CST transient solver with periodic boundary conditions

Hi all,
I want to simulate asymmetric SRR structures using transient solver. Because of the asymmetry I cant use PEC and PMC boundary conditions and while using Periodic boundary condition I get the following error "Waveguide port touches periodic boundary condition".
Is there anyway to get around that?
Also can we use plane wave excitation to get S parameters , if so then how ?

Thanks in advance
Tayyab

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