Transmitter and Receiver Windowing for OFDM Systems

Abstract

正交分頻多工(OFDM)系統的應用近來引起相當廣泛的興趣。在傳統OFDM系統的傳送與接收器使用的是方形窗，所以信號的頻譜在截止頻帶擁有較高能量。對收發端信號加非方型窗是一種減低其能量的做法。在此篇論文中，我們將設計兩類傳送窗，非重疊型窗與重疊型窗，使得傳送訊號在發送端的截止頻帶擁有最小的能量。非重疊窗使用非方型窗，以及後極處理矩陣，維持系統的ISI-free的特性。在此種情況，些微的訊號雜訊比將被犧牲以換取截止頻帶能量的改善。在另一類傳送窗，重疊型窗則是使用較多的循環延伸，以緩和窗的邊緣變化，因此，我們將犧牲一些資料傳輸率以換取截止頻帶能量的改善。在接收端框化，也是改善系統能的一種方法，我們將發現階梯狀的窗可使得接收到的AWGN能量最小，而且框化的接收器將對頻率偏移會較慢造成效能飽和。

There is a great interest in applications of OFDM recently. The conventional OFDM system uses rectangular windows at transceivers, so the spectrum has strong and slowly decreasing side-lobes. Windowing is a method to reduce the side-lobes. In this thesis, two types of transmitting windowing, non-overlapping and overlapping windowing, are designed to minimize the guard band energy of transmitter output. The non-overlapping windowing bases on non-rectangular function and a corresponding post-processing matrix is needed for restoring the ISI-free system. In this case, SNR loss has to be traded for the out-of-band improvement. In the other case, the overlapping windowing uses longer cyclic extension to smoothen the transition parts of windows. So we trade some data rate for the out-of-band improvement. Receiving windowing can also use the redundant samples not affected by channel to improve the reception of system. We will see that stair-case window is optimal for minimizing the received AWGN power. We will also see that the inclusion of a receiver window leads to a lower BER floor in the presence of frequency offset.