# Transport Block calculation in LTE Downlink

I would like to understand the relation between the TB size calculation in the TS36.213 document and in the simulator (https://www.nt.tuwien.ac.at/research/mobile-communications/vienna-lte-a-simulators/). I’m going to make an example so maybe you can help me to see this relation:

Method 1 (from the TS36.213)

Calculation Procedure for downlink(PDSCH) is as follows :

• i) refer to TS36.213 Table 7.1.7.1-1

ii) get I_TBS for using MCS value (Let’s assume MCS is 1. in this case, I_TBS is 1 )

iii) refer to TS36.213 Table7.1.7.2.1

iv) go to column header indicating the number of RB (Let’s assume that RB is 50)

v) go to row header ‘1’ which is I_TBS

vi) we would get 1800 (if the number of RB is 50 and I_TBS is 9)

vii) (This is Transport Block Size per 1 ms for one Antenna)

And this is method 2 (from the Vienna LTE System Level simulator):

• i) This is the formula:

TB_size_bits = max(8*round(1/8*(the_RB_grid.sym_per_RB_nosync .* num_assigned_RB .* modulation_order .* coding_rate * 2))-24,0);

• ii) if the_RB_grid.sym_per_RB_nosyn= 80

iii) num_assigned_RB=50

iv) modulation_order = 2

v) coding_rate=0.0762

vi) That gives TB_size_bits = 1195 bits

Do you know how I can go from one method to another or if there is any relation?

Thank you. Best regards,

Natalia.

I found this ppt: http://www.ece.drexel.edu/walsh/Gwanmo-Nov11-2.pdf (slides 9-12) where I saw a bit of light regarding on how the method 1 calculates TB size:

For data, there are:

10 OFDM symbols per subframe x 12 subcarriers = 120 (we are counting the 6 RS symbols and excluding the PDCCH symbols)

Therefore,

120 symbols x 50 RBs x 2 bits/symbol (QPSK) = 12000 bits

For QPSK we know that the efficiency is 0.15 (approx.), so:

12000 x 0.15 = 1800 bits, as in table Table7.1.7.2.1 3GPP TS 36.312 for 50 RBs and I_TBS=1 (sorry, in my question I made a mistake and wrote I_TBS 9 the second time: can’t edit it in the forum)

However, in the simulator, using method 2, we are considering:

Number of non-sync symbols per RB= 12×7 – 4 (RS symbols)=80 OFDM symbols per slot

80 non-sync symbols x 50 RBs x 2 bits/symbol x 0.0762 x 2 slots =1219 bits

1219 bits -24 bits for the CRC = 1195 bits :)

Please, let me know if you don’t agree with these methods.

Thanks,

BTS.