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LTE Event B1 / B2 Inter-RAT Measurement Procedure Call Flow

call-flow LTE | Measurements | Inter-RAT | RRC

LTE Event B1 and B2 inter-RAT measurement procedures are the LTE reporting paths used when the UE observes candidate target systems outside E-UTRA. They are the measurement side of many inter-RAT mobility decisions.

This page explains the inter-RAT report meaning before you move into a full Inter-RAT Handover Procedure analysis.

Introduction

The eNB configures the inter-RAT measurement scope, the UE monitors the target RAT, and the UE sends Measurement Report once the B1 or B2 trigger becomes true.

The main nodes are the UE and eNB, with the target RAT appearing as the external system named by the report.

What Is Event B1 / B2 Inter-RAT Measurement Procedure in Simple Terms?

What starts the procedure: The eNB configures inter-RAT measurement behavior for a non-LTE target system.
What the UE and network want to achieve: Detect when another RAT becomes suitable for later mobility or service continuity.
What success looks like: The UE sends the expected inter-RAT Measurement Report with the right RAT context and timing.
What failure means: The inter-RAT trigger is missing, late, unstable, or linked to the wrong target system.

Why this procedure matters

This page is useful when the later inter-RAT move looks surprising and the first question is whether the target RAT actually met the configured measurement condition.

Quick Fact Sheet

Procedure name	LTE Event B1 / B2 Inter-RAT Measurement Procedure
Domain	Connected-mode inter-RAT measurement reporting
Main trigger	B1 or B2 inter-RAT condition becomes true
Start state	UE is connected on LTE with inter-RAT measurement configuration active
End state	The eNB receives an inter-RAT measurement result
Main nodes	UE, eNB
Main protocols	RRC
Main success outcome	Valid inter-RAT report is available for later control handling
Main failure outcome	Missing or misleading inter-RAT measurement result
Most important messages	Measurement Control, Measurement Report
Main specs	TS 36.331

LTE Event B1 / B2 Inter-RAT Measurement Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is in connected LTE service.
Inter-RAT measurement configuration is active.
The target RAT is measurable in the current 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 target RAT   |
|--Measurement Report---->|
|   later inter-RAT use  |

Major Phases

Phase	What happens
1. Inter-RAT configuration	The eNB configures the target RAT measurement behavior.
2. Inter-RAT monitoring	The UE observes the target system.
3. Inter-RAT reporting	The UE sends Measurement Report once B1 or B2 is met.
4. Result use	The eNB uses the report for later inter-RAT control handling.

Step-by-Step Breakdown

Step 1: Configure inter-RAT measurements

Sender -> receiver: eNB -> UE

Message(s): Measurement Control

Purpose: Activate target-RAT measurement rules.

State or context change: The UE begins monitoring the non-LTE target system.

Note: This is where B1 and B2 become relevant in the trace.

Step 2: Observe the target RAT

Sender -> receiver: UE

Message(s): Inter-RAT measurement evaluation

Purpose: Check whether the configured target-system rule becomes true.

State or context change: The UE evaluates the target RAT against the B1 or B2 condition.

Note: B1 and B2 differ in how serving and target conditions are combined.

Step 3: Send the inter-RAT report

Sender -> receiver: UE -> eNB

Message(s): Measurement Report

Purpose: Return the target-RAT result to LTE.

State or context change: The eNB has a measured basis for later inter-RAT handling.

Note: This is the handoff point between radio observation and inter-RAT decision making.

Step 4: Use the result

Sender -> receiver: eNB

Message(s): Later inter-RAT mobility or service action

Purpose: Interpret the target-RAT result for a later procedure.

State or context change: The report becomes input to later inter-system logic.

Note: This page stops before the actual handover starts.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Control	RRC	eNB -> UE	Configures B1 or B2 target-RAT reporting behavior.	Check target RAT, threshold rule, and stability conditions.
Measurement Report	RRC	UE -> eNB	Returns the inter-RAT result once the rule is met.	Inspect the target RAT context and the timing of the report.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Target RAT	The non-LTE system being measured.	Measurement Control and Measurement Report	Shows which inter-RAT branch is relevant later.	Wrong target-system assumption.
B1 or B2 rule	The exact inter-RAT event logic being used.	Measurement Control	Explains why the report did or did not appear.	B1 and B2 are treated as the same condition.
Serving condition	The LTE serving context used in the inter-RAT comparison.	Configuration and trace timing	Needed to interpret B2 especially.	The serving side is ignored while reading the target report.
Target threshold	The inter-RAT threshold applied to the target system.	Measurement Control	Shows when the target should become reportable.	Unexpected target behavior is judged without checking the configured threshold.
Reported target identity	The actual target-system result returned by the UE.	Measurement Report	Links the report to the correct inter-RAT branch.	Wrong RAT or wrong target cell is assumed.

Successful Completion

Success means the LTE side receives the correct inter-RAT report with the right target-system context and timing.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Expected inter-RAT report is missing	The target RAT never met the rule or the configured rule does not match the expected scenario.	Measurement Control and target-RAT conditions.	Measurement Report	LTE Uu	Check whether the target RAT was actually configured and measurable.
Inter-RAT report appears but later handover looks wrong	The report may be valid, but the later target selection or mobility action is the real issue.	The report versus the later inter-RAT branch.	Measurement Report	LTE Uu	Separate report correctness from later inter-RAT decision logic.

What to Check in Logs and Traces

Check whether the report is B1 or B2 before interpreting it.
Match the target RAT in the report to the later inter-RAT branch.
Read the target threshold together with the serving-side condition when B2 is involved.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

B1 and B2 are inter-RAT observation procedures. They explain why LTE decided another RAT looked suitable before the actual inter-RAT move starts.

FAQ

What are LTE Event B1 and B2?

They are the LTE inter-RAT measurement event procedures used when the UE reports that a target system outside LTE meets the configured rule.

Does this page explain the full inter-RAT handover?

No. This page explains the inter-RAT report stage before the later mobility procedure.