  The scarcity of spectrum resources in current wireless communication systems
has sparked enormous research interest in the terahertz (THz) frequency band.
This band is characterized by fundamentally different propagation properties
resulting in different interference structures from what we have observed so
far at lower frequencies. In this paper, we derive a new expression for the
coverage probability of downlink transmission in THz communication systems
within a three-dimensional (3D) environment. First, we establish a 3D
propagation model which considers the molecular absorption loss, 3D directional
antennas at both access points (APs) and user equipments (UEs), interference
from nearby APs, and dynamic blockages caused by moving humans. Then, we
develop a novel easy-to-use analytical framework based on the dominant
interferer analysis to evaluate the coverage probability, the novelty of which
lies in the incorporation of the instantaneous interference and the vertical
height of THz devices. Our numerical results demonstrate the accuracy of our
analysis and reveal that the coverage probability significantly decreases when
the transmission distance increases. We also show the increasing blocker
density and increasing AP density impose different impacts on the coverage
performance when the UE-AP link of interest is in line-of-sight. We further
show that the coverage performance improvement brought by increasing the
antenna directivity at APs is higher than that brought by increasing the
antenna directivity at UEs.
