Energy Efficiency and Asymptotic Performance Evaluation of Beamforming Structures in Doubly Massive MIMO mmWave Systems

Future cellular systems based on the use of millimeter waves will heavily rely on the use of antenna arrays both at the transmitter and at the receiver. For complexity reasons and energy consumption issues, fully digital pre-coding and post-coding structures may turn out to be unfeasible, and thus suboptimal structures, making use of simplified hardware and a limited number of RF chains, have been investigated. This paper considers and makes a comparative assessment of several suboptimal pre-coding and post-coding beamforming structures for a cellular multiuser MIMO system with large number of antennas. Analytical formulas for the asymptotic achievable spectral efficiency and for the global energy efficiency of several beamforming structures are derived in the large number of antennas regime. Using the most recently available data for the energy consumption of phase shifters and switches, we show that fully digital beamformers may actually achieve a larger energy efficiency than lower-complexity solutions, as well as that low-complexity beamsteering purely analog beamforming may in some cases represent a good performance-complexity tradeoff solution.