Matching Items (4)
Description
Global Positioning System (GPS) is a navigation system widely used in civilian and military application, but its accuracy is highly impacted with consequential fading, and possible loss of communication due to multipath propagation and high power interferences. This dissertation proposes alternatives to improve the performance of the GPS receivers to obtain a system that can be reliable in critical situations. The basic performance of the GPS receiver consists of receiving the signal with an antenna array, delaying the signal at each antenna element, weighting the delayed replicas, and finally, combining the weighted replicas to estimate the desired signal. Based on these, three modifications are proposed to improve the performance of the system. The first proposed modification is the use of the Least Mean Squares (LMS) algorithm with two variations to decrease the convergence time of the classic LMS while achieving good system stability. The results obtained by the proposed LMS demonstrate that the algorithm can achieve the same stability as the classic LMS using a small step size, and its convergence rate is better than the classic LMS using a large step size. The second proposed modification is to replace the uniform distribution of the time delays (or taps) by an exponential distribution that decreases the bit-error rate (BER) of the system without impacting the computational efficiency of the uniform taps. The results show that, for a BER of 0.001, the system can operate with a 1 to 2 dB lower signal-to-noise ratio (SNR) when an exponential distribution is used rather than a uniform distribution. Finally, the third modification is implemented in the design of the antenna array. In this case, the gain of each microstrip element is enhanced by embedding ferrite rings in the substrate, creating a hybrid substrate. The ferrite rings generates constructive interference between the incident and reflected fields; consequently, the gain of a single microstrip element is enhanced by up to 4 dB. When hybrid substrates are used in microstrip element arrays, a significant enhancement in angle range is achieved for a given reflection coefficient compared to using a conventional substrate.
ContributorsRivera-Albino, Alix (Author) / Balanis, Constantine A (Thesis advisor) / Tepedelenlioğlu, Cihan (Committee member) / Kiaei, Sayfe (Committee member) / Aberle, James T (Committee member) / Arizona State University (Publisher)
Created2013
Description
Satellite communications employs circular polarization (CP) to circumvent thewell-known phenomenon known as Faraday Rotation, where the ionosphere rotates the
horizontal and vertical polarization components resulting in signal degradation especially
at lower frequencies, i.e., VHF and L-band, and in tropical regions of the earth. Satellite circularly polarized antenna feed technology commonly employs bulkyand lossy 90-degree hybrid combiners to convert linear polarization to circular
polarization, which results in a higher noise figure for receive applications and a less
repeatable and more difficult design to tune and manufacture. This thesis aims at designing, modeling and simulating a prototype S/X dual bandCP feed/polarizer utilizing a technique known as the “Spread-Squeeze” polarizer, which
offers the advantages of compact size, ease of manufacture, and lower loss and noise
figure, relative to the current technology that often employs an external 3-dB hybrid
combiner. Ansys High Frequency Structure Simulator (HFSS), a commercial
electromagnetic modeling and simulation tool, is used for the simulations. Further, this thesis aims to characterize the performance of the dual feed hornwith respect to aperture efficiency, that is, the degree to which the feed horn illuminates
the parabolic reflector.
ContributorsCowan, Brad (Author) / Aberle, James (Thesis advisor) / Pan, George (Committee member) / Trichopoulos, Georgios (Committee member) / Arizona State University (Publisher)
Created2021
Description
Impedance-modulated metasurfaces are compact artificially-engineered surfaces whose surface-impedance profile is modulated with a periodic function. These metasurfaces function as leaky-wave antennas (LWAs) that are capable of achieving high gains and narrow beamwidths with thin and light-weight structures. The surface-impedance modulation function for the desired radiation characteristics can be obtained using the holographic principle, whose application in antennas has been investigated extensively.
On account of their radiation and physical characteristics, modulated metasurfaces can be employed in automotive radar, 5G, and imaging applications. Automotive radar applications might require the antennas to be flush-mounted on the vehicular bodies that can be curved. Hence, it is necessary to analyze and design conformal metasurface antennas. The surface-impedance modulation function is derived for cylindrically-curved metasurfaces, where the impedance modulation is along the cylinder axis. These metasurface antennas are referred to as axially-modulated cylindrical metasurface LWAs (AMCLWAs). The effect of curvature is modeled, the radiation characteristics are predicted analytically, and they are validated by simulations and measurements.
Communication-based applications, like 5G and 6G, require the generation of multiple beams with polarization diversity, which can be achieved using a class of impedance-modulated metasurfaces referred to as polarization-diverse holographic metasurfaces (PDHMs). PDHMs can form, one at a time, a pencil beam in the desired direction with horizontal polarization, vertical polarization, left-hand circular polarization (LHCP), or right-hand circular polarization (RHCP). These metasurface antennas are analyzed, designed, measured, and improved to include the ability to frequency scan.
In automotive radar and other imaging applications, the performance of metasurface antennas can be impacted by the formation of standing waves due to multiple reflections between the antenna and the target. The monostatic RCS of the metasurface antenna is reduced by modulating its surface impedance with a square wave, to avert multiple reflections. These square-wave-modulated metasurfaces are referred to as checkerboard metasurface LWAs, whose radiation and scattering characteristics, for normal incidence parallel polarization, are analyzed and measured.
On account of their radiation and physical characteristics, modulated metasurfaces can be employed in automotive radar, 5G, and imaging applications. Automotive radar applications might require the antennas to be flush-mounted on the vehicular bodies that can be curved. Hence, it is necessary to analyze and design conformal metasurface antennas. The surface-impedance modulation function is derived for cylindrically-curved metasurfaces, where the impedance modulation is along the cylinder axis. These metasurface antennas are referred to as axially-modulated cylindrical metasurface LWAs (AMCLWAs). The effect of curvature is modeled, the radiation characteristics are predicted analytically, and they are validated by simulations and measurements.
Communication-based applications, like 5G and 6G, require the generation of multiple beams with polarization diversity, which can be achieved using a class of impedance-modulated metasurfaces referred to as polarization-diverse holographic metasurfaces (PDHMs). PDHMs can form, one at a time, a pencil beam in the desired direction with horizontal polarization, vertical polarization, left-hand circular polarization (LHCP), or right-hand circular polarization (RHCP). These metasurface antennas are analyzed, designed, measured, and improved to include the ability to frequency scan.
In automotive radar and other imaging applications, the performance of metasurface antennas can be impacted by the formation of standing waves due to multiple reflections between the antenna and the target. The monostatic RCS of the metasurface antenna is reduced by modulating its surface impedance with a square wave, to avert multiple reflections. These square-wave-modulated metasurfaces are referred to as checkerboard metasurface LWAs, whose radiation and scattering characteristics, for normal incidence parallel polarization, are analyzed and measured.
ContributorsRamalingam, Subramanian (Author) / Balanis, Constantine A. (Thesis advisor) / Aberle, James T. (Committee member) / Palais, Joseph C. (Committee member) / Trichopoulos, Georgios C. (Committee member) / Arizona State University (Publisher)
Created2020
Description
This work focuses on the analysis and design of large-scale millimeter-wave andterahertz (mmWave/THz) beamforming apertures (e.g., reconfigurable reflective surfaces–
RRSs). As such, the small wavelengths and ample bandwidths of these frequencies enable
the development of high-spatial-resolution imaging and high-throughput wireless
communication systems that leverage electrically large apertures to form high-gain
steerable beams.
For the rigorous evaluation of these systems’ performance in realistic application
scenarios, full-wave simulations are needed to capture all the exhibited electromagnetic
phenomena. However, the small wavelengths of mmWave/THz bands lead to enormous
meshes in conventional full-wave simulators. Thus, a novel numerical decomposition
technique is presented, which decomposes the full-wave models in smaller domains with
less meshed elements, enabling their computationally efficient analysis. Thereafter, this
method is leveraged to study a novel radar configuration that employs a rotating linear
antenna with beam steering capabilities to form 3D images. This imaging process requires
fewer elements to carry out high-spatial-resolution imaging compared to traditional 2D
phased arrays, constituting a perfect candidate in low-profile, low-cost applications.
Afterward, a high-yield nanofabrication technique for mmWave/THz graphene
switches is presented. The measured graphene sheet impedances are incorporated into
equivalent circuit models of coplanar switches to identify the optimum mmWave/THz
switch topology that would enable the development of large-scale RRSs.ii
Thereon, the process of integrating the optimized graphene switches into largescale mmWave/THz RRSs is detailed. The resulting RRSs enable dynamic beam steering
achieving 4-bits of phase quantization –for the first time in the known literature–
eliminating the parasitic lobes and increasing the aperture efficiency. Furthermore, the
devised multi-bit configurations use a single switch-per-bit topology retaining low system
complexity and RF losses. Finally, single-bit RRSs are modified to offer single-lobe
patterns by employing a surface randomization technique. This approach allows for the use
of low-complexity single-bit configurations to suppress the undesired quantization lobes
without residing to the use of sophisticated multi-bit topologies.
The presented concepts pave the road toward the implementation and proliferation
of large-scale reconfigurable beamforming apertures that can serve both as mmWave/THz
imagers and as relays or base stations in future wireless communication applications.
ContributorsTheofanopoulos, Panagiotis (Author) / Trichopoulos, Georgios (Thesis advisor) / Balanis, Constantine (Committee member) / Aberle, James (Committee member) / Bliss, Dan (Committee member) / Groppi, Christopher (Committee member) / Arizona State University (Publisher)
Created2021