Covert Communications in Uplink NOMA Systems with Channel Inversion Power Control

In this paper, we investigate covert communications in uplink nonorthogonal multiple access (NOMA) systems, where a private user tries to transmit information covertly with the shield of a public communication link and a warden tries to detect this transmission. In order to confuse the detection of the warden, we adopt channel inversion power control (CIPC) in the public communication link to fully utilize the channel uncertainty. Specifically, we derive the closed-form expressions of the transmission outage probability and the expected detection error probability from the conservative perspective of the private user. Our analysis shows that the expected detection error probability could be enhanced with the increase of the threshold parameter of CIPC. Furthermore, there is a trade-off between the interference introduction and the channel improvement caused by the threshold parameter of CIPC. Therefore, we optimize this parameter to achieve the maximum covert rate, and we find that the maximum covert rate approximates to a constant when the transmit power of public user is large enough.