在無線通信中，利用大氣做為傳播媒介是一項最經濟實惠的傳輸材料。然而這免付費的媒體材料特性卻相當不穩定，經常會受外在多項因素的影響，例如：大氣壓力、濕度、溫度及潮差等。大氣的不規則及漲退潮的變化，造成空氣中的折射率改變及反射波的相位變動，而使無線通信中斷的機率增加。
本論文以265公里超長距離4GHz無中繼視線跨海微波鏈路做為研究素材，資料蒐集將近7個月。分析結果顯示會造成通訊中斷的因素歸納如下，大約有67.56%是由於海面漲退潮影響、62.16%與濕度、大氣壓力變化有關及59.45%受折射率、溫度改變的關係。
長距離跨海微波通信經常會發生嚴重訊號衰落而損及系統的傳輸品質與可靠度，長久以來都是以訊號分集技術來克服衰落所帶來的影響。有兩種方式可得到一個較佳之接收訊號：第一種是透過空間分集將兩個訊號的相位混合，合成一個接收訊號。第二種是經過兩階的選擇器從兩個訊號中選擇出一個較大振幅者，當做所要之接收訊號。
第一種方式已非常普遍被使用且沿用相當多年，理論上應有幾分貝的混波增益存在。然而，當直線波與反射波兩波相位相差過大時，超過混波器的處理能力之外，其合成波的振幅會不增反降。為解決此問題，本論文提出以第二種方式來克服混波所帶來的困擾，分析結果顯示系統可用率會比使用第一種方式來得高，且當可用率低於99.95%時，使用第二種方式所得到的改善效果會更顯著。

In wireless communications, the atmosphere is used as a transmission medium. However, this free open medium varies frequently due to atmospheric anomalies. Recently, many technical papers have been reported the factors such as the meteorological conditions and reflectivity or refractivity in the effects of the performance of the radio system.
In this thesis we measure the signals received from a 265 kilometers line-of-sight over sea microwave radio link operating at 4 GHz. The data collection period is about 7 months. Our analytical results show that about 67.56 percent of transmission outage is related to the tidal variance, 62.16 percent correlated with the humidity and pressure, and 59.45 percent with the refractive index and temperature.
In long distance over sea propagation microwave links, severe fading occurs frequently, which often substantially impair the quality or reliability of the transmission system. The space diversity scheme has been used to deal with this problem.
In general, two methods are used in diversity schemes. The first one is to combine the two received signals by the phase combiner, and the other is to select the larger amplitude of the two received signals. The first method has been used prevalently for a long time. Although it obtained a combining gain, the combining signal amplitudes could be possibly pulled back with a long delay times between different paths. Hence even both signal magnitudes are strong, the effect of combing are limited. In order to solve this problem, we propose a second method to deal this problem by selecting largest amplitude. Our result shows that the improvement of the system availability is better than the first method, noticeably as the availability is below 99.95%.

Abstract in Chinese i
Abstract in English ii
Acknowledgment iv
Contents v
List of Figures and Tables vii
Chapter 1 Introduction 1
Chapter 2 Basic Principles 3
2.1 Radio rays reflection 4
2.2 Atmospheric Effects and Radio Ray bending 7
2.3 Anomalous propagation due to refractive index variated 10
2.4 Fresnel Zones 17
2.5 Multipath Fading 19
2.6 Designs of Microwave Radio Systems 21
2.6.1 Calculating the free-space loss 21
2.6.2 Calculating the flat fade margin 21
2.6.3 Evaluation of the antenna gain 22
2.6.4 Evaluation of the atmospheric gases attenuation 23
2.6.4.1 Evaluation of the attenuation of dry air 23
2.6.4.2 Evaluation of the attenuation of water vapor 24
2.6.5 Space-diversity improvement in radio systems 26
2.6.6 Outage prediction for systems using space diversity 28
2.6.7 Outage prediction of the systems using the frequency diversity 31
2.6.8 Prediction of the outage of using space combining with frequency diversities scheme 31
2.6.8.1 Two - receiver construction 31
2.6.8.2 Four - receiver construction 32
Chapter 3 Path Description and Equipment Parameters 34
Chapter 4 Measurement Results and Analysis 38
4.1 Definition of measurement criteria 38
4.2 Data analysis 39
4.3 The results of space diversity structures 60
4.4 The results of different diversity structures 65
Chapter 5 Conclusions 71
References 72
Appendix 74

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