M Werner - Böcker

The important role that satellites play in the field of communications will be manifested by a large number of new satellite systems to be implemented within the next few years. These new systems will basically belong to two categories: (i) satellite networks for mobile/personal communications, mainly with handheld terminals, and (ii) satellite networks for broadband multimedia communications, mainly for fixed but also for portable and mobile terminals. This book gives an overview of both families of satellite systems. In Part I, the basics of geostationary and non-geostationary satellite constellations are dealt with, as well as the principles of satellite communications. Part II deals with satellite systems for mobile/personal communications and ad­ dresses various aspects of networking (multiple access, cell structure, routing, etc.); it also deals with technology, regulation, and financing. Part III is ded­ icated to future satellite systems for broadband communications (Internet, multimedia) and discusses satellite-specific aspects of broadband communi­ cations, in particular on the basis of ATM and TCP /IP. A survey of existing and planned satellite systems completes the book. The authors of this book are scientists at the German Aerospace Center, DLR (Deutsches Zentrum fiir Luft- und Raumfahrt) and work in the Digital Networks group of the Institute for Communications and Navigation.

After almost half a century of existence the main question about quantum field theory seems still to be: what does it really describe? and not yet: does it provide a good description of nature? J. A. Swieca Ever since quantum field theory has been applied to strong int- actions, physicists have tried to obtain a nonperturbative und- standing. Dispersion theoretic sum rules, the S-matrix bootstrap, the dual models (and their reformulation in string language) and s the conformal bootstrap of the 70 are prominent cornerstones on this thorny path. Furthermore instantons and topological solitons have shed some light on the nonperturbati ve vacuum structure respectively on the existence of nonperturbative "charge" s- tors. To these attempts an additional one was recently added', which is yet not easily describable in terms of one "catch phrase". Dif- rent from previous attempts, it is almost entirely based on new noncommutative algebraic structures: "exchange algebras" whose "structure constants" are braid matrices which generate a ho- morphism of the infini te (inducti ve limi t) Artin braid group Boo into a von Neumann algebra.Mathematically there is a close 2 relation to recent work of Jones * Its physical origin is the resul t of a subtle analysis of Ei nstein causality expressed in terms of local commutati vi ty of space-li ke separated fields. It is most clearly recognizable in conformal invariant quantum field theories.