Chipless RFID

Chipless tag printed with inkjet printing conductive ink.

To understand the development of chipless RFID tags, it is important to view it in comparison to classic RFID and barcode. RFID benefits from a very wide spectrum of functionalities, related to the use of radio-frequency (RF) waves for data exchange. The acquisition of the identifier (ID) is made much easier and volumetric readings are possible, all on tags containing modifiable information. These functions are impossible to implement with a barcode, but in reality, 70% of the items manufactured worldwide are equipped with it. The reasons for this enthusiasm are simple: barcode functions very well and is extremely cheap, the label as well as the reader. This is why barcodes remain the uncontested benchmark in terms of identification, with a cost-to-simplicity-of-use ratio that remains unequalled.

It is also true that RFID contributes other significant functionalities, and the question is therefore one of imagining a technology based on RF waves as a communication vector that would retain some of the advantages of barcodes. Pragmatically speaking, the question of system cost, and particularly of the tags that must be produced in large numbers, remains the central point. Due to the presence of electronic circuits, these tags have a non-negligible cost that is a very great deal higher than that of barcodes. It is logical therefore that a simple solution consists of producing chipless RF tags. The high cost of RFID tags is actually one of the principal reasons that chipped RFID is rare in the market for tags for widely distributed products, a market that numbers in the tens of thousands of billions of units sold per year. In this market, optic barcodes are very widely used.

However, technically speaking, chipped RFID offers significant advantages including increased reading distance and the ability to detect a target outside the field of vision, whatever its position. The concept of chipless RF label has been developed with the idea of competing with barcodes in certain areas of application. RFID has many arguments in its favor in terms of functionality, the only problem remaining being the price. The barcode offers no other feature than the ID recovery; however, the technology is time-tasted, widespread, and extremely low cost.

Chipless RFID has also good arguments in terms of functionality. Some functionalities are degraded versions of what RFID can do (read range / reading flexibility are reduced ...), others seem to be even more relevant in chipless (discretion, product integrity of the tag). The main advantage is the cost of chipless tags. As compared to the barcodes, the chipless technology must bring other features that are impossible to implement with the optical approach, while remaining a very low cost approach, that is to say, a potentially printable one. This is why the writing / rewriting and sensor capabilities are crucial features for the large-scale development of such a technology. For instance, the development of very low cost sensor - tags is now eagerly awaited for, for application reasons.[2]

Chipless rfid operating principle. A. Vena, E. Perret, and S. Tedjini, 2013.

Like various existing RFID technologies, chipless RFID tags are associated with a specific RF reader, which questions the tag and recovers the information contained in it. The operating principle of the reader is based on the emission of a specific electromagnetic (EM) signal toward the tag, and the capture of the signal reflected by the tag. The processing of the signal received—notably via a decoding stage—makes it possible to recover the information contained in the tag.[3]

However, chipless RFID tags are fundamentally different from RFID tags. In the latter, a specific frame is sent by the reader[4] toward the tag according to a classic binary modulation schema. The tag demodulates this signal, processes the request, possibly writes data in its memory, and sends back a response, modulating its load.[5] Chipless RFID tags, on the other hand, function without a communication protocol. 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. They can be viewed as radar targets possessing a specific, stationary temporal or frequential signature. With this technology, the remote reading of an identifier consists of analyzing the radar signature of the tag.

Currently, one of main challenges of the chipless technology is the robustness of tag detection in different environments. It is useless to try to increase the quantity of information that a chipless tag can have if the tag ID cannot be read properly in real environments and without complex calibration techniques. The detection of a chipless tag in noisy environments is much more difficult in chipless than in UHF RFID due to the absence of modulation in time, that is, the absence of two different states in the backscattering signal.

Surface acoustic wave

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.

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.[16]

Paper chipless tag printed with flexography technique.

Many improvements have been done in the past few years on communication systems, based on electronic devices where an integrated circuit is at the heart of the whole system. The democratization of these chipped based systems like the RFID one have however given rise to environmental issues.

Lately, new research projects, such as European Research Council (ERC) funded project ScattererID,[18] have introduced the paradigm of RF communication system based on chipless labels, where new useful functionalities can be added. With comparable costs to a barcode, these labels should stand out by providing more functionalities than the optical approach. The objective of ScattererID project is to show that it is possible to associate the chipless label ID with other features like the ability to write and rewrite the information, to associate an ID with a sensor function and to associate an ID with gesture recognition.

The possibility of designing reconfigurable and low cost tags involves the development of original approaches at the forefront of progress, like the use of CBRAM from microelectronics, allowing to achieve reconfigurable elements based on Nano-switches.