Near-Field Wideband Beamforming for RIS Based on Fresnel Zone

TL;DR

This paper introduces a Fresnel zone-based beamforming method for RIS in near-field wideband systems, effectively mitigating beam split effects and improving signal focus across frequencies.

Contribution

It proposes a novel beamforming design leveraging Fresnel zones to eliminate beam split, and develops algorithms to optimize RIS phase shifts for near-field wideband communication.

Findings

The method reduces beam split effects in RIS-based systems.

Achieves near-optimal achievable rate bounds.

Simulation confirms improved beam focusing across frequencies.

Abstract

Reconfigurable intelligent surface (RIS) has emerged as a promising solution to overcome the challenges of high path loss and easy signal blockage in millimeter-wave (mmWave) and terahertz (THz) communication systems. With the increase of RIS aperture and system bandwidth, the near-field beam split effect emerges, which causes beams at different frequencies to focus on distinct physical locations, leading to a significant gain loss of beamforming. To address this problem, we leverage the property of Fresnel zone that the beam split disappears for RIS elements along a single Fresnel zone and propose beamforming design on the two dimensions of along and across the Fresnel zones. The phase shift of RIS elements along the same Fresnel zone are designed aligned, so that the signal reflected by these element can add up in-phase at the receiver regardless of the frequency. Then the expression of equivalent channel is simplified to the Fourier transform of reflective intensity across Fresnel zones modulated by the designed phase. Based on this relationship, we prove that the uniformly distributed in-band gain with aligned phase along the Fresnel zone leads to the upper bound of achievable rate. Finally, we design phase shifts of RIS to approach this upper bound by adopting the stationary phase method as well as the Gerchberg-Saxton (GS) algorithm. Simulation results validate the effectiveness of our proposed Fresnel zone-based method in mitigating the near-field beam split effect.