UE in ENDC (NSA) suddenly declaring RLF cause: MAX RETRX

5G NR
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1

Hi Experts.

Any idea why an UE in ENDC (NSA) mode on 5G connected mode with data download suddenly declares RLF in LTE with RLF cause: MAX RETRX?

Any suggestions or feedback on what to check here to root cause the reason for the RLF declared by UE?

2

Maximum number of ARQ retransmission at RLC level was reached.
Usually this is related to poor RF.

3

Check if the RLC retransmission is = t32.
This is the maximum.

4

Thanks I will check this.
Do you know how to check it on qcom UE side logs using xcap?

Thanks. Yes it is t32.

5

It is at MAC layer if you have user plane enabled.
Check if SN 3547 (for example) was retransmitted 4 times.
So at RLC there are 32 retransmissiosn while at MAC level there are 4 retransmissions for HARQ process.
It depends on parameters but should be like this.

6

Right. But do you know any specific parameters from a Qualcomm or Exynos (Samsung) UE logs to check (to track the retx)? This?

7

retx in DL are done by eNodeB I think you cannot see settings in UE logs.

8

Take a look at RLC-level packets on QXDM/QCAT and you will be able to find the ones where you have the indications to understand if the packet was retx’ed.

For instance, you have control PDUs pointing what SNs were ACK’ed.

9

Since max retx is reached by UE it has to be UL.

Sure, thanks.

10

Max retx works both for UL and DL with separate params for each.

11

Max retx on DL is for eNB and has a different criteria.

12

What is the acceptable value for retransmission?

13

It is usually t32 in UL.

14

The root cause for this behavior has been found.
RRC was configuring ANR-DRX for the UE while data transaction was going on. Due to this MAC Scheduler was not scheduling the UE in DL even though DL BO was reported by RLC to MAC (for STATUS PDU) in eNB. Due to this eNB was not transmitting the STATUS PDU to UE and hence RLF was reached in UE due to RLC MAX RETX.
Whether to schedule the UE in DL after DRX configuration from RRC, after having received SR Indication from UE and scheduling the UE in UL, is a design preference in eNB as Specification does not mention anything about it. In this case, eNB chose not to schedule the UE in DL even though it was being scheduled in UL when SR was received from UE.
Thanks all for your input and suggestions.

1 Like
15

How we can verify this conclusion
from field layer 3 message ?

16

Via SCGFailureInformationNR

17

@marcengo I would like to ask for 5G SA layer 3 massage for RLF.