V disertační práci je popsán heliový kryostat s experimentální válcovou konvekční celou pro studium proudění při přirozené konvekci za velmi vysokých Rayleigho čísel Ra (až do řádu 10^15) a Nusseltových čísel Nu (až do řádu 10^4). Pracovní látkou je studené 4He, které umožňuje díky výhodným fyzikálním vlastnostem dosažení velmi vysokých hodnot čísel Ra. Návrh kryostatu je založen na koncepci lázňových NMR kryostatů s nízkým odparem kryokapalin. Ve středu kryostatu je umístěna konvekční cela o průměru 300 mm a výšce 300 mm. Celu tvoří horní a spodní dno a výměnná střední část. Tyto díly jsou spojeny rozebíratelnými přírubami těsněnými indiovým drátem. Výměnná část umožňuje snadnou modifikaci geometrie cely. Hlavní přednost kryostatu spočívá v minimálním vlivu konstrukce cely a použitých materiálů na studovanou konvekci. Cela kryostatu je navržena pro pracovní tlaky do 250 kPa.

ITC/USA 2006 Conference Proceedings / The Forty-Second Annual International Telemetering Conference and Technical Exhibition / October 23-26, 2006 / Town and Country Resort & Convention Center, San Diego, California / The aeronautical channel is an air to ground channel characterized by multipath, high doppler shifts, Rayleigh fading and noise. Use of a channel sounder ensures proper estimation of the parameters associated with the impulse response of the channel. These estimates help us to characterize the radio channels associated with aeronautical telemetry. In order to have a satisfactory channel characterization, the amplitudes, phase shifts and delays associated with each multipath component in the channel model must be determined.

Les méthodes de surveillance et d'inspection des structures en béton sont indispensables pour évaluer les dégradations du béton. L'étendue des dommages peut entraîner des frais considérables pour l'entretien et la réparation. Les techniques d'évaluation non destructives du béton durci aident à contrôler l'état de dégradation des structures de béton et ces résultats fournissent l'information nécessaire pour mettre en place l'entretien éventuel et les réparations nécessaires. Cette étude concerne l'utilisation des méthodes d'investigation basées sur la propagation des ondes de surface Rayleigh pour la caractérisation non intrusive des structures de béton. Des simulations numériques ont été effectuées pour examiner la propagation des ondes élastiques sur une dalle de béton afin de caractériser cette structure en utilisant la méthode MASW ( Modal Analysis of Surface Waves ). La technique MASW a été développée à l'Université de Sherbrooke pour le domaine de la géotechnique et du contrôle des infrastructures civiles. Les résultats des simulations numériques ont démontré plusieurs aspects importants dans la formation et la propagation des ondes de Rayleigh pour caractériser les couches de béton en profondeur.

Nowadays, many environmental and climatic factors such as weathering actions, temperature variation, chemical attacks, abrasion and other degradation processes can cause near surface damage (0.0 m to 0.5 m) to most concrete structures exposed to severe environmental conditions. As such, the spread of such damage and, subsequently, the loss of mechanical properties of materials are very progressive in long term. The main purpose of Multichannel Analysis of Surface Waves (MASW) method is to 1) characterize near surface damage in concrete structures as a nondestructive testing procedure, 2) estimate the thickness of superficial layers and determine shear-wave velocity (V[subscript S]) profiles. The originality of this research is the application of MASW as a non-destructive method for the evaluation of near surface damage in concrete structures. Indeed, major applications of this method are concerning geotechnical applications. Experiments have been conducted on two concrete slabs (0.80 m, 3 m et 3.50 m) at IREQ with different typical simulated near surface pathologies in order to test the accuracy of MASW method. The results demonstrate that the MASW method has a potential to identify the homogeneity of concrete and estimate the thickness of superficial layers in concrete structures.

ITC/USA 2007 Conference Proceedings / The Forty-Third Annual International Telemetering Conference and Technical Exhibition / October 22-25, 2007 / Riviera Hotel & Convention Center, Las Vegas, Nevada / While investigating methods for more efficiently allocating the available spectrum researchers noticed that in many geographical locations, there are broad bands of frequencies that are lightly utilized. Such inefficiencies are inevitable with fixed spectral allocation rules. Cognitive Radios actively measure the spectral utilization and adapt their modulation, frequencies, bandwidths, power, etc. to take advantage of these lightly used “spectral holes” or “white spaces”. Much of the research work in cognitive radios has not taken into account some of the challenges faced in the telemetry community-including multipaths and a guaranteed quality of service. This paper highlights how some mathematical models of adaptive modulation discussed extensively in many research papers and textbooks can be used in Cognitive Radios as well.

International Telemetering Conference Proceedings / October 25-28, 1999 / Riviera Hotel and Convention Center, Las Vegas, Nevada / In this paper, we extended the matched filter bound (MFB) of time-discrete multipath Rayleigh fading channels derived in [1,2] for multiscale wavelet signaling communication.

ITC/USA 2005 Conference Proceedings / The Forty-First Annual International Telemetering Conference and Technical Exhibition / October 24-27, 2005 / Riviera Hotel & Convention Center, Las Vegas, Nevada / To correctly demodulate a signal sent through a multiple-input multiple-output (MIMO) channel, a receiver may use training to learn the channel parameters. The choice of training parameters can significantly impact system performance. Training too often yields low throughput while training infrequently produces poor channel estimates and increased transmission errors. Previous work on optimal training parameters has focused on the block fading Rayleigh model. This work examines a more general case; finding the training parameters that maximize throughput for a continuously varying channel. Training parameters that maximize a lower bound on channel capacity are determined via simulation, and general guidelines are presented for selecting optimal training parameters.

In a multiuser system, several signals are transmitted simultaneously within the same frequency band. This can result in significant improvements both in spectral efficiency and system capacity. However, a detrimental effect of the shared transmissions (both in time and bandwidth), is that the signal received at the base station (BS) or access point (AP) suffers from cochannel interference (CCI) and inter-symbol interference (ISI). This situation presents challenges to receiver design. To combat the destructive nature of multipath fading, a receiver often employs multiple antennas to collect the faded superimposed versions of the transmitted signals. The multiple signals are combined and processed in such a way that the effects of CCI and ISI are minimized and the desired information is reliably recovered. The situation is even more challenging when the system is operating under overload, i.e. when there are fewer receive antennas than there are transmitted signals. Multiuser detection (MUD) is used to simultaneously estimate the information sent by the transmitters. To do this, the receiver exploits differences among the cochannel signals (through unique spatial signatures in this case). We consider a cochannel communication system where multiple transmitted signals arrive at a receiver (equipped with multiple receive antennas) after propagating through a Rayleigh fading channel. It is assumed that the receiver is operating in an overloaded scenario. For such systems, an optimum maximum a posterior probability (MAP) detector estimates the transmitted signal by maximizing the probability of correct decision. The MAP detector reduces to the maximum likelihood (ML) detector when all the transmitted signals are equiprobable. The computational complexity of both MAP and ML detectors increases exponentially with the number of transmitted signals and the channel memory. For large systems suffering severe CCI and ISI, this is clearly not a good choice for real-time implementation due to the associated computational expenses. The main factors that influence the complexity of MAP / ML detection are: (i) the number of transmitted signals (or equivalently the number of users sharing the system resources), (ii) modulation alphabet size, and (iii) length of the channel memory. On the other hand, linear detection approaches fail to offer acceptable performance while other nonlinear sub-optimum approaches incur high computational costs for reasonably improved system performance and exhibit an irreducible error-floor at medium to high signal to noise ratio (SNR) values. We develop receiver signal processing techniques for the frequency-flat fading channel (where all the multipaths of the transmitted signal arrive at the receiver within a symbol period). We develop an ant colony optimization (ACO) assisted soft iterative detection approach for binary phase-shift keying (BPSK) modulated signals which employs a simplified MAP criteria to extract the most probable signals from the search space. The structure of the receiver is such that it can continue operating under overloaded conditions. The technique achieves near maximum likelihood (ML) performance in critically loaded cases using much lower complexity. For the challenging case of overload it still offers performance close to ML at low to moderate SNR values. Second, an integrated framework comprising of ACO metaheuristic and a recursively defined ML search criteria is developed to handle multilevel modulations. The proposed receiver is capable of achieving near-ML performance for the considered system with significant savings in computational complexity. The receiver framework is independent of the system loading condition, and therefore it remains suitable for overloaded scenarios. Due to the branch and bound nature of the algorithm, an exact expression for the complexity cannot be determined. Instead, an upper bound on computational complexity is developed.

This thesis describes the beach field-testing of a 2-degree-of-freedom (2-DOF) transverse line Rayleigh wave source for use in a seismo-acoustic SONAR for buried mine detection. The source is composed of six identical modules, each composed of two radial and two vertical inertial mass shakers, all mounted to a common base plate. The vertical and horizontal excitation of each module is independent. Electrically, the six modules are organized into three independent sets of two. The length of the source is 46 inches (1.17m), the width 6 inches (0.15m) and the weight 126 pounds (562N). Field tests were conducted using a 5-cycle, (smooth-)amplitude-modulated transient excitation signal. Based on past and present experience that the Rayleigh wave speed at the beach test site is approximately 90 m/s, a carrier frequency of 100 Hz was employed, producing a Rayleigh wavelength and surface penetration depth of approximately 1 m (approximately equal to the source aperture length), which is appropriate for the intended application. The source radiation directional characteristics were investigated for various relative amplitudes and phases of vertical and horizontal shaker excitation. A null result was obtained, that is, no unambiguous evidence of directivity was observed in any of the experiments conducted using this source. These included measurements wherein the source modules were removed from the common base plate and emplaced on the sand. The reason for this result is unknown, but it is suspected that a good possibility is that the beach sand medium just below its surface, whereupon the present source and receivers were deployed, is not homogeneous enough to support coherent spatial beamforming. Based upon the past success of a 4-element end-fire line array in producing directional Rayleigh wave radiation (10-15 dB front-to-back rejection), it is suggested that, if such a source can be made mobile, it may well be the best source candidate for a seismo-acoustic SONAR for buried mine detection.

A caracterização de feixes de laser é imprescindível para que suas aplicações sejam bem sucedidas. Nos métodos tradicionais, essa caracterização envolve um processo lento e exaustivo, uma vez que são necessárias múltiplas medições do diâmetro do feixe em diversas posições ao longo do eixo de propagação. Por esta razão, neste trabalho, apresenta-se um método novo de caracterização baseado no espalhamento Rayleigh. As vantagens deste método são o fato de permitir múltiplas medidas do diâmetro do feixe em apenas uma medição, com precisão igual ou melhor do que os métodos tradicionais, e de possibilitar sua execução em tempo real. Esta única medição é feita a partir da imagem do perfil lateral do feixe, propagando-se num meio espalhador. O desenvolvimento deste novo método envolveu estudos de adequação dos meios espalhadores e a adaptação de sistemas ópticos de obtenção de imagem. Os resultados teóricos e experimentais comprovaram a viabilidade do método baseado no espalhamento Rayleigh para a caracterização de um feixe do laser de HeNe.