CQI

A channel quality indicator is a numerical measurement that a wireless receiver transmits back to its transmitter to describe how well the signal arrived. In LTE and 5G networks, the receiver continuously evaluates its received signal and reports a CQI value, typically on a scale of 0 to 15, where 0 means the signal is too degraded to decode and 15 means near-perfect reception. The transmitter uses this feedback to adjust its transmission power, modulation scheme, and coding rate in real time.

The CQI calculation depends on several physical measurements made at the receiver. The receiver measures the signal-to-interference-plus-noise ratio (SINR) of the downlink channel, and from this single metric it infers which modulation and coding scheme (MCS) the transmitter can reliably use. A CQI of 1 permits only QPSK with a code rate of 1/5; a CQI of 15 permits 64-QAM with a code rate of 8/9. Different standards map SINR ranges to CQI levels slightly differently, but the principle is constant: the measurement tells the transmitter how much it can push.

CQI is distinct from other channel measurements. Signal strength alone (RSRP, received signal reference power) does not account for interference or noise. CQI is a link-adaptation metric; it assumes the receiver knows what throughput it can achieve. This is why CQI reporting is periodic and mandatory in cellular systems. A mobile device might report CQI every 5 or 10 milliseconds on the uplink, often using compressed or differential encoding to save uplink bandwidth.

Variants and practical limits

Wideband CQI reports a single value for the entire carrier bandwidth; subband CQI reports per-resource block or per-subcarrier, allowing the transmitter to use frequency-selective scheduling. In TDD systems, the transmitter can estimate the downlink channel from uplink sounding reference signals and infer CQI without waiting for explicit feedback, reducing overhead. In FDD, explicit CQI reporting is necessary because uplink and downlink channels are independent. Measurement reports can be delayed; a handset moving at highway speed may report stale CQI that no longer reflects the true channel, leading to retransmission and throughput loss.

CQI failures occur when the receiver miscalculates SINR, particularly in high-speed or multipath-rich environments. Overestimated CQI causes the transmitter to use too aggressive a scheme, leading to block error rates above 10 percent and retransmission storms. Underestimated CQI wastes bandwidth by requesting a more robust modulation than necessary. Serving cells and neighbor cells also affect CQI; a device in the overlap of two strong signals may report low CQI due to inter-cell interference even though the serving cell's signal is strong, forcing the transmitter to back off unnecessarily.