STAR-RIS-Aided NOMA for Secured xURLLC

Short packet-based advanced Internet of things (A-IoT) in six generation (6G) calls for not only the next generation of ultra-reliable low-latency communications (xURLLC) but also highly secured communications. In this paper, we aim to address this objective by developing a downlink non-orthogonal multiple access (NOMA) system with untrusted user. However, there exist two key problems: The confidential/private message for the far user will be exposed to the untrusted near user with successful SIC; The restrictive trade-off among reliability, security and latency poses a great challenge in achieving secured xURLLC. In order to solve these issues, we introduce simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS), which provides additional degree of freedom to enable a secure and fair decoding order and achieve a desired trade-off among reliability, security and latency. To fully reveal the trade-off among reliability, security and latency, we characterize the reliability and security via decoding error probabilities. A leakage probability minimization problem is modeled to optimize the passive beamforming, power allocation and blocklength subject to secure successive interference cancellation (SIC) order, reliability and latency constraints. To solve this complex problem, we explore its intrinsic properties and propose an algorithm based on majorization minimization (MM) and alternative optimization (AO). Simulation results demonstrate that our proposed scheme can concurrently fulfill the stringent reliability and latency requirements while significantly enhancing security.