Description
Sidelink Discovery Reference Signal Received Power (SD-RSRP) is a critical physical layer measurement in 3GPP's Device-to-Device (D2D) and Proximity Services (ProSe) framework, applicable to both LTE sidelink (from Rel-12) and NR sidelink (from Rel-16). It quantifies the power level of the received Discovery Reference Signal (DRS) transmitted by a nearby UE on the sidelink interface (PC5). Unlike cellular downlink RSRP which measures signals from a base station (eNodeB/gNB), SD-RSRP measures signals from peer devices, enabling direct discovery and link quality estimation. The measurement is performed by the UE's physical layer on configured discovery resources within the ProSe discovery carrier.
The technical procedure involves the transmitting UE broadcasting a Discovery Message, which includes associated Discovery Reference Signals. These signals have a known structure and sequence, allowing receiving UEs to detect them and measure their received power. The SD-RSRP measurement is defined as the linear average over the power contributions of the resource elements that carry the specific discovery reference signals within the measurement bandwidth. The UE's higher layers (RRC) configure the measurement and reporting criteria, which can be event-triggered or periodic. The measured SD-RSRP value is then used by the UE's ProSe protocol stack or application layer to make decisions, such as whether to announce the discovery of another device or to select the best peer for a potential direct communication link.
SD-RSRP plays a pivotal role in the sidelink discovery process, which operates in two main models: Model A ("I am here" announcements) and Model B ("who is there?"/"are you there?" requests and responses). In both models, the measured SD-RSRP can be used as a criterion for filtering discoveries—only devices with an SD-RSRP above a certain threshold might be considered valid or relevant. This prevents the UE from being overwhelmed by distant or weak signals and ensures reliable discovery only within a desired proximity range. It is a foundational metric for network-controlled or autonomous resource selection, power control, and mobility management in sidelink communications, directly impacting the performance and reliability of services like public safety communications, vehicle-to-everything (V2X), and commercial ProSe.
Purpose & Motivation
SD-RSRP was created to address the fundamental need for a standardized, quantitative measure of sidelink discovery signal strength in LTE and NR ProSe. Prior to ProSe standardization, direct device discovery and communication lacked a unified method for devices to assess the quality and viability of a direct radio link. The introduction of ProSe in LTE Rel-12 for public safety and commercial use cases required a mechanism for devices to autonomously or network-assistedly discover each other and estimate link quality before establishing communication.
The development of SD-RSRP was motivated by the limitations of simple binary discovery (detected/not detected). A power-based measurement allows for proximity estimation, link quality prediction, and intelligent discovery filtering. It solves the problem of determining whether a discovered device is within a useful communication range, which is essential for efficient resource utilization and service reliability. For network-controlled scenarios, the base station can use reported SD-RSRP measurements to manage discovery resources and interference. In autonomous scenarios, devices use SD-RSRP to make independent decisions about peer discovery and connection initiation, enabling robust direct communication in out-of-coverage or partial-coverage situations critical for public safety and V2X applications.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (203 CRs across 6 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 SD-RSRP function was introduced to support sidelink discovery for Proximity Services (ProSe), specifically enabling a Mission Critical service remote UE to discover a potential 5G ProSe UE-to-Network relay. This involved the introduction of time reference provisioning and corrections to the sidelink measurement periodic triggering condition to ensure reliable discovery signalling. These enhancements provided the foundational measurement procedures for direct device-to-device discovery in the 5G system.
- Introduction of time reference provision TS 36.331CR3341
- Signalling for euCA (Enhancing LTE CA Utilization) TS 36.331CR3391
- CR on signalling introduction of UE overheating support in NR SA scenario TS 38.331CR0729
- Additional capability signalling for 1024QAM support TS 36.331CR4031
- Additional capability signalling for 1024QAM support TS 38.331CR1120
- Correction on V2X sidelink communication in TS 36.300 TS 36.300CR1199
+ 48 more changes
In Release 16, the SD-RSRP function was introduced as part of the new 5G V2X with NR Sidelink capability, enabling enhanced proximity-based services for direct device-to-device communication. This function supports the 5G ProSe UE-to-Network Relay Discovery service, allowing remote UEs to discover potential relay UEs. The specification work for this, including the necessary signalling procedures, was detailed in updates to the Radio Resource Control protocols (TS 36.331 and TS 38.331).
- Introduction of 5G V2X with NR Sidelink TS 36.300CR1271
- Signalling UE capability Identity TS 36.300CR1294
- Introduction of 5G V2X with NR Sidelink in TS 36.331 TS 36.331CR4222
- CR for 36.331 for Power Savings TS 36.331CR4245
- Introduction of signalling for high-speed train scenarios TS 36.331CR4326
- Introduction of UE Power Saving in NR TS 38.300CR0193
+ 33 more changes
In Release 17, the SD-RSRP function was enhanced to support the newly introduced Sidelink Relay feature, enabling remote Mission Critical service UEs to discover and measure potential 5G ProSe UE-to-Network relay UEs. This was part of broader NR Sidelink enhancements, which included new UE capabilities and RRC signaling for relay-related configuration. These improvements facilitated more reliable device-to-device discovery and connection establishment for relay-assisted Mission Critical services.
- Introduction of Sidelink Relay TS 38.300CR0403
- Introduction of UE power saving enhancements In 38.300 TS 38.300CR0417
- RRC CR for NR Sidelink enhancement TS 38.331CR2902
- Introduction of RRC signaling for measurement gap enhancement TS 38.331CR2913
- CR to TS 38.331 on Network assistant signalling for Rel-17 CRS interference mitigation TS 38.331CR3021
- UE Security Capabilities signaling in E-UTRAN [UE_Sec_Caps] TS 36.300CR1359
+ 31 more changes
In Release 18, the SD-RSRP function was enhanced as part of the broader NR Sidelink Evolution to support improved discovery and selection for 5G ProSe UE-to-network relays, particularly for Mission Critical services. These enhancements enable more reliable relay discovery for remote UEs by improving the measurement and reporting of sidelink discovery signals. This supports service continuity procedures between MBS sessions and ProSe relay connections, as outlined in the updated architectural and functional models.
- MC service control signalling over 5G MBS TS 23.289CR0020
- Use of 5G ProSe UE-to-network relay service for MC services TS 23.289CR0026
- Architectural model over 5G ProSe TS 23.289CR0042
- Off-network functional model over 5G ProSe TS 23.289CR0043
- 5G ProSe UE-to-network relay for MC service TS 23.289CR0046
- Service continuity with a 5G ProSe UE-to-network relay for MBMS TS 23.289CR0047
+ 49 more changes
In Release 19, the SD-RSRP function was enhanced to support discovery and connectivity for Mission Critical service UEs through new 5G ProSe UE-to-Network relay capabilities, including multi-hop relaying. This enables remote UEs to discover relay UEs supporting MC services, as defined for ProSe Layer-2 and Layer-3 relaying techniques. The support for these relay procedures, including for multi-hop scenarios, provides the foundational discovery mechanism where SD-RSRP measurements are utilized.
- ProSe support in NPN [ProSe_NPN] TS 38.300CR0957
- Introduction of Low-Power Wake-Up Signal and Receiver for NR TS 38.300CR1015
- Introduction of NR sidelink multi-hop U2N Relay TS 38.300CR1028
- ProSe support in NPN [ProSe_NPN] TS 38.331CR5209
- Introduction of NR sidelink multi-hop relay TS 38.331CR5429
- Introduction of signaling support for intra-band non-collocated EN-DC/NR-CA deployment Phase 2: new receiver type(s) TS 38.331CR5479
+ 8 more changes
In Release 20, the SD-RSRP function was enhanced to support new 5G ProSe multi-hop relay configurations, specifically for UE-to-network and UE-to-UE relay scenarios. These enhancements enabled the discovery and measurement of sidelink reference signals across multiple relay hops to facilitate extended proximity services. The updates provided the necessary configurations for relay discovery and path establishment as part of the broader Mission Critical service architecture.
Explore further
Broader topics and technologies where SD-RSRP plays a role.
Defining Specifications
3GPP specifications that define or reference SD-RSRP, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.289 vk10 | Mission Critical services over 5G System | Rel-20 |
| TS 36.300 vj00 | E-UTRAN Radio Interface Protocol Architecture Overview | Rel-19 |
| TS 36.331 vj00 | LTE RRC Protocol Specification | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |