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RRC5GgNB -> UE3GPP TS 38.331
5G NR - System Information Block 3 (SIB3)
System Information Block 3 (SIB3) is an NR broadcast system information block used for intra-frequency cell reselection behavior and idle-state mobility decisions on the current NR carrier.
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 3 (SIB3)
Protocol
RRC
Technology
5G
Direction
gNB -> UE
Interface
Uu
Signaling bearer / channel
Broadcast transport / BCCH-DL-SCH
Typical trigger
Broadcast by the cell as part of additional system information so camped UEs can apply the intended intra-frequency reselection policy.
Main purpose
Provides intra-frequency reselection parameters so the UE can decide when to stay on the serving cell, when to search, and when to reselect another NR cell on the same frequency.
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, Intra-Frequency Cell Reselection, Idle Mobility, Inactive Mobility
Related timers
Reselection timing is driven by broadcast mobility parameters rather than one dedicated RRC transaction timer
Related cause values
SIB3 does not carry reject causes, Problems are inferred from missing SI acquisition or UE reselection behavior that does not match the configured intra-frequency policy
What is System Information Block 3 (SIB3) in simple terms?
System Information Block 3 (SIB3) is an NR broadcast system information block used for intra-frequency cell reselection behavior and idle-state mobility decisions on the current NR carrier.
Provides intra-frequency reselection parameters so the UE can decide when to stay on the serving cell, when to search, and when to reselect another NR cell on the same frequency.
Why this message matters
SIB3 is the NR broadcast block that mainly helps the UE decide how to behave with other cells on the same frequency.
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 mobility-information view of the current NR carrier.
Preconditions:
MIB and SIB1 have been acquired.
The UE knows the scheduling for additional system information.
Next likely message: Intra-frequency reselection evaluation
Intra-Frequency Cell Reselection
Call flow position: Used when the UE evaluates same-frequency neighbor cells while idle or inactive.
Typical state: UE is not in dedicated connected-mode signaling.
Preconditions:
Current cell broadcast information is valid.
Neighbor-cell measurements or search conditions are relevant.
Next likely message: Stay on current cell or reselect another NR cell on the same carrier
Idle / Inactive Mobility
Call flow position: Provides broadcast rules for mobility decisions on the serving frequency.
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, search, or reselection depending on radio conditions
Next message(s): Intra-frequency cell reselection evaluation, Idle mobility decisions, Later access on a reselected 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 MIB, SIB1, and the scheduling needed for additional SI
Security context: Broadcast information. It is cell-common and not protected by dedicated AS security.
Message Structure Overview
SIB3 is narrower in practical scope than SIB2. It is mainly about intra-frequency mobility rather than the broader additional-SI picture.
For engineering work, the key question is whether the announced same-frequency reselection policy matches the expected field behavior.
SIB3 is especially useful when UEs stay too long on a weak serving cell or bounce between same-frequency neighbors.
ASN.1 for 5G NR - System Information Block 3 (SIB3)
SIB3 is typically carried inside the broader SystemInformation container. In practice, engineers care less about the wrapper and more about whether the intra-frequency reselection values explain the observed camping behavior.
The key practical block is intraFreqCellReselectionInfo.
Search thresholds help explain when the UE begins checking same-frequency alternatives.
t-ReselectionNR and hysteresis values help explain whether the UE moves too quickly, too slowly, or not at all.
Important Information Elements
IE
Required
Description
cellReselectionInfoCommon
Optional
Common reselection context used with the serving cell's broadcast mobility policy.
intraFreqCellReselectionInfo
Yes
The most important SIB3 content. It drives same-frequency reselection behavior and explains why the UE searches, stays, or leaves.
s-IntraSearchP / s-IntraSearchQ
Optional
Search thresholds that influence when the UE should begin evaluating same-frequency neighbors.
t-ReselectionNR
Optional
Reselection timing parameter that affects how quickly the UE commits to a same-frequency cell change.
q-Hyst and related threshold behavior
Optional
Hysteresis and related mobility controls that stabilize or accelerate same-frequency reselection.
Detailed field explanation
cellReselectionInfoCommon
Common reselection context used with the serving cell's broadcast mobility policy.
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.
intraFreqCellReselectionInfo
The most important SIB3 content. It drives same-frequency reselection behavior and explains why the UE searches, stays, or leaves.
Presence: Required
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.
s-IntraSearchP / s-IntraSearchQ
Search thresholds that influence when the UE should begin evaluating same-frequency neighbors.
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.
t-ReselectionNR
Reselection timing parameter that affects how quickly the UE commits to a same-frequency cell change.
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-Hyst and related threshold behavior
Hysteresis and related mobility controls that stabilize or accelerate same-frequency reselection.
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 and SIB1 were acquired successfully before reviewing SIB3 behavior.
Verify that additional SI acquisition completed and that the UE actually has the current SIB3 content.
Check intra-frequency search thresholds against the observed radio conditions.
Inspect hysteresis and reselection timing against the actual move or no-move behavior.
Compare serving-cell SIB3 with same-frequency neighbor behavior instead of analyzing only one cell.
If users report unstable camping, correlate SIB3 with RF measurements and paging-area observations.
Common Issues and Troubleshooting
The UE stays too long on a weak serving cell.
Likely cause: SIB3 thresholds or hysteresis may discourage same-frequency reselection.
What to inspect: Check intraFreqCellReselectionInfo, q-Hyst, and t-ReselectionNR.
Next step: Compare the serving-cell policy with same-frequency neighbors and real RF conditions.
The UE reselects too aggressively between same-frequency cells.
Likely cause: Reselection timing or hysteresis may be too permissive.
What to inspect: Check t-ReselectionNR and any hysteresis-related parameters.
Next step: Correlate SIB3 with measurement fluctuations and field mobility logs.
Paging works, but idle mobility seems unstable.
Likely cause: The issue may be in SIB3 same-frequency mobility policy rather than in the paging procedure itself.
What to inspect: Check whether the current serving cell and candidate same-frequency cells broadcast a consistent reselection policy.
Next step: Analyze MIB, SIB1, SIB2, and SIB3 together as one broadcast mobility chain.
LTE / 5G / Variant Comparison
SIB3 versus SIB2
SIB2 gives broader additional mobility and reselection context. SIB3 is more focused on same-frequency reselection behavior.
SIB3 versus dedicated RRC
SIB3 is broadcast cell-common mobility information, not UE-specific connected-mode configuration.
FAQ
What is SIB3 in 5G NR?
SIB3 is System Information Block 3, an NR broadcast block used mainly for intra-frequency cell reselection behavior.
Who sends SIB3?
The gNB broadcasts SIB3 as additional system information.
What is the main purpose of SIB3?
To provide same-frequency reselection parameters for idle and inactive mobility.
On which channel is SIB3 sent?
SIB3 is carried in system information on BCCH-DL-SCH.
Why is SIB3 useful in troubleshooting?
Because it helps explain why the UE stays, searches, or reselects between NR cells on the same carrier.
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.