| 摘要: | 摘要： 隨著無線通訊鏈路使用日益頻繁，頻譜之擁擠，世界各國均傾向採用Ka (26~40GHz)波段，不過此波段在地表或大氣通道傳播時，容易受到自然環境影響，其中又以降雨造成的信號衰減情況，對通訊系統的品質影響最為嚴重，而預估降雨所造成的信號衰減將為重要的課題。 降雨衰減估計，即考慮電波在大氣通道中傳播時，雨滴粒子對電波能量所造成的吸收及散射之程度，因此雨滴－即雨滴粒徑分佈便成為估計降雨衰減之重要指標。雨滴粒徑分佈對應於降雨量之研究在多年前即被提出，Exponential、Gamma、Weibull、Lognormal分佈都曾被用來做為雨滴粒徑分佈模型的依據;主要是建立適用的雨滴粒徑分佈模型，才能方便準確的使用在雨衰估計的理論中。本研究分析中壢地區之雨滴粒徑分類統計，結果發現中壢之雨滴粒徑分佈為Gamma分佈；接著我們利用2002年及2003年，共二年的中壢地區雨滴譜儀量測資料，擬合（Fitting）降雨量及雨滴粒徑分佈係數μ、σ、N0的關係，並成功的擬合出本地Gamma雨滴粒徑分佈模型，其分佈模型參數分別為：μ=0.67364?R0.19476;σ=0.23637?R0.29347;N0=829.63?R0.30689；再將此模型代入雨衰估計理論－我們假設雨滴為球形，利用邁氏散射(Mie Scattering)來計算雨滴粒子的消散係數(Extinction Cross Section)，它是一個和電波頻率，雨滴粒徑大小有關的係數，利用此法便可以得到降雨量對應於降雨衰減之關係，以達雨衰估計之目的。 在降雨衰減估計的研究中，Crane. R. K.提供了降雨衰減模型(1978)，而國際電信聯盟也提供雨衰模型(ITU-R P838 1992) ，本文最後，分別以本實驗室之Ka波段地面鏈路量測系統之降雨衰減實驗量測資料、Crane 雨衰模型、ITU-R P838 雨衰模型及利用本地Gamma雨滴粒徑分佈模型計算降雨衰減結果做比較分析，分別分析在年際性及季節性不同下，其雨滴粒徑分佈對應於降雨衰減之影響；比較結果可証明本地Gamma雨滴粒徑分佈模型所估算的降雨衰減值較為正確，而Crane 雨衰模型則有明顯高估、ITU-R P838 雨衰模型則有明顯低估的現象。 Abstract: The rapid growth in demand for additional communication capacity has put pressure on engineers to develop microwave systems operating at higher frequencies. Many people tend to choice the Ka-band (26-40 GHz) for the applications. It turns out, however, that frequencies above 10 GHz attenuation caused by atmospheric particles can reduce the reliability and performance of radar and space communication links. Such particles include oxygen, ice crystals, rain ,fog, and snow. Prediction of the effect of these precipitates on the performance of a system becomes important. In this study, we will discussed the prediction of attenuation of an EM wave propagating through rain drops. The prediction of attenuation due to rain is considered the EM wave propagating through rain drops, the energy of the EMW will be absorbed and scattered by rain drops, then the raindrop-size distribution (DSD) is the most part in the prediction of attenuation due to rain. Many raindrop-size distribution have been reported. Exponential、Gamma 、Weibull、Lognormal distribution have been use to be the raindrop-size model. Because use the suitable raindrop-size distribution model, we can calculate the rain attenuation more accurately. In this study, we analysis the raindrop-size distribution in Chung-li, then we find the raindrop-size distribution in Chung-li follow the Gamma distribution; then a Gamma raindrop-size distribution is proposed by fitting the measurements of rainfall observed using a distrometer in Chung-li, the three parameter of raindrop-size is μ=0.67364?R0.19476;σ=0.23637?R0.29347;N0=829.63?R0.30689. Then we use Gamma raindrop-size distribution model to predict the rain attenuation due to rain. We will assume that rain drops are spherical and the scattering mechiasm follows the Mie scattering theory . In the passing research of rain attenuation prediction, Crane. R. K. proposed the “Global (Crane) Attenuation Model (1978)”and ITU proposed the “ITU-R P838”rain attenuation prediction methods, finally, we comparison of the rain attenuation measurements result using our “Ka band (28GHz) terrestrial link propagation system” and the prediction result from Chung-li raindrop-size distribution model and and Crane Global (Crane) Attenuation Model and ITU-R 838 rain attenuation prediction methods, then we find the prediction from our raindrop-size distribution have very good prediction, but Crane model have higher prediction and ITU-R model have lower prediction. |
