  Doubly-near-far problem in RF-powered networks can be mitigated by choosing
appropriate device-to-device (D2D) communication mode and implementing
energy-efficient information transfer (IT). In this work, we present a novel RF
energy harvesting architecture where each transmitting-receiving user pair is
allocated a disjoint channel for its communication which is fully powered by
downlink energy transfer (ET) from hybrid access point (HAP). Considering that
each user pair can select either D2D or cellular mode of communication, we
propose an optimized transmission protocol controlled by the HAP that involves
harvested energy-aware jointly optimal mode selection (MS) and time allocation
(TA) for ET and IT to maximize the sum-throughput. Jointly global optimal
solutions are derived by efficiently resolving the combinatorial issue with the
help of optimal MS strategy for a given TA for ET. Closed-form expressions for
the optimal TA in D2D and cellular modes are also derived to gain further
analytical insights. Numerical results show that the joint optimal MS and TA,
which significantly outperforms the benchmark schemes in terms of achievable
RF-powered sum-throughput, is closely followed by the optimal TA scheme for D2D
users. In fact, about $2/3$ fraction of the total user pairs prefer to follow
the D2D mode for efficient RF-powered IT.
