  Terahertz (THz) communication is now being considered as one of possible
technologies for the sixth generation (6G) communication systems. In this
paper, a novel three-dimensional (3D) space-time-frequency non-stationary
massive multiple-input multiple-output (MIMO) channel model for 6G THz indoor
communication systems is proposed. In this geometry-based stochastic model
(GBSM), the initialization and evolution of parameters in time, space, and
frequency domains are developed to generate the complete channel transfer
function (CTF). Based on the proposed model, the correlation functions
including time auto-correlation function (ACF), spatial crosscorrelation
function (CCF), and frequency correlation function (FCF) are investigated. The
results show that the statistical properties of the simulation model match well
with those of the theoretical model. The stationary intervals at different
frequencies are simulated. The non-stationarity in time, space, and frequency
domains is verified by theoretical derivations and simulations.
