Luca De Nardis Ranging and positioning in UWB ad- hoc networks Problem definition

10/08/2003NEuWb Internal Meeting2 Summary Positioning in UWB networks Ranging techniques Distributed positioning algorithms Open issues

10/08/2003NEuWb Internal Meeting3 Key assumptions: –No GPS available position must be obtained by lower layer information (ranging) –No guaranteed anchor nodes position must be built from scratch (but anchor nodes should be exploited if present) –Mobility frequent updates will be required, depending on mobility degree and requested accuracy Positioning in UWB networks

10/08/2003NEuWb Internal Meeting4 Angle of Arrival - Ranging techniques - Based on directional antennas (e.g.: linear arrays): Antenna elements Incident plane wave direction Two main measurement techniques: –Phase interferometry: the angle is estimated by phase differences in the signal received by antenna elements –Beamforming: the angle of arrival is estimated by moving the main beam of the array over the angular field of interest

10/08/2003NEuWb Internal Meeting5 Angle of Arrival - Ranging techniques - Drawbacks: –Highly coherent receiver (all channels must have the same effect on the received signal) –The cost of the receiver increases as the array size increases the number of elements required to obtain a given accuracy strongly depends on the radio environment The size should be reduced as much as possible BUT

10/08/2003NEuWb Internal Meeting6 Angle of Arrival - Ranging techniques - Angle measurements from two anchor nodes are required to determine a position in a 2-d environment: No distance estimation is required

10/08/2003NEuWb Internal Meeting7 Time of Arrival - Ranging techniques - Based on the evaluation of roundtrip time between transmitter and receiver: Requires coordination between transmitter and receiver

10/08/2003NEuWb Internal Meeting8 Time of Arrival - Ranging techniques - Issues: –The estimation is influenced by clock relative drifts between the two terminals: –A small time value (  t prop ) is obtained as the difference of two much larger time values (t 1 -t 0,  t sync ) the result is influenced by the clock accuracy in B Example:  t syncA -  t syncB = 5 nsec  d AB = 1.5 m

10/08/2003NEuWb Internal Meeting9 Time of Arrival - Ranging techniques - Range measurements from three anchor nodes are required to determine a position in a 2-d environment:

10/08/2003NEuWb Internal Meeting10 Received Signal Strength Indicator - Ranging techniques - Alternative solution to Time of Arrival in order to estimate distance between terminals +Lower requirements in terms of synchronization and clock precision -Requires accurate estimation of channel behavior -Distance estimation extremely sensible to propagation fluctuations and moving obstacles

10/08/2003NEuWb Internal Meeting11 A GPS-enabled protocol - Distributed positioning algorithms - The Picoradio approach: –Full connectivity: triangulation –Partial connectivity: cooperative ranging, divided in two phases: Start-up Maintenance

10/08/2003NEuWb Internal Meeting12 A GPS-enabled protocol - Distributed positioning algorithms - Start-up: two alternatives 1.Global Topology Discovery: Each terminal (including anchor nodes) performs its own triangulation at local level considering itself in position (0,0) and broadcasts the results through the network When a terminal receives localization information generated by an anchor node it rotates its own coordinate system 2.TERRAIN: Only anchor nodes start broadcasting localization information Not GPS-enabled terminals wait for localization information from 4 different anchor nodes. Maintenance: periodical triangulation to manage with terminal mobility

10/08/2003NEuWb Internal Meeting13 A GPS-free protocol - Distributed positioning algorithms - Self-Positioning Algorithm: –No anchor nodes –Each terminal starts its own topology discovery –A criterion must be given to establish which coordinate system will be adopted in the network: MAC address Speed (the lower, the better) Reliability (Available power) ….

10/08/2003NEuWb Internal Meeting14 Algorithm definition - Open issues - Algorithm convergence is a serious issue with or without anchor nodes In real world we must take into account –Ranging errors –Communication failures –Mobility Robustness is a key point in algorithm definition and testing phases

10/08/2003NEuWb Internal Meeting15 Effect on routing - Open issues - 1.Localization information is not available at network set-up 2.Periodic update packets will be requested to maintain localization information up to date Routing and localization protocol should be coordinated in broadcasting information Routing protocol must be independent from localization information

10/08/2003NEuWb Internal Meeting16 References “Position location using wireless communications on highways of the future,” Rappaport, T.S.; Reed, J.H.; Woerner, B.D. IEEE Communications Magazine, Volume: 34 Issue: 10, Oct Pages: “Location in distributed ad-hoc wireless sensor networks,” Savarese, C.; Rabaey, J.M.; Beutel, J., Acoustics, Speech, and Signal Processing, Proceedings IEEE International Conference on, 2001 Pages: vol.4 “GPS-free positioning in mobile Ad-Hoc networks,” S.Capkun, M.Hamdiand and J.P.Hubaux, Hawaii International Conference On System Sciences, HICSS- 34 January 3-6, 2001 Outrigger Wailea Resort.