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(2012)ThesisAs a well-established multilevel converter topology, the cascaded H-bridge (CHB) is suitable for large-scale PV grid-integration due to its high-voltage and high-power capability, high power quality, modular structure, and multiple DC-links which are able to incorporate a large number of PV generators. Since PV system grounding is required by most countries grid codes and the modulation process of the CHB introduces high potentials upon the DC-side of the converter, galvanic isolation between the PV and the grid is essential to avoid the potential induced degradation (PID) of the PV modules, and to prevent the leakage current, as a result of the PV panel-to-ground parasitic capacitance, from flowing through the converter and ground, posing serious safety concerns. PV isolation can be achieved by the implementation of isolated DC/DC converters. The DC/DC converters connect the PV sources to the CHB converter through DC-links, performing maximum power point tracking (MPPT) and necessary DC voltage amplification, while the imbedded high-frequency transformers isolate the PV from the AC-side. The thesis presents the system validation and documents the performance evaluation of the CHB converter with isolated DC/DC converters for large-scale PV grid-integration, featuring different power and voltage level configurations, tailored with two control approaches, and through specific case studies. Two isolated DC/DC converter topologies, namely the boost-half-bridge (BHB) and the flyback are implemented. The CHB converter provides good quality voltage and current waveforms and is capable of medium-voltage grid-connection of the PV system. The system can be well regulated with voltage-oriented control (VOC) schemes both in the synchronous frame using Proportional-Integral (PI) controllers, and in the stationary frame using Proportional-Resonant (PR) controllers. Both DC/DC converters are able to perform MPPT under Standard Test Conditions (STC) and track the varying input voltage reference signal in a wide range fast and accurately. Compared with the flyback, the BHB converter presents lower ripples on the control variables and offers better MPPT efficiency at a higher component count. As the building block of the multiphase DC/DC converters, which feature high-voltage and high-current capability with modular and interleaved structures, the BHB makes a competitive candidate for this application.
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(2012)ThesisIn this work the modeling and performance analysis are present for a proposed MIMO Free-Space system called "Phase-Fading" system, which is a single-user with multiple-antenna system, where the transmitters and receivers are randomly located in specific areas and the links are dominated by Line-of-Sight (LoS) components. Since Free-Space Phase-Fading MIMO system channel is stochastic, based on Telatar's research, its capacity is determined by singular eigenvalue distribution of the random correlation matrix corresponding to channel transmit matrix. Firstly, for proposed Free-Space MIMO system, the asymptotic analysis was performed for the ergodic capacity with respect to the infinite number of transmit and receive antennas while keeping the aspect ratio beta constant, for which the explicit expression of asymptotic capacity was uncovered. In addition, an unification property has been found for asymptotic capacity performances in Phase-fading and Rayleigh channel. Secondly, considering the finite order channel capacity, simulation plots were present and the Phase-Fading System has better capacity than the conventional Free-Space MIMO systems and Rayleigh channel. Moreover, the singular value distribution has been worked out for 2 by 2 Phase-fading channel as well the capacity formula. Besides, an essential part has been discovered in the joint probability density function of eigenvalues.
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(2012)ThesisThe global trend towards larger size ground mounted solar photovoltaic (PV) power plants is set to continue, with the development of several projects in the 200MW range and higher. This installation trend will challenge the current PV plant architectures by requiring power converters with a higher power rating and a higher voltage level at the point of common coupling (PCC), which can lead to higher ratio transformers or more transformation stages to be used for the connection of the solar farm with the electricity grid. Two possible solutions are proposed in this thesis to minimize the number of transformer stages and/or the transformer turns ratio of a grid-connected PV plant without changing the standard configuration of the system. The first solution is a multistring PV system architecture based on a high-voltage-gain DC/DC converter. By introducing a high-voltage-gain DC/DC converter, the PV system can be connected to a medium voltage grid through a single transformer stage and the turns ratio of transformer can be reduced, thus resulting in reduced cost and increased efficiency of the PV system. A 1MW section of a PV plant has been modeled and simulated using MATLAB/Simulink and PLECS Blockset. The simulation results of three different case studies are presented to evaluate the performance of the proposed PV system configuration. The second solution is a PV system based on a cascaded H-bridge (CHB) multilevel converter topology. Despite the fact that the CHB converter topology can deal with the aforementioned challenges, it faces the issue of leakage current that flows through the solar panel parasitic capacitance to ground which could damage the PV panels and pose safety problems. This thesis proposes a CHB topology with multiphase isolated DC/DC converter for a large-scale PV system which eliminates the leakage current issue. At the same time, the multiphase structure of the DC/DC converter helps to increase the power rating of the converter and to reduce the PV voltage and current ripples. A 0.54 MW rated seven-level CHB converter using multiphase isolated DC/DC converters has been modeled and simulated using MATLAB/Simulink and PLECS Blockset. Simulation results of different case studies are presented to evaluate the performance of the proposed PV system configuration. The proposed PV system based on a high-voltage-gain DC/DC converter has achieved satisfactory performance for boosting the voltage, thus the PV system is connected to a medium voltage grid through a single transformer with low turns ratio. Moreover the interleaved configuration of the high-voltage-gain DC/DC converter helps to increase the voltage gain and power rating of the converter. The medium voltage grid connection with a single transformer stage also has been achieved in the proposed PV system based on a CHB topology with multiphase isolated DC/DC converter. Moreover, the use of a multiphase isolate DC/DC converter has completely removed the leakage current issue and has resulted in better maximum power point tracking (MPPT) efficiency than the single-phase converter case.
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(2012)ThesisThe last decade has witnessed a rapid surge in application of body sensor net- works, especially in the domains of military, healthcare and fitness. In such networks, wearable wireless sensor devices are used to measure and communi- cate a subject’s vital signs (such as heart rate, body temperature, blood glucose level, etc.) Security, in this context, is a critical issue as these devices deal with personal health data, requiring strict privacy and confidentiality. However, tra- ditional secret-key generation mechanisms (such as Diffie-Hellman) are typically computation and power intensive and not suitable for resource-constrained sen- sor devices. This thesis aims at realizing a practical and low-cost secret key generation mechanism for wearable sensor devices. First, we investigate a secret key generation mechanism that extracts shared secret keys using properties of the near-body wireless channel between two com- municating parties. For a fully body-worn scenario, our experimental results, using off-the-shelf IEEE 802.15.4 devices, indicate that this approach is feasible for dynamic scenarios where communicating devices are placed in non-line-of- sight positions on the body. We also suggest an enhancement for existing key generation mechanisms using a filtering mechanism which considerably reduces bit mismatches. Second, we employ a channel hopping technique to decorrelate secret bit ex- traction. Due to fast sampling rates, successive samples of channel properties are correlated in time, yielding weak keys with reduced entropy. To overcome this, we use channel hopping to increase channel diversity. We conduct extensive experi- ments to show that channel hopping increases frequency diversity and effectively decorrelates successive channel samples and thereby dramatically improving the strength of the secret key. Furthermore, we identify key parameters affecting performance, namely channel spacing, the number of available channels, and user activity. We show that it is possible to devise an optimal hopping strategy to maximize the benefit by balancing between channel spacing and the number of channels for high frequency and temporal diversity. In this thesis, we describe, implement and validate our solutions in real body- worn scenarios. It is hoped that our effort contributes to the research and devel- opment of usable body sensor networks.
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(2012)ThesisImproving energy efficiency of Internet equipment is becoming an increasingly important research topic, motivated by the need to reduce energy costs (and Carbon footprint) for Internet Service Providers, as well as increase of power density to achieve more switching capacity per-rack. To clearly understand how power consumption of Internet equipment is determined by the network elements such as sleep or active states of the device components, number of active ports or links, traffic patterns and network architectures, researchers have profiled the energy demand of commercial routing platforms and proposed ways to reduce network power consumption. However, these early works usually profile only coarse-grained power models (i.e., at the granularity of per line-card or per port) and the proposed power saving solutions involve significant architectural and/or protocol changes in the network. The cost and risk associated with such drastic changes increase the barrier to adoption by network operators, thus stretching the time-horizon at which they become practical for wide-scale deployment. In this thesis we profile fine-grained power models for Internet equipment and propose a power saving scheme that requires minimal changes to existing router design, carries little risk of impacting network performance, is almost entirely transparent to network operators and is ready for incremental deployment. We first profile power consumption of NetFPGA, an increasingly popular routing platform for networking research due to its versatility and low-cost, by conducting several experiments that allow us to decompose the power consumption of the NetFPGA routing card into fine-grained per-packet and per-byte components with reasonable accuracy. This work opens the doors for estimating network-wide energy footprints at the granularity of traffic sessions and applications (e.g., due to TCP file transfers), and provides a benchmark against which energy improvements arising from new architectures and protocols can be evaluated. Second, we analyse the power consumption of Energy Efficient Ethernet (EEE) switches in several experiments and based on the results propose a power model to profile them. Energy Efficient Ethernet is an IEEE standard (802.3az) for improving energy efficiency in Ethernet devices, which was newly issued in Sep. 2010. Our work is the first evaluation of power consumption of EEE switches. The proposed model can be used to predict the energy savings when deploying the new switches and also for research on further power saving techniques such as energy efficient routing or dynamic link shutdown. Third, we propose a simple and practical algorithm for activating buffers in backbone router line-cards incrementally as needed and putting them to sleep when not in use. We evaluate our algorithm on traffic traces from carrier and enterprise networks, via simulations in ns2, and by implementing it on a programmable-router test-bed. Our study shows that much of the energy associated with off-chip packet buffers can be eliminated with negligible impact on traffic performance. Dynamic adjustment of active router buffer size provides a low-complexity low-risk mechanism of saving energy that is amenable for incremental deployment in networks today.
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(2013)ThesisDetecting the presence of a charge in a region of a materialâ or charge sensingâ is a crucial requirement for many solid state quantum computer implementations. Quantum point contacts are recognised as excellent charge sensing elements, and can also aid the study of the physics behind carrier transport in semiconductors. This thesis dissertation presents the fabrication and measurement results of a quantum point contact, based on silicon MOSFET technology. The device, employing a multiple gate stack, was fabricated with electron beam lithography and then measured at 4.2 Kelvin to explore the prospects for quantised conductance in a silicon nanostructure.
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(2013)ThesisPlasma-assisted ignition technology has been proposed to boost the combustion efficiency of hypersonic ramjets during high speed fight. One technique utilises high-voltage nanosecond-duration pulses, which can generate free radicals thereby initiating ignition earlier in the combustion chamber and improving fuel efficiency. A high-voltage nanosecond pulse generator is an integral part of the system. In this thesis, a high-voltage nanosecond pulse generation system, employing an inductive voltage adder, is designed, built and tested. The high-voltage nanosecond pulse generation system, utilising multiple high-speed, high-voltage MOSFETs, can generate width-adjustable pulses (from 20ns to 50ns) with fast rise time (< 5ns), fast fall time (< 20ns) and variable amplitude using multiple switch cells. Using the inductive voltage adder, the system is configured in three different ways: single switch cell, two switch cells coupled in parallel and two switch cells coupled in series. The performance and features of each configuration are simulated and experimentally validated. The parallel-coupled two switch configuration increases the peak current capability of the system for a given MOSFET current rating. The increased distributed capacitance is a dominant factor, which leads to mismatch of the drain-to-source voltage at turn-off which increases the output pulse width. The series-coupled two switch configuration doubles the peak voltage of the output pulse. However, the increased leakage inductance is a major contributor to increased rise and fall time of the output pulse and this is demonstrated experimentally. An investigation on the impact of residual flux density on the pulse shape is also presented in this thesis. Without resetting the core, the output pulse will experience increased transient time and decreased peak amplitude. The investigation demonstrates that changing the residual flux density is an alternative method of adjusting the output pulse width where core saturation is used to terminate the pulse. The transient response of the output pulse is determined by the time constant of the leakage inductance and the load resistance. The experimental result demonstrates that the parallel coupling configuration can be used to decrease the time constant thereby improving the quality of the output pulse; series coupling increases the time constant. It is shown that reducing the leakage inductance plays key role in delivering high-quality pulses and that when utilising series coupling it may well be a requirement to also parallel-couple cells to limit the increase in leakage inductance.
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(2010)ThesisIn traditional infrastructural wireless local area network (WLAN), the mobile node (MN) makes the decision of choosing an access point (AP). The MN creates a list of APs in range along with the received signal strength indicator (RSSI). It then solely uses the RSSI as a decision metric for selecting an AP to connect to as this is the only information available at the MN. We argue that a MN is not the correct entity in WLAN for making the choice of an AP when many APs are available as it does not have the complete view of the environment. Secondly, choosing an AP solely on the basis of RSSI is not an efficient algorithm. This can lead to concentration of MNs at single AP resulting in a decreased average throughput for every MN associated to that AP. In this thesis, we propose to transfer this decision to the AP in a transparent manner. While our solution exists for single administrative domain with a centralized controller, we propose a completely distributed architecture for personal WLANs where APs select to serve MNs based on the MN concentration, network load, throughput and effect of serving a new MN on the network. The APs in different networks collaborate among one another in a completely decentralized manner to provide unified network access to MNs with focus on maximizing the system capacity. The broadcast nature of wireless is used in an intelligent manner to select dynamic uplink and downlink paths between APs and MNs.
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(2012)ThesisRecently, Erdal Arikan proposed a method “channel polarization” and then introduced polar codes based on this method. Polar codes are a breakthrough in coding theory because they are the first kind of codes to be proved to achieve capacity for a wide range of channels with linear encoding and decoding complexity O(N logN), where N is the blocklength of the code. In this work we investigate the construction of polar codes under additive white Gaussian noise (AWGN) channel and then improve their performance. The first problem we consider is the construction of polar codes under AWGN channel. In specific, polar codes are constructed based on Gaussian approximation. The formula of calculating Bhattacharyya parameter is also derived. The performance and implementation complexity of our scheme and the existing schemes are compared. Results show that the polar codes we construct are efficient, practical and achieve a good tradeoff between decoding performance and implementation complexity. To further improve the error performance, turbo polar codes are proposed. Turbo polar codes are constructed by concatenating two polar codes parallelly. An iterative decoding method is adopted to decode turbo polar codes. The encoder and decoder of turbo polar codes are designed. Besides, we analyze the performance of turbo polar codes by considering the effect of iteration number, interleaver size and different decoding algorithms. Moreover, the interleaver structure is devised to improve the performance. Finally, we compare turbo polar codes with polar codes and show that turbo polar codes achieve a better performance.
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(2013)ThesisEvery day, millions of people ask what time it is. Depending on the person and their reason for asking, the required accuracy and precision of an answer varies. Humans are not the only ones that care about time though, (perhaps more importantly) countless electronic devices ranging from cell phones to satellites do as well. Without accurate and precise frequency references and timekeeping systems, many of the electronics in use today would function incorrectly. The primary goal of this thesis was to achieve both synchronization and syntonization using a Global Positioning System (GPS) receiver called Namuru, which was developed at the University of New South Wales (UNSW). In the former case, this means that the pulse per second (PPS) signals output by the receiver should be aligned with the second boundaries of GPS Time (GPST). In the latter case, this means that the frequency of the voltage-controlled temperature-compensated crystal oscillator (VC-TCXO) onboard the receiver should be equal to its setpoint. The secondary goal of this thesis was to develop a clock model that accurately simulated the synchronization and syntonization performance of the receiver. The tertiary goal of this thesis was to develop supplementary features for the receiver like position-hold and holdover modes. In order to achieve these goals, several new capabilities were added to the receiver and much work was done in a number of different areas, such as field-programmable gate array (FPGA) programming, writing firmware, modifying hardware, and modeling.