Reconfigurable antennas at base-station
I do not know what you mean by “reconfigurable antenna”, but a MIMO system has multiple antennas at the transmitter TX and also multiple antennas at the receiver RX. 

This can be used to kind of “form” the radiation pattern of the TX or to improve the data reception of the RX through spatial diversity. 

In short: Multiple antennas provide better communication channels. (But this does not scale endlessly with number of antennas.)