Retrodirective Frequency Diverse Array Focusing for Wireless Information and Power Transfer

Simultaneous wireless information and power transfer (WIPT) has so far mainly relied on a phased-array antenna, but only angle-focusing capability without range focusing can be achieved for phased-array antennas. In this paper, we depart from the phased-array antenna and propose the use of retrodirective frequency diverse array (FDA) focusing in base station for long-range WIPT applications. In the retrodirective FDA focusing, an additional small frequency offset is added across the phase-conjugating mixers in the retrodirective array to produce both range-dimension and angle-dimension focused beamforming for energy-efficient WIPT. The FDA beampattern is decoupled in range-angle dimension and can be independently controlled through properly designing the transmit weighting vector or frequency offsets. More importantly, the proposed WIPT scheme allows self-tracking the direction-of-arrival of the pilot signals without requirement of any a priori knowledge. A maximum efficiency optimization approach based on discrete prolate spheroidal sequence is also proposed for the beampattern synthesis. In addition, robust physical-layer security can be obtained by utilizing FDA angle-range-time-dependent beampattern characteristics. The harvested energy, achievable bit rate, and achievable secrecy rate at the user receivers are derived for the WIPT, along with extensive numerical results. Both the theoretical analysis and numerical results verify the superiority of FDA over phased array in energy efficiency and security for WIPT applications.