Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time. All wireless products with 802.11n support MIMO, which is part of the technology that allows 802.11n to reach much higher speeds than products without 802.11n.
In order to implement MIMO, either the station (mobile device) or the access point (AP) needs to support MIMO. For optimal performance and range, both the station and the AP must support MIMO.
MIMO technology takes advantage of a natural radio-wave phenomenon called multipath. With multipath, transmitted information bounces off walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times.
In the past, multipath caused interference and slowed down wireless signals. MIMO technology takes advantage of multipath behavior by using multiple, smart transmitters and receivers with an added spatial dimension, to dramatically increase performance and range.
MIMO makes antennas work smarter by enabling them to combine data streams arriving from different paths and at different times to effectively increase receiver signal-capturing power. Smart antennas use spatial diversity technology, which puts surplus antennas to good use. When there are more antennas than spatial streams, the antennas can add receiver diversity and increase range.
More antennas usually equate to higher speeds. A wireless adapter with three antennas can have a speed of 600 mbps while an adapter with two antennas has a speed of 300 Mbps. The router also needs to have multiple antennas, and fully support all of the features of 802.11n, to attain the highest speed possible.
Legacy wireless devices use Single-Input Single-Output (SISO) technology. These devices cannot take advantage of multipath, and can only send or receive one spatial stream at a time.