Comme toutes modulations multiporteuses, l'OFDM souffre d'une forte variation d'amplitudes qui se traduit par un PAPR élevé. Cette caractéristique de l'OFDM la rend très sensible aux non-linéarités de l'amplificateur de puissance. Pour faire face à cette problématique, on peut surdimensionner l'amplificateur de puissance (solution non efficace en terme de rendement énergétique), linéariser l'amplificateur via les techniques de pré-distorsions, ou réduire le PAPR du signal avant amplification. L'objectif de cette thèse s'inscrit dans cette dernière thématique et plus particulièrement sur les techniques dites d'ajout de signal.Dans cette thèse, après une étude sur l'état de l'art des techniques de réduction du PAPR et en particulier les techniques dites d'ajout de signal, nous avons développé et proposé de nouvelles techniques de réduction du PAPR. Ces contributions s'articulent principalement autour des techniques de Clipping et de la Tone Reservation. / One of the main drawbacks of the OFDM modulation scheme is its high Peak-To-Average Power variation (PAPR) which can induce poor power efficiency at the transmitter amplifier. The digital base band pre-distortion for linearisation of power amplifier and the PAPR mitigation are the most commonly used solution in order to deals with efficiency and linearisation at the high power amplifier. This thesis is focused on the PAPR mitigation solution, and particularly on the adding signal based techniques. The proposed solutions in this report are about improving the Tone Reservation method which is the most popular adding signal based technique for PAPR mitigation, and also the classical clipping method which is the most simple method (in terms of computational complexity) actually.

Innovations and process understanding is the current focus in pharmaceutical industry. The objective of this research was to explore application of high power ultrasound in the slurry crystallisation and application of low power ultrasound (3.5 MHz) as process analytical technology (PAT) tool to understand pharmaceutical processing such as hot melt extrusion. The effect of high power ultrasound (20 kHz) on slurry co-crystallisation of caffeine / maleic acid and carbamazepine / saccharin was investigated. To validate low power ultrasound monitoring technique, it was compared with the other techniques (PAT tools) such as in-line rheology and in-line NIR spectroscopy. In-line rheological measurements were used to understand melt flow behaviour of theophylline / Kollidon VA 64 system in the slit die attached to the hot melt extruder. In-line NIR spectroscopic measurements were carried out for monitoring any molecular interactions occurring during extrusion. Physical mixtures and the processed samples obtained from all experiments were characterised using powder X-ray diffraction, thermogravimetry analysis, differential scanning calorimetry, scanning Electron Microscopy, dielectric spectroscopy and high performance liquid chromatography, rotational rheology, fourier transform infrared spectroscopy and near infrared spectroscopy. The application of high power ultrasound in slurry co-crystallisation of caffeine / maleic acid helped in reducing equilibrium time required for co-crystal formation. During carbamazepine / saccharin co-crystallisation high power ultrasound induced degradation of carbamazepine was negligible. Low power ultrasound can be used as a PAT tool as it was found to be highly sensitive to the changes in processing temperatures and drug concentration.

by Anhui Liang. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1996. / Includes bibliographical references (p. D1-D4). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web.

The RF high power amplifier is a key module in the NMR spectrometer. Robustness, lower power consumption, and small size are requirements. In this thesis, devices are studied and different design approaches are considered. New ideas are introduced, and simulations are used to show if it these work. A real prototype is developed. Results from the prototype are satisfactory and in good agreement with the simulation results. This allows for the possibility of a real portable NMR spectrometer 'Lapspec'. Points of note: • Feedback to stabilize amplifier, • Hard bias to improve rise time of pulse, • A rugged device is chosen, • Power limiter technology is used to avoid overdrive amplifier, • Lower value attenuator at output of final stage to reduce load VSWR, • Reason of spike is studied, the solution to reduce spike is given, • The reason of instability of amplifier with NMR load is analyzed, • A method is introduced to ensure there is no oscillation while the High Power Amplifier (HPA) is connected with the NMR probe.

We have investigated the use of aluminum based amorphous metallic glass as the anode in lithium ion rechargeable batteries. Amorphous metallic glasses have no long-range ordered microstructure; the atoms are less closely packed compared to the crystalline alloys of the same compositions; they usually have higher ionic conductivity than crystalline materials, which make rapid lithium diffusion possible. Many metallic systems have higher theoretical capacity for lithium than graphite/carbon; in addition irreversible capacity loss can be avoided in metallic systems. With careful processing, we are able to obtain nano-crystalline phases dispersed in the amorphous metallic glass matrix. These crystalline regions may form the active centers with which lithium reacts. The surrounding matrix can respond very well to the volume changes as these nano-size regions take up lithium. A comparison study of various kinds of anode materials for lithium rechargeable batteries is carried out. / Singapore-MIT Alliance (SMA)

This PhD thesis addresses the threat posed to society by sources that can produce high power electromagnetic pulses (HEPM) and be used maliciously to disturb or damage electronic equipment. The vulnerability from intentional electromagnetic interference (IEMI) has increased in the recent decades due to the widespread dependence of the civil society on sensitive electronic systems and proliferation of radiation sources. As the characteristics of the disturbances associated with IEMI often have very high frequency content, the existing mitigation measures and protection components may not be adequate. It was seen that for ultra wideband (UWB) transients low voltage protection components may not work as intended, due to parasitic components that arises from the packaging of the device. The large spatial distribution of many civilian facilities and critical infra-structures (e.g., power generation, communications, train system, etc.) presents many unexpected ports for an attacker as the majority of the parts of these systems are not protected or secure. As the new European Rail Traffic Management System (ERTMS) will utilize wireless communication for communication and control of the trains the vulnerability from different radiating HPEM sources was investigated. Angles of incidence and frequencies that are a threat in a given situation are identified. Due to the possibility of unexpected ports, the propagation of differential mode ultra wideband transients in low voltage power networks, when injected into a power socket of a facility, was studied. The effects on the transient propagation from cable bends, switches and junctions were studied, both in a laboratory setup and in the network of a facility. Also, as modern electronic equipment and systems may not be tested for waveforms and disturbances other than standardized EMC tests, experiments on some common commercial-off-the-shelf (COTS) equipment were performed with non-standard test situation. It was seen that these could easily be disturbed or even permanently damaged. In addition, due to the inherent difficulties with IEMI, a new method for classifying facilities from IEMI is suggested. It is based on available terminology of accessibility (A), susceptibility (S) and consequence (C), but expands these and forms the so called IEMI/ASC-cube.

As modern satellite communication systems move toward multi-carrier high power communications, there is an increased need for high-power RF devices in the space industry. However, at high-power some RF devices have exhibited an electron plasma (multipactor electron avalanche discharge) that severely damages the RF device and could render it unusable. This is especially a problem in space where repairs to communication equipment is cost-prohibitive. As a result, a number of models have been developed in recent years to predict the onset of multipactor discharge. However, most existing models can only analyze selected geometries and they also require a large number of electrons to predict the power at which multipactor discharge will occur. This has placed a limitation on the types of RF structures that can be analysed for multipactor breakdown. This research work, uses a new generalized procedure to develop an efficient multipactor model that could be used to analyze the complex structures found in the commercial space industry, by coupling EM field information from established industry-standard EM solvers. A robust secondary emission model is also developed in order to model the advanced phenomenological characteristics of secondary emission that are not taken into account in other models. The result of the generalized approach taken in this research is a highly efficient multipactor model that requires far fewer electrons to be analysed in order to converge to accurate results, and the ability to analyse more complex RF structures than current models. Multipactor analysis for different structures were performed, and the breakdown results predicted by this model were in good agreement with other models where expected. However, for other cases where certain simplifying assumptions do not hold true, such as higher order waveguide multipaction and high impedance transmission line multipaction, results provided by this model were found to be more accurate and efficient when compared to other models.

As modern satellite communication systems move toward multi-carrier high power communications, there is an increased need for high-power RF devices in the space industry. However, at high-power some RF devices have exhibited an electron plasma (multipactor electron avalanche discharge) that severely damages the RF device and could render it unusable. This is especially a problem in space where repairs to communication equipment is cost-prohibitive. As a result, a number of models have been developed in recent years to predict the onset of multipactor discharge. However, most existing models can only analyze selected geometries and they also require a large number of electrons to predict the power at which multipactor discharge will occur. This has placed a limitation on the types of RF structures that can be analysed for multipactor breakdown. This research work, uses a new generalized procedure to develop an efficient multipactor model that could be used to analyze the complex structures found in the commercial space industry, by coupling EM field information from established industry-standard EM solvers. A robust secondary emission model is also developed in order to model the advanced phenomenological characteristics of secondary emission that are not taken into account in other models. The result of the generalized approach taken in this research is a highly efficient multipactor model that requires far fewer electrons to be analysed in order to converge to accurate results, and the ability to analyse more complex RF structures than current models. Multipactor analysis for different structures were performed, and the breakdown results predicted by this model were in good agreement with other models where expected. However, for other cases where certain simplifying assumptions do not hold true, such as higher order waveguide multipaction and high impedance transmission line multipaction, results provided by this model were found to be more accurate and efficient when compared to other models.

In wireless communication systems, multiple standards have been implemented to meet the past and present demands of different applications. This proliferation of wireless standards, operating over multiple frequency bands, has increased the demand for radio frequency (RF) components, and consequently power amplifiers (PA) to operate over multiple frequency bands. In this research work, a systematic approach for the synthesis of a novel dual-band matching network is proposed and applied for effective design of PA capable of maintaining high power efficiency at two arbitrary widely spaced frequencies. The proposed dual-band matching network incorporates two different stages. The first one aims at transforming the targeted two complex impedances, at the two operating frequencies, to a real one. The second stage is a dual-band filter that ensures the matching of the former real impedance to the termination impedance to 50 Ohm. Furthermore, an additional transmission line is incorporated between the two previously mentioned stages to adjust the impedances at the second and third harmonics without altering the impedances seen at the fundamental frequencies. Although simple, the harmonic termination control is very effective in enhancing the efficiency of RF transistors, especially when exploiting the Class J design space. The proposed dual-band matching network synthesis methodology was applied to design a dual-band power amplifier using a packaged 45 W gallium nitride (GaN) transistor. The power amplifier prototype maintained a peak power efficiency of about 68% at the two operating frequencies, namely 800 MHz and 1.9 GHz. In addition, a Volterra based digital predistortion technique has been successfully applied to linearize the PA response around the two operating frequencies. In fact, when driven with multi-carrier wideband code division multiple access (WCDMA) and long term evolution (LTE) signals, the linearized amplifier maintained an adjacent channel power ratio (ACPR) of about 50 dBc and 46 dBc, respectively.

The temperature and thermal stress distributions and variations of the high power LED module were studied in this work. The thermal-elastic-plastic 3D finite element models of MSC.marc software package are employed to simulate these performances for the high power LED module. Two high power white light LED module designs are investigated¡G one is the traditional phosphorescent silicone with blue LED module and the other is a phosphor glass lens with blue LED module. The distributions of temperature and thermal stress of in these two operating LED modules are compared and discussed. The effects of different packaging parameters¡Ge.g. bonding materials, substrate materials, lens materials on the temperature and thermal stress have also been studied in this work. The simulated results reveal that the serious thermal crack may occur for these two designs if the power of single die is over 10 watt. The simulated results also indicate that an attached fin cooler may improve these thermal crack disadvantaged significantly. The effect of fin design parameters on the peak temperature reduction has studied. A feasible fin design for the high power LED module has also been proposed.