Secure energy efficiency for NOMA based cognitive radio networks with nonlinear energy harvesting

In order to improve the spectrum efficiency and secrecy energy efficiency, in this paper, we propose a non-orthogonal multiple access (NOMA)-based secure scheme for cognitive radio networks. In the proposed scheme, the secondary users harvest energy from the radio-frequency signals to securely transmit the secondary privacy information with the NOMA technique. Unlike the conventional ideal linear energy harvesting, we employ the practical nonlinear energy harvesting model for energy harvesting. To implement the proposed scheme, the energy transmitter first broadcasts radio-frequency signals to power the secondary users. Then, the secondary users employ the NOMA technique to transmit the uplink privacy information, which is threatened by the eavesdropper. Considering two scenarios: two secondary users and more than two secondary users, we first provide comprehensive analysis of the secondary secrecy performances and derive the closed-form expressions of the secrecy outage probability for both scenarios. Following the above analysis, we develop the optimization problems to optimally allocate the time slot and the secondary transmit power such that the minimum secrecy energy efficiency is maximized under the constraints of the transmission security and reliability requirements. A two-stage algorithm is proposed to efficiently solve the above optimization problems. Numerical results are presented to verify the analysis in terms of the secrecy rate and secrecy energy efficiency.