In current 802.11 systems without Multiple-Input/Multiple-Output (MIMO) there is a single Radio Frequency (RF) chain on the Wi-Fi device. Multiple antennas use the same hardware to process the radio signal. So only one antenna can transmit or receive at a time as all radio signals need to go through the single RF chain. In MIMO there can be a separate RF chain for each antenna allowing multiple RF chains to coexist.
It is similar to a highway, imagine that the current systems are on a highway with only 1 lane and MIMO systems are on a highway with several lanes. The MIMO system can accommodate more traffic (i.e. data) and as more signals are processed the likelihood of getting a better signal increases. This process is also known as spatial multiplexing as multiple spatial streams (radio signals) are carried on a single frequency simultaneously.
When both the AP (access point) and station are using MIMO the amount of data able to be processed on both ends of the connection increases improving the speed and connectivity of the Wi-Fi network.
MIMO also uses spatial diversity to improve performance. Antenna diversity in current systems allows multiple antennas to receive signals then choose the “best” signal to be processed by the radio hardware.
More on MIMO and spatial chains can be found within the page What is MIMO?
This applies to: