28 (2) 5.12 (8) 1.28 (1) 1.28 (1) 6.4 (5) 2.56 (1) 2.56 (1) 7.68 (3) LTE Small Cell Optimization Table 15.4 Maximum time to determine if a measured cell satisfies cell reselection criterion Evaluation time for LTE intra-/inter-frequency Evaluation time for UTRA LTE idle mode DRX cycle neighbour cells, seconds FDD cells, seconds (DRX length, seconds (DRX cycles) cycles) 5.12 (16) 15.36 (48) 5.12 (8) 15.36 (24) 6.4 (5) 19.2 (15) 7.68 (3) 23.04 (9) What Is the Delay to Make a Cell Reselection Decision Once a Cell Becomes Suitable? If a non-serving cell in the measurement list becomes eligible for reselection the UE will have to be able to make the cell reselection decision within a time window defined in 3GPP [2,3]. The reselection decision should not be made too quickly but on the other hand it should not be delayed excessively either. The UE is not allowed to make cell reselection unless the candidate cell has fulfilled the reselection criteria for at least T reselection seconds, where reselection serving cell broadcast parameter that may be different for every RAT. It is also required that the minimum time between cell reselections is defined as 1 second even if T reselection and hence the 3GPP standard prohibits making two reselections within a window of less than second. On the other hand, assuming T reselection = 0, the maximum delay to trigger a cell reselection to a suitable cell must not be more than the value given in Table 15.4 multiplied by the number of layers. Comparing Tables 15.3 and 15.4 it can be deduced that for intra-LTE reselection the worst- case decision delay is five measurement intervals while for UTRA FDD measurements it is three measurement intervals. What Is the Total Time to Trigger Reselection to a Cell That Satisfies Cell Reselection Criteria? The UE can only measure RSRP and RSRQ of cells whose physical cell identity (PCI) and frame timing relative to serving cell are known. This is since the location of the reference signals 1 in time and frequency depends on the subframe boundaries,