Channel Quality Indicator (CQI)

CQI can be reported as a single wideband value for the entire system bandwidth or as multiple sub-band values for frequency-selective scheduling.

• Wideband CQI: Averages channel quality across all resource blocks, minimizing feedback overhead.
• Sub-band CQI: Provides per-sub-band quality metrics, enabling the scheduler to allocate resources in frequency regions with the best conditions.
• The configuration is an RRC parameter, balancing granularity against uplink control channel capacity.

The reported CQI index directly corresponds to a specific modulation order and code rate that the UE can decode with a transport block error probability not exceeding 0.1.

• A 4-bit CQI value (0-15) maps to combinations from QPSK to 256QAM.
• CQI 1: QPSK, very low code rate (78/1024).
• CQI 7: 16QAM, code rate 378/1024.
• CQI 15: 256QAM, code rate 948/1024.
• The eNB/gNB is not forced to follow the recommendation but uses it as an upper bound for scheduling decisions.

CQI derivation relies on Channel State Information Reference Signals (CSI-RS) in 5G NR or Cell-Specific Reference Signals (CRS) in LTE.

• The UE estimates the Signal-to-Interference-plus-Noise Ratio (SINR) from these known pilot symbols.
• It then selects the highest CQI index whose associated transport block error rate remains below 10% under the measured SINR.
• The accuracy of CQI is fundamentally limited by the density and interference profile of the reference signals.

CQI feedback can be triggered in two distinct modes to balance latency and resource usage.

• Periodic CQI: Configured via RRC, the UE transmits CQI on PUCCH at fixed intervals. Low latency overhead but less flexible.
• Aperiodic CQI: Triggered dynamically by a DCI format 0_1 grant, the UE transmits a detailed report on PUSCH. This allows on-demand, high-resolution feedback for bursty traffic.
• Aperiodic reporting can be multiplexed with data for efficient resource utilization.

To reduce feedback overhead in sub-band reporting, differential CQI encodes the quality of sub-bands relative to the wideband value.

• The wideband CQI is reported as an absolute 4-bit index.
• Each sub-band CQI is then reported as a 2-bit differential offset (-1, 0, +1, +2) relative to the wideband level.
• This exploits the frequency correlation of the channel, significantly compressing the payload without losing scheduling flexibility.

5G NR defines three distinct CQI tables to support different spectral efficiency targets and use cases.

• Table 1 (64QAM): Default table supporting up to 64QAM with a maximum efficiency of 5.55 bps/Hz.
• Table 2 (256QAM): Extends to 256QAM for high-SINR scenarios, reaching 7.41 bps/Hz.
• Table 3 (Low-SE): Designed for ultra-reliable low-latency communication (URLLC), targeting a 0.00001 BLER instead of 0.1.
• The active table is configured via higher-layer signaling.

A UE feedback parameter that indicates the number of usable independent spatial layers or streams that can be supported under current channel conditions. RI directly constrains the CQI reporting—a higher rank enables reporting CQI for multiple codewords simultaneously. In 5G NR, RI values range from 1 to 8, depending on the UE's antenna configuration. The UE estimates the channel matrix, computes its singular values, and determines how many eigenmodes exceed a signal-to-noise ratio threshold. A mismatch between reported RI and actual channel rank leads to either underutilized spatial capacity or excessive block errors.

A UE feedback index that recommends a specific precoding matrix from a predefined codebook for the transmitter to use in beamforming subsequent transmissions. PMI and CQI are jointly computed—the UE assumes the recommended precoder will be applied when calculating the CQI value. In 5G NR Type-I codebooks, PMI selects a DFT-based beam from an oversampled grid. Type-II codebooks provide higher resolution by reporting amplitude and phase coefficients for multiple beams. The gNB may override the PMI recommendation, but doing so invalidates the accompanying CQI assumption.

The known channel properties of a communication link, including scattering, fading, and power decay, used by a transmitter to adapt its signal to current propagation conditions. CQI is one component of the broader CSI report, alongside RI, PMI, and CSI-RS Resource Indicator (CRI). The full CSI framework enables link adaptation: the UE measures downlink reference signals (CSI-RS or SSB), estimates the channel matrix, and feeds back quantized recommendations. In FDD systems, CSI must be explicitly fed back due to lack of channel reciprocity, making CQI accuracy critical for spectral efficiency.

A predefined combination of modulation order and channel coding rate that determines the instantaneous data rate of a transmission. CQI directly maps to an MCS table—each CQI index corresponds to a specific modulation (QPSK, 16QAM, 64QAM, 256QAM) and target code rate. The gNB uses the reported CQI to select an MCS for the next downlink grant, though it may apply an outer loop adjustment based on HARQ statistics. 5G NR defines multiple MCS tables optimized for different spectral efficiency targets and reliability requirements (eMBB vs. URLLC).

The ratio of erroneously received transport blocks to the total number of transmitted transport blocks, serving as the primary link quality metric. CQI is defined relative to a target BLER—typically 10% for eMBB services in LTE and 5G NR. The UE estimates the highest MCS that would not exceed this BLER threshold given current SINR conditions and reports the corresponding CQI. An outer loop link adaptation algorithm at the gNB adjusts the MCS offset based on observed HARQ ACK/NACK statistics to maintain the target BLER despite CQI estimation errors.

The ratio of desired signal power to the sum of interference and noise power, representing the fundamental physical-layer metric that drives CQI estimation. The UE maps measured SINR to a CQI index through a lookup process that accounts for receiver capability (e.g., MMSE-IRC, ML detection). Effective SINR mapping techniques such as Exponential Effective SINR Mapping (EESM) or Mutual Information Effective SINR Mapping (MIESM) compress per-subcarrier SINR values into a single metric for wideband CQI reporting. Sub-band CQI provides finer granularity by reporting per-resource-block-group SINR.