The aim of this thesis is to compare various simulation models of orthogonal cutting process with each other as well as with various experiments. The effects of several process parameters, such as friction and separation criterion, on the results are analyzed. As simulation tool, commercial implicit finite element codes MSC.Marc, Deform2D and the explicit code Thirdwave AdvantEdge are used. Separation of chip from the workpiece is achieved either only with continuous remeshing or by erasing elements according to the damage accumulated. From the results cutting and thrust forces, shear angle, chip thickness and contact length between the chip and the rake face of the tool can be estimated. For verification of results several cutting experiments are performed at different cutting conditions, such as rake angle and feed rate. Results show that commercial codes are able to simulate orthogonal cutting operations within reasonable limits. Friction is found to be the most critical parameter in the simulation, since good agreement can be achieved for individual process variables by tuning it. Therefore, simulation results must be assessed with all process variables and friction parameter should be tuned according to the shear angle results. Plain damage model seems not appropriate for separation purposes of machining simulations. On the other hand, although remeshing gives good results, it leads to the misconception of crack generation at the tip of the tool. Therefore, a new separation criterion is necessary to achieve both good physical modeling and prediction of process variables.

The generalized moment of order k of a mass distribution sigma for a natural number k is given by integral of lambda to the power k with respect to mass distribution sigma and variable lambda. In extended moment problem, given a sequence of real numbers, it is required to find a mass distribution whose generalized moment of order k is k&#039 / th term of the sequence. The conditions of existence and uniqueness of the solution obtained by Hamburger are studied in this thesis by the use of orthogonal polynomials determined by a measure on real line. A chapter on the study of asymptotic behaviour of orthogonal functions on compact subsets of complex numbers is also included.

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Orthogonal frequency division multiplexing (OFDM) has become a popular modulation method in high-speed wireless communications. By partitioning a wideband fading channel into flat narrowband channels, OFDM is able to mitigate the detrimental effects of multipath fading using a simple one-tap equalizer. However, in the time domain OFDM signals suffer from large envelope variations, which are often characterized by the peak-to-average ratio (PAR). High PAR signals, like OFDM, require that transmission amplifiers operate at very low power efficiencies to avoid clipping. In this dissertation, we explore the problems associated with transmitted OFDM signals through peak limited channels. A large part of this work deals with analyzing different distortion metrics and determining which metrics are most useful. We find that the signal-to-noise-plus-distortion ratio (SNDR) is one of the most important metrics in assessing distortion in nonlinear channels. As part of this analysis, we compare sample-based SNDR and symbol-based SNDR and find that using the more comprehensive symbol-based metric as the objective in SNDR maximization algorithms leads to only marginal SNDR improvements. The SNDR perspective is also applied to existing PAR-reduction techniques to compare existing schemes and proposed new schemes. Part of this work involves deriving a SNDR maximizing adaptation of the popular PAR-reduction scheme, selected mapping (SLM). We also compare another popular PAR-reduction method, partial transmit sequence (PTS), to SLM through a variety of metrics including SNDR and found that for any given amount of complexity or side information SLM provided better performance. The next major piece of work in this dissertation addresses synchronization and channel estimation in peak-limited channels for OFDM. We build off of existing work that shows that embedded synchronization energy is a more bandwidth efficient means of synchronization than preamble-base methods. With this, we demonstrate a method for generating embedded sequences that have low PAR, and thus minimize the PAR of the combination OFDM symbol/embedded sequence among all embedded sequences. Next, we extend this work to sequences called joint synchronization-pilot sequences (JSPSs) by deriving the symbol-estimate mean squared error (MSE) pilot placements for the JSPSs and by showing how the JSPSs can be used with SLM for blind detection. Finally, the dissertation concludes with a derivation of the SNDR-optimal transmitter/receiver pairs. Using functional analysis, we show that the SNDR-optimal receivers for peak-limited transmitters are not linear. Instead they follow non-linear functions that depend on the noise and signal distributions.

This doctoral thesis deals with designs and developments of multi(wide)-band multifunctional antennas based on folded dipoles. In the beginning, the concept of Orthogonal Folded Dipoles (OFD) are put forward. Orthogonal folded dipoles are formed by two identical two-port folded dipoles orthogonally joining with each other at the center, and can be fed through different combinations of feed nodes to offer dual- and wide-band modes, respectively. The impedances of the 2 modes are studied both by analytical models and by commercial electromagnetic simulation tools. The properties of the linearly polarized radiation patterns in the two modes vary quite little, except for ±45◦  rotation of two principal planes. In this way, orthogonal folded dipoles can possibly provide pattern reconfigurability in a context of switchable types and orientations of polarizations. Next, the concept of Log-Periodic Folded Dipole Array (LPFDA) is proposed. It stems from the traditional log-periodic dipole array, whereas folded dipoles instead of straight dipoles are applied as the elements of the array. Two configurations, i.e. partly-scaled LPFDA and fully-scaled LPFDA, are studied through simulations and optimizations. The comparison shows that the latter outperforms the former in terms of higher directivity, reduced front-to-back ratio and lower crosspolar level. The key parts of this thesis focus on exploiting Eleven antennas, based on the LPFDAs, in a variety of applications. First, the 4-port L-band lab model for use in satellite terminals demonstrates that the radiation patterns for monopulse tracking can be achieved through different excitation combinations of the multiport Eleven antenna. Second, a decade bandwidth, an unchanged phase center and nearly constant directivities over the whole band can be regarded as the major figure-of-merit of the Eleven antenna, which makes it suitable as feed for prime-fed reflector antennas. Through endeavors of using Genetic Algorithms, the wideband Eleven antennas have been gradually optimized in term of matching as well as efficiencies. Besides, the rotationally symmetric circular Eleven feed is a very promising solution for improving the BOR1 efficiency and therefore the aperture efficiency by a fraction of dB. Last but not least, the two multiport L-band lab models measured in a reverberation chamber demonstrate that the Eleven antenna with MIMO diversity ports can possibly overcome narrowband multipath fading in a real radio link and improve the link quality in terms of a significant diversity gain and high maximum available capacity.

Block codes are one of the most widely used codes to improve reliability of data transmissions. They are used both independently as well as with other codes such as convolutional codes and have found many applications in many areas, ranging from space communications to digital versatile discs (DVD). More recently, powerful codes derived from block and iteratively decoded codes have also been adopted in serveral standards (e.g. DVB-S2). The first part of this thesis deals with the design of computationally efficient soft-decision decoding algorithms for block codes. / A bandwidth-efficient modulation technique called orthogonal frequency division multiplexing (OFDM) has been adopted in many international standards to achieve high speed data transmissions over frequency selective fading channels. OFDM signals however, have high amplitude fluctuations. This is known as the peak-to-average power ratio (PAPR) problem. The second part of this thesis focuses on designing a coding scheme to simultaneously correct errors and reduce the PAPR of OFDM signals. / Soft-decision decoding of block codes provide significant performance gains over hard-decision decoding. However, optimal soft-decision decoding is an NP-hard problem, where the decoding complexity grows exponentially with the code length. This thesis develops two computationally efficient sub-optimal soft-decision decoding algorithms by formulating soft-decision decoding as an optimisation problem. The two algorithms are based on a compact genetic algorithm and a k shortest paths algorithm, respectively. The performance and complexity of these algorithms are investigated and compared with various other known decoding schemes. The results obtained show that the proposed decoding algorithm achieves large performance gains over the known decoding schemes. It is also observed that the proposed algorithms achieve near-optimal performance with manageable complexity. / In addition to these soft-decision decoding algorithms, this thesis also proposes a coding technique and an efficient encoding algorithm for joint error-correction and PAPR reduction of OFDM signals. The proposed coding technique is first expressed as an optimisation problem and a computationally efficient sub-optimal algorithm is then proposed to solve this problem. The PAPR reduction and error-correction performance of the proposed algorithm are studied. The results show that the proposed algorithm significantly improves the system performance and also gives PAPR reductions comparable with other known PAPR reduction techniques. / Thesis (PhDTelecommunications)--University of South Australia, 2007.

The efficient use of radio resources is crucial in order for future wireless systems to be able to meet the demand for high speed data communication services. Orthogonal Frequency Division Multiplexing (OFDM) is an important technology for future wireless systems as it offers numerous advantages over other existing technologies, such as robust performance over multipath fading channels and the ability to achieve high spectral efficiency. Dynamic resource allocation can fully exploit the advantages of OFDM, especially in multiple user systems. In this thesis, we investigate a resource allocation problem in a multiple user, multiple cell OFDM cellular network focusing on downlink communications. (For complete abstract open document)

Thesis (Ph.D.)--Tufts University, 2003. / Adviser: Joseph P. Noonan. Submitted to the Dept. of Electrical Engineering. Includes bibliographical references (leaves 149-149). Access restricted to members of the Tufts University community. Also available via the World Wide Web;

Thesis (MScIng)--University of Stellenbosch, 2006. / Bibliography. Also available via the Internet.

Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008. / Includes bibliographical references (p. 59-63). Also available in electronic version.