Receiver, optical space communication system, and optical space communication receiving method

Abstract

The optical space communication transmitting terminal (2) is provided with: transmitters (24a to 24d) configured in a plurality of groups, which transmit optical signals of mutually different wavelengths; combiners (23a, 23b) provided for each group, which output wavelength multiplexed optical signals by combining the optical signals transmitted from the transmitters (24a to 24d) belonging to the group; optical amplifiers (22a, 22b) provided for each group, which amplify the wavelength multiplexed optical signals; and optical antennas (21a, 21b) provided for each group, which transmit the amplified optical signals to space, the optical antennas (21a, 21b) of each group transmitting the optical signals to the same direction.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a receiver, an optical space communication system, and an optical space communication reception method. BACKGROUND

[0002] Optical space communication is being studied for application to a feeder link of a next-generation geostationary satellite as a carrier wave of light has a short wavelength and enables efficient light transmission with suppressed beam broadening, compared to microwave wireless communication (for example, refer to Non-Patent Literature 1).

[0003] PRIOR ART DOCUMENT

[0004] NON-PATENT LITERATURE

[0005] Non-Patent Literature 1: P. Shubert et al., “System Design of a High Capacity Optical Relay Network”, Proc. ICSOS 2019, S3.3. SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION

[0007] As described in Non-Patent Literature 1, in order to realize a Tbps-level feeder link in optical space communication, an optical power of 100 W or more is required. However, it is difficult to realize a high-output optical amplifier of 100 W or more, and in the conventional optical space communication technology, there is a problem that the communication capacity decreases due to insufficient output of the optical amplifier. For example, when the output of the optical amplifier is insufficient, the transmission power of the optical space communication transmission terminal is insufficient, and along with this, the optical power reaching the optical space communication reception terminal is also insufficient, and the signal-to-noise ratio (SNR) of the signal demodulated from the reception signal decreases, and thus the communication capacity decreases.

[0008] The present disclosure is to solve the above problem, and aims to obtain a receiver, an optical space communication system, and an optical space communication reception method that can suppress a decrease in communication capacity due to insufficient output of an optical amplifier in optical space communication.

[0009] MEANS FOR SOLVING THE PROBLEM

[0010] The optical space communication system of the present disclosure has an optical space communication terminal and a receiving terminal, the optical space communication terminal has a plurality of transmitters which constitute a plurality of groups, transmits optical signals of mutually different wavelengths, a combiner which is provided for each group, outputs wavelength multiplexed optical signals by combining the optical signals transmitted from the transmitters belonging to the group, an optical amplifier which is provided for each group, amplifies the wavelength multiplexed optical signals, and an optical antenna which is provided for each group, transmits the amplified optical signals to the space, and the receiving terminal receives the optical signals transmitted by the optical space communication terminal, and the optical antennas of each group transmit the optical signals to the same optical antenna of the receiving terminal.

[0011] Effects of the Invention

[0012] According to the present disclosure, a plurality of wavelengths which are subjected to wavelength multiplexing are grouped, the optical signals of different wavelengths are combined for each group, the wavelength multiplexed optical signals are amplified, and the amplified optical signals are transmitted in the same direction for each group, and therefore, the transmission power of each wavelength can be increased. Thus, the optical power reaching the receiving side is also increased, the SNR of the signals demodulated from the received signals is improved, and therefore, the reduction in the communication capacity due to the insufficient output of the optical amplifier in optical space communication can be suppressed. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1 is a block diagram showing the structure of the optical space communication system of Embodiment 1.

[0014] Figure 2 is a block diagram showing the structure of the optical space communication terminal of Embodiment 2.

[0015] Figure 3 is a block diagram showing the structure of the receiver of Embodiment 2.

[0016] Figure 4 is a flowchart showing the optical space communication receiving method of Embodiment 2.

[0017] Figure 5A is a block diagram showing the hardware structure realizing the functions of the receiver of Embodiment 2, Figure 5B is a block diagram showing the hardware structure executing the software realizing the functions of the receiver of Embodiment 2.

[0018] Figure 6 is a block diagram showing the structure of the optical space communication system of Embodiment 3. DETAILED DESCRIPTION

[0019] Embodiment 1.

[0020] Figure 1 is a block diagram showing the structure of the optical space communication system 1 of Embodiment 1. InFigure 1 In the present embodiment, the optical space communication system 1 is a system that communicates optical signals obtained by wavelength-multiplexing light of four wavelengths λ1 to λ4 that are different from each other, for example, and includes an optical space communication transmission terminal 2 and an optical space communication reception terminal 3.

[0021] The optical space communication transmission terminal 2 is a transmission terminal that transmits an optical signal obtained by wavelength-multiplexing light of the wavelengths λ1 to λ4. The optical signal is a signal obtained by mapping a transmission signal for the optical space communication reception terminal 3 to a signal point for the optical signal. The optical space communication reception terminal 3 is a reception terminal that receives the optical signal transmitted by the optical space communication transmission terminal 2. The transmission signal is demapped from the above-described signal point of the optical signal received by the optical space communication reception terminal 3.

[0022] Structural elements of the optical space communication transmission terminal 2 are classified into a group (1) and a group (2). The optical antenna 21a, the optical amplifier 22a, the combiner 23a, the transmitter 24a, and the transmitter 24b belong to the group (1). The optical antenna 21b, the optical amplifier 22b, the combiner 23b, the transmitter 24c, and the transmitter 24d belong to the group (2). The group (1) is a group that transmits an optical signal obtained by wavelength-multiplexing adjacent wavelengths λ1 and λ2 among the four wavelengths λ1 to λ4, and the group (2) is a group that transmits an optical signal obtained by wavelength-multiplexing adjacent wavelengths λ3 and λ4.

[0023] Among the plurality of wavelengths in which wavelength-multiplexing is performed, a wavelength that does not exist in overlapping in the interval of the adjacent wavelengths in the spectral band of the optical signal is used. For example, in the group (1), there is no overlapping between the spectral band of the optical signal of the wavelength λ1 and the spectral band of the optical signal of the wavelength λ2, and in the group (2), there is no overlapping between the spectral band of the optical signal of the wavelength λ3 and the spectral band of the optical signal of the wavelength λ4. Further, there is also no overlapping between the spectral band of the optical signal of any wavelength in the group (1) and the spectral band of the optical signal of any wavelength in the group (2). Thereby, interference of the optical signals in which the wavelengths are close to each other is suppressed.

[0024] In the group (1), the transmitter 24a is a coherent transmitter (Tx(λ1)) that transmits the optical signal of the wavelength λ1, and the transmitter 24b is a coherent transmitter (Tx(λ2)) that transmits the optical signal of the wavelength λ2. The combiner 23a generates an optical signal obtained by wavelength-multiplexing the wavelength λ1 and the wavelength λ2 by combining the optical signal of the wavelength λ1 and the optical signal of the wavelength λ2. The optical amplifier 22a amplifies the wavelength-multiplexed signal generated by the combiner 23a.

[0025] In the group (2), the transmitter 24c is a coherent transmitter (Tx(λ3)) that transmits an optical signal of the wavelength λ3, and the transmitter 24d is a coherent transmitter (Tx(λ4)) that transmits an optical signal of the wavelength λ4. The wavelength multiplexing signal of the wavelengths λ3 and λ4 is generated by the multiplexer 23b by multiplexing the optical signal of the wavelength λ3 and the optical signal of the wavelength λ4. The wavelength multiplexing signal generated by the multiplexer 23b is amplified by the optical amplifier 22b.

[0026] The optical antenna 21a belonging to the group (1) transmits the wavelength multiplexing signal of the wavelengths λ1 and λ2 amplified by the optical amplifier 22a to the optical space communication receiving terminal 3. Also, the optical antenna 21b belonging to the group (2) transmits the wavelength multiplexing signal of the wavelengths λ3 and λ4 amplified by the optical amplifier 22b to the optical space communication receiving terminal 3. That is, the optical antenna 21a and the optical antenna 21b transmit optical signals to the same direction in which there is a common transmission target.

[0027] The output power of each of the optical amplifiers 22a and 22b has a limit, and therefore, the optical space communication transmitting terminal 2 divides the plurality of wavelengths subjected to wavelength multiplexing into N (N is an integer of 2 or more) groups, multiplexes optical signals of different wavelengths for each group, amplifies the wavelength multiplexing signal, and transmits the amplified optical signal to the same direction for each group. Thereby, the transmission power of each wavelength signal becomes N times, and for the optical space communication receiving terminal 3, a signal of N times the optical power is received for each wavelength, and therefore, in the optical space communication transmitting terminal 2, the SNR is improved by about N times, and a communication capacity of about N times can be achieved.

[0028] The wavelengths of the optical signal output from the optical antenna 21a and the optical signal output from the optical antenna 21b are different from each other, and therefore, interference is reduced. Thereby, an optical signal subjected to wavelength multiplexing in the same manner as the wavelength multiplexing signal transmitted by one optical antenna is transmitted to the optical space communication receiving terminal 3.

[0029] In the optical space communication transmitting terminal 2, the output power of each optical amplifier can be reduced without changing the communication capacity, compared to a case in which the optical signals of the wavelengths λ1 to λ4 are set as one wavelength multiplexing signal, and the amplified by one optical amplifier and transmitted by one optical antenna. That is, although the transmission power of each wavelength signal is increased, the transmission power of each optical antenna is reduced, and therefore, the optical space communication transmitting terminal 2 is effective from the viewpoint of eye safety.

[0030] In the optical spatial communication transmission terminal 2, the transmission system of the optical signals of which the wavelengths are close among the plurality of wavelengths subjected to wavelength multiplexing is classified into the same group. Thereby, the wavelength range supported by the optical amplifier and the optical antenna belonging to the same group becomes narrower than the case where the optical signals subjected to wavelength multiplexing of the plurality of wavelengths are uniformly amplified and transmitted, and thus the wavelength dependency of the gain in the optical amplifier becomes smaller.

[0031] In the optical amplifier having a filter inside for fixing the gain, the loss of the filter is reduced. By grouping the optical signals of the plurality of wavelengths, the wavelength range supported by the optical antenna of each group becomes narrower, and thus the optical spatial communication transmission terminal 2 allows a simpler optical system. For example, the number of pieces of lenses for suppressing chromatic aberration can be reduced, and the number of layers of dielectric multilayer films for preventing reflection can be reduced.

[0032] The optical spatial communication reception terminal 3 is provided with an optical antenna 31, an optical amplifier 32, a wavelength demultiplexer 33, a receiver 34a, a receiver 34b, a receiver 34c, and a receiver 34d. The optical antenna 31 converges the light transmitted from the optical spatial communication transmission terminal 2 and propagating in space. The optical amplifier 32 amplifies the light converged by the optical antenna 31. The wavelength demultiplexer 33 demultiplexes the optical signals of each wavelength in the light amplified by the optical amplifier 32.

[0033] The receiver 34a is a coherent receiver (Rx(λ1)) that receives the optical signal of the wavelength λ1 in the optical signals demultiplexed by the wavelength demultiplexer 33 by each wavelength. The receiver 34b is a coherent receiver (Rx(λ2)) that receives the optical signal of the wavelength λ2 in the optical signals demultiplexed by the wavelength demultiplexer 33 by each wavelength. The receiver 34c is a coherent receiver (Rx(λ3)) that receives the optical signal of the wavelength λ3 in the optical signals demultiplexed by the wavelength demultiplexer 33 by each wavelength. The receiver 34d is a coherent receiver (Rx(λ4)) that receives the optical signal of the wavelength λ4 in the optical signals demultiplexed by the wavelength demultiplexer 33 by each wavelength.

[0034] An angle formed by two straight lines respectively connecting the opening center of the optical antenna 21a and the opening center of the optical antenna 21b provided in the optical spatial communication transmission terminal 2 with the opening center of the optical antenna 31 of the optical spatial communication reception terminal 3 as a vertex is included in the allowable angle error range of the optical spatial communication reception terminal 3 with respect to the incident light. Thereby, the light transmitted by the optical antenna 21a and the optical antenna 21b provided in the optical spatial communication transmission terminal 2 is incident in the optical antenna 31 of the optical spatial communication reception terminal 3 with an incident angle within the allowable angle error range, and is received as the wavelength multiplexed signal transmitted by one optical antenna.

[0035] For example, in a case where the communication distance of the optical space communication transmission terminal 2 and the optical space communication reception terminal 3 is long, the angle formed by two straight lines respectively connecting the opening center of the optical antenna 21a and the opening center of the optical antenna 21b with the opening center of the optical antenna 31 of the optical space communication reception terminal 3 as a vertex becomes a very small angle. Therefore, as a result, the angle is within the allowable angle error range of the optical space communication reception terminal 3 with respect to the incident light.

[0036] In the explanation so far, a case where the optical space communication transmission terminal 2 multiplexes four wavelengths of optical signals has been shown, but it is also possible to multiplex two wavelengths of signals different from each other, and it is also possible to multiplex three or more wavelengths. Furthermore, in the explanation so far, a case where the structural elements of the optical space communication transmission terminal 2 are divided into group (1) and group (2) has been shown, but the structural elements of the optical space communication transmission terminal 2 can also be divided into three or more groups.

[0037] Furthermore, in the explanation so far, a case where the optical space communication transmission terminal 2 multiplexes two wavelengths different for each group of two signals has been shown, but the number of wavelengths multiplexed in each group can also be a different number. For example, it is also possible that, in a case where the number of wavelengths multiplexed is five, three wavelengths different for three signals are multiplexed in group (1), and two wavelengths different for two signals are multiplexed in group (2).

[0038] As described above, the optical space communication transmission terminal 2 of Embodiment 1 groups a plurality of wavelengths to be multiplexed, multiplexes optical signals of different wavelengths for each group, amplifies the optical signals having been multiplexed, and transmits the amplified optical signals in the same direction for each group. Thereby, the optical space communication transmission terminal 2 can increase the transmission power of each wavelength, and along with this, the optical power reaching the reception side also increases, the SNR of the signal demodulated from the reception signal improves, and thus, it is possible to suppress a decrease in communication capacity due to an insufficient output of an optical amplifier in optical space communication.

[0039] In the optical space communication system 1 of Embodiment 1, the angle formed by two straight lines respectively connecting the opening center of the optical antenna 21a and the opening center of the optical antenna 21b possessed by the optical space communication transmission terminal 2 with the opening center of the optical antenna 31 of the optical space communication reception terminal 3 as a vertex is included in the allowable angle error range of the optical space communication reception terminal 3 with respect to the incident light. Thereby, the optical space communication reception terminal 3 can receive the light transmitted by the optical antenna 21a and the optical antenna 21b as well as the light transmitted by one optical antenna.

[0040] Embodiment 2.

[0041] Figure 2is a block diagram showing a configuration of the optical space communication transceiver terminal 4 as the optical space communication terminal of Embodiment 2. In Figure 2 The optical space communication transceiver terminal 4 is an optical space communication terminal that transmits an optical signal obtained by wavelength multiplexing of the lights of wavelengths λ1 to λ4 and receives an optical signal obtained by wavelength multiplexing of the lights of wavelengths λ5 to λ8, and uses the optical antenna 41a and the optical antenna 41b to transmit and receive the optical signals.

[0042] Further, in the optical space communication transceiver terminal 4, as a structural element that transmits the optical signals, there are provided the optical amplifier 42a, the optical amplifier 42b, the combiner 43a, the combiner 43b, the transmitter 44a, the transmitter 44b, the transmitter 44c, and the transmitter 44d, and as a structural element that receives the optical signals, there are provided the optical amplifier 45a, the optical amplifier 45b, the splitter 46a, the splitter 46b, the receiver 47a, the receiver 47b, the receiver 47c, and the receiver 47d.

[0043] The structural elements that transmit the optical signals in the optical space communication transceiver terminal 4 are divided into Group (1) and Group (2) as in Embodiment 1. The optical antenna 41a, the optical amplifier 42a, the combiner 43a, the transmitter 44a, and the transmitter 44b belong to Group (1). The optical antenna 41b, the optical amplifier 42b, the combiner 43b, the transmitter 44c, and the transmitter 44d belong to Group (2). Group (1) is a group that transmits an optical signal obtained by wavelength multiplexing of the adjacent wavelengths λ1 and λ2 among the four wavelengths λ1 to λ4. Group (2) is a group that transmits an optical signal obtained by wavelength multiplexing of the adjacent wavelengths λ3 and λ4.

[0044] Among the plurality of wavelengths in which wavelength multiplexing is performed, as in Embodiment 1, a wavelength in which there is no overlap in the spectral band of the optical signal in the interval of the adjacent wavelengths is used. For example, in Group (1), there is no overlap between the spectral band of the optical signal of the wavelength λ1 and the spectral band of the optical signal of the wavelength λ2, and in Group (2), there is no overlap between the spectral band of the optical signal of the wavelength λ3 and the spectral band of the optical signal of the wavelength λ4. Further, there is also no overlap between the spectral band of the optical signal of any wavelength in Group (1) and the spectral band of the optical signal of any wavelength in Group (2). Thereby, interference of the optical signals in which the wavelengths are close to each other is suppressed.

[0045] In Group (1), the transmitter 44a is a coherent transmitter (Tx(λ1)) that transmits the optical signal of the wavelength λ1, and the transmitter 44b is a coherent transmitter (Tx(λ2)) that transmits the optical signal of the wavelength λ2. The combiner 43a generates an optical signal obtained by wavelength multiplexing of the wavelengths λ1 and λ2 by combining the optical signal of the wavelength λ1 and the optical signal of the wavelength λ2. The optical amplifier 42a amplifies the wavelength multiplexed signal generated by the combiner 43a.

[0046] In the group (2), the transmitter 44c is a coherent transmitter (Tx(λ3)) that transmits an optical signal of the wavelength λ3, and the transmitter 44d is a coherent transmitter (Tx(λ4)) that transmits an optical signal of the wavelength λ4. The coupler 43b generates an optical signal obtained by wavelength multiplexing the wavelength λ3 and the wavelength λ4 by coupling the optical signal of the wavelength λ3 and the optical signal of the wavelength λ4. The optical amplifier 42b amplifies the wavelength multiplexed signal generated by the coupler 43b.

[0047] The optical antenna 41a belonging to the group (1) transmits the optical signal obtained by wavelength multiplexing the wavelength λ1 and the wavelength λ2 amplified by the optical amplifier 42a. Also, the optical antenna 41b belonging to the group (2) transmits the optical signal obtained by wavelength multiplexing the wavelength λ3 and the wavelength λ4 amplified by the optical amplifier 42b. The optical antenna 41a and the optical antenna 41b transmit optical signals to the same direction in which there is a common transmission target.

[0048] The output power of each of the optical amplifiers 42a and 42b has a limit, and therefore, the optical space communication transceiver terminal 4 divides a plurality of wavelengths subjected to wavelength multiplexing into N (N is an integer of 2 or more) groups, couples optical signals of different wavelengths for each group, amplifies the optical signals subjected to wavelength multiplexing, and transmits the amplified optical signals to the same direction for each group. Thereby, the transmission power of each wavelength signal becomes N times, and at the receiving side, a signal of N times the optical power is received for each wavelength, and therefore, in the optical space communication transceiver terminal 4, the SNR is improved by about N times, and a communication capacity of about N times can be achieved.

[0049] In the optical signal receiving system of the optical space communication transceiver terminal 4, the optical antennas 41a and 41b converge light propagating in space. For example, in a case where the communication distance from the optical space communication transceiver terminal 4 is long, the light sufficiently diffuses by propagating in space to reach the optical space communication transceiver terminal 4, and therefore, is incident to a plurality of optical antennas.

[0050] The optical amplifier 45a amplifies light converged by the optical antenna 41b, and the optical amplifier 45b amplifies light converged by the optical antenna 41a. The wave divider 46a divides each wavelength optical signal subjected to wavelength multiplexing in the light amplified by the optical amplifier 45a, and the wave divider 46b divides each wavelength optical signal subjected to wavelength multiplexing in the light amplified by the optical amplifier 45b.

[0051] The receiver 47a is a coherent receiver (Rx(λ5)) that receives the optical signal of wavelength λ5 among the optical signals that are demultiplexed by the demultiplexer 46a by each wavelength and receives the optical signal of wavelength λ5 among the optical signals that are demultiplexed by the demultiplexer 46b by each wavelength. The receiver 47b is a coherent receiver (Rx(λ6)) that receives the optical signal of wavelength λ6 among the optical signals that are demultiplexed by the demultiplexer 46a by each wavelength and receives the optical signal of wavelength λ6 among the optical signals that are demultiplexed by the demultiplexer 46b by each wavelength.

[0052] The receiver 47c is a coherent receiver (Rx(λ7)) that receives the optical signal of wavelength λ7 among the optical signals that are demultiplexed by the demultiplexer 46a by each wavelength and receives the optical signal of wavelength λ7 among the optical signals that are demultiplexed by the demultiplexer 46b by each wavelength. The receiver 47d is a coherent receiver (Rx(λ8)) that receives the optical signal of wavelength λ8 among the optical signals that are demultiplexed by the demultiplexer 46a by each wavelength and receives the optical signal of wavelength λ8 among the optical signals that are demultiplexed by the demultiplexer 46b by each wavelength.

[0053] Figure 3 is a block diagram that shows the structure of the receivers 47a to 47d. In Figure 3 , the receivers 47a to 47d are coherent receivers that receive the optical signals that are demultiplexed by the demultiplexer 46a and the demultiplexer 46b. The receivers 47a to 47d have a coherent detector 471a, a coherent detector 471b, an ADC 472, an MISO equalization processing section 473, a phase difference compensation section 474, and a demapping section 475. In addition, the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475 are functional structural elements that are provided by a digital signal processing circuit that is connected to the ADC 472.

[0054] The coherent detector 471a and the coherent detector 471b are detectors that cause the optical signals of the wavelengths that are demultiplexed by the demultiplexer 46a and the demultiplexer 46b to interfere with the local oscillation light (laser light) that is output from a local oscillation light source and are converted into electrical signals. For example, in the receiver 47a, the coherent detector 471a causes the optical signal of wavelength λ5 that is demultiplexed by the demultiplexer 46a to interfere with the local oscillation light that is output from a local oscillation light source and is converted into an electrical signal. The coherent detector 471b causes the optical signal of wavelength λ5 that is demultiplexed by the demultiplexer 46b to interfere with the local oscillation light that is output from a local oscillation light source and is converted into an electrical signal.

[0055] The ADC 472 is an AD converter that converts an analog electric signal corresponding to an optical signal of a wavelength separated from the received optical signal into a digital signal using each electric signal subjected to coherent detection by the coherent detector 471a and the coherent detector 471b. For example, in the receiver 47a, the ADC 472 converts an analog electric signal corresponding to an optical signal of the wavelength λ5 into a digital signal using an electric signal of the optical signal of the wavelength λ5 subjected to detection by the coherent detector 471a and an electric signal of the optical signal of the wavelength λ5 subjected to detection by the coherent detector 471b.

[0056] The MISO equalization processing section 473 performs multiple input single output processing (MISO processing) and equalization processing with respect to a plurality of digital signals converted from analog electric signals by the ADC 472. For example, in a case where the MISO equalization processing section 473 is implemented by an FIR filter, filter coefficients are adaptively set so that an output of the MISO equalization processing section 473 approaches an expected signal point distribution. At the time of setting the filter coefficients, a least mean square (LMS) algorithm based on a known signal pattern inserted in advance into a part of the signal can be used, or a method such as a constant envelope reference algorithm or an LMS algorithm based on temporary determination that does not require a known signal pattern can be used.

[0057] The phase difference compensation section 474 compensates for a phase difference of a signal subjected to MISO processing and equalization processing by the MISO equalization processing section 473. For example, the phase difference compensation section 474 compensates for a frequency difference and a phase difference between a transmission signal output from the transmitter 44a to 44d and a local oscillation light with respect to a signal subjected to MISO processing and equalization processing by carrier estimation. A series of processing constituted by MISO processing, equalization processing, and phase compensation is demodulation of an optical signal.

[0058] The demapping section 475 decodes a signal whose phase difference is compensated for by the phase difference compensation section 474. For example, with respect to a signal subjected to MISO processing and equalization processing, a bit sequence is output as a reception signal in correspondence with mapping of the bit sequence and the signal in the transmission system. Processing of converting a signal point after demodulation into bit data is decoding of an optical signal.

[0059] In addition, the coherent detector 471a, the coherent detector 471b, and the ADC 472 can not be structural elements possessed by the receivers 47a to 47d, but can be structural elements possessed by the terminal. In this case, the receivers 47a to 47d only have a digital signal processing circuit connected to the ADC 472.

[0060] The optical space communication reception method of Embodiment 2 is as described below.

[0061] Figure 4This is a flowchart illustrating the optical space communication receiving method of Embodiment 2, showing the processing of the MISO equalization processing unit 473, the phase difference compensation unit 474, and the demapping unit 475 in receivers 47a-47d. The MISO equalization processing unit 473 performs MISO processing and equalization processing on multiple digital signals from the ADC 472 (step ST1). Next, the phase difference compensation unit 474 compensates for the phase difference of the signals processed by the MISO equalization processing unit 473 (step ST2). The demapping unit 475 demaps the signals whose phase difference has been compensated by the phase difference compensation unit 474 (step ST3). Receivers 47a-47d input signals from the ADC 472 and repeatedly execute steps ST1-ST3.

[0062] The hardware structure for implementing the functions of receivers 47a to 47d is described below.

[0063] Figure 5A This is a block diagram showing the hardware structure for implementing the functions of receivers 47a to 47d. Figure 5B This is a block diagram illustrating the hardware structure of the software that implements the functions of receivers 47a to 47d. Figure 5A and Figure 5B In the middle, the coherent detector 100 is Figure 3 The coherent detectors 471a and 471b shown are ADC101. Figure 3 The ADC472 shown.

[0064] The functions of the MISO equalization processing unit 473, phase difference compensation unit 474, and demapping unit 475 in receivers 47a-47d are implemented by a processing circuit (digital signal processing circuit). That is, receivers 47a-47d are equipped with functions for performing... Figure 4 The processing circuit shown in steps ST1 to ST3. The processing circuit can also be dedicated hardware, or it can be a CPU (Central Processing Unit) that executes the program stored in memory.

[0065] In the processing circuit Figure 5AThe processing circuit 102 of the illustrated dedicated hardware corresponds, for example, to a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a combination thereof. The functions of the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475 provided in the receivers 47a to 47d can be implemented by different processing circuits, or the functions thereof can be unified and implemented by one processing circuit.

[0066] In the case of the processing circuit being Figure 5B In the case of the processor 103 illustrated, the functions of the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475 provided in the receivers 47a to 47d are implemented by software, firmware, or a combination of software and firmware. In addition, the software or firmware is described in the form of a program and stored in the memory 104.

[0067] The processor 103 implements the functions of the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475 provided in the receivers 47a to 47d by reading out and executing the program stored in the memory 104. For example, the receivers 47a to 47d are provided with the memory 104 storing a program that, when executed by the processor 103, results in the execution of Figure 4 the processing of the steps ST1 to ST3 illustrated. These programs cause the computer to execute the steps or methods of the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475. The memory 104 can also be a computer-readable storage medium storing a program for causing a computer to function as the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475.

[0068] The memory 104 corresponds, for example, to a non-volatile or volatile semiconductor memory such as a RAM (Random Access Memory), a ROM (ReadOnly Memory), a flash memory, an EPROM (Erasable Programmable ROM), an EEPROM (Electrically-EPROM), or the like, a magnetic disk, a flexible disk, an optical disk, a high-density disk, a mini disk, a DVD, or the like.

[0069] It is also possible that part of the functions of the MISO equalization processing section 473, the phase difference compensation section 474, and the demapping section 475 provided in the receivers 47a to 47d is implemented by dedicated hardware, and the remaining part is implemented by software or firmware. For example, the MISO equalization processing section 473 and the phase difference compensation section 474 implement the functions by the processing circuit 102 as dedicated hardware, and the demapping section 475 implements the functions by the processor 103 reading and executing a program stored in the memory 104. In this way, the processing circuit can implement the above functions by hardware, software, firmware, or a combination thereof.

[0070] As described above, the optical space communication transceiving terminal 4 of Embodiment 2 has, in addition to the structure of the optical space communication transmitting terminal 2, a wavelength splitter 46a and 46b that splits the optical signal into each wavelength that is multiplexed, and a receiver 47a to 47d that receives the optical signal split by the wavelength splitter 46a and 46b, provided for each wavelength that is multiplexed. The optical antennas 41a and 41b converge the light that propagates in space, and the optical amplifiers 45a and 45b amplify the light converged by the optical antennas 41a and 41b. The wavelength splitter 46a and 46b splits the light amplified by the optical amplifiers 45a and 45b into each wavelength of the optical signal that is multiplexed in the light, and the receiver 47a to 47d demodulates and decodes the optical signal split by the wavelength splitter 46a and 46b. The optical space communication transceiving terminal 4 has the same structure as the optical space communication transmitting terminal 2, and thus the same effects as Embodiment 1 are obtained. Furthermore, the noise superimposed on each signal converged by the optical antennas 41a and 41b is mainly the noise generated from the optical amplifiers 45a and 45b located at the front stage of the wavelength splitter 46a and 46b, and is mutually independent noise. Therefore, the optical space communication transceiving terminal 4 coherently detects the signals converged by the optical antennas 41a and 41b, and performs MISO processing on the signals after the coherent detection, whereby the SNR of the received signal is improved, and reduction in communication capacity can be suppressed.

[0071] Embodiment 3.

[0072] Figure 6 is a block diagram showing the structure of the optical space communication system 1A of Embodiment 3. In Figure 6 , the optical space communication system 1A is, for example, a system that communicates an optical signal obtained by multiplexing four wavelengths λ1 to λ4 of light that are different from each other, and communicates an optical signal obtained by multiplexing four wavelengths λ5 to λ8 of light that are different from each other, and has an optical space communication transceiving terminal 4 and an optical space communication transceiving terminal 5.

[0073] Optical space communication transceiver terminal 4 is the first transceiver terminal, which sends optical signals obtained by wavelength multiplexing light with wavelengths λ1 to λ4 to optical space communication transceiver terminal 5, and receives optical signals obtained by wavelength multiplexing light with wavelengths λ5 to λ8 from optical space communication transceiver terminal 5. Optical space communication transceiver terminal 5 is the second transceiver terminal, which sends optical signals obtained by wavelength multiplexing light with wavelengths λ5 to λ8 to optical space communication transceiver terminal 4, and receives optical signals obtained by wavelength multiplexing light with wavelengths λ1 to λ4 from optical space communication transceiver terminal 4. That is, optical space communication transceiver terminal 4 and optical space communication transceiver terminal 5 engage in bidirectional communication.

[0074] like Figure 6 As shown, the optical space communication transceiver terminal 5 includes an optical antenna 51, optical amplifiers 52a and 52b, a multiplexer 53, a demultiplexer 54, transmitters 55a-55d, and receivers 56a-56d. Transmitters 55a-55d are coherent transmitters (Tx(λ5)-Tx(λ8)) that transmit optical signals with wavelengths λ5-λ8. The multiplexer 53 generates an optical signal by multiplexing the optical signals with wavelengths λ5-λ8. The optical amplifier 52a amplifies the wavelength-multiplexed signal generated by the multiplexer 53. The optical antenna 51 transmits the wavelength-multiplexed signal amplified by the optical amplifier 52a to the optical space communication transceiver terminal 4.

[0075] Optical antenna 51 converges light transmitted from optical space communication transceiver terminal 4 and propagating in space. Optical amplifier 52b amplifies the light converged by optical antenna 51. Demultiplexer 54 demultiplexes the optical signal of each wavelength that has been wavelength multiplexed in the light amplified by optical amplifier 52b. Receivers 56a to 56d are coherent receivers (Rx(λ1) to Rx(λ4)) that receive the optical signals of wavelengths λ1 to λ4 demultiplexed by demultiplexer 54.

[0076] The angle formed by the two straight lines connecting the center of the opening of the optical antenna 51 of the optical space communication transceiver terminal 5 and the center of the opening of the optical antenna 41a and the optical antenna 41b of the optical space communication transceiver terminal 4, with the center of the opening of the optical antenna 51 of the optical space communication transceiver terminal 5 as the vertex, is within the allowable angular error range of the optical space communication transceiver terminal 5 for incident light. Therefore, the light transmitted by the optical antennas 41a and 41b is incident at an angle within the allowable angular error range in the optical antenna 51 and is received in the same way as a wavelength multiplexed signal transmitted by a single optical antenna.

[0077] For example, in a case where the communication distance between the optical space communication transceiver terminal 4 and the optical space communication transceiver terminal 5 is long, the angle formed by two straight lines respectively connecting the opening center of the optical antenna 41a and the opening center of the optical antenna 41b with the opening center of the optical antenna 51 of the optical space communication transceiver terminal 5 as a vertex becomes a very small angle. As a result, the angle is within the allowable angle error range of the optical space communication transceiver terminal 5 with respect to incident light.

[0078] As above, the optical space communication system 1A of Embodiment 3 is provided with the optical space communication transceiver terminal 4 and the optical space communication transceiver terminal 5. The angle formed by two straight lines respectively connecting the opening center of the optical antenna 41a and the opening center of the optical antenna 41b provided in the optical space communication transceiver terminal 4 with the opening center of the optical antenna 51 of the optical space communication transceiver terminal 5 as a vertex is within the allowable angle error range of the optical space communication transceiver terminal 5 with respect to incident light. Thus, the optical space communication transceiver terminal 5 can receive light transmitted by the optical antenna 41a and the optical antenna 41b as well as light transmitted by one optical antenna.

[0079] In addition, combinations of the respective embodiments or modifications of any structural element of the respective embodiments can be made, or any structural element can be omitted in the respective embodiments.

[0080] Industrial Applicability

[0081] The optical space communication terminal of the present disclosure can be used for optical space communication with artificial satellites, for example.

[0082] Explanation of Reference Signs

[0083] 1, 1A optical space communication system, 2 optical space communication transmission terminal, 3 optical space communication reception terminal, 4, 5 optical space communication transceiver terminal, 21a, 21b, 31, 41a, 41b, 51 optical antenna, 22a, 22b, 32, 42a, 42b, 45a, 45b, 52a, 52b optical amplifier, 23a, 23b, 43a, 43b, 53 combiner, 24a to 24d, 44a to 44d, 55a to 55d transmitter, 33, 46a, 46b, 54 splitter, 34a to 34d, 47a to 47d, 56a to 56d receiver, 471a, 471b coherent detector, 473 MISO equalization processing section, 474 phase difference compensation section, 475 demapping section.

Claims

1. An optical space communication system, characterized in that, The optical space communication system includes an optical space communication terminal and a receiving terminal. This optical space communication terminal has the following features: Multiple transmitters, forming N groups, transmit optical signals of different wavelengths, where N is an integer greater than 2; N multiplexers are provided, with one multiplexer set for each group. The multiplexer outputs a wavelength-multiplexed optical signal by multiplexing the optical signals transmitted by the transmitters belonging to the group. N first optical amplifiers are configured, one for each group, and the first optical amplifier amplifies the wavelength-multiplexed optical signal; and N optical antennas are configured, with one antenna set up for each group. Each optical antenna transmits amplified optical signals into space. The receiving terminal receives the optical signal sent by the optical space communication terminal. Each of the groups of optical antennas transmits optical signals to the same optical antenna of the receiving terminal.

2. The optical space communication system according to claim 1, characterized in that, The optical space communication terminal also features: A wavelength division multiplexer, which divides the optical signal into multiplexed wavelengths; and A receiver, configured according to each multiplexed wavelength, receives the optical signal after it has been split by the wavelength divider. The optical antenna is also used to converge light propagating in space. The optical space communication terminal also features: The second optical amplifier amplifies the light focused by the optical antenna. The wavelength division unit (WDM) divides the light amplified by the second optical amplifier into optical signals of each wavelength that are multiplexed within the light. The receiver demodulates and decodes the optical signal after it has been split by the wavelength divider.

3. A receiver that demodulates and decodes multiple optical signals after wavelength division by the wavelength divisionr of the optical space communication terminal in the optical space communication system of claim 2, characterized in that, The receiver has: The multiple input single output equalization processing unit takes multiple digital signals as input and performs multiple input single output processing and equalization processing on the multiple input digital signals. The multiple digital signals are obtained by performing coherent detection and analog-to-digital conversion on multiple optical signals after being split by the wavelength divider. A phase difference compensation unit compensates for the phase difference of the signal processed by the multi-input single-output equalization processing unit; as well as The demapping unit decodes the signal after the phase difference has been compensated.

4. An optical space communication system, characterized in that, In the optical space communication system of claim 1 The angle formed by two straight lines connecting the opening centers of any two optical antennas of the optical space communication terminal, with the opening center of the receiving terminal's optical antenna as the vertex, is included within the allowable angle error range of the receiving terminal for incident light.

5. An optical space communication system, characterized in that, The optical space communication system includes: The first transceiver terminal is the optical space communication terminal as described in claim 2; as well as The second transceiver terminal transmits and receives optical signals with the first transceiver terminal. The angle formed by two straight lines connecting the opening center of the optical antenna of the second transceiver terminal to the opening centers of any two optical antennas of the first transceiver terminal, with the opening center of the second transceiver terminal as the vertex, is included within the allowable angle error range of the second transceiver terminal for incident light. The first transceiver terminal and the second transceiver terminal communicate bidirectionally.

6. A method for receiving optical space communication signals, comprising a receiver for receiving optical space communication signals, wherein the receiver demodulates and decodes multiple optical signals after being demultiplexed by the wavelength division unit of the optical space communication terminal described in claim 2, characterized in that, The optical space communication receiving method includes the following steps: The multiple input single output equalization processing unit takes in multiple digital signals and performs multiple input single output processing and equalization processing on the multiple input digital signals. The multiple digital signals are obtained by performing coherent detection and analog-to-digital conversion on the multiple optical signals after being split by the wavelength divider. The phase difference compensation unit compensates for the phase difference of the signal processed by the multi-input single-output equalization processing unit; as well as The demapping unit decodes the signal after the phase difference has been compensated. The series of processes performed by the multi-input single-output equalization processing unit, the phase difference compensation unit, and the demapping unit are repeatedly executed.

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