  We illustrate potential benefits of using massive antenna arrays for wireless
energy transfer (WET). Specifically, we analyze probability of outage in WET
over fading channels when a base station (BS) with multiple antennas beamforms
energy to a wireless sensor node (WSN). Our analytical results show that by
using massive antenna arrays, the range of WET can be increased for a given
target outage probability. We prove that by using multiple-antenna arrays at
the BS, a lower downlink energy is required to get the same outage performance,
resulting into savings of radiated energy. We show that for energy levels used
in WET, the outage performance with least-squares or minimum mean-square error
channel estimates is same as that obtained based on perfect channel estimates.
We observe that a strong line-of-sight component between the BS and WSN lowers
outage probability. Furthermore, by deploying more antennas at the BS, a larger
energy can be transferred reliably to the WSN at a given target outage
performance for the sensor to be able to perform its main tasks. In our
numerical examples, the RF power received at the input of the sensor is assumed
to be on the order of a mW, such that the rectenna operates at an efficiency in
the order of 50 %.
