very slow any other way to do it?

 clear, clc, close all;

N=1024; Cp=64; zf=[]; NTx=8; Npath=100; tap=randi([3 TAP]); ds=Cp; s=N+N-1; N1=Npath*Npath; 

N2=Npath*NTx;

for aa=1:Npath

    for bb=1:tap

        p(aa,bb)=randi([1 ds]);

        p(aa,:)=sort(p(aa,:),'descend');

        xx(aa,:)=(abs(randn(1,tap)+(randn(1,tap))*1i));

        xx(aa,:)=sort(xx(aa,:));

    end

end

% Loop for adding random positions & normalized impulse response (h_N)

for cc=1:Npath

    for dd=1:tap

        h(cc,p(cc,dd))=xx(cc,dd);

        U(cc)=sqrt(sum((h(cc,:).^2)));

        h_N(cc,:)=h(cc,:)./U(cc);

        h_N(cc,:)=circshift(h_N(cc,:),[0 -min(p(cc,:))+1]);

    end

end

bk= randi ([0,15],NTx,N);

Dk=C(bk+1);

d=ifft((Dk).*sqrt(N));

tx=[d(:,end-Cp+1:end) d];

* | _for ii=1:Npath

    for hh=1:N+Cp

        for jj=1:NTx           

            [Txx(jj,:),zf]=filter(h_N(ii,:),1,tx(jj,:),zf);

           Txxx(ii,hh)=sum(Txx(:,hh));

        end

    end    
end_ | *

[Cedric]

Your code doesn't work (undefined TAP, flawed calls to RANDI, RANDN, etc). You should use the profiler and identify the bottleneck(s):

 profile viewer

and execute your script from there.