 in networks and devices; the pace of evolution will obviously depend on the specific market. The focus in this chapter is on inter-layer mobility of a classical cellular network without small cells. Mobility in heterogeneous networks is beyond the scope of this chapter. Regarding terminology, the term 'layer' is defined by the combination of RAT and carrier frequency. For example, LTE2600, LTE2100, UMTS2100 and GSM900 are all considered as different layers. 15.2 Inter-layer Idle Mode Mobility and Measurements The purpose of this section is to elaborate on the rather rich details of the idle mode cell selection/reselection and measurement process in LTE. In contrast to standard references the focus of this section is on the details of the UE measurement process, with some examples of how commercial LTE UEs implement the sometimes mystifying 3GPP idle mode measurement requirements. 15.2.1 Initial Cell Selection and Minimum Criteria for UE to Camp on a Cell When the UE is switched on for the first time, it performs the public land mobile network (PLMN) and cell selection procedure, which involves sweeping UE-supported frequency bands for received total power and searching the strongest carrier frequencies for cells in non-forbidden PLMNs. The selection of PLMN can be automatic or manual. At subsequent PLMN selections, pre-stored frequency and RAT information can be used to accelerate the procedure. The order in which to search different RATs is not specified by 3GPP and is UE implementation specific, unless specific RAT priorities are encoded in universal subscriber identity module (USIM) for the PLMN [1]. The UE also enters the PLMN and cell selection procedure in order to recover from loss of coverage. Loss of coverage is defined as a two-step procedure. In the first step UE searches for cells on all known frequencies received in system information. This first search step is triggered if the serving cell does not fulfil the cell camping criteria for Nserv idle mode discontinuous reception (DRX) cycles, where Nser