Artificial-Noise-Aided Optimal Beamforming in Layered Physical Layer Security

In this letter, we develop a novel layered physical layer security model to secure multiple messages simultaneously. It maintains a hierarchical security structure, in which each message transmitted from the base station has a security level, and each user in the network has a security clearance correspondingly. Focusing on the basic two-layer unicast system, we propose an artificial-noise-aided optimal beamforming scheme to maximize the higher level information security performance while adhering to the low-level information secrecy rate constraint. Due to the nonconvexity of the problem formulated, a successive convex approximation-based algorithm is proposed to earn a safe convex approximation of the original. Afterward, a low-complexity zero-forcing beamforming scheme is proposed to achieve high efficiency by reducing the computational complexity. Simulation results validate the superiority of our schemes on the promotion of the overall secrecy performance.