Physical-Layer Security of the NOMA-Assisted ISAC Systems Under Near-Field Scenario

Integrated Sensing and Communication (ISAC) improves resource utilization efficiency by sharing resources between communication and radar sensing. With the development of ISAC technology, there is a growing focus on how to protect information at the physical layer to ensure communication security. In this context, the physical-layer security (PLS) of the near-field ISAC system is investigated, operating in nonorthogonal multiple access (NOMA) scenario. In our proposed system, the base station (BS) transmits confidential messages to multiple communication users (CUs) while wirelessly sensing a target. Consider a joint transmission beamforming design to support secure communication and ensure sensing requirements. Specifically, we derive the Cramér-Rao bound (CRB) for joint distance and angle sensing in the near-field, and maximise the secrecy rate for CUs under the CRB constraint imposed by target sensing. Since the beamforming problem is nonconvex and challenging to be solved, semi-definite relaxation (SDR) and successive convex approximation (SCA) algorithms are investigated to transform the objective problem into a convex one, providing local optimal solution after acceptable iterations. The beamforming and overall system performance are also evaluated and studied by means of analysis and simulation. Simulation results show that our proposed method leverages near-field joint angle and distance beamforming design, enhancing the performance of secure communication while meeting the sensing requirements in the ISAC system.