Modern satellite communication systems mainly rely on time sharing to optimize the throughput. Each receiver uses the channel during a given fraction of time. During this period, the transmission parameters (i.e., the modulation and the coding rate) are chosen in order to transmit as much information as possible. The scheme is easy to implement which explains its popularity. However, it is today well established that time sharing is not optimal in terms of spectrumefficiency offered to the receivers. Indeed, the scheme that consists in sending superposed data offers better performance than the time sharing. This thesis investigates the application of superposition coding in satellite communication systems. First of all, we study the performance of hierarchical modulation which is an implementation of superposition coding at the modulation level.We propose a performance evaluation method for such modulations.We also compare the performance of hierarchical and non hierarchical modulations in terms of spectrum efficiency and link unavailability. These two criteria are very important for broadcast system and we show that hierarchical modulations often offer better performance than non hierarchical modulations. Then, we study the performance improvement in terms of spectrum efficiency when using hierarchical modulation in satellite communication systems. Two issues are addressed. The first one is how to group the receivers in pairs in order to transmit data with a hierarchical modulation. The second issue is the computation of the spectrumefficiency.We show that significant gains are possible depending on the system configuration. The last part considers a system where multiple users communicate through a satellite. The satellite acts as a relay in our scenario.We propose a communication scheme where several users emit at the same time with appropriate transmitting power. Thus the signals naturally superpose and generate interference. The receivers use two mechanisms for decoding the signals : physical layer network coding and demodulation of superposed constellations. Finally, we explain how the performance improvements obtained by superposition coding in several scenarios open perspectives for future work.

In the domain of satellite networks, the emergence of low-cost interactive terminals motivates the need to develop and implement multiple access protocols able to support dierent user proles. In particular, the European Space Agency (ESA) and the German Aerospace Center (DLR) have recently proposed random access protocols such as Contention Resolution Diversity Coded ALOHA (CRDSA) and Irregular Repetition Slotted ALOHA (IRSA). These methods are based on physical-layer network coding and successive interference cancellation in order to attempt to solve the collisions problem on a return channel of type Slotted ALOHA. This thesis aims to provide improvements of existing random access methods. We introduce Multi-Slot Coded Aloha (MuSCA) as a new generalization of CRDSA. Instead of transmitting copies of the same packet, the transmitter sends several parts of a codeword of an error-correcting code ; each part is preceded by a header allowing to locate the other parts of the codeword. At the receiver side, all parts transmitted by the same user, including those are interfered by other signals, are involved in the decoding. The decoded signal is then subtracted from the total signal. Thus, the overall interference is reduced and the remaining signals are more likely to be decoded. Several methods of performance analysis based on theoretical concepts (capacity computation, density evolution) and simulations are proposed. The results obtained show a signicant gain in terms of throughput compared to existing access methods. This gain can be even more increased by varying the codewords stamping rate. Following these concepts, we also propose an application of physical-layer network coding based on the superposition modulation for a deterministic access on a return channel of satellite communications. We observe a gain in terms of throughput

In this paper, we study an interference relay network with a satellite as relay. We propose a cooperative strategy based on physical layer network coding and superposition modulation decoding for uni-directional communications among users. The performance of our solution in terms of throughput is evaluated through capacity analysis and simulations that include practical constraints such as the lack of synchronization in time and frequency. We obtain a significant throughput gain compared to the classical time sharing case.

The automatic identification system (AIS) is a system allowing ships and coast stations to exchange some information by VHF radio. This information includes the identifier, status, location, direction and speed of the emitter. The aim of this thesis is to allow the reception of AIS messages by low Earth orbit satellites without modifying the existing ship equipments. With this system, it becomes possible to know the position of all ships over the Earth. As a consequence, several new services become available, such as global traffic monitoring or determining boat location (for ship-owners). Satellite reception of AIS signals is subjected to a higher noise level when compared to ground level reception. This noise makes classical demodulation and decoding methods unusable. A first contribution of this thesis is to develop new demodulators using error correction methods. These demodulators take advantage of the presence of a cyclic redundancy check (CRC) block in the messages as well as known information about the structure of messages and data. Generalizations of the proposed receiver have also been studied in order to take into account the phase noise of the received signals and the possible collision of messages sent simultaneously by several vessels. The last part of this thesis is devoted to the study of localization methods for ships that do not transmit their location in AIS messages. This localization takes advantage of information contained in the received messages such as the propagation delay and the carrier frequency shift due to the Doppler effect, and a ship movement model.

The automatic identification system (AIS) is a system allowing ships and coast stations to exchange some information by VHF radio. This information includes the identifier, status, location, direction and speed of the emitter. The aim of this thesis is to allow the reception of AIS messages by low Earth orbit satellites without modifying the existing ship equipments. With this system, it becomes possible to know the position of all ships over the Earth. As a consequence, several new services become available, such as global traffic monitoring or determining boat location (for ship-owners). Satellite reception of AIS signals is subjected to a higher noise level when compared to ground level reception. This noise makes classical demodulation and decoding methods unusable. A first contribution of this thesis is to develop new demodulators using error correction methods. These demodulators take advantage of the presence of a cyclic redundancy check (CRC) block in the messages as well as known information about the structure of messages and data. Generalizations of the proposed receiver have also been studied in order to take into account the phase noise of the received signals and the possible collision of messages sent simultaneously by several vessels. The last part of this thesis is devoted to the study of localization methods for ships that do not transmit their location in AIS messages. This localization takes advantage of information contained in the received messages such as the propagation delay and the carrier frequency shift due to the Doppler effect, and a ship movement model.

This thesis focuses on jointly improving the spectral efficiency and the power efficiency of satellite transmission schemes. The emergence of new services and the increasing number of actors in this field involve higher transmission rates with increasingly limited resources. Recent progress in the embedded technologies and in digital communications offered to consider transmission schemes with higher spectral and power efficiency. Nevertheless, the major current challenge consists in making efficient use of resources. The study developed in this thesis explores the possibilities of jointly improving the spectral and power efficiency by offering a combination of the Cyclic- Code-Shift Keying modulation (CCSK), which power efficiency increases with the degree of modulation, with a multiplexing technique such as Code-Division Multiplexing (CDM) to offset the deterioration on the spectral efficiency due to the spread spectrum induced by CCSK. Two approaches based on the use of Gold sequences of length N are defined : • a multi-stream approach with an optimal receiver implemented through sphere decoding. The complexity due to the receiver optimality leads to limited spectral efficiencies but the study of performance, confirmed by simulations, shows an increase in power efficiency with spectral efficiency. • a single-stream approach justified by the appearance of redundancy in the patterns following the sequences multiplexing. The single-stream approach offers spectral efficiencies equivalent to the adopted schemes in the DVB-S2 standard, with improved power efficiency from a certain level of signal to noise ratio compared to those schemes. Subsequently, the study focuses on the implementation of several modulation symbols on the subcarriers of an OFDM modulator and the benefits and advantages of such an approach. It concludes with the contribution of channel coding based on nonbinary block codes such as Reed-Solomon and LDPC codes. The proposed waveform offers operating points with high spectral efficiency and high power efficiency with attractive perspectives. In the current context, its application is limited by its amplitude fluctuations but is possible in a multicarrier transmission context, as expected in the years to come.

This thesis focuses on jointly improving the spectral efficiency and the power efficiency of satellite transmission schemes. The emergence of new services and the increasing number of actors in this field involve higher transmission rates with increasingly limited resources. Recent progress in the embedded technologies and in digital communications offered to consider transmission schemes with higher spectral and power efficiency. Nevertheless, the major current challenge consists in making efficient use of resources. The study developed in this thesis explores the possibilities of jointly improving the spectral and power efficiency by offering a combination of the Cyclic- Code-Shift Keying modulation (CCSK), which power efficiency increases with the degree of modulation, with a multiplexing technique such as Code-Division Multiplexing (CDM) to offset the deterioration on the spectral efficiency due to the spread spectrum induced by CCSK. Two approaches based on the use of Gold sequences of length N are defined : • a multi-stream approach with an optimal receiver implemented through sphere decoding. The complexity due to the receiver optimality leads to limited spectral efficiencies but the study of performance, confirmed by simulations, shows an increase in power efficiency with spectral efficiency. • a single-stream approach justified by the appearance of redundancy in the patterns following the sequences multiplexing. The single-stream approach offers spectral efficiencies equivalent to the adopted schemes in the DVB-S2 standard, with improved power efficiency from a certain level of signal to noise ratio compared to those schemes. Subsequently, the study focuses on the implementation of several modulation symbols on the subcarriers of an OFDM modulator and the benefits and advantages of such an approach. It concludes with the contribution of channel coding based on nonbinary block codes such as Reed-Solomon and LDPC codes. The proposed waveform offers operating points with high spectral efficiency and high power efficiency with attractive perspectives. In the current context, its application is limited by its amplitude fluctuations but is possible in a multicarrier transmission context, as expected in the years to come.

This paper deals with the problem of detecting a collision target in ground clutter, using a long integration time. A single reception channel being available, classical space time adaptive processing (STAP) cannot be used. After range processing, ground clutter can be modeled as a known interference subspace in the Doppler domain depending on its radial and orthoradial speeds. We exploit this a priori knowledge to perform an adpative detection of a collision target supposed to lie in a known and different subspace. A GLRT detector is first derived for known clutter covariance matrix. Then, the unknown covariance matrix is adaptively estimated from the projection of the data onto the modeled clutter subspace, and is plugged in the GLRT to form a suboptimal detector. The proposed scheme can be viewed as a synthetic STAP, for which the space domain is replaced by a clutter orthoradial information and longer integration time.

Waveforms transmitted by the elements of a MIMO radar system may differ in power and bandwidth. This raises the question of optimal resource allocation among the radar elements. Specifically, we are asking, given constrained resources, what is the optimal bandwidth and optimal joint power and bandwidth allocation for best target localization performance. Using the Cramér-Rao Lower Bound (CRLB) as the figure of merit for localization accuracy, the resource allocation optimization problem turns out to be non-convex. We apply a Difference of Convex functions programming approach to develop quasi-optimal algorithms for solving the resource allocation problems. A lower bound is also developed to help assess the quality of our solutions. Numerical examples demonstrate that bandwidth allocation has considerably more impact on performance than power allocation, and that the best performance is obtained with joint power and bandwidth allocation.