mm-wave EM-imaging chipless RFID system

A new mm-wave imaging chipless RFID system is proposed. The system comprises a multi-bit tiny chipless RFID tag, a double sided printed dipole array (DSPDA) as the reader antenna and the synthetic aperture radar (SAR) image processing algorithm. The multi-bit tags comprise a series of meander line and strip line polarizers on a paper substrate. A linearly polarized (LP) incident wave illuminates the tag surface at 60 GHz instrumentation scientific and medical (ISM) band. The DSPDA is a four-element linear array of DSPD elements. One DSPDA illuminates the tag in vertical polarisation and another DSPDA receives the backscattered signal in horizontal polarisation. The backscattered signal in the cross-polar radar cross section (RCS) of the tag contains the data contents. The reader synthetically processes the cross-polar RCS to construct the image of the tag surface. This data is decoded in the reader’s synthetic aperture radar (SAR) processor. The developed mm-wave imaged tag is immune to noise and interference due to its cross polar nature of data processing. Since the tag does not operate on the frequency signature based principle as is for its predecessor tags, the new imaged tag provides high data capacity, compact design and immunity to printing errors. The advantage of the cross-polar RCS data encoding over the conventional co-polar RCS data encoding is analysed and then verified through a series of experiments in different practical scenarios such as tags attached: (i) to an object in cluttered and interfering environments; (ii) on liquid containers; (iii) on metallic surfaces such as aluminium cans; and finally, (iv) the tag is covered with thick paper, plywood and cardboard and read at non-line-of-sight (NLOS) conditions. Moreover, the radius of curvature of a bent tag is thoroughly investigated attaching the tag on tubes with varying radii. The proposed tag demonstrates robustness toward multipath and clutter interferences, provides a very reliable approach to tagging of bent surfaces and highly reflective items such as water bottles and aluminium cans. Moreover, the mm-wave tag can be read NLOS inside envelopes, cardboard and wooden boxes hence secure item tagging is also feasible in this approach. The achieved data encoding capacity of the tag is 2 bits/cm2 which is very promising for industrial applications. A credit card size tag offers more than 90 bits of data. This content capacity can be increased further with addition of range resolution image processing of the tag surface. The requirement for relative movement of the reader with respect to the tag is also addressed with a new multiple input multiple output (MIMO)-based SAR technique. A limited number of fixed transmit and receive antennas is design based on the MIMO theory to provide a fast imaging processing for the proposed mm-wave EM-imaging chipless RFID system. The MIMO signal processing results in significant throughput enhancements in terms of tag reading time and least hardware requirement.