Coordinated MIMO precoding for power minimization in femtocell systems

This paper addresses the problem of coordinated radio resource allocation in the downlink for a set of femtocells operating on the same band under OFDMA access. We focus on the minimization of the total transmitted power subject to minimum rate constraints for the users connected to the femtocells in the set. By using interference prices to represent the interference cost at each receiver, a distributed solution is proposed, where each transmitter solves a convex optimization problem. For the sake of generality, multiple antennas are considered at both transmitters and receivers. Therefore, the solution includes, as particular cases, the three possible antenna configurations: multiple-input multiple-output (MIMO), multiple-input single-output (MISO), and single-input single-output (SISO). In the solution obtained, the transmitter avoids allocating large power at those carriers and directions for which the interference caused to the users in other femtocells is more critical. If the power required to fulfill the target rates is low, with the consequent reduction of the caused interference, the solution tends to be the conventional waterfilling solution.