How the UE determines MCS that is reported as part of CQI feedback?

Hi Experts.

Does anyone know how the UE determines MCS that is reported as part of CQI feedback?

I recall it is related to the 10% BLER… but details of procedure is appreciated.

Admin note: this post was updated with image below.

CQI is from 0 to 15 MCS is from 0 to 27.

MCS is adapted based on CQI report and target BLER.

I mean how does UE determine the MCS that it feedbacks to gNB?

I’m interested in the procedure of obtaining the recommended MCS.

It feedbacks CQI, not MCS.

Based on CQI and target BLER and previous BLER values it adjusts the new MCS for next scheduling.

Just keep in mind that CQI reports comes like every 80,160 or 320 msec.

Yes, I meant CQI that includes MCS. But how does the ue use the target BLER?

I mean how is the target BLER used to obtain MCS?.

It is easy.

Let’s say that UE reports CQI 13.

This would mean that MCS had to be something like 23, 24 or 25.

If previous BLER values were lower than 10% than it will use MCS25.

If previous BLER values were higher than 10% it will use MCS23 or even lower than that it depends how high were previous BLER values.

I mean the NACK values.

There are very complicated algos how MCS is adjusted slow or fast), same for MIMO rank change, same for CA activation or not.

You need to read vendor documentation for this.

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DL/UL link adaptation step up/down can be configured from DU.

I believe typical config for step down >> step up (ex. 30 % vs. 3%).

3GPP tables for CQI to code-rate / efficiency and MCS to code rate efficiency are shown in the table.
UE manufacturers have a propriety table for SINR to CQI reporting. This CQI gives the range of efficiency and the code rate UE can support based on the current feedback.
gNB OEMs also have a proprietry mechanism to consider configured BLER target or how often change it, based on these inputs one of the MCS can be selected in the range of reported CQI.

eg: CQI 12 reported by UE then MCS 20 to 27 can be selected based on the link adaptation algorithm set by the gNB OEMs

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