TA and related basic concepts
TA：Tracking Area.TA is a newly established concept for the location management of the UE by the LTE system.
LA(Location Area; LAI = PLMN + LAC) is the concept of circuit domain in the era of 2G and 3G, which enables the mobile switch (MSC/SEVER) to know the location of the terminal in time. The mobile switching center does searching in all cells in the location area of the terminal when paging the terminal. The terminal does not need to update the location in a location area, while it needs to do so when moving across different location areas in order to let the network knows the location area of the terminal. The terminal needs to maintain a periodic location area update process in order to maintain close contact with the network side as well.
RA(Routing Area; RAI = PLMN+ LAC + RAC) is the concept of packet domain in the era of 2G and 3G, which enables the SGSN to know the location of the terminal in time. Before the terminal initiates data transmission, it must register with the SGSN and the HLR and page the terminal in the routing area. The terminal does not need to perform RA update in one RA; RA update will occur when moving across the routing area; and periodic RA update is required.
Tracking Area is a concept newly set up by the LTE/SAE system to manage the location of the UE. It is defined as an area in which UE doesn't need the service of updating and can move freely. Thr function of TA can be divided as paging management and location updating management, to accomplish managing the location of the UE. And EPC is informed the TA of UE through the Tracking Area Registration.
When UE is idle, MME is able to know the TA of the UE; when there is a paging for an idle UE, MME must paging in every cells of the Tracking Area that the UE is registered.
Tracking Area is a cell-level configuration. Multiple cells can be configured with a same TA and one cell belongs to one TA only.
TAI means Tracking Area Identity in LTE, and is combined with PLMN and TAC.
TAI = PLMN + TAC（Tracking Area Code）
Multiple TAs form a TA List which is allocated to a UE. To reduce frequent interaction to the wireless network, UE doesn't have to execute the TA Update when moving within a TA List. However, when UE comes into a new Tracking Area which is not on the TA List allocated, the TA Update needs to be executed: MME redistributing a TA List, in which there could be some TAs in the former list, to the UE.
The Influence of CSFB on TA
When CSFB is on, the location of UE needs to update simultaneously in both MME and MSC; thus a correspondence between the TA and LA needs to be guaranteed. Based on that, MME is unable to turn on the function of Intelligent TA List, otherwise, because no signaling would be transfered when UE moving within the TA List, the MSC would be unable to update the responded LA information, causing the voice pager message cannot be responded to in time.