Channel Quality Indicator
Understanding Channel Quality Indicator
Link Adaptation and Downlink Scheduling
In modern cellular networks (such as 4G LTE and 5G New Radio), the physical channel conditions between the base station (gNB) and the user equipment (UE) fluctuate constantly due to distance, obstacles, and interference. To maximize spectral efficiency, the base station must dynamically adjust its transmission parameters, a process called Adaptive Modulation and Coding (AMC). The key feedback parameter that drives this process is the Channel Quality Indicator (CQI).
The UE continuously measures the downlink signal quality (specifically the SINR) using reference signals. It maps the measured SINR to a specific CQI index value, typically ranging from 0 to 15. The UE then transmits this CQI value back to the base station in the uplink control channel. A high CQI value indicates a clean channel, prompting the base station to use high-order modulation (like 256-QAM) and a high coding rate to maximize data throughput. A low CQI value indicates a degraded channel, forcing a fallback to robust modulation (like QPSK) and heavy error correction.
CQI Mapping Tables and Reporting Modes
The mapping between the measured channel quality and the reported CQI index is defined by standardized tables in the 3GPP specifications. Each CQI index corresponds to a specific modulation scheme and code rate. The standard requires that the selected MCS must allow the UE to decode the downlink data with a Block Error Rate (BLER) not exceeding 10%. This ensures a reliable connection while pushing the data rate as close to the channel capacity as possible.
3GPP defines multiple CQI reporting modes to balance feedback overhead against scheduling accuracy. In wideband reporting, the UE reports a single CQI value for the entire channel bandwidth, which is suitable for flat fading. In sub-band reporting, the UE reports separate CQI values for different sub-bands, allowing the base station to perform frequency-selective scheduling, transmitting data only on the subcarriers with the highest quality. Reporting can be periodic (transmitted at fixed intervals) or aperiodic (triggered by a request from the base station).
Key Mathematical Relations
Technical Specifications Comparison
| CQI Index | Modulation Scheme | Approximate Code Rate | Spectral Efficiency (bps/Hz) | Typical SINR Range (dB) |
|---|---|---|---|---|
| 0 | Out of Range | N/A | N/A | < -6 dB (Connection lost) |
| 1 - 3 | QPSK (Robust) | 0.078 - 0.188 | 0.15 - 0.377 | -6 to -1 dB (Severe cell edge) |
| 4 - 6 | QPSK / 16-QAM | 0.300 - 0.490 | 0.60 - 1.96 | -1 to 5 dB (Moderate cell edge) |
| 7 - 9 | 16-QAM / 64-QAM | 0.478 - 0.601 | 2.40 - 3.90 | 5 to 11 dB (Average coverage) |
| 10 - 12 | 64-QAM | 0.650 - 0.852 | 4.21 - 5.11 | 11 to 17 dB (Good signal) |
| 13 - 15 | 64-QAM / 256-QAM | 0.754 - 0.925 | 5.55 - 7.40 | > 17 dB (Excellent line-of-sight) |
Frequently Asked Questions
What is the primary purpose of the Channel Quality Indicator (CQI)?
The primary purpose of CQI is to enable Adaptive Modulation and Coding (AMC) on the downlink. By reporting the channel conditions back to the base station, CQI allows the network to dynamically select the highest possible data rate that can be reliably decoded by the user, maximizing spectral efficiency.
How does the UE determine the CQI index to report?
The UE measures the Signal-to-Interference-plus-Noise Ratio (SINR) of the downlink reference signals. It then references a standard 3GPP mapping table to find the highest CQI index whose corresponding modulation and coding rate yields a block error rate (BLER) of less than 10% under those SINR conditions.
What is the difference between wideband and sub-band CQI reporting?
Wideband CQI reporting provides a single average channel quality index across the entire carrier bandwidth, which minimizes feedback overhead. Sub-band CQI reporting divides the bandwidth into sub-bands and reports quality for each, allowing the base station to schedule data on the best frequencies, boosting throughput in frequency-selective fading.