Strain estimation device, receiver, communication system, strain estimation method, and program

Abstract

This enables accurate detection of at least one of the distortion within the transmitter and the distortion within the receiver. [Solution] The distortion estimation device includes an equalization filter that includes at least one of a receiver-internal distortion compensation filter and a transmitter-internal distortion compensation filter; an initial value setting unit that sets an initial value of the receiver-internal distortion compensation filter based on the frequency response of a receiving device and sets an initial value of the transmitter-internal distortion compensation filter based on the frequency response of a transmitting device (at least one of these); a coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter; and a distortion detection unit that detects receiver-internal distortion based on the filter coefficients of the receiver-internal distortion compensation filter and detects transmitter-internal distortion based on the filter coefficients of the transmitter-internal distortion compensation filter (at least one of these).

Description

Technical Field

[0004] ,

[0001] The present disclosure relates to a distortion estimation device, a receiver, a communication system, a distortion estimation method, and a program.

Background Art

[0002] In an optical transmission system, in order to achieve high-speed communication of 1 terabit per second (bps) or more, modulation methods such as a high symbol rate and higher-order Quadrature Amplitude Modulation (QAM) modulation are essential. Since the introduction of digital coherent reception technology, flexible equalization signal processing on the receiving side by digital signal processing has become possible. However, generally, higher-order multilevel modulation signals are vulnerable to distortion. In an advanced multilevel modulation scheme, high-precision equalization processing is required to ensure performance.

[0003] As a related technique, Patent Document 1 discloses a coherent optical receiver in a polarization multiplexed digital coherent optical communication system. In Patent Document 1, the optical receiver has an equalization signal processing circuit having an adaptive multi-layer filter. The adaptive multi-layer filter has, in this order from the signal input side, an in-receiver distortion compensation filter, a chromatic dispersion compensation filter, a polarization separation filter, a carrier phase compensation filter, and an in-transmitter distortion compensation filter. In Patent Document 1, the coefficients of the in-receiver distortion compensation filter, the polarization separation filter, and the in-transmitter distortion compensation filter are adaptively controlled based on the output of the in-transmitter distortion compensation filter and a desired state.

[0004] The optical receiver described in Patent Document 1 has a transmitter strain detection unit and a receiver strain detection unit. The transmitter strain detection unit detects transmitter strain based on the coefficients of a transmitter strain compensation filter after the adaptive control of the coefficients of the adaptive multilayer filter converges. The receiver strain detection unit also detects receiver strain based on the coefficients of a receiver strain compensation filter after the adaptive control of the coefficients of the adaptive multilayer filter converges. In Patent Document 1, the transmitter strain detection unit and the receiver strain detection unit detect the imbalance between the in-phase (I) component and the quadrature (Q) component, the skew between the I component and the Q component, and the phase shift between the I component and the Q component as transmitter strain and receiver strain, respectively. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] International Publication No. 2022 / 091452 [Overview of the project] [Problems that the invention aims to solve]

[0006] In Patent Document 1, the adaptive multilayer filter in the receiver adaptively compensates for both the distortion generated in the transmitter device (hereinafter also referred to as transmitter distortion) and the distortion generated in the receiver device (hereinafter also referred to as receiver distortion). However, there is a problem that the stability of filter convergence deteriorates when there is an excessive amount of distortion that differs for each IQ (hereinafter also referred to as IQ distortion) or distortion that is common to all IQs (hereinafter referred to as common distortion). In Patent Document 1, if the residual error after filter convergence is large, it is not possible to detect the transmitter distortion and receiver distortion with high accuracy.

[0007] One exemplary object of this disclosure is to provide a strain estimation device, receiver, communication system, strain estimation method, and program that can accurately detect at least one of transmitter strain and receiver strain. [Means for solving the problem]

[0008] A strain estimation device according to a first aspect of the present disclosure includes an equalization filter that includes at least one of a receiver strain compensation filter for compensating for in-receiver strain occurring in the polarization multiplexed signal transmitted from a transmitter via a transmission line, and a transmitter strain compensation filter for compensating for transmitter strain occurring in the polarization multiplexed signal in the transmitter; an initial value setting unit that sets initial values ​​of the filter coefficients of the receiver strain compensation filter based on the frequency response of a receiving device in the receiver, and sets initial values ​​of the filter coefficients of the transmitter strain compensation filter based on the frequency response of a transmitting device in the transmitter, and at least one of these; a coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver strain compensation filter and the transmitter strain compensation filter included in the equalization filter; and a strain detection unit that, after the convergence of the filter coefficients, detects in-receiver strain based on the filter coefficients of the receiver strain compensation filter, and at least one of these detects in-transmitter strain based on the filter coefficients of the transmitter strain compensation filter.

[0009] A receiver according to a second aspect of the present disclosure includes a detector that coherently receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, and a distortion estimation device that detects distortion contained in the signal based on the coherently received polarization multiplexed signal. The distortion estimation device includes an equalization filter that includes at least one of an in-receiver distortion compensation filter for compensating in-receiver distortion occurring in the polarization multiplexed signal in the receiver, and an in-transmitter distortion compensation filter for compensating in-transmitter distortion occurring in the polarization multiplexed signal in the transmitter; an initial value setting unit that sets initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and sets initial values ​​of the filter coefficients of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter, and at least one of these; a coefficient update unit that adaptively controls the filter coefficients of at least one of the in-receiver distortion compensation filter and the in-transmitter distortion compensation filter included in the equalization filter; and a distortion detection unit that, after the convergence of the filter coefficients, detects in-receiver distortion based on the filter coefficients of the in-receiver distortion compensation filter, and detects in-transmitter distortion based on the filter coefficients of the in-transmitter distortion compensation filter.

[0010] A communication system according to a third aspect of this disclosure includes a transmitter that transmits a polarization multiplexed signal, a transmission line for transmitting the polarization multiplexed signal, a receiver that receives the polarization multiplexed signal via the transmission line, and a strain estimation device. The distortion estimation device includes an equalization filter that includes at least one of an in-receiver distortion compensation filter for compensating in-receiver distortion occurring in the polarization multiplexed signal in the receiver, and an in-transmitter distortion compensation filter for compensating in-transmitter distortion occurring in the polarization multiplexed signal in the transmitter; an initial value setting unit that sets initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and sets initial values ​​of the filter coefficients of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter, and at least one of these; a coefficient update unit that adaptively controls the filter coefficients of at least one of the in-receiver distortion compensation filter and the in-transmitter distortion compensation filter included in the equalization filter; and a distortion detection unit that, after the convergence of the filter coefficients, detects in-receiver distortion based on the filter coefficients of the in-receiver distortion compensation filter, and detects in-transmitter distortion based on the filter coefficients of the in-transmitter distortion compensation filter.

[0011] A distortion estimation method according to a fourth aspect of the present disclosure includes performing an equalization process on a polarization multiplexed signal received in a receiver using an equalization filter that includes at least one of a receiver distortion compensation filter for compensating for in-receiver distortion occurring in the polarization multiplexed signal in a receiver that receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in a transmitter, and performing at least one of setting initial values ​​of the filter coefficients of the receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting initial values ​​of the filter coefficients of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter, adaptively controlling the filter coefficients of at least one of the receiver distortion compensation filter and the transmitter distortion compensation filter included in the equalization filter, and after the convergence of the filter coefficients, detecting receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detecting transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter.

[0012] A fifth aspect of the present disclosure describes a program that causes a processor to perform an equalization process on a polarization multiplexed signal received in a receiver that receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, using an equalization filter that includes at least one of a receiver distortion compensation filter for compensating for in-receiver distortion occurring in the polarization multiplexed signal and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in the transmitter. The program performs at least one of the following: setting initial values ​​of the filter coefficients of the receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting initial values ​​of the filter coefficients of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. The program adaptively controls the filter coefficients of at least one of the receiver distortion compensation filter and the transmitter distortion compensation filter included in the equalization filter, and after the convergence of the filter coefficients, it performs at least one of the following: detecting in-receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detecting in-transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter. [Effects of the Invention]

[0013] The strain estimation device, receiver, communication system, strain estimation method, and program related to this disclosure can accurately detect at least one of the strain within the transmitter and the strain within the receiver. [Brief explanation of the drawing]

[0014] [Figure 1] This is a block diagram illustrating the communication system related to this disclosure. [Figure 2] This is a block diagram showing the schematic configuration of the receiver. [Figure 3] This is a block diagram showing an example configuration of the communication system related to this disclosure. [Figure 4] This is a block diagram showing an example configuration of the digital signal processing unit. [Figure 5] This is a block diagram showing an example of the configuration of the strain estimation unit. [Figure 6] It is a block diagram showing a configuration example of an adaptive equalization filter. [Figure 7] It is a graph showing initial values of filter coefficients of a receiver internal distortion compensation filter or a transmitter internal distortion compensation filter set in a general method. [Figure 8] It is a graph showing initial values of filter coefficients set by an initial value setting unit. [Figure 9] It is a graph showing the relationship between error and time in adaptive control of filter coefficients. [Figure 10] It is a flowchart showing an operation procedure of distortion estimation in an optical fiber communication system. [Figure 11] It is a diagram schematically showing acquisition of frequency response. [Figure 12] It is a constellation of an output signal of an optical receiver. [Figure 13] It is a constellation of an output signal of an optical receiver. [Figure 14] It is a constellation of an output signal of an optical receiver. [Figure 15] It shows the optical spectrum of a transmitted signal measured in simulation. [Figure 16] It shows the optical spectrum of a transmitted signal measured in simulation. [Figure 17] It shows the optical spectrum of a transmitted signal measured in simulation. [Figure 18] It shows a configuration example of a computer device.

Embodiments for Carrying Out the Invention

[0015] Prior to the description of embodiments of the present disclosure, an overview of the present disclosure will be described. FIG. 1 is a block diagram schematically showing a communication system according to the present disclosure. The communication system 10 includes a transmitter 11, a transmission line 13, and a receiver 15. The transmitter 11 and the receiver 15 are interconnected via the transmission line 13. The transmitter 11 transmits a polarization multiplexed signal to the transmission line 13. The receiver 15 receives the polarization multiplexed signal transmitted from the transmitter 11 from the transmission line 13.

[0016] Figure 2 is a block diagram showing the schematic configuration of the receiver 15. The receiver 15 includes a detector 21 and a distortion estimation device 30. The detector 21 coherently receives the polarization multiplexed signal transmitted from the transmitter. The distortion estimation device 30 detects the distortion contained in the signal based on the coherently received polarization multiplexed signal. In this disclosure, the distortion estimation device 30 does not necessarily have to be included in the receiver 15. The distortion estimation device 30 may be configured as a device independent of the receiver 15.

[0017] The distortion estimation device 30 includes an equalization filter 31, a coefficient update unit 32, an initial value setting unit 33, and a distortion detection unit 34. The equalization filter 31 performs equalization processing on the signal received in the receiver 15. The equalization filter 31 includes a receiver distortion compensation filter 36 and a transmitter distortion compensation filter 37. The receiver distortion compensation filter 36 compensates for receiver distortion occurring in the polarization multiplexed signal in the receiver 15. The transmitter distortion compensation filter 37 compensates for transmitter distortion occurring in the polarization multiplexed signal in the transmitter 11.

[0018] The coefficient update unit 32 adaptively controls the coefficients of the receiver's internal distortion compensation filter 36 and the transmitter's internal distortion compensation filter 37, respectively. After the coefficients of the equalization filter 31 converge, the receiver's internal distortion compensation filter 36 is considered to have information corresponding to the characteristics of the distortion generated in the receiver 15. Similarly, the transmitter's internal distortion compensation filter 37 is considered to have information corresponding to the characteristics of the distortion generated in the transmitter 11. After the filter coefficients converge, the distortion detection unit 34 detects the receiver's internal distortion based on the filter coefficients of the receiver's internal distortion compensation filter 36. The distortion detection unit 34 also detects the transmitter's internal distortion based on the filter coefficients of the transmitter's internal distortion compensation filter 37.

[0019] In this disclosure, the initial value setting unit 33 sets the initial values ​​of the filter coefficients of the receiver distortion compensation filter 36 based on the frequency response of the receiving device. The initial value setting unit 33 also sets the initial values ​​of the filter coefficients of the transmitter distortion compensation filter 37 based on the frequency response of the transmitting device. In this disclosure, it is considered that, in the adaptive updating of the filter coefficients, the distortion occurring in the receiver 15 can be accurately compensated by the receiver distortion compensation filter 36 from the initial stage. It is also considered that the distortion occurring in the transmitter 11 can be accurately compensated by the transmitter distortion compensation filter 37 from the initial stage. Therefore, it is considered that the filter coefficients of the receiver distortion compensation filter 36 and the transmitter distortion compensation filter 37 will converge appropriately. After the coefficients converge, the distortion detection unit 34 detects the receiver distortion and the transmitter distortion from the filter coefficients of the receiver distortion compensation filter 36 and the transmitter distortion compensation filter 37, respectively. In this way, the distortion detection unit 34 can accurately detect the receiver distortion and the transmitter distortion.

[0020] In this disclosure, the initial value setting unit 33 does not necessarily need to set initial values ​​for both the receiver distortion compensation filter 36 and the transmitter distortion compensation filter 37. The initial value setting unit 33 may set an initial value for either the receiver distortion compensation filter 36 or the transmitter distortion compensation filter 37 based on the frequency response of the device. For example, the initial value setting unit 33 may set an initial value for the receiver distortion compensation filter 36 based on the frequency response of the receiving device. For the transmitter distortion compensation filter 37, the initial value setting unit 33 may set an initial value determined by a general method. Alternatively, the initial value setting unit 33 may set an initial value for the transmitter distortion compensation filter 37 based on the frequency response of the transmitting device. For the receiver distortion compensation filter 36, the initial value setting unit 33 may set an initial value determined by a general method as the initial value.

[0021] Furthermore, in this disclosure, the equalization filter 31 only needs to have at least one of the receiver strain compensation filter 36 and the transmitter strain compensation filter 37, and does not necessarily need to have both the receiver strain compensation filter 36 and the transmitter strain compensation filter 37. The strain detection unit 34 does not detect receiver strain if the equalization filter 31 does not include the receiver strain compensation filter 36. Also, the strain detection unit 34 does not detect transmitter strain if the equalization filter 31 does not include the transmitter strain compensation filter 37.

[0022] The embodiments of this disclosure will be described in detail below. Note that the following descriptions and drawings have been omitted and simplified as appropriate for clarity of explanation. Furthermore, in each drawing, the same elements and similar elements are denoted by the same reference numerals, and redundant explanations have been omitted where necessary.

[0023] A first embodiment will be described. Figure 3 is a block diagram showing an example configuration of a communication system according to the present disclosure. In the first embodiment, the communication system is assumed to be an optical fiber communication system employing polarization multiplexing multilevel modulation and performing coherent reception. The optical fiber communication system 100 has an optical transmitter 110, a transmission line 130, and an optical receiver 150. The optical fiber communication system 100 constitutes, for example, an optical submarine cable system. The optical fiber communication system 100 corresponds to the communication system 10 shown in Figure 1. The optical transmitter 110 corresponds to the transmitter 11 shown in Figure 1. The transmission line 130 corresponds to the transmission line 13 shown in Figure 1. The optical receiver 150 corresponds to the receiver 15 shown in Figure 1.

[0024] The optical transmitter 110 converts multiple transmission data into polarization-multiplexed optical signals. The optical transmitter 110 includes an encoding unit 111, a pre-equalization unit 112, a digital-to-analog converter (DAC) 113, an optical modulator 114, and a laser diode (LD) 115. The encoding unit 111 encodes the data. The encoding unit 111 outputs four signals, for example, in-phase (I) components of X-polarization and Y-polarization, and quadrature (Q) components.

[0025] The pre-equalization unit 112 performs pre-equalization on the four encoded signal sequences to compensate for distortions in the devices within the optical transmitter. For example, the pre-equalization unit 112 has a 2x2 IQ multi-input multi-output (MIMO) filter for each polarization, which takes the I component and Q component as input and output. The 2x2 IQ MIMO filter compensates for distortions occurring within the optical transmitter 110, such as distortions in the I component and Q component in each polarization, and crosstalk between IQ. The pre-equalization unit 112 is also called a pre-equalization filter.

[0026] DAC113 converts the four pre-equalized signal sequences into analog electrical signals. DAC113 inputs these converted analog electrical signals to the optical modulator 114. An electrical amplifier is placed between DAC113 and the optical modulator 114, and the optical modulator 114 receives analog electrical signals whose amplitude has been amplified by the electrical amplifier.

[0027] The LD115 outputs continuous wave (CW) light. The optical modulator 114 modulates the CW light output from the LD115 according to four analog electrical signals input from the DAC113, generating a polarization-multiplexed optical signal such as a polarization-multiplexed QAM signal. The optical modulator 114 includes, for example, a Mach-Zehnder (MZ) modulator. The optical modulator 114 sends the generated polarization-multiplexed optical signal to the transmission line 130.

[0028] The transmission line 130 transmits the polarization-multiplexed optical signal output from the optical transmitter 110 to the optical receiver 150. The transmission line 130 includes an optical fiber 132 and an optical amplifier 133. The optical fiber 132 guides the optical signal transmitted from the optical transmitter 110. The optical amplifier 133 amplifies the optical signal and compensates for propagation loss in the optical fiber 132. The optical amplifier 133 is configured as, for example, an erbium-doped fiber amplifier (EDFA). The transmission line 130 may include multiple optical amplifiers 133.

[0029] The optical receiver 150 includes an LD 151, a coherent receiver 152, an analog-to-digital converter (ADC) 153, a digital signal processing unit 154, a decoding unit 155, and a distortion estimation unit 160. The LD 151 outputs CW light, which becomes the local oscillator light. The coherent receiver 152 is configured as a polarization diversity type coherent receiver. The coherent receiver 152 uses the CW light output from the LD 151 to perform coherent detection on the optical signal transmitted through the optical fiber 132. The coherent receiver 152 outputs four sequences of received signals (electrical signals) corresponding to the I and Q components of the coherently detected X and Y polarizations. The coherent receiver 152 corresponds to the detector 21 shown in Figure 2.

[0030] The ADC153 receives the received signal output from the coherent receiver 152 via an electrical amplifier. The ADC153 samples the received signal output from the coherent receiver 152 and converts the received signal into a digital signal. The ADC153 outputs the converted digital signal to the digital signal processing unit 154. The digital signal processing unit 154 performs digital signal processing on the four sequences of received signals sampled by the ADC153 and demodulates the received signal. The digital signal processing unit 154 is also called a digital signal processing circuit or an equalization signal processing circuit.

[0031] The digital signal processing unit 154 includes an adaptive equalization filter. The adaptive equalization filter included in the digital signal processing unit 154 compensates for various distortions contained in the digital signal. The decoding unit 155 decodes the demodulated signal and restores the transmitted data. The decoding unit 155 is also called a decoder. In the optical receiver 150, circuits such as the digital signal processing unit 154 and the decoding unit 155 can be configured using a device such as a digital signal processor (DSP).

[0032] Figure 4 is a block diagram showing an example configuration of the digital signal processing unit 154. The digital signal processing unit 154 includes 2×1 widely linear (WL) MIMO filters 181X and 181Y, arranged for X polarization and Y polarization respectively, wavelength dispersion compensation filters 182X and 182Y, and carrier phase compensation filters 184X and 184Y. The digital signal processing unit 154 also includes a 2×2 strictly linear (SL) MIMO filter 183. In this embodiment, MIMO filters are also called MIMO equalizers.

[0033] The digital signal processing unit 154 receives the complex number data of the X-polarized and Y-polarized signals, which are obtained by converting the IQ components of the X-polarized and Y-polarized signals output from the ADC 153 into complex number signals, respectively. In the digital signal processing unit 154, the 2×1 WL MIMO filters 181X and 181Y, the wavelength dispersion compensation filters 182X and 182Y, the 2×2 SL MIMO filter 183, and the carrier phase compensation filters 184X and 184Y are connected in tandem with respect to the input signal. The filters included in the digital signal processing unit 154 constitute an adaptive multilayer filter.

[0034] The 2×1 WL MIMO filters 181X and 181Y compensate for distortions that occur within the optical receiver 150. For example, the 2×1 WL MIMO filters 181X and 181Y compensate for distortions that occur within the optical receiver 150, such as distortions in the I and Q components for each polarization, and crosstalk between I and Q. The 2×1 WL MIMO filters 181X and 181Y correspond to in-receiver distortion compensation filters. Note that the 2×1 WL MIMO filter is equivalent to a 2×2 IQ MIMO filter that uses I and Q components as input and output, and a 2×2 IQ MIMO filter may be used to compensate for distortions that occur within the optical receiver 150. The fluctuations in in-receiver distortion are gradual and can be captured as quasi-dynamic fluctuations. In this embodiment, the 2×1 WL MIMO filters 181X and 181Y are treated as quasi-static filters.

[0035] The Chromatic Dispersion Compensation (CDC) filters 182X and 182Y compensate for signal distortion caused by chromatic dispersion during optical fiber transmission, for each polarization. The coefficients of the chromatic dispersion compensation filters 182X and 182Y are set based on a physical model of the distortion caused by chromatic dispersion. Since the distortion caused by chromatic dispersion is fixed, the chromatic dispersion compensation filters 182X and 182Y are treated as static filters.

[0036] The 2x2 SL MIMO filter 183 is configured as a complex coefficient MIMO filter with two inputs and two outputs of complex signals. The 2x2 SL MIMO filter 183 compensates for signal distortion caused by polarization state fluctuations and dispersion of polarization modes during optical fiber transmission. The carrier phase compensation filters 184X and 184Y compensate for signal distortion caused by frequency offset and phase offset between the carrier of the transmitted optical signal and the local oscillator light on the receiving side. For example, an SL 1x1 1-tap Finite Impulse Response (FIR) filter is used for the carrier phase compensation filters 184X and 184Y. The carrier phase compensation filters 184X and 184Y are also called Carrier Phase Recovery (CPR) filters.

[0037] The coefficients of the 2×2 SL MIMO filter 183 are adaptively updated using a coefficient update unit (not shown in Figure 4). More specifically, the coefficient update unit calculates the difference between the output of the carrier phase compensation filters 184X and 184Y, which are the final stage filters of the adaptive multilayer filter, and a predetermined value or desired state as a loss function. The coefficient update unit updates the coefficients of the 2×2 SL MIMO filter 183 to minimize the loss function. The phase compensated by the carrier phase compensation filter is calculated separately based on the output signal of the carrier phase compensation filter using a method not shown in the figure. A general M-square method or a digital phase-locked loop (PLL) using a provisional decision can be used to calculate the phase to be compensated.

[0038] In the digital signal processing unit 154, the 2×1 WL MIMO filters 181X and 181Y, the wavelength dispersion compensation filters 182X and 182Y, the 2×2 SL MIMO filter 183, and the carrier phase compensation filters 184X and 184Y can each be configured using hardware circuits. On the other hand, the function of the coefficient update unit can be realized by the processor in the digital signal processing unit 154 operating according to a program read from memory.

[0039] Returning to Figure 3, the distortion estimation unit 160 receives the digital signal output from the ADC 153. In other words, a signal equivalent to the signal input to the digital signal processing unit 154 is branched to the distortion estimation unit 160. The distortion estimation unit 160 uses the input digital signal to estimate the distortion within the transmitter, i.e., the distortion occurring in the optical transmitter 110. The distortion estimation unit 160 also uses the input digital signal to estimate the distortion within the receiver, i.e., the distortion occurring in the optical receiver 150. The distortion estimation unit 160 estimates the distortion within the transmitter and receiver, for example, at the time of factory shipment of the optical transmitter 110 and optical receiver 150, or during pre-operation testing of the optical fiber communication system 100. During operation of the optical fiber communication system 100, the distortion estimation unit 160 may estimate the distortion within the transmitter and receiver intermittently, rather than continuously.

[0040] Figure 5 is a block diagram showing an example configuration of the strain estimation unit 160. The strain estimation unit 160 includes an adaptive equalization filter 161, a coefficient update unit 162, an initial value setting unit 163, a strain detection unit 164, and a coefficient control unit 165. The strain estimation unit 160 includes, for example, one or more processors and one or more memories. At least a portion of the functions of the strain estimation unit 160 may be realized by the processor operating according to a program read from the memory. The strain estimation unit 160 corresponds to the strain estimation device 30 shown in Figure 2.

[0041] The adaptive equalization filter 161 performs equalization processing on the digital signal output from the ADC 153. In the equalization processing, the adaptive equalization filter 161 compensates for various distortions contained in the received signal. The adaptive equalization filter 161 includes a receiver distortion compensation filter that compensates for receiver distortion and a transmitter distortion compensation filter that compensates for transmitter distortion. The coefficient update unit 162 adaptively controls the coefficients of the adaptive equalization filter 161. For example, the coefficient update unit 162 adaptively controls or updates the coefficients of the adaptive equalization filter 161 based on the difference between the output signal of the adaptive equalization filter 161 and a predetermined value or desired state of the output signal. The adaptive equalization filter 161 corresponds to the equalization filter 31 shown in Figure 2. The coefficient update unit 162 corresponds to the coefficient update unit 32 shown in Figure 2.

[0042] The initial value setting unit 163 sets the initial values ​​of the filter coefficients in the adaptive control of the filter coefficients of the adaptive equalization filter 161. The initial value setting unit 163 sets the initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the optical receiver 150. The receiving device includes, for example, the coherent receiver 152, ADC 153, and a printed circuit board on which the device is mounted, as shown in Figure 3. The initial value setting unit 163 also sets the initial values ​​of the filter coefficients of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the optical transmitter 110. The transmitting device includes, for example, the DAC 113, optical modulator 114, and a printed circuit board on which the device is mounted, as shown in Figure 3.

[0043] Figure 6 is a block diagram showing an example configuration of the adaptive equalization filter 161. The adaptive equalization filter 161 receives the complex number data of the X-polarization and Y-polarization, which are obtained by converting the IQ components of the X-polarization and Y-polarization, respectively, output from the ADC 153 into complex number signals. The adaptive equalization filter 161 includes 2×1 WL MIMO filters 171X and 171Y, wavelength dispersion compensation filters 172X and 172Y, 2×2 SL MIMO filter 173, carrier phase compensation filters 174X and 174Y, and 2×1 WL MIMO filters 175X and 175Y. In the adaptive equalization filter 161, these filters are connected in tandem with respect to the input signal.

[0044] The 2×1 WL MIMO filters 171X and 171Y compensate for distortion generated within the optical receiver 150 for each polarization. The chromatic dispersion compensation filters 172X and 172Y compensate for signal distortion caused by chromatic dispersion during optical fiber transmission for each polarization. The chromatic dispersion compensation filters 172X and 172Y are static filters with fixed coefficients. The 2×2 SL MIMO filter 172 is configured as a complex coefficient MIMO filter with two inputs and two outputs of complex signals. The 2×2 SL MIMO filter 173 uses an FIR filter with more than one tap. The 2×2 SL MIMO filter 172 compensates for signal distortion caused by polarization state fluctuations and dispersion of polarization modes during optical fiber transmission. The 2×1 WL MIMO filters 171X and 171Y correspond to the in-receiver distortion compensation filter 36 shown in Figure 2.

[0045] The carrier phase compensation filters 174X and 174Y compensate for signal distortion caused by frequency and phase offsets between the carrier of the transmitted optical signal and the local oscillator light on the receiving side. For example, SL 1×1 1-tap FIR filters are used for the carrier phase compensation filters 174X and 174Y. The phase compensated by the carrier phase compensation filters 174X and 174Y is calculated separately using a general M-square method or a digital PLL with provisional decision. The 2×1 WL MIMO filters 175X and 175Y compensate for distortions that occur within the optical transmitter 110 for each polarization. The 2×1 WL MIMO filters 175X and 175Y correspond to the transmitter distortion compensation filter 37 shown in Figure 2.

[0046] The initial value setting unit 163 acquires, for example, the frequency response of one or more receiving devices, i.e., the frequency response data of one or more receiving devices. The initial value setting unit 163 also acquires the frequency response of one or more transmitting devices, i.e., the frequency response data of one or more transmitting devices. The frequency response data can be generated, for example, by measuring the frequency response of the devices using dedicated measuring equipment during the device design phase or shipment testing. The receiving device frequency response and the transmitting device frequency response each include at least one of intensity information and phase information. The intensity information and phase information may be IQ-specific information or IQ-common information. Furthermore, the intensity information and phase information may be polarization-common information. In other words, it is not necessary to have all the device frequency response data. Also, the frequency response data measured using dedicated measuring equipment during the device design phase or shipment testing may differ from the actual device frequency response due to degradation over time and temperature fluctuations. This device frequency response data is used as the initial value of the filter coefficients in the adaptive control of the filter coefficients of the adaptive equalization filter 161. Therefore, even with frequency response data from a partial device, or frequency response data with differences, the filter coefficients are expected to converge appropriately as long as the differences are not excessive.

[0047] The initial value setting unit 163 sets the filter coefficients that compensate for the receiving device frequency response as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 171X and 171Y, which are in-receiver distortion compensation filters. For example, the initial value setting unit 163 multiplies the frequency responses of two or more receiving devices in the frequency domain and sets the filter coefficients that compensate for the transfer function, i.e., the multiplied frequency response, as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 171X and 171Y. The initial value setting unit 163 also sets the filter coefficients that compensate for the transmitting device frequency response as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y, which are in-transmitter distortion compensation filters. For example, the initial value setting unit 163 multiplies the frequency responses of two or more transmitting devices in the frequency domain and sets the filter coefficients that compensate for the transfer function, i.e., the multiplied frequency response, as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y. The initial value setting unit 163 may also set the filter coefficients that compensate for the frequency response of one receiving device as the initial values ​​for the filter coefficients of the 2×1 WL MIMO filters 171X and 171Y. Similarly, the initial value setting unit 163 may also set the filter coefficients that compensate for the frequency response of one transmitting device as the initial values ​​for the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y.

[0048] The coefficient update unit 162 adaptively updates the coefficients of several filters included in the adaptive equalization filter 161 based on the difference between the output of the 2×1 WL MIMO filters 175X and 175Y and the desired state of the output of the 2×1 WL MIMO filters 175X and 175Y. In this embodiment, the coefficient update unit 162 adaptively updates the coefficients of the 2×1 WL MIMO filters 171X and 171Y, the 2×2 SL MIMO filter 173, and the 2×1 WL MIMO filters 175X and 175Y.

[0049] The coefficient update unit 162 calculates the difference between the output of the 2×1 WL MIMO filters 175X and 175Y and the desired state, i.e., the error, as the loss function. The coefficient update unit updates the coefficients of the 2×1 WL MIMO filters 171X and 171Y, the 2×2 SL MIMO filter 173, and the 2×1 WL MIMO filters 175X and 175Y to minimize the loss function. The coefficient update unit 162 updates the filter coefficients h of each filter to minimize the error E, using the following formula, with μ satisfying 0 < μ < 1 as the step size in the coefficient update. h→h-μE

[0050] The strain detection unit 164 detects or estimates receiver strain based on the coefficients of the 2×1 WL MIMO filters 171X and 171Y that compensate for receiver strain. The strain detection unit 164 also detects or estimates transmitter strain based on the coefficients of the 2×1 WL MIMO filters 175X and 175Y that compensate for transmitter strain. The strain detection unit 164 can estimate receiver strain and transmitter strain, respectively, using known algorithms for estimating compensated strain from filter coefficients. The strain detection unit 164 corresponds to the strain detection unit 34 shown in Figure 2.

[0051] The coefficient control unit 165 sets or controls the coefficients of the filters included in the digital signal processing unit 154 to compensate for the in-receiver distortion, based on the in-receiver distortion estimated by the distortion detection unit 164. That is, the coefficient control unit 165 sets or controls the coefficients of the 2×1 WL MIMO filters 181X and 181Y shown in Figure 4, based on the estimated in-receiver distortion. For example, the coefficient control unit 165 controls the coefficients of the 2×1 WL MIMO filters 181X and 181Y so that the in-receiver distortion estimated by the distortion detection unit 164 is compensated for.

[0052] Furthermore, the coefficient control unit 165 sets or controls the coefficients of the pre-equalization unit 112 included in the optical transmitter 110 shown in Figure 3, based on the internal transmitter strain estimated by the strain detection unit 164. The coefficient control unit 165 controls the coefficients of the pre-equalization unit 112, for example, so that the inverse characteristic of the internal transmitter strain estimated by the strain detection unit 164 is added to the signal output to the DAC 113. By controlling the filter coefficients of the pre-equalization unit 112 according to the internal transmitter strain estimated at the receiving end, the receiving end can receive a signal in which the internal transmitter strain has been compensated.

[0053] Here, the fluctuations in transmitter and receiver distortions are gradual, and these distortion fluctuations can be treated as quasi-dynamic fluctuations. Therefore, the coefficient control unit 165 does not need to constantly control the coefficients of the 2×1 WL MIMO filters 181X and 181Y, and the coefficients of the pre-equalization unit 112, for example, for every symbol.

[0054] In this embodiment, since the compensation for transmitter and receiver distortion targets quasi-dynamic fluctuations, the adaptive equalization filter 161 in the distortion estimation unit 160 does not need to be implemented with high-speed hardware circuitry. In this embodiment, the adaptive equalization filter 161 is implemented by software processing. In other words, the adaptive equalization processing used for distortion estimation is offloaded to software processing. When the estimation of transmitter distortion and receiver distortion is performed using the adaptive equalization filter 161 implemented by software processing, there is an effect of suppressing the increase in the hardware circuitry of the digital signal processing unit 154.

[0055] Figure 7 is a graph showing the initial values ​​of the filter coefficients of a receiver or transmitter distortion compensation filter set using a common method. In the example in Figure 7, the initial values ​​of the receiver or transmitter distortion compensation filter are set to "1" only in the time domain, with the values ​​of the other taps set to "0". In this case, the intensity characteristics of the initial filter coefficients show a flat characteristic with respect to frequency. Also, the phase characteristics of the initial filter coefficients show a characteristic that decreases at a constant rate with respect to frequency.

[0056] Figure 8 is a graph showing the initial values ​​of the filter coefficients set by the initial value setting unit 163. In the example in Figure 8, the initial values ​​of the receiver distortion compensation filter or the transmitter distortion compensation filter vary in the time domain. In the example in Figure 8, the intensity characteristics of the initial values ​​of the filter coefficients show a characteristic of varying with respect to frequency. Furthermore, the phase characteristics of the initial values ​​of the filter coefficients show a characteristic of decreasing while varying with respect to frequency.

[0057] Figure 9 is a graph showing the relationship between error and time in adaptive control of filter coefficients. In the graph shown in Figure 9, the vertical axis represents the magnitude of the error or loss function, and the horizontal axis represents time. In the graph, the error when initial values ​​set according to the general method shown in Figure 7 are used is shown by a dashed line. In the graph, the error when initial values ​​set by the initial value setting unit 163 shown in Figure 8 are used is shown by a solid line.

[0058] Referring to Figure 9, it can be seen that when the initial values ​​of the filter coefficients are set based on the frequency response of the device, the residual error after coefficient convergence is reduced compared to when the initial values ​​are set according to a general method. The reduced residual error indicates that the receiver distortion and transmitter distortion are accurately compensated in the adaptive equalization filter 161. In this embodiment, the distortion detection unit 164 detects the receiver distortion and transmitter distortion from the adaptive equalization filter 161, in which the receiver distortion and transmitter distortion are accurately compensated. Therefore, the distortion detection unit 164 can accurately detect the transmitter distortion and receiver distortion.

[0059] Next, the operating procedure will be explained. Figure 10 is a flowchart showing the operating procedure for strain estimation in the optical fiber communication system 100. The operating procedure for strain estimation corresponds to the strain estimation method. The optical transmitter 110 transmits a polarization multiplexed signal to the optical receiver 150 via the transmission line 130. The optical receiver 150 receives the polarization multiplexed signal transmitted from the optical transmitter 110 via the transmission line 130 (step S1).

[0060] In the optical receiver 150, the adaptive equalization filter 161 of the distortion estimation unit 160 receives complex number data for each polarization from the ADC 153. The initial value setting unit 163 sets the initial values ​​of the filter coefficients of the receiver distortion compensation filter and transmitter distortion compensation filter included in the adaptive equalization filter 161 based on the frequency response of the device (step S2). In step S2, the initial value setting unit 163 sets the initial values ​​of the filter coefficients of, for example, the 2×1 WL MIMO filters 171X and 171Y shown in Figure 6, based on the frequency response of the receiving device. The initial value setting unit 163 also sets the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y based on the frequency response of the transmitting device. The setting of the initial values ​​of the filter coefficients in step S2 may be performed before the reception of the polarization multiplexed signal in step S1.

[0061] In step S2, the initial value setting unit 163 does not necessarily need to set both the initial values ​​of the filter coefficients of the receiver distortion compensation filter and the transmitter distortion compensation filter based on the frequency response of each device. In step S2, the initial value setting unit 163 may set the initial values ​​of the filter coefficients of the receiver distortion compensation filter based on the frequency response of the receiving device and set the initial values ​​of the filter coefficients of the transmitter distortion compensation filter according to a general method. Alternatively, in step S2, the initial value setting unit 163 may set the initial values ​​of the filter coefficients of the transmitter distortion compensation filter based on the frequency response of the transmitting device and set the initial values ​​of the filter coefficients of the receiver distortion compensation filter according to a general method.

[0062] The coefficient update unit 162 adaptively controls the coefficients of the adaptive equalization filter 161 (step S3). In step S3, the coefficient update unit 162 adaptively updates the coefficients of, for example, the 2×1 WL MIMO filters 171X and 171Y, the 2×2 SL MIMO filter 173, and the 2×1 WL MIMO filters 175X and 175Y.

[0063] The strain detection unit 164 detects transmitter strain and receiver strain based on the coefficients of the adaptive equalization filter 161 after the coefficients of the adaptive equalization filter 161 have converged (step S4). In step S4, the strain detection unit 164 detects receiver strain from the coefficients of the adaptively updated 2×1 WL MIMO filters 171X and 171Y. The strain detection unit 164 also detects transmitter strain from the coefficients of the adaptively updated 2×1 WL MIMO filters 175X and 175Y.

[0064] The coefficient control unit 165 controls the coefficients of the filter for compensating for the transmitter distortion and the filter for compensating for the receiver distortion based on the transmitter distortion and receiver distortion detected in step S4 (step S5). In step S5, the coefficient control unit 165 controls the coefficients of the 2×1 WL MIMO filters 181X and 181Y included in the digital signal processing unit 154 shown in Figure 4, according to the detected receiver distortion. The coefficient control unit 165 also controls the coefficients of the pre-equalization unit 112 included in the optical transmitter 110 based on the detected transmitter distortion.

[0065] In this embodiment, the initial value setting unit 163 sets the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 171X and 171Y in adaptive control of the filter coefficients based on the frequency response of the receiving device. The initial value setting unit 163 also sets the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y in adaptive control of the filter coefficients based on the frequency response of the transmitting device. In this embodiment, the initial values ​​set based on the frequency response of the device are used in adaptive control of the filter coefficients. In this way, in adaptive updating of the filter coefficients, distortion occurring in the optical receiver 150 with the 2×1 WL MIMO filters 171X and 171Y can be accurately compensated from the initial stage. Also, distortion occurring in the optical transmitter 110 with the 2×1 WL MIMO filters 175X and 175Y can be accurately compensated. In this embodiment, the distortion detection unit 164 detects receiver distortion from the filter coefficients of the receiver distortion compensation filter, in which receiver distortion and transmitter distortion are accurately compensated. Furthermore, the strain detection unit 164 detects internal transmitter strain from the filter coefficients of the transmitter strain compensation filter, which accurately compensates for internal transmitter strain. Therefore, the strain detection unit 164 can accurately detect both internal transmitter strain and internal receiver strain.

[0066] Here, if the IQ distortion, which is a distortion that differs for each IQ, or the common distortion, which is a distortion that exists in common with all IQs, is excessive in the optical transmitter 110 or the optical receiver 150, it becomes difficult to derive the optimal step size for each filter coefficient in updating the coefficients of the adaptive equalization filter 161. If an appropriate step size cannot be derived, the stability of filter convergence in the adaptive equalization filter 161 deteriorates. Furthermore, regarding the common distortion that is common to all IQs, since the common distortion is commutative between the optical transmitter 110 and the optical receiver 150, it becomes difficult to separate the distortion in the optical transmitter 110 from the distortion in the optical receiver 150.

[0067] If the filter coefficients do not converge properly in the adaptive equalization filter 161, it becomes necessary to change the update size of each filter, i.e., the step size μ, and redo the adaptive control of the filter coefficients. In this embodiment, the initial values ​​of the filter coefficients are appropriately set according to the frequency response of the device in the adaptive control of the filter coefficients. Therefore, even if there is excessive distortion in the received signal, the filter coefficients are expected to converge properly. Accordingly, in this embodiment, it is not necessary to change the step size μ and redo the adaptive control of the filter coefficients many times, and the derivation of the optimal step size μ for each filter can be relaxed.

[0068] Furthermore, in the adaptive equalization filter 161, the transmitter IQ common distortion, receiver IQ common distortion, and transmission path IQ common distortion can be compensated by any of the 2×1 WL MIMO filters 171X and 171Y for receiver distortion compensation, the 2×2 SL MIMO filter 173 for polarization separation, and the 2×1 WL MIMO filters 175X and 175Y for transmitter distortion compensation. In this embodiment, the inverse characteristics of the device's frequency response are given as initial values ​​for the 2×1 WL MIMO filters 171X and 171Y and the 2×1 WL MIMO filters 175X and 175Y. In that case, it is considered that the receiver IQ common distortion is compensated by the 2×1 WL MIMO filters 171X and 171Y, and the transmitter IQ common distortion is compensated by the 2×1 WL MIMO filters 175X and 175Y. For this reason, this embodiment is considered to be able to improve the separation of transmitter IQ common distortion and receiver IQ common distortion.

[0069] Furthermore, in this embodiment, the coefficient control unit 165 controls the coefficients of the pre-equalization unit 112 and the coefficients of the filter included in the digital signal processing unit 154 for compensating for the receiver distortion, based on the estimated transmitter distortion and receiver distortion. In this embodiment, as described above, the transmitter distortion and receiver distortion can be estimated with high accuracy. Therefore, in this embodiment, the decoding unit 155 can be input with a signal in which the transmitter distortion and receiver distortion have been compensated with high accuracy.

[0070] Next, a second embodiment will be described. In the second embodiment, the initial value setting unit 163 acquires the IQ skew, that is, the skew between the I component and the Q component, for each polarization. The IQ skew is the delay difference between IQ and is the phase information for each IQ. The initial value setting unit 163 also acquires the intensity information of the frequency response common to the polarizations that corresponds to the common distortion. Based on the IQ skew and the intensity information of the frequency response common to the polarizations, the initial value setting unit 163 sets the initial values ​​of the in-receiver distortion compensation filter and the in-transmitter distortion compensation filter in the adaptive control of the filter coefficients.

[0071] The acquisition of IQ skew will now be explained. In the optical transmitter 110, a delay difference is introduced between the I component and the Q component of the X-polarized signal input to the DAC 113 shown in Figure 3. Similarly, a delay difference is introduced between the I component and the Q component of the Y-polarized signal input to the DAC 113. Furthermore, in the optical receiver 150, a delay difference is introduced between the I component and the Q component of both the X-polarized signal and the Y-polarized signal input from the ADC 153 to the digital signal processing unit 154.

[0072] The delay differences given in the optical transmitter 110 and the optical receiver 150 are swept, the signal quality for each given delay difference is measured, and the measured signal quality is recorded. The signal quality is highest when the IQ skew occurring in the optical transmitter 110 and the IQ skew occurring in the optical receiver 150 can be compensated for by the given delay difference. The initial value setting unit 163 acquires the delay difference on the transmitter side that results in the highest signal quality as the transmitter-side IQ skew. The initial value setting unit 163 also acquires the delay difference on the receiver side that results in the highest signal quality as the receiver-side IQ skew.

[0073] The derivation of common distortion will now be explained. Figure 11 schematically shows the acquisition of the frequency response. First, the output signal of the optical transmitter 110 is input to the optical spectrum analyzer (OSA), and the first frequency response is acquired in the OSA. The first frequency response corresponds to the transmitter frequency response (Tx frequency response). In Figure 11, the first frequency response is shown by a solid line. Next, the optical receiver 150 receives the polarization multiplexed signal transmitted from the optical transmitter 110, and the second frequency response is acquired from the digital signal output from the ADC 153. The second frequency response includes the transmitter frequency response and the receiver frequency response (Rx frequency response). In Figure 11, the second frequency response is shown by a dashed line.

[0074] The initial value setting unit 163 calculates the difference between the second frequency response and the first frequency response and obtains intensity information of the receiver frequency response. The obtained transmitter frequency response and receiver frequency response show intensity information common to polarization.

[0075] The initial value setting unit 163 sets the initial values ​​of the filter coefficients of the transmitter distortion compensation filter included in the adaptive equalization filter 161 based on the acquired transmitter-side IQ skew and the intensity information of the transmitter frequency response. More specifically, the initial value setting unit 163 multiplies the transmitter-side IQ skew, which is the phase information of each IQ, and the intensity information of the transmitter frequency response in the frequency domain. The initial value setting unit 163 sets the filter coefficients that compensate for the transfer function obtained by the multiplication as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 175X and 175Y shown in Figure 6.

[0076] Furthermore, the initial value setting unit 163 sets the initial values ​​of the filter coefficients of the receiver distortion compensation filter included in the adaptive equalization filter 161 based on the acquired receiver IQ skew and the intensity information of the receiver frequency response. More specifically, the initial value setting unit 163 multiplies the receiver IQ skew, which is the phase information of each IQ, and the intensity information of the receiver frequency response in the frequency domain. The initial value setting unit 163 sets the filter coefficients that compensate for the transfer function obtained by the multiplication as the initial values ​​of the filter coefficients of the 2×1 WL MIMO filters 171X and 171Y shown in Figure 6.

[0077] In this embodiment, the initial value setting unit 163 sets the initial values ​​of the receiver distortion compensation filter and the transmitter distortion compensation filter in adaptive control of the filter coefficients using information acquired from the optical transmitter 110 and the optical receiver 150. In the first embodiment, the frequency response data of the device acquired using dedicated measuring equipment during the device design stage or shipment testing was used to acquire the initial values. In the second embodiment, even if there is no frequency response data acquired using dedicated measuring equipment, the initial values ​​of the filter coefficients can be set using information acquired from the optical transmitter 110 and the optical receiver 150. The frequency response acquired in the second embodiment is considered to be inferior to the frequency response acquired using dedicated measuring equipment. However, the acquired frequency response is used for the initial values ​​of the filter coefficients, and the filter coefficients are updated adaptively. Therefore, as long as the frequency response acquired in the second embodiment is not excessively inferior to the frequency response acquired using dedicated measuring equipment, the filter coefficients are considered to converge appropriately.

[0078] The inventors conducted simulations to verify the effects of the above embodiments. The simulations used a 128GBaud polarization-multiplexed 64QAM signal. In the simulations, the optical transmitter was given an IQ skew of 0.5 symbols to the X-polarized signal. The optical receiver was given an IQ skew of 1.5 symbols to the Y-polarized signal. Furthermore, a Butterworth frequency response common to the IQ was applied to both the optical transmitter and the optical receiver. Receiver distortion and transmitter distortion were detected from the adaptive equalization filter on the receiving side, and the filter coefficients compensating for the detected receiver distortion were used as the filter coefficients of the fixed filter in the optical receiver. Similarly, the filter coefficients compensating for the detected transmitter distortion were used as the filter coefficients of the fixed filter in the optical transmitter.

[0079] Figures 12 to 14 show the constellation of the output signal of the optical receiver 150 obtained by simulation. Figure 12 shows the constellation when distortion compensation is not performed by the fixed filters in the optical transmitter and the optical receiver. Figure 13 shows the constellation when initial values ​​with the characteristics shown in Figure 7 are used for the adaptive control of the filter coefficients of the adaptive equalization filter. Figure 14 shows the constellation when initial values ​​based on the frequency response of the device are used for the adaptive control of the filter coefficients of the adaptive equalization filter.

[0080] Figures 15 to 17 show the optical spectra of the transmitted signals measured in the simulation. In Figures 15 to 17, the horizontal axis represents frequency and the vertical axis represents spectral intensity. Figure 15 shows the optical spectrum of the transmitted signal when distortion compensation is not performed by a fixed filter in the optical transmitter. Figure 16 shows the optical spectrum of the transmitted signal when initial values ​​with the characteristics shown in Figure 7 are used for the adaptive control of the filter coefficients of the adaptive equalization filter. Figure 17 shows the optical spectrum of the transmitted signal when initial values ​​based on the frequency response of the device are used for the adaptive control of the filter coefficients of the adaptive equalization filter.

[0081] Referring to Figure 12, it can be seen that if distortion in the optical transmitter and optical receiver is not compensated using a fixed filter, it is impossible to distinguish signal points, and the signal cannot be correctly decoded. Also, referring to Figure 15, it can be seen that the signal bandwidth of the optical spectrum of the transmitted signal is limited by the frequency response of the Butterworth characteristic common to the IQ of the transmitting side. Referring to Figure 13, when filter coefficients determined by a general method are used as the initial values ​​for the adaptive control of the filter coefficients of the adaptive equalization filter, the signal quality is somewhat improved compared to the case without compensation. However, the signal quality is not high. Also, referring to Figure 16, it can be seen that amplitude ripple occurs in the optical spectrum of the transmitted signal. This is thought to be the result of overcompensating the frequency response of the Butterworth characteristic common to the IQ of the optical transmitter and optical receiver with the transmitter distortion compensation filter. In contrast, referring to Figure 14, it can be seen that when filter coefficients based on the frequency response of the device are used as the initial values ​​for the adaptive control of the filter coefficients of the adaptive equalization filter, the signal quality does not deteriorate, and good signal characteristics are obtained. Furthermore, referring to Figure 17, it was confirmed that the optical spectrum of the transmitted signal exhibits a flatter frequency response compared to Figure 15 or Figure 16. This is thought to be a result of improving the separation between the transmitter IQ common distortion and the receiver IQ common distortion when initial values ​​based on the device's frequency response were used for the adaptive control of the filter coefficients of the adaptive equalization filter.

[0082] In the embodiments described above, an example was given in which the strain estimation unit 160 is included in the optical receiver 150. However, this disclosure is not limited thereto. The strain estimation unit 160 may be configured as a separate device from the optical receiver 150. In that case, the strain estimation unit 160 may be configured as a computer device such as a personal computer (PC). For example, the optical receiver 150 may have an interface for connecting to a computer device, and may output the digital signal output by the ADC 153 to the computer device through that interface.

[0083] Figure 18 shows an example configuration of a computer device that can be used as a strain estimation unit 160. The computer device 400 includes a processor 410 such as a Central Processing Unit (CPU), a storage unit 420, a Read Only Memory (ROM) 430, a Random Access Memory (RAM) 440, a communication interface (IF: Interface) 450, and a user interface 460.

[0084] The communication interface 450 is an interface used for communication with external devices. The communication interface 450 may be used to acquire the four received signals output by the ADC 153. The user interface 460 includes a display unit, such as a display. The user interface 460 also includes input units such as a keyboard, mouse, and touch panel.

[0085] The memory unit 420 is an auxiliary storage device capable of holding various types of data. The memory unit 420 does not necessarily have to be part of the computer device 400; it may be an external storage device or cloud storage connected to the computer device 400 via a network.

[0086] ROM 430 is a non-volatile memory device. A semiconductor memory device, such as a relatively small-capacity flash memory, is used for ROM 430. The program executed by the CPU 410 can be stored in the storage unit 420 or ROM 430. The storage unit 420 or ROM 430 stores various programs that cause the CPU 410 to perform strain compensation and detection processes.

[0087] The above program, when loaded into a computer, includes a set of instructions or software code for causing the computer to perform one or more of the functions described in the embodiments. The program may be stored in a non-temporary computer-readable medium or a physical storage medium. Examples, but not limited to, include RAM, ROM, flash memory, solid-state drive (SSD) or other memory technologies, Compact Disc (CD), digital versatile disc (DVD), Blu-ray® disc or other optical disc storage, magnetic cassette, magnetic tape, magnetic disk storage or other magnetic storage devices. The program may be transmitted over a temporary computer-readable medium or a communication medium. Examples, but not limited to, include, a temporary computer-readable medium or a communication medium that includes electrically, optically, acoustically, or otherwise propagating signals.

[0088] RAM440 is a volatile memory device. Various semiconductor memory devices such as Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM) can be used for RAM440. RAM440 can be used as an internal buffer for temporarily storing data. CPU410 loads a program stored in memory unit 420 or ROM430 into RAM440 and executes the loaded program. CPU410 may also have an internal buffer for temporarily storing data.

[0089] Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0090] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0091] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0092] [Note 1] An equalization filter comprising at least one of a receiver distortion compensation filter for compensating for in-receiver distortion occurring in the polarization multiplexed signal in a receiver that receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, and a transmitter distortion compensation filter for compensating for in-transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A strain estimation device comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

[0093] [Note 2] The distortion estimation apparatus according to Appendix 1, wherein the initial value setting unit performs at least one of the following: setting a filter coefficient for compensating the frequency response of the receiving device as the initial value of the filter coefficient of the in-receiver distortion compensation filter, and setting a filter coefficient for compensating the frequency response of the transmitting device as the initial value of the filter coefficient of the in-transmitter distortion compensation filter.

[0094] [Note 3] The distortion estimation device according to Appendix 1 or 2, wherein the initial value setting unit performs at least one of the following: multiplying the frequency responses of two or more receiving devices in the frequency domain and determining a filter coefficient that compensates the multiplied frequency response as the initial value of the filter coefficient of the in-receiver distortion compensation filter; and multiplying the frequency responses of two or more transmitting devices in the frequency domain and determining a filter coefficient that compensates the multiplied frequency response as the initial value of the filter coefficient of the in-transmitter distortion compensation filter.

[0095] [Note 4] The distortion estimation apparatus according to any one of the appendices 1 to 3, wherein the frequency response of the receiving device includes at least one of intensity information and phase information, and the frequency response of the transmitting device includes at least one of intensity information and phase information.

[0096] [Note 5] The distortion estimation device according to any one of the appendices 1 to 4, wherein the initial value setting unit obtains intensity information of the frequency response of the receiving device common to each polarization from the intensity information of the first frequency response obtained from an optical measuring instrument that receives the polarization multiplexed signal output from the transmitter and the intensity information of the second frequency response obtained from the polarization multiplexed signal received in the receiver, and derives initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the obtained intensity information of the frequency response of the receiving device common to each polarization and the IQ skew which is derived for each polarization and represents the delay difference between the in-phase component and the orthogonal component of the signal in the receiver.

[0097] [Note 6] The distortion estimation device according to any one of the appendices 1 to 5, wherein the initial value setting unit obtains frequency response intensity information of the transmitting device common to each polarization from the frequency response intensity information obtained from an optical measuring instrument that receives the polarization multiplexed signal output from the transmitter, and derives initial values ​​of the filter coefficients of the distortion compensation filter in the transmitter based on the obtained frequency response intensity information of the transmitting device common to each polarization and the IQ skew, which is derived for each polarization and represents the delay difference between the in-phase component and the orthogonal component of the signal in the transmitter.

[0098] [Note 7] The transmitting device includes a digital-to-analog converter that converts a digital signal into an analog signal, and a modulator that modulates a signal according to the output signal of the digital-to-analog converter. The receiving device is a distortion estimation apparatus according to any one of the appendices 1 to 6, comprising a detector that coherently receives the polarization multiplexed signal and an analog-to-digital converter that converts the analog signal output by the detector into a digital signal.

[0099] [Note 8] The receiver has an equalization signal processing circuit that includes an in-receiver distortion compensation filter for compensating for in-receiver distortion occurring within the receiver, The distortion estimation device according to any one of the appendices 1 to 7, further comprising a coefficient control unit that controls the coefficient of the receiver distortion compensation filter of the equalization signal processing circuit according to the detected receiver distortion.

[0100] [Note 9] The transmitter has a pre-equalization filter that pre-equals the polarization multiplexed signals output to the transmission line, The strain estimation device according to Appendix 8, wherein the coefficient control unit controls the coefficient of the pre-equalization filter according to the detected strain inside the transmitter.

[0101] [Note 10] An equalization filter, which includes an in-receiver distortion compensation filter for compensating for in-receiver distortion occurring in the polarization multiplexed signal in a receiver that receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, An initial value setting unit sets the initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, A coefficient update unit that adaptively controls the filter coefficients of the distortion compensation filter within the receiver, A strain estimation device comprising: a strain detection unit that detects the strain within the receiver based on the filter coefficients of the receiver strain compensation filter after the filter coefficients have converged.

[0102] [Note 11] An equalization filter including an internal distortion compensation filter that compensates for internal distortion occurring in the polarization multiplexed signal at the transmitter, which is included in the received signal transmitted from the transmitter and received at the receiver via the transmission line, An initial value setting unit sets the initial value of the filter coefficient of the distortion compensation filter within the transmitter based on the frequency response of the transmitting device in the transmitter, A coefficient update unit that adaptively controls the filter coefficients of the distortion compensation filter within the transmitter, A strain estimation device comprising: a strain detection unit that, after the convergence of the filter coefficients, detects the strain within the transmitter based on the filter coefficients of the transmitter strain compensation filter.

[0103] [Note 12] It is a receiver, A detector that coherently receives polarization-multiplexed signals transmitted from a transmitter via a transmission line, The system includes a distortion estimation device that detects distortion contained in the signal based on the coherently received polarization multiplexed signal, The aforementioned strain estimation device is An equalization filter including at least one of a receiver distortion compensation filter for compensating for receiver distortion occurring in the polarization multiplexed signal in the receiver, and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A receiver comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

[0104] [Note 13] The receiver has an equalization signal processing circuit that includes an in-receiver distortion compensation filter for compensating for in-receiver distortion occurring within the receiver, The receiver according to Appendix 12, further comprising a coefficient control unit that controls the coefficient of the receiver strain compensation filter of the equalization signal processing circuit according to the detected receiver strain.

[0105] [Note 14] A transmitter that transmits polarization multiplexed signals, A transmission path for transmitting the polarization multiplexed signal, A receiver that receives the polarization multiplexed signal via the transmission line, It has a strain estimation device, The aforementioned strain estimation device is An equalization filter including at least one of a receiver distortion compensation filter for compensating for receiver distortion occurring in the polarization multiplexed signal in the receiver, and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A communication system comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

[0106] [Note 15] The receiver has an equalization signal processing circuit that includes an in-receiver distortion compensation filter for compensating for in-receiver distortion occurring within the receiver, The communication system according to Appendix 14, wherein the distortion estimation device further includes a coefficient control unit that controls the coefficient of the receiver distortion compensation filter of the equalization signal processing circuit according to the detected receiver distortion.

[0107] [Note 16] The transmitter has a pre-equalization filter that pre-equals the polarization multiplexed signals output to the transmission line, The communication system as described in Appendix 15, wherein the coefficient control unit controls the coefficient of the pre-equalization filter according to the detected distortion inside the transmitter.

[0108] [Note 17] An equalization filter is used to perform an equalization process on the polarized multiplexed signal received by a receiver that receives a polarized multiplexed signal transmitted from a transmitter via a transmission line. The equalization filter includes at least one of a receiver distortion compensation filter that compensates for in-receiver distortion occurring in the polarized multiplexed signal, and a transmitter distortion compensation filter that compensates for in-transmitter distortion occurring in the polarized multiplexed signal in the transmitter. At least one of the following is performed: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. The filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter are adaptively controlled. A distortion estimation method that, after the convergence of the filter coefficients, performs at least one of the following: detecting receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detecting transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter.

[0109] [Note 18] An equalization filter is used to perform an equalization process on the polarized multiplexed signal received by a receiver that receives a polarized multiplexed signal transmitted from a transmitter via a transmission line. The equalization filter includes at least one of a receiver distortion compensation filter that compensates for in-receiver distortion occurring in the polarized multiplexed signal, and a transmitter distortion compensation filter that compensates for in-transmitter distortion occurring in the polarized multiplexed signal in the transmitter. At least one of the following is performed: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. The filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter are adaptively controlled. A program for causing a processor to perform a process that, after the filter coefficients have converged, detects receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detects transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter.

[0110] Some or all of the elements (e.g., configuration and function) described in Appendices 2 to 9 that are dependent on Appendice 1 may also be dependent on Appendices 10 and 11, respectively, in the same way as the dependencies between Appendices 2 to 9. Furthermore, some or all of the elements described in Appendices 2 to 9 that are dependent on Appendice 1 may also be dependent on Appendices 12, 14, 17, and 18, respectively, in the same way as the dependencies between Appendices 2 to 9. Some or all of the elements described in any appendice may apply to various hardware, software, recording means, systems, and methods for recording software. [Explanation of symbols]

[0111] 10: Communication Systems 11: Transmitter 13: Transmission line 15: Receiver 21: Detector 30: Strain estimation device 31: Equalization filter 32: Coefficient update section 33: Initial value setting section 34: Distortion detection unit 36: Distortion compensation filter in receiver 37: Transmitter distortion compensation filter 100: Fiber Optic Communication System 110: Optical Transmitter 111: Encoding section 112: Pre-equalization section 113: DAC 114: Optical modulator 115:LD 130: Transmission line 132: Optical fiber 133: Optical amplifier 150: Optical receiver 151:LD 152: Coherent receiver 153: ADC 154: Digital signal processing unit 155: Decoding section 160: Distortion Estimation Unit 161: Adaptive Equalization Filter 162: Coefficient update section 163: Initial value setting section 164: Distortion detection unit 165: Coefficient Control Unit 171X, 171Y: 2x1 WL MIMO filter 172X, 172Y: Wavelength dispersion compensation filters 173:2×2 SL MIMO filter 174X, 174Y: Carrier phase compensation filters 175X, 175Y: 2x1 WL MIMO filter 181X, 181Y: 2x1 WL MIMO filter 182X, 182Y: Wavelength dispersion compensation filters 183:2×2 SL MIMO filter 184X, 184Y: Carrier phase compensation filters 400: Computer equipment 410: Processor 420: Storage section 430:ROM 440: RAM 450: Communication Interface 460: User Interface

Claims

1. An equalization filter comprising at least one of a receiver distortion compensation filter for compensating for in-receiver distortion occurring in the polarization multiplexed signal in a receiver that receives a polarization multiplexed signal transmitted from a transmitter via a transmission line, and a transmitter distortion compensation filter for compensating for in-transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A strain estimation device comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

2. The distortion estimation device according to claim 1, wherein the initial value setting unit performs at least one of the following: setting a filter coefficient for compensating the frequency response of the receiving device as the initial value of the filter coefficient of the in-receiver distortion compensation filter, and setting a filter coefficient for compensating the frequency response of the transmitting device as the initial value of the filter coefficient of the in-transmitter distortion compensation filter.

3. The distortion estimation device according to claim 1 or 2, wherein the initial value setting unit performs at least one of the following: multiplying the frequency responses of two or more receiving devices in the frequency domain and determining a filter coefficient that compensates the multiplied frequency response as the initial value of the filter coefficient of the in-receiver distortion compensation filter; and multiplying the frequency responses of two or more transmitting devices in the frequency domain and determining a filter coefficient that compensates the multiplied frequency response as the initial value of the filter coefficient of the in-transmitter distortion compensation filter.

4. The distortion estimation apparatus according to claim 1 or 2, wherein the initial value setting unit obtains intensity information of the frequency response of the receiving device common to each polarization from the intensity information of the first frequency response obtained from an optical measuring instrument that receives the polarization multiplexed signal output from the transmitter and the intensity information of the second frequency response obtained from the polarization multiplexed signal received in the receiver, and derives initial values ​​of the filter coefficients of the in-receiver distortion compensation filter based on the obtained intensity information of the frequency response of the receiving device common to each polarization and the IQ skew which is derived for each polarization and represents the delay difference between the in-phase component and the orthogonal component of the signal in the receiver.

5. The receiver has an equalization signal processing circuit that includes an in-receiver distortion compensation filter for compensating for in-receiver distortion occurring within the receiver, The distortion estimation device according to claim 1 or 2, further comprising a coefficient control unit that controls the coefficient of the receiver distortion compensation filter of the equalization signal processing circuit according to the detected receiver distortion.

6. The transmitter has a pre-equalization filter that pre-equals the polarization multiplexed signals output to the transmission line, The strain estimation device according to claim 5, wherein the coefficient control unit controls the coefficient of the pre-equalization filter according to the detected strain inside the transmitter.

7. It is a receiver, A detector that coherently receives polarization-multiplexed signals transmitted from a transmitter via a transmission line, The system includes a distortion estimation device that detects distortion contained in the signal based on the coherently received polarization multiplexed signal, The aforementioned strain estimation device is An equalization filter including at least one of a receiver distortion compensation filter for compensating for receiver distortion occurring in the polarization multiplexed signal in the receiver, and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A receiver comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

8. A transmitter that transmits polarization multiplexed signals, A transmission path for transmitting the polarization multiplexed signal, A receiver that receives the polarization multiplexed signal via the transmission line, It has a strain estimation device, The aforementioned strain estimation device is An equalization filter including at least one of a receiver distortion compensation filter for compensating for receiver distortion occurring in the polarization multiplexed signal in the receiver, and a transmitter distortion compensation filter for compensating for transmitter distortion occurring in the polarization multiplexed signal in the transmitter, An initial value setting unit that performs at least one of the following: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. A coefficient update unit that adaptively controls the filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter, A communication system comprising a strain detection unit that, after the convergence of the filter coefficients, detects receiver strain based on the filter coefficients of the receiver strain compensation filter, and detects transmitter strain based on the filter coefficients of the transmitter strain compensation filter, at least one of the above.

9. An equalization filter is used to perform an equalization process on the polarized multiplexed signal received by a receiver that receives a polarized multiplexed signal transmitted from a transmitter via a transmission line. The equalization filter includes at least one of a receiver distortion compensation filter that compensates for in-receiver distortion occurring in the polarized multiplexed signal, and a transmitter distortion compensation filter that compensates for in-transmitter distortion occurring in the polarized multiplexed signal in the transmitter. At least one of the following is performed: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. The filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter are adaptively controlled. A distortion estimation method that, after the convergence of the filter coefficients, performs at least one of the following: detecting receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detecting transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter.

10. An equalization filter is used to perform an equalization process on the polarized multiplexed signal received by a receiver that receives a polarized multiplexed signal transmitted from a transmitter via a transmission line. The equalization filter includes at least one of a receiver distortion compensation filter that compensates for in-receiver distortion occurring in the polarized multiplexed signal, and a transmitter distortion compensation filter that compensates for in-transmitter distortion occurring in the polarized multiplexed signal in the transmitter. At least one of the following is performed: setting the initial value of the filter coefficient of the in-receiver distortion compensation filter based on the frequency response of the receiving device in the receiver, and setting the initial value of the filter coefficient of the in-transmitter distortion compensation filter based on the frequency response of the transmitting device in the transmitter. The filter coefficients of at least one of the receiver-internal distortion compensation filter and the transmitter-internal distortion compensation filter included in the equalization filter are adaptively controlled. A program for causing a processor to perform a process that, after the filter coefficients have converged, detects receiver distortion based on the filter coefficients of the receiver distortion compensation filter, and detects transmitter distortion based on the filter coefficients of the transmitter distortion compensation filter.

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