  The high spectral efficiency of massive MIMO (Multiple Input Multiple Output)
is mainly achieved through the exploitation of spatial multiplexing, i.e. by
using a high number of MIMO layers that are applied simultaneously to many
users. The power consumption of a massive MIMO base station is determined by
the hardware driving a high number of antenna ports and elements. This paper
focuses on practical deployment situations with varying user load. During hours
with low number of users a certain significant part of hardware power
consumption would remain with conventional massive MIMO processing, while the
full potential of spectral efficiency cannot be exploited due to the low number
of users, resulting in low power efficiency and cost. We investigate the impact
of different hybrid array architectures on spectral efficiency, average user
throughput and power consumption and show how to design a massive MIMO system
with significantly improved energy efficiency for a given target scenario,
while maintaining a targeted service quality.
