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.

/pdf/high-impedance-electromagnetic-surfaces-with-a-forbidden-3vtu72a5an.pdf

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

Part A of this report examines several high-frequency models fornonprincipal-plane scattering from a rectangular, perfectly conductingplate. Two methods, the Method of Equivalent Currents and cornerdiffraction coefficients, are considered. Formulations forsecond-order Physical Theory of Diffraction equivalent currents andfor corner diffracted fields are presented. Comparisons are madeamong the following plate models: first-order Physical Opticsequivalent currents, first-order Geometrical Theory of Diffractionequivalent currents, first-order Physical Optics/Physical Theory ofDiffraction equivalent currents, second-order Physical Theory ofDiffraction equivalent currents, corner diffraction coefficients,Moment Method, and experimental results. Results away from grazingare accurate using only first-order terms. Near grazing, second-orderand corner diffraction terms improve the results for many cases.Part B of the report investigates the pattern control of hornantennas using lossy materials to coat the inner walls of the horn.Integral Equation and Moment Method techniques are used to formulatethe problem. It is clearly demonstrated that side lobe levelreduction can be achieved using impedance surfaces on the inner wallsof the horn.-i-

Part a _

The authors present simplified, computationally faster, versions of the dielectric-wedge UTD (uniform geometrical theory of diffraction) coefficients for the case of a wedge with one perfectly conducting face. Expressions for several scattering configurations are considered. Included are plane-wave incidence, far-field observation; cylindrical-wave incidence from a finite distance (/spl rho/'), far-field observation; plane-wave incidence, observation at a finite distance (/spl rho/); and surface-wave scattering with the appropriate transition terms. These variations allow one to incorporate higher-order diffraction terms into the analysis of typical, practical target geometries such as a flat plate or dihedral corner reflector. A model for predicting the RCS (radar cross section) of a flat plate coated with an electrically thin, lossy dielectric using the UTD coefficients for a coated half plane is presented. >

Scattering from coated geometries

An approach is proposed which can be used in lieu of lossy, full-wave solutions to provide accurate and efficient data for the CAD (computer-aided design) of multilayer, multiconductor MIC and MMIC (monolithic microwave integrated circuits) structures. This application gives results that are as accurate as lossy full-wave techniques over a wide range of frequency, including the dispersive region. In addition to giving accurate results, this method is up to three times faster, depending on the number and type of substrates or superstrates. Results are shown for various symmetric and asymmetric, multiconductor, multilayer structures which have good agreement with the lossy, fully-wave approach and use significantly less computer time. >

Fast and accurate computation of dielectric losses in multi-layer, multi-conductor microstrip structures

This paper develops design guidelines, based on a nonuniform applied magnetic field and nonuniform magnetic field internal to the ferrite material, for ferrite-loaded circular cross-section CBS antenna that improves the gain and the tunable bandwidth by judiciously shaping and positioning of the ferrite specimen within the cavity.

Gain and bandwidth enhancement of ferrite-loaded circular cross-section CBS antenna

In VHF and UHF bands, antenna patterns for short range airborne communications are required to be omni-directional. Contributions from reflections and diffractions from the airframe usually distort the primary radiation patterns of antennas mounted on airframes. Moreover, these diffractions and reflections become increasingly important as the frequency of operation increases. Therefore, for this type of application, an antenna performance simulated without the airframe is not an accurate representation. In this study, an accurate and versatile finite element method (FEM) based on the discretization of Helmholtz's equation has been developed to analyze VHF and UHF antennas mounted on helicopter airframes, parts of which may include composite material and anisotropies. The FEM code was implemented with linear edge-based tetrahedral elements and first/second-order Absorbing Boundary Conditions. Possible material anisotropies in the computational domain are accounted for by using a permittivity and a permeability tensor. Radiation characteristics are predicted for absolute gain, spatial field distribution, input impedance and return loss, and then compared with measurements for verification.

Constantine A. Balanis

Papers

Part a _

Scattering from coated geometries

Fast and accurate computation of dielectric losses in multi-layer, multi-conductor microstrip structures

Gain and bandwidth enhancement of ferrite-loaded circular cross-section CBS antenna

Finite element analysis of VHF/UHF antennas on helicopter airframes