Near-Field Beam Training in THz Communications: The Merits of Uniform Circular Array

As the line-of-sight propagation component is dominant in Terahertz (THz) channels, beam training is regarded as an efficient beamforming strategy for THz communications. In the Fresnel region, the near-field effect boosts the codebook size for the beam training since each near-field beam merely concentrates the radiation in a small area, rather than along a direction regardless of distance. In this letter, we propose to use the uniform circular array (UCA) in THz near-field communications, benefiting from its geometric advantage in beam training. Specifically, an explicit model is proposed for characterizing the beam pattern of UCA. Based on this model, we propose a codebook design for the near-field beam training, where the predefined training beams aim at covering the user's area. Compared to the conventional uniform linear array (ULA), the UCA theoretically shows the advantage of reducing the codebook size in near-field communications. Numerical results validate the effectiveness of the beam-shape model and show the merits of our proposed near-field codebook of UCA.