I choose here random binary message. I divide this message into a certain number of bits. 1
And here I choose two divided it in packets of ten bits. 2
This means that each symbol represents ten bits with ten bits half 1024 possible combinations. 3
So there would be 1024 different symbols. 4
Here I have a representation of a LoRA modulation in SF10. 5
And if I had to use an SF12, I would have divided it by group of 12 bits. 6
And you can see in the first transmission representing the first packet that it has the specific symbol 7
. 8
On this first symbol, I start from a certain frequency, which is not the bottom one. 9
This is one symbol among the 1024 existing ones. 10
So when I take another packet of ten beats, I have a different symbols. 11
The second sweep started the higher frequency and the third one to an even higher one. 12
But they all have the same duration. 13
So the LoRa frame is simply a succession of symbols all centered around the channel frequency. 14
Let's see how it's represented when we received this message on a spectrum analyzer. 15
We obtained the following diagram. 16
Once again, we have the frequency on the y-axis and a time on the x-axis. 17
We already used this presentation earlier. 18
We can see here two different symbols. 19
First, there is a symbol which starts as a certain frequency which is not the lowest one since the 20
low frequency is at the bottom. 21
And then a new symbol starts. 22
And again, this new symbol starts at a different frequency. 23
Anyway, whatever the symbol, it will always sweep the other bandwidth, which is usually 125 kilohertz. 24
What are the frequencies at which these symbols could have started? 25
We use here spreading factor of 10. 26
So it could have been one of the 1024 frequency available along the axis. 27
But there is one thing you have to be very careful about when I remind you that the SF10 representation was 28
used. 29
We said that there were 10 bits in each symbol. 30
We must absolutely not imagine that the fact that they are more bits in the symbol will increase the 31
bitrate because it's exactly the opposite in reality. 32
Obviously, if I say that there are more bits in the symbol, we can state that more information will 33
be transmitted. 34
But in fact, there is one thing that I haven't mentioned so far. 35
This is the symbol time. 36
Of course. 37
Here I said that we were working in an SF10. 38
So we have the feeling that there are more binary elements than if we were working in SF8. 39
But in reality, there is a big difference between the symbol transmission time in SF10 and 40
the symbol transmission time in SF8. 41
So we cannot consider and it's completely false to say that having a packet of 10 bits will increase 42
the bitrate. 43
So we'll speak about that in the next video and once we'll have understood the time of a symbol and the 44
number of bit of which symbol, then we'll easily calculate the bitrate of a LoRa transmission.