E-SMLC

Enhanced Serving Mobile Location Centre

Core Network →
Introduced in Rel-9 Also in: Services, Core Network

E-SMLC is the core network entity in LTE and 5G networks that determines a UE's geographic location by managing positioning procedures and calculating location estimates for services like emergency calls.

Category
Core Network
Introduced
Rel-9
Where
Radio Access Network › NG-RAN (5G)
Also touches
2 segments
Specifications
22 specs
E-SMLC Description Purpose Related Classification Detected Changes Specifications

Description

The Enhanced Serving Mobile Location Centre (E-SMLC) is a key functional element within the Evolved Packet Core (EPC) and 5G Core (5GC) architecture, dedicated to control-plane positioning. It is the central node that orchestrates the positioning process for a target UE. The E-SMLC interfaces with the Mobility Management Entity (MME) in EPC or the Access and Mobility Management Function (AMF) in 5GC via the SLs interface (for LTE) or NLs interface (for 5G NR). Upon receiving a location request, the E-SMLC selects the appropriate positioning method based on factors like required accuracy, UE capabilities, and network conditions.

The E-SMLC supports multiple positioning techniques. For LTE, these include Observed Time Difference of Arrival (OTDOA), which uses reference signals from multiple eNodeBs; Assisted Global Navigation Satellite System (A-GNSS), which provides aiding data to the UE's GNSS receiver; Enhanced Cell ID (E-CID), which uses timing advance and signal measurements; and Uplink Time Difference of Arrival (UTDOA). For 5G NR, it supports similar methods like DL-TDOA (Downlink TDOA) and Multi-RTT (Round Trip Time). The E-SMLC communicates with the UE via the LPP (LTE Positioning Protocol) or its NR counterpart, NRPPa, to obtain measurements or deliver assistance data. It also communicates with base stations (eNodeBs/gNBs) and, if needed, a Secure User Plane Location (SUPL) Enabled Location Server (SLP) via the LPPa protocol to gather cell information or configure positioning reference signals.

Architecturally, the E-SMLC contains the positioning calculation function. It processes raw measurement data (e.g., time differences, signal strengths) received from the UE and/or the network to compute a latitude, longitude, and uncertainty estimate. The final location result is then delivered to the requesting entity, such as a Gateway Mobile Location Centre (GMLC) for external applications or the MME/AMF for emergency call handling. Its operation is defined in detail across 3GPP specifications covering service requirements (23.271), stage 2 procedures (36.305), and stage 3 protocols (36.355, 37.355).

Purpose & Motivation

The E-SMLC was introduced in 3GPP Release 9 to meet stringent regulatory requirements for emergency caller location, such as the E911 mandate in the United States and the E112 directive in Europe. Prior location methods in 2G/3G networks, often relying on Cell-ID or less standardized control-plane approaches, lacked the accuracy and reliability needed for rapidly locating individuals in distress. The E-SMLC provided a standardized, network-based control-plane architecture within the all-IP LTE network, ensuring reliable location determination independent of the UE's IP connectivity status.

Beyond emergency services, the creation of the E-SMLC enabled a wide array of commercial and operational location-based services (LBS). It allows mobile network operators to offer precise positioning to third-party application providers, support asset tracking, and enable network optimization tasks like coverage analysis. The E-SMLC addressed the limitations of user-plane solutions like SUPL, which required an IP connection and active UE participation, by providing a network-controlled alternative that could function even during limited-service states. Its design ensured scalability and support for multiple, high-accuracy positioning methods to cater to diverse use case requirements.

Classification

Part ofGMLC
Related approachesLPPOTDOAA-GNSSMME

Detected Changes Across Releases

from 3GPP Change Requests

Specific changes extracted from the „Change history“ tables of 3GPP specifications (115 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.

Studied in Rel-9, normative work from Rel-15.

Rel-15 11 changes

In Release 15, the E-SMLC's capabilities were expanded to support new positioning methods and technologies, including the introduction of IMU support for OTDOA and the support of OTDOA for NB-IoT. The enhancements also provided for higher accuracy location estimates and extended OTDOA assistance data to include new parameters like TDD UL/DL configuration and SFN offset for NR (New Radio). Furthermore, the release improved the granularity of location reporting by distinguishing the location source when sensor methods are used and by adding the PSCell to the User Location Information.

  • Introduction of IMU support for OTDOA TS 36.355CR0204
  • Support of Enhanced VoLTE Performance TS 36.413CR1547
  • Support of OTDOA in NB-IoT enhancement TS 36.455CR0093
  • Addition of High Accuracy location estimates TS 29.171CR0051
  • OTDOA Assistance Data Request for NR TS 36.355CR0222
  • Addition of TDD UL/DL configuration to OTDOA assistance data TS 36.355CR0213

+ 5 more changes

Rel-16 33 changes

In Release 16, the E-SMLC's enhancements included support for the BDS B1C signal for A-GNSS and the introduction of procedures for handling concurrent location requests. The release also completed the Unified Location Service Exposure Procedure and clarified various operational aspects, such as procedures for 5GC-MT-LR and periodic or triggered location reporting with exception support.

  • Support of Concurrent Location Request TS 23.273CR0006
  • CR for TS36.171, Introduction of BDS B1C in A-GNSS TS 36.171CR0020
  • Introduction of B1C signal in BDS system in A-GNSS TS 36.305CR0083
  • Introduction of B1C signal in BDS system in A-GNSS TS 37.355CR0248
  • Introduction of B1C signal in BDS system in A-GNSS TS 38.305CR0013
  • Correction and Improvement of Low Power Periodic and Triggered Location TS 23.273CR0011

+ 27 more changes

Rel-17 20 changes

In Release 17, the E-SMLC saw enhancements including the introduction of a Scheduled Location Time for deferred requests, support for location estimates in Local Coordinates and high-accuracy GNSS with NMEA data, and the addition of GNSS integrity requirements in location requests. It also gained the ability to handle Multiple QoS classes for deferred requests and to support location services for SNPNs. Furthermore, new procedures were added for unified location service exposure to Network Functions within the serving PLMN.

  • Add definition and usage for 5G positioning area and 5G enhanced positioning area TS 23.273CR0166
  • Introduction of the Scheduled Location Time TS 23.273CR0187
  • Addition of a Scheduled Location Time TS 23.273CR0151
  • CR to 36.401: Baseline CR for introducing Rel-17 Enhanced eNB Architecture Evolution TS 36.401CR0090
  • NMEA GGA sentence info in high accuracy GNSS location estimates [HA-GNSS-NMEA] TS 37.355CR0349
  • Location estimate in Local Coordinates TS 23.273CR0144

+ 14 more changes

Rel-18 38 changes

In Release 18, the E-SMLC's capabilities were enhanced to support new scenarios including Mobile Base Station Relay (MBSR) for Location Service and the introduction of network verification of UE location. The release also added functionality for more granular triggered location reporting, allowing reporting only when the UE is in a target area defined with finer granularity, and introduced support for sub-1-second location information reporting periodicity via LPP. Furthermore, it expanded location service continuity between EPS and 5GS for procedures like deferred MT-LR.

  • Multiple location report for MT-LR Immediate Location Request for the regulatory service TS 23.273CR0237
  • Support of Mobile Base Station Relay for Location Service TS 23.273CR0240
  • UE location reporting only when the UE locates in the target area defined with finer granularity TS 23.273CR0242
  • Privacy Check for NWDAF requesting UE location from GMLC TS 23.273CR0249
  • Introduce new feature: local LMF and GMLC selection TS 23.273CR0254
  • Verification of location for NTN access TS 23.273CR0258

+ 32 more changes

Rel-19 13 changes

In Release 19, the E-SMLC's capabilities were expanded to support the deferred 5GC-MT-LR procedure for periodic location events using NRPPa periodic measurement reports and to enable multiple location reports for next-generation emergency routing. Furthermore, A-GNSS support was enhanced through the introduction of new signals, specifically the BDS B2b signal and the NavIC L1 SPS signal, across LTE, NR, and LPP specifications. The release also clarified procedures for location services involving WAB-Nodes and MWAB-gNBs.

  • Adding Deferred 5GC-MT-LR Procedure for Periodic Location Events based NRPPa Periodic Measurement Reports TS 23.273CR0589
  • Multiple Location Report for Next Generation Emergency Routing TS 23.273CR0608
  • CR for TS 36.171 to introduce BDS B2b signal in A-GNSS TS 36.171CR0032
  • Introduction of NavIC L1 SPS A-GNSS in LTE Stage 2 specification TS 36.305CR0120
  • Introduction of BDS B2b in A-GNSS TS 36.305CR0121
  • Introduction of NavIC L1 SPS A-GNSS in LPP TS 37.355CR0532

+ 7 more changes

Explore further

Broader topics and technologies where E-SMLC plays a role.

Topics
Authentication (5G-AKA, EAP)EPC (Evolved Packet Core)Positioning & LocationLTE / LTE-AdvancedEmergency Services5G NR (New Radio)
Technologies
LTE5G

Defining Specifications

3GPP specifications that define or reference E-SMLC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.

SpecificationTitleRelease
TR 21.905 vj00 3GPP Technical Terms and Definitions Rel-19
TS 23.271 vj00 LCS Stage 2 Specification Rel-19
TS 23.273 vj50 5G Location Services Stage 2 Architecture Rel-19
TR 23.730 ve00 Study on extended CIoT architecture Rel-14
TS 29.171 vj00 LCS Application Protocol (LCS-AP) Specification Rel-19
TS 36.111 vj00 LMU Requirements for UTDOA Positioning Rel-19
TS 36.112 vj00 E-UTRAN LMU Conformance Requirements Rel-19
TS 36.171 vj10 A-GNSS Minimum Performance Requirements for UE Rel-19
TS 36.214 vj00 E-UTRA Physical Layer Measurements Rel-19
TS 36.305 vj00 UE Positioning in E-UTRAN Stage 2 Rel-19
TS 36.355 vj00 LTE Positioning Protocol (LPP) Rel-19
TS 36.401 vj00 E-UTRAN Overall Architecture Description Rel-19
TS 36.413 vj10 S1 Application Protocol (S1AP) Rel-19
TS 36.455 vj00 LTE Positioning Protocol Annex (LPPa) Rel-19
TS 36.456 vj00 SLm Interface Introduction Rel-19
TS 36.458 vj00 SLm Interface Signalling Transport Rel-19
TS 36.809 vc00 Study on RF Pattern Matching for LTE Positioning Rel-12
TS 36.855 vd00 E-UTRA Positioning Enhancements Study Rel-13
TS 36.896 ve00 Study on Flexible eNB-ID and Cell-ID in E-UTRAN Rel-14
TS 37.355 vj20 LTE Positioning Protocol (LPP) Rel-19
TS 38.305 vj00 NG-RAN UE Positioning Stage 2 Rel-19
TR 38.889 vg00 NR-based access to unlicensed spectrum study Rel-16