Telecommunications – Transmitter and receiver at separate stations – Short range rf communication
Reexamination Certificate
2000-08-17
2004-04-20
Appiah, Charles (Department: 2686)
Telecommunications
Transmitter and receiver at separate stations
Short range rf communication
C340S010200, C340S010320, C340S010420
Reexamination Certificate
active
06725014
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a field of radio frequency tag identification. More specifically, the present invention relates to a method for resolving contention in identification of a plurality of radio frequency tags.
BACKGROUND OF THE INVENTION
The object of any Radio Frequency Identifications (“RF ID”) system is to carry data in suitable transponders, generally known as tags, and to retrieve data, by machine-readable means, at a suitable time and place to satisfy particular application needs. The word “transponder”, derived from TRANSmitter/resPONDER, reveals the function of a tag, which usually responds to a transmitted request or a communicated request for data it carries. The data in the tag may provide identification information for an item, a location, an identity of a vehicle, an animal, a person or other information.
Radio frequency identification systems are typically categorized as either “active” or “passive”. In an active RF ID system, tags are powered by an internal battery, and data written into active tags may be rewritten and modified. In a passive RF ID system, tags operate without an internal power source and are usually programmed with a unique set of data that cannot be modified. A typical passive RF ID system comprises two components: an interrogator (reader) and a plurality of passive tags. The main component of every passive RF ID system is a plurality of information carrying tags that respond to a coded RF signals that are typically sent from the reader.
A passive tag typically includes an antenna and a semiconductor chip comprising radio frequency (“RF”) circuitry, logic and a non-volatile memory unit. Further, a time-varying electromagnetic RF signal (carrier signal) that is typically transmitted by a reader located at a base station energizes the passive tag. The reader usually comprises a microcontroller-based unit with a wound output coil and detector hardware. In a typical configuration, in addition to the reader, the base station comprises an antenna, an RF transmitter and an RF receiver. The reader may issue commands to the RF transmitter and receive commands from the RF receiver. The commands may serve to identify a plurality of tags present in a reader's field of view. The size of the reader's field of view depends primarily on a power level of the RF signal transmitted from the reader.
The RF transmitter at the base station may, for instance, encode a command from the reader, modulate the command to a radio frequency signal and, then, pass it to the antenna located at the base station. The RF receiver at the base station may receive, demodulate and pass to the reader return signals from the plurality of RF tags in the reader's field of view.
As known in the art, a passive tag is not a transmitter, yet a bi-directional communication is taking place in passive RF ID systems. The passive tag may employ a process called a backscatter modulation to send data to a reader. In a typical system, the reader continuously generates an RF carrier sine wave and senses for occurrences of a modulation, which would indicate a presence of a tag in a reader's field of view. When a tag enters the reader's field of view and, further, receives sufficient energy to operate, the tag typically divides down the RF carrier sine wave and, subsequently, starts clocking out identification data from a non-volatile memory unit, such as programmable registers.
The tag may clock out the identification data to an output transistor, which is typically connected to the tag's input coils. Consequently, the tag's output transistor may sequentially shunt the coil correspondingly to data that is clocked out from the non-volatile memory unit. Shunting the coil causes a momentary fluctuation of the carrier wave, which results in a slight change in an amplitude of the carrier wave and, thus, the reader may peak-detect the amplitude-modulated data and process the resulting bitstream according to encoding and modulation methods used in a particular RF ID system. This amplitude-modulation loading of the reader's transmitted field provides a communication path back to the reader.
In a modem business, maintaining an accurate inventory of merchandise is crucial. For instance, in a rental video store, a tag could be attached to each video tape in the video store, and one or more readers could then be used to maintain an inventory of video tapes. Further, it is often crucial to quickly determine what tags are in a given area.
There are many procedures that readers in RF ID systems may use to verify identification of a plurality of tags in readers' fields of view. In this regard, each tag in a reader's field of view may have an associated identification value that is stored cooperatively in a number of identification registers on the tag. Typically, for example, a tag may have a 36-bit identification value that is stored in three 12-bit identification registers. Further, each tag may have a corresponding set of counters (i.e., typically, three 12-bit counters) that can be controlled by carrier signals sent from a reader. A reader may send RF instruction signals to the tags in its field of view, which may cause actions to be taken with respect to the tags' counters and identification registers, so as to, in turn, modify the RF carrier and indicate the presence of the tags. One such instruction, for instance, may be an initialize instruction, which causes a recipient-tag to initialize a specified counter or counters. Another such instruction, for instance, may be an increment instruction, which causes a recipient-tag to increment a specified counter or counters.
According to one of the commonly used procedures for tag identification, a reader may first send an initialization instruction signal out to any tags in its field of view, directing the tags to initialize their first counters to zero. The reader may then sequentially transmit increment instructions to the tags, causing the tags to sequentially increment their first counters. Given 12 bits per register, in a typical arrangement, the reader may thus send out 4095 increment instructions.
When a count on the first counter of a tag matches a coded identification value on the tag's associated first identification register, the tag may then be arranged to respond to the reader by shifting out its complete identification value from all of its identification registers, thereby modifying the RF carrier to indicate the presence of the tag. Further, the tag may also shift out additional bits, such as a checksum (e.g., an 8-bit checksum), for instance. In this manner, if only one tag responds to the reader, the reader may properly detect that tag's identification and conclude that the tag is present.
However, this approach can suffer from a problem known as a contention. Contention occurs when more than one tag selects the same time slot for transmission of an identification coded value. This can occur in the process described above, for instance, if more than one tag's first counter matches the coded value in its first identification register. As a result, more than one tag will responsively send their full identification values and checksums to the reader. When is contention occurs, the reader is simultaneously bombarded with more than one tag's identification code and checksum in the same time slots. From the reader's perspective, a single identification code and checksum arrives, but the checksum will not be correct. Consequently, the reader will not be able to correctly detect the identification of each responding tag.
If contention occurs in the process above, according to common practice (known as the “partial read” method), the reader may keep the first counters of the tags in the field of view at the contending count (i.e., by not further incrementing the first counters) and may initiate counting on the second counters of the tags in the reader's field of view. In turn, when (i) the first contending count matches the tag's
Appiah Charles
Honeywell International , Inc.
McDonnell & Boehnen Hulbert & Berghoff
Perez-Gutierrez Rafael
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