  Radio frequency wireless power transfer (RF-WPT) is an emerging technology
that enables transferring energy from an energy access point (E-AP) to multiple
energy receivers (E-Rs), in a wireless manner. In practice, there are some
restrictions on the power level or the amount of energy that the E-AP can
transfer, which need to be considered in order to determine a proper power
transfer policy for the E-AP. In this paper, we formulate the problem of
finding the optimal policy for two practical scenarios of powerlimited and
energy-limited E-APs. The formulated problems are non-convex stochastic
optimization problems that are very challenging to solve. We propose optimal
and near-optimal policies for the power transfer of the E-AP to the E-Rs, where
the optimal solutions require statistical information of the channel states,
while the near-optimal solutions do not require such information and perform
well in practice. Furthermore, to ensure fairness among E-Rs, we propose two
fair policies, namely Max- Min Fair policy and quality-of-service-aware
Proportional Fair policy. MMF policy targets maximizing the minimum received
power among the E-Rs, and QPF policy maximizes the total received power of the
E-Rs, while guaranteeing the required minimum QoS for each E-R. Various
numerical results demonstrate the significant performance of the proposed
policies.
