Wireless power transfer optimization for nonlinear passive backscatter devices

We present a method for enhancing far-field wireless power transfer (WPT) to nonlinear, passive UHF RFID backscatter transponders using a multi-input multi-output (MIMO) base station. The proposed method does not require on-tag power measurements or on-tag channel estimation, either of which would add significant complexity and power consumption for microwatt-class wirelessly powered devices such as passive UHF RFID tags. We show in a measurement-based proof of concept that WPT optimization to nonlinear backscatter transponders is possible solely based on the backscatter signal, without knowledge of the incident power level at the tag or prior knowledge of the tag's characteristics. Using an 8×8 MIMO transceiver array to optimize power delivery, we observed an average WPT enhancement of 8.9 dB relative to an un-optimized 8-transmitter configuration across a 50m3 volume in an office/lab environment. We also show that power to a given tag can be selectively denied, with a notch depth of -106.4 dB (noise floor of the presented measurements). These results are comparable to values previously observed for linear backscatter transponders, and suggest that the use of MIMO interrogators could lead to improved forward-link performance and thus efficiently provide power for sensors or other new power-hungry functions on passive transponders.