Optimal Sequential Precoder Design for MIMO-Relay-ARQ Systems with Direct Links

This paper investigates the optimal retransmission precoder design in nonregenerative multi-input multi-output (MIMO) relay systems, where the direct links are considered. Aiming at maximizing the retransmission mutual information, we first derive the form of the optimal retransmission precoder by utilizing the matrix theory. After that, the precoder optimization problem, namely the power allocation, is formulated as a convex optimization problem, where the karush-kuhn-tucker conditions are used to obtain the optimal solution. Through extensive simulations, our proposed precoder scheme shows significant performance gains compared to the conventional retransmission precoder.