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LTE Cell Reselection Procedure Call Flow

call-flowLTE | Idle Mobility | RRC

LTE Cell Reselection is the idle-mode mobility procedure that moves the UE from one suitable cell to another without a connected handover. It operates while the UE is in idle behavior and directly affects later paging, tracking area update, and service return.

This is an idle mobility workflow, not a connected handover procedure.

Introduction

The cell reselection procedure runs while the UE is not in active connected signaling. The UE evaluates neighbor cells, applies idle mobility rules, and chooses a more suitable cell when needed.

The main node is the UE, with network behavior appearing through system information, neighbor definitions, and later idle-side continuation.

What Is LTE Cell Reselection in Simple Terms?

What starts the procedure: The UE in idle mode finds that another cell is more suitable than the current one.
What the UE and network want to achieve: Keep the UE on a suitable idle-serving cell for reachability and later access.
What success looks like: The UE camps on a better cell and later procedures continue from that new idle context.
What failure means: The UE remains on a poor cell, oscillates, or later paging and access behavior becomes unstable.

Why this procedure matters

This procedure explains many idle-side issues that later appear as paging misses, TAU surprises, or unstable return to connected mode.

Quick Fact Sheet

Procedure name	LTE Cell Reselection Procedure
Domain	Idle-mode mobility
Main trigger	Another cell becomes more suitable for camping
Start state	UE is in RRC_IDLE
End state	UE camps on a new suitable cell
Main nodes	UE, serving cell, candidate cell
Main protocols	RRC system information and idle mobility rules
Main success outcome	UE remains reachable and stable on the new idle cell
Main failure outcome	Poor idle camping, oscillation, or later service instability
Most important messages	System information and later follow-up branches such as TAU or Service Request
Main specs	TS 36.304, TS 36.331

LTE Cell Reselection Procedure call flow — Click the diagram to open the full-size in a new tab.

Preconditions

The UE is already in idle mode.
The UE has system information for the serving and candidate cells.
Idle mobility rules allow evaluation and camping on a new suitable cell.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Measures, evaluates, and chooses the next suitable idle cell.
Serving cell	Provides the current camped context and related system information.
Candidate cell	Becomes the next camped cell if selected.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> cells	Provides system information and the radio environment for idle camping.
Idle mobility rules	UE internal	Control measurement evaluation and reselection decisions.

End-to-End Call Flow

UE            Serving Cell         Candidate Cell
|   camped          |                    |
|-- evaluate radio quality ------------>|
|   compare idle criteria               |
|---------------------------- camp on new cell
|   later paging / TAU / access start on new cell |

Major Phases

Phase	What happens
1. Idle evaluation	The UE compares the current cell with candidate cells.
2. Reselection decision	The UE decides another cell is more suitable.
3. Camping	The UE camps on the new cell.
4. Later idle continuation	Paging, TAU, or later access begins from the new camped cell.

Step-by-Step Breakdown

Step 1: Idle evaluation and reselection

Sender -> receiver: UE internal mobility handling

Message(s): No dedicated connected-mode signaling

Purpose: Choose the most suitable idle-serving cell.

State or context change: The UE changes from one camped cell to another.

Note: Problems here often appear later as paging misses or unstable access rather than as a clear reselection failure message.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
System Information	RRC	Cell -> UE	Provides the idle mobility configuration used for reselection.	Cell-selection and reselection parameters, TAC, and neighbor-related context.
Tracking Area Update Request	NAS	UE -> MME	May appear later if the new cell places the UE in a different tracking area.	Whether reselection led to a mobility update.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Idle reselection criteria	The rules the UE uses to compare cells.	System information	Explain why the UE stayed or moved.	Wrong cell priorities or poor idle tuning.
TAI / TAC	The tracking-area identity of the new cell.	System information and later TAU behavior	Shows whether reselection should trigger a mobility update.	Unexpected TAU after cell change.
Camped cell stability	The behavior after the UE changes cells.	Later paging or access attempts	Confirms whether the new cell was actually usable.	Ping-pong camping or paging miss pattern.

Successful Completion

Success means the UE camps stably on the new suitable cell and later idle-side procedures continue normally from that context.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
UE camps on the wrong cell or oscillates	Idle mobility settings or radio quality conditions are unstable.	System information and later camping behavior.	System Information, later TAU or access messages	LTE Uu	Check idle tuning and whether the new cell really stayed suitable.
Paging fails after reselection	The UE is reachable on the wrong cell or later mobility update did not align with the new area.	Reselection timing and later paging or TAU behavior.	Paging, TAU Request	LTE Uu, NAS	Check whether the UE moved into a new tracking area and updated registration correctly.

What to Check in Logs and Traces

Check whether the UE was in idle mode before the cell change.
Inspect system information and the resulting camped cell behavior.
Follow the next idle-side branch such as paging, TAU, or fresh access.

Related sub-procedures

Related message reference pages

Tracking Area Update Request

Related troubleshooting pages

Notes

Cell reselection is idle mobility, not handover. It changes the camped cell without using a connected handover branch.

FAQ

What is LTE Cell Reselection?

It is the idle-mode procedure that moves the UE from one suitable LTE cell to another.

Can reselection trigger TAU?

Yes. If the new camped cell belongs to a different tracking area, a later TAU may follow.