What is CQI in LTE?

The Channel Quality Indicator (CQI) contains information sent from a UE to the eNode-B to indicate a suitable downlink transmission data rate, i.e., a Modulation and Coding Scheme (MCS) value. CQI is a 4-bit integer and is based on the observed signal-to-interference-plus-noise ratio (SINR) at the UE. The CQI estimation process takes into account the UE capability such as the number of antennas and the type of receiver used for detection. This is important since for the same SINR value the MCS level that can be supported by a UE depends on these various UE capabilities, which needs to be taken into account in order for the eNode-B to select an optimum MCS level for the transmission. The CQI reported values are used by the eNode-B for downlink scheduling and link adaptation, which are important features of LTE.

CQI reporting can be either periodic or aperiodic. A UE can be configured to have both periodic and aperiodic reporting at the same time.

0. Periodic CQI reporting is defined by the following characteristics:
   
- When the UE is allocated PUSCH resources in a subframe where a periodic CQI report is configured to be sent, the periodic CQI report is transmitted together with uplink data on the PUSCH. Otherwise, the periodic CQI reports are sent on the PUCCH.

Aperiodic CQI reporting is defined by the following characteristics:

• -  The report is scheduled by the eNB via the PDCCH;
  -  Transmitted together with uplink data on PUSCH.
• When a CQI report is transmitted together with uplink data on PUSCH, it is multiplexed with the transport block by L1 (i.e. the CQI report is not part of the uplink the transport block).
  The eNB configures a set of sizes and formats of the reports. Size and format of the report depends on whether it is transmitted over PUCCH or PUSCH and whether it is a periodic or aperiodic CQI report. 
  

  

LTE supports wideband and subband CQI reporting. A wideband CQI value is a single 4-bit integer that represents an effectiveSINR as observed by the UE over the entire channel bandwidth. With wideband CQI, the variation in the SINR across the channel due to frequency selective nature of the channel is masked out. Therefore, frequency selective scheduling where a UE is placed only in resource blocks with high SINR is not possible with wideband CQI reporting. To support frequency selective scheduling, each UE needs to report the CQI with a fine frequency granularity, which is possible with subband CQI reporting. A subband CQI report consists of a vector of CQI values where each CQI value is representative of the SINR observed by the UE over a subband. A subband is a collection of n adjacent Physical Resource Blocks (PRBs) where the value of n can be 2, 3, 4, 6, or 8 depending on the channel bandwidth and the CQI feedback mode.

In the plain words, CQI is an indicator carrying the information on how good/bad the communication channel quality is. 

CQI is the information that UE sends to the network and practically it implies the following two

i) Current Communication Channel Quality is (Value from 1 to 15)

ii) UE wants to get the data with transport block size, which in turn can be directly converted into throughput.

What if network sends a large transport block even though UE reports low CQI, it is highly probable that UE failed to decode it (cause CRC error on UE side) and UE send NACK to network and the network have to retransmit it which in turn cause waste of radio resources.

In LTE, there are 15 different CQI values randing from 1 to 15 and mapping between CQI and modulcation scheme, transport block size is defined as follows (36.213)

Reference: 3GPP TS 36.300