Filippo Capolino

Millimeter-wave massive MIMO: the next wireless revolution?

The combination of millimeter-wave communications, arrays with a massive number of antennas, and small cell geometries is a symbiotic convergence of technologies that has the potential to dramatically improve wireless access and throughput. This article outlines the benefits, challenges, and potential solutions associated with cellular networks that incorporate these technologies.

Array thinning by using antennas in a Fabry-Perot cavity for gain enhancement

A Fabry–Perot cavity (FPC) between a ground plane and a partially reflective surface (PRS) is used here to design array antennas with large distance between the radiating elements. This configuration provides some advantages: i) a reduction of the number of array elements to achieve high directivity; ii) large space between contiguous elements that may host a bulky feeding network as required for dual polarization or active antennas; iii) small coupling and easy feeding network designs because of the smaller number of elements with larger inter-element distance. We show that when designing the FPC antenna a frequency shift of the gain maximum may occur, especially in this sparse array configuration. We also show the existence of preferred distances between elements that controls both the directivity and the side lobe level, and how the presence of the FPC and the relaxed requirement of the interelement distance result in a lower interelement coupling. The presented dual polarized antenna comprises two interleaved 2 x 2 arrays placed in a 2-layer FPC, and exhibits a 19 dBi gain and 30 dB of isolation between the two ports over an operating bandwidth of approximately 5.7%, i.e., typical for patch antennas.

Analysis of directive radiation from a line source in a metamaterial slab with low permittivity

In this paper an investigation is presented of metamaterial structures excited by a line source aimed at producing narrow directive beams. The structure under consideration is a grounded slab made of a homogeneous metamaterial medium with a plasma-like dispersive permittivity; for low values of the slab permittivity an extremely directive beam pointing at broadside can be obtained. Conditions for the maximization of radiation at broadside are given and the narrow-beam effect is shown to be related to the excitation of a leaky mode supported by the slab, with radiation maximization corresponding to small and equal values of the phase and attenuation constants. The frequency bandwidth and directivity are expressed in a simple closed form in terms of the attenuation constant of the leaky mode. By increasing the slab height for a fixed frequency, the leaky mode is analytically shown to give rise to a beam that is scanned from broadside to the critical angle for plane-wave refraction, thus being confined to a narrow angular region around broadside. Numerical results are given that illustrate these features, and full-wave simulations of a metamaterial structure made of an array of metallic cylinders are presented that confirm the results of the analytical study. The case of a line source inside a semi-infinite metamaterial region is also considered and its radiation characteristics compared with those of the metamaterial slab

Graphene-based tunable hyperbolic metamaterials and enhanced near-field absorption

We explore the near-field radiative thermal energy transfer properties of hyperbolic metamaterials. The presence of unique electromagnetic states in a broad bandwidth leads to super-planckian thermal energy transfer between metamaterials separated by a nano-gap. We consider practical phonon-polaritonic metamaterials for thermal engineering in the mid-infrared range and show that the effect exists in spite of the losses, absorption and finite unit cell size. For thermophotovoltaic energy conversion applications requiring energy transfer in the near-infrared range we introduce high temperature hyperbolic metamaterials based on plasmonic materials with a high melting point. Our work paves the way for practical high temperature radiative thermal energy transfer applications of hyperbolic metamaterials.We investigate a novel implementation of hyperbolic metamaterial (HM) at far-infrared frequencies composed of stacked graphene sheets separated by thin dielectric layers. Using the surface conductivity model of graphene, we derive the homogenization formula for the multilayer structure by treating graphene sheets as lumped layers with complex admittances. Homogenization results and limits are investigated by comparison with a transfer matrix formulation for the HM constituent layers. We show that infrared iso-frequency wavevector dispersion characteristics of the proposed HM can be tuned by varying the chemical potential of the graphene sheets via electrostatic biasing. Accordingly, reflection and transmission properties for a film made of graphene-dielectric multilayer are tunable at terahertz frequencies, and we investigate the limits in using the homogenized model compared to the more accurate transfer matrix model. We also propose to use graphene-based HM as a super absorber for near-fields generated at its surface. The power emitted by a dipole near the surface of a graphene-based HM is increased dramatically (up to 5 × 10(2) at 2 THz), furthermore we show that most of the scattered power is directed into the HM. The validity and limits of the homogenized HM model are assessed also for near-fields and show that in certain conditions it overestimates the dipole radiated power into the HM.

Efficient computation of the 2-D Green's function for 1-D periodic structures using the Ewald method

The Ewald method is applied to accelerate the evaluation of the Greens function of an infinite periodic phased array of line sources. The Ewald representation for a cylindrical wave is obtained from the known representation for the spherical wave, and a systematic general procedure is applied to extend previous results. Only a few terms are needed to evaluate Ewald sums, which are cast in terms of error functions and exponential integrals, to high accuracy. Singularities and convergence rates are analyzed, and a recipe for selecting the Ewald splitting parameter /spl epsiv/ is given to handle both low and high frequency ranges. Indeed, it is shown analytically that the choice of the standard optimal splitting parameter /spl epsiv//sub 0/ will cause overflow errors at high frequencies. Numerical examples illustrate the results and the sensitivity of the Ewald representation to the splitting parameter /spl epsiv/.

Frequency-domain Green's function for a planar periodic semi-infinite phased array .I. Truncated floquet wave formulation

This two-part sequence deals with the derivation and physical interpretation of a uniform high-frequency solution for the field radiated at finite distance by a planar semi-infinite phased array of parallel elementary electric dipoles. The field obtained by direct summation over the contributions from the individual radiators is restructured into a double series of wavenumber spectral integrals whose asymptotic reduction yields a series encompassing propagating and evanescent Floquet waves (FWs) together with corresponding diffracted rays, which arise from scattering of the FW at the edge of the array. The formal aspects of the solution are treated in the present paper. They involve a sequence of manipulations in the complex spectral wavenumber planes that prepare the integrands for subsequent efficient and physically incisive asymptotics based on the method of steepest descent. Different species of spectral poles define the various species of propagating and evanescent FW. Their interception by the steepest descent path (SDP) determines the variety of shadow boundaries for the edge truncated FW. The uniform asymptotic reduction of the SDP integrals, performed by the Van der Waerden (1951) procedure and yielding a variety of edge-diffracted fields, completes the formal treatment.

Low Profile Fully Planar Folded Dipole Antenna on a High Impedance Surface

A fully planar antenna design incorporating a high impedance surface (HIS) is presented. The HIS is composed by a periodic array of subwavelength dogbone-shaped conductors printed on top of a thin dielectric substrate and backed by a metallic ground plane. First, the characteristics of a dipole over PEC or PMC layers, a dielectric slab, and the HIS are compared and studied in detail, highlighting the advantages provided by the use of the HIS. Then, the design of a low profile folded dipole antenna working at 5.5 GHz on top of the HIS is described. The surface provides close to 6% antenna impedance bandwidth and increased gain up to 7 dBi, while shielding the lower half space from radiation. The antenna structure comprises three metal layers without any vias between them, and its overall thickness is 0.059λ0. The dipole is fed by a balanced twin lead line through a balun transformer integrated in the same antenna layer. A prototype has been built and measurements confirming simulation results are provided.

Fundamental properties of the field at the interface between air and a periodic artificial material excited by a line source

An efficient algorithm based on a moment-method formulation is presented for the evaluation of the field produced by a line source at the interface between an air superstrate and a one-dimensional-periodic artificial-material slab. The formulation provides physical insight into the nature of the fields via path deformation in the complex wavenumber plane. From an asymptotic analysis in the complex wavenumber plane it is found that the space wave produced by a line source consists of an infinite number of space harmonics that decay algebraically as x/sup -3/2/. Guided modes may also exist and be excited, including leaky modes.

The Fundamental Physics of Directive Beaming at Microwave and Optical Frequencies and the Role of Leaky Waves

This review paper summarizes various aspects of directive beaming and explains these aspects in terms of leaky waves. Directive beaming occurs in antenna design where a narrow beam is obtainable by using fairly simple planar structures excited by a single source. These structures include Fabry-Pérot cavity structures as well as metamaterial structures made from artificial low-permittivity media. Directive beaming also occurs in the optical area where it has been observed that highly directive beams can be produced from small apertures in a metal film when an appropriate periodic patterning is placed on the film. One aspect that these phenomena all have in common is that they are due to the excitation of one or more weakly attenuated leaky waves, the radiation from which forms the directive beam. This is established in each case by examining the role of the leaky waves in determining the near-field on the aperture of the structure and the far-field radiation pattern of the structure.

Efficient computation of the 3D Green's function for the Helmholtz operator for a linear array of point sources using the Ewald method

The Ewald method is applied to accelerate the evaluation of the Green’s function (GF) of an infinite equispaced linear array of point sources with linear phasing. Only a few terms are needed to evaluate Ewald sums, which are cast in terms of error functions and exponential integrals, to high accuracy. It is shown analytically that the choice of the standard ‘‘optimal’’ Ewald splitting parameter E0 causes overflow errors at high frequencies (period large compared to the wavelength), and convergence rates are analyzed. A recipe for selecting the Ewald splitting parameter is provided.