US007321777B2
`
`a2) United States Patent
`US 7,321,777 B2
`(0) Patent No.:
`*Jan. 22, 2008
`(45) Date of Patent:
`Billhartz et al.
`
`(54)
`
`(75)
`
`WIRELESS COMMUNICATIONS SYSTEM
`INCLUDING A WIRELESS DEVICE
`LOCATOR AND RELATED METHODS
`
`Inventors: Thomas Jay Billhartz, Melbourne, FL
`(US); Vivek Krishna, Palm Bay, FL
`(US); Steve Kopman, Melbourne, FL
`(US)
`
`(73)
`
`Assignee:
`
`Harris Corporation, Melbourne, FL
`(US)
`
`(*)
`
`Notice:
`
`Subject to any disclaimer, the term ofthis
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`This patent is subject to a terminal dis-
`claimer.
`
`(21)
`
`Appl. No.: 11/531,487
`
`(22)
`
`Filed:
`
`Sep. 13, 2006
`
`(65)
`
`(63)
`
`(51)
`
`(52)
`
`(58)
`
`Prior Publication Data
`
`US 2007/0026873 Al
`
`Feb. 1, 2007
`
`Related U.S. Application Data
`
`Continuation of application No. 10/767,794, filed on
`Jan. 29, 2004, now Pat. No. 7,110,779.
`
`Int. Cl.
`
`(2006.01)
`H04Q 7/20
`UWS. Ch. ceecccceecseessesseeessen
`455/456.2; 455/456.1;
`455/456.5; 455/456.6; 342/457; 370/328
`Field of Classification Search ...... 455/41.2-41.3,
`455/456.1-457, 553.1, 63.4, 562.1, 67.11,
`455/67.13, 67.16, 402.2; 342/357.08-357.09,
`342/357.1, 450; 340/426.19, 426.22, 539.23,
`340/825.49; 370/310.2, 328, 331, 338; 701/201;
`702/149
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`5,526,357 A
`5,550,549 A
`5,687,196 A
`5,706,010 A
`6,292,665 Bl
`6,680,923 Bl
`6,865,394 B2
`
`6/1996 Jandrell oe 370/95.2
`8/1996 Procter, Jr. et al.
`........... 342/47
`11/1997 Proctor, Jr. et al.
`.......... 375/347
`1/1998 Franke oe we. 342/47
`9/2001 Hildebrand et al.
`........ 455/456
`1/2004 Leon oo... eeeeeeeereeee 370/328
`3/2005 Ogino et al. we. 455/456.1
`
`
`
`(Continued)
`OTHER PUBLICATIONS
`
`Yellowjacket-A, 802.lla Wi-Fi
`Varitronics Systems.
`
`Analysis System, Berkeley
`
`Primary Examiner—George Eng
`Assistant Examiner—Kamran Afshar
`(74) Attorney, Agent, or Firm—Allen, Dyer, Doppelt,
`Milbrath & Gilchrist, P.A.
`
`(57)
`
`ABSTRACT
`
`A wireless communications system may include a plurality
`of wireless communications devices and a wireless device
`
`locator. More particularly, the wireless device locator may
`include at least one antenna and a transceiver connected
`
`thereto, and a controller for cooperating with the transceiver
`for transmitting a plurality of location finding signals to a
`target wireless communications device from among the
`plurality thereof. The target device may transmit a respective
`reply signal for each of the location finding signals. Addi-
`tionally, the controller may also cooperate with the trans-
`ceiver for receiving the reply signals, and it may determine
`a propagation delay associated with the transmission of each
`location finding signal and the respective reply signal there-
`for based upon a known device latency of the target device.
`As such, the controller may estimate a range to the target
`device based upon a plurality of determined propagation
`delays.
`
`See application file for complete search history.
`
`25 Claims, 9 Drawing Sheets
`
` ‘YARGETUID
`KNOWN?
` ¥
`DETER
`RECEIVE UMSOLICTED
`SIGNAL(
`
`
`
`
`
`TRANSMIT LOCATION
`FINDING SIGHALS
`
`MON RECEIVEREPEY
`SIGRALS
`eeaeDENCE
`TRE
` 1
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`DEER
`
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`YES
`TYPEFROM REPLY
`
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`Wl
`
`
`
`
`
`
`DETERMINE
`PROPAGATION DELAYS
`WN
`
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`FESTA RANGETO
`"TARGET
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`
`APPLE 1001
`
`APPLE 1001
`
`1
`
`

`

`US 7,321,777 B2
`
`Page 2
`
`U.S. PATENT DOCUMENTS
`
`6,990,428 B1*
`1/2006 Kaiser et al.
`............ 455/456.1
`
`7,042,868 B2*
`5/2006 Runkle et al.
`..
`+ 370/347
`
`7,046,987 B2*
`5/2006 Siegeletal. ....
`455/404.2
`7,110,779 B2*
`.........
`9/2006 Billhartz et al.
`455/456.2
`
`2002/0080759 Al
`6/2002 Harringtonetal.
`......... 370/338
`
`sooooteve “i ooo. vairington et 7
`space
`
`..........
`artorana et al.
`..........
`2003/0025602 Al
`2/2003 Medemaet al.
`340/568.1
`
`2/2003 Crabtree etal.
`... 340/539
`2003/0034887 Al
`
`2003/0043073 Al
`3/2003 Gray et al.
`..
`342/465
`
`2003/0117320 A1l*
`6/2003 Kimetal.
`...
`342/457
`
`2003/0128163 Al*
`7/2003 Mizugaki et al.
`........... 342/464
`
`
`
`2003/0162550 Al
`2003/0182062 Al
`2003/0191604 Al
`2004/0072566 Al*
`2004/0081139 Al
`2004/0110514 Al
`2004/0203889 Al
`2004/0266348 Al
`2005/0037775 Al
`2005/0059411 Al
`s0G/0183488 AL*
`
`8/2003 Kuwaharaet al.
`.......... 455/456
`9/2003 DeLorneetal. ............ 701/201
`
`10/2003 Kuwaharaetal. .......... 702/150
`4/2004 Kuwahara et al.
`.......... 455/440
`
`4/2004. Beckmannetal
`... 370/352
`
`6/2004 Kimetall....
`.. 455/456.
`
`.......
`... 455/456.1
`10/2004 Karaoguz.
`12/2004 Deshpandeetal. ........ 455/A1.2
`........ 455/456.1
`2/2005 Moeglein et al.
`;
`
`w. 455/456.1
`3/2005 Zhengdi
`.........
`8/2006 Billhct
`4SS/A56 5
`MNNAIIZ,
`eevee ces ceececcees
`
`* cited by examiner
`
`2
`
`

`

`U.S. Patent
`
`Jan. 22, 2008
`
`Sheet 1 of 9
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`U.S. Patent
`
`Jan. 22, 2008
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`Sheet 2 of 9
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`US 7,321,777 B2
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`\
`43
` CONTROLLER _. TRANSCEIVER _
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`Jan. 22, 2008
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`U.S. Patent
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`Jan. 22, 2008
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`U.S. Patent
`
`Jan. 22, 2008
`
`Sheet 6 of 9
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`US 7,321,777 B2
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`
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`

`U.S. Patent
`
`Jan. 22, 2008
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`Jan. 22, 2008
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`U.S. Patent
`
`Jan. 22, 2008
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`Sheet 9 of 9
`
`US 7,321,777 B2
`
`10
`
`START
`
`112
`
`119
`
`TARGET UID
`KNOWN?
`
`
`
`RECEIVE UNSOLICTED
`
`SIGNALS) AND
`
`DETERMINE UID
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`113
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`TRANSMIT LOCATION
`FINDING SIGNALS
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`FINISH
`
`FIG. 1]
`
`11
`
`11
`
`

`

`US 7,321,777 B2
`
`1
`WIRELESS COMMUNICATIONS SYSTEM
`INCLUDING A WIRELESS DEVICE
`LOCATOR AND RELATED METHODS
`
`FIELD OF THE INVENTION
`
`The present application is a continuation of U.S. patent
`application serial No. 10/767,794 filed Jan. 29, 2004 now
`U.S. Pat. No. 7,110,779,
`the entire content of which is
`incorporated herein by reference.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`wireless location techniques are used in numerousappli-
`cations. Perhaps the most basic of these applications is for
`locating lost articles. By way of example, published U.S.
`patent application Ser. No. 2003/0034887 to Crabtree et al.
`discloses a portable article locator system for locating lost
`articles such as glasses, keys, pets, television remotes, etc.
`More particularly, a wireless transceiver is attached to a
`person, animal, or other object. A handheld locator transmits
`a locator signal to the wireless transceiver which includes a
`unique address code of the transceiver. If the received code
`matches that stored by the wireless transceiver, it sends a
`return signal back to the locator device. The locator device
`uses the return signal
`to determine the distance and/or
`direction to the wireless transceiver from the user’s location.
`The locator device includes an antenna array which
`includes a plurality of omni-directional antennas. The loca-
`tor unit determines the bearing to the wireless transceiver by
`switching between antennas in the antenna array and using
`Doppler processing to determine a direction of a wireless
`signal received from the transceiver. The distance to the
`wireless transmitter is also determined based upon the
`reception of the wireless signal at each of the antennas of the
`antenna array. Furthermore, in one embodiment, which is
`intendedto avoid interference between two or more locators
`
`in a commonarea, a plurality of locator signals may be sent
`from a locator at a standard repetition rate. The locator’s
`receiver then only listens for responses during predeter-
`mined windowsfollowing each transmission.
`In contrast, in some applications it is desirable to deter-
`mine the location of an unknownsignaltransmitter. U.S. Pat.
`No. 5,706,010 to Franke discloses such a system in which a
`transmitter locator receives a signal from the unknown
`signal transmitter and processes the signal to determine a
`bearing to the unknownsignal transmitter. The transmitter
`locator then sends an interrogating signal to the unknown
`signal transmitter. Upon receiving the interrogating signal,
`the unknown signal transmitter heterodynes the interroga-
`tion signal with its own carrier signal to generate an inter-
`modulation return signal. A processor of the transmitter
`locator measures the round-trip transit time from the trans-
`mission of the interrogation signal to the reception of the
`intermodulation return signal. A range to the unknownsignal
`transmitter is then calculated based upon the round-trip
`transit time.
`
`Still another application in which locating a wireless
`communications device is often necessary is in cellular
`telephone networks. That is, it may be necessary to locate
`particular cellular telephone users for law enforcement or
`emergency purposes, for example. U.S. Pat. No. 6,292,665
`to Hildebrand et al., which is assigned to the present
`assignee, discloses a method for geolocating a cellular
`phoneinitiating a 911 call. A base station transceiver trans-
`mits a supervisory audio tone (SAT), which is automatically
`looped back by the calling cellular phone. Returned SAT
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`signals are correlated with those transmitted to determine the
`range of the cellular phone. In addition, incoming signals
`from the cellular phone, such as the returned SAT signals,
`are received by a phased array antenna and subjected to
`angle of arrival processing to determine the direction of the
`cellular phonerelative to the base station. The cellular phone
`is geolocated based upon the angle of arrival and the range
`information. A correction factor provided by the manufac-
`turer of a given cellular telephone is used to account for the
`loopback path delay through the phone.
`Oneadditional area in which wireless device location can
`be important is in wireless networks, such as wireless local
`area networks (WLANs)or wide area networks (WANs), for
`example. A typical prior art approach to locating terminals
`within a WLANincludeslocating a plurality of receivers at
`fixed locations within a building, for example, and then
`determining (i.e., triangulating) the position of a terminal
`based upon a signal received therefrom at each of the
`receivers.
`
`Anotherprior art approach for wireless terminal location
`is to use a direction finding (DF) device which includes a
`directional antenna for receiving signals when pointed in the
`direction of a transmitting node. An example of a portable
`DF device for WLANs is the Yellowjacket 802.1la wi-fi
`analysis system from Berkeley Varitronics. This device uses
`a passive DF technique,i.e., it does not solicit any signals
`from a terminal but instead waits for the terminalto transmit
`signals before it can determine the direction of the trans-
`mission.
`Despite the advantages of such prior art wireless com-
`munications device locators, additional wireless location
`features may be desirable in various applications.
`
`SUMMARY OF THE INVENTION
`
`In view of the foregoing background, it is therefore an
`object of the present invention to provide a wireless com-
`munications device locator which provides enhanced loca-
`tion features and related methods.
`
`This and other objects, features, and advantages in accor-
`dance with the present invention are provided by a wireless
`communications system which may include a plurality of
`wireless communications devices each having a device type
`associated therewith from among a plurality of different
`device types. Further, each device type may have a known
`device latency associated therewith. The system may also
`include a wireless device locator. More particularly,
`the
`wireless device locator may includeat least one antenna and
`a transceiver connected thereto, and a controller for coop-
`erating with the transceiver for transmitting a plurality of
`location finding signals to a target wireless communications
`device from amongthe plurality of wireless communications
`devices. The target wireless communications device may
`transmit a respective reply signal for each of the location
`finding signals.
`Additionally, the controller of the wireless device locator
`may also cooperate with the transceiver for receiving the
`reply signals, and it may determine a propagation delay
`associated with the transmission of each location finding
`signal and the respective reply signal therefor. This may be
`done based upon the known device latency of the target
`wireless communications device. As such,
`the controller
`may estimate a range to the target wireless communications
`device based upon a plurality of determined propagation
`delays.
`the wireless device locator advanta-
`In other words,
`geously provides active range finding. In other words, the
`
`12
`
`12
`
`

`

`US 7,321,777 B2
`
`3
`wireless device locator prompts the target wireless commu-
`nications device to send reply signals using the location
`finding signals, rather than passively waiting until the target
`wireless communications device begins transmitting. This
`allows for quicker and more efficient device location.
`Furthermore, by estimating the range based upon a plu-
`rality of propagation delays,
`the wireless device locator
`mitigates the effects of variations in the device latency time.
`That is, while the target wireless communication device has
`a known device latency,
`there will necessarily be some
`amountof variance from one transmissionto the next. Using
`a plurality of propagation delays associated with different
`transmissions provides
`a
`significantly more
`accurate
`approximation of the device latency time and, thus, a more
`accurate range estimation. By way of example, the controller
`may estimate the range based upon an average (e.g., mean,
`median, mode, etc.) of the propagation delays.
`In addition, each wireless communications device may
`have a unique identifier (UID) associated therewith, and the
`controller may insert the UID for the target wireless com-
`munications device in each of the location finding signals.
`Furthermore,
`the target wireless communications device
`may generate respective reply signals based upon the UID in
`the location finding signals. That
`is,
`the target wireless
`communications device will act upon the location finding
`signals because these signals include its UID, whereas the
`other wireless communications device will not.
`The target wireless communications device may generate
`unsolicited signals including the UID thereof. As such, the
`controller may cooperate with the transceiver to receive at
`least one unsolicited signal from the target device, and the
`controller may also determine the UID for the target device
`from the at least one unsolicited signal. Thus, if the UID of
`a target wireless communications device is not already
`known, the wireless device locator may passively “listen”
`for unsolicited signals therefrom (1.e., signals that the wire-
`less communications device did not solicit) and determine
`the UID based thereon.
`
`4
`briefly above. The method may include transmitting a plu-
`rality of location finding signals to the target wireless
`communications device, and receiving a respective reply
`signal for each of the location finding signals therefrom. The
`method mayfurther include determining a propagation delay
`associated with the transmission of each location finding
`signal and the respective reply signal therefor based upon
`the known device latency of the target wireless communi-
`cations device. As such, a range to the target wireless
`communications device may be estimated based upon a
`plurality of determined propagation delays.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG.1 is schematic block diagram of a wireless commu-
`nications system in accordance with the present invention
`including a wireless local area network (WLAN) and wire-
`less device locator for locating WLAN devicesthereof.
`FIG.2 is a schematic block diagram generally illustrating
`the components of the wireless device locator of FIG. 1.
`FIG.3 is a graphillustrating the signal propagation delay
`and device latency components used by the controller of
`FIG.2 to estimate range.
`FIG. 4 is a schematic block diagram illustrating an
`embodiment of the wireless device locator of FIG. 2 for a
`
`WLANimplementation.
`FIG. 5 is a schematic block diagram illustrating in greater
`detail an embodiment of the transceiver of the wireless
`device locator of FIG. 4.
`FIGS. 6 and 7 are histogramsillustrating range estimation
`test results performed using the wireless device locator of
`FIG. 4.
`
`20
`
`25
`
`30
`
`40
`
`45
`
`50
`
`Additionally, the controller may also determine the device
`type of the target wireless communications device based
`upon the UID. By way of example, the UIDs may include
`media access control (MAC) addresses of respective wire-
`less communications devices. Accordingly, the controller
`may determine the device type of the target wireless com-
`The present invention will now be described more fully
`munications device based upon the MAC address in some
`hereinafter with reference to the accompanying drawings, in
`applications.
`which preferred embodiments of the invention are shown.
`In accordance with another advantageous aspect of the
`This invention may, however, be embodied in manydifferent
`invention, the at least one antenna may beaplurality of
`forms and should not be construed as limited to the embodi-
`antennas, and the controller may cooperate with theplurality
`of antennas to determine a bearing to the target wireless
`communications device based upon at
`least one of the
`received reply signals. More particularly, the bearing may be
`a three-dimensional bearing, which may be particularly
`useful for locating wireless communications devices within
`a multi-story building, for example. In particular, the anten-
`na(s) may be one or more directional antennas, for example.
`Further, the wireless device locator may further include a
`portable housing carrying the at
`least one antenna,
`the
`transceiver, and the controller.
`The wireless device locator may be used with numerous
`type of wireless communications device. For example, the
`wireless communications devices may be wireless local area
`network (WLAN) devices, mobile ad-hoc network (MA-
`NET) devices, and cellular communications devices.
`A method aspect of the invention is for locating a target
`wireless communications device from among a plurality of
`wireless communications devices, such as those discussed
`
`FIG. 8 is a graph illustrating bearing determination in
`accordance with the present invention.
`FIGS. 9 and 10 are schematic block diagramsillustrating
`alternate embodiments of the wireless communications sys-
`tem of FIG. 1 including a mobile ad-hoc network (MANET)
`and a cellular network, respectively.
`FIG. 11 is a flow diagram illustrating a wireless device
`location method in accordance with the present invention.
`
`DETAILED DESCRIPTION OF THE
`PREFERRED EMBODIMENTS
`
`ments set forth herein. Rather, these embodiments are pro-
`vided so that this disclosure will be thorough and complete,
`and will fully convey the scope of the invention to those
`skilled in the art. Like numbers refer to like elements
`throughout, and prime and multiple prime notation are used
`to indicate similar elements in different embodiments.
`
`Referring initially to FIGS. 1 and 2, a wireless commu-
`nications system 30 illustratively includes a wireless local
`area network (WLAN)31 and a wireless location device 32.
`The WLAN 31 illustratively includes an access point 33
`(e.g., a server) and a plurality of WLAN devices or terminals
`which communicate therewith wirelessly, such as the laptop
`computers 34, 35, and the desktop computer 36. Various
`WLAN protocols may be used in accordance with the
`present invention for such wireless communications(e.g.,
`TEEE 802.11, Bluetooth, etc.), as will be appreciated by
`those of skill in the art. Moreover, it will also be appreciated
`that additional access points and/or other numbers of wire-
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`US 7,321,777 B2
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`5
`less communications devices may be used, even though only
`a few numberthereof are shown for clarity of illustration.
`Further, numerous other types of WLAN enabled wireless
`communications devices(e.g., personal data assistants, etc.)
`may also be used, as will be further appreciated by those
`skilled in the art.
`Each wireless communications device 34-36 in the
`
`WLAN 31 has a device type associated therewith from
`among a plurality of different device types. More particu-
`larly, the device type may signify the particular manufac-
`turer and/or model of a given WLANcardor chip set used
`therein. In some embodiments,
`it may also signify the
`standard the device complies with (e.g., IEEE 802.11).
`The device type is importantin that different device types
`will have known device latencies associated therewith. For
`example, different WLAN cards or chip sets will have a
`certain latency associated with the time they take to process
`a received signal and generate an acknowledgement reply
`thereto. These delay times may be fairly consistent across
`different models from a same manufacturer, or they may
`vary significantly. Additionally, WLAN protocols such as
`TEEE 802.11 have a specified interframe spacing associated
`therewith, as will be appreciated by those skilled in the art.
`Thus,
`in circumstances where the interframe spacing
`requirements are closely adhered to, the latency of a given
`WLANcard or chip set will be substantially equal to the
`interframe spacing.
`The wireless device locator 32 illustratively includes an
`antenna 39 and a transceiver 41 connected thereto, as well
`as a controller 42 connected to the transceiver. These com-
`
`ponents may conveniently be carried by a portable housing
`43 in some embodiments, although they could be imple-
`mented in a more stationary embodiment, if desired. In the
`illustrated example, the antenna 39 is a directional antenna,
`although omni-directional antennas may also be used, as
`will be appreciated by those skilled in the art. It will also be
`appreciated that various antenna/transceiver combinations
`maybe used. As will be discussed further below, more than
`one antenna may beused in certain embodiments to provide
`bearing determination capabilities, and separate transceivers
`may optionally be used for respective antennas, if desired.
`Operation of the wireless device locator 32 will now be
`described with reference to FIG. 3. The controller 42 coop-
`erates with the transceiver 41 for transmitting a plurality of
`location finding signals to a target wireless communications
`device to be located from among the plurality of wireless
`communications devices. In the present example, the laptop
`34 is the target device.
`As will be appreciated by those skilled in the art, each
`WLANdevice 34-36 in the network 31 will have a unique
`identifier (UID) associated therewith which is used in sig-
`nals transmitted between the respective devices and the
`access point 33. The UID distinguishes the devices 34-36
`from one another so that each device only acts upon or
`responds to signals intended for it, and so the access point
`33 knows which device it is receiving signals from.
`Depending upon a given implementation,
`the wireless
`locator device 32 may or may not know the UIDofthetarget
`device 34 before hand. For example, in some embodiments
`the wireless device locator 32 could download the UID from
`the access point 33 (either wirelessly or over a wired
`network connection, for example). This may be the case
`when trying to locate a node in a LAN where the nodeis
`already registered with the network. However, if the UID is
`not known, the wireless device locator 32 may passively
`listen to the target device 34 for unsolicited signals being
`transmitted therefrom. This feature may be advantageousfor
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`law enforcement applications, or for locating an interfering
`node that is not registered with a particular network but
`causes interference therewith, for example. By “unsolicited”
`signals it is meant that these signals are not solicited by the
`wireless device locator 32 itself, although such signals may
`have been solicited from another source (e.g., the access
`point 33).
`The controller 42 cooperates with the transceiver 41 to
`receive one or more of the unsolicited signals, and the
`controller determines the UID for the target device 34
`therefrom. Of course, the method by which the controller 42
`determines the UID from the unsolicited signal will depend
`upon the given implementation, and whether or to what
`degree such signals are encrypted.
`Additionally, the controller 42 may also determine the
`device type of the target wireless communications device 34
`based upon the UID thereof. By way of example, the UIDs
`may include media access control
`(MAC) addresses of
`respective wireless communications devices. The MAC
`addresses may be specific to a particular type of device
`manufacturer, or indicate a particular operational protocol
`with which the device is operating, as will be appreciated by
`those skilled in the art. Accordingly,
`the controller may
`determine the device type of the target wireless communi-
`cations device 34 based upon the MAC address thereof in
`some applications.
`As such,
`to locate the target device 34, the controller
`inserts the UID therefor in each of the location finding
`signals. By way of example, the location finding signal may
`include the UID of the target device 34 in a header packet
`and a valid but empty data packet. This will force the target
`device 34 to generate a reply signal acknowledging receipt
`of the location finding signal (i.e, an ACK signal). Of
`course, various other location finding signals could be used
`to cause the target terminal 34 to generate an ACKsignal, as
`will be appreciated by those skilled in the art. The controller
`42 cooperates with the transceiver 41 for receiving the reply
`signals from the target device 34 via the antenna 39. The
`location finding signals and reply signals may be radio
`frequency (RF), microwave, optical, or other suitable types
`of signals, as will be appreciated by those skilledin theart.
`The controller 42 determines the propagation delay asso-
`ciated with the transmission of each location finding signal
`and the respective reply signal therefor, and it uses this
`propagation delay to estimate a rangeto the target device 34.
`However, the propagation delay has to first be determined
`based upon the total round trip time from the sending of the
`location finding signal to the reception of the respective
`reply signal.
`Thetotal roundtrip time will include several components.
`Referring more particularly to FIG.3, the first componentis
`the time associated with transmitting a location finding
`signal 45, whichis illustrated with an arrow. That is, this is
`the time from the beginning of the location finding signal
`transmission (timet,) to end thereof (time t,). Two time axes
`are shown in FIG.3. The top or upper axis represents events
`that occur at the target device 34, while the bottom or lower
`axis represents events that occur at
`the wireless device
`locator 32.
`
`The second component of the round trip time is the
`propagation delay or time t,,, it takes for the location
`finding signal 45 to travel from the wireless device locator
`32 to the target device 34 (i.e., from timet, to t,). The third
`componentof the roundtrip timeis the device latency t,, of
`the target device 34 (1.e., form timet, to t,). This is the time
`it takes the target device 34 to receive, process, and transmit
`a reply signal 46 responsiveto the location finding signal 45.
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`US 7,321,777 B2
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`7
`The final components of the roundtrip time are propagation
`delay t,,,, of the reply signal 46 (i.e., from time t, to t,), and
`the reception time thereof by the wireless device locator 32
`(i.e., from time t, to t).
`The controller 42 will know the times associated with the
`
`transmission ofthe location finding signal 45 (..e., from time
`ty to t,), as well as the time associated with the reception of
`the reply signal 46 (i.e., from time t, to t,) for each round
`trip, since these can be readily measured by the controller.
`The quantities that the controller 42 will not know are the
`propagation delays tp,,, tpp2 and the actual device latency
`tor:
`Yet, as noted above, the controller 42 will have access to
`the known device latency(i.e., a mean latency) for the given
`device type of the target device 34, which provides a close
`approximation of the actual device latency t,,. The known
`device latency could be a measured value based upon
`collected data, it could be provided by manufacturers, or it
`could be based upon a value set
`in a communications
`standard, as discussed above, for example.
`As will be appreciated by those skilled in the art, the
`actual device latency will likely vary somewhat from one
`transmission to the next for any wireless communications
`device, potentially by aslittle as a few nanosecondsto a few
`microseconds, depending upon device configurations, pro-
`cessing loads, etc. Accordingly, a close approximation ofthe
`total propagation delay (1e.,
`time tpp,,+time tpp.) may
`therefore be obtained by substituting the known device
`latency for the actual device latency t,,, and subtracting this
`value from the time between times t, and t,. Dividing the
`total propagation delay by two (since both propagation
`delays may be considered equal or substantially equal for a
`stationary or relatively slow moving target device 34) and
`multiplying this by the speed of light gives the estimated
`distance to the target device 39, based upon the single
`propagation delay associated with the signal pair 45, 46.
`Yet, as noted above, device latencies tend to vary from
`one transmission to the next. Since the location finding
`signals and reply signals are traveling at the speed oflight,
`such variances can make a significant difference in the
`estimated distances. Moreparticularly,light travels approxi-
`mately 1000 ft.
`in one microsecond. Thus, if the device
`latency varies by one microsecond from one transmission to
`the next, the estimated distanceto the target device 34 would
`similarly vary by 1000 ft. or so, which likely will be an
`unacceptable accuracy for many applications.
`In accordancewith the present invention, the controller 42
`advantageously estimates the range to the target device 34
`not solely based upon a single measured propagation delay,
`but rather upon a plurality thereof. More particularly, by
`estimating the range based upona plurality of propagation
`delays, the wireless device locator 32 mitigatesthe effects of
`the variations in the actual device latency time. This pro-
`vides a significantly more accurate approximation of the
`device latency time and, thus, a more accurate range esti-
`mation. By way of example, the controller 42 may estimate
`the range based upon an average of the propagation delays,
`though other suitable statistical functions may also be used
`(e.g., mean, median, mode, etc.). Of course,
`it should be
`noted that the average may be taken on the entire round trip
`delay instead of first subtracting out the known device
`latency as described above. That is, the same result may be
`obtained byfirst taking the average and then subtracting out
`the known device latency, as will be appreciated by those
`skilled in the art.
`An exemplary embodiment of the present invention is
`now described with reference to FIGS. 4 and 5. The wireless
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`device locator 32' may use a personal data assistant (PDA)
`as the controller 42', although a personal computer (PC) or
`other suitable computing device may also be used. More
`particularly, the PDA 42' illustratively includes a graphical
`user interface (GU1) 50', and a received signal strength
`indication (RSSI) processing module 51' for cooperating
`with the transceiver 41' to perform above described range
`estimation processing operations. More particularly,
`the
`RSSI module 51' may be im