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LTE Sidelink Relay UE Procedure Call Flow

call-flow LTE | Sidelink Relay | RRC

LTE Sidelink Relay UE Procedure is the sidelink-aware branch where the relay-capable UE exposes the context needed for relay-aware control and continuation.

The main anchors are RRC Connection Reconfiguration, Sidelink UE Information r12, and the later relay-aware behavior that should match the UE-reported context.

Introduction

This page narrows the broader sidelink family to the relay-capable UE branch. The core question is whether the relay-side UE context was configured, reported, and then reflected correctly in later handling.

Use it when the trace belongs to sidelink relay use and the analysis has to stay on the relay-capable UE side rather than on a generic sidelink branch.

What Is LTE Sidelink Relay UE Procedure in Simple Terms?

What starts the procedure: The LTE side enters a relay-capable sidelink branch and needs the relay UE context.
What the UE and network want to achieve: A relay-aware sidelink control state with the right UE-reported information behind it.
What success looks like: The relay UE branch is configured, the UE reports the needed context, and later behavior matches it.
What failure means: The relay-aware branch is configured but the later result does not match the UE report.

Why this procedure matters

Relay-capable sidelink traces are easy to flatten into generic sidelink unless the relay role stays visible. This page keeps the relay UE context at the center of the analysis.

Quick Fact Sheet

Procedure name	LTE Sidelink Relay UE Procedure
Domain	Relay-capable sidelink UE control
Main trigger	Need for relay-capable UE context in a sidelink-aware branch
Start state	A relay-aware sidelink branch is relevant in the current trace
End state	Relay-aware continuation matches the UE-reported relay context
Main nodes	UE, eNB
Main protocols	RRC
Main success outcome	Relay-capable UE context is reflected correctly in later handling
Main failure outcome	Later behavior does not align with the relay-aware UE report
Most important messages	RRC Connection Reconfiguration, Sidelink UE Information r12
Main specs	TS 36.331

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

Preconditions

The trace belongs to a relay-capable sidelink branch.
The LTE side can interpret relay-related sidelink context.
The UE can expose the needed sidelink relay information.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
Relay-capable UE	Applies relay-aware sidelink configuration and reports the relay-side context.
eNB	Configures the relay-aware branch and uses the report in later control.
Relay branch context	Represents the relay-capable sidelink meaning that should remain visible through the trace.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the relay-aware sidelink control and reporting path.
RRC	UE <-> eNB	Carries the configuration and UE report behind the relay-aware branch.

End-to-End Call Flow

Relay-capable UE          eNB
|<--RRC Reconfig----------|
|--Sidelink UE Info------>|
|==== relay-aware continuation ===>|

Major Phases

Phase	What happens
1. Enter the relay-aware branch	The LTE side decides that relay-capable UE context matters.
2. Deliver the relay-aware configuration	The UE receives the branch update.
3. Report the relay-side context	The UE exposes the relevant sidelink information.
4. Continue with relay-aware handling	The later branch uses that report as its context anchor.

Step-by-Step Breakdown

Start the relay-aware branch

Sender -> receiver: eNB -> UE

Message(s): RRC Connection Reconfiguration

Purpose: Deliver the control update that makes relay-capable UE context relevant.

State or context change: The trace is now in a relay-aware sidelink branch.

Note: This is the point where relay-aware reading should begin.

Report the relay-side UE context

Sender -> receiver: UE -> eNB

Message(s): Sidelink UE Information r12

Purpose: Expose the relay-capable UE information the branch depends on.

State or context change: The network now has explicit relay-side context for later handling.

Note: If later behavior looks wrong, compare it first against this UE report.

Use the report in later handling

Sender -> receiver: eNB

Message(s): Relay-aware control decision

Purpose: Carry the reported relay context into the next sidelink relay step.

State or context change: The later branch is now driven by relay-aware context instead of by generic sidelink assumptions.

Note: This explains why the next action may differ from ordinary sidelink behavior.

Validate the later branch outcome

Sender -> receiver: UE <-> eNB

Message(s): Relay-aware continuation

Purpose: Check whether later behavior matches the reported relay UE context.

State or context change: The branch is now complete enough to explain the later relay-aware result.

Note: This is where UE-report correctness and later decision 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 relay-aware sidelink branch.	Check whether the relay-capable meaning of the branch begins here.
Sidelink UE Information r12	RRC	UE -> eNB	Reports the relay-side UE context.	Check the exact relay-relevant information the UE exposed before later behavior is judged.
Remote UE Report	NAS	UE -> Network	Useful companion reference when relay-aware traces also expose remote-side status over NAS.	Check whether the trace is mixing relay-capable UE context with remote UE status handling.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Relay-aware entry point	The branch moment where relay-capable UE context becomes relevant.	RRC reconfiguration	Shows where relay-aware analysis should start.	The relay meaning is read too early or too late.
Reported relay context	The relay-relevant information the UE actually sent.	Sidelink UE Information r12	Provides the main fact the later relay branch should follow.	Later behavior is judged without the actual UE report.
Outcome alignment	Whether later relay-aware handling matches the report.	Post-report branch	Separates report quality from later branch logic.	The relay UE report is correct, but the branch still follows the wrong assumptions.
NAS overlap	Whether remote UE status signaling is also present in the same trace.	Across the relay window	Useful for separating relay-capable UE context from remote UE status handling.	The wrong relay-side message family is blamed.
Branch continuity	Whether the trace stays relay-aware after the report.	Whole procedure window	Prevents mixed traces from drifting into generic sidelink interpretation.	The analysis silently shifts into the wrong branch family.

Successful Completion

Success means the relay-aware branch is configured, the UE reports the relay-side context, and later behavior matches that report.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Later relay-aware behavior does not match the UE report	The later branch may be using stale or generic sidelink assumptions.	Sidelink UE Information r12 and the immediate next relay-aware action.	Sidelink UE Information r12	LTE Uu	Align the exact report with the exact next relay-aware action first.
The trace looks relay-aware, but the branch context is unclear	Generic sidelink control and relay-aware control may be mixed together.	The reconfiguration point and any remote UE NAS signaling around it.	RRC Connection Reconfiguration, Remote UE Report	LTE Uu, NAS	Separate relay-capable UE context from remote UE status signaling.

What to Check in Logs and Traces

Start from the branch entry point where relay-capable UE meaning becomes visible.
Use Sidelink UE Information r12 as the main proof of relay-side UE context.
Check whether NAS remote UE signaling overlaps the same trace window.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

This page is centered on the relay-capable UE branch. Use the remote UE relay page when the main trace anchor is the Remote UE Report family instead.

FAQ

What is the LTE Sidelink Relay UE Procedure?

It is the relay-capable sidelink branch where the UE reports the relay-side context needed for later relay-aware handling.

How is it different from Remote UE Relay Procedure?

This page centers on the relay-capable UE’s sidelink report, while the remote UE relay page centers on the remote UE status branch.