Received Signal Quality

Description

Received Signal Quality (RXQUAL) is a critical quality-of-link measurement in GSM and related 3GPP technologies. Unlike RXLEV, which measures signal strength, RXQUAL estimates the Bit Error Rate (BER) on the received digital signal prior to channel decoding. It is a direct indicator of the corruption experienced by the radio burst due to factors like interference, multipath fading, and noise. Architecturally, RXQUAL is measured by the physical layer of the receiver—in the Mobile Station (MS) for the downlink and in the Base Transceiver Station (BTS) for the uplink. The measurement is typically derived by comparing the known training sequence (midamble) embedded in every normal burst with the received version, calculating the error rate over this known bit pattern.

The operation involves processing the demodulated bits before they are passed through the channel equalizer and decoder. The calculated raw BER is then mapped to one of eight discrete RXQUAL levels (0 to 7), where RXQUAL 0 represents the best quality (BER < 0.2%) and RXQUAL 7 represents the worst (BER > 12.8%). This quantized value is reported to the higher layers (RR management) and included in measurement reports sent to the network. The Base Station Controller (BSC) uses these reports in a complementary fashion with RXLEV.

RXQUAL plays a vital role in network optimization and maintaining call quality. Its primary function is to inform handover decisions. A handover may be triggered due to bad quality (RXQUAL-based handover) even if the signal level (RXLEV) is acceptable, which is crucial in interference-limited scenarios. It is also a key input for power control algorithms; if RXQUAL is poor, the network may command an increase in transmission power to overcome interference, provided RXLEV is not already too high. Furthermore, persistent poor RXQUAL values while RXLEV is good are a classic diagnostic signature of co-channel or adjacent channel interference, guiding network planning and optimization efforts to improve frequency reuse and overall system capacity.

Purpose & Motivation

RXQUAL was introduced to address a key limitation of using only signal strength (RXLEV) for network control. A strong signal does not guarantee a good quality connection; it could be corrupted by high levels of interference from other cells using the same frequency (co-channel interference) or an adjacent frequency. Relying solely on RXLEV could lead to a mobile station being stuck on a strong but heavily interfered channel, resulting in a dropped call or poor voice quality.

The creation of RXQUAL was motivated by the need for a second, orthogonal metric that directly reflected the usability of the radio link. It solves the problem of distinguishing between a weak signal (poor coverage) and a strong but corrupted signal (poor quality due to interference). This distinction allows the network to apply the correct remedy: a handover to a different cell for coverage issues, or a handover to a different frequency channel or an adjustment in power control for interference issues. By providing a standardized measure of link quality, RXQUAL enabled more intelligent and robust radio resource management, which was essential for achieving the high capacity and quality targets of dense GSM network deployments.