Let me put some background - The number of Transmit antenna in the cell is determined by the UE itself, and this info is not broadcasted in the System Information ie SIB2. eNB transmit the RS using Transmit diversity if more than one antenna port is used, and the UE then does the channel estimates for all the probable antenna ports and determines if eNB had used more than one Antenna.
Now coming to the Antenna Info IE, we have two instances AntennaInfoCommon and AntennaInfoDedicated.
AntennaInfoCommon IE carries Antenna Port Info used in the serving cell, but this IE is carried only in the "MobilityControlInfo" as part of "RRC Connection Reconfiguration", which will be used during handover. Where as AntennaInfoDedicated is used in RRC Connection Setup/Reconfiguration Msg, which commands the UE to use which exact Transmission Mode (1 to 8) is being used during this radio connection.