2. 关于无线信道的多普勒谱，请回答以下问题：
(1)　　在某些无线信道中，多普勒频移会引起无线信道输出信号的混叠。请描述产生混叠的各种原因；
(2)　　无线信道中的多普勒谱有一种经典谱(classic spectrum)，请解释产生这种谱形状的机理；
(3)　　请用Simulink工具编写代码(S-Function方式)，产生一单径瑞利信道，其多普勒谱为经典谱，其中移动速率为120km/h。并请描述：自己用计算机产生的经典谱是正确的依据。(2. Regarding the doppler spectrum of the wireless channel, please answer the following questions:
(1) in some wireless channels, doppler frequency shift will cause aliasing of the output signals of the wireless channel.Describe the various causes of aliasing;
(2) there is a classic spectrum in the doppler spectrum in the wireless channel, please explain the mechanism of producing this spectrum shape;
(3) please use Simulink tool to write code (s-function mode) to generate a single-path Rayleigh channel, whose doppler spectrum is the classical spectrum, in which the moving rate is 120km/h.And please describe: your own computer generated classical spectrum is the correct basis.)