Seungwon Choi

Hanyang University, Sŏul, Seoul, South Korea

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Publications (87)108.91 Total impact

  • Hyunwook Yang, Gyuyoung Lee, Seungwon Choi
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    ABSTRACT: When Zero-Forcing (ZF) is adopted as a detector, decreasing the condition number of the channel matrix increases the BER performance. In this paper, we propose a new detection algorithm which reduces the condition number of channel matrix down to nearly 2 on average. Since the least singular value of the channel matrix is a major factor determining the condition number, we, first, project the received signal into a space spanned by singular vectors that are orthogonal to the one corresponding to the least singular value. Then, LR decomposition is performed to reduce further the condition number of the projected channel matrix. Computer simulations show that the performance of the proposed algorithm is comparable to that of the ML detector for both correlated and uncorrelated channels. And also the proposed algorithm provides an at least 2dB improvement compared to the conventional LR-based Ordered Successive Interference Cancellation (LR-OSIC) detector with a Bit Error Rate (BER) of 10-3 and a comparable computation load.
    IEICE Transactions on Communications 01/2015; E98.B(2):280-287. DOI:10.1587/transcom.E98.B.280 · 0.33 Impact Factor
  • Hyunwook Yang, Yeongyu Han, Seungwon Choi
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    ABSTRACT: In a multi-user multiple-input multiple-output (MU-MIMO) system that adopts zero-forcing (ZF) as a precoder, the best selection is the combination of users who provide the smallest trace of the inverse of the channel auto-correlation matrix. Noting that the trace of the matrix is closely related to the determinant, we search for users that yield the largest determinant of their channel auto-correlation matrix. The proposed technique utilizes the determinant row-exchange criterion (DREC) for computing the determinant-changing ratio, which is generated whenever a user is replaced by one of a group of pre-selected users. Based on the ratio computed by the DREC, the combination of users providing the largest changing ratio is selected. In order to identify the optimal combination, the DREC procedure is repeated until user replacement provides no increase in the determinant. Through computer simulations of four transmit antennas, we show that the bit error rate (BER) per signal-to-noise ratio (SNR) as well as the sum-rate performance provided by the proposed method is comparable to that of the full search method. Furthermore, using the proposed method, a partial replacement of users can be performed easily with a new user who provides the largest determinant.
    IEICE Transactions on Communications 07/2014; E97.B(7):1429-1434. DOI:10.1587/transcom.E97.B.1429 · 0.33 Impact Factor
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    ABSTRACT: Decoding latency of the turbo decoder has been a serious problem in real-time processing of communication systems. This paper presents a novel procedure of reducing the latency of the turbo decoder which has been implemented with GPU (Graphic Processing Unit). The main contribution of this paper is to present an efficient procedure of reducing the latency of GPU-based turbo decoder through an efficient parallel processing of maximum a posteriori (MAP). Through experimental tests, we have verified that the proposed turbo decoder reduces the latency from 34,767μs to 273μs per iteration.
    2014 International Symposium on Consumer Electronics (ICSE); 06/2014
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    ABSTRACT: This paper presents an optimum placement of Gateways operating in Wireless Sensor Networks (WSNs). The full search method of Gird deployment requires tremendous amount of computation. A novel algorithm for resolving the complexity problem is proposed. The new algorithm utilizes the Cross Entropy Method (CEM) for the maximum network performance with a lot less operations compared to the full search method. Applying the proposed method to various practical situations such as health care system, Electronic Shelf Labeling (ESL), etc, we have verified the superb throughput performance of the proposed algorithm using OPNET simulator. Compared to the full search, which is ideal method, the proposed method provides nearly 98% throughput in various experimental tests.
    2014 International Symposium on Consumer Electronics (ICSE); 06/2014
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    ABSTRACT: This paper introduces an implementation of GPU-based beamforming antenna module consisting of 4 elements. The proposed system can be mounted on top of the next generation base station system to reduce the radiating power at the base station. The transmitting power at base station is reduced proportionally to the number of antenna elements, due to the pencil beam provided between the base station and receiving terminal. Conversely, received signal at the terminal is enhanced by 4 times if the transmit power is maintained the same. Through various experimental tests, we have verified the proposed system consisting of 4 antenna elements provides nearly 12dB improvement in the receiving power at the terminal.
    2014 International Symposium on Consumer Electronics (ICSE); 06/2014
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    ABSTRACT: In this paper, we present an implementation of a long term evolution (LTE) system on a software defined radio (SDR) platform using a conventional personal computer that adopts a graphic processing unit (GPU) and a universal software radio peripheral2 (USRP2) with a URSP hardware driver (UHD) to implement an SDR software modem and a radio frequency transceiver, respectively. The central processing unit executes C++ control code that can access the USRP2 via the UHD. We have adopted the Ettus Research UHD due to its high degree of flexibility in the design of the transceiver chain. By taking advantage of this benefit, a simple cognitive radio engine has been implemented using libraries provided by the UHD. We have implemented the software modem on a GPU that is suitable for parallel computing due to its powerful arithmetic and logic units. A parallel programming method is proposed that exploits the single instruction multiple data architecture of the GPU. We focus on the implementation of the Turbo decoder due to its high computational requirements and difficulty in parallelizing the algorithm. The implemented system is analyzed primarily in terms of computation time using the compute unified device architecture profiler. From our experimental tests using the implemented system, we have measured the total processing time for a single frame of both transmit and receive LTE data. We find that it takes 5.00 and 8.58 ms for transmit and receive, respectively. This confirms that the implemented system is capable of real-time processing of all the baseband signal processing algorithms required for LTE systems.
    Analog Integrated Circuits and Signal Processing 03/2014; 78(3). DOI:10.1007/s10470-013-0229-1 · 0.40 Impact Factor
  • Source
    Donghyun Kum, Daegeun Kang, Seungwon Choi
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    ABSTRACT: This paper presents a novel user selection method based on the signal-to-interference-plus-noise ratio (SINR), which is approximated using limited feedback data at the base stations (BSs) of multiple user multiple-input multiple-output (MU-MIMO) systems. In the proposed system, the codebook vector index, the quantization error obtained from the correlation between the measured channel and the codebook vector, and the measured value of the largest singular value are fed back from each user to the BS. The proposed method not only generates precoding vectors that are orthogonal to the precoding vectors of the previously selected users and are highly correlated with the codebook vector of each user but also adopts the quantization error in approximating the SINR, which eventually provides a significantly more accurate SINR than the conventional SINR-based user selection techniques. Computer simulations show that the proposed method enhances the sum rate of the conventional SINR-based methods by at least 2.4 (2.62) bps/Hz when the number of transmit antennas and number of receive antennas per user terminal is 4 and 1(2), respectively, with 100 candidate users and an SNR of 30 dB.
    Etri Journal 02/2014; 36(1). DOI:10.4218/etrij.14.0113.0267 · 0.95 Impact Factor
  • Kyunghoon Kim, Hyunwook Yang, Seungwon Choi
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    ABSTRACT: This letter presents a novel method of user selection in a downlink multi-user multiple input multiple output (MU-MIMO) system employing the precoding procedures of zero forcing or block diagonalization (BD). The proposed technique utilizes the cross-entropy method (CEM) in order to maintain a performance level comparable to that of the full search (FS) method with reasonable complexity. With the CEM, the proposed system can select multiple users at once instead of selecting a single user at each step. From various computer simulations, it has been verified that the proposed method exhibits nearly 98 % of the sum-rate compared to the FS method, which implies that the proposed method far outperforms conventional methods such as semi-orthogonal user selection (SUS) or the capacity-based suboptimal user selection (CBSUS) algorithm. Compared to CBSUS, the proposed technique enhances the sum-rate by approximately 1.1 bps/Hz with about half the complexity when each user is equipped with two receiving antennas. In the case where each user is equipped with a single antenna, the proposed method outperforms the method of SUS by about 0.3 bps/Hz, at the expense of a complexity increase of $$O(2M)$$O(2M) times.
    Wireless Personal Communications 01/2014; 74(2):789-802. DOI:10.1007/s11277-013-1321-7 · 0.98 Impact Factor
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    ABSTRACT: In this work, a novel lattice reduction (LR) precoding method is proposed. The technique combines conventional LR precoding with a method of reducing the singular value coefficients of the LR-reduced basis matrix. The performance of the new technique was comparable to that of sphere encoding, while its complexity was lower than that of other sub-optimal methods.
    Wireless Personal Communications 05/2013; 70(1). DOI:10.1007/s11277-012-0694-3 · 0.98 Impact Factor
  • Hyunwook Yang, Seungwon Choi
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    ABSTRACT: We propose a novel precoding algorithm that is a zero-forcing (ZF) method combined with adaptive beamforming in the Worldwide Interoperability for Microwave Access (WiMAX) system. In a Multiuser Multiple-Input Multiple-Output (MU-MIMO) system, ZF is used to eliminate the Multiple Access Interference (MAI) in order to allow several users to share a common resource. The adaptive beamforming algorithm is used to achieve the desired SNR gain. The experimental system consists of a WiMAX base station that has 2 MIMO elements, each of which is composed of three-array antennas and two mobile terminals, each of which has a single antenna. Through computer simulations, we verified that the proposed method outperforms the conventional ZF method by at least 2.4 dB when the BER is 0.1%, or 1.7 dB when the FER is 1%, in terms of the SNR. Through a hardware implementation of the proposed method, we verified the feasibility of the proposed method for realizing a practical WiMAX base station to utilize the channel resources as efficiently as possible.
    International Journal of Antennas and Propagation 04/2013; 2013. DOI:10.1155/2013/976301 · 0.83 Impact Factor
  • Jaeho Chung, Yusuk Yun, Seungwon Choi
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    ABSTRACT: This paper presents field experiments on a Multi-Input Multi-Output (MIMO) system that combines Adaptive Beamforming (ABF) and Spatial Multiplexing (SM) procedures. The combination of SM signal processing with ABF is applied to WiBro, the South Korean Orthogonal Frequency Division Multiplexing (OFDM) system that follows the IEEE 802.16e standard. The field experimental results show that ABF-MIMO OFDM system outperforms a simple MIMO OFDM system by 2dB (1.5dB) in the signal to noise ratio (SNR) for 16-QAM (64-QAM) under low correlated fading channel and 4dB (2.5dB) in the SNR for 16-QAM (64-QAM) under highly correlated fading channel, respectively, at the frame error rate (FER) of 1%. Details on the implementation of ABF-MIMO OFDM system is also presented in this paper. Through the system implementation and its field experimental results, we verify that the combination of MIMO OFDM system with ABF provides improved performance over a simple MIMO OFDM system in real propagation channel environment and, in particular, it is more effective in highly correlated fading channel. KeywordsMIMO–Adaptive beamforming (ABF)–OFDM–IEEE 802.16e–WiBro
    Telecommunication Systems 04/2013; 52(4):1-14. DOI:10.1007/s11235-011-9475-7 · 1.16 Impact Factor
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    ABSTRACT: This paper introduces an anti-jamming technology for satellite communication that can cancel terrestrial jammers. In this paper, we first consider the Conjugate Gradient Method (CGM) as the first adaptive algorithm. Using this method, the attainment of the depth of the nulls and the rate of convergence are shown to be excellent. We also introduce a second novel adaptive algorithm, the hardware complexity of which for real-time implementation remains within the capacity of an off-the-shelf serial processor. A simulator based on the proposed adaptive algorithms is then illustrated to verify the performance of an adaptive system designed for implementation on a geostationary satellite. Finally, we demonstrate a hardware implementation of the adaptive system for canceling terrestrial jammers. Detailed hardware descriptions of an implemented adaptive system - such as the antenna array module, the beamforming module, and the controller module - are illustrated, along with the results of the performance obtained in laboratory tests.
    IEEE Antennas and Propagation Magazine 02/2013; 55(1):32-48. DOI:10.1109/MAP.2013.6474483 · 1.15 Impact Factor
  • Kyunghoon Kim, Hyunwook Yang, Seungwon Choi
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    ABSTRACT: This paper proposes a novel user selection algorithm to enhance the system performance of a multi-user, multiple-input, multiple-output (MU-MIMO) system. The proposed algorithm first selects a primary user set using a predetermined threshold, and selects sub-users for each primary user to define user groups that maximize the determinant of the channel matrix consisting of selected user(s) and a candidate user. The user group that provides the maximum sum rate is then determined. According to computer simulation results, the proposed algorithm improves the system sum rate, compared to the conventional algorithm, by approximately 0.5bps/Hz.
    ICT Convergence (ICTC), 2013 International Conference on; 01/2013
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    ABSTRACT: A novel calibration method using the Zadoff-Chu (ZC) sequence as a test signal is proposed. Using this technique, the timing information of the received test signal that contains the phase characteristics of each antenna path can be accurately estimated. Performance of the proposed method is first analyzed through computer simulation. Then, the proposed calibration algorithm is implemented on a Field Programmable Gate Array (FPGA) board to verify its excellent performance in terms of the estimation accuracy for both the amplitude characteristics and the phase characteristics of each antenna path of a given array system.
    ICT Convergence (ICTC), 2013 International Conference on; 01/2013
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    ABSTRACT: This paper presents an implementation of a 2 × 2 Multi-Input Multi-Output Software Defined Radio (SDR) Base Station system using a Message Passing Interface (MPI)-based Graphic Processing Unit (GPU) cluster as its modem processor for a high-speed data processing. Recently, GPUs have been widely researched especially for SDR systems because of their capability for exploiting parallel processing using a large number of Arithmetic Logic Units. MPI-based GPU clusters have been adopted in order to further increase performance capability. From our experimental results, it has been found that the implemented system consisting of three GPU nodes can enhance the modem speed by more than 2.5 times compared to a single GPU system. A dual-mode Mobile Device (MD) prototype supporting Worldwide Interoperability for Microwave Access and Long Term Evolution communications systems is implemented. In our design, one of the two waveforms can automatically be selected by the MD itself using a dual-mode controller that determines the reconfiguration of the MD modem depending on the received signal quality.
    Analog Integrated Circuits and Signal Processing 11/2012; 73(2). DOI:10.1007/s10470-012-9941-5 · 0.40 Impact Factor
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    ABSTRACT: Since H. Yao proposed the lattice reduction (LR)-aided detection algorithm for the MIMO detector, one can exploit the diversity gain provided by the LR method to achieve performance comparable to the maximum likelihood (ML) algorithm but with complexity close to the simple linear detection algorithms such as zero forcing (ZF), minimum mean squared error, and successive interference cancellation, etc. In this paper, in order to reduce the processing time of the LR-aided detector, a graphics processing unit (GPU) has been proposed as the main modem processor in such a way that the detections can be performed in parallel using multiple threads in the GPU. A 2X2 multiple input multiple output (MIMO) WiMAX system has been implemented using a GPU to verify that various MIMO detection algorithms such as ZF, ML, and LR-aided methods can be processed in real-time. From the experimental results, we show that GPUs can realize a 2X2 WiMAX MIMO system adopting an LR-aided detector in real-time. We achieve a processing time of 2.75 ms which meets the downlink duration specification of 3 ms. BER performance of experimental tests also indicates that the LR-aided MIMO detector can fully exploit diversity gain as well as ML detector.
    Analog Integrated Circuits and Signal Processing 11/2012; 73(2). DOI:10.1007/s10470-012-9870-3 · 0.40 Impact Factor
  • Source
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    ABSTRACT: This article introduces a novel flexible mobile device reconfiguration classes (MDRCs) framework which is expected to be applicable to future generation mass market radio devices. This framework sets the scene for a vision of a smooth evolution of reconfigurability features in mobile devices, paving the way from current, mainly static implementation choices toward a fully flexible device platform environment. For this flexible mobile device framework, baseband interfaces are introduced as they are currently discussed in ETSI reconfigurable radio systems standardization. Furthermore, this technical solution is positioned with respect to security requirements and the basic regulatory framework which is currently under revision in Europe. Indeed, the future revised Radio Equipment and Telecommunications Terminal Equipment Directive is expected to allow for such advanced reconfiguration, enabling users to acquire and install so-called RadioApps software components which may affect the compliance of a mobile device to the essential requirements of the Directive.
    IEEE Wireless Communications 08/2012; 19(4):9-16. DOI:10.1109/MWC.2012.6272418 · 6.52 Impact Factor
  • Hyunwook Yang, Seungwon Choi
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    ABSTRACT: In this paper, we present an algorithm for reducing the transmit normalization factor by perturbing the transmit signal in a Multi-User Multiple Input Multiple Output (MU-MIMO) system which uses the channel inverse matrix as its precoding matrix. A base station must normalize unnormalized transmit signals due to the limitation of the constant transmit power. This paper defines the norm of the unnormalized transmit signal as the transmit normalization factor used to normalize the transmit signal. Recalling that the transmit normalization factor consists of a combination of the singular values from the channel inverse matrix, we provide a codebook that successively reduces the coefficients of these singular values. Through computer simulations, the proposed algorithm is compared to sphere encoding in terms of the Bit Error Rate (BER) and the outage probability in a MU-MIMO signal environment. Sphere encoding is known to be an optimal solution amongst the perturbation methods that reduce the transmit normalization factor [1]. This work demonstrates that the proposed algorithm is has very good performance, comparable to that of sphere encoding, while its computational load is nearly 200 times less. Since the codebook in our algorithm depends only on the given channel, the difference in the computational complexity becomes even greater when the channel state is not changed, because the codebook can be reused. Furthermore, the codebook exhibits the characteristic of robustness to the maximum Doppler shift.
    IEICE Transactions on Communications 07/2012; E95.B(7):2405-2413. DOI:10.1587/transcom.E95.B.2405 · 0.33 Impact Factor
  • Jaeho Chung, Yusuk Yun, Seungwon Choi
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    ABSTRACT: This paper details the downlink performance analysis of a MIMO (Multi-Input Multi-Output) system that combines adaptive beamforming (ABF) and spatial multiplexing (SM) procedures. The combination of MIMO signal processing with ABF is applied to WiBro (Wireless Broadband), the South Korean orthogonal frequency division multiple access (OFDMA) system that follows the IEEE 802.16e standard. Performance analysis is based on the results of experiments and simulations obtained from a prototype system implementation and fixed-point simulator. Both the prototype experiments and simulations demonstrate that the ABF-MIMO OFDMA system improves the required signal to noise ratio (SNR) over the conventional MIMO OFDMA system by 3 dB (QPSK)/2.5 dB (16-QAM) for the frame error rate of 1% in the WiBro signal environments. From the implementation of the prototype system and its experimental results, we verify the feasibility of the ABF-MIMO technology for realizing a practical WiBro base station. Copyright © 2011 John Wiley & Sons, Ltd.
    International Journal of Communication Systems 12/2011; 24:1627-1646. DOI:10.1002/dac.1239 · 1.11 Impact Factor
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    ABSTRACT: Conventional communication systems have been implemented using digital signal processors (DSPs) and/or field programmable gate arrays (FPGAs), especially for software defined radio (SDR) functionality. We propose a scheme that uses a graphics processing unit (GPU) in place of the conventional DSPs or FPGAs for the implementation of an SDR-based communication system. The GPU, a high-speed parallel processor with multiple arithmetic logic units, is adopted for the signal processing of the physical layer required for the parallel processing in an SDR system. The compute unified device architecture (CUDA) based on the C language provides a software development kit (SDK) for the modem application of the GPU. Therefore we utilize the CUDA SDK to implement the real-time modem function. This paper presents an implementation of a 2×2 multiple-input multiple-output (MIMO) WiMAX system employing a GPU as the real-time modem. By installing a radio frequency module on top of the GPU modem, we implement a real-time transmission system for video data. The performance of the proposed GPU-based system is demonstrated by comparing its operation time against that of the conventional DSP-based system. KeywordsSDR–GPU–CUDA–WiMAX–MIMO
    Analog Integrated Circuits and Signal Processing 12/2011; 69(2):107-117. DOI:10.1007/s10470-011-9764-9 · 0.40 Impact Factor

Publication Stats

694 Citations
108.91 Total Impact Points

Institutions

  • 1993–2014
    • Hanyang University
      • • Department of Electronics and Computer Engineering
      • • Department of Electronic and Electrical Engineering
      Sŏul, Seoul, South Korea
  • 2011
    • Swansea University
      • College of Engineering
      Swansea, WLS, United Kingdom
  • 2002–2005
    • Yong-in Songdam College
      Eidō, North Chungcheong, South Korea
    • Feng Chia University
      • Department of Electrical Engineering
      臺中市, Taiwan, Taiwan
  • 2001
    • Kyungpook National University
      • Department of Electrical Engineering
      Sangju, North Gyeongsang, South Korea
  • 1997
    • Syracuse University
      • Department of Electrical Engineering and Computer Science
      Syracuse, NY, United States