JP5261771B2 - Chromatic dispersion compensation method and receiving apparatus - Google Patents

Description

  The present invention relates to an optical signal chromatic dispersion compensation method and a receiving apparatus in optical communication.

  An optical signal transmitted in optical communication has a difference in propagation speed for each frequency component in an optical fiber that is a propagation path. Therefore, an arrival time difference occurs in an optical signal received by a receiving apparatus, and chromatic dispersion occurs. Conventionally, such chromatic dispersion has been compensated by using a dispersion compensating fiber having a characteristic opposite to that of an optical fiber that is a propagation path (see, for example, Non-Patent Document 1). However, in the conventional technique using a dispersion compensating fiber, it is necessary to install a dispersion compensating fiber corresponding to the characteristic for each propagation path, which hinders flexible network design such as path change.

  Therefore, in recent years, a method for compensating for chromatic dispersion by digital signal processing has been proposed. FIG. 9 is a block diagram showing a configuration of a receiving apparatus that performs such a chromatic dispersion compensation method. The receiving apparatus includes an analog / digital conversion unit 901, a chromatic dispersion compensation unit 902, and a local light generation unit 910. The local light generation unit 910 outputs local light having a frequency determined according to the transmitted optical signal. The analog / digital conversion unit 901 converts the optical signal received by the receiving device into a digital signal by combining the optical signal in synchronization with the local light input from the local light generation unit 910. The chromatic dispersion compensator 902 performs Fourier transform on the received digital signal using an overlap save method or an overlap add method (for example, refer to Non-Patent Document 2) for each block having a predetermined number of Fourier transform points. Convert to frequency domain. The chromatic dispersion compensator 902 calculates the chromatic dispersion amount based on the characteristics of the optical fiber that is the propagation path and the transmission distance, multiplies the digital signal by the inverse characteristic of the calculated chromatic dispersion amount, and then converts the received signal again. Inverse Fourier transform in time domain. The chromatic dispersion compensator 902 removes the preceding and following digital signals from the digital signal converted into the time domain based on a predetermined inter-block interference removal interval, and outputs a digital signal with compensated chromatic dispersion. As a result, the influence of waveform distortion due to wavelength dispersion can be removed or reduced.

“Next-Generation Ultra-High-Speed Optical Communication Technology—Technical Issues and Overcoming Solutions for Optical Device Development”, First Edition, Technical Information Association, Inc., June 27, 2003, pp. 11-28. 112-118 John J. Shynk, “Frequency-domain and adaptive adaptive filtering”, Signal Processing Magazine, IEEE, 1992, pp. 196 14-37

  However, in the synchronous detection method using local light, there is a case where there is an error between the frequency of the optical signal and the frequency of local light, and the optical frequency shift based on this error may cause deterioration of the characteristics of chromatic dispersion compensation. . That is, if the optical frequency shift occurs in the signal converted into the frequency domain by performing the Fourier transform by the chromatic dispersion compensation unit 902, a shift occurs in the removal interval of the inter-block interference, and the inter-block interference occurs in the signal after the chromatic dispersion compensation. Remains to deteriorate the characteristics of chromatic dispersion compensation. As described above, in the chromatic dispersion compensation by digital signal processing, there is a problem that the optical dispersion between the input optical signal and the local light emission frequency may deteriorate the chromatic dispersion compensation characteristics.

  The present invention has been made in consideration of such circumstances, and its purpose is to have a shift between the optical frequency of the optical signal and the optical frequency of the local light when performing chromatic dispersion compensation in optical communication. It is an object of the present invention to provide a chromatic dispersion compensation method and a receiving apparatus that improve chromatic dispersion by minimizing optical frequency deviation and compensating chromatic dispersion.

In order to solve the above-described problem, one aspect of the present invention is a chromatic dispersion compensation method in optical communication, in which an analog-digital conversion step of converting a received optical signal into a digital signal using local light emission; A frequency estimation step for calculating an estimated value of the optical frequency deviation between the optical signal and the local light , a wavelength for compensating the optical frequency deviation of the digital signal based on the calculated estimated value, and compensating for the chromatic dispersion of the digital signal. A dispersion compensation step, and in the chromatic dispersion compensation step, the digital signal converted in the analog-digital conversion step is Fourier-transformed for each predetermined block, and the chromatic dispersion amount according to the characteristics of the optical signal propagation path After performing the inverse Fourier transform by compensating the chromatic dispersion based on the inverse characteristics of the signal, the signal contained in the block is determined according to the estimated value of the optical frequency deviation. Characterized in that to compensate for the optical frequency deviation of the digital signal by discarding before and after the signal is affected by inter-Chi blocks interference.

According to another aspect of the present invention, in the chromatic dispersion compensation step, the digital signal converted in the analog-digital conversion step is multiplied by the phase rotation corresponding to the estimated value of the optical frequency shift calculated in the frequency estimation step. Thus, the optical frequency shift of the digital signal is compensated.

One embodiment of the present invention is a chromatic dispersion compensation method in optical communication, which includes an analog-digital conversion step of converting a received optical signal into a digital signal using local light, and an optical signal and local light. A frequency estimation step for calculating an estimated value of the optical frequency shift, and a chromatic dispersion compensation step for compensating for the optical frequency shift of the digital signal based on the calculated estimated value and compensating for the chromatic dispersion of the digital signal. In the chromatic dispersion compensation step, the digital signal converted in the analog-digital conversion step is Fourier-transformed to calculate the chromatic dispersion amount based on the characteristics of the propagation path of the optical signal and the estimated value of the optical frequency shift. The optical frequency shift of the digital signal is compensated by compensating the chromatic dispersion based on the inverse characteristic of the chromatic dispersion amount.

One embodiment of the present invention is a chromatic dispersion compensation method in optical communication, which includes an analog-digital conversion step of converting a received optical signal into a digital signal using local light, and an optical signal and local light. A frequency estimation step for calculating an estimated value of the optical frequency shift, a chromatic dispersion compensation step for compensating for the optical frequency shift of the digital signal based on the calculated estimated value, and compensating for the chromatic dispersion of the digital signal, and chromatic dispersion It further comprises a decoding step of decoding the digital signal compensated for chromatic dispersion in the compensation step and feeding back the estimated value of the optical frequency shift used in the decoding to the chromatic dispersion compensation step.

Another embodiment of the present invention is a receiving device in optical communication, an analog-digital conversion circuit that converts a received optical signal into a digital signal using local light, and an optical frequency of the optical signal and the local signal a frequency estimation circuit for calculating the estimated value of the deviation, based on the calculated estimated value to compensate for the optical frequency deviation of the digital signal, and comprises a wavelength dispersion compensation circuit for compensating the chromatic dispersion of the digital signal, the wavelength The dispersion compensation circuit performs Fourier transform of the digital signal converted by the analog / digital conversion circuit for each specified block, and compensates for chromatic dispersion based on the inverse characteristic of the chromatic dispersion amount according to the propagation path characteristic of the optical signal. After performing the inverse Fourier transform, the digital signal is discarded by discarding the signals before and after the influence of interblock interference among the signals included in the block according to the estimated value of the optical frequency shift. Characterized in that to compensate for the optical frequency deviation of the signal.

Further, according to one aspect of the present invention, the chromatic dispersion compensation circuit multiplies the digital signal converted by the analog / digital conversion circuit by the phase rotation corresponding to the estimated value of the optical frequency shift calculated by the frequency estimation circuit, It is characterized by compensating for the optical frequency shift of the digital signal.

Another embodiment of the present invention is a receiving device in optical communication, in which an analog-digital conversion circuit that converts a received optical signal into a digital signal using local light, and an optical frequency of the optical signal and local light A frequency estimation circuit that calculates an estimated value of the shift, and a chromatic dispersion compensation circuit that compensates for the optical frequency shift of the digital signal based on the calculated estimated value and compensates for the chromatic dispersion of the digital signal. The dispersion compensation circuit Fourier transforms the digital signal converted by the analog-digital conversion circuit, calculates the amount of chromatic dispersion based on the characteristics of the propagation path of the optical signal and the estimated value of the optical frequency deviation, and calculates the chromatic dispersion The optical frequency shift of the digital signal is compensated by compensating the chromatic dispersion based on the inverse characteristic of the quantity.

Another embodiment of the present invention is a receiving device in optical communication, in which an analog-digital conversion circuit that converts a received optical signal into a digital signal using local light, and an optical frequency of the optical signal and local light Frequency estimation circuit for calculating an estimated value of deviation, chromatic dispersion compensation circuit for compensating for optical frequency deviation of a digital signal based on the calculated estimated value, and compensating for chromatic dispersion of the digital signal, and chromatic dispersion compensation circuit And a decoding circuit that decodes the digital signal compensated for chromatic dispersion and feeds back the estimated value of the optical frequency shift used for decoding to the chromatic dispersion compensating circuit.

  According to the present invention, an optical signal received in optical communication is converted into a digital signal using local light, an estimated value of an optical frequency shift between the optical signal and the local signal is calculated, and based on the calculated estimated value The optical frequency deviation of the digital signal is compensated and the chromatic dispersion of the digital signal is compensated. Therefore, when performing the chromatic dispersion compensation in the optical communication, the optical frequency of the optical signal and the optical frequency of the local light are shifted. In this case, the chromatic dispersion is compensated while minimizing the shift of the optical frequency to improve the communication quality.

It is a block diagram which shows the structural example of the receiver by the 1st Embodiment of this invention. It is a figure which shows the frequency versus level of the correlation vector of the received signal with an optical frequency shift of 1 GHz. It is a figure which shows the flow of the overlap save by the 1st Embodiment of this invention. It is a figure which shows the phase of the chromatic dispersion compensation coefficient by the 1st Embodiment of this invention. It is a figure showing distribution of inter-block interference of a received signal with an optical frequency shift of 1 GHz. 5 is a flowchart illustrating an operation example of the reception device according to the first embodiment of the present invention. It is a block diagram which shows the structural example of the receiver by the 2nd Embodiment of this invention. 6 is a flowchart illustrating an operation example of the receiving device according to the second embodiment of the present invention. It is a block diagram which shows the structural example of the receiver by a prior art.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
A. First Embodiment First, a first embodiment of the present invention will be described.
FIG. 1 is a block diagram showing a configuration of a receiving apparatus according to the first embodiment of the present invention. In the figure, the receiving apparatus includes an analog / digital conversion unit 101, a frequency estimation unit 102, a chromatic dispersion compensation unit 103, and a local light generation unit 110. The local light generation unit 110 outputs local light having a frequency determined according to the transmitted optical signal. When the optical signal is received by the receiving device, the analog / digital conversion unit 101 converts the optical signal into a digital signal based on the local light output from the local light generation unit 110 and inputs the digital signal to the frequency estimation unit 102. At this time, the optical signal may be directly converted into a digital signal, or the optical signal may be converted into an electric signal and then converted into a digital signal. The frequency estimation unit 102 converts an input signal into a frequency domain signal, estimates a center frequency from a power distribution in the frequency domain, and calculates an optical frequency shift that is a difference from the assumed center frequency. The chromatic dispersion compensation unit 103 performs chromatic dispersion compensation based on the input digital signal, information on the optical frequency shift, information on the type and distance of the optical fiber, and outputs a digital signal in which chromatic dispersion is compensated.

  Here, the influence of the optical frequency shift between the optical signal on the transmission side and the station signal on the reception side on the characteristics of chromatic dispersion compensation will be described in detail. If the optical frequency of the optical signal transmitted to the receiving device is fc (GHz) and the optical frequency of local light used for reception is fr (GHz), the optical frequency shift Δf can be expressed as Δf = fc−fr. . Here, the characteristics of the digital signal obtained in the case of Δf = 0 (GHz) and the characteristics of the digital signal obtained in the case of Δf = 1 (GHz) are compared, and a problem in chromatic dispersion compensation is obtained. To clarify.

  FIG. 2 shows a 25 Gb / s QPSK (quadrature phase shift keying) single carrier signal propagated to a single mode fiber with a transmission distance of 4000 km and a chromatic dispersion of 17 ps / nm, and an optical signal baseband signal of 1552.12 nm. The correlation vector of the received digital signal is converted into a digital signal by 25 GS / s, and the power distribution is displayed in the frequency domain. Here, the local light (laser) frequency of the synchronous detection output from the local light generation unit 110 is shifted from the frequency of the transmitted optical signal by 1 GHz, and the sampling frequency is 25 GHz to obtain data. From the figure, it can be confirmed that the signal intensity distribution is shifted in the positive direction because the DC component (0 GHz) is not the center of the signal distribution due to the influence of the optical frequency shift. Since a signal of −12.5 to 0 GHz is acquired at a sampling frequency of 25 GHz, it can be handled as a signal of 12.5 to 25 GHz. However, here, the received signal is −12.5 to 12.5 GHz. expressing.

FIG. 3 is a diagram showing a flow when the chromatic dispersion compensator 103 compensates the chromatic dispersion of the received signal after time T ₀ by removing the signal by the overlap save method. The chromatic dispersion compensation unit 103 extracts received signal data for each Fourier transform point number (Nf), and performs chromatic dispersion compensation by block processing. Wavelength dispersion compensator 103, the wavelength dispersion compensating a signal from time _{T 0} from _T 0 -Nd of signals back interblock interference cancellation interval (Nd) from the time _{T 0, T 0 + Nf-} Nd-1 The signals up to are extracted as the first block (step S1). The chromatic dispersion compensation unit 103 performs a Fourier transform on the extracted optical signal (step S2).

  Here, as described above, the reception signal received via the propagation path by the optical fiber causes a difference in arrival time for each frequency component due to chromatic dispersion. This phenomenon can be expressed by the chromatic dispersion coefficient, chromatic dispersion slope value, and optical fiber length (transmission distance) of the optical fiber (for example, “Govind P. Agrawal,“ Nonlinear fiber optics ”Academic press, 2006 pp. 63). -65, pp. 76-77)). Accordingly, the chromatic dispersion can be compensated by calculating the chromatic dispersion amount using the transmission distance information of the optical signal and the information on the type of the optical fiber. For example, if the amount of chromatic dispersion is expressed as a phase shift in the frequency domain, the phase rotation in the frequency domain caused by chromatic dispersion can be defined by the following equation (1) for the carrier of frequency f.

Here, L is the transmission distance [km], λ is the wavelength [nm], c is the speed of light 3 × 10 ⁻⁷ [km / ps], D is the chromatic dispersion coefficient [ps / nm / km], and D _slope is the dispersion slope. The coefficient [ps / nm ² / km], fc is the frequency of the optical signal. Since the chromatic dispersion coefficient and the dispersion slope can be given specific values depending on the type of optical fiber, if the type of optical fiber and the distance L are known, it is possible to calculate an estimated value of the phase rotation g (f). It is. In the formula (1), for a fiber that is greatly affected by chromatic dispersion, the chromatic dispersion slope can be ignored by setting D _slope = 0. Since it can be estimated that the digital signal converted from the optical signal is affected by the chromatic dispersion expressed by Equation (1), the digital signal is multiplied by the inverse characteristic of chromatic dispersion expressed by Equation (1). By doing so, the influence of chromatic dispersion can be removed or reduced. Here, when the chromatic dispersion compensation coefficient is calculated based on the chromatic dispersion coefficient and the transmission distance of the single mode fiber according to the equation (1), the phase of the chromatic dispersion compensation coefficient is calculated as shown in FIG. Since the amplitude value is 1, it is not shown. When Δf = 0, the chromatic dispersion compensation coefficient according to Equation (1) is the optimal solution, and chromatic dispersion can be compensated.

Returning to FIG. 3, the chromatic dispersion compensation unit 103 multiplies the chromatic dispersion compensation coefficient calculated in this way by the digital signal in the frequency domain converted in step S2 (step S3). The chromatic dispersion compensation unit 103 multiplies the converted digital signal by a chromatic dispersion compensation coefficient, and then performs inverse Fourier transform to convert the signal into a time domain signal (step S4). _Among the _{T 0 -Nd~T 0 + Nf-Nd} -1 in the time domain of the signals, signals before and after Nd characteristics due to the influence of the inter-block interference is deteriorated. Therefore, to discard the signals before and after _Nd (T _{0 -Nd~T} 0 -1 and _{T 0 + Nf-2Nd-1~T} 0 + Nf-Nd-1) ( step _{S5), T 0 ~T 0 +} Nf-2Nd- 1 digital signal is selected. Subsequently, the wavelength dispersion compensating unit 103, duplicate computes _{T 0 + Nf-3Nd~T 0 +} T 0 + discarding the Nd symbols before and after the block constituted by 2Nf-3Nd-1 Nf-2Nd~T 0 + 2Nf A signal of -4Nd-1 is obtained, the chromatic dispersion of the continuous optical signal is compensated without excess and deficiency, and the data block is restored (step S6).

  At this time, according to the flow of FIG. 3, assuming that Nf = 512, the distribution of inter-block interference generated in the signal in the time domain in which chromatic dispersion compensation is performed by the chromatic dispersion compensation coefficient shown in FIG. 4 is shown in FIG. In FIG. 5, the solid line represents Δf = 0 (GHz), that is, the case where the optical signal frequency on the transmission side and the frequency of the station signal on the reception side completely match. In this case, the distribution of residual inter-block interference is the same before and after the block, and a signal that does not cause inter-block interference can be obtained by simply selecting the central signal. That is, in the example of the solid line in FIG. 5, if Nd is set to 100 symbols or more, chromatic dispersion compensation can be performed without being affected by inter-block interference. However, the distribution of inter-block interference of signals in the case of Δf = 1 (GHz) indicated by a dotted line is not the same before and after. In this case, the block to be discarded is shifted in order to eliminate the influence of inter-block interference, and a signal different from the assumed signal distribution is multiplied by the chromatic dispersion compensation coefficient to be multiplied, thereby degrading the chromatic dispersion compensation characteristics. . As described above, when an optical frequency shift occurs, interblock interference remains in a signal subjected to chromatic dispersion compensation unless the magnitude of the signal to be deleted before and after is set larger than the actual size. As described above, the optical frequency shift Δf causes deterioration in the characteristics of chromatic dispersion compensation.

  In the present embodiment, such optical frequency shift is compensated to prevent deterioration in characteristics of chromatic dispersion compensation. Returning to FIG. 1, the frequency estimation unit 102 calculates a signal distribution in the frequency domain of the digital signal converted by the analog / digital conversion unit 101 and estimates an optical frequency shift of the optical signal. Here, assuming that the reception signal is r (t) and the reception time is t, the frequency estimation unit 102 performs Fourier transform of the reception signal for each number of Fourier transform points (Nf), and the obtained reception signal in the frequency domain. Calculate the expected value of the power value, calculate the position of the positive and negative frequency values that are X (dB) lower than the peak value of the expected power value in the frequency domain, and estimate the optical frequency deviation from the median value To do.

  For example, an example of estimating the optical frequency shift in the case shown in FIG. 2 will be described. In FIG. 2, 1000 received signal levels in the frequency domain obtained by Fourier transforming the correlation vector of the received signal are obtained and the expected value is obtained. If the number of correlation vectors to be averaged is Nv, the correlation vector ρ (i) (i = 1 to Nf) can be obtained by the following equation (2).

  Here, the superscript * indicates a complex conjugate. Here, since the absolute value of power has no significant meaning, the division term for averaging is omitted. Even if the correlation vector is not obtained in this way, the expected value of the received signal may be used instead of the correlation value as ρ (i) = Σr (j), the eigenvector of the correlation matrix may be used, or the correlation A vector having a high correlation with the eigenvector obtained by multiplying the correlation matrix by the vector may be used. FIG. 2 shows the power distribution using Fourier transform on the result obtained with Nv = 1000 in the above correlation vector. Here, the power distribution in FIG. 2 is expressed as p (f). f represents the frequency which is the horizontal axis in FIG. The optical frequency shift is obtained by evaluating how much the signal power distribution is shifted from 0 GHz (center of gravity) from this power distribution. Here, a method for obtaining the center of gravity of the signal will be described. The barycentric frequency Fx of the signal can be expressed by the following equation (3).

  Here, Fp is a frequency to be considered before and after 0 GHz, and can be set to 12.5 GHz at the maximum when applied to 25 GS / s. However, as can be seen from FIG. 2, in the frequency band of −11 GHz or less, it can be seen that power tends to increase due to signal folding. For this reason, it is desirable that Fp take a value slightly smaller than half of the sampling frequency. Here, when Fp = 10 GHz is calculated, Fx = 0.88 GHz is obtained. Although there is an error with respect to the actual optical frequency shift of 1 GHz, Δf can be reduced from 1 GHz to 0.12 GHz. In order to determine the optical frequency deviation, in addition, two frequencies that are below the specified value are obtained to obtain the median value, or the frequency Fmin that minimizes the power is calculated, and the appropriate value is obtained from Fmin ± Fs / 2. Δf can be obtained by selecting a center frequency. Further, Δf may be estimated by a conventionally proposed carrier synchronization method (for a method of estimating Δf by a carrier synchronization method, see, for example, “Digital Communication” by Proakis, Science and Technology Publishing, pp. 410-425 "). In this way, the frequency estimation unit 102 calculates an estimated value of the optical frequency shift (Δf) indicating how much the center position of the distribution is shifted from positive to negative from the DC component (0 GHz) in FIG.

  Alternatively, the optical frequency shift (Δf) is an algorithm such as a minimum mean square error (Maximum Mean Square Error), a maximum signal-to-noise ratio (SNR) method, a constrained output power minimization method, or a CMA (Constant Modulus Algorithm). , Using training symbols, or using blind prior knowledge about signals. Alternatively, an optical frequency shift (Δf) can be obtained by estimating a frequency shift with respect to received signals with respect to a plurality of polarization planes, using a plurality of these results, averaging and weighting, and adding.

  The chromatic dispersion compensator 103 is based on the information on the chromatic dispersion value, chromatic dispersion slope, and transmission distance of the optical fiber stored in its own storage area according to the estimated value of the optical frequency deviation calculated by the frequency estimator 102. The chromatic dispersion is compensated by using Equation (1). Here, the following three types of chromatic dispersion compensation methods performed by the chromatic dispersion compensation unit 103 in consideration of the optical frequency shift can be considered. That is, (a) a method of correcting the optical frequency shift of the input signal and then multiplying by the chromatic dispersion compensation coefficient based on Expression (1), and (b) the expression (1) for the input signal. ) Based on the chromatic dispersion compensation coefficient, and then performing inverse Fourier transform to select a signal position corresponding to the optical frequency shift. (C) Correcting Δf to fc in Equation (1) to compensate for chromatic dispersion compensation In this method, a coefficient is calculated and the chromatic dispersion is compensated based on the calculated chromatic dispersion compensation coefficient.

  First, the method (a) will be described. In this method, compensation for optical frequency deviation is performed in a state where the signal is not in the frequency domain before step S2 in FIG. When the chromatic dispersion compensator 103 indicates that the optical frequency shift calculated by the frequency estimator 102 is shifted by Y (GHz), the chromatic dispersion compensator 103 applies Y to the digital signal output from the analog / digital converter 901. Multiply the phase rotation corresponding to (GHz). However, in this case, both interference and noise power concentrated on the original DC component are frequency-converted. For this reason, it is necessary to perform processing such as removal of a DC component before performing frequency conversion. In response to this, for example, the expected value of the received signals in a plurality of time domains before and after is subtracted, the frequency channel information corresponding to DC is replaced with 0 when Fourier transform is performed, or a digital high-pass filter A method such as passing through can be considered.

  Next, the method (b) will be described. In this method, compensation for optical frequency deviation is performed after step S4 in FIG. For example, in FIG. 5, it can be seen that the distribution of inter-block interference is shifted corresponding to the optical frequency shift. FIG. 5 is an example in which an overlap save method is performed by 512-point FFT (Fast Fourier transform). Since the reception sampling frequency is 25 GHz, the number of FFT points corresponding to 1 GHz is about 20 points. Therefore, in the conventional chromatic dispersion compensation, the Nd signals before and after the Fourier transform block are deleted after the inverse Fourier transform in step S5 in FIG. Nd + 20 signals at the end of the signal and the block are removed, and the signal after the chromatic dispersion is continuously selected by the chromatic dispersion processing performed in duplicate in the preceding and succeeding blocks. Therefore, chromatic dispersion can be compensated.

  On the other hand, when the overlap add method is used for FFT, chromatic dispersion compensation can be continuously performed by adding 0 before and after the block of the digital signal. Here, the same number of 0s are added before and after the block by the conventional method and Fourier transform is performed, but the number of 0s added before and after the block is made asymmetric to prevent signal degradation due to interblock interference. be able to. As described above, when 512-point Fourier transform is performed at a sampling frequency of 25 GS / s, if a 1 GHz optical frequency shift is observed, a difference of 20 points to 0 added before and after the block. You can put on.

  In such an overlap save method and overlap add method, the sampling number Ns to be asymmetric is the sampling frequency Fs, the number of points used for the Fourier transform used for chromatic dispersion compensation, and the light calculated by the frequency estimation unit 102. If the frequency shift is Δf, it can be expressed by the following equation (4).

Here, c is the speed of light, L is the length of the optical fiber, Fs is the sampling frequency, and fc is the center frequency of the optical signal. “Integer (A)” indicates that an integer value based on the value of A is calculated. For example, for a positive number A, an integer value obtained by rounding off the decimal point, an integer value obtained by rounding down the decimal point, and rounding up the decimal point An integer value or an integer value close to A among one or more predetermined integers can be used.
Also in the case of (b), the DC component can be removed before and after multiplication by the coefficient of chromatic dispersion compensation as in the case of (a). In this case, by setting the DC component to 0 after Fourier transform, it is possible to remove noise / interference power caused by the DC component without performing a new calculation.

  In (c), in step S3 of FIG. 3, Δf is corrected to fc in Equation (1) to calculate a coefficient of chromatic dispersion compensation, and a new chromatic dispersion is compensated based on the calculated chromatic dispersion compensation coefficient. By calculating the coefficient, the chromatic dispersion can be appropriately compensated so that the distribution of inter block interference (IBI) does not shift.

  FIG. 6 is a flowchart illustrating an operation example of the receiving apparatus according to the present embodiment. When the optical signal is received by the receiving device, the analog / digital conversion unit 101 converts the optical signal into a digital signal based on the local light output from the local light generation unit 110 (step S11), and the frequency estimation unit 102 To enter. The frequency estimation unit 102 converts the input signal into a frequency domain signal, estimates a center frequency from the power distribution in the frequency domain, and calculates an optical frequency shift that is a difference from the assumed center frequency (step) S12). The chromatic dispersion compensator 103 performs chromatic dispersion compensation based on the input digital signal, information on the optical frequency shift, information on the type and distance of the optical fiber, and outputs a digital signal with compensated chromatic dispersion (step S13). ).

B. Second Embodiment Next, a second embodiment of the present invention will be described.
FIG. 7 is a block diagram showing a configuration of a receiving apparatus according to the second embodiment of the present invention. In this figure, the receiving apparatus includes an analog / digital conversion unit 201, a chromatic dispersion compensation unit 202, and a decoding unit 203. Each unit in the present embodiment has the same configuration as each unit in the first embodiment, and a configuration unique to the present embodiment will be described.

  After the optical signal is converted into a digital signal by the analog / digital converter 201, the chromatic dispersion compensator 202 receives the transmission distance and the characteristics of the optical fiber from the input signal, and the optical frequency fed back from the decoder 203 described later. Based on the shift Δf, the influence of chromatic dispersion is removed by signal processing and output to the decoding unit 203. The decoding unit 203 compensates for the optical frequency shift based on the pilot signal included in the input signal or the estimated value of the optical frequency shift calculated using the characteristics of the modulation method of the input signal, and calculates the reception weight. Then, the digital signal is decoded. The decoding unit 203 outputs the decoded signal, and high-accuracy frequency shift information is necessary for decoding the signal. Therefore, the frequency shift information estimated here is fed back to the chromatic dispersion compensation unit 202. To do.

  Here, the chromatic dispersion compensation unit 202 compensates the chromatic dispersion amount using the transmission distance information of the optical signal and the information of the type of the optical fiber. For example, the received signal can be transformed into the frequency domain by Fourier transform, multiplied by the inverse characteristic of Equation (1), and inverse Fourier transformed, or the inverse characteristic of the property expressed by Equation (1) can be converted into the time domain. And a convolution operation can be performed on the received digital signal. At this time, the chromatic dispersion compensator 202 considers the estimated value of the optical frequency shift input from the decoder 203, and similarly to the methods (a) to (c) of the first embodiment, Dispersion compensation can be performed. In the present embodiment, it is also possible to perform rough estimation of the frequency shift by using the frequency estimation unit 102 in the previous stage of the chromatic dispersion compensation unit 202 as in the first embodiment.

  FIG. 8 is a flowchart illustrating an operation example of the receiving apparatus according to the present embodiment. When the optical signal is received by the receiving device, the analog / digital conversion unit 201 converts the optical signal into a digital signal based on the local light output from the local light generation unit 110 (step S21), and the chromatic dispersion compensation unit Input to 202. At this time, when the optical frequency shift is estimated by the frequency estimation unit 102 as in the first embodiment, the rough estimation result of the frequency shift is set as the initial value of the optical frequency shift (step S22). The chromatic dispersion compensation unit 202 performs chromatic dispersion compensation based on the input digital signal, information on optical frequency deviation, optical fiber type, and distance information, and outputs the digital signal compensated for chromatic dispersion to the decoding unit 203. (Step S23). The decoding unit 203 decodes the input digital signal, updates Δf based on the frequency shift information used at this time, and feeds back the updated Δf to the chromatic dispersion compensation unit 202 (step S24). The chromatic dispersion compensation unit 202 calculates chromatic dispersion compensation based on the fed back Δf (step S23).

In the present embodiment, it has been described that the initial value (Δf) of the frequency shift is calculated in step S22 by the frequency estimation unit 102 similar to that in the first embodiment. However, a predetermined numerical value (for example, Δf = 0) may be used as an initial value.
As described above, according to the present invention, even when there is a difference in the frequency of an optical signal between reception and transmission, the influence of chromatic dispersion in optical communication is removed by digital arithmetic processing, and waveform deterioration due to chromatic dispersion is suppressed. It becomes possible.

  The embodiment of the present invention has been described above. Each functional unit according to the present embodiment is realized by dedicated hardware (for example, wired logic). The functional unit is configured by a memory and a CPU (central processing unit), and a program for realizing the function of each unit is stored in the memory. The function may be realized by loading and executing. In addition, a program for realizing the function of the processing unit in the present invention is recorded on a computer-readable recording medium, and the program recorded on the recording medium is read into a computer system and executed to perform chromatic dispersion compensation. You may go. Here, the “computer system” includes an OS and hardware such as peripheral devices. The “computer-readable recording medium” refers to a storage device such as a flexible medium, a magneto-optical disk, a portable medium such as a ROM and a CD-ROM, and a hard disk incorporated in a computer system. Further, the “computer-readable recording medium” refers to a volatile memory (RAM) in a computer system that becomes a server or a client when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In addition, those holding programs for a certain period of time are also included.

  The program may be transmitted from a computer system storing the program in a storage device or the like to another computer system via a transmission medium or by a transmission wave in the transmission medium. Here, the “transmission medium” for transmitting the program refers to a medium having a function of transmitting information, such as a network (communication network) such as the Internet or a communication line (communication line) such as a telephone line. The program may be for realizing a part of the functions described above. Furthermore, what can implement | achieve the function mentioned above in combination with the program already recorded on the computer system, what is called a difference file (difference program) may be sufficient.

DESCRIPTION OF SYMBOLS 101 Analog / digital conversion part 102 Frequency estimation part 103 Chromatic dispersion compensation part 110 Local light generation part 201 Analog / digital conversion part 202 Chromatic dispersion compensation part 203 Decoding part 901 Analog / digital conversion part 902 Chromatic dispersion compensation part 910 Local light generation part

Claims (6)

A chromatic dispersion compensation method in optical communication,
An analog-digital conversion step for converting the received optical signal into a digital signal using local light,
A frequency estimation step of calculating an estimated value of an optical frequency shift between the optical signal and the local light ;
Compensating optical frequency shift of the digital signal based on the calculated estimated value, and chromatic dispersion compensation step for compensating chromatic dispersion of the digital signal;
Equipped with a,
In the chromatic dispersion compensation step,
The digital signal converted in the analog-digital conversion step is Fourier-transformed for each predetermined block, and the chromatic dispersion is compensated based on the inverse characteristic of the chromatic dispersion amount according to the propagation path characteristic of the optical signal. After performing the inverse Fourier transform, according to the estimated value of the optical frequency deviation, the optical frequency deviation of the digital signal is reduced by discarding the signals before and after the influence of inter-block interference among the signals included in the block. chromatic dispersion compensation method characterized by compensating. A chromatic dispersion compensation method in optical communication,
An analog-digital conversion step for converting the received optical signal into a digital signal using local light,
A frequency estimation step of calculating an estimated value of an optical frequency shift between the optical signal and the local light;
Compensating optical frequency shift of the digital signal based on the calculated estimated value, and chromatic dispersion compensation step for compensating chromatic dispersion of the digital signal;
With
In the chromatic dispersion compensation step,
The digital signal converted in the analog-digital conversion step is subjected to Fourier transform, and the chromatic dispersion amount is calculated based on the propagation path characteristic of the optical signal and the estimated value of the optical frequency shift, and the calculated chromatic dispersion amount wavelength dispersion compensating method characterized by compensating the optical frequency deviation of the digital signal by compensating for chromatic dispersion based on the inverse characteristic of. A chromatic dispersion compensation method in optical communication,
An analog-digital conversion step for converting the received optical signal into a digital signal using local light,
A frequency estimation step of calculating an estimated value of an optical frequency shift between the optical signal and the local light;
Compensating optical frequency shift of the digital signal based on the calculated estimated value, and chromatic dispersion compensation step for compensating chromatic dispersion of the digital signal;
A decoding step of decoding the digital signal compensated for chromatic dispersion in the chromatic dispersion compensation step, and feeding back an estimated value of the optical frequency shift used in decoding to the chromatic dispersion compensation step;
Wavelength dispersion compensation method, characterized in that it comprises a. A receiving device in optical communication,
An analog / digital conversion circuit that converts the received optical signal into a digital signal using local light; and
A frequency estimation circuit for calculating an estimated value of an optical frequency shift between the optical signal and the local light ;
A chromatic dispersion compensation circuit that compensates for optical frequency shift of the digital signal based on the calculated estimated value and compensates for chromatic dispersion of the digital signal;
Equipped with a,
The chromatic dispersion compensation circuit is:
The digital signal converted by the analog-digital conversion circuit is Fourier-transformed for each predetermined block, and the chromatic dispersion is compensated based on the inverse characteristic of the chromatic dispersion amount according to the characteristic of the propagation path of the optical signal and reversed. After performing Fourier transform, according to the estimated value of the optical frequency deviation, the optical frequency deviation of the digital signal is compensated by discarding the signals before and after the influence of inter-block interference among the signals included in the block. receiving device which is characterized in that. A receiving device in optical communication,
An analog / digital conversion circuit that converts the received optical signal into a digital signal using local light; and
A frequency estimation circuit for calculating an estimated value of an optical frequency shift between the optical signal and the local light;
A chromatic dispersion compensation circuit that compensates for optical frequency shift of the digital signal based on the calculated estimated value and compensates for chromatic dispersion of the digital signal;
With
The chromatic dispersion compensation circuit is:
The digital signal converted by the analog / digital conversion circuit is Fourier-transformed to calculate the chromatic dispersion amount based on the propagation path characteristic of the optical signal and the estimated value of the optical frequency shift, and the calculated chromatic dispersion amount receiving apparatus characterized by compensating the optical frequency deviation of the digital signal by compensating for chromatic dispersion based on the inverse characteristic. A receiving device in optical communication,
An analog / digital conversion circuit that converts the received optical signal into a digital signal using local light; and
A frequency estimation circuit for calculating an estimated value of an optical frequency shift between the optical signal and the local light;
A chromatic dispersion compensation circuit that compensates for optical frequency shift of the digital signal based on the calculated estimated value and compensates for chromatic dispersion of the digital signal;
A decoding circuit that decodes the digital signal compensated for chromatic dispersion by the chromatic dispersion compensation circuit, and that feeds back the estimated value of the optical frequency shift used for decoding to the chromatic dispersion compensation circuit ;
Receiving apparatus characterized by comprising a.

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