Synchronization Signal based Reference Signal Received Power

Description

SS-RSRP (Synchronization Signal based Reference Signal Received Power) is a fundamental radio resource management (RRM) measurement defined in 3GPP New Radio (NR) standards. It is a linear average over the power contributions (in watts) of the resource elements that carry the secondary synchronization signals (SSS) within the synchronization signal/physical broadcast channel (SS/PBCH) block, commonly known as the SSB. The measurement is performed by the user equipment (UE) on the downlink signals transmitted by the gNodeB. The SSB is a critical signal structure that is periodically broadcast by the cell, containing the Primary Synchronization Signal (PSS), Secondary Synchronization Signal (SSS), and the Physical Broadcast Channel (PBCH). SS-RSRP measurement specifically focuses on the SSS, as it provides a stable and cell-specific reference. The UE measures the power of these specific resource elements after performing necessary synchronization and channel estimation procedures.

The architecture supporting SS-RSRP involves the UE's physical layer processing chain. Upon powering on or during mobility events, the UE scans for SSBs across different frequency bands and beam directions. Once an SSB is detected and its timing is acquired, the UE isolates the resource elements corresponding to the SSS within that SSB. The power of these REs is measured, typically involving filtering to average out fast fading and interference effects, resulting in a stable RSRP value reported in dBm. This measurement is then reported to the higher layers (Layer 3) of the UE's protocol stack and can be further reported to the network via measurement reports (e.g., in RRC_IDLE, RRC_INACTIVE, or RRC_CONNECTED states) to assist network decisions.

SS-RSRP plays a central role in several network functions. It is the primary metric for cell selection and reselection procedures, where the UE compares the SS-RSRP of the serving cell and neighboring cells against predefined thresholds (e.g., Squal, Srxlev). For handover and beam management, SS-RSRP measurements of different SSBs (which correspond to different beams in FR2) are used to determine the best cell or beam for connectivity. The network configures measurement objects, reporting configurations, and event triggers (like A3, A5 events) based on SS-RSRP. Its accuracy and reliability are paramount, as errors can lead to ping-pong handovers, call drops, or suboptimal cell attachment, directly impacting user experience and network performance.

Purpose & Motivation

SS-RSRP was introduced in 3GPP Release 15 as part of the 5G NR standardization to provide a standardized, accurate method for measuring downlink signal strength from synchronization signals. In previous generations like LTE, RSRP was measured based on Cell-Specific Reference Signals (CRS). However, 5G NR introduced a more flexible and beam-centric design, where the always-on CRS was eliminated to save overhead and energy. The SS/PBCH block became the primary signal for initial access and mobility. Therefore, a new measurement analogous to RSRP, but based on the SS block, was necessary. SS-RSRP solves the problem of providing a consistent reference for signal power in a network where reference signals are not continuously transmitted.

The creation of SS-RSRP was motivated by the need to support efficient mobility in diverse 5G deployment scenarios, including high-frequency bands (mmWave) with beamforming. In these scenarios, signal quality can vary significantly across different beams. SS-RSRP measurements per SSB (beam) allow the UE and network to identify the strongest beam for communication. It addresses the limitation of LTE's CRS-based RSRP, which was not suitable for the bursty, beam-swept transmission of SSBs in NR. By defining the measurement specifically on the SSS, it ensures a cell-specific and stable power estimate that is decoupled from potential power variations in other channels, enabling reliable radio link monitoring and mobility decisions in the 5G ecosystem.