CQI (Channel Quality Indicator) is an index from 0 to 15 reported by the mobile device (UE) to the base station, indicating the quality of the downlink radio channel. It determines the modulation and coding scheme (MCS) used to optimize throughput based on current radio conditions.
What is CQI?
Channel Quality Indicator: the index that drives real-time throughput adaptation in 4G and 5G networks.
Detailed explanation
CQI is a feedback mechanism from the device to the base station enabling dynamic link adaptation. The device continuously evaluates downlink channel quality by measuring SINR. It converts this measurement into a CQI index from 0 (no transmission possible) to 15 (optimal radio conditions).
The base station uses the received CQI to select the most appropriate MCS. A high CQI (12 to 15) enables 256-QAM with aggressive coding, maximizing throughput. A low CQI (1 to 4) forces QPSK with protective coding, reducing throughput but ensuring transmission reliability.
CQI reporting can be periodic (at regular intervals configured by the network) or aperiodic (on demand). In LTE, CQI can be reported for the entire bandwidth (wideband CQI) or per sub-band (subband CQI), enabling finer frequency-domain scheduling.
In 5G NR, the CQI mechanism is extended with additional tables for new modulations (uplink 256-QAM) and multi-beam configurations. CQI remains central to the Link Adaptation mechanism that continuously optimizes the throughput/reliability trade-off.
CQI and modulation mapping
| CQI | Modulation | Channel quality |
|---|---|---|
| 13 to 15 | 64-QAM / 256-QAM | Excellent |
| 10 to 12 | 64-QAM | Good |
| 7 to 9 | 16-QAM | Fair |
| 0 to 6 | QPSK | Poor |
How HiCellTek measures CQI
The RF Monitor module in HiCellTek displays CQI in real time as reported by the Qualcomm chipset. The value is correlated with SINR, active MCS and observed throughput, providing a complete view of the link adaptation mechanism. CQI values are GPS-geolocated and exportable to identify areas where channel quality is insufficient.
Frequently asked questions
What is a good CQI value in 4G LTE?
What is the relationship between CQI and throughput?
How does the device calculate CQI?
HiCellTek displays CQI, SINR, MCS and throughput in real time to optimize mobile network performance.