So now on one side we know number of RBs allocated and modulation order (bits per symbol) - from which we can calculate (physical) bits that can be transferred and on the other side we know transport block size; refer diagram below showing DL SCH processing at PHY [36.212:5.3.2].
MAC
DTCH (Transport block)
PHY
CRC (24 bit) attachment
Segmentation & CRC attach for coding
Channel coding
Rate matching
Modulation
To OFDM modulation
Fig 1.6.9
As coding is applied after 24 bit CRC attached, coding rate would be ratio of (TBS + 24) and (number of physical bits allocated in a subframe).
TB size determined above is when no spatial multiplexing is present. In case of two layer spatial multiplexing, TB size is calculated by looking up earlier table for 2. NPRB if NPRB ≤ 55 and by looking up value received from earlier table for NPRB in additional Table 7.1.7.2.2-1 otherwise.
