Pairing Inspired Covert Communication in Uplink NOMA Systems Against SIC Warden

In this letter, we investigate the covert communication in uplink non-orthogonal multiple access (NOMA) system against a warden who can also execute successive interference cancellation (SIC). To alleviate the user interference at the legitimate receiver, we propose a novel pairing scheme based on double-threshold (DT) of covert communication channel gain. For the proposed scheme, the covertness constraint, the transmission outage probability from covert and public users to legitimate receiver are derived, which reveal the impacts of SIC and pairing threshold on the communication covertness and reliability. Simulation results are provided to prove our analysis and demonstrate the superiority of the proposed DT pairing scheme in terms of effective covert rate (ECR). In addition, numerical results show the impacts of pairing threshold and warden's SIC on the covert communication design where the equal prior probabilities for covert user and the maximal transmit power for the public user are generally not optimal to maximize ECR.