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Environmental scatter radio sensors with RF energy harvesting

Daskalakis Spyridon-Nektarios

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Year 2016
Type of Item Master Thesis
Bibliographic Citation Spyridon-Nektarios Daskalakis, "Environmental scatter radio sensors with RF energy harvesting", Master Thesis, School of Electrical and Computer Engineering, Technical University of Crete, Chania, Greece, 2016
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Real-time monitoring of environmental parameters, such as soil moisture, with wireless sensor networks (WSN), is invaluable for precision agriculture. However, conventional radios in large-scale deployments are limited by power consumption, cost and complexity constraints. Scatter-radio, a promising technology, allows the development of large-scale and low-cost WSN. In this work, the development of an analog scatter-radio WSN for soil moisture is presented. The WSN consists of low-cost (6 Euro per sensor), low-power (in the order of 200 W per sensor) soil moisture scatter radio sensors, with high communication range (up to 250 m). The WSN utilizes analog frequency modulation (FM) in a bistatic network architecture, while the sensors operate simultaneously, using frequency division multiple access (FDMA). The network utilizes an ultra-low cost software-defined radio reader and proposes a custom microstrip capacitive sensor with a simple calibration methodology. Overall root mean squared error (RMSE) below 1% is observed, even for ranges of 250 m. In order to supply the sensor with energy, a radio frequency (RF) energy harvesting supply system is also presented. The system collects the unused ambient RF energy from scatter radio emitters and from one FM station. The design consists of a double diode rectifier with operation in the two frequencies of 900 MHz and 97,5 MHz, simultaneously, utilizing a low-cost, lossy FR-4 substrate and a low-complexity rectifier circuit. The achieved RF-to-DC rectification efficiency was 14,49% and 27,44% at 868 MHz and 97,5 MHz, respectively, and 20 dBm input power. The rectifier was connected to a commercial boost converter in order to manage and improve the output power of the rectifier. Finally, the end-to-end efficiency of the system was 21%, at 97,5 MHz and 15 dBm input power. This work offers a concrete example of ultra-low power and cost wireless sensor networking with (unconventional) novel scatter radio technology.

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