• The Journal of Korean Institute of Communications and Information Sciences
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• v.41 no.1
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• pp.23-36
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• 2016

We propose a new type of OFDM-CDMA scheme which allows large inter-user timing offset using zero correlation zone(ZCZ) code in conjunction with block spreading technique. Moreover to maximize spectral efficiency, the proposed OFDMA does not have guard time(GT). This is opposite to the trends in the conventional schemes where GT are supposed to be larger to allow larger inter-user timing offset. It is remarkable that the proposed GT-free OFDM-CDMA scheme completely cancels inter-user interference in the multipath fading simply by despreading process. This inter-user interference-free feature still remains even there exist inter-user timing offsets as large as multiple OFDM symbols. Although the self-user interference exists due to no GT, it can be effectively suppressed by simple successive interference cancellation(SIC) from the first symbol in spread block as it is free from inter symbol interference(ISI).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.37C no.10
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• pp.965-976
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• 2012

Multi-Platform Image and Intelligence Common data link(MPI-CDL) systems are designed to transmit the imaginary and signal intelligence data at an aeronautical to ground line of sight(LOS) link. This paper proposes a method to predict a link availability and analyzes the required link margin to satisfy a given link availability for MPI-CDL systems. To estimate a link availability the proposed method applies the conditional probability so that both a rain attenuation and a multipath fading are considered simultaneously. Link margins to meet the link availability for MPI-CDL systems are calculated according to an operating environment including frequencies, flight altitudes and transmission ranges. The required link margins for actual unmanned air vehicle systems are also given by simulation results.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.34 no.2A
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• pp.132-138
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• 2009

Cognitive radio is considered as a promising solution to scarce spectrum problem. The primary object of cognitive radio is to increase spectral efficiency, while causing limited interference to primary users who are using the spectrum. Hence, an essential part of cognitive radio systems is spectrum sensing which determines whether a particular spectrum is occupied or not by a primary user at a particular time. However, sensing decision of each individual secondary user alone may not be reliable enough due to shadowing and multipath fading of wireless channels. The so called hidden terminal problem makes the problem even worse, possibly yielding undesired interference to the primary users. Recently, cooperative spectrum sensing is emerging as a remedy to these problems of individual sensing. Cooperative sensing allows a group of secondary users to share local sensing information to extract a global decision with high fidelity. In this paper, we investigate a cooperative spectrum sensing algorithm based on hard decisions of local sensing outcomes. Specifically, we propose an effective scheme for combining local decisions by introducing weighting factors that reflect reliability of the corresponding secondary user. Through computer simulations, the performance of the proposed combining scheme is compared with that of the conventional scheme without weighting factors in various environments.

• Journal of Satellite, Information and Communications
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• v.6 no.2
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• pp.20-25
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• 2011

The authors have proposed novel TDD-CDMA systems with Pre-Rake transmit diversity schemes (system 1 and system 2) using multiple transmit antennas in [2] and have also evaluated the system performance through the theoretical analysis and computer simulation. However, the performance of system 2 which transmit a signal using all antennas has not been evaluated for multi-user environment. Therefore in this paper, we analyze the performance of system 2 for multi-user environment and compare the performance with that of the already proposed system 1 which chooses only one antenna. From the numerical results, it is found that system 2 outperforms system 1 as the number of users increases while system 1 outperforms system 2 at a small number of users. Therefore in order to achieve the best system performance, the Pre-Rake transmit diversity type should be selected at the base station according to the number of users.

• Journal of the Institute of Electronics and Information Engineers
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• v.52 no.4
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• pp.17-22
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• 2015

In this paper, we propose a cooperative vehicle communication scheme for high transmission efficiency and coverage extension under multipath fading environment of moving vehicle networks. The proposed scheme uses a Network coding scheme for improvement of receiving performance by using virtual Multiple-Input Multiple-Output(MIMO) transmit diversity. Simulation results have shown that the proposed scheme also provides alleviated Inter Symbol Interference(ISI) and Inter Channel Interference(ICI) as well as Signal-to-Noise Ratio(SNR) improvement and improve 3dB compared to the conventional scheme, since it can utilize the good properties of spatial diversity and coding gain by using virtual MIMO configuration. In this paper, we propose simulations of Ultra-Wideband(UWB) communication system to show validity by using the MATLAB.

• Journal of Advanced Navigation Technology
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• v.23 no.3
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• pp.258-263
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• 2019

OFDM is popular digital communication method due to its immunity to multipath fading and capability of high speed data transmission, but it has disadvantage of high PAPR in transmission signal when many subcarrier modulated signal are added to the same phase. When frequency domain high amplitude impulse is inserted before IFFT in OFDM transmitter, the PAPR of OFDM signal in tme domain can be effectively reduced. In this paper, the degree of PAPR improvement of OFDM communication system with CQAM subcarrier modulation is analysed by adopting impulse insertion technique before IFFT of transmitter. Furthermore, it is verified that additional PAPR performance improvement can be obtained by finding optimum position of impulse insertion for maximizing PAPR reduction. Through computer simulation, the degree of PAPR improvement according to amplitude and position of inserted impulse is superior to conventional technique in OFDM-CQAM system.

• 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 the Acoustical Society of Korea
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• v.26 no.7
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• pp.366-374
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• 2007

In underwater wireless communication, needs for long distance communication using the high frequency are surpassing ones of short range communication by ultrasonic wave, and demands for transmitting and receiving various data such as voice or high resolution image data are increasing as well. In this work, we studied the effects on the real underwater communication depending on the difference of digital modulation methods. Simulation shows that only the performance of GMSK among many other PSK based modulation schemes(BPSK, QPSK, MSK, GMSK) is significant. Test condition simulates the oceanographic conditions along the 207-survey line, 15Km south of Busan and SNR is maintained 35dB or below. Simulated tests are composed of both transmitting image data($3{\times}10^5$ pixel, 4 bit per pixel) and voice communication($10^{-2}$BER, channel capacity of 1Kbps). Test results show that there are gain of about 7 seconds in transmission time in image transmission case, where channel capacity for BPSK, QPSK, and MSK and for GMSK were 65 Kbps and 45 Kbps, respectively and gain of about 8Km in distances in voice communication case.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.25 no.5B
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• pp.879-888
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• 2000

In this paper, the performance of IS-95B CDMA system adopting multi-code scheme for data service on the forward link is obtained by theoretical analysis and computer simulation. The wireless channel is assumed to include AWGN, MUI(Multi-User Interference), and MPI(Multi-Path Interference), and multipath Rayleigh fading modeled by TDL(Tapped Delay Line). To obtain actual performance, IS-95B system using PN codes(M-sequence) concatenated with Walsh codes is considered. The BER performance of voice/data service satisfying QoS requirement and the maximum capacity of voice users is derived in terms of various system parameters(voice activity factor, required SNR of voice/data users, the number of multi-codes, the number of voice/data users.,etc.). As a result, under the condition that the maximum allowable number of voice user is 38 in voice only system, as the number of multi-code is decreased by 1, the maximum allowable number of voice user is increased by 5% for system with 1 data user and 22.3% for system with 4 data users. Therefore, it is possible to increase maximum allowable number of users in IS-95B CDMA system by appropriate control of the number of multi-codes and required SNR according to channel environment and traffic characteristic.

• Journal of Broadcast Engineering
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• v.5 no.2
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• pp.199-210
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• 2000

The IBAC (In-Band Adjacent-Channel) DAB (Digital Audio Broadcasting) system is to provide multichannel CD quality audio services and multimedia data services including text and picture in FM band (88~105 MHz). As the FM band is being used by the existing analog radio broadcasting, there must he an analysis of the interference effect between IBAC DAB and analog FM signal. Therefore, the protection ratio should be evaluated to verify the system compatibility and allocate the new IBAC DAB channel in FM band. In this paper, among the three types of interferences, FM-to-DAB, DAB-to-FM and DAB-to-DAB, that can be occurred, the Protection ratio of IBAC DAB system for co-channel FM interferer is analyzed by modeling the FM interferer and considering the multipath fading channel. The simulation results show that IBAC DAB system has far better sensitivity than Eureka 147 and needs a relatively high protection ratio for co-channel FM interferer, because of its narrow bandwidth, about one third of that of Eureka 147.