在低軌道衛星通訊系統的性能分析議題上，不論是採用不同的調變系統架構或是傳輸通道模型，在許多研究文獻中均有完整的探討。然而這些研究大多著重在假設某個固定場景(scenario) 的情況下來分析系統的性能表現，而沒有著重在考慮該場景是否有可能發生，或是發生的機率為何。由於低軌道衛星的通訊鏈路的仰角及方位角會隨時間改變，且根據以往對衛星通訊通道的研究結果顯示，不同鏈路的通道衰落統計特性也會有所不同，因此若要求得整體系統的性能表現，有必要將每個通訊鏈路發生的機率以及該鏈路的通道統計特性納入考量。 而在另一方面，許多傳輸通道的模型，例如仰角相依的通道模型，也未將電離層閃爍納入考量，其主要困難的原因是電離層閃爍的現象與電波頻率、地磁擾動、太陽的活動(以黑子數多寡表示)、時間、季節以及衛星鏈路的電離層穿透點所對應的地磁緯度等因素有關，因此需要一個時變且與地面-衛星幾何關係相依的通道模型。 本論文提出了兩個低軌道衛星鏈路的統計模型，其一是只有考慮仰角分佈的仰角機率密度函數模型，其二是同時考慮仰角及方位角不同的衛星鏈路機率分佈函數模型。所得結果則是用於探討以多波束低軌道衛星為基礎的DS-CDMA 通訊系統在不同的通道環境下，如仰角相依的Rice-lognormal通道以及與仰角及方位角有關的電離層閃爍通道，系統的位元錯誤率表現，並比較單一衛星以及在星系架構下採用衛星分集系統的性能比較。 System performance analysis of low earth orbiting (LEO) satellite based communication system, with different communication schemes and channel models, have been studied by many investigators. However, these studies have all been based on a fixed scenario for evaluating the system performance. The probability of occurrence of a given scenario due to the continuously changing elevation and azimuth angles has not been taken into consideration. To do so, appropriate probability distribution functions (PDFs) have to be studied. Also, it has become more evident that ionospheric scintillation has to be taken into account in the evaluation of the performance of such global satellite communication systems. For most investigations, however, ionospheric scintillation has not been included in the proposed propagation channel models. The major difficulty lies in the fact that scintillation in the ionosphere varies according to radio frequency, day of year, time of day, ionospheric latitude and longitude, sunspot number, and magnetic activity. This requires a time-varying channel model that depends on earth-satellite geometry. In this thesis, the probability distribution function for elevation angles and for satellite links consisting of both elevation and azimuth angles are derived based on earth-satellite geometry. The results are used to evaluate the bit error rate (BER) performance for LEO satellite based direct sequence-code division multiple access (DS-CDMA) communication systems under Rice-lognormal channel, which is characterized as elevation angle dependent, and under ionospheric scintillation channel, which is time-varying and is related to both the elevation and azimuth angles. Performance improvement due to satellite diversity is also examined.
