Communications: electrical – Selective – Interrogation response
Reexamination Certificate
2001-03-16
2004-04-27
Horabik, Michael (Department: 2635)
Communications: electrical
Selective
Interrogation response
C340S010100, C370S437000, C707S793000, C711S128000
Reexamination Certificate
active
06727803
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to devices and systems for radio frequency identification, and more particularly to interrogation devices and systems that identify radio frequency identification transponders.
BACKGROUND OF THE INVENTION
Radio Frequency Identification (RFID) is a technology that is used to locate, identify and track many different types of items, such as clothing, laundry, luggage, furniture, computers, parcels, vehicles, warehouse inventory, components on assembly lines, and documents. RFID transponders are used in much the same way as optical bar codes, identifying the item to which they are affixed as being a particular individual or as being part of a specific group. Unlike bar codes, RFID transponders, such as items
6
in
FIG. 1
, can be read even when they cannot be seen, and hence a “direct line of sight” for transmitted RF energy
4
and reflected RF energy
8
(
FIG. 1
) is not required between an interrogation device
2
and a transponder. Furthermore, the identification numbers of a multiplicity of transponders
6
can be read virtually simultaneously, with little or no effort on the part of the user to “aim” the interrogation device at each and every transponder. Some RFID transponders can store information in addition to that used for identification. This additional information may also be re-programmable by the user. Information within the transponder is typically accessed by a process variously referred to in the art as “scanning,” “reading,” or “interrogating.”
RFID transponders are typically interrogated by a radio transceiver with some added intelligence to enable it to send and receive data in accordance with a communication protocol designed into the transponder. When interrogating one or more transponders, the transceiver transmits RF energy
4
to the transponder, and encodes information on the outgoing signal by modulating the amplitude, phase and/or frequency of the signal. The RFID transponder can receive this signal and interpret the information sent by the interrogating device, and may also then respond by sending information contained in reflected RF energy
8
back to the interrogating device.
RFID transponders are often classified as either active or passive. An active transponder is continuously powered by a battery or alternate power source. In contrast, a passive transponder obtains its power from the RF field imposed upon it by an RFID transponder interrogation device. A passive RFID transponder, therefore, must remain close enough physically to the interrogating device to obtain adequate power to operate its circuits. Typically, the range for a passive transponder will be less than that of an active transponder, given that the interrogating device is transmitting the same amount of RF power at the same frequency for both types of transponders.
RFID transponders may be constructed from discrete components on a circuit board or they may be fabricated on a single silicon die, using integrated circuit (IC) techniques and needing only the addition of an antenna to function. Transponders are generally designed to operate in one of a number of different frequency bands. Popular frequency bands are centered around 125 kHz, 13.56 MHz, 915 MHz and 2.45 GHz. These particular frequencies are chosen primarily because regulations in many countries permit unlicensed operation in these bands, and the permitted transmission power levels are suitable for communicating with and/or providing power to the RFID transponders. Transponders operating at lower frequencies (e.g., 125 kHz and 13.56 MHz) generally require larger antennas, and typically employ inductive coupling via multiple-turn coils to achieve a small antenna size. Still other low-frequency transponders leverage capacitive coupling via large conductive surfaces. High frequency transponders typically utilize electric field coupling via simple half wavelength dipole antennas. For example, 2.45 GHz transponders can use simple paper-thin, printed-conductor antennas as small as 60 mm by 5 mm. In contrast, 125 kHz transponders typically use a coil antenna, usually either made of many loops of wire or of a foil spiral affixed to a substrate material. In low frequency transponders, coils, printed spirals, or conductive areas must be quite large in order to achieve an appreciable operating range. Examples of such transponders may be found in U.S. Pat. Nos. 4,654,658 and 4,730,188.
RFID transponders are typically identified by a number contained within a memory structure within each transponder. This memory structure may be programmed in a variety of ways, depending on the technology used to implement the memory structure. Some transponders may employ factory-programmable metal links to encode the ID. Others may employ one-time-programmable (OTP) methods, which allow the end user to program the ID. This is often referred to as Write Once, Read Many (WORM) technology, or as Programmable Read Only Memory (PROM). Both fusible links and anti-fuse technologies are used to implement this method of storage. Still other technologies allow the user to program and re-program the ID many times. Electrically Erasable Programmable Read Only Memory (EEPROM) and FLASH memory are examples of technologies that can be used to implement this type of access. The transponder ID number is typically stored in a binary format for ease of implementation, though other representations could be used.
When multiple RFID transponders are within range of the interrogating device, it is typically desired to be able to identify all of the transponders in the field. Once the transponders have been identified, their presence may be noted in a computer database. Following identification, each of the transponders may also be addressed individually to perform additional functions, such as the storing or retrieving of auxiliary data.
The ability of the system to efficiently identify the presence of a multiplicity of transponders is highly dependent upon the communications protocol used to interrogate the transponders. Among those familiar with the art, a protocol suitable for allowing multiple transponders to respond to an interrogation request is typically referred to as an “anti-collision protocol.” The process of singling out one transponder for communication is typically referred to as the process of “isolation.”
Most anti-collision protocols communicating between an interrogation device and a multiplicity of RFID transponders simultaneously present in an RF field have relied upon pseudo-random number (PN) generators. PN generators are typically used to vary the time during which the transponders may respond, so as to eventually allow a response from each transponder to reach the interrogation device without colliding destructively with the response from another transponder. Examples of such protocols can be found in U.S. Pat. Nos. 5,537,105, 5,550,547, and 5,986,570.
A drawback of using PN generators is that it is difficult to predict the time required to identify all of the transponders in the field, given that a certain number of transponders are in the field; hence, the time required is non-deterministic, even when the identities of the transponders being read are known. The use of random or pseudo-random intervals also necessitates the use of large time gaps between transponder transmissions to decrease the likelihood of collision between the transponder transmissions. This slows down the transponder communication process and drastically decreases the number of transponders which can be identified during a given amount of time. Previous anti-collision protocols utilizing PN generators have claimed to have the ability to achieve sustained read rates of up to approximately 80 transponders per second. Some protocols can read a single transponder in as little as 1 ms, but as the number of transponders in the field multiplies, PN generator-based protocols decline in performance, significantly increasing the average per-tag read time required.
The only known non-PN generator-based protocols available are des
Bangachon William
E-Tag Systems, Inc.
Horabik Michael
Loudermilk & Associates
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