5G Drop Rate – How To Tackle 5G Call Drops is the third session covering 5G Call Flows. It explains 5G Drops or SCG Failures in detail.
The 5G Drops can be categorized into 2 categories which are UE Initiated Drops and gNB Initiated Drops. Both of these drops are explained along with their causes and signalling flows.
References: 3GPP 36331 & 3GPP 38133
5G Call Flows (Session 3): 5G Drop Rate – How To Tackle 5G Call Drops
In case you have missed previous sessions and would like to view/read them before Session 3, please follow below URLs.
- As per 3GPP, the 5G Drops can be categorized into two broad categories
- The UE initiated SCG Failures which are mostly related to uplink problems
- The gNB initiated SCG Failures which are mostly related to downlink problems
- The signalling flow is different for both of these scenarios as shown on the right
- One of the causes of 5G drops can be related to RACH.
- It is interesting because a RACH failure usually means an accessibility failure in other technologies. However, a RACH failure in 5G NR (NSA) means a call drop
- This is due to the fact because a 5G NR RACH happens after the 5G NR leg has been successfully setup i.e. after a successful ENDC setup.
- So, a RACH failure will cause the UE to drop back to LTE and this will result as a call drop in the KPI
- If all the RACH attempts fail then the UE initiates a SCG Failure Information towards the anchor eNB which results in SCG Release. This is considered a 5G Drop on the gNB.
UPLINK RLC DROPS
- Another cause of 5G call drop can be related to data retransmissions.
- If the UE fails to send data successfully to the gNB or the gNB fails to receive the data successfully for a configurable number of times, then the UE will initiate a SCG Failure Information towards the anchor eNB which results in SCG Release. This is also pegged as a 5G Drop on the gNB.
POOR QUALITY DROPS
- The UE always keeps measuring the link adaptation channels to verify the channel quality.
- This means that the UE will keep monitoring the SSB channel and will always know about its SINR.
- If the SINR is low such that the UE’s estimated BLER for PDCCH exceeds 10% then it is considered a Qout. Duration of Qout is equal to the SSB period so if SSB period is 20ms then a Qout will be triggered after 20ms. (3GPP 38133)
- This means that if N310 is 3 then the total duration for 3xN310 would be around 60 ms.
- If consecutive N310 (pre-configured) events happen then the UE initiates T310 timer. If before T310 timer expires, the UE is able to retain the link (N311), then the UE stops the T310 timer.
- N311 is defined as the SINR where the estimated PDCCH BLER is below 2% (3GPP 38133)
- If the UE is unable to retain the link and T310 expires then the UE will initiate RLF.
DOWNLINK RLC DROPS
- Similarly, If the UE fails to receive data successfully from the gNB for a configurable number of times, then the gNB will initiate a SCG Release towards the anchor eNB. This is also pegged as a 5G Drop on the gNB.
- The difference here is that in Uplink retransmission problem, the UE sends the SCG Failure towards anchor eNB while in Downlink retransmission problem, the gNB identifies the drop and initiates SgNB Release towards the anchor eNB.
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