Optical reception device and optical communication system

Abstract

This optical reception device comprises a compensation function estimation unit that estimates a compensation function to be used when a digital signal processing unit, for performing digital signal processing on a transmission data sequence, performs digital signal processing, by performing, on a received optical signal, signal processing having a configuration simulating the configuration of the digital signal processing unit in an optical transmission device which transmits optical signals by using an analog multiplexing technology.

Description

Optical receiving device and optical communication system

[0001] The present invention relates to an optical receiving device and an optical communication system.

[0002] In the backbone networks of optical communication systems, there is a demand for an increase in transmission capacity per channel due to the recent expansion of communication traffic and the increase in the capacity of lines accommodating client signals such as Ethernet.

[0003] In order to expand the transmission capacity per channel, improvements in modulation speed are being considered. However, the limitation of the analog bandwidth of DACs poses a challenge in increasing the modulation speed. To date, in order to expand the analog bandwidth and generate high-speed electrical signals, technologies have been proposed that generate high-speed signals by analog-combining multiple low-speed signals using multiple DACs and analog multiplexing units (e.g., Patent Document 1 and Non-Patent Document 1).

[0004] Additionally, a technique has been proposed for compensating for signal waveform distortion characteristics that occur during analog multiplexing using digital signal processing. For example, Patent Document 2 discloses a technique for compensating for linear distortion of a signal using a transfer function in which a signal obtained by cyclically permuting the signal spectrum on the receiving side is used as an input signal. Furthermore, Patent Document 3 discloses a technique for compensating for linear and nonlinear distortion of a signal using a 2M×M filter in which a signal is divided into M parts on the frequency axis and M original divided components and M folded divided components obtained by folding the original parts on the frequency axis are used as input.

[0005] International Publication No. 2017 / 033446 Japanese Patent Application Laid-Open No. 2020-096264 International Publication No. 2020 / 054173

[0006] X. Chen, et al., “All-electronic 100-GHz Bandwidth Digital-to-Analog Converter Generating PAM Signals up to 190 Gbaud,” Journal of Lightwave Technology, Vol. 35, No. 3, pp. 411-417 (2017).DP Kingma and JL Ba. 2015, arXiv:1412.6980 (2014).

[0007] However, it has been unclear how to accurately determine the compensation function when linear and nonlinear waveform distortions exist.

[0008] In view of the above circumstances, an object of the present invention is to provide a technique that can estimate a compensation function with high accuracy.

[0009] One aspect of the present invention is an optical receiving device that is an optical transmitting device that transmits an optical signal using analog multiplexing technology, and that includes a compensation function estimating unit that performs signal processing on a received optical signal using a configuration that simulates the configuration of a digital signal processing unit that performs digital signal processing on a transmission data series, thereby estimating a compensation function that the digital signal processing unit uses when performing digital signal processing.

[0010] According to the present invention, the compensation function can be estimated with high accuracy.

[0011] FIG. 1 is a diagram showing an example of the configuration of an optical communication system. FIG. 2 is a diagram showing an example of the configuration of an optical transmitting device. FIG. 3 is a diagram showing an example of the configuration of a MIMO filter that performs MIMO signal processing. FIG. 4 is a diagram showing an example of the configuration of a high-speed signal generating unit. FIG. 5 is a diagram showing an example of the configuration of an optical receiving device. FIG. 6 is a diagram showing the signal-to-noise ratio (SNR) of a signal received by an optical receiving device in the present embodiment and a comparative example. FIG. 7 is a diagram showing an example of the configuration of an optical communication system. FIG. 8 is a diagram showing an example of the configuration of an optical communication system.

[0012] (Overall Configuration) Fig. 1 is a diagram showing an example of the configuration of an optical communication system 1. The optical communication system 1 includes an optical transmitting device 10, an optical receiving device 20, and an optical transmission path 30. The optical transmitting device 10 transmits an optical signal to the optical receiving device 20 via the optical transmission path 30 using analog multiplexing technology. The optical transmission path 30 may include an optical amplifier 310, which amplifies the optical signal transmitted by the optical transmitting device 10.

[0013] The optical receiving device 20 transmits a compensation function to the optical transmitting device 10. The optical transmitting device 10 processes a signal based on the compensation function transmitted from the optical receiving device 20. The means by which the optical receiving device 20 transmits the compensation function to the optical transmitting device 10 is not limited, and may be, for example, transmitted via the optical transmission path 30 or by electrical communication means. In the electrical communication means, any device or memory may be provided between the optical receiving device 20 and the optical transmitting device 10.

[0014] 2 is a diagram showing an example of the configuration of the optical transmitting device 10. The optical transmitting device 10 includes a digital signal processing unit 11, a high-speed signal generating unit 130, and an optical modulation unit 140.

[0015] The digital signal processing unit 11 performs digital signal processing on the transmission data sequence to generate a low-speed signal. The digital signal processing unit 11 includes a transmission signal generating unit 110 and a low-speed signal generating unit 120.

[0016] The transmission signal generation unit 110 generates a modulated signal sequence, which is a high-speed signal, based on the transmission data sequence. For example, if the optical transmission path 30 is a single-mode fiber and the communication system 1 performs coherent transmission, the transmission signal generation unit 110 assigns transmission bits to symbol points such as QAM (Quadrature Amplitude Modulation) for the transmission data sequence to generate a modulated signal sequence, which is a high-speed signal. The transmission signal generation unit 110 may perform filtering processing such as a root raised cosine filter on the modulated signal sequence, or any other signal processing such as pre-chromatic dispersion compensation and pre-nonlinear optical effect compensation to compensate for waveform distortion in the transmission path.

[0017] The low-speed signal generator 120 generates a low-speed signal based on the modulated signal sequence generated by the transmission signal generator 110 and a compensation function transmitted from the optical receiving device 20. The low-speed signal generator 120 performs, for example, a Hilbert transform on the modulated signal sequence generated by the transmission signal generator 110 to generate a Hilbert-transformed signal. The low-speed signal generator 120 performs MIMO (multiple-input / multiple-output) signal processing on the modulated signal sequence and Hilbert-transformed signal generated by the transmission signal generator 110, thereby compensating in advance for linear waveform distortion occurring in the signal and generating a low-speed signal.

[0018] 3 is a diagram showing an example of the configuration of a MIMO filter that performs MIMO signal processing. In the MIMO signal processing, two input signals (a modulated signal sequence and a Hilbert transform signal) are subjected to an intermediate frequency ω ＩＦ Multiply by a sinusoidal function with a frequency that is an integer multiple of the intermediate frequency ω. ＩＦ is a frequency that is set based on the band of the low-speed signal generated by the low-speed signal generating unit 120 and the band of the high-speed signal generated by the high-speed signal generating unit 130, and is not particularly limited. In the example shown in FIG. 3, 1, cos(ω ＩＦ t), sin(ω ＩＦ t), cos(2ω ＩＦ t) to generate four signals from one input signal. ＩＦ t), sin(ω ＩＦ t), cos(2ω ＩＦ In the combination of sinusoidal functions, the modulation rate of the transmitted signal is ω ＩＦ For example, when the input signal is four times as large as the input signal, it is possible to prevent signal distortion including DC components and double wave components that occur in the high-speed signal generating unit 130 (described later). The sine wave function multiplied by the input signal is sin(2ω) depending on the configuration of the high-speed signal generating unit 130 (described later). ＩＦ t), cos(3ω ＩＦ A sinusoidal function such as t may also be used.

[0019] In MIMO signal processing, a signal generated from two input signals is filtered to generate two output signals. The filter coefficients of the filter that filters the signal generated from the two input signals are determined by a compensation function transmitted from the optical receiving device 20. Ultimately, in MIMO signal processing, two output signals (first output signal and second output signal) are generated from the two input signals. In the example shown in FIG. 3, eight signals generated from the two input signals are filtered to generate two output signals. In the example shown in FIG. 3, ideally, the two output signals s(t) shown in equation (1) are generated from the signal s(t) input to the low-speed signal generating unit 120. Ａ (t) and s Ｂ (t) is output.

[0020] HT{s(t)} is a Hilbert transform signal obtained by Hilbert transforming the signal s(t). Equation (1) represents an ideal state in which no distortion occurs in analog multiplexing. In the case of equation (1), cos2ω is IF The distortion component corresponding to becomes 0, the MIMO filter coefficient becomes 0, and does not contribute to compensation.

[0021] The low-speed signal generating unit 120 may use digital signal processing to perform processing to compensate for nonlinear waveform distortion of the signal. The nonlinear waveform distortion is nonlinear waveform distortion that occurs in the high-speed signal after analog multiplexing in the analog domain, or nonlinear waveform distortion that occurs in the low-speed signal before analog multiplexing. The type of digital signal processing is, for example, a Volterra filter or a neural network.

[0022] The high-speed signal generating section 130 generates a high-speed signal based on the low-speed signal generated by the low-speed signal generating section 120. Fig. 4 shows an example of the configuration of the high-speed signal generating section 130. The high-speed signal generating section 130 includes analog multiplexers (high-speed linear selectors) 131 and 132. Ａ (t) and -s Ａ Switch (t) and s Ｂ (t) and -s Ｂ(t) is switched by shifting the phase by π / 2. Then, the high-speed signal generator 130 adds the two switched signals together to generate the signal s Ａ (t) cos(ω ＩＦ t)-s Ｂ (t) sin(ω ＩＦ The output signal is equal to s(t), which generates a high-speed signal.

[0023] 4, distortion occurs in the output signal due to skew between input signal lanes, frequency response differences, signal reflections, DC components and double wave components generated in the analog multiplexer, and nonlinearity generated in the low-speed signal or high-speed signal before and after analog processing. The low-speed signal generation unit 120 described above generates a signal that cancels out the distortion.

[0024] When the low-speed signal generating section 120 performs a different linear transformation, the high-speed signal generating section 130 can have the configuration shown in Fig. 5. The high-speed signal generating section 130 shown in Fig. 5 outputs a signal shown in equation (2).

[0025] The optical modulation section 140 converts the high-speed signal generated by the high-speed signal generation section 130 into an optical signal. The optical modulation section 140 transmits the optical signal to the optical receiving device 20.

[0026] 6 is a diagram showing an example of the configuration of the optical receiving device 20. The optical receiving device 20 includes an electrical signal converting section 210, a transmission path compensating section 220, a data decoding section 230, and a compensation function estimating section 240.

[0027] The electrical signal conversion unit 210 converts the optical signal received from the optical transmitting device 10 into an electrical signal. The electrical signal conversion unit 210 includes, for example, a laser module and an AD converter. The electrical signal conversion unit 210 converts the optical signal received from the optical transmitting device 10 into a baseband signal using local oscillation light from the laser module, and then converts the baseband signal into an electrical signal using the AD converter, thereby converting the optical signal into an electrical signal. The electrical signal conversion unit 210 may convert the optical signal received from the optical transmitting device 10 into an electrical signal using, for example, direct detection.

[0028] The transmission path compensator 220 compensates for waveform degradation of the electrical signal generated by the electrical signal converter 210. Waveform degradation occurs in the optical signal due to chromatic dispersion, polarization fluctuation, nonlinear optical effects, and the like in the optical transmission path 30.

[0029] The data decoding unit 230 decodes the electrical signal whose waveform degradation has been compensated for by the transmission path compensating unit 220. The data decoding unit 230 decodes the error correction code of the electrical signal whose waveform degradation has been compensated for by the transmission path compensating unit 220, for example, and restores the binary information transmitted by the optical transmitting device 10.

[0030] The compensation function estimation unit 240 estimates a compensation function based on the electrical signal whose waveform degradation has been compensated for by the transmission path compensation unit 220. The compensation function estimated by the compensation function estimation unit 240 is used to compensate for waveform degradation occurring in the optical transmitting device 10. The compensation function estimation unit 240 estimates the compensation function by performing signal processing with a configuration that simulates the digital signal processing unit 11 of the optical transmitting device 10.

[0031] When the low-speed signal generating unit 120 performs a Hilbert transform on the modulated signal sequence generated by the transmission signal generating unit 110 to generate a Hilbert transform signal, the compensation function estimating unit 240 performs a Hilbert transform on the electrical signal whose waveform distortion has been compensated for by the transmission path compensating unit 220 in the same manner as the low-speed signal generating unit 120. When the low-speed signal generating unit 120 performs MIMO (multiple-input / multiple-output) signal processing on the modulated signal sequence and Hilbert transform signal generated by the transmission signal generating unit 110, the compensation function estimating unit 240 performs MIMO (multiple-input / multiple-output) signal processing on the electrical signal whose waveform distortion has been compensated for by the transmission path compensating unit 220 and the Hilbert transform signal.

[0032] When the low-speed signal generating section 120 performs processing to compensate for nonlinear waveform distortion of the signal, the compensation function estimating section 240 also performs processing to compensate for nonlinear waveform distortion of the signal in the same manner as the low-speed signal generating section 120 .

[0033] The compensation function estimation unit 240 calculates a compensation function based on the electrical signal whose waveform distortion has been compensated for by the transmission path compensation unit 220 and the signal generated by the low-speed signal generation unit 120. The compensation function estimation unit 240 calculates, for example, the difference between the electrical signal whose waveform distortion has been compensated for by the transmission path compensation unit 220 and the signal generated by the low-speed signal generation unit 120, and calculates a compensation function so as to reduce the difference.

[0034] The compensation function estimation unit 240 sequentially calculates the gradient of each process from the output side using, for example, backpropagation, updates each process based on the gradient, and calculates a compensation function. The compensation function estimation unit 240 updates the filter components that perform each process based on, for example, the gradient. The compensation function includes filter coefficients used in MIMO signal processing. When the low-speed signal generation unit 120 performs processing to compensate for nonlinear waveform distortion of the signal, the compensation function includes filter coefficients used in the processing to compensate for the nonlinear waveform distortion of the signal.

[0035] As a method for updating each process based on the gradient, for example, there is a stochastic gradient descent method or a method using Adam disclosed in Non-Patent Document 2.

[0036] The compensation function estimating unit 240 transmits the calculated compensation function to the optical transmitting device 10. The compensation function estimating unit 240 may perform processing to correct a frequency shift that occurs in the filter coefficient due to the frequency difference, based on the frequency difference between the signal light and the local light when a high-speed signal is received, which is obtained in the waveform degradation compensation processing by the transmission path compensating unit 220.

[0037] The compensation function estimator 240 may include filter processing that simulates signal waveform distortion characteristics that occur in the high-speed signal generator 130, with the aim of reducing noise components that occur in analog parts such as the high-speed signal generator 130 and the transmission path 30. In this case, the compensation function estimator 240 includes filter processing that receives the low-speed signal generated by the low-speed signal generator 120 and outputs a simulation signal of the corresponding high-speed signal generated by the high-speed signal generator 130. In this case, the compensation function estimator 240 updates the filter coefficients in the low-speed signal processing so as to reduce the difference between the high-speed signal generated by inputting the modulated signal sequence generated by the transmission signal generator 110 to a compensation function and then inputting the low-speed signal generated by the low-speed signal to a simulation filter.

[0038] By performing such processing, the compensation function estimation unit 240 can eliminate the influence of noise generated in analog parts such as the high-speed signal generation unit 130 and the transmission path 30, compared to when updating the filter coefficients based on the difference between the electrical signal whose waveform degradation has been compensated for by the transmission path compensation unit 220 and the signal generated by the low-speed signal generation unit 120, thereby enabling higher quality compensation function estimation.

[0039] After the difference between the signal output by inputting the low-speed signal generated by the low-speed signal generation unit 120 and the corresponding modulated signal sequence generated by the transmission signal generation unit 110 becomes smaller than a predetermined value (after becoming sufficiently small), the compensation function estimation unit 240 performs high-speed signal processing, in which the high-speed signal generated by the high-speed signal generation unit 130 is input and the same high-speed signal is output. At this time, the compensation function estimation unit 240 updates the filter coefficients in the high-speed signal processing so that the difference between the signal generated by performing high-speed signal processing and low-speed signal processing on the high-speed signal generated by the high-speed signal generation unit 130 and the high-speed signal generated by the high-speed signal generation unit 130 becomes smaller.

[0040] (Experiment) An experiment using the optical communication system 1 of this embodiment will be described below. In the experiment, the high-speed signal generation unit 130 generates four signals (XI, XQ, YI, YQ) that independently handle the polarization degrees of freedom X and Y and the in-phase / quadrature degree of freedom of the coherent optical signal. The optical modulation unit 140 converts these signals into optical signals using a polarization multiplexed IQ optical modulator or the like.

[0041] In this experiment, the modulation rate is 132 GBaud and the signaling method is stochastically shaped 64 QAM.

[0042] In this embodiment, the low-speed signal generation unit 120 performs a Hilbert transform and MIMO signal processing. In the comparative example, neither the Hilbert transform nor the MIMO signal processing is performed. In this embodiment, the compensation function estimation unit 240 performs a Hilbert transform and MIMO signal processing. In the comparative example, the optical receiving device 20 does not include the compensation function estimation unit 240. In other respects, the optical communication system in the comparative example is equivalent to the optical communication system 1 in this embodiment.

[0043] 7 is a diagram showing the signal-to-noise ratio (SNR) of the signal received by the optical receiving device 20 in this embodiment and the comparative example. The SNR in the comparative example was 14.30 dB, while the SNR in this embodiment was 17.33 dB. The SNR of the received signal in this embodiment was improved by about 3 dB compared to the comparative example.

[0044] (Other Embodiments) One embodiment of the present invention has been described in detail above with reference to the drawings, but the specific configuration is not limited to that described above, and various design changes and the like can be made within the scope that does not deviate from the gist of the present invention.

[0045] In this embodiment, the optical receiving device 20 transmits a compensation function to the optical transmitting device 10, but the optical receiving device 20 does not have to transmit a compensation function to the optical transmitting device 10 that transmits the optical signal to be received. For example, as in the optical communication system 1-a shown in Figure 8, when a combination of an optical transmitting device 10-a, an optical receiving device 20-a, an optical transmission line 30-a, and an optical amplifier 310-a and a combination of an optical transmitting device 10-b, an optical receiving device 20-b, an optical transmission line 30-b, and an optical amplifier 310-b are arranged in the same position and the transmission and reception directions are opposite, the optical receiving device 20-a may transmit a compensation function to the optical transmitting device 10-b, and the optical receiving device 20-b may transmit a compensation function to the optical transmitting device 10-a.

[0046] Furthermore, as in the optical communication system 1-b shown in FIG. 9, when a combination of an optical transmitting device 10-a and an optical receiving device 20-a and a combination of an optical transmitting device 10-b and an optical receiving device 20-b are arranged in the same position, the transmitting and receiving directions are opposite, and each combination is connected via an optical transmission path 30 and signal multiplexing devices 40, 41, the optical receiving device 20-a may transmit a compensation function to the optical transmitting device 10-b, and the optical receiving device 20-b may transmit a compensation function to the optical transmitting device 10-a.

[0047] All or part of the functions of the compensation function estimator 240 of the optical receiving device 20 in the above-described embodiment may be implemented by a computer. In this case, a program for implementing this function may be recorded on a computer-readable recording medium, and the program may be loaded into a computer system and executed. Note that the term "computer system" as used herein includes an OS and peripheral hardware. Furthermore, the term "computer-readable recording medium" refers to portable media such as flexible disks, optical magnetic disks, ROMs, and CD-ROMs, as well as recording devices such as hard disks built into a computer system. Furthermore, the term "computer-readable recording medium" may include media that dynamically store programs for a short period of time, such as communication lines used when transmitting programs via networks such as the Internet or communication lines such as telephone lines, and media that store programs for a certain 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 part of the above-described functions, or may be one that can implement the above-described functions in combination with a program already stored in the computer system. Furthermore, all or part of the functions of the compensation function estimating section 240 of the optical receiving device 20 may be realized using a programmable logic device such as an FPGA (Field Programmable Gate Array).

[0048] 1 Optical communication system, 10 Optical transmitting device, 11 Digital signal processing unit, 110 Transmission signal generating unit, 120 Low-speed signal generating unit, 130 High-speed signal generating unit, 140 Optical modulation unit, 20 Optical receiving device, 210 Electrical signal converting unit, 220 Transmission path compensating unit, 230 Data decoding unit, 240 Compensation function estimating unit, 30 Optical transmission path, 310 Optical amplifier, 40 Signal multiplexing device

Claims

1. An optical receiving device comprising: an optical transmitting device that transmits optical signals using analog multiplexing technology; and a compensation function estimating unit that performs signal processing on received optical signals using a configuration that simulates the configuration of a digital signal processing unit that performs digital signal processing on a transmission data sequence, and estimates a compensation function used when the digital signal processing unit performs digital signal processing.

2. The optical receiving device according to claim 1, wherein the digital signal processing unit performs MIMO (multiple-input / multiple-output) signal processing, and the compensation function includes a filter coefficient in the MIMO signal processing.

3. The optical receiving device according to claim 1 or 2, wherein the digital signal processing unit performs processing to compensate for nonlinear waveform distortion of the optical signal, and the compensation function includes a filter coefficient used in processing to compensate for nonlinear waveform distortion of the optical signal.

4. An optical communication system comprising an optical transmitting device that transmits optical signals using analog multiplexing technology and an optical receiving device that receives the optical signals, wherein the optical transmitting device comprises a digital signal processing unit that performs digital signal processing on a transmission data series, and the optical receiving device comprises a compensation function estimation unit that estimates a compensation function used when the digital signal processing unit performs digital signal processing by performing signal processing on the received optical signal with a configuration that simulates the configuration of the digital signal processing unit.

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