IETE Journal of Research
In this paper, we consider the physical layer security of a downlink cooperative multiple-input single-output (MISO) non-orthogonal multiple access (NOMA) network over Nakagami-m fading. The network consists of a multi-antenna source, multiple decode-and-forward (DF) relays, and two users with different priority, in the presence of a passive eavesdropper. Specifically, we investigate the secure transmission from the source to the users via the help of the relays, in which the signals of the users might or might not be successfully decoded at the relays. Thus, we aim to design secure communication protocols for the considered system, where transmit antenna selection (TAS), maximal ratio transmission (MRT), and relay selection are employed based on the source-to-relays channel conditions. To characterize the secrecy performance, analytical and asymptotic expressions for the secrecy outage probability (SOP) of the overall system are derived. Our analytical results validated by Monte-Carlo simulations show that the secrecy performance of cooperative NOMA networks can be improved by increasing the number of antennas at the source and the number of relays. Comparing TAS and MRT, we conclude that TAS has lower complexity but can achieve the secrecy performane comparable with MRT. Finally, the results also indicate that our proposed protocol bring more general and practical insights with similar secrecy performance compared with the conventional solution.