Gi-Hong Im

Pohang University of Science and Technology, Geijitsu, Gyeongsangbuk-do, South Korea

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Publications (107)134.33 Total impact

  • [Show abstract] [Hide abstract]
    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.
    IEEE Transactions on Communications 04/2015; 63(4):1121-1135. DOI:10.1109/TCOMM.2015.2408606 · 1.99 Impact Factor
  • Seoulmin Lee · Tae-Kyoung Kim · Gi-Hong Im
    IEEE Communications Letters 01/2015; DOI:10.1109/LCOMM.2015.2472408 · 1.27 Impact Factor
  • 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.
    IEEE Communications Letters 12/2014; 18(12):2217-2220. DOI:10.1109/LCOMM.2014.2363461 · 1.27 Impact Factor
  • 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.
    IEEE Transactions on Wireless Communications 08/2014; 13(8):4401-4414. DOI:10.1109/TWC.2014.2317472 · 2.50 Impact Factor
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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.
    Journal of Communications and Networks 08/2014; 16(4):447-457. DOI:10.1109/JCN.2014.000075 · 1.01 Impact Factor
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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.
    IEEE Transactions on Communications 04/2014; 62(4):1178-1187. DOI:10.1109/TCOMM.2014.021714.130140 · 1.99 Impact Factor
  • 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.
    IEEE Communications Letters 02/2014; 18(2):193-196. DOI:10.1109/LCOMM.2013.112713.131767 · 1.27 Impact Factor
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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.
    IEEE Transactions on Communications 01/2014; 63(1):1-1. DOI:10.1109/TCOMM.2014.2382100 · 1.99 Impact Factor
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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.
    IEEE Transactions on Communications 10/2013; 61(10):4127-4138. DOI:10.1109/TCOMM.2013.090513.120403 · 1.99 Impact Factor
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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.
    IEEE Transactions on Wireless Communications 08/2013; 12(8):4142-4152. DOI:10.1109/TWC.2013.071913.121996 · 2.50 Impact Factor
  • 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.
    IEEE Transactions on Communications 04/2013; 61(4):1254-1263. DOI:10.1109/TCOMM.2013.012913.120133 · 1.99 Impact Factor
  • 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.
    IEEE Communications Letters 04/2013; 17(4):665-668. DOI:10.1109/LCOMM.2013.022713.122683 · 1.27 Impact Factor
  • 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.
    IEEE Transactions on Communications 01/2013; 61(1):312-324. DOI:10.1109/TCOMM.2012.092612.120024 · 1.99 Impact Factor
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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.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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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.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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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.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
  • Hyun-Myung Kim · Moonsik Min · Gi-Hong Im
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    ABSTRACT: In this paper, we propose a feedback granularity control scheme for multiuser multiple-input multiple-output orthogonal frequency-division multiple access (MIMO-OFDMA) systems. The proposed scheme resizes the unit and the resolution of the feedback to efficiently exploit channel correlations in both the time and the frequency domains. Two types (centralized and distributed) of granularity search methods are proposed, and their performance characteristics over practical wireless channels are investigated. Simulation results show that the proposed schemes (particularly the distributed method) effectively calculate the optimal granularity, and thus significantly improve the sum-rate of multiuser MIMO-OFDMA systems.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
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    ABSTRACT: This article presents experimental and simulation results for the use of TV band devices (TVBDs) in TV white space, considering the presence of interference by incumbent services. Digital TV (DTV) services are major incumbent services currently operating in the TV bands. With DTV service, co-channel and adjacent channel deployment scenarios of TVBD networks are introduced. To safely protect the incumbent service, a minimum separation distance from the DTV protected contour, which is called the keep-out distance, is required. We estimate the keep-out distance for different ranges of TVBD transmit antenna height by using several propagation models and measurements of ultra-high-frequency signals in Korea. We also investigate the hidden node problem for the spectrum sensing operation mode of TVBDs. According to the results of these measurements, the hidden node margin should be at least 38 dB in order to protect DTV service. Finally, the service coverage reduction of TVBD networks caused by neighboring DTV service is discussed. It is shown that the service coverage of a wireless local area network system decreases about 50 percent by co-channel interference from neighboring DTV service when the field strength of the DTV received signal is 41 dBu.
    IEEE Communications Magazine 12/2012; 50(12):88-94. DOI:10.1109/MCOM.2012.6384456 · 4.01 Impact Factor
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    ABSTRACT: In this letter, we consider user scheduling for single-carrier frequency division multiple access with iterative multiuser detection. To achieve high spectral efficiency, a scheduler should allocate a common set of subcarriers to as many users as possible, while guaranteeing the quality-of-service (QoS) of each user. To address this problem, we first investigate optimal user selection via an exhaustive search method. Based on this investigation, we then propose a pattern-aware user scheduling algorithm for the optimal user selection. Simulation results show that the proposed algorithm provides higher spectral efficiency than conventional algorithms and achieves the performance of optimal user selection with much reduced computational complexity.
    IEEE Communications Letters 10/2012; 16(10):1608-1611. DOI:10.1109/LCOMM.2012.081612.121001 · 1.27 Impact Factor
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    ABSTRACT: Reliable detection of primary user activity increases the opportunity to access temporarily unused bands and prevents harmful interference to the primary system. By extracting a global decision from local sensing results, cooperative sensing achieves high reliability against multipath fading. For the effective combining of sensing results, which is generalized by a likelihood ratio test, the fusion center should learn some parameters, such as the probabilities of primary transmission, false alarm, and detection at the local sensors. During the training period in supervised learning, the on/off log of primary transmission serves as the output label of decision statistics from the local sensor. In this paper, we extend unsupervised learning techniques with an expectation maximization algorithm for cooperative spectrum sensing, which does not require an external primary transmission log. Local sensors report binary hard decisions to the fusion center and adjust their operating points to enhance learning performance. Increasing the number of sensors, the joint-expectation step makes a confident classification on the primary transmission as in the supervised learning. Thereby, the proposed scheme provides accurate parameter estimates and a fast convergence rate even in low signal-to-noise ratio regimes, where the primary signal is dominated by the noise at the local sensors.
    Journal of Communications and Networks 10/2012; 14(5):471-480. DOI:10.1109/JCN.2012.00003 · 1.01 Impact Factor

Publication Stats

789 Citations
134.33 Total Impact Points


  • 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
  • 1998
    • University of Illinois, Urbana-Champaign
      • Department of Electrical and Computer Engineering
      Urbana, IL, United States
  • 1994–1996
    • AT&T Labs
      Austin, Texas, United States