MIB


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Today we will learn about MIB.

In order to understand the role of MIB, let us first see the steps that the device goes through when it is switched on.

What happens when a device is powered on?


When a device is powered on, it is not aware of the full system bandwidth. So it does a frequency search, and it first syncs with the DC sub-carrier. 

Using this information, the device reads central 6 RBs (72 subcarriers).

The primary and secondary synchronization signals (PSS and SSS) and the PBCH (containing the MIB) all are contained in the central 72 subcarriers (6 resource blocks) of the system bandwidth. Initially, the Network allows the device to demodulate just this central region. 

On demodulating this info, the device gets the RSSI value using which it initiates Initial Acquisition process. It decodes PSS and SSS for Time and Frame Synchronization. Using this info, the device also finds the PCI (Physical Cell ID). The device subsequently received MIB and SIB from the NW and then initiates the process of cell selection.



What is DC subcarrier?


DC here stands for Direct Current and it is a subcarrier that has no information sent on it. It is an important subcarrier in OFDM based systems as it is used by the device to locate the center of the OFDM frequency band. 

Here is a DC subcarrier that the mobile device uses to help find the LTE frequency.

Note: The algorithm to read the DC carriers differs for different chipset vendors as it depends on how that vendor has implemented the Physical layer protocol stack.
  

What is Initial Acquisition in LTE?


In order to initiate a registration with the NW, device needs to acquire MIB, SIB1, and SIB2. The technical term for this process is Initial Acquisition. 

Acquisition process step by step:


Device is powered on >> It searches for PLMN >> It scans the supported bands within  respective  PLMN >>It scans the EARFCN >> It makes a priority table using the RSSI values of all the previously scanned EARFCNs >> It acquires request from the priority table one-by-one if a suitable cell isn’t found>> It reads MIB and SIB1 >> If cell selection criteria is met, the device reads SIB 2 and other SIBs >> It initiates PRACH process.


What is System information broadcast?


In order the let the devices know some basic information about the network, the eNodeB broadcasts blocks of information called MIB and SIBs. This information is essential for the device to communicate with the network.

System Information broadcast is divided into the Master Information Block (MIB) and a number of System Information Blocks (SIBs).

What information does the MIB carry?


The MIB carries the following information elements:

v  System bandwidth (UE must know the bandwidth if it wants to decode any other physical layer channels) and Number of Transmit Antennae.

v  System Frame Number (SFN) (System frame number is transmitted in the MIB for mainly two reasons: initial synchronization and periodic sync that a UE can do to find out any drifts between its timing and the eNodeB’s timing.)

v  Physical Hybrid Automatic Repeat Request (HARQ) Indicator Channel (PHICH) Configuration (PHICH info, which is nothing but the Ng value (or the number of PHICH groups used in the first symbol) is important too. Unless the UE knows that, it will not be able to decode PDCCH and hence any DCI elements. This field also tells the device about the PHICH duration, whether it is normal or extended PHICH, which is also essential for the UE to decode the control channels completely.)

How many bits does the MIB carry?


The MIB carries 24 bits.

System frame number:  8 bits
System bandwidth:  3 bits
PHICH information:  3 bits 
Note: 1 bit is to indicate normal or extended PHICH and 2 bits to indicate the PHICH Ng value.
Reserved for future use:  10 bits

Why MIB retransmitted after 4 frames or SFN mod 4 = 0? 

The MIB corresponds to one BCH transport block. The BCH Transmission Time Interval (TTI), or the time required to transmit a single transport block, is 40msec or 4 frames. The BCH is transmitted in 4 parts, and each part is mapped to the first subframe (subframe 0) of a frame and it is possible that each transmission is independently decodable, depending on signal conditions. 

In order to ensure that subframe 0 is received, the capture needs to be at least 11 subframes long, to account for the possibility that the capture is started during subframe 0. For poor signal conditions, all 4 parts of the TTI may be required, in which case the capture should be at least 41 subframes long.

How the repetition of MIB schedule?


The MIB uses a fixed schedule with a periodicity of 40ms with repetitions within 40ms with different RV (SFN is transmits the redundancy versions like RV0, RV1, RV2).

Which subframe is used for transmitting MIB?


The first transmission of MIB is scheduled in subframe 0 of radio frames for which the SFN mode4 = 0, and the repetitions are also scheduled in subframe 0 of all other radio frames.

How we can know antenna-specific mask using MIB?

The MIB+CRC provides the number of transmission antennae used by the eNodeB. The MIB+CRC is scrambled using an antenna-specific mask.

Channel mapping of MIB
Broadcast Common Control Channel (Logical Channel – BCCH)
Broadcast Channel (Transport Channel – BCH)
Physical Broadcast Channel (Physical Channel – PBCH)

Supported IE of MIB?

Note: This is as per 3gpp TS36.331.
Master Information Block ::= SEQUENCE
{
dl-Bandwidth ENUMERATED { n6, n15, n25, n50, n75, n100},
phich-Config PHICH-Config,
system Frame Number BIT STRING (SIZE (8)),
spare BIT STRING (SIZE (10))
}

How many resource blocks are required for MIB?

6RBs in Frequency domain.

Which modulation is used for MIB?

QPSK

To decode PDCCH, how MIB used?


From the PHICH information obtained from MIB, the device decodes the place of Control Format Indicator (PCFICH demodulation for LTE), which indicates the Physical Downlink Control Channel (PDCCH) length.

PBCH contains the Master Information Block (MIB) part of the MAC layer channel BCH. The other part of BCH (System Information Block) is contained in a PDSCH allocation encoded with the SI-RNTI.
 


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