Power allocation for enhancing the physical layer secrecy performance of artificial noise-aided full-duplex cooperative NOMA system

This paper investigates the physical layer security (PLS) performance of full-duplex cooperative non-orthogonal multiple access (FD-CNOMA) network in the presence of an external passive eavesdropper. Firstly, we derive approximate analytical expressions for the secrecy outage probabilities (SOPs) of the downlink users and the system SOP (SSOP) of a single-cell FD-CNOMA network, considering the presence/absence of direct link from the base station (BS) to the far user, under imperfect successive interference cancellation conditions. We consider both perfect channel state information and imperfect CSI conditions. To enhance the PLS performance, we propose an artificial noise (AN)-aided framework and derive approximate analytical expressions for the SOPs of the downlink users and the SSOP of the AN-aided FD-CNOMA network. The asymptotic SOP and SSOP expressions are also derived, which are used to determine the secrecy diversity orders. The proposed AN-aided framework significantly reduces the SOPs of the users and the SSOP of the network compared to that without AN. To further enhance the PLS performance, we use the Polak-Ribiere conjugate gradient method to determine the optimal power allocation coefficient (OPAC) for the users at the BS that minimizes the SSOP of the AN-aided FD-CNOMA network. The proposed OPAC significantly reduces the SOPs of the users and the SSOP, compared to random selection/equal setting of the PACs. Finally, we extend the performance evaluations to a multi-cell scenario and demonstrate that the PLS performance of the FD-CNOMA network significantly deteriorates in the presence of co-channel interference.