To improve transmission over fading radio channels a diversity system is used. A
diversity system uses two or more channels. The idea of diversity is that these
channels should be as independent as possible and hopefully the fading dips of the
channels do not occur at the same time.
There are many types of diversity including Time diversity, Space diversity, Polarization Diversity; Frequency Diversity and Angle Diversity.
A MIMO-system (Multiple Input Multiple Output) is a system where the transmitter
and the receiver use multiple antennas. The antennas in the transmitter as well as the receiver are separated a certain distance from each other. The correlation between the individual channels depend on this distance.
The SNR is given simply as below:
(n×Es)\(m×N0)
Where "n" and "m" are the numbers of transmit and receive antennas respectively and "Es" is the signal energy and "N0" is the spectral density of the noise.
from this simple relation SNR can be controlled by controlling n,m and hece control the BER.
hope that this is helpfull.