Mobility Management and UE States
IDLE State Mobility Management
The initial entry procedure, described in Chapter 11, details PLMN selection and the cell selection steps required. Based on interactions between the NAS and the AS, the UE is able to identify the PLMN, and search the E-UTRA's frequency band to identify—for each carrier frequency—the strongest cell. The objective is to first a suitable cell for the UE to camp. If no suitable cells are available, the UE tries to find an acceptable cell—one through which it is able to initiate emergency calls, and receive ETWS and CMAS messages. Coming from RRC-CONNECTED, the UE would either camp on the cell through which it was connected, or the cell indicated by the RRC in the state transition.
Once in the RRC-IDLE, the UE continues reselection. UE makes measurements of the serving and neighboring cells to enable to the reselection process. If the UE is either “camped normally” or “camped on any cell”, it detects, synchronizes and monitors intra-frequency, inter-frequency and inter-RAT cells indicated by the serving cell. The network need not support this measurement activity, that is, through explicitly providing a list of neighboring cells and their frequencies and bandwidth information.
Intra-frequency reselection is based on ranking of cells in terms of the average signal quality. Inter-frequency reselection is based on frequency priorities set by the PLMN. The serving cell can provide a Neighboring Cell List (NCL) for intra- ...