On Optimal Energy-Efficient Multi-User MIMO

Energy efficiency is becoming increasingly important for mobile devices because battery technology has not kept up with the growing demand of ubiquitous multimedia communications. Since multi-user multiple-input multiple-output (MU-MIMO) is a key technology in next-generation wireless communications, this paper addresses optimal energy-efficient design for MU-MIMO. The energy efficiency is measured by a classic metric, "throughput per Joule", while both RF transmit power and device electronic circuit power are considered. We define the energy efficiency (EE) capacity for MU-MIMO and study the power allocation that achieves this capacity. We show that user antennas should be used only when the corresponding subchannels are sufficiently good and using them improves the overall network EE. Based on theoretical analysis, we further develop low-complexity yet globally optimal energy-efficient power allocation algorithms that converge to the optimum exponentially. Finally comprehensive simulation results are provided to demonstrate the significant gain in network energy efficiency.