  In this work, we study massive multiple-input multiple-output (MIMO)
precoders optimizing power consumption while achieving the users' rate
requirements. We first characterize analytically the solutions for narrowband
and wideband systems minimizing the power amplifiers (PAs) consumption in low
system load, where the per-antenna power constraints are not binding. After, we
focus on the asymptotic wideband regime. The power consumed by the whole base
station (BS) and the high-load scenario are then also investigated. We obtain
simple solutions, and the optimal strategy in the asymptotic case reduces to
finding the optimal number of active antennas, relying on known precoders among
the active antennas. Numerical results show that large savings in power
consumption are achievable in the narrowband system by employing antenna
selection, while all antennas need to be activated in the wideband system when
considering only the PAs consumption, and this implies lower savings. When
considering the overall BS power consumption and a large number of subcarriers,
we show that significant savings are achievable in the low-load regime by using
a subset of the BS antennas. While optimization based on transmit power pushes
to activate all antennas, optimization based on consumed power activates a
number of antennas proportional to the load.
