Conditional Handover

A Conditional Handover (CHO) is defined as a handover that is executed by the UE when one or more handover execution conditions are met. The UE starts evaluating the execution condition(s) upon receiving the CHO configuration, and stops evaluating the execution condition(s) once a handover is executed.

The following principles apply to CHO:

• The CHO configuration contains the configuration of CHO candidate cell(s) generated by the candidate gNB(s) and execution condition(s) generated by the source gNB.
• An execution condition may consist of one or two trigger condition(s) (CHO events A3/A5, as defined). Only single RS type is supported and at most two different trigger quantities (e.g. RSRP and RSRQ, RSRP and SINR, etc.) can be configured simultaneously for the evalution of CHO execution condition of a single candidate cell.
• Before any CHO execution condition is satisfied, upon reception of HO command (without CHO configuration), the UE executes the HO procedure as described below, regardless of any previously received CHO configuration.
• While executing CHO, i.e. from the time when the UE starts synchronization with target cell, UE does not monitor source cell.
• CHO is not supported for NG-C based handover is not allowed.

Conditional Handover-Control-Plane handling

As in intra-NR RAN handover, in intra-NR RAN CHO, the preparation and execution phase of the conditional handover procedure is performed without involvement of the 5GC; i.e. preparation messages are directly exchanged between gNBs. The release of the resources at the source gNB during the conditional handover completion phase is triggered by the target gNB. The figure below depicts the basic conditional handover scenario where neither the AMF nor the UPF changes:

Intra-AMF/UPF Conditional Handover Step:

• The source gNB decides to use CHO.
• The source gNB requests CHOfor one or more candidate cells belonging to one or more candidate gNBs. A CHO request message is sent for each candidate cell.
• The candidate gNB(s) sends CHO response(HO REQUEST ACKNOWLEDGE) including configuration of CHO candidate cell(s) to the source gNB. The CHO response message is sent for each candidate cell.
• The source gNB sends an RRCReconfiguration message to the UE, containing the configuration of CHO candidate cell(s)and CHO execution condition(s).
• CHO configuration of candidate cells can be followed by other reconfiguration from the source gNB.
• A configuration of a CHO candidate cell cannot contain a DAPS handover configuration.
• The UE sends an RRCReconfigurationComplete message to the source gNB.
• If early data forwarding is applied, the source gNB sends the EARLY STATUS TRANSFER message.
• The UE maintains connection with the source gNB after receiving CHO configuration, and starts evaluating the CHO execution conditions for the candidate cell(s). If at least one CHO candidate cell satisfies the corresponding CHO execution condition, the UE detaches from the source gNB, applies the stored corresponding configuration for that selected candidate cell, synchronises to that candidate cell and completes the RRC handover procedure by sending RRCReconfigurationComplete message to the target gNB. The UE releases stored CHO configurations after successful completion of RRC handover procedure.
• The target gNB sends the HANDOVER SUCCESS message to the source gNB to inform that the UE has successfully accessed the target cell. In return, the source gNB sends the SN STATUS TRANSFER message following the principles of Intra-AMF/UPF Handover.
• Late data forwarding may be initiated as soon as the source gNB receives the HANDOVER SUCCESS message.
• The source gNB sends the HANDOVER CANCEL message toward the other signalling connections or other candidate target gNBs, if any, to cancel CHO for the UE.

Conditional Handover User-plane handling

The U-plane handling for Conditional Handover follows the same principles for DAPS handover , if early data forwarding is applied, except that, in case of Full Configuration, HFN and PDCP SN are reset in the target gNB after the SN assignment is handed over to the target gNB. If late data forwarding is applied, the U-plane handling follows the RLC-AM or RLC-UM bearer principles.

Conditional Handover-Data Forwarding

If late data forwarding is applied, the source NG-RAN node initiates data forwarding once it knows which target NG-RAN node the UE has successfully accessed. In that case the behavior of the Conditional Handover data forwarding follows the same behavior as defined for the intra-system handover data forwarding, except the behavior for DRBs configured with DAPS handover.

If early data forwarding is applied instead, the source NG-RAN node initiates data forwarding before the UE executes the handover, to a candidate target node of interest. The behavior of early data forwarding for the Conditional Handover follows the same principles for DRBs configured with DAPS handover in the intra-system handover.