ction registration PLMN selected PLMN available Registration area change Cell selection and reselection Radio measurements Figure 7.19 Idle mode overview. candidate for reselection according to the reselection criteria (Section 7.7.4.1), it rese- lects onto that cell and camps on it and again checks if the cell is suitable for camping. If the cell where the UE camps in RRC_IDLE does not belong to at least one tracking area to which UE is registered, location registration needs to be performed. The overview is shown in Figure 7.19. Similarly, if the cell where the UE camps in RRC_INACTIVE does not belong to the RAN-based notification area to which the UE is registered, a RAN area update needs to be performed. By performing location registration procedures, the network can limit paging to a smaller area and avoiding paging overload. 7.7.4.1 Cell Selection and Reselection Process When the UE is first powered on, it will initiate the Initial Cell Selection procedure. The UE will scan all RF channels according to its capabilities to find a suitable cell. On each carrier frequency, the UE needs only search for the strongest cell. Once a suitable cell is found, this cell is selected. Initial cell selection is used to ensure that the UE gets into service (or back to the service area) as fast as possible. The UE may also have stored information about the available carrier frequencies and cells in the neighborhood. This information may be based on the SI or any other infor- mation the UE has acquired in the past - 3GPP specifications do not exactly define what kind of information the UE is required or allowed to use for SI cell selection. If the UE does not find a suitable cell using the stored information, the Initial Cell Selection procedure shall be started in order to ensure that a suitable cell is found. For the cell to be suitable, it has to fulfill the S-criterion: where: Srxlev = Qrxlevmeas - (Qrxlevmin + Qrxlevminoffset) - Pocpmensation - Qoffsettemp Squal = Qqualmeas - (Qqualmin + Qqualminoffset - Qoffsettemp Qrx