Description
Reference Signal Time Difference (RSTD) is a critical measurement defined for user equipment (UE) positioning in 3GPP LTE and 5G NR networks. It is the core measurement for the Observed Time Difference of Arrival (OTDOA) positioning method. RSTD is defined as the relative timing difference between the time a UE receives a downlink positioning reference signal (PRS) from a neighboring cell and the time it receives a PRS from its reference (typically serving) cell. Specifically, RSTD = T_subframe_Rx_neighbor - T_subframe_Rx_reference, where T_subframe_Rx is the time of arrival of the start of a subframe containing PRS. The UE performs this measurement on special, low-interference Positioning Reference Signals (PRS) that are periodically transmitted by base stations (eNBs in LTE, gNBs in NR). The network's Location Server (E-SMLC in LTE, LMF in NR) assists the UE by providing OTDOA assistance data via the LPP protocol. This data includes the PRS configuration (carrier frequency, bandwidth, periodicity, muting pattern) and the geographic coordinates of the candidate transmitter locations (eNBs/gNBs). The UE uses this information to measure the RSTD for multiple neighboring cells relative to its reference cell. These measured RSTD values are then reported back to the location server. The server, knowing the precise transmission timing relationships between the cells (provided by another network entity, e.g., the base station), uses the set of RSTD measurements to perform multilateration calculations. Each RSTD measurement defines a hyperbolic line of position (LOP). The intersection of multiple LOPs, derived from RSTD measurements to at least three geographically dispersed cells, pinpoints the UE's location. The accuracy of RSTD measurements, which can be on the order of nanoseconds, directly translates to positioning accuracy, often targeting sub-10 meter precision in ideal conditions.
Purpose & Motivation
RSTD and the OTDOA method were developed to provide a network-based, UE-assisted positioning solution that does not rely solely on Global Navigation Satellite Systems (GNSS) like GPS. GNSS signals are often weak or unavailable indoors and in urban canyons. While Cell-ID provides very coarse location, and Enhanced Cell-ID (E-CID) uses timing advance and angle measurements for moderate accuracy, OTDOA via RSTD was designed for higher precision. The creation of RSTD addressed the need for a standardized, scalable method to exploit the cellular network's own infrastructure for accurate positioning. It solves the problem of measuring subtle time-of-flight differences from multiple synchronized transmitters to a receiver. The use of dedicated PRS signals, introduced alongside RSTD, was motivated by the need for a clean, high-quality signal for timing measurements. Normal cell-specific reference signals (CRS) are subject to interference and are not optimized for time-of-arrival measurements. PRS are designed with specific sequences, increased density, and muting patterns to reduce interference, enabling the precise RSTD measurements necessary for accurate multilateration. This supports regulatory requirements like E911, as well as commercial location-based services.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (33 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.
In Release 15, the RSTD function for OTDOA was enhanced with new assistance data elements, including the TDD UL/DL configuration and SFN offset. Support was introduced for inter-RAT RSTD measurements with autonomous and measurement gaps, and for sensor-assisted positioning through the addition of IMU support and related methods. Furthermore, the release defined new signalling procedures between an LMF and NG-RAN node/UE and for OTDOA assistance data requests in NR.
- Introduction of IMU support for OTDOA TS 36.355CR0204
- OTDOA Assistance Data Request for NR TS 36.355CR0222
- Addition of TDD UL/DL configuration to OTDOA assistance data TS 36.355CR0213
- SFN offset for OTDOA TS 36.355CR0229
- Capture signalling flows where the last serving gNB moves the UE to RRC_IDLE TS 38.300CR0087
- Signalling between an LMF and NG-RAN node/UE TS 38.305CR0001
+ 3 more changes
In Release 16, the enhancements for the RSTD function primarily involved support for OTDOA assistance data when the serving cell is an NR cell, and provided clarifications on the quality and timestamp for RSTD measurements. This ensured positioning procedures were correctly defined for 5G New Radio deployments. Additionally, updates were made to the assistance data for the BDS (BeiDou) satellite system, including the introduction of the B1C signal.
- 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
- Corrections to 36.214 for Reference Point for eNB Rx – Tx time difference TS 36.214CR0057
- Update B1I signal ICD file to v3.0 in BDS system in A-GNSS TS 37.355CR0259
- Clarification of quality and time stamp for RSTD measurements TS 37.355CR0274
- Description on timestamp reference in NR positioning measurement report TS 37.355CR0311
+ 6 more changes
In Release 17, specific corrections were made to the RSTD measurement procedure regarding the applicability of the timing error margin for the Rx-Tx time difference measurement (RxTEG) within the NR-Multi-RTT-SignalMeasurementInformation field. These were part of miscellaneous corrections to ensure consistent technical implementation. Additionally, clarifications were introduced for the Galileo navigation message model within the GNSS assistance data to specify the signal reference for SSR clock corrections.
- UE Security Capabilities signaling in NG-RAN [UE_Sec_Caps] TS 38.300CR0427
- Corrections on applicability of timing error margin of RxTEG in NR-Multi-RTT-SignalMeasurementInformation field descriptions and other Miscellaneous corrections TS 37.355CR0431
- GNSS SSR BDS orbit emphemeris reference clarification to align with RTCM TS 37.355CR0461
- NRPPA corrections of references to RRC TS 38.455CR0100
- Clarifying Galileo NAV message in the GNSS Navigation model to clarify SSR clock correction signal reference TS 37.355CR0412
In Release 18, the key new development for RSTD-related functionality was the introduction of the NR UE Rx-Tx time difference measurement for use in NR Uplink Enhanced Cell ID (UL E-CID) positioning. This enhancement involved specific corrections and value definitions for the UE Rx-Tx Time Difference measurement to ensure accurate implementation.
- Introduction of NR UE Rx-Tx time difference measurement in NR UL E-CID TS 38.305CR0164
- Introduction of NR UE Rx-Tx time difference measurement in NR UL E-CID TS 38.455CR0124
- Value UE Rx-Tx Time Difference ASN.1 presence correction TS 38.455CR0160
- Correction of UE Rx-Tx Time difference measurement TS 38.455CR0169
- Reference for User Service Description TS 38.300CR0866
In Release 19, the RSTD (Reference Signal Time Difference) function was enhanced by the introduction of a B2b signal for the BDS (BeiDou) system within A-GNSS positioning. This addition expanded the suite of supported global navigation satellite systems for improved location accuracy. Furthermore, the release introduced a Low-Power Wake-Up Signal and Receiver for NR, aimed at reducing UE power consumption during positioning measurements.
Explore further
Broader topics and technologies where RSTD plays a role.
Defining Specifications
3GPP specifications that define or reference RSTD, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 36.133 vj20 | E-UTRA RRM Requirements | Rel-19 |
| TS 36.214 vj00 | E-UTRA Physical Layer Measurements | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 36.855 vd00 | E-UTRA Positioning Enhancements Study | Rel-13 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 37.571 vj00 | UE Conformance for Positioning | Rel-19 |
| TS 37.857 vd10 | Study on Indoor Positioning Enhancements | Rel-13 |
| TS 38.133 vj20 | 5G UE Radio Requirements for RRC_IDLE Mobility | Rel-19 |
| 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 |
| TR 38.857 vh00 | Study on NR Positioning Enhancements | Rel-17 |
| TR 38.889 vg00 | NR-based access to unlicensed spectrum study | Rel-16 |