How does UE knows which modulation scheme to use (i.e. QPSK/16QAM/64QAM)?

RAN
1

Hi Experts, I have one basic query.

How does UE knows which modulation scheme to use (i.e. QPSK/16QAM/64QAM)?
Any RRC message carrying info?

I am seeing for 256QAM there is IE altcqi-table-r12 but not able to find for lower modulation 64,16 QAM.

2

In DCI format for DL assignment or UL grant, there is MCS field, which denotes which MCS Index to use.
MCS Index correspond to modulation order or scheme to use (QPSK,16 QAM and 64 QAM, 256 QAM).
There are 2 table for MCS to modulation scheme mapping.
One is without 256 QAM and one with 256 QAM.
If UE has notified to the network that it supports 256 QAM in downlink during UE capability exchange, then eNB/gNB can instruct UE via RRC Signalling to use table which contain 256 QAM.
But which modulation scheme to use for particular transport block in instantaneously given in DCI format.

3

Thanks. So it means for lower modulation scheme there is no IE available at RRC layer.

4

Yes. If UE do not support 256 QAM, then no need for RRC signalling.
IE is used to activate new table for 256 QAM.

5

Nice explanation! :ok_hand:

6

Hi Experts,

Someone has tested the deactivation of 256Qam table in bad radio conditions ?

Thanks.