Reference Signal Receiving Quality

Description

Reference Signal Receiving Quality (RSRQ) is a fundamental Layer 1 measurement in both LTE and 5G NR networks that quantifies the quality of the received cell-specific reference signals. It is defined as the ratio N * RSRP / (E-UTRA carrier RSSI), where N is the number of resource blocks (RBs) of the E-UTRA carrier RSSI measurement bandwidth. In simpler terms, RSRQ compares the power of the desired reference signals (RSRP) to the total received power, including interference and noise, within the same measurement bandwidth. This yields a dimensionless metric, typically reported in dB, that approximates a narrowband signal-to-interference-plus-noise ratio (SINR) for the reference signals.

From an architectural perspective, RSRQ measurement is performed by the User Equipment (UE). The network configures measurement objects, reporting configurations, and measurement gaps via RRC signaling. The UE measures the RSRP on the cell-specific reference signals (CRS in LTE, SSB or CSI-RS in NR) and the Received Signal Strength Indicator (RSSI) across the configured bandwidth. The physical layer performs these measurements, and the results are filtered (using L1 and L3 filtering) before being reported to higher layers. In LTE, RSRQ is a primary measurement for Radio Resource Management (RRM), including cell selection/reselection and handover. In NR, while SS-RSRQ (based on SSB) remains important, CSI-RSRQ has been introduced for more flexible quality assessment, especially in beamformed scenarios.

The role of RSRQ in the network is multifaceted. It provides a critical input for mobility algorithms. While RSRP indicates signal strength, RSRQ indicates how 'clean' that signal is. A cell with high RSRP but poor RSRQ may be heavily congested or suffering from strong interference, making it a less desirable candidate for connection. Therefore, network algorithms often use RSRQ (or its derivatives) to trigger handovers, manage load balancing, and configure cell reselection parameters. It is a more reliable indicator of potential user throughput and connection stability than RSRP alone, especially at cell edges where interference is predominant.

Purpose & Motivation

RSRQ was introduced in LTE Release 8 to solve a fundamental limitation of using only RSRP for radio resource management. RSRP alone indicates the strength of the signal from a serving or neighboring cell but does not account for the level of interference or the overall load on the channel. A cell could have a strong RSRP but be unusable due to excessive interference from neighboring cells operating on the same frequency (co-channel interference). This could lead to poor handover decisions, where a UE connects to a strong but heavily interfered cell, resulting in degraded user experience and dropped calls.

The creation of RSRQ provided a standardized, network-controlled metric that combines signal strength and interference into a single quality indicator. This allowed for more intelligent cell selection and handover, improving overall network performance, capacity, and user-perceived quality. Its purpose expanded in subsequent releases to support new features like Carrier Aggregation (where secondary cell selection considers quality), dual connectivity, and, in NR, to operate in conjunction with beam measurements. RSRQ remains a cornerstone measurement because it addresses the classic trade-off in cellular networks between signal strength and interference, enabling algorithms that optimize for both connectivity and quality of service.