Last Visited Registered Tracking Area Identity

Description

The Last Visited Registered TAI (Last Visited Registered Tracking Area Identity) is a mobility management parameter that identifies the tracking area (TA) most recently visited by a User Equipment (UE) within its registered tracking area list. A TAI uniquely identifies a tracking area within a Public Land Mobile Network (PLMN) and consists of a PLMN ID and a Tracking Area Code (TAC). The Last Visited Registered TAI is stored in the UE and in the network (e.g., in the MME for LTE or AMF for 5G) and is used to optimize paging and mobility procedures.

In cellular networks, the tracking area is a set of cells that are grouped together for paging and location management. When a UE registers with the network, it receives a list of TAIs (the registration area) where it can move without performing a tracking area update (TAU). The UE stores the TAI of the last tracking area it visited, which is the Last Visited Registered TAI. This information is used by the network to determine the UE's approximate location for paging. When downlink data arrives for the UE, the network pages the UE in the tracking areas listed in the registration area, but it can prioritize the Last Visited Registered TAI to reduce paging load and latency.

The Last Visited Registered TAI is particularly important in 5G networks, where the registration area can be large and dynamic. In satellite NG-RAN cells, which may broadcast multiple TAIs, the UE selects the current TAI from the broadcast list. The Last Visited Registered TAI then reflects the TAI last selected by the UE as the current TAI. This ensures that the network has an up-to-date record of the UE's location for paging and mobility management.

The parameter is maintained through various procedures. When the UE performs a TAU or registration update, it includes the Last Visited Registered TAI in the request if it is moving to a new TA. The network may update the registration area and provide a new list of TAIs. The UE stores the Last Visited Registered TAI in its non-volatile memory and in the USIM. The network stores it in the subscriber context in the MME/AMF. During handover or cell reselection, the UE updates the Last Visited Registered TAI if it enters a new tracking area.

The Last Visited Registered TAI helps in reducing paging signaling. Instead of paging the UE in all cells of the registration area, the network can first page in the Last Visited Registered TAI and then expand if necessary. This is especially beneficial in networks with large registration areas, such as those with satellite coverage. It also aids in mobility management by providing the network with a hint of the UE's location for subsequent procedures. In summary, the Last Visited Registered TAI is a key mobility management parameter that enhances paging efficiency and location tracking in 3GPP networks.

Purpose & Motivation

The Last Visited Registered TAI was introduced to improve paging efficiency and mobility management in cellular networks. In early networks (e.g., GSM), location areas were used, and the network had to page the UE across the entire location area, which could be large and lead to significant signaling overhead. With the introduction of tracking areas in GPRS and later in LTE, the concept of a registration area with multiple TAIs allowed for more granular location management. However, paging across all TAIs in the registration area could still be inefficient. The Last Visited Registered TAI provides a more precise location hint, enabling the network to page first in the most likely tracking area.

The parameter solves the problem of paging load in networks with large registration areas. In 5G, registration areas can be very large to reduce TAU signaling, especially for stationary or low-mobility UEs. However, large areas increase paging cost. By storing the Last Visited Registered TAI, the network can prioritize paging in that TA, reducing the number of cells that need to be paged and thus saving radio resources and reducing latency. This is particularly important for delay-sensitive services like voice over NR (VoNR) and mission-critical communications.

Historically, the concept was first defined in 3GPP Rel-8 for LTE (TS 24.301). It was later adapted for 5G in Rel-15 (TS 24.501). The definition has evolved to accommodate new scenarios, such as satellite NG-RAN cells broadcasting multiple TAIs, which was addressed in Rel-17. The Last Visited Registered TAI continues to be a fundamental parameter for mobility management in modern cellular networks.