Gain adjustment method, optical receiving device, and computer program

Abstract

A gain adjustment method for an optical transmission system that communicates using a digital coherent method and comprises an optical transmission device and an optical reception device. The gain adjustment method: converts optical signals transmitted by the optical transmission device into electrical signals; converts the electrical signals from analog signals into digital signals; performs first signal processing on the digital signals; uses a digital filter on the digital signals that have undergone first signal processing and performs adaptive equalization processing; corrects the amplitude of the digital filter output signals, on the basis of the digital filter output signal amplitude and phase information and on the basis of the amplitude of known transmission signals; and performs second signal processing on the amplitude-corrected digital filter output signals.　

Description

[Technical field]

[0001] The present invention relates to a gain adjustment method, an optical receiving device, and a computer program. [Background technology]

[0002] In coherent optical communication, polarization / phase diversity transmission and reception has been realized, and digital signal processing utilizing phase information obtained on the receiving side has been realized (for example, see Non-Patent Document 1). Crosstalk and linear distortion between polarization multiplexed signals are equalized by adaptive coefficient control of a digital filter represented by a Finite Impulse Response Filter (FIR filter), and crosstalk and delay difference between In-Phase / Quadrature of a quadrature amplitude modulation (QAM) signal can also be equalized by adaptive coefficient control of an FIR filter. In this case, coefficient control that minimizes the mean square error with a reference signal can be used. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Seb J. Savory, “Digital filters for coherent optical receivers”, Vol. 16, Issue 2, pp. 804-817 (2008). Summary of the Invention [Problem to be solved by the invention]

[0004] However, when coefficient control is performed in a digital filter to minimize the mean square error in a noise-exposed environment, the filter coefficients converge so that the amplitude of the transmission signal component in the digital filter output deviates from the expected value. Therefore, the deviation of the amplitude of the transmission signal component in the digital filter output from the expected value affects the processing of the signal processing unit downstream of the digital filter, and in some cases, the accuracy of the signal processing is deteriorated.

[0005] In view of the above circumstances, an object of the present invention is to provide a technique capable of improving the accuracy of signal processing by a signal processing unit subsequent to a digital filter. [Means for solving the problem]

[0006] One aspect of the present invention is a gain adjustment method in an optical transmission system that communicates using a digital coherent method and includes an optical transmitting device and an optical receiving device, the gain adjustment method comprising the steps of: converting an optical signal transmitted from the optical transmitting device into an electrical signal, converting the electrical signal from an analog signal to a digital signal, performing a first signal processing on the digital signal, performing adaptive equalization processing on the digital signal that has been subjected to the first signal processing using a digital filter, correcting the amplitude of the output signal of the digital filter based on information on the amplitude and phase of the output signal of the digital filter and the amplitude of a known transmitted signal, and performing a second signal processing on the output signal of the digital filter whose amplitude has been corrected.

[0007] One aspect of the present invention is an optical receiving device in an optical transmission system for communicating using a digital coherent method, the optical receiving device comprising an optical transmitting device and an optical receiving device, the optical receiving device comprising: a coherent optical receiving unit that converts an optical signal transmitted from the optical transmitting device into an electrical signal; an analog-to-digital conversion unit that converts the electrical signal from an analog signal to a digital signal; a first signal processing unit that performs first signal processing on the digital signal; an adaptive equalization unit that performs adaptive equalization processing using a digital filter on the digital signal after the first signal processing; an amplitude correction unit that corrects the amplitude of the output signal of the digital filter based on information on the amplitude and phase of the output signal of the digital filter and the amplitude of a known transmitted signal; and a second signal processing unit that performs second signal processing on the output signal of the digital filter whose amplitude has been corrected.

[0008] One aspect of the present invention is a computer program for causing a computer to function as an optical receiving device in an optical transmission system that communicates using a digital coherent method and is equipped with an optical transmitting device and an optical receiving device, the computer program causing a computer to function as the optical receiving device in an optical transmission system that communicates using a digital coherent method, the computer program causing an optical signal transmitted from the optical transmitting device to be converted into an electrical signal, the electrical signal to be converted from an analog signal to a digital signal, a first signal processing being performed on the digital signal, and the digital signal that has been subjected to the first signal processing being subjected to adaptive equalization processing using a digital filter, and correcting the amplitude of the output signal of the digital filter based on information on the amplitude and phase of the output signal and the amplitude of a known transmitted signal. Effect of the Invention

[0009] According to the present invention, it is possible to improve the accuracy of signal processing by the signal processing section subsequent to the digital filter. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram illustrating a system configuration of an optical transmission system according to a first embodiment. [Diagram 2]FIG. 2 is a diagram illustrating an example of the configuration of a digital signal processing unit in the first embodiment. [Diagram 3] 5A to 5C are diagrams for explaining a process performed by an amplitude correction unit in the first embodiment. [Figure 4] 5A to 5C are diagrams for explaining a process performed by an amplitude correction unit in the first embodiment. [Diagram 5] 4 is a flowchart showing a flow of processing performed by the optical receiving device in the first embodiment. [Figure 6] 13A to 13C are diagrams for explaining a process performed by an amplitude correction unit in the second embodiment. [Figure 7] 13A to 13C are diagrams for explaining a process performed by an amplitude correction unit in the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 1 is a diagram showing a system configuration of an optical transmission system 100 in the first embodiment. The optical transmission system 100 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 transmits an optical signal transmitted by the optical transmitter 10 to the optical receiver 20. The optical transmission path 30 is composed of an optical fiber 31 that connects the optical transmitter 10 and the optical receiver 20, and an optical amplifier 32 that amplifies the optical signal. Note that the optical transmission path 30 may have devices such as an optical switch and a regenerator inserted in the middle of the path.

[0012] The optical transmitting device 10 includes an optical transmitting unit 11 that transmits an optical signal. The optical transmitting unit 11 includes an electrical signal generating unit 12 and an optical signal generating unit 13. The electrical signal generating unit 12 encodes transmission data, which is an information source, and converts the encoded transmission data into an electrical signal waveform to generate and output an electrical signal of the transmission data.

[0013] The optical signal generating unit 13 converts the electrical signal generated by the electrical signal generating unit 12 into an optical signal and transmits the optical signal to the optical receiving device 20 via the optical transmission path 30. The optical signal generating unit 13 includes a digital-to-analog converter, a driver amplifier, a modulator, a laser, etc. The optical signal generating unit 13 generates an optical signal using, for example, a QPSK (Quadrature Phase Shift Keying) modulation method.

[0014] The optical receiving device 20 includes an optical receiving section 21 that receives an optical signal. The optical receiving section 21 includes a coherent optical receiving section 22 and a digital signal processing section 23. The coherent optical receiving section 22 includes a 90-degree optical hybrid circuit, a local oscillator light source, a photodetector, and an optical fiber coupled thereto. An analog-to-digital converter may be provided in the coherent optical receiving section 22, or an analog-to-digital converter may be provided between the coherent optical receiving section 22 and the digital signal processing section 23.

[0015] The coherent optical receiver 22 separates the baseband optical signal into two optical signals with orthogonal polarization planes. These optical signals and the local light from the local oscillator light source are input to a 90-degree optical hybrid circuit, which produces a total of four output lights: one set of output lights caused by interference between the two lights in phase and antiphase, and one set of output lights caused by interference in orthogonal (90°) and anti-orthogonal (-90°). Each of these output lights is converted from an optical signal to an analog electrical signal by a photodiode. The analog-to-digital converter converts the analog signal into a digital signal and outputs it to the digital signal processor 23.

[0016] When an optical signal propagates through the optical transmission line 30, the signal waveform is distorted due to a nonlinear optical effect in which the phase of the signal rotates in proportion to the optical power of the signal. The digital signal processing unit 23 receives the digital signal output by the analog-to-digital converter as a received signal, and performs various compensation operations on the received signal.

[0017] 2 is a diagram showing an example of the configuration of the digital signal processing unit 23 in the first embodiment. The digital signal processing unit 23 includes a first signal processing unit 231, an adaptive equalization unit 232, an amplitude correction unit 233, and a second signal processing unit 234.

[0018] The first signal processing unit 231 performs signal processing on the input digital signal. For example, the first signal processing unit 231 compensates for chromatic dispersion occurring in the optical transmission path 30 in the input digital signal. Note that the signal processing performed by the first signal processing unit 231 is not limited to this, and other signal processing may be performed. For example, the first signal processing unit 231 may perform any signal processing that has been conventionally performed before the adaptive equalization processing by the adaptive equalization unit 24.

[0019] The adaptive equalization unit 232 compensates for distortion occurring in the waveform of the optical signal in the optical transmission path 30. That is, the adaptive equalization unit 232 corrects code errors occurring in the optical signal due to inter-symbol interference (inter-symbol interference) in the optical transmission path 30. The adaptive equalization unit 232 executes adaptive equalization processing using a digital filter such as an FIR filter (finite impulse response filter) according to the set tap coefficients.

[0020] The amplitude correction unit 233 corrects the amplitude of the received signal based on the four digital signals (digital filter outputs) that have been subjected to adaptive equalization processing and the known transmission signal. In this way, the amplitude correction unit 233 uses the known transmission signal as part of the process of correcting the amplitude of the received signal.

[0021] The second signal processing unit 234 performs signal processing on the four digital signals that have been subjected to adaptive equalization processing and correction by the amplitude correction unit 233. For example, the second signal processing unit 234 performs processing to compensate for a frequency offset, processing to compensate for a phase offset, and demodulation and decoding of the digital signal in the input digital signal. Note that the signal processing performed by the second signal processing unit 234 is not limited to this, and other signal processing may be performed. For example, the second signal processing unit 234 may perform any signal processing that has been conventionally performed after the adaptive equalization processing by the adaptive equalization unit 24.

[0022] 3 and 4 are diagrams for explaining the processing performed by the amplitude correction unit 233 in the first embodiment. FIG. 3 shows time series data of the output from the adaptive equalization unit 232. For example, FIG. 3 shows time series data of the output from the adaptive equalization unit 232 obtained at each of time k to time k+N-1. The time series data in the upper part of FIG. 3 shows a known transmission signal symbol KS and a filter output symbol. The amplitude correction unit 233 rotates the filter output symbol at each time from the current position to the first quadrant.

[0023] As a result, the time series data in the upper part of FIG. 3 is converted into the time series data shown in the lower part of FIG. 3. k ,S k+1 ,…,S k+N-1 ) represents the amplitude of the filter output symbols after rotating them to the first quadrant. k ,S k+1 ,…,S k+N-1 belongs to complex numbers. The reason for rotating the time series of filter output symbols on the complex plane and collecting them in one quadrant (for example, the first quadrant) is that the digital filter output is randomly arranged in the first to fourth quadrants, so if the filter output symbols are averaged without rotation, they will statistically converge to 0, and the signal component (x i +j·x q ) cannot be calculated.

[0024] Furthermore, a method equivalent to the first embodiment can be realized by taking the arithmetic average for each quadrant. However, in this case, a memory for storing the arithmetic average results for each of the four quadrants is required. Therefore, a method that collects the results in one quadrant can reduce memory.

[0025] The amplitude correction unit 233 calculates the amplitude x of the known transmission signal component based on the following equation (1) using the amplitude value at each time after rotating the filter output symbol into the first quadrant. i ,x qCalculate.

[0026]

number

[0027] In addition, N in equation (1) represents the number of symbols used for gain estimation. The amplitude x of the known transmission signal component obtained based on equation (1) is i ,x q 4. Furthermore, the amplitude correction unit 233 corrects the amplitude x i ,x q Using this, the gain g of the in-phase component of the received signal and the quadrature component is calculated based on the following equation (2). i ,g q Calculate the gain g i ,g q corresponds to the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature phase component of the received signal.

[0028]

number

[0029] In addition, h in formula (2) i ,h q represents the gain estimation result correction coefficient (In-Phase / Quadrature), and t i ,t q represents a known transmission signal QPSK amplitude (In-Phase / Quadrature) used for coefficient control of the adaptive equalizer 232.

[0030] FIG. 5 is a flowchart showing a flow of processing in the optical receiving device 20 in the first embodiment. The coherent optical receiving unit 22 receives an optical signal transmitted from the optical transmitting device 10 (step S101). The optical signal received by the coherent optical receiving unit 22 is converted into an electrical signal, and then converted from an analog signal to a digital signal by an analog-to-digital converter and input to the digital signal processing unit 23.

[0031] The first signal processing unit 231 performs a first signal processing on the input digital signal (step S102). The first signal processing unit 231 outputs the digital signal that has been subjected to the first signal processing to the adaptive equalization unit 232. The adaptive equalization unit 232 performs an adaptive equalization process on the digital signal that has been subjected to the first signal processing and that is output from the first signal processing unit 231 (step S103).

[0032] The digital signal that has been subjected to adaptive equalization processing by adaptive equalization unit 232 is input to amplitude correction unit 233. Amplitude correction unit 233 estimates the amount of amplitude correction for the received signal using the digital signal that has been subjected to adaptive equalization processing (step S104). Specifically, first, amplitude correction unit 233 rotates the symbol of the filter output at each time into the first quadrant in the time series data of the digital signal (filter output) that has been subjected to adaptive equalization processing. Next, amplitude correction unit 233 calculates the amplitude x i ,x q Then, the amplitude correction unit 233 calculates the amplitude x i ,x q Using the value of , the gain g in the in-phase component of the received signal and the quadrature component is calculated based on the above equation (2). i ,g q The in-phase component of the amplitude correction amount (gain g i ) and the quadrature component of the amplitude correction amount (g q ) is calculated as follows.

[0033] The amplitude correction unit 233 corrects the amplitude of the in-phase digital signal and the amplitude of the quadrature digital signal, which have been subjected to the adaptive equalization process, by using the calculated amplitude correction amount (step S105). Specifically, the amplitude correction unit 233 corrects the amplitude of the in-phase digital signal and the amplitude of the quadrature digital signal by multiplying the in-phase digital signal by the in-phase component of the amplitude correction amount and multiplying the quadrature digital signal by the quadrature component of the amplitude correction amount. The amplitude correction unit 233 outputs the corrected in-phase digital signal and the corrected quadrature digital signal to the second signal processing unit 234. The second signal processing unit 234 performs second signal processing on the corrected in-phase digital signal and the corrected quadrature digital signal output from the amplitude correction unit 233 (step S106). In this way, the second signal processing unit 234 restores the data transmitted from the optical transmission device 10.

[0034] According to the optical transmission system 100 configured as above, it is possible to improve the accuracy of signal processing by the signal processing unit subsequent to the digital filter. Specifically, in the optical transmission system 100, the amplitude of the received signal is corrected based on the output signal from the digital filter and a known transmission signal, and the signal is output to the subsequent signal processing unit (second signal processing unit 234). As a result, compared to conventional signal processing in which the output from the digital filter is directly passed to the subsequent signal processing unit, the amplitude of the received signal is corrected before the subsequent signal processing is performed, so that the symbol position can be determined more accurately in the decoding process. Therefore, it is possible to perform highly accurate signal processing. As a result, it is possible to improve the accuracy of signal processing by the signal processing unit subsequent to the digital filter.

[0035] A modification of the first embodiment will be described. In the above-described embodiment, the amplitude correction unit 233 calculates the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature component of the received signal based on the result of averaging the digital filter output for a predetermined period. The amplitude correction unit 233 may calculate the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature component of the received signal based on the result of averaging the absolute values ​​of the digital filter output for a predetermined period.

[0036] In the above-described embodiment, the amplitude correction unit 233 has been shown to rotate the symbol of the filter output at each time to the first quadrant. The amplitude correction unit 233 may rotate the symbol of the filter output at each time to a quadrant other than the first quadrant. For example, the amplitude correction unit 233 may rotate the symbol of the filter output at each time to any quadrant from the second quadrant to the fourth quadrant. In this way, the amplitude correction unit 233 can achieve the above-described effect by rotating the symbol of the filter output at each time to any quadrant from the first quadrant to the fourth quadrant.

[0037] Second Embodiment In the second embodiment, a configuration is described in which an amplitude correction amount is estimated based on a time series of an error between an output of a digital filter and a symbol of a known transmission signal, and the amplitude of a received signal is corrected. The configuration in the second embodiment is similar to that in the first embodiment. The processing in the amplitude correction unit 233 differs from that in the first embodiment. The differences from the first embodiment are described below.

[0038] 6 and 7 are diagrams for explaining the processing performed by the amplitude correction unit 233 in the second embodiment. FIG. 6 shows time series data of the output from the adaptive equalization unit 232. For example, FIG. 6 shows time series data of the output from the adaptive equalization unit 232 obtained at each of time k to time k+N-1. The time series data in the upper part of FIG. 6 shows a symbol KS of a known transmission signal and a symbol of a filter output. Furthermore, an arrow 51 shown in the time series data in the upper part of FIG. 6 represents an error between the symbol KS of a known transmission signal and a symbol of a filter output. The amplitude correction unit 233 rotates the arrow 51 indicating the error at each time from the current position to the first quadrant.

[0039] As a result, the time series data in the upper part of FIG. 6 is converted into the time series data shown in the lower part of FIG. 6. k ,e k+1 ,…,e k+N-1 ) represents the amount of rotation of the error between the digital filter output and the known transmitted signal into the first quadrant. Note that e k ,e k+1 ,…,e k+N-1 belongs to a complex number. The reason why the error time series is rotated on the complex plane and collected in one quadrant (for example, the first quadrant) is the same as that in the first embodiment. Furthermore, the advantage of taking the arithmetic average for each quadrant is also the same as that in the first embodiment.

[0040] The amplitude correction unit 233 uses the error value at each time after rotating the error between the digital filter output and the known transmission signal into the first quadrant to calculate an amount (In-Phase / Quadrature) a representing the difference between the known transmission signal component after rotating the digital filter output into the first quadrant and the known transmission signal based on the following equation (3): i ,a q Calculate.

[0041]

number

[0042] The difference quantity a obtained based on equation (3) i ,a q is shown in Figure 7.

[0043] Furthermore, the amplitude correction unit 233 calculates the amount a i ,a q Using this, the gain g of the in-phase component of the received signal and the quadrature component is calculated based on the following equation (4). i ,g q The in-phase component of the amplitude correction amount (gain g i ) and the quadrature component of the amplitude correction amount (g q ) Gain g i ,g q corresponds to the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature phase component of the received signal.

[0044]

number

[0045] The amplitude correction unit 233 multiplies the in-phase component digital signal by the in-phase component of the calculated amplitude correction amount, and multiplies the quadrature phase component digital signal by the quadrature phase component of the amplitude correction amount, thereby correcting the amplitude of the in-phase component digital signal and the amplitude of the quadrature phase component digital signal.

[0046] According to the optical transmission system 100 in the second embodiment configured as described above, it is possible to improve the accuracy of signal processing by the signal processing unit subsequent to the digital filter, as in the first embodiment. Furthermore, compared to the first embodiment, it is possible to narrow the range of values ​​by converting into the error between the digital filter output and a known transmission signal. Therefore, it is possible to effectively utilize the bit width of the arithmetic circuit, compared to the first embodiment.

[0047] A modification of the second embodiment will be described. In the above-described embodiment, the amplitude correction unit 233 calculates the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature component of the received signal based on the result of averaging the error between the digital filter output and the known transmission signal for a predetermined period. The amplitude correction unit 233 may calculate the amplitude correction amount of the in-phase component of the received signal and the amplitude correction amount of the quadrature component of the received signal based on the result of averaging the absolute value of the error between the digital filter output and the known transmission signal for a predetermined period.

[0048] In the above-described embodiment, the amplitude correction unit 233 rotates the value of the error between the digital filter output and the known transmission signal at each time to the first quadrant. The amplitude correction unit 233 may rotate the position of the error between the digital filter output and the known transmission signal at each time to a quadrant other than the first quadrant. For example, the amplitude correction unit 233 may rotate the error at each time to any quadrant from the second quadrant to the fourth quadrant. In this way, the amplitude correction unit 233 can achieve the above-described effect by rotating the error at each time to any quadrant from the first quadrant to the fourth quadrant.

[0049] A part of the functions of the optical receiving device 20 in the above-mentioned embodiment may be realized by a computer. In that case, a program for realizing the functions may be recorded in a computer-readable recording medium, and the program recorded in the recording medium may be read into a computer system and executed to realize the functions. The term "computer system" as used herein includes hardware such as an OS (Operating System) and peripheral devices. The term "computer-readable recording medium" refers to a portable medium such as a flexible disk, a magneto-optical disk, a ROM (Read Only Memory), and a CD-ROM, and a storage device such as a hard disk built into a computer system. The term "computer-readable recording medium" may also include a medium that dynamically holds a program for a short period of time, such as a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line, and a medium that holds a program for a certain period of time, such as a volatile memory inside a computer system that is a server or client in that case. The above-mentioned program may be a program for realizing a part of the above-mentioned functions, or may be a program that can realize the above-mentioned functions in combination with a program already recorded in the computer system, or may be a program that is realized using a programmable logic device such as an FPGA (Field-Programmable Gate Array).

[0050] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0051] The present invention can be applied to optical transmission system technology in which equalization processing is performed using a digital filter. [Explanation of symbols]

[0052] REFERENCE SIGNS LIST 10...optical transmitting device, 11...optical transmitting section, 12...electrical signal generating section, 13...optical signal generating section, 20...optical receiving device, 21...optical receiving section, 22...coherent optical receiving section, 23...digital signal processing section, 30...optical transmission path, 31...optical fiber, 32...optical amplifier, 231...first signal processing section, 232...adaptive equalization section, 233...amplitude correction section, 234...second signal processing section

Claims

1. A gain adjustment method in an optical transmission system that performs communication by a digital coherent system and includes an optical transmitting device and an optical receiving device, comprising: converting the optical signal transmitted from the optical transmitter into an electrical signal; converting the electrical signal from an analog signal to a digital signal; performing a first signal processing on the digital signal; performing adaptive equalization processing on the digital signal that has been subjected to the first signal processing using a digital filter; correcting the amplitude of the output signal of the digital filter based on information on the amplitude and phase of the output signal of the digital filter and the amplitude of a known transmission signal; performing a second signal processing on the output signal of the digital filter whose amplitude has been corrected; an amplitude correction amount for correcting the amplitude of the output signal of the digital filter is estimated based on an average result of an amplitude error between the output of the output signal of the digital filter for a predetermined period and a known transmission signal, or an absolute value of an amplitude error between the output of the output signal of the digital filter for a predetermined period and a known transmission signal; Gain adjustment method.

2. the amplitude correction amount is estimated based on an average result after rotating an amplitude error between an output of the output signal of the digital filter for a predetermined period and a known transmission signal into any one of the first quadrant to the fourth quadrant in a complex plane; The gain adjustment method according to claim 1 .

3. An optical transmission system for performing communication by a digital coherent system, comprising an optical transmitting device and an optical receiving device, a coherent optical receiving unit that converts an optical signal transmitted from the optical transmitting device into an electrical signal; an analog-to-digital converter for converting the electrical signal from an analog signal to a digital signal; a first signal processing unit that performs a first signal processing on the digital signal; an adaptive equalization unit that performs adaptive equalization processing on the digital signal that has been subjected to the first signal processing using a digital filter; an amplitude correction unit that corrects the amplitude of the output signal of the digital filter based on information on the amplitude and phase of the output signal of the digital filter and the amplitude of a known transmission signal; a second signal processing unit that performs second signal processing on the output signal of the digital filter whose amplitude has been corrected; Equipped with an optical receiving device, wherein the amplitude correction unit estimates an amplitude correction amount for correcting the amplitude of the output signal of the digital filter based on an average result of an amplitude error between an output of the output signal of the digital filter for a predetermined period of time and a known transmission signal, or an average result of an absolute value of an amplitude error between an output of the output signal of the digital filter for a predetermined period of time and a known transmission signal.

4. A computer program for causing a computer to function as an optical receiving device in an optical transmission system for performing communication by a digital coherent method, the computer program comprising: an optical transmitting device; and an optical receiving device, an optical signal transmitted from the optical transmitting device is converted into an electric signal, the electric signal is converted from an analog signal to a digital signal, a first signal processing is performed on the digital signal, and an adaptive equalization process is performed on the digital signal that has been subjected to the first signal processing using a digital filter; and based on information on the amplitude and phase of an output signal of the digital filter and the amplitude of a known transmission signal, the amplitude of the output signal of the digital filter is corrected; A computer program for estimating an amplitude correction amount for correcting the amplitude of an output signal of the digital filter, based on an average result of an amplitude error between an output of the output signal of the digital filter for a predetermined period and a known transmission signal, or an average result of an absolute value of an amplitude error between an output of the output signal of the digital filter for a predetermined period and a known transmission signal.

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