A cell needs to broadcast a lot of information but UE needs some important information in a quick manner without going through all the system information. First UE wants to know in how much bandwidth cell is working (based on RBs being used it comes to know cell bandwidth. UE wants to maintain timing SYNC with cell so it keeps track with system frame number i.e a rolling number 0 -255.
Based on the system frame number it makes out when MIB information can modify from network.
To decode DCI from PDCCH, UE need PHICH information so that it can decode DCI which transmits over PDCCH.
So there is limited space of 6 RBs irrespective of any bandwidth to broadcast UE must broadcast information. That's why system information is divided into two parts
- UE must minimal system information - MIB
- Other system information in which some information may be optional - SIBs
MIB is carried by PBCH while SIB is carried by PDSCH.
