Hybrid STAR-RIS: A Unified Approach to Localization, Communication, and Power Transfer

We propose a novel hybrid simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-based framework to enable integrated localization, communication, and wireless power transfer (WPT). Each STAR-RIS element operates in an active or passive mode to forward signals bidirectionally, while a low-cost sensor array on the reflective side enables target localization and direction-of-arrival (DoA) estimation. To facilitate distance estimation, frequency-modulated continuous wave (FMCW) signals are employed, while the Cramér-Rao bound (CRB) is derived to characterize the theoretical performance limits. DoA estimation is carried out using the multiple signal classification (MUSIC) algorithm. To capture the trade-off between the performance gains from active amplification and the increase in energy consumption, we investigate the energy efficiency (EE) of the proposed system. Numerical results demonstrate that a carefully chosen selection between the numbers of active and passive RIS elements could offer significant improvements in the communication throughput, harvested energy, and localization accuracy.