Digital signal processing circuit, method, receiver, and communication system

Abstract

The present invention makes it possible to accurately compensate for intra-transmitter distortion. A wavelength dispersion compensation filter (31) multiplies each of a real component and an imaginary component of each of a first polarization and a second polarization of a polarization multiplexed optical signal by a filter coefficient for compensating for wavelength dispersion. An adaptive equalizer (32) multiplies an input signal and its phase conjugation by a complex impulse response and adds together signals multiplied by the complex impulse response. The adaptive equalizer (32): applies, to each polarization, a phase rotation for carrier phase compensation including a frequency offset and the inverse rotation of the phase rotation; and adds together, for each polarization, a signal to which the phase rotation is applied and a signal to which the inverse rotation of the phase rotation is applied.

Description

[Technical Field]

[0001] The present disclosure relates to digital signal processing circuits, methods, receivers, and communication systems. [Background technology]

[0002] In optical fiber communications, multi-level modulation, such as high-order quadrature amplitude modulation (QAM), is being adopted to achieve high spectral efficiency. The introduction of coherent receiver technology has enabled flexible equalization signal processing at the receiver side, such as the simultaneous compensation of chromatic dispersion accumulated in the optical fiber transmission line. However, high-order multi-level modulation signals are generally vulnerable to distortion. Therefore, distortion caused by imperfections in components within the transmitter and receiver is becoming a new bottleneck in advancing high-level modulation. In particular, high symbol rates and high-level modulation methods are essential for realizing optical transmission systems with speeds of 1 Tbps (bit per second) or more. Therefore, high-precision equalization processing is required to ensure performance in such advanced modulation methods.

[0003] As a related technique, Non-Patent Document 1 discloses multilayer strictly linear (SL) and widely linear (WL) filters for compensating for linear distortion including distortion in a transmitter and distortion in a receiver. The multilayer SL and WL filters include a distortion compensation filter in a receiver, a chromatic dispersion compensation filter, a polarization separation filter, a carrier phase compensation filter, and a distortion compensation filter in a transmitter. A total of four real-valued received signal sequences, consisting of an in-phase (I) component and a quadrature (Q) component of two polarized waves, X and Y, with respect to local oscillator light, are input to the multilayer SL and WL filters.

[0004] The distortion compensation filter in the receiver, the chromatic dispersion compensation filter, the carrier phase compensation filter, and the distortion compensation filter in the transmitter compensate for the distortion in the receiver, chromatic dispersion, carrier phase noise, and distortion in the transmitter, respectively, for each polarization. In Non-Patent Document 1, the distortion compensation filter in the receiver and the distortion compensation filter in the transmitter use 2x1WL filters arranged for each polarization. The chromatic dispersion compensation filter and the carrier phase compensation filter use 1x1SL filters arranged for each polarization.

[0005] The polarization separation filter is a filter that performs polarization mode dispersion compensation and polarization separation, and handles both of the two polarized waves. In Non-Patent Document 1, a 2x2SL filter is used as the polarization separation filter. 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 using the output of the distortion compensation filter in the transmitter, which is the final filter stage.

[0006] Note that a 2×1WL filter is equivalent to a real-signal-input, real-coefficient 2×2 MIMO filter having 2×2=4 real-coefficient filters. In this disclosure, a complex-coefficient MIMO filter that receives a complex signal and its complex conjugate as input, and an equivalent real-signal-input, real-coefficient MIMO filter are also collectively referred to as a WL MIMO filter. In this context, a normal complex-signal-input, complex-coefficient MIMO filter is called an SL MIMO filter.

[0007] As another related technique, Patent Document 1 discloses a receiver having a demodulation digital signal processing unit. The demodulation digital signal processing unit receives the real component XI and imaginary component XQ of the X polarization of a received complex signal, and the real component YI and imaginary component YQ of the Y polarization of the received complex signal. The demodulation digital signal processing unit convolves an impulse response that compensates for the frequency characteristics of the receiver and a complex impulse response for chromatic dispersion compensation with each of the real component XI, imaginary component XQ, real component YI, and imaginary component YQ.

[0008] The digital signal processing unit further dynamically compensates for IQ imbalance, IQ lane skew, and bias deviation of the IQ modulator that occur in the transmitter, as well as impairments occurring in the optical fiber transmission line and the receiver, in an adaptive equalizer. The adaptive equalizer is configured as an 8x2 complex IQ WL MIMO (multiple-input and multiple-output) equalizer. The adaptive equalizer, which is a MIMO equalizer, receives eight signals: real component XI, imaginary component XQ, real component YI, and imaginary component YQ of a complex signal, and their respective phase conjugates. Furthermore, at the output of the adaptive equalizer, a signal that has undergone phase rotation for frequency offset compensation and a signal that has undergone phase rotation opposite to that for frequency offset compensation are added together. The adaptive equalizer described in Patent Document 1 can simultaneously compensate for effects occurring in a transmitting device (hereinafter also referred to as Tx load) and effects occurring in a receiving device (hereinafter also referred to as Rx load) using 8x2 complex IQ WL MIMO on the receiving side. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2020-141294 [Non-patent literature]

[0010] [Non-Patent Document 1] MANABU ARIKAWA, AND KAZUNORI HAYASHI, “Transmitter and receiver impairment monitoring using adaptive multi-layer linear and widely linear filter coefficients controlled by stochastic gradient descent”, Optics Express Vol. 29, Issue 8, pp. 11548-11561, 2021 Summary of the Invention [Problem to be solved by the invention]

[0011] In the adaptive equalizer described in Patent Document 1, a signal that has been subjected to phase rotation for frequency offset compensation is added to a signal that has been subjected to phase rotation that is opposite to the phase rotation for frequency offset compensation. However, the adaptive equalizer described in Patent Document 1 has a problem in that the phase noise of the light source affects the equalization accuracy of distortion in the transmitter.

[0012] In view of the above circumstances, one object of the present disclosure is to provide a digital signal processing circuit, method, receiver, and communication method that can improve the accuracy of equalizing distortion in a transmitter. [Means for solving the problem]

[0013] To achieve the above object, the present disclosure provides, as a first aspect, a digital signal processing circuit, which includes a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of a first polarization and a second polarization of a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver by a filter coefficient that compensates for chromatic dispersion, and a signal indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization output from the chromatic dispersion compensation filter, multiplies each of the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adds together the signals multiplied by the complex impulse responses, and applies a phase rotation for carrier phase compensation including a frequency offset to the added signals for each polarization. and an adaptive equalizer that performs phase rotation for carrier phase compensation and multiplies each of the input signals indicating phase conjugates of the real and imaginary components of the first polarization and the input signals indicating phase conjugates of the real and imaginary components of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal on which the phase rotation for carrier phase compensation has been performed and the signal on which the inverse rotation of the phase rotation for carrier phase compensation has been performed for each polarization, and outputs the added signals; and a filter coefficient update unit that uses an output of the adaptive equalizer to update the phase rotation for carrier phase compensation and the complex impulse response multiplied in the adaptive equalizer.

[0014] The present disclosure provides, as a second aspect, a receiver. The receiver includes a detector that coherently receives a polarization multiplexed optical signal transmitted from a transmitter via a transmission line, and a digital signal processing circuit that performs equalization signal processing on the coherently received received signal. The digital signal processing circuit includes a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of a first polarization and a second polarization of the received signal by a filter coefficient that compensates for chromatic dispersion, and receives signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization output from the chromatic dispersion compensation filter, multiplies the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse response, performs phase rotation for carrier phase compensation including a frequency offset on the added signals for each polarization, and performs equalization signal processing on the input signals. an adaptive equalizer that multiplies each of the signals indicating the phase conjugates of the real and imaginary components of the first polarization and the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, and performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, and outputs the added signal; and a filter coefficient update unit that updates the phase rotation for carrier phase compensation and the complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

[0015] The present disclosure provides, as a third aspect, a communication system including a transmitter that transmits a polarization multiplexed optical signal via a transmission path, a receiver including a detector that coherently receives the polarization multiplexed optical signal transmitted from the transmitter, and a digital signal processing circuit that performs equalization signal processing on the coherently received received signal. The digital signal processing circuit includes a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first polarization and the second polarization of the received signal by a filter coefficient that compensates for chromatic dispersion, and receives as input a signal indicating the real and imaginary components of the first polarization and a signal indicating the real and imaginary components of the second polarization output from the chromatic dispersion compensation filter, multiplies each of the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adds up the signals multiplied by the complex impulse responses, and performs phase rotation for carrier phase compensation including a frequency offset on the added signals for each polarization, and an adaptive equalizer that multiplies each of the signals indicating the phase conjugates of the real and imaginary components of the first polarization and the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, and performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, and outputs the added signal; and a filter coefficient update unit that updates the phase rotation for carrier phase compensation and the complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

[0016] The present disclosure provides, as a fourth aspect, a digital signal processing method, comprising: in a chromatic dispersion compensating filter, multiplying each of real and imaginary components of a first polarization and a second polarization of a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver by a filter coefficient that compensates for chromatic dispersion; in an adaptive equalizer to which signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization output from the chromatic dispersion compensating filter are input, multiplying each of the signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, and adding a frequency offset to the added signals for each polarization. the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that indicates the phase conjugate of the real component and the imaginary component of the first polarization and the signal that indicates the phase conjugate of the real component and the imaginary component of the second polarization by a complex impulse response, adding the signals that have been multiplied by the complex impulse responses, applying an inverse rotation of the phase rotation for carrier phase compensation to the added signals for each polarization, adding the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, outputting the added signals, and updating the phase rotation for carrier phase compensation and the complex impulse response that is multiplied in the adaptive equalizer using an output of the adaptive equalizer. [Effects of the Invention]

[0017] The digital signal processing circuit, method, receiver, and communication method according to the present disclosure can improve the accuracy of equalization of distortion in a transmitter. [Brief explanation of the drawings]

[0018] [Figure 1] 1 is a block diagram illustrating a schematic diagram of a communication system according to the present disclosure. [Figure 2] FIG. 2 is a block diagram showing a schematic configuration of a receiver. [Figure 3] FIG. 1 is a block diagram showing a signal transmission system according to a first embodiment of the present disclosure. [Figure 4] FIG. 2 is a block diagram showing an example of the basic configuration of a digital signal processing unit. [Figure 5] FIG. 2 is a block diagram showing a more detailed configuration example of a digital signal processing unit. [Figure 6] FIG. 1 is a block diagram showing general digital signal processing for performing Rx load compensation, chromatic dispersion compensation, carrier phase compensation, and Tx load compensation. [Figure 7] FIG. 1 is a block diagram showing an example of the configuration of digital signal processing used in the description. [Figure 8] FIG. 10 is a signal distribution diagram showing the signal distribution of the I-channel and Q-channel without Tx load and Rx load compensation. [Figure 9] FIG. 1 is a signal distribution diagram showing the signal distribution of the I-channel and Q-channel when equalization equivalent to the adaptive equalization described in Patent Document 1 is performed. [Figure 10] 4A and 4B are signal distribution diagrams showing the signal distributions of the I-channel and Q-channel when the digital signal processing unit according to the present embodiment is used. [Figure 11] FIG. 10 is a block diagram showing a configuration example of a digital signal processing unit used in a second embodiment of the present disclosure. [Figure 12] FIG. 1 is a block diagram showing an example of the configuration of an adaptive equalizer including a phase compensation filter. [Figure 13] FIG. 10 is a schematic diagram showing an example of a waveform after subcarrier synthesis. [Figure 14] FIG. 10 is a block diagram showing a configuration example of a digital signal processing unit used in a third embodiment of the present disclosure. [Figure 15] FIG. 2 is a block diagram showing a part of the configuration of an optical transmitter. [Figure 16] FIG. 10 is a block diagram showing an optical receiver used in a modified example. [Figure 17] FIG. 1 is a block diagram showing a part of the configuration of an optical receiver. DETAILED DESCRIPTION OF THE INVENTION

[0019] Prior to describing embodiments of the present disclosure, an overview of the present disclosure will be described. Fig. 1 schematically shows a communication system according to the present disclosure. The communication system 10 includes a transmitter 11 and a receiver 15. The transmitter 11 and the receiver 15 are connected to each other via a transmission path 13. The transmitter 11 transmits a polarization multiplexed optical signal via the transmission path 13. The receiver 15 receives the polarization multiplexed optical signal transmitted from the transmitter 11 via the transmission path 13.

[0020] 2 shows a schematic configuration of the receiver 15. The receiver 15 has a detector 21 and a digital signal processing circuit 22. The detector 21 coherently receives a polarization multiplexed optical signal transmitted from a transmitter. The digital signal processing circuit 22 performs equalization signal processing on the received signal coherently received by the detector 21.

[0021] The digital signal processing circuit 22 has a chromatic dispersion compensation filter 31, an adaptive equalizer 32, and a filter coefficient update unit 33. The chromatic dispersion compensation filter 31 compensates for chromatic dispersion in the received signal, which is a polarization multiplexed signal. The adaptive equalizer 32 is arranged after the chromatic dispersion compensation filter 31. The adaptive equalizer 32 compensates for distortion contained in the received signal. The filter coefficient update unit 33 updates the coefficients of the filter included in the adaptive equalizer 32 using the output of the adaptive equalizer 32.

[0022] The adaptive equalizer 32 receives a signal indicating the real and imaginary components of the first polarization output from the chromatic dispersion compensation filter 31 and a signal indicating the real and imaginary components of the second polarization. The adaptive equalizer 32 multiplies each of the input signals by a complex impulse response, adds the signals multiplied by the complex impulse responses, and applies phase rotation for carrier phase compensation, including a frequency offset, to the added signals for each polarization. The adaptive equalizer 32 also multiplies a signal indicating the phase conjugate of the input signal by the complex impulse response and adds the phase conjugate signals multiplied by the complex impulse response. The adaptive equalizer 32 applies inverse phase rotation for carrier phase compensation, including a frequency offset, to the added phase conjugate signals for each polarization. The adaptive equalizer 32 adds the signal subjected to phase rotation for carrier phase compensation and the signal subjected to inverse phase rotation for carrier phase compensation for each polarization, and outputs the added signal.

[0023] In the present disclosure, the adaptive equalizer 32 performs phase rotation for carrier phase compensation, including a frequency offset, on a signal obtained by adding an input signal multiplied by a complex impulse response. The adaptive equalizer 32 also performs the inverse of the phase rotation for carrier phase compensation, including a frequency offset, on a signal obtained by adding a phase conjugate signal multiplied by a complex impulse response. In the present disclosure, the adaptive equalizer 32 performs phase rotation for carrier phase compensation, including a frequency offset, and the inverse of the phase rotation. Therefore, the adaptive equalizer 32 can reduce the degree to which the phase noise of the light source affects the accuracy of equalization of distortion in the transmitter, thereby achieving highly accurate equalization of distortion in the transmitter.

[0024] Embodiments of the present disclosure will be described in detail below. FIG. 3 shows a signal transmission system according to a first embodiment of the present disclosure. In this embodiment, it is assumed that the signal transmission system is an optical fiber communication system that employs a polarization multiplexed QAM method and performs coherent reception. The optical fiber communication system 100 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 transmission line 130 corresponds to the transmission line 13 shown in FIG. 1. The optical receiver 150 corresponds to the receiver 15 shown in FIG. 1.

[0025] 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 DAC (Digital Analog Converter) 113, an optical modulator 114, and an LD (Laser Diode) 115. The encoder 111 encodes the data. The encoder 111 outputs four series of signals, for example, an in-phase (I) component of an X polarization (first polarization) and a Y polarization (second polarization), and a quadrature (Q) component.

[0026] The pre-equalization unit 112 performs pre-equalization on the coded four-sequence signal to compensate for distortions of devices in the optical transmitter in advance. The DAC 113 converts each of the four-sequence signals that have undergone pre-equalization into an analog electrical signal.

[0027] The LD 115 outputs a CW (Continuous wave) light. The optical modulator 114 modulates the CW light output from the LD 115 in accordance with the four-series signals output from the DAC 113 to generate a polarization-multiplexed optical signal. The optical modulator 114 generates, for example, a polarization-multiplexed QAM signal. The optical modulator 114 transmits the polarization-multiplexed optical signal to the transmission path 130.

[0028] 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 a plurality of optical amplifiers 133.

[0029] The optical receiver 150 includes an LD 151, a coherent receiver 152, an ADC (Analog digital converter) 153, a digital signal processing unit 154, and a decoding unit 155. In the optical receiver 150, circuits such as the digital signal processing unit 154 and the decoding unit (decoder) 155 can be configured using a device such as a DSP (digital signal processor).

[0030] The LD 151 outputs CW light to be used 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. The coherent receiver 152 corresponds to the detector 21 shown in FIG. 2.

[0031] The ADC 153 samples the received signal output from the coherent receiver 152 and converts the received signal into a signal in the digital domain. 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 may include one or more processors and one or more memories. At least a part of the functions of the digital signal processing unit 154 may be realized by the processor operating in accordance with a program read from the memory. The digital signal processing unit 154 corresponds to the digital signal processing circuit 22 shown in FIG. 2. The decoding unit 155 decodes the demodulated signal to restore the transmitted data.

[0032] 4 shows an example of the basic configuration of a digital signal processing unit 154 that implements a digital signal processing method. The digital signal processing unit 154 has a chromatic dispersion compensation filter 161, an adaptive equalizer 162, a phase compensation filter 163, and a filter coefficient updating unit 170. Note that, although the phase compensation filter 163 is shown in FIG. 4 as a block independent of the adaptive equalizer 162, it is assumed that the phase compensation filter 163 is incorporated into the adaptive equalizer 162.

[0033] In the digital signal processing unit 154, the chromatic dispersion compensation filter 161, adaptive equalizer 162, and phase compensation filter 163 are connected in cascade with respect to the input signal. The digital signal processing unit 154 may include, for example, one or more filters that compensate for distortion contained in the input signal, located before the chromatic dispersion compensation filter 161. The chromatic dispersion compensation filter 161 corresponds to the chromatic dispersion compensation filter 31 shown in FIG. 2. The adaptive equalizer 162 and phase compensation filter 163 correspond to the adaptive equalizer 32 shown in FIG. 2.

[0034] The filter coefficient update unit 170 monitors the input of the adaptive equalizer 162 and the output of the phase compensation filter 163. The filter coefficient update unit 170 also monitors the output of the adaptive equalizer 162, i.e., the input of the phase compensation filter 163. The filter coefficient update unit 170 updates the filter coefficients of the adaptive equalizer 162 and the phase compensation filter 163 using the output of the phase compensation filter 163. The filter coefficient update unit 170 adaptively controls the coefficients of the adaptive equalizer 162 and the phase compensation filter 163 using, for example, an error backpropagation algorithm based on a predetermined loss function. The loss function is calculated based on the difference between the output signal of the phase compensation filter 163, which is the final-stage filter, and a desired state. The filter coefficient update unit 170 corresponds to the filter coefficient update unit 33 shown in FIG. 2.

[0035] 5 shows a more detailed example of the configuration of the digital signal processing unit 154. In this example, the adaptive equalizer 162 is configured as a complex WL MIMO filter. The IQ components (XI and XQ) of the X polarization and the IQ components (YI and YQ) of the Y polarization are input to the chromatic dispersion compensation (CDC) filter 161. The chromatic dispersion compensation filter 161 multiplies the I components (real components) XI and YI and the Q components (imaginary components) XQ and YQ of the X polarization and the Y polarization, respectively, by a filter coefficient that compensates for chromatic dispersion. The chromatic dispersion compensation filter 161 outputs a complex signal X, compensated for chromatic dispersion. I , X Q , Y I , and Y Q Output.

[0036] The adaptive equalizer 162 includes a total of 16 complex coefficient filters that configure an 8×2 complex WL equalizer (hereinafter also referred to as an 8×2 WL MIMO filter), and a phase compensation filter 163. The 8×2 WL MIMO filter includes a real component X of the X polarization output from the chromatic dispersion compensation filter 161. I and the imaginary component X Q , and the real component of the Y polarization Y I and the imaginary component Y Q The imaginary component of each polarization, XQ and Y Q are multiplied by i, which represents the imaginary unit. In the 8x2 WL MIMO filter, each complex coefficient filter multiplies the input signal and the phase conjugate of the input signal by a complex impulse response. In the following description, the phase conjugate is referred to as " * " X multiplied by the complex impulse response I , X Q , Y I , and Y Q are added by the adder and output to the phase compensation filter 163. I , X Q , Y I , and Y Q The phase conjugates of the signals are added by an adder and output to the phase compensation filter 163 .

[0037] The phase compensation filter 163 is a filter (ph x and pH y ) and a filter (ph x * and pH y * ) The phase compensation filter 163 includes the signal X multiplied by the complex impulse response. I , X Q , Y I , and Y Q The phase compensation filter 163 performs phase rotation for carrier phase compensation on the sum of X multiplied by the complex impulse response. I , X Q , Y I , and Y Q The sum of the phase conjugates of the above is subjected to phase rotation for carrier phase compensation for each polarization.

[0038] The adaptive equalizer 162 adds, for each polarization, a signal that has been subjected to phase rotation for carrier phase compensation and a signal that has been subjected to the reverse rotation of the phase rotation for carrier phase compensation. x and a signal with a phase rotation of ph x *The phase-rotated signal is then converted into the output signal X Z The adaptive equalizer 162 outputs the Y polarization as ph y and a signal with a phase rotation of ph y * and the output signal Y Z Output as

[0039] The filter coefficient update unit 170 updates the coefficients (complex impulse responses) of each complex coefficient filter of the adaptive equalizer 162 so as to minimize the loss function described above. The filter coefficient update unit 170 updates the coefficients of each complex coefficient filter by, for example, stochastic gradient descent so as to minimize the loss function calculated based on the output of the phase compensation filter 163. The filter coefficient update unit 170 calculates the coefficients of the phase compensation filter 163, i.e., the amount of phase rotation in the phase compensation filter 163, based on the output of the phase compensation filter 163. The calculation of the phase compensation amount can be performed using a general M-th power method or a digital phase locked loop (PLL) using tentative decision. In this embodiment, carrier phase noise including a frequency offset is compensated for in the phase compensation filter 163. The calculation of the coefficients of the phase compensation filter 163 can be performed using, for example, a second-order PLL having two time constants.

[0040] Below, it will be explained that the adaptive equalizer 162 including the phase compensation filter 163 can compensate for distortion in the transmitter (Tx load), distortion in the receiver (Rx load), frequency offset, and phase noise of the light source. The 8×2 complex WL equalizer used in the adaptive equalizer 162 can be considered as an equalizer obtained by extending the 4×1 complex WL equalizer to polarization multiplexing. For this reason, the 4×1 complex WL equalizer will be used below to explain the distortion to be compensated.

[0041] 6 shows general digital signal processing for performing Rx load compensation, chromatic dispersion compensation, carrier phase compensation, and Tx load compensation. In this example, the digital signal processing includes a receiver-internal distortion compensation filter 501, a chromatic dispersion compensation filter 502, a carrier phase compensation filter 503, and a transmitter-internal distortion compensation filter 504. A non-polarization multiplexed received signal (digital signal) is input to the receiver-internal distortion compensation filter 501.

[0042] The receiver internal distortion compensation filter 501 is a filter that compensates for the Rx load. The chromatic dispersion compensation filter 502 is a filter that compensates for chromatic dispersion. The carrier phase compensation filter 503 is a filter that compensates for the phase noise of the light source. The transmitter internal distortion compensation filter 504 is a filter that compensates for the Tx load. It is assumed that a 2×1WL filter is used for the receiver internal distortion compensation filter 501 and the transmitter internal distortion compensation filter 504.

[0043] The input signal of the distortion compensation filter 501 in the receiver is represented by x, and the filter coefficient of the distortion compensation filter 501 in the receiver is represented by h. Also, the coefficient of the chromatic dispersion compensation filter 502 is represented by h. cd and the output signal of the chromatic dispersion compensation filter 502 is denoted by y. In this case, y is expressed by the following equation using x. TIFF0007736079000001.tif540

[0044] The coefficient of the carrier phase compensation filter 503 is e -iθ Then, the output signal y′ of the carrier phase compensation filter 503 is expressed by the following equation. TIFF0007736079000002.tif640 When the filter coefficient of the distortion compensation filter 504 in the transmitter is g and the output signal of the distortion compensation filter 504 in the transmitter is z, z is expressed by the following equation. TIFF0007736079000003.tif37125

[0045] When z expressed in the above equation 1 is modified for each IQ, the output signal of the transmitter distortion compensation filter 504 can be modified as follows: TIFF0007736079000004.tif94149

[0046] Fig. 7 shows an example of the configuration of digital signal processing used in the description. In Fig. 7, a 4x1 WL equalizer (4x1 complex WL MIMO filter) 190 is used for the digital signal processing. The 4x1 WL equalizer includes two complex conjugate transform units and four complex coefficient filters. The 8x2 complex WL equalizer included in the adaptive equalizer 162 has a configuration in which the 4x1 WL equalizer 190 is extended to polarization multiplexing.

[0047] Signals that have been subjected to chromatic dispersion compensation individually for I and Q are input to the 4×1WL equalizer 190. The input signal to the chromatic dispersion compensation filter is denoted by x, and the filter coefficient of the chromatic dispersion compensation filter is denoted by h. cd Furthermore, the input signal of the 4×1WL equalizer 190 is set to y, and the phase rotation in the phase compensation filter is set to e -iθ In this case, the output signal z of the 4×1WL equalizer 190 is expressed by the following equation. TIFF0007736079000005.tif3193 When z expressed in the above equation is transformed for each IQ, the output signal of the 4×1WL equalizer 190 can be transformed as follows: TIFF0007736079000006.tif47150

[0048] The first to fourth terms on the right-hand side of the above formula 2 and formula 3 are compared. From TIFF0007736079000007.tif37147, TIFF0007736079000008.tif2377. In other words, the above formulas 2 and 3 are the same. Therefore, it can be said that the digital signal processing using the two sets of 2×1 WL filters and wavelength dispersion filters (complex numbers) shown in Fig. 6 is equivalent to the digital signal processing using a 4×1 complex number WL filter and individual IQ wavelength dispersion filters.

[0049] Next, we will explain how to update the filter coefficients in the adaptive equalizer 162. If the input signal to the 4×1WL equalizer 190 is x and the inputs to the phase compensation filter are y and y*, then y and y* can be expressed by the following equations using x. TIFF0007736079000009.tif2347In the above equation, j represents the number of dimensions of the input, i represents the number of dimensions of the output, and k represents the number of samples. Also, m represents the number of taps of the filter (FIR (Finite Impulse Response) filter). The output z of the adaptive equalizer 162 is expressed as follows: The loss function φ used to update the filter coefficients is defined by the following equation, where d is a teacher signal representing the desired state. TIFF0007736079000011.tif1148

[0050] The filter coefficients of the 4×1 WL equalizer 190 are updated using the stochastic gradient descent method so as to minimize the loss function. TIFF0007736079000012.tif9977TIFF0007736079000013.tif1140TIFF0007736079000014.tif2171 TIFF0007736079000015.tif10176TIFF0007736079000016.tif1243TIFF0007736079000017.tif1967

[0051] From the above, each filter coefficient after updating is given by the following equation, where α is the step size that controls the magnitude of the update. TIFF0007736079000018.tif4175

[0052] The phase compensation coefficients in the phase compensation filter are e -iθi , e iθi is. θ iis calculated separately based on φ[k] using a method that will not be described in detail here. A digital PLL using a general teacher signal is used to calculate the phase compensation amount including the frequency offset and phase noise.

[0053] In this embodiment, the digital signal processing unit 154 includes an adaptive equalizer 162 (8×2 complex WL equalizer) and a phase compensation filter 163 included in the 8×2 complex WL equalizer. The filter coefficient update unit 170 updates the coefficients of the adaptive equalizer 162 using the output signal of the phase compensation filter 163. In this embodiment, the adaptive equalizer 162 performs phase compensation including frequency offset in the phase compensation filter 163. By adopting this configuration, the adaptive equalizer 162 can collectively compensate for the Tx load, the Rx load, polarization fluctuation (polarization mode dispersion), frequency offset, and light source phase noise. This embodiment can reduce the impact of light source phase noise on the equalization accuracy of the Tx load, thereby enabling accurate equalization of the Tx load.

[0054] The inventors performed a simulation to verify the effect of equalization in the digital signal processing unit 154. In the simulation, a polarization multiplexed 64QAM signal of 130 GB (Baud) was used. 100 kHz noise was added to this signal as phase noise to both the LD and the local oscillator light on the transmitting side. Furthermore, an IQ skew of 0.5 UI (Unit Interval) was added to the X-polarized Q signal at the transmitter, and an IQ skew of -0.5 UI was added to the Y-polarized Q signal at the receiver. The chromatic dispersion was set to 7.5 ns / nm.

[0055] Figure 8 shows the signal distribution of the I-channel and Q-channel when the Tx load and Rx load are not compensated for in the equalization digital signal processing. In the simulation, the signal converted to a digital signal by the ADC was equalized using a chromatic dispersion compensation filter, a polarization separation filter, and a carrier phase compensation filter. In this case, since the Tx load and Rx load are not compensated for in the equalization digital signal processing, it is difficult to distinguish the signal points in both the X polarization and the Y polarization.

[0056] Figure 9 shows the signal distributions of the I-channel and Q-channel when equalization equivalent to the adaptive equalization described in Patent Document 1 is performed in equalization digital signal processing. In the simulation, signals converted to digital signals by an ADC are equalized using a chromatic dispersion compensation filter, an 8x2 MIMO filter, a frequency offset compensation filter, and a carrier phase compensation filter. In this case, improvement in distortion is observed for the Y-polarized wave to which IQ skew is added at the receiver. However, for the X-polarized wave signal to which IQ skew is added at the transmitter, although the reception characteristics are improved compared to Figure 8, the reception characteristics are not sufficiently high.

[0057] Fig. 10 shows the signal distribution of the I-channel and Q-channel when the digital signal processing unit 154 according to this embodiment is used. Comparing Fig. 10 with Fig. 8 and Fig. 9, it can be seen that when the digital signal processing unit 154 is used, the reception characteristics can be improved for both X-polarized and Y-polarized signals. Thus, the simulation confirmed that this embodiment can accurately equalize the Tx load.

[0058] Next, a second embodiment of the present disclosure will be described. Fig. 11 shows an example configuration of a digital signal processing unit used in the second embodiment of the present disclosure. In this embodiment, the digital signal processing unit 154a has a subcarrier separation unit 164 in addition to the configuration of the digital signal processing unit 154 shown in Fig. 4. The filter coefficients in the digital signal processing unit 154a may be updated in the same manner as the filter coefficients described in the first embodiment.

[0059] In this embodiment, the received signal is subcarrier multiplexed in addition to being polarization multiplexed. The subcarriers include two subcarriers, namely, a first subcarrier SC1 and a second subcarrier SC2. The first subcarrier SC1 and the second subcarrier SC2 are a pair of subcarriers. The subcarriers may include four subcarriers, namely, first to fourth subcarriers SC1 to SC4. In this case, the first subcarrier SC1 and the fourth subcarrier SC4 are paired, and the second subcarrier SC2 and the third subcarrier SC3 are paired.

[0060] Subcarrier multiplexing on the transmitting side will now be described. The optical transmitter 110 (see FIG. 3) further includes a subcarrier multiplexed signal processing unit between the encoding unit 111 and the pre-equalization unit 112. The subcarrier multiplexed signal processing unit includes a plurality of FFT (fast Fourier transform) units corresponding to the number of subcarriers, a subcarrier allocation unit, and an IFFT (inverse FFT) unit.

[0061] In the subcarrier multiplexed signal processing unit, the transmission data is separated into multiple subcarrier signals, and the separated multiple subcarrier signals are input to multiple FFT units. Each FFT performs an FFT on the input subcarrier signal and converts the subcarrier signal into a subcarrier FFT signal in the frequency domain. The subcarrier arrangement unit frequency-shifts the frequency-domain subcarrier FFT signal by the frequency shift amount for each subcarrier, and generates a subcarrier-arranged signal by arranging the frequency-shifted signal in the frequency domain. The IFFT unit performs an IFFT on the frequency-domain subcarrier-arranged signal and converts the subcarrier-arranged signal into a subcarrier multiplexed signal in the time domain.

[0062] Next, subcarrier separation on the receiving side will be described. The subcarrier separation unit 164 includes an FFT unit, a separation unit, and multiple IFFT units corresponding to the number of subcarriers. A digital subcarrier multiplexed signal is input to the subcarrier separation unit 164 from the ADC 153 (see FIG. 3). The FFT unit performs an FFT on the input subcarrier multiplexed signal and converts the subcarrier multiplexed signal into a frequency-domain subcarrier multiplexed FFT signal. The subcarrier separation unit separates multiple subcarrier signals included in the frequency-domain subcarrier multiplexed FFT signal by subcarrier. The subcarrier separation unit generates multiple subcarrier separation signals corresponding to the number of subcarriers. Each IFFT unit converts the subcarrier separation signals, which are frequency-domain signals, into time-domain signals. Note that FIG. 11 shows an example in which the subcarrier separation unit 164 is arranged before the chromatic dispersion compensation filter 161, but this embodiment is not limited to this. The subcarrier separation unit 164 may be arranged between the chromatic dispersion compensation filter 161 and the adaptive equalizer 162.

[0063] 12 shows a configuration example of an adaptive equalizer 162 including a phase compensation filter 163. In this example, the adaptive equalizer 162 includes a 16×4 complex WL equalizer (hereinafter also referred to as a 16×4 WL MIMO filter). A signal in which chromatic dispersion has been compensated for individually for I and Q for each subcarrier by a chromatic dispersion compensation filter 161 is input to the 16×4 WL MIMO filter. Specifically, the 16×4 WL MIMO filter receives the I and Q components (xi SC1 , xq SC1 , yi SC1 , yq SC1 , xi SC2 , xq SC2 , yi SC2 , yq SC2 ) is entered.

[0064] In the 16×4 WL MIMO filter, each complex coefficient filter multiplies the input signal and the phase conjugate of the input signal by a complex impulse response. SC1, xq SC1 , yi SC1 , yq SC1 and the phase conjugate of the second subcarrier xi SC2 * , xq SC2 * , yi SC2 * , yq SC2 * and are added together in the adder, and the coefficient is ph xSC1 , ph xSC1* , ph ySC1、 and pH ySC1* The coefficient is output to the phase compensation filter 163. xSC1 The output of the phase compensation filter and the coefficients are xSC1* The output of is added to the output of the adder, and the added signal is the X-polarized signal of the first subcarrier X. SC1 The coefficient is output as ph ySC1 The output of the phase compensation filter and the coefficients are ySC1* The output of is added to the output of the adder, and the added signal is the Y-polarized signal of the first subcarrier, Y SC1 is output as

[0065] Also, the xi of the second subcarrier multiplied by the complex impulse response SC2 , xq SC2 , yi SC2 , yq SC2 and the phase conjugate of the first subcarrier xi SC1 * , xq SC1 * , yi SC1 * , yq SC1 * and are added together in the adder, and the coefficient is ph xSC2 , ph xSC2* , ph ySC2、 and pH ySC2* The coefficient is output to the phase compensation filter 163. xSC2 The output of the phase compensation filter and the coefficients are xSC2* The output of the second subcarrier is added to the output of the second subcarrier in the adder. SC2 The coefficient is output as ph ySC2The output of the phase compensation filter and the coefficients are ySC2* The output of is added to the output of the adder, and the added signal is the Y-polarized signal of the second subcarrier, Y SC2 is output as

[0066] Fig. 13 shows a schematic example of a waveform after subcarrier synthesis. In Fig. 13, the horizontal axis indicates frequency and the vertical axis indicates power. In a subcarrier-multiplexed signal, if the frequency characteristics of the I component and the Q component differ, the conjugate component SC2 of the second subcarrier is mixed with the first subcarrier SC1 by IQ mixing. * In addition, the conjugate component SC1 of the first subcarrier is added to the second subcarrier SC2 by IQ mixing. * That is, IQ mixing occurs due to the conjugate components of each subcarrier SC. The conjugate component of each subcarrier occurs in the paired subcarrier SC.

[0067] In the above 16×4 WL MIMO filter, the phase conjugate of the second subcarrier SC2 is added to the signal of the first subcarrier SC1, and the phase conjugate of the first subcarrier SC1 is added to the signal of the second subcarrier SC2. In this way, the adaptive equalizer 162 can correct distortion without being affected by IQ mixing. The number of subcarriers is not limited to two, and the received signal may be multiplexed onto four or more subcarriers. In this case, a 16×4 WL MIMO filter may be placed for each pair of two subcarriers. Other effects are the same as those described in the first embodiment.

[0068] Next, a third embodiment of the present disclosure will be described. Fig. 14 shows a configuration example of a digital signal processing unit used in the third embodiment of the present disclosure. In this embodiment, the digital signal processing unit 154b has a distortion estimation unit 165 in addition to the configuration of the digital signal processing unit 154 shown in Fig. 4. The distortion estimation unit 165 estimates the Tx load based on the filter coefficients of the adaptive equalizer 162. The filter coefficients in the digital signal processing unit 154b may be updated in the same manner as the filter coefficients described in the first embodiment. This embodiment can also be applied to a configuration using subcarrier multiplexing described in the second embodiment.

[0069] In this embodiment, the filter coefficients of the pre-equalization unit 112 (see FIG. 3) of the optical transmitter 110 are controlled based on the filter coefficients of the digital signal processing unit 154b on the receiving side. FIG. 15 shows a part of the configuration of the optical transmitter 110. The optical transmitter 110 has a 2×1WL filter 117 and an IQ separator 118 corresponding to each of the X polarization and the Y polarization. The 2×1WL filter 117 corresponds to the pre-equalization unit 112 shown in FIG. 3. The 2×1WL filter 117 arranged corresponding to the X polarization receives an X polarization complex signal (XI+iXQ). The output signal of the 2×1WL filter 117 is separated into an I component real signal and a Q component real signal by the IQ separator 118 and converted into an analog signal by the DAC 113. The 2×1WL filter 117 arranged corresponding to the Y polarization receives a Y polarization complex signal (YI+iYQ). The output signal of the 2×1WL filter 117 is separated into a real I-component signal and a real Q-component signal by the IQ separator 118 and converted into an analog signal by the DAC 113 .

[0070] The distortion estimation unit 165 (see FIG. 14) estimates the Tx load from the filter coefficients of the adaptive equalizer 162 after coefficient convergence. The Tx load can be calculated based on the coefficients of the filter that multiplies the complex impulse response shown in FIG. 5. In this embodiment, the filter coefficients of the 2×1WL filter in the pre-equalization unit 112 are set in the pre-equalization unit 112 so that the inverse characteristics of the Tx load estimated by the distortion estimation unit 165 are added to the transmitted signal. By setting the filter coefficients of the pre-equalization unit 112 according to the Tx load estimated on the receiving side, the Tx load can be compensated for on the transmitting side.

[0071] Note that a real signal input real coefficient MIMO filter may be used in the transmitting side pre-equalization unit 112. When a 2×2 Real MIMO filter is used in the pre-equalization unit 112, the inverse characteristics of the Tx load estimated from the 8×2 WL MIMO filter may be converted into the coefficients of the 2×2 Real MIMO filter.

[0072] In this embodiment, part or all of the digital signal processing shown in Fig. 4 or 5 may be implemented in hardware different from the digital signal processing unit 154b. Fig. 16 shows an optical receiver used in a modified example. In this modified example, the optical receiver 150 is connected to an external device 160. The external device 160 is configured as a computer device such as a personal computer (PC). In the optical receiver 150, the digital signal output by the ADC 153 is branched to the external device 160. The optical receiver 150 has an interface for connecting to the external device 160, and outputs the digital signal to the external device 160 via the interface.

[0073] The external device 160 reproduces the operations of the chromatic dispersion compensation filter 161, the adaptive equalizer 162, and the phase compensation filter 163 using simulation or the like, and updates the filter coefficients. In the external device 160, the chromatic dispersion compensation filter, the 8×2WL equalizer, and the phase compensation filter may be implemented as dedicated hardware. The external device 160 estimates the Tx load based on the updated filter coefficients of the 8×2WL equalizer. The external device 160 may transmit the filter coefficients of the pre-equalization unit 112 to the optical transmitter 110 and update the filter coefficients of the pre-equalization unit 112. Alternatively, the filter coefficients corresponding to the Tx load estimated in the external device 160 may be manually set in the pre-equalization unit 112. In this embodiment, when the Tx load estimation is performed in the external device 160, the digital signal processing unit 154 does not need to have a filter for compensating for the Tx load.

[0074] Furthermore, in this embodiment, the external device 160 (its distortion estimation unit) may estimate the Rx load from the filter coefficients of the 8×2WL equalizer after coefficient convergence. The Rx load can be calculated based on the coefficients of a filter that multiplies the complex impulse response shown in FIG. 5. In FIG. 16, the digital signal processing unit 154 includes a filter that compensates for distortion within the receiver. The filter coefficients of the distortion compensation filter within the receiver are set so that the inverse characteristics of the estimated Rx load are applied to the received signal.

[0075] 17 shows a portion of the configuration of the optical receiver 150. In the optical receiver 150, the digital signal processing unit 154 has an IQ combining unit 156 and a 2×1WL filter 157 corresponding to each of the X polarization and the Y polarization. The 2×1WL filter 157 is an equalizer that performs equalization processing on the polarization multiplexed optical signal coherently received in the optical receiver 150. The IQ combining unit 156 arranged corresponding to the X polarization combines the real signals XI and XQ converted into digital signals by the ADC 153 into an X polarization complex signal (XI+iXQ). The X polarization complex signal (XI+iXQ) is input to the 2×1WL filter 157 arranged corresponding to the X polarization. Furthermore, the IQ combining unit 156 arranged corresponding to the Y polarization combines the real signals YI and YQ converted into digital signals by the ADC 153 into a Y polarization complex signal (YI+iYQ). The complex signal (XI+iXQ) of the Y polarization is input to the 2×1WL filter 157 arranged corresponding to the Y polarization.

[0076] The external device 160 sets the filter coefficients of the 2×1WL filter 157 for each polarization based on the estimated Rx load. Alternatively, the filter coefficients according to the Rx load estimated by the external device 160 may be manually set in the 2×1WL filter 157 for each polarization. In this way, distortion within the transmitter can be compensated for in each polarization using the 2×1WL 157. In this case, an existing circuit can be used for the digital signal processing unit 154 used for receiving signals.

[0077] In this embodiment, the distortion estimation unit 165 estimates the Tx load from the filter coefficients of the adaptive equalizer 162 after coefficient convergence. The Tx load can be compensated for on the transmitting side by controlling the filter coefficients of the pre-equalization unit 112 included in the optical transmitter 110 based on the Tx load estimated on the receiving side. Furthermore, the distortion estimation unit 165 can estimate the Rx load from the filter coefficients of the adaptive equalizer 162 after coefficient convergence. The Rx load can be compensated for by controlling the filter coefficients of the receiver distortion compensation filter in the digital signal processing unit 154 included in the optical receiver 150 based on the estimated Rx load.

[0078] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the above-described embodiments, and changes and modifications to the above-described embodiments without departing from the spirit of the present disclosure are also included in the present disclosure.

[0079] For example, some or all of the above embodiments can be described as, but are not limited to, the following supplementary notes.

[0080] [Appendix 1] a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the polarization multiplexed optical signal transmitted from the transmitter and received by the receiver by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter, and a signal indicating the real and imaginary components of a second polarization; a complex impulse response is multiplied by each of the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization; the signals multiplied by the complex impulse responses are added together; and a phase rotation for carrier phase compensation including a frequency offset is performed on the added signals for each polarization; and an adaptive equalizer that multiplies each of a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, and outputs the added signal; a filter coefficient update unit that updates a phase rotation for carrier phase compensation and a complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

[0081] [Appendix 2] 2. The digital signal processing circuit of claim 1, wherein the adaptive equalizer includes a complex 8x2 Widely Linear (WL) MIMO filter.

[0082] [Appendix 3] the 8×2WL MIMO filter is a WL filter that receives as input a complex signal representing a real component and an imaginary component of the first polarization, a complex signal representing a real component and an imaginary component of the second polarization, a complex signal representing a phase conjugate of the real component and an imaginary component of the first polarization, and a complex signal representing a phase conjugate of the real component and an imaginary component of the second polarization, and outputs a complex signal of the first polarization and a complex signal of the second polarization.

[0083] [Appendix 4] the polarization multiplexed optical signal is a digital subcarrier multiplexed optical signal in which data is multiplexed onto a plurality of subcarriers, the plurality of subcarriers include a pair of first and second subcarriers; The adaptive equalizer for a first subcarrier, multiplying each of a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization by a complex impulse response; for the second subcarrier, multiplying each of a signal indicating a phase conjugate of the real component and an imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and an imaginary component of the second polarization by a complex impulse response; adding the first subcarrier signal and the second subcarrier signal multiplied by the complex impulse response; and performing a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation on the added signal for each subcarrier and each polarization; 2. The digital signal processing circuit of claim 1, wherein, for a second subcarrier, a signal indicating the real and imaginary components of the first polarization and a signal indicating the real and imaginary components of the second polarization are multiplied by a complex impulse response, for the first subcarrier, a signal indicating the phase conjugate of the real and imaginary components of the first polarization and a signal indicating the phase conjugate of the real and imaginary components of the second polarization are multiplied by a complex impulse response, the second subcarrier signal multiplied by the complex impulse response and the first subcarrier signal are added, and a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation are performed for each subcarrier and for each polarization.

[0084] [Appendix 5] 5. The digital signal processing circuit of claim 4, wherein the adaptive equalizer includes a complex 16x4 Widely Linear (WL) MIMO filter.

[0085] [Appendix 6] The 16×4WL MIMO filter includes a complex signal representing a real component of the first polarization of the first subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the second polarization of the first subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the first polarization of the second subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the second polarization of the second subcarrier and a complex signal representing an imaginary component, a complex signal representing a phase conjugate of the real component of the first polarization of the first subcarrier and a complex signal representing a phase conjugate of the imaginary component, and a complex signal representing a phase conjugate of the second polarization of the first subcarrier. 6. The digital signal processing circuit according to claim 5, wherein the WL filter receives as input a complex signal representing a phase conjugate of a real component of polarization and a complex signal representing a phase conjugate of an imaginary component, a complex signal representing a phase conjugate of a real component of the first polarization of the second subcarrier, and a complex signal representing a phase conjugate of a real component of the second polarization of the second subcarrier, and outputs a complex signal of the first polarization of the first subcarrier and a complex signal of the second polarization of the second subcarrier, and a complex signal of the first polarization of the second subcarrier and a complex signal of the second polarization of the second subcarrier.

[0086] [Appendix 7] 7. The digital signal processing circuit according to claim 1, further comprising a distortion estimation unit that estimates at least one of distortion occurring in the transmitter and distortion occurring in the receiver based on the complex impulse response in the adaptive equalizer.

[0087] [Appendix 8] The digital signal processing circuit according to any one of appendixes 1 to 7, wherein the adaptive equalizer compensates for distortion occurring in the transmitter, distortion occurring in the receiver, polarization mode dispersion, frequency offset, and phase noise of an optical source.

[0088] [Appendix 9] a detector that coherently receives a polarization multiplexed optical signal transmitted from a transmitter via a transmission line; a digital signal processing circuit that performs equalization signal processing on the coherently received received signal, The digital signal processing circuit a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the received signal by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter, and a signal indicating the real and imaginary components of a second polarization; a complex impulse response is multiplied by each of the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization; the signals multiplied by the complex impulse responses are added together; and a phase rotation for carrier phase compensation including a frequency offset is performed on the added signals for each polarization; and an adaptive equalizer that multiplies each of a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, and outputs the added signal; a filter coefficient update unit that updates a phase rotation for carrier phase compensation and a complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

[0089] [Appendix 10] 10. The receiver of claim 9, wherein the adaptive equalizer includes a complex 8x2 Widely Linear (WL) MIMO filter.

[0090] [Appendix 11] the polarization multiplexed optical signal is a digital subcarrier multiplexed optical signal in which data is multiplexed onto a plurality of subcarriers, the plurality of subcarriers include a pair of first and second subcarriers; The adaptive equalizer for a first subcarrier, multiplying each of a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization by a complex impulse response; for the second subcarrier, multiplying each of a signal indicating a phase conjugate of the real component and an imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and an imaginary component of the second polarization by a complex impulse response; adding the first subcarrier signal and the second subcarrier signal multiplied by the complex impulse response; and performing a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation on the added signal for each subcarrier and each polarization; 10. The receiver of claim 9, wherein, for a second subcarrier, the receiver multiplies each of a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization by a complex impulse response; for the first subcarrier, the receiver multiplies each of a signal indicating a phase conjugate of the real component and an imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and an imaginary component of the second polarization by a complex impulse response; adds the second subcarrier signal multiplied by the complex impulse response and the first subcarrier signal; and performs, for each subcarrier and for each polarization, a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation.

[0091] [Appendix 12] 12. The receiver of claim 11, wherein the adaptive equalizer includes a complex 16x4 Widely Linear (WL) MIMO filter.

[0092] [Appendix 13] a transmitter for transmitting a polarization multiplexed optical signal via a transmission line; a receiver including a detector that coherently receives the polarization multiplexed optical signal transmitted from the transmitter; a digital signal processing circuit that performs equalization signal processing on the coherently received received signal, The digital signal processing circuit a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the received signal by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter, and a signal indicating the real and imaginary components of a second polarization; a complex impulse response is multiplied by each of the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization; the signals multiplied by the complex impulse responses are added together; and a phase rotation for carrier phase compensation including a frequency offset is performed on the added signals for each polarization; and an adaptive equalizer that multiplies each of a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for carrier phase compensation and the signal that has been subjected to the inverse rotation of the phase rotation for carrier phase compensation for each polarization, and outputs the added signal; A communication system comprising: a filter coefficient update unit that updates a phase rotation for carrier phase compensation and a complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

[0093] [Appendix 14] 14. The communication system of claim 13, wherein the adaptive equalizer includes a complex 8x2 Widely Linear (WL) MIMO filter.

[0094] [Appendix 15] the polarization multiplexed optical signal is a digital subcarrier multiplexed optical signal in which data is multiplexed onto a plurality of subcarriers, the plurality of subcarriers include a pair of first and second subcarriers; The adaptive equalizer for a first subcarrier, multiplying each of a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization by a complex impulse response; for the second subcarrier, multiplying each of a signal indicating a phase conjugate of the real component and an imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and an imaginary component of the second polarization by a complex impulse response; adding the first subcarrier signal and the second subcarrier signal multiplied by the complex impulse response; and performing a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation on the added signal for each subcarrier and each polarization; the communication system of claim 13, wherein for a second subcarrier, a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization are multiplied by a complex impulse response, for the first subcarrier, a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization are multiplied by a complex impulse response, the second subcarrier signal multiplied by the complex impulse response and the first subcarrier signal are added, and a phase rotation for carrier phase compensation and an inverse rotation of the phase rotation for carrier phase compensation are performed for each subcarrier and for each polarization.

[0095] [Appendix 16] 16. The communication system of claim 15, wherein the adaptive equalizer includes a complex 16x4 Widely Linear (WL) MIMO filter.

[0096] [Appendix 17] the transmitter has a pre-equalization unit that pre-equalizes the polarization multiplexed optical signal, 17. A communication system according to any one of appendixes 13 to 16, wherein the filter coefficients of the pre-equalization unit are controlled in accordance with distortion occurring in the transmitter estimated based on the filter coefficients of the adaptive equalizer.

[0097] [Appendix 18] the receiver has an equalization unit that performs equalization processing on the coherently received polarization multiplexed optical signal, 18. The communication system according to any one of appendixes 13 to 17, wherein the filter coefficients of the equalization unit are controlled in accordance with distortion occurring in the transceiver estimated based on the filter coefficients of the adaptive equalizer.

[0098] [Appendix 19] in the chromatic dispersion compensating filter, multiplying the real components and imaginary components of the first polarization and the second polarization of the polarization multiplexed optical signal transmitted from the transmitter and received by the receiver by filter coefficients that compensate for chromatic dispersion; an adaptive equalizer to which the signals indicating the real and imaginary components of the first polarized wave output from the chromatic dispersion compensation filter and the signals indicating the real and imaginary components of the second polarized wave are input, multiplying each of the signals indicating the real and imaginary components of the first polarized wave and the signals indicating the real and imaginary components of the second polarized wave by a complex impulse response, adding up the signals multiplied by the complex impulse responses, and performing phase rotation for carrier phase compensation including a frequency offset on the added signals for each polarization; multiplying a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, performing an inverse rotation of the phase rotation for carrier phase compensation on the added signals for each polarization, adding the signal on which the phase rotation for carrier phase compensation has been performed and the signal on which the inverse rotation of the phase rotation for carrier phase compensation has been performed for each polarization, and outputting the added signals; A digital signal processing method using an output of the adaptive equalizer to update a phase rotation for carrier phase compensation and a complex impulse response to be multiplied in the adaptive equalizer. [Explanation of symbols]

[0099] 10:Communication Systems 11:Transmitter 15: Receiver 13: Transmission path 21: Detector 22: Digital signal processing circuit 31: chromatic dispersion compensation filter 32: Adaptive equalizer 33: Filter coefficient update unit 100: Optical fiber communication system 110: Optical transmitter 130: Transmission line 150: Optical receiver 111: Encoding section 112: Pre-equalization section 113:DAC 114: Optical modulator 115:LD 117:2 x 1WL filter 118:IQ separation section 132: Optical fiber 133: Optical amplifier 151:LD 152: Coherent receiver 153:ADC 154: Digital signal processing unit 155: Decryption unit 156:IQ synthesis part 157:2 x 1WL filter 160: External device 161: chromatic dispersion compensation filter 162: Adaptive equalizer 163: Phase compensation filter 164: Subcarrier separation unit 165: Distortion estimation unit 170: Filter coefficient update unit 190:4×1WL equalizer

Claims

1. a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the polarization multiplexed optical signal transmitted from the transmitter and received by the receiver by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter and a signal indicating the real and imaginary components of a second polarization, multiplying the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, and performing phase rotation for phase compensation on the added signals for each polarization to compensate for a frequency offset and phase noise of a light source; an adaptive equalizer that multiplies the input signals indicating phase conjugates of the real and imaginary components of the first polarization and the input signals indicating phase conjugates of the real and imaginary components of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for phase compensation and the signal that has been subjected to the inverse rotation of the phase compensation phase for each polarization, and outputs the added signal; a filter coefficient update unit that updates the phase rotation for phase compensation and the complex impulse response multiplied in the adaptive equalizer using the output of the adaptive equalizer.

2. The digital signal processing circuit of claim 1 , wherein the adaptive equalizer includes a complex 8×2 Widely Linear (WL) MIMO filter.

3. 3. The digital signal processing circuit according to claim 2, wherein the 8×2WL MIMO filter is a WL filter that receives as input a complex signal representing a real component and an imaginary component of the first polarization, a complex signal representing a real component and an imaginary component of the second polarization, a complex signal representing a phase conjugate of the real component and an imaginary component of the first polarization, and a complex signal representing a phase conjugate of the real component and an imaginary component of the second polarization, and outputs the complex signal of the first polarization and the complex signal of the second polarization.

4. the polarization multiplexed optical signal is a digital subcarrier multiplexed optical signal in which data is multiplexed onto a plurality of subcarriers, the plurality of subcarriers include a pair of a first subcarrier and a second subcarrier; The adaptive equalizer for a first subcarrier, multiplying each of a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization by a complex impulse response; for the second subcarrier, multiplying each of a signal indicating a phase conjugate of the real component and an imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and an imaginary component of the second polarization by a complex impulse response; adding the first subcarrier signal and the second subcarrier signal multiplied by the complex impulse response; and performing a phase rotation for phase compensation and an inverse rotation of the phase rotation for phase compensation on the added signal for each subcarrier and each polarization; 2. The digital signal processing circuit according to claim 1, wherein, for a second subcarrier, a signal indicating a real component and an imaginary component of the first polarization and a signal indicating a real component and an imaginary component of the second polarization are multiplied by a complex impulse response, for the first subcarrier, a signal indicating a phase conjugate of the real component and the imaginary component of the first polarization and a signal indicating a phase conjugate of the real component and the imaginary component of the second polarization are multiplied by a complex impulse response, the second subcarrier signal multiplied by the complex impulse response and the first subcarrier signal are added, and the phase rotation for phase compensation and the inverse rotation of the phase rotation for phase compensation are performed for each subcarrier and for each polarization.

5. The digital signal processing circuit of claim 4 , wherein the adaptive equalizer includes a complex 16×4 Widely Linear (WL) MIMO filter.

6. The 16×4WL MIMO filter includes a complex signal representing a real component of the first polarization of the first subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the second polarization of the first subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the first polarization of the second subcarrier and a complex signal representing an imaginary component, a complex signal representing a real component of the second polarization of the second subcarrier and a complex signal representing an imaginary component, a complex signal representing a phase conjugate of the real component of the first polarization of the first subcarrier and a complex signal representing a phase conjugate of the imaginary component, and a complex signal representing a phase conjugate of the second polarization of the first subcarrier.

6. The digital signal processing circuit according to claim 5, wherein the WL filter receives as input a complex signal representing a phase conjugate of a real component of the first polarization of the second subcarrier and a complex signal representing a phase conjugate of an imaginary component thereof, a complex signal representing a phase conjugate of a real component of the first polarization of the second subcarrier and a complex signal representing a phase conjugate of an imaginary component thereof, and outputs a complex signal of the first polarization of the first subcarrier and a complex signal of the second polarization of the second subcarrier.

7. 7. The digital signal processing circuit according to claim 1, further comprising a distortion estimation unit that estimates at least one of distortion occurring in the transmitter and distortion occurring in the receiver based on the complex impulse response in the adaptive equalizer.

8. a detector that coherently receives a polarization multiplexed optical signal transmitted from a transmitter via a transmission line; a digital signal processing circuit that performs equalization signal processing on the coherently received received signal, The digital signal processing circuit a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the received signal by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter and a signal indicating the real and imaginary components of a second polarization, multiplying the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, and performing phase rotation for phase compensation on the added signals for each polarization to compensate for a frequency offset and phase noise of a light source; an adaptive equalizer that multiplies the input signals indicating phase conjugates of the real and imaginary components of the first polarization and the input signals indicating phase conjugates of the real and imaginary components of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for phase compensation and the signal that has been subjected to the inverse rotation of the phase compensation phase for each polarization, and outputs the added signal; a filter coefficient update unit that updates a phase rotation for phase compensation and a complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

9. a transmitter for transmitting a polarization multiplexed optical signal via a transmission line; a receiver including a detector that coherently receives the polarization multiplexed optical signal transmitted from the transmitter; a digital signal processing circuit that performs equalization signal processing on the coherently received received signal, The digital signal processing circuit a chromatic dispersion compensation filter that multiplies each of the real and imaginary components of the first and second polarizations of the received signal by a filter coefficient that compensates for chromatic dispersion; a signal indicating the real and imaginary components of a first polarization output from the chromatic dispersion compensation filter and a signal indicating the real and imaginary components of a second polarization, multiplying the input signals indicating the real and imaginary components of the first polarization and the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, and performing phase rotation for phase compensation on the added signals for each polarization to compensate for a frequency offset and phase noise of a light source; an adaptive equalizer that multiplies the input signals indicating phase conjugates of the real and imaginary components of the first polarization and the input signals indicating phase conjugates of the real and imaginary components of the second polarization by a complex impulse response, adds the signals multiplied by the complex impulse responses, performs an inverse rotation of the phase rotation for phase compensation on the added signals for each polarization, adds the signal that has been subjected to the phase rotation for phase compensation and the signal that has been subjected to the inverse rotation of the phase compensation phase for each polarization, and outputs the added signal; A communication system comprising: a filter coefficient update unit that updates the phase rotation for phase compensation and the complex impulse response multiplied in the adaptive equalizer using an output of the adaptive equalizer.

10. in the chromatic dispersion compensating filter, multiplying the real components and imaginary components of the first polarization and the second polarization of the polarization multiplexed optical signal transmitted from the transmitter and received by the receiver by filter coefficients that compensate for chromatic dispersion; an adaptive equalizer to which the signals indicating the real and imaginary components of the first polarization output from the chromatic dispersion compensation filter and the signals indicating the real and imaginary components of the second polarization are input, multiplying the signals indicating the real and imaginary components of the first polarization and the signals indicating the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, and performing phase compensation for the added signals to compensate for a frequency offset and a phase noise of a light source for each polarization; and multiplying each of the signals indicating the phase conjugates of the real and imaginary components of the first polarization and the signals indicating the phase conjugates of the real and imaginary components of the second polarization by a complex impulse response, adding the signals multiplied by the complex impulse responses, applying an inverse rotation of the phase rotation for phase compensation to the added signals for each polarization, adding the signal that has been subjected to the phase rotation for phase compensation and the signal that has been subjected to the inverse rotation of the phase compensation phase for each polarization, and outputting the added signals; A digital signal processing method using an output of the adaptive equalizer to update a phase rotation for phase compensation and a complex impulse response to be multiplied in the adaptive equalizer.

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