Enhanced Effective Aperture Distribution Function for Characterizing Large-Scale Antenna Arrays

Accurate characterization of large-scale antenna arrays is growing in importance and complexity for the fifth-generation (5G) and beyond systems, as they feature more antenna elements and require increased overall performance. The full 3D patterns of all antenna elements in the array need to be characterized because they are in general different due to construction inaccuracy, coupling, antenna array's asymmetry, etc. The effective aperture distribution function (EADF) can provide an analytic description of an antenna array based on a full-sphere measurement of the array in an anechoic chamber. However, as the array aperture increases, denser spatial samples are needed for EADF due to large distance offsets of array elements from the reference point in the anechoic chamber, leading to a prohibitive measurement time and increased complexity of EADF. In this paper, we present the EADF applied to large-scale arrays and highlight issues caused by the large array aperture. To overcome the issues, an enhanced EADF is proposed with a low complexity that is intrinsically determined by the characteristic of each array element rather than the array aperture. The enhanced EADF is validated using experimental measurements conducted at 27-30 GHz frequency band with a relatively large planar array

Forward and Inverse Modeling of GPS Multipath for Snow Monitoring

Snowpacks provide reservoirs of freshwater, storing solid precipitation and delaying runoff to be released later in the spring and summer when it is most needed. The goal of this dissertation is to develop the technique of GPS multipath reflectometry (GPS-MR) for ground-based measurement of snow depth. The phenomenon of multipath in GPS constitutes the reception of reflected signals in conjunction with the direct signal from a satellite. As these coherent direct and reflected signals go in and out of phase, signal-to-noise ratio (SNR) exhibits peaks and troughs that can be related to land surface characteristics. In contrast to other GPS reflectometry modes, in GPS-MR the poorly separated composite signal is collected utilizing a single antenna and correlated against a single replica. SNR observations derived from the newer L2-frequency civilian GPS signal (L2C) are used, as recorded by commercial off-the-shelf receivers and geodetic-quality antennas in existing GPS sites. I developed a forward/inverse approach for modeling GPS multipath present in SNR observations. The model here is unique in that it capitalizes on known information about the antenna response and the physics of surface scattering to aid in retrieving the unknown snow conditions in the antenna surroundings. This physically-based forward model is utilized to simulate the surface and antenna coupling. The statistically-rigorous inverse model is considered in two parts. Part I (theory) explains how the snow characteristics are parameterized; the observation/parameter sensitivity; inversion errors; and parameter uncertainty, which serves to indicate the sensing footprint where the reflection originates. Part II (practice) applies the multipath model to SNR observations and validates the resulting GPS retrievals against independent in situ measurements during a 1-3 year period in three different environments - grasslands, alpine, and forested. The assessment yields a correlation of 0.98 and an RMS error of 6-8 cm, with the GPS under-estimating in situ snow depth by approximately 15%. GPS daily site averages were found effective in mitigating random noise without unduly smoothing the sharp transitions as captured in new snow events. This work corroborates the readiness of quality-controlled GPS-MR for snow depth monitoring, reinforcing its maturity for operational usage

Engineering of intelligent reflecting surfaces: Reflection locality and angular stability

Reconfigurable intelligent surfaces (RISs) are electromagnetically passive controllable structures, deflecting the incident wave beam in directions predefined by the control signal. A usual way to design RIS based on metasurfaces (MSs) is based on the application of the approximation in which the reflective properties of a uniform MS are attributed to a unit cell of the non-uniform one. We call this approximation the reflection locality. In the present paper, we show that this approximation may result in heavy errors. We also find a condition under which this approximation is applicable for a wide range of incidence and deflection angles. This condition is the angular stability of the reflection phase of a uniform MS based on which the non-uniform one is generated. We present an approximate analytical proof of the equivalence of the reflection locality and angular stability. As an example, we report theoretical and experimental results we obtained for a binary RIS whose generic uniform analogue has the angular stability. Meanwhile, for its counterpart without angular stability (the so-called mushroom MS) the same model fails

Electromagnetic Waves

This book is dedicated to various aspects of electromagnetic wave theory and its applications in science and technology. The covered topics include the fundamental physics of electromagnetic waves, theory of electromagnetic wave propagation and scattering, methods of computational analysis, material characterization, electromagnetic properties of plasma, analysis and applications of periodic structures and waveguide components, and finally, the biological effects and medical applications of electromagnetic fields

NASA Tech Briefs, December 1990

Topics: New Product Ideas; NASA TU Services; Electronic Components and Circuits; Electronic Systems; Physical Sciences; Materials; Computer Programs; Mechanics; Machinery; Fabrication Technology; Mathematics and Information Sciences; Life Sciences

Enabling 5G Technologies

The increasing demand for connectivity and broadband wireless access is leading to the fifth generation (5G) of cellular networks. The overall scope of 5G is greater in client width and diversity than in previous generations, requiring substantial changes to network topologies and air interfaces. This divergence from existing network designs is prompting a massive growth in research, with the U.S. government alone investing $400 million in advanced wireless technologies. 5G is projected to enable the connectivity of 20 billion devices by 2020, and dominate such areas as vehicular networking and the Internet of Things. However, many challenges exist to enable large scale deployment and general adoption of the cellular industries. In this dissertation, we propose three new additions to the literature to further the progression 5G development. These additions approach 5G from top down and bottom up perspectives considering interference modeling and physical layer prototyping. Heterogeneous deployments are considered from a purely analytical perspective, modeling co-channel interference between and among both macrocell and femtocell tiers. We further enhance these models with parameterized directional antennas and integrate them into a novel mixed point process study of the network. At the air interface, we examine Software-Defined Radio (SDR) development of physical link level simulations. First, we introduce a new algorithm acceleration framework for MATLAB, enabling real-time and concurrent applications. Extensible beyond SDR alone, this dataflow framework can provide application speedup for stream-based or data dependent processing. Furthermore, using SDRs we develop a localization testbed for dense deployments of 5G smallcells. Providing real-time tracking of targets using foundational direction of arrival estimation techniques, including a new OFDM based correlation implementation

Periodic Frequency Selective Surfaces for Reduction of Specular Scatter in Indoor Applications

This thesis investigates the use of a variety of passive frequency selective surfaces for specular scatter reduction. Motivation from this work stems from the increased interest in controlling propagation in indoor environments. Influencing the propagation environment using both passive and active structures is of current research interest due to the increased use of wireless devices inside building structures. This thesis aims to develop surfaces suitable for installation on corridor walls to work alongside existing solutions. An initial literature review of frequency selective surfaces; particularly for use inside buildings to create smart environments, suggests reducing the propagation down corridors could be beneficial in decreasing co-channel interference although no solutions have been offered. Development of the initial comb frequency selective surface (CR-FSS) enabled measurement systems and simulation models to be constructed and compared. Due to the various limitations of the CR-FSS, design modifications and evolutions are investigated to overcome issues with poor angular performance, polarisation dependant performance, and experimental manufacture. The initial challenge was to create a rotationally symmetrical surface which could reduce specular scatter from additional angles of incidence in the elevation plane. A pin reflection FSS (PR-FSS) was created, however investigation of the structure showed that it was ineffectual for TE polarisation. In a multipath environment this could be an issue which effects performance. Investigation of additional variations of the CR-FSS such as the slanted comb FSS (SC-FSS) and crenelated CR-FSS complete the analysis. A validation of a frequency selective comb structures is conducted with in-building multipath simulations. Statistical plots show that a comb structure can be used to significantly improve the signal-to-interference ratio (SIR) of co-channel transmitters at 2.4 GHz by reducing propagation down a corridor

Broadband Microwave and Millimeter-wave Metasurfaces

Metasurfaces have shown unprecedented control over electromagnetic waves. Utilizing structures with dimensions much smaller than the wavelength, they eliminate the need for bulky metamaterials and provide novel functionalities not shown in conventional diffractive devices. Among metasurface architectures demonstrated to date, thin patterned metallic layers modeled as surface impedances have robust design methods, enabling arbitrary wave-front control in a deeply sub-wavelength thickness. However, these metasurfaces are often limited to very narrow-band operation. In metasurfaces, resonant particles made from patterned metallic layers provide control over the wavefront by spatial variation of their physical geometry. In transmissive operation, three layers of metallic particles provide complete control of electric and magnetic responses by which the reflected wave can be annihilated. The spectral response of each resonance is generally dispersive, therefore a rigorous method to engineer the dispersion is necessary to achieve the wavefront manipulation in a broader bandwidth. This thesis provide a systematic study on the bandwidth of metasurfaces by first investigating their performance limits and then proposing a synthesis method based on engineering of surface impedances. Broadband achromatic and dispersive metasurfaces are designed and realized in microwave and millimeter wave regimes, and their aberrations are systematically analyzed. In Chapter 1, a brief introduction to the history and basic concepts of metasurfaces is given. In Chapter 2, to understand physical limits on the achievable operating bandwidth of achromatic metasurfaces, we apply Foster's reactance theorem to the surface impedances of the metasurfaces and derive general limits relating the bandwidth and total size of the metasurface. Having explored the general limits of achromatic metasurfaces, in Chapter 3, we develop a more realistic limit using the time bandwidth product of a single resonance, revealing the critical role of the substrate electrical thickness on the magnetic response. In Chapter 4, a rigorous design methodology is proposed to obtain a transmissive metasurface in which the Huygens' condition is maintained over a broad bandwidth and range of phase tuning. In Chapter 5, the performance of a broadband focusing metasurface operating at oblique incidence is systematically characterized. Chapter 6 summarizes the thesis, discusses the outlook and proposes possible follow-up projects for broadband metasurfaces