STAR-RIS Assisted Reliable and Secure Transmissions in Wireless-Powered Communications

This article investigates a reliable and secure wireless-powered communication system assisted by a simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) serving indoor and outdoor users. To align with practical application, we consider two eavesdropping conditions: mixed and indirect eavesdropping links. To improve the reliability and security of downlink energy transfer (ET) and uplink information transmission (IT), we propose four STAR-RIS schemes utilizing time-switching (TS) and energy-splitting (ES) protocols: 1) The dual-TS (DTS) scheme employs the TS protocol for both downlink ET and uplink IT; 2) The mixed ES-TS (MET) scheme switches from the ES protocol in downlink ET to the TS protocol in uplink IT; 3) The dual-ES (DES) scheme employs the ES protocol for both downlink ET and uplink IT; 4) The mixed TS-ES (MTE) scheme switches from the TS protocol in downlink ET to the ES protocol in uplink IT. Further, accurate and asymptotic connection outage probability, secrecy outage probability, and effective secrecy throughput are analyzed for each proposed scheme. Theoretical analysis and simulation results demonstrate that: 1) At high transmission power or with a large number of STAR-RIS elements, the MTE scheme achieves the highest reliability. Conversely, the DES scheme provides the best reliability at lower transmission power or with fewer STAR-RIS elements; 2) Under indirect eavesdropping links, the DES scheme achieves the highest security, followed by the MTE, MET, and DTS schemes. However, this performance order is reversed under mixed eavesdropping links; 3) The DES scheme achieves the best overall performance, followed by the MTE, MET, and DTS schemes, all of which outperform the benchmark schemes.