Location Information Request

Description

A Location Information Request (LIR) is a core procedural element within 3GPP architectures for obtaining the position of a mobile device. It is not a single protocol message but a conceptual service request that is realized through specific protocols and interfaces, such as the LTE Positioning Protocol (LPP) over the SLs interface, or the OMA SUPL protocol, or via core network interfaces like the Gateway Mobile Location Centre (GMLC) using the Le or SLg interfaces. An LIR is initiated by a Location Service (LCS) Client, which could be an external application (e.g., a mapping service), an internal network service (e.g., for lawful interception), or the UE itself. The request contains parameters such as the target UE's identifier (MSISDN, IMSI), the required Quality of Service (QoS) for the location (e.g., accuracy, response time), and the type of location information needed (e.g., geographic coordinates, civic address).

Upon receiving an LIR, the network's location system engages the appropriate positioning methods. For network-based positioning, the network may use measurements from base stations (e.g., Observed Time Difference of Arrival - OTDOA in LTE, Downlink Time Difference of Arrival - DL-TDOA in 5G). For UE-based positioning, the network provides assistance data (e.g., satellite ephemeris for A-GNSS) to the UE, which then computes its own fix and may report it back. Hybrid methods combine both. The control and coordination of this process are managed by network entities like the Evolved Serving Mobile Location Centre (E-SMLC) in LTE or the Location Management Function (LMF) in 5G. These entities select the positioning method based on the LIR's QoS, UE capabilities, and available network measurements.

The result of processing an LIR is a Location Information Response, which delivers the estimated location data to the requesting client. The entire procedure involves multiple network domains: the core network for client authorization and request routing, the radio access network for measurement collection, and potentially the UE. Privacy is a critical aspect; the network must verify that the LCS Client is authorized to request the location of the specific target UE, based on subscriber privacy settings and regulatory rules. The LIR mechanism is thus the triggering event for a complex, standardized sequence that transforms radio signals or satellite data into a usable geographic location for services.

Purpose & Motivation

The concept of the Location Information Request emerged to standardize how applications and services could programmatically obtain a mobile subscriber's location. Before its standardization, location services were proprietary and fragmented, hindering the development of widespread location-based applications. The primary problem it solves is providing a unified, secure, and reliable interface for authorized entities to request UE location, which is essential for commercial services (friend-finder, location-based advertising), emergency services (E-911/112), and network optimization.

Historically, the need was driven by regulatory mandates for emergency caller location and the commercial potential of location-based services. The LIR framework, evolving since 3GPP Release 99, created a clear architectural separation between the entity requesting location (the LCS Client) and the network's positioning capabilities. This addressed the limitations of ad-hoc solutions by defining standardized protocols (e.g., LPP, SUPL), interfaces (e.g., Le, SLg), and privacy control mechanisms. It enabled a ecosystem where network operators could expose location capabilities to third-party application providers in a controlled manner, fueling innovation in mobile services while ensuring subscriber privacy and network security.