Signal-to-Noise ratio, S/N
Larger SNR means it is easier to extract signal from noise (close to signal source point). For a given encoding scheme, SNR and BER is inverse proportional.
SNR versus BER tradeoffs
The strong the signal is, the more probability to extract original big from accepted signal with noise. Even we can encode signal using signal strength like under schemes. If you are out of coverage area, that means you are wasting signal strength.
Signal encoding schemes