How Transport Channel Work in LTE?
A transport channel defines how and with what characteristics the information is transmitted.
- Inherited from the WCDMA, data on the transport channel is organized into “Transport Blocks”, TBs.
- A Transport block can be transmitted every TTI = 1 ms
The “Transport Format”, TF, defines how the blocks can be transmitted:
- Transport block size, it depends on the MCS and the number of PRB allocated
- Allowed modulation scheme
- Antenna mapping
Note: In case of multi-antenna system, there can be 2 TBs for each TTI.
Each TTI, the scheduler decides which chunk to allocate to which user. The chunks are not standardized.
The following transport channels in DL have been defined by the 3GPP:
- Broadcast Channel (BCH) characterized by a fixed, pre-defined transport format with a robust modulation to be broadcast in the entire coverage area of the cell.
- Downlink Shared Channel (DL-SCH) characterized by: A dynamic link adaptation by varying the modulation, coding and transmit power support for H-ARQ (radio retransmission).
- Paging Channel (PCH) characterized by: Requirement to be broadcast in the entire cell.
- Multicast Channel (MCH) characterized by: requirement to be broadcast in the entire coverage area of the cell
The following transport channels in UL have been defined by the 3GPP:
- Uplink Shared Channel (UL-SCH) characterized by: support for dynamic link adaptation by varying the transmit power and potentially modulation and coding , support for H-ARQ ,support for both dynamic and semi-static resource allocation.
- Random Access Channel (RACH) characterized by limited control information, collision risk