Secure Downlink Transmission Strategies against Active Eavesdropping in NOMA Systems: A Zero-Sum Game Approach.

Non-orthogonal multiple access technology (NOMA), as a potentially promising technology in the 5G/B5G era, suffers fromubiquitous security threats due to the broadcast nature of the wirelessmedium. In this paper, we focus on artificial-signal-assisted and relay-assisted secure downlink transmission schemes against external eavesdropping in the context of physical layer security, respectively. To characterize the non-cooperative confrontation around the secrecy rate between the legitimate communication party and the eavesdropper, their interactions are modeled as a two-person zero-sum game. The existence of the Nash equilibrium of the proposed game models is proved, and the pure strategyNash equilibriumand mixed-strategyNash equilibriumprofiles in the two schemes are solved and analyzed, respectively. The numerical simulations are conducted to validate the analytical results, and showthat the two schemes improve the secrecy rate and further enhance the physical layer security performance of NOMA systems. [ABSTRACT FROM AUTHOR]