Preserving Covert Communication in Downlink NOMA Systems via Reradiating on Antennas

In this letter, we achieve covert communication for downlink non-orthogonal multiple access systems by reradiating on the public user's antenna. Specifically, we formulate the problem of maximizing the effective covert rate for the covert user under the constraints of covertness for the warden and quality of service for the public user. For the optimization problem, the power allocation factor at the transmitter and the power fraction of the reradiating processing at the public user, which have a dual effect on the system's performance, are jointly optimized. Numerical results indicate that: 1) the maximum effective covert rate tends to build up to a specific constant value as the transmit power increases; and 2) there exists an optimal value of the predetermined rate for the covert user to maximize the effective covert rate.