  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.
