There exist two cell selection procedures:
Initial Cell SelectionThe UE has to find a suitable cell of the PLMN, which was selected by the NAS. To do so, the mobile phone scans all radio frequency carriers of UTRA. Hereby, the UE focuses its cell search to the strongest cell on each carriers. As soon as the mobile phone has found a suitable cell, it selects it. In contrast to the cell selection procedure „stored information cell selection“, this procedure does not require knowledge in advance, which of the scanned radio frequency bands holds UTRA carriers.
Stored Information Cell SelectionTo speed up the cell selection process – for instance, when the UE is switched on again – information about UTRA carriers, even cell parameters such as cell scrambling codes can be stored in the UE. The UE uses this information to find a suitable cell of the PLMN, which was selected by the NAS. If the cell selection based on stored information in the UE fails – e.g. the selected PLMN cannot be found – the UE continues the cell selection process based on the Initial Cell Selection procedure.
Both for Initial Cell Selection and Stored Information Cell Selection, a cell is only suitable for the UE to camp on, if it fulfils the cell selection criterion S:
Srxlev > 0 AND Squal > 0 in the FDD mode
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