Overview of Radiolocation in Overview of Radiolocation in CDMA Cellular SystemsCDMA Cellular Systems

James J. Caffery, Jr.

Gordon L. Stuber

Georgia Institute of Technology

RadiolocationRadiolocation

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 Applications of Wireless LocationLocation

E-911Location-sensitive billingFraud detectionCellular system design and resource

managementFleet management and intelligent

transportation systems (ITS)

Accuracy RequirementsAccuracy 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 MethodsRadiolocation 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 StrengthSignal 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)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 SystemsTime-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.)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

TDOATDOA

Performance of TOA vs. AOAPerformance 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 ErrorSources of ErrorMultipath

– 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 effectThe near-far effect

ConclusionConclusion 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.The End.

Thank you!Thank you!