Backscatter radio frequency systems and devices for novel wireless sensing applications

Abstract

This thesis examines the use of backscatter radio frequency identification (RFID) in novel wireless sensing applications.<br />Backscatter RFID in sensor networks relies on the radio communication between an RFID reader, acting as a control unit, and a multitude of passive or semi-passive RFID transponders (tags), acting as sensor nodes. All power for the transmission of the sensor data is drawn from the electromagnetic field radiated by the reader. Hence, their low-power consumption makes backscatter tags appropriate for sensing applications that require small, light-weight, and low-maintenance sensor nodes.<br /> It is vital to ensure a reliable power transfer and wireless communication between the reader and the tags. Thus, the major design goal in this work is to realize backscatter RFID systems and devices which lead to high system performances.<br />Particular attention is paid to the design of a tag antenna at 864MHz for the wheel unit (WU) of an advanced tire monitoring system (ATMS).<br />One premise of the application is that the antenna is directly attached to a car tire. The tire's material parameters vary strongly with type and vendor of the tire. Thus, a tag antenna is prototyped which can cope with a change in the tire environment. An analysis shows that the antenna prototype assures a stable power supply to the tag's chip for different tire environments and leads to a good system performance.<br />In addition, an on-body backscatter RFID system is evaluated for remote health monitoring applications at 900MHz and 2.45GHz. The system performance is evaluated in a realistic indoor scenario through on-body channel measurements. An analysis of a state-of-the-art system example shows that the use of semi-passive chips leads to a reliable performance in the system's forward link. A strategy to overcome limitations in the system's backward link is to use a phase-modulated backscatter signal.<br />Finally, a backscatter RFID sensor tag is designed that monitors the varying curvature of an object at 5.8GHz. The backscatter sensor includes a transducer prototype which changes its impedance as a function of bending and directly modulates the carrier signal sent from the RFID reader. The transducer prototype is optimized with respect to the sensor's sensitivity to bend and with respect to the sensor tag's modulation efficiency. It is found that the prototype qualifies for integration in the sensor tag and assures a good RFID system performance.