Physical layer security of multi-antenna relay assisted NOMA networks

In this paper, the secrecy performance of one multi-antenna relay assisted non-orthogonal multiple access (NOMA) network is investigated when the relay employs either amplify-and-forward (AF) protocol or decode-and-forward (DF) protocol respectively. Selection combination (SC) and transmitting antenna selection (TAS) schemes are employed at the multi-antenna relay for the network. The impact of outdated channel state information (CSI) for the antenna selection is also considered for the model. The exact and asymptotic expressions for secrecy outage probability (SOP) and strictly positive secrecy capacity (SPSC) are derived over Rayleigh fading channels, and these results are also validated by Monte Carlo Simulations. The results show that the network with AF or DF scheme has almost the same tight approximate theoretical secrecy performance although the exact SOP performance of DF scheme is slightly better than AF scheme as shown by the Monte Carlo simulation results. SOP tends towards the performance floor with constant value as revealed by the asymptotic analysis, which means that the diversity order of SOP is zero regardless of different antenna number and outdated/perfect CSI. But more antennas and less outdated CSI can improve the secrecy performance in the lower and medium SNR region, and they also affect the optimal NOMA power allocation factor.