Performance analysis for security improvement in secondary NOMA networks

In order to improve the security of cognitive non-orthogonal multiple access (NOMA) systems, we propose a highly secure forwarding strategy for secondary networks, in which the relay uses the data sent by the primary transmitter to improve the security of its forwarding data. Specifically, two relay encoding strategies are designed to improve the security of the cognitive transmitter data, namely, power superposition (PS) encoding strategy and bit-level exclusive OR-PS (XOR-PS) encoding strategy. Considering the imperfect successive interference cancellation (SIC) technology, the exact closed-form expressions of the outage probabilities and intercept probabilities of the PS and XOR-PS schemes in Rayleigh fading scenarios are derived. Then, the corresponding approximate results in high signal-to-noise ratio (SNR) are also given and the correctness of the theoretical results is verified by simulations. Furthermore, two other conventional PS and XOR-PS schemes without using the data of the primary user, represented by NPS and NXOR-PS, respectively, are provided as the benchmark to compare the security and reliability of the proposed protocols. Finally, numerical results show that the security of the proposed PS and XOR-PS schemes is much better than that of the NPS and NXOR-PS schemes in the case of without degrading the outage performance.