Signal-to-Noise Ratio (SNR)
SNR is the difference between the received signal level and the noise floor and is expressed in dB: SNR = received signal (dBm) - noise floor (dBm). The quantity that determines whether the link will be established is not the absolute signal level but this difference. Every modulation scheme has a lower SNR threshold it can decode.
The same -100 dBm signal level can be measured on two different sites, but the outcome differs. If the noise floor is -120 dBm the SNR is 20 dB and the link works comfortably; if the floor has risen to -105 dBm the SNR drops to 5 dB and packets start to fail.
- There are two ways to raise SNR: increase the signal or reduce the noise.
- Reducing the noise is usually cheap: moving the antenna away from the noise source, separating and shielding cables.
- The choice of modulation changes the threshold: because FSK works independently of amplitude noise, it survives at a lower SNR than OOK.
In the field this means: to solve a range problem, look for the noise source first. Range dropping while an inverter or a welding machine is running and recovering when it stops is a clear indication that the problem is on the noise side, not the signal side.
Context in which this term is used: Wireless remote control installation: pairing, relay, safety