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MIMO |
In radio, multiple-input and multiple-output, or MIMO (pronounced mee-moh or my-moh), is the use of multiple antennas at both the transmitter and receiver to improve communication performance. It is one of several forms of smart antenna technology.
MIMO 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). Because of these properties, MIMO is a current theme of international wireless research. (Refer to: Research trends in MIMO literature)
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The earliest ideas in this field go back to work by A.R. Kaye and D.A. George (1970) and W. van Etten (1975, 1976). Jack Winters and Jack Salz at Bell Laboratories published several papers on beamforming related applications in 1984 and 1986.
Arogyaswami Paulraj and Thomas Kailath proposed the concept of Spatial Multiplexing using MIMO 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, which considers a configuration where multiple transmit antennas are co-located at one transmitter to improve the link throughput effectively.
Bell Labs was the first to demonstrate a laboratory prototype of spatial multiplexing (SM) in 1998, where spatial multiplexing is a principal technology to improve the performance of MIMO communication systems.
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 2005, Airgo Networks had developed a pre-11n version based on their patents on MIMO. Following that in 2006, several companies (Broadcom, Intel,Marvell..) have fielded a MIMO-OFDM solution based on a pre-standard for IEEE 802.11n WiFi standard. Also in 2006, several companies (Beceem Communications, Samsung, Runcom Technologies, etc.) have developed MIMO-OFDMA based solutions for IEEE 802.16e WIMAX broadband mobile standard. All upcoming 4G systems will also employ MIMO technology. Several research groups have demonstrated over 1 Gbit/s prototypes.
MIMO can be sub-divided into three main categories, precoding, spatial multiplexing or SM, and diversity coding.
Precoding is multi-layer beamforming in a narrow sense or all spatial processing at the transmitter in a wide-sense. In (single-layer) 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 input. The benefits of beamforming are to increase the signal gain from constructive combining and to reduce the multipath fading effect. 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 all of the receive antenna and precoding is used. Note that precoding requires knowledge of the channel state information (CSI) 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 a 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 used 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 precoding when the channel is known at the transmitter or combined with diversity coding when decoding reliability is in trade-off.
Up to now, multi-antenna MIMO (or Single user MIMO) technology has been mainly developed and is implemented in some standards, e.g. 802.11n (draft) products.
Recently, the research on multi-user MIMO technology is emerging. While full multi-user MIMO (or network MIMO) can have higher potentials, from its practicality the research on (partial) multi-user MIMO (or multi-user and multi-antenna MIMO) technology is more active.
Spatial multiplexing 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 to be used in Mobile radio telephone standards such as recent 3GPP and 3GPP2 standards. In 3GPP, High-Speed Packet Access plus (HSPA+) and Long Term Evolution (LTE) standards take MIMO into account. Moreover, to fully support cellular environments MIMO research consortia including IST-MASCOT propose to develop advanced MIMO techniques, i.e., multi-user MIMO (MU-MIMO).
In MIMO systems, a transmitter sends multiple streams by multiple transmit antennas. The transmit streams go through a matrix channel which consists of multiple paths between multiple transmit antennas at the transmitter and multiple receive antennas at the receiver. Then, the receiver gets the received signal vectors by the multiple receive antennas and decodes the received signal vectors into the original information. Here is a MIMO system model:
where 
and 
are the receive and transmit vectors, respectively. In addition, 
and 
are the channel matrix and the noise vector, respectively.
Referring to information theory, the average capacity of a MIMO system is as follows:
![C_\mathrm{CL} = E[\max_{\mathbf{Q}} \log_2 \det(\mathbf{I} + \mathbf{H}\mathbf{Q}\mathbf{H}^{H})]
= E[\log_2 \det(\mathbf{I} + \mathbf{U}\mathbf{S}\mathbf{U}^{H})]](http://upload.wikimedia.org/math/0/8/1/081bb28a3222402c257d16a6498208b6.png)
and 
. The functions of svd() and waterfilling() represent singular value decomposition and power allocation by the water filling rule, respectively.![C_\mathrm{OL} = \max_{\mathbf{Q}} E[\log_2 \det(\mathbf{I} + \mathbf{H}\mathbf{Q}\mathbf{H}^{H})]
= E[\log_2 \det(\mathbf{I} + \mathbf{H}\mathbf{H}^{H})]](http://upload.wikimedia.org/math/b/b/9/bb9281a427bf693fcfed569ed4307094.png)
can achieve the capacity of a open-loop MIMO system, which is mostly min(Nt,Nr) times larger than that of a SISO systemPapers by Gerard J. Foschini and Michael J. Gans2, Foschini3 and Emre Telatar have 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 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 has published an introduction to this area 4.
There exists a fundamental tradeoff between diversity and multiplexing in a MIMO system (Zheng and Tse, 2003) 5.
In the IEEE international VTC 2007 fall conference (30 September – 3 October 2007, Renaissance Harborplace Hotel, Baltimore), approximately 130 MIMO, or spatial processing, based papers were presented among 420 other wireless communication papers. Those about MIMO treat not only antenna processing but also various wireless technologies over MIMO configurations. Some of those papers take into account multi-user MIMO in addition to multi-antenna MIMO. Multi-user type techniques consider multiple active users as a basic unit of multiple element processing while multi-antenna type techniques consider multiple antenna elements.
Given the nature of MIMO, it is not limited to wireless communication. It can be used for wire line communication as well. For example, a new type of DSL technology (Gigabit DSL) has been proposed based on Binder MIMO Channels.
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