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Position Location using RadioFingerprints in Wireless Networks

Prashant KrishnamurthyGraduate Program in Telecom & Networking

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Agenda

Introduction Radio Fingerprints What Industry is Doing

Research

Conclusions

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Introduction

Why Position Location?

Location based services

Driving directions,concierge services,etc.

E-911 mandate

100m at least 67% ofthe time and within300m at least 95% ofthe time

Metrics

Accuracy (e.g., 100m)

Precision (e.g., 67%)

Delay

Coverage

Capacity

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Algorithms for Position

Location Association to a Point of Access (POA) Time or Time Difference of Arrival (TOA/TDOA)

Other distance or range based schemes

Angle or Direction of Arrival (AOA/DOA)

Radio Fingerprinting

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Algorithms Again

Tx-1 Tx-2

Tx-3

(c) TDOA basedposition location

Tx-1 Tx-2

Tx-3

Rx

(b) TOA basedposition location

. .Tx-A Tx-B

(d) AOA basedposition location

Rx

. .

Tx-1 Tx-2

Rx

(a) Cell-ID based

position location

Rx

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Remarks (1)

Cell-ID (POA)

43% of the time, a MS may associate itself witha base station that is NOT closest to it

Poor accuracy - 800m in NY area TOA/TDOA approaches

Several standards in cellular networks Provide reasonable accuracy

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Remarks (2)

AOA/DOA Techniques

Many cells use omnidirectional antennas 120o antennas have large beamwidths to

accurately estimate directions

Not part of any standard

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Radio Fingerprinting: Idea

Access Point Grid Point

AP1AP2

r1

r2

RSS from AP 1

RSSfromA

P

2

Fingerprint at grid location

Sample RSS vectorFingerprint

EstimatedLocation

Decision Boundary

Idea for WiFi (with some measurements) was first published by researchers from Microsoft

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What makes up a

Fingerprint? Any unique characteristic that differentiateslocation

Common to use RSS from multiple base stationsor access points Others: Signal-to-Interference, time delays, cell-

IDs seen, etc.

Match observed sample with entries in database toestimate location Exact matches are unlikely - errors

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Why Fingerprinting?

Multipath propagation Impacts error with TOA/TDOA and AOA techniques

Beneficial in the case of fingerprinting Software only approach

No new hardware, spectrum, or sensing technologiesoutside of what already exists Improved time to fix

Lower power consumption (compared to GPS)

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Why Not Fingerprinting?

Database of fingerprints is laborious to create Unclear how much information needs to be stored

Too much or too little? Censored data

Database may have to be regularly updated New cells, change in environment, etc. Self-healing?

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Fingerprinting in Cellular-

Only Networks Comparison withAssisted-GPS in mix of

indoor and outdoortest points

Blind trial in New YorkCity and Toronto by

operators Polaris Wireless judgedthe best

Accuracy Precision

< 50m 74%, 69%

< 100m 91%, 90%

< 150m 99%, 96%< 200, 300m 100%

Source: M. J. Feuerstein, "Urban and Indoor Location using Pattern Matching of Wireless Network Measurements," Invited

Workshop on Opportunistic RF Localization for Next Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

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Hybrid Positioning in

Cellular-Only Networks Combining

techniquesimprovesaccuracy andprecision

WLS = WirelessLocationSignature

Source: M. J. Feuerstein, "Urban and Indoor Location using Pattern Matching of Wireless Network Measurements," Invited

Workshop on Opportunistic RF Localization for Next Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

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Using WiFi with Cellular

and GPS Approach madepopular by Skyhookand iPhone

XPS - HybridPositioning

WPS - WiFiPositioning

Available for WindowsMobile devices as well

Source: Skyhook Wireless

Without Skyhook With Skyhook

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Combining WiFi with GPS

and Cellular Why? Over 50 million WiFi APs deployed

26 million in the US Downtown area - average of 10-18 APs

detected in any location

Ideal to use SSIDs & RSS as the radio fingerprints Use only 2 GPS satellites with the radiofingerprints

Source: F. Alizadeh, Opportunistic vs Hybrid positioning, Invited Workshop on Opportunistic RF Localization for Next

Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

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Performance of Skyhooks

WPS

Better accuracy than GPS

Better coverage than GPS

50% Prec 95% Prec

HTC Tilt

Indoor Outdoor

Source: F. Alizadeh, Opportunistic vs Hybrid positioning, Invited Workshop on Opportunistic RFLocalization for Next Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

Manhattan & San Francisco60 outdoor and 40 indoor points

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WPS/XPS Performance

Source: F. Alizadeh, Opportunistic vs Hybrid positioning, Invited Workshop on Opportunistic RF Localization for Next

Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

Technique WPS XPS

50% Prec. 68 m 44 m

95% Prec. 117 m 97 m

One Shot50% Tracking, 50% one-shot

Power Consumption (lower estimate)

Recent work at Skyhook

Prototype results

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Research

Goals Better understanding for Indoor-Only environments

with WiFi Can we develop a model that can predict the accuracy

and precision?

Factors: Number of access points, path-loss exponent,variability of RSS, how close should grid points be

Can we use the model to develop guidelines for systemdeployment?

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Challenges and Highlights

Lacked measurement data We took extensive measurements in the IS Building and

Hillman Library

Variability of the RSS is not Gaussian Even if it is assumed to be Gaussian, the constellation of

fingerprints is highly irregular

Employed concept of neighborhood graphs to improvemodels precision

We observe clustering - actually good news for systemdeployment

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People

Two Ph.D. students Kamol Kaemerungsi (2004) Nattapong Swangmuang (2008)

Future?

Impact of censored fingerprint data?

Extension to ad hoc/sensor networks? Impact of/on dynamic spectrum access?

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Thank You!