Secrecy Analysis of Ambient Backscatter NOMA Systems Under I/Q Imbalance

In this correspondence, we investigate the reliability and the security of the ambient backscatter (AmBC) non-orthogonal multiple access (NOMA) systems, where the source aims to communicate with two NOMA users in the presence of an eavesdropper. To be practical, we assume that all nodes and backscatter device (BD) suffer from in-phase and quadrature-phase imbalance (IQI). More specifically, some analytical expressions for the outage probability (OP) and the intercept probability (IP) are derived. In order to obtain more insights, the asymptotic behaviors for the OP in the high signal-to-noise ratio (SNR) regime are explored, and corresponding diversity orders are derived. Numerical results show that: 1) Although IQI reduces the reliability, it can enhance the security; 2) Compared with the orthogonal multiple access (OMA) systems, the considered AmBC NOMA systems can obtain better reliability when the SNR is lower; 3) There are error floors for the OP in the high SNR regime due to the reflection coefficient $\beta$; 4) There exists a trade-off between reliability and security.