Hi, ramani:
If you want to extract the input impedance Zin, you can just take DFT of the V(t) and I(t) to get V(f) and I(f). Then, Zin = V(f) / I(f). From Zin, you can get the 50-ohm normalized s-parameters. If you want to find the generalized s-parameters normalized to the port impedance, you should try to extract the incident wave and reflected wave. There should be places you can find different ways of doing so. If you can not find anyway, you can check it in my Ph.D. thesis (Jian-X. Zheng, Dept. of ECE of University of Colorado at Boulder, 1990. I implemented a few ways to solve it. I believe I have not published any paper on those schemes and my Ph.D. thesis might be the only place to find the schemes I implemented. Regards.