This thesis first presents a new cooperative wireless communication network strategy that incorporates energy cooperation and data cooperation. The model establishment, design goal formulations, and algorithms for throughput maximization of the proposed protocol are presented and illustrated using a three-node network with two energy harvesting (EH) user nodes and a destination node. Transmission models are established from the performance analysis for a total of four scenarios. Based on the models, we seek to find optimal energy management strategies by jointly optimizing time allocation for each user, power allocations over these time intervals, and data throughputs at user nodes so as to maximize the sum-throughput or, alternatively, the minimum throughput of the two users in all scenarios. An accelerated Newton barrier algorithm and an alternative algorithm based on local quadratic approximation of the transmission models are developed to solve the aforementioned optimization problems. Then the thesis extends the cooperative strategy to multi-source wireless communication network, where N source users communicate with the destination via one relay that harvests energy from the RF signals transmitted by the sources through time-division multiple access (TDMA). We characterize the Energy-Throughput (E-T) tradeoff regions between the maximum achievable average throughput of the sources and the total amount of saved energy in three circumstances. For the case N=1, all harvested energy will be used to forward the message. For the case N>1, we compare two transmission strategies: one is common PS ratio strategy that the relay adopts the same PS ratio for all sources; the other is individual PS ratio strategy that each source uses an individual PS ratio. Numerical experiments under practical settings provide supportive evidences to our performance analysis. / Graduate

Missing from extant conflict literature is an examination of both symmetric and asymmetric conflict ties. To address this void, network analysis was utilized to examine the responses (both symmetric and asymmetric conflict ties) of 140 employees and managers in four divisions of a large agency of the Federal Government. The study was limited to conflict over scarce resources. Conflict management methods were examined as well as the perceptions of how respondents both cope with and feel about conflict. The results indicate that when two people in a conflict setting are structurally equivalent they both report actions and feelings that are opposite from those of- the other person. This finding, an inverse contagion effect, has been termed diffusion resistance.

My dissertation focuses on the strategic implications of the link between organizational culture and social network structure. I study their role in the process of knowledge transfer and diffusion, organizational memory, and organizational design. More broadly, I examine the way that social structure influences the information environment, and what effect this has on organizational learning. I focus in particular on the process of cultural evolution. My dissertation leverages digitization as a phenomenon of inherent interest and as an empirical setting that can improve our theoretical understanding of both digital and non-digital communities. I have developed an expertise in computational methods, especially in machine learning techniques related to text and other unstructured data, and in the analysis of "big data," especially pertaining to large-scale networks. By combining these computational tools with organizational theory and the rich relational data generated by the explosion of digital records, my research grants insight into the dynamic process of learning in organizations and the implications for innovation and competitive advantage. I explore how digitization informs and develops our understanding of organizational culture, knowledge transfer, and the labor market. Specifically, I investigate how digitization has opened a window to observe network structure and language, providing a lasting record of these changes through time. Using these digital records to observe the structure of social relations and the language used to communicate can help deepen our theory of knowledge transfer for a wide range of organizations, not just those that operate in the digital sphere. This means that these studies also have implications for understanding organizations in non-digital settings. My dissertation contributes both theoretically and empirically to the knowledge theory of the firm. However, the mechanisms underlying knowledge transfer remain underdeveloped. I contribute by disentangling the related mechanisms of language and organizational structure, and I propose that common language directly impacts what knowledge may be efficiently transferred. Next, my dissertation contributes to the growing field of digitization. Digitization is salient for researchers both as a unique phenomenon and as an ever-expanding source of accessible data to test theory. Moreover, since one of the central contributions of digitization is to reduce the cost of information gathering, it is well-suited to my theoretical setting of knowledge transmission and organizational memory. Finally, my dissertation contributes to our understanding of culture in organizations. The focus on language as an aspect of culture allows both additional formalization as well as more specific empirical tests of the contribution of culture to organizational outcomes. In particular, a focus on dynamic settings in each of the chapters reveals the interplay between organizational structure, memory, and change. This helps us to understand how language evolves, how it is learned, and how it changes in response to information shocks.

Outsourcing, consolidation and cost savings of IT services, are increasingly becoming an imperative source of competitive advantage and a great challenge for most local and global businesses. These challenges not only affect consumers, but also the service providers’ community. As IT is slowly becoming commoditised, consumers, such as business organisations, are increasingly expecting IT services that will mimic other utility services such as water, electricity, and telecommunications.To this end, no one model has been able to emulate these utilities in the computing arena.Cloud Computing is the recent computing phenomenon that attempts to be the answer to most business IT requirements. This phenomenon is gaining traction in the IT industry, with a promise of advantages such as cost reduction, elimination of upfront capital outlay, pay per use models, shared infrastructure, and high flexibility allowing users and providers to handle high elasticity of demand. The critical success factor that remains unanswered for most IT organisations and its management is: What is the effect of the communication network factors on Internet based technology such as Cloud Computing, given the emerging market context.This study therefore, investigates the effect of four communication network factors (price, availability, reliability and security) in the adoption of Cloud Computing by IT managers in a South African context, including their propensity to adopt the technology. The study investigates numerous technology adoption theories, in which Technology, Organisation and Environment (TOE) framework is selected due to it having an organisational focus as opposed to an individual focus.Based on the results, this study proposes that Bandwidth (Pricing and Security) should be included into any adoption model that involves services running on the Internet. The study makes an attempt to contribute to the emerging literature of Cloud Computing, Internet in South Africa, in addition to offering organisations considering adoption and Cloud Providers’ significant ideas to consider for Cloud Computing adoption. / Dissertation (MBA)--University of Pretoria, 2012. / Gordon Institute of Business Science (GIBS) / unrestricted

Modern computer networks pose a great challenge for monitoring the network performance due to their large scale and high complexity. Directly measuring the performance of internal network elements is prohibitive due to the tremendous overhead. Alternatively, network tomography, a technique that infers the unobserved network characteristics (e.g., link delays) from a small number of measurements (e.g., end-to-end path delays), is a promising solution for monitoring the internal network state in an e cient and e ective manner. This thesis initiates two variants of network tomography: concentrated network tomography and bound-based network tomography. The former is motivated by the practical needs that network operators normally concentrate on the performance of critical paths; the latter is due to the need of estimating performance bounds whenever exact performance values cannot be determined. This thesis tackles core technical di culties in concentrated network tomography and bound- based network tomography, including (1) the path identi ability problem and the monitor deploy- ment strategy for identifying a set of target paths, (2) strategies for controlling the total error bound as well as the maximum error bound over all network links, and (3) methods of constructing measure- ment paths to obtain the tightest total error bound. We evaluate all the solutions with real-world Internet service provider (ISP) networks. The theoretical results and the algorithms developed in this thesis are directly applicable to network performance management in various types of networks, where directly measuring all links is practically impossible. / Graduate

No description available.

No description available.

Emerging large-scale modular power converters are pursuing high-performance distributed control systems. As opposed to the centralized control architecture, the distributed control architecture features shared computational burdens, pulse-width modulation (PWM) latency compensation, simplified fiber-optic cable connection, redundant data routes, and greatly enhanced local control capabilities. Modular multilevel converters (MMCs) with conventional control are subjected to large capacitor voltage ripples, especially at low-line frequencies. It is proved that with appropriate arm current shaping in the timescale of a switching period, referred as the switching-cycle control (SCC), such line-frequency dependence can be eliminated and MMCs are enabled to work even in dc-dc mode. Yet the SCC demands multiple times of arm current alternations in one switching period. To achieve the high-bandwidth current regulation, hybrid modulation approach incorporating both the carrier-based modulation and the peak-current-mode (PCM) modulation is adopted. The combined digital and analog control and the extreme time-sensitive nature together pose great challenges on the practical implementation that the existing distributed control systems cannot cope with. This dissertation aims to develop an optimized distributed control system for SCC implementation. The critical analog PCM modulation is enabled by the intelligent gate driver with integrated rogowski coil and field programmable gate array (FPGA). A novel distributed control architecture is proposed accordingly for SCC applications where the hybrid modulation function is shifted to the gate driver. The proposed distributed control solution is verified in the SCC-based converter operations. Accompanied by the growing availability of medium-voltage silicon carbide (SiC) devices, fast-switching-enabled novel control schemes raise a high synchronization requirement for the communication network. Power electronics system network (PESNet) 3.0 is a proposed next-generation communication network designed and optimized for a distributed control system. This dissertation presents the development of PESNet 3.0 with a sub-nanosecond synchronization error (SE) and a gigabits-per-second data rate dedicated for large-scale high-frequency modular power converters. The White Rabbit Network technology, originally developed for the Large Hadron Collider accelerator chain at the European Organization for Nuclear Research (CERN), has been embedded in PESNet 3.0 and tailored to be suited for distributed power conversion systems. A simplified inter-node phase-locked loop (N2N-PLL) has been developed. Subsequently, stability analysis of the N2N-PLL is carried out with closed-loop transfer function measurement using a digital perturbation injection method. The experimental validation of the PESNet 3.0 is demonstrated at the controller level and converter level, respectively. The latter is on a 10 kV SiC-MOSFET-based modular converter prototype, verifying ±0.5 ns SE at 5 Gbps data rate for a new control scheme. The communication network has an impact on the converter control and operation. The synchronicity of the controllers has an influence on the converter harmonics and safe operation. A large synchronization error can lead to the malfunction of the converter operation. The communication latency poses a challenge to the converter control frequency and bandwidth. With the increased scale of the modular converter and control frequency, the low-latency requirement of communication network becomes more stringent. / Doctor of Philosophy / Emerging silicon carbide (SiC) power devices with 10 times higher switching frequencies than conventional Si devices have enabled high-frequency high-density medium-voltage converters. In the meantime, the power electronics building block (PEBB) concept has continually benefited the manufacturing and maintenance of modular power converters. This philosophy can be further extended from power stages to control systems, and the latter become more distributed with greatly enhanced local control capabilities. In the distributed control and communication system, each PEBB is equipped with a digital controller. In this dissertation, a real-time distributed control architecture is designed to take the advantage of the powerful processing capability from all digital control units, achieving a minimized digital delay for the control system. In addition, pulse-width modulation (PWM) signals are modulated in each PEBB controller based on its own clock. Due to the uncontrollable latency among different PEBB controllers, the synchronicity becomes a critical issue. It is necessary to ensure the synchronous operation to follow the desired modulation scheme. This dissertation presents a synchronized communication network design with sub-ns synchronization error and gigabits-per-second data rate. Finally, the impact of the communication network on the converter operation is analyzed in terms of the synchronicity, the communication latency and fault redundancy.

A system composed of unreliable components can experience different levels of performance as its configuration changes due to failures and repairs. One approach used to measure overall system performance is to weight the level of performance measured for each system state by the probability that the system is in that state and then sum across all system states. Many performance measures have a transient behavior following a change in the state of the system. Because of the difficulty associated with transient analysis, the system is often assumed to be in steady state when measuring the performance for each system state. When this approach is used to analyze packet-switched communication networks, which consist of highly reliable high-speed links and switching nodes, it is argued that the steady-state assumption is justified on the basis of the large difference in rates of traffic-related events, such as call completions and packet transmissions, compared to component-related events, such as failures and repairs. To investigate the validity of this assumption, we define lower bounds for the length of the transient phase fol1owing link failures and repairs. For both cases, we obtain a distribution for the length of the lower bound. The transient phase is significant when its length exceeds a given fraction of the time until the next change in network state. Using the distributions for these lengths, we derive an expression for the probability that the transient phase is significant in terms of the amount of traffic on the link and the ratio of the rates for traffic-related events and network state changes. These results show that the difference in rates between traffic-related events and component related events is not enough by itself to justify the steady-state assumption. The amount of traffic carried on the link and the size of the network must also be considered. These results indicate some situations where the steady-state assumption is inappropriate. We also obtain sufficient conditions for transient-phase significance following link failures. Although these results do not indicate when it is safe to use the steady-state assumption, they provide a measure of the risk associated with using it. / Ph. D.

Numerous power electronics building blocks (PEBB) based power conversion systems have been developed to explore modular design, scalable voltage and current ratings, low-cost operations, etc. This paper further extends the modular concept from the power stage to the control system. The communication network in SiC-based modular power converters is becoming significant for distributed control architecture, with the requirements of tight synchronization and low latency. The influence of the synchronization accuracy on harmonics under the phase-shifted carrier pulse width modulation (PSC-PWM) is evaluated. When the synchronization is accurate, the influence of an increase in harmonics can be ignored. Thus, a synchronous distributed control and communication protocol with well-performed synchronization of 25 ns accuracy is proposed and verified for a 120 kHz SiC-based impedance measurement unit (IMU) with cascaded H-bridge PEBBs. An improved synchronization method with additional analog circuits is further implemented and verified with sub-ns synchronization accuracy. / The power electronics building block (PEBB) concept is proposed for medium-voltage converter applications in order to realize the modular design of the power stage. Traditionally, the central control architecture is popular in converter systems. The voltage and current are sensed and then processed in one central controller. The control hardware interfaces and software have to be customized for a specified number of power cells, and the scalability of controller is lost. In stead, in the distributed control architecture, a local controller in each PEBB can communicate with the sensors, gate drivers, etc. A high-level controller collects the information from each PEBB and conducts the control algorithm. In this way, the design can be more modular, and the local controller can share the computation burden with the high-level controller, which is good for scalability. In such distributed control architecture, a synchronous communication system is required to transmit data and command between the high-level controller and local controllers. A power converter always requires a highly synchronized operation to turn on or turn off the devices. In this work, a synchronous communication protocol is proposed and experimentally validated on a SiC-based modular power converter.