Low-complexity mitigation of multipath interference in PAM-4 IM/DD systems via recursive filtering

Multipath interference (MPI) severely degrades the performance of high-speed multi-level intensity-modulation/direct-detection (IM/DD) systems by inducing amplitude fluctuations in the received signal, and many digital signal processing (DSP)-based mitigation techniques with high computational complexity have been reported. In this paper, we propose a low-complexity MPI mitigation method based on first-order recursive filtering. The method consists of a coarse mitigation scheme to suppress slowly varying amplitude fluctuations dominated by carrier-carrier beating, and a fine mitigation scheme to both suppress these fluctuations and further reduce residual distortion arising from broadband crosstalk and imperfect coarse-stage suppression. Simulation and experimental results for 32 GBaud PAM-4 systems demonstrate a substantial reduction in bit-error rate (BER) over a wide range of signal-to-interference ratios (SIRs) and laser linewidths. BERs below the KP4-FEC threshold at an SIR of 22 dB are achieved, and the SIR tolerance is improved by over 8 dB using the proposed MPI mitigation method for both broad- and narrow-linewidth laser sources. The proposed scheme provides superior performance compared to previously reported digital filtering methods, while requiring only four multiplications, two additions, and two registers per symbol, thereby yielding ultra-low computational complexity for MPI mitigation.