Least-squares-assisted BCJR algorithm for severe ISI mitigation in bandwidth-limited IM/DD optical interconnection

To effectively mitigate the impairments of severe inter-symbol interference (ISI), a least-square-assisted Bahl, Cocke, Jelinek, and Raviv (LS-BCJR) detection algorithm is proposed and experimentally demonstrated in a bandwidth-limited intensity-modulation direct-detection (IM/DD) pulse amplitude modulation (PAM4) optical interconnection system. A training-based short-tap LS equalizer is utilized after the 1 + D post filter to dynamically reconstruct an effective channel response, thus assisting the BCJR estimator to optimize the transition metric calculation and obtain more accurate probabilistic information from the Markov for soft-symbol decision. The experimental results reveal the enhanced system performance with the 160 Gb/s PAM4 signals transmitted within a 19-GHz system bandwidth. The proposed LS-BCJR scheme improves the received optical power sensitivity by 1.2 and 0.9 dB under the validity comparison of the maximum likelihood sequence estimation (MLSE) and conventional BCJR schemes at the BER threshold of 2.4e-2. Furthermore, the LS-BCJR achieves the optimal receiver sensitivity improvement by 3 dB at the BER threshold of 3.8e-3, demonstrating a notable improvement over the conventional BCJR scheme.