CQI: Channel Quality Indicator

Basically the CQI in LTE is calculated for the downlink channel by the UE/Mobile and practically it implies:

• Current communication channel quality
• UE wants to get the data according to current channel quality transport block size, which in turn can be directly converted into throughput

There is no explicit description in any standard on the mechanism by which the CQI is calculated, but the following factors play important roles to CQI measurement:

• signal-to-noise ratio (SNR)
• signal-to-interference plus noise ratio (SINR)
• signal-to-noise plus distortion ratio (SNDR)

UE does this mostly by estimating the channel quality using the downlink cell specific reference signal. The UE gets to calculate the SINR based on the strength of the reference signal and it usually has a look up table as to tell what SINR maps to what CQI. Once done, UE reports this CQI over PUCCH or PUSCH.

In LTE, there is a mapping between CQI and modulation scheme. With the modulation scheme in the table, you would get a certain range of MCS you can use for each CQI index. But you cannot pinpoint a specific MCS and Number of RBs. You need another condition to get the proper MCS and N RBs and it is 'Code Rate'. But still there is not a single formula that would give you a single/determined value for MCS and NRB. 

The PUCCH Reporting Type determines whether Subband or Wideband CQI is being reported. [3] 

References

[1] http://www.sharetechnote.com/html/Handbook_LTE_CQI.html
[2] https://www.quora.com/How-Channel-Quality-Indicator-is-calculated-in-LTE-Technology
[3] http://www.sharetechnote.com/html/Handbook_LTE_CQI_PMI_RI_ReportingConfiguation.html