Estimation of position using WLAN access point radio propagation characteristics in a WLAN positioning system

• US 20070258421A1
• Filed: 05/08/2006
• Published: 11/08/2007
• Est. Priority Date: 05/08/2006
• Status: Active Grant

• Current Assignee: Skyhook Wireless Incorporated (Liberty Broadband Corp.)
• Original Assignee: Skyhook Wireless Incorporated (Liberty Broadband Corp.)
• Inventors: Pahlavan, Kaveh, Alizadeh-Shabdiz, Farshid
• Granted Patent: US 7,515,578 B2
• Time in Patent Office: Days
• US Class Current: 370/338
• CPC Class Codes: G01S 11/06 using intensity measurements
• CPC Class Codes: G01S 5/02521 using a radio-map
• CPC Class Codes: H04W 64/00 Locating users or terminals or network equipment for network management purposes, e.g. mobility management

Abstract

A method for estimating position using WLAN access point radio propagation characteristics in a WLAN location based service is provided. A location-based services system has a plurality of Wi-Fi access points in a target area. The Wi-Fi access points are positioned at geographic locations and have signal coverage areas. A method of characterizing at least one of the Wi-Fi access points comprises determining the geographic location of the Wi-Fi access point, dividing the signal coverage area of the Wi-Fi access point into at least one section, and determining radio propagation characteristics for each section. The radio propagation characteristics of each section characterize a radio channel of the Wi-Fi access point, and the characterization can be used in a location algorithm.

Claims

1. In a location-based services system having a plurality of Wi-Fi access points in a target area, the Wi-Fi access points being positioned at geographic locations and having signal coverage areas, a method of characterizing at least one of the Wi-Fi access points, comprising:

determining the geographic location of the Wi-Fi access point;

dividing the signal coverage area of the Wi-Fi access point into at least one section; and

determining radio propagation characteristics for each section;

wherein the radio propagation characteristics of each section characterize a radio channel of the Wi-Fi access point and wherein the characterization can be used in a location algorithm.

2. The method of claim 1, wherein the signal coverage area is one section.

3. The method of claim 1, wherein the sections are divided according to at least two radials from the Wi-Fi access point.

4. The method of claim 3, wherein the number of radials is six or less.

5. The method of claim 1, wherein the sections are divided according to at least one distance from the Wi-Fi access point.

6. The method of claim 5, wherein the number of distances is one.

7. The method of claim 1, wherein the sections are divided according to at least two radials and at least one distance from the Wi-Fi access point.

8. The method of claim 1, further comprising:

measuring a plurality of received signal power values within the signal coverage area, each received signal power value being measured at an associated position relative to the Wi-Fi access point; and

determining the sections based on the plurality of received signal power values and associated positions.

9. The method of claim 1, wherein the radio propagation characteristics include a signal power-distance gradient.

10. The method of claim 9, wherein the signal power-distance gradient for each section is determined by a method comprising:

measuring a plurality of received signal power values within the signal coverage area, each received signal power value being measured at an associated position relative to the Wi-Fi access point;

performing a linear regression on the plurality of received signal power values and associated positions; and

using a slope determined by the linear regression to calculate the signal power-distance gradient.

11. The method of claim 9, wherein the signal power-distance gradient for each section is determined by the method comprising:

measuring a plurality of received signal power values within the signal coverage area, each received signal power value being measured at an associated position relative to the Wi-Fi access point;

calculating a distance corresponding to each of the signal power values, the distances being measured from the associated positions of the signal power values to the geographic locations of the Wi-Fi access points;

estimating an average radius of signal coverage using the standard deviation of the distances;

using the average radius of signal coverage to calculate the signal power-distance gradient.

12. The method of claim 11, wherein the standard deviation of the distances, σ

, is calculated according to equations having the form

$\mathrm{\sigma <br><br>}=\sqrt{\frac{\underset{1}{\overset{N}{\sum <br><br>}}<br><br>{\left(d_i\right)}^2}{N-1}},N><br><br>1$ $\mathrm{\sigma <br><br>}=0,N=1,$

wherein;

d_i is the distance of the associated position of received power value i from the Wi-Fi access point; and

N is the number of signal power values.

13. The method of claim 11, wherein the signal power-distance gradient, α

, is calculated using equations having the form

$\mathrm{\alpha <br><br>}={\mathrm{\alpha <br><br>}}_{\min },\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\mathrm{\sigma <br><br>}><br><br>{\mathrm{\sigma <br><br>}}_{\max }\right)$ $\mathrm{\alpha <br><br>}={\mathrm{\alpha <br><br>}}_{\max },\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(\mathrm{\sigma <br><br>}<<br><br>{\mathrm{\sigma <br><br>}}_{\min }\right)$ $\mathrm{\alpha <br><br>}={\mathrm{\alpha <br><br>}}_{\max }+\left({\mathrm{\alpha <br><br>}}_{\min }-{\mathrm{\alpha <br><br>}}_{\max }\right)<br><br>\left(\frac{\log <br><br>\left(\mathrm{\sigma <br><br>}\right)-\log <br><br>\left({\mathrm{\sigma <br><br>}}_{\min }\right)}{\log <br><br>\left({\mathrm{\sigma <br><br>}}_{\max }\right)-\log <br><br>\left({\mathrm{\sigma <br><br>}}_{\min }\right)}\right),<br><br>\mathrm{if}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left({\mathrm{\sigma <br><br>}}_{\min }\le <br><br>\mathrm{\sigma <br><br>}\le <br><br>{\mathrm{\sigma <br><br>}}_{\max }\right)$

wherein;

σ

_min is a minimum signal coverage threshold;

σ

_max is a maximum signal coverage threshold;

σ

is the average radius of signal coverage;

α

_min is a minimum signal power-distance gradient; and

α

_max is a maximum signal power-distance gradient.

14. The method of claim 13, wherein:

a five-sigma value of σ

_min is about 60 meters;

a five-sigma value of σ

_max is about 700 meters;

α

_min is 2 dBWatts/dBMeters; and

α

_max is 6 dBWatts/dBMeters.

15. A method for estimating the position of a Wi-Fi enabled device, the method comprising:

the Wi-Fi enabled device communicating with Wi-Fi access points within range of the Wi-Fi enabled device to cause the Wi-Fi access points to transmit signals;

the Wi-Fi enabled device receiving the signals transmitted by the Wi-Fi access points;

identifying the Wi-Fi access points;

retrieving calculated locations and estimated radio propagation characteristics of the Wi-Fi access points from a reference database using Wi-Fi access point identifiers; and

using the calculated locations and the estimated radio propagation characteristics to estimate the position of the Wi-Fi enabled device.

16. The method of claim 15, wherein at least one of the Wi-Fi access points has a signal coverage area, the signal coverage area having at least one section, each section having corresponding radio propagation characteristics.