Telecom engineering reference for protocols, messages, call flows, troubleshooting, releases, and tools.
Menu
RRC5GgNB -> UE3GPP TS 38.331
5G NR - System Information Block 4 (SIB4)
System Information Block 4 (SIB4) is an NR broadcast system information block used for inter-frequency cell reselection behavior and idle-state mobility decisions across other NR carrier frequencies.
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 4 (SIB4)
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-frequency reselection policy across neighboring NR carriers.
Main purpose
Provides inter-frequency reselection parameters so the UE can evaluate neighboring NR frequencies correctly while idle or inactive.
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-Frequency 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
SIB4 does not carry reject causes, Problems are inferred from missing SI acquisition or inter-frequency mobility behavior that does not match the configured policy
What is System Information Block 4 (SIB4) in simple terms?
System Information Block 4 (SIB4) is an NR broadcast system information block used for inter-frequency cell reselection behavior and idle-state mobility decisions across other NR carrier frequencies.
Provides inter-frequency reselection parameters so the UE can evaluate neighboring NR frequencies correctly while idle or inactive.
Why this message matters
SIB4 is the NR broadcast block that mainly tells the UE how to behave with neighboring NR cells on other frequencies.
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 neighboring NR carriers.
Preconditions:
MIB and SIB1 have been acquired.
The UE knows the scheduling for additional system information.
Next likely message: Inter-frequency reselection evaluation
Inter-Frequency Cell Reselection
Call flow position: Used when the UE evaluates neighbor NR cells on other frequencies while idle or inactive.
Typical state: UE is not in dedicated connected-mode signaling.
Preconditions:
The serving cell broadcast information is valid.
Neighbor carrier search conditions are relevant.
Next likely message: Stay on the serving frequency or reselect a different NR carrier
Idle / Inactive Mobility
Call flow position: Provides broadcast rules for mobility decisions across alternate NR frequencies.
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, frequency search, or reselection depending on radio conditions
Next message(s): Inter-frequency cell reselection evaluation, Idle mobility decisions across other NR carriers, 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 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
SIB4 focuses on inter-frequency mobility rather than same-frequency behavior.
For engineering work, the main question is whether the neighbor-frequency policy matches the expected field behavior.
SIB4 becomes especially important when UEs do not move to a target NR carrier, leave too early, or keep selecting the wrong frequency.
ASN.1 for 5G NR - System Information Block 4 (SIB4)
SIB4 is typically carried inside the broader SystemInformation container. In practice, the important part is whether the inter-frequency carrier list and thresholds explain the observed mobility behavior.
The key practical block is interFreqCarrierFreqList.
carrierFreq tells you which alternate NR carrier the UE may evaluate.
Thresholds, offsets, and priority help explain why a candidate frequency is preferred, ignored, or delayed.
Important Information Elements
IE
Required
Description
interFreqCarrierFreqList
Yes
The main SIB4 content. It defines the alternate NR carrier frequencies and related reselection parameters.
carrierFreq
Optional
Identifies the neighboring NR carrier frequency the UE may evaluate.
threshX-HighP / threshX-LowP
Optional
Thresholds that influence when an inter-frequency candidate becomes attractive or should be deprioritized.
q-OffsetFreq
Optional
Frequency-specific offset that changes how attractive or unattractive the candidate carrier looks to the UE.
cellReselectionPriority
Optional
Priority information that helps explain why one frequency is preferred over another during idle mobility.
Detailed field explanation
interFreqCarrierFreqList
The main SIB4 content. It defines the alternate NR carrier frequencies and related reselection parameters.
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.
carrierFreq
Identifies the neighboring NR carrier frequency the UE may evaluate.
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-HighP / threshX-LowP
Thresholds that influence when an inter-frequency candidate becomes attractive or should be deprioritized.
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-OffsetFreq
Frequency-specific offset that changes how attractive or unattractive the candidate carrier looks to the UE.
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.
cellReselectionPriority
Priority information that helps explain why one frequency is preferred over another during idle mobility.
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 decoded successfully before reviewing SIB4 behavior.
Verify that the UE actually acquired the additional SI carrying SIB4.
Check the configured inter-frequency carriers against the expected neighbor plan.
Inspect thresholds, offsets, and priorities against the observed move or no-move behavior.
Compare the serving frequency and target frequencies together rather than analyzing only one carrier.
If users report poor camping or delayed mobility across carriers, correlate SIB4 with RF measurements and paging-area observations.
Common Issues and Troubleshooting
The UE does not move to the expected neighboring NR carrier.
Likely cause: Inter-frequency thresholds, offsets, or priorities may discourage reselection.
What to inspect: Check interFreqCarrierFreqList, threshX-HighP, threshX-LowP, and cellReselectionPriority.
Next step: Compare the configured target carrier against the actual neighbor-frequency plan and RF conditions.
The UE jumps too aggressively to another carrier.
Likely cause: Inter-frequency policy may be too permissive or too favorable toward the target frequency.
What to inspect: Check priority and offset values for the candidate carrier.
Next step: Correlate SIB4 values with measurement fluctuations and field mobility logs.
Same-frequency behavior looks fine, but cross-carrier mobility is wrong.
Likely cause: The issue may sit in SIB4 rather than in SIB3 or the serving-cell RF only.
What to inspect: Separate same-frequency and inter-frequency behavior clearly in the trace analysis.
Next step: Analyze SIB2, SIB3, and SIB4 together as one broadcast mobility chain.
LTE / 5G / Variant Comparison
SIB4 versus SIB3
SIB3 focuses on same-frequency reselection. SIB4 focuses on inter-frequency reselection across other NR carriers.
SIB4 versus dedicated RRC
SIB4 is broadcast cell-common mobility information, not UE-specific connected-mode configuration.
FAQ
What is SIB4 in 5G NR?
SIB4 is System Information Block 4, an NR broadcast block used mainly for inter-frequency cell reselection behavior.
Who sends SIB4?
The gNB broadcasts SIB4 as additional system information.
What is the main purpose of SIB4?
To provide inter-frequency reselection parameters for idle and inactive mobility across neighboring NR carriers.
On which channel is SIB4 sent?
SIB4 is carried in system information on BCCH-DL-SCH.
Why is SIB4 useful in troubleshooting?
Because it helps explain why the UE does or does not move between different NR carrier frequencies.
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.