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Absolute priority based cell reselection

In the UTRAN, inter-frequency and inter-RAT reselections were based on the same ranking as inter frequency reselections. It was seen that it is very difficult for the network to control as the measurement quantities of different RAT are different and the network needs to be able to control reselection between multiple RAT. And as it was seen that operator would like to control how UE prioritizes camping on different RAT or frequencies of EUTRAN, a new method for reselection handling between different RATs/frequencies (called layers from now) was chosen. The method is known as absolute priority based reselection. Each layer is given a priority and based on this information the UE tries to camp on the highest priority frequency/RAT if it can provide decent service. In order for the UE to decide if decent service can be provided the network allocates each frequency/RAT a threshold Tthrs,high, which has to be fulfilled before reselection to such a layer is performed. A similar Treselection as in intra frequency reselection is utilized, i e, the new layer need to fulfill the threshold for consecutive time of Treselection before reselection is performed. This is used to eliminate reselections if just temporary fading for the evaluated frequency occurs. To make reselection to a lower priority layer the UE will not reselect to that if the higher priority layer is still above the threshold or if the lower priority frequency is not above another threshold Tthres,low.

GSM: Priority based cell reselection

WCDMA: Priority based cell reselection