United States Patent [19]
`Herrick
`
`I IIIII IIIIIIII Ill lllll lllll lllll lllll lllll lllll lllll lllll 111111111111111111
`US005512908A
`Patent Number:
`Date of Patent:
`
`[11]
`
`[45]
`
`5,512,908
`Apr. 30, 1996
`
`[54] APPARATUS AND METHOD FOR
`LOCATING CELLULAR TELEPHONES
`
`Primary Examiner------Gregory C. Issing
`Attorney, Agent, or Firm-David W. Gomes
`
`[75]
`
`Inventor: David L. Herrick, Mont Vernon, N.H.
`
`[57]
`
`ABSTRACT
`
`[73] Assignee: Lockheed Sanders, Inc., Nashua, N.H.
`
`[21] Appl. No.: 272,725
`
`[22] Filed:
`
`Jul. 8, 1994
`
`Int. Cl.6
`.............................. GOlS 1/24; H04M 11/00
`[51]
`[52] U.S. Cl . ............................. 342/387; 342/451; 379/59
`[58] Field of Search ..................................... 342/387, 451;
`379/59
`
`[56]
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,433,335
`5,317,323
`5,327,144
`
`2/1984 Wind ....................................... 342/463
`5/1994 Kennedy et al ........................ 342/457
`7/1994 Stilp et al ............................... 342/387
`
`A method for determining the time difference of arrival,
`tdoa, of signals from a cellular telephone, includes deter(cid:173)
`mining tdoas for a plurality of channels, averaging the tdoas
`from the channels, subtracting the average tdoa from the
`individual channel tdoas to determine residual channel tdoas
`and determining an overall residual tdoa from the residual
`channel tdoas. An apparatus and method for collecting cell
`phone transmission signals for performing tdoa calculations,
`includes receiving, digitizing and temporarily storing wide(cid:173)
`band transmission signals while monitoring a known reverse
`control channel for a predetermined cell phone number to be
`located, and permanently storing the delayed reverse control
`channel signal once the predetermined number is identified.
`
`17 Claims, 4 Drawing Sheets
`
`16
`
`18
`
`16
`
`18
`
`GPS
`
`CELL PHONE
`Rx
`
`CELL PHONE
`Rx
`
`GPS
`
`12
`
`REMOTE
`BASEi
`
`13
`
`REMOTE
`BASE2
`
`16
`
`18
`
`19 _..
`MODEM
`7 MILES
`
`CELL PHONE
`GPS
`Rx.--~~~~---.
`
`-19
`
`MODEM
`7MILES
`
`10~
`
`CENTRAL
`BASE
`
`20
`
`TDOA
`CPU
`
`14
`
`24
`
`22
`
`D
`
`LAPTOP
`WITH
`CELLUAR
`MODEM
`
`Twitter Exhibit 1054
`Twitter, Inc. v. BlackBerry Ltd.
`Page 00001
`
`

`

`U.S. Patent
`
`Apr. 30, 1996
`
`Sheet 1 of 4
`
`5,512,908
`
`16
`
`18
`
`16
`
`18
`
`GPS
`
`CELL PHONE
`Rx
`
`13
`
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`
`16
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`18
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`CELL PHONE
`Rx
`
`GPS
`
`12
`
`REMOTE
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`
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`
`MODEM
`7 MILES
`
`10~
`
`CENTRAL
`BASE
`
`20
`
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`CPU
`
`14
`
`FIG. I
`
`-19
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`MODEM
`7MILES
`
`24
`
`22
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`D
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`WITH
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`MODEM
`
`Page 00002
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`Page 00003
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`

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`
`Page 00004
`
`

`

`U.S. Patent
`
`Apr. 30, 1996
`
`Sheet 4 of 4
`
`w
`
`Rab, i~
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`tdoa
`
`5,512,908
`
`Raby~ FIG. 4
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`FIG. 5
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`f
`
`RECONSTRUCTED WIDEBAND TIME DOMAIN PLOT OF MULTIPLE ARRIVALS
`16 FREQUENCIES UNIFORMLY SPACED OVER A I MHz BAND
`2 . - - - - - - -~ -~ - - - - - - -
`DIRECT
`ARRIVAL
`94 \.
`
`"'-96 MULTIPATH ARRIVAL
`
`FIG. 6
`
`5
`
`10
`tDELAYkk
`DELAY (MICROSECONDS)
`
`15
`
`20
`
`Page 00005
`
`

`

`5,512,908
`
`5
`
`25
`
`1. Field of the Invention
`The present invention generally relates to the function of
`locating cellular telephones and particularly, to a method and
`apparatus which take advantage of the multiple channels
`used by a cellular telephones for this purpose.
`2. Statement of the Prior Art
`The proliferation of cellular telephones, or cell phones,
`and their technology and usage, has revealed various appli(cid:173)
`cations for the ability to locate cell phones. These applica(cid:173)
`tions include "911" calls, tourist and travel information,
`tracking of unauthorized cell phone usage and illegal activi(cid:173)
`ties and the locating of commercial and government
`vehicles, to name a few. The basic cell phone systems can
`only determine the nearest cell phone base station which
`locates the cell phone to within 3 to 10 miles.
`A great deal of technology also exists concerning the
`tracking or locating of radio transmitters. A goal of any such
`system for use with cellular phones is the use of as much
`existing equipment as possible or at least compatibility with
`existing equipment. Examples of this are use with unmodi(cid:173)
`fied cellular phones and compatibility with existing cellular
`base stations.
`One known method for locating transmitters is time
`difference of arrival, tdoa, which has been used for many
`years, at least in such applications as LORAN and GPS. The
`application of this process to cell phones includes measuring
`the time of arrival of the same signal at a multiplicity of
`locations and comparing the times to determine how long
`the signal took to reach each receiver. Because it operates on
`the ordinary signal transmitted, it does not require any
`modifications of the transmitter. One such application of
`tdoa technology combined with direction finding and
`applied to cellular phones is described in U.S. Pat. No.
`5,317,323. The system described therein uses GPS timing
`signals at the receivers to determine the time of signal
`reception and generally concerns the use of direction finding
`to help eliminate multipath and co-channel interference.
`Unfortunately, the use of direction finding requires the use
`of a steerable antenna array and is thus not compatible with 45
`existing omnidirectional antenna structures. The patent does
`show the importance of removing multipath interference.
`For this reason, it is a goal of any such locating system for
`cellular telephones to distinguish between a direct signal
`from a cell phone and multipath reflections of the same 50
`signal from buildings and other reflectors. Reflected signals,
`which take a longer and unknown path to the receiver,
`provide less accurate location information.
`
`1
`APPARATUS AND METHOD FOR
`LOCATING CELLULAR TELEPHONES
`
`BACKGROUND OF THE INVENTION
`
`SUMMARY OF THE INVENTION
`
`2
`It is yet a further object of the present invention to
`perform such location finding while taking advantage of the
`frequency separation of a plurality of cell phone channels to
`improve the accuracy of the location finding.
`It is yet a further object of the present invention to
`perform such location finding using antennas and antenna
`structures which are normally used for typical cell phone
`operation.
`In one form, the present invention provides a method for
`10 determining the time difference of arrival, tdoa, of signals
`from a cellular telephone, cell phone, at a pair of base
`stations, comprising the steps of: substantially simulta(cid:173)
`neously sampling transmission signals from the cell phone at
`the pair of base stations in a plurality of cell phone channels
`15 having different frequencies; correlating the corresponding
`signals sampled at the base stations for each channel to
`determine a tdoa therebetween for each channel; averaging
`the tdoas determined for each channel; subtracting the
`average tdoa from each of the determined channel tdoas to
`20 determine a residual phase tdoa for each channel; determin(cid:173)
`ing an overall phase tdoa between the pair of base stations
`based upon the residual phase tdoas for each channel; and
`adding the average tdoa to the overall phase tdoa to deter(cid:173)
`mine an overall tdoa between the pair of base stations.
`In another form, the present invention provides a method
`for collecting transmission signals at a first cell phone base
`station from a particular cell phone for the purpose of
`determining location of the cell phone using time difference
`of arrival, tdoa, comprising the steps of: receiving signals
`transmitted over all channels of a wideband, cell phone base
`station, operating spectrum including a known reverse con(cid:173)
`trol channel thereof; digitizing the received wideband spec(cid:173)
`trum signals; storing the digitized signals as digitized data in
`a delay memory; monitoring the known reverse control
`channel signal for a cell phone transmission including a
`predetermined phone number of a cell phone to be located;
`identifying a predetermined number from the monitoring of
`the reverse control channel; and storing data from the delay
`40 memory in response to the identifying of the predetermined
`number, including the portion of the received signal bearing
`the identifying data of the predetermined number.
`In yet another form, the present invention provides an
`apparatus for collecting transmission signals at a first cell
`phone base station from a particular cell phone for the
`purpose of determining location of the cell phone using time
`difference of arrival, tdoa, comprising: means for receiving
`signals transmitted over all channels of a wideband, cell
`phone base station, operating spectrum including a known
`reverse control channel thereof; means for digitizing the
`received wideband spectrum signals; means for storing the
`digitized signals as digitized data in a delay memory; means
`for monitoring the known reverse control channel signal for
`a cell phone transmission including a predetermined phone
`55 number of a cell phone to be located; means for identifying
`a predetermined number from the monitoring of the reverse
`control channel; and means for storing data from the delay
`memory in response to the identifying of the predetermined
`number, including the portion of the received signal bearing
`the identifying data of the predetermined number.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`30
`
`35
`
`Accordingly, it is a object of the present invention to
`provide an apparatus and method for determining the loca(cid:173)
`tion of cellular telephones by using time difference of arrival
`measurements at a multiplicity of cell phone base stations.
`It is a further object of the present invention to perform
`such location finding even when the signals received at one
`or more of the base stations are indistinguishable from noise
`and other co-channel interference. _
`It is still a further object of the present invention to 65
`perform such location finding in the presence of high levels
`of multipath reflected signals.
`
`60
`
`The present invention is illustratively described in refer(cid:173)
`ence to the appended drawings in which:
`FIG. 1 is a system block diagram of an apparatus con(cid:173)
`structed in accordance with one embodiment of the present
`invention;
`
`Page 00006
`
`

`

`5,512,908
`
`3
`FIG. 2 is a block diagram of one embodiment of a portion
`of FIG. 1;
`FIG. 3 is a block diagram of a second embodiment of a
`portion of FIG. 1;
`FIG. 4 is a signal diagram of some of the results of the
`tdoa determination process;
`FIG. 5 is a diagram of additional results of the tdoa
`process; and
`FIG. 6 is another signal diagram of additional results of 1o
`the tdoa process.
`
`DETAILED DESCRIPTION OF THE DRAWINGS
`
`50
`
`30
`
`4
`signals received by antenna 16 on both the reverse control
`channel and the reverse voice channel. The received cell
`phone signals from down converter 42 are synchronously
`fed through an analog-to-digital, ND, converter 48 to the
`5 processing section 34 along with the time tag signals on line
`49. Additionally, AID converter 48 is triggered in response
`to the GPS signals so that the time tags correspond to the
`exact points in time that AID converter 48 performs sam-
`pling.
`Processing section 34 generally includes a delay memory
`50, a filter/decimator 52, a FIFO buffer 54 and a time tag
`decoder 56. Memory 50 receives digitized data from the cell
`phone bandwidth being received along with corresponding
`time tag data. Buffer 54 and decoder 56 output data to a
`control CPU 60 in control section 36 and receive an alarm
`15 or trigger signal therefrom. Filter/decimator 52 receives a
`tuning signal from CPU 60. The purpose of delay memory
`50 is to store data from the cell phone bandwidth while
`control CPU 60 determines if a portion of that data and its
`corresponding time tag should be recorded. Because delay
`20 memory 50 temporarily stores all data in the cell phone
`spectrum, it must operate at a fairly high speed such as 25
`MHz to cover a cell phone bandwidth of 10 MHz. The
`purpose of filter/decimator 52 is to digitally tune to only a
`single channel, 30 KHz, and filter out the remainder of the
`25 cell phone spectrum being fed from delay memory 50.
`Because the data then recorded in FIFO buffer 54 only
`corresponds to the single channel bandwidth, it can run at a
`much lower speed, i.e. 100 KHz, than delay memory 50.
`Control section 36 includes the control CPU 60 along
`with a communications CPU 62 which communicate with
`each other and with the remainder of the locator system 10
`using an Ethernet connection 64 over comm. lines 19.
`Comm. CPU 62 is also coupled to a cell phone receiver 66
`35 of sorts which solely monitors the forward control and voice
`channels and records voice channel assignments sent from
`the base station to individual cell phones. In another imple(cid:173)
`mentation voice channel assignment data may be hard wired
`or reported via a direct computer link from the cell phone
`40 system thereby making receiver 66 unnecessary.
`In operation, comms. CPU 62 receives data over the
`Ethernet 64 to monitor for a specific predetermined phone
`number to be located. Cell phone calls to specific telephones
`number may also be so designated and monitored. Comms.
`45 CPU 62 passes this information to control CPU 60 which
`monitors the reverse control channel signal from receiver 44.
`Control CPU 60 also sends a tuning signal to filter/decimator
`52 so that the data exiting therefrom is the contents of the
`reverse control channel. Whenever a cell phone places a call,
`it transmits both its own number and the number to be called
`over the reverse control channel. Likewise whenever a cell
`phone is called, its number is transmitted over the forward
`control channel and it responds over the reverse control
`channel using its phone number.
`Whenever control CPU 60 matches a cell phone trans-
`mission number with a predetermined number to be moni(cid:173)
`tored, control CPU 60 generates an alarm or trigger signal
`for FIFO buffer 54 and time tag decoder 56. This trigger
`causes buffer 54 to begin collecting data and decoder 56 to
`identify the specific time tag corresponding to the beginning
`of the data being so stored. The trigger signal is simulta(cid:173)
`neously sent to all of the immediately surrounding base
`stations so that simultaneously transmitted signals are col(cid:173)
`lected. Surrounding base stations will collect data even
`though the reverse control channel signals they received
`may not have been strong enough to enable identification of
`the predetermined cell phone number.
`
`FIG. 1 shows one embodiment of a cell phone locating
`system 10 which would take advantage of existing cell
`phone base stations and antennas. System 10 generally
`includes a multiplicity of existing cell base stations 12-14,
`to which the apparatus of the present invention is added.
`Base stations 12-14 make use of existing transmit and
`receive antennas 16 which may be identical in form and
`function. Added to each base station is an additional GPS
`(Global Positioning System) antenna 18 which receives
`signals from the existing civilian GPS for use in the locating
`function. Each of the base stations also includes further
`equipment for performing the locating function as described
`below. Communication lines 19 are shown connecting at
`least two remote base stations 12,13 with a central base
`station 14. Central base station 14 includes equipment not
`present in the remote base stations 12,13 in the form of tdoa
`CPU 20 for processing signals received by all base stations
`12-14. Tdoa CPU 20 may be located at any one of the base
`stations or may be located separately from such base sta(cid:173)
`tions. Such separate location would include the interconnec(cid:173)
`tion of all comm. lines 19 directly between the base stations
`and tdoa CPU 20. A cell phone 21, located within the range
`of the base stations 12-14, may be located by the equipment
`of the present embodiment.
`The base station 14, having tdoa CPU 20, is shown being
`accessed by a computer 22, such as a portable computer,
`using a telecommunications link, such as a cellular tele(cid:173)
`phone modem 24, for the purpose of requesting location
`information or the performance of a locating operation on a
`cell phone such as 21.
`FIG. 2 shows a system block diagram for installation at
`one of the remote base stations 12-14 of FIG. 1. The system
`30 shown generally includes a receiver section 32, a pro(cid:173)
`cessing section 34 and a control and communication section
`36. Receiver section 32 is coupled to the existing cell phone
`antenna 16 and the GPS antenna 18. Signals from the
`existing cell phone antenna 16 are coupled through a Filter
`and Low Noise Amplifier 40 to a frequency down converter
`42 and a receiver 44 for the reverse control channel of the
`cell phone system. Down converter 42 outputs signals from
`the full cell phone bandwidth being received. The analog 55
`output of receiver 44 is nominally fed through a TTL
`converter 45 for providing a digital format to the control
`section 36.
`Down converter 42 also receives demodulated GPS tim(cid:173)
`ing signals or time tags at one pulse per second and one
`kilopulse per second from a GPS receiver 46 connected to
`GPS antenna 18. Down converter 42 includes a counter (not
`shown) which is clocked by the kilopulse signal and reset by
`the pulse per second signal, thus producing a time tag for
`association with the received cell phone signals. Thus, the
`GPS time tags, which are substantially the same at all local
`base stations are used to measure the time of arrival of
`
`60
`
`65
`
`Page 00007
`
`

`

`5,512,908
`
`10
`
`25
`
`6
`pling. To control the AID converters 80,81 and time tag
`generation, a 10 MHz. signal from the GPS receiver 46 is
`converted to a 30 MHz clock signal by the down converter
`and fed to converters 80,81 and a time tag generator 82 in
`processor section 74.
`Processor section 74 further includes a delay memory 84,
`a filter/decimator 54, FIFO buffer 84 and an FM demodu(cid:173)
`lator 86. Control and communications section 76 includes a
`single CPU 88 to perform the functions of both CPUs of
`· FIG. 2. The type of signals passed between processor section
`74 and control and comm. section 76 are the same as FIG.
`2.
`
`5
`The delay in time between the receipt of a reverse control
`channel signal and the decoding of that signal by control
`CPU 60 to generate a trigger in the present embodiment
`amounts to approximately 20 ms. of data or less. This
`corresponds to the amount of data stored in delay memory 5
`50 which can therefore be captured in buffer 54 even after
`the number being monitored has been decoded therefrom. In
`response to a trigger, buffer 54 collects 82 milliseconds of
`data which may then be passed as a block or packet to
`control CPU 62 along with the corresponding decoded initial
`time tag from decoder 56.
`Once a monitored phone is identified from the reverse
`control channel from receiver 44, the forward control chan(cid:173)
`In operation, all of the channels in the wideband cell
`nel is further monitored by comms. CPU 62 and receiver 66
`phone spectrum are received, down converted and digitized
`to determine the voice channel assignment from the base
`station. This assignment is passed to control CPU 60. After 15
`in receiver section 72 and fed to processor section 74 as a
`the 82 milliseconds of reverse control channel data is
`pair of base band signals 90,91 along with clock and time tag
`collected in buffer 54, control CPU 60 re-tunes filter/
`signals synchronized to the digitization process. Processor
`decimator 52 to the assigned reverse voice channel and
`section 74 receives one of the digitized data signals 90,91,
`re-triggers buffer 54 and time tag decoder 56 to collect
`under the control of CPU 88 depending upon the spectrum
`another 82 milliseconds of data from the assigned reverse 20
`location of the reverse control channel being monitored. The
`voice channel. This data is likewise passed with its initial
`received signal 90,91 is fed directly into delay memory 82
`time tag to control CPU 60 and the locator system for
`and into the filter/decimator 54 through a shunt 92 around
`processing.
`delay memory 82. Filter/decimator 54 is initially set to
`During and after data collection from the reverse voice
`separate the reverse control channel and feeds that data to
`FM demodulator 86, which separates the data contained
`channel, receiver 66 and comms. CPU 62 also monitor the
`forward voice channel to detect further voice channel
`therein from the carrier signal so that the data can be
`monitored by CPU 88. This data is passed through a small
`assignments. When such assignments are detected, control
`portion of buffer 84 and fed to CPU 88 for monitoring.
`CPU 60, as well as surrounding base stations, are notified to
`change data collection channels. This subsequent reverse 30 When CPU 88 detects a response from a predetermined
`voice channel switching both insures that sufficient data is
`cell phone over the demodulated reverse control channel, it
`generates a trigger which switches the input of filter/deci-
`collected for each collection period and also provides addi-
`mator 54 from shunt 92 to the output of delay memory 82.
`tional data for improving the accuracy of the location
`function performed by tdoa CPU 20.
`Delay memory 82 holds sufficient data so that the predeter-
`This process of recording reverse control and voice chan- 35 mined number may be detected by CPU 88 while the same
`data is still present in delay memory 82. The trigger signal
`nel data with its corresponding time tags is performed
`also causes a time tag signal to be sent to CPU 88 to mark
`simultaneously at each of the base stations receiving the cell
`phone transmission. This may be done in response to iden-
`the beginning of the data being collected and causes the data
`output of filter/decimator to be stored in FIFO buffer 84.
`tification of the reverse control channel signal at each base
`station or it may be done in response to trigger signals 40 Once an amount of data (nominally 320 milliseconds) has
`been collected in buffer 84, it is passed to CPU 88. CPU 88
`generated from one or more of the base stations and passed
`to surrounding base stations via comms. CPU 62 and the
`then re-tunes the filter/decimator to the reverse voice chan-
`Ethemet connection 64. The latter triggering avoids the
`nel assigned to the predetermined cell phone and detected
`from receiver 66 and also triggers another time tag. This
`necessity that each of the base stations receive a signal of
`sufficient strength from which to identify the predetermined 45 causes data to be collected in buffer 84 from the reverse
`voice channel. Similar data collections are also made from
`number. The 82 milliseconds of recorded data at each base
`station provides sufficient identity between the
`three
`subsequent reverse voice channel assignments. Once data
`has been collected and sent to CPU 88, the data is sent with
`recorded sets of data to determine comparable times of
`its respective time tags to tdoa CPU 20 for tdoa calculations.
`arrival. Thereafter the recorded data is passed to tdoa CPU
`20 where calculations are made to determine relative trans- 50
`The method and apparatuses which function in accor-
`mission time to the base stations 12-14 and to apply that
`dance with the present invention are robust in that only one
`data to mapping software. These tdoa calculations are
`of the base stations monitoring the reverse control channel
`described in greater detail below.
`needs to clearly receive a signal from the predetermined cell
`FIG. 3 shows an alternate system embodiment 70 of the
`phone. Because it is possible to simply trigger all surround-
`base station system 30 of FIG. 2. Sections and components 55
`ing base stations to record and transmit data based upon one
`in FIG. 3 bearing the same reference numbers as those in
`or more identified control and voice channels, even signals
`FIG. 2 are identical in form and function. Generally included
`received by several of the base stations which are too weak
`are a receiver section 72, processing section 74 and control
`for identification can still be used for tdoa measurements.
`and communication section 76. Receiver section 72 differs
`The remote triggering is facilitated by the wideband nature
`from FIG. 2 in that it includes a dual down converter 78 and 60 of the data which is received and stored in the delay memory
`of each base station.
`a pair of AID converters 80,81. The purpose for this is to
`cover the entire 25 MHz cell phone spectrum with available
`The purpose of analyzing the signal data from each of the
`AID converters 80,81 which only operate at 30 MHz. This
`base stations is to calculate the tdoa between at least two
`is accomplished by splitting the cell phone spectrum into a
`pairs out of at least three base stations. This tdoa data is then
`pair of 12.5 MHz. base band signals with the dual down
`65 converted to relative distance data and the location of the
`converter 78 and then feeding each base band signal through
`transmitter can be calculated from the known locations of
`a separate 30 MHz. AID 80,81 to provide adequate sam-
`the base stations and coordinated with map software. For
`
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`5,512,908
`
`7
`purposes of identification and notation, base stations are
`referred to as a,b,c and cell phone transmission channels
`located at different frequencies in the cell phone spectrum
`are referred to as x,y,z.
`The signal data, S(t) for tdoa calculations comes in
`packets of digitized signals. Each packet, S, corresponds to
`one transmission channel x,y,z received at one base station,
`a,b,c as in Sax, Sal" Sa,, Sbx ... Each packet also includes a
`time tag, nominally for the first sample in the packet, and the
`sampling frequency or time between samples is known. Two
`or more channel transmissions are sampled at each base
`station. Each channel transmission is sampled at three base
`stations or more.
`The first step in the tdoa determination process is the
`transformation of each packet of data into the frequency
`domain by performing an FFT to produce S(f) as in Sax, Say•
`Sa,, Sbx• Sby• Sb, · · ·
`The overall tdoa between any two base stations, rab• is
`determined by first determining the tdoa function in each
`separate transmission channel, Rabx• Raby• Rabz· This is done
`in the frequency domain by multiplymg the signal data
`received at a base station in a transmission channel, Sax, by
`the complex conjugate of the same channel data received at
`another base station, S bx*,
`
`The same correlation is performed for each transmission
`channel, x,y,z, between the two base stations a,b.
`These calculated tdoa functions are converted back into
`the time domain by an inverse FFT and show an amplitude 30
`peak, as seen in FIG. 4, at the tdoa of the two correlated
`signals. The precise peak is usually not aligned with one of
`the points produced by the inverse FFT, so a quadratic
`interpolation is performed using the adjacent points to make
`a more accurate determination of the peak amplitude and the 35
`exact time delay, or tdoa, thereof for each transmission
`channel.
`The tdoa signals plotted in FIG. 4 vary between trans(cid:173)
`mission channels for each pair of base stations because of
`phase differences at the different frequencies. However, the 40
`tdoa amplitudes in the time domain are substantially equal in
`the absence of substantial multipath signal reception. Thus,
`variation of the tdoa amplitudes between transmission chan(cid:173)
`nels is an indication of multipath problems.
`The interpolated time delay or tdoa values for all trans- 45
`mission channels measured between two base stations are
`averaged to determine Avg.rab·
`The average tdoa is subtracted from each of the correlated
`tdoa functions in the frequency domain by using a multi(cid:173)
`plication factor to determine a residual phase tdoa, Ph.Rabx• 50
`due to phase in each channel.
`Ph.Rabx=Rab..xd2
`Pifl, where t= Avg.rab• and f is the center
`frequency of the respective channel, x.
`The resulting phase vectors in each of the frequency bins
`of the residual tdoa function, Ph.Rabx• Ph.Raby• . . . , are 55
`summed to determine the residual phase tdoa, Ph.Rab• for
`each transmission channel, x,y,z.
`The amplitude of the residual phase tdoas from all of the
`transmission channels between two base stations are plotted,
`as shown in FlG. 5, against their respective transmission 60
`channel frequencies and the angle of the plot determines the
`overall residual tdoa, Ph.rab• between two base stations. This
`residual phase tdoa is added to the average tdoa, Avg.rab• for
`the respective pair of base stations to determine the overall
`tdoa, rab• between those stations.
`If the plot of phase tdoa vs. frequency of FlG. 5 is not
`linear or if the peak values of the tdoa functions in the time
`
`65
`
`5
`
`25
`
`10
`
`8
`domain of FlG. 4 are not equal between a pair of base
`stations, multipath, or multiple signal reception is indicated.
`In such case, the residual phase tdoa values for each fre(cid:173)
`quency channel, as shown in FlG. 5, are combined with their
`respective interpolated amplitude values from Rabx• ... of
`FIG. 4 and are noted in the frequency domain. All non(cid:173)
`sample frequencies are set to a zero amplitude and zero
`phase. The resulting frequency domain data is converted
`back into the time domain by an inverse FFT and the
`resulting time domain data will show the receipt of multiple
`signals as shown in FlG. 7. The earlier signal 94 is used as
`the overall residual phase delay, Ph.rab• even though the later
`signal 96 may be stronger. With a minimum of three
`transmission channels unevenly spaced within the cell phone
`band, the earlier arrival may be readily ascertained. Distin-
`15 guishing the various multipath signals received is also
`enhanced zero padding in the frequency domain before
`taking the inverse FFT.
`The embodiments described above are intended to be
`taken in an illustrative and not a limiting sense. Various
`20 modifications and changes may be made to the above
`embodiments by persons skilled in the art without departing
`from the scope of the present invention as defined in the
`appended claims.
`What is claimed is:
`1. A method for determining the time difference of arrival,
`tdoa, of signals from a cellular telephone, cell phone, at a
`pair of base stations, comprising the steps of:
`substantially simultaneously sampling transmission sig(cid:173)
`nals from the cell phone at the pair of base stations in
`a plurality of cell phone channels having different
`frequencies;
`correlating the corresponding signals sampled at the base
`stations for each channel to determine a tdoa therebe(cid:173)
`tween for each channel;
`averaging the tdoas determined for each channel;
`subtracting the average tdoa from each of the determined
`channel tdoas to determine a residual phase tdoa for
`each channel;
`determining an overall phase tdoa