2D simulating On-chip differential trace

Hi, friends:

I am simulating a on-chip coplanar edge coupled stripline. There is no reference plane underneath the trace.

2D quasistatic EM simulator is used for simulation. I am wondering how to assign the ideal ground as reference plane?

More, I am wondering that what is going to happen is real chip. There is no ideal ground at all!

I am also interested.
What is going to happen if I set background (boudnary) to be ideal ground? Is that correct?

I found it is good to have different setting for impedance problem and admitance problems.