• Textile Coloration and Finishing
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• v.16 no.5
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• pp.8-18
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• 2004

In order to study on the color change of silk dyed with natural colorant due to light fading, and find out the effect of combination dyeing, colorant extracts of safflower red, safflower yellow and amur cork tree were used, either singly or in combination. In combination dyeing, safflower yellow or amur cork tree dyeing process was added on the top of the silk fabric was dyed with safflower red. Color change and light fastness were investigated by $L^*,\; a^*,\; b^*$ H, V/C, and Color difference. Brightness of silk fabric dyed with safflower red and safflower yellow increased gradually with increasing the radiation time of UV light, but amur cork tree was decreased and turned to dull. Color difference of dyed with Amur cork tree showed higher than the others. Combination dyeing of safflower red and amur cork tree provided better light fastness than the one of safflower red and safflower yellow.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.11 no.7
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• pp.1283-1288
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• 2007

An equalization technique for OFDM systems in tine variant fading environment is described. Time variant channels lead to interchannel interference which increases the bit mr rate. A frequency domain equalizer using pilot symbols is proposed. The equalizer decreases the interchannel interference affecting the error performance. The effectiveness of the proposed technique is analyzed via computer simulation.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.11 no.2
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• pp.273-279
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• 2007

I evaluate the BER performance of OFDM systems in frequency selective Doppler time variant fading channels, considering the pilot patterns for channel estimation. The performance of the systems is degraded due to channel estimation error. For the reduction of performance degradation in acceptable level, the optimum distance of pilot symbols in pilot pattern is 5 subcarriers in frequency domain and 6 OFDM block in time domain.

• International Journal of Internet, Broadcasting and Communication
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• v.11 no.1
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• pp.60-68
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• 2019

In this paper, we propose the adaptive beamforming algorithm for the MIMO (Multi-Input Multi-Out)-OFDMA(Orthogonal Frequency Division Multiplexing Access)system to improve the performance. The performance of MIMO-OFDMA systems is greatly decreased in the wireless channel environment with multiusers, because the received signals are much distorted by a cochannel interference (CCI) during the space-time decoding. The proposed approach can track the DOA of each signal from the multiple antennas of the desired user without being greatly dependent on the impinging angle. And beams are directed toward the multiple transmitters of the desired user while null beams are directed toward interference directions. Therefore, we can can effectively cancel CCI and mitigate the impairment of delay spread while preserving the STC(space time code) diversity. BER performance improvement is investigated through computer simulation by applying the proposed approach to MIMO-OFDMA system in a multipath fading channel with CCI.

• International Journal of Internet, Broadcasting and Communication
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• v.11 no.2
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• pp.76-83
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• 2019

In this paper, we propose the beamforming algorithm for the performance improvement of MIMO MC-CDMA system. The proposed multibeamforming of MIMO MC-CDMA structure having the same number of beamformer as the number of transmit antenna is derived by calculating the error signals between the coded pilot symbols and the corresponding received signals from the multiple transmitters of the desired user in the frequency domain, transforming the frequency-domain error signals into time-domain error signals, and updating the weights of the multibeamformer in the time-domain in the direction minimizing the mean squared error (MSE). The proposed approach can track each direction of arrival (DOA) of the signals from multi-antennas of a desired user. The performance improvement is investigated through computer simulation by applying the proposed approach to MIMO MC-CDMA system in a multipath fading channel with multiusers.

• ETRI Journal
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• v.43 no.2
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• pp.197-208
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• 2021

Single-carrier frequency division multiple access (SC-FDMA) is currently being used in long-term evolution uplink communications owing to its low peak-to-average power ratio (PAPR). This study proposes a new transceiver design for an SC-FDMA system based on Walsh-Hadamard transform (WHT). The proposed WHT-based SC-FDMA system has low-PAPR and better bit-error rate (BER) performance compared with the conventional SC-FDMA system. The WHT-based SC-FDMA transmitter has the same complexity as that of discrete Fourier transform (DFT)-based transmitter, while the receiver's complexity is higher than that of the DFT-based receiver. The exponential companding technique is used to reduce its PAPR without degrading its BER. Moreover, the performances of different ordered WHT systems have been studied in additive white Gaussian noise and multipath fading environments. The proposed system has been verified experimentally by considering a real-time channel with the help of wireless open-access research platform hardware. The supremacy of the proposed transceiver is demonstrated based on simulated and experimental results.

• Proceedings of the Korea Institute of Convergence Signal Processing
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• 2000.08a
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• pp.69-72
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• 2000

Multilevel modulation schemes are known to be highly bandwidth efficient. By varying modulation level adaptively according to channel conditions (i.e. adaptive QAM schemes or AQAM), high bandwidth efficiency can be achieved. This paper considers the effect of time delay on the adaptive QAM schemes in dispersive fading channels. In order to undertake investigations effectively, a simulator has been developed. The simulation results show that the BER performance obtained for the wideband channel with a delay spread of 0.75 $\mu$sec is better than 10$^{-4}$ with a SNR value of 40 ㏈ and ABPS is found to be approximately 5.5.

• The Journal of the Korea Contents Association
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• v.9 no.4
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• pp.65-71
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• 2009

In this paper a new space-time diversity relaying strategy using cooperative communication technique is proposed. More than one relaying terminals are included in one cooperative group to share their state information, such as frame error rate and channel state information. The best terminals are selected to send bit information using space-time diversity relay system. An implementation for the proposed scheme is also presented using the TDMA cooperative protocol. The resulting receive signal to transmit signal ratio and computer simulation demonstrate that the proposed strategy outperforms the conventional cooperative system.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.5A
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• pp.355-362
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• 2009

In real-time communication services, delay constraints are among the most important QoS (Quality of Service) factors. In particular, it is difficult to guarantee the delay requirement over wireless channels, since they exhibit dynamic time-varying behavior and even severe burst-errors during periods of deep fading. Channel throughput may be increased, but at the cost of the additional delays when ARQ (Automatic Repeat Request) schemes are used. For real-time communication services, it is very essential to predict data deliverability. This paper derives the delay distribution and the successful delivery probability within a given delay budget using a priori channel model and a posteriori information from the perspective of queueing theory. The Gilbert-Elliot burst-noise channel is employed as an a Priori channel model, where a two-state Markov-modulated Bernoulli process $(MMBP_2)$ is used. for a posteriori information, the channel parameters, the queue-length and the initial channel state are assumed to be given. The numerical derivation is verified and analyzed via Monte Carlo simulations. This numerical derivation is then applied to a rate control scheme for real-time video transmission, where an optimal encoding rate is determined based on the future channel capacity and the distortion of the reconstructed pictures.

• Journal of information and communication convergence engineering
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• v.4 no.2
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• pp.71-74
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• 2006

A basic access method about IEEE 802.11 MAC layer protocol using IEEE 802.11 wireless LANs is the DCF thatis based on the CSMA/CA. But, cause of IEEE 802.11 MAC layer uses original backoff algorithm (exponential backoff method), when collision occurred, the size of contention windows increases the double size Also, a time of packet transmission delay increases and efficienty is decreased by original backoff scheme. In this paper, we have analyzed TCP packet transmission time of IEEE 802.11 MAC DCF protocol for wireless LANs a proposed enhanced backoff algorithm. It is considered the transmission time of transmission control protocol (TCP) packet on the orthogonal frequency division multiplexing (OFDM) in additive white gaussian noise (A WGN) and Rician fading channel. From the results, a proposed enhanced backoff algorithm produces a better performance improvement than an original backoff in wireless LAN environment. Also, in OFDM/quadrature phase shift keying channel (QPSK), we can achieve that the transmission time in wireless channel decreases as the TCP packet size increases and based on the data collected, we can infer the correlation between packet size and the transmission time, allowing for an inference of the optimal packet size in the TCP layer.