4G Speeds

In this post we are going to teach you how to calculate the data rate in LTE. In order to follow the process, you need todownload the 3GPP document, TS 36.213.

Now, let’s see an example for the calculation procedure for downlink(PDSCH):

  1. refer to TS 36.213 Table 7.1.7.1-1
table 7.1.7.1-1

2. get I_TBS for using MCS value (Let’s assume MCS is 2. in this case, I_TBS is 1 )

You may wonder how we get this MCS index. Well, in the 3GPP document 36.213, the DCI contains 5 bits indicating MCS Index. Therefore, this value will be given by the DCI field.

    3. refer to TS 36.213 Table 7.1.7.2.1
    4. go to column header indicating the number of RB (Let’s assume that RB is 50)
Table 7.1.7.2.1

5. go to row header ‘1’ which is I_TBS

    6. we would get 1800 (if the number of RB is 50 and I_TBS is 1)

    7. (This is Transport Block Size per 1 ms for one Antenna)
 If we use 1 antenna, the throughput is 1800 bits * 1000 subframes/sec = about 1.8 Mbps 

Now, how these 1800 bits are obtained? What’s the formula behind this number? Have a look to the following set of Resource Blocks:

control and data region

Source: http://www.ece.drexel.edu/walsh/Gwanmo-Nov11-2.pdf

For data, there are:

10 OFDM symbols per subframe x 12 subcarriers = 120 (we are counting the 6 Reference Signal 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

Note that a more accurate way to do this calculation, which you can find in other sources as well, uses this approach:

other frame

Source: http://www.ece.drexel.edu/walsh/Gwanmo-Nov11-2.pdf

 

Here, 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 

The 0.0762 is the code rate, which is calculated from table 7.2.3-1: 4-bit CQI Table (3GPP document TS. 36.213):

table 7.2.3-1


The calculation procedure for uplink(PUSCH) is the same as the downlink: same steps as above except that you have to refer to 36.213 Table 8.6.1-1 at step 1.

Hope this was helpful!

Any feedback or comment on this post will be very welcomed! 🙂

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