Covert Communication of Active RIS-Assisted Uplink SGF-NOMA Systems

This paper investigates the covert performance of an active reconfigurable intelligent surface (A-RIS)-assisted uplink non-orthogonal multiple access (NOMA) system with semi-grant-free (SGF) transmission in the presence of an illegal user. Using SGF transmission, grant-free (GF) users bypass the authorization process and transmit signals by sharing resource blocks reserved for grant-based (GB) users. Among the K GF users, the one with the best channel conditions shares the resource block with the GB user, ensuring service quality and avoiding collisions. Based on this, we derive the exact expressions for detection error probability (DEP) and covert rate (CR) to evaluate the system’s covert performance. In conclusion, simulation results confirm the accuracy of our analysis and offer valuable insights.