5G(NR): Xn Based Handover

5G(NR): Xn Based Handover

 

Introduction:

The basic handover procedures is same in any networks, i.e. UE reports measurement report with neighbor cell PCI and signal strength to source cell, source cell take the decision to start handover procedure to best target cell and Target Cell completes the Handover procedure.

 

Impotent Pointers for Xn Handover:

1. Signal strength of both source gNB and target gNB should be reachable to UE during the HO. 
   
2. Xn Handover is similar X2 Handover in 4G LTE
   
3. XnAP interface must be established between source and Target gNB.
4. This type of Handover is only applicable for intra-AMF mobility, i.e. Xn handover cannot be used if Source and Target gNB is connected to different AMF
5. Xn Handover can be Intra Frequency HO and Inter Frequency HO
6. Source and Target gNB can be connected with two different UPFs
7. Tracking Area code should be same. Re-Registration is required after Successful Handover if the Source gNB and Target gNB belong to different Tracking Area (TAC)
8. Xn Handover is Faster as Compare to N2/NGAP Handover due to short signalling path and 5G Core involved in only for switch the PDU session path

High level setup diagram:

      where both the gNB is served by same AMF and UPF. and for XnHO, XnAP interface is active between source gNB and target gNB.

Signaling Exchange b/w Source gNB and target gNB.