logo supelec

Une grande école d'ingénieurs au cœur des sciences
de l'information, de l'énergie et des systèmes


Scientific objectives

The design of wireless communication systems dates from the days of Guglielmo Marconi (1874-1937). Traditionally, much of the engineering effort has focused on combating the adverse effect of multipath fading, i.e., the dramatic variation of channel strength over time and frequency. In the past decade, however, some authors have suggested a new point of view directly inspired by Shannon's information theory: that fading can be viewed as a benefit to be exploited, not necessarily a foe. Research based on this new viewpoint makes it possible to increase the spectral efficiency of wireless systems by an order of magnitude. Two specific developments along this approach are multiple antenna and opportunistic communications which have both stimulated theoretical analysis and experimentation over the last few years. In parallel, the field of channel coding has seen an extraordinary breakthrough with the advent of turbo codes (1991) and the rebirth (1995) of Gallager’s low-density parity check (LDPC) codes (1963). Even though it has been fewer than 15 years since the publication of Berrou and Glavieux landmark paper, turbo codes and related LDPC codes have already had a significant impact in practice. Indeed, almost all digital communication and storage system standards that involve error-correction are being upgraded to include these new capacity-approaching coding schemes. It is obvious from this explosion of academic and technological activities that turbo codes, and more generally turbo-inspired concepts throughout the rest of the digital communication chain (following the turbo principle), whose great success is not fully understood yet, are revolutionizing the way of information is transmitted (or stored) and make conceivable (and feasible) new communication strategies derived directly from information theory.

Wireless communication networks are typically naturally multiuser systems, often designed as centralized cellular systems. A multiuser communication environment differs from a basic single-user environment in several crucial aspects which makes it substantially more complicated. As multiple transmitters and receivers share the same communication medium, they cause mutual interference to each other. Unless properly processed, interference is typically detrimental to the system performance, so that multiuser environment is inherently competitive. On the other hand, interaction among the users creates opportunities for the multiple transmitters to cooperate, especially in the presence of fading (e.g., opportunistic communications, relaying, user cooperation etc.). Cooperation can also take place at the receiver side (e.g., multiuser detection). So, in fact, multiuser channels bring an interesting interplay between competition and cooperation, which gives raise to lots of unsolved questions. Further, cooperation often involves feedback, whose role has so far resisted rigorous information-theoretical treatment in most cases. Due to these complexities, multiuser information theory, which paves the way for practical system design, is still largely incomplete and multiuser channel capacity problems represent some of the most challenging open problems in information theory.

In the hierarchy of network design, the role of information processing has traditionally been relegated to two layers of the protocol stack: source coding at the application layer and technology to maintain link reliability at the physical layer. Such a layered conceptual decomposition is certainly one of the reasons that have led to the phenomenal success of the Internet and the existing wireless cellular systems. However, layering does not necessarily capture the inherent complexity of the operations inside the network. It is one of the basic motivations of the M2 SAR to explore the considerable improvements that a new integrated approach, often referred to as crosslayer design and optimization (which is systemic in nature and not unlike the turbo principle itself), could potentially bring compared to the conventional disjoint approach. Prominent among the critical issues to solve is the problem of the different time scales at which events occur in a typical multimedia mobile wireless network, e.g., the changes in topology or user traffic occur at much slower rate than signal-to-noise ratio variations due to fast fading. This implies that the physical and medium access layers should react much faster than the network and higher layers. The layers' behaviors, though, are clearly coupled, so engineers need to consider novel integrated protocols and mechanisms permitting fast adaptation at the local layer before information is exchanged across layers. Other open questions concern the type of information that should be exchanged across layers and the ways the different layers could exploit this information.

Naturally as multidisciplinary as the field of wireless communications is, the Master SAR is meant to provide students with the theoretical background (including recent mathematical tools developments in random matrix theory, game theory, graphical models and statistical physics) that will enable them to understand those important conceptual breakthroughs and new paradigms and to speculate about the best way they could be integrated into future (cooperative) wireless systems.

Last update : 05/05/2010