# TB size calculation in the simulator

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• #4128
Natalia
Participant

Hi everyone,

I would like to understand the relation between the TB size calculation in the TS36.213 document and in the simulator (file lteSheduler.m in folder “schedulers”). 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 (in the 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 = 11952 bits

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

Thank you. Best regards,

Natalia.

#4163
Natalia
Participant

Please, note that in method 2:

vi) That gives TB_size_bits = 1195 bits

thanks.

#4267
Natalia
Participant

Hi everyone,

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,

Natalia.

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