5G NR SCG Failure and Radio Link Failure

If the same situation occurs on the MN node, then the UE triggers the Re-establishment procedure whereas, in the case of the SN node, UE triggers the SCG failure with the proper value of failure type based on different conditions: 

SCG radio link failure: A UE mark the radio link failure on the SCG cell if any of the conditions are met:

• Expiry of T310 in Pcell: If the Radio link failure (RLF) occurs because of T310 expiry then the UE trigger SCG failure with failure type as t310-expiry. UE starts the T310, upon receiving N310 consecutive “out of sync” indications from the lower layer. 
• Random access problem indication from SCG MAC: if RLF occurs due to random access problem then failure type is set as random access problem. 
• Max RLC re-transmission reached: in this case failure type is rlc-MaxNumRetx. 

Failure of SCG reconfiguration with sync: If NR cell sync failed during NR cell addition the UE triggers the SCG failure with failure typeset as scg-ChangeFailure. 

SCG configuration failure for RRC message on SRB3: if reconfiguration message received over SRB3 is invalid then UE triggers SCG failure typeset as scg-reconfigFailure. If reconfiguration is received over SRB1 is invalid then UE triggers the re-establishment. 

SCG Integrity checks failure: if reconfiguration message received over SRB3 integrity check failed then UE triggers SCG failure with failure typeset as srb3-Integrity Failure.  

The following SCG failure cases are supported:

• SCG RLF
• SN change failure
• For EN-DC, NGEN-DC, and NR-DC, SCG configuration failure
• For EN-DC, NGEN-DC, and NR-DC, SCG RRC Integrity check failure
• Upon SCG failure the UE suspends SCG transmissions for all radio bearers and reports the SCG Failure information to the MN, instead of triggering re-establishment. 
• In all SCG failure cases, the UE maintains the current measurement configurations from both the MN and the SN and the UE continues measurements based on the configuration from the MN and the SN if possible. 
• The UE includes in the SCG failure information message the measurement results available according to the current measurement configuration of both the MN and the SN. 

Type of SCG failure reason

• T310 expiry
• synchReconfigFailure-SCG
• Random access problem
• rlc-MaxNumRetx
• srb3-IntegrityFailure
• scg-reconfigFailure 

The UE suspends SCG transmission for all Signalling radio bearers (SRB) and DRBs and reset its MAC entity in the 5GC. 

Afterward, the UE sends SCGFailureInformation message on its primary leg i.e to the Master Node. the SCGFailureInformation may be used by the UE’s Master Node to facilitate subsequent SCG configuration. The Master Node may alternatively decide not to add an SCG for the UE.