We're going to check some technical terms that are present in documentation, and we'll use this little 1
diagram as a support for the explanation. First of all, we transmit with a power Pₜ and the fraction 2
of this power will arrive to the receiver. 3
But along a transmission path, some noise has been added to the signal. 4
So at the receiver side, we have Pᵣ, the reserved power, plus Pₙ the noise power. 5
So what do we want? 6
We want to have the highest possible received power. 7
And of course, we also would like the noise to be as low as possible, ideally, and theoretically we would 8
like the received power equal to the transmitted power and the noise power equal to zero. 9
But that's obviously not possible, because that would mean that a transmission path is lossless and 10
noiseless. 11
Now that we have explained these three terms, we can explain almost everything. 12
First of all, the first term that we will see in the documentation is a term called RSSI. 13
RSSI is the abbreviation of receive signal strength indication and it's equivalent to the received power 14
Pᵣ. 15
Then they will speak about the sensitivity. 16
Sensitivity is something that is very important for a receiver because it's the minimum power that it 17
will be able to receive. 18
So a power Pₜ has been emitted. 19
There is an attenuation in the air because there are losses during transmission. 20
And finally, there is more residual power received. If this residual power is high enough for the receiver 21
to demodulate the signal. 22
It means that this power was higher than the receiver sensitivity. 23
Let's recap that one more time. 24
We receive a signal with a power called RSSI. 25
If the RSSI is greater than the sensitivity, then it's okay. 26
If it's lower than the sensitivity, then it's not okay. 27
The third term is related to the noise power. 28
Indeed, we receive Pᵣ, but we also receive Pₙ and we're going to make the ratio between the two of 29
them Pᵣ over Pₙ. 30
And of course, we want this ratio to be as high as possible to indicate that there is more useful signal 31
than noise. 32
These three definitions, they have units. 33
And again, we use decibels. 34
So dBm for RSSI and sensitivity because in both case we speak about power and dB for signal to 35
noise ratio because we speak about power ratio. 36
And the beauty of the decibel representation is that we will be able to add and subtract them each other.