近來，無論是在軍事領域或是民間，無人飛行器系統的應用越來越受到歡迎，如搜救、即時監控、偵查行動、交通監控以及危險區域之探勘等。
本論文的主要目的為設計一個是用於無人飛行器的通訊系統，目標為一個翼展6公尺之中型大小的無人飛機，其應用為在飛機行駛同時，將其拍攝之影像即時傳送到地面的基地台。飛機之最大時速為200公里，且需支援的最大距離為100公里，以及所需的資料傳輸速率需達每秒4百萬位元。首先，我們定義了航空通道模型與鏈路預算之分析。然後，我們呈現了一個基於正交多頻分工之通訊系統的設計流程，此系統能夠有效對抗都普勒頻率，以飛機最大時速200公里為計算達277.8赫茲，以及支援長距離100公里的傳輸。此正交多頻分工系統有512個子載波，其間距為15.6 千赫茲，且有密集分布之領航碼與達時間長度16 微秒之循環前置區段，因而能在時散與頻散之通道有良好效能。所提出的收發機架構由一根傳送天線與二根接收天線所組成，以使用最大比率合成，並且採用里德所羅門碼以及迴旋碼，以增進效能表現。時間與頻率之同步的設計也包含於此論文。最後，在使用頻寬8百萬赫茲和傳輸功率20瓦的情況下，達到了目標所需的資料傳輸速率，且能夠有約5.25分貝的系統增益。

Nowadays, unmanned aircraft systems (UAS) are becoming popular and find their places not only in military applications but also civilian ones. For example, they can be used in the search and rescue, real-time surveillance, reconnaissance operations, traffic monitoring, hazardous site inspection, range extension, and even agriculture field.
In this thesis, we aim to design a communication system for UAS. The target vehicle is a median unmanned aircraft with a wingspan of 6 m and the application is that the aircraft transmits the video filmed by the equipped camera to the ground station during the flight within a distance of 100 km. The maximum cruise speed is 200 km per hour and a data rate of 4 Mbps is required for the quality of the video. First, we define aeronautical channel models and analyze the link budget. Then, an OFDM based communication design process is presented, featuring robustness against Doppler frequency which is 277.8 Hz at the maximum speed 200 km/h of the aircraft and transmission in a long distance within 100 km. The OFDM system with the FFT size of 512 has a large subcarrier spacing 15.6 kHz and with the dense arrangement of pilots and the long duration of the cyclic prefix 16 μs, the system is robust in doubly dispersive channels. The proposed transceiver has one transmit antenna and two receive antennas which are used to perform maximal ratio combining. A concatenation of Reed Solomon (255, 239) codes and 1/2-rate convolutional codes is adopted to further enhance performance. Timing and frequency synchronization are also included. The required data rate is achieved in a bandwidth of 8 MHz, and the system margin is about 5.25 dB when a transmit power of 20 W is applied.

Chapter 1 Introduction 1
1.1 Background 1
1.2 Motivation 2
1.3 Main Contributions 2
1.4 Organization 3
Chapter 2 Overview of Air-to-Ground Communications 5
2.1 Aeronautical Data Network 6
2.2 L-DACS 6
2.2.1 Orthogonal Frequency Division Multiplexing 6
2.2.2 L-DACS1 7
2.3 Unmanned Aircraft System 8
Chapter 3 Aeronautical Channel Modeling 11
3.1 Wireless Channel 11
3.2 Link Budget 12
3.3 WSSUS Multipath Channel Model 15
3.4 Aeronautical Channel Model 17
3.5 Proposed Channel Model 22
3.6 Flying over a Ground Station 23
Chapter 4 System Design 25
4.1 System Parameter Determination 25
4.1.1 Target Performance 25
4.1.2 Parameter Evaluation 26
4.2 Pilot Arrangement and Channel Equalization 28
4.3 Antenna Diversity 33
4.3.1 Receive Diversity 33
4.3.2 STBC and SFBC 34
4.4 ECC 36
4.5 Time-varying Doppler frequency 39
4.6 Summary 40
Chapter 5 System Implementation and Simulation Results 45
5.1 System Architecture 45
5.2 Synchronization 48
5.2.1 Coarse Symbol Timing Detection 49
5.2.2 Fine Symbol Timing Detection 51
5.2.3 Carrier Frequency Offset 53
5.2.4 Integer Carrier Frequency Offset 54
5.2.6 Sampling Frequency Offset 56
5.2.5 Carrier Phase Offset 56
5.3 System Performance 57
Chapter 6 Future Works and Conclusion 59
6.1 Conclusion 59
6.2 Future Works 59
Bibliography 61
Appendix A Preamble Generation 65

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