Current Tracking Area Identity

Description

The Current Tracking Area Identity (TAI) is a concept introduced in 3GPP Release 19 to address the complexities of satellite NG-RAN cells that broadcast multiple Tracking Area Codes (TACs) of the selected PLMN. The TAI is a fundamental identifier in cellular networks, composed of a Public Land Mobile Network (PLMN) ID and a Tracking Area Code (TAC). It defines a tracking area, which is a set of cells within which a UE can move without performing a Tracking Area Update (TAU). The current TAI is the specific TAI that the UE's Non-Access Stratum (NAS) layer selects from the broadcast information of the serving cell, and it is used for mobility management procedures.

In traditional terrestrial networks, a cell broadcasts a single TAC, so the current TAI is simply that TAC combined with the PLMN ID. However, in satellite NG-RAN cells, due to the wide coverage and movement of satellites, a cell may broadcast multiple TACs of the selected PLMN. This is because a satellite beam may cover multiple tracking areas on the ground, or the satellite's movement may cause the tracking area to change over time. To handle this, the UE NAS layer must select one TAI as the current TAI from the multiple TACs broadcast, as specified in subclause 4.23.5 of the relevant specifications. This selection process ensures that the UE has a consistent TAI for registration and paging purposes.

The current TAI is used by the UE to determine whether it needs to perform a Tracking Area Update when it moves to a new cell. If the new cell's TAI is not in the UE's list of registered TAIs, the UE initiates a TAU procedure to update its location with the core network. The current TAI is also used by the network to page the UE within the tracking area. In satellite networks, the selection of the current TAI is critical to avoid unnecessary TAUs and to ensure that paging is efficient. The UE NAS layer applies rules to select the current TAI, which may consider factors such as the satellite's ephemeris, the UE's location, and the broadcast TACs.

The introduction of the current TAI concept in Rel-19 reflects the growing importance of satellite access in 5G and beyond. Satellite NG-RAN cells are expected to complement terrestrial networks, providing coverage in remote areas, on ships, and in aircraft. The multiple TAC broadcast is a unique feature of satellite cells, and the current TAI selection ensures that the UE can operate seamlessly across both terrestrial and satellite networks. The specifications 24.301 and 24.501 define the NAS procedures for EPS and 5GS, respectively, and they include the details of how the current TAI is selected and used.

Key components of the current TAI include the PLMN ID, which identifies the network operator, and the TAC, which identifies the tracking area within that PLMN. The UE stores the current TAI and uses it in NAS messages such as ATTACH REQUEST, TRACKING AREA UPDATE REQUEST, and SERVICE REQUEST. The network also uses the TAI to manage the UE's location and to page the UE. In satellite networks, the current TAI may change as the satellite moves, and the UE must update its current TAI accordingly. The selection process is designed to minimize signaling and to ensure that the UE is always reachable.

The current TAI is a relatively new term, introduced in Rel-19, and it is expected to evolve as satellite networks become more prevalent. Its definition is precise and tied to the specific scenario of satellite NG-RAN cells broadcasting multiple TACs. For terrestrial networks, the current TAI is simply the TAI of the serving cell, and the term may not be explicitly used. However, with the integration of satellite and terrestrial networks, the current TAI concept provides a unified way to handle location management across different access types.

Purpose & Motivation

The current TAI concept was introduced to solve a specific problem in satellite NG-RAN cells: the broadcasting of multiple Tracking Area Codes (TACs) of the selected PLMN. In terrestrial networks, a cell broadcasts a single TAC, so the TAI is unambiguous. However, in satellite networks, a single cell may cover multiple tracking areas on the ground, or the satellite's movement may cause the tracking area to change. Broadcasting multiple TACs allows the network to indicate all possible tracking areas that the cell serves, but it creates ambiguity for the UE: which TAI should the UE consider as its current TAI for registration and paging?

Without a clear definition, the UE might select an arbitrary TAC, leading to inconsistent location registration and paging failures. The current TAI concept, defined in Rel-19, provides a standardized way for the UE NAS layer to select the current TAI from the multiple TACs broadcast. This ensures that the UE and the network have a consistent view of the UE's location, enabling efficient paging and mobility management. It also avoids unnecessary Tracking Area Updates when the UE moves within the same tracking area.

The introduction of this term reflects the evolution of 3GPP networks towards non-terrestrial networks (NTN). As satellite communication becomes integrated with 5G, new challenges arise that require updates to long-standing concepts. The current TAI is a prime example of how 3GPP adapts its definitions to accommodate new technologies while maintaining backward compatibility and interoperability. It is a small but crucial piece in the puzzle of seamless global connectivity.