Why the minimal MCS chosen by the eNodeB for the user?

Question

As far as I know eNodeB calculates the number of Physical Resource Blocks allocated to each UE, based on the specific metric, then takes the minimal modulation and coding scheme to deduce the transport block size that must be allocated to each user, Why this minimal MCS ? Isn't it too sub-optimal?

Answer 1

It's not that eNodeB choose minimal MCS for the user. It assigns the MCS on basis of UE channel information which is gathered by CQI - that reflects Signal to Noise ratio measurement, which fulfills required BER for that. With this information eNodeB decides how many bits/modulation symbol can be used for the transmission with required number of physical resource blocks.

So minimal modulation coding scheme not have to be minimal, it depends upon the algorithm you used. As I defined eNodeB may assign the optimal MCS that corresponds to the received CQI, above on basis of CQI and if it’s too optimistic (i.e. receives always NACKs) then it adjust parameters and assigns also lower than optimal MCS.