Impact of Untrusted Relay on Physical Layer Security in Non-Orthogonal Multiple Access Networks

In this study, the wireless sensor network is investigated in scenario of untrusted relay required to user at far distance. In particular, an untrusted relay assists long distance transmission in situation of non-existence of the direct link between source and destination. This paper employs non-orthogonal multiple access (NOMA) scheme to serve large number of users at different allocated power levels to adapt secure criteria. Specifically , to evaluate the security performance we first examine the secure outage probability (SOP) and then strictly positive secrecy capacity (SPSC) is studied. To further characterize the trade-off between system security and other controlling coefficients, we then investigate the impacts of power allocation factors and power levels of the eavesdropper. In order to find tractable expressions to provide additional insights in term of the performance evaluation, the asymptotic expressions regarding both SOP and SPSC are performed in high signal-to-noise ratio (SNR) region. In addition, secure performance of considered NOMA network is compared in two modes related to untrusted relay, including Amplify-and-Forward and Decode-and-Forward mode. Finally, simulation results are presented to corroborate the proposed methodology.