Design and implementation of a downlink MC-CDMA receiver

Cette thèse présente une étude d'un système complet de transmission en liaison descendante utilisant la technologie multi-porteuse avec l'accès multiple par division de code (Multi-Carrier Code Division Multiple Access, MC-CDMA). L'étude inclut la synchronisation et l'estimation du canal pour un système MC-CDMA en liaison descendante ainsi que l'implémentation sur puce FPGA d'un récepteur MC-CDMA en liaison descendante en bande de base. Le MC-CDMA est une combinaison de la technique de multiplexage par fréquence orthogonale (Orthogonal Frequency Division Multiplexing, OFDM) et de l'accès multiple par répartition de code (CDMA), et ce dans le but d'intégrer les deux technologies. Le système MC-CDMA est conçu pour fonctionner à l'intérieur de la contrainte d'une bande de fréquence de 5 MHz pour les modèles de canaux intérieur/extérieur pédestre et véhiculaire tel que décrit par le "Third Genaration Partnership Project" (3GPP). La composante OFDM du système MC-CDMA a été simulée en utilisant le logiciel MATLAB dans le but d'obtenir des paramètres de base. Des codes orthogonaux à facteur d'étalement variable (OVSF) de longueur 8 ont été choisis comme codes d'étalement pour notre système MC-CDMA. Ceci permet de supporter des taux de transmission maximum jusquà 20.6 Mbps et 22.875 Mbps (données non codées, pleine charge de 8 utilisateurs) pour les canaux intérieur/extérieur pédestre et véhiculaire, respectivement. Une étude analytique des expressions de taux d'erreur binaire pour le MC-CDMA dans un canal multivoies de Rayleigh a été réalisée dans le but d'évaluer rapidement et de façon précise les performances. Des techniques d'estimation de canal basées sur les décisions antérieures ont été étudiées afin d'améliorer encore plus les performances de taux d'erreur binaire du système MC-CDMA en liaison descendante. L'estimateur de canal basé sur les décisions antérieures et utilisant le critère de l'erreur quadratique minimale linéaire avec une matrice' de corrélation du canal de taille 64 x 64 a été choisi comme étant un bon compromis entre la performance et la complexité pour une implementation sur puce FPGA. Une nouvelle séquence d'apprentissage a été conçue pour le récepteur dans la configuration intérieur/extérieur pédestre dans le but d'estimer de façon grossière le temps de synchronisation et le décalage fréquentiel fractionnaire de la porteuse dans le domaine du temps. Les estimations fines du temps de synchronisation et du décalage fréquentiel de la porteuse ont été effectués dans le domaine des fréquences à l'aide de sous-porteuses pilotes. Un récepteur en liaison descendante MC-CDMA complet pour le canal intérieur /extérieur pédestre avec les synchronisations en temps et en fréquence en boucle fermée a été simulé avant de procéder à l'implémentation matérielle. Le récepteur en liaison descendante en bande de base pour le canal intérieur/extérieur pédestre a été implémenté sur un système de développement fabriqué par la compagnie Nallatech et utilisant le circuit XtremeDSP de Xilinx. Un transmetteur compatible avec le système de réception a également été réalisé. Des tests fonctionnels du récepteur ont été effectués dans un environnement sans fil statique de laboratoire. Un environnement de test plus dynamique, incluant la mobilité du transmetteur, du récepteur ou des éléments dispersifs, aurait été souhaitable, mais n'a pu être réalisé étant donné les difficultés logistiques inhérentes. Les taux d'erreur binaire mesurés avec différents nombres d'usagers actifs et différentes modulations sont proches des simulations sur ordinateurs pour un canal avec bruit blanc gaussien additif

Optimization of technological parameters when polishing sic materials by magnetic compound fluid with the straight electromagnetic yoke

Crystallized silicon carbide (SiC) wafers are widely used in the field of integrated circuits as well as essential in the epitaxial growth of graphene and are one of the promising materials for applications in electronics at future high capacity. The surface quality of the required ultra-fine crystalline silicon wafer is the most essential factor in achieving graphene's desired electronic properties. Aiming to produce superfine surface quality SiC wafers, in this study, a new algorithm is developed to solve optimization problems with many nonlinear factors in ultra-precision machining by magnetic liquid mixture. The presented algorithm is a collective global search inspired by artificial intelligence based on the coordination of nonlinear systems occurring in machining processes. A new algorithm based on the optimization collaborative of multiple nonlinear systems (OCMNO) with the same flexibility and high convergence was established in optimizing surface quality when polishing the SiC wafers. To show the effectiveness of the proposed OCMNO algorithm, the benchmark functions were analyzed together with the established SiC wafers polishing optimization process. To give the best-machined surface quality, polishing experiments were set to find the optimal technological parameters based on a new algorithm and straight electromagnetic yoke polishing method. From the analysis and experimental results when polishing SiC wafers in an electromagnetic yoke field when using a magnetic compound fluid (MCF) with technological parameters according to the OCMNO algorithm for ultra-smooth surface quality with Ra=2.306 nm. The study aims to provide an excellent reference value in optimizing surface polishing SiC wafers, semiconductor materials, and optical device

On the geometry of the Coble-Dolgachev sextic

In this paper, we study the intersection of the Coble-Dolgachev sextic with special projective spaces. Let us recall that the Coble-Dolgachev sextic C_6 is the branch divisor of a double cover map. The adjunction of divisors is an involution of Pic^1(X) that lifts to a non-trivial involution. The fixed locus Fix(τ) is the disjoint union of two projective spaces P^4 and P^3

Spiritual factors in Vietnamese consumers' purchasing decisions

Consumer demand is a complex and dynamic phenomenon that exerts a profound influence on economic decision-making, market trends, and overall societal well-being. In this context, comprehending the determinants of consumer demand assumes paramount significance for various stakeholders, including businesses, policymakers, and scholars. Furthermore, the acknowledgment of spiritual factors as influential determinants in shaping the values and behaviors of a population introduces a nuanced dimension, particularly within culturally diverse societies such as Vietnam. This research endeavor seeks to engage with these pivotal dimensions by elucidating the role of spiritual factors in tandem with the determinants of consumer demand, with specific reference to the distinctive cultural milieu of Vietnam. Employing a comprehensive multidimensional methodology, encompassing qualitative interviews, surveys, quantitative data analysis, historical and cultural inquiries, logical reasoning, and empirical case studies, this study strives to unravel the intricate interplay between spirituality and consumer behavior. It aims to explicate the influence of deeply ingrained spiritual beliefs, firmly entrenched within Vietnamese culture, on various spheres of decision-making. These spheres span from the observance of auspicious birth rituals and naming conventions to the orchestration of significant life events and the valuation of high-value assets such as real estate and automobiles. Moreover, this research seeks to illuminate the enduring nexus between spirituality, cultural heritage, and the choices made by consumers. It endeavors to provide insights into the multifaceted dynamics governing decision-making processes within the Vietnamese context. Additionally, this inquiry extends its purview to the exploration of the economic implications and marketing strategies predicated upon spiritual underpinnings, underscoring the substantial role played by these spiritual beliefs in shaping the consumer landscape. Ultimately, this research aspires to furnish a holistic comprehension of how spirituality is interwoven into the fabric of daily existence in Vietnam, significantly molding the values and behaviors of its citizenry

Density-dependent phonoriton states in highly excited semiconductors

The dynamical aspects of the phonoriton state in highly-photoexcited semiconductors is studied theoretically. The effect of the exciton-exciton interaction and nonbosonic character of high-density excitons are taken into account. Using Green's function method and within the Random Phase Approximation it is shown that the phonoriton dispersion and damping are very sensitive to the exciton density, characterizing the excitation degree of semiconductors.Comment: ICTP preprint IC/95/226, Latex, 10 pages, 3 figure

EFFECT OF BLASTING ON THE STABILITY OF LINING DURING EXCAVATION OF NEW TUNNEL NEAR THE EXISTING TUNNEL

In recent years, experimental and numerical researches on the effect of blasting pressure on the stability of existing tunnels was widely obtained. However, the effect of the blasting pressure during excavation a new tunnel or expansion old tunnels on an existing tunnel has disadvantages and still unclear. Some researches were carried out to study the relationship of the observed Peak Particle Velocity (PPV) on the lining areas along the existing tunnel direction, due to either the lack of in situ test data or the difficulty in conducting field tests, particularly for tunnels that are usually old and vulnerable after several decades of service. This paper introduces using numerical methods with the field data investigations on the effect of the blasting in a new tunnel on the surrounding rock mass and on the existing tunnel. The research results show that not only predicting the tunnel lining damage zone under the impact of blast loads but also determination peak maximum of explosion at the same time at the surface of tunnel working

Effects of Frequency and Mass of Eccentric Balls on Picking Force of The Coffee Fruit for The As-Fabricated Harvesting Machines

Currently, Vietnam ranks second about the coffee export in the world. To match that position, the use of coffee harvesting tools/machines according to the automatic trend is essential. However, the most common forms of coffee harvesting in Vietnam are manual, improved manual-coffee picking machines that are imported from foreign countries. The above harvesting forms have low productivity and have some disadvantages such as labor cost, labor hiring, high labor cost, and long harvesting time, low harvesting, and post-harvesting quality. Studies of scientists around the world have applied the principle of vibration to produce the picking force for coffee fruits, and the picking force is known to be different from many factors in every region of the world growing coffee. The paper presents the method of design and manufacturing an automatic coffee harvesting machine based on the evaluation of picking force for coffee in Vietnam. The influences of eccentric ball mass and vibrating frequency on the magnitude of the picking force are carefully calculated. On that basis, the experimental programming is applied to find the optimal working point of the picking machine for coffee. The results showed that the eccentric ball mass of 8.5 kg, the rotation speed of the eccentric ball from 480 to 574 rpm would produce the picking force by the coffee fruit of Vietnam

Correlated Attention in Transformers for Multivariate Time Series

Multivariate time series (MTS) analysis prevails in real-world applications such as finance, climate science and healthcare. The various self-attention mechanisms, the backbone of the state-of-the-art Transformer-based models, efficiently discover the temporal dependencies, yet cannot well capture the intricate cross-correlation between different features of MTS data, which inherently stems from complex dynamical systems in practice. To this end, we propose a novel correlated attention mechanism, which not only efficiently captures feature-wise dependencies, but can also be seamlessly integrated within the encoder blocks of existing well-known Transformers to gain efficiency improvement. In particular, correlated attention operates across feature channels to compute cross-covariance matrices between queries and keys with different lag values, and selectively aggregate representations at the sub-series level. This architecture facilitates automated discovery and representation learning of not only instantaneous but also lagged cross-correlations, while inherently capturing time series auto-correlation. When combined with prevalent Transformer baselines, correlated attention mechanism constitutes a better alternative for encoder-only architectures, which are suitable for a wide range of tasks including imputation, anomaly detection and classification. Extensive experiments on the aforementioned tasks consistently underscore the advantages of correlated attention mechanism in enhancing base Transformer models, and demonstrate our state-of-the-art results in imputation, anomaly detection and classification

THE CORRELATION BETWEEN HIGH SCHOOL STUDENTS’ PHONOLOGICAL AWARENESS AND THEIR PRONUNCIATION OF ENGLISH INFLECTIONAL MORPHEMES –ED AND -S: A CASE IN THE MEKONG DELTA OF VIETNAM

This study examined the relationship between high school students' phonological awareness and their performance in pronouncing allomorphs of English inflectional morphemes -ed and -s. The study involved 31 high school students in Can Tho City in the Mekong Delta of Vietnam. Data of the study were collected through a pronunciation written test (PWT) and a pronunciation oral test (POT). The findings pointed out that all the students had phonological knowledge of the two morphemes; however, the majority of the participants made errors in pronouncing them which indicated their lack of pronunciation performance. In addition, there was no correlation between the students’ phonological awareness and their pronunciation performance detected in the study. Based on the results, pedagogical implications were suggested.  Article visualizations