Gi-Hong Im


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Publications (111)141.35 Total impact

  • No preview · Article · Jan 2016 · IEEE Transactions on Communications
  • Tae-Kyoung Kim · Wonseok Choi · Gi-Hong Im

    No preview · Article · Jan 2016 · IEEE Communications Letters
  • Seoulmin Lee · Tae-Kyoung Kim · Gi-Hong Im
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    ABSTRACT: We consider a power allocation algorithm in a network-coded system with iterative detection for single-carrier transmission. To achieve high network coding gain, the power fraction of each user should be allocated according to channel state information. To facilitate the power allocation, we first develop a density evolution technique for the iterative detection scheme. We then investigate the distribution of optimal power fraction by an exhaustive search. Based on the distribution, we propose a quantization-based power allocation algorithm with a low complexity. In simulations, the proposed algorithm significantly improves the network coding gain.
    No preview · Article · Nov 2015 · IEEE Communications Letters
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    ABSTRACT: This paper proposes an improved spectrum-sharing protocol for multiuser cooperation in cognitive radio (CR) networks. In CR networks, a secondary user (SU) can access the licensed bands of a primary user (PU) as compensation for cooperative transmission. During cooperative transmission, the SU concurrently transmits its own signal and network-coded signal from the PUs. However, detection errors at the SU cause error propagation which degrades the performance of the PU and SU. To address this problem, we develop a cooperative maximal-ratio combining scheme that mitigates the error propagation and achieves diversity gain. To evaluate the combining scheme, we derive a diversity order and closed-form bit error rate (BER) expression for arbitrary -QAM at high SNR. The analysis results show that the diversity order and BER depend on the fraction of the transmit power at the SU. Based on the dependency of the fraction factor, we propose an optimization problem to minimize the BER of the SU while guaranteeing the PU's BER. Further, we apply the BER-constrained optimization problem to the adaptive modulation system. Simulation results show that the proposed cooperation provides full diversity gain to the PU and thus improves its spectral efficiency by using the optimized fraction factor.
    No preview · Article · Apr 2015 · IEEE Transactions on Communications
  • Moonsik Min · Yo-Seb Jeon · Gi-Hong Im

    No preview · Article · Jan 2015 · IEEE Transactions on Wireless Communications

  • No preview · Article · Jan 2015 · IEEE Communications Letters
  • Yun-Seong Kang · Moonsik Min · Gi-Hong Im
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    ABSTRACT: We consider a subcarrier grouping algorithm for reducing feedback load in a multiple antenna downlink system with orthogonal frequency-division multiplexing. We first improve the sum rate of the conventional subcarrier grouping algorithm, which uses antenna combining at the receiver, by eliminating extra performance degradation induced by unnecessary mismatches between a representative quantization vector and effective channels. Then, we propose an algorithm that jointly performs selection of a representative quantization vector and calculation of receive combining vectors. The proposed scheme selects a more reliable quantization vector and achieves much higher sum rate than the conventional algorithm with the same feedback load.
    No preview · Article · Dec 2014 · IEEE Communications Letters
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    ABSTRACT: In this paper, we investigate the frequency domain channel estimation for multiple-input multiple-output (MIMO) single-carrier frequency-division multiple-access (SC-FDMA) systems. In MIMO SC-FDMA, code-division multiplexed (CDM) pilots such as cyclic-shifted Zadoff-Chu sequences have been adopted for channel estimation. However, most frequency domain channel estimation schemes were developed based on frequency-division multiplexing of pilots. We first develop a channel estimation error model by using CDM pilots, and then analyze the mean-square error (MSE) of various minimum MSE (MMSE) frequency domain channel estimation techniques. We show that the cascaded one-dimensional robust MMSE (C1D-RMMSE) technique is complexity-efficient, but it suffers from performance degradation due to the channel correlation mismatch when compared to the two-dimensional MMSE (2D-MMSE) technique. To improve the performance of C1D-RMMSE, we design a robust iterative channel estimation (RITCE) with a frequency replacement (FR) algorithm. After deriving the MSE of iterative channel estimation, we optimize the FR algorithm in terms of the MSE. Then, a low-complexity adaptation method is proposed for practical MIMO SC-FDMA systems, wherein FR is performed according to the reliability of the data estimates. Simulation results show that the proposed RITCE technique effectively improves the performance of C1D-RMMSE, thus providing a better performance-complexity tradeoff than 2D-MMSE.
    No preview · Article · Aug 2014 · Journal of Communications and Networks
  • Hyun-Myung Kim · Moonsik Min · Gi-Hong Im
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    ABSTRACT: This paper proposes feedback optimization schemes for multiple-input single-output orthogonal frequency-division multiple access (MISO-OFDMA) systems with heterogeneous users. The proposed scheme adjusts the unit and resolution of the feedback to efficiently exploit the long-term channel statistics, including the path loss, delay spread, and mobility. To develop an analytical optimization framework, we first derive the sum-rate of MISO-OFDMA systems when the channel-state information (CSI) at the transmitter is imperfect because of the quantization error, channel variation, and feedback delay. Based on the derived results, we present effective solutions to the sum-rate maximization problem for distributed and centralized scenarios. When the channel statistics of the users are not available at the base station, each user can improve the sum-rate of the system by minimizing the distortion on CSI feedback under a fixed per-user feedback rate. With the channel statistics of the users, the base station can further control the user set and the feedback rate in addition to the feedback granularity. We evaluate the performance of both distributed and centralized schemes over practical wireless environments, where each user has different long-term channel statistics. Simulation results show that both methods significantly improve the performance of MISO-OFDMA systems.
    No preview · Article · Aug 2014 · IEEE Transactions on Wireless Communications
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    ABSTRACT: In this paper, we propose a protocol for the coexistence of primary and secondary systems over block-fading channels. In the protocol, the primary system employs a hybrid automatic repeat request (HARQ). When the primary system retransmits the data signal, the secondary system serves as a relay for the retransmission of the primary system and simultaneously transmits its data signal. To efficiently accomplish the protocol, we analyze the average throughput of the primary and secondary systems by using the long-term average throughput (LAT). We formulate an optimization problem to maximize the LAT of the secondary system. The constraint of the optimization problem is that the LAT of the primary system with secondary system is not less than that of the primary system alone. Through the optimization problem, we obtain the closed-form solutions of the transmission rate of the secondary system and the fraction of the transmit power for relaying the primary system's data signal and transmitting the secondary system's data signal. Numerical results show that the primary system does not lose the average throughput, and rather achieves an additional throughput gain by adjusting the fraction of the transmit power of the secondary system.
    No preview · Article · Apr 2014 · IEEE Transactions on Communications
  • Wonseok Choi · Moon-Gun Song · Jaeha Ahn · Gi-Hong Im
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    ABSTRACT: We propose soft combining methods for cooperative spectrum sensing in cognitive radio networks over fast-fading channels. In fast-fading environment, it is difficult to obtain the exact probability distribution of the total received energy during an observation period. Therefore, we apply the mixture of gamma (MoG) approximation to obtain the distribution function of the total received energy. By exploiting the approximated distribution, we derive a soft combining method that reduces the probability of missed detection for a given probability of false alarm based on the Neyman—Pearson criterion. In addition, we propose a simplified soft combining method by using the dominant component of the MoG distribution. Simulation results show that the proposed combining methods outperform the conventional methods based on the central limit theorem, the equal gain combining, and the SNR weighted combining.
    No preview · Article · Feb 2014 · IEEE Communications Letters
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    ABSTRACT: In this paper, we study low-complexity detection schemes for cooperative multiple-input multiple-output (MIMO) systems with decode-and-forward (DF) relays. In fixed DF protocols, the error propagation effect that limits the diversity gain can be overcome by using joint maximum-likelihood (JML) detection, but its complexity increases exponentially with the number of streams and modulation orders. To develop a low-complexity detection scheme, we first extend the cooperative maximum-ratio combining (C-MRC) scheme to MIMO relay networks, and then investigate the diversity order of the system. We show that C-MRC achieves full diversity gain for orthogonal space-time code, but its diversity gain is limited when the deployed space-time code is non-orthogonal. To address this problem, we propose an effective detection scheme that is applicable to arbitrary space-time codes. On the basis of pairwise error probability approximation, max-log approximation, and the Chernoff bound, we construct an equivalent point-to-point MIMO signal model, and then apply tree-search detection techniques. Compared to JML, the proposed detection scheme significantly reduces the detection complexity while preserving diversity gain. Moreover, it can be readily extended to complex-field network-coding systems whose performance is superior to that of Galois-field network-coding systems. Analysis and simulation results confirm the efficiency of the proposed scheme.
    No preview · Article · Jan 2014 · IEEE Transactions on Communications
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    ABSTRACT: In cognitive radio networks, multiple secondary systems can access a licensed spectrum when none of the secondary transmitters cause harmful interference to the Primary Users (PUs). For spectrum sharing with coordination among co-located secondary systems, we propose both centralized and distributed beamforming algorithms. The proposed algorithms minimize the total transmit power of secondary systems, while maintaining the interference to PUs below a certain threshold and satisfying the Quality-of-Service (QoS) constraint for each secondary system. The centralized algorithm achieves the optimal transmit power by exploiting the virtual uplink-downlink duality using the knowledge of the channel state information for all the secondary links. However, the assumption of global channel knowledge at each secondary system may not be allowed in practical applications for multi-cell coordination. To address this problem, we design a distributed transceiver beamformer that satisfies the interference constraint to protect PUs. On the basis of this distributed beamformer, we also propose power allocation algorithms that guarantee the QoS for secondary systems. Distributed beamforming and power allocations operate iteratively to minimize the total transmit power. Simulation results show that the distributed algorithms achieve a near-optimal transmit power while satisfying both the QoS and interference constraints.
    No preview · Article · Oct 2013 · IEEE Transactions on Communications
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    ABSTRACT: In spectrum-sharing-based cognitive radio networks, multiple secondary systems can access a licensed spectrum to better utilize scarce radio resources. When the multiple secondary transmitters are co-located, the weighted sum-rate of the secondary users (SUs) is mainly limited by the inter-cell interference (ICI). With limited cooperation among co-located secondary transmitters, we propose an algorithm for decentralized beamforming with power allocation via dual decomposition. To maximize the weighted sum-rate of the SUs, the proposed decentralized algorithm efficiently mitigates the ICI by the undesired leakage power limitation at each secondary transmitter. Because the channel information is not perfectly known at the transmitter in practical applications, we also develop a decentralized robust beamformer. To efficiently design the robust beamformer, a convex problem is formulated by semi-definite relaxation. Simulation results show that the proposed algorithm with perfect channel state information (P-CSI) efficiently maximizes the weighted sum-rate performance by the undesired leakage power limitation. For an imperfect CSI with a small error bound, the proposed robust beamformer approaches the performance of a P-CSI case, without causing harmful interference to the primary user.
    No preview · Article · Aug 2013 · IEEE Transactions on Wireless Communications
  • Hyoung-Jin Lim · Moon-Gun Song · Gi-Hong Im
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    ABSTRACT: In this paper, we study a cooperation-based dynamic spectrum leasing mechanism via multi-winner auction of multiple bands. Based on a second-price auction mechanism, the primary users independently conduct auctions to determine winners who are then granted access to leased bands and prices for those bands. Before auctions, each secondary user jointly chooses bands which they want to lease and generates bids for those bands with a limited transmit power budget. To this end, we determine the feasibility conditions for band selection and for power and time allocation. Further, we propose a low complexity bidding algorithm that iteratively selects a band and calculates the bid for that band. For the paying price among winners after auctions, a multi-objective optimization problem is considered. We derive Pareto optimal solutions and propose paying schemes that prioritize the objectives with pre-defined weighting rules. In addition, the payment and secondary utility are investigated with regard to the number of winners. Simulation results show that secondary users achieve significantly increased utility as more winners are chosen, while primary users are guaranteed a non-negative benefit from spectrum leasing. The power consumed by the secondary users for cooperative transmission is compared for the proposed paying schemes.
    No preview · Article · Apr 2013 · IEEE Transactions on Communications
  • Moonsik Min · Hyun-Myung Kim · Gi-Hong Im
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    ABSTRACT: We consider the sum-rate performance of multiple-input/single-output broadcast channels in which each user has a different average signal-to-noise ratio (SNR) corresponding to their distance from the base station. Although there are already studies investigating the sum-rate growth under this channel assumption, their proofs are limited to the cases where the pathloss exponent is equal to one or two. However, the path-loss exponent is greater than two in realistic wireless environments. Therefore, by deriving the cumulative distribution function of the channel gain, which differs from the previous results, we can provide an asymptotic sum-rate and a general proof for the sum-rate growth regardless of the value of the pathloss exponent.
    No preview · Article · Apr 2013 · IEEE Communications Letters
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    ABSTRACT: In orthogonal frequency-division multiple access (OFDMA) downlink systems, user equipments over large delay spread channels reduce the cell spectral efficiency due to intersymbol interference (ISI). Channel shortening is an attractive filtering technique to mitigate ISI over such large delay spread scenarios, but the filter should be designed carefully to prevent subsequent spectrum null phenomenon. In this paper, we propose subband interference suppression (SIS) in channel shortening for OFDMA downlink systems. In our framework, we first develop the minimum subband interference (MSI) filter which reduces the interference and noise enhancement within the subcarriers of interest. To reduce its complexity, we present the delay preselection method, which predetermines significant delay candidates on the basis of estimation results. Simulation results show that the proposed SIS framework is effective over cellular scenarios, where conventional schemes do not work properly.
    No preview · Conference Paper · Jan 2013
  • Moonsik Min · Dongsik Kim · Hyun-Myung Kim · Gi-Hong Im
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    ABSTRACT: We propose an opportunistic two-stage feedback and scheduling algorithm that is based upon zero-forcing beamforming with semi-orthogonal user selection (ZFBF-SUS) to reduce its feedback load. In an SUS algorithm, a base station schedules semi-orthogonal users using the feedback of all users' channel information. However, such feedback overhead significantly increases with the number of users. To reduce the feedback load of ZFBF-SUS systems, the proposed two-stage feedback scheme opportunistically and separately exploits the signal-to-interference-plus-noise ratio and orthogonality in each stage such that only a fraction of users feed back their channel information. Based on this opportunistic feedback scheme, we decouple the entire process of the SUS into two sub-processes. The proposed two-stage feedback and scheduling scheme effectively exclude the inappropriate users for ZFBF transmission in both feedback and scheduling periods, and thus save the scarce resources consumed for the feedback process. We derive an analytical expression for the average number of feedback bits of the proposed system and optimize it in an averaged sense. Further, the sum-rate of the proposed system is theoretically investigated. Both analytical and simulation results show that the proposed algorithm achieves the performance of conventional ZFBF-SUS systems with a significantly reduced number of feedback bits.
    No preview · Article · Jan 2013 · IEEE Transactions on Communications
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    ABSTRACT: In this paper, we analyze the outage probability and the long-term average throughput (LAT) of a decode-and-forward (DF) cooperative scheme with hybrid automatic repeat request (HARQ) over Nakagami-m fading channels. To derive the outage probability, the distribution of the weighted sum of two independent gamma random variables is needed. We approximately obtain the distribution by using gamma approximation and derive a closed-form expression of the outage probability of the DF cooperative scheme with high SNR assumption. By exploiting the outage probability, we obtain the LAT of the DF cooperative scheme. Numerical results show that the derived LAT of the DF cooperative scheme is able to predict the average throughput and the DF cooperative scheme achieves higher LAT than the direct transmission scheme because of higher diversity gains.
    No preview · Conference Paper · Jan 2013
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    ABSTRACT: In this paper, we propose an efficient soft-detector for spatial multiplexing multiple-input multiple-output systems. Maximum a posteriori probability detection employing QR decomposition with M algorithm (QRM-MAP) achieves improved performance by adopting unbiased minimum-mean-square error (UMMSE) preprocessing. The efficiency of QRM-MAP can be further improved by using statistical information obtained from MMSE detection. Based on this information, the proposed detector orders tree branches and excludes insignificant branches in advance. To achieve low-and-fixed complexity, the node enumeration pattern that considers the greedy nature of the M algorithm is predetermined according to the node order. In addition, the soft information from MMSE can be used to compensate for the inherent performance loss caused by an insufficient number of candidates. Simulation results show that the proposed scheme provides an enhanced performance-complexity tradeoff as compared to the conventional schemes.
    No preview · Conference Paper · Jan 2013

Publication Stats

932 Citations
141.35 Total Impact Points


  • 2009-2015
      경산시, Gyeongsangbuk-do, South Korea
  • 2000-2014
    • Pohang University of Science and Technology
      • Department of Electronic and Electrical Engineering
      Geijitsu, Gyeongsangbuk-do, South Korea
  • 2007
    • SAIT Polytechnic
      Calgary, Alberta, Canada
  • 1994-1996
    • AT&T Labs
      Austin, Texas, United States