Signal processing method, signal processing device and communication system

Abstract

This signal processing method involves: converting, into frequency domain signals, real components and imaginary components of polarized waves of reception signals having undergone subcarrier multiplexing and polarization multiplexing; selecting frequency domain signals corresponding to subcarriers; performing a first equalization process for inputting, as input signals, the real components and the imaginary components of the polarized waves of the subcarriers having undergone frequency inversion and phase conjugation, multiplying, for each of the subcarriers and each of the polarized waves, a real component and an imaginary component of the polarized wave included in the corresponding input signal by a complex transfer function, then adding the results, and performing inverse transformation from the frequency domain signals to time domain signals; performing a second equalization process for performing frequency inversion on the real components of the polarized waves included in the input signals, performing frequency inversion on the imaginary components and signals of the real components having undergone the complex conjugate, multiplexing signals of the imaginary components having undergone complex conjugate by the complex transfer function, then adding the results, and performing inverse transformation from the frequency domain signals to time domain signals; and adding or subtracting a transmission data bias correction signal to or from a signal obtained by adding up a first addition signal and a second addition signal.　

Description

[Technical Field]

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

[0002] In digital coherent transmission, it is necessary not only to compensate for waveform distortion occurring in the optical fiber transmission line, but also to adaptively compensate for device imperfections in the optical transmitter and receiver. In adaptive equivalent circuits used in general signal processing, compensation is mainly performed for waveform distortion occurring in the transmission line, and compensation for device imperfections in the transmitter and receiver must be performed separately in subsequent signal processing. Therefore, there is a technology that simultaneously compensates for device imperfections in the transmitter and receiver (see, for example, Patent Document 1 and Non-Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2020-141294 [Non-patent literature]

[0004] [Non-Patent Document 1] Takayuki Kobayashi, et. al., “35-Tb / s C-Band Transmission Over 800 km Employing 1-Tb / s PS-64QAM Signals Enhanced by Complex 8 × 2 MIMO Equalizer”, Optical Fiber Communication Conference Postdeadline Papers 2019, Th4B.2 Summary of the Invention [Problem to be solved by the invention]

[0005] The adaptive equalization circuits in Patent Document 1 and Non-Patent Document 1 have a different configuration from 2x2 MIMO (Multiple Input Multiple Output) adaptive equalization circuits with complex inputs and complex outputs that are commonly used in conventional optical communications. In the adaptive equalization circuits in Patent Document 1 and Non-Patent Document 1, the generated tap coefficients and the like have no commonality or compatibility, and the total number of taps increases. As a result, the amount of calculations increases exponentially with the increase in the number of taps. Furthermore, the methods in Patent Document 1 and Non-Patent Document 1 have the problem of being unable to handle multi-carrier signals.

[0006] In view of the above circumstances, an object of the present invention is to provide a technique that can perform equalization processing on multi-carrier signals while reducing the amount of calculation in digital coherent optical transmission. [Means for solving the problem]

[0007] One aspect of the present invention includes a conversion step of converting real and imaginary components of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal into frequency-domain signals; a subcarrier selection step of selecting frequency-domain signals corresponding to subcarriers; a signal input step of inputting as input signals the frequency-domain signals of the real components and the imaginary components of each polarization of each selected subcarrier, and the converted frequency-domain signals obtained by performing frequency inversion with respect to the center frequency of the selected subcarrier on the frequency axis of the frequency-domain signals of the real components and the frequency-domain signals of the imaginary components of each polarization of a pair of subcarriers that are line-symmetric with respect to a DC component and then complex conjugating the converted signals; and a signal input step of applying a complex transfer function to each of the frequency-domain signals of the real components and the frequency-domain signals of the imaginary components of each polarization included in the input signal for each subcarrier and polarization. an equalization step of performing a first equalization process in which a frequency domain signal obtained by converting the real components of each polarization included in the input signal is multiplied and then added to inversely convert the frequency domain signal into a time domain signal; and a second equalization process in which a frequency domain signal obtained by converting the real components of each polarization included in the input signal is multiplied by a complex transfer function and then added to inversely convert the frequency domain signal into a time domain signal; and a compensation step of performing, for each subcarrier and each polarization, a phase rotation for frequency offset compensation on the time domain signal converted by the first equalization process to generate a first sum signal, a phase rotation that is opposite to the phase rotation for frequency offset compensation on the time domain signal converted by the second equalization process to generate a second sum signal, and adding or subtracting a transmit data bias correction signal to or from a signal obtained by summing the first sum signal and the second sum signal.

[0008] One aspect of the present invention includes an addition step of performing an imaginary unit multiplication process of multiplying an imaginary component of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal by an imaginary unit j, and then performing an addition process of adding the imaginary component multiplied by the imaginary unit j to a real component of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal; a conversion step of converting the signal obtained by the addition process of the imaginary component multiplied by the imaginary unit j and the real component into a frequency domain signal; and and for the frequency domain signalThe frequency domain signal after frequency inversion and complex conjugation a first computed frequency domain signal obtained by adding the frequency domain signal after compensation and the frequency domain signal after compensation of the converted frequency domain signal and then multiplying by 1 / 2; a computed frequency domain signal; From the frequency domain signal, The frequency domain signal after frequency inversion and complex conjugation a second computed frequency domain signal obtained by subtracting the frequency domain signal after compensation from the frequency domain signal after compensation has been performed on the converted frequency domain signal and then multiplying by 1 / 2j; a subcarrier selection step of inputting the calculated frequency domain signal and selecting a frequency domain signal corresponding to a subcarrier; a signal input step of inputting the frequency domain signal corresponding to the subcarrier selected in the subcarrier selection step as an input signal, either directly or after compensation; and No. 1 of the computed frequency domain signal and the imaginary component No. 1 A first equalization process in which each of the calculated frequency domain signals is multiplied by a complex transfer function and then added, and the frequency domain signal is inversely converted to a time domain signal; No. 2 of the computed frequency domain signal and the imaginary component No. 2 The signal processing method includes: an equalization step of multiplying each of the calculated frequency domain signals by a complex transfer function, adding the multiplied signals, and performing second equalization processing to inversely convert the frequency domain signals into time domain signals; and a compensation step of, for each subcarrier and each polarization, performing a phase rotation for frequency offset compensation on the time domain signals converted by the first equalization processing to generate a first sum signal, performing a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signals converted by the second equalization processing to generate a second sum signal, and adding or subtracting a transmit data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal.

[0009] One aspect of the present invention includes a frequency conversion unit that converts real and imaginary components of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal into frequency domain signals; a subcarrier selection step that selects frequency domain signals corresponding to subcarriers; a signal input unit that inputs as input signals the frequency domain signals of the real components and the imaginary components of each polarization of each selected subcarrier, and frequency domain signals after conversion in which the frequency domain signals of the real components and the imaginary components of each polarization of a pair of subcarriers that are line-symmetric with respect to a DC component are frequency-inverted with respect to the center frequency of the selected subcarrier on the frequency axis and complex conjugates are taken; and a complex transfer function that applies, for each subcarrier and polarization, to the frequency domain signals of the real components and the frequency domain signals of the imaginary components of each polarization included in the input signal. and a second equalization process that multiplies the frequency domain signals after converting the real components of each polarization included in the input signal by a complex transfer function and then adds the multiplied signals to inversely convert the frequency domain signals into time domain signals. The signal processing device includes: an equalization unit that performs a first equalization process that multiplies the frequency domain signals after converting the real components of each polarization included in the input signal by a complex transfer function and then adds the multiplied signals to inversely convert the frequency domain signals into time domain signals; and a compensation unit that performs, for each subcarrier and each polarization, a phase rotation for frequency offset compensation on the time domain signals converted by the first equalization process to generate a first sum signal, a phase rotation that is opposite to the phase rotation for frequency offset compensation on the time domain signals converted by the second equalization process to generate a second sum signal, and adds or subtracts a transmit data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal.

[0010] According to one aspect of the present invention, there is provided an adder that performs an imaginary unit multiplication process of multiplying an imaginary component of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal by an imaginary unit j, and then performs an addition process of adding the imaginary component multiplied by the imaginary unit j to a real component of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal; a frequency converter that converts the signal obtained by the addition process of the imaginary component multiplied by the imaginary unit j to a frequency domain signal; and and for the frequency domain signalThe frequency domain signal after frequency inversion and complex conjugation a first computed frequency domain signal obtained by adding the frequency domain signal after compensation and the frequency domain signal after compensation of the converted frequency domain signal and then multiplying by 1 / 2; a computed frequency domain signal; From the frequency domain signal, The frequency domain signal after frequency inversion and complex conjugation a second computed frequency domain signal obtained by subtracting the frequency domain signal after compensation from the frequency domain signal after compensation has been performed on the converted frequency domain signal and then multiplying by 1 / 2j; a subcarrier selection unit that receives the calculated frequency domain signal and selects a frequency domain signal corresponding to a subcarrier; a signal input unit that receives the frequency domain signal corresponding to the subcarrier selected by the subcarrier selection unit as an input signal, either directly or after compensation; and a signal input unit that receives the real component of each polarization included in the input signal for each subcarrier and polarization. No. 1 of the computed frequency domain signal and the imaginary component No. 1 a first equalization process in which each of the calculated frequency domain signals is multiplied by a complex transfer function and then added, and the frequency domain signal is inversely converted to a time domain signal; No. 2 of the computed frequency domain signal and the imaginary component No. 2 the signal processing device includes: an equalization unit that performs second equalization processing by multiplying each of the calculated frequency domain signals by a complex transfer function, adding the multiplied signals, and inversely converting the frequency domain signals into time domain signals; and a compensation unit that, for each subcarrier and each polarization, performs a phase rotation for frequency offset compensation on the time domain signals converted by the first equalization processing to generate a first sum signal, performs a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signals converted by the second equalization processing to generate a second sum signal, and adds or subtracts a transmit data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal.

[0011] One aspect of the present invention is a communication system including a transmitter that transmits a polarization multiplexed signal that has been subcarrier multiplexed and polarization multiplexed, and a receiver that has the above-described signal processing device. [Effects of the Invention]

[0012] According to the present invention, it is possible to reduce the amount of calculations and perform equalization processing even on multi-carrier signals in digital coherent optical transmission. [Brief explanation of the drawings]

[0013] [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. 10 illustrates an example of a coefficient calculation unit. [Figure 4] FIG. 10 illustrates an example of a coefficient calculation unit. [Figure 5] FIG. 10 illustrates an example of a coefficient calculation unit. [Figure 6] FIG. 10 illustrates an example of a coefficient calculation unit. [Figure 7] FIG. 10 is a diagram illustrating another example of a subcarrier selection unit. [Figure 8] FIG. 10 is a diagram illustrating another example of a subcarrier selection unit. [Figure 9] FIG. 10 is a diagram illustrating another example of a subcarrier selection unit. [Figure 10] FIG. 10 is a diagram for explaining the effects of the present invention. [Figure 11] FIG. 10 is a diagram for explaining the effects of the present invention. [Figure 12] FIG. 10 is a diagram illustrating an example of the configuration of a demodulation digital signal processing unit in the second embodiment. [Figure 13] FIG. 10 is a diagram illustrating an example of the configuration of a demodulation digital signal processing unit according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. (First embodiment) FIG. 1 is a diagram showing an example of the configuration of a digital coherent optical transmission system 1 according to the first embodiment. The digital coherent optical transmission system 1 includes a transmitter 10 and a receiver 50. The transmitter 10 transmits a polarization multiplexed signal. The polarization multiplexed signal transmitted by the transmitter 10 is a subcarrier-multiplexed signal. The receiver 50 receives the polarization multiplexed signal from the transmitter 10.

[0015] The transmitter 10 has at least one transmitting unit 100. The transmitting unit 100 outputs a polarization multiplexed signal of a specified wavelength to an optical fiber transmission line 30. The optical fiber transmission line 30 is provided with any number of optical amplifiers 31. Each optical amplifier 31 receives a polarization multiplexed 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. The receiver 50 has at least one receiving unit 500. The receiving unit 500 receives the polarization multiplexed signal.

[0016] First, the configuration of the transmitter 10 will be described. 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 conversion unit 114, a waveform shaping unit 115, a subcarrier multiplexing unit 116, a pre-equalization unit 117, and digital-to-analog converters (DACs) 118-1 to 118-4. To perform subcarrier multiplexing, the transmitting unit 100 includes signal generating units 119, each including a mapping unit 112, a training signal inserting unit 113, a frequency conversion unit 114, and a waveform shaping unit 115, the number of which corresponds to the number of subcarriers.

[0017] The encoding unit 111 performs FEC (forward error correction) encoding on the transmission bit string and outputs the resulting transmission signal.

[0018] Mapping section 112 maps the transmission signal output from encoding section 111 to symbols.

[0019] The training signal inserting unit 113 inserts a known training signal into the transmission signal symbol-mapped by the mapping unit 112 .

[0020] The frequency conversion unit 114 performs upsampling by changing the sampling frequency of the transmission signal into which the training signal has been inserted.

[0021] The waveform shaping unit 115 limits the band of the sampled transmission signal.

[0022] The subcarrier multiplexing unit 116 subcarrier-multiplexes the signals generated by the signal generating units 119 .

[0023] Pre-equalization section 117 compensates for distortion in the waveform of the transmission signal subcarrier-multiplexed by subcarrier multiplexing section 116, and outputs the signal to DACs 118-1 to 118-4.

[0024] DAC 118-1 converts the I (in-phase) component of the X polarization of the transmission signal input from pre-equalization unit 117 from a digital signal to an analog signal and outputs the signal to modulator driver 120. DAC 118-2 converts the Q (quadrature) component of the X polarization of the transmission signal input from pre-equalization unit 117 from a digital signal to an analog signal and outputs the signal to modulator driver 120. DAC 118-3 converts the I component of the Y polarization of the transmission signal input from pre-equalization unit 117 from a digital signal to an analog signal and outputs the signal to modulator driver 120. DAC 118-4 converts the Q component of the Y polarization of the transmission signal input from pre-equalization unit 117 from a digital signal to an analog signal and outputs the signal to modulator driver 120.

[0025] 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 118-i, and drives the modulator of the integrated module 140 with the amplified analog signal.

[0026] The light source 130 is, for example, a semiconductor laser (LD), and outputs light of a specified wavelength.

[0027] The integrated module 140 includes IQ modulators 141-1 and 141-2 and a polarization combining unit 142. The IQ modulator 141-1 generates an X-polarized optical signal by modulating the optical signal output by the light source 130 based on 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 generates a Y-polarized optical signal by modulating the optical signal output by the light source 130 based on 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 generated by the IQ modulator 141-1 and the Y-polarized optical signal generated by the IQ modulator 141-2. The polarization multiplexing unit 142 outputs the generated polarization multiplexed signal to the optical fiber transmission line 30 .

[0028] Next, the configuration of the receiver 50 will be described. 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).

[0029] The optical front end 520 converts the optical signal into an electrical signal while maintaining the phase and amplitude of the polarization-multiplexed phase-modulated 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.

[0030] The polarization separator 521 separates the input optical signal into an X-polarized optical signal and a Y-polarized optical signal. The polarization separator 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] Digital signal processing unit 530 includes analog-to-digital converters (ADC) 531-1 to 531-4, demodulation digital signal processing unit 532, demapping unit 533, and decoding unit 534. Note that some functional units that perform signal processing by demodulation digital signal processing unit 532 are provided in the number equal to the number of subcarriers.

[0035] 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 .

[0036] The demodulation digital signal processing unit 532 receives 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 phase noise on each of the input signals. The demodulation digital signal processing unit 532 also performs signal processing such as compensation for frequency characteristics and chromatic dispersion as necessary.

[0037] Whether or not the demodulation digital signal processing unit 532 performs signal processing such as compensation for frequency characteristics and compensation for chromatic dispersion depends on the configuration of the demodulation digital signal processing unit 532. Therefore, this will be explained in detail when explaining the configuration of the demodulation digital signal processing unit 532. The demodulation digital signal processing unit 532 is one aspect of a signal processing device.

[0038] 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.

[0039] 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.

[0040] 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).

[0041] Next, the configuration of the demodulation digital signal processing unit 532 will be described. 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 shown in Fig. 2 performs signal processing such as equalization processing and compensation for frequency offset and phase noise. Note that the demodulation digital signal processing unit 532 shown in Fig. 2 does not perform signal processing such as compensation for frequency characteristics and compensation for chromatic dispersion.

[0042] The demodulation digital signal processing unit 532 includes an adaptive equalization unit 54 and a frequency / phase offset compensation unit 55. The adaptive equalization unit 54 adaptively performs equalization processing on each input signal. The frequency / phase offset compensation unit 55 performs processing such as compensation for frequency offset and phase noise on the received signal that has been equalized by the adaptive equalization unit 54. The demodulation digital signal processing unit 532 includes components surrounded by dotted line 541 for the number of subcarriers. The configuration shown in FIG. 2 is a configuration that performs processing for the first subcarrier. The configuration that performs processing for the second subcarrier is the same as the configuration that performs processing for the first subcarrier, except that the subcarrier numbers are different. Therefore, the configuration that performs processing for the first subcarrier will be described as an example.

[0043] Next, the operation of demodulation digital signal processing unit 532 will be described. Adaptive equalization unit 54 of demodulation digital signal processing unit 532 receives real component XI and imaginary component XQ of the X-polarized received signal, which have been converted into digital signals by ADCs 531-1 to 531-4, and real component YI and imaginary component YQ of the Y-polarized received signal. Adaptive equalization unit 54 stores the received real component XI, imaginary component XQ, real component YI, and imaginary component YQ in the corresponding buffers. These buffers correspond to those used in the overlap save method described in Reference 1 below. (Reference 1: John J. Shynk, “Frequency-Domain and Multirate Adaptive Filtering”, January 1992.)

[0044] The adaptive equalizer 54 performs an N-point discrete Fourier transform or fast Fourier transform (corresponding to the "N-DFT" shown in FIG. 2) on each of the real component XI, imaginary component XQ, real component YI, and imaginary component YQ stored in the buffer. In this way, the adaptive equalizer 54 converts the real and imaginary components of each polarization into frequency-domain signals. That is, the adaptive equalizer 54 generates a frequency-domain signal for the real component XI, a frequency-domain signal for the imaginary component XQ, a frequency-domain signal for the real component YI, and a frequency-domain signal for the imaginary component YQ.

[0045] The frequency domain signal of real component XI, the frequency domain signal of imaginary component XQ, the frequency domain signal of real component YI, and the frequency domain signal of imaginary component YQ generated by the adaptive equalization unit 54 are input to a subcarrier selector. The subcarrier selector outputs frequency domain signals in frequency ranges corresponding to at least subcarriers 1 to K (K is an integer of 2 or greater). The subcarrier selector shown in FIG. 2 only selects frequencies for separating subcarrier signals and does not select compensation coefficients.

[0046] The subcarrier selector (first stage subcarrier selector) provided in the I lane of the X polarization selects a frequency domain signal XI1 in a frequency range corresponding to the first subcarrier of the real component XI and a frequency domain signal XI2 in a frequency range corresponding to the K-th subcarrier of the real component XI. K When the subcarrier selection unit outputs the second subcarrier, it selects and outputs the second subcarrier and the (K-1)th subcarrier. When the subcarrier selection unit outputs the kth subcarrier (where 1≦k≦K is an integer), it selects and outputs the kth subcarrier and the (K-k+1)th subcarrier.

[0047] The frequency domain signal XI1 of the first subcarrier of the real component XI output by the subcarrier selection unit is split into two, and the two split frequency domains are input directly to the coefficient calculation unit. On the other hand, the frequency domain signal XI1 of the Kth subcarrier of the real component XI output by the subcarrier selection unit Kis branched into two, and the two branched frequency domain signals are converted into inverted and complex conjugated frequency domain signals by the inversion and complex conjugation unit and input to the coefficient calculation unit.

[0048] Here, the inverted and complex conjugated frequency domain signal is a signal that has been inverted around DC in the frequency domain and complex conjugated in order to realize the generation of a complex conjugate signal in the time domain and the equivalent operation in the frequency domain. Consider a frequency domain signal X(f), and the inverted and complex conjugated signal is X † Hereinafter, the frequency domain signal of the real component converted by the inversion / complex conjugation unit will be referred to as the "real component inverted complex conjugate signal."

[0049] Similarly, the subcarrier selector (second-stage subcarrier selector) provided in the Q lane of the X polarization selects the frequency domain signal XQ1 of the first subcarrier of the imaginary component XQ and the frequency domain signal XQ of the K-th subcarrier of the imaginary component XQ. K The frequency domain signal XQ1 of the first subcarrier of the imaginary component XQ output by the subcarrier selection unit is branched into two, and the two branched frequency domains are input directly to the coefficient calculation unit.

[0050] On the other hand, the frequency domain signal XQ of the Kth subcarrier of the imaginary component XQ output by the subcarrier selector K is branched into two, and the two branched frequency domain signals are converted into inverted and complex conjugated frequency domain signals by the inversion and complex conjugation unit and input to the coefficient calculation unit. Hereinafter, the frequency domain signal of the imaginary component converted by the inversion and complex conjugation unit will be referred to as the "imaginary component inverted complex conjugate signal."

[0051] Similarly, the subcarrier selection unit (third-stage subcarrier selection unit) provided in the I lane of the Y polarization selects the frequency domain signal YI1 of the first subcarrier of the real component YI and the frequency domain signal YI KThe frequency domain signal YI1 of the first subcarrier of the real component YI output by the subcarrier selection unit is branched into two, and the two branched frequency domains are input directly to the coefficient calculation unit. On the other hand, the frequency domain signal YI1 of the Kth subcarrier of the real component YI output by the subcarrier selection unit is K is split into two, and the two split frequency domains are inverted and converted into complex conjugate frequency domain signals, which are then input to the coefficient calculation unit.

[0052] Similarly, the subcarrier selection unit (fourth stage subcarrier selection unit) provided in the Q lane of the Y polarization selects the frequency domain signal YQ1 of the first subcarrier of the imaginary component YQ and the frequency domain signal YQ of the Kth subcarrier of the imaginary component YQ. K The frequency domain signal YQ1 of the first subcarrier of the imaginary component YQ output by the subcarrier selection unit is branched into two, and the two branched frequency domains are input directly to the coefficient calculation unit. On the other hand, the frequency domain signal YQ of the Kth subcarrier of the imaginary component YQ output by the subcarrier selection unit is K is split into two, and the two split frequency domains are inverted and converted into complex conjugate frequency domain signals, which are then input to the coefficient calculation unit.

[0053] The coefficient calculation section calculates impulse responses H1 to H 16 The complex transfer function multiplication is performed for each subcarrier. That is, the impulse responses H1 to H 16 In FIG. 2, the coefficient calculation unit calculates impulse responses H1 to H 16 Although only the values ​​are shown, the specific configuration of the coefficient calculation unit will be described with reference to FIGS.

[0054] The adaptive equalization unit 54 outputs a frequency domain signal XI1 of the first subcarrier of the real component XI multiplied by the complex transfer function of the impulse response H1, a frequency domain signal XQ1 of the first subcarrier of the imaginary component XQ multiplied by the complex transfer function of the impulse response H5, a frequency domain signal YI1 of the first subcarrier of the real component YI multiplied by the complex transfer function of the impulse response H9, and a frequency domain signal YI2 of the first subcarrier of the impulse response H 13 The adaptive equalization unit 54 then adds the imaginary component YQ multiplied by the complex transfer function YQ1 to the frequency domain signal YQ1 of the first subcarrier to generate a sum signal. The sum signal generated by the adaptive equalization unit 54 is then folded in the frequency domain. The folding process involves folding back frequency components with absolute values ​​greater than half the symbol rate (Nyquist frequency) in a line-symmetric manner around the Nyquist frequency and adding them together. This process corresponds to downsampling in the time domain.

[0055] The adaptive equalization unit 54 performs an M-point (M is a natural number, and N≧K×M) inverse discrete Fourier transform or inverse fast Fourier transform on the sum signal that has undergone folding processing (corresponding to "M-IDFT" shown in FIG. 2). In this way, the adaptive equalization unit 54 converts the frequency domain signal into a time domain signal. Thereafter, the adaptive equalization unit 54 performs signal cutout processing on the time domain signal using the overlap save method (corresponding to "Cut" shown in FIG. 2).

[0056] In order to realize the above processing, the adaptive equalization unit 54 has a buffer, a Fourier transform unit, a branching unit, a coefficient calculation unit, an adding unit, a folding processing unit, an inverse Fourier transform unit, and a cutting unit.

[0057] Although the above describes a configuration in which folding, M-IDFT, and Cut processing are performed in that order, the processing may be performed in the order of M-IDFT, Cut, and downsampling.

[0058] The frequency / phase offset compensator 55 applies a frequency offset exp(jφ) to the added signal extracted by the adaptive equalizer 54 as described above. x,1(n)), where n represents the symbol interval.

[0059] The adaptive equalizer 54 outputs the real component inverted complex conjugate signal XI obtained by multiplying the complex transfer function of the impulse response H2. k † (-f) and the complex transfer function of the impulse response H6 are multiplied to obtain the imaginary component inverted complex conjugate signal XQ. k † (-f) and impulse response H 10 The real component inverted complex conjugate signal YI is multiplied by the complex transfer function k † (-f) and impulse response H 14 The imaginary component inverted complex conjugate signal YQ is multiplied by the complex transfer function k † (-f) to generate an added signal. The added signal generated by the adaptive equalization unit 54 is then subjected to folding, M-IDFT, and Cut processing.

[0060] The frequency / phase offset compensator 55 applies a frequency offset exp(-jφ) to the added signal extracted by the adaptive equalizer 54 as described above. x,1 The frequency / phase offset compensation unit 55 multiplies the frequency offset exp(jφ x,1 (n)) and the sum signal multiplied by the frequency offset exp(-jφ x,1 (n)) is added to obtain the received signal of the first subcarrier of the X polarization component.

[0061] The demodulation digital signal processing unit 532 adds a transmission data bias correction signal C for canceling the bias deviation of the X polarization component to the received signal of the first subcarrier of the obtained X polarization component. X1 The distortion-corrected received signal X of the first subcarrier of the X polarization component is 1,Rsig (n) is obtained.

[0062] On the other hand, the adaptive equalization unit 54 generates a real component XI1(f) multiplied by the complex transfer function of the impulse response H3, an imaginary component XQ1(f) multiplied by the complex transfer function of the impulse response H7, and an imaginary component XQ1(f) multiplied by the complex transfer function of the impulse response H 11 The real component YI1(f) is multiplied by the complex transfer function of 15 The sum signal generated by the adaptive equalization unit 54 is then subjected to folding, M-IDFT, and Cut processing. The frequency / phase offset compensation unit 55 applies a frequency offset exp(jφ y,1 Multiply by (n).

[0063] The adaptive equalizer 54 outputs the real component inverted complex conjugate signal XI obtained by multiplying the complex transfer function of the impulse response H4. K † (-f) and impulse response H 12 The imaginary component is inverted and the complex conjugate signal XQ is multiplied by the complex transfer function K † (-f) and impulse response H 16 The real component inverted complex conjugate signal YI is multiplied by the complex transfer function K † (-f) and impulse response H 14 The imaginary component inverted complex conjugate signal YQ is multiplied by the complex transfer function K † (-f) to generate an added signal. The added signal generated by the adaptive equalization unit 54 is then subjected to folding, M-IDFT, and Cut processing.

[0064] The frequency / phase offset compensator 55 applies a frequency offset exp(-jφ) to the added signal extracted by the adaptive equalizer 54 as described above. y,1 The frequency / phase offset compensation unit 55 multiplies the frequency offset exp(jφ y,1 (n)) and the sum signal multiplied by the frequency offset exp(-jφ y,1(n)) is added to obtain the received signal of the first subcarrier of the Y polarization component.

[0065] The demodulation digital signal processing unit 532 adds a transmission data bias correction signal C for canceling the bias deviation of the Y polarization component to the received signal of the first subcarrier of the obtained Y polarization component. Y1 The distortion-corrected received signal Y of the X polarization component is obtained by adding (or subtracting) Rsig (n) is obtained.

[0066] The values ​​of N, M, and impulse responses H1 to H 16 , and frequency offset exp(jφ x、k (n)), exp(-jφ x、k (n)), exp(jφ y、k (n)), exp(-jφ y、k (n)) are adaptively and dynamically changed. The receiver 50 obtains these values ​​by any method.

[0067] Next, the configuration and operation of the coefficient calculation unit will be described. FIGS. 3 to 6 are diagrams showing an example of the configuration of the coefficient calculation unit. As shown in FIGS. 3 to 6, the coefficient calculation unit provided in the demodulation digital signal processing unit 532 includes four coefficient calculation units. The coefficient calculation unit shown in FIG. 3 is a functional unit that calculates impulse responses H1, H3, H5, and H7. The coefficient calculation unit shown in FIG. 4 is a functional unit that calculates impulse responses H2, H4, H6, and H8. The coefficient calculation unit shown in FIG. 5 is a functional unit that calculates impulse responses H9, H10, H11, H12, H13, H14, H15, H16, H17, H18, H19, H20, H21, H22, H23, H24, H25, H26, H27, H28, H29, H30, H31, H32, H33, H34, H35, H36, H37, H38, H39, H40, H41, H42, H43, H44, H45, H46, H47, H48, H49, H50, H51, H52, H53, H54, H55, H56, H57, H58, H59, H60, H61, H62, H63, H64, H65, H66, H67, H68, H69, H70, H71, H72, H73, H74, H75, H76, H77, H78, H79, H80, H81, H81, H82, H83, H84, H85, H86, H87, H88, H89, H81, H81, H82, H83, H84, H85 11 ,H 13 ,H 15 The coefficient calculation unit shown in Figure 6 is a functional unit that calculates the impulse response H 10 ,H 12 ,H 14 ,H 16 The coefficient calculation unit includes a coefficient update unit that updates the value of the impulse response.

[0068] In the following description, the coefficient calculation unit shown in FIG. 3 will be referred to as the "first coefficient calculation unit," the coefficient calculation unit shown in FIG. 4 will be referred to as the "second coefficient calculation unit," the coefficient calculation unit shown in FIG. 5 will be referred to as the "third coefficient calculation unit," and the coefficient calculation unit shown in FIG. 6 will be referred to as the "fourth coefficient calculation unit." When there is no need to particularly distinguish between the first coefficient calculation unit to the fourth coefficient calculation unit, they will simply be referred to as the coefficient calculation unit. The operation of the coefficient calculation unit will be described below.

[0069] (Operation of the first coefficient calculation unit) The frequency domain signal of real component XI and the frequency domain signal of imaginary component XQ are input to the first coefficient calculation unit. The frequency domain signal of real component XI and the frequency domain signal of imaginary component XQ input to the first coefficient calculation unit are branched into a first path and a second path, respectively. In the first path, the frequency domain signal of real component XI and the frequency domain signal of imaginary component XQ are multiplied by a complex transfer function updated by the coefficient update unit.

[0070] In the second path, the frequency domain signal of the real component XI and the frequency domain signal of the imaginary component XQ are converted into inverted and complex conjugated frequency domain signals by the inversion and complex conjugation unit. As a result, the frequency domain signal of the real component XI input to the first coefficient calculation unit is converted into a real component inverted complex conjugate signal, and the frequency domain signal of the imaginary component XQ is converted into an imaginary component inverted complex conjugate signal.

[0071] In the first coefficient calculation unit, the real component inverted complex conjugate signal and the imaginary component inverted complex conjugate signal are multiplied by a signal based on the received signal. Here, the signal based on the received signal is a signal obtained based on the following processes (1) to (5).

[0072] (1): Reference signal (e.g., d x (n)) to the received signal (e.g., X Rsig Subtract (n) (2): A frequency offset (e.g., exp(-jφ)) is applied to the signal obtained by the process in (1). x Multiply (n))) (3): Add zeros to the signal obtained by processing (2) (corresponding to the "zero addition" shown in Figure 3) (4): The signal obtained in (3) is subjected to an M-point inverse discrete Fourier transform or inverse fast Fourier transform (corresponding to the "M-DFT" shown in Figure 3). (5): Copy the frequency domain signal obtained by processing (4) in the frequency domain (corresponding to the "foldback copy" shown in Figure 3).

[0073] A reference signal (e.g., d x (n) or d y (n)) is a pilot signal inserted in advance on the transmitting side or a received signal (e.g., X Rsig (n) or Y Rsig The value provisionally determined as (n) is used. The zero adding process shown in (3) is a process of adding M / N times the signal length to be cut in the overlap save method described in Reference 1 to the input signal. In the zero adding process, M / N times the signal length to be cut is continuously added to the input signal. The frequency domain copy shown in (5) is a process of copying the frequency domain signal axisymmetrically with the Nyquist frequency as the reference. The frequency domain copy shown in (5) corresponds to upsampling in the time domain.

[0074] Although the above describes a configuration in which zero addition, M-DFT, and foldback copying are performed, upsampling and N-DFT may be performed instead.

[0075] The real component inverted complex conjugate signal and the imaginary component inverted complex conjugate signal multiplied by the signal based on the received signal are input to a coefficient update unit. The coefficient update unit performs N-IDFT, Cut, zero addition, N-DFT, multiplication by a step size μ, and addition of the value of the previous impulse response on the real component inverted complex conjugate signal and the imaginary component inverted complex conjugate signal multiplied by the signal based on the received signal. As the step size μ, a normalized LMS (Reference 1) that normalizes the step size for each frequency bin by the input signal power may be used.

[0076] Taking the process of updating the impulse response H1 as an example of the process of the first coefficient calculation unit, the coefficient update unit first performs an N (e.g., N=256)-point inverse discrete Fourier transform or inverse fast Fourier transform on a real component inverted complex conjugate signal (here, signal A1) multiplied by a signal based on the received signal. This allows the coefficient update unit to convert the frequency-domain signal A1 into the time-domain signal A1. Next, the coefficient update unit performs a signal extraction process using the overlap-save method on the time-domain signal A1. Next, the coefficient update unit adds zeros to the extracted time-domain signal A1. Next, the coefficient update unit multiplies the zero-added time-domain signal A1 by a step size μ1. Next, the coefficient update unit updates the value of the impulse response H1 by adding the value of the previously obtained impulse response H1 to the time-domain signal A1 multiplied by the step size μ1.

[0077] The process of updating the impulse response H3 in the first coefficient calculation unit is the same as the process described above except that the step size value is different. Furthermore, the process of updating the impulse responses H5 and H7 in the first coefficient calculation unit is the same as the process described above except that the imaginary component inverted complex conjugate signal multiplied by the signal based on the received signal is input to the coefficient update unit and the step size value is different.

[0078] (Operation of the second coefficient calculation unit) The second coefficient calculation unit receives as input the inverted real component complex conjugate signal of the real component XI and the inverted imaginary component complex conjugate signal of the imaginary component XQ. The inverted real component complex conjugate signal of the real component XI and the inverted imaginary component complex conjugate signal of the imaginary component XQ input to the second coefficient calculation unit are branched into a first path and a second path, respectively. In the first path, the inverted real component complex conjugate signal of the real component XI and the inverted imaginary component complex conjugate signal of the imaginary component XQ are multiplied by the complex transfer function updated by the coefficient update unit.

[0079] In the second path, the inverted complex conjugate signal of the real component XI and the inverted complex conjugate signal of the imaginary component XQ are converted into inverted and complex conjugated frequency domain signals by the inverting and complex conjugating unit. As a result, the inverted complex conjugate signal of the real component XI input to the second coefficient calculation unit is converted into the frequency domain signal of the real component XI, and the inverted complex conjugate signal of the imaginary component XQ is converted into the frequency domain signal of the imaginary component XQ.

[0080] In the second coefficient calculation unit, the frequency signal of the real component XI and the frequency domain signal of the imaginary component XQ are multiplied by the signal based on the received signal. However, in the signal based on the received signal in the second coefficient calculation unit, a frequency offset exp(jφ x The signal obtained in the process of (1) is multiplied by (n). The frequency signal of real component XI and the frequency domain signal of imaginary component XQ multiplied by the signal based on the received signal are input to the coefficient update unit. The coefficient update unit performs N-IDFT, Cut, zero addition, N-DFT, multiplication by step size μ, and addition of the value of the previous impulse response on the frequency signal of real component XI and the frequency domain signal of imaginary component XQ multiplied by the signal based on the received signal. The process performed by the coefficient update unit is the same as the process described in Figure 3, so a description thereof will be omitted.

[0081] (Operation of the third coefficient calculation unit) The processing performed by the third coefficient calculation unit is based on the following points: the input signal is a Y-polarized signal, the step size used in the coefficient update unit is different, and in generating a signal based on the received signal, a frequency offset exp(jφ y (n)) is a reference signal (e.g., d y (n)) to the received signal (e.g., Y Rsig The process is the same as that performed by the first coefficient calculation unit, except that the signal obtained by subtracting (n) is multiplied by the coefficient.

[0082] (Operation of the fourth coefficient calculation unit) The processing performed by the fourth coefficient calculation unit is based on the following points: the input signal is a Y-polarized signal, the step size used in the coefficient update unit is different, and in generating a signal based on the received signal, a frequency offset exp(-jφ y (n)) is a reference signal (e.g., d y (n)) to the received signal (e.g., Y Rsig The process is the same as that performed by the second coefficient calculation unit, except that the signal obtained by subtracting (n) is multiplied by the coefficient.

[0083] The cut and zero-add processing in the coefficient update unit corresponds to multiplication of a rectangular window function in the time domain. By changing the window function in the time domain to a Cosine window and processing it as a convolution in the frequency domain, it is possible to omit the N-IDFT and N-DFT, and thus simplify the process.

[0084] The demodulation digital signal processor 532 configured as described above performs adaptive equalization in the frequency domain on a pair of frequency-domain signals of subcarriers separated in the frequency domain and frequency-inverted complex conjugate signals of the frequency-domain signals of subcarriers that are axisymmetric with respect to DC. This reduces the increase in the number of multiplications due to convolution operations of time-domain coefficients. This makes it possible to reduce the amount of calculations required for multicarrier signals. Furthermore, the reduced amount of calculations makes it possible to achieve power savings in the receiver 50 of the digital coherent optical transmission system.

[0085] (Modification of the first embodiment) The configuration of the subcarrier selection section included in demodulation digital signal processing section 532 does not need to be limited to the configuration shown in Fig. 2. For example, the subcarrier selection section may have any of the configurations shown in Figs. 7 to 9. Figs. 7 to 9 are diagrams showing other examples of the subcarrier selection section. First, a description will be given using Fig. 7.

[0086] In the configuration shown in Fig. 7, the subcarrier selection unit compensates for frequency characteristics before the frequency selection unit, and performs dispersion compensation for each subcarrier after the frequency selection unit. The receiver device characteristic compensation coefficient H RXX The "XX" in RXX is either "XI", "XQ", "YI", or "YQ". The frequency domain signal of the real component XI, the frequency domain signal of the imaginary component XQ, the frequency domain signal of the real component YI, and the frequency domain signal of the imaginary component YQ, whose frequency characteristics have been compensated, are input to the subcarrier selector. The subcarrier selector performs the same processing as described above. The frequency domain signals of the subcarriers output by the subcarrier selector are multiplied by the dispersion compensation coefficient H for each subcarrier. CD is multiplied.

[0087] When the subcarrier selection unit outputs the k-th subcarrier (1≦k≦K, an integer), the k-th subcarrier and the (K−k+1)-th subcarrier are selected and multiplied by the dispersion compensation coefficient corresponding to the k-th subcarrier. The processing after output from the subcarrier selection unit is the same as that described above.

[0088] In the configuration shown in Fig. 8, the subcarrier selection unit performs signal processing such as compensation for frequency characteristics and chromatic dispersion after the frequency selection unit. The frequency domain signal of real component XI, the frequency domain signal of imaginary component XQ, the frequency domain signal of real component YI, and the frequency domain signal of imaginary component YQ generated by the adaptive equalization unit 54 are input to the subcarrier selection unit. The subcarrier selection unit performs the same processing as described above. For each signal output from the frequency selection unit, a receiving-side device characteristic compensation coefficient H RXXk~(K-k+1) and compensation coefficient H´ CD is multiplied by H RXXk~(K-k+1) represents the receiver device characteristic compensation coefficient for the frequency range corresponding to subcarriers 1 to K. Compensation coefficient H' CD is the dispersion compensation coefficient H for each subcarrier CDare arranged in the frequency direction.

[0089] 9, the subcarrier selection unit performs signal processing such as compensation for frequency characteristics and chromatic dispersion before the frequency selection unit. The subcarrier selection unit calculates a receiving device characteristic compensation coefficient H RXX and compensation coefficient H´ CD The subcarrier selection unit performs the same processing as described above. The processing after output from the subcarrier selection unit is the same as described above.

[0090] Fig. 10 is a diagram showing the N-DFT size dependency (DFT is calculated by FFT) of the received SNR (Signal-Noise Ratio) for 2 subcarriers, 60 GBaud, and 16 QAM (Quadrature Amplitude Modulation) using the configuration of the demodulation digital signal processing unit 532 equipped with the subcarrier selection unit shown in Fig. 7. As shown in Fig. 10, it can be seen that increasing the DFT size increases the time response (frequency resolution) that can be compensated, and therefore improves the received SNR (Signal-to-Noise Ratio).

[0091] Fig. 11 shows a comparison of the number of multiplications between a conventional configuration (e.g., the configuration described in Patent Document 1) and the configuration of the demodulation digital signal processor 532 equipped with the subcarrier selector shown in Fig. 7. In Fig. 11, the input sampling rate is 240 GSample / a, the symbol rate (per subcarrier) is 60 GBaud, the number of subcarriers K is 2, the DFT size is N, the IDFT block size is M=N / (2K) (assuming that the DFT and IDFT are calculated using the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT)), and the overlap amount of the Overlap Save method is 1 / 2 (in this case, the time response length within the compensable range is (N / (2K) × sampling interval), resulting in the same compensation performance as the conventional configuration with a tap length of N / (2K)). Note that Fig. 11 does not include the amount of calculation required for the imperfection coefficients and dispersion compensation coefficients of the receiving device (only the calculation of the adaptive filter coefficients is considered).

[0092] The number of multiplications in the fast Fourier transform is 4 × (N / 2) × log2(N), the number of multiplications in the inverse fast Fourier transform is K × 4 × (N / 4 / K) × log2(N / 2 / K), and the number of multiplications for the adaptive filter coefficients is K × 16 × (N / K). Under these conditions, the number of symbols that can be output from one block is N / 4, so the number of multiplications per symbol is 2 × log2(N) + 4 × log2(N / 2) + 64. With the conventional configuration, it is sufficient to consider the number of multiplications for the convolution operation per symbol, so this is 16L where L is the number of taps in the adaptive filter.

[0093] (Second embodiment) In the second embodiment, a configuration that can reduce the number of discrete Fourier transforms or fast Fourier transforms compared to the first embodiment will be described. Note that in the second embodiment, the configuration of the adaptive equalization unit included in the demodulation digital signal processing unit is different from that of the first embodiment. Therefore, only the differences from the first embodiment will be described.

[0094] Fig. 12 is a diagram showing an example of the configuration of a demodulation digital signal processing unit 532a in the second embodiment. Note that Fig. 12 omits the configuration of the frequency / phase offset compensating unit 55 and subsequent units, which are the same as those in the first embodiment. The adaptive equalizer 54a of the demodulation digital signal processing unit 532a shown in Fig. 12 differs from the adaptive equalizer 54 in the configuration of the stage preceding the subcarrier selecting unit. Note that the demodulation digital signal processing unit 532a does not perform signal processing such as compensation for frequency characteristics and chromatic dispersion.

[0095] The adaptive equalizer 54a of the demodulation digital signal processor 532 receives the real component XI and imaginary component XQ of the X-polarized received signal, which have been converted into digital signals by the ADCs 531-1 to 531-4, and the real component YI and imaginary component YQ of the Y-polarized received signal. The adaptive equalizer 54a multiplies the received imaginary component XQ by an imaginary unit j to generate an imaginary component jXQ. The adaptive equalizer 54a adds the real component XI and the imaginary component jXQ to generate an addition signal. As a result, the adaptive equalizer 54a generates an addition signal of XI + jXQ. The adaptive equalizer 54a stores the generated addition signal in a buffer.

[0096] The adaptive equalizer 54a performs an N-point discrete Fourier transform or fast Fourier transform on the sum signal stored in the buffer (corresponding to "N-DFT" shown in FIG. 12), thereby converting the X-polarized sum signal into a frequency domain signal.

[0097] The frequency domain sum signal generated by the adaptive equalization unit 54a is branched into two. One of the branched frequency domain sum signals is converted into an inverted and complex conjugated frequency domain signal. In the following description, the frequency domain sum signal that is branched and converted into an inverted and complex conjugated frequency domain signal before the branching unit is referred to as the "frequency domain converted sum signal," and the frequency domain sum signal that is not branched and converted into an inverted and complex conjugated frequency domain signal is referred to as the "frequency domain pre-converted sum signal."

[0098] The frequency domain pre-conversion sum signal and the frequency domain post-conversion sum signal are each branched into two, and the adaptive equalization unit 54a adds the frequency domain pre-conversion sum signal and the frequency domain post-conversion sum signal together, and then multiplies the result by 1 / 2. This signal is equivalent to the frequency domain signal of real component XI in the first embodiment. The sum signal multiplied by 1 / 2 (the frequency domain signal of real component XI) is then branched into four by a branching unit, and two of the four branched signals are input directly to the coefficient calculation unit, while the remaining two signals are inverted and converted into complex conjugate frequency domain signals and input to the coefficient calculation unit.

[0099] Furthermore, the adaptive equalization unit 54a subtracts the frequency-domain post-conversion sum signal from the frequency-domain pre-conversion sum signal, and then multiplies the result by 1 / 2j. This signal is equivalent to the frequency-domain signal of the imaginary component XQ in the first embodiment. Thereafter, the signal multiplied by 1 / 2j (the frequency-domain signal of the imaginary component XQ) is branched into four by a branching unit, and two of the four branched signals are input directly to the coefficient calculation unit, while the remaining two signals are inverted and converted into complex conjugate frequency-domain signals and then input to the coefficient calculation unit. The above is the processing related to the X polarization.

[0100] Similarly, the adaptive equalization unit 54a multiplies the input imaginary component YQ by the imaginary unit j to generate an imaginary component jYQ. The adaptive equalization unit 54a adds the real component YI and the imaginary component jYQ together. As a result, the adaptive equalization unit 54a generates an added signal of YI + jYQ. The adaptive equalization unit 54a stores the generated added signal in a buffer.

[0101] The adaptive equalizer 54a performs an N-point discrete Fourier transform or fast Fourier transform on the sum signal stored in the buffer (corresponding to "N-DFT" shown in FIG. 12), thereby converting the Y-polarized sum signal into a frequency domain signal.

[0102] The frequency-domain sum signal generated by the adaptive equalization unit 54a is branched into two. One of the branched frequency-domain sum signals is converted into an inverted and complex-conjugated frequency-domain signal. The frequency-domain pre-conversion sum signal and the frequency-domain post-conversion sum signal are each branched into two, and the adaptive equalization unit 54a adds the frequency-domain pre-conversion sum signal and the frequency-domain post-conversion sum signal together and then multiplies the result by 1 / 2. This signal is equivalent to the frequency-domain signal of the real component YI in the first embodiment. The sum signal multiplied by 1 / 2 (the frequency-domain signal of the real component YI) is then branched into four by the branching unit, and two of the four branched signals are input directly to the coefficient calculation unit, while the remaining two signals are converted into frequency-domain signals that are inverted and complex-conjugated and then input to the coefficient calculation unit.

[0103] Furthermore, the adaptive equalization unit 54a subtracts the frequency-domain post-conversion addition signal from the frequency-domain pre-conversion addition signal, and then multiplies the result by 1 / 2j. This signal is equivalent to the frequency-domain signal of the imaginary component YQ in the first embodiment. Thereafter, the signal multiplied by 1 / 2j (the frequency-domain signal of the imaginary component YQ) is branched into four by a branching unit, and two of the four branched signals are input directly to the coefficient calculation unit, while the remaining two signals are inverted and converted into complex conjugate frequency-domain signals and then input to the coefficient calculation unit. The above is the processing related to Y polarization.

[0104] In the adaptive equalization unit 54a, the processing after the coefficient calculation unit is the same as in the first embodiment.

[0105] The demodulation digital signal processing unit 532 in the second embodiment configured as described above can reduce the number of discrete Fourier transforms or fast Fourier transforms compared to the first embodiment. Specifically, the demodulation digital signal processing unit 532 in the second embodiment performs a discrete Fourier transform or fast Fourier transform after adding the real component XI and the imaginary component XQ. This eliminates the need to perform a discrete Fourier transform or fast Fourier transform on each of the real component XI and the imaginary component XQ. Therefore, the number of discrete Fourier transforms or fast Fourier transforms can be reduced compared to the first embodiment.

[0106] (Modification of the second embodiment) The subcarrier selection section included in the adaptive equalization section 54a may have any of the configurations shown in FIGS.

[0107] (Third embodiment) In the third embodiment, the configuration of the adaptive equalizer included in the demodulation digital signal processing unit is different from that of the second embodiment, so the differences from the second embodiment will be described.

[0108] Fig. 13 is a diagram showing an example of the configuration of a demodulation digital signal processing unit 532b in the third embodiment. Note that Fig. 13 omits the configuration from the frequency / phase offset compensating unit 55 onwards, which is the same as in the second embodiment. The demodulation digital signal processing unit 532b includes an adaptive equalizer 54b and a frequency / phase offset compensating unit 55 (omitted in Fig. 13).

[0109] The adaptive equalizer 54b applies the receiving device characteristic H RXI and the receiving device characteristics H RXQ The sum of (1 / 2 x H CD * ) multiplied by

[0110] Similarly, the adaptive equalizer 54b applies the reception-side device characteristic H RXI From the receiving device characteristics HRXQ The value obtained by subtracting (1 / 2 × H CD * ) multiply by 1 / 2×H CD * The frequency domain pre-conversion sum signal of X polarization multiplied by 1 / 2×H CD * Each of the converted sum signals of the X-polarized frequency domain multiplied by is split into two.

[0111] The adaptive equalizer 54b is 1 / 2×H CD * The pre-conversion sum signal of the X polarization frequency domain multiplied by 1 / 2×H CD * The converted sum signal in the frequency domain of the X-polarized wave multiplied by is added to the converted sum signal. This sum signal is then input to the first-stage subcarrier selection unit.

[0112] Furthermore, the adaptive equalizer 54b is 1 / 2×H CD * The sum of the converted frequency domain signals of the X-polarized waves multiplied by 1 / 2×H CD * The pre-conversion sum signal in the frequency domain of the X-polarized wave multiplied by is then subtracted. This subtracted signal is then input to the second-stage subcarrier selection unit. The above is the processing related to the X polarization.

[0113] The adaptive equalizer 54b applies the receiving device characteristic H RYI and the receiving device characteristics H RYQ The sum of (1 / 2 x H CD * Similarly, the adaptive equalizer 54b multiplies the converted sum signal in the frequency domain of the Y-polarized wave by the receiving device characteristic H RYI From the receiving device characteristics H RYQ The value obtained by subtracting (1 / 2 × H CD * ) multiply by 1 / 2×H CD * The frequency domain pre-conversion sum signal of Y polarization multiplied by 1 / 2×H CD *Each of the converted sum signals of the Y-polarized wave in the frequency domain multiplied by is split into two.

[0114] The adaptive equalizer 54b is 1 / 2×H CD * The frequency domain pre-conversion sum signal of Y polarization multiplied by 1 / 2×H CD * The converted sum signal in the frequency domain of the Y-polarized wave multiplied by is added to the converted sum signal. This sum signal is then input to the subcarrier selection unit in the third stage.

[0115] Furthermore, the adaptive equalizer 54b is 1 / 2×H CD * The sum signal after the frequency domain transformation of the Y polarization multiplied by is 1 / 2×H CD * The pre-conversion sum signal in the frequency domain of the Y-polarized wave multiplied by is then subtracted. This subtracted signal is then input to the fourth-stage subcarrier selector. The above is the processing related to Y polarization.

[0116] In the adaptive equalizer 54b, the processing after the subcarrier selection section is the same as in the second embodiment.

[0117] The demodulation digital signal processing unit 532b in the third embodiment configured as above is different from the second embodiment in that it is possible to reduce the number of discrete Fourier transforms or fast Fourier transforms compared to the first embodiment. RXI -H RXQ ,H RYI -H RYQ If is small, it is possible to reduce the bit precision.

[0118] (Modification of the third embodiment) The subcarrier selection section included in the adaptive equalization section 54b may have the configuration shown in FIG.

[0119] (Modifications common to the first to third embodiments) In each of the above embodiments, a configuration that performs wavelength division multiplexing in addition to subcarrier multiplexing and polarization division multiplexing may be combined. In such a configuration, the digital coherent optical transmission system 1 shown in FIG. 1 differs from the system in FIG. 1 in the following respects. The transmitter 10 further includes transmitting units 100 equal to the number of WDM (Wavelength Division Multiplexing) channels. For example, if the number of WDM channels is 10, the transmitter 10 will include 10 transmitting units 100. Each transmitting unit 100 outputs an optical signal of a different wavelength. A WDM multiplexer, an optical fiber transmission line 30, and a WDM demultiplexer are provided between the transmitter 10 and the receiver 50. The WDM multiplexer multiplexes the optical signals output by each transmitting unit 100 and outputs the multiplexed optical signal to the optical fiber transmission line 30. The WDM demultiplexer demultiplexes the optical signals transmitted through the optical fiber transmission line 30 by wavelength. The receiver 50 further includes receiving units 500 equal to the number of WDM channels. For example, if the number of WDM channels is 10, the receiver 50 will include 10 receiving units 500. Each receiving unit 500 receives the optical signal demultiplexed by the WDM demultiplexer 40. The wavelengths of the optical signals received by each receiver 500 are different from each other. The processing executed by the receiver 500 is the same as the processing described above.

[0120] In each of the above embodiments, when N=K×M, folding processing in the adaptive equalizers 54, 54a, and 54b does not need to be performed.

[0121] 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.

[0122] 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).

[0123] 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]

[0124] The present invention can be applied to a technique for receiving subcarrier-multiplexed polarization-multiplexed signals in digital coherent optical transmission. [Explanation of symbols]

[0125] 1...Digital coherent optical transmission system 10...Transmitter 30...Optical fiber transmission line 31...Optical amplifier 50...Receiver 54, 54a, 54b...adaptive equalization section 55...Frequency / phase offset compensation section 56...Front-end correction and chromatic dispersion estimation unit 100...Transmitter 110...Digital signal processing unit 111...encoding section 112...Mapping section 113...Training signal insertion unit 114...Frequency conversion unit 115...Waveform shaping section 116...Subcarrier multiplexing unit 117...Pre-equalization section 118-1 to 118-4: Digital-to-analog converters 119...Signal generation unit 120...Modulator driver 121-1~121-4...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 523-1~523-4...BPD 524-1~524-4...Amplifier 530...Digital signal processing unit 531-1 to 531-4...Analog-to-digital converters 532, 532a, 532b...Demodulation digital signal processing section 533…Demapping section 534...Decoding unit

Claims

1. a conversion step of converting real and imaginary components of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal into frequency domain signals; a subcarrier selection step of selecting a frequency domain signal corresponding to the subcarrier; a signal input step of inputting, as input signals, the frequency domain signals of the real components and the frequency domain signals of the imaginary components of each polarized wave of each selected subcarrier, and the frequency domain signals after frequency inversion with respect to the center frequency of the selected subcarrier on the frequency axis of the frequency domain signals of the real components and the frequency domain signals of the imaginary components of each polarized wave of a pair of subcarriers that are line-symmetric with respect to a DC component and complex conjugate conversion; an equalization step of performing a first equalization process for each subcarrier and polarization, in which the frequency domain signals of the real components and the frequency domain signals of the imaginary components of each polarization included in the input signal are multiplied by a complex transfer function, and then added together to inversely convert the frequency domain signals into time domain signals; and a second equalization process for multiplying the frequency domain signals of the real components after conversion and the frequency domain signals of the imaginary components after conversion of each polarization included in the input signal by a complex transfer function, and then added together to inversely convert the frequency domain signals into time domain signals; a compensation step of performing a phase rotation for frequency offset compensation on the time domain signal converted by the first equalization processing for each subcarrier and each polarization to generate a first sum signal, performing a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signal converted by the second equalization processing to generate a second sum signal, and adding or subtracting a transmit data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal; A signal processing method comprising:

2. an addition processing step of performing an imaginary unit multiplication process of multiplying an imaginary component of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal by an imaginary unit j, and then performing an addition processing of adding the imaginary component multiplied by the imaginary unit j and a real component of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal; a transforming step of transforming the signal obtained by adding the imaginary component multiplied by the imaginary unit j and the real component into a frequency domain signal; a subcarrier selection step of selecting a frequency domain signal corresponding to a subcarrier by inputting a first computed frequency domain signal obtained by adding the frequency domain signal of each polarization to a converted frequency domain signal obtained by frequency inverting the frequency domain signal on the frequency axis and taking a complex conjugate, and then multiplying by ½; a first computed frequency domain signal obtained by adding the frequency domain signal after compensation has been performed on the frequency domain signal to a converted frequency domain signal obtained by compensating the converted frequency domain signal, and then multiplying by ½; a second computed frequency domain signal obtained by subtracting from the frequency domain signal a converted frequency domain signal obtained by frequency inverting the frequency domain signal on the frequency axis and taking a complex conjugate, and then multiplying by ½; or a second computed frequency domain signal obtained by subtracting from the compensated frequency domain signal a converted frequency domain signal obtained by compensating the frequency domain signal, and then multiplying by ½; a signal input step of inputting, as an input signal, a frequency domain signal corresponding to the subcarriers selected in the subcarrier selection step, either directly or after compensation; an equalization step for performing, for each subcarrier and polarization, a first equalization process in which a first calculated frequency domain signal of the real component and a first calculated frequency domain signal of the imaginary component of each polarization included in the input signal are multiplied by a complex transfer function, respectively, and then added together to inversely convert the frequency domain signal into a time domain signal; and a second equalization process in which a second calculated frequency domain signal of the real component and a second calculated frequency domain signal of the imaginary component of each polarization included in the input signal are multiplied by a complex transfer function, respectively, and then added together to inversely convert the frequency domain signal into a time domain signal; a compensation step of performing a phase rotation for frequency offset compensation on the time domain signal converted by the first equalization processing for each subcarrier and each polarization to generate a first sum signal, performing a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signal converted by the second equalization processing to generate a second sum signal, and adding or subtracting a transmit data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal; A signal processing method comprising:

3. a first signal processing for branching the frequency domain signal into a first path and a second path, adding the frequency domain signal branched into the first path and the frequency domain signal branched into the second path which has been frequency-inverted and complex-conjugated, and then multiplying the result by 1 / 2; and a second signal processing for subtracting the frequency domain signal branched into the second path which has been frequency-inverted and complex-conjugated, from the frequency domain signal branched into the first path, and then multiplying the result by 1 / 2j, for each polarization, and then performing selection in the subcarrier selection step.

3. The signal processing method according to claim 2.

4. a first signal processing for branching the frequency domain signal into a first path and a second path, adding the frequency domain signal branched into the first path after compensation for frequency characteristics and chromatic dispersion compensation and the frequency domain signal branched into the second path after frequency inversion and complex conjugation, compensation for frequency characteristics and chromatic dispersion compensation, and a second signal processing for subtracting the frequency domain signal branched into the second path after frequency inversion and complex conjugation, compensation for frequency characteristics and chromatic dispersion compensation, from the frequency domain signal branched into the first path after compensation for frequency characteristics and chromatic dispersion compensation, and then performing the selection in the subcarrier selection step, for each polarization; 3. The signal processing method according to claim 2.

5. In the subcarrier selection step, After the frequency characteristics are compensated, frequency domain signals corresponding to the subcarriers are selected, and dispersion compensation is performed for each subcarrier. or, After selecting the frequency domain signals corresponding to the subcarriers, compensation for frequency characteristics and dispersion compensation for each subcarrier are performed. or, After performing frequency characteristic compensation and dispersion compensation for each subcarrier, a frequency domain signal corresponding to the subcarrier is selected. A signal processing method according to any one of claims 1 to 4.

6. a frequency conversion unit that converts real and imaginary components of each polarization of the subcarrier-multiplexed and polarization-multiplexed received signal into frequency domain signals; a subcarrier selection step of selecting a frequency domain signal corresponding to the subcarrier; a signal input unit that inputs, as input signals, the frequency domain signal of the real component and the frequency domain signal of the imaginary component of each polarized wave of each selected subcarrier, and the frequency domain signals after conversion in which the frequency domain signal of the real component and the frequency domain signal of the imaginary component of each polarized wave of a pair of subcarriers that are line-symmetric with respect to a DC component are frequency-inverted with respect to the center frequency of the selected subcarrier on the frequency axis and complex conjugated; an equalization unit that performs, for each subcarrier and polarization, a first equalization process in which the frequency domain signals of the real components and the frequency domain signals of the imaginary components of each polarization included in the input signal are multiplied by a complex transfer function, and then the resulting signals are added together to inversely convert the frequency domain signals into time domain signals; and a second equalization process in which the frequency domain signals after the conversion of the real components and the frequency domain signals after the conversion of the imaginary components of each polarization included in the input signal are multiplied by a complex transfer function, and then the resulting signals are added together to inversely convert the frequency domain signals into time domain signals; a compensation unit that performs, for each subcarrier and each polarization, a phase rotation for frequency offset compensation on the time domain signal converted by the first equalization processing to generate a first sum signal, performs a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signal converted by the second equalization processing to generate a second sum signal, and adds or subtracts a transmission data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal; A signal processing device comprising:

7. an adder that performs an imaginary unit multiplication process of multiplying an imaginary component of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal by an imaginary unit j, and then performs an addition process of adding the imaginary component multiplied by the imaginary unit j and a real component of each polarization of a subcarrier-multiplexed and polarization-multiplexed received signal; a frequency transform unit that transforms the signal obtained by adding the imaginary component multiplied by the imaginary unit j and the real component into a frequency domain signal; a subcarrier selector configured to select a frequency domain signal corresponding to a subcarrier by inputting a first computed frequency domain signal obtained by adding the frequency domain signal of each polarization to a converted frequency domain signal obtained by frequency inverting the frequency domain signal on the frequency axis and taking a complex conjugate, and then multiplying by ½; a first computed frequency domain signal obtained by adding the frequency domain signal after compensation has been performed on the frequency domain signal to a converted frequency domain signal obtained by compensating the converted frequency domain signal, and then multiplying by ½; a second computed frequency domain signal obtained by subtracting from the frequency domain signal a converted frequency domain signal obtained by frequency inverting the frequency domain signal on the frequency axis and taking a complex conjugate, and then multiplying by ½; or a second computed frequency domain signal obtained by subtracting from the compensated frequency domain signal the converted frequency domain signal, the converted frequency domain signal, and then multiplying by ½; a signal input unit that inputs a frequency domain signal corresponding to the subcarrier selected by the subcarrier selection unit as an input signal, either directly or after compensation; an equalization unit that performs, for each subcarrier and polarization, a first equalization process in which the first calculated frequency domain signal of the real component and the first calculated frequency domain signal of the imaginary component of each polarization included in the input signal are multiplied by a complex transfer function, and then summed, thereby performing an inverse conversion from a frequency domain signal to a time domain signal; and a second equalization process in which the second calculated frequency domain signal of the real component and the second calculated frequency domain signal of the imaginary component of each polarization included in the input signal are multiplied by a complex transfer function, and then summed, thereby performing an inverse conversion from a frequency domain signal to a time domain signal; a compensation unit that performs, for each subcarrier and each polarization, a phase rotation for frequency offset compensation on the time domain signal converted by the first equalization processing to generate a first sum signal, performs a phase rotation opposite to the phase rotation for frequency offset compensation on the time domain signal converted by the second equalization processing to generate a second sum signal, and adds or subtracts a transmission data bias correction signal to or from a signal obtained by adding the first sum signal and the second sum signal; A signal processing device comprising:

8. 8. A communication system comprising: a transmitter for transmitting a polarization multiplexed signal that has been subcarrier multiplexed and polarization multiplexed; and a receiver having the signal processing device according to claim 6.

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