Question on Frequency in MIMO

I want to know the frequencies on each antenna in a MIMO system
to be same or different. I heard that array antenna will obtain both
increased spectral efficiency and reduced co-channel interfere.
If antennas use same frequency, it is easy to understand the spectral
efficiency can be increased. If antennas use different frequency, it
is natrual that CCI will be reduced.
Thank you.

same.
both for space multiplexing or space diversity