Traditional WLANs communicate using a single antenna to radiate and receive the RF signals. Since the WLAN functioning is half-duplex, the same antenna will radiate and receive signals at different times. If we have multiple antennas, both at the transmitting and the receiving stations, we can send multiple data streams from the sender to the receiver through each antenna, thereby increasing the effective bandwidth between the two systems. This is referred to as Multiple-Input multiple-output and is abbreviated as MIMO. A MIMO configuration is generally denoted as n x m, where n is the number of transmitting streams and m is the number of receiving streams.
The following figure shows a 2 x 2 MIMO configuration as an illustration: