RFIDs offer longer range and ability to be automated, unlike barcodes that require a human operator for interrogation. The main challenge to their adoption is the cost of RFIDs. The design and fabrication of ASICs needed for RFID are the major component of their cost, so removing ICs altogether can significantly reduce its cost. The major challenges in designing chipless RFID is data encoding and transmission.
Time-domain reflectometry vs frequency signature devices
Chipless RFID tags may use either time-domain reflectometry or frequency signature techniques. In time domain reflectometry the interrogator sends a pulse and listens for echoes. The timing of pulse arrivals encodes the data. In frequency signature RFIDs the interrogator sends waves of several frequencies, a broadband pulse or a chirp, and monitors the echoes' frequency content. 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.
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 a few data bits encoded in the presence or absence of certain chemicals.
Somark employed a dielectric barcode that may be read using microwaves. The dielectric material reflects, transmits and scatters the incident radiation; the different position and orientation of these bars affects the incident radiation differently and thus encodes the spatial arrangement in the reflected wave. The dielectric material may be dispersed in a fluid to create a dielectric ink. They were mainly used as tags for cattle, which were "painted" using a special needle. The ink may be visible or invisible according to the nature of the dielectric, Operating frequency of the tag may be changed by using different dielectrics.
CrossID nanometric ink
This system uses varying magnetism. Materials resonate at different frequencies when excited by radiation. The reader analyzes the spectrum of the reflected signal to identify the materials. 70 different materials were found. Each material's presence or absence may be used to encode a bit, enabling encoding up to 270 unique binary strings. They work on frequencies between three and ten gigahertz.
In 2004 Tapemark announced a chipless RFID that will have only a passive antenna with a diameter as small as 5 µm. The antenna consists of small fibers called nano-resonant structures. Spatial difference in structure encode data. The interrogator sends out a coherent pulse and reads back an interference pattern that it decodes to identify a tag. They work from 24 GHz–60 GHz. Tapemark later discontinued this technology.
Programmable magnetic resonance
Sagentia's devices are acousto-magnetic. They exploit the resonance features of magnetically soft magnetostrictive materials and the data retention capability of hard magnetic materials. Data is written to the card using the 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 device state as a spectral signature. Tags built by Sagentia for AstraZeneca fall into this category.
Magnetic data tagging
Flying Null technology uses a series of passive magnetic structures, much like the lines used in conventional barcodes. These structures are made of soft magnetic material. The interrogator contains two permanent magnets with like poles. The resulting magnetic field has a null volume in the centre. Additionally, interrogating radiation is used. 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.
Surface acoustic wave
Surface acoustic wave devices consists of a piezoelectric crystal-like lithium niobate on which transducers are made by single-metal-layer photolithographic technology. The transducers usually are Inter-Digital Transducers (IDT), which have a two-toothed comb-like structure. An antenna is attached to the IDT for reception and transmission. The transducers convert the incident radio wave to surface acoustic waves that travel on the crystal surface until it reaches the encoding reflectors that reflect some waves and transmit the rest. The IDT collects the reflected waves and transmits them to the reader. The first and last reflectors are used for calibration as the response may be affected by physical parameters such as temperature. A pair of reflectors may also be used for error correction. The reflections increase in size from nearest to farthest of the IDT to account for losses due to preceding reflectors and wave attenuation. Data is encoded using Pulse Position Modulation (PPM). The crystal is logically divided into groups, such that each group typically has a length equal to the inverse of the bandwidth. Each group is divided into slots of equal width. The reflector may be placed in any slot. 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 is equal to the system bandwidth. The reflector's slot position may be used to encode phase. The devices' temperature dependence means they can also act as temperature sensors.
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 signal dips. Each dipole antenna can encode one bit. The frequency swept will be determined by the antenna length.
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