e assigned to a UE are implementation dependent. An example will be shown in a later section. 15.2.3 Idle Mode Measurements The following topics, illustrated in Figure 15.1, are of some interest when troubleshooting and optimizing idle mode mobility: How often does UE measure the level and quality of detected neighbour cells? What is the delay to make a cell reselection decision? How often does UE search for new neighbour cells to be measured? In the beginning of the example in Figure 15.1 the UE is measuring only the serving cell since no other cells have been found by the cell search step. For the intra-frequency case the measurement interval is the same as the serving cell idle mode DRX cycle, to maximize UE sleep time. The search step is repeated periodically and if a new cell is found it is added to the list of measured cells (PCI22 in the example). Even though PCI22 is stronger than the serving cell, cell reselection cannot be triggered immediately. The reselection timer is started at the first instant when the neighbour cell ranks better than the serving cell. As long as the Inter-layer Mobility Optimization timer value is less than the broadcast value Treselection, reselection must not be triggered. The ranking criterion and the timer value are typically evaluated once every DRX cycle. It should be noted that the value of the Treselection timer is an integer number of seconds (0-7 seconds) while the idle mode DRX period in seconds is a non-integer. Therefore, reselection may not be triggered exactly at the instant of timer expiry but at the next DRX cycle after the timer expiry. The time from first measurement to reselection decision is called the evaluation time. The sum of search interval and evaluation time is called detection time. Search is initiated at cell reselection after reading the new neighbour information from BCCH. The UE idle mode measurement requirements are specified in terms of detection time, measurement interval and evaluation time; the search interval is not directly specified except 