PBCH stands for Physical Broadcast CHannel
The physical broadcast channel (PBCH) is a channel that carries essential information for the initial access of a cell in a radio access network. It is present in downlink signals only, and it is transmitted on symbols 0, 1, 2, and 3 of slot 1 and occupies the center 72 subcarriers. The PBCH is not transmitted on any resource elements assigned to cell-specific reference signals (C-RS) for antenna ports 0, 1, 2, and 3.
The PBCH carries a transport block called the broadcast channel (BCH), which contains a message called the master information block (MIB). The MIB is 14 bits long and it contains the following information:
• The system bandwidth, which indicates the number of resource blocks used by the cell
• The physical hybrid ARQ indicator channel (PHICH) configuration, which indicates the duration and resource allocation of the PHICH
• The system frame number (SFN), which is a 10-bit counter that identifies the radio frame
• The spare bits, which are reserved for future use
The MIB is coded with a low coding rate and mapped to four subframes within a 40 ms interval. Each subframe is self-decodable, which means that the user equipment (UE) can decode the PBCH from any subframe without waiting for the whole 40 ms period. However, the UE can also combine multiple subframes to improve the decoding performance. The PBCH is designed to be detectable without prior knowledge of the system bandwidth and to be accessible at the cell edge as well.
The PBCH is important for the UE to perform the initial cell search and synchronization, as well as to obtain the basic system information and parameters. The PBCH is also used by the network to broadcast its identity and availability to the UE. The PBCH is one of the first channels that the UE tries to decode when it enters a radio access network.