UE when it is switched on, it cannot directly establish a connection with eNB. It needs system information. This system information is divided into MIB and SIB.
As said above, there are 2 Types System Information Block available in 4G LTE.
1. MIB: Master Information Block
2. SIB: System Information Block
In this chapter we shall learn about MIB. In the next chapter we shall learn about SIB.
How MIB is transmitted:
Direction: eNB to UE
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: BCH
Physical Channel: PBCH
MIB is the most important information block. It is the first step of the UE to read about MIB after initial cell synchronization.
MIB will carry below information:
1. Downlink channel bandwidth in term of resource blocks (RBs)
2. PHICH configuration (PHICH duration and PHICH resource)
3. System Frame Number
MIB is a 24 bit information, it will carry below information:
3 bits for system bandwidth
3 bits for PHICH information,
1 bit to indicate normal or extended PHICH
2 bit to indicate the PHICH Ng value
8 bits for system frame number
10 bits are reserved for future use
TTI (Transit Time Interval) for MIB is 40ms. It means, for every 40ms MIB will be transmitted.
For every 10ms MIB will be retransmitted.
It means that the first transmission of the MIB is scheduled in subframe #0 of radio frames for which the SFNmod4 = 0.
Consider the image below:
MIB cannot be transmitted on any other channel because the UE cannot access PDSCH during the initial stages.
Hence there is a need for a broadcast channel during the initial access – PBCH is a Broadcast Channel which carries MIB information.
As the UE has no knowledge of the system bandwidth PBCH is used for transmission.