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3GPP mobility reference · 4G · 5G · NTN · IoT

Connected mode

Conditional handover (CHO)

The network pre-configures candidate target cells with execution conditions and the UE moves when a condition is met.

5GNTN

Overview

In conditional handover the UE receives conditional reconfigurations together with execution conditions, and only applies a stored target configuration (which includes reconfigurationWithSync) when its condition is fulfilled. The XnAP Handover Success message later tells the source node that the UE arrived, and Conditional Handover Cancel is the XnAP procedure for cancelling preparation.

Execution conditions reuse measurement events (condEventA3, A4, A5 and variants) and add two NTN-oriented kinds: condEventD1 (distance based) and condEventT1 (time based), both Rel-17, and condEventD2 (Rel-18). TS 38.331 states that CHO configured with only D1, D2 or T1 leaves the detection of the applicable cell to UE implementation.

Call flow

  1. 1
    UEtoSource gNB

    MeasurementReport

    Early report, before the normal handover threshold.

  2. 2
    Source gNBtoTarget gNB(s)

    HANDOVER REQUEST (CHO preparation)

    One or more candidates are prepared over XnAP.

  3. 3
    Source gNBtoUE

    RRCReconfiguration (conditional reconfiguration)

    Stored candidate configurations plus execution conditions. The UE stays on the source cell.

  4. 4
    UE

    Evaluate execution conditions

    When a condition is fulfilled the UE executes the stored target configuration.

  5. 5
    UEtoTarget gNB

    Random access, then RRCReconfigurationComplete

    T304 supervises this as for any reconfiguration with sync.

  6. 6
    Target gNBtoSource gNB

    HANDOVER SUCCESS (XnAP)

Timers and parameters

NameWhereRole
T304UESupervises execution, as for any reconfiguration with sync.

Full parameter and timer lookup →

Specifications

SpecWhat it covers
TS 38.300Conditional Handover. NTN specifics are in 16.14.3.2.2.(see clause 9.2.3.4)
TS 38.331Conditional reconfiguration IEs and the condEvent definitions.
TS 38.423Handover Success. See also 8.2.9 Conditional Handover Cancel.(see clause 8.2.8)

Changes by release

ReleaseChange
Rel-16Conditional reconfiguration (condRRCReconfig-r16).
Rel-17condEventA4, condEventD1 and condEventT1.
Rel-18condEventD2.
Rel-19Conditional LTM execution conditions (cltm-ExecutionCondition-r19 capability fields).

Failure behaviour in the specs

  • If execution fails, the reconfiguration-with-sync failure handling applies: re-establishment with handoverFailure (TS 38.331 5.3.5.8.3).

Symptom-based troubleshooting →

Related procedures

Connected mode

5G Xn-based handover

Inter-gNB handover prepared over the Xn interface, with an NG path switch toward the core.

5G

NTN

NR-NTN mobility and satellite switch

Satellite assistance in SIB19 and how NR uses it for synchronisation, measurement initiation, conditional handover and satellite switch.

5GNTN

Failure handling

Radio link failure and RRC re-establishment

Out-of-sync handling with N310, N311 and T310, then re-establishment supervised by T311 and T301.

4G5G