How does the UE detect a Radio Link Failure?
– Understanding this part is key, especially when troubleshooting or improving 4G & 5G retainability. It helps you know how the UE spots a radio link failure, which is vital for pinpointing issues. Plus, it gives you confidence when adjusting UE-related timers for better performance.
– It’s important to understand that the BTS and UE independently supervise radio procedures and events to detect RLF. Based on UE or BTS-related timer configurations, the UE or BTS can declare a Radio Link Failure (RLF) based on the network settings. This means that UE and BTS detection of out-of-sync can be asynchronous and independent events.
– There are various types of UE RLF detection methods. Below is the most common cases and UE will declare Radio Link Failure(RLF) if any of the below cases meet.
1- Out of Sync: UE Initiate Out-Of-Sync indications for a consecutive number based on configure parameters and this is when the radio link quality belonging to all the monitored Reference Signals is worse than Qout.(n310, n311, T310)
2- T304: Handover Failure will be declared when the maximum waiting time for Successful HO expire.
3- MAX UL RLC Retransmission: UE will declare RLF when the maximum RLC Retransmission is reached.(MAXULRLCTX)
4- Random Access Procedure: Random Access Procedure fails due to reaching the maximum no of preamble transmission.
Then, What happens after the RLF Declaration?
– The UE’s declaration of RLF results in the initiation of the RRC re-establishment procedure.
– Meanwhile, the BTS’s detection of Out of Sync results in a forced PDCCH procedure targeting UE to perform RRC re-establishment, and the BTS must wait for a configurable timer for the UE to execute RRC re-establishment.
Key Points & Important Notes:
– A UE facing Radio Link Failure quickly turns off its transmit power to avoid causing uplink interference after losing connection.
– 3GPP TS 38.133 states that a UE should power down within 40ms of detecting Radio Link Failure.
– The BTS must also detect this failure to prevent resource allocation towards an unreachable UE.
– These measures are designed to secure network capacity during RLF incidents.
– This also explain when UE/Network may have intermittent or complete silent interruptions. For instance, during the RLF procedure “Out of Sync Reporting,” the UE experiences voice interruptions until connection recovery.
