The Common datalink(CDL) is a communication system that transmits large capacity (up to 274Mbps) image and intelligence signal over long distance (more than 200km) air-ground environment. As a result of analyzing the air-ground communication system operating in an environment similar to the Common datalink where the narrow-beam ground station antenna is located in the open space, the curved earth 2-Ray channel model is suitable for the long distance air-ground environment.
The curved-earth 2-Ray channel model is used as a path loss model for analyzing the change of the received signal power of the narrow band in the link budget because the delay time of the ground reflection wave is very short. On the other hand, since the symbol duration of the common datalink is very short as several nanosecond, a performance analysis method for a wideband 2-Ray channel is required.
This paper distinguishes between the flat fading that changes the amplitude of the original symbol and the frequency selective fading that causes inter-symbol interference, using the characteristics of the raised cosine filter combined with the pulse shaping filter of the transmitter and matched filter of the receiver. We propose a mathematical analysis method for analyzing the probability that the flat and frequency selective fading cause symbol error in M-PSK of the common datalink and confirm that the proposed mathematical analysis method is valid through simulation test.
As a result of analyzing the communication distance in the operation environment of the common datalink using the proposed curved earth 2-Ray reception performance analysis method, it is confirmed that the communication distance is limited due to the ground reflection wave in the long distance operation. We confirmed that the communication distance analyzed at each operating altitude matches the actual communication distance of various UAVs currently in operation.
Finally, this paper proposes a method to control the ground station receiving antenna upwards to solve the problem that communication distance is limited because the effect of ground reflection wave increases when the common datalink is operated at long distance. The proposed antenna control method can increase the communication distance by more than 50km without increasing the operating altitude of the aircraft and without adding a channel compensation circuit to the communication equipment to eliminate wideband interference of the ground reflection wave.