Ephrem T. Bekele

4-D Arrays as Enabling Technology for Cognitive Radio Systems

Time-modulation (TM) in four-dimensional (4-D) arrays is implemented by using a set of radio-frequency switches in the beam forming network to modulate, by means of periodic pulse sequences, the static excitations and thus control the antenna radiation features. The on-off reconfiguration of the switches, that can be easily implemented via software, unavoidably generates harmonic radiations that can be suitably exploited for multiple channel communication purposes. As a matter of fact, harmonic beams can be synthesized having different spatial distribution and shapes in order to receive signals arriving on the antenna from different directions. Similarly, the capability to generate a field having different frequency and spatial distribution implies that the signal transmitted by time-modulated 4-D arrays is direction-dependent. Accordingly, such a feature is also exploited to implement a secure communication scheme directly at the physical layer. Thanks to the easy software-based reconfigurability, the multiple harmonic beamforming, and the security capability, 4-D arrays can be considered as an enabling technology for future cognitive radio systems. In this paper, these potentialities of time-modulated 4-D arrays are presented and their effectiveness is supported by a set of representative numerical simulation results.

Sparsening Conformal Arrays Through a Versatile

Sparsening conformal arrangements is carried out through a versatile Multi-Task Bayesian Compressive Sensing (MT-BCS) strategy. The problem, formulated in a probabilistic fashion as a pattern-matching synthesis, is that of determining the sparsest excitation set (locations and weights) fitting a reference pattern subject to user-defined geometrical constraints. Results from a set of representative numerical experiments are presented to illustrate the key-features of the proposed approach as well as to assess, also through comparisons, its potentials in terms of matching accuracy, element saving, and computational costs.

BCS

Time-modulated arrays (TMAs) are antenna systems where the transmitted or received signal is modulated by periodic time pulses. Several strategies have been proposed for defining the characteristics of the pulse sequence to control the antenna radiation pattern starting from the hypothesis of ideal radio-frequency (RF) switches with instantaneous rise and fall time. In this paper, the use of switches having a non-null transition between the on and off state is taken into account and the impact on the radiation features is studied and verified through a set of numerical results also in comparative fashion with ordinary rectangular pulses.

-Based Method

The use of inhomogeneous metamaterial lenses is proposed to enable suitable radiation properties for arbitrary-shape antenna arrays. Towards this end, the Quasi-Conformal Transformation Optics (QCTO) methodology is generalized to allow an arbitrary physical arrangement coated with a suitable lens to exhibit the same radiating features of an arbitrary reference virtual array in free space. A representative numerical example, concerned with a two-dimensional layout, is presented to assess the effectiveness of the proposed method as well as the enhanced features of the resulting metamaterial-coated arrays with respect to standard conformal arrangements.

Pulse-Shaping Strategy for Time Modulated Arrays—Analysis and Design

The array miniaturization problem is addressed by means of a material-by-design approach. More specifically, an innovative strategy that integrates a quasi-conformal transformation optics (QCTO) technique and a source inversion method is proposed to design radome-coated arrays exhibiting the same radiation properties of wider virtual arrangements comprising more antenna elements. Toward this end, the state-of-the-art QCTO theory is generalized to account for source constraints within the synthesis process. Representative numerical examples are provided to assess the reliability, the flexibility, and the effectiveness of the proposed synthesis approach as well as the possibility to realize suboptimal radomes with simplified, but cheaper/easier, structures (e.g., structures based on tiles of isotropic dielectrics).

Generalized QCTO for Metamaterial-Lens-Coated Conformal Arrays

Within the transformation electromagnetics (TE) framework, an innovative approach, based on the System-by-Design paradigm, is proposed for the synthesis of isotropic nonmagnetic metamaterial lenses. Selected numerical results, concerned with an application of the SbD-QCTO approach, are reported to give some insights on its advantages and current limitations in terms of computational efficiency, effectiveness, and flexibility.

Array Miniaturization Through QCTO-SI Metamaterial Radomes

In this paper, the design of metamaterial-enhanced antenna arrays is carried out exploiting a quasi-conformal coordinate transformation approach. More specifically, the transformation of a physically linear arrangement into a virtual curved array with a larger size is considered by means of an isotropic inhomogeneous lens comprising non-magnetic materials. The effectiveness of the considered approach for the enhancement of the scanning and beamwidth performance of linear arrangements is preliminarily assessed by means of numerical simulations.

An SbD-QCTO Approach to the Synthesis of Isotropic Metamaterial Lenses

The synthesis of compact metamaterial-loaded sectoral or conical horn antennas matching the radiation properties of larger layouts is addressed. Unlike state-of-the-art strategies, the metamaterial lens that fills the miniaturized horn is designed so that the radiated field coincides with that of the reference arrangement both in amplitude and phase. To this end, the synthesis is first recast as a 3-D transformation electromagnetics (TE) problem, then mapped into an equivalent 2-D one. This latter is numerically solved by means of a recently introduced 2-D quasi-conformal TE algorithm. Selected examples from a wide numerical validation and representative comparisons with state-of-the-art methodologies are illustrated.

Design and synthesis of innovative metamaterial-enhanced arrays

Evolutionary algorithms (EAs) have had a widespread diffusion and have demonstrated to be very effective techniques for the design of antenna arrays. However, the main limitations are the high computational burden and low convergence speed which stimulated researchers towards the development of even more effective approaches or the definition of suitable optimization frameworks. Two innovative EA-based method for the synthesis of large thinned arrays and of compromise sum-difference sub-arrayed antennas are presented and discussed in this work where the flexibility and performance of EAs have been exploited to obtain effective and efficient designs.

Transformation Electromagnetics Miniaturization of Sectoral and Conical Metamaterial-Enhanced Horn Antennas

Quasi Conformal Transformation Optics (QCTO) is applied in innovative antenna array enhancement applications. The QCTO is implemented numerically. Examples provided include miniaturization of a linear array using isotropic metamaterial lens and transformation of an arbitrary shaped conformal array to a well known (for example circular) array. A full-wave electromagnetic simulation is used to validate the approach.