Full-duplex jamming for physical layer security improvement in NOMA-enabled overlay cognitive radio networks

In this paper, we analyze the physical layer security (PLS) performance of nonorthogonal multiple access (NOMA)-enabled overlay cognitive radio networks (NOMA-OCRNs) in the presence of an external passive eavesdropper. Here PLS is expressed in terms of the secrecy outage probabilities (SOPs) experienced by the primary user (PU) and secondary user (SU). We obtain approximate expressions for the SOPs of both PU as well as SU assuming a jamming-free environment, where both primary and secondary destination nodes are half-duplex devices. To improve the SOP performance, we propose a jamming-assisted framework, where full-duplex destination nodes are employed, which are capable of transmitting jamming signals to confound the eavesdropper. Approximate expressions for the SOPs of PU and SU are derived for the jamming-assisted framework as well. It is demonstrated that the proposed jamming-assisted framework significantly reduces the SOPs compared to the jamming-free scenario. We also determine optimal power allocation coefficients (OPACs) for PU and SU at the secondary transmitter that maximizes the total secrecy throughput of the jamming-assisted NOMA-OCRN with FD destinations. It is shown that the suggested OPAC significantly enhances the total secrecy throughput, compared to the default selection of the PAC.