5G-NR Master Information Block (MIB)

MIB is mandatory system information that is broadcasted by the gNB at a set periodicity. During the initial cell selection procedure, UE assumes that the SSB is broadcasted every 20ms by the gNB. UE gets MIB by decoding the PBCH from the SS-PBCH block beam.

UE Successfully decodes MIB when it is able to successfully read SSB Block.

In the case of Stand, Alone UE assumes that SSB periodicity is 20ms and it performs Cell selection search with 20ms periodicity.

MIB and SIB1 are also known as ‘Minimum system information’ as they provide basic parameters that help UE during initial cell selection.

The MIB includes the system information transmitted on BCH.

  • Signaling radio bearer: N/A
  • RLC-SAP: TM
  • Logical channel: BCCH
  • Transport Channel : BCH
  • Physical Channel : PBCH
  • Direction: Network to UE

The Contents of Master Information Block (MIB) are shown in the image below. MIB Contains a total of 23 bits but an additional bit is used for BCH message type indication which makes a total of 24 bits for MIB.

Master Information Block (MIB)

systemFrameNumber:

  • The range of SFN is from {0-1023} which corresponds to 10.24 seconds.
  • SFN Consists of 10 Bits and the 6 most significant bits (MSB) of the 10-bit System Frame Number (SFN) are defined in MIB .
  • UE gets  the 4 LSB of the SFN which are conveyed in the PBCH transport block as part of channel coding (i.e. outside the MIB encoding)

subCarrierSpacingCommon

  • This IE provides subcarrier spacing for SIB1, Msg2, Msg4 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, the subcarrier spacing for SIB1 is the same as that for the corresponding SSB and this field instead is used for deriving the QCL relation between SS/PBCH blocks

ssb-SubcarrierOffset

  • The IE Corresponds to kSSB , which is the frequency domain offset between SSB and the overall resource block grid in number of subcarriers as shown in the image below .
  • For operation with shared spectrum channel access, this field corresponds to SSB, and kSSB is obtained from SSB; the LSB of this field is used also for deriving the QCL relation between SS/PBCH blocks .
  • The value range of this field may be extended by an additional most significant bit encoded within PBCH.
  • This field may indicate that this cell does not provide SIB1 and that there is hence no CORESET#0 configured in MIB 
  • In this case, the field pdcch-ConfigSIB1 may indicate the frequency positions where the UE may (not) find a SS/PBCH with a control resource set and search space for SIB1
5G-NR SSB Frequency Location Information

pdcch-ConfigSIB1

  • Determines a common ControlResourceSet (CORESET), a common search space and necessary PDCCH parameters.
  • If the field ssb-SubcarrierOffset indicates that SIB1 is present, the field pdcch-ConfigSIB1 indicates the frequency positions where the UE may find SS/PBCH block with SIB1
  • If the field ssb-SubcarrierOffset indicates that SIB1 is absent, then the field pdcch-ConfigSIB1 indicates.
  • That the network does not provide SS/PBCH block with SIB1.

intraFreqReselection

  • Controls cell selection/reselection to intra-frequency cells when the highest ranked cell is barred, or treated as barred by the UE.
  • This field is ignored by IAB-MT.
  • In LTE this information was provided in SIB1.

dmrs-TypeA-Position

This IE indicates the position of (first) DM-RS for downlink and uplink when using ‘Mapping Type A’

cellBarred :

  • This IE indicates whether the current cell is barred. This field is ignored by IAB-MT. In LTE this information was provided in SIB1
  • UE uses this field dmrs-TypeA-Position along with the ‘Time-domain resource assignment’  field received in DCI.
  • When UE receives a DCI , there is a field in DCI which indicates ‘Time-domain resource assignment’ which maps to a look up tables used by UE to determine the time domainPDSCH allocations.
  • The Lookup table can be provided in SIB1 or if absent UE uses the 3GPP standard table from TS 38. 214 Section 5.1.2.1.1

How can the UE determine whether the NR Cell is a SA cell or a NSA Cell ?

Since NR Can be deployed in Stand-Alone mode and Non-Stand Alone mode, How can the UE determine whether the NR Cell is a SA cell or an NSA Cell?. UE knows this when it reads the MIB.

When the UE scans and detects an SS/PBCH block, it reads the MIB and looks for the values present in the IE pdcch-ConfigSIB1.

In the case of Stand Alone(SA) the UE determines from MIB that a CORESET for Type0-PDCCH CSS set, is present if kSSB ≤ 23 for FR1 or if  kSSB ≤ 11 for FR2.

In the case of Non-Stand Alone(NSA) the UE determines from MIB that a CORESET for Type0-PDCCH CSS set is not present if kSSB >23 for FR1 or if kSSB >11 for FR2;

The CORESET for Type0-PDCCH CSS set may be provided by PDCCH-ConfigCommon.

If the UE determines that from MIB that a CORESET for Type0-PDCCH CSS set is present it will know that the NR cell is Stand Alone capable. The UE determines the number of consecutive resource blocks and a number of consecutive symbols for the CORESET of the Type0-PDCCH CSS set from controlResourceSetZero in pdcch-ConfigSIB1.

pdcch-ConfigSIB1 maps to an index in a list of  Tables 13-1 through 13-10 in 3GPP TS 38.213 .

The PDCCH monitoring occasions are obtained from searchSpaceZero in pdcch-ConfigSIB1, which is also included in MIB.

I add more details on how SIB1 is decoded from the information provided in MIB.

Further reading