A UE that has just found a cell knows its PCI from the PSS and SSS, but nothing else about it. The MIB is the first message the UE decodes, and it tells the UE just enough to read the rest of the system information.

MIB is special signal that carries the following information. As you see, you can get the System Bandwidth and SFN by decoding MIB.

i) DL Bandwidth, Number of Transmit Antenna

ii) System Frame Number (SFN)

iii) PHICH Configuration

iv) Transmit every 40 ms , repeat every 10 ms

The MIB is only 24 bits long, so it carries only what a UE cannot find any other way. Everything else, from the cell identity to the random access settings, comes later in SIB1 and the other SIBs.

Followings are the topics to be covered in this page.

• MIB Contents
• MIB Transmission and Scheduling
• Reference

MIB Contents

What exactly does the MIB carry today? The original Release 8 MIB had three fields and 10 spare bits. Later releases have used most of those spare bits for eMTC and NTN, and the listing below is the current one.

Following is based on 36.331 v19.3.0 (Release 19)

MasterInformationBlock ::=          SEQUENCE {
    dl-Bandwidth                        ENUMERATED {
                                            n6, n15, n25, n50, n75, n100},
    phich-Config                        PHICH-Config,
    systemFrameNumber                   BIT STRING (SIZE (8)),
    schedulingInfoSIB1-BR-r13           INTEGER (0..31),
    systemInfoUnchanged-BR-r15          BOOLEAN,
    partEARFCN-r17                  CHOICE {
        spare                           BIT STRING (SIZE (2)),
        earfcn-LSB                      BIT STRING (SIZE (2))
    },
    spare                               BIT STRING (SIZE (1))
}

Following is based on 36.331 v19.3.0 (Release 19)

PHICH-Config ::=                    SEQUENCE {
    phich-Duration                      ENUMERATED {normal, extended},
    phich-Resource                      ENUMERATED {oneSixth, half, one, two}
}

The dl-Bandwidth field gives the downlink bandwidth in resource blocks, from n6 for 1.4 MHz to n100 for 20 MHz. The UE needs it before it can find any channel that spans the whole carrier, such as the PCFICH, the PHICH and the PDCCH.

The phich-Config field tells the UE how many resources the PHICH takes. The UE needs this to know which REs of the control region are left for the PDCCH. The phich-Duration gives the number of symbols the PHICH spans, and phich-Resource gives the factor N_g that sets the number of PHICH groups.

The systemFrameNumber field carries only the 8 most significant bits of the 10-bit SFN. The UE gets the 2 least significant bits from the position of the frame within the 40 ms PBCH TTI. They are 00 in the first frame, 01 in the second, 10 in the third and 11 in the last, as the 36.331 v19.3.0 field descriptions state.

The remaining fields came with later releases. The field schedulingInfoSIB1-BR-r13 tells a BL UE or a UE in CE where to find SIB1-BR, and value 0 means SIB1-BR is not scheduled. The field systemInfoUnchanged-BR-r15 tells such a UE that the system information has not changed, so it can skip reading it again. The field partEARFCN-r17 carries the 2 least significant bits of the EARFCN for NTN bands that use a 100 kHz raster.

The total is still 24 bits, the same as in Release 8. The new fields only replaced spare bits, so a Release 8 UE decodes the MIB of a Release 19 cell without trouble and ignores the bits it does not know.

• 24 bits in every release : new fields took over spare bits.
• Bandwidth and PHICH first : the UE needs both before it can read the PDCCH.
• 8 of the 10 SFN bits : the other 2 come from the 40 ms PBCH timing.
• Later fields for eMTC and NTN : SIB1-BR scheduling and EARFCN bits.

MIB Transmission and Scheduling

How does a UE decode the MIB reliably, even at the edge of a cell? The answer is the PBCH, which spreads one MIB over 40 ms and repeats part of it in every radio frame. The diagram below shows that schedule next to that of SIB1.

Following is the cycles in which MIB and SIB is transmitted. (For the details of the function and contents of each SIBs, see this page)

The MIB changes every 4 frames and repeats in each frame between. SIB1 changes every 8 frames and repeats in every even frame with a different RV. The other SIBs follow the schedule given in SIB1.

36.331 v19.3.0 clause 5.2.1.2 fixes this schedule. The first transmission of the MIB is in subframe 0 of each frame with SFN mod 4 = 0, and it repeats in subframe 0 of every other frame. SIB1 is sent in subframe 5 of each frame with SFN mod 8 = 0, and it repeats in subframe 5 of every other even frame.

The diagram labels the MIB repetitions as the same as the original, which is true of the MIB content. The coded bits differ, however. The 24 MIB bits and a 16-bit CRC are coded and rate matched to 1920 bits with the normal CP. Each of the 4 frames carries a different quarter of them. Each quarter is still decodable on its own, so a UE with good signal can read the MIB from one frame. A UE at the cell edge can combine up to four frames. The PBCH page shows the mapping in detail.

The number of transmit antennas is not a field of the MIB, although the list above names it. The eNB scrambles the 16-bit CRC of the BCH with a mask that depends on the number of antenna ports. 36.212 v19.3.0 Table 5.3.1.1-1 gives all zeros for 1 port, all ones for 2 ports, and 0101... for 4 ports. The UE tries the three masks, and the one that passes the CRC tells it the antenna count.

• MIB every 40 ms : repeated in subframe 0 of every frame.
• SIB1 every 80 ms : repeated in subframe 5 of every even frame.
• 1920 coded bits over 4 frames : each frame carries a decodable quarter.
• Antenna count from the CRC mask : not from a MIB field.

Reference

[1] 3GPP TS 36.331 v19.3.0 - clause 5.2.1.2, Scheduling, and clause 6.2.2, MasterInformationBlock

[2] 3GPP TS 36.212 v19.3.0 - clause 5.3.1, Broadcast channel, and Table 5.3.1.1-1, CRC mask for PBCH

[3] 3GPP TS 36.211 v19.3.0 - clause 6.6, Physical broadcast channel