  In this paper, we design and experiment a far-field wireless power transfer
(WPT) architecture based on distributed antennas, so-called WPT DAS, that
dynamically selects transmit antenna and frequency to increase the output dc
power. Uniquely, spatial and frequency diversities are jointly exploited in the
proposed WPT DAS with low complexity, low cost, and flexible deployment to
combat the wireless fading channel. A numerical experiment is designed to show
the benefits using antenna and frequency selections in spatially and frequency
selective fading channels for single-user and multi-user cases. Accordingly,
the proposed WPT DAS for single-user and two-user cases is prototyped. At the
transmitter, we adopt antenna selection to exploit spatial diversity and adopt
frequency selection to exploit frequency diversity. A low-complexity
over-the-air limited feedback using an IEEE 802.15.4 RF interface is designed
for antenna and frequency selections and reporting from the receiver to the
transmitter. The proposed WPT DAS prototype is demonstrated in a real indoor
environment. The measurements show that WPT DAS can boost the output dc power
by up to 30 dB in single-user case and boost the sum of output dc power by up
to 21.8 dB in two-user case and broaden the service coverage area in a low
cost, low complexity, and flexible manner.
