  This paper studies a novel cooperation method in a two-user wireless powered
communication network (WPCN), in which one hybrid access point (HAP) broadcasts
wireless energy to two distributed wireless devices (WDs), while the WDs use
the harvested energy to transmit their independent information to the HAP. To
tackle the user unfairness problem caused by the near-far effect in WPCN, we
allow the WD with the stronger WD-to-HAP channel to use part of its harvested
energy to help relay the other weaker user's information to the HAP. In
particular, we exploit the use of backscatter communication during the wireless
energy transfer phase such that the helping relay user can harvest energy and
receive the information from the weaker user simultaneously. We derive the
maximum common throughput performance by jointly optimizing the time duration
and power allocations on wireless energy and information transmissions. Our
simulation results demonstrate that the backscatter-assisted cooperation scheme
can effectively improve the throughput fairness performance in WPCNs.
