直接序列超寬頻系統於密集路徑通道下之同步與等化技術之研究

Abstract

超寬頻技術近年來於高速室內無線通訊受到極大的矚目，此乃因為超寬頻系統的傳輸資料率在數公尺的距離下能達到數百Mbps。在這類技術當中，使用直接序列展頻是最被廣泛熟知的技術來符合超寬頻信號的規定。在本論文中，吾人提出一種具有增強型先-遲碼追蹤迴路(early-late code tracking loop)與選擇性前置犁耙分集結合(S-Pre-Rake diversity combining)能力之IEEE 802.15.3a 直接序列超寬頻(DS-UWB)系統。由於超寬頻系統所傳送的極短脈衝可解析出許多的路徑，因此通道響應將會由密集的多路徑所組成，並具有長延遲擴散(long delay spread)的特性。傳統的先-遲碼追蹤迴路的效能會因為緊密間隔的多路徑符元間干擾而大幅下降，為了克服此一問題，吾人可採用增強型先-遲碼追蹤迴路有效地消除干擾。另一方面，接收信號能量會散佈於長延遲擴散的多路徑上並造成嚴重的符元間干擾，雖然使用一長階數等化器可有效地還原通道所造成之效應，但其複雜度卻會太高。吾人所提出之傳送端選擇性前置犁耙分集結合器與接收端簡化等化器於之共同設計可大幅降低等化器複雜度並同時達成分集結合和符元間干擾消除之目的。最後，吾人藉由電腦模擬驗證上述架構在室內超寬頻環境中，具有可靠的傳輸效能。

Ultrawideband (UWB) technologies have attracted much attention in high speed indoor wireless communications recently since the transmission data rate of UWB can achieve hundreds of megabits per second over distances of several meters. Among them, direct sequence spread spectrum (DSSS) is the most well-known approach to meeting regulations of UWB signal. In this thesis, an IEEE 802.15.3a DS-UWB system with enhanced early-late (EL) code tracking loop and selective-Pre-Rake (S-Pre-Rake) diversity combining schemes is proposed. UWB system can resolve many paths due to the ultra short transmitted pulses; hence the channel response will be composed of dense multipaths and have a long delay spread. The performance of conventional EL code-tracking loop is typically significantly degraded due to the closely spaced multipath intersymbol interference (ISI). To mitigate this problem, the enhanced EL code tracking scheme is adopted to effectively remove the interference. On the other hand, the received energy is distributed over long delay spread multipaths which will induce serious ISI effect. Although an equalizer with a large number taps can restore the channel effect, its complexity may be probability high. The proposed joint design of S-Pre-Rake combiner at the transmitter and simplified equalizer at the receiver can greatly alleviate the equalizer complexity and achieve both diversity combining and ISI elimination simultaneously. We evaluate the performance of the proposed system and confirm that it works reliably in indoor UWB environments.