Hi Experts.
What is use of different MCS values for same QAM?
How to know which MCS to choose?
It is not just Modulation, Coding is different too.
This is chosen based on BLER too.
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.
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
