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The real question is how can PSpice do that? It's not included in PSpice as far as I know (I might be wrong though, I'm not referring to its user manual while speculating about this), because it's not designed for s parameter simulations. You can come up with a model that would work for PSpice but that might be even harder than just switching to ADS or AWR.
I'd suggest ADS because it's integrated well with the cadence virtuoso tools. And OrCAD is a part of cadence, so it might as well work well with OrCAD PSpice too. But I'm still just speculating.
The s-parameters being a small signal analysis can be easily calculated using any spice by means of AC analysis. The question is - the mos model you are using for LNA simulation is valid for RF range ?
The two small circuits in figure 1, 4 need to be connected to your original design.
From s-parameter basic theory you can easily derive the relationship and understand what these circuits are trying to do.
I verified the method with spectre results. They match perfectly.
Suppose there is port1 and port2 and you need to find S21 and S11, connect circuit in fig-4 at port1 and fig-1 at port2.
For S12 and S22, interchange the circuits.
You can observe there is small signal (excitation) provided in fig-4, with a terminating characteristic impedance (50-ohm). At receiver side, the port is terminated separately (with 50-ohm, see examples below in appnote).
If you are connecting these circuits directly on the schematic (and not generating the hidden pins as described in appnote), you don't need R_loop1 (in fig4) and R1 (in fig1).