Investigation of multipath interference impairments and a FEC-aided equalization solution in IM/DD PAM4 interconnects

This work investigates multipath interference (MPI) impairments and proposes a forward error correction (FEC) aided MPI equalization (FAME) solution for intensity-modulation direct-detection (IM/DD) PAM4 interconnects. We comprehensively analyze the impact of laser linewidth, signal-to-interference ratio (SIR), and reflection delay on MPI generation through both theoretical models and experiments. To mitigate these impairments, a single-iteration frame is integrated by a FEC decoder and a simplified error-magnitude-dependent (EMD) equalizer. The decoded bits from the FEC decoder serve as reference symbols for the convergence of the equalizer, and the equalized symbols are fed back into the FEC decoder, establishing a closed-loop iterative process. This iterative mechanism enhances reference symbol fidelity compared to classical decision-directed schemes, effectively mitigating error propagation. Experimental validation via 60 GBaud PAM4 transmission over 2 km of standard single-mode fiber demonstrates up to 2.2 dB SIR tolerance improvement at the 2.4 × 10^-4 FEC threshold.