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].

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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 FDTD method with the Berenger perfectly matched layer (PML) absorbing boundary conditions (ADCs) is used to analyze the radiation characteristics of an HF antenna on a metallic airframe of the AH-64 Apache helicopter. The antenna of interest is an experimental 14' "towel bar" used in the 2-30 MHz band. A companion 24' HF antenna is mounted on the opposite side of the helicopter tail boom. The FDTD predictions compare well with experimental results from an operational Apache helicopter and with measured data obtained using an approximate 10:1 scale model of the Apache.

FDTD analysis of Apache helicopter HF antennas

The exact solution for the interior impedance wedge was evaluation asymptotically to yield a geometrical theory of diffraction (GTD). The geometrical-optics terms correspond identically to simple ray-tracing results. The diffraction field is analogous to the perfectly conduction case with suitable multiplying factors to account for the lossy reflections. The surface-wave contribution and its associated transition field are included to account for complex poles of the auxiliary Maliuzhinets function. Numerical integration using an adaptive quadrature routine was used to verify the accuracy of the technique. The analysis of the interior wedge geometry extends the work of M.I. Herman and J.L. Volakis (1987) to allow the study of complex structures which may include many multiple reflections and diffractions within interior wedges. >

Reflections, diffractions, and surface waves for an interior wedge with impedance surfaces

The CPFDTD (contour-path finite-difference time-domain) method was used for analysis of pyramidal horn antennas with or without composite E-plane walls. For the metallic horn very accurate antenna gain patterns were computed compared to measurements. When the E-plane walls are coated with lossy magnetic material, for gain pattern control, the CPFDTD yields acceptable gain patterns compared to measurements. >

Contour path FDTD method for pyramidal horns with composite E-plane walls

A low-profile loop antenna positioned above a hybrid circular (HC) surface is presented. The HC consists of two different metallic rings, which are combined in a single ground plane to improve the radiation performance of the radiating element. The integration of the loop antenna with the HC surface leads to a directivity of 11 dB and a fractional bandwidth of 14%.

High Directive Loop Antenna for Low-Profile Applications

During this research period, we have effectively transferred existing computer codes from CRAY supercomputer to work station based systems. The work station based version of our code preserved the accuracy of the numerical computations while giving a much better turn-around time than the CRAY supercomputer. Such a task relieved us of the heavy dependence of the supercomputer account budget and made codes developed in this research project more feasible for applications. The analysis of pyramidal horns with impedance surfaces was our major focus during this research period. Three different modeling algorithms in analyzing lossy impedance surfaces were investigated and compared with measured data. Through this investigation, we discovered that a hybrid Fourier transform technique, which uses the eigen mode in the stepped waveguide section and the Fourier transformed field distributions across the stepped discontinuities for lossy impedances coating, gives a better accuracy in analyzing lossy coatings. After a further refinement of the present technique, we will perform an accurate radiation pattern synthesis in the coming reporting period.

/pdf/antenna-pattern-control-using-impedance-surfaces-4kpnk7js2o.pdf

Constantine A. Balanis

Papers

FDTD analysis of Apache helicopter HF antennas

Reflections, diffractions, and surface waves for an interior wedge with impedance surfaces

Contour path FDTD method for pyramidal horns with composite E-plane walls

High Directive Loop Antenna for Low-Profile Applications

Antenna pattern control using impedance surfaces