Signal processing device, signal processing method, and communication system

Abstract

This signal processing device is for compensating for a waveform distortion which occurs in an optical signal. The signal processing device comprises an adaptive equalization unit having: a carrier phase recovery part for estimating the relative phases of all light sources provided in a transmitter that wavelength-multiplexes and transmits M (M is an integer of 2 or more) spatially-multiplexed polarization multiplexed signals, and the relative phases of all local oscillators provided in a receiver that receives wavelength-multiplexed polarization multiplexed signals; and an adaptive filter part that, by using as input signals a set of a received signal and a phase conjugate signal of the received signal or a plurality of signals mathematically equivalent to the phase conjugate signal of the received signal and the received signal, multiplies the input signals by the relative phases of all the light sources provided in the transmitter estimated by the carrier phase recovery part and the complex conjugates of the relative phases of all the light sources provided in the transmitter, and the relative phases of all the local oscillators provided in the receiver and the complex conjugates of the relative phases of all the local oscillators provided in the receiver.　

Description

Signal processing device, signal processing method, and communication system

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

[0002] With the development of information technology, the demand for communications that supports it continues to expand. Among these, optical communication using optical fibers enables long-distance, high-capacity communication due to its wide available frequency band and low signal attenuation, and is widely used for modern fixed lines. In recent years, research has been conducted on space division multiplexing (SDM) systems, which transmit and receive signals using the core modes of multicore fiber (MCF), multimode fiber (MMF), and multicore multimode fiber (MCMMF) as independent channels.

[0003] Light propagating through an optical fiber is affected by polarization rotation, polarization mode dispersion, and polarization dependent loss due to the effects of random birefringence within the fiber. These transmission distortions within the fiber can be reduced on the receiving side by using digital signal processing called a 2x2 MIMO (Multiple-input multiple-output) adaptive filter, which uses each polarization component as an input and output.

[0004] Among multicore fibers, those with strong coupling between cores, such as coupled-core multicore fibers, and multimode fibers, experience crosstalk between SDM channels and core / mode dependent loss during propagation, but these can be reduced by using a 2M x 2M MIMO adaptive filter, which is an extension of the above processing (M: SDM multiplexing number). In a spatial multiplexing system with M spatial multiplexing, a total of M optical devices, such as a local oscillator light source, a polarization separator, a balanced detector, and an optical 90-degree hybrid coupler, are prepared, one set per channel, and signal light and local oscillator light are input to these devices.

[0005] In conventional signal processing methods, it is assumed that the phases of all channels of a local oscillator light source are synchronized. However, achieving optical synchronization requires the amplification of the local oscillator light using an optical amplifier, an optical-electrical phase-locked loop, or injection locking, which results in a problem of large-scale transmitter / receiver equipment. In contrast, a recently proposed MIMO carrier phase recovery configuration (see, for example, Non-Patent Document 1) uses a 2M × 2M MIMO adaptive filter to compensate for crosstalk and other issues that occur during optical fiber transmission, while simultaneously using digital signal processing such as an extended Kalman filter to estimate the relative phases of all light sources on the transmitting side and all local oscillator light on the receiving side, enabling transmission and reception in a system using one local oscillator light per mode.

[0006] K. Shibahara, T. Mizuno, and Y. Miyamoto, “MIMO carrier phase recovery for carrier-asynchronous SDM-MIMO reception based on the extended Kalman filter”, Optics Express 29.11 (2021): 17111-17124. A. Kawai, M. Nakamura, T. Kobayashi and Y. Miyamoto, “Digital Four-Dimensional Transceiver IQ Characterization”, Journal of Lightwave Technology, 41.5 (2023): 1389-1398. M. Nakamura, et al, “High Information Rate of 128-GBaud 1.8-Tb / s and 64-GBaud 1.03-Tb / s Signal Generation and Detection Using Frequency-Domain 8× 2 MIMO Equalization”, 2022 Optical Fiber Communications Conference and Exhibition (OFC). IEEE, 2022. A. Kawai, M. Nakamura, M. Takahashi, T. Kobayashi and Y. Miyamoto, “Partial Freezing for MIMO Adaptive Equalizers: Precise Demodulation in Dynamic Environments”, Journal of Lightwave Technology, early access (2023) doi: 10.1109 / JLT.2023.3251996.

[0007] Japanese Patent Application Laid-Open No. 2020-141294

[0008] However, previously proposed MIMO carrier phase recovery methods did not compensate for IQ waveform distortions, such as IQ skew, IQ imbalance, and IQ crosstalk, that exist between the transmitter and receiver. In other words, the presence of such IQ waveform distortions could result in a degradation of signal quality. Because the impact of IQ waveform distortion increases as the signal symbol rate increases, IQ waveform distortion could become a problem when high-symbol-rate signals are used. Therefore, IQ waveform distortion, like inter-channel crosstalk, needed to be compensated for. In receiver systems using conventional single-mode fiber, compensation methods have been proposed, including a method of compensating for signal distortions occurring within the transmitter / receiver using adaptive filters with special configurations, such as a 4x2 MIMO adaptive filter or an 8x2 MIMO adaptive filter (see, for example, Non-Patent Document 2), or a method of calculating signal distortion characteristics from filter coefficients and feeding them back to a fixed pre-equalization filter within the transmitter / receiver (see, for example, Non-Patent Document 3). However, there has been no method to extend this method to spatial multiplexing systems by combining it with MIMO carrier phase recovery.

[0009] In view of the above circumstances, an object of the present invention is to provide a technology that can perform at least one of compensation for IQ waveform distortion occurring in a signal at a transmitter or receiver or estimation of its characteristics when the phase of the local oscillator light on the receiving side is not synchronized in a system that uses spatially multiplexed signals.

[0010] One aspect of the present invention is a signal processing device for compensating for waveform distortion occurring in an optical signal, the signal processing device comprising: a carrier phase recovery unit that estimates the relative phases of all light sources provided in a transmitter that wavelength-multiplexes and transmits M (M is an integer of 2 or greater) spatially multiplexed polarization multiplexed signals, and the relative phases of all local oscillator light provided in a receiver that receives the wavelength-multiplexed polarization multiplexed signals; and an adaptive equalization unit that has an adaptive filter unit that takes as input signals a pair of a received signal and a phase conjugate signal of the received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signals of the received signal, and multiplies the input signals by the relative phases of all light sources provided in the transmitter estimated by the carrier phase recovery unit and the complex conjugates of the relative phases of all light sources provided in the transmitter, and by the relative phases of all local oscillator light provided in the receiver and the complex conjugates of the relative phases of all local oscillator light provided in the receiver.

[0011] One aspect of the present invention is a signal processing method performed by a signal processing device for compensating for waveform distortion occurring in an optical signal, the signal processing method estimating the relative phases of all light sources provided in a transmitter that wavelength-multiplexes and transmits M (M is an integer of 2 or greater) spatially multiplexed polarization multiplexed signals, and the relative phases of all local oscillator light provided in a receiver that receives the wavelength-multiplexed polarization multiplexed signals, and multiplying the input signals by a pair of a received signal and a phase conjugate signal of the received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal, and multiplying the input signals by the complex conjugates of the estimated relative phases of all light sources provided in the transmitter and the relative phases of all light sources provided in the transmitter, and by the complex conjugates of the relative phases of all local oscillator light provided in the receiver and the relative phases of all local oscillator light provided in the receiver.

[0012] One aspect of the present invention is a communication system comprising the above-mentioned signal processing device, and the transmitter and receiver having a filter function that compensates for signal waveform distortion based on the characteristics of the transmitter and the characteristics of the receiver obtained by the signal processing device.

[0013] According to the present invention, in a system using spatially multiplexed signals, when the phase of the local oscillator light on the receiving side is not synchronized, it becomes possible to perform at least one of compensation for IQ waveform distortion occurring in the signal at the transmitter or receiver or estimation of the characteristics.

[0014] FIG. 1 is a diagram illustrating an example of the configuration of a digital coherent optical transmission system in a first embodiment. FIG. 2 is a diagram illustrating an example of the configuration of a transmitter and a receiver in the first embodiment. FIG. 3 is a diagram for explaining an overview of processing performed by an adaptive equalizer in the first embodiment. FIG. 4 is a diagram illustrating an example of a detailed configuration including an adaptive equalizer in the first embodiment. FIG. 5 is a diagram illustrating an example of the configuration of a transmitter and a receiver in a second embodiment. FIG. 6 is a diagram for explaining an overview of processing performed by a characteristic function derivation unit in the second embodiment. FIG. 7 is a diagram illustrating an example of the configuration of a characteristic function derivation unit in the second embodiment. FIG. 8 is a flowchart illustrating the flow of processing in a receiver in the second embodiment.

[0015] An embodiment of the present invention will be described below with reference to the drawings. (First Embodiment) Fig. 1 is a diagram showing an example of the configuration of a digital coherent optical transmission system 1 according to the first embodiment. The digital coherent optical transmission system 1 includes a transmitter 10, a WDM multiplexer 20, an optical fiber transmission line 30, a WDM demultiplexer 40, and a receiver 50. The transmitter 10 transmits polarization multiplexed signals that are spatially multiplexed by M (M≧2). The spatially M-multiplexed polarization multiplexed signals are transmitted to the receiver 50 via, for example, a multicore fiber or a multimode fiber. The receiver 50 receives the spatially M-multiplexed polarization multiplexed signals from the transmitter 10. Hereinafter, because polarization multiplexing and spatial multiplexing can be treated equivalently in signal processing, it is assumed that polarization multiplexing and spatial multiplexing are performed simultaneously, and N is defined as N=2M, and it is considered that the signal is N-multiplexed overall (if polarization multiplexing is not performed, N=M should be read as such).

[0016] The WDM multiplexer 20 multiplexes the optical signals output from each transmitter 100 and outputs the multiplexed signal to the optical fiber transmission line 30. The optical fiber transmission line 30 is provided with any number of optical amplifiers 31. Each optical amplifier 31 receives an optical signal from the optical fiber transmission line 30 on the transmitter 10 side, amplifies the signal, and outputs the signal to the optical fiber transmission line 30 on the receiver 50 side. The WDM demultiplexer 40 demultiplexes the optical signals transmitted through the optical fiber transmission line 30 according to wavelength.

[0017] First, the configuration of the transmitter 10 will be described. The transmitter 10 has a plurality of transmitting units 100. In this embodiment, the transmitter 10 has the same number of transmitting units 100 as the number of WDM (Wavelength Division Multiplexing) channels. Each transmitting unit 100 outputs an optical signal of a different wavelength.

[0018] The transmitter 100 includes a digital signal processing unit 110, N integrated modules 120, and a spatial multiplexing unit 130. The digital signal processing unit 110 performs digital signal processing on the transmission bit string. Each integrated module 120 generates a polarization multiplexed signal of a different mode using a signal after digital signal processing by the digital signal processing unit 110. Each integrated module 120 outputs the generated polarization multiplexed signal to the spatial multiplexing unit 130. The spatial multiplexing unit 130 spatially multiplexes the polarization multiplexed signals of different modes output from each integrated module 120. The spatial multiplexing unit 130 outputs the spatially multiplexed polarization multiplexed signal to the WDM multiplexer 20.

[0019] Next, the configuration of the receiver 50 will be described. The receiver 50 has a plurality of receiving units 500. In this embodiment, the transmitter 10 has as many receiving units 500 as there are WDM channels. Each receiving unit 500 receives an optical signal of a different wavelength.

[0020] The receiver 500 includes a spatial demultiplexer 510, N optical front-end units 520, and a digital signal processor 530. The spatial demultiplexer 510 demultiplexes the polarization multiplexed signal demultiplexed by wavelength by the WDM demultiplexer 40 into a different mode. Each optical front-end unit 520 receives a polarization multiplexed signal of a different mode. Using the input polarization multiplexed signal, each optical front-end unit 520 generates electrical signals of I and Q components of the X and Y polarizations. In this case, the optical front-end unit 520 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization multiplexed signal. The digital signal processor 530 restores the received bit stream by performing digital signal processing on the electrical signals of I and Q components of the X and Y polarizations generated by each optical front-end unit 520.

[0021] Although the above embodiment describes an example of a single optical fiber transmission line, the same applies to a spatially multiplexed transmission system (for example, a multi-core fiber, a multi-mode fiber, and free space transmission).

[0022] The detailed configurations of the transmitting unit 100 and the receiving unit 500 will be described using FIG. 2. FIG. 2 is a diagram showing an example of the configuration of the transmitting unit 100 and the receiving unit 500 in the first embodiment. The multiple transmitting units 100 included in the transmitter 10 have the same configuration except for the wavelengths used as light sources, and therefore a light source outputting light of wavelength λ1 will be described as an example. The multiple receiving units 500 included in the receiver 50 have the same configuration except for the wavelengths used as local oscillation light sources, and therefore a local oscillation light source outputting light of wavelength λ1 will be described as an example. Note that FIG. 2 shows only one transmitting unit 100 and one receiving unit 500, and the WDM multiplexer 20, optical fiber transmission line 30, and WDM demultiplexer 40 provided between the transmitter 10 and the receiver 50 are omitted.

[0023] First, a description will be given of the configuration of the transmitting section 100. The digital signal processing section 110 includes an encoding section 111, a mapping section 112, a training signal inserting section 113, a frequency changing section 114, a waveform shaping section 115, and a pre-equalization section .

[0024] The encoding unit 111 performs FEC (forward error correction) encoding on the transmission bit string and outputs the resulting transmission signal. The mapping unit 112 maps the transmission signal output from the encoding unit 111 to symbols. The training signal insertion unit 113 inserts a known training signal into the transmission signal symbol-mapped by the mapping unit 112.

[0025] The frequency changer 114 performs upsampling by changing the sampling frequency of the transmission signal into which the training signal has been inserted. The waveform shaping unit 115 limits the band of the sampled transmission signal. The pre-equalization unit 116 compensates for distortion in the waveform of the transmission signal band-limited by the waveform shaping unit 115.

[0026] The integrated module 120 includes digital-to-analog converters (DACs) 121-1 to 121-4, a modulator driver 131, a light source 140, IQ modulators 150-1 and 150-2, and a polarization combining unit 160. The DAC 121-1 converts the I (in-phase) component of the X polarization of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the signal to the modulator driver 131. The DAC 121-2 converts the Q (quadrature) component of the X polarization of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131. The DAC 121-3 converts the I component of the Y polarization of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131. The DAC 121 - 4 converts the Q component of the Y polarization of the transmission signal input from the pre-equalization unit 116 from a digital signal to an analog signal, and outputs the analog signal to the modulator driver 131 .

[0027] The modulator driver 131 has amplifiers 131-1 to 131-4. The amplifier 131-i (i is an integer between 1 and 4) amplifies the analog signal output from the DAC 121-i and drives the IQ modulators 150-1 and 150-2 with the amplified analog signal. The light source 140 is, for example, an LD (semiconductor laser). The light source 140 in the j-th transmitter 100 outputs light of wavelength λj (j is an integer between 1 and the number of WDM channels).

[0028] The IQ modulator 150-1 outputs an X-polarized optical signal generated by modulating the optical signal output by the light source 140 with the I component of the X-polarized wave output by the amplifier 131-1 and the Q component of the X-polarized wave output by the amplifier 131-2. The IQ modulator 150-2 outputs a Y-polarized optical signal generated by modulating the optical signal output by the light source 140 with the I component of the Y-polarized wave output by the amplifier 131-3 and the Q component of the Y-polarized wave output by the amplifier 131-4. The polarization combining unit 160 generates a polarization multiplexed signal by polarization multiplexing the X-polarized optical signal output by the IQ modulator 150-1 and the Y-polarized optical signal output by the IQ modulator 150-2. The polarization combining unit 160 outputs the generated polarization multiplexed signal to the spatial multiplexing unit 130.

[0029] Next, we will explain the configuration of the receiving unit 500. The optical front-end unit 520 includes a local oscillation light source 521, a polarization splitter 522, optical 90-degree hybrid couplers 523-1 and 523-2, BPDs (Balanced Photo Diodes) 524-1 to 524-4, amplifiers 525-1 to 525-4, and analog-to-digital converters (ADCs) 526-1 to 526-4.

[0030] The local oscillation light source 521 is, for example, an LD. The local oscillation light source 521 outputs a local oscillation light (LO). The polarization separation unit 522 separates the input polarization multiplexed signal into an X-polarized optical signal and a Y-polarized optical signal. The polarization separation unit 522 outputs the X-polarized optical signal to the optical 90-degree hybrid coupler 523-1 and outputs the Y-polarized optical signal to the optical 90-degree hybrid coupler 523-2.

[0031] The optical 90-degree hybrid coupler 523-1 causes interference between the X-polarized optical signal and the local oscillator light output from the local oscillator light source 521, and extracts an I-component optical signal and a Q-component optical signal from the received optical field. The optical 90-degree hybrid coupler 523-1 outputs the extracted X-polarized I-component optical signal and Q-component optical signal to the BPDs 524-1 and 524-2.

[0032] The optical 90-degree hybrid coupler 523-2 causes interference between the Y-polarized optical signal and the local oscillation light output from the local oscillation light source 521, and extracts the I and Q components of the received optical field. The optical 90-degree hybrid coupler 523-2 outputs the extracted I and Q components of the Y-polarized wave to the BPD 524-3 and the BPD 524-4.

[0033] BPDs 524-1 to 524-4 are differential input photoelectric converters. BPD 524-i outputs the difference between the photocurrents generated in two photodiodes with matching characteristics to amplifier 525-i. BPD 524-1 converts the I component of the received signal of X polarization into an electrical signal and outputs it to amplifier 525-1. BPD 524-2 converts the Q component of the received signal of X polarization into an electrical signal and outputs it to amplifier 525-2. BPD 524-3 converts the I component of the received signal of Y polarization into an electrical signal and outputs it to amplifier 525-3. BPD 524-4 converts the Q component of the received signal of Y polarization into an electrical signal and outputs it to amplifier 525-4. The amplifier 525-i (i is an integer of 1 to 4) amplifies the electrical signal output from the BPD 524-i and outputs it to the ADC 526-i (i is an integer of 1 to 4).

[0034] The ADC 526 - i (i is an integer between 1 and 4) converts the electrical signal output from the amplifier 525 - i from an analog signal to a digital signal, and outputs the digital signal to the digital signal processing unit 530 .

[0035] The digital signal processing unit 530 includes a front-end correction unit 531 , a chromatic dispersion compensation unit 532 , an adaptive equalization unit 533 , a frequency phase offset compensation unit 534 , a demapping unit 535 , and a decoding unit 536 .

[0036] The front-end correction unit 531 receives the I component of the X-polarized received signal from the ADC 526-1, the Q component of the X-polarized received signal from the ADC 526-2, the I component of the Y-polarized received signal from the ADC 526-3, and the Q component of the Y-polarized received signal from the ADC 526-4. The front-end correction unit 531 uses the received signals to generate a received signal that has been compensated for the frequency characteristics in the optical front-end unit 520, and outputs the generated signal to the chromatic dispersion compensation unit 532.

[0037] The chromatic dispersion compensator 532 estimates the chromatic dispersion suffered in the optical fiber transmission line 30, compensates for the estimated chromatic dispersion on the electrical signal output from the front-end corrector 531, and outputs the signal to the adaptive equalizer 533. The adaptive equalizer 533 adaptively equalizes the received signal output from the chromatic dispersion compensator 532. The adaptive equalizer 533 outputs the received signal after equalization to the frequency phase offset compensator 534. The frequency phase offset compensator 534 performs processes such as compensation for frequency offset and phase noise on the received signal after equalization by the adaptive equalizer 533.

[0038] The demapping unit 535 determines the symbols of the received signal output by the frequency phase offset compensating unit 534 and converts the determined symbols into binary data. The decoding unit 536 performs error correction decoding processing such as FEC on the binary data demapped by the demapping unit 535 to obtain a received bit string.

[0039] Fig. 3 is a diagram illustrating an overview of the processing performed by the adaptive equalizer 533 in the first embodiment. Fig. 4 is a diagram illustrating a detailed configuration example including the adaptive equalizer 533 in the first embodiment. In Fig. 3, the adaptive equalizer 533 includes an adaptive filter unit 540 and a MIMO carrier phase recovery unit 541. In the first embodiment, the adaptive equalizer 533 performs adaptive equalization filtering by the adaptive filter unit 540, which has an increased internal degree of freedom, in combination with the MIMO carrier phase recovery unit 541, thereby compensating for IQ waveform distortion occurring in signals in the transmitter 10 or the receiver 50.

[0040] In FIG. 3 and FIG. 4, S Ｒ１ ,S ＊ Ｒ１ ,…,S ＲN ,S ＊ ＲN represents the received signal in an N-multiplexed system, with each signal component and its complex conjugate as each vector component, and S ＲＴ１ ,S ＊ Ｔ１ ,…,S ＴN ,S ＊ ＴNrepresents a transmission signal in which each signal component and its complex conjugate are used as vector components in an N-multiplexed system. Ｒ１ ,…,θ ＲN represents the relative phase of the receiver, and θ Ｔ１ ,…,θ ＴN represents the relative phase on the transmitting side.

[0041] Although omitted in FIG. 3, the received signal S Ｒ１ ,S ＊ Ｒ１ ,…,S ＲN ,S ＊ ＲN is input to a chromatic dispersion compensator 532 (not shown in FIG. 3) located before the adaptive equalizer 533. The received signal S Ｒ１ ,S ＊ Ｒ１ ,…,S ＲN ,S ＊ ＲN The received signals after chromatic dispersion compensation for the respective components are input to the adaptive equalizer 533. The received signals input to the adaptive equalizer 533 are input to the adaptive filter 540 and the MIMO carrier phase recovery unit 541.

[0042] The adaptive filter unit 540 has a configuration of a 4N×N MIMO adaptive equalization filter. The adaptive filter unit 540 receives an IQ signal and a set of phase conjugate signals of the IQ signal, or a set of signals mathematically equivalent thereto, as input signals, and calculates filter coefficients (h １ ,…,h ４Ｎ２ ) is obtained. ４Ｎ２ The "2" in is a superscript of N.

[0043] The MIMO carrier phase recovery unit 541 calculates the relative phases θ of all light sources on the transmitting side based on the input signal. Ｔ１ ,…,θ ＴN and the relative phase θ of all local light beams at the receiving side Ｒ１ ,…,θ ＲN The MIMO carrier phase recovery unit 541 estimates the receiving side relative phase θ Ｒ１ ,…,θ ＲN and the relative phase θ of the transmitting side Ｔ１ ,…,θ ＴNAn extended Kalman filter may be used as an algorithm for estimating the relative phase. The MIMO carrier phase recovery unit 541 may use the demodulation result output from the adaptive filter unit 540 to estimate the relative phase. The transmitting side relative phase θ estimated by the MIMO carrier phase recovery unit 541 Ｔ１ ,…,θ ＴN and the relative phase θ of the receiver Ｒ１ ,…,θ ＲN is input to the adaptive filter section 540.

[0044] The specific processing will be described with reference to Fig. 4. The wavelength dispersion compensator 532 receives the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN The chromatic dispersion compensator 532 receives the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) is divided into the first path and the second path. The received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) is the inverse characteristic of chromatic dispersion ("h" in FIG. 4) －１ ＣＤ The received signal S split into the second path is subjected to a convolution operation corresponding to Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) is the forward characteristic of chromatic dispersion ("h" in FIG. 4) ＣＤ The chromatic dispersion compensator 532 performs a convolution operation corresponding to the inverse characteristic of chromatic dispersion ("h －１ ＣＤ ") or forward characteristic ("h" in FIG. 4 ＣＤ ) may be mathematically equivalent to the convolution operation

[0045] Next, the wavelength dispersion compensator 532 compensates for the inverse characteristic of wavelength dispersion ("h" in FIG. 4). －１ ＣＤ ") or forward characteristic of chromatic dispersion ("h" in FIG. 4) ＣＤ Received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) are input to the adaptive equalizer 533. The adaptive equalizer 533 receives the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω), the receiving side relative phase obtained by the MIMO carrier phase recovery unit 541 is converted into the time domain (e ｉθＲ１ (t), e －ｉθＲ１ (t), ..., e ｉθＲＮ (t), e －ｉθＲＮ (t)).

[0046] Specifically, the adaptive equalizer 533 performs a convolution operation corresponding to the inverse characteristic of chromatic dispersion on the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) and the relative phase of the receiver (e －ｉθＲ１ (t), ..., e －ｉθＲＮ The adaptive equalizer 533 multiplies the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω) with respect to the receiving side relative phase e －ｉθＲ１ Multiply by (t).

[0047] Furthermore, the adaptive equalizer 533 performs a convolution operation corresponding to the forward characteristic of chromatic dispersion on the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) and the relative phase of the receiver (e ｉθＲ１ (t), ..., eｉθＲＮ The adaptive equalizer 533 multiplies the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω) with respect to the receiving side relative phase e ｉθＲ１ Multiply by (t).

[0048] The adaptive equalizer 533 calculates the relative phase on the receiving side in the time domain (e ｉθＲ１ (t), e －ｉθＲ１ (t), ..., e ｉθＲＮ (t), e －ｉθＲＮ (t)) may be used.

[0049] Next, the adaptive equalizer 533 multiplies the received signal S by the relative phase on the receiving side or performs an equivalent operation. Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω) is applied to a 4N×N MIMO adaptive filter to obtain the impulse response (h １ ~h ４Ｎ２ Finally, the adaptive equalizer 533 performs a convolution operation on the impulse response (h １ ~h ４Ｎ２ ) is convoluted with the received signal S Ｒ１ (ω), S ＊ Ｒ１ (ω),…,S ＲN (ω), S ＊ ＲN (ω), the transmitting side relative phase obtained by the MIMO carrier phase recovery unit 541 is converted into the time domain (e ｉθＴ１ (t), e －ｉθＴ１ (t), ..., e ｉθＴＮ (t), e －ｉθＴＮ The adaptive equalizer 533 multiplies the transmitting side relative phase in the time domain (e ｉθＴ１ (t), e －ｉθＴ１ (t), ..., e ｉθＴＮ (t), e －ｉθＴＮBy performing this operation, when the local oscillator light on the receiving side is not synchronized, it becomes possible to simultaneously compensate for IQ waveform distortions on the transmitting side and receiving side by signal processing on the receiving side.

[0050] The detailed principle will be explained below. In an N-multiplexed system, a transmission signal S is a vector component that is made up of each signal component and its complex conjugate. Ｔ (ω) is expressed as equation (1), and the received signal S Ｒ (ω) is expressed as equation (2). Ｔ (ω) and the received signal S Ｒ (ω) is a 2N-dimensional complex vector.

[0051]

[0052]

[0053] Transmission signal S Ｔ (ω) and the received signal S Ｒ If (ω) is the above equations (1) and (2), the transmission signal S Ｔ (ω) and the received signal S Ｒ The relationship (ω) can be expressed as the following equation (3).

[0054]

[0055] The symbols in equation (3) have the following meanings. Note that, for simplicity, nonlinear effects occurring in the transmitter 10, receiver 50, and fiber transmission line are ignored. FT: Fourier transform H Ｔ (ω): 2N × 2N complex matrix representing the IQ distortion characteristics on the transmitter side. H Ｔｐｈａｓｅ (T): 2N × 2N complex matrix representing the relative phase of the laser light on the transmitting side. H ＣＤ (ω): 2N × 2N complex matrix representing chromatic dispersion in the transmission line. H ＳＯＰ (ω): A 2N × 2N complex matrix that represents the characteristics of linear waveform distortions that occur in the optical domain during transmission other than chromatic dispersion, such as polarization rotation, polarization mode dispersion, and polarization dependent loss in the transmission path. H Ｒｐｈａｓｅ (T): 2N × 2N complex matrix representing the relative phase of the laser light on the receiving side. H Ｒ (ω): 2N × 2N complex matrix representing the IQ distortion characteristics on the receiving side

[0056] The above equation (3) can be transformed into the following equation (4).

[0057]

[0058] A(ω) and B(ω) in equation (4) are 2N×2N complex matrices.

[0059] The adaptive equalizer 533 uses equation (4) to perform an operation equivalent to the above equation for the entire filter by multiplying the received signal and its complex conjugate by the receiving side relative phase after compensating for chromatic dispersion in a 4N×N MIMO configuration in which the received signal and its complex conjugate are used as input signals. Ｒ Instead of (ω), S Ｒ For example, the signal shown in the following equation (5) may be input to the adaptive equalizer 533.

[0060]

[0061] A 4NxN MIMO adaptive filter may operate in the time domain or the frequency domain (see, for example, Non-Patent Document 4). The coefficient update algorithm of the MIMO adaptive filter may be, for example, the least mean squares (LMS) algorithm. Furthermore, in the above-described configuration, all operations related to input light that have undergone convolution operations corresponding to the forward characteristics of chromatic dispersion or equivalent operations may be ignored (calculated by replacing them with 0) (2NxN configuration). In this case, IQ waveform distortion characteristics on either the transmitting or receiving side can be compensated for, but the required amount of calculations can be reduced.

[0062] A 4NxN MIMO adaptive filter is equivalent to a configuration in which filters that individually compensate for transmitter IQ waveform distortion, receiver IQ waveform distortion, and transmission path waveform distortion are applied in a cascade manner. Therefore, in the above-described configuration, instead of the 4NxN configuration, etc., an adaptive filter configured to apply in a cascade manner any one or all of the equivalent filters that individually compensate for transmitter IQ waveform distortion, receiver IQ waveform distortion, and transmission path waveform distortion may be used.

[0063] According to the receiver 50 described above, when an independent local oscillator light is used for each channel, the 4N×N MIMO adaptive equalization filter processing combined with the MIMO carrier phase recovery unit 541 makes it possible to compensate for IQ waveform distortion occurring in the signal at the transmitter 10 or the receiver 50.

[0064] Second Embodiment In the second embodiment, the filter coefficients (h １ ,…,h ４Ｎ２ ) and the relative phase of the transmitter (θ Ｔ１ ,…,θ ＴＮ ) and the receiver relative phase (θ Ｒ１ ,…,θ ＲＮ This paper describes a configuration for estimating the characteristics of the transmitter and receiver when the phase of the local oscillator light on the receiving side is not synchronized, by utilizing the fact that the IQ characteristics of the receiving system (corresponding to a frequency-dependent 2N × 2N matrix) including crosstalk between lanes are contained in the BERT.

[0065] In the second embodiment, the system configuration is the same as in the first embodiment. The second embodiment differs from the first embodiment in that the receiver includes a characteristic function derivation unit that estimates the characteristics of the transmitter and receiver. The following description will focus on the differences.

[0066] 5 is a diagram showing an example of the configuration of the transmitter 100 and receiver 500a according to the second embodiment. The receiver 500a includes a spatial demultiplexer 510, N optical front-end units 520, and a digital signal processor 530a. The digital signal processor 530a includes a front-end corrector 531, a chromatic dispersion compensator 532, an adaptive equalizer 533, a frequency phase offset compensator 534, a demapping unit 535, a decoder 536, and a characteristic function deriving unit 537.

[0067] The characteristic function derivation unit 537 calculates the IQ waveform distortion characteristic H Ｔ －１ (ω) and the IQ waveform distortion characteristic H Ｒ －１ The characteristic function derivation unit 537 derives the derived IQ waveform distortion characteristic HＴ －１ (ω) to the pre-equalization unit 116 of the transmitter 10, and the IQ waveform distortion characteristic H Ｒ －１ (ω) to the front-end correction unit 531. This makes it possible to compensate for IQ waveform distortion with a low amount of calculation using the front-end correction unit 531 and a static digital filter or the like inside the pre-equalization unit 116. At this time, the front-end correction unit 531 and the pre-equalization unit 116 convert the IQ waveform distortion characteristic H Ｒ －１ (ω) and the IQ waveform distortion characteristic H Ｔ －１ (ω) may be converted.

[0068] The pre-equalization unit 116 calculates the IQ waveform distortion characteristic H Ｔ －１ The front-end correction unit 531 compensates for distortion in the waveform of the transmission signal using the IQ waveform distortion characteristic H Ｒ －１ (ω) is used to compensate for the frequency characteristics in the optical front end section 520 .

[0069] 6 is a diagram for explaining an outline of the processing performed by the characteristic function derivation unit 537 in the second embodiment. In the second embodiment, the filter coefficients (h １ ,…,h ４Ｎ２ ), and the transmitting side relative phase (θ Ｔ１ ,…,θ ＴＮ ) and the receiver relative phase (θ Ｒ１ ,…,θ ＲＮ ) contains complete information on the IQ characteristics of the receiving system including crosstalk between lanes (corresponding to a frequency-dependent 2N × 2N matrix), and the characteristic function derivation unit 537 estimates the characteristics of the transmitter 10 and the receiver 50 when the phase of the local oscillator light on the receiving side is not synchronized.

[0070] Although omitted in FIG. 6, the received signal S Ｒ１ ,S＊ Ｒ１ ,…,S ＲN ,S ＊ ＲN is input to a chromatic dispersion compensator 532 (not shown in FIG. 6) located before the adaptive equalizer 533. The chromatic dispersion compensator 532 then outputs a received signal S Ｒ１ ,S ＊ Ｒ１ ,…,S ＲN ,S ＊ ＲN The received signals after chromatic dispersion compensation for the respective components are input to the adaptive equalizer 533. The received signals input to the adaptive equalizer 533 are input to the adaptive filter 540 and the MIMO carrier phase recovery unit 541.

[0071] The characteristic function derivation unit 537 calculates the filter coefficients (h １ ,…,h ４Ｎ２ ), and the transmitting side relative phase (θ Ｔ１ ,…,θ ＴＮ ) and the receiver relative phase (θ Ｒ１ ,…,θ ＲＮ ) and the input filter coefficient (h １ ,…,h ４Ｎ２ ), the relative phase of the transmitter (θ Ｔ１ ,…,θ ＴＮ ) and the receiver relative phase (θ Ｒ１ ,…,θ ＲＮ ) based on the IQ waveform distortion characteristic H Ｔ －１ (ω) and the IQ waveform distortion characteristic H Ｒ －１ Derive (ω).

[0072] The specific processing will be described. The characteristic function derivation unit 537 uses the (h １ ,…,h ４Ｎ２ ) to calculate the matrix A(ω) defined by the following equation (6).

[0073]

[0074] Thereafter, the characteristic function derivation unit 537 calculates the IQ waveform distortion characteristic H on the receiving side based on the following equation (7) using each element of the matrix A(ω). Ｒ －１ Derive (ω).

[0075]

[0076] By the calculation shown in equation (7), the waveform distortion components generated in the transmitter 10 and the transmission path included in the filter coefficients are divided, and as a result, the IQ waveform distortion characteristic H Ｒ －１ Furthermore, the characteristic function derivation unit 537 uses each element of the matrix A(ω) to obtain the IQ waveform distortion characteristic H Ｔ －１ Derive (ω).

[0077]

[0078] By the calculation shown in equation (8), the waveform distortion components generated in the receiver 50 and the transmission path included in the filter coefficients are divided, and as a result, the IQ waveform distortion characteristic H Ｔ －１ (ω) can be obtained. In addition, the IQ waveform distortion characteristic H Ｒ －１ (ω) and the IQ waveform distortion characteristic H Ｔ －１ (ω) is compared with the characteristics inherent to the transmitter 10 and receiver 50 due to the existence of a relative phase, and is expressed as exp(-iθ Ｔ１ ), ..., exp(-iθ ＴＮ ), exp(iθ Ｒ１ ), ..., exp(iθ ＲＮ ) in the time domain. Therefore, the characteristic function derivation unit 537 calculates exp(-iθ Ｔ１ ), ..., exp(-iθ ＴＮ ), exp(iθ Ｒ１ ), ..., exp(iθ ＲＮ ) obtained by the MIMO carrier phase recovery unit 541 is used to correct Ｔ１ ), ..., exp(iθ ＴＮ ), exp(-iθ Ｒ１ ), ..., exp(-iθ ＲＮ) may be folded.

[0079] 7 is a diagram showing an example of the configuration of the characteristic function derivation unit 537 in the second embodiment. In FIG. 7, the characteristic function derivation unit 537 calculates exp(iθ Ｔ１ ), ..., exp(iθ ＴＮ ), exp(-iθ Ｒ１ ), ..., exp(-iθ ＲＮ ) in FIG. －１ represents the formula (7), and Y in FIG. －１ X represents Equation (8), and conv represents the convolution operation. In FIG. 7, the IQ waveform distortion characteristic H Ｒ －１ (ω) is the value obtained by equation (7) expressed as exp(-iθ Ｒ１ ), ..., exp(-iθ ＲＮ 7 shows the configuration calculated by convolving the IQ waveform distortion characteristic H Ｔ －１ (ω) is the value obtained by equation (8) expressed as exp(iθ Ｔ１ ), ..., exp(iθ ＴＮ 7 shows a configuration calculated by convolving exp(-iθ) as described above. Ｔ１ ), ..., exp(-iθ ＴＮ ), exp(iθ Ｒ１ ), ..., exp(iθ ＲＮ ) is convoluted in the time domain.

[0080] The characteristic function derivation unit 537 calculates the filter coefficients (h １ ,…,h ４Ｎ２ ) and exp(iθ) obtained by the MIMO carrier phase recovery unit 541 Ｔ１ ), ..., exp(iθ ＴＮ ), exp(-iθ Ｒ１ ), ..., exp(-iθ ＲＮ ) is input. The characteristic function derivation unit 537 performs calculation processing to obtain the IQ waveform distortion characteristic H Ｔ －１(ω) and the IQ waveform distortion characteristic H Ｒ －１ As in the first embodiment, the signals input to the adaptive filter unit 540 may be signals mathematically equivalent to the IQ signals and the phase conjugate signals of the IQ signals. In this case, the M －１ That is, when the signals input to the adaptive filter unit 540 are IQ signals and phase conjugate signals of the IQ signals, M shown in FIG. －１ There is no need for a basis change operation in the sense of linear algebra, such as that expressed by

[0081] 8 is a flowchart showing the processing flow of the receiver 50 in the second embodiment. The optical front-end unit 520 receives an optical signal (polarization multiplexed signal) transmitted through the optical fiber transmission line 30 (step S101). The functional units in the optical front-end unit 520 perform polarization separation on the received optical signal using the polarization separation unit 522, extraction of the I and Q components of the X polarization using the optical 90-degree hybrid coupler 523-1, extraction of the I and Q components of the Y polarization using the optical 90-degree hybrid coupler 523-2, conversion to electrical signals, and amplification of the electrical signals.

[0082] The ADC 531-i converts the electrical signal output from the amplifier 525-i from an analog signal to a digital signal (step S102). The front-end correction unit 531 uses the input signals to generate a received signal that has been compensated for the frequency characteristics of the optical front-end unit 520 (step S103). The chromatic dispersion compensation unit 532 performs chromatic dispersion compensation on the electrical signal output from the front-end correction unit 531 (step S104).

[0083] The adaptive equalizer 533 performs equalization processing on the received signal output from the chromatic dispersion compensator 532 (step S105). The adaptive equalizer 533 outputs filter coefficients obtained during the equalization processing to the characteristic function derivation unit 537. The characteristic function derivation unit 537 uses the filter coefficients output from the adaptive equalizer 533 to calculate the IQ waveform distortion characteristic H Ｒ －１Furthermore, the characteristic function derivation unit 537 adds the IQ waveform distortion characteristic H Ｔ －１ (ω) is derived (step S106).

[0084] The receiver 50 configured as described above makes it possible to estimate the characteristics of the transmitter 10 and the receiver 50. Therefore, by inputting inverse characteristic functions to the equivalent filters of the transmitter 10 and the receiver 50 to correct communication distortion, a highly efficient and highly reliable optical communication system can be realized.

[0085] (Modification of the Second Embodiment) The characteristic function derivation unit 537 does not need to be provided inside the receiver 50. For example, the characteristic function derivation unit 537 may be provided in an external computer having a communication function with the receiver 50. In this case, the characteristic function derivation unit 537 provided in the external computer may derive the IQ waveform distortion characteristic H Ｔ －１ (ω) and the IQ waveform distortion characteristic H Ｒ －１ The information required to derive (ω) is received from the receiver 50, and the IQ waveform distortion characteristic H Ｔ －１ (ω) and the IQ waveform distortion characteristic H Ｒ －１ The characteristic function derivation unit 537 derives the derived IQ waveform distortion characteristic H Ｔ －１ (ω) is transmitted to the pre-equalization unit 116, and the IQ waveform distortion characteristic H Ｒ －１ (ω) is transmitted to the front-end correction unit 531. This reduces the amount of real-time calculations in the receiver 50.

[0086] Some of the functional units of the receiver 50 in the above-described embodiment may be implemented by a computer. In this case, a program for implementing the functions may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed. Note that the term "computer system" here includes hardware such as an OS and peripheral devices.

[0087] Furthermore, "computer-readable recording media" refers to portable media such as flexible disks, optical magnetic disks, ROMs (Read Only Memory), and CD-ROMs, as well as storage devices such as hard disks built into computer systems. Furthermore, "computer-readable recording media" may also include devices that dynamically store programs for a short period of time, such as communication lines used when transmitting programs over networks like the Internet or communication lines like telephone lines, or devices that store programs for a fixed period of time, such as volatile memory within the computer systems that serve as servers or clients. Furthermore, the programs may be designed to implement some of the aforementioned functions, or may be capable of implementing the aforementioned functions in combination with programs already stored in the computer system, or may be implemented using programmable logic devices such as FPGAs (Field-Programmable Gate Arrays).

[0088] Although an embodiment of the present invention has been described above in detail with reference to the drawings, the specific configuration is not limited to this embodiment, and includes designs within the scope of the gist of the present invention.

[0089] The present invention can be applied to a technique for receiving spatially multiplexed polarization multiplexed signals in digital coherent optical transmission.

[0090] 10... transmitter, 20... WDM multiplexer 20, 30... optical fiber transmission line, 40... WDM demultiplexer, 50... receiver, 100... transmitting unit, 110... digital signal processing unit, 111... encoding unit, 112... mapping unit, 113... training signal insertion unit, 114... frequency change unit, 115... waveform shaping unit, 116... pre-equalization unit, 120... integrated module, 121-1 to 121-4... digital-to-analog converter (DAC), 130... spatial multiplexing unit, 131... modulator driver, 131-1 to 131-4... amplifier, 140... light source, 150-1, 150-2... IQ modulator, 160... polarization combining unit, 500, 500a... receiving unit, 510... spatial demultiplexer, 520...optical front end unit, 521...local oscillation light source, 522...polarization separation unit, 523-1, 523-2...optical 90-degree hybrid couplers, 524-1 to 524-4...BPDs, 525-1 to 525-4...amplifiers, 526-1 to 526-4...analog-to-digital converters, 530, 530a...digital signal processing unit, 531...front end correction unit, 532...chromatic dispersion compensation unit, 533...adaptive equalization unit, 534...frequency phase offset compensation unit, 535...demapping unit, 536...decoding unit, 537...characteristic function derivation unit

Claims

1. A signal processing device for compensating for waveform distortion occurring in an optical signal, comprising: a carrier phase recovery unit that estimates the relative phases of all light sources provided in a transmitter that wavelength-multiplexes and transmits M (M is an integer of 2 or more) spatially multiplexed polarization multiplexed signals, and the relative phases of all local light beams provided in a receiver that receives the wavelength-multiplexed polarization multiplexed signal; and an adaptive equalization unit having an adaptive filter unit that takes as input a pair of a received signal and a phase conjugate signal of the received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal, and multiplies the input signal by the complex conjugate of the relative phases of all light sources provided in the transmitter estimated by the carrier phase recovery unit and the relative phases of all light sources provided in the transmitter, and the complex conjugate of the relative phases of all local light beams provided in the receiver and the relative phases of all local light beams provided in the receiver.

2. The signal processing device according to claim 1, further comprising a characteristic function derivation section that estimates a transmitter characteristic and a receiver characteristic based on the filter coefficients obtained by the adaptive equalization section.

3. The signal processing device according to claim 2, wherein the characteristic function derivation section estimates the characteristics of the transmitter and the characteristics of the receiver based on the filter coefficients, the relative phases of all light sources on the transmitting side, and the relative phases of all local light beams on the receiving side.

4. A signal processing method performed by a signal processing device for compensating for waveform distortion occurring in an optical signal, comprising: estimating the relative phases of all light sources provided in a transmitter that wavelength-multiplexes and transmits M (M is an integer of 2 or more) spatially multiplexed polarization multiplexed signals, and the relative phases of all local light beams provided in a receiver that receives the wavelength-multiplexed polarization multiplexed signals; and using a pair of a received signal and a phase conjugate signal of the received signal, or a plurality of signals mathematically equivalent to the received signal and the phase conjugate signal of the received signal, as input signals, multiplying the input signals by the complex conjugates of the estimated relative phases of all light sources provided in the transmitter and the relative phases of all light sources provided in the transmitter, and the complex conjugates of the relative phases of all local light beams provided in the receiver and the relative phases of all local light beams provided in the receiver.

5. A communication system comprising the signal processing device according to claim 2, and the transmitter and receiver having a filter function for compensating for signal waveform distortion based on the characteristics of the transmitter and the characteristics of the receiver obtained by the signal processing device.

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