Base station for distance determination

Abstract

A base station transmits a first spread spectrum signal having a first code. It receives and analyzes an impulse response of multipath components of a second spread spectrum signal having a second code to determine a first received component. The second signal is time synchronized with the first spread spectrum signal. A distance determination is made based on in part a timing difference between the second code of the received second signal and the first code of the base station's transmitted first signal and the determined first received components for that base station's received second signal.

Description

BACKGROUND

[0002] 1. Field of the Invention

[0003] 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.

[0004] 2. Description of the Prior Art

[0005] 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.

[0006] Another subscriber location technique is disclosed in U.S. Pat. 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.

[0007] 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.

[0008] There exists a need for an accurate mobile subscriber unit location system that uses data already available in an existing CDMA communication system.

SUMMARY

[0009] A base station transmits a first spread spectrum signal having a first code. It receives and analyzes an impulse response of multipath components of a second spread spectrum signal having a second code to determine a first received component. The second signal is time synchronized with the first spread spectrum signal. A distance determination is made based on in part a timing difference between the second code of the received second signal and the first code of the base station's transmitted first signal and the determined first received components for that base station's received second signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

[0010] FIG. 1 is an illustration of a simplified, prior art CDMA system.

[0011] FIG. 2 is an illustration of a prior art CDMA system.

[0012] FIG. 3 is a block diagram of major components within a prior art CDMA system.

[0013] FIG. 4 is a block diagram of components within a prior art CDMA system.

[0014] FIG. 5 is an illustration of a global pilot signal and an assigned pilot signal being communicated between a base station and a subscriber unit.

[0015] FIG. 6 is a block diagram of a first embodiment of the present invention using at least three base stations.

[0016] FIG. 7 is an illustration of locating a subscriber unit using the first embodiment of the present invention with at least three base stations.

[0017] FIG. 8 is a block diagram of a second embodiment of the present invention showing components used in a subscriber unit.

[0018] FIG. 9 is an illustration of locating a subscriber unit using the second embodiment of the present invention with two base stations.

[0019] FIG. 10 is an illustration of locating a subscriber unit using the second embodiment of the present invention with more than two base stations.

[0020] FIG. 11 is a detailed illustration of the third embodiment of the present invention having a base station with multiple antennas.

[0021] FIG. 12 is an illustration of the third embodiment having a base station with multiple antennas.

[0022] FIG. 13 is a block diagram of components used in the third embodiment.

[0023] FIG. 14 is an illustration of multipath.

[0024] FIG. 15 is a graph of a typical impulse response of multipath components.

[0025] FIG. 16 is a block diagram of components within a fourth embodiment correcting for multipath.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0026] The preferred embodiments will be described with reference to the drawing figures where like numerals represent like elements throughout.

[0027] Shown in FIG. 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.

[0028] 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.

[0029] 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.

[0030] FIG. 2 illustrates a CDMA communication system 30. The communication system 30 comprises a plurality of base stations 361, 362 . . . 36n. Each base station 361, 362 . . . 36n is in wireless communication with a plurality of subscriber units 401, 402 . . . 40n, which may be fixed or mobile. Each subscriber unit 401, 402 . . . 40n communicates with either the closest base station 361 or the base station 361 which provides the strongest communication signal. Each base station 361, 362 . . . 36n is in communication with other components within the communication system 30 as shown in FIG. 3.

[0031] A local exchange 32 is at the center of the communications system 30 and communicates with a plurality of network interface units (NIUs) 341, 342 . . . 34n. Each NIU is in communication with a plurality of radio carrier stations (RCS) 381, 382 . . . 38n or base stations 361, 362 . . . 36n. Each (RCS) 381, 382 . . . 38n or base station 361, 362 . . . 36n communicates with a plurality of subscriber units 401, 402 . . . 40n within its communicating range.

[0032] FIG. 4 depicts a block diagram of the pertinent parts of an existing spread spectrum CDMA communication system. Each independent base station 361, 362 . . . 36n generates a unique global pilot signal using a global pilot chip code generating means 421 and spread spectrum processing means 441. The global pilot chip code generating means 421 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 401 by the base station's transmitter 461.

[0033] A receiver 481 at a subscriber unit 401 receives available signals from a plurality of base stations 361, 362 . . . 36n. As shown in FIG. 5, the global pilot 501 travels from the base station 361 to the subscriber unit 401 and can be represented as: 1$\begin{array}{cc}{\tau }_1=\frac{d_1}{c}.& \mathrm{Equation}\left(1\right)\end{array}$

[0034] The time the signal travels from the base station 361 to the subscriber unit 401, τ1, equals the distance between the base station 361 and subscriber unit 401, d1, divided by the speed of light, c.

[0035] Referring back to FIG. 4, a global pilot chip code recovery means 541 within the subscriber unit 401 can receive global pilot chip code sequences from a plurality of base stations 361, 362 . . . 36n. The subscriber unit 401 generates a replica of a global pilot chip code sequence and synchronizes the generated replica's timing with the received global pilot 501. The subscriber unit 401 also has a processor 821 to perform the many analysis functions of the subscriber unit 401.

[0036] The subscriber unit 401 generates an assigned pilot signal 521 using assigned pilot chip code generating means 561 and spread spectrum processing means 581. The assigned pilot chip code generating means 561 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 τ1 with respect to the base station 361, 362 . . . 36n. The spread spectrum processing means 581 generates the assigned pilot signal 521 by modulating the assigned pilot chip code sequence up to a desired center frequency. The assigned pilot signal 521 is transmitted to all base stations 361, 362 . . . 36n within range to receive the assigned pilot signal 521.

[0037] The base station 361 receives the assigned pilot signal 521 with the base station's receiver 621. The received assigned pilot 521 travels the same distance d, as the global pilot signal 501 as shown in FIG. 5. Accordingly, the received assigned pilot signal will be delayed by c with respect to the mobile unit 401 and by 2τ1 with respect to the global pilot 501 generated at the base station 361.

[0038] Since the chip code sequence of the assigned pilot 521 received at the base station 361 will be delayed by 2τ1 with respect to the chip code sequence of the global pilot signal 501 generated at the base station 361, the round trip propagation delay, 2τ1, can be determined by comparing the timing of the two chip code sequences. Using the round trip propagation delay, 2τ1, the distance d1, between the base station 361 and subscriber unit 401 can be determined by: 2$\begin{array}{cc}{d}_1=c·\frac{2{\tau }_1}{2}.& \mathrm{Equation}\left(2\right)\end{array}$

[0039] If a spreading sequence having a chipping rate of at least 80 ns is used and the communication system has the ability to track {fraction (1/16)}th of a chip, the distance d1 can be measured to within 2 meters.

[0040] FIG. 6 is a block diagram of a first embodiment of the present invention. No additional hardware is required in the subscriber unit 401. The only changes are implemented by software within the subscriber unit's processor 821 and the processors 661, 662 . . . 66n, 68, 701, 702 . . . 70n located within the base station 361, NIU 341 or Local Exchange 321, Precincts 741, 742 . . . 74n and Ambulance Dispatcher 76.

[0041] The subscriber unit 401 is sent a signal by a base station 361 indicating that a 911 call was initiated and to begin the subscriber location protocol. Upon receipt, the subscriber unit 401 will sequentially synchronize its transmission chip code sequence to at least three base stations' chip code sequences. To allow reception by the base stations 362, 363 . . . 36n 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.

[0042] A processor 661 within each base station 361, 362 . . . 36n is coupled to the assigned pilot chip code recovery means 641 and the global pilot chip code generator 421. The processor 661 compares the two chip code sequences to determine the round trip propagation delay τ1, τ2 . . . τn and the respective distance d1, d2 . . dn between the subscriber unit 401 and the respective base station 361, 362 . . . 36n. Within either a NIU 341 or the local exchange 32, a processor 68 receives the distances d1, d2 . . . dn from the processors 661, 662 . . . 66n within all the base stations 361, 362 . . . 36n. The processor 68 uses the distances d1, d2 . . . dn to determine the location of the subscriber unit 401 as follows.

[0043] By using the known longitude and latitude from three base stations 361, 362, 363 and distances d1, d2, d3, the location of the subscriber unit 401 is determined. As shown in FIG. 7 by using the three distances d1, d2, d3, three circles 781, 782, 783 with radiae 801, 802, 803 are constructed. Each circle 781, 7822, 783 is centered around a respective base station 361, 362, 363. The intersection of the three circles 781, 782, 783 is at the location of the subscriber unit 401.

[0044] Using the Cartesian coordinates, the longitude and latitude corresponding with each base station 361, 362 . . . 36n is represented as Xn, Yn, where Xn is the longitude and Yn is the latitude. If X, Y represents the location of the subscriber unit 401, using the distance formula the following equations result:

(X1−X)2+(Y1−Y)2=d12  Equation (3)

(X2−X)2+(Y2−Y)2=d22  Equation (4)

(X3−X)2+(Y3−Y)2=d32  Equation (5)

[0045] In practice due to small errors in calculating the distances d1, d2, d3, 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 364, 365 . . . 36n can be used to calculate additional distances for inclusion in the estimation analysis.

[0046] The subscriber unit's location is sent through the communication system 30 to at least one precinct 741, 742 . . . 74n and an ambulance dispatcher 76. A processor 701 within each precinct 741, 742 . . . 74n 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 721. The display comprises a listing of all 911 calls and addresses on a geographic map.

[0047] 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.

[0048] FIG. 8 is a second embodiment of a location system. At least two base stations 361, 362 . . . 36n have their internal timing synchronized with each other and transmit their respective global pilot signals 521, 522 . . . 52n with time synchronized chip code sequences. The subscriber unit 401 receives the global pilots 521, 522 . . . 52n. However, the received global pilots 521, 522 . . . 52n are not synchronized. The global pilot 521 from a first base station 361 will travel distance d1, and is delayed by τ1. The global pilot 522 from a second base station 362 travels distance d2 and is delayed by τ2. The subscriber unit 401 recovers each base station's global pilot chip code sequence with its global pilot chip code recovery means 541. A processor 821 within the subscriber unit 401 is coupled to each global pilot chip code recovery means 541, 522 . . . 54n. The processor 821 compares the chip code sequences of each pair of pilot chip code sequences and calculates the time differences Δt1, Δt2 . . . Δtn between the sequences as follows.

[0049] Within the subscriber unit 401, the chip code sequences used by each base station 361, 362 . . . 36n are stored. After synchronizing with the first base station's pilot 361, the processor 821 will store where within the sequence synchronization was obtained. This process is repeated for the other base stations 362, 363 . . . 36n. 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).

[0050] By using the relative time difference between τ1, τ2, . . . τn 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:

Δt1=τ2−τ1  Equation (6)

Δt2=τ3−τ2  Equation (7)

[0051] The time differences Δt1, Δt2 . . . Δtn are transmitted to at least one of the base stations 361.

[0052] At least one base station 361 recovers the time difference data from the received signals using time difference recovery means 841. The time difference data is sent with the distance data d1 through the communications system to a processor 68. The processor 68 determines the location of the subscriber unit 401 using the time difference data Δt1, Δt2 . . . Δtn and the distance data d1, d2 . . . dn as follows.

[0053] Using information from only two base stations 361, 362 as shown in FIG. 9, the processor uses distances d1, d2 to create two circles 781, 782. Using the time difference, Δt1, a hyperbola 861 can be constructed as follows.

[0054] All the points along the hyperbola 861 receive the global pilot signals 521, 522 from the synchronized base stations 361, 362 with the same time difference, Δt1. The time difference Δt1 can be converted to a distance difference Δd1 by substituting Δt1 for t1 and Δd1 for d1 in Equation 1. Using the distance formula and X, Y as the location of the subscriber unit 401, the following equation results:

Δd1={square root}{square root over ((X1−X)2+(Y1−Y)2)}−{square root}{square root over ((X2−X)2+(Y2−Y)2)}  Equation (8)

[0055] By using Equation 8 with Equations 3 and 4 in a maximum likelihood estimation, the location of the subscriber unit 401 can be determined. The subscriber unit's location is subsequently sent to the nearest police precinct 741, 742 . . . 74n and ambulance dispatcher 76 in the cellular area.

[0056] For improved accuracy, additional base stations 361, 362 . . . 36n are used. FIG. 10 shows the invention used with three base stations 361, 362, 363. The distances d1, d2, d3 are used to create three circles 781, 782, 783. Using time differences Δt1, Δt2, two intersecting hyperbolas 861, 862 are constructed. With maximum likelihood estimation, the subscriber units' location calculated with two hyperbolas 861, 862, and three circles 781, 782, 783 yields greater accuracy.

[0057] As shown in FIG. 8, the subscriber unit 401 is required to process each global pilot chip code sequence to determine the time differences Δt1, Δt2 . . . Δtn. An alternate approach removes the processing from the subscriber unit 401.

[0058] With reference to FIG. 6, the mobile unit 401 will synchronize the assigned pilot to one of the base station's global pilot chip code sequences, such as the nearest base station 361 with a delay of τ1. The assigned pilot 501 is transmitted to all base stations 361, 362 . . . 36n. The assigned pilot 501 will be received at each base station with a respective delay, τ1+τ1+τ1, τ1+τ2, τ1+3. Each base station 361, 362 . . . 36n will send the delayed chip code sequence along with the calculated distance to a processor 68 located in a NIU 341 or local exchange 32. The processor 68 will calculate the time differences Δt1, Δt2 . . . Δtn by comparing the received assigned pilot chip code sequences. Since all received assigned pilot chip code sequences are delayed by τ1, the τ1 delay will cancel out of the resultant time differences Δt1, Δt2 . . . Δtn. Accordingly, the subscriber unit 401 can be located using hyperbolas 861, 862 as previously described.

[0059] Another embodiment shown in FIGS. 11, 12 and 13 uses a base station 361 with multiple antennas 881, 882 . . . 88n. Two of the antennas 881, 882 lie along a centerline 92 at a known distance, l, apart as shown in FIG. 11. Both antennas 881, 882 receive the assigned pilot signal 901, 902 from the subscriber unit 401. However, the antenna 882 further away from the subscriber unit 401 receives the signal over a slightly longer distance d1′ and with a slight delay with respect to the nearer antenna 881. This delay results in a carrier phase difference, φ, between the signals received at each antenna as shown on FIG. 13. A processor 66 using the received carrier phase difference and the chip code sequence recovered by each assigned pilot chip code recovery means 961, 962 . . . 96n can determine the location of the subscriber unit 401 as follows.

[0060] As shown in FIG. 12, the subscriber unit 401 is located at distance d1 at angle a from the centerline 92 of the antennas 881, 882. As seen at the scale of FIG. 12 both received assigned pilot signals 901, 902 appear to be coincident. However, as shown in FIG. 11, the received assigned pilot signals 901, 902 are slightly separated. The received assigned pilot signal 901 returning to the first antenna 881 travels a distance d1. The received assigned pilot signal 902 returning to the second antenna 882 travels a slightly longer distance d1′. As shown in FIG. 11, the difference between the two distances d1, d1′ is a distance m.

[0061] Since the distances d1, d1′ between the antennas 881, 882 and the subscriber unit 401 are much larger than the distance l between the antennae 881, 882 both received assigned pilot signals 901, 902 follow approximately parallel paths. By constructing a right triangle using a point 94 which is distance d1 from the subscriber unit 401 as shown in FIG. 11, the angle ∝ can be determined by the following geometric relationship:

∝=COS−1 (m/l).  Equation (9)

[0062] The distance m can be determined by using the carrier phase difference, φ, between the two received signals 901, 902 as follows: 3$\begin{array}{cc}m=\frac{\phi ·\lambda }{2\pi }& \mathrm{Equation}\left(10\right)\end{array}$

[0063] 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 f of the assigned pilot signal as follows:

λ=c/f.  Equation (11)

[0064] The processor 68 also compares the chip code sequences of the global pilot generating means 421 with the recovered assigned pilot chip code sequence to determine the distance d1 as shown in FIG. 6. Using both the angle ∝ and distance d1, the processor 661 locates the subscriber unit 401 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 881, 882. One such technique is using antennas employing sectorization. Subsequently, the subscriber unit's location is sent to the precincts 741, 742 . . . 74n and ambulance dispatcher 76. Additional antennas may be used to improve on the accuracy of the system.

[0065] An alternate embodiment uses more than one base station 361, 362 . . . 36n. A processor 68 located within either a NIU 341 or the local exchange 32 collects distance and angle information from more than one base station 361, 362 . . . 36n as well as the time differences Δt1, Δt2 . . . Δtn, between the base stations 361, 362 . . . 36n. Using the maximum likelihood estimation technique, the processor 68 determines a more accurate location of the subscriber unit 401.

[0066] A fourth embodiment corrects for multipath. FIG. 14 illustrates multipath. A signal such as a global pilot signal is transmitted from a base station 361. The signal follows a multitude of paths 981, 982 . . . 98n between the base station 361 and subscriber unit 401.

[0067] FIG. 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 981, 982 . . . 98n. The impulse response 106 shows the collective signal magnitude of all the multipath components received at each propagation delay.

[0068] The previously described subscriber unit location techniques assumed the subscriber unit 401 synchronizes with the line of sight multipath component 981 traveling distance d1. However, if the subscriber unit synchronizes with a non-line of sight multipath component 981, 982 . . . 98n, the distance calculation will be in error due to the delay MD1 as shown in FIG. 15.

[0069] FIG. 16 is a system correcting for errors resulting from multipath. The global pilot 501 is sent from the base station 361 to subscriber unit 401. The subscriber unit 401 collects all of the multipath components using a multipath receiver 1021 such as disclosed in U.S. patent application Ser. No. 08/669,769, Lomp et al., incorporated here by reference. A processor 821 within the subscriber unit 401 analyzes the impulse response 100 of the received global pilot signal 501.

[0070] Since the line of sight multipath component 981 travels the shortest distance d1, the first received component 981 is the line of sight component. If the line of sight component is not received, the first received component 981 will be the closest and, accordingly, the best available estimate for the line of sight component. The processor 821 compares the chip code sequence of the first received component 981 with the chip code sequence used to synchronize the assigned pilot chip code sequence. This comparison determines the delay due to multipath, MD1. The multipath delay, MD1, is transmitted to the base station 361.

[0071] A processor 661 and multipath receiver 1041 within the base station 361 perform the same analysis on the received assigned pilot signal. As a result, the multipath delay, MD2, of the assigned pilot signal is determined. Additionally, multipath delay recovery means 1061 recovers the transmitted global pilot signal's multipath delay MD1 for use by the processor 661. The processor 661 compares the generated global pilot chip code sequence to the recovered assigned pilot chip code sequence to determine the round trip propagation delay 2τ1. To correct for multipath, the processor 661 subtracts both the global pilot signal's multipath delay MD1 and the assigned pilot signals multipath delay MD2 from the calculated round trip propagation delay, 2τ1. The corrected round trip propagation delay is used to determine the subscriber unit's location in one of the techniques as previously described.

[0072] 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

1. A base station comprising: means for transmitting a first spread spectrum signal having a first code; means for receiving and analyzing an impulse response of multipath components of a second spread spectrum signal having a second code to determine a first received component, the second signal time synchronized with the first spread spectrum signal; and means for making a distance determination based on in part a timing difference between the second code of the received second signal and the first code of the base station's transmitted first signal and the determined first received component for that base station's received second signal.

2. The base station of claim 1 wherein the first spread spectrum signal is a pilot signal.

3. The base station of claim 1 wherein the distance determination is determined by dividing the timing difference by two times a speed of light.

4. The base station of claim 1 wherein a chipping rate of the first spread spectrum signal is at least 80 ns and the timing difference is tracked with a precision of at least {fraction (1/16)} of a chip.

5. A base station comprising: a plurality of antennas, each of the antennas separated by a known distance; means for transmitting a first spread spectrum signal having a first code; means for receiving, using the plurality of antennas, and analyzing an impulse response of multipath components of a second spread spectrum signal having a second code to determine a first received component, the second signal time synchronized with the first spread spectrum signal; means for making a distance determination based on in part a timing difference between the second code of the received second signal and the first code of the base station's transmitted first signal and the determined first received component for that base station's received second signal; means for comparing a phase difference of a carrier signal of the second spread spectrum signal as received by each of the plurality of antennas; and means for determining an angle of the received second signal using the distance determination and the phase difference.

6. The base station of claim 1 wherein the first spread spectrum signal is a pilot signal.

7. The base station of claim 1 wherein two of the antennas are separated by a distance 1, the carrier phase difference is φ, a wave length of the carrier is λ and a value m is per