How Transport Channel Processing in LTE?
Transport channels are block oriented transmission services and transmit one or more transport blocks per transmission time interval (TTI).
The TTI is 1 ms (e.g. one sub-frame). Transport blocks may in general have variable size. Typically the number of transport blocks that can be transmitted in a TTI depends on whether spatial multiplexing is done.
The physical layer now encodes the transport block via a series of coding units which are the following:
CRC (Cyclic Redundancy Check): Each transport block is protected with
checksum calculated as cyclic redundancy check. The checksum is a 24 bit long CRC. The block error rate that is measured from this checksum will be a criteria of the system.
Channel Coding and Rate Matching: The forward error check that is applied to the transport block with its CRC is based on three available coding algorithms: turbo coding rate 1/3, convolutional coding rate 1/3 or a 32-to-2 block coding. ULDSCH, DL-DSCH, MCH and PCH will always use turbo coding, only the BCH uses convolutional coding. The block coder is not for transport channels, it is used by PFCICH for example. Rate matching is done directly after coding, it can puncture or repeat bits.
Interleaving: The coded bit sequence from rate matching is then interleaved to randomize highly correlated noise introduced on air at the receiver side.
Data Modulation: Finally the binary data must be brought in the form suitable for the complex arithmetics of OFDMA/SC-FDMA. Therefore always 1, 2, 4 or 6 bits are taken together to build an OOK, BPSK, QPSK, 16QAM or 64QAM symbol.
Resource Mapping: One of the key features of EUTRAN is the completely
dynamic implemented resource handling. So instead of having a fixed subcarrier/time index for each OFDM symbol generated before, the MAC
scheduler assigns subcarrier/time index dynamically for each symbol.
Antenna Mapping: The last part is to bring the symbols for one antenna together and modulate the signal (via IFFT, or SC-FDMA) to the RF modulator. This process might include to apply additional phase factors and weighting matrices to optimize MIMO.