Dual UAV–NOMA Based Covert Communications: A Perspective on Spatio-Temporal-Energy Domain Cooperative Differentiation

This paper investigates covert communications in an uncrewed aerial vehicle (UAV) assisted cellular system in the presence of a ground eavesdropper(Eav). To enhance the secrecy performance of a secure user (SU) while guaranteeing the quality-of-service (QoS) requirements of common users, a dual-UAV cooperative non-orthogonal multiple access (NOMA) framework with a two-slot transmission scheme, referred to as Spatio-Temporal-Energy Domain Cooperative NOMA (STEDC-NOMA), is proposed. In the considered architecture, an aerial base station (UAV-BS) and an aerial relay (UAV-Relay) collaboratively serve ground users, where the transmission is divided into two time slots and different common users are paired with the SU in each slot for NOMA superposition. By jointly exploiting temporal-domain two-slot cooperation, spatio-domain dual-UAV diversity, and energy-domain NOMA superposition, additional degrees of freedom are introduced for covert communication with fairer access. Based on this framework, a joint optimization problem of user pairing and resource allocation is formulated to maximize the overall secrecy capacity of the SU under the QoS constraints of common users. To support the proposed scheduling strategy, closed-form power allocation solutions are derived for the two slot respectively. Numerical results demonstrate that, compared with the previous benchmark schemes, the proposed STEDC-NOMA significantly improves the secrecy performance while satisfying the QoS demands and enhancing the access fairness of all users.