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  • (2005) Patwary, Mohammad Nuruzzaman
    Increasing demand for wireless communication challenges the availability of limited radio resources, such as bandwidth and power etc. Limited resources cause a trade off with the quality of service. The work presented in this thesis is intended to develop algorithms those can be used to demodulate information with optimal amount of resources (Signal to Noise Ratio, Processing memory requirement, Computational complexity etc ). In the first part of the thesis a decision feedback sequence detection algorithm has been proposed that provides exactly the same bit error rate as in standard maximum likelihood sequence estimation but with 95% lowmomputational complexity. Besides that the proposed algorithm achieves 2 dB signal to noise ratio (SNR) gain over the existing decision feedback algorithms. The proposed algorithm is applicable in multiple input multiple output (MIMO) as well as single input single output (SISO) wireless communication systems. In the second part of the thesis an adaptive blind sequence detection algorithm has been proposed where a novel reference channel has been exploited. The problem of bit-shift ambiguity in blind sequence detection is completely eliminated exploiting the proposed algorithm. A 3 dB SNR gain is achieved against the existing blind sequence detection algorithms for the system without error correction code. The BER performance is highly scalable with the variation of segmentation window size. In the third part of the thesis, two different sequence detection algorithms have been proposed to track rapidly time varying channels. One of the algorithms, called extended window survivor processing (EWSP), requires lower computational complexity than that of Per-survivor processing (PSP) sequence detection process. The other algorithm, called bi-directional survivor processing reduces 17% of channel misacquisation than that of PSP. Consequently, both of these algorithms reduce the probability of error propagation in the detection process. In the final part of the thesis, the capacity and coverage of the UMTS urban network has been analyzed while the Repeaters are inserted. It has been found that the system capacity with repeaters is doubled in an environment with the propagation constant 3.7-3.8. As a by product, 10% increase in the cell coverage was also found.

  • (2005) Trajkovic, Vladimir D.
    Turbo codes discovered in 1993 by Berrou et al. [1] were the first coding technique that approached the Shannon theoretical limit of information transmission to within 0.5 dB. Soon after this discovery, it was recognized that the same Turbo principle could be applied to a variety of detection/decoding problems such as equalization, multi-user detection, joint channel estimation and decoding etc. The computational complexity of such schemes remains a challenging issue since many Turbo detectors require computationally complex Maximum Likelihood (ML) detection in combination with channel decoding. Consequently, a class of low complexity Turbo detectors employing linear and decision feedback filtering instead of ML have been invented recently in order to solve this problem. This dissertation describes low complexity adaptive turbo detection methods for wireless communications, namely Turbo equalization and Turbo Multi-user detection. The adaptive turbo detectors are shown to outperform their conventional Minimum Mean Squared Error (MMSE) counterparts regarding the SNR-BER performance. For Turbo equalization the most remarkable improvement has been achieved for highly frequency-selective channels. For Turbo Multi-user detection most of the gain is obtained for overloaded DS-CDMA systems where the number of users exceeds the processing gain. A theoretical analysis of Turbo equalization provides a new set of MMSE coefficients. The proposed new detector is shown to outperform all turbo detectors of similar computational complexity. The second major contribution of this thesis is a proposed adaptive method for user ordering for Successive Decision Feedback multi-user Detectors (S-DFD). The method is shown to outperform all previously proposed ordering methods delivering significant improvement in SNR-BER characteristic. The analysis of S-DFD has revealed that a proper user ordering can significantly improve the performance of the system. The proposed ordering has also been implemented in the adaptive iterative S-DFD improving the BER performance especially after the first turbo iteration since this is the most critical part, which determines the SNR floor at which the Turbo effect starts.

  • (2002) Kaenton, Julaporn
    The effects of anisotropy of thermal conductivity and natural convection on solidification have been studied numerically. A fixed grid enthalpy-based formulation was developed to model convection and anisotropic conduction during solidification of pure materials and alloys in a rectangular cavity. The time dependent governing equations, describing the conservation of mass, momentum, energy and concentration were solved using a vorticity-stream function formulation. A finite difference-finite volume method was employed, incorporating an improved discretization method and a modified Samarskii-Andreyev ADI scheme with internal iterations. The interface was tracked with the use of an interfacial energy equation. A monotonic second-order upwind scheme (MSOU) was used for convective fluxes with central differences for the diffusion terms of concentration. Comparisons between the present calculations, analytical solutions, existing experimental results and other numerical methods are very good. The improved discretisation method is shown to have an excellent performance as it can solve the discontinuity of temperature, velocity, vorticity and stream function across the solid-liquid interface. Effects of anisotropic conduction on the temperature distribution through a gallium crystal are examined. The results show that anisotropy distorts the isotherms, especially at the adiabatic boundaries, and also decreases the overall heat transfer at the isothermal walls. Effects of aspect ratio, Stefan number, liquid superheat and boundary conditions and anisotropy during solidification are investigated. A study of solidification from either the side wall or the top wall of a cavity containing pure gallium show that natural convection has a significant effect on rate of solidification and the shape of the solid-liquid interface. The results, covering a range of values of Rayleigh number, aspect ratio and anisotropy characteristics, show how anisotropy affects the growth morphology and the flow structure. The effects of liquid aspect ratio on oscillatory convective flow during solidification are studied and compared with those for pure natural convection. Solidification from the side wall of a cavity containing a gallium-0.5% wt indium alloy was considered. The results show that anisotropy distorts the interface shape, and hence the interface shape has an effect on solute redistribution and flow patterns. The code was also used for natural convection driven melting problems of pure gallium where the interface shape is more irregular than in solidification problems. A correlation of the melting rate is given in terms of non-dimensional time, Rayleigh number, Stefan number and aspect ratio.