Stacked Intelligent Metasurfaces and STAR-RIS-Enabled Terahertz ISAC System

In this letter, a novel terahertz (THz) band integrated sensing and communication (ISAC) framework is proposed, which synergistically integrates stacked intelligent meta-surfaces (SIM) with simultaneously transmitting and reflecting reconfigurable intelligent surfaces (STAR-RIS). In this system, SIM enables high-precision wave-based beamforming with reduced complexity, while STAR-RIS provides full-space coverage and facilitates partitioning sensing and communication spaces. Together, they address the key challenges of multi-target sensing and multi-user communication in the THz band. The Cramér-Rao bound (CRB) is derived for two-dimensional directionof-arrival (DoA) estimation, and an optimization problem is formulated to maximize energy efficiency (EE) under this CRB constraint. To address this coupled non-convex optimization problem, we propose the meta soft actor-critic (Meta-SAC) algorithm, a meta-learning-based solution that jointly optimizes power allocation, SIM phase shifts, and STAR-RIS transmissionreflection (T-R) coefficients. To efficiently solve this problem, the generative adversarial networks (GANs) are integrated with the Meta-SAC algorithm. Simulation results demonstrate significant improvements in EE and sensing accuracy, showcasing the transformative potential of SIM and STAR-RIS integration for next-generation ISAC networks.