/* for longer time than Treselection */ S < Thresh serv, low If serving layer drops below this threshold, AND if there is a lower-priority layer that Reselect to highest-ranked is above Thresh cell (which is of lower Reselect to highest-ranked cell reselect to that (lower or priority than current) (if there is any cell equal priority) cell. above Thresh x, low) Figure 4.21 A high-level view and the usage of the parameters for interfrequency and inter-RAT cell reselection. The left-hand side flow diagram illustrates the UE behavior when the UE camps on its highest-priority RAT and frequency layer. The right-hand side diagram shows the case when the UE camps on a RAT/frequency layer that is not the UE's highest-priority layer. already received a UE-specific priority list (by RRC or nonaccess stratum [NAS] signaling), then the UE ignores this parameter set. Figure 4.21 summarizes the usage of the interfrequency and inter-RAT system information broadcast parameters. When the UE is camping on its highest-priority cell, it periodically measures the S value (e.g., the RSRP) of the currently serving cell and compares this value with Thresh S value below thresh- serving, high old, the UE starts to look for interfrequency and inter-RAT cells. The cells whose measured S value is above Thresh x,low will be ranked (taking into account their associated priorities). When the serving cell's S value is below the Thresh serving, lowvalue, the UE reselects to the highest-ranked such cell. The CR procedure is a bit differ- ent when the UE is camping on a cell that is not its highest-priority cell. In this case, the UE constantly looks for availability for higher-priority cells by performing © 2009 by Taylor & Francis Group, LLC Architecture and Protocol Support for RRM inter-RAT and interfrequency measurements. If it finds any such (higher-priority) cell, it will reselect to that cell. In addition, similarly to the previous case, if the cur- rently serving cell drops below Thresh low' the UE reselects serving, lower-priority cell if