Massive MIMO and Small Cells: Improving Energy Efficiency by Optimal Soft-Cell Coordination

To improve the cellular energy efficiency, without sacrificing Quality-of-Service (QoS) at the users, the network topology must be densified to enable higher spatial reuse. The authors analyze a combination of two densification approaches, namely "Massive" Multiple-Input Multiple-Output (MIMO) base stations and small-cell access points. If the latter are operator-deployed, a spatial soft-cell approach can be taken where the multiple transmitters serve the users by joint non-coherent beamforming. They minimize the total power consumption (both emitted power and hardware power usage) while satisfying QoS constraints. This problem is proved to have a hidden convexity that enables efficient solution algorithms.