Mixer NF design in Cadence

I have a question when try to estimate a mixer's NF.
How dose Cadence spectre calculate the NF of a mixer?
I mean, if we are designing a mixer with onchip input from an LNA, then the input impedance of the mixer surely won't be 50Ω.
So, when calculate the NF, which needs a value of Rs, dose it use the Zin of the mixer?
If not, what is the Rs?
Thanks.

I think if you simulate only the mixer in Cadence, then u have to specify the source impedance properly. But if you simulate LNA and mixer as a whole, then it takes into account the intermediate impedance.