Multitransmitter Wireless Power Transfer Optimization for Backscatter RFID Transponders

We present an efficient method for enhancing far-field wireless power transfer (WPT) to highly power-constrained mobile devices such as UHF and microwave RFID transponders using a multiple-input-multiple-output (MIMO) base station system. Existing techniques for channel estimation are prohibitively complex and consume too much power to be integrated into milliwatt- or microwatt-class wirelessly powered devices. We show that power transfer can be optimized using a transponder's backscatter signal alone, without requiring channel estimation and power measurement circuitry on the transponder. A measurement-based proof of concept is presented, illustrating that for any linear backscatter transponder, power optimization based on the backscattered signal is equivalent to local measurement at the mobile device itself. Using an 8 x 8 MIMO transceiver array to optimize power delivery across 100 randomly chosen locations in our office environment, we observe an average WPT enhancement of 8.1 dB relative to an unoptimized transmitter configuration. Transponders with nonlinear power harvesters can easily be supported with the addition of a single RF switching transistor.