Nonuniform Error Distributions Induced by Multipath Interference in Optical IM/DD PAM4 Systems: Implications for FEC Performance

This paper presents a comprehensive analysis of multipath interference (MPI) in intensity modulation direct detection (IM/DD) 4-level pulse amplitude modulation (PAM4) systems, focusing on its impact on post-FEC performance. We show that MPI induces nonuniform error distributions and therefore critically undermines forward error correction (FEC) performance, due to the fluctuation characteristics of MPI noise and the interactions among multiple reflection paths. Simulation results reveal that MPI can cause unacceptable post-FEC performance even when pre-FEC performance appears satisfactory. Specifically, while pre-FEC metrics indicate an MPI penalty of merely 0.5dB, cases involving multiple reflection paths show a penalty exceeding 1.5dB in terms of post-FEC performance. Additionally, we analyze the effects of chirp on MPI impairment in electro-absorption modulated laser (EML) - and directly modulated laser (DML) -based systems. Moreover, we investigate the effects of power distribution among reflection paths on MPI severity, showing that emulating MPI effects with a single interfering path significantly underestimates MPI penalty in the perspective of post-FEC performance. Furthermore, the effects of laser linewidth on MPI severity are studied. We show broader laser linewidths inherently tend to enhance post-FEC performance, and discuss the impact of the adopted signal processing schemes. The effects of interleaving on MPI-induced nonuniform error distribution are also discussed. Finally, as an example of a system-level holistic MPI impact assessment, we analyze the joint effects of high-pass filtering (HPF) and interleaving. Our findings underscore that to ensure reliable IM/DD PAM4 implementation, the threat of MPI should be evaluated and managed carefully from the perspective of post-FEC performance.