  A general three-dimensional (3D) non-stationary massive multiple-input
multiple-output (MIMO) geometry-based stochastic model (GBSM) for the sixth
generation (6G) communication systems is proposed in the paper. The novelty of
the model is that the model is designed to cover a variety of channel
characteristics, including space-time-frequency (STF) non-stationarity,
spherical wavefront, spatial consistency, channel hardening, etc. Firstly, the
introduction of the twin-cluster channel model is given in detail. Secondly,
the key statistical properties such as space-time-frequency correlation
function (STFCF), space cross-correlation function (CCF), temporal
autocorrelation function (ACF), frequency correlation function (FCF), and
performance indicators, e.g., singular value spread (SVS), and channel capacity
are derived. Finally, the simulation results are given and consistent with some
measurements in relevant literatures, which validate that the proposed channel
model has a certain value as a reference to model massive MIMO channel
characteristics.
