Secrecy performance analysis of UAV-assisted CR-NOMA wireless communication systems

Aerial access networks are considered by one of the promising solutions for 6G technology with Unmanned Aerial Vehicles (UAVs) acting as aerial relays to fill coverage gaps, enhance line-of-sight communication, and improve network performance and user experience. To improve spectrum utilization, this paper studies a UAV-assisted wireless communication network based on Cognitive Radio (CR) and Non-Orthogonal Multiple Access (NOMA) technologies. Considering the presence of potential malicious eavesdroppers in complex practical communication scenarios, multiple transmitting nodes cooperate through UAVs. To enhance physical layer security performance, an efficient low-complexity transmitter-relay selection scheme is proposed, which selects the optimal transmitting node based on link channel quality, referred to as the Suboptimal Node Transmission Strategy (SNTS). To further enhance system security performance, another scheme is proposed, which selects the optimal transmitting node by maximizing the transmission capacity of the far user, known as the Optimal Node Transmission Strategy (ONTS). Under the decode-and-forward relay strategy, the security performance of the network under both schemes is studied. Under the joint constraints imposed by the primary interference power and secondary maximum transmission power, the secrecy outage probability for users is derived and validated through simulations. The results indicate that under identical conditions, as the number of transmitting nodes increases, the probability of establishing a line-of-sight link in ground-air communications rises, leading to a decrease in the system’s secrecy outage probability. Furthermore, in terms of multi-user diversity gain, ONTS significantly outperforms SNTS. However, the computational overhead of ONTS is relatively high. Therefore, when meeting the actual communication needs of users, a trade-off between complexity and performance must be considered.