Max-Min Secrecy Rate Optimization through Beam Focusing in Near-Field Communications

We propose a framework for near-field secure transmission, wherein a multi-antenna base station (BS) employs hybrid beamforming to transmit confidential information to multiple users (UEs), while mitigating the information leakage to multiple eavesdroppers (EVs). We solve the problem of maximizing the minimum UE's secrecy rate via beam focusing in the near-field, subject to the transmit power budget constraint and unit-modulus phase shift constraints of the analog beamformer. We propose a computationally efficient way to solve this problem by employing successive convex approximation and alternating optimization techniques. Numerical results demonstrate the significant performance superiority of the proposed near-field beam focusing algorithm over the existing state-of-the-art techniques. We also discuss new interesting insights arising from the coexistence of multiple EVs and multiple UEs in the same angular direction.