A Study on Joint Maximum-Likelihood Detection for Downlink LEO Satellite Communications with Different 8QAM Constellations

In order to improve the spectral efficiency during the wireless communications, a power-domain non-orthogonal multiple access (NOMA) technique has attracted great attention in recent. In this paper, we consider a joint maximum-likelihood (ML) detector to investigate the feasibility of power-domain NOMA in downlink satellite networks. When either satellite adopts constellation with a modulation order of 8, the constellation structure highly affects the decoding performance of both streams at the same time. In order to compare the decoding performance, we perform extensive MATLAB simulations and measure the decoding performance from the viewpoint of bit error rate (BER) in the scenario where two satellites transmit their independent streams to the ground base station (GBS) at the same time. From the results, we verify that the constellation structure affects the decoding performance of joint ML detector, which implies that the careful design of constellation is required when higher order modulation scheme is applied with the joint ML detector.