  This work analyzes a mmWave single-cell network, which comprises a macro base
station (BS) and an overlaid tier of small-cell BSs using a wireless backhaul
for data traffic. We look for the optimal number of antennas at both BS and
small-cell BSs that maximize the energy efficiency (EE) of the system when a
hybrid transceiver architecture is employed. Closed-form expressions for the
EE-optimal values of the number of antennas are derived that provide valuable
insights into the interplay between the optimization variables and hardware
characteristics. Numerical and analytical results show that the maximal EE is
achieved by a 'close-to' fully-digital system wherein the number of BS antennas
is approximately equal to the number of served small cells.
