说明：  基于随机过程的莱斯表达式产生窄带随机过程： 2、掌握窄带随机过程的特性，包括均值（数学期望）、方差、相关函数及功率谱密度等。
(Rice random process based on the expression of narrow-band random process generated: 2, grasp the characteristics of narrow-band random process, including the mean (mathematical expectation), variance, correlation function and power spectral density and so on.)