Artificial Noise Aided Covert Communications in Multi-Relaying FSO-RF NOMA Systems: Analysis and Optimization

This paper investigates covert communications for a multi-beam multi-relaying free-space optical-radio frequency (FSO-RF) non-orthogonal multiple access (NOMA) system. To achieve both high reliability and covertness against multiple cooperative wardens, a joint design incorporating transmit laser selection (TLS) and artificial noise (AN) is proposed. Specifically, the multi-beam source first activates the optimal relay via a TLS scheme, and the selected multi-antenna relay then forwards the superimposed NOMA signals to both public and covert users. To disrupt the wardens’ detection without harming legitimate links, AN is dynamically injected into the null space of the legitimate channels. Specifically, the system’s detection performance is comprehensively characterized by deriving the analytical expressions of the detection error probability (DEP), the optimal detection threshold, the minimum DEP, and the average minimum DEP, followed by the derivation of closed-form outage probability to evaluate the transmission reliability of both users. To fully exploit the potential of this architecture, a joint optimization problem is formulated to maximize the covert rate via jointly designing the power allocation coefficient, the information beamforming vector, and the AN beamforming vector. An alternating optimization algorithm is adopted to solve the non-convex coupled problem efficiently through tractable subproblems. Finally, Monte-Carlo simulations validate the theoretical analysis and quantify the impacts of key system parameters.