Multi-RIS-Aided Satellite-Terrestrial Downlink RSMA Communication for User Fairness Exploiting Statistical CSI

Abstract

As the role of satellites in sixth generation mobile communications system becomes increasingly well defined, satellite-terrestrial communications face an urgent need to improve spectral efficiency while ensuring user fairness. In this paper, we propose a multiple transmissive reconfigurable intelligent surfaces (RISs)-aided satellite-terrestrial downlink transmission scheme with rate-splitting multiple access. Based on statistical channel state information, we formulate a max-min user ergodic rate problem by jointly optimizing the satellite precoding, the phase shifts of multiple RISs, and the common-rate allocation. To tackle the resulting non-convex problem, we derive approximate closed-form expressions of multi-user ergodic rates and equivalently reformulate the problem via fractional programming. We then design a block coordinate descent-based algorithm to solve the transformed problem. Simulation results verify that the designed transmission scheme achieves significant performance gains in improving minimum user ergodic rate.