To get service from the selected PLMN, the UE performs two types of  procedures: LTE cell selection and LTE cell reselection

CELL SELECTION in LTE

Upon PLMN selection, LTE UE uses “cell selection” for fast cell searching to camp on. To receive system information LTE UE tunes to the control channels. This procedure is known as “camping on the cell”.

The UE will then register its presence in the registration area of the chosen cell by NAS (Non Access Stratum) registration procedure. NAS registration  procedure means the LTE upper layer information is transmitted from UE to CN via AS (Access Stratum). The NAS offers the E-UMTS service to the users. The cell will be decided as suitable if it fulfils the cell selection criteria.

The purpose of camping on a cell is:

• To enable LTE UE reception of system information from the selected PLMN

• To allow LTE UE an RRC connection, accessing the network on the cell control
channel.

• To receive paging and respond to paging messages on a tuned control channel in the registration area. The PLMN knows the tracking area of the cell in which the registered UE is camped.

CELL RESELECTION in LTE

If the UE finds a “better” cell, UE reselects it and camps on it. After camping on, UE monitors the system information to get the quality threshold and performs
measurements for the cell reselection evaluation procedure. The UE evaluates
whether or not a better cell exists.

The E-UTRAN controls the quality measurements for LTE cells to be reselected. The UE measurements are triggered according to the serving cell quality level and the threshold indicated in the system information. The measurement must satisfy different requirements for intra frequency, inter frequency or inter RAT (Radio Access Technology) quality estimations.

Cell selection is performed in RRC idle mode. The camping on a cell in idle mode enables the UE to receive information from the LTE network. UE stays in idle mode until it transmits a request to establish an RRC connection. After receiving the RRC connection set up, the mode changes into connected mode.