Why PHICH comes in MIB?
To decode PDCCH (after decoding MIB UE has to has to decode PDCCH for other SIBs - MIB has the most essential physical - PHY - parameters to access the cell, like DL bandwidth, SFN and PHICH configuration), the exact number of OFDM symbols thath are being used in the system is required.
Location of PCFICH can be identified from physical layer cell id which can be deduced from PSS and SSS.
So before reading PDCCH, UE has to know about RE occupied by PHICH and hence PHICH information is included into MIB.
So the number of OFDM symbols are figure out by PCFICH and PHICH.
PCFICH and PHICH both are placed at first OFDM symbol for each downlink frame.
PHICH Info consist of phich-Duration & phich-Resource which comes in MIB.
This is the down link physical channel which is used to send the ACK/NACK for the uplink data coming from UE.
When UE sends any data then ENODEB uses this PHICH resource to send acknowledgement for the uplink data.
As MIB is most frequently transferred so it is part of this.
Also, MIB holds the information whether the PHICH is Normal or Extended Duration.
Normal Duration --> 1st OFDMA symbols belonging to a subframe
Extended Duration --> 1st 3 OFDMA symbols to a subframe
PHICH is used to signal Ack or Nack for UL data transferred on PUSCH
0 +ve Ack
1-ve Ack [Nack]