STAR-RIS-NOMA Assisted Power Constrained Covert Communication Systems

In this paper, a covert communication system assisted by a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) in non-orthogonal multiple access (NOMA) networks is proposed. When the maximum channel power gain from the transmitter in the main network to the users in adjacent networks is smaller than the threshold, the transmitter sends the NOMA signals to the users with the assistance of STAR-RIS. Moreover, channel estimation errors and imperfect successive interference cancellation (SIC) are considered in the system analysis. To characterize the covert performance, the detection error probability (DEP) and the average minimum detection error probability (AMDEP) are first derived. Then, the closed-form solutions of the outage probability (OP) for the users are obtained. Finally, the maximum effective covert rate (ECR) subject to the covertness constraint and non-interruption constraint is studied. The numerical simulation results show that: 1) the detection performance of Willie (warden) deteriorates as the channel estimation errors increase; 2) when the transmitter power exceeds the threshold, communication in the main network is interrupted; 3) under a certain transmit power, the maximum ECR increases with the increasing of the number of STAR-RIS reflection elements.