Physical Layer Security in Near-Field Communications

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 internal eavesdropper (E) in the near field. Based on the spherical-wave channel state information of U and E, a two-stage algorithm is proposed to maximize the near-field secrecy capacity. In the first stage, the fully-digital beamformers are optimized. Then, the optimal analog beamformers and baseband digital beamformers are 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.