Enhanced wireless node location using differential signal strength metric

• US 7,286,835 B1
• Filed: 09/10/2004
• Issued: 10/23/2007
• Est. Priority Date: 09/10/2004
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Airespace, Inc. (Cisco Systems, Inc.)
• Inventors: Davi, Gregg Scott, Dietrich, Paul F.
• Primary Examiner(s): Perez-Gutierrez; Rafael
• Assistant Examiner(s): Au; Gary
• Application Number:
  US10/938,460
• Time in Patent Office: 1,138 Days
• Field of Search: 455/456.1, 455/67.01, 342/357.01, 342/450
• US Class Current: 455/456.1
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management

Abstract

A wireless node location mechanism that employs a differential signal strength metric to reduce the errors caused by variations in wireless node transmit power, errors in signal strength detection, and/or direction-dependent path loss. As opposed to using the absolute signal strength or power of an RF signal transmitted by a wireless node, implementations of the location mechanism compare the differences between signal strength values detected at various pairs of radio receivers to corresponding differences characterized in a model of the RF environment. One implementation searches for the locations in the model between each pair of radio receivers where their signal strength is different by an observed amount.

Claims

1. A method for estimating the location of a wireless node relative to a plurality of radio receivers operative to detect the strength of radio frequency (RF) signals, wherein a RF coverage map, corresponding to each of the radio receivers, characterizes the signal strength values for locations in a physical region, comprising

collecting signal strength values, detected at a plurality of radio receivers, corresponding to signals transmitted by a wireless node;

and

determining a differential error surface comprising, for all unique pairs of radio receivers of the plurality of radio receivers, the sum of the squares of the difference between the signal strength values detected by a pair of radio receivers less the difference between the signal strength values in the RF coverage maps associated with the pair of radio receivers; and

estimating the location of the wireless node using the differential error surface wherein estimating the location of the wireless node further comprises;

computing, for each radio receiver, an individual error surface based on the RF coverage map associated with the radio receiver and the signal strength detected by the radio receiver;

aggregating the individual error surfaces to create a total absolute error surface;

adding the total absolute error surface and the differential error surface; and

selecting a location based on the results of the adding step.

2. The method of claim 1 wherein estimating the location comprises finding the minimum of the differential error surface.

3. The method of claim 1 further comprising

detecting, at a plurality of radio transceivers, the strength of signals transmitted by a wireless node.

4. The method of claim 1 wherein the RF coverage maps each comprise a plurality of location coordinates associated with corresponding signal strength values.

5. The method of claim 4 wherein the RF coverage maps are heuristically constructed.

6. The method of claim 4 wherein the RF coverage maps are based on a mathematical model.

7. The method of claim 1 wherein the signals transmitted by the wireless nodes are formatted according to a wireless communications protocol.

8. The method of claim 7 wherein the wireless communications protocol is the IEEE 802.11 protocol.

9. The method of claim 1 wherein only signal strength values above a threshold signal strength value are used to compute the estimated location of the wireless node.

10. The method of claim 1 wherein the wireless node comprises an RFID tag.

11. The method of claim 1 wherein estimating the location of the wireless node further comprises finding the locations of the minimum of the results of the adding step.

12. An apparatus facilitating the location of a wireless node in an RF environment, comprising

a plurality of radio receivers comprising at least one antenna, the plurality of radio receivers operative to detect the strength of signals transmitted by wireless nodes and provide the detected signal strengths to a wireless node location model;

wherein a RF coverage map, corresponding to each of the radio receivers, characterizes the signal strength values for locations in a physical region, and

a wireless node location module operative to determine a differential error surface comprising, for all unique pairs of radio receivers of the plurality of radio receivers, the sum of the squares of the difference between the signal strength values detected by a pair of radio receivers less the difference between the signal strength values in the RF coverage maps associated with the pair of radio receivers; and

estimate the location of the wireless node using the differential error surface wherein estimating the location of the wireless node further comprises;

compute, for each radio receiver, an individual error surface based on the RF coverage map associated with the radio receiver and the signal strength detected by the radio receiver;

aggregate the individual error surfaces to create a total absolute error surface;

add the total absolute error surface and the differential error surface; and

select a location based on the results of the adding step.

13. The apparatus of claim 12 wherein the wireless node location module is operative to find the minimum of the differential error surface.

14. The apparatus of claim 12 wherein the RF coverage maps each comprise a plurality of location coordinates associated with corresponding signal strength values.

15. The apparatus of claim 14 wherein the RF coverage maps are heuristically constructed.

16. The apparatus of claim 14 wherein the RF coverage maps are based on a mathematical model.

17. The apparatus of claim 12 wherein the signals transmitted by the wireless nodes are formatted according to a wireless communications protocol.

18. The apparatus of claim 17 wherein the wireless communications protocol is the IEEE 802.11 protocol.

19. A wireless node location mechanism operating in association with a wireless network environment comprising a plurality of radio receivers operative to detect the signal strength of signals transmitted by wireless nodes, wherein a RF coverage map, corresponding to each of the radio receivers, includes signal strength values for locations in a physical region, comprising:

a wireless node location module operative to

receive, from at least some of the plurality of radio receivers, the detected signal strength of RF signals transmitted by a wireless node; and

determine a differential error surface comprising for all unique pairs of radio receivers of the plurality of radio receivers, the sum of the squares of the difference between the signal strength values detected by a pair of radio receivers less the difference between the signal strength values in the RF coverage maps associated with the pair of radio receivers; and

estimate the location of the wireless node using the differential error surface wherein estimating the location of the wireless node further comprises;

compute, for each radio receiver, an individual error surface based on the RF coverage map associated with the radio receiver and the signal strength detected by the radio receiver;

aggregate the individual error surfaces to create a total absolute error surface;

add the total absolute error surface and the differential error surface; and

select a location based on the results of the adding step.

20. The wireless node location mechanism of claim 19 wherein the wireless node location module is operative to find the minimum of the differential error surface.

21. The wireless node location mechanism of claim 19 wherein the RF coverage maps each comprise a plurality of location coordinates associated with corresponding signal strength values.

22. The wireless node location mechanism of claim 21 wherein the RF coverage maps are heuristically constructed.

23. The wireless node location mechanism of claim 21 wherein the RF coverage maps are based on a mathematical model.

24. The wireless node location mechanism of claim 19 wherein the signals transmitted by the wireless nodes are formatted according to a wireless communications protocol.

25. The wireless node location mechanism of claim 24 wherein the wireless communications protocol is the IEEE 802.11 protocol.

26. The wireless node location mechanism of claim 19 further comprising a plurality of radio receivers operative to detect the signal strength of signals transmitted by wireless nodes.

27. A wireless network system facilitating the location of a wireless node, comprising

a plurality of access elements for wireless communication with at least one remote client element and for communication with a central control element;

wherein a RF coverage map, corresponding to each of the access elements, characterizes the signal strength values for locations in a physical region,

wherein the access elements are each operative to

establish and maintain, in connection with a central control element, wireless connections with remote client elements;

detect the strength of received signals;

append a signal strength value to frames received from wireless nodes; and

transmit received frames to a central control element;

at least one central control element for supervising the access elements, wherein the central control element is operative to

manage wireless connections between the access elements and corresponding remote client elements, and

store signal strength data appended to frames transmitted by the plurality of access elements in association with wireless node identifiers; and

a wireless node location module operative to

determine a differential error surface comprising, for all unique pairs of radio receivers of the plurality of radio receivers, the sum of the squares of the difference between the signal strength values detected by a pair of radio receivers less the difference between the signal strength values in the RF coverage maps associated with the pair of radio receivers; and

estimate the location of the wireless node using the differential error surface wherein estimating the location of the wireless node further comprises;

compute, for each access element, an individual error surface based on the RF coverage map associated with the access element and the signal strength detected by the access element;

aggregate the individual error surfaces to create a total absolute error surface;

add the total absolute error surface and the differential error surface; and

select a location based on the results of the adding step.

28. The system of claim 27 wherein the wireless node location module is further operative to find the minimum of the differential error surface.

29. The system of claim 27 wherein the wireless node location module resides in a network management system.

30. The system of claim 27 wherein the wireless node location module resides in the central control element.

31. The system of claim 27 wherein the wireless node location module maintains a signal strength matrix including values representing the strength of signals detected between the access elements.

32. The system of claim 27 wherein the frames are 802.11 frames.

33. The system of claim 32 wherein the wireless node identifiers are MAC addresses.

34. The system of claim 27 wherein the RF coverage maps each comprise a plurality of location coordinates associated with corresponding signal strength values.