In addition to the answer of sony ....
MIB has only 24 bits information but at the transmitter (because it is the most important info of broadcast channel) we made it of 1920 bits by repeating using some algorithm of channel coding,rate matching,interleaving,scrambling etc. and the after QPSK modulation it becomes 960 modulation symbol.
After that we divide this 960 symbols in 4 parts of 240 (which contains the same info but differ only by scrambling with the PN sequence)and placing it in the four symbol of 6 RB length (6*12=72-12 reference symbol=60*4=240 symbols)int second slot of first subframe of four continuous frames of 40 ms time. And then transmit.
At the UE side if he switched on his device in between the four continuous Frame then he descrambles the 240 bits of that frames which he detects first with the PN sequence which he already knows and if it is zero then it knows the correct frame no and a/c to that it waits for the next arrival of first frame and then decoding process starts at UE side and finally gets 24 bits of transport block in which first three bits shows the No. of RB assigned to that system. And UE decides the configuration on which eNodeB is running ac to this...
000--> 6 RB --> 1.4 MHz
001--> 15 RB --> 3 MHz
010--> 25 RB --> 5 MHz
011--> 50 RB --> 10 MHz
100--> 75 RB --> 15 MHz
101--> 100 RB --> 20 MHz