RFID—radio frequency identification—is a means of identifying objects via a wireless communication protocol, a capability that enables myriad advantages and breakthroughs in applications ranging from supply chain management to asset tracking to authentication of frequently counterfeited pharmaceuticals.
Some have likened RFID to an electronic barcode, which is an oversimplification. RFID technology goes far beyond the capability of passive, non-unique printed graphics (barcodes) that require line-of-sight to readers, lack interactivity, cannot be used to identify unique objects, and are easily counterfeited or otherwise compromised. RFID tags are sophisticated, intelligent devices that carry unique, field-updatable information. They leverage huge efficiencies not only in time, cost, and labor, but in unprecedented supply chain visibility. These are just a few of the reasons driving major retailers, the Department of Defense, the FDA, and other organizations and entities to mandate the deployment of RFID technology.
A typical RFID system consists of three primary components: tags, readers, and reader antennas. Let's take a closer look at each of these components.
RFID Tag Manufacturing
An RFID tag generally comprises an integrated circuit (IC or chip) that has been mounted on a flexible PET (PolyEthyleneTherephtalate) or paper substrate, which has been preprinted with conductive ink (or assembled with an etched, stamped, or vapor-deposited antenna pattern), according to the particular antenna design. The resulting inlay assembly is then converted or sandwiched between a printed label and its adhesive backing, yielding a smart label. This label can be programmed with a unique tracking identifier called an electronic product code (EPC) and attached to an item, case, or pallet. For some applications, tag chips can also be factory programmed before they are assembled.
RFID Tag Operation
How do RFID tags work? Fundamentally, they operate on the same basis as other wireless devices like your cordless or cellular phones or wireless LAN: an antenna attached to the tag chip's electrical circuits radiates electromagnetic waves through the air in a manner defined by a particular communications protocol. The specific frequency of those waves is expressed in Hertz (cycles per second). The protocol of interest to this discussion is the EPCglobal ultra-high frequency (UHF) Gen 2 standard—a single worldwide standard that defines tag and reader communications operating in the 860 MHz to 960 MHz frequency band.
UHF Gen 2 tags are passive devices that operate without the use of a battery. Instead, they "harvest" the electromagnetic energy emitted by an in-range reader, converting that energy into the DC power required to operate the chip. Being thus powered, the chip can communicate with the reader.
Communication flow is in either the reader-to-tag or tag-to-reader direction. The tag transmits digital information to the reader by reflecting, or backscattering, part of the energy transmitted to it by the reader. This backscatter reflection is the mechanism by which the tag "talks back" to the reader; the reflected signal is modulated according to the appropriate communications protocol to transmit data to the reader.
The Gen 2 protocol involves a series of commands and tag responses that control the communications between readers and what might potentially be a large population of tags within the readers' zones of operation.
RFID Tag Antennas
Convenient as it would be, there is no single best tag antenna design for every application operating in every region of the world. As a result, there has been a proliferation of specially optimized forms and configurations, whether they're designed for palletized cases or small items. Tag size, form factor, cost, orientation sensitivity, and range are a few of the tradeoffs to consider when making a tag selection. Some tags might be optimized for a particular frequency band, while others might be tuned for good performance when attached to materials with particular dielectric characteristics (cardboard versus glass, for example). Others still might be more general purpose in scope, working reasonably well across the entire UHF spectrum, in free space or affixed to an item. Inlays and converted labels powered by Impinj technology are available with a multitude of antenna form factors from our partners.
The reader, also known as an interrogator, is a device that provides network connectivity between tag data and enterprise system software. The reader communicates with the RFID tags within its field of operation, performing any number of operations including simple continuous inventorying, filtering (searching for tags that meet certain criteria), writing to selected tags, etc.
The reader uses its antenna to send information encoded in a modulated waveform as well as the tone that the tag uses to power itself. A receiver circuit on the tag is able to detect the modulated field, decode the information, and use its own antenna to send (backscatter) a response.
Readers may be fixed (dock door or shelf installations), mobile (installed on a forklift or hand-held), or in the form of a module contained within a printer/encoder.
RFID Reader Antennas
Reader antennas, like tag antennas, may also assume form factors appropriate to their application. For example, antennas may be fitted to dock doors, embedded in store shelves or racks, fitted within a point-of-sale terminal, or integrated into the guide rails of conveyor equipment.
How do reader antennas work? In basic terms, an antenna converts electrical current into electromagnetic waves that are then radiated into space in a particular pattern at a given level of intensity. The parameters of greatest interest to the tag are polarization (or the reader antenna wave's electric field vector, orientation, and direction) and the power level of the transmission.
A linearly polarized antenna radiates entirely in one plane in the direction of signal propagation, while with a circularly polarized antenna, the plane of polarization rotates in a circular fashion (effectively a corkscrew when considered in time), making a complete revolution during one period of the wave.
System performance is also greatly impacted by the configuration of the reader antenna ports (where the antennas connect to the reader), the choice of which is generally driven by tradeoffs in cost and performance. Two schemes of choice are monostatic and bistatic. A monostatic system uses the same antenna to both transmit and receive, while a bistatic system uses separate, dedicated antennas for the transmit and receive operations. A four-port monostatic reader requires four antennas; a four-port bistatic reader requires eight antennas. Deployment costs and complications notwithstanding, some reader suppliers have opted for the bistatic scheme because they haven't developed the technology to maintain the high receive sensitivity required in a monostatic antenna configuration. Because both transmit and receive signals share the same antenna in a monostatic system, this arrangement is subject to signal reflections back into the receiver path, raising the noise floor, lowering dynamic range, and lowering reader sensitivity.
Technology from Impinj, however, changed all that. Recognizing that a monostatic system is inexpensive, simple to deploy, and exhibits better data collection and processing efficiency over bistatic solutions, Impinj engineered a series of patent pending innovations that culminated in INR™ (Impinj Noise Reduction) to enable monostatic's benefits without compromising sensitivity. INR not only lowers the noise floor by as much as 20 dB compared to traditional monostatic configurations, it has the added benefit of rejecting interference from readers operating in adjacent channels.
What's more, monostatic antennas greatly simplify the extension of UHF Gen 2 to item-level tagging (ILT). ILT is difficult and impractical for bistatic systems, because the transmit and receive antennas must be positioned so closely together that the signal from the transmit antenna couples strongly back into the receive antenna. The reason that some readers use bistatic antennas is to provide isolation between the transmit and receive signals in typical portal applications; they don't get this isolation in ILT environments. Simply put, monostatic readers work better for ILT.