  In this paper, we design, prototype, and experiment a closed-loop radiative
wireless power transfer (WPT) system with adaptive waveform and beamforming
using limited feedback. Spatial and frequency domains are exploited by jointly
utilizing multi-sine waveform and multi-antenna beamforming at the transmitter
in WPT system to adapt to the multipath fading channel and boost the output dc
power. A closed-loop architecture based on a codebook design and a low
complexity over-the-air limited feedback using an IEEE 802.15.4 RF interface is
proposed. The codebook consists of multiple codewords where each codeword
represents particular waveform and beamforming. The transmitter sweeps through
the codebook and then the receiver feeds back the index of the optimal
codeword, so that the waveform and beamforming can be adapted to the multipath
fading channel to maximize the output dc power without requiring explicit
channel estimation and the knowledge of accurate Channel State Information. The
proposed closed-loop WPT with adaptive waveform and beamforming using limited
feedback is prototyped using a Software Defined Radio equipment and measured in
a real indoor environment. The measurement results show that the proposed
closed-loop WPT with adaptive waveform and beamforming can increase the output
dc power by up to 14.7 dB compared with the conventional single-tone and
single-antenna WPT system.
