Dispersive source models in wireless communications subscriber location
- Autores: René Patricio Játiva Espinoza
- Directores de la Tesis: José Vidal Manzano (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2017
- Idioma: inglés
- Tribunal Calificador de la Tesis: Gonzalo Seco Granados (presid.), Alba Pagès Zamora (secret.), Carlos Fernández Prades (voc.)
- Programa de doctorado: Programa de Doctorado en Teoría de la Señal y Comunicaciones por la Universidad Politécnica de Catalunya
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
Mobile subscriber positioning is an issue in permanent revision due to the new possibilities of relation that the knowledge of the position introduces, among users and an increasing number of devices. This problem is boarded in this thesis in the context of wireless communications and from the perspective of statistical signal processing. This research provides a quite complete description of this task both from the theoretical viewpoint and through intensive simulations, by using stochastic models that conceive signal as a dispersive source characterized for their spatial and temporal probability density functions. These models are justified from experimental measurements, reduce the mathematical complexity and are suited for studying the problem of positioning.
Signal is studied in the framework of Direct Sequence ¿ Spread Spectrum (DS ¿SS), but the study is quite general and could be applied to other infrastructures. It introduces the positioning technologies, and discusses the problems and possible solutions appearing when these schemes are applied to wireless communications systems. It particularly studies the degradation associated with the Non Line Of Sight (NLOS) condition between the transmitter and the receiver, and mechanisms for its mitigation. For achieving realistic simulation scenarios, the Greenstein¿s gain-delay propagation model has been used, and a simulation platform to evaluate positioning accuracy has been developed. Furthermore, the use of some important statistics to perform NLOS mitigation on timing ¿ based positioning algorithms have been proposed. As a result of the derivation of these statistics from the Greenstein¿s model, it was concluded that the quality of the timing measures decays more strongly with the link distances between transmitter and receiver than the suggested when the propagation model is not taken account. Therefore, proper weightings have been provided. Moreover, the weighted linear least squares algorithm has been revisited and a new two-stage solution that includes geometrical restrictions has been proposed and successfully implemented.
Since the signal in a wireless channel is affected by scattering, the use of dispersive models for the theoretical study of these signals in the context of the positioning problem is justified. Therefore the use of Cramer-Rao bounds (CRB) derived from these models is proposed to extract pondered conclusions about the quality of timing estimates. CRBs are derived for Time of Arrival estimation for both Rice and Rayleigh propagation. To the best of our knowledge, it is in fact the most complete model of its kind in the literature, since it incorporates a way to take into account spatial and temporal correlation among channel estimates, the impact of the roll-off factor, the number of sensors and the number of channel estimates, and also because it assumes an exponential dispersion from delays, which it is characteristic of mobile channels, instead of two or three paths, typical in literature. This information model has been integrated to the simulation platform providing a useful approach to evaluate both qualitative and quantitative the benefits of using space-time diversity in terms of the positioning accuracy.
Finally, this thesis proposes a two-stage procedure to acquire the required improved timing estimates for enhancing the positioning accuracy. Therefore, signal is discriminated from noise at the first stage using a Generalized Likelihood Ratio Test (GLRT). Signal detectability is evaluated using a model developed to put in evidence the optimum operation point from the viewpoint of the quality of the timing detection. A high resolution timing estimation has been proposed for the second stage to reduce timing uncertainty from a chip time at the first stage to a small fraction of this value. Final results show as subscriber location may be performed with a high degree of accuracy from network-based architectures using this procedure
