a=0.05;         %0.05 
Rs = 14e9;      %14e9 
bw=(1+a)*Rs   %14.7e9 --> real value with -50dB constraint: 15.298e9

Ts=1/Rs;
t1=-64*Ts;  %maximum 8Tb
t2=64*Ts;
N=512;
t=linspace(t1, t2, N);
N=16*N;
f=(-N/2:N/2-1)/(16*(t2-t1));
freq=1/Ts;
%freq=1;


x=sinc(freq*t).*(cos(pi*a*freq*t)./(1-(2*a*freq*t).^2));
X=fft(x, N);

XX=abs(fftshift(X))*(t2-t1)/N;
XX_=20*log10(XX/max(XX));
plot(f, XX_, 'b-');