In Release-16, 3GPP Introduced a new Handover procedure that allows UE to decide to perform handover when certain conditions are met, In the Legacy handover procedure network was in charge of making the decision whether the handover should be performed or not and it was a reactive process and prone to resulting in handover failures.
So in 5G-NR Release-16, 3GPP Introduced Conditional handover feature which allows UE to decide wether to perform handover when certain conditions are met.
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 the UE stops evaluating the execution condition(s) once a handover is executed (legacy handover or conditional handover execution).
The Condition Handover will contain information regarding the CHO configuration and 3GPP Specifications has 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 on when to trigger CHO.
- The execution condition may consist of one or two trigger condition(s).
- Only a Single Reference signal type is supported for CHO and at most two different trigger quantities (e.g. RSRP and RSRQ, 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 CHO Configuration and before the CHO execution condition is satisfied another HO Command is received from the gNB, then UE will trigger Handover based on the 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 (if configured).
- While executing CHO, i.e. from the time when the UE starts synchronization with the target cell, 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
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-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 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).
- Note1: CHO configuration of candidate cells can be followed by other reconfiguration from the source gNB
- Note2: 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 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 RRCReconfigurationComplete message to the target gNB. The UE releases stored CHO configurations after successful completion of 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 UE is configured with Conditional Handover and gNB sends Legacy Handover configuration?
There might be scenarios where while the UE is evaluating Conditional Handover scenarios it might receive 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 the abandon evaluating the Conditional Handover conditions and proceed with the latest Handover Configuration received?
3GPP specifications has outlined the scenario and specfied the UE behaviour in this scenario.
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 38.300 126.96.36.199, regardless of any previously received CHO configuration.3GPP TS 38.300 188.8.131.52,
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 UE will evaluate the 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.
- And 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 at the same time Multiple target cells have met the associated execution condition, then it is up to UE implementation to decide which target SpCell to select and proceed with the Conditional Handover procedure.
- gNB sends ConditionalReconfiguration IE in the RRCReconfiguration message to 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