System and method for clock distribution and synchronization and radio location

• US 7,499,423 B1
• Filed: 06/10/2004
• Issued: 03/03/2009
• Est. Priority Date: 06/10/2004
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Inventors: Carty, Clark, Amos, James A.
• Primary Examiner(s): Dao; Minh D
• Application Number:
  US10/865,545
• Time in Patent Office: 1,727 Days
• Field of Search: 370/324, 370/503, 370/507, 370/509, 370/512, 370/520, 342/387, 455/456, 455/404, 455/522, 455/425, 455/41.2
• US Class Current: 370/324
• 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: H04J 3/0682 by delay compensation, e.g. by compensation of propagation delay or variations thereof, by ranging
• CPC Class Codes: H04L 7/0008 Synchronisation information channels, e.g. clock distribution lines
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management

Abstract

A system for locating a client in a wireless network. The system includes at least three access points. Each access point is adapted to wirelessly communicate with a client having a location using packeted data, be placed in wired data communication within a selected data network; and to share a common precise time reference using the selected data network. The system further includes a switch adapted to translate the relative position of the access points based on a shared common precise time reference and to calculate the location of the client based on a transmission from the client.

Claims

1. A method for determining location in a wireless network having at least three access points, comprising the steps of:

determining the relative position of the at least three access points;

distributing a clock to synchronize clocking in the at least three access points;

receiving a packet at a first access point;

oversampling a correlation of the packet;

obtaining a first correlation peak; and

adjusting the relative position of the first access point using the correlation peak to represent a first coplanar point.

2. The method for determining location in a wireless network having at least three access points of claim 1, further comprising the steps of:

receiving a packet at a second access point;

obtaining a second correlation peak; and

adjusting the relative position of the second access point using the second correlation peak to represent a second coplanar point.

3. The method for determining location in a wireless network having at least three access points of claim 2, further comprising the steps of:

receiving a packet at a third access point;

obtaining a third correlation peak; and

adjusting the relative position of the third access point using the third correlation peak to represent a third coplanar point.

4. The method for determining location in a wireless network having at least three access points of claim 3, further comprising the step of determining the equation of a circle using the three coplanar points, wherein the center of the circle corresponds to the location of the packet.

5. The method for determining location in a wireless network having at least three access points of claim 1, wherein the oversampling takes place at least 6 Megahertz (MHz).

6. A system for locating a client in a wireless network, comprising:

at least three access points, each access point adapted to;

(i) wirelessly communicate with at least one client having a location using packeted data;

(ii) be placed in wired data communication within a selected data network; and

(iii) to share a common precise time reference using the selected data network;

a switch adapted to translate the relative position of the at least three access points based on a shared common precise time reference and to calculate the location of the at least one client based on a transmission from the client;

wherein the switch sends out a timing synchronization pulse packet, the packet is detected by each access point, and the detected packets are used to globally synchronize timing information across the network; and

wherein the detected packets are oversampled to provide additional location accuracy by adjusting a relative position of a first of the at least three access points using a correlation peak obtained from oversampling.

7. The system for locating a client in a wireless network of claim 6, wherein the switch includes a precision time reference shared by the at least three access points.

8. The system for locating a client in a wireless local area network of claim 7, wherein the switch includes a respective physical interface card for each of the at least three access points, the precision time reference coupled to the physical interface cards, and the physical interface cards generating synchronization packets that are used by the at least three access points in sharing the common precise time reference.

9. The system for locating a client in a wireless network of claim 8, wherein each physical interface card includes time domain reflectometry circuitry capable of determining the distance to a respective access point.

10. The system for locating a client in a wireless network of claim 6, wherein time stamps are generated based on the oversampled detected packets.

11. The system for locating a client in a wireless network of claim 10, wherein the time stamps are used in the location calculation.

12. The system for locating a client in a wireless network of claim 6, wherein each of the at least three access points includes a precision clock and, wherein the precision clocks are synchronized via the wired data network.

13. The system for locating a client in a wireless network of claim 12, wherein the client transmits packeted data, the at least three access points receive and time stamp the packeted data, and the switch uses a time difference of arrival technique to calculate the location of the client based on the time stamps.

14. An apparatus for locating a client in a wireless network including at least three access points, comprising:

means for determining the relative position of at least three access points;

means for translating the relative position of the at least three access points, the means for translating the relative position further comprises means for oversampling a correlation of a packet received at a first of the at least three access points and means for obtaining a correlation peak;

means for synchronizing clocking in the at least three access points;

means for receiving a time of receipt of a client transmission from the least three access points; and

means for calculating the location of the client based on the translated positions of the access points;

wherein the means for translating uses a correlation peak obtained by the means for obtaining a correlation peak to represent a first coplanar point.

15. The apparatus for locating a client in a wireless network including at least three access points of claim 14, further comprising:

means for generating a time synchronization pulse packet.

16. The apparatus for locating a client in a wireless network including at least three access points of claim 14, wherein the oversampling takes place at least 500 Megahertz (MHz).

17. The apparatus for locating a client in a wireless network including at least three access points of claim 14, wherein the correlation peak is used to synchronizing clocking in the at least three access points.

18. The apparatus for locating a client in a wireless network including at least three access points of claim 14, further comprising

means for determining a distance to an access point.

19. The apparatus for locating a client in a wireless network including at least three access points of claim 14, wherein the distance to an access point is used in translating the relative position of the at least three access points.

20. The apparatus for locating a client in a wireless network including at least three access points of claim 14, wherein calculating the location of the client includes a time difference of arrival technique to calculate the location.

21. The apparatus for locating a client in a wireless network including at least three access points of claim 20, wherein time difference of arrival technique uses the equation of a circle and the center of the circle relates to the location of the client.