Covert Communication for UAV-Assisted Satellite-Terrestrial Systems With STAR-RIS and RSMA

In this paper, we investigate an unmanned aerial vehicle (UAV)-assisted satellite-terrestrial system with simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) and rate splitting multiple access (RSMA). More specially, the satellite employs RSMA to send messages over a STAR-RIS-assisted UAV to a pair of legitimate users located on the earth in order to enable better use of spectrum resources. Serving as a mobile aerial base station, the UAV transmits information to the users through a fixed-gain amplify-and-forward (AF) or decode-and-forward (DF) relay. Also, we use the STAR-RIS to enhance the link from satellite to terrestrial. In order to evaluate the covert performance of the considered system, we derive closed-form expressions of covert performance metrics including the outage probability (OP), the covert transmission rate, and the detection error probability (DEP). Then, we analyze the impact of changes in the system parameters on the system performance, and provide some useful insights. Additionally, we present numeric simulations to validate the correctness of the theoretical results and show the considered system performs better in terms of the covert performance than the system using the non-orthogonal multiple access (NOMA) protocol.