LTE Broadcast
Channel
The LTE Broadcast Channel is carried on the PBCH and PDSCH.
The PBCH carries the Master Information Block of the BCH. The UE reads the MIB
at the time of cell (re)selection. The SIBs are carried on the PDSCH and are
read by the UE when required.
The information carried in the MIB is crucial in the initial
cell selection process, and consists of:
- DL Bandwidth (3 bits) – tells the System bandwidth (1.4MHz …. 20 MHz)
- PHICH Configuration (3 bits)
- System Frame Number (8 bits)
- Spare bits (10 bits)
The MIB also indirectly communicates one of the three Antenna
Configuration in use in the DL. The CRC of the MIB data transmitted undergoes a
special mask to encode this information. The UE does trial and error method to
determine the mask applied and thus knows the DL antenna configuration in use.
Mapping of PBCH
onto the OFDM Resource Grid
- The Physical BCH channel is restricted to the 72 sub carriers about the DC in the resource grid.
- The PBCH is transmitted in the first four OFDMA symbols of the 1st slot of each radio frame.
- The PBCH uses QPSK modulation
- The scrambling code of PBCH is a cell specific code determined by the UE from the P-SCH/S-SCH.
- The TTI of PBCH is 40 ms, i.e. the UE has to decode PBCH in 4 consecutive radio frames to get the information transmitted in the MIB.
- The Downlink Reference Signals override the PBCH in Resource Elements reserved for them.
- The PBCH is transmitted on each antenna port.
 |
| LTE Physical Broadcast Channel |