The idea of electromagnetic (EM) imaging for data encoding in chipless radiofrequency identification (RFID) systems was briefly introduced in Chapter 2. It was shown that imaging technique differs from other conventional methods as it provides extra spatial diversity. In imaging approach, each small section of the tag is separately scanned while it independently carries data. Providing fine scanning resolution governs the frequency band of operation and mandates the synthetic aperture radar (SAR) technique. However, the next issue that shall be answered is how to encode data in a small area of millimeter order.

Earth imaging system, the first and most important application of EM imaging, may suggest an approach for data encoding. In earth imaging, the earth's surface is interrogated by a radar system that is normally mounted on an air-/spacecraft. Based on the reflected signal, the radar system forms the related EM image of the earth. The strength of the reflected signal from each section of the earth's surface is linked to the image and shows the terrain of the earth. Rough surfaces result in higher reflection and therefore a brighter part in the EM image, for example, a building or mountain. An impinged signal on a flat surface is reflected in the opposite direction, by Snell's law; hence, radar receives no energy and a dark section in the image is formed. Figure 3.1 shows the principle of image brightness ...

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