s-parameters to pulse

Hi,
I have used FDTD method for simulation small signal microwave amplifier and applied modulated gaussian pulse as a source. I need some information:
1- pulse is determined as follow:
dt=4.41e-13; %time step
fm=10e9 %frequency
T=-1;
for n=1:10000
T=T+1;
pulse(n)=A*exp(-B*(((t0-T)/tw)^2))*sin(2*pi*fm*T*dt);
end
how A,B,t0 and tw parameres can be determined?

2- I know that to calculate the S-parameters of amplifier, it needs to take the Fourier transform of the time responses, therefore I used FFT in matlab, but results were not right!
thanks.