Antenna diversity uses multiple antennas to compensate for multipath interference and improve wireless connectivity. Diversity can be in the receiving or transmitting antennas. As the radio signal is transmitted multipath causes the signal to separate into multiple signals. Because the signals take different paths they reach the receiver at different times. If there is diversity at the receiving end both antennas receive signals and the "best" signal is used. For each transmission there can be a different "best" signal and the antenna chosen as "best" can change with each transmission. Only one antenna can process the signal at a time since there is only one wireless processing device for all antennas.
Multiple-Input/Multiple-Output (MIMO ) uses spatial diversity to accept multiple "best" signals simultaneously. Each antenna is able to
transmit or receive signals. Where the legacy system can only accept the single "best" signal, MIMO can accept multiple signals through its multiple antennas. A MIMO access point (AP) sends multiple spatial streams from its antennas to a MIMO station that receives the signals through its multiple antennas. Performance is improved on both the transmit and receive sides of the signal.
As you increase the number of antennas on each side that can transmit or receive a spatial stream the maximum rate increases. The best scenario is to have three transmit antennas and three receive antennas using 40 MHz channel bonding at 5 GHz. With this setup the maximum theoretical speed is 450 Mbps. Actual data throughput will always be less due to WiFi protocol overhead, interference, obstructions, etc.