Below diagram shows simplistic view of DTCH processing at PHY [36.212:5.3.2].
MAC
DTCH (Transport block)
PHY
CRC attachment
Segmentation & CRC attach for coding
Channel coding
Rate matching
Modulation
To OFDM modulation
Fig 1.3.4
Let us look at redundancy added each stage.
For DTCH, 24 bit CRC is added to transport block [36.212:5.3.2.1]. Before channel coding is applied, transport block is segmented if more than 40 bits [36.212:5.1.2]. Each segment is appended with 24 bit CRC, which acts as a code block. DL-SCH use Turbo 1/3 coding [36.212:5.1.3]. Each code block goes through Turbo 1/3 coding and interleaving, generating 3 streams - each of size (code block size + 4) bits. Rate matching collects bits from these streams, forms virtual circular buffer and then - based on redundancy version, resource assignment, and modulation scheme - chooses certain number of (consecutive) bits for transmission. Modulation does not add to overhead.
