The probability density function (PDF) of a signal denoted by the symbol , is defined as follows: is the fraction of the total time spent by between the levels and . See Figure 1.11
Figure 1.11: Definition of Probability Density Function.
Thus,
Clearly and . The quantity is called zero crossing probability. It indicates the time spent by the signal around time
denoted by the symbol 
, is defined as follows: 
is the fraction of the total time spent by 
and 
. See Figure 1.11
and 
. The quantity 
is called zero crossing probability. It indicates the time spent by the signal around time 
