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5G Inter-RAT Handover Procedure Explained

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

Introduction

In 5G networks, Inter-RAT (Radio Access Technology) Handover occurs when a User Equipment (UE) moves from a 5G NR network to another radio technology such as LTE or vice versa.

This procedure ensures that the UE maintains connectivity even when 5G coverage is not available or when mobility requires switching between technologies.

Typical transitions include:

5G NR -> LTE
LTE -> 5G NR
5G NR -> 3G (legacy networks in some deployments)

The procedure is defined by the 3rd Generation Partnership Project in:

3GPP TS 23.502 - 5G System Procedures
3GPP TS 36.300 - LTE Architecture
3GPP TS 38.300 - NR Architecture

Why Inter-RAT Handover Is Needed

Although 5G networks are expanding rapidly, many deployments still rely on multi-technology coverage.

Inter-RAT mobility ensures:

continuous service outside 5G coverage
seamless fallback to LTE
compatibility with legacy networks

Common use cases include:

Scenario	Description
Coverage fallback	UE moves out of 5G coverage
Voice fallback	VoNR unavailable -> fallback to LTE
Network optimization	Load balancing across RATs

Network Elements Involved

UE (User Equipment)

Measures neighboring cells belonging to different radio technologies.

Source gNB

The 5G base station currently serving the UE.

Target eNB

The LTE base station that will serve the UE after handover.

AMF / MME

Depending on the handover direction, mobility coordination may involve:

AMF for 5G core
MME for LTE core

Interfaces Used

Interface	Description
NR-Uu	Radio interface between UE and gNB
LTE-Uu	Radio interface between UE and eNB
N2	Interface between gNB and AMF
S1-MME	Interface between eNB and MME
Xn / S1	Mobility coordination interfaces

Inter-RAT Handover Call Flow

Below is the simplified signaling sequence for 5G -> LTE handover.

UE        Source gNB        AMF/MME        Target eNB
 |             |                |              |
 |--Measurement Report-------->|
 |             |                |              |
 |             |--Handover Required---------->|
 |             |                |--Handover Request-->|
 |             |                |              |
 |             |                |<--Handover Ack------|
 |<--RRC Handover Command------|
 |                              |
 |====== UE switches to LTE cell ======|
 |                              |
 |---RRC Reconfiguration Complete----->|

During this process, the UE transitions from NR radio access to LTE radio access.

Step-by-Step Explanation

Step 1: Measurement Reporting

The UE measures neighboring LTE cells while connected to the 5G gNB.

Measurements include:

RSRP
RSRQ
Signal quality of LTE cells

The UE sends a Measurement Report to the source gNB.

Important parameters to check

Engineers should verify:

inter-RAT measurement configuration
LTE neighbor cell list
measurement event thresholds

Step 2: Handover Decision

The source gNB determines that LTE provides better coverage or service availability.

The gNB initiates the Inter-RAT handover procedure.

Important parameters to check

Check:

inter-RAT thresholds
LTE neighbor configuration
load balancing policies

Step 3: Handover Preparation

The core network coordinates the handover between the NR and LTE systems.

The target eNB allocates resources for the UE.

Important parameters to check

Verify:

target LTE cell availability
bearer context transfer
security context transfer

Step 4: Handover Command

The source gNB sends an RRC Reconfiguration message containing the handover command.

This message instructs the UE to connect to the LTE target cell.

Important parameters to check

Look for:

LTE cell identity
frequency parameters
mobility control information

Step 5: Handover Execution

The UE disconnects from the 5G NR cell and synchronizes with the LTE eNB.

The UE sends RRC Reconfiguration Complete to confirm the handover.

Data Path After Inter-RAT Handover

Once the handover completes, the data path becomes:

UE -> LTE eNB -> Core Network -> Data Network

User sessions continue without interruption.

Common Inter-RAT Handover Types

Type	Description
NR -> LTE	Fallback when leaving 5G coverage
LTE -> NR	Upgrade to 5G coverage
NR -> 3G	Rare legacy fallback scenario

Troubleshooting Inter-RAT Handover

Handover Failure

Possible causes:

incorrect LTE neighbor configuration
missing inter-RAT measurement setup
target cell resource shortage

Radio Link Failure

Possible reasons:

UE cannot synchronize with LTE cell
handover command lost
signal quality issues

Session Interruption

Possible causes:

bearer context transfer failure
core network signaling issues
routing configuration errors

Key Messages in Inter-RAT Handover

Message	Purpose
Measurement Report	UE reports LTE neighbor measurements
Handover Required	Source gNB initiates handover
Handover Request	Core network requests resources
Handover Command	UE instructed to move to LTE
RRC Reconfiguration Complete	UE confirms handover

Relevant 3GPP Specifications

The Inter-RAT Handover procedure is defined by the 3rd Generation Partnership Project in:

3GPP TS 23.502 - 5G System Procedures
3GPP TS 36.300 - LTE Architecture
3GPP TS 38.300 - NR Architecture

Summary

The Inter-RAT Handover procedure enables seamless mobility between 5G NR and other radio technologies such as LTE.

The process includes:

measurement reporting by the UE
handover decision by the network
preparation of the target RAT resources
UE synchronization with the new radio technology

This mechanism ensures continuous connectivity across heterogeneous mobile networks.