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

Abstract

The present invention makes it possible, with a small number of taps, to carry out compensation and polarisation separation for polarisation mode dispersion, and transmitter-internal distortion compensation. A detector (21) coherently receives a polarisation-multiplexed optical signal transmitted from a transmitter via a transmission path. A digital signal processing circuit (22) carries out equalisation signal processing on a coherently received reception signal. A carrier phase compensation filter (31) compensates for a carrier phase in the reception signal. A 4 × 2 WL filter (32) is located downstream of the carrier phase compensation filter (31) and compensates for distortion in the polarisation-multiplexed optical signal. A filter coefficient updating unit (33) uses the output of the 4 × 2 WL filter (32) to update a filter coefficient of the carrier phase compensation filter (31) and a filter coefficient of the 4 × 2 WL filter (32).

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 receiving technology has enabled flexible equalization signal processing at the receiving end using digital signal processing, such as the collective compensation of chromatic dispersion accumulated in optical fiber transmission lines. 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 higher multi-level modulation.

[0003] As a related technique, Non-Patent Document 1 discloses equalization digital signal processing on the receiver side. Fig. 14 shows the equalization digital signal processing. The digital signal processing includes filters used in a general polarization multiplexing coherent communication system. The filters include a distortion compensation filter 501 in the receiver, a chromatic dispersion compensation filter 502, a polarization separation filter 503 that performs polarization mode dispersion compensation and polarization separation, a carrier phase compensation filter 504, and a distortion compensation filter 505 in the transmitter.

[0004] The above filters receive a total of four real-valued received signal sequences, each consisting of an in-phase (I) component and a quadrature (Q) component of two polarized waves, X and Y, relative to the local oscillator light. The receiver distortion compensation filter 501, chromatic dispersion compensation filter 502, carrier phase compensation filter 504, and transmitter distortion compensation filter 505 compensate for receiver distortion, chromatic dispersion, carrier phase, and transmitter distortion for each polarized wave. Meanwhile, the polarization separation filter 503 handles both polarized waves.

[0005] Of the above filters, the receiver-internal distortion compensation filter 501 and chromatic dispersion compensation filter 502 are quasi-static filters. The receiver-internal distortion compensation filter 501 and chromatic dispersion compensation filter 502 use coefficients that are prepared based on a physical model of distortion, etc. On the other hand, the polarization separation filter 503 and the transmitter-internal distortion compensation filter 505 are filters whose coefficients are adaptively controlled based on their respective outputs. The carrier phase compensation filter 504 is calculated separately using a method that uses a pilot signal, etc.

[0006] As another related technique, Non-Patent Document 2 discloses the correction and monitoring of transmitter / receiver impairments using multilayer strictly linear (SL) filters and widely linear (WL) filters. In Non-Patent Document 2, 2×1WL filters arranged for each polarization are used for the distortion compensation filters in the receiver and the transmitter. 1×1SL filters arranged for each polarization are used for the chromatic dispersion compensation filters and carrier phase compensation filters. 2×2SL filters are used for the polarization separation filters. The coefficients of the distortion compensation filters in the receiver, chromatic dispersion compensation filters, polarization separation filters, carrier phase compensation filters, and distortion compensation filters in the transmitter are adaptively controlled using the output of the distortion compensation filter in the transmitter, which is the final filter stage. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] Chris RS Fludger and Theo Kupfer, “Transmitter impairment mitigation and monitoring for high baud-rate, high order modulation systems”, ECOC 2016; 42nd European Conference on Optical Communication, 18-22 September, 2016 [Non-patent document 2] 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]

[0008] To realize an optical transmission system of 1 Tbps (bit per second) or more, a high symbol rate and a high multi-level modulation scheme are essential, and to ensure performance in such an advanced modulation scheme, high-precision equalization processing is required. In Non-Patent Document 1, the effect generated in the transmitting device (hereinafter also referred to as Tx load) is compensated for on the receiving side. However, the performance of carrier phase compensation in the carrier phase compensation filter 504 arranged in the preceding stage of the distortion compensation filter 505 in the transmitter depends on the Tx load. Therefore, there is a problem in that the performance of carrier phase compensation ultimately affects the equalization accuracy of the Tx load.

[0009] In contrast to the above, in Non-Patent Document 2, the coefficients of the filters at each stage are updated using the output of the transmitter internal distortion compensation filter, which is the final stage filter. Therefore, it is thought that Patent Document 2 can compensate the carrier phase in the carrier phase compensation filter without depending on the Tx load. However, in Patent Document 2, a polarization separation filter is arranged in the stage preceding the carrier phase compensation. Filters with a relatively large number of taps are used for the polarization separation filter and the transmitter distortion compensation filter, which poses a problem of large circuit scale for equalization digital signal processing.

[0010] In view of the above circumstances, one of the objects of the present disclosure is to provide a digital signal processing circuit, method, receiver, and communication method that can perform polarization mode dispersion compensation, polarization separation, and distortion compensation in a transmitter with a small number of taps. [Means for solving the problem]

[0011] To achieve the above object, the present disclosure provides, as a first aspect, a digital signal processing circuit including: a first carrier phase compensation filter that compensates for a carrier phase in a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver; a 4×2 Widely Linear (WL) filter disposed downstream of the first carrier phase compensation filter that compensates for distortion contained in the polarization multiplexed optical signal; and a filter coefficient update unit that updates the filter coefficients of the first carrier phase compensation filter and the 4×2 WL filter using an output of the 4×2 WL filter.

[0012] 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 path, and a digital signal processing circuit that performs equalization signal processing on the coherently received received signal. The digital signal processing circuit includes a carrier phase compensation filter that compensates for a carrier phase in the received signal, a 4×2 Widely Linear (WL) filter that is arranged downstream of the carrier phase compensation filter and compensates for distortion contained in the polarization multiplexed optical signal, and a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the 4×2 WL filter using an output of the 4×2 WL filter.

[0013] The present disclosure provides, as a third aspect, a communication system. The communication system includes a transmitter that transmits a polarization multiplexed optical signal via a transmission path and a receiver that receives the transmitted polarization multiplexed optical signal. The receiver includes 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 carrier phase compensation filter that compensates for a carrier phase in the received signal, a 4×2 Widely Linear (WL) filter that is disposed downstream of the carrier phase compensation filter and compensates for distortion contained in the polarization multiplexed optical signal, and a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the 4×2 WL filter using an output of the 4×2 WL filter.

[0014] The present disclosure provides, as a fourth aspect, a digital signal processing method, comprising: compensating, in a carrier phase compensation filter, a carrier phase in a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver; compensating, in a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter, for distortion contained in the polarization multiplexed optical signal; and updating, using an output of the 4×2 WL filter, a filter coefficient of the carrier phase compensation filter and a filter coefficient of the 4×2 WL filter. [Effects of the Invention]

[0015] The digital signal processing circuit, method, receiver, and communication method according to the present disclosure can perform polarization mode dispersion compensation and polarization separation, and distortion compensation in a transmitter, with a small number of taps. [Brief explanation of the drawings]

[0016] [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 an example of the configuration of digital signal processing used in the description. [Figure 7] Graph showing the signal distribution of the I-channel and Q-channel without Tx load compensation. [Figure 8] Graph showing the signal distribution of the I-channel and Q-channel when the Tx load is compensated. [Figure 9] 6 is a graph showing the signal distribution of the I-channel and Q-channel when the digital signal processing unit according to the present embodiment is used. [Figure 10] 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 11] FIG. 2 is a block diagram showing a part of the configuration of an optical transmitter. [Figure 12] FIG. 11 is a block diagram showing a configuration example of a digital signal processing unit used in a third embodiment of the present disclosure. [Figure 13] FIG. 10 is a block diagram showing an optical receiver used in a modified example. [Figure 14] FIG. 1 is a block diagram illustrating equalization digital signal processing. DETAILED DESCRIPTION OF THE INVENTION

[0017] 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.

[0018] 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.

[0019] The digital signal processing circuit 22 has a carrier phase compensation filter 31, a 4×2WL filter 32, and a filter coefficient update unit 33. The carrier phase compensation filter 31 compensates for the carrier phase in the received signal, which is a polarization multiplexed signal. The 4×2WL filter 32 is arranged after the carrier phase compensation filter 31. The 4×2WL filter 32 compensates for distortion contained in the received signal. The filter coefficient update unit 33 updates the filter coefficients of the carrier phase compensation filter 31 and the 4×2WL filter 32 using the output of the 4×2WL filter 32.

[0020] In the present disclosure, when carrier phase compensation is performed before the 4×2WL filter 32, the 4×2WL filter 32 can simultaneously perform compensation for distortion in the transmitter (Tx load), polarization mode dispersion compensation, and polarization separation. This allows for a reduction in the number of taps compared to when separate filters are provided for polarization mode dispersion and polarization separation. Therefore, the present disclosure allows for a reduction in the circuit size of the digital signal processing circuit.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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).

[0027] 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.

[0028] 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.

[0029] 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 includes a carrier phase compensation filter 161, a 4×2WL equalizer (WL filter) 162, and a filter coefficient updating unit 170. In the digital signal processing unit 154, the carrier phase compensation filter 161 and the 4×2WL equalizer 162 are arranged in cascade connection 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 carrier phase compensation filter 141. The carrier phase compensation filter 161 corresponds to the carrier phase compensation filter 31 shown in FIG. 2. The 4×2WL equalizer 162 corresponds to the 4×2WL filter 32 shown in FIG. 2.

[0030] The filter coefficient update unit 170 monitors the output of the 4×2WL equalizer 162 and the input of the carrier phase compensation filter 161. The filter coefficient update unit 170 also monitors the output of the carrier phase compensation filter 161, i.e., the input of the 4×2WL equalizer 162. The filter coefficient update unit 170 updates the filter coefficients of the carrier phase compensation filter 161 and the 4×2WL equalizer 162 using the output of the 4×2WL equalizer 162. The filter coefficient update unit 170 adaptively controls the coefficients of the 4×2WL equalizer 162 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 4×2WL equalizer 162, 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.

[0031] 5 shows a more detailed example of the configuration of the digital signal processing unit 154. The carrier phase compensation filter 161 receives complex data of the X polarization and complex data of the Y polarization, which are obtained by converting the IQ components of the X polarization and the IQ components of the Y polarization into complex signals. The carrier phase compensation filter 161 compensates for signal distortion caused by a frequency offset and a phase offset between the carrier of the transmitted optical signal and the local oscillator light on the receiving side. The carrier phase compensation filter 161 includes a carrier phase compensation filter 161X corresponding to the X polarization and a carrier phase compensation filter 161Y corresponding to the Y polarization. For example, an SL 1×1 1-tap FIR (Finite Impulse Response) filter is used for the carrier phase compensation filter 161X and the carrier phase compensation filter 161Y.

[0032] The 4×2WL equalizer 162 includes complex conjugate transformation units 181 arranged corresponding to each polarization, and a total of eight complex coefficient filters 182 constituting a WL MIMO (multiple-input and multiple-output) filter. The complex conjugate transformation units 181 convert the complex data of the X polarization and the complex data of the Y polarization output from the carrier phase compensation filter 161 into complex conjugate data of the X polarization and complex conjugate data of the Y polarization, respectively. The complex data of the X polarization, the complex data of the Y polarization, the complex conjugate data of the X polarization, and the complex conjugate data of the Y polarization are input to the complex coefficient filters 182 constituting the WL MIMO filter. Each complex coefficient filter 182 is configured as an FIR filter. In the 4×2WL equalizer 162, the number of taps of the FIR filter is selected depending on the distortion to be compensated.

[0033] Note that a WL 4×2 MIMO filter is equivalent to a real-signal-input, real-coefficient 4×4 MIMO filter having 4×4=16 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 collectively referred to as a WL MIMO filter. In this context, a normal complex-signal-input, complex-coefficient MIMO filter is referred to as an SL MIMO filter.

[0034] The filter coefficient update unit 170 updates the coefficients of the complex coefficient filter 182 of the 4×2WL equalizer 162 so as to minimize the above-mentioned loss function. The filter coefficient update unit 170 updates the coefficients of each filter by, for example, stochastic gradient descent so as to minimize the loss function calculated based on the filter output of the 4×2WL equalizer 162. The filter coefficient update unit 170 calculates the coefficients of the carrier phase compensation filter 161, i.e., the amount of phase compensation in the carrier phase compensation filter 161, based on the output of the 4×2WL equalizer 162. The phase compensation amount can be calculated using a general M-th power method or a digital phase locked loop (PLL) using tentative decision.

[0035] The following describes updating of the filter coefficients of the 4×2WL equalizer 162 in the digital signal processing unit 154. In the following description, updating of the filter coefficients in the case of one polarization will be described. FIG. 6 shows an example of the configuration of digital signal processing used in the description. In FIG. 6, a 2×1WL equalizer (2×1WL MIMO filter) 190 is used for the digital signal processing instead of the 4×2WL equalizer 162 (see FIG. 5). The 2×1WL equalizer 190 includes a complex conjugate transformation unit 191 and two complex coefficient filters 192. The 4×2WL equalizer 162 has a configuration in which the 2×1WL equalizer 190 is extended to polarization multiplexing. Therefore, the operating principle of updating the coefficients in the 4×2WL equalizer 162 is the same as the operating principle of updating the filter coefficients in the 2×1WL equalizer 190.

[0036] If the input of the carrier phase compensation filter 161 is x (=I+iQ), the output of the carrier phase compensation filter 161 is y i [k] is expressed by the following formula. TIFF0007718499000001.tif654 where j represents the input dimensionality, i represents the output dimensionality, and k represents the sample. -iθj denotes the phase compensation coefficient. The output z of the 2×1WL equalizer i [k] is expressed by the following formula. TIFF0007718499000002.tif7107In the above formula, y i * Yes i where m is the number of taps in the FIR filter. The loss function φ[k] used to update the filter coefficients is the complex conjugate of d i [k] is the teacher signal and is defined by the following equation. TIFF0007718499000003.tif553

[0037] The filter coefficients of the 2×1 WL equalizer 190 are updated using the stochastic gradient descent method so as to minimize the above loss function. TIFF0007718499000004.tif7297TIFF0007718499000005.tif4837

[0038] 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. TIFF0007718499000006.tif4648

[0039] Phase compensation coefficient e -iθj θ in j represents the amount of phase compensation. j is calculated separately based on φ[k]. The phase compensation amount includes a frequency offset and a phase error. For example, a digital PLL using a general teacher signal is used to calculate the phase compensation amount.

[0040] In this embodiment, the digital signal processing unit 154 has a carrier phase compensation filter 161 and a 4×2WL equalizer 162. The filter coefficient update unit 170 updates the coefficients of the carrier phase compensation filter 161 and the 4×2WL equalizer 162 using the output signal of the 4×2WL equalizer 162. In this embodiment, the carrier phase compensation filter 161 is arranged in the preceding stage of the 4×2WL equalizer 162, and the 4×2WL equalizer 162 equalizes the signal whose carrier phase has been compensated. By employing such a configuration, the digital signal processing unit 154 can simultaneously perform compensation for polarization mode dispersion (PDM), polarization demultiplexing, and compensation for the Tx load.

[0041] For example, in the equalization digital signal processing shown in FIG. 14 , the polarization separation filter 503 compensates for polarization fluctuation, the carrier phase compensation filter 504 compensates for carrier phase, and the transmitter distortion compensation filter 505 compensates for Tx load. In this case, to achieve highly accurate Tx compensation and adaptive equalization that accommodates high PMD, each filter needs to have a long tap length. For example, if the number of taps in the polarization separation filter 503 is m and the number of taps in the transmitter distortion compensation filter 505 is m, the tap length of the filter required for polarization mode dispersion compensation and Tx load compensation is 2m. In contrast, in this embodiment, polarization mode dispersion compensation and Tx load compensation can be performed simultaneously in the 4×2WL equalizer 162. Therefore, in this embodiment, the tap length of the filter required for polarization mode dispersion compensation and Tx load compensation is m. In this way, this embodiment can integrate two adaptive filters with long tap lengths into a single filter, thereby achieving equalization digital signal processing with a small circuit scale.

[0042] 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. To this signal, 100 kHz noise was added to the LD and local oscillator light on the transmitting side as phase noise, and an IQ skew of 0.5 UI (Unit Interval) was added to the Q signal of X polarization in the transmitter.

[0043] Figure 7 shows the signal distribution of the I-channel and Q-channel when the Tx load is 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 the polarization separation filter 503 and carrier phase compensation filter 504 shown in Figure 14. In this case, since the Tx load is not compensated for in the equalization digital signal processing, it is difficult to distinguish the signal point in the X polarization.

[0044] Figure 8 shows the signal distributions of the I-channel and Q-channel when the Tx load is compensated for in the equalization digital signal processing. In the simulation, signals converted to digital signals by the ADC were equalized using the polarization separation filter 503, carrier phase compensation filter 504, and transmitter distortion compensation filter 505 shown in Figure 14. In this case, no distortion is observed in the Y-polarized signal to which no skew is added. However, for the X-polarized signal, although the reception characteristics are improved compared to Figure 7, they are still not sufficiently high.

[0045] Fig. 9 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. 9 with Fig. 7 and Fig. 8, it can be seen that the reception characteristics of the X-polarized signal can be improved when the digital signal processing unit 154 is used. In this way, the simulation confirmed that the polarization fluctuation and the Tx load can be compensated for collectively in the digital signal processing unit 154, even without the need for a separate polarization separation filter.

[0046] Next, a second embodiment of the present disclosure will be described. Fig. 10 shows a configuration example 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 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 4x2WL equalizer 162. 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.

[0047] In this embodiment, the filter coefficients of the pre-equalization unit 112 of the optical transmitter 110 are controlled based on the filter coefficients of the digital signal processing unit 154a on the receiving side. FIG. 11 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 Y polarization complex signal (YI+iYQ) is input to the 2×1WL filter 117 arranged corresponding to the Y polarization. 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 .

[0048] In this embodiment, the distortion estimation unit 165 (see FIG. 10) estimates the Tx load from the filter coefficients of the 4×2WL equalizer 162 after coefficient convergence. The Tx load can be calculated based on the complex coefficient filter 182 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.

[0049] Note that, if the filter types differ between the digital signal processing unit 154a on the receiving side and the pre-equalization unit 112 on the transmitting side, coefficient conversion can be performed. For example, consider a case where a 4×2 WL MIMO filter is used in the digital signal processing unit 154a and a 2×2 Real MIMO filter is used in the pre-equalization unit 112. In this case, the inverse characteristics of the Tx load estimated from the 4×2 WL MIMO filter can be converted into coefficients of the 2×2 Real MIMO filter.

[0050] Next, a third embodiment of the present disclosure will be described. Fig. 12 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 carrier phase compensation filter 163 at a stage subsequent to the 4x2WL equalizer 162 in addition to the configuration of the digital signal processing unit 154 shown in Fig. 4.

[0051] In this embodiment, the filter coefficient update unit 170 updates the filter coefficient of either the carrier phase compensation filter (first carrier phase compensation filter) 161 or the carrier phase compensation filter (second carrier phase compensation filter) 163. The filter coefficient update unit 170 does not cause the other of the carrier phase compensation filter 161 and the carrier phase compensation filter 163 to perform carrier phase compensation, and outputs the input signal as is. In other words, the filter coefficient update unit 170 operates either the carrier phase compensation filter 161 or the carrier phase compensation filter 163, but does not operate the other.

[0052] When the filter coefficient update unit 170 operates the carrier phase compensation filter 161, the digital signal processing unit 154b performs carrier phase compensation in a stage before the 4×2WL equalizer 162. On the other hand, when the filter coefficient update unit 170 operates the carrier phase compensation filter 163, the digital signal processing unit 154b performs carrier phase compensation in a stage after the 4×2WL equalizer 162. In this way, the digital signal processing unit 154b is configured to be able to select whether carrier phase compensation is performed in a stage before the 4×2WL equalizer 162 or in a stage after the 4×2WL equalizer 162.

[0053] Distortion compensation in the 4×2WL equalizer 162 will be described in the cases where carrier phase compensation is performed before the 4×2WL equalizer 162 and where carrier phase compensation is performed after the 4×2WL equalizer 162. Below, the 4×2WL equalizer 162 (see FIG. 5) is replaced with a 2×1WL equalizer 190 (see FIG. 6), and distortion compensation in the case of one polarization will be described. As described above, the 4×2WL equalizer 162 has a configuration in which the 2×1WL equalizer 190 is extended to polarization multiplexing. Therefore, the operating principle of distortion compensation in the 4×2WL equalizer 162 is the same as the operating principle of distortion compensation in the 2×1WL equalizer 190.

[0054] First, a case where carrier phase compensation is performed in the carrier phase compensation filter 163 arranged after the 2×1WL equalizer will be described. The input of the 2×1WL equalizer is x (complex signal), the output of the carrier phase compensation filter 163 is y, and the phase compensation coefficient is e -iθ In this case, y and x satisfy the following equation. TIFF0007718499000007.tif643TIFF0007718499000008.tif641The above ye iθ The IQ display is shown below: TIFF0007718499000009.tif1041y I and y Q and x I and x Q The relationship is expressed by the following formula: TIFF0007718499000010.tif1051From the above formula, TIFF0007718499000011.tif1061. In this case, IQ distortion is compensated for, followed by phase error compensation. In this case, the 2x1WL equalizer compensates for the effects occurring in the receiving device (hereinafter also referred to as Rx load).

[0055] Next, a case where carrier phase compensation is performed in the carrier phase compensation filter 161 arranged in the front stage of the 2×1WL equalizer will be described. The input of the carrier phase compensation filter 161 is x (complex signal), the output is x', the output of the 2×1WL equalizer is y, and the phase compensation coefficient is e -iθ In this case, y and x satisfy the following equation. TIFF0007718499000012.tif544TIFF0007718499000013.tif1557In this case, TIFF0007718499000014.tif1061. In this case, the phase error is compensated, then the IQ distortion is compensated. In this case, with a 2x1WL equalizer, the Tx load is compensated.

[0056] For example, a person such as a system operator determines whether to use carrier phase compensation filter 161 or carrier phase compensation filter 163, depending on the ratio of Rx load to Tx load in the system. The filter coefficient update unit 170 outputs a phase compensation amount θ=0 to the carrier phase compensation filter for which it has been determined that carrier phase compensation is not to be performed.

[0057] For example, when the Tx load is dominant, the filter coefficient update unit 170 fixes the phase compensation amount to be output to the carrier phase compensation filter 163 to θ=0. The filter coefficient update unit 170 updates the filter coefficient of the carrier phase compensation filter 161 using the output of the 4×2WL equalizer 162. On the other hand, when the Rx load is dominant, the filter coefficient update unit 170 fixes the phase compensation amount to be output to the carrier phase compensation filter 161 to θ=0. The filter coefficient update unit 170 updates the filter coefficient of the carrier phase compensation filter 163 using the output of the 4×2WL equalizer 162. In this way, the digital signal processing unit 154b can select whether to compensate for the Rx load or the Tx load.

[0058] In the second embodiment, an example has been described in which it is assumed that the filters in the digital signal processing are implemented in the digital signal processing unit 154a. However, the present disclosure is not limited to this. As a modified example, 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 154a.

[0059] 13 shows an optical receiver used in a modified example. In this modified example, optical receiver 150 is connected to external device 160. External device 160 is configured as a computer device such as a personal computer (PC). In optical receiver 150, a digital signal output by ADC 153 is branched to external device 160. Optical receiver 150 has an interface for connecting to external device 160, and outputs the digital signal to external device 160 via the interface.

[0060] The external device 160 reproduces the operations of the carrier phase compensation filter 161 and the 4×2WL equalizer 162 using simulation or the like, and updates the filter coefficients. In the external device 160, the carrier phase compensation filter and the 4×2WL equalizer may be implemented by dedicated hardware. The external device 160 estimates the Tx load based on the updated filter coefficients of the 4×2WL 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 the second 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 the Tx load.

[0061] The above describes the embodiments of the present disclosure in detail, but the present disclosure is not limited to the above-described embodiments, and changes and modifications to the above-described embodiments that do not deviate from the spirit of the present disclosure are also included in the present disclosure.

[0062] [Appendix 1] a first carrier phase compensation filter that compensates for a carrier phase in a polarization multiplexed optical signal transmitted from the transmitter and received by the receiver; a 4×2 Widely Linear (WL) filter disposed after the first carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the first carrier phase compensation filter and the filter coefficients of the 4×2WL filter using the output of the 4×2WL filter.

[0063] [Appendix 2] a first polarization signal and a second polarization signal are multiplexed in the polarization multiplexed optical signal; 2. The digital signal processing circuit of claim 1, wherein the first polarized signal and the second polarized signal are input to the first carrier phase compensation filter as complex signals, and the first carrier phase compensation filter compensates for the carrier phase in each of the first polarized wave and the second polarized wave.

[0064] [Appendix 3] The digital signal processing circuit of claim 2, wherein the 4×2 WL filter is a WL filter that receives the complex signal of the first polarization, the complex conjugate signal of the first polarization, the complex signal of the second polarization, and the complex conjugate signal of the second polarization as inputs, and outputs the complex signal of the first polarization and the complex signal of the second polarization.

[0065] [Appendix 4] Further, a second carrier phase compensation filter is arranged after the 4×2WL filter, 4. The digital signal processing circuit according to claim 1, wherein the filter coefficient update unit causes one of the first carrier phase compensation filter and the second carrier phase compensation filter to perform carrier phase compensation, and does not cause the other of the first carrier phase compensation filter and the second carrier phase compensation filter to perform carrier phase compensation.

[0066] [Appendix 5] the filter coefficient update unit updates the filter coefficient of one of the first carrier phase compensation filter and the second carrier phase compensation filter using the output of the 4×2WL filter, and fixes the filter coefficient of one of the first carrier phase compensation filter and the second carrier phase compensation filter to a phase compensation amount of 0.

[0067] [Appendix 6] 6. The digital signal processing circuit according to claim 1, further comprising a distortion estimation unit that estimates distortion of the transmitter based on the filter coefficients of the 4×2WL filter.

[0068] [Appendix 7] 7. The digital signal processing circuit according to claim 1, wherein the 4×2WL filter compensates for distortion and polarization mode dispersion that occurs in the transmitter.

[0069] [Appendix 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 carrier phase compensation filter that compensates for a carrier phase in the received signal; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter.

[0070] [Appendix 9] a first polarization signal and a second polarization signal are multiplexed in the polarization multiplexed optical signal; 9. The receiver of claim 8, wherein the first polarization signal and the second polarization signal are input as complex signals, and the carrier phase compensation filter compensates for the carrier phase in each of the first polarization and the second polarization.

[0071] [Appendix 10] 10. The receiver described in Supplementary Note 9, wherein the 4×2 WL filter is a WL filter that receives the complex signal of the first polarization, the complex conjugate signal of the first polarization, the complex signal of the second polarization, and the complex conjugate signal of the second polarization as inputs, and outputs the complex signal of the first polarization and the complex signal of the second polarization.

[0072] [Appendix 11] a transmitter for transmitting a polarization multiplexed optical signal via a transmission line; a receiver for receiving the transmitted polarization multiplexed optical signal, The receiver includes: 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 carrier phase compensation filter that compensates for a carrier phase in the received signal; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter.

[0073] [Appendix 12] a first polarization signal and a second polarization signal are multiplexed in the polarization multiplexed optical signal; 12. The communication system of claim 11, wherein the first polarization signal and the second polarization signal are input as complex signals to the carrier phase compensation filter, and the carrier phase compensation filter compensates for the carrier phase in each of the first polarization and the second polarization.

[0074] [Appendix 13] 13. The communication system of claim 12, wherein the 4×2 WL filter is a WL filter that receives the complex signal of the first polarization, the complex conjugate signal of the first polarization, the complex signal of the second polarization, and the complex conjugate signal of the second polarization as inputs, and outputs the complex signal of the first polarization and the complex signal of the second polarization.

[0075] [Appendix 14] the transmitter has a pre-equalization unit that pre-equalizes the polarization multiplexed optical signal, 14. The communication system according to claim 11, wherein the filter coefficients of the pre-equalization unit are controlled in accordance with the distortion of the transmitter estimated based on the filter coefficients of the 4×2WL filter.

[0076] [Appendix 15] a carrier phase compensation filter for compensating for a carrier phase in a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; A digital signal processing method comprising: updating the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter. [Explanation of symbols]

[0077] 10:Communication Systems 11:Transmitter 15: Receiver 13: Transmission path 21: Detector 22: Digital signal processing circuit 31: Carrier phase compensation filter 32:4×2WL filter 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 161, 163: Carrier phase compensation filter 162:4×2WL equalizer 165: Distortion estimation unit 170: Filter coefficient update unit 181: Complex conjugate transform unit 182: Complex coefficient filter 190:2×1WL equalizer 191: Complex conjugate transform unit 192: Complex coefficient filter

Claims

1. a first carrier phase compensation filter that compensates for a carrier phase in a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver; a 4×2 Widely Linear (WL) filter disposed after the first carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the first carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter.

2. a signal of a first polarization and a signal of a second polarization are multiplexed in the polarization multiplexed optical signal; 2. The digital signal processing circuit according to claim 1, wherein the first polarized signal and the second polarized signal are input to the first carrier phase compensation filter as complex signals, and the first carrier phase compensation filter compensates for the carrier phase in each of the first polarized wave and the second polarized wave.

3. 3. The digital signal processing circuit according to claim 2, wherein the 4×2 WL filter is a WL filter that receives the complex signal of the first polarization, the complex conjugate signal of the first polarization, the complex signal of the second polarization, and the complex conjugate signal of the second polarization as inputs, and outputs the complex signal of the first polarization and the complex signal of the second polarization.

4. a second carrier phase compensation filter disposed after the 4×2 WL filter; 4. The digital signal processing circuit according to claim 1, wherein the filter coefficient update unit causes one of the first carrier phase compensation filter and the second carrier phase compensation filter to perform carrier phase compensation, and does not cause the other of the first carrier phase compensation filter and the second carrier phase compensation filter to perform carrier phase compensation.

5. 5. The digital signal processing circuit according to claim 4, wherein the filter coefficient update unit updates the filter coefficient of one of the first carrier phase compensation filter and the second carrier phase compensation filter using the output of the 4×2 WL filter, and fixes the filter coefficient of one of the first carrier phase compensation filter and the second carrier phase compensation filter to a phase compensation amount of 0.

6. 6. The digital signal processing circuit according to claim 1, further comprising a distortion estimation unit that estimates distortion of the transmitter based on the filter coefficients of the 4x2WL filter.

7. 7. The digital signal processing circuit according to claim 1, wherein the 4x2 WL filter compensates for distortion and polarization mode dispersion occurring in the transmitter.

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 carrier phase compensation filter that compensates for a carrier phase in the received signal; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter and compensating for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter.

9. a transmitter for transmitting a polarization multiplexed optical signal via a transmission line; a receiver for receiving the transmitted polarization multiplexed optical signal, The receiver includes: 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 carrier phase compensation filter that compensates for a carrier phase in the received signal; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter and compensating for distortion contained in the polarization multiplexed optical signal; a filter coefficient update unit that updates the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4×2WL filter using an output of the 4×2WL filter.

10. a carrier phase compensation filter for compensating for a carrier phase in a polarization multiplexed optical signal transmitted from a transmitter and received by a receiver; a 4×2 Widely Linear (WL) filter disposed downstream of the carrier phase compensation filter, which compensates for distortion contained in the polarization multiplexed optical signal; A digital signal processing method comprising: updating the filter coefficients of the carrier phase compensation filter and the filter coefficients of the 4x2WL filter using an output of the 4x2WL filter.

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