说明：  用余弦法仿真的瑞利衰落，包括自相关互相关、功率谱密度等等。依据是Chengshan Xiao2003年的文章。
(Cosine method for simulation of Rayleigh fading, including the auto-correlation cross-correlation, power spectral density and so on. Based on the the Chengshan Xiao2003 article.)