Safeguarding NOMA Networks via Reconfigurable Dual-Functional Surface Under Imperfect CSI

This paper investigates the use of reconfigurable dual-functional surface to guarantee full-space secure transmission in non-orthogonal multiple access (NOMA) networks. In presence eavesdroppers, downlink communication from base station legitimate users is safeguarded by simultaneously transmitting and reflecting intelligent (STAR-RIS), where three practical operating protocols, namely energy splitting (ES), mode selection (MS), time (TS), are studied. The joint optimization power allocation, active passive beamforming investigated maximize secrecy efficiency (SEE), taking into account imperfect channel state information (CSI) all channels. For ES, approximating semi-infinite constraints with $\mathcal {S}$ -procedure general sign-definiteness, problem solved an alternating framework. Besides, proposed algorithm extended MS protocol solving a mixed-integer non-convex problem. While for TS, two-layer iterative method proposed. Simulation results show that: 1) STAR-RIS assisted NOMA networks able provide up 33.6% higher SEE than conventional RIS counterparts; 2) TS ES protocols generally preferable low high domain, respectively; 3) accuracy CSI estimation bit resolution consumption crucial reap benefits offered STAR-RIS.