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LTE V2X Sidelink Procedure Call Flow

call-flow LTE | V2X | Sidelink | RRC

LTE V2X Sidelink Procedure is the vehicle-oriented sidelink branch where LTE control and sidelink-aware context are used for V2X continuation rather than for ordinary LTE-only behavior.

The practical anchors are RRC Connection Reconfiguration, Sidelink UE Information r12, and the later V2X-oriented branch that should reflect that reported context.

Introduction

This page keeps the trace in the V2X-aware sidelink branch instead of in the broader sidelink family. The practical question is whether the vehicle-oriented branch was configured and whether the network had the correct UE-reported context behind it.

Use it when the trace clearly belongs to a V2X sidelink case and the analysis has to stay inside that specialized branch.

What Is LTE V2X Sidelink Procedure in Simple Terms?

What starts the procedure: The LTE side enters a V2X-oriented sidelink branch where vehicle-related sidelink behavior matters.
What the UE and network want to achieve: A usable V2X-aware control state with the right sidelink context behind it.
What success looks like: The V2X sidelink branch is configured, reported, and reflected correctly in later handling.
What failure means: The branch exists, but the UE context, timing, or later V2X handling does not match it.

Why this procedure matters

V2X traces are easy to misread if they are treated as generic LTE or even as generic sidelink only. The branch meaning changes once the trace becomes vehicle-oriented, so the UE report and later action have to be matched carefully.

Quick Fact Sheet

Procedure name	LTE V2X Sidelink Procedure
Domain	Vehicle-oriented sidelink control
Main trigger	Need to use a V2X-aware sidelink branch under LTE control
Start state	UE is in a trace branch where V2X-oriented sidelink behavior matters
End state	The V2X branch is configured and later behavior aligns with it
Main nodes	UE, eNB
Main protocols	RRC
Main success outcome	The V2X-oriented sidelink branch is interpreted and used correctly
Main failure outcome	The branch is treated like generic LTE-only behavior or the reported context is ignored
Most important messages	RRC Connection Reconfiguration, Sidelink UE Information r12
Main specs	TS 36.331

LTE V2X Sidelink Procedure Call Flow — Click the diagram to open the full-size in a new tab.

Preconditions

A V2X-relevant sidelink branch is in use or being configured.
The LTE side can interpret V2X-oriented sidelink context.
The UE can expose the relevant sidelink information for the branch.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Applies V2X-aware sidelink configuration and reports the relevant branch context.
eNB	Configures the V2X-oriented sidelink branch and uses the UE report in later handling.
V2X branch context	Represents the specialized sidelink branch that should not be read as generic LTE-only control.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the V2X-oriented sidelink control and reporting branch.
RRC	UE <-> eNB	Carries the configuration and reporting messages behind the V2X branch.

End-to-End Call Flow

UE                     eNB
|<--RRC Reconfig-------|
|--Sidelink UE Info--->|
|==== V2X sidelink continuation ===>|

Major Phases

Phase	What happens
1. Enter the V2X-aware branch	The LTE side marks the current branch as vehicle-oriented sidelink handling.
2. Deliver the branch configuration	The UE receives the control update behind that branch.
3. Report the relevant UE context	The UE exposes the sidelink information the branch depends on.
4. Continue under V2X-aware handling	The LTE side uses that context in later V2X-oriented control.

Step-by-Step Breakdown

Start the V2X-aware branch

Sender -> receiver: eNB -> UE

Message(s): RRC Connection Reconfiguration

Purpose: Deliver the connected-mode or control update that makes the V2X-oriented sidelink branch active.

State or context change: The trace is now in a vehicle-oriented sidelink branch rather than in generic LTE-only control.

Note: This is the key boundary between ordinary LTE interpretation and V2X-oriented interpretation.

Report the sidelink context

Sender -> receiver: UE -> eNB

Message(s): Sidelink UE Information r12

Purpose: Expose the UE-side context the V2X branch depends on.

State or context change: The eNB now has the explicit branch context for later action.

Note: If later handling looks wrong, this message is the main fact to compare it against.

Use the V2X branch context

Sender -> receiver: eNB

Message(s): V2X-aware control decision

Purpose: Read the reported context into the later branch decision.

State or context change: The branch is now driven by explicit sidelink context rather than by assumption alone.

Note: This often explains why the next action does or does not match expectations.

Validate the outcome

Sender -> receiver: UE <-> eNB

Message(s): Later V2X-oriented continuation

Purpose: Check whether the later branch really reflects the reported V2X context.

State or context change: The procedure is now complete enough to explain the later V2X path.

Note: This is where branch identity and outcome identity should be checked together.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
RRC Connection Reconfiguration	RRC	eNB -> UE	Starts or updates the V2X-aware sidelink branch.	Check whether the V2X branch actually begins at this point.
Sidelink UE Information r12	RRC	UE -> eNB	Reports the UE-side context the V2X branch depends on.	Check what information the UE really exposed before later V2X behavior is judged.
RRC Connection Reconfiguration Complete	RRC	UE -> eNB	Confirms the UE accepted the V2X-aware branch update.	Check whether the later UE report follows a real accepted branch configuration.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
V2X branch entry point	The exact control update where the trace becomes V2X-aware.	RRC reconfiguration	Defines when later behavior should be read through a V2X branch lens.	The trace is treated as V2X too early or too late.
Reported V2X-side context	The sidelink-related information the UE exposed for this branch.	Sidelink UE Information r12	Useful for matching the later action to a real UE report.	Later behavior is judged on assumption instead of on the actual report.
Outcome alignment	Whether the next V2X-aware behavior fits the report.	Post-report branch	Separates report problems from later decision problems.	The report is present, but the branch still follows the wrong context.
Branch identity continuity	Whether the trace stays in the V2X branch after the report.	Whole procedure window	Prevents mixed traces from drifting back into generic sidelink or LTE-only interpretation.	The analysis silently shifts into the wrong branch family.
Decision timing	How close the report is to the later action it should influence.	Around the report window	Useful when multiple V2X-related updates appear close together.	The wrong report instance is used to explain the next step.

Successful Completion

Success means the V2X-oriented sidelink branch is configured, the UE reports the right context, and later behavior matches that branch.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
The later V2X action does not match the report	The branch may be using stale context or the wrong branch assumptions.	The exact sidelink report and the exact next V2X-oriented action.	Sidelink UE Information r12	LTE Uu	Align one report instance to one later action before diagnosing the branch logic.
The trace looks vehicle-oriented, but no useful branch report appears	The configuration may not have been accepted or the report window is missing.	The reconfiguration and the first expected report point.	RRC Connection Reconfiguration, Sidelink UE Information r12	LTE Uu	Check whether the V2X branch was really active first.

What to Check in Logs and Traces

Start from the branch entry point where the trace becomes V2X-aware.
Use the sidelink UE report as the main proof of UE-side context.
Keep branch identity visible while checking later V2X behavior.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

Use this page when the trace is clearly V2X-oriented. If the trace only shows a generic sidelink branch, start from the broader LTE sidelink page.

FAQ

What is the LTE V2X Sidelink Procedure?

It is the vehicle-oriented sidelink branch where LTE control and sidelink-aware context shape the later V2X behavior.

Why not just use the generic sidelink page?

Because V2X traces are easier to read when the branch identity stays explicitly vehicle-oriented from start to finish.