0
1
00:00:01,260 --> 00:00:05,160
For this example, we will again consider a transmitter and a receiver.
1

2
00:00:05,730 --> 00:00:09,210
The transmitter sends a message with a power of 13 dBm.
2

3
00:00:10,440 --> 00:00:14,760
It's transmitted through an antenna with a gain of 2 dBi.
3

4
00:00:15,330 --> 00:00:17,910
These are specific decibels for antennas.
4

5
00:00:18,570 --> 00:00:21,750
They are isotropic decibels, but they are decibels.
5

6
00:00:21,750 --> 00:00:27,570
And again, you can add or subtract them with the other decibels value dB or dBm.
6

7
00:00:28,440 --> 00:00:31,260
So you don't have to worry about the i in the dBi.
7

8
00:00:31,590 --> 00:00:35,700
And if it bothers you, just forget it and consider it simply as dB.
8

9
00:00:36,240 --> 00:00:40,140
So here we have a gain of 2 dB which is associated with the antenna.
9

10
00:00:40,800 --> 00:00:46,110
Then we have the signal going into the transmission path in the air, which has an estimated loss of
10

11
00:00:46,110 --> 00:00:47,190
60 dB.
11

12
00:00:47,910 --> 00:00:49,160
This is a loss.
12

13
00:00:49,170 --> 00:00:57,270
So in the calculation, don't forget to use -60 dB and not 60 dB and the antenna at a receiver side has a
13

14
00:00:57,270 --> 00:00:58,440
gain of 2 dB.
14

15
00:00:59,430 --> 00:01:06,360
Finally, the receiver has a sensitivity of -80 dBm, so it would be necessary that the received signal
15

16
00:01:06,360 --> 00:01:10,230
is higher than -80 dBm for the message to be received.
16

17
00:01:10,860 --> 00:01:11,910
Is it going to work?
17

18
00:01:12,360 --> 00:01:22,230
We just have to add all gains and attenuation, the 13 dB power, the 2 dB gain, the air losses and a 2 dB
18

19
00:01:22,230 --> 00:01:23,850
gain of the receiver's antenna.
19

20
00:01:24,450 --> 00:01:26,460
And we compare it to the sensitivity.
20

21
00:01:27,090 --> 00:01:29,940
Is it greater than -80 dBm or not?
21

22
00:01:30,540 --> 00:01:31,590
If so, it's good.
22

23
00:01:31,980 --> 00:01:33,600
If not, it doesn't work.
23

24
00:01:34,800 --> 00:01:45,600
If we do the calculation 13 + 2 - 60 + 2, we find -43 dBm and this is indeed higher than
24

25
00:01:45,600 --> 00:01:47,220
the -80 dBm.
25

26
00:01:47,730 --> 00:01:53,400
This -43 dBm is the received power, so it's the RSSI.
26

27
00:01:54,540 --> 00:01:56,130
So we've answered a first question.
27

28
00:01:56,400 --> 00:01:58,050
The transmission is successful.
28

29
00:01:58,710 --> 00:02:00,810
Let's go now on the second part of the problem.
29

30
00:02:01,830 --> 00:02:06,960
For the second part, we're interested in SNR, signal of noise ratio.
30

31
00:02:07,650 --> 00:02:11,700
And we consider that a noise power as a value of -50 dBm.
31

32
00:02:12,750 --> 00:02:16,500
As we saw earlier, a ratio in decibels become a subtraction.
32

33
00:02:17,310 --> 00:02:22,860
So the SNR is simply the received signal power minus the noise power.
33

34
00:02:23,670 --> 00:02:31,560
It's therefore the RSSI, -43 dBm, minus the noise power, -50 DB.
34

35
00:02:32,280 --> 00:02:33,780
This is an easy calculation.
35

36
00:02:33,780 --> 00:02:35,250
It gives 7 dB.
36

37
00:02:36,180 --> 00:02:37,830
Are we satisfied with this value?
37

38
00:02:37,860 --> 00:02:42,250
Well, I can tell. You will need to check the documentation of yout LoRa transceiver.
38

39
00:02:42,270 --> 00:02:47,580
And it will tell you the minimum SNR needed for a successful reception.
39

40
00:02:48,660 --> 00:02:52,200
We can come back to the speaker and listener analogy.
40

41
00:02:52,200 --> 00:02:55,110
Now we can consider that a speaker use a very loud voice.
41

42
00:02:56,010 --> 00:02:57,540
Does the listener understand?
42

43
00:02:58,170 --> 00:03:04,050
Again, I can't be sure, because even if we assume that the voice will be above the listener sensitivity
43

44
00:03:04,050 --> 00:03:07,740
level, then what happens if it's a noisy environment?
44

45
00:03:08,040 --> 00:03:13,170
If the noise exceeds a certain threshold, the signal over noise ratio will go down.
45

46
00:03:13,380 --> 00:03:19,110
And the listener, in spite of the fact that the speaker speaks loudly, might not get the message.
46

47
00:03:23,480 --> 00:03:26,960
And finally we can calculate the link budget.
47

48
00:03:27,860 --> 00:03:31,200
This is the transmitted power minus the sensitivity.
48

49
00:03:31,820 --> 00:03:39,110
So that's the difference between 13 dBm and -80 dBm, and the result is 93 dBm.
49

50
00:03:40,310 --> 00:03:41,750
So we have a three notions here.
50

51
00:03:42,350 --> 00:03:48,050
First, it's sensitivity and a signal to noise ratio that are both important to have a successful transmission
51

52
00:03:48,890 --> 00:03:54,140
and the link budget, which is very useful to characterize the potential of a radio transmission.
52

53
00:03:54,920 --> 00:04:01,220
Now that we've understood what's the RSSI, what's the sensitivity, the signal over noise and finally
53

54
00:04:01,220 --> 00:04:05,390
the link budget, we'll be able to go and see a real LoRa frame on a gateway.
54

55
00:04:06,050 --> 00:04:10,460
After that, thanks to what we've learned, we will study a little bit of a LoRa transceiver
55

56
00:04:10,460 --> 00:04:11,240
documentation.