PMI stands for Precoding Matrix Indicator.
PMI – UE reports PMI, RI in PUCCH (or PUSCH) in which it indicates to eNB , which precoding matrix should be used for downlink transmission which is determined by RI.
RI stands for Rank Indicator
RI – UE indicates to eNB, the number of layers that should be used for downlink transmission to the UE. RI-ConfigIndex determines how often the UE is supposed to report RI (Rank Indicator) on the PUCCH. Rank Indication is one of the important input to eNB , in selection of the transmission layer in downlink data transmission. Even though the system is configured in transmission mode 3 (or open loop spatial multiplexing) for a particular UE and if the same UE report the Rank Indication value 1 to eNB, eNB will start sending the data in Tx diversity mode to UE . If UE report Rank Indication 2 , eNB will start sending the downlink data in MIMO mode (Transmission Mode 3).