How to set parameters to find Correlation Gain in Matlab

i am trying to find Target Range and Correlation Gain using Sinusoidal FM signal mixed with Random Noise.


fm=1e3;

B=180kHz
fs=360kHz;

Brn=2*fm;

Ntap=1000;

xsfm=exp(j*beta*sin(2*pi*fm));
xxrn=randn(1,Nfft+2*Ntap)+j*randn(1,Nfft+2*Ntap);

ht=fir1(Ntap,Brn/fs);
nxrn1=filter(ht,1,xxrn);
xrn=nxrn1(Ntap+1+(1:Nfft));
xrn=xrn/std(xrn);

xfmrn=xsfm.*xrn;

nd=round(taur/Ts)
xrfmrn=circshift(xfmrn, [0 nd]);

xft=fft(xfmrn);
xrft=fft(xrfmrn);
xref=conj(xft);
xb=xref.*xrft;
Rx(ii,=ifft(xb);

from Wiener Kintchin Theorem
Rx(tau)=ifft(|X(f)|^2)

I want to know Why there is a difference when i take
i) Correlation mean value then power
ii)Correlation Power then mean value

Why there is ambiguity after 63km from actual target.
How can i match theoretical and simulation results???
Is this correct approach to use circshift(xfmrn, [0 nd]) to use my received signal???