Master Information Block (MIB)

The MIB is carried exclusively on the Physical Broadcast Channel (PBCH) within the Synchronization Signal Block (SSB). It is transmitted on a fixed 72-subcarrier bandwidth centered on the carrier frequency, regardless of the total channel bandwidth. This fixed allocation ensures that a UE with no prior knowledge of the cell's configuration can reliably decode the MIB immediately after detecting the PSS and SSS.

The MIB carries a compact set of mandatory parameters essential for decoding the System Information Block Type 1 (SIB1). The payload includes:

• systemFrameNumber: The 6 most significant bits of the 10-bit System Frame Number (SFN).
• subCarrierSpacingCommon: The subcarrier spacing used for SIB1, Msg2/4, and broadcast SI messages.
• ssb-SubcarrierOffset: The frequency domain offset between the SSB and the overall resource grid.
• dmrs-TypeA-Position: The position of the first DMRS symbol.
• pdcch-ConfigSIB1: The configuration for the Control Resource Set (CORESET#0) and search space required to schedule SIB1.

The pdcch-ConfigSIB1 field is the most critical element within the MIB, consisting of 8 bits that define the configuration of CORESET#0 and the Type0-PDCCH search space. The first 4 bits indicate the controlResourceSetZero index from a predefined table, specifying the CORESET's resource blocks and number of symbols. The remaining 4 bits indicate the searchSpaceZero index, defining the monitoring occasions and slot periodicity for the PDCCH scheduling SIB1.

The MIB carries only the 6 most significant bits (MSBs) of the 10-bit SFN. The remaining 4 least significant bits (LSBs) are derived implicitly from the PBCH transmission timing within the 80 ms TTI. The PBCH payload is scrambled with a sequence that depends on the 2nd and 3rd LSBs, while the SSB index embedded in the PBCH DMRS provides the remaining LSBs. This layered encoding allows the UE to reconstruct the full SFN without explicit transmission of all bits.

The ssb-SubcarrierOffset field defines the frequency offset in subcarriers between the lowest subcarrier of the SSB and the common resource block grid. This offset is essential for the UE to locate Point A, the absolute reference point for all resource block indexing. In FR1, this field is 5 bits for offsets below 15 kHz SCS and 4 bits for higher SCS, with the remaining values used to indicate the absence of a paired SIB1 in unlicensed spectrum.

The MIB content is updated with a modification period of 80 ms, during which the same information is repeatedly transmitted. A change in the MIB is indicated by an increment of the systemInfoValueTag in SIB1, or by a paging message for UEs in RRC_IDLE/RRC_INACTIVE. The UE re-acquires the MIB at the boundary of the next modification period, ensuring synchronized system information updates across all connected devices.

A 10-bit counter that identifies the radio frame within a 1024-frame hyperframe cycle. The 6 most significant bits (MSBs) of the SFN are carried explicitly in the MIB payload. The 4 least significant bits (LSBs) are implicitly encoded in the PBCH scrambling sequence initialization, requiring the UE to perform blind descrambling to recover the full SFN. This two-part encoding ensures robust timing acquisition during initial cell search.

The second synchronization signal decoded after PSS acquisition. The SSS is an m-sequence that provides the physical-layer cell identity group (N_ID1) and enables radio frame synchronization. Together with the PSS-derived N_ID2, the SSS completes the Physical Cell Identity (PCI) calculation: PCI = 3 * N_ID1 + N_ID2. The PCI is essential for PBCH descrambling and MIB decoding.

The next system information message scheduled after MIB acquisition. The MIB provides the downlink bandwidth and configuration for the Control Resource Set (CORESET #0) where the UE searches for the Physical Downlink Control Channel (PDCCH) scheduling SIB1. SIB1 carries cell selection parameters, PLMN identity, and scheduling information for remaining SIBs. Without successful MIB decoding, the UE cannot locate or decode SIB1.