Central RandD Laboratory

Yuseong gu, South Korea

Central RandD Laboratory

Yuseong gu, South Korea
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Lee B.M.,Central RandD Laboratory | Kim Y.,Kwangwoon University
Circuits, Systems, and Signal Processing | Year: 2013

In recent years, many peak-to-average power ratio (PAPR) reduction techniques have been proposed for orthogonal frequency division multiplexing (OFDM) signals. Among various techniques, the iterative clipping and filtering (ICAF) technique has been considered as a practical scheme, and widely used owing to its non-expansion of bandwidth, low computational complexity, and simplicity in implementation without receiver-side cooperation. However, the performance of conventional ICAF technique is degraded, because the same signals are iteratively clipped with a fixed clipping threshold (CT) in every clipping operation. In this paper, we analyze the performance of conventional ICAF technique, and then propose an adaptive ICAF scheme, which clips the signal with an adaptively modified CT in every clipping operation to achieve enhanced PAPR reduction of OFDM signals. Simulation results show that the proposed scheme significantly outperforms the conventional scheme, in PAPR reduction of OFDM signals at the same number of iterations. © 2012 Springer Science+Business Media New York.


Kim D.-H.,Kangwon National University | Lee D.-W.,Kangwon National University | Gil G.-T.,Central RandD Laboratory
Eurasip Journal on Wireless Communications and Networking | Year: 2010

We deal with a cost-based adaptive handover hysteresis scheme for the horizontal handover decision strategies, as one of the self-optimization techniques that can minimize the handover failure rate (HFR) in the 3rd generation partnership project (3GPP) long-term evolution (LTE) system based on the network-controlled hard handover. Especially, for real-time operation, we propose an adaptive hysteresis scheme with a simplified cost function considering some dominant factors closely related to HFR performance such as the load difference between the target and serving cells, the velocity of user equipment (UE), and the service type. With the proposed scheme, a proper hysteresis value based on the dominant factors is easily obtained, so that the handover parameter optimization for minimizing the HFR can be effectively achieved. Simulation results show that the proposed scheme can support better HFR performance than the conventional schemes. Copyright © 2010 Doo-Won Lee et al.


Kim B.,Ajou University | Kim K.,Ajou University | Koo H.-L.,Ajou University | Roh B.-H.,Ajou University | And 2 more authors.
2010 Long Island Systems, Applications and Technology Conference, LISAT 10 | Year: 2010

For tactical networks, rapid reconfiguration and recovery of network topology is highly required. In this paper, we propose an effective tactical network architecture with WMN(Wireless Mesh Network)-based backbone. To reduce the computational and maintenance complexities of tactical networks, a hierarchical architecture is considered in the proposed architecture. The proposed architecture is called called TMB (Tactical Mesh Backbone). In addition, we develop an OPNET simulator not only to evaluate the performances, but also to develop and improve core algorithms for the TMB architecture such as routing, topology management, mobility support and so on. © 2010 IEEE.


Lee B.M.,Central RandD Laboratory | Kim Y.,Kwangwoon University
Wireless Personal Communications | Year: 2013

In the cognitive radio (CR) systems supporting multiple wireless services, the performance of a communication system can be significantly degraded by an adjacent channel interference (ACI) from a neighbor wireless system located in the adjacent channel. A guard band (GB) is generally used to avoid the ACI, but its bandwidth is carefully decided for efficient usage of spectrum. Furthermore, an adjustable GB instead of the conventional fixed GB can be a more effective scheme since the services in the adjacent channel can be frequently changed in the CR systems. In this paper, a practically adjustable GB decision algorithm based on the information of spectrum masks is proposed for the Orthogonal frequency division multiplexing-based CR systems. The spectrum splatter from the neighbor system is analyzed and the proposed scheme is derived with the analytic results and the inverse closed-form of well-known theoretical BER performance. The effectiveness of proposed algorithm is verified by simulation results. © 2012 Springer Science+Business Media, LLC.


Lee B.M.,Central RandD Laboratory | Kim Y.,Kwangwoon University
Wireless Personal Communications | Year: 2013

The recently introduced impulse postfix OFDM (IP-OFDM) system, which performs channel estimation in time-domain by exploiting the IP instead of pilot tones, can achieve the enhanced bit error rate (BER) performance compared to that of conventional OFDM systems. Since there is a peak-to-average power ratio (PAPR) issue of using the IP, however, the decision criteria for determining the power boosting factor (PBF) of IP were proposed to resolve the PAPR issue while achieving the optimum BER performance in the IP-OFDM system with an ideal high power amplifier (HPA). In this paper, a practical IP-OFDM system with a nonlinear HPA is considered and the impact of nonlinear HPA on the determination of the PBF of IP as well as the BER performance are analyzed. The BER lower bound is also derived in the case of QPSK and 16QAM. It is shown that the approximated lower bound approaches to the simulation results as the PBF is increased. According to the analytical results, due to the nonlinearity of the HPA, 1-3dB increased PBF than the ideal PBF is required to achieve the optimum BER performance, which is even deteriorated compared to that of IP-OFDM with an ideal soft envelope limiter. © 2012 Springer Science+Business Media, LLC.


Lee B.M.,Central RandD Laboratory | Kim Y.,Kwangwoon University
International Journal of Communication Systems | Year: 2011

The impulse postfix OFDM (IP-OFDM) system exploits the IP, which consists of a high power impulse sample and several zero samples at the end of a zero padded-OFDM symbol block, to estimate channel impulse response (CIR) in time domain. In this paper, the impact of IP length on the BER performance of the IP-OFDM system is analyzed. According to the analytic results, the BER performance can be significantly degraded with both a shorter length of IP as well as a longer length of IP than that of the CIR. Thus, an adaptive IP scheme, which adjusts the length of IP adaptively depending on the length of CIR, is proposed to enhance the BER performance of IP-OFDM systems and its effectiveness is demonstrated by computer simulations. The BER performance of the IP-OFDM systems with the proposed adaptive scheme is compared with that of the conventional IP-OFDM system over various modulation schemes. Simulation results show that the IP-OFDM with the proposed scheme can achieve about 2 dB performance enhancement compared with that of conventional systems at BER=10-2. Copyright © 2010 John Wiley & Sons, Ltd.


Ahn H.,Central RandD Laboratory | Kim K.,Kyonggi University
Telecommunication Systems | Year: 2014

As a tool of item management, the primary goal of RFID middleware is to transmit a series of filtered tag data to the RFID-based applications, and so it has to provide a means for specifying event-constraints to refine and filter the raw data from the associated RF readers. However, the current specifications, such as EPCIS's ALE, ISO/IEC's SSI and other vendor-specific specifications, are too circumstantial to be understood by the application developers, and also they have to get acquainted with the technological details of those various types of specifications. To alleviate these difficulties, this paper proposes a referential framework consisting of unified RFID event-constraint specification and its managerial mechanisms, and we dub it a policy-driven RFID event management framework. The essential components of the framework are an XML-based RFID event management policy definition language and a policy exchanging protocol. Through the proposed framework, it is expected for the RFID-based application developers to be able to easily specify their event-constraints without acquainting with the technological details of the current specifications. © 2013 Springer Science+Business Media New York.


Lee B.M.,Central RandD Laboratory | De Figueiredo R.J.P.,University of California at Irvine | Kim Y.,Kwangwoon University
Wireless Personal Communications | Year: 2012

The high peak-to-average power ratio (PAPR) of time domain signals has been a major problem in orthogonal frequency division multiplexing (OFDM) systems, and thus various PAPR reduction algorithms have been introduced. Partial transmit sequence (PTS) is one of the most attractive solutions because of its good performance without distortion. However, it is considered as an impractical solution for the realization of high-speed data transmission systems due to its high computational complexity. In this paper, a novel PAPR reduction algorithm based on a tree-structured searching technique is proposed to reduce the PAPR with low complexity. In the proposed scheme, the computational complexity of searching process is decreased by adjusting the size of tree with two parameters, width and depth, while preserving good performance. The simulation results showthat proposed scheme provides similar performance with optimum case with remarkably reduced computational complexity. © Springer Science+Business Media, LLC. 2010.

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