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ABSTRACT: The spectra, magnitude and spatial distribution of low-frequency (ω ωci) density fluctuation have been measured by two independent experimental methods in the edge plasma of the TEXT tokamak. Good agreement between far-infrared laser scattering and Langmuir probe measurements has been achieved and the strengths of each technique are evaluated. Langmuir probes are used to directly determine the particle flux induced by edge fluctuations (Γ ×B) and collective Thomson scattering permits an extension of these observations to the plasma interior. Results are presented for typical discharge conditions in a tokamak.
Nuclear Fusion 01/2011; 27(7):1125. · 4.09 Impact Factor
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ABSTRACT: The results of a study of the statistical space/time properties of the turbulence observed in the edge region of the Pretext tokamak are reported. A local estimate of the wavenumber frequency spectra S(κ,ω) for poloidal (ky)an d toroidal (kz) wavenumbers is determined for the range 0.5 cm−1 < k < 7.5 cm−1, for both potential and density fluctuations obtained from spatially fixed Langmuir probe pairs. The S(ky, ω) spectra aredominated by low frequencies ( < 200 kHz) and small wavenumbers (< 3 cm−1) and appear broadened about an approximately linear statistical dispersion relation, (ω). The broadening is characterized by a spectral width σk (ω) (root-mean-square deviation about (ω)) which is of the order of (ω). The turbulence appears to propagate poloidally with an apparent mean phase velocity of (1–2) × 105 cms−1 in the ion diamagnetic drift direction. Measurements of the fluctuation-induced particle transport reveal the particle flux to be outward and resulting primarily from the low-frequency, long-wavelength components of the turbulence. A particle diffusion coefficient Dx, estimated from the measured particle flux and density profiles, is of the order of the coefficient estimated for Bohm diffusion, but appears to increase with density.
Nuclear Fusion 01/2011; 24(5):527. · 4.09 Impact Factor
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ABSTRACT: In Orthogonal Frequency Division Multiplexing, a simple clipping method is widely used in order to reduce the peak-to-average power ratio since it is easy to implement. The performance analysis of the clipping approach has been previously introduced in the literature. Clipping, however, is a nonlinear process and may cause two major undesirable effects: (i) spectral regrowth, which causes unacceptable out-of-band radiation; and (ii) distortion of the desired signal, which increases bit-error-rate (BER). The out-of-band radiation can and often is suppressed by filtering, which leads to peak regrowth. Therefore, iterative clipping and filtering is required until the desired clipping level is achieved. However, this iterative process makes BER estimation difficult. This letter provides expressions and analytical techniques for estimating the attenuation factor, error vector magnitude, and BER, using a noise enhancement factor that is obtained by simulation. Simulation results show strong agreement with our semi-analytical results for 1024 subcarriers.
IEEE Transactions on Wireless Communications 06/2010; · 2.59 Impact Factor
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ABSTRACT: The active constellation extension (ACE) technique is attractive for reducing peak power in OFDM systems. It is simple and easy to implement and provides suboptimal gain in peak power reduction. However, its peak power reduction capacity is sensitive to low target clipping ratios. In this paper, we propose a novel ACE algorithm using pilot symbols. In contrast to the existing ACE techniques, our proposed algorithm can reach any desired target clipping level with low complexity, even when the clipping level is set below the initially unknown optimum value. Simulation results are utilized to compare our proposed algorithm with the existing ACE techniques. It is shown that our proposed algorithm provides better peak-to-average power ratio (PAR) reduction performance for low clipping ratios.
Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
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ABSTRACT: For PAR reduction in OFDM systems, the clipping-based Active Constellation Extension (ACE) technique is simple and attractive for practical implementation. However, we observe it cannot achieve the minimum PAR when the target clipping level is set below an initially unknown optimum value. To overcome this low clipping ratio problem, we propose a novel ACE algorithm with adaptive clipping control. Simulation results demonstrate that our proposed algorithm can reach the minimum PAR for severely low clipping ratios. In addition, we present the tradeoff between PAR and the loss in E<sub>b</sub>/N<sub>o</sub> over an AWGN channel in terms of the clipping ratio.
IEEE Communications Letters 02/2010; · 0.98 Impact Factor
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ABSTRACT: In this paper, we utilize joint time-frequency analysis to inspect frequency-varying currents at the grid connection of a variable speed wind turbine system. Through simulation experiments based on a PSCAD dynamic model, frequency-varying characteristics of the currents are investigated and compared with the wind speed. Of particular note is the appearance of nonharmonic frequencies when the wind speed drops below a certain critical value, which is potentially harmful to the grid power quality. A comparison method based on joint time-frequency analysis is also provided to estimate the onset wind speed corresponding to the appearance of the extraneous frequencies.
Power Electronics and Machines in Wind Applications, 2009. PEMWA 2009. IEEE; 07/2009
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ABSTRACT: Space-time block codes (STBCs) from coordinate interleaved orthogonal designs (CIODs) offer several advantages including full-diversity and single-symbol decodability. In an effort to assess their performance in quasi-static frequency nonselective i.i.d. Nakagami-m fading channels, we analyze the error rate, outage capacity, and information outage probability. First, based on an accurate closed-form formula for the average symbol pairwise error rate (SPER), we derive tight union upper and lower bounds on the symbol-error rate (SER). Second, we apply Gaussian and Gamma approximations to provide closed form expressions for the outage capacity. Third, using high signal-to-noise ratio (SNR) and moment-matching approximation techniques, we also derive accurate closed-form approximations for the information outage probability (IOP). Finally, we show that STBCs from CIODs provide full-diversity by deriving SER based and IOP-based asymptotic and instantaneous diversity orders. Monte-Carlo simulations show that the analytical results agree with simulation experiments.
IEEE Transactions on Communications 04/2009; · 1.68 Impact Factor
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ABSTRACT: International standard bodies have specified an instrument capable of detecting and assessing the severity of lighting flicker, which is called a flickermeter. However, a deficiency with regard to flicker caused by high-frequency interharmonics has been reported in the recent literature. In this paper, another deficiency of the International Electrotechnical Commission (IEC) flickermeter with regard to low-frequency interharmonics, whose frequencies are below the difference between the fundamental frequency and the cutoff frequency of a bandpass filter of the IEC standard flickermeter, will be discussed and illustrated based on analysis and numerical experiments. A new approach based on down-up sampling in the discrete-time sample domain is also proposed to address the flicker-detection problem associated with interharmonics. We demonstrate the efficacy of the proposed method by a comparison with the current flickermeter standard in the presence of interharmonics.
IEEE Transactions on Instrumentation and Measurement 02/2009; · 1.21 Impact Factor
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ABSTRACT: We analyze the effect of time-selective fading on the information outage probability and diversity order of Alamouti transmit diversity with joint maximum likelihood, symbolwise linear maximum likelihood, zero forcing, and decision feedback detectors. In contrast to the existing bit error rate (BER) and bit error outage (BEO) analyses, we evaluate the information outage probabilities and corresponding asymptotic diversity orders for each detector. From the theoretical and simulation results, we demonstrate that, when the channel is time selective, the asymptotic diversity orders for the four detectors approach two, zero, one, and one, respectively.
IEEE Transactions on Vehicular Technology 12/2008; · 1.92 Impact Factor
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ABSTRACT: This paper develops a signal processing, time-frequency analysis-based analytic solution to locate transient capacitor switching disturbances. The flow of transient disturbance energy caused by the capacitor switching is determined by the time and frequency localized phase difference. Cross time-frequency analysis provides time- and frequency-localized phase difference between the transient voltage and current disturbance waveforms which determine the direction of transient disturbance energy flow. The time and frequency localization properties of the proposed scheme allows one to expand the application to complicated power distribution systems without knowledge of system parameters, capacitor size, and configuration. The proposed scheme has been verified by the electromagnetic transients program simulation for all possible spatial locations of the capacitor switching.
IEEE Transactions on Power Delivery 11/2008; · 1.35 Impact Factor
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ABSTRACT: Per-flow congestion control helps endpoints fairly and efficiently share network resources. Better utilization of network resources can be achieved, however, if congestion management algorithms can determine when two different flows share a congested link. Such knowledge can be used to implement cooperative congestion control or improve the overlay topology of a P2P system. Previous techniques to detect shared congestion either assume a common source or destination node, drop-tail queueing, or a single point of congestion. We propose in this paper a novel technique, applicable to any pair of paths on the Internet, without such limitations. Our technique employs a signal processing method, wavelet denoising , to separate queueing delay caused by network congestion from various other delay variations. Our wavelet-based technique is evaluated through both simulations and Internet experiments. We show that, when detecting shared congestion of paths with a common endpoint, our technique provides faster convergence and higher accuracy while using fewer packets than previous techniques, and that it also accurately determines when there is no shared congestion. Furthermore, we show that our technique is robust and accurate for paths without a common endpoint or synchronized clocks; more specifically, it can tolerate a synchronization offset of up to one second between two packet flows.
IEEE/ACM Transactions on Networking 09/2008; · 2.03 Impact Factor
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ABSTRACT: By combining multiple-input-multiple-output (MIMO) communication with block transmission using a cyclic prefix (CP), MIMO block transmission systems can not only achieve high data rates but also simplify channel estimation and equalization. In this paper, to provide a bandwidth-efficient solution for channel estimation in MIMO block transmission systems with a CP, we present a framework for blind channel estimation based on a general non-redundant precoding. Using this framework, we propose a blind estimator exploiting a simplified non-redundant precoding that is robust against overestimates of a true MIMO channel order. Furthermore, in the case with the number of transmit antennas greater than that of receive antennas, we show under specific system conditions that the proposed blind algorithms can be used for estimation of the MIMO channel without oversampling the received signals. With the simplified precoding conditions established in this paper, the proposed method does not impose the strict identifiability conditions on the MIMO channel, which are required for the existing methods. Furthermore the proposed estimator achieves accurate channel estimation with a small number of symbol blocks. In addition, we derive a simplified precoder that is optimized in the sense of minimizing the impact of unknown additive noise. We discuss the tradeoff between channel estimation accuracy and bit error rate performance associated with the simplified precoder. Finally, we develop a technique for resolving the channel ambiguity in the proposed blind estimator to create a semi-blind channel estimator relying on only a few known symbols.
IEEE Transactions on Signal Processing 07/2008; · 2.63 Impact Factor
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ABSTRACT: The current IEC flickermeter standard only considers light flicker of incandescent lamps. Hence, the standard flicker-meter has limitations regarding assessing flicker of lamp types other than the incandescent lamp, particularly in the presence of interharmonics. In this paper, the flicker responses of various lamps (e.g., incandescent, compact fluorescent, and LED (light-emitting-diode) lamps) are investigated using a laboratory interharmonic flicker station and a photo detector. The experimental results demonstrate that LED lamps exhibit flicker response similar to that of compact fluorescent lamps, which are sensitive to high-frequency interharmonics located close to the odd order harmonics. Our analysis indicates that a diode bridge rectifier, which converts AC to DC for both compact fluorescent and LED lamps, is a source of flicker due to high-frequency interharmonics.
Instrumentation and Measurement Technology Conference Proceedings, 2008. IMTC 2008. IEEE; 06/2008
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ABSTRACT: Space-time block codes (STBCs) from coordinate interleaved orthogonal designs (CIODs) have attracted considerable attention lately due to their full-diversity and single-symbol decodability for multiple antenna systems. In this letter, we analyze the information outage probability of STBCs from symmetric structured CIODs in quasi-static frequency-nonselective Rayleigh fading channels. By using the probability density function (PDF) and the cumulative distribution function (CDF) of the product of two i.i.d. chi-square random variables and then exploiting a moment-matching approximation technique, we derive an accurate closed-form approximation for the information outage probability of symmetric CIODs and a corresponding asymptotic diversity order, the results of which can be used to theoretically predict the performance of symmetric CIODs with various antenna configurations in non-ergodic Rayleigh fading channels. Some simulations are performed to verify the accuracy of the derived formulas and illustrate the theoretical results.
IEEE Transactions on Wireless Communications 06/2008; · 2.59 Impact Factor
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ABSTRACT: Selected mapping (SLM) techniques have been used to reduce the high peak-to-average power ratio (PAR) in orthogonal frequency division multiplexing (OFDM) systems. The capacity of PAR reduction can be increased by improving the PAR statistics at the cost of complexity of the OFDM transmitter. However, it is difficult to clarify the performance degradation in the presence of nonlinearities, such as a power amplifier (PA), with the PAR metric because the statistical distribution of PAR focuses only on the highest peak, even though PAR reduction can help to reduce the required PA backoff. The nonlinear distortion is related to the amount of signal greater than the saturated threshold value; therefore, it is desirable to know the envelope power distribution. In this paper, we derive a closed form expression for the envelope power distribution in SLM/OFDM systems with nonlinearity, and verify the derived expression with simulation results.
Acoustics, Speech and Signal Processing, 2008. ICASSP 2008. IEEE International Conference on; 05/2008 · 4.63 Impact Factor
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ABSTRACT: Despite their many advantages, orthogonal frequency division multiplexing (OFDM) systems have been limited by nonlinear devices such as power amplifiers and DACs due to OFDM's inherently high peak-to-average power ratio (PAR). This high PAR limits the use of high efficiency amplification devices. To reduce the PAR in OFDM systems, some low complexity tone reservation (TR) algorithms have been proposed. However, we find the existing low complexity TR algorithms can not achieve the minimum PAR when the clipping level is set below the initially unknown optimum value. To overcome this problem, in this paper we propose a robust PAR reduction technique by combining low complexity TR and adaptive clipping control. Simulation results demonstrate the robustness of the proposed method to severely low clipping levels.
Signals, Systems and Computers, 2007. ACSSC 2007. Conference Record of the Forty-First Asilomar Conference on; 12/2007
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ABSTRACT: We find that placing each pilot tone in an equally spaced manner according to the conventional placement scheme is not suitable for doubly selective channel estimation. In this paper, we propose an efficient pilot tone placement scheme enabling accurate channel estimation in OFDM systems regardless of time variations of a channel. Since the number of channel impulse response taps to be estimated is typically much greater than the number of pilot tones, linear minimum mean square error (LMMSE) estimation schemes for time-invariant channels cannot be straightforwardly extended to doubly selective channel estimation. To overcome this problem, we propose an accurate LMMSE channel estimator that exploits a small number of pilot tones located according to the derived pilot placement. To achieve performance close to the LMMSE estimator but with lower complexity, an approximate LMMSE (ALMMSE) channel estimator is also proposed. Finally, we propose a novel iterative ALMMSE channel estimator that achieves better performance than the LMMSE and ALMMSE estimators, while having complexity in between the two.
IEEE Transactions on Wireless Communications 11/2007; · 2.59 Impact Factor
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ABSTRACT: Since a damaged or abnormal-state machine often generates highly nonlinear signals, it is desirable to use a tool that can effectively detect and analyze nonlinear signatures. The bicoherence has been proposed for such nonlinear analysis since it is a measure of the phase coupling between interacting frequency components. However, the bicoherence has some difficulties in machine condition monitoring due to the challenge of distinguishing between the intrinsic nonlinear signature of a healthy machine and the nonlinear signature of a faulted machine. To address this issue, we propose a novel method exploiting the bispectral change detection (BCD) to detect and analyze the machine faults. The principal advantages of the proposed BCD method are that it can suppress the intrinsic nonlinear signature of healthy machines and emphasize the fault-induced nonlinearities. Therefore, the proposed BCD method can discriminate between fault-induced nonlinearities and intrinsic nonlinearities, and thus can be a strong and sensitive diagnostic for machine faults. The usefulness and statistical robustness of this method are demonstrated via some experimental results.
Power Engineering Society General Meeting, 2007. IEEE; 07/2007
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ABSTRACT: In this paper, we propose a novel condition monitoring method for adjustable speed drives (ASD) and ASD-driven motors. The key idea is to diagnose the status of the load side of an ASD from observations made at the source side. The use of a higher-order spectral metric, the cross bicoherence, is proposed for the detection and analysis of anomalies associated with nonlinear devices in ASD systems. In this paper, we apply our method to detect phase imbalance at the load side of an ASD. Our experimental results demonstrate that the proposed method provides a unique signature of phase imbalance for detection and classification of asymmetric impedance associated with the phase imbalance. Furthermore, the proposed method shows a more sensitive detection performance compared to the conventional imbalance measurement method, which enables early detection of potential faults.
Electric Ship Technologies Symposium, 2007. ESTS '07. IEEE; 06/2007
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ABSTRACT: In this paper, we survey different formulations of the reconfiguration problem and the different techniques used for its solution. We also compare the problem of reconfiguration of shipboard power systems to that of large scale systems. The reconfiguration of closely coupled power systems offers some unique challenges and limits the applicability of previously developed analysis. Although the number of switches is small in comparison to terrestrial power systems, the demands on the shipboard power system can be more severe. Further, because of space limitations the safety through redundancy of systems is not always feasible. Stability and service restorative capabilities are key objectives in closely coupled networks. After reviewing research on shipboard power systems, we motivate the system theoretical approach to solve reconfiguration problem in this context.
Electric Ship Technologies Symposium, 2007. ESTS '07. IEEE; 06/2007
