| 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 SIB6 - System Information Block Type 6 |
|---|---|
| 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 SIB6 when idle mobility may involve UMTS, during initial camping, inter-RAT reselection evaluation, system-information refresh, or other LTE-to-UTRA mobility analysis. |
| Main purpose | Provides the UTRA frequency priorities, thresholds, and neighbor-frequency detail the UE needs when deciding whether to leave the serving LTE frequency and camp on a UMTS carrier. |
| 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 |
Broadcast LTE RRC system information block carried inside System Information that provides inter-RAT reselection information for UTRA frequencies and UTRA neighboring cells in idle mode.
Provides the UTRA frequency priorities, thresholds, and neighbor-frequency detail the UE needs when deciding whether to leave the serving LTE frequency and camp on a UMTS carrier.
SIB6 is the LTE broadcast block that tells an idle UE how to judge UMTS frequencies for reselection.
Call flow position: Inter-RAT mobility preparation step used after the UE already has the LTE serving-frequency context and now needs UTRA reselection information.
Typical state: UE is camped in idle mode on LTE and is building the broader mobility view toward UMTS frequencies.
Preconditions:
Next likely message: Continued camping or UTRA candidate-frequency ranking
Call flow position: Inter-RAT candidate-frequency evaluation step where the UE compares the current LTE serving frequency against UTRA frequencies.
Typical state: UE is idle and is deciding whether a UMTS carrier offers a better camping choice than the serving LTE carrier.
Preconditions:
Next likely message: Stay on LTE or camp on a better UTRA frequency
Call flow position: Refresh step where the UE reacquires SIB6 because LTE-to-UTRA reselection data may have changed.
Typical state: UE must refresh stored UTRA mobility assumptions before continuing to rely on them.
Preconditions:
Next likely message: Idle mobility continuation using refreshed SIB6 inter-RAT data
Previous message(s): MasterInformationBlock, SystemInformationBlockType1, System Information carrying SystemInformationBlockType3, System Information carrying SystemInformationBlockType5
Next message(s): Inter-RAT candidate-frequency ranking and reselection decision, Idle continuation on the serving LTE frequency, Camping on a better UTRA frequency
Security context: Broadcast message with no dedicated SRB or AS security. The UE reads it after MIB, SIB1, and the earlier LTE broadcast context.
SystemInformationBlockType6 ::= SEQUENCE {
carrierFreqListUTRA-FDD CarrierFreqListUTRA-FDD OPTIONAL,
carrierFreqListUTRA-TDD CarrierFreqListUTRA-TDD OPTIONAL,
t-ReselectionUTRA T-Reselection,
t-ReselectionUTRA-SF SpeedStateScaleFactors OPTIONAL,
lateNonCriticalExtension OCTET STRING OPTIONAL,
nonCriticalExtension SystemInformationBlockType6-v890-IEs OPTIONAL
}
CarrierFreqListUTRA-FDD ::= SEQUENCE (SIZE (1..maxUTRA-FDD-Carrier)) OF CarrierFreqUTRA-FDD
CarrierFreqUTRA-FDD ::= SEQUENCE {
carrierFreq UTRA-CarrierFreq,
cellReselectionPriority CellReselectionPriority OPTIONAL,
threshX-High ThreshX-High,
threshX-Low ThreshX-Low,
q-RxLevMin Q-RxLevMin,
p-MaxUTRA P-MaxUTRA OPTIONAL,
q-QualMin Q-QualMin OPTIONAL
}
This page focuses on the stable Release 18 SIB6 reading model used in practice: which UTRA frequencies are advertised, how they are prioritized, and what thresholds govern LTE-to-UTRA reselection.
BCCH-DL-SCH-Message
message c1 : systemInformation : {
criticalExtensions systemInformation-r8 : {
sib-TypeAndInfo {
sib6 {
carrierFreqListUTRA-FDD {
{
carrierFreq {
uarfcn-DL 10613
},
cellReselectionPriority 4,
threshX-High 6,
threshX-Low 4,
q-RxLevMin -60,
p-MaxUTRA 21,
q-QualMin -18
}
},
t-ReselectionUTRA 2
}
}
}
}
| IE | Required | Description |
|---|---|---|
carrierFreqListUTRA-FDD | Optional | List of UTRA FDD carrier frequencies with their inter-RAT reselection parameters. |
carrierFreqListUTRA-TDD | Optional | List of UTRA TDD carrier frequencies with their inter-RAT reselection parameters where relevant. |
cellReselectionPriority | Optional | Priority used when ranking the UTRA frequency against LTE and other candidate RAT or frequency choices. |
threshX-High / threshX-Low | Optional | Thresholds that govern movement between the serving LTE frequency and the candidate UTRA frequency. |
q-RxLevMin / q-QualMin | Optional | Minimum level and quality assumptions for the candidate UTRA frequency. |
p-MaxUTRA | Optional | Optional maximum UE transmit power assumption for the candidate UTRA frequency. |
lateNonCriticalExtension | Optional | Late extension branch for later release additions. |
nonCriticalExtension | Optional | Release-extension branch used for newer inter-RAT mobility additions. |
carrierFreqListUTRA-FDDList of UTRA FDD carrier frequencies with their inter-RAT reselection parameters.
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.
carrierFreqListUTRA-TDDList of UTRA TDD carrier frequencies with their inter-RAT reselection parameters where relevant.
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.
cellReselectionPriorityPriority used when ranking the UTRA frequency 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.
threshX-High / threshX-LowThresholds that govern movement between the serving LTE frequency and the candidate UTRA frequency.
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.
q-RxLevMin / q-QualMinMinimum level and quality assumptions for the candidate UTRA frequency.
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.
p-MaxUTRAOptional maximum UE transmit power assumption for the candidate UTRA frequency.
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.
lateNonCriticalExtensionLate 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.
nonCriticalExtensionRelease-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.
Likely cause: The SIB6 priorities or threshX values may legitimately favor staying on LTE longer than expected.
What to inspect: Check the UTRA carrier list, cellReselectionPriority, threshX-High, threshX-Low, and the measured radio conditions.
Next step: Validate the LTE-to-UTRA mobility policy before blaming the UE behavior.
Likely cause: The advertised UTRA frequency may carry higher priority or more favorable thresholds than assumed.
What to inspect: Compare the LTE serving-frequency view from SIB3 with the UTRA policy in SIB6.
Next step: Use SIB6 as the main explanation path for the inter-RAT move.
Likely cause: The refreshed SIB6 may have updated UTRA priorities, thresholds, or minimum level and quality assumptions.
What to inspect: Compare the old and new SIB6 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.
SIB5 explains movement toward other LTE frequencies. SIB6 extends the mobility view further toward UTRA.
SIB3 is about the current LTE serving frequency. SIB6 is about leaving LTE and evaluating UTRA candidates.
System Information is the container message. SIB6 is the LTE-to-UTRA mobility payload inside that container.
It is the LTE broadcast block that gives the UE inter-RAT reselection information for UTRA frequencies.
Because it often explains why an idle UE stayed on LTE or moved to UMTS.
Start with the advertised UTRA frequencies, their priorities, and the threshX values.
SIB5 is about other LTE carriers. SIB6 extends the mobility view further toward UTRA.
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.