ABSTRACT OF THE THESIS

Traffic offloading in HetNet using power biasing considering different path loss exponent

By

Jyotica Yadav

Master of Science in Electrical Engineering

University of California, Irvine, 2019

Professor Ender Ayanoglu, Chair

With the advent of new cellular technology, the number of users is expected to grow at a drastic rate that comes with the cost of an increase in data traffic. Therefore, to provide enough capacity for all the users, node density must be increased. One way to achieve that is through cell splitting but that would lead to higher chances of intercell interference along with incurring higher costs of installing high power macro nodes. Another way to attain a higher network capacity is to overlay a network of small low power nodes over an existing network thereby creating a heterogeneous network, i.e., HetNet [1]. In HetNet, the user is made available with more than one type of base station differing in the transmit power level and physical size. But, even with multiple options at hand, the user terminal always tends to associate itself to the macro base station i.e., the station with the highest transmit power. The reason is, that associativity is based on the received signal strength values i.e., RSS values, therefore, irrespective of the distance between the user and the station, the user will always have a preference towards macro station over low tier stations. Because of the above-mentioned claim, the overall reason behind shifting towards low tier station nullifies, therefore, it is essential to offload the data traffic from the macro tier. Some of the techniques to offload data traffic are offloading via power control, offloading via femtocell deployment and offloading via biasing, out of which, the latter is of concern to the thesis [2]. The research so far focuses on the stationary user and its connectivity to a base station. With user velocity in the picture, comes the concept of handover and its associated delays. Handover delays in dense networks lower the effect of an overall increase in network capacity as it includes the transmission of control signals rather than effective data transmission thereby raising the overhead over the network. Few handover skipping schemes have been proposed to avoid frequent handovers whose underlying assumption is same path loss exponents for all the tiers and unbiased power levels [3]. In this thesis, the effect of different path loss exponents and biased power levels has been investigated and the effect on coverage probability has been studied. In the end, the simulation results obtained are compared with the work done by other researchers so far.

It is historically proven that artificial heterostructures are of paramount importance for both fundamental research and technological application. One distinguishable example is superlattices and quantum-well heterostructures of conventional semiconductors (III-V). Several fundamental observations such as two-dimensional electron gas, quantum confinement effect, quantum Hall effect and fractional quantum Hall effect, were first realized in artificial heterostructures of conventional semiconductors. On the technological front, artificial heterostructures of conventional semiconductors were found to revolutionize optoelectronic and high-speed electronic industry. Given the capability of artificial heterostructures to enable new physical phenomena, with an appropriate choice of material system, these heterostructures hold the potential to uncover hidden physical phenomena and exotic phases, which are otherwise not observed in bulk systems. One choice of such a system could be a strongly correlated material where the presence of strong electronic correlations has been known to enable unique electronic and magnetic properties. Complex oxides, in particular, are known to exhibit a strong interaction between various degrees of freedom available such as spin, charge, orbital and lattice degree, and thus offer a suitable choice of material system. In literature, one can find plenty of examples from different areas of research such as superconductivity, magnetism, ferroelectricity and thermoelectricity, where artificial superlattices led to the observation of fundamentally different behavior, compared to bulk superlattice constituents. These artificial superlattices and heterostructures continue to be the most promising candidate for exploring new phenomena and enhancing physical properties in complex oxide material systems. In this dissertation, artificial superlattices of complex oxides were synthesized in a thin-film geometry to enable the observation of fundamentally new physical phenomena, compared to their bulk counterparts, in few selected areas of investigation.

First, I will present the experimental results on heat transport across superlattice structures composed of insulating perovskite oxides. The measured thermal conductivity of these artificial superlattices exhibited a unique phonon transport, i.e. coherent phonon transport, phenomenon that is extremely rare to observe in bulk form. The key element for enabling the coherent transport of phonon in these superlattice systems is the range of phonon wavelengths, which carry most of the heat in the material. The critical range being between 1-3 nanometers, the size of the system needs to reduce to this length scale in order to observe the effect of the wave nature of phonons on heat transport. Thus, superlattice structures offer an ideal candidate to search for this novel phenomenon. Several theoretical studies predicted the existence of these phenomena, but the experimental evidence of their existence remained largely absent or inconclusive. By synthesizing the superlattice of perovskite oxides, I’ve observed unambiguous evidence of the coherent transport of phonons at short-period superlattices of SrTiO3-CaTiO3 and BaTiO3-SrTiO3. In contrast to conventional heat conduction mechanisms, where phonon transport can be described by energy-carrying particles, the uniqueness of this phenomenon is highlighted by the fact that the wave aspect of phonons needs to be invoked to understand their transport behavior. After observing the wave nature of phonons in dictating heat conduction across periodic superlattices, other artificial heterostructures were studied to understand the nature of coherent phonon transport at the superlattice interfaces. Designed superlattice-like sequences, where the structural order can be controlled from a periodic sequence to a completely random sequence, were synthesized to further understand the role of the coherent scattering of phonons in thermal transport across superlattice structures.

Next, I will discuss the experimental observation of another unique phenomenon, which was enabled by artificial superlattices of complex oxides in a different subject of research, i.e. ferroelectricity. Several theoretical studies on nanostructured ferroelectric systems such as nanodisc, nanocomposites, superlattices etc., predicted the stabilization of novel ferroelectric ground states. A number of different topologies of electrical polarization such as vortices and skyrmions, were predicted in ferroelectric nanostructures, which showed a strong resemblance to spin topologies such as skyrmion, merons etc. found in magnetic systems. Experimental confirmation for the existence of these exotic polarization states, however, remained absent. By leveraging the competition among charge, orbital and lattice degrees of freedom in superlattices of a paraelectric (SrTiO3) and ferroelectric (PbTiO3) material, vortex-antivortex structures of electrical polarization were stabilized in ferroelectric (PbTiO3) layers of PbTiO3-SrTiO3 superlattice. Only for a narrow range of superlattice periods, the polarization vortices are stabilized with a balance between one gradient energy associated with the non-uniform polarization profile of the vortex structure and the other’s electrostatic and elastic energies associated with depolarization fields and epitaxial constraints from the substrate, respectively.

In the last section, I will describe unique and unusual phenomena associated with the existence of polarization vortices in paraelectric/ferroelectric superlattices. Superlattices of PbTiO3-SrTiO3 with varying periodicity showed a rich spectrum of characteristically different ferroelectric domains. Specifically, in the short-period regime, domain size was observed to evolve with a negative scaling coefficient, which is unusual for typical ferroelectrics. When the superlattice periodicity was increased, the average domain size decreased, suggesting an opposite behavior to the universal Kittel’s Law, where in the latter, an increase in ferroelectric thickness leads to an increase in the average domain size. Second, I will present intriguing results on the fundamental characteristics of polarization vortices as revealed from X-ray circular dichroism studies. The finite difference in absorption spectra of left-circular vs. right-circular polarized light from vortex structures, suggested that vortex-antivortex arrays are chiral in nature. The presence of chirality in polarization vortex structures is a characteristically different behavior compared to bulk ferroelectric systems where uniformly polarized regions are expected to exhibit a linear X-ray dichroism (i.e. a difference in the absorption spectra of linearly polarized light in a direction parallel and perpendicular to ferroelectric order). Lastly, a possibility of manipulating phonon dispersion using vortex ordering or vortex “lattice” is discussed, along with a hypothesis of inducing localization in propagative phonons.

The term internet of things has evolved by many folds due to the convergence of multiple technologies like machine learning, embedded systems, wireless-sensors, real-time analytics, etc. A growing number of IoT devices are being developed for consumer use, including connected vehicles, home automation, wearable technology, connected health, and remote monitoring devices. We want to apply some of these technologies and techniques to make commercial spaces smarter.

Buildings are responsible for a significant portion of energy consumption in the US, accounting for more than 40% of US primary energy consumption. Heating, ventilation, and air-conditioning (HVAC) accounts for nearly 50% of that use. Conditioning buildings is important since people spend 87% of their time in the place they live (residential) and the place they work (commercial). Despite this massive expense, many users are dissatisfied with the thermal conditions in buildings.

In this thesis, we explore the tradeoff between commercial building HVAC energy consumption and the quality of thermal conditioning provided to users. The framework has several components that help to address the current HVAC control systems shortcomings, including (a) occupancy sensing in real-time (b) occupancy prediction models based on historical occupancy data (c) human-in-the-loop comfort feedback (d) data-driven thermodynamic building models, and (e) weather forecasting data

All these components provide the necessary input to our model predictive control optimization framework that minimizes monetary costs in energy use while maintaining quality comfort bounds for the building's users based on real-time user's feedback. We tested our framework OFFICE in a real LEED Gold certified university building with over 20 workers performing their daily tasks for 4 weeks, and we showed that we could obtain monetary costs savings of more than 10% while at the same time reducing the users' dissatisfaction levels with thermal comfort from 25% to 0% dissatisfaction, significantly improving the quality of thermal service provided to the building's users.

Transition metal dichalcogenides based on transition metals (like Mo, W, Ta) and chalcogens (like S, Se, Te) are important two-dimensional (2D) materials in materials research for a range of applications including but not limited to electronics, photonics, optoelectronics, sensing, and photovoltaics. At the scale of the dimensions of these materials, the properties differ from their properties at larger dimensions. To exploit these interesting properties for novel applications, the growth methods of the materials are an important aspect. While materials research has advanced over several decades in this area, substantial challenges exist in their controlled and rapid growth. MoTe2 is an especially challenging material to grow among the TMD materials and needs more investigation given its superior properties. In this thesis, I report the study of the parameters of growth mechanisms of materials like MoTe2, MoSe2, and MoS2 on SiO2, Si, and GaN substrates. I present reproducible growth of MoTe2 in an ultra-high vacuum (UHV) environment using co-deposition of transition metal and chalcogen. The phase diagram developed for this system shows that both semimetallic and semiconducting phases of MoTe2 can be grown on SiO2 substrates. With the addition of a MoS2 underlayer, the phase of MoTe2 is affected significantly. A similar growth mechanism can be used to grow MoSe2. The quality of the materials is investigated using Raman spectroscopy, photoluminescence spectroscopy, cross-sectional transmission electron microscopy, scanning electron microscopy, low energy electron diffraction, X-ray photospectroscopy, energy dispersive spectroscopy, and electrical characteristics.

In cancer, autophagy has been shown to promote resistance to chemotherapy by inducing pro-survival mechanisms. This observation has led to basic and clinical studies combining chloroquine, an autophagy inhibitor, with standard chemotherapies. However, the range of particular chemotherapies likely to benefit from this combination remains unclear. Here we apply DrugCell, a deep learning model of the molecular pathways that govern cancer therapeutic response, to systematically prioritize drugs that promote cell survival through the activation of autophagy. Using this approach, we analyze the response of tumor cell lines to 684 drugs, identifying 21 for which resistance or sensitivity is modulated by genetic mutations in autophagy pathways. We systematically screen these prioritized drugs against a fluorescent readout of autophagic flux, confirming that 14 stimulate this pathway and 1 leads to pathway suppression, for a hit rate of 15/22 or 71%. In contrast, only 1 in 6 of a control set of drugs is found to affect autophagy activity. Finally, we test eight of the autophagy activators for synergy with chloroquine treatment, finding that four are highly synergistic, producing a BLISS score greater than 10. These results suggest that, for these drugs (JQ1, tosedostat, leptomycin B, vorinostat), chloroquine inhibition of autophagic flux interferes with autophagy as a survival response.