5G NR - System Information Block 8 (SIB8)

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 8 (SIB8)
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 cdma2000 where that legacy mobility path still matters.
Main purpose	Provides cdma2000-related reselection information so the UE can make correct idle or inactive mobility decisions when neighboring cdma2000 systems remain part of the operator 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 driven by broadcast mobility parameters rather than a dedicated UE-specific RRC transaction timer
Related cause values	SIB8 does not carry reject causes, Problems are inferred from missing SI acquisition or inter-RAT behavior that does not match the configured cdma2000 mobility policy

What is System Information Block 8 (SIB8) in simple terms?

System Information Block 8 (SIB8) is an NR broadcast system information block used for inter-RAT mobility context toward cdma2000 systems where that legacy mobility path still exists.

Provides cdma2000-related reselection information so the UE can make correct idle or inactive mobility decisions when neighboring cdma2000 systems remain part of the operator mobility strategy.

Why this message matters

SIB8 is the NR broadcast block that mainly tells the UE how to behave with cdma2000 targets when that legacy RAT is still part of the mobility design.

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 legacy radio technologies.

Preconditions:

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

Next likely message: cdma2000 reselection evaluation

Inter-RAT Cell Reselection to cdma2000

Call flow position: Used when the UE evaluates mobility from NR toward cdma2000.

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

Preconditions:

The serving-cell broadcast information is valid.
cdma2000 neighbors are part of the mobility design.

Next likely message: Stay on NR or reselect to cdma2000

Idle / Inactive Mobility

Call flow position: Provides broadcast rules for mobility decisions from NR toward cdma2000.

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 5 (SIB5), System Information Block 6 (SIB6), System Information Block 7 (SIB7)

Next message(s): cdma2000 reselection evaluation, Idle mobility decisions toward cdma2000, Later access on a reselected cdma2000 cell

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

SIB8 focuses on NR-to-cdma2000 mobility rather than NR-only mobility or other inter-RAT paths.
For engineering work, the main question is whether the legacy cdma2000 mobility policy matches the intended deployment strategy.
SIB8 matters mostly in networks where cdma2000 still exists as part of coverage, roaming, or continuity planning.

ASN.1 for 5G NR - System Information Block 8 (SIB8)

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 {
    sib8 SystemInformationBlockType8,
    ...
  }
}

SystemInformationBlockType8 ::= SEQUENCE {
  ...
}

How to read this ASN.1

SIB8 is typically carried inside the broader SystemInformation container. In practice, engineers care most about whether the cdma2000 target list and thresholds explain the observed legacy inter-RAT mobility behavior.

Decoded Message Dump

BCCH-DL-SCH-Message
  message: c1
    systemInformation
      criticalExtensions
        systemInformation
          sib-TypeAndInfo
            sib8
              cdma2000-CarrierList
                bandClass: bc0
                arfcn: 384
                cellReselectionPriority: 3
                threshX-High: 7
                threshX-Low: 4
                q-OffsetFreq: dB0

How to read this dump

The key practical block is the cdma2000 carrier list.
Band class and frequency information tell you whether cdma2000 is really part of the mobility strategy.
Thresholds and offsets help explain why the UE stays on NR or moves toward cdma2000.

Important Information Elements

IE	Required	Description
`cdma2000 carrier list`	Yes	The main SIB8 content. It defines the candidate cdma2000 carriers or neighbor information relevant for mobility from NR toward cdma2000.
`band class and frequency information`	Optional	Identifies the target cdma2000 system and helps explain whether it is really part of the mobility design.
`priority and threshold-related parameters`	Optional	Help explain when cdma2000 should become attractive enough to be considered for reselection.
`offset or equivalent biasing information`	Optional	Changes how favorable or unfavorable the cdma2000 target appears in idle mobility decisions.
`search and timing controls`	Optional	Operationally important parameters that affect when the UE searches and when it commits to a move toward cdma2000.

Detailed field explanation

`cdma2000 carrier list`

The main SIB8 content. It defines the candidate cdma2000 carriers or neighbor information relevant for mobility from NR toward cdma2000.

Presence: Required