We now look at several LoRa's transceiver documentation, and we will start by looking at the 1
SX1272. 2
It's not a recent transceiver, but never mind. 3
We'll see a new one after this. 4
You'll see the differences. 5
So, we'll do a quick Google search for the SX1272 and the first result heads us to Semtech website. 6
And we can have a lot of information about this company. 7
So we'll immediately download the documentation in PDF format. 8
In this documentation, very quickly in the first page, we have the key product features. 9
In this key product features, we learn that it is a LoRa modem and that's exactly what we wanted. 10
We also learn that the maximum link budget is 157 dB, means that this transceiver will have a maximum transmitted 11
power of 20 dBm, and that if we want to respect the standard, we'll use the maximum power of 14 dBm 12
and finally we'll also learn that the sensitivity is -137 dBm. And what can we notice ? 13
We can notice that the link budget that has been calculated here has simply been found by subtracting 14
the maximum transmitted power with the sensitivity that is 20 dBm minus -137 dBm, which is 157 dB. 15
Let's now see a component of the same range, but a little more recent, the SX1276. 16
So we do another quick search on Google SX1276. Of course again, we fall on Semtech's website. And 17
this time we'll not download the documentation but we'll simply note that we're still dealing with a LoRa 18
modem, 19
but the link budget is better. 20
We went from 157 dB to 168 dB. 21
Indeed, the transmitted power is still 20 dBm and if we want to respect the standard, it's still 14 dBm. 22
But this time the sensitivity is -148 dBm. 23
And of course if we make the same calculation, substracting the transmitted power with the sensitivity, 24
we find 20 dBm minus -148, which is in that case 168 dB of link budget. 25
Now we'll see what is said about the Signal Over Noise Ratio. So we can go on any transceivers documentation 26
and we'll see a table with the following information. 27
This table present the Signal Over Noise Ratio according to the Spreading Factor. 28
We've already talked about this Spreading Factor and we've said that we could take any value between 5 29
and 12. In this specific transceiver, 30
the lowest Spreading Factor is 6. 31
But anyway, when we'll speak about LoRaWAN, the Spreading Factor will have to be between 7 to 12. 32
And obviously all LoRa transceiver will be fine. 33
So no worries. 34
So let's take a first example. 35
If I take a Spreagin Factor of 8, the LoRa demodualtor will be able to work at -10 36
dB. 37
So what's -10dB ? 38
-10dB is ten times more noise than signal. 39
It means that the LoRa demodulator is able to find a transmitted signal in spite of the fact that 40
there is ten times more noise than signal, which is quite considerable. 41
Just imagine you're talking to someone and there is noise around and the noise is ten times louder 42
than the person talking to you. 43
And yet, you can understand what's being said. 44
If you now take the spreading factor 12 you can demodulate a signal with SNR ratio of -20 dB. 45
This means that you able to find a signal that is drawn in a noise that is 100 times stronger. 46
You're talking to someone, you have noise around. 47
This noise is 100 times louder. 48
And yet, you can consider that 49
you can understand what's being said. 50
That is really amazing. 51
Of course, there is some counterpart to that. 52
Actually, in the second column there is another information and we'll get back to it later. 53
But what it does basically say is that every time we use the highest Spreading Factor, we send more information 54
in the frame. 55
So in other words, that we'll have an influence on the bitrate. 56
But once again, we'll see that in the next chapter.