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LTE Inter-Frequency Handover Call Flow

call-flow LTE | Mobility | Cross EARFCN | RRC | X2 | S1AP

LTE inter-frequency handover moves the UE between LTE cells that use different LTE frequency layers. It is the connected mobility path used when the best target cell is not on the current serving EARFCN.

This page focuses on the frequency-layer change itself and how it affects preparation, execution, and post-move continuity.

Introduction

The procedure begins after connected measurements show that another LTE carrier layer offers the preferred target. The source side prepares the target cell, the UE receives the handover command, retunes to the target layer, and the target side completes the serving-cell switch.

The main nodes are the UE, source eNB, target eNB, MME, and SGW.

What Is Inter-Frequency Handover in Simple Terms?

What starts the procedure: The target LTE cell is on a different frequency layer and the network decides to move the UE there.
What the UE and network want to achieve: Change both the serving cell and the LTE carrier layer while keeping service active.
What success looks like: The UE reaches the target LTE frequency, accesses the target cell, and traffic continues there.
What failure means: The retune or target-access leg fails, or the post-move continuity path breaks.

Why this procedure matters

This case explains why some mobility problems are tied to a frequency-layer change rather than a simple neighbor-cell swap. It is also a useful bridge to inter-RAT mobility analysis.

Quick Fact Sheet

Procedure name	LTE Inter-Frequency Handover
Domain	Connected LTE mobility across different LTE carrier layers
Main trigger	Measurement-driven target selection on another LTE frequency
Start state	UE is connected on the source LTE frequency
End state	UE is connected on the target LTE frequency
Main nodes	UE, source eNB, target eNB, MME, SGW
Main protocols	RRC, X2AP or S1AP, GTP-U
Main success outcome	Serving cell and frequency layer change cleanly
Main failure outcome	Interrupted mobility during retune or target access
Most important messages	Measurement Report, RRC Connection Reconfiguration, Path Switch Request
Main specs	TS 36.331, TS 36.300, TS 23.401

LTE Inter-Frequency Handover 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.

Detect target layer to Switch continuity. The new serving layer becomes the active delivery path.

Preconditions

The UE is already in connected LTE service.
Inter-frequency measurements are configured and valid.
The target LTE layer is allowed and accessible.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Measures another LTE layer, retunes, and accesses the target cell.
Source eNB	Chooses the target layer and starts preparation.
Target eNB	Admits the UE on the new frequency layer and becomes serving.
MME	Handles the later path-switch update.
SGW	Continues user-plane anchoring after the move.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries measurement, command, retune, and target access signaling.
X2 or S1-MME	Source side <-> target side / MME	Carries the control path used to prepare and finalize the move.
S1-U	eNB <-> SGW	Carries user-plane continuity after target takeover.

End-to-End Call Flow

UE          Source eNB         Target eNB           MME
|--Inter-freq Measurement->|             |                 |
|                          |--HO prep--->|                 |
|                          |<--HO ack----|                 |
|<--HO command-------------|             |                 |
|==== retune to target LTE frequency =====================>|
|                          |             |--Path Switch--->|
|                          |<--Release source context-----|

Major Phases

Phase	What happens
1. Inter-frequency trigger	Measurements show the preferred target is on another LTE layer.
2. Target preparation	The target-frequency cell receives UE context.
3. Retune and access	The UE changes serving frequency and reaches the target cell.
4. EPC continuity update	The target-side path becomes active and the source side is released.

Step-by-Step Breakdown

Step 1: Inter-frequency trigger

Sender -> receiver: UE -> source eNB

Message(s): Measurement Report

Purpose: Show that the preferred target is on another LTE frequency.

State or context change: The UE is still on the source layer while the target is selected.

Note: This is where frequency-layer change becomes visible.

Step 2: Target prep

Sender -> receiver: Source eNB -> target eNB

Message(s): Handover Request and acknowledgment

Purpose: Prepare the target layer before retuning the UE.

State or context change: The target cell is ready on the new EARFCN.

Note: Retune should not start before the target is prepared.

Step 3: Retune and target access

Sender -> receiver: Source eNB -> UE -> target eNB

Message(s): RRC Connection Reconfiguration

Purpose: Move the UE to the target LTE frequency and cell.

State or context change: The serving frequency changes as the target cell becomes active.

Note: This is the main difference from same-frequency mobility.

Step 4: Path switch

Sender -> receiver: Target eNB -> MME

Message(s): Path Switch Request

Purpose: Complete EPC-side continuity through the target layer.

State or context change: Downlink delivery follows the target LTE frequency layer.

Note: Check this even when the radio move looked clean.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source eNB	Shows that a different LTE layer should be selected.	Inspect the reported EARFCN and event result.
RRC Connection Reconfiguration	RRC	source eNB -> UE	Carries the target-frequency command to the UE.	Check the target-frequency details and timing.
Path Switch Request	S1AP	target eNB -> MME	Completes EPC continuity after the move.	Inspect the final target eNB and bearer continuity.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Source and target EARFCN	The LTE layer before and after the move.	Measurement and command	Confirms this is an inter-frequency case.	Layer confusion or wrong target mapping.
PCI and ECGI	Target cell identity on the new layer.	Measurement and command	Shows which target cell actually won.	Wrong target correlation.
Measurement event	The trigger behind the move.	Measurement Report	Explains why the source side decided to leave the current layer.	Threshold mismatch or oscillation.
Retune timing	Time between command and target access.	Air trace	Helps explain late access or missed execution.	Retune delay or incomplete access.
Path switch result	EPC continuity outcome after target access.	S1AP	Confirms bearer continuity after the layer change.	Radio move succeeded but EPC update failed.

Successful Completion

Success means the UE reaches the target LTE frequency and cell, then the target-side path becomes the active continuity path.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Wrong-layer oscillation	Measurements are unstable across frequency layers.	Measurement history before and after the move.	Measurement Report	LTE Uu	Check inter-frequency thresholds and neighbor design.
Retune succeeds but continuity does not	The UE reached the target layer but the bearer path did not fully switch.	Target-side S1AP and bearer mapping.	Path Switch Request	S1-MME, S1-U	Inspect EPC continuity before blaming RF alone.

What to Check in Logs and Traces

Check both the source and target EARFCN values.
Confirm the UE retuned to the intended LTE layer after the command.
Verify whether the target-side path switch followed the retune.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Inter-frequency handover is still LTE-to-LTE mobility. The key difference is that the UE changes serving LTE layer as well as cell.

FAQ

What is inter-frequency handover?

It is LTE handover to a target cell on another LTE carrier frequency.

Does inter-frequency handover mean inter-RAT?

No. Inter-RAT means moving to another radio technology such as UTRAN, GERAN, or CDMA2000.