In this chapter, we discuss the fundamental limit – the Shannon limit – on data rate for a communications link. We motivate this limit by providing a sketch of the derivation. To construct this sketch, we discuss the idea of the ratio of hypersphere volumes and how the radius of Gaussian vectors converges to a known radius as the dimensionality goes to infinity. We provide a second approach to think about the Shannon limit, or equivalently channel capacity, of a data link by considering mutual information and entropy. By using a simple line-of-sight channel, we discuss the resulting link theoretical capacity and an approach to estimating the practical limit on data rate. Finally, we provide an overview of source coding approaches.

Digital communication systems are ubiquitous. Examples of digital communication systems include cell phones, Bluetooth, WiFi, and cable modems. This book explores in depth how these communication systems work and the fundamental limits on the performance of digital communication systems. We begin in this chapter with a high-level description of digital communication systems so as to understand the trade-offs in designing a communication system. We also explore the fundamental limits that can be achieved in terms of the data rate possible for a given bandwidth and the energy needed for a given level of noise.

In this chapter, differences between magnetic communication and electromagnetic wave-based communication are summarized and major advantages of magnetic communication are discussed, which provides a big picture of the applicable scenarios of magnetic communication. In addition, the physical circuit for magnetic communications is introduced. The fundamental performance metrics, such as path loss, bandwidth, capacity, and connectivity are discussed.

Communication stack

For convenience in design, the operations of radios are often broken into a number of functional layers. The standard version of this stack is referred to as the open systems interconnection (OSI) model [291], as seen in Figure 4.1. The model has two groups of layers: host and media. The host layers are the application, presentation, session, and transport layers. The media layers are the network, data-link, and physical layers. In many radio systems, some of these layers are trivial or the division between the layers may be blurred. The OSI stack is commonly interpreted in terms of wired networks such as the internet. Depending upon the details of an implementation, various tasks may occupy different layers. Nonetheless, the OSI layered architecture is useful as a common reference for discussing radios. In this text, the media layers are of principal importance.

The network layer indicates how data are routed from an information source to a sink node, as seen in Figure 4.2. In the case of a network with two nodes, this routing is trivial. In the case of an ad hoc wireless network, the routing may be both complicated and time varying. The network layer may break a data sequence at the source node into smaller blocks and then reassemble the data sequence at the sink node. It also may provide notification of errors to the transport layer.

In this chapter, we have provided an overview of ambient backscatter communication systems. Firstly, we have introduced the fundamentals of modulated backscatter and its three main configurations, i.e., monostatic, bistatic, and ambient backscatter communication systems. Then, key channel-coding and modulation techniques in modulated backscatter communication systems are discussed. Two major types of backscatter communication channels and their link budgets are also introduced. Next, theoretical analyses and experimental measurements of backscatter channels are reviewed. Finally, we have discussed some research challenges of backscatter communication systems, especially ambient backscatter communication systems.

Chapter 3 discusses the fundamentals of backscatter radio communications, analyzes the RFID backscatter channel, its major limitations and mitigation approaches, and presents recent advances including novel RFID quadrature backscatter modulation techniques.

The initial Community treaties establishing the ECSC, the EEC and EURATOM did not contain any fundamental rights provisions at all. The 1953 Draft Treaty embodying the Statute of a European Political Community envisaged human rights protection as a major task and proposed to incorporate the European Convention on Human Rights (ECHR), a treaty concluded by many European states in 1950 under the auspices of the Council of Europe and enforced by the European Court of Human Rights (ECtHR) in Strasbourg. After the plans for a European Defence Community were buried by the French National Assembly in 1954, this idea also became obsolete. With the resurgence of the ‘functionalist approach’, culminating in the 1957 Rome Treaties, the view prevailed that the economic integration now pursued did not warrant the inclusion of human rights guarantees.

With the growth of Community activities, however, the likelihood of infringement of fundamental rights also increased. Clearly, the extension of Community law into many fields beyond the core aspects of the four freedoms was not a wholly unintended ‘spill-over effect’ of economic integration. This tendency was reinforced by the specific development of EC law, in particular of direct effect and supremacy in such landmark cases as Van Gend en Loos and Costa v. ENEL. Both direct effect and supremacy increase the probability that it is EC law itself and not any national implementation of Community obligations that may infringe human rights.