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LTE Mobility From E-UTRA Procedure Call Flow

call-flow LTE | RRC | Inter-RAT | Mobility From E-UTRA Command

Mobility From E-UTRA is the LTE-side procedure that commands the UE to leave LTE and continue on another radio technology. It is the LTE exit branch inside many outbound inter-RAT handovers.

This page focuses on the LTE-side command and transition point rather than the full target-RAT completion logic.

Introduction

The procedure begins after LTE decides that service should move to another technology. The source eNB delivers Mobility From E-UTRA Command, and the UE leaves the current LTE serving path to continue on the target system.

This page is often the cleanest bridge between LTE radio traces and target-system mobility traces.

What Is Mobility From E-UTRA Procedure in Simple Terms?

What starts the procedure: The source LTE side decides the UE should leave LTE toward another RAT.
What the UE and network want to achieve: Move the UE out of LTE with the target-system information needed for continuation.
What success looks like: The UE receives the command, exits LTE, and begins the target-system continuation branch.
What failure means: The UE never executes the exit command cleanly or the target branch never starts.

Why this procedure matters

This procedure is the exact LTE transition point for outbound inter-RAT mobility. It is useful when the key question is not the whole inter-RAT procedure, but the moment when LTE stops being the serving system.

Quick Fact Sheet

Procedure name	LTE Mobility From E-UTRA Procedure
Domain	LTE-side outbound inter-RAT transition
Main trigger	Policy or measurements require a move away from LTE
Start state	UE is in connected LTE service
End state	UE has left LTE and is continuing on the target RAT branch
Main nodes	UE, source eNB, target system
Main protocols	RRC and target-RAT signaling
Main success outcome	LTE exit command is applied and target continuation starts
Main failure outcome	LTE exit or target continuation breaks
Most important messages	Mobility From E-UTRA Command
Main specs	TS 36.331 and target-RAT mobility specs

LTE Mobility From E-UTRA Procedure 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.

Build the exit branch to Start target branch. The next visible messages now belong to the target technology rather than LTE.

Preconditions

The UE is connected on LTE.
Another RAT has been selected as the next serving technology.
The outbound inter-RAT preparation is complete enough for the UE to leave LTE.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Receives the exit command and leaves LTE.
Source eNB	Delivers the target-system payload that tells the UE how to continue.
Target RAT	Receives the UE after LTE exit and continues the service branch.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> source eNB	Carries the outbound mobility command.
Target RAT interface	UE <-> target system	Carries the continuation after LTE exit.

End-to-End Call Flow

UE              Source eNB               Target RAT
|<--Mobility From EUTRA Command--|                  |
|==== leave LTE ================================>>|
|---------------- target access and continuation ->|

Major Phases

Phase	What happens
1. LTE exit decision	The source side decides that the UE should leave LTE.
2. Command delivery	The source eNB sends Mobility From E-UTRA Command.
3. LTE exit execution	The UE stops using the source LTE cell.
4. Target continuation	The target technology takes over.

Step-by-Step Breakdown

Step 1: Exit decision

Sender -> receiver: source eNB

Message(s): Inter-RAT mobility decision

Purpose: Choose a target system outside LTE.

State or context change: LTE is still serving the UE before command delivery.

Note: The page starts just before the explicit LTE exit point.

Step 2: Command delivery

Sender -> receiver: source eNB -> UE

Message(s): Mobility From E-UTRA Command

Purpose: Carry the target-system information that tells the UE how to continue.

State or context change: The UE has the payload it needs to leave LTE.

Note: This is the key message to inspect in outbound inter-RAT cases.

Step 3: Leave LTE

Sender -> receiver: UE

Message(s): LTE exit execution

Purpose: Stop the current LTE serving path and begin the move to the target system.

State or context change: The UE is no longer continuing normal LTE connected service.

Note: This is where LTE trace visibility often ends.

Step 4: Continue on target

Sender -> receiver: UE -> target RAT

Message(s): Target access and completion

Purpose: Resume service on the target system.

State or context change: The target technology becomes active if the exit succeeded.

Note: The next messages depend on the target RAT.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Mobility From E-UTRA Command	RRC	source eNB -> UE	Commands the UE to leave LTE toward another RAT.	Inspect the target-system payload and the exact timing of LTE exit.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Target system	The RAT selected after LTE.	Mobility From E-UTRA Command	Determines which target branch should appear next.	Wrong target expectation.
Exit timing	The time when LTE service actually stops.	Air trace around the command	Useful for trace correlation across systems.	Target trace looks delayed because the LTE exit time was misread.
Command payload	The target-system instructions delivered by LTE.	Mobility From E-UTRA Command	Needed for valid target continuation.	Decode gap or incomplete payload interpretation.
Next target message	The first visible message on the target system.	Target trace	Confirms that the command really led to continuation.	No target follow-up appears.
Recovery path	What happened if target continuation failed.	Later trace	Shows whether the UE recovered, returned, or lost service.	Inter-RAT failure looks like generic radio loss.

Successful Completion

Success means the UE applies Mobility From E-UTRA Command, stops following the LTE serving path, and begins the correct target-system continuation branch.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Command appears but target branch is missing	The UE received the LTE exit message but the target-side continuation did not start.	Mobility command timing and the first expected target message.	Mobility From E-UTRA Command	LTE Uu and target-RAT interface	Open the target-specific page and check whether the right target system was prepared.

What to Check in Logs and Traces

Inspect Mobility From E-UTRA Command first.
Correlate the last LTE message with the first target-system message.
Check whether the target system in the command matches the target trace you are following.

Related sub-procedures

Related message reference pages

Mobility From E-UTRA Command

Related troubleshooting pages

Notes

Mobility From E-UTRA is the LTE-side exit point. After this, the target technology becomes the main place to continue the trace.

FAQ

What is Mobility From E-UTRA?

It is the LTE-side command and transition path used when the UE leaves LTE toward another radio technology.