function x1=generate_x1(numbits)
Nc=1600;
x1(1)=1;       % sequence x1 initializition
for i=2:31     % x1(0)=1,x1(n)=0,n=1,2,...,30;
    x1(i)=0;    
end

for i=1:(Nc+numbits-31)                   % generate sequence x1
    x1(i+31)=mod((x1(i+3)+x1(i)),2);
end