正交多載波分頻多工系統 通道估測技術之研究通道估測技術之研究 ;A study of channel estimation for OFDM systems

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題名:	正交多載波分頻多工系統通道估測技術之研究通道估測技術之研究;A study of channel estimation for OFDM systems
作者:	葉哲勝;Che-Shen Yeh
貢獻者:	資訊工程研究所
關鍵詞:	正交多載波分頻多工系統;通道估測技術;channel estimation;OFDM
日期:	2000-12-27
上傳時間:	2009-09-22 11:25:41 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	OFDM 系統由於傳輸性能好，被廣泛應用在各種寬頻通信服務。例如：ADSL、 DAB、DVB-T、Japanese ISDB、 ETSI HiperLAN2、以及 IEEE802.11.。傳統OFDM 系統的通道估測皆採用頻域引導波信號，接收端也在頻域利用此引導波信號作通道估測，作者為了使OFDM 系統在行動接收特性更為出色，本文提出三種時域通道估測方法與傳統通道估測方式在『靜態多路徑干擾』、『慢速衰落多路徑干擾』以及『行動接收造成快速衰落』三種通道環境進行估測的研究比較。本文提出三種通道估測方法如下： lFrequency domain Pilot Time domain Average (FPTA) lTime domain Pilot Time Cross-correlation (TPTC) lTime domain Pilot Time Cross-correlation and Time varying Estimation. (TPTCTE) 從理論分析及電腦模擬驗証，OFDM信號經過multipath fading加上AWGN組合的無線通道在時域作通道估測方法比頻域處理明顯改進傳輸性能。本文也針對行動接收因素作廣泛研究，包含行動接收時引入Doppler frequency shift 與 long term fading 以及short term fading，加上short term fading中各多路徑干擾信號因為入射角不同導致通道模式中各個多路徑干擾信號相位變化的角速度皆被考慮在內。經電腦模擬證實本文提出三種通道估測方法傳輸性能比傳統方式更佳，其研究成果歸納如下： 1.FPTA方法的優點也是技術簡單，pilot to noise ratio大，適用於fading channel，尤其對於小範圍廣播，例如LMDS (Local Multipoint Distribution Service) 通道響應短者，可採用較小的pilot ratio (例如1/32或是1/64)不但節省pilot之overhead，同時通道模式估測特性相當好。但是本方法缺點是pilot ratio較大(例如1/4)時，時域引導信號大，浪費發射功率。 2.TPTC方法具有FPTA方法的優點，卻沒有FPTA的缺點，可適應各種不同通道模式且適合於fading channel。 3.TPTCTE方法不只考慮OFDM symbol內channel的時變特性，利用在時域上有較佳pilot to noise特性精準估測通道模式的時變量，使得在fast fading情形下仍然有出色的表現。所以，TPTCTE方法在Slow-fading與fast-fading中均有最佳的表現 4.當level cross rate高者，fading dip深且多，表示burst error時常出現，可加強TCM correct功能改善性能；若average duration of fade較長時，表示有較長串的burst error，可利用深度較大的interleaving技術，將length長的burst error均勻平配各個TCM packet中來改善性能。 5.TPTCTE對於通道模式時變性(time varying)較嚴重有顯著的改善效果，再配合interleaving and TCM correct功能，可降低Symbol error rate的error-floor現象，提昇OFDM system的傳輸性能。 The orthogonal frequency division multiplexing (OFDM) system has been widely adopted in both wire line and wireless communication systems, such as ADSL, DAB, DVB-T, Japanese ISDB, ETSI HiperLAN2, IEEE802.11. In OFDM systems channel fading effects are eliminated by the equalization of received signals, where the equalizer coefficients are determined according to results of channel estimation process. The channel estimation is usually accomplished in the frequency domain by inserting pilot tones in OFDM symbols. In this dissertation, channel estimations of OFDM systems in the time-domain for both slow fading and fast fading channels are investigated. Three time-domain channel estimation methods are proposed in this dissertation as follows: 1.Frequency domain Pilot Time domain Average (FPTA) 2.Time domain Pilot Time Cross-correlation(TPTC) 3.Time domain Pilot Time Cross-correlation with Time varying Estimation. (TPTCTE) Theoretical analysis reveals that a higher P/N ratio can be obtained with proposed estimation methods. It is also demonstrated from computer simulations, under assumptions of multipath distortion and AWGN channel, the proposed estimation techniques can outperform the conventional estimation techniques in which the channel response is estimated in the frequency domain. The simulations in this dissertation employ some practical fading channel models of digital transmission. One of these models used is obtained by experimental results of the field-testing of the Digital Television (DTV) related activities of Taiwan. Other channel models used are recommended by the Advanced Television Test Center (ATTC) in the U. S. A. and by the Digital Audio Broadcasting (DAB) project in Europe, with both static and dynamic multipath distortions. The simulation results reveal that the proposed time-domain channel estimation techniques can perform better than the conventional frequency domain channel estimation technique. The proposed FPTA and TPTC techniques have advantages of low computational complexity and high P/N ratio. The simulation results also demonstrate that the TPTCTE method can improve the performance especially for time varying channel and can reduce the SER error floor. In TPTCE a novel channel estimation and compensation scheme for fast fading channel is employed for OFDM systems using time domain training sequence. Which results in a better performance for both slow fading and fast fading environments.
顯示於類別:	[資訊工程研究所] 博碩士論文

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