Thursday, 19 March 2009
LTE: MIB and SIB transmission
One very interesting change that has been done is that in LTE the system information is not all transmitted on the same channel unlike UMTS/HSPA. Logically all BCH info is mapped on BCCH. On the transport channel side, the information is mapped either on BCH or DL-SCH. (DL-SCH = Downlink Shared Channel). Ofcourse they map onto different physical channels as can be seen in the diagram.
Once the UE has achieved synchronisation, it will read the MIB to camp on the cell. In LTE, MIB contains very limited information. It contains information about the cell bandwidth, some information about PHICH (Physical HARQ Indicator Channel) and the SFN.
The SIB's as I mentioned are transmitted on DL-SCH mapped on PDSCH. To receive information about SIB's the UE needs information about PHICH which is read from the MIB.
The BCH channel has a TTI of 40ms (which is quite a lot for LTE, if you think about it). Also it has very small transport block size and protected with 1/3 convolutional code and 16bit CRC.
All these help to keep the overhead in LTE system to minimum.