Delay Minimizing User Association in Cellular Networks via Hierarchically Well-Separated Trees

We study downlink delay minimization within the context of cellular user association policies that map mobile users to base stations. We note the delay minimum user association problem fits within a broader class of network utility maximization and can be posed as a non-convex quadratic program. This non-convexity motivates a split quadratic objective function that captures the original problem's inherent tradeoff: association with a station that provides the highest signal-to-interference-plus-noise ratio (SINR) vs. a station that is least congested. We find the split-term formulation is amenable to linearization by embedding the base stations in a hierarchically well-separated tree (HST), which offers a linear approximation with constant distortion. We provide a numerical comparison of several problem formulations and find that with appropriate optimization parameter selection, the quadratic reformulation produces association policies with sum delays that are close to that of the original network utility maximization. We also comment on the more difficult problem when idle base stations (those without associated users) are deactivated.Comment: 6 pages, 5 figures. Submitted on 2013-10-03 to the 2015 IEEE International Conference on Communications (ICC). Accepted on 2015-01-09 to the 2015 IEEE International Conference on Communications (ICC

Long-term field and laboratory leaching tests of cemented radioactive wastes

Experiments with real and simulated radioactive cementitious wasteforms were set up to compare the leaching behaviour of cementitious wasteforms containing nuclear power plant operational waste in field and laboratory test conditions. Experiments revealed that the average annual Cs-137 leach rate in deionised water was about thirty-five times greater compared with the measured average value for the 1st year of the field test. Cumulative leached fraction of Cs-137 for 1st year (3.74%) was close to values reported in literature for similar laboratory experiments in deionised water, however more than two orders of magnitude higher than the 1st year leached fraction of Cs-137 in the repository test (0.01%). Therefore, to compare field and laboratory test results, a scaling factor is required in order to account for surface to volume factor difference, multiplied by a temperature factor and a leach rate decrease coefficient related to the ground water composition. (C) 2011 Elsevier B.V. All rights reserved

On the Applications of Metric Trees and Metric Labeling to Hard Combinatorial Optimization Problems

Matching of metric distributions is a fundamental problem in computer science having numerous real life applications including computer vision, pattern recognition, and natural language processing. The problem can be reduced to graph matching since arbitrary metrics can be represented using graphs.Exact graph matching is known to be computationally intractable which motivates inexact matching approaches. Although graphs structurally represent metric data without losing information, processing data over graphs is prone to entail performance problems. Specifically, running time of many graph algorithms depend on the number of edges and vertices of the input graph which increases quadratically by the number of nodes on the extreme case of complete graphs. Thus, it is desirable to obtain a sparse representation of the data while preserving the quality of information. A common technique to achieve this is through representing graphs by metric trees which recently became defacto metric structures for embedding problems. In this dissertation, we focus on problems involving data that can be represented by graphs. As a general theme, we concentrate on approximation of graphs by trees for improving the performance of certain algorithms using topological structure of trees. We also focus on the fundamental problem of inexact graph matching, efficient approximation algorithms for the problem and its applications. Specifically, we present an inexact graph matching problem referred to as multilayer matching, which utilizes the structure of hierarchical metric trees. We represent graphs as trees and achieve matching over the trees to improve the performance and accuracy of inexact matching. We also establish a relationship between the well known metric labeling problem and inexact graph matching. We propose efficient approximation algorithms for both multilayer matching and metric labeling using the primal-dual approximation scheme. We provide application of the proposed methods to image matching, pattern recognition, and question answering problems. Finally, we present a novel motion segmentation method utilizing metric trees which provides tracking of objects in a video sequence without a priori knowledge of number of objects in the scene.Ph.D., Computer Science -- Drexel University, 201

Progress in cementation of reactor resins

Effect of bentonite and ash additives on the leaching and strength properties of cement-resin matrix was investigated. Three groups of samples have been prepared by cementation of low-level-radioactive resin. These are (A) cement-resin-ash matrix, (B) cement-resin-bentonite matrix and (C) cement-resin matrix. Each of the samples has different amounts of resin, cement, water and additives. Water/Cement ratio of the samples was arranged in the range of 0.4-0.5. Group (A) samples included fly ash as an additive in the range of 10-30% (w/w). Group (B) samples included bentonite as an additive in the range of 3-12% (w/w). Group (C) samples were prepared without any additives. Leaching rates of radionuclides (Co-60, Cs-137) and stabilities of these waste forms were compared to each other. Although long-term leach tests showed that bentonite additives significantly decrease the leaching rates (4.85 x 10(-8) kg m(2)/s for Co-60 and 2.68 x 10(-8) kg m(2)/s for Cs-137), compressive strength of these samples was not sufficient for handling. Optimum values have been measured by the tests of samples B4 and B5 in this group. Bentonite contents of these samples were between 5 and 7% (w/w). Evaluation of test results showed that optimum ratio of resin to matrix material was between 0.36 and 0.67. (c) 2006 Elsevier Ltd. All rights reserved

Immobilization of radioactive borate liquid waste using natural diatomite

In this article, influence of diatomite as a natural adsorbent in immobilization of radioactive borate liquid waste in the cement-diatomite matrix was investigated. Cylindrical concrete mixtures were prepared with different amounts of diatomite additives at three different w/c values. Mechanical strength tests were carried out to determine the uniaxial compressive strength of the samples. A new dynamic column type leaching tests were applied to the solidified waste forms, and leach rates of Cs-137 and Co-60 were determined according to different diatomite additives. The experimental results indicated that the increasing of the diatomite additive amounts resulted high isolation performance of the solidified waste form. However, the results revealed that mechanical strength decreases with increasing borate diatomite amount. Results show that 9% amount of natural diatomite is effective as an admixture of the cementation process for achieving long-term stability of solidified borate waste. Optimum strength and leach values were determined as 11.5MPa with leaching rates of 7.5x10(-4) and 3.2x10(-5)cm/d for Cs-137 and Co-60, respectively