We saw in the previous video that the real bitrate, when sending one byte LoRa payload is 309.3 bits per 1
second at SF7 and 9.6 bit per second at SF12. 2
So we're far from the documentation values. The temperature used, as the LoRa payload, is called the phy payload in the 3
specification. 4
That's what the gateway is interested in. The gateway takes the phy payload and sends it to the server. 5
So if you are using a point to point LoRa transmission, everything is fine and you can keep a one byte 6
payload. 7
But what we want to do is not a LoRa transmission. 8
It's a LoRaWAN transmission. 9
And there is a big difference. 10
LoRaWAN will set up new services for the device. 11
For example, there will be special fields relative to its security, to its identification or to its application 12
parameters. 13
This means that there will be a LoRaWAN header. 14
This LoRaWAN header will increase the number of bit transmitted through the network. And the user 15
data, 16
our temperature, will be encapsulated into the LoRa Header and the LoRaWAN Header. 17
Obviously the extra data in the LoRaWAN Header will necessarily increase the transmission time of 18
the whole frame. The previous phy payload is now the user data, our temperature, and the LoRaWAN header. 19
So now to calculate the new bitrate, we need to know the new Time On Air of the entire LoRaWAN frame. 20
For that, I could dive into the specifications and find out the size of the LoRaWAN Header, but we 21
haven't spoken much about 22
the LoRaWAN specifications so far. So let's do it in another way. 23
I'm going to send a LoRaWAN frame with one byte user payload for my device. One time with SF7, 125 kHz 24
and a second time with SF12. 25
And on my gateway log, I'm going to take the Time On Air. 26
Let's do that. 27
I go on my gateway log and I send a frame. 28
We can see here that the first frame uses SF7, 125 kHz bandwidth and four over five coding 29
rate. 30
And the second one, SF 12, 125 kHz bandwidth and four over five coding rate. 31
And what do we see? 32
We see that the airtime, 33
that's just another word for Time On Air, 34
is 46.3 milliseconds, when we use SF 7 and 1155.1 milliseconds when we use SF 35
12. 36
These are the two time I will use to calculate my LoRaWAN bitrate right after. When I transmit it with 37
my LoRaWAN device, 38
I transmitted only one byte as we agreed, so you can be very surprised to see that the payload size 39
is 14 bytes. 40
What do these 14 bytes include? 41
These 14 bytes are the fee payload. 42
The include my one byte temperature and of course, the LoRaWAN Header. 43
We can therefore said that the LoRaWAN Header has in that case a size of 13 bytes. 44
In the LoRaWAN Header, there will be the device address, integraty message, the application port and 45
so on, but no stress, 46
we'll see that soon. 47
So same calculation as before, 48
but we take into consideration the new frame duration. Here for SF 7, I transmitted 8 bits divided 49
by 46.3 millisecond and that gives 172.7 bits per second. 50
And when we use SF 12, I still have 8 bits transmitted, but a duration of 1155.1 milliseconds, 51
and that gives a result of 6.9 bits per second. 52
So again, the bitrate has been significantly reduced. 53
Unfortunately, it's not over yet because the European standard on the 868 MHz band has another 54
restriction which will reduce even more the final bitrate. That will be the last step, and we're 55
going to see that in the next video.