GNSS Signal processing and spatial diversity exploitation
Authors:   , García Molina José Antonio 
Published: October 2023

Global Navigation Satellite Systems (GNSS) signals are broadly used for positioning, navigation and timing (PNT) in many different applications and use cases, Athough different PNT technologies are available, GNSS is expected to be a key player in the derivation of positioning and timing for many future applications, including those in the context of the Internet of Things (IOT) or autonomous vehicles, since it has the imponant advantage of being open access and worldwide available. Indeed, GNSS is performing very well in mild propagation conditions, achieving position and time synchronization accuracies down to the cm and ns levels, respectively. Nevertheless, the exploitation of GNSS in harsh propagation conditions typical of urban and indoor scenarios is very challenging, resulting in position errors of up to tens or even hundreds of meters, and timing accuracies of hundreds of ns.
This thesis deals with the processing of GNSS signals for positioning and timing in harsh propagation conditions. In particular, the focus is on signal processing techniques exploiting the spatial diversities present both at transmission and reception levels when multiple GNSS satellites are in view by multiple receiver antennas, which form a multiple-input multiple-output (MIMO) system. In this context, three problems or research areas open in the GNSS literature are targeted. The first research area is the unambiguous estimation of and positioning with high-order binary offset carrier (BOC) signals. The second research area is the time synchronization in indoor conditions. And the third research area is the positioning with co-located and distributed receiver antennas.
In the first research area, this thesis shows that the robust unambiguous positioning with high-order BOC signals in harsh propagation conditions is possible when jointly exploiting these signals in the position domain and taking advantage of the spatial diversity introduced by arrays of antennas. The proposed estimators introduce an important benefit with respect to singlesatellite-based unambiguous techniques (operating at pseudorange level) thanks to the processing gain introduced by the MIMO-GNSS system formed. Indeed, when multiple antennas are featured by the receiver, the proposed approach allows the exploitation of high-order BOC signals even in indoor conditions, achieving positioning accuracies of few meters in propagation conditions for which BPSK(I) signals can only achieve accuracies of tens of meters.
In the second research area, this thesis proposes a joint time and channel estimation approach for static indoor GNSS receivers featuring an array of antennas in order to improve the timing accuracy in indoor propagation conditions. This approach exploits both the structure of the diffuse multipath components of the indoor channel and the MIMO system formed by all the GNSS signals received via an array of antennas, Simulation results with a wideband satellite-to-indoor channel model show that the proposed timing estimators allow an important mitigation of the dominant indoor multipath conditions.
Finally, in the third research area, this thesis proposes the exploitation of co-located and distributed receiver antennas for positioning in harsh propagation conditions. In order to improve the performance achieved with co-located antennas, a distributed array processing approach for collaborative GNSS-based snapshot positioning is proposed in the MIMO-GNSS framework. In this solution, one of the receivers is used as anchor and a distributed array is formed, allowing to transform the positioning problem into an angle estimation problem in order to reduce the computational burden.
La señales transmitidas por los sistemas globales de navegación por satélite (Global Navigation Satellite Systems, GNSS) son comúnmente utilizadas para posicionamiento y sincronización en muchas y diversas aplicaciones. De hecho, se espera que los sistemas GNSS sean clave en futuras aplicaciones como el Internet de las cosas o los vehículos autónomos, ya que su uso es abierto, gratuito y global. No obstante, aunque las prestaciones que se pueden obtener con las señales GNSS son excelentes en condiciones de propagación favorables, la explotación de estas señales en condiciones más adversas típicas de entornos urbanos o de interior sigue siendo un problema complejo en la actualidad, Esta tesis aborda el procesado de señales GNSS para posicionamiento y sincronización en condiciones de propagación adversas, centrándose en la explotación de la diversidad espacial disponible cuando un array de antenas es utilizado para recibir las señales de varios satélites GNSS en vista. En concreto, esta tesis tiene como objetivo tres problemas o áreas de investigación abiertas en la literatura. La primera área de investigación trata la estimación no ambigua de la posición con señales BOC (Binary Offset Carrier) de alto orden. La segunda área de investigación se centra en el problema de sincronización basado en GNSS en entornos de interior. Y la tercera área de investigación aborda el problema de posicionamiento con arrays de antenas y arrays de receptores distribuidos.