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LTE RRCLTEeNodeB -> UE3GPP TS 36.331
LTE RRC SIB8 - System Information Block Type 8
Broadcast LTE RRC system information block carried inside System Information that provides inter-RAT reselection information for cdma2000 1xRTT and HRPD idle mobility.
Message Fact Sheet
Protocol
lte-rrc
Network
lte
Spec
3GPP TS 36.331
Spec Section
5.2.2, 5.2.4, 6.2.2
Direction
eNodeB -> UE
Message Type
Broadcast System Information
Full message name
LTE RRC SIB8 - System Information Block Type 8
Protocol
LTE-RRC
Technology
LTE
Direction
eNodeB -> UE
Interface
Uu
Signaling bearer / channel
Broadcast system information transport / BCCH mapped to DL-SCH
Typical trigger
The UE reads System Information carrying SIB8 when idle mobility may involve cdma2000, during initial camping, inter-RAT reselection evaluation, system-information refresh, or other LTE-to-cdma2000 mobility analysis.
Main purpose
Provides cdma2000 priorities, thresholds, search-related information, and timing context the UE needs when deciding whether to leave the serving LTE frequency and camp on a cdma2000 system.
Main specification
3GPP TS 36.331, 5.2.2, 5.2.4, 6.2.2
Release added
Release 8
Procedures where used
System Information Acquisition, Idle-mode Camping, Cell Reselection, Inter-RAT Mobility Evaluation, System Information Change Notification
What is System Information Block Type 8 in simple terms?
Broadcast LTE RRC system information block carried inside System Information that provides inter-RAT reselection information for cdma2000 1xRTT and HRPD idle mobility.
Provides cdma2000 priorities, thresholds, search-related information, and timing context the UE needs when deciding whether to leave the serving LTE frequency and camp on a cdma2000 system.
Why this message matters
SIB8 is the LTE broadcast block that tells an idle UE how to judge cdma2000 frequencies for reselection.
Where this message appears in the call flow
LTE idle-mode camping
In LTE idle-mode camping, System Information Block Type 8 gives the UE the cdma2000 mobility view it needs when considering 1xRTT or HRPD systems beyond the serving LTE carrier.
Call flow position: Inter-RAT mobility preparation step used after the UE already has the LTE serving-frequency context and now needs cdma2000 reselection information.
Typical state: UE is camped in idle mode on LTE and is building the broader mobility view toward cdma2000 1xRTT or HRPD systems.
Preconditions:
MIB, SIB1, SIB2, and the relevant LTE mobility SIBs have already been acquired.
System Information containing SIB8 is present and decodable.
Next likely message: Continued camping or cdma2000 candidate-frequency ranking
LTE cell reselection
In LTE cell reselection, the UE combines the serving-frequency LTE policy from SIB3 with the cdma2000 data from SIB8 to decide whether to camp on 1xRTT or HRPD.
Call flow position: Inter-RAT candidate-frequency evaluation step where the UE compares the current LTE serving frequency against cdma2000 frequencies.
Typical state: UE is idle and is deciding whether a cdma2000 carrier offers a better camping choice than the serving LTE carrier.
Preconditions:
The UE is already camped on an LTE serving frequency.
Reselection evaluation has already been triggered.
Next likely message: Stay on LTE or camp on a better cdma2000 frequency
System information change handling
When system information changes, the UE refreshes SIB8 so that LTE-to-cdma2000 mobility decisions follow the updated priorities, thresholds, and target-system assumptions.
Call flow position: Refresh step where the UE reacquires SIB8 because LTE-to-cdma2000 reselection data may have changed.
Typical state: UE must refresh stored cdma2000 mobility assumptions before continuing to rely on them.
Preconditions:
A system-information refresh was triggered.
Refreshed SIB1 and the relevant later System Information are available.
Next likely message: Idle mobility continuation using refreshed SIB8 inter-RAT data
Next message(s): Inter-RAT candidate-frequency ranking and reselection decision, Idle continuation on the serving LTE frequency, Camping on a better cdma2000 frequency
Message direction and transport
Sender and receiver: eNodeB -> UE
Interface: Uu
Domain: Access-side radio control for LTE-to-cdma2000 idle mobility and inter-RAT reselection
Signaling bearer: Broadcast system information transport
Logical channel: BCCH mapped to DL-SCH
Transport / encapsulation: System Information carried on BCCH and transmitted on DL-SCH with SIB8 included in sib-TypeAndInfo
Security context: Broadcast message with no dedicated SRB or AS security. The UE reads it after MIB, SIB1, and the earlier LTE broadcast context.
Message Structure Overview
System Information Block Type 8 extends LTE idle mobility away from LTE and toward cdma2000.
Its practical meaning comes from which cdma2000 systems are supported, how they are prioritized, and what thresholds and timing assumptions govern movement toward them.
In troubleshooting, SIB8 is most useful when the mobility question is about why the UE stayed on LTE or moved to cdma2000 rather than another LTE, UTRA, or GERAN option.
ASN.1 for LTE RRC SIB8 - System Information Block Type 8
This page focuses on the stable SIB8 reading model used in practice: which cdma2000 systems are advertised, how they are prioritized, and what thresholds, search, and timing assumptions govern LTE-to-cdma2000 reselection.
LTE RRC SIB8 - System Information Block Type 8 - Example Dump
Priority and threshold groups used when ranking cdma2000 candidates against LTE and other candidate RAT or frequency choices.
searchWindowSize
Optional
Search-window assumption used when preparing measurements or candidate evaluation toward cdma2000.
csfb-RegistrationParam1XRTT
Optional
Optional registration-related context for LTE interworking toward cdma2000 1xRTT.
longCodeState1XRTT
Optional
1xRTT timing-related value used in interworking support.
systemTimeInfo
Optional
cdma2000 system-time context that helps align interworking behavior.
lateNonCriticalExtension
Optional
Late extension branch for later release additions.
nonCriticalExtension
Optional
Release-extension branch used for newer inter-RAT mobility additions.
Detailed field explanation
parametersHRPD / parameters1XRTT
Parameters used for LTE interworking toward cdma2000 HRPD or 1xRTT systems.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
Priority and threshold groups used when ranking cdma2000 candidates against LTE and other candidate RAT or frequency choices.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
searchWindowSize
Search-window assumption used when preparing measurements or candidate evaluation toward cdma2000.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
csfb-RegistrationParam1XRTT
Optional registration-related context for LTE interworking toward cdma2000 1xRTT.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
longCodeState1XRTT
1xRTT timing-related value used in interworking support.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
systemTimeInfo
cdma2000 system-time context that helps align interworking behavior.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
lateNonCriticalExtension
Late extension branch for later release additions.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
nonCriticalExtension
Release-extension branch used for newer inter-RAT mobility additions.
Presence: Optional
In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.
What to check in logs and traces
Confirm MIB, SIB1, and the relevant LTE mobility SIBs were acquired before interpreting SIB8.
Check whether SIB8 is present in the relevant System Information message.
Inspect whether the page carries 1xRTT parameters, HRPD parameters, or both.
Inspect band classes, priorities, threshX values, and any search-window or system-time context relevant to the case.
Compare SIB8 against SIB6 and SIB7 when explaining why the UE stayed on LTE or moved to a different RAT.
Common Issues and Troubleshooting
The UE stays on LTE even though cdma2000 coverage looks available.
Likely cause: The SIB8 priorities, thresholds, or cdma2000-specific assumptions may legitimately favor staying on LTE longer than expected.
What to inspect: Check the advertised cdma2000 mode, band class data, cellReselectionPriority, threshX-High, threshX-Low, and the measured radio conditions.
Next step: Validate the LTE-to-cdma2000 mobility policy before treating the behavior as unexpected.
The UE reselects to cdma2000 earlier than expected.
Likely cause: The advertised cdma2000 system may carry higher priority or more favorable thresholds than assumed.
What to inspect: Compare the LTE serving-frequency view from SIB3 with the cdma2000 policy in SIB8.
Next step: Use SIB8 as the main explanation path for the inter-RAT move.
Inter-RAT mobility changed after a system-information refresh.
Likely cause: The refreshed SIB8 may have updated cdma2000 priorities, thresholds, timing, or target-system assumptions.
What to inspect: Compare the old and new SIB8 content and correlate the changed values with the new mobility outcome.
Next step: Treat it as a broadcast inter-RAT mobility-policy change until proven otherwise.
Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.
