5G Master Information Block (NR-MIB)

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Over all structure of MIB and SIB in NR is almost similar to LTE and other legacy technologies. However there is a major difference related to the periodical transmission of SIBs. In LTE and other legacy technologies the SIBs are broadcasted priodically regardless of whether UE wants it or not. But, in New Radio (NR) there are two versions of SIBs. One is being transmitted periodically like LTE and other is being transmitted only on-Demand i.e. when requested by UE.

NR MIB characteristics 

  • MIB is transmitted over BCH transport channel and PBCH physical channel
  • It is QPSK modulated
  • It included the necessary parameters required to decode SystemInformationBlockType1 (SIB1)
  • Its priodicity is about 80 ms and with in this 80 ms repetitive transmission happens
  • It is transmitted on OFDM symbol 1,2,3 .
  • It uses 0 to 239 sub carrier number on symbol 1 & 3, where as on symbol 2 it uses subcarried number 0 to 47 and 192 to 239 as per TS 38.211.

MIB/SIB signalling call flow:

  • Signalling radio bearer: N/A
  • RLC-SAP: TM
  • Logical channel: BCCH
  • Direction: Network to UE

RRC Parameters for Master Information Block:

MIB ::= SEQUENCE {

  • systemFrameNumber                         BIT STRING (SIZE (6)),
  • subCarrierSpacingCommon               ENUMERATED {scs15or60, scs30or120},
  • ssb-SubcarrierOffset                            INTEGER (0…15),
  • dmrs-TypeA-Position                          ENUMERATED {pos2, pos3},
  • pdcch-ConfigSIB1                                 INTEGER (0…255),
  • cellBarred                                               ENUMERATED {barred, notBarred},
  • intraFreqReselection                           ENUMERATED {allowed, notAllowed},
  • spare                                                        BIT STRING (SIZE (1))

}

Parameters Description:

  • systemFrameNumber : Similar to LTE, NR does have a 10 bit System Frame Number start from 0 to 1023. MIB carries 6 most significant bit (MSB) of the 10 bits and remaining 4 LSB of the SFN are conveyed in the PBCH transport block as part of channel coding (i.e. outside the MIB encoding).
  • subCarrierSpacingCommon: Indicates the Subcarrier spacing for SIB1, Message 2/4 for initial access and System Information-messages. It can have a values 15 and 30 kHz valid for carrier frequencies < 6GHz; Values 60 and 120 kHz are applicable for carrier frequencies > 6GHz
  • ssb-subcarrierOffsetindicates the frequency domain offset between SSB and the overall resource block grid in number of subcarriers. This field may indicate that this cell does not provide SIB1 and that there is hence no common CORESET. 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
  • dmrs-TypeA-Position: Indicates Position of (first) DL DM-RS. It corresponds to L1 parameter ‘DL-DMRS-typeA-pos’
  • pdcchConfigSIB1: It is corresponds to RMSI-PDCCH-Config in TS 38.213 [13], section 4.1. Determines a bandwidth for PDCCH/SIB, a common ControlResourceSet (CORESET) a common search space and necessary PDCCH parameters. If the field ssb-SubcarrierOffset  indicates that SIB1 is not present, the field pdcch-ConfigSIB1 indicate 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], section 13).
  • cellBarred indicates whether the cell allows UEs to camp on this cell as per specification TS 38.304
  • intraFreqReselection indicates if Intra frequency cell reselection is Allowed or notAllowed. It controls cell 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

MIB Acquiring  and  Processing 

MIB / SIB acquisition process  vary case to case. Here we will dicsuss overall MIB/SIB acquisition process as defined in TS 38.331 NR-RRC specification.

Case 1 : No On-Demand System Information, No previously stored SIB and UE just powered ON

  • UE Powered ON
  • Cell Search (PSS and SSS) and PBCH decode to get MIB
  • Decode and store the MIB
  • check if cellBarred= barred,  stop here   cellBarred != barred, move to next to process further information
  • Decode SIB1 using parameters provided by MIB and store the results
  • As SIB1 indicate no demand SI, decode other System Information (SIBs)

Case 2 : On-Demand System Information Indication, No previously stored SIB and UE just powered ON

  • UE Powered ON
  • Cell Search (PSS and SSS) and PBCH decode to get MIB
  • Decode and store the MIB
  • check if cellBarred= barred,  stop here   cellBarred != barred, move to next to process further information
  • Decode SIB1 using parameters provided by MIB and store the results
  • As SIB1  indicate on demand SI, check RRC status

If UE RRC status is in RRC-IDLE or RRC-INACTIVE state

  • Trigger UE lower layers to initate RACH procedure
  • Acquire the on-Demand SI messages when Acknowledgement for SI request is recieved

If UE RRC status is in RRC-Connected state

  • Behaviour not yet defined by specification

LTE and NR MIB Comparision

Parameters Long Term Evolution (LTE) New Radio (NR)
Broadcast Channel
  • Transport – BCH
  • Physical- PBCH
  • Transport – BCH
  • Physical- PBCH
Periodicity 40 ms generation periodicity with 10 ms re-transmission periodicity  80 ms generation periodicity with repetitions made within 80 ms
Channel Coding Tail Bit Convolution encoding Polar Coding
Modulation QPSK QPSK
Resource Allocation
  • 6 RBs (72 subcarriers) in Frequency domain
  • 4 symbols of first subframe second slot symbol 0, 1, 2 and 3.
  • It is transmitted on OFDM symbol 1,2,3 .
  • It uses 0 to 239 sub carrier number on symbol 1 & 3, where as on symbol 2 it uses subcarried number 0 to 47 and 192 to 239

Computation SFN During Cell Search in NR 5G

Similar to LTE, 5G NR also have System Frame Number (SFN) 0 to 1023 and requires 10 bits to represent SFN. the 6 bits for SFN can be obtained from MIB and remain 4 bits are derived from PBCH payload.

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