Abstract
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.