Nonlinear Quantization for Power-Domain Non-orthogonal Multiple Access Passive Optical Network

A passive optical network (PON) system is required to accommodate multi-services represented by fifth/sixth (5G/6G) mobile communication systems. While maintaining low cost, to meet these requirements (which are high capacity, low latency, and high reliability), the PON system must accommodate a large number of optical network units (ONUs). The use of power-domain non-orthogonal multiple access (PD-NOMA) for PON has been studied to increase the number of accommodatable ONUs. However, with an increase in the ratio of transmitted power between multiplexed ONUs, the effect of quantization noise in the analog-to-digital converter (ADC) is more significant. This study proposes to apply an optimal quantizer based on the Lloyd-Max algorithm (which is a nonlinear quantization algorithm) to PD-NOMA-PON. In general, the Lloyd-Max algorithm requires continuous probability density function when setting the partitions. We adopt a continuous function estimated from the discrete probability distribution extracted by the low-resolution ADC using an expectation maximization algorithm. We conduct experiments to verify the feasibility of the proposed quantizer for the downlink and confirm the improvement of the optical signal-to-noise ratio penalty for all power ratios.