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LTE: How transport block size (TBS) is calculated for an UE (unicast) or group of UEs (multicast) ?

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In unicast mode, MCS is very dynamic in nature and depends on feedback from UE while in multicast or broadcast there is no feedback from UE then how a particular MCS index is chosen ? And is there any possibility when MCS for an already configured MBSFN can be updated ?

posted Nov 3, 2016 by Harshita

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2 Answers

+1 vote

Hi,

As you said in multicast ENB will not consider the single UE feedback, But ENb can take decision based on overall UE feedback(average). Yes ENB can change the MCS even if it is con figured before. It is left to MAC to select an MCS at run time.

answer Nov 3, 2016 by Jaganathan
+1 vote

Hi,

eNodeB have to consider overall channel condition of all UEs and selects medium level mcs (By averaging) which can be accepted by all UEs(cell center or edge). Noramlly for Broadcast Data eNB selects medium level MCS values due to retransmissions not possible for broadcast data.

answer Nov 3, 2016 by Suchakravarthi Sripathi
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0 votes

What could be the reason for defining logical channel grouping concept in LTE ? Why logical channels can't be used independently ?

+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?

0 votes

On what parameter basis, eNodeB changes its current transmission mode to different transmission mode for an UE ?
What parameters are considered to take decision on transmission mode ? Does the eNodeB inform to UE about the selected TM ?

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