A near-field secure transmission framework is proposed. Employing the hybrid beamforming architecture, a multi-antenna base station (BS) transmits confidential information to a multi-antenna legitimate user (U) against a multi-antenna eavesdropper (E) in the near field. A two-stage algorithm is proposed to maximize the near-field secrecy capacity. Based on the fully-digital beamformers obtained in the first stage, the optimal analog beamformers and baseband digital beamformers can be alternatingly derived in the closed-form expressions in the second stage. Numerical results demonstrate that in contrast to the far-field secure communication relying on the angular disparity, the near-field secure communication mainly relies on the distance disparity between U and E.

翻译：提出了一种近场安全传输框架。采用混合波束赋形架构，多天线基站（BS）在近场中向多天线合法用户（U）传输机密信息，同时抵御多天线窃听者（E）的攻击。设计了一种两阶段算法以最大化近场安全容量。基于第一阶段得到的全数字波束赋形器，第二阶段可交替推导出最优模拟波束赋形器和基带数字波束赋形器的闭式解。数值结果表明，与依赖角度差异的远场安全通信不同，近场安全通信主要依赖U与E之间的距离差异。