Télédiffusion | EPFL Graph Search

La télédiffusion, en anglais : broadcasting et en allemand Rundfunk désigne une technique de retransmission de signaux de télévision vers un grand nombre de récepteurs ou téléviseurs. La télédiffusion est un moyen unidirectionnel pour propager des signaux de télévision ne dépendant pas du nombre de récepteurs en fonction et en mode de retransmission numérique, ne subissant pas les limites ou dysfonctionnements éventuels, en termes de débit ou d'infrastructure de réseau local. La télédiffusion exploitée pour la transmission de signaux analogiques ou numériques, vidéo, audio et des données associées comme le télétexte et le guide électronique de programmes (EPG). La télédiffusion est définie conformément à ses caractéristiques de norme et standard de télévision. Principe général Un site d'émission peut être capté par les récepteurs figurant dans sa zone de couverture. L'émetteur reste opérationnel, que les récepteurs soient en fonctionnement ou pas. Le contrôle de la

Throughput Analysis of Large Networks

Serj Haddad

While wired infrastructure constitutes the backbone of most wireless networks, wireless systems appeal the most to the dynamic and rapidly evolving requirements of today's communication systems because of their ease of deployment and mobility, not to mention the high cost of building a wired infrastructure. This led to an increased interest in the so called wireless ad hoc networks formed of a group of users, known as nodes, capable of communicating with each other through a shared wireless channel. Needless to say, these nodes are asked to use the shared wireless medium in the most efficient fashion, which is not an easy task given the absence of wired backbone. This requires a profound understanding of the wireless medium to establish a decentralized cooperation scheme, if needed, that best utilizes the resources available in the wireless channel. A significant part of this thesis focuses on the properties of the shared wireless channel, whereby we are interested in studying the spatial diversity and the beamforming capabilities in large wireless networks which are crucial in analyzing the throughput of ad hoc networks. In this thesis, we mainly focus on the problem of broadcasting information in the most efficient manner in a large two-dimensional ad hoc wireless network at low SNR and under line-of-sight propagation. A new communication scheme, which we call multi-stage back-and-forth beamforming, is proposed, where source nodes first broadcast their data to the entire network, despite the lack of sufficient available power. The signal's power is then reinforced via successive back-and-forth beamforming transmissions between different groups of nodes in the network, so that all nodes are able to decode the transmitted information at the end. This scheme is shown to achieve asymptotically the broadcast capacity of the network, which is expressed in terms of the largest singular value of the matrix of fading coefficients between the nodes in the network. A detailed mathematical analysis is then presented to evaluate the asymptotic behavior of this largest singular value. We further characterize the maximum achievable broadcast rate under different sparsity regimes. Our result shows that this rate depends negatively on the sparsity of the network. This is to be put in contrast with the number of degrees of freedom available in the network, which have been shown previously to increase with the sparsity of the network. In this context, we further characterize the degrees of freedom versus beamforming gain tradeoff, which reveals that high beamforming gains can only be obtained at the expense of reduced spatial degrees of freedom. Another important factor that impacts the throughput in wireless networks is the transmit/receive capability of the transceiver at the nodes. Traditionally, wireless radios are half-duplex. However, building on self-interference cancellation techniques, full-duplex radios have emerged as a viable paradigm over the recent years. In the last part of this thesis, we ask the fundamental question: how much can full-duplex help? Intuitively, one may expect that full-duplex radios can at most double the capacity of wireless networks, since they enable nodes to transmit and receive at the same time. However, we show that the capacity gain can indeed be larger than a factor of 2; in particular, we construct a specific instance of a wireless network where the the full-duplex capacity is triple the half-duplex capacity.

EPFL2017

Broadcast, relay and feedback in Gaussian channels

Sibi Raj Bhaskaran Pillai

Network information theory explores the fundamental data transport limits over communication networks. Broadcasting and relaying are two natural models arising in communication contexts where multiple users share a common communication medium. Number of relay and broadcasting models are considered in this thesis. We first collect the existing methodologies of communication over a discrete memoryless relay channel. A new communication strategy over this channel is also proposed, which achieves the same data-rates as the best known schemes in literature, incurring a lower delay at the same time. We then consider the Gaussian broadcast channel (GBC), and show that its capacity region is enlarged by the availability of feedback, even if the feedback is from only one of the receivers. We further show a low complexity, explicit capacity achieving strategy over a GBC with degraded receivers. A combination of relay and broadcast models, called the relay-broadcast channel (RBC) is analyzed next. Achievable regions are proposed for different RBC scenarios. For the Gaussian version of an RBC, we characterize the complete capacity region when the receivers are degraded with respect to the relay. The final part of this thesis investigates communication over fading channels. We propose upper bounds for non-coherent, as well as lower bounds for coherent communication over a MIMO fading channel.

EPFL2007

Cooperative Data Exchange with Weighted Cost based on d-Basis Construction

Michael Christoph Gastpar, Su Li, Abhin Shah

We consider the cooperative data exchange problem, in which nodes are fully connected with each other. Each node initially only has a subset of the K packets making up a file and wants to recover the whole file. Node i can make a broadcast transmission, which incurs cost w_i and is received by all other nodes. The goal is to minimize the total cost of transmissions that all nodes have to send, which is also called weighted cost. Following the same idea of our previous work which provided a method based on d-Basis construction to solve cooperative data exchange problem without weighted cost, we present a modified method to solve cooperative data exchange problem with weighted cost. We present a polynomial-time deterministic algorithm to compute the minimum weighted cost and determine the rate vector and the packets that should be used to generate each transmission. By leveraging the connection to Maximum Distance Separable codes, the coefficients of linear combinations of the optimal coding scheme can be efficiently generated. Our algorithm has significantly lower complexity than the state of the art. In particular, we prove that the minimum weighted cost function is a convex function of the total number of transmissions for integer rate cases.

2017

Throughput Analysis of Large Networks

Serj Haddad

EPFL2017