VoNR to VoLTE handover during a call

Inter-RAT Handover During a Live Call

When a UE on an active VoNR call moves to an area with only LTE coverage, an inter-RAT handover transfers the call seamlessly from 5G to 4G. The gNB detects degrading 5G signal quality from measurement reports indicating LTE is a better option. It sends Handover Required to the AMF, which coordinates with the target MME via the N26 interface. The MME prepares the target eNB, allocating resources including a QCI=1 bearer. The critical QoS mapping converts 5QI=1 (5G voice) to QCI=1 (4G voice), preserving the same delay and bitrate guarantees for uninterrupted call quality.