april 20, 2008emmett nicholas ece 256 1 drive-by localization of roadside wifi networks anand prabhu...

April 20, 2008 Emmett NicholasECE 256

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Drive-by Localization of Roadside WiFi Networks

Anand Prabhu Subramanian, Pralhad Deshpande, Jie Gao, Samir R. Das

Accepted in INFOCOM 2008, Phoenix, Arizona, April 2008


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Motivation

• Learn about the nature of WiFi networks– Density, connectivity,

interference properties– LOCATION of the APs– Provide datasets for

research on Internet topology

• Localization of infrastructure nodes (APs)


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Existing Technologies• GPS

– Not available on most wireless clients today• RADAR

– Uses WIFI fingerprints for indoor localization• VORBA

– Rotating directional antennas are used in APs– Signal strength and angle of arrival (AoA) used to localize clients

indoors• War-driving databases

– Locations where APs are heard with a sniffer• MobiSteer

– Steerable beam directional antenna with a WiFi client mounted on a moving car


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Drive-by Localization (DriveByLoc)

• Use MobiSteer– Gather frames from roadside APs on different

directional beams– Estimate the AoA of the frames– Many samples are collected from different

locations• Passive approach

– Based on “sniffing”– APs are unaware of localization effort


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Hardware/Software Setup

• Multi-beam 2.4 GHz antenna– 1 omnidirectional beam– 16 directional beams

• 45⁰ half-power beam-width• Rotated 22.5⁰ with respect to adjacent beam

– Electronically steerable• GPS receiver• Each received frame is logged with the tuple:

<AP, location, orientation, beam, SNR>


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Experimental Scenarios

Parking lot(2 APs)

Apartment complex(17 APs)

Office building(2 APs)


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Data Collection

• Ideally, measurements for each AP are taken on all beams at many points– Beam with highest SNR is pointing towards AP

• Complications…– Each channel/beam combination takes ≈ 100ms– Determining orientation– Non-zero beamwidth– Reflections


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Localization Algorithm

• Estimate AoA of frames from a given AP at each measurement point– Average SNR for frames on each directional beam

• Beam with strongest average SNR is expected to point directly to AP– Orientation information & strongest beam used to

position AP– Sum-square of angular error from all strongest

beam directions is minimized


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Non-zero Beamwidth


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Reflections


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Understanding Reflections

Parking lot Office building


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Interesting observation:CLUSTERING


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New approach…1. Use the k-means algorithm to group the measurement points into k clusters2. Determine which one these k images is the real AP


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Modeling Reflections by k-Means Clustering

• For any given value of k, assume L1,…,Lk are the k locations of the AP (real and the images)

– Li’s are chosen randomly within “feasible region”

– Each measurement mapped to some Li that provides minimum angular error


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Modeling Reflections by k-Means Clustering

1. Compute a point for each cluster, Ci, in the feasible region that minimizes intra-cluster sum-square of angular errors

2. Ci’s become new Li’s

3. Go to Step 1.


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Choosing Real AP Location from k Images

• Impossible to know for sure• But a certain heuristic helps:

– Each measurement ranks k images based on distance to itself– “The nearest image is ranked 1st and the next is ranked 2nd and so on.”– Choose image with least sum of ranks


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Learning k for Clustering

• Use the idea from the G-means algorithm• Start with k=1, and successively increment k

– Perform k-means clustering for each k– Check whether error values in each cluster satisfy

statistical test for normality• If YES, stop.• If NO, increment k and repeat.


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Performance Evaluation


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Benefit of Using Directional Antennas and AOA

“DrivebyLoc is about an order of magnitude better than trilateration”


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Benefit of Modeling Reflection Using Clustering

“Overall it should be recommended that DrivebyLoc be used with modeling beamwidth”


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Impact of GPS Accuracy


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Impact of Car Speed


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Conclusions

• Contributions– Completely passive– Realization that signal reflections can cause

significant localization errors• Development of clustering method to solve this

problem

• Enables accurate WiFi map of urban APs with minimum effort

• What about 3D?


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Thank You

april 20, 2008emmett nicholas ece 256 1 drive-by localization of roadside wifi networks anand prabhu...

Documents

emmett nicholas ece

directional beam beam

clustering slide

beamwidth slide

real ap slide

minimized slide

roadside aps

omnidirectional beam