A 10.7-Gb/s DPSK receiver with 4000-ps/nm dispersion
tolerance using a shortened MZDI and 4-state MLSE
Citation for published version (APA):
Al Fiad, M. S. A. S., Borne, van den, D., Napoli, A., Hauske, F. N., Koonen, A. M. J., & Waardt, de, H. (2008). A 10.7-Gb/s DPSK receiver with 4000-ps/nm dispersion tolerance using a shortened MZDI and 4-state MLSE. In 2008 Conference on Optical Fiber Communication/National Fiber Optic Engineers Conference : [2008
OFC/NFOEC] ; San Diego, CA, 24 - 28 February 2008 (pp. owt3-1/3). Institute of Electrical and Electronics Engineers. https://doi.org/10.1109/OFC.2008.4528775
DOI:
10.1109/OFC.2008.4528775
Document status and date: Published: 01/01/2008 Document Version:
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A 10.7-Gb/s DPSK Receiver with 4000-ps/nm Dispersion
Tolerance using a Shortened MZDI and 4-state MLSE
M.S. Alfiad (1), D. van den Borne (1), A. Napoli (2), F. N. Hauske (3), A.M.J. Koonen (1), H. de Waardt (1) 1: COBRA institute, Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands (m.s.alfiad@tue.nl)
2: Nokia Siemens Networks, Hofmannstr. 51, D-81379 Munich, Germany (antonio.napoli@nsn.com) 3: Federal Armed Forces University Munich, EIT-3, D-85577 Neubiberg Germany (fabian.hauske@unibw.de) Abstract: We experimentally demonstrate a 10.7-Gb/s NRZ-DPSK receiver with 4000-ps/nm
chromatic dispersion tolerance by using phase demodulation with a half-bit delay in combination with a simple 4-state maximum likelihood sequence estimation (MLSE) equalizer.
2008 Optical Society of America
OCIS codes: (060.4510) Optical Communications; (060.5060) Phase modulation;
1. Introduction
Maximum likelihood sequence estimation (MLSE) is a powerful technology to enhance the robustness of optical receivers against chromatic dispersion (CD) [1], polarization mode dispersion [2, 3], and narrowband optical filtering [4]. Most notably, the combination of non-return-to-zero (NRZ) modulation with a 4-state MLSE receiver approximately doubles the CD tolerance [1].
To further increase the CD tolerance, the number of states in the MLSE receiver has to be increased. This allows the compensation of nearly arbitrary amounts of CD, but the receiver complexity grows exponentially, making this approach less practical [5]. Duobinary modulation together with a MLSE receiver is another attractive solution, as it results in a CD tolerance of approximately 4000 ps/nm [6]. However, Duobinary modulation has the disadvantage that it requires a 2-4 dB higher optical signal-to-noise ratio (OSNR), which limits its applications for long-haul transmission systems.
In order to realize both a high CD tolerance and a low ONSR requirement, the combination of differential phase shift keying (DPSK) and a MLSE receiver would be ideal. DPSK requires 3 dB less ONSR than non-return-to-zero (NRZ) on-off-keying (OOK) and >5 dB less ONSR than Duobinary modulation. However, prior investigations have shown that the combination of DPSK and MLSE has a negligible improvement in terms of CD tolerance [2, 3]. In [7] it was shown that this can be partially alleviated by a MLSE receiver employing joint-symbol estimation on the constructive and destructive ports of a DPSK receiver. In this paper we present a simpler approach that results in a larger CD tolerance. We show that the use of a Mach-Zehnder delay interferometer (MZDI) with a <1-bit delay [8-10], together with MLSE, results in a ~4000 ps/nm CD tolerance for 10.7-Gb/s NRZ-DPSK modulation.
2. Experimental setup
Fig. 1 depicts the DPSK transmitter and receiver setup. DPSK is generated by modulating the output of a distributed feed-back (DFB) laser with a zero-biased Mach-Zehnder modulator (MZM) driven at a data rate of 10.7 Gb/s (PRBS 215-1). Afterwards, standard single mode fibers (SSMF) with gradually increased lengths are used to vary the accumulated CD between 0 ps/nm and 4500 ps/nm. The launch power into the SSMF is set to 0 dBm, in order to minimize the impact of fiber nonlinearity.
At the receiver, a variable optical attenuator (VOA) along with an erbium doped fiber amplifier (EDFA) is used to set the OSNR to the desired value (measured within 0.1 nm resolution bandwidth). Afterwards the signal passes through an optical band pass filter (OBPF) with a 3-dB bandwidth of 50 GHz. A second EDFA and OBPF ensure a constant power into the receiver. Then, the signal is input to a MZDI which realizes the phase-to-amplitude conversion. Two different MZDIs are employed; one with a 1-bit delay, the other with a 0.5-bit delay (S-MZDI), followed by balanced photodiode. Subsequently, the output of the balanced photodiode is used as an input signal for (I) a commercial MLSE receiver [1], (II) a digital storage oscilloscope (DSO) for off-line processing or (III) a hard decision receiver (HDR). The commercial MLSE-receiver samples at 2 sample/bit, with a 3-bit vertical resolution and a 4-state Viterbi decoder. For the off-line processing a TDS 6804B DSO with a sampling rate of 20 Gsample/s is used. To obtain exactly 2 sample/bit, the signal is re-sampled to 21.4 Gsample/s using a software for clock recovery based on the digital filter and square timing recovery algorithm [11]. The electrical bandwidth of the DSO is 8 GHz. Afterwards, MLSE equalization with 3-bit vertical resolution, and 4-state Viterbi decoder is applied to the data by off-line processing. Sequences of 106 bits are used for the off-line processing to achieve an accuracy of 99.9% for a
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BER of 10-4 [12]. A bit error rate tester (BERT) is used for HDR and real-time MLSE measurements. Finally, as a reference for the CD tolerance that can be obtained for binary modulation formats we use 10.7-Gb/s Duobinary modulation, generated by a commercial Duobinary transponder with pre-coder [6]. At the receiver side a single-ended photodiode is used in the case of Duobinary and the CD tolerance is measured in combination with both the HDR and the real-time MLSE.
Fig. 1 Experimental setup; OSA: optical spectrum analyzer Fig. 2 Off-line and real-time MLSE sampling phase, eye diagrams after balanced detection.
3. Off-line Experimental results
Fig. 3 depicts the measured CD tolerance for both the HDR and off-line MLSE equalizer, when using either the S-MZDI or 1-bit delay MZDI. Comparing the 1-bit delay MZDI with and without MLSE equalization confirms the inefficiency of MLSE for DPSK modulation, which has been previously reported using both experiments [2] and simulations [3]. This has been attributed in [7] to the difference in performance between the constructive and destructive outputs of the MZDI. The use of the S-MZDI, on the other hand, considerably improves the CD tolerance. This can be explained by considering the difference between the constructive and destructive output of the MZDI, which are Duobinary and alternating mark inversion (AMI) modulated, respectively. For a <1-bit differential delay the Duobinary coded constructive output of the MZDI contains a larger fraction of the signal, whereas the destructive output port contains a smaller fraction. As the contribution of the destructive output limits the MLSE performance in the CD limited regime [7], using a <1 bit delay improves the CD tolerance. Combining the S-MZDI with the MLSE receiver further extends the CD tolerance and allows a nearly constant OSNR requirement for a CD up to 3500 ps/nm, with a 2-dB OSNR penalty at 4000 ps/nm.
HDR, S-MZDI MLSE, S-MZDI HDR, standard MZDI MLSE, standard MZDI
Duobinary, HDR
DPSK, short (0.5-bit) MZDI + MLSE Duobinary, MLSE DPSK, short (0.65-bit) MZDI + MLSE (sim)
Fig. 3 Required OSNR for different receiver types Fig. 4 Comparison between DPSK and Duobinary
To further point out the efficiency of the proposes scheme in the CD limited regime, Fig. 4 compares the CD tolerance of the S-MZDI/MLSE combination (square) with Duobinary modulation (diamond). This shows a clear OSNR improvement (~2.5 dB) and a better CD tolerance when using DPSK modulation together with the S-MZDI and MLSE. We point out that the OSNR requirement of S-MZDI/MLSE for low CD can be further improved by optimizing the bit-delay of the S-MZDI. A trade-off between CD tolerance and back-to-back OSNR sensitivity can be obtained by varying the differential delay of the S-MZDI. Simulations show that the optimum value is ~0.65 bit-delay, which is reported in Fig.4 (circle). With a ~0.65 bit-bit-delay, the S-MZDI requires at back-to-back only 1 dB more OSNR than the conventional MZDI, while maintaining nearly the same CD tolerance.
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4. Real-time experimental results
To show the feasibility of the MLSE and S-MZDI combination we compare in Fig. 5 the performance of the off-line processed MLSE with a commercial, real-time MLSE [1]. The real-time MLSE shows a slight OSNR penalty for low CD, but the same OSNR requirement for higher CD. This can be attributed to the difference in the sampling phase chosen in the two methods. In [10] it has been shown that due to the deterministic interference between consecutive symbols in the S-MZDI, the output is close to an inverted return-to-zero (RZ) signal. Because the real-time MLSE is optimized for NRZ-OOK modulation, it samples the signal in a different part of the symbol period resulting in our setup in a non optimal back-to-back performance, as explained in Fig. 2a. On the other hand Fig. 2b shows that in the CD limited regime, the eye diagram losses the RZ shape, which makes the choice in sampling phase less critical. This explains the convergence of the two curves for a CD in excess of 2500 ps/nm.
In order to show the sensitivity with respect to the choice of the sampling phase, we simulated a DPSK system with 0.5 bit delay S-MZDI, and used two different sampling phases for its balanced output. Sampling phases were either suboptimal (25% and 75% of the symbol period) or optimal (0% and 50% of the symbol period) as depicted in Fig. 6. The simulations point out that sampling at 0% and 50% results in the optimal OSNR sensitivity. Moreover, we observe that our simulations with suboptimal sampling phase (triangle) are nearly identical to the measured CD tolerance with the real-time MLSE (square) for both low and higher CD. Note that the real-time MLSE receiver could be easily modified to sample at the optimal sampling point. We would also expect a further improvement of the back-to-back sensitivity for a MLSE receiver that would sample at 25%, 50% and 75%, but this requires a more complex MLSE receiver.
Off-line MLSE Real-time MLSE
Real-time MLSE
Simulation, suboptimal sampling
Offline processing, optimal sampling Simulation, optimal sampling
Fig. 5 Off-line and real-time MLSE equalization Fig. 6 Comparison between experimental and simulation results for different sampling instants
5. Conclusion
We showed that a shortened MZDI (S-MZDI) together with a MLSE receiver enables a 4000-ps/nm dispersion tolerance for 10.7-Gb/s NRZ-DPSK modulation. This combines a low OSNR requirement and high CD tolerance into a single solution. We further compared the performance of 0.5 bit-delay S-MZDI to Duobinary, and showed that it has ~2.5 dB better OSNR sensitivity while being more tolerant to CD.
6. References
[1] A. Faebert et al., “Performance of a 10.7 Gb/s receiver with digital equalizer using MLSE”, ECOC’04, paper Th4.1.5. [2] I. Lobato et al., “Comparison of MLSE equalizer performance with OOK and DPSK at 10.7 Gb/s”, ECOC’06, paper We2.5.3. [3] C. Xia et al., “Electrical dispersion compensation for different modulation formats with optical filtering”, OFC’06, paper OWR2.
[4] J. Dowine et al., “Experimental measurements of the effectiveness of MLSE against narrowband optical filtering distortion”, OFC’07, paper OMG4.
[5] P. Poggiolini et al., “1,040 km Uncompensated IMDD Transmission over G.652 Fiber at 10 Gbit/s using a Reduced-State SQRT-Metric
MLSE Receiver”,ECOC’06, paper Th4.4.6.
[6] J. P. Elbers et al., “Measurement of the dispersion tolerance of optical duobinary with an MLSE receiver at 10.7Gbit/s”,OFC’05, paper OTHJ4.
[7] M. S. Alfiad et al., “Robust detection of 10.7Gbit/s DPSK using joint decision MLSE”,ECOC’07, paper Th9.1.2. [8] B. Mikkelsen et al., “Partial DPSK with Excellent Filter and OSNR Sensitivity”, Elec. Lett., pp.1363-1364, 2006.
[9] N. Yoshikane et al., “Benifit of Half-Bit Delay Demodulation for Severely Bandlimited RZ-DPSK Signal”, ECOC’03, paper We3.5.2. [10] F. Lize et al., “Free spectral range optimization of Return-to-Zero differential phase shift keyed demodulation in the Presence of Chromatic
Dispersion”, Opt. Expr, pp.6817-6822, 2007.
[11] M. Oerder, “Digital Filter and Square Timing Recovery,” IEEE transactions on communications, Vol.36, No.5, May 1988, pp. 605–612. [12] M. Jeruchim, “Techniques for estimating the bit error rate in the simulation of digital communication systems”, IEEE JSAC, pp.153-170,
1984.
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