User Cooperation in Wireless Powered Communication Networks With a Pricing Mechanism.

This paper considers user cooperation and a pricing mechanism in a wireless-powered communication network (WPCN) in which two users harvest energy from a dedicated hybrid access point (H-AP), which has a constant power supply and acts as a power station during a downlink (DL). They also independently transmit their information to the H-AP [ which acts as an information receiving station during an uplink (UL)] using the individually harvested energy. Based on the "doubly near-far" problem, this paper proposes a cooperative scheme among users in the WPCN. Compared with the source user (SU), the channel conditions for a helping user (HU), which is closer to the H-AP, is usually better for DL energy harvesting and for transmitting UL information. Thus, the HU can use its harvested energy to forward the SU's information to the H-AP. Furthermore, energy is usually scarce for each user in a WPCN; therefore, the HU is under no obligation to accept the SU's cooperative request and can choose to act selfishly to conserve resources. This paper presents a new pricing strategy to motivate the HU to sell its excess energy to help an SU complete a UL information transfer. Two transmission protocols are investigated: in the ideal case, the energy expenditure of the SU equals the energy used by the HU to relay information; in the normal case, the HU seeks additional profit. This paper formulates the SU's expenditures and relay data in the ideal case as an optimization problem. An investigation of relay placement and the SU communication mode selection problem are also discussed. The numerical results show that the proposed pricing strategy can significantly reduce the expected costs to the SU and improve the reliability of user UL communications in a WPCN.