Collaborative Relay Beamforming with Direct Links in Wireless Powered Communications

In this work, we exploit the signal and energy cooperation in wireless powered multi-user networks. In particular, multiple relays are employed to assist data transmissions from a multi-antenna hybrid access point (HAP) to a distant receiver. The HAP also transfers wireless power to the relays in either a power-splitting (PS) or time-switching (TS) protocol. With dense user deployment, the direct links from the HAP to the receivers are short and can contribute considerably to the overall throughput. To account for the direct links, we propose a throughput maximization problem by jointly optimizing the HAP's beamforming strategy to control the information and power transfer to the relays as well as individual relays' energy harvesting and collaborative beamforming strategies. The main challenge lies in that the direct links require the beamforming design to balance the performances of relay and direct transmissions. Though the throughput maximization problem is non-convex and the globally solution may not be available, we obtain two feasible lower performance bounds corresponding to the PS and TS protocols. Our simulation results also verify that the new design with direct links achieves significant performance improvement compared with the conventional scheme that ignores the direct links.