CQI (Channel Quality Indicator) is a key parameter in LTE (Long Term Evolution) networks that reflects the quality of the radio channel between a User Equipment (UE) and the eNodeB (base station). It is used to inform the eNodeB about the downlink channel conditions so that it can select the most appropriate Modulation and Coding Scheme (MCS) to maximize throughput while maintaining reliable communication.
CQI is measured and reported by the UE and helps the network dynamically adapt transmission parameters for each user, improving spectral efficiency and overall network performance. The higher the CQI value, the better the channel quality, and the more aggressive the MCS that can be used.
How CQI is Determined
The process of CQI determination involves several stages that occur within the UE. These steps are repeated periodically or based on certain events such as channel condition changes.
1. Downlink Channel Measurement
The UE constantly monitors the downlink signals transmitted by the eNodeB, particularly the Reference Signals (RS). These signals are known sequences embedded in the LTE downlink resource grid, which allow the UE to estimate the quality of the channel.
- RSRP (Reference Signal Received Power): Measures the average power of reference signals.
- RSRQ (Reference Signal Received Quality): Ratio of RSRP to total received power (including interference and noise).
- SINR (Signal to Interference plus Noise Ratio): Calculated from reference signals and used to infer channel quality.
While RSRP and RSRQ are also used for cell selection and mobility, SINR is directly tied to CQI estimation.
2. SINR to CQI Mapping
Once the SINR is measured, the UE maps this value to a CQI index using a standardized or operator-defined lookup table. The mapping is not universal and can vary slightly depending on implementation, but 3GPP provides general guidelines.
| CQI Index | Modulation | Code Rate | Estimated SINR (dB) |
|---|---|---|---|
| 1 | QPSK | Low (e.g. 0.076) | ~ -6.7 |
| 5 | QPSK | Medium (e.g. 0.3) | ~ -1.0 |
| 9 | 16QAM | Higher (e.g. 0.6) | ~ 5.5 |
| 15 | 64QAM | High (e.g. 0.93) | ~ 18.7 |
The UE selects the highest CQI value for which the current SINR meets or exceeds the required threshold. A CQI of 15 indicates very good channel conditions, while lower values indicate poorer channel quality.
3. CQI Reporting
After determining the appropriate CQI value, the UE sends this information back to the eNodeB through the Physical Uplink Control Channel (PUCCH) or the Physical Uplink Shared Channel (PUSCH). The reporting can be:
- Periodic: Sent at regular intervals defined by network configuration.
- Aperiodic: Sent only when requested by the network or based on events like rapid channel degradation.
The format and scheduling of CQI reporting are configured by the network, and the UE adheres to these instructions.
4. Usage by eNodeB
Once the eNodeB receives the CQI report, it uses it to determine:
- The Modulation and Coding Scheme (MCS) for upcoming downlink transmissions.
- Which resource blocks (RBs) to allocate to the UE.
- Whether link adaptation is needed (e.g. fall back to lower MCS if CQI drops).
This feedback loop allows the LTE system to adapt rapidly to changing radio conditions, such as movement of the UE or interference from neighboring cells.
Relation Between CQI and MCS
Each CQI index corresponds to a specific MCS. Below is a simplified mapping:
| CQI | MCS Index | Modulation | Efficiency (bits/symbol) |
|---|---|---|---|
| 1-3 | 0-3 | QPSK | ~0.15 – 0.38 |
| 4-9 | 4-11 | 16QAM | ~0.6 – 1.5 |
| 10-15 | 12-28 | 64QAM | ~1.6 – 5.55 |
A good CQI allows higher order modulation like 64QAM or even 256QAM (in LTE-Advanced Pro) to be used, which significantly increases the data rate.
Factors Affecting CQI
- Distance from the eNodeB: The farther the UE is, the lower the SINR and CQI.
- Interference: From other cells or external sources reduces signal quality.
- Mobility: High-speed movement can cause fading and Doppler effects.
- Obstacles: Buildings, trees, and walls can attenuate the signal.
- Frequency Band: Higher frequencies suffer more from attenuation.
The UE continuously monitors these changing conditions and updates the CQI reports accordingly to ensure optimal performance.
In summary, CQI determination in LTE is a dynamic and continuous process that enables adaptive modulation and coding, ensuring efficient use of radio resources based on real-time channel conditions. It is essential for maintaining high throughput and reliability in modern mobile networks.