.
`
`United States P?t?llt [19]
`Stilp et a1.
`
`IlllllllllllllllllllllIllllllllllllllllllllllllllllllllllllllllllilllllllll
`[11] Patent Number:
`5,327,144
`[45] Date of Patent:
`Jul. 5, 1994
`
`US00532714-4A
`
`[54] CELLULAR TELEPHONE LOCATION
`SYSTEM
`
`.
`[75] Inventors: Louise A. Stilp, Broomall, Pa.;Curt1s
`A. Knight, Washington, DC; John
`C. Webber, Herndon, Va.
`_
`_
`_
`[73] 458181191: Associated RT, Inc., Pmsburgh, Pa-
`
`5,008,679 4/1991 E?land et a1. .................... .. 342/353
`5,023,809 6/1991 Spackman et a1. ..
`364/516
`5,023,900 6/1991 Tayloe et a1. . . . .
`. . . . .. 379/32
`5,055,851 10/1991 Sheffer ....... ..
`.. 342/457
`5,095,500 3/1992 Tayloe et a1. ............ .. 379/32
`5,101,501 3/ 1992 Gilhousen et al. ..
`.... .. 455/33
`5,126,748 6/1992 Ames et a1. ........ ..
`342/353
`5,128,623 7/1992 Gilmore . . . . . . .
`. . . . . . .. 328/1
`
`,
`
`[21] Appl' N°" 59’2“
`[22] Filed:
`May 7, 1993
`
`[51] Int. Cl.5 ........................ .. G018 1/24; GOlS 3/02;
`1104M 11/00
`
`Us. Cl. .................................. .
`
`5,153,902 10/1992 Buhl et a1. . . . . . . . .
`
`342/457
`5,166,694 11/1992 Russell et a1.
`364/449
`5,208,756 5/1993 Song ........... ..
`5,218,618 6/1993 Sagey ................................ .. 342/457
`
`. . . . .. 379/60
`
`FOREIGN PATENT DOCUMENTS
`
`Inf]
`
`v
`
`[56]
`
`OTHER PUBLICATIONS
`Smith, William w., “Passive Location of Mobile Cellu
`lar Telephone Terminals,” IEEE, 1991, pp. 221-225.
`Primary Examiner-—Gregory C. Issing
`Anor'fe? 5488"’, 0' Firm-Woodcock washbum K1111!
`Machewlcz & Norms
`
`[58] Field of Search ....................... .. 342/387, 457, 35;
`379/58’ 59’ 6°, 62; 364/449
`References Cited
`U.S. PATENT DOCUMENTS
`3,384,891 5/1968 Anderson ........................... .. 343/65
`3,646,580 2/1972 Fuller et a1, . . . .
`. . . .. 342/457
`3,680,121 7/1972 Anderson et a1.
`....... .. 342/387
`ABSTRACT
`[57]
`4,177,466 12/1979 Reagan
`343/112 TC
`4,297,701 10/1931 HQIII'iQUCS
`343/6-5 LC A cellular telephone location system for automatically
`4’433’335 2/1984 wmd ' ' ' ' ‘ ' '
`' ' ' " 343/463
`recording the location of one or more mobile cellular
`4,596,988 6/1986 Wanka
`343/457
`.
`.
`4 638 321 V1987 Drogin .
`342/444
`telephones comprises three or more cell site systems 12.
`4:639:73 1/1987 King ct
`342/424
`Each cell site system is located at a cell site of a cellular
`4,651,156 3/1987 Martinez
`342/457
`telephone system. Each cell site system includes an
`4,651,157 3/1987 Gray et a1. .... ..
`342/457
`antenna that may be mounted on the same tower or
`4,728,959 3/1933 Malone)’ 6131-
`342/457
`building as the antenna employed by the cellular tele
`lséi?ect a1‘
`phone system and equipment that may be housed in the
`4:791:572 ‘D1988 Greemyn'i'gzL
`equipment enclosure of the corresponding cell site. The
`4,797,679 1/1989 Cusdin a a1. ____ “
`cell site systems are coupled via Tl commumcatron
`4,818,998 4/1989 Apscll et a1, _____ ,_
`links 14 to a central site 16. The central site may be
`4,870,422 9/1989 Counselrnan, III
`collocated with the cellular telephone system’s MTSO.
`1%;
`?g‘gman =1 3L
`The central site 16 is further coupled to a database 20,
`4,908,629 M1990 Apscn et al-
`wlgch rcrliay be1 rglmctmtelyblocaged from the central site
`4,926,161 5/1990 Cupp ........ ..
`a“ m e “an e o 5“ 5°“ 6“
`
`“
`
`,
`
`,
`
`C Cr ............. ..
`
`364/449
`342/387
`342/44
`342/357
`
`342/457
`
`~
`
`5
`
`.... .. 342/442
`4,975,7l0 12/1990 Baghdady
`5,003,317 3/1991 Gray et a]. ........................ .. 342/457
`
`45 Claims, 15 Drawing Sheets
`
`I211
`
`I411
`
`12b
`
`14b
`
`16
`
`CENTRAL
`SITE
`
`a 141
`
`141:
`
`18
`
`22
`
`20\
`
`111111155
`
`-
`J z"
`J
`
`"
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 1
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 1 of 15
`
`5,327,144
`
`PRIOR ART
`Fig. M
`
`PRIOR ART
`Fig. IB
`
`LOCAL
`OFFICE
`
`@ .
`
`WI
`
`CE
`
`PRIOR ART
`F'g. I6‘
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 2
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 2 of 15
`
`5,327,144
`
`Ion
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 3
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 4
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 5
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 6
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 7
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 8
`
`

`
`US. Patent
`
`July 5,1994
`
`Sheet 8 of 15
`
`5,327,144
`
`RECEIVE 1 FRAME OF
`DATA FROM ALL
`CELL SITES
`
`l
`CRDSS-CDRRELATE
`DATA
`l
`/
`GENERATE
`5'6"“
`TABLE
`A
`i————< 5
`
`
`
`TDOADATA l
`
`.
`
`FILTER
`
`CALCULATE LAT., LONG.
`FOR EACH SIGNAL
`
`CELL SITE
`3
`4------~N
`
`1
`
`2
`
`AT1 AT2 AT3 AT4~~ ‘
`8T1
`BT2""
`
`C.“ 0T2 ......... . . GT3 . . . .
`
`Fig_ 7
`
`DECODE PHONE N0.
`roa EACH SIGNAL,USING
`STRONGEST SAMPLE
`
`STORE LOCATION PHONE
`no. DATA m DATABASE
`
`i
`l
`
`SEND TO USER
`GENERATE BILLING DATA
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 9
`
`

`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 10
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 10 of 15
`
`5,327,144
`
`IS RECEIVED POWER
`ABOVE THRESHOLD AT
`ANY CELL SITE?
`
`N
`
`SET CORRELATOR INPUTS iTO
`PROCESS THAT CELL SITE S
`DATA AS AN AUTOCORRELATION
`(BOTH INPUTS ON SAME SITE DATAI
`
`(E CORRELATOR nmsnsm)"
`LY
`FOURIER TRANSFORM TO
`POWER SPECTRUM
`I
`
`DETERMINE WHICH SICNALLINC
`CHANNELS HAVE-TRANSMISSIONS
`SAVE RESULT
`l
`CLEAR TIME INDEX
`
`s51 CORRELATOR INPUT DATA '8' TO
`ANOTHER CELL sns's DATA LEAVING
`'A“ (IN THE SITE WITH POWER DETECTION
`
`(Is CORRELATOR FINISHEO?>-N—
`lY
`
`SAVE CORRELATOR RESULTS
`
`OBTAIN CORRELATIONS
`
`INCREMENT TIME INDEX
`
`£______
`I
`START OVER PROCESSING
`ALL '8" CHANNEL CELL SITES
`L
`.__1
`IS THERE A'B' CELL SITE THAT Y
`HASN'T BEEN PROCESSED YET?
`
`IS POWER STILL
`BEINC RECEIVED?
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 11
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 11 of 1s .
`
`5,327,144
`
`SET INDEXI T0 SITE INDEX
`OF SITE WITH POWER DETECTION
`
`'
`
`STEP 2
`OBTAIN DELAYS AND
`FREQUENCY DIFFERENCES
`
`SET TIME INDEX TO FIRST TIME
`
`STORE CORRELATION IN ROW OF
`2-D ARRAY WHERE ROW NUMBER
`CORRESPONOS TO TIME INDEX
`
`ANOTHER TIME?
`
`INOREMENT TIME INDEX
`
`OBTAIN 2-D FOURIER TRANSFORM
`I
`SEARCH TRANSFORM FOR
`HIGHEST AMPLITUDE
`I
`INTERPOLATE TO PEAK
`I
`SAVE DELAY, FREQUENCY
`OFFSET RESULTS
`\r—l——\v
`
`ANOTHER INDEX 2?,
`N
`
`Fig. 8B
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 12
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 12 of '15
`
`5,327,144
`
`S
`GET OBSERVED DELAYS, FREQUENCIES
`I
`GET CORRESPDNDINIl PHONE INFORMATION
`
`SET LATITUDE TO START LATITUDE
`
`SET LONCITUDE TO START LONCITUDE
`
`Fig. 86‘
`STEP 3
`ESTIMATE Loom"
`
`CIVEN LATITUDE, LONCITUDE CALCULATE
`THEORETICAL VALUES DE DELAYS
`(INCLUDING SITE BIAS)
`
`I
`
`GET SUM OF‘ SQUARES OF OBSERVED
`MINUS COMPUTED DELAYS 'X'
`
`IS THIS THE SMALLEST “X”
`OBTAINED SO FAR?
`
`N
`
`LY
`
`SAVE LATITUDE IN BEST_LAT
`SAVE LONCITUDE IN BESLLON
`
`(TRY ANOTHER LoNcTTunE?)l—-> INCREMENT LDNGITUOE
`N
`Y
`
`QTY ANOTHER LATTTUTTE?§—Y—> INCREMENT LATITUDE
`N
`
`V
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 13
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 13 of 15
`
`5,327,144
`
`Fig. 8D
`
`STARTING AT BESLLAT AND BESLLDN
`PERFORM A LINEARIZED—VIEIGHT-LEAST-
`SQUARES ITERATION'STEP, DETERMINING
`LAT_CORRECTION AND L0N_CORRECTIDN
`
`ADD LAT_CORRECTION T0 BEST_LAT
`ADD LON_CORRECTION T0 BEST_LON
`}
`
`IS HAGNITUDE OE LAT_CORRECTION_ N
`LESS THAN 0.0001?
`
`LY
`lY
`
`IS MAGNITUDE OF LON_CORRECTION
`LESS THAN 0.0001?
`
`U
`
`0
`
`SET SPEED TO ZERO
`
`SET DIRECTION TO ZEROINORTHI
`
`GIVEN SPEED, DIRECTION CALCULATE
`THEORETICAL VALUES OF FREQUENCIES,
`TAKING SITE BIAS INTO ACCOUNT
`
`I
`
`GET SUM OF SQUARES 0F OBSERVED
`NINUS COMPUTED FREQUENCIES 'Y'
`
`(Is nus THE SHALLEST ‘Y’ OBTAINED so FAR?}N——
`TY
`SAVE SPEED IN BEST_SPEED
`SAVE DIRECTION IN BEST_DIRECTION
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 14
`
`

`
`US. Patent
`
`July 5, 1994
`
`Sheet 14 of 15
`
`5,327,144
`
`QTY ANOTHER DIRECTION?>Y——+ INCREMENT omecnou j
`
`TRY ANOTHER SPEED?
`
`INGREMENT SPEED
`
`STARTING AT BESLSPEED AND
`BEST_DIREGT|DN, PERFORM A LINEARIZED
`WEIGHTED-LEAST-SOUARES STEP DETERMINING
`SPEED_GDRREGTIDN AND DIREGTION_GORREGTION
`
`ADD SPEEO.GORREGTIDN TO BESLSPEED
`ADD DIREGTION_GDRREGTION
`TD BESLDIREGTION
`
`IS MAGNITUDE OF SPEED_GORREGTIDN
`LESS THAN I MPH?
`
`IS MAGNITUDE OF DIREGTIDN_CORREGTIDN
`LESS THAN 1° ANGLE?
`
`N
`
`N
`
`lY
`iY
`OUTPUT PHONE INFORMATION
`BESLLAT BESLLON BEST SPEED,
`DESLDIRECIION
`@
`
`Fig. 8E
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 15
`
`

`
`US. Patent
`
`July 5,1994
`
`Sheet 15 of 15
`
`5,327,144
`
`CELLULAR
`MTSO
`
`BILLING
`
`'
`
`-
`
`‘
`
`‘TAPE
`
`RADIO
`
`IIIIIIII‘
`
`MERGE
`PROCESS
`
`MODIFIED
`BILLING
`TAPE
`
`LOCATION
`TAPE
`
`Fig. .9
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 16
`
`

`
`1
`
`5,327,144
`
`CELLULAR TELEPHONE LOCATION SYSTEM
`
`FIELD OF THE INVENTION
`The present invention relates generally to the ?eld of
`mobile cellular telephone systems (including both ana
`log and digital cellular systems) and more particularly
`relates to a system for automatically locating mobile
`cellular telephones operating within a prescribed geo
`graphic area.
`
`5
`
`25
`
`2
`bands, one of which is available to wire line common
`carriers only and the other of which is available to
`non-wire line common carriers only. In any given sys
`tem, the non-wire line service provider operates within
`the “A side” of the spectrum and the wire line provider
`operates within the “B side" of the spectrum Cellular
`channels are 30 KHz wide and include control channels
`and voice channels. In particular, the twenty-one con
`trol channels for “A” systems are numbered 313
`through 333 and occupy a 30 KHz band of frequencies
`834.390 MHz to 834.990 MHz. The control channels for
`“B” systems are numbered 334 through 354 and occupy
`835.020 MHz to 835.620 MHz. Each cell site (or, where
`a cell site is “sectored” as described below, each sector
`of that cell site) uses only a single control channel. The
`control channel from a cell site to a mobile unit is called
`the “forward” control channel and the control channel
`from the cellular telephone to the cell site is called the
`“reverse” control channel. Signals are continuously
`broadcast over a forward control channel by each cell
`site. In contrast, signals are discontinuously (periodi
`cally) broadcast by the cellular telephones over a re
`verse control channel. If the cell sites are so close to one
`another that control channels using the same frequency
`interfere with each other, the control channel at each
`cell site is further quali?ed by a digital color code rang
`ing from zero to three. This allows each cell site to be
`uniquely identi?ed, for example, within a range of
`twenty to thirty miles.
`'
`Directional cell site antennas may be used to reduce
`co-channel and adjacent-channel interference. FIG. 1B
`illustrates how sectored antennas may be used to reduce
`such interference. The circles represent cell sites and
`the broken lines represent the azimuthal edges of the
`front lobes of 120° directional antennas. The labels “A”
`, “B” , and “C” refer to channel sets, cells, and cell sites
`simultaneously. The labels “1” , “2” , and “3” refer to
`directional antennas and sectors of cells simultaneously.
`Thus, for example, if a particular channel is assigned to
`sector 1 of cell B and adjacent channels are assigned to
`cells A and C, these adjacent channels should be as
`signed to sector 1 in cells A and C.
`'
`When a cellular telephone is ?rst turned on, it scans
`all forward control channels, listening for the channel
`with the strongest signal. The telephone then selects the
`forward control channel with the strongest signal and
`listens for system overhead messages that are broadcast
`periodically, for example, every 0.8 seconds. These
`overhead messages contain information regarding the
`access parameters to the cellular system. One such ac
`cess parameter is the frequency of registration, which
`refers to how often a given telephone must inform the
`system that the telephone is within the system’s geo
`graphic con?nes. Registration frequencies typically
`range from once per minute to once per thirty minutes.
`The overhead messages also contain busy/idle bits
`that provide information about the current availability
`of the reverse control channel for that cell. When the
`reverse control channel becomes free, as indicated by
`the busy/idle bit, the cellular telephone attempts to
`register itself by seizing the reverse control channel.
`Cellular telephones re-register themselves at the rate
`determined by the cellular system. Registration parame
`ter requirements are determined by each cellular sys
`tem. For example, the options include (1) 7-digit NXX
`XXXX, (2) 3-digit NPA, and (3) 32-bit electronic serial
`number. Each of these options constitutes a digital
`
`BACKGROUND OF THE INVENTION
`Prior to the invention disclosed herein, there has been
`no known system for automatically tracking mobile
`cellular telephones. Although related technologies
`(radio navigation systems such as direction ?nding and
`LORAN, emergency location devices for aircraft, satel
`lite tracking and surveillance, and the like) have been
`extant for many years, none of these technologies has
`been applied to automatically locate cellular telephones
`as described herein. Accordingly, the background in
`formation most pertinent to gaining an understanding of
`the present invention relates to a cellular telephone
`system itself, as opposed to the peripherally related
`radio navigation and location technologies. The follow
`ing discussion refers to FIGS. lA-lC in providing an
`overview of a cellular telephone technology. In addi
`tion, it should be noted that the inventive concepts
`disclosed herein are applicable to both analog and digi
`tal (for example, TDMA) cellular systems that employ
`analog control channels.
`Cellular telephone systems typically include many
`cell sites and a centrally-located cellular switch, called
`a Mobile Telephone Switching Office (MTSO). There
`are typically sixty to one hundred cell sites in large
`cities and ?fteen to thirty cell sites in smaller cities. Cell
`sites are usually spaced at distances of one-half to
`twenty miles. Each cell’site generally comprises one or
`more antennas mounted on a triangular platform. The
`platform is placed on a tower or atop a tall building,
`40
`preferably ?fty to three hundred feet above the sur
`rounding terrain.
`The fundamental idea behind a cellular system is
`frequency reuse. This concept of frequency reuse is
`implemented by employing a pattern of overlapping
`cells, with each cell conceptually viewed as a hexagon.
`This concept is illustrated in FIG. 1A, which depicts a
`layout for a cellular system, employing seven distinct
`sets of frequencies. In this ?gure, each shading pattern
`represents a unique frequency set. FIG. 1C schemati
`cally depicts the main components and arrangement of
`cellular telephone system. As discussed above, fre
`quency reuse allows the cellular system to employ a
`limited number of radio channels to serve many users.
`For example, FIG. 1A depicts an area served by 14
`cells, divided into two clusters. Each cluster contains
`seven cells. A separate set of channels is assigned to
`each cell in a cluster. However, the sets used in one
`cluster are reassigned in the other cluster, thus reusing
`the available spectrum. The signals radiated from a cell
`in channels assigned to that cell are powerful enough to
`provide a usable signal to a mobile cellular telephone
`within that cell, but preferably not powerful enough to
`interfere with co-channel signals in distant cells. All
`cellular telephones can tune to any of the channels.
`65
`The Federal Communications Commission (FCC)
`has allocated a 25 MHz spectrum for use by cellular
`systems. This spectrum is divided into two 12.5 MHz
`
`45
`
`55
`
`60
`
`T-Mobile / TCS / Ericsson EXHIBIT 1011
`T-Mobile / TCS / Ericsson v. TracBeam
`Page 17
`
`

`
`20
`
`25
`
`5,327,144
`4
`3
`the dispatcher his location. Prior art systems are unable
`word. Because of sync bits and error correction tech
`niques, each digital word is 240 bits long. With an initial
`to trace a call from a cellular telephone. Therefore, a
`48-bit sync stream, each cellular telephone transmission
`cellular telephone user in such a situation would be in a
`dire predicament. Once again, it would be highly ad
`is a minimum of 288 bits long, and as long as 1488 bits.
`vantageous for the system to be able to ascertain the
`Moreover, each discontinuous transmission by a cellu
`lar telephone includes a period of unmodulated carrier.
`user’s location and provide this information to emer
`gency medical personnel. There would be numerous
`Therefore, an average transmission on the reverse con
`other applications for a system that could automatically
`trol channel lasts about 100 milliseconds. Cellular tele
`phones also transmit in response to pages by the cellular
`locate a cellular telephone.
`system, as well as in response to user-initiated calls. The
`term "paging” is used to describe the process of deter
`mining a mobile telephone’s availability to receive an
`incoming call. The complementary function of initiat
`ing a call by the mobile telephone is called “access.”
`The paging and access functions occur on the control
`channels.
`When turned on but not in active use, a mobile cellu
`lar telephone periodically scans the control channels
`assigned to the system and marks for use the strongest
`carrier found. With the mobile receiver tuned to this
`strongest carrier, the cellular telephone continuously
`decodes a digital modulating data stream, looking for
`incoming calls. Any call to a mobile terminal is initiated
`like a normal telephone call. A seven- or ten-digit num
`ber is dialed and the telephone network routes the call
`to a central computer. The number is broadcast on the
`control channels of every cell in the system. When a
`called telephone detects its number in the incoming data
`stream, it sends its identification back to the system. The
`system uses a digital message on the control channel to
`30
`designate a channel for the telephone to use. The tele
`phone tunes to this channel and the user is then alerted
`to the incoming call. A similar sequence is involved
`when a cellular telephone user originates a call. The
`user dials the desired telephone number into a register in
`35
`the telephone. This number is transmitted over the con
`trol channel to the nearest cell (i.e., the cell with the
`strongest carrier). The system computer then designates
`a channel for the call and the mobile unit is automati
`cally tuned to that channel.
`The cellular telephone industry has enjoyed wide
`spread success in its relatively brief lifetime. New sub
`scribers, apparently recognizing the many advantages
`in being able to initiate and receive calls while away
`from home, are being enrolled in ever-increasing num
`45
`bers. Indeed, in many cities, the competition between
`the A and B sides to enlist new subscribers is ?erce.
`Accordingly, there is a great need for new services to
`offer current and potential subscribers. The present
`invention sprang from the recognition that mobility, the
`main advantage offered by a cellular system, is also a
`disadvantage in certain situations. For example, a lost or
`stolen cellular telephone is dif?cult to recover. Thus, a
`system that could automatically locate the telephone
`would be quite bene?cial to users. In addition, if the
`cellular telephone were in an automobile and the auto
`mobile were stolen, a system that could locate the tele
`phone would also be able to locate the automobile, thus
`providing a valuable service to users. Moreover, there
`are situations where the user of a cellular telephone may
`60
`become lost. An example of such a situation is where
`the user is driving in an unknown area at night with his
`telephone in the car. Again, it would be a great advan
`tage for the system to be able to automatically locate the
`telephone and, upon request, inform the user of his
`location. Similarly, a cellular telephone user experienc
`ing a medical emergency who dials an emergency tele
`phone number (for example, 91 1) may not be able to tel]
`
`SUMMARY OF THE INVENTION
`The present invention provides a cellular telephone
`location system for determining the locations of multi
`ple mobile cellular telephones each initiating periodic
`signal transmissions over one of a prescribed set of
`control channels. The invention may be embodied in a
`system that employs much of the existing infrastructure
`of a cellular system. For example, as described below in
`greater detail, a cellular telephone location system in
`accordance with the present invention may employ the
`cellular system’s towers and cell site enclosures. In this
`sense, the cellular telephone location system may be
`overlaid on the cellular system.
`There are numerous advantages provided by moni
`toring control channels to track the locations of cellular
`telephones. First, a voice channel is an expensive and
`relatively scarce resource. Cellular systems typically
`require approximately six to eight seconds to allocate a
`voice channel to a speci?c telephone. If voice channels
`were employed for location tracking, the cellular tele
`phone would have to be called and commanded to
`initiate a voice channel call every time a location sample
`were to be taken. This would be both expensive and
`time consuming. Thus, it would be extremely inef?cient
`for a location system to require the telephone to initiate
`periodic voice channel transmissions. Second, each
`voice channel transmission adds a call record in an
`associated billing system. Therefore, a large burden
`would be placed on the billing system if the location
`system were to require periodic voice channel transmis
`sions. In contrast, control channel transmissions already
`occur periodically in cellular systems. Thus, the present
`invention is compatible with existing cellular telephone
`protocols and would not require the cellular system or
`the individual cellular telephones to be modi?ed. Third,
`since the frequency of control channel transmissions is
`software controllable, a location system in accordance
`with the present invention could control the frequency
`of control channel transmissions and offer different
`subscribers different location information update rates.
`Fourth, another advantage afforded by monitoring con
`trol channel transmissions is in connection with energy
`ef?ciency. Control channel transmissions are very short
`and require little power in comparison to voice channel
`transmissions. Accordingly, requiring periodic voice
`channel transmissions would cause a signi?cant battery
`drain in the individual cellular telephones. This is
`avoided by monitoring control channels.
`Accordingly, there are significant advantages af
`forded by monitoring periodic control channel trans
`missions to automatically locate mobile cellular tele
`phones. However, monitoring control channels requires
`detection of such weak, short duration signals that have
`travelled large distances (for example, twenty-?ve
`miles). The present inventors have developed highly
`sophisticated signal processing methods and apparatus
`to detect extremely brief, low power control channel
`signals. Both the concept of monitoring periodic con
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`5
`trol channel transmissions, as opposed to voice channel
`transmissions, and the particular way in which this
`function is carried out represent signi?cant technologi
`cal advancements.
`An exemplary embodiment of the present invention
`comprises at least three cell site systems and a central
`site system. Each cell site system comprises an elevated
`ground-based antenna; a baseband convertor for receiv
`ing cellular telephone signals transmitted by the cellular
`telephones and providing baseband signals derived from
`the cellular telephone signals; a timing signal receiver
`for receiving a timing signal common to all cell sites;
`and a sampling subsystem for sampling the baseband
`signal and formatting the sampled signal into frames of
`digital data. Each frame includes a prescribed number
`15
`of data bits and time stamp bits, wherein the time stamp
`bits represent the time at which the cellular telephone
`signals were received. The central site system com
`prises means for processing the frames of data from the
`cell site systems to generate a table identifying individ
`ual cellular telephone signals and the differences in
`times of arrival of the cellular telephone signals among
`the cell site systems; and means for determining, on the
`basis of the times of arrival, the locations of the cellular
`telephones responsible for the cellular telephone sig
`nals.
`In one preferred embodiment of the invention, the
`central site system comprises a correlator for cross-cor
`relating the data bits of each frame from one cell site
`with the corresponding data bits of each other cell site.
`In addition, this preferred embodiment comprises a
`database for storing location data identifying the cellu
`lar telephones and their respective locations, and means
`for providing access to the database to subscribers at
`remote locations. The system also comprises means for
`providing location data to a speci?c cellular telephone
`user upon request by using, for example, CPDP without
`setting up a voice call (“CPDP” represents the Cellular
`Packet Data Protocol, which involves sending data
`over voice channels when the voice channels would not
`otherwise be in use). The latter feature is especially
`useful in connection with laptop or handheld computers
`having cellular modems and mapping software.
`Embodiments of the invention may also advanta
`geously include means for merging the location data
`with billing data for the cellular telephones and generat
`ing modified billing data. In this embodiment, the billing
`data indicates the cost for each telephone call made by
`the cellular telephones within a certain time period, the
`cost being based upon one or more predetermined bill
`ing rates, and the modi?ed billing data is based upon a
`different rate for calls made from one or more pre
`scribed locations. For example, the system may apply a
`lower billing rate for telephone calls made from a user’s
`home or office or other geographic locale.
`Embodiments of the invention may also advanta
`geously include means for transmitting a signal to a
`selected cellular telephone to cause the selected tele
`phone to transmit a signal over a control channel. Such
`capability would allow the system to immediately 10
`cate that telephone without waiting for one of its peri
`odic control channel transmissions.
`In addition, embodiments of the invention may com
`prise means for automatically sending location informa
`tion to a prescribed receiving station in response to
`receiving a distress signal from a cellular telephone.
`With this capability, emergency assistance may be pro
`vided to a user in distress. For example, when a user
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`6
`dials “911” the system would automatically tell an
`emergency dispatcher the user’s location.
`Another element of a preferred embodiment is a
`means for comparing the current location of a given
`telephone with a prescribed range of locations and indi
`cating an alarm condition when the current location is
`not within the prescribed range. Such an element could
`be used, for example, to notify a parent when the child,
`who borrowed the parent’s car and cellular telephone
`to “go to the mall,” has in fact gone somewhere else. Of
`course, many other applications of such an alarm func
`tion are possible.
`Yet another element of a preferred embodiment is a
`means for detecting a lack of signal transmissions by a
`given telephone and in response thereto automatically
`paging the given telephone to cause it to initiate a signal
`transmission. This would allow the system to locate a
`telephone that has failed to register itself with the cellu
`lar system. Such a lack-of-signal-transmission detection
`feature could be used, for example, to generate an alarm
`for subscribers at remote locations.
`In addition, preferred embodiments may also include
`means for estimating a time of arrival of a given tele
`phone at a prespeci?ed location. This would be useful,
`for example, in connection with a public transportation
`system to provide quasi-continuous estimated times of
`arrival of busses along established routes. Of course,
`many other applications of this feature are also possible.
`Embodiments of the present invention may also com
`prise means for continuously tracking a given telephone
`by receiving voice signals transmitted by the given
`telephone over a voice channel and determining the
`location of the given telephone on the basis of the voice
`signals. This voice channel tracking could be used as an
`adjunct to control channel tracking. This feature would
`require the location system to track the channel assign
`ment of each telephone whose location is to be deter
`mined. The tracking of channel assignments by the
`location system could employ the dynamic channel
`assignment protocol employed by the cellular system.
`The present invention also provides methods for
`determining the location of one or more mobile cellular
`telephones. Such methods comprise the steps of: (a)
`receiving the signals at at least three geographically
`separated cell sites; (b) processing the signals at each
`cell site to produce frames of data, each frame compris
`ing a prescribed number of data bits and time stamp bits,
`the time stamp bits representing the time at which the
`frames were produced at each cell site; (c) processing
`the frames of data to identify individual cellular tele
`phone signals and the differences in times of arrival of
`the cellular telephone signals among the cell sites; and
`(d) determining, on the basis of the times of arrival, the
`locations of the cellular telephones responsible for the
`cellular telephone signals.
`One preferred embodiment of the inventive method
`comprises estimating the location of a cellular telephone
`by performing the following steps: (1) creating a grid of
`theoretical points covering a prescribed geographic
`area, the theoretical points being spaced at prescribed
`increments of latitude and longitude; (2) calculating
`theoretical values of time delay for a plurality of pairs of
`cell sites; (3) calculating a least squares difference
`(LSD) value based on the theoretical time delays and
`measured time delays for a plurality of pairs of cell sites;
`(4) searching the entire grid of theoretical points and
`determining the best theoretical latitude and longitude
`for which the value of LSD is minimized; and (5) start
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`ing at the best theoretical latitude and longitude, per
`forming another linearized-weighted-least-squares iter
`ation to resolve the actual latitude and longitude to
`within a prescribed number of degrees or fraction of a
`degree. Preferably, the calculating step (2) comprises
`accounting for any known site biases caused by me
`chanical, electrical, or environmental factors, the site
`biases determined by periodically calculating the posi
`tions of reference cellular transmitters at known loca
`tions.
`In addition, the least squares difference is preferably
`given by:
`-
`
`15
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`20
`
`where, Delay__T,,y represents the theoretical delay be
`tween cell sites ); and y, x and y being indices represen
`tative of cell sites; Delaye_.Oxy represents the observed
`delay between cell sites it and y; Qxy represents a quality
`factor for the delay measurement between cell sites it
`and y, the quality factor being an estimated measure of
`the degree to which multipath or other anomalies may
`have affected a particular delay measurement.
`25
`Further, the inventive method may advantageously
`include detecting a ?rst leading edge of a cellular tele
`phone signal and rejecting subsequent leading edges of
`the cellular tele