Mobile Terminated Location Request

Description

Mobile Terminated Location Request (MT-LR) is a standardized procedure within the 3GPP Location Services (LCS) architecture. It allows an external LCS Client (e.g., an emergency call center, a logistics application, or a lawful interception entity) to request and receive the location of a target User Equipment (UE). The architecture involves several key entities: the LCS Client, the Gateway Mobile Location Center (GMLC), the Home Subscriber Server (HSS), the Mobility Management Entity (MME) in LTE, the Serving GPRS Support Node (SGSN) in UMTS, or the Access and Mobility Management Function (AMF) in 5GC, and the location determination functions within the Radio Access Network (RAN) and core network (e.g., Enhanced Serving Mobile Location Center - E-SMLC in LTE, Location Management Function - LMF in 5G). The procedure is initiated by the LCS Client sending a location request to the GMLC. The GMLC acts as the gateway, performing authorization and privacy checks based on the client's identity and the subscriber's privacy profile stored in the HSS. If authorized, the GMLC queries the HSS to find the serving node (MME/SGSN/AMF) for the target UE. The request is then forwarded to this serving node. The serving node, in coordination with the RAN and the specific location server (E-SMLC/LMF), triggers the location determination process. This can involve various positioning methods such as Observed Time Difference of Arrival (OTDOA), Uplink Time Difference of Arrival (UTDOA), Assisted GNSS (A-GNSS), or Enhanced Cell ID (E-CID). The calculated location estimate is then routed back through the chain (serving node -> GMLC) to the requesting LCS Client. The entire process adheres to strict privacy regulations, requiring subscriber consent (either pre-provisioned or real-time) unless overridden by legal authority (e.g., for emergency services). The MT-LR procedure is defined across multiple protocol layers and network generations, ensuring consistent service delivery from GSM to 5G.

Purpose & Motivation

MT-LR exists to provide a standardized, secure, and privacy-compliant mechanism for authorized external entities to locate mobile subscribers. It solves the problem of enabling valuable location-based services (e.g., navigation, friend-finder, asset tracking) and critical services (like Enhanced 911/E112 emergency location) without exposing subscriber location arbitrarily. Before its standardization, proprietary solutions or limited network capabilities made widespread, interoperable location services difficult. The creation of MT-LR was motivated by regulatory requirements (e.g., emergency caller location), commercial opportunities for Location-Based Services (LBS), and operational needs (e.g., fleet management). It addressed the limitations of earlier, non-standardized methods by defining clear architectural roles, interfaces (e.g., Le, Lg, SLg, NLs), and privacy control procedures. This allows network operators to offer location as a controlled service to third-party providers while protecting subscriber privacy. The procedure ensures that location retrieval can be performed regardless of the UE's state (idle or connected) and across different radio access technologies.