Signal processing device and signal processing method

Abstract

One aspect of the present invention pertains to a signal processing device equipped with a number of filter processing units which corresponds to the number of spatial modes of a received optical signal, a total calculation unit for outputting the total of the output from the plurality of filter processing units, an IFFT processing unit for subjecting said total to an inverse Fourier transform, an output signal selection unit for outputting a signal containing only a pre-set prescribed portion of the output from the IFFT processing unit, an error signal output unit for outputting the difference between a desired signal and the output from the output signal selection unit as an error signal, and an FFT processing unit for subjecting the signal containing the error signal to a Fourier transform, wherein: the filter processing unit updates a filter weighting coefficient by using a signal obtained by multiplying only a pre-set prescribed portion of the output from the FFT processing unit and of a signal obtained by subjecting the received optical signal to processing which involves a Fourier transform by one another; and the filter processing unit outputs a signal obtained by multiplying only a pre-set prescribed portion of the filter weighting coefficient and of the received optical signal by one another.

Description

[Technical Field]

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

[0002] In recent years, with the launch of 5G (5th Generation) services, high-definition video service distribution, and the development of IoT (Internet of Things) services, the amount of communication traffic flowing through optical networks has been steadily increasing. Various measures have been implemented in optical networks to meet the demand for increasing communication traffic. Some of these measures can be implemented without changing the structure of the optical fiber used as the transmission path. A specific example of such a measure is the enhancement of the functionality of optical communication system equipment installed in optical network terminal stations. Other examples include the introduction of optical amplifiers and optical switches.

[0003] The optical fibers that form the basis of current high-capacity optical networks are mostly single-mode fibers (SMF), except for short-distance local networks such as LANs (Local Area Networks). Single-mode fibers have a single core within the cladding that serves as the pathway for optical signals. Single-mode fibers are designed to allow only single-mode propagation in wavelength bands such as the C-band and L-band used in high-capacity long-distance optical networks. This makes it possible to stably transmit information reaching several terabits per second over long distances.

[0004] In optical networks such as those mentioned above, digital coherent transmission technology has been commercially introduced in 100 Gbps-class optical transmission equipment. Digital coherent transmission technology combines coherent receiving technology with ultra-high-speed digital signal processing technology. Coherent receiving technology is a receiving technology that detects the interference light between light and local oscillator light on the receiving side. Ultra-high-speed digital signal processing technology reproduces the envelope waveform of the optical signal in the digital domain and performs tasks such as equalizing waveform distortion that occurs in the transmission line and within the transmitter / receiver.

[0005] By using digital coherent transmission technology, it is possible to effectively remove waveform distortion based on the physical mechanisms that cause it. This has led to the realization of optical transceivers that are small, inexpensive, and have low power consumption. The advent of digital coherent transmission technology has made it possible to improve the receiving sensitivity of optical transmission in large-capacity optical networks. Furthermore, digital coherent transmission technology makes it possible to dramatically improve information transmission efficiency by encoding information in the amplitude, phase, and polarization of the optical carrier wave.

[0006] Polarization-multiplexed optical transmission is a specific example of a transmission method using digital coherent transmission technology that transmits information via polarization. Polarization-multiplexed optical transmission uses two orthogonal polarization modes in a single-mode fiber. Polarization-multiplexed optical transmission can transmit different information via orthogonal polarizations. When polarization-multiplexed optical transmission is performed, orthogonal polarizations are mixed in a complex manner along the optical transmission path, causing the orthogonal axes of the polarization modes to fluctuate rapidly. Therefore, tracking such polarizations using optical devices is difficult. Therefore, a polarization-diversity-compatible receiver receives a polarization-multiplexed optical signal containing orthogonal polarizations, converts the received polarization-multiplexed optical signal into a digital signal, and separates each polarization using digital signal processing. This separation process can be modeled as a 2x2 MIMO (Multiple-Input Multiple-Output) system used in wireless communication systems. This enables the extraction of information for each polarization from the separated signal. As a result, communication is established between communication devices.

[0007] Another specific example of a transmission method using digital coherent transmission technology is mode-multiplexed optical transmission, which uses multiple spatial modes (hereinafter referred to as "modes") in a multimode optical fiber. In mode-multiplexed optical transmission, a fiber with a wider core diameter than a single-mode fiber is used as the transmission medium. This makes it possible to excite multiple modes even in existing wavelength bands such as the C band. Therefore, different information can be carried in each mode. In mode-multiplexed optical transmission, as in polarization-multiplexed optical transmission, mode-multiplexed optical signals are mixed in a complex manner during propagation through a multimode optical fiber. A receiver compatible with mode diversity receives the mode-multiplexed optical signals and converts them into digital signals. The optical signals are then demultiplexed using MIMO signal processing on a scale corresponding to the number of excited modes.

[0008] As a more specific example, consider a few-mode fiber that excites two linearly polarized (LP) modes. In a few-mode fiber for two LP modes, the fundamental mode, LP01, and a higher-order mode, LP11, are excited. Furthermore, two degenerate modes of the LP11 mode (referred to as LP11a and LP11b, respectively) and the polarization modes of each mode (referred to as X-polarized and Y-polarized, respectively) are utilized. This allows a few-mode fiber for two LP modes to carry different information in six spatial modes: LP01X, LP01Y, LP11aX, LP11aY, LP11bX, and LP11bY. Therefore, ignoring the nonlinear optical effects of optical fiber, a few-mode fiber for two LP modes can, in principle, achieve a transmission capacity three times that of existing single-mode fibers.

[0009] In this way, by carrying different and independent information on the light propagating in each spatial mode in a multimode optical fiber, the transmission capacity per optical fiber can be increased by the number of spatial modes excited.

[0010] MIMO signal processing requires compensation not only for coupling between spatial modes but also for dispersion, a phenomenon caused by differential delays of signal pulses along the time axis. Dispersion is a phenomenon resulting from differences in group delay between guided modes. Specific examples of dispersion include polarization mode dispersion, which occurs in single-mode optical fibers, and modal dispersion, which occurs in multimode optical fibers. Dispersion generally accumulates over transmission distance. Therefore, MIMO signal processing for optical signals transmitted over long distances requires MIMO signal processing with a finite impulse response (FIR) with a number of multipliers (taps) that sufficiently accommodates the temporal spread of signal pulses due to dispersion. Hereinafter, this type of MIMO signal processing will be referred to as MIMO-FIR signal processing. As such, the number of required taps increases with transmission distance. Therefore, there is a risk that the size of the signal processing circuit will increase with transmission distance.

[0011] Frequency-domain MIMO-FIR signal processing, which performs time-domain signal processing in the frequency domain, is known as an effective method for reducing the scale of signal processing circuits for MIMO-FIR signal processing (see Non-Patent Documents 1 and 2). Frequency-domain MIMO-FIR signal processing is performed based on the fact that circular convolution operations are equivalent to element-product operations in the frequency domain. Frequency-domain MIMO-FIR signal processing effectively enables a reduction in the signal processing scale of MIMO-FIR signal processing by applying processing via fast Fourier transform. MIMO-FIR signal processing makes it possible to simultaneously compensate for the separation of spatial mode coupling, including polarization, and dispersion occurring in the transmission fiber. [Prior art documents] [Non-patent literature]

[0012] [Non-Patent Document 1] Mansour, D., & Gray, A. (1982). Unconstrained frequency-domain adaptive filter. IEEE Transactions on Acoustics, Speech, and Signal Processing, 30(5), 726-734. [Non-patent document 2] Md. Saifuddin Faruk and Kazuro Kikuchi, "Adaptive frequency-domain equalization in digital coherent optical receivers," Opt. Express 19, 12789-12798 (2011) Summary of the Invention [Problem to be solved by the invention]

[0013] In frequency-domain MIMO-FIR signal processing proposed so far (for example, the method described in Non-Patent Document 2), sampled signals on the spatial and time axes are handled independently as digital input signals for fast Fourier transform. For example, an input signal sampled at a 2x oversampling rate is divided into four types: u_ox, u_ex, u_oy, and u_ey, and fast Fourier transform processing is performed on each. u_ox, u_ex, u_oy, and u_ey represent the X-polarized signal at odd sampling timings, the X-polarized signal at even sampling timings, the Y-polarized signal at odd sampling timings, and the Y-polarized signal at even sampling timings, respectively.

[0014] However, dividing the input signal into odd and even sampling timings, as in the above process, does not fully utilize known information such as signal placement in the frequency domain. For example, with the development of digital coherent transmission technology, optical signal spectrum shaping is now commonly performed in current high-speed optical signal generation circuits. In particular, spectral shaping based on Nyquist filtering with a roll-off rate approaching zero enables high-density placement of wavelength-multiplexed signals. Therefore, for signals sampled at an oversampling rate of 1 or higher, independently processing signals with different sampling timings has the disadvantage of losing information about the localization of signal power on the frequency axis.

[0015] In view of the above circumstances, an object of the present invention is to provide a technique that enables a reduction in the signal processing scale of frequency domain MIMO-FIR type signal processing. [Means for solving the problem]

[0016] One aspect of the present invention is a signal processing device comprising: a number of filter processing sections corresponding to the number of spatial modes of a received optical signal; a summation calculation section that outputs the summation of outputs from the plurality of filter processing sections; an IFFT processing section that performs an inverse Fourier transform on the summation; an output signal selection section that outputs a signal that includes only a predetermined portion of the output of the IFFT processing section; an error signal output section that outputs the difference between the output of the output signal selection section and a desired signal as an error signal; and an FFT processing section that performs a Fourier transform on the signal that includes the error signal, wherein the filter processing section updates filter weight coefficients using a signal obtained by multiplying only the predetermined portion of the output of the FFT processing section and a signal obtained by performing processing including a Fourier transform on the received optical signal, and the filter processing section outputs a signal obtained by multiplying only the predetermined portion of the filter weight coefficients and the received optical signal.

[0017] One aspect of the present invention is a signal processing method performed by a signal processing device having filter processing units, the number of which corresponds to the number of spatial modes of a received optical signal, the signal processing method including: outputting a sum of outputs from a plurality of filter processing units; performing an inverse Fourier transform on the sum; outputting an output signal including only a predetermined portion of the output of the inverse Fourier transform; outputting a difference between the output signal and a desired signal as an error signal; performing a Fourier transform on the signal including the error signal; the filter processing unit updates filter weighting coefficients using a signal obtained by multiplying only a predetermined portion of the result of performing the Fourier transform on the signal including the error signal by a signal obtained by performing processing including a Fourier transform on the received optical signal; and the filter processing unit outputs a signal obtained by multiplying only a predetermined portion of the filter weighting coefficients by the received optical signal. [Effects of the Invention]

[0018] According to the present invention, it is possible to reduce the signal processing scale of frequency domain MIMO-FIR type signal processing. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram showing an outline of the present invention. [Figure 2] 1 is a diagram showing an outline of the configuration of a signal processing device 100 according to the present invention. [Figure 3] 1 is a diagram illustrating an outline of the configuration of a filter processing unit 11 in a signal processing device 100. FIG. [Figure 4] FIG. 10 is a diagram illustrating the amount of calculation. [Figure 5] FIG. 10 is a diagram illustrating the effect of reducing the amount of calculation depending on the transmission distance. [Figure 6] 10 is a diagram showing a reduction rate of the amount of calculation according to the present invention. [Figure 7] FIG. 1 is a diagram showing an outline of the configuration of a conventional signal processing device 900. [Figure 8] FIG. 1 is a diagram showing an outline of the configuration of a filter processing unit 92 in a conventional signal processing device 900. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention is a technology applied to an optical signal receiving device that receives an optical signal. The optical signal receiving device includes, for example, an optical front end, an analog-to-digital converter, and a DSP processing unit. The optical front end converts an optical signal that arrives via a transmission path into an analog electrical signal. The analog-to-digital converter converts the analog electrical signal into a digital electrical signal. The DSP processing unit decodes the digital electrical signal by performing DSP (Digital Signal Processing). The DSP processing unit includes, for example, an adaptive filter equalization unit (adaptive filter equalization circuit). The signal processing device of the present invention may be provided in the DSP processing unit as such an adaptive filter equalization unit.

[0021] In the following description, the number of spatial modes is assumed to be D (D is a natural number). For simplicity, the sampling rate for the received optical signal to the digital signal is assumed to be an oversampling rate of 2. However, any oversampling rate of 1 or greater (e.g., fractional sampling) may also be used. The frequency-domain MIMO-FIR signal processing uses the overlap-save method, with a block length of N (N is a natural number) and an overlap rate of 50%. However, these configurations are merely examples, and other configurations may also be applied. Generally, N is a power of 2 greater than 1 that can efficiently perform Fourier transform processing using fast Fourier transform processing, and this assumption is used in the following description. Furthermore, the notation "x_a" indicates that the subscript "a" is added to the lower right of x, and the notation "x^a" indicates that the subscript "a" is added to the upper right of x.

[0022] First, to understand the configuration of the present invention, the configuration of a conventional signal processing device will be described first. Fig. 7 is a diagram showing an outline of the configuration of a conventional signal processing device 900. The signal processing device 900 shown in Fig. 7 is part of a device that performs frequency-domain MIMO-FIR signal processing. The signal processing device 900 performs frequency-domain MIMO-FIR signal processing on input signals u_1(k) to u_D(k). Through this processing, the signal processing device 900 outputs an estimate v_1(k) for spatial channel 1.

[0023] Here, u_i is the received signal for spatial mode i (1≦i≦D). k represents a block number, but if its meaning is clear, its description will be omitted. Signal processing device 900 outputs an estimated value v_i for spatial channel i (1≦i≦D). A signal processing device that performs frequency-domain MIMO-FIR signal processing has as many signal processing devices 900 as shown in FIG. 7, the number of which is i. Since signal processing device 900 has the same configuration regardless of the value of i, the following explanation will be given taking signal processing device 900 for i=1 as an example.

[0024] A parallelization unit 91 is provided for each received signal u_i. The parallelization unit 91_i divides the received signal u_i into u_i^o corresponding to odd-numbered sampling timings and u_i^e corresponding to even-numbered sampling timings. One filter processing unit 92 is provided for each odd-numbered sampling timing of the received signal u_i and one for each even-numbered sampling timing of the received signal u_i. The filter processing unit 92 performs filtering. A summation calculation unit 93 outputs the sum of the output signals from all filter processing units 92.

[0025] The IFFT processing unit 94 performs an inverse Fourier transform on the output signal of the summation calculation unit 93. The processing block size of the inverse Fourier transform in the IFFT processing unit 94 is N / 2. The output signal selection unit 95 saves components with index numbers from 1+N / 4 to N / 2 of the output of the IFFT processing unit 94, and discards other components. The output signal selection unit 95 finally outputs v_1. In general, the saved components are those for which the results of circular convolution and linear convolution match.

[0026] The error signal output unit 96 outputs the difference between the output signal v_1 and the desired signal as an error signal. The zero adder 97 adds N / 4 "0"s to the beginning of the error signal. The FFT processor 98 performs Fourier transform processing of size N / 2 and outputs a frequency domain error signal E_1. The filter processor 92 updates the filter weighting coefficients using the frequency domain error signal E_1.

[0027] Fig. 8 is a diagram showing an outline of the configuration of a filter processing unit 92 in a conventional signal processing device 900. The input signal is u_1^o as an example. The configuration of the filter processing unit 92 shown in Fig. 8 is common to the filter processing units 92_i_1 and 92_i_2 (1≦i≦D) shown in Fig. 7. The filter processing unit 92 performs filtering on the input signal u_1^o. The filter processing unit 92 performs updating of the filter weight coefficient for the input signal u_1^o.

[0028] The FFT processing unit 921 converts the input signal u_1^o into a frequency domain signal. The FFT processing unit 921 performs a Fourier transform of size N / 2. The complex conjugate processing unit 922 converts the frequency domain signal output from the FFT processing unit 921 into a complex conjugate signal. The multiplication processing unit 923 outputs the element product of the complex conjugate signal and the frequency domain error signal E_1. The multiplication processing unit 924 outputs the product of the element product and a step size parameter μ. The update unit 925 updates the filter weight coefficients using the sum of W(k), which is the filter weight coefficient of the previous block number, and the output of the multiplication processing unit 924. The delay processing unit 926 applies a delay to the updated filter weight coefficients. The multiplication processing unit 927 outputs the element product of the updated filter weight coefficients and the output of the FFT processing unit 921. The output of the multiplication processing unit 927 is the output signal of the filter processing unit 92.

[0029] In the above description, an unconstrained frequency domain LMS (Least Mean Square) method (Non-Patent Document 1) is used as an example of the filter coefficient update algorithm of the filter processing unit 92. However, any filter coefficient update algorithm may be applied to the filter processing unit 92. For example, a constrained frequency domain LMS method or a frequency domain RLS (Recursive Least Square) method (Reference) may be applied. Reference: Zhiqun Yang, Jian Zhao, Neng Bai, Ezra Ip, Ting Wang, Zhihong Li, and Guifang Li, "Experimental demonstration of adaptive VFF-RLS-FDE for long-distance mode-division multiplexed transmission," Opt. Express 26, 18362-18367 (2018)

[0030] Next, an overview of the present invention will be described. A signal processing device according to the present invention is applied to FDE (Frequency-Domain Equalization). FIG. 1 is a diagram showing an overview of the present invention. In the conventional FDE technology that uses a 2x oversampled signal as input, as described above, processing is performed separately for signals with odd sampling timings and signals with even sampling timings. Therefore, prior information on the signal band cannot be utilized. The signal processing device of the present invention performs processing by utilizing the characteristics of the signal on the frequency axis. Specifically, as shown in FIG. 1, processing is performed for bands 71 ​​and 73 where a signal exists, but not for band 72 where no signal exists. This configuration makes it possible to reduce the calculation load.

[0031] Next, a signal processing device of the present invention will be described. Fig. 2 is a diagram showing an outline of the configuration of a signal processing device 100 of the present invention. The signal processing device 100 shown in Fig. 2 is part of a device that performs frequency-domain MIMO-FIR signal processing. The signal processing device 100 performs frequency-domain MIMO-FIR signal processing on input signals u_1(k) to u_D(k). Through this processing, the signal processing device 100 outputs an estimated value v_1(k) for spatial channel 1. In this way, the signal processing device 100 outputs an estimated value v_i for spatial channel i (1≦i≦D). A signal processing device that performs frequency-domain MIMO-FIR signal processing has as many signal processing devices 100 shown in Fig. 2 as there are i. Since the signal processing device 100 has the same configuration regardless of the value of i, the following description will be given taking the signal processing device 100 for i=1 as an example.

[0032] A filter processing unit (filter) 11 is provided for each received signal u_i. The filter processing unit 11 performs filtering on the received signal u_i. The summation unit 12 outputs the sum of the output signals from all filter processing units 11. The IFFT processing unit 13 performs inverse Fourier transform processing of a block size N on the output signal of the summation unit 12. The output signal selection unit 14 saves a predetermined portion of the output of the IFFT processing unit 13 and discards the other components. The output signal selection unit 14 finally outputs v_1. The components saved in the output signal selection unit 14 (the predetermined portion of the components to be output) are, for example, components with index numbers selected every (m-1)th from index numbers 1+N / 2 to N where the results of the circular convolution and the linear convolution match (m is the oversampling rate). When the oversampling rate is 2, components with index numbers selected every other are saved in the output signal selection unit 14. In this case, a signal having components with N / 4 index numbers is saved and output as an output signal. For example, when N=64, the output signal selection unit 14 selects and stores only 16 components with index numbers 33, 35, 37, . . . , 63, and outputs them as the output signal v_1.

[0033] The error signal output unit 15 outputs the difference between the output signal v_1 and the desired signal as an error signal. The zero adder 16 adds N / 2 "0"s to the beginning of the error signal. The zero adder 16 further allocates the components of the error signal with index numbers 1 to N / 4 to every other index number. A "0" is allocated to index numbers to which no error signal components are allocated (skipped index numbers). The FFT processor 17 performs Fourier transform processing of size N and outputs a frequency domain error signal E_1. The filter processor 11 updates the filter weighting coefficients using the frequency domain error signal E_1.

[0034] Fig. 3 is a diagram showing an outline of the configuration of the filter processing unit 11 in the signal processing device 100. The input signal is u_1 as an example. The configuration of the filter processing unit 11 shown in Fig. 3 is common to the filter processing units 11_i (1 ≦ i ≦ D) shown in Fig. 2. The filter processing unit 11 performs filtering on the input signal u_1. The filter processing unit 11 performs updating processing on the filter weight coefficients for the input signal u_1.

[0035] The FFT processing unit 111 transforms the input signal u_1 into a frequency domain signal. The FFT processing unit 111 performs a Fourier transform of size N. The complex conjugate processing unit 112 transforms the frequency domain signal output from the FFT processing unit 111 into a complex conjugate signal. The selective multiplication processing unit 113 outputs an element product of the complex conjugate signal and the frequency domain error signal E_1. The frequency domain multiplication processing in the selective multiplication processing unit 113 is selectively performed only on frequency domain signals excluding components with M (M≦N) index numbers. For example, the selective multiplication processing unit 113 performs multiplication processing only on N / 2 signals excluding components with index numbers from 1+N / 4 to 3N / 4. The selective multiplication processing unit 113 does not calculate element products for index numbers from 1+N / 4 to 3N / 4, but represents them using predetermined values. For example, the selective multiplication processing unit 113 sets the value of these element products to 0.

[0036] The selective multiplication unit 114 outputs the product of the element product output from the selective multiplication unit 113 and the step size parameter μ. The update unit 115 updates the filter weight coefficients by using the sum of W(k), which is the filter weight coefficient of the previous block number, and the output of the selective multiplication unit 114. The delay unit 116 imparts a delay to the updated filter weight coefficients.

[0037] The selective multiplication processor 117 outputs the element products of the updated filter weight coefficients and the output of the FFT processor 111. The output of the selective multiplication processor 117 is the output signal of the filter processor 11. The frequency domain multiplication process in the selective multiplication processor 117 is selectively performed only on the frequency domain signal excluding the components of M (M≦N) index numbers. The element products for the components of the excluded M index numbers are not calculated but are represented using predetermined values. For example, the selective multiplication processor 117 sets the values ​​of these element products to 0.

[0038] A supplementary note will be made regarding the selection of M index numbers in the selective multipliers 113, 114, and 117. When a value that asymptotically approaches 0 (e.g., 0.01) is used as the roll-off rate of the Nyquist filter processing in optical signal spectrum shaping, the number of index numbers for which element multiplication is not performed in the selective multipliers is M=N / 2, ranging from 1+N / 4 to 3N / 4. This is because the signal components corresponding to index numbers from 1+N / 4 to 3N / 4 correspond to components outside the signal band in the frequency domain. For this reason, the above-described processing is appropriate for processing a signal that has been spectrum shaped in advance.

[0039] The M indexes that perform the multiplication process are common to the selection type multiplication processors 113, 114, and 117. In the above description, the unconstrained frequency domain LMS method (Non-Patent Document 1) is used as an example of the filter coefficient update algorithm of the filter processor 11. However, any filter coefficient update algorithm may be applied to the filter processor 11. For example, the constrained frequency domain LMS method or the frequency domain RLS method (Reference) may be applied. The same is also applicable to the function of selecting index numbers for performing the multiplication process of each selection type multiplication processor (113, 114, and 117).

[0040] FIG. 4 is a diagram showing the amount of calculation. The column for "Conventional Method" shows the number of multiplication operations and the number of Fourier transform operations (including inverse Fourier transform operations) required for each process (input signal conversion, output calculation, error calculation, and update) in the signal processing device 900 of the conventional technology described with reference to FIGS. 7 and 8. The column for "Proposed Method" shows the number of multiplication operations and the number of Fourier transform operations (including inverse Fourier transform operations) required for each process (input signal conversion, output calculation, error calculation, and update) in the signal processing device 100 described with reference to FIGS. 2 and 3. N is the block size, and D is the number of spatial modes (including polarization). This amount of calculation is the amount for unconstrained FDE-LMS. The amount of calculation shown as the number of multiplications does not include multiplications performed in the Fourier transform and inverse Fourier transform. When converting the value shown as the total to the number per mode or symbol, it is divided by ND / 4. As shown in FIG. 4, the signal processing device 100 of the present invention can reduce the amount of calculation compared to the conventional method. Furthermore, it is possible to reduce the amount of calculation more significantly in a multi-spatial mode multiplexing system.

[0041] Figure 5 shows the effect of reducing the computational complexity according to the transmission distance. As shown in Figure 5, the 4-core coupled fiber achieved a reduction in computational complexity of approximately 16%, and the 12-core coupled fiber achieved a reduction in computational complexity of approximately 35%. The number of complex multiplications was used as a parameter representing the computational complexity, and the number of complex multiplications required for fast (inverse) Fourier transform processing of a signal with block size N (N is a power of 2) was set to N / 2*log2(N). Furthermore, the optical fiber types constituting the optical transmission lines were assumed to be coupled multicore fibers with four cores (transmission line 1) and twelve cores (transmission line 2). The signal symbol rate, oversampling ratio, and spatial mode dispersion coefficient were set to 10 GBaud, 2, and 20 ps / (km)^1 / 2, respectively.

[0042] FIG. 6 is a diagram showing the reduction rate of the amount of calculation according to the present invention. The reduction rate of the amount of calculation is defined as the signal volume required for the proposed method divided by the signal processing scale of the conventional method. The step-like transition in the reduction rate of the amount of calculation indicates the transition of the block length based on the fast Fourier transform processing as a power of 2. It can be seen that the amount of calculation can be reduced by the signal processing device 100 of the present invention in both transmission path 1 and transmission path 2. In particular, when the number of spatial modes D is large, the effect of the reduction rate of the amount of calculation according to the signal processing device 100 of the present invention is significant.

[0043] The above-described processing of the signal processing device 100 may be implemented using a processor such as a CPU (Central Processing Unit) and memory, or may be implemented in hardware. When a processor and memory are used, the processing is implemented by the processor executing a program. When implemented in hardware, all or part of the above-described processing may be implemented using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above-described program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as a flexible disk, a magneto-optical disk, a ROM, a CD-ROM, and a semiconductor storage device (e.g., a solid-state drive (SSD)), as well as storage devices such as a hard disk or semiconductor storage device built into a computer system. The above-described program may be transmitted via a telecommunications line.

[0044] 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]

[0045] The present invention is applicable to optical signal receivers that use frequency domain MIMO-FIR type signal processing. [Explanation of symbols]

[0046] 100... signal processing device, 11... filter processing unit, 12... summation calculation unit, 13... IFFT processing unit, 14... output signal selection unit, 15... error signal output unit, 16... zero addition unit, 17... FFT processing unit, 111... FFT processing unit, 112... complex conjugate processing unit, 113... selection type multiplication processing unit, 114... selection type multiplication processing unit, 115... update unit, 116... delay processing unit, 117... selection type multiplication processing unit

Claims

1. a number of filter processing units corresponding to the number of spatial modes of the received optical signal; a summation calculation unit that outputs the sum of outputs from a plurality of filter processing units; an IFFT processing unit that performs an inverse Fourier transform on the sum; an output signal selection unit that outputs a signal that includes only a predetermined portion of the output of the IFFT processing unit; an error signal output unit that outputs a difference between the output of the output signal selection unit and a desired signal as an error signal; an FFT processing unit that performs a Fourier transform on a signal including the error signal, the filter processing unit updates the filter weight coefficients using a signal obtained by multiplying only a predetermined part of the output of the FFT processing unit and a signal obtained by performing processing including a Fourier transform on the received optical signal; The signal processing device wherein the filter processing unit outputs a signal obtained by multiplying only a predetermined part of the received optical signal by a filter weighting coefficient.

2. The signal processing device according to claim 1 , wherein the filter processing unit multiplies a portion corresponding to a component of a signal band in a frequency domain by the predetermined portion.

3. 3. The signal processing device according to claim 1, wherein the output signal selection unit outputs a signal including components with index numbers every (m-1) when an oversampling rate of the received optical signal is m.

4. A signal processing method performed by a signal processing device having filter processing units, the number of which corresponds to the number of spatial modes of a received optical signal, outputting the sum of the outputs of the plurality of filter processing units; performing an inverse Fourier transform on the sum; outputting an output signal including only a predetermined portion of the output of the inverse Fourier transform; outputting a difference between the output signal and a desired signal as an error signal; performing a Fourier transform on the signal including the error signal; the filter processing unit updates the filter weight coefficients using a signal obtained by multiplying only a predetermined part of a result of performing a Fourier transform on a signal including the error signal and a signal obtained by performing processing including a Fourier transform on the received optical signal; The signal processing method, wherein the filter processing unit outputs a signal obtained by multiplying only a predetermined part of the received optical signal by a filter weighting coefficient.

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