From Focusing to Con-Focusing: Optimal Power Transfer in Line-of-Sight Near-Field MIMO

2026-06-26

AI Summary

This work addresses the significant degradation in power transmission efficiency observed in line-of-sight near-field MIMO systems when both transmitter and receiver employ large-aperture antennas, a limitation stemming from conventional focusing strategies that neglect aperture effects. The study is the first to elucidate this failure mechanism and introduces a novel con-focusing approach that requires neither channel state information nor iterative refinement—only a single beam refinement step. Leveraging the Fresnel diffraction model, the authors derive a closed-form expression for power gain under fully analog unit-modulus beamforming and employ the Fresnel number and aperture ratio for generalized analysis. Theoretically, the proposed con-focusing scheme is shown to asymptotically approach the rank-one eigenvalue bound across all Fresnel numbers; notably, in symmetric-aperture systems, it surpasses far-field beamforming at a Fresnel number of 1.947 and consistently achieves near-optimal performance, thereby circumventing the 10 dB per decade gain loss inherent to traditional focusing at high frequencies.

Abstract

Beamfocusing is the established near-field strategy for a large array serving a single-antenna user. We consider the single-user line-of-sight MIMO link, free of multipath, in which the user, too, carries an extended aperture, and show that the focusing prescription inverts: beyond a modest Fresnel number, focusing on the user is outperformed by far-field steering. Under fully analog, unit-modulus beamforming, we derive closed-form power gains for focusing (each aperture phase-matched to the other's center) and for steering (a planar phase ramp) in the Fresnel regime, and prove that their comparison is governed by two dimensionless quantities: the link Fresnel number, the product of the two aperture lengths normalized by wavelength and link distance, and the aperture ratio, irrespective of how many elements discretize the apertures. For equal apertures the two gains cross exactly once, at the universal value 1.947; beyond it, focusing loses ten dB per decade of Fresnel number, and the advantage celebrated in the MISO literature survives only as the receive aperture vanishes. We then derive the strategy that is order-optimal at every Fresnel number, con-focusing: both apertures aim at the common point from which they subtend equal angles. It attains the rank-one eigenbound in leading constant, needs no channel knowledge, degenerates to plain steering for equal apertures, and is acquirable within one beam-refinement round with no geometry exchange between the terminals.

Problem

• near-field MIMO
• optimal power transfer
• beamfocusing
• Fresnel number
• large aperture

Innovation

• con-focusing
• near-field MIMO
• Fresnel number
• beamforming
• extended aperture