適用於移動式WiMAX系統之疊代式通道估計與載波間干擾消除技術

Abstract

正交分頻多工系統為新一代無線通訊系統最常使用的技術，如IEEE 802.11a/g/n、 IEEE 802.16、IEEE 802.20、數位電視、數位廣播等許多系統均採用此技術。傳統的正交分頻多工系統並不適用在高速移動的環境中，然而移動傳輸是未來無線通訊系統的趨勢之一，如IEEE 802.16-2005可支援到車速120 km/hour，而IEEE 802.20更可支援到車速 250 km/hour。傳統的正交分頻多工系統可以有效率地使用在非時變通道中，且僅需簡單的等化器，即可修正通道效應，但在移動的環境中，通道隨著時間改變，使得接收端的子載波失去正交性，因而導致子載波之間的相互干擾，使得解調變後的效能降低。在本論文中，吾人設計出一個疊代式通道估計與載波間干擾消除技術，其利用通道估計結果將ICI 以數學模型趨近表示之，使接收端得以有效併行地消去子載波間的干擾，而能在移動的環境下運作。吾人藉由電腦模擬驗證此一新演算法在移動的環境中並使用IEEE 802.16-2005的規格下，可有效改善位元錯誤率。

Orthogonal Frequency Division Multiplexing (OFDM) is a popular technique in modern wireless communications. There are many systems adopting the OFDM technique, such as IEEE 802.11 a/g/n, IEEE 802.16, Digital Video Broadcasting, etc. On the other hand, mobile transmission is a trend in future wireless communications. For example, IEEE 802.16-2005 supports vehicle speed up to 120 km/hour. OFDM systems can be used efficiently in time invariant environments with one-tap equalizers. However, subcarriers are no longer orthogonal to each other in time-varying channels, and this causes the intercarrier interference (ICI) and degrades the system performance. To alleviate this problem, we propose an iterative ICI mitigation and time-varying channel estimation scheme, which can obtain the ICI information from the channel estimate using piece-wise linear or Taylor series approximate and then cancel it in parallel fashion. The proposed PIC structure and the overall iterative system structure designed are discussed in detail. Finally, we evaluate the performance of the proposed system under mobility using IEEE 802.16-2005 standard and confirm that it achieves good BER performance.