正交分頻多工通信之通道估測技術研究及效能分析

Abstract

本論文主要利用均方差及運算複雜度,來比較慢衰減性通道中,不同的通道估測技術如線性,二次方,三次木條曲線內插等等。考慮實用性,我們研究了這些技術對即將到來的802.16a無線都會區域網路的適用性外,也建議其中較為合適的一個。 至於動態性通道,我們知道因為通道的時變破壞正交分頻多工(Orthogonal Frequency Division Multiplexing, OFDM) 系統的正交性。而與都普勒頻率成正比的載波間干擾(ICI)將增加位元錯誤率地平(BER floor)。為了減輕這個效應的影響,我們檢視快衰減性效應外,也介紹了幾個基於假設通道衝脈響應在一個正交分頻多工符元的時間內為線性變化的方法。同樣的,為了實用性,我們拿802.16e (802.16a的行動版本)來做評估的例子。從討論和模擬看來,我們發現,現存的方法因為導引信號分配結構, 資料量速率的要求, 硬體複雜度等考慮, 並不適合802.16e使用。最後,基於一個簡單的想法,我們提出了一個針對時變通道且假設等距導引信號分配結構的估測方法。

In this thesis, we focus on the Mean Square Error (MSE) performance and computation complexity comparisons of various channel estimation techniques, like linear, second order, cubic spline interpolations and etc. in slowly fading channel. For practicality, we investigate their applicability to channel estimation of the new coming 802.16a Wireless Metropolitan Area Network (WMAN), and suggest feasible ones. As for mobile channel, we find that time-varying channels destroy inter-carrier orthogonality in OFDM (Orthogonal Frequency Division Multiplexing) transmission. The introduced inter-carrier interference (ICI) increases BER error floor in proportion to Doppler frequency. For mitigating this effect, we survey the fast fading effect and introduce several approaches based on the assumption that the channel impulse response varies linearly during an OFDM symbol. Similarly, for practical consideration, we consider 802.16e (the mobile vision of 802.16a) for evaluation example. From discussions and simulations, we find that they are not suitable for 802.16e for the reasons of pilot allocation scheme, data rate, and hardware complexity. Finally, based on a simple idea, we propose one approach for time-variant channel estimation assuming equally spaced pilot allocation scheme.