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LTE Conditional Handover / Conditional Reconfiguration Call Flow

call-flow LTE | Mobility | Conditional Handover | RRC

LTE conditional handover is a mobility approach where the network prepares one or more candidate targets in advance and the UE executes the move only when the configured condition becomes true. The preparation and the execution trigger are separated in time.

This page focuses on the prepared-but-not-yet-executed nature of conditional mobility.

Introduction

The source side builds conditional reconfiguration toward one or more candidate targets, sends the condition-bearing mobility configuration to the UE, and the UE later executes the move only if the configured condition is met during connected service.

This page is useful when the trace shows handover preparation long before the actual mobility execution moment.

What Is Conditional Handover / Conditional Reconfiguration in Simple Terms?

What starts the procedure: The network wants to prepare mobility early but delay execution until a condition is met.
What the UE and network want to achieve: Reduce late preparation delay by separating target preparation from final execution.
What success looks like: The UE stores the prepared target information and later executes the right target branch when the condition is met.
What failure means: The prepared branch expires, the wrong target is chosen, or execution never starts when expected.

Why this procedure matters

Conditional handover is different from normal immediate handover because the command and the actual move do not happen at the same time. This page helps explain that gap.

Quick Fact Sheet

Procedure name	LTE Conditional Handover / Conditional Reconfiguration
Domain	Prepared LTE mobility with delayed execution
Main trigger	Conditional mobility policy and later UE-side condition match
Start state	UE is connected and receives a prepared mobility configuration
End state	UE executes the prepared target move or the prepared branch is not used
Main nodes	UE, source eNB, candidate target eNBs
Main protocols	RRC and handover preparation signaling
Main success outcome	Prepared target is used when the configured trigger becomes true
Main failure outcome	Condition never matches or execution fails when triggered
Most important messages	Conditional Reconfiguration, Measurement Report, later mobility execution
Main specs	TS 36.331 and related LTE mobility behavior

LTE Conditional Handover / Conditional Reconfiguration call flow — Click the diagram to open the full-size in a new tab.

Handover Concept

This illustration shows the basic handover concept used in this procedure: the UE leaves the serving side after the mobility decision and continues on the target side once the target path is ready.

Prepare candidates to Execute prepared move. The UE uses the stored target data once the condition is satisfied.

Preconditions

The UE is already in connected LTE service.
The network has candidate targets it can prepare in advance.
Conditional mobility support is available in the scenario being traced.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Stores the prepared target data and later executes the move only when the condition becomes true.
Source eNB	Prepares candidate targets and sends the conditional mobility configuration.
Candidate target eNBs	Receive early preparation so execution can start faster later.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> source eNB	Carries the conditional configuration and later execution trigger.
Preparation path	source eNB <-> candidate targets	Carries the early preparation toward candidate targets.

End-to-End Call Flow

UE             Source eNB           Candidate targets
|                |--prepare targets----->|
|<--Conditional Reconfiguration----------|
|==== wait for condition =================|
|---- execute chosen target ------------->|

Major Phases

Phase	What happens
1. Candidate preparation	The source side prepares potential targets in advance.
2. Conditional configuration	The UE receives stored target information instead of an immediate move.
3. Trigger wait	The UE stays on the source side until the condition becomes true.
4. Execution	The UE executes the prepared target branch when the condition is met.

Step-by-Step Breakdown

Step 1: Prepare candidate targets

Sender -> receiver: source eNB -> candidate targets

Message(s): Conditional target preparation

Purpose: Make one or more targets ready before the move is actually needed.

State or context change: The targets are prepared while the UE is still on the source side.

Note: This is the main design difference from immediate handover.

Step 2: Send conditional configuration

Sender -> receiver: source eNB -> UE

Message(s): Conditional Reconfiguration

Purpose: Give the UE the prepared target information and the condition that must be met before moving.

State or context change: The UE stores the prepared mobility branch.

Note: The UE does not move immediately after this message.

Step 3: Wait for execution trigger

Sender -> receiver: UE

Message(s): Condition evaluation during connected service

Purpose: Delay the actual move until the configured trigger is satisfied.

State or context change: The UE continues to use the source side while monitoring the trigger.

Note: This gap can make the trace look unusual if you expect immediate execution.

Step 4: Execute the prepared move

Sender -> receiver: UE -> chosen target

Message(s): Prepared handover execution

Purpose: Start the target move when the condition becomes true.

State or context change: The chosen target branch begins using the already prepared data.

Note: After this point, the page hands off to the normal execution branch.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source eNB	Provides the measurement context behind the conditional decision.	Inspect which target conditions led to candidate preparation.
RRC Connection Reconfiguration	RRC	source eNB -> UE	Carries the conditional or prepared mobility configuration.	Check whether the message prepares a later move rather than an immediate one.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Candidate targets	Prepared target cells stored before execution.	Conditional config and prep state	Show which targets are available when the trigger occurs.	Wrong candidate set or stale target prep.
Condition rule	The exact rule that must become true before moving.	Conditional configuration	Explains why the UE waited or executed later.	Condition never met or misread in analysis.
Execution timing	The delay between configuration and actual move.	Full trace	Shows whether the conditional branch behaved as intended.	Assuming immediate execution when none was expected.
Chosen target	The final target used when the condition was met.	Later execution branch	Links the prepared set to the actual move.	Wrong target chosen from the prepared set.
Expiry or cancellation	Whether the prepared branch was later invalidated.	Later source-side control state	Explains why a prepared move was never used.	Prepared state silently expired.

Successful Completion

Success means the UE stores the prepared target data, waits on the source side, and later executes the correct target branch when the condition becomes true.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Prepared branch never executes	The condition was never met or the prepared target became invalid before execution.	Conditional configuration and later source-side state.	Conditional Reconfiguration and later mobility history	LTE Uu and preparation path	Check whether the branch expired or was superseded by another mobility decision.

What to Check in Logs and Traces

Check whether the early reconfiguration was conditional rather than an immediate move.
Separate target preparation time from actual execution time.
Confirm which prepared target was finally chosen when the trigger became true.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Conditional handover separates preparation from execution. That timing gap is the main reason these traces look different from ordinary immediate handover.

FAQ

What is LTE conditional handover?

It is a mobility approach where the target is prepared first and the UE executes the move later only when the configured condition is met.