JP6378649B2 - Chromatic dispersion estimation apparatus and optical transmission system - Google Patents

Description

  The present invention relates to a chromatic dispersion estimation device and an optical transmission system.

  In an optical fiber which is a propagation path for optical communication, a difference in propagation speed occurs in an optical signal propagating in the optical fiber due to the wavelength dependence of the refractive index of the optical fiber. For this reason, the optical signal propagated through the optical fiber and received by the receiving device includes waveform distortion. This waveform distortion is called chromatic dispersion. The chromatic dispersion has been compensated by using a dispersion compensating fiber having a characteristic opposite to that of an optical fiber used as a propagation path (for example, Non-Patent Document 1).

  However, when using a dispersion-compensating fiber, the characteristics of the optical fiber used in the propagation path are measured in advance, the characteristics of chromatic dispersion in the transmission path are specified, and the opposite characteristics to the specified characteristics are obtained. It is necessary to select a dispersion compensating fiber having the same. When there are a plurality of propagation paths, the compensation amount must be adjusted by selecting a compensation dispersion fiber for each transmission path, and the operability is impaired. Further, when switching the transmission line, it is necessary to specify the characteristics of chromatic dispersion again, or to exchange the dispersion compensating fiber, and maintainability is also impaired.

  Therefore, in recent years, a method for compensating for chromatic dispersion by digital signal processing has been proposed and put into practical use. In this method, the chromatic dispersion generated in the optical signal is estimated from the optical signal received by the receiver, and the waveform distortion is obtained by applying a characteristic opposite to the estimated chromatic dispersion characteristic to the received optical signal. Can be removed or reduced.

  In order to estimate the chromatic dispersion generated in the optical signal from the optical signal propagated through the optical fiber that is the propagation path, a method for adding a redundant signal to the optical signal on the transmission side and a redundant signal to the optical signal are provided. There is a way to not.

  The former method, for example, gives a known signal such as a training signal on the transmission side to the information sequence to be transmitted, and estimates the chromatic dispersion by extracting the known signal from the optical signal received on the reception side, To compensate. An example of such a method is described in Non-Patent Document 2, and according to the described method, a cumulative chromatic dispersion of 40,000 ps / nm of a single mode fiber of 2,000 km or more is estimated within ± 200 ps / nm. It is described that it can be compensated. However, in this method, in order to transmit data to be transmitted within a predetermined time, it is necessary to increase the bit rate, and there is a problem that the optical signal is deteriorated due to the influence of electricity and optical filtering. If the bit rate is not increased, there is a problem that the effective transmission rate decreases.

  In the latter method, a variance value is estimated from filter coefficients of an adaptive equalizer used in digital signal processing in optical communication. An example of such a method is described in Patent Document 1. In Patent Document 1, a signal processing system compensates for signal distortion that does not depend on polarization including chromatic dispersion in a signal, and a signal that depends on polarization with respect to the output of the first filter unit. A second filter unit that compensates for distortion using an adaptive equalization filter, and a transfer function corresponding to signal distortion independent of polarization is calculated based on tap coefficients used in the adaptive equalization filter of the second filter unit. And a feedback unit that updates the tap coefficient in the first filter unit based on the calculated transfer function. That is, the estimation of the dispersion value of chromatic dispersion is limited to the number of taps of the second filter unit, and there is a problem that the estimation range of the dispersion value is limited.

JP 2014-2333039 A

“Next-Generation Ultra-High-Speed Optical Communication Technology—Technical Issues and Overcoming Solutions for Optical Device Development”, 1st Edition, Technical Information Association, June 27, 2003, p. 112-118 Suzuki Ota, et al., “Research and development of digital coherent signal processing technology for increasing capacity of optical communication network”, IEICE Journal, IEICE, 2012, Vol. 95, no. 12, p. 1100-1116

  In view of the above circumstances, it is an object of the present invention to provide a chromatic dispersion estimation apparatus and an optical transmission system that can estimate chromatic dispersion without limiting the estimation range without using a training signal for estimating chromatic dispersion. .

  One embodiment of the present invention includes a Fourier transform unit that transforms a received optical signal into a frequency domain signal by Fourier transform on the electrical signal obtained by opto-electrical conversion, and a certain period of time for the frequency domain signal. An averaging unit that performs averaging, a spectrum inversion unit that inverts the frequency domain signal averaged by the averaging unit symmetrically with respect to a reference point, a signal obtained by the spectrum inversion unit, and the frequency A chromatic dispersion estimation device comprising: a chromatic dispersion estimation unit that estimates a frequency response of chromatic dispersion generated in the optical signal from a multiplication result with a signal in a region.

  According to another aspect of the present invention, the chromatic dispersion estimation apparatus includes a compensation unit that compensates the chromatic dispersion of the electrical signal based on a frequency response of the chromatic dispersion estimated by the chromatic dispersion estimation unit.

  Further, according to one aspect of the present invention, in the chromatic dispersion estimation apparatus, the chromatic dispersion estimation unit generates a compensation signal based on the estimated frequency response of the chromatic dispersion, and the compensation unit is a signal in the frequency domain. Is multiplied by the compensation signal to compensate for chromatic dispersion.

  According to another aspect of the present invention, in the above chromatic dispersion estimation apparatus, the electrical signal in which chromatic dispersion is compensated by performing inverse Fourier transform on a multiplication result of the signal in the frequency domain and the compensation signal. The inverse Fourier transform part which produces | generates is provided.

  According to another aspect of the present invention, an optical transmission system includes a transmission device that outputs an optical signal corresponding to data to be transmitted to an optical fiber transmission line, and a reception device that receives the optical signal propagated through the optical fiber transmission line. The transmission device includes a one-dimensional light modulation unit that performs one-dimensional modulation on the unmodulated light based on the data and generates the optical signal, and the reception device receives the received light An opto-electric conversion unit that converts a signal into an electrical signal, a Fourier transform unit that converts the signal into a frequency domain signal by Fourier transform on the electrical signal, and an average that averages the frequency domain signal over a certain period A spectrum inversion unit that inverts the frequency domain signal averaged by the averaging unit and the averaging unit symmetrically with respect to a reference point, the signal obtained by the spectrum inversion unit, and the frequency domain A wavelength dispersion estimation unit for estimating a frequency response of chromatic dispersion generated in the optical signal from a multiplication result of the signal of the signal, and a wavelength of the electrical signal based on the frequency response of chromatic dispersion estimated by the chromatic dispersion estimation unit An optical transmission system comprising: a compensation unit that compensates for dispersion.

  According to the present invention, it is possible to estimate chromatic dispersion without limiting the estimation range without using a training signal for estimating chromatic dispersion.

It is a block diagram which shows the structural example of the optical transmission system in this embodiment. It is a figure which shows an example of one-dimensional modulation. It is a block diagram which shows the structural example of the frequency domain processing circuit in this embodiment, and a chromatic dispersion estimation / compensation circuit. It is a figure which shows the characteristic of the optical transmission system in this embodiment.

  Hereinafter, a chromatic dispersion estimation compensation device and an optical transmission system according to embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration example of an optical transmission system according to the present embodiment. The optical transmission system includes a transmission device 1, an optical fiber transmission line 2, and a reception device 3. The transmission device 1 generates a transmission optical signal based on the input transmission data, and outputs the generated transmission optical signal to the optical fiber transmission line 2. The transmission optical signal is propagated through the optical fiber transmission line 2 and reaches the reception device 3. The receiving device 3 receives a transmission optical signal propagated through the optical fiber transmission line 2 as a reception optical signal. The receiving device 3 acquires received data by converting the received optical signal received into an electrical signal and performing signal processing on the electrical signal. The received data matches the transmitted data when the transmission quality in the optical transmission system is a certain level or higher.

  The transmission device 1 includes a one-dimensional light modulation unit 11. The one-dimensional light modulator 11 receives transmission data and applies one-dimensional modulation based on the transmission data to unmodulated light having a certain wavelength. The one-dimensional light modulator 11 outputs a transmission optical signal obtained by the one-dimensional modulation to the optical fiber transmission line 2. Here, one-dimensional modulation will be described. FIG. 2 is a diagram illustrating an example of one-dimensional modulation. In FIG. 2, the horizontal axis represents the in-phase component of the optical signal, and the vertical axis represents the quadrature component of the optical signal. One-dimensional modulation is, for example, binary phase shift keying (BPSK) shown in FIG. 2 (a), 4-level amplitude shift keying (4ASK) shown in FIG. 2 (b), There is a 4-level pulse amplitude modulation (4PAM) shown in FIG. As shown in FIG. 2, the one-dimensional modulation is a method of performing modulation such that signal points are arranged on a straight line in complex signal display. Further, as the one-dimensional modulation, other modulation such as intensity modulation (On-Off Keying: OOK) that is not shown in FIG. 2 may be used.

  Returning to FIG. 1, the description of the optical transmission system will be continued. The receiving device 3 includes an optical / electrical conversion circuit 31 and a digital signal processing unit 32. The optical / electrical conversion circuit 31 converts the received optical signal received via the optical fiber transmission path 2 into an electrical signal, and acquires a signal having an in-phase component and a quadrature component from the electrical signal. The optical / electrical conversion circuit 31 outputs signals of the in-phase component and the quadrature component to the digital signal processing unit 32. When the receiving device 3 is a coherent receiver, the optical / electrical conversion circuit 31 includes, for example, a local light source, a polarization beam splitter, an optical 90-degree hybrid, a photodetector, a transimpedance amplifier, an analog-digital converter, and the like. .

  The digital signal processing unit 32 includes a frequency domain processing circuit 33, a chromatic dispersion estimation / compensation circuit 34, a time domain processing circuit 35, and a symbol determination circuit 36. The frequency domain processing circuit 33 inputs in-phase component and quadrature component signals from the optical / electrical conversion circuit 31 and performs processing in the frequency domain on the input signals. The frequency domain processing circuit 33 outputs a frequency domain signal obtained from the in-phase component signal and the quadrature component signal to the chromatic dispersion estimation / compensation circuit 34. Further, the frequency domain processing circuit 33 outputs a signal subjected to processing in the frequency domain to the time domain processing circuit 35. The chromatic dispersion estimation / compensation circuit 34 estimates the chromatic dispersion in the received optical signal based on the signal input from the frequency domain processing circuit 33. The chromatic dispersion estimation / compensation circuit 34 generates a compensation signal based on the estimated chromatic dispersion and outputs it to the frequency domain processing circuit 33. The time domain processing circuit 35 performs signal processing in the time domain such as adaptive equalization and timing recovery on the signal input from the frequency domain processing circuit 33 and outputs the signal to the symbol determination circuit 36. The symbol determination circuit 36 generates reception data based on a signal input from the time domain processing circuit 35, and outputs the generated reception data to an external device or a higher-level device.

  FIG. 3 is a block diagram showing a configuration example of the frequency domain processing circuit 33 and the chromatic dispersion estimation / compensation circuit 34 in the present embodiment. In the following, symbols (^: hat, ￣: overline) described above variables in FIG. 3 and mathematical expressions are described before the variables. For example, it is written as ^ A (ω). The frequency domain processing circuit 33 includes an FFT processing unit 331, a multiplication unit 332, and an IFFT processing unit 333. The FFT processing unit 331 performs FFT on the signal r (n) input to the frequency domain processing circuit 33 to convert it to a frequency domain signal R (ω). The FFT processing unit 331 outputs the signal R (ω) to the multiplication unit 332 and the chromatic dispersion estimation / compensation circuit 34. The multiplier 332 multiplies the signal R (ω) input from the FFT processor 331 by the compensation signal ^ D (ω) input from the chromatic dispersion estimation / compensation circuit 34, and outputs the multiplication result to the IFFT processor 333. To do. The IFFT processing unit 333 performs IFFT on the multiplication result and converts it to a time-domain signal ^ a (n). The IFFT processing unit 333 outputs the signal ^ a (ω) to the time domain processing circuit 35.

  The chromatic dispersion estimation / compensation circuit 34 includes an averaging unit 341, a spectrum inversion unit 342, a multiplication unit 343, and a chromatic dispersion estimation unit 344. The averaging unit 341 averages the signal R (ω) input from the FFT processing unit 331 over a certain period, and calculates a signal ￣R (ω) obtained by averaging the signal R (ω). Averager 341 outputs signal ￣R (ω) to spectrum inverter 342 and multiplier 343. The spectrum inversion unit 342 generates a signal ￣R (−ω) obtained by inverting the signal ￣R (ω) input from the averaging unit 341 symmetrically with respect to the reference point. The spectrum inversion unit 342 outputs the generated signal ￣R (−ω) to the multiplication unit 343. The reference point (frequency) used in the spectrum inversion unit 342 is determined in advance based on, for example, the frequency difference between the transmission device 1 and the reception device 3.

  The multiplication unit 343 multiplies ￣R (ω) input from the averaging unit 341 and ￣R (−ω) input from the spectrum inversion unit 342, and the multiplication result ￣R (ω) · ￣R ( -Ω) is output to the chromatic dispersion estimation unit 344. The chromatic dispersion estimation unit 344 estimates the chromatic dispersion in the optical fiber transmission line 2 from the input multiplication result ￣R (ω) · ￣R (−ω). The chromatic dispersion estimation unit 344 outputs the compensation signal ^ D (ω) generated based on the chromatic dispersion estimation result to the frequency domain processing circuit 33.

Hereinafter, signal processing in the frequency domain processing circuit 33 and the chromatic dispersion estimation / compensation circuit 34 will be described. The electric signal (received signal) r (n) obtained by the optical / electrical conversion circuit 31 can be expressed by the equation (1).

Here, a (n) is transmission data and is a real signal. d (n) is a time response of chromatic dispersion generated in the optical fiber transmission line 2. w (n) is additive noise generated by amplification or the like in the transmission device 1 or the reception device 3. A symbol with a cross (x) inside a circle (◯) is a convolution operator. The received signal r (n) is converted into a signal R (ω) by the FFT processing unit 331, and Equation (2) is obtained.

Here, D (ω) can be written by equation (3). Here, δ represents the total amount of chromatic dispersion generated in the optical fiber transmission line 2.

By averaging by the averaging unit 341, W (ω) corresponding to the additive noise w (n) in Equation (2) cancels each other and can be ignored, and the average ￣R (ω) in Equation (2) is 4).

Since the transmission data a (n) is a real signal, Expression (5) is established. A ^* represents the complex conjugate of A.

Moreover, the output of the spectrum inversion part 342 becomes Formula (6).

As described above, the multiplication result obtained by the multiplication unit 343 is expressed by Expression (7).

Since the phase portion in Equation (7) includes only the frequency response due to chromatic dispersion, the frequency response of chromatic dispersion can be estimated by specifying the phase in Equation (7). When ^ δ (ω) in equation (8) is an estimated phase, the received signal R (ω) multiplied by the compensation signal ^ D (ω) represented by equation (9) is expressed by equation (10). Thus, A (ω) of the discrete Fourier transform result of the transmission data a (n) can be obtained.

  As described above, the chromatic dispersion estimation unit 344 calculates the estimated phase ^ δ (ω) from the multiplication result ￣R (ω) · ￣R (−ω) output from the multiplication unit 343, and the equation (9). Is output to the frequency domain processing circuit 33. In the frequency domain processing circuit 33, the multiplying unit 332 multiplies the discrete Fourier transform result R (ω) of the received signal r (n) by the compensation signal ^ D (ω), whereby the transmission data a (n) is discrete. A (ω) of the Fourier transform result can be obtained. That is, the multiplication unit 332 operates as a compensation unit that compensates for chromatic dispersion in the signal r (n). The IFFT processing unit 333 obtains an estimated value a (n) of the transmission data a (n) by performing inverse Fourier transform on A (ω) of the discrete Fourier transform result and transforming it into a time domain signal. be able to. In FIG. 3, the processing for one signal r (n) is described. However, the processing for each of the signal of the in-phase component and the signal of the quadrature component is performed by the frequency domain processing circuit 33 and the wavelength dispersion. This is performed in the estimation / compensation circuit 34.

  According to the chromatic dispersion estimation / compensation circuit 34 of the present embodiment, by estimating the frequency response of the chromatic dispersion using the symmetry of the signal spectrum, the estimation of the chromatic dispersion and the estimation result without using the training signal. A compensation signal can be generated based on By multiplying the received signal by this compensation signal, chromatic dispersion can be compensated. Further, the estimation of the chromatic dispersion by the chromatic dispersion estimation / compensation circuit 34 uses the symmetry of the signal spectrum of the one-dimensional modulation, so that the chromatic dispersion can be estimated without limiting the estimation range. it can.

  In addition, the optical transmission system of the present embodiment includes the frequency domain processing circuit 33 and the chromatic dispersion estimation / compensation circuit 34, so that chromatic dispersion can be estimated and compensated without using a training signal. That is, according to the optical transmission system of this embodiment, it is possible to estimate and compensate for a wide range of chromatic dispersion without reducing the effective transmission rate. In addition, it is not necessary to determine the coefficient in the filter as compared with the estimation and compensation of chromatic dispersion using a filter or the like. Therefore, even when the optical fiber transmission line 2 is changed, the chromatic dispersion is estimated and compensated immediately. It is possible to perform processing with high responsiveness.

  FIG. 4 is a diagram illustrating characteristics of the optical transmission system according to the present embodiment. Here, the case where 4ASK which can take the value of (-3, -1, +1, +3) is shown as one-dimensional modulation. In FIG. 4, the horizontal axis indicates the optical signal-to-noise power ratio (OSNR), and the vertical axis indicates the error rate (BER). The solid line graph in the figure shows the theoretical value of 4ASK. The error rate in the optical transmission system when the total amount of chromatic dispersion generated in the optical fiber transmission line is 150 ns / nm is indicated by diamonds (♦). As can be seen from the figure, the error rate and the theoretical value in the optical transmission system of this embodiment are almost the same, and estimation of a wide range of chromatic dispersion is possible without using a training signal for estimating chromatic dispersion. It can be seen that and compensation can be realized.

  You may make it implement | achieve all or one part of the digital signal processing part 32 in the receiver of embodiment mentioned above with a computer. For example, it is realized by recording a program for realizing each component included in the digital signal processing unit 32 on a computer-readable recording medium, causing the computer system to read and execute the program recorded on the recording medium. May be. 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” is a program that dynamically holds a program for a short time, like a communication line when a program is transmitted via a network such as the Internet or a communication line such as a telephone line. In this case, a volatile memory inside a computer system serving as a server or a client in that case may be included and a program held for a certain period of time. Further, the program may be for realizing a part of the above-described constituent elements, and may be realized by combining the above-described constituent elements with a program already recorded in a computer system. It may be realized by using hardware such as PLD (Programmable Logic Device) or FPGA (Field Programmable Gate Array).

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and includes designs and the like that do not depart from the gist of the present invention.

  For example, the FFT processing unit 331, the averaging unit 341, the spectrum inversion unit 342, the multiplication unit 343, and the chromatic dispersion estimation unit 344 as the Fourier transform unit are the frequency domain processing circuit 33 and the chromatic dispersion estimation / compensation circuit. However, the configuration may be configured as a chromatic dispersion estimation apparatus including the FFT processing unit 331, the averaging unit 341, the spectrum inversion unit 342, the multiplication unit 343, and the chromatic dispersion estimation unit 344.

  The present invention can also be applied to applications in which it is indispensable to perform estimation and compensation of chromatic dispersion whose estimation range is not limited without using a training signal for estimating chromatic dispersion.

DESCRIPTION OF SYMBOLS 1 ... Transmission apparatus 2 ... Optical fiber transmission line 3 ... Reception apparatus 11 ... One-dimensional light modulation part 31 ... Optical / electrical conversion circuit 32 ... Digital signal processing part 33 ... Frequency domain processing circuit 34 ... Chromatic dispersion estimation / compensation circuit 35 ... Time domain processing circuit 36 ... Symbol determination circuit 331 ... FFT processing unit 332 ... Multiplication unit (compensation unit)
333 ... IFFT processing unit 341 ... averaging unit 342 ... spectrum inversion unit 343 ... multiplication unit 344 ... wavelength dispersion estimation unit

Claims (5)

A Fourier transform unit for transforming the received optical signal into a frequency domain signal by Fourier transform on the electrical signal obtained by photoelectric conversion;
An averaging unit that performs averaging over a certain period with respect to the signal in the frequency domain;
A spectrum inversion unit for inverting the frequency domain signal averaged by the averaging unit symmetrically with respect to a reference point;
A chromatic dispersion estimating unit that estimates a frequency response of chromatic dispersion generated in the optical signal from a multiplication result of the signal obtained by the spectrum inverting unit and the signal in the frequency domain;
A chromatic dispersion estimation apparatus comprising: Based on the frequency response of the chromatic dispersion estimated by the chromatic dispersion estimation unit, the compensation unit for compensating the chromatic dispersion of the electrical signal,
The chromatic dispersion estimation apparatus according to claim 1. The chromatic dispersion estimation unit generates a compensation signal based on the estimated frequency response of chromatic dispersion,
The compensation unit compensates chromatic dispersion by multiplying the frequency domain signal and the compensation signal.
The chromatic dispersion estimation apparatus according to claim 2. An inverse Fourier transform unit that generates the electrical signal in which chromatic dispersion is compensated by performing an inverse Fourier transform on a multiplication result of the signal in the frequency domain and the compensation signal;
The chromatic dispersion estimation apparatus according to claim 3. An optical transmission system comprising: a transmission device that outputs an optical signal corresponding to data to be transmitted to an optical fiber transmission line; and a reception device that receives the optical signal propagated through the optical fiber transmission line,
The transmitter is
A one-dimensional light modulator that performs one-dimensional modulation on the unmodulated light based on the data and generates the optical signal;
With
The receiving device is:
A photoelectric conversion unit that converts the received optical signal into an electrical signal;
A Fourier transform unit for transforming the electrical signal into a frequency domain signal by Fourier transform;
An averaging unit that performs averaging over a certain period with respect to the signal in the frequency domain;
A spectrum inversion unit for inverting the frequency domain signal averaged by the averaging unit symmetrically with respect to a reference point;
A chromatic dispersion estimating unit that estimates a frequency response of chromatic dispersion generated in the optical signal from a multiplication result of the signal obtained by the spectrum inverting unit and the signal in the frequency domain;
Based on the frequency response of the chromatic dispersion estimated by the chromatic dispersion estimation unit, a compensation unit that compensates for the chromatic dispersion of the electrical signal;
Comprising
Optical transmission system.

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