0
1
00:00:01,050 --> 00:00:06,690
The first two bitrates we found, depending on two parameters, the spreading factor and a bandwidth.
1

2
00:00:07,200 --> 00:00:13,910
These two parameters are those we will have to configure in the Lora device to make it work. Actually, there is an additional
2

3
00:00:13,920 --> 00:00:16,410
that is more parameters that will influence the bitrate.
3

4
00:00:16,950 --> 00:00:19,950
And this additional parameter is what we call the coding rate.
4

5
00:00:20,610 --> 00:00:24,000
So what is it and how does it really influence the bitrate?
5

6
00:00:24,820 --> 00:00:31,050
The coding rate says that if, for example, we choose the value 4/5, then each time we
6

7
00:00:31,050 --> 00:00:35,250
transmit 4 bits, then in a reality we'll transmit 5.
7

8
00:00:36,120 --> 00:00:42,810
If we take a coding rate of 4/8, then each time we transmit 4 bits, we will in reality
8

9
00:00:42,810 --> 00:00:43,710
transmit 8.
9

10
00:00:44,370 --> 00:00:45,600
What's the purpose of that?
10

11
00:00:46,260 --> 00:00:51,660
Because here we have the impression that we are just adding bits, but in fact these additional bits
11

12
00:00:51,660 --> 00:00:53,910
would improve the robustness of the communication.
12

13
00:00:54,300 --> 00:00:54,930
Why is that?
13

14
00:00:55,440 --> 00:01:00,510
Because these additional bits will be able to detect and possibly even correct error.
14

15
00:01:01,110 --> 00:01:02,350
That's the good part.
15

16
00:01:02,850 --> 00:01:07,650
But a bad one is that these additional elements will produce what we call an overhead ratio.
16

17
00:01:08,370 --> 00:01:14,130
This means that they will increase the number of bits without increasing the useful information, and
17

18
00:01:14,130 --> 00:01:17,820
so the useful bit for the user will be deteriorated.
18

19
00:01:18,480 --> 00:01:20,640
Let's take the two cases we've seen so far.
19

20
00:01:21,030 --> 00:01:27,720
First at SF7, with 125 kilohertz bandwidth and with the coding rate of 4/5.
20

21
00:01:28,230 --> 00:01:32,790
If I choose 4/5 coding rates, it's because it's the most commonly used.
21

22
00:01:33,120 --> 00:01:35,490
But we must keep in mind that it's not the worst case.
22

23
00:01:36,330 --> 00:01:41,100
On a second case, we'll be at SF12 with the same bandwidth and the same coding rate 4/5
23

24
00:01:41,100 --> 00:01:41,760
.
24

25
00:01:42,210 --> 00:01:47,850
This time we have a deterioration of the data rate, which obviously correspond to the overhead ratio.
25

26
00:01:48,810 --> 00:01:57,840
And if we do the calculation, that's 6 836 kbps divided by a ratio of 1.25, and we find
26

27
00:01:57,870 --> 00:02:00,570
5469 bits per second.
27

28
00:02:01,380 --> 00:02:07,890
For software, we find 293 bits per second, which has significantly reduced its initial value.