Near-Field Secure Beamfocusing With Receiver-Centered Protected Zone

TL;DR

This paper explores near-field secure communication by designing beamfocusing strategies that create protected zones around the receiver, using optimization techniques to enhance secrecy performance against eavesdroppers.

Contribution

It introduces a max-min optimization framework for beamfocusing with protected zones, including a low-complexity solution and a virtual zone approach using full-duplex receivers.

Findings

Protected zones improve secrecy performance.

The proposed methods achieve near-optimal secrecy under various conditions.

Virtual protected zones can be effective when physical zones are infeasible.

Abstract

This work studies near-field secure communications through transmit beamfocusing. We examine the benefit of having a protected eavesdropper-free zone around the legitimate receiver, and we determine the worst-case secrecy performance against a potential eavesdropper located anywhere outside the protected zone. A max-min optimization problem is formulated for the beamfocusing design with and without artificial noise transmission. Despite the NP-hardness of the problem, we develop a synchronous gradient descent-ascent framework that approximates the global maximin solution. A low-complexity solution is also derived that delivers excellent performance over a wide range of operating conditions. We further extend this study to a scenario where it is not possible to physically enforce a protected zone. To this end, we consider secure communications through the creation of a virtual protected zone using a full-duplex legitimate receiver. Numerical results demonstrate that exploiting either the physical or virtual receiver-centered protected zone with appropriately designed beamfocusing is an effective strategy for achieving secure near-field communications.