Ranging and positioning in UWB adhoc networks Problem

Ranging and positioning in UWB adhoc networks Problem definition Luca De Nardis 10/08/2003 NEu. Wb Internal Meeting

Summary • • Positioning in UWB networks Ranging techniques Distributed positioning algorithms Open issues 10/08/2003 NEu. Wb Internal Meeting 2

Positioning in UWB networks • 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 10/08/2003 NEu. Wb Internal Meeting 3

Angle of Arrival - Ranging techniques - • Based on directional antennas (e. g. : linear arrays): Incident plane wave direction Antenna elements • 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 10/08/2003 NEu. Wb Internal Meeting 4

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 size should be reduced as much as possible BUT the number of elements required to obtain a given accuracy strongly depends on the radio environment 10/08/2003 NEu. Wb Internal Meeting 5

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 10/08/2003 NEu. Wb Internal Meeting 6

Time of Arrival - Ranging techniques - • Based on the evaluation of roundtrip time between transmitter and receiver: • Requires coordination between transmitter and receiver 10/08/2003 NEu. Wb Internal Meeting 7

Time of Arrival - Ranging techniques - • Issues: – The estimation is influenced by clock relative drifts between the two terminals: Example: Dtsync. A- Dtsync. B= 5 nsec Dd. AB= 1. 5 m – A small time value (Dtprop) is obtained as the difference of two much larger time values (t 1 -t 0, Dtsync) the result is influenced by the clock accuracy in B 10/08/2003 NEu. Wb Internal Meeting 8

Time of Arrival - Ranging techniques - • Range measurements from three anchor nodes are required to determine a position in a 2 -d environment: 10/08/2003 NEu. Wb Internal Meeting 9

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 10/08/2003 NEu. Wb Internal Meeting 10

A GPS-enabled protocol - Distributed positioning algorithms - • The Picoradio approach: – Full connectivity: triangulation – Partial connectivity: cooperative ranging, divided in two phases: • • 10/08/2003 Start-up Maintenance NEu. Wb Internal Meeting 11

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 10/08/2003 NEu. Wb Internal Meeting 12

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: • • 10/08/2003 MAC address Speed (the lower, the better) Reliability (Available power) …. NEu. Wb Internal Meeting 13

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 10/08/2003 NEu. Wb Internal Meeting 14

Effect on routing - Open issues - 1. Localization information is not available at network set-up Routing protocol must be independent from localization information 2. Periodic update packets will be requested to maintain localization information up to date Routing and localization protocol should be coordinated in broadcasting information 10/08/2003 NEu. Wb Internal Meeting 15

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. 1996 Pages: 33 -41. • “Location in distributed ad-hoc wireless sensor networks, ” Savarese, C. ; Rabaey, J. M. ; Beutel, J. , Acoustics, Speech, and Signal Processing, 2001. Proceedings. 2001 IEEE International Conference on , 2001 Pages: 2037 -2040 vol. 4 • “GPS-free positioning in mobile Ad-Hoc networks, ” S. Capkun, M. Hamdiand J. P. Hubaux, Hawaii International Conference On System Sciences, HICSS 34 January 3 -6, 2001 Outrigger Wailea Resort. 10/08/2003 NEu. Wb Internal Meeting 16