適用於時變正交分頻多工系統之干擾消除技術

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 802.16e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。雖然最小均方差等化器可以改善此種干擾問題，然而其複雜度過高，不利於正交分頻多工系統採用。在本論文中，吾人利用移動傳輸通道的特性對共軛梯度法做最佳事先處理，來降低最小均方差等化器所需反矩陣運算的複雜度，使得接收端得以有效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等化器在移動的環境中，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, IEEE 802.20, Digital Video Broadcasting, Digital Audio Broadcasting, etc. On the other hand, mobile transmission is a trend in future wireless communications. For example, IEEE 802.16e supports vehicle speed up to 120 km/hour, and IEEE 802.20 supports vehicle speed up to 250 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. Although the minimum mean square error (MMSE) equalizer can be employed to solve this problem, it requires a high complexity and is impractical for an OFDM receiver to be implemented. To alleviate this problem, we propose a conjugate gradient based method with optimal precondition by employing the property of the mobile channel, which can reduce the complexity of the matrix inversion problem in the MMSE equalizer. Finally, we evaluate the performance of the proposed system and confirm that it achieves good BER performance in mobile environments.