• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.8 no.1
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• pp.22-34
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• 1997

In LEO system, the signal degradation is mainly due to fading and Doppler shift, so that the analysis of the signal degradation and compensation techniques are very important. As the Doppler shift compensator, the block demodulator has been known to be useful in compensating for the time-varing Doppler shift, but its compensating ability is about several hundreds Hz in 32 ksymbols / s QAM (QPSK) signal transmission. Therefore, in this paper, to compensate for severe fading and Doppler shift more than several kHz, we use a conventional pilot symbol-aided fading compensator, and propose the Doppler shift compensator. It is shown that the proposed compensator is able to compensate for Doppler shift more than several kHz. And a pilot symbol-aided fading compensator is shown to be a suitable scheme for severe Rician fading ($K{\leq}10 dB$) as well as Rayleigh fading. Also, it is shown that the equal gain combiner improves greatly the QAM performance even if the fading or Doppler shift becomes deeper or larger.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.14 no.7
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• pp.3482-3486
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• 2013

In the case of the Pilot Symbol Assisted Modulation (PSAM) method predicting and compensating amplitude and phases caused on fading channels, there can be severe performance deterioration by Doppler spread on fast fading channels. In this paper, the fading compensation method suggested so as to improve occurring problems as well as analyze them. Doppler spread is the major cause of the bit error rate(BER) performance deterioration. Compared to the existing PSAM method, the more performance deterioration occurs, the larger Doppler spread appears but performance shows well its less $10^{-2}BER$ performance than the existing PSAM method in the suggested method whereas the existing PSAM method has about $10^{-1}BER$ its considerable performance deterioration that caused by Doppler spread within a symbol cycle with the level of delay wave interference.

• Journal of Advanced Navigation Technology
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• v.13 no.4
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• pp.544-550
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• 2009

In this paper, we proposed the radio wave propagation characteristics of the next-generation ultrafast wireless communication system in millimeter-wave fading channel. For considering doppler shift and Rayleigh fading simultaneously, the fading simulator of Jakes model implemented and analyzed the performance of the next-generation wireless communication system. In addition, the error rate characteristics of DS-CDMA system analyzed in the millimeter-wave fading channel and the system performance improved by coding technique and diversity technique.

• Proceedings of the Korean Institute of Communication Sciences Conference
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• 1996.10a
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• pp.911-914
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• 1996

• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.7 no.1
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• pp.71-82
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• 1996

In this paper, the Doppler phase error due to the relative velocity between a satellite and the earth station in communications using a low earth orbit mobile satellite is detected. The performance of BPSK system in the presence of Rician fading with Doppler phase error and interference, noise is compared with that of the system disturbed by Doppler phase error and noise only. The expression of error rate performance of BPSK system is derived as the type of complementary error function. The numerical calculation of the induced equation are performed in terms of satellite-height, orbit-eccentricity, the velocity of the earth, Rician fading parameter, signal to interference rateio (SIR), the ratio of carrier frequency and base band bit rate. The main conclusion that can be drawn from this analysis is that Rician fading channel environment with Doppler phase error and interference effect yields severe performance degradation than Do- ppler phase error and noise effect in satellite communication channel. And using the numerical calculation, we give a quantitative insight how much the satellite communication channel parameters degrade the system performance. Furthermore it is shown that an appropriate transmission power control for the performance enhancement is beneficial to the new satellite communication system planning.

• Proceedings of the Korea Institute of Convergence Signal Processing
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• 2001.06a
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• pp.33-36
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• 2001

In MC-CDMA systems effects of delay spread of the channel are reduced with increased symbol duration by simultaneously transmitting data symbols on the parallel subcarriers. However, the increased symbol duration causes the system to be more vulnerable to time selective fading. In this paper, we investigate the effects of time selective fading characteristics of the mobile channel from the viewpoint of desired signal power to inter-carrier interference power ratio at the combiner output of the MC-CDMA receiver.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 1998.05a
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• pp.383-388
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• 1998

이동통신에서, 신호의 페이딩을 보상하기 위하여 pilot symbol assisted modulation(PSAM)에 대한 많은 연구가 진행 되어왔다. 본 논문에서는 최대비합성다이버시티를 사용하는 PSA-QAM에서 전력증폭기 비선형성의 효과를 분석하였다. 먼저, PSAM성능의 한계를 구하기 위하여, 페이딩에 대한 완벽한 정보가 주어진 상태에서 최대비합성다이버시티를 갖는 QAM의 이론적 성능을 분석하였다. 실제 PSAM에서, 페이딩에 대한 정보는 파일럿 심볼의 보간에 의하여 얻어진다. 오차의 평균전력을 최소화하는 보간필터를 사용하여 필터의 탭수, 파일럿 심볼프레임 주기, 도플러 주파수가 성능에 미치는 영향을 분석하였다. 비선형 전력증폭기 AB, B, C급의 AM/AM, AM/PM 특성을 성능분석에 고려하였다. 비선형 보상기술인 Cartesian Feedback Loop (CFB) 이용하여 AWGN 채널과 Rayleigh 페이딩 채널에서 비선형 전력증폭기 종류에 따른 성능변화를 나타내었다.

• Journal of the Korean Institute of Telematics and Electronics S
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• v.35S no.6
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• pp.9-18
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• 1998

In this paper, an equalization technique for OFDM(orthogonal frequency division multiplexing) in a time-variant multipath fading environment is described. A loss of subchannel orthogonality due to time-varying multipath fading channels leads to interchannel interference (ICI) which increases the error floor in proportion to Doppler frequency. A simple frequency-domain equalizer which can compensate the effect of ICI caused by time variation of multipath fading channel is proposed by modifying the previous frequency-domain equalization technique with taking into account only the ICI terms significantly affecting the error performance. The effectiveness of the proposed approach is demonstrated via computer simulation by applying it to OFDM systems when the multipath fading channel is slowly time variant.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.10 no.5
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• pp.798-805
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• 2006

I evaluate the BER performance of downlink OFDMA systems in time variant Rayleigh Doppler fading channel, considering the carrier frequency offset and the nonlinear high power amplifier. I obtain the required output back-off of the nonlinear amplifier and value of frequency offset for good BER performance. And I also analyze the BER degradation upon Doppler fading channel.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.11 no.12
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• pp.2391-2397
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• 2007

I evaluate the BER performance of uplink OFDMA systems in time variant Rayleigh Doppler fading channel, considering the carrier frequency offset and the nonlinear high power amplifier. I obtain the required output back-off of the nonlinear amplifier and value of frequency offset for good BER performance. And I also analyze the BER degradation upon Doppler fading channel.