The RF-based wireless power transfer has been recently proposed as a cost-effective way to sustain wireless communications among billions of user devices. The recent development of ambient backscatter radio in supports persistent wireless communications between two symmetric user devices that are solely powered by the energy harvested from ambient RF signals. The backscatter radio operates in the passive mode that transmits information by modulating and reflecting the incident RF signals.
A basic difficulty related to wireless power transfer is how to tell the "power"wave where it should deliver its energy. Unless you are using near field power transfer (Limited to about one wavelength in distance but usually much less) you either need a rather high directivity transmitting antenna or the radiated power goes everywhere .This may lead to undesired electromagnetic interference. There were some discussions related to old ideas of TESLA for this kind of power transfer, but the fundamental problem to tell the energy where to go and where NOT to go always persists.