Optimal Progressive Precoder Design for ARQ Packet Retransmissions in Nonregenerative MIMO Relay Systems

This paper investigates the precoder design for packet retransmissions in nonregenerative multi-input multi-output (MIMO) relay systems. Assuming the channel state information (CSI) only available at the relay and the destination, the precoder design performed at the relay is considered. To fully utilize the time diversity provided by automatic repeat request (ARQ), progressive precoders are designed with the objective of maximizing the mutual information delivered by multiple transmissions of the same packet. We derive the form of the optimal ARQ precoder by restating the optimization problem as a matrix diagonalization problem, then the power allocation is formulated as a convex optimization problem where the karush-kuhn-tucker conditions are used to obtain the optimal solution. Numerical results show that the proposed precoders can improve the system performance significantly.