| Recent years, the demand of mobile communication is not only voice communication, but also multi-rate multimedia communications. Next generation cellular systems must employ advanced techniques which not only increase the data rate but also enable the system to satisfy the quality of service(QoS). In order to combat the fading and variations of wireless channel, cooperative communication is proposed, which deals with the limitation of terminals'size and function.This thesis focuses on the study of power allocation of cooperative communication aiming at SER minimization. Firstly, multi-relay system model and simplified single-relay system model are introduced. In case of AF and DF cooperation systems, closed-form SER formulation is formulated with MPSK and MQAM signals, using moment generating function(MGF). Moreover, an SER approximation is obtained which is asymptotically tight at high SNR.Secondly, optimum power allocation is derived for both AF and DF systems based on the asymptotical approximation using Lagrange multiplier approach.Thirdly, the SER performance of AF and DF systems are compared by simulation with different modulation methods and power allocation schemes under various channel circumstances. Simulation results show that optimum strategy outperforms equal power allocation method in both AF and DF cooperation systems. And it appears that the OPA (optimum power allocation) scheme does not depend on the link between source and destination, but only depends on the channel links related to relay. When it comes to the comparison of AF and DF, the performance is almost the same at low SNR. As the SNR increases, AF is outperformed.Finally, an adaptive transmission scheme is proposed. Proper modulation, number of relays and power allocation scheme are chosen according to various channel states aiming at SER minimization. Simulation results show that the proposed adaptive protocol using OPA provides a lower SER compared with existing protocols. |
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