In this video, I'm going to show you the LoRa modulation and the LoRa frame. 1
We know that the modulation is made with Chirp. It's a sweep frequency, 2
and the difference between the bottom frequency and the top one represents the Bandwidth. 3
In the LoRa frame, 4
there is first a preamble, and an optional Header. At the tail of the frame, 5
there is a CRC to check if there was an error in the frame. 6
And finally, the most important part is what we call the PHY Payload. 7
This is the LoRaWAN message carried by the frame. 8
The preambule part, header, the presence or not of the CRC can be configured the way we want. 9
The only requirement is that this information should be the same on the transmitter and on the receiver. 10
So if you want to use the LoRa modulation and comply with the LoRa frame format, then you can do your own 11
configuration of these fields. 12
However, if you aim to use the LoRaWAN protocol on top of the LoRa modulation, then you don't have the 13
choice and these fields must have a specific value. 14
Right. 15
So for this transmission, I have programmed a simple LoRa message of the string: 16
"Hello". I'm using SF12, 125kHz bandwidth, 868.1Mhz channel, 17
And for the frame, I've selected the preamble of 8 symbols, with a header and a CRC. About the 18
preamble. 19
We just need to remember that the specification says that there is always 4.25, so 4 plus a quarter 20
of a symbol in addition to the number of preambule configuration. 21
So in our case, that will make 12.25 symbols overall. 22
There is probably a good reason for this additional 4.25 symbols, but I've got no idea where it comes from. 23
To be exact, 24
in these 4.25 symbols, there will be 2 upchirps and 2.25 downchirps. 25
That makes indeed 4.25. 26
Great. 27
So first to try this LoRa modulation, I'm going to use the transceiver of a LoRa device to send a hello 28
characters. 29
And on the other side, I'm going to use an SDR. An SDR is a Software Defined Radio managed by an application. 30
The one I'm using for this demonstration is the ADAM-PLUTO mainly used for education, and the software 31
that I use is the Open Source SDR Angel. 32
Okay, so in the SDR Angel software, I need to configure my receiver. 33
So 868.1 Mhz. 34
And I will decrease the gain because in my situation, my LoRa device is very close from my receiver. 35
I start it. 36
Here we go. 37
We can perfectly see the LoRa modulation here. 38
I will try to stop the visualization when the frame starts. 39
Great. 40
Perfect. 41
Here we can see the preambules. 42
They are 8 upchirps. 43
One, two, three, four, five, six, seven and eight. 44
Then there are another 2 upchirps. 45
One, two. And finally two. 46
And a quarter downchips. 47
One, two. 48
And this little bit here. Right, 49
of course, it's very difficult with this representation to analyze the frame and its content. 50
So we're going to use a little pluggin. 51
This plugin is called ChirpChat demodulator. 52
The first thing I need to do is to configure the parameters used for the transmission: 53
The most important one are the SF12 and the bandwidth. 54
Then, now, when I run the demodulator, I can see the Raw Payload of the LoRa packet. 55
This is exactly the same "Hello" string that I sent with my device and we can check on each transmission the 56
CRC status to see if there was some errors during the modulation process. 57
That obviously happened when the RSSI, 58
so the power of the signal received is very low, or if the SNR is low as well. 59
But that's not my case here. 60
Okay. 61
So now I would like to present a second demonstration. 62
Let's say that for some reason we would like to debug a transmission. 63
For example, you have doubts about the value sent by your device and you want to visualize the LoRa 64
packet around you. 65
It's a kind of LoRa sniffer that we want with the same purpose as an oscilloscope when we do some electronic 66
debugging. 67
And what is a LoRa packet sniffer? a gateway? 68
Yes, because a gateway is able to demodulate any LoRa packet. 69
And the nice thing with a gateway is that it can demodulate many channels and all spreading factors at 70
the same time. 71
So, for this last demonstration, I'm going to use a gateway and look at the Log to see what happens. 72
On my gateway, 73
to see the Log I need to connect thanks to a serial link. 74
And I can now see all packet. 75
It's uplink, 76
if it comes from my device, and it's downlink, if it's sent to my device. 77
Of course, if I send the "hello" string then my gateway can demodulate the packet and print the characters 78
in the Log. 79
Great. 80
So now we don't just want to send "hello" with a LoRa modulation, but instead we need to send all the information 81
of the LoRaWAN protocol. 82
That's what we're going to do in the next video.