Optical communication system, slave station device and optical communication method

Abstract

To extend the reception range of an RF signal in an optical communication system.SOLUTION: An optical communication system includes a branch section that splits an RF signal received via an antenna, a first receiving circuit that generates a first analog signal based on the RF signal, a second receiving circuit that generates a second analog signal based on the RF signal, an optical signal generating circuit that transmits an optical signal including the first analog signal and the second analog signal, an analog signal acquiring circuit that extracts the first analog signal and the second analog signal from the optical signal, a first ADC that digital-converts the first analog signal to generate a first digital signal, and a second ADC that digital-converts the second analog signal to generate a second digital signal. The first receiving circuit and the second receiving circuit have different reception gains and noise figures.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to an optical communication system, a slave station device, and an optical communication method. [Background technology]

[0002] Conventionally, technologies have been developed for optically modulating and transmitting analog signals. For example, Non-Patent Document 1 (Yoshihiro Imasa, "Basics and Use of RoF Devices," RF World, CQ Publishing Co., Ltd., October 29, 2011, No. 16, pp. 107-108) discloses an optical communication system using RoF (Radio over Fiber) technology. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Yoshihiro Imasu, "The Fundamentals and Use of RoF Devices", RF World, CQ Publishing Co., Ltd., October 29, 2011, No. 16, p. 107-108 Summary of the Invention [Problem to be solved by the invention]

[0004] There is a demand for a technique that goes beyond the technique described in Non-Patent Document 1 and that is capable of expanding the reception range of RF (Radio Frequency) signals in an optical communication system.

[0005] The present disclosure has been made to solve the above-mentioned problems, and has as its purpose to provide an optical communication system, a slave station device, and an optical communication method that are capable of expanding the reception range of an RF signal in an optical communication system. [Means for solving the problem]

[0006] An optical communication system according to the present disclosure is an optical communication system used for mobile communication, and includes: a branching unit that branches an RF signal received via an antenna; a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit; a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit; an optical signal generation circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit and transmits the generated optical signal via an optical fiber; an analog signal acquisition circuit that acquires the first analog signal and the second analog signal from the optical signal transmitted by the optical signal generation circuit; a first ADC (Analog to Digital Converter) that generates a first digital signal by digitally converting the first analog signal acquired by the analog signal acquisition circuit; and a second ADC that generates a second digital signal by digitally converting the second analog signal acquired by the analog signal acquisition circuit, and the first receiving circuit and the second receiving circuit have different reception gains and noise figures.

[0007] One aspect of the present disclosure may be realized not only as a master station device having such a characteristic processing unit, but also as an optical 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 master station device.

[0008] One aspect of the present disclosure may be realized not only as a slave station device having such a characteristic processing unit, but also as an optical 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 slave station device. Effect of the Invention

[0009] According to the present disclosure, it is possible to expand the receiving range of an RF signal in an optical communication system. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment of the present disclosure. [Diagram 2] FIG. 2 is a diagram illustrating a configuration of a communication system according to a comparative example. [Diagram 3] FIG. 3 is a diagram for explaining the receiving range of an RF signal in a communication system according to a comparative example. [Figure 4] FIG. 4 is a diagram illustrating configurations of a slave station device and a master station device according to the first embodiment of the present disclosure. [Diagram 5] FIG. 5 is a diagram illustrating a configuration of a base station device according to the first embodiment of the present disclosure. [Figure 6] FIG. 6 is a diagram illustrating a configuration of a receiving circuit in a slave station device according to the first embodiment of the present disclosure. [Figure 7] FIG. 7 is a diagram for explaining a receiving range of an RF signal in the optical communication system according to the first embodiment of the present disclosure. [Figure 8] FIG. 8 is a diagram for explaining a receiving range of an RF signal in the optical communication system according to the first embodiment of the present disclosure. [Figure 9] FIG. 9 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure. [Figure 10] FIG. 10 is a diagram illustrating a configuration of an optical communication system according to the second embodiment of the present disclosure. [Figure 11] FIG. 11 is a diagram illustrating configurations of a slave station device and a master station device according to the second embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] First, the contents of the embodiments of the present disclosure will be listed and described.

[0012] (1) An optical communication system according to an embodiment of the present disclosure is an optical communication system used for mobile communication, comprising: a branching unit that branches an RF signal received via an antenna; a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit; a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit; an optical signal generation circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit and transmits the generated optical signal via an optical fiber; an analog signal acquisition circuit that acquires the first analog signal and the second analog signal from the optical signal transmitted by the optical signal generation circuit; a first ADC that generates a first digital signal by digitally converting the first analog signal acquired by the analog signal acquisition circuit; and a second ADC that generates a second digital signal by digitally converting the second analog signal acquired by the analog signal acquisition circuit, wherein the first receiving circuit and the second receiving circuit have different reception gains and noise figures.

[0013] In this way, by using multiple receiver circuits with different receiver gains and noise figures to generate multiple analog signals based on an RF signal received via an antenna, in an optical communication system with a high noise figure for the entire system, it is possible to realize a wider reception range by adding up the multiple reception ranges corresponding to the multiple receiver circuits as the reception range of an RF signal where the analog signal has a desired SNR against thermal noise and the level of the analog signal falls within the allowable input range of an ADC, thereby expanding the reception range of an RF signal in an optical communication system.

[0014] (2) In the above (1), the optical communication system may further include a first demodulation unit that generates a first demodulated signal by demodulating the first digital signal generated by the first ADC, a second demodulation unit that generates a second demodulated signal by demodulating the second digital signal generated by the second ADC, and a selection unit that makes a decision regarding the selection of any one of the multiple demodulated signals based on quality of the multiple demodulated signals including the first demodulated signal generated by the first demodulation unit and the second demodulated signal generated by the second demodulation unit.

[0015] With this configuration, it is possible to communicate with the communication terminal using a demodulated signal of higher quality among a plurality of demodulated signals.

[0016] (3) In the above (1) or (2), the first receiving circuit may generate the first analog signal in a first frequency band, the second receiving circuit may generate the second analog signal in a second frequency band that does not overlap with the first frequency band, and the optical signal generating circuit may generate the optical signal including an electrical signal in which the first analog signal and the second analog signal are frequency-multiplexed, and transmit the optical signal to the analog signal acquiring circuit.

[0017] With this configuration, it is possible to use a smaller number of conversion devices for converting electrical signals into optical signals, and transmit a plurality of analog signals included in the optical signal, each corresponding to a plurality of receiving circuits.

[0018] (4) In the above (1) or (2), the first receiving circuit may generate the first analog signal in a first frequency band, the second receiving circuit may generate the second analog signal in a third frequency band that at least partially overlaps with the first frequency band, and the optical signal generating circuit may generate the optical signal in which a first optical signal including the first analog signal and a second optical signal including the second analog signal are optically wavelength-multiplexed, and transmit the optical signal to the analog signal acquiring circuit.

[0019] With this configuration, a plurality of analog signals corresponding to a plurality of receiving circuits can be transmitted in an optical signal having a narrower wavelength band.

[0020] (5) A slave station device according to an embodiment of the present disclosure is a slave station device used for mobile communications, and includes a branching unit that branches an RF signal received via an antenna, a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit, a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit, and an optical signal generating circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit, wherein the first receiving circuit and the second receiving circuit have different receiving gains and noise figures.

[0021] In this way, by using multiple receiver circuits with different receiver gains and noise figures to generate multiple analog signals based on an RF signal received via an antenna, in an optical communication system with a high noise figure for the entire system, it is possible to realize a wider reception range by adding up the multiple reception ranges corresponding to the multiple receiver circuits as the reception range of an RF signal where the analog signal has a desired SNR against thermal noise and the level of the analog signal falls within the allowable input range of an ADC, thereby expanding the reception range of an RF signal in an optical communication system.

[0022] (6) An optical communication method according to an embodiment of the present disclosure is an optical communication method in an optical communication system used for mobile communication, comprising the steps of: branching an RF signal received via an antenna; generating a first analog signal and a second analog signal based on the branched RF signal; generating an optical signal including the generated first analog signal and the second analog signal; transmitting the generated optical signal via an optical fiber; acquiring the first analog signal and the second analog signal from the transmitted optical signal; generating a first digital signal by digitally converting the acquired first analog signal; and generating a second digital signal by digitally converting the acquired second analog signal, wherein in the step of generating the first analog signal and the second analog signal, the first analog signal and the second analog signal are generated using a first receiving circuit and a second receiving circuit having different receiving gains and noise figures from each other.

[0023] In this way, by using a method of generating a plurality of analog signals based on an RF signal received via an antenna using a plurality of receiving circuits with different receiving gains and noise figures, it is possible to realize a wider receiving range by adding up the plurality of receiving ranges respectively corresponding to the plurality of receiving circuits as the receiving range of an RF signal in which the analog signal has a desired SNR against thermal noise and the level of the analog signal falls within the allowable input range of an ADC in an optical communication system with a high noise figure of the entire system, thereby expanding the receiving range of an RF signal in the optical communication system.

[0024] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and the description thereof will not be repeated. In addition, at least some of the embodiments described below may be arbitrarily combined.

[0025] <First embodiment> [Configuration and basic operation] FIG. 1 is a diagram illustrating a configuration of an optical communication system according to a first embodiment of the present disclosure. Referring to FIG. 1, the optical communication system 501 includes a master station device 101, a slave station device 201, and a base station device 301. The master station device 101 and the slave station device 201 are connected to each other via an optical fiber 191. The optical communication system 501 is used as a fronthaul for mobile communication. The optical communication system 501 may include a plurality of slave station devices 201. In this case, for example, the plurality of slave station devices 201 are connected to the master station device 101 via one optical fiber 191 and an optical coupler.

[0026] The master station device 101 and the slave station device 201 transmit optical signals, including analog signals, via an optical fiber 191 .

[0027] More specifically, the slave station device 201 receives an RF signal SRu including communication data from the communication terminal 111. For example, the bandwidth of the RF signal SRu is 400 MHz. The communication terminal 111 is not limited to a mobile terminal, and may be a fixed terminal. The slave station device 201 generates an analog signal SAu in, for example, an intermediate frequency (IF) band by frequency-converting the RF signal SRu received from the communication terminal 111, and transmits an optical signal including the generated analog signal SAu to the master station device 101 via the optical fiber 191.

[0028] The master station device 101 receives an optical signal from the slave station device 201 via the optical fiber 191. The master station device 101 acquires an analog signal SAu from the received optical signal, and transmits the acquired analog signal SAu to the base station device 301.

[0029] The base station device 301 generates a digital signal by digitally converting the analog signal SAu received from the master station device 101, and processes the generated digital signal.

[0030] Also, for example, the base station device 301 transmits a digital signal including communication data to the master station device 101 .

[0031] The master station device 101 converts the digital signal received from the base station device 301 into an analog signal SAd, for example, in the IF band, and transmits an optical signal including the generated analog signal SAd to the slave station device 201 via the optical fiber 191.

[0032] The slave station device 201 receives an optical signal from the master station device 101 via the optical fiber 191. The slave station device 201 acquires an analog signal SAd from the optical signal received from the master station device 101, and generates an RF signal SRd by frequency-converting the acquired analog signal SAd. The slave station device 201 transmits the RF signal SRd to the communication terminal 111.

[0033] For example, the slave station device 201 transmits broadcast information for specifying the frequency of the RF signal SRu, the bandwidth of the RF signal SRu, the transmission power of the RF signal SRu, and the like, to the communication terminal 111 within its communication coverage area.

[0034] The communication terminal 111 that has received the broadcast information from the slave station device 201 transmits an RF signal SRu with a power level according to the broadcast information for synchronizing with the optical communication system 501 in accordance with the received broadcast information.

[0035] The level of the RF signal SRu that the slave station device 201 receives from the communication terminal 111 is not uniform but varies due to the influence of the position of the communication terminal 111 relative to the slave station device 201 and variations in propagation path loss of the analog signal SAu in the optical communication system 501. In the optical communication system 501, the transmission power of the RF signal SRu can be specified to the communication terminal 111, while the RF signal SRu received from the communication terminal 111 is amplified by a predetermined gain.

[0036] [assignment] In the optical communication system 501, the lower limit of the receiving range Rg of the RF signal SRu is the level of the RF signal SRu at which the analog signal SAu has a desired SNR (Signal to Noise Ratio) against thermal noise. For example, the lower limit of the receiving range Rg of the RF signal SRu is the level of the RF signal SRu at which the BLER (Block Error Rate) of the analog signal SAu that has been multi-level modulated using 64QAM (Quadrature Amplitude Modulation) or the like is 0.1%.

[0037] Moreover, the upper limit of the reception range Rg of the RF signal SRu is the level of the RF signal SRu when the level of the analog signal SAu is a level obtained by subtracting a predetermined margin from the upper limit of the allowable input range of the ADC that digitally converts the analog signal SAu. Hereinafter, the lower limit of the reception range Rg of the RF signal SRu is also referred to as the "minimum reception power Pmin," and the upper limit of the reception range Rg of the RF signal SRu is also referred to as the "maximum reception power Pmax."

[0038] Fig. 2 is a diagram showing a configuration of a communication system according to a comparative example. Fig. 2 shows a communication system 601 that does not use an optical signal for transmitting an analog signal SAu, as compared with the optical communication system 501. Referring to Fig. 2, the communication system 601 includes an antenna 611, an RF circuit 612, an IF circuit 613, and an ADC 614.

[0039] The RF circuit 612 receives an RF signal SRu from the communication terminal 111 via the antenna 611, generates an analog signal SAu in the IF band by frequency-converting the received RF signal SRu, and outputs the generated analog signal SAu to the IF circuit 613. The IF circuit 613 amplifies the analog signal SAu received from the RF circuit 612, and outputs the amplified analog signal SAu to the ADC 614. The ADC 614 generates a digital signal by digitally converting the analog signal SAu received from the IF circuit 613, and outputs the generated digital signal to a subsequent circuit.

[0040] The allowable input range of the ADC 614 is assumed to be 58 dB, from −53 dBm to +5 dBm. The upper input limit of the RF circuit 612 is assumed to be 0 dBm, and the upper input limit of the IF circuit 613 is assumed to be +5 dBm.

[0041] Fig. 3 is a diagram for explaining the receiving range of an RF signal in a communication system according to a comparative example. In Fig. 3, the vertical axis represents power [dBm]. Fig. 3 shows the receiving range Rg of an RF signal SRu in a communication system 601.

[0042] 3, when the bandwidth of the RF signal SRu that the RF circuit 612 receives from the communication terminal 111 is 400 MHz, the thermal noise is −88 dBm. For example, when the desired SNR of the analog signal SAu relative to the thermal noise is 20 dB and the overall noise figure of the RF circuit 612 and the IF circuit 613 is designed to be 6 dB, the minimum received power Pmin of the RF signal SRu in the communication system 601 is −62 dBm.

[0043] The RF circuit 612 and the IF circuit 613 are designed so that the level of the analog signal SAu output from the IF circuit 613 to the ADC 614 falls within the allowable input range of the ADC 614. Specifically, for example, the RF circuit 612 is designed to have a gain of 12 dB and a noise figure of 6 dB. Also, for example, the IF circuit 613 is designed to have a gain of -3 dB and a noise figure of 3 dB. As a result, the maximum received power Pmax of the RF signal SRu in the communication system 601 is -7 dBm. Also, the receiving range Rg of the RF signal SRu in the communication system 601 can be set to 55 dB, which is 3 dB smaller than the allowable input range of the ADC 614.

[0044] On the other hand, unlike the communication system 601, the optical communication system 501 uses an optical signal for transmitting an analog signal, and therefore includes a conversion device D1 that converts an electrical signal into an optical signal, and a conversion device D2 that converts an optical signal into an electrical signal. For example, the conversion device D1 is provided in the slave station 201, and the conversion device D2 is provided in the master station 101.

[0045] Generally, the conversion efficiency of the conversion device D1 is set low in consideration of a trade-off with characteristics such as output response, and therefore the noise figure is high. Therefore, if the optical communication system 501 is configured to include the conversion devices D1 and D2 instead of the IF circuit 613 in the communication system 601, the noise figure of the entire system will be higher than that of the communication system 601, so the minimum received power Pmin will be higher and the receiving range Rg will be narrower. In other words, if the optical communication system 501 is configured to include the conversion devices D1 and D2 instead of the IF circuit 613 in the communication system 601, the receiving sensitivity of the RF signal SRu will be degraded.

[0046] Furthermore, in the optical communication system 501, in order to keep the noise figure of the entire system low, it is possible to use an RF circuit with a larger gain instead of the RF circuit 612 in the communication system 601. However, when an RF circuit with a larger gain is used, while it is possible to suppress deterioration in the receiving sensitivity of the RF signal SRu, the maximum receiving power Pmax determined according to the upper limit of the allowable input range of the ADC becomes lower, and the receiving range Rg becomes narrower.

[0047] Therefore, the optical communication system 501 according to the first embodiment of the present disclosure solves the above problem by adopting the following configuration.

[0048] (Configuration of optical communication system) Fig. 4 is a diagram showing the configuration of a slave station device and a master station device according to the first embodiment of the present disclosure. Referring to Fig. 4, the slave station device 201 includes an antenna 11, a branching unit 12, receiving circuits 20A and 20B, and an optical signal generating circuit 30. The receiving circuit 20A is an example of a first receiving circuit. The receiving circuit 20B is an example of a second receiving circuit. Hereinafter, each of the receiving circuits 20A and 20B is also referred to as a receiving circuit 20. The optical signal generating circuit 30 includes a diplexer 31 and an optical modulation unit 32. The optical modulation unit 32 is an example of a conversion device D1.

[0049] The master station 101 includes an analog signal acquiring circuit 40. The analog signal acquiring circuit 40 includes an optical demodulator 41, a diplexer 42, and transmitters 43A and 43B. The optical demodulator 41 is an example of a conversion device D2. Hereinafter, each of the transmitters 43A and 43B will also be referred to as a transmitter 43.

[0050] For example, a circuit consisting of the optical signal generating circuit 30 and the analog signal acquiring circuit 40 has a gain of −3 dB, a noise figure of 37 dB, and an upper input limit of +5 dBm.

[0051] 5 is a diagram showing a configuration of a base station device according to a first embodiment of the present disclosure. The base station device 301 includes ADCs 51A and 51B, demodulators 52A and 52B that are demodulators 52, and a selector 53. The ADC 51A is an example of a first ADC. The ADC 51B is an example of a second ADC. The demodulator 52A is an example of a first demodulator. The demodulator 52B is an example of a second demodulator. Hereinafter, each of the ADCs 51A and 51B is also referred to as an ADC 51. The allowable input range of the ADC 51 is 58 dB, from -53 dBm to +5 dBm.

[0052] 4 again, the branching unit 12 branches the RF signal SRu received via the antenna 11. More specifically, the branching unit 12 branches the RF signal SRu received from the communication terminal 111 via the antenna 11 into two systems, and outputs the RF signals SRu branched into the two systems to the receiving circuits 20A and 20B, respectively. Note that the slave station device 201 may be configured to include an amplifier that amplifies the RF signal SRu received via the antenna 11, in a stage preceding the branching unit 12.

[0053] The receiving circuit 20A generates an analog signal SAu1 in the IF band based on the RF signal SRu branched by the branching unit 12. For example, the receiving circuit 20A generates an analog signal SAu1 in a frequency band fb1. The frequency band fb1 is an example of a first frequency band. The analog signal SAu1 is an example of a first analog signal.

[0054] The receiving circuit 20B generates an analog signal SAu2 in the IF band based on the RF signal SRu branched by the branching unit 12. For example, the receiving circuit 20B generates an analog signal SAu2 in a frequency band fb2 that does not overlap with the frequency band fb1. The frequency band fb2 is an example of a second frequency band. The analog signal SAu2 is an example of a second analog signal.

[0055] Fig. 6 is a diagram showing a configuration of a receiving circuit in a slave station device according to the first embodiment of the present disclosure. With reference to Fig. 6, a receiving circuit 20A includes amplifiers 21A and 22A, an attenuator 23A, a frequency conversion unit 24A, and a BPF 25A. A receiving circuit 20B includes attenuators 21B and 23B, an amplifier 22B, a frequency conversion unit 24B, and a BPF 25B. The amplifiers 21A, 22A, and 22B are, for example, low noise amplifiers (LNAs).

[0056] The receiving circuit 20A is a circuit provided with a multi-stage amplifier to increase the gain. In the receiving circuit 20A, the amplifier 21A receives the RF signal SRu from the branching unit 12, amplifies the received RF signal SRu, and outputs the amplified RF signal SRu to the amplifier 22A. The amplifier 22A amplifies the RF signal SRu received from the amplifier 21A, and outputs the amplified RF signal SRu to the attenuator 23A. The attenuator 23A attenuates the RF signal SRu received from the amplifier 22A, and outputs the attenuated RF signal SRu to the frequency conversion unit 24A. For example, the frequency conversion unit 24A generates an analog signal SAu1 in a frequency band fb1 by multiplying the RF signal SRu received from the attenuator 23A by a local signal received from a local oscillator (not shown), and outputs the analog signal SAu1 to the BPF 25A. BPF 25A attenuates frequency components outside a predetermined passband of analog signal SAu1 received from frequency conversion unit 24A.

[0057] The receiving circuit 20B is a circuit provided with attenuators 21B and 23B having a predetermined attenuation ratio so that the output signal of the amplifier 22B and the output signal of the frequency conversion unit 24B are not distorted. In the receiving circuit 20B, the attenuator 21B receives the RF signal SRu from the branching unit 12, attenuates the received RF signal SRu, and outputs the attenuated RF signal SRu to the amplifier 22B. The amplifier 22B amplifies the RF signal SRu received from the attenuator 21B, and outputs the amplified RF signal SRu to the attenuator 23B. The attenuator 23B attenuates the RF signal SRu received from the amplifier 22B, and outputs the attenuated RF signal SRu to the frequency conversion unit 24B. For example, the frequency conversion unit 24B generates an analog signal SAu2 of the frequency band fb2 by multiplying the RF signal SRu received from the attenuator 23B by a local signal received from a local oscillator (not shown), and outputs the analog signal SAu2 to the BPF 25B. BPF 25B attenuates frequency components outside a predetermined passband of analog signal SAu2 received from frequency conversion unit 24B.

[0058] The receiving circuits 20A and 20B have different receiving gains and noise figures. More specifically, the receiving circuit 20A has a gain of 40 dB, a noise figure of 6 dB, and an upper input limit of zero dBm. The receiving circuit 20B has a gain of 12 dB, a noise figure of 10 dB, and an upper input limit of +5 dBm. In the optical communication system 501, it is necessary to adjust the gain in each circuit under the constraint of the noise figure of the entire system. Therefore, in the optical communication system 501, the noise figure of the entire system is appropriately designed by adjusting the gains in the receiving circuits 20A and 20B.

[0059] 4 again, the optical signal generating circuit 30 generates an optical signal including the analog signals SAu1, SAu2 generated by the receiving circuits 20A, 20B, and transmits the generated optical signal to the master station 101 via the optical fiber 191. For example, the optical signal generating circuit 30 generates an optical signal including an electrical signal into which the analog signals SAu1, SAu2 are frequency-multiplexed, and transmits the optical signal to the analog signal acquiring circuit 40 in the master station 101.

[0060] More specifically, the diplexer 31 combines an analog signal SAu1 that has passed through the BPF 25A in the receiving circuit 20A and an analog signal SAu2 that has passed through the BPF 25B in the receiving circuit 20B to generate an electrical signal including the analog signals SAu1 and SAu2, and outputs the electrical signal to the optical modulation section 32.

[0061] The optical modulation unit 32 receives the electrical signal from the diplexer 31 and generates an optical signal with a wavelength of λ1 by optically modulating the received electrical signal. The optical modulation unit 32 transmits the generated optical signal to the parent station 101 via an optical coupler (not shown) and an optical fiber 191.

[0062] The analog signal acquiring circuit 40 in the master station 101 acquires the analog signals SAu1 and SAu2 from the optical signal transmitted by the optical signal generating circuit 30.

[0063] More specifically, the optical demodulator 41 in the analog signal acquisition circuit 40 receives an optical signal from the slave station device 201 via the optical fiber 191 and an optical coupler (not shown), and outputs an electrical signal at a level corresponding to the intensity of the received optical signal.

[0064] The diplexer 42 receives an electrical signal from the optical demodulator 41 and outputs analog signals SAu1 and SAu2 included in the received electrical signal to the transmitters 43A and 43B. More specifically, the diplexer 42 outputs, of the electrical signal received from the optical demodulator 41, analog signal SAu1, which is a frequency component of the frequency band fb1, to the transmitter 43A, and outputs analog signal SAu2, which is a frequency component of the frequency band fb2, to the transmitter 43B.

[0065] The transmitter 43A transmits the analog signal SAu1 received from the diplexer 42 to the base station device 301. The transmitter 43B transmits the analog signal SAu2 received from the diplexer 42 to the base station device 301.

[0066] 7 and 8 are diagrams for explaining the receiving range of an RF signal in an optical communication system according to a first embodiment of the present disclosure. In Fig. 7 and Fig. 8, the vertical axis is power [dBm]. Fig. 7 shows a receiving range Rg1 of an RF signal SRu passing through a receiving circuit 20A in the optical communication system 501. Fig. 8 shows a receiving range Rg2 of an RF signal SRu passing through a receiving circuit 20B in the optical communication system 501.

[0067] 7, the minimum reception power Pmin of the RF signal SRu passing through the receiver circuit 20A in the optical communication system 501 is −62 dBm, and the maximum reception power Pmax of the RF signal SRu passing through the receiver circuit 20A in the optical communication system 501 is −35 dBm. Also, the reception range Rg1 of the RF signal SRu passing through the receiver circuit 20A in the optical communication system 501 is 27 dB.

[0068] 8, the minimum reception power Pmin of the RF signal SRu passing through the receiver circuit 20B in the optical communication system 501 is −43 dBm, and the maximum reception power Pmax of the RF signal SRu passing through the receiver circuit 20B in the optical communication system 501 is −7 dBm. Also, the reception range Rg2 of the RF signal SRu passing through the receiver circuit 20B in the optical communication system 501 is 36 dB.

[0069] The receiving circuit 20A has a lower minimum received power Pmin than the receiving circuit 20B, and is a circuit for increasing the receiving sensitivity of the RF signal SRu in the optical communication system 501. On the other hand, the receiving circuit 20B has a higher maximum received power Pmax than the receiving circuit 20A, and is a circuit for preventing the analog signal SAu from exceeding the upper limit of the allowable input range of the ADC 51 in the base station device 301 when the RF signal SRu is a strong input.

[0070] The receiving ranges Rg1 and Rg2 overlap. In the optical communication system 501, by selectively using the analog signal SAu1 based on the RF signal SRu that has passed through the receiving circuit 20A and the analog signal SAu2 based on the RF signal SRu that has passed through the receiving circuit 20B, the receiving range Rg of the RF signal SRu in the optical communication system 501 becomes the sum of the receiving ranges Rg1 and Rg2. Therefore, the receiving range Rg of the RF signal SRu in the optical communication system 501 can be expanded.

[0071] 5 again, the ADC 51A in the base station device 301 generates a digital signal SDu1 by digitally converting the analog signal SAu1 acquired by the analog signal acquisition circuit 40 in the parent station device 101. The digital signal SDu1 is an example of a first digital signal. More specifically, the ADC 51A receives the analog signal SAu1 from the transmission unit 43A in the analog signal acquisition circuit 40, and generates the digital signal SDu1 by digitally converting the received analog signal SAu1. The ADC 51A outputs the generated digital signal SDu1 to the demodulation unit 52A.

[0072] The ADC 51B in the base station device 301 generates a digital signal SDu2 by digitally converting the analog signal SAu2 acquired by the analog signal acquiring circuit 40 in the parent station device 101. The digital signal SDu2 is an example of a second digital signal. More specifically, the ADC 51B receives the analog signal SAu2 from the transmitting unit 43B in the analog signal acquiring circuit 40, and generates the digital signal SDu2 by digitally converting the received analog signal SAu2. The ADC 51B outputs the generated digital signal SDu2 to the demodulation unit 52B.

[0073] The demodulation unit 52A demodulates the digital signal SDu1 generated by the ADC 51A to generate a demodulated signal Dem1, which is a demodulated signal Dem. The demodulation signal Dem1 is an example of a first demodulated signal. The demodulation unit 52A outputs the generated demodulated signal Dem1 to the selection unit 53.

[0074] The demodulation unit 52B demodulates the digital signal SDu2 generated by the ADC 51B to generate a demodulated signal Dem2, which is the demodulated signal Dem. The demodulation signal Dem2 is an example of a second demodulated signal. The demodulation unit 52B outputs the generated demodulated signal Dem2 to the selection unit 53.

[0075] The selection unit 53 makes a decision regarding the selection of one of the demodulated signals Dem1 and Dem2 based on the quality of the demodulated signals Dem1 and Dem2 generated by the demodulation units 52A and 52B. More specifically, the selection unit 53 receives the demodulated signal Dem1 from the demodulation unit 52A and calculates the BLER of the demodulated signal Dem1. The selection unit 53 also receives the demodulated signal Dem2 from the demodulation unit 52B and calculates the BLER of the demodulated signal Dem2. For example, the selection unit 53 selects one of the demodulated signals Dem1 and Dem2 based on the calculated BLER of the demodulated signals Dem1 and Dem2. The selection unit 53 transmits the selected demodulated signal Dem from the demodulated signals Dem1 and Dem2 to a subsequent circuit (not shown) that processes the demodulated signal Dem. On the other hand, the selection unit 53 discards the unselected demodulated signal Dem from the demodulated signals Dem1 and Dem2.

[0076] For example, when the BLER of the demodulated signal Dem1 becomes equal to or greater than a predetermined value while the demodulated signal Dem1 is being transmitted to a downstream circuit, the selector 53 switches the demodulated signal Dem to be transmitted to the downstream circuit to the demodulated signal Dem2.

[0077] Also, for example, when the BLER of the demodulated signal Dem2 becomes equal to or greater than a predetermined value while the demodulated signal Dem2 is being transmitted to a downstream circuit, the selection unit 53 switches the demodulated signal Dem to be transmitted to the downstream circuit to the demodulated signal Dem1.

[0078] The selection unit 53 does not need to select either one of the demodulated signals Dem1 and Dem2. More specifically, when the BLER of the demodulated signal Dem1 is equal to or greater than a predetermined value and the BLER of the demodulated signal Dem2 is equal to or greater than a predetermined value, the selection unit 53 discards the demodulated signals Dem1 and Dem2 without selecting the demodulated signal Dem to be transmitted to a downstream circuit (not shown). That is, the selection unit 53 has a squelch function.

[0079] [Operation flow] FIG. 9 is a diagram illustrating an example of a communication sequence in the optical communication system according to the first embodiment of the present disclosure.

[0080] 9, first, the slave station device 201 receives an RF signal SRu from the communication terminal 111 via the antenna 11 (step S11).

[0081] Next, the slave station device 201 branches the received RF signal SRu into two systems (step S12).

[0082] Next, the slave station device 201 generates analog signals SAu1 and SAu2 based on the branched RF signal SRu. More specifically, the slave station device 201 generates the analog signal SAu1 based on the RF signal SRu using the receiving circuit 20A. Also, the slave station device 201 generates the analog signal SAu2 based on the RF signal SRu using the receiving circuit 20B (step S13).

[0083] Next, the slave station device 201 generates an optical signal including the generated analog signals SAu1 and SAu2 (step S14).

[0084] Next, the slave station device 201 transmits the generated optical signal to the master station device 101 via the optical fiber 191 (step S15).

[0085] Next, the master station device 101 receives an optical signal from the slave station device 201 via the optical fiber 191, and acquires the analog signals SAu1 and SAu2 from the received optical signal (step S16).

[0086] Next, the master station 101 transmits the acquired analog signals SAu1 and SAu2 to the base station 301 (step S17).

[0087] Next, the base station device 301 generates digital signals SDu1 and SDu2 by digitally converting the analog signals SAu1 and SAu2 from the master station device 101 (step S18).

[0088] Next, the base station device 301 demodulates the generated digital signals SDu1 and SDu2 to generate demodulated signals Dem1 and Dem2 (step S19).

[0089] Next, the base station device 301 selects one of the demodulated signals Dem1 and Dem2 based on the quality of the demodulated signals Dem1 and Dem2. More specifically, the base station device 301 calculates the BLER of the demodulated signals Dem1 and Dem2, selects one of the demodulated signals Dem1 and Dem2 based on the calculated BLER, and transmits the selected demodulated signal Dem to a downstream circuit (step S20).

[0090] Although the optical communication system 501 according to the first embodiment of the present disclosure has been described as including the base station device 301, the present invention is not limited to this. The optical communication system 501 may not include the base station device 301.

[0091] In addition, in the optical communication system 501 according to the first embodiment of the present disclosure, the base station device 301 is configured to include the ADCs 51A and 51B, the demodulators 52A and 52B, and the selector 53, but this is not limiting. Instead of the base station device 301, the master station device 101 may be configured to include the ADCs 51A and 51B. Instead of the base station device 301, the master station device 101 may be configured to include the ADCs 51A and 51B and the demodulators 52A and 52B. Instead of the base station device 301, the master station device 101 may be configured to include the ADCs 51A and 51B, the demodulators 52A and 52B, and the selector 53.

[0092] In the optical communication system 501 according to the first embodiment of the present disclosure, the slave station device 201 is configured to include two receiving circuits 20, but is not limited to this. The slave station device 201 may be configured to include three or more receiving circuits 20. The master station device 101 may be configured to include three or more transmitting units 43. That is, the optical communication system 501 may be configured to include three or more circuits, not limited to two circuits, for the RF signal SRu. In this case, the base station device 301 includes three or more demodulation units 52. The selection unit 53 in the base station device 301 makes a decision regarding the selection of any one of the three or more demodulation signals Dem based on the quality of the three or more demodulation signals Dem generated by the three or more demodulation units 52, respectively.

[0093] In the optical communication system 501 according to the first embodiment of the present disclosure, the slave station device 201 is configured to transmit an optical signal including analog signals SAu1 and SAu2 to the master station device 101, but the present invention is not limited to this. The slave station device 201 may be configured to transmit an optical signal including RF signals SRu1 and SRu2 to the master station device 101 instead of the analog signals SAu1 and SAu2. More specifically, the receiving circuit 20A in the slave station device 201 generates an RF signal SRu1 based on the RF signal SRu branched by the branching unit 12. The receiving circuit 20B in the slave station device 201 generates an RF signal SRu2 based on the RF signal SRu branched by the branching unit 12. The optical signal generating circuit 30 generates an optical signal including the RF signals SRu1 and SRu2 generated by the receiving circuits 20A and 20B, and transmits the generated optical signal to the analog signal acquiring circuit 40 in the master station device 101.

[0094] Next, another embodiment 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 characters and the description thereof will not be repeated.

[0095] <Second embodiment> This embodiment is different from the optical communication system 501 according to the first embodiment in that it relates to an optical communication system 502 that optically wavelength-multiplexes and transmits an optical signal including an analog signal SAu1 and an optical signal including an analog signal SAu2. The contents other than those described below are the same as those of the optical communication system 501 according to the first embodiment.

[0096] Fig. 10 is a diagram illustrating a configuration of an optical communication system according to a second embodiment of the present disclosure. With reference to Fig. 10, compared to the optical communication system 501, an optical communication system 502 includes a master station device 102 instead of the master station device 101, and includes a slave station device 202 instead of the slave station device 201.

[0097] 11 is a diagram illustrating the configuration of a slave station device and a master station device according to a second embodiment of the present disclosure. With reference to FIG. 11, compared to the slave station device 201, the slave station device 202 includes receiving circuits 50A and 50B instead of the receiving circuits 20A and 20B, and includes an optical signal generating circuit 60 instead of the optical signal generating circuit 30. The receiving circuit 50A is an example of a first receiving circuit. The receiving circuit 50B is an example of a second receiving circuit. Compared to the optical signal generating circuit 30, the optical signal generating circuit 60 includes optical modulation units 61A and 61B and an optical coupler 62 instead of the diplexer 31 and the optical modulation unit 32. The optical modulation units 61A and 61B are an example of a conversion device D1.

[0098] Compared to the master station device 101, the master station device 102 includes an analog signal acquiring circuit 70 instead of the analog signal acquiring circuit 40. Compared to the analog signal acquiring circuit 40, the analog signal acquiring circuit 70 includes an optical coupler 71 and optical demodulators 72A and 72B instead of the optical demodulator 41 and the diplexer 42. The optical demodulators 72A and 72B are examples of the conversion device D2.

[0099] For example, a circuit consisting of the optical signal generating circuit 60 and the analog signal acquiring circuit 70, like the circuit consisting of the optical signal generating circuit 30 and the analog signal acquiring circuit 40, has a gain of -3 dB, a noise figure of 37 dB, and an upper input limit of +5 dBm.

[0100] The receiving circuit 50A generates an analog signal SAu3 in the IF band based on the RF signal SRu branched by the branching unit 12. For example, the receiving circuit 50A generates an analog signal SAu3 in a frequency band fb3. The frequency band fb3 is an example of a first frequency band. The analog signal SAu3 is an example of a first analog signal. The receiving circuit 50A has a gain of 40 dB, a noise figure of 6 dB, and an upper input limit of zero dBm, similar to the receiving circuit 20A.

[0101] The receiving circuit 50B generates an analog signal SAu4 in the IF band based on the RF signal SRu branched by the branching unit 12. For example, the receiving circuit 50B generates an analog signal SAu4 in a frequency band fb4 that at least partially overlaps with the frequency band fb3. The frequency band fb4 is an example of a second frequency band. The analog signal SAu4 is an example of a second analog signal. The receiving circuit 50B has a gain of 12 dB, a noise figure of 10 dB, and an upper input limit of +5 dBm, similar to the receiving circuit 20B.

[0102] The optical signal generating circuit 60 generates an optical signal including the analog signals SAu3 and SAu4 generated by the receiving circuits 50A and 50B. For example, the optical signal generating circuit 60 generates an optical signal in which an optical signal including the analog signal SAu3 and an optical signal including the analog signal SAu4 are optically wavelength-multiplexed, and transmits the optical signal to the analog signal acquiring circuit 70 in the master station 102. The optical signal including the analog signal SAu3 is an example of a first optical signal. The optical signal including the analog signal SAu4 is an example of a second optical signal.

[0103] More specifically, the optical modulation unit 61A generates an optical signal with a wavelength of λ1 by optically modulating the analog signal SAu3 received from the receiving circuit 50A. The optical modulation unit 61A outputs the generated optical signal to the optical coupler 62.

[0104] The optical modulation unit 61B also generates an optical signal with a wavelength of λ2 by optically modulating the analog signal SAu4 received from the receiving circuit 50B. The optical modulation unit 61B outputs the generated optical signal to the optical coupler 62.

[0105] The optical coupler 62 optically wavelength-multiplexes the optical signals of wavelengths λ1 and λ2 received from the optical modulation units 61A and 61B, and transmits the optical signal obtained by optically wavelength-multiplexing the optical signals of wavelengths λ1 and λ2 to the parent station 102 via an optical coupler (not shown) and an optical fiber 191.

[0106] The analog signal acquiring circuit 70 in the master station 102 acquires the analog signals SAu3 and SAu4 from the optical signal generated by the optical signal generating circuit 60.

[0107] More specifically, the optical coupler 71 in the analog signal acquisition circuit 70 receives an optical signal from the slave station device 202 via the optical fiber 191 and an optical coupler (not shown), and separates the received optical signal according to its wavelength. Of the received optical signals, the optical coupler 71 outputs an optical signal with a wavelength λ1 to the optical demodulation unit 72A, and outputs an optical signal with a wavelength λ2 to the optical demodulation unit 72B.

[0108] The optical demodulator 72A outputs an analog signal SAu3, which is an electrical signal having a level corresponding to the intensity of the optical signal received from the optical coupler 71, to the transmitter 43A. The optical demodulator 72B outputs an analog signal SAu4, which is an electrical signal having a level corresponding to the intensity of the optical signal received from the optical coupler 71, to the transmitter 43B.

[0109] The transmitter 43A transmits the analog signal SAu3 received from the optical demodulator 72A to the base station device 301. The transmitter 43B transmits the analog signal SAu4 received from the optical demodulator 72B to the base station device 301.

[0110] The above-described embodiments should be considered as illustrative and not restrictive in all respects. 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.

[0111] Each process (each function) of the above-mentioned embodiment is realized by a processing circuit (circuitry) including one or more processors. The above-mentioned processing circuit may be composed of an integrated circuit or the like in which one or more memories, various analog circuits, and various digital circuits are combined in addition to the above-mentioned one or more processors. The above-mentioned one or more memories store programs (instructions) that cause the above-mentioned one or more processors to execute each of the above-mentioned processes. The above-mentioned one or more processors may execute each of the above-mentioned processes according to the programs read from the above-mentioned one or more memories, or may execute each of the above-mentioned processes according to a logic circuit designed in advance to execute each of the above-mentioned processes. The above-mentioned processors may be 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). The above-mentioned physically separated processors may execute each of the above-mentioned processes in cooperation with each other. For example, the processors mounted on each of a plurality of physically separated computers may cooperate with each other via a network such as a LAN (Local Area Network), a WAN (Wide Area Network), the Internet, etc. The program may be installed in the memory from an external server device or the like via the network, or may be distributed in a state stored in 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 installed in the memory from the recording medium.

[0112] The above description includes the following additional features. [Appendix 1] An optical communication system for use in mobile communication, comprising: a branching unit that branches an RF signal received via an antenna; a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit; a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit; an optical signal generating circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit, and transmits the generated optical signal via an optical fiber; an analog signal acquisition circuit that acquires the first analog signal and the second analog signal from the optical signal transmitted by the optical signal generation circuit; a first ADC that generates a first digital signal by digitally converting the first analog signal acquired by the analog signal acquisition circuit; a second ADC that generates a second digital signal by digitally converting the second analog signal acquired by the analog signal acquisition circuit; the first receiving circuit and the second receiving circuit have different receiving gains and noise figures; An optical communication system, wherein at least one of the first analog signal and the second analog signal is an intermediate frequency band signal. [Explanation of symbols]

[0113] 11 Antenna 12 Branch 20A, 20B Receiving circuit 21A Amplifier 22A Amplifier 23A Attenuator 24A Frequency conversion unit 25A BPF 21B Attenuator 22B Amplifier 23B Attenuator 24B Frequency conversion section 25B BPF 30 Optical signal generation circuit 31 Diplexer 32 Optical modulation section 40 Analog signal acquisition circuit 41 Optical demodulation section 42 Diplexer 43A, 43B Transmitter 50A, 50B Receiving circuit 51A, 51B ADC 52A, 52B Demodulation section 53 Selection section 60 Optical signal generation circuit 61A, 61B Optical modulation section 62 Optical Coupler 70 Analog signal acquisition circuit 71 Optical Coupler 72A, 72B Optical demodulation section 111 Communication terminal 101,102 Master station device 191 Optical Fiber 201,202 Slave station device 301 Base station equipment 501,502 Optical communication systems 601 Communication Systems 611 Antenna 612 RF circuit 613 IF circuit 614 ADC

Claims

1. An optical communication system for use in mobile communication, comprising: a branching unit that branches an RF signal received via an antenna; a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit; a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit; an optical signal generating circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit, and transmits the generated optical signal via an optical fiber; an analog signal acquisition circuit that acquires the first analog signal and the second analog signal from the optical signal transmitted by the optical signal generation circuit; a first ADC (Analog to Digital Converter) that generates a first digital signal by digitally converting the first analog signal acquired by the analog signal acquisition circuit; a second ADC that generates a second digital signal by digitally converting the second analog signal acquired by the analog signal acquisition circuit; An optical communication system, wherein the first receiving circuit and the second receiving circuit have different receiving gains and noise figures.

2. The optical communication system further comprises: a first demodulation unit that demodulates the first digital signal generated by the first ADC to generate a first demodulated signal that is a demodulated signal; a second demodulation unit that demodulates the second digital signal generated by the second ADC to generate a second demodulated signal that is a demodulated signal; and a selection unit that makes a decision regarding the selection of any one of the plurality of demodulated signals based on quality of the plurality of demodulated signals including the first demodulated signal generated by the first demodulation unit and the second demodulated signal generated by the second demodulation unit.

3. the first receiver circuit generates the first analog signal in a first frequency band; the second receiver circuit generates the second analog signal in a second frequency band that does not overlap with the first frequency band; 3. The optical communication system according to claim 1, wherein the optical signal generating circuit generates the optical signal including an electrical signal in which the first analog signal and the second analog signal are frequency-multiplexed, and transmits the optical signal to the analog signal acquiring circuit.

4. the first receiver circuit generates the first analog signal in a first frequency band; the second receiver circuit generates the second analog signal in a third frequency band that at least partially overlaps with the first frequency band; 3. The optical communication system according to claim 1, wherein the optical signal generation circuit generates an optical signal in which a first optical signal including the first analog signal and a second optical signal including the second analog signal are optically wavelength-multiplexed, and transmits the optical signal to the analog signal acquisition circuit.

5. A slave station device for use in mobile communications, comprising: a branching unit that branches an RF signal received via an antenna; a first receiving circuit that generates a first analog signal based on the RF signal branched by the branching unit; a second receiving circuit that generates a second analog signal based on the RF signal branched by the branching unit; an optical signal generating circuit that generates an optical signal including the first analog signal generated by the first receiving circuit and the second analog signal generated by the second receiving circuit; The first receiving circuit and the second receiving circuit have different receiving gains and noise figures.

6. An optical communication method in an optical communication system used for mobile communication, comprising: branching an RF signal received via an antenna; generating a first analog signal and a second analog signal based on the branched RF signal; generating an optical signal including the generated first analog signal and the generated second analog signal; transmitting the generated optical signal over an optical fiber; obtaining the first analog signal and the second analog signal from the transmitted optical signal; generating a first digital signal by digitally converting the acquired first analog signal; and generating a second digital signal by digitally converting the acquired second analog signal; In the step of generating the first analog signal and the second analog signal, the first analog signal and the second analog signal are generated using a first receiving circuit and a second receiving circuit having mutually different receiving gains and noise figures.

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