JP2017028534A - Coherent optical communication system and communication method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To properly compensate for wavelength dispersion by improving the estimation accuracy of the amount of wavelength dispersion.SOLUTION: An optical transmission device 10 generates a plurality of signal sequences having power concentrated on a plurality of frequency bands, the power being concentrated on a different frequency band from each other, multiplexes two or more training signal sequences among the generated training signal sequences on a main signal, then converts the signal into an optical signal, and transmits the optical signal to an optical fiber 50. An optical reception device 20 converts the optical signal received from the optical fiber 50 into an electric signal, then converts the same into a digital signal, detects a training signal sequence with a different frequency band every digital signal obtained by conversion, estimates the amount of wavelength dispersion from a frequency component included in the detected training signal sequence, and compensates for signal distortion due to the wavelength dispersion of the digital signal obtained by conversion according to the estimated amount of wavelength dispersion.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a coherent optical communication system and a communication method for compensating for chromatic dispersion by estimating a chromatic dispersion amount using a training signal sequence.

  In recent years, in optical fiber communication, an optical receiver is provided with a local oscillation laser, and coherent reception is performed to mix an optical signal received from an optical transmitter via an optical fiber (optical transmission line) and an electric field. Yes. In this coherent optical communication system, a chromatic dispersion estimation technique that compensates for chromatic dispersion described later is applied.

  Chromatic dispersion is a phenomenon in which the propagation speed varies depending on the wavelength of an optical signal. If the wavelength is the same, the propagation speed is the same, but if the wavelength is different, the propagation time is different. The optical signal modulated on the transmission side has a certain wavelength width. However, since the propagation time of the optical signal varies depending on the bandwidth in the optical fiber, if the signal is a baseband signal, the waveform of the signal is distorted and deteriorates. The chromatic dispersion is expressed in units of [ps / nm] indicating how much the propagation time is different when the 1 nm wavelength is different.

  The chromatic dispersion estimation technique is a technique for compensating the above-described signal distortion. First, an optical signal including a training signal sequence necessary for performing the compensation is transmitted from the optical transmission apparatus to the optical fiber, and this transmission signal is transmitted. Is received by the optical receiver. Next, the chromatic dispersion is compensated by estimating the chromatic dispersion amount in the optical fiber from the training signal sequence in the received signal. Thus, this is a technique for compensating for signal distortion caused by chromatic dispersion in which the pulse width of an optical signal spreads in time during propagation through an optical fiber.

  One example of a coherent optical communication system using this chromatic dispersion estimation technique is disclosed in Patent Document 1. In this system, the optical transmission apparatus generates a plurality of signal sequences having power concentrated in a plurality of frequency bands, which will be described later, each having a frequency spectrum having a different shape as a training signal sequence. The generated plurality of training signal sequences are selectively switched according to the optical transmission path, and transmitted as a signal including a signal sequence obtained by time-multiplexing the selected training signal sequence. In the optical receiving apparatus, the receiving unit corresponding to the selected training signal sequence enables estimation of the chromatic dispersion amount from the arrival time difference at the frequency where the power of the training signal sequence included in this signal is concentrated. By compensating for the calculated amount of chromatic dispersion, the accuracy of decoding the received signal can be improved.

In addition, the above-described “a plurality of signal sequences (training signal sequences) having power concentrated in a plurality of frequency bands” is, in other words, “a training signal sequence in which power is concentrated in a plurality of specific frequencies”.
“The power concentrates on a plurality of specific frequencies” means, as shown in FIG. 10A, when the frequency f is plotted on the horizontal axis and the power Pw is plotted on the vertical axis, the specific frequencies f1, f2 on the f axis. This means that the power component shown by the bar graph protrudes. The power component is the peak frequency.
To explain further, for example, -S, S, -S, S,... Of a BPSK (Binary Phase Shift Keying) signal is converted into a time waveform (= signal on the I and Q time axes as shown in FIG. When the time waveforms S and -S are subjected to FFT (Fast Fourier Transform) and viewed in the frequency domain, the power components are obtained. This power component (peak frequency) is expressed as “power concentrates on a specific frequency”.

International Publication No. 2014/112516

  In the system of Patent Document 1 described above, at least one training signal sequence is selected and transmitted after generating a training signal sequence of a plurality of frequency bands by the optical transmission device, and this is selected by the optical reception device and the chromatic dispersion amount is selected. Estimating. However, if the power of the training signal sequence received by the optical receiver is reduced due to the distortion of the transfer function of the optical transmission path between the transmitter and receiver, the estimation accuracy of the chromatic dispersion will deteriorate, and the chromatic dispersion will be compensated appropriately. There was a problem that it was impossible.

  The present invention has been made in view of such circumstances, and provides a coherent optical communication system and a communication method capable of appropriately compensating for chromatic dispersion by improving the estimation accuracy of the chromatic dispersion amount. Let it be an issue.

  As means for solving the above-mentioned problems, the invention according to claim 1 is directed to optical conversion after multiplexing a main signal obtained by modulating polarization with transmission information and a signal sequence in which power is concentrated at a specific frequency in the polarization. A coherent optical communication system in which the transmitted optical signal is received from the optical transmission path, and a plurality of signal sequences having power concentrated in a plurality of frequency bands, and A plurality of signal sequences each of which power is concentrated in different frequency bands are generated as training signal sequences, and two or more training signal sequences among these generated training signal sequences are multiplexed onto the main signal and then converted into an optical signal. An optical transmission device that converts and transmits the optical signal to the optical transmission line; and converts the optical signal received from the optical transmission line into an electric signal, and then converts the signal into a digital signal. A training signal sequence having a different frequency band for each of the signal signals is detected, a chromatic dispersion amount is estimated from a frequency component included in the detected training signal sequence, and the conversion is performed according to the estimated chromatic dispersion amount. An optical receiver that compensates for signal distortion due to chromatic dispersion of a digital signal, and a coherent optical communication system.

  The invention according to claim 8 is an optical transmission line in which an optical transmission device performs optical conversion after multiplexing a main signal whose polarization is modulated by transmission information and a signal sequence in which power is concentrated at a specific frequency in the polarization. Is a communication method in which the transmitted optical signal is received from the optical transmission path by the optical receiver, wherein the optical transmitter is a plurality of signal sequences having power concentrated in a plurality of frequency bands, In addition, a plurality of signal sequences each of which power is concentrated in different frequency bands are generated as training signal sequences, and two or more training signal sequences among the generated training signal sequences are multiplexed on the main signal, and then the light is transmitted. The signal is converted into a signal and transmitted to the optical transmission line, and the optical receiver converts the optical signal received from the optical transmission line into an electric signal, and then converts the signal into a digital signal. A training signal sequence having a different frequency band for each signal is detected, a chromatic dispersion amount is estimated from a frequency component included in the detected training signal sequence, and the converted digital signal is converted according to the estimated chromatic dispersion amount. A communication method that compensates for signal distortion caused by chromatic dispersion of a signal.

  According to these claims 1 and 8, the following operational effects can be obtained. In the optical transmission apparatus, two or more types of training signal sequences having different frequency spectra in different frequency bands are multiplexed and transmitted on the main signal. Thereby, for example, even when the transfer function of the optical transmission line has a component that significantly deteriorates any training signal sequence, the chromatic dispersion amount is selected by selecting the training signal sequence whose characteristics are not deteriorated in the optical receiver. Thus, it is possible to appropriately compensate for chromatic dispersion.

  According to a second aspect of the present invention, the optical transmission device trains a plurality of signal sequences having power concentrated in a plurality of frequency bands, each of which has power concentrated in a different frequency band. A generation unit that generates a signal sequence; a selection unit that selects two or more training signal sequences from a plurality of training signal sequences generated by the generation unit in a time division manner; and the selected training signal sequence as the main signal A signal multiplexing unit that generates a multiplexed signal sequence that is time-multiplexed to the optical signal, and an electro-optical conversion unit that converts the generated multiplexed signal sequence into an optical signal and transmits the optical signal to the optical transmission line, The photoelectric conversion unit that converts an optical signal received from the optical transmission path into an electrical signal, the AD conversion unit that converts the converted electrical signal into a digital signal, and the AD conversion unit The digital signal is duplicated and divided by the number selected by the selection unit, a training signal sequence having a different frequency band is extracted for each of the divided digital signals, and the frequency components included in the extracted training signal sequence An estimator for estimating the amount of chromatic dispersion from the above, and a compensator for compensating for signal distortion caused by chromatic dispersion of the digital signal converted by the AD converter in accordance with the estimated amount of chromatic dispersion. The coherent optical communication system according to claim 1.

  According to this configuration, it is possible to transmit two or more types of training signal sequences having different frequency spectra in different frequency bands, and to receive and appropriately extract these training signal sequences. For this reason, since it is possible to select a training signal sequence whose characteristics are not deteriorated, it is possible to improve the estimation accuracy of the chromatic dispersion amount and appropriately compensate the chromatic dispersion.

  In the invention according to claim 3, when the selection unit selects the training signal sequence generated by the generation unit, the selection unit outputs selection number information indicating the number of the selected training signal sequences to the signal multiplexing unit. The signal multiplexing unit further multiplexes the selection number information to generate a multiplexed signal sequence when the selected training signal sequence is time-multiplexed with the main signal, and the estimation unit includes the AD conversion The digital signal converted by the unit is duplicated and divided by the number corresponding to the selection number information included in the digital signal, and training signal sequences having different frequency bands are extracted for each of the divided digital signals. The coherent optical communication system according to claim 2, wherein a chromatic dispersion amount is estimated from a frequency component included in the extracted training signal sequence.

  According to this configuration, the number of training signal sequences that are two or more between transmission and reception is transmitted and received while arbitrarily changing according to the characteristics of the optical transmission path, and the wavelength based on the arbitrary number of training signal sequences. The chromatic dispersion can be corrected by estimating the amount of dispersion. For this reason, it is possible to estimate the amount of chromatic dispersion with higher accuracy and appropriately compensate for waveform distortion due to chromatic dispersion.

  According to a fourth aspect of the present invention, the selection unit includes at least frequency characteristics of the optical transmission device or the optical reception device, narrowing of the optical transmission path, frequency characteristics of a transmission channel of the optical transmission path, and chromatic dispersion amount. 4. The coherent optical communication system according to claim 2, wherein two or more training signal sequences are selected from a plurality of training signal sequences generated by the generation unit based on one.

  According to this configuration, when two or more training signal sequences multiplexed on the main signal are optically transmitted from the optical transmission apparatus to the optical reception apparatus via the optical transmission path, the optical transmission path is within the compatible range of the transfer function. It is possible to transmit a training signal sequence that fits. For this reason, it is possible to improve the estimation accuracy of the chromatic dispersion amount by using the training signal sequence whose characteristics are not deteriorated and appropriately compensate for the chromatic dispersion.

  The invention according to claim 5 is characterized in that the estimation unit is a chromatic dispersion amount having a small temporal variation among each chromatic dispersion amount estimated for each of the extracted training signal sequences in different frequency bands, or each wavelength. The coherent optical communication system according to any one of claims 2 to 4, wherein a chromatic dispersion amount with a small deviation from an average value of the dispersion amount is selected.

  According to this configuration, it is possible to select a chromatic dispersion amount of a training signal sequence with little characteristic degradation or no characteristic degradation and use it for chromatic dispersion compensation. Therefore, the estimation accuracy of the chromatic dispersion amount is improved, and the chromatic dispersion is improved. It can be compensated appropriately.

  In the invention according to claim 6, the estimation unit compares the extracted power values of the training signal sequences of the different frequency bands, selects a training signal sequence having the maximum power value, and selects the selected training signal. 5. The coherent optical communication system according to claim 2, wherein a process of estimating a chromatic dispersion amount from a frequency component included in a signal sequence and outputting the chromatic dispersion amount to the compensation unit is performed.

  According to this configuration, it is possible to select a training signal sequence having the maximum power value, that is, a training signal sequence having no characteristic degradation, estimate the chromatic dispersion amount, and use it for chromatic dispersion compensation. Accordingly, it is possible to improve the estimation accuracy of the chromatic dispersion amount and appropriately compensate for the chromatic dispersion.

  In the invention according to claim 7, the estimation unit compares the extracted signal power to noise power ratios of the extracted training signal sequences of different frequency bands, and determines a training signal sequence having a maximum signal power to noise power ratio. 5. The processing according to claim 1, wherein a process of selecting, estimating a chromatic dispersion amount from a frequency component included in the selected training signal sequence, and outputting the estimated chromatic dispersion amount to the compensation unit is performed. It is a coherent optical communication system.

  According to this configuration, it is possible to select a training signal sequence having a maximum signal power-to-noise power ratio, that is, a training signal sequence having no characteristic deterioration, estimate the chromatic dispersion amount, and use it for chromatic dispersion compensation. Accordingly, it is possible to improve the estimation accuracy of the chromatic dispersion amount and appropriately compensate for the chromatic dispersion.

  According to the present invention, it is possible to provide a coherent optical communication system and a communication method capable of improving the estimation accuracy of the chromatic dispersion amount and appropriately compensating for the chromatic dispersion.

It is a block diagram which shows the structure of the coherent optical communication system which concerns on embodiment of this invention. It is a block diagram which shows the structure of the optical transmitter of the coherent optical communication system of this embodiment. It is a figure which shows the multiplex signal arrangement | sequence of the multiplex signal series of an optical transmitter. It is a frequency spectrum figure of the multiplex signal sequence at the time of carrying out time multiplexing of the specific frequency band signal sequence (training signal sequence) of a different frequency band. It is a block diagram which shows the structure of the optical receiver of the coherent optical communication system of this embodiment. It is a block diagram which shows the structure of the chromatic dispersion estimation part of an optical receiver. It is a flowchart for demonstrating the communication operation | movement of the optical transmitter of the coherent optical communication system of this embodiment. It is a flowchart for demonstrating communication operation | movement of the optical receiver of the coherent optical communication system of this embodiment. It is a block diagram which shows the other structure of the chromatic dispersion estimation part of an optical receiver. (A) Frequency spectrum diagram showing that power concentrates on two or more specific frequencies, (b) Diagram showing BPSK signals -S, S on the IQ plane.

Embodiments of the present invention will be described below with reference to the drawings.
<Configuration of Embodiment>
FIG. 1 is a block diagram showing a configuration of a coherent optical communication system according to an embodiment of the present invention.
In the coherent optical communication system 100 shown in FIG. 1, each multiplexing device 40 that is spaced apart from a remote place is connected by an optical fiber 50, and the optical transmission device 10 and the optical receiving device 20 are connected to each multiplexing device 40. Configured.

  The optical transmission device 10 on the left side of FIG. 1 modulates binary series transmission information to generate an optical signal, and outputs this optical signal to the multiplexing device 40. This optical signal is multiplexed with another optical signal (not shown) in the multiplexer 40 and transmitted to the right multiplexer 40 via the optical fiber 50. Note that the multiplexing device 40 has a function of executing, for example, wavelength multiplexing or time division multiplexing. The right multiplexer 40 extracts the transmitted signal light by demultiplexing and outputs it to the optical receiver 20. The optical receiver 20 includes a local oscillation laser (not shown), performs coherent reception, and acquires original transmission information from the received optical signal.

<Optical transmitter>
The optical transmission device 10 has a function of performing parallel transmission (or MIMO: Multiple-Input Multiple-Output transmission) using a parallel or orthogonal X polarization and Y polarization. As illustrated in FIG. 2, the optical transmission device 10 includes a transmission signal modulation unit 11, a signal multiplexing unit 12, an electro-optical conversion unit 13, n training signal sequence generation units 14 a to 14 n, and a training signal sequence. And a selector 15.
In addition, when each component part 11-15 is provided with 2 series for X polarization and Y polarization, since 2 series is the same structure, only 1 series is shown in FIG. The transmission signal modulation unit 11 is also referred to as a modulation unit 11, the training signal sequence generation units 14a to 14n are also referred to as generation units 14a to 14n, and the training signal sequence selection unit 15 is also referred to as a selection unit 15.

  The transmission signal modulation unit 11 modulates the polarization with a binary sequence (transmission information) of data to be transmitted, and outputs a transmission symbol sequence (main signal) obtained thereby. Examples of the modulation scheme include BPSK, QPSK (Quadrature Phase Shift Keying), and QAM (Quadrature Amplitude Modulation). However, other modulation schemes may be used.

  Each of the training signal sequence generation units 14a to 14n is a specific band signal sequence having power concentrated in two or more specific frequency bands (also referred to as specific bands), and each of the specific band signal sequences having different frequency spectrums is a training signal. A series is generated and output to the selection unit 15.

  The training signal sequence selection unit 15 selects two or more of the n types of training signal sequences input from the generation units 14 a to 14 n and outputs the selected training signal sequence to the signal multiplexing unit 12. Specifically, the selection unit 15 sets at least one parameter among the frequency characteristics of the optical transmitter 10 or the optical receiver 20, the narrowing in the optical fiber 50, the frequency characteristics of the transmission channel of the optical fiber 50, and the chromatic dispersion amount. Based on the n types of training signal sequences, two or more training signal sequences are selected.

  The frequency characteristics of the optical transmission device 10 or the optical reception device 20 include frequency characteristics of a digital band limiting filter (not shown) in each of the transmission / reception devices 10, 20, a power amplifier, a digital / analog converter, and an analog / digital converter. , Frequency characteristics in a front end unit such as an analog low-pass filter, received signal power for each frequency component, or a received signal power to noise power ratio.

  For the transmission symbol sequence input from the modulation unit 11, the signal multiplexing unit 12 converts an arbitrary number of two or more training signal sequences selected by the training signal sequence selection unit 15 to an arbitrary signal period Ns (Ns ≧ 1 and Ns are positive numbers), and a multiplexed signal sequence obtained as a result is output to the electro-optical converter 13.

  The electro-optical conversion unit 13 converts the multiplexed signal sequence that is an electrical signal input from the signal multiplexing unit 12 into an optical signal, and outputs the optical signal. The output optical signal is transmitted to the optical fiber 50 via the left multiplexer 40 shown in FIG. 1 and further transmitted to the optical receiver 20 via the left multiplexer 40. When the signal multiplexing unit 12 outputs a plurality of multiplexed signal sequences, the electro-optical conversion unit 13 polarizes each multiplexed signal sequence with a different polarization plane and outputs an optical signal.

  An example of the multiplexed signal sequence is shown in FIG. 3 and will be described. As shown in FIG. 3, for the transmission data signal DG that is a transmission symbol sequence output from the modulation unit 11, a training signal composed of Nt symbols (Nt ≧ 1, Nt is a positive number) for each Ns symbols. A multiplexed signal sequence G is generated by time-multiplexing TG1 and TG2. The training signals TG1 and TG2 may be referred to as reference signals, pilot signals, known signals, and the like.

  Here, as the specific band signal series that are the training signals TG1 and TG2, for example, an alternating signal having a point symmetry with respect to the origin on the IQ plane can be used. That is, a BPSK signal is generated, and -S, S, -S, S, ..., -S, S and two signal points are alternately used. Further, a QPSK signal is generated, and (S, S), (-S, -S), (S, S), (-S, -S), ..., (S, S), (-S, -S ), (S, -S), (-S, S), (S, -S), (-S, S), ..., (S, -S), (-S, S). Can be used. Here, S represents an arbitrary complex number. This complex number is represented as (α, β). (Α, β) represents the signal component of (real part, imaginary part), and can be represented as α + jβ as a complex number. j is an imaginary unit.

  Also, one signal is transmitted M times (M is an arbitrary positive number) such as -S, -S, S, S, -S, -S, S, S, ..., -S, -S, S, S. Here, M = 2) Repeated alternating signals can also be used. Moreover, a specific band signal having peaks in four or more frequency bands can be generated by mixing or convolving signals corresponding to a plurality of repetition times. Furthermore, it is possible to generate a band signal having two or more specific frequencies by generating and adding a plurality of sine waves having different periods. Furthermore, a band signal of a specific frequency can be generated by transmitting a signal only to a specific subcarrier using an Orthogonal Frequency Division Multiplexing (OFDM) system.

  In such a method, the generation units 14a to 14n generate a plurality of training signal sequences that are specific band signal sequences, and the selection unit 15 selects two or more training signals TG1 from the generated training signal sequences. , TG2 are selected and time-multiplexed with respect to the transmission data signal DG.

  Here, as an example, it is assumed that there are two generation units 14a and 14b (14b is not shown) in transmission signal transmission of 30 Gbaud (L = 2), and (S, S) and (−S) of the QPSK signal , -S) is generated by the generation unit 14a as the first specific band signal sequence. Further, the QPSK signal is converted into (S, S), (S, S), (-S, -S), (-S, -S), (S, S), ..., (S, S), ( A signal sequence obtained by repeating one signal twice, such as (S, S), (-S, -S), (-S, -S), is generated by the generation unit 14b as the second specific band signal sequence. Assume a case.

  The specific band signal sequence will be described with reference to FIG. FIG. 4 is a frequency spectrum diagram of a multiplexed signal sequence when the specific band signal sequence (training signal sequence) in this embodiment is time-multiplexed. In FIG. 4, the horizontal axis indicates the frequency F (GHz), the vertical axis indicates the power Pw (dB), and an example of generating two types of training signals (training signal series) TG1 and TG2 is given.

  As shown in FIG. 4, two training signals TG1, which are peak components of a frequency spectrum having a 30 GHz interval, are generated on the high frequency side of 15 GHz and the low frequency side of −15 GHz as the first specific band signal series. Yes. Further, as the second specific band signal series, two training signals TG2, which are peak components of a frequency spectrum having a 15 GHz interval, are generated on the high frequency side of 7.5 GHz and the low frequency side of -7.5 GHz. .

  That is, the power of the frequency spectrum of the training signal TG2 as the second specific band signal sequence is concentrated in a frequency band different from the band where the power of the frequency spectrum of the training signal TG1 as the first specific band signal sequence is concentrated. doing. Thus, even when the transfer function of the optical fiber 50 has a component that significantly deteriorates any of the training signals TG1 or TG2, for example, by time-multiplexing two types of training signals TG1 and TG2 having different frequency peak components, By selecting the training signal TG1 or TG2 whose characteristics are not deteriorated, it is possible to stably estimate the amount of chromatic dispersion and compensate the chromatic dispersion of the signal.

<Optical receiver>
Next, the optical receiver 20 will be described with reference to FIG.
The optical receiver 20 includes a photoelectric conversion unit 21, an AD (analog / digital) conversion unit 22, a chromatic dispersion estimation unit 23, a chromatic dispersion compensation unit 24, an adaptive equalization unit 25, and a carrier phase recovery unit 26. And a decoding unit 27. The chromatic dispersion estimating unit 23 is also referred to as an estimating unit 23, the chromatic dispersion compensating unit 24 is also referred to as a compensating unit 24, and the carrier phase reproducing unit 26 is also referred to as a reproducing unit 26.

  The photoelectric conversion unit 21 converts an optical signal transmitted from the optical transmission device 10 via the optical fiber 50 into an electrical signal. For example, the photoelectric conversion unit 21 separates the optical electric field of the coherently received signal light into orthogonal components using a local oscillation laser and converts it into an electrical analog signal.

  The AD conversion unit 22 converts the analog electrical signal converted by the photoelectric conversion unit 21 into a digital signal (also referred to as a digital reception signal) and outputs the digital signal to the estimation unit 23 and the compensation unit 24.

  The chromatic dispersion estimating unit 23 extracts the training signals TG1 and TG2 from the digital reception signal as described later, estimates the chromatic dispersion amount, and inputs the estimated chromatic dispersion amount to the compensating unit 24. The estimated amount of chromatic dispersion is a value for compensating for signal distortion due to chromatic dispersion of the digital received signal.

  The chromatic dispersion compensation unit 24 compensates for signal distortion due to chromatic dispersion of the digital reception signal according to the estimated dispersion compensation amount, and outputs the compensated digital reception signal to the adaptive equalization unit 25.

The adaptive equalization unit 25 separates the polarization multiplexed signal from the digital received signal compensated by the compensation unit 24, and compensates for the signal distorted in the polarization mode dispersion and transmission / reception or transmission path in the separated polarization multiplexed signal. The compensated signal is output to the reproducing unit 26.
Note that the compensation unit 24 and the adaptive equalization unit 25 may be implemented by either time domain or frequency domain compensation.

The carrier phase reproduction unit 26 reproduces the phase of the carrier wave in the signal compensated by the adaptive equalization unit 25.
The decoding unit 27 uses the phase information of the reproduced carrier wave to convert the compensated signal into a binary signal to obtain a transmission source symbol sequence (transmission information).

  Next, the configuration of the chromatic dispersion estimation unit 23 will be described with reference to FIG. As illustrated in FIG. 6, the estimation unit 23 includes a signal division unit 31, m (m is a positive integer greater than or equal to 2) chromatic dispersion amount calculation units (also referred to as calculation units) 32 a to 32 m, and chromatic dispersion amounts. And a selector 33.

  The signal division unit 31 duplicates and divides the digital reception signal input from the AD conversion unit 22 into m pieces, and outputs the divided digital reception signal to the calculation units 32a to 32m.

  Each of the chromatic dispersion amount calculation units 32 a to 32 m calculates the chromatic dispersion amount from two or more (m types) training signal sequences transmitted from the optical transmission device 10. That is, each of the calculation units 32a to 32m has one of m types of BPF (Band pass filter) (not shown) that passes m types of training signal sequences, and the training signal sequence that has passed through the BPF. The amount of chromatic dispersion is calculated. In this calculation, the chromatic dispersion amount is obtained from the arrival time difference at the frequency where the power of the training signal sequence is concentrated.

  As a specific example, when m types of training signal sequences are two types of training signals TG1 and TG2, the respective chromatic dispersion amount calculation units 32a to 32m pass the two types of training signals TG1 and TG2 through each BPF. The amount of chromatic dispersion of each training signal TG1, TG2 that has passed is calculated.

  Note that the division number m in the signal division unit 31 and the number m of each of the chromatic dispersion amount calculation units 32a to 32m are the same number and are selected by the selection unit 15 (FIG. 2) of the optical transmission apparatus 10. It is set corresponding to the number m of two or more training signal sequences.

  The chromatic dispersion amount selector 33 selects the chromatic dispersion amount calculated by each of the calculators 32a to 32m. Here, the chromatic dispersion amount having a large time fluctuation (fluctuation) is removed from the m (two or more) chromatic dispersion amounts. Specifically, the chromatic dispersion amount is accumulated for the designated frame after estimation by calculation is started in each of the calculation units 32a to 32m, and the one with the small fluctuation of the accumulated chromatic dispersion amount is selected. As a fluctuation evaluation method, standard deviation, variance, or the like can be used. It is also possible to select a chromatic dispersion amount with a small deviation from the average value. The chromatic dispersion amount may be selected using other statistical values.

<Operation of Embodiment>
Next, communication operations by the optical transmission device 10 and the optical reception device 20 of the coherent optical communication system 100 of the present embodiment will be described.
First, the communication operation of the optical transmitter 10 (see FIG. 2) will be described with reference to the flowchart shown in FIG.

  In step S1, the modulation unit 11 modulates the polarization with a binary sequence (transmission information) of data to be transmitted, and outputs a transmission symbol sequence (main signal) obtained thereby.

  Next, in step S2, each of the generation units 14a to 14n is a specific band signal sequence having power concentrated in two or more specific bands, and each of the specific band signal sequences having different frequency spectrums is converted into a training signal sequence (n Are generated as a type of training signal sequence) and output to the selection unit 15.

  Next, in step S3, the selection unit 15 selects two or more training signal sequences (for example, two training signals TG1 and TG2) from the n types of training signal sequences input from the generation units 14a to 14n, The two selected training signals TG 1 and TG 2 are output to the signal multiplexing unit 12.

  Next, in step S4, the signal multiplexing unit 12 applies the two training signals TG1 and TG2 selected by the selection unit 15 to the transmission symbol sequence input from the modulation unit 11 every arbitrary signal period Ns. Time multiplexing is performed, and the resulting multiplexed signal sequence is output to the electro-optical converter 13.

  In step S5, the electro-optical converter 13 converts the multiplexed signal sequence input from the signal multiplexer 12 into an optical signal, and outputs this optical signal. The output optical signal is transmitted to the optical fiber 50 via the left multiplexer 40 shown in FIG. 1 and further transmitted to the optical receiver 20 via the left multiplexer 40.

Next, the communication operation of the optical receiver 20 (see FIG. 5) will be described with reference to the flowchart shown in FIG.
In step S <b> 6, the photoelectric conversion unit 21 uses the local oscillation laser (not shown) to optically transmit the optical signal transmitted from the optical transmission device 10 via the optical fiber 50. The field is separated into orthogonal components and converted into electrical analog signals.

  Next, in step S <b> 7, the AD conversion unit 22 converts the converted analog signal into a digital signal and outputs the digital signal to the estimation unit 23 and the compensation unit 24.

  In step S8, the estimation unit 23 extracts two or more training signal sequences (here, two training signals TG1 and TG2) from the digital signal to estimate the chromatic dispersion amount, and the estimated wavelength. The amount of dispersion is input to the compensation unit 24.

  In step S <b> 9, the compensation unit 24 compensates the signal distortion due to the chromatic dispersion of the digital signal according to the dispersion compensation amount estimated in step S <b> 8, and outputs the compensated digital signal to the adaptive equalization unit 25.

  In step S10, the adaptive equalization unit 25 separates the polarization multiplexed signal from the digital signal compensated by the compensation unit 24, and the separated polarization multiplexed signal is transmitted through the transmission path (including polarization mode dispersion and transmission / reception). ) Is compensated for, and the compensated signal is output to the reproducing unit 26.

In step S <b> 11, the reproduction unit 26 reproduces the phase of the carrier wave in the signal after compensation by the adaptive equalization unit 25.
In step S12, the decoding unit 27 uses the phase information of the reproduced carrier wave to convert the signal after the compensation in step S10 into a binary signal to obtain a transmission source symbol sequence (transmission information). .

<Effect of embodiment>
As described above, the coherent optical communication system 100 according to the present embodiment performs optical conversion after multiplexing a main signal whose polarization is modulated by transmission information and a training signal sequence in which power is concentrated at a specific frequency in the polarization. Then, the optical signal transmitted to the optical fiber 50 is received from the optical fiber 50.

  The feature of this embodiment is that the optical transmission apparatus 10 is a plurality of signal sequences having power concentrated in a plurality of frequency bands, and a plurality of signal sequences each having power concentrated in different frequency bands is used as a training signal sequence. Then, two or more of the generated training signal sequences are multiplexed into the main signal, converted into an optical signal, and transmitted to the optical fiber 50. Further, the optical receiver 20 converts the optical signal received from the optical fiber 50 into an electric signal and then converts it into a digital signal, and detects a training signal sequence having a different frequency band for each of the converted digital signals. The chromatic dispersion amount is estimated from the frequency components included in the training signal sequence, and signal distortion due to the chromatic dispersion of the converted digital signal is compensated according to the estimated chromatic dispersion amount.

  That is, in the optical transmission device 10, two or more types of training signal sequences having different frequency spectra in different frequency bands are multiplexed and transmitted on the main signal. For this reason, for example, even when the transfer function of the optical fiber 50 has a component that significantly degrades any training signal sequence, the optical receiver 20 selects a training signal sequence that has not deteriorated in characteristics, thereby obtaining a chromatic dispersion amount. Thus, it is possible to appropriately compensate for chromatic dispersion. As a result, the optical receiver 20 can properly decode the transmission information transmitted from the optical transmitter 10.

  Further, the optical transmission device 10 generates a plurality of signal sequences having power concentrated in a plurality of frequency bands and a plurality of signal sequences each having power concentrated in a different frequency band as a training signal sequence. To 14n and a selection unit 15 that selects two or more training signal sequences in a time division manner from the plurality of training signal sequences generated by the generation units 14a to 14n. Further, a signal multiplexing unit 12 that generates a multiplexed signal sequence obtained by time-multiplexing the selected training signal sequence with a main signal, and electric light that converts the generated multiplexed signal sequence into an optical signal and transmits the optical signal to the optical fiber 50. The conversion unit 13 is provided.

  The optical receiver 20 includes a photoelectric converter 21 that converts an optical signal received from the optical fiber 50 into an electrical signal, and an AD converter 22 that converts the converted electrical signal into a digital signal. Further, the digital signal converted by the AD conversion unit 22 is duplicated and divided by the number selected by the selection unit 15, and a training signal sequence having a different frequency band is extracted for each of the divided digital signals. The estimator 23 for estimating the chromatic dispersion amount from the frequency components included in the training signal sequence and the signal distortion due to the chromatic dispersion of the digital signal converted by the AD converter 22 are compensated according to the estimated chromatic dispersion amount. The compensator 24 is provided.

  According to this configuration, it is possible to transmit two or more types of training signal sequences having different frequency spectra in different frequency bands, and to receive and appropriately extract these training signal sequences. For this reason, since it is possible to select a training signal sequence whose characteristics are not deteriorated, it is possible to improve the estimation accuracy of the chromatic dispersion amount and appropriately compensate the chromatic dispersion.

  The selection unit 15 generates the generation unit based on at least one of the frequency characteristics of the optical transmission device 10 or the optical reception device 20, the narrowing in the optical fiber 50, the frequency characteristics of the transmission channel of the optical fiber 50, and the chromatic dispersion amount. A process of selecting two or more training signal sequences from the plurality of training signal sequences generated in 14a to 14n is performed.

  According to this configuration, when two or more training signal sequences multiplexed on the main signal are optically transmitted from the optical transmission device 10 to the optical reception device 20 via the optical fiber 50, the transfer function adaptation range of the optical fiber 50 is used. It is possible to transmit a training signal sequence that falls within. For this reason, it is possible to improve the estimation accuracy of the chromatic dispersion amount by using the training signal sequence whose characteristics are not deteriorated and appropriately compensate for the chromatic dispersion.

  In addition, the estimation unit 23 is the chromatic dispersion amount estimated for each of the extracted training signal sequences of different frequency bands, the chromatic dispersion amount having a small time variation, or a deviation from the average value of the chromatic dispersion amounts. A process of selecting a small amount of chromatic dispersion is performed.

  According to this configuration, it is possible to select a chromatic dispersion amount of a training signal sequence with little characteristic degradation or no characteristic degradation and use it for chromatic dispersion compensation. Therefore, the estimation accuracy of the chromatic dispersion amount is improved, and the chromatic dispersion is improved. It can be compensated appropriately.

  Further, the estimation unit 23 compares the extracted power values of the training signal sequences of different frequency bands, selects the training signal sequence of the maximum power value, and the frequency component included in the selected training signal sequence From this, a process of estimating the chromatic dispersion amount and outputting it to the compensation unit 24 is performed.

  According to this configuration, it is possible to select a training signal sequence having the maximum power value, that is, a training signal sequence having no characteristic degradation, estimate the chromatic dispersion amount, and use it for chromatic dispersion compensation. Accordingly, it is possible to improve the estimation accuracy of the chromatic dispersion amount and appropriately compensate for the chromatic dispersion.

<Modification of chromatic dispersion estimation unit>
FIG. 9 shows a block diagram of a modification of the chromatic dispersion estimation unit in the present embodiment.
The chromatic dispersion estimating unit 23A illustrated in FIG. 9 includes a signal dividing unit 31, BPFs 35a to 35m, a power calculating unit 36, and a chromatic dispersion amount calculating unit 37. The signal dividing unit 31 is the same as that described above.

  The BPFs 35a to 35m are arranged in the same number as the digital signal division number m in the signal division unit 31, and the frequency passbands of the individual BPFs 35a to 35m are all different. In other words, each of the BPFs 35a to 35m has a frequency band that allows any one of m types of training signal sequences to pass therethrough. For example, the BPF 35a passes only the first training signal sequence of the m types of training signal sequences, and the BPF 35m passes only the mth training signal sequence.

The power calculation unit 36 compares the power values in each of the training signal sequences that have passed through the BPFs 35a to 35m, and selects the training signal sequence having the maximum power component.
The chromatic dispersion amount calculation unit 37 calculates the chromatic dispersion amount from the arrival time difference at the frequency where the power of the selected training signal sequence is concentrated.

  In this way, by selecting the training signal sequence having the maximum power component from the plurality of training signal sequences in the power calculation unit 36 and calculating the chromatic dispersion amount from the selected training signal sequence, the following is performed. Effects can be obtained. That is, the frequency characteristics of the digital band limiting filter in the optical transmitter 10 and the optical receiver 20 connected by the optical fiber 50, the power amplifier, the digital / analog converter, the analog / digital converter, the analog low-pass filter, etc. It is possible to stably estimate the amount of chromatic dispersion regardless of the change in distortion of the temporal transfer function of the frequency characteristic in the front end portion.

  When the power values compared by the power calculator 36 are the same, a training signal sequence having the same power value is output to the chromatic dispersion amount calculator 37. The calculation unit 37 averages the training signal sequences having the same power value and estimates the chromatic dispersion amount as one training signal sequence. By averaging in this way, it is possible to improve the estimation accuracy of the amount of chromatic dispersion.

In addition, the power calculation unit 36 compares the signal power to noise power ratio (S / N ratio) instead of comparing the power values in each of the training signal sequences that have passed through each of the BPFs 35a to 35m, and the S / N ratio. The largest training signal sequence may be selected.
Also in this case, it is possible to stably estimate the chromatic dispersion amount regardless of the change in the distortion of the time-related transfer function of the frequency characteristic in the front end portion described above.

  In addition, about a concrete structure, it can change suitably in the range which does not deviate from the main point of this invention. For example, in the estimation unit 23 of the optical reception device 20 illustrated in FIG. 6, the number m of divisions in the signal division unit 31 and the number m of the calculation units 32 a to 32 m are generated in the optical transmission device 10 illustrated in FIG. The same number as the parts 14a to 14n. At this time, an output terminal that outputs each signal after the division by the signal dividing unit 31 and an input terminal of each of the calculation units 32a to 32m are connected on a one-to-one basis. Further, when the selection unit 15 (FIG. 2) selects the training signal sequence generated by the generation units 14a to 14n, the information on the selection number k (selection number information) is converted into a signal frame by the signal multiplexing unit 12. In other words, it is inserted in at least the front side of the first training signal sequence, such as being inserted in the first region of the preamble of the first.

  When the digital signal received by the optical receiver 20 (FIG. 6) is input to the signal dividing unit 31, the signal dividing unit 31 detects the selection number k inserted at the head of the digital signal, and this selection is performed. Divide k by several k and output to each of the calculation units 32a to 32k. Each of the calculation units 32a to 32k calculates and estimates a chromatic dispersion amount from the training signal sequence that has passed through each BPF.

  According to this configuration, even if a number of training signal sequences are selected after generating a plurality of training signal sequences by the optical transmission device 10, the optical reception device 20 appropriately extracts the training signal sequence by the selected number and estimates the chromatic dispersion amount. be able to. In other words, the number of training signal sequences that are two or more between transmission and reception is transmitted and received while arbitrarily changing according to the characteristics of the optical transmission path, etc., and the chromatic dispersion amount is estimated based on the arbitrary number of training signal sequences Thus, chromatic dispersion can be corrected. For this reason, it is possible to estimate the amount of chromatic dispersion with higher accuracy and appropriately compensate for waveform distortion due to chromatic dispersion.

  Further, the estimation unit 23 of the optical receiver 20 feeds back the estimated chromatic dispersion amount to the optical transmission device 10, and the selection unit 15 of the optical transmission device 10 minimizes the fed back chromatic dispersion amount. Alternatively, the selected number k of two or more training signal sequences may be changed and transmitted to the optical receiver 20. In this way, it is possible to appropriately compensate for waveform distortion due to chromatic dispersion.

DESCRIPTION OF SYMBOLS 10 Optical transmitter 11 Transmission signal modulation part 12 Signal multiplexing part 13 Electro-optical conversion part 14a-14n Training signal sequence generation part (generation part)
15 Training signal sequence selector (selector)
20 optical receiver 21 photoelectric conversion unit 22 AD conversion unit 23 chromatic dispersion estimation unit (estimation unit)
24 Chromatic dispersion compensator (compensator)
25 adaptive equalization unit 26 carrier phase recovery unit 27 decoding unit 31 signal division unit 32a to 32m, 37 chromatic dispersion amount calculation unit 33 chromatic dispersion amount selection unit 35a to 35m BPF
36 Power Calculation Unit 40 Multiplexer 50 Optical Fiber 100 Coherent Optical Communication System

In order to solve the above problems, the invention according to claim 1, the optical transmission apparatus, multiplexed with a main signal modulated polarization in transmission information, and a signal sequence power to a particular frequency in the polarization is concentrated after transmitted to the optical transmission line is optically converted to a coherent optical communication system for receiving from the optical transmission line by an optical receiving apparatus the transmitted optical signal, the optical transmission apparatus, concentrated on a plurality of frequency bands A plurality of signal sequences having power to be generated and generating as a training signal sequence a plurality of signal sequences each having power concentrated in different frequency bands, and a plurality of training signals generated by the generation unit A selection unit that selects two or more training signal sequences from the sequence in a time-sharing manner, and selects selection number information indicating the number of the selected training signal sequences; A training signal sequence and the selected number information are time-multiplexed with the main signal to generate a multiplexed signal sequence, and the generated multiplexed signal sequence is converted into an optical signal and transmitted to the optical transmission line. An optical / electrical converter, and the optical receiver converts an optical signal received from the optical transmission path into an electrical signal, and an AD converter that converts the converted electrical signal into a digital signal. And the digital signal converted by the AD conversion unit is duplicated and divided by the number corresponding to the selection number information included in the digital signal, and training signal sequences having different frequency bands for each of the divided digital signals Of the extracted chromatic dispersion amounts estimated for each of the extracted training signal sequences in different frequency bands, and select a chromatic dispersion amount with a small temporal variation to estimate the chromatic dispersion amount. An estimation unit, a signal distortion due to wavelength dispersion of the digital signal converted by the AD conversion unit, characterized by comprising a compensator for compensating in accordance with the estimated amount of wavelength dispersion.

The invention according to claim 5 is an optical transmission line in which an optical transmission device performs optical conversion after multiplexing a main signal whose polarization is modulated with transmission information and a signal sequence in which power is concentrated at a specific frequency in the polarization. And transmitting the transmitted optical signal from the optical transmission line by the optical receiver, wherein the optical transmitter is a plurality of signal sequences having power concentrated in a plurality of frequency bands. And generating as a training signal sequence a plurality of signal sequences each having power concentrated in different frequency bands, and selecting two or more training signal sequences from the generated plurality of training signal sequences in a time division manner And selecting the selection number information indicating the number of the selected training signal sequences, and selecting the selected training signal sequence and the selection number information. A step of time-multiplexing the signal to generate a multiplexed signal sequence, and a step of converting the generated multiplexed signal sequence into an optical signal and transmitting the optical signal to the optical transmission line, A step of converting an optical signal received from a transmission path into an electrical signal; a step of converting the converted electrical signal into a digital signal; and converting the converted digital signal into the selection number information included in the digital signal. The number of chromatic dispersions estimated by each of the extracted training signal sequences of different frequency bands is extracted by duplicating and dividing the corresponding number, extracting the training signal sequences having different frequency bands for each of the divided digital signals. Among them, a step of selecting a chromatic dispersion amount having a small temporal variation and estimating it as a chromatic dispersion amount, and a signal distortion due to the chromatic dispersion of the converted digital signal, And executes a step of compensating in accordance with the estimated amount of wavelength dispersion.

According to the first and fifth aspects, the following operational effects can be obtained. In the optical transmission apparatus, two or more types of training signal sequences having different frequency spectra in different frequency bands are multiplexed and transmitted on the main signal. Thereby, for example, even when the transfer function of the optical transmission line has a component that significantly deteriorates any training signal sequence, the chromatic dispersion amount is selected by selecting the training signal sequence whose characteristics are not deteriorated in the optical receiver. Thus, it is possible to appropriately compensate for chromatic dispersion. In addition, the number of training signal sequences that are two or more between transmission and reception is transmitted and received while changing arbitrarily according to the characteristics of the optical transmission path, etc., and the chromatic dispersion amount is estimated based on the arbitrary number of training signal sequences Thus, chromatic dispersion can be corrected. For this reason, it is possible to estimate the amount of chromatic dispersion with higher accuracy and appropriately compensate for waveform distortion due to chromatic dispersion. Furthermore, since the chromatic dispersion amount of the training signal sequence with little or no characteristic degradation can be selected and used for chromatic dispersion compensation, the estimation accuracy of the chromatic dispersion amount is improved and the chromatic dispersion is compensated appropriately. be able to.

According to a second aspect of the present invention, the selection unit includes at least frequency characteristics of the optical transmission device or the optical reception device, narrowing of the optical transmission path, frequency characteristics of a transmission channel of the optical transmission path, and chromatic dispersion amount. based on one, a coherent optical communication system according to claim 1, characterized by selecting two or more training signal sequence from a plurality of training signal sequence generated by the generation unit.

In the invention according to claim 3 , the estimation unit compares the extracted power values of the training signal sequences of the different frequency bands, selects the training signal sequence of the maximum power value, and selects the selected training signal. 3. The coherent optical communication system according to claim 1, wherein a process of estimating a chromatic dispersion amount from a frequency component included in a signal sequence and outputting the estimated chromatic dispersion amount to the compensation unit is performed.

According to a fourth aspect of the present invention, the estimation unit compares the extracted signal power to noise power ratios of the training signal sequences of the different frequency bands, and determines a training signal sequence having a maximum signal power to noise power ratio. 3. The coherent optical communication system according to claim 1, wherein a process of selecting, estimating a chromatic dispersion amount from a frequency component included in the selected training signal sequence, and outputting the estimated chromatic dispersion amount to the compensation unit is performed. is there.

Claims (8)

A main signal obtained by modulating the polarization with transmission information and a signal sequence in which power is concentrated at a specific frequency in the polarization are multiplexed and then optically converted and transmitted to the optical transmission path. A coherent optical communication system received from a transmission line,
A plurality of signal sequences having power concentrated in a plurality of frequency bands and a plurality of signal sequences each having power concentrated in a different frequency band are generated as training signal sequences, and 2 of these generated training signal sequences are generated. An optical transmitter that multiplexes two or more training signal sequences into the main signal, converts the signal into an optical signal, and transmits the optical signal to the optical transmission path;
The optical signal received from the optical transmission path is converted into an electrical signal and then converted into a digital signal, and a training signal sequence having a different frequency band is detected for each converted digital signal, and included in the detected training signal sequence An optical receiver that estimates a chromatic dispersion amount from a frequency component to be compensated, and compensates for a signal distortion due to the chromatic dispersion of the converted digital signal according to the estimated chromatic dispersion amount;
A coherent optical communication system comprising: The optical transmitter is
A plurality of signal sequences having power concentrated in a plurality of frequency bands, and generating a plurality of signal sequences each having power concentrated in a different frequency band as a training signal sequence;
A selection unit that selects two or more training signal sequences in a time division manner from a plurality of training signal sequences generated by the generation unit;
A signal multiplexing unit for generating a multiplexed signal sequence obtained by time-multiplexing the selected training signal sequence with the main signal;
An electro-optical converter that converts the generated multiplexed signal sequence into an optical signal and transmits the optical signal to the optical transmission line;
The optical receiver is
A photoelectric conversion unit that converts an optical signal received from the optical transmission path into an electrical signal;
An AD converter for converting the converted electrical signal into a digital signal;
The digital signal converted by the AD conversion unit is duplicated and divided by the number selected by the selection unit, and a training signal sequence having a different frequency band is extracted for each of the divided digital signals. An estimation unit that estimates the amount of chromatic dispersion from the frequency components included in the signal sequence;
The coherent optical communication system according to claim 1, further comprising: a compensation unit that compensates for signal distortion due to chromatic dispersion of the digital signal converted by the AD conversion unit according to the estimated chromatic dispersion amount. . When the selection unit selects the training signal sequence generated by the generation unit, the selection unit outputs selection number information indicating the number of the selected training signal sequences to the signal multiplexing unit,
The signal multiplexing unit generates a multiplexed signal sequence by further multiplexing the selection number information when the selected training signal sequence is time-multiplexed with the main signal.
The estimation unit replicates and divides the digital signal converted by the AD conversion unit by the number corresponding to the selection number information included in the digital signal, and the frequency band is different for each of the divided digital signals. The coherent optical communication system according to claim 2, wherein a training signal sequence is extracted, and a chromatic dispersion amount is estimated from a frequency component included in the extracted training signal sequence. The selection unit generates the generation based on at least one of frequency characteristics of the optical transmission apparatus or the optical reception apparatus, narrowing in the optical transmission path, frequency characteristics of a transmission channel of the optical transmission path, and chromatic dispersion amount. The coherent optical communication system according to claim 2 or 3, wherein two or more training signal sequences are selected from a plurality of training signal sequences generated by the unit. The estimator is one of the chromatic dispersion amounts estimated for each of the extracted training signal sequences in different frequency bands, the chromatic dispersion amount having a small time variation, or a deviation from the average value of the chromatic dispersion amounts. The coherent optical communication system according to any one of claims 2 to 4, wherein a chromatic dispersion amount having a small amount is selected. The estimation unit compares the extracted power values of the training signal sequences of the different frequency bands, selects a training signal sequence of the maximum power value, and selects from the frequency components included in the selected training signal sequence The coherent optical communication system according to any one of claims 2 to 4, wherein a process of estimating a chromatic dispersion amount and outputting the estimated chromatic dispersion amount to the compensation unit is performed. The estimation unit compares the extracted signal power to noise power ratios of the extracted training signal sequences of different frequency bands, selects a training signal sequence having a maximum signal power to noise power ratio, and selects the selected training signal sequence. The coherent optical communication system according to any one of claims 2 to 4, wherein a process of estimating a chromatic dispersion amount from a frequency component included in a signal sequence and outputting the estimated chromatic dispersion amount to the compensation unit is performed. The optical transmission device multiplexes the main signal obtained by modulating the polarization with transmission information and the signal sequence in which power is concentrated at a specific frequency in the polarization, and then optically converts and transmits the optical signal to the optical transmission line. A communication method for receiving an optical signal from an optical transmission line by an optical receiver,
The optical transmission device generates a plurality of signal sequences having power concentrated in a plurality of frequency bands, and a plurality of signal sequences each concentrated in power in a different frequency band, as training signal sequences. Two or more training signal sequences in the training signal sequence are multiplexed with the main signal, converted to an optical signal, and transmitted to the optical transmission line.
The optical receiver converts an optical signal received from the optical transmission path into an electric signal and then converts it into a digital signal, detects a training signal sequence having a different frequency band for each of the converted digital signals, and detects the detected signal. A chromatic dispersion amount is estimated from frequency components included in the training signal sequence, and signal distortion due to chromatic dispersion of the converted digital signal is compensated according to the estimated chromatic dispersion amount. .