  The accurate estimation of Channel State Information (CSI) is of crucial
importance for the successful operation of Multiple-Input Multiple-Output
(MIMO) communication systems, especially in a Multi-User (MU) time-varying
environment and when employing the emerging technology of Reconfigurable
Intelligent Surfaces (RISs). Their predominantly passive nature renders the
estimation of the channels involved in the user-RIS-base station link a quite
challenging problem. Moreover, the time-varying nature of most of the realistic
wireless channels drives up the cost of real-time channel tracking
significantly, especially when RISs of massive size are deployed. In this
paper, we develop a channel tracking scheme for the uplink of RIS-enabled MU
MIMO systems in the presence of channel fading. The starting point is a tensor
representation of the received signal and we rely on its PARAllel FACtor
(PARAFAC) analysis to both get the initial estimate and track the channel time
variation. Simulation results for various system settings are reported, which
validate the feasibility and effectiveness of the proposed channel tracking
approach.
