Sum-Rate Maximization in Two-Way MIMO Cooperative Networks With an Energy Harvesting Relay

This study considers a two-way amplify-and-forward relay system wherein two users exchange information with the assistance of an energy-harvesting MIMO relay node. With two-time-slot transmission, the relay receives the transmitted signal from both users in the first time-slot and splits it into two parts for information forwarding and energy harvesting, respectively, in accordance with a particular power splitting ratios. The phases of the RF signals used for energy harvesting at each relay antenna are shifted before being combined to achieve the highest power. Meanwhile, the RF signals used for information forwarding are passed through a linear precoder and then broadcast to the users in the second time-slot. We aim to optimize the power splitting ratios, precoding matrix, and RF phase shifting factors such that the sum-rate is maximized. Since joint optimization is intractable, the parameters are partitioned into three sub-optimization problems. For each parameter, a sub-optimal solution is derived in a (semi-)closed form. It shows numerically that the proposed sub-optimal solutions for the optimal phase shift and power split ratio are close to the global optimal solutions. Moreover, the proposed solution for the precoding matrix outperforms the so-called 'General Power Iterative' algorithm proposed in the literature.