Normally LTE UE makes radio measurement in two stages which are idle and connected states. In idle state mode the measurement is used for cell selection and cell re-selection. In connected state mode, the measurement is used for handover and redirection scenarios, of course here we mean only mobility cases. The measurement (RSRP, RSRQ, SINR etc.) can also be used for other calculations such as eNodeB resource allocation.
UE receives idle related measurement parameters via SIB messages from eNodeB, but for connected related measurement parameters are send to UE through RRC Connection Reconfiguration messages by eNodeB.
In LTE connected mode, to start measurement and make a handover decision is done by defined actions which is called “Event”. There are several types of events in LTE: A1, A2, A3, A4, A5, B1, B2. General overview was described for each of event on the following table, for more details need to check these links:
• A1, A2 for Inter-Frequency (these events are used for start or stop measurement)
• A1, A2 for Inter-Technology (these events are used for start or stop measurement)
• A3 for Intra-Frequency and Inter-Frequency
• A4 for Inter-Frequency
• A5 for Inter-Frequency
• B1 for Inter-Technology (Inter-RAT)
• B2 for Inter-Technology (Inter-RAT)