number (a string of binary values) or some other identifying characteristic, and (2) an antenna to radiate or reradiate radio frequency (RF) energy, modulated by the identification number, to an apparatus that can detect that modulation and thus the identification value. Many variations of these two elements are possible, giving RFID tags quite a wide range of capabilities, as discussed below.

RFID VARIABLES

This section provides an overview of some of the components of an RFID system. It is important to note that, although many variations are available in each of the elements of an RFID tag, the variations are not necessarily available in all combinations. For example, a tag that can be read from a long way off will most likely require its own power source; a tag with no battery may be limited to a range of a few tens of meters. Some of the parameters to consider when evaluating or analyzing RFID systems are power requirements, the method of coupling between readers and tags, the receiving sensitivity and power output of antennas, the power requirements of the RFID tag chip (if the identifying tag uses a chip), and the frequency of operation. Several choices are usually available for each of these parameters, but the field of available tags is not simply the outer-product of all these options, because some combinations are not technically feasible or cost-effective.

Tags

The Basic RFID Tag

The simplest version of an RFID tag is a passive identification (ID) tag. It does not contain its own power source but instead harvests the power it needs from the reader’s RF emissions.3 It holds only a unique identifier and no other state information. When a reader reads the tag’s ID, it typically uses the ID to index a database that contains more expansive information about the object. For example, a tag on a package of pharmaceuticals may point to a database entry about the provenance of the drugs in the package, the distribution history of the package, and its final destination. As another example, electronic-article surveillance (EAS) systems currently employed extensively in libraries use the physical characteristics of a magnetic ribbon to backscatter a unique signature.

The coupled design of the reader and the tag antenna determines the range at which the tag’s ID can be read. Since this basic RFID tag is entirely dependent on getting power from the reader, the range with today’s technology tends to be quite limited, varying from near contact (so-called contactless technologies—such as some smart cards) to a maximum of about 15 meters (for many of the tags currently in supply-chain trials). Other complications in the reading process include the presence of multiple tags, interference from other radio sources (and in particular other readers), the absorption of radio energy by different materials between the tag and the reader,4 and the fact that reader transmission power is limited by regulatory bodies (see below).

3  

A reader communicates information to a tag by modulating an RF waveform, typically using amplitude-shift-keying (ASK) modulation. A reader receives information from a tag by transmitting a continuous-wave (CW) RF signal to the tag; the tag responds by modulating the radar cross section (the impedance match) of its antenna, thereby backscattering an information signal to the reader.

4  

RFID systems use either reader-talks-first or tag-talks-first operation. In reader-talks-first operation, tags wait to receive commands from a reader before backscattering. A tag responds with an information signal by modulating its antenna impedance only after being directed to do so by a reader. In tag-talks-first operation, tags backscatter information to a reader as soon as the tag enters an energizing RF field. In this latter case, a tag modulates its antenna impedance with an information signal until being directed to stop doing so by a reader. In addition, RFID systems can be half-duplex or full-duplex, with the former being



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