Enhanced Signal Transmission in Integrated Satellite-Terrestrial Networks: A Double Reconfigurable Intelligent Surfaces Approach

This paper presents a comprehensive investigation of the use of double reconfigurable intelligent surfaces (RISs) in an integrated satellite-terrestrial network (ISTN), where the ground base station (GBS) and the satellite respectively serve the users within and outside the coverage of the GBS. However, nonnegligible co-channel interference can be induced, especially during the downlink transmission. To address the issue, two RISs are employed to assist the ISTN in enhancing the desired signal power and mitigating the interference, with one deployed on the satellite and the other near the GBS users. Specifically, our goal is to maximize the weighted sum rate of all users by jointly optimizing the active beamforming vectors of the satellite and GBS, and the passive reflection coefficient vectors of the two RISs. An alternating optimization algorithm based on fractional programming is then derived, which only requires the calculations of closed form or semi-closed form expressions in each iteration. Simulation results validate the performance advantage of the proposed double RIS-aided ISTN with the joint beamforming designs, which demonstrate the potential of integrating double RISs in ISTN to enhance the system performance.