  Multiple antenna techniques, that allow energy beamforming, have been looked
upon as a possible candidate for increasing the efficiency of the transfer
process between the energy transmitter (ET) and the energy receiver (ER) in
wireless energy transfer. This paper introduces a novel scheme that facilitates
energy beamforming by utilizing Received Signal Strength Indicator (RSSI)
values to estimate the channel. Firstly, in the training stage, the ET will
transmit sequentially using each beamforming vector in a codebook, which is
pre-defined using a Cramer-Rao lower bound analysis. The RSSI value
corresponding to each beamforming vector is fed back to the ET, and these
values are used to estimate the channel through a maximum likelihood analysis.
The results that are obtained are remarkably simple, requires minimal
processing, and can be easily implemented. Also, the results are general and
hold for all well known fading models. The paper also validates the analytical
results numerically, as well as experimentally, and it is shown that the
proposed method achieves impressive results in wireless energy transfer.
