自相关函数Rxx（tao）可以了解不同时刻同一随机样本间的波形相似程度。

　　自功率谱密度函数Sxx（f）：反映相关函数在时域内表达随机信号自身与其他信号在不同时刻的内在联系。当随机信号均值为零时，自相关函数和自功率谱密度函数互为傅立叶变换对。

-Autocorrelation function Rxx (tao) can learn the same random sample at different times the degree of similarity between the waveforms. Since the power spectral density Sxx (f): reflects the correlation function of random signals in time domain expression of itself and other signals at different times of internal relations. When the random signal is zero mean, autocorrelation function and power spectral density function of each other since the Fourier transform pair.