• The Journal of Korean Institute of Electromagnetic Engineering and Science
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• v.23 no.2
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• pp.235-243
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• 2012

In this paper, we propose a method to improve performance of pilot symbol assisted modulation(PSAM) channel estimation method for OFDM systems over frequency selective channel. When channel values are estimated, the low pilot density used for channel estimation increases not only the effective data rate but also power efficiency. Thus, the lower pilot density which is used for channel estimation is better for OFDM system. At first, we estimate the channel values which are located at the middle of adjacent pilots, and then all of the possible channel values are estiamted by using original pilot values and previously estimated pilot values. Furthermore, the error of estimated channel values is reduced by introducing guard interval which is designed acccording to maximum channel delay. Performance achieved with the proposed method is illustrated by simulation experiments in comparison with the existing methods in terms of mean squared error(MSE).

• The Journal of Korean Institute of Communications and Information Sciences
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• v.36 no.5A
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• pp.441-449
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• 2011

Sample timing offset (STO) and carrier frequency offset (CFO) are caused by inter-symbol interference (ISI), inter-carrier interference (ICI) and phase error in orthogonal frequency division multiplexing (OFDM) system. OFDM characteristic is sensitive about STO and CFO. So when ICI occurs, compensation is hard and complex equalizer is needed. In this paper, we propose an effective correction method using feedback process with pilot and synchronization symbol. After feedback with estimated value in frequency domain, STO and CFO are corrected by control sample & and holder and oscillator. As a result of simulation, we confirm that STO and CFO can be corrected without equalizer through feedback.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.33 no.4A
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• pp.467-472
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• 2008

One of the maindisadvantages of Orthogonal Frequency Division Multiplexing (OFDM) is the Peak to Average Power Ratio (PAPR) problem. To reduce PAPR and improve the error performance of the OFDM signal over a fading channel, an OFDM system combining an Adaptive Symbol Selection Scheme (ASSS) and channel estimation can be used. Because of the side information of ASSS, however, the data rate decreases in the conventional ASSS OFDM system. In this letter, to overcome this impairment, we propose a technique to transmit side information by using pilot tones, and demonstrate that the proposed technique can give reasonable bit error rate (BER) performance.

• Proceedings of the KIEE Conference
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• 2004.07d
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• pp.2610-2612
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• 2004

The polyphase filter-based orthogonal frequency division multiplexing (PF-OFDM) is proposed in [1]. It provides more efficient data transmission mechanism than the classical OFDM method. However, the channel estimation mechanism in the classical OFDM system such as the cyclic prefix can not be applied straightforwardly, since the received signal contains unpredictable terms. Therefore, the PF-OFDM system requires a complicated channel estimation scheme when it works on the multipath fading communication channel. In this paper, we proposed a pilot-symbol aided channel estimation algorithm suitable for the PF-OFDM system which efficiently deals with the unpredictable terms and verified its performance through a series of computer simulations.

• Journal of electromagnetic engineering and science
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• v.1 no.2
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• pp.113-119
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• 2001

Conventional channel estimation methods for orthogonal frequency division multiplexing (OFDM) system don't show the good characteristics in terms of fast fading channels. To solve this drawback in conventional methods, we propose the channel estimation method fur OFDM, assisted pilot for improvement and convergence in mobile system (APIM), which has a good performance and computational complexity in consideration of other methods. APIM uses the more developed concept of conventional methods and a block frame structure within a whole channel. This concept prevents overall performance from diverging or showing a poor one. The simulation results demonstrate the APIM outperforms pilot symbol assisted modulation (PSAM) and extended symbol aided estimation (ESAE) in terms of mean square error (MSE) and bit error rate (BER) performance under all Rayleigh fading environment. Considering the simulation performance and computational complexity, we can see APIM shows better characteristics than conventional methods for OFDM and has not any error floor even in a fast Rayleigh fading environment.

• The Transactions of the Korean Institute of Electrical Engineers D
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• v.50 no.8
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• pp.400-405
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• 2001

Quadrature Amplitude Modulation(QAM) is well known a bandwidth efficient modulation scheme. However, its use for mobile communications has been limited due to noise and signal distortion. Recently, the QAM scheme is emerging as a new modulation scheme for a band-limited wireless system. In this paper, we propose an iterative decoding algorithm assuming QAM signal for a narrow-band mobile channel. The Algorithm is based on the EM(Expectation Maximization) Algorithm, and the performances of the proposed decoder are assessed using computer simulation. The simulation results show that the proposed decoder perform better compared to that of other schemes.

• Journal of the Institute of Electronics Engineers of Korea TC
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• v.48 no.11
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• pp.17-21
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• 2011

In this paper, we propose a pilot symbol assisted low complexity LS channel estimation method for OFDM in fast time varying channels. The proposed method shows low complexity characteristics in terms of memory space and processing time compared with conventional BEM channel model LS estimation methods.

• The Journal of Korean Institute of Communications and Information Sciences
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• v.26 no.6A
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• pp.913-922
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• 2001

Multi-Carrier CDMA 시스템에서 불완전한 채널 예측과 다중사용자 간섭은 시스템의 성능 저하를 유발한다. 본 논문에서는 역방향 링크에서 파일럿 심볼을 이용하여 채널을 예측하는 Multi-Carrier CDMA BPSK 시스템에서 불완전한 채널 예측과 다중사용자 간섭의 영향을 연구하였다. 더욱이, 라이시안 페이딩 채널의 역방향 링크에서 최대비 합성 공간 다이버시티와 선택 합성 공간 다이버시티 기법을 적용한 성능 개선에 대하여 연구하였다. 수치계산 결과, 파일럿 심볼을 이용하여 채널을 예측하는 Multi-Carrier CDMA BPSK 시스템의 BER 성능은 등가 잡음 대역폭의 분산($\sigma$$^2$$_{c}$)에 매우 민감하나 무선 채널에서의 신호 전력 대 잡음 전력비에는 그리 심하지 않았다. 불완전한 채널 예측에 의한 BER 열화와 파일럿 심볼 간격의 최적화는 라이시안 페이딩 채널에서 Multi-Carrier CDMA BPSK 시스템의 BER 계산을 통하여 얻을 수 있었다. 그리고 불완전한 채널 예측에 의한 BER 열화는 신호 전력 대 잡음 전력비와 채널 예측 필터의 등가 잡음 대역폭에 관계하고 있음을 알 수 있었다.

• ETRI Journal
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• v.26 no.5
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• pp.397-404
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• 2004

In this paper, we provide a new detection scheme for a pilot symbol assisted interference nulling and cancellation operation to reduce unexpected effects owing to parallel transmission in orthogonal frequency division multiplexing (OFDM)-based spatial multiplexing systems. We have shown that the investigated OFDM vertical Bell laboratories layered space time (VBLAST) detection based on hybrid processing performs better than ordinary OFDM-VBLAST detections based on serial processing and parallel processing, respectively.

• 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.