Recent years have witnessed the emergence of novel application paradigms such as the Wireless Sensor Network and Context Aware computing. Among the challenges posed by these applications, localization—i.e. the process of locating people and/or devices—has emerged as a key problem that has found only partial answers. Although GPS receivers are common on many consumer electronic devices, alternative solutions are needed when locating devices that strive to be small and inexpensive, as in sensor networks, or when supporting indoor positioning. This dissertation focuses on radio-based positioning schemes suitable for applications where GPS is not a viable solution.

The first part of this work addresses schemes that use proximity constraints inferred from radio connectivity. A novel solution based on the Self-Organizing Map (SOM) formalism is proposed. Using extensive simulations, the SOM approach is shown to achieve a low localization error using limited computational resources. Comparison with other schemes demonstrate favorable results, especially in sparse deployments and when few (or none) of the nodes are located at known positions. The second part focuses on theoretical analysis of the results. Two broad families of positioning schemes are analyzed: (1) Range-free schemes that use radio proximity information, as in the SOM approach; and (2) Range-based schemes that measure the attenuation of the Radio-Frequency (RF) signal to estimate inter-node distances. First, analysis of the Fisher Information and the Cramér-Rao bound are used to investigate the theoretical limits that bound the localization error in the two cases. Then, general design criteria are proposed to reduce the error of range-free schemes and determine in which operative conditions they can outperform range-based solutions.

In the final part of this work, the theoretical results are used to design an improved variant of the SOM algorithm that combines the best traits of proximity and RSS ranging localization. Validation using measurements from real deployments shows significant improvements over the original SOM version and other localization schemes. Practical implementation of RF-based positioning systems is further investigated by using directional antennas for Angle of Arrival (AOA) estimation. A novel angle-based system that uses a single anchor is described and validated using experimental results. Additionally, a SOM variant capable of exploiting AOA information in collaborative localization is investigated using simulations.