  User cooperation is an effective technique to tackle the severe near-far user
unfairness problem in wireless powered communication networks (WPCNs). In this
paper, we consider a WPCN where two collaborating wireless devices (WDs) first
harvest wireless energy from a hybrid access point (HAP) and then transmit
their information to the HAP. The WD with the stronger WD-to-HAP channel helps
relay the message of the other weaker user. In particular, we exploit the use
of ambient backscatter communication during the wireless energy transfer phase,
where the weaker user backscatters the received energy signal to transmit its
information to the relay user in a passive manner. By doing so, the relay user
can reuse the energy signal for simultaneous energy harvesting and information
decoding (e.g., using an energy detector). Compared to active information
transmission in conventional WPCNs, the proposed method effectively saves the
energy and time consumed by the weaker user on information transmission during
cooperation. With the proposed backscatter-assisted relaying scheme, we jointly
optimize the time and power allocations on wireless energy and information
transmissions to maximize the common throughput. Specifically, we derive the
semi-closed-form expressions of the optimal solution and propose a
low-complexity optimal algorithm to solve the joint optimization problem. By
comparing with some representative benchmark methods, we simulate under
extensive network setups and demonstrate that the proposed cooperation method
effectively improves the throughput performance in WPCNs.
