1, The signal only in certain discrete functions are given the instantaneous value and become discrete-time signal. It can come from a sample of the analog signal, or the rate of time and not in a row of discrete signals. In the discrete signals analysis and processing , Usually by a certain order in time in a row is not a group of several collections, called the series. 2, two different sequences can be the same as the corresponding Z transform, and the convergence of different domains, in order to make the series and Z-one mapping is a transformation in the transformation of the Z series is given at the same time, we must specify the convergence domain. 3, random signal is not precise mathematical description of the relationship between the type of signal, but it's subject to change in the law of statistics, probability and statistics can be used to describe characteristics. Immediately to the signal time course of doing all the long record of the observation function as the sample. 4, random signals in the time domain and frequency domain characteristics can be used statistical methods to study, the amplitude of the signal characteristics of the mean, mean square value and probability density function. The time-domain signal characteristics can also be used to describe auto-correlation function, the frequency characteristics can be used to describe the power spectral density function.