Optical communication system, earth station device, satellite station device, and communication method

Abstract

This optical communication system comprises an earth station device, and a satellite station device that performs relay processing for relaying information transmitted and received between a communication terminal and the earth station device, wherein, in the relay processing, the satellite station device performs information transmission with the communication terminal using a radio frequency (RF) signal, and performs information transmission with the earth station device using an optical signal including an analog signal.

Description

Optical communication system, earth station equipment, satellite station equipment, and communication method

[0001] This application claims priority based on Japanese Patent Application No. 2023-122576, filed on July 27, 2023, the disclosure of which is incorporated herein in its entirety by reference.

[0002] Non-Patent Document 1 discloses a communication system conforming to a direct access method, in which a radio repeater is mounted on a HAPS (High Altitude Platform Station), which is an example of a satellite station device.

[0003] Yuki Hokazono and two others, "Efforts towards practical application of HAPS in space RAN," NTT Technical Journal, Nippon Telegraph and Telephone Corporation, October 1, 2022, Vol. 34, No. 10, pp. 17-22

[0004] The optical communication system disclosed herein comprises an earth station device and a satellite station device that performs a relay process to relay information transmitted and received between a communication terminal and the earth station device, and in the relay process, the satellite station device transmits information between the communication terminal and the satellite station device using an RF (Radio Frequency) signal, and transmits information between the satellite station device and the earth station device using an optical signal including an analog signal.

[0005] One aspect of the present disclosure may be realized not only as an earth station device having such a characteristic processing unit, but also as a communication method including such characteristic processing steps, or as a program for causing a computer to execute such steps. Furthermore, one aspect of the present disclosure may be realized as a semiconductor integrated circuit that realizes part or all of the earth station device.

[0006] One aspect of the present disclosure may be realized not only as a satellite station device having such a characteristic processing unit, but also as a communication method having such characteristic processing steps, or as a program for causing a computer to execute such steps. Furthermore, one aspect of the present disclosure may be realized as a semiconductor integrated circuit that realizes part or all of the satellite station device.

[0007] Fig. 1 is a diagram showing a configuration of an optical communication system according to a first embodiment of the present disclosure. Fig. 2 is a diagram showing a configuration of an earth station device according to the first embodiment of the present disclosure. Fig. 3 is a diagram showing a configuration of a satellite station device according to the first embodiment of the present disclosure. Fig. 4 is a diagram showing an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure. Fig. 5 is a diagram showing a configuration of an optical communication system according to a second embodiment of the present disclosure. Fig. 6 is a diagram showing a configuration of an earth station device according to the second embodiment of the present disclosure. Fig. 7 is a diagram showing a configuration of a satellite station device according to the second embodiment of the present disclosure.

[0008] 2. Description of the Related Art Conventionally, in order to expand the coverage area of ​​a communication system, a technique of relaying radio waves using a satellite station device has been used.

[0009] [Problem to be Solved by the Present Disclosure] There is a need for a technology that goes beyond the technology described in Non-Patent Document 1 and that can increase the capacity of the feeder link between the earth station equipment and the satellite station equipment while realizing miniaturization and low power consumption of the satellite station equipment.

[0010] The present disclosure has been made to solve the above-mentioned problems, and its purpose is to provide an optical communication system, an earth station device, a satellite station device, and a communication method that can increase the capacity of the feeder link between the earth station device and the satellite station device while realizing a smaller satellite station device with lower power consumption.

[0011] Effect of the Present Disclosure According to the present disclosure, it is possible to increase the capacity of the feeder link between the earth station device and the satellite station device while realizing miniaturization and low power consumption of the satellite station device.

[0012] [Description of Embodiments of the Present Disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0013] (1) An optical communication system according to an embodiment of the present disclosure includes an earth station device and a satellite station device that performs a relay process to relay information transmitted and received between a communication terminal and the earth station device, and in the relay process, the satellite station device transmits information between the communication terminal and the earth station device using an RF signal, and transmits information between the satellite station device and the earth station device using an optical signal including an analog signal.

[0014] In this way, by using optical signals containing analog signals for information transmission between the satellite station equipment and the earth station equipment, the capacity of the feeder link between the earth station equipment and the satellite station equipment can be increased compared to a configuration using RF signals for information transmission between the satellite station equipment and the earth station equipment. Furthermore, compared to a configuration using optical signals containing digital signals for information transmission between the satellite station equipment and the earth station equipment, digital-to-analog conversion processing is not required in the satellite station equipment, and optical signals can be transmitted using fewer optical transceivers, which makes it possible to reduce the size and power consumption of the satellite station equipment and reduce delays in relay processing. Therefore, it is possible to achieve a reduction in the size and power consumption of the satellite station equipment while increasing the capacity of the feeder link between the earth station equipment and the satellite station equipment.

[0015] (2) In the above (1), the earth station device may generate a first optical signal including the first analog signal and transmit the generated first optical signal to the satellite station device, and the satellite station device may acquire the first analog signal from the first optical signal received from the earth station device and transmit a first RF signal based on the acquired first analog signal to the communication terminal.

[0016] This configuration increases the capacity of the downstream feeder link between the earth station and the satellite station. Furthermore, compared to a configuration in which optical signals containing digital signals are used to transmit information from the earth station to the satellite station, this configuration eliminates the need for digital-to-analog conversion in the satellite station and allows optical signals to be received using fewer optical transceivers, thereby enabling the satellite station to be smaller, consume less power, and reduce relay processing delays.

[0017] (3) In the above (2), the earth station equipment may generate the first optical signal that is intensity-modulated, phase-modulated, or frequency-modulated based on the first analog signal, and transmit the generated first optical signal to the satellite station equipment.

[0018] With this configuration, when generating a first optical signal that is intensity-modulated based on the first analog signal, the first optical signal can be generated with a simple configuration. Furthermore, when generating a first optical signal that is phase-modulated or frequency-modulated based on the first analog signal, the first optical signal can be transmitted and received with higher sensitivity between the earth station and the satellite station than when using an intensity-modulated first optical signal.

[0019] (4) In the above (2) or (3), the earth station equipment may generate a plurality of the first optical signals having different wavelengths and transmit the generated plurality of first optical signals to the satellite station equipment.

[0020] With this configuration, it is possible to further increase the capacity of the feeder link for downstream communication between the earth station equipment and the satellite station equipment.

[0021] (5) In any of (1) to (4) above, the satellite station device may receive a second RF signal from the communication terminal, generate a second optical signal including a second analog signal based on the received second RF signal, and transmit the generated second optical signal to the earth station device, and the earth station device may obtain the second analog signal from the second optical signal received from the satellite station device.

[0022] This configuration increases the capacity of the feeder link for upstream communication between the earth station and the satellite station. Furthermore, compared to a configuration in which optical signals containing digital signals are used to transmit information from the satellite station to the earth station, this configuration eliminates the need for digital conversion of RF signals in the satellite station and allows optical signals to be transmitted using fewer optical transceivers, thereby enabling the satellite station to be made smaller, consume less power, and reduce relay processing delays.

[0023] (6) In the above (5), the satellite station equipment may generate the second optical signal that is intensity-modulated, phase-modulated, or frequency-modulated based on the second analog signal, and transmit the generated second optical signal to the earth station equipment.

[0024] With this configuration, when generating a second optical signal that is intensity-modulated based on the second analog signal, the second optical signal can be generated with a simple configuration. Furthermore, when generating a second optical signal that is phase-modulated or frequency-modulated based on the second analog signal, the second optical signal can be transmitted and received with higher sensitivity between the earth station equipment and the satellite station equipment than when using an intensity-modulated second optical signal.

[0025] (7) In the above (5) or (6), the satellite station equipment may generate a plurality of the second optical signals having different wavelengths and transmit the generated second optical signals to the earth station equipment.

[0026] With this configuration, it is possible to further increase the capacity of the feeder link for upstream communication between the earth station equipment and the satellite station equipment.

[0027] (8) In the above (1), the earth station device may generate a first optical signal including the first analog signal and transmit the first optical signal to the satellite station device via a first optical antenna; the satellite station device may receive the first optical signal from the earth station device via a second optical antenna, obtain the first analog signal from the received first optical signal, and transmit the first RF signal based on the obtained first analog signal to the communication terminal; the satellite station device may receive a second RF signal from the communication terminal, generate a second optical signal including the second analog signal based on the received second RF signal, and transmit the second optical signal to the earth station device via the second optical antenna; and the earth station device may receive the second optical signal from the satellite station device via the first optical antenna and obtain the second analog signal from the received second optical signal.

[0028] With this configuration, optical signals can be transmitted and received between the earth station equipment and the satellite station equipment using a smaller number of optical antennas.

[0029] (9) In the above (1), the earth station device may generate a first optical signal including the first analog signal and transmit the generated first optical signal to the satellite station device; the satellite station device may acquire the first analog signal from the first optical signal received from the earth station device and transmit the first RF signal based on the acquired first analog signal to the communication terminal; the satellite station device may receive a second RF signal from the communication terminal, generate a second optical signal including the second analog signal based on the received second RF signal and transmit the generated second optical signal to the earth station device; the earth station device may acquire the second analog signal from the second optical signal received from the satellite station device; the satellite station device may relay the first analog signal and the second analog signal between the earth station device and the communication terminal according to a bent pipe method and transmit and receive the first RF signal and the second RF signal to and from the communication terminal according to a direct access method.

[0030] With this configuration, relay processing between the communication terminal and the earth station equipment can be performed with a simple configuration.

[0031] (10) An earth station device according to an embodiment of the present disclosure includes an optical communication unit that transmits and receives signals to and from a communication terminal via a satellite station device that transmits and receives RF signals to and from the communication terminal, and the optical communication unit transmits optical signals including analog signals to the satellite station device and / or receives optical signals including analog signals from the satellite station device.

[0032] In this way, by transmitting optical signals containing analog signals between the earth station and the satellite station, the capacity of the feeder link between the earth station and the satellite station can be increased compared to a configuration in which RF signals are transmitted between the earth station and the satellite. Furthermore, compared to a configuration in which optical signals containing digital signals are transmitted between the earth station and the satellite, digital-to-analog conversion processing is not required in the satellite station, and optical signals can be transmitted using fewer optical transceivers, which makes it possible to reduce the size and power consumption of the satellite station and reduce delays in relay processing. Therefore, it is possible to reduce the size and power consumption of the satellite station while increasing the capacity of the feeder link between the earth station and the satellite.

[0033] (11) A satellite station device according to an embodiment of the present disclosure is a satellite station device that performs a relay process to relay information transmitted and received between a communication terminal and an earth station device, and includes an RF communication unit that transmits information using RF signals with the communication terminal, and an optical communication unit that transmits information using optical signals including analog signals with the earth station device.

[0034] In this way, by using optical signals containing analog signals for information transmission between the satellite station equipment and the earth station equipment, the capacity of the feeder link between the earth station equipment and the satellite station equipment can be increased compared to a configuration using RF signals for information transmission between the satellite station equipment and the earth station equipment. Furthermore, compared to a configuration using optical signals containing digital signals for information transmission between the satellite station equipment and the earth station equipment, digital-to-analog conversion processing is not required in the satellite station equipment, and optical signals can be transmitted using fewer optical transceivers, which makes it possible to reduce the size and power consumption of the satellite station equipment and reduce delays in relay processing. Therefore, it is possible to achieve a reduction in the size and power consumption of the satellite station equipment while increasing the capacity of the feeder link between the earth station equipment and the satellite station equipment.

[0035] (12) A communication method according to an embodiment of the present disclosure is a communication method in an optical communication system including an earth station device and a satellite station device that performs a relay process to relay information transmitted and received between a communication terminal and the earth station device, and includes a step in which the satellite station device transmits information to and from the communication terminal using an RF signal, and a step in which the satellite station device transmits information to and from the earth station device using an optical signal including an analog signal.

[0036] In this way, by using optical signals containing analog signals to transmit information between a satellite station and an earth station, the capacity of the feeder link between the earth station and the satellite station can be increased compared to a method using RF signals to transmit information between the satellite station and the earth station. Furthermore, compared to a method using optical signals containing digital signals to transmit information between a satellite station and an earth station, digital-to-analog conversion processing is not required in the satellite station and optical signals can be transmitted using fewer optical transceivers, which makes it possible to reduce the size and power consumption of the satellite station and reduce delays in relay processing. Therefore, it is possible to reduce the size and power consumption of the satellite station while increasing the capacity of the feeder link between the earth station and the satellite station.

[0037] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, identical or corresponding parts are designated by the same reference numerals, and their description will not be repeated. Furthermore, at least some of the embodiments described below may be combined in any manner.

[0038] 1 is a diagram showing the configuration of an optical communication system according to a first embodiment of the present disclosure. Referring to FIG. 1, the optical communication system 301 includes an earth station 101 and a satellite station 201.

[0039] The earth station device 101 is installed on the earth. More specifically, the earth station device 101 is installed in the troposphere, for example, on the ground. The earth station device 101 may be installed on the sea or may be a portable device that can be mounted on a moving object such as an aircraft. The earth station device 101 and one or more communication terminals 121 transmit and receive communication data. For example, the communication terminal 121 is a mobile communication terminal located on the earth.

[0040] The satellite station device 201 is an artificial satellite station located outside the troposphere. For example, the satellite station device 201 is located in the stratosphere. The satellite station device 201 performs a relay process to relay information transmitted and received between the communication terminal 121 and the earth station device 101. In the relay process, the satellite station device 201 transmits information to and from the communication terminal 121 using RF signals, and transmits information to and from the earth station device 101 using optical signals including analog signals.

[0041] More specifically, the earth station 101 receives an analog signal SAd including communication data from the base station 111. The earth station 101 generates a downstream optical signal SPd1 including the received analog signal SAd, and transmits the generated downstream optical signal SPd1 to the satellite station 201. The analog signal SAd is an example of a first analog signal. The downstream optical signal SPd1 is an example of a first optical signal.

[0042] For example, the earth station 101 generates a plurality of downstream optical signals SPd1 having different wavelengths. More specifically, the earth station 101 receives a plurality of analog signals SAd from the base station 111 via a plurality of transmission paths. The earth station 101 generates downstream optical signals SPd1a and SPd1b including the received analog signals SAd, and transmits the generated downstream optical signals SPd1a and SPd1b to the satellite station 201. The downstream optical signal SPd1a is the downstream optical signal SPd1 having a wavelength λ1a. The downstream optical signal SPd1b is the downstream optical signal SPd1 having a wavelength λ1b. The earth station 101 may generate and transmit three or more downstream optical signals SPd1 having different wavelengths.

[0043] The satellite station 201 acquires a plurality of analog signals SAd from the downstream optical signals SPd1a and SPd1b received from the earth station 101, and transmits an RF signal SRd based on the acquired plurality of analog signals SAd to the communication terminal 121.

[0044] The satellite station equipment 201 also receives an RF signal SRu containing communication data from the communication terminal 121. The satellite station equipment 201 generates an upstream optical signal SPu1 containing an analog signal SAu based on the received RF signal SRu, and transmits the generated upstream optical signal SPu1 to the earth station equipment 101. The analog signal SAu is an example of a second analog signal. The upstream optical signal SPu1 is an example of a second optical signal.

[0045] For example, the satellite station equipment 201 generates a plurality of upstream optical signals SPu1 having different wavelengths. More specifically, the satellite station equipment 201 receives a plurality of RF signals SRu from a plurality of communication terminals 121, respectively, and generates a plurality of analog signals SAu based on the received RF signals SRu. The satellite station equipment 201 generates upstream optical signals SPu1a and SPu1b including the generated analog signals SAu, and transmits the generated upstream optical signals SPu1a and SPu1b to the earth station equipment 101. The upstream optical signal SPu1a is the upstream optical signal SPu1 having a wavelength λ2a. The upstream optical signal SPu1b is the upstream optical signal SPu1 having a wavelength λ2b. Note that the satellite station equipment 201 may generate and transmit three or more upstream optical signals SPu1 having different wavelengths. For example, the wavelengths λ1a and λ1b of the downstream optical signals SPd1a and SPd1b and the wavelengths λ2a and λ2b of the upstream optical signals SPu1a and SPu1b are different from each other.

[0046] The earth station 101 acquires a plurality of analog signals SAu from the upstream optical signals SPu1a and SPd1b received from the satellite station 201. The earth station 101 transmits the acquired plurality of analog signals SAu to the base station 111.

[0047] The satellite station equipment 201 relays the analog signals SAd and SAu in accordance with the bent pipe method between the earth station equipment 101 and the communication terminal 121. More specifically, for example, the satellite station equipment 201 performs frequency conversion and amplification of the signals, but transmits an RF signal SRd based on the analog signal SAd to the communication terminal 121 without modulating or demodulating the analog signals SAd and SAu, and transmits an upstream optical signal SPu1 including the analog signal SAu to the earth station equipment 101.

[0048] Furthermore, the satellite station device 201 transmits and receives RF signals SRd and SRu to and from the communication terminal 121 in accordance with the direct access method. More specifically, the satellite station device 201 transmits the RF signal SRd directly to the communication terminal 121 and receives the RF signal SRu directly from the communication terminal 121 without going through another base station.

[0049] 2 is a diagram illustrating a configuration of an earth station device according to the first embodiment of the present disclosure. Referring to FIG. 2, the earth station device 101 includes an analog transceiver unit 10 and an optical communication unit 20. The optical communication unit 20 includes an optical modulator unit 21, an optical demodulator unit 22, optical amplifiers 23A and 23B, an optical separator unit 24, an optical antenna 25, and a tracking unit 26. The optical antenna 25 is an example of a first antenna. The analog transceiver unit 10 and the tracking unit 26 are partly or entirely realized by a processing circuit including one or more processors, for example.

[0050] The optical communication unit 20 transmits and receives signals to and from the communication terminal 121 via the satellite station equipment 201. The optical communication unit 20 transmits a downstream optical signal SPd1 to the satellite station equipment 201 and receives an upstream optical signal SPu1 from the satellite station equipment 201. The optical antenna 25 is formed of, for example, a lens.

[0051] FIG. 3 is a diagram illustrating the configuration of a satellite station device according to the first embodiment of the present disclosure. Referring to FIG. 3 , the satellite station device 201 includes an RF communication unit 30 and an optical communication unit 40. The RF communication unit 30 includes an RF antenna 31, an RF separation unit 32, amplifiers 33A and 33B, and frequency conversion units 34A and 34B. The optical communication unit 40 includes an optical demodulation unit 41, an optical modulation unit 42, optical amplifiers 43A and 43B, an optical separation unit 44, an optical antenna 45, and a tracking unit 46. The optical antenna 45 is an example of a second antenna. Part or all of the tracking unit 46 is implemented, for example, by a processing circuit including one or more processors.

[0052] The RF communication unit 30 transmits information using RF signals SRd and SRu to and from the communication terminal 121. The optical communication unit 40 transmits information using a downstream optical signal SPd1 including an analog signal SAd and an upstream optical signal SPu1 including an analog signal SAu to and from the earth station equipment 101. The optical antenna 45 is formed of, for example, a lens.

[0053] 2 and 3 , the tracker 26 in the earth station device 101 periodically or irregularly transmits a laser beam for a beacon with a predetermined intensity via the optical antenna 25. Also, the tracker 46 in the satellite station device 201 periodically or irregularly transmits a laser beam for a beacon with a predetermined intensity via the optical antenna 45. Note that the tracker 26 may transmit the laser beam for a beacon via an optical antenna other than the optical antenna 25. Also, the tracker 46 may transmit the laser beam for a beacon via an optical antenna other than the optical antenna 45.

[0054] The tracking unit 46 in the satellite station 201 acquires the position of the earth station 101 in accordance with a predetermined control algorithm for acquiring and tracking the earth station 101, and adjusts the transmission direction of the laser light from the optical antenna 45 so as to track the earth station 101. More specifically, the tracking unit 46 measures the amount of laser light received by the optical antenna 45 from the laser light transmitted from the optical antenna 25, and detects the position of the earth station 101 based on the measurement result. The tracking unit 46 adjusts the orientation of the optical antenna 45 so as to be able to transmit the laser light toward the detected position of the earth station 101.

[0055] Furthermore, the tracker 26 in the earth station 101 acquires the position of the satellite station 201 in accordance with a predetermined control algorithm for acquiring and tracking the satellite station 201, and adjusts the transmission direction of the laser light from the optical antenna 25 to track the satellite station 201. More specifically, the tracker 26 measures the amount of laser light received by the optical antenna 25 from the laser light transmitted from the optical antenna 45, and detects the position of the satellite station 201 based on the measurement result. The tracker 26 adjusts the orientation of the optical antenna 25 so that the laser light can be transmitted toward the detected position of the satellite station 201.

[0056] For example, the optical communication system 301 employs a TDD (Time Division Duplex) system. More specifically, in the optical communication system 301, a downlink communication period CPd, in which downlink communication from the base station device 111 to the communication terminal 121 is performed, and an uplink communication period CPu, in which uplink communication from the communication terminal 121 to the base station device 111 is performed, are switched and alternately repeated. For example, the satellite station device 201 performs relay processing according to TDD, which uses the timing of the clock of the satellite station device 201 as a reference. More specifically, the downlink communication period CPd and the uplink communication period CPu are set based on the transmission and reception timing of RF signals SRd and SRu by the RF antenna 31 in the satellite station device 201.

[0057] The earth station 101 transmits downstream optical signals SPd1a and SPd1b to the satellite station 201 during a downstream communication period CPd. The satellite station 201 also transmits upstream optical signals SPu1a and SPu1b to the earth station 101 during an upstream communication period CPu.

[0058] (Downstream Communication) The analog transceiver 10 in the earth station device 101 receives an analog signal SAd containing communication data from the base station device 111. The analog transceiver 10 outputs the received analog signal SAd to the optical modulator 21. The analog signal SAd may be an IF band signal or an RF band signal.

[0059] The optical modulation unit 21 receives the analog signal SAd from the analog transceiver 10 and generates downstream optical signals SPd1a and SPd1b by optically modulating the received analog signal SAd. The optical modulation unit 21 outputs the generated downstream optical signals SPd1a and SPd1b to the optical amplifier 23A.

[0060] For example, the optical modulation unit 21 generates downstream optical signals SPd1a and SPd1b that are intensity-modulated based on the analog signal SAd. More specifically, the optical modulation unit 21 is a direct modulation optical modulator including an LD (Laser Diode). The LD outputs downstream optical signals SPd1a and SPd1b to the optical amplifier 23A, the intensity of which corresponds to the level of the analog signal SAd received from the analog transceiver 10.

[0061] The optical modulation unit 21 may be an external modulation type optical modulator including an LD and an external modulator. In this case, the LD outputs light of a constant intensity to the external modulator. The external modulator uses the light received from the LD to output downstream optical signals SPd1a and SPd1b to the optical amplifier 23A, the intensity of which corresponds to the level of the analog signal SAd received from the analog transceiver 10. The external modulator may be, for example, an electroabsorption modulator or a Mach-Zehnder modulator.

[0062] The optical amplifier 23A amplifies the downstream optical signals SPd1a and SPd1b received from the optical modulation unit 21 and outputs the amplified signals to the optical separation unit 24.

[0063] The optical separator 24 outputs the downstream optical signals SPd1a and SPd1b received from the optical amplifier 23A to the optical antenna 25. For example, the optical separator 24 is a multiplexer / demultiplexer that outputs the light of wavelengths λ1a and λ1b received from the optical amplifier 23A to the optical antenna 25 and outputs the light of wavelengths λ2a and λ2b received from the optical antenna 25 to the optical amplifier 23B.

[0064] The earth station 101 transmits the downstream optical signals SPd1a and SPd1b to the satellite station 201 via the optical antenna 25. More specifically, the optical amplifier 23A transmits the amplified downstream optical signals SPd1a and SPd1b via the optical separator 24 and the optical antenna 25 to the satellite station 201.

[0065] The satellite station 201 receives the downstream optical signals SPd1a and SPd1b from the earth station 101 via the optical antenna 45. More specifically, the optical antenna 45 receives the downstream optical signals SPd1a and SPd1b from the earth station 101.

[0066] The optical separator 44 outputs the downstream optical signals SPd1a and SPd1b received by the optical antenna 45 to the optical amplifier 43A. For example, the optical separator 44 is a multiplexer / demultiplexer that outputs the light of wavelengths λ1a and λ1b received from the optical antenna 45 to the optical amplifier 43A and outputs the light of wavelengths λ2a and λ2b received from the optical amplifier 43B to the optical antenna 45.

[0067] The optical amplifier 43A receives the downstream optical signals SPd1a and SPd1b from the earth station 101 via the optical antenna 45 and the optical demultiplexer 44. The optical amplifier 43A amplifies the received downstream optical signals SPd1a and SPd1b and outputs them to the optical demodulator 41.

[0068] The optical demodulator 41 generates an analog signal SAd, which is an electrical signal having a level corresponding to the intensity of the downstream optical signals SPd1a and SPd1b received from the optical amplifier 43A. More specifically, the optical demodulator 41 is an optical demodulator including a photodiode (PD). The PD outputs the analog signal SAd, which is an electrical signal having a level corresponding to the intensity of the downstream optical signals SPd1a and SPd1b received from the optical amplifier 43A, to the frequency converter 34A. The PD may be, for example, a PIN photodiode or an avalanche photodiode.

[0069] The frequency converter 34A receives the analog signal SAd from the optical demodulator 41 and generates an RF signal SRd based on the received analog signal SAd. More specifically, the frequency converter 34A generates the RF signal SRd by up-converting or down-converting the frequency of the analog signal SAd. The frequency converter 34A outputs the generated RF signal SRd to the amplifier 33A.

[0070] Amplifying unit 33A amplifies RF signal SRd received from frequency converting unit 34A and outputs the amplified signal to RF separating unit 32.

[0071] During the downstream communication period CPd, the RF separator 32 outputs the RF signal SRd received from the amplifier 33A to the RF antenna 31. More specifically, the RF separator 32 is a switch capable of switching the connection between the amplifiers 33A and 33B and the RF antenna 31. In accordance with a control signal received from a switch controller (not shown), the RF separator 32 connects the amplifier 33A and the RF antenna 31 during the downstream communication period CPd, and connects the amplifier 33B and the RF antenna 31 during the upstream communication period CPu.

[0072] The amplifier 33 A transmits the amplified RF signal SRd to the communication terminal 121 via the RF separator 32 and the RF antenna 31 .

[0073] (Upstream Communication) The RF antenna 31 receives an RF signal SRu containing communication data from the communication terminal 121 .

[0074] During the upstream communication period CPu, the amplifier 33B receives the RF signal SRu from the communication terminal 121 via the RF antenna 31 and the RF separator 32. The amplifier 33B amplifies the received RF signal SRu and outputs it to the frequency converter 34B.

[0075] The frequency conversion unit 34B receives the RF signal SRu from the amplification unit 33B and generates an analog signal SAu based on the received RF signal SRu. More specifically, the frequency conversion unit 34B generates the analog signal SAu by down-converting or up-converting the frequency of the RF signal SRu. The analog signal SAu may be an IF band signal or an RF band signal. The frequency conversion unit 34B outputs the generated analog signal SAu to the optical modulation unit 42.

[0076] The optical modulator 42 receives the analog signal SAu from the frequency converter 34B, and generates upstream optical signals SPu1a and SPu1b by optically modulating the received analog signal SAu. The optical modulator 42 outputs the generated upstream optical signals SPu1a and SPu1b to the optical amplifier 43B.

[0077] For example, the optical modulation unit 42 generates intensity-modulated upstream optical signals SPu1a and SPu1b based on the analog signal SAu. More specifically, the optical modulation unit 42 is a direct modulation optical modulator including an LD. The LD outputs the upstream optical signals SPu1a and SPu1b to the optical amplifier 43B, with intensities corresponding to the level of the analog signal SAu received from the frequency conversion unit 34B.

[0078] The optical modulation unit 42 may be an external modulation type optical modulator including an LD and an external modulator. In this case, the LD outputs light of a constant intensity to the external modulator. The external modulator uses the light received from the LD to output upstream optical signals SPu1a and SPu1b to the optical amplifier 43B, the intensity of which corresponds to the level of the analog signal SAu received from the frequency conversion unit 34B. The external modulator may be, for example, an electroabsorption modulator or a Mach-Zehnder modulator.

[0079] The optical amplifier 43B amplifies the upstream optical signals SPu1a and SPu1b received from the optical modulation unit 42 and outputs the amplified signals to the optical separation unit 44.

[0080] The optical separator 44 outputs the upstream optical signals SPu1a and SPu1b received from the optical amplifier 43B to the optical antenna 45.

[0081] The satellite station 201 transmits the upstream optical signals SPu1a and SPu1b to the earth station 101 via the optical antenna 45. More specifically, the optical amplifier 43B transmits the amplified upstream optical signals SPu1a and SPu1b via the optical separator 44 and the optical antenna 45 to the earth station 101.

[0082] The earth station 101 receives the upstream optical signals SPu1a and SPu1b from the satellite station 201 via the optical antenna 25. More specifically, the optical antenna 25 receives the upstream optical signals SPu1a and SPu1b from the satellite station 201.

[0083] The optical separator 24 outputs the upstream optical signals SPu1a and SPu1b received by the optical antenna 25 to the optical amplifier 23B.

[0084] The optical amplifier 23B receives the upstream optical signals SPu1a and SPu1b from the satellite station equipment 201 via the optical antenna 25 and the optical demultiplexer 24. The optical amplifier 23B amplifies the received upstream optical signals SPu1a and SPu1b and outputs them to the optical demodulator 22.

[0085] The optical demodulator 22 generates an analog signal SAu, which is an electrical signal having a level corresponding to the intensities of the upstream optical signals SPu1a and SPu1b received from the optical amplifier 23B. More specifically, the optical demodulator 22 is an optical demodulator including a PD. The PD outputs an analog signal SAd, which is an electrical signal having a level corresponding to the intensities of the upstream optical signals SPu1a and SPu1b received from the optical amplifier 23B, to the analog transceiver 10. The PD may be, for example, a PIN photodiode or an avalanche photodiode.

[0086] The analog transceiver 10 transmits the analog signal SAu received from the optical demodulator 22 to the base station device 111 .

[0087] In conventional communication systems that use satellite station equipment, propagation paths are longer than in communication systems that do not use satellite station equipment, resulting in larger transmission delays. In contrast, the optical communication system 301 uses optical signals containing analog signals to transmit information between the earth station equipment 101 and the satellite station equipment 201, which makes it possible to reduce transmission delays caused by the satellite station equipment 201 compared to systems that use optical signals containing digital signals.

[0088] [Operation Flow] FIG. 4 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure.

[0089] Referring to FIG. 4, first, the earth station device 101 receives an analog signal SAd containing communication data from the base station device 111 (step S11).

[0090] Next, the earth station equipment 101 generates downstream optical signals SPd1a and SPd1b that are intensity-modulated based on the analog signal SAd (step S12).

[0091] Next, the earth station 101 transmits the amplified downstream optical signals SPd1a and SPd1b to the satellite station 201 via the optical antenna 25 (step S13).

[0092] Next, the satellite station 201 transmits information using the downstream optical signals SPd1a and SPd1b with the earth station 101. More specifically, the satellite station 201 receives the downstream optical signals SPd1a and SPd1b from the earth station 101 via the optical antenna 45, and acquires the analog signal SAd from the downstream optical signals SPd1a and SPd1b (step S14).

[0093] Next, the satellite station equipment 201 generates an RF signal SRd by up-converting the frequency of the analog signal SAd (step S15).

[0094] Next, the satellite station device 201 transmits information using the RF signal SRd to the communication terminal 121. More specifically, the satellite station device 201 transmits the amplified RF signal SRd to the communication terminal 121 via the RF antenna 31 during the downlink communication period CPd (step S16).

[0095] Next, the satellite station device 201 transmits information using the RF signal SRu to and from the communication terminal 121. More specifically, the satellite station device 201 receives the RF signal SRu containing communication data from the communication terminal 121 during the uplink communication period CPu (step S17).

[0096] Next, the satellite station equipment 201 generates an analog signal SAu by down-converting the frequency of the amplified RF signal SRu (step S18).

[0097] Next, the satellite station equipment 201 generates intensity-modulated upstream optical signals SPu1a and SPu1b based on the analog signal SAu (step S19).

[0098] Next, the satellite station 201 transmits information using the upstream optical signals SPu1a and SPu1b to the earth station 101. More specifically, the satellite station 201 transmits the amplified upstream optical signals SPu1a and SPu1b to the earth station 101 via the optical antenna 45 (step S20).

[0099] Next, the earth station 101 receives the upstream optical signals SPu1a and SPu1b from the satellite station 201 via the optical antenna 25, and acquires the analog signal SAu from the upstream optical signals SPu1a and SPu1b (step S21).

[0100] Next, the earth station equipment 101 transmits the analog signal SAu to the base station equipment 111 (step S22).

[0101] Although the optical communication system 301 according to the first embodiment of the present disclosure is configured to perform both downstream communication and upstream communication, this is not limiting. For example, the optical communication system 301 may be configured to perform downstream communication but not upstream communication. In this case, the satellite station device 201 does not receive the RF signal SRu from the communication terminal 121 or transmit the upstream optical signal SPu1 to the earth station device 101. Also, for example, the optical communication system 301 may be configured to perform upstream communication but not downstream communication. In this case, the satellite station device 201 does not receive the downstream optical signal SPd1 from the earth station device 101 or transmit the RF signal SRd to the communication terminal 121.

[0102] Furthermore, in the optical communication system 301 according to the first embodiment of the present disclosure, the earth station device 101 is configured to transmit a downstream optical signal SPd1 including an analog signal SAd to the satellite station device 201, and the satellite station device 201 is configured to transmit an upstream optical signal SPu1 including an analog signal SAu to the earth station device 101. However, this is not limited to this. Either the earth station device 101 or the satellite station device 201 may be configured to generate and transmit an optical signal including a digital signal. Specifically, for example, the earth station device 101 receives a digital signal including communication data from the base station device 111, generates a downstream optical signal including the received digital signal, and transmits the generated downstream optical signal to the satellite station device 201. Also, for example, the satellite station device 201 receives an RF signal SRu including communication data from the communication terminal 121, generates an upstream optical signal including a digital signal based on the received RF signal SRu, and transmits the generated upstream optical signal to the earth station device 101.

[0103] Furthermore, in the optical communication system 301 according to the first embodiment of the present disclosure, the satellite station device 201 is configured to perform relay processing in accordance with TDD based on the timing of the clock of the satellite station device 201. However, this is not limited to this. The satellite station device 201 may also be configured to perform relay processing in accordance with TDD based on the timing of the clock of the communication terminal 121. That is, the downlink communication period CPd and the uplink communication period CPu may be set based on the timing of transmission and reception of RF signals SRd and SRu by an RF antenna in the communication terminal 121. In this case, due to transmission delay between the satellite station device 201 and the communication terminal 121, it may be necessary for the satellite station device 201 to process the RF signals SRd and SRu in parallel. In this case, the satellite station device 201 may include, for example, a circulator instead of the RF separator 32, which is a switch.

[0104] In addition, in the optical communication system 301 according to the first embodiment of the present disclosure, the wavelengths λ1a and λ1b of the downstream optical signals SPd1a and SPd1b and the wavelengths λ2a and λ2b of the upstream optical signals SPu1a and SPu1b are different from each other, but this is not limited to this. For example, the wavelengths λ1a and λ2a may be the same, and the wavelengths λ1b and λ2b may be the same. In this case, for example, the earth station 101 includes an optical circulator instead of the optical demultiplexer 24, which is a multiplexer / demultiplexer, and the satellite station 201 includes an optical circulator instead of the optical demultiplexer 44, which is a multiplexer / demultiplexer.

[0105] In the optical communication system 301 according to the first embodiment of the present disclosure, the earth station device 101 is configured to generate a downstream optical signal SPd1a having a wavelength λ1a and a downstream optical signal SPd1b having a wavelength λ1b, but this is not limiting. The earth station device 101 may be configured to receive one analog signal SAd from the base station device 111 and generate a downstream optical signal SPd1 having a single wavelength that includes the received analog signal SAd.

[0106] In the optical communication system 301 according to the first embodiment of the present disclosure, the satellite station device 201 is configured to generate an upstream optical signal SPu1a having a wavelength λ2a and an upstream optical signal SPu1b having a wavelength λ2b, but this is not limiting. The satellite station device 201 may also be configured to receive one RF signal SRu from the communication terminal 121 and generate an upstream optical signal SPu1 having a single wavelength including an analog signal SAu based on the received RF signal SRu.

[0107] Furthermore, although the earth station device 101 according to the first embodiment of the present disclosure is configured to transmit downstream optical signals SPd1a and SPd1b to the satellite station device 201 and receive upstream optical signals SPu1a and SPu1b from the satellite station device 201 via a common optical antenna 25, this is not limitative. The earth station device 101 may be configured to include an optical antenna 25A that is an optical antenna 25 for transmitting the downstream optical signals SPd1a and SPd1b, and an optical antenna 25B that is an optical antenna 25 for receiving the upstream optical signals SPu1a and SPu1b. In this case, the earth station device 101 does not include the optical separator 24.

[0108] Furthermore, although the satellite station device 201 according to the first embodiment of the present disclosure is configured to receive downstream optical signals SPd1a and SPd1b from the earth station device 101 and transmit upstream optical signals SPu1a and SPu1b to the earth station device 101 via a common optical antenna 45, the configuration is not limited to this. The satellite station device 201 may be configured to separately include an optical antenna 45A that is an optical antenna 45 for receiving the downstream optical signals SPd1a and SPd1b, and an optical antenna 45B that is an optical antenna 45 for transmitting the upstream optical signals SPu1a and SPu1b. In this case, the satellite station device 201 does not include the optical separator 44.

[0109] Furthermore, although the satellite station device 201 according to the first embodiment of the present disclosure is configured to relay the analog signals SAd and SAu between the earth station device 101 and the communication terminal 121 according to the bent-pipe method, the present invention is not limited to this. The satellite station device 201 may be configured to perform some kind of signal processing, such as modulation and demodulation of the analog signals SAd and SAu.

[0110] Furthermore, although the satellite station device 201 according to the first embodiment of the present disclosure is configured to transmit and receive RF signals SRd and SRu to and from the communication terminal 121 according to the direct access method, the present disclosure is not limited to this. The satellite station device 201 may be configured to transmit the RF signal SRd to the communication terminal 121 via another base station, or may be configured to receive the RF signal SRu from the communication terminal 121 via another base station.

[0111] Furthermore, although the satellite station device 201 according to the first embodiment of the present disclosure has been described as including the frequency conversion units 34A and 34B, this is not limitative. The satellite station device 201 may also be configured without including the frequency conversion units 34A and 34B.

[0112] Next, other embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference numerals and their description will not be repeated.

[0113] Second Embodiment This embodiment relates to an optical communication system 302 in which a phase-modulated optical signal is transmitted, as compared with the optical communication system 301 according to the first embodiment. Except for the contents described below, the optical communication system 302 is the same as the optical communication system 301 according to the first embodiment.

[0114] 5 is a diagram illustrating a configuration of an optical communication system according to a second embodiment of the present disclosure. Compared to the optical communication system 301, the optical communication system 302 includes an earth station 102 instead of the earth station 101, and a satellite station 202 instead of the satellite station 201.

[0115] For example, the earth station 102 generates a plurality of downstream optical signals SPd2 having different wavelengths. More specifically, the earth station 102 generates a downstream optical signal SPd2a having a wavelength λ1a, and a downstream optical signal SPd2b having a wavelength λ1b, and transmits the generated downstream optical signals SPd2a and SPd2b to the satellite station 202. Note that the earth station 102 may generate and transmit three or more downstream optical signals SPd2 having different wavelengths.

[0116] Furthermore, for example, the satellite station equipment 202 generates a plurality of upstream optical signals SPu2 having different wavelengths. More specifically, the satellite station equipment 202 generates an upstream optical signal SPu2a that is an upstream optical signal SPu2 having a wavelength λ2a and an upstream optical signal SPu2b that is an upstream optical signal SPu2 having a wavelength λ2b, and transmits the generated upstream optical signals SPu2a and SPu2b to the earth station equipment 102. Note that the satellite station equipment 202 may generate and transmit three or more upstream optical signals SPu2 having different wavelengths.

[0117] 6 is a diagram illustrating a configuration of an earth station device according to a second embodiment of the present disclosure. Referring to FIG. 6, compared to the earth station device 101, the earth station device 102 includes an optical communication unit 50 instead of the optical communication unit 20, and further includes a processing unit 60. Compared to the optical communication unit 20, the optical communication unit 50 includes an optical modulation unit 51 instead of the optical modulation unit 21, and an optical demodulation unit 52 instead of the optical demodulation unit 22.

[0118] 7 is a diagram showing the configuration of a satellite station device according to the second embodiment of the present disclosure. Referring to FIG. 7 , satellite station device 202, compared to satellite station device 201, includes an RF communication unit 70 instead of RF communication unit 30 and an optical communication unit 80 instead of optical communication unit 40. Compared to RF communication unit 30, RF communication unit 70 further includes a processing unit 71. Compared to optical communication unit 40, optical communication unit 80 includes an optical demodulation unit 81 instead of optical demodulation unit 41 and an optical modulation unit 82 instead of optical modulation unit 42.

[0119] 6 and 7, for example, the optical modulation unit 51 in the earth station device 102 generates downstream optical signals SPd2a and SPd2b that are phase-modulated based on the analog signal SAd. More specifically, the optical modulation unit 51 is an external modulation optical modulator including an LD and an external modulator. In this case, the LD outputs light of a constant intensity at a frequency ω1a to the external modulator. The external modulator uses the light received from the LD to output downstream optical signals SPd2a and SPd2b to the optical amplifier 23A, the phases of which correspond to the level of the analog signal SAd received from the analog transceiver 10. The external modulator is, for example, a lithium niobate modulator.

[0120] The optical amplifier 23 A amplifies the downstream optical signals SPd 2 a and SPd 2 b received from the optical modulation unit 51 , and transmits the amplified downstream optical signals SPd 2 a and SPd 2 b via the optical separator 24 and the optical antenna 25 to the satellite station equipment 202 .

[0121] The optical amplifier 43A in the satellite station equipment 202 receives the downstream optical signals SPd2a and SPd2b from the earth station equipment 102 via the optical antenna 45 and the optical separator 44. The optical amplifier 43A amplifies the received downstream optical signals SPd2a and SPd2b and outputs them to the optical demodulator 81.

[0122] The optical demodulator 81 generates a phase-modulated signal, which is an electrical signal having a phase corresponding to the phase of the downstream optical signals SPd2a and SPd2b received from the optical amplifier 43A. More specifically, the optical demodulator 81 is an optical demodulator including a PD. The PD receives a beat signal obtained by interfering the downstream optical signals SPd2a and SPd2b with a local optical signal having a frequency ω2a, and outputs a phase-modulated signal whose carrier frequency corresponds to the absolute value of the difference between the frequencies ω1a and ω2a to the processing unit 71. The PD may be, for example, a PIN photodiode, an avalanche photodiode, or a balanced photodiode.

[0123] The processing unit 71 receives the phase-modulated signal from the optical demodulation unit 81 and generates an analog signal SAd, which is an electrical signal with a level corresponding to the phase of the received phase-modulated signal. The processing unit 71 outputs the generated analog signal SAd to the frequency conversion unit 34A.

[0124] The frequency conversion unit 34A receives the analog signal SAd from the processing unit 71, generates an RF signal SRd based on the received analog signal SAd, and outputs the generated RF signal SRd to the amplification unit 33A.

[0125] Amplifying unit 33A amplifies RF signal SRd received from frequency converting unit 34A, and transmits the amplified RF signal SRd to communication terminal 121 via RF separating unit 32 and RF antenna 31.

[0126] (Uplink Communication) During the uplink communication period CPu, the amplifier 33B receives the RF signal SRu from the communication terminal 121 via the RF antenna 31 and the RF separator 32. The amplifier 33B amplifies the received RF signal SRu and outputs it to the frequency converter 34B.

[0127] The frequency conversion unit 34B receives the RF signal SRu from the amplification unit 33B, generates an analog signal SAu based on the received RF signal SRu, and outputs the generated analog signal SAu to the optical modulation unit 82.

[0128] The optical modulation unit 82 generates phase-modulated upstream optical signals SPu2a and SPu2b based on the analog signal SAu. More specifically, the optical modulation unit 82 is an external modulation optical modulator including an LD and an external modulator. In this case, the LD outputs light of a constant intensity at a frequency ω1b to the external modulator. The external modulator uses the light received from the LD to output upstream optical signals SPu2a and SPu2b to the optical amplifier 43B, the phases of which correspond to the level of the analog signal SAd received from the frequency conversion unit 34B. The external modulator is, for example, a lithium niobate modulator.

[0129] The optical amplifier 43 B amplifies the upstream optical signals SPu 2 a and SPu 2 b received from the optical modulator 82 , and transmits the amplified upstream optical signals SPu 2 a and SPu 2 b via the optical separator 44 and the optical antenna 45 to the earth station equipment 102 .

[0130] The optical amplifier 23B in the earth station 102 receives the upstream optical signals SPu2a and SPu2b from the satellite station 202 via the optical antenna 25 and the optical demultiplexer 24. The optical amplifier 23B amplifies the received upstream optical signals SPu2a and SPu2b and outputs them to the optical demodulator 52.

[0131] The optical demodulator 52 generates a phase-modulated signal, which is an electrical signal having a phase corresponding to the phase of the upstream optical signals SPu2a and SPu2b received from the optical amplifier 23B. More specifically, the optical demodulator 52 is an optical demodulator including a PD. The PD receives a beat signal obtained by interfering the upstream optical signals SPu2a and SPu2b with a local optical signal having a frequency ω2b, and outputs a phase-modulated signal whose carrier frequency corresponds to the absolute value of the difference between the frequencies ω1b and ω2b to the processing unit 60. The PD may be, for example, a PIN photodiode, an avalanche photodiode, or a balanced photodiode.

[0132] The processing unit 60 receives the phase-modulated signal from the optical demodulation unit 52 and generates an analog signal SAu, which is an electrical signal with a level corresponding to the phase of the received phase-modulated signal. The processing unit 60 outputs the generated analog signal SAu to the analog transceiver unit 10.

[0133] The analog transceiver 10 transmits the analog signal SAu received from the processor 60 to the base station device 111 .

[0134] In the optical communication system 302 according to the second embodiment of the present disclosure, the earth station 102 is configured to generate downstream optical signals SPd2a and SPd2b that are phase-modulated based on the analog signal SAd and transmit the downstream optical signals SPd2a and SPd2b to the satellite station 202. However, this is not limiting. The earth station 102 may be configured to generate downstream optical signals SPd3a and SPd3b that are frequency-modulated based on the analog signal SAd instead of the downstream optical signals SPd2a and SPd2b and transmit the downstream optical signals SPd3a and SPd3b to the satellite station 202.

[0135] Alternatively, the earth station 102 may be configured to generate downstream optical signals SPd1a and SPd1b intensity-modulated based on the analog signal SAd, instead of the downstream optical signals SPd2a and SPd2b, and transmit the downstream optical signals SPd1a and SPd1b to the satellite station 202. In this case, the earth station 102 includes the optical modulator 21 described above, instead of the optical modulator 51.

[0136] Furthermore, in the optical communication system 302 according to the second embodiment of the present disclosure, the satellite station device 202 is configured to generate upstream optical signals SPu2a and SPu2b that are phase-modulated based on the analog signal SAu and transmit the upstream optical signals SPu2a and SPu2b to the earth station device 102. However, this is not limiting. The satellite station device 202 may be configured to generate upstream optical signals SPu3a and SPu3b that are frequency-modulated based on the analog signal SAd instead of the upstream optical signals SPu2a and SPu2b and transmit the upstream optical signals SPu3a and SPu3b to the earth station device 102.

[0137] Alternatively, the satellite station equipment 202 may be configured to generate upstream optical signals SPu1a and SPu1b that are intensity-modulated based on the analog signal SAd, instead of the upstream optical signals SPu2a and SPu2b, and transmit the upstream optical signals SPu1a and SPu1b to the earth station equipment 102. In this case, the satellite station equipment 202 includes the optical modulation unit 42 described above, instead of the optical modulation unit 82.

[0138] In the optical communication system 302 according to the second embodiment of the present disclosure, the earth station 102 is configured to generate the downstream optical signal SPd2a having a wavelength λ1a and the downstream optical signal SPd2b having a wavelength λ1b, but this is not limiting. The earth station 102 may be configured to generate the downstream optical signal SPd2 having a single wavelength.

[0139] In the optical communication system 302 according to the second embodiment of the present disclosure, the satellite station device 202 is configured to generate an upstream optical signal SPu1a having a wavelength λ2a and an upstream optical signal SPu2b having a wavelength λ2b, but this is not limiting. The satellite station device 202 may also be configured to generate an upstream optical signal SPu2 having a single wavelength.

[0140] The above-described embodiments should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0141] Each process (each function) in the above-described embodiments is realized by a processing circuit (circuitry) including one or more processors. The processing circuit may be configured as an integrated circuit or the like that combines one or more memories, various analog circuits, and various digital circuits in addition to the one or more processors. The one or more memories store programs (instructions) that cause the one or more processors to execute each of the processes. The one or more processors may execute each of the processes according to the program read from the one or more memories, or may execute each of the processes according to a logic circuit designed in advance to execute each of the processes. The processor may be any of various processors suitable for computer control, such as a CPU (Central Processing Unit), a GPU (Graphics Processing Unit), a DSP (Digital Signal Processor), an FPGA (Field Programmable Gate Array), and an ASIC (Application Specific Integrated Circuit). Note that the physically separated processors may cooperate with each other to execute the processes. For example, the processors installed in the physically separated computers may cooperate with each other via a network such as a LAN (Local Area Network), a WAN (Wide Area Network), or the Internet to execute the processes. The program may be installed into the memory from an external server device or the like via the network, or may be distributed in a state stored on a recording medium such as a CD-ROM (Compact Disc Read Only Memory), a DVD-ROM (Digital Versatile Disc Read Only Memory), or a semiconductor memory, and then installed into the memory from the recording medium.

[0142] The above description includes the following additional features: [Supplementary Note 1] An optical communication system comprising: an earth station device; and a satellite station device that performs a relay process for relaying information transmitted and received between a communication terminal and the earth station device, wherein, in the relay process, the satellite station device transmits information using an RF signal with the communication terminal, and transmits information using an optical signal including an analog signal with the earth station device, the earth station device is located in the troposphere, and the satellite station device is located in the stratosphere, and the satellite station device performs the relay process in accordance with a TDD method that uses the timing of a clock in the satellite station device as a reference.

[0143] DESCRIPTION OF SYMBOLS 10 Analog transceiver 20 Optical communication unit 21 Optical modulation unit 22 Optical demodulation unit 23A, 23B Optical amplifier 24 Optical separation unit 25 Optical antenna 26 Tracking unit 30 RF communication unit 31 RF antenna 32 RF separation unit 33A, 33B Amplification unit 34A, 34B Frequency conversion unit 40 Optical communication unit 41 Optical demodulation unit 42 Optical modulation unit 43A, 43B Optical amplifier 44 Optical separation unit 45 Optical antenna 46 Tracking unit 50 Optical communication unit 51 Optical modulation unit 52 Optical demodulation unit 60 Processing unit 70 RF communication unit 71 Processing unit 80 Optical communication unit 81 Optical demodulation unit 82 Optical modulation unit 101, 102 Earth station equipment 111 Base station equipment 121 Communication terminal 201, 202 Satellite station equipment 301, 302 Optical communication system

Claims

1. An optical communications system comprising: an earth station; and a satellite station that performs a relay process to relay information transmitted and received between a communication terminal and the earth station, wherein, in the relay process, the satellite station transmits information between the communication terminal and the satellite station using an RF (Radio Frequency) signal, and transmits information between the satellite station and the earth station using an optical signal including an analog signal.

2. The optical communication system of claim 1, wherein the earth station equipment generates a first optical signal including the first analog signal and transmits the generated first optical signal to the satellite station equipment, and the satellite station equipment acquires the first analog signal from the first optical signal received from the earth station equipment and transmits a first RF signal based on the acquired first analog signal to the communication terminal.

3. The optical communication system according to claim 2, wherein the earth station equipment generates the first optical signal that is intensity modulated, phase modulated or frequency modulated based on the first analog signal, and transmits the generated first optical signal to the satellite station equipment.

4. An optical communication system according to claim 2 or 3, wherein said earth station equipment generates a plurality of said first optical signals having different wavelengths and transmits said generated plurality of said first optical signals to said satellite station equipment.

5. An optical communication system as described in any one of claims 1 to 4, wherein the satellite station equipment receives a second RF signal from the communication terminal, generates a second optical signal including a second analog signal based on the received second RF signal, and transmits the generated second optical signal to the earth station equipment, and the earth station equipment obtains the second analog signal from the second optical signal received from the satellite station equipment.

6. The optical communication system according to claim 5, wherein the satellite station equipment generates the second optical signal which is intensity modulated, phase modulated or frequency modulated based on the second analog signal, and transmits the generated second optical signal to the earth station equipment.

7. An optical communication system according to claim 5 or 6, wherein the satellite station equipment generates a plurality of second optical signals having different wavelengths and transmits the generated plurality of second optical signals to the earth station equipment.

8. The optical communication system of claim 1, wherein the earth station device generates a first optical signal including the first analog signal and transmits the first optical signal to the satellite station device via a first optical antenna; the satellite station device receives the first optical signal from the earth station device via a second optical antenna, acquires the first analog signal from the received first optical signal, and transmits a first RF signal based on the acquired first analog signal to the communication terminal; the satellite station device receives a second RF signal from the communication terminal, generates a second optical signal including the second analog signal based on the received second RF signal, and transmits the second optical signal to the earth station device via the second optical antenna; and the earth station device receives the second optical signal from the satellite station device via the first optical antenna and acquires the second analog signal from the received second optical signal.

9. The optical communication system according to claim 1, wherein the earth station device generates a first optical signal including the first analog signal and transmits the generated first optical signal to the satellite station device, the satellite station device acquires the first analog signal from the first optical signal received from the earth station device and transmits the first RF signal based on the acquired first analog signal to the communication terminal, the satellite station device receives a second RF signal from the communication terminal, generates a second optical signal including the second analog signal based on the received second RF signal and transmits the generated second optical signal to the earth station device, the earth station device acquires the second analog signal from the second optical signal received from the satellite station device, and the satellite station device relays the first analog signal and the second analog signal between the earth station device and the communication terminal in accordance with a bent pipe method, and transmits and receives the first RF signal and the second RF signal to and from the communication terminal in accordance with a direct access method.

10. An earth station device comprising an optical communication unit that transmits and receives signals to and from a communication terminal via a satellite station device that transmits and receives RF signals to and from the communication terminal, wherein the optical communication unit performs at least one of transmitting optical signals including analog signals to the satellite station device and receiving optical signals including analog signals from the satellite station device.

11. A satellite station device that performs relay processing to relay information transmitted and received between a communication terminal and an earth station device, comprising: an RF communication unit that transmits information using RF signals with the communication terminal; and an optical communication unit that transmits information using optical signals including analog signals with the earth station device.

12. A communication method in an optical communication system having an earth station device and a satellite station device that performs relay processing to relay information transmitted and received between a communication terminal and the earth station device, the communication method including: a step in which the satellite station device transmits information between the communication terminal and the satellite station device using an RF signal; and a step in which the satellite station device transmits information between the earth station device and the satellite station device using an optical signal including an analog signal.

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