Dual-pilot phase recovery with pair-wise maximum-ratio combining for coherent PONs

Non-orthogonal multiple access (NOMA) combined with digital subcarrier multiplexing (DSCM) enables dense and flexible coherent passive optical networks (PONs). However, NOMA signal superposition biases pilot-aided carrier-phase recovery (CPR), while bandwidth-limited transceivers impose penalties on edge subcarriers. In this work, we propose a unified digital signal processing (DSP) framework that incorporates dual-pilot CPR with pair-wise maximum-ratio combining (MRC) across adjacent center-edge subcarrier pairs. By inserting pilots into both constituent signals of the superposed NOMA stream, the receiver builds a composite pilot reference that suppresses interference-induced phase bias. The resulting reliability estimate is then used to weight subcarrier branches in MRC, turning quality imbalance into diversity gain. In an experimental demonstration with an 80-Gbaud NOMA-DSCM coherent PON, the dual-pilot CPR achieves more robust phase recovery than conventional pilot-based and blind CPR baselines under NOMA-induced interference. With MRC enabled, the framework further delivers up to 2.29 dB SINR gain over benchmarks at low optical power, reducing average BER by 75.89% for ONU 1 and 30.33% for ONU 2.