Receiving device and receiving method

There is provided a receiving device including a hardware processor configured to demodulate a signal into which a first signal and a second signal are wavelength-multiplexed, into a first baseband signal and a second baseband signal corresponding to the first signal and the second signal, respectively, extract, from the second baseband signal, a signal component of crosstalk from the second signal to the first signal, shift a frequency of the extracted signal component, and compensate for the crosstalk from the second signal to the first signal, based on the extracted signal component shifted by the frequency.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2017-020553, filed on Feb. 7, 2017, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a receiving device and a receiving method.

BACKGROUND

A technique for performing a crosstalk compensation by MIMO processing on a wavelength division multiplexed optical signal propagating through an optical fiber transmission line having wavelength dispersion is known in the related art. A technique for applying MIMO processing to compensate for a crosstalk between spatially multiplexed signals and separate them from each other is also known. MIMO is an acronym for multiple input multiple output.

Related techniques are disclosed in, for example, Japanese Laid-Open Patent Publication No. 2016-100815 and International Publication Pamphlet No. WO 2015/052895.

SUMMARY

According to an aspect of the invention, a receiving device includes a hardware processor configured to demodulate a signal into which a first signal and a second signal are wavelength-multiplexed, into a first baseband signal and a second baseband signal corresponding to the first signal and the second signal, respectively, extract, from the second baseband signal, a signal component of crosstalk from the second signal to the first signal, shift a frequency of the extracted signal component, and compensate for the crosstalk from the second signal to the first signal, based on the extracted signal component shifted by the frequency.

DESCRIPTION OF EMBODIMENTS

In the above-described related art, the scale of a circuit for compensating a crosstalk may increase in some cases. For example, a process of compensating the crosstalk is performed based on a signal to be compensated and an adjacent signal. For this reason, for example, a wide signal band is required for a frequency shifter that performs frequency shift of the adjacent signal and a circuit that performs MIMO processing, which may result in the increase in the scale of the circuit. The compensation of the crosstalk is, for example, to remove at least a portion of a crosstalk component included in a signal (a signal component leaked out from another signal).

Embodiments of a technique capable of compensating a crosstalk while suppressing an increase in a circuit scale will be described in detail below with reference to the accompanying drawings.

First Embodiment

(Receiving Device According to First Embodiment)

FIG. 1is a view illustrating an example of a receiving device according to a first embodiment. As illustrated inFIG. 1, the receiving device100according to the first embodiment includes, for example, coherent receiving FEs111to113, ADCs121to123, and adaptive equalization units131to133. The receiving device100further includes, for example, carrier wave phase synchronization units141to143, delay compensation units151to153, and crosstalk compensation units161to163. FE is an acronym for front-end. ADC is an acronym for analog/digital converter.

The receiving device100receives signal light101, for example. The signal light101is an optical signal transmitted from a counterpart of the receiving device100via an optical transmission line. In addition, the signal light101is signal light obtained by multiplexing a plurality of optical signals having different wavelengths. In the example illustrated inFIG. 1, optical signals of channels ch1to ch3are wavelength-multiplexed in the signal light101. The center frequency of the optical signal of the channel ch1is a frequency f1. The center frequency of the optical signal of the channel ch2is a frequency f2. The center frequency of the optical signal of the channel ch3is a frequency f3.

For example, the signal light101is signal light obtained by wavelength-multiplexing the respective optical signals of the channels ch1to ch3at an optical stage. Alternatively, the signal light101may be signal light obtained by wavelength-multiplexing the signals (electric signals) of the channels ch1to ch3in an electric stage and converting them into an optical signal. As another alternative, the signal light101may be signal light obtained by combining wavelength multiplexing in the optical stage and wavelength multiplexing in the electric stage.

In addition, the band of the optical signal of the channel ch1and the band of the optical signal of the channel ch2partially overlap with each other. In addition, the band of the optical signal of the channel ch2and the band of the optical signal of the channel ch3partially overlap with each other. Therefore, a crosstalk between the channels occurs in the signal light101. The receiving device100accurately decodes each signal included in the signal light101by compensating a crosstalk of each signal included in the received signal light101. The compensation of the crosstalk of the signal is, for example, to remove at least a portion of a crosstalk component included in the signal (e.g., a signal component leaked out from another signal).

The signal light101is branched by, for example, a branching unit such as an optical coupler and is incident into each of the coherent receiving FEs111to113. The coherent receiving FE111is an optical front end that coherently receives the optical signal of the channel ch1included in the incident signal light101using local light emission. For example, the coherent receiving FE111mixes the incident signal light101and the local light emission of the frequency f1, and receives the mixture. As a result, a baseband signal of the channel ch1included in the signal light101is obtained. The coherent receiving FE111outputs the obtained signal of the channel ch1to the ADC121.

Similarly, the coherent receiving FE112coherently receives the optical signal of the channel ch2included in the incident signal light101using the local light emission of the frequency f2and outputs a signal of the channel ch2obtained by the coherent reception to the ADC122. In addition, the coherent receiving FE113coherently receives the optical signal of the channel ch3included in the incident signal light101using the local light emission of the frequency f3and outputs a signal of the channel ch3obtained by the coherent reception to the ADC123.

The ADC121converts the signal of the channel ch1output from the coherent receiving FE111from an analog signal to a digital signal and outputs the obtained digital signal to the adaptive equalization unit131. The ADC122converts the signal of the channel ch2output from the coherent receiving FE112from an analog signal to a digital signal and outputs the converted digital signal to the adaptive equalization unit132. The ADC123converts the signal of the channel ch3output from the coherent receiving FE113from an analog signal to a digital signal and outputs the converted digital signal to the adaptive equalization unit133.

The adaptive equalization units131to133, the carrier wave phase synchronization units141to143, and the delay compensation units151to153are demodulation processing units that demodulate signals for the different channels in the baseband.

The adaptive equalization unit131performs an adaptive equalization process on the signal of the channel ch1output from the ADC121. As a result, an X polarized signal and a Y polarized signal included in the signal of the channel ch1are obtained. The X polarized signal and the Y polarized signal are, for example, polarized components whose polarization directions are orthogonal to each other. The adaptive equalization unit131outputs the obtained X polarized signal and Y polarized signal of the channel ch1to the carrier wave phase synchronization unit141.

Similarly, the adaptive equalization unit132performs an adaptive equalization process on the signal of the channel ch2output from the ADC122to obtain an X polarized signal and a Y polarized signal of the channel ch2, which are then output to the carrier wave phase synchronization unit142. In addition, the adaptive equalization unit133performs an adaptive equalization process on the signal of the channel ch3output from the ADC123to obtain an X polarized signal and a Y polarized signal of the channel ch3, which are then output to the carrier wave phase synchronization unit143.

The carrier wave phase synchronization unit141performs a carrier wave phase synchronization process on the X polarized signal and the Y polarized signal of the channel ch1output from the adaptive equalization unit131. The carrier wave phase synchronization process may be performed, for example, by performing timing synchronization, polarization separation, and carrier wave phase reproduction and restoring a constellation. The carrier wave phase synchronization unit141outputs the respective signals subjected to the carrier wave phase synchronization process to the delay compensation unit151.

Similarly, the carrier wave phase synchronization unit142performs a carrier wave phase synchronization process on the X polarized signal and the Y polarized signal of the channel ch2output from the adaptive equalization unit132. Then, the carrier wave phase synchronization unit142outputs the X polarized signal and the Y polarized signal of the channel ch2subjected to the carrier wave phase synchronization process to the delay compensation unit152. In addition, the carrier wave phase synchronization unit143performs a carrier wave phase synchronization process on the X polarized signal and the Y polarized signal of the channel ch3output from the adaptive equalization unit133. Then, the carrier wave phase synchronization unit143outputs the X polarized signal and the Y polarized signal of the channel ch3subjected to the carrier wave phase synchronization process to the delay compensation unit153.

The delay compensation unit151performs a delay compensation on the X polarized signal and the Y polarized signal of the channel ch1output from the carrier wave phase synchronization unit141, and outputs the respective signals subjected to the delay compensation to the crosstalk compensation units161and162. As a result, the demodulated X polarized signal and Y polarized signal of the channel ch1are output to the crosstalk compensation units161and162. The delay compensation may be performed, for example, by compensating a group delay difference between the channels based on a frequency spacing between the optical signals and a dispersion value in an optical transmission line through which the signal light101is transmitted.

Similarly, the delay compensation unit152performs delay compensation on the X polarized signal and the Y polarized signal of the channel ch2output from the carrier wave phase synchronization unit142, and outputs the respective signals subjected to the delay compensation to the crosstalk compensation units161to163. As a result, the demodulated X polarized signal and Y polarized signal of the channel ch2are output to the crosstalk compensation units161to163.

In addition, the delay compensation unit153performs delay compensation on the X polarized signal and the Y polarized signal of the channel ch3output from the carrier wave phase synchronization unit143, and outputs the respective signals subjected to the delay compensation to the crosstalk compensation units162and163. As a result, the demodulated X polarized signal and Y polarized signal of the channel ch3are output to the crosstalk compensation units162and163.

The crosstalk compensation unit161compensates for a crosstalk from each signal of the channel ch2with respect to each signal of the channel ch1(e.g., the X polarized signal and the Y polarized signal) output from the delay compensation unit151, based on each signal of the channel ch2output from the delay compensation unit152. Then, the crosstalk compensation unit161outputs each signal compensated for the crosstalk.

The crosstalk compensation unit162compensates for a crosstalk from each signal of the channels ch1and ch3with respect to each signal of the channel ch2output from the delay compensation unit152, based on each signal of the channels ch1and ch3output from the delay compensation units151and153. Then, the crosstalk compensation unit162outputs each signal compensated for the crosstalk.

The crosstalk compensation unit163compensates for a crosstalk from each signal of the channel ch2with respect to each signal of the channel ch3output from the delay compensation unit153, based on each signal of the channel ch2output from the delay compensation unit152. Then, the crosstalk compensation unit163outputs each signal compensated for the crosstalk.

The adaptive equalization units131to133, the carrier wave phase synchronization units141to143, the delay compensation units151to153, and the crosstalk compensation units161to163illustrated inFIG. 1may be implemented by a hardware processor (a digital circuit) such as DSP or FPGA. DSP is an acronym for digital signal processor. FPGA is an acronym for field programmable gate array.

In addition, the receiving device100may include a decoding unit that performs a decoding by determining each signal output from the crosstalk compensation units161to163. Decoding of a signal may be carried out, for example, by determining a symbol corresponding to a combination of phase and amplitude of the signal. This decoding unit may be implemented by a hardware processor such as DSP or FPGA.

The coherent reception of signal light (e.g., the signal light101) obtained by wavelength-multiplexing a first signal and a second signal may be implemented by, for example, the coherent receiving FEs111to113. A demodulation unit that demodulates the first signal and the second signal based on a result of the coherent reception may be implemented by, for example, the adaptive equalization units131to133, the carrier wave phase synchronization units141to143, and the delay compensation units151to153.

The configuration in which the receiving device100receives three wavelength-multiplexed optical signals has been described in the example illustrated inFIG. 1, but the present disclosure is not limited to such a configuration. For example, the receiving device100may be configured to receive two or four or more wavelength-multiplexed optical signals. For example, in a case of receiving N wavelength-multiplexed optical signals, the receiving device100includes N coherent receiving FEs, N ADCs, N adaptive equalization units, N carrier wave phase synchronization units, N delay compensation units, and N crosstalk compensation units.

(Crosstalk Compensation Unit (ch1) According to First Embodiment)

FIG. 2is a view illustrating an example of the crosstalk compensation unit (ch1) according to the first embodiment. The crosstalk compensation unit161that compensates for a crosstalk from each signal of the channel ch2with respect to each signal of the channel ch1(e.g., the X polarized signal and the Y polarized signal) will be described below. Referring toFIG. 2, the crosstalk compensation unit161may include BPFs211and212, frequency shift units221and222, multiplication units231to234, addition units241and242, and subtraction units251and252. BPF is an acronym for band pass filter.

The X polarized signal of the channel ch2output from the delay compensation unit152illustrated inFIG. 1is input to the BPF211. The BPF211extracts a signal component on the low frequency side of the input X polarized signal of the channel ch2to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch1in the X polarized signal of the channel ch2. Then, the BPF211outputs the extracted signal component to the frequency shift unit221.

The Y polarized signal of the channel ch2output from the delay compensation unit152illustrated inFIG. 1is input to the BPF212. The BPF211extracts a signal component on the low frequency side of the input Y polarized signal of the channel ch2to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch1in the Y polarized signal of the channel ch2. Then, the BPF212outputs the extracted signal component to the frequency shift unit222.

The frequency shift unit221frequency-shifts the signal component output from the BPF211. For example, the frequency shift unit221shifts the frequency of the signal component output from the BPF211to the high frequency side by f2-f1. The frequency shift unit221outputs the frequency-shifted signal component to the multiplication units231and233.

The frequency shift unit222frequency-shifts the signal component output from the BPF212. For example, the frequency shift unit222shifts the frequency of the signal component output from the BPF212to the high frequency side by f2-f1. The frequency shift unit222outputs the frequency-shifted signal component to the multiplication units232and234.

The multiplication units231to234, the addition units241and242, and the subtraction units251and252are compensation units that perform a crosstalk compensation by MIMO.

The multiplication unit231multiplies the signal component output from the frequency shift unit221by a coefficient W2xx and outputs the multiplied signal component to the addition unit241. The multiplication unit232multiplies the signal component output from the frequency shift unit222by a coefficient W2yx and outputs the multiplied signal component to the addition unit241. The multiplication unit233multiplies the signal component output from the frequency shift unit221by a coefficient W2xy and outputs the multiplied signal component to the addition unit242. The multiplication unit234multiplies the signal component output from the frequency shift unit222by a coefficient W2yy and outputs the multiplied signal component to the addition unit242.

The addition unit241adds the signal components output from the multiplication units231and232and outputs the added signal components to the subtraction unit251. When the coefficients W2xx and W2yx are optimally adjusted, the signal component output from the addition unit241to the subtraction unit251indicates a crosstalk component due to the signal of the channel ch2with respect to the X polarized signal of the channel ch1.

The addition unit242adds the signal components output from the multiplication units233and234and outputs the added signal components to the subtraction unit252. When the coefficients W2xy and W2yy are optimally adjusted, the signal component output from the addition unit242to the subtraction unit252indicates a crosstalk component due to the signal of the channel ch2with respect to the Y polarized signal of the channel ch1.

The X polarized signal of the channel ch1output from the delay compensating unit151illustrated inFIG. 1and the signal component output from the addition unit241are input to the subtraction unit251. The subtraction unit251subtracts the signal component output from the addition unit241from the input X polarized signal of the channel ch1. This makes it possible to compensate for a crosstalk due to the signal of the channel ch2with respect to the X polarized signal of the channel ch1. The subtraction unit251outputs the X polarized signal of the channel ch1compensated for crosstalk.

The Y polarized signal of the channel ch1output from the delay compensating unit151illustrated inFIG. 1and the signal component output from the addition unit242are input to the subtraction unit252. The subtraction unit252subtracts the signal component output from the addition unit242from the input Y polarized signal of the channel ch1. This makes it possible to compensate for a crosstalk due to the signal of the channel ch2with respect to the Y polarized signal of the channel ch1. The subtraction unit252outputs the Y polarized signal of the channel ch1compensated for crosstalk.

The crosstalk compensation unit161controls the coefficients W2xx, W2yx, W2xy, and W2yy in the multiplication units231to234based on an error rate of the signal of the channel ch1obtained by the decoding unit at the subsequent stage of the crosstalk compensation unit161. For example, the crosstalk compensation unit161monitors the error rate while changing a combination of coefficients, specifies each combination of coefficients with the lowest error rate, and sets the specified combination of coefficients in the multiplication units231to234.

Further, the crosstalk compensation unit161may calculate the crosstalk amount for the signal of the channel ch1based on the coefficients W2xx, W2yx, W2xy, and W2yy. For example, the crosstalk compensation unit161calculates the crosstalk amount for the signal of the channel ch1by W2xx^2+W2yx^2+W2xx^2+W2yy^2. Then, the crosstalk compensation unit161outputs information indicating the calculated crosstalk amount.

For example, the receiving device100transmits the signal output from the crosstalk compensation unit161to the transmitting device of the transmission source of the signal light101. As a result, the transmitting device of the transmission source of the signal light101may perform a process such as correcting a deviation of the frequency spacing of each channel. The deviation of the frequency spacing is caused by, for example, a frequency fluctuation of laser. Alternatively, the receiving device100may output the signal output from the crosstalk compensating unit161to the administrator of the receiving device100. As a result, the administrator of the receiving device100may monitor a crosstalk in the signal received by the receiving device100.

(Crosstalk Compensation Unit (ch2) According to First Embodiment)

FIG. 3is a view illustrating an example of the crosstalk compensation unit (ch2) according to the first embodiment. The crosstalk compensation unit162that compensates for a crosstalk from each signal of the channels ch1and ch3with respect to each signal of the channel ch2(e.g., the X polarized signal and the Y polarized signal) will be described below. Referring toFIG. 3, the crosstalk compensation unit162may include BPFs311to314, frequency shift units321to324, multiplication units331to338, addition units341to344, and subtraction units351and352.

The X polarized signal of the channel ch1output from the delay compensation unit151illustrated inFIG. 1is input to the BPF311. The BPF311extracts a signal component on the high frequency side of the input X polarized signal of the channel ch1to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch2in the X polarized signal of the channel ch1. Then, the BPF311outputs the extracted signal component to the frequency shift unit321.

The Y polarized signal of the channel ch1output from the delay compensation unit151illustrated inFIG. 1is input to the BPF312. The BPF312extracts a signal component on the high frequency side of the input Y polarized signal of the channel ch1to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch2in the Y polarized signal of the channel ch1. Then, the BPF312outputs the extracted signal component to the frequency shift unit322.

The frequency shift unit321frequency-shifts the signal component output from the BPF311. For example, the frequency shift unit321shifts the frequency of the signal component output from the BPF311to the low frequency side by f2-f1. The frequency shift unit321outputs the frequency-shifted signal component to the multiplication units331and333.

The frequency shift unit322frequency-shifts the signal component output from the BPF312. For example, the frequency shift unit322shifts the frequency of the signal component output from the BPF312to the low frequency side by f2-f1. The frequency shift unit322outputs the frequency-shifted signal component to the multiplication units332and334.

The multiplication units331to338, the addition units341to344, and the subtraction units351and352are compensation units that perform a crosstalk compensation by MIMO.

The multiplication unit331multiplies the signal component output from the frequency shift unit321by a coefficient W1xx and outputs the multiplied signal component to the addition unit341. The multiplication unit332multiplies the signal component output from the frequency shift unit322by a coefficient W1yx and outputs the multiplied signal component to the addition unit341. The multiplication unit333multiplies the signal component output from the frequency shift unit321by a coefficient W1xy and outputs the multiplied signal component to the addition unit342. The multiplication unit334multiplies the signal component output from the frequency shift unit322by a coefficient W1yy and outputs the multiplied signal component to the addition unit342.

The addition unit341adds the signal components output from the multiplication units331and332, and outputs the added signal components to the subtraction unit351. When the coefficients W1xx and W1yx are optimally adjusted, the signal component output from the addition unit341to the subtraction unit351indicates a crosstalk component due to the signal of the channel ch1with respect to the X polarized signal of the channel ch2.

The addition unit342adds the signal components output from the multiplication units333and334, and outputs the added signal components to the subtraction unit352. When the coefficients W1xy and W1yy are optimally adjusted, the signal component output from the addition unit342to the subtraction unit352indicates a crosstalk component due to the signal of the channel ch1with respect to the Y polarized signal of the channel ch2.

The X polarized signal of the channel ch3output from the delay compensating unit153illustrated inFIG. 1is input to the BPF313. The BPF313extracts a signal component on the low frequency side of the input X polarized signal of the channel ch3to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch2in the X polarized signal of the channel ch3. Then, the BPF313outputs the extracted signal component to the frequency shift unit323.

The Y polarized signal of the channel ch3output from the delay compensation unit153illustrated inFIG. 1is input to the BPF314. The BPF314extracts a signal component on the low frequency side of the input Y polarized signal of the channel ch3to extract a signal component that becomes a crosstalk with respect to the signal of the channel ch2in the Y polarized signal of the channel ch3. Then, the BPF314outputs the extracted signal component to the frequency shift unit324.

The frequency shift unit323frequency-shifts the signal component output from the BPF313. For example, the frequency shift unit323shifts the frequency of the signal component output from the BPF313to the high frequency side by f3-f2. The frequency shift unit323outputs the frequency-shifted signal component to the multiplication units335and337.

The frequency shift unit324frequency-shifts the signal component output from the BPF314. For example, the frequency shift unit324shifts the frequency of the signal component output from the BPF314to the high frequency side by f3-f2. The frequency shift unit324outputs the frequency-shifted signal component to the multiplication units336and338.

The multiplication unit335multiplies the signal component output from the frequency shift unit323by a coefficient W3xx and outputs the multiplied signal component to the addition unit343. The multiplication unit336multiplies the signal component output from the frequency shift unit324by a coefficient W3yx and outputs the multiplied signal component to the addition unit343. The multiplication unit337multiplies the signal component output from the frequency shift unit323by a coefficient W3xy and outputs the multiplied signal component to the addition unit344. The multiplication unit338multiplies the signal component output from the frequency shift unit324by a coefficient W3yy and outputs the multiplied signal component to the addition unit344.

The addition unit343adds the signal components output from the multiplication units335and336, and outputs the added signal components to the subtraction unit351. When the coefficients W3xx and W3yx are optimally adjusted, the signal component output from the addition unit343to the subtraction unit351indicates a crosstalk component due to the signal of the channel ch3with respect to the X polarized signal of the channel ch2.

The addition unit344adds the signal components output from the multiplication units337and338, and outputs the added signal components to the subtraction unit352. When the coefficients W3xy and W3yy are optimally adjusted, the signal component output from the addition unit344to the subtraction unit352indicates a crosstalk component due to the signal of the channel ch3with respect to the Y polarized signal of the channel ch2.

The X polarized signal of the channel ch2output from the delay compensating unit151illustrated inFIG. 1and the signal components output from the addition units341and343are input to the subtraction unit351. The subtraction unit351subtracts the signal components output from the addition units341and343from the input X polarized signal of the channel ch2. This makes it possible to compensate for a crosstalk due to the signals of the channels ch1and ch3with respect to the X polarized signal of the channel ch2. The subtraction unit351outputs the X polarized signal of the channel ch2compensated for crosstalk.

The Y polarized signal of the channel ch2output from the delay compensating unit151illustrated inFIG. 1and the signal components output from the addition units342and344are input to the subtraction unit352. The subtraction unit352subtracts the signal components output from the addition units342and344from the input Y polarized signal of the channel ch2. This makes it possible to compensate for a crosstalk due to the signals of the channels ch1and ch3with respect to the Y polarized signal of the channel ch2. The subtraction unit352outputs the Y polarized signal of the channel ch2compensated for crosstalk.

The crosstalk compensation unit162controls the coefficients W1xx, W1yx, W1xy, W1yy, W3xx, W3yx, W3xy, and W3yy in the multiplication units331to338based on an error rate of the signal of the channel ch2obtained by the decoding unit at the subsequent stage of the crosstalk compensation unit162. For example, the crosstalk compensation unit162monitors the error rate while changing a combination of coefficients, specifies each combination of coefficients with the lowest error rate, and sets the specified combination of coefficients in the multiplication units331to338.

Further, the crosstalk compensation unit162may calculate the crosstalk amount for the signal of the channel ch2based on the coefficients W1xx, W1yx, W1xy, W1yy, W3xx, W3yx, W3xy, and W3yy. For example, the crosstalk compensation unit162calculates the crosstalk amount for the signal of the channel ch2by W1xx^2+W1yx^2+W1xx^2+W1yy^2+W3xx^2W3yx^2+W3xx^2+W3yy^2. Then, the crosstalk compensation unit162outputs information indicating the calculated crosstalk amount.

For example, the receiving device100transmits the signal output from the crosstalk compensation unit162to the transmitting device of the transmission source of the signal light101. As a result, the transmitting device of the transmission source of the signal light101may perform a process such as correcting a deviation of the frequency spacing of each channel. Alternatively, the receiving device100may output the signal output from the crosstalk compensating unit162to the administrator of the receiving device100. As a result, the administrator of the receiving device100may monitor a crosstalk in the signal received by the receiving device100.

An extraction unit that extracts a signal component of crosstalk from the second signal to the first signal from a signal obtained by demodulation of the second signal may be implemented by, for example, the BPFs211and212illustrated inFIG. 2or the BPFs311to314illustrated inFIG. 3. A shift unit that frequency-shifts the extracted signal component may be implemented by, for example, the frequency shift units221and222illustrated inFIG. 2or the frequency shift units321to324illustrated inFIG. 3.

A compensation unit that compensates for a crosstalk on a signal obtained by demodulation of the first signal based on the frequency-shifted signal component may be implemented by, for example, the multiplication units231to234, the addition units241and242, and the subtraction units251and252illustrated inFIG. 2. Alternatively, this compensation unit may be implemented by the multiplication units331to338, the addition units341to344, and the subtraction units351and352illustrated inFIG. 3.

(Crosstalk Compensation Unit (ch3) According to First Embodiment)

The crosstalk compensation unit163that compensates for a crosstalk from the channel ch2with respect to each signal of the channel ch3has the same configuration as the crosstalk compensation unit161that compensates for the crosstalk from the channel ch2with respect to each signal of the channel ch1(seeFIG. 2).

(Crosstalk Compensation by Receiving Device According to First Embodiment)

FIG. 4is a view illustrating an example of crosstalk compensation by the receiving device according to the first embodiment. InFIG. 4, the same parts and units as inFIG. 1are denoted by the same reference numerals, and explanation of which will not be repeated. The compensation of crosstalk from the signals of the channels ch1and ch3with respect to each signal (X polarized signal and Y polarized signal) of the channel ch2by the crosstalk compensation unit162will be described below by way of an example with reference toFIG. 4.

In the signal light101, BW represents a signal band per optical signal. Since the signal light101includes three optical signals of channels ch1to ch3, the signal band of the signal light101is 3*BW. A crosstalk component411in the signal light101is a crosstalk component between the optical signals of the channels ch1and ch2. A crosstalk component412in the signal light101is a crosstalk component between the optical signals of the channels ch2and ch3.

Processing stages401to404inFIG. 4are the stages of a receiving process by the receiving device100. The signals of the channels ch1to ch3included in the signal light101are demodulated by the coherent receiving FEs111to113, the ADCs121to123, the adaptive equalization units131to133, the carrier wave phase synchronization units141to143, and the delay compensation units151to153.

As a result, as shown in the processing stage401, baseband channels ch1to ch3are obtained. At this time, the crosstalk component411between the signal of the channel ch2and the signal of the channel ch1, and the crosstalk component412between the signal of the channel ch2and the signal of the channel ch3, are included in the signal of the channel ch2to be subjected to the crosstalk compensation.

Next, as shown in the processing stage402, the crosstalk compensation unit162extracts a crosstalk component421of the signal of the channel ch1with respect to the signal of the channel ch2by the BPFs311and312illustrated inFIG. 3. In addition, the crosstalk compensation unit162extracts a crosstalk component422of the signal of the channel ch3with respect to the signal of the channel ch2by the BPFs313and314illustrated inFIG. 3.

Next, as shown in the processing stage403, the crosstalk compensation unit162frequency-shifts the crosstalk component421of the channel ch1to the low frequency side by the frequency shift units321and322illustrated inFIG. 3. As a result, the frequency of the crosstalk component421of the channel ch1matches the frequency of the crosstalk component411of the channel ch2. In addition, the crosstalk compensation unit162frequency-shifts the crosstalk component422of the channel ch2to the high frequency side by the frequency shift units323and324illustrated inFIG. 3. As a result, the frequency of the crosstalk component422of the channel ch2matches the frequency of the crosstalk component412of the channel ch2.

The crosstalk component421of the channel ch1in the processing stage403may be expressed as E′(1) of the following equation (1), for example. In this equation (1), h(1) is an impulse response of the BPFs311and312illustrated inFIG. 3, for example. E(1) is a result of demodulation of the signal of the channel ch1by the adaptive equalization unit131, the carrier wave phase synchronization unit141, and the delay compensation unit151. Δf is a frequency spacing (e.g., f2-f1) between the channel ch1and an adjacent channel.
E′(1)=(h(1)*E(1))×exp(−j2nπΔft)  (1)

The signal of the channel ch2in the processing stage403may be expressed as E′(2) of the following equation (2), for example. In this equation (2), E(2) is a result of demodulation of the signal of the channel ch2by the adaptive equalization unit132, the carrier wave phase synchronization unit142, and the delay compensation unit152.
E′(2)=E(2)  (2)

The crosstalk component422of the channel ch3in the processing stage403may be expressed as E′(3) of the following equation (3), for example. In this equation (3), h(3) is an impulse response of the BPFs313and314illustrated inFIG. 3, for example. E(3) is a result of demodulation of the signal of the channel ch3by the adaptive equalization unit133, the carrier wave phase synchronization unit143, and the delay compensation unit153. Δf is a frequency spacing (e.g., f3-f2) between the channel ch3and an adjacent channel.
E′(3)=(h(3)*E(3))×exp(+j2nπΔft)  (3)

Next, as shown in the process stage404, the crosstalk compensation unit162weights the respective coefficients on the crosstalk components421and422by the multiplication units331to338and the addition units341to344illustrated inFIG. 3. Then, the crosstalk compensation unit162compensates for the crosstalk components411and412of the signal of the channel ch2by subtracting a result of the weighting from the signal of the channel ch2by the subtraction units351and352illustrated inFIG. 3.

The crosstalk compensation in the process stage404may be expressed by the following equation (4), for example. In this equation (4), E″(2x) and E″(2y) are X and Y polarized signals of the channel ch2after the crosstalk compensation, respectively. W1xx, W1yx, W1xy, W1yy, W3xx, W3yx, W3xy, and W3yy are the respective coefficients in the multiplication units331to338illustrated inFIG. 3. E′(1x) and E′(1y) are X and Y polarized signals included in E′(1) shown in the above equation (1), respectively. E′(2x) and E′(2y) are X and Y polarized signals included in E′(2) shown in the above equation (2). E′(3x) and E′(3y) are X and Y polarized signals included in E′(3) shown in the above equation (3).

FIG. 5is a view illustrating another example of the crosstalk compensation unit (ch1) according to the first embodiment. InFIG. 5, the same parts and units as inFIG. 2are denoted by the same reference numerals, and explanation of which will not be repeated. As illustrated inFIG. 5, in addition to the configuration illustrated inFIG. 2, the crosstalk compensation unit161may include tentative determination units511and512and nonlinear processing units521and522.

The X polarized signal of the channel ch2output from the delay compensation unit152illustrated inFIG. 1is input to the tentative determination unit511. The tentative determination unit511performs a tentative determination (e.g., tentative decoding) of the input X polarized signal of the channel ch2. The signal tentative determination may be performed, for example, by determining a symbol corresponding to a combination of phase and amplitude of the signal. The tentative determination unit511outputs the tentatively determined signal to the nonlinear processing unit521.

The Y polarized signal of the channel ch2output from the delay compensation unit152illustrated inFIG. 1is input to the tentative determination unit512. The tentative determination unit512performs a tentative determination of the input Y polarized signal of the channel ch2and outputs the tentatively determined signal to the nonlinear processing unit522.

The nonlinear processing unit521performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit511. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit521is a nonlinear distortion given to the X polarized signal of the channel ch2by the transmitting device that transmits the signal light101to the receiving device100. The predetermined nonlinear distortion may be obtained, for example, by calculation based on the design of the transmitting device, actual measurement using the transmitting device, or the like. The nonlinear processing unit521outputs the filtered signal to the BPF211.

The nonlinear processing unit522performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit512. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit522is a nonlinear distortion given to the Y polarized signal of the channel ch2by the transmitting device that transmits the signal light101to the receiving device100. The nonlinear processing unit522outputs the filtered signal to the BPF212.

The BPF211extracts a signal component to become a crosstalk with respect to the signal of the channel ch1, out of the signal output from the nonlinear processing unit521. The BPF212extracts a signal component to become a crosstalk with respect to the signal of the channel ch1, out of the signal output from the nonlinear processing unit522.

(Another Example of Crosstalk Compensation Unit (ch2) According to First Embodiment)

FIG. 6is a view illustrating another example of the crosstalk compensation unit (ch2) according to the first embodiment. InFIG. 6, the same parts and units as inFIG. 3are denoted by the same reference numerals, and explanation of which will not be repeated. As illustrated inFIG. 6, in addition to the configuration illustrated inFIG. 3, the crosstalk compensation unit162may include tentative determination units611to614and nonlinear processing units621to624.

The X polarized signal of the channel ch1output from the delay compensation unit151illustrated inFIG. 1is input to the tentative determination unit611. The tentative determination unit611performs a tentative determination of the input X polarized signal of the channel ch1and outputs the tentatively determined signal to the nonlinear processing unit621. The Y polarized signal of the channel ch1output from the delay compensation unit151illustrated inFIG. 1is input to the tentative determination unit612. The tentative determination unit612performs a tentative determination of the Y polarized signal of the channel ch1which is input and outputs the tentatively determined signal to the nonlinear processing unit622.

The nonlinear processing unit621performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit611. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit621is a nonlinear distortion given to the X polarized signal of the channel ch1by the transmitting device that transmits the signal light101to the receiving device100. The nonlinear processing unit621outputs the filtered signal to the BPF311. The nonlinear processing unit622performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit612. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit622is a nonlinear distortion given to the Y polarized signal of the channel ch1by the transmitting device that transmits the signal light101to the receiving device100. The nonlinear processing unit622outputs the filtered signal to the BPF312.

The BPF311extracts a signal component to become a crosstalk with respect to the signal of the channel ch2, out of the signal output from the nonlinear processing unit621. The BPF312extracts a signal component to become a crosstalk with respect to the signal of the channel ch2, out of the signal output from the nonlinear processing unit622.

The X polarized signal of the channel ch3output from the delay compensation unit153illustrated inFIG. 1is input to the tentative determination unit613. The tentative determination unit613performs a tentative determination of the X polarized signal of the channel ch3which is input and outputs the tentatively determined signal to the nonlinear processing unit623. The Y polarized signal of the channel ch3output from the delay compensation unit153illustrated inFIG. 1is input to the tentative determination unit614. The tentative determination unit614performs tentative determination of the Y polarized signal of the channel ch3which is input and outputs the tentatively determined signal to the nonlinear processing unit624.

The nonlinear processing unit623performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit613. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit623is a nonlinear distortion given to the X polarized signal of the channel ch3by the transmitting device that transmits the signal light101to the receiving device100. The nonlinear processing unit623outputs the filtered signal to the BPF313. The nonlinear processing unit624performs filtering to give a predetermined nonlinear distortion to the signal output from the tentative determination unit614. The predetermined nonlinear distortion given to the signal by the nonlinear processing unit624is a nonlinear distortion given to the Y polarized signal of the channel ch3by the transmitting device that transmits the signal light101to the receiving device100. The nonlinear processing unit624outputs the filtered signal to the BPF314.

The BPF313extracts a signal component to become a crosstalk with respect to the signal of the channel ch2, out of the signal output from the nonlinear processing unit623. The BPF314extracts a signal component to become a crosstalk with respect to the signal of the channel ch2, out of the signal output from the nonlinear processing unit624.

(Another Example of Crosstalk Compensation Unit (ch3) According to First Embodiment)

Another example of the crosstalk compensation unit163that compensates for the crosstalk from the channel ch2for each signal of the channel ch3is the same as the example of the crosstalk compensation unit161that compensates for the crosstalk from the channel ch2for each signal of the channel ch1(seeFIG. 5).

In this manner, according to the receiving device100of the first embodiment, it is possible to frequency-shift a signal component of crosstalk from the second signal to the first signal, which is extracted from a signal obtained by demodulation of the second signal. Then, it is possible to compensate for a crosstalk for a signal obtained by demodulation of the first signal, based on the frequency-shifted signal component.

This makes it possible to narrow the band of a signal input to a circuit that compensates for a crosstalk by MIMO processing, for example, so that a signal band (pass band) of the circuit that compensates for the crosstalk may be narrowed. Therefore, it is possible to compensate for the crosstalk while suppressing an increase in circuit scale.

Further, since it is possible to narrow the band of a signal input to a circuit that frequency-shifts a signal component of crosstalk, for example, it is possible to narrow the signal band (pass band) of the circuit that shifts the frequency of the crosstalk signal component. Therefore, it is possible to compensate for the crosstalk while suppressing an increase in circuit scale.

For example, the above-mentioned first signal and second signal are signals which are wavelength-multiplexed by frequencies adjacent to each other in the signal light. However, the first signal and the second signal are not limited to the respective signals wavelength-multiplexed by the frequencies adjacent to each other in the signal light but may have a relationship that a crosstalk from the second signal to the first signal occurs. Further, the first signal and the second signal may be signals wavelength-multiplexed in an electric stage or signals wavelength-multiplexed in an optical stage.

For example, the receiving device100frequency-shifts the extracted crosstalk signal component by an amount corresponding to a difference between the center frequencies of the first signal and the second signal in the signal light. This makes it possible to compensate for a crosstalk by combining the band of the crosstalk signal component in the signal obtained by demodulation of the first signal and the band of the extracted crosstalk signal component.

In addition, the receiving device100weights the frequency-shifted crosstalk signal component and performs a MIMO processing to compensate for the crosstalk based on the weighted crosstalk signal components. In this case, the receiving device100may output information indicating the amount of crosstalk from the second signal to the first signal based on the weighting coefficients for the crosstalk signal component in the crosstalk compensation (MIMO processing). This makes it possible to monitor the crosstalk amount using a circuit that compensates for the crosstalk.

In addition, the receiving device100may determine a signal obtained by demodulation of the second signal, give a nonlinear distortion to a signal obtained by the determination, and extract a crosstalk signal component from the signal given the nonlinear distortion. For example, the receiving device100gives the signal a nonlinear distortion given to the second signal by the transmitting device that transmits the second signal. This makes it possible to accurately extract a crosstalk signal component corresponding to a nonlinear distortion on the transmission side of the signal light and hence accurately compensate for the crosstalk.

Second Embodiment

Next, a second embodiment will be described with emphasis placed on parts different from those in the first embodiment. The second embodiment addresses a case where the receiving device100receives an optical signal of Nyquist frequency division multiplexing (NFDM). NFDM is, for example, a method of Nyquist-pulsing a signal of a symbol rate of several Gbaud and electrically multiplexing the signal.

(Communication System According to Second Embodiment)

FIG. 7is a view illustrating an example of a communication system according to the second embodiment. InFIG. 7, the same parts and units as inFIG. 1are denoted by the same reference numerals, and explanation of which will not be repeated. Referring toFIG. 7, a communication system700according to the second embodiment may include transmitting devices710,720and730, an optical multiplexing unit740, an optical transmission path750, and a receiving device100.

The transmitting device710generates and transmits an optical signal. For example, the transmitting device710includes mappers711(#1to #n), Nyquist filters712(#1to #n), an electrical multiplexing unit (MUX)713, a DAC714, and an E/O front end715. DAC is an acronym for digital/analog converter. n represents the number of subcarriers included in the optical signal transmitted by the transmitting device710.

Data to be transmitted to the receiving device100is input to the mappers711(#1to #n). Each of the mappers711(#1to #n) performs mapping of the input data to a signal. For example, each mapper711(#1to #n) maps data to the amplitude and phase (symbol) of the signal. The mappers711(#1to #n) output the signals obtained by the mapping to the Nyquist filters712(#1to #n), respectively.

The Nyquist filters712(#1to #n) Nyquist-pulses the signals output from the mappers711(#1to #n), respectively, by filtering. A Nyquist pulse is a pulse which satisfies, for example, the Nyquist conditions (zero crossing at equally spaced intervals except peak). The Nyquist filters712(#1to #n) output the respective Nyquist-pulsed signals to the electric multiplexing unit713.

The electrical multiplexing unit713electrically wavelength-multiplexes the signals (Nyquist pulses) output from the Nyquist filters712(#1to #n). For example, the electrical multiplexing unit713performs wavelength multiplexing by setting the frequencies of the signals to different frequencies and combining the signals (subcarriers) having the set frequencies. Then, the electrical multiplexing unit713outputs a signal obtained by the wavelength multiplexing to the DAC714.

The mappers711(#1to #n), the Nyquist filters712(#1to #n), and the electrical multiplexing unit713may be implemented by a hardware processor such as DSP or FPGA.

The DAC714converts the signal output from the electric multiplexing unit713from a digital signal to an analog signal which is then output to the E/O front end715. The E/O front end715converts the signal output from the DAC714from an electrical signal to an optical signal. Then, the E/O front end715transmits the optical signal obtained by the conversion to the optical multiplexing unit740. The E/O front end715may be implemented by, for example, a light source such as a laser diode (LD) and its driving circuit.

The transmitting device720generates an optical signal of a frequency band different from that of the transmitting device710and transmits the generated optical signal to the optical multiplexing unit740. The transmitting device720has the same configuration as the transmitting device710, for example. The transmitting device730generates an optical signal of a frequency band different from those of the transmitting devices710and720, and transmits the generated optical signal to the optical multiplexing unit740. The transmitting device730has, for example, the same configuration as the transmitting device710.

The optical multiplexing unit740wavelength-multiplexes the optical signals having different frequency bands transmitted from the transmitting devices710,720, and730. Then, the optical multiplexing unit740transmits a signal light obtained by the wavelength multiplexing to the receiving device100via the optical transmission line750.

The optical transmission line750is, for example, an optical fiber such as SSMF. SSMF is an acronym for standard single mode fiber (standard type optical fiber). Further, the optical transmission line750may include an optical repeater such as an amplifier. The receiving device100receives the signal light transmitted from the optical multiplexing unit740via the optical transmission line750.

(Nyquist Waveform in Case Where Nonlinear Distortion in Transmitting Device According to Second Embodiment is Small)

FIG. 8is a view illustrating an example of a Nyquist waveform in a case where a nonlinear distortion in the transmitting device according to the second embodiment is small. InFIG. 8, the horizontal axis represents a frequency [GHz] and the vertical axis represents normalized power [dB]. Nyquist waveforms801and802illustrated inFIG. 8indicate the frequency spectra of subcarriers included in the optical signal transmitted from the transmitting device710in a case where the nonlinear distortion in the transmitting device710is relatively small.

The Nyquist waveform801is the frequency spectrum of the optical signal in a case where nonlinear response compensation by signal processing is performed (with nonlinear response compensation). The Nyquist waveform802is the frequency spectrum of the optical signal in a case where nonlinear response compensation by signal processing is not performed (without nonlinear response compensation). As illustrated inFIG. 8, when the nonlinear distortion in the transmitting device710is relatively small, the distortion of the Nyquist waveform is small and may be suppressed by the nonlinear response compensation by signal processing.

(Nyquist Waveform in Case where Nonlinear Distortion in Transmitting Device According to Second Embodiment is Large)

FIG. 9is a view illustrating an example of a Nyquist waveform in a case where a nonlinear distortion in the transmitting device according to the second embodiment is large. InFIG. 9, the same portions as inFIG. 8are denoted by the same reference numerals, and explanation of which will not be repeated. Nyquist waveforms801and802illustrated inFIG. 9indicate the frequency spectra of subcarriers included in the optical signal transmitted from the transmitting device710in a case where the nonlinear distortion in the transmitting device710is relatively large.

As illustrated inFIG. 9, when the nonlinear distortion in the transmitting device710is relatively large, the distortion of the Nyquist waveform is large and may be suppressed, but hardly completely, by the nonlinear response compensation by signal processing. Therefore, leakage of signal components outside the signal band increases and a crosstalk to other channels increases. Further, if the wavelength spacing between the channels is increased in order to suppress the crosstalk, the frequency utilization efficiency decreases.

In contrast, the receiving device100may compensate for the generated crosstalk, so that, even when the nonlinear distortion in the transmitting device710is large, it is possible to suppress deterioration of reception quality due to the crosstalk.

Although the optical signal transmitted by the transmitting device710has been illustrated inFIGS. 8 and 9, the same applies to the optical signals transmitted by the transmitting devices720and730.

(Signal Light Received by Receiving Device According to Second Embodiment)

FIG. 10is a view illustrating an example of signal light received by the receiving device according to the second embodiment. InFIG. 10, the horizontal axis represents a frequency and the vertical direction represents signal strength. The receiving device100illustrated inFIG. 7receives, for example, signal light1000illustrated inFIG. 10. The signal light1000includes optical signals1010,1020, and1030. The optical signal1010is an optical signal transmitted by the transmitting device710illustrated inFIG. 7. The optical signal1010includes a plurality of Nyquist pulsed subcarriers1011.

The optical signal1020is an optical signal transmitted by the transmitting device720illustrated inFIG. 7. The frequency band of the optical signal1020is set on the higher frequency side than the frequency band of the optical signal1010. The optical signal1020includes a plurality of Nyquist pulsed subcarriers1021. The optical signal1030is an optical signal transmitted by the transmitting device730illustrated inFIG. 7. The frequency band of the optical signal1030is set to the higher frequency side than the frequency bands of the optical signals1010and1020. The optical signal1030includes a plurality of Nyquist pulsed subcarriers1031.

In the signal light1000, a crosstalk between the subcarriers in the optical signals or a crosstalk between the optical signals occurs due to, for example, nonlinear distortions in the transmitting devices710,720, and730. For example, an inter-optical signal crosstalk1001illustrated inFIG. 10is a crosstalk in which a signal component of the highest frequency subcarrier of the optical signal1010leaks into a signal component of the lowest frequency subcarrier of the optical signal1020. In addition, inter-subcarrier crosstalks1002and1003illustrated inFIG. 10are crosstalks in which signal components of subcarriers leak into signal components of adjacent subcarriers in the optical signal1020.

The receiving device100compensates for at least one of the crosstalk between the subcarriers in the optical signal and the crosstalk between the optical signals.

FIG. 11is a view illustrating another example of the signal light received by the receiving device according to the second embodiment. InFIG. 11, the same portions as inFIG. 10are denoted by the same reference numerals, and explanation of which will not be repeated. As illustrated inFIG. 11, if the spacing between the optical signals1010,1020, and1030and the spacing between the subcarriers in these optical signals are increased, a crosstalk may be suppressed. However, this requires a wider frequency band, resulting in deterioration of frequency use efficiency.

In contrast, the receiving device100may compensate for the crosstalk between the subcarriers in the optical signals and the crosstalk between the optical signals, so that, even when the spacing between the optical signals and the spacing between the subcarriers are not increased, it is possible to suppress deterioration of reception performance.

(Receiving Device according to Second Embodiment)

FIG. 12is a view illustrating an example of the receiving device according to the second embodiment. InFIG. 12, the same parts and units as inFIG. 1orFIG. 10are denoted by the same reference numerals, and explanation of which will not be repeated. Referring toFIG. 12, the receiving device100may include coherent receiving FEs111to113, ADCs121to123, and receiving units1211to1213.

In the example illustrated inFIG. 12, the receiving device100receives the signal light1000including the optical signals1010,1020, and1030illustrated inFIG. 10. In the example illustrated inFIG. 12, the optical signals1010,1020, and1030are assumed as channels ch1to ch3, respectively.

The coherent receiving FE111coherently receives the optical signal1010(channel ch1) and a portion of subcarriers of the optical signal1020. Such coherent reception may be implemented by using the local light emission having the same center frequency as the optical signal1010for the local light emission of the coherent receiving FE111and by designing the band of the coherent receiving FE111wider than the band of the optical signal1010. A portion of subcarriers of the optical signal1020coherently received by the coherent receiving FE111are, for example, subcarriers having a frequency closest (lowest frequency side) to the frequency of the optical signal1010, among the subcarriers1021.

The coherent receiving FE112coherently receives the optical signal1020(channel ch2), a portion of subcarriers of the optical signal1010, and a portion of subcarriers of the optical signal1030. Such coherent reception may be implemented by using the local light emission having the same center frequency as the optical signal1020for the local light emission of the coherent receiving FE112and by designing the band of the coherent receiving FE112wider than the band of the optical signal1020. A portion of subcarriers of the optical signal1010coherently received by the coherent receiving FE112are, for example, subcarriers having a frequency closest (highest frequency side) to the frequency of the optical signal1020, among the subcarriers1011of the optical signal1010. A portion of subcarriers of the optical signal1030coherently received by the coherent receiving FE112are, for example, subcarriers having a frequency closest (lowest frequency side) to the frequency of the optical signal1020, among the subcarriers1031of the optical signal1030.

The coherent receiving FE113coherently receives the optical signal1030(channel ch3) and a portion of subcarriers of the optical signal1020. Such coherent reception may be implemented by using the local light emission having the same center frequency as the optical signal1030for the local light emission of the coherent receiving FE113and by designing the band of the coherent receiving FE113wider than the band of the optical signal1030. A portion of subcarriers of the optical signal1020coherently received by the coherent receiving FE113are, for example, subcarriers having a frequency closest (highest frequency side) to the frequency of the optical signal1030, among the subcarriers1021.

The receiving unit1211performs a process of receiving the optical signal1010(channel ch1) based on a signal coherently received by the coherent receiving FE111and converted from analog to digital by the ADC121. A signal input to the receiving unit1211includes the optical signal1010and a portion of subcarriers1021of the optical signal1020adjacent to the optical signal1010. The receiving unit1211outputs the signal of the channel ch1obtained by the reception process.

The receiving unit1212performs a process of receiving the optical signal1020(channel ch2) based on a signal coherently received by the coherent receiving FE112and converted from analog to digital by the ADC122. A signal input to the receiving unit1212includes the optical signal1020, a portion of subcarriers1011of the optical signal1010adjacent to the optical signal1020, and a portion of subcarriers1031of the optical signal1030adjacent to the optical signal1020. The receiving unit1212outputs the signal of the channel ch2obtained by the reception process.

The receiving unit1213performs a process of receiving the optical signal1030(channel ch3) based on a signal coherently received by the coherent receiving FE113and converted from analog to digital by the ADC123. A signal input to the receiving unit1213includes the optical signal1030and a portion of subcarriers1021of the optical signal1020adjacent to the optical signal1030. The receiving unit1213outputs the signal of the channel ch3obtained by the reception process.

The receiving units1211to1213illustrated inFIG. 12may be implemented by, for example, a hardware processor such as DSP or FPGA. In addition, the receiving device100may include a decoding unit that performs decoding by determining each signal output from the receiving units1211to1213. This decoding unit may also be implemented by digital circuits such as the above-mentioned DSP and FPGA.

(Receiving Unit (ch1) of Receiving Device According to Second Embodiment)

FIG. 13is a view illustrating an example of the receiving unit (ch1) of the receiving device according to the second embodiment. Referring toFIG. 13, the receiving unit1211that performs the process of receiving the optical signal1010(channel ch1) may include an adjacent signal separation unit1310, a subcarrier separation unit1320, and frequency shift units1331(#1to #n) and1332. The receiving unit1211may further include adaptive equalization units1341(#1to #n) and1342, carrier wave phase synchronization units1351(#1to #n) and1352, delay compensation units1361(#1to #n) and1362, and a crosstalk compensation unit1370. The symbol n represents the number of subcarriers included in the optical signal1010.

The signal output from the ADC121illustrated inFIG. 12is input to the adjacent signal separation unit1310. This signal includes the optical signal1010and a portion of subcarriers1021of the optical signal1020adjacent to the optical signal1010. The adjacent signal separation unit1310separates the subcarriers1021of the optical signal1020from the input signal.

For example, the adjacent signal separation unit1310uses a training sequence (TS) included in the optical signal1020to estimate the frequency offset amount of the optical signal1020. Then, the adjacent signal separation unit1310may separate the subcarriers1021of the optical signal1020by digital filtering processing based on the estimated frequency offset amount.

Then, the adjacent signal separation unit1310outputs the separated subcarriers1021of the optical signal1020to the frequency shift unit1332. Further, the adjacent signal separation unit1310outputs the input signal to the subcarrier separation unit1320. In addition, the adjacent signal separation unit1310may perform a process of compensating for the frequency offset of the subcarriers1021of the optical signal1020based on the estimated frequency offset amount.

The subcarrier separation unit1320separates n subcarriers (subcarriers1011) of the optical signal1010included in the signal output from the adjacent signal separation unit1310. For example, the subcarrier separation unit1320uses the training sequence (TS) included in the optical signal1010to estimate the frequency offset amount of the optical signal1010. Then, the subcarrier separation unit1320separates the n subcarriers of the optical signal1010by performing digital filtering processing based on the estimated frequency offset amount.

Then, the subcarrier separation unit1320outputs the separated n subcarriers to the frequency shift units1331(#1to #n), respectively. Further, the subcarrier separation unit1320may perform a process of compensating the frequency offset of each subcarrier of the optical signal1010based on the estimated frequency offset amount.

The n frequency shift units1331(#1to #n) perform frequency shifting for the subcarriers output from the subcarrier separation unit1320so that the subcarriers become basebands. For example, the frequency shift units1331(#1to #n) perform frequency shifting based on the frequency spacing of each signal (each subcarrier) set by the electrical multiplexing unit713of the transmitting device710illustrated inFIG. 7. Then, the frequency shift units1331(#1to #n) output the frequency-shifted subcarriers to the adaptive equalization units1341(#1to #n), respectively.

The frequency shift unit1332performs frequency shifting for the subcarriers output from the adjacent signal separation unit1310so that the subcarriers become basebands. For example, the frequency shift unit1332performs frequency shifting based on the spacing between the center frequency of the optical signal1010transmitted by the transmitting device710and the center frequency of a subcarrier on the lowest frequency side among the subcarriers1021of the optical signal1020transmitted by the transmitting device720. Then, the frequency shift unit1332outputs the frequency-shifted subcarriers to the adaptive equalization unit1342.

The n adaptive equalization units1341(#1to #n) perform adaptive equalization processing on the subcarriers output from the frequency shift units1331(#1to #n), respectively. As a result, an X polarized signal and a Y polarized signal included in each subcarrier of the optical signal1010are obtained. The adaptive equalization units1341(#1to #n) output the obtained X polarized and Y polarized signals to the carrier wave phase synchronization units1351(#1to #n), respectively.

Similarly, the adaptive equalization unit1342performs adaptive equalization processing on the subcarriers output from the frequency shift units1332. As a result, an X polarized signal and a Y polarized signal included in a portion of subcarriers of the optical signal1020are obtained. The adaptive equalization unit1342outputs the obtained X polarized and Y polarized signals to the carrier wave phase synchronization unit1352.

The n carrier wave phase synchronization units1351(#1to #n) perform carrier wave phase synchronization processing on the X polarized and Y polarized signals output from the adaptive equalization units1341(#1to #n), respectively. Then, the carrier wave phase synchronization units1351(#1to #n) output the signals subjected to the carrier wave phase synchronization processing to the delay compensation units1361(#1to #n), respectively.

Similarly, the carrier wave phase synchronization unit1352performs carrier wave phase synchronization processing on the X polarized and Y polarized signals output from the adaptive equalization unit1342. Then, the carrier wave phase synchronization unit1352outputs the X polarized and Y polarized signals subjected to the carrier wave phase synchronization processing to the delay compensation unit1362.

The n delay compensation units1361(#1to #n) perform delay compensation on the X polarized and Y polarized signals output from the carrier wave phase synchronization units1351(#1to #n), respectively, and output the delay-compensated signals to the crosstalk compensation unit1370. As a result, the X polarized and Y polarized signals demodulated for each subcarrier of the optical signal1010are output to the crosstalk compensation unit1370.

Similarly, the delay compensation unit1362performs a delay compensation on the X polarized and Y polarized signals output from the carrier wave phase synchronization unit1352and outputs the delay-compensated signals to the crosstalk compensation unit1370. As a result, the X polarized and Y polarized signals demodulated for a portion of subcarriers of the optical signal1020are output to the crosstalk compensation unit1370.

The crosstalk compensation unit1370compensates for a crosstalk to each signal (X polarized signal and Y polarized signal) of each subcarrier of the optical signal1010based on each signal output from the delay compensation units1361(#1to #n) and1362. Then, the crosstalk compensation unit1370outputs each signal of each subcarrier of the optical signal1010compensated for the crosstalk.

(Crosstalk Compensation Unit (ch1) of Receiving Device According to Second Embodiment)

FIG. 14is a view illustrating an example of the crosstalk compensation unit (ch1) of the receiving device according to the second embodiment. Referring toFIG. 14, the crosstalk compensation unit1370may include n inter-subcarrier crosstalk compensation units1411to141n. n represents the number of subcarriers included in the optical signal1010.

For example, the inter-subcarrier crosstalk compensation unit1411receives each polarized signal (X polarized and Y polarized signals) of the first and second subcarriers #1and #2of the optical signal1010(main signal) from the delay compensation units1361(#1and #2) illustrated inFIG. 13. The inter-subcarrier crosstalk compensation unit1411compensates for a crosstalk from each polarized signal of the subcarrier #2to each polarized signal of the subcarrier #1and outputs each polarized signal of the subcarrier #1compensated for the crosstalk.

The inter-subcarrier crosstalk compensation unit1412receives each polarized signal of the first to third subcarriers #1to #3of the optical signal1010(main signal) from the delay compensation units1361(#1to #3) illustrated inFIG. 13. The inter-subcarrier crosstalk compensation unit1412compensates for a crosstalk from each polarized signal of the subcarriers #1and #3to each polarized signal of the subcarrier #2and outputs each polarized signal of the subcarrier #2compensated for the crosstalk.

The inter-subcarrier crosstalk compensation unit1413receives each polarized signal of the second to fourth subcarriers #2to #4of the optical signal1010(main signal) from the delay compensation units1361(#2to #4) illustrated inFIG. 13. The inter-subcarrier crosstalk compensation unit1413compensates for a crosstalk from each polarized signal of the subcarriers #2and #4to each polarized signal of the subcarrier #3and outputs each polarized signal of the subcarrier #3compensated for the crosstalk.

Similarly, the inter-subcarrier crosstalk compensation unit141nreceives each polarized signal of the (n−1)thand nthsubcarriers #n−1 and #n of the optical signal1010(main signal) from the delay compensation units1361(#n−1 and #n) illustrated inFIG. 13. In addition, the inter-subcarrier crosstalk compensating unit141nreceives each polarized signal of a subcarrier having a frequency closest to the frequency of the optical signal1010among the subcarriers of the optical signal1020(adjacent signal) from the delay compensating unit1362illustrated inFIG. 13. The inter-subcarrier crosstalk compensation unit1413compensates for a crosstalk from the subcarrier #n−1 and the subcarrier of the optical signal1020to each polarized signal of the subcarrier #n and outputs each polarized signal of the subcarrier #n compensated for the crosstalk.

The inter-subcarrier crosstalk compensation unit1411illustrated inFIG. 14may have the same configuration as the crosstalk compensation unit161illustrated inFIG. 2, for example. The inter-subcarrier crosstalk compensation units1412to141nillustrated inFIG. 14may have the same configuration, for example, as the crosstalk compensation unit162illustrated inFIG. 3.

(Receiving Unit (ch2) of Receiving Device According to Second Embodiment)

FIG. 15is a view illustrating an example of the receiving unit (ch2) of the receiving device according to the second embodiment. Referring toFIG. 15, the receiving unit1212that performs the process of receiving the optical signal1020(channel ch2) may include an adjacent signal separation unit1510, a subcarrier separation unit1520, and frequency shift units1531(#1to #n),1532, and1533. The receiving unit1212may further include adaptive equalization units1541(#1to #n),1542, and1543, carrier wave phase synchronization units1551(#1to #n),1522, and1553, delay compensation units1561(#1to #n),1562, and1563, and a crosstalk compensation unit1570.

The receiving unit1212has the same configuration as the receiving unit1211illustrated inFIG. 13. However, the signal output from the ADC122illustrated inFIG. 12is input to the adjacent signal separation unit1510. This signal includes the optical signal1020, a portion of subcarriers1011of the optical signal1010adjacent to the optical signal1020, and a portion of subcarriers1031of the optical signal1030adjacent to the optical signal1020.

The adjacent signal separation unit1510separates a portion of subcarriers1011of the optical signal1010adjacent to the optical signal1020and a portion of subcarriers1031of the optical signal1030adjacent to the optical signal1020from the input signal. Then, the adjacent signal separation unit1510outputs the separated subcarriers1011of the optical signal1010to the frequency shift unit1532. Further, the adjacent signal separation unit1510outputs the separated subcarriers1031of the optical signal1030to the frequency shift unit1533.

The frequency shift unit1532performs a frequency shifting for the subcarriers output from the adjacent signal separation unit1510so that the subcarriers become basebands, and outputs the frequency-shifted subcarriers to the adaptive equalization unit1542. The frequency shift unit1533performs frequency shifting for the subcarriers output from the adjacent signal separation unit1510so that the subcarriers become basebands, and outputs the frequency-shifted subcarriers to the adaptive equalization unit1543.

The adaptive equalization unit1542performs an adaptive equalization processing on the subcarriers output from the frequency shift unit1532and outputs the obtained X polarized and Y polarized signals to the carrier wave phase synchronization unit1552. The adaptive equalization unit1543performs an adaptive equalization processing on the subcarriers output from the frequency shift unit1533and outputs the obtained X polarized and Y polarized signals to the carrier wave phase synchronization unit1553.

The carrier wave phase synchronization unit1552performs a carrier wave phase synchronization processing on the X polarized and Y polarized signals output from the adaptive equalization unit1542, and outputs the signals subjected to the carrier wave phase synchronization processing to the delay compensation unit1562.

The delay compensation unit1562performs delay compensation on the X polarized and Y polarized signals output from the carrier wave phase synchronization unit1552, and outputs the delay-compensated signals to the crosstalk compensation unit1570. As a result, the X polarized and Y polarized signals demodulated for a portion of subcarriers1011of the optical signal1010are output to the crosstalk compensation unit1570.

The delay compensation unit1563performs a delay compensation on the X polarized and Y polarized signals output from the carrier wave phase synchronization unit1553, and outputs the delay-compensated signals to the crosstalk compensation unit1570. As a result, the X polarized and Y polarized signals demodulated for a portion of subcarriers1031of the optical signal1030are output to the crosstalk compensation unit1570.

The crosstalk compensation unit1570compensates for a crosstalk to each signal (X polarized signal and Y polarized signal) of each subcarrier of the optical signal1020based on each signal output from the delay compensation units1561(#1to #n),1562, and1563. Then, the crosstalk compensation unit1570outputs each signal of each subcarrier of the optical signal1020compensated for the crosstalk.

(Receiving Unit (ch3) of Receiving Device According to Second Embodiment)

The receiving unit1213that performs the process of receiving the optical signal1030(channel ch3) may have the same configuration as the receiving unit1212(see, e.g.,FIG. 14) that performs the process of receiving the optical signal1020(channel ch2).

(Crosstalk Compensation Unit (ch2) of Receiving Device According to Second Embodiment)

FIG. 16is a view illustrating an example of the crosstalk compensation unit (ch2) of the receiving device according to the second embodiment. Referring toFIG. 16, the crosstalk compensation unit1570may include n inter-subcarrier crosstalk compensation units1611to161n. The symbol n represents the number of subcarriers included in the optical signal1020.

The inter-subcarrier crosstalk compensation units1611to161nare the same as the inter-subcarrier crosstalk compensation units1411to141nillustrated inFIG. 14. However, the inter-subcarrier crosstalk compensation unit1611receives each polarized signal (X polarized and Y polarized signals) of the first and second subcarriers #1and #2of the optical signal1020(main signal) from the delay compensation units1561(#1and #2) illustrated inFIG. 15. In addition, the inter-subcarrier crosstalk compensation unit1611receives each polarized signal of a subcarrier having a frequency closest to the frequency of the optical signal1020among the subcarriers of the optical signal1010(adjacent signal) from the delay compensation unit1562illustrated inFIG. 15. The inter-subcarrier crosstalk compensation unit1611compensates for a crosstalk from the subcarrier #2and the subcarrier of the optical signal1010to each polarized signal of the subcarrier #1, and outputs each polarized signal of the subcarrier #1compensated for the crosstalk.

The inter-subcarrier crosstalk compensation units1611to161nillustrated inFIG. 16may have the same configuration as the crosstalk compensation unit162illustrated inFIG. 3, for example.

(Another Example of Crosstalk Compensation Unit (ch1) According to Second Embodiment)

FIG. 17is a view illustrating another example of the crosstalk compensation unit (ch1) according to the second embodiment. InFIG. 17, the same parts and units as inFIG. 5are denoted by the same reference numerals, and explanation of which will not be repeated. The second embodiment may also use the configuration of the crosstalk compensation unit161that performs the tentative determination as illustrated inFIG. 5. In this case, as illustrated inFIG. 17, the crosstalk compensation unit161may include Nyquist filters1711and1712in addition to the configuration illustrated inFIG. 5. The crosstalk compensation unit161illustrated inFIG. 17may be used, for example, in the inter-subcarrier crosstalk compensation unit1411illustrated inFIG. 14.

The tentative determination unit511outputs the tentatively determined signal to the Nyquist filter1711. The Nyquist filter1711Nyquist-pulses the signal output from the tentative determination unit511by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit521. The nonlinear processing unit521performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1711.

The tentative determination unit512outputs the tentatively determined signal to the Nyquist filter1712. The Nyquist filter1712Nyquist-pulses the signal output from the tentative determination unit512by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit522. The nonlinear processing unit522performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1712.

For example, the Nyquist filters1711and1712are filters having the same filtering characteristics as the Nyquist filter included in the transmitting device720illustrated inFIG. 7. With the configuration illustrated inFIG. 17, the signal of the channel ch2which is Nyquist-pulsed by the Nyquist filter of the transmitting device720and is affected by the nonlinear distortion of the transmitting device720is reproduced, and the crosstalk may be compensated based on the reproduced signal.

(Another Example of Crosstalk Compensation Unit (ch2) According to Second Embodiment)

FIG. 18is a view illustrating another example of the crosstalk compensation unit (ch2) according to the second embodiment. InFIG. 18, the same parts and units as inFIG. 6are denoted by the same reference numerals, explanation of which will not be repeated. The second embodiment may also use the configuration of the crosstalk compensation unit162that performs the tentative determination as illustrated inFIG. 6. In this case, as illustrated inFIG. 18, the crosstalk compensation unit162may include Nyquist filters1811to1814in addition to the configuration illustrated inFIG. 5. The crosstalk compensation unit162illustrated inFIG. 18may be used, for example, in the inter-subcarrier crosstalk compensation units1412to141nillustrated inFIG. 14and the inter-subcarrier crosstalk compensation units1611to161nillustrated inFIG. 16.

The tentative determination unit611outputs the tentatively determined signal to the Nyquist filter1811. The Nyquist filter1811Nyquist-pulses the signal output from the tentative determination unit611by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit621. The nonlinear processing unit621performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1811.

The tentative determination unit612outputs the tentatively determined signal to the Nyquist filter1812. The Nyquist filter1812Nyquist-pulses the signal output from the tentative determination unit612by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit622. The nonlinear processing unit622performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1812.

The tentative determination unit613outputs the tentatively determined signal to the Nyquist filter1813. The Nyquist filter1813Nyquist-pulses the signal output from the tentative determination unit613by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit623. The nonlinear processing unit623performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1813.

The tentative determination unit614outputs the tentatively determined signal to the Nyquist filter1814. The Nyquist filter1814Nyquist-pulses the signal output from the tentative determination unit614by filtering and outputs the Nyquist-pulsed signal to the nonlinear processing unit624. The nonlinear processing unit624performs filtering that gives a predetermined nonlinear distortion to the signal output from the Nyquist filter1814.

The Nyquist filters1811and1812are filters having the same filtering characteristics as the Nyquist filters712(#1to #n) included in the transmitting device710illustrated inFIG. 7. As a result, the signal of the channel ch1which is Nyquist-pulsed by the Nyquist filters712(#1to #n) of the transmitting device710and is affected by the nonlinear distortion of the transmitting device710is reproduced, and the crosstalk may be compensated based on the reproduced signal.

In addition, the Nyquist filters1813and1814are filters having the same filtering characteristics as the Nyquist filter included in the transmitting device730illustrated inFIG. 7. As a result, the signal of the channel ch3which is Nyquist-pulsed by the Nyquist filter of the transmitting device730and is affected by the nonlinear distortion of the transmitting device730is reproduced, and the crosstalk may be compensated based on the reproduced signal.

(Another Example of Receiving Unit (ch1) of Receiving Device According to Second Embodiment)

FIG. 19is a view illustrating another example of the receiving unit (ch1) of the receiving device according to the second embodiment. InFIG. 19, the same parts and units as inFIG. 13are denoted by the same reference numerals, and explanation of which will not be repeated. Referring toFIG. 19, the receiving unit1211may include a crosstalk compensation unit1910in place of the crosstalk compensation unit1370illustrated inFIG. 13. The crosstalk compensation unit1910may have the same configuration as the crosstalk compensation unit161illustrated inFIG. 2 or 5, for example.

The receiving unit1211outputs each signal output from the delay compensation units1361(#1to #n−1). The crosstalk compensation unit1910receives each signal output from the delay compensation units1361(#n) and1362. The crosstalk compensation unit1910compensates for a crosstalk from the channel ch2with respect to each signal (X and Y polarized signals) of the highest frequency side (nth) subcarrier of the optical signal1010based on each signal output from the delay compensation units1361(#n) and1362. Then, the crosstalk compensation unit1910outputs each signal of the highest frequency side (nth) subcarrier of the optical signal1010compensated for the crosstalk.

As illustrated inFIG. 19, the receiving unit1211may be configured to compensate for a crosstalk between optical signals and not compensate for a crosstalk between subcarriers in the optical signals. Thus, for example, demodulation processing of subcarriers may be reduced and the circuit scale may also be reduced. In this case, for example, the crosstalk between the subcarriers of the channel ch1may be compensated (pre-compensated) by the transmitting device710illustrated inFIG. 7.

(Another Example of Receiving Unit (ch2) of Receiving Device According to Second Embodiment)

FIG. 20is a view illustrating another example of the receiving unit (ch2) of the receiving device according to the second embodiment. InFIG. 20, the same parts and units as inFIG. 15are denoted by the same reference numerals, and explanation of which will not be repeated. Referring toFIG. 20, the receiving unit1212may include crosstalk compensation units2011and2012in place of the crosstalk compensation unit1570illustrated inFIG. 15. The crosstalk compensation units2011and2012may have the same configuration as the crosstalk compensation unit161illustrated inFIG. 2 or 5, for example. The receiving unit1212outputs each signal output from the delay compensation units1561(#2to #n−1).

The crosstalk compensation unit2011receives each signal output from the delay compensation units1561(#1) and1562. The crosstalk compensation unit2011compensates for a crosstalk from the channel ch1with respect to each signal (X and Y polarized signals) of the lowest frequency side (first) subcarrier of the optical signal1020based on each signal output from the delay compensation units1561(#1) and1562. Then, the crosstalk compensation unit2011outputs each signal of the lowest frequency side (first) subcarrier of the optical signal1020compensated for the crosstalk.

The crosstalk compensation unit2012receives each signal output from the delay compensation units1561(#n) and1563. The crosstalk compensation unit2012compensates for a crosstalk from the channel ch3with respect to each signal (X and Y polarized signals) of the highest frequency side (nth) subcarrier of the optical signal1020based on each signal output from the delay compensation units1561(#n) and1563. Then, the crosstalk compensation unit2012outputs each signal of the highest frequency side (nth) subcarrier of the optical signal1020compensated for the crosstalk.

As illustrated inFIG. 20, the receiving unit1212may be configured to compensate for a crosstalk between optical signals and not compensate for a crosstalk between subcarriers in the optical signals. Thus, for example, demodulation processing of subcarriers may be reduced and the circuit scale may also be reduced. In this case, for example, the crosstalk between the subcarriers of the channel ch2may be compensated (pre-compensated) by the transmitting device720illustrated inFIG. 7.

(Another Example of Receiving Unit (ch3) of Receiving Device According to Second Embodiment)

The receiving unit1213that receives the signal of the channel ch3may also be configured to compensate for a crosstalk between optical signals and not compensate for a crosstalk between subcarriers in the optical signals, for example, as the receiving unit1211illustrated inFIG. 19. Thus, for example, demodulation processing of subcarriers may be reduced and the circuit scale may also be reduced. In this case, for example, the crosstalk between the subcarriers of the channel ch3may be compensated (pre-compensated) by the transmitting device730illustrated inFIG. 7.

(Still Another Example of Receiving Unit (ch1) of Receiving Device According to Second Embodiment)

FIG. 21is a view illustrating still another example of the receiving unit (ch1) of the receiving device according to the second embodiment. InFIG. 21, the same parts and units as inFIG. 13are denoted by the same reference numerals, explanation of which will not be repeated. Referring toFIG. 21, the receiving unit1211may include a carrier wave phase synchronization unit2110in place of the carrier wave phase synchronization units1351(#1to #n) and1352illustrated inFIG. 13.

The carrier wave phase synchronization unit2110has the same configuration as the carrier wave phase synchronization units1351(#1to #n) and1352illustrated inFIG. 13, for example. However, the carrier wave phase synchronization unit2110performs carrier wave phase synchronization processing between the adjacent signal separation unit1310and the subcarrier separation unit1320. In addition, the carrier wave phase synchronization unit2110performs phase synchronization of adjacent signals using a phase noise of the main signal.

As in the example illustrated inFIG. 21, various modifications may be made to the processing unit that demodulates the optical signals in the receiving unit1211. A modification of the processing unit that demodulates the optical signals in the receiving unit1211has been described, but the same applies to the processing unit that demodulates the optical signals in the receiving units1212and1213.

In this manner, according to the receiving device100of the second embodiment, in the configuration in which the signal light including each signal wavelength-multiplexed in the electric stage is coherently received, it is possible to compensate for a crosstalk while suppressing an increase in circuit scale, similarly to the first embodiment.

In addition, the receiving device100may coherently receive the signal light obtained by wavelength-multiplexing the first optical signal based on the first signal and the second optical signal based on the signal obtained by electrically wavelength-multiplexing the plurality of signals including the second signal. In this case, the receiving device100demodulates some signals of the second optical signal as the second signal and extracts a signal component of crosstalk from the signal obtained by the demodulation to the first signal. This makes it possible to suppress the circuit scale as compared with a configuration of demodulating all the signals of the second optical signal for compensation of a crosstalk of the first signal.

In addition, some signals of the second optical signal include a signal wavelength-multiplexed with a frequency closest to the frequency of the first signal among the signals of the second optical signal. A signal having the largest crosstalk amount with respect to the first signal among the signals of the second optical signal may be demodulated and used for compensation of the crosstalk of the first signal. As a result, it is possible to accurately compensate for a crosstalk while suppressing the circuit scale.

In addition, similarly to the first optical signal, the second optical signal may also be an optical signal based on a signal obtained by electrically wavelength-multiplexing a plurality of signals. In this case, the receiving device100may be configured to compensate for a crosstalk of the second signal of the second optical signal with respect to the first signal of the first optical signal and not compensate for a crosstalk between the signals included in the first optical signal. As a result, it is possible to suppress the circuit scale of the receiving device100. In this case, the compensation of the crosstalk between the signals included in the first optical signal may be performed, for example, in a transmitting device that transmits the first optical signal.

In addition, the receiving device100may determine a signal obtained by demodulation of the second signal, Nyquist-pulse the signal obtained by the determination, give a nonlinear distortion to the Nyquist-pulsed signal, and extract a crosstalk signal component from the signal given the nonlinear distortion. For example, the receiving device100Nyquist-pulses the signal obtained by the determination by giving a filter to the second signal when the transmitting device that transmits the second signal Nyquist-pulses the second signal. As a result, it is possible to accurately extract a signal component of a crosstalk corresponding to a distortion of a Nyquist waveform due to the nonlinear distortion on the transmission side of the signal light and accurately compensate for the crosstalk.

The configuration in which the receiving device100receives the NFDM optical signal has been described in the second embodiment, but the present disclosure is not limited to such a configuration. For example, in the second embodiment, the receiving device100may be configured to receive an optical signal of orthogonal frequency division multiplexing (OFDM).

(Processing by Receiving Device According to Each Embodiment)

FIG. 22is a flowchart illustrating an example of a process by the receiving device according to each embodiment. The receiving device100according to the above-described first and second embodiments executes each operation illustrated inFIG. 22, for example. First, the receiving device100performs coherent reception of the signal light transmitted via the optical transmission line (operation S2201). The operation S2201is executed by, for example, the coherent receiving FEs111to113illustrated inFIG. 1 or 12, for example.

Next, the receiving device100performs demodulation of each signal included in the signal light based on a result of the coherent reception in the operation S2201(operation S2202). The operation S2202is executed by each adaptive equalization unit, each carrier wave phase synchronization unit, and each delay compensation unit illustrated inFIG. 1, for example. Alternatively, the operation S2202is executed by the adjacent signal separation unit1310, the subcarrier separation unit1320, each frequency shift unit, each adaptive equalization unit, each carrier wave phase synchronization unit, and each delay compensation unit, for example, illustrated in one ofFIGS. 13, 15, and19to21.

Next, the receiving device100extracts a crosstalk component to the main signal in the adjacent signal of the main signal targeted for crosstalk compensation among the signals obtained by the demodulation process in the operation S2202(operation S2203). The operation S2203is executed by each BPF, for example, illustrated inFIG. 2, 3, 5, 6, 17, or18.

Next, the receiving device100frequency-shifts the crosstalk component extracted in the operation S2203(operation S2204). The operation S2204is executed by each frequency shift unit, for example, illustrated inFIG. 2, 3, 5, 6, 17, or18.

Next, the receiving device100compensates for a crosstalk of the main signal by MIMO processing based on the crosstalk component frequency-shifted in the operation S2204(operation S2205). The operation S2205is executed by each multiplication unit, each addition unit, and each subtraction unit illustrated inFIG. 2, 3, 5, 6, 17, or18.

Next, the receiving device100decodes the main signal compensated for the crosstalk in the operation S2205(operation S2206) and ends the series of processing. The operation S2206is executed by the decoding unit at the subsequent stage of the crosstalk compensation unit, for example, illustrated in one ofFIGS. 1, 13, 15, and 19 to 21.

As illustrated inFIG. 22, extracting the crosstalk component of the adjacent signal before frequency shifting and MIMO processing of the adjacent signal makes it possible to narrow the signal band in a circuit that performs frequency shifting and MIMO processing (e.g., to the aforementioned BW or so). Therefore, it is possible to compensate for a crosstalk while suppressing an increase in circuit scale.

As described above, according to the receiving device and the receiving method, a crosstalk may be compensated while suppressing an increase in circuit scale.

For example, with the recent increase in data traffic volume, there is a demand for further increase in capacity of an optical fiber transmission system. In order to meet such a demand, there has been proposed a technique in which the spectrum of an optical signal is made rectangular and narrowed by OFDM, Nyquist filtering, or the like to reduce the occupied band of the optical signal. Using the OFDM or Nyquist filtering technique makes it possible to densely multiplex the frequency spacings of wavelength multiplexed signals to the order of a symbol rate.

However, in some cases, incompleteness may occur in a Nyquist waveform due to, for example, degradation of the linearity in a transmitting device circuit. In such a case, leakage of a signal out of a signal band due to pulse spreading may increase and a crosstalk between channels may become large. The influence of this crosstalk increases with the increase in the number of multi-levels of modulation. For example, in 32 quadrature amplitude modulation (QAM) or 64 QAM, a crosstalk between channels becomes large.

Even when nonlinear response compensation is performed on such a crosstalk, for example, by signal processing, it is difficult to completely suppress the crosstalk since there is a limitation in the calculation accuracy of a signal processing circuit. In the meantime, it is conceivable to widen a channel spacing in order to reduce the influence of the crosstalk, but which may result in deterioration of the frequency utilization efficiency.

In addition, since a circuit for compensating the crosstalk requires processing in a wide signal band including a signal to be compensated and a signal of an adjacent channel to act as a crosstalk to the signal to be compensated, the circuit scale becomes large.

In contrast, according to each of the above-described embodiments, a signal component to act as a crosstalk is extracted from a signal demodulated from a result of the coherent reception and may be frequency-shifted and used for crosstalk compensation. This makes it possible to narrow the band of a frequency shifter that frequency-shifts a signal component acting as a crosstalk or a compensation circuit that performs crosstalk compensation and accordingly, compensate for the crosstalk while suppressing an increase in circuit scale.