Zhengqing Yun

University of Hawaiʻi at Mānoa, Honolulu, Hawaii, United States

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Publications (72)36.83 Total impact

  • Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: This paper reviews the basic concepts of rays, ray tracing algorithms, and radio propagation modeling using ray tracing methods. We focus on the fundamental concepts and the development of practical ray tracing algorithms. The most recent progress and a future perspective of ray tracing are also discussed. We envision propagation modeling in the near future as an intelligent, accurate, and real-time system in which ray tracing plays an important role. This review is especially useful for experts who are developing new ray tracing algorithms to enhance modeling accuracy and improve computational speed.
    No preview · Article · Jan 2015
  • Soo Lim · Anwuna Awelemdy · Zhengqing Yun · Magdy Iskander
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    ABSTRACT: As terrestrial wireless communication systems become more ubiquitous, there is a rising need to study the propagation characteristics of every habitable environment so that effective communication systems can be established. Past research showed that open-trench drains, whose presence is prevailing in many Asian cities, make up an additional channel in which radio signal can adequately travel. In this paper, we have further our studies in the propagation characteristics of an open-trench drain to delve into areas that have not been previously reported. Specifically, we present field measurement results of an open-trench drain for line-of-sight (LoS) and non-line-of-sight (NLoS) portions at three frequencies (900 MHz, 2.4 and 5.8 GHz) for three different polarizations, namely, vertical-vertical (VV), horizontal-horizontal (HH), and horizontal-vertical (HV) polarizations. Subsequently, we have adopted K-means clustering approach to process the obtained data in deriving path loss exponent n and their associated standard deviation. Comparison is made of the n values, as well as the standard deviation values, before and after employing K-means clustering.
    No preview · Article · Jan 2015 · IEEE Antennas and Wireless Propagation Letters
  • S.Y. Lim · Zhengqing Yun · M.F. Iskander
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    ABSTRACT: The propagation characteristics of indoor multi-floor environments have been studied extensively and empirical models for many scenarios are available. These studies usually do not concern about stair structures. In this paper we study radio propagation in four typical stairwells through measurement at two frequencies (2.4 GHz and 5.8 GHz). Values of path loss exponent $n$ have been derived for vertical and horizontal polarizations. These $n$-values for stairwells are found to be higher than the $n$ -values for multi-floor environments. We also propose a new path loss model based on the so-called “accumulative distance” the receiver has traveled, in addition to the conventional separation distance between transmitting and receiving antennas. The new path loss model has lower $n$ values and, most importantly, smaller standard deviation and can thus be considered a better model fitting the measurement data. The results in this study can be useful for designers of small cell wireless communications system such as pico- and femto-cells.
    No preview · Article · Sep 2014 · IEEE Transactions on Antennas and Propagation
  • Darcy A. Bibb · Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: The source localization using time reversal in urban environments is studied for single and array receive antennas. Ray tracing is used for simulation of the EM wave propagation because full wave methods (e.g., the FDTD method) are not able to deal with the large size of the environments.
    No preview · Conference Paper · Jul 2014

  • No preview · Conference Paper · Jul 2014
  • Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: Diffractions from multiple-wedge structures are important in many wireless communications environments. For irregular terrain, the diffraction from ridges are commonly calculated using a two-dimensional height profile. Due to the fact that such a profile does not have the information about the slope and orientation of the ridges, and the interior angle is often incorrectly represented, three dimensional method is needed to improve the accuracy of the diffracted field. Furthermore, the ray paths determined using the height profile is in general wrong because they are not the least-time paths. In this paper we compare the path loss differences between the two- and three-dimensional methods.
    No preview · Conference Paper · Apr 2014
  • Nuri Celik · Hao Xu · Trevor Wilkey · Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: Cognitive radio systems adaptively adjust their radio parameters based on the feedback from the electromagnetic environment. Therefore, it is expected that cognitive radio systems will have better performance if they are integrated with accurate site-specific propagation modeling. Our group has long standing expertise in propagation modeling utilizing geo-spatial tools and indoor building models, and has focus on integrating these models as part of the environment awareness engine in cognitive radios. In this paper, we present the preliminary results for a multi-antenna based spectrum sensing algorithm with adaptive thresholds. The results show that, an almost fixed detection rate of 85% can be achieved for a mobile cognitive radio in an urban environment when the thresholds are set based on the path loss information provided by site-specific propagation modeling. On the other hand, the spectrum sensing algorithm with a fixed threshold shows a great variability in the detection performance, going as low as 65% in regions with heavy fading, which leads to harmful interference.
    No preview · Conference Paper · Jul 2013
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    ABSTRACT: Radio propagation in smaller radio cells is significantly affected by the surrounding environments such as the geometry of buildings, terrain, foliage, and others. In this work, focus is given on radio propagation in environment where drains are present - both covered and partially open. Ray tracing method was utilized to examine the differences between the ground-reflected rays from these two scenarios at 900 MHz. Discussions are given to offer an insight into the propagation of radio wave in drains environment.
    No preview · Conference Paper · Jul 2013
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    ABSTRACT: The full spatial covariance matrix of the multiple-input multiple-output channel is an important quantity that is used in channel modeling, communication system signal processing, and performance analysis. This paper formulates a closed-form expression for this covariance matrix based on the average multipath propagation characteristics and the radiation patterns of the antennas in the transmit and receive arrays. Simplifications to the formulation are established for cases where the propagation can be represented using discrete plane waves or where the power angular spectrum can be modeled using clusters that satisfy uniform, truncated Gaussian, or truncated Laplacian distributions. Numerical studies demonstrate application of the formulation as well as its accuracy under different scenarios.
    No preview · Article · Feb 2013 · IEEE Transactions on Antennas and Propagation
  • Nobutaka Omaki · Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: The SSPE algorithm is derived from two-dimensional wave equation through different approximation methods. Different expressions of SSPE are used in different references. It is important to understand the differences for better implementation of SSPE. Our numerical results show mat all these different formulations provide similar path loss results for a flat ocean surface. Detailed explanations and derivations will be presented in the conference.
    No preview · Conference Paper · Nov 2012
  • Nobutaka Omaki · Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: Modeling radio propagation over ocean surface is important for homeland security applications. Due to its fast simulation speed, the parabolic equation (PE) method is a good candidate for and has been used in such simulations for decades. Since PE method is based on paraxial approximation of wave equation, its accuracy is not guaranteed for short ranges and larger propagation angles. On the other hand, Finite Difference Time Domain (FDTD) method is well known as an accurate method in short range but not suitable for large regions (in terms of wavelengths). For homeland security applications, it is necessary to deal with both short and long range propagation problems and it is natural to integrate PE and FDTD for simulating such scenarios. To exploit the advantages of PE and FDTD, quantifying the ranges suitable for these two methods is important and practically useful. In this paper, we examine the accuracy of PE method as a function of range (in short range regime) for smooth/flat ocean surfaces.
    No preview · Conference Paper · Jul 2012
  • Zhengqing Yun · Nobutaka Omaki · Magdy F. Iskander
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    ABSTRACT: When diffraction of ridges is required for predicting radio signal in mountainous regions, the realistic shape of ridges is often simplified (e.g., as a knife edge or a wedge). Also, the ridge orientation is usually ignored and the EM waves are assumed to be normally incident on the ridge. As more accurate radio propagation prediction is needed for advanced wireless communications, it is necessary to take into account more realistic features of ridges. For example, the slopes and orientation of ridges should be considered as two key features since they directly affect the values of diffraction coefficients. In this paper we present methods to extract these features of ridges based on digital terrain elevation data. The effect of the ridge orientation on the diffracted field is then investigated.
    No preview · Conference Paper · Jul 2012
  • Soo Yong Lim · Zhengqing Yun · Magdy F. Iskander
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    ABSTRACT: The propagation characteristics in indoor stairwell differ from those of the conventional indoor signal propagation across different floors and hence, deserve independent scrutiny. In this work, the propagation characteristics in four indoor stairwells have been examined by means of extensive field measurements at two frequency bands - 2.4 and 5.8 GHz respectively. It has been found the values of the path loss exponent n for stairwell are higher than those for other indoor environments. More importantly, the standard deviation of the n-values are high, leading to less accurate propagation models. This paper proposes the use of K-means clustering for the path loss data and values of path loss exponent n of stairwells are subsequently computed. The newly-computed path loss exponents n are lowered and their standard deviation values are significantly reduced. The path loss characteristics are useful for understanding radio propagation in pico- or femto-cells that have great potential in increasing the capacity and reducing the operational cost for wireless communications systems.
    No preview · Conference Paper · Jul 2012
  • Zhengqing Yun · M.F. Iskander · N. Omaki · A. Barrios
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    ABSTRACT: This paper describes an overview of recent advances in propagation modeling with some focus on contributions in characterizing challenging communications environments in urban and costal communications scenarios. MIMO capacity is estimated for typical communication scenarios, and impact of mutual coupling and wall type effects on the estimated MIMO capacities are quantified for various numbers of antenna array elements.
    No preview · Conference Paper · Jan 2012
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    ABSTRACT: In this paper, a low complexity hybrid smart antenna system with directional elements and reduced-size digital beamformer is proposed to combat the inter-symbol interference (ISI) problem over frequency-selective fading channels. The conventional smart antenna systems with omni-directional elements utilize the full-size digital beamformer to suppress interference and obtain the optimum performance. However, the proposed hybrid smart antenna system with directional elements can split the linear receive array into two subarrays. One subarray contains the elements with no ISI, which can be processed with a simple maximum ratio combiner (MRC). The second subarray includes the elements with the ISI interference, which require the use of a reduced-size optimum beamformer for interference suppression. Finally, the outputs of the two subarrays are combined for optimal detection of the transmitted signals. Simulation results confirm that the proposed low complexity hybrid smart antenna system can provide robust performance under the multipath fading channel with ISI, and outperform the conventional smart antenna systems due to the use of directional antennas and utilization of multipath diversity gain.
    No preview · Article · Dec 2011
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    ABSTRACT: Emerging homeland security applications require low-cost and rapid installation of HF radar systems in challenging terrain environments. One example concerns hills with natural slopeandsurfaceirregularityandroughness.Inthisletter,through the investigation of radio-wave propagation in such an environ- ment, we examine the effect of the slope and surface roughness of the terrain on the performance of HF radars. It is shown that the slope and surface irregularity affect the effective electrical spacing between array elements and cause beamforming and scanning errors, which lead to misdetection of targets. It is also shown by an investigation of an actual HF radar site in the area of Koko Head in Hawaii that scattering from surface roughness of the terrain adds extra error to the phase difference between antenna array elements , hence degrades the radar performance further. canoperatewithnegligible site preparation and provide multiple operating frequencies in the HF band. Here, and denote the free-space wavenumber and the radius of the sphere enclosing the antenna, respectively. Another difficulty is associated with operating HF radars in challenging propagation environments.Thisincludesdeploying HF radars on sloping sites, in areas within dense vegetation, and in proximity to buildings near coastal areas. These issues result in the deterioration of radar performance and the need for time-consuming and possibly expensive calibrations. In this letter, we specifically address the propagation issues and errorsinthe performanceofan HF radarplaced on asloping site and quantify phase errors between the array elements. The effect of the slope and the surface roughness is then incorpo- rated in evaluating the beamforming performance of radar on a sloping site with rough surface.
    No preview · Article · Dec 2011 · IEEE Antennas and Wireless Propagation Letters
  • Source
    N. Omaki · Zhengqing Yun · N. Celik · Hyoung-Sun Youn · M.F. Iskander
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    ABSTRACT: Emerging homeland security applications require low-cost and quickly deployable HF radar systems. In order to overcome the significant costs/unavailability of land lease in coastal areas for fixed applications, deployment of HF radar systems in challenging terrain environments is necessary. One such environment is the hilltops which naturally have slope and roughness. In this paper, through the investigation of radio wave propagation in such environments, we examined the effect of the slope and roughness of the terrain on the performance of HF radars. It is shown that the slope of the terrain will affect the effective inter-element spacing of the array and causes beamforming and scanning errors. It is also shown that surface roughness of the terrain also changes the effective inter-element phase differences and hence array performance. While the effect of the slope could be very significant diffraction effects from surface roughness resulted in a much smaller error of about 18°.
    Full-text · Conference Paper · Aug 2011
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    ABSTRACT: Emerging homeland security applications require low-cost and fast, deployable, high-frequency (HF) radar systems and the ability to operate in challenging terrain environments. With the need to cover as many border and coastal areas as possible, taking advantages of available transmitter resources to track targets using passive radar technologies is yet another area of research of considerable interest. In this paper, we describe the development of an HF radar system that meets these operational challenges, and we also highlight some recent implementation of the passive radar technology for homeland security applications. Specifically, we describe the design of a novel, electrically small HF antenna system consisting of three helical elements, one connected to the feed port while the other two are folded arms terminated with switchable loads. The antenna is 0.90-m (
    No preview · Article · May 2011 · Marine Technology Society Journal
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    ABSTRACT: The detection and classification of buried targets such as unexploded ordnance (UXO) using ground penetrating radar (GPR) technology involves complex qualitative features and 2-D scattering images. These processes are often performed by human operators and are thus subject to error and bias. Artificial intelligence (AI) technologies, such as neural networks (NN) and fuzzy systems, have been applied to develop autonomous classification algorithms and have shown promising results. Genetic programming (GP), a relatively new AI method, has also been examined for these classification purposes. In this letter, the results of a comparison between the classification performances of NN versus the GP techniques for GPR UXO data are presented. Simulated 2-D scattering patterns from one UXO target and four non-UXO objects are used in this comparison. Different levels of noise and cases of untrained data are also examined. Obtained results show that GP provides better performance than NN methods with increasing problem difficulty. Genetic programming also showed robustness to untrained data as well as an inherent capability of providing global optimal searching, which could minimize efforts on training processes.
    No preview · Article · Feb 2011 · IEEE Antennas and Wireless Propagation Letters
  • J. Kobashigawa · Hyoung-sun Youn · M. Iskander · Zhengqing Yun
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    ABSTRACT: In this paper, we present the results of our next step effort in comparison of classification performances between the NN and the GP techniques based on the simulated scattering patterns of UXO-like object and non-UXO objects. For this comparative study, 2 dimensional scattering images from one UXO target and four non-UXO objects were generated by numerical simulation tool (FEKO). For non-UXO objects, the most challenging targets to discriminate from UXO, since all these objects produce resonance signal as UXO-like targets do [6], were selected. Classification performances of both techniques (NN vs. GP) in different level of noise and in the case of presence of untrained data were examined and the results and observations are discussed.
    No preview · Conference Paper · Aug 2010

Publication Stats

788 Citations
36.83 Total Impact Points

Institutions

  • 2002-2015
    • University of Hawaiʻi at Mānoa
      • • College of Engineering
      • • Hawai‘i Center for Advanced Communications
      Honolulu, Hawaii, United States
  • 2005-2012
    • Honolulu University
      Honolulu, Hawaii, United States
  • 2004-2006
    • Hawaii Pacific University
      Honolulu, Hawaii, United States
  • 2000-2002
    • University of Utah
      • Department of Electrical and Computer Engineering
      Salt Lake City, Utah, United States