5G(NR): Xn Based Handover

Introduction:

The basic handover and any type of mobility procedures are same in any type networks, i.e. UE measures the nearby signals and select the some good signals and make the report based on these signal strength and quality and after that sent this report to source cell, source cell take the decision to start handover procedure to best cell that is called Target Cell.  and then target cell completes the Handover procedure.

 

There are some basic Impotent Pointers for XN Handover:

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• Signal strength of both source gNodeB and target gNodeB should be reachable to UE during the handover, because during handover signaling are required with source gNB and target gNB also.
  
• Xn-based Handover is very similar X2-based Handover in LTE
  
• Xn-based handover is only possible if XnAP interface is established between source and Target gNBs.
• This type of Handover is only applicable for intra-AMF mobility (with in same AMF ), i.e. Xn handover cannot be used if Source and Target gNB is connected to different AMFs.
• Xn-based Handover can be both Intra Frequency handover and Inter Frequency handovers.
• It is possible that source and target gNB can be connected with two different UPFs(user plane functions)
• Tracking Area code should be same. RRC Re-Registration is requirred after Successful Handover if the Source gNB and the Target gNB belongs to different Tracking Area code (TAC).
• Xn-based Handover is much faster as Compare to NGAP Handover due to short signaling root and 5G Core involved in only for switch the data path and PDU session.

High level setup diagram:

      Both source  gNB  and target gNB is serving by  same AMF and UPF. and source gNB and target gNB should have the active XNAP interface and active NGAP interface with AMF.

Signaling Exchange b/w Source gNB and target gNB is as shown in picture below..

Key Steps in Xn-Based Handover

1. Measurement Reporting

• UE sends Measurement Report to Source gNB.
• IEs:
  • MeasResults: Contains signal strength, quality, etc.
  • ReportConfigId: Identifies the reporting configuration.

2. Handover Decision

• Source gNB decides to initiate handover based on measurements.
• IEs:
  • TargetCellId: Identifies the target cell.
  • Cause: Reason for handover (e.g., signal degradation).

3. Handover Request (XnAP: Handover Request)

• Source gNB → Target gNB via Xn interface.
• IEs:
  • UE Context Information: Includes UE ID, security context.
  • RRC Context: RRC configuration for UE.
  • Bearer Contexts: QoS flows and data bearers.
  • Target Cell ID: Target gNB cell.

4. Handover Request Acknowledge (XnAP)

• Target gNB → Source gNB.
• IEs:
  • RRC Configuration: For UE to access target cell.
  • Admitted Bearers: Confirmed bearers for handover.
  • Target to Source Transparent Container: it contains RRC reconfiguration info that goes to UE.

5. Handover Command (RRC: RRCConnectionReconfiguration)

• Source gNB → UE.
• IEs:
  • MobilityControlInfo: Target cell info.
  • RadioBearerConfig: Setup for new bearers.
  • MeasurementConfig: New measurement settings.

6. Random Access Procedure

• UE accesses target cell using contention-free or contention-based RA.
• IEs:
  • RA-RNTI, PreambleIndex, TimingAdvance.

7. RRC Reconfiguration Complete

• UE → Target gNB.
• IEs:
  • ReconfigComplete: Confirms successful reconfiguration.

8. Path Switch Request (Optional if UPF changes)

• Target gNB → AMF (if UPF needs update).
• IEs:
  • UE Context, Bearer Info, New Tunnel Info.

9. Handover Notify (XnAP)

• Target gNB → Source gNB.
• IEs:
  • UE ID, Handover Status.

10. Resource Release Command

• Source gNB releases UE resources.
• IEs:
  • UE ID, Release Cause.

Benefits of Xn-Based Handover

• Low latency and minimal disruption.
• No AMF/UPF involvement, reducing signaling overhead.
• Efficient resource usage and load balancing.