Secure Cooperative Relay Transmissions Against Eavesdropping in UAV-Aided NOMA Networks

Unmanned aerial vehicles (UAVs) have been widely adopted as aerial relays, to facilitate seamless coverage through their flexible deployment and high maneuverability. However, the limited power and spectral efficiency of UAVs remain a critical challenge in UAV-aided networks. Additionally, the inherent broadcast nature of wireless channels exposes UAVs to the risk of interception by eavesdroppers. To tackle these issues, this paper proposes a novel secure cooperative relay transmission scheme for UAV-aided non-orthogonal multiple access (NOMA) downlink networks to mitigate eavesdropping threats. Firstly, a network framework is delineated wherein UAVs function as relays to connect ground users and fortify against malicious eavesdropping. To this end, closed-form expressions for secrecy rate and UAV energy consumption are derived. Subsequently, the system performance is characterized by the secrecy rate-energy ratio (SER) which is maximized, subject to constraints on user clustering, UAV serving range, and transmitting power. The optimization problem is disaggregated into two subproblems: the user clustering association subproblem and a hybrid subproblem that encompasses UAV trajectory planning and NOMA transmitting power allocation. The former subproblem is addressed using a conditioned K-means clustering algorithm, whereas the latter is resolved through a deep Q-network (DQN)-based approach. Ultimately, the effectiveness of the proposed optimization method is validated through simulation results.