| 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。本研究將嘗試建立一個能較適用於台灣北部地區的Ka波段降雨衰減模式 許多研究文獻中指出，雨滴粒徑分佈模型(DSD)是估計降雨衰減的最重要因子之一。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，但我們發現此些模型在台灣的適用性上均較不足。因此，本文中利用位於中壢及安坑的兩部二維光學式雨滴譜儀進行長期的降雨粒徑觀測，並利用兩年(2002~2003)的資料進行統計分析。我們先將觀測資料分為不同季節及不同降雨率(R<5mm/hr, R=5~10mm/hr, R=10~20mm/hr, R=20~40mm/hr, R>40mm/hr)統計其機率分佈函數(PDF)，再將其PDF與已知之統計分佈：Gamma、Lognormal及Weibull分佈做相似性比較。經過RMS的分析之後發現使用Gamma分佈做為基礎模型最適用於本地區。於是再將其所需之三個參數：平均值μ，標準差σ，單位體積內之雨滴顆粒數N0，根據Power-Law的型式對觀測進行擬合(curve fit)。結果發現：μ=0.6736R0.1947，σ=0.2364R0.2935，N0=58.992R0.274。此即為之後估算降雨衰減時之DSD模型。 根據理論，降雨造成之衰減可視為天線波束內之所有雨滴粒子的消散係數(extinction coefficient)總合。本文中利用兩種方法計算消散係數：第一，假設雨滴為球型並利用米式散射近似(Mie scattering approximation)估算。第二，利用T-Matrix方法計算。最後結果發現利用米式散射近似計算的降雨衰減估計較接近觀測衰減值，利用T-Matrix計算的降雨衰減估計則誤差較大。 為了測試本研究的降雨衰減模型，我們在中壢中央大學內架射了一組Ka波段信號量測系統，做為量測實際降雨衰減值之用。比較後發現，本文中所提出的降雨衰減模型相當符合觀測結果。為了對照，我們亦與其它兩個廣泛使用的Crane及ITU-R降雨衰減模型比較。結果發現，本文提出的模式仍然與觀測值有最好的相符結果。Crane及ITU-R模型則會較為高估衰減，表示此兩模型較不適用於本地區。 文末，我們亦對於降雨模型的年際性及季節性差異做了評估。最後並對未來的研究提出一些展望。 As the communication services are increasingly demanding more access for higher frequencies up to Ka-band and beyond, a model to predict the propagation through rain is required in order to estimate the link budget and the communication performance. The rain drop size distribution (DSD) is the most important parameter in the rain attenuation prediction model. In this paper, we establish the DSD model from measurements, followed by presenting a rain attenuation model using the DSD model. A two year observation (2002-2003) of rain drop size distribution using two two-dimensional optical distrometers at different location were recorded. The DSD were measured and analyzed for different seasons under various rain rates. The variability of DSD in both space and time was clearly shown even in the not so large area of North Taiwan. It follows that a relationship between rain rate and DSD was established. By applying statistical regression, it was also found that, in most cases, the DSD follows the Gamma distribution best. By applying three parameters (μ:mean, σ:standard deviation, : drop numbers per unit volume) into the Gamma model, the DSD model can be established. Long-term rain attenuation measurements using a Ka band (28GHz) CW system at vertical polarization were conducted in northern Taiwan. An optical rain gauge, which has resolution of 0.01mm and can collect the rain rate every 5 seconds, measured the rain rate at the same location as the Ka band CW system. The attenuation due to the rain can be estimated by calculating the extinction coefficient over all of the rain drops within the antenna beam volume. Two methods were used to estimate the extinction coefficient. First, assuming that the scattering mechanism follows the Mie scattering approximation, and the rain drops are all sphere. Second, using T-Matrix method and the oblateness of the rain drops that vary from 1.0 to 0.8. Making use of the DSD model, a semi-empirical rain attenuation model was then developed. To validate the model, we compared it with the measured data. We further compared the results to the ITU-R and Crane models, and the comparisons show that the proposed model matched very well with in-situ measurement from a two-year data set. Both the Crane model and the ITU-R model were inadequate, as expected, for a correct interpretation of the accumulated measurement data producing overestimates. |
