  We present a hybrid signaling approach to position energy-neutral devices.
Our method combines the best of two worlds: instant RF signals for both
communication and energy transfer, and slower propagating acoustic waves for
accurate distance measurements. We introduce advanced energy loading of an
'E-buffer' through beamformed RF signals. The scientific contribution of our
approach is twofold, advancing both the positioning performance and the energy
harvesting efficiency. On the one hand, we overcome current distance
limitations in RF backscattering-based indoor localization. On the other hand,
it enhances the energy harvesting by using the calculated position and
beamforming a higher amount of directed RF energy into the mobile node. We
provide a functional architecture for both sides of the system and present a
proof-of-concept. Practical measurements in representative use cases show an
update rate of 10 positions per hour within the regulatory constraints in the
868MHz band for distances up to 4.5 m. An energy and power model are drawn up
to provide insights into the performance trade-offs.
