Multiple-input multiple-output communications

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Multiple-input multiple-output, or MIMO, is an abstract mathematical model for multi-antenna communication systems where the transmitter has multiple antennas capable of transmitting independent signals and the receiver is equipped with multiple receive antennas. Special, degenerated cases of MIMO are SIMO, when the transmitter is constrained to have only a single antenna, and MISO when the receiver is constrained to have a single receive antenna.

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 without any increase in bandwidth or overall transmit power expenditure. In general, MIMO technology increases the spectral efficiency (the number of information bits you can transmit per second of time and per Hertz of bandwidth) of a wireless communication system by exploiting the space domain (since multiple antennas are physically separated in space).

Wireless MIMO communication can be sub-divided into three main categories, spatial multiplexing for enhancing the peak data transmission rate, transmit diversity methods such as space-time coding for enhancing the robustness of the transmission and beamforming technologies for improving received signal gain and reducing interference to other users. However, these MIMO transmission techniques are not mutually exclusive, it is possible to construct a MIMO system with both spatial multiplexing and diversity benefits.

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[edit] Development of MIMO in wireless communications

In the 1990s, multiple antennas for wireless communication systems were researched all over the world with the aim to perform beamforming towards the receiver/mobile phone. The term smart antenna was coined for a system where the beam adaptively (i.e. smartly) tracked the receiver (i.e. the mobile phone) as the person carrying it moved around, just like the beam from a flashlight can be used to track a person moving in the dark. The shown benefits of smart antennas was increased signal gain from constructive interference in the pointing direction of the beam (where the intended receiver is) and at the same time destructive interference to receivers in other directions than the pointing direction for the beam. To create the beam, a narrow antenna spacing between the multiple antennas on the transmitter is used. Commonly the physical antenna spacing is selected as half the wavelength of the transmitted signal to fulfill the spatial version of the Nyquist–Shannon sampling theorem and thereby avoid grating lobes, the spatial equivalent to aliasing.

The drawback of the beamforming technology is that in an urban environment, where the signal is commonly scattered towards buildings and moving cars etc, the focusing property (constructive interference) of the beams gets blurred and most of the signal gain and interference reduction properties are lost. However, in the end of the 1990s, this drawback was suddenly turned into an advantage when space-time codes and spatial multiplexing technologies were developed´. These methods are created to exploit the multipath propagation phenomena to increase data throughput and range, or reduce bit error rates. Commonly the physical antenna spacing in these type of systems are selected to be greater than the wavelength of the transmitted signal to ensure low correlation between the MIMO channels and ensure high diversity order.

MIMO technologies is best compatible with flat fading channels to ensure low complexity and power consumption in the receiever. Therefore MIMO is mostly used in conjunction with OFDM, a modulation technology that 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. There exists also standardization of MIMO in WCDMA systems such as HSDPA, a process that is currently under way.

[edit] Important achievements in MIMO for wireless 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. 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 MIMOTM" for 802.11n. However, it is unclear whether hardware based on this chipset will be compatible with other devices once the 802.11n standard is ratified.

[edit] MIMO and information theory

It has been shown by Telatar 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.

[edit] Promises of MIMO technology

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 five times the (theoretical maximum) speed of existing 802.11g hardware.


[edit] See also

[edit] External links

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