Aircraft-to-Aircraft Communications: Channel Modeling and Data Link Design

Abstract

Aircraft-to-aircraft (A2A) communications are poised to play a crucial role in the future of air traffic. The saturation of airspace in many parts of the world is leading to the need for air traffic management systems that can operate as autonomously as possible. This autonomy can only be achieved if aircraft are able to make decisions without involving the already overburdened air traffic controllers. To do this, aircraft must be able to communicate directly with each other in order to be fully aware of the air traffic around them, resolve conflicts, and coordinate trajectories directly with each other. A2A communications are also gaining a lot of attention in other areas, such as the military, where new air combat systems require reliable communications between a large number of flying vehicles. The stringent requirements set for the new A2A communication systems imply the need for an optimal design of the data links. This is only possible if the characteristics of the radio propagation channel are known accurately and if realistic models are available to reproduce the channel faithfully. Both aspects have not been thoroughly investigated in the literature and prevent the optimal design and testing of new A2A communication systems.

In this thesis we embark on a comprehensive study of A2A channels, analyzing a wealth of channel measurements and deriving numerous statistics of the different channel components for various configurations. These configurations encompass multiple ground surfaces, including forest, fields, and lakes, different geometries between aircraft and the Earth's surface, and diverse antenna configurations. Analyzing the measured amplitude of the channel components in the different scenarios allows us to provide very valuable statistics not yet measured in A2A channels in such detail. In our quest for an accurate model and for a deeper understanding of the scattering components, we develop and validate a novel theoretical technique to characterize the scattering components with a particular emphasis on their delay/Doppler distribution, an aspect that has been largely disregarded in the literature. Building upon this foundation, a geometry-based stochastic A2A channel model is proposed. This model not only merges the information acquired from the measurements and the novel technique to characterize the scattering components, but also offers a significant advancement in the field by providing a practical tool for simulating the propagation channels in any arbitrary A2A scenario. Its practicability is demonstrated by applying it to the design of the A2A extension of the L-band Digital Aeronautical Communications System (LDACS). By using the developed channel model, we are able to determine the optimal design of the physical layer through computer simulations. Among others, we investigate the performance of orthogonal frequency-division multiplexing (OFDM) and of different forward error correction schemes, including low-density parity-check (LDPC) and polar codes used in 5G communications, in A2A scenarios.