Backscatter Sensing with Single-tag Path Variation Cancelling

Backscatter sensing with battery-less tag sensors has gained popularity in low-cost wireless sensing for IoT applications. This paper proposes a backscatter sensing method with a single tag for canceling the effect of path variation (PV), such as unknown reader-sensor distance and orientation. A reference/sensing (Ref/Sen) mode selection in the tag sensor is applied to generate the differential backscattered signals, which selects a reference/sensing capacitor by a mode-selection switch and causes the reflection coefficient variation. A Ref-Sen differential (RSD) variable ∆r is defined and calculated by the interrogator based on two signal parameters: power and phase. ∆r is a PV-independent variable corresponding to the sensing information. An RFID sensing system with a liquid-level tag sensor has been designed and evaluated. Experimental results demonstrate a 0.5-mm resolution in liquid-level sensing. The robustness-test results under different PVs, including 18 dBm power variation and 0.6 m distance variation, have proved the proposed PV-cancelling method’s validity. The standard deviations for the amplitude and phase variations are 0.00344 and 1.65°.