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LTE to 5G Inter-RAT Handover / Redirection Call Flow

call-flow LTE | 5G NR | Return to NR | Inter-RAT Mobility

LTE to 5G Inter-RAT Handover / Redirection is the return path that moves a UE from LTE back onto NR when 5G once again becomes the better serving option.

The real engineering question is whether the move back to 5G creates stable service, not just whether the UE briefly reacquires NR.

Introduction

This page covers the reverse of NR-to-LTE inter-RAT movement: the LTE-side decision to return to 5G, the execution branch used, and the validation of stable NR service afterward.

In live networks, the biggest trap is mistaking a successful re-entry attempt for a stable and worthwhile return to NR.

What Is LTE to 5G Inter-RAT Handover / Redirection in Simple Terms?

What starts the procedure: LTE determines that the UE should move back toward NR.
What the UE and network want to achieve: Restore 5G service cleanly and keep it stable after the move.
What success looks like: The UE reaches NR and continues service there without immediate oscillation.
What failure means: The return branch is mistimed, weak, or unstable after NR re-entry.

Why this procedure matters

Return-to-NR mobility can look healthy in a single trace snapshot while still being a bad user experience if it immediately bounces back to LTE.

Quick Fact Sheet

Procedure name	LTE to 5G Inter-RAT Handover / Redirection
Domain	Mobility from LTE back toward 5G NR
Main trigger	UE on LTE is steered or handed back to NR for better service or capacity
Start state	UE is active or camped on LTE while NR becomes desirable again
End state	UE moves from LTE to NR by redirection or handover and service continues on 5G
Main nodes	UE, source eNB, target gNB, MME/EPC and AMF/5GC or interworking core path
Main protocols	LTE measurements, redirection or handover signaling, NR access, bearer continuity
Main success outcome	UE leaves LTE and successfully returns to NR with usable service on 5G
Main failure outcome	The return path is triggered but NR access or continuity fails
Most important messages	Measurement triggers, LTE-side mobility command, NR access and continuity validation
Main specs	TS 23.502, TS 36.300, TS 38.300, TS 36.331, TS 38.331

LTE to 5G Inter-RAT Handover or Redirection procedure flow

Preconditions

The UE is currently on LTE.
NR has become available or preferable again.
The serving LTE side can steer the UE back toward a valid NR target.
The target NR side can hold service after re-entry.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Moves from LTE back to NR when the network decides 5G should serve again.
Source eNB	Owns the LTE-side decision to steer the UE toward NR.
Target gNB	Accepts the UE and becomes the new 5G serving node.
LTE core side	Supports context continuity from the LTE serving environment.
5G core side	Anchors service once the UE is back on NR.

Interfaces used

Interface	Path	Role
LTE-Uu	UE <-> eNB	Carries the LTE-side measurements and redirection or handover branch.
NR-Uu	UE <-> gNB	Carries the final 5G access and resumed service.
Interworking mobility interfaces	LTE side <-> NR side	Support the return from LTE to NR.

End-to-End Call Flow

UE                eNB / LTE side          Target gNB / NR side
|                     |                        |
|..... service on LTE ........................ |
|<-- return-to-NR command --------------------|
|==== leave LTE and access NR ===============>|
|==== resumed service on NR if stable =======>|

Major Phases

Phase	What happens
1. LTE-serving state	The UE is currently on LTE even though NR may now be available again.
2. Return-to-NR decision	The LTE side decides that the UE should be moved back to 5G.
3. Redirection or handover execution	The UE is instructed to leave LTE toward a target NR path.
4. NR access and continuity	The UE synchronizes to NR and resumes service under the gNB.
5. Stable 5G re-entry	The UE continues on NR without immediate fallback or instability.

Step-by-Step Breakdown

The LTE side sees NR as the better serving target again

Sender -> receiver: UE -> eNB

Message(s): LTE measurements and return trigger

Purpose: Start the move back from LTE toward 5G.

State or context change: The serving LTE path becomes a temporary state rather than the final home for service.

Note: This is the mirror image of NR-to-LTE inter-RAT movement, but the access and core assumptions are different.

The network chooses handover or redirection toward NR

Sender -> receiver: eNB with core interworking support

Message(s): LTE-side mobility decision

Purpose: Decide how the UE should leave LTE and attempt NR re-entry.

State or context change: The return-to-NR branch becomes a concrete mobility action.

Note: Redirection and handover have different continuity quality, so classify the branch correctly.

The UE leaves LTE and seeks the target NR path

Sender -> receiver: UE -> target gNB

Message(s): LTE release or move command followed by NR access

Purpose: Return the UE to 5G radio service.

State or context change: The UE transitions from LTE back into NR serving conditions.

Note: A move back to NR is only useful if the target NR path is truly strong enough to hold the service.

NR service is restored

Sender -> receiver: UE <-> gNB

Message(s): NR access and resumed traffic

Purpose: Prove the UE can actually continue service on 5G again.

State or context change: The UE exits the fallback or LTE-serving branch and re-enters 5G operation.

Note: This should be validated with traffic, not only with the fact that the UE attached to NR.

Stability after return is checked

Sender -> receiver: UE, gNB, mobility monitoring

Message(s): Post-return measurements and continuity observation

Purpose: Ensure the move did not simply create fast ping-pong back toward LTE.

State or context change: The re-entry to 5G becomes a stable mobility outcome.

Note: Immediate fallback after return is a sign that the return trigger was too aggressive or badly timed.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
LTE-side mobility command	LTE RRC	eNB -> UE	Starts the move back toward NR.	Classify whether it was redirection or handover-like continuity.
NR access behavior	NR RRC / access	UE -> gNB	Shows whether the UE successfully re-entered 5G.	Primary radio success proof.
Resumed traffic on NR	User plane	UE <-> gNB	Validates usable service after the move.	Needed to go beyond access success.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Return trigger	Why LTE decided to hand the UE back to NR.	Decision stage	Explains whether the move was coverage, capacity, or policy driven.	Wrong trigger timing creates instability.
Redirection vs handover path	How the UE was told to return to NR.	Execution branch	Determines expected continuity quality.	Misclassification hides the correct failure branch.
Target NR quality	Whether the returned NR cell is actually strong enough.	Access and post-return stage	Core predictor of stable re-entry.	Poor target quality causes immediate oscillation.
Post-return service continuity	Whether real traffic works on NR again.	After access	Best final proof that the return was useful.	Access alone is not enough.

Success Criteria

The return-to-NR decision is well timed and justified.
The execution branch is correctly understood as redirection or handover.
The UE re-enters NR successfully.
Traffic stays stable on NR after the move.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Return to NR is triggered too early	The UE is pushed back to 5G before the NR path is truly stable.	Decision timing and post-return measurements.	Decision and post-return stage	Cross-layer	Often appears as quick ping-pong between LTE and NR.
UE leaves LTE but cannot establish stable NR service	NR access succeeds poorly or continuity is weak after return.	NR access and first packets on NR.	Execution and post-return stage	NR-Uu	This is the main operational failure branch.
Redirection is mistaken for seamless handover	The expected continuity level is overestimated.	Exact mobility branch used from LTE.	Execution classification	LTE RRC	Always classify the return path first.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is LTE to 5G Inter-RAT Handover / Redirection?

It is the return path that moves a UE from LTE back toward NR when 5G becomes preferable again.

Is it always a true handover?

No. Some deployments use redirection-like behavior instead of seamless handover continuity.

What proves success?

The UE reaches NR and service remains stable there afterward.

What should I inspect first?

Start with why LTE chose NR again, then classify the exact branch, then validate NR stability and traffic.