CDMA geolocation system

• US 6,665,332 B1
• Filed: 09/09/1999
• Issued: 12/16/2003
• Est. Priority Date: 09/09/1998
• Status: Expired due to Fees

• Current Assignee: Allen Telecom LLC (CommScope Holding Company, Inc.)
• Original Assignee: Allen Telecom Inc. (CommScope Holding Company, Inc.)
• Inventors: Gravely, Thomas B., Carlson, John P., Sullivan, Mark C.
• Primary Examiner(s): Chin, Stephen
• Assistant Examiner(s): CHANG, EDITH M
• Application Number:
  US09/392,965
• Time in Patent Office: 1,559 Days
• Field of Search: 375/356, 375/133, 375/267, 455/435, 455/456, 370/503, 370/320, 370/335, 370/280, 370/336, 342/357, 342/363, 342/457
• US Class Current: 375/130
• CPC Class Codes: G01S 11/02 using radio waves G01S19/00 takes precedence
• CPC Class Codes: G01S 5/02213 Receivers arranged in a network for determining the position of a transmitter
• CPC Class Codes: G01S 5/06 Position of source determined by co-ordinating a plurality of position lines defined by path-difference measurements G01S5/12 takes precedence
• CPC Class Codes: G01S 5/14 Determining absolute distances from a plurality of spaced points of known location
• CPC Class Codes: H04B 1/70757 with increased resolution, i.e. higher than half a chip H04B1/70758 takes precedence
• CPC Class Codes: H04B 1/7077 Multi-step acquisition, e.g. multi-dwell, coarse-fine or validation
• CPC Class Codes: H04B 2201/70715 with application-specific features

Abstract

A geolocation system for geolocating a mobile transceiver operating in a CDMA communication system is disclosed having improved time of arrival extraction which allows the extracting of time of arrival information of weak CDMA emissions. The improved time of arrival extraction is accomplished by breaking the received CDMA emission into M identical processing stages. Each stage performs despreading/demodulating at over sampled chip offsets from the next processing stage. The P-point fast Fourier transform of the M stages is taken and in effect a two dimensional time versus frequency cross ambiguity function is created. The peak of the function may be interpolated to create an accurate estimate of the time of arrival of the emission from the mobile transceiver, thus improving the accuracy of time of arrival measurements and adjusting for doppler frequency shifts that may otherwise corrupt the measurements when integrating over a long period of time.

Claims

1. A geolocation system for providing geolocation of a mobile transceiver capable of spread spectrum coded radio frequency emissions and communication, said geolocation system comprising:

a plurality of base stations comprising a primary base station and one or more secondary base stations for receiving the spread spectrum coded radio frequency emissions, the primary base station being in communication with the mobile transceiver and the secondary base stations for conveying selected coded information bits from the mobile transceiver to the secondary base stations via the primary base station;

means for synchronizing the plurality of base stations to the mobile transceiver in time;

means for determining the geolocation of the mobile transceiver based on times of arrival of the spread spectrum coded radio frequency emissions at the plurality of base stations;

wherein each of the plurality of base stations comprises;

means for identifying a signal of interest in a spread spectrum coded radio frequency emission received at the base station, a portion of the signal of interest including the selected coded information bits conveyed from the mobile transceiver to the secondary base stations via the primary base station;

means for determining a time of arrival of the spread spectrum coded radio frequency emission received at the base station;

and wherein said means for determining the time of arrival of the spread spectrum coded radio frequency emission received at the one or more secondary base stations comprises;

means for dividing the spread spectrum coded radio frequency emission received at the one or more secondary base stations into a plurality of stages as a function of an estimated propagation distance;

despreader/demodulator means for despreading/demodulating each of the plurality of stages into the selected coded information bits; and

calculating means for calculating the fast Fourier transform of each of the plurality of stages to produce a time-frequency cross ambiguity function to filter and isolate strongest peak.

2. The geolocation system of claim 1 further comprising a geolocation control system for determining a primary base station from the plurality of base stations, the primary base station being in active communication with the mobile transceiver.

3. The geolocation system of claim 2 wherein the geolocation control system further comprises means for routing said identified information bits of interest from the primary base station to secondary base stations, the secondary base stations comprising all of the plurality of base stations except the primary base station.

4. The geolocation system of claim 3 wherein upon receiving the routed identified information bits of interest, each of the secondary base stations is configured for using the routed identified information bits of interest for identifying the information bits of interest in the spread spectrum coded radio frequency emission received at the secondary base station.

5. The geolocation system of claim 4 wherein the means for determining time of arrival in each of the secondary base stations further comprises despreader/demodulator means for despreading/demodulating each of the plurality of stages into the plurality of information bits at over sampled chip offsets.

6. The geolocation system of claim 1 wherein said means for dividing divides the spread spectrum coded radio frequency emission into M stages wherein:

$M=\frac{\left(2<br><br>d/c\right)}{\mathrm{Tc}}*N$

Tc=Chip Duration (seconds)

c=speed of light (3*10⁸ m/s)

d=maximum expected propagation distance (meters)

N=chip over sampling rate.

7. The geolocation system of claim 1 further comprising means for interpolating a selected peak in the time-frequency cross ambiguity function for determining the time of arrival.

8. The geolocation system of claim 1 wherein the means for identifying information bits of interest from the spread spectrum coded radio frequency emission received at the base station comprises means for extracting information bits from the spread spectrum coded radio frequency emission received at the bast station, each information bit having an amplitude;

and means for identifying the information bit having the largest amplitude.

9. The geolocation system of claim 2 wherein said geolocation control unit includes means for instructing each of the base stations to store spread spectrum coded radio frequency emissions received at the base station.

10. The geolocation system of claim 1 wherein each of the base stations is divided into a plurality of sectors, each sector being capable of receiving a spread spectrum coded radio frequency emission from the mobile transceiver, the geolocation system further comprising means for determining which sectors of the base stations receives the spread spectrum coded radio frequency emission.

11. The geolocation system of claim 10 wherein a spread spectrum coded radio frequency emission is received by each of a first base station and a second base station, each of the first and second base stations having position coordinates and the first base station being designated the origin, such that the estimated position of the mobile transceiver is determined as follows:

$\tilde{y}=\frac{4<br><br>b<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\gamma <br><br>}\pm <br><br>\sqrt{16<br><br>b^2<br><br>{\mathrm{\gamma <br><br>}}^2+16<br><br>\left(a^2+b^2\right)<br><br>\left(4<br><br>a^2<br><br>r_1^2-{\mathrm{\gamma <br><br>}}^2\right)}}{-8<br><br>\left(a^2+b^2\right)}$ 

{overscore (x)}=±

{square root over (r₁²−

{overscore (y)})}²

21. The geolocation system of claim 1, wherein the primary base station of said plurality of base stations comprises the base station closest to the mobile transceiver and in communication therewith for conveying the selected coded information bits to the secondary base stations.

22. The geolocation system of claim 1, wherein the primary base station is in communication with the secondary base stations for conveying Walsh symbols as the selected coded information bits from the mobile transceiver to the secondary base stations via the primary base station.

23. The geolocation system of claim 22, where the selected coded information bits are Walsh symbols, and wherein the coded information extraction at the primary site comprises a Walsh Transform.

24. The geolocation system of claim 1, wherein the means for determining time of arrival comprises extractor means for extracting the signal of interest from the spread spectrum coded radio frequency emission received at each of the base stations wherein the divider means divides each emission into M identical processing stages for timing extraction performed on each of the processing stages by the extractor means to jointly extract the signal of interest and time of arrival of the spread spectrum coded radio emission.

25. The geolocation system of claim 24, where the selected coded information bits are Walsh symbols, and wherein the coded information extraction at the primary site comprises a Walsh Transform.

26. The geolocation system of claim 24, where the selected coded information bits are Walsh symbols, and wherein the timing extraction comprises joint multistage time domain correlation, frequency domain and Walsh transforms to extract the timing of arrival information.

27. The geolocation system of claim 24, wherein the despreader/demodulator means operates over each of the M identical processing stages at oversampled chip offsets.

28. The geolocation system of claim 27, wherein the despreader/demodulator means comprises an interpolation for timing extraction in a time-frequency cross ambiguity function.

29. The geolocation system of claim 28, wherein the despreader/demodulator means uses a one-half chip offset.

30. The geolocation system of claim 28, wherein the extractor means provides time dimension information and further comprises a frequency domain transform to separate frequency content to enhance time information accuracy.

31. The geolocation system of claim 28, wherein the frequency transform function comprises a P-point fast Fourier transform.

32. The geolocation system of claim 30, wherein the time-frequency cross ambiguity function provides immunity to doppler frequency domain shifts to preserve the time of arrival information throughout the frequency domain transform stages.

12. A method of ascertaining the geolocation of a mobile transceiver capable of spread spectrum coded radio frequency emissions and which is in communication with a plurality of base stations, said method comprising the steps of:

designating a primary base station and one or more secondary base stations from the plurality of base stations;

receiving spread spectrum coded radio frequency emissions from the mobile transceiver at the of primary base stations;

establishing the primary base station in communication with the mobile transceiver and the secondary base stations;

conveying selected coded information bits from the mobile transceiver to the secondary base stations via the primary base station;

synchronizing the plurality of base stations to the mobile transceiver in time;

extracting a signal of interest from a spread spectrum coded radio frequency emission received at each of the base stations;

determining the primary base station based on the extracted signal of interest and identifying a portion of the signal of interest including the selected coded information bits conveyed from the mobile transceiver to the secondary base stations via the primary base station;

forwarding the selected coded information bits of interest from the primary base station to the secondary base stations, the secondary base stations comprising all of the plurality of base stations except the primary base station;

determining times of arrival of the spread spectrum coded radio frequency emission received at one or more of the secondary base stations;

determining a time of arrival of the spread spectrum coded radio frequency emission received at the primary base station;

determining the geolocation of the mobile transceiver utilizing the determined times of arrival.

13. The method of claim 12 wherein the step of determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the secondary base stations further comprises the steps of:

dividing the spread spectrum coded radio frequency emission received at each of the secondary base stations into a plurality of stages;

despreading/demodulating each of the plurality of stages into the extracted information bits;

calculating the fast Fourier transform of each of the stages to produce a time-frequency cross ambiguity function; and

determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the secondary base stations based on the time-frequency cross ambiguity function.

14. The method of claim 12 wherein the step of determining the geolocation of the mobile transceiver further comprises the steps of:

determining the ranges between the mobile transceiver and the plurality of base stations utilizing the determined times of arrival; and

calculating the geolocation of the mobile transceiver based on the ranges between the mobile transceiver and the plurality of base stations.

15. The method of claim 13 further comprising the step of despreading/demodulating each of the plurality of stages into the extracted information bits at over sampled chip offsets for each of the secondary base stations.

16. The method of claim 13 wherein the step of dividing comprises dividing the spread spectrum coded radio frequency emission received at each of the secondary base stations into M stages wherein:

$M=\frac{\left(2<br><br>d/c\right)}{\mathrm{Tc}}*N$

Tc=Chip Duration (seconds)

c=speed of light (3*10⁸ m/s)

d=maximum expected propagation distance (meters)

N=chip over sampling rate.

17. The method of claim 13 wherein the step of determining the times of arrival of the spread spectrum coded radio frequency emission received at each of the secondary base stations further comprises the step of interpolating a selected peak in the time-frequency cross ambiguity function and determining the times of arrival based on the selected peak.

18. The method of claim 12 wherein the step of identifying from the extracted information bits information bits of interest in the spread spectrum coded radio frequency emission received at each of the base stations further comprises identifying the extracted information bits having the largest amplitude.

19. The method of claim 12 wherein each of the plurality of base stations is divided into a plurality of sectors, each sector being capable of receiving a spread spectrum coded radio frequency emission from the mobile transceiver, the method further comprising the step of determining which sector of the base station receives the spread spectrum coded radio frequency emission.

20. The method of claim 19 wherein a spread spectrum coded radio frequency emission is received by each of a first base station and a second base station, each of the first and second base stations having position coordinates and the first base station being designated the origin, the method further comprising the step of estimating the position of the mobile transceiver as follows:

$\tilde{y}=\frac{4<br><br>b<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\gamma <br><br>}\pm <br><br>\sqrt{16<br><br>b^2<br><br>{\mathrm{\gamma <br><br>}}^2+16<br><br>\left(a^2+b^2\right)<br><br>\left(4<br><br>a^2<br><br>r_1^2-{\mathrm{\gamma <br><br>}}^2\right)}}{-8<br><br>\left(a^2+b^2\right)}$ 

{overscore (x)}=±

{square root over (r₁²−

{overscore (y)})}²