50G PON FEC Evaluation using Markov Models to Generate Correlated Errors due to Equalization

Channel measurements for a 50G passive optical network (PON) show a significant amount of intersymbol interference (ISI) due to chromatic dispersion (CD) and the use of low-cost, bandwidth-limited unit[25]{Gbaud}-class data center components. The channel characteristics at the input of the forward error-correction (FEC) decoder are measured for several receiver equalization schemes, and the error correlation at the output of each equalizer is characterized using a sub-class of a Fritchman's Markov model. This model is used to generate error sequences that match the observed correlation statistics, and these sequences are then used to evaluate the performance of a candidate low-density parity-check (LDPC) code with lifting factor 256. It is shown that for a bandwidth-limited 50G PON system with unit[83]{ps/nm} dispersion, the optical power penalty (OPP) due to a reduction in error-correction performance caused by correlated errors is unit[0.3--0.6]{dB}. Precoding, i.e., differential encoding of the input data and differential decoding, helps to reduce the OPP for some equalizers but increases the penalty for other equalizers. The application of a 256-bit segment interleaver is seen to provide a negligible improvement.