Thesis title: Exploiting the Wireless Channel for Communications
Thesis abstract: The recent development in digital communications has made wireless networks one of the most promising areas of research. Fueled by the advances in digital processing, high data rate connections have recently become the focus of research in the communication domain. The growth of the Internet and the introduction of a multitude of applications culminated in a new era of communications in which wireless networks play a very important role. However, the wireless environment still offers some challenges that need to be addressed before meeting next generation mobile requirements. Due to imperfect channel characterization and modeling, much of the potential of the wireless environment is wasted. Furthermore, the need of multiple simultaneous connections lead to the use of multiple access schemes that were not designed to cope with some of the wideband/multiple antenna wireless characteristics. The objective of the thesis is to treat these two points. The first part of this thesis is dedicated to the use of bayesian probability theory tools that enable the derivation of models based only on partial knowledge of the environment. Using Jaynes' maximum entropy principle, we present a framework that allows us to infer on the channel characteristics by choosing probability distributions that maximize entropy under the constraints that represent the actual state of knowledge. This technique is at the heart of scientific inference and has applications in many other fields. Probabilistic channel Models for two different types of environment are derived: wideband channels and multiple-input multiple-output (MIMO) channels. In the second part, the multiple access problem for ultra wideband (UWB) systems is considered. Despite the large amount of work conducted during recent years on UWB technology, no scheme can cope with the high dispersion of UWB channels and still offer reasonable spectral efficiency. An innovative scheme that exploits the users' channels to guarantee multiple access is introduced, entitled channel division multiple access (ChDMA).