Sidelink Reference Signal Received Power

Description

SL-RSRP is a physical layer measurement defined for NR and evolved LTE sidelink, analogous to the downlink RSRP measurement on the Uu interface but applied to the PC5 interface. It quantifies the average received power of resource elements that carry sidelink-specific reference signals, such as the Demodulation Reference Signal (DM-RS) within the Physical Sidelink Shared Channel (PSSCH) or the Channel State Information Reference Signal (CSI-RS) configured for sidelink. The measurement is performed by a receiving UE on signals transmitted by a peer UE, typically over a configured measurement resource set. The UE filters the measurements in time and frequency to produce a reliable estimate, reported in dBm. This measurement is critical for several sidelink procedures. For beam management in NR sidelink, especially at higher frequencies, SL-RSRP measurements on different beam pairs are used to select the optimal transmit and receive beam, a process known as beam refinement. In resource allocation, particularly for autonomous resource selection modes (e.g., Mode 2), a UE senses the channel and measures SL-RSRP from other UEs' transmissions. If the measured SL-RSRP for a given resource is above a configured threshold, the UE considers that resource as occupied or reserved by a high-interference neighbor and excludes it from its own candidate resource set, implementing a sensing-based semi-persistent scheduling (SPS) algorithm. Furthermore, SL-RSRP serves as an input for power control algorithms, helping UEs adjust their transmit power to maintain a target received power at the receiver, optimizing interference and battery life. The accuracy and reporting mechanisms for SL-RSRP are specified in detail to ensure interoperability and reliable sidelink performance.

Purpose & Motivation

SL-RSRP was introduced to provide a standardized, quantifiable metric for assessing the quality of the direct radio link between UEs, a capability absent in early sidelink specifications. Initial D2D and V2X releases focused on basic connectivity without sophisticated link adaptation or interference-aware resource selection. As sidelink use cases expanded to include advanced V2X requiring ultra-reliable low-latency communication (URLLC) and NR sidelink operating in mmWave bands, the need for precise channel state information became paramount. SL-RSRP addresses this by enabling UEs to measure link strength, which is fundamental for several key functions: enabling efficient beam management for directional communications in high-frequency NR sidelink, replacing the simpler omni-directional assumptions of LTE sidelink; facilitating intelligent, interference-aware resource selection in autonomous modes, which reduces packet collisions and improves reliability in dense scenarios; and supporting closed-loop power control to minimize interference and save UE battery. It essentially brings the mature channel measurement and management paradigms of the cellular Uu link to the device-to-device domain, which is essential for achieving the performance targets of advanced V2X, public safety, and commercial D2D applications.

Key Features

• Measurement of received power from sidelink reference signals (e.g., DM-RS, CSI-RS)
• Fundamental for sidelink beam management and beam selection procedures
• Key input for sensing-based autonomous resource selection (Mode 2) algorithms
• Used for sidelink power control to optimize interference and power consumption
• Supports both LTE and NR sidelink air interfaces
• Enables link quality estimation for mobility and handover decisions in sidelink

Evolution Across Releases

Defining Specifications