Secure communication for UAV two-way relay networks^☆

Non-Orthogonal Multiple Access (NOMA) in Unmanned Aerial Vehicle (UAV)-assisted communications is a promising technique in future wireless networks. However, for UAV-assisted communication systems, secure and covert communication is crucial for maintaining confidentiality in battlefield environments. This study focuses on a novel two-way relaying system assisted by the UAV, leveraging Power Domain NOMA (PD-NOMA), trajectory design, and power allocation strategies to enhance secure communication rates. A PD-NOMA scheme is proposed for the half-duplex two-way UAV relay, utilizing the advantage of Air-to-Ground (A2G) channel. The covert communication scheme is proposed based on the proposed NOMA scheme. A method using permutation matrices is proposed to dynamically adjust the NOMA decoding order based on the UAV trajectory and communication power levels, to reduce complexity while ensuring information causality constraints. A low-interference jamming strategy is proposed for the system for covertness communication. Because of the non-convexity of the problem, the power allocation and trajectory design problem are solved with Difference of Convex (DC) programming and Successive Convex Approximation (SCA). The schemes of jointly designing the NOMA order, allocating the communication power, and designing the trajectory are proposed to maximize the minimum secure communication data rate. Simulation results show that the proposed NOMA-UAV secure communication schemes outperform the benchmarks of the conventional Orthogonal Multiple Access (OMA) method.