Near-Field Wideband Beam Training Based on Distance-Dependent Beam Split

TL;DR

This paper tackles the high pilot overhead of near-field beam training in XL-MIMO by revealing a distance-dependent beam split (DDS) phenomenon that exploits a TD-PS array to distribute multi-frequency beams across multiple angle-distance strips. It introduces an on-grid wideband training scheme that searches angles and distances simultaneously in a single pilot set, and adds two off-grid refinements—auxiliary beam pair and match-filter methods—to improve accuracy for users not aligned with grid points. The authors provide practical TD-PS parameter design rules, interleaving strategies to ensure full angle-distance coverage, and hardware-friendly implementations with fixed TDs. Simulation results show that the proposed DDS-based method achieves near-optimal rates with substantially reduced training overhead compared with near-field rainbow and exhaustive approaches, especially at moderate-to-low SNR and for off-grid users.

Abstract

Near-field beam training is essential for acquiring channel state information in 6G extremely large-scale multiple input multiple output (XL-MIMO) systems. To achieve low-overhead beam training, existing method has been proposed to leverage the near-field beam split effect, which deploys true-time-delay arrays to simultaneously search multiple angles of the entire angular range in a distance ring with a single pilot. However, the method still requires exhaustive search in the distance domain, which limits its efficiency. To address the problem, we propose a distance-dependent beam-split-based beam training method to further reduce the training overheads. Specifically, we first reveal the new phenomenon of distance-dependent beam split, where by manipulating the configurations of time-delay and phase-shift, beams at different frequencies can simultaneously scan the angular domain in multiple distance rings. Leveraging the phenomenon, we propose a near-field beam training method where both different angles and distances can simultaneously be searched in one time slot. Thus, a few pilots are capable of covering the whole angle-distance space for wideband XL-MIMO. Theoretical analysis and numerical simulations are also displayed to verify the superiority of the proposed method on beamforming gain and training overhead.

Figures (9)

• Figure 1: TD-PS precoding structure.
• Figure 2: Comparison of distance-dependent beam split with traditional beam split with different settings: (a) Beams of different subcarriers in proposed distance-dependent beam split; (b) Beams of different subcarriers in traditional beam split with $\alpha_p'+\frac{2q}{d}=0$; (c) Beams of different subcarriers in traditional beam split with $\alpha_p'+\frac{2q}{d}\neq 0$.
• Figure 3: Add another pilot to guarantee coverage of possible regions.
• Figure 4: Hardware implementations of TD by fixed time delay network.
• Figure 5: Auxiliary beam pair-assisted method.