在正交分頻多工系統上之載波頻率偏移的估計

Abstract

正交分頻多工系統在現在的有線及無線通訊系統中都很常被使用到，它主要的概念就是將一個大的頻寬分成很多個小的頻寬，利用其正交性來克服干擾。但這樣的系統對於載波頻率偏移相當的敏感，因此在正交分頻多工系統當中，如果正確的估測出載波頻率偏移是一個非常重要的問題所在。 在本篇論文當中我們將會提出一個估測載波頻率偏移的方法，首先在傳送端我們將會使用虛擬載波來幫助我們推導出要最小化的函數，接著我們會將問題轉換成尋找最大公因式的問題，並且使用子空間演算法來求解最大公因式，然後求得載波頻率偏移的估測值，最後我們會把效能跟直接最小化目標函數所得到的效能來作一個比較。

The OFDM system has been a widely adopted technique for both wired and wireless transmission. It is well-known that one of the most important problem for the OFDM system is the carrier frequency mismatch between the transmitter and the receiver. The frequency mismatch is always modeled as a CFO(carrier frequency offset) effect. Therefore how to estimate the CFO correctly is one of the most important problems for the OFDM systems. In this thesis, we will use a semiblind method for estimating the CFO in the OFDM systems. At the transmitter, some tones of the transmitted signal before applied to the normalized IDFT block are chosen to be the virtual carriers. The method is semiblind because we only need to know the locations of these virtual carriers but no need to know what modulation scheme that the transmitted data uses. Using these virtual carriers, we can get our cost function. By minimizing the cost function, the CFO can be estimated. We convert the problem of minimizing the cost function into the problem of finding the GCD(greatest common divisor) polynomial. The method we use for finding the GCD polynomial is the subspace method. Finally the performance of the proposed method will be compared with the method that minimizes the cost function directly.