Near-Field Beamfocusing for Secure Transmission with Receiver-Centered Protected Zone

This work studies near-field secure communications empowered by beamfocusing and demonstrates, for the first time, the benefit of having a protected eavesdropper-free zone around the legitimate receiver. We consider the worst-case secrecy performance against an eavesdropper potentially located anywhere outside the protected zone. Under this consideration, a max-min optimization problem for beamfocusing design is formulated, which can be interpreted as a two-player sequential game between the transmitter and eavesdropper. Despite the NPhardness of the problem, we propose a synchronous gradient descent ascent framework that approximates the global maximin solution. Moreover, we present a low-complexity heuristic beamfocusing solution that delivers excellent performance over a wide range of scenarios. Numerical results demonstrate that exploiting the receiver-centered protected zone with appropriately designed beamfocusing is an effective strategy for achieving near-field secure communications.