Overcoming the Shadow: Bending Airy Beams for Radiative Near-Field Multi-User Access in Half-Space Blockage Scenarios

TL;DR

This paper addresses radiative near-field MU-MIMO in ELAA systems under half-space blockage, where traditional focusing suffers rank deficiency and outages. It introduces a physics-aware Airy beamforming framework that edge-rides diffraction edges, restoring channel rank and connectivity. The contributions include a Green's-function RNF model revealing singular-value collapse, an edge-riding Airy-analog analog beamforming strategy, and an Airy-null steering approach for mixed shadow-bright scenarios, achieving over 20 dB SNR gains and up to 35% spectral-efficiency improvements without RIS. These results suggest Airy beams as a hardware-efficient alternative for blockage-resilient RNF networks, with potential for extension to 3D geometries and learning-based real-time adaptation.

Abstract

The move to next-generation wireless communications with extremely large-scale antenna arrays (ELAAs) brings the communications into the radiative near-field (RNF) region, where distance-aware focusing is feasible. However, high-frequency RNF links are highly vulnerable to blockage in indoor environments dominated by half-space obstacles (walls, corners) that create knife-edge shadows. Conventional near-field focused beams offer high gain in line-of-sight (LoS) scenarios but suffer from severe energy truncation and effective-rank collapse in shadowed regions, making hardware remedies such as reconfigurable intelligent surfaces (RIS) impractical. We propose a beamforming strategy that exploits the auto-bending property of Airy beams to mitigate half-space blockage without additional hardware. The Airy beam is designed to ``ride'' the diffraction edge, accelerating its main lobe into the shadow to restore connectivity. Our contributions are threefold: (i) a Green's function-based RNF multi-user channel model that analytically reveals singular-value collapse behind knife-edge obstacles; (ii) an Airy analog beamforming scheme that optimizes the bending trajectory to recover the effective channel rank; and (iii) an Airy null-steering method that aligns oscillatory nulls with bright-region users to suppress interference in mixed shadow/bright scenarios. Simulations show that the proposed edge-riding Airy strategy achieves an SNR improvement of over 20 dB and restores full-rank connectivity in shadowed links compared to conventional RNF focusing, virtually eliminating outage in geometric shadows and increasing multi-user spectral efficiency by approximately 35\% under typical indoor ELAA configurations. These results demonstrate robust RNF multi-user access in half-space blockage scenarios without relying on RIS.

Figures (7)

• Figure 1: Conceptual illustration of the proposed Airy beamforming strategy versus traditional near-field focusing in a radiative near-field (RNF) half-space blockage scenario. The traditional focused beam (red trajectory) propagates linearly and is severely truncated by the knife-edge obstacle, resulting in a communication outage for the shadowed user (UE-1). In contrast, the proposed Airy beam (blue trajectory) exploits its unique auto-bending property to "ride" the diffraction edge, curving into the geometric shadow zone to re-establish a reliable link for UE-1, while simultaneously serving the line-of-sight user (UE-k).
• Figure 2: Baseline performance of the RNF-MU system with conventional focusing in free space. (a) System Geometry: UE1 fixed at $(-5\lambda, 250\lambda)$ and UE2 scanning at $z=300\lambda$. (b) Condition Number $\kappa(\mathbf{H}_{\text{eff}})$: A sharp singularity ($\kappa_{\max} \approx 158$) occurs at $x_2 = -6\lambda$, corresponding to angular alignment with UE1. (c) Common SINR: Equalized SINR profile showing degradation at the angular collision. (d) Sum-Rate: Minimum throughput observed at the singularity point. (e)--(f) Angular Analysis: SINR and $\kappa$ plotted against UE2's angle $\theta_2$, confirming the overlap occurs at $\theta_2 \approx \theta_1 \approx -1.15^\circ$. (g)--(i) Field Intensity Distributions: Normalized field power $|E(x,z)|^2$ for UE2 located at $x_2 = -10\lambda$, $0\lambda$, and $+10\lambda$, respectively, visualizing the distinct focal spots.
• Figure 3: Performance comparison in the double-shadow scenario ($x_1=-5\lambda$, fixed; $x_2$ scanning inside shadow). (a) System geometry showing the knife-edge blockage at $z=150\lambda$. (b) Sum-rate performance: Airy beams (red) maintain a robust rate floor in the deep shadow where traditional beams (blue) suffer outage. (c) Sum-rate gain of Airy over traditional focusing, showing gains up to 18 bps/Hz. (d) Common SINR: Airy beams provide a usable SINR floor despite interference, whereas traditional SINR collapses. (e) Condition number: Note that while Airy has a higher $\kappa$ near collision, it provides non-zero singular values, enabling communication. (f) Steering angles used for the Airy beams, following simple geometric logic.
• Figure 4: Normalized field intensity distributions $|E(x,z)|^2$ (dB). Top Row (a, b): Deep shadow scenario ($x_2 = -11\lambda$). Traditional beams (a) are severed, while Airy beams (b) bend significantly to serve the distant user. Bottom Row (c, d): Angular collision scenario ($x_2 \approx x_1 = -5\lambda$). Traditional beams (c) fail completely, while Airy beams (d) successfully deliver power to both co-located users despite the spatial overlap. Parameters: $B=-25, F \approx 163\lambda$.
• Figure 5: Airy-Null optimization results in the mixed Shadow-Bright scenario. (a) Geometry showing UE1 in shadow and UE2 in bright region. (b) Sum-rate vs. steering angle $\Delta \theta$: The peak occurs at $\Delta \theta \approx -2.8^\circ$, not at the geometric zero. (c) Condition number $\kappa$ vs. $\theta$: A sharp dip in $\kappa$ coincides with the sum-rate peak, indicating improved orthogonality. (d)-(e) Comparison of Sum-Rate and Condition Number for Trad-All, Airy-Geo, and Airy-Opt strategies. (f) Analog coupling strengths, highlighting the interference suppression capability of the optimized design.