為了應付OFDM於時變通道中所造成之通道效應，通道追蹤已成為重要之探討課題。通常會有兩種方法來解決此通道效應之問題，第一種方法將於傳輸訊號中插入領航訊號(Pilot)，不過如此將造成頻寬效率下降。第二種方法為利用Preamble加上決策迴授機制的方法，於頻寬效率上是優於第一種方式，不過第二種方法的缺點為，在兩個連續的Payloads之中還是需要插入一個Preamble，假若我們的Frame長度太小，同樣會降低頻寬效率。因此，我們提出半盲目通道追蹤法來提升頻寬效率。傳統的通道追蹤是利用決策迴授機制的方法，不過此法最大的缺點是會有錯誤傳播情形發生，造成系統性能降低。因此於本論文中，我們提出半盲目通道追蹤法並結合兩種不同的切換機制與傳統的通道追蹤方法利用BER(Bit Error Rate)於性能上做一比較，從模擬的結果中可得知，我們所提出的通道追蹤方法可得到較佳之性能。 To deal with the channel effect in an OFDM time-varying channel, channel tracking is an important issue. We can have two methods to solve the channel effect problem. First, we use pilots to be inserted in transmitted signals, which significantly reduce the bandwidth efficiency. Second, we use a preamble along with decision feedback methods, which has better bandwidth efficiency than the first method. However, the drawback of the latter method is the requirement of a preamble between two successive data payloads. If the length of the frame is small, the bandwidth efficiency is also restricted. Therefore, we propose a semi-blind channel tracking method to improve the bandwidth efficiency. It is well-known that the decision feedback channel tracking method has the error propagation problem which can degrade the system performance. In contrast, the semi-blind channel tracking method applies a decision error criterion to change the use of the error control signal without the error propagation problem. Simulation results show that the proposed semi-blind channel tracking scheme has better BER performance than the decision feedback method.
