Techniques de coopération dans les réseaux à radio cognitive : conception et évaluation des performances

Cognitive radio technology allows unlicensed users (called secondary users) to transmit on the licensed frequency bands, without degrading the quality-of-service of the licensed users' transmissions (called primary users). In order to avoid undesirable interference at the primary receivers, the secondary transmitters have to limit their transmit power. This limitation may cause performance degradation for the secondary transmissions. Combining cognitive radio with other emerging technologies, such as user cooperation and multiple antennas may have many benefits on both the primary and secondary transmissions. In this dissertation, we propose and investigate different cooperation techniques for the cognitive radio network. Some of the proposed techniques exploit antenna selection or precoding at one multi-antenna relay node, while the other proposed techniques use relay(s) selection when several single-antenna relay nodes are available to assist either the primary, the secondary or both transmissions simultaneously. The associated power allocation problem or pre-coding problem is investigated and the exact expressions of the primary and secondary outage probabilities or the secondary ergodic capacity are derived. For some of the proposed techniques, the imperfect knowledge of the channel states at the secondary transmitters is taken into account, hence modifying the power allocation or pre-coding process. We show by analysis and simulation that each of the proposed cooperation technique is able to improve significantly the secondary performance with respect to the primary Quality-of-Service. The realized performances outperform those of conventional transmission techniques used in the context of cognitive radio networks.