In GSM, mobiles are tracked

In GSM, mobiles are tracked in Location Areas and Routeing Areas. In UMTS, mobiles can be tracked in UTRAN Registration Areas as well as Las and Ras. whereas in LTE Tracking Area is used as a generic name for LA, RA and URA.

In LTE network location of a UE in IDLE state is known by the network on a Tracking Area granularity.

When paging the UE, the EPC indicates a 'geographical area' that is translated within E-UTRAN to the actual cells that shall be paged. A 'geographical area' could be a tracking area or a list of tracking areas.

The MME initiates the paging procedure by sending the PAGING message to each eNB with cells belonging to the tracking area(s) in which the UE is registered. Each eNB can contain cells belonging to different tracking areas, whereas each cell can only belong to one TA.

Regards
Pankaj