%圓周卷積
function y=circonvt(x1,x2,N)
if(length(x1)>N)
    error('N should bigger than or equal to the length of x1!');
end
if(length(x2)>N)
    error('N should bigger than or equal to the length of x2!');
end
x1=[x1,zeros(1,N-length(x1))];
x2=[x2,zeros(1,N-length(x2))];
m=[0:1:N-1];
H=zeros(N,N);
for n=1:1:N
    H(n,:)=cirshftt(x2,n-1,n);
end
y=x1*H';