Ce travail de thèse porte sur la modélisation des phénomènes de propagation affectant les signaux de navigation par satellite en environnement urbain dense avec une focalisation particulière sur les multitrajets et l'aspect large bande du canal de propagation espace/Terre. Le simulateur de canal pseudo temps-réel développé, SCHUN (Simplified CHannel for Urban Navigation), repose sur une approche hybride physico-statistique. La composante statistique de la modélisation permet essentiellement de générer une ville virtuelle à partir de distributions de bâtiments connues. Le reste de la modélisation s'appuie sur une approche physique simplifiée où les interactions ondes électromagnétiques/ville virtuelle reposent d'une part sur un modèle de macro-diffusion à l'échelle des façades, (3CM (Three Component Model)), et d'autre part sur un modèle physique de masquage du trajet direct par les bâtiments. Les principales méthodes numériques sous-jacentes sont l'optique physique et la théorie uniforme de la diffraction. Le simulateur de canal SCHUN ouvre aujourd'hui des perspectives intéressantes pour la modélisation large bande du canal de propagation espace/Terre. Optimisé pour des temps de calcul raisonnables, alliant une composante statistique à une composante physique simplifiée, ce simulateur a été conçu et validé par des mesures expérimentales pour répondre à des besoins de simulation des systèmes à diversité de satellite, diversité de réception, diversité de polarisation ou encore diversité de fréquence pour des applications de navigation par satellite. / This PhD work deals with land mobile satellite channel modelling and addresses the specific issue of satellite navigation systems in urban environments with a particular focus on multipath modelling and wide-band representation of the channel. The developed land mobile satellite channel simulator, SCHUN (Simplified CHannel for Urban Navigation), is based on a hybrid physical-statistical approach satisfying fast computation requirements. The statistical component of the modelling is mainly used during the virtual city synthesis step based on known statistical distributions of building height and street width. The rest of the modelling comes from deterministic methods using simplified electromagnetic interaction models reproducing building macro-scattering (3CM model (Three Component Model)) and building blockage of the direct path. The main underlying electromagnetic methods are the physical optics and the uniform theory of diffraction. The SCHUN simulator now opens interesting perspectives for the modelling of wide-band land mobile satellite propagation channel in dense urban environments. Optimised for pseudo real-time constraints, it uses both physical and statistical approaches. Furthermore, the SCHUN simulator has been designed and validated against measurements to answer specific needs of satellite diversity, receiving diversity, polarisation diversity or frequency diversity for satellite navigation applications.

Mobile communication systems are undergoing significant development on increasingly wide frequency bands (5G in particular). To support this development, a detailed characterization of the propagation of electromagnetic waves in L, S and C band between a station (satellite, airborne or ground) and a mobile platform is necessary to analyze and model the phenomena that have a decisive impact on the performance, availability and operability of systems. The environments of interest are complex (urban environment for example) and include a wide range of different elements (buildings, pylons, trees etc.) that will have an impact on the signal received by a mobile in reception. These needs motivated the development of a simulator using an enhanced hybrid physicalstatistical model for Land Mobile Satellite (LMS) propagation Channel. This simulator has been developed by a PhD student and presented in [Ait+13]. This study has been conducted by the ONERA on behalf of the CNES. The Simplified CHannel for Urban Navigation (SCHUN) ensure a wideband characterization of the channel, with realistic multipath modelling and is dedicated to the testing of GNSS systems. However, this model must be validated in S and C bands and measurement campaigns have been carried out to compare simulated and experimental data. In this work, the data corresponding to a trajectory with alternatively LOS and NLOS situation in an urban canyon have been derived from a measurement campaign carried out in Saint-Lary in S and C frequency bands. The post-processing of the data performed in the S frequency band has been performed using a pre-existing algorithm implemented at ONERA relying upon acquisition and tracking signal processing principles of GNSS. This trajectory has been simulated along with its surroundings using SCHUN. The Channel Impulse Response has been computed to derive multipath characteristics. Comparisons have been made between simulations and experimental data and have shown great concordance. Future works would be first to extend this comparison to C-bands and then to statistically simulate a virtual city corresponding to the town in which the experiment has been carried out to complete the validation. / Mobila kommunikationssystem genomgår en betydande utveckling på allt bredare frekvensband (särskilt 5G). För att stödja denna utveckling krävs en detaljerad karakterisering av utbredningen av elektromagnetiska vågor i L-, S- och C-banden mellan en station (satellit, luftburen eller markbaserad) och en mobil plattform för att analysera och modellera de fenomen som har en avgörande inverkan på systemens prestanda, tillgänglighet och funktionsduglighet. De intressanta miljöerna är komplexa (t.ex. stadsmiljöer) och innehåller en mängd olika element (byggnader, master, träd osv.) som påverkar den signal som tas emot av en mobil i mottagning.  Dessa behov motiverade utvecklingen av en simulator som använder en förbättrad fysisk-statistisk hybridmodell för landmobila satellituppbredningskanaler (LMS). Denna simulator har utvecklats av en doktorand och presenteras i [Ait+13]. Denna studie har genomförts av ONERA på uppdrag av CNES. Simplified CHannel for Urban Navigation (SCHUN) säkerställer en bredbandig karakterisering av kanalen med realistisk multipath-modellering och är avsedd för testning av GNSS-system. Denna modell måste dock valideras i S- och C-banden och mätkampanjer har genomförts för att jämföra simulerade och experimentella data. I detta arbete har data som motsvarar en bana med alternativt LOS- och NLOS-situation i en urban canyon tagits fram från en mätkampanj som utfördes i Saint-Lary i S- och C-banden. Efterbearbetningen av data från S-frekvensbandet har utförts med hjälp av en befintlig algoritm som implementerats vid ONERA och som bygger på GNSS-signalbehandlingsprinciperna för förvärv och spårning. Denna bana har simulerats tillsammans med dess omgivningar med hjälp av SCHUN. Kanalimpulsresponsen har beräknats för att få fram egenskaperna för multipelväg. Jämförelser har gjorts mellan simuleringar och experimentella data och har visat stor överensstämmelse. Framtida arbeten skulle vara att först utvidga denna jämförelse till C-bandet och sedan statistiskt simulera en virtuell stad som motsvarar den stad där experimentet utfördes för att slutföra valideringen.

Les systèmes de communications par satellite sont utilisés dans le contexte de la radiodiffusion, de la navigation, du sauvetage et du secours aux sinistrés, car ils permettent de fournir des services sur une large zone de couverture. Cependant, cette zone de couverture est limitée par l'effet de masquage provoqué par des obstacles qui bloquent la liaison directe entre le satellite et un utilisateur terrestre. L'effet de masquage devient plus sévère en cas de satellites à faibles angles d'élévation ou lorsque l'utilisateur est à l'intérieur. Pour résoudre ce problème, les Systèmes Coopératifs Hybride Satellite-Terrestre (HSTCS) ont été proposés. Dans un système HSTCS, l'utilisateur mobile peut profiter de la diversité spatiale en recevant des signaux à la fois du satellite et des relais terrestres. Les gap-fillers fixes ou mobiles sont utilisés pour relayer le signal satellite. La plupart des systèmes de diffusion par satellite utilisent les gap-fillers fixes alors que les gap-fillers mobiles sont nécessaires en cas de communications d'urgence lorsque l'infrastructure fixe n'est pas disponible. Dans les scénarios d'urgence (incendie, tremblement de terre, inondations, explosion) l'infrastructure terrestre existante est endommagée, donc les HSTCSs sont appropriés pour mettre à jour des informations qui permettent aux sauveteurs d'intervenir efficacement et en toute sécurité. En particulier, une mise en œuvre rapide et souple est nécessaire, ce qui pourrait être fourni par le déploiement de gap-fillers mobiles (véhicule ou portable). Plusieurs scénarios coopératifs et techniques de transmission ont déjà été proposés et étudiés. Cependant, la plupart des méthodes proposées ne fournissent qu'une analyse de performance fondée sur la simulation alors que les expressions analytiques de la probabilité de coupure et de la Probabilité d'Erreur Symbole (SEP) n'ont pas encore été établies. Cette thèse se focalise sur l'analyse de performances des systèmes HSTCS. La probabilité de coupure et SEP du système utilisant le schéma de transmission Selective Decode-and-Forward (SDF), avec ou sans sélection de relais, est évaluée dans le cas des modulations MPSK et MQAM. Cette expression analytique permet de concevoir le système HSTCS. Ces résultats sont applicables aux cas des relais fixes ou mobiles. La seconde partie de cette thèse est consacrée à des problèmes de synchronisation (décalage en temps et en fréquence ainsi que l'étalement Doppler). La mobilité des utilisateurs crée l'étalement Doppler qui détruit l'orthogonalité des sous-porteuses dans les signaux de type Orthogonal Frequency Division Multiplexing (OFDM). Cette perte d'orthogonalité engendre de l'interférence entre sous-porteuses (ICI) et donc une dégradation des performances du système en termes de SEP. Dans ce cas, on présente les conditions dans lesquelles cette dégradation peut être compensée par une augmentation du Rapport Signal sur Bruit (SNR) du côté de l'émetteur. Le résultat dépend du schéma de modulation et aussi de la vitesse des utilisateurs. / Satellite communication systems are used in the context of broadcasting, navigation, rescue, and disaster relief since they allow the provision of services over a wide coverage area. However, this coverage area is limited by the masking effect caused by obstacles that block the Line-Of-Sight (LOS) link between the satellite and a terrestrial user. The masking effect becomes more severe in case of low satellite elevation angles or when the user is indoor. To address this issue, Hybrid Satellite-Terrestrial Cooperative Systems (HSTCSs) have been proposed. In an HSTCS, the mobile user can exploit the diversity advantages by receiving signals from both satellite and terrestrial components. Fixed or mobile gap-fillers are used to relay the satellite signal. Most of satellites broadcasting systems have been implemented using fixed gap-fillers while mobile gap-fillers are needed in emergency cases when the fixed infrastructure is not available. In emergency scenarios (e.g., fire, earthquake, flood and explosion), the existing terrestrial infrastructure has been destroyed. So, an HSTCS is appropriate for transmitting the information between the rescuers and the central office. This allows the rescuers to operate efficiently. In particular, a fast and flexible implementation is needed and this could be provided by deploying mobile gap fillers (vehicle or mobile handheld). Recently, the topic of HSTCSs has gain interest in the research community. Several cooperative scenarios and transmission techniques have been proposed and studied. However, most of existing approaches only provide a performance analysis based on simulation results and the analytical expression of the exact Symbol Error Probability (SEP) is generally not provided. This dissertation focuses on the performance analysis of HSTCSs. The exact closed-form outage probability and SEP of Selective Decode-and-Forward (SDF) transmission scheme with and without relay selection are derived for both M-ary phase shift keying (MPSK) and M-ary quadrature amplitude modulation (MQAM) schemes. This analytical SEP helps in designing and dimensioning HSTCSs. Our results are applicable to both fixed and mobile relaying techniques. Another part of the dissertation is dedicated to synchronization issues (time, frequency shifting/spreading). The mobility of users induces a Doppler spread in the Orthogonal Frequency Division Multiplexing (OFDM) signal that destroys the orthogonality of subcarriers. The loss of orthogonality produces Inter-subCarrier Interference (ICI) and hence a degradation of the system performance in terms of SEP. In this case, we present the conditions in which this degradation can be compensated for by an increase in the Signal to Noise Ratio (SNR) at the transmitter side. The result depends on both the modulation scheme and the speed of the mobile users.

Effort to estimate unknown time-varying channels as a part of high-speed mobile communication systems is of interest especially for next-generation wireless systems. The high computational complexity of the optimal Wiener estimator usually makes its use impractical in fast time-varying channels. As a powerful candidate, the adaptive least mean squares (LMS) algorithm offers a computationally efficient solution with its simple first-order weight-vector update equation. However, the performance of the LMS algorithm deteriorates in time-varying channels as a result of the eigenvalue disparity, i.e., spread, of the input correlation matrix in such chan nels. In this work, we incorporate the L MS algorithm into the well-known bidirectional processing idea to produce an extension called the bidirectional LMS. This algorithm is shown to be robust to the adverse effects of time-varying channels such as large eigenvalue spread. The associated tracking performance is observed to be very close to that of the optimal Wiener filter in many cases and the bidirectional LMS algorithm is therefore referred to as near-optimal. The computational complexity is observed to increase by the bidirectional employment of the LMS algorithm, but nevertheless is significantly lower than that of the optimal Wiener filter. The tracking behavior of the bidirectional LMS algorithm is also analyzed and eventually a steady-state step-size dependent mean square error (MSE) expression is derived for single antenna flat-fading channels with various correlation properties. The aforementioned analysis is then generalized to include single-antenna frequency-selective channels where the so-called ind ependence assumption is no more applicable due to the channel memory at hand, and then to multi-antenna flat-fading channels. The optimal selection of the step-size values is also presented using the results of the MSE analysis. The numerical evaluations show a very good match between the theoretical and the experimental results under various scenarios. The tracking analysis of the bidirectional LMS algorithm is believed to be novel in the sense that although there are several works in the literature on the bidirectional estimation, none of them provides a theoretical analysis on the underlying estimators. An iterative channel estimation scheme is also presented as a more realistic application for each of the estimation algorithms and the channel models under consideration. As a result, the bidirectional LMS algorithm is observed to be very successful for this real-life application with its increased but still practical level of complexity, the near-optimal tracking performa nce and robustness to the imperfect initialization.