Doubt in the design of mimo antenna

I am designing an MIMO antenna.in my design the patch is present at both at the front side and backside side of the substrate .I dont Know whether i have to create groundplane or not.
In the picture i have attached they have specified cavity of electric wall.what is meant by cavity of electric wall.
Second doubt :For the structure shown in the figure ,which arms are to be connected to the coaxial feed