Sidelink Reference Signal Received Power

Description

Sidelink Reference Signal Received Power (S-RSRP) is a physical layer measurement defined for sidelink (SL) communication in 3GPP LTE (starting from Release 12) and NR (from Release 16 onwards). Sidelink refers to the direct device-to-device (D2D) communication interface, known as PC5, used in Proximity Services (ProSe) and Vehicle-to-Everything (V2X) applications. S-RSRP is the sidelink counterpart to the downlink RSRP measurement. It is defined as the linear average over the power contributions (in Watts) of the resource elements that carry sidelink demodulation reference signals (DM-RS) within a specified measurement bandwidth and time duration.

The measurement is performed by a receiving UE on the DM-RS transmitted by a potential transmitting UE. These DM-RS are embedded within the Physical Sidelink Shared Channel (PSSCH) for data or the Physical Sidelink Control Channel (PSCCH) for control information, depending on the specific sidelink transmission. The UE measures the power of these reference signals to estimate the path loss and channel quality of the sidelink. The accuracy of S-RSRP is vital as it feeds into higher-layer algorithms for resource management. The UE typically reports measured S-RSRP values to its protocol stack, which uses them for criteria such as triggering a measurement report to the network (in network-scheduled modes) or for autonomous decisions (in UE autonomous resource selection modes).

From an architectural perspective, S-RSRP measurement is a function of the UE's physical layer. The procedure involves the UE synchronizing to the sidelink synchronization signals (S-SS), identifying the DM-RS positions within the received sidelink subframes, and performing power averaging. The specific resources to measure are configured by higher layers via RRC signaling (in mode 1 or mode 3) or determined by pre-configured parameters (in mode 2 or mode 4). S-RSRP is a key input for the Sidelink Radio Link Monitoring (S-RLM) procedure, where the UE monitors the quality of an established sidelink to detect radio link failure. It also plays a central role in the sensing-based semi-persistent scheduling (SPS) algorithm used in LTE V2X mode 4, where UEs sense the channel and measure S-RSRP of reservations from other UEs to select resources that are likely to be free and have acceptable interference levels.

In NR Sidelink (introduced in Release 16), the concept of S-RSRP is extended and refined. NR supports more flexible reference signal structures and wider bandwidths. S-RSRP measurements in NR can be performed on different reference signal types, such as Phase-Tracking Reference Signals (PT-RS) in addition to DM-RS, and across the activated bandwidth part (BWP) for the sidelink. The measurement is crucial for advanced NR-V2X features like multi-antenna transmission, beam management for sidelink, and enhanced resource allocation schemes. The UE uses S-RSRP to select the best beam pair link in beamformed sidelink operations and to assist in power control mechanisms, ensuring reliable direct communication while minimizing interference to other sidelink users.

Purpose & Motivation

S-RSRP was introduced in 3GPP Release 12 to support the new LTE Direct (D2D) feature, known as Proximity Services (ProSe). The fundamental problem it addresses is the need for a standardized, reliable method for a UE to measure the signal strength and quality of a direct radio link from another UE. Prior to sidelink, cellular measurements like RSRP were only defined for the Uu interface (UE-to-network). For direct communication, UEs require a similar metric to assess link viability, perform handover between direct and infrastructure paths, and manage interference. S-RSRP provides this essential physical layer measurement, enabling UEs and the network to make informed decisions about sidelink communication.

The primary motivation was to enable discovery and communication services for public safety and commercial ProSe. Public safety users often operate in areas without network coverage, necessitating direct UE-to-UE communication. S-RSRP allows a UE to evaluate the proximity and channel conditions of other discovered UEs, which is critical for deciding whether to establish a direct link. It also supports network-controlled operation, where the eNB uses UE-reported S-RSRP measurements to manage sidelink resource pools, perform mode selection (whether to communicate via the network or directly), and manage interference between cellular and sidelink transmissions.

With the expansion to V2X in Releases 14 and 15, the role of S-RSRP became even more critical. For autonomous resource selection (LTE Mode 4), UEs must sense the channel to select transmission resources. They decode the control information (SA) from other vehicles and measure the S-RSRP of the associated DM-RS. If the measured S-RSRP is above a certain threshold, the resource is considered reserved and possibly experiencing high interference, so the sensing UE will exclude it from its own candidate resource set. This interference avoidance mechanism is foundational to the distributed scheduling in V2X and relies entirely on accurate S-RSRP measurements. Therefore, S-RSRP evolved from a simple link quality indicator into a core enabler of distributed, reliable, and low-latency direct communication for safety-critical automotive applications.