  Wireless energy transfer, namely RF-based energy harvesting, is a potential
way to prolong the lifetime of energy-constrained devices, especially in
wireless sensor networks. However, due to huge propagation attenuation, its
energy efficiency is regarded as the biggest bottleneck to widely applications.
It is critical to find appropriate transmission policies to improve the global
energy efficiency in this kind of systems. To this end, this paper focuses on
the sensor networks scenario, where a mobile control center powers the sensors
by RF signal and also collects information from them. Two related schemes,
called as harvest-and-use scheme and harvest-store-use scheme, are
investigated, respectively. In harvest-and-use scheme, as a benchmark, both
constant and adaptive transmission modes from sensors are discussed. To
harvest-store-use scheme, we propose a new concept, the best opportunity for
wireless energy transfer, and use it to derive an explicit closed-form
expression of optimal transmission policy. It is shown by simulation that a
considerable improvement in terms of energy efficiency can be obtained with the
help of the transmission policies developed in this paper. Furthermore, the
transmission policies is also discussed under the constraint of fixed
information rate. The minimal required power, the performance loss from the new
constraint as well as the effect of fading are then presented.
