SuRLLC: Secure Ultra-Reliable and Low Latency Communication in NOMA-UAV Intelligent Transportation Systems

The ultra-reliable and low-latency communication (uRLLC) plays a vital role in road safety and traffic management of intelligent transportation systems (ITS). Integrating unmanned aerial vehicles (UAVs) and non-orthogonal multiple access (NOMA) has become a prospective strategy for uRLLC by separately establishing line-of-sight (LoS) link and simultaneous transmissions. The confidentiality and security of information transmission in the context of uRLLC faces serious challenges, due to the openness nature of the wireless medium. Different from traditional encryption methods, featured by high computing complexity and serious processing delay, physical layer security (PLS) exploits the inherent properties of the wireless medium to prevent transmitted messages from being eavesdropped. In addition, NOMA-UAV systems pose unique challenges for secure transmission due to potential stronger LoS channels between the source and the eavesdroppers. Therefore, in this paper, we investigate the confidentiality and security of a NOMA-UAV uRLLC system, under which an UAV serves two types of users with different requirements, i.e., secret users (both security and reliability) and public users (only reliability). Based on the tools of stochastic geometry, we address the challenges of secure communications caused by the mobility of UAV, and derive the closed-form expression of secrecy rate maximization, a key metric for observing the key system parameters on the security and reliability. Furthermore, joint design of UAV trajectory and transmit power is proposed to enhance the system security. Finally, compared with popular PLS scheme empowered by UAV trajectory design, the proposed scheme is evaluated by numerical simulations for theoretical analyses and effectiveness in practice.