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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 19, Iss. 26 — Dec. 12, 2011
  • pp: B728–B735
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Experimental demonstration of adaptive digital monitoring and compensation of chromatic dispersion for coherent DP-QPSK receiver

Robert Borkowski, Xu Zhang, Darko Zibar, Richard Younce, and Idelfonso Tafur Monroy  »View Author Affiliations


Optics Express, Vol. 19, Issue 26, pp. B728-B735 (2011)
http://dx.doi.org/10.1364/OE.19.00B728


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Abstract

We experimentally demonstrate a digital signal processing (DSP)-based optical performance monitoring (OPM) algorithm for in-service monitoring of chromatic dispersion (CD) in coherent transport networks. Dispersion accumulated in 40 Gbit/s QPSK signal after 80 km of fiber transmission is successfully monitored and automatically compensated without prior knowledge of fiber dispersion coefficient. Four different metrics for assessing CD mitigation are implemented and simultaneously verified proving to have high estimation accuracy. No observable penalty is measured when the monitoring module drives an adaptive digital CD equalizer.

© 2011 OSA

1. Introduction

In literature different approaches to CD monitoring directly from the received data (blind monitoring) have been presented, such as: parameter extraction from FIR filter coefficients [2

2. F. Hauske, J. Geyer, M. Kuschnerov, K. Piyawanno, T. Duthel, C. Fludger, D. van den Borne, E. Schmidt, B. Spinnler, H. de Waardt, and B. Lankl, “Optical performance monitoring from FIR filter coefficients in coherent receivers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OThW2.

], time- [3

3. M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive chromatic dispersion equalization for non-dispersion managed coherent systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OMT1.

] or frequency-domain [4

4. F. N. Hauske, C. Xie, Z. P. Zhang, C. Li, L. Li, and Q. Xiong, “Frequency domain chromatic dispersion estimation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JThA11.

] monitors placed before timing recovery stage and delay-tap sampling technique [5

5. D. Wang, C. Lu, A.P.T Lau, and S. He, “Adaptive chromatic dispersion compensation for coherent communication systems using delay-tap sampling technique,” IEEE Photon. Technol. Lett. 14, 1016–1018 (2011). [CrossRef]

].

On the other hand, as to the authors’ knowledge, no experimental trial of a monitoring module preceding timing recovery has been performed so far. In this paper we report on a successful experimental demonstration of CD estimation using the aforementioned method. We show that the receiver can adaptively adjust in order to mitigate signal degradation due to CD, even without prior knowledge of the fiber dispersion coefficient.

2. Blind chromatic dispersion monitoring for digital coherent receivers

We consider a polarization-division-multiplexing (PDM) quadrature phase-shift keying (QPSK) receiver consisting of the optical front-end and a DSP part, as shown in Fig. 1. The incoming optical signal is photodetected and sampled by analog-to-digital converters (ADCs) operating at twice the symbol rate. The CD monitor and equalizer block, which is of interest to this paper, is used for estimating and performing adaptive equalization of chromatic dispersion. Next, timing recovery takes place. A short, 7-tap finite impulse response (FIR) filter is then used for polarization demultiplexing and mitigation of residual impairments. Finally, carrier recovery is performed.

Fig. 1 Typical structure of a digital coherent receiver with CD monitoring and equalization block. The receiver in the figure monitors CD from time domain samples.

2.1. Generic scanning algorithm

The CD monitor and equalizer block in Fig. 1 uses a variable, transversal, frequency domain equalizer (FDE) for cancelation of inter-symbol interference (ISI) due to CD. The incoming signal p[k] representing received complex optical field is first divided into blocks of length N, indexed consecutively with k = 0...N – 1, and transformed to frequency domain. The resulting signal is then multiplied with the HCD digital filter
HCD=exp(jf2πλ2cCD),
(1)
where f is the clock frequency, λ the signal wavelength, c the speed of light and CD the value of CD. The rationale behind using an FDE is the number of required complex multiplications as compared to time domain approach [6

6. B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE J. Sel. Topics Quantum Electron. 5, 1180–1192 (2010). [CrossRef]

]. Next, the signal Q[k] obtained after multiplication is transformed back to time domain as q[k] and constitutes the output of the equalizer. Signals p and q (or Q, depending on particular method) are fed to a CD DSP OPM module which computes the metric J. Since the value of CD present in the channel is unknown, the transfer function HCD may not be computed. However, due to the fact that HCD has only one degree of freedom, adaptation can be performed by sweeping over a range of CD parameter, every time updating HCD, until an optimal operating point is found. An interest range of CD values shall be specified, which in general will be different for each optical network and may depend on the topology and traffic characteristic. The CD parameter can be initialized as to coincide with the most probable CD value of the received signal in order to increase the convergence speed. The space of CD parameter is then gradually searched with a given resolution and the metric J [CD] is computed for every value of CD under test.

The algorithm can be further extended to take average of the metric over multiple blocks or include multiple passes, each time narrowing the scanning range and increasing the scanning resolution. An example of a two-pass scan based on experimental data is shown in Fig. 2. Range between 0 ps/nm and 4000 ps/nm is swept with a step of 200 ps/nm (coarse scan). Once this initial estimate is obtained, a second pass with a resolution of only 20 ps/nm sets in and ranges 800 ps/nm centered around the previously found estimate (fine scan). This allows to increase the accuracy of the estimation. In theory, if no other impairment than CD is present in the signal, the maximum estimation error should be half of the scanning resolution. In practice, estimation error resulting from a single run is much greater because metrics are computed from blocks of finite length. Nonetheless, the accuracy of CD estimation at this stage is not strictly important because residual CD is compensated in the MIMO FIR filter that follows the CD equalizer in the receiver’s DSP chain (comp. Fig. 1).

Fig. 2 Comparison of all metrics after normalization to [0,1] range. Pav metric was subtracted from 1 for clearness of comparison. Bold dashed vertical line shows the actual value of CD present in the channel (1280 ps/nm).

2.2. Dispersion metrics

This section provides a brief overview of algorithms for metrics computation. Four different metrics were implemented and experimentally verified in a transmission experiment. Figure 2 shows a comparison of those metrics generated from an experimental transmission of 20 Gbaud QPSK signal in a channel with 1280 ps/nm CD.

2.2.1. CMA metric

The first evaluated metric is described in [3

3. M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive chromatic dispersion equalization for non-dispersion managed coherent systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OMT1.

, 7

7. M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, M.S. Alfiad, A. Napoli, and B. Lankl, “DSP for coherent single-carrier receivers,” J. Lightwave Technol. 27, 3614–3622 (2009). [CrossRef]

] and references provide its evaluation in computer simulations. The algorithm for this metric is based on a modified constant modulus algorithm (CMA) where a deviation from a constant power R2 is the error function (metric). Since the received signal is sampled at twice the symbol rate, another normalization constant R1 has to be used. Both R1 and R2 have to be constantly estimated from the power of odd and even samples of the received signal. The metric J is then computed
J[CD]=1Nk=0N1(||q[2k+1]|2R1|+||q[2k]|2R2|).
(2)
The required normalization constants R1 and R2 are determined for each block. First, the mean power of odd and even samples, 1 and 2, is calculated
q¯1=1Nk=0N1(q[2k+1])2q¯2=1Nk=0N1(q[2k])2.
(3)
Based on the ratio of 2 to 1, R1 and R2 normalization constants are determined as follows:
[R1R2]={[RaRc]ifq¯2q¯1>ξ[RbRb]ifξ1q¯2q¯1ξ[RcRa]ifq¯2q¯1<ξ1
(4)
with proposed empirically adjusted parameters ξ = 1.25, Ra = 0.6, Rb = 1.5 and Rc = 2 for the received complex signal power normalized to 1.

The curve of the metric presented in Fig. 2 was averaged over 8 different realizations, each having N = 256 samples.

2.2.2. Mean signal power

The second implemented metric is a simplified variant of the CMA-based metric. The mean power of samples at the input to the equalizer
P=1Nk=0N1|p[k]|2
(5)
is compared with the mean power of the signal after CD equalization and decimation to 1 sample/symbol stage. The post-decimation mean power, expressed in terms of signal q at the output of the CD equalizer is given by
P^=2Nk=0N21|q[2k]|2.
(6)
Next, J metric is found according to
J[CD]=|PP^|,
(7)
and the estimated value of CD parameter is indicated by the maximum of the metric.

In Fig. 2 the estimated CD value is found at a minimum as the metric was mirrored along the horizontal line at 0.5. Block size chosen for this metric was N = 2048.

2.2.3. Eigenvalue spread

An alternative metric, operating with time domain samples, relies on inspection of eigenvalue spread of the autocorrelation matrix. The concept, reviewed in [8

8. S. Haykin, Adaptive filter theory (Prentice-Hall, 2002).

], has not been used previously in relation to CD monitoring.

This metric uses samples from the CD equalizer after performing decimation to 1 sample/symbol.

An engineering rule for the autocorrelation matrix size producing good results was found to be
L=CDmaxCDmin75,
(11)
where in the numerator the maximum and minimum values of CD parameter (expressed in ps/nm) in the range of interest are used (units neglected). It should be noted that eigenvalues computation is an expensive task in terms of required processing power and, therefore, practical use of this metric might be limited.

In the curve shown in Fig. 2, block size was chosen to be N = 16384 and matrix size L = 53.

2.2.4. Frequency spectrum autocorrelation

The last metric, frequency spectrum autocorrelation, uses post-equalization samples before the inverse fast Fourier transform (IFFT) block. This method was studied in simulation in [4

4. F. N. Hauske, C. Xie, Z. P. Zhang, C. Li, L. Li, and Q. Xiong, “Frequency domain chromatic dispersion estimation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JThA11.

]. It uses signal Q, which is the frequency domain representation of the CD equalizer output after multiplication with the filtering function HCD as presented in Fig. 1. First, a discrete circular autocorrelation is computed
U[m]=1Nk=0N1csgn(m(Q[k]))Q*[k],
(12)
where 〇m (Q) is a circular shift operator that circularly shifts vector Q by m positions (mN) and csgn is a complex extension of the sign function sgn, defined as csgn(x) = sgn[ℜ (x)] + ιsgn[ℑ (x)], with ℜ and ℑ denoting, respectively, real and imaginary part of a complex number. It is not necessary for m to cover all possible shifts and thus m ranging from 0.7N2 up to 0.7N2 has been used.

The metric function J[CD] for a single CD value under test is then calculated as
J[CD]=m|U[m]|2,
(13)
where summation over m covers all applied circular shifts.

The metric curve shown in Fig. 2 was obtained after averaging 20 realizations, each calculated from a block Q of size N = 256 samples as to smoothen the obtained curve.

3. Experimental setup

Fig. 3 Experimental setup of the transmission system. CW: continuous wave laser, PRBS: pseudorandom binary sequence generator, PC: polarization controller, PBC: polarization beam combiner, EDFA: erbium-doped fiber amplifier, ASE loading: amplified spontaneous emission noise loading, VOA: variable optical attenuator.

4. Experimental results

Figure 4 shows bit error rate (BER) curves after demodulation of the traces using an equalizer driven by the monitoring module, where Preset is a reference line showing performance of the receiver when CD filter is manually set with an a priori known CD value of either 0 ps/nm (Fig. 4a) or 1280 ps/nm (Fig. 4b). Remaining lines show the performance of the receiver for different CD metrics as OSNR is varied for both transmission distances. It may be observed that regardless of the CD distortion present in the channel, lines depart only to a very small extent from the reference Preset line. This shows that both: each metric and the CD DSP monitor itself are reliable enough as not to introduce any penalty when compared to a CD filter with a fixed CD value. The FIR filter used for polarization demultiplexing is too short to compensate CD after 80 km of fiber transmission; effectively only residual CD is mitigated via the FIR filter, while bulk of the dispersion is removed by the FDE CD equalizer driven by the monitoring module. It is necessary to point out that this proof-of-concept works satisfactory with single polarization QPSK signal and it should be scalable to PDM-QPSK as CD affects both polarizations equally.

Fig. 4 BER vs. OSNR curves for each metric under investigation. Preset: reference line based on a priori known CD filter, CMA: constant modulus algorithm, Pav: mean signal power, Eig. spread: eigenvalue spread, Freq. AC: frequency spectrum autocorrelation.

5. Conclusions

We experimentally demonstrated the use of DSP OPM algorithms for CD compensation and estimation module preceding timing recovery stage. This allows for an autonomous operation of a digital coherent receiver in a dispersion non-compensated coherent transport network. To the best of our knowledge this is the first demonstration showing feasibility of the presented receiver arrangement and algorithms in experimental setting. We found out that the four different metrics for assessing dispersion mitigation provide reliable estimations of CD so as no penalty is observed when compared to a CD filter whose value was fixed prior to the transmission.

We thank Teraxion for providing PureSpectrum™-NLLs for this experiment. The research leading to these results is partially supported by the CHRON project (Cognitive Heterogeneous Reconfigurable Optical Network) with funding from the European Community’s Seventh Framework Programme [FP7/2007–2013] under grant agreement no. 258644.

References and links

1.

S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express 16, 804–817 (2008). [CrossRef] [PubMed]

2.

F. Hauske, J. Geyer, M. Kuschnerov, K. Piyawanno, T. Duthel, C. Fludger, D. van den Borne, E. Schmidt, B. Spinnler, H. de Waardt, and B. Lankl, “Optical performance monitoring from FIR filter coefficients in coherent receivers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OThW2.

3.

M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive chromatic dispersion equalization for non-dispersion managed coherent systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OMT1.

4.

F. N. Hauske, C. Xie, Z. P. Zhang, C. Li, L. Li, and Q. Xiong, “Frequency domain chromatic dispersion estimation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JThA11.

5.

D. Wang, C. Lu, A.P.T Lau, and S. He, “Adaptive chromatic dispersion compensation for coherent communication systems using delay-tap sampling technique,” IEEE Photon. Technol. Lett. 14, 1016–1018 (2011). [CrossRef]

6.

B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE J. Sel. Topics Quantum Electron. 5, 1180–1192 (2010). [CrossRef]

7.

M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, M.S. Alfiad, A. Napoli, and B. Lankl, “DSP for coherent single-carrier receivers,” J. Lightwave Technol. 27, 3614–3622 (2009). [CrossRef]

8.

S. Haykin, Adaptive filter theory (Prentice-Hall, 2002).

OCIS Codes
(060.1660) Fiber optics and optical communications : Coherent communications
(060.2330) Fiber optics and optical communications : Fiber optics communications

ToC Category:
Backbone and Core Networks

History
Original Manuscript: October 3, 2011
Revised Manuscript: November 25, 2011
Manuscript Accepted: November 26, 2011
Published: December 6, 2011

Virtual Issues
European Conference on Optical Communication 2011 (2011) Optics Express

Citation
Robert Borkowski, Xu Zhang, Darko Zibar, Richard Younce, and Idelfonso Tafur Monroy, "Experimental demonstration of adaptive digital monitoring and compensation of chromatic dispersion for coherent DP-QPSK receiver," Opt. Express 19, B728-B735 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-26-B728


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References

  1. S. J. Savory, “Digital filters for coherent optical receivers,” Opt. Express16, 804–817 (2008). [CrossRef] [PubMed]
  2. F. Hauske, J. Geyer, M. Kuschnerov, K. Piyawanno, T. Duthel, C. Fludger, D. van den Borne, E. Schmidt, B. Spinnler, H. de Waardt, and B. Lankl, “Optical performance monitoring from FIR filter coefficients in coherent receivers,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2008), paper OThW2.
  3. M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, A. Napoli, and B. Lankl, “Adaptive chromatic dispersion equalization for non-dispersion managed coherent systems,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2009), paper OMT1.
  4. F. N. Hauske, C. Xie, Z. P. Zhang, C. Li, L. Li, and Q. Xiong, “Frequency domain chromatic dispersion estimation,” in Optical Fiber Communication Conference, OSA Technical Digest (CD) (Optical Society of America, 2010), paper JThA11.
  5. D. Wang, C. Lu, A.P.T Lau, and S. He, “Adaptive chromatic dispersion compensation for coherent communication systems using delay-tap sampling technique,” IEEE Photon. Technol. Lett.14, 1016–1018 (2011). [CrossRef]
  6. B. Spinnler, “Equalizer design and complexity for digital coherent receivers,” IEEE J. Sel. Topics Quantum Electron.5, 1180–1192 (2010). [CrossRef]
  7. M. Kuschnerov, F. N. Hauske, K. Piyawanno, B. Spinnler, M.S. Alfiad, A. Napoli, and B. Lankl, “DSP for coherent single-carrier receivers,” J. Lightwave Technol.27, 3614–3622 (2009). [CrossRef]
  8. S. Haykin, Adaptive filter theory (Prentice-Hall, 2002).

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