The very first step for UE to gain initial access to the network after completing initial cell synchronization is to read the Master information block (MIB) on BCCH (Logical channel), BCH (Transport channel) and PBCH (Physical channel). Resource elements used by MIB are the first 4 OFDMA symbols of second slot of first subframe of a radio frame. On frequency domain it occupies 72 subcarriers. MIB carries very little but most important information for UE initial access. The content of MIB includes
· Downlink channel bandwidth in term of resource blocks (RBs)
· PHICH configuration (PHICH duration and PHICH resource)
· System Frame Number
New MIB is broadcasted every radio frame for which SFN mod 4 = 0 (40ms repetition) while its copies are broadcasted in the middle 10ms radio frames as shown in the figure below
Why PHICH is carried by MIB?
After decoding MIB, UE has to decode PDCCH to read other system information blocks (SIBs). PDCCH, PHICH and PCFICH share the resources in the control region of a subframe. So to find the available resources for PDCCH, UE has to know the PHICH configuration only, as PCFICH resources are fixed and known