  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 precoding
and postcoding 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, both from a spectral efficiency and energy efficiency point of
view, of several suboptimal precoding and postcoding beamforming structures for
a cellular multiuser MIMO (MU-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 beam-steering purely
analog beamforming may in some cases represent a good performance-complexity
trade-off solution.
