MIMO

MIMO stands for multiple-input multiple-output, an abstract mathematical model for some systems. In radio communications if multiple antennas are employed, the MIMO model naturally arises. MIMO exploits phenomena such as multipath propagation to increase throughput, or reduce bit error rates, rather than attempting to eliminate effects of multipath.

MIMO can also be used in conjunction with OFDM and it will be part of the IEEE 802.11n High-Throughput standard, which is expected to be finalized in early 2007.

History of MIMO in radio communications

Jack Winters at Bell Laboratories filed a patent on wireless communications using multiple antennas in 1984. Jack Salz, also of Bell Laboratories published a paper on MIMO in 1985, based on Winters' research. Winters and many others published articles on MIMO in the period from 1986 to 1995.

In 1996, Greg Raleigh and Gerald J. Foschini invented new approaches to MIMO which increased its efficiency. Greg Raleigh is the founder of Airgo Networks, which claims to be the inventor of MIMO OFDM, offering a "pre-N" chipset called "True MIMO". However, it is unlikely that hardware based on this chipset will be compatible with other devices once the 802.11n-standard is ratified.

MIMO and information theory

It has been shown that the channel capacity (a theoretical measure of throughput) for a MIMO system is increased as the number of antennas is increased, proportional to the minimum of number of transmit and receive antennas. This basic result in information theory is what led to a spur of research in this area.

Benefits of MIMO

MIMO, as used by the expected 802.11n standard, will offer up to eight times the coverage, and up to six times the speed, of current 802.11g networks. A few wireless networking manufacturers have released "pre-N" hardware in anticipation of an eventual standard. As of August 2005, there are several non-compatible submissions being considered for 802.11n.

The future of wireless networks

The 802.11n ('MIMO') standard is still being discussed, but one prototype can offer up to (under optimal conditions) 250 Mbit/second. This is over four times the speed of existing 802.11g hardware.

Other new enhancements will include the arrival of 802.11e and 802.11i. 802.11e will prioritize important information on the network (i.e. a voice message takes precedence over email or a webpage). 802.11i will give an increase in security by using WPA2.

MIMO has just been added to the latest draft version of Mobile WiMAX (802.16e).