Secrecy performance analysis for STAR-RIS-aided NOMA 6G networks

The marriage of the non-orthogonal multiple access (NOMA) and simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) techniques is envisioned to be a key enabler for improving both the energy efficiency and spectrum efficiency for evolving wireless networks. This paper investigates the physical layer security (PLS) for a downlink STAR-RIS-aided NOMA wireless communications networks, in which a STAR-RIS is employed to assist the downlink transmissions between base station and users. The energy splitting (ES) protocol is utilized by the STAR-RIS to serve a pair of NOMA users located on both sides of it. Under the ES protocol, each reconfigurable element is capable of operating in both transmission and reflection modes concurrently. Two scenarios are considered in this paper: one is the scenario with only external eavesdroppers, and another is the scenario where both external and internal eavesdroppers exist. For the scenario with only external eavesdropping, an optimal phase selection scheme is introduced to improve the system PLS. A random phase selection scheme is introduced to further improve the PLS for the second scenario. To evaluate the secrecy performance for the two scenarios, we establish a closed-form expressions of secrecy outage probability (SOP) for STAR-RIS-aided NOMA downlink system. For gleaning further insights, the asymptotic expression for the derived SOP is obtained. Moreover, secrecy diversity order (SDO) is derived. As a further advance, the system parameter optimization is carried out with the goal of minimizing the SOP. The analytical results show the superior secrecy performance of the proposed scheme. However, it has also been shown that when there is internal eavesdropping, the SDO of cell-edge users is zero.