• KSII Transactions on Internet and Information Systems (TIIS)
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• v.4 no.3
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• pp.341-357
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• 2010

Forward Error Correction (FEC) techniques have been adopted to overcome packet losses and to improve the quality of video delivery. The efficiency of the FEC has been significantly compromised, however, due to the characteristics of the wireless channel such as burst packet loss, channel fluctuation and lack of Quality of Service (QoS) support. We propose herein an Adaptive Cross-layer FEC mechanism (ACFEC) to enhance the quality of video streaming over 802.11 WLANs. Under the conventional approaches, FEC functions are implemented on the application layer, and required feedback information to calculate redundancy rates. Our proposed ACFEC mechanism, however, leverages the functionalities of different network layers. The Automatic Repeat reQuest (ARQ) function on the Media Access Control (MAC) layer can detect packet losses. Through cooperation with the User Datagram Protocol (UDP), the redundancy rates are adaptively controlled based on the packet loss information. The experiment results demonstrate that the ACFEC mechanism is able to adaptively adjust and control the redundancy rates and, thereby, to overcome both of temporary and persistent channel fluctuations. Consequently, the proposed mechanism, under various network conditions, performs better in recovery than the conventional methods, while generating a much less volume of redundant traffic.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• v.9 no.1
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• pp.528-532
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• 2005

As the current IEEE 802.11 wireless LAN standard provides the unified channel access procedure for each traffic service without considering its own traffic characteristics, it is difficult to get its differentiated service qualities at IEEE 802.11 wireless LAN system. To solve this limit, IEEE 802.11e standard that can provide differentiated service to each traffic is under preparation. In this study, we implemented IEEE 802.11e EDCA channel access scheme with NS-2 simulator, and showed its numerical results in various system environments with using average delay and throughput as performance measures.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.7 no.4
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• pp.800-818
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• 2013

Supporting video streaming over multi-hop wireless networks is particularly challenging due to the time-varying and error-prone characteristics of the wireless channel. In this paper, we propose a joint optimization scheme for video streaming over multi-hop wireless networks. Our coding scheme, called Joint Source/Network Coding (JSNC), combines source coding and network coding to maximize the video quality under the limited wireless resources and coding constraints. JSNC segments the streaming data into generations at the source node and exploits the intra-session coding on both the source and the intermediate nodes. The size of the generation and the level of redundancy influence the streaming performance significantly and need to be determined carefully. We formulate the problem as an optimization problem with the objective of minimizing the end-to-end distortion by jointly considering the generation size and the coding redundancy. The simulation results demonstrate that, with the appropriate generation size and coding redundancy, the JSNC scheme can achieve an optimal performance for video streaming over multi-hop wireless networks.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.40 no.6
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• pp.225-236
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• 2003

In this paper, we propose a wireless scheduling algorithm to provide the Internet delay proportional differentiated services in wireless networks. For considering network environments that have burst and location-dependent channel errors, our proposed WDPS(Wireless Delay Proportional Service) scheduling algorithm adaptively serves packets in class queues based on the delivered delay performance for each class. The remarkable characteristics of WDPS scheduler are supporting a fair relative delay service, providing graceful throughput and delay compensation, and avoiding class queue blocking problem. Through simulations, we show that the algorithm achieves the desirable properties to provide delay proportional services in wireless networks.

• Journal of the Institute of Electronics and Information Engineers
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• v.51 no.8
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• pp.3-10
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• 2014

Millimeter wave (mmWave) has attracted great interest recently and the necessity of Millimeter Mobile Broadband (MMB) system has appeared based on the 4 Generation Long Term Evolution-Advanced (LTE-A) Specification. Currently, there are many studies about the mmWave communication channel. And it is subject of interest to analyze the performance in MMB channel environments. In this paper, we design the MMB system for 5th Generation mobile communication and propose channel models through the analysis of the mmWave propagation characteristics. Also, we have analyzed the performance of the MMB system of 28 GHz band in MMB channel environments.

• Journal of Advanced Navigation Technology
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• v.8 no.1
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• pp.73-80
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• 2004

In the case of Non-Line-of-Sight (NLOS), common telecommunication systems typically have Rayleigh distributed amplitude characteristics. However measurement result of Ultra Wideband (UWB) Multi-Band Orthogonal Frequency Division Multiplexing (MB-OFDM) system which is proposed as one of candidate standard in IEEE 802. 15. 3a for Wireless Personal Area Network (WPAN) shows that it has independent log normal fading in each cluster as well as in each ray within the cluster. Based on this clustering phenomenon observed, MB-OFDM channel model derived from Saleh-Valenzuela model with a couple of slight modifications. In this paper, channel remodeling for RF frequency hopping in MB-OFDM system is achieved, and performances of MB-OFDM system for each channel mode and data rate are verified using modified channel model.

• IEIE Transactions on Smart Processing and Computing
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• v.3 no.3
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• pp.142-152
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• 2014

Vehicular Ad-hoc Networks (VANETs) are comprised of wireless mobile nodes characterized by a randomly changing topology, high mobility, availability of geographic position, and fewer power constraints. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidate for the physical layer of VANET because of the inherent characteristics of the spectral efficiency and robustness to channel impairments. The susceptibility of OFDM to Inter-Carrier Interference (ICI) is a challenging issue. The high mobility of nodes in VANET causes higher Doppler shifts, which results in ICI in the OFDM system. In this paper, a frequency domain com-btype channel estimation was used to cancel out ICI. The channel frequency response at the pilot tones was estimated using a Least Square (LS) estimator. An efficient interpolation technique is required to estimate the channel at the data tones with low interpolation error. This paper proposes a robust interpolation technique to estimate the channel frequency response at the data subcarriers. The channel induced noise tended to degrade the Bit Error Rate (BER) performance of the system. Parallel concatenated Convolutional codes were used for error correction. At the decoding end, different decoding algorithms were considered for the component decoders of the iterative Turbo decoder. A performance and complexity comparison among the various decoding algorithms was also carried out.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37C no.9
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• pp.791-801
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• 2012

Currently, SEAMCAT has been widely used as a tool to evaluate the effects of interference among a variety of wireless communication systems. This supports various channel propagation models, all of which are based on some statistical models for the channel propagation and do not exploit any specific terrain characteristics. Thus it is not appropriate for assessing the effects of interference between wireless systems, given some specific terrain features. In order to overcome this limit in its use, it is necessary to extend the capability of SEAMCAT to support a channel propagation model which takes into account terrain informations. The ITU-R P.526 is a familiar channel propagation model which calculates the path loss considering the terrain features. In this paper, we present an enhanced version of SEAMCAT which supports the ITU-R P.526 and provide a few examples of interference evaluation using it.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.16 no.10
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• pp.2095-2100
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• 2012

In this paper, we propose the channel sounding scheme which is made for ideal communication between some application as well as the short distance of high speed data transmission in MIMO-OFDM system for Wireless PAN. This method is able to perceive the duration of the impulse response through the delaying of power delay profile, modeled a power delay profile which has an attenuate characteristic, and obtained the coefficient of channel response by ML (maximum likelihood). Through the amplitudes, phases and delays associated with each multipath component which were acquired from this channel sounding scheme, we can describe the wave propagation characteristics of channels between the transmitter and receiver so that the receiver could enhance not only the reliability but also the ability of communication link. Multi agent system models can be used to analyze the path of the system within any time frame. Further, parameter values can be perturbed to examine how the path of the system changes in response to exogenous shocks.

• Proceedings of the IEEK Conference
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• 2004.06a
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• pp.3-6
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• 2004

This paper. we investigate the OFDM-based wireless LAN systems operating in the 60 GHz frequency band as part of the fourth-generation (4G) systems. The 60 GHz band is of much interest since this is the band in which a massive amount of spectral space (5 GHz) has been allocated worldwide for dense wireless local communications. This paper gives an overview of 60 GHz indoor wireless channel characteristics and an effect on phase noise. The performance of OFDM system is severely degraded by the local oscillator phase noise, which causes both common phase error and inter-carrier interference. We provide the exact analysis of the phase noise effect on the OFDM system.