[Paper Review] Near-Field Wireless Power Transfer with Dynamic Metasurface Antennas

Haiyang Zhang, Nir Shlezinger+4 more|arXiv (Cornell University)|Oct 10, 2021

Energy Harvesting in Wireless Networks4 citations

TL;DR

This paper proposes a dynamic metasurface antenna (DMA)-based near-field wireless power transfer (WPT) system that enables focused energy beams for multi-user charging in the radiating near-field (Fresnel) region. By jointly optimizing digital precoding and DMA element weights via alternating optimization on a Riemannian manifold, the system achieves up to 13.4 μW at a target receiver—48% higher than far-field beamforming—while minimizing energy pollution and enabling priority-based charging of multiple devices.

ABSTRACT

Radio frequency wireless power transfer (WPT) enables charging low-power mobile devices without relying on wired infrastructure. Current existing WPT systems are typically designed assuming far-field propagation, where the radiated energy is steered in given angles, resulting in limited efficiency and possible radiation in undesired locations. When large arrays at high frequencies, such as DMA, are employed, WPT might take place in the radiating near-field (Fresnel) region where spherical wave propagation holds, rather than plane wave propagation as in the far-field. In this paper, we study WPT systems charging multiple devices in the Fresnel region, where the energy transmitter is equipped with an emerging DMA, exploring how the antenna configuration can exploit the spherical wavefront to generate focused energy beams. In particular, after presenting a mathematical model for DMA-based radiating near-field WPT systems, we characterize the weighted sum-harvested energy maximization problem of the considered system, and we propose an efficient solution to jointly design the DMA weights and digital precoding vector. Simulation results show that our design generates focused energy beams that are capable of improving energy transfer efficiency in the radiating near-field with minimal energy pollution.

Motivation & Objective

• To address the inefficiency and energy pollution of conventional far-field WPT systems in dense 6G Internet of Everything (IoE) environments.
• To explore the potential of dynamic metasurface antennas (DMAs) in enabling beam focusing in the radiating near-field (Fresnel) region for improved WPT efficiency.
• To jointly optimize digital precoding and DMA weights to maximize weighted sum-harvested energy under realistic DMA hardware constraints.
• To demonstrate that near-field beam focusing enables intelligent, priority-aware multi-user charging, unlike conventional far-field beam steering.

Proposed method

• Formulated a mathematical model for DMA-based radiating near-field WPT, incorporating spherical wavefront propagation and Lorentzian response of metamaterial elements.
• Proposed an alternating optimization algorithm to solve the non-convex joint optimization of digital precoding and DMA weights.
• Derived a closed-form solution for the digital precoding vector under fixed DMA configuration.
• Reformulated the DMA weight optimization as a Riemannian manifold optimization problem, solved efficiently using the Riemannian conjugate gradient method.
• Incorporated practical constraints such as maximum transmit power (1 W), RF-to-DC conversion efficiency (ζ = 0.5), and inter-element spacing of λ/2.
• Used a 30 cm × 30 cm DMA with N_d = N_e = ⌊2D/λ⌋ elements to simulate beam focusing at 28 GHz (near-field) and 1.2 GHz (far-field).

Experimental results

Research questions

• RQ1 Can dynamic metasurface antennas (DMAs) enable focused energy beams in the radiating near-field region to improve WPT efficiency compared to far-field beamforming?
• RQ2 How does the joint optimization of digital precoding and DMA weights enhance the weighted sum-harvested energy in multi-user near-field WPT?
• RQ3 To what extent can near-field beam focusing reduce energy pollution compared to conventional far-field beam steering?
• RQ4 Can the system intelligently prioritize energy delivery to multiple users with similar angular directions, even when they are in close proximity?

Key findings

• At 28 GHz (near-field), the system achieved 13.4 μW of harvested power at the target receiver, a 48% improvement over the far-field case at 1.2 GHz, despite the latter's lower path loss.
• The near-field system concentrated energy into a focused beam around the target, while the far-field system produced a wider beam, resulting in lower power concentration and higher energy pollution.
• By adjusting weighting coefficients, the system could prioritize energy delivery: increasing α₂ from 0.1 to 0.9 raised harvested power at the second receiver from 2.5 μW to 4.7 μW, while reducing power to the first receiver from 30.3 μW to 18.7 μW.
• The proposed algorithm successfully focused energy beams on desired focal points, demonstrating effective beam focusing in the radiating near-field with minimal energy leakage.
• The Riemannian conjugate gradient method efficiently solved the non-convex DMA weight optimization problem, enabling practical implementation of near-field beamforming.
• The results confirm that near-field beam focusing via DMAs enables intelligent, multi-user WPT with higher efficiency and lower interference than far-field alternatives.