Joint source and relay precoder design for MIMO relay systems with direct link

A new precoding scheme based on mutual information maximization for amplify-and-forward MIMO relay systems with considering the direct link is proposed. We study the mutual information of the system and set a total power constraint on the sum power of the source and the relay. The problem we formulate is too difficult to solve directly. By using properly designed precoder structure and the Hardamard inequality, we finally change the original problem into scalar form. We solve it in an iterative fashion and by employing the Karash-Kuhn-Tucker conditions, we give out closed-form solutions to each iteration step. Simulation results demonstrate that the proposed design method converges rapidly and significantly outperforms the existing methods.