On the Performance of NOMA Assisted Semi-Grant-Free Transmission in Satellite Systems

This paper investigates a semi-grant-free (SGF) access scheme for satellite communication systems, where one earth station (ES) and many mobile terminals (MTs) desire to access the satellite network simultaneously via uplink non-orthogonal multiple access (NOMA). We first propose two NOMA assisted SGF transmission schemes according to the availability of different channel state information (CSI). When perfect CSI is available, the satellite broadcasts an instantaneous access threshold to all MTs to enable dynamic power control (DPC), thus guaranteeing the quality of service of the ES. Otherwise, with the introduction of a statistical access threshold, a novel DPC with probability constraint is proposed to fulfill a satisfactory transmission. Next, we propose a new probability density function to characterize the shadowed-Rician distribution with estimation error, with which closed-form system throughput expressions under the proposed schemes are derived. Finally, computer simulations validate the effectiveness and superiority of our analysis and reveal the impact of CSI errors on the system performance.