Optimization of 50G-PON APD-based receivers

In this paper, we approach the optimization of IM-DD systems leveraging APD-based receivers and Feed-Forward Equalization (FFE) at the receiver side (RX). We review the role of APD-based receivers in recent evolutions of IEEE and ITU-T standards for high-speed Passive Optical Networks (PON) and in particular in 50G-PON as discussed in the recently approved Recommendation ITU-T G.9804.3. We analyze different optimization methods for PAM-2 (NRZ) and PAM-4 modulation formats, based on grid search methods and analytical closed-form solutions for decision thresholds and PAM-4 internal levels. We develop our study on a simulated optical IM-DD system, transmitting at a Baud Rate Rs=50 GBaud (i.e., 50 Gbps for NRZ, 100 Gbps for PAM-4). We assess power budget gain in terms of Optical Path Loss (OPL) with respect to nominal thresholds and levels, determining that optimizations are effective (i.e., approximately 1 dB OPL gain) when the system has moderate bandwidth limitations ($B_{3dB} \gt 0.5 \cdot R_{s}$). Our study can be of interest in the future for a better definition of the TDEC parameter for APD-based receivers.