“5G NR Bearer Drop” with SCG failure “randomAccessProblem”

5G NR
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1

Experts…
In 5G DT analysis facing frequent “5G NR Bearer Drop” with SCG failure information pointed towards “randomAccessProblem”
Any insights on this type of failure?

2

So you mean ENDC setup is happening but sometimes SCG failure is seen?

3

If RLF occur due to random access problem then failureType set as randomAccessProblem.
Ex. Lets say UE is in connected mode but not uplink synchronized i.e. timeAlignmentTimer expired before RRC Inactivity timer.
In this case if there is some uplink traffic then UE will start RACH procedure and during this time if RACH gets failed on NR cell then UE will trigger the SCG failure.

4

During Mobility, this will happen due to Beam Failure or RACH failures due to UL Data Arrival.

5

I think this is when you have 5G coverage holes you will get this type of messages quite often.

6

Just curious to know, is your NR drop happening when LTE anchor is changed and the new LTE CELL is not configured to be an anchor for the initial NR cell?

7

Hi,

There could be many reason of SCG failure with cause: Random Access problem but while log analysis
our 1st step should be to check UL BLER status ,if there are High UL BLER seen and at same time UL data sent by UE then this problem can come. other reason possible during NR aggregation , after msg 2 , msg3 not reached to N/W or reached but no MSG 4 . hence UE can try many times msg1–> msg2–>msg3 , finally it reached to max retransmission count and then Trigger SCG failure with Cause: Random Access Problem

8

Probably DL/UL mismatch. You may need to increase B1 threshold

9

SCG failure triggered by UE can be caused by rlc-MaxNumRetx,t310-Expiry, randomAccessProblem,scg-reconfigFailure or synchReconfigFailure-SCG during mobility in case of beam failure or in bad NR coverage because RA failures .

in your case you get SCG failure because of randomAccessProblem, so you need to take action to solve the original reason of the failures in addition to configure the SCG failure handling that can handle this failure & allowing the UE to re-synch for serving cell / instant measurements for serving cell NBR cell/ inter-RAT measurements

10

If randomAccess is the problem, few points to check.

SSB is decoded for sure since the UE know which and where to “Random Access”
If SSB isn’t decoded during SSB-Periodicity and during B1 Activation, then different failure happens.

  1. Uplink problem
    -TA calculation (Timing Advance calculation) also uplink overhead calculation for NR access have been off or wrong. Decoded SSB RSRP in accurately tells how much uplink power is required to “Random Access” gNB. If SSB0 , SSB1 , SSB2 or so on antenna Line of Sight is obscured when or during Random Access, failure happens also. It depends how UE calculated SSB-RSRP and Uplink power is usually average value of beams of SSB decoded during “SMTC window”.

  2. Timing Issue
    -If eNB and gNB isn’t synced up in Real Time, this could be causing the problem, FDD LTE to TDD 5G require to know SN for PDCP and also exact CP system time when to Random Access. SN for PDCP isn’t exactly related to RACH process. But as I say in every possible error scenarios, because of “Timing” issue. There’s problem with 5G TDD gNB. GPS lock has to be always in sync or in margin of GPS holdover.

Unless the parameters for gNB and eNB is all checked up, you shouldn’t face RACH problem unless it’s bad signal issue.

11

Agreed, should be a major reason.
Besides this, sometimes it is also reported if cell radius set too short for SCG cell, and user is farther than configured cell radius.

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12

start first by comparing your TA and cell Radius, if you find that you TA>= Cell Radius, increase your cell Radius to 14.5km this is the max I guess for all vendors, if your Cell Radius is into its max value 14.5km than try downtilt your NR Cell, let me know if this help also if u can share some snapshoots for DT if you have to analyze and support you further