SIW TRANSITION mode conversion

Hi

This is regarding Conventional Microstrip to SIW Taper transition.

How does the mode conversion take place? i.e. Quasi TEM to TE mode conversion.
Which parameter plays significant role in doing mode conversion?
Could you please explain me with diagrams?
I am looking for an equation / theoretical explanation about role of taper structure specific to the mode conversion.

Thanks.

theory of small reflections