  This paper focuses on wireless energy transfer (WET) to a pair of low complex
energy receivers (ER), by only utilizing received signal strength indicator
(RSSI) values that are fed back from the ERs to the energy transmitter (ET).
Selecting the beamformer that maximizes the total average energy transfer
between the ET and the ERs, while satisfying a minimum harvested energy
criterion at each ER, is studied. This is a nonconvex constrained optimization
problem which is difficult to solve analytically. Also, any analytical solution
to the problem should only consists of parameters that the ET knows, or the ET
can estimate, as utilizing only RSSI feedback values for channel estimation
prohibits estimating some channel parameters. Thus, the paper focuses on
obtaining a suboptimal solution analytically. It is proven that if the channels
between the ET and the ERs satisfy a certain sufficient condition, this
solution is in fact optimal. Simulations show that the optimality gap is
negligibly small as well. Insights into a system with more than two ERs are
also presented. To this end, it is highlighted that if the number of ERs is
large enough, it is possible to always find a pair of ERs satisfying the
sufficient condition, and hence, a pairwise scheduling policy that does not
violate optimality can be used for the WET.
