Receiving apparatus and receiving method

Abstract

This reception device comprises: a conversion unit that converts a received optical signal to an electrical signal; a demodulation unit that generates a carrier signal on the basis of the electrical signal; a physical quantity measurement unit that measures the physical quantity of the electrical signal and the physical quantity of the carrier signal; and a control unit that adjusts the physical quantity of the received optical signal when the physical quantity of the electrical signal is below a first threshold value, or when the physical quantity of the carrier signal is below a second threshold value. The physical quantity of the electrical signal is the intensity level of the received optical signal. The control unit amplifies the intensity level of the received optical signal when the physical quantity of the electrical signal is below the first threshold value. The physical quantity of the carrier signal is at least one of the carrier-to-noise ratio, the amount of composite second-order distortion, or the amount of composite third-order distortion. The control unit amplifies the intensity level of the received optical signal when the physical quantity of the carrier signal is below the second threshold value.

Description

Technical Field

[0001] The present invention relates to a receiving apparatus and a receiving method. This application claims priority based on PCT / JP2021 / 005816 filed as an international application on February 17, 2021, the contents of which are incorporated herein by reference.

Background Art

[0002] In an optical transmission system, the transmission apparatus (Transmission Amplifier: TA) on the uppermost upstream side, the receiving apparatus (Video-Optical Network Unit: V-ONU) on the lowermost downstream side, the transmission apparatus, the relay apparatus (Video-Optical Line Terminal: V-OLT), the branching apparatus, and the receiving apparatus may be connected using a coaxial cable, an optical fiber, or the like.

[0003] In an optical transmission system, the transmission apparatus may transmit a video signal to the receiving apparatus using an optical signal generated by frequency modulation (FM) batch conversion and optical modulation. Here, the optical signal (video signal) transmitted from the transmission apparatus is relayed to the branching apparatus by the relay apparatus. The branching apparatus distributes the optical signal relayed by the relay apparatus to a plurality of receiving apparatuses. Each receiving apparatus generates a carrier signal (video signal) based on an electrical signal by performing demodulation processing on the electrical signal corresponding to the optical signal input from the branching apparatus. The receiving apparatus outputs the generated carrier signal to the video playback apparatus.

[0004] For the carrier signals output from the receiving device, threshold values for each of the carrier-to-noise ratio (CNR), the amount of composite second order (CSO), and the amount of composite triple beat (CTB) are determined for each type of carrier signal. Thereby, the quality for video reproduction is guaranteed. In an optical transmission system, the quality of the carrier signal output from the receiving device needs to be guaranteed based on these respective threshold values (see Non-Patent Document 1).

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] In order to guarantee the quality of the carrier signal output from the receiving device, it is necessary to guarantee the quality of the carrier signal (original carrier signal) input to the transmitting device. However, even if the quality of the carrier signal input to the transmitting device is guaranteed, the intensity level and quality of the optical signal deteriorate according to the transmission distance of the optical signal between the relay device and the receiving device.

[0007] Therefore, when the quality of the carrier wave signal input to the transmission device is constant, the transmission distance of the optical signal is limited. In other words, the range of the transmission distance is determined according to the respective thresholds for the intensity level of the optical signal input from the relay device to the branching device, the intensity level of the optical signal input from the branching device to the receiving device, and the quality of the carrier wave signal output from the receiving device. Thus, when the transmission distances of the optical signals are different, if receiving devices with different receiving performances for each transmission distance are not prepared, it may not be possible to guarantee the quality of the carrier wave signal.

[0008] In view of the above circumstances, an object of the present invention is to provide a receiving device and a receiving method capable of improving the possibility of guaranteeing the quality of a carrier wave signal without preparing receiving devices with different receiving performances for each transmission distance.

Means for Solving the Problem

[0009] One aspect of the present invention is a receiving device including a conversion unit that converts a received optical signal into an electrical signal, a demodulation unit that generates a carrier wave signal based on the electrical signal, a physical quantity measurement unit that measures a physical quantity of the electrical signal or a physical quantity of the carrier wave signal, and a control unit that adjusts a physical quantity of the received optical signal when the physical quantity of the electrical signal is less than a first threshold or when the physical quantity of the carrier wave signal is less than a second threshold.

[0010] One aspect of the present invention is a receiving method executed by a receiving device, the receiving method including a conversion step of converting a received optical signal into an electrical signal, a demodulation step of generating a carrier wave signal based on the electrical signal, a physical quantity measurement step of measuring a physical quantity of the electrical signal or a physical quantity of the carrier wave signal, and a control step of adjusting a physical quantity of the received optical signal when the physical quantity of the electrical signal is less than a first threshold or when the physical quantity of the carrier wave signal is less than a second threshold.

Effects of the Invention

[0011] According to the present invention, it is possible to improve the quality of a carrier wave signal without preparing a receiving device having different reception performances for each transmission distance.

Brief Description of the Drawings

[0012]

Figure 1

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Modes for Carrying Out the Invention

[0013] Embodiments of the present invention will be described in detail with reference to the drawings. (First Embodiment) FIG. 1 is a diagram showing a configuration example of an optical transmission system 1 (video optical transmission network) in each embodiment. The optical transmission system 1 is a system that transmits an optical signal. The optical signal is generated, for example, according to a video signal. The optical transmission system 1 includes a transmission device 2, a plurality of relay devices 3, a plurality of branching devices 4, and a plurality of reception devices 5.

[0014] A carrier signal such as a video signal is input to the transmission device 2 (TA). The transmission device 2 generates an optical signal by frequency modulation batch conversion and optical modulation for the carrier signal. That is, the transmission device 2 batch-converts the input carrier signal (frequency multiplexed signal) (Radio Frequency signal) into a broadband frequency modulation signal (frequency modulation signal). The transmission device 2 modulates the intensity of the optical signal using the broadband frequency modulation signal. The transmission device 2 transmits an optical signal addressed to one or more reception devices 5 (V-ONU) to the relay device 3-0.

[0015] The relay device 3 is an optical subscriber line terminal device (OLT). Hereinafter, the relay device 3 is a video signal-relaying optical subscriber line terminal device (V-OLT). An optical signal is input to the relay device 3-0 from the transmission device 2. The relay device 3-0 outputs the optical signal to the relay device 3-n (n is an integer of 1 or more) using an optical fiber. The optical signal output from the relay device 3-0 is relayed by a plurality of relay devices 3-n.

[0016] An optical signal is input to the relay device 3-n from the relay device 3-0. The relay device 3-n relays the optical signal to the branching device 4-n using an optical fiber. The relay device 3-n outputs an optical signal having an intensity level of "P V-OLT " [dBm] to the branching device 4-n. Note that the relay device 3 may perform dispersion compensation processing for reducing the influence of non-linear optical effects in the optical signal on the optical signal as necessary.

[0017] An optical signal is input to the branching device 4-n from the relay device 3-n. The branching device 4-n distributes the optical signal relayed by the relay device 3-n to a plurality of receiving devices 5-n.

[0018] Hereinafter, the lower limit of the intensity level of the optical signal input to the receiving device 5-n is denoted as "Pdn". Hereinafter, the upper limit of the intensity level of the optical signal input to the receiving device 5-n is denoted as "Pun".

[0019] The receiving device 5 is an optical network unit (ONU). Hereinafter, the receiving device 5 is a video optical network unit (V-ONU) that outputs a video signal. An optical signal having a predetermined intensity level [dBm] is input to the receiving device 5-n from the branching device 4-n. For example, an optical signal having an intensity level ranging from "Pd1" to "Pu1" according to the transmission distance is input to the receiving device 5-1 from the branching device 4-1. For example, an optical signal having an intensity level ranging from "Pd2" to "Pu2" according to the transmission distance is input to the receiving device 5-2 from the branching device 4-2.

[0020] The receiving device 5-n generates a carrier signal based on an electrical signal by performing demodulation processing on the electrical signal corresponding to the optical signal input from the branching device 4-n. The receiving device 5 outputs the generated carrier signal to a user terminal (not shown). The user terminal is, for example, an information processing device that plays back video.

[0021] Next, the details of the receiving device 5a will be described. FIG. 2 is a diagram showing a configuration example of the receiving device 5a in the first embodiment. The receiving device 5a corresponds to the receiving device 5 shown in FIG. 1. The receiving device 5a includes a level measurement unit 50, a physical quantity measurement unit 51a, a control unit 52a, a level amplification unit 53a, an optical processing unit 54a, a demodulation unit 55, and a carrier signal amplification unit 56a. The optical processing unit 54a includes a conversion unit 540, a branching unit 541, and a preamplifier 542. The carrier signal amplification unit 56a includes a filter 560 and a post amplifier 561.

[0022] The optical processing unit 54a may further include an electrical amplifier in at least one of the front stage and the rear stage of the branching unit 541. Thereby, even when the intensity level of the electrical signal decreases due to the branching of the electrical signal by the branching unit 541, the intensity level of the electrical signal can be compensated.

[0023] An optical signal is input to the level measurement unit 50 from the branching device 4. The level measurement unit 50 measures the intensity level of the optical signal input from the branching device 4. The level measurement unit 50 outputs a control signal representing the measured intensity level of the optical signal to the control unit 52a. The level measurement unit 50 outputs the optical signal input from the branching device 4 to the level amplifier unit 53a.

[0024] An electrical signal (frequency modulation batch conversion signal) is input to the physical quantity measurement unit 51a from the branching unit 541. Hereinafter, the frequency modulation batch conversion signal is referred to as an "FM batch conversion signal". The physical quantity measurement unit 51a measures the intensity level of the electrical signal (FM batch conversion signal) input from the branching unit 541. The physical quantity measurement unit 51a outputs a control signal representing the measured intensity level of the electrical signal (FM batch conversion signal) to the control unit 52a.

[0025] The control unit 52a acquires a control signal representing the intensity level "Pm" of the optical signal from the level measurement unit 50. The control unit 52a acquires a control signal representing the intensity level "Em" of the electrical signal (FM batch conversion signal) from the physical quantity measurement unit 51a.

[0026] The control unit 52a holds in advance data representing the range "Pd~Pu" [dBm] of the intensity level of the optical signal that the conversion unit 540 can receive, and data representing the intensity level "E" of the electrical signal required for the demodulation process executed by the demodulation unit 55.

[0027] The control unit 52a adjusts the amplification gain of the optical signal in the level amplifier 53a so that the intensity level "Em" of the electrical signal is equal to or higher than the intensity level "E", and the intensity level of the optical signal input to the conversion unit 540 falls within the range from the lower limit "Pd" to the upper limit "Pu". The control unit 52a outputs a control signal representing the amplification gain to the level amplifier 53a.

[0028] The control unit 52a determines whether the intensity level "Em" of the electrical signal is lower than the intensity level "E" of the electrical signal. When the intensity level "Em" of the electrical signal is lower than the intensity level "E" of the electrical signal, the control unit 52a adjusts the amplification gain of the optical signal in the level amplifier 53a so that the level amplifier 53a outputs an optical signal with an intensity level that is higher than the intensity level "Pm" of the optical signal by a fixed width "Pw" to the conversion unit 540.

[0029] The control unit 52a determines again whether the intensity level "Em" of the electrical signal is lower than the intensity level "E" of the electrical signal. When the intensity level "Em" of the electrical signal is lower than the required intensity level "E" of the electrical signal, the control unit 52a readjusts the amplification gain of the optical signal in the level amplifier 53a. The control unit 52a readjusts the amplification gain of the optical signal in the level amplifier 53a until it is determined that the intensity level "Em" of the electrical signal is equal to or higher than the intensity level "E" of the electrical signal. However, when the intensity level of the optical signal exceeds the upper limit "Pu", the control unit 52a aborts the readjustment process of the amplification gain so that the intensity level of the optical signal does not exceed the upper limit "Pu".

[0030] The control algorithm executed by the control unit 52a is not limited to a specific algorithm. The control unit 52a executes the adjustment process of the amplification gain using an optimization method such as the binary search method or the non - linear least squares method. Note that if the quality of the carrier signal (e.g., carrier - to - noise ratio) reaches the quality standard (threshold value) for video playback, the intensity level "Em" of the electrical signal may not be optimized.

[0031] An optical signal corresponding to the carrier signal is input from the level measurement unit 50 to the level amplification unit 53a (variable amplification unit). The level amplification unit 53a acquires a control signal representing the amplification gain from the control unit 52a. The level amplification unit 53a amplifies the intensity level of the optical signal based on the amplification gain indicated by the control signal. The level amplification unit 53a outputs the optical signal to the conversion unit 540.

[0032] The optical processing unit 54a is, for example, a Receiver Optical Sub-Assembly (ROSA). The conversion unit 540 (photoelectric conversion unit) is a photodiode. An optical signal is input from the level amplification unit 53a to the conversion unit 540. The conversion unit 540 converts the optical signal into an electrical signal (FM integrated conversion signal). The conversion unit 540 outputs the converted electrical signal to the branching unit 541.

[0033] The branching unit 541 acquires the electrical signal from the conversion unit 540. The branching unit 541 distributes the electrical signal (FM integrated conversion signal) to the preamplifier 542 and the physical quantity measurement unit 51a.

[0034] The preamplifier 542 is, for example, a Trans-impedance amplifier (TIA). The preamplifier 542 acquires the electrical signal (FM integrated conversion signal) from the branching unit 541. The preamplifier 542 amplifies the intensity level of the electrical signal based on a predetermined amplification gain. The preamplifier 542 outputs an electrical signal with an intensity level of "E" or higher to the demodulation unit 55.

[0035] The demodulation unit 55 acquires an electrical signal (FM integrated conversion signal) with an intensity level of "E" or higher from the preamplifier 542. The demodulation unit 55 generates a carrier signal (video signal) based on the electrical signal by performing demodulation processing on the electrical signal (FM integrated conversion signal). The demodulation unit 55 outputs the carrier signal to the filter 560.

[0036] The carrier signal amplification unit 56a amplifies the intensity level of the carrier signal. The filter 560 acquires the carrier signal (video signal) from the demodulation unit 55. The filter 560 is a low-pass filter (LPF). The filter 560 allows the low-frequency carrier signal (video signal) to pass through. That is, the filter 560 removes noise from the carrier signal (video signal). The post-amplifier 561 amplifies the intensity level of the carrier signal (video signal) with noise removed based on a predetermined amplification gain. The post-amplifier 561 outputs the carrier signal (video signal) with the amplified intensity level to a user terminal (not shown).

[0037] In this way, the quality of the carrier signal with the amplified intensity level is equal to or higher than a predetermined threshold value. That is, in the carrier signal, the carrier-to-noise ratio, the amount of composite second-order distortion, and the amount of composite third-order distortion are guaranteed.

[0038] Next, an operation example of the optical transmission system 1 will be described. FIG. 3 is a flowchart showing an operation example of the optical transmission system 1 in the first embodiment. The optical transmission system 1 executes the operations shown in the flowchart in FIG. 3 at a predetermined cycle.

[0039] The level measurement unit 50 measures the intensity level of the input optical signal. The level amplification unit 53a amplifies the intensity level of the optical signal based on the amplification gain instructed from the control unit 52a using a control signal (step S101). The conversion unit 540 converts the optical signal into an electrical signal (FM integrated conversion signal). The branching unit 541 distributes the electrical signal (FM integrated conversion signal) to the pre-amplifier 542 and the physical quantity measurement unit 51a. The physical quantity measurement unit 51a measures the intensity level of the electrical signal (FM integrated conversion signal) input from the branching unit 541 (step S102).

[0040] The physical quantity measurement unit 51a measures the intensity level of the electrical signal (FM batch conversion signal) input from the branch unit 541 (step S103). The control unit 52a determines whether the intensity level "Em" of the electrical signal is equal to or higher than the intensity level "E" of the electrical signal (threshold or higher) (step S104). When it is determined that the intensity level "Em" of the electrical signal is equal to or higher than the intensity level "E" of the electrical signal (step S104: YES), the optical transmission system 1 ends the operation shown using the flowchart in FIG. 3.

[0041] When it is determined that the intensity level "Em" of the electrical signal is less than the intensity level "E" of the electrical signal (step S104: NO), the control unit 52a generates a control signal so that the level amplification unit 53a further amplifies the intensity level of the optical signal by a fixed width "Pw". The level amplification unit 53a further amplifies the intensity level of the optical signal by a fixed width "Pw" (step S105).

[0042] The control unit 52a determines whether the intensity level further amplified by the fixed width "Pw" exceeds the upper limit "Pu" (step S106). When it is determined that the intensity level further amplified by the fixed width "Pw" is equal to or less than the upper limit "Pu" (step S106: NO), the optical transmission system 1 ends the operation shown using the flowchart in FIG. 3. When it is determined that the intensity level further amplified by the fixed width "Pw" exceeds the upper limit "Pu" (step S106: YES), the control unit 52a generates a control signal to stop the process of the level amplification unit 53a further amplifying the intensity level of the optical signal by a fixed width. The level amplification unit 53a amplifies the intensity level of the optical signal with the upper limit amplification gain (step S107).

[0043] As described above, the conversion unit 540 converts the received optical signal (FM batch conversion signal) into an electrical signal. The demodulation unit 55 generates a carrier signal based on the electrical signal. The physical quantity measurement unit 51a measures the physical quantity of the electrical signal output from the conversion unit 540. When the physical quantity of the electrical signal (intensity level "Em") is less than the first threshold value (intensity level "E"), the control unit 52a adjusts the physical quantity of the received optical signal. For example, the physical quantity of the electrical signal is the intensity level of the electrical signal. When the physical quantity of the electrical signal is less than the first threshold value, the control unit 52a amplifies the intensity level of the received optical signal.

[0044] Thereby, it is possible to improve the quality of the carrier signal without preparing reception devices with different reception performances for each transmission distance. That is, it is possible to improve the quality of the carrier signal without having to properly select reception devices according to the transmission distance.

[0045] In the first embodiment, the reception device 5a improves the quality of the output carrier signal by adjusting the intensity level of the input optical signal. Thereby, the transmission distance between the relay device 3-n and the reception device 5a can be increased, so that it is possible to use the reception device 5a with the same reception performance in a wider area.

[0046] The deterioration of the carrier-to-noise ratio (CNR) is affected by both the noise in the transmission path of the optical signal and the noise inside the reception device 5a (V-ONU). When the intensity level of the optical signal input to the reception device 5a is low, the influence of the noise inside the reception device 5a is greater than the influence of the noise in the transmission path of the optical signal. Therefore, by amplifying the intensity level of the optical signal input to the reception device 5a, the influence of the noise inside the reception device 5a is mitigated, so that it is possible to suppress the deterioration of the carrier-to-noise ratio (CNR).

[0047] (Second Embodiment) In the second embodiment, the difference from the first embodiment is that the quality of the carrier signal (video signal) output from the post amplifier is measured. In the second embodiment, the description will focus on the differences from the first embodiment.

[0048] FIG. 4 is a diagram showing a configuration example of the receiving apparatus 5b in the second embodiment. The receiving apparatus 5b corresponds to the receiving apparatus 5 shown in FIG. 1. The receiving apparatus 5b includes a level measurement unit 50, a physical quantity measurement unit 51b, a control unit 52b, a level amplification unit 53b, an optical processing unit 54b, a demodulation unit 55, and a carrier signal amplification unit 56b.

[0049] The control unit 52b, the level amplification unit 53b, and the optical processing unit 54b correspond to the control unit 52a, the level amplification unit 53a, and the optical processing unit 54a shown in FIG. 2. The optical processing unit 54b includes a conversion unit 540 and a preamplifier 542. The carrier signal amplification unit 56b includes a filter 560, a post amplifier 561, and a branching unit 562.

[0050] The carrier signal amplification unit 56b may further include an electric amplifier at at least one of the front stage and the rear stage of the branching unit 562. Thereby, even when the intensity level of the electric signal decreases due to the branching of the electric signal by the branching unit 562, the intensity level of the electric signal can be compensated.

[0051] An electric signal (video signal) is input to the physical quantity measurement unit 51b from the branching unit 562. The physical quantity measurement unit 51b measures the carrier-to-noise ratio (CNR) of the electric signal (video signal) input from the branching unit 562. The physical quantity measurement unit 51b may measure the amount of composite second-order distortion and the amount of composite third-order distortion of the electric signal input from the branching unit 562. The physical quantity measurement unit 51b outputs a control signal representing the measured carrier-to-noise ratio "CNRm(i)" of the electric signal (video signal) to the control unit 52b. This symbol "i" is an integer greater than 0 and less than the integer "N" when the integer "N" represents the total number of video channels.

[0052] The control unit 52b acquires a control signal representing the intensity level "Pm" of the optical signal from the level measurement unit 50. The control unit 52b acquires a control signal representing the carrier-to-noise ratio of the electrical signal (video signal) from the physical quantity measurement unit 51b.

[0053] The control unit 52b pre-holds data representing the range "Pd~Pu" [dBm] of the intensity level of the optical signal that the conversion unit 540 can receive, and data representing the carrier-to-noise ratio "CNR(i)" of the electrical signal required for video playback.

[0054] The control unit 52b adjusts the amplification gain of the optical signal in the level amplification unit 53b so that the intensity level "Em" of the electrical signal is equal to or higher than the intensity level "E", and the intensity level of the optical signal input to the conversion unit 540 falls within the range from the lower limit "Pd" to the upper limit "Pu". The control unit 52b outputs a control signal representing the amplification gain to the level amplification unit 53b.

[0055] The control unit 52b determines whether the carrier-to-noise ratio "CNRm(i)" of the electrical signal is lower than the carrier-to-noise ratio "CNR(i)" of the electrical signal (video signal). When the carrier-to-noise ratio "CNRm(i)" of the electrical signal is lower than the carrier-to-noise ratio "CNR(i)" of the electrical signal, the control unit 52b adjusts the amplification gain of the optical signal in the level amplification unit 53b so that the level amplification unit 53b outputs an optical signal with an intensity level that is higher than the intensity level "Pm" of the optical signal by a fixed width "Pw" to the conversion unit 540.

[0056] The control unit 52b determines again whether the carrier-to-noise ratio "CNRm(i)" of the electrical signal is lower than the carrier-to-noise ratio "CNR(i)" of the electrical signal (video signal). When the carrier-to-noise ratio "CNRm(i)" of the electrical signal is lower than the required carrier-to-noise ratio "CNR(i)" of the electrical signal, the control unit 52b readjusts the amplification gain of the optical signal in the level amplification unit 53b. The control unit 52b readjusts the amplification gain of the optical signal in the level amplification unit 53b until it is determined that the carrier-to-noise ratio "CNRm(i)" of the electrical signal is equal to or higher than the carrier-to-noise ratio "CNR(i)" of the electrical signal. However, when the intensity level of the optical signal exceeds the upper limit "Pu", the control unit 52b stops the readjustment process of the amplification gain so that the intensity level of the optical signal does not exceed the upper limit "Pu". Also, when the intensity level of the optical signal is lower than the lower limit "Pd", the control unit 52b stops the readjustment process of the amplification gain so that the intensity level of the optical signal does not fall below the lower limit "Pd".

[0057] The control algorithm executed by the control unit 52b is not limited to a specific algorithm. The control unit 52b executes the adjustment process of the amplification gain using an optimization method such as the binary search method or the non-linear least squares method. Note that if the quality of the carrier signal (e.g., the carrier-to-noise ratio) reaches the quality standard (threshold value) at which video can be reproduced, the carrier-to-noise ratio "CNRm(i)" of the electrical signal does not have to be optimized.

[0058] An optical signal corresponding to the carrier signal is input from the level measurement unit 50 to the level amplification unit 53b (variable amplification unit). The level amplification unit 53b acquires a control signal representing the amplification gain from the control unit 52b. The level amplification unit 53b amplifies the intensity level of the optical signal based on the amplification gain instructed using the control signal. The level amplification unit 53b outputs the optical signal to the conversion unit 540.

[0059] The optical processing unit 54b is, for example, a receiver optical subassembly. An optical signal is input from the level amplifier 53b to the conversion unit 540. The conversion unit 540 converts the optical signal into an electrical signal (FM integrated conversion signal). The conversion unit 540 outputs the converted electrical signal to the preamplifier 542.

[0060] The post amplifier 561 outputs the carrier signal (video signal) with the amplified intensity level to the branching unit 562. The branching unit 562 acquires the carrier signal (video signal) from the post amplifier 561. The branching unit 562 distributes the carrier signal (video signal) to a user terminal (not shown) and the physical quantity measurement unit 51b.

[0061] Next, an operation example of the optical transmission system 1 will be described. FIG. 5 is a flowchart showing an operation example of the optical transmission system 1 in the second embodiment. The optical transmission system 1 executes the operations shown in the flowchart of FIG. 5 at a predetermined cycle. The operations from step S201 to step S202 are the same as the operations from step S101 to step S102 shown in FIG. 3.

[0062] The demodulation unit 55 generates a carrier signal (video signal) based on the electrical signal by performing demodulation processing on the electrical signal (FM integrated conversion signal) (step S203). The physical quantity measurement unit 51b measures at least one of the carrier-to-noise ratio (CNR) of the electrical signal input from the branching unit 562, the amount of composite second-order distortion of the carrier signal, and the amount of composite third-order distortion of the carrier signal (step S204).

[0063] The control unit 52b determines, for example, whether the carrier-to-noise ratio is equal to or higher than the threshold value of the carrier signal (above the quality standard) (step S205). When it is determined that the carrier-to-noise ratio is equal to or higher than the threshold value of the carrier signal (above the quality standard) (step S205: YES), the optical transmission system 1 ends the operations shown in the flowchart of FIG. 5.

[0064] When it is determined that the carrier-to-noise ratio is less than the threshold value of the carrier signal (less than the quality standard) (step S205: NO), the control unit 52b proceeds to the process of step S206. The operations from step S206 to step S208 are the same as the operations from step S105 to step S107 shown in FIG. 3.

[0065] As described above, the conversion unit 540 converts the received optical signal into an electrical signal. The demodulation unit 55 generates a carrier signal based on the electrical signal. The physical quantity measurement unit 51b measures the physical quantity of the carrier signal output from the post-amplifier 561. The physical quantity of the carrier signal is at least one of, for example, the carrier-to-noise ratio, the amount of composite second-order distortion, and the amount of composite third-order distortion. When the physical quantity of the carrier signal is less than the second threshold value (less than the quality standard), the control unit 52b adjusts the physical quantity of the received optical signal. For example, when the physical quantity of the carrier signal is less than the second threshold value, the control unit 52b amplifies the intensity level of the received optical signal.

[0066] This makes it possible to further improve the quality of the carrier signal without preparing receiving devices with different receiving performances for each transmission distance. Also, even if the intensity level "Em" of the electrical signal (frequency modulation integrated conversion signal) is initially equal to or higher than the intensity level "E", it is possible to improve the quality of the carrier signal.

[0067] (Third Embodiment) In the third embodiment, the difference from the second embodiment is that the waveform distortion of the optical signal due to wavelength dispersion is compensated. In the third embodiment, the description will be centered on the difference from the second embodiment.

[0068] FIG. 6 is a diagram showing a configuration example of the receiving device 5c in the third embodiment. The receiving device 5c corresponds to the receiving device 5 shown in FIG. 1. The receiving device 5c includes a physical quantity measurement unit 51c, a control unit 52c, a level amplification unit 53c, an optical processing unit 54c, a demodulation unit 55, a carrier signal amplification unit 56b, and a compensation unit 57.

[0069] The control unit 52c, the level amplifier 53c, and the optical processing unit 54c correspond to the control unit 52b, the level amplifier 53b, and the optical processing unit 54b shown in FIG. 4. The optical processing unit 54c includes a conversion unit 540 and a preamplifier 542. The carrier signal amplifier 56c includes a filter 560, a post amplifier 561, and a branching unit 562.

[0070] The carrier signal amplifier 56c may further include an electrical amplifier at at least one of the front stage and the rear stage of the branching unit 562. Thereby, even when the intensity level of the electrical signal decreases due to the branching of the electrical signal by the branching unit 562, the intensity level of the electrical signal can be compensated.

[0071] An optical signal is input to the compensation unit 57 from the branching device 4. The compensation unit 57 acquires a control signal representing the compensation amount from the control unit 52c. The compensation unit 57 has, for example, a dispersion compensating fiber (Dispersion Compensating Fiber). The dispersion compensating fiber is, for example, a slope compensating and dispersion compensating fiber (SC-DCF). The compensation unit 57 compensates for the waveform distortion of the optical signal due to wavelength dispersion based on the compensation amount instructed from the control unit 52c using the control signal. The compensation unit 57 outputs the optical signal with the waveform distortion compensated to the level amplifier 53c.

[0072] The control unit 52c preliminarily holds data representing the intensity level range “Pd~Pu” [dBm] of the optical signal that the conversion unit 540 can receive, and data representing the carrier-to-noise ratio “CNR(i)” of the electrical signal required for video reproduction.

[0073] The control unit 52c adjusts the amplification gain of the optical signal in the level amplifier 53c so that the intensity level “Em” of the electrical signal is equal to or higher than the intensity level “E” and the intensity level of the optical signal input to the conversion unit 540 falls within the range from the lower limit “Pd” to the upper limit “Pu”. The control unit 52c outputs a control signal representing the amplification gain to the level amplifier 53c.

[0074] The control unit 52c determines whether the carrier-to-noise ratio "CNRm(i)" of the electrical signal is smaller than the carrier-to-noise ratio "CNR(i)" of the electrical signal (video signal). When the carrier-to-noise ratio "CNRm(i)" of the electrical signal is smaller than the carrier-to-noise ratio "CNR(i)" of the electrical signal, the control unit 52c adjusts the compensation amount of the waveform distortion of the optical signal in the compensation unit 57 so that the compensation unit 57 compensates the waveform of the optical signal with a compensation amount that is a fixed width "Vw" more than the current compensation amount.

[0075] The control unit 52c determines again whether the carrier-to-noise ratio "CNRm(i)" of the electrical signal is smaller than the carrier-to-noise ratio "CNR(i)" of the electrical signal (video signal). When the carrier-to-noise ratio "CNRm(i)" of the electrical signal is smaller than the required carrier-to-noise ratio "CNR(i)" of the electrical signal, the control unit 52c readjusts the compensation amount of the waveform distortion of the optical signal in the compensation unit 57. The control unit 52c may readjust the amplification gain of the optical signal in the level amplification unit 53c.

[0076] The control unit 52c readjusts the compensation amount of the waveform distortion of the optical signal in the compensation unit 57 until it is determined that the carrier-to-noise ratio "CNRm(i)" of the electrical signal is equal to or greater than the carrier-to-noise ratio "CNR(i)" of the electrical signal. The control unit 52c may readjust the amplification gain of the optical signal in the level amplification unit 53c. However, when the compensation amount of the waveform distortion exceeds the upper limit of the compensation amount, the control unit 52c aborts the readjustment process of the compensation amount so that the compensation amount of the waveform distortion does not exceed the upper limit of the compensation amount.

[0077] The control algorithm executed by the control unit 52c is not limited to a specific algorithm. The control unit 52c executes the adjustment process of the compensation amount using an optimization method such as the binary search method or the non-linear least squares method, for example.

[0078] Next, an operation example of the optical transmission system 1 will be described. FIG. 7 is a flowchart showing an operation example of the optical transmission system 1 in the third embodiment. The optical transmission system 1 executes the operations shown in the flowchart of FIG. 7 at a predetermined cycle.

[0079] The compensation unit 57 compensates for the waveform distortion of the optical signal due to chromatic dispersion based on the compensation amount instructed from the control unit 52c using a control signal (step S301). The operations from step S302 to step S305 are the same as the operations from step S202 to step S205 shown in FIG. 5.

[0080] When it is determined that the carrier-to-noise ratio is less than the threshold value of the carrier signal (less than the quality standard) (step S305: NO), the control unit 52c generates a control signal so that the compensation unit 57 further increases the compensation amount of the waveform distortion of the optical signal by a certain width "Vw". The compensation unit 57 further increases the compensation amount of the waveform distortion of the optical signal by a certain width "Vw" (step S306).

[0081] The control unit 52c determines whether the compensation amount further increased by a certain width "Vw" exceeds the upper limit (step S307). When it is determined that the compensation amount further increased by a certain width "Vw" is below the upper limit (step S307: NO), the optical transmission system 1 ends the operation shown using the flowchart in FIG. 7. When it is determined that the compensation amount further increased by a certain width "Vw" exceeds the upper limit (step S307: YES), the control unit 52c generates a control signal to stop the process of further increasing the compensation amount of the waveform distortion of the optical signal by the compensation unit 57. The compensation unit 57 compensates for the waveform distortion of the optical signal with the upper limit compensation amount (step S308).

[0082] As described above, the compensation unit 57 compensates for the waveform distortion of the received optical signal. The conversion unit 540 converts the received optical signal into an electrical signal. The demodulation unit 55 generates a carrier signal based on the electrical signal. The physical quantity measurement unit 51c measures the physical quantity of the carrier signal. For example, it is at least one of the carrier-to-noise ratio, the amount of composite second-order distortion, and the amount of composite third-order distortion. When the physical quantity of the carrier signal is less than the second threshold value (less than the quality standard), the control unit 52c adjusts the physical quantity of the received optical signal. For example, when the physical quantity of the carrier signal is less than the second threshold value, the control unit 52c increases the compensation amount of the waveform distortion of the received optical signal.

[0083] Accordingly, it is possible to improve the quality of the carrier signal without preparing a receiving device having different receiving performances for each transmission distance. Even when the waveform of the input optical signal is deteriorated due to chromatic dispersion, it is possible to improve the quality of the carrier signal.

[0084] In the third embodiment, the receiving device 5c improves the quality of the output carrier signal by compensating for the waveform distortion of the input optical signal. Accordingly, the transmission distance between the relay device 3-n and the receiving device 5c can be increased, so that it is possible to use the receiving device 5c having the same receiving performance in a wider area.

[0085] Since the transmitting device 2 (base station side) does not have to compensate for the waveform distortion due to the chromatic dispersion of the optical signal, it is possible to improve the quality of the carrier signal even if the access distance between the receiving device 5 (port of the optical amplifier for access) and the user terminal (not shown) varies. That is, since the receiving device 5 compensates for the waveform distortion due to the chromatic dispersion of the optical signal, it is possible to improve the quality of the carrier signal even when the access distance between the receiving device 5 and the user terminal (not shown) varies.

[0086] (Fourth Embodiment) In the fourth embodiment, the difference from the second embodiment is that the receiving device 5 determines whether or not the relay device 3 compensates for the waveform distortion of the optical signal based on a measured value such as the carrier-to-noise ratio of the optical signal. In the fourth embodiment, the description will be centered on the difference from the second embodiment.

[0087] FIG. 8 is a diagram showing a configuration example of the relay device 3d-n (n is an integer of 1 or more) in the fourth embodiment. The relay device 3d corresponds to the relay device 3 shown in FIG. 1. The relay device 3d-n includes a relay optical amplification unit 30, a housing unit 31, a compensation unit 32, two level amplification units 33, two couplers 34, and a switching unit 35.

[0088] Hereinafter, the system including the compensation unit is referred to as the "first system". For example, the system (transmission line) of the compensation unit 32, the level amplifier 33-1, and the coupler 34-1 is the first system. Hereinafter, the system without the compensation unit is referred to as the "second system". For example, the system of the level amplifier 33-2 and the coupler 34-2 is the second system. The relay device 3d-n includes the first system and the second system in parallel.

[0089] The optical amplifier 30 for relaying amplifies the intensity level of the optical signal to be relayed. The optical amplifier 30 for relaying outputs the downstream optical signal to the accommodating unit 31. The optical amplifier 30 for relaying acquires the upstream optical signal from the accommodating unit 31.

[0090] The accommodating unit 31 accommodates the communication of the receiving device 5d by using the optical signal transmitted through the first system or the second system. That is, the accommodating unit 31 acquires the upstream optical signal from the first system or the second system connected to the switching unit 35. The accommodating unit 31 outputs the downstream optical signal to both the first system and the second system. Among the downstream optical signal output to the first system and the downstream optical signal output to the second system, only the downstream optical signal of the system connected (conducted) to the switching unit 35 is transmitted downstream by the switching unit 35.

[0091] The compensation unit 32 has, for example, a dispersion compensation fiber. The compensation unit 32 compensates for the waveform distortion of the optical signal due to wavelength dispersion. The compensation unit 32 outputs the downstream optical signal with the waveform distortion compensated to the level amplifier 33. The compensation unit 32 outputs the upstream optical signal with the waveform distortion compensated to the accommodating unit 31.

[0092] The level amplifier 33-1 amplifies the intensity level of the optical signal. The level amplifier 33-1 outputs the downstream optical signal to the coupler 34-1. The level amplifier 33-1 outputs the upstream optical signal to the compensation unit 32. The level amplifier 33-2 amplifies the intensity level of the optical signal. The level amplifier 33-2 outputs the downstream optical signal to the coupler 34-2. The level amplifier 33-2 outputs the upstream optical signal to the accommodating unit 31.

[0093] The coupler 34 (optical coupler) branches the downstream optical signal to each switching unit 35. The coupler 34-1 combines the upstream optical signals acquired from each switching unit 35. The coupler 34-1 outputs the combined upstream optical signal to the level amplifier 33-1. The coupler 34-2 combines the upstream optical signals acquired from each switching unit 35. The coupler 34-2 outputs the combined upstream optical signal to the level amplifier 33-2.

[0094] The switching unit 35 switches the connection destination to the first system or the second system according to the control (feedback) by the switching instruction signal of the receiving device 5d. The receiving device 5d corresponds to the receiving device 5 shown in FIG. 1. The switching unit 35 transmits the downstream optical signal transmitted through the first system or the second system according to the switching instruction signal to the receiving device 5d via the branching device 4. The switching unit 35 outputs the upstream optical signal acquired from the receiving device 5d via the branching device 4 to the first system or the second system according to the switching instruction signal.

[0095] The transmission method of the switching instruction signal transmitted from the receiving device 5d to the switching unit 35 is not limited to a specific method. For example, a control line for the switching instruction signal may be separately prepared between the switching unit 35 and the receiving device 5d. For example, when the receiving device 5d is a GV-ONU (Gigabit Video - Optical Network Unit), the GV-ONU transmits the switching instruction signal to the relay device 3d using the line for communication of GE-PON (Gigabit Ethernet-Passive Optical Network). The GV-ONU is a receiving device in which a receiving device (ONU) of GE-PON and an image receiving device (V-ONU) are integrated.

[0096] The switching unit 35-1 of the relay device 3d-1 transmits the optical signal to the receiving device 5-1-1 via the branching device 4-1. The switching unit 35-1 of the relay device 3d-1 acquires the optical signal from the receiving device 5-1-1 via the branching device 4-1. The switching unit 35-2 of the relay device 3d-1 transmits the optical signal to the receiving device 5-1-2 (not shown). The switching unit 35-2 of the relay device 3d-1 acquires the optical signal from the receiving device 5-1-2 (not shown) via the branching device 4-1.

[0097] FIG. 9 is a diagram showing a configuration example of the receiving apparatus 5d in the fourth embodiment. The receiving apparatus 5d includes a physical quantity measurement unit 51d, a control unit 52d, an optical processing unit 54d, a demodulation unit 55, a carrier signal amplification unit 56d, and a memory 58.

[0098] The control unit 52d and the optical processing unit 54d correspond to the control unit 52b and the optical processing unit 54b shown in FIG. 4. The optical processing unit 54d includes a conversion unit 540 and a preamplifier 542. The carrier signal amplification unit 56d includes a filter 560, a post amplifier 561, and a branching unit 562.

[0099] The branching unit 562 outputs the carrier signal output from the post amplifier 561 to the physical quantity measurement unit 51d. The physical quantity measurement unit 51d measures the carrier-to-noise ratio (CNR) of the electrical signal (video signal) input from the branching unit 562. The physical quantity measurement unit 51d records the quality information (measurement value of the carrier-to-noise ratio) of the carrier signal in the memory 58. The memory 58 (buffer) stores the quality information of the carrier signal in association with the system to which the connection destination of the switching unit 35 corresponds.

[0100] The control unit 52d switches the connection destination of the switching unit 35 to the first system or the second system using a switching instruction signal. The control unit 52d determines whether or not both the quality information of the carrier signal when the connection destination of the switching unit 35 is the first system and the quality information of the carrier signal when the connection destination of the switching unit 35 is the second system are stored in the memory 58. The control unit 52d switches the connection destination of the switching unit 35 of the relay device 3d to the system with higher carrier signal quality.

[0101] Next, an operation example of the optical transmission system 1 will be described. FIG. 10 is a flowchart showing an operation example of the optical transmission system 1 in the fourth embodiment. The switching unit 35 of the relay device 3d switches the connection destination of the switching unit 35 to the first system in the relay device 3d (step S401). The conversion unit 540 converts the optical signal received from the relay device 3d via the branching device 4 into an electrical signal (step S402). The demodulation unit 55 generates a carrier signal based on the electrical signal (step S403). The physical quantity measurement unit 51d measures the carrier-to-noise ratio of the carrier signal output from the post amplifier 561 (step S404). The physical quantity measurement unit 51d records the quality information (measurement value of the carrier-to-noise ratio) of the carrier signal in the memory 58 (step S405).

[0102] The control unit 52d determines whether or not both the quality information of the carrier signal when the connection destination of the switching unit 35 is the first system and the quality information of the carrier signal when the connection destination of the switching unit 35 is the second system are stored in the memory 58 (step S406).

[0103] If at least one of the quality information of the carrier signal when the connection destination is the first system and the quality information of the carrier signal when the connection destination is the second system is not stored in the memory 58 (step S406: NO), the control unit 52d switches the connection destination of the switching unit 35 to the second system (the system of the compensation unit 32, the level amplifier 33-1, and the coupler 34-1) in the relay device 3d (step S407). The control unit 52d returns the process to step S402.

[0104] If both the quality information of the carrier signal when the connection destination is the first system and the quality information of the carrier signal when the connection destination is the second system are stored in the memory 58 (step S406: YES), the control unit 52d switches the connection destination of the switching unit 35 of the relay device 3d to the system with higher quality of the carrier signal (step S408).

[0105] As described above, the switching unit 35 of the relay device 3d-n (n is an integer of 1 or more) switches the connection destination to the first system or the second system according to the control by the control unit 52d. The switching unit 35 of the relay device 3d-n transmits the optical signal transmitted through the connection destination system to the receiving device 5-n via the branching device 4. The control unit 52d switches whether to compensate for the waveform distortion of the optical signal before transmission by the relay device 3d based on the physical quantity of the carrier signal. That is, the switching unit 35 switches whether to compensate for the waveform distortion of the optical signal according to the control by the control unit 52d. The control unit 52d switches the connection destination of the switching unit 35 to the system with higher reception quality between the first system and the second system. When the optical signal is transmitted through the first system, the compensation unit 32 compensates for the waveform distortion of the optical signal.

[0106] As a result, it is possible to improve the quality of the carrier signal without preparing receiving devices with different reception performances for each transmission distance.

[0107] In an actual transmission environment, the signal quality is not determined only based on the transmission distance. For example, due to additional fusion caused by a disconnection that occurred in the optical fiber in the past, or the existence of a transmission line using a poor-quality optical fiber, signal quality degradation may occur at irregular points in the transmission line. If it is determined which of the first system or the second system the switching unit 35 conducts to only based on whether the distance from the relay device 3d-n to the receiving device 5 is less than a threshold distance (for example, 10 km), although the reception quality would be improved by using the compensation unit 32, the receiving device 5 may be accommodated in the second system because the distance from the relay device 3 is slightly shorter than the threshold distance. Also, although the reception quality would be improved by not using the compensation unit 32, the receiving device 5 may be accommodated in the first system because the distance from the relay device 3 is slightly longer than the threshold distance (boundary distance). If it is not possible to switch whether to compensate for the waveform distortion of the optical signal, the effect of improving the reception quality in a receiving device (V-ONU) whose distance from the relay device (V-OLT) is close to the threshold distance (for example, 10 km) cannot be expected.

[0108] On the other hand, since the control unit 52d determines whether to use the compensation unit 32 of the relay device 3d-n based on the measured value "CNRm(i)" of the carrier-to-noise ratio, in particular, an effect of improving the reception quality in the receiving device 5 whose distance from the relay device 3 is close to the threshold distance (boundary distance) can be expected.

[0109] For example, although the reception quality is improved by using the compensation unit 32, the receiving device 5 that was accommodated in the second system because the distance from the relay device 3 is slightly shorter than the threshold distance is accommodated in the first system in the relay device 3d-n based on the measured value "CNRm(i)". As a result, an effect of improving the reception quality in the receiving device 5 can be expected.

[0110] For example, although the reception quality is improved by not using the compensation unit 32, the receiving device 5 that was accommodated in the first system because the distance from the relay device 3 is slightly longer than the threshold distance (boundary distance) is accommodated in the second system in the relay device 3d-n based on the measured value "CNRm(i)". As a result, an effect of improving the reception quality in the receiving device 5 can be expected.

[0111] (Modification Example of the Fourth Embodiment) In the modification example of the fourth embodiment, the difference from the fourth embodiment is that the receiving device 5 includes a compensation unit and a switching unit. In the modification example of the fourth embodiment, the description will be centered on the difference from the fourth embodiment.

[0112] FIG. 11 is a diagram showing a configuration example of a relay device 3e-n (n is an integer of 1 or more) in a modification example of the fourth embodiment. The relay device 3e corresponds to the relay device 3 shown in FIG. 1. The relay device 3e-n includes a relay optical amplifier unit 30, an accommodation unit 31, and a coupler 34. The relay optical amplifier unit 30, the accommodation unit 31, and the coupler 34 correspond to the relay optical amplifier unit 30, the accommodation unit 31, and the coupler 34 shown in FIG. 8.

[0113] The accommodation unit 31 accommodates the communication of the receiving device 5d using the optical signal transmitted through the first system or the second system. The level amplification unit 33 amplifies the intensity level of the optical signal. The level amplification unit 33 outputs the downstream optical signal to the coupler 34. The level amplification unit 33 outputs the upstream optical signal to the accommodation unit 31. The coupler 34 branches the downstream optical signal to each receiving device 5e. The coupler 34 combines the upstream optical signals acquired from each receiving device 5e. The coupler 34 outputs the combined upstream optical signal to the level amplification unit 33.

[0114] FIG. 12 is a diagram showing a configuration example of the receiving device 5e in a modification of the fourth embodiment. The receiving device 5e corresponds to the receiving device 5 shown in FIG. 1. The receiving device 5e includes a switching unit 35, a physical quantity measurement unit 51e, a control unit 52e, an optical processing unit 54e, a demodulation unit 55, a carrier signal amplification unit 56e, a compensation unit 57, and a memory 58.

[0115] The switching unit 35 corresponds to the switching unit 35 shown in FIG. 8. The compensation unit 57 corresponds to the compensation unit 57 shown in FIG. 6 or the compensation unit 32 shown in FIG. 8.

[0116] Next, an operation example of the optical transmission system 1 will be described. FIG. 13 is a flowchart showing an operation example of the optical transmission system 1 in a modification of the fourth embodiment. The switching unit 35 of the receiving device 5e switches the connection destination of the switching unit 35 to the first system in the receiving device 5d (step S501). Each step from step S502 to step S506 is the same as each step from step S402 to step S406 shown in FIG. 10.

[0117] When at least one of the quality information of the carrier signal when the connection destination is the first system and the quality information of the carrier signal when the connection destination is the second system is not stored in the memory 58 (step S506: NO), the control unit 52e switches the connection destination of the switching unit 35 of the receiving device 5e to the second system (the system of the compensation unit 32) in the receiving device 5e (step S507). The control unit 52e returns the process to step S502.

[0118] When both the quality information of the carrier signal when the connection destination is the first system and the quality information of the carrier signal when the connection destination is the second system are stored in the memory 58 (step S506: YES), the control unit 52e switches the connection destination of the switching unit 35 of the receiving device 5e to the system with higher carrier signal quality (step S508).

[0119] As described above, the switching unit 35 of the receiving device 5e inputs an optical signal to the compensation unit 57 or the conversion unit 540 according to the control by the control unit 52e. That is, the switching unit 35 switches the connection destination to the first system or the second system according to the control by the control unit 52e. The control unit 52e switches whether to compensate for the waveform distortion of the optical signal after reception by the receiving device 5e based on the physical quantity of the carrier signal. That is, the switching unit 35 switches whether to compensate for the waveform distortion of the optical signal according to the control by the control unit 52e. The control unit 52e switches the connection destination of the switching unit 35 to the system with higher reception quality between the first system and the second system. When the optical signal is transmitted through the first system, the compensation unit 32 compensates for the waveform distortion of the optical signal. That is, when the optical signal is input from the switching unit 35, the compensation unit 57 compensates for the waveform distortion of the input optical signal.

[0120] As a result, it is possible to improve the quality of the carrier signal without preparing receiving devices with different reception performances for each transmission distance. In the fourth embodiment, a failure of the switching unit 35 of the relay device 3d affects each of the subordinate receiving devices 5. On the other hand, in a modification of the fourth embodiment, a failure of the switching unit 35 affects only the receiving device 5e including the switching unit 35. That is, the optical transmission system 1 of the modification of the fourth embodiment has a special effect that the influence range of the failure of the switching unit 35 is small.

[0121] (Hardware configuration example) FIG. 14 is a diagram showing an example of the hardware configuration of the receiving apparatus 5 in each embodiment. Some or all of the functional units of the receiving apparatus 5 are realized as software by a processor 100 such as a CPU (Central Processing Unit) executing a program stored in a storage device 102 having a nonvolatile recording medium (non-transitory recording medium) and a memory 101. The program may be recorded on a computer-readable recording medium. A computer-readable recording medium is a non-transitory recording medium such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), a portable medium such as a CD-ROM (Compact Disc Read Only Memory), or a storage device such as a hard disk incorporated in a computer system.

[0122] Some or all of the functional units of the receiving apparatus 5 may be realized using hardware including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).

[0123] The above embodiments may be combined with each other.

[0124] As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and designs and the like within the scope not departing from the gist of the present invention are also included.

Industrial Applicability

[0125] The present invention is applicable to an optical transmission system that transmits a video signal or the like using an optical signal.

Explanation of Signs

[0126] 1…Optical transmission system, 2…Transmission device, 3, 3d, 3e…Relay device, 4…Branching device, 5, 5a, 5b, 5c, 5d, 5e…Receiving device, 30…Optical amplifier for relay, 31…Housing part, 33…Level amplifier, 34…Coupler, 35…Switching part, 50…Level measurement part, 51a, 51b, 51c, 51d, 51e…Physical quantity measurement part, 52a, 52b, 52c, 52d, 52e…Control part, 53a, 53b, 53c…Level amplifier, 54a, 54b, 54c, 54d, 54e…Optical processing part, 55…Demodulation part, 56a, 56b, 56c, 56d, 56e…Carrier wave signal amplifier, 57…Compensation part, 58…Memory, 100…Processor, 101…Memory, 102…Storage device, 540…Conversion part, 541…Branching part, 542…Preamplifier, 560…Filter, 561…Post amplifier, 562…Branching part

Claims

1. a conversion unit that converts a received optical signal into an electrical signal; a demodulation unit that generates a carrier signal based on the electrical signal; a physical quantity measurement unit that measures a physical quantity of the electrical signal or a physical quantity of the carrier signal; a control unit that adjusts the physical quantity of the received optical signal by a binary search method or a non-linear least squares method when the physical quantity of the electrical signal is less than a first threshold value or when the physical quantity of the carrier signal is less than a second threshold value; a branching unit that branches the electrical signal to the demodulation unit and the physical quantity measurement unit; an electrical amplifier that amplifies the electrical signal in at least one of a stage before and a stage after the branching unit A receiving apparatus comprising:

2. The physical quantity of the electrical signal is an intensity level of the received optical signal, The control unit amplifies the intensity level of the received optical signal when the physical quantity of the electrical signal is less than the first threshold value. The receiving apparatus according to claim 1.

3. The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, The control unit amplifies the intensity level of the received optical signal when the physical quantity of the carrier signal is less than the second threshold value. The receiving apparatus according to claim 1.

4. A compensation unit that compensates for waveform distortion of the received optical signal is provided, The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, The control unit increases a compensation amount for waveform distortion of the received optical signal when the physical quantity of the carrier signal is less than the second threshold value. The receiving apparatus according to claim 1.

5. The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, The control unit switches whether to compensate for waveform distortion of the optical signal before transmission or after reception based on the physical quantity of the carrier signal. The receiving apparatus according to claim 1.

6. A receiving method executed by a receiving apparatus, comprising: a conversion step of converting a received optical signal into an electrical signal; a demodulation step of generating a carrier signal based on the electrical signal; a physical quantity measurement step of measuring a physical quantity of the electrical signal or a physical quantity of the carrier signal; When the physical quantity of the electrical signal is less than the first threshold value, or when the physical quantity of the carrier signal is less than the second threshold value, a control step of adjusting the physical quantity of the received optical signal by a binary search method or a non-linear least squares method; A branching step of branching the electrical signal to a demodulation unit that executes the demodulation step and a physical quantity measurement unit that executes the physical quantity measurement step; An electrical amplifier step of amplifying the electrical signal in at least one of the front stage and the rear stage of the branching unit that executes the branching step A receiving method including.

7. The physical quantity of the electrical signal is the intensity level of the received optical signal, In the control step, when the physical quantity of the electrical signal is less than the first threshold value, the intensity level of the received optical signal is amplified. The receiving method according to claim 6.

8. Including a compensation step of compensating for the waveform distortion of the received optical signal, The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, In the control step, when the physical quantity of the carrier signal is less than the second threshold value, the intensity level of the received optical signal is amplified. The receiving method according to claim 6.

9. The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, In the control step, when the physical quantity of the carrier signal is less than the second threshold value, the amount of compensation for the waveform distortion of the received optical signal is increased. The receiving method according to claim 6.

10. The physical quantity of the carrier signal is at least one of a carrier-to-noise ratio, an amount of composite second-order distortion, and an amount of composite third-order distortion, In the control step, whether to compensate for the waveform distortion of the optical signal before transmission or after reception is switched based on the physical quantity of the carrier signal. The receiving method according to claim 6.

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