Overview of Radiolocation in CDMA Cellular Systems

1. Overview of Radiolocation in CDMA Cellular Systems James J. Caffery, Jr. Gordon L. Stuber Georgia Institute of Technology

2. Radiolocation • Measures parameters of radio signals that travel between an MS and a set of fixed transceivers. • Can be implemented in two ways: • The MS calculates its own position eg. in GPS • The BSs relay signals to a central site for processing. • Advantage – no modifications required in the MS handset.

3. Applications of Wireless Location • E-911 • Location-sensitive billing • Fraud detection • Cellular system design and resource management • Fleet management and intelligent transportation systems (ITS)

4. Accuracy Requirements • Application-specific • E-911: Location accuracy to within 125m • Fleet management and ITS services: Knowledge of general vicinity will do. • Methods: • Cellular network as proximity location system • Signal strength, AOA and TOA • Monitoring handoffs

5. Radiolocation Methods • Based on signal strength, angle of arrival (AOA), or time of arrival (TOA) measurements, or their combinations. • This article only considers the case where the signal measurements are made at the BSs.

6. Signal Strength • Uses a known mathematical model describing the path loss attenuation with distance. • The MS lies on a circle centered at the BS. Location of the MS can be determined by using multiple BSs. • Problems: Errors due to multipath and shadowing

7. Angle of Arrival (AOA) • Estimate the MS location by measuring the AOAs of a signal from an MS at several BSs. • Problems: Accuracy diminishes with increasing distance between the MS and BS. • Useful for macrocells, may be impractical for microcells.

8. The scattering model for propagation in macrocells. • The MS is a distance d from the BS and is surrounded by a scattering ring of radius a.

9. Time-Based Systems • TOA - Estimate the TOAs of a signal transmitted by the MS and received at multiple BSs. The MS’s position is given by the intersection of the circles. • TDOA – Estimate the time differences of arrival (TDOAs) of a signal received at multiple pairs of BSs. The MS’s position is given by the intersection of the hyperbolae.

10. Time-Based Systems (contd.) • Problems: • Require high-resolution timing measurements • LOS propagation conditions are necessary to to achieve high accuracy • Requires synchronization at three or more BSs

11. TDOA

12. Performance of TOA vs. AOA • Performance of the TOA and AOA location methods for a varying number of BSs and scatterer radii in macrocells. • Solid lines denote results for TOA, dashed lines results for AOA.

13. Sources of Error • Multipath • Effects of reflected signals associated with signal propagation • NLOS • Signal arriving from at the BS from the MS is reflected or diffracted, and takes a longer path than the direct path. • Multiple Access Interference • Co-channel interference

14. The near-far effect

15. Conclusion • Radiolocation fits naturally with wireless networks. • It would allow location determination with existing handsets without modification, but might need considerable modification on the network side. • TOA or TDOA most appropriate for high accuracy; signal strength measurements for low accuracy. • Need for techniques to mitigate multipath and NLOS propagation and multiple access interference.

16. The End.Thank you!