Abstract
摘要：近年來，有許多關於OFDM系統的問題值得注意，隨著多媒體使用無線通訊傳輸的需求增加，無線寬頻技術變得愈來愈重要。在眾多無線寬頻技術中，OFDM是最值得信賴且獲得廣泛的注意。然而，OFDM系統包含了一些缺點：傳送端DFT濾波器的頻帶分割能力不佳而且以1/f的衰減速率衰減。為了避免個別使用者之間的互擾，在OFDM傳送端需要昂貴的類比濾波器。甚且，另一個眾所皆知的問題是當傳送通道在頻譜上有零點時，OFDM系統會有效能飽和的問題。而這個三年計劃的目的便是嘗試解決這些問題，我們的訴求是在不變動OFDM系統的主要架構的大前提下修改一些OFDM系統。在每一年度結束前，我們會有具體的相關比較模擬。
第一年，我們將會把重心放在OFDM系統頻帶漏損的問題上，我們將不採用IEEE std. 802.11a上所提供簡單但頻譜包含能力差的raised cosine window函式，而是試著使用具有較佳的停止帶衰減的除窗式濾波，為了降低此方法的成本，我們將會探討接收端個別通道擉立運做的可行性。
第二年，我們將會把重心放在效能衰減上，並且計劃利用在前編碼的技術。不若以往的前置作業，我們將會學習
Abstract: Recently there have been considerable interests in orthogonal frequency division multiplexing (OFDM) systems. Due to the increasing demands on the transmission of vast amount of multimedia signals over wireless channel, broadband wireless communication technology has become more and more important. Among different techniques for broadband wireless transmission, OFDM is the most promising technology and it has drawn a lot of attention. However the popular OFDM systems suffer from a number of drawbacks. The frequency separation ability of the transmitting DFT filters is rather poor and their stopband attenuation decays at a rate of 1/f. To avoid causing interference to other users, costly analog filter is needed at the OFDM transmitter. Moreover it is well know that when the transmission channel has some spectral nulls, the OFDM systems suffer from the problem of performance flooring. It is the purpose of this three-year project to attempt to solve these problems. Modification techniques without changing the main structure of OFDM systems will be proposed. At the end of each year, simulations will be carried out for performance comparison.
In the first year, we will address the issue of spectral leakage of OFDM systems. Instead of the simple raised cosine function in IEEE std. 802.11a which provides little spectral containment ability, we will consider windows that have a better stopband attenuation. To reduce the cost of removing ISI caused by windowing, we will explore the possibility of channel independent processing technique at the receiver.
In the second year, we will address the issue of performance flooring. We propose to use the precoding technique.Unlike previous work, we will study nonredundant precoding technique that is suitable for the conventional popular OFDM systems. To reduce the cost of the receiver, we will employ channel independent processing technique. Moreover to further reduce the cost of transmitter, low cost precoders will be applied. If possible, we will explore the possibility of designing channel independent precoders that minimize the bit error rate (BER) and/or mean square error.
In the third year project, we will apply the spread spectrum codes to OFDM systems. The attractive spread signature codes proposed by Wornell will be considered for this application. In addition, we will use the theory of multirate signal processing to construct new spread spectrum codes.
Keyword(s)
多載波
OFDM
調變系統
無線傳輸
寬頻通訊
mutlicarrier
OFDM
wireless transmission
broadband communication
