Digital signal processing device, communication system, digital signal processing method, and program

Abstract

The present invention makes it possible to achieve highly accurate distortion compensation while suppressing overcompensation for IQ common distortion. This digital signal processing device comprises: a first multilayer adaptive filter that includes a first receiver distortion compensation filter which compensates for receiver distortion included in a polarization multiplexed signal and a first transmitter distortion compensation filter which compensates for transmitter distortion included in the polarization multiplexed signal; a distortion extraction unit that extracts the reception-side IQ distortion difference and the transmission-side IQ distortion difference on the basis of a coefficient of the first receiver distortion compensation filter and a coefficient of the first transmitter distortion compensation filter; and a coefficient control unit that controls a coefficient of a second receiver distortion compensation filter on the basis of the extracted reception-side IQ distortion difference and that controls a coefficient of a second transmitter distortion compensation filter on the basis of the extracted transmission-side IQ distortion difference.

Description

Digital signal processing device, communication system, digital signal processing method, and program

[0001] The present disclosure relates to a digital signal processing device, a communication system, a digital signal processing method, and a program.

[0002] In optical transmission systems, high symbol rates and modulation methods such as high-order quadrature amplitude modulation (QAM) are essential to achieve high-speed communications of 1 Tbit / s (bps) or more. Since the introduction of digital coherent receiving technology, flexible equalization signal processing on the receiving side has become possible using digital signal processing. However, high-order multilevel modulation signals are generally vulnerable to distortion. High-order multilevel modulation methods require highly accurate equalization processing 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 with an adaptive multilayer filter. The adaptive multilayer filter has, in this order from the signal input side, a distortion compensation filter in the receiver, a chromatic dispersion compensation filter, a polarization separation filter, a carrier phase compensation filter, and a distortion compensation filter in the transmitter. In Patent Document 1, the coefficients of the distortion compensation filter in the receiver, the polarization separation filter, and the distortion compensation filter in the transmitter are adaptively controlled based on the output of the distortion compensation filter in the transmitter and a desired state.

[0004] The coherent optical receiver described in Patent Document 1 has a transmitter distortion detector and a receiver distortion detector. The transmitter distortion detector detects transmitter distortion based on the coefficients of the transmitter distortion compensation filter after convergence of adaptive control of the adaptive multilayer filter coefficients. The receiver distortion detector detects receiver distortion based on the coefficients of the receiver distortion compensation filter after convergence of adaptive control of the adaptive multilayer filter coefficients. In Patent Document 1, the transmitter distortion detector and receiver distortion detector detect IQ imbalance, IQ skew, and IQ phase shift as transmitter distortion and receiver distortion, respectively.

[0005] International Publication No. 2022 / 091452

[0006] In Patent Document 1, an adaptive multilayer filter in a receiver adaptively compensates for distortion in a transmitter and distortion in a receiver collectively. However, distortion (hereinafter also referred to as common distortion) that exists in common between the in-phase (I) component and the quadrature (Q) component of a polarization multiplexed signal contained in a received signal is interchangeable between the transmitter, the transmission path, and the receiver. For this reason, it is difficult to separate the common distortion in the transmitter from the common distortion in the receiver. If the common distortion were compensated for in both the transmitter and the receiver, the common distortion would be overcompensated, resulting in degradation of reception characteristics.

[0007] An exemplary object of the present disclosure is to provide a digital signal processing device, a communication system, a digital signal processing method, and a program that can achieve highly accurate distortion compensation while suppressing overcompensation of IQ common distortion.

[0008] A digital signal processing device according to a first aspect of the present disclosure includes a first adaptive multilayer filter including a first receiver distortion compensation filter that compensates for receiver distortion included in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and a first transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal; a distortion extraction unit that extracts a receiver-side IQ distortion difference, which is the difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter; and a coefficient control unit that controls a coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted receiver-side IQ distortion difference, and controls a coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted transmitter IQ distortion difference.

[0009] A communication system according to a second aspect of the present disclosure includes the digital signal processing device described above, a transmitter that transmits the polarization multiplexed signal, and a receiver that receives the polarization multiplexed signal transmitted from the transmitter.

[0010] A digital signal processing method according to a third aspect of the present disclosure includes compensating for receiver distortion included in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and transmitter distortion included in the polarization multiplexed signal, using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; extracting a receiver-side IQ distortion difference, which is the difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter; controlling a coefficient of a second receiver distortion compensation filter that compensates for the receiver distortion included in the polarization multiplexed signal, based on the extracted receiver-side IQ distortion difference; and controlling a coefficient of a second transmitter distortion compensation filter that compensates for the transmitter distortion included in the polarization multiplexed signal, based on the extracted transmitter IQ distortion difference.

[0011] A program according to a fourth aspect of the present disclosure causes a processor to execute processing including: compensating for receiver distortion included in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and transmitter distortion included in the polarization multiplexed signal, using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; extracting, based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter, a receiver-side IQ distortion difference which is the difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter; controlling, based on the extracted receiver-side IQ distortion difference, a coefficient of a second receiver distortion compensation filter that compensates for the receiver distortion included in the polarization multiplexed signal; and controlling, based on the extracted transmitter IQ distortion difference, a coefficient of a second transmitter distortion compensation filter that compensates for the transmitter distortion included in the polarization multiplexed signal.

[0012] A digital signal processing device, a communication system, a digital signal processing method, and a program according to the present disclosure can achieve highly accurate distortion compensation while suppressing overcompensation of IQ common distortion.

[0013] 1 is a block diagram showing a schematic configuration example of a communication system according to the present disclosure. FIG. 2 is a block diagram showing a schematic configuration example of a digital signal processing device. FIG. 3 is a block diagram showing a configuration example of a communication system according to the present disclosure. FIG. 4 is a block diagram showing a configuration example of a pre-equalization unit. FIG. 5 is a block diagram showing a configuration example of an adaptive equalization filter included in a digital signal processing unit. FIG. 6 is a block diagram showing a configuration example of a distortion estimation unit. FIG. 7 is a flowchart showing an operation procedure for extracting an IQ distortion difference in an optical fiber communication system. FIG. 8 is a flowchart showing an operation procedure for extracting an IQ distortion difference and an IQ common distortion in an optical fiber communication system. FIG. 9 is a diagram showing signal distributions of the I-channel and Q-channel of a received signal measured in a simulation. FIG. 10 is a diagram showing signal distributions of the I-channel and Q-channel of a received signal measured in a simulation. FIG. 11 is a block diagram showing a configuration example of a computer device.

[0014] Prior to describing embodiments of the present disclosure, an overview of the present disclosure will be described. Fig. 1 is a block diagram showing a schematic configuration example of a communication system according to the present disclosure. The communication system 10 shown in Fig. 1 includes a transmitter 11, a receiver 15, and a digital signal processing device 20. The transmitter 11 and the receiver 15 are connected to each other via a transmission path. The transmitter 11 transmits a polarization multiplexed signal to the transmission path. The receiver 15 receives the polarization multiplexed signal transmitted from the transmitter 11 via the transmission path. The receiver 15 coherently receives the polarization multiplexed signal transmitted from the transmitter 11.

[0015] 2 is a block diagram showing a schematic configuration example of a digital signal processing device 20. The digital signal processing device 20 has an adaptive multilayer filter 21, a distortion extraction unit 24, and a coefficient control unit 25. The adaptive multilayer filter 21 includes a receiver distortion compensation filter 22 and a transmitter distortion compensation filter 23. The receiver distortion compensation filter 22 is also called a first receiver distortion compensation filter. The transmitter distortion compensation filter 23 is also called a first transmitter distortion compensation filter.

[0016] The receiver distortion compensation filter 22 compensates for receiver distortion contained in the polarization multiplexed signal received by the receiver 15. The transmitter distortion compensation filter 23 compensates for transmitter distortion contained in the polarization multiplexed signal received by the receiver 15. The distortion extraction unit 24 extracts a difference in receiving-side IQ distortion, which is the difference between the distortion of the I component and the distortion of the Q component of the polarization multiplexed signal generated in the receiver 15, based on the coefficients of the receiver distortion compensation filter 22. The distortion extraction unit 24 also extracts a difference in transmitting-side IQ distortion, which is the difference between the distortion of the I component and the distortion of the Q component of the polarization multiplexed signal generated in the transmitter 11, based on the coefficients of the transmitter distortion compensation filter 23.

[0017] The coefficient control unit 25 controls the coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted difference in receiver IQ distortion. Also, the coefficient control unit 25 controls the coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted difference in transmitter IQ distortion. For example, the coefficient control unit 25 controls the coefficient of the second receiver distortion compensation filter so that the difference in receiver IQ distortion is compensated. Also, the coefficient control unit 25 controls the coefficient of the second transmitter distortion compensation filter so that the difference in transmitter IQ distortion is compensated.

[0018] In the present disclosure, the distortion extraction unit 24 extracts the difference between the receiving-side IQ distortion and the difference between the transmitting-side IQ distortion based on the coefficients of the receiver distortion compensation filter 22 and the coefficients of the transmitter distortion compensation filter 23. The coefficient control unit 25 controls the coefficients of the second receiver distortion compensation filter and the coefficients of the second transmitter distortion compensation filter based on the difference between the receiving-side IQ distortion and the difference between the transmitting-side IQ distortion. The receiver distortion and the transmitter distortion include IQ distortion, and the IQ distortion includes IQ-specific distortion that is specific to the I component and the Q component, respectively, and IQ-common distortion that is common to the I component and the Q component. The IQ-common distortion is interchangeable between the transmitter and the receiver. In the present disclosure, the coefficient control unit 25 controls the coefficients of the second receiver distortion compensation filter and the coefficients of the second transmitter distortion compensation filter so as to compensate for the difference between the receiving-side IQ distortion and the difference between the transmitting-side IQ distortion. In this manner, it is possible to achieve high-precision distortion compensation while avoiding the problem of overcompensation of the IQ-common distortion.

[0019] Hereinafter, embodiments of the present disclosure will be described in detail. Note that the following description and drawings have been omitted and simplified as appropriate for clarity of explanation. In addition, in each drawing, the same or similar elements are designated by the same reference numerals, and duplicate explanations are omitted as necessary.

[0020] FIG. 3 is a block diagram showing an example configuration of a communication system according to the present disclosure. A first embodiment of the present disclosure will be described with reference to FIG. 3. In this embodiment, it is assumed that the communication system is an optical fiber communication system that employs a polarization multiplexed multilevel modulation scheme and performs coherent reception. The optical fiber communication system 100 shown in FIG. 3 includes 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 FIG. 1. The optical transmitter 110 corresponds to the transmitter 11 shown in FIG. 1. The optical receiver 150 corresponds to the receiver 15 shown in FIG. 1.

[0021] The optical transmitter 110 converts multiple transmission data into a polarization multiplexed optical signal. The optical transmitter 110 includes an encoder 111, a pre-equalizer 112, a digital-to-analog converter (DAC) 113, an optical modulator 114, and a laser diode (LD) 115. The encoder 111 encodes the data. The encoder 111 outputs, for example, four series of signals including in-phase (I) components and quadrature (Q) components of X and Y polarizations.

[0022] The pre-equalization unit 112 performs pre-equalization on the coded four-sequence signal to compensate in advance for distortions of devices within the optical transmitter. FIG. 4 is a block diagram showing an example configuration of the pre-equalization unit 112. In this example, the pre-equalization unit 112 has 2×2 IQ Multiple Input Multiple Output (MIMO) equalizers 171X and 171Y arranged for each polarization. The IQ MIMO equalizer is also called an IQ MIMO filter. The pre-equalization unit 112 is also called a pre-equalization filter.

[0023] The 2×2 IQ MIMO equalizer 171X is an IQ MIMO filter that receives as input and output the I and Q component signals of X polarization. The 2×2 IQ MIMO equalizer 171Y is an IQ MIMO filter that receives as input and output the I and Q component signals of Y polarization. The 2×2 IQ MIMO equalizers 171X and 171Y compensate for distortions occurring within the optical transmitter 110, such as distortions occurring in the I and Q components of the X and Y polarizations, respectively. The pre-equalization unit 112 corresponds to a second transmitter distortion compensation filter.

[0024] The DAC 113 converts each of the four series of pre-equalized signals into an analog electrical signal, and inputs the converted analog electrical signal to the optical modulator 114. An electrical amplifier is disposed between the DAC 113 and the optical modulator 114, and the analog electrical signal whose amplitude has been amplified by the electrical amplifier is input to the optical modulator 114.

[0025] The LD 115 outputs continuous wave (CW) light. The optical modulator 114 modulates the CW light output from the LD 115 in accordance with four analog electrical signals input from the DAC 113 to generate 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 transmits the generated polarization multiplexed optical signal to the transmission path 130.

[0026] The transmission path 130 transmits the polarization multiplexed optical signal output from the optical transmitter 110 to the optical receiver 150. The transmission path 130 has 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 path 130 may include multiple optical amplifiers 133.

[0027] 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 that serves as local oscillator light. The coherent receiver 152 is configured as a polarization diversity 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 series of received signals (electrical signals) corresponding to the I and Q components of the coherently detected X and Y polarizations.

[0028] The ADC 153 receives the received signal output from the coherent receiver 152 via an electrical amplifier. The ADC 153 samples the received signal output from the coherent receiver 152 and converts it into a digital signal. The ADC 153 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 series of received signals sampled by the ADC 153 and demodulates the received signal. The digital signal processing unit 154 is also referred to as a digital signal processing circuit. The digital signal processing unit 154 includes an adaptive equalization filter that compensates for various distortions contained in the digital signal. The decoding unit 155 decodes the demodulated signal to restore the transmitted data. The decoding unit 155 is also referred to as 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 devices such as a digital signal processor (DSP).

[0029] Fig. 5 is a block diagram showing an example configuration of an adaptive equalization filter included in the digital signal processing unit 154. The adaptive equalization filter 180 shown in Fig. 5 includes 2x2 IQ MIMO equalizers 181X and 181Y, chromatic dispersion compensation (CDC) filters 182X and 182Y, and carrier phase compensation filters 184X and 184Y, which are arranged for the X polarization and the Y polarization, respectively. The adaptive equalization filter 180 also includes a 2x2 strictly linear (SL) MIMO equalizer 183.

[0030] The adaptive equalization filter 180 receives the IQ component signals of X polarization and the IQ component signals of Y polarization output from the ADC 153. In the adaptive equalization filter 180, 2×2 IQ MIMO equalizers 181X and 181Y, CDC filters 182X and 182Y, a 2×2 SL MIMO equalizer 183, and carrier phase compensation filters 184X and 184Y are arranged in cascade with respect to the input signals. The adaptive equalization filter 180 corresponds to a second adaptive multilayer filter.

[0031] The 2×2 IQ MIMO equalizers 181X and 181Y compensate for distortions occurring within the optical receiver 150, such as distortions occurring in the I and Q components of each polarization and crosstalk occurring between I and Q. The 2×2 IQ MIMO equalizers 181X and 181Y correspond to second receiver distortion compensation filters. The 2×2 IQ MIMO equalizer is equivalent to a widely linear (WL) MIMO equalizer, which inputs and outputs complex data obtained by converting I and Q components into complex signals. A WL MIMO equalizer may be used to compensate for distortions occurring within the optical receiver 150. The receiver distortions vary slowly and are perceived as quasi-dynamic variations. In this embodiment, the 2×2 IQ MIMO equalizers 181X and 181Y are treated as quasi-static filters.

[0032] The CDC filters 182X and 182Y compensate for signal distortion caused by chromatic dispersion during optical fiber transmission for each polarization. The coefficients of the CDC filters 182X and 182Y are set based on a physical model of distortion caused by chromatic dispersion. The distortion caused by chromatic dispersion is fixed, and the CDC filters 182X and 182Y are treated as static filters.

[0033] The 2×2 SL MIMO equalizer 183 is configured as a complex-coefficient MIMO filter with two inputs and two outputs for complex signals. The 2×2 SL MIMO equalizer 183 compensates for signal distortion caused by polarization state fluctuations and polarization mode dispersion during optical fiber transmission. The 2×2 SL MIMO equalizer 183 corresponds to the second 2×2 SL MIMO equalizer. The carrier phase compensation filters 184X and 184Y compensate for signal distortion caused by frequency offsets 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 finite impulse response (FIR) filters are used as the carrier phase compensation filters 184X and 184Y. The carrier phase compensation filters 184X and 184Y are also called carrier phase recovery (CPR) filters.

[0034] The coefficients of the 2×2 SL MIMO equalizer 183 are adaptively updated or controlled using a coefficient update unit not shown in FIG. 5 . More specifically, the coefficient update unit calculates, as a loss function, the difference between the outputs of carrier phase compensation filters 184X and 184Y, which are the final-stage filters of the adaptive multilayer filter, and a predetermined value or a desired state. The coefficient update unit updates the coefficients of the 2×2 SL MIMO equalizer 183 so as to minimize the loss function. The phase compensated by the carrier phase compensation filter 174 is calculated separately based on the output by a method not shown. The phase to be compensated can be calculated using a general M-th power method or a digital phase locked loop (PLL) using tentative decision making.

[0035] In the digital signal processing unit 154, the 2×2 IQ MIMO equalizers 181X and 181Y, the CDC filters 182X and 182Y, the 2×2 SL MIMO equalizer 183, and the carrier phase compensation filters 184X and 184Y may each be configured using hardware circuits. The function of the coefficient update unit may also be configured using hardware circuits. The function of the coefficient update unit may be realized by a processor included in the digital signal processing unit 154 operating in accordance with a program read from a memory. The filter configuration shown in FIG. 5 is an example, and the configuration of the adaptive equalization filter included in the digital signal processing unit 154 is not necessarily limited to the configuration shown in FIG. 5. For example, the adaptive equalization filter 180 may include a filter that compensates for nonlinear distortion. The digital signal processing unit 154 does not necessarily have to include all of the filters shown in FIG. 5.

[0036] Furthermore, the 2×2 IQ MIMO equalizers 181X and 181Y, which are receiver distortion compensation filters, and the 2×2 IQ MIMO equalizers 171X and 171Y, which are transmitter distortion compensation filters, only need to be inserted in a signal path from when the polarization multiplexed signal is transmitted from the optical transmitter 110 until when the polarization multiplexed signal is decoded in the optical receiver 150, and the positions at which these filters are arranged are not limited to specific positions. For example, the digital signal processing unit 154 may have a 2×2 IQ MIMO equalizer used as a transmitter distortion compensation filter.

[0037] Returning to FIG. 3 , the distortion estimator 160 receives as input a digital signal output from the ADC 153. In other words, a signal equivalent to the signal input to the digital signal processor 154 is branched off into the distortion estimator 160. The distortion estimator 160 estimates or extracts the difference in IQ distortion in the transmitter and the difference in IQ distortion in the receiver using the input digital signal. The distortion estimator 160 extracts the difference in IQ distortion in the transmitter and the difference in IQ distortion in the receiver, for example, when the optical transmitter 110 and the optical receiver 150 are shipped from a factory or during a test before operation of the optical fiber communication system 100. The distortion estimator 160 may extract the difference in IQ distortion in the transmitter and the difference in IQ distortion in the receiver intermittently, rather than constantly, during operation of the optical fiber communication system 100.

[0038] 6 is a block diagram showing an example configuration of the distortion estimation unit 160. The distortion estimation unit 160 includes an adaptive multilayer filter 161, a filter coefficient update unit 162, a distortion extraction unit 163, and a coefficient control unit 164. The distortion estimation unit 160 is physically configured as a device including, for example, one or more processors and one or more memories. At least a part of the functions of the distortion estimation unit 160 may be realized by the processor operating in accordance with a program read from the memory. The distortion estimation unit 160 corresponds to the digital signal processing device 20 shown in FIG. 1.

[0039] The adaptive multilayer filter 161 includes 2×1 WL Multiple Input Single Output (MISO) equalizers 191X and 191Y, CDC filters 192X and 192Y, carrier phase compensation filters 194X and 194Y, and 2×1 WL MISO equalizers 195X and 195Y, which are arranged for the X and Y polarizations, respectively. The adaptive multilayer filter 161 also includes a 2×2 SL MIMO equalizer 193.

[0040] The adaptive multilayer filter 161 receives complex data of the X polarization and complex data of the Y polarization, which are obtained by converting the IQ components of the X polarization and the IQ components of the Y polarization output from the ADC 153 into complex signals. In the adaptive multilayer filter 161, the 2×1 WL MISO equalizers 191X and 191Y, the CDCs 192X and 192Y, the 2×2 SL MIMO equalizer 193, the carrier phase compensation filters 194X and 194Y, and the 2×1 WL MISO equalizers 195X and 195Y are arranged in cascade with respect to the input signals. The adaptive multilayer filter 161 is a first adaptive multilayer filter and corresponds to the adaptive multilayer filter 21 shown in FIG. 2 .

[0041] The 2×1 WL MISO equalizers 191X and 191Y compensate for distortion occurring in the optical receiver 150, such as distortion occurring in the I and Q components in each polarization and crosstalk occurring between I and Q. The 2×1 WL MISO equalizers 191X and 191Y correspond to the receiver distortion compensation filter 22 shown in FIG. 2 , which is a first receiver distortion compensation filter. Note that the 2×1 WL MISO equalizer is equivalent to an IQ MIMO equalizer that receives the I and Q components as inputs and outputs. An IQ MIMO equalizer may be used to compensate for distortion occurring in the optical receiver 150.

[0042] The CDC filters 192X and 192Y compensate for signal distortion caused by chromatic dispersion during optical fiber transmission for each polarization. The coefficients of the CDC filters 192X and 192Y are set based on a physical model of distortion caused by chromatic dispersion. The distortion caused by chromatic dispersion is fixed, and the CDC filters 192X and 192Y are treated as static filters.

[0043] The 2×2 SL MIMO equalizer 193 is configured as a complex-coefficient MIMO filter with two inputs and two outputs for complex signals. The 2×2 SL MIMO equalizer 193 compensates for signal distortion caused by polarization state fluctuations and polarization mode dispersion during optical fiber transmission. The 2×2 SL MIMO equalizer 193 corresponds to the first 2×2 SL MIMO equalizer. The carrier phase compensation filters 194X and 194Y compensate for signal distortion caused by frequency offsets 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 as the carrier phase compensation filters 194X and 194Y.

[0044] The 2×1 WL MISO equalizers 195X and 195Y compensate for distortion occurring in the optical transmitter 110, such as distortion occurring in the I component and Q component in each polarization and crosstalk occurring between I and Q. The 2×1 WL MISO equalizers 195X and 195Y correspond to the transmitter distortion compensation filter 23 shown in FIG. 2 , which is a first transmitter distortion compensation filter. Note that the 2×1 WL MISO equalizer is equivalent to an IQ MIMO equalizer that receives the I component and the Q component as input and output, and an IQ MIMO equalizer may be used to compensate for distortion occurring in the optical transmitter 110.

[0045] The filter coefficient update unit 162 adaptively controls or updates the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y, the filter coefficients of the 2×2 SL MIMO equalizer 183, and the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y. For example, the filter coefficient update unit 162 monitors the input signals and output signals of the 2×1 WL MISO equalizers 191X and 191Y. The filter coefficient update unit 162 also monitors the input signals and output signals of the 2×2 SL MIMO equalizer 183. The filter coefficient update unit 162 also monitors the input signals and output signals of the 2×1 WL MISO equalizers 195X and 195Y. The filter coefficient update unit 162 calculates a predetermined loss function based on, for example, the difference between the output signals of the 2×1 WL MISO equalizers 195X and 195Y, which are final-stage filters, and the desired state. The filter coefficient update unit 162 adaptively controls or updates the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y, the filter coefficients of the 2×2 SL MIMO equalizer 183, and the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y using an error backpropagation algorithm based on the predetermined loss function.

[0046] Furthermore, the filter coefficient update unit 162 controls the filter coefficients of the carrier phase compensation filters 194X and 194Y, i.e., the phases compensated by the carrier phase compensation filters 194X and 194Y. The filter coefficient update unit 162 calculates the phases compensated by the carrier phase compensation filters 194X and 194Y based on their outputs. The phases to be compensated can be calculated using a general M-th power method or a digital PLL using tentative decision making.

[0047] The distortion extraction unit 163 extracts or estimates the difference in IQ distortion on the receiving side based on the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y after coefficient convergence. The distortion extraction unit 163 also extracts or estimates the difference in IQ distortion on the transmitting side based on the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y after coefficient convergence. The coefficient control unit 164 controls the coefficients of the pre-equalization unit 112 (see FIG. 4 ) of the optical transmitter 110 based on the extracted difference in IQ distortion on the transmitting side. The coefficient control unit 164 also controls the coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y (see FIG. 5 ) included in the digital signal processing unit 154 of the optical receiver 150 based on the extracted difference in IQ distortion on the receiving side. The distortion extraction unit 163 corresponds to the distortion extraction unit 24 shown in FIG. 2 . The coefficient control unit 166 corresponds to the coefficient control unit 25 shown in FIG.

[0048] Extraction of the difference in IQ distortion will be explained below. First, conversion from the filter coefficients of the 2×1 WL MISO equalizer used in the adaptive multilayer filter 161 to the filter coefficients of the 2×2 IQ MIMO equalizer used in the digital signal processing unit 154 will be explained. In the following explanation, a complex number (its set) is defined as C, and a real number (its set) is defined as R. "*" represents a complex conjugate. A complex signal x(t)∈C is a real signal x Ｉ (t), x Ｑ (t)∈R, x(t)=x Ｉ (t) + ix Ｑ (t). Also, the complex signal y(t) ∈ C can be expressed as the real signal y Ｉ (t), y Ｑ (t)∈R, y(t)=y Ｉ (t) + iy Ｑ Furthermore, the complex filter h(τ) can be expressed as Ｉ (τ), h Ｑ (τ), h(τ) = h Ｉ (τ) + ih Ｑ (τ).

[0049] Let x(t) be the input complex signal of the 2×1 WL MISO equalizer, and y(t) be the output complex signal of the 2×1 WL MISO equalizer. In this case, the output complex signal y(t) can be expressed by the following equation (1) using the input complex signal x(t). When the above equation (1) is expressed using real signals, the following equation (2) is obtained. When the above formula (2) is converted into a determinant, the following formulas (3) and (4) are obtained.

[0050] From the above formulas (3) and (4), the following formula (5) is obtained. From the above formula (5), the following formula (6) is obtained. As explained above, the complex filter coefficients of a 2×1 WL MISO equalizer can be converted into real filter coefficients of a 2×2 IQ MIMO equalizer.

[0051] Conversion from the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y used as transmitter distortion compensation filters to filter coefficients that compensate for the IQ characteristic difference on the transmitting side, i.e., the IQ distortion difference, can be derived as follows.

[0052] The complex signal distortions α(ω) and β(ω) of the complex signals Y(ω) and X(ω) of each polarization on the transmitting side can be expressed in the frequency domain as follows: In equation (7), when β(ω) = 0, Y(ω) = α(ω)X(ω), and only the IQ common distortion remains. When α(ω) = 1 and β(ω) = 0, Y(ω) = X(ω), and all distortions, including the IQ common distortion, disappear.

[0053] From the above equation (1), the complex filter coefficients of the 2×1 WL MISO equalizer can be expressed in the frequency domain as follows: The frequency response of the filter coefficients obtained from the 2×1 WL MISO equalizer is ideally the inverse response of the signal distortion. Therefore, by multiplying the transmit signal by the coefficients in advance, the complex signal distortions α(ω) and β(ω) can be compensated for. In other words, substituting Equation (8) into Equation (7) gives α(ω) = 1, β(ω) = 0, and Y(ω) = X(ω).

[0054] From the above formulas (10) and (11), the following formula (12) is obtained. Furthermore, from equations (11) and (12), the following equation (13) is obtained.

[0055] The filter coefficients that compensate for only the IQ characteristic difference are derived as follows. From equations (7) and (14), the following equation is obtained: In this case, the common distortion remains, but the complex conjugate signal disappears, and the IQ characteristic difference can be compensated for.

[0056] From the above equations (12), (13), and (14), a complex conjugate signal that compensates only for the IQ characteristic difference from the complex filter coefficients of the 2×1 WL MISO equalizer is obtained. The filter coefficients to be multiplied are as follows: The above frequency domain filter coefficients are converted into time domain filter coefficients that compensate for only the IQ characteristic difference as follows:

[0057] The tap coefficients may be shifted so that only the center tap coefficient is 1 and the others are 0. The tap coefficients are shifted by the same value.

[0058] When the complex filter coefficients of the WL MISO equalizer are converted to real filter coefficients of the IQ MIMO equalizer, the following is obtained from equations (6) and (16):

[0059] Next, the conversion from the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y used as receiver distortion compensation filters to filter coefficients that compensate for the IQ characteristic difference on the receiving side, i.e., the difference in IQ distortion, can be derived as follows:

[0060] The complex signal distortions α(ω) and β(ω) of the complex signals Y(ω) and X(ω) of each polarization at the receiving side can be expressed in the frequency domain as follows: In equation (17), when β(ω) = 0, Y(ω) = α(ω)X(ω), and only the IQ common distortion remains. When α(ω) = 1 and β(ω) = 0, Y(ω) = X(ω), and all distortions, including the IQ common distortion, disappear.

[0061] From the above equation (1), the complex filter coefficients of the 2×1 WL MISO equalizer can be expressed in the frequency domain as follows: The frequency response of the filter coefficients obtained from the 2×1 WL MISO equalizer is ideally the inverse response of the signal distortion. Therefore, by multiplying the received signal by the coefficients, the complex signal distortions α(ω) and β(ω) can be compensated. In other words, by substituting Equation (17) into Equation (18), α(ω) = 1, β(ω) = 0, This becomes:

[0062] From the above equations (20) and (21), the following equation (22) is obtained. Furthermore, from equations (21) and (22), the following equation (23) is obtained.

[0063] The filter coefficients that compensate for only the IQ characteristic difference are derived as follows. From equations (17) and (24), the following equation is obtained: In this case, the common distortion remains, but the complex conjugate signal disappears, and the IQ characteristic difference can be compensated for.

[0064] From the above equations (22), (23), and (24), a complex conjugate signal that compensates only for the IQ characteristic difference from the complex filter coefficients of the 2×1 WL MISO equalizer is obtained. The filter coefficients to be multiplied are as follows: The above frequency domain filter coefficients are converted into time domain filter coefficients that compensate for only the IQ characteristic difference as follows:

[0065] The tap coefficients may be shifted so that only the center tap coefficient is 1 and the others are 0. The tap coefficients are shifted by the same value.

[0066] When the complex filter coefficients of the WL MISO equalizer are converted to real filter coefficients of the IQ MIMO equalizer, the following is obtained from equations (6) and (26):

[0067] The coefficient control unit 164 controls the filter coefficients of the 2×2 IQ MIMO equalizers 171X and 171Y included in the pre-equalization unit 112. More specifically, the coefficient control unit 164 derives coefficients of an FIR filter for compensating for the difference in IQ distortion for each polarization, and controls the coefficients of the FIR filters included in the 2×2 IQ MIMO equalizers 171X and 171Y using the derived filter coefficients.

[0068] The coefficient control unit 164 controls the filter coefficients of the 2×2 IQ MIMO equalizers 171X and 171Y so that, for example, the inverse characteristics of the difference in transmitting-side IQ distortion calculated by the distortion extraction unit 163 are added to the signal output from the pre-equalization unit 112. In this case, the 2×2 IQ MIMO equalizers 171X and 171Y function as filters that compensate for the difference in IQ distortion in each polarization. By controlling the filter coefficients of the pre-equalization unit 112 in accordance with the difference in transmitting-side IQ distortion extracted on the receiving side, it is possible to receive a signal on the receiving side in which the difference in IQ distortion has been compensated.

[0069] Furthermore, the coefficient control unit 164 controls the filter coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154. More specifically, the coefficient control unit 164 derives coefficients of an FIR filter for compensating for the difference in IQ distortion for each polarization, and controls the coefficients of the FIR filters included in the 2×2 IQ MIMO equalizers 181X and 181Y using the derived filter coefficients.

[0070] The coefficient control unit 164 controls the filter coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y, for example, so as to compensate for the difference in the receiving-side IQ distortion calculated by the distortion extraction unit 163. In this case, the 2×2 IQ MIMO equalizers 181X and 181Y function as filters that compensate for the difference in the IQ distortion for each polarization. By controlling the filter coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y in accordance with the difference in the receiving-side IQ distortion, the digital signal processing unit 154 can compensate for the difference in the IQ distortion.

[0071] Here, the fluctuations in the transmitter distortion and the receiver distortion are gradual and can be treated as quasi-dynamic fluctuations. Therefore, the coefficient control unit 164 does not need to constantly control the filter coefficients of the 2×2 IQ MIMO equalizers 171X and 171Y and the filter coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y, for example, for each symbol.

[0072] In this embodiment, since the compensation for transmitter distortion and the compensation for receiver distortion are performed to compensate for quasi-dynamic variations, the adaptive multilayer filter 161 in the distortion estimation unit 160 does not need to be implemented as a hardware circuit that operates at high speed. In this embodiment, the adaptive multilayer filter 161 is implemented by software processing. In other words, the adaptive equalization processing used for distortion extraction is offloaded to software processing. In contrast, the filters included in the pre-equalization unit 112 and the digital signal processing unit 154 are implemented by hardware circuits. When the difference between the transmitting-side IQ distortion and the difference between the receiving-side IQ distortion are extracted using the adaptive multilayer filter 161 implemented by software processing, there is an advantage in that the increase in the hardware circuitry of the digital signal processing unit 154 can be suppressed.

[0073] Next, the operation procedure will be described. Fig. 7 is a flowchart showing the operation procedure for extracting the difference between IQ distortions in the optical fiber communication system 100. The operation procedure for extracting the difference between IQ distortions corresponds to a digital signal processing method.

[0074] The optical receiver 150 receives the optical signal transmitted from the optical transmitter 110 via the transmission path 130 (step S11). In the optical receiver 150, the complex data of each polarization output from the ADC 153 is input to the adaptive multilayer filter 161 of the distortion estimator 160. The filter coefficient updater 162 adaptively controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y, the 2×2 SL MIMO equalizer 193, and the 2×1 WL MISO equalizers 195X and 195Y included in the adaptive multilayer filter 161 (step S12). In the adaptive multilayer filter 161, the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y corresponding to the first receiver distortion compensation filter are adaptively controlled to compensate for the receiver distortion. The filter coefficients of the 2×2 SL MIMO equalizer 193 are adaptively controlled to perform polarization separation. The filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y corresponding to the first transmitter distortion compensation filters are adaptively controlled to compensate for the distortion in the transmitter.

[0075] After the filter coefficients have converged, the distortion extraction unit 163 extracts the difference in IQ distortion on the receiving side and the difference in IQ distortion on the transmitting side (step S13). In step S13, the distortion extraction unit 163 extracts the difference in IQ distortion on the receiving side from the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y. The distortion extraction unit 163 also extracts the difference in IQ distortion on the transmitting side from the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y.

[0076] The coefficient control unit 164 controls the filter coefficients based on the difference in IQ distortion detected in step S13 (step S14). In step S14, the coefficient control unit 164 controls the filter coefficients of the pre-equalization unit 112 in the optical transmitter 110, which corresponds to the second transmitter distortion compensation filter, based on the difference in the transmitting-side IQ distortion. Also, the coefficient control unit 164 controls the filter coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154 in the optical receiver 150, which corresponds to the second receiver distortion compensation filter, based on the difference in the receiving-side IQ distortion.

[0077] Thereafter, the optical transmitter 110 transmits the optical signal, in which the difference in IQ distortion on the transmitting side has been compensated for in the pre-equalization unit 112, to the transmission path 130. Furthermore, the optical receiver 150 compensates for the difference in IQ distortion on the receiving side in 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154. The decoding unit 155 decodes the signal input via the digital signal processing unit 154 to restore the transmitted data.

[0078] In this embodiment, the coefficients of the 2×1 WL MISO equalizers 191X and 191Y after coefficient convergence and the coefficients of the 2×1 WL MISO equalizers 195X and 195Y after coefficient convergence include IQ common distortion and IQ distortion. The IQ common distortion is commutative between the transmitter and the receiver, and the difference in IQ distortion is non-commutative between the transmitter and the receiver. The distortion extraction unit 163 extracts the difference in receiving-side IQ distortion from the coefficients of the 2×1 WL MISO equalizers 191X and 191Y after coefficient convergence. The distortion extraction unit 163 also extracts the difference in receiving-side IQ distortion from the coefficients of the 2×1 WL MISO equalizers 195X and 195Y after coefficient convergence.

[0079] The coefficient control unit 164 derives filter coefficients for the 2×2 IQ MIMO equalizers 171X and 171Y arranged for each polarization and included in the pre-equalization unit 112 of the optical transmitter 110, based on the extracted difference in the transmitting IQ distortion. The coefficient control unit 164 feeds back the derived filter coefficients to the optical transmitter 110, causing the pre-equalization unit 112 to compensate for the difference in the transmitting IQ distortion. Furthermore, the coefficient control unit 164 derives filter coefficients for the 2×2 IQ MIMO equalizers 181X and 181Y arranged for each polarization and included in the digital signal processing unit 154 of the optical receiver 150, based on the extracted difference in the receiving IQ distortion. The coefficient control unit 164 outputs the derived filter coefficients to the digital signal processing unit 154, causing the digital signal processing unit 154 to compensate for the difference in the receiving IQ distortion.

[0080] IQ common distortion is difficult to separate because it is interchangeable between the transmitter, the transmission path, and the receiver. On the other hand, IQ common distortion can be compensated for at either the transmitter or the receiver. Furthermore, IQ common distortion is adaptively compensated for by a 2×2 SL MIMO equalizer used for polarization separation and polarization mode dispersion (PMD) compensation on the receiving side. Therefore, it is not necessary to compensate for IQ common distortion on the transmitting side and the receiving side using the 2×2 IQ MIMO equalizers 171X and 171Y in the transmitter or the 2×2 IQ MIMO equalizers 181X and 181Y in the receiver.

[0081] In this embodiment, the filter coefficients of the pre-equalization unit 112 are controlled based on the difference in IQ distortion on the transmitting side, and the pre-equalization unit 112 compensates for the difference in IQ distortion on the transmitting side in the optical transmitter 110. Furthermore, the filter coefficients of the MIMO equalizer that compensates for receiver distortion included in the digital signal processing unit 154 are controlled based on the difference in IQ distortion on the receiving side, and the digital signal processing unit 154 compensates for the difference in IQ distortion on the receiving side. In this manner, it is possible to selectively compensate for the difference in IQ distortion on the transmitting side and the difference in IQ distortion on the receiving side. In this case, the digital signal processing unit 154 can achieve highly accurate distortion compensation while avoiding overcompensation of the IQ common distortion.

[0082] Next, a second embodiment of the present disclosure will be described. In this embodiment, the distortion estimation unit 160 extracts a difference in transmitting-side IQ distortion and a difference in receiving-side IQ distortion in a first stage. After extracting the difference in transmitting-side IQ distortion and the difference in receiving-side IQ distortion, the distortion estimation unit 160 extracts or estimates an IQ common distortion in a second stage. The distortion estimation unit 160 controls the filter coefficients of the pre-equalization unit 112 and the MIMO equalizer included in the digital signal processing unit 154 based on the difference in transmitting-side IQ distortion and the difference in receiving-side IQ distortion extracted in the first stage and the IQ common distortion extracted in the second stage.

[0083] In the first stage, the filter coefficient update unit 162 adaptively controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y, the filter coefficients of the 2×2 SL MIMO equalizer 183, and the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y. After the filter coefficients converge, the distortion extraction unit 163 extracts a difference in receiving-side IQ distortion from the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y. The distortion extraction unit 163 also extracts a difference in transmitting-side IQ distortion from the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y. The extraction of the transmitting-side IQ distortion difference and the receiving-side IQ distortion difference in the first stage may be similar to the extraction of the transmitting-side IQ distortion difference and the receiving-side IQ distortion difference described in the first embodiment.

[0084] In the second stage, the coefficient control unit 164 controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y so that the difference in IQ distortion on the receiving side is compensated for in the 2×1 WL MISO equalizers 191X and 191Y. The coefficient control unit 164 also controls the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y so that the difference in IQ distortion on the transmitting side is compensated for in the 2×1 WL MISO equalizers 195X and 195Y.

[0085] In the second stage, the filter coefficient update unit 162 adaptively controls the filter coefficients of the 2×2 SL MIMO equalizer 183. In the second stage, the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y are fixed to filter coefficients for compensating for the difference in IQ distortion on the receiving side. Also, the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y are fixed to filter coefficients for compensating for the difference in IQ distortion on the transmitting side. In this case, the filter coefficients of the 2×2 SL MIMO equalizer 193 are adaptively controlled to compensate for the remaining IQ common distortion in addition to polarization separation and PMD compensation.

[0086] In the second stage, the distortion extraction unit 163 estimates or extracts the IQ common distortion from the filter coefficients of the 2×2 SL MIMO equalizer 193 after the coefficients have converged. The coefficient control unit 164 distributes the extracted IQ common distortion between the transmitting side and the receiving side using an arbitrary method. For example, the coefficient control unit 164 distributes the IQ common distortion between the transmitting side and the receiving side so as to maximize the received signal quality.

[0087] The coefficient control unit 164 sets filter coefficients obtained by adding a filter coefficient for compensating for the IQ common distortion allocated to the transmitting side to a filter coefficient for compensating for the difference in IQ distortion on the transmitting side to the pre-equalization unit 112. The coefficient control unit 164 also sets filter coefficients obtained by adding a filter coefficient for compensating for the IQ common distortion allocated to the receiving side to a filter coefficient for compensating for the difference in IQ distortion on the receiving side to the digital signal processing unit 154.

[0088] Next, the operation procedure will be described. FIG. 8 is a flowchart showing the operation procedure for extracting the IQ distortion difference and the IQ common distortion in the optical fiber communication system 100. The optical receiver 150 receives the optical signal transmitted from the optical transmitter 110 via the transmission path 130 (step S21). The filter coefficient update unit 162 adaptively controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y, the 2×2 SL MIMO equalizer 193, and the 2×1 WL MISO equalizers 195X and 195Y included in the adaptive multilayer filter 161 (step S22). After the filter coefficients converge, the distortion extraction unit 163 extracts the difference between the IQ distortion on the receiving side and the difference between the IQ distortion on the transmitting side (step S23). Steps S21 to S23 may be similar to steps S11 to S13 shown in FIG. 7. Steps S21 to S23 correspond to the first stage of operation.

[0089] The coefficient control unit 164 controls the filter coefficients included in the adaptive multilayer filter 161 based on the difference in receiving-side IQ distortion and the difference in transmitting-side IQ distortion extracted in step S23 (step S24). In step S24, the coefficient control unit 164 controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y based on the difference in receiving-side IQ distortion. The coefficient control unit 164 also controls the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y based on the difference in transmitting-side IQ distortion.

[0090] The filter coefficient update unit 162 adaptively controls the filter coefficients of the 2×2 SL MIMO equalizer 193 (step S25). In step S25, the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y are fixed to filter coefficients for compensating for the difference in IQ distortion on the receiving side. Also, the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y are fixed to filter coefficients for compensating for the difference in IQ distortion on the transmitting side. In this case, the filter coefficients of the 2×2 SL MIMO equalizer 193 after coefficient convergence include IQ common distortion that is interchangeable between the transmitter and the receiver. After coefficient convergence, the distortion extraction unit 163 extracts the IQ common distortion from the filter coefficients of the 2×2 SL MIMO equalizer 193 (step S26). Steps S24 to S26 correspond to the second stage of operation.

[0091] The coefficient control unit 164 controls the coefficients of the filters included in the pre-equalization unit 112 and the digital signal processing unit 154 based on the difference between the transmitting-side IQ distortion and the receiving-side IQ distortion extracted in step S23 and the IQ common distortion extracted in step S26 (step S27). In step S27, the coefficient control unit 164 allocates the IQ common distortion extracted in step S26 to the transmitting side and the receiving side. The coefficient control unit 164 controls the filter coefficients of the pre-equalization unit 112 based on the difference between the transmitting-side IQ distortion and the IQ common distortion allocated to the transmitting side. In addition, the coefficient control unit 164 controls the filter coefficients of the MIMO equalizer included in the digital signal processing unit 154 that compensates for receiver distortion based on the difference between the receiving-side IQ distortion and the IQ common distortion allocated to the receiving side.

[0092] The optical transmitter 110 then transmits the optical signal, in which the difference in IQ distortion on the transmitting side and part of the IQ common distortion have been compensated for in the pre-equalization unit 112, to the transmission path 130. The optical receiver 150 also compensates for the difference in IQ distortion on the receiving side and part of the IQ common distortion in 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154. The decoding unit 155 decodes the signal input via the digital signal processing unit 154 to restore the transmitted data.

[0093] In this embodiment, the coefficient control unit 164 controls the filter coefficients of the 2×1 WL MISO equalizers 191X and 191Y and the filter coefficients of the 2×1 WL MISO equalizers 195X and 195Y using the difference between the transmitting-side IQ distortion and the difference between the receiving-side IQ distortion extracted in the first stage. In the second stage, the filter coefficient update unit 162 adaptively controls the filter coefficients of the 2×2 SL MIMO equalizer 193. The distortion extraction unit 163 extracts the IQ common distortion from the filter coefficients of the 2×2 SL MIMO equalizer 193. The coefficient control unit 164 controls the coefficients of the filters included in the pre-equalization unit 112 and the digital signal processing unit 154 based on the extracted difference between the transmitting-side IQ distortion and the receiving-side IQ distortion and the extracted IQ common distortion.

[0094] In this embodiment, the coefficient control unit 164 controls the filter coefficients of the pre-equalization unit 112 in the optical transmitter 110 so that a part of the IQ common distortion is compensated for in addition to the difference between the IQ distortions on the transmitting side. Also, the coefficient control unit 164 controls the filter coefficients of the filter that compensates for receiver distortion included in the digital signal processing unit 154 in the optical receiver 150 so that a part of the IQ common distortion is compensated for in addition to the difference between the IQ distortions on the receiving side. In this way, the pre-equalization unit 112 and the digital signal processing unit 154 can compensate for the IQ common distortion in addition to the difference between the IQ distortions, thereby improving signal quality.

[0095] As described above, the IQ common distortion can be adaptively compensated for by the 2×2 SL MIMO equalizer 183 included in the digital signal processing unit 154 on the receiving side. However, in this case, in order to compensate for the IQ common distortion with high accuracy, the 2×2 SL MIMO equalizer 183 requires a filter with a long tap length. In this embodiment, the IQ common distortion is compensated for by the pre-equalization unit 112 and the 2×2 IQ MIMO equalizers 181X and 181Y. Therefore, the required tap length can be shorter than when the IQ common distortion is compensated for in the 2×2 SL MIMO equalizer 183, and the circuit size of the 2×2 SL MIMO equalizer 183 can be reduced.

[0096] The above description has been given of an example in which extraction of the IQ common distortion is performed using the adaptive multilayer filter 161 of the distortion estimation unit 160. In other words, the description has been given of an example in which extraction of the IQ common distortion is performed in adaptive equalization processing offloaded to software processing. However, the present embodiment is not limited to this. Extraction of the IQ common distortion may be performed using a signal in which the difference between the transmitter IQ distortion and the receiver IQ distortion extracted in the first stage has been compensated for, and extraction of the IQ common distortion may be performed using the adaptive equalization filter 180 included in the digital signal processing unit 154. In other words, extraction of the IQ common distortion may be performed using the adaptive equalization filter 180 implemented as a hardware circuit.

[0097] When the IQ common distortion is extracted using the adaptive equalization filter 180, the coefficient control unit 164 controls the coefficients of the pre-equalization unit 112 based on the difference between the IQ distortions on the transmitting side. The coefficient control unit 164 also controls the coefficients of the 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154 based on the difference between the IQ distortions on the receiving side. In the digital signal processing unit 154, the coefficients of the 2×2 SL MIMO equalizer 183 are adaptively controlled. After the coefficients converge, the distortion extraction unit 163 extracts the IQ common distortion based on the coefficients of the 2×2 SL MIMO equalizer 183.

[0098] Alternatively, extraction of the IQ common distortion may be performed by combining the adaptive equalization processing offloaded to software processing and the adaptive equalization filter 180 implemented as a hardware circuit. For example, the coefficient control unit 164 controls the coefficients of the pre-equalization unit 112 based on the difference between the IQ distortions on the transmitting side. The coefficient control unit 164 also controls the coefficients of the 2×1 WL MISO equalizers 191X and 191Y included in the adaptive multilayer filter 161 based on the difference between the IQ distortions on the receiving side. In this case, compensation for the IQ distortions on the transmitting side is performed in the pre-equalization unit 112 implemented as a hardware circuit, and compensation for the IQ distortions on the receiving side is performed in the adaptive equalization processing offloaded to software processing. Even in this case, the IQ common distortion can be extracted based on the coefficients of the 2×2 SL MIMO equalizer 193 after coefficient convergence.

[0099] Alternatively, the output signals of the 2×2 IQ MIMO equalizers 181X and 181Y included in the digital signal processing unit 154 may be branched to the adaptive multilayer filter 161 of the distortion estimation unit 160. In this case, the difference between the IQ distortion on the transmitting side and the difference between the IQ distortion on the receiving side are calculated using a hardware circuit, and the IQ common distortion can be extracted from the coefficients of the 2×2 SL MIMO equalizer 193 implemented in software.

[0100] The inventors conducted a simulation to verify the effects of the above embodiment. In the simulation, a 96 GB (Baud) polarization-multiplexed 16QAM signal was used. The light source linewidth was 100 kHz, and the frequency offset was 1 GHz. The optical signal-to-noise ratio (OSNR) was 30 dB / 0.1 nm, and the chromatic dispersion was 6,000 ps / nm. In the adaptive equalization filter 180 included in the digital signal processing unit 154, a 2×1 WL MISO equalizer was used to compensate for the difference in IQ distortion on the receiving side and the difference in IQ distortion on the transmitting side. The tap lengths of the 2×1 WL MISO equalizer and the 2×2 SL MIMO equalizer 183 included in the adaptive equalization filter 180 were 25. On the transmitting side, an IQ skew of 0.25 symbols (2.6 ps) was applied in the X polarization. On the receiving side, an IQ skew of 0.25 symbols (2.6 ps) was applied in the Y polarization.

[0101] 9 to 11 show signal distributions of the I-channel and Q-channel of a received signal measured in a simulation. FIG. 9 shows the signal distributions of the I-channel and Q-channel of a received signal when IQ distortion compensation by the 2×1 WL MISO equalizers 191X and 191Y is not performed in the digital signal processing unit 154. FIG. 10 shows an example of the signal distributions of the I-channel and Q-channel of a received signal after coefficient convergence of the adaptive multilayer filter 161 shown in FIG. 6. FIG. 11 shows the signal distributions of the I-channel and Q-channel of a received signal when the coefficients of the pre-equalization unit 112 are controlled based on the difference in IQ distortion on the transmitting side, and the coefficients of the 2×1 WL MISO equalizers 191X and 191Y are controlled based on the difference in IQ distortion on the receiving side.

[0102] 9 , when the difference in the IQ distortion on the transmitting side is not compensated for in the pre-equalization unit 112 and the difference in the IQ distortion on the receiving side is not compensated for in the 2×1 WL MISO equalizers 191X and 191Y, signal points cannot be identified in the X polarization and the Y polarization. In contrast, when the coefficients of the adaptive multilayer filter 161 are adaptively controlled in the distortion estimation unit 160, as shown in FIG. 10 , the signal points in the output signal of the adaptive multilayer filter 161 are separated in the X polarization and the Y polarization, and good reception characteristics are obtained. Furthermore, when the difference in the IQ distortion on the transmitting side and the difference in the IQ distortion on the receiving side are compensated for in the pre-equalization unit 112 and the digital signal processing unit 154, good reception characteristics are obtained, similar to the case of FIG. 10 . Thus, simulations have confirmed that the present disclosure enables highly accurate distortion compensation.

[0103] In the above-described embodiments, an example has been described in which the distortion estimation unit 160 is included in the optical receiver 150. However, the present disclosure is not limited to this. The distortion estimation unit 160 may be configured as a device separate from the optical receiver 150. In this case, the distortion 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 the computer device, and the digital signal output by the ADC 153 may be output to the computer device via the interface.

[0104] 12 is a block diagram showing an example configuration of a computer device that can be used as the distortion estimation unit 160. The computer device 400 includes a central processing unit (CPU) 410, a storage unit 420, a read only memory (ROM) 430, a random access memory (RAM) 440, a communication interface (IF) 450, and a user interface 460.

[0105] The communication interface 450 is an interface used for communication with external devices. The communication interface 450 can be used to acquire the four systems of 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, a mouse, and a touch panel.

[0106] The storage unit 420 is an auxiliary storage device that can store various types of data. The storage unit 420 does not necessarily have to be a part of the computer device 400, but may be an external storage device or cloud storage connected to the computer device 400 via a network.

[0107] The ROM 430 is a non-volatile storage device. For example, a semiconductor storage device with a relatively small capacity, such as a flash memory, is used for the ROM 430. The programs executed by the CPU 410 can be stored in the storage unit 420 or the ROM 430. The storage unit 420 or the ROM 430 stores various programs for causing the CPU 410 to perform processes for updating filter coefficients, extracting distortion, and controlling filter coefficients.

[0108] The program includes instructions or software code that, when loaded into a computer, causes the computer to perform one or more functions described in the embodiments. The program may be stored in a non-transitory computer-readable medium or a tangible storage medium. By way of example and not limitation, computer-readable media or tangible storage media include RAM, ROM, flash memory, solid-state drive (SSD) or other memory technologies, compact discs (CDs), digital versatile discs (DVDs), Blu-ray discs or other optical disc storage, magnetic cassettes, magnetic tapes, magnetic disk storage or other magnetic storage devices. The program may also be transmitted on a transitory computer-readable medium or communication medium. By way of example and not limitation, transitory computer-readable media or communication media include electrical, optical, acoustic, or other forms of propagated signals.

[0109] The RAM 440 is a volatile storage device. Various semiconductor memory devices such as dynamic random access memory (DRAM) or static random access memory (SRAM) are used for the RAM 440. The RAM 440 can be used as an internal buffer for temporarily storing data and the like. The CPU 410 loads a program stored in the storage unit 420 or the ROM 430 into the RAM 440 and executes the program. The CPU 410 may have an internal buffer for temporarily storing data and the like.

[0110] Although the present disclosure has been described above with reference to the embodiments, the present disclosure is not limited to the above-described embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present disclosure within the scope of the present disclosure.

[0111] Each drawing is merely an example for describing one or more embodiments. Each drawing may not relate to only one particular embodiment, but may also relate to one or more other embodiments. As will be understood by those skilled in the art, 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 to create, for example, an embodiment not explicitly shown or described. Not all features or steps shown in any one drawing are necessary to describe an exemplary embodiment, and some features or steps may be omitted. The order of steps described in any drawing may be changed as appropriate.

[0112] Some or all of the above-described embodiments can be described as, but are not limited to, the following supplementary notes.

[0113] [Supplementary Note 1] A first adaptive multilayer filter including a first receiver distortion compensation filter that compensates for receiver distortion contained in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and a first transmitter distortion compensation filter that compensates for transmitter distortion contained in the polarization multiplexed signal; a distortion extraction unit that extracts a receiving-side IQ distortion difference, which is the difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitting-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter; a coefficient control unit that controls a coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted difference in receiving-side IQ distortion, and controls a coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted difference in transmitting-side IQ distortion.

[0114] [Supplementary Note 2] The digital signal processing device according to Supplementary Note 1, wherein the second receiver distortion compensation filter and the second transmitter distortion compensation filter are filters inserted in a signal path from when the polarization multiplexed signal is transmitted from the transmitter to when the polarization multiplexed signal is decoded in the receiver.

[0115] [Supplementary Note 3] The digital signal processing device according to Supplementary Note 1 or 2, wherein the first receiver distortion compensation filter and the first transmitter distortion compensation filter are implemented by software processing, and the second receiver distortion compensation filter and the second transmitter distortion compensation filter are implemented by hardware circuits.

[0116] [Supplementary Note 4] The digital signal processing device according to any one of Supplementary Notes 1 to 3, wherein in the first adaptive multilayer filter, the first receiver distortion compensation filter and the first transmitter distortion compensation filter are cascaded with respect to an input signal, and the first receiver distortion compensation filter is arranged in a stage preceding the second transmitter distortion compensation filter.

[0117] [Supplementary Note 5] The digital signal processing device according to Supplementary Note 4, wherein the first adaptive multilayer filter includes a first 2x2 strictly linear (SL) Multiple Input Multiple Output (MIMO) equalizer arranged after the first receiver distortion compensation filter.

[0118] [Supplementary Note 6] The digital signal processing device according to Supplementary Note 5, further comprising a filter coefficient update unit that adaptively controls coefficients of the first receiver distortion compensation filter, coefficients of the first transmitter distortion compensation filter, and coefficients of the first 2×2 SL MIMO equalizer, wherein the distortion extraction unit extracts the difference in IQ distortion on the receiving side and the difference in IQ distortion on the transmitting side based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter after the coefficients have converged.

[0119] [Supplementary Note 7] In a first stage, the filter coefficient update unit adaptively controls the filter coefficients of the receiver distortion compensation filter, the filter coefficients of the transmitter distortion compensation filter, and the filter coefficients of the 2×2 SL MIMO equalizer, and the distortion extraction unit extracts the difference in receiving-side IQ distortion and the difference in transmitting-side IQ distortion based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter, and in a second stage subsequent to the first stage, the coefficient control unit controls the coefficients of the first receiver distortion compensation filter based on the extracted difference in receiving-side IQ distortion and controls the coefficients of the first transmitter distortion compensation filter based on the extracted difference in transmitting-side IQ distortion, and the filter coefficient update unit adaptively controls the filter coefficients of the first 2×2 SL MIMO equalizer, and the distortion extraction unit extracts the difference in receiving-side IQ distortion and the difference in transmitting-side IQ distortion based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter. The digital signal processing device according to claim 6, further comprising: extracting an IQ common distortion, which is a distortion that is commonly present in the I component and the Q component, based on a filter coefficient of a MIMO equalizer; and wherein the coefficient control unit divides the extracted IQ common distortion into a receiving-side IQ common distortion and a transmitting-side IQ common distortion, controls the coefficient of the second receiver distortion compensation filter based on the difference between the extracted receiving-side IQ distortions and the receiving-side IQ common distortion, and controls the coefficient of the second transmitter distortion compensation filter based on the extracted transmitting-side IQ distortion difference and the transmitting-side IQ common distortion.

[0120] [Supplementary Note 8] The digital signal processing device according to any one of Supplementary Notes 1 to 7, wherein the second receiver distortion compensation filter is included in a second adaptive multilayer filter inserted in a signal path from when the polarization multiplexed signal is received in the receiver until when the polarization multiplexed signal is decoded.

[0121] [Supplementary Note 9] The digital signal processing device according to Supplementary Note 8, wherein the second adaptive multilayer filter has a second 2x2 SL MIMO equalizer whose coefficients are adaptively controlled and is arranged after the second receiver distortion compensation filter.

[0122] [Supplementary Note 10] The digital signal processing device according to any one of Supplementary Notes 1 to 9, wherein the second transmitter distortion compensation filter is a pre-equalization filter arranged in the transmitter.

[0123] [Supplementary Note 11] The digital signal processing device according to any one of Supplementary Notes 1 to 10, wherein the coefficient control unit controls coefficients of the second receiver distortion compensation filter so as to compensate for the difference in the receiving-side IQ distortion, and controls coefficients of the second transmitter distortion compensation filter so as to compensate for the difference in the transmitting-side IQ distortion.

[0124] [Supplementary Note 12] A communication system comprising: a digital signal processing device according to any one of Supplementary Notes 1 to 11; a transmitter that transmits the polarization multiplexed signal; and a receiver that receives the polarization multiplexed signal transmitted from the transmitter.

[0125] [Supplementary Note 13] A receiver distortion included in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and a transmitter distortion included in the polarization multiplexed signal, are compensated for using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; and a receiver-side IQ distortion difference, which is a difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is a difference between distortion of an I component and distortion of a Q component of the polarization multiplexed signal generated at the transmitter, are extracted based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter; a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal, based on the extracted difference in receiving-side IQ distortion; and a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal, based on the extracted difference in transmitting-side IQ distortion.

[0126] [Supplementary Note 14] A receiver distortion included in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and a transmitter distortion included in the polarization multiplexed signal, are compensated for using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; and a receiver-side IQ distortion difference, which is a difference between distortion of an in-phase (I) component and distortion of a quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is a difference between distortion of an I component and distortion of a Q component of the polarization multiplexed signal generated at the transmitter, are extracted based on coefficients of the first receiver distortion compensation filter and coefficients of the first transmitter distortion compensation filter; a program that causes a processor to execute processing including controlling a coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted difference in receiving-side IQ distortion, and controlling a coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted difference in transmitting-side IQ distortion.

[0127] Some or all of the elements (e.g., configurations and functions) described in Supplementary Notes 2 to 11 that are dependent on Supplementary Note 1 may also be dependent on Supplementary Notes 13 and 14 in the same dependency relationship as Supplementary Notes 2 to 11. Some or all of the elements described in any Supplementary Note may be applied to various hardware, software, recording means for recording software, systems, and methods.

[0128] 10: Communication system 11: Transmitter 15: Receiver 20: Digital signal processing device 21: Adaptive multilayer filter 22: Receiver distortion compensation filter 23: Transmitter distortion compensation filter 24: Distortion extraction unit 25: Coefficient control unit 100: Optical fiber communication system 110: Optical transmitter 111: Encoding unit 112: Pre-equalization unit 113: DAC 114: Optical modulator 115: LD 130: Transmission path 132: Optical fiber 133: Optical amplifier 150: Optical receiver 151: LD 152: Coherent receiver 153: ADC 154: Digital signal processing unit 155: Decoding unit 160: Distortion estimation unit 161: Adaptive multilayer filter 162: Filter coefficient update unit 163: Distortion extraction unit 164: Coefficient control unit

Claims

1. A first adaptive multilayer filter including a first receiver distortion compensation filter that compensates for receiver distortion contained in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and a first transmitter distortion compensation filter that compensates for transmitter distortion contained in the polarization multiplexed signal; a distortion extraction unit that extracts a receiver-side IQ distortion difference, which is the difference between distortion of the in-phase (I) component and distortion of the quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter; a coefficient control unit that controls a coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted difference in receiving-side IQ distortion, and controls a coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted difference in transmitting-side IQ distortion.

2. The digital signal processing device according to claim 1, wherein the second receiver distortion compensation filter and the second transmitter distortion compensation filter are filters inserted in a signal path from when the polarization multiplexed signal is transmitted from the transmitter to when the polarization multiplexed signal is decoded in the receiver.

3. A digital signal processing device according to claim 1 or 2, wherein the first receiver distortion compensation filter and the first transmitter distortion compensation filter are implemented by software processing, and the second receiver distortion compensation filter and the second transmitter distortion compensation filter are implemented by hardware circuits.

4. A digital signal processing device according to any one of claims 1 to 3, wherein in the first adaptive multilayer filter, the first receiver distortion compensation filter and the first transmitter distortion compensation filter are cascaded to an input signal, and the first receiver distortion compensation filter is arranged in a stage preceding the second transmitter distortion compensation filter.

5. The digital signal processing device according to claim 4, wherein the first adaptive multi-layer filter comprises a first 2x2 strictly linear (SL) Multiple Input Multiple Output (MIMO) equalizer arranged after the first receiver distortion compensation filter.

6. The digital signal processing device according to claim 5, further comprising a filter coefficient update unit that adaptively controls the coefficients of the first receiver distortion compensation filter, the coefficients of the first transmitter distortion compensation filter, and the coefficients of the first 2x2 SL MIMO equalizer, and wherein the distortion extraction unit extracts the difference in IQ distortion on the receiving side and the difference in IQ distortion on the transmitting side based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter after the coefficients have converged.

7. In a first stage, the filter coefficient update unit adaptively controls the filter coefficients of the receiver distortion compensation filter, the filter coefficients of the transmitter distortion compensation filter, and the filter coefficients of the 2×2 SL MIMO equalizer, and the distortion extraction unit extracts the difference in receiver IQ distortion and the difference in transmitter IQ distortion based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter, and in a second stage subsequent to the first stage, the coefficient control unit controls the coefficients of the first receiver distortion compensation filter based on the extracted difference in receiver IQ distortion and controls the coefficients of the first transmitter distortion compensation filter based on the extracted difference in transmitter IQ distortion, and the filter coefficient update unit adaptively controls the filter coefficients of the first 2×2 SL MIMO equalizer, and the distortion extraction unit extracts the difference in receiver IQ distortion and the difference in transmitter IQ distortion based on the coefficients of the first 2×2 SL MIMO equalizer.

7. The digital signal processing device according to claim 6, wherein an IQ common distortion, which is distortion that is commonly present in the I component and the Q component, is extracted based on a filter coefficient of a MIMO equalizer, and the coefficient control unit divides the extracted IQ common distortion into a receiving-side IQ common distortion and a transmitting-side IQ common distortion, controls the coefficient of the second receiver distortion compensation filter based on the extracted difference between the receiving-side IQ distortions and the receiving-side IQ common distortion, and controls the coefficient of the second transmitter distortion compensation filter based on the extracted difference between the transmitting-side IQ distortions and the transmitting-side IQ common distortion.

8. A digital signal processing device according to any one of claims 1 to 7, wherein the second receiver distortion compensation filter is included in a second adaptive multilayer filter inserted in a signal path from when the polarization multiplexed signal is received in the receiver until when the polarization multiplexed signal is decoded.

9. The digital signal processing device according to claim 8, wherein the second adaptive multilayer filter is arranged after the second receiver distortion compensation filter and has a second 2x2 SL MIMO equalizer whose coefficients are adaptively controlled.

10. A digital signal processing device according to any one of claims 1 to 9, wherein the second transmitter distortion compensation filter is a pre-equalization filter disposed in the transmitter.

11. A digital signal processing device as claimed in any one of claims 1 to 10, wherein the coefficient control unit controls the coefficients of the second receiver distortion compensation filter so as to compensate for the difference in the receiving-side IQ distortion, and controls the coefficients of the second transmitter distortion compensation filter so as to compensate for the difference in the transmitting-side IQ distortion.

12. A communication system comprising: a digital signal processing device according to any one of claims 1 to 11; a transmitter for transmitting the polarization multiplexed signal; and a receiver for receiving the polarization multiplexed signal transmitted from the transmitter.

13. Compensating for receiver distortion contained in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and transmitter distortion contained in the polarization multiplexed signal, using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; extracting a receiver-side IQ distortion difference, which is the difference between distortion of the in-phase (I) component and distortion of the quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter; a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal, based on the extracted difference in receiving-side IQ distortion; and a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal, based on the extracted difference in transmitting-side IQ distortion.

14. Compensating for receiver distortion contained in a polarization multiplexed signal transmitted from a transmitter and received by a receiver, and transmitter distortion contained in the polarization multiplexed signal, using a first receiver distortion compensation filter and a first transmitter distortion compensation filter included in an adaptive multilayer filter, respectively; extracting a receiver-side IQ distortion difference, which is the difference between distortion of the in-phase (I) component and distortion of the quadrature (Q) component of the polarization multiplexed signal generated at the receiver, and a transmitter-side IQ distortion difference, which is the difference between distortion of the I component and distortion of the Q component of the polarization multiplexed signal generated at the transmitter, based on the coefficients of the first receiver distortion compensation filter and the coefficients of the first transmitter distortion compensation filter; a program that causes a processor to execute processing including controlling a coefficient of a second receiver distortion compensation filter that compensates for receiver distortion included in the polarization multiplexed signal based on the extracted difference in receiving-side IQ distortion, and controlling a coefficient of a second transmitter distortion compensation filter that compensates for transmitter distortion included in the polarization multiplexed signal based on the extracted difference in transmitting-side IQ distortion.

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