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Cooperative Multi-relay Assisted Multicast Beamforming in Wirelessly Powered Communications

This paper studied downlink multicasting in a wirelessly powered communication network, where multiple energy-constrained relay nodes collaboratively assist the transmissions from a multi-antenna hybrid access point (HAP) to multiple receivers in the downlink. Each relay is equipped with a single antenna and capable of energy harvesting (EH) from the HAP's RF signals in a power splitting protocol. Due to the confliction between EH and information transmission at the relays, they need to optimally control the power splitting ratio, according to the relays' channel conditions and energy status, with the aim of improving the signal-to-noise ratio (SNR) at all receivers. We propose a SNR maximization problem by jointly optimizing the HAP's signal beamforming and the relays' EH parameters. Though it is non-convex and difficult to solve optimally, we design an iterative algorithm by efficiently solving semi-definite programs successively. Simulation results show the efficacy of the proposed algorithm and validate our findings that relays will choose not to broadcast signal when its channel condition is poor.