# Two-Stage Hierarchical Beam Training for Near-Field Communications

## TL;DR

This paper introduces a two-stage hierarchical beam training method for near-field XL-array communications, significantly reducing training overhead while maintaining high data rates.

## Contribution

It proposes a novel two-stage hierarchical beam training approach that efficiently searches for user direction and distance in near-field scenarios, reducing overhead compared to exhaustive methods.

## Key findings

- Achieves over 99% reduction in training overhead.
- Maintains comparable rate performance to exhaustive search.
- Effective in near-field XL-array communication environments.

## Abstract

Extremely large-scale array (XL-array) has emerged as a promising technology to improve the spectrum efficiency and spatial resolution of future wireless systems. However, the huge number of antennas renders the users more likely to locate in the near-field (instead of the far-field) region of the XL-array with spherical wavefront propagation. This inevitably incurs prohibitively high beam training overhead since it requires a two-dimensional (2D) beam search over both the angular and distance domains. To address this issue, we propose in this paper an efficient two-stage hierarchical beam training method for near-field communications. Specifically, in the first stage, we employ the central sub-array of the XL-array to search for a coarse user direction in the angular domain with conventional far-field hierarchical codebook. Then, in the second stage, given the coarse user direction, we progressively search for the fine-grained user direction-and-distance in the polar domain with a dedicatedly designed codebook. Numerical results show that our proposed two-stage hierarchical beam training method can achieve over 99% training overhead reduction as compared to the 2D exhaustive search, yet achieving comparable rate performance.