應用離散弦波轉換對正交分頻多工系統之 通道估測研究

Studies of Discrete Sinusoidal Transform in The Channel Estimation for OFDM System

柯鴻均(Hung-Chung Ke)

指導教授 ： 許超雲

大同大學/電機資訊學院/通訊工程研究所/碩士(2008年)

摘要

在無線通訊系統裡，衰減是一個影響訊號傳送的主要因素，在高速行動傳輸系統中衰減補償是首先要解決的問題。
將正交分頻多工系統的部份子載波中加上前導碼(pilot)，再輔以內插方式可以有效地得到通道估測值，這個機制提供有效解決訊號衰減所造成的問題。它藉著這些通道估測值，有效的找回經過通道衰減的信號。目前常用的內插方法有DFT、DCT等方法，本論文模擬以802.11a所傳送的訊號，經不同之調變方式(BPSK、 QPSK、 16 QAM)，及通過不同衰減參數(AWGN、Doppler frequency、Jakes model)下，在接收端可透過前導碼得到不同的頻率響應，我們使用所得到的頻率響應可以去除訊號衰減的影響並正確估測原始訊號。從上述中，使用不同的通道估測方法而得到誤碼結果，得知DFT-base比混合型 DCT-base 通道估測方法好。為何要使用混合型?因為當使用DCT 第二型做通道估測時，發現放大倍率(up-sampling factor)為偶數時，前導碼(pilot)的值會跑掉而奇數時則不會，因此吾人使用DCT第二型做正轉換(forward transform)後，再採用DCT第一型做逆轉換(inverse transform)以便解決放大倍率(up-sampling factor)為偶數所導致前導碼(pilot)資料不正確的問題。

關鍵字

通道估測

並列摘要

In wireless communications system, fading is one of the main problems to affect the transmitting signal, hence the fading compensation is the most important issue to be solved especially in high-speed mobile communication.
We can place pilot in some sub-carriers of OFDM system to get estimation signals by using interpolation. It provides good effect to recover the destructed signals to the original ones. DFT-base and DCT-base methods are popular interpolation for channel estimation. In this thesis, we use different modulations (BPSK, QPSK, 16 QAM) to transmit OFDM symbol according specification of IEEE 802.11a. The OFDM symbol pass through non-ideal channel (with AWGN and Rayleigh fading) and we can get frequency response by extracting pilots. We use the frequency response to estimate the fading effect and remove the estimated fading effect then the signals will be recovered.
From above mention, we use different interpolation and get DFT-base method’s symbol error rate is better than hybrid DCT-base method. Why do we adopt hybrid DCT-base method? When we perform DCT type II to do interpolation, the even up-sampling factor would not recover the original pilot value but the odd up-sampling factor can recover it. We propose forward transform by DCT type II, inverse transform by DCT Type I to overcome this problem.

並列關鍵字

OFDM ； DFT ； DCT ； channel estimation ； 802.11a

參考文獻

[3] S. Haykin and M. Moher, Modern Wireless Communications, Prentice Hall, 2005.

[5] T. S. Rappaport, Wireless communications, Prentice Hall Press, 1996.

[6] Y. R. Zheng and C. Xiao, “Improved models for the generation of multiple uncorrelated Rayleigh fading waveforms,” IEEE Communications Letters, vol. 6, pp.256-258, June 2002.

[7] B. Sklar and E. Okamoto, “Rayleigh fading channels in mobile digital communication systems .I. Characterization,” IEEE Communications Magazine, vol. 35, pp.90-100, July 1997.

[8] B. Sklar and E. Okamoto, “Rayleigh fading channels in mobile digital communication systems .II. Characterization,” IEEE Communications Magazine, vol. 35, pp.148-155, September 1997.