We study the design of a DVB-S2 system in order to maximise spectrum efficiency. This task is usually challenging due to channel variability. Modern satellite communications systems such as DVB-SH and DVB-S2 rely mainly on a time sharing strategy to optimise the spectrum efficiency. Recently, we showed that combining time sharing with hierarchical modulation can provide significant gains (in terms of spectrum efficiency) compared to the best time sharing strategy. However, our previous design does not improve the DVB-S2 performance when all the receivers experience low or large signal-to-noise ratios. In this article, we introduce and study a hierarchical QPSK and a hierarchical 32-APSK to overcome the previous limitations. We show in a realistic case based on DVB-S2 that the hierarchical QPSK provides an improvement when the receivers experience poor channel condition, while the 32-APSK increases the spectrum efficiency when the receivers experience good channel condition.

This paper argues that algorithmic skeletons are a suitable programming model for multi-core architectures. The high-level abstractions offered by algorithmic skeletons provide a simple way for non-parallel programmers to address parallel programming. Previous algorithmic skeleton frameworks and libraries have addressed distributed computing environments such as Clusters and Grids. This paper proposes a parallel skeleton library, Skandium; and concludes, after an experimental evaluation, that algorithmic skeletons are an effective methodology to program multi-core architectures.

Omniscient debuggers make it possible to navigate backwards in time within a program execution trace, drastically improving the task of debugging complex applications. Still, they are mostly ignored in practice due to the challenges raised by the potentially huge size of the execution traces. This paper shows that omniscient debugging can be realistically realized through the use of different techniques addressing efficiency, scalability and usability. We present TOD, a portable Trace-Oriented Debugger for Java, which combines an efficient instrumentation for event generation, a specialized distributed database for scalable storage and efficient querying, support for partial traces in order to reduce the trace volume to relevant events, and innovative interface components for interactive trace navigation and analysis in the development environment. Provided a reasonable infrastructure, the performance of TOD allows a responsive debugging experience in the face of large programs.

In new distributed systems, object mobility is usually allowed and is sometimes used by the underlying object manager system to benefit from object access locality. On the other hand, in-transit references to objects can exist at any moment in asynchronous distributed systems. In the presence of object mobility and in-transit references, many garbage collector (GC) algorithms fail to operate correctly. Others need to use the system's object finder to find the objects while performing their work. As a general principle, a GC should never interfere with object manager polices (such as forcing migration or fixing an object to a given processor). However, if the GC uses the object finder, it will change the access pattern of the system, and eventually it could foul the global allocation policy. In this article we propose a new GC family, Indirect Garbage Collectors, allowing to separate the problems of object management (placement, replication, and retrieval) from garbage collection. This property allows our algorithms to be implemented on top of almost any existent distributed object system, without having to use the object finder.

Distributed symbolic computations involve the existence of remote references allowing an object, local to a processor, to designate another object located on another processor. To reclaim inaccessible objects is the non trivial task of a distributed Garbage Collector (GC). We present in this paper a new distributed GC algorithm which (i) is fault-tolerant, (ii) is largely independent of how a processor garbage collects its own data space, (iii) does not need centralized control nor global stop-the-world synchronization, (iv) allows for multiple concurrent active GCs, (v) does not require to migrate objects from processor to processor and (vi) eventually reclaims all inaccessible objects including distributed cycles.