STAR-RIS-Assisted Physical-Layer Key Generation

This paper investigates the application of simultaneously transmitting and reflecting reconfigurable surface (STAR-RIS) in physical layer key generation (PKG), and proposes a novel STAR-RIS-assisted multiuser PKG framework. We formulate a sum secret key rate maximization problem for the proposed framework, by optimizing phase shift and amplitude coefficients for both independent and coupled phase-shift models. To tackle this non-convex problem, we derive the optimal phase shift in the closed form and demonstrate that the coupled phase shifts do not compromise key generation performance. The original problem is thus simplified into an optimization of amplitude coefficients, for which we use the alternating optimization scheme. Simulation results validate the effectiveness of the proposed algorithm by comparing the key generation performance with that of STAR-RISs using the random configuration and conventional reflecting/transmitting-only RISs.