Sidelink Discovery Reference Signal Received Power

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.