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LTE Sidelink Procedure Call Flow

call-flow LTE | Sidelink | RRC

LTE Sidelink Procedure is the LTE control path used when the UE enters or updates a sidelink-aware branch and the network needs sidelink-specific context rather than ordinary LTE-only signaling assumptions.

The practical anchors are RRC Connection Reconfiguration, Sidelink UE Information r12, and the later sidelink-aware continuation that should match the reported context.

Introduction

This page is the broad LTE sidelink reference path. It is useful when the trace includes sidelink-aware configuration or reporting and later behavior has to be explained through that branch instead of through standard LTE mobility or bearer logic.

Use it when the UE is clearly exposing sidelink-related information and the main question is whether the surrounding LTE control path reflects that context correctly.

What Is LTE Sidelink Procedure in Simple Terms?

What starts the procedure: The network enters a sidelink-aware branch and needs sidelink-specific UE context or continuation.
What the UE and network want to achieve: A usable sidelink-aware LTE control state with the right UE-reported context behind it.
What success looks like: The sidelink-aware branch is configured, the UE reports the expected context, and later handling matches it.
What failure means: The branch is configured but the reported sidelink context is missing, stale, or not reflected later.

Why this procedure matters

Sidelink traces become hard to read when they are treated like ordinary LTE-only control. The value of this page is keeping the trace centered on the sidelink-aware branch and on the UE report that feeds it.

Quick Fact Sheet

Procedure name	LTE Sidelink Procedure
Domain	Sidelink-aware LTE control
Main trigger	Need for sidelink-related configuration or UE reporting
Start state	UE is in a connected or relevant LTE state where sidelink context matters
End state	The sidelink-aware branch is configured and later behavior reflects the correct context
Main nodes	UE, eNB
Main protocols	RRC
Main success outcome	Sidelink-aware handling matches the reported UE context
Main failure outcome	The LTE side works from incomplete or stale sidelink assumptions
Most important messages	RRC Connection Reconfiguration, Sidelink UE Information r12
Main specs	TS 36.331

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

Preconditions

A sidelink-aware branch is relevant to the current LTE trace.
The LTE side can configure and interpret sidelink-related behavior.
The UE can expose sidelink-related context toward the network.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Applies sidelink-aware configuration and reports sidelink-related context.
eNB	Configures the sidelink-aware branch and interprets the UE report.
Sidelink context	Represents the side branch of LTE behavior that differs from ordinary LTE-only signaling.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the sidelink-aware configuration and reporting branch.
RRC	UE <-> eNB	Carries the main sidelink-aware control exchange.

End-to-End Call Flow

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

Major Phases

Phase	What happens
1. Enable sidelink-aware handling	The LTE side decides that sidelink-related behavior matters in the current branch.
2. Deliver the sidelink-aware configuration	The UE receives the control update tied to that branch.
3. Report the sidelink context	The UE exposes the relevant sidelink-related information.
4. Continue with the sidelink-aware branch	The LTE side uses that context in later handling.

Step-by-Step Breakdown

Start the sidelink-aware branch

Sender -> receiver: eNB -> UE

Message(s): RRC Connection Reconfiguration

Purpose: Provide the connected-mode or control update that makes sidelink context relevant.

State or context change: The trace has now moved from LTE-only behavior into a sidelink-aware branch.

Note: This is the best anchor when the sidelink question starts inside a wider connected-mode trace.

Report the sidelink context

Sender -> receiver: UE -> eNB

Message(s): Sidelink UE Information r12

Purpose: Expose the UE’s sidelink-related information to the LTE side.

State or context change: The eNB now has explicit sidelink context for the next decision or continuation.

Note: This is the main message that proves what the UE actually told the network.

Read the report into later handling

Sender -> receiver: eNB

Message(s): Sidelink-aware control decision

Purpose: Use the reported sidelink information in the next procedure branch.

State or context change: The LTE side is now working from explicit sidelink context rather than from assumption alone.

Note: If later handling ignores the report, the issue may be decision logic rather than missing signaling.

Validate the branch outcome

Sender -> receiver: UE <-> eNB

Message(s): Later sidelink-aware continuation

Purpose: Check whether the next behavior matches the reported sidelink context.

State or context change: The procedure is complete enough to explain the later sidelink-aware outcome.

Note: This is where report correctness and outcome correctness should be separated.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
RRC Connection Reconfiguration	RRC	eNB -> UE	Starts or updates the sidelink-aware branch.	Check whether the sidelink question really begins at this connected-mode update.
Sidelink UE Information r12	RRC	UE -> eNB	Reports the UE’s sidelink-related context.	Check what sidelink information was actually reported before later behavior is judged.
RRC Connection Reconfiguration Complete	RRC	UE -> eNB	Confirms the UE accepted the sidelink-aware configuration.	Check whether the sidelink-aware report followed after a real accepted configuration.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Sidelink-aware config point	The LTE-side update that made sidelink context relevant.	RRC reconfiguration	Shows where the trace should start being interpreted as sidelink-aware.	The branch is read as sidelink before it was actually configured that way.
Reported sidelink context	The information the UE exposed about the sidelink branch.	Sidelink UE Information r12	It is the core fact the later handling should follow.	Later behavior is judged without checking what the UE really reported.
Outcome alignment	Whether later behavior matches the reported sidelink context.	Post-report branch	Separates good reporting from good later handling.	The report is correct, but the later branch still follows the wrong assumptions.
Branch timing	How close the report was to the later decision it should influence.	Around the report window	Useful when multiple sidelink-related updates exist in one trace.	The wrong report instance is matched to the later outcome.
LTE-only overlap	Whether ordinary LTE-only logic is being mixed into the sidelink branch interpretation.	Whole procedure window	Prevents ordinary control assumptions from obscuring the sidelink-specific meaning.	The trace is analyzed as if it were ordinary mobility or bearer control.

Successful Completion

Success means the sidelink-aware branch is configured, the UE reports the needed context, and later handling matches that context.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
The branch looks sidelink-aware, but later behavior does not fit the UE report	The network may be ignoring the report or using the wrong report instance.	Sidelink UE Information r12 and the immediate next sidelink-aware action.	Sidelink UE Information r12	LTE Uu	Align the exact report with the exact later outcome before diagnosing policy.
A sidelink problem is visible, but no useful UE report appears	The branch may not have been configured correctly, or the trace window missed the report.	The earlier reconfiguration and the report timing.	RRC Connection Reconfiguration, Sidelink UE Information r12	LTE Uu	Prove the branch was actually set up for sidelink-aware reporting first.

What to Check in Logs and Traces

Anchor the analysis on the reconfiguration that made the branch sidelink-aware.
Use Sidelink UE Information r12 as the main proof of UE-side context.
Separate report correctness from later decision correctness.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is the broad sidelink reference path. Use the V2X or relay-specific pages when the trace clearly belongs to one of those narrower sidelink branches.

FAQ

What is the LTE Sidelink Procedure?

It is the LTE control path where sidelink-aware configuration and UE reporting shape the later branch behavior.

Which message matters most in sidelink analysis?

Sidelink UE Information r12 is usually the clearest proof of what the UE reported to the network.