How MCS varies for each transmission? What factors taken into consideration for deciding MCS?

Question

Let suppose, for initial transmission MCS=28, this transmission got nack.... for retransmission what should be MCS 27 or something else ???? If retransmission_1 also nacked, what will be next MCS ?

Answer 1

Hi sripathi,

As soon as Enode recieve CQI report from UE , It will apply MCS in further transmission. Till that point it is EnodeB Scheduler implemtationto select MCS. But in most case while attach procedure itself ENodeb will configure CQI periodicity to recieve CQI and whenever datat flow starts ENodeb will apply.
Cqi range from 0 to 15, there is corresponding MCS mapping is there in Physical layer SPEC.

Comments

HI jaganathan, yes i agree with your explanation. But i want to know how MCS value varies for each transmission (if first value is 28 .... for next retransmission can i select 27 ) or any other parameters should consider?

---

It is implmentation specific, ENB can select in any way but standard scheduler implementation is scheduler will adjust only when CQI changes , IF retrasnmission means scheduler will play only with RV. Because IF you change the CQI at retx time then You need more RB's than original transmission Rb's, so most of the scheduler avoid's it. but if really CQI reoort is less than original then it can adjust.

---

if some data transmitted and its not decoded successfully, so we have to decrease no of bits transmitting in a symbol (MCS) and retransmit that data .... if we not decrease the MCS, How can we assure data will be reached properly in next transmission?

---

Hi sripathi,

It is not mandatory that we should reduce the MCS, When you do retx based on CQI reoport you can adjust, even CQI to MCS mapping you should not change frequently, say examble user is moving so he will frequently send different CQI but at ENB side we should have some mechanism that average of all cqi er should consider, so likewise when doing retx user may comes to good channel quality area so instead of going and reducing blindly mcs enb should have intelligence to tx it.