Adaptive and Intelligent Software-Radio Design for Robust Wireless Communications in Challenging Environments
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The proliferation of wireless devices and the communication networks that enable them has created profound societal impact in recent years. As engineers and researchers look to build more capable and reliable communication systems, many questions remain open, such as how to best utilize the limited radio spectrum and how to maximize the efficiency of wireless devices and networks. While terrestrial communication capabilities continue to expand, even more ambitious frontiers remain to be explored towards realizing the vision of connecting humans, machines, and the world's information across land, air, space, and sea. Indeed, a great many transformative applications can be made possible by extending wireless network capabilities now commonplace in terrestrial systems to the oceans, skies, and cosmos. The wealth of information gathered can be made available in real-time and connectivity of people and devices can be enabled anytime, anywhere. However, legacy technologies and systems are not able to adequately address the challenges related to the unique characteristics of a wireless channel in these environments and the dynamic nature of its conditions. At the same time, increased system complexity makes effective control and decision making by a human operator almost impossible. To enable robust communications, it is imperative that wireless devices are aware of and able to adapt to changing conditions. Such decisions must occur autonomously, without human intervention, in order to adequately respond to time-varying radio environments. The potentially large performance gains realized by autonomous and intelligent adaptation of system parameters are key for the efficiency and resilience of the wireless networks of the future. This dissertation presents adaptive and intelligent techniques across the physical, medium access control, and network layers that enable cognitive, reliable, and efficient communications in challenging environments. Our implementation platform is software-defined radio. Spectrum access and cross-layer optimization in the presence of multi-user interference are key challenges. Theoretical developments for airborne communications are evaluated through extensive simulation studies and field testing of a hybrid ground-air cognitive radio network. Adaptive techniques to overcome the challenges of underwater acoustic communications are proposed and validated through simulation. Promising directions for agile and efficient space communications are investigated. Intelligence, autonomy, and adaptation are characteristics common to all systems and algorithms proposed.