Communications system | EPFL Graph Search

A communications system or communication system is a collection of individual telecommunications networks systems, relay stations, tributary stations, and terminal equipment usually capable of interconnection and interoperation to form an integrated whole. The components of a communications system serve a common purpose, are technically compatible, use common procedures, respond to controls, and operate in union. Telecommunications is a method of communication (e.g., for sports broadcasting, mass media, journalism, etc.). Communication is the act of conveying intended meanings from one entity or group to another through the use of mutually understood signs and semiotic rules. Types By media An optical communication system is any form of telecommunication that uses light as the transmission medium. Equipment consists of a transmitter, which encodes a message into an optical signal, a communication channel, which carries the signal to its destination, and a rece

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

Video Multicast in (large) local area networks

Martin Vetterli, Jean-Paul Wagner

We consider the problem of distributing high-quality video signals over IP multicast in large Local Area Networks (LANs), under real-time delay constraints, and with software-only processing. In a large LAN (such as the network of a university campus, or the network of a large company), the source of channel heterogeneity that the video communications system must cope with is not that of different bandwidth constraints available to each receiver, but it is essentially variations in the available CPU power that each receiver will have in order to decode the incoming signal. In this paper we propose a new architecture for a video multicast system, present the design of the different components of this system, and show results obtained in a real implementation. Our feeds consist of video encoded at about 3 Mbits/sec and 16 frames/sec, capable of tolerating the loss of about 300 Kbits/sec worth of data. Decoding is performed on Linux PCs, and the quality of our reconstructed signals degrades gracefully with the speed of the CPU on which the receiver runs.

2002

Throughput Analysis of Large Networks

Serj Haddad

EPFL2017