CQI stands for Channel Quality Indicator.
- Definition: CQI is a value that indicates the quality of the downlink communication channel as experienced by the User Equipment (UE). It is a measure of the channel conditions and helps the eNodeB (base station) in making decisions regarding modulation and coding schemes for data transmission.
- Range: The CQI value ranges from -1 to 15.
- –1: Represents the poorest channel quality.
- 15: Represents the best channel quality.
Below picture represents CQI Plot & Threshold during which is derived from LTE drive test log files.
CQI Importance:
- Indicates Channel Quality: A higher CQI value indicates a higher quality channel, which can support higher data rates with more efficient modulation and coding schemes. Conversely, a lower CQI value suggests a poorer channel quality.
- Impacts Throughput: Better CQI values allow for higher throughput as the eNodeB can use more advanced modulation schemes (e.g., 64QAM) and higher coding rates. Poor CQI values result in the use of more robust but less efficient schemes (e.g., QPSK).
- Dedicated Mode: CQI is measured by the UE and reported to the eNodeB while in dedicated mode, where a specific connection is established between the UE and the network.
Dependencies and Measurement:
- RF Conditions: CQI is dependent on various radio frequency conditions, including signal strength, interference, and noise.
- Feedback Mechanism: For downlink data transmission, CQI feedback from UEs helps the eNodeB understand the interference status and optimize resource allocation.
- HARQ (Hybrid Automatic Repeat Request): CQI values, along with HARQ retransmission information, help in predicting the interference on uplink subcarriers.
In summary, CQI plays a critical role in LTE networks by providing essential feedback on channel conditions, enabling efficient resource management, and ensuring optimal throughput based on current radio conditions.
Read Also: What is RSRP, RSRQ, SINR, RSSI in LTE?