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LTE Intra-LTE Handover Procedure Call Flow

call-flow LTE | Mobility | RRC | X2 | S1AP | GTP-U

LTE Intra-LTE handover is the connected-mode mobility procedure that moves the UE from a source LTE cell to a target LTE cell while preserving ongoing service. It covers the normal LTE-to-LTE mobility path before the flow branches into interface-specific variants such as X2 or S1-based handover.

If this path does not complete cleanly, the UE may lose continuity, trigger RRC re-establishment, or fall into a broader recovery branch.

Introduction

The procedure begins after measurement-driven mobility evaluation while the UE is already in RRC_CONNECTED. The source side prepares the target cell, the UE receives the handover command, accesses the target, and the target side completes the path switch toward the EPC.

The main nodes are the UE, source eNB, target eNB, MME, and SGW. In practice, this is the baseline reference flow used to interpret more specific cases such as intra-frequency, inter-frequency, X2, and S1 handover.

What Is Intra-LTE Handover Procedure in Simple Terms?

What starts the procedure: A connected UE is moving and the source side decides another LTE cell should become the serving cell.
What the UE and network want to achieve: Keep the session active while moving the serving radio path to the target LTE cell.
What success looks like: The UE accesses the target cell, the target eNB confirms completion, and the EPC switches the downlink path to the target side.
What failure means: The UE loses connected continuity, target access fails, or later recovery replaces the intended mobility path.

Why this procedure matters

This is the core LTE connected-mobility flow. It explains how measurement-driven decisions become a real serving-cell move and where continuity can break between radio execution and EPC path update.

Quick Fact Sheet

Procedure name	LTE Intra-LTE Handover Procedure
Domain	Connected LTE-to-LTE mobility
Main trigger	Measurement-driven connected mobility decision
Start state	UE is in RRC_CONNECTED on the source LTE cell
End state	UE is in RRC_CONNECTED on the target LTE cell with user-plane continuity restored
Main nodes	UE, source eNB, target eNB, MME, SGW
Main protocols	RRC, X2AP or S1AP, GTP-U
Main success outcome	Target cell becomes serving cell and the EPC path switches cleanly
Main failure outcome	Interrupted mobility or later recovery branch
Most important messages	Measurement Report, Handover Request, RRC Connection Reconfiguration, Path Switch Request
Main specs	TS 36.300, TS 36.331, TS 23.401

LTE Intra-LTE Handover 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.

Measure to Switch. The target eNB informs the EPC so downlink delivery and bearer anchoring move to the target side.

Preconditions

The UE already has an active connected LTE context.
Source and target LTE cells are known and mobility policy allows connected handover.
Measurement information and target-cell admission logic support the move.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Reports measurements, receives the handover command, accesses the target cell, and completes the move.
Source eNB	Decides or initiates the handover, prepares the target, and releases the source-side path after success.
Target eNB	Admits the UE, creates target-side context, and becomes the new serving eNB after execution.
MME	Carries S1-side control handling and receives the later path-switch update.
SGW	Anchors the user-plane path and applies the downlink switch toward the target eNB.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> source and target eNB	Carries the measurement reports, handover command, and target access leg.
X2	Source eNB <-> target eNB	Often carries target preparation and status exchange for intra-LTE handover.
S1-MME	eNB <-> MME	Carries EPC control continuity and later path-switch handling.
S1-U	eNB <-> SGW	Carries user-plane tunneling before and after the move.

End-to-End Call Flow

UE          Source eNB          Target eNB             MME              SGW
|--Measurement Report->|                  |                  |                |
|                      |--Handover Request------------------>|                |
|                      |<--Handover Request Ack--------------|                |
|<--RRC Reconfiguration|                  |                  |                |
|==== target access and sync ===============================================>|
|                      |                  |--Path Switch Req->|                |
|                      |                  |<--Path Switch Ack-|                |
|                      |<--UE Context Release--------------- |                |

Major Phases

Phase	What happens
1. Mobility trigger	The source side evaluates measurement conditions and chooses a target LTE cell.
2. Target preparation	The target eNB receives UE context, admission details, and bearer information.
3. Handover execution	The UE receives the command, leaves the source side, and accesses the target cell.
4. Path switch and cleanup	The target eNB updates the EPC path and the source side releases the old context.

Step-by-Step Breakdown

Step 1: Measurement-driven trigger

Sender -> receiver: UE -> source eNB

Message(s): Measurement Report

Purpose: Show that the target cell now meets the handover condition.

State or context change: The UE remains on the source side while the mobility decision is made.

Note: Start here if the handover seems unexpected or too early.

Step 2: Target preparation

Sender -> receiver: Source eNB -> target eNB

Message(s): Handover Request and Handover Request Acknowledge

Purpose: Prepare target resources and copy the UE context to the target side.

State or context change: The target eNB is ready before the UE is told to move.

Note: Admission failure here stops the handover before UE execution starts.

Step 3: Handover command

Sender -> receiver: Source eNB -> UE

Message(s): RRC Connection Reconfiguration

Purpose: Deliver the target-side mobility information to the UE.

State or context change: The UE leaves the source serving path and begins target access.

Note: This is the critical transition between preparation and real execution.

Step 4: Target access

Sender -> receiver: UE -> target eNB

Message(s): Random access and handover confirm handling

Purpose: Create a working radio path on the target LTE cell.

State or context change: The target cell becomes the active radio side for the UE.

Note: If this leg fails, the issue becomes a mobility failure rather than a preparation problem.

Step 5: Path switch

Sender -> receiver: Target eNB -> MME -> SGW

Message(s): Path Switch Request and path update handling

Purpose: Move the EPC downlink path to the target eNB.

State or context change: User-plane continuity follows the target serving side.

Note: A clean target access does not guarantee the path switch also completed.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source eNB	Triggers the source-side mobility decision.	Check the event result and the target cell identity.
Handover Request	X2AP or S1AP	source eNB -> target side	Prepares the target cell with UE context.	Inspect bearer list, target admission, and forwarding context.
Handover Request Ack	X2AP or S1AP	target side -> source eNB	Confirms target readiness.	Check whether admission was granted and whether transparent container data is present.
RRC Connection Reconfiguration	RRC	source eNB -> UE	Carries the handover command toward the UE.	Inspect the mobility command container and target details.
Path Switch Request	S1AP	target eNB -> MME	Moves EPC control and downlink path to the target side.	Check bearer mapping and the new serving eNB identity.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
PCI / ECGI	Target cell identifiers used during the move.	Measurement results, handover command	They show exactly which target cell was selected.	Wrong target correlation or neighbor mismatch.
E-RAB list	Bearer set carried into the target preparation.	Handover Request, path switch handling	Shows which bearers should survive the move.	Missing bearer continuity or partial setup.
Target container	Radio configuration passed to the UE for target access.	RRC Connection Reconfiguration	Needed for a valid move to the target LTE cell.	Invalid target data or decode gap.
MME and S-GW path context	Core-side mobility continuity identifiers.	Path Switch Request and response	Shows whether the EPC moved the bearer path to the target side.	Path switch missing or stale bearer context.
Cause and timing	Mobility reason and execution timing pattern.	Measurement, command, target access, release	Helps separate bad preparation from bad execution.	Late command, rushed execution, or replay confusion.

Successful Completion

Success means the UE is stably served by the target LTE cell, the target side has accepted the context, and the EPC path has switched to the target eNB.

After successful completion, the source-side context is released and the user-plane continuity follows the target cell.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Target prep fails	The target cell rejects admission or cannot create the required resources.	Handover Request and acknowledgment result.	Handover Request, Handover Request Ack	X2 or S1-MME	Check target admission cause and neighbor relation before looking at the UE.
UE receives the command but never reaches the target	Execution failed during the source-to-target move.	RRC command, target random access, and any later recovery branch.	RRC Connection Reconfiguration, later recovery messages	LTE Uu	Open handover failure and recovery.
Target access succeeds but traffic does not continue	The EPC path switch did not complete or bearers were not mapped correctly.	Target-side S1AP and path-switch result.	Path Switch Request, Path Switch Request Ack	S1-MME, S1-U	Compare the E-RAB list before and after the move.

What to Check in Logs and Traces

Correlate the Measurement Report with the target cell named in the handover command.
Confirm whether target preparation finished before the UE received the handover command.
Check whether the UE actually accessed the target side or dropped into recovery.
Verify that the path switch happened after target access, not just the radio move.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Intra-LTE handover is the umbrella mobility view. X2 and S1 handover are execution variants of the same basic LTE-to-LTE move.

A good target-access trace is only part of the success path. The later path switch is what makes the EPC follow the UE to the new cell.

FAQ

What is LTE Intra-LTE handover?

It is the connected procedure that moves a UE from one LTE cell to another while keeping the service active.

Is every intra-LTE handover X2-based?

No. Some cases use direct X2 signaling, while others use S1 through the MME.

What confirms success?

Target access plus a clean path switch is the practical success pattern.