关于系统辨识程序的问题。

(t)=1.5y(t-1)-0.7(t-2)+u(t-1)=0.5(t-2)+e(t)-0.9e(t-1)+0.1e(t-2)
t=1....256
u(t)is white noise, e(t) is a zero mean uncorrelated random variable.
noise ratio(S/N)is four.

how to :
plot input output data u(t) y(t)

What i should do?
I only know get the whith noise x=randon(1,256)
another i donot know to fit S/N is four ,please tell me how to do or read which books?
Thank you very much!
请告诉我如何过滤噪音的，各位有没有作系统辨识的方面资料。如何比较信号等。