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LTE Handover to UTRAN Call Flow

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

LTE handover to UTRAN is the outbound inter-RAT mobility path used when the UE leaves LTE and continues connected service on UMTS. It is one of the most common legacy-target interworking cases.

This page focuses on the LTE-to-UTRAN branch after the outbound move has been selected.

Introduction

The LTE side prepares the UTRAN branch, delivers Mobility From E-UTRA Command, and the UE continues with the target-side UTRAN access sequence.

Use this page when the target system is specifically UTRAN rather than GERAN or CDMA2000.

What Is Handover to UTRAN in Simple Terms?

  • What starts the procedure: The network chooses UMTS as the target system for connected service continuation.
  • What the UE and network want to achieve: Leave LTE and continue on UTRAN without falling back to idle recovery first.
  • What success looks like: The UE exits LTE and reaches the expected UTRAN continuation branch.
  • What failure means: The target UTRAN branch does not start or does not complete.

Why this procedure matters

This page is the target-specific version of outbound inter-RAT mobility when the destination system is UTRAN.

Quick Fact Sheet

Procedure name LTE Handover to UTRAN
Domain Outbound inter-RAT mobility from LTE to UMTS
Main trigger UTRAN selected as the target RAT
Start state UE is connected on LTE
End state UE continues on UTRAN
Main nodes UE, source eNB, target UTRAN
Main protocols LTE RRC and UTRAN target control signaling
Main success outcome Service leaves LTE and continues on UMTS
Main failure outcome Inter-RAT transition to UTRAN fails
Most important messages Mobility From E-UTRA Command, UTRAN target access messages
Main specs TS 36.331 and UTRAN mobility specs
LTE Handover to UTRAN 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.

Handover Concept Concept illustration of a UE moving from the serving side to the target side. Source eNB UTRAN UE moving to target Serving side Target side
Prepare UTRAN to Continue on UMTS. The target UMTS side becomes active if the move succeeds.
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Preconditions

  • The UE is in connected LTE service.
  • UTRAN has been chosen as the target system.
  • The target UMTS branch is prepared.

Nodes and Interfaces

Nodes involved

Node Role in this procedure
UE Applies the LTE exit command and continues on UMTS.
Source eNB Sends the outbound mobility command toward UTRAN.
UTRAN Takes over service after LTE exit.

Interfaces used

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

End-to-End Call Flow

UE             Source eNB             UTRAN
|<--Mobility From EUTRA Command--|      |
|==== leave LTE =======================>|
|----------- UTRAN access and completion ->|

Major Phases

Phase What happens
1. Prepare UTRAN branch The LTE side builds the target UMTS move.
2. Deliver exit command The UE receives the outbound LTE command.
3. Leave LTE The UE stops following the source LTE cell.
4. Continue on UTRAN The UMTS target branch becomes active.

Step-by-Step Breakdown

Step 1: UTRAN target preparation

Sender -> receiver: LTE side

Message(s): Inter-RAT preparation

Purpose: Prepare the UMTS target branch.

State or context change: LTE still serves the UE.

Note: This stage is still LTE-side preparation.

Step 2: LTE exit command

Sender -> receiver: source eNB -> UE

Message(s): Mobility From E-UTRA Command

Purpose: Tell the UE to continue on UTRAN.

State or context change: The UE has the UMTS target information.

Note: Inspect this message first in LTE traces.

Step 3: Exit LTE

Sender -> receiver: UE

Message(s): LTE exit execution

Purpose: Stop the source LTE serving path.

State or context change: The UE is no longer continuing as an LTE-connected UE.

Note: This is the cross-system trace pivot point.

Step 4: Start UTRAN branch

Sender -> receiver: UE -> UTRAN

Message(s): UTRAN access and completion

Purpose: Continue service on UMTS.

State or context change: UTRAN becomes active if the move succeeds.

Note: The next visible control path is UMTS-specific.

Important Messages

Message Protocol Direction Purpose in this procedure What to inspect briefly
Mobility From E-UTRA Command RRC source eNB -> UE Carries the UTRAN target move. Inspect the target-system payload and the LTE exit timing.

Important Parameters to Inspect

Parameter What it is Where it appears Why it matters Common issues
Target RAT UTRAN as the destination system. Mobility command Confirms this page is the right target-specific branch. Wrong target interpretation.
Exit timing The last LTE control moment before UMTS continuation. Air trace Useful for correlating LTE and UMTS traces. Timing mismatch across systems.
UTRAN follow-up The first target-side control message. UTRAN trace Shows whether the move actually reached UMTS. No target continuation appears.
Service type The service expected to continue on UTRAN. Mobility context Helps interpret whether the move preserved the expected branch. Wrong expectation about service continuity.
Recovery result What happens if UTRAN entry fails. Later trace Shows whether the UE recovers or loses service. Failure hidden as generic loss.

Successful Completion

Success means LTE exit completes and the target UTRAN continuation starts as expected.

Common Failures and Troubleshooting

Symptom Likely cause Where to inspect Relevant message(s) Relevant interface(s) Likely next step
LTE exit happens but UTRAN continuation is missing The outbound move reached the system boundary but the UMTS target branch did not complete. The LTE exit point and the first expected UTRAN message. Mobility From E-UTRA Command LTE Uu and UTRAN target interface Check the target UTRAN trace immediately after the LTE exit command.
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What to Check in Logs and Traces

  • Check that the target system in the LTE command is UTRAN.
  • Use the LTE exit command as the cross-system correlation point.
  • Confirm whether the first UTRAN message appears after the LTE exit.

Related Pages

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Handover to UTRAN is a target-specific continuation. Use the LTE exit command as the handoff point between LTE and UMTS traces.

FAQ

What is LTE handover to UTRAN?

It is the inter-RAT mobility path that moves connected service from LTE to UMTS.