Secrecy performance of underlay cooperative cognitive network using non-orthogonal multiple access with opportunistic relay selection

Abstract

In this paper, an underlay cooperative cognitive network using a non-orthogonal multiple access (UCCN-NOMA) system is investigated, in which the intermediate multiple relays help to decode and forward two signals x1 and x2 from a source node to two users D1 and D2, respectively, under wiretapping of an eavesdropper (E). We study the best relay selection strategies by three types of relay selection criteria: the first and second best relay selection is based on the maximum channel gain of the links Ri-D1, Ri-D2, respectively; the third one is to ensure a minimum value of the channel gains from the Ri-E link. We analyze and evaluate the secrecy performances of the transmissions x1 and x2 from the source node to the destination nodes D1, D2, respectively, in the proposed UCCN-NOMA system in terms of the secrecy outage probabilities (SOPs) over Rayleigh fading channels. Simulation and analysis results are presented as follows. The results of the (sum) secrecy outage probability show that proposed scheme can realize the maximal diversity gain. The security of the system is very good when eavesdropper node E is far from the source and cooperative relay. Finally, the theoretical analyses are verified by performing Monte Carlo simulations.