Reference of signals in electromagnetic (PCB/Package) simulation

[spices]

Dear experts
Possibly some of you have simulated a lot of pcb or package with full-wave electromagnetic solvers, such as HFSS.

But there is a question from me.

In most cases, I would like to take the VSS (ground plane) as the reference for signals. This is easy to deal with if the signals are between two ground planes (stripline) or on the top layer as ustrip.
However, in some cases, for example, if the first layer is power, second the signal and third the ground plane. Should we take both the top power layer and third ground layer as reference planes? And how to model in such as HFSS?

Any idea is welcome!

thanks a lot

 

[FvM]

The question makes only sense if you are modeling all involved PCB elements, specifically all power planes along with all bypass capacitors connecting the planes. Do you?

I believe most transmission line simulations will restrict itself to a simplified model with ideal ground planes, treating adjacent power planes as ground.