Understanding the Optimal Power Allocation Structure in AF-Based MIMO Cooperative Networks with Multiple Relays

Cooperative networking is known to have significant potential in increasing network capacity and transmission reliability. Although there have been extensive studies on applying cooperative networking in multi-hop ad hoc networks, most works on cooperative network optimization are limited to the basic three-node relay scheme and single-antenna systems. These two limitations are interconnected and both due to a limited understanding of the optimal power allocation structure in MIMO cooperative networks (MIMO-CN). In this paper, we study in depth the structural properties of the optimal power allocation in MIMO-CN. More specifically, we show that the optimal power allocation at the source and each relay follows a matching structure in MIMO-CN. This result generalizes the power allocation result under the basic three-node setting to the multi-relay setting, for which the optimal power allocation structure has been heretofore unknown. We further quantify the performance gain due to cooperative relay and establish a connection between cooperative relay and pure relay. Finally, based on these structural insights, we reduce the MIMO-CN rate maximization problem to an equivalent scalar formulation. We then propose a global optimization method to solve this simplified and equivalent problem.