Signal processing device, signal processing method, and computer program

Abstract

This signal processing device comprises: a batch adaptive equalization unit that uses a first adaptive filter to subject polarized digital signals obtained via the analog-to-digital conversion of polarized and multiplexed signals to a waveform distortion equalization process; a matrix conversion unit that acquires at least the filter coefficient of the first adaptive filter from batch adaptive equalization unit and converts the acquired filter coefficient by a matrix operation; a degree-of-freedom separation adaptive equalization unit that uses a second adaptive filter including a filter coefficient converted by the matrix conversion unit to subject the polarized digital signals to an equalization process for waveform distortion due to polarization state fluctuation in the transmission path; and a switching control unit that selects a signal subjected to the equalization process by the batch adaptive equalization unit or by the degree-of-freedom separation adaptive equalization unit as a reception signal.　

Description

[Technical Field]

[0001] The present invention relates to a signal processing device, a signal processing method, and a computer program. [Background technology]

[0002] Optical communication using optical fiber (hereinafter referred to as "optical transmission system") has the advantages of a wide available frequency band and little signal attenuation. As a result, optical transmission systems are capable of long-distance, high-capacity communication and are widely used in modern fixed lines. Optical transmission systems achieve highly reliable optical communication by compensating for signal distortions generated in optical transmitters and receivers and optical fiber transmission lines using digital signal processing.

[0003] In recent long-distance, high-capacity optical fiber transmission systems, it is common to adopt polarization multiplexing transmission, which transmits independent signals for each polarization component of light. In polarization multiplexing transmission, dynamic polarization state fluctuations such as polarization rotation and polarization mode dispersion in the optical fiber transmission line cause interference between symbols, which limits the transmission capacity. However, polarization multiplexing transmission can be achieved by compensating for dynamic polarization state fluctuations using 2x2 MIMO (Multiple Input Multiple Output) adaptive filtering in the digital signal processing circuit at the receiving end.

[0004] Increasing the symbol rate of signals is being considered as a means of further improving system capacity. However, in high-symbol-rate transmission, in addition to polarization state fluctuations, IQ waveform distortions occurring within the transmitter and receiver, i.e., distortions caused by relative differences in the characteristics between the in-phase and quadrature components of the signal, become a capacity-limiting factor. Examples of IQ waveform distortions include IQ skew, which is the difference in lane length between the IQ lanes of the transmitter and receiver, amplitude or phase imbalance, which is the difference in amplitude or phase characteristics, and crosstalk between IQ lanes. While it is possible to measure the characteristics of the IQ waveform distortions before the transmitter and receiver are turned on and compensate for them with fixed values, adaptive compensation has practical advantages because the distortions fluctuate due to temperature changes and aging of the transmitter and receiver.

[0005] These IQ waveform distortions can also be compensated for by using adaptive filters with special configurations. Examples of such adaptive filters include a 4x2 MIMO configuration (see, for example, Non-Patent Document 1) and an 8x2 MIMO configuration (see, for example, Patent Document 1). By increasing the internal degrees of freedom of the adaptive part of the adaptive filter, the 4x2 MIMO configuration can compensate for IQ waveform distortion on the receiving side, and the 8x2 MIMO configuration can compensate for IQ waveform distortion on both the transmitting and receiving sides, along with fluctuations in the polarization state. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2020-141294 [Non-patent literature]

[0007] [Non-Patent Document 1] R. Rios-Muller, J. Renaudier, and G. Charlet, “Blind Receiver Skew Compensation and Estimation for Long-Haul Non-Dispersion Managed Systems Using Adaptive Equalizer”, Journal of Lightwave Technology, Vol. 33, no. 7, pp. 1315-1318, 2015. Summary of the Invention [Problem to be solved by the invention]

[0008] However, existing methods that increase the internal degrees of freedom of the filter as described above have the problem of poorer tracking ability to dynamic polarization fluctuations compared to conventional 2x2 MIMO. While IQ waveform distortion does not fluctuate rapidly, it is known that the polarization state fluctuates rapidly in transmission lines made of optical fiber when subjected to electromagnetic or mechanical shocks such as those caused by lightning strikes or facility construction. Therefore, adaptive filters must be able to track such rapid fluctuations in the polarization state due to external factors.

[0009] However, in order to track dynamic polarization fluctuations, it is necessary to increase the step size set when updating the filter coefficients of the adaptive filter. However, in the MIMO structure with increased internal degrees of freedom shown in Non-Patent Document 1 and Patent Document 1, the trace of the covariance matrix of the input signal vector increases. Therefore, there is a problem that the upper limit of the step size at which the filter operates stably decreases, and if a step size exceeding this limit is set, the filter coefficients diverge to infinity. Therefore, conventionally, there has been a problem in that it is not possible to achieve adaptive equalization that can track high-speed dynamic fluctuations in the polarization state while maintaining tolerance to IQ waveform distortion in the transmitter and receiver.

[0010] In view of the above circumstances, the present invention aims to provide a technology capable of adaptive equalization that maintains tolerance to IQ waveform distortion in transmitters and receivers while also being able to track high-speed dynamic fluctuations in the polarization state originating from the transmission path. [Means for solving the problem]

[0011] One aspect of the present invention is a signal processing device comprising: a collective adaptive equalization unit that performs waveform distortion equalization processing on digital signals of each polarization obtained by analog-to-digital conversion of a polarization-multiplexed signal using a first adaptive filter; a matrix transformation unit that acquires filter coefficients of the first adaptive filter from at least the collective adaptive equalization unit and transforms the acquired filter coefficients by matrix calculation; a degree-of-freedom separate adaptive equalization unit that performs waveform distortion equalization processing on digital signals of each polarization caused by fluctuations in the polarization state in a transmission path using a second adaptive filter including the filter coefficients converted by the matrix transformation unit; and a switching control unit that selects, as a received signal, the collective adaptive equalization unit or a signal that has been equalized by the degree-of-freedom separate adaptive equalization unit.

[0012] One aspect of the present invention is a signal processing method that uses a first adaptive filter to perform waveform distortion equalization processing on digital signals of each polarization obtained by analog-to-digital conversion of a polarization-multiplexed signal, obtains at least filter coefficients of the first adaptive filter, converts the obtained filter coefficients by a matrix operation, and uses a second adaptive filter including the converted filter coefficients to perform waveform distortion equalization processing on the digital signals of each polarization due to fluctuations in the polarization state in a transmission path, and selects, as a received signal, a signal that has been equalized using the first adaptive filter or the second adaptive filter.

[0013] One aspect of the present invention is a computer program for causing a computer to execute a process of performing equalization processing of waveform distortion using a first adaptive filter on digital signals of each polarization obtained by analog-to-digital conversion of a polarization-multiplexed signal, obtaining at least filter coefficients of the first adaptive filter, converting the obtained filter coefficients by a matrix operation, performing equalization processing of waveform distortion due to fluctuations in the polarization state in a transmission path on the digital signals of each polarization using a second adaptive filter including the converted filter coefficients, and selecting, as a received signal, a signal that has been subjected to equalization processing using the first adaptive filter or the second adaptive filter. [Effects of the Invention]

[0014] The present invention makes it possible to achieve adaptive equalization that maintains tolerance to IQ waveform distortion in the transmitter and receiver while also being able to follow high-speed dynamic fluctuations in the polarization state originating from the transmission path. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a diagram illustrating an example of the configuration of a digital coherent optical transmission system according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating an example of the configuration of a demodulation digital signal processing unit according to the first embodiment. [Figure 3] FIG. 2 is a diagram for explaining an outline of processing by a demodulation digital signal processing unit in the first embodiment. [Figure 4] 5A to 5C are diagrams illustrating experimental results of a demodulation digital signal processing unit according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) 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 and a receiver 50. The transmitter 10 transmits a polarization multiplexed signal. The receiver 50 receives the polarization multiplexed signal from the transmitter 10.

[0017] First, the configuration of the transmitter 10 will be described. The transmitter 10 has a transmitting unit 100. The transmitting unit 100 outputs an optical signal of a specified wavelength to an optical fiber transmission line 30. The optical fiber transmission line 30 is provided with one or more 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 it to the optical fiber transmission line 30 on the receiver 50 side.

[0018] The transmitting unit 100 includes a digital signal processing unit 110, a modulator driver 120, a light source 130, and an integrated module 140. The digital signal processing unit 110 includes an encoding unit 111, a mapping unit 112, a training signal inserting unit 113, a frequency changing unit 114, a waveform shaping unit 115, a pre-equalization unit 116, and digital-to-analog converters (DACs) 117-1 to 117-4.

[0019] 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. The frequency change unit 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.

[0020] Pre-equalization unit 116 compensates for distortion in the waveform of the transmission signal band-limited by waveform shaping unit 115, and outputs the resulting signal to DACs 117-1 to 117-4. DAC 117-1 converts the I (in-phase) component of the X polarization of the transmission signal input from pre-equalization unit 116 from a digital signal to an analog signal, and outputs the resulting signal to modulator driver 120. DAC 117-2 converts the Q (quadrature) component of the X polarization of the transmission signal input from pre-equalization unit 116 from a digital signal to an analog signal, and outputs the resulting signal to modulator driver 120. DAC 117-3 converts the I component of the Y polarization of the transmission signal input from pre-equalization unit 116 from a digital signal to an analog signal, and outputs the resulting signal to modulator driver 120. DAC 117-4 converts the Q component of the Y polarization of the transmission signal input from pre-equalization unit 116 from a digital signal to an analog signal, and outputs the resulting signal to modulator driver 120.

[0021] The modulator driver 120 has amplifiers 121-1 to 121-4. The amplifier 121-i (i is an integer between 1 and 4) amplifies the analog signal output from the DAC 117-i and drives the modulator of the integrated module 140 with the amplified analog signal. The light source 130 is, for example, an LD (semiconductor laser). The light source 130 outputs light of a specified wavelength.

[0022] The integrated module 140 includes IQ modulators 141-1 and 141-2 and a polarization combining unit 142. The IQ modulator 141-1 outputs an X-polarized optical signal generated by modulating the optical signal output by the light source 130 with the I-component of the X-polarized wave output by the amplifier 121-1 and the Q-component of the X-polarized wave output by the amplifier 121-2. The IQ modulator 141-2 outputs a Y-polarized optical signal generated by modulating the optical signal output by the light source 130 with the I-component of the Y-polarized wave output by the amplifier 121-3 and the Q-component of the Y-polarized wave output by the amplifier 121-4. The polarization combining unit 142 generates a polarization multiplexed signal by polarization multiplexing the X-polarized optical signal output by the IQ modulator 141-1 and the Y-polarized optical signal output by the IQ modulator 141-2. The polarization multiplexing unit 142 outputs the generated polarization multiplexed signal to the optical fiber transmission line 30 .

[0023] Next, the configuration of the receiver 50 will be described. The receiver 50 has a receiving unit 500. The receiving unit 500 receives a polarization multiplexed signal propagated through the optical fiber transmission line 30. The receiving unit 500 includes a local oscillation light source 510, an optical front end 520, and a digital signal processing unit 530. The local oscillation light source 510 is, for example, an LD. The local oscillation light source 510 outputs local oscillation light (LO).

[0024] The optical front end 520 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization multiplexed signal. The optical front end 520 includes a polarization splitter 521, optical 90-degree hybrid couplers 522-1 and 522-2, BPDs (Balanced Photo Diodes) 523-1 to 523-4, and amplifiers 524-1 to 524-4.

[0025] The polarization separation unit 521 separates the input polarization multiplexed signal into an X-polarized optical signal and a Y-polarized optical signal. The polarization separation unit 521 outputs the X-polarized optical signal to the optical 90-degree hybrid coupler 522-1 and outputs the Y-polarized optical signal to the optical 90-degree hybrid coupler 522-2.

[0026] The optical 90-degree hybrid coupler 522-1 causes interference between the X-polarized optical signal and the local oscillator light output from the local oscillator light source 510, and extracts an I-component optical signal and a Q-component optical signal from the received optical field. The optical 90-degree hybrid coupler 522-1 outputs the extracted X-polarized I-component optical signal and Q-component optical signal to the BPDs 523-1 and 523-2.

[0027] The optical 90-degree hybrid coupler 522-2 causes interference between the Y-polarized optical signal and the local oscillation light output from the local oscillation light source 510, and extracts the I and Q components of the received optical field. The optical 90-degree hybrid coupler 522-2 outputs the extracted I and Q components of the Y-polarized wave to the BPDs 523-3 and 523-4.

[0028] BPDs 523-1 to 523-4 are differential input photoelectric converters. BPD 523-i outputs the difference between the photocurrents generated in two photodiodes with matching characteristics to amplifier 524-i. BPD 523-1 converts the I component of the X-polarized received signal into an electric signal and outputs it to amplifier 524-1. BPD 523-2 converts the Q component of the X-polarized received signal into an electric signal and outputs it to amplifier 524-2. BPD 523-3 converts the I component of the Y-polarized received signal into an electric signal and outputs it to amplifier 524-3. BPD 523-4 converts the Q component of the Y-polarized received signal into an electric signal and outputs it to amplifier 524-4. Amplifier 524-i (i is an integer between 1 and 4) amplifies the electric signal output from BPD 523-i and outputs it to digital signal processing unit 530.

[0029] The digital signal processing unit 530 includes analog-to-digital converters (ADCs) 531-1 to 531-4, a demodulation digital signal processing unit 532, a demapping unit 533, and a decoding unit 534.

[0030] The ADC 531 - i (i is an integer between 1 and 4) converts the electrical signal output from the amplifier 524 - i from an analog signal to a digital signal, and outputs the digital signal to the demodulation digital signal processing unit 532 .

[0031] The demodulation digital signal processing unit 532 receives as input the I component of the X-polarized received signal from the ADC 531-1, the Q component of the X-polarized received signal from the ADC 531-2, the I component of the Y-polarized received signal from the ADC 531-3, and the Q component of the Y-polarized received signal from the ADC 531-4. The demodulation digital signal processing unit 532 performs signal processing such as at least equalization processing and compensation for frequency offset and chromatic dispersion on each of the input signals. The demodulation digital signal processing unit 532 is one aspect of a signal processing device.

[0032] The demapping unit 533 determines the symbols of the received signal output by the demodulation digital signal processing unit 532, and converts the determined symbols into binary data.

[0033] The decoding unit 534 performs error correction decoding processing such as FEC on the binary data demapped by the demapping unit 533 to obtain a received bit string.

[0034] 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).

[0035] Next, we will explain the configuration of the demodulation digital signal processing unit 532. Fig. 2 is a diagram showing an example of the configuration of the demodulation digital signal processing unit 532 in the first embodiment. The demodulation digital signal processing unit 532 includes a collective adaptive equalization unit 5321, a matrix transformation unit 5322, a degree-of-freedom separation adaptive equalization unit 5323, and a switching control unit 5324.

[0036] The collective adaptive equalizer 5321 receives as input the digital signals output from the ADC 531-i. The collective adaptive equalizer 5321 adaptively performs equalization processing on each input signal (each digital signal). For example, the collective adaptive equalizer 5321 performs equalization processing on waveform distortion using a first adaptive filter. The first adaptive filter is an adaptive filter used in any of the conventional 2×2 MIMO configurations, 4×2 MIMO configurations, and 8×2 MIMO configurations. The collective adaptive equalizer 5321 outputs filter coefficients obtained in the adaptive equalization processing process to the matrix transformation unit 5322.

[0037] The matrix conversion unit 5322 appropriately converts the transmitter / receiver characteristics obtained by calculation using the filter coefficients output from the collective adaptive equalization unit 5321 into a form applicable to the degree-of-freedom separating adaptive equalization unit 5323, and outputs the converted form to the degree-of-freedom separating adaptive equalization unit 5323. Here, the form applicable to the degree-of-freedom separating adaptive equalization unit 5323 refers to the determinants of equations (9) to (11) described below. That is, the matrix conversion unit 5322 converts the transmitter / receiver characteristics obtained by calculation using the filter coefficients output from the collective adaptive equalization unit 5321 into the determinants of equations (9) to (11) described below. In this way, the matrix conversion unit 5322 acquires, from at least the collective adaptive equalization unit 5321, the filter coefficients of the adaptive filter used by the collective adaptive equalization unit 5321, and converts the acquired filter coefficients by matrix calculation.

[0038] The degree-of-freedom separating adaptive equalizer 5323 receives as input the digital signal output from the ADC 531-i and the transmitter / receiver characteristics output from the matrix conversion unit 5322. The degree-of-freedom separating adaptive equalizer 5323 performs adaptive equalization processing on the digital signal, which has high tracking capability and can simultaneously compensate for transmitter / receiver IQ distortion characteristics, using the transmitter / receiver characteristics input from the matrix conversion unit 5322. For example, the degree-of-freedom separating adaptive equalizer 5323 performs equalization processing on the digital signal of each polarization to remove waveform distortion caused by fluctuations in the state of polarization in the transmission path, using a second adaptive filter including filter coefficients converted by the matrix conversion unit 5322.

[0039] The switching control unit 5324 selects either the signal that has been equalized by the collective adaptive equalization unit 5321 or the signal that has been equalized by the degree-of-freedom separation adaptive equalization unit 5323 as the final received signal based on the elapsed time, the quality of the signal after equalization, or a signal from outside the receiver 50, and outputs the selected signal to the demapping unit 533.

[0040] The switching control unit 5324 may select, as the final received signal, either the signal that has been equalized by the collective adaptive equalization unit 5321 or the signal that has been equalized by the degree-of-freedom separation adaptive equalization unit 5323, based on the following conditions, for example: Note that the following is just an example, and the selection may be made by other methods. Basically, to increase the tracking speed for polarization fluctuations, signals that have been equalized by the degree of freedom separation adaptive equalizer 5323 are preferentially selected. Immediately after the start-up of the transmitter / receiver when the filter coefficients have not yet been obtained, the signal that has been equalized by the collective adaptive equalization unit 5321 is selected.

[0041] 3 is a diagram illustrating an overview of the processing of the demodulation digital signal processing unit 532 in the first embodiment. The demodulation digital signal processing unit 532 in the first embodiment expresses the coefficients of the adaptive equalization filter as the product of a matrix representing the degrees of freedom of the polarization state fluctuation and a matrix representing the degrees of freedom of the transmitter / receiver characteristics excluding the polarization state fluctuation, and updates the elements of the matrix representing the degrees of freedom of the polarization state fluctuation. This reduces the effective degrees of freedom during updating and increases the maximum step size for stable operation, making it possible to follow high-speed dynamic fluctuations in the polarization state.

[0042] An 8x2 MIMO configuration operating in the frequency domain is shown as an example in Figure 3. The specific configuration of the 8x2 MIMO configuration operating in the frequency domain is described in Reference 1, so a description thereof will be omitted. (Reference 1: M. Nakamura, T. Kobayashi, F. Hamaoka, and Y. Miyamoto, “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”, in Proceedings of Optical Fiber Communication Conference 2022, San Diego, United States, 2022, paper M3H.1.)

[0043] In Figure 3, S x,in (ω) represents the X-polarized input signal, and S y,in (ω) represents the Y-polarized input signal, and S x,out (ω) represents the output signal of the X polarization, and S y,out (ω) represents the output signal of the Y polarization. The 8×2 MIMO configuration can be expressed as shown in the upper part of Figure 3 (configuration representing the collective adaptive equalization unit). Note that the 4×2 MIMO configuration is obtained by ignoring the lower branch (branch to CD) in the upper part of Figure 3, and S x,out (ω) and S y,out Ignoring the phase conjugate components of (ω) (the second and fourth components of the input vector) is equivalent to 2×2 MIMO. Here, CD represents chromatic dispersion compensation, and Δω, -Δω represent offset frequency compensation of the local oscillator. The collective adaptive equalization unit 5321 is equipped with adaptive filters shown in the following equations (1) and (2).

[0044]

number

[0045]

number

[0046] The adaptive filter H shown in equations (1) and (2) 8×2,1(ω) and adaptive filter H 8×2,2 (ω) can be rewritten as the following equations (3) and (4) through calculation.

[0047]

number

[0048]

number

[0049] Here, P in equation (4) represents a permutation matrix, which is expressed as in equation (5) below.

[0050]

number

[0051] H' shown in equations (3) and (4) SOP (ω) represents a matrix that indicates the degree of freedom of the polarization state fluctuation, and is expressed as the following equation (6).

[0052]

number

[0053] H' shown in equation (4) T (ω) represents a matrix obtained by excluding the degree of freedom representing the polarization state fluctuation from the degrees of freedom of the transmitter 10, and is expressed as the following equation (7).

[0054]

number

[0055] H' shown in equations (3) and (4) R (ω) represents a matrix obtained by excluding the degree of freedom representing the polarization state fluctuation from the degrees of freedom of the receiver 50, and is expressed as the following equation (8).

[0056]

number

[0057] H´ SOP (ω),H´ T (ω) and H´ R Each component of (ω) is H 8×2,1 (ω),H 8×2,2 Using each component of (ω), it can be calculated as shown in the following equations (9) to (11).

[0058]

number

[0059]

number

[0060]

number

[0061] By using the above equations (9) to (11), adaptive equalization can be performed that is both tolerant to IQ waveform distortion and responsive to high-speed polarization fluctuations, as follows:

[0062] 2 and 3, the processing flow of demodulation digital signal processing unit 532 in the first embodiment will be described. Here, a situation is assumed in which high-speed fluctuations in the polarization state may occur due to factors such as lightning strikes or shocks associated with facility construction. First, ADCs 531-1 to 531-4 output digital signals obtained by analog-to-digital conversion of received signals to demodulation digital signal processing unit 532. Demodulation digital signal processing unit 532 receives as input real component XI and imaginary component XQ of the X-polarized received signal and real component YI and imaginary component YQ of the Y-polarized received signal, which have been converted into digital signals by ADCs 531-1 to 531-4.

[0063] The demodulation digital signal processing unit 532 stores the input real component XI, imaginary component XQ, real component YI, and imaginary component YQ in the corresponding buffers. The demodulation digital signal processing unit 532 generates a complex signal S using the real component XI, imaginary component XQ, real component YI, and imaginary component YQ stored in the buffers. x,out and S y,out , and the complex signal S x,out and S y,out The complex conjugate of S * x,out and S * y,out Calculate where S x corresponds to XI+iXQ, and S y corresponds to YI+iYQ. * x,out ,S * y,out Each of them is a complex conjugate signal S * x,out ,S * y,out It is written as follows.

[0064] The demodulation digital signal processor 532 outputs the calculated complex signal S x,out and S y,out , and the complex conjugate signal S * x,out and S * y,out The demodulation digital signal processing unit 532 performs a discrete Fourier transform or a fast Fourier transform of N (N is a natural number) points on each of the complex signals S. As a result, the demodulation digital signal processing unit 532 converts the real and imaginary components of each polarized wave into signals in the frequency domain. That is, the demodulation digital signal processing unit 532 converts the complex signal S x,out and the frequency domain components of S y,out The frequency domain components of S * x,out and the frequency domain components of S * y,out The demodulation digital signal processing unit 532 generates the frequency domain components of the generated complex signal S x,out and the frequency domain components of S y,out The frequency domain components of S * x,out and the frequency domain components of S * y,outInput the frequency domain components of

[0065] The demodulation digital signal processing unit 532 divides each input frequency domain signal and performs calculations to compensate for chromatic dispersion for each frequency domain signal ("CD" in FIG. 3). -1 For each frequency domain signal, a CD is multiplied. -1 The signal multiplied by is output to the collective adaptive equalizer 5321. 8×2,1 The adaptive equalization process is performed at (ω). The signal obtained by multiplying each frequency domain signal by CD is output to the collective adaptive equalizer 5321. 8×2,2 Adaptive equalization processing is performed at (ω). The collective adaptive equalization unit 5321 outputs the filter coefficients obtained in the process of adaptive equalization processing to the matrix conversion unit 5322.

[0066] The matrix conversion unit 5322 calculates transmitter and receiver characteristics using the filter coefficients output from the collective adaptive equalization unit 5321. The matrix conversion unit 5322 appropriately converts the calculated transmitter and receiver characteristics into a form applicable to the degree-of-freedom separating adaptive equalization unit 5323. Specifically, the matrix conversion unit 5322 converts the calculated transmitter and receiver characteristics from equation (9) to equation (11) above. The matrix conversion unit 5322 outputs information on the converted transmitter and receiver characteristics (information from equations (9) to (11)) to the degree-of-freedom separating adaptive equalization unit 5323. The degree-of-freedom separating adaptive equalization unit 5323 uses the transmitter and receiver characteristics input from the matrix conversion unit 5322 to perform adaptive equalization processing on the digital signal that has high tracking capability and can simultaneously compensate for transmitter and receiver IQ distortion characteristics. Based on the elapsed time, the quality of the signal after equalization processing, signals from outside the receiver, etc., the switching control unit 5324 switches between the signal equalized by the batch adaptive equalization unit 5321 and the signal equalized by the degree-of-freedom separation adaptive equalization unit 5323 to be the final received signal, and outputs the signal to the subsequent processing stage of the receiver's digital signal processing.

[0067] The collective adaptive equalization unit 5321 operates an adaptive filter using a conventional 8×2 MIMO configuration (hereinafter referred to as "collective equalization"). The collective adaptive equalization unit 5321 outputs the equalized signal to the switching control unit 5324, and outputs the filter coefficient to the matrix conversion unit 5322. Note that the collective adaptive equalization unit 5321 may stop operating when the degree of freedom separation adaptive equalization unit 5323 is performing signal equalization processing.

[0068] Although the degree of tracking is lower with batch equalization compared to the equalization processing by the degree-of-freedom separation adaptive equalization unit 5323, it is possible to obtain the characteristics of the transmitter and receiver while equalizing fluctuations in the polarization state. For this reason, it is expected that batch equalization will be operated immediately after the transmitter 10 and receiver 50 are started up and the characteristics of the transmitter 10 and receiver 50 have not yet been obtained, or that batch equalization will be operated to obtain the characteristics of the transmitter 10 and receiver 50 in the background while the degree-of-freedom separation adaptive equalization unit 5323 is operating.

[0069] The matrix transformation unit 5322 converts the adaptive filter H obtained by the collective equalization 8×2,1 (ω),H 8×2,2 From each component of (ω), H' is calculated using the above equations (9) to (11). SOP (ω), H´ T (ω), H´ R The matrix transformation unit 5322 calculates each component of (ω). SOP (ω), H´ T (ω), H´ R The information of each component of (ω) is output to the degree of freedom separation adaptive equalization unit 5323. Furthermore, the matrix transformation unit 5322 outputs the information of each component of H' SOP (ω), H´ T (ω), H´ R Based on each component of (ω), H 8×2,1 (ω),H 8×2,2 The matrix transformation unit 5322 calculates each component of the calculated H 8×2,1 (ω),H 8×2,2The information on each component of (ω) is output to the collective adaptive equalization unit 5321. As a result, the matrix conversion unit 5322 converts the filter coefficients into formats that are compatible with the collective adaptive equalization unit 5321 and the degree-of-freedom separating adaptive equalization unit 5323. The collective adaptive equalization unit 5321 and the degree-of-freedom separating adaptive equalization unit 5323 perform equalization processing using the filter coefficients in the formats converted by the matrix conversion unit 5322.

[0070] The degree of freedom separation adaptive equalization unit 5323 increases the update step size and then SOP The degree of freedom separating adaptive equalization unit 5323 operates while adaptively updating the degrees of freedom of (ω) (hereinafter referred to as "degree of freedom separating equalization"). By operating with degree of freedom separating equalization, it is possible to improve the ability to follow polarization fluctuations when an external factor such as a lightning strike is expected. The degree of freedom separating adaptive equalization unit 5323 outputs the signal after equalization processing to the switching control unit 5324, and outputs the filter coefficients to the matrix conversion unit 5322. Note that the degree of freedom separating adaptive equalization unit 5323 may stop operating when the collective adaptive equalization unit 5321 is performing signal equalization processing.

[0071] If the IQ waveform distortion characteristics in the transmitter and receiver change due to the passage of time or aging, H´ T (ω), H´ R In such a case, the degree-of-freedom separating adaptive equalization unit 5323 may update the degree-of-freedom separating adaptive equalization unit 5323 again using information obtained from the collective equalization via the matrix transformation unit 5322. The degree-of-freedom separating adaptive equalization unit 5323 may independently calculate H' by using the gradient descent method or the like. T (ω), H´ R (ω) can be updated. H´ T (ω), H´ R (ω)H´ R The timing for updating (ω) may be after a certain period of time has elapsed. The signal quality after equalization may be monitored and the timing for updating may be determined based on the results. The coefficients in the degree-of-freedom separation equalization may be updated using a gradient descent method or the like.

[0072] FIG. 4 is a diagram showing experimental results of the demodulation digital signal processing unit 532 in the first embodiment. The example shown in Figure 4 shows the results of an experiment conducted under the following conditions: (Conditions for sender) An optical signal with a modulation speed of 128 GBaud, modulation method QPSK (Quadrature Phase Shift Keying), and number of symbols per frame of 65,336 was generated, and polarization disturbance was added to the optical signal using a polarization scrambler at a maximum speed of 11.01 Mrad / s.

[0073] (receiving side conditions) Demodulation was performed using an 8x2 MIMO adaptive filter operating with 2x2, 4x2, and 8x2 degrees of freedom separation equalization in the frequency domain. For the degree of freedom separation MIMO operation, H' was calculated in advance using 50 frames of signals. T (ω), H´ R The signal-to-noise ratio (SNR) was evaluated for each frame, and the lowest SNR among 10 consecutive frames was defined as the measured SNR.

[0074] In the example shown in Figure 4, the FFT size used during demodulation was fixed at 1024, and demodulation was performed with various step sizes for comparison. Generally, if the step size is too small, the SNR is low because it cannot track polarization fluctuations. Increasing the step size improves the SNR, but if it is too large, the filter operation becomes unstable and the SNR begins to decrease again. With the 8x2 MIMO system operating with degree-of-freedom equalization, the step size at the limit where instability occurs can be increased by about twice as much as with conventional 8x2 MIMO. Compared to 2x2 MIMO and 4x2 MIMO configurations, it is possible to compensate for IQ waveform distortion at both the transmitting and receiving ends. Therefore, it can be seen that the proposed configuration, 8x2 MIMO with degree-of-freedom equalization, achieves the highest SNR.

[0075] The receiver 50 configured as described above includes a collective adaptive equalization unit 5321 that uses an adaptive filter to equalize waveform distortion caused by the transmitter / receiver and the transmission path, a degree-of-freedom separating adaptive equalization unit 5323 that equalizes waveform distortion caused by fluctuations in the transmission path, a matrix conversion unit 5322 that acquires filter coefficients of the adaptive filter from the collective adaptive equalization unit 5321, converts the acquired filter coefficients into a format compatible with the degree-of-freedom separating adaptive equalization unit 5323 by matrix calculation, and outputs the converted filter coefficients to the degree-of-freedom separating adaptive equalization unit 5323, and a switching control unit 5324 that selects a signal that has been equalized by either the collective adaptive equalization unit 5321 or the degree-of-freedom separating adaptive equalization unit 5323. This allows the degrees of freedom of the adaptive equalizer to be decomposed into the form of a matrix product, and by appropriately fixing the matrix portion corresponding to the IQ characteristics of the transmitter and receiver in the degree of freedom separation adaptive equalization unit 5323 and adaptively updating the matrix portion representing the polarization state fluctuation components of the transmission path, it becomes possible to achieve adaptive equalization that is resistant to IQ waveform distortion of the transmitter and receiver while also being able to follow high-speed dynamic fluctuations in the polarization state that are mainly caused by the transmission path.

[0076] (Modification 1 of the first embodiment) To reduce the influence of noise, the matrix conversion unit 5322 may average the adaptive equalization filter coefficients obtained at multiple times before performing the matrix calculation.

[0077] (Modification 2 of the first embodiment) The adaptive filters used by the collective adaptive equalization unit 5321 and the degree-of-freedom separation adaptive equalization unit 5323 are filters that operate in the frequency domain, but may also be filters that operate in the time domain.

[0078] (Modification 3 of the first embodiment) In the above-described embodiment, the 8×2 MIMO configuration may be replaced with another adaptive equalization configuration having a function of compensating for IQ waveform distortion in the transmitter and receiver, such as a 4×2 MIMO configuration.

[0079] (Fourth modification of the first embodiment) In the above-described embodiment, the demodulation digital signal processing unit 532 receives the complex signal S x,out ,S y,outand the complex conjugate signal S * x,out ,S * y,out However, the demodulation digital signal processing unit 532 may use the real component XI, the imaginary component XQ, the real component YI, and the imaginary component YQ as they are. In this case, the above-mentioned equations cannot be used as they are, but equivalent equations can be obtained by performing a basis conversion operation corresponding to the vectors and matrices of the above-mentioned equations.

[0080] (Second embodiment) In the second embodiment, a configuration will be described in which, in order to improve the SNR, coefficients corresponding to the degrees of freedom of polarization state fluctuations are updated with low frequency resolution. Note that the second embodiment differs from the first embodiment in the processing performed by the degrees of freedom separation adaptive equalization unit. The differences will be described below.

[0081] When there is signal reflection in the electronic circuitry of the transmitter / receiver, the frequency characteristics of the transmitter / receiver, i.e., H' T (ω), H´ R (ω) is H´ SOP In some cases, the signal has a finer structure in the frequency domain compared to (ω). In that case, to fully compensate for the effect, it is necessary to increase the frequency resolution of the adaptive filter, i.e., to compensate using a signal within a wider time window in the time domain. In an adaptive filter that operates in the frequency domain, this corresponds to increasing the FFT size.

[0082] However, if the time window is made larger, more noise components will enter the time window, resulting in a trade-off in that the SNR of the equalized signal will decrease. T (ω), H´ R While maintaining high frequency resolution of (ω), we can obtain H´ with low frequency resolution. SOPBy updating (ω), the SNR of the signal after equalization is improved. When there is reflection, the required time width may actually be about 10 to 100 times wider. Therefore, for example, the degree of freedom separation adaptive equalization unit 5323 calculates H' with a frequency resolution of at least 1 / 100 to 1 / 10. SOP Update (ω).

[0083] Hereinafter, we consider operating an adaptive filter in the frequency domain. We assume that the gradient descent method is used to update the coefficients in the degree of freedom separation equalization. The update formula in this case can be expressed as the following formula (12).

[0084]

number

[0085] The degree-of-freedom separating adaptive equalizer 5323 performs an inverse Fourier transform on the second term on the right side of equation (12) before updating it, and sets an appropriate time window in the time domain. That is, the degree-of-freedom separating adaptive equalizer 5323 multiplies an appropriate window function such as a step function or a triangular window, and then performs a Fourier transform. This allows H' to be calculated with low frequency resolution. SOP (ω) can be updated. When operating in the time domain, the resolution can be limited by directly multiplying e, s1, and s2 by a window function.

[0086] According to the receiver 50 of the second embodiment configured as described above, the coefficients corresponding to the degree of freedom of the polarization state fluctuation are updated with low frequency resolution, thereby limiting noise components during the update, thereby enabling the SNR to be improved.

[0087] 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.

[0088] 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 over communication lines like telephone lines, or devices that store programs for a fixed period of time, such as volatile memory within a computer system that serves as a server or client in such cases. Furthermore, the program may be one that implements some of the aforementioned functions, or one that can realize the aforementioned functions in combination with a program already stored in the computer system, or one that can be implemented using a programmable logic device such as an FPGA (Field-Programmable Gate Array).

[0089] Although an embodiment of the present invention has been described in detail above 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. [Industrial Applicability]

[0090] The present invention can be applied to a technique for receiving a polarization multiplexed signal in digital coherent optical transmission. [Explanation of symbols]

[0091] 10...Transmitter, 30...Optical fiber transmission line, 50...Receiver, 100...Transmitter, 110...Digital signal processing unit, 111...Encoder, 112...Mapping unit, 113...Training signal insertion unit, 114...Frequency change unit, 115...Waveform shaping unit, 116...Pre-equalization unit, 117-1 to 117-4...Digital-to-analog converter (DAC), 120...Modulator driver, 121-1 to 121-4...Amplifier, 130...Light source, 140...Integrated module, 141-1, 141-2...IQ modulator, 142...Polarization combining unit, 500...Receiver, 510...Local oscillator light source, 520...Optical front end, 521...Polarization separation unit, 522-1, 522-2...Optical 90-degree hybrid coupler, 523-1 to 523-4...BPD, 524-1 to 524-4...Amplifier, 530...Digital signal processing unit, 531-1 to 531-4...Analog-to-digital converter, 532...Demodulation digital signal processing unit, 533...Demapping unit, 534...Decoding unit, 5321...Batch adaptive equalization unit, 5322...Matrix transformation unit, 5323...Degree of freedom separation adaptive equalization unit, 5324...Switching control unit

Claims

1. a collective adaptive equalization unit that performs waveform distortion equalization processing on digital signals of each polarization obtained by analog-to-digital conversion of the polarization multiplexed signal using a first adaptive filter; a matrix transformation unit that acquires filter coefficients of the first adaptive filter from at least the batch adaptive equalization unit and transforms the acquired filter coefficients by matrix calculation; a degree-of-freedom separating adaptive equalization unit that performs equalization processing on the digital signals of each polarization using a second adaptive filter including filter coefficients converted by the matrix conversion unit to remove waveform distortion caused by fluctuations in the polarization state in the transmission path; a switching control unit that selects, as a received signal, a signal that has been equalized by the collective adaptive equalization unit or the degree-of-freedom separation adaptive equalization unit; A signal processing device comprising:

2. 2. The signal processing device according to claim 1, wherein the degree-of-freedom separating adaptive equalization unit performs equalization processing using, as the second adaptive filter, a filter whose filter coefficients are expressed as a product of a matrix representing degrees of freedom of a polarization state variation and a matrix representing degrees of freedom of a transmitter / receiver characteristic excluding the polarization state variation.

3. 3. The signal processing device according to claim 1, wherein the degree-of-freedom separating adaptive equalization unit updates the filter coefficients of the second adaptive filter with low frequency resolution when updating the coefficients corresponding to the degrees of freedom of the polarization state fluctuation.

4. 3. The signal processing device according to claim 1, wherein the switching control unit selects, as the received signal, either the signal that has been equalized by the batch adaptive equalization unit or the signal that has been equalized by the degree-of-freedom separation adaptive equalization unit, based on any one of an elapsed time, a quality of the signal after equalization processing, and a signal from an external source.

5. performing waveform distortion equalization processing on the digital signals of each polarization obtained by analog-to-digital conversion of the polarization multiplexed signal using a first adaptive filter; Acquire filter coefficients of at least the first adaptive filter, and convert the acquired filter coefficients by a matrix operation; performing equalization processing of waveform distortion due to fluctuations in the polarization state in the transmission path on the digital signals of each polarization using a second adaptive filter including the converted filter coefficients; selecting, as the received signal, a signal that has been subjected to equalization processing using the first adaptive filter or equalization processing using the second adaptive filter; Signal processing methods.

6. On the computer, performing waveform distortion equalization processing on the digital signals of each polarization obtained by analog-to-digital conversion of the polarization multiplexed signal using a first adaptive filter; Acquire filter coefficients of at least the first adaptive filter, and convert the acquired filter coefficients by a matrix operation; performing equalization processing of waveform distortion due to fluctuations in the polarization state in the transmission path on the digital signals of each polarization using a second adaptive filter including the converted filter coefficients; A computer program for executing a process of selecting, as a received signal, a signal that has been subjected to equalization processing using the first adaptive filter or equalization processing using the second adaptive filter.

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