How does chipless RFID work and what RF encoding techniques does it use?

Chipless RFID works by encoding data in the RF signature (spectral response or time-domain response) of a tag that contains no semiconductor chip, relying instead on passive resonant structures printed on a substrate. The tag is interrogated by a wideband RF reader, and the tag's data is extracted from the characteristics of the reflected (backscattered) signal. RF encoding techniques: spectral signature encoding (the tag contains multiple resonant elements, each tuned to a different frequency. When illuminated by a wideband signal: each resonator creates a notch (dip) or peak in the backscattered spectrum at its resonant frequency. The presence/absence of each notch encodes one bit. For N resonators: N bits of data can be encoded. Example: a tag with 20 resonators (each representing one bit of a unique code) operating across 2-10 GHz), time-domain encoding (the tag contains multiple reflective structures (short-circuited microstrip stubs of different lengths). Each stub creates an echo at a different time delay (proportional to its length). The time positions of the echoes encode the data), phase encoding (the tag's resonant elements modulate the phase of the backscattered signal at specific frequencies. The phase pattern encodes data), and polarization encoding (the tag's elements have different polarization responses (horizontal, vertical, 45 degrees). The polarization signature at each frequency encodes data). The advantage of chipless RFID: extremely low cost (the tag is just a printed pattern on paper or plastic, no chip, no assembly; potentially $0.01-0.05 per tag vs. $0.05-0.15 for chipped RFID tags). The challenges: low data capacity (current chipless RFID: 5-40 bits vs. 96+ bits for EPC Gen2 chipped tags), shorter read range (1-20 cm vs. 1-15 m for UHF RFID), and larger tag size (the resonant elements occupy more area than a chip-based tag).