METHOD AND SYSTEM FOR LOCATING A MOBILE SUBSCRIBER IN A CDMA COMMUNICATION SYSTEM

Abstract: The invention determines the geographic location of a subscriber unit within a CDMA communication system. At least one base station transmits a spread spectrum signal with a chip code sequence unique to that base station. A subscriber unit receives the base station signal and transmits a spread spectrum signal wiht a unique chip code sequence time synchronized with the chip code sequence of the received base station signal. The base station receives the subscriber unit signal and compares the chip code sequence of the received subscriber unit singal with the chip code sequence signal transmitted by the base station to determine the location of the subscriber unit.

Description: METHOD AND SYSTEM FOR LOCATING A MOBILE SUBSCRIBER IN A CDMA COMMUNICATION SYSTEM Field of the Invention This invention generally relates to spread spectrum code division multiple access (CDMA) communication systems. More particularly, the present invention relates to a system and method that determines the geographic location of a subscriber unit within a CDMA communication system. Description of the Prior Art Wireless systems capable of locating a subscriber are presently known in the art. One wireless technique uses the global positioning system (GPS). In GPS, the communication handset receives data transmitted continuously from the 24 NAVSTAR satellites. Each satellite transmits data indicating the satellite' s identity, the location of the satellite and the time the message was sent. The handset compares the time each signal was received with the time it was sent to determine the distance to each satellite. Using the determined distances between the satellites and the handset along with the location of each satellite, the handset can triangulate its location and provide the information to a communication base station. However, the incorporation of a GPS within a subscriber unit increases its cost. Another subscriber location technique is disclosed in U.S. Patent No. 5,732,354. A mobile telephone using time division multiple access (TDMA) as the air interface is located within a plurality of base stations. The mobile telephone measures the received signal strength from each of the base stations and transmits each strength to each respective base station. At a mobile switching center, the received signal strengths from the base stations are compared and processed. The result yields the distance between the mobile telephone and each base station. From these distances, the location of the mobile telephone is calculated. Wireless communication systems using spread spectrum modulation techniques are increasing in popularity. In code division multiple access (CDMA) systems, data is transmitted using a wide bandwidth (spread spectrum) by modulating the data with a pseudo random chip code sequence. The advantage gained is that CDMA systems are more resistant to signal distortion and interfering frequencies in the transmission path than communication systems using the more common time division multiple access (TDMA) or frequency division multiple access (FDMA) techniques. There exists a need for an accurate mobile subscriber unit location system that uses data already available in an existing CDMA communication system. SUMMARY OF THE INVENTION The invention determines the geographic location of a subscriber unit within a CDMA communication system. At least one base station transmits a spread spectrum signal with a chip code sequence unique to that base station. A subscriber unit receives the base station signal and transmits a spread spectrum signal with a unique chip code sequence time synchronized with the chip code sequence of the received base station signal. The base station receives the subscriber unit signal and compares the chip code sequence of the received subscriber unit signal with the chip code sequence signal transmitted by the base station to determine the location of the subscriber unit. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of a simplified, prior art CDMA system. Figure 2 is an illustration of a prior art CDMA system. Figure 3 is a block diagram of major components within a prior art CDMA system. Figure 4 is a block diagram of components within a prior art CDMA system. Figure 5 is an illustration of a global pilot signal and an assigned pilot signal being communicated between a base station and a subscriber unit. Figure 6 is a block diagram of a first embodiment of the present invention using at least three base stations. Figure 7 is an illustration of locating a subscriber unit using the first embodiment of the present invention with at least three base stations. Figure 8 is a block diagram of a second embodiment of the present invention showing components used in a subscriber unit. Figure 9 is an illustration of locating a subscriber unit using the second embodiment of the present invention with two base stations. Figure 10 is an illustration of locating a subscriber unit using the second embodiment of the present invention with more than two base stations. Figure 11 is a detailed illustration of the third embodiment of the present invention having a base station with multiple antennas. Figure 12 is an illustration of the third embodiment having a base station with multiple antennas. Figure 13 is a block diagram of components used in the third embodiment. Figure 14 is an illustration of multipath. Figure 15 is a graph of a typical impulse response of multipath components. Figure 16 is a block diagram of components within a fourth embodiment correcting for multipath. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout. Shown in Figure 1 is a simplified CDMA communication system. A data signal with a given bandwidth is mixed with a spreading code generated by a pseudo random chip code sequence generator producing a digital spread spectrum signal. Upon reception, the data is reproduced after correlation with the same pseudo random chip code sequence used to transmit the data. Every other signal within the transmission bandwidth appears as noise to the signal being despread. For timing synchronization with a receiver, an unmodulated pilot signal is required for every transmitter. The pilot signal allows respective receivers to synchronize with a given transmitter, allowing despreading of a traffic signal at the receiver. In a typical CDMA system, base stations send global pilot signals to all subscriber units within their communicating range to synchronize transmissions in a forward direction. Additionally, in some CDMA systems, for example a B- CDMA™ system, each subscriber unit sends a unique assigned pilot signal to synchronize transmissions in a reverse direction. Figure 2 illustrates a CDMA communication system 30. The communication system 30 comprises a plurality of base stations 36 x , 36 2 ... 36 n . Each base station 36 1; 36 2 ... 36 n is in wireless communication with a plurality of subscriber units 40 l5 40 2 ... 40 n , which may be fixed or mobile. Each subscriber unit 40 α , 40 2 ... 40 n communicates with either the closest base station 36j or the base station 36 j which provides the strongest communication signal. Each base station 36 36 2 ... 36 n is in communication with other components within the communication system 30 as shown in Figure 3. A local exchange 32 is at the center of the communications system 30 and communicates with a plurality of network interface units (NIUs) 34 x , 34 2 ... 34 n . Each NIU is in communication with a plurality of radio carrier stations (RCS) 38 38 2 ... 38 n or base stations 36 l5 36 2 ... 36 n . Each (RCS) 38 l5 38 2 ... 38 n or base station 36 l5 36 2 ... 36 n communicates with a plurality of subscriber units 40 l5 40 2 ... 40 n within its communicating range. Figure 4 depicts a block diagram of the pertinent parts of an existing spread spectrum CDMA communication system. Each independent base station 36 l5 36 2 ... 36 n generates a unique global pilot signal using a global pilot chip code generating means 42 x and spread spectrum processing means 44j. The global pilot chip code generating means 42 x generates a unique pseudo random chip code sequence. The unique pseudo random chip code sequence is used to spread the resultant signals bandwidth such as to 15 MHZ as used in the B-CDMA™ air interface. The spread spectrum processing means modulates the global pilot chip code sequence up to a desired center frequency. The global pilot signal is transmitted to all subscriber units 40 j by the base station's transmitter 46-,. A receiver 48 x at a subscriber unit 40 x receives available signals from a plurality of base stations 36 x , 36 2 ... 36 n . As shown in Figure 5, the global pilot 50 x travels from the base station 36 x to the subscriber unit 40 x and can be represented as: τ 7 = d j . c Equation (1) The time the signal travels from the base station 36 α to the subscriber unit 40 x , τ 7 , equals the distance between the base station 36 x and subscriber unit 40 1? d j , divided by the speed of light, c. Referring back to Figure 4, a global pilot chip code recovery means 54 x within the subscriber unit 40 x can receive global pilot chip code sequences from a plurality of base stations 36 x , 36 2 ... 36 n . The subscriber unit 40 x generates a replica of a global pilot chip code sequence and synchronizes the generated replica' s timing with the received global pilot 50j. The subscriber unit 40 x also has a processor 82 x to perform the many analysis functions of the subscriber unit 40 x . The subscriber unit 40 x generates an assigned pilot signal 52 x using assigned pilot chip code generating means 56 x and spread spectrum processing means 58 x . The assigned pilot chip code generating means 56 x generates a pseudo random chip code sequence with its timing synchronized with the recovered global pilot chip code sequence. As a result, the assigned pilot chip code sequence is delayed by τ x with respect to the base station 36 x , 36 2 ...36 n . The spread spectrum processing means 58 x generates the assigned pilot signal 52 x by modulating the assigned pilot chip code sequence up to a desired center frequency. The assigned pilot signal 52 x is transmitted to all base stations 36 1? 36 2 ... 36 n within range to receive the assigned pilot signal 52 x . The base station 36 x receives the assigned pilot signal 52 x with the base station's receiver 62 x . The received assigned pilot 52 x travels the same distance d l as the global pilot signal 50 x as shown in Figure 5. Accordingly, the received assigned pilot signal will be delayed by τ 1 with respect to the mobile unit 40 x and by 2τ 7 with respect to the global pilot 50 x generated at the base station 36 x . Since the chip code sequence of the assigned pilot 52 x received at the base station 36 x will be delayed by 2τ with respect to the chip code sequence of the global pilot signal 50 x generated at the base station 36 x , the round trip propagation delay, 2τ 2 , can be determined by comparing the timing of the two chip code sequences. Using the round trip propagation delay, 2τ 7 , the distance d 1 between the base station 36 x and subscriber unit 40 x can be determined by: d 1 - c 2τ 7 . 2 Equation (2) If a spreading sequence having a chipping rate of at least 80ns is used and the communication system has the ability to track 1/16 th of a chip, the distance d j can be measured to within 2 meters. Figure 6 is a block diagram of a first embodiment of the present invention. No additional hardware is required in the subscriber unit 40 x . The only changes are implemented by software within the subscriber unit's processor 82 x and the processors 66 α , 66 2 ...66 n , 68, 70 x , 70 2 ...70 n located within the base station 36 x , NIU 34 x or Local Exchange 32 x , Precincts 74 α , 74 2 ... 74 n and Ambulance Dispatcher 76. The subscriber unit 40 x is sent a signal by a base station 36 x indicating that a 911 call was initiated and to begin the subscriber location protocol. Upon receipt, the subscriber unit 40 x will sequentially synchronize its transmission chip code sequence to at least three base stations' chip code sequences. To allow reception by the base stations 36 2 , 36 3 ... 36 n outside of the subscriber unit's normal communicating range, these transmissions will be sent at a higher than normal power level temporarily over-riding any adaptive power control algorithms. A processor 66 x within each base station 36 l5 36 2 ... 36 n is coupled to the assigned pilot chip code recovery means 64 x and the global pilot chip code generator 42 x . The processor 66 x compares the two chip code sequences to determine the round trip propagation delay τ x , τ 2 ... τ n and the respective distance d d 2 ... d n between the subscriber unit 40 x and the respective base station 36 α , 36 2 ... 36 n . Within either a NIU 34 x or the local exchange 32, a processor 68 receives the distances d lt d 2 ... d n from the processors 66 x , 66 2 ... 66 n within all the base stations 36 x , 36 2 ... 36 n . The processor 68 uses the distances d v d 2 ... d n to determine the location of the subscriber unit 40 x as follows. By using the known longitude and latitude from three base stations 36 l5 36 2 , 36 3 and distances d d 2 , d 3 , the location of the subscriber unit 40 x is determined. As shown in Figure 7 by using the three distances d v d 2 , d 3 , three circles 78 x , 78 2 , 78 3 with radiae 80 1? 80 2 , 80 3 are constructed. Each circle 78 x , 78 2 , 78 3 is centered around a respective base station 36 x , 36 2 , 36 3 . The intersection of the three circles 78 x , 78 2 , 78 3 is at the location of the subscriber unit 40 x . Using the Cartesian coordinates, the longitude and latitude corresponding with each base station 36 x , 36 2 ... 36 n is represented as X n , Y n , where X n is the longitude and Y n is the latitude. If X, Y represents the location of the subscriber unit 40 x , using the distance formula the following equations result: (X -X) 2 +(Xι-Y 2 = d t 2 Equation (3) (X 2 -X) 2 +(Y 2 -Y) 2 = d 2 3 Equation (4) (X 3 -X) 2 +(Y 3 -Y) 2 = d 3 2 Equation (5) In practice due to small errors in calculating the distances d d 2 , d 3 , Equations 3, 4 and 5 cannot be solved using conventional algebra. To compensate for the errors, a maximum likelihood estimation is used to determine the location and are well known to those skilled in the art. For increased accuracy, additional base stations 36 4 , 36 5 ... 36 n can be used to calculate additional distances for inclusion in the estimation analysis. The subscriber unit's location is sent through the communication system 30 to at least one precinct 74 x , 74 2 ... 74 n and an ambulance dispatcher 76. A processor 70 x within each precinct 74 x , 74 2 ...74 n and the ambulance dispatcher 76 receives the location of all 911 calls originating in the system and displays the location on a conventional computer monitor 72 x . The display comprises a listing of all 911 calls and addresses on a geographic map. An alternate approach reduces the number of processors by transmitting raw data through the communication system 30 and processing the raw data at a single site. Figure 8 is a second embodiment of a location system. At least two base stations 36 x , 36 2 ... 36 n have their internal timing synchronized with each other and transmit their respective global pilot signals 52 x , 52 2 ... 52 n with time synchronized chip code sequences. The subscriber unit 40 x receives the global pilots 52 l5 52 2 ... 52 n . However, the received global pilots 52 x , 52 2 ... 52 n are not synchronized. The global pilot 52 x from a first base station 36 x will travel distance dj and is delayed by τ ; . The global pilot 52 2 from a second base station 36 2 travels distance d 2 and is delayed by τ 2 . The subscriber unit 40 x recovers each base station' s global pilot chip code sequence with its global pilot chip code recovery means 54 x . A processor 82 x within the subscriber unit 40 x is coupled to each global pilot chip code recovery means 54 x , 52 2 ... 54 n . The processor 82 x compares the chip code sequences of each pair of pilot chip code sequences and calculates the time differences Δt x , Δtj ... Δt n between the sequences as follows. Within the subscriber unit 40 l5 the chip code sequences used by each base station 36 α , 36 2 ... 36 n are stored. After synchronizing with the first base station's pilot 36 , the processor 82 x will store where within the sequence synchronization was obtained. This process is repeated for the other base stations 36 2 , 36 3 ... 36 n . The synchronization process can be done sequentially (synchronizing to the first base station's chip code sequence then the second, etc.) or in parallel (synchronizing to all base stations at the same time). By using the relative time difference between τ l5 τ 2 , ... τ n each base station's chip code sequence and knowing that each base station's pilot was sent at the same time, with two base stations the time differences are calculated as follows: Atj =τ 2 -Tj Equation (6) Δt 2 = τ 3 -τ 2 Equation (7) The time differences Δt x , Δtj ... Δt n are transmitted to at least one of the base stations 36 x . At least one base station 36 x recovers the time difference data from the received signals using time difference recovery means 84 x . The time difference data is sent with the distance data dj through the communications system to a processor 68. The processor 68 determines the location of the subscriber unit 40 x using the time difference data Δt x , Δt^ ... Δt n and the distance data d j , d 2 ... d„ as follows. Using information from only two base stations 36 x , 36 2 as shown in Figure 9, the processor uses distances d v d 2 to create two circles 78 x , 78 2 . Using the time difference, Atj, a hyperbola 86 x can be constructed as follows. All the points along the hyperbola 86 x receive the global pilot signals 52 x , 52 2 from the synchronized base stations 36 l5 36 2 with the same time difference, At j . The time difference At, can be converted to a distance difference Adj by substituting At, for t j and Ad 1 for d x in Equation 1. Using the distance formula and X, Y as the location of the subscriber unit 40 x , the following equation results: Δ j ^(X. -Xf+W^YΫ-^-Xf+^-Y) 2 Equation (8) By using Equation 8 with Equations 3 and 4 in a maximum likelihood estimation, the location of the subscriber unit 40 x can be determined. The subscriber unit's location is subsequently sent to the nearest police precinct 74 l5 74 2 ... 74 n and ambulance dispatcher 76 in the cellular area. For improved accuracy, additional base stations 36 x , 36 2 ... 36 n are used. Figure 10 shows the invention used with three base stations 36 l5 36 2 , 36 3 . The distances d 2 , d 2 , d 3 are used to create three circles 78 x , 78 2 , 78 3 . Using time differences Δt l5 Δtj, two intersecting hyperbolas 86 x , 86 2 are constructed. With maximum likelihood estimation, the subscriber units' location calculated with two hyperbolas 86 l5 86 2 , and three circles 78 x , 78 2 , 78 3 yields greater accuracy. As shown in Figure 8, the subscriber unit 40 x is required to process each global pilot chip code sequence to determine the time differences Δt x , Δt 2 ... Δt n . An alternate approach removes the processing from the subscriber unit 40 x . With reference to Figure 6, the mobile unit 40 x will synchronize the assigned pilot to one of the base station's global pilot chip code sequences, such as the nearest base station 36 x with a delay of τ 7 . The assigned pilot 50 x is transmitted to all base stations 36 x , 36 2 ... 36 n . The assigned pilot 50 x will be received at each base station with a respective delay, τ^ + τ 7 , τ x + τ 2 , τ ; + τ 3 . Each base station 36 l5 36 2 ... 36 n will send the delayed chip code sequence along with the calculated distance to a processor 68 located in a NIU 34 x or local exchange 32. The processor 68 will calculate the time differences Δt x , Δ^ ... Δt n by comparing the received assigned pilot chip code sequences. Since all received assigned pilot chip code sequences are delayed by τ /5 the τ } delay will cancel out of the resultant time differences Δt x , Δ^ ... Δt n . Accordingly, the subscriber unit 40 x can be located using hyperbolas 86 x , 86 2 as previously described. Another embodiment shown in Figures 11, 12 and 13 uses a base station 36 x with multiple antennas 88 l5 88 2 ... 88 n . Two of the antennas 88 1? 88 2 lie along a centerline 92 at a known distance, /, apart as shown in Figure 11. Both antennas 88 j , 88 2 receive the assigned pilot signal 90 x , 90 2 from the subscriber unit 40 x . However, the antenna 88 2 further away from the subscriber unit 40 x receives the signal over a slightly longer distance d and with a slight delay with respect to the nearer antenna 88 x . This delay results in a carrier phase difference, φ, between the signals received at each antenna as shown on Figure 13. A processor 66 using the received carrier phase difference and the chip code sequence recovered by each assigned pilot chip code recovery means 96 96 2 ... 96 n can determine the location of the subscriber unit 40 x as follows. As shown in Figure 12, the subscriber unit 40 x is located at distance d j at angle α from the centerline 92 of the antennas 88 x , 88 2 . As seen at the scale of Figure 12 both received assigned pilot signals 90 x , 90 2 appear to be coincident. However, as shown in Figure 11, the received assigned pilot signals 90 , 90 2 are slightly separated. The received assigned pilot signal 90 1 returning to the first antenna 88 x travels a distance d The received assigned pilot signal 90 2 returning to the second antenna 88 2 travels a slightly longer distance d . As shown in Figure 11, the difference between the two distances d v d is a distance m. Since the distances d d/ between the antennas 88 x , 88 2 and the subscriber unit 40j are much larger than the distance I between the antennae 88 x , 88 2 both received assigned pilot signals 90 α , 90 2 follow approximately parallel paths. By constructing a right triangle using a point 94 which is distance d t from the subscriber unit 40 x as shown in Figure 11, the angle « can be determined by the following geometric relationship: o = COS 1 (mil). Equation (9) The distance m can be determined by using the carrier phase difference, φ, between the two received signals 90 x , 90 2 as follows: -λ m = Equation (10) 2π The distance m equals the phase difference between the two signals, φ, in radians multiplied by the wavelength of the signal, λ, divided by 2π. The wavelength, λ, can be derived from the known frequency / of the assigned pilot signal as follows: λ = c/f. Equation (11) The processor 68 also compares the chip code sequences of the global pilot generating means 42 x with the recovered assigned pilot chip code sequence to determine the distance d 2 as shown in Figure 6. Using both the angle « and distance d j , the processor 66 x locates the subscriber unit 40 x using simple geometry. There are many techniques well known to those skilled in the art to eliminate the ambiguity between locations above and below the antennas 88 x , 88 2 . One such technique is using antennas employing sectorization. Subsequently, the subscriber unit's location is sent to the precincts 74 x , 74 2 ... 74 n and ambulance dispatcher 76. Additional antennas may be used to improve on the accuracy of the system. An alternate embodiment uses more than one base station 36 x , 36 2 ... 36 n . A processor 68 located within either a NIU 34 x or the local exchange 32 collects distance and angle information from more than one base station 36 x , 36 2 ... 36 n as well as the time differences Δt x , Δtj ... Δt n , between the base stations 36 x , 36 2 ...36 n . Using the maximum likelihood estimation technique, the processor 68 determines a more accurate location of the subscriber unit 40 x . A fourth embodiment corrects for multipath. Figure 14 illustrates multipath. A signal such as a global pilot signal is transmitted from a base station 36 x . The signal follows a multitude of paths 98 x , 98 2 ... 98 n between the base station 36 x and subscriber unit 40 x . Figure 13 is a graph showing the impulse response 136 of the received multipath components. Since each received multipath component travels a unique path, it arrives at a receiver with a propagation delay determined by the length of the path 98 x , 98 2 ... 98 n . The impulse response 106 shows the collective signal magnitude of all the multipath components received at each propagation delay. The previously described subscriber unit location techniques assumed the subscriber unit 40 x synchronizes with the line of sight multipath component 98 x traveling distance d However, if the subscriber unit synchronizes with a non-line of sight multipath component 98 x , 98 2 ... 98„, the distance calculation will be in error due to the delay MD X as shown in Figure 15. Figure 16 is a system correcting for errors resulting from multipath. The global pilot 50 x is sent from the base station 36 x to subscriber unit 40 x . The subscriber unit 40 x collects all of the multipath components using a multipath receiver 102 x such as disclosed in U.S. Patent Application No. 08/669,769, Lomp et al., incorporated here by reference. A processor 82 x within the subscriber unit 40 x analyzes the impulse response 100 of the received global pilot signal 50 x . Since the line of sight multipath component 98 x travels the shortest distance d j , the first received component 98 x is the line of sight component. If the line of sight component is not received, the first received component 98 x will be the closest and, accordingly, the best available estimate for the line of sight component. The processor 82 x compares the chip code sequence of the first received component 98 x with the chip code sequence used to synchronize the assigned pilot chip code sequence. This comparison determines the delay due to multipath, MD X . The multipath delay, MD X , is transmitted to the base station 36 x . A processor 66 x and multipath receiver 104 x within the base station 36 x perform the same analysis on the received assigned pilot signal. As a result, the multipath delay, MD 2 of the assigned pilot signal is determined. Additionally, multipath delay recovery means 106 x recovers the transmitted global pilot signal's multipath delay MD X for use by the processor 66 x . The processor 66 x compares the generated global pilot chip code sequence to the recovered assigned pilot chip code sequence to determine the round trip propagation delay 2τ . To correct for multipath, the processor 66 x subtracts both the global pilot signal's multipath delay MD X and the assigned pilot signals multipath delay MD 2 from the calculated round trip propagation delay, 2τ x . The corrected round trip propagation delay is used to determine the subscriber unit's location in one of the techniques as previously described. Although the invention has been described in part by making detailed reference to certain specific embodiments, such detail is intended to be instructive rather than restrictive. It will be appreciated by those skilled in the art that many variations may be made in the structure and mode of operation without departing from the scope of the invention as disclosed in the teachings herein.

Claims: CLAIMS 1. A method for geographically locating a subscriber unit within a CDMA communication system having base stations with fixed locations, the method comprising: , (a) transmitting a first spread spectrum signal with a first pseudo random chip code sequence from a base station; (b) receiving of said first spread spectrum signal including first signal multipath components at said subscriber unit; (c) determining a first received component of the received first signal multipath components; (d) transmitting a second spread spectrum signal with a second pseudo random chip code sequence from said subscriber unit, said second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal; (e) receiving said second spread spectrum signal including second signal multipath components at said base station; (f) determining a first received component of the received second signal multipath components; (g) determining a distance between said base station and said subscriber unit based on the first received component of said second signal; and (h) determining said subscriber unit's geographic location based on said distance determination. 2. The method of claim 1 wherein steps (a)-(g) are performed respectively for each of three base stations, each base station associated with a different fixed location, and step (h) determines said subscriber unit's geographic location based on said distance determinations for each of said three base stations. 3. The method of claim 1 further comprising the steps of: performing steps (a)-(g) for both a first and a second base station, each base station associated with a different fixed location; time synchronizing said first base station's first spread spectrum signal with said second base station's first spread spectrum signal; and determining a time difference between said first base station' s and said second base station's first spread spectrum signals received at said subscriber unit; and wherein step (h) determines said subscriber unit's geographic location using said distance determinations and said determined time difference. 4. The method of claim 1 further comprising the steps of: performing steps (a)-(g) for both a first and a second base station, each base station associated with a different fixed location; receiving at said second base station said second spread spectrum signal which is time synchronized with said first base station' s spread spectrum signal, said first base station's second signal received at said second base station including first base station's second signal multipath components; determining a first received component of said received first base station's second signal multipath components at said second base station; and determining a time difference between said first base station' s second spread spectrum signal received at said first base station and at said second base station; and wherein step (h) determines said subscriber unit's geographic location using said distance determinations and said determined time difference. 5. The method of claim 1 wherein said base station has first and second antennas each associated with a different fixed location and said second spread spectrum signal is received separately by said first and second antennas, further comprising the steps of: determining the phase difference between the respective second spread spectrum signals received by said first and second antennas; and calculating an angle associated with said base station and said subscriber unit based on said phase difference and the distance between the first and second antenna's fixed locations; and wherein step (h) determines said subscriber unit's geographic location based on said distance determination and said calculated angle. 6. The method of claim 1 wherein said first signal and said second signal are pilot signals. 7. The method of claim 1 wherein said base station's fixed location is associated with a latitude and a longitude and step (h) determines said subscriber unit's geographical location based on said latitude, said longitude, and said distance determination. 8. The method of claim 1 further comprising the step of displaying said subscriber unit's geographic location on a map. 9. The method of claim 1 further comprising the step of displaying said subscriber unit's geographic location as an address. 10. The method of claim 1 wherein step (h) determines said subscriber unit's geographic location using said distance determination in a maximum likelihood estimation. 11. A method for determining a distance between a subscriber unit and a base station in a CDMA communication system, the method comprising: (a) transmitting a first spread spectrum signal with a first pseudo random chip code sequence from a base station; (b) receiving of said first spread spectrum signal including first signal multipath components at said subscriber unit; (c) determining a first received component of the received first signal multipath components; (d) transmitting a second spread spectrum signal with a second pseudo random chip code sequence from said subscriber unit, said second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal; (e) receiving said second spread spectrum signal including second signal multipath components at said base station; (f) determining a first received component of the received second signal multipath components; and (g) determining a distance between said base station and said subscriber unit based on the first received component of the second signal. 12. A spread spectrum CDMA communication system capable of geographically locating a subscriber unit, the system comprising: means for determining said subscriber unit's geographic location based on a distance determination; and at least one base station, each of said at least one base stations comprising: means for transmitting a first spread spectrum signal with a first pseudo random chip code sequence; means for receiving a second spread spectrum signal including second signal multipath components; means for determining a first received component of said received second signal multipath components; and means for determining a distance between said base station and said subscriber unit as said distance determination based on the first received component of said second signal; and said subscriber unit comprising: means for receiving said first spread spectrum signal including first signal multipath components; means for determining a first received component of said received first signal multipath components; and means for transmitting a second spread spectrum signal with a second pseudo random chip code sequence, said second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal. 13. The system of claim 12 wherein : said at least one base station is three base stations, each base station associated with a different fixed location; said subscriber unit receiving means receives from each of said three base stations said respective first spread spectrum signal including respective first signal multipath components; said subscriber unit first component determining means determines a respective first received component for each of said respective received first signal multipath components; said subscriber unit transmitting means transmits three second spread spectrum signals, each associated with one of said three base stations, each with a respective second pseudo random chip code sequence, said respective second pseudo random chip code sequence being time synchronized with said respective received first spread spectrum signal from said respective one of said three base stations; and said geolocation determining means determines said subscriber unit's geographic location based on said distance determination from each of said three base stations 14. The system of claim 12 wherein: said at least one base station is two base stations, each base station associated with a different fixed location; said subscriber unit receiving means receives from each of said two base stations said respective first spread spectrum signal including respective first signal multipath components; said subscriber unit first component determining means determines a respective first received component for each respective received first signal multipath components; said subscriber unit transmitting means transmits two second spread spectrum signals, each associated with one of said two base stations, each with a respective second pseudo random chip code sequence, said respective second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal from said respective base station; said subscriber unit further comprising time difference means for determining a time difference between said first base station's and said second base station's first spread spectrum signals received at said subscriber unit; said geographic location determining means determines said subscriber unit's geographic location using said distance determinations and said time difference; and said first base station's first spread spectrum signal transmitted at said first base station is time synchronized with said second base station's first spread spectrum signal transmitted at said second base station. 15. The system of claim 12 wherein: said at least one base station is two base stations, each base station associated with a different fixed location; said first base station's first spread spectrum signal transmitted at said first base station is time synchronized with said second base station's first spread spectrum signal transmitted at said second base station; said subscriber unit receiving means receives from each of said two base stations said respective first spread spectrum signal including respective first signal multipath components; said subscriber unit first component determining means determines a respective first received component for each of said respective received first signal multipath components; said subscriber unit transmitting means transmits two second spread spectrum signals, each associated with one of said two base stations, each with a respective second pseudo random chip code sequence, each of said respective second pseudo random chip code sequences being time synchronized with said received first spread spectrum signal from said respective one of said two base stations; and said second base station further comprising: means for receiving said second spread spectrum signal which is time synchronized with said first base station' s spread spectrum signal, said received first base station's second signal including first base station's second signal multipath components; means for determining a first received component of said received first base station's second signal multipath components at said second base station; and said geographic location determining means determines said subscriber unit' s geographic location using said distance determinations and a determined time difference; and the system further comprising: means for determining a time difference between said first base station's second spread spectrum signal received at said first base station and at said second base station as said determined time difference. 16. The system of claim 12 wherein: said base station further comprising: a first and second antenna, each associated with a different fixed location and said second spread spectrum signal is received separately by said first and second antennas; means for determining the phase difference between the respective second spread spectrum signals received by said first and second antennas; and means for calculating an angle associated with said base station and said subscriber unit based on said phase difference and the distance between the first and second antennas fixed locations; and wherein said geographic location determining means determines said subscriber unit's geographic location based on said distance determination and said calculated angle. 17. The system of claim 12 further comprising means for displaying said subscriber unit's geographic location on a map. 18. The system of claim 12 further comprising means for displaying said subscriber unit's geographic location as a street address. 19. A CDMA communication system capable of determining a distance between a subscriber unit and a base station, the system comprising: means for determining a distance between said base station and said subscriber unit based on the first received component of the second signal; said base station comprising: means for transmitting a first spread spectrum signal with a first pseudo random chip code sequence; means for receiving a second spread spectrum signal including second signal multipath components; and means for determining a first received component of the received second signal multipath components; said subscriber unit comprising: means for receiving said first spread spectrum signal including first signal multipath components; means for determining a first received component of said received first signal multipath components; and means for transmitting a second spread spectrum signal with a second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal. 20. A subscriber unit for use in a CDMA communication system capable of geographically locating said subscriber unit, said subscriber unit comprising: means for receiving a first spread spectrum signal with a first pseudo random chip code sequence, said first signal including first signal multipath components; means for determining a first received component of said received first signal multipath components; means for determining a delay associated with said first received component; and means for transmitting a second spread spectrum signal with a second pseudo random chip code sequence and a signal including said determined delay, said second pseudo random chip code sequence being time synchronized with said received first spread spectrum signal.

Inventors: MESECHER DAVID K, MESECHER, DAVID, K.
Applicants: INTERDIGITAL TECH CORP, INTERDIGITAL TECHNOLOGY CORPORATION
Application year: 1999
Main classification: H04Q7
IPC codes: G01S5, H04B1, H04B7, H04W64