Radio-frequency identification

Radio-frequency identification uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder called a tag, a radio receiver, and a transmitter. When triggered by an electromagnetic interrogation pulse from a nearby RFID reader device, the tag transmits digital data, usually an identifying inventory number, back to the reader. This number can be used to track inventory goods.
Passive tags are powered by energy from the RFID reader's interrogating radio waves. Active tags are powered by a battery and thus can be read at a greater range from the RFID reader, up to hundreds of meters.
Unlike a barcode, the tag does not need to be within the line of sight of the reader, so it may be embedded in the tracked object. RFID is one method of automatic identification and data capture.
RFID tags are used in many industries. For example, an RFID tag attached to an automobile during production can be used to track its progress through the assembly line, RFID-tagged pharmaceuticals can be tracked through warehouses, and implanting RFID microchips in livestock and pets enables positive identification of animals. Tags can also be used in shops to expedite checkout, and to prevent theft by customers and employees.
Since RFID tags can be attached to physical money, clothing, and possessions, or implanted in animals and people, the possibility of reading personally linked information without consent has raised privacy concerns. These concerns resulted in standard specifications development addressing privacy and security issues.
In 2014, the world RFID market was worth US$8.89 billion, up from US$7.77 billion in 2013 and US$6.96 billion in 2012. This figure includes tags, readers, and software/services for RFID cards, labels, fobs, and all other form factors. The market value is expected to rise from US$12.08 billion in 2020 to US$16.23 billion by 2029.
In 2024, about 50 billion tag chips were sold, according to Atlas RFID and RAIN Alliance webinars in July 2025.

History

In 1945, Leon Theremin invented the "Thing", a listening device for the Soviet Union which retransmitted incident radio waves with the added audio information. Sound waves vibrated a diaphragm which slightly altered the shape of the resonator, which modulated the reflected radio frequency. Even though this device was a covert listening device, rather than an identification tag, it is considered to be a predecessor of RFID because it was passive, being energised and activated by waves from an outside source.
Similar technology, such as the Identification friend or foe transponder, was routinely used by the Allies and Germany in World War II to identify aircraft as friendly or hostile. Transponders are still used by most powered aircraft. An early work exploring RFID is the landmark 1948 paper by Harry Stockman, who predicted that "Considerable research and development work has to be done before the remaining basic problems in reflected-power communication are solved, and before the field of useful applications is explored."
Mario Cardullo's device, patented on January 23, 1973, was the first true ancestor of modern RFID, as it was a passive radio transponder with memory. The initial device was passive, powered by the interrogating signal, and was demonstrated in 1971 to the New York Port Authority and other potential users. It consisted of a transponder with 16 bit memory for use as a toll device. The basic Cardullo patent covers the use of radio frequency, sound and light as transmission carriers. The original business plan presented to investors in 1969 showed uses in transportation, banking, security and medical.
In 1973, an early demonstration of reflected power RFID tags, both passive and semi-passive, was performed by Steven Depp, Alfred Koelle and Robert Freyman at the Los Alamos National Laboratory. The portable system operated at 915 MHz and used 12-bit tags. This technique is used by the majority of today's UHFID and microwave RFID tags.
In 1983, the first patent to be associated with the abbreviation RFID was granted to Charles Walton.
In 1996, the first patent for a batteryless RFID passive tag with limited interference was granted to David Everett, John Frech, Theodore Wright, and Kelly Rodriguez.

Design

A radio-frequency identification system uses tags, or labels attached to the objects to be identified. Two-way radio transmitter-receivers called interrogators or readers send a signal to the tag and read its response.

Readers

RFID systems can be classified by the type of tag and reader. There are 3 types:

A passive reader active tag system has a passive reader which only receives radio signals from active tags. The reception range of a PRAT system reader can be adjusted from, allowing flexibility in applications such as asset protection and supervision.
An active reader passive tag system has an active reader, which transmits interrogator signals and also receives authentication replies from passive tags.
An active reader active tag system uses active tags activated with an interrogator signal from the active reader. A variation of this system could also use a battery-assisted passive tag which acts like a passive tag but has a small battery to power the tag's return reporting signal.

Fixed readers are set up to create a specific interrogation zone which can be tightly controlled. This allows a highly defined reading area for when tags go in and out of the interrogation zone. Mobile readers may be handheld or mounted on carts or vehicles.

Frequencies

Signaling

Signaling between the reader and the tag is done in several different incompatible ways, depending on the frequency band used by the tag. Tags operating on LF and HF bands are, in terms of radio wavelength, very close to the reader antenna because they are only a small percentage of a wavelength away. In this near field region, the tag is closely coupled electrically with the transmitter in the reader. The tag can modulate the field produced by the reader by changing the electrical loading the tag represents. By switching between lower and higher relative loads, the tag produces a change that the reader can detect. At UHF and higher frequencies, the tag is more than one radio wavelength away from the reader, requiring a different approach. The tag can backscatter a signal. Active tags may contain functionally separated transmitters and receivers, and the tag need not respond on a frequency related to the reader's interrogation signal.
An Electronic Product Code is one common type of data stored in a tag. When written into the tag by an RFID printer, the tag contains a 96-bit string of data. The first eight bits are a header which identifies the version of the protocol. The next 28 bits identify the organization that manages the data for this tag; the organization number is assigned by the EPCGlobal consortium. The next 24 bits are an object class, identifying the kind of product. The last 36 bits are a unique serial number for a particular tag. These last two fields are set by the organization that issued the tag. Rather like a URL, the total electronic product code number can be used as a key into a global database to uniquely identify a particular product.
Often more than one tag will respond to a tag reader. For example, many individual products with tags may be shipped in a common box or on a common pallet. Collision detection is important to allow reading of data. Two different types of protocols are used to "singulate" a particular tag, allowing its data to be read in the midst of many similar tags. In a slotted Aloha system, the reader broadcasts an initialization command and a parameter that the tags individually use to pseudo-randomly delay their responses. When using an "adaptive binary tree" protocol, the reader sends an initialization symbol and then transmits one bit of ID data at a time; only tags with matching bits respond, and eventually only one tag matches the complete ID string.
Both methods have drawbacks when used with many tags or with multiple overlapping readers.

Bulk reading

"Bulk reading" is a strategy for interrogating multiple tags at the same time, but lacks sufficient precision for inventory control. A group of objects, all of them RFID tagged, are read completely from one single reader position at one time. However, as tags respond strictly sequentially, the time needed for bulk reading grows linearly with the number of labels to be read. This means it takes at least twice as long to read twice as many labels. Due to collision effects, the time required is greater.
A group of tags has to be illuminated by the interrogating signal just like a single tag. This is not a challenge concerning energy, but with respect to visibility; if any of the tags are shielded by other tags, they might not be sufficiently illuminated to return a sufficient response. The response conditions for inductively coupled HF RFID tags and coil antennas in magnetic fields appear better than for UHF or SHF dipole fields, but then distance limits apply and may prevent success.
Under operational conditions, bulk reading is not reliable. Bulk reading can be a rough guide for logistics decisions, but due to a high proportion of reading failures, it is not currently suitable for inventory management. However, when a single RFID tag might be seen as not guaranteeing a proper read, multiple RFID tags, where at least one will respond, may be a safer approach for detecting a known grouping of objects. In this respect, bulk reading is a fuzzy method for process support. From the perspective of cost and effect, bulk reading is not reported as an economical approach to secure process control in logistics.