MIMO doesn't work well in line-of-sight environments
Q: Can you explain multi-input multi-output (MIMO) so that anyone can understand it?
A: MIMO means multiple antennas are transmitting and multiple antennas are receiving the signals. In other words, multiple antennas are inputting signals into the radio frequency (RF) medium and multiple antennas are outputting signals from the RF medium.
When multiple signals are transmitted in the same frequency and at the same time, the signal that arrives at the receiving antennas is the summation of all the transmitted signals. In other words, the receiving antennas each receive a combination of the transmitted signals.
If the different receiving antennas each receive very different combinations of signals, the receiver can determine what signals would have caused these combined signals. This then enables the receiver to estimate what the transmitted signals were. However, if the received signals are very similar at the receiving antennas, the receiver can't guess what the transmitted signals were.
The received signals will be similar at the receiving antennas if they followed very similar paths over the air. For this reason, MIMO doesn't work well in line-of-sight environments.
MIMO works best in environments where the signal between the Access Point (AP) and the client are reflected off of walls, filing cabinets, and even your body. When signals reflect off of walls and other surfaces, there's a very high probability that the signals arriving at each of the receiving antennas will be quite different. The receiver can then determine the transmitted signals.