Telecom engineering reference for protocols, messages, call flows, troubleshooting, releases, and tools.
Menu
LTE RRCLTEeNodeB -> UE3GPP TS 36.331
LTE RRC SIB4 - System Information Block Type 4
Broadcast LTE RRC system information block carried inside System Information that provides intra-frequency neighbor-cell detail used during idle-mode camping and same-frequency cell reselection.
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 SIB4 - System Information Block Type 4
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 SIB4 when same-frequency idle mobility needs candidate-cell-specific detail, during initial camping, intra-frequency reselection, system-information refresh, or other idle-mobility evaluation on the serving LTE carrier.
Main purpose
Provides the neighbor-cell-specific intra-frequency reselection information the UE needs after SIB3 so it can interpret same-frequency candidate cells, cell-specific offsets, blacklists, and reserved CSG identity ranges.
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, Same-frequency Mobility Evaluation, System Information Change Notification
What is System Information Block Type 4 in simple terms?
Broadcast LTE RRC system information block carried inside System Information that provides intra-frequency neighbor-cell detail used during idle-mode camping and same-frequency cell reselection.
Provides the neighbor-cell-specific intra-frequency reselection information the UE needs after SIB3 so it can interpret same-frequency candidate cells, cell-specific offsets, blacklists, and reserved CSG identity ranges.
Why this message matters
SIB4 is the LTE broadcast block that tells an idle UE how to treat specific same-frequency neighbor cells.
Where this message appears in the call flow
LTE idle-mode camping
In LTE idle-mode camping, System Information Block Type 4 adds the same-frequency neighbor detail the UE uses while staying camped and evaluating nearby LTE cells.
Call flow position: Neighbor-cell interpretation step used after the UE already has serving-frequency reselection rules and now needs the same-frequency candidate-cell detail.
Typical state: UE is camped in idle mode and is evaluating same-frequency neighboring LTE cells relative to the serving cell.
Preconditions:
MIB, SIB1, SIB2, and SIB3 have already been acquired.
System Information containing SIB4 is present and decodable.
Next likely message: Continued camping or same-frequency candidate ranking
LTE cell reselection
In LTE cell reselection, the UE combines the serving-cell policy from SIB3 with the same-frequency neighbor detail from SIB4 to decide which LTE candidate cell to camp on.
Call flow position: Same-frequency candidate-cell evaluation step where the UE applies SIB4 neighbor detail on top of the serving-frequency rules from SIB3.
Typical state: UE is idle and is comparing specific same-frequency candidate cells against the serving-cell policy.
Preconditions:
The UE is already camped on the LTE serving frequency.
Reselection evaluation has already been triggered.
Next likely message: Stay on the serving cell or camp on a better same-frequency neighbor
System information change handling
When system information changes, the UE refreshes SIB4 so that same-frequency neighbor-cell interpretation follows the updated broadcast mobility data.
Call flow position: Refresh step where the UE reacquires SIB4 because same-frequency neighbor-cell interpretation may have changed.
Typical state: UE must refresh stored same-frequency 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 SIB4 neighbor detail
Next message(s): Same-frequency candidate-cell ranking and reselection decision, Idle continuation on the serving cell, Camping on a better same-frequency LTE cell
Message direction and transport
Sender and receiver: eNodeB -> UE
Interface: Uu
Domain: Access-side radio control for same-frequency idle mobility and neighbor-cell interpretation
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 SIB4 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 serving-frequency mobility context.
Message Structure Overview
System Information Block Type 4 adds same-frequency candidate-cell detail on top of the serving-frequency mobility rules provided by SIB3.
Its practical meaning comes from which neighbor cells receive specific offsets, which cells are blacklisted, and whether reserved CSG identity ranges affect the interpretation.
In troubleshooting, SIB4 is most useful when the question is not whether reselection should happen at all, but why the UE preferred or ignored a particular same-frequency candidate cell.
ASN.1 for LTE RRC SIB4 - System Information Block Type 4
This page focuses on the stable top-level SIB4 structure that remains operationally important in Release 18 LTE troubleshooting: same-frequency neighbor cells with specific offsets, blacklisted cells, and optional CSG identity ranges.
LTE RRC SIB4 - System Information Block Type 4 - Example Dump
The first useful question is which same-frequency neighbors actually receive explicit q-OffsetCell treatment.
If a candidate cell is ignored unexpectedly, check whether it is blacklisted or whether another neighbor gets a more favorable offset.
SIB4 rarely stands alone in analysis; read it with SIB3 because serving-frequency policy and candidate-cell detail belong together.
Important Information Elements
IE
Required
Description
intraFreqNeighCellList
Optional
List of same-frequency neighboring LTE cells with cell-specific reselection offsets.
intraFreqBlackCellList
Optional
List of same-frequency cells that should be treated as blacklisted for reselection handling.
csg-PhysCellIdRange
Optional
Range of physical cell identities reserved for CSG cells on the frequency where this information was received.
lateNonCriticalExtension
Optional
Late extension branch for later release additions.
nonCriticalExtension
Optional
Release-extension branch used for newer same-frequency mobility additions.
Detailed field explanation
intraFreqNeighCellList
List of same-frequency neighboring LTE cells with cell-specific reselection offsets.
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.
intraFreqBlackCellList
List of same-frequency cells that should be treated as blacklisted for reselection handling.
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.
csg-PhysCellIdRange
Range of physical cell identities reserved for CSG cells on the frequency where this information was received.
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 same-frequency 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, SIB2, and SIB3 were acquired before interpreting SIB4.
Check whether SIB4 is present in the relevant System Information message.
Inspect intraFreqNeighCellList for cell-specific q-OffsetCell treatment.
Inspect intraFreqBlackCellList if a same-frequency candidate is unexpectedly ignored.
Compare SIB4 against SIB3 when explaining why a specific same-frequency reselection happened or did not happen.
Common Issues and Troubleshooting
The UE ignores a same-frequency neighbor that looks radio-strong.
Likely cause: The candidate cell may be blacklisted or may not receive a favorable cell-specific offset in SIB4.
What to inspect: Check intraFreqNeighCellList, intraFreqBlackCellList, and the serving-frequency rules from SIB3.
Next step: Validate the candidate-cell-specific reselection treatment before blaming measurements alone.
The UE prefers one same-frequency neighbor over another.
Likely cause: SIB4 may assign different q-OffsetCell values that tilt candidate ranking even when raw radio conditions look similar.
What to inspect: Compare the q-OffsetCell values for the competing candidates and correlate them with observed reselection behavior.
Next step: Treat SIB4 as the neighbor-detail layer on top of SIB3 rather than analyzing the candidates in isolation.
Same-frequency mobility changed after a system-information refresh.
Likely cause: The refreshed SIB4 may have updated neighbor-cell offsets, blacklists, or reserved identity ranges.
What to inspect: Compare the old and new SIB4 content and then correlate the changed neighbor treatment with the new idle mobility outcome.
Next step: Use SIB4 as the explanation path for changed same-frequency candidate handling.
LTE / 5G / Variant Comparison
Compared with System Information Block Type 3
SIB3 explains the serving-frequency reselection policy. SIB4 adds same-frequency neighbor-cell detail such as q-OffsetCell and blacklists.
Compared with System Information Block Type 5
SIB4 stays on the current LTE carrier and focuses on same-frequency neighbors. SIB5 extends the reselection view toward other LTE frequencies.
Compared with System Information
System Information is the container message. SIB4 is the same-frequency neighbor-detail payload inside that container.
FAQ
What is System Information Block Type 4 in LTE?
It is the LTE broadcast block that gives the UE same-frequency neighbor-cell detail for idle-mode reselection.
Why is LTE SIB4 important?
Because it often explains why a specific same-frequency neighbor cell was preferred, ignored, or blacklisted even when the serving-frequency policy from SIB3 looked reasonable.
What should I inspect first in LTE SIB4?
Start with intraFreqNeighCellList, q-OffsetCell, intraFreqBlackCellList, and then compare those values with the serving-frequency rules from SIB3.
How does SIB4 relate to SIB3?
SIB3 gives the serving-frequency policy. SIB4 gives the candidate-cell-specific same-frequency detail used on top of that policy.
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.
