  In this paper, we propose a quadruple-UPA architecture for realizing the
beamforming with wide coverage in terahertz (THz) communication. Considering
the severe path loss suffered in the THz wave propagation, the pilot signals of
conventional channel estimation strategies may not be effectively detected. In
sight of this, we propose a fast 3D beam training strategy to jointly realize
the angle estimation and the beamforming. Specifically, we first develop a
novel hierarchical codebook that predefines some codewords (including narrow
beams and wide beams) stage by stage. The narrow beams are designed as the
beamforming solution candidates, whereas the wide beams are designed for
reducing the training complexity. Then, we propose a 3D beam training procedure
to find the optimal beamforming solution, i.e., the optimal narrow-beam pair,
in a fast manner. Numerical results are presented to validate the effectiveness
of the proposed scheme.
