• The Journal of The Korea Institute of Intelligent Transport Systems
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• v.17 no.5
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• pp.135-142
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• 2018

This paper compares and analyzes advantages and disadvantages of CDMA(code division multiple access) and OFDM(orthogonal frequency division multiplexing) transmission techniques for underwater acoustic channel environments. Computer simulations were carried out in various underwater acoustic channels with varying r.m.s.(root mean square) delay, doppler frequency and the number of multipaths. When r.m.s. delay and doppler frequency are within the tolerance of designed transmission schemes, the computer simulations show that CDMA has better BER performance than that of the OFDM. However, when the doppler frequency exceeds the tolerance, BER performance of the CDMA decreases.

• Journal of Satellite, Information and Communications
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• v.7 no.2
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• pp.59-66
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• 2012

In a frequency selective fading channel, Pre-Rake diversity technique provides simple receiver structure and sufficient diversity effect for mobile terminals. However, conventional Pre-Rake systems are more severely affected than Rake systems with regard to multiple access interference since the number of multipaths received at the mobile unit is nearly twice that of the Rake systems. In order to overcome the problem, a Pre-Rake TDD-CDMA system using zero correlation duration (ZCD) spreading code is proposed. Modified ZCD codes including guard interval (GI) is also proposed and the performance is evaluated by computer simulation under the condition of various channel environment.

• Journal of the Korea Society of Computer and Information
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• v.11 no.3
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• pp.271-279
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• 2006

This study proposes a new diversity algorithm to improve the signal-to-noise ratio. In the wireless channel, if fading occurs due to the multipaths, the performance of the system is apparently reduced. One of the methods to reduce fadings like this is the diversity method, and this study aims to improve the performance of the system by proposing a new diversity algorithm. This study applied rake receiver It applied QPSK and OQPSK modulation methods and applied the convolutional codes, where the code rate is 1/3 and the constraint length is 9, and the turbo code where the constraint length is 4. Under these conditions, this study compared and analyzed the average error probability of Multicarrier CDMA system.

• Journal of Communications and Networks
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• v.18 no.5
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• pp.735-743
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• 2016

This paper proposes a direction-of-arrival (DOA)-based beamforming approach for multi-cell massive multiple-input multiple-output systems with uniform rectangular arrays (URAs). The proposed approach utilizes the steering vectors of the URA to form a basis of the spatial space and selects the partial space for beamforming according to the DOA information. As a result, the proposed approach is of lower computational complexity than the existing methods which utilize the channel covariance matrices. Moreover, the analysis demonstrates that the proposed approach can eliminate the interference in the limit of infinite number of the URA antennas. Since the proposed approach utilizes the multipaths to enhance the signal rather than discarding them, the proposed approach is of better performance than the existing low-complexity method, which is verified by the simulation results.

• Journal of the Korea Society of Computer and Information
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• v.11 no.5 s.43
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• pp.203-209
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• 2006

In this paper, proposed diversity algorithm that decrease fading. In the wireless channel, if fading occurs due to the multipaths the performance of the system is apparently reduced. This study applied tap-delay receiver. It applied QPSK and OQPSK modulation methods and applied the convolutional codes, where the code rate is 1/2 and 1/3 and the constraint length is 11 and the turbo code where the constraint length is 6. The diversity algorithm proposed in this paper could be compared and analyzed the average error probability of modulation method variable of fading factor to uplink and downlink channels.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.11B
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• pp.1546-1555
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• 2001

Conventionally, in a CDMA system analysis, we assume only a single path within one chip duration. But, in this paper, we assume various number of multipaths within one chip duration according to spreading bandwidth in fixed channel model. Thus we take into account of the effects of autocorrelation and relative phases among multipath components within one chip duration according to different bandwidth, and analyze fading effects. And we derive the pdf of output signal. Then, we derive the average error probability versus the number of users from derived pdf. We use a Maximal Ratio Combining (MRC) RAKE receiver under the JTC channel model which is one of the popular realistic wideband channel models. And we employ hybrid FDMA/CDMA systems to compare the performance of W-CBMA system for the same occupied total bandwidth. Then, we compare and analyze them for different bandwidth based on the number of users as a parameter. From the simulation results for different bandwidth, better performance can be obtained for wider bandwidth system where more resolvable multipath components are available.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.27 no.4B
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• pp.298-307
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• 2002

This paper concentrates on developing and analyzing an advanced space-time multiuser receiver for WCDMA forward link. Through exploring the interference caused by co-channel users and delayed multipaths in CDMA, a constrained linear adaptive filter is adopted to cancel the interference. Furthermore, by utilizing the space-time diversity, an optimum diversity receiver, called a constrained adaptive space-time interference canceller (C-ASTIC), is proposed. For comparison, the statistical analysis of multiuser performance for WCDMA system with space-time diversity is described. The result of the simulation showed that C-ASTIC performs better than both the space-time diversity combining maximal ratio combiner (MRC) and the single-antenna adaptive interference canceller in a multipath fading channel. Also, the efficiency of the C-ASTIC in the multimedia communication environment is investigated under the multiuser, multi-transmit antenna, and multirate WCDMA system in a multipath fading channel. From the results, the C-ASTIC was validated to be useful for multi-rate WCDMA system through improving the performance gain by more than 3 dB at BER of 10$\^$-3/ in a half or more loaded system.

• Journal of Broadcast Engineering
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• v.10 no.1 s.26
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• pp.4-13
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• 2005

An equalization algorithm is proposed to guarantee a stable performance in fast fading channels for digital television (DTV) systems from the advanced television system committee (ATSC) standard. In channels with high Doppler shifts, the conventional equalization algorithm shows severe performance degradation. Although the conventional equalizer compensates poor channel conditions to some degree, long filter taps required to overcome long delay profiles are not suitable for fast fading channels. The Proposed sparse equalization algorithm is robust to the multipaths with long delay Profiles as well as fast fading by utilizing channel estimation and equalizer initialization. It can compensate fast fading channels with high Doppler shifts using a filter tap selection technique as well as variable step-sizes. Under the ATSC test channels, the proposed algorithm is analyzed and compared with the conventional equalizer. Although the proposed algorithm uses small number of filter taps compared to the conventional equalizer, it is stable and has the advantages of fast convergence and channel tracking.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.1
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• pp.9-15
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• 2011

Communication environments in the context of underwater channel are characterized to be bad by the characteristics of multipath. Multipaths are affected by various factors e.g. the temperature and the salinity of the ocean. In this paper, the representative sound speed profiles were calculated in the southern part of Baengnyeoung island so that the eigen-ray paths with the channel impulse responses were determined using the average sound speed profile of last decade. The performance of underwater communication was analyzed using the BPSK modulation and time reversal method. The significant differences of results were shown according to the change of season and carrier frequency by using computer simulation. In addition, improved performance is obtained using preprocess channel impulse response for the better comparison of two cases of summer and autumn.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.46 no.1
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• pp.40-47
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• 2009

This paper presents the method of measuring the feedback interference channel, which is developed between the transmit and receive antennas of a wireless repeater by receiving the transmit signal at the receive antenna of the identical repeater, and experiment results obtained by analyzing the measurements. This experiment uses 2 GHz WCDMA signal and is carried out near a highway. The high-speed mobiles on highways cause reflected signals with high Doppler frequencies and large energy. In order to characterize the feedback channel, the power delay profile and the scattering function are estimated by identifying the delay spread, the Doppler spread, the number of fingers, and the attenuation with delay. Since the feedback interference channel is constructed between the fixed TX and RX antennas, which is dependent upon the multipaths developed by moving or fixed objects around the antennas, the channel shows different properties comparing to the conventional channels between the base station and the mobile station. Therefore, the results presented in the paper are expected to provide guidelines for designing and evaluating wireless repeater systems.