采用直接法或间接法（通过自相关函数）编制matlab程序，计算信号cos（2πf1t）+cos（2πf2t）叠加高斯白噪声后的功率谱密度。(Matlab program is compiled by direct method or indirect method (through autocorrelation function) to calculate the power spectral density of signal cos (2 PI f1t) +cos (2 PI f2t) s