A near-field secure transmission framework is proposed. Employing the hybrid beamforming architecture, a base station (BS) transmits the confidential information to a legitimate user (U) against an 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-filed 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，基站在近场中向合法用户传输机密信息，同时防范窃听者的窃听。为最大化近场保密容量，提出了一种两阶段算法。基于第一阶段获得的数字波束赋形器，可在第二阶段通过闭式表达式交替推导出最优模拟波束赋形器和基带数字波束赋形器。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合法用户与窃听者之间的距离差异。