Beyond 100G optical channel noise modeling for optimized soft ...

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Beyond 100G Optical Channel Noise Modeling for Optimized Soft-Decision FEC Performance Yu Zhao1, Juan Qi1, Fabian N. Hauske1, Changsong Xie1, Doris Pflueger2, Gerhard Bauch2 1) Huawei Technologies Duesseldorf GmbH, European Research Center, Riesstrasse 25, D-80992 Munich 2) Institute of Information Processing, UniBw Munich, Werner-Heisenberg-Weg 39, 85577 Neubiberg E-mail: [email protected]

Abstract: Deviations away from ideal AWGN statistics degrading FEC performance are demonstrated caused by the nonlinear optical channel and by receiver-side DSP. An according noise model is demonstrated for FEC test and optimization. OCIS codes: (060.0060) Fiber optics and optical communications; (060.1660) Coherent communications

1. Introduction Advanced forward error correction (FEC) codes are desirable for increasing the transmission distance of coherent optical transmission systems [1-2]. Besides enabling enhanced digital signal processing (DSP) algorithms, highspeed analogue-to-digital converters (ADCs) also facilitate the use of soft-decision FEC (SD-FEC) in digital coherent receivers. Paid by larger complexity, SD-FEC codes result in higher net coding gain (NCG) compared to hard decision FEC (HD-FEC) codes. Thanks to fast chip technology development, the implementation of complex SD-FEC codes for 100 Gb/s optical transmission and beyond becomes reality [3]. Based on investigations in the past decade, quasi-cyclic low-density parity-check (QC-LDPC) codes seem to be most promising candidates for SD-FEC codes in high-speed optical systems. Most FEC algorithm investigations and performance evaluations are based on the additive white Gaussian noise (AWGN) channel model, assuming ideal channel conditions and ideal pre-processing prior to the decoder. In [4], 100G real-time experiments proved performance degradation for HD-FEC. Recently, different kinds of DSP algorithms were investigated for equalization of linear and nonlinear channel distortions such as chromatic dispersion (CD) compensation (CDC), 2x2 multiple-input multiple-output (MIMO) polarization-mode dispersion (PMD) filters and fiber nonlinearity mitigation. Furthermore, digital synchronization for timing recovery (TR) and carrier phase estimation (CPE) can be applied [5]. No full compensation of all deterministic channel distortions within a reasonable implementation complexity is known leaving a non-negligible fraction of inter-symbol interference after DSP. Furthermore, averaging is applied in synchronization algorithms adding correlation and causing deformation of the noise distribution. In particular, differential decoding resulting from the phase ambiguity within the carrier recovery stage is known to affect the decoder performance [2]. Clearly all deviations away from the ideal AWGN channel model cause a degradation of the SD-FEC performance. Therefore, an optical channel noise (OCN) model including realistic residual signal distortions after DSP is strongly demanded for test and optimization of SD-FEC codes. In this paper, we investigate the noise statistics for 128 Gb/s polarization-division multiplexed (PDM) quaternary phase-shift keying (QPSK) transmission prior to SD-FEC based on offline data. The noise correlation and distribution is evaluated for linear and nonlinear channel conditions combined with various configurations of the carrier-phase recovery and extracted into an enhanced noise model for test and optimization of SD-FEC performance. A 0.3 dB OSNR penalty is observed for a 20% QC-LDPC code in typical wavelength-division multiplexing (WDM) channel conditions degrading the large NCG of the SD-FEC. 2. 100G LDPC Offline Experiments and Analysis

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We tested the QC-LDPC code of [1] in 128Gbps PDM-QPSK modulated offline experiments including typical DSP for synchronization and equalization (Fig.1). Four lines of PRBS data sequences (XI/XQ/YI/YQ) were LDPC encoded, QPSK modulated and transmitted in 100G/10G hybrid channel. The BER was calculated prior and after decoding leading to BERin and BERout respectively. Fig. 2 compares the experimental result to the widely used AWGN scenario, demonstrating the obvious degradation of the LDPC performance due to channel influence and DSP. The observed NCG penalty was about 0.6~0.7 dB. Facing the severe performance degradation under real optical channel conditions and DSP, a comprehensive analysis of the input data to the SD-FEC with respect to distribution, auto-correlation and power spectral density (PSD) has been performed. After removing the modulated data, the remaining noise process has been evaluated for the center channel of 100G WDM transmission (9 channels) and 100G/10G hybrid transmission (1x100G surrounded by 8x10G OOK channels), as shown in Fig. 3. Clearly a larger averaging window inside the CPE induces a stronger correlation, which refers to a smaller bandwidth in the PSD. For 100G WDM transmission, the noise prior to the CPE can be assumed rather white with a uniform noise density. The CPE alters the parts of the PSD according to the bandwidth of the carrier recovery, which refers to 0.7 GHz, 1.4 GHz and 3.5 GHz for averaging over 40, 20 and 8 symbols respectively. For 100G/10G hybrid transmission, nonlinear channel interference adds colored nonlinear phase noise with a bandwidth around 2 GHz such that only the short averaging window of 8 symbols effectively mitigates the colored phase noise. For long averaging windows, strong colored phase noise remains, which severely affects the subsequent FEC [6]. Furthermore, implementation constraints like processing latency and parallelized processing have been identified to alter the noise statistics (not in this paper). Apart from the correlation, also the distribution of the noise is affected, where short averaging leads to more “egg-shape” distributions while long averaging closely approximates Gaussian noise, as shown in Fig. 3. In contrast to the correlation properties, the distributions are less affected by the channel condition. However, based on these selected examples it can be clearly observed that both, channel and DSP conditions, affect the noise statistics.

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Fig. 4. Temporary phase-slip evolution for 100G WDM (left top) and for 100G/10G hybrid (left bottom) transmission. Block diagram of combined channel and DSP noise model (right).

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Finally, the phase ambiguity in the CPE results in bursts with whole-number multiples of /2. Figure 4, left, shows a low phase-slip probability with long bursts and high phase-slip probability with short bursts for 100G WDM and 100G/10G hybrid transmission respectively. This undermines the detrimental influence of colored, nonlinear phase noise from 10G neighbors [6]. The phase-slip probability is a measure if differential decoding is required to overcome frequent phase-slips or if pilot sequences or framing information is sufficient for phase correction to avoid large error bursts. In order to predict the expected SD-FEC performance, an according noise model is derived from the evaluated offline data, as discussed in the next section 3. Channel Noise Modeling and Results From the experimental phase noise analysis, we derive that components of AWGN n(t) (0 mean, σn deviation) and nonlinear phase distortion φp(t) affect a modulated symbol s(t), which yields to the input signal of the FEC, j ( t ) y(t )  s(t )  e p  n(t ) , as shown in Fig. 4, right. For φp(t), we first generate white, Gaussian noise (0 mean, σp deviation), which is passed through a phase noise filter stage composed by a low-pass filter (provides colored phase noise for scenarios with residual channel impairments) and a CPE filter (subtracts spectral components according to CPE implementation) to emulate the correlation properties. Adding phase-slips modeled by a Markov process we finally obtain φp(t), which is combined with the colored amplitude noise n(t). In Fig. 5, the modeled noise statistics (top two) for an exemplarily linear 100G WDM (CPE average 20 symbols) scenario prove a good first-order matching with according offline results (bottom two). The QC-LDPC code of [1] has been tested based on this noise model predicting a performance degradation about 0.3dB (Fig. 6). Similar investigations for hybrid transmission result in even larger penalties. 4. Conclusion QC-LDPC performance degradation due to channel impairments and DSP configuration is demonstrated for 128Gb/s PDM-QPSK transmission. Based on experimental evaluation of the noise statistics, a model taking into account signal correlation and distribution has been derived, which allows to predict the expected OSNR penalty reducing the given NCG of SD-FEC. The use of whitening filters and LDPC LLR initialization optimization are potential candidates to mitigate this penalty. Alternatively, DSP algorithms and configuration could be optimized. 5. References [1] D. Chang, et al., “FPGA Verification of a Single QC-LDPC Code for 100 Gb/s Optical Systems without Error Floor down to BER of 10 -15”, in OFC 2011, paper OTuN2. [2] F. Yu, et al., “Soft-Decision LDPC Turbo Decoding for DQPSK Modulation in Coherent Optical Receivers,” in ECOC 2011, paper We.10.P1.70. [3] S. S. Iyer and E. J. Nowak, “45nm SOI and beyond – getting to a general purpose technology,” in IEEE Internat. SOI Conf., pp.1-4 (2007). [4] D. Ogasahara, et al., “Real-Time Evaluation of Optical Nonlinear Effects on 112Gbps PM-QPSK Signal in Dispersion Managed Links,” in OFC 2011, paper OMR3. [5] C. R. S. Fludger, et al., “Coherent Equalization and POLMUX-RZ-DQPSK for Robust 100-GE Transmission,” J. of Lightwave Technol., 26, 64-72 (2008). [6] D. van den Borne, et al, “Carrier phase estimation for coherent equalization of 43-Gb/s POLMUX-NRZ-DQPSK transmission with 10.7 Gb/s NRZ neighbors”, in ECOC 2007, paper We 7.2.3.

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