Direct-Conversion Sensor for Wireless Sensing Networks

This paper introduces a new class of microwave sensor architecture using direct conversion principle to eliminate data processing and provide low-power smart sensor nodes. Using a six-port circuit as the modulator, the sensing data are up-converted directly to a microwave frequency and sent by an antenna. At the receiver, a six-port circuit is used as a demodulator to down-convert the received signal and extract the sensing data. To predict the communication channel of the sensor network and provide a robust connection, a pilot sequence is sent along with the sensor data which is used in the calibration procedure at the receiver for accurate extraction of sensing data and making sensors smarter. Any kind of microwave sensors based on the variation of impedance can be used in the proposed structure. In this paper, the capacitive loading is used as a material sensor. By variation of material in the sensing area, the impedance of the load and reflection coefficient ( $\Gamma$ ) are changed. Then, the amplitude and phase of $\Gamma$ are transmitted via the six-port circuit, received in the network base station, and analyzed to find the material changes. As an example, a system is fabricated and evaluated at 2.45 GHz.