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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^[1], Foschini^[2] 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>

References

^ Gerard J. Foschini and Michael. J. Gans (January 1998). "On limits of wireless communications in a fading environment when using multiple antennas". Wireless Personal Communications. 6 (3): 311–335.
^ Gerard J. Foschini (autumn 1996). "Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas". Bell Labs Technical Journal. 1 (2): 41–59. {{cite journal}}: Check date values in: |date= (help)

External links

[1] Gerard J. Foschini and Michael. J. Gans (January 1998). "On limits of wireless communications in a fading environment when using multiple antennas". Wireless Personal Communications. 6 (3): 311–335.

[2] Gerard J. Foschini (autumn 1996). "Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas". Bell Labs Technical Journal. 1 (2): 41–59. {{cite journal}}: Check date values in: |date= (help)

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[2]

@@ Line 1: / Line 1: @@
-'''Multiple-input multiple-output''', or '''MIMO''', is an abstract mathematical model for multi-[[antenna (radio)|antenna]] communication [[system analysis|systems]] where the transmitter has multiple antennas capable of transmitting independent signals and the receiver is equipped with multiple receive antennas. Special, [[degenerate|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.
+'''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 last few years, MIMO technology has attracted a lot of attention in the area of [[wireless| 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).
+During the past ten years, Smart Antennas technology has attracted attention in [[wireless| 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]].
-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_code|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.
+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.
-==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 [[adaptive|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]].
+[[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.
-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_code|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.
+[[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.
-MIMO technologies is best compatible with [[flat fading]] channels to ensure low complexity and power consumption in the receiver. 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.
+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.
-==Important achievements in MIMO for wireless communications==
-The earliest publications on MIMO (called "Multiple Channel Transmission Systems") are by A.R. Kaye and D.A. George (1970) and W. van van Etten (1975, 1976).
+==Application of Smart Antennas==
-Jack Winters at [[Bell Labs|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.
+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.
-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<sup><small>TM</small></sup>" 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.
+==History of Smart Antennas ==
-==MIMO and information theory==
-Papers by Gerard J. Foschini and Michael J. Gans<ref>{{cite journal|author=Gerard J. Foschini and Michael. J. Gans|title=On limits of wireless communications in a fading environment when using multiple antennas|journal=Wireless Personal Communications|pages=311–335|volume=6|issue=3|date=January 1998}}</ref>, Foschini<ref>{{cite journal|author=Gerard J. Foschini|title=Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas|journal=Bell Labs Technical Journal |pages=41–59|volume=1|number=2|date=autumn 1996}}</ref> and Emre Telatar
-have shown [http://web.mit.edu/6.454/www/www_fall_2004/alex_o/Telatar99.pdf#search=%22telatar%22 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.
+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).
-==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) 270 [[Mbit]]/second.  This is just under five times the theoretical maximum speed of existing 802.11g hardware (54 [[Mbit]]/second).
+Jack Winters at [[Bell Labs|Bell Laboratories]] and Jack Salz  at Bell Labs  published several papers on beamforming related applications in the mid eighties [[1984]], [[1986]].
-Upcoming [[4G]] systems also employ MIMO technology. Japanese [[NTT DoCoMo]] has demoed MIMO technology using a 100 MHz channel that can transfer up to 5 Gbit/s using 12 antennas.<ref>[http://www.physorg.com/news90256260.html NTT DoCoMo Achieves World's First 5Gbps Packet Transmission in 4G Field Experiment]</ref>
+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<ref>{{cite journal|author=Gerard J. Foschini and Michael. J. Gans|title=On limits of wireless communications in a fading environment when using multiple antennas|journal=Wireless Personal Communications|pages=311–335|volume=6|issue=3|date=January 1998}}</ref>, Foschini<ref>{{cite journal|author=Gerard J. Foschini|title=Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas|journal=Bell Labs Technical Journal |pages=41–59|volume=1|number=2|date=autumn 1996}}</ref> and Emre Telatar
+have shown [http://web.mit.edu/6.454/www/www_fall_2004/alex_o/Telatar99.pdf#search=%22telatar%22 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>{{cite book|author=A. Paulraj, R. Nabar and D. Gore|title= Introduction to Space-time Communications|book= Cambridge University Press=2003}}<ref>
 ==See also==
@@ Line 41: / Line 51: @@
 ==External links==
-*802.11 Wireless Networks: The Definitive Guide, Second Edition. [http://www.oreilly.com/catalog/802dot112/chapter/ch15.pdf Chapter 15: A Peek Ahead at 802.11n: MIMO-OFDM] (PDF)
 [[Category:Wireless communications]]