1. A method for transmitting data from a transmitter unit to a receiver unit in a multiple-input multiple-output (MIMO) communication system, comprising: at the receiver unit, receiving a plurality of signals via a plurality of receive antennas, wherein the received signal from each receive antenna comprises a combination of one or more signals transmitted from the transmitter unit, processing the received signals to derive channel state information (CSI) indicative of characteristics of a plurality of transmission channels used for data transmission, and transmitting the CSI back to the transmitter unit; and at the transmitter unit, receiving the CSI from the receiver unit, and processing data for transmission to the receiver unit based on the received CSI.