Multiple Input, Multiple Output
IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) 11n a (PHY) Physical Layer 1 of the OSI model. 40 MHz channels to the PHY (physical layer), and frame aggregation to the MAC layer.
With MIMO, each physical radio card had multiple radio chains - currently up to three per physical radio card. There will be up to four radio chains per radio card. A radio chain is an RF radio
connected to an RF antenna.
MIMO is a technology which uses multiple antennas to coherently resolve more information than possible using a single antenna. In 802.11a,b, and g systems multiple transmissions are not possible without significant impact to throughput and link integrity.