隨著電信服務需求的持續增加，愈來愈多的無線通訊系統設計者考慮使用Ka波段，因為它擁有寬頻與高傳輸率等特性，也因此地面多點分配系統（Local Multipoint Distribution System ,LMDS）在陸地微波站的應用日趨重要。然而降雨衰減是造成此波段通訊系統效能退化的最主要成因，因為其信號會受豪雨吸收與散射而衰減，而且位處亞熱帶之台灣地區的年平均降雨量很大，故有必要對此波段在LMDS中受雨衰影響下的電波特性作分析研究。 本論文乃考慮在典型的細胞規劃環境下，使用美國Hughes Network Systems公司所提供的LMDS系統規格，並且引用Crane全球降雨衰減修正模型以及國際電信聯合會提供之最新降雨衰減模型 (ITU-R 530-8)，來預估此系統在台灣地區其通道受降雨衰減影響下的累積分佈特性。另外，再藉由動態頻寬配置的基本概念，將動態調變技術 (Dynamic Modulation Scheme) 應用在此系統中以求得最大的頻寬效益 (Bandwidth Efficiency)。 本論文將提供完整的分析通道降雨衰減程序之運作流程，並針對通道使用效能有詳盡的討論，並進行電腦模擬作效能的評估。 Regulatory agencies from several nations are currently licensing or allocating significant blocks of spectrums near 28 GHz and 40GHz for use by terrestrial wireless broadband access systems such as LMDS (Local Multipoint Distribution Systems) which supported include bandwidth intensive video distribution, data services such as high-speed internet and several emerging interactive multimedia communications. However, attenuation due to rainfall can severely degrade the radio wave propagation at frequencies above 10 GHz. Since the subtropical climate in Taiwan is characterized as high intensity rainfall, the knowledge of the rain attenuation at the frequency of operation is extremely required for the design of a reliable communication system at a particular location. The thesis, based on a typical LMDS architecture and Hughes Network Systems’ specification for the system, uses modified Crane model as well as ITU-R 530-8 model to predict the channel characteristics in terms of cumulative distribution function and further studies the channel utilization of the system by making use of the DMS (Dynamic modulation Scheme) technique to achieve the best bandwidth efficiency.
