Description
Uplink Relative Time of Arrival (UL-RTOA) is a measurement defined for positioning purposes in 3GPP networks. It represents the measured time of arrival of a specific uplink signal from a target User Equipment (UE) at a network reception point (e.g., a gNB or LMU), relative to the reception point's internal timing reference. The core principle is that multiple, geographically dispersed reception points simultaneously listen for a known uplink signal transmitted by the UE, such as a Sounding Reference Signal (SRS) configured for positioning. Each point records the precise time (the RTOA) at which the signal arrives. Because the signal travels at the speed of light, differences in these arrival times (Time Differences of Arrival - TDOA) correspond to differences in the distance from the UE to each reception point. The network collects these UL-RTOA measurements from multiple points and forwards them to a location server, typically the Location Management Function (LMF). The LMF then calculates the TDOA values between pairs of reception points. Using multilateration algorithms, the LMF determines hyperbolic lines of position; the intersection of these hyperbolas estimates the UE's location. The accuracy of UL-RTOA depends on factors like signal bandwidth (which affects time resolution), synchronization accuracy between the network reception points, and the geometry of those points relative to the UE. The measurement procedure involves coordination where the LMF requests the UE to transmit a specific uplink positioning reference signal, and configures the relevant gNBs to perform and report the UL-RTOA measurement. This method forms the basis for the Uplink Time Difference of Arrival (UL-TDOA) positioning technique.
Purpose & Motivation
UL-RTOA was introduced to provide a network-based, uplink-centric positioning method that does not rely on Global Navigation Satellite System (GNSS) signals. This is crucial for locating devices indoors, in urban canyons, or in other environments where GNSS coverage is poor or unavailable. It addresses the need for regulatory compliance (e.g., emergency caller location) and supports commercial location-based services. Prior to its standardization, network-based positioning often relied on downlink methods (like OTDOA) requiring UE measurement capabilities, or less accurate cell-ID techniques. UL-RTOA enables a method where the computational burden and measurement complexity reside primarily in the network infrastructure, which can be advantageous for low-complexity IoT devices or when the UE's battery or processing power is limited. Its development from Rel-13 onwards was driven by the growing demand for reliable and accurate positioning across all deployment scenarios.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (17 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-13, normative work from Rel-15.
In Release 15, the UL-RTOA (Uplink Relative Time of Arrival) function itself is not explicitly detailed in the provided grounding context. The context focuses on OTDOA (Observed Time Difference of Arrival) performance evaluations, enhancements like PRS transmission improvements, and studies for indoor positioning. Based on the given Change Request titles, the specific new introduction related to uplink positioning assistance was the "Addition of TDD UL/DL configuration to OTDOA assistance data."
In Release 16, the enhancements for UL-RTOA (Uplink Relative Time of Arrival), often discussed under OTDOA positioning, included specific corrections to uplink transmission procedures and assistance data handling. Key updates involved support for OTDOA assistance data specifically for the case of an NR serving cell, refining the data provided to the UE. Furthermore, corrections were made to the descriptions of relative cartesian coordinate handling within the positioning support framework to improve accuracy.
- Mapping of Uplink Traffic to Backhaul RLC Channels TS 38.300CR0255
- Correction on uplink transmission allowed without TA TS 38.300CR0343
- Support OTDOA assistance data for case of NR serving cell TS 38.305CR0062
- Correction on relative cartesian coordinate TS 38.455CR0034
- Correction on relative cartesian coordinate TS 38.473CR0766
- Correction to OTDOA positoning support descriptions in R16 TS 38.305CR0048
+ 2 more changes
In Release 17, the UL-RTOA (Uplink Relative Time of Arrival) function was introduced alongside the new "uplink GapFR2" capability. This enhancement was complemented by a correction to the semantic description of the Burst Arrival Time parameter to ensure clarity and alignment within the specification.
In Release 18, the UL-RTOA function was not explicitly mentioned as a new feature within the provided grounding context or the listed Change Request titles. The documentation focuses on enhancements for OTDOA (Observed Time Difference of Arrival), including evaluations of positioning error with synchronization errors and studies on PRS transmission enhancements, but does not specify any new developments for UL-RTOA itself.
- Introduction of MIMO evolution for Downlink and Uplink TS 38.300CR0742
- Support of oversize UL SDT Data Arrival [Large SDT Uplink Data] TS 38.300CR0748
- Switching from SDT to RRC connected state [Large SDT Uplink Data] TS 38.473CR1213
- Correction on burst arrival time in handover TS 38.300CR0875
- Introduction of separate uplink and downlink PDU set QoS parameters TS 38.473CR1278
Explore further
Broader topics and technologies where UL-RTOA plays a role.
Defining Specifications
3GPP specifications that define or reference UL-RTOA, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 37.857 vd10 | Study on Indoor Positioning Enhancements | Rel-13 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.305 vj00 | NG-RAN UE Positioning Stage 2 | Rel-19 |
| TS 38.455 vj10 | NR Positioning Protocol A (NRPPa) | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |
| TR 38.857 vh00 | Study on NR Positioning Enhancements | Rel-17 |