Multiple users compete for a common resource like bandwidth to communicate data in interference networks. Existing approaches in dealing with interference limit the rate of communication due to paucity of shared resources. This limitation in the rate gets more glaring as the number of users in the network increases. For example, existing wireless systems either choose to orthogonalize the users (for example, Frequency Division Multiple Access (FDMA) systems or Code Division Multiple Access (CDMA) systems) or treat interference as Gaussian noise at the receivers. It is well known that these approaches are sub-optimal in general. Orthogonalization of users limit the number of available interference-free channels (known as degrees of freedom, abbreviated as DoF) and treating interference as noise means that the receiver cannot make use of the structure in the interfering signals. This motivates the need to analyze alternate transmit and decoding schemes in interference networks. This thesis mainly analyzes transmit schemes that use linear precoding for various configurations of interference networks with some practical constraints imposed by the use of finite input constellations, propagation delays, and channel state availability at the transmitters. The main contributions of this thesis are listed below. Achievable rates using precoding with finite constellation inputs in Gaussian Interference Channels (GIC) is analyzed. A metric for finding the approximate angle of rotation to maximally enlarge the Constellation Constrained (CC) capacity of two-user Gaussian Strong Interference Channel (GSIC) is proposed. Even as the Gaussian alphabet FDMA rate curve touches the capacity curve of the GSIC, with both the users using the same finite constellation, we show that the CC FDMA rate curve lies strictly inside the CC capacity curve at high powers. For a K-user MIMO GIC, a set of necessary and sufficient conditions on the precoders under which the mutual information between between relevant transmit-receive pairs saturate like in the single user case is derived. Gradient-ascent based algorithms to optimize the sum-rate achieved by precoding with finite constellation inputs and treating interference as noise are proposed. For a class of Gaussian interference networks with general message demands, identified as symmetrically connected interference networks, the expected sumspectral efficiency (in bits/sec/Hz) is shown to grow linearly with the number of transmitters at finite SNR, using a time-domain Interference Alignment (IA) scheme in the presence of line of sight (LOS) channels. For a 2×2 MIMO X-Network with M antennas at each node, we identify spacetime block codes that could be coupled with an appropriate precoding scheme to achieve the maximum possible sum-DoF of 4M 3 , for M = 3, 4. The proposed schemes are shown to achieve a diversity gain of M with SNR-independent finite constellation inputs. The proposed schemes have lower CSIT requirements compared to existing schemes. This thesis also makes an attempt to guarantee a minimum throughput when the zero-interference conditions cannot be satisfied in a wireline network with three unicast sessions with delays, using Precoding Based Network Alignment (PBNA). Three different PBNA schemes namely PBNA with time-varying local encoding coefficients (LECs), PBNA using transform approach and time-invariant LECs, and PBNA using transform approach and block time-varying LECs are proposed and their feasibility conditions analyzed.

Abstract The goal of this thesis is to provide a unified concept of lossy-forwarding from the theoretical analysis to practical scheme design for the decode-and-forward-based multiple access relay channel (MARC) system. To improve the performance of MARC with the relay subject to resources or/and time constraints, the erroneous estimates output from simple detection schemes are used at the relay are forwarded and exploited. A correlation is then found between two sequences: one is the network-coded sequence sent from the relay, and the other is their corresponding exclusive-OR-ed information sequence. Several joint network-channel coding (JNCC) techniques are provided in which the correlation is utilized to update the log-likelihood ratio sequences during the iterative decoding process at the destination. As a result, the bit error rate (BER) and frame error rate (FER) are improved compared with those of MARC with select DF strategy (SDF-MARC). The MARC proposed above is referred to as erroneous estimates-exploiting MARC (e-MARC). To investigate the achieved FER performance of the e-MARC system, the outage probability for e-MARC with two source nodes is theoretically derived. We re-formulate the e-MARC system and identify its admissible rate region according to the Slepian-Wolf theorem with a helper. Then, the outage probability is obtained by a set of integral over the rate region with respect to the probability density functions of all the links' instantaneous signal-to-noise power ratios. It is found through simulations that, as one of the source nodes is far away from both the relay and destination, e-MARC is superior to SDF-MARC in terms of outage performance. Furthermore, a joint adaptive network-channel coding (JANCC) technique is then proposed to support e-MARC with more source nodes. A vector is constructed at the destination in JANCC to identify the indices of the incorrectly decoded source node(s), and re-transmitted to the relay for requesting additional redundancy. The relay performs network-coding only over the estimates specified by the vector upon receiving the request. Numerical results show that JANCC-aided e-MARC is superior to e-MARC in terms of FER and goodput efficiency. In addition, compared iterative decoding is performed at relay with SDF-MARC, the use of differential detection with JANCC-aided e-MARC significantly reduces the computational complexity and latency with only a small loss in the FER. / Tiivistelmä Tämän väitöskirjan tarkoituksena on tuottaa yhtenäinen kokonaisuus häviöllisestä lähetyksestä pura-ja-lähetä (DF) -pohjaisessa monikäyttörelejärjestelmässä (MARC) sekä teoreettisesta että käytännöllisestä näkökulmasta. Parantaakseen resurssi- tai aikarajoitetun MARC-järjestelmän suorituskykyä, vastaanotin hyödyntää riippuvuussuhdetta releen välittämien informaatiosekvenssien virheellisten estimaattien ja suoraan lähteestä tulevien informaatiosekvenssien välillä (e-MARC). Työssä ehdotetaan useita yhdistetyn verkko -ja kanavakoodauksen menetelmiä (JNCC), joissa log-uskottavuussuhdesekvenssit iteratiivisen purkamisprosessin aikana päivitetään hyödyntämällä sekvenssien riippuvuussuhdetta vastaanottimessa. Tämän tuloksena sekä bittivirhe- että kehysvirhesuhdetta saadaan parannettua verrattuna selektiiviseen pura-ja-lähetä menetelmää käyttävään MARC-strategiaan (SDF-MARC). Kehysvirheen suorituskyvyn tarkastelua varten työssä johdetaan teoreettinen epäkäytettävyyden todennäköisyys e-MARC-menetelmälle kahden lähettimen tapauksessa. Lisäksi e-MARC-menetelmälle määritetään tiedonsiirtonopeusalue Slepian-Wolf -teoreeman mukaisesti. Tämän jälkeen saadaan epäkäytettävyyden todennäköisyys kaikkien linkkien signaalikohinasuhteen todennäköisyystiheysfunktion integraalina tiedonsiirtonopeusalueen yli. Simulointitulokset osoittavat e-MARC-menetelmän paremman epäkäytettävyyden todennäköisyyden verrattuna SDF-MARC-menetelmään silloin kun yksi lähettimistä on kaukana sekä releestä että vastaanottimesta. Mahdollistaakseen useamman lähteen käytön e-MARC-menetelmässä, työssä ehdotetaan lisäksi adaptiivinen yhdistetyn verkko-ja kanavakoodauksen menetelmä (JANCC). Siinä vastaanotin määrittää väärin purettujen sekvenssien lähettimet ja ilmoittaa ne vektorimuodossa takaisin releelle pyytääkseen näiden lähettimien informaation uudelleenlähetystä. Tämän jälkeen rele suorittaa verkkokoodauksen vain tunnistusvektorin määrittämien informaatiosekvenssien estimaatteihin perustuen. Tulokset näyttävät, että JANCC-menetelmää käyttävä e-MARC saavuttaa paremman kehysvirheen ja hyödyllisen läpäisyn tehokkuuden verrattuna e-MARC-menetelmään.

In this thesis, we investigate physical-layer network coding in an L × M × K relay network, where L source nodes want to transmit messages to K sink nodes via M relay nodes. We focus on the information processing at the relay nodes and the compute-and-forward framework. Nested lattice codes are used, which have the property that every linear combination of codewords is a valid codeword. This property is essential for physical-layer network coding. Because the actual network coding occurs on the physical layer, the network coding coefficients are determined by the channel realizations. Finding the optimal network coding coefficients for given channel realizations is a non-trivial optimization problem. In this thesis, we provide an algorithm to find network coding coefficients that result in the highest data rate at a chosen relay. The solution of this optimization problem is only locally optimal, i.e., it is optimal for a particular relay. If we consider a multi-hop network, each potential receiver must get enough linear independent combinations to be able to decode the individual messages. If this is not the case, outage occurs, which results in data loss. In this thesis, we propose a new strategy for choosing the network coding coefficients locally at the relays without solving the optimization problem globally. We thereby reduce the solution space for the relays such that linear independence between their decoded linear combinations is guaranteed. Further, we discuss the influence of spatial correlation on the optimization problem. Having solved the optimization problem, we combine physical-layer network coding with physical-layer secrecy. This allows us to propose a coding scheme to exploit untrusted relays in multi-user relay networks. We show that physical-layer network coding, especially compute-and-forward, is a key technology for simultaneous and secure communication of several users over an untrusted relay. First, we derive the achievable secrecy rate for the two-way relay channel. Then, we enhance this scenario to a multi-way relay channel with multiple antennas. We describe our implementation of the compute-and-forward framework with software-defined radio and demonstrate the practical feasibility. We show that it is possible to use the framework in real-life scenarios and demonstrate a transmission from two users to a relay. We gain valuable insights into a real transmission using the compute-and-forward framework. We discuss possible improvements of the current implementation and point out further work. / In dieser Arbeit untersuchen wir Netzwerkcodierung auf der Übertragungsschicht in einem Relay-Netzwerk, in dem L Quellen-Knoten Nachrichten zu K Senken-Knoten über M Relay-Knoten senden wollen. Der Fokus dieser Arbeit liegt auf der Informationsverarbeitung an den Relay-Knoten und dem Compute-and-Forward Framework. Es werden Nested Lattice Codes eingesetzt, welche die Eigenschaft besitzen, dass jede Linearkombination zweier Codewörter wieder ein gültiges Codewort ergibt. Dies ist eine Eigenschaft, die für die Netzwerkcodierung von entscheidender Bedeutung ist. Da die eigentliche Netzwerkcodierung auf der Übertragungsschicht stattfindet, werden die Netzwerkcodierungskoeffizienten von den Kanalrealisierungen bestimmt. Das Finden der optimalen Koeffizienten für gegebene Kanalrealisierungen ist ein nicht-triviales Optimierungsproblem. Wir schlagen in dieser Arbeit einen Algorithmus vor, welcher Netzwerkcodierungskoeffizienten findet, die in der höchsten Übertragungsrate an einem gewählten Relay resultieren. Die Lösung dieses Optimierungsproblems ist zunächst nur lokal, d. h. für dieses Relay, optimal. An jedem potentiellen Empfänger müssen ausreichend unabhängige Linearkombinationen vorhanden sein, um die einzelnen Nachrichten decodieren zu können. Ist dies nicht der Fall, kommt es zu Datenverlusten. Um dieses Problem zu umgehen, ohne dabei das Optimierungsproblem global lösen zu müssen, schlagen wir eine neue Strategie vor, welche den Lösungsraum an einem Relay soweit einschränkt, dass lineare Unabhängigkeit zwischen den decodierten Linearkombinationen an den Relays garantiert ist. Außerdem diskutieren wir den Einfluss von räumlicher Korrelation auf das Optimierungsproblem. Wir kombinieren die Netzwerkcodierung mit dem Konzept von Sicherheit auf der Übertragungsschicht, um ein Übertragungsschema zu entwickeln, welches es ermöglicht, mit Hilfe nicht-vertrauenswürdiger Relays zu kommunizieren. Wir zeigen, dass Compute-and-Forward ein wesentlicher Baustein ist, um solch eine sichere und simultane Übertragung mehrerer Nutzer zu gewährleisten. Wir starten mit dem einfachen Fall eines Relay-Kanals mit zwei Nutzern und erweitern dieses Szenario auf einen Relay-Kanal mit mehreren Nutzern und mehreren Antennen. Die Arbeit wird abgerundet, indem wir eine Implementierung des Compute-and-Forward Frameworks mit Software-Defined Radio demonstrieren. Wir zeigen am Beispiel von zwei Nutzern und einem Relay, dass sich das Framework eignet, um in realen Szenarien eingesetzt zu werden. Wir diskutieren mögliche Verbesserungen und zeigen Richtungen für weitere Forschungsarbeit auf.

WCDMA is typically characterised as a system capable of providing mobile users with data rates up to 2 Mb/s and beyond. It has been termed an ultra high-speed, ultra high-capacity radio technology that will be able to carry a new range of fast, colourful media, such as colour graphics, video, animations, digital audio, Internet and e-mail that consumers will be able to access over their mobiles devices. This current study has researched on the various existing Multiuser detection (MUD) processes or proposals conducted by various research institutions around the world. It has identified the advantages that the past work offers, and it is these advantages that form the basis of the current research into the improvement techniques. The proposed Partial Parallel Pipelined Multiuser Detector (PPPMUD or P3MUD) has come about from two main flavours or directions of research. The first one seeks to promote the Soft Parallel Interference Cancellation technique as an effective bias mitigation technique. This bias occurring in the second stage decision statistics, exhibits a very harmful effect on system Bit Error Rate, (BER), particularly for large system loads. This current study goes further by carefully analysing the Soft Cancellation Factor, SCF behaviour to eventually derive and determine the optimum SCF value which exhibits positive characteristics when varied with the increasing system load (number of users). This optimum SCF value is called the universal SCF or SCFUNV, as it is theoretically supposed to perform favourably under various system loads. A favourable or acceptable performance would be characterised by low observed or measured BER during the system processing stages. A further enhancement to the operational performance of the SCFUNV algorithm is the SCFUNV Compensator, which is basically a compensation mechanism created by modelling the behaviour of the SCF values, and adjusts the SCFUNV depending on the system load, (number of simultaneous users). Thus, the SCFUNV is adaptively adjusted in order to perform acceptably under all load conditions. The second direction of research, as regards improvements in MUD techniques, involves the conventional Bit-Streaming, Pipelined Multiuser Detector. This came about due to the computational complexity as well as matrix inversions which affected earlier asynchronous multiuser detection techniques. This detector has a pipelined architecture which avoids multishot (block-based) detection and instead, processes the bits in a streaming fashion. The architecture consists of a matched filter followed by three stages of parallel interference cancellation, (PIC). This present study extends that research by outlining the advantages of incorporating the soft parallel interference cancellation technique, by way of the universal soft cancellation factor, (SCFUNV), into the conventional pipelined multiuser detector architecture to form the P3MUD architecture, which includes the compensator. The contributions of the proposed P3MUD system is that the observed BER output simulations are promising, with an observed overall decrease in the error rate for the P3MUD process, as compared to the conventional pipelined detection method. This decrease signifies an improvement offered by the proposed (P3MUD) algorithm. Further observed analysis indicates the possibility of decreasing the number of parallel interference cancellation stages from three to at least two, after the matched filter detection stage, without an observable change in system BER. Hence, the proposal of the two-stage P3MUD. / Dissertation (MEng (Electronic))--University of Pretoria, 2006. / Electrical, Electronic and Computer Engineering / unrestricted

No description available.

In this thesis, we analyze the performance of the Post Detection Integration (PDI) techniques used for detection of weak DS/CDMA signals in the presence of uncertainty in the frequency, noise variance and data bits. Such weak signal detection problems arise, for example, in the first step of code acquisition for applications such as the Global Navigation Satellite Systems (GNSS) based position localization. Typically, in such applications, a combination of coherent and post-coherent integration stages are used to improve the reliability of signal detection. We show that the feasibility of using fully coherent processing is limited due to the presence of unknown data-bits and/or frequency uncertainty. We analyze the performance of the two conventional PDI techniques, namely, the Non-coherent PDI (NC-PDI) and the Differential-PDI (D-PDI), in the presence of noise and data bit uncertainty, to establish their robustness for weak signal detection. We show that the NC-PDI technique is robust to uncertainty in the data bits, but a fundamental detection limit exists due to uncertainty in the noise variance. The D-PDI technique, on the other hand, is robust to uncertainty in the noise variance, but its performance degrades in the presence of unknown data bits. We also analyze the following different variants of the NC-PDI and D-PDI techniques: Quadratic NC-PDI technique, Non-quadratic NC-PDI, D-PDI with real component (D-PDI (Real)) and D-PDI with absolute component (D-PDI (Abs)). We show that the likelihood ratio based test statistic derived in the presence of data bits is non-robust in the presence of noise uncertainty. We propose two novel PDI techniques as a solution to the above mentioned shortcomings in the conventional PDI methods. The first is a cyclostationarity based sub-optimal PDI technique, that exploits the periodicity introduced due to the data bits. We establish the exact mathematical relationship between the D-PDI and cyclostationarity-based signal detection methods. The second method we propose is a modified PDI technique, which is robust against both noise and data bit uncertainties. We derive two variants of the modified technique, which are tailored for data and pilot channels, respectively. We characterize the performance of the conventional and proposed PDI techniques in terms of their false alarm and detection probabilities and compare them through the receiver operating characteristic (ROC) curves. We derive the sample complexity of the test-statistic in order to achieve a given performance in terms of detection and false alarm probabilities in the presence of model uncertainties. We validate the theoretical results and illustrate the improved performance that can be obtained using our proposed PDI protocols through Monte-Carlo simulations.

Le but principal des réseaux mesh sans fil (Wireless Mesh Networks-WMNs) est de fournir une dorsale de communication pour un grand nombre d'utilisateurs, car les WMNs doivent supporter un trafic énorme. Dans cette thèse, nous visons la maximisation d'utilisation et la répartition équitable de la bande passante dans les WMNs. Nous considérons deux environnements : WMN utilisant la norme IEEE 802.11 MAC, qui est caractérisée par son déploiement répandu et peu cher, et WMN utilisant les antennes directionnelles, qui représentent une technologie clé pour la réutilisation spatiale dans les réseaux sans fil. Pour les WMMs basés sur IEEE 802.11, nous concevons NICC, un protocole de contrôle de congestion qui reconnaît la congestion comme un problème lié au voisinage, et non pas au lien. NICC gère la congestion par une collaboration entre les nœuds d’un voisinage sans fil. En faisant usage de certains champs sous-exploités dans l'en-tête IEEE 802.11, NICC fournit un retour de congestion implicite et multi-bit. Ceci assure un contrôle précis du trafic sans affecter la bande passante. Pour les WMNs utilisant les antennes directionnelles, nous concevons FreeDMAC, un protocole MAC basé sur la technologie TDMA. FreeDMAC garantit que chaque nœud est conscient de toutes les transmissions dans son voisinage, ce qui évite les problèmes MAC causés par les antennes directionnelles, et ainsi, améliore l'utilisation de la bande passante. En outre, FreeDMAC est capable de fournir deux niveaux d’équité: équité entre les liens et équité entre les flux / The main purpose of Wireless Mesh Networks (WMNs) is to provide a communication backbone for a high number of end-users, thus WMNs have to support heavy traffic load. In this thesis, we intend to maximize utilization and achieve fair allocation of the bandwidth resources in WMNs. We consider two WMN environments: WMN using the IEEE 802.11 MAC standard, which is characterized by its cheap devices and widespread deployment, and WMN using directional antennas, which are emerged as an attractive technology to enhance the spatial reusability in wireless networks. For WMM based on IEEE 802.11, we design NICC, a congestion control scheme that recognizes congestion as neighborhood-related problem, and not a link-based one. Indeed, complex interference among neighboring nodes is the main starvation cause in WMNs. Therefore, NICC handles congestion using mutual cooperation within a wireless neighborhood. NICC makes use of some underexploited fields in the IEEE 802.11frame header in order to provide an implicit multi-bit congestion feedback, and thus ensure accurate rate control without generating overhead, making efficient use of bandwidth. For WMN with directional antennas, we design FreeDMAC, a TDMA-based MAC scheme with contention-free scheduling. FreeDMAC guarantees that each node is aware of all ongoing transmissions in its neighborhood, and thus avoids directional-related problems such as deafness, making efficient use of bandwidth. Moreover, FreeDMAC presents a link-slot assignment that provides two levels of fairness: Per-link and per-flow fairness

Les réseaux mesh sans fil (Wireless Mesh Networks-WMNs) sont des réseaux facilement déployables et à faible coût qui peuvent étendre l’Internet dans des zones où les autres réseaux peuvent difficilement accéder. Cependant, plusieurs problèmes de qualité de service (QoS) et de sécurité freinent le déploiement à grande échelle des WMNs. Dans cette thèse, nous proposons un modèle de contrôle d’admission (CA) et un système de réputation afin d’améliorer les performances du réseau mesh et de le protéger des nœuds malveillants. Notre système de CA vise à assurer la QoS des flux admis dans le réseau en termes de bande passante et de délai tout en maximisant l’utilisation de la capacité du canal. L’idée de notre solution est d’associer au contrôle d’admission une planification de liens afin d’augmenter la bande passante disponible. Nous proposons également un système de réputation ayant pour but de détecter les nœuds malveillants et de limiter les fausses alertes induites par la perte de paquets sur les liens du réseau. L’idée de notre solution est d’utiliser des tests statistiques comparant la perte de paquets sur les liens avec un modèle de perte préétabli. De plus, il comprend un système de surveillance composé de plusieurs modules lui permettant détecter un grand nombre d’attaques. Notre CA et notre système de réputation ont été validés, les résultats montrent qu’ils atteignent tous deux leurs objectifs / Wireless mesh networks (WMNs) are a very attractive new field of research. They are low cost, easily deployed and high performance solution to last mile broadband Internet access. However, they have to deal with security and quality of service issues which prevent them from being largely deployed. In order to overcome these problems, we propose in this thesis two solutions: an admission control with links scheduling and a reputation system which detects bad nodes. These solutions have been devised in order to further merge into a secure admission control. Our admission control schedules dynamically the network’s links each time a new flow is accepted in the network. Its goal is to accept only flows which constraints in terms of delay and bandwidth can be respected, increase the network capacity and decrease the packet loss. Our reputation system aims at assigning each node of the network a reputation which value reflects the real behavior of the node. To reach this goal this reputation system is made of a monitoring tool which can watch many types of attacks and consider the packet loss of the network. The evaluations of our solutions show that they both meet their objectives in terms of quality of service and security

Cette thèse est dédié à l’étude de la problématique des interférences multi utilisateurs dans le système Argos et à la proposition des diverses techniques pour réduire les effets de ces interférences. Le système Argos est un système mondial de localisation et de collecte de données géo positionnées par satellite. Il permet à l’échelle mondiale de collecter et de traiter les données émises par des émetteurs installés sur la surface de terre. Ces émetteurs sont connus sous le nom de balises. Ces balises sont installées sur des voiliers, des stations météo, des bouées, ainsi que sur quelques animaux (phoques, penguins, etc.…). Le système Argos a été créé en 1978 par le Centre National des Études spatiales (CNES), l’agence spatiale américaine (NASA) et l’agence américaine d’étude de l’atmosphère et de l’océan (NOAA). Depuis sa création, le nombre de balises Argos n’a cessé d’augmenter afin de couvrir au mieux la couverture mondiale. Nous sommes orientés ainsi à la saturation de la bande d’émission et à la présence des interférences multi utilisateurs (MUI) provenant de la réception simultanée de plusieurs signaux émis par les balises. Cette MUI limite la capacité du système Argos et dégrade les performances en termes de Taux Erreur Bit (TEB). Actuellement, le système Argos n’est capable de traiter qu’un seul signal reçu à un instant donné. D’où, l’intérêt d’implanter des techniques au niveau du récepteur capable de réduire les effets des interférences et de traiter les signaux émis par toutes les balises. Plusieurs techniques de détection multi utilisateurs (MUD) ont été développées dans le cadre de cette problématique. Ces techniques sont principalement implantées dans les systèmes CDMA où des codes d’étalement sont utilisés afin de différencier entre les différents signaux. Ceci n’est pas le cas du système Argos où les signaux ne présentent pas des séquences d’étalement et que les bandes de fréquences pour ces différents signaux ne sont pas disjointes à cause de l’effet Doppler et donc, un recouvrement spectral au niveau du récepteur est très probable. Dans ce contexte, l’objectif du travail présenté dans cette thèse est d’étudier différentes techniques MUD appliquées au système Argos et d’évaluer ces techniques au niveau des performances en termes de TEB et de complexité d’implantation. Dans ce travail, nous présentons les différentes composantes du système Argos ainsi que son mode de fonctionnement. Ensuite, nous présentons la problématique dans le système Argos ainsi que les différentes solutions proposées. Parmi ces solutions, nous montrons celle basant sur l’implantation des techniques MUD au niveau du récepteur. Ces différentes techniques MUD sont alors présentées ainsi que les avantages et les inconvénients de chacune d’elles. Parmi les techniques possédant un bon compromis entre les performances d’une part et la complexité d’autre part, nous notons la technique d’annulation par série d’interférence (SIC). Dans cette technique, les signaux sont démodulés successivement suivant l’ordre décroissant des puissances. Cette technique nécessite une étape d’estimation des paramètres des signaux à chaque étape. L’impact d’une estimation imparfaite des différents paramètres est aussi étudié. Après l’étude des impacts des erreurs d’estimation, nous proposons des estimateurs adaptables au système Argos. Les performances de ces estimateurs sont obtenues en comparant les variances de leurs erreurs aux bornes de Cramer Rao (CRB). Enfin, nous terminons le travail par une conclusion générale des résultats obtenus et nous envisageons les perspectives des prochains travaux. / In our thesis, we investigate the application of multi user detection techniques to a Low Polar Orbit (LPO) satellite used in the Argos system. Argos is a global satellite-based location and data collection system dedicated for studying and protecting the environment. User platforms, each equipped with a Platform Transmitter Terminal (PTT), transmit data messages to a 850 km LPO satellite. An ARGOS satellite receives, decodes, and forwards the signals to ground stations. All PTTs transmit at random times in a 100 kHz bandwidth using different carrier frequencies. The central carrier frequency f0 is 401.65 MHz. Due to the relative motion between the satellite and the platforms, signals transmitted by PTTs are affected by both a different Doppler shift and a different propagation delay. Thus, the Argos satellite receives overlapping signals in both frequency and time domains inducing Multiple Access Interference (MAI). One common approach to mitigate the MAI problem is to implement Multi User Detection (MUD) techniques at the receiver. To tackle this problem, several MUD techniques have been proposed for the reception of synchronous and asynchronous users. In particular, the Successive Interference Cancelation (SIC) detector has been shown to offer a good optimality-complexity trade-off compared to other common approaches such as the Maximum Likelihood (ML) receiver. In an Argos SIC receiver, users are decoded in a successive manner, and the signals of successfully decoded users are subtracted from the waveform before decoding the next user. This procedure involves a parameter estimation step and the impact of erroneous parameter estimates on the performance of Argos SIC receiver has been studied. Argos SIC receiver has been shown to be both robust to imperfect amplitude and phase estimation and sensitive to imperfect time delay estimation. The last part of our work focuses on the implementation of digital estimators for the Argos system. In particular, we propose a time delay estimator, a frequency estimator, a phase estimator and an amplitude estimator. These estimators are derived from the ML principle and they have been already derived for the single user transmission. In our work, we adapt successfully these estimators for the multi user detector case. These estimators use the Non Data Aided (NDA) cases in which no a priori information for the transmitted bits is required. The performance of these different estimators are compared to the Cramer Rao Bound (CRB) values. Finally, we conclude in our work by showing the different results obtained during this dissertation. Also, we give some perspectives for future work on Argos system.

Les futures générations de réseaux sans fil posent beaucoup de défis pour la communauté de recherche. Notamment, ces réseaux doivent être en mesure de répondre, avec une certaine qualité de service, aux demandes d'un nombre important de personnes et d'objets connectés. Ce qui se traduit par des exigences assez importantes en termes de capacité. C'est dans ce cadre que les méthodes d'accès multiple non orthogonaux (NOMA) ont été introduit. Dans cette thèse, nous avons étudié et proposé une méthodes d'accès multiple basé sur la technique compute and forawrd et sur les réseaux de point (Lattice codes) tout en considérant différentes constructions de lattice. Nous avons également proposé des amélioration de l'algorithme de décodage de la méthode SCMA (Sparse code multiple access) basé sur les réseaux de points. Afin de simplifier les décodeurs multi-niveaux utilisés, nous avons proposé des expressions simplifiées de LLRs ainsi que des approximations. Finalement, nous avons étudié la construction D des lattices en utilisant les codes polaires. Cette thèse était en collaboration avec le centre de recherche de Huawei France. / Future generations of wireless networks pose many challenges for the research community. In particular, these networks must be able to respond, with a certain quality of service, to the demands of a large number of connected people and objects. This drives us into quite important requirements in terms of capacity. It is within this framework that non-orthogonal multiple access methods (NOMA) have been introduced. In this thesis, we have studied and proposed a multiple access method based on the compute and forward technique and on Lattice codes while considering different lattice constructions. We have also proposed improvements to the algorithm for decoding the Sparse code multiple access (SCMA) method based on Lattice codes. In order to simplify the multi-stage decoders used in here, we have proposed simplified expressions of LLRs as well as approximations. Finally, we studied the construction D of lattices using polar codes. This thesis was in collaboration with the research center of Huawei France.