In this procedure, the UE informs the network about the new TA it has entered. This difference triggers the UE to perform a Registration update procedure toward the network. For example, when a UE that was previously assigned a RA with TAs 1 and 2 moves into a cell that is broadcasting TA 3, the UE will notice that the broadcast information includes a different TA than those it has previously stored as part of the RA. If the broadcasted Tracking Area is not part of the assigned RA the UE starts a procedure – called a Registration procedure – toward the network to inform it that it is now in a different location. The gNB/ng-eNB broadcast TA identity in each cell and the UE compares this information with the one or more TA it has previously stored as part of the assigned RA. RA is used as a base for the network to search for the UE and for the UE to report its location. In order to create efficiencies, therefore, cells are grouped together into Tracking Areas (TA), and one or more Tracking Areas may be assigned to the UE as a Registration Area (RA). It is not practical to keep track of a UE in idle mode every time it moves between different cells due to the amount of signaling it would cause, nor to search for a UE across the entire network for every terminating event (e.g. Radio networks are built by cells that range in size from tens and hundreds of meters to tens of kilometers as discussed in Chapter 3 and the UE updates the network about its location on a regular basis. Idle-mode Mobility Management for 5GS using NR and E-UTRA is built on similar concepts to LTE/E-UTRAN (EPS), GSM/WCDMA, and CDMA. Catherine Mulligan, in 5G Core Networks, 2020 7.2.2 Registration and Mobility

No suitable cells in tracking area