This paper presents an overview of progress in the area of multiple input multiple output (MIMO) space-time coded wireless systems. After some background on the research leading to the discovery of the enormous potential of MIMO wireless links, we highlight the different classes of techniques and algorithms proposed which attempt to realize the various benefits of MIMO including spatial multiplexing and space-time coding schemes. These algorithms are often derived and analyzed under ideal independent fading conditions. We present the state of the art in channel modeling and measurements, leading to a better understanding of actual MIMO gains. Finally, the paper addresses current questions regarding the integration of MIMO links in practical wireless systems and standards.

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From theory to practice: an overview of MIMO space-time coded wireless systems

Spatial modulation (SM) is a recently developed transmission technique that uses multiple antennas. The basic idea is to map a block of information bits to two information carrying units: 1) a symbol that was chosen from a constellation diagram and 2) a unique transmit antenna number that was chosen from a set of transmit antennas. The use of the transmit antenna number as an information-bearing unit increases the overall spectral efficiency by the base-two logarithm of the number of transmit antennas. At the receiver, a maximum receive ratio combining algorithm is used to retrieve the transmitted block of information bits. Here, we apply SM to orthogonal frequency division multiplexing (OFDM) transmission. We develop an analytical approach for symbol error ratio (SER) analysis of the SM algorithm in independent identically distributed (i.i.d.) Rayleigh channels. The analytical and simulation results closely match. The performance and the receiver complexity of the SM-OFDM technique are compared to those of the vertical Bell Labs layered space-time (V-BLAST-OFDM) and Alamouti-OFDM algorithms. V-BLAST uses minimum mean square error (MMSE) detection with ordered successive interference cancellation. The combined effect of spatial correlation, mutual antenna coupling, and Rician fading on both coded and uncoded systems are presented. It is shown that, for the same spectral efficiency, SM results in a reduction of around 90% in receiver complexity as compared to V-BLAST and nearly the same receiver complexity as Alamouti. In addition, we show that SM achieves better performance in all studied channel conditions, as compared with other techniques. It is also shown to efficiently work for any configuration of transmit and receive antennas, even for the case of fewer receive antennas than transmit antennas.

Spatial Modulation

Accurate and tractable channel modeling is critical to realizing the full potential of antenna arrays in wireless communications. Current approaches represent two extremes: idealized statistical models representing a rich scattering environment and parameterized physical models that describe realistic scattering environments via the angles and gains associated with different propagation paths. However, simple rules that capture the effects of scattering characteristics on channel capacity and diversity are difficult to infer from existing models. We propose an intermediate virtual channel representation that captures the essence of physical modeling and provides a simple geometric interpretation of the scattering environment. The virtual representation corresponds to a fixed coordinate transformation via spatial basis functions defined by fixed virtual angles. We show that in an uncorrelated scattering environment, the elements of the channel matrix form a segment of a stationary process and that the virtual channel coefficients are approximately uncorrelated samples of the underlying spectral representation. For any scattering environment, the virtual channel matrix clearly reveals the two key factors affecting capacity: the number of parallel channels and the level of diversity. The concepts of spatial zooming and aliasing are introduced to provide a transparent interpretation of the effect of antenna spacing on channel statistics and capacity. Numerical results are presented to illustrate various aspects of the virtual framework.

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Deconstructing multiantenna fading channels

Multiple-input-multiple-output (MIMO) wireless systems use multiple antenna elements at transmit and receive to offer improved capacity over single antenna topologies in multipath channels In such systems, the antenna properties as well as the multipath channel characteristics play a key role in determining communication performance This paper reviews recent research findings concerning antennas and propagation in MIMO systems Issues considered include channel capacity computation, channel measurement and modeling approaches, and the impact of antenna element properties and array configuration on system performance Throughout the discussion, outstanding research questions in these areas are highlighted

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A review of antennas and propagation for MIMO wireless communications

Find the most up-to-date and comprehensive treatment of classical and modern antennas and their related technologies in Modern Antenna Handbook. Have access to current theories and practices in the field of antennas, with topics like metamaterials, microelectromechanical systems (MEMS), frequency selective surfaces (FSS), radar cross sections (RCS), and advanced numerical and computational methods targeted primarily for the analysis and design of antennas. Written by leading international experts, this book will help you understand recent developments in antenna-related technology and the future direction of this fast-paced field.

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Modern Antenna Handbook

A study of the capacity of multiple element antenna systems is presented, with particular emphasis on the effect that mutual coupling between the antenna elements has on the capacity. The results presented here show, contrary to some earlier claims, that correlation between different channel coefficients as a function of antenna spacing, can in fact decrease when the mutual coupling effect is accounted for. As a consequence, capacity also improves. A realistic channel model is used to perform simulations to support these claims.

Mutual coupling effects on the capacity of multielement antenna systems

The mutual coupling in a uniform linear array (ULA) of dipoles is calculated using basic electromagnetic concepts. Since the coupling often is unknown and needs to be estimated, a simpler model is proposed based on the electromagnetic analysis. The parameterization of this model is shown to be locally unambiguous. A necessary condition for the joint solution of directions and coupling parameters to be unique is also derived. Finally, the directions and coupling parameters are estimated using a maximum likelihood method. It is found that the simpler coupling model with just a few parameters well describes the full electromagnetic model.

Modeling and estimation of mutual coupling in a uniform linear array of dipoles

A novel way of exploiting higher modes of antennas as diversity branches in multiple-input-multiple-output (MIMO) systems is introduced. Essentially, antennas employing multiple modes offer characteristics similar to an antenna array, through multiple modes and using only a single element. The physical mechanism that yields different received signals is the fact that each mode has a different radiation pattern. Analytical expressions for the correlation between signals received by different modes are presented for a biconical and a circular microstrip antenna that employs higher order modes. It is found that the correlation is low enough to yield a significant diversity gain. Furthermore, the channel capacity of a MIMO system using a multimode antenna, i.e., an antenna employing multiple modes, is found to be comparable to the capacity of an array. Since only one element is needed, the multimode antenna offers several advantages over traditional arrays, and is an interesting antenna solution for future high capacity MIMO systems.

Correlation and channel capacity of MIMO systems employing multimode antennas

This paper provides an analytical framework useful for assessing the use of all six electric and magnetic electromagnetic field polarizations for multiantenna communications systems. The approach uses a mapping between the induced signal currents and the received electromagnetic field in order to formulate a diversity interpretation of the six-polarization problem. Application of the framework to a simple, yet representative, channel model demonstrates that for full multipath elevation and azimuthal angle spread, six communication modes are theoretically possible. However, to implement the system requires more antenna design work since a straightforward implementation is found to reduce the potential number of modes to three.

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Analysis of electromagnetic field polarizations in multiantenna systems

The area of sensor array processing has attracted considerable interest in the signal processing community. The focus of this work has been on high resolution Direction Of Arrival (DOA) estimation algorithms that detect and locate aircrafts using radar systems. These algorithms exploit the fact that an electromagnetic wave that is received by an array of antenna elements reaches each element at di erent time instants. In practical antennas the elements of the array a ect each other through mutual coupling and this reduces the direction nding ability. The e ects of mutual coupling on direction nding are examined in this thesis. To be able to analyze the e ects of mutual coupling, the coupling needs to be calculated. Here, the coupling in a Uniform Linear Array (ULA) of thin and nite dipoles is calculated using basic electromagnetic concepts. The e ects of coupling on the estimation of the DOA is studied by calculating a lower bound on the variance of the DOA estimates, the Cram er-Rao lower Bound (CRB). It is found that a known coupling does not a ect the CRB signi cantly. Several methods of estimating the DOAs in the presence of a known coupling are derived and analyzed in simulations. However, in some cases the coupling might not be known, and one way of mitigating the e ects of an unknown coupling is to estimate the coupling along with the DOAs. The CRB for this case is derived. Several methods of estimating the directions and coupling parameters are also derived, and it is found that estimating the coupling along with the DOAs mitigates the e ects of an unknown coupling.

T. Svantesson

Papers

Mutual coupling effects on the capacity of multielement antenna systems

Modeling and estimation of mutual coupling in a uniform linear array of dipoles

Correlation and channel capacity of MIMO systems employing multimode antennas

Analysis of electromagnetic field polarizations in multiantenna systems

Direction finding in the presence of mutual coupling