222 research outputs found
Super-Directive Antenna Arrays: How Many Elements Do We Need?
Super-directive antenna arrays have faced challenges in achieving high
realized gains ever since their introduction in the academic literature. The
primary challenges are high impedance mismatches and resistive losses, which
become increasingly more dominant as the number of elements increases.
Consequently, a critical limitation arises in determining the maximum number of
elements that should be utilized to achieve super-directivity, particularly
within dense array configurations. This paper addresses precisely this issue
through an optimization study to design a super-directive antenna array with a
maximum number of elements. An iterative approach is employed to increase the
array of elements while sustaining a satisfactory realized gain using the
differential evolution (DE) algorithm. Thus, it is observed that
super-directivity can be obtained in an array with a maximum of five elements.
Our results indicate that the obtained unit array has a higher
realized gain than a uniform linear array with conventional excitation. For
these reasons, these results make the proposed architecture a strong candidate
for applications that require densely packed arrays, particularly in the
context of massive multiple-input multiple-output (MIMO)
Analysis of a Waveguide-Fed Metasurface Antenna
The metasurface concept has emerged as an advantageous reconfigurable antenna
architecture for beam forming and wavefront shaping, with applications that
include satellite and terrestrial communications, radar, imaging, and wireless
power transfer. The metasurface antenna consists of an array of metamaterial
elements distributed over an electrically large structure, each subwavelength
in dimension and with subwavelength separation between elements. In the antenna
configuration we consider here, the metasurface is excited by the fields from
an attached waveguide. Each metamaterial element can be modeled as a
polarizable dipole that couples the waveguide mode to radiation modes. Distinct
from the phased array and electronically scanned antenna (ESA) architectures, a
dynamic metasurface antenna does not require active phase shifters and
amplifiers, but rather achieves reconfigurability by shifting the resonance
frequency of each individual metamaterial element. Here we derive the basic
properties of a one-dimensional waveguide-fed metasurface antenna in the
approximation that the metamaterial elements do not perturb the waveguide mode
and are non-interacting. We derive analytical approximations for the array
factors of the 1D antenna, including the effective polarizabilities needed for
amplitude-only, phase-only, and binary constraints. Using full-wave numerical
simulations, we confirm the analysis, modeling waveguides with slots or
complementary metamaterial elements patterned into one of the surfaces.Comment: Original manuscript as submitted to Physical Review Applied (2017).
14 pages, 14 figure
Computational polarimetric microwave imaging
We propose a polarimetric microwave imaging technique that exploits recent
advances in computational imaging. We utilize a frequency-diverse cavity-backed
metasurface, allowing us to demonstrate high-resolution polarimetric imaging
using a single transceiver and frequency sweep over the operational microwave
bandwidth. The frequency-diverse metasurface imager greatly simplifies the
system architecture compared with active arrays and other conventional
microwave imaging approaches. We further develop the theoretical framework for
computational polarimetric imaging and validate the approach experimentally
using a multi-modal leaky cavity. The scalar approximation for the interaction
between the radiated waves and the target---often applied in microwave
computational imaging schemes---is thus extended to retrieve the susceptibility
tensors, and hence providing additional information about the targets.
Computational polarimetry has relevance for existing systems in the field that
extract polarimetric imagery, and particular for ground observation. A growing
number of short-range microwave imaging applications can also notably benefit
from computational polarimetry, particularly for imaging objects that are
difficult to reconstruct when assuming scalar estimations.Comment: 17 pages, 15 figure
The Road to 6G: Ten Physical Layer Challenges for Communications Engineers
While the deployment of 5G cellular systems will continue well in to the next
decade, much interest is already being generated towards technologies that will
underlie its successor, 6G. Undeniably, 5G will have transformative impact on
the way we live and communicate, yet, it is still far away from supporting the
Internet-of-Everything (IoE), where upwards of a million devices per
(both terrestrial and aerial) will require ubiquitous,
reliable, low-latency connectivity. This article looks at some of the
fundamental problems that pertain to key physical layer enablers for 6G. This
includes highlighting challenges related to intelligent reflecting surfaces,
cell-free massive MIMO and THz communications. Our analysis covers theoretical
modeling challenges, hardware implementation issues and scalability among
others. The article concludes by delineating the critical role of signal
processing in the new era for wireless communications.Comment: IEEE Communications Magazine, Accepte
Computational Microwave Imaging Using 3D Printed Conductive Polymer Frequency-Diverse Metasurface Antennas
A frequency-diverse computational imaging system synthesized using
three-dimensional (3D) printed frequency-diverse metasurface antennas is
demonstrated. The 3D fabrication of the antennas is achieved using a
combination of PolyLactic Acid (PLA) polymer material and conductive polymer
material (Electrifi), circumventing the requirement for expensive and
time-consuming conventional fabrication techniques, such as machine milling,
photolithography and laser-etching. Using the 3D printed frequency- diverse
metasurface antennas, a composite aperture is designed and simulated for
imaging in the K-band frequency regime (17.5-26.5 GHz). The frequency-diverse
system is capable of imaging by means of a simple frequency-sweep in an-all
electronic manner, avoiding mechanical scanning and active circuit components.
Using the synthesized system, microwave imaging of objects is achieved at the
diffraction limit. It is also demonstrated that the conductivity of the
Electrifi polymer material significantly affects the performance of the 3D
printed antennas and therefore is a critical factor governing the fidelity of
the reconstructed images.Comment: Original manuscript as submitted to IET Microwaves, Antennas &
Propagation (2017). 17 pages, 8 figure
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