Scatter radio sensor network with analog frequency modulation principles

Abstract

Scatter radio communication is implemented with very simple, low-power and low-cost front-ends that only consist of a single radio frequency (RF) switch. This work develops a bistatic scatter radio wireless sensor network (WSN) with analog energy-assisted tags that monitor relative humidity percentage (%RH) and consume less than 1 mWatt power. Particularly, the tags em- ploy a capacitance-to-frequency converter, that is implemented with a 555 timer and modulates the capacitance of the HCH-1000 %RH sensor. The frequency-modulated pulses are routed to the tag’s RF front-end which is designed to increase communication range. In order to convert the out- put frequency of the tags to %RH, a transfer function is estimated using careful polynomial surface fitting calibration and including the ambient tem- perature. Frequency division multiple access (FDMA) networking is im- plemented with the utilization of different passive components on each tag. Moreover the receiver that is implemented on a software defined radio (SDR) platform exploits carefully designed software filters based on histogram and Savitsky-Golay smoothing techniques. The achieved communication range is over 130 meters at an end-to-end root mean squared error (RMSE) of less than 5 %RH. For the network evaluation, a testbed is calibrated and deployed in a tomato greenhouse demonstrating a novel analog bistatic scatter radio WSN. Finally, an over the air programmable (OTAP) testbed was developed, employing nodes that utilize both an active radio front-end and scatter radio front-end in order to facilitate remote monitoring and debugging.