Double-Notch Frequency-Coded Corner Reflectors for sub-THz Chipless RFID Tags

Frequency-coded corner reflectors (CR) have been proposed as an essential component of chipless indoor localization systems at sub-terahertz frequencies. These tags operate by encoding a notch in the frequency domain of the otherwise stable backscattered power spectrum by a CR. The current state-of-the-art in the WR-3 band (220 GHz to 330 GHz) is based on single notch frequency selective surfaces (FSS), whose maximum number of implementable frequency codes is too few for indoor localization applications. This work proposes the coding of the CR with a single layer FSS implementing two notches, using split ring resonators (SRRs) with two different resonance frequencies. The concept is demonstrated by encoding the response of a 3-cm-edge CR with an FSS resonating at 242 GHz and 308 GHz. The tag achieves a maximum radar cross-section (RCS) of 3 dBsqm at 275 GHz. Four more tags are demonstrated by simply varying the radii of the SRRs. It is estimated that this approach can implement up to 15 non-overlapping codes within the WR-3 band, three times more than the state-of-the-art.