April 20, 2008Emmett Nicholas ECE 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

April 20, 2008Emmett Nicholas ECE 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)

April 20, 2008Emmett Nicholas ECE 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

April 20, 2008Emmett Nicholas ECE 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

April 20, 2008Emmett Nicholas ECE 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:

April 20, 2008Emmett Nicholas ECE Experimental Scenarios Parking lot (2 APs) Apartment complex (17 APs) Office building (2 APs)

April 20, 2008Emmett Nicholas ECE 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

April 20, 2008Emmett Nicholas ECE 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

April 20, 2008Emmett Nicholas ECE Non-zero Beamwidth

April 20, 2008Emmett Nicholas ECE Reflections

April 20, 2008Emmett Nicholas ECE Understanding Reflections Parking lotOffice building

April 20, 2008Emmett Nicholas ECE Understanding Reflections Parking lotOffice building Interesting observation: CLUSTERING

April 20, 2008Emmett Nicholas ECE Understanding Reflections Parking lotOffice building New approach… 1.Use the k-means algorithm to group the measurement points into k clusters 2.Determine which one these k images is the real AP

April 20, 2008Emmett Nicholas ECE Modeling Reflections by k-Means Clustering For any given value of k, assume L 1,…,L k are the k locations of the AP (real and the images) –L i ’s are chosen randomly within “feasible region” –Each measurement mapped to some L i that provides minimum angular error

April 20, 2008Emmett Nicholas ECE 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.C i ’s become new L i ’s 3.Go to Step 1.

April 20, 2008Emmett Nicholas ECE 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 1 st and the next is ranked 2 nd and so on.” –Choose image with least sum of ranks

April 20, 2008Emmett Nicholas ECE 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.

April 20, 2008Emmett Nicholas ECE Performance Evaluation

April 20, 2008Emmett Nicholas ECE Benefit of Using Directional Antennas and AOA “DrivebyLoc is about an order of magnitude better than trilateration”

April 20, 2008Emmett Nicholas ECE Benefit of Modeling Reflection Using Clustering “Overall it should be recommended that DrivebyLoc be used with modeling beamwidth”

April 20, 2008Emmett Nicholas ECE Impact of GPS Accuracy

April 20, 2008Emmett Nicholas ECE Impact of Car Speed

April 20, 2008Emmett Nicholas ECE 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?

April 20, 2008Emmett Nicholas ECE Thank You