Enhancing secrecy performance of energy harvesting NOMA systems using IoT user scheduling under untrusted users

In this work, multiple energy-limited Internet of Things (IoT) users are intended to communicate with their destination (Du$D_{u}$). The scheduled IoT user harvests the energy from a source signal and uses this energy to forward a mixture of its message and the source message to the Du$D_{u}$ and the source destination based on non-orthogonal multiple access (NOMA). Since the messages are transmitted at the same resource block, the far untrusted user (source destination) has the opportunity to access the message destined for the near user (Du$D_{u}$) by successive interference cancellation. Therefore, an IoT user scheduling strategy is proposed to achieve the reliability and security of transmission in the network. To indicate the superiority of the proposed scheduling scheme and the impact of different parameters on the network performance under untrusted users, the secrecy outage probability of the system is derived by tracking exact and asymptotic analyses and then it is compared with that of benchmarks through simulations. The results reveal the crucial role of the near user threshold secrecy rate as the system's bottleneck and the negligible impact of the power allocation coefficients and the untrusted far user threshold rate on the secrecy performance with increasing the signal-to-noise-ratio.