  This paper studies a novel user cooperation method in a wireless powered
communication network (WPCN), where a pair of closely located devices first
harvest wireless energy from an energy node (EN) and then use the harvested
energy to transmit information to an access point (AP). In particular, we
consider the two energy-harvesting users exchanging their messages and then
transmitting cooperatively to the AP using space-time block codes.
Interestingly, we exploit the short distance between the two users and allow
the information exchange to be achieved by energy-conserving backscatter
technique. Meanwhile the considered backscatter-assisted method can effectively
reuse wireless power transfer for simultaneous information exchange during the
energy harvesting phase. Specifically, we maximize the common throughput
through optimizing the time allocation on energy and information transmission.
Simulation results show that the proposed user cooperation scheme can
effectively improve the throughput fairness compared to some representative
benchmark methods.
