Covert Communication for STAR-RIS-Assisted Ambient Backscatter Communication Systems With RSMA

In this paper, we investigate covert communications in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted ambient backscatter communication (AmBC) system with rate-splitting multiple access (RSMA). Specifically, we use STAR-RIS as a backscatter device to enhance the backscatter link against interference from the direct link. In addition STAR-RIS utilizes the RSMA protocol to transmit information to both covert and public users, enabling more efficient use of spectrum resources. In order to evaluate the system performance, we derive expressions for detection error probability (DEP), outage probability (OP), covert rate (CR) and spectral efficiency (SE). Then, we perform an optimal detection threshold analysis for DEP, an asymptotic analysis for OP, as well as a lower bound analysis for CR, and analyze the impact of system parameter changes on the system performance. The numerical results show that the STAR-RIS-RSMA assisted ambient backscatter communication (AmBC) system performs better in terms of the covert performance than the system using tags as well as the system using the non-orthogonal multiple access (NOMA) protocol.