Multiple-input multiple-output, or MIMO, is an abstract mathematical model for multi-antenna communication systems. During the last few years, MIMO technology has attracted a lot of attention in the area of wireless communications, since significant increases in throughput and range are possible at the same bandwidth and same overall transmit power expenditure. In general, MIMO technology increases the spectral efficiency of a wireless communication system.

Wireless MIMO communication exploits phenomena such as multipath propagation to increase data throughput and range, or reduce bit error rates, rather than attempting to eliminate effects of multipath propagation as traditional SISO (Single-Input Single-Output) communication systems seek to do.

MIMO can also be used in conjunction with OFDM, and is part of the IEEE 802.16 standard and will also be part of the IEEE 802.11n High-Throughput standard, which is expected to be finalized in mid 2007. Standardization of MIMO to be used in 3G standards such as HSDPA is currently under way.

History of MIMO in radio communications

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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. Notably, Dr Winters is now Chief Scientist at Motia Inc. which produces a 'beamforming amplifier' chip, that is designed to operate independently of any MIMO implementation.

In 1996, Greg Raleigh and Gerard 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^TM" for 802.11n. 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

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

Promises of MIMO technology

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As an example, 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.

Encoding

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In a MIMO system with $N\_t$ transmitter antennas and $N\_r$ receiver antennas, the input-output relationship can be described as

$\backslash mathbf\{y\}=\backslash mathbf\{HWx\}+\backslash mathbf\{n\}$

where $\backslash mathbf\{x\}$ is the $N\_s\backslash times\; 1$ vector of transmitted symbols, $\backslash mathbf\{y,n\}$ are the $N\_r\backslash times\; 1$ vectors of received symbols and noise respectively, $\backslash mathbf\{H\}$ is the $N\_r\backslash times\; N\_t$ matrix of channel coefficients and $\backslash mathbf\{W\}$ is the $N\_t\backslash times\; N\_s$ linear precoding matrix. The transmitter symbols are thus $N\_s$-fold spatially multiplexed over the MIMO channel, or in other words, $N\_s$ streams are transmitted in parallel, leading to a theoretically $N\_s$-fold increase in spectral efficiency. Linear precoding implies that a precoding matrix $W$ is used to precode the symbols in the vector to enhance the performance. The column dimension $N\_s$ of $W$ can be selected smaller than $N\_t$ which is useful if the channel can not support $N\_s$ streams.

See also

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• Antenna diversity
• Beamforming
• Space–time code
• Space–time block code
• 802.11
• 802.16

External links

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• 802.11 Wireless Networks: The Definitive Guide, Second Edition. Chapter 15: A Peek Ahead at 802.11n: MIMO-OFDM (PDF)

• MIMO: A better way to implement video over WLAN

Wireless communications | IEEE 802 | Information theory

MIMO | Multiple-input multiple-output | MIMO