  In this paper, a novel three-dimensional (3D) space-time-frequency (STF)
non-stationary geometry-based stochastic model (GBSM) is proposed for the sixth
generation (6G) terahertz (THz) wireless communication systems. The proposed
THz channel model is very general having the capability to capture different
channel characteristics in multiple THz application scenarios such as indoor
scenarios, device-to-device (D2D) communications, ultra-massive multiple-input
multiple-output (MIMO) communications, and long traveling paths of users. Also,
the generality of the proposed channel model is demonstrated by the fact that
it can easily be reduced to different simplified channel models to fit specific
scenarios by properly adjusting model parameters. The proposed general channel
model takes into consideration the non-stationarities in space, time, and
frequency domains caused by ultra-massive MIMO, long traveling paths, and large
bandwidths of THz communications, respectively. Statistical properties of the
proposed general THz channel model are investigated. The accuracy and
generality of the proposed channel model are verified by comparing the
simulation results of the relative angle spread and root mean square (RMS)
delay spread with corresponding channel measurements.
