High CQI; 10% BLER; Low MCS

5G NR
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1

Hi Experts,

I am doing some data-collection on commercial T-mobile SA network in DFW area. I only have access to the UE logs and not the NW side logs.

I have noticed a case in which the CQI at the UE side is high (~12) but the MCS is quite low (2-3) and BLER is at 10%.

What could be the reason for this wierd behavior?

High CQI; 10% BLER; Low MCS

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could be low DL buffered traffic in enodB side

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Thanks for the response.

Can you elaborate a bit on why we would expect low MCS when DL buffered traffic at gNB is low?

My initial thinking was that - regardless of the amount of buffered traffic - highest possible MCS should be used granted the BLER can remain <=10%.

5

BufferOccupancy should give effect on RB allocation, not on MCS.
CQI AND OLRC Output → SINR->MCS needs to be followed for MCS. Now as per that MCS and BufferOccupancy , calculate corresponding RB allocation and TBSize.

6

Thank you for the response.

Based on my quick search I could not find this table. Can you please point me to this table, i.e., 3GPP document number and table number. Thanks.

7

PUCCH power control can be the culprit due to buffered DL traffic. If PDSCH are very apart from each other there are high chances that PUCCH power is not properly maintained, so many HARQ Acks will not get received at gNodeB.
Periodically Dummy TA command(Value:31) can be sent to UE, to be in sync for PUCCH power maintenance.

8

I have similar understanding. Thanks.