You can use the technique of transmitting pilot signals in a predetermined procedure to estimate the channel (CSI) at the receiver end. Since it difficult to obtain channel information at the transmitter end (which actually helps in beam forming) you feedback the CSI from the receiver end.
CSI is refers to channel state information .which gives the current state of the channel,the Base station will estimate the CSI in the received signals on the uplink .then this CSI is used bty the BS for precoding process.which is useful to estimate the channel more accurately and also for power controlling process