Hyuncheol Park

Korea Advanced Institute of Science and Technology, Sŏul, Seoul, South Korea

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Publications (84)33.36 Total impact

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    ABSTRACT: This paper presents a closed form of an optimal cooperative amplify-and-forward (AF) relay amplifying matrix for a distributed relay network of M-source-M-destination pairs and N relays, called a cooperative distributed AF relay network. The objective of this paper is to derive closed forms of minimum mean square error (MMSE)-based and zero-forcing (ZF)-based optimal AF relay amplifying matrices for the cooperative distributed AF relay network under the transmitter power constraint (TPC) at the relays, the receiver power constraint (RPC) at the destinations, and the no-power constraint (NPC) condition. Additionally, by substituting the derived optimum AF relay amplifying matrices into the original cost functions (CFs), the behavior of the optimum CFs and the total optimum signal component power (SCP) at the destinations are compared to each other for different cases. Finally, using the MMSE criterion, a novel relay selection scheme is proposed for the cooperative distributed AF relay network.
    IEEE Transactions on Wireless Communications 01/2014; 13(5):2741-2755. · 2.42 Impact Factor
  • Yeonggyu Shim, Hyuncheol Park
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    ABSTRACT: In this paper, we propose the optimal time allocation for two-way relay using decode-and-forward (DF) multiple access broadcast (MABC) protocol. We analytically determine the optimal transmit time durations for the first and the second time slots, which maximize the achievable sum rate under a sum time constraint for a given squared magnitude of the channel coefficients and transmit power values. Numerical results show that the achievable sum rate of the optimal time allocation is greater than or equal to that of the equal time allocation.
    IEEE Communications Letters 01/2014; 18(5):721-724. · 1.16 Impact Factor
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    ABSTRACT: This paper studies both cooperative and noncooperative amplify-and-forward (AF) wireless relay strategies with power constraints at the source and at the relays. The main objective is to design jointly and iteratively the closed form of a source scaling factor and a relay amplifying matrix, based on the minimum mean square error (MMSE) criterion under the conditions of both no- channel and channel phase misalignments. With the derived optimal source scaling factors and relay amplifying matrices, impacts on the system performance of both no-channel and channel phase misalignment are investigated by presenting the bit error rate and cost function behavior. Finally, this paper presents the iterative algorithm to solve the constrained Lagrangian optimization problem with a low computational complexity.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
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    ABSTRACT: This paper proposes amplify-and-forward (AF) relay matrices for a one-source-one-destination pair and cooperative distributed N relay nodes under three adverse wireless relay communication environments: (1) broadband noise jamming (BNJ), (2) node geometry (NG), and (3) channel uncertainty (CU). The main objective of this paper is to analytically derive the minimum mean square error (MMSE)-based optimal relay amplifying matrix. Another objective of this paper is to analyze the MMSE cost function, relay power usage, and signal component power of the received signal at the destination using the derived optimal relay amplifying matrix. Finally, the impacts on these three wireless communication environments for the wireless relay networks will be observed by presenting bit error rate (BER) performance.
    Vehicular Technology Conference (VTC Spring), 2013 IEEE 77th; 01/2013
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    ABSTRACT: This paper presents both cooperative and noncooperative amplify-and-forward (AF) relay networks consisting of a one-source-one-destination pair and N-relays under receiver power constraint (RPC) at the destination. The optimal AF relay amplifying matrices for the relays by minimizing the mean square error are analytically derived. This paper also shows that the performance of the optimum cooperative relay network is identical to that of the optimum noncooperative network, as long as the same receiver power constraint is applied. The purpose of the RPC is to reduce the interference level for users located in neighboring cells. Finally, using the derived optimal relay amplifying matrices, the achievable rate, the pairwise error probability, and the outage probability are investigated.
    Vehicular Technology Conference (VTC Spring), 2013 IEEE 77th; 01/2013
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    ABSTRACT: An amplify-and-forward (AF) relay network has been studied extensively, but a closed form of an optimal AF relaying matrix is not available yet for the distributed AF relay network with the destination power constraint (DPC) and the following adverse environments: (a) partial-band noise jamming (PBNJ), (b) node geometry (NG), and (c) channel uncertainty (CU). Hence, this paper presents a closed form of an optimum AF relaying matrix consisting of a one-source-one-destination pair and N relay nodes under such adverse PBNJ, NG, CU, and DPC environments, based on the minimum mean square error (MMSE). By adopting the optimal relay amplifying matrix, the MMSE cost function behaviors are analytically and numerically studied.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
  • Yeonggyu Shim, Hyuncheol Park, Hyuck M. Kwon
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    ABSTRACT: This paper proposes an optimal power allocation method for two-way decode-and-forward (DF) relay networks when transmit power values at source nodes are the same. In this paper we consider the multiple access (MAC) capacity for DF relaying scheme. Using case studies, it analytically determines the optimal power values for the two source nodes and one relay node. The achievable sum rate is maximized under a sum power constraint for given squared magnitude of the channel coefficients. Finally, numerical results show that the achievable sum rate for proposed optimum power allocation is greater than or equal to that for equal power allocation.
    Wireless Communications and Networking Conference (WCNC), 2013 IEEE; 01/2013
  • Kwanghoon Kim, Hyuncheol Park, H.M. Kwon
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    ABSTRACT: Inter-carrier interference (ICI) and multiuser interference (MUI) occur in an uplink orthogonal frequency division multiple access system if a channel between a mobile user and a base station is rapidly time-varying. To reduce the ICI and MUI, this letter proposes a subcarrier allocation method. The key idea is to allocate a high-speed user's subcarrier near a low-speed user's one. Then, a wide Doppler spread of a high-speed user can be spilled over a low-speed user's subcarrier band with insignificant influence, and the signal-to-interference-plus-noise ratio (SINR) at each high-speed user's subcarrier can be maximized. This can provide a significant performance improvement of multiuser detection in an uplink-OFDMA.
    IEEE Transactions on Communications 01/2013; 61(1):7-12. · 1.75 Impact Factor
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    ABSTRACT: A cooperative amplify-and-forward (AF) wireless relay scheme consisting of M sources, N relays, and L destinations all equipped with a single antenna is studied in this paper. The main objective is to design jointly and iteratively the closed-form of minimum mean square error (MMSE)-based source precode and relay amplifying matrices under a jamming environment with transmit power constraints and aggregate power constraints. With the derived optimal source precode and relay amplifying matrices, the jamming influence on system performance with both transmit and aggregate power constraints is examined numerically by using Monte-Carlo simulations.
    Vehicular Technology Conference (VTC Spring), 2013 IEEE 77th; 01/2013
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    ABSTRACT: This paper addresses a more practical relay network consisting of a mobile source, multiple distributed mobile amplify-and-forward (AF) relays, and a mobile destination node. It considers an appropriate channel model between nodes, which is a cascaded Rayleigh fading channel. The objective of this paper is to determine an optimum AF relay vector (or matrix) using the minimum mean square error (MMSE) criteria for these distributed AF relay networks. The effects of jamming and imperfect channel state information (ICSI) on network performance are included in the analysis. Additionally, by substituting the derived amplifying relay vectors into the MMSE cost functions, it can be predicted which case will yield a better BER performance.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
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    ABSTRACT: This paper proposes a cooperative wireless relay network consisting of a one-source-one-destination node pair and N relay nodes in three wireless communication environments, i.e., jamming, node geometry, and channel uncertainty. The main objective of this paper is to analytically derive the optimal cooperative amplify-and-forward (AF) wireless relay matrix under three conditions with the relay power constraint based on the minimum mean square error (MMSE) criterion. Using the derived amplifying relay matrix, the MMSE cost function and achievable rate will be investigated numerically and analytically.
    Communications (ICC), 2013 IEEE International Conference on; 01/2013
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    ABSTRACT: This paper assumes a practical channel model, called a cascaded Rayleigh fading channel, for a noncooperative distributed mobile relay network, which consists of one mobile source and one mobile destination, and multiple mobile amplify-and-forward (AF) relays. Then, an optimum AF relay amplifying vector (or matrix) is analytically designed using the minimum mean square error (MMSE) criteria and uses it for an efficient mobile relay selection. Power is intentionally not constrained. Instead, this paper presents a scaling factor scheme to meet a target signal-to-noise ratio (SNR_{TGT} ) at the destination. This SNR_{TGT} strategy can implicitly embrace the power constraint problems and be a more practical implementation.
    Vehicular Technology Conference (VTC Spring), 2013 IEEE 77th; 01/2013
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    ABSTRACT: This paper studies amplify-and-forward (AF) relay strategies for two-way wireless relay networks consisting of two sources with a single antenna and multiple relays with a single antenna. The closed form of relay amplifying matrices (vectors) with the perfect cancelation of self-interference under both no channel uncertainty and channel uncertainty is presented based on the minimum mean square error (MMSE) criterion. With the derived optimal relay amplifying matrices, the MMSE cost function behavior, the relay power usage, and the sum of the achievable rate are analyzed. Additionally, a filter for each destination will be provided to remove the influences of noises at the receivers.
    Vehicular Technology Conference (VTC Fall), 2013 IEEE 78th; 01/2013
  • Kwanghoon Kim, Hyuncheol Park, H.M. Kwon
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    ABSTRACT: As channel time-variation increases, orthogonality among subcarriers in an orthogonal frequency division multiplexing (OFDM) symbol can be destroyed because of the relatively long symbol period, whereupon intercarrier interference (ICI) appears, and hence an irreducible error floor occurs. It was shown recently that grouping pilot tones into a number of equally spaced clusters can yield better channel estimation against the doubly selective channel than placing each pilot tone in an equally spaced manner. However, a random pilot sequence was used in the literature, and an optimal sequence has not yet been studied. This paper presents how to optimize the clustered pilot sequence. First, this paper (a) proves the existence of an optimum pilot sequence and (b) suggests a guideline for finding an optimum sequence. Second, this paper shows that an optimum sequence is independent of the signal-to-noise ratio and Doppler rate, and the sequence is generally optimum for any channels. Third, this paper verifies through analysis and simulation that the coded-OFDM system with an optimized cluster pilot sequence can yield a smaller mean square channel estimation error and lower bit error rate than the system with an equidistance pilot or a random cluster pilot under a rapidly time-varying channel.
    IEEE Transactions on Communications 01/2012; 60(5):1357-1370. · 1.75 Impact Factor
  • Taedong Shin, Gibum Kim, Hyuncheol Park, H.M. Kwon
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    ABSTRACT: In a multiple-input multiple-output (MIMO) system of a large number of antennas, the requirement of an analog to digital converter (ADC) in each antenna string causes high implementation cost and excessive signal processing. Hence, a hybrid beamforming has been proposed in the literature as a solution. However, still quantization errors introduced at the analog beamformer part can lead to performance degradation. This paper is to present a quantization error reduction scheme for a hybrid beamformer in a 2 by 2 MIMO system. The key idea is to sacrifice the data rate from a full rate to a half rate, estimate all user data in a digital baseband, and then restore the received signal at each antenna string. By using the restored received signal and the residual (which is calculated in the estimator), quantization error can be reduced. Simulation results verify that the proposed scheme can improve the bit error rate performance with a resolution of smaller number of bits.
    Communications (APCC), 2012 18th Asia-Pacific Conference on; 01/2012
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    ABSTRACT: This paper studies minimum mean square error (MMSE)-based full-duplex relay processing matrices and source/destination beamforming vectors under perfect channel state information by imposing constraints on the transmit power of source, relay, both, separately, and in various combinations. The main contribution of this paper is the derivation of a set of relay processing and source/destination beamforming vectors under diverse conditions of transmit power constraints on the source and relay. By comparing the bit error rate (BER) performance of each case, an efficient design of a full-duplex amplify-and-forward relay system is presented.
    Vehicular Technology Conference (VTC Fall), 2012 IEEE; 01/2012
  • Gibum Kim, Cheolkyu Shin, Hyuncheol Park
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    ABSTRACT: Demands for high throughput and stable service quality are increasing. Frame aggregation mechanisms in IEEE 802.11n wireless local area networks (WLANs) can provide improved throughput, but the effect of A-MSDU decreases significantly in error-prone channels. Therefore, adaptive frame size estimation (FSE) depending on the channel condition is required to maintain the improved throughput. In this paper, we proposed frame error rate (FER) based FSE scheme in error-prone and time-varying channel such as a high-stressed network. A tight FER bound is derived to obtain instantaneous link condition, and extended Kalman filter (EKF) is used to estimate frame size optimally for next transmission with current channel information. Our simulation results show that the proposed FSE scheme improves the throughput two times higher than a nonadaptation approach in high-stressed network condition.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2012 IEEE 23rd International Symposium on; 01/2012
  • Jinwoo Kim, Kwonhyung Lee, Hyuncheol Park
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    ABSTRACT: Visible light communication (VLC) is a short range optical wireless communication using the light emitting diode (LED) lights, which can provide both illumination and communication. In this paper, we characterize the indoor multi-path dispersion for VLC and propose multiple pulse position modulation (MPPM) as a power efficient modulation. Due to the reflections in indoor environment, equalization process is essential to mitigate inter-symbol interference (ISI). Frequency domain equalizer (FDE) is implemented to reduce computational complexity. We evaluate communication performance in terms of bit error rate (BER) and the spectral efficiency. Simulation results show that the proposed system yields a comparable BER performance to other equalization systems with lower complexity.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2012 IEEE 23rd International Symposium on; 01/2012
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    ABSTRACT: This paper presents an amplify-and-forward (AF) relay scheme for M-source-M-destination pairs and N relay nodes. Cooperative minimum mean square error (MMSE) and zero-forcing (ZF) strategies for wireless relay networks under the relay power constraint are investigated. The main contribution of this paper is the derivation of both the MMSE-based and the ZF-based amplifying relay matrices (ARMs) under the relay power constraint. By adopting the ARMs derived, the cost function and the total signal component power (SCP) of the received signals at the destination nodes are investigated. Finally, the system bit error rate (BER) performance is evaluated theoretically by using the cost function and total SCP behaviors, and numerically by using Monte-Carlo simulations.
    Signals, Systems and Computers (ASILOMAR), 2012 Conference Record of the Forty Sixth Asilomar Conference on; 01/2012
  • Cheolkyu Shin, Hyuncheol Park
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    ABSTRACT: This paper describes the exploitation of physical layer confidence in bits (SoftPHY) in order to improve the rate selection in a bit-interleaved coded orthogonal frequency division multiplexing (BIC-OFDM) system. Instead of relying on a measured instantaneous bit error rate (I-BER) from received packets, we built a closed-form I-BER expression that gives accurate and simple functional mapping between the instantaneous channel state and the selected set of modulation level and coding rate. The accuracy of the rate selection and the enhancement of the proposed scheme are confirmed through Monte-Carlo simulations.
    Personal Indoor and Mobile Radio Communications (PIMRC), 2012 IEEE 23rd International Symposium on; 01/2012