Conditional Handover (CHO) in 5G NR Release 16

In Release-16, 3GPP introduced a new handover procedure that allows the User Equipment (UE) to decide to perform a handover when certain conditions are met. In the legacy handover procedure, the network was in charge of making the decision whether the handover should be performed or not. This legacy approach was a reactive process and was prone to resulting in handover failures.

So in 5G-NR Release-16, 3GPP introduced the Conditional Handover (CHO) feature, which empowers the UE to decide whether to perform a handover when defined conditions are satisfied.

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 it stops evaluating the execution condition(s) once a handover is executed (either a legacy handover or a conditional handover execution).

Principles of Conditional Handover

The Conditional Handover will contain information regarding the CHO configuration. 3GPP specifications have defined the following principles for CHO:

• The Candidate gNB(s) or the Potential Target gNB(s) provide the CHO configuration.
• The Source gNB provides the execution condition(s) to the UE indicating when to trigger CHO.
• The execution condition may consist of one or two trigger conditions.
• Only a single reference signal type is supported for CHO, and at most two different trigger quantities (e.g., RSRP and RSRQ, or RSRP and SINR, etc.) can be configured simultaneously for the evaluation of CHO execution conditions of a single candidate cell.
• If a UE is configured with a CHO Configuration and, before the CHO execution condition is satisfied, another HO Command is received from the gNB, then the UE will trigger Handover based on the new HO Command received and will not wait for any of the CHO Conditions to satisfy. (In short, the Legacy HO Configuration takes precedence over the CHO Configuration).
• While executing CHO (i.e., from the time when the UE starts synchronization with the target cell), the UE does not monitor the source cell.
• Condition Handover (CHO) is not supported for NG-C based handover in Release-16 of the specification.

Conditional Handover Call Flow from UE Perspective

In the case of 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:

Step-by-Step Call Flow

• Step-0: The UE context within the source gNB contains information regarding roaming and access restrictions which were provided either at connection establishment or at the last TA update.
• Step-1: The source gNB configures the UE measurement procedures and the UE reports according to the measurement configuration.
• Step-2: The source gNB decides to hand over the UE, based on MeasurementReport and RRM information.
• Step-3: The source gNB requests CHO for one or more candidate cells belonging to one or more candidate gNBs. A CHO request message is sent for each candidate cell.
• Step-4: Admission Control may be performed by the target gNB. Slice-aware admission control shall be performed if the slice information is sent to the target gNB. If the PDU sessions are associated with non-supported slices, the target gNB shall reject such PDU Sessions.
• Step-5: The candidate gNB(s) sends a CHO response (HO REQUEST ACKNOWLEDGE) including the configuration of the CHO candidate cell(s) to the source gNB. The CHO response message is sent for each candidate cell.
• Step-6: The source gNB sends an RRCReconfiguration message to the UE, containing the configuration of CHO candidate cell(s) and CHO execution condition(s).
  • Note 1: CHO configuration of candidate cells can be followed by other reconfigurations from the source gNB.
  • Note 2: A configuration of a CHO candidate cell cannot contain a DAPS handover configuration.
• Step-7: The UE sends an RRCReconfigurationComplete message to the source gNB.
• Step-7a: If early data forwarding is applied, the source gNB sends the EARLY STATUS TRANSFER message.
• Step-8: The UE maintains the connection with the source gNB after receiving the 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, synchronizes to that candidate cell, and completes the RRC handover procedure by sending an RRCReconfigurationComplete message to the target gNB. The UE releases stored CHO configurations after successful completion of the RRC handover procedure.
• Step-8a: The target gNB sends the HANDOVER SUCCESS message to the source gNB to inform that the UE has successfully accessed the target cell.
• Step-8b: In return, the source gNB sends the SN STATUS TRANSFER message.
• Step-8c: The source gNB sends the HANDOVER CANCEL message toward the other signaling connections or other candidate target gNBs, if any, to cancel CHO for the UE.

What Happens if a UE is Configured with CHO and Receives a Legacy HO Command?

There might be scenarios where, while the UE is evaluating Conditional Handover conditions, it receives a Legacy Handover command with the corresponding Handover configuration of the target gNB. In this scenario, what should be the behavior of the UE?

• Shall it drop the Legacy Handover configuration and continue evaluating the Conditional handover conditions? Or,
• Shall it abandon evaluating the Conditional Handover conditions and proceed with the latest Handover Configuration received?

Conditional Handover Fallback to Legacy Handover Call Flow

3GPP specifications have outlined this scenario and specified the UE behavior. According to 3GPP TS 38.300 9.2.3.2:

Before any CHO execution condition is satisfied, upon reception of the HO command (without CHO configuration), the UE executes the HO procedure as described in clause 3GPP TS 38.300 9.2.3.2, regardless of any previously received CHO configuration.

So, in short, the Legacy HO Configuration takes precedence over the CHO Configuration (if configured), and the Conditional Handover condition evaluation is abandoned by the UE.

How the UE Evaluates Conditional Handover Conditions

• The network might configure the UE with one or more candidate target SpCells in the ConditionalReconfiguration message.
• The UE evaluates the condition of each configured candidate target SpCell.
• If any one of the target cells fulfills the associated execution condition, then the UE applies the conditional reconfiguration associated with the fulfilled target SpCell.
• If multiple target cells meet the associated execution condition at the same time, then it is up to UE implementation to decide which target SpCell to select and proceed with the Conditional Handover procedure.
• The gNB sends the ConditionalReconfiguration Information Element (IE) in the RRCReconfiguration message to the UE, which contains information related to the target cell(s) for CHO.

3GPP Spec References

• 3GPP TS 38.300: NR; NR and NG-RAN Overall Description
• 3GPP TS 38.331: NR; Radio Resource Control (RRC); Protocol specification