For this example, we will again consider a transmitter and a receiver. 1
The transmitter sends a message with a power of 13 dBm. 2
It's transmitted through an antenna with a gain of 2 dBi. 3
These are specific decibels for antennas. 4
They are isotropic decibels, but they are decibels. 5
And again, you can add or subtract them with the other decibels value dB or dBm. 6
So you don't have to worry about the i in the dBi. 7
And if it bothers you, just forget it and consider it simply as dB. 8
So here we have a gain of 2 dB which is associated with the antenna. 9
Then we have the signal going into the transmission path in the air, which has an estimated loss of 10
60 dB. 11
This is a loss. 12
So in the calculation, don't forget to use -60 dB and not 60 dB and the antenna at a receiver side has a 13
gain of 2 dB. 14
Finally, the receiver has a sensitivity of -80 dBm, so it would be necessary that the received signal 15
is higher than -80 dBm for the message to be received. 16
Is it going to work? 17
We just have to add all gains and attenuation, the 13 dB power, the 2 dB gain, the air losses and a 2 dB 18
gain of the receiver's antenna. 19
And we compare it to the sensitivity. 20
Is it greater than -80 dBm or not? 21
If so, it's good. 22
If not, it doesn't work. 23
If we do the calculation 13 + 2 - 60 + 2, we find -43 dBm and this is indeed higher than 24
the -80 dBm. 25
This -43 dBm is the received power, so it's the RSSI. 26
So we've answered a first question. 27
The transmission is successful. 28
Let's go now on the second part of the problem. 29
For the second part, we're interested in SNR, signal of noise ratio. 30
And we consider that a noise power as a value of -50 dBm. 31
As we saw earlier, a ratio in decibels become a subtraction. 32
So the SNR is simply the received signal power minus the noise power. 33
It's therefore the RSSI, -43 dBm, minus the noise power, -50 DB. 34
This is an easy calculation. 35
It gives 7 dB. 36
Are we satisfied with this value? 37
Well, I can tell. You will need to check the documentation of yout LoRa transceiver. 38
And it will tell you the minimum SNR needed for a successful reception. 39
We can come back to the speaker and listener analogy. 40
Now we can consider that a speaker use a very loud voice. 41
Does the listener understand? 42
Again, I can't be sure, because even if we assume that the voice will be above the listener sensitivity 43
level, then what happens if it's a noisy environment? 44
If the noise exceeds a certain threshold, the signal over noise ratio will go down. 45
And the listener, in spite of the fact that the speaker speaks loudly, might not get the message. 46
And finally we can calculate the link budget. 47
This is the transmitted power minus the sensitivity. 48
So that's the difference between 13 dBm and -80 dBm, and the result is 93 dBm. 49
So we have a three notions here. 50
First, it's sensitivity and a signal to noise ratio that are both important to have a successful transmission 51
and the link budget, which is very useful to characterize the potential of a radio transmission. 52
Now that we've understood what's the RSSI, what's the sensitivity, the signal over noise and finally 53
the link budget, we'll be able to go and see a real LoRa frame on a gateway. 54
After that, thanks to what we've learned, we will study a little bit of a LoRa transceiver 55
documentation.