Semi-blind AF transmission in secure NOMA systems

In the wireless channel state information (CSI)-assisted amplify-and-forward (AF) networks, an instantaneous CSI of the first hop is required to scale the amplification gain. However, the deployment of instantaneous CSI always remains difficult in real applications because it increases the CSI overhead and causes resources wastage such as power and bandwidth. In order to reduce the CSI overhead and the system complexity, we suggest the integration of semi-blind relay in secure non-orthogonal multiple access (NOMA) systems where only a statistical CSI of the first hop is used to generate the amplification gain. This paper addresses the performance analysis of the secure semi-blind AF-NOMA (S-SBAF-NOMA) schemes in which the base station communicates with a pair of users via a semi-blind relay node in the presence of one eavesdropper. First, we provide the expressions for the end-to-end signal-to-noise ratio (SNR) at each receiver node. We then derive new analytical and asymptotic expressions for strictly positive secrecy capacity (SPSC) and secrecy outage probability (SOP). To ensure the exactness and the tractability of mathematical analysis, we provide some numerical results obtained through simulation rounds in Matlab, and we compare them with those of secure CSI-assisted AF-NOMA (S-CSIAF-NOMA) networks. Our results show that the proposed S-SBAF-NOMA scheme achieves comparable secrecy performance/same performance bounds as compared to S-CSIAF-NOMA scheme at the gain of a decrease in processing complexity and system overhead. Numerical results also demonstrate that S-SBAF-NOMA networks achieve superior secrecy performance for lower values of target data rates and SNR of the illegal link.