Mobility-based Time References for Wireless Sensor Networks

Abstract

Wireless Sensor Networks require small low-cost radios to enable communication among its nodes. Since those radios must be fully integrated to reduce cost and size, integration is required also for their on-board time references, which are needed to achieve synchronization with the other nodes. To deal with the lower accuracy of integrated references with respect to standard quartz-crystal references, this thesis proposes the use of a duty-cycled wake-up radio and an impulse-radio modulation scheme to relax the allowed inaccuracy up to 1%. After a review of the fully integrated references with such level of inaccuracy and a power consumption low enough to be compliant with typical Wireless-Sensor-Networks energy sources, an oscillator referenced to the electron mobility in a MOS transistor is chosen as a viable candidate. To show that such references can achieve an inaccuracy of 1%, sources of errors for such oscillators have been analyzed and minimized. Prototypes built in a 65-nm CMOS process and in a 0.16-um CMOS process demonstrate proper operation over a wide temperature range while drawing less than 50 uW from a 1.2-V supply. Furthermore, to compensate the temperature dependence of the mobility, which affects the oscillator output frequency, a temperature compensation scheme has been devised. The compensation is based on a low-power low-voltage 65-nm CMOS temperature sensor with an inaccuracy of 0.2 °C (3 sigma) over the military temperature range. Finally, it is shown that the temperature-compensated mobility-based time reference can achieve the performance required by Wireless Sensor Nodes.