We know that Lorawan targets long range transmissions. 1
At our university we set up three gateways on rooftops. And we've achieved a successful 57 kilometres LoRa 2
transmission. 3
This is a huge distance if you consider that the end device transmits with a maximum power of 14 dBm. 4
Obviously, your gateway coverage depends on many parameters. 5
It depends on your environment on the weather, on your gateway location and so on. 6
When we look at our university campus, we are pretty sure that any end device deployed in this area will 7
be able to reach one of the three gateways. 8
But let's take an example. 9
If we consider only one gateway, let's say this one. We now imagine that we want to deploy a device 10
a little bit further than the campus area, let's say in the city on the other side of the lake, it's 11
about ten kilometers from the campus. 12
And of course, I would like to connect this end device to my LoRaWAN network. 13
What are my options? 14
The first idea is to increase the power transmission. 15
That's something obvious. 16
You can do it in your own device configuration, but you can only do it up to 14 dBm . 17
That's because the European regulation limits the transmission to 14 dBm in the 868 megahertz band. 18
Obviously, if you increase the power, you will also reduce your device battery life. 19
But this option works. 20
There is also another possibility you can increase the spreading factor. 21
So I've designed some circles on the picture and each circle represents the coverage area for a specific 22
spreading factor. 23
You can notice that if I increase the spreading factor, then I increase my gateway coverage. 24
Actually, when I increase the spreading factor during transmission, it becomes easier for the gateway 25
to demodulate the signal. 26
So we can say that the gateway can receive a LoRa frame with a lower power transmission if the spreading 27
factor is higher. 28
Let's take a very simple example. 29
If you transmit a message from the other side of the lake, then the gateway can demodulate it. 30
If you use an SF12 transmission but with the same power, it doesn't work if you use an SF11, 10 or 31
lower. 32
Let's use again the analogy we've previously used with one speaker and one listener. 33
We imagine that the speaker uses his usual voice, so with a certain level. 34
Then if you are a listener, it will be always easier for you to understand the speaker if he speaks 35
clearly. 36
Taking time to articulate each syllables of the sentence and on the contrary, with the same voice, 37
it would be harder for you to understand. 38
The Speaker If he talks very quickly. 39
The way the speaker talks is exactly what the spreading factor is. 40
So to make things easier to understand. 41
You can just consider that sending a message to a higher spreading factor is just taking time to send 42
clear message bit after bit. 43
That's exactly what happened on our gateway. 44
If the signal is received with a very low power, that's the case. 45
If the device is far away on the other side of the lake, then the gateway can understand it only if 46
it was clearly and slowly modulated, which is the case with the spreading factor. 47
12. 48
So one could say, why don't we always use spreading factor 12 so we can reach further devices? 49
Yes, but if you think that, then you forget that an end device using a spreading factor 12 will take 50
longer to transmit. 51
Because if you speak clearly, then it takes longer to say your message. 52
And obviously the longer your transmission time, what we call the time on air, the more energy the 53
device consumes.