SNR Vs Fade Margin

A radio receiver tuned to a particular frequency channel will receive whatever is transmitted on that channel plus any receive whatever is transmitted on that channel plus any background noise. If the strength of a transmission is significantly stronger than the noise, then the receiver is able to effectively ignore the noise – the transmission (normally called the “signal”) has a good-signal-to-noise ratio. If the signal is of similar strength as the background noise, the receiver will not be able to discriminate the signal from the noise – this is a poor-signal-to-noise ration or SNR.

When a data message is modulated onto a RF signal, the ability to demodulate the message depends on the background noise. As a signal gets closer to the noise level, the demodulated data has more errors, as the noise makes it harder to determine if a demodulated bit of data is a 1 or 0. These errors are called “bit-errors”, and the error rate (errors per total bits) is the bit-error-ratio or “BER”.

The sensitivity of a radio receiver is the lowest RF signal that it can detect reliably – generally quoted at a specified BER. The “data sensitivity” is the lowest RF signal that the receiver can demodulate a data message with very low level of external noise.

Data sensitivity is normally expressed at a particular BER.

A transmitted signal can vary in strength. During rain or fog, the radio signal is attenuated (decreased) by the denser air. In a thunder storm, the transmitted signal will decrease, and the noise level will increase – this could turn a low BER into a high BER.

A SNR of 5dB means that the average signal measurement is 5dB stronger than the average noise level. Unfortunately radio noise is often much less than receiver sensitivity, so SNR is not particularly relevant.

A more common term that is relevant is the “Fade-Margin” – how much can a radio signal decrease (or “fade”) before the receiver can no longer demodulate data. Fade margin is the difference between the transmitted signal and noise or receiver data sensitivitiy, whichever is greater. It gives a measurement on how much the signal needs to "fade" before it becomes unreliable. The fade margin should be measured on a fine day.

For modern industrial wireless products, a fade margin of 10dB is adequate - this gives enough margin for loss of signal or increased noise during poor weather of high solar activity. An installation will work reliably with a lower fade margin, however not all of the time.