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HartFi : An Energy-Efficient Localization System

HartFi : An Energy-Efficient Localization System. Song Han, Tianji Li, Chen Qian , Douglas Leith , Aloysius K. Mok , Simon S. Lam. Introduction. Location-based applications (LBA) are becoming increasingly popular

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HartFi : An Energy-Efficient Localization System

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  1. HartFi: An Energy-Efficient Localization System Song Han, Tianji Li, Chen Qian, Douglas Leith, Aloysius K. Mok, Simon S. Lam

  2. Introduction • Location-based applications (LBA) are becoming increasingly popular • Foursquare, Facebook Places, Gowalla, MicroBlog, TrafficSense, Pothole Patrol, PeopleNet … • Most LBAs are running on mobile devices. • Many LBAs benefit from having Loc info available all the time. • Tradeoff among existing localization techniques • Energy efficiency, estimation accuracy, coverage …

  3. Introduction (Cont.) • Pros and Cons of existing localization techniques • GPS: • Most accurate localization • Energy hungry, does not work in indoor environment • Cellular network: • Less energy consumption • Coverage in both indoor/outdoor environment • Estimation error is large • Wi-Fi: • Estimation accuracy and energy efficiency lie between those of GPS and cellular network

  4. Motivation • Use low-power interface to propagate location information • Features of 802.15.4-based interface: • Radio consumes up to two orders of magnitude less energy than Wi-Fi . (1mW for 802.15.4-based radio and 32-200mW for Wi-Fi). • Data rate (250 kbps) is sufficient for location-based applications. • Tree or mesh structure can help achieve large coverage.

  5. Motivation • HartFi: An energy-efficient localization system • Collaborative: • Devices have Wi-Fi and WirelessHART interfaces combined together • Energy efficient: • Only a few devices acquire location info through Wi-Fi directly • Using WirelessHART interface to propagate <loc, rss> info • Accurate: • A novel approach for reducing error propagation • Coverage: • A multiple-hop mesh network is formed among WirelessHART interfaces

  6. HartFi Infrastructure • A device in HartFi system has multiple interfaces • GPS, Wi-Fi, Bluetooth, WirelessHART… • WH interfaces form a mesh network • All other interfaces work as usual • A unique coordinator in the system • Manages the network and schedules all the transmissions • As an option, reports location information to the location server for other applications

  7. Localization Algorithm HartFi defaults to Wi-Fi localization system if there is no WH interface. Support incremental deployment.

  8. Reducing Estimation Error Propagation lb ld Devices a, b, c ,d have locations. Hence there are 4 combinations and 4 confidences values for u. u lc la • Using confidence-based iterative localization • Run localization algorithms for multiple rounds • In each round, each node, say u, computes a confidence value for every three devices that have locations.

  9. Reducing Estimation Error Propagation Product of confidences of references Error of the distance measurement • Confidence of a trilateration: • Node u uses the three devices with max confidence value for trilateration. Then it sets that value as the confidence of its estimated location. • Confidence value <= 1. An AP’s confidence is set to 1. If device u computes its location using node v’s location, Cu<Cv.

  10. HartFi Hardware and Testbed Setup Asus netbook (Atheroes AR5007EG wireless network adapter) WirelessHART interface

  11. Energy Consumption Measurements Using WirelessHART interface leads to 75.23% power saving • Experiment Setup • Connect the netbook to an AP through Wi-Fi interface. Ping the AP every 1-second. • Connect the netbook to a WH coordinator through WH interface. Send “hello” messages with the same size as the ping packet. • Measure the time (in minutes) taken to consume half of the full battery capacity.

  12. Inferring Distance From RSS (Cont.) Comparable coverage More accurate RSS model

  13. Simulation Results • 100 nodes in 100m x 100m square area and 12 Wi-Fi APs • Pure WiFi localization also requires node sharing location information, because not every node can find 3 nearby APs. • Iterative localization of HartFi runs 5 rounds. • Errors of distance measurements are based on experiment traces

  14. A unit of energy cost is the cost to send a WirelessHART msg. • When the number of nodes increases, less portion of nodes requires Wi-Fi localization. Hence the average cost decreases.

  15. Future Work • Enhance WH network with Ad-hoc mode • How to select the new coordinator in face of dynamics? • How to make communication/configuration consistent? • How to keep synchronization when the coordinator changes? • Simulations in large-scale HartFi system • Testbed experiments in outdoor/indoor environment • Comparison of localization accuracy between Wi-Fi and HartFi system • Performance on HartFi incremental deployment

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