Modelling and Analysis of Power Imbalance Impact on Digital Subcarrier Multiplexed Coherent PON

Coherent transmission is strongly considered for very-high speed PON (the next PON generation as defined by ITU-T) to meet the ever-increasing bandwidth demands (at least 200 Gbps), benefiting from higher modulation formats and improved DSP-based receiver sensitivity. Digital subcarrier multiplexing (DSCM), as a variant of coherent PON, can further provide enhanced flexibility and lower latency, making it a promising solution for 5G/6G fronthaul applications. However, existing point-to-point DSCM systems must be adapted to PON-specific challenges, particularly the large power dynamic range inherent to point-to-multipoint shared infrastructure. In this paper, we investigate the impact of upstream subcarrier power imbalance (PI) in DSCM-PON systems considering two physical impairments: transmitter pulse-shaping filter and subcarrier frequency offset. We adapt and employ the frequency-resolved semi-analytical model to quantify the DSCM PON performance degradation due to PI and further validate it against time-domain numerical simulations. The proposed model shows high accuracy, with a discrepancy below 0.5 dB compared to numerical results. Our findings highlight that PI can become a critical limitation in DSCM-PON performance and should be carefully considered in system dimensioning and physical-layer design.