• Proceedings of the Acoustical Society of Korea Conference
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• 1993.06a
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• pp.273-275
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• 1993

The throughput of conventional ARQ protocols can be improved by dynamically adapting the packet length. This protocol transmits packets with the length to maximize the transmission efficiency, based on the dynamic estimation of time-varying channel condition. A very simple adaptive scheme is presented. The results of a simulation show that the scheme performs well.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2005.11a
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• pp.65-66
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• 2005

This paper presents an adaptive satellite communication systems to adapt channel environment. High performance coding and modulation techniques are applied to poor channel condition, otherwise, bandwidth efficient coding and modulation techniques are applied to good channel condition to obtain high transmission rate.

• Journal of Korea Society of Digital Industry and Information Management
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• v.10 no.4
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• pp.143-150
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• 2014

A high power amplifier(HPA) has widely been used in wireless communications. In order to have a maximum power efficiency, the HPA uses a saturation domain having nonlinear characteristics, which generates nonlinear distortions to decrease the transmission efficiency. In order to solve this problem, various methods to linearize HPA have been reported until now, and the pre-distorter method is very attractive among them. In this paper, a new enhanced adaptive pre-distorter method was proposed to be applied for compensating nonlinear distortions of HPA. Through computer simulations as well as the analytical analysis, it could be shown that it is possible for both conventional adaptive pre-distorter and proposed adaptive pre-distorter, to be applied for linearizing HPA. Also, the simulations results demonstrated that the proposed adaptive pre-distorter may have faster convergence speed and better capability of compensating the nonlinear distortion than the conventional adaptive pre-distorter with nearly equal computation complexity.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.29 no.4A
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• pp.370-377
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• 2004

In this paper, we present an adaptive modulation technique for orthogonal frequency division multiplexing (OFDM) for broadband wireless communications. Also, using improved channel prediction, we enhance the performance of adaptive OFDM in high mobility environments. Adaptive modulation technique has been shown to achieve reliable high-rate data transmission over frequency-selective fading channel when OFDM is employed. This scheme requires the accurate channel information between two stations for a better performance. In an outdoor high mobility environment, most of adaptive OFDM systems have to be given the channel information transmitted from the receiver. Even if it is possible, there is some delay. Moreover, the channel impulse response between two stations is very rapidly varied. If the channel information is obsolete at the time of transmission, then poor system performance will result. In order to solve this problem, we propose adaptive OFDM with improved channel predictor. The proposed bit allocation algorithm has a lower complexity and the proposed scheme mitigates the effect of channel delay. Robust approach is less sensitive to outdated channel information. Performance results show that the proposed scheme can achieve considerable performance enhancement.

• The Journal of the Korea institute of electronic communication sciences
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• v.10 no.1
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• pp.103-110
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• 2015

WBAN(Wireless Body Area Network) is a Wireless Sensor Network for supporting various applications around body within 2~3m which consists of medical and non-medical device. MAC in WBAN environment should satisfy requirements such as low power consumption, various transmission rate, QoS, and duty-cycle, efficiently distribute frequency band, be strong at traffic load and save energy. This paper proposes AQ(Adaptive Queuing) MAC superframe structure for efficient energy use, considering the increase of traffic load. The simulation result also show that transmission rate and average MAC delay rate is improved comparing IEEE 802.15.4 MAC with AQ MAC.

• Journal of the Korea Institute of Military Science and Technology
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• v.20 no.2
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• pp.187-196
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• 2017

The size of raw data has dramatically increased due to the recent trend of Synthetic Aperture Radar(SAR) development plans for high resolution and high definition image acquisition. The large raw data has an impact on satellite operability due to the limitations of storage and transmission capacity. To improve the SAR operability, the SAR raw data shall be compressed before transmission to the ground station. The Block Adaptive Quantization (BAQ) algorithm is one of the data compression algorithm and has been used for a long time in the spaceborne SAR system. In this paper, an optimization method of BAQ threshold table is introduced using real SAR raw data to prevent the degradation of signal quality caused by data compression. In this manner, a new variation estimation strategy and a new threshold method for block type decision are introduced.

• Management & Information Systems Review
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• v.3
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• pp.201-214
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• 1999

Adaptive correlator receiver with neural network based on complex multilayer perceptron is persented for suppressing interference of narrow-band of direct spread spectrum communication systems. Recursive least square algorithm with backpropagation error is used for fast convergence and better performance in adaptive correlator scheme. According to signal noise and transmission power, computer simulation results show that bit error ratio of adaptive correlator using neural network improved that of adative transversal filter of direct sequence spread spectrum considering of jamming and narrow-band interference. Bit error ratio of adaptive correlator with neural network is reduced about 10-1 than that of adaptive transversal filter where interference versus signal ratio is 5dB.

• The Transactions of the Korean Institute of Electrical Engineers A
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• v.52 no.12
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• pp.698-704
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• 2003

Autoreclosure provides a mean of improving power transmitting ability and system stability. The conventional reclosure adopts the fixed dead time interval strategy, that is, the reclosure is activated after a time delay to restore the system to normal as quickly as possible without regard to the system conditions, however, these simple techniques cannot give the optimal operating performance. For this reason, various adaptive reclosure algorithms have been proposed recently, This paper presents an adaptive autoreclosure algorithm including the variable dead time, optimal reclosure, sequential reclosure and emergency extended equal-area criterion (EEAC) algorithm in order to improve the system stability. The reclosure algorithm performs out the operations that are attuned to the power system conditions. The proposed adaptive reclosure algorithm is verified and tested by using EMTP MODELS, and the simulation results show that the system oscillations are reduced and the transient stability is enhanced by employing the proposed adaptive reclosure algorithm.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.18 no.9
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• pp.2117-2123
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• 2014

To cope with the explosive growth of traffic which is considered as one of the most biggest threat to current mobile communication systems, various solutions such as small cell and device-to-device communication have been exploited. Directional transmission in which transmission power of base station is focused onto the direction where the mobile station is located, can be used to increase throughput of the system. In this work, we develop a system-level simulator for cellular mobile communication systems using directional transmission and adaptive interference estimation scheme for directional transmission has been proposed. By using the developed simulator, the performance of cellular mobile communication systems with directional transmission is examined. Moreover, it is shown that the overall throughput of cellular system can be improved by utilizing directional transmission.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.7B
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• pp.706-718
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• 2002

In bluetooth system, there are two kinds of interference. One is the frequency static interference, for example 802.11 direct sequence, the interferer uses fixed frequency band. Another is frequency dynamic interference, for example other piconets or 802.11 frequency hopping, the interferer uses dynamic frequency channel and cant be estimated. In this paper we introduce a novel solution of hybrid method of Listen-Before-Talk (LBT) and Adaptive Frequency Hopping (AFH) to address the coexistence of bluetooth and Direct Sequence of wireless local area network (WLAN). Before any bluetooth packet transmission, in the turn around time of the current slot, both the sender and receiver sense the channel whether there is any transmission going on or not. If the channel is busy, packet transmission is withdrawn until another chance. This is the LBT in Bluetooth. Because of asymmetry sense ability of WLAN and bluetooth, AFH is introduced to combat the left front-edge packet collisions. In monitor period of AFH, LBT is performed to label the channels with static interference. Then, all the labeled noisy channels are not used in the followed bluetooth frequency hopping. In this way, both the frequency dynamic and frequency static interference are effectively mitigated. We evaluate the solution through packet collision analysis and a detail realistic simulation with IP traffic. It turns out that the hybrid method can combat both the frequency dynamic and frequency static interference. The packet collision analysis shows it almost doubles the maximal system aggregate throughput. The realistic simulation shows it has the least packet loss.