System and method for determining signal source location in wireless local area network

• US 9,274,207 B2
• Filed: 01/30/2007
• Issued: 03/01/2016
• Est. Priority Date: 02/01/2006
• Status: Active Grant

• Current Assignee: Zebra Technologies Corporation
• Original Assignee: ZIH Corp. (Zebra Technologies Corporation)
• Inventors: Boyd, Robert W.
• Primary Examiner(s): Cai, Wayne
• Application Number:
  US11/668,569
• Publication Number:
  US 20070176749A1
• Time in Patent Office: 3,318 Days
• Field of Search: 455130-132, 4554561-4566, 3405721-5724, 340/539.11, 340/539.13, 340/539.15, 340/10.1, 340/10.3
• US Class Current: 1/1
• CPC Class Codes: G01S 5/021 Calibration, monitoring or correction G01S5/0252 takes precedence
• CPC Class Codes: G01S 5/0215 Interference
• CPC Class Codes: G01S 5/0218 Multipath in signal reception
• CPC Class Codes: G01S 5/02216 Timing or synchronisation of the receivers
• 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: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management
• CPC Class Codes: H04W 84/12 WLAN [Wireless Local Area Networks]

Abstract

A system and method locates a wireless local area network (WLAN) signal source. A wireless LAN signal as an RF signal is propagated from a WLAN signal source at a fixed, known location to a plurality of receivers having unsynchronized clocks. The time-of-arrival is measured at each receiver and the timing relationship from the fixed, known location of the signal source is used to relate time-of-arrival measurements from a signal arriving from an unknown location.

Claims

1. A system for locating a signal source in a wireless local area network (WLAN), comprising:

at least one WLAN signal source positioned at a known location that transmits a wireless radio frequency (RF) signal;

a plurality of receivers, wherein each receiver is configured to;

receive the RF signal from the at least one WLAN signal source, and

generate, and transmit to a processor, a time-of-arrival measurement for the RF signal relative to a clock of the receiver;

wherein the clocks of the plurality of receivers are unsynchronized; and

the processor configured to;

receive the time-of-arrival measurements from the plurality of receivers, and

for each receiver, define a respective time relationship between the clock of the receiver and a system time based on the received time-of-arrival measurement from the receiver.

2. A system according to claim 1, and further comprising a tag transmitter positioned at each WLAN signal source.

3. A system according to claim 2, wherein each tag transmitter transmits an RF signal having data identifying an object to which the tag transmitter is associated.

4. A system according to claim 1, wherein each receiver comprises a wireless access point of the WLAN.

5. A system according to claim 1, and further comprising a plurality of WLAN signal sources at fixed, known locations.

6. A system according to claim 1, and further comprising a tag transmitter embedded at a receiver and operative as a WLAN beacon.

7. A system according to claim 1, wherein said processor is operative for determining clock timing relationships for clocks based on one of at least propagation or processing delays in the receivers for synchronizing receiver performance.

8. A system according to claim 1, wherein each WLAN signal source includes a circuit for transmitting a pseudo-random spread spectrum RF signal.

9. A system according to claim 1, wherein said processor is operative for processing first-to-arrive signals at each receiver.

10. The system of claim 1, wherein each of the receivers is further configured to:

receive a second wireless RF signal from a WLAN signal source at an unknown location; and

generate, and transmit to the processor, a second time-of-arrival measurement for the second RF signal relative to the clock of the receiver; and

wherein the processor is configured to receive the second time-of-arrival measurements from each of the receivers and, for each time-of-arrival measurement, apply the respective time relationship between the clock of the receiver from which the time-of-arrival measurement was received and the system time to determine a time-of-arrival measurement relative to the system time.

11. The system of claim 1, wherein the system time is defined as a mean or a median of the unsynchronized clocks of the plurality of receivers.

12. The system of claim 1, wherein the system time is unsynchronized with at least all but one of the clocks of the receivers.

13. The system of claim 1, wherein the processor configured to define the respective time relationship includes being configured to define the respective time relationship between the clock of the receiver and the system time based on a known distance between the WLAN signal source and the receiver.

14. A system for locating a wireless station in communication with a wireless local area network (WLAN) comprising:

a plurality of wireless mobile devices each forming a WLAN signal source that transmits RF signals;

a plurality of cells defining a WLAN and each having a receiver forming an access point in the WLAN and configured to communicate with WLAN signal sources, wherein at least one of the WLAN signal sources is positioned at a known location, and wherein each of the receivers is configured to;

receive an RF signal from the at least one WLAN signal source positioned at the known location, and

generate, and transmit to a processor, a time-of-arrival measurement for the RF signal relative to a clock of the receiver;

wherein the clocks of the of receivers are unsynchronized; and

the processor configured to;

receive the time-of-arrival measurements from the receivers, and

for each receiver, define a respective time relationship between the clock of the receiver and a system time based on the received time-of-arrival measurement from the receiver.

15. A system according to claim 14, and further comprising a tag transmitter positioned at a signal source.

16. A system according to claim 15, wherein each tag transmitter transmits an RF signal having data identifying an object to which the tag transmitter is associated.

17. A system according to claim 15, wherein each tag transmitter includes a circuit for transmitting a pseudo-random spread spectrum RF signal from signal sources.

18. A system according to claim 15, and further comprising a tag transmitter embedded at a receiver and operative as a WLAN beacon.

19. A system according to claim 14, wherein said processor is operative for determining clock timing relationships for clocks based on one of at least propagation or processing delays in the receivers for synchronizing receiver performance.

20. A system according to claim 14, wherein said processor is operative for processing first-to-arrive signals at each of the receivers.

21. A method for locating a signal source in a wireless local area network (WLAN), the method comprising:

propagating a wireless radio frequency (RF) signal from a WLAN signal source at a known location to a plurality of receivers; and

at each receiver, generating time-of-arrival measurements of the RF signal relative to a clock of the receiver, wherein the clocks of the receivers are unsynchronized;

at each receiver, transmitting the time-of-arrival measurements of the RF signal relative to the clock of the receiver to a processor;

receiving the time-of-arrival measurements at the processor; and

defining a respective time relationship between the clock of each receiver with a system time based on the time-of-arrival measurements.

22. A method according to claim 21, further comprising transmitting RF signals from tag transmitters positioned at the WLAN signal sources at known and unknown locations.

23. A method according to claim 21, further comprising receiving the RF signal within receivers as access points of the WLAN.

24. A method according to claim 21, further comprising propagating RF signals from a plurality of WLAN signal sources at fixed, known locations.

25. A method according to claim 21, further comprising propagating an RF signal from a tag transmitter embedded at a receiver and operative as a WLAN beacon.

26. A method according to claim 21, further comprising determining clock timing relationships for clocks based on one of at least propagation or processing delays in the receivers for synchronizing receiver performance.

27. A method according to claim 21, further comprising maintaining a model of timing relationships of clocks at the receivers.

28. A method according to claim 21, further comprising transmitting an RF signal from at least one tag transmitter having data identifying the object to which the tag is associated.

29. A method according to claim 21, further comprising transmitting a pseudo random spread spectrum RF signal from WLAN signal sources.

30. A method according to claim 21, further comprising processing first-to-arrive signals at each receiver.

31. A method according to claim 21, further comprising forming a tracking loop to track clock relationships that occur naturally over time.