  This paper studies a novel user cooperation method in a wireless powered
cooperative communication network (WPCN) in which a pair of distributed
terminal users first harvest wireless energy broadcasted by one energy node
(EN) and then use the harvested energy to transmit information to a destination
node (DN). In particular, the two cooperating users exchange their independent
information with each other so as to form a virtual antenna array and transmit
jointly to the DN. By allowing the users to share their harvested energy to
transmit each other's information, the proposed method can effectively mitigate
the inherent user unfairness problem in WPCN, where one user may suffer from
very low data rate due to poor energy harvesting performance and high data
transmission consumptions. Depending on the availability of channel state
information at the transmitters, we consider the two users cooperating using
either coherent or non-coherent data transmissions. In both cases, we derive
the maximum common throughput achieved by the cooperation schemes through
optimizing the time allocation on wireless energy transfer, user message
exchange, and joint information transmissions in a fixed-length time slot. We
also perform numerical analysis to study the impact of channel conditions on
the system performance. By comparing with some existing benchmark schemes, our
results demonstrate the effectiveness of the proposed user cooperation in a
WPCN under different application scenarios.
