Description
A Location Measurement Unit (LMU) is a critical component in 3GPP networks that facilitates the determination of a User Equipment's (UE) geographical position for Location-Based Services (LCS). It functions by making precise radio measurements on uplink signals from the UE (UE-assisted mode) or downlink signals from base stations (NodeBs, eNBs, gNBs) depending on the positioning method. Architecturally, LMUs can be standalone units, integrated into base station sites, or implemented as software functions within network nodes. For Observed Time Difference of Arrival (OTDOA) in LTE and NR, LMUs measure the Reference Signal Time Difference (RSTD) between neighboring cells, providing timing data to the Evolved Serving Mobile Location Centre (E-SMLC) or Location Management Function (LMF). The E-SMLC/LMF then calculates the UE's position using multilateration algorithms. In UTRA (UMTS), LMUs support Uplink Time Difference of Arrival (U-TDOA) by measuring the arrival time of UE signals at multiple geographically dispersed units. Key components include high-precision timing receivers (often synchronized via GPS or network timing protocols), measurement processing units, and interfaces to the core LCS architecture (e.g., SLm interface in LTE). LMUs ensure measurement accuracy and reliability, which are paramount for emergency services (E911), commercial location services, and network optimization. Their operation is coordinated by the network's positioning server, which instructs them on what to measure and when, based on service requests.
Purpose & Motivation
The LMU was created to meet regulatory and commercial demands for accurate mobile device positioning. Initial driver was emergency services mandates (like E911 in the US) requiring network-based location determination even when GPS is unavailable in the handset. Prior to LMU-enhanced methods, basic Cell ID positioning offered poor accuracy (hundreds of meters to kilometers), insufficient for emergency response. LMUs enabled advanced network-based techniques like OTDOA and U-TDOA, which provide much finer accuracy (tens of meters) by utilizing timing measurements from multiple cell sites. This solved the limitation of relying solely on UE-based GPS, which has indoor coverage challenges and depends on device capability. Historically introduced in 3GPP Release 99, LMUs provided the infrastructure needed for standardized, network-assisted positioning across GSM, UMTS, LTE, and 5G NR. They addressed the need for a universal positioning solution that works independently of UE hardware, ensuring compliance with safety regulations and enabling a wide array of commercial location-based applications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (25 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the LMU function was enhanced to support new measurement capabilities, including UE carrier phase measurements and NPBCH-based NRSRP measurements for positioning. The release also introduced corrections and refinements to existing procedures, such as SFTD measurements, and expanded inter-RAT measurement support to include NR. Furthermore, it added mechanisms for location reporting with effective PSCell ID and revised UE measurement report definitions to accommodate NB-IoT devices.
- CR on 36.214 for UE carrier phase measurements TS 36.214CR0044
- Inter-RAT Measurements of NR TS 36.214CR0048
- Introduction of QoE Measurement Collection for LTE TS 36.300CR1073
- Introduction of QoE Measurement Collection for MTSI services TS 36.300CR1140
- Introduction of NPBCH based NRSRP measurement TS 36.214CR0052
- Correction on SFTD measurement in TS 36.214 TS 36.214CR0053
+ 7 more changes
In Release 16, the LMU function was enhanced to introduce RSRP measurement reporting based on the Received Signal Strength (RSS), alongside UE E-CID measurement reporting for improved location accuracy. The release also provided specific clarifications for triggering RSRP measurements for a number of cells, particularly to address the requirements for Unmanned Aerial Vehicles (UAVs). These updates built upon the existing LMU framework, which supports various positioning mechanisms like Time of Arrival (TOA) and Enhanced Observed Time Difference (E-OTD).
In Release 17, the LMU (Location Measurement Unit) function was enhanced with a correction on measurement reporting specifically for interference detection in Unmanned Aerial Vehicles (UAVs). This update refined the procedures for how LMUs report measurements, such as Time of Arrival (TOA) or Enhanced Observed Time Difference (E-OTD) data, to the Serving Mobile Location Center (SMLC) in scenarios involving UAVs. The change aimed to improve the accuracy and reliability of location estimates for aerial users by addressing unique interference challenges in those environments.
- Correction on measurement reporting for interference detection in UAV TS 36.300CR1373
In Release 18, the LMU function was enhanced to support new IoT and Non-Terrestrial Network (NTN) scenarios, including corrections for IoT NTN neighbor cell measurements and mobility between terrestrial and NTN cells. The release also introduced specific procedures for coarse UE location information reporting from the MME to the eNB for NB-IoT UEs. Furthermore, it provided corrections for UE location information reporting and location-based measurements specifically for IoT-NTN operations.
- CR on E-UTRAN measurement in IoT NTN TS 36.214CR0058
- IoT NTN measurement corrections TS 36.300CR1401
- Correction for IoT NTN neighbour cell measurements and TN to NTN mobility TS 36.300CR1405
- Correction on UE Location Information Reporting in IoT-NTN TS 36.300CR1410
- Correction on measurement TS 36.300CR1414
- Coarse UE Location Information Reporting from MME to eNB for NB-IoT UEs TS 36.300CR1415
+ 1 more changes
In Release 19, the LMU function was updated with corrections for neighbour cell measurement procedures specifically for Internet of Things (IoT) devices operating in Non-Terrestrial Networks (NTN) using Time Division Duplex (TDD). This enhancement ensures accurate synchronization and timing measurements, which are critical for positioning mechanisms like E-OTD and TOA that rely on LMU and base station coordination. The update refines the LMU's role in supporting mobile-assisted positioning within these new network architectures.
- Corrections on neighbour cell measurement for IOT NTN TDD TS 36.300CR1443
Explore further
Broader topics and technologies where LMU plays a role.
Defining Specifications
3GPP specifications that define or reference LMU, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 03.071 v7b0 | Location Services (LCS) Stage 2 Description | Rel-7 |
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.171 v1300 | LCS Stage 2 Specification for UMTS | Rel-4 |
| TS 23.271 vj00 | LCS Stage 2 Specification | Rel-19 |
| TS 25.111 vj00 | LMU RF Characteristics for UTRA FDD | Rel-19 |
| TS 25.305 vj00 | UTRAN UE Positioning Stage 2 | Rel-19 |
| TS 32.102 vj00 | Telecom Management Physical Architecture Framework | Rel-19 |
| TS 32.808 v1800 | Common User Profile Storage Framework | Rel-8 |
| TS 36.111 vj00 | LMU Requirements for UTDOA Positioning | Rel-19 |
| TS 36.112 vj00 | E-UTRAN LMU Conformance Requirements | Rel-19 |
| TS 36.214 vj00 | E-UTRA Physical Layer Measurements | Rel-19 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.305 vj00 | UE Positioning in E-UTRAN Stage 2 | Rel-19 |
| TS 36.456 vj00 | SLm Interface Introduction | Rel-19 |
| TS 36.459 vj00 | SLmAP for E-UTRAN Positioning | Rel-19 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |