Longitudinal Structuring of Artificial Noise Electromagnetic Field for Enhanced Physical-Layer Security in Line-of-Sight Wireless Communications

Securing data transmission against eavesdropping is a critical requirement in modern communication systems. Beyond traditional encryption methods, physical-layer techniques that exploit the physics of electromagnetic wave propagation have emerged as promising solutions for enhancing security. However, existing physical-layer security approaches typically do not inherently prevent eavesdropping from listeners positioned along the line-of-sight path. Here, we propose, simulate, and experimentally demonstrate a line-of-sight physical-layer security scheme that employs a longitudinally structured artificial-noisecarrying beam co-propagating with the data-carrying signal beam. This noise beam spatially overlaps the signal beam throughout the propagation path but undergoes significant spatial broadening only near the legitimate receiver, resulting in a low-intensity noise region at the receiver's aperture. Consequently, the signal-to-noise ratio (SNR) exhibits a tailored distance dependence, enabling successful data recovery only at the intended receiver's location while significantly degrading reception elsewhere. We demonstrate the feasibility of this approach across diverse electromagnetic carrier frequencies through proof-of-concept wireless experiments: a 120-Gbit/s 16-QAM optical link at 1550 nm and a 20-Gbit/s QPSK sub-terahertz link at 300 GHz. The measured SNR values along the propagation path outside the designed receiver region remain at least 17 dB lower than those at the receiver's aperture.