1IntroductionThe problem of determining the location of a person or an object is an ancient one.Many different methods were employed over the recent centuries. For example, thenavigation of ships has been supported by referencing to the celestial map of stars,lighthouses or even by transportable devices known as sextants. More recently,tracking the location of vehicles is primarily done with the support of satellite basedsystems. The first deployed of these systems is the well known GPS system. Fromair planes over ships to cars, nearly every modern vehicle today is able to determineits position with an accuracy down to a few meters. But the need for localizationsolutions is not just confined to vehicles of transportation services. For example,due to the now ubiquitous availability of powerful mobile computing devices, therealization of personalized context- and location-aware applications has become anactive field of research. But the natural habitats of human individuals, the indoorenvironments, are dark zones for the signals of the GPS satellites.The lack of a comparable efficient indoor localization method motivates the researchactivities into alternative localization systems that are specifically adapted to theseenvironments. Therefore, indoor localization solutions have been based on variousinformation sources that reflect the constraints of the different use-cases. Whereas ahypothesized domestic robot can be specifically designed to carry multiple sensors asoptical cameras, ultra-sound or infra-red devices, this degree of freedom is not givenfor the localization of human beings. There, the sensors need also to be unobtrusivewhich can be ensured by sensing signals of communication networks.This thesis will focus on the signals of IEEE 802.11 wireless networks as the pri-mary source of information to approach the localization problem. The importantadvantage of Wi-Fi, in contrast to other technologies, is the inexpensive hardwareand the already dense deployment of Wi-Fi Access Points(APs) in urban areas. Forexample, at the RWTH Aachen University it is most likely to be in range of at least 5APs across the campus side1. Widespread interest into these signal-strength based1Although it has to be noted, that RWTH is an university with a strong technical background,and thus probably a site with a high saturation of APs. But by interpolating the history it canalso be expected that the density of deployed Wi-Fi infrastructure still increases.