Location Server

Description

The Location Server is a central architectural component in 3GPP location services (LCS). It is defined as a physical or logical entity that manages positioning for a target device—such as a UE—by obtaining measurements and other location information from one or more positioning units and providing assistance data to those units to help determine the location. The Location Server may also compute or verify the final location estimate. This definition encompasses several specific instantiations across different generations of 3GPP technology, including the Enhanced Serving Mobile Location Center (E-SMLC) for LTE, the Secure User Plane Location (SUPL) Location Platform (SLP) for user-plane positioning, and the Location Management Function (LMF) for 5G NR.

In the control-plane architecture, the Location Server interacts with the Radio Access Network (RAN) and the Core Network (CN) to orchestrate positioning procedures. For LTE, the E-SMLC communicates with the eNodeB via the LCS-AP protocol over the SLs interface, requesting and receiving measurement reports. The E-SMLC then computes the final location using methods such as Assisted GNSS (A-GNSS), Observed Time Difference of Arrival (OTDOA), and Enhanced Cell ID. In 5G, the LMF performs a similar role, interfacing with the gNodeB and the Access and Mobility Management Function (AMF) via the NL1 and NL2 interfaces. The LMF supports additional positioning methods such as NR-based OTDOA, multi-RTT, and angle-of-arrival/departure techniques.

In the user-plane architecture, the SUPL SLP enables positioning over the user plane, bypassing the control-plane signaling of the core network. This is particularly useful for commercial location-based services where the network operator may not be directly involved in every positioning request. The SLP communicates with the UE using the SUPL protocol over an IP connection, exchanging assistance data and measurement results. The SLP can operate in either proxy mode, where it interacts with the control-plane Location Server, or in non-proxy mode, where it directly computes the location.

The Location Server also plays a critical role in privacy and security. It enforces authorization policies, ensuring that location requests are only fulfilled for authorized clients and that the target subscriber has consented to the positioning. It may also apply quality-of-positioning (QoP) parameters, such as horizontal and vertical accuracy, response time, and priority, to tailor the positioning procedure to the needs of the requesting application. The Location Server is specified in a number of 3GPP documents, including TS 29.998 (LCS-AP), TS 36.355 (LTE Positioning Protocol), TS 36.855 (LTE positioning enhancements), TS 37.355 (NR Positioning Protocol), and TS 37.857 (NR positioning enhancements).

The definition of Location Server has evolved over time. In Rel-8, the term was defined simply as "See location service," which was a placeholder. By Rel-9, a full definition was introduced, listing E-SMLC and SUPL SLP as examples. This remained unchanged through Rel-15. In Rel-16, the definition was updated to include the LMF, reflecting the introduction of 5G NR and the need to encompass the new 5G positioning architecture. This expanded definition carried forward through Rel-17, Rel-18, and Rel-19, demonstrating the ongoing relevance of the Location Server concept across multiple radio generations.

Purpose & Motivation

The Location Server exists to centralize and standardize the complex task of determining a mobile device's geographic position. Before its introduction, positioning was often handled by proprietary solutions embedded in individual network elements, leading to interoperability issues and limited flexibility. The Location Server abstracts the positioning logic into a dedicated entity, allowing the network to support multiple positioning methods, manage assistance data, and compute location estimates in a consistent manner. This separation of concerns enables operators to deploy and upgrade positioning capabilities independently of the radio access and core network functions.

The need for a dedicated Location Server became acute with the rise of regulatory requirements for emergency calling (e.g., E911, E112) and the growing commercial demand for location-based services. Emergency services require highly accurate and reliable location information, often within seconds, which necessitates sophisticated positioning techniques like A-GNSS and OTDOA. These techniques require the network to provide assistance data to the UE and to collect and process measurements from multiple cells—tasks that are best handled by a specialized server. The Location Server also addresses the need for privacy and authorization, providing a single point where location requests can be validated against subscriber consent and regulatory policies.

Furthermore, the evolution from control-plane to user-plane positioning (SUPL) and the introduction of 5G NR positioning (LMF) reflect the changing landscape of mobile communication. The Location Server concept has proven adaptable, accommodating new radio technologies and new service requirements without requiring a fundamental redesign. This adaptability is a testament to the foresight of the original 3GPP architecture, which anticipated the need for a flexible, scalable positioning framework.