Position Location using Radio Fingerprints in Wireless Networks
Prashant KrishnamurthyGraduate Program in Telecom & Networking
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?