Mitigated multipath interference in PAM4 IM/DD optical links using instantaneous mean intensity addressing

Abstract

Multipath interference (MPI) poses a significant challenge for 4-level pulse amplitude modulation (PAM4) signals in short-reach intensity modulation and direct detection (IM/DD) interconnect systems. MPI, arising from signal reflections due to the multiple fiber connectors, introduces irregular intensity fluctuations across PAM4 levels, severely degrading the system performance. This work proposes an MPI noise mitigation scheme, instantaneous mean intensity addressing (IMIA), which suppresses the MPI noise by utilizing an instantaneous mean intensity table and an MPI noise table for real-time noise extraction and removal. Simulation results in a 50-Gbps PAM4 transmission system under single and multiple forward-traveling reflected light (FTRL) scenarios demonstrate the effectiveness of the proposed IMIA scheme across varying MPI conditions. In both scenarios, the IMIA scheme achieves over 6 dB and over 4 dB MPI tolerance for the 7% hard-decision forward error correction (HD-FEC) and KP4-FEC criteria, respectively. Moreover, the integration of the IMIA scheme with a noise whitening filter (NWF) and maximum likelihood sequence estimation (MLSE) provides an additional 2 dB of MPI tolerance improvement.