Will Element Grouping Enhance Covert Communication Performance in ARIS-Aided NOMA Systems?

This paper investigates the impacts of element grouping on the covert communication performance of an active reconfigurable intelligent surface (ARIS)-aided non-orthogonal multiple access (NOMA) system. Through element grouping, each element of the ARIS works in either the reflecting mode to reflect the information signal or the jamming mode to generate jamming signal. Optimizing the NOMA transmit power, jamming power, ARIS beamforming, and receive beamforming jointly is necessary to maximize the covert communication rate. To tackle the unsolvable covert communication rate maximization problem, we decouple the original problem into four sub-problems of optimizing NOMA transmit power, jamming power, ARIS beamforming, and receive beamforming, respectively. To tackle the mixed-integer non-linear programming for element grouping, we introduce the arithmetic and geometric mean-based penalty term and apply the Dinkelbach transform to reformulate the optimization problem. Next, we propose an alternating optimization algorithm to optimize the system parameters. The numerical results demonstrate the effectiveness of the element grouping in improving the covert communication rate. However, the element grouping scheme achieves a lower covert communication rate performance compared to the scheme without element grouping, indicating that using element grouping for covert communications in the ARIS-aided NOMA system is not the preferred option.