Optical receiver and optical receiving method

Abstract

To provide an optical receiver capable of accurately compensating for signal distortion without increasing the number of tap stages of an adaptive equalizer.SOLUTION: An optical receiver includes: a receiver to receive an optical signal input via an optical transmission path; a converter to convert an electrical analog signal corresponding to the optical signal into a digital signal; a first compensator to fixedly compensate for a first signal distortion of the digital signal based on a first tap coefficient; a second compensator to adaptively compensate for a second signal distortion of the digital signal that has been compensated by the first compensator, based on a second tap coefficient with a second tap stage number different from a first tap stage number of the first compensator; and a controller to acquire the digital signal output from the first compensator before being input to the second compensator, calculate a third tap coefficient that adaptively compensates for the second signal distortion with the first tap stage number, and update the first tap coefficient of the first compensator based on the first tap coefficient and the third tap coefficient.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an optical receiving device and an optical receiving method. [Background technology]

[0002] Coherent optical communications and coherent optical receivers are known (see, for example, Patent Documents 1 to 3). In coherent optical communications, distortion in a transmission signal is compensated for on the receiving side by digital signal processing. Digital signal processing mainly involves processes such as chromatic dispersion compensation, frequency control and phase adjustment, polarization multiplexing / demultiplexing, and polarization dispersion compensation. The polarization multiplexing / demultiplexing and polarization dispersion compensation processes are mainly performed by adaptive equalization.

[0003] In digital signal processing, an adaptive equalizer generally uses a digital filter. The adaptive equalizer can compensate for the transmission signal by setting tap coefficients calculated to cancel out distortion in the transmission signal in the digital filter. The tap coefficients are updated sequentially to adapt to conditions that change over time. This allows the adaptive equalizer to perform compensation that tracks fluctuations in the polarization state (see, for example, Patent Document 4). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2023-506565 [Patent Document 2] US Patent Application Publication No. 2022 / 0393772 [Patent Document 3] International Publication No. 2017 / 091393 [Patent Document 4] Patent Publication No. 2021-190787 Summary of the Invention [Problem to be solved by the invention]

[0005] Incidentally, it is desirable to reduce the number of tap stages of the digital filter used in the adaptive equalizer described above in order to ensure the ability to follow fluctuations in the state of polarization. For example, reducing the number of tap stages not only ensures the ability to follow fluctuations in the state of polarization, but also has the potential to reduce the power consumed by the adaptive equalizer.

[0006] However, reducing the number of tap stages may lead to a decrease in the compensation accuracy of the adaptive equalizer. If the compensation accuracy decreases, distortion in the transmission signal (hereinafter referred to as signal distortion) will not be sufficiently compensated, and there is a risk of signal errors occurring in the transmission signal. However, if the number of tap stages of the digital filter used in the adaptive equalizer is increased in order to improve the compensation accuracy, another problem arises: the power consumed by the adaptive equalizer will increase.

[0007] Therefore, in one aspect, an object of the present invention is to provide an optical receiving device and an optical receiving method that accurately compensate for signal distortion without increasing the number of tap stages of an adaptive equalizer. [Means for solving the problem]

[0008] In one embodiment, an optical receiving device includes a receiving unit that receives an optical signal input via an optical transmission path, a converting unit that converts an electrical analog signal corresponding to the optical signal into a digital signal, a first compensating unit that fixedly compensates for a first signal distortion of the digital signal based on a first tap coefficient, a second compensating unit that adaptively compensates for a second signal distortion of the digital signal after compensation by the first compensating unit based on a second tap coefficient with a second tap stage that is different from the first tap stage of the first compensating unit, and a control unit that acquires the digital signal output from the first compensating unit and before input to the second compensating unit, calculates a third tap coefficient that adaptively compensates for the second signal distortion with the first tap stage, and updates the first tap coefficient of the first compensating unit based on the first tap coefficient and the third tap coefficient. [Effects of the Invention]

[0009] Signal distortion can be compensated for with high precision without increasing the number of tap stages of the adaptive equalizer. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an example of an optical transmission system. [Figure 2] 1 is an example of a hardware configuration of an optical receiving device. [Figure 3] 10 is an example of a functional configuration of an RxDSP and a reception control unit. [Figure 4] 10 is a flowchart illustrating an example of processing executed by a reception control unit. [Figure 5] 10 is an example of a graph illustrating the effect of the optical transmission system. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0012] As shown in Fig. 1, the optical transmission system ST includes an optical transmitter 10 and an optical receiver 20. The optical transmitter 10 and the optical receiver 20 are connected via an optical transmission path 30. The optical transmission path 30 includes, for example, an optical fiber and an optical repeater. Examples of optical repeaters include a ROADM (Reconfigurable Optical Add / Drop Multiplexer) and an ILA (In-Line Amplifier). The optical transmitter 10 receives an electrical client signal in a digital format from a client network.

[0013] The client signal is, for example, an Ethernet (registered trademark) signal. The client signal may be a main signal, or may be a control signal that includes only parameters for adjusting transmission characteristics, etc. The optical transmitting device 10 converts the client signal into an optical signal 40 and transmits it to the optical transmission path 30. As a result, the optical signal 40 propagates through the optical transmission path 30. The optical receiving device 20 receives the optical signal 40 from the optical transmission path 30. Upon receiving the optical signal 40, the optical receiving device 20 converts the optical signal 40 into a client signal and transmits it to a client network.

[0014] Next, the hardware configuration of the optical receiving device 20 will be described with reference to FIG.

[0015] 2, the optical receiving device 20 includes an RxDSP (Rx Digital Signal Processor) 210, an ADC (Analogue to Digital Converter) 220, and an ICR (Integrated Coherent Receiver) 230. The optical receiving device 20 also includes an ITLA (Integrable Tunable Laser Assembly) 240 and a reception control unit 250. The RxDSP 210 is a DSP mounted on the optical receiving device 20. The reception control unit 250 is provided independently of the RxDSP 210.

[0016] Here, the ICR 230 includes a 90° optical hybrid circuit (simply indicated as 90° in FIG. 2) 231, a BPD (Balanced Photo Diode) 232, and a TIA (Transimpedance Amplifier) ​​233. The ICR 230 is an integrated circuit in which the 90° optical hybrid circuit 231, the BPD 232, and the TIA 233 are housed in a single package. The ICR 230 or the 90° optical hybrid circuit 231 is an example of a receiving unit. Although not shown, the ITLA 240 includes a local light source that outputs local light (specifically, laser light).

[0017] The 90° optical hybrid circuit 231 receives an optical signal 40 transmitted from the optical transmitter 10 and propagated through the optical transmission line 30. The 90° optical hybrid circuit 231 receives the optical signal 40 using the local oscillator light output from the ITLA 240 and outputs it to the BPD 232. The BPD 232 converts the optical signal 40 into a current signal and outputs it to the TIA 233. The TIA 233 converts the current signal output from the BPD 232 into a voltage signal, amplifies the voltage signal to an amplitude suitable for the ADC 220, and outputs the amplified voltage signal to the ADC 220 as an electrical analog signal.

[0018] In this way, the ICR 230 receives the input optical signal 40 and converts it into an analog signal using the 90° optical hybrid circuit 231, the BPD 232, and the TIA 233. The ADC 220 is an example of a conversion unit, and converts the analog signal into a digital signal and outputs it to the RxDSP 210. The RxDSP 210 receives the digital signal output from the ADC 220 based on the baud rate set by the reception control unit 250.

[0019] The RxDSP 210 is an example of a processing unit and performs various types of digital signal processing. For example, the RxDSP 210 performs symbol demapping on data symbols included in a digital signal based on the baud rate and multi-level modulation method set by the reception control unit 250. Specifically, the RxDSP 210 converts the data symbols into a binary data bit string. The RxDSP 210 then reproduces a transfer frame corresponding to the binary data bit string. The transfer frame includes, for example, an OTU (Optical channel Transport Unit) frame. After reproducing the transfer frame, the RxDSP 210 extracts a client signal from the transfer frame and transmits the extracted client signal to a client network. Details of the digital signal processing performed by the RxDSP 210 will be described later.

[0020] The reception control unit 250 includes a processor and memory, and controls the operations of the RxDSP 210 and the ITLA 240. The processor includes, for example, a CPU (Central Processing Unit). The memory includes volatile memory such as RAM (Random Access Memory) and non-volatile memory such as ROM (Read Only Memory). The reception control unit 250 may be an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0021] The reception control unit 250 performs various settings on the RxDSP 210 in accordance with instructions from the operation terminal, and adjusts the frequency of the ITLA 240. The operation terminal may be a PC (Personal Computer) or a smart terminal (e.g., a tablet terminal). For example, when a signal type including a baud rate, a multi-level modulation method, etc. is input from the operation terminal to the reception control unit 250, the reception control unit 250 sets the baud rate, the multi-level modulation method, etc. in the RxDSP 210.

[0022] Next, the functional configuration of the RxDSP 210 and the reception control unit 250 will be described with reference to FIG.

[0023] First, the functional configuration of the RxDSP 210 will be described. The RxDSP 210 includes an FEQ (Fixed Equalizer) 211 and an AEQ (Adaptive Equalizer) 212. The RxDSP 210 also includes an FOC (Frequency Offset Compensation) 213, a CPR (Carrier Phase Recovery) 214, and a monitor 215. Although not shown, the RxDSP 210 also includes a demodulation unit and the like that performs symbol demapping processing after the CPR 214 and processes to output a client signal.

[0024] The FEQ 211 is an example of a first compensator, and performs fixed compensation for signal distortions occurring in the optical transmitting device 10, the optical receiving device 20, and the optical transmission path 30 for the digital signal output from the ADC 220. Specifically, the FEQ 211 performs chromatic dispersion compensation, skew compensation, and bandwidth characteristic compensation for analog devices such as the ADC 220. The AEQ 212 is an example of a second compensator, and adaptively compensates for signal distortions of the optical signal 40 caused by polarization mode dispersion and polarization dependent loss occurring in the optical transmission path 30 for the digital signal output from the FEQ 211.

[0025] The FOC 213 estimates an optical frequency offset for the digital signal output from the AEQ 212 and compensates for the estimated optical frequency offset. The optical frequency offset is the difference between the frequency of the transmission light output from the ITLA (not shown) of the optical transmitting device 10 and the frequency of the local light output from the ITLA 240 of the optical receiving device 20. The CPR 214 compensates for phase noise of the ITLA 240 and fluctuation components of the high-speed residual frequency offset that could not be fully compensated for by the FOC 213. The monitor 215 is an example of a transfer unit, and when the optical receiving device 20 is started up, it observes the digital signal output from the FEQ 211 and before it is input to the AEQ 212, and transfers the signal to the reception control unit 250.

[0026] Here, both FEQ211 and AEQ212 are realized by FIR (Finite Impulse Response) filters. An FIR filter is a type of digital filter. More specifically, FEQ211 is realized by an FIR filter including a fractionally spaced filter, and AEQ212 is realized by an FIR filter including a butterfly filter. However, the first tap stage, which is the number of tap stages of the FIR filter that realizes FEQ211, and the second tap stage, which is the number of tap stages of the FIR filter that realizes AEQ212, are different.

[0027] For example, the number of second tap stages is less than the number of first tap stages. As a result, the FEQ 211 compensates for most of the signal distortion, such as chromatic dispersion and skew, as the first signal distortion, and the AEQ 212 compensates for the signal distortion remaining in the digital signal after compensation by the FEQ 211 as the second signal distortion.

[0028] Furthermore, the FEQ 211 compensates for most of the signal distortion based on tap coefficients (hereinafter referred to as first tap coefficients) set in the FEQ 211 itself before the optical receiving device 20 is started up. The first tap coefficients are determined, for example, based on prior experiments or design. Meanwhile, the AEQ 212 calculates tap coefficients (hereinafter referred to as second tap coefficients) to be set in the AEQ 212 itself based on a predetermined control method, for example, a constant modulus algorithm (CMA) method or a decision-directed least mean square (DD-LMS) method. After calculating the second tap coefficients, the AEQ 212 compensates for the remaining signal distortion based on the calculated second tap coefficients.

[0029] During the manufacturing stage of the optical receiving device 20, the optical receiving device 20 is manufactured within a specific environmental temperature range that is adjusted in advance, so the first tap coefficients can be determined in advance. However, during the operational stage of the optical receiving device 20, it may be operated outside the specific environmental temperature range. In this case, there is a possibility that the FEQ 211 will not be able to fully compensate for the signal distortion. For this reason, the AEQ 212 adaptively compensates for the signal distortion that the FEQ 211 has not been able to fully compensate for, based on the second tap coefficients.

[0030] In this way, the roles of the FEQ 211 and the AEQ 212 are different. The FEQ 211 performs fixed compensation for signal distortion contained in a digital signal based on the first tap coefficient, while the AEQ 212 adaptively compensates for signal distortion that cannot be fully compensated for by the FEQ 211 due to, for example, fluctuations in the ambient temperature, based on the second tap coefficient.

[0031] Next, the reception control unit 250 will be described. First, the reception control unit 250 includes a calculation unit 251 and an update unit 252. The calculation unit 251 acquires the digital signal transferred from the monitor 215 when the optical receiving device 20 is started up. Upon acquiring the digital signal, the calculation unit 251 calculates a third tap coefficient based on the digital signal.

[0032] Here, calculation unit 251 is realized by a butterfly filter with a larger number of tap stages than the butterfly filter that realizes the above-mentioned AEQ 212. For example, calculation unit 251 is realized by a butterfly filter with a first number of tap stages that is larger than a second number of tap stages. In other words, calculation unit 251 is realized by a virtual AEQ (not shown) in which the number of tap stages of the fractionally spaced filter that realizes FEQ 211 (i.e., the first number of tap stages) is applied to the butterfly filter that realizes AEQ 212.

[0033] This allows calculation unit 251 to calculate a third tap coefficient that is different from both the first tap coefficient and the second tap coefficient, based on the digital signal and the virtual AEQ to which the first tap stage number is applied. After calculating the third tap coefficient, calculation unit 251 outputs the third tap coefficient to update unit 252.

[0034] The update unit 252 acquires the third tap coefficient calculated by the calculation unit 251. Upon acquiring the third tap coefficient, the update unit 252 acquires the first tap coefficient set in the FEQ 211 from the FEQ 211. Upon acquiring the first tap coefficient, the update unit 252 updates the first tap coefficient of the FEQ 211 based on the first tap coefficient and the third tap coefficient.

[0035] More specifically, the updating unit 252 generates a fourth tap coefficient based on the first tap coefficient, the third tap coefficient, and a convolution operation, and updates the first tap coefficient of the FEQ 211 based on the fourth tap coefficient. For example, the updating unit 252 can update the first tap coefficient to the fourth tap coefficient. That is, the updating unit 252 can replace the first tap coefficient with the fourth tap coefficient. This allows the updating unit 252 to reset the first tap coefficient set in the FEQ 211 during shipping test adjustment to the fourth tap coefficient when the optical receiving device 20 is started up. Therefore, during operation of the optical transmission system ST, the FEQ 211 compensates for signal distortion in a fixed manner based on the fourth tap coefficient.

[0036] Here, the convolution operation can be expressed by the following formula: In the formula, X[n] represents the fourth tap coefficient, h[n] represents the third tap coefficient, and x[n] represents the first tap coefficient. Also, M represents the number of the first tap stage, n represents the tap number in X[n], h[n], and x[n], and m represents the tap number in h[m].

number

[0037] Next, the operation of the reception control unit 250 will be described with reference to FIG.

[0038] First, the calculation unit 251 acquires a digital signal (step S1). More specifically, the calculation unit 251 acquires a digital signal transferred from the monitor 215. There is a possibility that signal distortion that has not been fully compensated for by the FEQ 211 remains in the digital signal acquired by the calculation unit 251.

[0039] After acquiring the digital signal, the calculation unit 251 then calculates the third tap coefficient (step S2). More specifically, as described above, the calculation unit 251 calculates the third tap coefficient based on the digital signal and the virtual AEQ to which the first tap stage number is applied.

[0040] After calculating the third tap coefficient, the update unit 252 then acquires the first tap coefficient (step S3). Specifically, the update unit 252 accesses the FEQ 211 and acquires from the FEQ 211 the first tap coefficient set in the FEQ 211 itself. After acquiring the first tap coefficient, the update unit 252 generates a fourth tap coefficient (step S4). As described above, the update unit 252 generates the fourth tap coefficient based on the third tap coefficient output from the calculation unit 251, the first tap coefficient, and the convolution operation.

[0041] After generating the fourth tap coefficient, the update unit 252 updates the first tap coefficient (step S5) and ends the process. More specifically, as described above, the update unit 252 updates the first tap coefficient of the FEQ 211 based on the fourth tap coefficient and ends the process.

[0042] Next, with reference to FIG. 5, the effects of the optical transmission system ST including the optical receiving device 20 will be described.

[0043] In the operation of the optical transmission system ST, the optical transmission path 30 described above often includes a ROADM as an optical repeater. Fig. 5 shows the relationship between the number of ROADM stages included in the optical transmission path 30 and the ROSNR (Required Optical Signal to Noise Ratio) for a comparative example and an example. The number of ROADM stages may be one or N, where N is a natural number equal to or greater than two. The ROSNR represents the limit value of the optical signal-to-noise ratio at which error-free transmission, i.e., no bit errors, can be achieved when the optical receiving device 20 receives the optical signal 40 transmitted from the optical transmitting device 10.

[0044] Here, the optical signal-to-noise ratio is defined as the ratio of signal components to noise components in the optical signal 40. The smaller the optical signal-to-noise ratio, the greater the amount of noise components superimposed on the optical signal 40. As the transmission distance of the optical signal 40 increases, the amount of noise superimposed on the optical signal 40 tends to increase. In other words, the optical signal-to-noise ratio decreases. Therefore, the larger the ROSNR, the smaller the amount of noise components that can be tolerated in the transmission of the optical signal 40. In other words, the smaller the ROSNR, the greater the amount of noise components that can be tolerated in the transmission of the optical signal 40.

[0045] When the optical transmission path 30 includes a ROADM, an optical filter installed in the ROADM narrows the passband of the optical signal 40. This narrowing of the passband causes noise components to be superimposed on the optical signal 40. Therefore, the narrowing of the passband becomes a factor limiting the transmission distance of the optical signal 40. Since the passband narrowing occurs for each ROADM, the amount of noise components superimposed on the optical signal 40 increases as the number of ROADM stages included in the optical transmission path 30 increases. Therefore, the optical signal 40 received by the optical receiving device 20 includes a large amount of noise components.

[0046] As shown in Figure 5, in both the example and the comparative example, the ROSNR increases as the number of ROADM stages increases. However, the rate at which the ROSNR increases differs between the example and the comparative example. For example, in the comparative example, the ROSNR increases more rapidly as the number of ROADM stages increases. On the other hand, in the example, the ROSNR increases as the number of ROADM stages increases, but not as rapidly as in the comparative example, and increases more slowly. For example, when the number of ROADM stages is N, the difference D2 in ROSNR between the comparative example and the example is more than twice (specifically, nearly four times) the difference D1 when the number of ROADM stages is one.

[0047] In this way, even if the number of ROADM stages increases, the increase in ROSNR according to the embodiment is suppressed when the increase in ROSNR according to the comparative example is used as a reference. That is, by including the optical receiving device 20 in the optical transmission system ST, the compensation performance for the optical transmission line 30 is improved. This allows the optical transmission system ST to include a large number of ROADM stages. As a result, the optical transmission system ST can extend the transmission distance of the optical signal 40.

[0048] As described above, the RxDSP 210 according to this embodiment includes a monitor 215 that transfers the digital signal output from the FEQ 211 and before input to the AEQ 212 to the reception control unit 250. The reception control unit 250 according to this embodiment also includes a calculation unit 251 and an update unit 252. The calculation unit 251 calculates a third tap coefficient based on a butterfly filter with a first number of tap stages and the digital signal. The update unit 252 updates the first tap coefficient based on the third tap coefficient and the first tap coefficient set in the FEQ 211.

[0049] This allows signal distortion to be compensated for with high accuracy without increasing the number of tap stages of the AEQ 212. In this way, the RxDSP 210 and the reception control unit 250 according to this embodiment work together to compensate for signal distortion with high accuracy, thereby improving the tolerance of the optical signal-to-noise ratio.

[0050] Furthermore, since the reception control unit 250 is provided independently from the RxDSP 210 including the FEQ 211 and the AEQ 212, it is possible to reduce the processing load on the RxDSP 210. This makes it possible to reduce the power consumption of the RxDSP 210 compared to when the RxDSP 210 includes the calculation unit 251 and the update unit 252.

[0051] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the specific embodiments, and various modifications and variations are possible within the scope of the gist of the present invention as described in the claims.

[0052] In addition, the following supplementary notes are provided in relation to the above description. (Supplementary Note 1) An optical receiving device comprising: a receiving unit that receives an optical signal input via an optical transmission line; a converting unit that converts an electrical analog signal corresponding to the optical signal into a digital signal; a first compensating unit that fixedly compensates for a first signal distortion of the digital signal based on a first tap coefficient; a second compensating unit that adaptively compensates for a second signal distortion of the digital signal after compensation by the first compensating unit based on a second tap coefficient, the second tap coefficient being different from the first tap stage number of the first compensating unit; and a control unit that acquires the digital signal output from the first compensating unit and before input to the second compensating unit, calculates a third tap coefficient that adaptively compensates for the second signal distortion with the first tap stage number, and updates the first tap coefficient of the first compensating unit based on the first tap coefficient and the third tap coefficient. (Supplementary Note 2) The optical receiving device according to Supplementary Note 1, wherein the control unit is provided independently from a processing unit including the first compensation unit and the second compensation unit. (Appendix 3) The optical receiving device described in Appendix 2, characterized in that the processing unit includes a transfer unit that acquires the digital signal output from the first compensation unit and before input to the second compensation unit and transfers it to the control unit. (Appendix 4) The optical receiving device described in Appendix 1 or 2, characterized in that the control unit generates a fourth tap coefficient based on the first tap coefficient, the third tap coefficient, and a convolution operation, and updates the first tap coefficient of the first compensation unit based on the fourth tap coefficient. (Appendix 5) The optical receiving device described in Appendix 1 or 2, characterized in that the control unit generates a fourth tap coefficient based on the first tap coefficient, the third tap coefficient, and a convolution operation, updates the first tap coefficient of the first compensation unit to the fourth tap coefficient, and the first compensation unit compensates for the first signal distortion in a fixed manner based on the fourth tap coefficient. (Supplementary Note 6) The optical receiving device according to Supplementary Note 1 or 2, characterized in that the first compensation unit is a first FIR filter including a fractionally spaced filter, and the second compensation unit is a second FIR filter including a butterfly filter. (Supplementary Note 7) An optical receiving method comprising: receiving an optical signal input via an optical transmission line; converting an electrical analog signal corresponding to the optical signal into a digital signal; fixedly compensating for a first signal distortion of the digital signal based on a first tap coefficient; adaptively compensating for a second signal distortion of the digital signal after the first signal distortion has been compensated for, based on a second tap coefficient, with a second tap stage number different from a first tap stage number of a first compensating unit that compensates for the first signal distortion; acquiring the digital signal after the first signal distortion has been compensated for but before the second signal distortion has been compensated for; calculating a third tap coefficient that adaptively compensates for the second signal distortion with the first tap stage number; and updating the first tap coefficient when compensating for the first signal distortion, based on the first tap coefficient and the third tap coefficient. [Explanation of symbols]

[0053] ST Optical Transmission System 10 Optical transmitter 20 Optical receiving device 30 Optical transmission line 40 Optical Signal 210 RxDSP 211 FEQ 212 AEQ 215 monitors 250 Reception control section 251 Calculation Unit 252 Update Department

Claims

1. a receiving unit that receives an optical signal input via an optical transmission line; a conversion unit that converts an electrical analog signal corresponding to the optical signal into a digital signal; a first compensating unit that compensates for a first signal distortion of the digital signal in a fixed manner based on a first tap coefficient; a second compensating unit that adaptively compensates for a second signal distortion of the digital signal after the compensation by the first compensating unit, based on a second tap coefficient, by a second tap number different from the first tap number of the first compensating unit; a control unit that acquires the digital signal output from the first compensator and before being input to the second compensator, calculates third tap coefficients that adaptively compensate for the second signal distortion with the first number of tap stages, and updates the first tap coefficients of the first compensator based on the first tap coefficients and the third tap coefficients; An optical receiving device having:

2. the control unit is provided independently from a processing unit including the first compensation unit and the second compensation unit.

2. The optical receiving device according to claim 1.

3. the processing unit includes a transfer unit that acquires the digital signal output from the first compensation unit and before being input to the second compensation unit, and transfers the digital signal to the control unit.

3. The optical receiving device according to claim 2.

4. the control unit generates a fourth tap coefficient based on the first tap coefficient, the third tap coefficient, and a convolution operation, and updates the first tap coefficient of the first compensation unit based on the fourth tap coefficient.

3. The optical receiving device according to claim 1 or 2.

5. receiving an optical signal input via an optical transmission line; converting an electrical analog signal corresponding to the optical signal into a digital signal; fixedly compensating for a first signal distortion of the digital signal based on a first tap coefficient; adaptively compensating for a second signal distortion of the digital signal after the first signal distortion has been compensated for, based on a second tap coefficient, with a second number of tap stages different from a first number of tap stages of a first compensating unit that compensates for the first signal distortion; acquiring the digital signal after the first signal distortion has been compensated for but before the second signal distortion has been compensated for, calculating third tap coefficients for adaptively compensating for the second signal distortion with the first number of tap stages, and updating the first tap coefficients when compensating for the first signal distortion based on the first tap coefficients and the third tap coefficients; Optical receiving method.

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