Cooperative diversity has been proposed to enhance the performance of high-rate communications over wireless channels. It realizes spatial diversity advantages in a distributed manner where two or more nodes share their antennas to mimic a virtual antenna array. The current research on cooperative diversity assumes the availability of perfect channel state information at the receiver. However, in practice, the fading channel coefficients need to be first estimated and then can be used in the detection process. This book first presents a comparative performance evaluation of cascaded channel estimation and disintegrated channel estimation techniques proposed for amplify-and-forward (AaF) relaying. Afterwards, the error rate performance of AaF relaying with partially-coherent and mismatched-coherent receivers is analyzed and the achievable diversity orders for different protocols are demonstrated. Finally, an optimal power allocation scheme is proposed for AaF cooperative systems in the presence of channel estimation. Taking SNR ratio at the destination node as the objective function, the optimum allocation of training/data power to broadcasting and relaying phases is determined.