Channel State Information at the Transmitter (CSIT) is a downlink CSI available at the transmitter (such as at the base station). The base station acquires CSIT by the feedback signaling from users in the network where each user estimates CSIT using receiving transmit pilot signal.
CSIT for MIMO[]
CSIT is optional for the performance improvement in single user MIMO systems while CSIT is crucial to achieve the maximum allowable throughput performance in multiuser MIMO systems.