Optimum Power Allocation for Cooperative Systems with Orthogonal Space-Time Transmissions

We investigate the rate maximizing power allocation strategy for a system where the source and the decode-and-forward (DF) relays cooperate by utilizing a distributed space-time block coding strategy. Under the assumption that all transmitters have perfect knowledge of the link gains, we analyze two limit cases - the case of no-collaboration among relays and the case of ideal collaboration among relays - and we provide a polynomial-time algorithm that computes the maximum achievable rate for the distributed space-time coding system despite the fact that the optimization problem is not convex and it belongs to a class of problems with variational inequality constraints. Due to the orthogonal structure of the space-time block code designs, the proposed algorithm can also be used, with minor modifications, in the computation of the optimum power allocation for the case of DF relays with orthogonal transmissions