  This paper studies a relay-assisted wireless powered communication network
(R-WPCN) consisting of multiple source-destination pairs and a hybrid relay
node (HRN). We consider a "charge-then-forward" protocol at the HRN, in which
the HRN with constant energy supply first acts as an energy transmitter to
charge the sources, and then forwards the information from the sources to their
destinations through time division multiple access (TDMA) or frequency division
multiple access (FDMA). Processing costs at the wireless-powered sources are
taken into account. Our goal is to maximize the sum-rate of all transmission
pairs by jointly optimizing the time, frequency and power resources. The
formulated optimization problems for both TDMA and FDMA are non-convex. For the
TDMA scheme, by appropriate transformation, the problem is reformulated as a
convex problem and be optimally solved. For the FDMA case, we find the
asymptotically optimal solution in the dual domain. Furthermore, suboptimal
algorithms are proposed for both schemes to tradeoff the complexity and
performance. Finally, the simulation results validate the effectiveness of the
proposed schemes.
