Abstract
在一無線行動分時多工(TDMA)通訊系統中，我們必須在接收端設計一等化 器來抵抗因多重路徑傳輸所造成的碼際干擾(ISI)及快速的時變特性，以 維持通訊的品質；在本論文中，我們將採用決策回授式的等化器架構，利 用以區塊處理為基礎的適應性濾波演算法，以美國蜂巢式電話通訊系統( IS-54)為例子，來驗証我們所提出的雙向外插等化技術，雙向外插等化技 術在等化過程中分為二個步驟，在通道嚴重衰頹之前，由適應性演算法來 更新決策回授式等化器中的係數，而在通道嚴重衰頹時，利用其線性變化 的特性，以線性外插的方式來預測所對應的濾波器係數，和傳統的雙向等 化技術相比，此方法所需的運算複雜度較低，且在快速衰頹(車速大於 60 Km/hour)的通道中其位元錯誤率(BER)也較低，並遠遠超過IS-54所需的要 求，如果只為了滿足IS-54的規格，我們又提出以雙向外插為基礎的固定 雙向外插等化技術，其雖無法預測何時產生嚴重衰頹，但具有運算複雜度 固定且低的優點。 Equalization is one of the most important parts in a time division multipleaccess (TDMA) wireless mobile communication system. Its purpose is to combatintersymbol interference caused by multipath fading channel during thetransmission process. In this thesis, we present a new equalization approach,called bi-directional linear extrapolation equalization (BDLEE) for thedown-link transmission in a TDMA-based mobile communication. Unlike theconventional bi- directional equalization (BDE) technique which updates theequalizer's coefficients based on a fixed adaptive algorithm throughput theinterval of data symbols, the BDLEE technique adopts two different strategiesfor updating the equalizer's coefficients; the coefficients are determinedbased on an adaptive algorithm before the channel fading exceeds a specifiedlevel, and are determined by a simple extrapolation scheme after that. It isclear that a BDLEE-based equalizer requires less computational complexity andless power than a BDE-based equalizer, especially for applications where acomplicated adaptive algorithm is required. This is an attractive feature forthe mobile station design of a TDMA-based wireless system. To demonstrate theperformance of the proposed approach, computer simulations based on a blockadaptive decision-feedback equalization scheme are performed for the NorthAmerican IS-54 (or IS-136) digital cellular system under the worst casetwo-ray Rayleigh fading channel. The simulation results show that the BDLEEtechnique reaches better bit error rate (BER) performance than the BDEtechnique when the speed of the mobile station is more than 60 Km/hour; whenthe speed lower than 60Km/hour, both systems have comparable BER performance.Besides, the performance exceeds the requirements of the IS-54 system greatly.To further reduce the computational complexity, the fixed bi- directionallinear extrapolation equalization (FBDLEE) which comes from BDLEE has fixedcomplexity but worse performance. If we just want to achieve the requirementof IS-54, we can select the suitable parameters of FBDLEE by computersimulations to achieve the BER specification of IS-54. Besides IS-54 system, these two proposed equalization methods can also be applied to all TDMA-basedcommunications.