Enhancing Secrecy Energy Efficiency in THz MIMO Two-Way Relay Systems With SWIPT and Precoding

This paper investigates the enhancement of secrecy energy efficiency (SEE) in a THz MIMO two-way relay system utilizing simultaneous wireless information and power transfer (SWIPT) and precoding techniques. We first model the SEE metric of the system under random SWIPT and precoding settings, and subsequently formulate the SEE maximization problem as a non-convex programming problem. By employing techniques such as Dinkelbach transformation, alternating optimization, and difference of convex programming, we then develop a multi-level theoretical framework to jointly optimize the energy splitting factor and precoding matrices. As a special case, we further demonstrate that the proposed theoretical framework can also be applied to solving the secrecy rate maximization problem. Finally, with the help of orthogonal-triangular decomposition and the Newton-Raphson method, we provide the optimal structure of the relay precoding matrix and a low-complexity suboptimal solution scheme for SEE maximization. Extensive numerical results validate our theoretical findings and demonstrate that the proposed schemes can significantly improve the system’s SEE compared to the conventional microwave-based schemes and the random baseline.