Hi,
Based on the DCI which is scheduled by ENB. There will be different DCI formats in LTE. If DCI is 2 or 2A then it is MIMO.
UE will get to know the transmission mode from rrc reconfiguration/rrc connection setup. So from transmission mode ue will know that can use MIMO or not
eNodeB will send uplink timing advance to UE periodically. So whether UE also has to send PRACH periodically? If not then how eNodeB will calculate the uplink timing advance without receiving PRACH periodically?