5G NR - System Information Block 12 (SIB12)

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.2.1, 5.2.2, 6.3.1
Direction	gNB -> UE	Message Type	Broadcast System Information

Full message name	5G NR - System Information Block 12 (SIB12)
Protocol	RRC
Technology	5G
Direction	gNB -> UE
Interface	Uu
Signaling bearer / channel	Broadcast transport / BCCH-DL-SCH
Typical trigger	Broadcast by the cell as additional system information when UEs supporting NR sidelink communication or discovery need sidelink-related broadcast configuration.
Main purpose	Provides sidelink-related broadcast configuration so the UE can perform or prepare NR sidelink communication and discovery according to the serving cell's policy.
Main specification	3GPP TS 38.331, 5.2.1, 5.2.2, 6.3.1
Release added	Release 16
Procedures where used	System Information Acquisition, NR Sidelink Communication, NR Sidelink Discovery, Sidelink-Related Access Decisions
Related timers	SIB12 does not use a dedicated UE-specific transaction timer, Interpretation depends on SI acquisition timing and the validity of the broadcast sidelink configuration
Related cause values	SIB12 does not carry reject causes, Problems are usually inferred from missing acquisition, wrong sidelink pool selection, or sidelink behavior that does not match the broadcast configuration

What is System Information Block 12 (SIB12) in simple terms?

System Information Block 12 (SIB12) is an NR broadcast system information block used to provide sidelink communication and discovery configuration to UEs that support NR sidelink.

Provides sidelink-related broadcast configuration so the UE can perform or prepare NR sidelink communication and discovery according to the serving cell's policy.

Why this message matters

SIB12 is the 5G NR broadcast block that mainly gives the UE the sidelink frequencies and pool configuration needed for NR sidelink communication or discovery.

Where this message appears in the call flow

System Information Acquisition

Call flow position: Read as additional broadcast information after the essential access layer is already available.

Typical state: UE is camped and expanding the broadcast-information context beyond access and mobility.

Preconditions:

MIB and SIB1 have been acquired.
The UE knows the scheduling for additional system information.
The UE supports or is configured for NR sidelink communication or discovery.

Next likely message: UE applies the sidelink-related broadcast configuration locally

NR Sidelink Communication / Discovery

Call flow position: Used when the UE needs cell-provided sidelink configuration for communication or discovery operation.

Typical state: UE is not necessarily in a dedicated UE-specific RRC transaction.

Preconditions:

The UE is sidelink-capable.
The serving cell broadcasts SIB12 or equivalent sidelink SI.

Next likely message: UE uses the sidelink frequency and pool configuration for communication or discovery behavior

RRC Connection Trigger for Sidelink

Call flow position: Relevant when sidelink-related conditions and broadcast configuration influence whether RRC connection establishment should occur.

Typical state: UE may be camped and evaluating whether to move into connected signaling.

Preconditions:

SIB12 has been acquired successfully.
Upper-layer sidelink conditions are present.

Next likely message: RRC connection establishment or continued sidelink preparation depending on scenario

Call flow position

Previous message(s): MIB, System Information Block 1 (SIB1), SystemInformation scheduling

Next message(s): NR sidelink communication preparation, NR sidelink discovery preparation, Later RRC connection establishment if sidelink-related conditions require it

Message direction and transport

Sender and receiver: gNB -> UE

Interface: Uu

Domain: Access-side radio control and broadcast sidelink information

Signaling bearer: Broadcast transport

Logical channel: BCCH-DL-SCH

Transport / encapsulation: RRC system information carried on BCCH-DL-SCH after the UE has acquired the essential system information and the scheduling for additional SI

Security context: Broadcast information. It is cell-common and not protected by dedicated AS security.

Message Structure Overview

SIB12 is not mainly about access, mobility, naming, or timing. Its practical purpose is broadcast sidelink configuration.
For engineering work, the main question is whether the sidelink frequencies and pools announced in SIB12 match the intended sidelink deployment and UE behavior.
SIB12 becomes especially important when NR sidelink communication or discovery does not start, uses the wrong pool, or behaves inconsistently.

ASN.1 for 5G NR - System Information Block 12 (SIB12)

BCCH-DL-SCH-Message ::= SEQUENCE {
  message CHOICE {
    c1 CHOICE {
      systemInformationBlockType1 SystemInformationBlockType1,
      systemInformation          SystemInformation,
      ...
    },
    ...
  }
}

SystemInformation ::= SEQUENCE {
  criticalExtensions CHOICE {
    systemInformation SystemInformation-IEs,
    ...
  }
}

SystemInformation-IEs ::= SEQUENCE {
  sib-TypeAndInfo SEQUENCE (SIZE (1..maxSIB)) OF CHOICE {
    sib12 SystemInformationBlockType12-r16,
    ...
  }
}

SystemInformationBlockType12-r16 ::= SEQUENCE {
  sl-FreqInfoList   SEQUENCE (SIZE (1..maxSL-Freq-r16)) OF SL-FreqInfo-r16,
  ...
}

How to read this ASN.1

SIB12 is typically carried inside the broader SystemInformation container. The most important practical content is the sidelink frequency and transmission/discovery pool configuration, especially sl-FreqInfoList and the related pool-selection fields.

Decoded Message Dump

BCCH-DL-SCH-Message
  message: c1
    systemInformation
      criticalExtensions
        systemInformation
          sib-TypeAndInfo
            sib12
              sl-FreqInfoList
                [0]
                  carrierFreq-r16: 520000
                  sl-TxPoolSelectedNormal: present
                  sl-DiscTxPoolSelected: present

How to read this dump

The key practical block is sl-FreqInfoList.
The sidelink frequency list explains whether the UE should use the serving-cell broadcast information for sidelink operation on a given frequency.
Pool-selection fields help explain why sidelink communication or discovery uses one pool, another pool, or fails to start at all.

Important Information Elements

IE	Required	Description
`sl-FreqInfoList`	Yes	The core SIB12 payload. It identifies the sidelink-related frequency information relevant to NR sidelink communication or discovery.
`sl-TxPoolSelectedNormal`	Optional	Broadcast sidelink transmission-pool selection information used for normal sidelink transmission behavior.
`sl-DiscTxPoolSelected`	Optional	Broadcast discovery-transmission pool configuration used when the UE performs sidelink discovery.
`sl-DRX-ConfigCommonGC-BC`	Optional	Additional sidelink-related broadcast configuration that may affect common sidelink operation.
`sidelink frequency and pool selection context`	Optional	Operationally important configuration that explains whether the UE can use a given sidelink frequency and which pool applies.

Detailed field explanation

`sl-FreqInfoList`

The core SIB12 payload. It identifies the sidelink-related frequency information relevant to NR sidelink communication or discovery.

Presence: Required