United States Patent (19)
`Shober
`
`54) IN-BUILDING MODULATED BACKSCATTER
`SYSTEM
`
`75 Inventor: R. Anthony Shober, Red Bank, N.J.
`
`73 Assignee: Lucent Technologies Inc., Murray Hill,
`N.J.
`
`*
`
`Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent term provisions of 35 U.S.C.
`154(a)(2).
`
`21 Appl. No.: 08/775,701
`
`Dec. 31, 1996
`22 Filed:
`(51) Int. Cl. ............................................... G08B 13/14
`52 U.S. Cl. ................. 340,572.4, 340/505.340,825.34;
`340/825.54; 340/572.7; 343/878
`• a
`s
`•
`s
`58 Field of Search ..................................... 340/572, 505,
`340/825.54, 825.34, 825.31, 825.49, 825.69,
`825.72, 572.4,572.7; 455/45, 106; 343/872,
`878; 23.5/585, 380, 375; 705/28; 342/42
`s
`s
`s
`s
`s 44 5i
`s
`
`56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`3,944,928 3/1976 Augenblick et al. ............... 340/572 X
`3,984,835 10/1976 Kaplan et al. ............................ 342/44
`4,075,632 2/1978 Baldwin et al. .......................... 342/.51
`4,360,810 11/1982 Landt ........................................ 342/44
`4,471,345 9/1984 Barrett, Jr. ...
`... 340/572
`4,510,495 4/1985 Sigrimis et al. ..
`340/825.54
`4.584,534 4/1986 Lijphart et al. ......................... 329/308
`4,641,374 2/1987 Oyama .................................. 342/.51 X
`(List continued on next page.)
`FOREIGN PATENT DOCUMENTS
`0670558 9/1995 European Pat. Off..
`0724351 7/1996 European Pat. Off..
`0750200 12/1996 European Pat. Off..
`52062 3/1988 Japan.
`
`USOO5952922A
`Patent Number:
`11
`(45) Date of Patent:
`
`5,952,922
`*Sep. 14, 1999
`
`2098.431 11/1982 United Kingdom.
`2193359 2/1988 United Kingdom.
`2202415 9/1988 United Kingdom.
`O5549 6/1989 WIPO.
`19781 9/1994 WIPO.
`E
`Th
`Mull
`Pri
`rimary Examiner-nomas Mullen
`57
`ABSTRACT
`57
`In accordance with the present invention, a radio commu
`nications System is disclosed that can operate in one of
`Several Modes which integrate in-building Security, location
`determination, messaging, and data communications capa
`bilities. The radio communication System includes at least
`one Interrogator for generating and transmitting a radio
`Signal. One or more Tags of the System receive the radio
`signal. A Backscatter Modulator modulates the reflection of
`he radio signal using a Subcarrier Signal, therebv forming a
`t
`9.
`ga
`gnal,
`y Ic
`9.
`reflected modulated signal. The Interrogator, receives and
`demodulates the reflected modulated Signal. The Interroga
`tor can also transmit a first information signal to one or more
`X
`tags, Specifying which tags should respond using the Back
`scatter Modulator. In the Interrogation Mode, the Interroga
`t
`determine the identitv of the Tags in th
`ding field
`or can determine Ine Identity of une 1ags in une reading neia,
`and can exchange data with those Tags that have been
`identified. In the Location Mode, the radio communications
`System can instruct Some or all Interrogators to determine
`the location of a Tag or Tags within the building, regardless
`of whether the Tag or Tags are in radio range of the
`Interrogation Mode. In the MeSSaging, or low Speed data
`communications Mode, the radio communications system
`can instruct Some or all Interrogators to transmit a command
`addressed to a particular Tag or Tags, regardless of whether
`the Tag or Tags are in the reading field of the Interrogation
`Mode, questing that Tag or Tags perform Some action. Other
`embodiments of the Messaging Mode allow the Interrogator
`to transmit data in addition to a command to the Tagor Tags,
`and also for the Tag or Tags to transmit a signal back to the
`Interrogator. This radio communications System can be
`interconnected with other communications capabilities Such
`as electronic mail, voice mail, location, inventory
`management, and other Systems.
`
`38 Claims, 9 Drawing Sheets
`
`212
`S.
`SUBCARRIER DEMODULATOR
`
`
`
`1505
`HIGH DATA
`RATE 7 DATA RATE
`SIGNAL
`DEMODULATOR
`
`1506
`7.
`
`LOW DATA
`RATE
`SIGNAL
`
`OW
`DATA RATE
`OEMODULATOR
`
`1720 R
`EMERGENCY
`EMERGENCY
`CHANNEL s-H CHANNEL
`SIGNAL
`DEMODULATOR
`
`
`
`
`
`FILTER
`fS1
`
`1502 2
`Fly
`S2
`
`17 O 2
`FILTER
`S3
`
`
`
`
`
`211
`
`Page 1 of 24
`
`GOOGLE EXHIBIT 1017
`
`

`

`5,952,922
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`2.E. E". et t - - - - - - - - - - - - - - - - - - - - - - 340.
`C
`F. E. et al. ...
`4816,839
`343,:5
`4.888,591 12/1989 Landt etal- - - - - - -
`342/44
`4.937581 6/1990 Baldwin et al.
`- - - - 342/44
`4,963,887 10/1990 Kawashima et al
`... 342/44
`5,030,807
`7/1991 Landt et all
`235/375
`5.214.409
`5/1993 Beige.
`... 34.0/572
`5,227.803
`7/1993 O'Connor et al.
`... 342/442
`5,252,979 10/1993 Nysen ............
`... 342/50
`5,305,008 4/1994 Turner et al. ..
`... 342/44
`5,305,459 4/1994 Rydel .............
`235/375 X
`5,317.309 5/1994 Vercellotti et al.
`340/825.54
`5,339,073 8/1994 Dodd et al. ...
`... 340/825.31
`5,347,263 9/1994 Carroll et al. .......................... 340/572
`5,390,339 2/1995 Bruckert et al. ........................ 455/440
`5,400,949 3/1995 Hirvonen et al. .
`228/180.22
`5,420,757 5/1995 Eberhardt et al. ...................... 361/813
`5,423,056
`6/1995 Linguist et al. ........................ 455/458
`
`5,426,667 6/1995 Van Zon ................................. 375/219
`5,448,242 9/1995 Sharpe et al. ............................. 342/42
`5,455,575 10/1995 Schuermann .............................. 342/42
`5,461,385 10/1995 Armstrong .
`... 342/42
`5,463,402 10/1995 Wairath et al. ......................... 342/359
`5,477,215 12/1995 Mandelbaum ..................... 340/825.34
`5,479,160 12/1995 Koelle .................................. 340/825.7
`5,479,416 12/1995 Snodgrass et al. .................. 371/37.12
`5,521,944 5/1996 Hegeler et al. .
`... 375/329
`5,523,749 6/1996 Cole et al. ..
`340/825.54
`5,525,993 6/1996 Pobanz et al. ............................ 342/.51
`5,525,994 6/1996 Hurta et al. ............................... 342/.51
`5,530,202 6/1996 Dais et al. ............................ 174/35 R
`5,581,576 12/1996 Lanzetta et al. ...
`... 455/45 X
`5,590,158 12/1996 Yamaguchi et al.
`... 375/331
`5,600,538 2/1997 Xanthopoulos
`361/683
`5,640,683 6/1997 Evans et al. .............................. 455/45
`5,649,295 7/1997 Shober et al. .....
`... 455/106 X
`5,649,296 7/1997 MacLellan et al.
`455/106 X
`5,686,920 11/1997 Hurta et al. ............................... 342/42
`
`
`
`Page 2 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 1 of 9
`
`5,952,922
`
`
`
`101
`
`APPLICATION
`PROCESSOR
`
`102
`
`
`
`
`
`FIG. 1
`
`103
`INTERROGATOR
`
`INTERROGATOR
`
`2 N/
`
`105
`
`\/
`
`N/
`
`107
`
`- - - - - - - - - - - - - - - - - m- - - S.
`
`103
`
`
`
`201
`RADIO
`SIGNAL
`SOURCE
`
`
`
`
`
`|
`
`}
`
`2O2
`
`
`
`203
`
`- - - -
`
`TRANSMITTER
`
`ANTENNA
`
`MODULATOR
`
`TRANSMITTER
`
`204
`
`INFORMATION
`SIGNAL
`
`SIGNAL
`STRENGTH
`214
`
`RF
`SOURCE
`
`RECEIVE
`ANTENNA
`
`|
`
`102
`
`
`
`
`
`
`
`
`
`Page 3 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 2 of 9
`
`5,952,922
`
`FIG. 3
`
`co
`308
`
`
`
`DETECTOR
`MODULATOR
`
`305
`
`
`
`
`
`
`
`103
`
`
`
`V/
`
`INTERROGATOR
`
`
`
`INTERROGATION
`MODE
`RANGE
`
`
`
`LOCATION,
`MESSAGING
`MODE
`RANGE
`
`Page 4 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 3 of 9
`
`5,952,922
`
`FIG. 5
`
`INTERROGATOR
`
`
`
`501
`UPLINK RANGE FOR
`INTERROGATION
`MODE
`
`
`
`
`
`503
`DOWNLINK RANGE
`FOR ALL THREE
`MODES
`
`FIG. E.
`
`ANTENNA ANTENNA
`CABLE "So
`
`CEILING
`—? -
`A - \ GROUND
`606
`PLANE
`RADOME
`
`
`
`
`
`5O2
`UPLINK RANGE
`FOR LOCATION,
`MESSAGING
`MODES
`RANGE
`
`
`
`- - - - -
`
`N
`
`I
`
`;
`
`'',
`
`ANTENNA
`PATTERN
`
`---------
`
`- - - - - - - -
`
`FLOOR
`
`Page 5 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 4 of 9
`
`5,952,922
`
`FIG. 7
`
`ANTENNA
`CABLE
`
`ANTENNA
`
`:
`
`;
`|
`
`704
`
`RADOME Go
`PLANE
`
`ANTENNA
`PATTERN
`
`?'
`
`,
`y
`
`/
`
`
`
`...'
`
`,
`w
`
`c.
`ROP
`702
`
`w
`
`y
`
`?
`
`FLOOR
`-----------
`----------- -
`—? -
`FIG. B.
`
`f
`f
`
`,
`
`”
`
`,
`
`
`
`DOWNLINK
`ANTENNA
`OMNIDIRECTIONAL
`801
`
`UPLINK
`ANTENNA
`PATTERN
`804
`S. - - -
`
`u
`
`,
`y
`',
`,
`
`--
`
`802
`UPLINK
`ANTENNA -
`DIRECTIONAL
`
`'s
`
`s
`
`DOWNLINK
`ANTENNA
`PATTERN
`
`-:
`
`f
`
`?
`
`- - - - - - - - -
`
`w
`
`-
`
`Page 6 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 5 of 9
`
`5,952,922
`
`FIG. 9
`OMNIDIRECTIONAL
`RECEIVE
`ANTENNA
`902
`
`13
`
`DIRECTIONAL
`RECEIVE
`ANTENNA
`903
`
`
`
`
`
`
`
`
`
`
`
`FIG 10
`OMNIDIRECTIONAL
`
`103
`
`-- - - - - - -m- - - -2
`
`
`
`
`
`200
`
`
`
`"EVE
`
`ANTENNA
`1003
`
`DIRECTIONAL
`RECEIVE
`ANTENNA
`1004
`
`101
`
`"DOWNLINK". OR INTERROGATOR TO TAG COMMUNICATIONS
`
`1103
`
`1102
`
`1104
`
`UA
`: TIME
`"UPLINK". OR TAG TO INTERROGATOR COMUNICATIONS
`-Ho
`FRAME A
`FRAMEB
`1110
`
`UB
`
`Page 7 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 6 of 9
`
`5,952,922
`
`FIG. 12
`
`
`
`
`:
`:
`:
`
`1240
`1230
`1220
`1210
`DATA
`IAG
`SYNC : COMMAND ;
`MESSAGE
`:
`: ADDRESS
`:
`:
`DOWNLINK DA
`
`1250
`ERROR:
`DETECI:
`AND/OR
`CORRECT:
`
`TIME
`
`FIG. 13
`
`1306
`1302
`SLOT : SLOT : SLOT
`SLOT : SLOT : SLOT
`;
`5 ;
`G
`:
`7
`2 :
`3 ;
`4
`;
`;
`VPLINK UA
`:
`1103
`
`; SLOT
`1
`:
`
`;
`;
`|
`
`SLOT
`8
`
`:
`
`
`
`INTERROCATION
`MODE
`
`MESSAGING
`MODE
`
`My 1313
`:
`
`|
`
`LOCATION
`MODE
`
`:
`
`UL
`
`1314
`
`Page 8 of 24
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 1999
`Sep. 14, 1999
`
`Sheet 7 of 9
`Sheet 7 0f 9
`
`5,952,922
`5,952,922
`
`Ma:
`
`
`20:53g053mm:22%we:2:
`
`-5232;@5382MS:
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`
`85::zezggmé
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`3.mE
`
`Page 9 of 24
`
`Page 9 of 24
`
`

`

`U.S. Patent
`
`Sep. 14, 1999
`
`Sheet 8 of 9
`
`5,952,922
`
`FIG. 15
`
`21?
`SUBCARRIER DEMODULATOR
`501
`
`1503
`
`H DATA RATE
`DEMODULATOR
`
`LOW
`EAAA's
`DEMODULATO
`
`
`
`
`
`HIGH DATA
`RATE
`SIGNAL
`
`to Data 1595)
`RATE
`SIGNAL
`1507
`
`SIGNAL
`STRENGTH
`
`FIG. 17
`
`21?
`SUBCARRIER DEMODULATOR
`1501
`
`
`
`1503
`
`DATA RATE
`DEMODULATOR
`
`HIGH DATA
`RATE
`SIGNAL
`
`1504
`to
`Low DATA 5)
`RATE
`H DATA RATE
`SIGNAL
`DEMODULATOR
`
`
`
`EMERGENCY
`CHANNEL
`SIGNAL
`
`
`
`EMERGENCY
`CHANNEL
`DEMODULATOR
`
`
`
`|
`
`211
`
`211
`
`Page 10 of 24
`
`

`

`U.S. Patent
`US. Patent
`
`Sep. 14, 1999
`mS
`
`Sheet 9 of 9
`
`5,952,922
`5,952,922
`
`A1,,2:LVWWHLVmeme
`9Emeggfig0%was
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`Page 11 of 24
`
`Page 11 of 24
`
`
`

`

`1
`IN-BUILDING MODULATED BACKSCATTER
`SYSTEM
`
`FIELD OF THE INVENTION
`This invention relates to wireleSS communication Systems
`and, more particularly, to an in-building or campus area
`wireleSS communication System using modulated backScat
`ter technology.
`
`RELATED APPLICATIONS
`Related Subject matter is disclosed in the following appli
`cations filed concurrently here with and assigned to the same
`ASSignee hereof: U.S. patent applications “Shielding Tech
`nology In Modulated Backscatter System,” Ser. No. 08/777,
`770, “Encryption for Modulated Backscatter Systems,” Ser.
`No. 08/777,832; “QPSK Modulated Backscatter System,”
`Ser. No. 08/775,694; “Modulated Backscatter Location
`System,” Ser. No. 08/777,643; “Antenna Array In An RFID
`System,” Ser. No. 08/775,217; “Subcarrier Frequency Divi
`sion Multiplexing Of Modulated Backscatter Signals,” Ser.
`No. 08/777,834; “IQ Combiner Technology In Modulated
`Backscatter System,” Ser. No. 08/775,695; “In-Building
`Personal Pager And Identifier.” Ser. No. 08/775,738, and
`referred to below as the “Shober-Pager” application; "Inex
`pensive Modulated Backscatter Reflector,” Ser. No. 08/774,
`499; “Passenger, Baggage, And Cargo Reconciliation
`System,” Ser. No. 08/782,026. Related subject matter is also
`disclosed in the following applications assigned to the same
`assignee hereof: U.S. patent application Ser. No. 08/504,
`188, now U.S. Pat. No. 5,640,683, entitled “Modulated
`BackScatter Communications System Having An Extended
`Range”; U.S. patent application Ser. No. 08/492,173, now
`U.S. Pat. No. 5,649,295, entitled “Dual Mode Modulated
`Backscatter System”; U.S. patent application Ser. No.
`08/492,174, now U.S. Pat. No. 5,649,296, entitled “Full
`Duplex Modulated Backscatter System”; and U.S. patent
`application Ser. No. 08/571,004, entitled “Enhanced Uplink
`Modulated Backscatter System.”
`BACKGROUND OF THE INVENTION
`Radio Frequency Identification (RFID) systems are used
`for identification and/or tracking of equipment, inventory, or
`living things. RFID Systems are radio communication SyS
`tems that communicate between a radio transceiver, called
`an Interrogator, and a number of inexpensive devices called
`Tags. In RFID Systems, the Interrogator communicates to
`the Tags using modulated radio Signals, and the Tags
`respond with modulated radio signals. After transmitting a
`message to the Tag (called the Downlink), the Interrogator
`then transmits a Continuous-Wave (CW) radio signal to the
`Tag. The Tag modulates the CW Signal using modulated
`backScattering where the antenna is electrically Switched, by
`the modulating Signal, from being an absorber of RF radia
`tion to being a reflector of RF radiation. This modulated
`backScatter allows communications from the Tag back to the
`Interrogator (called the Uplink). Conventional MBS systems
`are designed a) to identify an object passing into range of the
`Interrogator, and b) to store data onto the Tag and then
`retrieve that data from the Tag at a later time in order to
`manage inventory or perform another useful application.
`Let us concentrate our attention to applications in a
`campus environment or inside a building, and first consider
`the use of RFID technology for “Security” applications.
`RFID is today used in the security industry to facilitate
`building access, for example, the use of an RFID Tag to
`automatically authorize entrance to a building, or to record
`
`15
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`5,952,922
`
`2
`that an individual passed by a particular location. This
`operation is called the Interrogation Mode-a mode of
`operation where the Interrogator transmits a Signal to all
`Tags in the reading field, requesting those Tags to respond
`with data which identifies this Tag. The Tag then transmits
`this information back to the Interrogator using MBS.
`In addition, “Location' applications exist within a build
`ing or campus. (For the remainder of this disclosure, we will
`use the term “building” or “in-building” to mean either
`within a building or within a campus environment which
`could include a building.) For example, it would be benefi
`cial to know the location of a specific Tag within the
`building. This could be of benefit in high-security locations.
`Other applications of this capability include the ability of the
`in-building telephone System to route telephone calls to a
`phone close to where an individual is presently located.
`There have been prototype Systems, using infrared
`transmitters, to address this application; however, there are
`no commercial products, and infrared technology Suffers
`from lack of range and an inability to pass through objects.
`Thus, if the infrared transmitter is placed inside a perSon's
`Shirt pocket, the communications path is blocked. Thus,
`today, there are no cost-effective Solutions to the location
`problem.
`In addition, low speed data “Communications' applica
`tions also exist. An example of a current System to provide
`low-speed data communications is Paging. Some Paging
`Systems Suffer from poor in-building wireleSS coverage, and
`Some Paging Systems require paying usage charges to a
`Service provider on a per-transaction basis. Another alter
`native is to deploy a wireleSS data LAN in the building;
`however these products are Still relatively expensive. In
`addition, none of the current low speed data communica
`tions alternatives address the Security or Location issues
`discussed above. Our goal is a System that will Simulta
`neously address the Security, Location, and low-Speed data
`Communications applications.
`In this disclosure, we disclose how a radio frequency
`identification System, utilizing modulated backScatter, can
`be used to integrate the functions of Security, Location, and
`low Speed data Communications in a single System with a
`Single infrastructure. This invention can provide improved
`Security as well as cost-effective in-building or campus area
`location and communications Services.
`
`SUMMARY OF THE INVENTION
`In accordance with the present invention, a radio com
`munications System is disclosed that can operate in one of
`Several Modes which integrate in-building Security, location
`determination, messaging, and data communications capa
`bilities. The radio communication System includes at least
`one Interrogator for generating and transmitting a radio
`Signal. One or more Tags of the System receive the radio
`signal. A Backscatter Modulator modulates the reflection of
`the radio Signal using a Subcarrier Signal, thereby forming a
`reflected modulated Signal. The Interrogator receives and
`demodulates the reflected modulated Signal. The Interroga
`tor can also transmit a first information signal to one or more
`tags, Specifying which tags should respond using BackScat
`ter Modulator means. In the Interrogation Mode, the Inter
`rogator can determine the identity of the Tags in the reading
`field, and can exchange data with those Tags that have been
`identified. In the Location Mode, the radio communications
`System can instruct Some or all Interrogators to determine
`the location of a Tag or Tags within the building, regardless
`of whether the Tag or Tags are in radio range of the
`
`Page 12 of 24
`
`

`

`3
`Interrogation Mode. In the MeSSaging, or low Speed data
`communications Mode, the radio communications System
`can instruct Some or all Interrogators to transmit a command
`addressed to a particular Tag or Tags (regardless of whether
`the Tag or Tags are in the reading field of the Interrogation
`Mode) requesting that Tag or Tags perform Some action.
`Other embodiments of the Messaging Mode allow the
`Interrogator to transmit data in addition to a command to the
`Tagor Tags, and also for the Tagor Tags to transmit a signal
`back to the Interrogator. This radio communications System
`can be interconnected with other communications capabili
`ties Such as electronic mail, Voice mail, location, inventory
`management, and other Systems.
`
`BRIEF DESCRIPTION OF THE DRAWING
`In the drawing,
`FIG. 1 shows a block diagram of an illustrative Radio
`Frequency Identification (RFID) system;
`FIG. 2 shows a block diagram of an illustrative Interro
`gator Unit used in the RFID system of FIG. 1;
`FIG. 3 shows a block diagram of a Tag Unit used in the
`RFID system of FIG. 1;
`FIG. 4 shows the relationships among the Interrogation
`Mode, Location Mode, and MeSSaging Mode Ranges,
`FIG. 5 shows the relationships among the Uplink Range
`for the Interrogation Mode, the Uplink Range for the Loca
`tion and MeSSaging Modes, and the downlink Range for all
`three Modes;
`FIG. 6 shows a mounting arrangement for an omnidirec
`tional antenna in the ceiling of a building;
`FIG. 7 shows an alternate arrangement for an omnidirec
`tional antenna in the ceiling of a building;
`FIG. 8 shows the relationship between the Uplink
`Antenna pattern and the Downlink Antenna pattern;
`FIG. 9 shows an arrangement for combining the inputs
`from two receive antennas;
`FIG. 10 shows an arrangement for Switching between the
`inputs from two receive antennas;
`FIG. 11 shows the Frame structure of Downlink and
`Uplink Signals,
`FIG. 12 shows one embodiment of how the Downlink
`Signal could be Subdivided;
`FIG. 13 shows one embodiment of how the Uplink signal
`could be Subdivided;
`FIG. 14 shows where the Messaging Mode Uplink
`Response could be located;
`FIG. 15 shows how the Subcarrier Demodulator of FIG.
`2 can Support demodulation on two Subcarrier channels,
`FIG. 16 shows how the RFID System of FIG. 1 can be
`interconnected to electronic mail and Voice mail Systems,
`FIG. 17 shows how the Subcarrier Demodulator of FIG.
`2 can also Support demodulation on an Emergency Channel.
`DETAILED DESCRIPTION
`MBS Operation
`We now describe how a typical RFID system, utilizing
`MBS, operates. With reference to FIG. 1, there is shown an
`overall block diagram of a traditional RFID system. An
`Applications Processor 101 communicates over a Local
`Area Network (LAN, 102), which could be wired or
`wireless, to a plurality of Interrogators (103, 104). The
`Interrogators may then each communicate with one or more
`of the Tags (105, 107). For example, the Interrogator 103
`receives an information signal, typically from an Applica
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`tions Processor 101. The Interrogator 103 takes this infor
`mation signal and Processor 200 properly formats a Down
`link message (Information Signal200a) to be sent to the Tag.
`With joint reference to FIGS. 1 and 2, Radio Signal Source
`201 synthesizes a radio signal, the Modulator 202 modulates
`this Information Signal 200a onto the radio signal, and the
`Transmitter 203 sends this modulated signal via Antenna
`204, illustratively using amplitude modulation, to a Tag. The
`reason amplitude modulation is a common choice is that the
`Tag can demodulate Such a Signal with a single, inexpensive
`nonlinear device (Such as a diode).
`In the Tag 105 (see FIG. 3), the Antenna 301 (frequently
`a loop or patch antenna) receives the modulated Signal. This
`Signal is demodulated, directly to baseband, using the
`Detector/Modulator 302, which, illustratively, could be a
`single Schottky diode. The result of the diode detector is
`essentially a demodulation of the incoming Signal directly to
`baseband. The Information Signal 200a is then amplified, by
`Amplifier 303, and synchronization recovered in Clock
`Recovery Circuit 304. The Clock Recovery Circuit 304 can
`be enhanced by having the Interrogator Send the amplitude
`modulated Signal using Manchester encoding. The resulting
`information is sent to a Processor 305. The Processor 305 is
`typically an inexpensive 4 or 8bit microprocessor; the Clock
`Recovery Circuits 304 can be implemented in an ASIC
`(Application Specific Integrated Circuit) which works
`together with or is incorporated within the integrated circuit
`containing Processor 305. This Processor 305 can also serve
`as the driver for an optional Display Unit 309 should this Tag
`require a display. The Processor 305 generates an Informa
`tion Signal 306 to be sent from the Tag 105 back to the
`Interrogator (e.g., 103). This Information Signal 306 is sent
`to a Modulator Control Circuit 307, which uses the Infor
`mation Signal 306 to modulate a Subcarrier frequency gen
`erated by the Frequency Source 308. The Frequency Source
`308 could be a crystal oscillator separate from the Processor
`305, or a signal derived from the output of a crystal
`oscillator, or it could be a frequency Source derived from
`signals present inside the Processor 305-such as a multiple
`of the fundamental clock frequency of the Processor. The
`Modulated Subcarrier Signal 311 is used by Detector/
`Modulator 302 to modulate the modulated signal received
`from Tag 105 to produce a modulated backscatter (i.e.,
`reflected signal). This is accomplished by Switching on and
`off the Schottky diode using the Modulated Subcarrier
`Signal 311, thereby changing the reflectance of Antenna 301.
`A Battery 310 or other power supply provides power to the
`circuitry of Tag 105.
`There are a variety of techniques for using Modulated
`Backscatter (MBS) to send information from the Tag to the
`Interrogator. In some MBS technologies, the Modulator
`Circuit 307 of the Tag generates a modulated signal, which
`is amplitude modulated by an Information Signal 306 at
`frequency f. If the Radio Signal Source 201 generates an
`unmodulated frequency f, then the Interrogator receives
`Signals inside of the range (f-f) to (f+f), and generally
`filters out Signals outside of that range. This could be termed
`the “MBS at baseband' approach. Another approach would
`be for the Tag to generate two different Subcarrier frequen
`cies. The information could be conveyed in a frequency-shift
`keyed (FSK) fashion with the subcarrier frequency transi
`tioning between these two frequencies. Other modulation
`Schemes are possible as well, Such as Phase Shift Keying
`(PSK) of a single subcarrier frequency (e.g., BPSK, QPSK)
`or other complex modulation schemes (e.g., MFSK, MASK,
`etc.).
`Returning to FIG. 2, the Interrogator 103 receives the
`reflected and modulated Signal with the Receive Antenna
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`206, amplifies the signal with a Low Noise Amplifier 207,
`and demodulates the Signal using homodyne detection in a
`Quadrature Mixer 208. (In some Interrogator designs, a
`single Transmit (204) and Receive (206) Antenna is used. In
`this event, an electronic method of canceling the transmitted
`Signal from that received by the receiver chain is needed;
`this could be accomplished by a device Such as a Circulator.)
`Using the same Radio Signal Source 201 as used in the
`transmit chain means the demodulation to baseband is done
`using Homodyne detection; this has advantages in that it
`greatly reduces phase noise in the receiver circuits. The
`Mixer 208 then sends the Demodulated Signal 209 (if a
`Quadrature Mixer, it would send both I (in phase) and Q
`(quadrature) signals) to the Filter/Amplifier 210. The result
`ing filtered Signal-which in this invention is an Information
`Signal 211 carried on a Subcarrier—is then demodulated
`from the Subcarrier in the Subcarrier Demodulator 212,
`which then sends the Information Signal 213 to a Processor
`200 to determine the content of the message. The I and Q
`channels of Signal 209 can be combined in the Filter/
`Amplifier 210, or in the Subcarrier Demodulator 212, or
`they could be combined in the Processor 200.
`Using, e.g., the above techniques, a short-range, bidirec
`tional digital radio communications channel is implemented.
`A relatively inexpensive implementation is achieved using,
`as exemplary components, a Schottky diode, an amplifier to
`boost the Signal Strength, bit and frame Synchronization
`circuits, an inexpensive 4 or 8 bit microprocessor, Subcarrier
`generation circuits, and a battery. Most of these items are
`already manufactured in quantities of millions for other
`applications, and thus are not overly expensive. The circuits
`mentioned above for bit and frame synchronization and for
`Subcarrier generation can be implemented in custom logic
`Surrounding the microprocessor core; thus, except for a
`relatively Small amount of chip real estate, these functions
`come almost “for free.”
`Narrowband Operation
`Using the above procedures, a two-way digital radio
`communications channel can be constructed. We desire to
`extend the range of this two-way digital radio communica
`tions channel as much as possible. This involves both
`extending the range of the Downlink and also extending the
`range of the Uplink.
`Extending the range of the Downlink involves several
`factors. The Downlink is generally an amplitude modulated
`Signal, which is easily and inexpensively detected by a
`Single nonlinear device, Such as a microwave diode. It is
`important to match the impedances between the antenna and
`the diode to avoid gratuitous Signal attenuation. The data
`rate of the Downlink must be limited in order to reduce the
`noise bandwidth of the Downlink signal. We now discuss
`how the Tag can filter out unwanted Signals without
`increased cost. The Antenna (301) not only performs the
`tasks of receiving the RF signal, but it also filters RF signals
`outside of the antenna bandwidth. For example, at 2.45 GHZ,
`allowable RF carrier frequencies are from 2.400-2.485
`GHz. The design of the antenna, frequently a patch antenna,
`covers this frequency band but filters out frequencies beyond
`this range. An ideal frequency response would be for the
`antenna sensitivity to be within 3 dB across the allowable
`frequency range, but to fall off rapidly beyond this range. In
`addition, the Amplifier (303) also acts as a filter in the sense
`that the Amplifier is designed to only pass Amplitude
`Modulated (AM) signals that are within a certain passband
`around the expected Downlink data rate, which is typically
`a few kilobits per Second. Therefore, although the Tag is
`relatively simple, it has filtering capability to filter out both
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`RF signals whose frequency is outside the Antenna
`bandwidth, and also to filter out AM signals whose fre
`quency is outside of the Amplifier passband. This Tag design
`is also not greatly Sensitive to RF transmissions, inside the
`band of the antenna, whose modulation Scheme is primarily
`constant envelope. Thus, this design allows a robust Tag
`which is resistant to many potential interfering Signals.
`Extending the range of the Uplink also involves Several
`factors. First, the noise bandwidth of the Uplink signal must
`be reduced as much as possible. A number of useful appli
`cations can be implemented even if the data rate of the
`Uplink Signal is limited to a few bits per Second. Indeed, this
`limitation of the data rate can be taken to the extreme in
`which there is no data modulated onto the Single Subcarrier
`frequency; in this case, the mere presence or absence of a
`Signal received at this Subcarrier frequency indicates an
`“acknowledgment' or “no acknowledgment” to a previously
`received message.
`We further note that the Subcarrier frequency can be
`relatively accurately determined. For example, commer
`cially available crystals exist with a frequency of 32 kHz,
`and an accuracy of t100 ppm. Thus, the frequency of this
`crystal is known to t3.2 Hz. The Tag thus generates a
`Subcarrier frequency, f, of great accuracy. The Interrogator
`receives the reflected Signal, and demodulates it as discussed
`above using Homodyne detection.
`The Filter Amplifier (210) and Subcarrier Demodulator
`(213) function could then be implemented, together, inside
`a processor Such as a DSP. Narrowband filtering algorithms
`exist in the literature which can perform digital filtering of
`the signal with a bandwidth of less than 10 Hz, and where
`the first sidelobes are depressed 60 dB. Then, the signal
`Strength of the signal received through this digital filter is
`measured, and that Strength is compared to a reference Signal
`Strength which is Sufficiently above the average noise in that
`channel when no signal is present Such that Spurious noise
`Spikes are not misinterpreted as actual signals.
`In this manner, very weak Uplink Signals can be reliably
`detected. It has been found that, using these techniques,
`roughly equivalent range in the Downlink and the Uplink
`can be achieved.
`We now discuss the location of the Subcarrier frequency
`f. MBS systems exhibit noise in the Uplink signals due to
`reflections of the RF source from any number of reflectors.
`Walls and metal objects reflect RF radiation; these reflected
`signals are received by the Interrogator 103 at the same
`carrier frequency as they were transmitted. The Quadrature
`Mixer 208 is operated as a Homodyne Detector and thus is
`used to cancel these reflections. However, other reflectors
`generate reflected noise at frequencies away from the main
`carrier frequency-either from Doppler shifts or, more
`likely, from reflections off of electronic equipment operating
`at frequencies near the Subcarrier Frequency.
`One particularly difficult Source of noise is fluorescent
`lights, which have been shown to produce noise not only at
`their fundamental 60 Hz (in the United States) frequency,
`but also at Overtone frequencies well up into the tens of
`thousands of Hertz. It has been found especially helpful to
`locate the Subcarrier frequency f. Such that it falls between
`multiples of the fundamental 60 Hz frequency. From the 32
`kHz crystal, Simple circuits can generate the appropriate
`Subcarrier frequency.
`Multiple Mode Operation
`The basic features of multiple mode operation are that a)
`the Tag must be capable of receiving a Downlink message;
`b) the Tag must be told what type of Uplink message it is to
`transmit, whether it be an actual data message (higher bit
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`rate mode) or a simple acknowledgment message (long
`range mode), based upon information received in the Down
`link message; c) the Tag transmits the requested type of
`Uplink message; and d) the Interrogator interprets the
`Uplink message received in a proper manner. Several dif
`ferent types of acknowledgment messages in the long range
`mode can exist. Generally, an acknowledgment message has
`a data rate which is much less than the data rate of an actual
`data message (the higher bit rate mode), thus allowing
`filtering over a much Smaller frequen