In recent years, there has been a growing research interest in throughput optimization problems in a multi-hop wireless network. MIMO (multiple-input multiple-output), as an advanced physical layer technology, has been employed in multi-hop wireless networks to increase throughput with a given bandwidth or transmit power. It exploits the use of multiple antennas at the transmitter and receiver to increase spectral efficiency by leveraging its spatial multiplexing (SM) and interference cancellation (IC) capabilities. Instead of carrying complex manipulations on matrices, degree-of-freedom(DoF) based MIMO models, which require only simple computations, are widely used in networking research to exploit MIMO's SM and IC capabilities. In this thesis, we employ a new DoF model, which can ensure feasible solution and achieve a higher DoF region than previous DoF-based models. Based on this model, we study the DoF scheduling for a multi-hop MIMO network. Specifically, we aim to maximize the minimum rate among all sessions in the network. Some researches have been done based on this model to solve throughput optimization problems with the assumption that the route of each session is given priori. Although the fixed routing decreases the size of the problem, it also limits the performance of the network to a great extent. The goal of this thesis is to employ this new model to solve the throughput maximization problem by jointly considering flow routing, scheduling, and DoF allocation for SM and IC. We formulate it as a mixed integer linear program (MILP), which cannot be solved efficiently by commercial softwares even for moderate sized networks. Thus, we develop an efficient polynomial time algorithm by customizing the sequential fixing framework. Through simulation results, we show that this algorithm can efficiently provide near-optimal solutions for networks with different sizes. / Master of Science

Turbulent flows are encountered in nearly every case where fluid motion is involved. Turbulent flow in pipes is of great practical interest, and the problem of flow in the entry region is of special importance since the great majority of applications are in the realm of developing flow.

When entering into the subject of turbulent flow, it is essential to understand that the kind of flow with which we deal belongs to a particular class known as shear flow. These types of flow comprise flow fields in which relative velocities have been induced by shear stresses rather than by the action of pressure forces. In pipe flow, when the fluid enters through the well-rounded bell from a reservoir or from the calm open air, a uniform velocity distribution occurs at the pipe entrance. Immediately down stream from the entrance of the pipe, the flow is structured with a boundary layer near the wall, and is of uniform velocity profile in the central part. Due to the action of wall friction, the boundary layer grows thicker and thicker downstream. As the mass flux is constant throughout the pipe,’ the central stream must accelerate to compensate for this retardation of the flow near the wall. Finally, the boundary layer thickness reaches the value of pipe radius. The free stream, therefore, disappears from the central part of the pipe. Furthermore, Barbin and Jones (1)* pointed out that following the disappearance of the free stream, further changes in the velocity profile and turbulence structure occur before a fully developed condition is reached* The flow in the inlet region of a pipe is, therefore, a transition from a boundary layer type flow at the entrance to a fully developed flow downstream. The change of the free stream velocity in the entry region causes a greater reduction of the static pressure than that in the fully developed region.

Shape synthesis (also called topological synthesis or inverse design in other disciplines) has the potential to provide antenna engineers with a useful addition to their design tools. Transmitarray antennas, which consist of a feed antenna plus a printed planar structure that emulates a lens, are able to provide high directivity antenna performance, and have been the subject of sustained research over the past ten years. The transmitarray lens consists of a lattice of cells, with each cell occupied by an element that includes conductors of specific shape. The feed field incident on each element on the input surface side of the transmitarray is transformed by each element into a field of different amplitude and phase on the output surface side of each element, providing some desired aperture distribution on the output surface. In this thesis we develop a technique, and the overall computational tool to implement it, that fundamentally allows the electromagnetics to dictate how the conducting portions of a 3-layer element must be shaped in order to obtain some specific transmission coefficient. Such shape synthesis of the elements offers the possibility of obtaining elements that have properties not obtainable using conventional elements. These techniques were applied to the shape synthesis of dual-band elements (18 GHz and 24 GHz). A transmitarray using these elements was designed and fabricated, its performance measured and compared to simulated results. An in-depth discussion of the outcome experimentally validates the shape synthesis procedure.

This work addresses the problem of detecting devices which are stealthily attached to the computer for logging keystrokes from keyboards. These devices are known as hardware keyloggers (HKL). When an HKL is attached to the keyboard, certain electrical characteristics of the keyboard signal are altered. Based on these characteristics (features), differences have been identified in an accurate assertion was made about the presence of HKL. The characteristics from which the differences were obtained were used to make distributions and compared with distance-measuring methods. An experiment was done to collect data from a number of keyboards and form two distributions (training and test) to perform the comparison. It was possible to detect the presence of HKL in the keyboard with a minimum of 4 to 100 keystrokes. For justifying the stability of the features, the temperature of the surroundings was obtained and the dependence of the features on temperature was obtained. Also, an experiment was done to see whether the keyboards were uniquely affected by the HKLs. This was done without using any training data, i.e., the distribution of features which was used did not come from a known state of the system (either with HKL or not with HKL).

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1968. / Vita. / Bibliography: leaves 109-117. / by Rory Thompson. / Ph.D.

The research on transition in supersonic and hypersonic boundary layers has been reinvigorated in the last decades because of the increased interest in high-speed flight. The receptivity to environmental disturbances of high-speed boundary layers developing over flat plates or curved surfaces is a very important problem because the transition process is directly impacted by it. The main objective of the research is to determine the effect of small steps on laminar high-speed boundary-layers that are excited by freestream disturbances in the form of vorticity and acoustic waves. Both supesonic and hypersonic regimes are analyzed using a high-order compressible Navier-Stokes numerical algorithm. It is found that both the backward and the forward steps are capable of stabilizing the disturbances that propagate inside the boundary layer. This will potentially delay the formation of three-dimensional disturbances that are precursors to transition into turbulence.

The spatiotemporal evolution of temperature of leaves-on-branchlet carbon based electric double layer capacitors (EDLCs) under imposed constant current was studied using a continuum thermal model. The hot spot aggregated at the tips of graphene petals (GPs), particularly at the high concave surface, at the beginning of the charging step. As the charging proceeded, the overall temperature rose continuously, and the temperature distribution was likely uniform throughout the graphene petals due to an increasingly uniform distribution of ions on GPs surfaces. To elucidate the effects of electrode geometry on the change of temperature, several simple two-dimensional structures were also simulated in the charging step. Concave and planar structures contributed to high temperature change, while a convex structure tended to alleviate the hot spot. An insight into geometric effects on the thermal behavior may lead engineers to develop a new class of nanomaterials for supercapacitors.

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