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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Quality of service for voice over next generation networks.

Perumal, Eugene Govindhren. January 2004 (has links)
The Global communications transformation is currently in progress. Packet switched technology has moved from data - only applications into the heart of the network to take up the functions of traditional circuit - switched equipment. Voice over A TM(VoATM) and voice over IP(VoIP) are the two main alternatives for carrying voice packets over NGN' s. A TM offers the advantage of its built in quality of service mechanisms. IP on the other hand could not provide QoS guarantees in its traditional form. IP QoS mechanisms evolved only in recent years. There are currently no QoS differences between Next Generation Networks based on VoATM or VoIP. However non QoS agreements are more in favour of VoIP instead of VoA TM. This gives VoIP the leading edge bet the Voice over packet technologies. In this thesis the E - Model was optimized and used to study the effects of delay, utilization and coder design on voice quality. The optimization was used to choose a coder and utilization levels given certain conditions. An optimization algorithm formed through the E - Model was used to assist with the selection of parameters important to VoIP networks. These parameters include the link utilization, voice coder and allowable packet loss. This research also shows us that different utilization, voice coder and packet loss levels are optimal in different situations. A remote and core VoIP Network simulation model was developed and used to study the complex queuing issues surrounding VoIP networks. The models look at some of the variables that need to be controlled in order to minimize delay. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2004.

Video object segmentation and tracking.

Murugas, Themesha. 31 March 2014 (has links)
One of the more complex video processing problems currently vexing researchers is that of object segmentation. This involves identifying semantically meaningful objects in a scene and separating them from the background. While the human visual system is capable of performing this task with minimal effort, development and research in machine vision is yet to yield techniques that perform the task as effectively and efficiently. The problem is not only difficult due to the complexity of the mechanisms involved but also because it is an ill-posed problem. No unique segmentation of a scene exists as what is of interest as a segmented object depends very much on the application and the scene content. In most situations a priori knowledge of the nature of the problem is required, often depending on the specific application in which the segmentation tool is to be used. This research presents an automatic method of segmenting objects from a video sequence. The intent is to extract and maintain both the shape and contour information as the object changes dynamically over time in the sequence. A priori information is incorporated by requesting the user to tune a set of input parameters prior to execution of the algorithm. Motion is used as a semantic for video object extraction subject to the assumption that there is only one moving object in the scene and the only motion in the video sequence is that of the object of interest. It is further assumed that there is constant illumination and no occlusion of the object. A change detection mask is used to detect the moving object followed by morphological operators to refine the result. The change detection mask yields a model of the moving components; this is then compared to a contour map of the frame to extract a more accurate contour of the moving object and this is then used to extract the object of interest itself. Since the video object is moving as the sequence progresses, it is necessary to update the object over time. To accomplish this, an object tracker has been implemented based on the Hausdorff objectmatching algorithm. The dissertation begins with an overview of segmentation techniques and a discussion of the approach used in this research. This is followed by a detailed description of the algorithm covering initial segmentation, object tracking across frames and video object extraction. Finally, the semantic object extraction results for a variety of video sequences are presented and evaluated. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005

Traffic modeling in mobile internet protocol : version 6.

Mtika, Steve Davidson. January 2005 (has links)
Mobile Internet Protocol Version 6 (lPv6) is the new version of the Internet Protocol (IP) born out of the great success of Internet Protocol version 4 (IPv4). The motivation behind the development of Mobile IPv6 standard stems from user's demand for mobile devices which can connect and move seamlessly across a growing number of connectivity options. It is both suitable for mobility between subnets across homogenous and inhomogeneous media. The protocol allows a mobile node to communicate with other hosts after changing its point of attachment from one subnet to another. The huge address space available meets the requirements for rapid development of internet as the number of mobile nodes increases tremendously with the rapid expansion of the internet. Mobility, security and quality of service (QoS) being integrated in Mobile TPv6 makes it the important foundation stone for building the mobile information society and the future internet. Convergence between current network technologies: the intern et and mobile telephony is taking place, but the internet's IP routing was designed to work with conventional static nodes. Mobile IPv6 is therefore considered to be one of the key technologies for realizing convergence which enables seamless communication between fixed and mobile access networks. For this reason, there is numerous works in location registrations and mobility management, traffic modeling, QoS, routing procedures etc. To meet the increased demand for mobile telecommunications, traffic modeling is an important step towards understanding and solving performance problems in the future wireless IP networks. Understanding the nature of this traffic, identifying its characteristics and developing appropriate traffic models coupled with appropriate mobility management architectures are of great importance to the traffic engineering and performance evaluation of these networks. It is imperative that the mobility management used keeps providing good performance to mobile users and maintain network load due to signaling and packet delivery as low as possible. To reduce this load, Intemet Engineering Task Force (IETF) proposed a regional mobility management. The load is reduced by allowing local migrations to be handled locally transparent from the Home Agent and the Correspondent Node as the mobile nodes roams freely around the network. This dissertation tackles two major aspects. Firstly, we propose the dynamic regional mobility management (DRMM) architecture with the aim to minimize network load while keeping an optimal number of access routers in the region. The mobility management is dynamic based on the movement and population of the mobile nodes around the network. Most traffic models in telecommunication networks have been based on the exponential Poisson processes. This model unfortunately has been proved to be unsuitable for modeling busty IP traffic. Several approaches to model IP traffic using Markovian processes have been developed using the Batch Markovian Alrival Process (BMAP) by characterizing arrivals as batches of sizes of different distributions. The BMAP is constructed by generalizing batch Poisson processes to allow for non-exponential times between arrivals of batches while maintaining an underlying Markovian structure. The second aspect of this dissertation covers the traffic characterization. We give the analysis of an access router as a single server queue with unlimited waiting space under a non pre-emptive priority queuing discipline. We model the arrival process as a superposition of BMAP processes. We characterize the superimposed arrival processes using the BMAP presentation. We derive the queue length and waiting time for this type of queuing system. Performance of this traffic model is evaluated by obtaining numerical results in terms of queue length and waiting time and its distribution for the high and low priority traffic. We finally present a call admission control scheme that supports QoS. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.

Experiments in thin film deposition : plasma-based fabrication of carbon nanotubes and magnesium diboride thin films.

Coetsee, Dirk. January 2004 (has links)
A simple, low-cost plasma reactor was developed for the purpose of carrying out thin film deposition experiments. The reactor is based largely on the Atmospheric Pressure Nonequilibrium Plasma (APNEP) design with a simple modification. It was used in an attempt to fabricate magnesium diboride thin films via a novel, but unsuccessful CVD process where plasma etching provides a precursor boron flux. Carbon nanotubes were successfully synthesised with the apparatus using a plasma-based variation of the floating catalyst or vapour phase growth method. The affect of various parameters and chemicals on the quality of nanotube production was assessed. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2004.

Super-orthogonal space-time turbo codes in Rayleigh fading channels.

Pillai, Jayesh Narayana. January 2005 (has links)
The vision of anytime, anywhere communications coupled by the rapid growth of wireless subscribers and increased volumes of internet users, suggests that the widespread demand for always-on access data, is sure to be a major driver for the wireless industry in the years to come. Among many cutting edge wireless technologies, a new class of transmission techniques, known as Multiple-Input Multiple-Output (MIMO) techniques, has emerged as an important technology leading to promising link capacity gains of several fold increase in data rates and spectral efficiency. While the use of MIMO techniques in the third generation (3G) standards is minimal, it is anticipated that these technologies will play an important role in the physical layer of fixed and fourth generation (4G) wireless systems. Concatenated codes, a class of forward error correction codes, of which Turbo codes are a classical example, have been shown to achieve reliable performance which approach the Shannon limit. An effective and practical way to approach the capacity of MIMO wireless channels is to employ space-time coding (STC). Space-Time coding is based on introducing joint correlation in transmitted signals in both the space and time domains. Space-Time Trellis Codes (STTCs) have been shown to provide the best trade-off in terms of coding gain advantage, improved data rates and computational complexity. Super-Orthogonal Space-Time Trellis Coding (SOSTTC) is the recently proposed form of space-time trellis coding which outperforms its predecessor. The code has a systematic design method to maximize the coding gain for a given rate, constellation size, and number of states. Simulation and analytical results are provided to justify the improved performance. The main focus of this dissertation is on STTCs, SOSTTCs and their concatenated versions in quasi-static and rapid Rayleigh fading channels. Turbo codes and space-time codes have made significant impact in terms of the theory and practice by closing the gap on the Shannon limit and the large capacity gains provided by the MIMO channel, respectively. However, a convincing solution to exploit the capabilities provided by a MIMO channel would be to build the turbo processing principle into the design of MIMO architectures. The field of concatenated STTCs has already received much attention and has shown improved performance over conventional STTCs. Recently simple and double concatenated STTCs structures have shown to provide a further improvement performance. Motivated by this fact, two concatenated SOSTTC structures are proposed called Super-orthogonal space-time turbo codes. The performance of these new concatenated SOSTTC is compared with that of concatenated STTCs and conventional SOSTTCs with simulations in Rayleigh fading channels. It is seen that the SOST-CC system outperforms the ST-CC system in rapid fading channels, whereas it maintains performance similar to that in quasi-static. The SOST-SC system has improved performance for larger frame lengths and overall maintains similar performance with ST-SC systems. A further investigation of these codes with channel estimation errors is also provided. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.

ATM performance in rural areas of South Africa.

Mbatha, Sakhiseni J. January 2005 (has links)
Rural areas in developing countries span vast areas with a variety of climatic zones, vegetation and terrain features, which are hostile to the installation and maintenance of telecommunication infrastructures. Provision of telecommunications services to these areas using traditional wired and existing wiring telephone system with centralized network architecture becomes prohibitively expensive and not viable in many cases, because there is no infrastructure and the area is sparsely populated. Applications of wireless systems seem to provide a cost-effective solution for such a scenario. However, deployment of ATM in rural areas as a backbone technology wide area network (WAN) has not been thoroughly investigated so far. The dissertation investigates the feasibility of deployment of ATM backbone network (WAN) to be implemented in the rural. ATM is a digital transmission service for wide area networks providing speeds from 2 Megabits per second up to 155 Megabits per second. Businesses and institutions that transmit extremely high volumes of virtually error-free information at high speeds over wide area network with high quality and reliable connections currently use this service. For the purpose of saving the utilization of more bandwidth, the network should support or have a high forward bit rate, i.e. it must convey high traffic from base station to the user (i.e. upstream) than from the user to the base station (down stream). This work also investigates the features from the rural areas that degrade the performance of the networks and have a negative impact in the deployment of the telecommunications networks services. Identification of these features will lead to the suggestion of the least cost-effective telecommunication service. For the purpose of evaluating the performance and feasibility of the network, modeling of the ATM network is accomplished using Project Estimation (ProjEstim) Simulation Tool as the comprehensive tool for simulating large communication networks with detailed protocol modeling and performance analysis. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.

Opportunistic scheduling algorithms in downlink centralized wireless networks.

Yin, Rui. January 2005 (has links)
As wireless spectrum efficiency is becoming increasingly important with the growing demands for wideband wireless service scheduling algorithm plays an important role in the design of advanced wireless networks. Opportunistic scheduling algorithms for wireless communication networks under different QoS constraints have gained popularity in recent years since they have potentials of achieving higher system performance. In this dissertation firstly we formulate the framework of opportunistic scheduling algorithms. Then we propose three new opportunistic scheduling schemes under different QoS criteria and situations (single channel or multiple channel). 1. Temporal fairness opportunistic scheduling algorithm in the short term. We replicate the temporal fairness opportunistic scheduling algorithm in the long term. From simulation results we find that this algorithm improves the system performance and complies with the temporal fairness constraint in the long term. However, the disadvantage of this algorithm is that it is unfair from the beginning of simulation to 10000 time slot on system resource (time slots) allocation - we say it is unfair in the short term. With such a scheme, it is possible that some users with bad channel conditions would starve for a long time (more than a few seconds) , which is undesirable to certain users (say, real-time users). So we propose the new scheme called temporal fairness opportunistic scheduling algorithm in the short term to satisfy users ' requirements of system resource in both short term and long term. Our simulation results show that the new scheme performs well with respect to both temporal fairness constraint and system performance improvement. 2. Delay-concerned opportunistic scheduling algorithm. While most work has been done on opportunistic scheduling algorithm under fairness constraints on user level, we consider users' packet delay in opportunistic scheduling. Firstly we examine the packet delay performance under the long term temporal fairness opportunistic scheduling (TFOL) algorithm. We also simulate the earliest deadline-first (EDF) scheduling algorithm in the wireless environment. We find that the disadvantage of opportunistic scheduling algorithm is that it is unfair in packet delay distribution because it results in a bias for users with good channel conditions in packet delay to improve system performance. Under EDF algorithm, packet delay of users with different channel conditions is almost the same but the problem is that it is worse than the opportunistic scheduling algorithm. So we propose another new scheme which considers both users' channel conditions and packet delay. Simulation results show that the new scheme works well with respect to both system performance improvement and the balance of packet delay distribution. 3. Utilitarian fairness scheduling algorithm in multiple wireless channel networks. Existing studies have so far focused on the design of scheduling algorithm in the single wireless communication network under the fairness constraint. A common assumption of existing designs is that only a single user can access the channel at a given time slot. However, spread spectrum techniques are increasingly being deployed to allow multiple data users to transmit simultaneously on a relatively small number of separate high-rate channels. Not much work has been done on the scheduling algorithm in the multiple wireless channel networks. Furthermore in wire-line network, when a certain amount of resource is assigned to a user, it guarantees that the user gets some amount of performance, but in wireless network this point is different because channel conditions are different among users. Hence, in wireless channel the user's performance does not directly depend on its allocation of system resource. Finally the opportunistic scheduling mechanism for wireless communication networks is gaining popularity because it utilizes the "multi-user diversity" to maximize the system performance. So, considering these three points in the fourth section, we propose utilitarian fairness scheduling algorithm in multiple wireless channel networks. Utilitarian fairness is to guarantee that every user can get its performance requirement which is pre-defined. The proposed criterion fits in with wireless networks. We also use the opportunistic scheduling mechanism to maximize system performance under the utilitarian fairness constraint. Simulation results show that the new scheme works well in both utilitarian fairness and utilitarian efficiency of system resource in the multiple wireless channel situation. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2005.

Modeling and torque control implementtion for an 8/6 switched reluctance motor.

Wang, Sen. 30 May 2013 (has links)
This thesis begins with a brief introduction of the basic principles of operation of SRMs, and explains how flux characteristics are derived from voltage and current measurements, and presents results obtained from an 8/6 SRM. Torque characteristics are derived from these flux characteristics using both the inductance and co-energy methods. Comparison of these results with direct torque measurements shows that the co-energy method is significantly more accurate than the inductance method. Electrical and mechanical simulation models are derived from inductance and torque characteristics, and implemented in Matlab/Simulink. Simulated results are shown to agree with measurements obtained from physical locked and free rotor alignment experiments. These models are also used to illustrate the need for sophisticated commutation strategies and high performance current control loops to achieve low ripple torque control. The Matlab/Simulink models are transferred to PSCAD to compare the current control abilities, cost, complexity and robustness of the Asymmetrical Half Bridge (AHB), n+ 1 switch, and C-dump SRM converter topologies. The relatively high cost of the AHB converter is justified in terms of its robustness, simplicity and superior capabilities for current and torque control. The torque sharing function commutation strategy for low ripple torque control is presented and simulated with hysteresis current control for the 8/6 SRM fed from a four phase AHB converter. A DSP implementation of the current and torque control loops is also presented and tested under various dynamic speed and load conditions and recommendations are made for future work. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006

Real-time simulator studies and model development for time-down voltage stability analysis.

Makasa, Joseph Kangombe. January 2006 (has links)
Problems of voltage stability and voltage collapse have become a major concern in power system planning and operation in recent years, often as a result of power systems being operated under much more stressed conditions than was usual in the past. Factors that are responsible for this trend include: environmental pressures on transmission expansion; increased electricity consumption in concentrated heavy loads where installation of new generation is not feasible; new system loading patterns. Voltage stability problems are characterised by either slow or sudden voltage drops, sometimes escalating further to a collapse in voltage, leading, in some cases, to system wide blackouts. The power engineering community has devoted significant effort to developing new analysis tools and methods to control this type of instability. The main methods that have been developed and used for analysis of voltage stability are steady-state methods (power flow: analysis via PV and Q-V curves); dynamic analysis (time-domain simulations); modal analysis of system jacobian matrices and optimization (special optimal power flow). This thesis investigates the use of a particular tool, real time simulation, as a method for voltage stability analysis and testing of voltage control strategies. The particular simulator used is the Real-Time Digital Simulator (RTDS) from RTDS Technologies. The real-time simulator software environment provides generalized models of generation, transmission, distribution plant and loads that can be used to develop accurate models of power systems for analysis in real-time. The broad objectives of this thesis are to assess the suitability of the RTDS as a tool for time domain voltage stability analysis and to develop additional real-time models of particular power system controllers that are known to playa key role in voltage stability phenomena. In particular the thesis considers development of custom real-time models of a transformer on-load tap changer (OLTC) controller, detailed generator excitation controls (automatic voltage regulators), a static var compensator (SVC) controller and a synchronous condenser reactive output controller. The thesis then describes the development of real-time models of two benchmark systems for the voltage stability studies: a well known II-bus voltage stability benchmark system and a smaller (4-bus) benchmark system. These two benchmark systems are used to establish the validity and correctness of the custom real-time models and to investigate simple compensation and control strategies for voltage stabilization. In particular the thesis considers the following stabilizing techniques on the II-bus system: switched shunt capacitor compensation, voltage control using a synchronous condenser and finally the use of an SVc. Finally, the thesis demonstrates the ability of RTDS to investigate the performance of actual hardware controllers on the plant in the real-time model of the 11 bus system in a full closed loop arrangement. The custom-developed real-time software model of the OLTC controller in the II-bus benchmark system is replaced with an actual external hardware controllers connected in closed loop with the real-time simulation. This thesis has successfully confirmed the known characteristics of individual power system plant using the models provided in the RTDS environment and developed additional customized software models of controllers for voltage stability studies on the RTDS. The results of the RTDS simulations of voltage stability benchmark systems have been found to agree with documented results of these systems. The thesis has shown that the RTDS provides a suitable platform on which time-domain voltage stability studies can be conducted. The thesis has also shown that real-time digital simulation is a practicable technique for the analysis and investigation of control strategies for voltage stability, particularly when interactions between real hardware controllers and their impact on system stability are of concern. / Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, 2006

Multiple antenna systems : channel capacity and low-density parity-check codes.

Byers, Geoffrey James. January 2005 (has links)
The demand for high data rate wireless communication systems is evident today as indicated by the rapid growth in wireless subscribers and services. High data rate systems are bandwidth intensive but bandwidth is an expensive and scarce commodity. The ability of future wireless systems to efficiently utilise the available bandwidth is therefore integral to their progress and development. The wireless communications channel is a harsh environment where time varying multipath fading, noise and interference from other users and systems all contribute to the corruption of the received signal. It is difficult to overcome these problems and achieve the high data rates required using single antenna technology. Multiple-input-multipleoutput (MIMO) systems have recently emerged as a promising technique for achieving very large bandwidth efficiencies in wireless channels. Such a system employs multiple antennas at both the transmitter and the receiver. These systems exploit the spatial dimension of the wireless channel to achieve significant gains in terms of capacity and reliability over single antenna systems and consequently achieve high data rates. MIMO systems are currently being considered for 3rd generations cellular systems. The performance of MIMO systems is heavily dependent on the environment in which the system is utilised. For this reason a realistic channel model is essential for understanding the performance of these systems. Recent studies on the capacity of MIMO channels have focused on the effect of spatial correlation but the joint effect of spatial and temporal correlation has not been well studied. The first part of this thesis proposes a new spatially and temporally correlated MIMO channel model which considers motion of the receiver and nonisotropic scattering at both ends of the radio link. The outage capacity of this channel is examined where the effects of antenna spacing, array angle, degree of scattering and receiver motion are investigated. It is shown that the channel capacity still increases linearly with the number of transmit and receive antennas, despite the presence of both spatial and temporal correlation. The capacity of MIMO channels is generally investigated by simulation. Where analytical expressions have been considered for spatially correlated channels, only bounds or approximations have been used. In this thesis closed form analytical expressions are derived for the ergodic capacity of MIMO channels for the cases of spatial correlation at one end and both ends of the radio link. The latter does not lend itself to numerical integration but the former is shown to be accurate by comparison with simulation results. The proposed analysis is also very general as it is based on the transmit and receive antenna correlation matrices. Low-density parity-check (LDPC) codes have recently been rediscovered and have been shown to approach the Shannon limit and even outperform turbo codes for long block lengths. Non-binary LDPC codes have demonstrated improved performance over binary LDPC codes in the AWGN channel. Methods to optimise non-binary LDPC codes have not been well developed where only simulation based approaches have been employed, which are not very efficient. For this reason, a new approach is proposed which is based on extrinsic information transfer (EXIT) charts. It is demonstrated that by performing curve matching on the EXIT chart, good non-binary LDPC codes can be designed for the AWGN channel. In order to approach the theoretical capacity of MIMO channels, many space-time coded, multiple antenna (MA) systems have been considered in the literature. These systems merge channel coding and antenna diversity and exploit the benefits of both. Binary LDPC codes have demonstrated good performance in MA systems but nonbinary LDPC codes have not been considered. Therefore, the application of non-binary LDPC codes to MA systems is investigated where the codes are optimised for the system of interest, using a simulation and EXIT chart based design approach. It is shown that non-binary LDPC codes achieve a small gain in performance over binary LDPC codes in MA systems. / Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2005.

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