Energy Beamforming Design and User Cooperation for Wireless Powered Communication Networks

We investigate user cooperation (UC) for wireless powered communication networks, where two single-antenna users are first wirelessly powered by a multi-antenna hybrid access point (H-AP), and then cooperatively transmit information signals to this H-AP via the harvested energy. To incorporate user fairness in the system performance, we formulate the weighted sum-rate maximization problem for both users to jointly optimize the energy beamforming vector, time allocation, and power allocation. Due to the non-convexity of this problem, we propose two UC schemes by employing a series of efficient variable substitutions and semidefinite programming relaxation to solve this problem. In addition, we show that the solution of the proposed schemes can achieve the global optimum by exploiting the rank-one characteristic of the energy beamforming matrix. Finally, numerical results are provided to validate our proposed schemes, which quantifies the impact of UC schemes by comparing them with the non-cooperative scheme.