Application of network coding in satellite broadcast and multiple access channels

Satellite broadcasting and relaying capabilities enable mobile broadcast systems over wide geographical areas, which opens large market possibilities for handheld, vehicular and fixed user terminals. The geostationary (GEO) satellite orbit is highly suited for such applications, as it spares the need for satellite terminals to track the movement of the spacecraft, with important savings in terms of complexity and cost. The large radius of the GEO orbit (more than 40000 km) has two main drawbacks. One is the large free space loss experienced by a signal traveling to or from the satellite, which limits the signal-to-noise ratio (SNR) margins in the link budget with respect to terrestrial systems. The second drawback of the GEO orbit is the large propagation delay (about 250 msec) that limits the use of feedback in both the forward (satellite to satellite terminal) and the reverse (satellite terminal to satellite) link. The limited margin protection causes loss of service availability in environments where there is no direct line of sight to the satellite, such as urban areas. The large propagation delay on its turn, together with the large terminal population size usually served by a GEO satellite, limit the use of feedback, which is at the basis of error-control. In the reverse link, especially in the case of fixed terminals, packet losses are mainly due to collisions, that severely limit the access to satellite services in case a random access scheme is adopted. The need for improvements and further understanding of these setups lead to the development of our work. In this dissertation we study the application of network coding to counteract the above mentioned channel impairments in satellite systems. The idea of using network coding stems from the fact that it allows to efficiently exploit the diversity, either temporal or spatial, present in the system. In the following we outline the original contributions included in each of the chapters of the dissertation. Chapter 3. This chapter deals with channel impairments in the forward link, and specifically with the problem of missing coverage in Urban environments for land mobile satellite (LMS) networks. By applying the Max-flow Min-cut theorem we derive a lower bound on the maximum coverage that can be achieved through cooperation. Inspired by this result, we propose a practical scheme, keeping in mind the compatibility with the DVB-SH standard. We developed a simulator in Matlab/C++ based on the physical layer abstraction and used it to test the performance gain of our scheme with a benchmark relaying scheme that does allow coding at packet level. Chapter 4. The second chapter of contributions is devoted to the information theoretical study of real-time streaming transmissions over fading channels with channel state information at the transmitter only. We introduce this new channel model and propose several transmission schemes, one of which is proved to be asymptotically optimal in terms of throughput. We also provide an upper bound on the achievable throughput for the proposed channel model and compare it numerically with the proposed schemes over a Rayleigh fading channel. Chapter 5. Chapter 5 is devoted to the study of throughput and delay in non-real-time streaming transmission over block fading channels. We derive bounds on the throughput and the delay for this channel and propose different coding techniques based on time-sharing. For each of them we carry out an analytical study of the performance. Finally, we compare numerically the performance of the proposed schemes over a Rayleigh fading channel. Chapter 6. In the last technical chapter we propose a collision resolution method for the return link based on physical layer network coding over extended Galois field (EGF). The proposed scheme extracts information from the colliding signals and achieves important gains with respect to Slotted ALOHA systems as well as with respect to other collision resolution schemes.Una de les característiques mes importants de les plataformes de comunicacions per satèl.lit és la seva capacitat de retransmetre senyals rebuts a un gran número de terminals. Això es fonamental en contextes com la difusió a terminals mòbils o la comunicació entre màquines. Al mateix temps, la disponibilitat d’un canal de retorn permet la creació de sistemes de comunicacions per satèl.lit interactius que, en principi, poden arribar a qualsevol punt del planeta. Els satèl.lits Geoestacionaris son particularment adequats per a complir amb aquesta tasca. Aquest tipus de satèl.lits manté una posició fixa respecte a la Terra, estalviant als terminals terrestres la necessitat de seguir el seu moviment en el cel. Per altra banda, la gran distància que separa la Terra dels satèl.lits Geoestacionaris introdueix grans retrassos en les comunicacions que, afegit al gran número de terminals en servei, limita l’ús de tècniques de retransmissió basades en acknowledgments en cas de pèrdua de paquets. Per tal de sol.lucionar el problema de la pèrdua de paquets, les tècniques més utilitzades son el desplegament de repetidors terrestres, anomenats gap fillers, l’ús de codis de protecció a nivell de paquet i mecanismes proactius de resolució de col.lisions en el canal de retorn. En aquesta tesi s’analitzen i s’estudien sol.lucions a problemes en la comunicació per satèl.lit tant en el canal de baixada com el de pujada. En concret, es consideren tres escenaris diferents. El primer escenari es la transmissió a grans poblacions de terminals mòbils en enorns urbans, que es veuen particularment afectats per la pèrdua de paquets degut a l’obstrucció, per part dels edificis, de la línia de visió amb el satèl.lit. La sol.lució que considerem consisteix en la utilització de la cooperació entre terminals. Una vegada obtinguda una mesura del guany que es pot assolir mitjançant cooperació en un model bàsic de xarxa, a través del teorema Max-flow Min-cut, proposem un esquema de cooperació compatible amb estàndards de comunicació existents. El segon escenari que considerem es la transmissió de vídeo, un tipus de tràfic particularment sensible a la pèrdua de paquets i retards endògens als sistemes de comunicació per satèl.lit. Considerem els casos de transmissió en temps real i en diferit, des de la perspectiva de teoria de la informació, i estudiem diferents tècniques de codificació analítica i numèrica. Un dels resultats principals obtinguts es l’extensió del límit assolible de la capacitat ergòdica del canal en cas que el transmissor rebi les dades de manera gradual, enlloc de rebre-les totes a l’inici de la transmissió. El tercer escenari que considerem es l’accés aleatori al satèl.lit. Desenvolupem un esquema de recuperació dels paquets perduts basat en la codificació de xarxa a nivell físic i en extensions a camps de Galois, amb resultats molt prometedors en termes de rendiment. També estudiem aspectes relacionats amb la implementació pràctica d’aquest esquema

Seek and Decode: Random Multiple Access with Multiuser Detection and Physical-Layer Network Coding

We present a novel random multiple access scheme that combines joint multiuser detection (MUD) with physical-layer network coding (PLNC) over extended Galois fields (EGF). We derive an analytical bound to the throughput at the system level and present simulation results for the decoding at the physical level in both fast fading and block fading channels. We adopt a cross layer approach in which a non-binary joint multiuser decoder is used in combination with PLNC at slot level, while the use of EGF increases the system diversity at frame level. The results we present are encouraging and suggest that the combination of these two interference management techniques can significantly enhance the performance of random multiple access systems

Cardiac pathology and modern therapeutic approach in Behçet disease

Behçet disease (BD) is an enigmatic inflammatory disorder with multisystemic complications which is endemic in some countries but can be seen in the entire world. Valid diagnostic criteria are available. The pathology is related to a specific perivasculitis with involvement of both arteries and veins of all sizes. Minor arterial and cardiac involvement is frequent in BD but is usually asymptomatic. In exceptional cases cardiac symptoms may be the 1st manifestation of BD. The prevalence of severe cardiac complications (cardio-Behçet) should be < 10%. An impressive therapeutic improvement has been achieved by using appropriate catheterization techniques, coronary and intra-arterial stents, colchicine, drug-response modifying drugs and immunotherapy but, still cardio-Behçet has a poor prognosis. Efforts are undertaken to improve morbidity and prognosis with the use of newer drugs. An important part of the complications in BD are related to the frequent thromboembolic complications and there is high possibility that newer oral anticoagulants will be superior to the classical anticoagulants presently used. Available biologic agents have already been frequently used and seem to have improved the prognosis, but efforts are undertaken to find newer biologic agents with better therapeutic performance and less side-effects. Summarizing as much as possible the effects of the presently used biotherapy in BD, interferon-alpha is effective against many ocular, genital and perhaps vascular manifestations, but its effectiveness is limited by frequent adverse-effects (even if not dangerous for the cardiovascular system). Infliximab is a valid option in the therapy of ocular and cutaneous manifestations but it is less convincing in the therapy of vascular manifestations in vascular- and neuro-Behçet; furthermore, side-effects, including severe cardiovascular complications, are seen in a minority of patients; perhaps worse, infliximab seems to loose efficacy in the long-term therapy, while pharmacogenetics and receptor polymorphism may explain the existence of non-responders and the occurrence of resistance. Adalimumab might be a promising alternative for infliximab and seems to exert a good effect in an eurysmatic and other vascular complications. However, we lack long-term studies. Other biologic agents have been used only in few cases and it is too early to say if they offer new therapeutic perspectives

Assessing the benefits of natriuretic peptides-guided therapy in chronic heart failure

Heart failure (HF) is a major public heart burden among the ageing population. Optimizing management of patients remains challenging despite many advances in therapy for this patho­logy. Natriuretic peptides (NPs) are related to cardiac morbidity and mortality and their use in guiding treatment might help. Most data on the value of NPs-guided therapy in chronic HF came from centers with high experience in the therapy of HF. Ninety percent of patients had preserved left ventricular function. The story is just too complex to have the final answer. The numbers of treated patients is insufficient to allow a final decision. Most data derive from centers with high skills and were obtained with different assays, different protocols. Many questions are open. Can similar results be obtained in less specialized centers? It is undecided which NP should be used and how high should be the levels to guide the therapy. Which patients might especially benefit from this approach? Is the approach useful in patients with reduced systolic function? Is the strategy as useful in the elderly as in younger patients? In spite of these limitations, available data suggest that it is reasonable to consider the use of NPs to guide the therapy of HF with preserved systolic function. In order to answer some of the questions, a multicenter, prospective study began in January 2013. However, NP guided therapy in chronic HF will only find acceptance in clinical practice if its use results in therapeutic consequences

Angina pectoris in patients without flow-limiting coronary artery disease (cardiac syndrome X). A forest of a variety of trees

Coronary heart disease (CHD) represents an important problem worldwide. At present, more women than men are evaluated for CHD and it has been recognized that the prevalence of this pathology in women is at least the same as in men. We have learned that cardiac syndrome X (CSX) is frequent because worldwide each year millions of people (mostly women) with angina pectoris without flow-limiting epicardial pathology are identified. Data from large myocardial infarction registries suggest a 5% to 25% prevalence of cases without flow-limiting coronary pathology. It must, however, be considered that these people are said to have normal coronary arteries by visual analysis of biplane coronarography. On the other hand, as demonstrated from autopsy, and in vivo by ultrasound intravascular studies, it would be more appropriate to say that in the majority of these cases no obstructive or flow-limiting coronary pathology was detected by coronarography. In CSX, endothelial dysfunction and microvascular dysfunction, sometimes with coronary microvascular spams and epicardial coronary artery spasm, have been recognized as pathophysiologic mechanisms. In CSX, symptoms and pathologic signs are the same in patients with flow-limiting coronary pathology. The difference lies in the fact that the mechanisms of myocardial ischemia are microvascular and flow-limiting epicardial coronary pathology is absent. By interplay, the pathologic entities at work in CSX are linked with poor long-term outcome. The prevalence of these outcomes is probably smaller than in patients with flow-limiting coronary pathology but we lack precise values. Nonetheless, severe cardiovascular complications are frequent in CSX and it is thus the pathology is not benign. Drugs used in coronary ischemic disease are empirically prescribed to treat CSX, but we lack data from specific trials. It seems that statins and ranolazine might exert positive effects. However, specific research to target interventions in CSX would be necessary

New concepts in the therapy of atrial fibrillation

Atrial fibrillation is a frequent arrhythmia with increasing prevalence. The paper reviews the most important present aspects and paradigms in the treatment of the arrhythmia. The main aim of treatment is directed to improve the quality of life while reducing morbidity and mortality. A large experience derived from epidemiological registers and clinical research, impressive advances in interventional electrophysiological therapies and the introduction of non-vitamin K antagonists had a dramatic impact on the medical approach. Recommended steps to classify and treat atrial fibrillation are presented and discussed

Interference Cancellation and Joint Decoding for Collision Resolution in Slotted ALOHA

We present a novel decoding scheme for slotted ALOHA which is based on concepts from physical-layer network coding (PNC) and multi-user detection (MUD). In addition to recovering individual user packets from a packet collision as it is usually done with MUD, the receiver applies PNC to decode packet combinations that can be used to retrieve the original packets using information available from other slots. We evaluate the novel scheme and compare it with another scheme based on PNC that has been proposed recently and show that both attain important gains compared to basic successive interference cancellation. This suggests that combining PNC and MUD can lead to significant gains with respect to previously proposed methods on either one or the other