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Yanggu, South Korea

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. Source

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. Source

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. Source

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. Source

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. Source

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