Retour au site du LAAS-CNRS

Abstract

SDMA (Spatial Division Multiple Access) is a principle of radio resource sharing that relies on the division of the space dimension into separated communication channels. It can be used with common Frequency DivisionMultiple Access (FDMA), Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) techniques. Main terrestrial communication standards already implement SDMA. SDMA basically relies on adaptive and dynamic beam-forming associated to a clever algorithm in charge of resource allocation. As satellite communication systems move towards an increasing number of users and a larger throughput for each of them, SDMA is one of the most promising techniques that can reach these two goals. This paper studies static Frequency Allocation Problems (FAP) in a satellite communication system involving a gateway connected to a terrestrial network and some user terminals located in a service area. Two scenarios are considered: one based on SDMA and the other based on usual spot coverage. We propose original integer linear programming formulations and greedy allocation algorithms for the FAP which involve unusual cumulative interference constraints. By considering the link budget of each user, the objective is to maximize the number of users that the system can serve. We show through computational experiments on realistic data that the FAP associated with the SDMA system can be solved efficiently, yielding substantial improvement compared to the traditional system.

Mots-Clés / Keywords

Abstract

In this paper, we study Discrete Events Dynamic Systems (DEDS) that can be modeled by a linear representation on (max,+) algebra. This class corresponds to Timed Event Graphs (TEG). A linear system theory has been developed for these particular systems. Strong analogies then appear between the classical linear system theory and the (max,+)-linear system theory. Some control problems for these systems have previously been tackled. In all these works, the problematic was to compute an optimal solution in regard to the just-in-time criterion, indeed the proposed control laws satisfy some given control objectives while delaying as much as possible occurrences of input or internal events. For our concern, the aim of the control is to delay as less as possible the system while ensuring some given specifications. For example, in a railway network, one can aim at limiting the number of trains in a path while minimizing the induced delays. Another possible application concerns production systems subject to critical time constraints, in which sojourn times of pieces must not exceed a given value at some stages. We may then be interested at bounding the sojourn times while delaying as less as possible the system. We consider two control structures: the control is firstly formalized as a state feedback on state and then as a state feedback on inputs.