US007667575B2
`
`(12) United States Patent
`I-Iusak et al.
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,667,575 B2
`Feb. 23, 2010
`
`(54)
`
`(75)
`
`LOCATION VIRTUALIZATION IN AN RFID
`SYSTEM
`
`Inventors :
`
`David J. Husak, Windham, NH (US),
`Pattabhiraman Krishna, Westborough,
`MA (US); Peter Spreadborough,
`Haverhill, MA (US); Jeffrey H. Fischer,
`Boston, MA
`
`Assignee: Reva Systems Corporation,
`Chelmsford, MA (US)
`
`Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 836 days.
`
`Appl. No.: 11/194,144
`
`Filed:
`
`Jul. 29. 2005
`
`Prior Publication Data
`
`US 2006/0170565 A1
`
`Aug. 3, 2006
`
`Related U.S. Application Data
`
`Provisional application No. 60/592,933, filed o11 Jul.
`30, 2004.
`
`Int. Cl.
`H04Q 5/22
`(2006.01)
`(2006.01)
`G08B 13/14
`(2006.01)
`G08C 19/00
`U.S. Cl.
`.............. .. 340/10.2; 340/1031; 340/572.4;
`340/825.69; 370/314; 708/250; 708/255
`Field of Classification Search .............. .. 340/10.2,
`340/1031, 572.1, 825.69; 370/314; 708/250,
`708/255; 707/1, 3
`See application file for complete search history.
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,636,950 A
`
`1/ 1987 Caswcll ct al.
`
`(55)
`
`5,745,037 A *
`
`4/1998 Guthrie eta].
`
`......... .. 340/573.4
`
`(Continued)
`FOREIGN PATJNI DOCUM
`
`1 727 042 A1
`EP
`W0 W0 2006/109700 A1
`
`11/2006
`10/2006
`
`OTHER PUBLICATIONS
`
`“Passive RFID Basics”. Pete Sorrells. © 1998 Microchip Technology
`111:.
`
`Primary Examiner—Brian A linnnennan
`Assistant E.mminer—Nam V Nguyen
`(74) Attorney, Agent, or Firm—Weingarten, Schurgin,
`Gagnebin & Lebovici LLP
`
`(57)
`
`ABSTRACT
`
`A system and method ofdetennining locations ofone or 111ore
`RFID tags within an RFID environment. The system includes
`a plurality of RFID readers, each operative to transmit and
`receive RF signals for scanning a tag disposed within an RF
`coverage region associated with the reader, and for receiving
`tag data in response to the scanning of the tag. A plurality of
`sub-locations is determined within the environment, each
`corresponding to at least a portion of at least one of a plurality
`of RF coverage regions associated with the readers. The sub-
`locatio11s are mapped to a plurality of predefined locations
`Within the environment. A reader scans a tag, and receives tag
`scan data from the tag in response to the scanning of the tag.
`The tag scan data includes a tag identifier associated with the
`scanned tag. The tag scan data is mapped to the sub-locations
`based on the RF coverage region associated with the reader.
`The location of the scanned tag is then determined with ref-
`erence to the predefined locations Within the enviromnent,
`based on the tag identifier included in the tag scan data, the
`mapping of the tag scan data to the sub-locatio11s, a11d the
`mapping of the sub-locations to the predefined locations.
`
`94 Claims, 27 Drawing Sheets
`
`Host Computerts)
`.’lA..1;1A.L
`
`Raw Tag Data
`Processed Tag Data
`Locations
`Business Events
`System Status
`Notifications
`
`RFC - Exhibit 1009
`
`1
`
`

`
`US 7,667,575 B2
`Page 2
`
`11/2007 Ishiguro et al.
`7,298,270 B2
`8/2008 Strong et al.
`.............. .. 370/328
`7,411,921 B2*
`3/2008 gate
`7,416,122 B2
`7,463,142 132* 12/2008 Lindsay ............... .. 340/539.12
`2005/0109844 A1
`5/2005 Hilliard
`2005/0154570 A1
`7/2005 Sweeney
`2005/0154572 A1
`7/2005 Sweeney, 11
`2005/0222999 A1
`10/2005 Nihei
`2005/0231370 A1
`10/2005 Tagato
`2005/0237157 A1
`10/2005 Cooper etal.
`2006/0049944 A1
`3/2006 Ishiguro et al.
`2006/0082444 A1
`4/2006 Sweeney, 11 et al.
`2006/0158313 A1
`7/2006 Satou
`2006/0175407 A1
`3/2006 Kinoshita
`2003/0022290 A1
`1/2008 Ochiaj et 31.
`2003/0234862 A1
`9/2003 Funada et 31.
`2008/0250117 A1
`10/2008 Nihei
`
`............ .. 340/10.2
`
`* cited by examiner
`
`U.S. PATENT DOCUMENTS
`
`1/2000 Kubo ....................... .. 348/725
`6,011,597 A
`3/2000 Schepps
`~~ 340/5721
`6,040,774 A *
`.... ..
`11/2000 Werb et al
`340/10.1
`6,150,921 A
`1/2001 MacLe11aneta1
`340/10.1
`6,177,861 B1
`6,480,108 B2* 11/2002 McDonald ...... ..
`340/505
`6,608,551 B1
`8/2003 Anderson et al.
`. 340/10.51
`6,714,121 B1
`3/2004 Moore ......... ..
`340/10.3
`6,724,308 B2
`4/2004 Nicholson
`340/5721
`6,784,789 B2
`8/2004 Eroglu et al.
`............ .. 340/10.6
`6,804,537 B1
`10/2004 Fujii
`6,828,902 B2
`12/2004 Casden .................... .. 340/10.3
`.
`6,860,422 B2*
`3/2005 Hull et al.
`235/376
`
`7,039,359 B2*
`5/2006 Martinez .
`455/41.2
`7/2006 Kabala ........................ .. 707/1
`7,080,061 B2*
`7,132,948 B2
`11/2006 Sweeney, 11
`7,253,717 B2*
`8/2007 Armstrong et al.
`7,256,682 B2
`8/2007 Sweeney, 11
`
`
`
`340/10.2
`
`2
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 1 of 27
`
`US 7,667,575 B2
`
`Users
`
`100
`
`14.1 - 14r
`
`Host Computer(s)
`
`Raw Tag Data
`Processed Tag Data
`Locations
`Business Events
`System Status
`Notifications
`
`RFID
`Reader
`Comm‘
`
`RFID
`
`Reader
`Control
`
`
`
`Mid Level Processor(s)
`102.1—102n
`
`Tag Data
`
`3
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 2 of 27
`
`US 7,667,575 B2
`
`Transmit
`
`Data
`
`Processor
`
`Sequencer
`346
`
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`Data
`
`Control
`
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`
`Receive
`Data
`
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`
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`
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`
`Receive
`Data
`Processor
`Zfl
`
`Fig. 2a
`(Prior Art)
`
`4
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 3 of 27
`
`US 7,667,575 B2
`
`220 X
`
`E
`
`
`
`Tx Data
`
`Processor
`
`
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`
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`Processor
`
`2.351.
`
`Receive
`
`
`Data
`
`Fig. 2b
`
`5
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 4 of 27
`
`US 7,667,575 B2
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`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 6 of 27
`
`US 7,667,575 B2
`
`«K2
`
`ALE
`
`Custom
`
`Services
`
`EPCIS,
`
`JDBC, ODBC
`
`RP
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`Service Interface
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`
`'TDDP' + HTTP/TCP/IP
`RFID Switch
`402-2
`
`Fig, 4a
`
`8
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 7 of 27
`
`US 7,667,575 B2
`
`Direct Reader Reader
`Access
`Configuration
`
`Location
`Unified
`AP"."°a“°” Definitions
`Requlrments
`
`402
`
`Service Interfaces
`
`flfi
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`Fig- 43
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`i
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`flfl
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`
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`
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`
`
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`
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`
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`10.1
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`10.2
`
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`SNMP/UDP/IP + HTTP/TCP/IP
`
`10.q
`
`Fig. 4b
`
`9
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 8 of 27
`
`US 7,667,575 B2
`
`CLI
`
`HTTP
`
`402
`
`Reader Coordination
`
`E
`
`Constraint Sets
`
`Time Latency
`
`I Multistatic Operation I
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`
`Power Consumption
`
`Data Patterns
`
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`
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`
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`Transmitter Vectors
`
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`Time
`
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`
`Singulation
`Tag Search
`
`Schedule/Operate
`Reader Array
`
`10
`
`10
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 9 of 27
`
`US 7,667,575 B2
`
`L1
`
`B1
`
`S1
`
`S2
`
`S3
`
`Fig. 5
`
`
`
`1006
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`1002
`
`11
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 10 of 27
`
`US 7,667,575 B2
`
`Determine a plurality of segments from interrogation zones associated
`with a plurality of readers
`6_0Z
`
`Map the plurality of segments to predefined locations within an
`environment in which the readers are deployed
`@
`
`Collect tag data over -a predetermined time interval by the plurality of
`readers
`
`§_lE
`
`Map the tag data to the plurality of segments based on a reader identifier
`included in the tag data and the interrogation zone associated with the
`respective reader
`Q03;
`
`Determine the location of each tag within the environment using tag
`identifiers included in the tag data, the mapping of the tag data to the
`plurality of segments, and the mapping of the segments to the
`predefined locations within the environment
`610
`
`Fig. 6
`
`12
`
`12
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 11 of 27
`
`US 7,667,575 B2
`
`13
`
`13
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 12 of 27
`
`US 7,667,575 B2
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`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 13 of 27
`
`US 7,667,575 B2
`
`Reader 10.1
`
`Reader 10.2
`
`Reader 10.q
`
`Time framing
`
`Po[er
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`
`Fig. 15a
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`US 7,667,575 B2
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`U.S. Patent
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`Feb. 23, 2010
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`Sheet 16 of 27
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`US 7,667,575 B2
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`1400
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`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 17 of 27
`
`US 7,667,575 B2
`
`For each logical reader, compute a probability band around
`the reader
`
`1602
`
`Compute the areas or volume of overlap and non-overlap
`of RF coverage of the logical readers
`1604
`
`Identify a plurality of connected reader sets (CRSs)
`1606
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`Store segment information including segment identifiers
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`the segments as observation vectors in each CRS
`1608
`
`For each CRS, compute the likelihood of the segment for
`the different values of the observation vector
`1610
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`Fig. 16
`
`19
`
`19
`
`

`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 18 of 27
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`US 7,667,575 B2
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`
`U.S. Patent
`
`Feb. 23, 2010
`
`Sheet 19 of 27
`
`US 7,667,575 B2
`
`Power
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`Fig. 183
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`U.S. Patent
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`US 7,667,575 B2
`
`1
`LOCATION VIRTUALIZATION IN AN RFID
`SYSTEM
`
`CROSS REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims benefit of U.S. Provisional Patent
`Application No. 60/592,933 filed Jul. 30, 2004 entitled
`ACCESSING RFID TAG DATA USING A READER
`ARRAY.
`
`STATEMENT REGARDING FEDERALLY
`SPONSORED RESEARCH OR DEVELOPMENT
`
`N/A
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to a radio frequency
`identification (RFID) system including a plurality of RFID
`readers, and more specifically to a system and method of
`determining locations of one or more RFID tags within an
`environment in which the plurality of readers is deployed.
`In recent years, radio frequency identification (RFID) sys-
`tems have been employed in an ever increasing range of
`applications. For example, RFID systems have been used in
`supply chain management applications to identify and track
`merchandise throughout manufacture, warehouse storage,
`transportation, distribution, and retail sale. RFID systems
`have also been used in security applications to identify and
`track personnel for controlling access to restricted areas of
`buildings and plant facilities, thereby prohibiting access to
`such areas by individuals without the required authorization.
`Accordingly, RFID systems
`have been increasingly
`employed in diverse applications to facilitate the identifica-
`tion and tracking of merchandise, personnel, and other items
`and/or individuals that need to be reliably monitored and/or
`controlled within a particular environment.
`A conventional RFID system typically includes at least one
`RFID transponder or tag, at least one RFID reader, and at least
`one controller or host computer. For example, in a manufac-
`turing environment, RFID tags can be attached to selected
`items of manufacture or equipment, and at least one RFID
`reader can be deployed in the environment to interrogate the
`tags as the tagged items pass predefined points on the manu-
`facturing floor. In a typical mode of operation, the reader
`transmits a radio frequency (RF) signal in the direction of a
`tag, which responds to the transmitted RF signal with another
`RF signal containing information identifying the item to
`which the tag is attached, and possibly other data acquired
`during the manufacture of the item. The tag may also include
`at least one integrated transducer or environmental sensor for
`providing data such as the temperature or humidity level of
`the ambient environment. The reader receives the information
`
`and data transmitted by the tag, and provides the tag data to
`the host computer for subsequent processing. In this typical
`operating mode, the reader can be configured as a peripheral
`connected to a serial port of the host computer.
`More recently, RFID readers have been developed that are
`capable of being connected via a communications network to
`enterprise computer resources running one or more RFID-
`enabled client software applications. Such readers have been
`deployed in complex systems including many readers (e.g.,
`greater than 10) connected via one or more communications
`networks to a number of host computers, which may be part
`of an enterprise network server. Such ho st computers can run
`client applications for processing tag data to control access to
`
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`building and plant facilities, the movement of personnel and
`property,
`the operation of lighting/heating/ventilation/air
`conditioning facilities, and/or other diverse functions.
`Whether implemented as computer peripherals or net-
`worked devices, conventional RFID readers generally collect
`data from RFID tags much like optical barcode readers collect
`data from barcode labels. However, whereas an optical bar-
`code reader typically requires a direct line of sight to a bar-
`code label to read the data imprinted on the label, the RF
`signals employed by the typical RFID reader can penetrate
`through and/or diffract around objects obstructing an RFID
`tag from the RF field of view of the reader, thereby allowing
`the reader to access data from a tag that, for example, might be
`buried beneath one or more boxes of merchandise. In addi-
`
`tion, unlike the optical barcode reader, the conventional RFID
`reader can operate on and distinguish between multiple RFID
`tags within the field of the reader.
`In the conventional RFID system, each RFID tag typically
`includes a small antenna operatively connected to a micro-
`chip. For example, in the UHF band, the tag antenna can be
`just several inches long and can be implemented with con-
`ductive ink or etched in thin metal foil on a substrate of the
`
`microchip. Further, each tag can be an active tag powered by
`a durable power source such as an internal battery, or a passive
`tag powered by inductive coupling, receiving induced power
`from RF signals transmitted by an RFID reader. For example,
`an RFID reader may transmit a continuous unmodulated RF
`signal (i.e., a continuous wave, CW) or carrier signal for a
`predetermined minimum period of time to power a passive
`tag. The volume of space within which a reader can deliver
`adequate power to a passive tag is known as the power cou-
`pling zone of the reader. The internal battery of active tags
`may be employed to power integrated environmental sensors,
`and to maintain data and state information dynamically in an
`embedded memory of the tag. Because passive tags do not
`have a durable power source, they do not include active semi-
`conductor circuitry and must therefore maintain data and
`state information statically within its embedded memory. In
`addition, passive tags have an essentially unlimited life span,
`while the life span of active tags is typically limited by the
`lifetime of the internal battery, which in some implementa-
`tions may be replaceable.
`In conventional RFID systems that employ passive tags,
`each RFID reader typically follows a predefined sequence or
`protocol to interrogate and retrieve data from one or more
`RFID tags within the RF field ofthe reader (also known as the
`interrogation zone of the reader). It is noted that the interro-
`gation zone of a reader is generally determined by the physi-
`cal positioning and orientation of the reader relative to the
`tags, and the setting of various parameters (e.g., the transmit
`power) employed by the reader during the interrogation
`sequence. In systems employing passive tags, the interroga-
`tion zone is typically defined by the power coupling zone. For
`example, a typical interrogation sequence performed by an
`RFID reader includes transmitting a CW to one or more
`passive tags within the reader’s interrogation zone to power
`the tags, and transmitting a message packet (e.g., a request or
`command) by modulating the carrier signal. The passive tag
`then reads the message packet while tapping some of the
`energy of the CW to maintain its power. The message packet
`typically identifies one or a subset of the tags within the
`interrogation zone as the designated target of the message
`packet, and provides a request or command that the desig-
`nated tag is expected to perform. After the passive tag reads
`the information carried by the modulated carrier signal, the
`tag appropriately modulates the CW, and reflects a portion of
`the modulated wave back to the reader by changing the reflec-
`
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`

`
`US 7,667,575 B2
`
`3
`tion characteristics of its antenna via a technique known as
`backscatter modulation.
`In the event
`the interrogation
`sequence is employed in a system including active tags, the
`target active tag generates and transmits an appropriate
`response to the reader.
`During the typical interrogation sequence described above,
`the reader is tuned to detect changes in the small signals
`reflected from the antennae of the passive tags, or to receive
`the responses generated and transmitted by the active tags. In
`the event the reader detects changes in signal reflections or
`receives responses from more than one tag in response to a
`message packet, the reader refines the identification (e.g., the
`address) of the target of the message in an iterative manner
`until only one tag provides data or information in response to
`the request or command contained within the packet. For
`example, the tag address may be an electronic product code
`(EPC). This process of iterative refinement of the communi-
`cation between an RFID reader and a selected one of a plu-
`rality of RFID tags within the reader’s interrogation zone is
`known as singulation. Conventional singulation algorithms
`typically employ techniques similar to binary tree searches or
`randomized transmission delay techniques.
`After the reader has confirmed the presence ofand received
`data from the targeted tag,
`it may send another message
`packet to a next tag until all ofthe tags within its interrogation
`zone have been addressed. It is noted that some conventional
`
`interrogation protocols allow the creation of alias addresses
`for tags so that the reader is not required to transmit the actual
`tag address, which may carry private information. For
`example, a tag can indicate to the reader how its alias tag
`address is related to its actual tag address via the backscatter
`transmission. Further,
`the relationship between the alias
`address and the actual address can change each time the
`reader addresses that tag. The reader then typically sends the
`data provided by the tags to the ho st computer for sub sequent
`processing.
`However, the conventional RFID systems described above
`have a number of drawbacks. For example, in the event the
`system employs a single RFID reader, various factors such as
`(l) multi-path signal reflections from items, individuals, and/
`or other tags in the vicinity of a targeted tag, (2) dielectric
`loading of the tag antenna caused by the item or individual to
`which the tag is attached, and (3) shadowing of the RF signal
`transmitted by the reader caused by shielding and/or absorb-
`ing material near the tag, may prevent the single reader from
`successfully addressing and accessing data from each tag
`within its interrogation zone. The ability of an RFID reader to
`address RFID tags that may be at least partially obscured
`within its interrogation zone is known as the penetration
`ability ofthe reader. For an RFID system that includes a single
`RFID reader, the penetration ability of the reader is typically
`limited by the reader’s maximum transmit power.
`In addition, the interrogation zone of an RFID reader often
`changes depending upon the RF signal propagation charac-
`teristics in the environment in which the reader is deployed. It
`may therefore be virtually impossible to infer the actual inter-
`rogation zone of the reader directly from the reader’s trans-
`mission and reception settings and the transmission/reception
`capabilities of the tags. For example, a particular transmitter
`setting of an RFID reader may result in significantly different
`qualities of reception for two different RFID tags disposed in
`substantially the same location. Moreover, the interrogation
`zone of an RFID reader often fails to match the space that
`needs to be monitored in the RFID application.
`In addition, the conventional RFID system employing a
`single RFID reader is generally incapable of determining the
`location and/or direction of a moving RFID tag with preci-
`
`4
`
`sion. As described above, RFID systems have been used to
`identify and track merchandise, equipment, or personnel
`within a particular environment. Further, the tags attached to
`these items or individuals may be mobile or stationary, i.e.,
`moving or disposed at fixed locations in the environment. For
`example, in the event an RFID tag is attached to a forklift
`truck at a warehouse storage facility, a single RFID reader
`may be unable to determine whether the forklift is leaving or
`entering the facility at it passes the reader, which may be
`disposed at a dock bay. For this reason, a single RFID reader
`is often accompanied by one or more photoelectric (electric
`eye) detectors for determining the location and/or direction of
`movement of mobile tags. It is noted that an RFID reader can
`also be mobile (e.g., the reader may be a hand-held device or
`mounted to a vehicle) or stationary (e.g., the reader may be
`attached to a door, a wall, shelving, scaffolding, etc.).
`Conventional RFID systems that employ multiple RFID
`readers also have drawbacks. Such systems have included
`tens to hundreds of RFID readers and/or other input/output
`devices connected to a communications network controlled
`
`by tens to hundreds of host computers running specialized
`client applications. Such systems are frequently employed in
`applications that require many readers to perform tag moni-
`toring within a relatively large space, to provide adequate
`resolution for determining the physical locations of tags
`within the designated space, and/or to provide frequent or
`continuous operation as tagged items or individuals move
`briskly through the interrogation zones of the readers.
`However, such dense deployments of RFID readers are
`problematic due to RF interference and potentially conflict-
`ing charmel assignments of the readers. For example, the
`space within which the transmissions of a first reader may
`interfere with the reception of a second RFID reader operat-
`ing on the same channel as the first reader can be greater than
`the interrogation zones of the respective readers. Significant
`RF interference may also occur when readers operate on
`adjacent charmels. Even if multiple readers operate on chan-
`nels sufficiently far apart to avoid reader-to-reader interfer-
`ence, reader-to-tag interference may still occur since RFID
`tags are broadband devices capable ofreceiving RF transmis-
`sions from more than one reader at a time, which can confuse
`the tag circuitry.
`In addition, when multiple RFID readers are deployed
`within a conventional RFID system, the system is typically
`incapable of successfully coordinating the operation of the
`readers. Further, each ofthe multiple readers is limited in time
`and frequency usage to the transaction paradigms typically
`employed by conventional standalone readers. For example,
`each reader may only have access to its own information, and
`may therefore be unable to react to events detected by the
`other readers within the system. As a result, the multiple
`readers are generally incapable ofmanaging the large amount
`of potentially redundant information collected by the readers
`from the same group of tags within the same environment
`over an extended period of time. Further, such conventional
`systems are generally unable to discriminate between signifi-
`cant events and essentially meaningless movements of items
`and/or individuals within the RFID environment. Moreover,
`such conventional systems are generally incapable ofkeeping
`track of the various locations and functions of the many
`readers deployed within the environment, and assuring that
`all of the readers within the system are properly maintained
`and operational.
`It would therefore be desirable to have an improved system
`and method of accessing RFID tag data within an RFID
`environment. Such a system and method would have the
`capability of identifying and tracking tagged merchandise,
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`

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`US 7,667,575 B2
`
`5
`personnel, and other items and/or individuals within the envi-
`ronment with increased reliability. It would also be desirable
`to have a system and method of determining locations of one
`or more tags within the environment in which the system is
`deployed.
`
`BRIEF SUMMARY OF THE INVENTION
`
`In accordance with the present invention, a system and
`method is provided that can identify and track tagged mer-
`chandise, personnel, and other items and/or individuals
`within an RFID environment with increased reliability. The
`presently disclosed system and method can also be employed
`to determine locations of one or more tags within the envi-
`ronment.
`
`In one embodiment, the system includes a plurality of
`RFID readers. Each of the readers is operative to transmit and
`receive RF signals for interrogating at least one RFID tag
`disposed within an RF coverage region associated with the
`reader, and to receive tag data in response to the interrogation
`of the tag. In one mode of operation, a plurality of sub-
`locations is determined within an RFID environment in which
`
`the plurality ofreaders is deployed. Each of the sub-locations
`corresponds to at least a portion ofat least one of a plurality of
`RF coverage regions associated with the readers. Next, the
`plurality of sub-locations is mapped to a plurality of pre-
`defined locations within the environment. At least one of the
`
`readers then interrogates at least one tag within the environ-
`ment, and receives tag data from the tag in response to the
`interrogation of the tag by the reader. The tag data includes a
`tag identifier associated with the interrogated tag. Next, the
`tag data is mapped to the plurality of sub-locations based on
`the RF coverage region associated with the reader that inter-
`rogated the tag. The location of the interrogated tag is then
`determined with reference to the predefined locations within
`the environment, based on the tag identifier included in the tag
`data, the mapping of the tag data to the sub-locations, and the
`mapping of the sub-locations to the predefined locations.
`Other features, functions, and aspects of the invention will
`be evident from the Detailed Description ofthe Invention tha