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Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power ki...
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Airborne wind turbines (AWTs) represent a radically new and fascinating concept for future harnessing of wind power. This concept consists of realizing only the blades of a conventional wind turbine (CWT) in the form of a power kite flying at high speed perpendicular to the wind. On the kite are mounted a turbine, an electrical generator, and a power electronics converter. The electric power generated is transmitted via a medium voltage cable to the ground. Because of the high flight speed of the power kite, several times the actual wind speed, only a very small swept area of the turbine is required according to Betz's Law and/or a turbine of low weight for the generation of a given electric power. Moreover, because of the high turbine rotational speed, no gear transmission is necessary and the size of the generator is also reduced. For takeoff and landing of the power kite, the turbines act as propellers and the generators as motors, i.e., electric power is supplied so that the system can be maneuvered like a helicopter. In the present work, the configuration of power electronics converters for the implementation of a 100 kW AWT is considered. The major aspect here is the trade-off between power-to-weight ratio (W/kg) and efficiency. The dependence of cable weight and cable losses on the voltage level of power transmission is investigated, and a comparison is made between low voltage (LV) and medium voltage (MV) versions of generators. Furthermore, the interdependence of the weight and efficiency of a bidirectional dual active bridge dc–dc converter for coupling the rectified output voltage of a LV generator to the MV cable is discussed. On the basis of this discussion, the concept offering the best possible compromise of weight and efficiency in the power electronics system is selected and a model of the control behavior is derived for both the power flow directions. A control structure is then proposed and dimensioned. Furthermore, questions of electrom- gnetic compatibility and electrical safety are treated. In conclusion, the essential results of this paper are summarized, and an outlook on future research is given. To enable the reader to make simplified calculations and a comparison of a CWT with an AWT, the aerodynamic fundamentals of both the systems are summarized in highly simplified form in an Appendix, and numerical values are given for the 100 kW system discussed in this paper.
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This paper reviews the current state of loudspeaker-based spatial sound reproduction methods from technical perspective as well as perceptual perspective. A nomenclature is developed that allows for a strict separation between the...
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This paper reviews the current state of loudspeaker-based spatial sound reproduction methods from technical perspective as well as perceptual perspective. A nomenclature is developed that allows for a strict separation between these two perspectives. The physical fundamentals, practical realization, and results from perceptual studies are discussed for a number of well-established and emerging reproduction techniques. Further, the paper outlines novel approaches to spatial sound evaluation in terms of perceived quality and provides a comparison of current approaches.
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One- and two-way communication with digital compressed visual signals is now an integral part of the daily life of millions. Such commonplace use has been realized by decades of advances in visual signal compression. The design of...
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One- and two-way communication with digital compressed visual signals is now an integral part of the daily life of millions. Such commonplace use has been realized by decades of advances in visual signal compression. The design of effective, efficient compression and transmission strategies for visual signals may benefit from proper incorporation of human visual system (HVS) characteristics. This paper overviews psychophysics and engineering associated with the communication of visual signals. It presents a short history of advances in perceptual visual signal compression, and describes perceptual models and how they are embedded into systems for compression and transmission, both with and without current compression standards.
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Practical mobile communication systems suffer from time-variant channel fading and cochannel interference, both of which give rise to challenges concerning strategy design and performance analysis in wireless relay networks. Altho...
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Practical mobile communication systems suffer from time-variant channel fading and cochannel interference, both of which give rise to challenges concerning strategy design and performance analysis in wireless relay networks. Although these two aspects have attracted concerns in recent studies, most researchers still focused on dealing with a single problem and avoided intricate joint considerations. In this paper, we investigate the cooperative strategies for the communications of two cochannel pairs over time-correlated Rayleigh channels, and the system consists of a relay-assisted pair (Pair 1) along with a direct transmission pair (Pair 2). We begin with a theoretical evaluation on a conventional relay selection (RS) algorithm that maximizes outdated signal-to-interference-plus-noise ratio (M-SINR), and then develop a more sophisticated method that involves extra statistical and interference information based on the concept of minimizing outage probability (MOP). Closed-form outage probability expressions are derived for both pairs with each RS algorithm, and the MOP-type selection is confirmed to outperform M-SINR in robustness against channel fluctuation when the network operates on medium-to-high signal-to-noise ratio (SNR). Implementation issues of the RS methods are discussed to clarify their overhead and to support their practicability. Further, an interference control scheme is proposed to guarantee Pair 2 communication quality under either RS algorithm by disqualifying the relays that probably cause severe cochannel interference to Pair 2. The interference control mechanism is carried out via a novel threshold parameter $varepsilon$ that represents the tolerance of outage performance degradation of Pair 2 resulting from interference. The closed-form analysis validates that the given $varepsilon$ is rational for an arbitrar- network scenario, and it can balance the two pairs' outage behavior under a proper configuration. We reveal the influence of factors such as average SNR, transmit rate, and network topology on system performance to make a comprehensive comparison between diverse cooperative strategies.
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The latest H.264/AVC video coding standard achieves high compression rates in exchange for high computational complexity. Nowadays, however, many application scenarios require the encoder to meet some complexity constraints.
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Interleave-division multiple access (IDMA) has recently been proposed as a promising alternative to code-division multiple access (CDMA). In this paper, we consider the use of IDMA within a multiple-input multiple-output orthogona...
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Interleave-division multiple access (IDMA) has recently been proposed as a promising alternative to code-division multiple access (CDMA). In this paper, we consider the use of IDMA within a multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) multiuser system employing higher-order modulation and transmitting over frequency-selective MIMO channels. Based on a factor graph/message passing framework and the sum-product algorithm, we devise an iterative receiver that jointly performs pilot-aided channel estimation, multiuser detection, and channel decoding. The use of Gaussian message approximations results in a receiver complexity that scales only linearly with the number of users. A further complexity reduction is obtained by a novel selective message updating scheme. We also present a simulation-based comparison of the maximum rate achievable with our MIMO-OFDM-IDMA receiver with the information-theoretic capacity of the multiple-access channel. Finally, we provide simulation results illustrating the bit error rate (BER) performance of our receiver. It is observed that the proposed turbo-like integration of channel estimation in the iterative multiuser detection and channel decoding scheme yields a dramatic BER reduction, and that the proposed selective message updating scheme results in a significant reduction of complexity.
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Establishing bounds on the accuracy achievable by localization techniques represents a fundamental technical issue. Bounds on localization accuracy have been derived for cases in which the position of an agent is estimated on the ...
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Establishing bounds on the accuracy achievable by localization techniques represents a fundamental technical issue. Bounds on localization accuracy have been derived for cases in which the position of an agent is estimated on the basis of a set of observations and, possibly, of some a priori information related to them (e.g., information about anchor positions and properties of the communication channel). In this paper, new bounds are derived under the assumption that the localization system is map-aware, i.e., it can benefit not only from the availability of observations, but also from the a priori knowledge provided by the map of the environment where it operates. Our results show that: a) map-aware estimation accuracy can be related to some features of the map (e.g., its shape and area) even though, in general, the relation is complicated; b) maps are really useful in the presence of some combination of low SNRs and specific geometrical features of the map (e.g., the size of obstructions); c) in most cases, there is no need of refined maps since additional details do not improve estimation accuracy.
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Convexity properties of error rates of a class of decoders, including the maximum-likelihood/min-distance one as a special case, are studied for arbitrary constellations, bit mapping, and coding. Earlier results obtained for the a...
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Convexity properties of error rates of a class of decoders, including the maximum-likelihood/min-distance one as a special case, are studied for arbitrary constellations, bit mapping, and coding. Earlier results obtained for the additive white Gaussian noise channel are extended to a wide class of noise densities, including unimodal and spherically invariant noise. Under these broad conditions, symbol and bit error rates are shown to be convex functions of the signal-to-noise ratio (SNR) in the high-SNR regime with an explicitly determined threshold, which depends only on the constellation dimensionality and minimum distance, thus enabling an application of the powerful tools of convex optimization to such digital communication systems in a rigorous way. It is the decreasing nature of the noise power density around the decision region boundaries that ensures the convexity of symbol error rates in the general case. The known high/low-SNR bounds of the convexity/concavity regions are tightened and no further improvement is shown to be possible in general. The high-SNR bound fits closely into the channel coding theorem: all codes, including capacity-achieving ones, whose decision regions include the hardened noise spheres (from the noise sphere hardening argument in the channel coding theorem), satisfy this high-SNR requirement and thus has convex error rates in both SNR and noise power. We conjecture that all capacity-achieving codes have convex error rates. Convexity properties in signal amplitude and noise power are also investigated. Some applications of the results are discussed. In particular, it is shown that fading is convexity-preserving and is never good in low dimensions under spherically invariant noise, which may also include any linear diversity combining.
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Over the last several decades, developments in underwater laser line scan (LLS) serial imaging sensors have resulted in significant improvements in turbid water imaging performance. In the last few years, there has been renewed in?Pub>...
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Over the last several decades, developments in underwater laser line scan (LLS) serial imaging sensors have resulted in significant improvements in turbid water imaging performance. In the last few years, there has been renewed interest in distributed, truly multistatic time-varying intensity (TVI) (i.e., multiple transmitter nonsynchronous LLS) sensor configurations. In addition to being capable of high-quality image acquisition through tens of beam attenuation lengths, while simultaneously establishing a non-line-of-sight free-space communications link, these system architectures also have the potential to provide a more synoptic image coverage of larger regions of seabed and the flexibility to simultaneously examine a target from different perspectives. A related issue worth investigation is how to utilize these capabilities to improve rendering of the underwater scenes. In this regard, light field rendering (LFR)—a type of image-based rendering (IBR) technique—offers several advantages. Compared to other IBR techniques, LFR can provide signal-to-noise ratio (SNR) improvements and the ability to image through obscuring objects in front of the target. On the other hand, multistatic nonsynchronous LLS can be readily configured to acquire image sequences needed to generate LFR. This paper investigates the application of LFR to images taken from a distributed bistatic nonsynchronous LLS imager using both line-of-sight and non-line-of-sight imaging geometries to create multiperspective rendering of an unknown underwater scene. The issues related to effectively applying this technique to underwater LLS imagery are analyzed and an image postprocessing flow to address these issues is proposed. The results from a series of experiments at the Harbor Branch Oceanographic Institute at the Florida Atlantic University (HBOI–FAU, Fort Pierce, FL, USA) optical imaging test tank demonstrated the capability of using bistatic/multistatic nonsynch- onous LLS system to generated LFR and, therefore, verify the proposed image processing flow. The benefits of LFR to underwater imaging in challenging environments were further demonstrated via imaging against a variety of obstacles such as mesh screens, bubbles, and water at different turbidity. Image quality metrics based on mutual information and texture features were used in the analysis of the experimental results.
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This paper reviews the fundamentals and latest progress of modeling, analysis, and design technologies for signal integrity and electromagnetic compatibility on PCB and package in the past decades. Most results in this field are b...
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This paper reviews the fundamentals and latest progress of modeling, analysis, and design technologies for signal integrity and electromagnetic compatibility on PCB and package in the past decades. Most results in this field are based on the very rich and highly educational literature produced by Prof. C. Paul in his long scientific career. The inclusion of parameters variability effects is also considered, and it is demonstrated how statistical simulations can become affordable by means of recently-introduced stochastic methods. Finally, the necessity of practical training of designers is mentioned, and an experience relying on realistic PCB demonstrators is illustrated.
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