E.J. Powers

University of Texas at Austin, Austin, Texas, United States

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Publications (289)273.87 Total impact

  • Kitaek Bae · Changyong Shin · E.J. Powers
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    ABSTRACT: In the presence of nonlinearity, we analyze the impact of the selected mapping (SLM) technique on bit-error-rate (BER) performance of orthogonal frequency division multiplexing (OFDM) systems in an additive white Gaussian noise channel. The peak-to-average-ratio (PAR) reduction gain of SLM can be increased by improving the PAR statistics at the cost of complexity in the OFDM transmitter, thereby helping to decrease the required power amplifier (PA) output backoff (OBO). However, since the PAR statistics focus only on the statistical distribution of the highest peak in an OFDM symbol, the statistics cannot be used to quantify BER performance degradation in the presence of nonlinearity such as that caused by a PA or digital-to-analog converter (DAC). We first derive a closed-form expression for the envelope power distribution in an OFDM system with SLM. Then, using this derived envelope power distribution, we investigate the BER performance and the total degradation (TD) of OFDM systems with SLM under the existence of nonlinearity. We discuss peak backoff (PBO) and the clipping ratio, which determine the operating point of the PA and dynamic range of the DAC, respectively. Lastly, we consider the total degradation (TD), which indicates the tradeoff between the OBO and the E_b/N_o penalty due to nonlinearity, and numerically compute the PBO and clipping ratio that minimize the TD. The TD-minimizing PBO and clipping ratio are given as functions of the number of candidate signals in SLM.
    No preview · Article · May 2013 · IEEE Transactions on Wireless Communications
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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.
    No preview · Article · Jan 2011 · Nuclear Fusion
  • S.J. Levinson · J.M. Beall · E.J. Powers · R.D. Bengtson
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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.
    No preview · Article · Jan 2011 · Nuclear Fusion
  • Kitaek Bae · Jeffrey G. Andrews · Edward J. Powers
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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.
    No preview · Article · Jun 2010 · IEEE Transactions on Wireless Communications
  • Matthew Rylander · W. Mack Grady · Ari Arapostathis · Edward J. Powers
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    ABSTRACT: This paper develops a new large-signal transient load model to represent the composite power electronic load at a network bus. Traditional load models do not account for the transient responses of power electronic loads which occur at the onset and clearing of voltage sags. The new model is supported with actual utility fault response data and laboratory controlled tests. This paper shows that power electronic loads have a unique impact on generator rotor angle swings compared to those using conventional load models.
    No preview · Article · May 2010 · IEEE Transactions on Power Systems
  • Kitaek Bae · E.J. Powers
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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.
    No preview · Conference Paper · Apr 2010
  • Source
    Woutijn J Baars · Charles E Tinney · Edward J Powers
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    ABSTRACT: A new and unique analytical approach, capable of identifying both linear and higher-order coherences in multiple-I/O systems, is presented here in the context of turbulent flows. The technique is formed by combining two well-established methods: Proper Or-thogonal Decomposition (POD) and Higher-Order Spectra Analysis; both of which were developed independently in the field of turbulence and system identification, respectively. The latter of these is based on known methods for characterizing nonlinear systems by way of Volterra functional series. In that, both linear and higher-order kernels are formed to quantify the nonlinear spectral transfer of energy between the system's input and out-put. This reduces essentially to spectral Linear Stochastic Estimation (LSE) when only the first-order terms are considered, and is therefore presented in the context of stochastic esti-mation as spectral Higher-Order Stochastic Estimation (HOSE). However, the trade-off to seeking higher-order transfer kernels is that the increased complexity restricts the analysis to single-I/O systems. Low-dimensional (POD based) analysis techniques are inserted to alleviate this void as the POD coefficients represent the dynamics of the so-called spatial structures (modes) of a multi degree-of-freedom system. At first, a Monte Carlo Simulation is performed to demonstrate the validity and characteristics of the higher-order Volterra series model. Next, the POD based spectral HOSE method is applied to an experimental data set comprising synchronous measures of the near-field pressure and far-field acoustics of a coaxial jet flow. Both near-field (line array of microphones) and far-field (arc array of microphones) signatures are decomposed independently using POD in order to obtain frequency dependent POD coefficients. The linear and quadratic kernels are computed using different combinations of POD coefficients. The results indicate that the acoustic signatures in the far-field of the jet are linearly related to the pressure signatures in the near-field pressure, as is expected in a subsonic Mach number jet flow.
    Full-text · Article · Jan 2010
  • Kitaek Bae · Jeffrey G. Andrews · Edward J. Powers
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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.
    No preview · Article · Jan 2010 · IEEE Communications Letters
  • Wonjin Cho · Edward J. Powers · Surya Santoso
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    ABSTRACT: This paper presents the use of dithered sigma-delta modulation (ΣΔM) to mitigate low and high frequency harmonics of pulse-width modulated (PWM) inverters without increasing switching losses. A PWM bit stream generated by a 1 bit- 1st-order discrete ΣΔM is converted into a continuous pulse train which controls the inverter operation. The minimum pulse width of the pulse train is strictly defined by the sampling ratio, which prevents the inverter from missing switching operations. The spectrum of the pulse train demonstrates reduction of low and high frequency harmonics compared with the conventional sinusoidal PWM (SPWM), dithered SPWM, and carrier frequency modified SPWM spectra. The dithered ΣΔM bounds all spectral densities including harmonics and noise below -30dB with respect to the fundamental density without increasing the number of switching, while comparable SPWM methods produce harmonic peaks at a particular frequency or increase the number of switching.
    No preview · Conference Paper · Jan 2010
  • Wonjin Cho · Edward J. Powers · Surya Santoso
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    ABSTRACT: This paper presents the effect of dithering on both harmonics and interharmonics in adjustable speed drives (ASDs). Using a voltage source configuration, a six-pulse rectifier and a sinusoidal pulse width modulated (SPWM) inverter are modeled in the PSCAD/EMTDC simulation platform. The pulse width modulated (PWM) signal for the inverter is modified by means of the additive dither noise. The dither randomizes the timing of rising and falling edges of PWM signals, which eventually reduces periodic harmonics of the inverter output voltage. The reduced harmonics also affect the source voltage of the rectifier and ultimately reduce interharmonics. The mitigated harmonics and interharmonics in the ASD are quantified in conventional power spectrum. Furthermore, frequency coupling is also analyzed using cross bicoherence to investigate causes of interharmonics.
    No preview · Article · Jan 2010 · Conference Record - IEEE Instrumentation and Measurement Technology Conference
  • Source
    Wonjin Cho · E.J. Powers
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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.
    Preview · Conference Paper · Jul 2009
  • Hoojin Lee · Jeffrey G. Andrews · Robert W. Heath · Edward J. Powers
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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.
    No preview · Article · Apr 2009 · IEEE Transactions on Communications
  • Taekhyun Kim · Edward J. Powers · W. Mack Grady · Ari Arapostathis
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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.
    No preview · Article · Feb 2009 · IEEE Transactions on Instrumentation and Measurement
  • Taekhyun Kim · Edward J. Powers · W. Mack Grady · Ari Arapostathis

    No preview · Article · Jan 2009
  • Hoojin Lee · Edward J. Powers
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    ABSTRACT: In this letter, we derive a very accurate closed-form approximate formula for the average achievable rate of stacked orthogonal space-time block code (OSTBC) in Rayleigh fading channels. Some simulations are performed to demonstrate that the derived formula shows better agreement with Monte-Carlo simulation results than the existing closed-form approximate expressions.
    No preview · Article · Dec 2008 · IEICE Transactions on Communications
  • Hoojin Lee · Robert W. Heath · Edward J. Powers
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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.
    No preview · Article · Dec 2008 · IEEE Transactions on Vehicular Technology
  • Source
    Yong-June Shin · Edward J. Powers · W. Mack Grady · Ari Arapostathis
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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.
    Preview · Article · Nov 2008 · IEEE Transactions on Power Delivery
  • Ronald Stearman · Taekhyun Kim · Edward J. Powers
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    ABSTRACT: The influence of damage on the performance of the lifting surface of an aircraft has been an area of interest to the military for over half a century. Some of the earlier studies of this subject focused on the impact that structural damage resulting in the loss of lifting surface structural strength, stiffness, and mass might have on imparting catastrophic aeroelastic failures to the lifting surface. Discouraging mixed results arose over the next 30 years in these investigations that employed only structural, damage models. As time passed, it was finally noticed that if the often neglected, usually second-order, drag term was now included (as aerodynamic damage often creates a high, unsteady, localized drag rise) in the earlier aeroelastic damaged-wing analyses, more encouraging results were obtained. That is, the realistic asymmetric damaged lifting surfaces flutter and divergence analysis did show lowered, and not raised, flutter and divergence speeds over that of the undamaged lifting surface. This occurred both in the presence of or in the absence of structural damage as in the case of icing or external wing stores. In essence, it was determined at this time that the resulting high and unsteady drag rise caused by the damage could do more than just cause an energy deficiency in the aircraft mission. It could, in fact, cause shorter term catastrophic or stiffness failure events in terms of reduced flutter and divergence speeds as well as give rise to limit cycle oscillations embedded within what was shown to be that part of the damaged-aircraft flight envelope free from classic flutter and divergence. The present experimental wind-tunnel study was carried out to confirm these past theoretical findings by employing a one-twelfth-scale F-111 aeroelastic model and (third-order) higher-order statistical signal processing to identify the frequency ordering that occurs in these additional damaged-induced limit cycle oscillations. These limit cycle oscillations were also found to exhibit the classic lock-in/lock-out phenomenon that occurs in the well-known F/A-16 and F/A-18 wing store induced limit cycle oscillations.
    No preview · Article · Sep 2008 · Journal of Aircraft
  • Source
    Min Sik Kim · Taekhyun Kim · Yong-June Shin · Simon S. Lam · Edward J. Powers
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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.
    Preview · Article · Sep 2008 · IEEE/ACM Transactions on Networking
  • Changyong Shin · R.W. Heath · E.J. Powers
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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.
    No preview · Article · Jul 2008 · IEEE Transactions on Signal Processing

Publication Stats

6k Citations
273.87 Total Impact Points


  • 1967-2013
    • University of Texas at Austin
      • • Department of Electrical & Computer Engineering
      • • Fusion Research Center
      Austin, Texas, United States
  • 2007
    • Texas A&M University
      • Department of Electrical and Computer Engineering
      College Station, TX, United States
  • 2005
    • University of Texas System
      Austin, Texas, United States
  • 2004
    • Soongsil University
      • Department of Electronic Engineering
      Seoul, Seoul, South Korea
  • 2002
    • Princeton University
      • Princeton Plasma Physics Laboratory
      Princeton, NJ, United States
  • 2001
    • Universitetet i Tromsø
      • Department of Physics and Technology
      Tromsø, Troms, Norway
  • 2000
    • University of Tasmania
      Hobart Town, Tasmania, Australia
  • 1996
    • Trimble Navigation
      Sunnyvale, California, United States
  • 1993
    • Virginia Polytechnic Institute and State University
      • Department of Engineering Science and Mechanics
      Blacksburg, Virginia, United States
  • 1988
    • University of Nevada, Las Vegas
      Las Vegas, Nevada, United States