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

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.

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

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

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.

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

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.

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.

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.

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

TDOA

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.

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

The near-far effect

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.

The End. Thank you!