5G NR - System Information Block 5 (SIB5)

Message Fact Sheet

Protocol	rrc	Network	5g
Spec	3GPP TS 38.331	Spec Section	5.2.1, 5.2.2, 5.4.3, 6.3.1
Direction	gNB -> UE	Message Type	Broadcast System Information

Full message name	5G NR - System Information Block 5 (SIB5)
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 so camped UEs can apply inter-RAT reselection policy toward neighboring technologies.
Main purpose	Provides inter-RAT reselection information so the UE can make correct idle or inactive mobility decisions when neighboring non-NR technologies are part of the mobility strategy.
Main specification	3GPP TS 38.331, 5.2.1, 5.2.2, 5.4.3, 6.3.1
Release added	Release 15
Procedures where used	System Information Acquisition, Inter-RAT Cell Reselection, Idle Mobility, Inactive Mobility
Related timers	Reselection timing is controlled by broadcast mobility parameters rather than a dedicated UE-specific RRC transaction timer
Related cause values	SIB5 does not carry reject causes, Problems are inferred from missing SI acquisition or inter-RAT mobility behavior that does not match the configured policy

What is System Information Block 5 (SIB5) in simple terms?

System Information Block 5 (SIB5) is an NR broadcast system information block used for inter-RAT mobility context, helping the UE evaluate reselection toward other radio access technologies such as LTE.

Provides inter-RAT reselection information so the UE can make correct idle or inactive mobility decisions when neighboring non-NR technologies are part of the mobility strategy.

Why this message matters

SIB5 is the NR broadcast block that mainly tells the UE how to behave with other radio technologies such as LTE.

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 building the broader mobility view across other radio technologies.

Preconditions:

MIB and SIB1 have been acquired.
The UE knows the scheduling for additional system information.

Next likely message: Inter-RAT reselection evaluation

Inter-RAT Cell Reselection

Call flow position: Used when the UE evaluates mobility from NR toward another access technology such as LTE.

Typical state: UE is not in dedicated connected-mode signaling.

Preconditions:

The serving-cell broadcast information is valid.
Inter-RAT candidates are relevant to the deployment.

Next likely message: Stay on NR or reselect to another RAT

Idle / Inactive Mobility

Call flow position: Provides broadcast rules for mobility decisions across technologies.

Typical state: UE is camped or inactive and monitoring mobility-related broadcast information.

Preconditions:

The UE is already operating with valid serving-cell system information.

Next likely message: Paging monitoring, RAT search, or reselection depending on conditions

Call flow position

Previous message(s): MIB, System Information Block 1 (SIB1), System Information Block 2 (SIB2), System Information Block 3 (SIB3), System Information Block 4 (SIB4)

Next message(s): Inter-RAT reselection evaluation, Idle mobility decisions toward LTE or other configured RATs, Later access on a reselected target system

Message direction and transport

Sender and receiver: gNB -> UE

Interface: Uu

Domain: Access-side radio control and broadcast mobility 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

SIB5 focuses on inter-RAT mobility rather than NR-only same-frequency or inter-frequency behavior.
For engineering work, the main question is whether the target-RAT mobility policy matches the intended fallback or coverage strategy.
SIB5 becomes especially important when UEs do not leave NR for LTE when expected, or leave too early when NR should remain preferred.

ASN.1 for 5G NR - System Information Block 5 (SIB5)

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 {
    sib5 SystemInformationBlockType5,
    ...
  }
}

SystemInformationBlockType5 ::= SEQUENCE {
  ...
}

How to read this ASN.1

SIB5 is typically carried inside the broader SystemInformation container. In practice, engineers care most about whether the inter-RAT candidate list and thresholds explain the observed fallback or reselection behavior.

Decoded Message Dump

BCCH-DL-SCH-Message
  message: c1
    systemInformation
      criticalExtensions
        systemInformation
          sib-TypeAndInfo
            sib5
              interRAT-CarrierList
                rat-Type: eutra
                carrierFreq: 1800
                cellReselectionPriority: 5
                threshX-High: 8
                threshX-Low: 4
                q-OffsetFreq: dB0

How to read this dump

The key practical block is the inter-RAT carrier list.
RAT type and target frequency tell you which non-NR system is part of the mobility strategy.
Thresholds, offsets, and priorities help explain why the UE stays on NR or moves away from it.

Important Information Elements

IE	Required	Description
`interRAT carrier list`	Yes	The main SIB5 content. It defines the non-NR candidate systems the UE may evaluate.
`target RAT frequency information`	Optional	Identifies which target technology frequencies or carriers are relevant for reselection.
`priority and threshold-related parameters`	Optional	Help explain when another RAT should be considered attractive enough for reselection.
`q-Offset or equivalent RAT biasing information`	Optional	Changes how favorable or unfavorable a target RAT appears in the idle mobility decision.
`reselection timing / search behavior controls`	Optional	Operationally important parameters that affect when the UE searches and when it commits to a RAT change.