Precoder design for non-regenerative MIMO relay systems with partial channel state information

Abstract

The source and relay precoders for multiple-input multiple-output (MIMO) systems with a non-regenerative relay can be designed to maximize the capacity between the source and destination when the full channel state information (CSI) is available at the source and relay. In a practical system with a limited feedback rate, however, it is hard to obtain the full CSI at the transmitter. We design the source and relay precoders with partial CSI such as mean and covariance information of the relay-destination (R-D) channel. To solve the precoder design problem with partial CSI, new partial order relations for MIMO channels are obtained in terms of the outage probability and ergodic capacity. Using the partial ordering, the optimal structure of a relay precoder is derived to maximize the ergodic capacity with covariance information of the R-D link. The suboptimal structure of a relay precoder is also proposed with mean and covariance information of the R-D link using Wishart approximation. Furthermore, the optimal structures of the source and relay are jointly obtained when the covariance information of the R-D link is available at the relay. With the optimal and suboptimal precoder structures, the relay power allocation problem is solved by several methods. The optimal power allocation method using the difference of convex functions (DC) programming is presented but it has a high computational complexity. Jensen approach is simple and gives the closed-form solution but it is suboptimal. The suboptimal modified Jensen method is proposed with the low complexity and near optimal performance. For joint precoding scheme, the iterative power allocation is used. In numerical results, it is shown that the modified Jensen method has the nearly optimal performance. The performance gap between the partial CSI schemes and the full CSI scheme decreases as the correlation coefficient or Rician factor increases. It is also shown that the joint precoding gives the performance gai...