Aerial-terrestrial communications for next generation mobile and wireless networks
Creators: name: HEWA RANCHAGODA, Nirmani
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400608
subject: Wireless communication systems and technologies (incl. microwave and millimetrewave)
classificationCode: 400899
subject: Electrical engineering not elsewhere classified

description: Hundred years ago, wireless communication was only used for radio communication, but today the number of wirelessly connected devices has exceeded the global population. On average, one person has at least three wireless connected devices. Wireless connection has provided us with the ability to communicate with the whole world, and now we cannot imagine a time without it. Early wireless communication was mainly related to cellular systems. However, currently, mobile networks are having a difficult time fulfilling the demand for data rates.
 
Moreover, during emergencies and temporary international events, fixed cellular systems are inadequate to provide the required coverage. Therefore researchers around the world are working towards next-generation networks where all these demands for capacity and rates can be met. Air-to-Ground communication plays a unique role in Fifth Generation (5G) communication where they can be used as aerial base stations or relays to provide coverage during emergencies or large scale international events. Air-to-ground communication takes place between an aerial platform and a ground user. These aerial platforms can be manned/unmanned aerial vehicles or balloons, and ground users can be cellular towers or mobile users. The channel between them is known as the air-to-ground channel. Established understanding of the channel is essential before using it for any of the future applications.
 
In this thesis, we focus on modelling and performance analysis of the air-to-ground channel. We devote the first part of this research to understand and model the air-to- ground channel characteristics. The air-to-ground channel is different from the terrestrial channel due to the high probability of line-of-sight. Furthermore, the altitude or the elevation angle act as an additional parameter in air-to-ground communication. Since the platform is in the sky, the environmental structures such as buildings have a substantial impact on the channel. Besides, ground reflected component becomes stronger compared to all other multi-path components and hence the ground matters here than in terrestrial channels. In our first contribution, we propose a Gaussian mixture model to characterise urban air-to-ground channel and reveal how the mean building height can affect the received signal. We further analyse the line-of-sight and non-line-of-sight components in such environments. For the environments with fewer scatterers, we propose an elevation angle based two-ray ground reflection mean path loss model in our second contribution. We further reveal how outages can occur at specific elevation angles regardless of the aerial platform altitude. In many of the works mentioned in this research we verify results via ray-tracing or Monte-Carlo simulations.

The second part of this research focuses on improving coverage in air-to-ground networks using diversity approaches. Air-to-ground literature suggests many coverage improvement approaches such as beam-forming, power control, coordinated and controlled transmission, placing aerial platform in an optimal location etc. However, the use of diversity approaches is minimal in the literature. In diversity approaches, multiple signals that contain signal information are used jointly to achieve a better signal at the receiver. In the air-to-ground channels that follow the two-ray model, we show that frequency and polarisation diversity can be used to improve coverage.

In the final part of the thesis, we analyse the interference in air-to-ground networks. Interference in air-to-ground networks is caused by other neighbouring cochannel aerial base stations that are not connected to the user. We use stochastic geometric approaches that allow demonstrating the spatial randomness of the nodes to analyse the interference. We randomly deploy the drone base stations in the air while maintaining a constrained distance between each other. We investigate how this constrained distance, drone base station density and path loss model can impact the results. We further analyse how diversity combining techniques can be used to improve coverage in such instances.

The results presented in the thesis have been mostly published, partly submitted for publication in top venues, or currently undergoing a review process. These publications are highlighted and identified as contributions in each chapter of the thesis. Finally, we wish the readers will enjoy and the work in this thesis will help in the advancement of air-to-ground communication in next generation networks.