 must always be higher than the minimum threshold (this does not apply to UTRA TDD). It is not allowed to have the same priority for different RATs [3]. However, different frequency layers within a RAT can have the same priority. The quality-based cell reselection to UTRA TDD, GSM EDGE radio access network (GERAN) and CDMA2000 is not supported. Table 15.2 summarizes the reselection criteria for LTE, UTRA FDD and GERAN. In the table the minimum level (qRxLevMin) is based on either RSRP, RSCP or RSSI for LTE, UTRA FDD and GERAN, respectively. Similarly, qRxQualMin means either RSRQ or Ec/NO, depending on the measured system. The underlying idea of the absolute priority-based cell reselection is that the UE cannot camp on a layer that does not fulfil a minimum level requirement. Moreover, from all the layers satisfying the minimum level requirement the UE camps on the one with the highest priority. This makes planning of mobility thresholds intuitive and simple, which is especially useful since there may be several different RATs, carrier frequencies and bandwidths in the network. A mandatory criterion for exiting to a lower priority layer (level/quality threshold low) becomes hence the minimum required performance on the given layer, rather than the relative level/quality with respect to other layers, as with pre-Rel8 cell reselection. LTE Small Cell Optimization Search interval Treselection Trigger reselection Measurement Evaluation time Serving interval Detects PCI22 PCI22 Figure 15.1 Illustration of cell search, measurement and reselection process Besides broadcasting the serving and neighbour cell priorities, it is also possible to assign dedicated priorities for a UE in RRC connection release. Such mechanism is useful for load balancing purposes, for example. UE uses the dedicated layer priorities in cell reselection until either a new RRC connection is set up or the LTE timer T320 expires. The dedicated priorities can also include neighbour layers that are not broadcast on BCCH. When and what priorities can b