The NR Master Information Block (MIB) is the first essential broadcast RRC information a UE decodes from PBCH after synchronization. It gives the UE the minimal system information needed to continue toward SIB1 acquisition and initial access.
Message Fact Sheet
Protocol
rrc
Network
5g
Spec
3GPP TS 38.331
Spec Section
5.2.1, 5.2.2, 6.2.1, 6.3.1
Direction
gNB -> UE
Message Type
Broadcast System Information
Full message name
5G NR - Master Information Block (MIB)
Protocol
RRC
Technology
5G
Direction
gNB -> UE
Interface
Uu
Signaling bearer / channel
Broadcast transport / BCCH-BCH
Typical trigger
Periodically broadcast by the cell as part of the synchronization and PBCH path so any UE can acquire the first essential system information.
Main purpose
Provides the initial broadcast control information that anchors early access, including core scheduling and cell-level context needed before the UE can read further system information and attempt access.
Main specification
3GPP TS 38.331, 5.2.1, 5.2.2, 6.2.1, 6.3.1
Release added
Release 15
Procedures where used
Cell Search, Initial Access, System Information Acquisition, Cell Selection
What is Master Information Block (MIB) in simple terms?
The NR Master Information Block (MIB) is the first essential broadcast RRC information a UE decodes from PBCH after synchronization. It gives the UE the minimal system information needed to continue toward SIB1 acquisition and initial access.
Provides the initial broadcast control information that anchors early access, including core scheduling and cell-level context needed before the UE can read further system information and attempt access.
Why this message matters
MIB is the first essential 5G broadcast message the UE gets after synchronization. It gives the UE the minimum information needed to continue toward SIB1 and access.
Where this message appears in the call flow
Cell Search and Synchronization
Call flow position: Decoded immediately after the UE detects the SSB and successfully receives PBCH.
Typical state: UE is in the earliest access preparation phase and has not yet acquired the broader cell broadcast context.
Preconditions:
The UE has detected the serving cell.
The UE has synchronized to the SSB timing and frequency.
PBCH reception is successful enough to recover the MIB.
Next likely message: SystemInformationBlockType1 (SIB1)
Initial Access
Call flow position: Used before the UE can continue to SIB1 and then toward PRACH and RRC setup.
Typical state: UE is still outside connected-mode signaling and is building the first layer of cell context.
Preconditions:
The UE has a valid SSB/PBCH decode path.
Next likely message: SystemInformationBlockType1 (SIB1)
For trace work, the most useful reading is practical: does the UE have enough correct MIB information to locate and decode SIB1 and determine whether the cell is actually usable?
5G NR - Master Information Block (MIB) - Example Dump
The most critical operational field is often pdcch-ConfigSIB1 because it determines how the UE finds SIB1 scheduling.
cellBarred and intraFreqReselection immediately influence whether the UE should use the cell.
MIB is short by design, so even a small decode error can block the whole path toward SIB1 and access.
Important Information Elements
IE
Required
Description
systemFrameNumber
Yes
Carries the broadcast system frame number bits that help the UE align the radio-frame timing context.
subCarrierSpacingCommon
Yes
Broadcast common subcarrier spacing indication used by the UE as part of the early access and system-information interpretation path.
ssb-SubcarrierOffset
Yes
Provides offset information related to SSB and frequency positioning needed during early access interpretation.
dmrs-TypeA-Position
Yes
Broadcast common DMRS Type A position information used as part of the initial common cell configuration.
pdcch-ConfigSIB1
Yes
Tells the UE how to find the PDCCH scheduling information for SIB1, which is one of the most operationally important MIB fields.
cellBarred
Yes
Indicates whether the cell is barred for access.
intraFreqReselection
Yes
Indicates whether intra-frequency reselection is allowed.
Detailed field explanation
systemFrameNumber
Carries the broadcast system frame number bits that help the UE align the radio-frame timing context.
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.
subCarrierSpacingCommon
Broadcast common subcarrier spacing indication used by the UE as part of the early access and system-information interpretation path.
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.
ssb-SubcarrierOffset
Provides offset information related to SSB and frequency positioning needed during early access interpretation.
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.
dmrs-TypeA-Position
Broadcast common DMRS Type A position information used as part of the initial common cell configuration.
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.
pdcch-ConfigSIB1
Tells the UE how to find the PDCCH scheduling information for SIB1, which is one of the most operationally important MIB fields.
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.
cellBarred
Indicates whether the cell is barred for access.
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.
intraFreqReselection
Indicates whether intra-frequency reselection is allowed.
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.
What to check in logs and traces
Confirm the UE detected SSB and successfully decoded PBCH before reading MIB values.
Check that the systemFrameNumber and common timing context look stable across repeated captures.
Verify pdcch-ConfigSIB1 is plausible and allows the UE to progress toward SIB1 acquisition.
Inspect cellBarred and intraFreqReselection when the UE does not continue with normal access or camping behavior.
Correlate MIB with the later SIB1 decode and confirm the UE is following the same serving-cell context.
Common Issues and Troubleshooting
The UE detects the cell but never reaches SIB1.
Likely cause: PBCH/MIB decode may be failing, or pdcch-ConfigSIB1 may not be interpreted correctly.
What to inspect: Check PBCH quality, MIB field stability, and whether the UE knows how to locate SIB1 scheduling.
Next step: Correlate SSB, PBCH, MIB, and the first SIB1 acquisition attempt in one trace.
The UE sees the cell but does not treat it as usable.
Likely cause: The cell may be barred or reselection behavior may be restricted.
What to inspect: Check cellBarred and intraFreqReselection in the decoded MIB.
Next step: Compare the cell's broadcast policy with a known-good neighboring cell.
Initial access appears to fail before PRACH begins.
Likely cause: The issue may be earlier than PRACH and rooted in incomplete or incorrect MIB/PBCH processing.
What to inspect: Check whether MIB was decoded consistently and whether SIB1 acquisition followed.
Next step: Troubleshoot the broadcast-information chain, not only the PRACH procedure.
LTE / 5G / Variant Comparison
MIB versus SIB1
MIB is the minimal first broadcast anchor from PBCH, while SIB1 carries the broader essential operational cell information needed for access, PLMN selection, and cell use.
MIB versus dedicated RRC
MIB is cell-common broadcast information, not a dedicated UE-specific control-plane message like RRC Setup or RRC Reconfiguration.
FAQ
What is MIB in 5G NR?
MIB is the Master Information Block, the first essential broadcast RRC information the UE decodes from PBCH after synchronization.
Who sends the 5G NR MIB?
The gNB broadcasts MIB to all UEs through PBCH.
On which channel is MIB sent?
MIB is carried in the BCCH-BCH message and transmitted on PBCH.
What comes after MIB?
The UE uses MIB to continue toward SIB1 acquisition and then initial access behavior.
Why is pdcch-ConfigSIB1 important?
Because it tells the UE how to find the scheduling information needed to decode SIB1.
Can MIB explain why access never starts?
Yes. If MIB or PBCH processing fails, or if the cell is barred, the UE may never proceed toward SIB1, PRACH, or RRC setup.
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.