# In LTE, the transport block size does not depend on the CFI, so if the CFI is changed, what will happen?

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In LTE, the transport block size is a mapping of the CQI, MCS and the allocated PRB and does not depend on the CFI. So for the same TBSize (which means same CQI, MCS, allocated PRB), if the CFI is changed (as in case of dynamic CFI) what will happen? is the code rate will be increased or decreased?

posted Nov 2, 2017

+1 vote

Hi,

CFI will not decide TB size, as you said TB size will be calculated from PRB and MCS. CFI will indicate no of symbol used for PDCCH. If you increase CFI value that means more no of PDCCH symbol, so your channel coding rate will get decrease ,if you decrease CFI then channel coding rate will increase.

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+1 vote

I would like to understand the relation between the TB size calculation in the TS36.213 document and in the Vienna 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 (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?