LTE : Why master information block in transmitted over PBCH channel ?

Question

When an UE switch on, first it synchronizes (PSS and SSS) with cell and start reading system information. My question is what UE does after getting system bandwidth information from MIB ? What is the significance of this for an UE ? Does UE take different actions based on cell bandwidth ?

Answer 1

MIB cannot be transmitted on any other channel PDSCH because the UE cannot access PDSCH during the initial stages.

What are the other alternatives ? The UE cannot use signals - PSS and SSS are signals and they carry no information. So is the C-RS which is like a pilot signal used for both Channel Estimation and Downlink Demodulation.

Hence there is a need for a broadcast channel during the initial access - PBCH is a Broadcast Channel which carries MIB information.

Master Information block contains some crucial information which is needed for accessing the PDSCH.

This crucial information which is being sent must be decoded considering the fact that the receiver (i.e. the UE) has no knowledge of the system bandwidth, That's why the PBCH is centered around the center bandwidth.

There are just a few bits (i think 14 bits) for the MIB. Which includes the n_dl_rb and ng. These are transmitted using robust coding schemes like 1/3 code rate so that the likelihood of the receiver making a mistake is very low.