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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.
1

Network and system support for interactive embedded applications

Brown, James January 2011 (has links)
Recent advances in a number of relevant technologies have enabled wireless embedded systems to become both smaller and more capable. This has led to a massive expansion in their use and many novel applications are emerging. Many new applications require direct user interaction and in this way they differ from embedded applications currently deployed. User interaction requires sporadic availability of resources for event handling. Furthermore, processing cannot be delayed for long, as smooth system operation must be ensured. Existing wireless embedded systems are designed to conserve resources such that energy saving goals are met and systems can support extended deployment periods. Resource conservation is primarily achieved via prediction of system usage patterns and by delaying processing. However, these strategies cannot be applied in the context of interactive applications as usage patterns are less predictable and processing cannot be delayed. Thus, existing methods are not best suited to novel interactive wireless embedded systems, and novel strategies are required. This thesis examines a set of interactive embedded applications derived from the NEMO project which are shown to include the requirements and traits discussed above. In this thesis it is shown through the design and implementation of the modular NEMO device that interactive embedded systems with a modular design and exclusive use of interrupt driven inputs are able to support interactive application requirements in terms of responsiveness and resource consumption. Core contributions of this thesis are a novel wake-up radio required for practical implementation of interrupt driven operation and a set of communication primitives required to operate a dual radio architecture. The thesis shows that in the target application scenario all existing communication protocols are out-performed by communication protocols using a dual radio. This is shown in general via theoretical comparison of state-of-the-art and dual radio communication and by evaluation based on the constructed prototype system.
2

Mobility, AAA, security, privacy : a unified architecture to enable real-world host and network mobility

Georgopoulos, Panagiotis January 2012 (has links)
Supporting the requirements of mobile networking is a very exciting and challenging task that has the potential to positively influence people's every day communication and Internet experience. The exponential increase in the number of commuters requiring Internet connectivity and a variety of services at their fingertips, on their mobile devices on-the-go, highlights the importance and attention that this problem domain requires from the research community. Host and network mobility protocols have been developed in recent years to provide constant connectivity to commuters' mobile devices, moving individually or as a group, but are yet to be widely deployed in real-world scenarios, because many requirements in this problem domain remain unresolved. Commuters' mobile devices require quick, unobtrusive and reliable connectivity from the Wi-Fi access points they encounter as they move. Individuals require constant and secure connectivity without having to congure their devices as they roam from one Access Network to another. Obtaining secure and authenticated access and developing trust dynamically with the wireless Access Network they connect to is very challenging, especially due to the Access Network's own conflicting requirements. Wi-Fi Access Networks need to support Authentication, Authorization and Accounting (AAA) for mobile devices, whilst avoiding compromising their security policies. On the other hand mobile devices have privacy concerns and they may not want to reveal their identity to the Access Network they get connectivity from. Roaming Mobile Networks, such as the group of mobile IP devices that people carry in the bus as the bus moves, have the same and additional, even more complicated, requirements to the ones individual roaming Mobile Hosts have. The mobile router that facilitates all the devices within the Mobile Network must ensure services' provision to them in an unobtrusive manner and avoid exhausting their resources. A vast range of problems arise when Mobile Networks interconnect to one another to proliferate connectivity, as current network mobility protocols introduce routing and tunnelling inefficiencies in the communication of these networks. In addition, the lack of dynamic trust establishment and secure data transmission for interconnected Mobile Networks, requirements of paramount importance in mobile networking, make their real-world deployment even more difficult. This research thesis presents a Unied Architecture that facilitates the Mobility, AAA, Security and Privacy requirements of Mobile Hosts and Mobile Networks in an efficient and modular fashion for real-world deployments. The novelty and originality of our research is on addressing the requirements of all the parties involved in a holistic and non-conflicting manner, and allowing true mobility to be achieved in real-world scenarios. Our work contributes in the field of mobile networking by providing a AAA and Security model that allows Mobile Hosts and Mobile Networks to obtain quick, secure and constant connectivity, even in the most complex mobility topologies that can arise. In addition, the Unied Architecture provides a unique collaborative AAA and Privacy service for mobile networking deployments, that allows Access Networks to oer private but accountable connectivity, by carefully considering their AAA and security policies. Our research work also provides an original data security and trust establishment model for interconnected Mobile Networks, that opens the way for their establishment in real-world deployments. As part of this research body of work we also developed, and present as contribution, an experimental implementation of the devised Unied Architecture, by bringing together state of the art protocol solutions and by creating implementation components that efficiently satisfy requirements in the problem domain. Finally, we thoroughly evaluated qualitatively and quantitatively the implementation of our Unied Architecture on a real live IPv6 testbed with multiple Access Network links. Our results highlight the efficiency and suitability of our approach, and demonstrate its capabilities for achieving real-world IP mobility in every day scenarios.
3

Understanding spontaneous device association

Chong, Ming Ki January 2013 (has links)
Wireless devices nowadays support flexible formation of ad hoc networks. This allows users to establish virtual connections between their devices and devices they serendipitously encountered in their environment, and in a spontaneous manner. This process is known as spontaneous device association. Spontaneous device association has been an ongoing research topic for many years. Much existing work examined technical aspects, such as suggesting new techniques using special hardware sensors and supporting robust security. However, users aspects were seldom investigated. By considering user aspects, researchers can create association techniques that are better-suited for users and enhance usability. This thesis examines aspects of how users are involved in device association, the action that people spontaneously choose for associating devices, and what researchers need to consider for creating a usable association process. This thesis takes two research approaches: conceptual analysis and action elicitation. The first part of this thesis reviews existing work and examines the past and the current trends of spontaneous device association. It presents a taxonomy structure for classifying association techniques and an in-depth summary of existing device association research. It reveals the state of the art and discusses design considerations for building usable association techniques. The second part of this thesis elicits ideas for device association. By applying a methodology where people were asked to come up with actions for associating devices without the concern of technology, it answers the question of how people intuitively associate devices, in single-operator and group scenarios. The combined knowledge from the two parts forms the core contributions of this thesis. The contributions include presenting new conceptual models, identifying a set of factors that influence usability, and discovering the rationale of how non-technical users associate devices. Together, they provide knowledge for considering users, which potentially help researchers to create usable techniques that are in-line with people's preferences and expectation in spontaneous device association.
4

Orthogonal frequency division multiplexing for optical access networks

Omomukuyo, O. O. January 2013 (has links)
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme with numerous advantages that has for years been employed as the leading physical interface in many wired and wireless communication systems. Recently, with advancements made in digital signal processing, there has been a surge of interest in applying OFDM techniques for optical communications. This thesis presents extensive research on optical OFDM and how it is being applied in access networks. With the aid of theoretical analysis, simulations and experiments, it is shown that the system performance of direct-detection optical OFDM (DD-OOFDM) in the presence of MZM non-linear distortion can be improved by proper biasing and selection of appropriate drive to the MZM. Investigations are conducted to illustrate how a variation in the number of subcarriers and the modulation format influences the sensitivity of the DD-OOFDM system to the MZM non-linear distortion. The possibility of improving the spectral efficiency by reduction of the width of the guard band is also investigated. This thesis also looks into the radio-over-fibre (RoF) transmission of Multiband OFDM UWB as a transparent and low-cost solution for distributing multi-Gbit/s data to end-users in FTTH networks. Due to relaxed regulatory requirements and the wide bandwidth available, UWB operation in the 60-GHz band is also considered for this FTTH application scenario. Four techniques for enabling MB-OFDM UWB RoF operation in the 60-GHz band are experimentally demonstrated. The impacts of various parameters on the performance of the techniques as well as the limitations imposed by fibre distribution are illustrated. Finally, a digital pre-distorter is proposed for compensating for the MZM non-linearity. Experimental demonstration of this digital pre-distortion in an UWB over fibre transmission system shows an increased tolerance to the amplitude of the driving OFDM signal as well as an increase in the optimum modulation index of the OFDM signal.
5

Rapid prototyping of a fixed-complexity sphere decoder and its application to iterative decoding of turbo-MIMO systems

Barbero Liñan, Luis G. January 2007 (has links)
This thesis concentrates on the analysis of the sphere decoder (SD) for MIMO detection. It provides optimal maximum likelihood (ML) performance with reduced complexity compared to the maximum likelihood detector (MLD). However, a field-programmable gate array (FPGA) implementation of the algorithm presents several disadvantages due to its variable complexity and the sequential nature of its tree search. This research proposes a fixed-complexity sphere decoder (FSD) to overcome the drawbacks of the SD. It provides a fixed complexity and achieves quasi-maximum likelihood (ML) performance, combining a search through a small subset of the transmitted constellation with a novel channel matrix ordering. This represents a novel approach compared to most optimizations of the SD in the literature, which concentrate on reducing the average complexity of the algorithm. As a result, an implementation of the FSD is shown to provide the same error performance using less FPGA resources and achieving a considerably higher (and constant) throughput compared to previous SD hardware implementations. The same FSD concept is applied to a large MIMO system with 4 antennas at both ends of the link and 64-quadrature amplitude modulation (QAM). A list extension of the FSD (LFSD) combines the same channel matrix ordering and an extended fixed search to generate a list of candidates for short-value calculation. Depending on the size of the extended search, different levels of performance and complexity can be achieved making the algorithm suitable for reconfigurable architectures. Its FPGA implementation shows how soft-value information can be obtained with a fully pipelined architecture. It provides a constant throughput which is considerably higher than previously presented soft-MIMO detector implementations.
6

Coding of synthetic aperture radar data

Bhattacharya, Sujit January 2009 (has links)
Synthetic Aperture Radar (SAR) is a dedicated high-resolution sensor with imaging capability in all weather and day-night conditions and has been employed in several earth and interplanetary observation applications. A significant characteristic of this system is the generation of a large amount of data that involves major problems related to on-board data storage and downlink transmission. The near future SAR satellite missions planned would be pushing downlink data bandwidths to prohibitive levels, which dictate efficient on-board compression of raw data. Due to the limitation of the on-board resources in the satellite, it is desirable to have computationally efficient encoder. In this thesis we address the compression of complex-valued SAR raw data in the Compressed Sensing (CS) framework, in which the encoder is simple whereas the decoder is computational expensive. CS is an emerging technique for signal measurement and reconstruction that takes advantage of the fact that many signals are sparse under some basic or frame. The measurement of the signal in the CS framework is obtained by taking a small number of projections of the signal onto an incoherent basis. For the SAR raw data compression here we have considered a simple encoder, with a 2D-FFT followed by a random sampler. The reconstruction of the sparse coefficients of the signal from these projections is then based on the sparsity induced optimization techniques like Orthogonal Matching Pursuit (OMP) and iterative reconstruction methods. We demonstrate empirically that the CS framework for compression of complex-valued SAR raw data is effective for the cases when the SAR image is sparse in the spatial domain. We also address the limitations of this framework while dealing with actual satellite images due to lack of good sparsifying transforms for the complex-valued data. In this thesis, we present a new algorithm based on regularized iterative algorithm for finding sparse solution for the complex-valued data in which the regularization parameter is adaptively computed in each iteration. The effectiveness of the new algorithm is compared with existing methods like Basis Pursuit, OMP, etc with both real and complex data set.
7

Spread spectrum receiver architectures for mobile channels subject to multipath fading

Povey, Gordon J. R. January 1992 (has links)
Recently there has been considerable interest in spread spectrum techniques for use in commercial communication systems. One particular area where there may be considerable benefit is in mobile radio systems. Many of the difficulties presented by mobile radio communications result from the multipath fading which is present on the channel. This can lead to large reductions in the available channel capacity if measures are not taken to combat its effects. The use of diversity is highly effective in ameliorating the effects of fading. This research is concerned with an anti-multipath receiver architecture, the RAKE receiver, which uses the inherent multipath diversity of the received spread spectrum signal. As a precursor to this work we describe a spread spectrum correlator architecture, the serial-parallel receiver, which is capable of resolving the multipath diversity signals which are subsequently used in the RAKE receiver. A digital serial-parallel correlator is developed which has negligible implementation loss, fast PN code synchronisation, and can provide very large processing gains. The performance obtained from this spread spectrum receiver architecture is demonstrated using a DSP based implementation, and is shown to improve upon previous analogue implementations in most respects. It is particularly attractive for mobile communications since it can resolve the multipath signal components without the need for a large time-bandwidth product matched filter, or multiple active correlators. The performance of an adaptive RAKE receiver, using simple alpha tracker profile estimation filters, is analysed for a simulated UFG mobile channel (vehicular and hand-held) for a typical urban environment. Bit error rate graphs indicate that the performance approaches the theoretical upper bound when the signal fading is slow relative to the bit rate of the system (e.g. hand-held operation).
8

Perceptually motivated blind source separation of convolutive audio mixtures

Guddeti, Ram Mohana Reddy January 2005 (has links)
The first objective of this thesis is to apply psycho-acoustic principles to the spatial processing of speech signals in noisy and reverberant environment. The key assumption that will be adopted is that modern signal processing has failed to mimic the <i>cock-tail party effect</i> because there has been no attempt to adequately incorporate the psycho acoustical phenomenon of audio masking to aid source separation. A quasi linear mechanism for mimicking <i>simultaneous frequency</i> masking and<i> temporal masking</i> (post masking) techniques are developed. This frame work is used to construct blind source separation algorithms that exploit audio masking prior to source separation (preprocessor) and after source separation (postprocessor). The final objective of this thesis is to exploit the perceptual irrelevancy of some of the input speech spectrum using the perceptual masking techniques before utilizing the subspace method as a preprocessor of the frequency-domain ICA (FDICA) which reduces the effect of room reflections in advance and the remaining direct sounds then being separated by ICA. Incorporating the perceptual masking techniques prior to the application of FDICA with the subspace method as preprocessor not only reduces the computational complexity of similarity measure for solving the permutations but also avoids the so-called permutation problem by targeting a specific speech signal more intelligible than the available microphone signals.
9

Statistical single channel source separation

Darsono, Abd Majid January 2012 (has links)
Single channel source separation (SCSS) principally is one of the challenging fields in signal processing and has various significant applications. Unlike conventional SCSS methods which were based on linear instantaneous model, this research sets out to investigate the separation of single channel in two types of mixture which is nonlinear instantaneous mixture and linear convolutive mixture. For the nonlinear SCSS in instantaneous mixture, this research proposes a novel solution based on a two-stage process that consists of a Gaussianization transform which efficiently compensates for the nonlinear distortion follow by a maximum likelihood estimator to perform source separation. For linear SCSS in convolutive mixture, this research proposes new methods based on nonnegative matrix factorization which decomposes a mixture into two-dimensional convolution factor matrices that represent the spectral basis and temporal code. The proposed factorization considers the convolutive mixing in the decomposition by introducing frequency constrained parameters in the model. The method aims to separate the mixture into its constituent spectral-temporal source components while alleviating the effect of convolutive mixing. In addition, family of Itakura-Saito divergence has been developed as a cost function which brings the beneficial property of scale-invariant. Two new statistical techniques are proposed, namely, Expectation-Maximisation (EM) based algorithm framework which maximizes the log-likelihood of a mixed signals, and the maximum a posteriori approach which maximises the joint probability of a mixed signal using multiplicative update rules. To further improve this research work, a novel method that incorporates adaptive sparseness into the solution has been proposed to resolve the ambiguity and hence, improve the algorithm performance. The theoretical foundation of the proposed solutions has been rigorously developed and discussed in details. Results have concretely shown the effectiveness of all the proposed algorithms presented in this thesis in separating the mixed signals in single channel and have outperformed others available methods.
10

OFDM base T-transform for wireless communication networks

Ahmed, Mohammed Shweesh January 2012 (has links)
The prominent features associated with orthogonal frequency division multiplexing (OFDM) have been exploited in the area of high-speed communication networks. However, OFDM is prone to impairments such as frequency selective fading channel, high peak-to-average power ratio (PAPR) and heavy-tailed distributed impulsive noise, all of which can have negative impacts on its performance. These issues have received a great deal of attention in recent research. To compensate for these transmission impairments, a T-OFDM based system is introduced using a low computational complexity T-transform that combines the Walsh-Hadamard transform (WHT) and the discrete Fourier transform (DFT) into a single fast orthonormal unitary transform. The key contribution in this thesis is on the use of the T-transform along with three novel receiver designs. Additionally, new theoretical bit error rate (BER) formulae for the T-OFDM system are derived over communications channels using zero forcing (ZF) and minimum mean square error (MMSE) detectors, that are validated via simulation and shown to have close performance with the obtained performance results. It has been found that the T-OFDM outperformed the conventional OFDM based systems in the investigated channel models by achieving a signal-to-noise ratio (SNR) gain range of between 9dB and 16dB measured at 10−4 BER. In addition, the sparsity and block diagonal structure of the T-transform, along with its lower summation processes are exploited in this study to reduce the superposition of the subcarriers, leading to reduce the peak of the transmitted signals by a range of 0.75 to 1.2 dB with preserved average power. Furthermore, these attractive features of T-transform are employed with the conventional selective mapping (SLM) and partial transmitted sequences (PTS) schemes to propose three low cost novel techniques; T-SLM, T-PTS-I, and T-PTS-II. Compared to the conventional schemes, the T-SLM and T-PTS-I schemes have achieved a considerable reduction in both computational complexity and in PAPR, further increasing multipath resilience, even in the presence of high power amplifier (HPA). Whereas using the T-PTS-II scheme, the complexity ratio has been significantly reduced by approximately 80%, as well as reducing the SI bits further by two, with negligible PAPR degradation. Moreover, the effect of the independent sections of T-transform on the performance of T-OFDM system over the impulsive channel is addressed in this work, by deriving a new theoretical BER formula over such a transmission media. Furthermore, two novel II schemes WHT-MI-OFDM and WHT-MI-OFDM incorporating nonlinear blanking, both of which utilise the WHT and a matrix interleaver (MI) with the OFDM system, are proposed to suppress the deleterious effects of a severe impulsive noise burst on the T-OFDM system performance. Comparing with the traditional MI-OFDM system, the proposed schemes are much more robust to disturbances arising from the impulsive channel.

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