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5G Inter-RAT Handover Procedure Call Flow

call-flow 5G NR | LTE | Inter-RAT Mobility | Handover

5G Inter-RAT Handover is the mobility procedure used when service must move off NR onto another radio technology such as LTE.

The key engineering challenge is not just the radio move itself, but preserving usable service across the RAT boundary.

Introduction

This page explains the generic inter-RAT handover story: why the move starts, how the target RAT is prepared, how the UE leaves NR, and how service continuity is validated afterward.

When analyzing real traces, always separate the reason for leaving NR from the later target-RAT execution result.

What Is Inter-RAT Handover in Simple Terms?

What starts the procedure: Coverage, quality, policy, or service rules say another RAT should serve the UE.
What the UE and network want to achieve: Move the UE cleanly to the target RAT while preserving service continuity.
What success looks like: The UE accesses the target RAT and service remains usable.
What failure means: Interworking prep, target access, or bearer continuity breaks during the RAT change.

Why this procedure matters

Inter-RAT mobility is where access, core interworking, and bearer continuity all intersect. That makes it one of the easiest places to misdiagnose the wrong layer.

Quick Fact Sheet

Procedure name	5G Inter-RAT Handover
Domain	Mobility between 5G NR and LTE or other RATs
Main trigger	The network decides service should continue on a different radio technology
Start state	UE is active on 5G NR and another RAT becomes the better or required serving choice
End state	UE continues service on the target RAT with preserved context as far as possible
Main nodes	UE, source gNB, target eNB or other RAT node, AMF, MME/EPC where relevant
Main protocols	RRC measurement and command handling, interworking mobility coordination, bearer continuity
Main success outcome	UE moves off NR onto the target RAT and service remains available
Main failure outcome	Interworking coordination, target access, or bearer continuity fails during the RAT change
Most important messages	Measurement Report, Handover Required, RRC Reconfiguration, target-RAT access and completion
Main specs	TS 23.502, TS 38.300, TS 36.300, TS 38.331, TS 36.331

Preconditions

The UE is active on NR and another RAT is available as a valid destination.
Inter-RAT measurements and target-RAT configuration are present.
Core interworking or bearer continuity support is available for the move.
The target RAT can accept the UE after it leaves NR.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Measures inter-RAT targets, executes the move, and re-enters service on the new RAT.
Source gNB	Decides when NR is no longer the best or allowed serving technology.
Target eNB or RAT node	Accepts the UE on the new technology and anchors the radio side after the move.
AMF	Coordinates the NR-side mobility and interworking decision.
MME / EPC or other target-core anchor	Support the target-RAT mobility and bearer continuity where needed.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> source gNB	Carries measurement reporting and the last NR-side mobility command.
LTE-Uu or target RAT air interface	UE <-> target access node	Carries target-RAT access after leaving NR.
N2	Source gNB <-> AMF	Supports the NR-side mobility decision and coordination.
Interworking mobility interfaces	NR side <-> LTE/EPC or other RAT side	Transfer context and bearer handling across RAT boundaries.

End-to-End Call Flow

UE                Source gNB            Core / Target RAT            Target Node
|                     |                        |                    |
|-- Measurement Report ----------------------->|                    |
|                     |-- inter-RAT prep ----->|------------------->|
|<-- RRC Reconfiguration ----------------------|                    |
|==== leave NR and access target RAT =============================>|
|==== service continues on target RAT if bearer continuity holds =>|

Major Phases

Phase	What happens
1. Inter-RAT target becomes preferable	Measurements, coverage, policy, or service rules say NR should no longer carry the UE.
2. Interworking preparation	The network prepares the target RAT and coordinates context continuity.
3. NR-side command	The source sends the UE the information needed to leave NR toward the target RAT.
4. Target-RAT access	The UE synchronizes to the target technology and re-establishes the radio side there.
5. Service continuity validation	Bearer and application continuity are checked on the new RAT.

Step-by-Step Breakdown

The source gNB sees a target RAT as the right next serving choice

Sender -> receiver: UE -> source gNB

Message(s): Measurement Report

Purpose: Show that another RAT should now take over service.

State or context change: The mobility branch becomes inter-RAT rather than pure NR mobility.

Note: This may be driven by coverage, policy, service needs, or fallback logic such as voice continuity.

The network prepares the target RAT path

Sender -> receiver: Source side -> target RAT side

Message(s): Handover Required and target preparation

Purpose: Coordinate context and bearer continuity across technology boundaries.

State or context change: The target RAT becomes a viable destination before the UE leaves NR.

Note: Inter-RAT problems often start in interworking preparation rather than on the air interface itself.

The UE receives the NR-side move command

Sender -> receiver: Source gNB -> UE

Message(s): RRC Reconfiguration

Purpose: Tell the UE which target RAT and target cell it should move to.

State or context change: The UE is instructed to leave NR and enter the target technology.

Note: At this point the engineer should already know whether the move was a generic inter-RAT handover or part of a more specific flow like EPS fallback.

The UE accesses the target RAT

Sender -> receiver: UE -> target RAT node

Message(s): Target-RAT synchronization and access

Purpose: Re-establish radio continuity on the destination technology.

State or context change: The UE leaves NR and becomes active on the target RAT.

Note: Target-RAT access failure is often the most visible symptom even when the root cause was earlier preparation logic.

Service continuity is validated on the new RAT

Sender -> receiver: UE <-> target core and user plane

Message(s): Bearer continuity and first live packets

Purpose: Prove the move preserved usable service beyond the radio transition itself.

State or context change: The inter-RAT mobility branch closes in stable service on the target RAT.

Note: A clean radio move without working bearer continuity is still not a true success.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Measurement Report	RRC	UE -> source gNB	Shows why the inter-RAT branch started.	Use it to understand whether the trigger was coverage, quality, or another policy-driven reason.
Handover Required	NGAP / interworking control	Source side -> core / target side	Starts interworking preparation toward the target RAT.	Helpful for finding the first real transition point.
RRC Reconfiguration	RRC	Source gNB -> UE	Carries the command to leave NR toward the target RAT.	Check target RAT details carefully.
First target-RAT service packets	User plane	UE <-> target path	Validate bearer continuity on the new technology.	This is the real success measure.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Target RAT and cell	Which technology and cell the UE was told to enter.	Preparation and command stage	Defines the entire rest of the mobility branch.	Wrong target means instant downstream confusion.
Inter-RAT measurement quality	Why the target RAT was chosen.	Measurement Report	Explains whether the move was justified by radio conditions.	Important when the move feels unnecessary.
Bearer/context transfer state	How user sessions were preserved across RAT change.	Preparation and post-move stage	Separates radio success from service success.	A major root cause area for inter-RAT trouble.
Target access timing	How quickly the UE entered the new RAT.	Execution stage	Shows whether the move was clean or strained.	Long gaps degrade user experience.
Post-move service quality	How well traffic behaved on the new RAT.	After access	Determines whether the inter-RAT move was actually worthwhile.	A completed move can still be a poor service outcome.

Success Criteria

The target RAT is selected for a valid and traceable reason.
Interworking preparation completes before the UE leaves NR.
The UE accesses the target RAT successfully.
Bearer and application continuity remain usable after the move.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Interworking preparation is weak or incomplete	The target RAT path is not truly ready before the UE leaves NR.	Handover Required and target-preparation state.	Preparation stage	N2 and interworking interfaces	Often causes later failure that looks like RF trouble.
UE cannot access the target RAT	The command is delivered but the target technology cannot be entered successfully.	RRC command and target-RAT access behavior.	RRC Reconfiguration	Air interface	Main execution failure branch.
Move succeeds but sessions break	Radio continuity worked but bearer continuity did not.	Post-move traffic, bearer handling, and first application packets.	Post-move service stage	User plane	This is not a clean handover success.
The wrong inter-RAT branch is diagnosed	A specific use case such as EPS fallback or redirection is misread as generic handover.	Trigger reason and service context.	Measurement and decision stage	Cross-layer correlation	Always classify the exact inter-RAT use case before deep debugging.

What to Check in Logs and Traces

Write down why NR was left before reading the rest of the trace.
Inspect target-RAT preparation state before blaming the target RF alone.
Use RRC Reconfiguration to confirm the exact target RAT and target cell.
Validate the result with first service packets on the target RAT.
When voice is involved, check whether the branch was really EPS Fallback rather than generic inter-RAT mobility.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is 5G Inter-RAT Handover?

It is the mobility procedure where service moves between NR and another radio technology such as LTE.

Is it always NR to LTE?

No. That is the most common case here, but inter-RAT mobility can cover other technology transitions too.

What proves success?

The UE enters the target RAT and service continues with usable bearer continuity.

What should I inspect first?

Start with the trigger reason, target RAT, preparation state, and first service packets after the move.

How is it related to EPS Fallback?

EPS Fallback is a more specific inter-RAT mobility use case focused on voice continuity toward LTE.