Chipless RFID

Chipless RFID tags are RFID tags that do not require a microchip in the transponder.

RFIDs may be viable replacement for barcodes because of longer range and ability to be automated, unlike barcodes which need a human operator for interrogation. The main challenge in this respect is the cost of RFIDs. The design and fabrication of ASICs needed for RFID are the major component of an RFID's cost so removing IC altogether from an RFID can significantly reduce its cost. Because of the absence of ICs the major challenges in design of chipless RFID is the encoding and transmission of data.^[1]

Time-domain reflectometry vs frequency signature devices

Chipless RFID tags may use either time-domain reflectometry or frequency signature techniques. In time domain reflectometry type RFIDs the interrogator sends a pulse and listens for echoes. The timing of pulse arrivals encodes the data. In frequency signature type RFIDs the interrogator sends waves of several frequencies, a broad band pulse, or a chirp, and monitors the frequency content of the returned echoes. The presence or absence of certain frequency components in the received waves encodes the data. They may use chemicals, magnetic materials or resonant circuits to attenuate or absorb radiation of a particular frequency.^[2]

Chemical-based RFID tags

Siemens self-generating ceramic mixtures

In 2001, Roke Manor Research centre announced materials that emit characteristic radiation when moved. These may be exploited for storage of few bits of data encoded in the presence or absence of certain chemicals.^[3]

Somark Innovations biocompatible RFID ink

Somark employed a dielectric barcode that may be read using microwave. The dielectric materials reflects transmits and scatters the incident radiation different position and orientation of these bars affects the incident radiation differently and thus encoding the spatial arrangement in the reflected wave. The dielectric material may be dispersed in a fluid to create a dielectric ink.^[4] They were mainly used as tags for cattle. Cattle were "painted" using a special needle. The ink may be visible or invisible according the nature of the dielectric used, Operating frequency of the tag may be changed by using different dielectrics.^[5]

CrossID nanometric ink

This system uses materials exhibiting varying magnetism. They resonate at different frequencies when excited by radiation. The reader analyzes the spectrum of the reflected signal to make out which materials are present. 70 different materials were found. Each material's presence or absence may be used to encode one bit thus enabling encoding up to 2⁷⁰ unique binary strings. They work on frequencies between three and ten gigahertz.^[6]

Tapemark's chipless ID

In 2004 Tapemark announced a chipless RFID that will have only a passive antenna with diameter as small as 5 µm. The antenna consists of small fibers called nano-resonant structures. Spatial difference in structure may be used to encode data. The interrogator sends out a coherent pulse and reads back an interference patterns which it decodes to identify a tag. They work from 24 GHz-60 GHz.^[7] Tapemark has since discontinued development/manufacturing of this technology.

Magnetism-based RFID tags

Sagentia's programmable magnetic resonance

They are Acoustomagnetic devices. They exploit the resonance features of magnetically soft magnetostrictive materials and the data retention capability of hard magnetic material. Data is written to the card using contact method. The resonance of the magnetostrictive material is altered by the data stored in the hard material. Harmonics may be enabled or disabled corresponding to the state of the hard material, thus encoding the state of the device as a spectral signature. Tags built by Sagentia for AstraZeneca fall into this category.^{[8] [9] [10]}

Flying null's magnetic data tagging

The Flying Null technology uses a series of passive magnetic structures, much like the lines used in conventional bar codes. These structures are made of soft magnetic material. The interrogator contains two permanent magnets with like poles pointing each other. The resulting magnetic field will therefore have a null volume in the centre. Additionally an interrogating radiation is shun. The magnetic field created by the interrogator is such that it drives the soft material to saturation except when it is at the null volume. When in the null volume the soft magnet interacts with the interrogating radiation thus giving away the position of the soft material. Spatial resolution of more than 50 μm may be attained^[11][12]

Surface acoustic wave based RFIDs

Illustration of a simple SAW RFID encoding 013 in base 4. The first and last reflectors are used for calibration. The second and second last for error detection. The data is encoded in the remaining three groups. Each group contains 4 slots and an empty slot followed by another group.

It consists of a piezoelectric crystal like lithium niobate on which transducers are made by single-metal-layer photo-lithographic technology. The transducers used usually are Inter-Digital Transducers (IDT) which have a two-toothed comb like structure. There is an antenna attached to the IDT for reception and transmission. The transducers convert the incident radio wave to surface acoustic wave which travel on the surface of the crystal until it reaches the encoding reflectors that reflect some of the waves and transmit the rest. The IDT collects the reflected waves and transmits it to the reader. The first and last reflectors are used for calibration as the response may be affected by physical parameters like temperature. A pair of reflectors may also be used for error correction. The reflects increase in size from nearest to farthest of the IDT to account for losses due to preceding reflectors and attenuation to the acoustic wave. Data is encoded using Pulse Position Modulation technique. The crystal is logically divided into groups each group typically having a length equal to inverse of bandwidth of the system each group is divided into a number of slots of equal width and the reflector may be placed in any of the slots. The last slot in each group is usually unused leaving n-1 positions for the reflector thus encoding n-1 states. The repetition rate of the PPM will be equal to the bandwidth of the system. Additionally the position of the reflector in a slot may be used to encode phase. The temperature dependence of these devices means they can also act as temperature sensors along with providing RFID functions.^[13][14]

Capacitively tuned split microstrip resonators

They employ a grid of dipole antennas that are tuned to different frequencies. The interrogator generates a frequency sweep signal and scans for dips in the signal. It may be used to encode as many bits as there are dipole antenna. The frequency swept will be determined by the length of the antennas used. The coupling between the dipoles poses a challenge.^[15]

References