Location Services User Plane Protocol

Description

The Location Services User Plane Protocol (LCS-UPP) is the specific protocol that realizes the LCS User Plane (LCS-UP) framework. It is an application-layer protocol that rides on top of the standard 5G user plane transport stack (IP, UDP, GTP-U). LCS-UPP defines the structure of Protocol Data Units (PDUs) that carry all positioning-related information exchanged directly over the data channel, enabling efficient, high-volume transfers that would be prohibitive over control plane signaling.

Technically, LCS-UPP PDUs encapsulate payloads from higher-layer positioning protocols. The most significant of these is the LTE Positioning Protocol (LPP), which is repurposed for 5G. In the LCS-UP context, LPP messages (which contain measurement requests, measurement reports, assistance data delivery, and location information) are not sent over NAS but are instead packaged as the payload within an LCS-UPP PDU. The LCS-UPP layer adds its own header, which includes control information such as a PDU Type (e.g., Data, Acknowledge), a Sequence Number for reliable delivery if needed, and identifiers to associate the PDU with a specific positioning session or transaction. This encapsulated LPP-in-LCS-UPP PDU is then sent over the established PDU Session's user plane bearer.

The protocol operates between two primary endpoints: the LCS-UPP layer in the UE and the LCS-UPP layer in the Location Management Function (LMF). The gNB and UPF are transparent to the LCS-UPP content; they merely forward the IP packets containing the GTP-U-encapsulated LCS-UPP PDUs. LCS-UPP supports both connection-oriented and connectionless modes. For a sustained positioning session with frequent data exchange (e.g., continuous tracking), a connection-oriented context can be established, managing sequence numbers and acknowledgments. For one-off assistance data delivery, a simple connectionless datagram can be used. The protocol is designed to be lightweight, minimizing overhead to preserve the low-latency advantage of the user plane. It works in concert with control plane protocols (like LPP over NAS) which are still used for session establishment, capability negotiation, and activation/deactivation of the user plane positioning data transfer.

Purpose & Motivation

LCS-UPP was developed to provide a standardized, efficient encapsulation and delivery mechanism for positioning protocol data over the 5G user plane. Before its introduction, positioning protocols like LPP were solely carried over the control plane (NAS), which was not designed for the data intensity of advanced positioning. The need for a user plane protocol became apparent with 5G-Advanced use cases requiring the transfer of large assistance data files (e.g., real-time kinematic (RTK) corrections, precise point positioning (PPP) data) and high-frequency streams of raw sensor measurements from the UE.

The protocol solves the problem of how to reliably and efficiently transport structured positioning messages within the unstructured flow of user plane IP packets. It provides the necessary session management, message framing, and optional reliability mechanisms at the application layer without relying on TCP, which may introduce unacceptable latency and jitter. By defining LCS-UPP, 3GPP created a purpose-built conduit that allows the rich feature set of LPP (and future positioning protocols) to be leveraged in a high-performance data plane context. This enables network operators and service providers to offer differentiated, high-accuracy positioning-as-a-service with guaranteed QoS, a key enabler for monetizing 5G network capabilities in vertical markets.