• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.7A
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• pp.720-726
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• 2008

In this paper, we propose a multi-cell cooperation method for broadcast packet data services in the orthogonal frequency division multiplexing (OFDM)-based cellular system with multiple transmit antennas. In the proposed method, to transmit two streams of spatially demultiplexed or transmit diversity coded symbols over a number of transmit antennas, we divide a coded packet into multiple subparts to which different cell groups and antenna pairs are assigned. The proposed method enhances the diversity order by transforming the channel frequency responses of two symbol streams in each subpart of the broadcast packet. The increase in diversity of the proposed method is shown with the outage probability under various configurations.

• Journal of The Korean Association For Science Education
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• v.32 no.6
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• pp.1050-1062
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• 2012

Concepts of science have been developed by occupying 'ecological niche' within conceptual ecology. The ecological niche is determined from the mutual effect between intellectual environmental of the learner and new concept, which few studies have been conducted. This study examined how the ecological niche of the concept of photosynthesis in $7^{th}$ grade is changed by instruction. The ecological niche was analyzed using 2 methods: (1) the change in the diversity of concepts, and (2) the change in the proximity of concepts based on the frequency and the relativeness score of the concepts. The concept of photosynthesis was analyzed in the 4 domains in the place of photosynthesis, products of photosynthesis, reactants of photosynthesis, and environmental factors. The results of this study are as follows: (1) reduced diversity of concepts, (2) increased frequency and relativeness score of the scientific concepts, and (3) increased proximity of the scientific concepts by instruction. With these results, the mutual effects of the concepts within the conceptual ecology have become active by class to differentiate the relationships between the concepts, which accordingly displayed their changes in status.

• Journal of Communications and Networks
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• v.9 no.1
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• pp.75-83
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• 2007

Orthogonal frequency division multiplexing (OFDM) systems with multiple transmit antennas can exploit space-time block coding on each subchannel for reliable data transmission. Spacetime coded OFDM systems, however, are very sensitive to time variant channels because the channels need to be static over multiple OFDM symbol periods. In this paper, we propose to mitigate the channel variations in the frequency domain using a linear filter in the frequency domain that exploits the sparse structure of the system matrix in the frequency domain. Our approach has reduced complexity compared with alternative approaches based on time domain block-linear filters. Simulation results demonstrate that our proposed frequency domain block-linear filter reduces computational complexity by more than a factor of ten at the cost of small performance degradation, compared with a time domain block-linear filter.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.35 no.4A
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• pp.307-314
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• 2010

In this paper, a receive diversity combining technique is proposed for single-carrier frequency division multiple access (SC-FDMA)-based cooperative relay systems when discrete Fourier transform (DFT) spreading sizes for mobile station (MS) and relay station (RS) are different. The proposed technique is composed of a DFT spreading size adjustment block, a phase rotation compensation block, a channel phase compensation block, and a receive diversity combining block. The proposed technique is robust to multipath channels and can be operated with a relatively small computational complexity because receive diversity combining is performed with scalar operations in the frequency-domain. It is shown by computer simulation that the proposed receive diversity combining techniques achieve a performance gain over the conventional maximal ratio combining (MRC) techniques for SC-FDMA-based cooperative relay systems.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.3A
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• pp.197-204
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• 2009

Cyclic delay diversity (CDD) is considered a simple approach to exploit the frequency diversity, to improve the system performance in orthogonal frequency division multiplexing (OFDM) systems. Also, the linear preceding technique can significantly improve the performance of communication systems by exploiting the channel state in formation (CSI). In order to achieve enhanced performance, we propose applying linear preceding to the conventional CDD-OFDM transmit diversity schemes over Rayleigh fading channels. The proposed scheme works effectively with the accurate CSI in time-division-duplex (TDD) OFDM systems with CDD, where the reciprocity is ass umed instead of channel state feedback. For a BER of $10^{-4}$ and the mobility of 3 km/h, simulation results show that a gain of 6 dB is achieved by the proposed scheme over both flat fading and Pedestrian A (Ped A) channels, compared to the conventional CDD-OFDM system. On the other hand, for a mobility of 120 km/h, a gain of 2.7 dB and 3.8 dB is achieved in flat fading and Vehicular A (Veh A) channels, respectively.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.23 no.11
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• pp.1434-1442
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• 2019

In this paper, the channel change was continuously measured for 24 hours from July 5, 2017 on the coast near Deokjeok-do, Incheon. The underwater channel has various channel environment characteristics as the change in the time axis and the change in the frequency axis occurs in real time, and the underwater communication performance decreases due to the multipath fading and the Doppler effect. Therefore, in this study, we performed the OFDM system performance analysis in the underwater channel environment by applying the repetitive transmission diversity scheme in the time and frequency domain to improve the communication performance in the real-world underwater communication environment. Using the collected data, we compared the channel environment in the time and frequency domain and analyzed the BER performance according to the pilot spacing and the number of repetitive transmissions in the time and frequency axis.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.31 no.3A
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• pp.267-273
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• 2006

This paper improves the effects of clustered OFCDM scheme considering the frequency diversity effect over a frequency selective fading channel. In OFCDM with frequency domain spreading compared to OFDM, we can increase uncorrelated symbols by frequency allocation method of correspondent symbols over the same antenna and different antenna after spreading. The simulation results show that the performance of proposed system is improved by approximately 4 dB in ${\sigma}=0.02{\mu}sec$, the performance is improved by approximately 2.5dB in large delay spread in a 12-path Rayleigh fading channel with overall the root mean squared delay spread and the maximum Doppler frequency of 20 Hz. Also, the required average received Eb/No at the average BER of $10^{-3}$ by optimum method is improved by approximately 2.0 dB, compared to that of STA-OFCDM with frequency rearrange. The new method does not require any bandwidth expansion any feedback from the receiver to the transmitter and its computation complexity is similar to clustered OFCDM.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 2001.10a
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• pp.345-348
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• 2001

A new OFDM scheme, CI/OFDM using the interference of multiple carriers, is introduced. CI/OFDM system uses the N orthogonal carriers equally spaced in frequency, but each information bit is simultaneously modulated onto all carriers. The performance of CI/OFDM systems in a typical frequency selective fading channel environment, are analyzed. And it is shown that the frequency diversity benefits of this system result in significant performance enhancements relative to traditional OFDM system without causing bandwidth expansion.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.11 no.2
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• pp.122-136
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• 1986

In this paper, the new modulation system, the digital phase-frequency hybrid modulation system is proposed for mobile radio communications. The error rate and the outage equation of PFSK(Phase-Frequency Shift Keying) signal transmitted through the fading channel has been derived considiering deversity techniques. The error rate and the outgae rate performances of PFSK system have been evaluated and shown in figures in terms of carrier-to-noise power ratio(CNR), fading figure, numbers of diversity branches, correlation coefficient among the diversity branches. And the performance of PFSK system is superior to that of QDPSK system. And by using diversity techniques, system performances can be improved 13dB above in CNR.

• ETRI Journal
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• v.33 no.3
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• pp.310-319
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• 2011

In this paper, a new soft-fusion approach for multiple-receiver wireless communication systems is proposed. In the proposed approach, each individual receiver provides the central receiver with a confidence level rather than a binary decision. The confidence levels associated with the local receiver are modeled by means of soft-membership functions. The proposed approach can be applied to wireless digital communication systems, such as amplitude shift keying, frequency shift keying, phase shift keying, multi-carrier code division multiple access, and multiple inputs multiple outputs sensor networks. The performance of the proposed approach is evaluated and compared to the performance of the optimal diversity, majority voting, optimal partial decision, and selection diversity in case of binary noncoherent frequency shift keying on a Rayleigh faded additive white Gaussian noise channel. It is shown that the proposed approach achieves considerable performance improvement over optimal partial decision, majority voting, and selection diversity. It is also shown that the proposed approach achieves a performance comparable to the optimal diversity scheme.