Harmonic Transponders for Tracking and Sensing

This chapter is dedicated to the description of wireless-sensing systems based on harmonic transponders. Harmonic transponders are a special type of backscatter radios, which leverage a nonlinear component to “distort” the backscattered signal. This way, unlike traditional backscatter radios, the signal backscattered by harmonic transponders lies in a different frequency band (i.e. a harmonic frequency) with respect to the interrogating signal. This way, contributions such as transmitter leakage and signal reflections from the environments can be easily filtered out, thereby improving the sensitivity of the receiver and reducing the risk of self-jamming. After a brief introduction, the chapter will describe the basic principles of harmonic systems and the main research areas dedicated to extend the read range of the transponders and reduce their form factor. Then, some of the latest advancements in harmonic-based wireless sensors will be illustrated, focusing on their performance in harsh environments, such as industrial and outdoor environments. An overview of multi-bit harmonic transponders and harmonic transponders for localization purposes will be also provided. Indeed, harmonic transponders can be used in a variety of applications, ranging from electronic article surveillance to the tracking of items moving in environments characterized by strong clutter, and represent a valid solution also to implement energy-autonomous or even passive wireless sensing. For these reasons, harmonic transponders can operate in environments characterized by strong reflections and metal parts, which are particularly severe for signal propagation, as the contribution from the tags can be easily separated from the rest of the reflections. Additionally, due to their simple circuitry (they are usually based on single diodes), they are particularly robust, which makes them good candidates to operate in environments characterized by harsh conditions (such as low or high temperature, high humidity, and so forth), thereby expanding the possible fields of application of wireless sensors.