MIMO

Multiple-input multiple-output, or MIMO, refers to the use of multiple antennas both at the transmitter and receiver. Another common term for this technology is Smart Antennas. Special, degenerate cases of MIMO are SIMO, when the transmitter has a single antenna, and MISO when the receiver has a single antenna.

During the past ten years, Smart Antennas technology has attracted attention in wireless communications, since it offers significant increases in data throughput and link range without additional bandwidth or transmit power. It achieves this by higher spectral efficiency (more bits per second per Hertz of bandwidth) and link reliability or diversity (reduced fading).

Smart antennas can be sub-divided into three main categories, Beamforming, Spatial multiplexing and Diversity Coding.

In Beamforming the same signal is emitted from each of the transmit antennas with appropriate phase (and sometimes gain) weighting such that the signal power is maximized at the receiver antenna. The benefits of beamforming are increased signal gain from constructive combining and reduced fading (diversity). In the absence of scattering, beamforming results in a well defined directional pattern, but in typical cellular conventional beams are not a good analogy. When the receiver has multiple antennas, the transmit beamforming cannot simultaneously maximize the signal level at every receive antenna and a technique called dominant mode beamforming is used. Note that beamforming requires knowledge of the channel at the transmitter.

Spatial Multiplexing requires MIMO antenna configuration. In spatial multiplexing, a high rate signal is split into multiple lower rate streams and each stream is transmitted from a different transmit antenna in the same frequency channel. If these signals arrive at the receiver antenna array with sufficiently different spatial signatures, the receiver can separate these streams, creating parallel channels for free. Spatial multiplexing is very powerful technique for increasing channel capacity at higher ‘’’Signal to Noise Ratio (SNR)’’’. The maximum number of spatial streams is limited by the lesser in the number of antennas at the transmitter or receiver. Spatial multiplexing can be use with or without transmit channel knowledge.

Diversity Coding techniques are used when there is no channel knowledge at the transmitter. In diversity methods a single stream (unlike multiple streams in spatial multiplexing) is transmitted, but the signal is coded using techniques called Space-time Coding. The signal is emitted from each of the transmit antennas using certain principles of full or near orthogonal coding. Diversity exploits the independent fading in the multiple antenna links to enhance signal diversity. Because there is no channel knowledge, there is no beamforming or array gain from diversity coding.

Spatial multiplexing can also be combined with beam forming when the channel is known at the transmitter or combined with diversity coding when this is lacking. The physical antenna spacing are selected to be large - multiple wavelengths at the base station. The antenna separation at the receiver is heavily space constrained in hand sets, though at least 0.3 wavelength is needed.

Application of Smart Antennas

SM techniques makes the receivers very complex, and therefore it is typically combined with Orthogonal frequency-division multiplexing (OFDM) or with Orthogonal Frequency Division Multiple Access (OFDMA) modulation, where the problems created by multi-path channel are handled efficiently. The IEEE 802.16e standard incorporates MIMO-OFDMA. The IEEE 802.11n standard, which is expected to be finalized soon, recommends MIMO-OFDM. MIMO is also planned for 3GPP Long Term Evolution standard.

History of Smart Antennas

The earliest ideas in this field go back to work by A.R. Kaye and D.A. George (1970) and W. van van Etten (1975, 1976).

Jack Winters at Bell Laboratories and Jack Salz at Bell Labs published several papers on beamforming related applications in the mid eighties 1984, 1986.

Arogyaswami Paulraj and Thomas Kailath proposed the concept of Spatial Multiplexing using MIMO concept in 1993. Their US Patent No. 5,345,599 issued 1994 on Spatial Multiplexing emphasized applications to wireless broadcast.

In 1996, Greg Raleigh and Gerard J. Foschini refine new approaches to MIMO technology.

Bell Labs was the first to demonstrate a laboratory prototype of SM in 1998.

In the commercial arena, Iospan Wireless Inc. developed the first commercial system in 2001 that used MIMO-OFDMA technology. Iospan technology supported both diversity coding and spatial multiplexing. In 2006, several companies (Beceem Communications, Samsung,..) have developed MIMO-OFDMA based solutions for IEEEE 16e WIMAX broadband mobile standard. Also in 2006, several companies (Broadcom, Intel,..) have fielded a MIMO-OFDM solution based on a pre-stnadard for IEEE 11n WiFi standard. Airgo had developed a pre-11n version in 2005.

All upcoming 4G systems will also employ MIMO technology. Several research groups have demonsarte < 1 Gbps prototypes.

Smart Antenna Literature

Papers by Gerard J. Foschini and Michael J. Gans, Foschini and Emre Telatar have 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 finding in information theory is what led to a spurt of research in this area. A text book by A. Paulraj, R. Nabar and D. Gore have published an introduction to this area <ref>A. Paulraj, R. Nabar and D. Gore. Introduction to Space-time Communications. {{cite book}}: Unknown parameter |book= ignored (help)<ref>

See also

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