  Reconfigurable intelligent surface (RIS) is a promising technology for RF
wireless power transfer (WPT) as it is capable of beamforming and beam focusing
without using active and power-hungry components. In this paper, we propose a
multi-tile RIS beam scanning (MTBS) algorithm for powering up
internet-of-things (IoT) devices. Considering the hardware limitations of the
IoT devices, the proposed algorithm requires only power information to enable
the beam focusing capability of the RIS. Specifically, we first divide the RIS
into smaller RIS tiles. Then, all RIS tiles and the phased array transmitter
are iteratively scanned and optimized to maximize the receive power. We
elaborately analyze the proposed algorithm and build a simulator to verify it.
Furthermore, we have built a real-life testbed of RIS-aided WPT systems to
validate the algorithm. The experimental results show that the proposed MTBS
algorithm can properly control the transmission phase of the transmitter and
the reflection phase of the RIS to focus the power at the receiver.
Consequently, after executing the algorithm, about 20 dB improvement of the
receive power is achieved compared to the case that all unit cells of the RIS
are in OFF state. By experiments, we confirm that the RIS with the MTBS
algorithm can greatly enhance the power transfer efficiency.
