Le LETI possède un savoir-faire permettant de réaliser des micro modulateurs spatiaux de lumière (Spatial Light Modulators, SLMs) réalisés en technologie microélectronique sur silicium, ayant des très grandes résolutions et de très petits pas de pixels. Le LETI souhaite utiliser une technologie LCD particulière (la technologie IPS) dont d’une part les performances électro-optiques dans le domaine des micro-écrans ne sont pas connues mais d’autre part permet de garder des coûts réduits et de réduire le nombre d’étapes de fabrication. L’objectif de cette thèse est d’évaluer sur la base de simulations puis de réalisations techniques, les performances d’un micro-SLM IPS utilisé en modulation de phase et d’amplitude.Les recherches effectuées au cours de cette thèse portent sur 4 axes d’études. Le premier axe consiste à la recherche et à l’essai d’un logiciel spécifique (LCDMaster) permettant de simuler des cellules à cristaux liquides ainsi que des électrodes flottantes. Le second axe consiste à étudier en fonction du design du pixel et des différents modes d’alignements des cristaux liquides (Homogène, vertical et twisté) avec un pilotage de type IPS, les performances en modulation de phase. Les meilleurs résultats sont obtenus pour un alignement vertical fournissant un déphasage de 2 Pi. De plus, les performances d’un dispositif d’optique adaptative intégrant le SLM étudié ont été investiguées. Le troisième axe d’étude, consiste avec le même logiciel à investiguer sur les performances d’un SLM à cristaux liquide avec un pilotage de type IPS utilisant un alignement homogène en modulation d’amplitude (étude statique et dynamique). Cette étude permet également d’étudier les possibilités de diminution du temps de réponse de la cellule. Cette étude a aboutie à deux designs de SLM. Le premier design avec deux niveaux d’électrodes séparés par un isolant permettant de reproduire un effet type « cage de faraday ». La seconde a permis de diminuer de manière drastique le temps de réponse. Le quatrième axe de recherche consiste à tester des prototypes basés sur ces deux designs pour vérifier les performances obtenues en simulation grâce à un banc de caractérisation électro-optique. Des résultats concordants ont étés obtenus et de nouveaux types de SLM à pilotage IPS proposant un temps de rafraichissement rapides se rapprochant de l’état de l’art ont donc pu être désignés et caractérisés. / LETI possesses an expertise allowing to realize Spatial Light Modulators (SLMs) made in microelectronic technology on silicon, having very high resolutions and very small pixel steps. LETI wishes to use a specific LCD technology (IPS technology), on the one hand the electro-optical performances in the field of micro-display had to be investigated but on the other hand allows to keep costs down and reduce the number of manufacturing steps. The objective of this thesis is to evaluate on the basis of simulations and technical achievements, the performance of a micro-SLM IPS used in phase modulation and amplitude.Our investigations have been carried out on four major areas of study. The first axis is the search and testing of a specific software (LCDMaster) to simulate liquid crystal cells and floating electrodes. The second axis according to different designs of pixel and the different liquid crystal alignment modes (Homogeneous, vertical and twisted) with an IPS type control, to investigate the performances in modulation phase. The best results were obtained for a vertical alignment providing a phase shift of 2π. In addition, the performance of an adaptive optics device integrating the studied SLM has been investigated. The third axis of study focused on the performance investigation of a liquid crystal SLM with an IPS-type control using a homogeneous alignment in amplitude modulation (static and dynamic study). This study also makes it possible to study the possibilities of reducing the response time of the cell. It resulted in two SLM designs. The first design with two levels of electrodes separated by an insulator to reproduce a Faraday type effect. The second has drastically reduced the response time. The fourth area of research consists in testing prototypes based on these two designs to validate the performances obtained in simulation. Concordant results have been obtained and new types of IPS-controlled SLMs with state-of-the-art refreshing times have thus been designated and characterized.

Mel-frequency cepstral coefficients (MFCCs) are widely adopted in speech recognition as well as speaker recognition applications. They are extracted to primarily characterize the spectral envelope of a quasi-stationary speech segment. It was shown that cepstral features are closely related to the linguistic content of speech. Besides the magnitude-based cepstral features, there are resources in speech, e.g, the phase and excitation source, are believed to contain useful properties for speaker discrimination. Moreover, in real situations, there are large variations exist between the development and application scenarios for a speaker recognition system. These include channel mismatch, recording apparatus mismatch, environmental variation, or even change of emotional/healthy state of speakers. As a consequence, the magnitude-based features are insufficient to provide satisfactory and robust speaker recognition accuracy. Therefore, the exploitation of complementary features with MFCCs may provide one solution to alleviate the deficiency, from a feature-based perspective. / Speaker recognition (SR) refers to the process of automatically determining or verifying the identity of a person based on his or her voice characteristics. In practical applications, a voice can be used as one of the modalities in a multimodal biometric system, or be the sole medium for identity authentication. The general area of speaker recognition encompasses two fundamental tasks: speaker identification and speaker verification. / Wang, Ning. / Adviser: Pak-Chung Ching. / Source: Dissertation Abstracts International, Volume: 73-06, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references (leaves 177-193). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Dai, Yongheng. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

by Fu Sai-chuen. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1994. / Includes bibliographical references (leaves 102-111). / Acknowledgements / List of Abbreviation / Abstract / Prologue --- p.1 / Chapter / Chapter 1. --- Introduction --- p.2 / Chapter 1.1 --- Historical background of Ling Zhi (靈芝） --- p.4 / Chapter 1.2 --- Biological description of Ling Zhi (靈芝） --- p.6 / Chapter 1.3 --- Chemical composition of Ganoderma lucidum --- p.6 / Chapter 1.3.1 --- The triterpenoids --- p.7 / Chapter 1.3.2 --- Matrix polysaccharides --- p.8 / Chapter 1.4 --- Medicinal properties and biological activities --- p.9 / Chapter 1.4.1 --- Medicinal properties --- p.9 / Chapter 1.4.2 --- Biological activities --- p.10 / Chapter 1.4.2.1 --- Anti-tumour activity --- p.10 / Chapter 1.4.2.2 --- Enhancement of protein and nucleic acid synthesis --- p.11 / Chapter 1.4.2.3 --- Effects on nervous system --- p.11 / Chapter 1.4.2.4 --- Effects on respiratory system --- p.12 / Chapter 1.4.2.5 --- Effects on cardiovascular system --- p.12 / Chapter 1.4.2.6 --- Effects on immune system --- p.14 / Chapter 1.4.2.7 --- Hepatoprotection and detoxicant actions --- p.14 / Chapter 1.4.3 --- Summary --- p.15 / Chapter 1.5 --- The relationship between antioxidant defense/immune system and disease --- p.15 / Chapter 2. --- Materials and Methods --- p.20 / Chapter 2.1 --- Cell culture reagents / Chapter 2.1.1 --- Preparation of RPMI-1640 medium --- p.20 / Chapter 2.1.2 --- Preparation of fetal calf serum (FCS) --- p.20 / Chapter 2.1.3 --- Preparation of phosphate-buffered saline (PBS) --- p.20 / Chapter 2.1.4 --- Mitogens and test samples --- p.21 / Chapter 2.1.5 --- Trypan blue exclusion test --- p.21 / Chapter 2.1.6 --- Liquid scintillation counting --- p.21 / Chapter 2.2 --- Animal care --- p.22 / Chapter 2.3 --- Fractionation of Ganoderma lucidum extracts --- p.22 / Chapter 2.4 --- Carbon tetrachloride (CC14)- induced hepatotoxicity in mice --- p.23 / Chapter 2.4.1 --- Pretreatment scheme and CCl4 treatment --- p.23 / Chapter 2.4.2 --- Preparation of liver homogenates --- p.23 / Chapter 2.4.3 --- Biochemical assays for assessing hepatotoxicity --- p.25 / Chapter 2.4.3.1 --- Determination of plasma alanine aminotransferase (ALT) --- p.25 / Chapter 2.4.3.2 --- Determination of tissue glutathione (GSH) content --- p.25 / Chapter 2.4.3.3 --- Depletion of tissue GSH in liver homogenates by tert- butylhydroperoxide (tBHP) --- p.26 / Chapter 2.4.3.4 --- Determination of tissue malondialdehyde (MDA) content --- p.26 / Chapter 2.5 --- Preparation of murine hepatocytes --- p.27 / Chapter 2.5.1 --- Liver perfusion --- p.27 / Chapter 2.5.2 --- Isolation of murine hepatocytes --- p.29 / Chapter 2.5.3 --- Incubation protocol --- p.29 / Chapter 2.5.4 --- Determination of cellular glutathione by HPLC method --- p.30 / Chapter 2.6 --- Preparation of splenocytes and lymphocytes --- p.31 / Chapter 2.6.1 --- Isolation of murine splenocytes --- p.31 / Chapter 2.6.2 --- Mitogenic assay by measuring 3H-thymidine uptake --- p.31 / Chapter 2.6.3 --- B cell- and T cell- enrichment in splenocyte preparation --- p.32 / Chapter 2.6.4 --- Isolation of human peripheral blood lymphocytes (HPBL) from cord blood --- p.34 / Chapter 2.7 --- Chromatographic methods --- p.34 / Chapter 2.7.1 --- Fast Protein Liquid Chromatography (FPLC) with Superose 12 column --- p.34 / Chapter 2.7.2 --- Removal of lipopolysaccharide (LPS) with Detoxi Gel --- p.35 / Chapter 2.7.3 --- Desalting by Sephadex G-25 --- p.35 / Chapter 2.7.4 --- Determination of FDNB-conjugated GSH and GSSG by HPLC reverse phase chromatography --- p.36 / Chapter 2.8 --- Measurement of in vivo antibody production --- p.39 / Chapter 2.8.1 --- Immunization and pretreatment scheme --- p.39 / Chapter 2.8.2 --- Direct plaque assay --- p.39 / Chapter 2.9 --- Biochemical analysis --- p.39 / Chapter 2.9.1 --- Determination of protein content --- p.40 / Chapter 2.9.2 --- Determination of hexose content --- p.40 / Chapter 2.9.3 --- Determination of uronic acid content --- p.40 / Chapter 2.9.4 --- Determination of sulphate content --- p.41 / Chapter 2.9.5 --- Determination of hexosamine content --- p.42 / Chapter 2.10 --- Statistical analysis --- p.42 / Chapter 3. --- Results --- p.44 / Chapter 3.1 --- Extraction and fractionation of Ganoderma lucidum --- p.44 / Chapter 3.2 --- Hepatoprotective effect of Ganoderma lucidum fractions aganist CCl4-induced hepatotoxicity --- p.47 / Chapter 3.2.1 --- Hepatoprotection of Ganoderma lucidum fractions --- p.47 / Chapter 3.2.2 --- Effect of Ganoderma lucidum fraction pretreatment on hepatic malondialdehyde (MDA) level in CCl4-treated mice --- p.47 / Chapter 3.2.3 --- Effect of Ganoderma lucidum fraction pretreatment on hepatic glutathione (GSH) level in CCl4-treated mice --- p.51 / Chapter 3.3 --- Effect of GL2 on isolated hepatocytes --- p.51 / Chapter 3.3.1 --- Protection against phorone-induced hepatocytotoxicity --- p.51 / Chapter 3.3.2 --- Protection against menadione-induced hepatocytotoxicity --- p.55 / Chapter 3.4 --- Immunomodulatory effect of Ganoderma lucidum fractions --- p.55 / Chapter 3.4.1 --- Discovery of a mitogenic principle from Ganoderma lucidum fractions --- p.55 / Chapter 3.4.2 --- B cell-specific mitogenic activity --- p.58 / Chapter 3.4.3 --- Mitogenic activity in Ganoderma lucidum distinguishable from possible LPS contamination --- p.65 / Chapter 3.4.3.1 --- The use of polymyxin B sulphate --- p.65 / Chapter 3.4.3.2 --- The use of Detoxi Gel --- p.69 / Chapter 3.4.3.3 --- The combined use of polymyxin B sulphate and Detoxi Gel --- p.69 / Chapter 3.5 --- Enhancement of humoral response to sheep red blood cells (SRBC) by the mitogenic GL2 fraction --- p.72 / Chapter 3.6 --- Physical-chemical characterization of GL2 fraction --- p.76 / Chapter 3.6.1 --- Estimation of molecular size by FPLC size exclusion chromatography with Superose 12 column --- p.76 / Chapter 3.6.2 --- Sugar composition of GL2 fraction --- p.76 / Chapter 3.6.3 --- Comparison of elution profiles in Superose12 column with respect to mitogenic activity and sugar composition --- p.80 / Chapter 4. --- Discussions --- p.83 / Chapter 4.1 --- Enhancement of antioxidant status by Ganoderma lucidum fractions --- p.83 / Chapter 4.2 --- The significance of enhancement of GSH status --- p.89 / Chapter 4.3 --- Immunomodulatory actions of Ganoderma lucidum --- p.95 / Conclusions --- p.99 / Appendix --- p.100 / Reference --- p.102

To Kin Fai. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2000. / Includes bibliographical references (leaves 99-104). / Abstracts in English and Chinese. / Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Multi-carrier Communication Systems --- p.1 / Chapter 1.2 --- Objective of the Thesis --- p.4 / Chapter 1.3 --- Thesis Outline --- p.5 / Chapter 2 --- Wavelet Packet Division Multiplexing (WPDM) --- p.7 / Chapter 2.1 --- "Wavelets, Wavelet Packets and Multiresolution Analysis (MRA)" --- p.8 / Chapter 2.2 --- Application of Wavelet Packets in Multiple Signal Transmission --- p.14 / Chapter 2.3 --- Summary --- p.20 / Chapter 3 --- Nonlinear System Theories --- p.22 / Chapter 3.1 --- Characteristics of Memoryless Nonlinearities --- p.23 / Chapter 3.1.1 --- Memoryless Baseband Nonlinearities --- p.23 / Chapter 3.1.2 --- Memoryless Bandpass Nonlinearities --- p.24 / Chapter 3.2 --- Volt err a Series for Nonlinearities with Memory --- p.26 / Chapter 3.2.1 --- Baseband Nonlinearities with Memory --- p.26 / Chapter 3.2.2 --- Bandpass Nonlinearities with Memory --- p.27 / Chapter 3.3 --- High Power Amplifier (HPA) Models --- p.28 / Chapter 3.3.1 --- Traveling Wave Tube Amplifier (TWTA) --- p.28 / Chapter 3.3.2 --- Solid State Power Amplifier (SSPA) --- p.28 / Chapter 3.3.3 --- Input and Output Backoff Ratios --- p.29 / Chapter 3.4 --- Summary --- p.29 / Chapter 4 --- WPDM in the Presence of Amplifier Nonlinearities --- p.30 / Chapter 4.1 --- System Model --- p.31 / Chapter 4.2 --- Derivation of Channel Models --- p.32 / Chapter 4.2.1 --- Single-carrier WPDM --- p.32 / Chapter 4.2.2 --- Multi-carrier WPDM --- p.34 / Chapter 4.3 --- Performance Analysis --- p.35 / Chapter 4.3.1 --- Conditional Mean --- p.36 / Chapter 4.3.2 --- Conditional Variance --- p.41 / Chapter 4.3.3 --- Power Spectral Density (PSD) --- p.44 / Chapter 4.4 --- Probability of Symbol Error --- p.48 / Chapter 4.5 --- Simulation Results --- p.52 / Chapter 4.6 --- Summary --- p.56 / Chapter 5 --- Nonlinear Compensation (The pth-Order Inverse) --- p.57 / Chapter 5.1 --- Data Predistortion --- p.58 / Chapter 5.2 --- Predistorter Structure --- p.62 / Chapter 5.3 --- Complexity --- p.63 / Chapter 5.4 --- Simulation Results --- p.66 / Chapter 5.5 --- Summary --- p.78 / Chapter 6 --- Conclusions and Suggestions for Future Research --- p.79 / Chapter 6.1 --- Conclusions --- p.79 / Chapter 6.2 --- Suggestions for Future Research --- p.82 / Appendices --- p.83 / Chapter A --- Derivation of (4.14) --- p.83 / Chapter B --- Derivation of (4.16) --- p.85 / Chapter C --- Evaluation of higher order expectations --- p.86 / Chapter D --- Derivation of φ ss(T) in (4.32) --- p.90 / Chapter E --- Derivation of φsi(T) in(4.32) --- p.93 / Chapter F --- Derivation of φ is(T) in (4.32) --- p.95 / Chapter G --- Derivation of φii(T) in (4.32) --- p.97 / Bibliography --- p.99

Lee Sim Heung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2005. / Includes bibliographical references. / Abstracts in English and Chinese. / ABSTRACT --- p.i / ACKNOWLEGEMENT --- p.vi / LIST OF FIGURES --- p.xi / Chapter 1. --- INTRODUCTION --- p.1 / Chapter 1.1. --- New challenges in telecommunication --- p.2 / Chapter 1.2. --- Introduction to optical non-linearity and its impact to optical network --- p.3 / Chapter 1.3. --- Applications of optical nonlinearity --- p.5 / Chapter 1.3.1. --- Extinction ratio enhancement and wavelength conversion --- p.5 / Chapter 1.3.2. --- All optical ASK to DPSK converter --- p.6 / Chapter 1.3.3. --- All-optical RZ to NRZ format converter --- p.8 / Chapter 1.3.4. --- RF clock component enhancement for NRZ data --- p.10 / Chapter 1.3.5. --- Multi-wavelength converter with multi-channel broadcasting --- p.11 / Chapter 1.4. --- Overview --- p.12 / Chapter 2. --- PRINCIPLES AND THEORIES --- p.18 / Chapter 2.1. --- Optical nonlinearity --- p.19 / Chapter 2.2. --- Self-phase modulation (SPM) --- p.21 / Chapter 2.3. --- Cross-phase modulation (XPM) --- p.25 / Reference: --- p.28 / Chapter 3. --- EXTINCTION RATIO ENHANCEMENT AND WAVELENGTH CONVERSION USING CROSS-PHASE MODULATION IN A DISPERSION-SHIFTED FIBER --- p.29 / Chapter 3.1. --- Introduction --- p.30 / Chapter 3.2. --- Experimental details of extinction ratio enhancement --- p.31 / Chapter 3.3. --- Result and Discussions --- p.32 / Chapter 3.4. --- Conclusion --- p.36 / References: --- p.37 / Chapter 4. --- ALL-OPTICAL ASK TO DPSK FORMAT CONVERSION --- p.38 / Chapter 4.1. --- All Optical ASK to DPSK Format Conversion Using Cross-Phase Modulation in a Non-linear Fiber --- p.39 / Chapter 4.1.1. --- Introduction --- p.40 / Chapter 4.1.2. --- Experimental Details of ASK to DPSK format conversion by XPM in DSF --- p.42 / Chapter 4.1.3. --- Results and Discussion --- p.44 / Chapter 4.1.4. --- Experimental Details of ASK to DPSK format conversion by XPM in PCF --- p.47 / Chapter 4.1.5. --- Result and Discussion of XPM in PCF --- p.49 / Chapter 4.1.6. --- Comparison of DSF and PCF --- p.51 / References: --- p.54 / Chapter 4.2. --- All Optical ASK to ASK/DPSK Orthogonal Code Format Conversion Using Cross-Phase Modulation in a Dispersion-Shifted Fiber --- p.56 / Chapter 4.2.1. --- Introduction --- p.57 / Chapter 4.2.2. --- Experimental Details of ASK to ASK/DPSK format conversion --- p.58 / Chapter 4.2.3. --- Results and Discussion --- p.60 / Chapter 4.2.4. --- Conclusion --- p.63 / References: --- p.64 / Chapter 5. --- ALL-OPTICAL RZ TO NRZ FORMAT CONVERSION --- p.65 / Chapter 5.1. --- Introduction --- p.65 / Chapter 5.2. --- All-Optical RZ to NRZ Data Format Conversion Using Spectral Broadening Effect in a Dispersion-Shifted Fiber --- p.67 / Chapter 5.2.1. --- Principle of RZ-to-NRZ format conversion using SPM --- p.68 / Chapter 5.2.2. --- Experiment Details --- p.71 / Chapter 5.2.3. --- Results and Discussion --- p.72 / Chapter 5.2.4. --- Conclusion --- p.77 / Chapter 5.3. --- Spectral Filtering from a Cross-Phase Modulated Signal for RZ to NRZ Format and Wavelength Conversion --- p.78 / Chapter 5.3.1. --- Principle of RZ to NRZ format conversion by XPM --- p.79 / Chapter 5.3.2. --- Experiment --- p.81 / Chapter 5.3.3. --- Results and Discussion --- p.83 / Chapter 5.3.4. --- Conclusions --- p.87 / References: --- p.88 / Chapter 6. --- ALL-OPTICAL CLOCK COMPONENT EXTRACTION FROM NRZ DATA SIGNALS USING SELF-PHASE MODULATION IN A DISPERSION-SHIFTED FIBER --- p.90 / Chapter 6.1. --- Introduction --- p.91 / Chapter 6.2. --- Experimental Details --- p.92 / Chapter 6.3. --- Results and Discussion --- p.93 / Chapter 6.4. --- Conclusion --- p.96 / References: --- p.97 / Chapter 7. --- ALL-OPTICAL WAVELENGTH MULTICASTING USING SELF- PHASE MODULATION IN A NONLINEAR PHOTONIC CRYSTAL FIBER --- p.98 / Chapter 7.1. --- Introduction --- p.99 / Chapter 7.2. --- Simulation results --- p.100 / Chapter 7.3. --- Experimental Details --- p.103 / Chapter 7.4. --- Result and Discussion --- p.104 / Chapter 7.5. --- Conclusions --- p.109 / References: --- p.110 / Chapter 8. --- CONCLUSION AND FUTURE WORK --- p.111 / Chapter 8.1. --- Conclusion --- p.111 / Chapter 8.2. --- Possible Future Work --- p.114 / References: --- p.116 / APPENDIX --- p.117 / List of Publications --- p.117

No description available.

Due to constraints on spectrum availability and transmitter power, both bandwidth and power efficient communication techniques are desirable for mobile radio. Continuous phase modulated (CPM) signals have gained attention because of their attractive power spectra (Steele, 1992). It has been shown that the trellis coded modulation (TCM) schemes could provide better bit error rate performances compared to the uncoded schemes (Ungerboeck, 1982). Therefore, the combination of TCM which improves error probability and CPM signals which yield low spectral occupancy is expected to provide good coding and modulation over bandwidth and power limited channels such as the one encountered in mobile radio communications.In this research, a Trellis Coded Modulation (TCM) scheme, which combines convolutional coding and partial response Continuous Phase Modulation (CPM) such as Gaussian Minimum Shift Keying (GMSK), is investigated. Also, this study concentrates on the use of rate-half convolutional codes, and GMSK (B(subscript)0T=0.3). The latter has been adopted in the Global System for Mobile Communications (GSM) system.Appropriate codes are selected assuming Maximum Likelihood Sequence Detection (MLSD) based on the Viterbi algorithm using an extensive computer search. The bit-error-rate (BER) performances of the selected trellis coded GMSK schemes are theoretically evaluated in the presence of additive white Gaussian noise (AWGN) and frequency-flat fading. In the case of fading, the analysis is simplified to assume only amplitude-fading, and without considering the effect of fading on the phase of the received signal.Computer simulations are used to evaluate the BER performances of the proposed trellis coded GMSK schemes in the presence of AWGN and practical impairments, such as sample timing offset and carrier phase errors. Coding gains of up to 2.2dB at a BER of 10(subscript)-3 ++ / are obtained under ideal sample timing and carrier recovery conditions. This has been achieved without increasing the receiver complexity based on the number of states in the Viterbi decoder, compared to the uncoded GMSK scheme. Furthermore, these coded schemes are more tolerant to sample timing and carrier phase impairments.Also, the BER performances of the proposed trellis coded GMSK schemes have been extensively investigated by computer simulations for frequency-flat and frequency-selective fading channels. In the case of frequency-selective fading, the Viterbi decoding is made adaptive to cater for the channel impulse response variations with time. With this adaptive receiver, the irreducible BERs of the coded scheme is found to be lower than that of the uncoded. Performance improvements are obtained with a trellis coded GMSK scheme using a constraint length 2 code with a Viterbi decoder of 16 states compared to the 128 states required for the uncoded scheme. Further, the coded scheme has shown less sensitivity to carrier phase errors, compared to the uncoded.

<p>Crystallography today is by many seen as merely a tool for determining the structure of a material on the atomic level. It is expected that whatever comes out of the tool is the indisputable truth, a fact. This thesis is based on five publications illustrating that this is very far from the real world of materials research. The experiences drawn from the structural work in the papers are put together to illustrate when to be alert, and how to proceed with a structural investigation using non-standard crystallography. The focus is on interpreting the signs of additional order being present in a structure. The signs may be weak, such as extreme thermal vibration, or unit cell deformation. Or the signs can be strong, with superstructure reflections indicating the presence of either commensurate or incommensurate superstructures.</p>

Ce mémoire présente les travaux effectués au sein de l'équipe ETSN de Supélec, campus de Rennes, sur la réduction des effets des non linéarités dans une modulation OFDM, à l'aide de réseaux de neurones.<br />Tout d'abord, le mémoire commence par une introduction aux communications numériques et en particulier à la modulation OFDM. Aujourd'hui, plusieurs standards reposent sur cette technique de transmission, en particulier en raison de la simplicité de l'égalisation du canal, et donc la possibilité de transmettre avec plus d'efficacité des données sur des canaux multitrajets. Cependant le signal OFDM temporel est particulièrement sensible aux non-linéarités présentes dans l'amplificateur d'émission et diverses techniques sont étudiées pour diminuer ces effets.<br />Ensuite, les réseaux de neurones sont présentés, ainsi que leur utilisation dans le domaine de l'approximation de fonctions. Après avoir décrit les deux modèles de réseaux de neurones les plus courants, les réseaux d'ordre supérieur, tels que le RPN, sont introduits. Les techniques d'apprentissage de ces différentes architectures de réseaux de neurones sont également décrites.<br />Dans les différents correcteurs étudiés dans cette thèse, le réseau de neurones est placé dans le récepteur, après l'égalisation de canal. Son objectif est de corriger le signal reçu afin de compenser les effets des non-linéarités. Dans un premier temps le réseau de neurones est placé dans le domaine fréquentiel. Dans un système OFDM à 4 porteuses avec une modulation MAQ16, un amplificateur de type SSPA, un recul de 0 dB et pour un taux d'erreur binaire de 10-2, le correcteur avec un réseau RPN apporte un gain de 1,5 dB de rapport signal sur bruit. Cependant des difficultés apparaissent durant la phase d'apprentissage du réseau de neurones avec un nombre de porteuses supérieur.<br />Pour palier ce défaut, les réseaux de neurones décrits précédemment sont simplifiés en étant placés dans le domaine temporel. Ce système est plus proche des solutions déjà proposées pour la compensation des non-linéarités dans une modulation monoporteuse, avec toutefois des différences au niveau de l'égalisation du canal et de la nature de la fonction que doit accomplir le réseau de neurones. Un correcteur basé sur un réseau RPN a montré de très bonnes performances, même en augmentant le nombre de porteuses. Un gain de 8 dB a été mesuré pour un taux d'erreur binaire de 10-2 dans un système OFDM à 48 porteuses, une modulation MAQ16 et un amplificateur de type SSPA avec un recul de 0 dB. Le système présenté permet donc dans ces conditions de diviser la puissance de l'amplificateur, et donc sa consommation d'énergie, par un facteur supérieur à 4 tout en conservant la même qualité de transmission.<br />Le correcteur à RPN dans le domaine temporel est ensuite simulé sur un canal multitrajet, afin de vérifier que la compensation reste efficace dans le cas d'un canal sévère. Enfin les deux approches proposées (fréquentielle et temporelle) sont comparées, au niveau des performances obtenues et de la puissance de calcul nécessaire dans le récepteur. Une comparaison avec une autre approche proposée dans la littérature est également présentée. Le correcteur temporel basé sur un RPN est bien moins complexe que le système cité, au détriment d'une légère dégradation des performances.<br />Ce mémoire se conclut par quelques perspectives de recherche pouvant prolonger les travaux accomplis durant cette thèse.