Secrecy Outage Performance Enhancement for Three Secondary Users CR-NOMA Systems with TAS over Nakagami-m Channels

Cognitive radio combined with non-orthogonal multiple access (CR-NOMA) networks is a research hotspot in the wireless communication field. All the time, secure communication of wireless networks has become a pressing issue due to the openness of the wireless channel. Secondary outage is one of the important secrecy performance for the wireless communication. Secrecy outage performance (SOP) for a CR-NOMA network with three secondary users (SUs) was firstly investigated in our last work, which is still very rare so far. However, an assumption is only Rayleigh fading for the last work, which is not a practical model for the wireless communication networks. In the paper, we firstly introduce a CR-NOMA networks with three SUs over Nakagami-m channels, which utilizes multi-antennas technology at the base station (BS) to enhance SOP performance. In particularly, we propose two appropriate transmitter antenna selection schemes which are respectively a maximum channel capacity (MCC) scheme and a maximum-minimum (MM) scheme to optimize resources and mitigate common eavesdropping scenarios. The MCC scheme enhances system SOP performance under passive eavesdropping, while the MM scheme improves system SOP performance under active eavesdropping. Subsequently, we derive precise closed-form SOP expressions under the above two schemes. In order to obtain deeper insights, we analyze their asymptotic SOP performance. The asymptotic results indicate that the power allocation factor significantly influences system SOP performance. We utilize a simulated annealing algorithm to find the optimal power allocation factor which further enhance the system SOP performance. The results reveal that the optimal power allocation factor is not fixed, which changes with the transmit power of the BS. Finally, we verify the theoretical analysis through Monte Carlo simulations. Simulation results demonstrate that the MM and MCC schemes achieve higher SOP performance (6%-55% when transmit power is 20 dBm) and secrecy throughput than the single antenna. Meanwhile, the system SOP performance under the Nakagami-m channel is better than that under the Rayleigh channel.