Performance Investigation of Pilot-Aided Log-Likelihood Ratios for LDPC Coded CO-OFDM

Pilot-subcarrier and pilot-tone aided log-likelihood ratios (PA-LLR, PT-LLR), proposed for LDPC coded coherent optical OFDM (CO-OFDM) system, is reviewed in this paper. The knowledge of common phase error based on pilot-subcarriers or pilot-tone is incorporated into the new PA-LLR or PT-LLR metric, which eliminates the need for prior common phase error estimation and compensation. The formulation of both metrics is presented in a unified way. The performance of both metrics, in their approximate versions, is compared against the approximate conventional LLR (AC-LLR) for different modulation formats using different LDPC codes in a back-to-back case. APA-LLR or APT-LLR outperforms AC-LLR for higher-order QAM, with smaller number of pilot-subcarriers (PSCs) or at smaller pilot-tone-to-signal power ratios (PSRs). A time-domain blind intercarrier interference (BL-ICI) mitigation algorithm is employed to improve the performance, which eliminates the error floor at large laser linewidth. Furthermore, we examine the tolerance of different LLR metrics to linear fiber impairment (chromatic dispersion) or nonlinear phase noise (self-phase modulation), in which case, our metrics still outperform the conventional one. Iterative demodulation using new tentative-decision-based phasor offers almost 1-dB OSNR improvement for smaller number of PSCs. Finally, we also analytically prove that APA-LLR or APT-LLR converges to AC-LLR as the number of PSCs or PSR value increases. The optimal OSNR for calculating noise power is found to be around 10 dB for actual OSNR values beyond 10 dB in either the back-to-back case or after 13 600-ps/nm/km dispersion with channel compensation. APA-LLR and APT-LLR offer better performance than AC-LLR without any increase in complexity.