Two-Stage 3D Beam Training for Wideband THz MIMO with Frequency-Dependent Beamforming

The frequency-dependent beamforming technology has demonstrated outstanding performance in two-dimensional beam training for wideband THz systems, owing to its synchronized and high-resolution scanning capabilities. However, extending this technique to three-dimensional beam training poses challenges, as linearly split beams cannot provide comprehensive angular coverage in the candidate area. To this end, we propose a two-stage 3D beam training scheme, consisting of an initial stage and a refinement stage. In the initial stage, the angular coverage of frequency-dependent beamforming is modeled as a controllable rectangular region, and the codebook is designed by arranging this rectangular region within the candidate area. In the refinement stage, frequency-dependent beamforming is densely arranged around the candidate positions estimated in the initial stage to achieve precise localization. Additionally, we design a true-time delay network to facilitate frequencydependent beamforming for uniform planar arrays. Simulation results show that the proposed method approaches near-optimal performance in terms of achievable sum-rate while maintaining acceptable pilot overhead.