  Wireless-powered communication network (WPCN) is a key technology to power
energy-limited massive devices, such as on-board wireless sensors in autonomous
vehicles, for Internet-of-Things (IoT) applications. Conventional WPCNs rely
only on dedicated downlink wireless power transfer (WPT), which is practically
inefficient due to the significant energy loss in wireless signal propagation.
Meanwhile, ambient energy harvesting is highly appealing as devices can
scavenge energy from various existing energy sources (e.g., solar energy and
cellular signals). Unfortunately, the randomness of the availability of these
energy sources cannot guarantee stable communication services. Motivated by the
above, we consider a generalized WPCN where the devices can not only harvest
energy from a dedicated multiple-antenna power station (PS), but can also
exploit stored energy stemming from ambient energy harvesting. Since the
dedicated WPT consumes system resources, if the stored energy is sufficient,
WPT may not be needed to maximize the weighted sum rate (WSR). To analytically
characterize this phenomenon, we derive the condition for WPT activation and
reveal how it is affected by the different system parameters. Subsequently, we
further derive the optimal resource allocation policy for the cases that WPT is
activated and deactivated, respectively. In particular, it is found that when
WPT is activated, the optimal energy beamforming at the PS does not depend on
the devices' stored energy, which is shown to lead to a new unfairness issue.
Simulation results verify our theoretical findings and demonstrate the
effectiveness of the proposed optimal resource allocation.
