I'm a CS graduate, but to my shame have very limited knowledge of electrical engineering and especially antenna theory.
As far as I understand, RSSI determines quality of how measurer "hears" the object being measured. Noise determines environment conditions that affects measurer. And SNR is simply how much RSSI is better than Noise. This theory (assuming I got the basics right) raises only single question:
- How is it even possible for a single fixed measurer to determine both RSSI and Noise?
Now some practice. Let's say measurer is my Macbook Air running builtin Wireless Diagnostic tool. And the object being measured is my WiFi Router. Observed values are −60 dBm for RSSI and −92 dBm for Noise. Therefore SNR is 32 dB. What I completely cannot understand is:
- Why both values are negative and measured in dBm?
As far as I understand, −60 dBm means 10−9 W while −92 dBm means 10−12 W. But who radiate that power? Maybe that theory represents Noise as another "antenna"? But why is its value so small then? Or I miss some very key points here? I'll be thankful for an intuitive explanation of this stuff.