Secure transmission design for cell-edge user in RIS-aided NOMA network with hybrid SWIPT

Abstract To address the growing demands for energy-efficient and high-capacity sixth-generation (6 G) networks, reconfigurable intelligent surface (RIS) provides an innovative approach that enhances both spectral and energy efficiency. This paper investigates the physical layer security (PLS) of a downlink multiple-input single-output non-orthogonal multiple access (MISO-NOMA) system serving two users in the presence of multiple eavesdroppers. To enhance the security of legitimate communications against eavesdropping, we propose a RIS-assisted cooperative transmission scheme that integrates hybrid simultaneous wireless information and power transfer (H-SWIPT) and transmit antenna selection (TAS). In this framework, the cell-center user, acting as a decode-and-forward (DF) relay, assists the cell-edge user by leveraging hybrid time switching (TS) and power splitting (PS) SWIPT protocols. To evaluate secrecy performance, we derive a closed-form upper bound for the secrecy outage probability (SOP) and further analyze asymptotic SOP behavior and secrecy diversity order (SDO). Simulations confirm that our proposed scheme outperforms conventional orthogonal multiple access (OMA) and non-cooperative NOMA benchmarks in achievable secrecy performance.