CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Phase of International Application No. PCT/JP2019/033474 filed on Aug. 27, 2019, which claims priority to Japanese Application No. 2018-180887 filed on Sep. 26, 2018. The entire disclosures of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a chromatic dispersion compensation device, a chromatic dispersion compensation method, and an optical reception device.

BACKGROUND ART

In an optical communication, it is known that waveforms of optical signals are distorted due to the chromatic dispersion effect of an optical fiber in a process of propagation of the optical signals in the optical fiber. Chromatic dispersion is a phenomenon that occurs when a propagation velocity in the optical fiber varies depending on wavelengths, and the longer the optical fiber, the greater the effect of chromatic dispersion.

Conventionally, compensation of waveform distortion caused by the chromatic dispersion has been performed using a dispersion compensation fiber having chromatic dispersion characteristics opposite to that of the optical fiber of an optical transmission line. On the other hand, in coherent optical communication in recent years, chromatic dispersion compensation has been performed by digital signal processing by utilizing phase information obtained on a receiving side.

The coherent optical communication is performed by an optical transmission system S as shown inFIG. 15, for example. The optical transmission system S includes an optical transmission device300, an optical reception device400, and an optical transmission line1000. The optical transmission line1000is formed of an optical fiber that connects the optical transmission device300and the optical reception device400. In the optical transmission device300, an electric signal generation unit310takes in transmission information given from an external information source, and encode the taken transmission information to generate an electric signal. An optical signal generation unit311converts the electric signal generated by the electric signal generation unit310into an optical signal and transmits the optical signal.

The optical transmission line1000transmits the optical signal transmitted from the optical transmission device300to the optical reception device400. In the optical reception device400, a coherent optical reception unit401includes a 90° optical hybrid circuit, a local oscillation light source, a photodetector, an optical fiber that connects these components, and receives an optical signal using a coherent detection scheme.

In the digital signal processing unit402, an analog-digital conversion unit411quantizes an electric analog signal output from the coherent optical reception unit401and converts it into a digital signal. A chromatic dispersion compensation processing unit412takes in the digital signal as a reception signal, and performs chromatic dispersion compensation on the taken reception signal. An adaptive equalization processing unit413performs adaptive equalization processing on the reception signal subjected to the chromatic dispersion compensation. A phase compensation unit414performs compensation on a phase offset generated between light sources of a transmitter and a receiver. A frequency offset compensation unit415performs compensation of a frequency offset generated between the light sources of the transmitter and the receiver. An error correction unit416performs error correction on the reception signal output from the frequency offset compensation unit415based on an error correction code scheme, and outputs the error-corrected reception signal to the outside.

FIG. 16is a diagram showing an example of a procedure of chromatic dispersion compensation processing in the optical transmission system S. The optical transmission device300transmits, as a transmission signal, an optical signal having a certain occupied band in both directions of a plus direction and a minus direction of a frequency with respect to a center frequency. When a transmission signal is shown on a coordinate axis having a horizontal axis as a time and a vertical axis as a frequency, the transmission signal is represented by a rectangular shape (step S101).

As described above, when the transmission signal propagates through the optical fiber of the optical transmission line1000, the waveform distortion occurs because the propagation velocity of each frequency of the transmission signal is different due to the chromatic dispersion effect of the optical fiber. The waveform distortion can be represented, for example, as a phase delay that occurs in the waveform, and the phase delay can be represented as an arithmetic operation of multiplying the transmission signal by a transfer function that represents chromatic dispersion characteristics approximated by a quadratic function (step S102).

The coherent optical reception unit401of the optical reception device400receives the optical signal transmitted from the optical transmission line1000by coherent detection. The analog-digital conversion unit411converts the electric analog signal output from the coherent optical reception unit401into a digital signal.

The digital signal obtained by the conversion is represented in a shape of a parallelogram because the phase delay causes a substantial linear delay as the frequency increases (step S103).

As a chromatic dispersion compensation method using the digital signal processing performed by the chromatic dispersion compensation processing unit412, there is a method of equalizing a frequency domain. Such a method is performed by converting a time-domain reception signal into a frequency-domain reception signal by Fourier transform and returning to the time-domain reception signal by inverse Fourier transform after multiplying by a transfer function (hereinafter, also referred to as inverse transfer function) having a transfer function representing the characteristics of the chromatic dispersion received in the optical transmission line1000.

In addition, since the chromatic dispersion compensation in the frequency domain is block processing, inter-block interference (hereinafter, also referred to as IBI) occurs in which distortion remains in signals near both ends of the block. In order to prevent such an IBI, for example, a frequency domain equalizing method combined with overlap processing is used.

As shown inFIG. 17, the overlap processing is performed by a procedure including a process of selecting a section including an overlapping part having a length based on a length of the overlap part, a process of equalizing the frequency domain, and a process of removing the overlap part. Here, as the process of equalizing the frequency domain, chromatic dispersion compensation will be described as an example.

The chromatic dispersion compensation processing unit412takes in the digital signal output from the analog-digital conversion unit411as a reception signal, and selects, for the taken reception signal, sections500,501,502,503,504,505, . . . in time-series order. Here, it is assumed that a length of each of the sections500,501,502, . . . is predetermined to be10boxes. Each of the boxes includes a fixed number of reception signal samples.

The chromatic dispersion compensation processing unit412selects each of the sections500,501,502, . . . such that overlapping parts700,701,702, . . . having lengths determined based on the lengths of the overlap parts occur in adjacent sections overlap part.

The chromatic dispersion compensation processing unit412sets partial reception signals selected from the entire reception signals according to the selected sections500,501,502, . . . as processing blocks600,601,602,603,604,605, . . . , respectively. The chromatic dispersion compensation processing unit412performs Fourier transform on each of the processing blocks600,601,602, . . . , and converts time-domain reception signals into frequency-domain reception signals. The chromatic dispersion compensation processing unit412multiplies each of the processing blocks600,601,602, . . . including the frequency-domain reception signals by an inverse transfer function to perform a process of compensating distortion due to chromatic dispersion (step S104).

The chromatic dispersion compensation processing unit412performs an inverse Fourier transform on the frequency-domain signals multiplied by the inverse transfer function to return it to the time-domain signal. Thus, a rectangular signal similar to the transmission signal can be obtained as the reception signal subjected to the chromatic dispersion compensation (step S105).

As described above, since the chromatic dispersion compensation in the frequency domain is block processing, IBI is generated near both ends of the processing blocks600,601,602, . . . subjected to the inverse Fourier transform. In order to prevent such an IBI, the chromatic dispersion compensation processing unit412predetermines, as overlapping parts, parts having predetermined lengths at both ends of each of the processing blocks600,601,602, . . . .

For example, as shown inFIG. 18, 10boxes of each of the processing blocks600,601, and602are indicated as boxes600-1to600-10,601-1to601-10, and602-1to602-10, respectively.

In the chromatic dispersion compensation processing unit412, it is assumed that one box at both ends of each of the processing blocks600,601,602, . . . is predetermined as an overlap part. Then, in the case of the processing block600, the boxes600-1and600-10become overlap parts600OL-L and600OL-R, respectively. In the case of the processing block601, the boxes601-1and601-10become overlap parts601OL-L and601OL-R, respectively. In the case of the processing block602, the boxes602-1and602-10become overlap parts602OL-L and602OL-R, respectively.

At this time, regarding the overlapping part700, the box600-10of the overlap part600OL-R of the processing block600overlaps with the box601-2of the processing block601. On the other hand, the box601-1of the overlap part601OL-L of the processing block601overlaps with the box600-9of the processing block600. Accordingly, even when the overlap part600OL-R of the processing block600is removed, samples of the reception signal of the overlap part600OL-R remain in the box601-2of the processing block601. In addition, even when the overlap part601OL-L of the processing block601is removed, samples of the reception signal of the overlap part601OL-L remain in the box600-9of the processing block600.

In other words, even when the overlap parts600OL-L,600OL-R,601OL-L,601OL-R, . . . are removed in any of the processing blocks600,601, . . . , samples corresponding to the removed overlap parts600OL-L,600OL-R,601OL-L,601OL-R, . . . remain in the other processing blocks600,601, . . . . Therefore, in the overlapping parts700,701, . . . , the length of each of the overlapping parts700,701, . . . is determined as, for example, two boxes as shown inFIG. 18, so that the overlap parts600OL-L,600OL-R,601OL-L,601OL-R, . between the different processing blocks600,601, . . . do not become the same reception signal sample.

In the first and last processing blocks, for example, the first processing block600shown inFIG. 17, the left end overlap part600OL-L does not overlap with other processing blocks601,602, . . . . For this reason, regarding the overlap part600OL-L, samples do not remain as an exception due to the removal, but the IBI is prevented by the removal.

For example, as shown inFIG. 19, it is assumed that the overlap part of the processing block601is set to have a length twice that inFIG. 18. In this case, the overlap part601OL-L of the processing block601becomes the boxes601-1and601-2, and the overlap part601OL-R becomes the boxes601-9and601-10. Accordingly, the lengths of the overlapping parts700and701also need to be doubled.

The length of each of the overlapping parts700,701, . . . is not limited to twice the length of each of the overlap parts600OL-L,600OL-R,601OL-L,601OL-R, . . . , and may be twice or more. For example, in the overlapping parts700,701, . . . shown inFIG. 19, the box of the overlap part may be one box at both ends of each of the processing blocks600,601, . . . , that is,600-1,600-10,601-1,601-10, . . . may be the overlap parts.

Returning toFIG. 16, the chromatic dispersion compensation processing unit412removes the overlap parts6000R-L,6000R-R,6010R-L,6010R-R, . . . of the processing blocks600,601,602, . . . for the reception signals (step S106).

The chromatic dispersion compensation processing unit412combines the receptions signals, from which the overlap parts600OL-L,600OL-R,601OL-L,601OL-R, . . . are removed, to be a continuous signal sequence. Thus, the IBI is prevented, and the reception signals subjected to chromatic dispersion compensation can be obtained.

By the way, the amount of chromatic dispersion capable of being compensated by the chromatic dispersion compensation processing unit412depends on the number of samples that can be removed in the overlap processing. For example, the larger the number of samples that can be removed, that is, the longer the overlap parts6000R-L,6000R-R,6010R-L,6010R-R, . . . , the larger the amount of chromatic dispersion that can be compensated.

Since the amount of phase delay caused by the chromatic dispersion also increases as the length of the optical fiber of the optical transmission line1000becomes longer, it is necessary to increase the amount of chromatic dispersion that can be compensated accordingly. For this reason, it is necessary to increase the lengths of the overlap parts6000R-L,6000R-R,6010R-L,6010R-R, . . . . In order to increase the lengths of the overlap part6000R-L,6000R-R,6010R-L,6010R-R, . . . , it is necessary to increase the lengths of the sections500,501,502, . . . and increase the sizes of the processing blocks600,601,602, . . . .

However, the increase in size of the processing blocks600,601,602, . . . may cause a problem of increasing the circuit scale and power consumption of the digital signal processing unit402. To solve such a problem, there has been proposed a method of expanding the amount of chromatic dispersion capable of being compensated without changing the size of the processing block (for example, see Patent Literature 1).

A chromatic dispersion compensation processing unit412ashown inFIG. 20will be described below as an example of a configuration for expanding the amount of chromatic dispersion capable of being compensated without changing the size of the processing block. The chromatic dispersion compensation processing unit412ais a function unit applied in place of the chromatic dispersion compensation processing unit412of the digital signal processing unit402in the optical reception device400shown inFIG. 15.

A procedure of processing to be performed by the chromatic dispersion compensation processing unit412ashown inFIG. 20will be described with reference toFIG. 21. In the chromatic dispersion compensation processing unit412a,a section selection unit800takes in a digital signal output from an analog-digital conversion unit411as a reception signal. Since the reception signal is affected by the chromatic dispersion, the reception signal has a shape of a parallelogram in which the delay occurs substantially linearly as the frequency increases (step S201).

The section selection unit800performs section selection processing in the overlap processing described with reference toFIG. 17. In other words, the section selection unit800selects, for the taken reception signal, sections in which the overlapping parts700,701, . . . are generated between sections adjacent to each other in time-series order, and sequentially outputs the processing blocks600,601, . . . for each of the sections to a Fourier transform unit801.

The Fourier transform unit801converts time-domain reception signals included in the processing blocks600,601, . . . determined by each of the sections selected by the section selection unit800into frequency-domain reception signals. In addition, the Fourier transform unit801divides each of the processing blocks600,601, . . . including the frequency-domain reception signal into two frequency bands of a positive frequency band and a negative frequency band with a center frequency as a reference, for example, and outputs it as a division processing block.

Each of chromatic dispersion compensators (hereinafter, also referred to as CDC)812-1and812-2takes in a division processing block of a corresponding frequency band, and multiplies an inverse transfer function with a center of the frequency band as a phase rotation axis to perform chromatic dispersion compensation (step S202). A phase rotation axis900shown in step S202ofFIG. 21is, for example, a phase rotation axis of the inverse transfer function applied to one chromatic dispersion compensator812-1, and a phase rotation axis901is a phase rotation axis of the inverse transfer function applied to the other chromatic dispersion compensator812-2.

Inverse Fourier transform units813-1and813-2transform the reception signals included in the division processing blocks subjected to the chromatic dispersion compensation by the chromatic dispersion compensators812-1and812-2into time-domain reception signals (step S203). Overlap cut units814-1and814-2performs a process of removing the overlap parts in the overlap processing. In other words, each of the overlap cut units814-1and814-2removes the overlap part having a predetermined length from both ends of the division processing block output from each of the inverse Fourier transform units813-1and813-2(step S204).

Delay units815-1and815-2give a delay to the division processing block, from which the overlap part is removed, for each of the frequency bands, eliminate the shift caused by the chromatic dispersion effect, and adjust a timing on a time axis (step S205).

Fourier transform units820-1and820-2of the combination processing unit803transform reception signals included in the division processing block subjected to the timing adjustment into frequency-domain reception signals. The combination unit821combines the division processing blocks of the two frequency bands so as to be continuous on a frequency axis and generates the processing blocks600,601, . . . . An inverse Fourier transform unit822transforms reception signals included in the processing blocks600,601, . . . combined by the combination unit821into time-domain reception signals, and outputs the reception signals to an adaptive equalization processing unit413.

The chromatic dispersion compensation processing unit412adivides the frequency-domain band into two bands, performs chromatic dispersion compensation on the reception signal in each of the divided frequency bands, and then adjusts the amount of delay between the frequency bands in the time domain. Thus, the maximum amount of phase delay caused by the chromatic dispersion for each frequency band is reduced to half the amount in the case of not dividing into two frequency bands. The fact that the amount of phase delay can be reduced to ½ means that the length of the overlap part can also be halved.

Therefore, if the length of the overlap part is set to the same length as in the case of not dividing into two frequency bands, it is possible to make the amount of chromatic dispersion capable of being compensated double without changing the sizes of the processing blocks600,601,602, . . . . In terms of generalization, when the Fourier transform unit801divides the block into N frequency bands, the amount of phase delay and the length of the overlap part can be reduced to 1/N as compared with the case where the block is not divided, and the amount of chromatic dispersion capable of being compensated can be increased to N times.

CITATION LIST

Patent Literature

Patent Literature 1: Japanese Patent Laid-Open No. 2018-093280

SUMMARY OF THE INVENTION

Technical Problem

However, in the chromatic dispersion compensation processing unit412ashown inFIG. 20, there is a problem that the signal quality is deteriorated due to the distortion of the waveform at both ends of the processing blocks600,601, . . . of the output signal of the inverse Fourier transform unit822by the combination processing of the processing blocks for each frequency band in the combination processing unit803.

In view of the above circumstances, an object of the present invention is to provide a technique capable of reducing deterioration of signal quality caused by division of the frequency band in the chromatic dispersion compensation process involving the process of dividing the frequency band.

Means for Solving the Problem

An aspect of the present invention is to provide a chromatic dispersion compensation device that divides a reception signal obtained by receiving an optical signal using a coherent detection scheme into a plurality of frequency bands, adjusts a timing on a time axis of the reception signal for each of the divided frequency bands, performs combination processing for combining the reception signals included in the plurality of frequency bands, and performs chromatic dispersion compensation on the reception signal at any timing before or after the combination processing, the device including: a plurality of pre-combination section selection unit provided for each of the frequency bands and configured to select, before the combination processing, sections in which overlapping parts determined based on lengths of overlap parts predetermined between sections adjacent to each other in time-series order with respect to the reception signal for each of the frequency bands are generated, and to output the reception signal for each of the selected sections as a division processing block; and a post-combination overlap cut unit configured to remove the overlap parts from both ends of a processing block generated by combination of the division processing blocks in the combination processing so as to be continuous on a frequency axis.

In the chromatic dispersion compensation device according to the aspect of the present invention, the chromatic dispersion compensation device further includes: a section selection unit configured to select sections in which a first overlapping part determined based a length of a first overlap part determined between sections adjacent to each other in time-series order with respect to the reception signal are generated and to output the reception signals included in the selected sections as processing blocks; a Fourier transform unit configured to transform the output reception signal for each of the processing blocks into a frequency domain, to divide the processing block including the transformed reception signal into a plurality of frequency bands, and to output the processing block as the division processing block; a chromatic dispersion compensation unit configured to perform chromatic dispersion compensation on the division processing block of the frequency band and to remove the first overlap part from both ends of the division processing block; and a combination processing unit configured to perform the combination processing, wherein the pre-combination section selection unit uses the overlapping part as a second overlapping part, arranges the division processing blocks in time-series order, selects sections in which the second overlapping part is generated between sections adjacent to each other in time-series order, and outputs the reception signals included in the selected sections as the division processing blocks, the combination processing unit combines the division processing blocks to generate the processing block, and the post-combination overlap cut unit uses the overlap part as a second overlap part, and removes the second overlap part from both ends of the processing block.

In the chromatic dispersion compensation device according to the aspect of the present invention, the chromatic dispersion compensation unit does not remove the first overlap part from both ends of the division processing block, the pre-combination section selection unit arranges the division processing blocks in time-series order and outputs the division processing blocks directly, and the post-combination overlap cut unit removes the first overlap part from both ends of the processing block.

An aspect of the present invention is to provide a chromatic dispersion compensation method of dividing a reception signal obtained by receiving an optical signal using a coherent detection scheme into a plurality of frequency bands, adjusting a timing on a time axis of the reception signal for each of the divided frequency bands, performing combination processing for combining the reception signals included in the plurality of frequency bands, and performing chromatic dispersion compensation on the reception signal at any timing before or after the combination processing, the method including: selecting, before the combination processing, sections in which overlapping parts determined based on lengths of overlap parts predetermined between sections adjacent to each other in time-series order with respect to the reception signal for each of the frequency bands are generated; outputting the reception signal for each of the selected sections as a division processing block; and removing the overlap parts from both ends of a processing block generated by combination of the division processing blocks in the combination processing so as to be continuous on a frequency axis.

An aspect of the present invention is to provide an optical reception device including: a coherent optical reception unit configured to receive an optical signal using a coherent detection scheme; and a digital signal processing unit, wherein the digital signal processing unit includes an analog-digital conversion unit configured to convert an electric analog signal output from the coherent optical reception unit into a digital signal and to output the digital signal, and a chromatic dispersion compensation processing unit serving as a chromatic dispersion processing unit that is the chromatic dispersion compensation device according to any one of claims1to3and configured to take in the digital signal output from the analog-digital conversion unit as the reception signal.

Effects of the Invention

According to the present invention, it is possible to reduce the deterioration of the signal quality caused by division of the frequency band in the chromatic dispersion compensation process involving the process of dividing the frequency band.

