r_xx=randn(2*47+1);
M_w=41; N_w=41;
M_h=7;  N_h=7;
  tic
  for k=-M_w+1:M_w-1
    for l=-N_w+1:N_w-1
      R_xx=r2R(r_xx,M_h,N_h,k,l);
    end
  end
  toc
