Modulation and Coding is determined based on IMCS, CRC scrambling ID, and (type of) DCI format [36.213:7.1.7]. As we are yet to see CRC scrambling UE ID and DCI formats, here we will see determination based entirely on IMCS.
First step is to lookup table 7.1.7.1-1 (DL) or 8.6.1-1 (UL) - which maps IMCS - to get modulation order and TBS Index. The modulation order (number of bits per symbol =) is 2, 4, or 6 i.e. QPSK, 16QAM, and 64QAM. So from modulation order, we ger Modulation scheme.
Table 7.1.7.1-1/8.6.1-1: Modulation and TBS index table for PDSCH/PUSCH
MCS Index IMCS (0-31)
Modulation Order Qm (2,4,6)
TBS Index ITBS (0-26)
. . . .
As the name suggests, TBS index, would give us Transport Block Size. The Table 7.1.7.2.1-1 maps ITBS and NPRB (number of RBs allocated) to transport block size in bits.
Table 7.1.7.2.1-1: Transport block size table (dimension 27x110)
