Covert communications for dual-hop FSO-RF systems with NOMA

In this study, a covert hybrid free-space optical-radio frequency system with non-orthogonal multiple access (NOMA) is proposed. Specifically, in this proposed system, the source first incorporates both public and covert messages into one optical NOMA signal and then sends it to two legitimate users via an intermediate relay under the surveillance of a warden attempting to detect whether the covert information is transmitted or not. In addition, the warden may be located inside or outside the laser beam divergence region of the source, leading to two scenarios where the first-hop transmission may or may not be detected by the warden. Under those two scenarios, the analytical expressions of detection error probability for the warden and the outage probability for both legitimate users are first presented, and then the optimal detection threshold from the point of warden's perspective is also obtained. Moreover, the maximum effective covert rate subject to the covertness and outage constraints is presented. Finally, Monte Carlo simulations are performed to confirm the accuracy of those theoretical analyses.