摘要： 随着6G向超大规模天线阵列（ELAA）与高频段发展，通信场景中的近场区域显著扩大。然而，现有关于智能反射面（RIS）辅助非正交多址接入（NOMA）系统物理层安全的研究多局限于远场通信场景，且通常计算复杂度较高，这限制了其在近场大规模系统中的实际应用。针对RIS辅助近场上行NOMA系统，该文考虑一个接入点（AP）、RIS、较远用户、较近用户、窃听者（Eve）组成的上行系统，通过联合优化AP波束赋形与RIS相移，实现系统的保密和速率最大化。该问题受欧几里德范数和单位模约束的限制，是一个复杂的非凸问题，需要采用合理的资源分配方案。为此，该文提出一种低复杂度的块坐标下降（BCD）算法，通过将原问题分解为两个子问题进行求解。首先推导出AP波束赋形的闭式解，然后采用流形优化求解RIS相移。MATLAB仿真结果表明，在默认参数设置下，相较于随机相移、最大比传输（MRT）、正交多址接入（OMA）方案，所提方案使系统的保密和速率分别提升了约4.4bps/Hz、10%和15%。此外，所提方案在达到与半定松弛（SDR）方案相近性能的同时，实现了较低的计算复杂度。

Abstract: As 6G evolves toward extremely large antenna arrays (ELAA) and high-frequency bands, the near-field region in communication scenarios expands significantly. However, existing research on physical layer security for reconfigurable intelligent surface (RIS)-assisted non-orthogonal multiple access (NOMA) systems is largely confined to far-field communication scenarios. Moreover, the computational complexity is often high, which limits practical application in near-field large-scale systems. For RIS-assisted near-field uplink NOMA systems, this paper considers an uplink system comprising an access point (AP), RIS, a far user, a near user, and an eavesdropper (Eve). By jointly optimizing AP beamforming and RIS phase shifts, the system achieves maximum secrecy sum-rate. This problem is non-convex and challenging due to the Euclidean norm and unit modulus, necessitating an efficient resource allocation strategy. To address this, this paper proposes a low-complexity block coordinate descent (BCD) algorithm that decomposes the original problem into two subproblems. First, a closed-form solution for AP beamforming is derived, and then the manifold optimization is applied to obtain RIS phase shifts. MATLAB simulation results demonstrate that, under the default parameter settings, compared to random phase shift, maximum ratio transmission (MRT), and orthogonal multiple access (OMA) schemes, the proposed scheme enhances the secrecy sum-rate of the system by approximately 4.4bps/Hz, 10%, and 15% respectively. Furthermore, the proposed scheme achieves comparable performance to semi-definite relaxation (SDR) schemes while exhibiting lower computational complexity.