This letter investigates a cognitive wireless powered communication (WPC) system coexisting with a primary wireless communication system, in which a cognitive or secondary wireless device (S-WD) uses its harvested energy from a multi-antenna secondary hybrid access point (S-HAP) and the primary transmitter to communicate with the S-HAP. Based on the channel conditions at each fading state, the cognitive WPC system can adaptively switch the transmission mode between the forward-link energy transfer from the S-HAP to the S-WD and the reverse-link information transfer. Under this setup, we maximize the average throughput from the S-WD to the S-HAP by jointly optimizing the transmission mode, the transmit/receive beamformers at the S-HAP, and the power allocation at the S-WD over different fading states, subject to their respective power constraints, and an interference temperature constraint imposed to protect the reliable communication of the primary system. We employ zero-forcing beamforming for the S-HAP to cancel the interference from/to primary users, and develop an efficient algorithm for mode switching and power allocation.
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