Position Location using Radio Fingerprints in Wireless Networks

Prashant KrishnamurthyGraduate Program in Telecom & Networking

Agenda

• Introduction

• Radio Fingerprints

• What Industry is Doing

• Research

• Conclusions

Introduction• Why Position Location?

• Location based services

• Driving directions, concierge services, etc.

• E-911 mandate

• 100m at least 67% of the time and within 300m at least 95% of the time

• Metrics

• Accuracy (e.g., 100m)

• Precision (e.g., 67%)

• Delay

• Coverage

• Capacity

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

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 basedposition location

Rx

Remarks (1)

• Cell-ID (POA)

• 43% of the time, a MS may associate itself with a 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

Remarks (2)

• AOA/DOA Techniques

• Many cells use omnidirectional antennas

• 120o antennas have large beamwidths to accurately estimate directions

• Not part of any standard

Radio Fingerprinting: Idea

Access Point Grid Point

AP1AP2

r1

r2

RSS from AP 1

RS

S f

rom

AP

2

Fingerprint at grid location

Sample RSS vectorFingerprint

Estimated Location

Decision Boundary

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

What makes up a Fingerprint?

• Any unique characteristic that differentiates location

• Common to use RSS from multiple base stations or access points

• Others: Signal-to-Interference, time delays, cell-IDs seen, etc.

• Match observed sample with entries in database to estimate location

• Exact matches are unlikely - errors

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 technologies outside of what already exists

• Improved time to fix

• Lower power consumption (compared to GPS)

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?

Fingerprinting in Cellular-Only Networks

• Comparison with Assisted-GPS in mix of indoor and outdoor test points

• Blind trial in New York City and Toronto by operators

• Polaris Wireless judged the 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.

Hybrid Positioning in Cellular-Only Networks

• Combining techniques improves accuracy and precision

• WLS = Wireless Location Signature

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.

Using WiFi with Cellular and GPS

• Approach made popular by Skyhook and iPhone

• XPS - Hybrid Positioning

• WPS - WiFi Positioning

• Available for Windows Mobile devices as well

Source: Skyhook Wireless

Without Skyhook With Skyhook

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 radio fingerprints

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

Performance of Skyhook’s 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 RF Localization for Next Generation Wireless Devices, Worcester Polytechnic Institute, June 2008.

Manhattan & San Francisco60 outdoor and 40 indoor points

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

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 system deployment?

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 improve model’s precision

• We observe clustering - actually good news for system deployment

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?

Thank You!