What is the proper distance between antenna elements for MIMO?

Hi, everyone. I am designing dual band dual element MIMO antenna for WLAN operations for my thesis.
I got stuck on one thing. How can we choose proper distance between antenna elements for designing MIMO antenna in order to obtain high isolation and low mutual coupling? I searched the literature and I saw, almost every design the distance was chosen as 10mm but I don't know why. Is there any formula for calculation of the proper distance?

To my knowledge, a MIMO system should use wave paths that are "statistically independent". This can mean that a good isolation between antennas should be used.
On the other hand, a constant "crosstalk" is not an issue in statistics. So try to use "some" degree of isolation, say 10...20 dB. If you cannot achieve it by separation, then use grounded shields to increase isolation.
Try to see how others did it, and if the isolation caused the system a problem.



Elements close to each other form an "array", and a specific radiation pattern results from it. Usually array directivity increases.

Thanks for your advice, I will try that :)