麻烦解释一下下列程序是什么意思。实现什么功能？

a=0.003;
b=0.001;
D=4.84;
A=0.1;           
fs=800;            
h=1/fs;         
N=800;         
f0=100/5;      
t=0:h:N*h;


x1=A*cos(2*pi*f0*t);
figure(1);
plot(t,x1);


fs=400;            
h=1/fs;      
N=800;              
t=0:h:N*h;
x2=randn(1,801);
x2=x2-mean(x2);
x2=sqrt(a)*x2/sqrt(var(x2));
x2=sqrt(2*D)*x2;
x3=x1+x2;
figure(2);
plot(t,x3);      
axis([0,0.1,-0.4,0.4]);
xlabel('time/s');
ylabel('amplitude/V');
title('the waveform of the input in the time domain');