Low-Complexity Near-Field Beam Training with DFT Codebook based on Beam Pattern Analysis

Extremely large antenna arrays (ELAAs) operating in high-frequency bands have spurred the development of near-field communication, driving advancements in beam training and signal processing design. This paper proposed an efficient near-field beam training method using the discrete Fourier transform (DFT) codebook that is conventionally used for far-field users (FUs). We begin by analyzing the received beam pattern and deriving a closed-form expression for its width and central beam gain, which are validated through simulations. Using these derivations, we define a modified Rayleigh distance to distinguish between near-field and far-field users. Building on this, we propose a beam training method capable of simultaneously estimating user angle and distance with a complexity of O(1). Simulation results confirm the effectiveness of our proposed approach, demonstrating its capability for low-complexity near-field beam training while achieving high estimation accuracy.