KR101688789B1 - Nonlinearity-tolerant OSNR estimation method and apparatus for coherent communication systems - Google Patents
Nonlinearity-tolerant OSNR estimation method and apparatus for coherent communication systems Download PDFInfo
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- H04B10/616—Details of the electronic signal processing in coherent optical receivers
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- H04B10/614—Coherent receivers comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers
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Abstract
An optical signal-to-noise ratio estimation method and apparatus for a nonlinear optical transmission system are disclosed. The most significant feature of the present invention is to compensate for the influence of optical fiber nonlinearity in estimating the optical signal-to-noise ratio of a received signal in a coherent-based long-distance optical transmission system operating in the presence of nonlinearity. According to the present invention, not only can the optical signal-to-noise ratio be accurately monitored even in the presence of optical fiber non-linearity, but also in order to estimate the degree of signal distortion due to optical fiber nonlinearity, the number of transmission distances or wavelength division multiplexed signals, , Optical power per channel, and optical signal-to-noise ratio, it is possible to easily apply the present invention to a modern optical transmission network in which the environment of a system is changed by frequent network reconfiguration without prior information on the system environment .
Description
The present invention relates to a method for estimating the optical signal-to-noise ratio of a polarization division multiplexed signal, and more particularly to a coherent-based long-haul optical transmission system in which optical fiber nonlinearity exists and corrects the influence of optical fiber non- And more particularly,
Polarization-division-multiplexed signal (PDM signal) and digital coherent reception technology have become key technologies for high-speed and long-distance optical transmission systems. It is very important to monitor optical signal-to-noise ratios to efficiently operate and manage long-haul optical transmission networks where these technologies are used. This is because the optical signal-to-noise ratio can be used not only for estimating the bit-error rate (BER) of an optical signal but also for optimizing a digital signal processing algorithm in a coherent receiver.
Recently, various optical signal-to-noise ratio estimation methods have been proposed for coherent-based optical communication systems [1-7]. In order to estimate the optical signal-to-noise ratio (SNR), this method uses an error vector magnitude or a statistical average value of the amplitude noise to estimate the change of the received signal due to ASE (amplified spontaneous emission) . However, since signal distortion due to optical fiber nonlinearity at the receiving end is not different from ASE noise, the optical signal-to-noise ratio is estimated to be lower than the actual value when this method is applied to a long-haul optical transmission system operating in a region having nonlinearity .
In order to solve this problem, a method of estimating the optical signal-to-noise ratio more accurately at the receiver has been reported by using the characteristic that the fiber nonlinearity generates the correlation between the amplitudes of adjacent symbols [4]. However, since the correction parameters for the optical fiber nonlinearity used in the optical signal-to-noise ratio estimation method vary depending on the environment of the system such as the transmission distance or the number of wavelength division multiplexed signals, it is difficult to be utilized in a modern optical transmission network, .
[Non-Patent Document 1] D. J. Ives, B. C. Thomsen, R. Maher, and S. J. Savory, "Estimating OSNR of equalized QPSK signals, Opt. Express, vol. 19, no. B661-B666, 2011.
[2] C. Zhu, AV Tran, S. Chen, LB Du, CC Do, T. Anderson, AJ Lowery, and E. Skafidas, "Statistical moments-based OSNR monitoring for coherent optical systems, "Opt. Express, vol. 20, no. 16, pp. 17711-17721, 2012.
(Non-Patent Document 3) [3] M. S. Faruk, Y. Mori, and K. Kikuchi, "In-band estimation of optical signal-to-noise ratio from digital coherent receivers," IEEE Photon. J., vol. 6, no. 1, p. 7800109, 2014.
(Non-Patent Document 4) [4] Z. Dong, A. P. T. Lau, and C. Lu, "OSNR monitoring for QPSK and 16-QAM systems in presence of fiber nonlinearities for digital coherent receivers, Opt. Express, vol. 20, no. 17, pp. 19520-19534, 2012.
[5] F. Pittal, F. N. Hauske, Y. Ye, N. G. Gonzalez, and I. T. Monroy, "Joint PDL and in-band OSNR monitoring supported by data-aided channel estimation," in Proc. of Optical Fiber Communication 2012, paper OW4G.
(Non-Patent Document 6) [6] C. Do, A. V. Tran, C. Zhu, D. Hewitt, and E. Skafidas, "Data-aided OSNR estimation for QPSK and 16-QAM coherent optical system," IEEE Photon. J., vol. 5, no. 5, p. 6601609, 2013.
[7] D. Zhao, L. Xi, X. Tang, W. Zhang, Y. Qiao and X. Zhang, "Periodic training sequence aided in-band OSNR monitoring in digital coherent receiver, IEEE Photon. J., vol. 6, no. 4, p. 7902009, 2014.
It is an object of the present invention to provide an optical signal-to-noise ratio estimation method and an optical signal-to-noise ratio estimation method of the present invention, in which optical signal-to- And a method for estimating the position of the target object with high accuracy.
Another object is to provide a method and apparatus for estimating the degree of signal distortion due to fiber nonlinearity.
The optical signal-to-noise ratio estimation apparatus for a nonlinear optical transmission system according to the present invention includes a coherent receiver for receiving a polarization division multiplexed optical signal by a coherent detection method, and a demultiplexer for polarization-demultiplexing an electrical output signal of the coherent receiver, And a digital signal processor for calculating a signal-to-noise-and-distortion ratio (SNDR) of power due to noise and nonlinearity.
In the optical signal-to-noise ratio estimation apparatus for a nonlinear optical transmission system according to the present invention, a digital signal processor includes a resampler for normalizing and resampling the electrical output signal, a chromatic dispersion compensator for compensating cumulative chromatic dispersion of the resampled signal, A demultiplexer for demultiplexing the polarization of the polarized light signal with chromatic dispersion compensation, and an SNDR estimator for estimating the ratio of the power of the signal to the distortion power due to noise and nonlinearity using the polarization demultiplexed signal .
In the optical signal-to-noise ratio estimation apparatus for a nonlinear optical transmission system according to the present invention, a digital signal processor is connected to the SNDR estimation unit to correct power of distortion caused by optical fiber nonlinearity in a ratio of power of distortion due to noise and non- And a correlation function calculator for calculating a correlation function between the adjacent symbols in order to calculate the correlation function.
The optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention includes the steps of: (a) receiving a polarization division multiplexed optical signal by a coherent reception method using a coherent reception unit; and (b) (SNDR) of the power of the signal and the power of distortion caused by noise and nonlinearity by performing polarization demultiplexing of the received electrical output signal.
As described above, the optical signal-to-noise ratio estimation apparatus and method for a nonlinear optical transmission system according to the present invention compensates for the influence of optical fiber nonlinearity in a coherent-based long-distance optical transmission system operating in a non- The accuracy of the signal-to-noise ratio estimation can be greatly improved.
In order to estimate the degree of signal distortion due to optical nonlinearity in the use of the optical signal-to-noise ratio estimation method and apparatus according to the present invention, the number of transmission distances or wavelength division multiplexed signals, section length, optical power per channel, Since the information about the signal-to-noise ratio is not needed, the optical signal-to-noise ratio can be monitored in a modern optical transmission network in which the environment of the system changes due to frequent network reconfiguration without information on the system environment.
Further, by using the optical signal-to-noise ratio estimation method and apparatus according to the present invention, the power of signal distortion due to optical fiber nonlinearity and the power due to ASE noise generated in the optical fiber amplifier can be independently estimated, There is an effect that information on the optimum optical power of the optical transmission network for providing the optical power can be provided.
1 is a system configuration diagram including an optical signal-to-noise ratio estimation apparatus for a nonlinear optical transmission system according to the present invention.
FIG. 2 is an overall flowchart of a method of estimating an optical signal-to-noise ratio for a nonlinear optical transmission system according to the present invention.
3 is a detailed flowchart of step S10 in the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
4 is a detailed flowchart of step S20 in the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
5 is a detailed flowchart of step S24 of the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
6 is a graph showing amplitude correlation functions measured after transmitting a 112-Gb / s PDM-QPSK signal through a single mode optical fiber at 1600 km.
FIG. 7 is a graph showing amplitude correlation functions measured after transmitting a 112-Gb / s PDM-QPSK signal through a single mode optical fiber at 1600 km while changing channel power.
8 is a graph showing the relationship between power and amplitude correlation function of signal distortion due to optical fiber nonlinearity measured using optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
9 is a graph showing an RMS error of an optical signal-to-noise ratio estimated using the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
10 is a graph showing the influence of polarization mode dispersion on the optical signal-to-noise ratio estimation performance in the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to the present invention.
Hereinafter, a method and apparatus for estimating optical signal-to-noise ratio for a nonlinear optical transmission system according to the present invention will be described in detail.
1 is a system configuration including an optical signal-to-noise ratio estimation apparatus for a nonlinear optical transmission system according to the present invention. The optical signal-to-noise ratio estimation apparatus includes a
The
The
The
The
The ratio of the power of the signal to the distortion power due to noise and nonlinearity is expressed by the following equation (1).
Where P ch denotes the channel power, P ASE and P NID denote the power of the ASE noise received by the coherent receiver and the power of the signal distortion due to fiber nonlinearity, respectively.
In the present invention, the ratio of the power of the signal to the distortion power due to the noise and nonlinearity is estimated based on statistical moments, and the distortion of the estimated signal due to power, noise, and nonlinearity Is expressed by the following equation (2). &Quot; (2) "
Where r is the amplitude of the polarization demultiplexed signal and E [·] is the statistical expectation.
In the present invention, the correlation function between neighboring symbols due to optical fiber nonlinearity is expressed by Equation (3).
Here, R intra and R inter mean an amplitude correlation function and are defined by Equation (4), and? Is a weighting factor and is preferably set to a value of 0.6 to 1. [
Here, x and y mean x polarization and y polarization, respectively, and Δr x (k) represents the amplitude noise of the kth x polarization symbol among the outputs of the polarization demultiplexing block and expressed by the following equation (5).
The correlation function (R eff ) between the adjacent symbols is used to correct the power of the signal distortion due to the optical fiber non-linearity in the SNDR.
The optical signal-to-noise ratio estimation method using the optical signal-to-noise ratio estimation apparatus according to the present invention will now be described.
FIG. 2 is a flowchart illustrating an overall optical signal to noise ratio estimation method according to the present invention. Referring to FIG. 2, the
3, the step S11 of detecting the electric field of the optical signal using the local oscillator laser and the
4, the step S21 of normalizing and resampling the electrical output signal using the
As shown in FIG. 5, the step S24 may include estimating a carrier phase of a polarization demultiplexed signal (S241) and calculating a size of an error vector in an output signal on which the carrier phase is estimated (S242 ).
After the step S24, the correlation
The details of the SNDR estimation of S24 and the calculation of the correlation function R eff between the adjacent symbols in step S25 are described in detail in the
FIG. 6 is a graph showing the relationship between the amplitude correlation functions R intra (m) and R inter (m) measured after transmitting the 112-Gb / s PDM-QPSK signal through a single mode optical fiber without optical dispersion compensation It can be seen that when there is optical fiber nonlinearity, a negative correlation occurs between adjacent symbols.
In the case of R intra (m), the correlation was greatest when m was 1, and the correlation was greatest when m was 0 in R inter (m). As shown in FIG. 2, since R intra (m) and R inter (m) converge to zero rapidly as m increases, R intra (1) and R inter (0) are used to estimate P NID .
FIG. 7 is a graph showing R intra (1) and R inter (0) measured after transmitting a 112-Gb / s PDM-QPSK signal through a single mode optical fiber 1600 km while changing channel power. From this result, it can be seen that R intra (1) is more sensitive to the change of channel power than R inter (0) in the case of a single channel system in which only intra-channel nonlinearity exists.
Between the channel non-linearity (inter-channel nonlinearity) is dominant in the 50 GHz channel spacing to take into account the system of 3 after the optical power of the central channel low enough set to -5 dBm in the channel system of the central channel intra R (1 ) And R inter (0) were measured. In this case, it can be seen that the inter R (0) is therefore more sensitive to changes in intra-channel power than R (1), inter R (0) is more effective to estimate the nonlinear Effects of between channels. Therefore, it is effective to define the parameter R eff for nonlinearity in the channel and the degree of distortion of the signal due to nonlinearity between
Simulation was performed to investigate the relationship between R eff and P NID . A 112-Gb / s PDM-QPSK signal with a channel spacing of 50 GHz was generated using an optical transmitter. The channel power was varied in the range of -4 to 4 dBm. The optical transmission network consists of a single-mode optical fiber with a section length of 80 or 100 km and without dispersion compensation. The loss and nonlinear parameters of the optical fiber were set to 0.2 dB / km, 17 ps / nm / km, and 1.3 W -1 km -1 , respectively. The optical loss of each section was compensated by using an optical fiber amplifier, and the optical signal to noise ratio was varied in the range of 10 to 24 dB by inserting an optical attenuator in front of the optical fiber amplifier. At the receiving end, the center channel is separated by using the wavelength division demultiplexer, and then the reception and the digital signal processing are performed using the coherent receiving unit. First, the detected signal is normalized and resampled, the chromatic dispersion is compensated using an FIR filter, and then polarization demultiplexing is performed using a CMA-based equalizer.
8 shows the relationship between power (P NID / P ch ) of signal distortion due to normalized non-linearity and R eff . In the embodiment of the present invention, the weighting factor? Of Equation (3) is set to 0.85. As shown in FIG. 8, the relationship between P NID / P ch and R eff is correctly expressed by one quadratic polynomial regardless of the transmission distance, the number of channels, the distance, and the channel power. Therefore, it can be seen that R eff can be used as an accurate measure to estimate P NID .
9 is a graph illustrating an RMS error of an optical signal-to-noise ratio estimated using the optical signal-to-noise ratio estimation method for a nonlinear optical transmission system according to an embodiment of the present invention. The RMS error of the estimated optical signal-to-noise ratio is shown in Figs. 9 (a), 9 (b), and 9 (c) according to the number of channels, transmission distance and section length. Here, the estimated optical signal-to-noise ratio is expressed by Equation (6).
Where B ref is the reference noise bandwidth, typically 0.1 nm, and R s is the symbol rate. Α 1 and α 2 are the first order and second order coefficients of P NID / P ch , respectively, approximated by a quadratic polynomial for R eff . In the embodiment of the present invention,? 1 and? 2 are set to -10.5 and 563, respectively.
Also, the results obtained by using a conventional statistical moment-based estimation method for comparison are shown together with a dotted line in FIG. Compared with the conventional statistical moment based estimation method, it can be seen that the estimation accuracy is much better when the optical signal to noise ratio estimation method for the nonlinear optical transmission system according to the present invention is used. For example, when the conventional method is used, the maximum value of the estimation error according to the number of channels, transmission distance, and section length is 4.7, 7.8, and 4.1 dB, respectively. dB.
In order to evaluate the effect of polarization mode dispersion (PMD) on the optical signal-to-noise ratio estimation performance for the nonlinear optical transmission system according to the present invention, the wavelength division multiplexed three-channel 112-Gb / s PDM- The average error of optical signal to noise ratio estimation was measured after transmitting 1600 km through optical fiber with differential group delay (DGD) value. At this time, the channel power was set to 2 dBm. The results are shown in Fig. As the DGD increases, the estimated optical signal-to-noise ratio value decreases gradually. This is because R inter (0) decreases when there is a polarization mode dispersion. Nevertheless, even when the DGD is as large as 20 ps, the optical signal-to-noise ratio estimation error remains within 1 dB.
As described above, when the optical signal to noise ratio estimation method for nonlinear optical transmission systems according to the present invention is applied, even when the optical signal is affected by the optical fiber nonlinearity, the transmission distance, the number of wavelength division multiplexed channels, The optical signal-to-noise ratio can be estimated with high accuracy without prior knowledge of the length.
Further, the effect of estimating the degree of signal distortion due to optical fiber nonlinearity can be obtained.
In applying the optical signal-to-noise ratio estimation method according to the present invention, the step of estimating the SNDR may include a step of performing a carrier phase estimation on the polarization demultiplexed signal and then calculating an error vector magnitude Can be substituted for the step of calculating In this case, the amplitude correlation function for estimating the power of signal distortion due to optical fiber nonlinearity can be replaced with a correlation function of amplitude noise between adjacent symbols of the signal to which the carrier phase estimation process is applied.
Further, in applying the optical signal-to-noise ratio estimation method according to the present invention, the modulation method of the optical signal is not limited to the PDM-QPSK signal, and those skilled in the art will appreciate that the optical signal- It can be applied to an optical signal using a polarization division multiplexed modulation scheme such as a PDM m-ary PSK or a PDM m-QAM by using the estimation method, so that detailed description will be omitted.
However, the technical idea of the present invention is not limited to the above embodiments, and various optical signal-to-noise ratio estimation methods and apparatuses can be implemented in a range that does not depart from the technical idea of the present invention.
1: optical transmitter 3: optical transmission network
5: wavelength division demultiplexer 10: coherent receiver
11: electric field detecting unit 12: signal converting unit
20: digital signal processor 21: resampling unit
22: chromatic dispersion compensation unit 23: demultiplexing unit
24: SNDR estimating unit 25: Correlation function calculating unit
Claims (13)
A coherent receiver for receiving the polarization division multiplexed optical signal by a coherent detection method and
A digital signal processor (digital) for demultiplexing the electrical output signal of the coherent receiver to estimate a signal-to-noise-and-distortion ratio (SNDR) signal processor,
The digital signal processor uses a correlation function R eff between neighboring symbols calculated by calculating a correlation function between neighboring symbols to correct the power of distortion caused by optical fiber nonlinearity in a ratio of power due to noise and nonlinearity, Wherein the correlation function (R eff ) between neighboring symbols is calculated by Equation (3) and Equation (4).
&Quot; (3) "
Here, R intra and R inter mean the amplitude correlation function, and α is a weighting coefficient and is set to a value of 0.6 to 1.
&Quot; (4) "
Where E [·] is a mean statistical expectation, and m is the relative symbol index, α is a weighting factor, x and y are the means of x-polarized and y-polarized light, respectively, △ r x (k) is the polarization demultiplexing block Represents the amplitude noise of the kth < th > x polarization symbol.
Wherein the coherent receiver comprises:
An electric field detector for detecting an electric field of the optical signal using a local oscillator laser;
And a signal converter for converting the detected analog electrical signal into a digital signal.
The digital signal processor includes:
A resampler for normalizing and resampling the electrical output signal;
A chromatic dispersion compensator for compensating for cumulative chromatic dispersion of the normalized and resampled signals;
A demultiplexer for demultiplexing the polarization of the polarized light signal with chromatic dispersion compensated;
And an SNDR estimator for estimating a ratio of the power of the signal to the distortion power due to noise and nonlinearity using the polarization demultiplexed signal.
The SNDR estimator
Wherein the SNDR is estimated using Equation (2) based on statistical moments of the polarization demultiplexed signal.
&Quot; (2) "
Where r is the amplitude of the polarization demultiplexed signal and E [·] is the statistical expectation.
Wherein the chromatic dispersion compensator comprises:
And estimating the optical signal-to-noise ratio using the FIR-finite impulse response filter.
(b) polarization demultiplexing the received electrical output signal using a digital signal processor to estimate the ratio of the power of the signal to the distortion power due to noise and non-linearity (SNDR); and
(C) calculating a correlation function (R eff ) between adjacent symbols to correct power of distortion due to fiber nonlinearity in a ratio of power of distortion due to noise and non-linearity using the digital signal processor , Wherein the correlation function (R eff ) between adjacent symbols is calculated by [Equation (3)] and [Equation (4)].
&Quot; (3) "
Here, R intra and R inter mean the amplitude correlation function, and α is a weighting coefficient and is set to a value of 0.6 to 1.
&Quot; (4) "
Where E [·] is a mean statistical expectation, and m is the relative symbol index, α is a weighting factor, x and y are the means of x-polarized and y-polarized light, respectively, △ r x (k) is the polarization demultiplexing block Represents the amplitude noise of the kth < th > x polarization symbol.
The step (a)
(a-1) detecting an electric field of the optical signal using a local oscillator laser, and
and (a-2) converting the detected analog electrical signal into a digital signal by using the signal converting unit.
The step (b)
(b-1) normalizing and resampling the electrical output signal using a resampler;
(b-2) compensating for cumulative chromatic dispersion of the normalized and resampled signals using the chromatic dispersion compensating unit;
(b-3) demultiplexing the polarization of the polarized light signal with the chromatic dispersion compensated using the demultiplexing unit; and
(b-4) estimating a ratio of the power of the signal to the distortion power due to noise and non-linearity using the polarization demultiplexed signal using the SNDR estimator.
The step (b-4)
(b-4-1) estimating the carrier phase of the polarization demultiplexed signal and
(b-4-2) calculating an error vector magnitude in an output signal on which the carrier phase is estimated.
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