Space-Air-Ground FSO Transmission System Design for Reliable and Very-High-Throughput Satellite Communications




Zaghloul, Ramy

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Recent advances demonstrate satellite communications (SatComs) as a potent enabler for future Terabits/s wireless networks and bridging the digital divide. Existing SatCom systems, however, are mostly dependent on radio frequency (RF) with limited available bandwidth, which is the main bottleneck for further data rate increases. Free-space optical (FSO) communications, with huge license-free bandwidth, have emerged as a candidate alternative. Despite their ability to deliver high-throughput transmissions, FSO communications are weather-dependent and susceptible to atmospheric turbulence effects. Therefore, improving the FSO-link usability is crucial for better exploring its higher transmission rate. The goal of this research is to develop effective FSO-based SatCom solutions that can achieve very high throughput while enjoying high reliability and, as such, provide trustworthy support for future global connectivity. In this thesis, we adopt novel space-air-ground (SAG) FSO transmission approaches in the design of advanced SatCom systems for next-generation wireless networks. We first propose a reliable Terabits satellite feeder link solution. In particular, we propose a new SAG-FSO network with a strategically deployed high-altitude platform (HAP) relay to successfully remedy the atmospheric turbulence effects. We show that such a design can substantially mitigate the effects of atmospheric turbulence. Then, we integrate the proposed SAG-FSO network and hybrid single-hop (SH) FSO/RF transmission to create a SatCom feeder link with significantly improved performance and reliability. The numerical results show that the integrated transmission system achieves about 10 dB performance gain over existing solutions for both downlink and uplink scenarios. To mitigate weather effects and increase the reliability of SAG-FSO networks even further, we propose to combine SAG-FSO transmission with site diversity. With the recent technological advancements in solar cells and batteries, HAP-based relays can operate continuously for several months. A SatCom system with multiple HAP relays can enable a much more flexible design than a conventional one with multiple ground station sites. More precisely, we consider switching-based HAP relays with a hybrid SAG-FSO/RF transmission for SatCom. Our proposed system switches between HAPs based on the ground-HAP channel quality, as there are more atmospheric turbulence and weather effects. Meanwhile, the ground-HAP links corresponding to different HAP relays may experience correlated atmospheric turbulence. The obtained results illustrate that, despite the correlation adversely affecting performance, the transmission system still maintains a considerable gain over hybrid FSO/RF and single HAP systems. To increase the transmission rate for end users in a particular hot-spot area with higher traffic demand while maintaining ubiquitous coverage, we propose parallel RF and FSO transmissions to explore their complementary properties in beamwidth and bandwidth. In particular, RF transmissions serve the users over a large geographical area, while the FSO link is employed to increase the throughput to a particular hot-spot area with higher capacity demand through an access point. Independent data streams are adaptively sent over both links to satisfy capacity and availability requirements. Such a transmission strategy can effectively provide a high-speed connection to a centralized location. In addition, it can maintain ubiquitous coverage for numerous Internet-of-Things devices dispersed over a large geographical area via an RF link. We adopt the analytical system performance evaluation approach and develop efficient analytical expressions for important performance metrics for the proposed SatCom systems. Selected numerical examples and their discussions provide useful insights for engineering applications.



Free-space optics, satellite communications, hybrid FSO/RF systems, space-air-ground networks, high-altitude platforms, performance analysis