-signal. The sub-signal demodulator 332 outputs the demodulated sub-signal to the main signal demodulator 333. The main signal demodulator 333 uses the demodulated sub-signal to perform a demodulation process on the intermediate frequency band signal to generate a demodulated main signal. The main signal demodulator 333 outputs the demodulated main signal to a predetermined information processing device (not shown).

[0069] As described above, the second optical modulator 232 generates an optical signal corresponding to the sub-signal by superimposing the sub-signal on an optical signal of a predetermined wavelength. The second optical modulator 232 outputs the optical signal corresponding to the sub-signal to the collimator 231. The demodulator 33d generates a demodulated main signal by executing a demodulation process on the intermediate frequency band signal using the demodulated sub-signal (e.g., an encryption key).

[0070] This makes it possible to further reduce the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal demodulates the wireless signal. It is possible to ensure a more secure communication path. In addition, key information and the like required for decryption may be transmitted by free space optical communication that enables secure communication.

[0071] (First modified example of the second embodiment) In the first modified example of the second embodiment, the difference from the second embodiment is that the demodulator has a functional unit that decrypts the encrypted main signal. In the first modified example of the second embodiment, the difference from the second embodiment will be mainly described.

[0072] 7 is a diagram showing a configuration example of a communication system 1e in a first modified example of the second embodiment. The communication system 1e is a system that performs wireless communication. The communication system 1e includes a transmitter 2e and a receiver 3e.

[0073] The receiver 3e includes a radio signal receiving unit 30, a lightwave signal receiving unit 31e, a mixer 32, and a demodulation unit 33d. The mixer 32 may be included in the lightwave signal receiving unit 31e. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31e includes a collimator 311, a photoelectric conversion element 312e, an amplifier 313, and a multiplier 314. The demodulation unit 33e includes a separation unit 331, a sub-signal demodulation unit 332, a main signal demodulation unit 333, and a main signal decoding unit 334.

[0074] The sub-signal demodulation unit 332 performs a demodulation process on the undemodulated sub-signal. The sub-signal demodulation unit 332 outputs the demodulated sub-signal to the main signal decoding unit 334. The main signal demodulation unit 333 performs a demodulation process on the intermediate frequency band signal using the demodulated sub-signal to generate a demodulated main signal. The main signal demodulation unit 333 outputs the demodulated main signal to the main signal decoding unit 334. The main signal decoding unit 334 performs a decoding process on the demodulated main signal using the demodulated sub-signal (e.g., an encryption key). The main signal decoding unit 334 outputs the decoded main signal to a predetermined information processing device (not shown).

[0075] As described above, the primary signal decoder 334 executes a decryption process on the demodulated primary signal using the demodulated secondary signal (e.g., an encryption key). This makes it possible to further reduce the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal demodulates or decodes the wireless signal. It is also possible to ensure a more secure communication path.

[0076] (Second modified example of the second embodiment) The second modification of the second embodiment differs from the first modification of the second embodiment in that a local oscillation signal in which high frequency components have been cut (suppressed) is used for demodulating the sub-signal. The second modification of the second embodiment will be described focusing on the differences from the first modification of the second embodiment.

[0077] 8 is a diagram showing a configuration example of a communication system 1f in a second modified example of the second embodiment. The communication system 1f is a system that performs wireless communication. The communication system 1f includes a transmitter 2f and a receiver 3f.

[0078] The receiver 3f includes a radio signal receiving unit 30, a lightwave signal receiving unit 31f, a mixer 32, a demodulation unit 33f, a separation unit 34, and a filter 35. The mixer 32 may be included in the lightwave signal receiving unit 31f. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31f includes a collimator 311, a photoelectric conversion element 312f, an amplifier 313, and a multiplier 314. The demodulation unit 33f includes a sub-signal demodulation unit 332, a main signal demodulation unit 333, and a main signal decoding unit 334.

[0079] The separator 34 receives a local oscillation signal (electrical signal) from the multiplier 314. The separator 34 separates an undemodulated sub-signal (e.g., an encryption key) from the input locally oscillating signal. The separator 34 outputs the locally oscillating signal from which the sub-signal has been separated to the mixer 32. The filter 35 (low-pass filter) cuts (suppresses) high-frequency components of the undemodulated sub-signal. The filter 35 outputs the sub-signal from which the high-frequency components have been cut (undemodulated sub-signal) to the sub-signal demodulator 332. The sub-signal demodulator 332 performs demodulation processing on the undemodulated sub-signal.

[0080] As described above, the separator 34 separates the unmodulated sub-signal (e.g., the encryption key) from the locally oscillating signal. The filter 35 outputs the sub-signal from which the high frequency components have been cut to the sub-signal demodulator 332. The sub-signal demodulator 332 performs demodulation processing using the sub-signal from which the high frequency components have been cut. The sub-signal demodulator 332 outputs the demodulated sub-signal to the main signal decoder 334.

[0081] This makes it possible to further reduce the possibility that a receiver other than the intended receiver of the wireless signal will demodulate the wireless signal, thereby ensuring a more secure communication path.

[0082] Third embodiment The third embodiment differs from the first embodiment in that the frequency of the radio wave and the frequency of the light wave vary over time. The third embodiment will be described focusing on the differences from the first embodiment.

[0083] 9 is a diagram showing an example of the configuration of a communication system 1g in the third embodiment. The communication system 1g is a system that performs wireless communication. The communication system 1g includes a transmitter 2g and a receiver 3g.

[0084] The transmitter 2g includes a generator 20, a first signal generator 21g, a radio signal transmitter 22, and a lightwave signal transmitter 23g. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23g includes a collimator 231.

[0085] The receiver 3g includes a radio signal receiving unit 30, a lightwave signal receiving unit 31g, a mixer 32, and a demodulator 33g. The mixer 32 may be included in the lightwave signal receiving unit 31g. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31g includes a collimator 311, a photoelectric conversion element 312g, an amplifier 313, and a multiplier 314.

[0086] Next, the transmitter 2g will be described in detail. Optical frequency comb generator 201 outputs a multi-wavelength optical signal with wavelength spacing "Δλ" to optical filter 202. Here, the clock frequency of optical frequency comb generator 201 changes over time.

[0087] The optical filter 202 outputs an optical signal of wavelength "λ5" to the first optical modulator 221. The optical filter 202 outputs an optical signal of wavelength "λ6" to the photoelectric conversion element 222. The difference "λ6-λ5" between the wavelengths "λ5" and "λ6" corresponds to the frequency "f1". The optical filter 202 outputs an optical signal of wavelength "λ7" and an optical signal of wavelength "λ8" to the collimator 231. The difference "λ8-λ7" between the wavelengths "λ7" and "λ8" corresponds to the frequency "f2". The frequency "f2" changes with the passage of time. In the third embodiment, the difference "f1-f2" between the frequencies "f1" and "f2" is constant.

[0088] The antenna 223 generates a radio signal (terahertz signal) using at least one of millimeter waves and terahertz waves as a carrier wave of the main signal in response to an electrical signal in a frequency band including a frequency "f1" that changes over time. The antenna 223 transmits the radio signal using radio waves such as terahertz waves as a carrier wave of the main signal as a wideband radio signal. That is, the antenna 223 transmits radio waves corresponding to the main signal as a radio signal in a frequency band including a frequency "f1" that changes over time.

[0089] An optical signal with a wavelength of "λ7" and an optical signal with a wavelength of "λ8" are input to the collimator 231 from the optical filter 202. The collimator 231 outputs the optical signal with a wavelength of "λ7" and the optical signal with a wavelength of "λ8" as light waves corresponding to a local oscillation signal into free space. Here, the collimator 231 outputs light waves with a beam width equal to or smaller than a predetermined width into free space.

[0090] Fig. 10 is a diagram showing a first example of frequency change in the third embodiment. In Fig. 10, the frequency "f1" and the frequency "f2" change in a step-like manner as time passes, as an example.

[0091] Fig. 11 is a diagram showing a second example of frequency change in the third embodiment. In Fig. 11, the frequency "f1" and the frequency "f2" change over time in a sinusoidal shape, for example.

[0092] Next, the receiver 3g will be described in detail. The photoelectric conversion element 312g performs photoelectric conversion using light waves corresponding to the local oscillation signal. As a result of the photoelectric conversion, the photoelectric conversion element 312g generates an electrical signal corresponding to the local oscillation signal. The photoelectric conversion element 312g converts an optical signal of frequency "f2" corresponding to the difference "λ8-λ7" between the wavelengths "λ7" and "λ8" into a local oscillation signal (electrical signal) of frequency "f2". The amplifier 313 amplifies the amplitude (intensity) of the local oscillation signal of frequency "f2", which changes over time.

[0093] The mixer 32 generates an intermediate frequency band signal in a frequency band including a certain frequency "f1-f2" by multiplying (mixing) an RF signal in a frequency band including a frequency "f1" with a local oscillation signal of a frequency "f2". The mixer 32 outputs the intermediate frequency band signal in a frequency band including the certain frequency "f1-f2" to the demodulation unit 33g.

[0094] As described above, the radio signal transmitter 22 generates a radio signal using at least one of millimeter waves and terahertz waves as a carrier wave of a main signal in response to an electrical signal in a frequency band including the frequency "f1" that changes over time. The collimator 231 outputs an optical signal with a wavelength "λ7" and an optical signal with a wavelength "λ8" as light waves in response to a local oscillation signal into free space. The amplifier 313 amplifies the amplitude (intensity) of the local oscillation signal with the frequency "f2" that changes over time. The mixer 32 generates an intermediate frequency band signal in a frequency band including a constant frequency "f1-f2" by multiplying an RF signal in a frequency band including the frequency "f1" by a local oscillation signal with the frequency "f2".

[0095] This makes it possible to further reduce the possibility that a receiver other than the intended receiver of the wireless signal will demodulate the wireless signal, thereby ensuring a more secure communication path.

[0096] (Fourth embodiment) The fourth embodiment differs from the first modified example of the second embodiment in that data is distributed to a main signal or a sub-signal depending on the importance (type) of the data to be transmitted. The fourth embodiment will be described focusing on the differences from the first modified example of the second embodiment.

[0097] 12 is a diagram showing an example of the configuration of a communication system 1h in the fourth embodiment. The communication system 1h is a system that performs wireless communication. The communication system 1h includes a transmitter 2h and a receiver 3h.

[0098] The transmitter 2h includes a generator 20, a first signal generator 21h, a radio signal transmitter 22, a lightwave signal transmitter 23h, and a second signal generator 24h. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The first signal generator 21h includes a distributor 211, a main signal generator 212, and a sub-signal generator 213. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23h includes a collimator 231 and a second optical modulator 232.

[0099] The allocating unit 211 acquires data to be transmitted. The allocating unit 211 allocates the data to be transmitted to a predetermined communication path (radio signal transmitting unit, light wave signal transmitting unit) according to the importance (priority) of the data to be transmitted. Here, the allocating unit 211 determines the importance of the data according to, for example, a bit value at a specific position in an Ethernet (registered trademark) frame that includes the data to be transmitted. The allocating unit 211 may determine the importance of the data according to, for example, a communication port in the application layer.

[0100] In the fourth embodiment, the allocating unit 211 allocates data to be transmitted to the main signal generating unit 212 or the sub signal generating unit 213 depending on the importance of the data to be transmitted. For example, the allocating unit 211 allocates data whose importance is less than a threshold to the main signal generating unit 212. For example, the allocating unit 211 allocates data whose importance is equal to or greater than a threshold to the sub signal generating unit 213. Data whose importance is equal to or greater than a threshold is, for example, control data, log data, or personal data.

[0101] The main signal generating unit 212 obtains data whose importance is less than the threshold value from the allocating unit 211. The main signal generating unit 212 outputs to the first optical modulator 221 the main signal including the data whose importance is less than the threshold value.

[0102] The sub-signal generation unit 213 acquires data whose importance is equal to or greater than a threshold value from the allocation unit 211. The sub-signal generation unit 213 outputs to the second optical modulator 232 a sub-signal including the data whose importance is equal to or greater than a threshold value.

[0103] As described above, the allocating unit 211 acquires data to be transmitted. The allocating unit 211 allocates the data to be transmitted to the radio signal transmitting unit 22 or the light wave signal transmitting unit 23h according to the importance (priority) of the data to be transmitted.

[0104] This makes it possible to further reduce the possibility that a receiver other than the intended receiver of the wireless signal will demodulate the wireless signal. Since data is assigned to the main signal or the sub-signal depending on the importance of the data to be transmitted, it is possible to ensure a more secure communication path.

[0105] (Modification of the fourth embodiment) The difference between the fourth embodiment and the modified example of the fourth embodiment is that a local oscillation signal in which high frequency components have been cut (suppressed) is used for demodulating the sub-signal. The difference between the fourth embodiment and the modified example of the fourth embodiment will be mainly described.

[0106] 13 is a diagram showing a configuration example of a communication system 1i in a modification of the fourth embodiment. The communication system 1i is a system that performs wireless communication. The communication system 1i includes a transmitter 2i and a receiver 3i.

[0107] The receiver 3i includes a radio signal receiving unit 30, a lightwave signal receiving unit 31i, a mixer 32, a demodulation unit 33i, a separation unit 34, and a filter 35. The mixer 32 may be included in the lightwave signal receiving unit 31i. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31i includes a collimator 311, a photoelectric conversion element 312i, an amplifier 313, and a multiplier 314. The demodulation unit 33i includes a sub-signal demodulation unit 332, a main signal demodulation unit 333, and a main signal decoding unit 334.

[0108] The local oscillation signal is input to the separator 34 from the multiplier 314. The separator 34 separates the undemodulated sub-signal (e.g., an encryption key, personal data) from the local oscillation signal. The separator 34 outputs the local oscillation signal from which the sub-signal has been separated to the mixer 32. The filter 35 (low-pass filter) cuts (suppresses) high-frequency components of the undemodulated sub-signal. The filter 35 outputs the sub-signal from which the high-frequency components have been cut (the undemodulated sub-signal) to the sub-signal demodulation unit 332.

[0109] As described above, separator 34 separates the undemodulated sub signal from the local oscillation signal. Separator 34 outputs the local oscillation signal from which the sub signal has been separated to mixer 32. Filter 35 cuts high frequency components from the undemodulated sub signal. Filter 35 outputs the sub signal from which the high frequency components have been cut (undemoulated sub signal) to sub signal demodulator 332.

[0110] This makes it possible to further reduce the possibility that a receiver other than the intended receiver of the wireless signal will demodulate the wireless signal, thereby ensuring a more secure communication path.

[0111] Fifth embodiment The fifth embodiment differs from the fourth embodiment in that a local oscillation signal is transmitted using radio waves or light waves as a carrier wave depending on the atmospheric conditions in free space, and a main signal is transmitted using radio waves or light waves as a carrier wave depending on the atmospheric conditions in free space, etc. The fifth embodiment will be described focusing on the differences from the fourth embodiment.

[0112] 14 is a diagram showing an example of the configuration of a communication system 1j in the fifth embodiment. The communication system 1j is a system that performs wireless communication. The communication system 1j includes a transmitter 2j and a receiver 3j.

[0113] The transmitter 2j includes a generator 20, a first signal generator 21j, a radio signal transmitter 22, a lightwave signal transmitter 23j, a second signal generator 24j, and a switch 25. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23j includes a collimator 231 and a second optical modulator 232.

[0114] The receiver 3j includes a radio signal receiving unit 30, a lightwave signal receiving unit 31j, a mixer 32, and a demodulator 33j. The mixer 32 may be included in the lightwave signal receiving unit 31j. The radio signal receiving unit 30 includes an antenna 301 and an amplifier 302. The lightwave signal receiving unit 31j includes a collimator 311, a photoelectric conversion element 312j, and an amplifier 313.

[0115] Depending on the atmospheric conditions in free space and the conditions of use of the carrier frequency band, the losses of radio waves such as millimeter waves and terahertz waves and light waves may vary. For example, in free space optical communication, light waves are easily dispersed due to the effects of scintillation and the atmosphere in free space. Therefore, the switching unit 25 acquires information indicating each condition from, for example, an external information processing device (not shown). The switching unit 25 may switch between the communication path of the local oscillation signal and the communication paths of the main signal and the sub signal depending on each condition.

[0116] For example, the switching unit 25 may output the local oscillation signal output from the second signal generating unit 24j to the first optical modulator 221 so that the local oscillation signal is transmitted using radio waves as a carrier wave. When the local oscillation signal is transmitted using radio waves as a carrier wave, the amplifier 302 outputs the local oscillation signal, the amplitude of which has been amplified, to the mixer 32.

[0117] For example, the switching unit 25 may output the main signal and the sub signal output from the first signal generating unit 21j to the second optical modulator 232 so that the main signal and the sub signal are transmitted using a light wave as a carrier wave. When the main signal and the sub signal are transmitted using a light wave as a carrier wave, the photoelectric conversion element 312j generates an electrical signal corresponding to the local oscillation signal as a result of the photoelectric conversion.

[0118] As described above, the switching unit 25 switches between the communication path of the local oscillation signal and the communication paths of the main signal and the sub signal according to the atmospheric conditions in free space and the use conditions of the carrier frequency band. This makes it possible to further reduce the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal demodulates the wireless signal. It is also possible to ensure a more secure communication path.

[0119] Sixth embodiment The sixth embodiment differs from the first embodiment in that a photoelectric conversion element that operates at high speed at the frequency of an RF signal (carrier frequency) is used in the receiver. In the modification of the fifth embodiment, the differences from the first embodiment will be mainly described.

[0120] 15 is a diagram showing an example of the configuration of a communication system 1k in the sixth embodiment. The communication system 1k is a system that performs wireless communication. The communication system 1k includes a transmitter 2k and a receiver 3k.

[0121] The transmitter 2k includes a generator 20, a first signal generator 21k, a radio signal transmitter 22, and a lightwave signal transmitter 23k. The generator 20 includes an optical frequency comb generator 201 and an optical filter 202. The radio signal transmitter 22 includes a first optical modulator 221, an opto-electrical conversion element 222, and an antenna 223. The lightwave signal transmitter 23k includes a collimator 231.

[0122] The receiver 3k includes a radio signal receiving section 30, a light wave signal receiving section 31k, and a demodulation section 33k. The radio signal receiving section 30 includes an antenna 301 and an amplifier 302. The light wave signal receiving section 31k includes a collimator 311 and a photoelectric conversion element 312k.

[0123] The photoelectric conversion element 312k is a photoelectric conversion element that operates at high speed at the frequency of the RF signal (carrier frequency), and is, for example, a photodiode or a photoconductor. An RF signal (received signal) in a frequency band including the frequency "nf0" is input from the amplifier 302 to the photoelectric conversion element 312k. A light wave corresponding to a local oscillation signal of the frequency "mf0" is input from the collimator 311 to the photoelectric conversion element 312k.

[0124] The photoelectric conversion element 312k performs photoelectric conversion on an RF signal in a frequency band including the frequency "nf0" using a light wave corresponding to a local oscillation signal of frequency "mf0". The photoelectric conversion element 312k (down converter) generates an intermediate frequency band signal in a frequency band including the frequency "(nm)f0" as a result of the photoelectric conversion. The photoelectric conversion element 312k outputs the intermediate frequency band signal in a frequency band including the frequency "(nm)f0" to the demodulation unit 33k.

[0125] As described above, the photoelectric conversion element 312k performs photoelectric conversion on the RF signal using the lightwave corresponding to the local oscillation signal. This makes it possible to further reduce the possibility that a receiver other than the receiver designated as the communication destination of the wireless signal demodulates the wireless signal using a down converter with a simple device configuration in which the lightwave corresponding to the local oscillation signal is applied to the RF signal as a lightwave. It is also possible to ensure a more secure communication path.

[0126] FIG. 16 is a diagram showing an example of a hardware configuration of the communication device 100 in each embodiment. The communication device 100 corresponds to at least one of the transmitter and the receiver in each of the above-mentioned embodiments. Some or all of the functional units of the communication device 100 are realized as software by a processor 101 such as a CPU (Central Processing Unit) executing a program stored in a memory 103 having a non-volatile recording medium (non-transient recording medium). The program may be recorded in a computer-readable recording medium. The computer-readable recording medium is, for example, a portable medium such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a CD-ROM (Compact Disc Read Only Memory), or a non-transient recording medium such as a storage device 102 such as a hard disk built into a computer system. The communication unit 104 may communicate data and programs using a communication line.

[0127] Some or all of the functional units of communication device 100 may be realized using hardware including electronic circuits (electronic circuits or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0128] In the above-mentioned embodiment, the program implemented in the above-mentioned form does not depend on a single device, but may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed to perform image processing. Note that the term "computer system" here includes hardware such as an OS and peripheral devices. The term "computer system" also includes a WWW system equipped with a homepage providing environment (or display environment). The term "computer-readable recording medium" also includes a memory that holds a program for a certain period of time, such as a volatile memory (RAM) inside a computer system that becomes a server or client when a program is transmitted via a network such as the Internet or a communication line such as a telephone line.

[0129] The above program may also be transmitted from a computer system in which the program is stored in a storage device or the like to another computer system via a transmission medium, or by transmission waves in the transmission medium. Here, the "transmission medium" that transmits the program refers to a medium that has the function of transmitting information, such as a network (communication network) such as the Internet or a communication line (communication line) such as a telephone line. The above program may also be for realizing part of the above-mentioned functions. Furthermore, it may be a so-called difference file (difference program) that can realize the above-mentioned functions in combination with a program already recorded in the computer system.

[0130] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included.

[0131] The above embodiments may be combined. [Industrial Applicability]

[0132] The present invention is applicable to wireless communication systems. [Explanation of symbols]

[0133] 1a,1b,1c,1d,1e,1f,1g,1h,1i,1j,1k...Communication system, 2a,2b,2c,2d,2e,2f,2g,2h,2i,2j,2k...Transmitter,3a,3b,3c,3d , 3e, 3f, 3g, 3h, 3i, 3j, 3k...receiver, 20...generation section, 21a, 21b, 21c, 21d, 21e, 21f, 21g, 21h, 21i, 21j, 21k...first signal generation section, 22... Radio signal transmitting unit, 23a, 23b, 23c, 23d, 23e, 23f, 23g, 23h, 23i, 23j, 23k... Light wave signal transmitting unit, 24b, 24c, 24d, 24e, 24f, 24h, 24i... Second signal generating unit, 25... Switching unit, 30... Radio signal receiving unit, 31a, 31b, 31c, 31d, 31e, 31f, 31g, 31h, 31i, 31j, 31k... Light wave signal receiving unit, 32... Mixer, 33a, 33b, 33c, 33d, 33e, 33f, 33g, 33h, 33i, 33j, 33k... demodulation unit, 34... separation unit, 35... filter, 100... communication device, 101... processor, 102... storage device, 103... memory, 104... communication unit, 201... optical frequency comb generation unit, 202... optical filter, 211... distribution unit, 212... main signal generation unit, 213... sub-signal generation unit, 221... first optical modulator, 222... photoelectric conversion element child, 223...antenna, 231...collimator, 232...second optical modulator, 301...antenna, 302...amplifier, 311...collimator, 312a, 312b, 312c, 312d, 312e, 312f, 312g, 312h, 312i, 312j, 312k...photoelectric conversion elements, 313...amplifier, 314...multiplier, 331...separator, 332...sub-signal demodulator, 333...main signal demodulator, 334...main signal decoder

Claims

1. a radio signal transmitting unit that transmits a main signal using radio waves as a first carrier wave; a lightwave signal transmitting section for transmitting a local oscillation signal used for frequency conversion of the main signal in the receiver by using a lightwave as a second carrier wave; A transmitter comprising:

2. a radio signal receiving unit that receives a main signal transmitted using radio waves as a first carrier wave; a lightwave signal receiving unit that receives a local oscillation signal transmitted using a lightwave as a second carrier wave and performs frequency conversion of the main signal using the local oscillation signal; a demodulation unit that performs demodulation processing on the frequency-converted main signal; A receiver comprising:

3. A communication system comprising a transmitter and a receiver, The transmitter includes: a radio signal transmitting unit that transmits a main signal using radio waves as a first carrier wave; a lightwave signal transmitting unit that transmits a local oscillation signal used for frequency conversion of the main signal in the receiver by using a lightwave as a second carrier wave, The receiver includes: a radio signal receiving unit for acquiring the main signal transmitted using the radio wave as the first carrier wave; a lightwave signal receiving unit that receives the local oscillation signal transmitted using the lightwave as the second carrier wave and performs frequency conversion of the main signal using the local oscillation signal; A demodulation unit that performs demodulation processing on the frequency-converted main signal. Communication systems.

4. the radio wave is at least one of a millimeter wave and a terahertz wave, The light waves are infrared light. The communication system according to claim 3.

5. The light wave signal transmitting unit transmits the light wave using a collimator. The communication system according to claim 3 or 4.

6. A method of communication performed by a transmitter, comprising: a radio signal transmitting step of transmitting a main signal using radio waves as a first carrier wave; a lightwave signal transmitting step of transmitting a local oscillation signal used for frequency conversion of the main signal in a receiver by using a lightwave as a second carrier wave; A communication method including:

7. A receiver-implemented communication method, comprising: a radio signal receiving step of acquiring a main signal transmitted using radio waves as a first carrier wave; a local oscillation signal acquisition step of acquiring a local oscillation signal transmitted using the light wave as a second carrier wave; receiving a lightwave signal, performing a frequency conversion of the main signal using the local oscillator signal; a demodulation step of performing a demodulation process on the frequency-converted main signal; A communication method including:

8. A communication method performed by a communication system including a transmitter and a receiver, comprising: The transmitter includes: Transmitting a main signal using radio waves as a first carrier wave; a local oscillation signal used for frequency conversion of the main signal in the receiver is transmitted using a light wave as a second carrier wave; The receiver includes: Acquire the main signal transmitted using the radio wave as the first carrier wave; Acquire the local oscillation signal transmitted using the light wave as the second carrier wave; performing frequency conversion of the main signal using the local oscillator signal; performing a demodulation process on the frequency-converted main signal; Communication methods.

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