Link adaptation is the terminology used to describe techniques that improve multicarrierudcommunication systems performance by dynamically adapting the transmission parameters,udi.e., transmit power and number of bits per subcarrier, to the changing quality ofudthe wireless link. The research literature has focused on single objective optimizationudtechniques to optimize the multicarrier communication systems performance, e.g., maximizingudthe throughput/capacity or minimizing the transmit power subject to a set ofudconstraints. In this dissertation, we adopt a novel optimization concept, namely multiobjectiveudoptimization, where our objective is to simultaneously optimize the conflictingudand incommensurable throughput and power objectives.udMore specifically, in Chapters 2 and 3, we propose novel algorithms that jointly maximizeudthe multicarrier system throughput and minimize its total transmit power subjectudto quality-of-service, total transmit power, and maximum allocated bits per subcarrierudconstraints. The proposed algorithms require prior knowledge about the importance ofudthe competing objective functions in terms of pre-determined weighting coefficients, orudthey can adapt the weighting coefficients during the solution process while meeting theudconstraints, in order to reduce the computational complexity. Simulation results show significantudperformance gains in terms of the achieved throughput and transmit power whenudcompared to single optimization approaches, at the cost of no additional complexity.udMotivated by the obtained results, in Chapter 4 the problem is extended to the cognitive radio environment where the multicarrier unlicensed/secondary user, with limitedudsensing capabilities, needs to satisfy additional constraints for the leaked interference toudexisting licensed/primary users. In Chapter 5, a multiobjective optimization problem isudformulated to balance between the SU capacity and the leaked interference to existing primaryudusers, where the effect of the imperfect channel-state-information on the links fromudthe secondary user transmitter to the primary users receivers is considered. Simulationudresults show improvements of the energy efficiency of the secondary user when comparedudto its counterparts of the works in the literature, with reduced computational complexity.udIn Chapter 6 we investigate the optimal link adaptation problem to optimize theudenergy efficiency of secondary users while considering the effect of imperfect channelstate-udinformation on the links between the secondary user transmitter and receiver pairsudand the limited sensing capabilities of the secondary user. The proposed link adaptationudalgorithm guarantees minimum required rate for the secondary user and statisticaludinterference constraints to the existing primary users.udFinally, conclusions and possible extensions to the optimal link adaptation problem isuddiscussed in Chapter 7.
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