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Modulation Techniques (modulation + techniques)

Distribution by Scientific Domains

Engineering	75%

Selected Abstracts

Structures and modulation algorithms of direct ac/ac converters with two legs and three legs for two-phase systems

EUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 4 2010
Sangshin Kwak
Abstract Two-phase ac/ac matrix converter configurations are presented based on direct power conversion processes from three-phase input ac supplies to two-phase output ac loads. Two types of two-phase matrix converter topologies, utilizing the standard matrix converter configuration, are introduced with two-leg and three-leg based structures. Pulse width modulation (PWM) strategies with high-frequency composition for the two-leg and the three-leg based two-phase matrix converters are developed to synthesize sinusoidal output waveforms of two-phase loads from time-varying input ac voltages. Grounded on the developed modulation algorithms, the presented matrix converters directly generate controllable two-phase output voltages from sinusoidal input voltages with no intermediate dc conversion and concomittant dc-link elements. The proposed matrix converters can provide sinusoidal output current/voltage waveforms with independently controlled magnitude and 90° phase angle to two-phase loads. Thus, the converters are effective for both balanced and unbalanced two-phase loads. The presented two-phase operations with the standard matrix converter structures add the versatility for the matrix converter systems to function in multi-phase loads as a universal converter. Simulation and experimental results are presented to verify the developed modulation techniques and the converter configurations for the two-phase matrix converters. Copyright © 2009 John Wiley & Sons, Ltd. [source]

An adaptive predistorter using modified neural networks combined with a fuzzy controller for nonlinear power amplifiers

INTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 1 2004
Hong-min Deng
Abstract In digital radio systems, high data transmission rates require the use of spectrally efficient linear modulation techniques; however, these techniques are generally sensitive to nonlinearity caused by the high-power amplifier (HPA) employed in transmitter systems. The nonlinearity of HPA is potentially responsible for spectral spreading, adjacent channel interference (ACI), and degradation of bit-error rates (BERs). This article proposes an adaptive predistortion scheme to compensate for the HPA's nonlinearity by combining adaptive structure-varying neural networks and a fuzzy controller. Simulations show that this predistortion scheme can very effectively prevent the warping of the signal constellations, thus reducing the system's BER and learning time. © 2003 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14: 15,20, 2004. [source]

Channel estimation and physical layer adaptation techniques for satellite networks exploiting adaptive coding and modulation

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKING, Issue 2 2008
Stefano Cioni
Abstract The exploitation of adaptive coding and modulation techniques for broadband multi-beam satellite communication networks operating at Ka-band and above has been shown to theoretically provide large system capacity gains. In this paper, the problem of how to accurately estimate the time-variant channel and how to adapt the physical layer taking into account the effects of estimator errors and (large) satellite propagation delays is analyzed, and practical solutions for both the forward and the reverse link are proposed. A novel pragmatic solution to the reverse link physical layer channel estimation in the presence of time-variant bursty interference has been devised. Physical layer adaptation algorithms jointly with design rules for hysteresis thresholds have been analytically derived. The imperfect physical layer channel estimation impact on the overall system capacity has been finally derived by means of an original semi-analytical approach. Through comprehensive system simulations for a realistic system study case, it is showed that the devised adaptation algorithms are able to successfully track critical Ka-band fading time series with a limited impact on the system capacity while satisfying the link outage probability requirement. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Actively Q-switched all-fiber lasers

LASER PHYSICS LETTERS, Issue 2 2008
M.V. Andrés
Abstract Q-switching of fiber lasers using bulk elements has important drawbacks as reduced mechanical stability and high insertion losses. The development of efficient all-fiber modulation techniques is the key to obtain robust, compact and efficient Q-switched all-fiber lasers. Certainly, the development of fiber Bragg gratings (FBG) has been crucial to make progress on fiber lasers. FBGs permit a simple way to assemble all-fiber laser cavities and can be written in the active fiber itself. The Q-factor of this type of cavities is determined by the reflectivity of the FBGs and the losses of the fiber. Here, we focus on the use of magnetostrictive materials and the acousto-optic interaction to develop efficient Q-factor modulators. Most of these modulators include an FBG and take advantage of the specific interaction of the magnetostrictive materials or the acoustic wave with the FBG itself. Fiber optic technologies permit the development of a rather unique type of fiber lasers, i.e., actively Q-switched distributed feedback (DFB) fiber lasers. In this case, both the use of magnetostrictive materials and the acousto-optic interaction permit the generation of dynamic defects in an FBG that has been previously written in a highly Er-doped fiber. (© 2007 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source]