Connected mode
5G Xn-based handover
Inter-gNB handover prepared over the Xn interface, with an NG path switch toward the core.
Overview
The source gNB prepares the target directly over XnAP (Handover Preparation). The UE is told to move with an RRCReconfiguration containing reconfigurationWithSync; applying it starts timer T304 for the cell group. The AMF is involved at the end through a Path Switch Request.
Handover decisions are driven by measurement reports. Event A3 (neighbour better than serving by an offset, held for a configured TimeToTrigger) is the typical trigger. Data is forwarded between gNBs, with PDCP sequence state handed over by SN Status Transfer.
Call flow
- 1UEtoSource gNB
MeasurementReport
Triggered by a configured event such as A3.
- 2Source gNBtoTarget gNB
HANDOVER REQUEST (XnAP)
- 3Target gNBtoSource gNB
HANDOVER REQUEST ACKNOWLEDGE
Carries the handover command prepared by the target.
- 4Source gNBtoUE
RRCReconfiguration (reconfigurationWithSync)
The UE starts T304.
- 5Source gNBtoTarget gNB
SN STATUS TRANSFER + data forwarding
- 6UEtoTarget gNB
Random access, then RRCReconfigurationComplete
T304 is stopped when the reconfiguration with sync completes.
- 7Target gNBtoAMF
N2 PATH SWITCH REQUEST
Core-side switch of the user plane to the target.
- 8Target gNBtoSource gNB
UE CONTEXT RELEASE (XnAP)
Timers and parameters
| Name | Where | Role |
|---|---|---|
| T304 | UE | Started with reconfigurationWithSync. Expiry is a reconfiguration-with-sync failure. |
| TimeToTrigger | UE | Duration the event entering condition must hold before the report is triggered. |
Specifications
| Spec | What it covers |
|---|---|
| TS 38.300 | Handover in RRC_CONNECTED.(see clause 9.2.3.2) |
| TS 38.331 | T304 expiry (reconfiguration with sync failure).(see clause 5.3.5.8.3) |
| TS 38.423 | XnAP Handover Preparation. See also 8.2.2 SN Status Transfer and 8.2.7 UE Context Release.(see clause 8.2.1) |
| TS 23.502 | Xn based inter NG-RAN handover at the core-network level.(see clause 4.9.1.2) |
Changes by release
| Release | Change |
|---|---|
| Rel-16 | Conditional handover (condRRCReconfig-r16) and DAPS handover IEs appear in TS 38.331. |
| Rel-17 | NTN support: SIB19 and the conditional events condEventD1 and condEventT1. |
| Rel-18 | L1/L2 triggered mobility (ltm-Config-r18). |
| Rel-19 | Further LTM enhancements, including conditional LTM and LTM with NR-DC (-r19 fields). |
Failure behaviour in the specs
- On T304 expiry for the MCG the UE initiates RRC re-establishment, and sets reestablishmentCause to handoverFailure (TS 38.331 5.3.5.8.3 and 5.3.7).
Related procedures
Connected mode
5G NG-based (N2) handover
Handover whose preparation is signalled through the AMF over N2 instead of directly between gNBs.
Connected mode
Conditional handover (CHO)
The network pre-configures candidate target cells with execution conditions and the UE moves when a condition is met.
Failure handling
Radio link failure and RRC re-establishment
Out-of-sync handling with N310, N311 and T310, then re-establishment supervised by T311 and T301.