How to know which MCS to choose?

Specs & Protocol Testing
1

Hi Experts.

What is use of different MCS values for same QAM?

How to know which MCS to choose?

2

It is not just Modulation, Coding is different too.

This is chosen based on BLER too.

3

Radio conditions impact user bit rates.
The better radio conditions the higher throughput and vice-versa.

eNB selects MCS based on current radio conditions.
UE measures CQI and send it to eNB.
Therefore eNB selects MCS based on it.

For example CQI 12 will give you a 256QAM modulation with a code rate of 0.6943 in Rel.12.

4

MCS tells us about code rate to be used.
In a single QAM, there can be different options for coding.
Code rate can be understand as ratio of useful bits to total bits transferred.
So, higher the code rate - higher the spectral efficiency & lower the code rate - higher the redundancy.
Better the radio conditions, higher MCS (code rate) assigned by Link adaptation algorithm.

Coming to second part of question - We usually assign initial MCS & after that link adaptation algorithm takes over.
Link adaptation may increase or decrease MCS based on radio link feedback such as CQI/BLER/Power headroom.
Following is eg of 256 QAM MCS Table

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