3 Novembre 2011 « Utilisation de la modulation hiérarchique dans l’allocation des ressources dans les systèmes OFDMA ».

14h00 à l’Université Pierre et Marie Curie – Paris 6, amphi Herpin, batiment Escalangon.

Le Jury :

– M. Guy Pujolle, Président, Université Pierre et Marie Curie, France M. – Farouk Kamoun, Rapporteur, Ecole Nationale des Sciences de l’Informatique, Tunisie
– M. Fabio Martignon, Rapporteur, Université Paris-Sud 11, France
– M. Eitan Altman, Examinateur, INRIA Sophia Antipolis, France
– Mme. Berna Sayrac, Examinatrice, Orange Labs, Issy-Les-Moulineaux, France
– M. Tijani Chahed, Directeur de thèse, Telecom SudParis, France

Abstract :

We investigate, in this thesis, the use of Hierarchical Modulation (HM), a physical layer technique that enables to exploit multiuser diversity, for resource allocation in OFDMA-based systems, so as to improve the system capacity.

HM allows the sharing of the resources, namely subcarriers and power, between users of different radio conditions by sending an additional stream to a user with good radio conditions on a subcarrier that was initially allocated to carry an original stream to a user with lower radio conditions. And this, without affecting the original user’s rate nor the total amount of power assigned to the shared subcarrier.

Our study will be carried out at the flow-level, for a dynamic configuration where users come to the system at random times and leave it after a finite time duration corresponding to the completion of their services. This enables us to evaluate the system performance in terms of system-level metrics, such as mean transfer time and blocking probability, which are meaningful both to the user and to the network operator/provider.

We, first, focus on the flow-level modeling of the use of HM in an OFDMA-based cell, where resources are shared between users of different classes based on time division multiplexing, and quantify the gains achieved both individually and globally. We also propose and evaluate two novel extensions to the use of HM in such a setting and show the larger gains they permit.

We, second, study the use of HM with proportional fairness, an algorithm which belongs to the so-called alpha-fair allocation strategies and which is known to achieve a good trade-off between efficiency and fairness among those strategies. We evaluate the system performance and show that, with HM, a simple round robin mechanism achieves larger gains at an appreciably lower implementation complexity cost.

We, third, study the use of HM in relay-based OFDMA systems, considering both cooperative and non-cooperative relaying schemes. In the cooperative case, we propose a scheduling scheme which uses jointly HM and relaying to enable the relay to produce a more robust copy of the original signal. We also make use of link adaptation between the relay and the destination in order to minimize the cost of the resources needed for relaying. We propose an enhancement that takes advantage of the good radio conditions of users who are close to the base station to send them an additional stream using HM.

For the non-cooperative case, we propose a joint use of HM and relaying in the case where the relay is not needed for the successful decoding of the signal at the destination. Without HM, the use of relays in this case would be both inefficient and useless. With HM, users, both individually and globally, achieve a better performance, in terms of mean transfer times and blocking probabilities.