Secure Beamforming and Power Allocation for RIS-Assisted NOMA-ISAC Systems With Internal and External Eavesdroppers

This work investigates secure transmission in non-orthogonal multiple access (NOMA)-integrated sensing and communication (ISAC) systems, where legitimate users may act as internal eavesdroppers alongside external eavesdroppers. We propose a reconfigurable intelligent surface (RIS)-assisted channel selection scheme to degrade channels of internal eavesdroppers and enhance legitimate links. To counter external eavesdroppers, we minimize communication power while satisfying constraints on communication rates, security and sensing performance. Then, we formulate a secure beamforming and power allocation optimization problem, and then propose a semidefinite relaxation (SDR) and penalty-based alternating optimization (AO) algorithm to solve it. The convergence and the computational complexity of the proposed algorithm are analyzed and numerical results are provided to demonstrate the superior secure performance of the proposed scheme.