Wide Band Reference Signal Received Quality

Description

Wide Band Reference Signal Received Quality (WB-RSRQ) is a key Layer 3 measurement defined in 3GPP specifications for User Equipment (UE) in LTE (E-UTRA) and NR (New Radio) systems. It is an extension of the traditional RSRQ (Reference Signal Received Quality) measurement, designed to evaluate the quality of received reference signals over a wider bandwidth. The measurement is calculated as the ratio of the wideband RSRP (Reference Signal Received Power) to the wideband RSSI (Received Signal Strength Indicator), specifically N times RSRP divided by RSSI, where N is the number of resource blocks (RBs) over which the RSSI is measured. This provides a signal-to-interference-plus-noise ratio (SINR)-like metric that reflects the quality of the reference signals across a broader frequency range.

Architecturally, WB-RSRQ is performed by the UE's physical layer and reported to higher layers (RRC) for use in mobility and radio resource management procedures. The network configures the measurement through RRC signaling, specifying parameters such as the measurement bandwidth, reporting criteria (periodic or event-triggered), and filtering coefficients. The UE continuously measures the power of the Cell-specific Reference Signals (CRS in LTE) or Synchronization Signal Blocks/CSI-RS (in NR) within the configured bandwidth to compute RSRP, while simultaneously measuring the total received power (including serving cell, co-channel interference, and thermal noise) across the same bandwidth to compute RSSI.

The primary role of WB-RSRQ in the network is to enhance mobility and load balancing decisions, especially in heterogeneous networks (HetNets) with small cells. Unlike narrowband RSRQ, which might only reflect quality on a subset of resource blocks, WB-RSRQ gives a more averaged and representative view of the channel quality across the entire carrier bandwidth. This is particularly important for wideband carriers (e.g., 20 MHz, 100 MHz) where interference conditions can vary significantly across frequency. The measurement is used in events like A3 (neighbor becomes offset better than serving) for handover, and in cell reselection for idle mode mobility. It helps the network identify cells with better overall radio quality, leading to improved user throughput and reduced call drops.

Key components involved in WB-RSRQ include the UE's RF receiver chain, baseband processing units for signal measurement, and the RRC protocol for configuration and reporting. The measurement is standardized to ensure consistency across different UE implementations and network equipment. In NR, WB-RSRQ can be measured on SS/PBCH blocks or CSI-RS resources, providing flexibility for different deployment scenarios. The accuracy and reporting latency of WB-RSRQ are critical for real-time radio resource management, making it a fundamental aspect of LTE and NR radio interface performance.

Purpose & Motivation

WB-RSRQ was introduced in 3GPP Release 11 to address the limitations of traditional narrowband RSRQ in wideband LTE deployments and emerging heterogeneous networks. Prior to WB-RSRQ, RSRQ measurements were typically performed over a narrow bandwidth (e.g., 6 resource blocks), which could be unrepresentative of the overall carrier quality, especially in frequency-selective fading or interference scenarios. This narrowband measurement could lead to suboptimal handover decisions, where a cell might appear better on a specific subband but perform poorly across the entire bandwidth, resulting in degraded user experience after handover.

The motivation for WB-RSRQ stemmed from the increasing carrier bandwidths in LTE-Advanced (up to 20 MHz initially, and later aggregated carriers) and the deployment of small cells (pico, femto) alongside macro cells. In such HetNets, interference patterns are complex and can vary significantly across frequency. A wideband quality measurement provides a more holistic view, enabling better load balancing, interference coordination, and mobility robustness. For example, in a co-channel HetNet, a UE at the cell edge might experience high interference from a neighboring macrocell on some resource blocks but not others; WB-RSRQ captures this averaged effect, guiding the network to handover the UE to a cell with consistently better wideband quality.

Furthermore, WB-RSRQ supports enhanced radio resource management algorithms in the network. By providing a quality metric over the entire bandwidth, it aids in frequency-selective scheduling, inter-cell interference coordination (ICIC), and carrier aggregation management. It also facilitates the operation of features like enhanced Inter-Cell Interference Coordination (eICIC) and Further Enhanced ICIC (FeICIC) in Release 10 and 11, where accurate wideband quality assessment is crucial for almost blank subframe (ABS) selection and range expansion in small cells. Thus, WB-RSRQ was created to improve network performance, user throughput, and mobility reliability in advanced LTE and NR systems.

Key Features

• Measures reference signal quality over a wide bandwidth (configurable by network)
• Calculated as N*RSRP/RSSI, where N is number of RBs in measurement bandwidth
• Supports both LTE (CRS-based) and NR (SSB/CSI-RS-based) measurements
• Configurable via RRC signaling for periodic or event-triggered reporting
• Used for handover decisions, cell reselection, and radio resource management
• Provides averaged quality metric, reducing impact of frequency-selective interference

Evolution Across Releases

Defining Specifications