Signal-to-Noise Ratio (SNR)
Signal-to-Noise Ratio (SNR) is a measure used to directly compare the strength of a signal relative to background noise.  SNR is often expressed in decibles using the equation
represents the total signal power and
represents the total noise power. 
SNR is related to the maximum achievable BER (also referred to as the channel capacity C) and the channel bandwidth B by the Shannon-Hartley Theorem:
where S/N is the SNR not in decibles.