ency and inter-RAT mobility, other factors should also be supported-most © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM UE Switched ON (recovery from lack of coverage) User (manual) information PLMN and RAT Information Selection in USIM (e.g., last registered) (Initial) Cell Selection Registration Required? Measurements Registration Procedure (NAS) Cell Reselection New Cell Selected? Figure 4.19 A schematic view of the UE behavior in LTE. importantly, idle mode load balancing involving network control of idle mode UE distribution. Such load balancing has been motivated by two factors. First, the UE in idle mode does consume radio resources in the form of control plane signaling when responding to paging and when sending tracking area update messages to the network [14]. Second, some services (notably high bit rate point-to-point as well as multicast and broadcast services) can only be provided on LTE, which means that only UEs camping on LTE have immediate access to such services (i.e., without having to perform a handover). Accordingly, the UE behavior is designed to meet the most important idle mobility drivers that are identified in 3GPP TS 36.300 [14]. When the UE is pow- ered on or upon recovery from lack of coverage, the UE needs to select an appropri- ate PLMN and RAT. The main input to PLMN and RAT selection algorithms is provided manually by the human user (e.g., preferred RAT) or fetched from the user subscription identity module (USIM) (e.g., the most recently used PLMN). At cell selection, the UE searches for a suitable cell of the selected PLMN and RAT and chooses that cell to (initially) obtain available services by tuning to its © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology control channels. (We say that the UE camps on that cell.) According to 3GPP TS 36.304 [15], the UE will now, if necessary, register its presence in the tracking area of the chosen cell and RAT and, as an outcome of a successful location registr