Overview of Radiolocation in CDMA Cellular Systems

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Overview of Radiolocation in CDMA Cellular Systems

• James J. Caffery, Jr.
• Gordon L. Stuber
• Georgia Institute of Technology

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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.

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Applications of Wireless Location

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

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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

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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.

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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

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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.

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• 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.

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Time-Based Systems

• TOA - Estimate the TOAs of a signal transmitted
  by the MS and received at multiple BSs. The MSs
  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 MSs position is given by the
  intersection of the hyperbolae.

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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

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TDOA
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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.

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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

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The near-far effect
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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.

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The End.Thank you!