Signal processing device, communication system, and signal processing method
Patent Information
- Application Number
- JP2025541279
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2023-08-24
- Publication Date
- 2025-02-27
Abstract
Description
Signal processing device, communication system, and signal processing method
[0001] The present invention relates to a signal processing device, a communication system, and a signal processing method.
[0002] Optical communication has a wide available frequency band and little signal attenuation, making it possible to communicate over long distances and with large capacity. For these reasons, optical communication is widely used for fixed lines. In optical communication, optical single-mode fiber is sometimes used as the optical fiber transmission line.
[0003] In recent years, research into space division multiplexing (SDM) systems has been conducted. In SDM systems, multi-core fiber (MCF), multi-mode fiber (MMF), and multi-core-multimode fiber (MCMMF) are sometimes used as optical fiber transmission lines. SDM systems communicate optical signals using core modes as independent channels.
[0004] Optical signals propagating through an optical fiber transmission line are affected by polarization rotation, polarization mode dispersion, and polarization-dependent loss due to the effects of random birefringence in the core of the optical fiber. The receiver can reduce the effects of waveform distortion that occurs in optical signals propagating through the core through digital signal processing. Such digital signal processing uses a "2x2 MIMO (Multiple-input Multiple-output) adaptive filter" that uses each polarization component as an input and output. Birefringence fluctuates randomly over time due to mechanical factors (e.g., vibration) and electromagnetic factors (e.g., lightning strikes). Therefore, the equalization process must be updated to track these fluctuations.
[0005] Furthermore, in coupled-core multicore fibers and multimode fibers, crosstalk between spatial division multiplexed channels, core dependent loss (CDL), and mode dependent loss (MDL) occur in propagating optical signals. The crosstalk, core dependent loss, and mode dependent loss are reduced by equalization processing using a 2M×2M MIMO (Multiple-input multiple-output) adaptive filter, where "M" represents the number of multiplexed spatial division multiplexed optical signals.
[0006] The time width of the impulse response in a space-division multiplexing channel is significantly longer than that of a single-mode fiber channel. This increases the number of taps required for convolution filtering in the time domain, resulting in an increase in the amount of computation. Therefore, the use of a frequency-domain equalization adaptive filter to perform equalization processing has been considered (see Non-Patent Document 1). In equalization processing using a frequency-domain equalization adaptive filter, the time-domain convolution operation is converted into a simple multiplication in the frequency domain, thereby reducing the amount of computation.
[0007] Furthermore, when adaptive equalization processing is performed in real time, delays in the feedback loop for processing to update the filter coefficients of the adaptive equalization filter (hereinafter referred to as the "update processing feedback loop") become a problem. If the delay is long, the ability to follow time fluctuations such as birefringence in the optical fiber transmission line decreases, and the applicability of the transmitter and receiver to optical fiber transmission lines with significant time fluctuations decreases. Therefore, to shorten the delay, it is necessary to reduce the amount of calculation in the update processing feedback loop.
[0008] Therefore, it has been proposed to execute the optical carrier phase recovery process, which is normally executed in the update processing feedback loop, outside the update processing feedback loop (see Non-Patent Document 2). Here, the optical carrier phase recovery process is executed outside the update processing feedback loop using a training sequence (TS) signal. This reduces the amount of calculation in the update processing feedback loop.
[0009] K. Shibahara et al., "Dense SDM (12-Core × 3-Mode) Transmission Over 527 km With 33.2-ns Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization," in Journal of Lightwave Technology, vol.34, no.1, pp.196-204, 1 Jan.1, 2016, doi: 10.1109 / JLT.2015.2463102.R. Babaee, SO Gharan and M. Bouchard, "Feed-Forward Frequency Domain Equalizer for Fast SOP Transient Tracking in Coherent Optical Modems," in Journal of Lightwave Technology, doi: 10.1109 / JLT.2023.3253383.
[0010] However, this method assumes that the transfer function of the optical fiber transmission line is unitary (i.e., there is no polarization-dependent loss, core-dependent loss, or mode-dependent loss) when calculating the filter coefficients (tap coefficients). Therefore, in long-distance transmission and space-division multiplexing channels where the polarization-dependent loss, core-dependent loss, and mode-dependent loss become large, degradation of transmission quality may occur, narrowing the range of applicability of transmitters and receivers.
[0011] The least mean square (LMS) method is effective even when there are polarization-dependent losses, core-dependent losses, and mode-dependent losses. Therefore, an optical carrier phase recovery process that performs the least mean square method using only a training sequence signal may be performed outside the update process feedback loop.
[0012] However, instead, computationally intensive Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT) are required in the update feedback loop, so if the optical carrier phase recovery process implementing the least mean square method is performed outside the update feedback loop, another delay source, the Fast Fourier Transform and the Inverse Fast Fourier Transform, will be provided in the update feedback loop.
[0013] 7 is a diagram showing an example of the configuration of an adaptive equalizer that performs gradient unconstrained least mean squares algorithm. The adaptive equalizer illustrated in FIG. 7 includes two Fourier transform units, two multipliers, an inverse Fourier transform unit, two resampling units, a deletion unit, a complex conjugate unit, a subtractor, a zero adder, a multiplier, and an adder. In the update processing feedback loop, the fast Fourier transform may be performed after resampling (replication of elements) or before resampling.
[0014] The optical carrier phase recovery process (phase synchronization process) is performed outside the update process feedback loop, and the adaptive equalizer illustrated in Fig. 7 performs the least mean square method in the frequency domain in the update process feedback loop.
[0015] However, since a fast Fourier transform (FFT) and an inverse fast Fourier transform (IFFT) must be performed once each in the update processing feedback loop to compare the training sequence signal and the equalized signal, the adaptive equalization processing cannot be performed with low delay. As such, when there is polarization-dependent loss, core-dependent loss, or mode-dependent loss, there is a problem that the adaptive equalization processing cannot be performed with low delay.
[0016] In view of the above circumstances, the present invention aims to provide a signal processing device, a communication system, and a signal processing method that are capable of performing adaptive equalization processing with low delay even in the presence of polarization-dependent loss, core-dependent loss, or mode-dependent loss.
[0017] One aspect of the present invention is a signal processing device comprising a Fourier transform unit that generates a received signal in the frequency domain, which is the result of a fast Fourier transform of an input signal; an update processing feedback loop that updates filter coefficients based on the complex conjugate of the received signal in the frequency domain, a training sequence signal in the frequency domain, and a step size; a multiplication unit that generates a multiplication result of the received signal in the frequency domain and the filter coefficients; an inverse Fourier transform unit that generates a received signal in the time domain, which is the result of an inverse fast Fourier transform of the multiplication result; a resampling unit that performs resampling on the received signal in the time domain; and a deletion unit that deletes overlapping portions of the received signal in the time domain, which has been resampled.
[0018] One aspect of the present invention is a communication system comprising: a transmitter having a transmission unit that transmits a spatially multiplexed polarization multiplexed signal; a Fourier transform unit that generates a received signal in a frequency domain representation that is a fast Fourier transform result of a signal based on the spatially multiplexed polarization multiplexed signal; an update processing feedback loop that updates filter coefficients based on the complex conjugate of the received signal in the frequency domain representation, a training sequence signal in the frequency domain representation, and a step size; a multiplication unit that generates a multiplication result of the received signal in the frequency domain representation and the filter coefficients; an inverse Fourier transform unit that generates a received signal in a time domain representation that is a result of an inverse fast Fourier transform of the multiplication result; a resampling unit that performs resampling on the received signal in the time domain representation; and a deletion unit that deletes overlapping portions of the resampled received signal in the time domain representation.
[0019] One aspect of the present invention is a signal processing method executed by a signal processing device, the signal processing method including the steps of: generating a received signal in the frequency domain, which is a result of a fast Fourier transform of an input signal; updating filter coefficients based on the complex conjugate of the received signal in the frequency domain, a training sequence signal in the frequency domain, and a step size; generating a result of multiplying the received signal in the frequency domain and the filter coefficients; generating a received signal in the time domain, which is a result of an inverse fast Fourier transform of the multiplication result; performing resampling on the received signal in the time domain; and deleting overlapping portions of the received signal in the time domain after resampling.
[0020] The present invention makes it possible to perform adaptive equalization processing with low delay even in the presence of polarization-dependent loss, core-dependent loss, or mode-dependent loss.
[0021] FIG. 1 is a diagram showing an example of the configuration of a digital coherent optical transmission system in a first embodiment. FIG. 2 is a diagram showing an example of the configuration of an adaptive equalizer in the first embodiment. FIG. 3 is a diagram showing an example of a simulation result of signal quality with respect to filter update delay and interval in the first embodiment. FIG. 4 is a diagram showing an example of the configuration of an adaptive equalizer in a second embodiment. FIG. 5 is a diagram showing an example of the configuration of an adaptive equalizer in a third embodiment. FIG. 6 is a diagram showing an example of the hardware configuration of a digital signal processing unit in each embodiment. FIG. 7 is a diagram showing an example of the configuration of an adaptive equalizer that executes a least mean square method without gradient constraint.
[0022]
[0023] Embodiments of the present invention will be described in detail 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 (communication system) in 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.
[0023] The transmitter 10 includes one or more transmitting units 100. Hereinafter, the transmitter 10 includes the same number of transmitting units 100 as the number of wavelength division multiplexing (WDM) channels. The transmitting units 100 include a digital signal processing unit 110, an integrated module 120, and a spatial multiplexing unit 130.
[0024] The digital signal processing unit 110 includes an encoding unit 111, a mapping unit 112, a training signal insertion unit 113, a sampling frequency changing unit 114, a waveform shaping unit 115, and a pre-equalization unit 116. The integrated module 120 includes digital-to-analog converters 121-1 to 121-4 (DACs), amplifiers 122-1 to 122-4, IQ modulators 124-1 and 124-2, and a polarization combining unit 125.
[0025] The optical fiber transmission line 30 includes an optical fiber. The optical fiber may be a single-mode fiber or a spatially multiplexed transmission system (e.g., a multi-core fiber and a multi-mode fiber). In the following, the optical fiber is referred to as a spatially multiplexed transmission system as an example. The optical fiber transmission line 30 includes any number of optical amplifiers 31. Note that the digital coherent optical transmission system 1 may transmit an optical signal using free space transmission in a part of the optical fiber transmission line 30.
[0026] The receiver 50 includes one or more receiving units 500. As an example, the receiver 50 includes the same number of receiving units 500 as the number of wavelength division multiplexing (WDM) channels. The receiving units 500 include a spatial demultiplexer 510, an optical front end 520, and a digital signal processing unit 530.
[0027] The optical front end 520 includes a local oscillator light source 521, a polarization splitter 522, optical 90-degree hybrid couplers 523-1 to 523-2, balanced photodiodes 524-1 to 524-4 (BPD: Balanced Photo Diodes), amplifiers 525-1 to 525-4, and analog-to-digital converters 526-1 to 526-4 (ADC).
[0028] 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 offset compensation unit 534 , a demapping unit 535 , and a decoding unit 536 .
[0029] Next, the transmitter 10 will be described. The transmitter 10 transmits M (M≧2) spatially multiplexed polarization multiplexed signals (spatially multiplexed polarization multiplexed signals) to the receiver 50 using optical signals. Here, each transmitter 100 outputs optical signals of mutually different wavelengths to the WDM multiplexer 20.
[0030] The encoding unit 111 performs forward error correction (FEC) encoding on the transmission bit string. The encoding unit 111 outputs a transmission signal generated by performing the forward error correction encoding to the mapping unit 112. The mapping unit 112 maps the transmission signal output from the encoding unit 111 to a symbol.
[0031] The training signal inserting unit 113 inserts a known training signal into the transmission signal mapped to symbols by the mapping unit 112. The sampling frequency changing unit 114 changes the sampling frequency of the transmission signal into which the training signal has been inserted. Here, the sampling frequency changing unit 114 performs upsampling on the transmission signal into which the training signal has been inserted. The waveform shaping unit 115 limits the band of the upsampled transmission signal.
[0032] The pre-equalizer 116 compensates for waveform distortion in the transmission signal whose band has been limited by the waveform shaping unit 115. The pre-equalizer 116 outputs the transmission signal whose waveform distortion has been compensated for to the DACs 117-1 to 117-4.
[0033] The DAC 121-1 converts into an analog signal the I component (in-phase component) (real component signal) of the X polarization of the transmission signal (digital signal) input from the pre-equalization unit 116. The DAC 121-1 outputs this analog signal to the amplifier 122-1.
[0034] The DAC 121-2 converts into an analog signal the Q component (quadrature component) (imaginary component signal) of the X-polarized wave of the transmission signal (digital signal) input from the pre-equalization unit 116. The DAC 121-2 outputs this analog signal to the amplifier 122-2.
[0035] The DAC 121-3 converts into an analog signal the I component of the Y polarization of the transmission signal (digital signal) input from the pre-equalization unit 116. The DAC 121-3 outputs this analog signal to the amplifier 122-3.
[0036] The DAC 121-4 converts into an analog signal the Q component of the Y polarization of the transmission signal (digital signal) input from the pre-equalization unit 116. The DAC 121-4 outputs this analog signal to the amplifier 122-4.
[0037] Amplifier 122-i (i is an integer equal to or greater than 1 and equal to or less than 4) amplifies the power of the analog signal output from DAC 121-i. Amplifiers 122-1 to 122-2 use the analog signal whose power has been amplified to drive IQ modulator 124-1 of integrated module 120. Amplifiers 122-3 to 122-4 use the analog signal whose power has been amplified to drive IQ modulator 124-2 of integrated module 120.
[0038] The light source 123 is, for example, a laser diode (LD). The light source 123 provided in the j-th transmitter 100 emits a light beam having a wavelength of “λ ｊ (where the subscript "j" is an integer equal to or greater than 1 and equal to or less than the number of WDM channels) to the IQ modulator 124-1 and the IQ modulator 124-2.
[0039] IQ modulator 124-1 generates an X-polarized optical signal by modulating the light output from light source 123 based on the I component of the X-polarized wave output from amplifier 122-1 and the Q component of the X-polarized wave output from amplifier 122-2. IQ modulator 124-1 outputs the generated X-polarized optical signal to polarization combiner 125.
[0040] IQ modulator 124-2 generates a Y-polarized optical signal by modulating the light output from light source 123 based on the I component of the Y-polarized wave output from amplifier 122-3 and the Q component of the Y-polarized wave output from amplifier 122-4. IQ modulator 124-2 outputs the generated Y-polarized optical signal to polarization combiner 125.
[0041] The polarization combining unit 125 performs polarization combining (polarization multiplexing) on the X-polarized optical signal output from the IQ modulator 124-1 and the Y-polarized optical signal output from the IQ modulator 124-2. The polarization combining unit 125 outputs the polarization-combined optical signal (polarization multiplexed signal) to the spatial multiplexing unit 130.
[0042] 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.
[0043] Next, the WDM multiplexer 20, the optical fiber transmission line 30, and the WDM demultiplexer 40 will be described. The WDM multiplexer 20 multiplexes multiple optical signals (spatially multiplexed polarization multiplexed signals) output from each transmitter 100. The WDM multiplexer 20 outputs the multiplexed optical signal to the optical fiber transmission line 30. The optical fiber transmission line 30 transmits the multiplexed optical signal. Here, the optical signals are input to each optical amplifier 31. Each optical amplifier 31 amplifies the optical power of the input optical signal. Each optical amplifier 31 transmits the optical signal with amplified optical power toward the WDM demultiplexer 40. The WDM demultiplexer 40 demultiplexes the optical signals transmitted through the optical fiber transmission line 30 according to wavelength.
[0044] Next, the receiver 50 will be described. The receiver 50 receives the spatially multiplexed polarization multiplexed signal from the transmitter 10 via the WDM multiplexer 20, the optical fiber transmission line 30, and the WDM demultiplexer 40. Here, each receiver 500 receives the optical signal demultiplexed by the WDM demultiplexer 40. The wavelength of the optical signal received by each receiver 500 is different for each receiver 500.
[0045] The spatial demultiplexer 510 demultiplexes, by mode, the polarization multiplexed signal that has been demultiplexed by wavelength by the WDM demultiplexer 40. Each optical front end 520 acquires a polarization multiplexed signal of a different mode from the spatial demultiplexer 510. The optical front end 520 (coherent receiver) converts the received optical signal into an electrical signal (a received signal of the polarization multiplexed signal) using local oscillator light while maintaining the phase and amplitude of the polarization multiplexed signal.
[0046] The local oscillator light source 521 is, for example, a laser diode (LD), and outputs local oscillator light to optical 90-degree hybrid couplers 523-1 and 523-2.
[0047] The polarization separator 522 separates the received optical signal into an X-polarized optical signal and a Y-polarized optical signal. The polarization separator 522 outputs the X-polarized optical signal to the optical 90-degree hybrid coupler 523-1. The polarization separator 522 outputs the Y-polarized optical signal to the optical 90-degree hybrid coupler 523-2.
[0048] The optical 90-degree hybrid coupler 523-1 extracts the I and Q components of the received optical signal from the optical signal by causing interference between the local oscillation light output from the local oscillation light source 521 and the X-polarized optical signal. The optical 90-degree hybrid coupler 523-1 outputs the extracted I and Q components of the X-polarized wave to balanced photodiodes 524-1 and 524-2.
[0049] The optical 90-degree hybrid coupler 523-2 extracts the I and Q components of the received optical signal from the optical signal by causing interference between the local oscillation light output from the local oscillation light source 521 and the Y-polarized optical signal. The optical 90-degree hybrid coupler 523-2 outputs the extracted I and Q components of the Y-polarized wave to the balanced photodiodes 524-3 and 524-4.
[0050] The balance photodiodes 524-1 to 524-4 are differential input photoelectric converters. The balance photodiode 524-i includes two photodiodes (not shown) with the same characteristics. Each photodiode (not shown) generates a photocurrent. The balance photodiode 524-i outputs a current corresponding to the difference between the photocurrents to the amplifier 525-i.
[0051] The balanced photodiode 524-1 converts the I component of the X polarization of the received optical signal into an electrical signal. The balanced photodiode 524-1 outputs this electrical signal to the amplifier 525-1. The balanced photodiode 524-2 converts the Q component of the X polarization of the received optical signal into an electrical signal. The balanced photodiode 524-2 outputs this electrical signal to the amplifier 525-2. The balanced photodiode 524-3 converts the I component of the Y polarization of the received optical signal into an electrical signal. The balanced photodiode 524-3 outputs this electrical signal to the amplifier 525-3. The balanced photodiode 524-4 converts the Q component of the Y polarization of the received optical signal into an electrical signal. The balanced photodiode 524-4 outputs this electrical signal to the amplifier 525-4.
[0052] The amplifier 525-i amplifies the power of the electrical signal output from the balance photodiode 524-i, and outputs the power-amplified electrical signal to the ADC 526-i.
[0053] The ADC 526-i converts the electrical signal (analog signal) output from the amplifier 525-i into a digital signal, and outputs the digital signal to the front-end correction unit 531.
[0054] The front-end correction unit 531 receives an I component signal (real component signal) "XI" of the X polarization of the received signal from the ADC 526-1. The front-end correction unit 531 receives an Q component signal (imaginary component signal) "XQ" of the X polarization of the received signal from the ADC 526-2. The front-end correction unit 531 receives an I component signal "YI" of the Y polarization of the received signal from the ADC 526-3. The front-end correction unit 531 receives a Q component signal "YQ" of the Y polarization of the received signal from the ADC 526-4.
[0055] The front-end correction unit 531 uses each input component to generate a received signal in which the frequency characteristics in the optical front-end 520 have been compensated for. The front-end correction unit 531 also outputs the received signal in which the frequency characteristics in the optical front-end 520 have been compensated for to the chromatic dispersion compensation unit 532.
[0056] The chromatic dispersion compensator 532 estimates the chromatic dispersion that has occurred in the optical signal in the optical fiber transmission line 30. The chromatic dispersion compensator 532 performs processing to compensate for the estimated chromatic dispersion on the received signal output from the front-end corrector 531. The chromatic dispersion compensator 532 outputs the received signal that has been subjected to processing to compensate for the chromatic dispersion to the adaptive equalizer 533.
[0057] The adaptive equalization unit 533 adaptively performs equalization processing on the received signal output from the chromatic dispersion compensator 532 using an adaptive equalization filter. In frequency domain equalization (FDE), a least mean square method without gradient constraint or a least mean square method with gradient constraint may be performed. In the following, as an example, the least mean square method without gradient constraint is performed.
[0058] The frequency offset compensating unit 534 performs processing to compensate for a frequency offset on the received signal on which equalization processing has been adaptively performed by the adaptive equalization unit 533. The frequency offset compensating unit 534 may also perform processing to compensate for phase noise on the received signal on which equalization processing has been adaptively performed by the adaptive equalization unit 533.
[0059] The demapping unit 535 determines the symbols of the received signal output from the frequency offset compensating unit 534. The demapping unit 535 converts the determined symbols into binary data. The decoding unit 536 performs error correction decoding processing (e.g., forward error correction) on the binary data converted by the demapping unit 535. As a result, the decoding unit 536 generates a received bit string.
[0060] Next, the adaptive equalization unit 533 will be described. Fig. 2 is a diagram showing an example configuration of the adaptive equalization unit 533 in the first embodiment. The adaptive equalization unit 533 includes a Fourier transform unit 51, a multiplication unit 52, an inverse Fourier transform unit 53, a resampling unit 54, a deletion unit 55, a complex conjugate unit 56, a subtraction unit 58, a multiplication unit 59, and an addition unit 60. The adaptive equalization unit 533 may further include a training sequence signal filter 57.
[0061] The adaptive equalization filter update processing feedback loop includes a subtractor 58, a multiplier 59, and an adder 60. The update processing feedback loop may include a multiplier 52. The update processing feedback loop may include a training sequence signal filter 57.
[0062] The Fourier transform unit 51 (acquisition unit) converts the input signal (a signal based on the spatially multiplexed polarization multiplexed signal) from the chromatic dispersion compensator 532 into a received signal represented in the frequency domain. That is, the Fourier transform unit 51 generates a received signal represented in the frequency domain, which is the result of fast Fourier transform of the input signal. The Fourier transform unit 51 outputs the received signal represented in the frequency domain to the multiplier unit 52 and the complex conjugate unit 56.
[0063] The multiplication unit 52 multiplies the received signal represented in the frequency domain by the updated filter coefficient, and outputs the signal resulting from the multiplication of the received signal represented in the frequency domain by the updated filter coefficient (equalization processing result) to the inverse Fourier transform unit 53.
[0064] If adaptive equalization unit 533 does not include training sequence signal filter 57, multiplication unit 52 outputs the signal resulting from multiplying the received signal represented in the frequency domain by the updated filter coefficient to subtraction unit 58. If adaptive equalization unit 533 includes training sequence signal filter 57, multiplication unit 52 outputs the signal resulting from multiplying the received signal represented in the frequency domain by the updated filter coefficient to training sequence signal filter 57.
[0065] The inverse Fourier transform unit 53 performs an inverse Fourier transform on the signal resulting from multiplying the received signal in the frequency domain by the updated filter coefficients, thereby generating a received signal in the time domain (hereinafter referred to as the "time domain signal").
[0066] The resampling unit 54 resamples the time domain signal. The deletion unit 55 deletes overlapping portions of the resampled time domain signal. The deletion unit 55 outputs the received signal, which has been adaptively equalized, to the frequency offset compensation unit 534.
[0067] The complex conjugate unit 56 receives the received signal in the frequency domain from the Fourier transform unit 51. The complex conjugate unit 56 calculates the complex conjugate of the received signal in the frequency domain. The complex conjugate unit 56 outputs the complex conjugate of the received signal in the frequency domain to the multiplication unit 59.
[0068] The training sequence signal filter 57 is a filter with fixed filter coefficients. The training sequence signal filter 57 performs, for example, a fixed convolution filter operation on the signal resulting from multiplying the frequency domain representation of the received signal by the updated filter coefficients. The filter function of the training sequence signal filter 57 is, for example, a function that divides a sine function by its variable (sinc function).
[0069] The training sequence signal is a known signal that is not affected by waveform distortion or noise. The frequency representation of the training sequence signal is determined in advance by Fourier transforming the known signal that is not affected by waveform distortion or noise. The training sequence signal is, for example, a random signal that has been subjected to quadrature amplitude modulation (QAM) or quadrature phase-shift keying (QPSK). The training sequence signal may also be, for example, a constant amplitude zero auto-correlation (CAZAC) sequence signal.
[0070] The signal resulting from multiplying the received signal in the frequency domain by the updated filter coefficient is input to training sequence signal filter 57 from multiplication unit 52 or training sequence signal filter 57. Training sequence signal filter 57 attenuates the signal resulting from multiplying the received signal in the frequency domain by the updated filter coefficient at positions other than the position near where the training sequence signal, which is a known signal, is inserted. Training sequence signal filter 57 outputs the signal resulting from multiplying the received signal in the frequency domain by the updated filter coefficient to subtraction unit 58.
[0071] The subtractor 58 receives the signal resulting from multiplying the received signal represented in the frequency domain by the updated filter coefficient from the multiplier 52 or the training sequence signal filter 57. The subtractor 58 also receives the training sequence signal represented in frequency. The training sequence represented in frequency may be input to the subtractor 58 at a constant interval from the transmitter 10, or may be input to the subtractor 58 in bursts from the transmitter 10.
[0072] The subtraction unit 58 subtracts the frequency domain signal affected by waveform distortion and noise from the training sequence signal represented in frequency. That is, the subtraction unit 58 calculates the difference (subtraction result) between the frequency domain signal affected by waveform distortion and noise and the training sequence signal represented in frequency.
[0073] The multiplication unit 59 receives the complex conjugate of the received signal in the frequency domain representation from the Fourier transform unit 51. The subtraction result is input to the multiplication unit 59 from the subtraction unit 58. The multiplication unit 59 also receives the step size "μ". The step size "μ" is a positive constant. The multiplication unit 59 multiplies the result of the subtraction by the subtraction unit 58 by the complex conjugate of the received signal in the frequency domain representation and the step size "μ". The multiplication unit 59 outputs the result of multiplication of the subtraction result to the addition unit 60.
[0074] The adder 60 adds the filter coefficient of the adaptive equalization filter in the previous step to the result of multiplication of the subtraction result. The adder 60 outputs the result of addition to the previous filter coefficient as an updated filter coefficient to the multiplier 52. In this way, the adaptive equalization unit 533 adaptively performs equalization processing.
[0075] Next, the simulation results will be described. Fig. 3 is a diagram showing an example of the simulation results of signal quality versus filter update delay and interval in the first embodiment. The horizontal axis represents the delay amount (number of FFT blocks) of the update processing feedback loop, "filter update delay." The vertical axis represents the filter coefficient update interval (number of FFT blocks), "filter update interval." The contour lines are lines connecting the signal quality (Q value) of each demodulated received signal.
[0076] In the simulation results illustrated in FIG. 3, the received optical signal, which is a polarization multiplexed QPSK signal with a modulation rate of 1 Gbaud and polarization rotation of 25 kHz, is demodulated by frequency domain equalization with an FFT size of 1024.
[0077] If a delay equivalent to the signal length of, for example, three FFT blocks occurs in the fast Fourier transform, and a delay equivalent to the signal length of, for example, three FFT blocks occurs in the inverse fast Fourier transform, the delay equivalent to the signal length of a total of six FFT blocks is improved, and the Q value is improved according to the update interval.
[0078] As described above, the Fourier transform unit 51 generates a received signal in the frequency domain, which is the result of a fast Fourier transform of the input signal. The input signal is a signal based on a spatially multiplexed polarization multiplexed signal. The update processing feedback loop includes a subtractor 58, a multiplier 59, and an adder 60. The update processing feedback loop may further include a training sequence signal filter 57. The update processing feedback loop updates the filter coefficients of the adaptive equalization filter based on the complex conjugate of the received signal in the frequency domain, the training sequence signal in the frequency domain, and the step size "μ". The multiplier 52 generates a multiplication result of the received signal in the frequency domain and the filter coefficients. The inverse Fourier transform unit 53 generates a received signal in the time domain, which is the result of an inverse fast Fourier transform of the multiplication result. The resampling unit 54 performs resampling on the received signal in the time domain. The deletion unit 55 deletes overlapping portions of the resampled received signal in the time domain.
[0079] In this way, since the training sequence signal in frequency representation is input to the update processing feedback loop, the amount of calculation for the fast Fourier transform and the inverse fast Fourier transform added to the update processing feedback loop is reduced, which makes it possible to perform adaptive equalization processing with low delay even when there is polarization-dependent loss, core-dependent loss, or mode-dependent loss.
[0080] Second Embodiment The second embodiment is mainly different from the first embodiment in that the adaptive equalization unit includes a buffer in the update processing feedback loop. The second embodiment will be described focusing on the differences from the first embodiment.
[0081] 4 is a diagram showing an example of the configuration of an adaptive equalization unit 533a in the second embodiment. The adaptive equalization unit 533a includes a Fourier transform unit 51, a multiplication unit 52, an inverse Fourier transform unit 53, a resampling unit 54, a deletion unit 55, a complex conjugation unit 56, a subtraction unit 58, a multiplication unit 59, an addition unit 60, and a buffer 61. The adaptive equalization unit 533 may further include a training sequence signal filter 57. The buffer 61 may be provided either before or after the complex conjugation unit 56.
[0082] When the buffer 61 is provided before the complex conjugate unit 56, the buffer 61 temporarily stores the received signal represented in the frequency domain. When the buffer 61 is provided after the complex conjugate unit 56, the buffer 61 temporarily stores the complex conjugate of the received signal represented in the frequency domain. In this way, the buffer 61 delays the timing at which the complex conjugate of the received signal represented in the frequency domain is input to the update processing feedback loop. In other words, the buffer 61 adjusts the difference between the delay of the update processing feedback loop and the delay of the inverse Fourier transform unit 53.
[0083] By providing the buffer 61 on the input side of the update processing feedback loop, it is possible to reduce the effect of delays outside the update processing feedback loop relative to the update processing feedback loop. In other words, the mismatch in the amount of delay that occurs when the filter coefficients are calculated in the update processing feedback loop is eliminated, improving the speed at which the optical fiber transmission line 30 follows time fluctuations.
[0084] As described above, the update processing feedback loop includes the subtractor 58, the multiplier 59, and the adder 60. The update processing feedback loop may also include the training sequence signal filter 57. The buffer 61 adjusts the difference between the delay of the update processing feedback loop and the delay of the inverse Fourier transformer 53. This improves the ability to follow time fluctuations in the optical fiber transmission line 30, and makes it possible to perform adaptive equalization processing with low delay even when polarization-dependent loss, core-dependent loss, or mode-dependent loss is present.
[0085] (Third Embodiment) In the third embodiment, the main difference from the second embodiment is that the adaptive equalization unit also includes a buffer outside the update processing feedback loop (main signal system). In the third embodiment, the differences from the second embodiment will be mainly described.
[0086] 5 is a diagram showing an example of the configuration of an adaptive equalization unit 533b in the third embodiment. The adaptive equalization unit 533b includes a Fourier transform unit 51, a multiplication unit 52, an inverse Fourier transform unit 53, a resampling unit 54, a deletion unit 55, a complex conjugation unit 56, a subtraction unit 58, a multiplication unit 59, an addition unit 60, a buffer 61, a buffer 62, and a multiplication unit 63. The adaptive equalization unit 533 may further include a training sequence signal filter 57. The buffer 61 may be provided either before or after the complex conjugation unit 56.
[0087] The Fourier transform unit 51 replicates the input signal in the frequency domain. The Fourier transform unit 51 outputs the received signal in the frequency domain to the complex conjugate unit 56 and the buffer 62. The buffer 62 temporarily stores the received signal in the frequency domain. The effect of delay can be further reduced by determining a delay outside the update processing feedback loop to match the delay of the update processing feedback loop.
[0088] The multiplication unit 63 receives the frequency domain received signal from the buffer 62. The multiplication unit 63 multiplies the frequency domain received signal by the updated filter coefficient. This performs equalization processing on the frequency domain received signal. The multiplication unit 63 outputs a signal (equalization processing result) obtained by multiplying the frequency domain received signal by the updated filter coefficient to the inverse Fourier transform unit 53.
[0089] As described above, the update processing feedback loop includes the multiplication unit 52, the subtraction unit 58, the multiplication unit 59, and the addition unit 60. The update processing feedback loop may also include the training sequence signal filter 57. The buffers 61 and 62 adjust the difference between the delay of the update processing feedback loop and the delay of the inverse Fourier transform unit 53. This further improves the ability to follow time fluctuations in the optical fiber transmission line 30, and makes it possible to perform adaptive equalization processing with low delay even when polarization-dependent loss, core-dependent loss, or mode-dependent loss is present.
[0090] (Hardware Configuration) FIG. 6 is a diagram showing an example of the hardware configuration of the digital signal processing unit 530 (signal processing device) in each embodiment. The digital signal processing unit 530 is realized as software by a processor 101, such as a CPU (Central Processing Unit), executing a program stored in a storage device 103 having a non-volatile recording medium (non-transitory recording medium) and a memory 102. The 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 (Read Only Memory), and a CD-ROM (Compact Disc Read Only Memory), and non-transitory recording media such as a hard disk or a solid-state drive (SSD) built into a computer system. The communication unit 104 executes predetermined communication processing.
[0091] The digital signal processing unit 530 may be realized using hardware (accelerator) including an electronic circuit (electronic circuit or circuitry) using, for example, an LSI (Large Scale Integrated circuit), an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array).
[0092] 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.
[0093] The present invention is applicable to optical transmission systems.
[0094] 1...digital coherent optical transmission system, 5...demodulation digital signal processing unit, 10...transmitter, 20...WDM multiplexer, 30...optical fiber transmission line, 31...optical amplifier, 40...WDM demultiplexer, 50...receiver, 51...Fourier transform unit, 52...multiplication unit, 53...inverse Fourier transform unit, 54...resampling unit, 55...deletion unit, 56...complex conjugation unit, 57...training sequence signal filter, 58...subtraction unit, 59...multiplication unit, 60...addition unit, 61...buffer, 62...buffer, 63...multiplication unit, 100...transmission unit, 101...processor, 102...memory, 103...storage device, 104...communication unit, 110...digital signal processing unit, 111...encoding unit, 112...mapping unit, 113...training signal insertion unit, 114...sampling frequency change unit, 115...waveform shaping unit, 116...pre-equalization unit, 12 0...integrated module, 121-1 to 121-4...digital-analog converters, 122-1 to 122-4...amplifiers, 123...light source, 124-1 to 124-2...IQ modulators, 125...polarization combining unit, 130...spatial combining unit, 500...receiving unit, 510...spatial demultiplexer, 520...optical front end, 521...local oscillation light source, 522...polarization separation unit, 523-1, 523-2...optical 90-degree hybrid photodiodes, 524-1 to 524-4...balanced photodiodes, 525-1 to 525-4...amplifiers, 526-1 to 526-4...analog-to-digital converters, 530...digital signal processing unit, 531...front-end correction unit, 532...chromatic dispersion compensation unit, 533, 533a, 533b...adaptive equalization unit, 534...frequency offset compensation unit, 535...demapping unit, 536...decoding unit
Claims
1. A signal processing device comprising: a Fourier transform unit that generates a received signal in the frequency domain, which is the result of a fast Fourier transform of an input signal; an update processing feedback loop that updates a filter coefficient based on the complex conjugate of the received signal in the frequency domain, a training sequence signal in the frequency domain, and a step size; a multiplication unit that generates a multiplication result of the received signal in the frequency domain and the filter coefficient; an inverse Fourier transform unit that generates a received signal in the time domain, which is the result of an inverse fast Fourier transform of the multiplication result; a resampling unit that performs resampling on the received signal in the time domain; and a deletion unit that deletes overlapping portions of the received signal in the time domain after resampling.
2. The signal processing device according to claim 1, wherein the input signal is a signal based on a spatially multiplexed polarization multiplexed signal.
3. The signal processing device according to claim 1, further comprising a buffer for adjusting the difference between the delay of said update processing feedback loop and the delay of said inverse Fourier transform unit.
4. A communication system comprising: a transmitter having a transmission unit that transmits a spatially multiplexed polarization multiplexed signal; a Fourier transform unit that generates a received signal represented in the frequency domain, which is a fast Fourier transform result of a signal based on the spatially multiplexed polarization multiplexed signal; an update processing feedback loop that updates a filter coefficient based on the complex conjugate of the received signal represented in the frequency domain, a training sequence signal represented in the frequency domain, and a step size; a multiplication unit that generates a multiplication result of the received signal represented in the frequency domain and the filter coefficient; an inverse Fourier transform unit that generates a received signal represented in the time domain, which is a result of an inverse fast Fourier transform of the multiplication result; a resampling unit that performs resampling on the received signal represented in the time domain; and a deletion unit that deletes overlapping portions of the resampled received signal represented in the time domain.
5. A signal processing method executed by a signal processing device, comprising: a step of generating a received signal in the frequency domain, which is a result of a fast Fourier transform of an input signal; a step of updating a filter coefficient based on the complex conjugate of the received signal in the frequency domain, a training sequence signal in the frequency domain, and a step size; a step of generating a multiplication result of the received signal in the frequency domain and the filter coefficient; a step of generating a received signal in the time domain, which is a result of an inverse fast Fourier transform of the multiplication result; a step of performing resampling on the received signal in the time domain; and a step of deleting overlapping portions of the received signal in the time domain after resampling.