The MIMO abbreviation stands for Multiple Input Multiple Output. What does it mean? The MIMO concept implies that Wi-Fi devices may have several independent antennas for transmitting and receiving. The outgoing stream of data is distributed among the transmitting antennas and is put on the air at the same frequency, on the standard 20MHz radio channel. The MIMO method is based on the fact that the radio signal doesn’t spread out uniformly indoors but is reflected a number of times from walls, ceilings and various objects. Using a special modulation and sophisticated algorithms of processing the sent and received signals, it is possible to establish multiple data streams at the same frequency (the so-called spatial multiplexing) with reliable reconstruction of the original stream out of the mess of signals and reflections. Thus, the Wi-Fi channel bandwidth grows up in a linear manner depending on the number of send/receive pairs on the channel’s ends. Besides that, the physical principles of MIMO help improve coverage and reliability of Wi-Fi networks indoors because signal reflections do not hinder, but even facilitate the operation of the wireless connection.
MIMO looks a very promising technology and is expected to bring great improvements into performance, reliability, spectrum efficiency and coverage of future Wi-Fi networks. Not surprisingly it is one of the key features of the new 802.11n standard currently under development. Although there’s still a long way to ratification (there’s only a draft version of 802.11n available), Wi-Fi solutions developers are actively promoting MIMO products, different and usually incompatible with each other. Moreover, some manufacturers put a different meaning into the word MIMO that has nothing to do with the technology described above. For example, Atheros uses the send/receive couples not to do spatial multiplexing (i.e. to expand the channel bandwidth), but to increase the distance the signal can travel by focusing the transmitter energy in the direction of the receivers using the so-called Intelligent Antenna technology.
You should be aware that the abbreviation MIMO in a device description may mean anything (at least until an official ratification of the 802.11n standard). You can only be sure that the device has more than one transceiver. When purchasing a MIMO-supporting product, you may want to ask the seller what the manufacturer means in this particular case.
Summarizing this section I would like to stress the fact that most of technologies for improving Wi-Fi performance are proprietary and are not officially specified. For you to enjoy the advantages of such a technology, it must be supported by both the access point and the Wi-Fi adapter (this is usually only possible when the access point and adapter are based on chipsets from the same manufacturer).