hello sir i am calculating mean square for my two signals but its giving me constant mean square error..?

clc 
clear all
N=1024;
k=8;
d=16;
a=1;
b=5;
X=zeros(1,N);
X_p=randperm(N,k);
X_v1=rand(1,k);
X_v=a*X_v1+b;
X(X_p)=X_v;
x=ifft(X);
x1=0.1;
x2=0.5;
%X_v=[b,a+b];
X_re=zeros(1,N);
len=length(x);
% figure 
% plot(x)
% 
sss=4;
for l=1:sss;
    xd(l,:)=downsample(x(l:len),sss);
    XD(l,:)=fft(xd(l,:));
end 
% plot(abs(xd(1,:)));
k_zero=[];
k_one=[];
k_two=[];
for kd=1:N/d
    if XD(:,kd).^2<x1;
        k_zero=[k_zero,kd];
    else 

         if abs(XD(1,kd).^2)-(XD(2,kd).^2)<x2;
             k_one=[k_one,kd];
      X_k=[k_zero,k_one];
      X_value1=XD(1,kd);
      X_rel(X_k)=X_value1;
        X_po=mean(abs(XD(1,kd)-(XD(2,kd))));

         else 

          k_poss=[kd,kd+N/d];
  %         k_pair=[poss,kd+(kd-1)*(N/d)];

ss=[0,1,2,3];
kk=0;
poss=[];
for  ii=1:N/d
 poss=[poss,kd+(kd-1)*(N/d)];

       for ii=1:length(poss)
      k1=poss(ii);

      for jj=kd+1:length(poss)
          k2=poss(jj);
            kk=kk+1;
           k_p(kk,:)=[k1,k2];
     A=[1,  1 ; exp(1i*2*pi*(k1-1)*(ss(2)/N)), exp(1i*2*pi*(k2-1)*(ss(2)/N));exp(1i*2*pi*(k1-1)*(ss(3)/N)) , exp(1i*2*pi*(k2-1)*(ss(3)/N)); exp(1i*2*pi*(k1-1)*(ss(4)/N)), exp(1i*2*pi*(k2-1)*(ss(4)/N))];
         Aa(kk).A=A;
     XF=pinv(A)*(XD(:,1));
     xf(kk).XF=XF;       

               MSE(kk)=sum((XD(:,1))-(A*XF).^2);

      end 
       end 

end   
       end 
       end 
    end 

   plot(abs(MSE));

[irfan]

can anyone answer my question i am waiting for your answer