On Secure NOMA-Aided Semi-Grant-Free Systems

Semi-grant-free (SGF) transmission scheme enables grant-free (GF) users to utilize resource blocks allocated for grant-based (GB) users while maintaining the quality of service of GB users. This work investigates the secrecy performance of non-orthogonal multiple access (NOMA)-aided SGF systems. First, analytical expressions for the exact and asymptotic secrecy outage probability (SOP) of NOMA-aided SGF systems with a single GF user are derived. Then, the SGF systems with multiple GF users and a best-user scheduling scheme is considered. By utilizing order statistics theory, closed-form expressions for the exact and asymptotic SOP are derived. Monte Carlo simulation results demonstrate the effects of system parameters on the SOP of the considered system and verify the accuracy of the developed analytical results. The results indicate that both the outage target rate for GB and the secure target rate for GF are the main factors of the secrecy performance of SGF systems.