Physical Layer Security for STAR-RIS-Assisted NOMA Backscatter Communications against Multiple Eavesdroppers

Simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS) have emerged as a transformative technology for achieving omnidirectional coverage in smart radio environments, enabling energy-efficient and spectrally efficient wireless communications. When integrated with ambient backscatter communication (AmBC) and non-orthogonal multiple access (NOMA), STAR-RIS facilitates the concurrent exploitation of ambient radio frequency signals and spectrum resources, offering significant potential for scalable Internet of Things (IoT) networks. This paper conducts a comprehensive analysis of the physical layer security performance of a STAR-RIS-assisted AmBC system employing NOMA in the presence of multiple eavesdroppers. Specifically, the STAR-RIS serves as an active backscatter device to enhance the backscatter link by mitigating direct link interference, while NOMA optimizes spectrum utilization through power-domain multiplexing. We derive closed-form expressions for critical performance metrics, including outage probability (OP), intercept probability, throughput, and energy efficiency, under realistic channel fading models. Asymptotic analysis of the OP is provided to reveal insights into high signal-to-noise ratio regimes. Furthermore, we investigate the impact of key system parameters. Numerical results validate that the proposed STAR-RIS-assisted AmBC-NOMA framework significantly enhances secrecy performance compared to conventional AmBC systems, demonstrating its robustness against eavesdropping threats and its suitability for secure IoT applications.