Joint Optimization of Wireless Power Transfer and Collaborative Beamforming for Relay Communications

In this work, multiple relays are employed to assist data transmissions from a multi-antenna hybrid access point (HAP) to a distant receiver underlaying a cellular system. The relays are powered by energy harvesting (EH) from the HAP's signal beamforming, following a power splitting (PS) protocol. We aim to maximize the throughput subject to interference constraints at the cellular user equipments (CUEs). Each relay is allowed to adjust the PS ratio and transmit power to improve the data rate, meanwhile restrict the aggregate interference to the CUEs in a probabilistic manner. We also optimize the HAP's beamforming strategy to control the information and energy transfer to the relays. We show that the proposed throughput maximization problem, though non-convex, can be solved optimally by exploiting its monotonicity and designing a successive approximation algorithm. Numerical results verify the close coupling between the relays' power control and the HAP's energy transfer, which calls for a joint optimization to ensure the throughput-optimal performance.