Skip to content

MIB

Summary

  • MIB (Master Information Block) is the actual content carried inside the PBCH portion of the SSB — see SSB for how PBCH fits into the overall SSB structure.
  • It gives the UE just enough information to:
    • Finish deriving frame timing (SFN)
    • Know the subcarrier spacing used for SIB1
    • Locate and decode CORESET#0 / PDCCH for SIB1

Think of MIB as:

"Here are the last few settings you need before you can find and decode the rest of my system information (SIB1)."

MIB Structure

MIB and PBCH are often used interchangeably, but they aren't the same thing: PBCH is the physical channel — the "container" — while MIB is the RRC information element carried inside it.1 Its structure, in ASN.1 terms:

MIB ::=                             SEQUENCE {
    systemFrameNumber                   BIT STRING (SIZE (6)),
    subCarrierSpacingCommon             ENUMERATED {scs15or60, scs30or120},
    ssb-SubcarrierOffset                INTEGER (0..15),
    dmrs-TypeA-Position                 ENUMERATED {pos2, pos3},
    pdcch-ConfigSIB1                    PDCCH-ConfigSIB1,
    cellBarred                          ENUMERATED {barred, notBarred},
    intraFreqReselection                ENUMERATED {allowed, notAllowed},
    spare                               BIT STRING (SIZE (1))
}


PDCCH-ConfigSIB1 ::=                SEQUENCE {
    controlResourceSetZero              INTEGER (0..15),
    searchSpaceZero                     INTEGER (0..15)
}

pdcch-ConfigSIB1 isn't a flat 8-bit integer — it's itself a SEQUENCE of two 4-bit sub-fields, controlResourceSetZero and searchSpaceZero, each indexing separate tables in TS 38.213 §13.2

QXDM Tip

In a QXDM (or QCAT) log, a decoded MIB shows up under the BCCH_BCH logical channel, since MIB is mapped onto the BCCH logical channel and carried on the BCH transport channel before being sent over PBCH.

Field
 Bits Meaning
systemFrameNumber 6 The 6 most significant bits (MSB) of the 10-bit System Frame Number (SFN). The 4 LSB of the SFN are conveyed in the PBCH transport block as part of channel coding (i.e. outside the MIB encoding), as defined in clause 7.1 in TS 38.212 [17].
subCarrierSpacingCommon 1 Subcarrier spacing for SIB1, Msg.2/4 and MsgB for initial access, paging and broadcast SI-messages. If the UE acquires this MIB on an FR1 carrier frequency, the value scs15or60 corresponds to 15 kHz and the value scs30or120 corresponds to 30 kHz. If the UE acquires this MIB on an FR2 carrier frequency, the value scs15or60 corresponds to 60 kHz and the value scs30or120 corresponds to 120 kHz. For operation with shared spectrum channel access in FR1 (see 37.213 [48]) and for operation in FR2-2, the subcarrier spacing for SIB1, Msg.2/4 and MsgB for initial access, paging and broadcast SI-messages is same as that for the corresponding SSB. For operation with shared spectrum channel access, this field instead is used for deriving the QCL relation between SS/PBCH blocks as specified in TS 38.213 [13], clause 4.1.
ssb-SubcarrierOffset 4 Corresponds to \( k_{SSB} \) (see TS 38.213 [13]), which is the frequency domain offset between the SSB and the overall resource block grid in number of subcarriers (see TS 38.211 [16], clause 7.4.3.1). For operation with shared spectrum channel access in FR1 (see TS 37.213 [48]), this field corresponds to \( \bar{k}_{SSB} \), and \( k_{SSB} \) is obtained from \( \bar{k}_{SSB} \) (see TS 38.211 [16], clause 7.4.3.1); the LSB (Least Significant Bit) of this field is used also for deriving the QCL (Quasi Co-Location) relation between SS/PBCH blocks as specified in TS 38.213 [13], clause 4.1. The value range of this field may be extended by an additional MSB (Most Significant Bit) encoded within the PBCH as specified in TS 38.213 [13]. This field may indicate that this cell does not provide SIB1 and that there is hence no CORESET#0 configured in the MIB (see TS 38.213 [13], clause 13). In this case, the field pdcch-ConfigSIB1 may indicate the frequency positions where the UE may (or may not) find an SS/PBCH block with a control resource set and search space for SIB1 (see TS 38.213 [13], clause 13).
dmrs-TypeA-Position 1 Position of (first) DM-RS for downlink (see TS 38.211 [16], clause 7.4.1.1.2) and uplink (see TS 38.211 [16], clause 6.4.1.1.3).
pdcch-ConfigSIB1 8 (4+4) Determines a common ControlResourceSet (CORESET), a common search space and necessary PDCCH parameters. If the field ssb-SubcarrierOffset indicates that SIB1 is absent, the field pdcch-ConfigSIB1 indicates the frequency positions where the UE may find SS/PBCH block with SIB1 or the frequency range where the network does not provide SS/PBCH block with SIB1 (see TS 38.213 [13], clause 13).
cellBarred 1 Value barred means that the cell is barred, as defined in TS 38.304 [20]. This field is ignored by IAB-MT. This field is ignored for connectivity to NTN.
intraFreqReselection 1 Controls cell selection/reselection to intra-frequency cells when the highest ranked cell is barred, or treated as barred by the UE, as specified in TS 38.304 [20]. This field is ignored by IAB-MT
spare 1 Reserved for future use

Table 1. MIB Parameter Description

That's 23 bits for the MIB IE itself; add the 1-bit BCCH-BCH-Message choice bit — which selects between mib and an (unused, in current releases) messageClassExtension — and the RRC layer hands 24 bits down to the physical layer.1

Quick Parameter Explanation

subCarrierSpacingCommon
scs15or60 scs30or120
FR1 15 kHz 30 kHz
FR2 60 kHz 120 kHz

Table 2. SCS-Common Options

The same 1-bit enumerated value maps to a different pair of subcarrier spacings depending on which frequency range the cell operates in.

ssb-SubcarrierOffset (k_SSB)

This field, corresponding to \( k_{SSB} \), tells the UE how far the SSB is offset from the start of the common resource block grid — information it needs before it can locate SIB1's resources. In the baseline behavior, \( k_{SSB} \) ranges 0–23 for FR1 and 0–11 for FR2; values outside that range are used by the network to signal that this cell does not provide SIB1, in which case pdcch-ConfigSIB1 instead points to where (if anywhere) a neighboring SSB with SIB1 can be found. Later 3GPP releases extended this field's semantics with an additional MSB carried in the PBCH payload — always check the current TS 38.331 IE description for the exact rule in your target release.

pdcch-ConfigSIB1

controlResourceSetZero (4 bits) determines the number of resource blocks and symbols for CORESET#0, and searchSpaceZero (4 bits) determines the PDCCH monitoring occasions — both by indexing pre-defined tables in TS 38.213 §13. Together they let the UE fully locate and decode SIB1's PDCCH without any prior RRC configuration.

cellBarred & intraFreqReselection
  • cellBarred: if barred, UEs must not camp on or access this cell (per TS 38.304).3
  • intraFreqReselection: only meaningful when the cell is barred — controls whether the UE may still reselect to other cells on the same frequency, or must avoid the whole frequency layer.

Transmission Timing

  • MIB is mapped onto the BCCH logical channel and carried on the BCH transport channel, which is in turn carried on PBCH.
  • MIB content updates with an 80 ms periodicity, but is repeated within that window — retransmitted on every SSB occasion so the UE can combine repetitions to improve decoding reliability.
  • For initial cell selection, a UE may assume that half-frames carrying SSBs repeat with a period of 2 frames (20 ms).

Useful Resources

  1. Ryu, J. (n.d.). 5G/NR – MIB / SIB. ShareTechnote. https://www.sharetechnote.com/html/5G/5G_Mib_Sib.html ↩↩

  2. 3GPP. (n.d.). NR; Radio Resource Control (RRC) protocol specification (Technical Specification TS 38.331). 3rd Generation Partnership Project. https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3197 ↩

  3. 3GPP. (n.d.). NR; User Equipment (UE) procedures in idle mode and in RRC inactive state (Technical Specification TS 38.304). 3rd Generation Partnership Project. https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3192 ↩