  This paper presents a method to seamlessly extend the coverage of energy
supply field for wireless sensor networks in order to free sensors from wires
and batteries, where the multi-point scheme is employed to overcome path-loss
attenuation, while the carrier shift diversity is introduced to mitigate the
effect of interference between multiple wave sources. As we focus on the energy
transmission part, sensor or communication schemes are out of scope of this
paper. To verify the effectiveness of the proposed wireless energy
transmission, this paper conducts indoor experiments in which we compare the
power distribution and the coverage performance of different energy
transmission schemes including conventional single-point, simple multi-point
and our proposed multi-point scheme. To easily observe the effect of the
standing-wave caused by multipath and interference between multiple wave
sources, 3D measurements are performed in an empty room. The results of our
experiments together with those of a simulation that assumes a similar antenna
setting in free space environment show that the coverage of single-point and
multi-point wireless energy transmission without carrier shift diversity are
limited by path-loss, standing-wave created by multipath and interference
between multiple wave sources. On the other hand, the proposed scheme can
overcome power attenuation due to the path-loss as well as the effect of
standing-wave created by multipath and interference between multiple wave
sources.
