Covert Communications for AAV-Aided STAR-RIS Systems With NOMA

This study investigates covert communication for a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted system with non-orthogonal multiple access (NOMA). In particular, a source first transmits a NOMA signal consisting of both public and covert information to an autonomous aerial vehicle (AAV) relay, which then forwards it to both the covert and the public users via a STAR-RIS in the presence of a warden, which attempts to detect whether the covert information is forwarded to the covert user or not. Under practical communication scenarios, the warden may locate at different positions in order to detect the covert transmission in the dual-hop systems, which may lead to several different detecting scenarios. Out of those possible scenarios, in this study, three typical ones are considered: 1) The warden can only detect the signal from the source in the first-hop transmission; 2) Both the signals from the source and the relay in the first-hop and second-hop transmissions could be detected; 3) The warden can detect the direct as well as the reflected transmission via the STAR-RIS from the relay in the second-hop transmission. Taking those three scenarios into consideration, the analytical expressions of detection error probability of the warden are derived and verified with Monte-Carlo simulations. In addition, the optimal detection threshold as well as the maximum effective covert rate is presented and discussed.