E.J. Powers

The University of Texas at Austin, Texas City, TX, United States

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Publications (263)251.74 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.
    IEEE Transactions on Wireless Communications 01/2013; 12(5):2314-2322. · 2.42 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. · 2.73 Impact Factor
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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. · 2.73 Impact Factor
  • K. Bae, J.G. Andrews, E.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.
    IEEE Transactions on Wireless Communications 06/2010; · 2.42 Impact Factor
  • 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.
    Acoustics Speech and Signal Processing (ICASSP), 2010 IEEE International Conference on; 04/2010 · 4.63 Impact Factor
  • 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.
    Conference Record - IEEE Instrumentation and Measurement Technology Conference 01/2010;
  • 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.
    10th International Conference on Information Sciences, Signal Processing and their Applications, ISSPA 2010, Kuala Lumpur, Malaysia, 10-13 May, 2010; 01/2010
  • 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.
    IEEE Communications Letters 01/2010; 14:39-41. · 1.16 Impact Factor
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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.
    IEEE Transactions on Power Systems 01/2010; 25(2):914-921. · 2.92 Impact Factor
  • 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.
    Power Electronics and Machines in Wind Applications, 2009. PEMWA 2009. IEEE; 07/2009
  • Hoojin Lee, J. Andrews, R. Heath, E.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.
    IEEE Transactions on Communications 04/2009; · 1.75 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; · 1.36 Impact Factor
  • IEEE T. Instrumentation and Measurement. 01/2009; 58:152-160.
  • Hoojin Lee, R.W. Heath, E.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.
    IEEE Transactions on Vehicular Technology 12/2008; · 2.06 Impact Factor
  • Source
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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; · 1.52 Impact Factor
  • Source
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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; · 2.01 Impact Factor
  • 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.
    IEEE Transactions on Signal Processing 07/2008; · 2.81 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
  • Hoojin Lee, J.G. Andrews, E.J. Powers
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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; · 2.42 Impact Factor
  • Kitaek Bae, E.J. Powers
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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

Publication Stats

4k Citations
251.74 Total Impact Points

Institutions

  • 1977–2011
    • The University of Texas at Austin
      • • Fusion Research Center
      • • Department of Electrical & Computer Engineering
      Texas City, TX, United States
  • 2009
    • Freescale Semiconductors, Inc
      Austin, Texas, United States
  • 2008
    • University of South Carolina
      • Department of Electrical Engineering
      Columbia, SC, United States
    • Washington State University
      • School of Electrical Engineering and Computer Science
      Pullman, WA, United States
  • 2005–2008
    • Samsung Advanced Institute of Technology
      Usan-ri, Gyeonggi Province, South Korea
  • 2004–2005
    • Soongsil University
      • Department of Electronic Engineering
      Seoul, Seoul, South Korea
  • 2003
    • Yonsei University
      Sŏul, Seoul, South Korea
  • 1995–2003
    • Hanyang University
      • • Department of Electronics and Computer Engineering
      • • Department of Electronic and Electrical Engineering
      Ansan, Gyeonggi, 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
      Blacksburg, Virginia, United States
  • 1987
    • University of Nevada, Las Vegas
      Las Vegas, Nevada, United States
  • 1986
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States