Last Known Location

Description

The Last Known Location (LKL) is a concept in 3GPP location services (LCS) that refers to the most recent location estimate of a target User Equipment (UE) along with the time at which that estimate was obtained. It is stored in the LCS Server (e.g., GMLC, E-SMLC, or SMLC) and is used to provide location information to requesting applications when a current location fix is not necessary or cannot be obtained. The LKL is updated whenever a new location estimate is computed for the UE, either through periodic reporting or on-demand positioning procedures.

In the 3GPP architecture, location services are provided by the LCS Server, which may be a Gateway Mobile Location Centre (GMLC) in the core network or a Secure User Plane Location (SUPL) Location Platform (SLP). The LKL is typically stored in the GMLC or in the UE's home subscriber server (HSS) as part of the subscriber's location information. When an external LCS client requests the location of a UE, the GMLC can either initiate a new positioning procedure to obtain a fresh location or return the LKL if the request allows for a less accurate or older location. The LKL includes a timestamp, which indicates the age of the location estimate, allowing the client to assess its validity.

The LKL is particularly useful in scenarios where the UE is not reachable, is in idle mode, or when the requested quality of service (QoS) does not require a real-time fix. For example, in emergency calls, the LKL can provide an initial location estimate while a more accurate position is being determined. In location-based services (LBS), such as weather or traffic updates, the LKL may suffice. The LKL is also used in network optimization and analytics, where historical location data is valuable.

The process of maintaining the LKL involves several network elements. The UE may report its location periodically or upon request. The Radio Access Network (RAN) may provide cell ID or timing advance information. The LCS Server computes the location estimate using positioning methods such as Assisted GPS (A-GPS), Observed Time Difference of Arrival (OTDOA), or Enhanced Cell ID (E-CID). Once computed, the estimate and timestamp are stored as the LKL. The LKL is associated with the target UE's identity (e.g., IMSI, MSISDN) and can be retrieved by authorized LCS clients.

Security and privacy are critical aspects of LKL management. Access to the LKL is controlled by the LCS privacy profile, which defines which clients can obtain location information and under what conditions. The LKL is subject to the same privacy requirements as real-time location data. Additionally, the LKL may be purged after a certain period to comply with data retention policies. In summary, the Last Known Location is a fundamental concept in 3GPP location services, providing a balance between accuracy, latency, and resource efficiency.

Purpose & Motivation

The Last Known Location concept was introduced to address the need for location information in scenarios where a real-time location fix is either not possible or not required. In early 3GPP releases (R99), location services were primarily designed for emergency calls, where obtaining a current location is critical. However, as location-based services expanded, it became clear that many applications could tolerate a slightly stale location estimate, especially if it reduced signaling load and latency. The LKL provides a cached location that can be used immediately, improving response times and reducing network resource consumption.

The LKL solves the problem of location availability when the UE is unreachable or when the positioning procedure fails. For example, if a UE is in a coverage hole or has its radio turned off, the network cannot obtain a fresh location. In such cases, the LKL can still provide a useful estimate, albeit with a timestamp indicating its age. This is particularly important for emergency services, where any location information is better than none. The LKL also enables periodic location reporting without the need for continuous positioning, saving battery life and network resources.

Historically, the LKL was first defined in 3GPP TS 23.171 for R99, primarily for emergency call location. Over time, it was incorporated into TS 23.271 for general location services and later into TS 23.273 for 5G location services. The definition has remained largely consistent, with minor updates to reflect the transition from 2G/3G to LTE and 5G. The LKL continues to be a valuable tool for balancing accuracy, latency, and resource efficiency in location-based applications.