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
– 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 …
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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
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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.
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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
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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
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Localization Algorithm
HartFi defaults to Wi-Fi localization system if there is
no WH interface. Support incremental deployment.
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Reducing Estimation Error Propagation
• 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.
lb
ld
u
lc
Devices a, b, c ,d have
locations. Hence there are 4
combinations and 4
confidences values for u.
la
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Reducing Estimation Error Propagation
– Confidence of a trilateration:
Product of confidences of references
Error of the distance measurement
– 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.
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HartFi Hardware and Testbed Setup
• Asus netbook (Atheroes AR5007EG
wireless network adapter)
• WirelessHART interface
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Energy Consumption Measurements
• 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.
Netbook 1
Both Off
WH On
Wi-Fi On
Saving
92
82
58
70.588%
Netbook 2 115 interface
107 leads to
8475.23% power
87%
Using WirelessHART
saving
Netbook 3
123
117
88
80%
Netbook 4
91
81
61
63.33%
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Inferring Distance From RSS (Cont.)
Comparable coverage
More accurate RSS model
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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
– 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.
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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