Análisis de la modulación por ruido en las señales de tiempo de sistemas DVB-T para su utilización en radioposicionamiento

Abstract

Measurement of time has always been a human necessity, in all stages of human civilization. All activities have a chronogram, and all planifications are associated to some time scale. Measurement of this magnitude permits its being applied to uses which require more and more stable clocks. The exact measurement of time and the characterization of clock signals in order to evaluate its uncertainty have given rise to a branch of metrology of huge importance. The relation between space and time, which identifies the relative speeds of objects, reaches a special meaning when it affects radioelectric signals. Two circumstances are particularly significant. The first one is the fact that the speed of radioelectric signals in the void is an absolute magnitude, well known by everyone. The second one is the fact that it is time we are able to measure with the highest accuracy. The attempt to extend this accuracy to the measurement of space is almost immediate. This is the foundation for the positioning systems based on the measurement of time. Global positioning systems based on satellite allow receivers to know their positions thanks to the exact measurement of the time of flight of a radioelectric signal. But here accuracy is never absolute, and uncertainty in all original magnitudes conditions the positioning results. There are local radiating systems with fixed-position transmitters, whose location can be known with little uncertainty and which transmit clock signals of great accuracy. However, these are not designed for positioning issues. Mobile telephony and television broadcasting stations spin incredibly dense networks. The question is, if it is posible to build local positioning systems which are exact enough, by using signals transmitted by the nodes in these networks. This study is aimed at the characterization of the time signals obtained from frames transmitted by broadcasting DVB-T systems, which meet the standard European specifications. The results will allow us to estimate the uncertainty of position associated to that obtained from the measurement of time. But evaluating the accuracy in time signals is an end in itself which allows us to think of other applications, including time, space, and consequently, speed. The tools to characterize high stability clocks have been developed mainly in the last sixty years. The patterns of noise linked to these signals have been identified. They have been characterized in the domain of frequency, by studying their power density spectrum, and in the domain of time, being various specific dispersion estimators able to separate the types of noise. In order to characterize the domain of time, we will use Allan’s variance which, fifty years after its introduction, and having other variances for specific purposes been developed, remains as the most used tool.