Multiple-input multiple-output (MIMO) technology is maturing and is being incorporated into emerging wireless broadband standards like long-term evolution (LTE) [1]. For example, the LTE standard allows for up to eight antenna ports at the base station. Basically, the more antennas the transmitter/receiver is equipped with, and the more degrees of freedom that the propagation channel can provide, the better the performance in terms of data rate or link reliability. More precisely, on a quasi static channel where a code word spans across only one time and frequency coherence interval, the reliability of a point-to-point MIMO link scales according to Prob(link outage) ` SNR-ntnr where nt and nr are the numbers of transmit and receive antennas, respectively, and signal-to-noise ratio is denoted by SNR. On a channel that varies rapidly as a function of time and frequency, and where circumstances permit coding across many channel coherence intervals, the achievable rate scales as min(nt, nr) log(1 + SNR). The gains in multiuser systems are even more impressive, because such systems offer the possibility to transmit simultaneously to several users and the flexibility to select what users to schedule for reception at any given point in time [2].

/pdf/scaling-up-mimo-opportunities-and-challenges-with-very-large-puqp9v7gzk.pdf

Scaling Up MIMO: Opportunities and Challenges with Very Large Arrays

A new type of metallic electromagnetic structure has been developed that is characterized by having high surface impedance. Although it is made of continuous metal, and conducts dc currents, it does not conduct ac currents within a forbidden frequency band. Unlike normal conductors, this new surface does not support propagating surface waves, and its image currents are not phase reversed. The geometry is analogous to a corrugated metal surface in which the corrugations have been folded up into lumped-circuit elements, and distributed in a two-dimensional lattice. The surface can be described using solid-state band theory concepts, even though the periodicity is much less than the free-space wavelength. This unique material is applicable to a variety of electromagnetic problems, including new kinds of low-profile antennas.

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High-impedance electromagnetic surfaces with a forbidden frequency band

http://www.csun.edu/~andrzej/COMP529-S05/papers/Mesh%20Networks%20Survey.pdf

Wireless mesh networks: a survey

This paper surveys recent advances in the area of very large MIMO systems. 
With very large MIMO, we think of systems that use antenna arrays with an order of magnitude more elements than in systems being built today, say a hundred antennas or more. Very large MIMO entails an unprecedented number of antennas simultaneously serving a much smaller number of terminals. The disparity in number emerges as a desirable operating condition and a practical one as well. The number of terminals that can be simultaneously served is limited, not by the number of antennas, but rather by our inability to acquire channel-state information for an unlimited number of terminals. Larger numbers of terminals can always be accommodated by combining very large MIMO technology with conventional time- and frequency-division multiplexing via OFDM. Very large MIMO arrays is a new research field both in communication theory, propagation, and electronics and represents a paradigm shift in the way of thinking both with regards to theory, systems and implementation. The ultimate vision of very large MIMO systems is that the antenna array would consist of small active antenna units, plugged into an (optical) fieldbus.

/pdf/scaling-up-mimo-opportunities-and-challenges-with-very-large-1gai688763.pdf

Scaling up MIMO: Opportunities and Challenges with Very Large Arrays

Almost all mobile communication systems today use spectrum in the range of 300 MHz-3 GHz. In this article, we reason why the wireless community should start looking at the 3-300 GHz spectrum for mobile broadband applications. We discuss propagation and device technology challenges associated with this band as well as its unique advantages for mobile communication. We introduce a millimeter-wave mobile broadband (MMB) system as a candidate next generation mobile communication system. We demonstrate the feasibility for MMB to achieve gigabit-per-second data rates at a distance up to 1 km in an urban mobile environment. A few key concepts in MMB network architecture such as the MMB base station grid, MMB interBS backhaul link, and a hybrid MMB + 4G system are described. We also discuss beamforming techniques and the frame structure of the MMB air interface.

An introduction to millimeter-wave mobile broadband systems

The investigation of mutual coupling is of great importance and can contribute to the development of improved and reliable communication systems. In this paper, the coupling between wire antennas, e.g., monopoles, "towel-bar" and inverted-L antennas is analytically computed. Furthermore, the finite-difference time-domain (FDTD) method is used to calculate the coupling between these antennas. Both analytical and FDTD results are validated by comparison with measurements. Finally, the mutual coupling between wire antennas mounted on helicopter airframes is computed using FDTD and compared with measurements.

Coupling between wire antennas: analytic solution, FDTD, and measurements

The demand for increased capacity in wireless networks motivated recent research toward wireless systems that exploit space selectivity. The paper presents a comprehensive effort on smart antennas that examines and integrates antenna array designs and beamforming, and the impact of these designs on the network throughput. Fading and coding channel modeling are also examined to improve the system BER. The antenna design that has been chosen is a planar array of rectangular microstrip elements, The frequency of operation is 20 GHz, and both 4/spl times/4 and 8/spl times/8 designs are examined. Tchebyscheff and adaptively beamformed designs are considered, and their impact on network throughput and system BER is examined. From the results, it is possible to provide guidelines for the design of smart antenna systems for optimum capacity in mobile ad hoc networks.

Impact of smart antenna designs on network throughput and BER

Although there has been extensive research and application of the finite-difference time-domain (FDTD) method to electromagnetic scattering and penetration problems, only recently has FDTD been used to predict the radiation characteristics of antennas. The FDTD method has not been applied to low frequency or electrically small radiating structures. The development of higher-order absorbing boundary conditions (ABC) has allowed the use of very small cell sizes in the FDTD computational space by placing the absorbing boundaries closer than the traditional one wavelength from the scatterer or radiating structure. In this study, the FDTD method with higher-order absorbing boundary conditions is used to model and predict the far-field radiation of electrically short antennas. Results are presented and compared with measurement for HF loop or "towel bar" and inverted-L antenna elements used at 10 MHz mounted on a helicopter-like body, a square cylinder. The computed radiation patterns compare well with measurements. An FDTD cell size of /spl lambda//120 was used to model the smallest dimension of the problem, the height of the antenna.

Finite-difference time-domain of HF antennas

: The major objective of this dissertation is to design simple low-dispersion Finite-Difference Time-Domain (FDTD) methods for electromagnetics. Literature review indicated that the Nonstandard Finite Difference (NSFD) method exhibits great potentials in dispersion reduction. Different from the Standard Finite Difference (SFD) methods, the NSFD methods are derived directly based upon dispersion analysis. In this dissertation, the basic concepts of the NSFD methods are generalized to various extended finite-difference stencils. Furthermore, several improved NSFD Numerical simulations show that these schemes significantly reduce the dispersion error of their standard counterparts. Many technical issues in practical implementations, such as absorbing boundary conditions, stability conditions, and Gauss's Laws, are discussed and justified. Moreover, two special conditions are proposed for the extended stencils in the vicinity of the dielectric material discontinuities. It was demonstrated that the accuracy of the fourth-order stencil is fully restored by applying these conditions.

Nonstandard and Higher-Order Finite-Difference Methods for Electromagnetics

With recent technological advancements, antenna elements have become smaller whereas the platforms they operate on, e.g., helicopter airframes, become electrically larger. These problems yield large computational domains and require significant computational resources. Traditional finite methods (FDTD and FEM) are second-order accurate thereby restricting the size of the domains that can be handled efficiently. We propose an approach which combines a subgridding technique with a higher-order scheme. FDTD subgridding techniques divide the simulation space into two separate grids; a fine one and a coarse one. The standard FDTD(2,2) is used to handle any of the fine features of the structure, whereas on the coarse grid FDTD(2,4), which is second-order accurate in time and fourth-order accurate in space, is used. Thus existing successfully-applied techniques in FDTD(2,2) are available for use on the fine grid. On the coarse mesh, away from phenomena associated with the complex structure, FDTD(2,4) is used mainly to simulate wave propagation in homogeneous media. With this approach, high accuracy is obtained both around fine geometric features, such as thin wires, thin slots, etc., as well as in the wave propagation.

Constantine A. Balanis

Papers

Coupling between wire antennas: analytic solution, FDTD, and measurements

Impact of smart antenna designs on network throughput and BER

Finite-difference time-domain of HF antennas

Nonstandard and Higher-Order Finite-Difference Methods for Electromagnetics

A hybrid method of FDTD(2,4) and subgrid FDTD(2,2) for modeling of coupling