Outage probability is a crude measure of performance. It is defined as the probability when mutual information is less than the given threshold.



The importance of outage probability is that when outage occurs, there is more likely to have decoding failure. In other words, it is a typical error.

In fading channels, the received signal has no constant power which be depending on the channel, can be described by probability models. Thus, signal to noise ratio will also becomes a random variable. and Thus the maximum capacity of the channel becomes a random variable. Outage probability says according to the variable signal to noise ratio at the received end, what is the probability that a rate is not supported due to variable signal to noise raito.