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LTE Event A5 Measurement Procedure Call Flow

call-flow LTE | Measurements | RRC

LTE Event A5 Measurement Procedure is the LTE measurement event procedure used when the UE reports that the serving cell becomes worse than one threshold while a neighbor becomes better than another threshold. It is one event-specific branch inside the broader measurement reporting procedure.

This page focuses on the Event A5 trigger, how the report appears, and what that result usually means for later mobility or optimization handling.

Introduction

The eNB first configures the relevant measurement object and report criteria. The UE monitors the configured cells, evaluates the Event A5 condition, and sends Measurement Report when the event becomes true.

The main nodes are the UE and eNB. The result can feed later handover logic, coverage tuning, or neighbor-layer interpretation depending on the scenario.

What Is LTE Event A5 Measurement Procedure in Simple Terms?

What starts the procedure: The eNB configures measurement reporting and the UE starts evaluating the event condition in connected mode.
What the UE and network want to achieve: Detect when the serving cell becomes worse than one threshold while a neighbor becomes better than another threshold so the network can use that result for more selective mobility decisions that need both serving degradation and neighbor suitability.
What success looks like: The UE sends Measurement Report when the paired serving and neighbor threshold condition is met.
What failure means: The event does not trigger when expected, triggers too early, or produces unstable or misleading reports.

Why this procedure matters

This event helps explain why the network does or does not take a later action. It is useful when the question is whether the reported radio condition actually matched the expected Event A5 behavior.

Quick Fact Sheet

Procedure name	LTE Event A5 Measurement Procedure
Domain	LTE connected-mode measurement event reporting
Main trigger	Event A5 condition becomes true
Start state	UE is in RRC_CONNECTED with active measurement configuration
End state	Measurement result is available to the eNB for later control use
Main nodes	UE, eNB
Main protocols	RRC
Main success outcome	Correct event report is delivered at the correct time
Main failure outcome	Missing, late, early, or unstable event reporting
Most important messages	Measurement Control, Measurement Report
Main specs	TS 36.331

LTE Event A5 Measurement Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is already in connected LTE service.
The eNB has configured measurement objects, report configuration, and the event rule.
The relevant serving or neighbor cells are measurable in the active scenario.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Applies measurement rules, performs the configured observations, and sends the relevant result back to the network.
eNB	Configures measurement behavior, interprets results, and uses them for mobility, optimization, or analytics decisions.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries measurement configuration, reporting, and any later control action based on the result.

End-to-End Call Flow

UE                    eNB
|<--Measurement Control--|
|   monitor Event A5       |
|--Measurement Report---->|
|   later decision use    |

Major Phases

Phase	What happens
1. Configuration	The eNB activates the measurement object and event rule.
2. Monitoring	The UE evaluates whether the Event A5 condition becomes true.
3. Reporting	The UE sends Measurement Report once the configured trigger is met.
4. Result use	The eNB uses the report for later mobility, optimization, or analysis.

Step-by-Step Breakdown

Step 1: Configure the event

Sender -> receiver: eNB -> UE

Message(s): Measurement Control

Purpose: Deliver the measurement object, report configuration, and event rule to the UE.

State or context change: The UE starts monitoring the required serving or neighbor context.

Note: The event can only be interpreted correctly if the report configuration is known first.

Step 2: Evaluate the condition

Sender -> receiver: UE

Message(s): LTE Event A5 Measurement Procedure evaluation

Purpose: Check whether the serving cell becomes worse than one threshold while a neighbor becomes better than another threshold.

State or context change: The UE keeps monitoring until the rule becomes true and any timing conditions are satisfied.

Note: This is where hysteresis and time-to-trigger shape the final report timing.

Step 3: Send the report

Sender -> receiver: UE -> eNB

Message(s): Measurement Report

Purpose: Return the event result to the eNB.

State or context change: The eNB now has a concrete event outcome for later use.

Note: The report timing is often more important than the event name alone.

Step 4: Use the result

Sender -> receiver: eNB

Message(s): Later control action or analytics use

Purpose: Interpret the Event A5 result for more selective mobility decisions that need both serving degradation and neighbor suitability.

State or context change: The report becomes input to later control-plane logic.

Note: The event page stops at the measurement result, not the later handover or optimization branch.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Control	RRC	eNB -> UE	Configures the event rule and reporting behavior.	Check the report configuration, thresholds, hysteresis, and time-to-trigger.
Measurement Report	RRC	UE -> eNB	Returns the event result once the rule is met.	Inspect the event result, target identity, and the timing relative to expected radio behavior.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Event rule	The exact Event A5 reporting condition.	Measurement Control	Explains why the report did or did not appear.	The intended event is confused with another event type.
Threshold and offset	The configured trigger values behind the event.	Measurement Control	They define when the report should start.	The report looks early or late because the configured values were not checked.
Time-to-trigger	Delay the UE must satisfy before reporting.	Measurement Control	Important for distinguishing stable reporting from brief radio fluctuation.	Short fluctuation is misread as a valid event trigger.
Serving and target measurements	The measured radio condition used in the event comparison.	Measurement Report	Show why the event became true.	The reported result does not match the assumed serving or neighbor context.
PCI / EARFCN / RAT context	The identity of the cell or system involved in the event.	Measurement Report	Links the event to the correct serving or target context.	Wrong neighbor, wrong layer, or wrong RAT is assumed.

Successful Completion

Success means the UE reports the Event A5 event when the configured condition really becomes true and the eNB can use that result for later control logic.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Expected event report never appears	The event condition was never met, the report configuration is wrong, or the trace starts too late.	Measurement Control and later radio conditions.	Measurement Control, Measurement Report	LTE Uu	Check whether the configured rule actually matches the expected field condition.
Event report appears too early or too often	Threshold, hysteresis, or time-to-trigger values are too loose for the scenario.	Configuration values and repeated report timing.	Measurement Report	LTE Uu	Compare the trigger timing against the configured offsets and stability conditions.
Wrong target or wrong interpretation	The event result is valid, but the cell or later action being assumed is not.	Reported cell identity and the later control branch.	Measurement Report	LTE Uu	Match the reported target identity before interpreting the later procedure.

What to Check in Logs and Traces

Start with the Measurement Control values before interpreting the Measurement Report.
Check whether the reported result really matches the expected Event A5 condition.
Correlate the reported cell or RAT identity before linking the event to a later mobility action.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

LTE Event A5 Measurement Procedure is an event-specific reporting branch. The page explains the event result itself, not the full later mobility action that may follow it.

FAQ

What is LTE Event A5 Measurement Procedure?

It is the LTE measurement event procedure used when the serving cell becomes worse than one threshold while a neighbor becomes better than another threshold.

Does this page explain the later handover too?

No. This page stops at the event report and the meaning of that result.