MULTIPLE-INPUT MULTIPLE-OUTPUT COMMUNICATIONS
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'Multiple-input and multiple-output (antennas)', or 'MIMO', (pronounced ''mee-moh'' or ''mai-moh'') refers to the use of multiple antennas both at the transmitter and receiver to improve the performance of radio communication systems. It is one of several forms of smart antenna (SA), or intelligent antenna (IA).
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).
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 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.
Bell Labs was the first to demonstrate a laboratory prototype of spatial multiplexing (SM) in 1998, where spatial multiplexing is a principle 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 2006, several companies (Beceem Communications, Samsung, Runcom Technologies, etc.) have developed MIMO-OFDMA based solutions for IEEE 802.16e WIMAX broadband mobile standard. Also in 2006, several companies (Broadcom, Intel,..) have fielded a MIMO-OFDM solution based on a pre-standard for IEEE 802.11n WiFi standard. Airgo had developed a pre-11n version in 2005.
All upcoming 4G systems will also employ MIMO technology. Several research groups have 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 (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 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.
★ SISO/SIMO/MISO are degenerate cases of MIMO
★
★ Multiple-input and single-output (MISO) is a degenerate case when the receiver has a single antenna.
★
★ Single-input and multiple-output (SIMO) is a degenerate case when the transmitter has a single antenna.
★
★ single-input single-output (SISO) is a radio system where neither the transmitter nor receiver have multiple antenna.
★ Single-user MIMO
★
★ Bell Laboratories Layered Space-Time (BLAST), Gerard. J. Foschini (1996)
★
★ Per Antenna Rate Control (PARC), Varanasi, Guess (1998), Chung, Huang, Lozano (2001)
★
★ Selective Per Antenna Rate Control (SPARC), Ericsson (2004)
★ Multi-user MIMO/MAIMO-SDMA
★
★ In 3GPP/3GPP2, there have been active discussions with many companies including Samsung, Qualcomm, Ericsson, TI, Huawei, Philipse, Lucent-Alcatal, Freescale, et al.
★
★ PU2RC allows the network to allocate each antenna to the different users instead of allocating only single user as in single-user MIMO scheduling. The network can transmit user data through a codebook-based spatial beam or physical antenna. Efficient user scheduling, such as pairing spatially distinguishable users with codebook based spatial beams, are additionally discussed for the simplification of wireless networks in terms of additional wireless resource requirements and complex protocol modification.
★
★ Enhanced multiuser MIMO: 1) Employ advanced decoding techniques, 2) Employ advanced precoding techniques
★
★ MAIMO represents many-input and multiple-output, which means that the transmitter will have much more than two transmit antennas. SDMA represents either space-division multiple access or super-division multiple access where ''super'' emphasises that orthogonal division such as frequency and time division is not used but non-orthogonal approaches such as super-position coding are used.
★ Cooperative MIMO
★
★ Utilizes distributed antennas which belong to other users.
★
★ Of analogical interest may be the comparison between the evolution of computing cores and mobile antennas. To wit, a single high performance core is the first generation of CPU core evolution, progressing to a few cores, then to many cores in a centralized fashion as the second step -- the recent environment. Many computing initiatives anticipate that cooperative work from multiple cores owned by different users will be made available to the individual user, in return for help with others' information processing, such as ambient intelligence, wireless ubiquitous computing and semantic web (web 3.0).
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.
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 develope enhanced techniques for advanced MIMO communications
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.
The average capacity of a MIMO system is as follows:
:
which is min(N_t, N_r) times larger than that of a SISO system
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 has published an introduction to this area [3]
★ Intelligent antenna
★ Multiple antenna research
★ Smart Antennas
★ Phased array
★ Spatial multiplexing
★ Antenna diversity
★ Space–time code
★ Space–time block code
★ Beamforming
★ Precoding
★ Dirty paper coding (DPC)
★ 802.11
★ 802.16
1. On limits of wireless communications in a fading environment when using multiple antennas, Gerard J. Foschini and Michael. J. Gans, , , Wireless Personal Communications,
2. Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas, Gerard J. Foschini, , , Bell Labs Technical Journal,
3. Introduction to Space-time Communications, A. Paulraj, R. Nabar and D. Gore, , , , ,
★ Computerworld QuickStudy MIMO
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).
History of MIMO
Background technologies
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 and Jack Salz at Bell Laboratories published several papers on beamforming related applications in 1984 and 1986.
Principle technologies
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.
Bell Labs was the first to demonstrate a laboratory prototype of spatial multiplexing (SM) in 1998, where spatial multiplexing is a principle technology to improve the performance of MIMO communication systems.
Commercial areas
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, Runcom Technologies, etc.) have developed MIMO-OFDMA based solutions for IEEE 802.16e WIMAX broadband mobile standard. Also in 2006, several companies (Broadcom, Intel,..) have fielded a MIMO-OFDM solution based on a pre-standard for IEEE 802.11n WiFi standard. Airgo had developed a pre-11n version in 2005.
All upcoming 4G systems will also employ MIMO technology. Several research groups have demonstrated over 1 Gbit/s prototypes.
Categories of MIMO
Function based categories
MIMO can be sub-divided into three main categories, precoding, spatial multiplexing, or SM, and diversity coding.
'Precoding' is multi-layer beamforming. 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 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.
Object based categories
★ SISO/SIMO/MISO are degenerate cases of MIMO
★
★ Multiple-input and single-output (MISO) is a degenerate case when the receiver has a single antenna.
★
★ Single-input and multiple-output (SIMO) is a degenerate case when the transmitter has a single antenna.
★
★ single-input single-output (SISO) is a radio system where neither the transmitter nor receiver have multiple antenna.
★ Single-user MIMO
★
★ Bell Laboratories Layered Space-Time (BLAST), Gerard. J. Foschini (1996)
★
★ Per Antenna Rate Control (PARC), Varanasi, Guess (1998), Chung, Huang, Lozano (2001)
★
★ Selective Per Antenna Rate Control (SPARC), Ericsson (2004)
★ Multi-user MIMO/MAIMO-SDMA
★
★ In 3GPP/3GPP2, there have been active discussions with many companies including Samsung, Qualcomm, Ericsson, TI, Huawei, Philipse, Lucent-Alcatal, Freescale, et al.
★
★ PU2RC allows the network to allocate each antenna to the different users instead of allocating only single user as in single-user MIMO scheduling. The network can transmit user data through a codebook-based spatial beam or physical antenna. Efficient user scheduling, such as pairing spatially distinguishable users with codebook based spatial beams, are additionally discussed for the simplification of wireless networks in terms of additional wireless resource requirements and complex protocol modification.
★
★ Enhanced multiuser MIMO: 1) Employ advanced decoding techniques, 2) Employ advanced precoding techniques
★
★ MAIMO represents many-input and multiple-output, which means that the transmitter will have much more than two transmit antennas. SDMA represents either space-division multiple access or super-division multiple access where ''super'' emphasises that orthogonal division such as frequency and time division is not used but non-orthogonal approaches such as super-position coding are used.
★ Cooperative MIMO
★
★ Utilizes distributed antennas which belong to other users.
★
★ Of analogical interest may be the comparison between the evolution of computing cores and mobile antennas. To wit, a single high performance core is the first generation of CPU core evolution, progressing to a few cores, then to many cores in a centralized fashion as the second step -- the recent environment. Many computing initiatives anticipate that cooperative work from multiple cores owned by different users will be made available to the individual user, in return for help with others' information processing, such as ambient intelligence, wireless ubiquitous computing and semantic web (web 3.0).
MIMO limitation
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 MIMO
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 develope enhanced techniques for advanced MIMO communications
Enhanced techniques for advanced MIMO communications
| Type | Description |
|---|---|
| Many-input Many-output (MAIMAO) ★ Many-input Multiple-output (MAIMO) | Much more than 2 Tx/Rx-antennas, e.g., 16x16 ★ Much more than 2 Tx, e.g., 16x2, 16x4 .. |
| Cross-layer MIMO | Scheduling, etc. |
| Advanced decoding MIMO | MLD-like techniques such as multiuser shpere decoding |
| Multi-user MIMO/MAIMO-SDMA | Precoding: spatial beamforming/coding (SB/SC) |
| Cooperative MIMO ★ Computing techniques based MIMO | Wireless network optimization ★ DPC, Wyner-Ziv, Game theory, User cooperation, .. |
Mathematical description
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.
The average capacity of a MIMO system is as follows:
:
which is min(N_t, N_r) times larger than that of a SISO system
MIMO 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 has published an introduction to this area [3]
See also
Intelligent antenna technology
★ Intelligent antenna
★ Multiple antenna research
★ Smart Antennas
★ Phased array
Spatial techniques
★ Spatial multiplexing
★ Antenna diversity
★ Space–time code
★ Space–time block code
★ Beamforming
★ Precoding
★ Dirty paper coding (DPC)
Wireless standards
★ 802.11
★ 802.16
References
1. On limits of wireless communications in a fading environment when using multiple antennas, Gerard J. Foschini and Michael. J. Gans, , , Wireless Personal Communications,
2. Layered space-time architecture for wireless communications in a fading environment when using multi-element antennas, Gerard J. Foschini, , , Bell Labs Technical Journal,
3. Introduction to Space-time Communications, A. Paulraj, R. Nabar and D. Gore, , , , ,
External links
★ Computerworld QuickStudy MIMO
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