JP5992808B2 - Phase compensation device, optical receiver, optical transmission system, and phase compensation method - Google Patents

Description

  The present invention relates to a phase compensation device, an optical receiver, an optical transmission system, and a phase compensation method, and more specifically, generates an optical modulation signal on which a pilot tone signal is superimposed in an optical transmitter and transmits the optical modulation signal on an optical fiber transmission line. The pilot tone signal is extracted in the optical receiver, the phase noise generated in the optical transceiver and the optical fiber transmission line is estimated using the pilot tone signal, and the phase noise compensation is performed on the optical modulation signal based on the estimation. It relates to the technology to be performed.

  The backbone optical transmission system is required to economically accommodate high-speed client signals and transmit large amounts of information. To achieve the above object, a digital coherent transmission method combining coherent detection and digital signal processing has been studied from the viewpoint of improving frequency utilization efficiency, and high-speed and large-capacity information can be obtained by wavelength multiplexing transmission using the transmission method. Realization of transmission is expected. In the transmission method, phase synchronization is realized by digital signal processing.

S. L. Jansen, I. Morita, Itsuro Morita and Hideaki Tanaka, “20-Gb / s OFDM Transmission over 4,160km SSMF Enabled by RF-Pilot Tone Phase Noise Compensation,” PDP 15, OFC / NFOEC 2007

  There is a multi-level modulation method as a technique for improving frequency use efficiency for realizing high-speed and large-capacity information transmission. In this multilevel modulation system, a highly accurate phase synchronization technique is essential. Regarding this phase synchronization technology, the pilot tone signal is superimposed on the main signal as a reference signal on the transmission side, and phase noise generated in the optical transceiver and optical fiber transmission line is estimated on the reception side to compensate for phase fluctuations in the main signal Techniques to do this are being studied. The method is more advantageous in terms of phase estimation accuracy than a method that does not use a reference signal. The effectiveness of phase estimation and compensation processing using a pilot tone signal as a reference signal is disclosed in Non-Patent Document 1, for example.

  However, conventionally, in an optical fiber transmission line, there has been a problem that an error in phase estimation increases due to deterioration of a signal-to-noise ratio (SNR) of a pilot tone signal. Also, as a conventional method of superimposing the pilot tone signal on the main signal, there are a method of inserting the pilot tone signal outside the signal band of the main signal or a method of using some subcarriers for the pilot tone signal. According to the method, there is a problem that the frequency utilization efficiency decreases.

  The present invention has been made in view of such a background, and provides a phase compensation device, an optical receiver, an optical transmission system, and a phase compensation method capable of improving the accuracy of phase estimation of a pilot tone signal. With the goal.

According to the invention, this object is achieved by the means indicated in the claims.
In other words, one aspect of the phase compensation apparatus according to the present invention is a pilot tone separation unit that separates a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulation signal received via an optical transmission line. A matching unit that separates the pilot tone signal components separated by the pilot tone separation unit for each frequency to obtain a plurality of pilot tone signals having different frequencies, and a plurality of pilot tone signals obtained by the matching unit A plurality of phase synchronization circuits that are respectively phase-synchronized with each other , a phase estimation unit that estimates each phase of a pilot tone signal multiplexed with the modulation signal from output signals of the plurality of phase synchronization circuits, and the phase estimation unit was based on the respective phases, the phase noise of the main signal separated from said modulated signal Having a configuration of a phase compensation apparatus characterized by comprising: a phase noise compensation section for amortization, the.

  One aspect of the phase compensation apparatus according to the present invention is a pilot tone separation unit that separates a single pilot tone signal and a main signal from a modulation signal received via an optical transmission line, and is separated by the pilot tone separation unit. A matching unit that performs parallel processing on the acquired pilot tone signal to obtain a plurality of pilot tone signals that are parallelized, and a plurality of phase synchronization circuits that are respectively phase-synchronized with the plurality of pilot tone signals obtained by the matching unit, A phase estimation unit that estimates a phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits; and a phase estimation unit that is separated from the modulation signal based on the phase estimated by the phase estimation unit And a phase noise compensation unit for compensating for the phase noise of the main signal.

An aspect of the phase compensation apparatus according to the present invention includes a pilot tone separation unit that separates a main signal and a pilot tone signal component including a plurality of pilot tone signals having different frequencies from a modulated signal received via an optical transmission line, plural said pilot by performing tone separation unit parallelization process on the pilot tone signal component separated by, temporally interleaving between different frequencies in which a plurality of pilot tone signals or parallel having different parallel frequency A pilot tone signal , a plurality of phase synchronization circuits that are phase-synchronized with the plurality of pilot tone signals acquired by the matching unit, and multiplexed from the output signals of the plurality of phase synchronization circuits to the modulation signal a phase estimator for estimated pilot tone signal phase respectively, said phase estimation Based on the respective phase estimated by having a configuration of a phase compensation apparatus characterized by comprising a phase noise compensation section for compensating the phase noise of the main signal separated from the modulated signal.

  In one aspect of the phase compensation apparatus, for example, the pilot tone signal is assigned to a free frequency generated by limiting a signal band of a main signal on which the pilot tone signal is multiplexed.

  One aspect of the optical receiver according to the present invention is an optical receiver that receives a modulated signal via an optical transmission line, and as one of the phase compensators for compensating the phase of the modulated signal. It has the structure of the optical receiver characterized by having such an aspect.

  One aspect of the optical transmission system according to the present invention includes an optical transmitter that transmits a modulated signal on which a pilot tone signal is superimposed, an optical transmission path that transmits the modulated signal transmitted from the optical transmitter, and the optical transmission. An optical receiver that receives the modulated signal via a path, and the optical receiver is an optical receiver including any aspect of the phase compensation device. It has a configuration.

According to one aspect of the phase compensation method of the present invention, a step of separating a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulated signal received via an optical transmission line is performed. Separating the pilot tone signal components for each frequency to obtain a plurality of pilot tone signals having different frequencies, and providing a plurality of phase synchronization circuits for the plurality of pilot tone signals obtained by separating for each frequency. Performing phase synchronization, estimating each phase of a pilot tone signal multiplexed with the modulation signal from output signals of the plurality of phase synchronization circuits, and based on the estimated respective phases, from the modulation signal A phase compensation method comprising: a step of compensating for phase noise of the separated main signal. That.

  An aspect of the phase compensation method according to the present invention includes a step of separating a single pilot tone signal and a main signal from a modulated signal received via an optical transmission line, and a parallel processing to the separated pilot tone signal Obtaining a plurality of parallel pilot tone signals, phase-synchronizing a plurality of phase synchronization circuits to the obtained plurality of pilot tone signals, and output signals of the plurality of phase synchronization circuits Estimating a phase of a pilot tone signal multiplexed on the modulation signal from the phase and compensating a phase noise of the main signal separated from the modulation signal based on the estimated phase. The structure of the phase compensation method characterized by the above.

One aspect of the phase compensation method according to the present invention includes the steps of separating the main signal and the pilot tone signal component comprising a plurality of pilot tone signals with different frequencies from the modulated signal received via the optical transmission path, before Symbol min isolated on applying parallel processing to the pilot tone signal component, obtaining a plurality of pilot tone signals which are temporally interleaved between different frequencies in which a plurality of pilot tone signals or parallel having different parallel frequency Performing phase synchronization of a plurality of phase synchronization circuits with the acquired plurality of pilot tone signals, respectively, and phases of pilot tone signals multiplexed from the output signals of the plurality of phase synchronization circuits to the modulation signal, respectively a step of estimating, based on the respective phase in which the estimated, the separated from the modulated signal Having a configuration of a phase compensation method characterized by comprising the steps of compensating for phase noise of the signal.

  In one aspect of the phase compensation method, for example, a pilot tone signal multiplexed on the modulation signal is assigned to a vacant frequency generated by limiting a signal band of the modulation signal.

(Related invention)
An aspect of the optical transmission system according to the related invention is an optical transmitter including a pilot tone multiplexing unit that multiplexes one or more pilot tone signals with an optical modulation signal, and an optical transmission line that transmits the optical modulation signal. And an optical receiver that receives the optical modulation signal via the optical transmission path, and the optical receiver extracts a pilot tone signal from the received optical modulation signal; A matching unit that performs parallel processing on the pilot tone signal, a phase synchronization circuit that is phase-synchronized with the pilot tone signal that has been subjected to the parallel processing, and a multiplexed signal from the output signal of the phase synchronization circuit to the optical modulation signal. A phase estimation unit for estimating the phase of the pilot tone signal, and a phase noise for compensating for phase noise of the optical modulation signal based on the phase estimated by the phase estimation unit. Having a configuration of a phase compensation apparatus characterized by comprising a compensation unit.

In one aspect of the optical transmission system according to the related invention, for example, the pilot tone multiplexing unit is configured such that -B [Hz] to -B / 2 [Hz] with reference to the center frequency of the optical modulation signal of the baud rate B [Baud]. Or a means for multiplexing one or more pilot tone signals in a frequency region from B / 2 [Hz] to B [Hz].
In one aspect of the optical transmission system according to the related invention, for example, the matching unit has an adaptive parallel number according to a phase variation in which one or more pilot tone signals are received by the optical transmitter / receiver and the optical fiber transmission line. And an adaptive processing unit that changes to

In one aspect of the optical transmission system according to the related invention, for example, the adaptive processing unit reduces the parallel number when the phase fluctuation received in the optical fiber transmission line is high speed, and receives the signal in the optical fiber transmission line. Means is provided for increasing the parallel number when the phase fluctuation is slow.
In one aspect of the optical transmission system according to the related invention, for example, the matching unit includes a weighting circuit that weights the parallel pilot tone signals according to a chromatic dispersion amount or an amplitude of the pilot tone signal. And

In one aspect of the optical transmission system according to the related invention, for example, the matching unit parallelizes pilot tone signals after chromatic dispersion compensation.
In one aspect of the optical transmission system according to the related invention, for example, the phase estimation unit performs maximum likelihood estimation using distributions of output signals of a plurality of phase synchronization circuits.
In one aspect of the optical transmission system according to the related invention, for example, the phase estimation unit obtains an array gain and a diversity gain by an addition process using the weighting circuit and the phase synchronization circuit.

  According to the present invention, the accuracy of phase estimation of a pilot tone signal can be improved.

It is a figure which shows an example of a structure of the optical transmission system by 1st Embodiment of this invention. It is a block diagram which shows an example of a structure of the transmission circuit with which the optical transmitter of the optical transmission system by 1st Embodiment of this invention is provided. It is a functional block diagram of the signal processing part with which the optical receiver of the optical transmission system by 1st Embodiment of this invention is provided. It is a functional block diagram of the signal processing part (phase compensation part) with which the optical receiver of the transmission system by 1st Embodiment of this invention is provided. It is a functional block diagram of the signal processing part (matching part which comprises a phase compensation part) with which the optical receiver of the transmission system by 1st Embodiment of this invention is provided. It is a figure for demonstrating the example of the design method of the electric power of the pilot tone signal multiplexed by the main signal in the transmission circuit with which the optical transmitter of the optical transmission system by 1st Embodiment of this invention is equipped. It is a figure for demonstrating the effect in the opposing characteristic of the optical transmitter / receiver of the transmission system by 1st Embodiment of this invention. It is a functional block diagram of the signal processing part (phase compensation part) with which the optical receiver of the transmission system by 2nd Embodiment of this invention is provided. It is a functional block diagram of the signal processing part (matching part which comprises a phase compensation part) with which the optical receiver of the transmission system by 2nd Embodiment of this invention is provided. It is a functional block diagram of the modification of the signal processing part (matching part which comprises a phase compensation part) with which the optical receiver of the transmission system by 2nd Embodiment of this invention is provided. It is a functional block diagram of the signal processing part with which the optical receiver of the transmission system by 4th Embodiment of this invention is provided. It is a functional block diagram of the signal processing part (matching part which comprises a phase compensation part) with which the optical receiver of the transmission system by 4th Embodiment of this invention is provided. It is a figure for demonstrating the relationship between Q penalty and the insertion position (offset frequency) of a pilot tone signal in the transmission system by 5th Embodiment of this invention.

First to fifth embodiments of the present invention will be described.
Each embodiment of the present invention described below has a common feature that a pilot tone signal parallelized by decimation processing in an optical receiver is diversity-received, and thereby, between signal points associated with multi-level signal processing. It is possible to estimate and compensate for phase noise with accuracy according to distance reduction.

(First embodiment)
[Description of configuration]
FIG. 1 shows an example of the configuration of an optical transmission system 1 according to the first embodiment of the present invention.
As shown in FIG. 1, the optical transmission system 1 includes an optical transmitter 100, an optical transmission path 200, and an optical receiver 300. Among these, the optical transmitter 100 transmits an optical modulation signal (modulated light) in which the pilot tone signal PT is multiplexed (superposed), and includes a signal light source 110, an optical modulator 120, and a transmission circuit 130. . The signal light source 110 generates continuous light (CW light). The optical modulator 120 optically modulates the continuous light generated by the signal light source 110 based on the electric signal S input from the transmission circuit 130 to generate an optical modulation signal (modulated light).

  The transmission circuit 130 generates a main signal including symbols corresponding to the bit pattern of the data series DT composed of logical values “0” and “1” in accordance with a modulation scheme (for example, QAM: Quadrature Amplitude Modulation). is there. In the present embodiment, the transmission circuit 130 multiplexes the main signal with the pilot tone signal PT and outputs it as an electric signal S. The pilot tone signal PT is a reference signal that serves as a reference for the phase of the main signal.

  FIG. 2 shows an example of the configuration of the transmission circuit 130 provided in the optical transmitter 100. As shown in the figure, the transmission circuit 130 includes a symbol mapping unit 131 and a pilot tone multiplexing unit 132. Symbol mapping section 131 generates a main signal including symbols corresponding to the bit pattern of data series DT by symbol mapping data series DT composed of logical values “0” and “1” to corresponding complex points. It is.

  The pilot tone multiplexing unit 132 multiplexes the pilot tone signal PT with the main signal generated by the symbol mapping unit 131 to generate the electric signal S. In the present embodiment, pilot tone signal PT multiplexed on the main signal by pilot tone multiplexer 132 is a single pilot tone signal having a predetermined frequency. By driving the optical modulator 120 using the electrical signal S output from the transmission circuit 130, an optical modulation signal (modulated light) including the main signal on which the pilot tone signal PT is superimposed is obtained.

  Returning to FIG. The optical transmission line 200 transmits an optical modulation signal sent from the optical transmitter 100. In the present embodiment, an optical fiber transmission line is assumed as the optical transmission line 200, but the optical transmission line 200 is arbitrary as long as it can transmit an optical modulation signal, and may be a space, for example.

  The optical receiver 300 receives an optical modulation signal from the optical transmitter 100 via the optical transmission line 200. The optical receiver 300 includes an optical 90 ° hybrid 310, a photoelectric converter 320, an analog / digital converter 330, a signal processing unit 340, and a local oscillation light source 350. Among these, the optical 90 ° hybrid 310 synthesizes an optical modulation signal received via the optical transmission line 200 and the local oscillation light from the local oscillation light source 350. The photoelectric converter 320 converts the combined light output from the light 90 ° hybrid 310 into an electric signal (analog signal). The analog-to-digital converter 330 converts the analog signal into a data series DR that is a digital signal by sampling the analog signal output from the photoelectric converter 320 at a sampling interval T [sec]. The data series DR represents a digital modulation signal corresponding to the optical modulation signal received via the optical transmission line 200.

  The signal processing unit 340 performs processing for restoring the data series DT input to the optical transmitter 100 from the data series DR (digital quantity modulation signal) input from the analog-digital converter 330. is there. In the present embodiment, the signal processing unit 340 is composed of, for example, a digital signal processor (DSP). However, the present invention is not limited to this example, and the signal processing unit 340 may be realized by software, for example.

  FIG. 3 shows an example of functional blocks of the signal processing unit 340 included in the optical receiver 300. As shown in the figure, the signal processing unit 340 includes a phase compensation unit 341, a chromatic dispersion compensation unit 342, an adaptive equalizer 343, a carrier phase synchronization unit 344, and a symbol identification unit 345. Among these, the phase compensation unit 341 is a feature of the present embodiment, and is a phase compensation device for compensating the phase of the data series DR (digital modulation signal) input from the analog-digital converter 330 described above. Configure. Details thereof will be described later.

  The chromatic dispersion compensation unit 342 performs equalization processing on the data series DR phase-compensated by the phase compensation unit 341 (phase compensation device). The adaptive equalizer 343 performs processing for polarization separation of the polarization multiplexed signal on the data series DR on which equalization processing has been performed. The carrier wave phase synchronization unit 344 synchronizes the phase of the local oscillation light source of the optical receiver 300 with the phase of the signal light source 110 of the optical transmitter 100. The symbol identifying unit 345 identifies symbols from the phase-synchronized data series DR. The data sequence DT on the transmission side is restored from the identified symbol.

FIG. 4 shows an example of functional blocks of the phase compensation unit 341 of the signal processing unit 340 included in the optical receiver 300. As shown in the figure, the phase compensation unit 341 includes a pilot tone separation unit 10, a matching unit 20, N (N is an integer of 2 or more) phase-connected circuits 30 ₁ to 30 _N connected in parallel, and a phase estimation unit. 40 and a phase noise compensation unit 50. Among these, the pilot tone separation unit 10 receives a single pilot tone signal PT having a predetermined frequency and a main signal from a data sequence DR which is a digital modulation signal corresponding to the optical modulation signal received via the optical transmission line 200. It separates the signal DM.

The matching unit 20 performs parallel processing on the pilot tone signal PT separated by the pilot tone separation unit 10 to obtain _N pilot tone signals PT _{1 to} PTN that are parallelized. N phase synchronizing circuit ₃₀ 1 to 30 _N is for each phase with the plurality of pilot tone signals _PT 1 _{~PT N} obtained by the matching section 20. In the present embodiment, the characteristics of the _N phase synchronization circuits 30 ₁ to 30 _N are equal to each other. However, as will be described later, the characteristics of the _N phase synchronization circuits 30 ₁ to 30 _N are different from each other. It may be allowed. The phase estimation unit 40 estimates the phase of the pilot tone signal PT from the output signals of the _N phase synchronization circuits 30 ₁ to 30 _N. The phase noise compensation unit 50 compensates the phase noise of the main signal DM separated from the data series DR (modulation signal) based on the phase estimated by the phase estimation unit 40.

FIG. 5 shows a functional block of the matching unit 20 constituting the phase compensation unit 341. In the example shown in the figure, _N , which represents the number of phase synchronization circuits 30 ₁ to 30 _N , is 3. As shown in the figure, the matching unit 20 includes a weighting circuit 21 and an adaptive processing unit 22. Here, the weighting circuit 21 performs amplitude adjustment and phase adjustment by weighting the pilot tone signal PT multiplexed on the optical modulation signal that is a reception signal. Adaptive processing unit 22, by thinning out data of the weighted pilot tone signal PT, is to parallelize the single pilot tone signal PT on a plurality of pilot tone signals PT ₁ ~PT _N. In the example shown in the figure, since N is 3, the pilot tone signal PT is parallelized with three pilot tone signals PT _{1 to} PT _N (N = 3).

The parallelization of the pilot tone signal PT by the matching unit 20 is adaptively performed according to the frequency component of the pilot tone signal PT, for example. In the example of FIG. 5, by thinning out the data string "0,1,2,3,4,5" that constitute the pilot tone signal PT sequentially with two units, for example a pilot tone signal PT ₁ as a data string "3 ', 0 '". In addition to this example, for example, the data string “0, 1, 2, 3, 4, 5,. The rule for thinning out the data sequence of the pilot tone signal PT is not limited to this example, and is arbitrarily set so that, for example, phase synchronization by the phase synchronization circuits 30 ₁ to 30 _N is promoted.

[Description of operation]
Next, the operation of the optical transmission system 1 according to the present embodiment will be described.
In the optical transmitter 100 of FIG. 1, the transmission circuit 130 outputs to the optical modulator 120 an electric signal S in which the main signal generated from the data series DT is multiplexed with the pilot tone signal PT. The optical modulator 120 optically modulates the continuous light (CW light) generated by the signal light source 110 with the electric signal S to generate an optical modulation signal that is modulated light, and the optical modulation signal is optically digitized. It is sent to the reception optical transmission line 200 as a transmission signal. In this embodiment, the pilot tone signal PT multiplexed to the main signal is an electrical signal, but the optical transmitter 100 transmits the continuous light that is phase-synchronized with the continuous light generated by the signal light source 110 as the pilot tone signal TP. A configuration of sending to the path 200 may be used.

Here, the power of the pilot tone signal PT, which is an electrical signal, can be designed using a pilot tone signal power to signal power ratio (PSR).
With reference to FIG. 6, an example of a method for designing the power of pilot tone signal PT will be described. FIG. 6 is a diagram for explaining an example of a method for designing the power of pilot tone signal PT multiplexed on the main signal in transmission circuit 130. In FIG. 6, L11 indicates the relationship between the Q penalty and the PSR when the power of the pilot tone signal PT is relatively increased with respect to the power of the main signal. Contrary to L11, L12 represents the relationship between the Q penalty and PSR when the power of pilot tone signal PT is made relatively small with respect to the power of the main signal. L13 indicates the relationship between the total Q penalty and PSR when L11 and L12 are added together.

  As illustrated below, in an optical transmission system in which the influence of phase noise of nonlinear optical effect is significant in the light source and optical transmission line of an optical transceiver, by quantitatively evaluating the relationship between phase fluctuation and error rate, The power of pilot tone signal PT can be designed. For example, assume that the modulation scheme is polarization multiplexed 16QAM. In the optical transmission system 1 of the present embodiment, since the pilot tone signal PT is multiplexed and transmitted in the main signal in the optical transmitter 100, when the power of the pilot tone signal PT increases (that is, when the PSR increases), the relative The power of the main signal is reduced. For this reason, as indicated by L11 in FIG. 6, when the PSR increases, the Q penalty, which is an indicator of deterioration from a desired error rate, increases. For example, to make the Q penalty less than 0.1 dB, the PSR must be less than -18 dB.

  On the other hand, when the power of pilot tone signal PT becomes small (that is, when PSR becomes small), the estimation accuracy deteriorates. For this reason, as indicated by L12 in FIG. 6, when the PSR becomes smaller, the Q penalty increases. For example, when estimating the phase noise of a signal light source and a local oscillation light source of a 500 kHz optical transceiver, a PSR of −22 dB or more is required to estimate the phase with a Q penalty of 0.1 dB or less. Thus, the Q penalty tends to increase both when the power of the pilot tone signal PT is increased and decreased.

  Therefore, it is necessary to design the PSR so as to suppress both an increase in Q penalty when the power of the pilot tone signal PT is increased and an increase in Q penalty when the power of the pilot tone signal PT is decreased. For this purpose, the Q penalty when the power of the pilot tone signal PT is relatively increased (L11 in FIG. 6) and the Q penalty when the power of the pilot tone signal PT is relatively decreased (L12 in FIG. 6). Is added to calculate the total Q penalty (L13 in FIG. 6), and the PSR of the pilot tone signal PT may be designed so as to minimize the total Q penalty. In the example of FIG. 6, the PSR may be set to a value in the range of about −22 dB to about −18 dB. Transmitting circuit 130 multiplexes pilot tone signal PT with power satisfying the PSR set based on the above design method on the main signal.

Returning to FIG. An optical modulation signal (modulated light) transmitted from the optical transmitter 100 to the optical transmission line 200 is subjected to waveform degradation due to chromatic dispersion, polarization mode dispersion, and nonlinear optical effect, and the main signal and pilot tone included in the optical modulation signal The signal PT is subjected to substantially the same phase fluctuation.
The optical receiver 300 receives the optical modulation signal transmitted from the optical transmitter 100 to the optical transmission line 200 as an optically digitized reception signal, performs coherent detection on the optical modulation signal, and performs signal processing. At 340, the waveform deterioration equalization process in the optical transmission line 200 or the like is performed.

  Schematically, the phase compensation unit 341 in FIG. 3 constituting the signal processing unit 340 performs phase compensation on the modulation signal represented by the data series DR obtained by the analog-digital converter 330 in FIG. The chromatic dispersion compensator 341 performs equalization processing of chromatic dispersion (waveform deterioration caused by arrival time difference for each wavelength) generated in the optical transmission line 200 with respect to the phase-compensated modulated signal. The adaptive equalizer 343 separates the polarization multiplexed signal on the transmission side. The carrier phase synchronization circuit 344 synchronizes the phase of the signal light source 110 of the optical transmitter 100 with the phase of the local oscillation light source of the optical receiver 300. The symbol identification unit 345 identifies a symbol from the phase-synchronized modulation signal.

Next, the operation of the phase compensation unit 341 that is a characteristic part of the present embodiment will be described in detail.
The pilot tone separation unit 10 constituting the phase compensation unit 341 separates the pilot tone signal PT and the main signal DM from the data sequence DR that is the received modulation signal, for example, using a digital filter. The pilot tone signal PT separated by the pilot tone separation unit 10 is input to the matching unit 20 at a sampling interval T [sec]. The matching unit 20 performs weighting processing by the weighting circuit 21 (FIG. 5) and parallel processing by the adaptive processing unit 22 on the separated pilot tone signal PT. Among these, in the weighting process, amplitude adjustment and phase adjustment of the pilot tone signal PT are performed. In the parallel processing, the pilot tone signal PT subjected to amplitude adjustment and phase adjustment is parallelized with _N pilot tone signals PT _{1 to} PTN.

Pilot tone signal _PT 1 _{~PT N} in parallel by the adaptive processing section 22 of the matching unit 20 are respectively input to the N phase synchronizing circuit ₃₀ 1 to 30 _N. Phase synchronizing circuit ₃₀ 1 to 30 _N, respectively, to extract the phase of the pilot tone signal _PT 1 _{~PT N} by the phase synchronization for the N phase locked loop ₃₀ 1 to 30 _N, the pilot tone signal PT ₁ and it outputs a signal corresponding to the phase of ~PT _N. The phase estimation unit 40 estimates the phase of the pilot tone signal PT from the output signals of the phase synchronization circuits 30 ₁ to 30 _N. The phase noise compensation unit 50 uses the phase estimated by the phase compensation unit 40 to compensate for the phase variation of the main signal DM separated by the pilot tone separation unit 10 and outputs the main signal DMR in which the phase variation is compensated. To do.

Here, with reference to FIG. 5, the process regarding the parallelization by the adaptive process part 22 of the above-mentioned matching part 20 is demonstrated in detail.
Here, a case where a single pilot tone signal PT is parallelized will be described. However, as will be described in other embodiments described later, a similar parallelization process is also applied to two or more pilot tone signals PT. Is possible. That is, parallel processing can be performed on one or more pilot tone signals PT.

The adaptive processing unit 22 of the matching unit 20 thins out the PT signal input from the pilot tone separation unit 10 at the sampling interval T [sec] at a sampling interval NT [sec] every N samples, thereby obtaining a single pilot. The tone signal PT is paralleled with _N pilot tone signals PT _{1 to} PTN. Then, the adaptive processing section 22, a parallel pilot tone signal _PT 1 _{~PT N,} respectively, enter the subsequent stage to the plurality of phase synchronizing circuit ₃₀ 1 to 30 _N. In the following, it is assumed that _{N representing} the number of pilot tone signals PT _{1 to} PTN, that is, the number of phase synchronization circuits 30 ₁ to 30 _N represents “the number of parallelization” as appropriate.

In the present embodiment, for example, the optical transmitter 100, the optical transmission line 200, the light is transmitted by an external or phase synchronization circuit 30 ₁ to 30 _N, or a monitor unit (not shown) provided in the phase estimation unit 40 or the phase noise compensation unit 50. The magnitude of the phase fluctuation speed U [Hz] received at the receiver 300 or the like is monitored. The adaptive processing unit 22 adaptively changes the parallelization number N according to the magnitude of the phase fluctuation speed U [Hz] obtained by the monitor. For example, the adaptive processing unit 22 performs parallelization by setting the maximum integer at which the phase variation speed U is equal to or lower than the parallelized Nyquist frequency (1 / (2NT)) as the parallelization number N.

By the decimation process in the adaptive processing unit 22, the phase variation speed U [Hz] becomes N times NU “Hz”, but the phase variation frequency U [Hz] before decimation is 1 / N or less of the Nyquist frequency. If there is, the phase synchronization circuits 30 ₁ to 30 _N can follow the phase fluctuation.

Next, processing related to phase estimation by the phase estimation unit 40 will be described in detail.
Phase estimating unit 40 estimates the phase of the pilot tone signal PT from the pilot tone signal _PT 1 _{~PT N} of a plurality of streams outputted from the above-described phase synchronizing circuit ₃₀ 1 to 30 _N. The phase error variance (σ ² _single ) of the phase synchronization circuits 0 ₁ to 30 _N is expressed by the equation (1), and the smaller the phase error variance (σ ² _single ), the more accurate phase estimation becomes possible. .

Here, C1, C2, the modulation scheme, the phase design parameters of the synchronous circuit ₃₀ 1 to 30 _N (e.g., when using the PLL to phase lock circuit ₃₀ 1 to 30 _N, damping factor and natural frequency), the sampling frequency Is a constant determined by. γ is the SNR of the pilot tone signal PT after the thinning process before parallelization (i.e., the individual SNR of the pilot tone signal _PT 1 _{~PT N).} δ is a phase fluctuation before decimation processing before parallelization (that is, phase fluctuation of pilot tone signal PT).
Due to the thinning process in the adaptive processing unit 22, δ becomes N times that before the thinning process. Variance of each estimated error decimation after a a previous parallelized pilot tone signal ^{_{PT 1 ~PT N (σ 2 decimation}} ) is represented by the formula (2), the variance of the estimation error is increased.

However, since the number of parallelization is N, the variance (σ ² _parallel ) of the estimation error after _{parallelization} is 1 / N of the variance (σ ² _decimation ) of the phase error before parallelization, and Expression (3) It is expressed as As can be understood by comparing Equations (1) and (3), the effect of γ is 1 / N, resulting in a reduction in estimation error variance and an improvement in SNR of pilot tone signal PT. Is done.

  However, it is assumed that the phase fluctuation speed NU [Hz] after thinning is sufficiently smaller than the bandwidth of white noise. This assumption holds because the fluctuation frequency of the phase noise is assumed to be approximately 100 [kHz] to 100 [MHz], and is therefore one digit or more smaller than the bandwidth of white noise. The precondition for improving the SNR in this embodiment is that “a sufficient sampling point exists in a time width in which the value of the pilot tone signal PT can be regarded as constant”. For example, when the sampling is 100 GSps, the phase variation may be 10 GHz or less, whereas the phase noise variation frequency is sufficiently low. Under this condition, if the signal s (k) at the kth sampling point, the noise is n (k), and the expected value is E [•], the SNR is expressed by the following equation, and the SNR is improved N times. .

Series after sampling: s (k) + n (k)
Signal power: E [(N * S) ^ 2] = (N ^ 2) P
Noise power: E [(Σn (k)) ^ 2] = N (σ) ^ 2
(However, P = E [s ^ 2], σ ^ 2 = E [n ^ 2])
SNR = (signal power) / (noise power) = N (P / σ ^ 2)

In the parallel processing by the adaptive processing unit 22, as shown in the example of FIG. 5 (N = 3), three series of pilot tone signals PT _{1 to} PT _N are obtained by thinning out the data sequence of the pilot tone signal PT at equal intervals. (N = 3) is generated. To improve the SNR of the pilot tone signal _PT 1 _{~PT N} due to the effect of the above-mentioned parallel, in the phase estimation section 40, it is possible to obtain the effect of improving the SNR of the pilot tone signal PT. In the present embodiment has described the thinning processing in the sampling interval NT [sec] As an example, continuous K (K is an integer of 1 or more) enter the number of samples in the first phase lock loop circuit 30 _1, the following the block processing may be performed such as to enter the K samples in a second phase-locked loop 30 _2.

In this embodiment, the parallel phase synchronization circuits 30 ₁ to 30 _{N have} the same characteristics, but the phase synchronization circuits 30 ₁ to 30 _N are designed to have different characteristics. A specific phase synchronization circuit may be preferentially selected according to the characteristics of phase fluctuation. To select a particular phase synchronization circuit preferentially, for example matching unit 20, a weighting circuit for weighting in accordance with the amplitude of the pilot tone signal to the parallel pilot tone signal PT1~PT _N (not shown) May be further provided. As processing in this weighting circuit, “selective synthesis” for selecting a series having the maximum amplitude, SNR value, etc., “equal ratio synthesis” for summing each series, and depending on the amplitude, SNR value, etc. for each series A process such as “maximum ratio composition” may be used in which multiplication is performed by multiplying the coefficients. Further, for example, sample points with a low SNR may be deleted, and the other points may be selectively combined. For example, maximum ratio combining that is weighted according to the SNR may be performed. For example, the thinning cycle may be changed so that the frequency component synchronized for each phase synchronization circuit changes according to the pattern effect, and the weighting corresponding to the frequency may be combined. For example, it may be synthesized by performing thinning so that singular points such as discontinuities due to a difference in period between an analog-digital converter and fast Fourier transform (FFT) in the case of performing digital processing are eliminated.

Here, from the pilot tone signal PT ₁ ~PT _N of a plurality of streams which are weighted in the adaptive processing unit 22 described above, one sequence generated by summing the phase estimation section 40, generally that to estimate This is referred to as diversity processing, and the gain at this time is referred to as diversity gain. In the present embodiment, an effect obtained by paralleling one pilot tone signal PT multiplexed on the transmission side is referred to as diversity gain. On the other hand, an effect obtained by arraying a plurality of pilot tone signals PT multiplexed on the transmission side as in the second and third embodiments described later is referred to as an array gain.

Next, the process in the phase estimation unit 40 will be described in the case where a PLL (Phase Locked Loop) is used for the phase synchronization circuits 30 ₁ to 30 _N.
One of the main features of the present invention is that the phase compensation unit 341 includes a plurality of phase synchronization circuits 30 ₁ to 30 _N. When each of the phase synchronization circuits 30 ₁ to 30 _N is a PLL, the phase error (difference between the true phase and the PLL output phase) φi output from the i-th (1 ≦ i ≦ N) PLL is expressed by the equation (4 The Tikhonov distribution shown in FIG. However, .alpha.i is a constant determined by the SNR and the phase deviation of the pilot tone signal _PT 1 _{~PT N} after the thinning to be inputted to the PLL.

  Since it is possible to estimate a rough phase variation at the time of transmission design, the estimation accuracy is improved by performing maximum likelihood estimation by selecting the phase φest that maximizes the amount shown in Equation (5) from the output of each PLL. be able to. In equation (5), φi is ψi−φest, and ψi is the output phase of the i-th phase synchronization circuit. Specifically, ψi may be substituted into Equation (5) and φest may be calculated as a brute force, or the extreme value may be calculated analytically, and may be directly determined without being brute force.

  FIG. 7 is a diagram for explaining an example of the effect on the opposing characteristics of the optical transmitter 100 and the optical receiver 300 of the transmission system 1. In FIG. 7, L21 represents a theoretical bit error rate, L22 represents a bit error rate by phase estimation according to the present embodiment, and L23 represents a bit error rate when a single phase synchronization circuit is used. Represents. This example shows the effect on the opposing characteristics of the optical transceiver when the present embodiment is applied to a system in which 64QAM is used as the modulation system and the light source line width of the optical transceiver is 100 kHz. As understood from the figure, according to the present embodiment, an error rate of 10 −4 is realized as compared with the case where the phase estimation of the pilot tone signal PT is performed using a single phase synchronization circuit. EbN0 (ratio of signal power per bit to noise density) required for this is improved by about 1 dB, and the phase estimation accuracy is improved.

According to the first embodiment described above, a plurality of phase synchronization circuits 30 ₁ to 30 _N connected in parallel in the optical receiver 300 to a single pilot tone signal superimposed on the main signal in the optical transmitter. The phase gain can be realized with a simple circuit configuration, and phase estimation can be performed with high accuracy, by improving the phase estimation of a single pilot tone signal multiplexed on the main signal. Can do.

  In this embodiment, the description has been made focusing on the main signal (carrier) having a single optical frequency. However, in multicarrier transmission, pilot tone signals multiplexed on different carriers may be used. For example, a pilot tone signal multiplexed on the main signal is transmitted with an offset to a frequency separated from the main signal by a frequency F1 [Hz], and a pilot tone signal multiplexed on an adjacent carrier is transmitted from the main signal to the frequency F2 [ [Hz] is offset to a frequency separated by [Hz], and these two pilot tone signals are received at the receiver. At this time, the SNR of the pilot tone signal multiplexed at the frequency F1 is compared with the SNR of the pilot tone signal multiplexed at the frequency F2, and a pilot tone signal having a high SNR is used. This is suitable for preventing excessive compensation which becomes a problem when a pilot tone signal having a low SNR is used.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with appropriate reference to the drawings of the first embodiment described above.
The difference between the first embodiment and this embodiment resides in the pilot tone multiplexing unit 132 included in the transmission circuit 130 of the optical transmitter 100. That is, in this embodiment, the pilot tone multiplexing unit 132 multiplexes a plurality of different pilot tone signals PT ₁ ~PT _N frequency to the main signal. Inputting a plurality of pilot tone signals on the transmission side will be referred to as arraying. The matching unit 20 included in the phase compensation unit 341 of the optical receiver 300 performs weighting and adaptive processing on the plurality of pilot tone signals PT having different frequencies.

In the present embodiment, the pilot tone multiplexing unit 132 of the optical transmitter 100 functions as a unit that multiplexes (superimposes) a plurality of pilot tone signals having different frequencies on the main signal. In the present embodiment, as shown in FIG. 8, the pilot tone separation unit 10 of the optical receiver 300 has a plurality of pilot tone signals PT _{1 to} PT _{N having} different frequencies from the optical modulation signal received via the optical transmission path. Functions to separate pilot tone signal components including the. Matching portion 8 20, as shown in FIG. 9, a plurality of pilot tone signals PT ₁ ~ in parallel the pilot tone signal component comprising a pilot tone signal PT ₁ ~PT _N separated by the pilot tone separation section 10 to function as intended to get the PT _N. Other configurations are the same as those of the first embodiment.

In the present embodiment, γ in the above equation (1) representing the dispersion (σ ² _single ) of the phase errors of the phase synchronization circuits 30 ₁ to 30 _N is the pilot tone signals PT _{1 to} PT _N after frequency separation. SNR, and δ is the phase variation. Since the number of arrays is N, as shown in Equation (6), the variance of the estimation error (σ ² _parallel ) is the variance of the phase error (σ ² _single ) shown in Equation (1) due to the effect of arraying. Of 1 / N. As a result, the SNR of the pilot tone signal PT is improved and the phase variation is also reduced.
Further, to improve the SNR of the pilot tone signal PT ₁ ~PT _N by the effect of arraying described above, since the reduced equivalently phase change, the phase estimation section 40, it is possible to obtain the effect of reducing the estimation error.

Also in this embodiment, the parallel phase-locked loops 30 ₁ to 30 _N are designed to have different characteristics, and a specific phase-locked loop is preferentially selected according to the characteristics of phase fluctuation. Also good. To preferentially selected, for example, matching unit 20, with respect to the pilot tone signal PT ₁ ~PT _N which is the amount of wavelength dispersion or arrayed, a weighting circuit for weighting according to the amplitude of the pilot tone signal What is necessary is just to prepare further.

  For example, in order to perform maximum ratio combining from multiple sequences for pilot tone signals PT having different propagation delays due to chromatic dispersion, the sequence amplitude and delay are detected, and phase shifts due to amplitude multiplication and delay for each sequence This can be realized by adjusting the above and performing the summation process. In order to perform equal gain synthesis, for example, a delay of a sequence may be detected, a phase shift due to the delay may be adjusted for each sequence, and a summation process may be performed.

Next, a case where a PLL (Phase Locked Loop) is used for the phase synchronization circuits 30 ₁ to 30 _N will be described with respect to the processing in the phase estimation unit 40 of the present embodiment. One of the main features of the present invention is that the phase compensation unit 341 includes a plurality of phase synchronization circuits 30 ₁ to 30 _N. When each of the phase synchronization circuits 30 ₁ to 30 _N is a PLL, the phase error (difference between the true phase and the PLL output phase) φi output from the i-th (1 ≦ i ≦ N) PLL is expressed by the above equation ( 4) Follow the Tikhonov distribution. However, αi is a constant determined by the SNR and phase fluctuation amount of the pilot tone signal PT after decimation input to the PLL. Since it is possible to estimate the approximate phase fluctuation at the time of transmission design, the estimation accuracy can be improved by performing maximum likelihood estimation from the output of each PLL as in the first embodiment.

  For example, when the phase variation of the pilot tone signal PT can be ignored, the adaptive processing unit 22 of the present embodiment transmits a small number of pilot tone signals PT, performs synchronization processing on only a part of the phase synchronization circuits, and the remaining phase synchronization circuits You may pause.

According to the second embodiment described above, a plurality of stages of phase synchronization circuits 30 ₁ to 30 30 connected in parallel in the optical receiver with respect to a plurality of pilot tone signals having different frequencies superimposed on the main signal in the optical transmitter. By synchronizing the phase of _N , it is possible to realize array gain and diversity gain, which is a technology that improves the phase estimation of multiple pilot tone signals multiplexed on the main signal, with a simple circuit configuration, and phase estimation with high accuracy It can be performed.

In this embodiment, the case where the number of pilot tone signals to be transmitted is equal to the number of parallel phase synchronization circuits has been described. However, even if the numbers are different, it can be expected to obtain the same effect.
Further, although the present embodiment has been described focusing on a single optical frequency main signal (carrier), pilot tone signals multiplexed on different carriers may be used in multicarrier transmission. For example, a pilot tone signal multiplexed on the main signal is transmitted with an offset to a frequency separated from the main signal by a frequency F1 [Hz], and a pilot tone signal multiplexed on an adjacent carrier is transmitted from the main signal to the frequency F2 [ [Hz] is offset to a frequency separated by [Hz], and these two pilot tone signals are received at the receiver. At this time, phase compensation of the main signal is performed using the pilot tone signal multiplexed at the frequency F1 and the pilot tone signal multiplexed at the frequency F2. This is suitable in a system in which only one pilot tone signal is inserted into the main signal and multicarrier multiplexing is performed after optical modulation. Further, as illustrated in FIG. 10, when the adaptive processing unit 22 selects the parallel phase synchronization circuits 30 ₁ to 30 _N , the pilot tone signal is sent to a phase synchronization circuit different from the phase synchronization circuit that is originally input. May be provided with an interleaving function for inputting. In the example of FIG. 10, by the interleaving function, for example, data "1" of the pilot tone signal PT ₁ is input to a different phase synchronization circuit 30 _N of the phase synchronization circuit 30 ₁ to input original as a data "1 '" .

(Third embodiment)
Next, a third embodiment of the present invention will be described with appropriate reference to the drawings of the first embodiment.
This embodiment corresponds to a combination of the first embodiment and the second embodiment described above. In the present embodiment, the pilot tone multiplexing unit 132 of the optical transmitter 100 multiplexes a plurality of pilot tone signals PT having different frequencies into the main signal, and the matching unit 20 of the optical receiver 300 uses the plurality of pilot tones having different frequencies. The signal PT and the main signal are separated and thinned out and parallelized. In the present embodiment, as in the first and second embodiments, the estimation error of the pilot tone signal PT can be improved by the effect of parallelization and arraying.

  In the present embodiment, the pilot tone multiplexing unit 132 of the optical transmitter 100 functions as a unit that multiplexes (superimposes) a plurality of pilot tone signals having different frequencies on the main signal. The pilot tone separation unit 10 of the optical receiver 300 functions as a component that separates a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from an optical modulation signal received via the optical transmission line 200. To do. The matching unit 20 functions as a unit that separates the pilot tone signal components separated by the pilot tone separation unit 10 for each frequency and acquires a plurality of pilot tone signals having different frequencies. Other configurations are the same as those in the first embodiment or the second embodiment.

  For example, when the phase variation of the pilot tone signal PT can be ignored, the adaptive processing unit 22 of the present embodiment transmits a small number of pilot tone signals PT, performs synchronization processing on only a part of the phase synchronization circuits, and the remaining phase synchronization circuits You may pause. If the phase fluctuation is large, a large number of pilot tone signals may be transmitted, the number of parallelizations may be reduced, only a part of the phase synchronization circuits may be synchronized, and the remaining phase synchronization circuits may be suspended.

Also in this embodiment, the parallel phase-locked loops 30 ₁ to 30 _N are designed to have different characteristics, and a specific phase-locked loop is preferentially selected according to the characteristics of phase fluctuation. Also good. In order to preferentially select, for example, the matching unit 20 may further include a weighting circuit that weights the chromatic dispersion amount or the parallelized pilot tone signal PT according to the amplitude of the pilot tone signal. .

Next, a case where a PLL (Phase Locked Loop) is used for the phase locked loops 30 ₁ to 30 _N will be described for the processing in the phase estimation unit 40 of the present embodiment. One of the main features of the present invention is that the phase compensation unit 341 includes a plurality of phase synchronization circuits 30 ₁ to 30 _N. When each of the phase synchronization circuits 30 ₁ to 30 _N is a PLL, the phase error (difference between the true phase and the PLL output phase) φi output from the i-th (1 ≦ i ≦ N) PLL is expressed by the above equation ( 4) Follow the Tikhonov distribution. However, αi is a constant determined by the SNR and phase fluctuation amount of the pilot tone signal PT after decimation input to the PLL. Since it is possible to estimate approximate phase fluctuations during transmission design, estimation accuracy can be improved by performing maximum likelihood estimation from the output of each PLL in the same manner as in the first or second embodiment. .

According to the third embodiment described above, as in the second embodiment, a plurality of stages connected in parallel in the optical receiver to a plurality of pilot tone signals having different frequencies superimposed on the main signal in the optical transmitter. To realize diversity gain and array gain, which is a technique for improving the phase estimation of a plurality of pilot tone signals multiplexed on the main signal, with a simple circuit configuration by synchronizing the phase synchronization circuits 30 ₁ to 30 _N And phase estimation can be performed with high accuracy.

  In this embodiment, the description has been made focusing on the main signal (carrier) having a single optical frequency. However, in multicarrier transmission, pilot tone signals multiplexed on different carriers may be used. For example, a pilot tone signal multiplexed on the main signal is transmitted with an offset to a frequency separated from the main signal by a frequency F1 [Hz], and a pilot tone signal multiplexed on an adjacent carrier is transmitted from the main signal to the frequency F2 [ [Hz] is offset to a frequency separated by [Hz], and these two pilot tone signals are received at the receiver. At this time, phase compensation of the main signal is performed using the pilot tone signal multiplexed at the frequency F1 and the pilot tone signal multiplexed at the frequency F2. This is suitable in a system in which only one pilot tone signal is inserted into the main signal and multicarrier multiplexing is performed after optical modulation. Similarly to the example shown in FIG. 10 described above, when the adaptive processing unit selects the parallel phase synchronization circuit, the pilot tone signal is input to a phase synchronization circuit different from the phase synchronization circuit that is originally input. An interleaving function may be provided.

(Fourth embodiment)
Next, a fourth embodiment of the present invention will be described with appropriate reference to the drawings of the first embodiment.
FIG. 11 is a functional block diagram of the signal processing unit 340 included in the optical receiver 300 of the transmission system according to the present embodiment. In the first embodiment described above, the phase compensation unit 341 is provided before the chromatic dispersion compensation unit 342 as shown in FIG. 3, but in this embodiment, instead of the phase compensation unit 341, as shown in FIG. In addition, a phase compensation unit 341A is provided after the chromatic dispersion compensation unit 342. In the present embodiment, the chromatic dispersion compensation unit 342 is provided on the upstream side of the matching unit 20A as described above, so that the weighting process in the matching unit 20A included in the phase compensation unit 341A is the same as that of the first to third embodiments. Different from the matching unit 20. Other configurations are the same as those in the first to third embodiments.

  FIG. 12 is a functional block diagram of the matching unit 20A constituting the phase compensation unit 341A according to the present embodiment. The matching unit 20A includes a weighting circuit 21A and an adaptive processing unit 22. Here, the weighting circuit 21A is different from the weighting circuit 21 of the first embodiment shown in FIG. 5 in that it has only an amplitude adjustment function and does not have a phase adjustment function.

  In the present embodiment, the chromatic dispersion compensation unit 342 can adjust the phase shift due to the delay necessary for realizing each diversity combination. That is, in the present embodiment, the chromatic dispersion compensation unit 342 can also adjust the phase variation due to the delay in the optical transmission line 200 in the process of performing equalization processing of the chromatic dispersion generated in the optical transmission line 200. . For this reason, it is not necessary to adjust the phase shift in the weighting circuit 21A constituting the matching unit 20A. Therefore, according to the present embodiment, a phase shift adjustment circuit is not required in the matching unit 20A, and the circuit scale of the matching unit 20A can be reduced.

  Thus, in the fourth embodiment, instead of the phase compensation unit 341 of the first to third embodiments, a phase compensation unit 341A is provided in the subsequent stage of the chromatic dispersion compensation unit 342, as shown in FIG. The pilot tone separation unit 10 that separates the pilot tone signal PT and the main signal DM is arranged at the subsequent stage of the chromatic dispersion compensation unit 342. As a result, the processing of the weighting circuit 21A constituting the matching unit 20A can be simplified, and the circuit scale of the matching unit 20A can be reduced.

Further, according to the fourth embodiment, a plurality of stages of phase synchronization circuits 30 ₁ to 30 _N connected in parallel in the optical receiver to one or more pilot tone signals superimposed on the main signal in the optical transmitter. The phase gain can be realized with a simple circuit configuration, and the phase estimation can be performed with high accuracy, by improving the phase estimation of one or more pilot tone signals multiplexed on the main signal. be able to.

In this embodiment, the weighting circuit 21A is arranged in the preceding stage of the adaptive processing unit 22, but the weighting circuit 21A is in the subsequent stage of the chromatic dispersion compensating unit 342, and performs the summation process in the phase estimation unit 40 described above. What is necessary is just to be arrange | positioned in the front | former stage of the process part for performing.
The same configuration as that of the present embodiment can also be applied to the first to third embodiments. That is, in the first to third embodiments, the phase compensation unit 341 may be provided in the subsequent stage of the chromatic dispersion compensation unit 342.

(Fifth embodiment)
Next, a fifth embodiment of the present invention will be described with appropriate reference to the drawings of the first embodiment.
This embodiment is different from the configuration of the second embodiment in which the drawing of the first embodiment is used, in that the pilot tone multiplexing unit 132 of the optical transmitter 100 is different. In the present embodiment, the pilot tone signal PT is assigned to a free frequency generated by limiting the signal band of the main signal on which the pilot tone signal PT is multiplexed. In the present embodiment, the pilot tone multiplexing unit 132 uses the center frequency of the optical modulation signal with the baud rate B [Baud] as a reference, from −B [Hz] to −B / 2 [Hz], or B / 2 [ One or more pilot tone signals are multiplexed on the main signal in the frequency range from [Hz] to B [Hz].

  In general, as a technique for multiplexing the pilot tone signal PT with the main signal, there is a technique for inserting the pilot tone signal PT outside the signal band in the optical transmitter, or a technique for using some subcarriers as the pilot tone signal PT. Used. However, according to such a method, the frequency utilization efficiency decreases.

  Therefore, in the present embodiment, the pilot tone multiplexing unit 132 shown in FIG. 2 compensates for incompleteness of the optical transmitter 100, and at the same time, narrows the bandwidth of the main signal (signal occupation bandwidth) (band limitation). ) To secure the insertion position of the pilot tone signal PT within the signal band. Here, the compensation of imperfection of the optical transmitter 100 is, for example, pre-equalization processing of the frequency characteristics of the transmission circuit 130 that drives the optical modulator 120, or predistortion that improves the linearity of the optical modulator 120. It is processing. These compensation processes are not necessarily required in the present embodiment, and can be omitted.

  For example, the pilot tone multiplexing unit 132 uses a filter or the like for the main signal in the electrical signal processing stage before modulating the continuous light or the optical modulation signal modulated with the main signal when the pilot tone signal PT is multiplexed with the main signal. Waveform shaping is performed to narrow the signal occupation bandwidth (band limitation) described above. Thereby, pilot tone multiplexing section 132 ensures the insertion position of pilot tone signal PT including the bandwidth of the main signal before band narrowing.

  Next, pilot tone multiplexing section 132 electrically generates one or more pilot tone signals PT in addition to the main signal, and superimposes it on the main signal in the electric signal processing stage before modulating the continuous light. Alternatively, the pilot tone multiplexing unit 132 may modulate by superimposing on the optical modulation signal modulated by the main signal. Alternatively, the pilot tone multiplexing unit 132 may superimpose the light, which is separately modulated with the pilot tone signal PT of the electrical stage, with the light modulated with the main signal, the phase, the frequency, and the polarization in synchronization. Hereinafter, a method for electrically generating the pilot tone signal PT will be described. However, any configuration may be used as long as continuous light that is phase-synchronized with the continuous light to be modulated is transmitted from the optical transmitter 100 to the optical transmission line 200.

  When the pilot tone signal PT is electrically generated, the insertion interval F [Hz] of the pilot tone signal PT satisfies F> W when the phase fluctuation band to be compensated in the optical receiver 100 is W [Hz]. Is set. If the insertion bandwidth secured by pilot tone multiplexing section 132 is A [Hz], the maximum number of pilot tone signals PT that can be inserted (K is an integer) is the maximum integer that does not exceed A / W.

FIG. 13 shows an example of the relationship between the Q penalty and the insertion position (offset frequency) of the pilot tone signal in the optical transmission system according to the present embodiment, and shows the relationship when the pilot tone signal PT is inserted in the signal band. When modulation is performed at 16 GBaud, if the present embodiment is not applied, there is a signal occupation bandwidth from the center frequency of the signal to about 32 GHz (cutoff frequency f _CUT ). Further, according to the Nyquist theorem, the band can be narrowed up to the Nyquist frequency f _N (16 GHz in this embodiment) without causing intersymbol interference.

In this embodiment, for example, a raised cosine filter (coefficient 0.1) that satisfies the Nyquist theorem is used. At this time, the cut-off frequency f _CUT is about 17.6 GHz. In order to suppress the Q penalty to 1 dB or less, the pilot tone signal PT can be inserted with the designed Q penalty or less by separating the offset of the cutoff frequency f _CUT or more from the center frequency of the signal. Assuming that the secured insertion band A is about 14 GHz and the phase fluctuation band W to be compensated is 1 GHz, according to the present embodiment, up to 14 pilot tone signals PT are inserted into the main signal without reducing the frequency utilization efficiency. It will be possible. In this embodiment, the square cosine filter is used for waveform shaping. However, any other waveform shaping filter can be used as long as it satisfies the Nyquist theorem.

Also in the present embodiment, the parallel phase synchronization circuits 30 ₁ to 30 _N are designed to have different characteristics, and even if a specific phase synchronization circuit is preferentially selected according to the characteristics of the phase fluctuation. Good. In order to preferentially select, for example, the matching unit 20 may further include a weighting circuit that weights the paralleled pilot tone signals according to the amplitude of the pilot tone signals. As the processing of the weighting circuit, the above-described processes such as “selective synthesis”, “equivalent ratio synthesis”, and “maximum ratio synthesis” may be used. Also, for example, the pilot tone signal separated by the pilot tone separation unit by the frequency distribution of the pilot tone signal PT from the center frequency of the main signal due to the statistical distribution of modulation sidebands and the frequency characteristics of the optical transceiver and transmission line A synthesis method may be used in which PT is weighted by a ratio that is inversely proportional to the frequency difference from the center frequency of the main signal or a coefficient that takes into account the shape of the matched filter.

  In this embodiment, the description has been made focusing on the main signal (carrier) having a single optical frequency. However, in multicarrier transmission, pilot tone signals multiplexed on different carriers may be used. For example, a pilot tone signal multiplexed on the main signal is transmitted with an offset to a frequency separated from the main signal by a frequency F1 [Hz], and a pilot tone signal multiplexed on an adjacent carrier is transmitted from the main signal to the frequency F2 [ [Hz] is offset to a frequency separated by [Hz], and these two pilot tone signals are received at the receiver. At this time, phase compensation of the main signal is performed using the pilot tone signal multiplexed at the frequency F1 and the pilot tone signal multiplexed at the frequency F2. This is suitable in a system in which only one pilot tone signal is inserted into the main signal and multicarrier multiplexing is performed after optical modulation. Similarly to the example shown in FIG. 10, the adaptive processing unit inputs a pilot tone signal to a phase synchronization circuit different from the originally input phase synchronization circuit when selecting the parallel phase synchronization circuit. An interleave function may be provided.

According to the fifth embodiment described above, it is possible to suppress a decrease in frequency utilization efficiency by inserting one or more pilot tone signals in the signal band of the main signal in the optical transmitter. Further, the phase synchronization circuits 30 ₁ to 30 _N connected in parallel in the optical receiver are phase-synchronized with one or more pilot tone signals multiplexed in the main signal in the optical transmitter, thereby obtaining the main Diversity gain, which is a technique for improving the phase estimation of one or more pilot tone signals multiplexed on a signal, can be realized with a simple circuit configuration, and phase estimation can be performed with high accuracy.

  Note that the same configuration as that of this embodiment is applicable not only to the second embodiment but also to the first embodiment, the third embodiment, and the fourth embodiment. That is, the pilot tone multiplexing unit 132 in the first embodiment, the third embodiment, and the fourth embodiment may restrict the signal band of the main signal and secure the insertion position of the pilot tone signal PT.

  In the first to fifth embodiments described above, the characteristic portion of the present invention is expressed as phase compensation units (phase compensation devices) 341 and 341A of the optical transmission system. However, the characteristic portion of the present invention is expressed as a phase compensation method. You can also In this case, the phase compensation method corresponding to the first embodiment separates a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulated signal received via an optical transmission line. Separating the separated pilot tone signal components for each frequency to obtain a plurality of pilot tone signals having different frequencies; and a plurality of phases for the plurality of pilot tone signals obtained separately for each frequency Respectively, synchronizing a phase of the synchronization circuit, estimating a phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits, and based on the estimated phase, the modulation signal And compensating for phase noise of the main signal separated from the main signal. Can.

  In addition, the phase compensation method corresponding to the second embodiment includes a step of separating a single pilot tone signal and a main signal from a modulated signal received via an optical transmission line, and the separated pilot tone signal. Obtaining a plurality of parallel pilot tone signals by performing parallel processing, phase-synchronizing a plurality of phase synchronization circuits to the obtained plurality of pilot tone signals, and the plurality of phase synchronization circuits Estimating a phase of a pilot tone signal multiplexed on the modulated signal from the output signal of the output signal, compensating a phase noise of the main signal separated from the modulated signal based on the estimated phase, It can be expressed as a phase compensation method characterized by including

  A phase compensation method corresponding to the third embodiment includes a step of separating a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulation signal received via an optical transmission path; Obtaining a plurality of parallel pilot tone signals by performing parallel processing on the pilot tone signal components separated by the pilot tone separation unit; and a plurality of phase synchronization circuits for the obtained plurality of pilot tone signals Respectively, phase estimating a phase of a pilot tone signal multiplexed with the modulation signal from output signals of the plurality of phase synchronization circuits, and separation from the modulation signal based on the estimated phase Compensating for the phase noise of the main signal generated, and expressing as a phase compensation method, Kill.

  In the above phase compensation method, for example, the pilot tone signal PT is assigned to a free frequency generated by limiting the signal band of the main signal on which the pilot tone signal PT is multiplexed.

  Although the embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and can be arbitrarily modified or modified without departing from the gist of the present invention.

1 ... optical transmission system, 10 ... pilot tone separation unit, 20, 20A ... matching section, 21, 21A ... weighting circuit, 22 ... adaptive processing section, ₃₀ 1 to 30 N _... phase synchronization circuit, 40 ... phase estimating unit, 50 DESCRIPTION OF SYMBOLS ... Phase noise compensation part, 100 ... Optical transmitter, 110 ... Signal light source, 120 ... Optical modulator, 130 ... Transmission circuit, 131 ... Symbol mapping part, 132 ... Pilot tone multiplexing part, 200 ... Optical transmission line, 300 ... Light Receiver 310, optical 90 ° hybrid, 320 photoelectric converter, 330 analog-digital converter, 340 signal processing unit (DSP), 341, 341A phase compensation unit, 342 wavelength dispersion compensation unit, 343 adaptive Equalizer, 344, carrier wave phase synchronization unit, 345, symbol identification unit, 350, local oscillation light source.

Claims (10)

A pilot tone separation unit that separates a main signal from a pilot tone signal component including a plurality of pilot tone signals having different frequencies from a modulated signal received via an optical transmission line;
A matching unit that separates the pilot tone signal components separated by the pilot tone separation unit for each frequency to obtain a plurality of pilot tone signals having different frequencies;
A plurality of phase synchronization circuits that are respectively phase-synchronized with a plurality of pilot tone signals acquired by the matching unit;
A phase estimation unit that estimates each phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits;
A phase noise compensator for compensating phase noise of the main signal separated from the modulated signal based on the respective phases estimated by the phase estimator;
A phase compensation device comprising: A pilot tone separation unit that separates a single pilot tone signal and a main signal from a modulated signal received via an optical transmission line;
A matching unit that performs parallel processing on the pilot tone signal separated by the pilot tone separation unit to obtain a plurality of pilot tone signals that have been parallelized;
A plurality of phase synchronization circuits that are respectively phase-synchronized with a plurality of pilot tone signals acquired by the matching unit;
A phase estimation unit for estimating a phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits;
A phase noise compensation unit that compensates for phase noise of the main signal separated from the modulation signal based on the phase estimated by the phase estimation unit;
A phase compensation device comprising: A pilot tone separation unit that separates a main signal from a pilot tone signal component including a plurality of pilot tone signals having different frequencies from a modulated signal received via an optical transmission line;
Plural said pilot by performing tone separation unit parallelization process on the pilot tone signal component separated by, temporally interleaving between different frequencies in which a plurality of pilot tone signals or parallel having different parallel frequency A matching unit for acquiring a pilot tone signal of
A plurality of phase synchronization circuits that are respectively phase-synchronized with a plurality of pilot tone signals acquired by the matching unit;
A phase estimation unit for estimating the phase of each of the pilot tone signals multiplexed on the modulation signal from the output signals of the plurality of phase synchronization circuits;
A phase noise compensator for compensating phase noise of the main signal separated from the modulated signal based on the respective phases estimated by the phase estimator;
A phase compensation device comprising: The pilot tone signal is
4. The phase compensation apparatus according to claim 1, wherein the phase compensation apparatus is assigned to a vacant frequency generated by limiting a signal band of a main signal to which the pilot tone signal is multiplexed. An optical receiver that receives a modulated signal via an optical transmission line,
An optical receiver comprising the phase compensation device according to claim 1 as a phase compensation unit for compensating the phase of the modulation signal. An optical transmitter for transmitting a modulated signal on which a pilot tone signal is superimposed;
An optical transmission line for transmitting the modulated signal transmitted from the optical transmitter;
An optical receiver for receiving the modulated signal via the optical transmission line;
With
The optical transmission system according to claim 5, wherein the optical receiver is an optical receiver according to claim 5. Separating a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulated signal received via an optical transmission line;
Separating the separated pilot tone signal components for each frequency to obtain a plurality of pilot tone signals having different frequencies;
Phase-synchronizing a plurality of phase synchronization circuits to a plurality of pilot tone signals obtained separately for each frequency; and
Estimating each phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits;
Compensating for phase noise of the main signal separated from the modulated signal based on the estimated respective phases;
Including a phase compensation method. Separating a single pilot tone signal and a main signal from a modulated signal received via an optical transmission line;
Obtaining a plurality of parallel pilot tone signals by performing parallel processing on the separated pilot tone signals;
Phase-synchronizing a plurality of phase-locked circuits to the plurality of pilot tone signals obtained, respectively;
Estimating a phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits;
Compensating for phase noise of the main signal separated from the modulated signal based on the estimated phase;
Including a phase compensation method. Separating a pilot tone signal component including a plurality of pilot tone signals having different frequencies and a main signal from a modulated signal received via an optical transmission line;
Subjected to a parallel treatment before Symbol fraction separated pilot tone signal component, parallelized different pilot tone signal or parallelized different time-interleaved plurality of pilot tones between frequency of the frequency Acquiring a signal ;
Phase-synchronizing a plurality of phase-locked circuits to the plurality of pilot tone signals obtained, respectively;
Estimating each phase of a pilot tone signal multiplexed on the modulation signal from output signals of the plurality of phase synchronization circuits;
Compensating for phase noise of the main signal separated from the modulated signal based on the estimated respective phases;
Including a phase compensation method. The pilot tone signal is
10. The phase compensation method according to claim 7, wherein the phase compensation method is assigned to a vacant frequency generated by limiting a signal band of a main signal on which the pilot tone signal is multiplexed.

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