5G Handovers – Connected Mode Mobility
When UE is in RRC Connected state and it roam from one gNB to another gNB to another gNB, it performs handovers. Handover is essential for data and session connectivity. Based on the mobility route of a UE, there can be different types handover possible and some of these are listed below.
5G Mobility Types
Here is the list of some of very much known 5G Mobility Types.
- Intra gNB Handover
- Inter gNB Xn Handover
- Inter gNB N2 Handover
- Inter gNB Inter AMF N14 Based Handover
- Inter RAT N26 Based Handover
Intra gNB Handover: This type of handover occurs in gNBs when UE is moving from one cell to other cell connected to the same gNB. It does not require to change the AS security algorithms during intra gNB handover as the security termination point remains the same. If the UE does not receive an indication of new AS security algorithms during an intra gNB handover, the UE can continue to use the same algorithms as before.
Inter gNB Xn-Handover: The handover of UE from a source gNB to a target gNB over Xn is known as Xn-handover. It does required active Xn interface between Source and Target gNB. This Types of Handover is very fast as it requires minimum interaction with the 5G Core network. with respect to security concers the source gNB includes the UE security capabilities in the handover request message containing the ciphering and integrity algorithms used in the source cell. The target gNB can selects the algorithm with highest priority from the received security capabilities of the UE according to the prioritized local configured algorithms. The selected algorithms are indicated to the UE in the Handover Command message if the target gNB selects different algorithms.
Inter gNB N2-Handover: When the source and Target gNB does not have active Xn interface or handover over Xn interface is not allowed, then gNB can decide to perform handover over N2 interface. This Types of handover can be Intra AMF HO or Inter AMF HO. In this type of handover AMF play the anchor role for co-ordinating between Source and Target gNB to make Handover as a success. For N2-handover, the source gNB includes AS algorithms used in the source cell in the source to target transparent container that is sent to the target gNB.
Inter AMF N14 Based Handover: 3GPP has defined N14 interface to connects two AMF belonging to two different Operators or serving two PLMNs. When Source and Target gNBs are connected to different AMF, then handover can be triggered over N14 interface. By default the Inter AMF handover is considered as N2 Handover but in some case N14 interface can also be used for mobility.
Inter RAT N26 Based Handover: To support Inter RAT (4G-5G Interworking) mobility the 3GPP has defined N26 inteface which connect 5G AMF to 4G MME. The Inter-RAT Mobility is useful in early stages of SA deployment when operators do not have full coverage of 5G and coverage gaps are filled with 4G coverage. Under this condition UE may need to perform handover from 5G to 4G or visa versa then it can take a path of N26 interface.
- 3GPP TS 38 300- 5G NR Overall description Stage-2
- 3GPP TS 38.401 – 5G NG-RAN Architecture Description
- 3GPP TS 38.412 – 5G NG-RAN NG signalling transport