As we saw in a previous chapter. 1
LoRa is a spread spectrum modulation. 2
There are may ways, 3
to produce spread spectrum modulation. 4
And a specific one we use is called chirp spread spectrum. 5
That's because we use a special symbol to create the radio frequency message. 6
It's a chirp. 7
So what is it exactly? 8
I've simulated a chirp here in MATLAB. 9
This is simply a sinusoidal that switched from 0 to 125 kilohertz. 10
125 kilohertz is the most common situation used in LoRaWAN. 11
But in reality, it can also go up to 500 kilohertz. 12
That is what we call the bandwidth and the higher the bandwidth, the more information we can transmit. 13
So that increases the bitrate. 14
Producing a higher bandwidth also has its counterpart, because if you use a larger band for one transmission, 15
then we have less room for the others. 16
This symbol, as it's represented here, is not very convenient. 17
Actually, we are not really interested in the amplitude, but we would rather know the frequency. 18
We therefore use another representation. 19
This representation draws the chirp frequency instead of its amplitude. 20
The 125 kilohertz band is centered of what we called the channel, and I remind you that we are in the 868 MHz band 21
. 22
So there I've choosen a random channel of 868.1 megahertz. We'll see later that this specific channel must 23
be known by all LoRaWAN devices in Europe. 24
So how a frequency sweep starts at 868.1 megahertz minus half of 125 kHz, so -62.5 kilohertz. 25
That gives this frequency to 868.1 megahertz plus 62.5 kilohertz. 26
And that gives this frequency. 27
So the chip starts from the low frequency and sweeps until reaching this high frequency. 28
Soon we'll see that this chirp, even if it has the same shape, it doesn't always start at the same frequency. 29
So the basic shape of the chirp will be renamed a symbol when we come to talk about all the different 30
form it can have. And a time to send a symbol, 31
so a specific chirp, is represented here on the graph. 32
Now, what we want to know is what does this symbol represent exactly? 33
How can we use these symbols to represent the binary data of our messages? 34
That's what we will see in the next video.