for the higher layer search in good radio conditions (i.e. at most 60-second interval per higher priority layer). Nonetheless, it is convenient to think of the reselection process executed by the UE as a continuous cycle of search, measurement and evaluation steps. How Often Does UE Measure Neighbour Cells? Once the search procedure has produced a list of neighbour cells, the UE starts the collection of level and quality measurements of these cells. The measurement interval depends on the idle mode DRX cycle, the RAT being measured and the number of inter-frequency layers within the RAT. A summary of minimum requirements is given in Table 15.3. From the table it can be seen that the serving cell must be measured at least once every DRX cycle, while neighbour cell measurements have more relaxed requirements. Specifically, other RATs need to be measured less frequently than LTE layers. The time requirements in the two right-most columns in Table 15.3 for non-serving cells are per layer. In other words, if there are two layers to measure the interval between layers measurement would be twice the value given in the table. Example: Assuming that DRX cycle is 0.64 seconds and that there are two LTE inter- frequency layers and one 3G layer to measure, UE would measure one LTE inter-frequency layer once every 1.28 seconds and the 3G layer once every 5.12 seconds. Since there are two different LTE frequency layers, the time between measurements of cells on one frequency would be 2.56 seconds. These are the minimum requirements that every compliant UE in the market should satisfy. Table 15.3 Time period per layer to measure cells that have already been found in the search procedure, minimum requirement from 3GPP TS 36.133 Measurement interval for LTE intra-/inter- Measurement interval Measurement interval frequency neighbour for GERAN and UTRA LTE idle mode DRX for LTE serving cell, cells, seconds (DRX FDD cells, seconds cycle length, seconds seconds (DRX cycles) cycles) (DRX cycles) 0.32 (1) 1.28 (4) 5.12 (16) 0.64 (1) 1.