L’Internet des objets (ou IoT, pour Internet of Things) regroupe un ensemble de systèmes variés, tant par leurs contraintes que par leurs utilisations. Dans le cadre de cette thèse, nous allons nous intéresser aux LPWAN (Low-Power Wide-Area Network), les réseaux sans fil à grande couverture à faible consommation énergétique, en se basant sur le standard NB-IoT. Ces réseaux ont pour but de connecter des objets ou terminaux qui partagent certaines caractéristiques précises. Leur autonomie est optimisée pour durer le plus longtemps possible, ils ont de faibles quantités de données à transmettre régulièrement, et il s’agit d’équipements bas couts. L’objectif de cette thèse est de concevoir et d’étudier l’hybridation d’un système terrestre LPWAN avec une constellation de satellites en orbite basse, dite LEO (Low Earth Orbit) afin de proposer une extension de couverture. Dans un premier temps, le système satellite proposé est décrit. Il repose sur un lien unidirectionnel des terminaux vers le satellite. En l’absence de lien du satellite vers les terminaux, le schéma d’accès retenu est l’Aloha Temps Fréquence, ou aléatoire en temps et en fréquence. Ce schéma, propice à l’utilisation de terminaux à faible cout, impose cependant la mise en place d’une stratégie de réception dédiée. En effet, il est nécessaire de compenser l’absence d’information sur la localisation temporelle et fréquentielle des messages, qui sont reçus à un niveau de bruit élevé par le satellite. De plus, l’utilisation de satellites défilants impose une forte variation des paramètres fréquentiels des transmissions, ce qui complexifie la démodulation des messages. Une chaine de réception est proposée et évaluée ; l’estimation des paramètres fréquentiels nécessite la mise en place de méthodes spécifiques. En outre, l’utilisation d’un schéma aléatoire rend possible la réception par le satellite de plusieurs messages au même instant. Le couplage d’un turbocodage, d’un codage à répétition et de telles collisions mène à l’apparition de phénomènes d’interférences particuliers. L’impact de ces collisions d’abord sur les symboles reçus (taux d’erreur binaire) puis sur la décodabilité du message entier (taux d’erreur paquet) est décrit. Dans la dernière partie de cette thèse, les performances globales du système sont évaluées. Les modèles des performances du récepteur sont agrégés dans un simulateur qui modélise le traitement des messages tels qu’ils sont reçus par le satellite. Une méthode d’annulation successive des interférences (SIC pour Successive Interference Cancellation) est utilisée. -------/------- The Internet of Things relates to various systems, designed for diverse uses. In this thesis, the focus is made on Low-Power Wide-Area Networks (LPWAN), especially on NB-IoT. LPWA networks stand out from others by being designed to address low cost devices, or terminals, for low power and low bit rate communications. The aim of this thesis is to propose and study a hybrid system of a LPWA network by using a Low Earth Orbit (LEO) satellite constellation, in order to extend its coverage. First, the proposed satellite system is described. This system relies on a unidirectional link from the users to the satellite. By having the link from the satellite to the terminal removed, the selected transmission scheme is Time Frequency Aloha, or random in time and frequency. Whereas this scheme is convenient for low cost terminals, it complicates the transmission reception: no information on the timing nor the frequency of the transmissions are shared to the receiver. Additionnaly, using satellites implies strong distortions of the messages, and jeopardize their demodulation. A strategy to detect, estimate their parameters and demodulate the messages is proposed, and evaluated. The estimation of a message parameters required the creation of specific models. Moreover, using a random scheme implies the possibility of receiving overlapping messages. The impact of those collisions on the Bit Error Rate (BER) and on the Packet Error Rate (PER) has been studied, especially when using a turbocode and a repetition coding scheme. Finally, the whole system performance is estimated using simulations. The proposed abstraction methods of the transmission detection and demodulation are nested in a simulator. Transmissions are created as seen from the satellite, and are inputed in the modeled receiver. A Successive Interference Cancellation (SIC) method is implemented to improve the system performance.

The design of a new GNSS signal is always a trade-off between several figures of merit such as the position accuracy, the sensitivity or the Time To First Fix (TTFF). However, if the goal of the new signal design is to improve the acquisition process, the sensitivity and the TTFF have a higher relevance as figures of merit. Considering that, the main goal of this work is to present the joint design of a GNSS signal and the message structure to propose a new Galileo 2nd Generation (G2G) signal, which provides a higher sensitivity in the receiver and reduces the TTFF, in order to improve the acquisition process. Besides that, since this work has focused on the Galileo E1 Open Service (E1-OS), the signal must be compatible with those signals already presented in the same radio frequency spectrum. However, many of the concepts and methodologies can be easily extended to any GNSS signal. In order to present the join design of a GNSS signal and the message structure, several aspects such as the spreading modulation, the pseudorandom noise (PRN) codes, the channel coding or the signal multiplexing must be addressed.

This research modified the best approach of the state of art for cardiac motion estimation in 2-D ultrasound images. The motion estimation problem is composed of three terms, the data fidelity term, the spatial smoothness constraint, and regularization for the dictionary. We study the implications of using a convolutional dictionary instead of the standard dictionary. We evaluate the method in terms of motion estimation accuracy and strain errors and compare the performance with state of art algorithms. The results show that the proposed method is comparable with the current state of art method.

Satellite transmissions can suffer from high channel impairments, especially on the link between a satellite and a mobile end user. To cope with these errors, physical and link layer reliability schemes have been introduced at the price of an increased end-to-end delay seen by the transport layer (e.g. TCP). By default, TCP enables Delayed Acknowledgment (DelAck), that might increase the end-to-end delay when performing over satellite link-layer recovery schemes. As a matter of fact, even if this option enables to decrease the feedback path load and the stack processing overhead, it might be counterproductive in a satellite context. This motivates the present paper that aims at quantifying the impact of such TCP option in the context of Low Earth Orbit (LEO) satellite constellations. We perform several simulation measurements with two well-deployed TCP variants and show that DelAck should be disabled when used over link-layer HARQ schemes particularly when these schemes enable reordering buffer.

Random Access (RA) protocols have considerably evolved in satellite communications, especially after the introduction of Contention Resolution Diversity Slotted Aloha (CRDSA). However, CRDSA finds itself in a deadlock when the number of users is important. A complementary treatment Multireplica Decoding using Correlation based Localization (MARSALA) has hence been proposed to unlock CRDSA. This is fulfilled by localizing then combining replicas of the same undecoded packets using correlations. Based on a prior knowledge of the potential frame content by the receiver, a random Shared POsition Technique for Interfered random Transmissions (SPOTiT) is proposed to reduce MARSALA’s localization complexity. As a matter of fact, random SPOTiT highlights a manner for the receiver to be aware of time slot positions and the preamble used by each subscriber. Then it uses this information to target a lower number of slots for localization correlations. In this paper we propose a hybrid solution that mixes both DAMA and Random Access in order to lower the Packet Loss Ratio (PLR) floor. In fact, a centralized computing can manage replicas positions and preambles to use, in a way that no loops are created. This also allows to keep a simple packet localization as in SPOTiT. Hereafter, we provide an optimal distribution of frame content using two replicas per packet which is evaluated through simulation.

Reducing the Time To First Fix (TTFF) and improving the resilience of future Galileo signals are two important characteristics, especially when considering urban environments. To reach these goals, we studied two new advanced techniques based on the co-design of the message structure and the channel coding scheme. The first technique proposes a reduction of time needed to retrieve the data by reinforcing the parity check matrix structure constraints. The second technique provides an enhancement of the retrieved data error rate in parallel to a reduction of the time needed to retrieve the data thanks to a new co-design requirement based on a family of codes inspired from the rate-compatible root LDPC codes. The results obtained are promising, since the time to retrieve the data (and thus the TTFF) is significantly reduced, while keeping a good level of demodulation performance.